CATALYST SUPPORT FOR HIGH TEMPERATURE APPLICATIONS AND CATALYSTS AND CATALYTIC PROCESSES EMPLOYING SAME

Abstract: This invention is a HfO2-containing catalyst support material for use in high temperature catalytic processes, such as catalytic combustion, wherein the HfO2 is present in a sufficient amount to impart thermal stability on the catalyst support at temperatures in excess of 1,000 DEG C, as well as, heterogeneous, supported-platinum group metal catalyst employing the HfO2-containing support material and monolithic catalyst structures in which the HfO2-containing support material is used as a substrate washcoat for the active catalyst material. Also described are high temperature catalytic processes, such as catalytic combustion, employing catalysts and catalyst structures which utilize the HfO2-containing support material.

Description: CATALYST SUPPORT FOR HIGH TEMPERATURE APPLICATIONS AND CATALYSTS AND CATALYTIC PROCESSES EMPLOYING SAME FIELD OF THE INVENTION This invention relates to a stable refractory support material for high temperature catalytic reactions; to catalysts and catalytic structures employing said refractory support material and to high temperature catalytic processes which utilize the aforesaid catalysts and/or catalytic structures. More particularly, this invention is directed to improved catalytic support materials containing a thermally stable hafnium compound, e.g., HfO 2 , in amounts sufficient to impart thermal stability on the support in process applications where reaction temperatures exceed 1,000°C; to catalysts, typically containing one or more platinum group metals deposited on the support or a metal substrate coated with the support; and to high temperature catalytic processes such as partial oxidation or catalytic fuel combustion processes wherein said catalysts or resulting catalyst structures are employed to afford stable operation at high temperatures and other operating advantages. BACKGROUND OF THE INVENTION A variety of commercially important catalytic processes operate at high temperatures, for example, steam reforming of methane to CO and H , partial oxidation of hydrocarbons to synthesis gas, complete oxidation of hydrocarbons for emissions control, including automotive missions control, and catalytic combustion of fuels for further use in gas turbines, furnaces, boilers and the like. In each case, the catalyst employed is typically a heterogeneous catalyst in which the catalytically active species or material is supported on an inert or refractory metal oxide support. To optimize their catalytic activity, these heterogeneous catalysts are generally designed to afford a high exposed surface area ofthe active catalytic species. In addition to increasing the activity ofthe catalyst, the achievement of this design objective optimizes the utilization ofthe catalyst components which, of course, can be of significant economic benefit when the catalyst components include the very expensive precious metals. In the case of heterogeneous catalysts comprising a catalytically active metal, including precious metals, supported on an refractory oxide support, the surface area ofthe support is very important in obtaining a large exposed surface area ofthe catalytic species. In this regard, the large surface area and porous structure ofthe support oxide will allow the catalytic elements to be physically separated, with the actual particles ofthe catalytic species being widely spaced over the large surface area ofthe oxide support. When heterogeneous catalysts are operated at high reaction temperatures, the catalytic elements such as metal or metal oxide particles tend to sinter and, as a result, the particles ofthe catalytic species grow larger and the exposed catalyst area is reduced. Further, if the support surface area decreases in the high temperature environment, the active catalyst components will sinter even further. This phenomenon is quite undesirable since it will reduce catalytic activity and the ultimate cost effectiveness of catalyst usage. For this reason it is highly desirable to have a thermally stable, high surface area refractory support on which the active catalytic components are deposited. Materials problems typically encountered with heterogeneous, supported catalysts in these high temperatures applications include: (1) loss of support and/or catalytically active species surface area at high temperatures; (2) severe sintering ofthe catalyst in steam; and (3) poisoning of catalytic activity due to support-catalyst interactions. Zirconia (ZrO 2 ) has been employed in the past as a support material in certain high temperature catalytic processes (catalytic combustion and automotive emissions control) because of its refractory properties at elevated temperatures and compatibility with the active catalytic species. For example, see U.S. Patent No. 5,259,754 to Dalla Betta et. al. where a zirconium-containing support material for an active catalytic species comprising palladium is used in a metal foil catalyst structure, with the refractory support material and catalytically active metal being coated on at least some ofthe metal substrate surfaces, to stabily limit the catalyst temperature in the combustion process. However, the high surface area ZrO loses substantial surface area when subjected to high temperature treatment in air. This loss of surface area is greatly accelerated when water vapor is present. The addition of silica (SiO 2 ) to ZrO 2 can increase the thermal stability of ZrO 2 somewhat but this added stability is not retained at temperatures in the range of 1000°C where the SiO 2 -stabilized ZrO 2 loses most of its surface area and if steam is present the loss of surface area is even more severe. In the past, hafnium oxide or hydroxide has been mentioned in a general way among a shopping list of possible refractory oxidic materials as a support candidate for a variety of catalytic applications. For example, see U.S. Patent Nos. 2,375,402; 4,189,405; 4,240,983; 4,284,531; 4,648,975; 4,681,867; 4,880,764 and 5,204,308, as well as European Patent Application No. 0257 983; Japanese Patent Application No. 57-18,639 and British Patent No. 1,377,063. However, none ofthe prior disclosures of hafnium oxide have distinguished it from zirconium oxide nor pointed towards its unique stability in high temperature catalytic applications such as catalytic combustion of fuels. SUMMARY OF THE INVENTION This invention, in its broadest aspects, is directed to a refractory metal oxide support material for use in heterogeneous catalysis comprising HfO 2 , in admixture with SiO 2 or SiO 2 and ZrO 2 wherein the HfO 2 is present in an amount sufficient to impart thermal stability on support material at temperatures in excess of about 1000°C. In addition, this invention includes improved heterogeneous catalysts for high temperature catalytic reactions comprising a catalytically effective amount of a platinum group metal or mixture of platinum group metals deposited on the above-described refractory support material comprising HfO 2 in combination with SiO 2 or mixtures of SiO 2 and ZrO 2 . Also within the scope ofthe invention are catalytic structures comprising a monolithic substrate having a series of longitudinal passageways for the passage of a flowing reaction mixture, wherein at least a portion ofthe passageways are coated with a HfO 2 -containing refractory support material selected from HfO 2 or HfO 2 in admixture with either SiO 2 or SiO 2 and ZrO 2 as a passageway washcoat and a catalytically effective amount of a platinum group metal or mixture thereof is deposited on the washcoat in at least some ofthe passageways coated with the washcoat. A preferred aspect ofthe invention is directed to catalytic structures which are particularly useful for partial oxidation or complete combustion of gaseous or vaporizable fuels comprising a metallic foil substrate made up of a series of adjacently disposed catalyst-coated and catalyst-free channels for passage of a flowing reaction mixture, wherein the channels coated with catalyst are in heat exchange relationship with the adjacent catalyst-free channels and wherein the catalyst-coated channels are coated with a catalytically effective amount of a platinum group metal or mixture of platinum group metals and have a washcoat ofthe previously described refractory support material comprising HfO 2 , optionally in combination with SiO 2 or mixtures of SiO 2 and ZrO 2 , on which the platinum group metal catalyst is deposited. Also within the scope ofthe invention are improved high temperature catalytic processes employing the catalysts and catalyst structures ofthe invention including, specifically, an improved process for the combustion of a combustible fuel mixture using the above-described catalyst structure, which is a preferred embodiment ofthe invention. The refractory supports ofthe present invention are advantageous in that they not only provide surprisingly superior thermal stability in high temperature environments containing oxygen and/or water vapor, as compared, for example to a ZrO 2 or SiO 2 stabilized ZrO 2 support, but further, that the increased sintering observed at high temperatures when a platinum group metal such as palladium is supported on ZrO 2 or SiO 2 stabilized ZrO 2 is not observed with the refractory supports ofthe invention. In addition, the refractory supports ofthe invention appear to be highly compatible with platinum group metal catalysts in that no catalyst poisoning attributable to the support appears to occur. Finally, when used as washcoats in the catalyst structures ofthe invention employing platinum group metals (particularly palladium), the HfO 2 -containing refractory support materials ofthe invention provide superior performance in the catalytic combustion application including light-off temperature(s) (LOT) which are lower than those obtained with ZrO 2 or SiO 2 stabilized ZrO 2 washcoat which has been used previously. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a temperature vs. surface area graph showing the stability ofthe ZrO 2 - containing supports ofthe prior art. FIGS. 2A, 2B, 3A and 3B are temperature graphs which compare the stability of the ZrO 2 -containing supports ofthe prior art with the refractory supports ofthe invention in terms of surface area and crystallite size. FIG. 4 is a bar graph showing the change in surface area which occurs for various refractory support materials including prior art materials and the inventive materials when they are calcined for a fixed period of time at two different temperatures. FIG. 5 is a time graph showing the surface area stability of various refractory support materials including both prior art materials and the inventive materials over extended time periods at high temperature in the presence of steam. FIG. 6 is a bar graph which compares the surface area of a supported Pd catalyst of the invention with a supported Pd catalyst ofthe prior art before and after calcination for a fixed period of time. FIG. 7 is a time graph showing the surface area stability of a refractory support and supported Pd catalyst ofthe invention compared to a prior art refractory support and catalyst over extended periods of time at high temperature in the presence of steam. DESCRIPTION OF THIS INVENTION The haf ia-containing refractory support materials ofthe invention are characterized by surprising and exceptional surface area stability at high temperatures, e.g., temperatures exceeding 1000°C, in reaction environments which can include the presence of significant amounts of steam or water vapor. Further, this high temperature stability is retained when one or more platinum group metals, e.g., platinum or palladium, are loaded on the support thus facilitating extended catalyst operating lifetimes in high temperature processes such as combustion or partial combustion processes. To afford the desired surface area stability in high temperature applications, the HfO 2 content ofthe refractory support materials ofthe invention (in addition to the SiO 2 and/or ZrO 2 components as disclosed above), should be no less than about 5 mol percent based on total metal oxide content and preferably greater than about 10 mol percent. In this regard, in cases where SiO 2 is present as an adjunct metal oxide component in the refractory support, the amount of SiO 2 is preferably in the range of about 5 to 15 mol percent ofthe support oxide and, in cases where ZrO 2 is present, together with SiO 2 ,in the refractory oxide support, the amount of ZrO 2 present preferably ranges between about 5 and 80 mol percent ofthe support oxide. To assure sufficient surface area for stability in high temperature reactions, particularly when a catalytically-active species comprising a platinum group metal is deposited on the refractory oxide support, it is important that the hafhia-containing refractory support materials ofthe invention have a surface area of at least about 5 square meters per gram (m 2 /g) after calcination for 10 hours at a temperature of about 1100°C. Preferably, the hafhia-containing refractory support has a surface area in excess of about 10 m 2 /g and most preferably greater than about 20 m 2 /g after calcination at about 1100°C for about 10 hours. The surface areas recited in this paragraph are measured by BET or equivalent methods. This minimum surface area for stable high temperature operation is obtained though careful control ofthe micro-crystallite size ofthe HfO 2 or HfO *ZrO 2 solid solutions used to prepare the refractory oxide materials of this invention (see below), including the micro-crystallites which are impregnated with a SiO 2 precursor to prepare the SiO 2 stabilized hafhia-containing refractory support materials ofthe invention. The hafnia and hafhia-zirconia solid solutions used in the refractory oxide supports ofthe invention can be prepared from suitable oxide precursor compounds by means of precipitation or hydrolysis to form a solid oxide which is subsequently calcined to afford micro-crystallites of HfO 2 or HfO 2 *ZrO 2 in the desired size range for subsequent use in the refractory oxide support materials ofthe invention. In cases where silica is included in the oxide support materials ofthe invention, the calcined HfO 2 or HfO 2 »ZrO 2 micro- crystallite material is impregnated with a solution of a SiO 2 precursor and the impregnated solid is subject to hydrolysis and calcination to yield the desired silica stabilized HfO 2 or HfO 2 *ZrO 2 solid solution. The desired crystallite size range for the HfO 2 or HfO 2 *ZrO 2 crystallites, expressed in terms of average crystallite diameter, will vary somewhat dependent on the intended process application for the refractory oxide support ofthe invention, but is typically in the 5 to 15 nanometer (nm) size range prior to stabilization with silica. In cases where higher surface areas are desired, or greater sintering is expected, it is desirable to employ crystallite sizes between about 5 and about 10 nm, whereas, in cases where little or no sintering is expected and a more constant operation is desired, it is preferred to use crystallites in the range of about 10 to about 15 nm. The compounds of hafnium (and zirconium) which may be employed as precursors to the refractory oxides used in the invention include water soluble metal salts such as oxyhalides, nitrates, and sulfates, as well as soluble organo metallic compounds such as acetates and alkoxides. Preferred precursor compounds include hydrates of oxychlorides, e.g., HfOCl 2 and ZrOCl 2 and oxynitrates, e.g., HfO(NO 3 ) and ZrO(NO 3 ) 2 . In the case of the silica-stabilized refractory oxide support materials ofthe invention, the silica source can be any silicon compound which is soluble in the impregnating solution, typically aqueous or alcohol based, and which will provide a soluble form of silicon in solution that will subsequently form an oxide upon hydrolysis and calcination. In this regard suitable sources ofthe silica include alkylsilicates and organosilanes with alkyl silicates such as tetra ethyl ortho silicate and tetramethylorthosilicate being preferred. Most preferably, the silica precursor is tetraethyl orthosilicate (TEOS). As mentioned above, the HfO 2 or HfO 2 *ZrO 2 solid solutions employed in the refractory oxide support materials ofthe invention are suitably prepared by precipitation or hydrolysis from a variety of starting materials. In the case of precipitation (or coprecipitation when HfO 2 -ZrO 2 solid solutions are prepared), a suitable precipitant, typically a base such as ammonium hydroxide or an alkali metal hydroxide, is added to an aqueous solution ofthe oxide precursor compound or compounds, e.g., HfOCl 2 or an intimate mixture of HfOCl 2 and ZrOCl Preferably, the precipitant is selected such that undesirable or unnecessary compounds or products ofthe precipitation are volatilizable and decomposable upon calcination as set forth below. Prefened precipitants, in this regard, are ammonium compounds such as ammonium hydroxide and ammonium carbonate. The precipitation may be carried out in dilute or concentrated aqueous solution. Further, the rate at which precipitant is added and the degree of agitation used will vary depending upon the desired properties ofthe precipitate. More dilute precipitation solutions, slow addition and vigorous agitation is prefened since it will favor a coarser precipitate within the desired microcrystallite size range. The temperature during addition ofthe precipitant may be from about 15 to about 90°C. Higher temperatures generally produce a courser precipitate. The precipitant is typically added in stoichiometric excess, that is, until a pH of between about 9 and about 11 is reached. Subsequent to precipitation, the precipitate (or coprecipitated mixture in the case of Hf0 2 and ZrO 2 solid solutions) is recovered from the slurry which forms by filtration, centrifugation, etc. and then washed, if desired, prior to calcination. In the case where hydrolysis is used to obtain the HfO 2 and HfO 2 *ZrO 2 solid solution employed in the refractory support materials ofthe invention, the hydrolysis is typically carried out under hydrothermal conditions by heating a aqueous solution ofthe oxide precursor or precursor, to a temperature of between about 100°C and 250°C and corresponding pressure of about 1 to 40 atm. Subsequent to hydrolysis, the product slurry containing the solid oxide particles is subject to a conventional liquid/solids separation such as filtration or centrifugation to recover the solid product which can be washed, if desired, prior to calcination. To obtain microcrystallites of HfO 2 or HfO 2 -ZrO 2 solid solutions in the size range desired for the refractory support materials ofthe invention, the recovered solid products of 8 precipitation or hydrolysis are calcined in air at a temperature of from about 400 to about 1000°C for about 5 to about 15 hours. The desired calcination temperature will depend on the final use temperature and the degree of stability required. Preferably the calcination is conducted in air at about 1000°C for a period of about 10 hours. The product of calcination can be used directly in the refractory support materials ofthe invention or, preferably, be stabilized by the addition of a controlled amount of silica prior to such use. To incorporate the desired amount of silica into the calcined HfO 2 or HfO 2 *ZrO 2 solid solution, the calcined solid is contacted with a solution containing the silica precursor compound in water or a mixed water/alcohol solvent together with a suitable inorganic or organic acid in amounts sufficient to affect hydrolysis ofthe silica precursor. During the hydrolysis, the silica is absorbed on the surface ofthe calcined solid in ionic form which upon completion ofthe hydrolysis is converted to an oxide or hydroxide of silicon. The hydrolysis is suitably a carried out by heating the calcined solid in contact with the silica precursor at about 60-100°C for about 6 to 16 hours. While any conventional technique for contacting the calcined solid with the silica precursor solution may be used it is prefened to apply the silica precursor solution to the calcined solid by the incipient wetness impregnation technique. Following impregnation, the HfO 2 or HfO 2 *ZrO 2 solid solution stabilized with SiO 2 is calcined at about 800 to 1100°C in air depending on the final use temperature. The catalysts ofthe invention comprise a catalytically effective amount of a platinum group metal, e.g., platinum, ruthenium, palladium, iridium and rhodium, deposited on a hafhia-containing refractory metal oxide support selected from HfO 2 , SiO 2 stabilized HfO 2 and SiO 2 stabilized HfO 2 *ZrO 2 solid solution, wherein the HfO is present in a sufficient quantity to impart thermal stability on the catalyst at temperatures in excess of 1000°C. Preferably, the platinum group metals is selected from platinum or palladium or mixtures thereof and the refractory metal oxide support is selected from HfO 2 in admixture with either SiO or SiO 2 and ZrO 2 wherein the Hf0 2 is present in sufficient amounts to impart thermal stability on the catalyst at temperatures in excess of 1000°C. Most preferably, the catalyst comprises a catalytically effective amount of palladium on a refractory metal oxide support selected from HfO 2 in admixture with either SiO 2 or SiO 2 and ZrO 2 wherein the support contains at least about 20 mol percent of HfO 2 based on total metal oxide content and has a surface area of at least about 5 m 2 /g after calcination at about 1100°C for 10 hours. To form the catalysts ofthe invention the appropriate amount ofthe platinum group metal or mixtures of platinum group metals is deposited on the hafhia-containing refractory oxide support using metal deposition techniques which are conventional in the catalytic art. The quantity of platinum group metal added to the calcined support is an amount which is sufficient to provide catalytic activity in the desired end use. The specific amount added to the hafhia-containing support is dependent on a variety of factors or requirements, e.g., the reaction which is intended to be catalyzed, the feedstocks used, economics, activity, life, contaminants present, etc. The theoretical maximum amount of metal is suitably enough to cover the maximum amount of support without causing undue catalyst sintering and concomitant loss of activity. These clearly are competing factors: maximum catalytic activity per unit weight of final catalyst requires higher surface coverage ofthe support by the platinum group metal but higher surface coverage can promote sintering or growth ofthe particle size ofthe platinum group metal. Furthermore, the form ofthe catalyst support must be considered. If the support is used in a high space velocity environment, the catalyst loadings should be high to maintain sufficient conversion even thought the residence time is low. Economics has, as its general goal, the use ofthe smallest amount of catalytic metal which will do the required task. Finally, the presence of contaminants in the reaction feedstock, e.g., fuel in the case of catalytic combustion, would mandate the use of higher catalyst loadings to offset deterioration in the catalyst due to deactivation. In the typical case, the amount of platinum group metal used in the supported catalysts ofthe invention is a minor portion ofthe supported catalyst and generally does not exceed about 25% by weight ofthe calcined support. The amount may be about 0.01% to about 20% to economically maintain high activity with extended use. These percentages are based on the weight of calcined support, thus if the supported catalyst is used on an inert substrate, such as a metal foil substrate (discussed below), the supported catalyst may be, for example, about 10% to about 25% ofthe weight ofthe substrate and the percent weight of platinum group metal relative to the total weight of substrate and supported catalyst will be conespondingly less. The platinum group metal or mixture of platinum group metals which is employed will be governed largely by the intended catalytic application, that is, the specific chemical reaction to be catalyzed, and the desired reaction conditions, and economics. The platinum group metals which may be suitably used include platinum, ruthenium, palladium, iridium and rhodium as well as mixtures thereof. In the case of catalytic combustion or partial combustion, platinum or palladium or mixtures of platinum and palladium are preferred with palladium being most prefened. The platinum group metal may be incoφorated onto the hafhia-containing support in a variety of different methods using platinum-group metals containing complexes, compounds or dispersions of metal. The compound or complexes may be soluble in water or hydrocarbon solvents. The platinum group metal may be precipitated from solution. The liquid carrier generally needs only to be removable from the catalyst carrier by volatilization or decomposition while leaving the platinum group metal in dispersed form on the support. Examples of suitable platinum group metal complexes and compounds are platinum group metal chlorides, oxides, sulfides and nitrates, palladium diamine dinitrate, platinum tetramine hydroxide, chloroplatinic acid, palladium tetramine chloride, sodium palladium chloride, palladium 2-ethyl-hexanoic acid, hexamine rhodium chloride, hexamine iridium chloride, potassium platinum chloride and a variety of other platinum group metal salts or complexes. Although the chloride compounds produce catalysts which are typically quite active, chlorides are not an excellent choice when the catalyst is used in a combustor for a gas turbine. Chlorides, even in very small amounts, cause significant turbine blade and bucket conosion. Consequently, nitrogen-containing palladium or platinum precursors are most desirable when catalysts are being prepared for use in gas turbine combustors. After deposition on the hafhia-containing support, the platinum group metal complex or compound is treated, for example, by calcination or upon use to convert essentially all ofthe platinum group metal to its elemental or oxidic form. The supported catalysts ofthe invention can be employed in any conventional form depending on the desired end use and reaction conditions. For example, the supported catalysts ofthe invention can be in the form of a particulate such as spheres, pellets, rings and the like having diameters of 0.25 in. or less. Preferably, in the case of combustion or partial combustion, the supported catalysts ofthe invention are coated on the surfaces of a metallic or ceramic substrate fabricated in the form of a unitary or monolithic structure made up of an array of longitudinally disposed channels for passage ofthe combustible gas mixture with the supported catalyst being deposited on at least a portion ofthe longitudinally disposed channels. Most preferably the monolithic substrate is metallic with metallic substrate in the form as honeycombs, spiral rolls of corrugated sheet (which may be interspersed with flat separator sheets), columnar (or "handful of straws"), or other configurations having longitudinal channels or passageways permitting high space velocities with a minimal pressure drop being desirable in this service. They are malleable, can be mounted and attached to surrounding structures more readily, and offer lower flow resistance due to the thinner walls than can be readily manufactured in ceramic supports. Another practical benefit attributable to metallic substrates is the ability to survive thermal shock. Such thermal shocks occur in gas turbine operations when the turbine is started and stopped and, in particular, when the turbine must be rapidly shut down. In this latter case, the fuel is cut off or the turbine is "tripped" because the physical load on the turbine — e.g., a generator set — has been removed. Fuel to the turbine is immediately cut off to prevent overspeeding. The temperature in the combustion chambers (where the inventive process takes place) quickly drops from the temperature of combustion to the temperature ofthe compressed air. This drop could span more than 1000°C in less than one second. In any event, the supported catalyst is deposited (or otherwise placed) on the walls within the channels or passageways ofthe metal substrate in the amounts specified above. Several types of substrate materials are satisfactory in this service: aluminum, aluminum-containing or aluminum-treated steels, and certain stainless steels or any high temperature metal alloy, including cobalt or nickel alloys where a catalyst layer can be deposited on the metal surface. The prefened materials are aluminum-containing steels such as those found in U.S. Patent Nos. 4,414,023 to Aggen et al.; 4,331,631 to Chapman et al.; and 3,969,082 to Cairns et al. These steels, as well as other sold by Kawasaki Steel Corporation (River Lite R20-5SR), Vereinigte Deutsche Metallwerke AG (Alumchrom I RE), and Allegheny Ludlum Steel (Alia-IV) contain sufficient dissolved aluminum so that, when oxidized, the aluminum forms alumina whiskers or crystals on the steel's surface to provide a rough and chemically reactive surface for better adherence ofthe washcoat ofthe supported catalyst. The metallic substrates may be coated in the appropriate areas with the supported catalysts ofthe invention using a variety of techniques, either before or after fabrication into their final monolithic forms, as described above. For example, the sheets making up the catalyst substrate structure may be coated on one or both sides either before or after being molded or formed into the appropriate configuration for winding together to afford a monolithic structure in the form of a spiral roll. In this regard, in addition to alternating flat and corrugated sheets, the spiral roll can also be comprised of corrugated sheets having a herringbone pattern of corrugations which are wound together in a non-nesting fashion. The supported catalyst ofthe invention may be applied to the metallic substrate using a stepwise procedure where the hafhia-containing refractory support material is first applied either as a fine dispersion ofthe finished support material or by using the appropriate support precursors (see above) followed by hydrolysis and/or calcination to afford the desired hafhia-containing refractory support coating or washcoat on the metal substrate surface. When the hafhia-containing refractory support material is applied as a washcoat to the metal substrate, it may be applied in the same fashion as one would apply paint to a surface, e.g., by spraying, direct application dipping the substrate into the washcoat material, etc. After application ofthe refractory support coating or washcoat, the platinum group metal may be deposited on the washcoat as set forth above to yield the final catalyst structure ofthe invention on calcination. Alternatively, the final supported catalyst composition may be separately prepared using the techniques described above and applied to the metal substrate surface as a fine dispersion or suspension in the appropriate liquid carrier by spraying, dipping or the like. A prefened embodiment ofthe invention, which is particularly useful for catalytic combustion or partial combustion of combustible fluids, comprises a catalyst structure made up of a metallic foil substrate formed into a series of longitudinal adjacently disposed catalyst-coated and catalyst-free channels for passage of a flowing gaseous reaction mixture, wherein the channels coated with catalyst are in heat exchange relationship with the adjacent catalyst-free channels and in which the catalyst-coated channels are coated on at least a portion of their internal surface with a catalytically effective amount of a platinum group metal, preferably palladium, deposited on a washcoat of refractory support material comprising HfO 2 , optionally in combination with SiO 2 and ZrO 2 , with HfO 2 /SiO 2 and SiO 2 /HfO 2 *ZrO mixtures being prefened. For instance, the spiral corrugated structure noted above may be coated on one side with the refractory support washcoat and catalyst. The treated corrugated structure may then be rolled into a monolith. A separator sheet of similar material may also be coated on one side with the supported catalytic material and rolled along with the corrugated sheet into the spiral monolith. In any event, the surface in the monolith having the catalyst placed thereon produces heat during the combustion process. This heat may pass to the gas flowing by or may be conducted through the catalyst structure to the adjacent non-catalytic, and hence, cooler surface. From there the heat would pass into the non-combusted gas passing along that surface. This allows control ofthe temperature ofthe catalytic surface ofthe catalyst structure by an integral heat exchange without resorting to such measures as air dilution or extraneous heat exchange structures. Such a control might be desirable where, for instance, the preheat temperature ofthe inlet gas in quite high and the gas flow rate is unstable. The catalyst structure should be made in such a size configuration that the average linear velocity ofthe gas through the longitudinal channels in the catalyst structure is greater than about 0.2 m/second throughout the catalytic structure and no more than about 60 m/second. This lower limit is greater than the flame front speed for methane and the upper limit is a practical one for the type of supports currently commercially available. These average velocities may be somewhat different for fuels other than methane. The processes in which the catalysts and catalyst structures ofthe invention are useful include those processes in which a supported or heterogeneous platinum group metal catalyst is employed and where reaction temperatures are sufficiently high and/or the reaction is sufficiently exothermic that it is possible, if not probable, that the catalyst will be exposed to temperatures approaching or even exceeding 1000°C through normal operation or process upsets for times sufficient to cause catalyst sintering or deactivation. Such catalytic processes include the oxidation of ammonia to afford nitric acid, steam reforming of methane to CO and H 2 , complete oxidation of hydrocarbons for emission control, including automotive emissions control, and combustion or partial combustion of a combustible fuel. In a prefened embodiment, the process ofthe invention is directed to a process for the combustion or partial combustion of a combustible fuel. In this prefened embodiment, the fuel in gaseous or vaporous form is combined with an oxygen-containing gas, e.g., air and passed through a catalyst structure made up of a metallic foil substrate formed into a series of longitudinal, adjacently disposed catalyst-coated and catalyst-free channels wherein the channels coated with catalyst are in heat exchange relationship with the adjacent catalyst-free channels and in which the catalyst-coated channels are coated on at least a portion of their internal surface with a catalytically effective amount of palladium deposited on a washcoat of refractory support material comprising HfO 2 in admixture with either SiO 2 or SiO 2 and ZrO 2 , said refractory support material having a HfO 2 content of at least about 5 mol percent based on its total metal oxide content and a surface area of at least about 5 m 2 /g after calcination at 1000°C for 10 hours, whereby the gaseous mixture of fuel and oxygen-containing gas undergoes at least partial combustion on passage through the catalyst-coated channels ofthe catalyst structure. The catalytic combustion can be followed by an optional homogeneous combustion zone in which combustion ofthe fuel is completed at temperatures below those at which significant NO x will form, e.g., below 1500 or 1600°C. This process may be used with a variety of fuels and a broad range of process conditions. Typical fuels may include hydrocarbonaceous fuels as well as H 2 and CO/H 2 mixtures. Although normally gaseous hydrocarbons, e.g., methane, ethane and propane are highly desirable as a source of fuel for the process, most hydrocarbonaceous fuels capable of being vaporized at the process temperatures discussed below are suitable. For instance, the fuels may be liquid or gaseous at room temperature and pressure. Examples include the low molecular weight aliphatic hydrocarbons mentioned above as well as butane, pentane, hexane, heptane, octane; gasoline; aromatic hydrocarbons, such as benzene, toluene, ethylbenzene and xylene; naphthals, diesel fuel and kerosene; jet fuels; other middle distillates; heavier fuels (preferably hydrotreated to remove nitrogenous and sulfurous compounds); oxygen-containing fuels such as alcohols including methanol, ethanol, isopropanol, butanol, or the like; and ethers such as diethlyether, ethyl phenyl ether, MTBE, etc. Low BTU gas such as town gas or syngas may also be used as fuels. The fuel is typically mixed into the combustion air in an amount to produce a mixture having an adiabatic combustion temperature greater than the temperature achieved by the process of this prefened embodiment. Preferably the adiabatic combustion temperature is above 900°C, most preferably above 1000°C. Nongaseous fuels should be at least partially vaporized prior to their contacting the catalyst zone. The combustion air may be at atmospheric pressure or lower or may be compressed to a pressure of 35 atm or more. Stationary gas turbines, which ultimately could use the gas produced by this process, often operate at gauge pressures in the range of 8 to 35 atm. Consequently, this process may operate at a pressure between about 0 and about 35 atm, preferably between 0 to 17 atm. The fuel/air mixture supplied to the catalyst should be well mixed and the gas inlet temperature may be varied depending on the fuel used, this temperature may be achieve by preheating the gas through heat exchange or by adiabatic compression. The process uses a catalytic amount of a palladium-containing material deposited on a hafhia-containing refractory support which in turn is coated on a metal monolith substrate with low resistance to gas flow. The bulk outlet temperature ofthe partially combusted gas leaving the zone containing the catalyst and the wall temperature ofthe catalyst will be at temperatures significantly lower than the adiabatic temperature or the temperature the gas would obtain if the fuel were fully combusted. Generally, the gas temperature exiting the catalyst will be in the range of 720 to 950°C. The catalyst substrate temperature will generally be less than 1100°C and preferably will not exceed 950°C. These temperatures will depend on a variety of factors including the pressure of the system, the partial pressure ofthe oxygen, the calorific value ofthe fuel, the specific design and performance requirement ofthe gas turbine, and the like. Nevertheless, the catalyst will partially oxidize the fuel but will be limited to temperatures below the adiabatic temperature ofthe fuel-air mixture. EXAMPLES These examples show the preparation ofthe refractory catalyst support materials of the invention, supported catalysts ofthe invention employing these refractory support materials and catalyst structures wherein the refractory support materials ofthe invention are employed as washcoats on a monolithic metal substrate with a catalytically-active platinum-group metal being deposited on the washcoat, as well as tests which demonstrate the high temperature stability and other desirable properties ofthe refractory support material ofthe invention as used in the catalysts and catalyst structures ofthe invention. Comparative refractory supports, catalysts and catalyst structures are also shown. To test for high temperature stability in air or steam, two different procedures were used. For heat treatment in air, the sample materials were heated in a stagnant air muffle furnace at a 10°C/minute ramp rate to the desired temperature then held at this temperature for the required time. For heat treatment in air plus steam, the samples were heated in a tube furnace with a flowing gas of air plus 10% water vapor with a 3°C/minute ramp rate. The stability ofthe sample materials was determined by examining their surface area and/or crystallite particle size after high temperature treatment since a key characteristic of the better support materials is the retention of high surface area and small crystallite particle size at high temperature, higher surface area being conelated with smaller particle size. All surface areas were measured by the BET method, which is conventionally reported in m /g. However, it should be noted that HfO 2 has a density 1.7 times larger than that of ZrO 2 . Thus, for a ZrO 2 and HfO 2 material of equivalent particle size, the ZrO 2 would have a BET surface area 1.7 times larger. In addition, in this application it is the substrate or washcoat layer thickness that is important and for equivalent washcoat thicknesses, approximately 1.7 times as much HPO 2 by weight, must be used. Thus, for comparison purposes, BET surface areas are converted to Zirconia Equivalent Surface Areas or ZESA by conecting for the material density difference to a pure Zrθ2 basis. The ZESA was calculated in each case using the formula ZESA = (d x /dz r θ2) x S x where d x and S x are the density and BET surface area ofthe material under test and d x /dz r θ s ^ e density of Zrθ2- Tests of performance ofthe catalyst structures were carried out in a high pressure reactor system designed to simulate conditions in a gas turbine combustor. The test rig consisted of a steel pressure vessel that was lined with insulation ceramic and with a valve at the outlet to adjust pressure. High pressure air was passed through an electric resistance heater to obtain a hot air stream. Methane fuel was injected and then mixed by passage through a static mixer. This well mixed fuel air mixture was then passed over the catalyst and then through a post catalyst section. Thermocouples were placed upstream ofthe catalyst to measure the temperature ofthe gas entering the catalyst, within the catalyst and in contact with the substrate surface to measure the substrate temperature and downstream ofthe catalyst to measure the temperature ofthe gas in the post catalyst homogeneous combustion zone. In a light-off test to determine LOT, fuel is added to the air and temperature ofthe gas inlet to the catalyst is increased at a constant rate. Depending on the activity ofthe catalyst, at some inlet temperature the catalyst will begin reacting the fuel and the catalyst temperature and the outlet gas temperature will rise above the inlet gas temperature. The LOT is taken as the inlet gas temperature where the outlet gas temperature is changing most rapidly, i.e., the inflection point in the plot of inlet temperature versus outlet gas temperature. For stability measurements, the reactor conditions are held constant and the catalyst substrate and outlet gas temperatures are monitored over time periods of one or more hours to assess stability. Example 1 - Sample Preparation Samples were either prepared or obtained directly from suppliers and given letter designations for identification purposes in subsequent examples where stability testing, etc. is reported. Sample A (ZτG 2 Comparative Samplei ZrO 2 was obtained from DKK (DAIICHI KIGENSO KAGAKU KOGYO) as RC- 100P ZrO 2 powder with an initial BET surface are of 100 m /g and a pore volume of about 0.3 cc/g. Sample B fSiO 2 /ZrO 2 Comparative Sampled ZrO 2 powder (100 g) obtained from DKK (RC-100) was impregnated using the incipient wetness technique with a solution containing 21.1g of tetraethylorthosilicate (TEOS), 6.0g of 2 mM HNO 3 and ethanol to make up 30 ml solution. The resulting wet mixture was placed in a closed container and heated at 60°C for about 18 hours to effect hydrolysis ofthe TEOS precursor. After hydrolysis, the solid was dried at 110°C and subsequently calcined by heating at a ramp rate of 10°C/min to the desired temperatures set forth in the examples below for 10 hours. The final sample contained 11 mol percent SiO 2 . Sample C (HfO 2 ) HfO 2 was obtained from Teledyne Wah Chang (Albany, Oregon) as HfO 2 powder with an initial BET surface area of 20 m 2 /g (ZESA of 34 m 2 /g) and pore volume of 0.12 cc/g. Sample P (Si Q 2 Ω 2 ) SiO 2 /HfO 2 powder was prepared using the same procedure as employed for Sample B above except HfO 2 from Teledyne Wah Chang (Albany, Oregon) was substituted for the ZrO 2 and differing amounts of TEOS (12.37g) and 2 mM HNO 3 (4.0 g) were used. The dried sample in this case contained 11 mol percent SiO 2 . Sample E (SiO 2 ZHfQ 2 ) A second SiO 2 /HfO 2 sample was prepared as described above for Sample D except Teledyne HfO 2 wet cake (an intermediate of Teledyne 's commercial RGS HfO 2 ) was used. This wet cake was dried in an oven at 110°C for ~20 hours and precalcined at 400°C for 10 hours before SiO 2 impregnation, which resulted in monoclinic phase crystallites of 5 nm in average diameter. The dried SiO 2 /HfO 2 sample in this case contained 13 mol percent SiO 2 . Sample F (SiO 2 ZZ∑Ω 2 -»HiΩ 2 ) A HfO 2 -ZrO 2 solid solution (containing 10 mol percent HfO 2 ) was prepared by dissolving ZrOCl 2 • 8H 2 O (40 g) and HfOCl 2 • 8H 2 O (5.6 g) in distilled water and sealing the resulting solution in a Teflon ® lined Parr bomb which was then heated at 200°C for 17 hours at which time the resulting solid phase was recovered and dried at 110°C in a drying oven for 20 hours. This dried sample was then impregnated with SiO 2 using the procedure described for Sample B above except the quantities of stating materials were adjusted to give a dried solid containing 7 mol percent SiO 2 . Sample Q (SiO 2 ZZ∑Ω 2- » HiΩ 2 ) A HfO 2 -ZrO 2 solid solution (containing 17 mol percent HfO 2 ) was prepared by dissolving HfOCl 2 <»8H 2 O (38.12g) and ZrOCl 2 <»8H 2 O (150g) in 1000 ml of distilled water and precipitating with NH 2 OH until a pH of 10-11 was reached. The precipitated gel was rinsed several times with distilled water until free of chloride ions and then dried at 60°C in a Rota Vap (~0.5 mmHg for ~200 minutes) to afford a dried powder. This powder was precalcined at 600°C for 10 hours and impregnated with SiO using the procedure described for Sample B except the final Mol percent of SiO 2 was 13%. Sample H (SiO 2 ZZrQ 2 gHjQ 2 ) A HfO 2 -ZrO 2 solid solution (containing 57 mol percent HfO 2 ) was prepared by dissolving HfOCl 2 • 8H 2 O (211 g) and ZrOCl 2 • H 2 O (123 g) in 500 ml of distilled water and precipitating with NH 4 OH until a pH of 10-11 was reached. The precipitated gel was rinsed three times with distilled water and one time with ethanol by stir-centrifuge cycles and then dried at 60°C in a Roto Vap (-0.5 mm Hg for ~200 minutes) to afford a dried powder. This powder was precalcined at 500°C and impregnated with SiO 2 using the procedure given for Sample B above except on this occasion the final dried product contained 15 mol percent SiO 2 . Sample I (Td on SiO 2 /ZrO 2 Comparative Sample) A sample of SiO 2 /ZrO 2 prepared as described above for Sample B was impregnated with a solution of Pd (NH 3 ) 2 (NO 2 ) 2 dissolved in excess concentrated nitric acid and dried at 110°C for -10 hours to afford a Pd on SiO 2 /ZrO 2 catalyst with a loading of 20% by weight Pd, the SiO 2 being present at 11 mol percent ofthe SiO 2 /ZrO support material. Sample J CPd on SiO-JHffV. Using the procedure described for Sample I above, a sample of SiO 2 /HfO 2 prepared as described above for Sample D (in this case containing 11 mol percent SiO 2 ) was impregnated with Pd to afford, on drying, a Pd on SiO 2 /HfO 2 catalyst with a loading of 10% by weight Pd. Example 2 - Comparative Sample Stability Samples ZrO 2 (Sample A) and SiO 2 /ZrO 2 (Sample B) were calcined in air at various temperatures ranging from 600 to 1100°C for 10 hours and the BET surface area of the calcined material was determined. The results are shown in FIG. 1. As illustrated by FIG. 1, SiO 2 stabilized ZrO 2 is more stable than ZrO 2 alone but severe sintering still occurs above the 1000°C calcination with resulting loss of surface area to values below 5 m Ig. Example 3 - Stability of HfQ 2 and SiO 2 ZH O_ 2 Compared to ZrO^and SiO /ZrO ; HfO 2 (Sample C) and SiO 2 /HfO 2 (Sample D) and comparative ZrO 2 containing materials (Samples A and B) were calcined in air at various temperatures ranging from 600 to 1100°C for 10 hours and the stability ofthe calcined materials was determined by examining the ZESA surface area and crystallite size of each ofthe calcined samples. The results are shown in FIGS. 2 A and 2B. As shown in FIGS. 2 A and 2B, HfO 2 is somewhat more stable than ZrO 2 at temperatures above 800°C; however, SiO 2 stabilized HfO 2 is fully stable up to 1100°C while SiO 2 stabilized ZrO 2 shows severe sintering at the highest temperatures. Comparison of FIGS. 2A and 2B shows that for SiO 2 stabilized HfO 2 the crystallite size does not change but the surface area decreases somewhat suggesting that sintering ofthe silica is responsible for the surface area loss. After calcination of 1100°C, where the surface area of SiO 2 becomes essentially negligible, the measured surface area is essentially that ofthe starting SiO 2 stabilized HfO 2 , and is the same as that ofthe raw HfO 2 material. The observed stability of crystallite size at temperatures up to 1100°C is remarkable. Example 4 - Stabilitv of SiO,/HfO, Compared to SiO^ZrO-, Two different samples of SiO 2 stabilized HfO 2 (Samples D and E) and a comparative sample of SiO 2 stabilized ZrO 2 (Sample B) were calcined in air at various temperatures ranging from 800 to 1100°C for 10 hours and the stability ofthe samples was determined at each calcination temperature by examining their ZESA surface area and crystallite size. The results, which are illustrated in FIGS. 3A and 3B, shows that both HfO -containing samples have superb stability at the higher temperatures whereas the SiO 2 stabilized ZrO 2 shows a dramatic loss in surface area and increase in crystallite size at temperatures approaching 1100°C. Example 5 - Stability of SiO^tabilized H 0 2 «ZrO Solid Solutions A series of SiO 2 stabilized solid solutions of HfO 2 *ZrO 2 containing various amounts of HfO 2 (Samples F, G and H) as well as samples of ZrO 2 and HfO 2 stabilized with SiO 2 (Samples B and D, respectively) were calcined in air at 1000°C to 1100°C for 10 hours each and their ZESA surface area were measured to determine their stability at the calcination temperature used. The results are shown in FIG. 4. Here it is shown that SiO 2 stabilized HfO 2 exhibits exceptional stability at 1100°C compared to SiO 2 stabilized ZrO 2 and that SiO 2 stabilized ZrO 2 -HfO 2 solid solutions have excellent stability (similar to that exhibited by the SiO 2 stabilized HfO 2 ) provided HfO 2 comprises more than 10 mol percent ofthe stabilized composition. Example 6 - Stabilitv of SiO 2 /HfO, versus SiO ZZιΩ 2, m S e m Two different SiO 2 stabilized HfO 2 refractory supports (Samples D and E), a SiO 2 stabilized ZrO 2 -HfO 2 solid solution (Sample H) and a SiO 2 stabilized ZrO 2 comparative support material (Sample B) were aged in 10% H 2 O and air at 1035°C for up to about 500 hours and portions ofthe samples were periodically (every 100 hours) tested for changes in surface area. The results ofthe surface area determinations over the course ofthe aging experiment are shown in FIG. 5. As illustrated in FIG. 5, the silica stabilized HfO 2 compositions, including the ZrO 2 -HfO 2 solid solution, maintained essentially the same surface area over the duration ofthe test whereas the SiO 2 stabilized ZrO 2 showed a dramatic reduction in surface area within the first 100 hours ofthe aging test. This clearly illustrates the advantage ofthe refractory supports ofthe invention over the prior art materials in a high temperature, steam-containing environment. Example 7 - Stability of SiO Stabilized ZrO 2 or HfO 2 Supports Loaded with Palladium Samples of supported palladium catalysts wherein the Pd is deposited on SiO 2 stabilized ZrO 2 (Sample I) or SiO 2 stabilized HfO 2 (Sample J) were calcined for 10 hours in air at 1050°C and the surface area ofthe calcined catalysts was compared to the surface area ofthe supports which had been calcined in air for 10 hours at 1000°C prior to deposition ofthe Pd metal. The results ofthe tests are shown graphically in FIG. 6. Here the results illustrated in FIG. 6 show that there was essentially no change in the surface area ofthe SiO 2 stabilized HfO 2 , before and after catalyst deposition and calcination, while the surface area ofthe SiO 2 stabilized ZrO was decreased substantially. Example 8 - Stabilitv of SiC HiO^and SiO^ZrO-, and Supported Pd Catalysts Employing the Supports in Steam Samples of supported palladium catalysts wherein Pd is deposited on SiO 2 stabilized ZrO 2 (Sample I) or on SiO 2 stabilized HfO 2 (Sample J) as well as the supports themselves (Sample B and Sample D) were aged in 10% H 2 O and air at 1035°C for periods of time ranging up to 500 hours with portions ofthe aged samples being tested periodically (every 100 hours) for changes in surface area. The results are recorded in graphic form in FIG. 7. Here the results show that the Pd on SiO 2 /HfO 2 catalyst retains much of its surface areas for 300 hours in steam while the surface area ofthe Pd on SiO 2 stabilized ZrO 2 drops to a very low level after the first 100 hours of aging in steam. These results are consistent with the stability ofthe supports themselves in steam, since as shown in FIG. 7, the SiO 2 stabilized HfO 2 shows almost no change in surface area over a 500 hour aging time in steam while the SiO 2 stabilized ZrO 2 loses essentially all of its surface area in the first 100 hours of aging in steam at 1035°C. Example 9 - Use of Monolithic Catal v t Structures Having HfO 2 or SiO, Stabilized HfO 2 A series of laboratory scale catalyst structures employing a monolithic substrate made up of Fe/Cr/Al metal foil and catalytically active Pd deposited on a substrate washcoat selected from HfO 2 or SiO 2 stabilized HfO 2 (both according to the invention) or ZrO or SiO 2 stabilized ZrO 2 (comparative structures) were prepared and tested for LOT as set forth above. To prepare the catalyst test structures the metal foil was corrugated in a herringbone pattern and then oxidized at 900°C in air to form alumina whiskers on the foil surface. Collodoidal sols of HfO 2 , SiO 2 /HfO 2 , ZrO 2 or SiO 2 /ZrO 2 in water were sprayed on one side ofthe corrugated foil and the coated foils were air calcined at the temperatures indicated below in Table 1 for 10 hours to afford the final washcoated foil in each case. The palladium metal was deposited on the washcoat foil surface using the appropriate concentration of a Pd precursor (Pd [NH 3 ] 2 [NO ] dissolved in water with excess HNO 3 to lay down the desired quantity of Pd metal after calcination on the washcoat surface. The Pd precursor solution was sprayed onto the surface ofthe metal foil which had been washcoated as described above and the foil was then calcined in air at the temperatures given below in Table 1 to give the Pd concentrations per cm of foil set forth in Table 1. After calcination the corrugated foil was rolled so that the corrugations did not mesh to form a final metal structure of two-inch diameter and two- or three-inch length with longitudinal channels running axially through the structure. The structures prepared as described above were then tested for LOT using the general procedure given above. The tests were performed using a methane/air mixture having an adiabatic combustion temperature of 1300°C and at a pressure of 11.9 atmospheres. The results ofthe tests including further characterization ofthe catalyst structures used is given in Table 1 below. (a) Washcoat contained 15 mol % SiO 2 (b) Washcoat contained 11 mol % SiO 2 (c) Washcoat contained 15 mol % SiO 2 (d) Washcoat contained 11 mol % SiO 2 (e) Catalyst also contained 0.357 mg Pt per cm 2 foil (Pd/Pt atomic ratio of 6/1). As indicated in Table 1 above, in all cases the catalyst structures ofthe invention employing a HfO 2 -containing washcoat gave catalyst performance which is equal or better than the ZrO 2 -containing washcoated catalyst structures of the prior art in terms of a Lower LOT. For example, the 0.62 mg Pd/cm 2 catalyst on a SiO 2 stabilized HfO 2 support washcoat (6.2 mg/cm 2 ) gave a LOT of 460°C whereas using a SiO 2 stabilized ZrO 2 washcoat (5 mg/cm 2 ) at a Pd loading of 1.0 mg/cm 2 resulted in a LOT of 500°C. Further, a Pd catalyst loading of 0.185 mg/cm 2 on a SiO 2 stabilized HfO 2 washcoat (7.4 mg/cm ) gave a LOT of 510°C while a higher Pd loading of 0.25 mg/cm 2 on a SiO 2 stabilized ZrO 2 support washcoat (5 mg/cm 2 ) yielded a LOT of 540°C. Thus, in each case the HfO 2 - containing washcoat catalyst structure gave a lower LOT even though the active catalyst concentration was less than that present on the conesponding catalyst stracture with a ZrO 2 -containing refractory support washcoat. The invention has been disclosed both by description and by the use of examples. The examples are only examples and must not be used to limit the invention in any way. Furthermore, one having ordinary skill in this art would be able to determine equivalents to the invention described here but outside the literal scope ofthe appended claims. We also consider those equivalents to be part of our invention.

Claims: We claim as our invention: 1. A refractory metal oxide support material for a catalyst comprising HfO , in admixture with SiO 2 or SiO 2 and ZrO 2 wherein the HfO 2 is present in an amount sufficient to impart thermal stability on the support material at temperatures in excess of lOOO°C. 2. The refractory support material of Claim 1 wherein the HfO 2 is present in an amount which is at least about 5 mol percent based on the total metal oxide content ofthe support material . 3. The refractory support material of Claim 1 or 2 wherein the support material has a 2 B BEETT ssuurrffaaccee aarreeaa ooff aatt lleeaasstt aabbout 5 m /g after calcination for 10 hours at a temperature of about 1100°C. 4. The refractory support material of Claim 3 wherein the metal oxide comprises HfO 2 in admixture with SiO 2 . 5. The refractory support material of Claim 4 wherein the SiO 2 is present in an amount ranging from about 5 to about 15 mol percent ofthe total metal oxide content ofthe support material. 6. The refractory support material of Claim 3 wherein the metal oxide comprises HfO 2 in admixture with SiO 2 and ZrO 2 . 7. The refractory support material of Claim 6 wherein the SiO 2 is present in an amount ranging from about 5 to about 15 mol percent and the ZrO 2 is present in an amount ranging from about 5 to about 80 mol percent in each case ofthe total metal oxide content ofthe support material. 8. The refractory support material of Claim 7 wherein the HfO 2 is present in an amount which is greater than about 10 mol percent ofthe total metal oxide content ofthe support material. 9. The refractory support material of Claim 3 wherein the HfO 2 or HfO 2 admixture with ZrO 2 is present as microcrystallites in the size range of from about 5 to about 15 nm prior to the addition ofthe SiO 2 . 10. A heterogeneous catalyst having high temperature stability which comprises a catalytically effective amount of a platinum group metal or mixture of platinum group metals deposited on a refractory metal oxide support material comprising HfO 2 , in admixture with SiO 2 or SiO 2 and ZrO 2 , wherein the HfO 2 is present in an amount sufficient to impart thermal stability on the support material at temperatures in excess of 1000°C. 11. The heterogeneous catalyst of Claim 10 wherein the HfO 2 is present in an amount which is at least about 5 mol percent based on the total metal oxide content ofthe support material. 12. The heterogeneous catalyst of Claim 10 or 11 wherein the support material has a BET surface area of at least about 5 m /g after calcination for 10 hours at a temperature of about 1100°C. 13. The heterogeneous catalyst of Claim 12 wherein the metal oxide support material comprises HfO 2 in admixture with SiO 2 . 14. The heterogeneous catalyst of Claim 13 wherein the SiO 2 is present in an amount ranging from about 5 to about 15 mol percent ofthe total metal oxide content ofthe support material. 15. The heterogeneous catalyst of Claim 12 wherein the metal oxide support material comprises HfO 2 in admixture with SiO 2 and ZrO 2 16. The heterogeneous catalyst of Claim 15 wherein the SiO 2 is present in an amount ranging from about 5 to about 15 mol percent and the ZrO 2 is present in an amount ranging from about 5 to about 80 mol percent in each case ofthe total metal oxide content ofthe support material. 17. The heterogeneous catalyst of Claim 16 wherein the HfO 2 is present in an amount which is greater than about 10 mol percent ofthe total metal oxide content ofthe support material. 18. The heterogeneous catalyst of Claim 12 wherein the HfO 2 or HfO 2 admixture with ZrO 2 is present as microcrystallites in the size range of from about 5 to about 15 nm prior to the addition ofthe SiO 2 . 19. The heterogeneous catalyst of Claim 12 wherein the platinum group metal is selected from platinum, palladium and mixtures of platinum and palladium. 20. The heterogeneous catalyst of Claim 19 wherein the platinum group metal is palladium. 21. A catalyst structure made up of a high temperature resistant ceramic or metal substrate formed into a monolithic structure having a multitude of adjacently disposed longitudinal channels for passage of a flowing reaction mixture in which at least a portion ofthe channels are coated on their internal surface with a washcoat comprising a refractory metal oxide support material made up of HfO 2 or HfO 2 in admixture with either SiO 2 or SiO 2 and ZrO 2 , wherein the HfO 2 is present in an amount sufficient to impart thermal stability on the support material at temperatures in excess of 1000°C and wherein a catalytically effective amount of a platinum group metal is deposited on the washcoat in at least some ofthe channels coated with washcoat. 22. The catalyst structure of Claim 21 wherein the HfO 2 is present in the washcoat in an amount which is at least about 5 mol percent based on the total metal oxide content ofthe support material in the washcoat. 23. The catalyst structure of Claim 21 or 22 wherein the washcoat support material has a 2 B BEETT ssuurrffaaccee aarreeaa ooff a att lleeaasstt aalbout 5 m /g after calcination for 10 hours at a temperature of about 1100°C. 24. The catalyst structure of Claim 23 wherein the metal oxide washcoat comprises HfO 2 in admixture with SiO 2 . 25. The catalyst structure of Claim 24 wherein the SiO 2 is present in an amount ranging from about 5 to about 15 mol percent ofthe total metal oxide in the washcoat. 26. The catalyst structure of Claim 23 wherein the metal oxide washcoat comprises HfO 2 in admixture with SiO 2 and ZrO 2 . 27. The catalyst structure of Claim 26 wherein the SiO 2 is present in an amount ranging from about 5 to about 15 mol percent and the ZrO 2 is present in an amount ranging from about 5 to about 80 mol percent in each case ofthe total metal oxide content of the washcoat support material. 28. The catalyst structure of Claim 27 wherein the HfO 2 is present in an amount which is greater than about 10 mol percent ofthe total metal oxide content ofthe washcoat support material. 29. The catalyst structure of Claim 23 wherein the HfO 2 or HfO 2 admixture with ZrO 2 in the washcoat is present as microcrystallites in the size range of from about 5 to about 15 nm prior to the addition ofthe SiO 2 . 30. The catalyst structure of Claim 23 wherein the substrate is a high temperature resistant metal. 31. The catalyst structure of Claim 23 wherein the platinum group metal is selected from platinum, palladium and mixtures of platinum and palladium. 32. The catalyst structure of Claim 31 wherein the platinum group metal is palladium. 33. A catalyst structure for partial or complete oxidation of a gaseous or vaporizable fuel made up of a metallic foil substrate formed into a series of adjacent longitudinally disposed channels for passage of a reaction mixture comprising the fuel in admixture with an oxygen-containing gas in which at least a portion ofthe channels are coated on their interior surface with a combustion catalyst and the remaining channels are not coated and wherein the channels coated with catalyst are in heat exchange relationship with the adjacent catalyst-free channels, said catalyst-coated channels being coated on their interior surface with a washcoat comprising a refractory metal oxide support material made up of HfO 2 or HfO 2 in admixture with either SiO 2 or SiO 2 and ZrO 2 in which the HfO 2 is present in an amount sufficient to impart thermal stability on the support material at temperatures in excess of 1000°C and wherein a catalytically effective amount of a platinum group metal is deposited on the washcoat in the channels coated with catalyst. 34 The catalyst structure of Claim 33 wherein the HfO 2 is present in the washcoat in an amount which is at least about 5 mol percent based on the total metal oxide content ofthe support material in the washcoat. 35. The catalyst structure of Claim 33 or 34 wherein the washcoat support material has a BET surface area of at least about 5 m2/g after calcination for 10 hours at a temperature of about 1100°C. 36. The catalyst structure of Claim 35 wherein the metal oxide washcoat comprises HfO 2 in admixture with SiO 2 . 37. The catalyst structure of Claim 36 wherein the SiO 2 is present in an amount ranging from about 5 to about 15 mol percent ofthe total metal oxide in the washcoat. 38. The catalyst structure of Claim 35 wherein the metal oxide washcoat comprises HfO 2 in admixture with SiO 2 and ZrO 2 . 39. The catalyst structure of Claim 38 wherein the SiO 2 is present in an amount ranging from about 5 to about 15 mol percent and the ZrO 2 is present in an amount ranging from about 5 to about 80 mol percent in each case ofthe total metal oxide content of the washcoat support material. 40. The catalyst structure of Claim 39 wherein the HfO 2 is present in an amount which is greater than about 10 mol percent ofthe total metal oxide content ofthe washcoat support material. 41. The catalyst structure of Claim 35 wherein the HfO 2 or HfO 2 admixture -with ZrO 2 in the washcoat is present as microcrystallites in the size range of from about 5 to about 15 nm prior to the addition ofthe SiO 2 . 42. The catalyst structure of Claim 35 wherein the substrate is a high temperature resistant metal is an aluminum-containing steel. 43. The catalyst structure of Claim 35 wherein the platinum group metal is selected from platinum, palladium and mixtures of platinum and palladium. 44. The catalyst structure of Claim 43 wherein the platinum group metal is palladium. 45. A process for the catalytic conversion of reactants in a flowing reaction mixture to reaction products in a high temperature catalytic reaction catalyzed by a heterogeneous platinum group metal catalyst which comprises: (a) mixing the reactants in the proper proportions to form a flowing reaction mixture; and (b) contacting the mixture at reaction temperatures with a catalyst structure made up of a high temperature resistant ceramic or metal substrate formed into a monolithic structure having a multitude of adjacently disposed longitudinal channels for passage of a flowing reaction mixture wherein at least a portion ofthe channels are coated on their intemal surface with a washcoat comprising a refractory metal oxide support material made up of HfO 2 or HfO 2 in admixture with either SiO 2 or SiO 2 and ZrO 2 , wherein the HfO 2 is present in an amount sufficient to impart thermal stability on the support material at temperatures in excess of 1000°C and wherein a catalytically effective amount of a platinum group metal is deposited on the washcoat in at least some ofthe channels coated with washcoat. 46. A process for the combustion of a combustible fuel mixture comprising the steps of: (a) mixing a fuel and an oxygen-containing gas to form a combustible mixture; (b) contacting the mixture with a catalyst structure made up of a metallic foil substrate formed into a series of adjacent longitudinally disposed channels for passage of a reaction mixture comprising the fuel in admixture with an oxygen- containing gas wherein at least a portion ofthe channels are coated on their interior surface with a combustion catalyst and the remaining channels are not coated and wherein the channels coated with catalyst are in heat exchange relationship with the adjacent catalyst-free channels, said catalyst-coated channels being coated on their interior surface with a washcoat comprising a refractory metal oxide support material made up of HfO 2 or HfO 2 in admixture with either SiO 2 or SiO 2 and ZrO 2 in which the HfO 2 is present in an amount sufficient to impart thermal stability on the support material at temperatures in excess of 1000°C and wherein a catalytically effective amount of a platinum group metal is deposited on the washcoat in the channels coated with catalyst. 47. The process of Claim 46 wherein the HfO 2 is present in an amount which is at lest about 5 mol percent based on the total metal oxide content ofthe support material. 48. The process of Claim 46 or 47 wherein the support material in the washcoat has a B BEETT ssuurrffaaccee aarreeaa ooff aatt lleeaasstt aalbout 5 m 2 /g after calcination for 10 hours at a temperature of about 1100°C. 49. The process of Claim 48 wherein the metal oxide support material in the washcoat comprises HfO 2 in admixture with SiO 2 . 50. The process of Claim 49, wherein the SiO 2 is present in an amount ranging from about 5 to about 15 mol percent ofthe total metal oxide content ofthe washcoat support material. 51. The process of Claim 48 wherein the metal oxide support material in the washcoat comprises HfO 2 in admixture with SiO 2 and ZrO 2 52. The process of Claim 51 wherein the SiO 2 is present in an amount ranging from about 5 to about 15 mol percent and the ZrO 2 is present in an amount ranging from about 5 to about 80 mol percent in each case ofthe total metal oxide content ofthe washcoat support material. 53. The process of Claim 52 wherein the HfO 2 is present in an amount which is greater than about 10 mol percent ofthe total metal oxide content ofthe washcoat support material. 54. The process of Claim 46 or 47 wherein the HfO 2 admixture with ZrO 2 in the washcoat is present as microcrystallites in the size range of from about 5 to about 15 nm prior to the addition ofthe SiO 2 . 55. The process of Claim 48 wherein the HfO 2 is present in an amount which is at least about 20 mol percent ofthe total metal oxide content ofthe washcoat support material. 56. The process of Claim 46 wherein the platinum group metal is selected from platinum, palladium and mixtures of platinum and palladium. 57. The process of Claim 56 wherein the platinum group metal is palladium

Inventors: WANG RUIPING, WANG, RUIPING
Applicants: CATALYTICA INC, TANAKA PRECIOUS METAL IND, CATALYTICA, INC., TANAKA KIKINZOKU KOGYO K.K.
Application year: 1996
Main classification: C01B3
IPC codes: B01J21, B01J23, B01J32, C01B3, C04B41, F01N3, F23C13, F23D14