U.S. patent application number 14/128483 was filed with the patent office on 2014-05-15 for device for the purification of exhaust gases from a heat engine, comprising a ceramic carrier and an active phase mechanically anchored in the carrier.
This patent application is currently assigned to L'Air Liquide Societe Anonyme Pour L'Etude Et L'Exploitation Des Procedes Georges Claude. The applicant listed for this patent is Claire Bonhomme, Thierry Chartier, Pascal Del-Gallo, Raphael Faure, Sebastien Goudalle, Fabrice Rossignol. Invention is credited to Claire Bonhomme, Thierry Chartier, Pascal Del-Gallo, Raphael Faure, Sebastien Goudalle, Fabrice Rossignol.
Application Number | 20140130482 14/128483 |
Document ID | / |
Family ID | 46397167 |
Filed Date | 2014-05-15 |
United States Patent
Application |
20140130482 |
Kind Code |
A1 |
Del-Gallo; Pascal ; et
al. |
May 15, 2014 |
Device for the Purification of Exhaust Gases from a Heat Engine,
Comprising a Ceramic Carrier and an Active Phase Mechanically
Anchored in the Carrier
Abstract
Device for the purification of exhaust gases from a thermal
combustion engine comprising: one or several ceramic catalyst
carriers comprising an arrangement of crystallites of the same
size, same isodiametric morphology and same chemical composition,
or approximately the same size, same isodiametric morphology and
same chemical composition in which each crystallite is in point or
quasi-point contact with the surrounding crystallites, and one or
several active phases for chemical destruction of impurities in the
exhaust gas comprising metallic particles mechanically anchored in
said catalyst carrier such that coalescence and mobility of each
particle are limited to a maximum volume corresponding to the
volume of a crystallite of said ceramic catalyst carrier.
Inventors: |
Del-Gallo; Pascal; (Dourdan,
FR) ; Rossignol; Fabrice; (Verneuil Sur Vienne,
FR) ; Chartier; Thierry; (Feytiat, FR) ;
Faure; Raphael; (Villebon-Sur-Yvette, FR) ; Goudalle;
Sebastien; (Sens, FR) ; Bonhomme; Claire;
(Panazol, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Del-Gallo; Pascal
Rossignol; Fabrice
Chartier; Thierry
Faure; Raphael
Goudalle; Sebastien
Bonhomme; Claire |
Dourdan
Verneuil Sur Vienne
Feytiat
Villebon-Sur-Yvette
Sens
Panazol |
|
FR
FR
FR
FR
FR
FR |
|
|
Assignee: |
L'Air Liquide Societe Anonyme Pour
L'Etude Et L'Exploitation Des Procedes Georges Claude
Paris
FR
Centre National De La Recherche Scientifique
Paris
FR
Universite De Limoges
Limoges Cedex
FR
|
Family ID: |
46397167 |
Appl. No.: |
14/128483 |
Filed: |
June 8, 2012 |
PCT Filed: |
June 8, 2012 |
PCT NO: |
PCT/EP2012/060904 |
371 Date: |
December 20, 2013 |
Current U.S.
Class: |
60/274 ;
60/299 |
Current CPC
Class: |
B01D 2255/1023 20130101;
B01J 23/005 20130101; B01D 2255/106 20130101; B01D 2258/014
20130101; B01J 21/005 20130101; C04B 38/06 20130101; Y02T 10/22
20130101; B01J 23/63 20130101; F01N 3/2832 20130101; C04B 2111/0081
20130101; B01D 2255/2092 20130101; Y02A 50/20 20180101; B01D
2255/1025 20130101; B01J 35/006 20130101; B01D 2255/20746 20130101;
B01J 37/0215 20130101; B01J 37/0242 20130101; B01D 2255/20715
20130101; Y02T 10/12 20130101; B01D 2255/1028 20130101; B01D
2255/20753 20130101; B01D 2255/1026 20130101; B01J 23/83 20130101;
B01D 2255/1021 20130101; B01D 2258/012 20130101; B01D 2255/2063
20130101; F01N 3/2825 20130101; B01D 2255/2065 20130101; B01D
2255/104 20130101; B01J 37/0045 20130101; F01N 3/18 20130101; Y02A
50/2324 20180101; B01D 2255/102 20130101; B01D 2255/9202 20130101;
B01D 2255/20761 20130101; B01D 53/945 20130101; C04B 38/06
20130101; C04B 35/00 20130101; C04B 38/0006 20130101; C04B 38/0051
20130101; C04B 38/0074 20130101; C04B 38/0096 20130101 |
Class at
Publication: |
60/274 ;
60/299 |
International
Class: |
F01N 3/18 20060101
F01N003/18 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 27, 2011 |
FR |
1155688 |
Claims
1-8. (canceled)
9. A device for the purification of exhaust gases from a thermal
combustion engine comprising: one or several ceramic catalyst
carriers comprising an arrangement of crystallites of the same
size, same isodiametric morphology and same chemical composition,
or approximately the same size, same isodiametric morphology and
same chemical composition in which each crystallite is in point or
quasi-point contact with the surrounding crystallites; the average
equivalent diameter of said crystallites being between 2 and 20 nm;
and one or several active phases for chemical destruction of
impurities in the exhaust gas comprising metallic particles
mechanically anchored in said catalyst carrier such that
coalescence and mobility of each particle are limited to a maximum
volume corresponding to the volume of a crystallite of said ceramic
catalyst carrier; the average equivalent diameter of said metallic
particles being between 2 and 20 nm.
10. The device of claim 9, wherein the arrangement is made of a
material selected from the group consisting of: i) alumina
(Al.sub.2O.sub.3) or ceria (CeO.sub.2) optionally stabilised with
gadolinium oxide; ii) zirconia (ZrO.sub.2) optionally stabilised
with yttrium oxide or spinel phase; iii) lanthanum oxide
(La.sub.2O.sub.3); and mixtures thereof.
11. The device of claim 9, wherein the metallic particles are
chosen from among: (i) noble metals selected from the group
consisting of Ruthenium, Rhodium, Palladium, Osmium, Iridium,
Platinum, and an alloy of one to three thereof; (ii) transition
metals selected from the group consisting of Nickel, Silver, Gold,
Cobalt, Copper, and an alloy of one to three thereof; and (iii) an
alloy of one to three of the noble metals and one to three of the
transition metals.
12. The device of claim 9, wherein the average equivalent diameter
of crystallites is between 5 and 15 nm, and the average equivalent
diameter of metallic particles is less than 10 nm.
13. The device of claim 9, wherein the arrangement of crystallites
is optimally a compact hexagonal or face-centred cubic stack in
which each crystallite is in point or quasi-point contact with not
more than 12 other crystallites within a 3-dimensional space.
14. A method for purification of exhaust gases from a thermal
combustion engine, comprising circulating exhaust gases through the
device of claim 9.
15. The method of claim 14, wherein the thermal combustion engine
is an automobile vehicle engine.
16. The method of claim 15, wherein the automobile vehicle engine
is a diesel engine.
17. The method of claim 15, wherein the thermal combustion engine
is a gasoline engine.
Description
[0001] The invention relates to a device for the purification of
exhaust gases from a thermal combustion engine, commonly called a
"catalytic converter", particularly for an automobile vehicle,
device comprising a carrier on which at least one catalyst is
deposited for the chemical destruction of impurities in exhaust
gases. The function of such a device is to at least partly
eliminate the polluting gases contained in the exhaust gases,
particularly carbon oxide, hydrocarbons and nitrogen oxides, by
transforming them through reduction or oxidation reactions.
[0002] In particular, the invention discloses exhaust gas purifying
devices comprising ceramic oxide carriers and active metal
particles, for which the structural characteristics and anchoring
of particles in the carrier improve performances over those of
conventional catalyst oxide carriers.
[0003] Synergies have been observed between different chemical and
petrochemical industrial applications and the operating conditions
of an automobile engine. It is observed that the method that uses a
temperature and gaseous atmosphere (H.sub.2, CO, CO.sub.2, residual
CH.sub.4, H.sub.2O) most similar to the method of an engine
operating at full load is the Steam Methane Reforming (SMR) method.
This is particularly true for catalytic materials for active phase
selection aspects (noble metals, Ni, etc.), for oxide carrier
and/or active phase degradation mechanisms, for operating
temperature zones (600-1000.degree. C.) and to a certain extent for
spatial velocities particularly in the framework of structured SMR
reactors-exchangers. The principal consequence is particularly very
similar physical degradation phenomena (temperature inducing
coalescence of nanoparticles, delamination of deposits, etc.).
[0004] A heterogeneous gas-solid catalyst is usually an inorganic
material composed of at least one oxide or other ceramic carrier on
which several active phases are dispersed that convert reagents
into products through repeated and uninterrupted cycles of
elementary phases (adsorption, dissociation, diffusion,
reaction-recombination, diffusion, desorption). In some cases, the
carrier may act not only physically (large porous volume and large
BET surface area to improve dispersion of active phases), but also
chemically (for example to accelerate adsorption, dissociation,
diffusion and desorption of specific molecules). The catalyst
participates in conversion by returning to the original state
thereof at the end of each cycle throughout the service life
thereof. A catalyst modifies/accelerates the reaction mechanism(s)
and the associated reaction rate(s) but does not change the
thermodynamics.
[0005] Access of reagents to active particles must be maximised if
the conversion rate by the carried catalysts is to be maximised. We
will start by summarising the main steps in a heterogeneous
catalysis reaction, to help understand the advantage of a carrier
such as that developed herein. A gas composed of molecules A passes
through a catalytic bed and reacts on the catalyst surface to form
a species B gas.
[0006] These various elementary steps are:
[0007] a) Transport of reagent A (volume diffusion), through a gas
layer as far as the external surface of the catalyst,
[0008] b) Diffusion of the species A (volume or molecular diffusion
(Knudsen)), through the porous lattice of the catalyst, as far as
the catalytic surface,
[0009] c) Adsorption of species A on the catalytic surface,
[0010] d) Reaction of A to form B on catalytic sites present on the
surface of the catalyst,
[0011] e) Desorption of product B from the surface,
[0012] f) Diffusion of species B through the porous lattice,
[0013] g) Transport of product B (volume diffusion) from the
external surface of the catalyst, through the gas layer, as far as
the gas flow.
[0014] European standard EURO 5 applicable since 1 Sep. 2009 (and
shortly EURO 6 that will become applicable on 1 Sep. 2014) obliges
motor vehicle manufacturers to drastically limit emissions of toxic
gases (CO, NOx, unburned hydrocarbons). Optimisation of catalytic
converters is now largely related to the optimisation of catalysts
(efficiency, service life).
[0015] As a reminder, a catalytic converter is composed of a
stainless steel conversion chamber into which exhaust gases are
introduced. These gases pass through a ceramic structure usually
composed of an oxide type (cordierite, mullite, etc.) of ceramic
honeycomb substrate. A so-called three-way catalyst (TWC) is
deposited on the walls of the ceramic substrate (in the form of a
honeycomb). The catalyst accelerates the transformation rate of
reagents into products. The objective in catalytic converters is to
limit emissions of toxic gases (CO, NOx and unburned hydrocarbons)
by transforming them mainly into water, CO.sub.2 and nitrogen.
[0016] By definition, a three-way catalyst is capable of performing
3 types of reactions simultaneously: [0017] reduction of nitrogen
oxides into nitrogen and in carbon dioxide:
2NO+2C).fwdarw.N.sub.2+2CO.sub.2 [0018] oxidation of carbon
monoxides into carbon dioxide: 2CO+O.sub.2.fwdarw.2CO.sub.2 and
[0019] oxidation of unburned hydrocarbons (HC) into carbon dioxide
and water:
4C.sub.xH.sub.y+(4x+y)O.sub.2.fwdarw.4xCO.sub.2+2yH.sub.2O
[0020] Oxidation reactions (requiring a high partial pressure of
oxygen) and reduction reactions (low partial pressure of oxygen)
add constraints. They require a very precise quantity of air to be
added into the fuel. A lambda probe placed on the exhaust measures
the output oxygen quantity. A control loop very precisely controls
the air/fuel ratio, keeping it at an ideal value.
[0021] Note that: [0022] The catalytic converter is only efficient
starting from about 250-300.degree. C. This is why short journeys
are problematic. [0023] The following parasitic reaction can occur
at high temperatures: 2NO+CO.fwdarw.N.sub.2O+CO.sub.2
[0024] The ceramic architectures of catalytic converters for
automobile depollution are usually honeycomb substrates and most
are composed of cordierite (2 MgO-2 Al.sub.2O.sub.3-5 SiO.sub.2) or
mullite. These architectures develop a low specific area (a few
m.sup.2/g) with a bulk porosity of 20% to 40%.
[0025] Oxides are classical active phase carriers: alumina for the
thermochemical stability thereof at low temperatures
(<800.degree. C.), ceria for the redox properties thereof with
oxygen, and zirconia for the chemical affinity thereof with
rhodium. For a long time, research to increase the specific area
focussed on .gamma., .delta. and .theta. forms of alumina (from 50
to 250 m.sup.2/g). Since then, ceria and zirconia carriers
developing 20 to 100 m.sup.2/g have been made. However, in all
cases, the carrier will thermally collapse after a few cycles
inducing a drop in the specific surface area, a drop in the porous
volume and an acceleration of metallic nanoparticle
migration/diffusion/coalescence phenomena. Oxide carriers were
stabilised by the addition of elements such as yttrium, gadolinium,
lanthanum, etc., in order to minimise these thermal collapse
phenomena of oxide carriers under operating conditions. Thus
La-Al.sub.2O.sub.3, CeGdO, ZrYO, CeZrYO, etc. are used, which
limits thermal collapse but does not minimise metallic particle
migration/sintering phenomena.
[0026] Many studies have been carried out on deactivation of
three-way catalysts, but they do not take account of problems
related to properties of the mechanical strength of the cordierite
structure (breakage due to vibrations). Deactivation phenomena may
be classified as shown in FIG. 1.
[0027] Reversible deactivation phenomena occur at low temperatures
(<300.degree. C.): [0028] Physisorption of products and
reagents, for example CO.sub.2 [0029] Chemisorption of products and
reagents (for example sulphur oxide on an oxide)
[0030] Deactivation phenomena that occur at high temperatures
(600-1000.degree. C.) are irreversible and are often reactions
between: [0031] Elements of the active phase carrier oxide(s)
[0032] Noble metals leading to the formation of unwanted alloys
[0033] Noble metals and the active phase oxide carrier (for example
migration of the Rh.sup.3+ ion in a .gamma. Al.sub.2O.sub.3
structure).
[0034] However, the phenomena that have the largest impact on
performances of high temperature catalysts are (i) sintering of the
active phase carrier oxide and (ii) coalescence of active phase
metallic particles (nanoparticle diffusion/segregation/coalescence
phenomena), the second phenomenon being accelerated by the first,
as is the case in the Steam Methane Reforming (SMR) method.
[0035] As such, the problem arises of providing a device for
purification of exhaust gases from a thermal combustion engine
comprising an improved catalyst capable of stabilising active phase
nanometric particles under conditions similar to those encountered
during steam methane reforming, so as to improve the performances
thereof.
[0036] One solution according to the invention is a purification
device for exhaust gases from a thermal combustion engine
comprising: [0037] a ceramic catalyst carrier comprising an
arrangement of crystallites of the same size, same isodiametric
morphology and same chemical composition, or approximately the same
size, same isodiametric morphology and same chemical composition in
which each crystallite is in point or quasi-point contact with the
surrounding crystallites, and [0038] an active phase for chemical
destruction of impurities in the exhaust gas comprising metallic
particles mechanically anchored in said catalyst carrier such that
coalescence and mobility of each particle are limited to a maximum
volume corresponding to the volume of a crystallite of said ceramic
catalyst carrier.
[0039] The first advantage of the proposed solution concerns the
ultra-divided meso-porous ceramic catalyst carrier of the active
phase(s). This carrier develops a large available specific area
greater than or equal to 20 m.sup.2/g, due to the size of the
constituent nanometric particles thereof and the arrangement
thereof. Furthermore, the carrier is stable under operating
conditions of catalytic converters; in other words, the carrier is
stable at temperatures of between 600.degree. C. and 1000.degree.
C. in an atmosphere containing a mix of exhaust gases (CO,
H.sub.2O, NO, N.sub.2, C.sub.xH.sub.y, O.sub.2, N.sub.2O . . . ).
This thermal stability is directly related to the microstructure of
the synthesised material (arrangement of crystallites of the same
size, the same isodiametric morphology and same chemical
composition, or approximately the same size, the same isodiametric
morphology and same chemical composition, in which each crystallite
is in point or quasi-point contact with the surrounding
crystallites) and related to the associated synthesis
method(s).
[0040] The particular architecture of the catalyst carrier has a
direct influence on the stability of metallic nanoparticles. The
arrangement of crystallites and the porosity is sufficient to
develop mechanical anchorage of said metallic nanoparticles on the
carrier surface.
[0041] At the same time, the excellent dispersion of active phases
thus obtained can result in a large reduction in the quantity of
noble metals used without any loss of catalytic performances.
[0042] FIG. 2 shows the mechanical blockage of metallic particles
by the ceramic catalyst carrier. Firstly, it is quite clear that
the elementary active particles will be no larger than the size of
a carrier crystallite. Secondly, the movement thereof under the
combined effect of high temperature and a steam-enriched atmosphere
is nevertheless limited to the potential wells materialised by the
space between two crystallites. The arrows represent the only
possible movement of metallic particles.
[0043] Finally, note that the mechanical blockage created by the
ceramic catalyst carrier limits possible coalescence of the active
particles.
[0044] The device according to the invention may have one or
several of the following features, depending on the case: [0045]
said arrangement is made of alumina (Al.sub.2O.sub.3) optionally
stabilised with lanthanum, cerium or zirconium, or made of ceria
(CeO.sub.2) optionally stabilised with gadolinium oxide, or made of
zirconia (ZrO.sub.2) optionally stabilised with yttrium oxide or
made of spinel phase or lanthanum oxide (La.sub.2O.sub.3) or a mix
of one or several of these compounds; [0046] metallic particles are
chosen from among:
[0047] (i) noble metals chosen from among Ruthenium, Rhodium,
Palladium, Silver, Osmium, Iridium, Platinum or an alloy of one,
two or three of these noble metals, or
[0048] (ii) transition metals chosen from among Nickel, Silver,
Gold, Cobalt and Copper, or an alloy of one, two or three of these
transition metals, or
[0049] (iii) an alloy of one, two or three of these noble metals,
and one, two or three of these transition metals; [0050] the
average equivalent diameter of crystallites is between 2 and 20 nm,
preferably between 5 and 15 nm, and the average equivalent diameter
of metallic particles is between 2 and 20 nm, and preferably less
than 10 nm; [0051] the arrangement of active phase carrier
crystallites is optimally a compact hexagonal or face-centred cubic
stack in which each crystallite is in point or quasi-point contact
with not more than 12 other crystallites within a 3-dimensional
space.
[0052] Preferably, the catalytic (substrate+catalyst) assembly used
in the purification device according to the invention may comprise
a substrate with various architectures such as cellular structures,
drums, monoliths, honeycomb structures, spheres, multi-scale
structured reactors-exchangers (.mu.reactors), etc., with a ceramic
or metallic or ceramic-coated metallic nature, on which the active
phase carrier can be deposited (washcoat).
[0053] This invention also relates to a purification method for
exhaust gases from a thermal combustion engine in which said
exhaust gases are circulated through a device according to the
invention.
[0054] The thermal combustion engine is preferably an automobile
vehicle engine, and particularly a diesel engine or a gasoline
engine.
[0055] We will now describe in detail how the ceramic
carrier-active phase assembly (catalyst) used in the purification
device according to the invention is synthesised.
[0056] A method for preparing a ceramic carrier-active phase
assembly may comprise the following steps:
[0057] a) preparation of a ceramic catalyst carrier comprising an
arrangement of crystallites with the same size, same morphology and
same chemical composition, or approximately the same size,
morphology and same chemical composition, in which each crystallite
is in point or quasi-point contact with the surrounding
crystallites,
[0058] b) impregnation of the ceramic catalyst carrier with a
precursor solution of the metallic active phase(s);
[0059] c) calcination under air of the impregnated catalyst at a
temperature of between 350.degree. C. and 1000.degree. C.,
preferably at a temperature of between 450.degree. C. and
700.degree. C., and even more preferably at a temperature of
500.degree. C. so as to obtain one of the oxidised active phase(s)
deposited on the surface of the ceramic catalyst carrier(s);
and
[0060] d) optional reduction in the oxidised active phase(s)
between 300.degree. C. and 1000.degree. C., preferably at a
temperature of between 300.degree. C. and 600.degree. C., and even
more preferably at a temperature of 300.degree. C.
[0061] Note that this method may comprise one or several of the
following features: [0062] the impregnation step b) is performed in
a vacuum throughout a period of between 5 and 60 minutes; [0063] in
step b), the active phase solution is a solution of rhodium nitrate
(Rh(NO.sub.3).sub.3, 2H.sub.2O) or a solution of nickel nitrate
(Ni(NO.sub.3).sub.2, 6H.sub.2O) or palladium nitrate
((Pd(NO.sub.3).sub.3, 2 H.sub.2O) or platinum nitrate
((Pt(NO.sub.3).sub.x),yH.sub.2O) or a mix of these solutions.
Carbonate, chloride precursors, etc., or a mix of various
precursors (nitrates, carbonates, etc.) containing noble metals
(Rh, Pt, Ir, Ru, Re, Pd) and/or transition metals (Ni, Cu, Co, . .
. ) may also be used; [0064] after step d), said method may also
comprise an aging step e) under operating conditions or conditions
similar to operating conditions of the catalyst. The first
operating cycle (stop/start) may be considered as an aging
step.
[0065] The ceramic catalyst carrier described in step a) of the
preparation method for the ceramic carrier-active phase assembly
used in the purification device according to the invention may be
prepared using two methods.
[0066] A first method will lead to a ceramic catalyst carrier
comprising a substrate and a film on the surface of said substrate
comprising an arrangement of crystallites of the same size, same
isodiametric morphology and same chemical composition, or
approximately the same size, same isodiametric morphology and same
chemical composition in which each crystallite is in point or
quasi-point contact with the surrounding crystallites.
[0067] A second method will lead to a ceramic catalyst carrier
containing pellets comprising an arrangement of crystallites of the
same size, same isodiametric morphology and same chemical
composition, or approximately the same size, the same isodiametric
morphology and same chemical composition in which each crystallite
is in point or quasi-point contact with the surrounding
crystallites.
[0068] Note that the pellets are approximately spherical.
[0069] The first method for preparing this ceramic catalyst carrier
includes the following steps:
[0070] i) Preparation of a sol comprising aluminium and/or
magnesium and/or cerium and/or zirconium and/or yttrium and/or
gadolinium and/or lanthanum nitrate and/or carbonate salts, a
surfactant and solvents such as water, ethanol and ammonia;
[0071] ii) Dipping of a substrate into the sol prepared in step
i);
[0072] iii) Drying of the substrate impregnated with the sol so as
to obtain a gelled composite material comprising a substrate coated
with a gelled film; and
[0073] iv) Calcination of the composite material gelled in step
iii) at a temperature typically between 500.degree. C. and
1000.degree. C. in air.
[0074] Preferably, the substrate used in this first method for
preparation of the ceramic catalyst carrier is made of dense
alumina.
[0075] The second method for preparation of the ceramic catalyst
carrier comprises the following steps:
[0076] i) Preparation of a sol comprising aluminium and/or
magnesium and/or cerium and/or zirconium and/or yttrium and/or
gadolinium and/or lanthanum nitrate and/or carbonate salts, a
surfactant and solvents such as water, ethanol and ammonia;
[0077] ii) Atomisation of the sol under hot air flow so as to
evaporate the solvent and form a micronic powder;
[0078] iii) Calcination of the powder at a temperature of between
500.degree. C. and 1000.degree. C.
[0079] The sol prepared in the two ceramic catalyst carrier
preparation methods preferably comprises four main constituents:
[0080] Inorganic precursors: for cost limitation reasons, we have
chosen to use magnesium and aluminium, cerium, zirconium, yttrium
nitrates or a mix of these nitrate salts. Other inorganic
precursors could be used (carbonates, sulphonates, chlorides, etc.)
alone or mixed in the method. The stoichiometry of nitrates in the
example may be verified by Inductively Coupled Plasma (ICP) before
the solubilisation thereof in osmosed water. [0081] The surfactant.
A Pluronic F127 triblock copolymer of the EO-PO-EO type could be
used. It has two hydrophilic blocks (EO) and a central hydrophobic
block (PO). [0082] The solvent (absolute ethanol). [0083]
NH.sub.3,H.sub.2O (28% by mass). The surfactant is solubilised in
an ammonia solution that creates hydrogen bonds between the
hydrophilic blocks and the inorganic species.
[0084] The first step consists of solubilising the surfactant (0.9
g) in absolute ethanol (23 mL) and in an ammonia solution (4.5 mL).
The mix is then heated under reflux for 1 h. The previously
prepared nitrate solution (20 mL) is then added to the mix drop by
drop. The whole is heated under reflux for 1 h and is then cooled
to ambient temperature. The sol thus synthesised is aged in a
ventilated drying oven in which the ambient temperature (20.degree.
C.) is precisely controlled.
[0085] In the case of the first synthesis method, dipping consists
of immersing a substrate into the sol and then removing same at
constant speed. The substrates used for our study are alumina
plates sintered at 1700.degree. C. for 1 h30 in air (relative
density of substrates=97% of the theoretical density). This
invention is applicable to substrates with various architectures
such as cellular structures, drums, monoliths, honeycomb
structures, spheres, multi-scale structured reactors-exchangers
(.mu.reactors), etc., of a ceramic or metallic type, or
ceramic-coated metallic type, and on which said carrier can be
deposited (wash coat).
[0086] When the substrate is removed, movement of the substrate
entrains the liquid forming a surface layer. This layer separates
into two, the inner part moves with the substrate while the outer
part drops into the receptacle. Progressive evaporation of the
solvent leads to the formation of a film on the surface of the
substrate.
[0087] The thickness of the deposit obtained can be estimated as a
function of the viscosity of the sol and the pulling rate (Equation
1):
Equation 1
e.infin.k v.sup.2/3 [0088] where k is the deposition constant that
depends on the viscosity and density of the sol and the
liquid-vapour surface tension, and v is the pulling rate.
[0089] Thus, the thickness deposit increases as the pulling rate
increases.
[0090] The dipped substrates are then oven dried at between
30.degree. C. and 70.degree. C. for several hours. A gel is then
formed. Calcination of the substrates in air can eliminate nitrates
and also decompose the surfactant and thus release porosity.
[0091] In the case of the second synthesis method, the atomisation
technique can transform a sol into a solid dry form (powder) by the
use of a hot intermediary (FIG. 3).
[0092] The principle is based on spraying fine droplets on the sol
3, in a chamber 4 under a hot air flow 2 in order to evaporate the
solvent. The powder obtained is entrained by the heat flux 5 as far
as a cyclone 6 that will separate air 7 from the powder 8.
[0093] The instrument that can be used within the scope of this
invention is a commercial model with the reference "190 Mini Spray
Dryer" made by Buchi.
[0094] The powder recovered at the end of atomisation is dried in a
drying oven at 70.degree. C. and is then calcined.
[0095] Calcination at 900.degree. C. destroys the mesostructure of
the deposit that was present at 500.degree. C. Crystallisation of
the phase (spinel in this example) leads to local disorganisation
of the porosity. Nevertheless, the result is a ceramic catalyst
carrier according to the invention, in other words an ultra-divided
and highly porous deposit with almost spherical particles in point
contact with each other (FIG. 4). FIG. 4 shows 3 high resolution
SEM micrographs of the catalyst carrier with three different
magnifications.
[0096] These active phase carrier particles with a size of the
order of about ten nanometres have a very narrow size grading
distribution centred at about 12 nm. The average size of
crystallites, spinel in this example, is 12 nm (measured by
small-angle X-ray diffraction, FIG. 5). This size corresponds to
the size of elementary particles observed in scanning electron
microscopy indicating that elementary particles are
monocrystalline.
[0097] Small-angle X-ray diffraction (angle 2.theta. values between
0.5 and 6.degree.): we can use this technique to determine the size
of crystallites of the catalyst carrier. The diffractometer used in
this study based on a Debye-Scherrer geometry is equipped with a
curved position sensitive detector (Inel CPS 120) at the centre of
which the sample is placed. The sample is a monocrystalline
sapphire substrate on which the sol was dip-coated. The Scherrer
formula is used to correlate the width of diffraction peaks at
mid-height with the size of the crystallites (Equation 2).
D = 0.9 .times. .lamda. .beta. cos .theta. Equation 2
##EQU00001##
[0098] D is the size of the crystallites (nm)
[0099] .lamda. is the wavelength of the K.alpha. line of Cu (1.5406
.ANG.)
[0100] .beta. is the width of the line at mid-height (in rad)
[0101] .theta. is the diffraction angle.
[0102] In the catalyst preparation method according to the
invention, the ceramic catalyst carrier is then impregnated with a
solution of Rh, and/or Pt, and/or Pd and/or Ni precursor. The
studied catalyst is the three-way catalyst for use in catalytic
converters.
[0103] Impregnation in the case of an active phase comprising
rhodium carried by a spinel carrier (catalyst called AlMg+Rh) is
performed in a vacuum for 15 minutes. A nitrate of Rh
(Rh(NO.sub.3).sub.3, 2H.sub.2O) was selected as the inorganic
precursor of Rh.
[0104] The concentration of Rh in the nitrate solution was fixed at
0.1 g/L. After impregnation, the catalyst is calcined in air at
500.degree. C. for 4 h. At this stage, we have a rhodium oxide
deposited on the surface of the ultra-divided mesoporous carrier.
The active phase is reduced under Ar--H.sub.2 (3% vol) at
300.degree. C. for 1 h.
[0105] Sizes and metallic dispersion at the carrier surface were
observed by transmission electron microscopy (FIG. 6a). These
observations reveal the presence of Rh particles in the elementary
state with a size of the order of one nanometre. These small
particles are concentrated around spinel particles in the
carrier.
[0106] After aging to simulate the conditions of this catalyst
(900.degree. C., 48 h) in a catalytic converter, Rh particles
coalesce to a size of 5 nm (FIG. 6b). At this stage, an Rh particle
is stabilised on a spinel carrier particle, which strongly reduces
the possibility of future coalescence of metallic particles during
operation of the catalyst.
[0107] In the case of an active phase comprising nickel (catalyst
called AlMg+Ni), the carrier is impregnated with a solution of Ni
nitrate (Ni(NO.sub.3).sub.2, 6H.sub.2O). The concentration of Ni in
this solution may be fixed at 5 g/L. After impregnation, the
catalyst may be calcined in air at 500.degree. C. for 4 h and then
reduced under Ar--H.sub.2 (3% vol) at 700.degree. C. for 2 h.
[0108] Results similar to those obtained with the AlMg+Rh catalyst
are obtained with the AlMg+Ni catalyst.
[0109] In the case of active phase(s) comprising rhodium, platinum
and palladium (catalyst called AlMg+RhPtPd), the carrier is
impregnated with a solution of nitrates containing said
elements.
[0110] Note that the study for the ultra-divided mesoporous ceramic
carrier only concerned spinel (MgAl.sub.2O.sub.4). The two
described carrier synthesis methods may for example be extrapolated
to ceria optionally doped with gadolinium, or zirconia optionally
doped with yttrium oxide.
[0111] The catalyst according to the invention was stabilised over
time.
[0112] The AlMg+Rh catalyst was aged for 20 days, after being
exposed to a temperature of the order of 650.degree. C., and
another sample was exposed to a temperature of the order of
850.degree. C.
[0113] The microstructure of catalysts following aging was observed
by scanning electron microscopy. Since the plates for the two
temperatures are similar, we will present the characteristics of
catalysts exposed to aging at 850.degree. C. (FIG. 7). The
atmospheres are very similar to the atmospheres in catalytic
converters.
[0114] The ultra-divided spinel phase carrier (ceramic catalyst
carrier) is preserved after aging and the increase in the size of
spinel particles is very limited.
[0115] The size of metallic particles after aging remains globally
less than or equal to the size of the elementary crystallites of
the spinel carrier.
[0116] The advantage of developing an ultra-divided carrier to
facilitate mechanical anchoring of active phases is demonstrated on
these micrographs (FIG. 7a). In this figure, we see that metallic
dispersion is better on the ultra-divided deposit than on an
alumina grain not coated with the deposit, visible on the
photograph on the left. It is impossible to anchor metallic
particles mechanically at locations at which there is no deposit
and coalescence is natural.
[0117] The catalyst according to the invention will preferably be
used for Three-Way Catalysts (TWC) in catalytic converters for
automobile depollution.
[0118] In the framework of this study, the reaction concerns
depollution of exhaust gases. This invention may be extended to
various applications in heterogeneous catalysis provided that the
active phase(s) is (are) adapted to the required catalytic reaction
(SMR, chemical, petrochemical, environmental reactions, etc.), on
an ultra-divided ceramic catalyst carrier based on spinel, alumina,
ceria, zirconia (optionally stabilised with yttrium) or a mix of
these compounds.
* * * * *