U.S. patent application number 09/851090 was filed with the patent office on 2001-11-01 for combustion catalyst and combustion process using such a catalyst.
Invention is credited to Euzen, Patrick, Mabilon, Gil, Rebours, Stephane, Tocque, Eric.
Application Number | 20010036433 09/851090 |
Document ID | / |
Family ID | 26231538 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010036433 |
Kind Code |
A1 |
Euzen, Patrick ; et
al. |
November 1, 2001 |
Combustion catalyst and combustion process using such a
catalyst
Abstract
A combustion catalyst comprising a monolithic substrate, a
porous support based on refractory inorganic oxide and an active
phase formed by cerium, iron and at least one metal selected from
the group formed by palladium and platinum, the content of porous
support being between 100 and 400 g per liter of catalyst; the
content of cerium being between 0.3 and 20% by weight with respect
to the porous support; the content of iron being between 0.01 and
3.5 % of iron by weight with respect to the porous support; and the
content of palladium and/or platinum being between 3 and 20 g per
liter of catalyst. The catalyst of the invention is used in
particular in processes for the catalytic combustion of
hydrocarbons, carbon monoxide, hydrogen or mixtures thereof, in
processes involving one or more catalytic stages.
Inventors: |
Euzen, Patrick; (Rueil
Malmaison, FR) ; Tocque, Eric; (Rueil-Malmaison,
FR) ; Rebours, Stephane; (Rueil Malmaison, FR)
; Mabilon, Gil; (Carrieres Sur Seine, FR) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
2200 CLARENDON BLVD.
SUITE 1400
ARLINGTON
VA
22201
US
|
Family ID: |
26231538 |
Appl. No.: |
09/851090 |
Filed: |
May 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09851090 |
May 9, 2001 |
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08857072 |
May 15, 1997 |
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09851090 |
May 9, 2001 |
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08558188 |
Nov 15, 1995 |
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Current U.S.
Class: |
423/245.3 ;
423/247; 423/248; 502/262; 502/304 |
Current CPC
Class: |
F23C 13/00 20130101;
B01J 23/894 20130101 |
Class at
Publication: |
423/245.3 ;
502/304; 502/262; 423/247; 423/248 |
International
Class: |
B01J 023/10; B01D
053/86 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 15, 1994 |
FR |
94/13.739 |
Claims
What is claimed:
1. A combustion catalyst comprising a monolithic substrate, a
porous support based on refractory inorganic oxide and an active
phase formed by cerium, iron and at least one metal selected from
the group formed by palladium and platinum; the content of porous
support being between 100 and 400 g per liter of catalyst; the
content of cerium being between 0.3 and 20% by weight with respect
to the porous support; the content of iron being between 0.01 and
3.5% of iron by-weight with respect to the porous support; and the
content of palladium and/or platinum being between 3 and 20 g per
liter of catalyst.
2. A catalyst according to claim I characterised in that the
content of porous support is between 200 and 350 g per liter of
catalyst, the content of cerium is between 2 and 15% by weight with
respect to the porous support, the content of iron is between 0.1
and 2! of iron by weight with respect to the support and the
content of palladium and/or platinum is between 5 and 15 g per
liter of catalyst.
3. A catalyst according to claim 1 characterised in that the porous
support based on the refractory inorganic oxide is selected from
the group formed by alpha alumina, delta alumina, eta alumina,
gamma alumina, kappa alumina, khi alumina, rho alumina, theta
alumina, silica, silica-aluminas, titanium oxide, zirconia and
mixtures thereof.
4. A catalyst according to claim 1 characterised in that said
porous support has a specific surface area between 20 and 250
m2/g.
5. A catalyst according to claim 1 characterised in that the porous
support based on a refractory inorganic oxide is selected from the
group formed by alpha alumina, delta alumina, eta alumina, gamma
alumina, kappa alumina, khi alumina, rho alumina and theta
alumina.
6. A catalyst according to claim 5 characterised in that said
support has been thermally stabilized by the introduction of at
least one compound selected from the group formed by oxides of
trivalent rare earths, oxides of alkaline earth metals and
silica.
7. A catalyst according to claim 6 characterised in that said
support has been thermally stabilized by silica.
8. A catalyst according to claim 7 characterised in that the
content of silica is from 1 to St by weight with respect to the
porous support.
9. A catalyst according to claim 1 characterised in that said
substrate is metallic or ceramic.
10. A process for the catalytic combustion of at least one
combustible substance selected from hydrocarbons, carbon monoxide,
hydrogen or mixtures thereof in any proportions, characterised by
using a catalyst according to claim 1.
11. A process for the catalytic combustion of at least one
combustible substance selected from hydrocarbons, carbon monoxide,
hydrogen or mixtures thereof in any proportions, said process
comprising a plurality of catalytic stages of which at least one
functions at temperatures of less than 1100.degree. C.,
characterised in that a catalyst according to claim 1 is used in
said stage or stages.
12. A process according to claim 10 for abating the pollution
produced by exhaust gases of vehicles with controlled richness that
run on natural gas.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of pending U.S.
application Ser. No. 08/558,188 filed Nov. 15, 1995 (Attorney
Docket: PET 1341), incorporated by reference herein.
SPECIFICATION
[0002] The present invention concerns a non-selective oxidation
catalyst and the use thereof in the catalytic combustion of
hydrocarbons, carbon monoxide, hydrogen or mixtures thereof.
[0003] Conventional combustion conducted in the presence of a
flame, which is usually employed in processes for the combustion of
hydrocarbons such as methane is a procedure which is difficult to
control. It occurs in a well-defined range of air/hydrocarbon
concentrations and, besides the formation of carbon dioxide and
water, it results in the production of pollutants such as carbon
monoxide and nitrogen oxides. Catalytic combustion produces few
pollutants such as NO.sub.x and CO. In addition the introduction of
a catalyst permits better control of total oxidation in a wide
range of values in respect of the air/hydrocarbon ratio. They can
be outside the limits of inflammability of conventional combustion.
It may also be mentioned that it results in more compact
apparatuses and that it makes it possible to burn a very wide
variety of compounds.
[0004] As described in particular by D Reay in `Catalytic
Combustion: Current Status and Implications for Energy Efficiency
in the Process Industries. Heat Recovery Systems & CHP, 13, No
5, pages 383-390, 1993` and D Jones and S Salfati in `Rev Gen Therm
Fr No 330-331, pages 4101-406, June-July 1989`, there are multiple
applications of catalytic combustion: radiant panels and tubes,
catalytic heaters, gas turbines, co-generation, burners, catalytic
sleeves for vapour-reforming tubes, production of hot gases in the
field of heating by direct contact and reactors with catalytic
plates. Because the standards relating to the NO.sub.x emitted by
combustion processes are becoming more severe at an ever quickening
rate, the catalytic combustion chamber can advantageously replace
conventional burners which are the origin of high proportions of
NO.sub.x. The operating conditions--highly oxidising medium--of a
catalytic combustion chamber are veil remote from the applications
of automobile post-combustion: treatment of the exhaust gases from
petrol vehicles operating at the Level of richness 1 with a high
content of NO.sub.x and treatment of the exhaust gases from diesel
vehicles with a high proportion of particles and NO.sub.x. Those
fundamental differences involve seeking dedicated formulations for
combustion catalysts.
[0005] Combustion catalysts are generally prepared from a
monolithic substrate of ceramic or metal, on which there is
deposited a fine support layer formed by one or more refractory
oxides of a surface area and porosity greater than those of the
monolithic substrate. The active phase which is composed
essentially of metals of the platinum group is dispersed on that
oxide.
[0006] Thermal stability, low-temperature catalytic activity and
stability of catalytic activity generally constitute the three main
criteria for selection of the catalyst.
[0007] There are combustion catalysts which are more resistant to
high temperature. In some combustion processes the catalysts may be
subjected to very high temperatures which are often higher than
1000.degree. C. In the course of their use at such high
temperatures however it is found that the catalysts suffer from
degradation which reduces their levels of catalytic performance.
Sintering of the support and sintering of the active phase and/or
encapsulation thereof by the support are part of the most generally
quoted causes for explaining such degradation. In the case of such
catalysts which operate at high temperature thermal resistance may
become the predominant criterion, to the detriment of catalytic
activity. The supports of those catalysts are generally
alumina-based. It is known to the man skilled in the art that the
drop in specific surface area can be effectively stabilized by a
suitable doping agent. Rare earths and silica are often mentioned
as being among the stabilising agents with the best levels of
performance in respect of the alumina. Catalysts prepared by that
procedure are described inter alla in U.S. Pat. No. 4,220,559. In
that document the catalyst comprises metals from the group of
platinum or transition metals which are deposited on alumina, an
oxide of a metal selected from the group formed by barium,
lanthanum and strontium and an oxide of a metal selected from the
group formed by tin, silicon, zirconium and molybdenum.
[0008] In addition, in order to limit sintering of the active
metallic phase, it has been proposed that various stabilising
agents based essentially on oxides of transition metals may be
added.
[0009] Thus, in US patent U.S. Pat. No. 4,857,499 the catalyst
comprises a porous support in which the diameter of the pores is
between 150 and 300 .ANG. and of which the proportion by weight
with respect to the substrate is preferably between 50 and 200 g/l,
an active phase including at least 10% by weight, with respect to
the porous support, of a precious metal selected from the group
formed by palladium and platinum; a first promoter including at
least one element selected from the group formed by lanthanum,
cerium, praseodymium, necdymium, barium, strontium, calcium and
oxides thereof, of which the proportion by weight, with respect to
the porous support, is between 5 and 20%; a second promoter
including at least one element selected from the group formed by
magnesium, silicon and oxides thereof, of which the proportion by
weight, with respect to the active phase, is less than or equal to
10%, and a third promoter including at least one element selected
from the group formed by nickel, zirconium, cobalt, iron and
manganese and oxides thereof, of which the proportion by weight,
with respect to the active phase, is less than or equal to 10%. In
addition said catalyst can be deposited on a monolithic substrate
belonging to the group formed by cordierite, mullite, alpha
alumina, zirconia and titanium oxide; the proportion by weight of
porous support with respect to the volume of substrate being
between 50 and 200 g/l.
[0010] In U.S. patent U.S. Pat. No. 4,793,797 the catalyst
comprises an Inorganic support selected from the group formed by
oxides, carbides and nitrides of elements belonging to groups IIa,
IIIa and IV of the periodic system of elements or selected from the
group formed by La-.beta.-Al.sub.2O.sub.3,
Nd-.beta.-Al.sub.2O.sub.2, Ce-.beta.-Al.sub.2O.sub.3 or
Prop-.beta.-Al.sub.2O.sub.3, at least one precious metal selected
from the group formed by palladium, platinum,rhodium and ruthenium,
and at least one oxide of a base metal selected from the group
formed by magnesium, manganese, cobalt, nickel, strontium, niobium,
zinc, tin, chromium and zirconium, such that the atomic ratio of
the base metal to the precious metal is between 0.1 and 10.
[0011] In regard to some thereof those catalysts exhibit increased
durability with respect to the active metallic phase alone. However
the doping agents used are adapted to very severe temperature
conditions which may exceed 1000.degree. C. They do not make it
possible effectively to limit the deterioration in the levels of
performance of the catalyst which occurs at moderate temperatures
and which may be due to various causes that are different from
those which are the origin of the deterioration at high
temperatures.
[0012] Moreover, combustion catalysts have also been proposed,
based on hexaaluminates containing manganese, affording a good
compromise in terms of catalytic activity/thermal stability, as
described particular in U.S. patent U.S. Pat. No. 4,788,174. The
oxidation catalyst which is thus proposed may be represented by the
following formula:
A.sub.1-zC.sub.zB.sub.zAl.sub.12-yO.sub.19-.alpha.'
[0013] wherein:
[0014] A is at least one element selected from the group formed by
Ba, Ca and Sr with (0.0.ltoreq.0.ltoreq.0.4);
[0015] B is at least one element selected from the group formed by
Mn, Fe, Co, Ni, Cu and Cr with (x.ltoreq.y.ltoreq.2x);
[0016] C is K and/or Rb; and
[0017] .alpha.=11/2 {X-z (X-Y)+xZ-3Y) in which X, Y and Z
respectively represent the valencies of the elements A, C and
B.
[0018] Such catalysts however are found to exhibit activity at low
temperature which is inadequate to meet the requirements of a
combustion process. In order to remedy that disadvantage it has
been proposed that a precious metal can be added to such catalysts,
as described in particular in U.S. patent U.S. Pat. No. 4,959,339.
The catalyst proposed in that way is represented by the
formula:
A.sub.1-zC.sub.zB.sub.xD.sub.uAl-.sub.12-y-uO.sub.19-.alpha.
[0019] wherein
[0020] A is at least one element selected from the group formed by
Ba, Ca and Sr with (0.0.ltoreq.z.ltoreq.0.4);
[0021] B is at least one element selected from the group formed by
Mn, Fe, Co, Ni, Cu and Cr with (x.ltoreq.y.ltoreq.2x);
[0022] C is at least one element selected from the group formed by
K, Rb and the rare earths,
[0023] D is at least one element selected from the group formed by
Au, Ag, Pd, Pt and another precious metal of the group of platinum
with x+u.ltoreq.4; and
[0024] .alpha.=11/2 {X-z (X-Y)+xz+uU-3y-3u} wherein X, Y, Z and U
respectively represent the valencies of the elements A, C, B and
D.
[0025] In regard to some thereof those catalysts have a level of
low-temperature activity which is increased in relation to a
catalyst without a metallic phase.
[0026] It has also been proposed that a plurality of different
catalysts can be juxtaposed in a reactor having catalytic stages;
the first catalysts being more specifically dedicated to starting
the combustion reaction, the following catalysts serving to
stabilise the high temperature combustion reaction, and the number
of catalytic stages (or zones J being adjusted in dependence on the
conditions imposed by the use envisaged. Thus the following systems
are known:
[0027] First catalytic zone: Pd and Pt and NiO; and second
catalytic zone: Pt and Pd; for example as described in European
patent application EP-A-198 948;
[0028] First catalytic zone: Pd and/or Pt; second catalytic zone:
Sr.sub.0.8La.sub.0.2 MnAl.sub.11O.sub.19.alpha. and third catalytic
zone: Sr.sub.0.8La.sub.0.2M.sub.nAl.sub.110.sub.19-.alpha.; for
example as described in Japanese patent application JP-A-04/197
443; and
[0029] First catalytic zone; Pd and (Pt or Ag); second catalytic
zone: Pd and (Pt or Ag); and third catalytic zone: perovskite
ABO.sub.3 or oxide of metal of group V (Nb or V), group VI (Cr) or
group VIII (Fe, Co, Ni); for example as described in international
patent applications WO-A-9219848 and WO-A-92/9849.
[0030] Furthermore, it is known that as regards motor vehicles that
are powered by natural gas, natural gas is a promising fuel that
responds to the growing concerns regarding environmental
protection. It is a fuel that is used today by more than one
million vehicles in the world (270,000 in Italy, 250,000 in Russia,
150,000 in Argentina, 50,000 in New Zealand, 40,000 in the United
States, and 40,000 in Canada). Private and commercial vehicles run
on gasoline or natural gas bicarburation. Vehicles with diesel
engines (in particular buses) have been adapted to run on natural
gas. Limited development of these types of vehicles is planned in
several European countries (the Netherlands, Denmark, Switzerland,
Belgium) and more energetically in Canada, the United States, and
Australia. Methane represents on the order of 5 to 10% (about
100-200 ppmC) of emissions from gasoline-powered vehicle
hydrocarbons. This proportion increases to 10-20% after passage
over a catalyst because the methane is less well eliminated than
the other hydrocarbons. It represents more than 95% of the
hydrocarbons (about 1500 ppmC, or 5 to 10 times more than the
emissions of gasoline engines) that are contained in the exhaust
gases of vehicles with regulated richness that run on natural gas.
Thus, for the standardized 13-mode European cycle of a bus engine
that runs on natural gas in a lean mixture, several measurements of
the composition of the exhaust gases by gas chromatography on line
show that the latter does not vary significantly either as a
function of richness or as a function of load and conditions (see
Tables A and B below).
1TABLE A Influence of richness on the composition for the full-load
point of 1260 rpm 1000 m.N Lacq. meth- ethyl- pro- pro- iso-bu-
acet- bu- Gas ane eth-ane ene pane pyl-ene tane ylene tane 97.4
1.87 -- 0.11 -- -- -- 0.21 R = 97.23 1.88 0.69 0.09 0.02 0.02 0.04
0.03 0.66 R = 97.43 2.02 0.33 0.1 0.01 0.03 0.08 0.00 0.60 R =
96.28 2.51 0.99 0.09 0.02 0.04 0.06 0.00 0.58
[0031]
2TABLE B Comparison of the composition at iso-richness 0.60 for two
points with low load, 1260 rpm 100 m.N and 2100 rpm 90 m.N meth-
ethyl- pro- pro- iso-bu- acet- bu- ane eth-ane ene pane pyl-ene
tane ylene tane 1260 96.51 2.29 0.93 0.09 0.02 0.02 0.08 0.04 rpm
2100 95.46 3.08 1.19 0.09 0.03 0.06 0.07 0.0 rpm
[0032] In spite of the numerous improving works which have already
been produced, there is still an attraction in seeking catalysts
which enjoy an increased level of activity and stability, in
particular at low temperature. Indeed the solutions proposed such
as formulations based on hexaaluminates which are doped by a
precious metal or the use of different formulations in a reactor
having a plurality of catalytic stages do not solve the problem of
stability of the active phase at low temperature which is also the
cause of a deterioration in the levels of performance. Among the
causes envisaged for that deterioration in the levels of
low-temperature performance, sintering and/or poisoning of the
metallic phase as well as a modification in the oxidation state of
the active phase are part of those which are most generally
referred to.
[0033] Moreover, in accordance with European patent EP-B-27069,
catalysts are known for the treatment of internal combustion engine
exhaust gases, comprising iron and cerium which are associated with
metals from the group of platinum, which are deposited on a
refractory inorganic oxide.
[0034] The research work carried on by the applicants led them to
discover that surprisingly catalysts containing both iron, cerium
and precious metals, while remedying the disadvantages exhibited by
the prior-art catalysts, are found to exhibit an excellent degree
of activity as well as remarkable stability in the course of
time.
[0035] The present invention therefore proposes a combustion
catalyst characterised in that it comprises a monolithic substrate,
a porous support based on refractory inorganic oxide and an active
phase formed by cerium, iron and at least on metal selected from
the group formed by palladium and platinum; the content of porous
support being between 100 and 400 g per liter of catalyst; the
content of cerium being between 0.3 and 20% by weight with respect
to the porous support; the content of iron being between 0.01 and
3.5% of iron by weight with respect to the support; and the content
of palladium and/or platinum being between 3 and 20 g per liter of
catalyst.
[0036] In accordance with preferred features of the catalyst of the
present invention the proportion of porous support is between 200
and 350 g per liter of catalyst; the proportion of cerium is
between 2 and 15% by weight with respect to the porous support; the
proportion of iron is between 0.1 and 2% of iron by weight with
respect to the support; and the proportion of palladium and/or
platinum is between 5 and 15 g per liter of catalyst.
[0037] The proportion of porous support in the catalyst according
to the invention preferably varies between 100 and 400 g per liter
of catalyst and still more preferably between 200 and 350 g/l. If
the content of porous support is less than 100 g the level of
catalytic activity is not adequate. Conversely a content of porous
support of higher than 400 g/l is also harmful in terms of
catalytic activity as it results in the passages in the monolith
becoming blocked.
[0038] In the catalysts of the invention the monolithic substrate
may consist of a monolith with a cellular structure of ceramic or
metal (winding or stacking of metallic strips or association of
metallic fibres or metallic wires in the form of a monolith with a
fibrous structure). The ceramic used may be mullite, cordierite,
alpha alumina, zirconia, aluminium titanate, silicon carbide,
silicon nitride or mixtures thereof. Those monolithic substrates
are produced by extrusion. The metallic alloys used most preferably
have refractory properties. They may be composed for example of
iron, chromium, aluminium and cerium or yttrium such as the steel
Gilphal 135.RTM. from the company Imphy. The metallic substrate may
be previously subjected to an oxidising treatment at a temperature
of between 700.degree. C. and 1200.degree. C., preferably between
800 and 1000.degree. C. The density of cells, that is to say the
number of cells per section of monolith, is generally between 50
and 600 cells per square inch (7.75 to 93 cells per cm.sup.2).
[0039] The catalysts according to the invention provide improved
levels of performance in particular in processes for the catalytic
combustion of hydrocarbons, carbon monoxide, hydrogen or mixtures
thereof. They can also be used however in all catalytic oxidation
processes which operate at elevated temperatures.
[0040] Preparation and shaping of the support may constitute the
first step in preparation of the catalysts. The support based on
refractory oxide which is used according to the invention is
generally selected from the group formed by refractory oxides of
the metals from groups IIa, IIIa, IVa and IVb of the periodic
system of elements and mixtures thereof in any proportions.
[0041] In most cases aluminium oxide of the general formula
Al.sub.2O.sub.3, nH.sub.2O is used. Its specific surface area is
between 10 and 500 m /g. That oxide in which n is between 0 and 0.6
is conventionally obtained by controlled hydration of hydroxides in
which 1.ltoreq.n.ltoreq.3. Those hydroxides are themselves prepared
by precipitation in an aqueous medium of aluminium salts by bases
or acids. The precipitation and ageing conditions define a number
of forms of hydroxides of which the most common are boehmite (n=1),
gibbsite and bayerite (n=3). In dependence on the hydrothermal
treatment conditions those hydroxides give a plurality of
transition aluminas or oxides. The forms are thus denoted alpha,
delta, eta, gamma, kappa, khi, rho and theta. They are essentially
differentiated by the organisation of their crystalline structures.
When heat treatments are carried out those different forms are
capable of developing amongst each other in accordance with a
complex relationship which depends on the operating treatment
conditions. The alpha form which has a very low specific surface
area is stable at higher temperature. The preference is to use
aluminas which have a specific surface area of between 20 and 250
m/g and in particular gamma and/or delta alumina.
[0042] In order to enhance the thermal stability of the oxide or
oxides in question, various compounds may be incorporated in the
porous support, either directly in the form of pigments or in the
form of precursor compounds of oxides. Rare earths, alkaline-earth
metals and silica which are among the stabilising agents which
afford the highest levels of performance for alumina may
advantageously be incorporated in the porous support.
[0043] In general those supports used in accordance with the
present invention may advantageously have been treated, as is well
known to the man skilled in the art, by porogenic agents such as
those based on cellulose, naphthalene, natural gums or synthetic
polymers, in such a way as to impart desired porosity properties to
them.
[0044] The content of metal of the group formed by platinum and
palladium in the catalyst according to the invention varies between
3 and 20 g per liter of catalyst and preferably between 5 and 15
g/l. If the content of precious metal is less than 3 g, the
catalytic activity is not sufficiently high to satisfy the
requirements of a combustion process. Conversely, when the content
of precious metal exceeds 20 g, a further increase in the content
of precious metal does not permit a significant increase in the
level of catalytic activity. According to the invention palladium
is preferred. However platinum may advantageously be used for a
combustion stage operating at relatively low temperatures, for
example at about 500.degree. C., or in combination with
palladium.
[0045] The presence of iron and cerium which are deposited
simultaneously on the one or more refractory inorganic oxides makes
it possible to enhance the activity and stability of the catalyst,
in the course of time.
[0046] The content of iron in the catalysts according to the
invention is between 0.01 and 3.5% by weight with respect to the
support and more particularly between 0.1 and 2%. If the content of
iron exceeds 3.5% the iron can then greatly accelerate the drop in
specific surface area of,the porous alumina-based support.
[0047] The content of cerium in the catalysts of the present
invention is between 0.3 and 20% by weight with respect to the
support and preferably between 2 and 15% by weight with respect to
the porous support. If the content of cerium is lower than 0.3%
this does not satisfactorily promote catalytic activity. Conversely
when the cerium content exceeds 20% by weight with respect to the
porous support a further increase in the cerium content does not
permit a significant increase in catalytic activity.
[0048] Preparation of those catalysts which are deposited on a
substrate consists of a coating step, in the course of which the
substrate is immersed in a suspension containing the precursors of
the components of the catalyst, and is then dried and roasted after
evacuation of the excess of the suspension. A second step referred
to as an impregnation step permits the active metals to be
deposited. For that, the coated substrate is brought into contact
with one or more solutions of the precursor or precursors of the
active metals. After having been possibly drained the substrate
which is thus coated and impregnated is dried and subjected to a
heat treatment.
[0049] The deposit of iron and cerium on the catalyst support of
the present invention can be produced using any of the procedures
known to the man skilled in the art and may be effected at any
moment in preparation of the catalyst. They may be introduced in
the form of solid compounds (oxides, hydroxides, carbonates,
hydroxycarbonates or insoluble salts) or soluble compounds
(nitrates, sulphates, chlorides, alcoholates) into the coating
suspension and/or pre-impregnated on to one of the constituents of
the coating suspension and/or deposited on the porous support prior
to impregnation of the metals and/or coimpregnated with the metals
depending on the procedure envisaged. In the situation where the
iron and cerium are deposited after shaping of the aluminas
possibly containing other metals the methods used may be for
example dry impregnation, impregnation by an excess of solution or
ion exchange. When using a support which has already been shaped, a
preferred method of introducing that additional element is
impregnation in an aqueous medium, using an excess of solution. To
remove the impregnation solvent the impregnation operation is
followed by a drying operation and a roasting step in air at a
temperature of between 300 and 900.degree. C.
[0050] In accordance with a particular mode of operation the
support is successively impregnated with a solution containing
compounds containing iron and cerium and then with a solution or
solutions containing compounds of the precious metals which are to
be introduced.
[0051] As compounds of iron and cerium which can be used, mention
will be made in particular of the salts of iron and cerium and more
particularly ferric nitrate, ammoniacal iron citrate, ferric
chloride and cerous nitrate, cerous acetate, cerous chloride and
ammoniacal ceric nitrate.
[0052] The precursors of the metals of the group formed by platinum
and palladium are those which are conventionally used for the
preparation of catalysts, in particular chlorides, chloro
complexes, nitrates, ammino complexes and acetylacetonates. By way
of example mention may be made of chloroplatinic acid, palladium
chloride, platinum, tetrammine chloride, dinitrodiamminoplatinum
and palladium nitrate.
[0053] The depth of impregnation may advantageously be regulated by
the use of methods known to the man skilled in the art and in
particular by the addition to the solution of precious metals of a
certain amount of organic or inorganic acid. Nitric, hydrochloric
and hydrofluoric acids or acetic, citric and oxalic acids are
generally used.
[0054] The catalysts according to the invention provide improved
levels of performance, especially in processes for the catalytic
combustion of hydrocarbons such as methane, carbon monoxide,
hydrogen or mixtures thereof. They can also be used however in any
catalytic processes requiring elevated temperatures.
[0055] In addition the catalytic combustion reactors may comprise
one or more catalytic stages, the formulations of which may be
different. The catalysts of the present invention may be used in
reactors having one stage or a plurality of catalytic stages. In
the latter case they are preferably used in the catalytic stage or
stages which operates or operate at temperatures below 1100.degree.
C.
[0056] The following Examples illustrate the invention without
however limiting it:
[0057] The various precursors used are commercial products from
PROLABO.RTM.. The elementary composition of the catalysts was
determined by X-ray fluorescence (PHILIPS PW 1480.RTM.).
EXAMPLE 1
[0058] Preparation of a Catalyst C1 According to the Invention
[0059] Iron and cerium are deposited on gamma alumina by
impregnation of 700 g of alumina with an aqueous solution of cerous
nitrate and ferric nitrate. That solution contains the equivalent
of 45 g of cerium oxide (CeO.sub.2) and 15 g of iron oxide
(Fe.sub.2O.sub.3).
[0060] The impregnated alumina is then dried at 150.degree. C. and
then roasted in air at 600.degree. C. for a period of 3 hours.
[0061] A coating suspension is prepared from two liters of
deionised water to which there are added the equivalent of 12 g of
nitric acid, 600 g of alumina of gamma type which has been
previously impregnated with iron and cerium, and 140 g of
pseudo-boehnite with 72% of dry matter. That suspension is crushed
in such a way that the size of the particles is less than 10
microns.
[0062] In a first step referred to as the coating step a ceramic
monolith (cordierite) of 0. 84 liter having 62 cells per cm (400
cells per square inch) is immersed in the suspension and then
drained before the excess of suspension is removed by blowing. The
support is then dried and then roasted in an oven in which the
temperature is maintained at 600.degree. C. for 2 hours. Those
immersion, blowing and roasting steps are repeated a second time in
order to deposit the equivalent of 120 g of porous support per
liter of catalyst (substrate).
[0063] In a second step referred to as the impregnation step the
coated monolith is immersed in a solution of palladium nitrate in
such a way that the amount of palladium fixed after drying and
roasting at 500.degree. C. for a period of 2 hours is 3% by weight
of palladium with respect to the porous support, that is to say,
expressed with respect to the catalyst: 3.6 g of palladium per
liter of catalyst.
[0064] This catalyst C1 prepared in that way contains by weight
with respect to the porous support 4.13% of cerium, 1.31% of iron
and 3% of palladium.
EXAMPLE 2 (comparative)
[0065] Preparation of a Catalyst C2
[0066] To show the effect of the cerium on catalytic activity, a
coating suspension is prepared from two liters of deionised water
to which there are added the equivalent of 12 g of nitric acid, 600
g of alumina of gamma type which has been previously impregnated
with iron and 140 g of pseudo-boehmite with 72% of dry matter. That
suspension is crushed in such a way that the size of the particles
is less than 10 microns.
[0067] A ceramic monolith of 0.84 liter is coated by that alumina
suspension using the process of Example 1 so as to deposit 120 g of
porous support per liter of catalyst (substrate).
[0068] The monolith is then impregnated by a solution of palladium
so as to deposit by weight 3% of palladium with respect to the
coated porous support, that is to say expressed with respect to the
catalyst: 3.6 g of palladium per liter of catalyst.
[0069] The catalyst C2 prepared in that way contains by weight with
respect to the porous support 1.31% of iron and 3% of
palladium.
EXAMPLE 3 (comparative)
[0070] Preparation of a Catalyst C3
[0071] To show the effect of the iron on catalytic activity a
coating suspension is prepared from two liters of deionised water
to which there are added the equivalent of 12 g of nitric acid, 600
g of alumina of gamma type which has been previously impregnated
solely with cerium and 140 g of pseudo-boehmite with 72% of dry
matter. That suspension is crushed in such a way that the size of
the pacles is less than 10 microns.
[0072] A ceramic monolith of 0.84 liter is coated with that alumina
suspension using the process of Example I so as to deposit 120 g of
porous support per liter of catalyst (substrate).
[0073] The monolith is then impregnated with a solution of
palladium so as to deposit by weight 3% of palladium with respect
to the coated porous support, that is to say expressed with respect
to the catalyst: 3.6 g of palladium per liter of catalyst.
[0074] The catalyst C3 prepared in that way contains by weight with
respect to the porous support 4.15% of cerium and 3% of
palladium.
EXAMPLE 4 (comparative)
[0075] Preparation of a Catalyst C4
[0076] To show the effect of the iron and cerium on catalytic
activity, a coating suspension is prepared from two liters of
deionised water to which there are added the equivalent of 12 g of
nitric acid, 600 g of alumina of gamma type without iron or cerium
and 140 g of pseudoboehmite with 72% of dry matter. That suspension
is crushed in such a way that the size of the particles is less
than 10 microns. A ceramic monolith of 0.84 liter is coated with
that alumina suspension using the process of Example 1 so as to
deposit 120 g of alumina per liter of catalyst (substrate).
[0077] The monolith is then impregnated with a solution of
palladium so as to deposit by weight 3% of palladium with respect
to the coated porous support, that is to say expressed with respect
to the catalyst: 3.6 g of palladium per liter of catalyst.
EXAMPLE 5
[0078] Preparation of a catalyst C5 according to the invention
coating suspension is prepared from two liters of deionised water
to which there are added the equivalent of 12 g of nitric acid, 600
g of alumina of gamma type previously impregnated with cerium and
iron and 140 g of pseudo-boehmite with 72% of dry matter. That
suspension is crushed in such a way that the size of the particles
is less than 10 microns.
[0079] A ceramic monolith of 0.84 liter is coated with that alumina
suspension using the process of Example 1 so as to deposit 120 g of
porous support per liter of catalyst (substrate).
[0080] The monolith is then impregnated with a solution of
palladium so as to deposit by weight 3% of palladium with respect
to the coated porous support, that is to say expressed with respect
to the catalyst: 3.6 g of palladium per liter of catalyst.
[0081] The catalyst C5 prepared in that way contains by weight with
respect to the porous support 8.15% of cerium, 1.3% of iron and 3%
of palladium.
EXAMPLE 6 (comparative)
[0082] Preparation of a Catalyst C6
[0083] A coating suspension is prepared from two liters of
dejonised water to which there are added the equivalent of 12 g of
nitric acid, 600 g of alumina of gamma type previously impregnated
with cerium and 140 g of pseudo-boehmite with 72% of dry matter.
That suspension is crushed in such a way that the size of the
particles is less than 10 microns.
[0084] A ceramic monolith of 0.84 liter is coated with that alumina
suspension using the process of Example 1 so as to deposit 120 g of
porous support per liter of catalyst (substrate). The monolith is
then impregnated with a solution of palladium so as to deposit by
weight 3% of palladium with respect to the coated porous support,
that is to say expressed with respect to the catalyst: 3.6 g of
palladium per liter of catalyst.
[0085] The catalyst C6 prepared in that way contains by weight with
respect to the porous support 40% of cerium and 3% of
palladium.
EXAMPLE 7
[0086] Catalytic Activity of Catalysts C1 to C6.
[0087] The performances of the catalysts are compared in terms of
the reaction for the combustion of methane, the main constituent of
natural gas. Taking the prepared catalysts (references C1 to C6),
cylinders measuring 1.5 cm in diameter and 5 cm in length are cut
out, in the longitudinal direction of the passages.
[0088] The tests are conducted in a laboratory reactor comprising a
tube into which the catalyst is introduced. The tube is disposed at
the centre of a cylindrical oven which can be raised to a
temperature of 1500.degree. C. An air-methane mixture with 3.5% by
volume of methane is prepared by means of mass flow rate regulators
and passed to the intake of the reactor. The hourly flow rate of
the gases is 50,000 times greater than the volume of the substrate
(VVH=50,000 h.sup.-1). The concentration of methane at the intake
and the discharge of the reactor is determined by means of a flame
ionisation detector (JUM engineering analyser model FID 3-300).
Conversion into methane is the ratio in percentage between the
difference in concentration of methane between the intake and the
discharge and the concentration at the intake.
[0089] After a rise in temperature in the reaction mixture at
5.degree. C./min from 250.degree. C. to 530.degree. C., the intake
temperature of the reaction mixture is fixed at that temperature.
Conversion of the methane is ascertained after 36 hours of
operation under steady-state operating conditions. That period of
time makes it possible significantly to discriminate the
formulations involved, in dependence on their capacity for
stabilising the combustion of methane.
[0090] Table 1 sets out the elementary compositions of the
catalysts C1 to C6 and the levels of conversion obtained after 36
hours of operation in a steady-state condition. Table 1 clearly
shows the synergy effect as between the iron and the cerium, which
results in better stability of catalytic activity for the catalysts
which are prepared according to the invention.
3TABLE 1 Elementary composition of the catalysts C1 to C6 and
conversions of those catalysts obtained after 36 hours of operation
under steady-state conditions. Content of Pd Content of Fe (in (in
% by wt Conversion Content of Ce (In % by wt with with respect
Content of Pd in % after Number of Reference of % by wt with
respect respect to the to the (in g/l of 36 hours of the Example
the catalyst to the support) support) support) catalyst) operation
Example 1 C1 4.13 1.31 3 3.6 95 according to the invention Example
2 C2 0 1.31 3 3.6 45 (comparative) Example 3 C3 4.15 0 3 3.6 85
(comparative) Example 4 C4 0 0 3 3.6 45 (comparative) Example 5 C5
8.15 1.3 3 3.6 >90 according to the invention Example 6 C6 40 0
3 3.6 45 (comparative)
EXAMPLE 8
[0091] Preparation of Catalyst C7 According to the Invention
[0092] A coating suspension is prepared from two liters of
deionised water to which there are added the equivalent of 12 g of
nitric acid, 600 g of alumina of gamma type previously impregnated
with iron and cerium using the operating procedure described in
Example 1 and 140 g of pseudo-boehmite with 72% of dry matter. That
suspension is crushed in such a way that the size of the particles
is less than 10 microns.
[0093] A ceramic monolith of 0.85 liter is coated with that
suspension using the operating procedure of Example I so as to
deposit 120 g of porous support per liter of catalyst
(substrate).
[0094] The catalyst C7 prepared in that way contains by weight with
respect to the porous support 4.13% of cerium, 1.31% of iron and
10% of palladium.
EXAMPLE 9 (comparative)
[0095] Preparation of a Catalyst C8 in Accordance with the Prior
Art
[0096] A catalyst C8 based on iron, cerium and palladium is
prepared using the operating procedure of Example 1 of U.S. Pat.
No. 4,857,499 so as to deposit 120 g of porous support per liter of
catalyst.
[0097] That catalyst C8 contains 4.13% of cerium and 1.31% of iron
by weight with respect to the porous support and by weight with
respect to the volume of catalyst: 12 g/l of palladium.
EXAMPLE 10
[0098] Catalytic Activity of Catalysts C7 and C8
[0099] Taking the prepared catalysts (references C8 and C7),
cylinders measuring 1.5 cm in diameter and 5 cm in length are cut
out in the longitudinal direction of the passages.
[0100] The evaluation procedure of Example 7 is adopted in order to
compare catalysts C7 and C8.
[0101] Table 2 sets out the elementary compositions of catalysts C7
and C8 and the degrees of conversion obtained after 36 hours of
operation in a steady-state condition.
4TABLE 2 Elementary composition of catalysts C7 and C8 and
conversions of such catalysts obtained after 36 hours of operation
under steady-state conditions. Content Reference of Pd (in %
Content of Conversion of Content Content by wt with Pd (in g/l in %
the of of respect to of after 36 hours catalyst Ce (%) Fe (%) the
support) catalyst) of operation C7 4.13 1.31 10 12 >95 according
to the invention C8 com- 4.13 1.31 10 12 75 parative
EXAMPLE 11
[0102] Preparation of Catalysts C9 and C10 According to the
Invention
[0103] A new suspension is prepared from two liters of deionised
water to which there are added the equivalent of 12 g of nitric
acid, 600 g of alumina of gamma type and 140 g of pseudo-boehmite
with 72% of dry matter.
[0104] Two monoliths of ceramic of 0.84 liter are coated with that
suspension so as to deposit 120 g per liter of catalyst
(substrate).
[0105] Each coated monolith is then impregnated with an aqueous
solution of cerous nitrate and ferric nitrate. It is then dried at
120.degree. C. and roasted at 500.degree. C. for a period of 2
hours.
[0106] Each monolith is then impregnated separately by a solution
of palladium so as respectively to deposit 10% and 5% by weight of
palladium with respect to the impregnated coated layer, that is to
say expressed with respect to the catalyst: 12 g and 6 g
respectively of palladium per liter of catalyst.
[0107] The catalysts C9 and C10 prepared in that way contain by
weight with respect to the impregnated coated layer 4.13% of
cerium, 1.31% of iron and 10% of palladium for C9 and 5% of
palladium for C10 respectively.
EXAMPLE 12 (comparative)
[0108] Preparation of Catalysts C11 and C12 According to the Prior
Art
[0109] A monolith of 0.84 liter is coated by an alumina suspension
using the mode of operation of Example 11 so as to deposit 120 g of
alumina per liter.
[0110] That monolith is then impregnated with iron and cerium using
the procedure described in Example 11.
[0111] The monolith is then impregnated with a palladium solution
so as respectively to deposit 1% and 0.5% by weight of palladium
with respect to the impregnated coated layer, that is to say
expressed with respect to the catalyst: 1.2 g and 0.6 g of
palladium per liter of catalyst respectively. The catalysts C11 and
C12 prepared in that way contain by weight with respect to the
impregnated coated layer 4.13% of cerium, 1.31% of iron and 1% of
palladium for C11 and 0.5% of palladium for C12 respectively.
EXAMPLE 13
[0112] Catalytic Activity of Catalysts C9 to C12
[0113] Taking the prepared catalysts (references C9 to C12)
cylinders measuring 1.5 cm in diameter and 5 cm in length are cut
out in the longitudinal direction of the passages.
[0114] The evaluation procedure of Example 7 is adopted to compare
catalysts C9 to C12 having different palladium contents.
[0115] Table 3 sets out the elementary compositions of the
catalysts C9 to C12 and the degrees of conversion obtained after 36
hours of operation under steady-state conditions.
5TABLE 3 Elementary composition of catalysts C9 to C12 and
conversions of such catalysts obtained after 36 hours of
steady-state conditions. Content of Pd Content of Pd Conversion in
% Reference of Content of Content of (% with respect (in g/l of
after 36 hours the catalyst Ce (%) Fe (%) to the support) catalyst)
of operation C9 according 4.13 1.31 10 12 >95 to the invention
C10 according 4.13 1.31 5 6 >95 to the invention C11 comparative
4.13 1.31 1 1.2 .apprxeq.85 C12 comparative 4.13 1.31 0.5 0.6
50
[0116] This Table clearly shows that a content of precious metal
higher than those which are generally used in a post-combustion
situation is necessary to satisfy the requirements of a catalytic
combustion process. In contrast an excessively high content of
precious metal does not significantly improve the levels of
performance.
EXAMPLE 14 (comparative)
[0117] Preparation of a Catalyst C13
[0118] In order to observe the effect of lanthanum, a good
inhibitor for sintering of alumina, on the level of stability of
catalytic activity, a coating suspension is prepared from two
liters of deionised water to which there are added the equivalent
of 12 g of nitric acid, 600 g of alumina of gamma type previously
impregnated with lanthanum (21 g of La.sub.2O.sub.3) and 140 g of
pseudo-boehmite with 72% of dry matter. That suspension is crushed
in such a way that the size of the particles is less than 10
microns.
[0119] A ceramic monolith of 0.84 liter is coated with that
suspension using the process of Example 1 so as to deposit 120 g of
porous support per liter of catalyst (substrate).
[0120] The monolith is then impregnated with a solution of
palladium so as to deposit by weight 3% of palladium with respect
to the coated porous support, that is to say with respect to the
catalyst: 3.6 g of palladium per liter of catalyst.
[0121] The catalyst C13 prepared in that way contains by weight
with respect to the porous support 3% of La.sub.2O.sub.3 and 3% of
palladium.
EXAMPLE 15 (comparative)
[0122] Preparation of a Catalyst C14 According to the Prior Art
[0123] In order to observe the effect of silica, which is a good
inhibitor for sintering of alumina, on the level of stability of
catalytic activity, a coating suspension is prepared from two
liters of deionised water to which there are added the equivalent
of 12 g of nitric acid, 600 g of alumina of gamma type previously
impregnated with silicon and 140 g of pseudo-boehmite with 72% of
dry matter. That suspension is crushed in such a way that the size
of the particles is less than 10 microns. The evaluation procedure
of Example 7 is adopted in order to compare catalysts C1 and C13 to
C15 so as to evaluate the effect of the stabilising agents for
alumina on the level of stability of catalytic activity under
steady-state conditions.
[0124] Table 4 sets out the elementary compositions of the
catalysts C1 and C13 to C15 and the degrees of conversion obtained
after 36 hours of operation under steady-state conditions.
[0125] Table 4 clearly shows that the promoters based on rare
earth, alkaline earth metals or silica which are adapted to inhibit
sintering of the alumina at high temperature do not make it
possible effectively to limit the drop in catalytic activity
observed in steady-state conditions (catalysts C13 to C15). On the
other hand catalyst C1 according to the invention retains its
activity.
6TABLE 4 Composition of catalysts C1 and C13 to C15 and conversion
of such catalysts obtained after 36 hours of operation under
steady-state conditions. Content of Content of Conversion in %
Reference of Reference of Content of Content of Pd (in g/l
stabilising after 36 hours the Example the catalyst Ce (%) Fe (%)
of catalyst) oxide (%) of operation Example 1 according C1 4.13
1.31 3.6 0 >95 to the invention Example 14 C13 0 0 3.6
La.sub.2O.sub.3 .apprxeq.46 (comparative) (3%) Example 15 C14 0 0
3.6 SiO.sub.2 (4%) .apprxeq.69 (comparative) Example 16 C15 0 0 3.6
BaO (3%) .apprxeq.45 (comparative)
EXAMPLE 18
[0126] Preparation of a Catalyst C16 According to the Invention
[0127] A coating suspension is prepared from two liters of
deionised water to which there are added the equivalent of 12 g of
nitric acid, 600 g of alumina of gamma type previously impregnated
with iron and cerium using the operating procedure described in
Example 1 and 140 g of pseudoboehmite with 72% of dry matter. That
suspension is crushed in such a way that the size of the particles
is less than 10 microns.
[0128] A ceramic monolith of 0.84 liter is coated with that
suspension using the process of Example 1 so as to deposit 120 g of
porous support per liter of catalyst (substrate).
[0129] The monolith is then impregnated with a solution of
palladium so as to deposit by weight 5! of palladium with respect
to the coated porous support, that is to say with respect to the
catalyst: 6 g of palladium per liter of catalyst.
[0130] The catalyst C16 prepared in that way contains by weight
with respect to the porous support 4.13% of cerium, 1.31% of iron
and 5% of palladium.
EXAMPLE 19 (comparative)
[0131] Preparation of a Catalyst C17
[0132] A ceramic monolith of 0.84 liter is coated with a suspension
prepared as described in Example 14 using the process of Example 1
so as to deposit 120 g of porous support per liter of catalyst
(substrate).
[0133] That monolith is then impregnated with a solution of
palladium so as to deposit by weight 5% of palladium with respect
to the coated porous support, that is to say with respect to the
catalyst: 6 g of palladium per liter of catalyst.
[0134] The catalyst C17 prepared in that way contains by weight
with respect to the porous support 3% of La.sub.2O.sub.3 and 5% of
palladium.
EXAMPLE 20 (comparative)
[0135] Preparation of a Catalyst C18
[0136] A ceramic monolith of 0.84 liter is coated with a suspension
prepared as described in Example 18 using the process of Example 1
so as to deposit 120 g of porous support per liter of catalyst
(substrate).
[0137] That monolith is then to deposit by weight 5% of support,
which is equivalent In a second step impregnated by a solution of
5% of manganese with respect 6 g of manganese per liter of
catalyst.
[0138] The monolith is then impregnated by a solution of palladium
so as palladium with respect to the coated porous to 6 g of
palladium per liter of catalyst. that coated and impregnated
monolith is manganese nitrate so as to deposit by weight to the
coated porous support, that is to say 6 g of manganese per liter of
catalyst.
[0139] The catalyst C18 prepared in that way contains by weight
with respect to the porous support 3% of La.sub.2O.sub.3, 5% of
palladium and 5% of manganese.
EXAMPLE 21 (comparative)
[0140] Preparation of a Catalyst C19
[0141] A ceramic monolith of 0.84 liter is coated by a suspension
prepared as described in Example 18 using the process of Example 1
so as to deposit 120 g of porous support per liter of catalyst
(substrate).
[0142] That monolith is then impregnated by a solution of palladium
so as to deposit by weight 5% of palladium with respect to the
coated porous support, that is to say with respect to the catalyst:
6 g of palladium per liter of catalyst.
[0143] In a second step the coated and impregnated monolith is
impregnated by a solution of zinc nitrate in such a way that the
amount of zinc deposited is equal to 5% by weight with respect to
the coated layer, that is to say with respect to the catalyst: 6 g
of zinc per liter of catalyst.
[0144] The catalyst C19 prepared in respect to the porous support
3% of La.sub.2O.sub.3, 5% of palladium and 5% of zinc.
EXAMPLE 22
[0145] Catalytic Activity of the Catalysts C16 to C19
[0146] Taking the prepared catalysts (references C16 to C18)
cylinders measuring 1.5 cm in diameter and 5 cm in length are cut
out in the longitudinal direction of the passages.
[0147] The evaluation procedure of Example 7 is adopted in order to
compare catalysts C16 to C18 in order to value the effect of the
stabilising agents for the metallic phase on the level of stability
of catalytic activity.
[0148] Table 5 sets out the elementary compositions of the
catalysts C16 to C18 and the degrees of conversion obtained after
36 hours of operation under steady-state conditions.
7TABLE 5 Elemetary composition of catalysts C16 to C19 and
conversions of those catalysts obtained after 36 hours of operation
under steady- state conditions Conver- sion in % after Refer- Con-
Con- Content Con- 36 Reference ence tent tent of tent Content of
hours of the of the of Ce of Fe La.sub.2O.sub.3 of Pd stabilising
of op- Example catalyst (%) (%) (%) (g/l) agent (g/l) eration
Example C16 4.18 1.31 0 6 0 >95 18 accord- ing to the invention
Example C17 0 0 3 6 0 69 19 (com- parative) Example C18 0 0 3 6 Mg
(6 g) 60 20 (com- parative) Example C19 0 0 3 6 Zn (6 g) 60 21
(com- parative)
[0149] Table 5 clearly shows that the doping agents for stabilizing
the metallic phase which are suitable for inhibiting sintering of
the metallic phase at high temperature do not permit an effective
limitation on the drop in catalytic activity observed in the
steady-state condition (catalysts C17 to C19). In contrast catalyst
C16 retains its activity.
EXAMPLE 23
[0150] Preparation of Catalysts C20 to C25 According to the
Invention
[0151] In order to evaluate the impact of different doping agents
for the alumina (Si, La, Ba) on the stability of the catalyst
according to the invention, three identical suspensions of alumina
with 30% of dry matter are prepared. Added to those three
suspensions are respectively a solution of silicon, a solution of
lanthanum and a solution of barium, in such a way that the (doping
cation/Al.sub.total) atomic ratio=0.01.
[0152] Three ceramic monoliths of 0.84 l are coated with those
suspensions using the process of Example 1 so as to deposit 250 g
of porous support per liter of catalyst (substrate).
[0153] Those monoliths are then impregnated with a solution of iron
and cerium using the procedure described in Example 11.
[0154] Finally each of those three monoliths is impregnated with a
solution of palladium so as to deposit by weight 2.4% of palladium
with respect to the coated porous support, that is to say with
respect to the catalyst: 6 g of palladium per liter of
catalyst.
[0155] The catalyst C20 prepared in that way contains by weight
with respect to the porous support 4.13% of cerium, 1.31% of iron,
0.55% of Si and 5% of palladium, the catalyst C21 prepared in that
way contains by weight with respect to the porous support 4.13% of
cerium, 1.31% of iron, 2.7% of La and 5% of palladium, and the
catalyst C22 prepared in that way contains by weight with respect
to the porous support 4.13% of cerium, 1.31% of iron, 2.7% of Ba
and 5% of palladium.
[0156] In addition catalysts C23, C24 and C25 containing a higher
proportion of silicon, namely 1%, 2% and 3% respectively, are
prepared in the same manner as catalyst C20.
EXAMPLE 24
[0157] Thermal Stability of Catalysts C20 to C25 According to the
Invention
[0158] The hydrothermal ageing test is conducted in a laboratory
reactor comprising a tube into which the catalyst is introduced.
The tube is disposed within a cylindrical oven which can be raised
to a temperature of 1200.degree. C. An air/1% water vapour mixture
is passed to the intake of the reactor. The flow rate is 11/h/gram
of catalyst. The temperature is fixed at 900.degree. C., measured
by means of a thermocouple, and the duration of the treatment is 4
hours. Those operating conditions were chosen as they are
representative of the conditions of operation of a combustion
catalyst in a first stage of a catalytic combustion reactor. The
surface area of the catalyst was measured after such a treatment in
dependence on the nature of the doping agents. Table 6 sets out the
elementary compositions involved and the measured surface area.
8TABLE 6 Elementary composition of catalysts C16 and C20 to C25 and
surface area after hydrothermal ageing at 900.degree. C.-4h-1%
water. Refer- Con- Con- Content of Measured surface ence tent tent
Content of stabilising after hydrothermal of the of CE of Fe Pd (in
g/l agent aging catalyst (%) (%) of catalyst) (% by wt) (m.sup.2/g)
C16 4.13 1.31 6 0 131 C20 4.13 1.31 6 Si (0.55%) 154 C21 4.13 1.31
6 La (2.7%) 139 C22 4.13 1.31 6 Ba (2.7%) 131 C23 4.13 1.31 6 Si
(1%) 161 C24 4.13 1.31 6 Si (2%) 165 C25 4.13 1.31 6 Si (3%)
163
[0159] Table 6 shows that it may be particularly advantageous to
add silicon in order to improve the resistance to sintering of the
support. The preferred content of silicon is between 1 and 3%. In
contrast lanthanum and barium which are rather attractive doping
agents for inhibiting the transformation of alumina occurring at
about 1000.degree. C.-1200.degree. C.: theta alumina.fwdarw.alpha
alumina (see the article by D L Trium entitled: `Thermal stability
of catalysts supports` in the review Stud Surf Sci Catal Vol 68,
29-51 (1991) are found to be less effective than silicon.
EXAMPLE 25
[0160] Preparation of Catalysts C26, C27, C28, C29 and C30
According to the Invention
[0161] In order to show the effect of the content of porous support
on the stability of catalytic activity, a suspension is prepared as
in Example 11. Five monoliths of ceramic, of 0.84 liter, are coated
with that suspension using the process described in Example 1 so as
to deposit respectively 200 g, 250 g, 300 g, 350 g and 400 g of
porous support per liter of catalyst (substrate).
[0162] Those five monoliths when thus coated are impregnated with a
solution of iron and cerium using the procedure described in
Example 11.
[0163] Those five monoliths are then impregnated to an iso-content
of palladium with respect to the substrate, that is to say 6 g of
palladium per liter of catalyst, corresponding respectively to 3%,
2.4%, 2%, 1.71% and 1.5% by weight of palladium with respect to the
porous support.
[0164] The catalysts prepared in that way are respectively numbered
C26, C27, C28, C29 and C30.
EXAMPLE 26 (comparative)
[0165] Preparation of a Catalyst C31
[0166] In order to evaluate the effect of the increase in the
content of porous support to an iso-content of metal on the level
of stability of catalytic activity of a formulation which is
representative of the prior art, a ceramic monolith of 0.84 liter
is coated by a suspension prepared as described in Example 14 so as
to deposit 200 g of porous support per liter of catalyst
(substrate).
[0167] The monolith when so coated is impregnated with a solution
of iron and cerium using the procedure described in Example 11.
[0168] That monolith is then impregnated by a solution of palladium
so as to deposit 6 g of palladium with respect to a liter of
catalyst.
[0169] The catalyst C31 prepared in that way contains by weight
with respect to the porous support 3% of La.sub.2O.sub.3 and 3% of
palladium.
EXAMPLE 27 (comparative)
[0170] Preparation of a Catalyst C32
[0171] A ceramic monolith of 0.84 liter is coated by a suspension
containing iron, cerium and palladium, so as to deposit 200 g of
porous support per liter of catalyst (substrate). The contents of
iron and cerium are identical to those of catalysts C26 to C30.
[0172] The catalyst C32 prepared in that way contains by weight
with respect to the porous support 4.13% of cerium, 1.31% of iron
and 0.7% of palladium, that is to say 1.4 g of palladium per liter
of catalyst.
EXAMPLE 28
[0173] Catalytic Activity of the Catalysts C10, C17 and C26 to
32
[0174] Taking the prepared catalysts (references C10, C17, C26,
C27, C28, C29, C30, C31 and C32), cylinders measuring 1.5 cm in
diameter and 5 cm in length are cut out in the longitudinal
direction of the passages. The evaluation procedure of Example 7 is
adopted to compare those catalysts in order to evaluate the effect
of the content of porous support on the level of stability of
catalytic activity.
[0175] Table 7 sets out the elementary compositions of catalysts
C10, C17 and C26 to C32 and the degrees of conversion obtained
after 36 hours of operation under steady-state conditions.
9TABLE 7 Elementary composition of catalysts C10, C17 and C26 to
C32 and conversions of the catalysts obtained after 36 hours of
operation under steady-state conditions. Content of Content of % of
conversion Reference porous Content of Content of Content of Pd (in
g/l after 36 hours of of the catalyst support g/l Ce (%) Fe (%)
La.sub.2O.sub.3 (%) of catalyst) operation C10 according 120 4.18
1.31 0 6 >95 to the invention C26 according to 200 4.18 1.31 0 6
>98 the invention C27 according to 250 4.18 1.31 0 6 >98 the
invention C28 according to 300 4.18 1.31 0 6 >98 the invention
C29 according to 350 4.18 1.31 0 6 >95 the invention C30
according to 400 4.18 1.31 0 6 >90 the invention C17 comparative
120 0 0 3 6 .apprxeq.46 C31 comparative 200 0 0 3 6 .apprxeq.46 C32
comparative 200 4.18 1.31 0 1.4 .apprxeq.80
[0176] Table 7 clearly shows that the increase in the content of
alumina to iso-content of palladium for the catalyst of the present
invention increases the level of stability of catalytic activity of
the catalyst. A content of higher than 200 g/l significantly
improves that stability, but an excessively high content of porous
support, that is to say higher than 400 g/l, is found to be harmful
by virtue in particular of blockage of the passages in the monolith
in the coating operation. On the other hand, for the catalyst C31
of the prior art, the increase in the content of porous support
does not improve the stability of the catalyst in comparison with
catalyst C17 of Example 19. FIG. 1 which shows the developments in
the conversion of methane in dependence on time for the catalyst
C26 according to the invention and the catalyst C31 of the prior
art clearly shows that the catalytic activity of the catalyst C31
begins to oscillate after several hours of operation while the
catalyst C26 retains a high level of activity (>98%) after 36
hours. As regards catalyst C32 whose contents of palladium and
alumina are representative of automobile post-combustion catalysts,
this does not retain sufficient stability in respect of catalytic
combustion of methane, an application in which the operating
conditions are greatly removed from the operating conditions of
post-combustion.
EXAMPLE 29 (comparative)
[0177] Catalytic Activity in Post-Combustion of the Catalyst C26
According to the Invention
[0178] Taking the prepared catalyst C26 a cylinder measuring 30 mm
in diameter and 76 mm in length is cut out in the longitudinal
direction of the passages. That catalyst is tested on a laboratory
assembly as described in Example 10 of patent application FR-A-90
15750, lodged by the present applicants, in order to determine its
behavior in relation to the oxidation of carbon monoxide,
hydrocarbons and the reduction of nitrogen monoxide. The
proportions of the mixture studied, which are characteristic of
petrol vehicle exhaust gases, are as follows:
[0179] CO: 9000 ppm
[0180] NO: 200 ppm
[0181] CH.sub.4: 97 ppm.sup.-
[0182] C.sub.2H.sub.2: 102 ppm (methane equivalent)
[0183] C.sub.2H.sub.4: 581 ppm (methane equivalent)
[0184] C.sub.3H.sub.3: 720 ppm (methane equivalent)
[0185] CO.sub.2: 10% H.sub.2O:
[0186] H.sub.2O: 7%
[0187] O.sub.2: 0.6%
[0188] N.sub.2: balance
[0189] FIG. 2 shows the respective developments in the conversion
of CO and hydrocarbons and the reduction of NO in dependence on the
richness of the mixture. It is noted in particular that the
reduction in NO is not total in contrast to a conventional
post-combustion catalyst and that the NO reduction range is much
narrower than for a conventional post-combustion catalyst (FIG.
3).
[0190] This Example 29 shows that a catalyst according to the
invention is not suitable for treating the exhaust gases from a
petrol vehicle operating at richness 1.
EXAMPLE 30
[0191] Preparation of a Catalyst C33 According to the Invention
[0192] To evaluate the effect of platinum on the combustion of
methane a catalyst C33 is prepared in the same manner as catalyst
C10 but with the palladium being replaced by platinum, to an
iso-content of precious metal. The catalyst C33 contains 6 g of
platinum per liter of catalyst.
[0193] The catalyst C33 prepared in that way contains by weight
with respect to the porous support 4.13% of cerium, 1.31% of iron
and 5% of platinum.
EXAMPLE 31
[0194] Catalytic Activity of Catalysts C10 and C33 According to the
Invention
[0195] Taking the prepared catalysts (references C10 and C33),
cylinders measuring 1.5 cm in diameter and- 5 cm in length are cut
out in the longitudinal direction of the passages.
[0196] The tests are conducted in a laboratory assembly as
described in Example 6. The reaction mixture is raised at a rate of
5.degree. C./min from 250.degree. C. to 875.degree. C. The hourly
flow rate of the gases is 50,000 times higher than the volume of
the substrate (VVH=50,000.sup.h-1) The concentration of methane at
the intake and at the discharge of the reactor is determined by
means of a flame ionisation detector. The methane conversion is the
ratio in percentage between the difference in concentration of
methane between the intake and the discharge of the reactor and the
concentration at the intake. FIG. 4 shows the developments in the
methane conversion in dependence on the intake temperature of the
mixture in dependence either on catalyst C10 or catalyst C33.
[0197] Platinum is found to afford a lower level of performance
than palladium for starting combustion of the methane: the
half-convention tempertature is about 300.degree. C. for catalyst
C10 as against about 470.degree. C. for catalyst 33.
EXAMPLE 32
[0198] Catalytic Activity of Catalysts C1, C5, C7, C9, C10 and C16
for Abating the Pollution Produced by the Exhaust Gases from Motor
Vehicles that are Powered by Natural Gas
[0199] In the prepared catalysts (references C1, C2, C3, C4, C5, C6
and C7), cylinders are cut that are 1.5 cm in diameter and 5 cm in
length in the longitudinal direction of the channels.
[0200] The tests are carried out in a laboratory reactor that
contains a pipe into which the catalyst is introduced. This pipe is
placed in the center of a cylindrical furnace that can be brought
to a temperature of 900.degree. C. An oxygen-nitrogen-methane
mixture at 0.15% in volume of methane, 0.30% oxygen, and 99.55%
nitrogen is prepared with mass flow regulators and sent to the
intake of the reactor. With this methane content being
representative of the composition of the exhaust gases of motor
vehicles that are powered by natural gas. The hourly flow rate of
the gases is 50,000 times greater than the volume of the substrate
(VVH=50,000 h.sup.-1). The concentration of methane at the intake
and at the outlet of the reactor is determined with a flame
ionization detector (analyst JUM ENGINEERING model FID 3-300). The
conversion into methane is the ratio in percent between the
difference in methane concentration between the intake and the
outlet and the intake concentration.
[0201] After a rise in temperature with a reaction mixture at
5.degree. C./min from 250.degree. C. to 530.degree. C., the intake
temperature of the reaction mixture is set at this temperature. The
conversion of the methane after 50 hours of operation under
stabilized conditions is determined. This length of time makes it
possible to distinguish in a significant way the catalytic
formulations from the standpoint of their stability to convert the
methane into carbon dioxide and water.
[0202] The conversion of the methane obtained after 50 hours of
operation under stabilized conditions is in each case higher than
95%.
[0203] The preceding examples can be repeated with similar success
by substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
[0204] The entire disclosure of all applications, patents and
publications, cited above and below, and of corresponding French
Application No. 94/13.739, are hereby incorporated by
reference.
[0205] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention,
and without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *