U.S. patent application number 12/848392 was filed with the patent office on 2010-12-09 for nanostructured titanium oxide material and its synthesis procedure.
This patent application is currently assigned to INSTITUTO MEXICANO DEL PETROLEO. Invention is credited to Fernando Alvarez Ramirez, Carlos Angeles Chavez, Maria Antonia Cortes Jacome, Gerardo Ferrat Torres, Luis Francisco Flores Ortiz, Esteban Lopez Salinas, Marcelo Lozada y Cassou, Yosadara Ruiz Morales, Jose Antonio Toledo Antonio.
Application Number | 20100311576 12/848392 |
Document ID | / |
Family ID | 36145719 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100311576 |
Kind Code |
A1 |
Toledo Antonio; Jose Antonio ;
et al. |
December 9, 2010 |
Nanostructured titanium oxide material and its synthesis
procedure
Abstract
Nanomaterials of the JT phase of the titanium oxide TiO.sub.2-x,
where 0.ltoreq.x.ltoreq.1 having as a building block a crystalline
structure with an orthorhombic symmetry and described by at least
one of the space groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb. These
nanomaterials are in the form of nanofibers, nanowires, nanorods,
nanoscrolls and/or nanotubes. The nanomaterials are obtained from a
hydrogen titanate and/or a mixed sodium and hydrogen titanate
precursor compound that is isostructural to the JT crystalline
structure. The titanates are the hydrogenated, the protonated, the
hydrated and/or the alkalinized phases of the JT crystalline phase
that are obtained from titanium compounds such as titanium oxide
with an anatase crystalline structure, amorphous titanium oxide,
and titanium oxide with a rutile crystalline structure, and/or
directly from the rutile mineral and/or from ilmenite. The
titanates are submitted to dynamic thermal treatment in an inert,
oxidizing or reducing atmosphere to produce the JT phase of the
TiO.sub.2-x, where 0.ltoreq.x.ltoreq.1 with an orthorhombic
structure.
Inventors: |
Toledo Antonio; Jose Antonio;
(Mexico City, MX) ; Angeles Chavez; Carlos;
(Mexico City, MX) ; Cortes Jacome; Maria Antonia;
(Mexico City, MX) ; Alvarez Ramirez; Fernando;
(Mexico City, MX) ; Ruiz Morales; Yosadara;
(Mexico City, MX) ; Ferrat Torres; Gerardo;
(Mexico City, MX) ; Flores Ortiz; Luis Francisco;
(Mexico City, MX) ; Lopez Salinas; Esteban;
(Mexico City, MX) ; Lozada y Cassou; Marcelo;
(Mexico City, MX) |
Correspondence
Address: |
ROYLANCE, ABRAMS, BERDO & GOODMAN, L.L.P.
1300 19TH STREET, N.W., SUITE 600
WASHINGTON,
DC
20036
US
|
Assignee: |
INSTITUTO MEXICANO DEL
PETROLEO
Mexico City
MX
|
Family ID: |
36145719 |
Appl. No.: |
12/848392 |
Filed: |
August 2, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12623993 |
Nov 23, 2009 |
7799313 |
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12848392 |
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11121178 |
May 4, 2005 |
7645439 |
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12623993 |
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PCT/MX2004/000035 |
May 26, 2004 |
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11121178 |
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PCT/MX2003/000081 |
Oct 10, 2003 |
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PCT/MX2004/000035 |
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60646973 |
Jan 27, 2005 |
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Current U.S.
Class: |
502/344 ;
423/610; 502/400; 977/773 |
Current CPC
Class: |
B82Y 30/00 20130101;
C01G 23/047 20130101; Y10S 977/811 20130101; Y10T 428/256 20150115;
Y10T 428/2982 20150115; C01P 2002/76 20130101; C01P 2002/77
20130101; C01P 2004/04 20130101; C01G 23/043 20130101; C01P 2004/03
20130101; C01P 2004/16 20130101; Y10T 428/298 20150115; C01P
2004/13 20130101; C01G 23/04 20130101; Y10T 428/2975 20150115 |
Class at
Publication: |
502/344 ;
502/400; 423/610; 977/773 |
International
Class: |
B01J 23/04 20060101
B01J023/04; B01J 20/04 20060101 B01J020/04; C01G 23/047 20060101
C01G023/047; B01J 21/06 20060101 B01J021/06 |
Foreign Application Data
Date |
Code |
Application Number |
May 4, 2004 |
MX |
PA/A/2004/004265 |
Claims
1. Nanomaterial of hydrogen titanate and/or mixed sodium and
hydrogen titanate, having the formula HTiO.sub.2 and/or
Na.sub.xH.sub.1-xTiO.sub.2, wherein x is from 0 to less than 1, the
hydrogen and sodium atoms are in the interlayer regions of the
orthorhombic structure and said nanomaterial has nanofibrilar
and/or nanotubular morphology constituted of piled structural
layers that are folded or rolled inwards into themselves, or formed
by overlapped semitubes.
2. The nanomaterial of claim 1, wherein said nanomaterial has an
orthorhombic structure, which is isostructural with TiO.sub.2-x,
where 0.ltoreq.x.ltoreq.1, and whose unit cell is described by at
least one of the space groups 59 Pmmn, 63 Amma, 71 Immm or 63
Bmmb.
3. The nanomaterial of claim 1, wherein the cell parameters of the
crystalline lattice which is orthorhombic are: a from 0.263 nm to
0.331 nm; b from 0.332 nm to 0.448 nm, and c from 0.635 nm to 0.902
nm, for the case of the 59 Pmmn space group, and from 1.368 nm to
1.905, for the case of the 63 Amma, 71 Immm, and 63 Bmmb space
groups; with .alpha.=.beta.=.gamma.=90.degree.
4. The nanomaterial of claim 1, wherein said nanomaterial has a
disordered mesoporosity with an average pore diameter between 3 and
25 nm and a specific area between 100 and 600 m.sup.2/g.
5. The nanomaterial of claim 1, wherein said nanomaterial has a
nanofibrilar and/or a nanotubular morphology with dimensions of 3
to 50 nm in diameter and lengths from 0.1 to 100 .mu.m.
6. The nanomaterial of claim 1, wherein said nanomaterial has a
nanotubular morphology constituted by 1 to 50 structural layers
with an inter-layer spacing in the range of 0.635 nm to 0.902 nm
for the 59 Pmmn space group, and from 0.684 nm to 0.953 for the 63
Amma, 71 Immm, and 63 Bmmb space groups.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of Ser. No.
12/623,993, filed Nov. 23, 2009, which, in turn, is a divisional
application of Ser. No. 11/121,178, filed May 4, 2005, now U.S.
Pat. No. 7,645,439. application Ser. No. 11/121,178 is a
continuation-in-part of International Application No.
PCT/MX2004/000035, with an international filing date of May 26,
2004, and International Application No. PCT/MX2003/000081, with an
international filing date of Oct. 10, 2003. This application also
claims the benefit under 35 U.S.C. .sctn.119(a) of Mexican Patent
Application No. PA/a/2004/004265, filed May 4, 2004, and under 35
U.S.C. .sctn.119(e) of provisional application Ser. No. 60/646,973,
filed Jan. 27, 2005. The disclosures of all of the foregoing
applications are incorporated herein by reference in their
entirety.
[0002] Reference is made to application Ser. No. 11/121,179, filed
May 4, 2005, of Jose Antonio Toledo Antonio et al., entitled
"Selective Adsorbent Material And Its Use", now U.S. Pat. No.
7,416,655, which issued on Aug. 26, 2008.
FIELD OF THE INVENTION
[0003] The present invention relates to nanomaterials of titanium
oxide (TiO.sub.2-x, where 0.ltoreq.x.ltoreq.1), which have as a
building block a crystalline structure with an orthorhombic
symmetry. The new crystalline structure is the basic unit of
construction of nanomaterials which are nanofibers, nanowires,
nanorods, nanoscrolls and/or nanotubes. These nanomaterials are
obtained from a precursor that is isostructural to the new
crystalline structure. The precursor is composed of hydrogen
titanate and/or a mixed sodium and hydrogen titanate. These
titanates are the hydrogenated, protonated, hydrated and/or the
alkalinized phases of the new crystalline structure. In addition,
this invention also relates to the procedure of synthesis of the
nanomaterials.
BACKGROUND OF THE INVENTION
[0004] Researchers in the catalysis and materials fields have
focused considerable efforts on the design of new porous materials,
either synthetic or natural with enhanced textural properties,
through innovating synthesis procedures as the molecular molding.
Generally the porous structure of such solids is formed during its
crystallization or during further treatments.
[0005] The porous materials are classified, depending on their
predominating pore size, as: 1) microporous, with pore sizes
<1.0 nm; 2) mesoporous, with pore sizes between 1.0 and 50.0 nm,
and 3) macroporous, with pore sizes surpassing 50.0 nm. Of all of
them, the macroporous solids have a limited use as adsorbers or
catalysts due to the fact that they generally present a low surface
area and their large pores are not uniform. On the other hand, the
microporous and mesoporous solids are widely used in the
technologies of adsorption, separation and catalysis, particularly
for the processing and refining of oil. For such applications,
nowadays, there is an increase in the demand of new materials with
a well defined and homogeneous pore distribution, thermally stable,
with a high specific area and large pore volumes; in order to make
more efficient the physical and/or chemical processes in which
these materials are used.
[0006] The porous materials can have an amorphous or
nanocrystalline structure. The amorphous materials, such as silica
gel or alumina gel, do not have any crystallographic order, while
nanocrystalline solids such as transition alumina, gamma or eta,
present a partially ordered structured. Generally, these two kinds
of materials display a very wide pore distribution, which limits
their effectiveness as catalysts, adsorbents and/or ionic exchange
systems. The wide pore distribution limits mainly the use of these
materials in oil refining processes.
[0007] Zeolites and the molecular sieves are a clear example of
uniformity in the pore sizes that have to be rigorously
established. However, the pore size distribution is limited to the
microporous region, due to the fact that the pores are formed from
the cavities and/or channels that form the structure itself;
therefore, molecules of big dimensions cannot be processed in this
type of materials. On the other hand, these materials are generally
synthesized under hydrothermal conditions in the presence of a
porogen agent that engineers the porous structure.
[0008] The need to expand the uniformity and the homogeneity of the
pore sizes from the microporous region to the mesoporous region,
thus allowing the adsorption and processing of bigger molecules,
has led to the search of new organic agents capable of engineering
new structures. This has given origin to molecular sieves with
bigger pore size as the aluminophosphates, galliophosphates, etc.
(Nature, vol. 352, 320-323 (1991); J. Chem. Soc. Chem. Commun.,
875-876 (1991)). However, these structures are not thermally
stable.
[0009] With the discovery of the mesoporous silicates and
aluminosilicates in 1992 (U.S. Pat. Nos. 5,098,684 and 5,102,643),
a new stage in the development of ordered mesoporous materials
started. This type of materials, called M41S, have a uniform pore
size, which is adjustable to an interval between 1.3 and 10.0 nm.
Such materials display a pore wall with a thickness between 0.8 and
1.2 nm and a crystal size over 50.0 nm. On the other hand,
depending on the general conditions of synthesis, in particular on
the concentration of the organic porogen agent, the M41S materials
can have an hexagonal morphology (MCM-41), a cubic morphology
(MCM-48), or a laminar structure (J. Am, Chem. Soc., vol. 114,
834-843 (1992)). This implies a formation mechanism based on strong
electrostatic interactions and/or the ionic pairing between the
oligomer silicate precursor and the structure engineering agent,
making the removal of the later difficult.
[0010] The discovery of the carbon fullerene structure (C.sub.60)
during the 80s, which consists of a hollow sphere whose wall is
made up of sixty carbon atoms (Nature, vol. 318, 162-163 (1985)),
led to a new materials era of great discoveries as, for example,
the carbon nanotubes (Nature, vol. 354, 56-58 (1991)). These
structures and/or the nanotubular morphologies present interesting
physical and chemical properties, making them suitable for the
construction of nanoelectronic innovating devices, among other
applications. Due to this, the synthesis of nanomaterials of carbon
and inorganic materials has boomed in the past few years. In 1992
the first nanotubes and/or structures fullerene type of MoS.sub.2
and WS.sub.2 (Nature, vol. 360, 444-446 (1992)), were obtained.
Since then a great variety of nanomaterials includes: inorganic
oxides such as: VO.sub.2, ZrO.sub.2, TiO.sub.2, SiO.sub.2,
Al.sub.2O.sub.3, ZnO and TeO.sub.2, sulphides, selenides,
telurides, nitrates and transition metal carbides; among others
(Dalton Trans., 1-24 (2003)).
[0011] On the other hand, a series of studies in confined fluids
(M. Lozada-Cassou et al. J. Chem. Phys., vol. 80, 3344-3349 (1984);
J. Chem. Phys., vol. 92, 1194-1210 (1990); J. Chem. Phys., vol. 98,
1436-1450 (1993); Mol. Phys., vol. 86, 759-768 (1995); Phys., Rev.
E., vol. 53, 522-539 (1996); Phys. Rev. Letts., vol. 77, 4019-4022
(1996); Phys. Rev. E., vol. 56, 2958-2965 (1997); Phys. Rev.
Letts., vol. 79, 3656-3659 (1997)), showed that the confinement and
curvature at nano-scale produces electric fields and molecular
strengths of outstanding intensity. These studies show, for
example, that in nano-confinement a separation of charge in the
ionic fluid can occur (Phys. Rev. Letts., vol. 79 656-659 (1997)),
which implies confinement pressures in the order of 1,250 atm and
intermolecular repulsion forces of 5.times.10.sup.-9 N. This result
highlights the importance of the confinement for the molecular
separation and it oriented the present invention towards the search
of tubular structures at nanometric scale and to the development of
new materials with enhanced catalytic properties, semiconductive
properties, etc.
[0012] The nanotubes are materials that are applied, for example,
in processes involving adsorption phenomena, as they increase the
contact area by exposing the internal surface, the external
surface, the surface in the vertex and the surface in the
interlayer regions that compose the walls. This together with the
increase of the intensity in the force fields, due to curvature and
confinement of the nanotubes, enhance the catalytic activity of
catalysts or of active phase materials supported on nanotubes.
According to the porous materials classification, the nanotubes
present mesopores which are homogeneous with a size between 1 to 50
nm and with a high pore volume. These characteristics make the
nanotubes potentially useful as catalytic supports or as
catalysts.
[0013] In the past it has been possible to synthesize nanotubes
with walls composed of zirconium oxide, alumina, titania with
anatase structure, and transition metal sulfides among others, by
means of methods involving the addition of a structure engineering
agent, consisting of a cationic, anionic and/or neutral tensoactive
agent. However, the tensoactive elimination through calcinations,
leads in most cases to the collapse of the nanotubular
structure.
[0014] Other procedures in the nanotubes synthesis consist in the
application of porous membranes, organic or inorganic, to guide the
nanotube formation; however, they are generally applied for the
case of materials whose structure is compact or tridimensional
(3D). The materials with bidimensional structures (2D), like plates
and/or sheets, can form unidimensional materials (1D) of the
nanotube type and/or nanofibers, by the direct bending and/or
rolling of its structure, due to temperature effect, to pressure or
to the application of an electric potential, etc.
[0015] Titanium oxide is commonly presented as a tridimensional
structure material (3D) and it is basically used as a semiconductor
material in the construction of electronic and optoelectronic
devices, in the manufacturing of pigments and coatings, as catalyst
and/or catalyst support in several processes, as photocatalyst in
the degradation of organic compounds during environmental
protection processes, as photosensitive material in the
construction of fuel cells and solar cells, etc.
[0016] Titanium dioxide is known to exist in three crystalline
phases, anatase, rutile and brookite, as well as an amorphous
phase. There are other phases but these ones are the most common.
The anatase and rutile phases have a tetragonal crystal lattice,
and the brookite phase has an orthorhombic crystal lattice or
structure. This information is well known in the area. The anatase
and rutile phases which have a tetragonal crystal lattice are
different even though they both have a tetragonal crystal lattice.
The differences stem from the position of the atoms, the
surroundings of the atoms, the lattice parameters, and the space
group inside the tetragonal crystal lattice, and because these
parameters are different these two phases are differentiated with
different names (anatase and rutile). Each phase presents different
properties and among all of the phases anatase is the one that has
most applications, due to the fact that it can be obtained easily
through a conventional chloride or sulfide process.
[0017] On the other hand, nanotubes and/or titanium oxide
nanofibers with the anatase structure have been obtained, improving
in this way the textural properties of the titanium oxide. In this
direction, published U.S. Application No. 2004/0265587 describes a
procedure to obtain tubular TiO.sub.2 particles with the anatase
structure, with an external diameter of 5 to 40 nm, with lengths of
50 to 1,000 nm and a specific area of 450 m.sup.2/g if only one
hydrothermal treatment is carried out and a specific area in the
range of 400 m.sup.2/g to 500 m.sup.2/g, if two hydrothermal
treatments are carried out; thus in general the synthesis requires
two stages of hydrothermal treatment which involves an alkaline
metal and an organic alkaline base. The inventors apply such
tubular particles as photocatalysts and/or materials for the
construction of photoelectric cells showing good results.
[0018] U.S. Pat. No. 6,537,517 refers to a process for titanium
oxide production with tubular morphology and anatase structure,
with or without the presence of silicon oxide, by means of a
hydrothermic treatment involving an alkaline metal hydroxide. The
TiO.sub.2 nanotubes with anatase structure present specific surface
areas between 200 and 500 m.sup.2/g. It has been published in the
literature (Ma, R.; Bando, Y.; Sasaki, T. Chemical Physics Letters,
2003, 380, 577-582) that it is possible that the so claimed anatase
nanotubes in the aforementioned patent might have a
lepidocrocite-type structure instead of the so claimed anatase
structure. The lepidocrocite structure is defined for an iron oxide
compound (iron (III) oxide hydroxide, also known as
.gamma.-(FeOOH)). A lepidocrocite-type structure would mean that
the so claimed anatase-TiO.sub.2 nanotubes would have the same
structure as the iron (III) oxide hydroxide and the same space
group; however, the cell parameters cannot be exactly the same
neither the atoms positions because in one case the titanium atom
is involved and in the other case the iron atom is involved. Thus
it is clear that the state of the art is that the crystalline
structure, space group(s) and atomic positions in the unit cell
that composes the so claimed nanotubular anatase-TiO.sub.2
structure is not known.
[0019] A synthesized nanostructure with a phase different to
anatasa is given in the Korean laid-open patent application No.
P2003-0026268 where the synthesis of nanoparticles (balls or
spherical crystal with a nanometric size), mostly with the brookite
phase, which is known to have an orthorhombic crystal lattice, and
some rutile phase, with tetragonal crystal lattice, is reported.
The starting materials for the synthesis are TiCl1.sub.4 and
HNO.sub.3.
[0020] In the case of the U.S. Pat. No. 6,537,517 the starting
material is a powder of crystalline titanium oxide (crystalline
titania powder with the anatase or rutile phase) with an average
particle size between 2 to 100 nm, preferably from 2 to 30 nm (the
size of the crystallites that compose the particles is not
provided). The starting material is subjected to a hydrothermal
treatment, in the presence of an alkali metal hydroxide, that
comprises one step. However, as it is already mentioned in the
published U.S. Application No. 2004/0265587, the use of a titania
powder as starting material does not produce a high yield of the
titania nanotubes with anatase phase. On the contrary spherical
particles are synthesized in a higher yield than the nanotubes and
the final product presents a large residual amount of sodium that
hinders the efficiency of the nanotubes as possible catalyst. Also
in U.S. Pat. No. 6,537,517 it is mentioned that the nanotube
titania obtained from the alkali hydrothermal treatment may further
be heat-treated at from 200 to 1,200.degree. C. to improve the
crystallinity of TiO.sub.2 and to increase the catalytic activity
and that the nanotube does not collapse through this heat
treatment. It is not mentioned how the heat treatment is performed.
It is assumed that it was done as a regular well known heat
treatment which would involve a static, non-dynamic air atmosphere
by placing the product in an oven. It is claimed that the heat
treatment is expected to improve the crystallinity and activity of
the nanotubes. However, there is no table or data comparing the
properties of the nanotubes before and after such heat treatment.
Very recently it has appeared in the literature a paper entitled
"Regulation of the Physical Characteristics of Titania Nanotube
Aggregates Synthesized from Hydrothermal Treatment" (Chien-Cheng
Tsai and Hsisheng Teng, Chemistry of Materials 2004, 16, 4352-4358)
where the precursor used is a commercial TiO.sub.2 powder with a
composition 70% anatase and 30% rutile, and a primary particle size
of 21 nm (same method of synthesis as the reported in U.S. Pat. No.
6,537,517). In this paper the authors study how the stability and
pore structure (surface area) of the obtained nanotubes vary with
subsequent calcination at different temperatures (they do not give
any specifics about the calcination procedure, thus it is assumed
to be static air in an oven). The authors of the mentioned
literature paper found that the as-synthesized anatase nanotubes
remain tube-like at 400.degree. C. but these nanotubes have a sharp
surface area decrease with the calcination temperature sintering
(collapsing) at 600.degree. C. to form anatase rodlike structures.
Subsequently the rodlike structure agglomerates at 800.degree. C.,
forming anatase cylindrical particles, and at 900.degree. C. these
particles go through a phase transformation to the rutile phase.
These results contradict use of thermal treatment over the interval
(200.degree. C. to 1200.degree. C.) in U.S. Pat. No. 6,537,517 to
improve crystallinity without collapse or phase transformation at
the high temperature.
[0021] In published U.S. Application No. 2004/0265587, titanium
oxide sol is used as a starting material in which particles (no
powders) with specific average particle diameters (2 to 100 nm,
preferably 5 to 80 nm) are dispersed in water to prepare a water
dispersion sol which is used as starting material. The synthesis
method outlined in published U.S. Application No. 2004/0265587 to
obtain tubular titanium oxide particles involves preparing the
water dispersion sol (this step requires heating and many steps).
Then the water dispersion sol of titanium oxide particles is
subjected to a one step of hydrothermal treatment followed by
washing and calcining, or the water dispersion sol of titanium
oxide particles is subjected to a two hydrothermal treatments
instead of one. The first hydrothermal treatment is carried out in
the presence of an alkali metal hydroxide together with ammonium
hydroxide and/or an organic base. The presence of the ammonium
hydroxide and/or an organic base is claimed to reduce the alkali
metal impurities in the tubular titanium oxide particles. The
hydrothermal treatment is carried out at temperatures between 80 to
250.degree. C. (which is a higher temperature than the required in
the U.S. Pat. No. 6,537,517). While in the second hydrothermal
treatment the presence of a cation is required and the temperature
is the same as in the first hydrothermal treatment. The synthesis
presented in published U.S. Application No. 2004/0265587 involves
many steps, many reactants, high temperatures and possibly a second
hydrothermal treatment, and consequently the method becomes
industrially of high cost; also high temperatures are required in
both hydrothermal treatments. The method in published U.S.
Application No. 2004/0265587 also involves a heating treatment
(named as "reduction treatment") in an inert gas atmosphere, under
reduced pressure or in a reducing gas atmosphere. It is not said if
this so called reduction treatment is done in a dynamic flow or
static flow of the gas that composes the atmosphere, thus it is
assumed that it is carried out in a static way. The formula of the
final product given in published U.S. Application No. 2004/0265587
includes nitrogen and another transition metal different to
titanium in the case of preparing mixed metal compounds or M=Ti if
not mixed metal synthesis is carried out. The given formula of the
claimed synthesized tubular titanium oxide particles with anatase,
or rutile, or brookite phase is Ti.sub.aM.sub.bO.sub.xN.sub.y.
Experimental evidence on all the cell parameters to support the
indication that the tubular titanium oxide particles have anatase,
or rutile, or brookite phase is not provided in published U.S.
Application No. 2004/0265587.
SUMMARY OF THE INVENTION
[0022] The present invention relates to nanomaterials of titanium
oxide (TiO.sub.2-x, where 0.ltoreq.x.ltoreq.1), which have as a
building block a crystalline structure with an orthorhombic
symmetry described by at least one of the space groups 59 Pmmn, 63
Amma, 71 Immm or 63 Bmmb. The positions of the atoms, their
surroundings, the crystal parameters and space groups in the
crystalline materials of the present invention do not match any of
the known phases of titanium dioxide (anatase, brookite, rutile) or
the not so well known phases (beta and others). The new crystalline
structure forms the basic unit of construction of nanomaterials
which are nanofibers, nanowires, nanorods, nanoscrolls and/or
nanotubes. The nanomaterials are obtained from a precursor that is
isostructural to the new crystalline structure and is a hydrogen
titanate and/or a mixed sodium and hydrogen titanate. These
titanates are the hydrogenated, protonated, hydrated and/or the
alkalinized phases of the new crystalline structure. The new
crystalline structure is named the "JT phase" of the TiO.sub.2-x.
The term "JT phase" as used herein means a crystalline structure or
crystalline phase with an orthorhombic symmetry having the formula
TiO.sub.2-x, wherein 0.ltoreq.x.ltoreq.1, and has at least one of
the space groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb.
[0023] Also, this invention further involves the procedure of
synthesis of the nanomaterials, which have as a building block the
new JT phase. The new nanomaterials after being subjected to
dynamic thermal treatment in an inert, oxidizing or reducing
atmosphere have a stable nanotubular structure that is preserved
intact without collapse, thus maintaining their fibrilar morphology
and high specific area. It has been discovered that the hydrogen
titanate and mixed sodium and hydrogen titanate nanotubes, that are
unstable when heated higher than 300.degree. C. in air in an oven
can be stabilized by heating in a dynamic oxidizing, inert or
reducing flowing atmosphere, which transforms the titanate
nanotubes into the stable, oxygen deficient JT phase nanotubes.
[0024] The nanomaterials of the present invention are useful mainly
as a support for catalysts and/or as catalysts, as photocatalysts,
as adsorbents, as semiconductors in the construction of electronic
devices, in photoelectric cells, in pigments and cosmetics, among
other applications.
[0025] Thus, one aspect of the present invention relates to
nanomaterials of titanium oxide (TiO.sub.2-x, where
0.ltoreq.x.ltoreq.1), which have as building block a crystalline
structure with an orthorhombic symmetry described by at least one
of the space groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb. The new
crystalline structure is the basic unit of construction of
nanomaterials which are nanofibers, nanowires, nanorods,
nanoscrolls and/or nanotubes. The nanomaterials are obtained from a
precursor that is isostructural to the new crystalline structure.
The precursor is a hydrogen titanate and/or a mixed sodium and
hydrogen titanate. These titanates are the hydrogenated,
protonated, hydrated and/or the alkalinized phases of the new
crystalline structure.
[0026] Another aspect of this invention relates to a procedure for
the synthesis of the nanomaterials, which involves among other
stages: an alkaline treatment of the starting materials followed by
a thermal treatment under either reflux conditions or hydrothermal
treatment at atmospheric pressure, controlled or autogenous and
continuous stirring; then a treatment of ionic exchange is carried
out, which can be done in aqueous media with different levels of
acidity or in alcoholic media or in aqueous media, to obtain the
hydrogen titanates or the mixed sodium and hydrogen titanates.
Finally, the titanates are subjected to a dynamic thermal treatment
stage in an inert, oxidizing or reducing atmosphere to produce a
nanomaterial with an orthorhombic lattice described by at least one
of the space groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb.
[0027] A further aspect of the present invention is to provide a
procedure to synthesize the nanomaterials with the structure
TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1, and that presents
a fibrilar morphology and/or a tubular morphology depending on the
general conditions during the synthesis procedure.
[0028] A further aspect of the present invention is to provide
nanomaterials of titanium oxide (TiO.sub.2-x, where
0.ltoreq.x.ltoreq.1), which have a new crystalline structure and
are obtained from an isostructural precursor composed of hydrogen
titanate and/or a mixed sodium and hydrogen titanate which
correspond to the hydrogenated, protonated, hydrated and/or the
alkalinized phases of the new crystalline structure.
[0029] Another aspect of the present invention is to provide
nanomaterials of titanium oxide (TiO.sub.2-x, where
0.ltoreq.x.ltoreq.1), which have a new crystalline structure that
presents a fibrilar morphology or nanotubular morphology obtained
by a process which comprises an stage of thermal treatment in a
controlled dynamic atmosphere composed of air, O.sub.2, N.sub.2,
He, Ar or a mixture thereof at any concentration or in a controlled
dynamic atmosphere composed of a mixture of inert gas and H.sub.2
in a concentration of 5% or 30% by volume of H.sub.2 based on the
total volume of gas, in a dynamic heating regime starting from an
isostructural precursor composed of hydrogen titanate and/or a
mixed sodium and hydrogen titanate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] With the objective of obtaining an understanding of the
crystalline structure, which is the building block of the
nanomaterials of titanium oxide of the present invention, called
TiO.sub.2-x JT phase; where x varies between 0 and 1 due to the
fact that this crystalline structure presents a high oxygen
deficiency, and is described by at least one of the space groups 59
Pmmn, 63 Amma, 71 Immm or 63 Bmmb, reference is made to the
following Figures:
[0031] FIGS. 1a, 1b and 1c are scanning electron microscopy images
(SEM) where it is observed the nanofibrilar and/or nanotubular
morphology of the hydrogen titanate, of the mixed sodium and
hydrogen titanate and of the TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1 in which:
[0032] FIG. 1a shows nanofibers and/or nanotubes obtained at low
temperature, from 50 to 130.degree. C.,
[0033] FIG. 1b shows nanofibers and/or nanotubes obtained at medium
temperature, from 130 to 160.degree. C. and
[0034] FIG. 1c shows nanofibers and/or nanotubes of TiO.sub.2-x JT
phase, where 0.ltoreq.x.ltoreq.1, obtained after dynamic thermal
treatment of the hydrogen titanates, or the mixed sodium and
hydrogen titanates synthesized at high temperature, between 160 and
180.degree. C.;
[0035] FIGS. 2a, 2b and 2c are X-ray dispersive energy spectra
(EDX) which show the quantitative chemical composition of the
nanotubes and/or nanofibers, a) hydrogen titanate, b) mixed sodium
and hydrogen titanate and c) TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1, respectively;
[0036] FIGS. 3a-3f show transmission electron microscopy (TEM)
images where FIGS. 3a, 3b and 3c correspond to nanotubes of
hydrogen titanates and mixed sodium and hydrogen titanates, and
FIGS. 3d-3f correspond to nanotubes of TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1. FIGS. 3a-3f are illustrative examples of the
typical transmission electron microscopy images obtained for the
present hydrogen titanate and/or mixed sodium and hydrogen titanate
and the TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1.
[0037] FIG. 4 shows the X-ray diffraction patterns: (a) titanium
compound, meaning TiO.sub.2 with anatase structure or phase; (b)
and (c) hydrogen titanates and mixed sodium and hydrogen titanates
synthesized at 100.degree. C. (b) and at 160.degree. C. (c),
respectively; which are precursors of the TiO.sub.2-x JT phase,
where 0.ltoreq.x.ltoreq.1. FIG. 4 is an illustrative example of the
typical X-ray diffraction patterns obtained for the hydrogen
titanate and/or mixed sodium and hydrogen titanate. The intensity
of the peaks might change but not their position;
[0038] FIG. 5 shows the characteristic nitrogen adsorption isotherm
obtained for the hydrogen titanate nanomaterials, for the mixed
sodium and hydrogen titanate and for the TiO.sub.2-x JT phase,
where 0.ltoreq.x.ltoreq.1;
[0039] FIG. 6 shows the characteristic pores distribution of the
hydrogen titanate nanostructures, of the mixed sodium and hydrogen
titanate and of the TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1;
[0040] FIG. 7 shows the X-ray diffraction pattern of the
TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1, which is object of
the present invention;
[0041] FIG. 8 shows the model of the formation of the tubular
structures starting from an inwards rolled layer and/or sheet;
[0042] FIGS. 9a and 9b show the basic structural crystalline cell
or unit of the TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1. The
crystalline unit has an orthorhombic symmetry, and is described by
at least one of the space groups 59 Pmmn, 63 Amma, 71 Immm or 63
Bmmb, and it is the building block of the nanomaterials, which are
obtained from the synthesis procedure of the present invention.
FIG. 9(a) shows a three-dimensional (3D) view of the unit cell.
FIG. 9(b) shows a bi-dimensional (2D) view of the unit cell in the
(a, b) crystalline plane. As indicated, the positions of the atoms,
their surroundings, the crystal parameters and space groups in our
materials do not match with any of the known titanium dioxide
phases (anatase, brookite, rutile, amorphous) or with the not so
well known phases (beta and others);
[0043] FIG. 10a illustrates schematically: (a) the cell parameters
of the unit cell of the TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1 with tubular structure which is obtained from
the synthesis procedure of the present invention; FIGS. 10b and
10c, respectively, present the X-ray diffraction patterns simulated
for the model presented in FIG. 10(a) and for the structures shown
in Tables 2 and 9 of the titanates (hydrogen titanate and/or mixed
sodium and hydrogen titanate), and of TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1, respectively;
[0044] FIGS. 11a-11d show the following images: FIG.
11a--transmission electron microscopy (TEM) image, which presents
the morphology of the nanotubes of TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1; FIG. 11b--experimental electron diffraction
pattern of the nanotube or nanofiber presented in FIG. 11a and
where the reflections (200) and (020) associated with cell
parameters a and b respectively are shown as well as the angle
formed; FIG. 11c--the theoretically calculated electron diffraction
pattern, for the theoretical model of the JT phase, with
orthorhombic structure that is presented in FIGS. 9a and 9b; FIG.
11d--experimental electron diffraction pattern of a nanotube where
the reflections (001) and (020) associated with cell parameters c
and b respectively are shown, as well as the angle formed;
[0045] FIG. 12 shows a high resolution transmission electron
microscopy (HRTEM) of a fiber and/or tube where an inter-planar
spacing of approximately 0.7 nm is observed and it is associated
with the c cell parameter of the orthorhombic unit cell of the
TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1; and
[0046] FIG. 13 illustrates a dynamic heating system for forming the
nanomaterial TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1,
starting from the dynamic thermal treatment of the hydrogen
titanates and/or the mixed sodium and hydrogen titanates of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0047] The nanomaterials of titanium oxide of the present invention
have as a building block a crystalline structure with an
orthorhombic symmetry, and are referred to as the JT phase of the
TiO.sub.2-x, where 0.ltoreq.x 1 and preferably x is between 0.1 and
0.95. The new crystalline structure is the basic unit of
construction of nanomaterials which are nanofibers, nanowires,
nanorods, nanoscrolls and/or nanotubes. The nanomaterials are
obtained from a precursor that is isostructural to the new
crystalline structure. The precursor is a hydrogen titanate and/or
a mixed sodium and hydrogen titanate. These titanates are the
hydrogenated, protonated, hydrated and/or the alkalinized phases of
the new crystalline JT phase. In the structure of the TiO.sub.2-x
JT phase, x can vary form 0 to 1 due to the fact that the structure
has an oxygen deficiency, which makes it a useful material for
applications that involve surface adsorption phenomenon, among
other applications.
[0048] Likewise, this invention is related to the procedure of
synthesis of the present nanomaterials, which involves among other
stages: an alkaline treatment of the starting materials followed by
a thermal treatment under either reflux conditions or hydrothermal
treatment at atmospheric pressure, controlled or autogenous; then a
treatment of ionic exchange is carried out, which can be done in
alcoholic and/or in aqueous media or in aqueous media with
different levels of acidity, to thus obtaining the hydrogen
titanates or the mixed titanates of hydrogen and sodium. Finally,
the titanates are subjected to a dynamic thermal treatment stage in
an inert, oxidizing or reducing atmosphere to producing a material
with an orthorhombic lattice denominated as the JT phase of the
TiO.sub.2-x, where 0.ltoreq.x.ltoreq.1.
[0049] The calculated cell parameters of the unit cell of the
TiO.sub.2-x JT, where 0.ltoreq.x.ltoreq.1, are reported in Table 9
and they vary between the following intervals: a from 0.283 to
0.324 nm, b from 0.354 to 0.395 nm and c from 0.695 to 0.735 nm,
for the case of the 59 Pmmn space group, and from 1.408 nm to
1.453, for the case of the 63 Amma, 71 Immm, and 63 Bmmb space
groups; with .alpha.=.beta.=.gamma.=90.degree.. The cell parameters
depend on the general synthesis conditions of the hydrogen
titanates and/or the mixed sodium and hydrogen titanates and on the
conditions during the dynamic thermal treatment of them, under a
controlled dynamic atmosphere and a temperature in the range of
200.degree. C. and 500.degree. C., preferably between 200.degree.
C. and 450.degree. C., and most preferably between 200.degree. C.
and 400.degree. C.
[0050] In the present invention, the unit cell of TiO.sub.2-x JT,
where 0.ltoreq.x.ltoreq.1, (FIGS. 9a and 9b) represents the basic
unit of construction of the plates and/or sheets that compose the
nanotubes, nanofibers, nanowires, nanorods and/or nanoscrolls
(FIGS. 8 and 10a).
[0051] The nanomaterials with the TiO.sub.2-x JT structure, where
0.ltoreq.x.ltoreq.1, present a nanofibrilar morphology and/or a
nanotubular morphology, which is constituted of piled structural
layers that are rolled inwards (FIG. 8) into themselves. These
layers present a great deficiency of oxygen and their basic
structural unit, which is represented in FIGS. 9a and 9b, consist
of two oxygen atoms per one of titanium in a basic cell that
repeats thus forming the piled sheets that compose the nanotubes,
nanofibers, nanowires, nanorods and/or nanoscrolls by rolling
inwards into themselves. The unit cell of the structure TiO.sub.2-x
JT, where 0.ltoreq.x.ltoreq.1, presents an orthorhombic symmetry,
which is described by several space groups (59 Pmmn, 63 Amma, 71
Immm, or 63 Bmmb) defined in the "International Tables for
Crystallography" (International Tables for Crystallography Volumen
A, Space-Group Symmetry. Theo Hahn, editor, Kluwer Academic
Publisher: Netherlands, 1989). The atomic coordinates are presented
in Table 9.
[0052] The cell parameters of the phase called in this invention as
JT, whose cell parameters and relative atomic coordinates do not
match with any of the known titania phases, were determined
experimentally by characterization techniques such as: the
transmission electron microscopy (TEM), see FIG. 11a. The electron
diffraction patterns of isolated nanofibers, see FIG. 11b, were
used to obtain the a and b cell parameters. The c parameter of the
unit cell was obtained with high resolution transmission electron
microscopy (HRTEM). An HRTEM image is presented in FIG. 12. The
observed experimental cell parameters are a=0.317 nm, b=0.360 nm,
and c=0.700 nm. The experimental results agree with the same
results obtained by theoretical simulation (see Table 9). In FIG.
11c it is shown the theoretically calculated electron diffraction
pattern, for the theoretical model of the JT phase, with
orthorhombic structure that is presented in FIGS. 9a and 9b, and
the theoretical pattern agrees well with the experimental electron
diffraction pattern presented in FIG. 11b. From the experimental
electron diffraction pattern presented in FIG. 11b it can be seen
that the angle formed between the cell parameters a and b is equal
to 90.degree.. In FIG. 11d, which corresponds to an experimental
electron diffraction pattern of a nanotube and where the
reflections (001) and (020) are shown, it can be seen that the
angle formed between the cell parameters b and c is equal to
90.degree.. Thus the structure corresponds to a crystalline lattice
which is orthorhombic and that has been confirmed by theoretical
modeling using the structure presented in FIGS. 9a and 9b, whose
cell parameters and angles agree with the experimental data (see
Table 9). The positions of the atoms inside the orthorhombic
lattice do not match with the position of the atoms in any known
phase of titania.
[0053] The nanostructures and/or aggregates of nanotubes and
nanofibers with the TiO.sub.2-x JT structure, where
0.ltoreq.x.ltoreq.1, present the following characteristics: a pore
diameter of 2 to 30 nm, 0.01 .mu.m to 100 .mu.m in length and an
specific area of 5 to 500 m.sup.2/g, preferably 100 to 400
m.sup.2/g; depending on specific surface area of the titanate
precursor used (hydrogen titanate and/or mixed sodium and hydrogen
titanate).
[0054] The hydrogen titanate or titanium oxide hydrated and/or the
mixed sodium and hydrogen titanate, with the general formula
HTiO.sub.2 and Na.sub.xH.sub.1-xTiO.sub.2, where
0.ltoreq.x.ltoreq.1; which are also aims of the present invention,
have a structure with an orthorhombic symmetry similar to the
structure TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1. The
structure of these titanates is described by several space groups
(59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb) of the "International Tables
for Crystallography" (International Tables for Crystallography
Volumen A, Space-Group Symmetry. Theo Hahn, editor, Kluwer Academic
Publisher: Netherlands, 1989) and their relative atomic coordinates
are presented in Table 2, which correspond to the hydrogenated,
hydrated, protonated and/or alkalinized phases. The calculated
lattice parameters presented in Table 2 agree well with the
observed experimental data. The a b c parameter of the titanates
unit cell (hydrogen titanate and mixed sodium and hydrogen
titanate), which are precursors of TiO.sub.2-x JT phase, were
obtained with high resolution transmission electron microscopy
(HRTEM). The observed experimental cell parameters for the hydrogen
titanate and/or mixed sodium and hydrogen titanate are a=0.301 nm,
b=0.378 nm, and c=0.735 nm. In general the lattice parameters in
the hydrogen titanate and/or mixed sodium and hydrogen titanate are
larger than the cell parameters of TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1. This is mainly due to the presence of the
hydrogen and/or sodium atoms which are located in the inter-sheet
spacing (see figures in Table 2).
[0055] The relative atomic coordinates vary depending on both the
concentration of the alkaline metal in the mixed sodium and
hydrogen titanate, and on the crystalline array (see Table 2);
which in turn depend on the general synthesis conditions. The cell
parameters of the unit cell of the orthorhombic structure, that
conforms the nanotubes and/or nanofibers of the hydrogen titanates
and/or the mixed sodium and hydrogen titanate, vary within the
following interval: a from 0.263 nm to 0.331 nm; b from 0.332 nm to
0.448 nm, and c from 0.635 nm to 0.902 nm, for the case of the 59
Pmmn space group, and from 1.368 nm to 1.905, for the case of the
63 Amma, 71 Immm, and 63 Bmmb space groups; with
.alpha.=.beta.=.gamma.=90.degree.. In the case of the 59 Pmmn space
group the c parameter of the unit cell is akin to the spacing
between the layers that constitute the walls of the nanotube and/or
nanoscroll. For the case of the 63 Amma, 71 Immm, and 63 Bmmb space
groups the interlayer spacing is related to the c cell parameter
divided by two, because in these cases the unit cell is constituted
by two layers (see Table 2). The interplanar space ranges from
0.635 to 0.902 nm for the case of the 59 Pmmn space group, and from
0.684 nm to 0.953, for the case of the 63 Amma, 71 Immm, or 63 Bmmb
space groups; with .alpha.=.beta.=.gamma.=90.degree..
[0056] Likewise, this invention is related with the procedure of
synthesis of the titanate materials, which involves an alkaline
treatment of the starting materials followed by a thermal treatment
under either reflux conditions, at atmospheric pressure, or
hydrothermal treatment, at autogenous pressure(in an interval of 1
to 150 atm), with continuous stirring between 10 and 1,000 rpm,
preferably 100-500 rpm, to obtain the hydrogen titanate and/or the
mixed sodium and hydrogen titanate. Finally, the titanates are
submitted to a stage of thermal treatment in a controlled dynamic
atmosphere composed of air, O.sub.2, N.sub.2, He, Ar or a mixture
thereof at any concentration or in a controlled dynamic atmosphere
composed of a mixture of inert gas and H.sub.2 in a concentration
of 5% or 30% by volume of H.sub.2 based on the total volume of gas,
preferably in a dynamic atmosphere of air or nitrogen at a
temperature in the range of 200.degree. C. and 500.degree. C.,
preferably between 200.degree. C. and 450.degree. C.; most
preferably between 200.degree. C. and 400.degree. C. to thus obtain
the titanium oxide material with the structure (JT), which is an
aim or an object of the present invention.
[0057] The method to obtain nanomaterials such as nanofibers,
nanowires, nanorods, nanoscrolls and/or nanotubes of TiO.sub.2-x JT
phase, where 0.ltoreq.x.ltoreq.1, produces a nanomaterial with high
specific area, in an interval of 5 to 500 m.sup.2/g.
[0058] Initially, the process comprises treatment of a titanium
compound such as: titanium oxide with a crystalline structure of
the anatase type, amorphous titanium oxyhydroxide and/or amorphous
titanium hydroxide, titanium oxide with a crystalline structure
rutile type and/or directly from the mineral called rutile and/or
ilmenite; with an alkaline solution in a concentration inside the
range of 1 to 50 M, preferably of 5 to 20 M, of sodium hydroxide or
sodium carbonate, potassium hydroxide or ammonium hydroxide;
preferably of sodium hydroxide or potassium hydroxide. The
NaOH/TiO.sub.2 molar ratio used is in the range of 7 to 70 M,
preferably 10 to 60 M, which includes a ratio H.sub.2O/TiO.sub.2 of
50 to 410 M, preferably 80 to 300 M.
[0059] The resulting suspension can be subjected to a treatment
under hydrothermal conditions in a closed system without stirring
or with continuous stirring between 10 and 1,000 rpm, preferably
100-500 rpm at a temperature in the range of 50.degree. C. to
180.degree. C., preferably between 80.degree. C. and 160.degree. C.
under autogenous pressure in a range of 1 to 150 atm during a time
range of 1 to 100 hours, preferably of 1 to 80 hours.
Alternatively, the first formed suspension may be subjected to a
thermal treatment under reflux conditions at atmospheric pressure
without or with continuous stirring between 10 and 1,000 rpm,
preferably 100-500 rpm; at a temperature in the range of 50.degree.
C. to 150.degree. C., preferably in the range of 80.degree. C. to
120.degree. C.; during a period of time in the range of 1 to 100
hours, preferably in the range of 1 to 80 hours.
[0060] Next, the resulting materials are subjected to a treatment
of ionic exchange with a diluted acid solution with a concentration
in the range of 0.1 and 1 M in aqueous media, and using acids such
as the following: chlorhidric acid, sulfuric acid, nitric acid,
fluoric acid, boric acid and/or phosphoric acid, or ammonium
chloride, ammonium carbonate, or any ammonium salt capable of
exchanging sodium; preferably a solution of chlorhidric acid or
nitric acid; until the pH of the suspension is reduced to a value
between 1 and 7, preferably 2 and 4. Then, the suspension is aged
for an interval of time between 1 to 24 hours, preferably 3 to 18
hours; at room temperature, approximately 20.degree. C. Once the
aging process is finished, the suspension is separated by a
filtration procedure; the obtained solid is washed with sufficient
deionized water and it is dried in a stove with a temperature in
the range of 60.degree. C. to 120.degree. C., preferably between
80.degree. C. and 110.degree. C., during a time period of 4 to 24
hours, preferably 12 to 18 hours.
[0061] The ionic exchange treatments can also be done directly by
several successive washings with water or with alcohol (ethanol,
n-propanol, i-propanol, n-butanol, etc) or with mixtures of
alcohol-water in any proportion. Depending on the extent of the
washing and/or the decrease on the pH of the suspension, the
hydrogen titanates and/or the mixed sodium and hydrogen titanates,
which are aims of the present invention, are produced.
[0062] The hydrogen titanates and/or the mixed sodium and hydrogen
titanates, which are the precursors of the TiO.sub.2-x JT phase,
where 0.ltoreq.x.ltoreq.1, and aims of the present invention,
present: disordered mesoporosity, average pore diameter in the
range of 3 to 25 nm, and a specific area in the range of 100 to 600
m.sup.2/g.
[0063] The specific area of the hydrogen titanates and/or the mixed
sodium and hydrogen titanates, depends on their radial and
longitudinal dimensions; the general synthesis conditions during
the hydrothermal treatment or the thermal treatment under reflux
conditions; and on the prevailing conditions during the ionic
exchange treatment.
[0064] With the same titanium compound, such as the TiO.sub.2 with
anatase structure, and at constant hydrothermal reaction time and
in a closed system, the synthesis carried out at reaction
temperatures lower than 150.degree. C. (with a concentrated
solution of an alkaline metal hydroxide and/or an alkaline metal
carbonate in the range of 1 to 50 M, preferably from 5 to 20M)
produces small nanofibers and/or nanotubes with a piling level of 1
to 10 layers. The longitude of the nanofibers and/or nanotubes vary
between 0.1 and 1 .mu.m. Whereas the synthesis carried out at a
temperature between 150.degree. C. and 180.degree. C. produce
nanofibers and/or nanotubes of higher dimensions, with piling
levels between 10 to 50 layers and a length between 1 and 50
.mu.m.
[0065] Another parameter controlling nanofiber and/or nanotubes
growth of the hydrogen titanates and/or the mixed sodium and
hydrogen titanates, which are the precursors of the TiO.sub.2-x JT
phase, where 0.ltoreq.x.ltoreq.1, and that are aims of the present
invention, is the crystal size (size of crystallites) of the
starting material, and the average particle diameter of particles
composed of starting material crystallite aggregates, used in the
synthesis of the titanates. The average particle diameter used is
in the range of 2.5 to 8 .mu.m (2500 nm to 8000 nm). At a same
reaction temperature and at same reaction timing for the
hydrothermal reaction and or thermal reaction under reflux
conditions, large crystalline aggregates of the starting material
produce nanofibers and/or nanotubes of small dimensions, with
piling levels of 1 to 10 structural layers and a length between 0.1
and 5 .mu.m.
[0066] A titanium starting material with small average particle
diameter (<1 .mu.m) and/or with a small crystal size, such as
between 3 and 10 nm, preferably 5 to 10 nm for anatase, produces
large, stable (at room temperature) nanofibers and/or nanotubes of
hydrogen titanate and/or mixed sodium and hydrogen titanates with
lengths ranging from 1 to 10 .mu.m, and a piling level of 1 to 10
structural layers. According to the present invention, an anatase
precursor crystal size of between 5 and 10 nm is used in order to
reach its complete transformation into nanotubes and/or nanofibers
with dimensions in the nanoscale region, which exhibit high
specific surface area, and have a chemical composition of hydrogen
titanate or mixed sodium and hydrogen titanate but not anatase or
brookite. If amorphous titania is used to form the titanate, a
crystal size of 0.1 nm to 5 nm, preferably 1 to 3 nm may be
used.
[0067] Another parameter ruling the growth of the nanofibers and/or
nanotubes of the hydrogen titanates and/or the mixed sodium and
hydrogen titanates, which are the precursors of the TiO.sub.2-x JT
phase, where 0.ltoreq.x.ltoreq.1, and that are aims of the present
invention, is the time of the thermal reaction at reflux or
hydrothermal conditions. Using the same raw starting material, a
reaction time lower to 6 hours produces small nanofibers and/or
nanotubes with a length between 50 to 500 nm, with staking levels
from 2 to 5 layers, while a reaction time between 7 and 72 hours
produces a nanofibers and/or nanotubes growth of 0.1 to 10 .mu.m in
length, and with staking levels from 1 to 10 layers.
[0068] The control of dimensions of the hydrogen titanate and/or
the mixed sodium and hydrogen titanate is important, due to the
fact that they control the dimensions of the TiO.sub.2-x JT phase,
where 0.ltoreq.x.ltoreq.1, of the present invention.
[0069] The hydrogen titanates and/or the mixed sodium and hydrogen
titanates synthesized are subjected to a dynamic thermal treatment
stage in an oxidizing, or reducing, or inert controlled atmosphere
composed of air, O.sub.2, N.sub.2, He, Ar, or a mixture of them in
any concentration, or in a controlled dynamic atmosphere composed
of a mixture of inert gas and H.sub.2 in a concentration of 5% or
30% by volume of H.sub.2 based on the total volume of gas,
preferably in an oxidizing or inert atmosphere such as air or
nitrogen; in a dynamic flow of such gas, at a temperature between
200.degree. C. and 500.degree. C., preferably between 200.degree.
C. or 300.degree. C. and 450.degree. C.; most preferably between
200.degree. C. and 400.degree. C., at a suitable flowing gas rate,
such as between 0.1 to 1.0 liter per minute, preferably a flowing
gas rate of 0.3 to 0.5 liter per minute to obtain and stabilize the
TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1, and maintain the
nanotubular morphology and/or nanofibrilar morphology of the
titanate precursor.
[0070] The surface area exposed by such titanate nanotubes
generally drops when calcined up to 300.degree. C. when the
calcination is carried out under a steady, non-dynamic, air
atmosphere by placing the titanate nanotubes in an oven, and the
nanotubes collapse to form nanoparticles of anatase. The dynamic
heating method of the present invention provides a thermal
treatment methodology which avoids the sintering of the nanotubes,
while keeping their nanotubular morphology and high surface area
after calcination even at 400.degree. C. The nanotubular structure
is stabilized after the dynamic thermal treatment through the
formation of the layered structure of the JT phase.
[0071] The dynamic thermal treatment may be conducted, for example,
in a sealed, tubular chamber 10, which is shown in FIG. 13, which
may comprise a tube of quartz or sintered silica in which a sample
11 of the titanate nanotubes is placed and then sealed. Chamber 10
is connected by conduit 12, a mass flow controller 13, and conduit
15 to tank 14 which contains an oxidative gas, inert gas, or
reducing gas supply. Conduit 16 connects chamber 10 to water trap
18. Chamber 10 is surrounded by electric heater 20. Dynamic flow
during heating of the titanates is ensured by observing bubbles in
the water trap 18, which releases the gas via conduit 22.
[0072] The titanate nanotube sample 11 is heated to a temperature
of 120.degree. C. at a heating rate between 0.5.degree. C. to
20.degree. C./min, preferably between 1 to 10.degree. C./min. The
sample remains at this temperature between 0.5 to 5 hours, in order
to slowly eliminate the absorbed water. Thereafter, the temperature
may be increased, for example, up to 400.degree. C., at the same
heating rate. The sample remains at this temperature for between 1
to 24 hours, preferably between 2 and 10 hours. The samples are
heated under a dynamic flow of an inert or oxidizing gas, such as a
gaseous flow of air, oxygen, nitrogen, helium, or argon, or any
mixture of such gases in any concentration or in a controlled
dynamic atmosphere composed of a mixture of inert gas and H.sub.2
in a concentration of, for example, 5 to 30 volume percent of
H.sub.2 based on the total gas mixture. Preferably, air, nitrogen
or helium is used.
[0073] The dynamic thermal treatment, under controlled atmosphere
converts the hydrogen titanate and/or the mixed sodium and hydrogen
titanate into the TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1;
which maintains a unit cell with orthorhombic symmetry, that is
described by the different space groups contained in Table 9,
according to the space groups description in the "International
Tables for Crystallography" (International Tables for
Crystallography Volumen A, Space-Group Symmetry. Theo Hahn, editor,
Kluwer Academic Publisher: Netherlands, 1989). The cell parameters
of the unit cell vary within the following intervals: a from 0.283
to 0.324 nm, b from 0.354 to 0.395 nm and c from 0.695 to 0.735 nm,
for the case of the 59 Pmmn space group, and from 1.408 nm to
1.453, for the case of the 63 Amma, 71 Immm, and 63 Bmmb space
groups; with .alpha.=.beta.=.gamma.=90.degree. (see Table 9);
depending on the synthesis general conditions. Likewise, the
resulting material has a pore diameter in the interval of 3 to 50
nm and a specific area of 5 to 500 m.sup.2/g, preferably from 100
to 400 m.sup.2/g; and it is characterized by tubular structures
and/or fibrilar structures with a length between 0.1 .mu.m and 100
.mu.m.
[0074] The textural properties, namely, the surface area, pore
volume and pore diameter, of the nanostructures of TiO.sub.2-x JT
phase, where 0.ltoreq.x.ltoreq.1, decrease as the temperature of
the thermal treatment increases under controlled dynamic
atmosphere; however, the observed nitrogen adsorption and
desorption isotherm is characteristic of the mesoporous nature of
the nanotubes, which is confirmed through transmission electron
microscopy as it is shown in FIGS. 3d, 3e and 3f.
[0075] In FIGS. 1a-1c, the images from scanning electron microscopy
(SEM) are presented and where it is observed the nanofibrilar
and/or nanotubular morphology of the hydrogen titanate, of the
mixed sodium and hydrogen titanate and of the TiO.sub.2-x JT phase,
where 0.ltoreq.x.ltoreq.1. The length of the fibers and/or tubes
depends on the temperature of the hydrothermal or thermal treatment
reflux conditions; the higher the temperature the higher the
dimension of the fibers and/or tubes. FIG. 1a shows aggregates of
nanofibers and/or nanotubes of the hydrogen titanate and/or the
mixed sodium and hydrogen titanate, which were obtained at low
temperature, from 50.degree. C. to 130.degree. C. FIG. 1b presents
nanofibers and/or nanotubes with dimensions between 1 and 50 .mu.m,
which were obtained at temperatures between 130.degree. C. and
160.degree. C. FIG. 1c displays the fibrilar and/or tubular
morphology of the TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1,
obtained by the dynamic thermal treatment, between 300 and
500.degree. C. and in controlled dynamic atmosphere, of the
hydrogen titanates and/or mixed sodium and hydrogen titanates
synthesized between 160 and 180.degree. C. As it is observed, the
fibrilar morphology is kept.
[0076] In FIGS. 2a-2c, the X-ray dispersive energy spectra (EDX)
are presented. In FIG. 2a, it is observed that the spectrum of the
materials that were not submitted to thermal treatment is basically
composed of Ti and O, with an O/Ti atomic ratio between 2.0 and
2.8, which suggests the presence of a hydrogen titanate. Likewise,
in FIG. 2b, the spectrum of the mixed sodium and hydrogen titanate
material indicates that its composition is Ti, O and Na, with an
O/Ti atomic ratio between 2.0 and 2.8. In FIG. 2c, the spectrum of
the materials which were thermally treated in a dynamic oxidizing
or reducing or inert atmosphere is shown. It is observed that the
chemical composition is Ti and O, with an O/Ti atomic ratio between
1 and 1.9, which indicates a high oxygen deficiency.
[0077] In FIGS. 3a-3f, the transmission electron microscopy is
shown, where FIGS. 3a, 3b and 3c correspond to nanotubes and/or
nanofibers of hydrogen titanates and/or mixed sodium and hydrogen
titanates and FIGS. 3d, 3e and 3f correspond to nanotubes and/or
nanofibers of TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1. In
FIGS. 3a and 3d, it is observed that both materials basically have
nanotubular and/or nanofibrilar structures with a length of several
micrometers, between 0.01 and 1 .mu.m, and they have diameters
between 2 nm and 15 nm and between 3 and 10 nm for the titanates
(hydrogen and/or mixed sodium and hydrogen titanate) and the
TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1, respectively. The
nanotubes are generally opened at the ends.
[0078] FIGS. 3b and 3e present the hollow transversal section of
the nanotubes displaying an open internal spacing with diameters
between 5 and 15 nm. In FIG. 3c, it is shown that the walls of the
nanotubes of hydrogen titanate and/or mixed sodium and hydrogen
titanate are composed of approximately 1 to 5 structural layers. In
FIG. 3f, it is observed that the nanotubes of the TiO.sub.2-x JT
phase, where 0.ltoreq.x.ltoreq.1, are as well composed of 1 to 5
structural layers.
[0079] By comparing the images corresponding to the hydrogen
titanate and the mixed sodium and hydrogen titanate with the ones
corresponding to the TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1 (FIGS. 3a-3f) it is established that even after
a dynamic thermal treatment in controlled atmosphere, at a
temperature between 300.degree. C. and 500.degree. C.; the fibrilar
morphology and/or nanotubular morphology of the materials is kept.
Thus proving that the nanotubular and/or nanofibrilar structure
obtained through the procedure, aim of the present invention, is
thermally stable, keeping its high specific area.
[0080] In FIG. 4, the X-ray diffraction patterns of the nanofibers,
nanotubes, nanoscrolls, nanorods, nanowires and/or microfibers are
displayed, which correspond to the hydrogen titanate and/or mixed
sodium and hydrogen titanate obtained under the preferred
modalities of the present invention at two temperatures of
synthesis. The intensity and the position of the diffraction signal
corresponding to the crystalline surface (001) vary depending on
the size of the fibers and/or tubes. The spectrum label as (c) in
FIG. 4 corresponds to an X-ray diffraction pattern of big fibers
obtained at high temperature (160.degree. C.), while the spectrum
label as (b) in FIG. 4 corresponds to an X-ray diffraction pattern
of small fibers obtained at low temperature (100.degree. C.). The
signals are broaden and less defined as the diameter and length of
fibers decrease. Also in FIG. 4, the X-ray diffraction pattern of a
titanium compound used for the synthesis, meaning TiO.sub.2 with
anatase structure, is shown in the spectrum label as (a), with the
aim of illustrating the structural changes that take place during
the hydrothermal synthesis or thermal synthesis with reflux
according with the procedure which is aim of the present invention.
It is clear from these X-ray diffraction patterns that the phase of
the titanates is not anatase.
[0081] In FIG. 5, it can be seen that the nanostructures,
nanofibers and/or nanotubes present a type IV isotherm
(classification of the International Union of Pure and Applied
Chemistry (IUPAC)), a histeresis is observed at a relative pressure
(P/Po) of 0.4-0.6, indicating the existence of mesoporous
nanostructures. The isotherm presented in this Figure is
characteristic of both the hydrogen titanate and/or mixed sodium
and hydrogen titanate and the TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1. Through the application of the BET method
(Brunauer Emmett Teller) it was determined that the nanotubes
and/or nanofibers of both the titanates (hydrogen titanate and/or
mixed sodium and hydrogen titanate) and TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1, display a specific area between 100 and 500
m.sup.2/g, which indicates that the obtained titanates (hydrogen
titanate and/or mixed sodium and hydrogen titanate) of the present
invention maintain their nanotubular structure and therefore their
mesoporosity and high specific area when are submitted to a dynamic
thermal treatment in an atmosphere that can be oxidative, reductive
or inert.
[0082] In FIG. 6, it is observed the porous distribution obtained
from the application of the BJH mathematic model (Barrer Joyner
Halenda) to the desorption isotherms shown in FIG. 5, from which it
is possible to determine that the nanomaterials present a very
homogeneous distribution of the pore size and in the range of 3 and
4 nm. The pore size distribution is associated to the internal
diameter of the nanofibers and/or nanotubes, it was also determined
in the transmission electron microscopy studies. Besides the
nanomaterials, aims of the present invention, present a high pore
volume, with a value between 0.3 and 1.5 cm.sup.3/g.
[0083] FIG. 7 shows the X-ray diffraction pattern characteristic of
the orthorhombic TiO.sub.2-x JT structure, where
0.ltoreq.x.ltoreq.1, that comes from the hydrogen titanate and/or
the mixed sodium and hydrogen titanate submitted to thermal
treatment at a temperature between 300.degree. C. and 500.degree.
C. and under a dynamic oxidizing, or reducing, or inert
atmosphere.
[0084] FIG. 8 shows a schematic model of the nanotubes and/or
nanoscrolls formation, which consists in the rolling and/or folding
of laminar nanostructures having an overlapping of semitubular and
semicircular structures. It is believed that this mechanism
illustrates the formation of the nanotubes of hydrogen titanates
and/or mixed sodium and hydrogen titanates, which occurs during the
procedure according to the present invention.
[0085] FIGS. 9a and 9b present the unit cell of the structure
TiO.sub.2JT, where 0.ltoreq.x.ltoreq.1, constituted of oxygen and
titanium, with orthorhombic symmetry that constitutes the basic
unit of construction of the layers and/or sheets that roll and/or
fold to form the nanotubes and/or nanoscrolls, by means of the
mechanism illustrated in FIG. 8. FIG. 9a shows a tridimensional
view of the unitary cell and FIG. 9b shows a bidimensional view
along the a, b crystalline plane.
[0086] In FIG. 10a, a nanotube and/or nanoscroll is presented and
built with the unit cell of TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1, presented in FIGS. 9a and 9b; by following the
formation model mechanism of nanotubes and nanoscrolls presented in
FIG. 8. Also schematically illustrated is the spatial position of
the unit cell and its cell parameters in the nanotubes or
nanoscrolls. In the case of the 59 Pmmn space group the c parameter
of the unit cell is akin to the spacing between the layers that
constitute the walls of the nanotube and/or nanoscroll. For the
case of the 63 Amma, 71 Immm, and 63 Bmmb space groups the
interlayer spacing is related to the c cell parameter divided by
two, because in these cases the unit cell is constituted by two
layers (see Table 9). FIGS. 10b and 10c present the X-ray
diffraction patterns simulated for the model presented in the FIG.
10a and for the structures shown in the Tables 2 and 9,
respectively, which are characteristic of the nanotubes and/or
nanofibers of the hydrogen titanates and/or of the mixed sodium and
hydrogen titanates as well as of the TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1.
[0087] In FIGS. 11a-11d, the following images are shown:
11a--transmission electron microscopy (TEM) image, which presents
the morphology of the nanotubes of TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1; 11b--experimental electron diffraction pattern
of the nanotube or nanofiber presented in FIG. 11a; 11c shows the
simulated electron diffraction pattern for the theoretical model of
the unit cell of the TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1, that is presented in FIGS. 9a and 9b and that
has to be compared with 11b. The (200) reflection is associated
with the a cell parameter of the unit cell and it has a value in
the range of 0.283 nm and 0.324 nm. The (020) reflection is
associated with the b cell parameter of the unit cell and it has a
value in the range of 0.354 and 0.395 nm. In addition, it is also
observed that the nanofibers and/or nanotubes preferably grow in
the b axis direction (see FIG. 10a). 11d shows the experimental
electron diffraction pattern of a nanotube where the reflections
(001) and (020) associated with cell parameters c and b
respectively are shown, as well as the angle formed.
[0088] FIG. 12 shows a high resolution transmission electron
microscopy (HRTEM) of a fiber and/or tube where an inter-planar
spacing of approximately 0.7 nm is observed, and that corresponds
to the crystalline plane (001) of the structure of TiO.sub.2-x JT
phase, where 0.ltoreq.x.ltoreq.1. In the case of the 59 Pmmn space
group the c parameter of the unit cell is akin to the spacing
between the layers that constitute the walls of the nanotube and/or
nanoscroll. For the case of the 63 Amma, 71 Immm, and 63 Bmmb space
groups the interlayer spacing is related to the c cell parameter
divided by two, because in these cases the unit cell is constituted
by two layers (see Table 9). The interplanar space ranges from
0.695 to 0.735 nm for the case of the 59 Pmmn space group, and from
0.704 nm to 0.727, for the case of the 63 Amma, 71 Immm, or 63 Bmmb
space groups; with .alpha.=.beta.=.gamma.=90.degree..
EXAMPLES
[0089] The following examples show production of titanium oxide
nanomaterials with the crystalline structure TiO.sub.2-x JT phase,
where 0.ltoreq.x.ltoreq.1, that are thermally stable and that are
produced from a precursor of hydrogen titanate and/or mixed sodium
and hydrogen titanate with orthorhombic structure with our
synthesis procedure. The molar ration NaOH/TiO.sub.2 used is in the
range of 7 to 70 M, preferably 10 to 60 M, which includes a ratio
H.sub.2O/TiO.sub.2 of 50 to 410 M, preferably 80 to 300 M. The
examples are illustrative of the present invention, and are not
intended to limit the scope of the invention.
Examples 1 to 7
[0090] Examples 1 to 7 illustrate the preparation of the nanotubes
of hydrogen titanate and/or mixed sodium and hydrogen titanate
which are isostructural to the structure TiO.sub.2-x JT phase,
where 0.ltoreq.x.ltoreq.1, starting from titanium oxihydroxide
(with a crystal size smaller than 3 nm, a specific area of 190
m.sup.2/g, a pore volume of 0.22 cm.sup.3/g, and an average pore
diameter of 4.7 nm), prepared by the sol-gel method and under the
synthesis modality involving a hydrothermal treatment, aim of the
present invention.
[0091] 1.5 g of amorphous titanium oxihydroxide, obtained by the
sol-gel method, is placed in contact with 100 cm.sup.3 of a 5 to 20
M alkaline solution of sodium hydroxide. Each of the prepared
suspensions in examples 1 to 7 was poured in a closed vessel. In
Table 1, the different temperatures used in the synthesis for each
example are specified, within an interval from 100.degree. C. to
180.degree. C., and during a reaction time between 12 and 96
hours.
[0092] The resulting materials from each example is submitted to a
ionic exchange treatment that involves a washing process with a 1M
solution of chlorhidric acid until the pH of the suspension lowers
to a value between 1 and 7, then all the solutions are aged for a
period of time ranging from 12 to 18 hours. After the aging, each
suspension is filtered; then the thus obtained solids are washed
with abundant deionized water and dried at 110.degree. C.
[0093] As it can be seen in FIGS. 1(a) and 1(b), the materials
obtained are basically constituted of nanofibers and/or nanotubes
and/or aggregates of nanofibers and/or nanotubes. In FIG. 4, lines
(b) and (c), it can be observed that the X-ray diffraction patterns
correspond to a hydrogen titanate and/or to a mixed sodium and
hydrogen titanate with orthorhombic lattice whose unit cell is
described by the space groups 59 Pmmn, 63 Amma, 71 Immm, or 63 Bmmb
and the atomic positions presented in Table 2. The intensity and
the position of the X-ray diffraction signals corresponding to
(001) surface, varies according to the fibers size and the piling
level among the nanotubes layers respectively. FIG. 4 is a
representative X-ray diffraction pattern of the hydrogen titanate
and/or mixed sodium and hydrogen titanate.
TABLE-US-00001 TABLE 1 Textural properties of the nanotubes of
hydrogen titanate and/or the mixed sodium and hydrogen titanates,
obtained from amorphous titanium oxihydroxide as starting material.
Reaction Reaction Pore Temperature Time Area Pore Volume Diameter
Example (.degree. C.) (h) (m.sup.2/g) (cm.sup.3/g) (nm) 1 100 48
410 0.51 3.5 2 120 48 333 0.65 4.7 3 140 48 342 0.71 5.0 4 160 48
278 0.74 6.7 5 180 48 44 0.42 21.4 6 100 12 272 0.37 4.0 7 100 96
43 0.15 6.7
[0094] As it can be seen in Table 1, depending on the temperature
and the hydrothermal reaction time, the nanotubes and/or nanofibers
present a specific area between 40 and 500 m.sup.2/g and an average
pore diameter between 2 and 25 nm. At high reaction temperatures,
above 160.degree. C., Example 5, the specific area of the materials
decreases due to the nanofibers and/or nanotubes growth; this same
effect is produced with a long reaction time, Example 7. The
nanotubes are opened at the ends, as it can be seen in FIGS. 3a-3b,
with an internal diameter between 3 and 10 nm, and 1 to 50 layers
with a spacing of 0.6 to 1.0 nm between layers.
TABLE-US-00002 TABLE 2 Crystalline arrays of the orthorhombic phase
of the hydrogen titanate and/or mixed sodium and hydrogen titanate,
which are precursors of the TiO.sub.2-x JT Phase, where 0 .ltoreq.
.times. .ltoreq. 1. System JT_H JT_1_H JT_2_H JT_3_H Formula
HTiO.sub.2 HTiO.sub.2 HTiO.sub.2 HTiO.sub.2 Configuration Basic
unit. One sheet per unit cell. Two sheets per unit cell. Phase JT
Two sheets per unit cell. Phase JT Two sheets per unit cell. Phase
JT with one sheet moved half unit cell with rotation of one of the
sheets by with rotation of one of the sheets by along the b axis
180.degree.. 180 .degree. and moved half cell along the b axis.
Figure ##STR00001## ##STR00002## ##STR00003## ##STR00004## Space
Group 59 Pmmn 63 Amma 71 Immm 63 Bmmb Cell Parameters a (nm)
0.298679 0.310556 0.302282 0.309246 b (nm) 0.365239 0.368659
0.374325 0.351809 c (nm) 0.881823 1.885037 1.764735 1.696627
.alpha.=.beta.=.gamma..degree. 90 90 90 90 Relative Atomic
Positions Atom a b c a b c a b c a b c Ti 0.50000 0.50000 0.37112
0.25000 0.00000 0.80928 0.00000 0.00000 0.31273 0.00000 0.25000
0.32078 O 0.50000 1.00000 1.44423 0.75000 0.00000 0.72025 0.00000
-0.50000 0.12945 0.00000 0.25000 0.72085 O 0.50000 1.00000 1.74856
0.75000 0.00000 0.85989 0.00000 -0.50000 0.27881 0.00000 0.25000
0.87794 H 0.50000 0.00000 1.97883 0.00000 0.00000 0.00000 0.00000
-0.39978 0.50000 0.00000 0.75000 0.50016
Examples 8 to 15
[0095] Examples 8 to 15 illustrate the preparation of the nanotubes
and/or nanofibers of hydrogen titanate and/or mixed sodium and
hydrogen titanate; starting from a titanium oxide (with anatase
crystalline structure, a crystal size of 8 nm, an average particle
diameter between 0.5 and 2.5 .mu.m, a specific area of 102
m.sup.2/g, a pore volume of 0.51 cm.sup.3/g, and a pore diameter of
11.3 nm) and using a synthesis procedure which involves a thermal
treatment under reflux conditions, aim of the present
invention.
[0096] 7.5 g of titanium oxide with anatase structure is placed in
contact with 500 cm.sup.3 of a 5 to 20 M aqueous solution of sodium
hydroxide. Each of the prepared suspensions in Examples 8 to 14 was
submitted to a thermal treatment, under reflux conditions, at a
temperature between 80.degree. C. and 110.degree. C. In Table 3, it
is specified the reaction time and temperature used in each
example. The time interval is between 3 and 48 hours. Only in the
case of the Example 15 the formed suspension, containing the
titanium precursor and the alkaline metal hydroxide solution, was
submitted to a hydrothermal treatment at 160.degree. C. in a closed
system under autogenous pressure.
[0097] The resulting material from each example, are submitted to a
ionic exchange treatment through a washing process with a 1 M
solution of chlorhydric acid until the pH of the suspension lowers
to a value between 1 and 7. Then all the solutions are aged for a
period of time between 12 to 18 hours. Finally, each of the
obtained suspensions is filtered, and the thus obtained solids are
washed with abundant deionized water and dried at 110.degree.
C.
[0098] As it is observed in FIGS. 1a and 1b and in FIGS. 3a, 3b and
3c, as well as in the X-ray diffraction, FIG. 4 lines (b) and (c),
the obtained materials are basically constituted of nanofibers
and/or nanotubes and their structure correspond to a hydrogen
titanate and/or mixed sodium and hydrogen titanate, which present
an structural array with orthorhombic symmetry with a phase whose
unit cell is described by the space groups: 59 Pmmn, 63 Amma, 71
Immm or 63 Bmmb and with the cell parameters and atomic positions
presented in Table 2. FIG. 4 is a representative X-ray diffraction
pattern of the hydrogen titanate and/or mixed sodium and hydrogen
titanate and of the anatase precursor.
TABLE-US-00003 TABLE 3 Textural Properties of the nanotubes of
hydrogen titanate and/or mixed sodium and hydrogen titanate
obtained from anatase titanium oxide as starting material with a
crystal size of 8 nm and a average particle diameter between 0.5
and 2.5 .mu.m (500 nm to 2500 nm). Reaction Reaction Pore
Temperature Time Area Pore Volume Diameter Sample (.degree. C.) (h)
(m.sup.2/g) (cm.sup.3/g) (nm) 8 100 3 330 0.83 5.6 9 100 6 331 0.87
5.6 10 100 12 401 0.93 5.3 11 100 24 314 1.0 5.9 12 100 48 198 0.61
6.5 13 85 48 301 0.73 6.2 14 110 6 287 0.64 5.6 15 160* 48 181 0.75
11.3 *Hydrothermal treatment.
[0099] As it can be seen in Table 3, depending on the temperature
and reaction time fixed during the thermal treatment, the resulting
materials present: a specific area between 150 and 500 m.sup.2/g,
an average pore diameter between 4 and 12 nm and a pore volume
between 0.5 and 1.2 cm.sup.3/g. The nanotubes are opened at the
ends and have 1 to 50 layers with a spacing of 0.6 to 1.0 nm,
between layers.
[0100] From Examples 8 to 12 in Table 3 it is observed that the
optimal thermal treatment time, under reflux conditions, is of 12
hours. A lower or upper time decreases the displayed specific area
of the materials. On the other hand, it can be observed that for
the experiments carried out at the same reaction time and at a
temperature above 100.degree. C., the specific area decreases
(Examples 14 compared with example 9); while the decrease of the
temperature at long reaction times, Examples 12 and 13, favors the
specific area of the nanostructures.
Example 16
[0101] This example illustrates the preparation of nanotubes and/or
nanofibers of hydrogen titanate and/or mixed sodium and hydrogen
titanate starting from a titanium oxide compound with a rutile type
crystalline structure with a crystal size of 15 nm with an average
particle diameter of 0.25 .mu.m.
[0102] 1.5 g of a titanium oxide compound with a rutile type
crystalline structure is placed in contact with 100 cm.sup.3 of a 5
to 20 M aqueous solution of sodium hydroxide. The formed suspension
is submitted to a hydrothermal treatment at a temperature of
180.degree. C. during 48 hours and under autogeneous pressure in a
closed system.
[0103] The resulting material is submitted to an ionic exchange
treatment with a 1M solution of chlorhydric acid until the pH of
the suspension lowers to a value between 1 and 7. Then the solution
is aged for a period of time between 12 and 18 hours. After aging,
the suspension is filtered and the thus obtained solid is washed
with abundant deionized water and dried at 110.degree. C.
[0104] The material obtained in this example showed similar
characteristics to the materials obtained in the former Examples 1
to 15. It is basically constituted of nanofibers and/or nanotubes
FIGS. 1(a and b). Its X-ray diffraction pattern, FIG. 4 lines (b)
and (c), shows that its structure corresponds to a hydrogen
titanate and/or to a mixed sodium and hydrogen titanate with
orthorhombic structure whose unit cell is described by any of the
following space groups: 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb,
reported in Table 2. FIG. 4 is a representative X-ray diffraction
pattern of the hydrogen titanate and/or mixed sodium and hydrogen
titanate.
[0105] The nanotubes and/or nanofibers are constituted by 1 to 50
layers, presenting a specific area between 50 and 100 m.sup.2/g and
an average pore diameter between 2 and 15 nm.
Example 17
[0106] This example illustrates the preparation of nanotubes of
hydrogen titanate and/or mixed sodium and hydrogen titanate
starting directly from a rutile mineral with an average particle
diameter between 20 and 50 .mu.m.
[0107] 1.5 g of highly crystalline rutile mineral is placed in
contact with 100 cm.sup.3 of a 5 to 20 M aqueous solution of sodium
hydroxide. The formed suspension is submitted to a hydrothermal
treatment, at a temperature of 180.degree. C., during 72 hours and
under autogeneous pressure in a closed system.
[0108] The resulting material is submitted to an ionic exchange
treatment with a 1M solution of chlorhydric acid until the pH of
the suspension lowers to a value between 2 and 6. Then the solution
is aged for a period of time between 12 and 18 hours. After aging,
the suspension is filtered and the thus obtained solid is washed
with abundant deionized water and dried at 110.degree. C.
[0109] The material obtained in this example showed similar
characteristics to the materials obtained in the former Examples 1
to 16. It is basically constituted of nanofibers and/or nanotubes
of FIGS. 1a and 1b. Its X-ray diffraction pattern, FIG. 4 line-type
(c), shows that its structure corresponds to a hydrogen titanate
and/or to a mixed sodium and hydrogen titanate with orthorhombic
structure whose unit cell is described by any of the following
space groups: 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb, reported in
Table 2. We recall that FIG. 4 is a representative X-ray
diffraction pattern of the hydrogen titanate and/or mixed sodium
and hydrogen titanate and of anatase.
[0110] The nanotubes and/or nanofibers are constituted by several
layers, presenting a specific area between 3 and 50 m.sup.2/g and
an average pore diameter between 2 and 15 nm.
Examples 18 to 20
[0111] Examples 18 to 20 illustrate the preparation of the
nanotubes of hydrogen titanate and/or mixed sodium and hydrogen
titanate, starting from TiO.sub.2 compounds with anatase structure
with different crystal size and different average particle diameter
and different textural characteristics, which are presented in
Table 4.
[0112] 150 g of a TiO.sub.2 compound with anatase structure, sample
A to C in Table 4, are placed in contact with 3 liters of a 5 to 20
M aqueous solution of sodium hydroxide. Each of the formed
suspensions, for each titanium oxide sample with anatase structure
from the Examples 18 to 20, is submitted to a hydrothermal
treatment at a temperature of 100.degree. C. under autogeneous
pressure in a closed system and during 24 hours.
[0113] The resulting material was put into contact with a 1 M
solution of chlorhydric acid, to perform the ionic exchange where
the sodium is replace by hydrogen, until the pH of the suspension
lowered its value between 1 and 7. Then the suspension is kept
under constant stirring for a period of time between 1 and 24
hours. Finally, the suspension was filtered, it was washed with
abundant deionized water and the solid was dried at 110.degree.
C.
[0114] The resulting materials in each of the examples are
basically composed of nanofibers and/or nanotubes, according to
what is observed in FIGS. 1a and 1b and FIGS. 3a, 3b and 3c. The
X-ray diffraction pattern, FIG. 4 lines (b), shows that the
structure corresponds to a hydrogen titanate and/or to a mixed
sodium and hydrogen titanate with orthorhombic structure and a unit
cell described by the space groups presented in Table 2. Also, in
FIG. 4, it is observed that the intensity and the position of the
X-ray diffraction peak that corresponds to the (001) plane, varies
depending on the size of the fibers and depending on the piling
level between the layers of the nanotubes, which are ruled by the
textural and morphological characteristics of the TiO.sub.2 used as
starting material. FIG. 4 is a representative X-ray diffraction
pattern of the hydrogen titanate and/or mixed sodium and hydrogen
titanate and of anatase.
[0115] Under the same synthesis conditions the nanofibers and/or
nanotubes growth depends on the textural and morphological
characteristics of the TiO.sub.2 compound used as starting
material. The nanotubes are constituted of 1 to 50 layers, with a
space between layers of 0.6 to 1.7 nm. The nanotubes are opened by
the ends as it is observed in FIG. 3a with an internal diameter
between 3 and 10 nm, presenting a specific area between 300 and 450
m.sup.2/g and an average pore diameter between 4 and 10 nm, as it
can be seen in Table 5.
TABLE-US-00004 TABLE 4 Textural properties of the titanium oxide
with anatase structure used as starting material for the synthesis
of the hydrogen titanates and/or mixed sodium and hydrogen
titanates. Average Average Average Anatase Crystal Particle or
Specific Pore Pore Phase TiO.sub.2 Size aggregate Area Volume
Diameter Compound (nm) Diameter (.mu.m) (m.sup.2/g) (cm.sup.3/g)
(nm) A 8 0.5 a 2.5 102 0.42 16.5 B 8 2.5 a 8 101 0.37 14.5 C 5
<1 324 0.33 4.0
TABLE-US-00005 TABLE 5 Textural properties of titanium oxide with
tubular structure obtained for Examples 18 to 20. TiO.sub.2 Anatase
Average Phase Pore Compound Specific Area Pore Volume Diameter
Example Table 4 (m.sup.2/g) (cm.sup.3/g) (nm) 18 A 316 0.72 9.1 19
B 389 0.70 7.2 20 C 401 0.77 7.7 Conditions: 100.degree. C.,
autogenous pressure, reaction time of 24 hours, exchange of 18
hours.
EXAMPLE 21 to 25
[0116] Examples 21 to 25 illustrate the influence of the
hydrothermal reaction time in the synthesis of nanotubes and/or
nanofibers of hydrogen titanate and/or mixed sodium and hydrogen
titanate starting from a TiO.sub.2 compound with anatase phase
whose characteristics are described in Table 4, samples A to C.
[0117] In each case, 150 g of compounds A and C in Table 4, which
are titanium oxide with anatase structure, were put into contact
with 3 liters of a 5 to 20 M aqueous solution of sodium hydroxide.
The suspension formed was submitted to a hydrothermal treatment, at
a 100.degree. C. temperature under autogenous pressure in a closed
system during a period of time between 3 and 14 hours.
[0118] The resulting materials of each example were put in contact
with a 1M chlorhydric acid solution, to exchange the sodium ions
for hydrogen, until the pH of the suspension lowered to a value
between 1 and 7. Then the suspension was left on constant stirring
for a period of time between 1 and 24 hours. After aging, the
suspension was filtered and the obtained solid was washed with
sufficient deionized water and dried at 110.degree. C.
[0119] The resulting materials of each of the examples presented
similar characteristics to the former Examples 1 to 25. They are
basically constituted of nanofibers and/or nanotubes, according to
what is observed in FIGS. 1a and 1b and FIGS. 3a, 3b and 3c. In
FIG. 4 curves b, it is observed that the structure corresponds to a
hydrogen titanate and/or to a mixed sodium and hydrogen titanate
with orthorhombic structure, whose unit cell is described by the
space groups and atomic positions presented in Table 2. Also, in
FIG. 4, it is observed that the intensity and the position of the
X-ray diffraction peak that corresponds to the (001) plane, varies
depending on the size of the fibers and depending on the piling
level between the layers of the nanotubes. Thus, the growth of the
nanotubes both in the radial direction and the longitudinal
direction is also ruled by the time of the hydrothermal reaction
and the ionic exchange time with an acid solution. FIG. 4 is a
representative X-ray diffraction pattern of the hydrogen titanate
and/or mixed sodium and hydrogen titanate and of anatase.
[0120] The nanotubes are composed of 1 to 50 layers, with a 0.6 to
1.7 nm of spacing between layers. The nanotubes are opened, as it
can be seen in FIG. 3(b) with an internal diameter between 3 and 10
nm, presenting a specific area between 380 and 470 m.sup.2/g and an
average pore diameter between 5 and 8 nm, as can be observed in
Table 6.
TABLE-US-00006 TABLE 6 Textural properties of the nanotubes of
hydrogen titanate and/or mixed sodium and hydrogen titanates,
obtained at different reaction times and ionic exchange according
to examples 21 to 25. TiO.sub.2 Anatase Reaction Ionic Specific
Average Pore Average Pore Phase Compound Time Exchange Area Volume
Diameter Example Table 4 (h) Time (h) (m.sup.2/g) (cm.sup.3/g) (nm)
21 A 3 1 414 0.78 7.6 22 A 3 18 450 0.72 6.4 23 A 6 18 386 0.63 6.6
24 C 3 1 464 0.67 5.8 25 C 6 1 464 0.77 6.6 Conditions: 100.degree.
C., autogenous pressure.
[0121] As it can be seen from Table 6, using the same starting
material and at the same temperature of synthesis, the growth of
the nanofibers and/or nanotubes depends on the time of both the
hydrothermal reaction and the ionic exchange; the larger the
hydrothermal reaction time, the bigger the nanotubes size; while as
the ionic exchange time is increased, with the acid solution at a
constant pH, the size of the nanofibers decreases.
Examples 26 to 29
[0122] Examples 26 to 29 illustrate the synthesis of the nanotubes
of hydrogen titanate and/or mixed sodium and hydrogen titanate
starting from the A to C compounds of Table 4, which comprise
different compounds of TiO.sub.2 anatase phase, used as starting
material, and a synthesis procedure consisting of a thermal
treatment under reflux conditions.
[0123] In each case, 150 g of titanium oxide with anatase
structure, compounds A to C in Table 4, were put into contact with
3 litters of a 5 to 20 M aqueous solution of sodium hydroxide. The
suspension formed was submitted to a synthesis procedure consisting
of a thermal treatment under reflux condition, at a temperature of
100.degree. C. and at atmospheric pressure, in a system with
continuous stirring between 10 and 1,000 rpm, preferably 100-500
rpm, during a 3 hour reaction period.
[0124] The resulting material was put in contact with a 1M solution
of chlorhydric acid to perform a sodium ionic exchange for hydrogen
until the pH of the suspension lowers to a value between 1 and 7.
Then the suspension was aged for a 3 hour period for Examples 26 to
28 and for an 18 hour period for the case of Example 29. After
aging the suspension was filtered and the obtained solid was washed
with sufficient deionized water and dried at 110.degree. C.
[0125] The resulting materials of each of the examples presented
similar characteristics to the former Examples 1 to 25. They are
basically constituted of nanofibers and/or nanotubes, according to
what is observed in FIGS. 1a and 1b and FIGS. 3a, 3b and 3c. The
X-ray diffraction patterns obtained are similar to those shown in
FIG. 4, curve b, which correspond to the structure of the hydrogen
titanate with orthorhombic structure, whose unit cell is described
by the space groups and atomic positions presented in Table 2.
Also, in FIG. 4, it is observed that the intensity and the position
of the X-ray diffraction peak that corresponds to the (001) plane
varies depending on the size of the fibers and depending on the
piling level between the layers of the nanotubes. The textural and
morphological characteristics of the nanotubes resemble those of
the TiO.sub.2 used as starting material.
TABLE-US-00007 TABLE 7 Textural properties of the nanotubes of
hydrogen titanate and/or mixed sodium and hydrogen titanate,
obtained from different anatase compounds using a procedure that
involves thermal treatment under reflux conditions, with different
ionic exchange time according to examples 26 to 29. TiO.sub.2
Anatase Phase Reaction Ionic Specific Average Pore Average Pore
Compound Time Exchange Area Volume Diameter Example Table 4 (h)
Time (h) (m.sup.2/g) (cm.sup.3/g) (nm) 26 A 3 3 414 0.83 8.0 27 B 3
3 369 0.58 6.3 28 C 3 3 417 0.65 6.3 29 B 3 18 346 0.61 7.0
Conditions: 100.degree. C., autogenous pressure.
[0126] As it can be seen from Table 7, at constant synthesis
conditions, the nanofibers and /or nanotubes growth depends on the
textural and morphological characteristics of the TiO.sub.2 anatase
phase used as starting material, when the later is submitted to a
thermal treatment under reflux conditions. The nanotubes are
composed of 1 to 50 layers, with a space between layers of 0.6 to
1.7 nm. The nanotubes are opened, as it can be seen in FIG. 3b with
an internal diameter between 5 and 9 nm, presenting a specific area
between 340 and 420 m.sup.2/g and an average pore diameter between
2 and 10 nm. FIG. 3 is an example of a typical TEM image of our
hydrogen titanate and/or mixed sodium and hydrogen titanate
material.
Examples 30 and 31
[0127] Examples 30 and 31 illustrate the synthesis of the nanotubes
of hydrogen titanate and/or mixed sodium and hydrogen titanate
starting from the B compound in Table 4. There is no need of acid
treatment to perform the ionic exchange of sodium by hydrogen.
[0128] 150 g of the B compound in Table 4, which is a titanium
oxide with anatase structure, was put in contact with 3 liters of a
5 to 20 M aqueous solution of sodium hydroxide. The suspension
formed was submitted to a synthesis procedure comprising a
hydrothermal treatment under reflux conditions, at a 100.degree. C.
temperature, autogenous pressure in a closed system, with
continuous stirring between 10 and 1,000 rpm, preferably 100-500
rpm, during a period of 3 hours of reaction time.
[0129] In comparison with the former examples, in the case of
Example 30 the resulting material was not put in contact with a
chlorhydric acid solution to perform the ionic exchange of sodium
by hydrogen, but in this case the solution was exhaustively washed
with ethyl alcohol until de suspension pH decreased to a value
between 6 and 7. It was filtered, and dried at 110.degree. C.
[0130] For the case reported in Example 31, the resulting material
from the hydrothermal treatment stage was exhaustively washed with
abundant bi-distilled water until the suspension pH decreased to a
value between 7 and 8, then it was filtered and dried at
110.degree. C.
[0131] The resulting materials in each of the examples presented
similar characteristics to the former Examples 1 to 29. They are
basically constituted of nanofibers and/or nanotubes according to
FIGS. 1a and 1b and FIG. 3b. The obtained X-ray diffraction
patterns are similar to those presented in FIG. 4 line b, which
shows that the structure corresponds to a hydrogen titanate and/or
to a mixed sodium and hydrogen titanate with orthorhombic structure
whose unit cell is described by any of the following space groups:
59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb, reported in Table 2.
[0132] The nanotubes and/or nanofibers are constituted by 1 to 50
layers with a space between layers of 0.6 to 1.7 nm. The nanotubes
are opened at the end (FIG. 3b) and they present an internal
diameter between 3 and 10 nm, a specific area between 180 and 310
m.sup.2/g and an average pore diameter between 4 and 8 nm, as it
can be seen in Table 8.
TABLE-US-00008 TABLE 8 Textural properties of the hydrogen titanate
and/or mixed sodium and hydrogen titanate obtained by the procedure
of washings with water or ethyl alcohol. Washing Specific Area Pore
Volume Pore Diameter Example Solvent (m.sup.2/g) (cm.sup.3/g) (nm)
30 ethyl alcohol 303.96 0.544 7.16 31 water 185 0.203 4.38
[0133] From Examples 30 and 31, it can be said that the ionic
exchange of sodium by hydrogen can be done with different washing
solvents.
Examples 32 to 43
[0134] The nanotubes with hydrogen titanate structure obtained
through the procedures described in the former examples, were
submitted to a thermal treatment process at a temperature between
200.degree. C. and 400.degree. C. in an dynamic oxidizing or inert
or reducing atmosphere, thus obtaining the nanomaterial TiO.sub.2-x
JT phase, where 0.ltoreq.x.ltoreq.1, with an orthorhombic structure
whose structural characteristics are described by any of the
following space groups: 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb; and
the cell parameters and atomic positions presented in Table 9.
[0135] The materials obtained in the Examples 32 to 43 present an
orthorhombic structure of TiO.sub.2-x JT phase, where
0.ltoreq.x.ltoreq.1, aim of the present invention, and that keep
the nanofibers and/or nanotubes morphology that is present in the
hydrogen titanates and/or mixed sodium and hydrogen titanates,
which originated these materials after a thermal treatment in a
dynamic oxidizing or inert or reducing atmosphere as it is shown in
the scanning electron microscopy images (SEM) in FIG. 1c and FIGS.
3d, 3e and 3f.
[0136] Also, the nanotubes obtained after the thermal treatment in
a dynamic oxidizing or inert or reducing atmosphere, present an
orthorhombic structure, with a crystalline phase named as the JT
phase of titanium oxide TiO.sub.2-x, where 0.ltoreq.x.ltoreq.1. The
term "JT phase" means "a crystalline structure or crystalline phase
with an orthorhombic symmetry having the formula TiO.sub.2-x,
wherein 0.ltoreq.x.ltoreq.1, and has at least one of the space
groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb." As it can be seen in
FIG. 7, the X-ray diffraction peak corresponding to the (001) plane
is kept. This peak is also observed in the case of the precursor
materials, FIG. 4 curves b and c, which are the hydrogen titanate
and/or the mixed sodium and hydrogen titanate.
[0137] Through the X-ray dispersive energy (EDX) spectrum presented
in FIG. 2c, it is concluded that the nanostructures TiO.sub.2-x JT
phase, where 0.ltoreq.x.ltoreq.1, present a high oxygen deficiency.
In this spectrum is observed that the chemical composition is Ti
and O, with an O/Ti atomic ratio between 1 and 1.9, which indicates
a high oxygen deficiency.
[0138] The materials with the orthorhombic structure, with a
crystalline phase TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1,
aims of the present invention, present a morphology of nanofibers
and/or nanotubes as it can be observed in FIG. 1c and FIGS. 3d, 3e
and 3f. The cell parameters of the phase called in this invention
as JT, whose cell parameters and relative atomic coordinates do not
match with any of the known titania phases, were determined
experimentally. The electron diffraction patterns of isolated
nanofibers, see FIGS. 11a, 11b and 11d, were used to obtain the a
and b cell parameters. The c parameter of the unit cell was
obtained with high resolution transmission electron microscopy
(HRTEM). An HRTEM image is presented in FIG. 12. In FIG. 12, the
nanotubes obtained in the Examples 32 to 41 with orthorhombic
structure are shown. An interlayer space of 0.7 nm that corresponds
to the (001) plane and that is related to the c parameter of the
unit cell, which represents the space between the layers that
constitute the nanotube walls, as observed in FIG. 10. The observed
experimental cell parameters are a=0.317 nm, b=0.360 nm, and
c=0.700 nm. The experimental results agree with the same results
obtained by theoretical simulation (see Table 9). In FIG. 11c it is
shown the theoretically calculated electron diffraction pattern,
for the theoretical model of the JT phase, with orthorhombic
structure that is presented in FIGS. 9a and 9b, and as it can be
seen the theoretical pattern agrees well with the experimental
electron diffraction pattern presented in FIG. 11b. From the
experimental electron diffraction pattern presented in FIG. 11c it
can be seen that the angle formed between the cell parameters a and
b is equal to 90.degree.. In FIG. 11d, which corresponds to an
experimental electron diffraction pattern of a nanotube and where
the reflections (001) and (020) are shown, it can be seen that the
angle formed between the cell parameters b and c is equal to
90.degree.. Thus, the structure corresponds to a crystalline
lattice which is orthorhombic and that has been confirmed by
theoretical modeling using the structure presented in FIGS. 9a and
9b, whose cell parameters and angles agree with the experimental
data (see Table 9). The positions of the atoms inside the
orthorhombic lattice of the JT phase do not match with the position
of the atoms in brookite, which also has an orthorhombic lattice
but has a different, single space group 61 Pbca, or in any other
known phase of titania.
TABLE-US-00009 TABLE 9 Crystalline arrays of the unit cell of
TiO.sub.2-x phase JT, where 0 .ltoreq. .times. .ltoreq. 1. System
JT JT_1 JT_2 JT_3 Configuration Basic unit. One sheet per unit
cell. Two sheets per unit cell. Phase JT Two sheets per unit cell.
Phase JT phase Two sheets per unit cell. Phase JT with one sheet
moved half unit cell with rotation of one of the sheets by with
rotation of one of the sheets by along the b axis 180.degree.. 180
.degree. and moved half cell along the b axis. Figure ##STR00005##
##STR00006## ##STR00007## ##STR00008## Space Group 59 Pmmn 63 Amma
71 Immm 63 Bmmb Cell Parameters a (nm) 0.303772 0.304158 0.303414
0.303326 b (nm) 0.373553 0.373715 0.374958 0.37423 c (nm) 0.715056
1.427495 1.43262 1.427982 .alpha.=.beta.=.gamma..degree. 90 90 90
90 Relative Atomic Coordiantes Atom a b c a b c a b c a b c Ti
0.00000 0.00000 -0.15479 0.25000 0.00000 0.67265 -1.00000 0.00000
0.67361 0.00000 0.25000 0.82710 O 0.00000 0.50000 0.29540 0.25000
0.50000 0.89758 -1.00000 -0.50000 -0.10362 0.00000 0.75000 0.60200
O 0.00000 0.50000 -0.07264 0.25000 0.50000 0.71363 -1.00000
-0.50000 0.71351 0.00000 0.75000 0.78604
[0139] The calculated cell parameters of the unit cell of the
TiO.sub.2-x JT, where 0.ltoreq.x.ltoreq.1, are reported in Table 9
and they vary between the following intervals: a from 0.283 to
0.324 nm, b from 0.354 to 0.395 nm and c from 0.695 to 0.735 nm,
for the case of the 59 Pmmn space group, and from 1.408 nm to
1.453, for the case of the 63 Amma, 71 Immm, and 63 Bmmb space
groups; with .alpha.=.beta.=.gamma.=90.degree.. It is observed that
the nanofibers and/or nanotubes grow preferably in the b axis
direction. In Table 9 the atoms positions for each space group of
the JT phase and its variations are given. All the information
given in Table 9 compare very well with the experimental data and
as it can be seen the information given in Table 9 do not match
with any of the known phases of titania.
[0140] Also, the titanium oxide nanotubes with structure
TiO.sub.2-x JT phase, where 0.ltoreq.x.ltoreq.1, present a specific
area between 100 and 400 m.sup.2/g, with a distribution of pore
size that presents a pore average diameter between 4 and 10 nm, as
shown in Table 10. The titanium oxide nanotubes with TiO.sub.2-x
phase JT, where 0.ltoreq.x.ltoreq.1, are originated from the
thermal treatment, in a dynamic oxidizing or inert or reducing
atmosphere, of the hydrogen titanates and/or mixed sodium and
hydrogen titanates, keeping the nanotubular structure and the high
specific area after the thermal treatment, which means that this
material, the titanium oxide nanotubes with structure TiO.sub.2-x
JT phase, where 0.ltoreq.x.ltoreq.1, is thermally stable with
potential applications as catalysts and/or as a catalysts support
and in other processes involving the adsorption phenomena.
[0141] From Examples 32 to 41 (see Table 10) it can be said that
while the initial area of the titanate precursor determines the
area of the JT material, an increase in the temperature lowers the
JT area. In Examples 32 and 33 the areas of the titanate starting
materials are 342 m.sup.2/g and 278 m.sup.2/g, respectively;
whereas in Examples 34 through 39 the area of the titanate starting
material is around 400 m.sup.2/g, hence for this examples it is
obtained a JT material with a larger area. In example 40 the area
of the titanate starting material is 369 m.sup.2/g, which is larger
but not much from that in Example 32. Nevertheless the difference
in the JT area is quite important. This can be explained because
the thermal treatment was different. In example 32 the thermal
treatment was done in a static oxidizing atmosphere. Examples 42
and 43 are originated from the thermal treatment in a dynamic
reducing atmosphere composed of a mixture of 5% or 30% by volume of
H.sub.2 in N.sub.2, respectively; of the hydrogen titanates and/or
mixed sodium and hydrogen titanates. As it can be seen in Table 10
the starting materials for examples 42 and 43 were the same used in
examples 39 and 36 respectively. The only difference is the dynamic
atmosphere used and as it can be seen the use of a reducing
atmosphere for thermal treatment can be successfully applied. The
obtained specific surface areas for examples 42 and 39 are quite
similar and the same happens when comparing examples 43 and 36. In
general de difference in the area is approximately 10 m.sup.2/g
less for the examples under dynamic reducing atmosphere.
TABLE-US-00010 TABLE 10 Textural properties of the nanotubes with
structure TiO.sub.2-x JT phase, where 0 .ltoreq. x .ltoreq. 1,
prepared from the indicated examples and after heat treatment. Pore
Pore Example Precursor Temperature Time Area Volume Diameter
Example (specific area, m.sup.2/g) (.degree. C.) (h) (m.sup.2/g)
(cm.sup.3/g) (nm) 32.sup.b 3 (342) 400 4 214 0.74 7.7 33.sup.b 4
(278) 400 4 127 0.44 9.2 34.sup.a 19 (389) 200 4 348 0.73 6.3
35.sup.a 19 (389) 300 4 339 0.64 7.6 36.sup.a 19 (389) 400 4 289
0.70 9.7 37.sup.a 20 (401) 400 4 319 0.54 4.8 38.sup.a 20 (401) 400
4 323 0.54 4.8 39.sup.a 26 (414) 400 4 326 0.54 4.7 40.sup.a 27
(369) 400 4 325 0.54 4.7 41.sup.c 27 (369) 400 4 286 0.68 7.1
42.sup.d 26 (414) 400 4 313 0.64 6.0 43.sup.e 19 (389) 400 4 279
0.70 7.5 .sup.aThermal treatment temperature in dynamic oxidizing
atmosphere. .sup.bThermal treatment by calcining in a static
oxidizing atmosphere, i.e. in an oven. .sup.cThermal treatment in
dynamic nitrogen atmosphere. .sup.dThermal treatment in dynamic
reducing atmosphere composed of a mixture of 5% by volume of
hydrogen in N.sub.2 .sup.dThermal treatment in dynamic reducing
atmosphere composed of a mixture of 30% by volume of hydrogen in
N.sub.2
* * * * *