CA1197197A - Composition and method for exhaust gas treatment - Google Patents

Abstract

COMPOSITION AND METHOD FOR EXHAUST GAS TREATMENT
Abstract of the Disclosure Suspended particles in exhaust gases, such as carbon and lead particles in internal combustion engine exhaust gases, are removed by passing the gases through a coarse filter and then through a fine filter. The filters may comprise a ceramic foam material and a catalyst material effective for the conversion of the carbon particles and/or gaseous pollutants in the exhaust gases to innocuous entities may be deposited on the filters.

Description

,~ 3t7 Thls invention relates to exhau~t gas treatment ana, more particularly, to the removal of suspended lead and carbon partlcles and saseous pollutants from internal combustion engine exhaust gases.
Filters have been used to remove suspended solids from exhaust gases such as the lead and carbon particles in internal combustion engine exhaust gases. As particles accumulate in the filter, the resulting restriction of exhaust gas flow increases the back pressure and reduces filtration and engine efficiency. In order to restore normal operation, the filter must be periodically regenerated, for example, by mechanically cleaning the filter or by heating it ana combusting the trapped carbon particles.
Diesel particulate emissions are evidenced by the occasional visible smoke discharges that occur during acceleration or maximum power operation. The large quantities of the very small and light carbon particles in diesel exhaust gases present substantial difficulties in achieving a high degree of particulate removal and avoiding exce~sive back pressure.
In accordance ~ith the method of this invention, carbon and lead particles are removeà from internal combustion engine e~haust gases by passing the gases through a coarse filter and then through a fine filter.
I'he filters comprise a refractory material effective to trap the particles. The use of a fine filter permits trapping of a high percentage of the particles and substantially reduces particulate emissions. The use of a relatively coarse filter to remove larger particles ~efore the gases reach the second fine filter extends ~he useful life of t~e fine filter and reduces the rate at which the back pressure increases as the particles accumulate in the filters. The selective filtration and particulate trapping distri~ution of this invention thus provide high trapping efficiency with low increases in back press~re and eftective ~iltration for longer periods before regeneration is required.
The composition of this invention for convertinq one or more pollutants in an exhaust gas to innocuous entities and removing suspenaed particles from the gas comprises a catalyst material effective for the conversion deposited on a coarse filter and on a fine filter. The filters comprise a refractory material effective to trap the particles in the gas and are positioned so that the gas flows in succession through the coarse filter and the fine filter. W~en the catalyst material is effective for the conversion of carbon and gaseous pollutants in the exhaust gases, the exotherm from the conversion of gaseous pollutants occurs in the fine filter because of lo~
conversion resulting from mass transfer limitations in the coarse filter. The exotherm enhances combustion of carbon particles trapped in the fine filter and regeneration of the filtration capacity.
~5 The filters r,~ay comprise any material ~hich is effective for trapping the particle~ in the gases, for example, as a result of inertial impact or electrostatic attraction. The filters are generally made of a porous, refractory material which is resistant to the temperatures of the gases and of catalytic conversion of the pollutants. Suitable materials which have affinity for t~

the partlcles an~ to ~nich the particles adhere inc~ude refractory ceramic or metallic materials which have sufficient thermal and mechanical stability for use in a catalytic reactor. The metal may be, for example, steel, stainless steel, aluminum, copper, or nickel. The ceramic material may be a refractory metal oxide, such as alu~ina, silica, magnesia, zirconia, titania, chromia, or combinations thereof such as cordierite or a refractory metal silicate or carbide.
The filters may be in the form of refractory inorganic oxide beads, such as ceramic spheres or cylinders.
Preferably, the filters are unitary structures of relatively large size such as ceramic monoliths, metal wools, or metal meshes. An open cell filter structure - 15 having a plurality of interconnected voids is especially preferred. The continuous cells of such a structure provide convoluted gas flow paths so that there is a greater proba~ility that a particle will be trapped and not pass through the filter. This structure has a larger particulate retention capacity and higher filtration efficiency than other filters.
An especially preferrea filter having a continuous cellular structure is a ceramic foam. In addition to larger particulate retention capacities and higher filtration efficiencies, ceramic foams are particularly useful in the treatment of diesel exhaust emissions because of their lower pressure loss, higher self-agitation, larger geometricai surface area, and lo~er density than ceramic monoliths and other filters having 3n laterally extending flow paths. I'he ~oams also have ~ ~'7~

substantial re~lstance to heat and chemical and physical ~eg r aa ation.
The ceran,ic foan filter preferably used in the present invention is prepared from an open cell, flexible foam material having a plurality of interconnected voids surrounded by a web of the flexible foam material, such as a polyurethane foam or cellulosic foam. The foam material i~ impregnated with an aqueous ceramic slurry so that the slurry coats the ~eb. The coated foam is dried and heated to burn out the organic foam and sinter the ceran,ic coating. The product is a fused ceramic foan, having a plurality of interconnected voids surrounded by a ~eb of bonoed or fusea ceramic in the configuration of the organic foam. Suitable cordierite foams ot various cell sizes are commercially available from Bridgestone Tire Co., Ltd., Tokyo, Japan and may be prepared in accordance with t~e method oescribed in Japanese Kokai 77/77,11~, published June 29, 1977. Other ceramic foams which are suitable for use in this invention are described in U. S.
Patents 3,893,917 and 3,962,081.
The degree to ~hich the filter permits the passage of particles suspended in exhaust gases or traps them depends upon the void volume or porosity and the pore size of the filter. The porosity of the filter may comprise voids in ~5 a unitary structure or voids between individual components of a particulate filter medium, such as ceramic beads.
The pore size and porosity of the filters used in this invention may be varied to suit the particular gases being ~iltered. The filters generally have a pore size of from about 2 to about 50 pores per 25 millimeters in length.

Generally, the filters have a porosity, i.e., a void volum~, of from about 80 to about 95 percent of the total volume occupied by the filter. The porosity is obtained from measurements of the density of the filter material and the weight using the formula Porositv (~ weight of filter ~ x 100 density of filter material x volume of filter Suitable filters also generally have an air permeability of from about 400 to about 8000 x 10 7 square centimeters.
Tne coarse filter is located upstream in the flow of the gases through the composition and the fine filter is located downstream from the coarse filter in the flo~ of gases through the composition. The fine filter has a greater num~er of cells per unit length and a smaller cell size than the coarse filter. The respective pore sizes ana permeabilities may vary in accordance with the particular nature of the gas under treatment. The cell siæe~ o~ each filter are selecte~ to optimize the relative degree of particulate trapping in each of the filters and distribute the trappea particles between the filters so that the pressure drop is minimized while good trappins e~ficiency is maintained. The upstream tilter generally has a relatively coarse pore size from about 2 to ab~ut 20 pores per 25 millimeters in length and an air permea~ility of ~rom about 2500 to about 8000 x 10 7 square centimeters. The downstream relatively fine filter generally has a pore size of from about 15 to about 50 7 ~

pores per 25 milllmeters in length and an air permeabilit~
ot ~rom about 400 to about 2500 x 10 7 square centimeters. Preferably, the coarse filter has from about 6 to about 2~ pores per 25 millimeters in length and the fine filter has ~rom about 17 to about 30 pores per 25 millimeters in length.
Multiple coarse filters of the same or different cell sizes may be employed in combination with multiple fine filters of the same or different cell sizes to vary the selective filtration and balance the pressure loss and trapping efficiency needed for a particular qas treatment application.
A catalyst material may be deposited on the filters.
The catalyst material is a catalytically active metal or metal compound that is effective for the conversion of one or more pollutants in the exhaust gases to innocuous entltles. ~he pollutants may be the particulate and/or gaseous pollutants present in the exhaust gases.
Generally, the catalyst material is an oxidation catalyst. In internal combustion engine exhaust gas treatment, the catalyst material ~,ay be effective for the combustion of carbon particles. Suitable carbon comb~stion catalyst materials include an element of the first transition series, silver, hafnium, and mixtures thereof. As used in this application, the elements of the first transition series are vanadium, chromium, manganese, iron, cobalt, nickel, copper, and zinc. The material may be present in the form of the metal, metal o~ide, mixed metal oxide, such as copper chromite or a perovskite, or other catalytically-active metal compounds. Copper oxide and chromium oxide are preferred.

ll ) When used in the treatment: of internal comhu~tion engine exhaust gases~ the catalyst material is preferably also effective for the convercion of hydrocarbons, carbon monoxide, and/or nitrogen oxicle pollutants. Such catalyst materials inclu~e a noble metal, an element of the first transition series, and mixtures thereof. The noble metals are gold, silver, and the platinum gro~p metals are platinum, palladium, rhodium, ruthenium, iridium, and osmium. The material may be in the form of the metal, the metal oxide, or other catalytically active compounds of the metal.
Platinum, palladi~m, and clhromium oxide are preferred because of their high hydrocarbon oxidation activity at relatively low temperatures. t'hromiu~ oxide is highly preferred because it also is e~,pecially effective in the combustion of diesel exhaust carbon particles. In an especially preferred embodiment of this invention, the catalyst material comprise~ a pl2tinum group metal suc~ as platinum, palladium, or mixtures thereof and chromium 20 oxide. The com~ination of the platinum group metal and chromium oxide catalyzes carbon combustion at a significantly lo~er temperature lthan either component alone.
The catalyst material may be deposited on the filters in any desirable n,anner from aqueous or organic sol~tions of a metal compound or complex or slurries of the me~al or metal oxide~ Generally, deposiltion of this component is effected by impregnating the fi:Lter ~ith an aqueous solution of a water soluble, thermally decomposable inorganic salt or complex of the particular metal or - 8 ~

~.

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metals ana arying the impregnatea filter at a temperature of from about 90 to about 250C. for about 2 to about 20 hours. The arlea filter may then be calcined at a tem~erature of from about 300 to about 700C. for about 1 to about 3 hours. The calcination can be conducted in air or other oxidizing gases or in a reducing gas such as hyarogen if the metal form of the catalyst is ~esired.
Typical thermally decomposable ~water soluble metal com~ounàs include the acetate, chloride, and nitrate.
Preferably, the platinum group metal component is depositea in the ~orm of a sulfito complex as descrihed in U. S. Patent 3,850,847 of Graham et al. to enhance its disp~rsion ana surface area.
If a surface area higher than that of the fllter is desired, the catalyst material may be supported on a porous, refractory inorganic oxide. These oxides have a high total pore volume and surface area. Generally, the surface area of the refractory oxiae is at least about 75 square meters per gram, preferably from about 100 to about 300 square meters per gram, and the total pore volume i5 at least about 0.4 cubic centimeters per gram, preferably from about 0.5 to ahout 2.0 cubic centimeters per gram.
The surface areas referred to throughout this specification are determined by the nitrogen BET method.
The total pore volumes are determined by adding ~ater to a po~cier sample to the point where incipient ~etness just occurs.
Generally, the refractory oxide is composed predominantly of oxides of one or more metals of Groups II, III, and IV having atomic numbers not exceeding 40.

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S~itable porous re~rac~ory inorganic oxides ca be prepared by dehyarating, p~eferably s~b~tantially co~,pletely, the hydrate form of th~ oxide by calcination generally at temperatures ~f about 150 to about 800~C. for perio~s of from about 1/2 to aboùt 6 ~o~rs. The preferred refractory oxide is a transitional alumina, such as chi, rho, kappa, gamma, delta, eta, and theta a~uminas, especially gam~la alumina. A particularly pr~ferred gamma al~mina may be prepared by calcining a boehmite-pseudoboehn,ite intermediate al~mina prepared in accordance~ith U. S. Patent No. 4,154,812 of S3~chez et al~ at a temperature of about 650C. for about 1 hour. Other suitable oxides include, for example, calcined heryllia, zirconia, magnesia, and mixtures of metal oxides such as boria-alumina, silica-alumina, and the like.
In a highly preferred embodiment of t~is invention, the catalyst ~,aterial compri~es the diesel exhaust catalyst composition of U. S. Patent No. 4,303,55~ by Ernest and Welsh ~0 entitled '`Composite Diesel Exhaust Catalyst'. This catalyst comprise~ a mixture of catalytically-effective amounts of at least one material selected from the group consisting o~ a nobl~ metal, chromium, and catalytically-active rompounds thereof supported on ~ porous refractory ~5 inorg~nic oxide and at least one bu~k material ~e1ected from the group consi~ting o~ an element of the first tran~ition series, silv~r, ~afnium, and catalytically-active compounds thereof. The catalyst material ma~ compri~e a mixture of from about 4n to about 60 weight percent of a supported material comprising a platinum group metal, chromium oxide, or mixtures thereof supported on a transitional alumina and from about 40 to about 60 weight percent of a bulk material comprising copper oxide. The bulk material may be prepared by thermal decomposition of a compound of the desired metal.
35 Typically, the acetate, nitrate, carbonate, hycroxi~e, or chloride is heateo at a temperature of from about 450 to about 800C. for a period of from 1 to a~o~t 5 hours. The bulk material is slurried with the supported rnaterial and deposited on the filters.
The refractory oxlde may be coated on the filter and then the catalyst material deposited on the filter.
Pre~erably, ho~ever, the catalyst material is ~eposited on the refractory oxide and then the supported catalyst is deposited on the filter. For example, a suitable catalytic component may be added to an aqueous slurry of the oxide and the mixture deposited on the filter by conventional methods, such as dipping or spraying.
The coatea tilter is then dried at a temperature of from about 90C to about 250C. for about 1 to about 4 15 hours to remove the solvent and deposit the solids in an adherent film on the filter. The dried filter may be ca~cineu a~ Iro~, about 250C. to about 800C. for about 1 to about 4 hours.
The amount of the catalyst material that is coated on 20 the filter depends on economics, size limitations, and design characteristics. The catalyst material generally comprises about 1 to about 50 and preferably from about 2 to a~out 30 percent based upon the weight of the filter.
During use, the catalyst composition is typically 25 disposed so that it occupies the major part of the cross-sectional area cf a housing having a gas inlet and a gas outlet. The composition typically has the general shape o~ the housing and is positioned in the housing with the general direction of gas flo~ between the inlet and ~g~ 7 .

outlet. The filters nay ~e adhered together or spaced apart.
In employing the composltion of this invention in the treatment of internal combustion engine exha~st gases, the gases are contacted with the composition and the ~ead and carbon particles are trapped in the filters. The carbon particles are co~,b~sted along with the gaseo~s pollutants in the exhaust gases. The accumulated carbon partic~late deposits may be periodically removed by throttling the engine to reduce the air flow with fuel flow remaining constant an~ increase the exhaust temperature. At the resulting higher exhaust temperatures, the combustion of the particulates will be achievea quite rapidly in the presence of the catalytic filter of this invention.
In addition to the filtration of lead and~or carbon particles from internal combustion engine èxhaust emissions, t~e ~ilter of this invention may he used, for example, to reauce particulate emissions from other mobile po~er plants as well as stationary sources, such as gas turbine catalytic combustors, which utiliæe fuels which produce partic~late pollutant~.
This invention is illustrated by the following examples in which all parts and percentages are by weight unless otherwise indicate~.
Example 1 A composition of t~is invention which comprised the "Diesel Exhaust Catalyst" of U. S~ Patent No. 4,303,552 by Ernest and ~elsh deposited on comn,ercially available ceramic foam monoliths o~ Bridgestone Tire Co., I,td,, Tokyo, Japan was prepared as follows.

i ~ ~7~7 A boe~mite-pseuaoboeh~,ite intermediate hyarated alumina powder prepared in accordance with the process of U. S. Patent 9,154,812 of Sanchez et al. was calcined in air at 649C. for one hour. The resulting gamma al~mina had a total pore volume of 1.56 to 1.68 cubic centimeters per gram and a total volatiles content (loss in weight a~ter heating for 1 hour at 954C.) of 1.8 to 3.3 percent.
3000 grams of the calcined alumina po~der were in,pregnated with 840 milliliters of a solution of 260.29 grams oE chromic acetate in 840 milliliters of deionized water and 5 milliliters of glacial acetic acid. The impregnated powder was allowed to dry in air for 1/2 hour and then drie~ for 16 hours at 135C. The powder was screened t~rough a 20 mesh UO S. Standard Sieve and calcined at 843-871C. for 1 hour. The calcined chromia-alumina powder had a surface area of 107 square meters per gram and containea nominally 10 weight percent chromia.
150 grams of copper oxi~e freshly prepared by decomposing cupric acetate for 3 hours at 538C. in a muffle furnace and 150 grams of the chromia-alumina powder were separately ball milled with deionized water for 16 hours. The resulting slurries were combined in 3 1 to 1 ratio (solids basis) and homogenized. The pH of the combined slurry was adjusted to 3.5 with nitric acid. The procedure was repeated four times using a 1 to 1 weight ratio of the copper oxide and the chromia-alumina but varying the solids content of the slurry by adding additional water. Each slurry was coated on a Bridgestone ceramic foam monolith of 19.37 to 14.50 centimeters in ~_,9~

diameter anà 7.30 to 7.67 centimeters in length. The excess slurry was blown out of the c~ated monolithc and the monoliths were the~ ariea at 135CC. for 16 hours and activated for 1 hour at 428C. The number of cells per 25 millimeters in lengt~ of the monoliths, the solids co~tent of the slurries, and the amounts and percentages of the catal~st material coatea on the monoliths are shown in Table I.
Table I
10 Nominal No. ofSolids Pickup-cells/25 mm. Content ~ qrams ~Coating 6 27 44.4 9.1 13 23 45.2 g.l 14 35.9 6.1 14 41.1 7.7 Three of the 13 size monoliths and one of the 30 size monoliths were joined together and placed in a cy~indrical container. The 13 size monoliths were positioned as the 20 inlet structure, the first cer,tral section adjacent the ir,let structure, and the second central structure adjacent the outlet structure and the 30 size monolith ~as positioned as the outlet structure. The composition was tested in the treatr,~ent of an exhaust gas from one bank (4 cylinders) of a 5.7 liter Oldsmobile diesel engine at a gas flow rate of 90 cubic feet per minute. I~he test was run for about 6 hours and measurements of back pressure and weight of emissions were taken approximately every hour. Good average trapping efficiency and relatively low 30 pressure increase over the duration of the test were observed.

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Another confiq~ration was prepared in which the 6 size monoliths were positioned as the inlet section and the first and second central section and the 20 size was used as the outlet section and tested ~y the same method. It was Eound that there was a much lower pressure increase but a much lower average trapping efficiency than in the ~irst conflguration.
Example 2 The proceaure of Example 1 was repeated except that platinum and pallaàium were incorporated in the chromia-alumina powder as follows.
320 grams of the chromia-alumina powder of Example 1 were impregnated with a mixed solution of platinu~ and pallladium prepared by bubbling sulfur dioxide gas at 2 millimoles per minute into 250 milliliters of deionized water for 11.0 minutes and adding 4.022 milliliters of palla~ium nitrate solution having a titer o~ 129.27 grams of pallaaium per liter of solution. 112.012 grams of (NH4)6Pt(SO3)4 solution having a titer of 92.85 grams of platinum per kilogram of solution and 3.2 grams of dibasic ammonium citrate were then added to the solution. The total volume of the solution was increased to 434 milliliters by addition of deionized water. ~he powder was impregnated with this volume of solution, air d;ied for one hour, and then oven dried for 16 hours at 135C. The powder was finally activated in air for one hour at 538C. T~e powder nominally contained 3.30 percent of platinum and palladium in a 20 to 1 weight ratio.

339 grarr's of the ac:ivated pow~er and 300 grarns of freshly prepared copper oxide as in Example 1 were separately hall n,illed with ~eionized water at 28~ solids content for 16 hours. The two slips were combined in a 1 to 1 ratio (solids basis) and homogenized. Other slurries were prepared using a 1 to 1 ratio of the powders but varying the solids content by adding additional water.
The pH of the slurries was adjusted to 3.5 with nitric acid an~ the slurries were coated on Bridqestone ceramic foam monoliths of varying cell sizes. The coated monoliths were dried at 135C. for 16 hours and then activated for one hour at 428C. The pertinent data are shown in Table II.

lrL O ~-L 60-~ ~1 0 5L-0 L-8 Z9-5~ ~1 0 9ZL'0 S-L 86-~ 91 oz LL-0 S'L ~9'~ 91 oz 19L 0 ~-8 51-9~ ~Z 1 S9L 0 5 8 8-9~ ~Z 1 61L-0 9-L 55'~ 8Z 9 18L-0 L'8 9'Lb 8Z 9 ;Inlpellea ~~ 6uT~eo;) s~e~g dn~ld ~; ~u~uo;~ S~llos ~ 5z/s~ o wnul~ela ~o swe~g N leul~o~d e,~

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The monoliths may be joinea in the con~igurations of Example 1 and used in the treatment of ~iesel exhaust gases.

Claims (19)

WHAT IS CLAIMED IS:

1. A composition for converting one or more pollutants in an exhaust gas to innocuous entities and removing suspended particles from the gas which comprises a catalyst material effective for the conversion deposited on a coarse ceramic foam filter having a pore size of from about 2 to about 20 pores per 25 millimeters in length and on a fine ceramic foam filter having a pore size of from about 15 to about 50 pores per 25 millimeters in length, said filters positioned so that the gas flows in succession through the coarse filter and the fine filter.

2. The composition of claim 1 in which the filters have a porosity of from about 80 to about 95 percent.

3. The composition of claim 1 in which the filters have an air permeability of from about 400 to about 8000 x 10-7 square centimeters.

4. The composition of claim 1 in which the coarse filter has a pore size of from about 6 to about 20 pores per 25 millimeters in length and the fine filter has a pore size of from about 17 to about 30 pores per 25 millimeters in length.

5. The composition of claim 1 in which the coarse filter has an air permeability of from about 2500 to about 8000 x 10-7 square centimeters and the fine filter has an air permeability of from about 400 to about 2500 x 10 7 square centimeters.

6. The composition of claim 1 in which the catalyst material is effective for the combustion of carbon particles.

7. The composition of claim 6 in which the catalyst material comprises an element of the first transition series, silver, hafnium, or mixtures thereof.

8. The composition of claim 1 in which the catalyst material comprises a mixture of catalytically-effective amounts of at least one supported material selected from the group consisting of a no~le metal, chromi~m, and catalytically-active compounas thereof, said material supported on a porous refractory inorganic oxide, and a least one bulk material selected from the group consisting of an element of the first transition series, silver, hafni~m, and catalytically-active compounds thereof.

9. The composition of c~aim 8 i~ ~hich the supported material comprises a platinum group metal, chromium o~i~e, or n,ixtures thereof.

1~. The compo~ition of claim 8 in which the bulk material comprises copper oxide, chromium oxide, or mixtures thereof.

11. The composition of claim 8 in which the catalyst material comprises a mixture of from about 40 to about 60 weight percent of a s~pported material comprising a platinum group metal, chromium oxide, or mixtures thereof su~ported on a transitional alumina and from about 40 to about 60 weight percent of a bulk material comprisinq ~opper oxide.

12. A ~omposition for collecting and disposing of carbon particulates in exhaust gases of internal com~ustion engines whic~ comprises a carbon co~hustion catalyst material deposited on a coarse ceramic foam filter havinq a pore size of from about 2 to aho~t 20 pores per 25 millimeters in length and a fine ceranlic foam ilter having a larger pore size of from about 15 to about 50 pores per 25 millimeters in length, said filters positionea so that the gases flo~ in succession through the coarse filter and the fine filter.

13. The composltion of claim 12 in which the catalyst material com~rises an element of the firs~ transition series, silver, hafnium, or mixtures thereof.

14. The composition of claim 12 in which the catalyst material comprises a mixture of catalytically-effective amounts of at least one supported material selected from the group consisting of a noble metal, chromium, and catalytically-active compounds thereof, said material supported on a porous refractory inorganic oxide, and at least one bulk material selected from the group consisting of an element of the first transition series, silver, hafnium, and catalytically-active compounds thereof.

15. A method for removing carbon and lead particles from internal combustion engine exhaust gases comprising passing the gases through a coarse ceramic foam filter having a pore size of from about 2 to about 20 pores per 25 millimeters in length and then through a fine ceramic foam filter having a pore size of from about 15 to about 50 pore per 25 millimeters in length.

16. The method of claim 15 in which the coarse filter has a pore size of from about 6 to about 20 pores per 25 millimeters in length and the fine filter has a pore size of from about 17 to about 30 pores per 25 millimeters in length .

17. The method of claim 15 in which the filters further comprise a carbon combustion catalyst material deposited on the filters.

18. The method of claim 17 in which the catalyst material comprises a noble metal, an element of the first transition series, hafnium, or mixtures thereof.

19. The method of claim 17 in which the catalyst material comprises a mixture of catalytically effective amounts of at least one supported material selected from the group consisting of a noble metal, chromium, and catalytically-active compounds thereof, said material supported on a porous refractory inorganic oxide, and at least one bulk material selected from the group consisting of an element of the first transition series, silver, hafnium, and catalytically-active compounds thereof.