U.S. patent application number 09/825824 was filed with the patent office on 2002-03-21 for enhanced oxidation of air contaminants on an ultra-low density uv-accessible aerogel photocatalyst.
Invention is credited to Harwell, Jeffrey H., Lobban, Lance, Newman, Gerard K..
Application Number | 20020035162 09/825824 |
Document ID | / |
Family ID | 22066891 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020035162 |
Kind Code |
A1 |
Newman, Gerard K. ; et
al. |
March 21, 2002 |
Enhanced oxidation of air contaminants on an ultra-low density
UV-accessible aerogel photocatalyst
Abstract
A method of enhancing oxidation of air contaminants on an
ultra-low density, UV light accessible aerogel photocatalyst is
provided. The method includes the steps of providing a
photocatalytic reactor system broadly comprising a photocatalytic
reactor cell, a UV light source, and a pump to force the
contaminated air stream through the photocatalytic reactor cell.
The photocatalytic reactor cell includes glass cell. A catalyst bed
formed of a titanium dioxide aerogel is provided in the glass cell
whereby a high fraction of the titanium dioxide aerogel is
accessible to UV light and gas. The catalyst bed is exposed to UV
light from the UV light source and a contaminated air stream is
introduced into the photocatalytic reactor cell such that the air
stream passes through the catalyst bed causing oxidation of the
contaminants of the air stream.
Inventors: |
Newman, Gerard K.; (Oklahoma
City, OK) ; Harwell, Jeffrey H.; (Norman, OK)
; Lobban, Lance; (Norman, OK) |
Correspondence
Address: |
Dunlap, Codding & Rogers, P.C.
9400 North Broadway, Suite 420
Oklahoma City
OK
73114
US
|
Family ID: |
22066891 |
Appl. No.: |
09/825824 |
Filed: |
April 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09825824 |
Apr 4, 2001 |
|
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|
09189705 |
Nov 11, 1998 |
|
|
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6241856 |
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60066041 |
Nov 11, 1997 |
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Current U.S.
Class: |
516/98 ; 422/4;
423/245.1 |
Current CPC
Class: |
B01J 21/063 20130101;
B01J 35/0026 20130101; B01J 35/002 20130101; B01J 35/004
20130101 |
Class at
Publication: |
516/98 ;
423/245.1; 422/4 |
International
Class: |
B01J 013/00; B01J
008/00; B01D 021/01; A61L 009/00; C07C 011/24 |
Claims
What is claimed is:
1. A method of removing contaminants from a contaminated air
stream, comprising the steps of: providing a photocatalytic reactor
cell having an inlet, an outlet, a cavity extending between the
inlet and the outlet, a transparent side which permits light to be
emitted into the cavity, and a catalyist bed formed of a titanium
dioxide aerogel provided in the cavity such that a high fraction of
the titanium dioxide aerogel is accessible to UV light and gas;
exposing the catalyst bed to UV light; and introducing the
contaminated air stream into the photocatalytic reactor cell such
that the air stream passes through the catalyst bed causing
oxidation of the contaminants of the air stream.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 60/066,041, filed Nov. 11, 1997.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates generally to a method of
mineralizing air contaminants by oxidation, and more particularly,
but not by way of limitation, to an improved method of removing
contaminants from an air stream by passing the air stream through
an ultra-low density, UV-accessible aerogel photocatalyst.
[0005] 2. Brief Description of the Related Art
[0006] Environmental studies of the deleterious effects of air
contaminants are pushing the clean air issue to the forefront of
governmental legislation. The direct result of this is that the
ability to remove air pollutants quickly, safely, and economically
is now a recognized goal of many governmental and industrial
organizations. Over the last three decades, in response to this
recognized need, the scientific community has examined many
different and quite novel remediation technologies as cost
efficient candidates for maintaining clean air environments. Of
these attempts, it has been the use of solar radiation of
photocatalysts that has been seen as one of the most promising
candidates for remediation of air contaminants. This perceived
viability of photocatalysis for air remediation is due to its
ability to catalyze the complete destruction of almost any
hydrocarbon-based molecule ranging from common everyday solvents,
odors, fragrances, proteins, mildew, viruses, bacteria and other
organic vapors. This perspective has resulted in photocatalysis
being investigated from many different perspectives: from
identifying the most promising catalytic substrate to elucidating
its oxidation mechanism. It has been found that titanium oxide and
in particular its anatase crystal form is the most robust and
catalytically active photocatalyst tested. As a result, efforts to
commercialize titanium oxide photocatalytic processes have long
been ongoing with some systems already being put in place.
[0007] The reality of photocatalysis is that it has stayed at the
edge of being widely used by industry despite the fact that the
energy driving photocatalysis is freely available solar radiation
(or even that of an ultraviolet lamp) as well as the fact that
complete destruction of even the most toxic organic compounds can
be catalyzed. The primary factor slowing commercialization is seen
as the need to increase the effectiveness of the
photocatalysts.
[0008] Yet, increasingly, environmental concerns are overriding the
wait for improvements in photocatalysts and are themselves becoming
the driving force behind increasing implementation of
photocatalytic processes "as is" in industry. Today, many major
hotels are competitively advertising environmentally clean
environments such as high quality air, nontoxic carpets, less
polluting wall papers and other paraphernalia as well as financing
improved air filtration and scrubbing systems. Many air
conditioning vendors are experiencing requests for room systems
that not only cool the air but also remove the smell of smoke and
other odors or solvents. New products are thus being proposed that
will attempt to clean the air we breathe and, ultimately, increase
the competitiveness of the U.S. air conditioning industry by
integrating photocatalytic air cleaners into commercial air
conditioning systems. Such a system may be seen as analogous to the
catalytic converter on an automobile, yet different in that not
heat but light catalyzes the burning of organic pollutants. An
added advantage is that photocatalysts also clean indoor air at
ambient temperatures, allowing building managers to cut heating,
ventilation and air conditioning costs by reducing the rate of air
exchange or venting. Despite the advantages of photocatalysts,
there are several significant problems which have limited the
development of photocatalytic technologies for air
decontamination.
[0009] First, known photocatalytic materials display a limited
surface area to incident light. That is, for photocatalysis to
occur, the photocatalytic surface must be accessible to both
incident light and to the gases to be reacted. For any catalyst to
be efficient, large active surface areas are desirable.
Unfortunately, in high surface materials, including titanium oxide,
internal pores usually account for nearly all the surface area.
While these pores are accessible to gases via gas diffusion, they
are not easily irradiated with incident light. It has been found
that relevant UV wavelengths penetrate only approximately 4.5 .mu.m
deep in titania. Thus, only pore surface area relatively near the
external surface becomes photoactivated which results in low
activity.
[0010] Second, photocatalysis may be limited by the competition of
organic (contaminant) molecules for active surface sites.
Particularly with very small pores (typical of high surface area
materials), gas diffision, and adsorption may be highly restricted
and become the limiting step in the oxidation process, thus again
decreasing the usefulness of the catalyst.
[0011] Therefore, a need exist for an improved method of oxidizing
air contaminants with a photocatalyst that is substantially
transparent and has an ultra low density which permits most of its
high surface area to be jointly accessible to both incident
radiation and to the air contaminants. It is to such a method that
the present invention is directed.
BRIEF SUMMARY OF THE INVENTION
[0012] The present invention is directed to a method for removing
contaminants from an air stream by oxidating the contaminants on an
ultra-low density, UV light accessible aerogel photocatalyst. The
method includes the steps of providing a photocatalytic reactor
system broadly comprising a photocatalytic reactor cell, a UV light
source, and a pump to force the contaminated air stream through the
photocatalytic reactor cell. The photocatalytic reactor cell
includes a UV-accessible cell frame. A catalyst bed formed of a
titanium dioxide aerogel is provided in the cell such that a high
fraction of the titanium dioxide aerogel is accessible to both UV
light and gas. The catalyst bed is exposed to UV light from the UV
light source and the contaminated air stream is introduced into the
photocatalytic reactor cell such that the air stream passes through
the catalyst bed causing oxidation of the contaminants of the air
stream.
[0013] The objects, features and advantages of the present
invention will become apparent from the following detailed
description when read in conjunction with the accompanying drawings
and appended claims.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0014] FIG. 1 is a diagrammatic view of a photacatalytic reaction
system.
[0015] FIG. 2 is an exploded, perspective view of a photocatalytic
reactor cell used in the photacatalytic reaction system of FIG.
1.
[0016] FIG. 3 is an exploded, perspective view of another
embodiment of a photocatalytic reactor cell.
[0017] FIG. 4 is a graphical representation showing the IR scan of
the gas phase before the UV light was switched on.
[0018] FIG. 5 shows the IR scan 46 hours after UV light was
initiated
[0019] FIG. 6A is a graphical representation illustrating the
GC-Calibration curve for methane.
[0020] FIG. 6B is a graphical representation illustrating the
GC-Calibration curve for acetone.
[0021] FIG. 7 is a graphical representation illustrating acetone
adsorption for aerogel and anatase powder catalysts.
[0022] FIG. 8A is a graphical representation illustrating methane
concentration dependence on time for aerogel catalyst.
[0023] FIG. 8B is a graphical representation illustrating methane
concentration dependence on time for anatase catalyst.
[0024] FIG. 9 is a graphical representation of methane
concentration dependence on UV illumination and reaction time.
[0025] FIG. 10A is a graphical representation of methane
concentration dependence on time at different initial concetrations
for anatase powder.
[0026] FIG. 10B is a graphical representation of methane
concentration dependence on time at different initial concetrations
for aerogel.
[0027] FIGS. 11A-11D are graphical representations comparing the
change in methane concentration with time for aerogel and anatase
at different initial concentrations.
[0028] FIGS. 12A-12D are graphical representations comparing the
change in reaction rates dependence on time for aerogel and anatase
at different initial concentrations.
[0029] FIGS. 13A-13D are graphical representations comparing the
initial rates for methane at different initial concentration based
on mass, volume, illuminated area, and available surface area for
aerogel and anatase.
[0030] FIGS. 14A-14B are graphical representations for determining
the methane rate constants for aerogel and anatase and values for
rate constants based on mass, volume, illuminated area, and total
available, surface area.
[0031] FIGS. 15A-15B are graphical representations showing the
change in acetone gas phase concentration with aerogel and anatase
catalysts at different initial acetone concentrations.
[0032] FIGS. 16A-16C are graphical representations comparing
anatase and aerogel regarding change in acetone gas phase
concentration with time at different initial acetone
concentrations.
[0033] FIGS. 17A-17C are graphical representations showing change
of acetone gas phase concentration with time for aerogel with
initial feed of air and pure oxygen.
[0034] FIGS. 18A-18B are graphs for determining the acetone
adsorption constants for anatase and aerogel.
[0035] FIGS. 19A-19B are graphs for determining the initial rate
parameter for anatase powder and aerogel.
DETAILED DESCRIPTION OF THE INVENTION
[0036] With respect to oxidation, essentially all organic compounds
are thermodynamically unstable, yet condensed phase oxidation
processes are not yet widely used for destruction of organic
contaminants in air, water, wastewater or solid phases. Part of the
reason is that oxidation processes are rate limited by the chemical
kinetics, resulting in target organics reacting too slowly with the
common oxidants such as oxygen or even ozone, which has a highly
favorable redox potential. Another competing factor is that other
non-target contaminants present may consume unacceptable amounts of
the oxidants. This oxidant consumption load creates difficulty in
maintaining oxidant concentrations at an acceptable level due to
mass transfer problems. The end result is that the oxidation
process becomes economically less feasible. Nevertheless, even with
these problems, oxidation processes remain an attractive
alternative for air remediation and waste treatment and are an area
of intense research. The fact that oxidation processes are
destructive of organic compounds and do not simply separate them
from one phase to another as most treatment processes do is a
particularly attractive feature for indoor air applications in that
maintenance requirements are decreased in an era when waste
reduction is the goal.
[0037] The ideal process for destruction of indoor air contaminants
would consist of oxidation at ambient temperature and pressure.
Such a process in which dioxygen is converted efficiently to atomic
oxygen via a catalyst would be a very economical and effective
treatment for air contaminants.
[0038] In attempts to optimize oxidation processes, recent research
and development work indicate that oxidation rate limitations may
be removed and lowered if conventional oxidants are replaced by
combination of oxidants as well as combinations of oxidants with
ultraviolet radiation. Such mixed oxidation systems have been
labeled Advanced Oxidation Systems (AOS). Advanced Oxidation
Systems display enhanced reactivity for many reasons, one of which
is the higher generation of a variety of reactive free radicals. Of
these free radicals, the most important is the hydroxyl radical
which is an extremely powerful oxidant whose rate constant for
attacking organic molecules is in the range of
1.times.10.sup.8.fwdarw.10- M.sup.-1s.sup.-1. (See Table I.)
1TABLE I SOME SAMPLE OH RADICAL RATE CONSTANTS COMPOUND M
k.sub.M,OH10.sup.9M.sup.-1s.sup.-1 benzene 7.8 1-butanol 4.6 formic
acid 0.2 pyridine 3.8 2-chloroenthanol 0.9 vinyl chloride 7.1
toluene 6.8 tetrachloroethylene 2.3
[0039] The very high reaction rate constant governing OH radical
attack on organic compounds is noteworthy. It indicates that even
if the steady-state concentration of OH radicals is only 10.sup.-10
to 10.sup.-12M, oxidation treatment of an organic substrate in an
advanced oxidation process will be practical. This fact was
illustrated by Glaze for the example of tetrachloroethylene whose
rate constant with OH radicals is
2.3.times.10.sup.9M.sup.-1s.sup.-1. The pseudo-first order rate
constant k.sub.o, is given by
k.sub.o=-dln[M]/dt=(2.3.times.10.sup.9)[OH].sub.steady-state
[0040] The rate constant is so high that even with a steady-state
concentration of only [10.sup.-10M] OH radicals, the half life of
tetrachloroethylene is only 30 seconds. Such OH radicals can be
produced in water and/or water vapor and films either by
ultraviolet radiation, ozonation or photocatalysis.
[0041] With respect to photocatalysis, the low energy requirements
and utility of semiconductors as photocatalysts have been
demonstrated in numerous reports for the total oxidative
degradation of a wide range of organic contaminants, including even
odorants and complex molecules. Photocatalysis with irradiated
semiconductors leads to highly effective, spatially controlled
oxidations and reductions of organic and inorganic substrates,
respectively. TiO.sub.2, SnO.sub.2, and their compounds with other
such semiconducting metal oxides have a wide band gap and absorb
light in the UV range (e.g., for TiO.sub.2, light of
wavelength<3700 angstroms is absorbed). TiO.sub.2 in the anatase
form is the most commonly used semiconductor for photocatalysis.
When irradiated with the proper wavelength of UV light (which is
present in ambient sunlight), electrons in the valence band of
TiO.sub.2 are excited into the conduction band. The reaction of UV
light can be written as 1
[0042] The electron holes (h+) which are produced by the excitation
are powerful oxidizing agents--calculations show that their
oxidation potential is sufficient for complete oxidation of nearly
any contaminant. (This complete destruction is termed
mineralization.) The oxidation is believed to occur through a
hydroxyl radical:
OH.sup.-.sub.ads+h.sup.+.fwdarw.OH..sub.ads
[0043] Studies have shown that in the presence of air or oxygen,
UV-irradiated TiO.sub.2 is capable of complete destruction of
methane, ethane, toluene, carbon monoxide, acetone, butanol,
xylene, formaldehyde, and butyraldehyde, and odor compounds
including acetaldehyde, isobutyric acid, toluene, methylmercaptan,
hydrogen sulfide, and trimethylamine. Other studies have shown that
in aqueous solutions TiO.sub.2 photocatalyzes the complete
oxidation of phenol, chlorinated phenols, dioxins, and
polychlorinated biphenyls (PCBs), DDT, surfactants, saturated
aliphatic hydrocarbons, and s-triazine. Contaminants are
effectively decomposed by their interation with photoactivated
surfaces, giving innocuous products. Other aqueous system studies
have shown that photocatalysts are capable of inactivation of
biological agents and complete destruction of pesticides. These
contaminants may also be present in indoor air. These studies have
proven the potential of photocatalytic oxidation to improve the
quality of indoor air by destruction of chemical compounds from a
wide variety of classes including volatile organic compounds
(VOC's), and hydrocarbons (aromatics and nonaromatics) halocarbons,
oxygenates (including carbon monoxide, alcohols, ketones, and
eldehydes), sulfur-containing compounds, and amines. The results of
studies on a phage, a pesticide, and on other complex molecules
suggest that photocatalytic oxidation has promising potential to
inactivate or destroy components of environmental tobacco smoke
(ETS) and biological aerosols. Thus, photocatalysts, and in
particular TiO.sub.2, have great potential and the following
specific advantages for both air and water decontamination:
[0044] 1. The oxidation process occurs under ambient conditions,
i.e., room temperature and pressure, with oxygen as the
oxidant.
[0045] 2. The formation of photocyclized intermediate products,
unlike in the case of direct photolysis techniques, is avoided. In
fact, while intermediates have been isolated in several of the
studies cited above, the intermediates are also eventually
mineralized. Thus, given sufficient residence time, complete
mineralization can be achieved.
[0046] 3. Oxidation of substrates is complete at high enough
residence times (which depends on the catalyst's activity,
available surface area, and availability to gases).
[0047] 4. TiO.sub.2 is inexpensive, nontoxic, and has a long
catalyst life.
[0048] Titanium oxide aerogels can be made via a sol-gel process
followed by hypercritical or supercritical drying. The process
involves an appropriate titanium organometallic compound undergoing
hydrolysis and then condensation in the appropriate solvent to form
a particulate polymeric structure or a cluster aggregate structure,
depending on whether the process is acid- or base-catalyzed.
Acid-catalyzed gels are composed of interconnecting lattices of
extremely small and uniform particles. Particle sizes as small as 1
nanometer are obtainable.
[0049] Unlike many metal alkoxides, titanium alkoxides undergo a
very easy hydrolysis reaction, initiated even by atmospheric
moisture. Hydrolysis and polycondensation of dilute solutions of
titanium alkoxides have been investigated by several authors. Boyd
obtained essentially linear polymers by the following partial
hydrolysis reaction: 2
[0050] in which R is an alkyl group. Various condensed products
having the general formula Ti.sub.3(x+1)O.sub.4x(OR).sub.4(x+3)
were identified. It was also indicated that hydrolysis with water
in an amount higher than shown above leads to precipitation of
crystalline titanium compounds. However, it has been established
that the polymerization rate is much higher between --OR and --OH
than between two hydroxyl groups. As a result, TiO.sub.2 gels with
a highly polymerized network can be obtained by controlled
hydrolysis and polycondensation of titanium alkoxides. It is
preferable to delay polycondensation by using titanium alkoxides
that have high molecular weight alkyl groups which create steric
hindrance and by using acid catalysts that considerably decreases
the rate of hydrolysis. These considerations allow the formation of
transparent, blue tinted, monolithic TiO.sub.2 gels that can be
hypercritically dried to their aerogel form.
[0051] The actual steps of the synthesis involve titanium
tetraisopropoxide in 2-propanol (23-25 vol. %) hydrolyzed by a
solution containing acidified (HCl) water and 2-propanol.
Homogenous gelation is achieved in a few minutes to several hours
depending on the temperature. By using appropriate HCl
concentration, the gel structure can be made anatase. The gel must
then be hypercritically dried. First, solvent exchange is carried
out with acetone followed by solvent exchange with liquid CO.sub.2.
Heating the TiO.sub.2 gel filled with liquid C0.sub.2 above its
critical temperature creates a single phase inside and outside the
gel and allows one to bleed off the C0.sub.2 without collapsing the
gel's high surface area structure.
[0052] Most reported aerogels of titanium oxide have surface areas
quite low compared to silica aerogels. Surface areas of 80
m.sup.2/g up to 110 m.sup.2/g are typical of those reported. Unlike
silicon oxide, the titanium oxide contracts upon undergoing
annealing at high temperatures. To date, only slight improvements
in these values have been reported for annealed titania aerogels.
Recently, Schneider and Baiker reported 200 m.sup.2/g for a
TiO.sub.2 aerogel prepared from acid-catalyzed tetrabutoxytitanium
and calcined in flowing air at 623 K.
[0053] While still in solution, both silica and titania gels
possess extremely high surface area (>1500 m.sup.2/g). Recently,
ultra-low density (0.01 g/cm.sup.3) silica aerogels with surface
areas of about 2000 m.sup.2/g have been produced at Sandia National
Laboratory by first refluxing a silicon alkoxide with only 65% of
the stoichiometric amount of water required. This procedure
produces a condensed silica oil consisting of long linear polymers
similar to the linear polymers that Boyd achieved. The condensed
silica along with the silicon alkoxide is used as a precursor for
forming ultra high surface area gels. Based on Boyd's work, this
technology should apply to titanium oxide aerogels.
[0054] The main obstacle in obtaining a dry, high surface area
TiO.sub.2 aerogel has been unnecessary annealing, probably because
TiO.sub.2 has been considered almost exclusively for thermal
catalysis for which only its low surface area form is stable.
Aerogels are annealed to make them thermally stable and to
eliminate surface tension effects. At high temperatures silica
aerogels lost their surface hydroxyl groups. The aerogels are thus
transformed into aerogel glass. In contrast, titania aerogels
require surface hydroxyl groups to be active for photocatalytic
oxidation, which is in any case a low (ambient) temperature
process. Annealing titania aerogels is thus an unnecessary step.
Recently, Dagan prepared a non-annealed titanium dioxide aerogel
with a surface area of 600 m.sup.2/g [4]. When compared to an
annealed TiO.sub.2 aerogel, it showed both an order of magnitude
higher adsorption of an organic compound and an order of magnitude
increase in the rate of photooxidation of that molecule. It is
apparent that much like silicon dioxide, high surface area titanium
dioxide aerogels can be obtained with the following
characteristics: photocatalytic anatase form; surface area greater
than 600 m.sup.2/g; and pore volume greater than or equal to
85%.
[0055] In recent work by Ruckenstein et al., surfactant modified
hydrous titanium oxide gels were synthesized. Their gels exhibited
improved adsorption capacity, pore volume, and mechanical strength.
Small amounts of surfactants changed, in major ways, the
characteristics and properties of these gels, such as the number of
surface hydroxyl groups (which represent the main adsorption sites)
as well as particle size and concentration of surface Ti.sup.3+. As
discussed earlier, these factors are important for photocatalysis.
And, of critical significance, these factors are all affected by
the surfactant-based synthesis technique. Thus, the following two
characteristics were added to the aerogels: high density of
hydroxyl groups (adsorption and photocatalyst sites); and even
higher adsorption capacity due to surfactants.
[0056] A liquid surfactant is added to the sol-gel process in an
attempt to template its morphology. All components including water
can be solubilized by the surfactant. The surfactant is a nonionic
ethoxylated compound known as Igepal CO-660. This surfactant, as
well as other members of its family, is an effective solvent in the
sol-gel process. Aerogels formed using these solvents exhibited
reduced cracking upon drying. In addition, particle size and
reaction rate are strongly dependent on the choice and
concentration of surfactant.
[0057] The nonionic surfactant used leads to an isotropic gel and
does not force the polymerization process into a hexagonal or
lamellar geometry. It does, however, tailor the particle size and
narrows the pore size distribution. The nonionic surfactant solvent
for the sol-gel process represents a changeable parameter in
determining the morphology of aerogels.
[0058] Alternative means of creating TiO.sub.2 surfaces with high
or ultra-high surface areas may include a one step hydrolysis and
condensation of mixtures of silicon alkoxides and titanium
alkoxides, and using a previously-formed ultra-high surface area
silica aerogel as a template for a second hydrolysis and
condensation of a titanium dioxide layer over the aerogel surface.
A third method involves only a monolayer of TiO.sub.2 laid down on
the silica oxide lattice of the aerogel.
[0059] With respect to the one step binary aerogel, aerogels have
been formulated from mixtures of 2 to 5 components. The essential
barrier to multicomponent aerogels is whether the individual metal
alkoxides share solubility in either a single or mixed solvent. For
a TiO.sub.2--SiO.sub.2 binary aerogel, a single solvent system is
all that is required. Solutions of Si(OC.sub.2H.sub.5).sub.4 and
TI(OC.sub.2H.sub.5).sub.4 in ethanol can be used to prepare
aerogels. A strong acid catalyzes the hydrolysis reaction. Dilution
and gelling temperatures are similar to those applicable to a pure
SiO.sub.2 system, but the polymerization rate is considerably
increased by the addition of Ti(OC.sub.2H.sub.5).sub.4 and is
further accelerated by heating. It is recognized that titanium
alkoxides show a higher rate of polymerization than silicon based
alkoxides. Besides the mixed Si--O--Ti bonds created during the
polycondensation reaction, an increased number of Ti--O--Ti
linkages should be expected when TiO.sub.2 content increases.
[0060] The fast polymerization of titanium alkoxide under
experimental conditions results in partly crystalline gels. The
crystalline structure is identified as anatase. Results of several
investigators using the same system show that the final structure
of TiO.sub.2--XiO.sub.2 gels is strongly related to the processing
parameters and particularly to the amount of the hydrolysis
water.
[0061] In synthesizing aerogels whose compositions range from 5 to
50% TiO.sub.2 in the binary aerogel, titanium isopropoxide and
silicon tetraethoxide (TEOS) are used as precursors. Samples are
prepared by a pre-hydrolysis method using nitric acid as the
catalyst. The sol-gel is prepared by first adding 1/4 of the
stoichiometric amount of water into the desired amount of TEOS.
Titanium isopropoxide is then added to the mixture under vigorous
stirring. After mixing, additional water is added to complete
gelation with a final water to alkoxide ratio of 16. The resulting
clear solution is then aged two days before solvent exchange with
acetone and then liquid CO.sub.2 followed by hypercritical drying
to form the aerogel.
[0062] With respect to the two step binary aerogels, the silicon
alkoxide gel is first polymerized followed by flushing the gel with
anhydrous ethanol followed by flushing with dilute titanium
alkoxide in anhydrous ethanol. Alternatively, the silica can be
completely fabricated in its ultra-high surface area form, heat
treated to 135.degree. C. to remove physically adsorbed water thus
leaving a hydroxylated and partial alkoxylated surface and then
immersed in a dilute solution of titanium alkoxide in anhydrous
ethanol. There is no water present and the only reaction sites are
surface sites. This procedure allows a slow hydrolysis reaction of
titanium alkoxide at the surface. A previous experiment following
this procedure, but using 10.3 wt. % of Ti(OC.sub.2H.sub.5).sub- .4
in anhydrous ethanol, showed a 37% decrease in the surface area of
the silica aerogel used. The surface area decrease was due to heavy
coating of the surface by polymerized TiO.sub.2.
[0063] A second aerogel-coating technique utilizes coupling agents.
Tetra-functional organometallic compounds based on titanium,
silicon, and zirconium make useful coupling agents because the
central metal's tetravalency is conducive to electron sharing.
Titanium ester-derived coupling agents have an advantage in their
relative ease of building, thus providing specific function for a
wide scope of composite applications. Only one example of coupling
agents being used with an aerogel, in that case SiO.sub.2, has been
reported in the literature. Silane coupling to the aerogel surface
created a hydrophobic organic layer which was pyrolyzed to create a
carbon aerogel with significant electrical conductivity. It is
proposed we use a similar technique to form a titanium oxide layer
on an ultra-high surface area silica aerogel. Instead of silane
coupling, titanium coupling agents are used to form a monomolecular
film, which instead of being pyrolyzed, will be oxidized to
TiO.sub.2. By repeating this procedure, complete coverage of the
silica surface by TiO.sub.2 should be possible with coating
thicknesses ranging from monolayer to multilayer. The silica
aerogel can be made with surface area up to 2000 m.sup.2/g. Surface
modification with a TiO.sub.2 monolayer should not significantly
lower the surface area if pores are sufficiently large that
blockage does not occur. Surfactant-templated silica aerogels
should be ideal for this purpose.
EXAMPLES
[0064] Two catalysts have been studied. First, anatase titanium
dioxide powder (Aldrich, 99.9% anatase titanium IV oxide) was used
as supplied. A reactor cell (described below) contained 3.74 g of
TiO.sub.2 (BET=2.3 m.sup.2/g) which corresponds to a catalyst bed
volume of 48.5 cm.sup.2. The second catalyst was a pure titanium
dioxide aerogel which had been synthesized. Unlike the silicon
alkoxides that usually require a catalyst to drive their reaction
with water, the high reactivity of titanium alkoxides with water
poses significant problems to creating titanium aerogels of the
required form. The addition of the smallest amounts of water to the
titanium alkoxides formed precipitates and not the uniform gelation
required to create aerogels. However, addition of a specific amount
of the acetic acid to the solution resulted in a continuous
monolithic titanium gel that could then be supercritically dried to
its aerogel form. A pure titanium oxide low density aerogel was
synthesized using the excess water method. Titanium (IV)
iso-propoxide (97%, Aldrich), ethanol (anhydrous, Aldrich), and
acetic acid (99.5%, Fisher Scientific) were mixed in a beaker
according to a molar ration of Ti:EtOH:AAc=1:12:9. Then, under
constantly stirring, deionized water was added at a ratio of
Ti:H.sub.2O=1:40. The gelation time was 4-5 days forming a
transparent gel with slightly cloudy appearance. After washing with
acetone, the sample was the supercritically dried using carbon
dioxide as supercritical fluid. A reactor cell was loaded with 1.33
g of aerogel (BET=423 m.sup.2/g). The properties of both catalysts
are summarized in Table II.
2TABLE II PROPERTIES OF ANATASE AND AEROGEL ANATASE AEROGEL Mass
3.74 g 1.33 g Illuminated Area 48.5 cm.sup.2 26.6 cm.sup.2 Catalyst
Volume 3.85 cm.sup.3 2.11 cm.sup.3 Specific Surface Area 80
m.sup.2/g 423 m.sup.2/g
[0065] FIG. 1 illustrates a reactor system 10 used to study the
catalysts. The primary components of the reactor system 10 are as
follows: a photocatalytic reactor cell 12, a UV light 14 (300 Watt,
Ace Glass), GC-analyzer 16 (Sigma 300, FID, Perkin Elmer)+Varian
Integrator 4270, an infra-Red Cell and Spectrophotometer 18 (Model
500, Buck Scientific), a pump 20 (Metal Bellows Corp. Model MB-41,
stainless steel bellows), a pair of flow indicators 22 (Cole
Parmer), a pressure gauge 24, a pair of quick connects with filter
26 (Cajun 316 VCR), six valves 28 (Nupro), and stainless steel
tubing 30 (1/4 in OD).
[0066] A bypass around the photocatalytic reactor cell 12 was
implemented to avoid a high pressure drop across the catalyst bed
formed in the photocatalyst reactor cell 12. The quick connects 26
with filter gaskets were placed before and after the reactor cell
12 to prevent catalyst powder from being purged into the system
tubing and to allow for an easy change of the reactor cell. The
high capacity pump 20 was implemented to ensure faster mixing of
the contaminant after introduction to the reactor loop.
[0067] FIG. 2 shows the photocatalyst reactor cell 12 in greater
detail. The photocatalyst reactor cell 12 includes a quartz glass
cell 32 with a space 34 formed therethrough for receiving a
catalyst bed represented by the reference numeral 35 in FIG. 1. The
glass cell 32 is 3 in .times.5 in with the space 34 being {fraction
(1/32)} wide. The glass cell 32 is sealed with glass on both sides,
is open at the bottom, and has glass spacers 36 on top to provide
for an even flow distribution. The glass cell contains small layers
of glass wool (not shown) at both ends to hold the catalyst powder
in place.
[0068] The glass cell 32 is supported by a pair of aluminum frames
38a and 38b. Each frame 38a and 38b includes a first portion 40
that has a cavity 41 positionable over a corresponding end portion
the glass cell 32 and a silicone seal 42. Each first portion 40 is
further provided with an opening 43 in communication with the
cavity 41. It should be appreciated that the opening 43 of the
frame 38a serves as an inlet and the opening 43 of the frame 38b
serves as an outlet or vice versa. Each frame 38a and 38b further
includes a second portion 44 that is provided with a raised edge 46
for engagement with the silicon seal 42. Upon pressing together of
both portions 40 and 44 (using 14 screws) the silicon seal 42 is
pressed against the glass cell 32 and provides sealing of the
components.
[0069] An alternative reactor cell 48 is illustrated in FIG. 3. In
cell 48, the sealed glass cell is replaced with two glass plates 50
separated by a gasket 52 of variable thickness and held together by
two clamp-like frames 54a and 54b. This design will be used for
experiments to optimize the catalyst bed thickness.
[0070] The IR (Buck Scientific M-500) scans the wavelengths from
4000 cm.sup.-1 to 600 cm.sup.-1 during a standard scan of the gas
in the main system loop. The IR can also continuously scan and
record data at a single wavelength. The IR cell (Buck Scientific
6802) is 5 cm long with NaCl windows and is connected directly to
the main system loop with stainless steel flexible tubing (Cajon
321-4-X-24B2). The IR is controlled by an IBM Personal Computer
(IBM PS2 Model 50). The software used to view and analyze the IR
data is "Spectra Calc," by Galactic Industries Corporation.
[0071] The purpose of the IR is to enhance the detection
capabilities of the GC and the Mass Spectrometer. It is believed
that the results of the combined GC data, mass spectrometer data,
and IR data will give a better description of the reaction systems
under study than the GC data and mass spectrometer data alone.
[0072] The IR system has come on line recently and is undergoing
calibration. It has taken considerable effort to calibrate the IR
system thus far. Calibration of the IR system includes but is not
limited to:
[0073] 1. Determining the optimum IR cell size to use;
[0074] 2. Isolating computer interface problems; and
[0075] 3. Calibration of the IR beam.
[0076] Determination of the optimal IR cell size involves selecting
an IR cell that is not too small nor too big. If the IR cell is too
small, gases at very low concentrations may not be detected. If the
IR cell is too large, it will be very difficult to obtain accurate
kinetic data because the volume of the system will be so big that
gas phase concentration changes will be difficult to detect over
short time periods. The 5 cm cell was chosen over the 3 meter cell
for this reason.
[0077] The IR beam was checked periodically to insure that there
was no wavelength shift in the IR scans. If the wavelengths are
shifted, it is much more difficult to identify the chemical bonds
present.
[0078] The initial IR results are shown in FIGS. 4 and 5 for the
aerogel catalyst. FIG. 4 shows the IR scan of the gas phase before
the UV light was switched on. FIG. 5 shows the scan 46 hours after
the UV lamp was initiated. The peaks at 3432, 3000, 2572, 2430,
1772, 1430, 1201, 1090, 882, and 775 cm.sup.-1 are considered
nonexistent 46 hours after turning on the UV light, as seen in FIG.
5. The product peaks in FIG. 5 at 3684, 3579, 2374, 2297, 689
cm.sup.-1 have not been positively identified yet.
[0079] The gas from the system is automatically injected into the
GC (Perkin-Elmer Sigma 300 Gas Chromatograph) to a determined time
interval, i.e., every hour, by a pneumatically actuated valve. To
an adjusted time interval, the following events occur for the
automatic sampling:
[0080] 1. The timer sends a signal to the integrator (Varian 4270)
to begin analysis.
[0081] 2. The integrator then triggers an electrically controlled
valve (Humphrey 41E1), which then triggers a pneumatic valve (Valco
AH90).
[0082] 3. The pneumatic valve then remains in the "sampling"
position for 15 seconds to thoroughly fill a sampling loop with the
system gas.
[0083] 4. The integrator then shuts the electrically controlled
valve, which in turn, causes the pneumatic valve to inject the gas
from the sampling loop into the GC column.
[0084] 5. 15 minutes later, the integrator ends analysis and prints
the results.
[0085] 6. The process repeats after a predetermined time interval,
i.e., 60 minutes.
Experimental Results
[0086] GC-Calibration
[0087] The GC-Analyzer was calibrated for methane and acetone. This
was done by successive injection of known amounts of acetone (or
methane) into the reactor loop not containing the photocatalytic
reactor cell 12. The GC readings (area) were recorded and plotted
versus the calculated concentrations in the reactor loop. Linear
regression was applied to obtain the calibration equations as shown
in FIGS. 6A and 6B.
[0088] Reactor Volume Determination
[0089] The volume of the system, V.sub.SYS, was determined by
helium expansion. The reactor system was initially purged with
helium and then evacuated using a vacuum pump (Fisher Scientific
Mod. 5KH32FG 115E). The pressure, P.sub.vac, was recorded. An
external cylinder of known volume, V.sub.cyl, was filled with
helium at atmospheric pressure, P.sub.2, was recorded. By applying
a simple mass balance, the volume of the reactor system was
determined using the pressure difference of the expanded gas and
applying the ideal gas law (equation [1]):
PV=nRT [1]
[0090] where
[0091] P=Pressure
[0092] V=Volume
[0093] T=Temperature
[0094] n=number of moles
[0095] R=Universal gas constant
[0096] Considering the fact that the number of moles, n.sub.1, in
the cylinder before expansion is the same as the number of moles in
cylinder and reactor system after expansion, n.sub.2, we find by
applying of equation [1] that
n.sub.1 =n.sub.2 [2]
[0097] or
P.sub.1V.sub.cyl=P.sub.2(V.sub.cyl+V.sub.sys)-P.sub.vac(V.sub.sys)
[3]
[0098] Because it is not possible to draw an absolute vacuum to the
system, the term (-P.sub.vac(V.sub.sys)) in [3] had to be added to
the equation to account for the mass of helium that is left after
evacuation. The only unknown in [3] is V.sub.sys which can be
easily solved for. The volume of the reactor system was found to be
about 300 ml. The exact value varied due to alterations in the
reactor loop set-up and was redetermined after every change of the
reactor set-up.
[0099] Adsorption Studies
[0100] Since acetone adsorbs strongly on the catalyst, an
adsorption study was carried out on both catalysts, anatase powder
and aerogel. These data were needed to calculate the reaction rates
and reaction rate constants.
[0101] Successive increments of acetone were injected into the
reactor loop which included the reactor cell. The amount of acetone
adsorbed on the catalyst, m.sub.ads, was determined from the
difference of the gas phase concentrations if no adsorption
occurred, C.sub.exp, and the actual measured concentrations,
C.sub.meas.
C.sub.exp=V.sub.a.sup.inj* .left brkt-top..sub.a/V.sub.sys [4]
m.sub.ads=(C.sub.exp-C.sub.meas)V.sub.sys [5]
[0102] where
[0103] V.sub.a.sup.inj=Volume of acetone injected
[0104] V.sub.sys=Volume of system
[0105] P.sub.a=Density of acetone
[0106] FIG. 7 demonstrates the higher adsorption capacity of the
aerogel compared to the anatase. Methane adsorption was very low
and could not be accurately measured with this apparatus. The
concentration reading from the GC after injection of methane into
the reactor system was identical (within experimental error) to
that which would occur in the absence of adsorption. The low
adsorption of methane, combined with its chemical stability,
account for its much slower rate of oxidation compared to acetone
(as described later).
Studies of Photocatalytic Activity
[0107] Methane
[0108] The catalytic reactor cell was placed one foot away directly
in front of the UV-lamp. Before starting each run, the entire
reactor system was purged with dry air. The pump continuously
circulated the gas throughout the system. GC measurements were
taken for about one hour to assure that no other components are
left in the system. Then, methane vapor was injected to the system.
GC readings were observed until a constant baseline was established
typically one to two hours. Then, the UV light was switched on. An
air fan circulated room air around the photo cell which established
temperatures at the cell varying from 27-29.degree. C. The pressure
in the system was P=1 atm. The flow rate across the cell was 65-75
ml/min. Automated GC readings were recorded every hour until no
methane peak was observed or the concentration remained nearly
constant. Experiments with five different initial concentrations of
methane were performed by injecting 0.5, 1, 2, 6, and 10 ml of
methane at P=1 atm which corresponds to concentrations varying from
0.07 to 1.5 mmol/L. The maximum timer per experimental run was 50
hours.
[0109] FIGS. 8A and 8B show the change of methane concentration
with time for two runs at the same initial concentration. The data
are highly repeatable and the catalyst is reusable with little or
no alteration in its activity. A different study was performed
where the UV light was shut off during an experimental run. FIG. 9
indicates that no reaction occurs during UV off times. Moreover,
upon reinitializing of the UV light the change in concentration
continues at the same trend as it was before the interruption. From
this, it can be concluded that UV light is necessary for activation
of each reaction site. In addition, the catalyst activity does not
change with the duration of UV light exposure. An earlier study, in
which methane was circulated through a reactor cell containing no
catalyst but with the UV light on, showed that methane oxidation is
not catalyzed by UV light in the absence of the catalyst.
[0110] FIGS. 10A and 10B summarize the results for the studies on
anatase titanium powder and aerogel. FIG. 10A includes the data for
the change in methane concentration with time at different initial
concentrations on anatase titanium oxide powder, whereas FIG. 10B
summarizes the data for the aerogel. In order to compare the
performance of both catalysts the change in concentration with time
for both catalysts was plotted for each individual concentration in
FIGS. 11A-11D. This shows clearly that the gas phase methane
concentration decreases much faster on the aerogel catalyst than on
the anatase powder. Also, at the end of the experimental run, the
value at which the methane concentration does not decrease
significantly is much lower (up to 50%) for the aerogel catalyst.
Note that at the end of the run the methane concentration is still
decreasing, but slowly due to its low surface concentration. The
dependence of the reaction rates on time are shown in FIGS.
12A-12D. At all concentrations the initial rate on aerogel is
higher than that of anatase. Because methane adsorption is so low,
the rates were simply calculated by the ratio of change in methane
gas phase concentration over change in time: 1 - r = C t [ 6 ]
[0111] Moreover, it should be noted that the aerogel bed volume is
only about two thirds that of the anatase powder. Despite lower
volume and lower mass of the aerogel bed (1.33 g aerogel, 3.76 g
anatase) the performance of the titanium dioxide aerogel exceeds
that of the anatase titanium dioxide powder.
[0112] In order to evaluate reaction rate constants, initial rate
data were determined based on mass, volume, illuminated area and
total surface area of the catalyst bed. The Langmuir Hinshelwood
rate expression was applied by assuming that only m ethane adsorbs
on the catalyst surface. 2 r = kK c C c 1 + K c C c + K w C w [ 7
]
[0113] where
[0114] k=Rate constant
[0115] K.sub.c=Adsorption constant for contaminant
[0116] K.sub.w=Adsorption constant for water
[0117] C.sub.c=Concentration of contaminant
[0118] C.sub.w=Concentration of water
[0119] The water adsorption term in the denominator can be
eliminated since no water is present at the beginning of the run.
Hence, expression [7] for methane as contaminants becomes 3 r = kK
m C m 1 + K m C m [ 8 ]
[0120] where
[0121] K.sub.m=Adsorption constant for methane
[0122] C.sub.m=Concentration of methane
[0123] The initial rate data are summarized in Table III.
3TABLE III INITIAL REACTION RATES PER UNIT MASS, VOLUME,
ILLUMINATED AREA AND TOTAL AVAILABLE SURFACE AREA OF CATALYSTS
ANATASE AND AEROGEL Initial Rate per unit Initial per unit per unit
cat. gas phase per unit cat. illumin. surface concen- cat. mass
volume area area tration mmol mmol mmol mmol mmoI/L hr g-cat hr
cm.sup.3-cat hr cm.sup.2-ill. hr m.sup.2-cat ANATASE 0.06987
0.38478 0.37379 0.02967 0.00481 0.14591 0.91489 0.88875 0.07055
0.01144 0.29145 1.38877 1.34909 0.10709 0.01736 0.89571 3.29837
3.20413 0.25435 0.04123 1.40745 2.56865 2.49526 0.19808 0.03211
AEROGEL 0.06564 1.88193 1.18624 0.09417 0.00445 0.14400 3.13836
1.97821 0.1 5704 0.00742 0.28394 4.01841 2.53293 0.20107 0.00950
0.75381 6.76080 4.26155 0.33829 0.01598 1.50987 9.35851 5.89897
0.46828 0.02212
[0124] FIGS. 13A-13D illustrate that the initial rate for aerogel
based on mass, volume, and illuminated area is two to three times
higher than that of anatase. On the basis of the total catalyst
surface area the anatase demonstrates a better performance. This
implies that the actual reaction site of the anatase has a higher
activity than the site of the aerogel. This is expected since the
synthesized aerogel has a very low crystallinity. There lies a
potential of greatly improving the performance of aerogel by
increasing its crystallinity.
[0125] By rearranging the equation of [8] to 4 1 r = 1 kK m 1 C m +
1 k [ 9 ]
[0126] the rate constants based on mass, volume, illuminated area,
and surface area can be determined from the slope and intercept of
the plot of 1/r versus 1/C.sub.m, where 5 Intercept = 1 k Slope = 1
kK m
[0127] The plots and resulting parameters k and K.sub.m are
summarized in FIG. 14. The rate constants for the anatase powder
are higher except for the one based on the mass of catalyst. The
adsorption constant K.sub.m is larger for the aerogel which is
apparently responsible for its faster oxidation rate of
methane.
[0128] Acetone
[0129] Experiments with acetone as the contaminant were performed
similar to those described for methane. However, in order to
observe how much the adsorption has an effect on the actual gas
concentration, the initial set-up was performed as follows. The two
valves before and after the catalytic cell were completely closed
and the cell-bypass valve completely opened (see FIG. 1). A
predetermined amount of acetone was injected into the system and
allowed to circulate and mix in the reactor loop without being
exposed to the catalyst. The two valves disconnecting the reactor
cell from the system were then opened and the acetone gas mixture
circulated across the catalyst. Due to the very high adsorption of
acetone on the catalyst, the experiments were performed at much
higher injected amounts of acetone than for methane for better gas
phase measurability.
[0130] Four experimental runs at lower concentrations were
conducted with an initial desired acetone gas concentration ranging
from 0.5 to 3 mmol/L by injecting 0.015, 0.025, 0.050, and 0.075 ml
of liquid acetone into the reactor system of about 300 ml volume.
However, as already shown in FIG. 7, due to acetone adsorption on
the catalyst the actual gas concentration dropped by about 30-40%
for the anatase and by up to 90% for the aerogel. Because of the
strong adsorption of aerogel, the acetone gas phase concentration
at 0.015 ml injection was only measurable for two hours after the
beginning of the reaction. FIGS. 15A and 15B summarize the
experimental runs on aerogel and anatase by plotting acetone
concentrations versus time. The sharp drop in concentration shown
in these two graphs, e.g., at t=5 hours for run A10 on the aerogel
catalyst corresponds to the mixing in the additional reactor volume
and adsorption on the catalyst upon opening the valves to the
catalytic cell. In FIG. 15A, only three experimental runs are
summarized. The gas phase concentration upon injection of 0.015 ml
acetone was too low to collect sufficient number of data.
[0131] FIGS. 16A-16C compare the performances of aerogel and
anatase catalysts by plotting the acetone gas phase concentration
versus time after mixing and adsorption have occurred. At time t=10
hours, the UV light was switched on to initiate the photocatalytic
reaction. The gas phase concentration of acetone in the aerogel
catalyst system is very low already due to the high adsorption.
This demonstrates that the aerogel acts as an excellent adsorbent
which immediately removes a high percentage of the contaminant from
the air stream. As a result, the time required to eliminate the
remaining acetone from the gas phase is shorter than for the
anatase powder, but in both cases the acetone concentration
eventually decreases to zero.
[0132] The data in FIG. 17A-17C show two interesting features.
First, when the UV light is switched on at t=10 hours, the gas
phase acetone concentration sharply increases before decreasing.
The increase is particularly noticeable in FIG. 17A, i.e., with
0.075 ml of injected acetone. The concentration increase is due to
the small but rapid warming of the aerogel catalyst by the UV
light, causing desorption of some of the absorbed acetone. The
decrease, of course, is due to the rapid photocatalytic oxidation.
The second interesting feature is that in the case of the largest
initial concentration of acetone (FIG. 17A) the acetone
concentration does not decrease to zero but levels out at a low but
unchanging value. The same phenomenon occurred with the anatase
catalyst under same conditions (see FIG. 16A). Calculations showed
that 0.075 ml of acetone requires more oxygen for complete
oxidation than is available in the reactor system. Thus, the point
at which the acetone concentration is unchanging corresponds to the
depletion of all available oxygen. By using pure oxygen instead of
air in the system, we confirmed that even these high initial
concentrations of acetone could be removed by photocatalytic
oxidation (FIG. 17A). The depletion of oxygen also corresponded
with an increase in the concentration of partial oxidation products
in the system.
[0133] Partial oxidation products were observed at very low
concentrations by gas chromatography, mass spectrometry, and
infrared spectroscopy. Other researchers have not noted the
appearance of acetone partial oxidation products, but that may be
due to the larger reactor residence times typically used in other
experiments. (The possibility of partial oxidation products was one
of the reasons methane was used as a contaminant, since methane is
more difficult to oxidize than most hydrocarbons. That methane
would be completely oxidized is evidence of the strong oxidizing
potential of the aerogel system). At the very low concentrations
observed, identification of partial oxidation products is difficult
and is still tentative.
[0134] The evaluation of reaction rate constants was based on the
method described in a paper by Sauer and Ollis.sup.4. First, the
adsorption equilibrium constants for acetone were determined. This
was done by linear plots based on a Langmuir equation assuming no
water is present: 6 = m a ads . a monol . = K a C a 1 + K a C a [
10 ]
[0135] where
[0136] m.sub.a.sup.ads,=amount of acetone adsorbed
[0137] m.sub.a.sup.monol=amount of acetone in a monolayer
[0138] K.sup.a=adsorption constant of acetone
[0139] C.sup.a=concentration of acetone in gas phase
[0140] .THETA.=Fraction of surface covered with acetone
[0141] The results for anatase and aerogel are summarized in FIGS.
18A-18B. The reaction rate of acetone, r.sub.a, is the sum of the
concentration change in the gas phase and change of concentration
of acetone adsorbed on the catalyst, or: 7 r a V cat = V sys C a t
+ m a ads . t [ 11 ]
[0142] where
[0143] V.sub.cat=catalyst volume
[0144] V.sub.sys=system volume.
[0145] Taking into account that the rate without significant
adsorption is 8 r a = kKC a 1 + KC a = V sys V cat C a t [ 12 ]
[0146] we can substitute ma.sub.ads from [10] and r.sub.a from [12]
into [11] to get after rearranging 9 - kKC a 1 + KC a = d ( V sys C
a + a K a C a 1 + K a C a ) / t [ 13 ]
[0147] If one defines 10 = V sys C a + a K a C a 1 + K a C a [ 14
]
[0148] as the total mass of acetone in the system equation [13] can
be arranged to give 11 - 1 t = 1 kKV cat C a + 1 kV cat [ 15 ] A
plot of - 1 t versus 1 C a should give a straight line with slope =
1 kKV cat and [ 16 ] intercept = 1 kV cat . [ 17 ]
[0149] FIGS. 19A-19B give the results of this plot for anatase and
aerogel, respectively. The data for anatase powder give a linear
plot with R.sup.2=0.9988. The reaction constant was found to be
k.sup.anatase=0.786 mg/(min cm.sup.3-cat). However, as shown in
FIG. 19B the data for the aerogel are quite scattered. For this
purpose, the data at low concentrations (0.025 ml acetone injected)
which contributed mainly to the large scattering were eliminated
for the determination of m.sub.a and k to give an estimate of the
reaction rate constant. This resulted in a rate constant value of
k.sup.aerogel=1.0475 mg/(min cm.sup.3-cat). This value is about 30%
higher than that of the anatase. However, it should be noted that
the data of aerogel have a high uncertainty due to the data
scatter. Also, these data are on a reactor volume basis. On a
surface area basis, the k.sup.anatase is much larger than
k.sup.aerogel, again indicative of the higher intrinsic activity of
the crystalline anatase versus the noncrystalline aerogel.
Conclusions
[0150] The titania aerogel photocatalyst shows remarkable promise
for complete oxidation processes. The aerogel successfully
catalyzed the complete oxidation of acetone as well as methane, a
very stable hydrocarbon. On either a catalyst mass, catalyst
volume, or illuminated external surface area basis, the aerogel
significantly outperformed a 99.9% anatase titania powder. This
superior performance is despite the very low crystallinity of the
aerogel (only crystalline TiO.sub.2 exhibits the necessary band gap
properties for photoactivity, with the anatase form much more
active than the rutile form of titania). That the low-crystallinity
aerogel performed so well has significance both for ease of
utilization of the aerogel and for improvements in photocatalytic
activity.
[0151] One of the challenges in development of a photocatalytic
reactor is ensuring that a high fraction of the catalyst surface
area is accessible to UV light. UV light penetrates only a few
microns into titania, thus layers of titania powder must be spread
very thin over a non-catalytic support (e.g., a monolith) or
titania powder must be continuously stirred or mixed such that
particles are periodically exposed to UV light (e.g., in a
fluidized bed reactor). These requirements adds cost and complexity
to the process. The aerogel, on the other hand, has extremely think
pore walls, perhaps on the order of a few tens of angstroms. UV
light may penetrate through hundreds of these pore walls,
activating a large surface area without the necessity of think
layers or constant agitation. Possessing very low crystallinity,
the aerogel has only a small fraction of photoactive sites. Those
sites that are activated by UV radiation, however, necessarily lie
very close to the solid-gas interface; thus, absorbed UV photons
lead much more efficiently to contaminant oxidation. Probably all
of the UV light is absorbed by the anatase powder catalyst (even in
a {fraction (1/32)} inch thick layers), but only a relatively small
fraction of the absorbed photons lead to oxidation. With thicker
pore walls, the electron holes created by UV absorption have a much
longer diffusion path to the sold-gas surface, and are much more
likely to recombine with electrons in the bulk solid before
reaching the surface. Moreover, even in the {fraction (1/32)} inch
thick layer, much of the anatase powder is probably not utilized at
all because complete absorption of the UV light is accomplished
very close to the surface of the bed. The characteristic of the
aerogel enabling the efficient use of thicker beds makes the
technology much more likely for commercialization for relatively
small scale applications such as indoor air decontamination.
[0152] Another characteristic of the aerogel which should be noted
is its high adsorption capacity. As was already noted, this
capacity suggests that sudden high concentrations (`spikes`) of a
strongly adsorbing contaminant such as acetone are quickly removed
from the gas stream, to be more slowly oxidized on the surface.
[0153] From the above description it is clear that the present
invention is well adapted to carry out the objects and to attain
the advantages mentioned herein as well as those inherent in the
invention. While presently preferred embodiments of the invention
have been described for purposes of this disclosure, it will be
understood that numerous changes may be made which will readily
suggest themselves to those skilled in the art and which are
accomplished within the spirit of the invention disclosed and as
defined in the appended claims.
* * * * *