MXPA97005871A - Catalyz - Google Patents

Abstract

The present invention relates to an oxidation and / or combustion catalyst, characterized in that it comprises at least one metal oxide selected from the group consisting of palladium, platinum, nickel, cobalt and iron, which is a combination with zinc metal oxide or metal, the catalyst has molar ratio of metal oxide to zinc metal or zinc metal oxide of 1: 2 and the catalyst is in the form of an intimate mixture or alloy, the catalyst further comprises metal oxide of earth

Description

C A T A L I Z A D O R BACKGROUND OF THE INVENTION This invention relates to a catalyst for oxidation and reduction reactions. In addition, the invention relates to a new catalyst composition.

Combustion catalysts include, for example, the generic catalysts present in car exhaust systems and which reduce the output of nitrogen oxides (NOx) and carbon monoxide (CO) gases as components of exhaust fumes that are discharged to the atmosphere. Conventionally such a system of auto discharge consists of platinum together with rhodium. While these catalysts are effective in converting nitrogen oxides and carbon monoxide to molecular nitrogen (N2) and carbon dioxide (C02), platinum metal is expensive and therefore increases the cost of manufacturing exhaust catalysts.

Moreover, the platinum / rhodium catalyst system is not catalytically active until it reaches its characteristic 'off' temperature. Under laboratory conditions the 'off' temperature of a conventional Pd / Rh auto catalyst is approximately 157 * C. However, under practical conditions the 'off' temperature under normal operating criteria is in the region of 200-250'C. Increasing the car's catalyst temperature to approximately 200-250'C requires a heating period of approximately 8-10 minutes during which the catalyst is ineffective and undesirable NO * and CO gases are released into the atmosphere. There is therefore a strong motivation to produce a catalyst system for car exhaust that has a low shutdown temperature.

Another disadvantage of the conventional platinum / rhodium car exhaust catalyst system is that if a machine failure occurs (in which unused fuel is available for combustion within the exhaust system), the catalyst temperature can be increased as much as possible. as 550-600 * C. At these temperatures the conventional catalyst begins to deteriorate irreversibly.

One of the largest research areas in vehicle exhaust catalysis is to develop a palladium-based system as an alternative to the expensive platinum / rhodium system used in today's conventional car catalysis. However, to date, the greatest difficulties in achieving a palladium-based catalyst for controlling exhaust emissions from vehicles has been that palladium metal is willing to react with oxygen at moderately high temperatures resulting in the elution of palladium metal. of the catalyst coating, and also in the catalyst deactivation. Processes invoking deactivation of the catalyst include 1) deposition of non-labile materials on the surface of the catalyst, 2) restructuring of the catalyst surface and in particular sintering the function of the supported metal and 3) oxidation of the function of the catalyst. metal supported through the chemical bond of the active species generated during the catalysis process. The deactivation of the catalyst through the oxidation of the metal function yields a change in the metal character of the catalyst from its zero valence state to a localized state of positive charge.

U.S. Patent Application No. 9404802.2 discloses the use of a catalytic system for catalysis by replacing halogen in halo-substituted hydrocarbons. The catalyst of the reaction is the removal of a halogen atom and its replacement by a hydrogen atom, and is therefore classified as a hydrogenation reaction.

It is known in chemistry that catalysts tend to catalyze only very specific reactions. In particular, oxidation and hydrogenation reactions are seen as being very different indeed.

Contrary to expectations, it has now been found that the catalyst disclosed in UK 9404802.2 is also effective in combustion and oxidation reactions.

DESCRIPTION OF THE INVENTION The present invention therefore provides a catalyst for oxidation and combustion reactions, said catalyst comprising a catalytic metal selected from palladium, nickel, platinum, rhodium, silver, ruthenium, cobalt, iron, moiibdene and tungsten in combination with a material that is a electron donor or a precursor of an electron donor.

The function of the material is to stabilize the catalytic metal. Ideally the material should be able to stabilize the catalytic metal in its zero oxidation state.

The material can be, for example, a metal or an oxide thereof. The metal contained in the material may be different from the catalytic metal in the catalyst. Alternatively the material may be an organic moiety with the ability to donate electrons, for example a ligand or polymer with electron donor groups. Examples of suitable metals and metal oxides that can be used as the material of the invention include (but are not limited to) gallium, zinc, aluminum, gold, silver, platinum, nickel, mercury, cadmium, indium or thallium or the oxides of any of those metals.

In a preferred embodiment the catalyst comprises a metal plus zinc (for example palladium / zinc).

Desirably the catalyst material is zinc or zinc oxide.

Palladium in combination with zinc or zinc oxide is found to be especially effective in catalyzing oxidation and combustion reactions.

The catalytic metal: material ratio can be varied as required. A ratio of 1: 2 has been found to be satisfactory, however other relationships can also be useful and the optimization of these relationships is mere routine.

The catalyst components may be in the form of an alloy or other close mixture, but other types of combination may also prove useful.

The presence of the material in the catalyst of the invention suppresses the catalytic system from oxidation or other reactions that could destroy the catalyst. Thus, it has been found that the useful catalytic lifetime of the catalyst of the present invention is much greater than the lifetime of conventional combustion or oxidation catalysts.

Without being bound by the theory it seems that a synergetic relationship exists between metals forming the catalyst alloy in which each metal component offers chemical stability over the metal function of the companion.

Catalysts according to the present invention, especially a palladium-containing catalyst system, have been shown to exhibit a lower quench temperature than conventional catalysts.

Also in contrast to conventional platinum / rhodium car catalyst systems, a catalyst according to the present invention, especially a palladium-containing catalyst system, has been shown to exhibit catalytic activity after 'firing' the catalyst in 600 * air. C.

In further experimental work, it has been shown that in the Pd / ZnO catalyst of the present invention the ionization energy of the palladium component is merely 333.4 eV. This value compares very favorably with the ionization energy of 335 eV for conventional catalysts. Thus, in the catalyst of the present invention the catalytic metal (here, Pd) is capable of being ionized more easily compared to the conventional catalyst. In addition it has been shown that the environment of the catalytic metal is highly homogeneous and that a high level of intimacy with the material is achieved.

The catalyst can be used according to the present invention to catalyze the combustion of combustible hydrocarbons, for example propane. In experiments involving a propane / O? Combustion reaction? a Pd / ZnO catalyst was observed to lower the required quench temperature from 290 * C of the conventional catalyst at 180 ° C.

The present invention therefore provides a catalytic exhaust system for use in engines, said catalytic system comprising catalysts as defined above.

It has further been found that if the catalyst of the present invention (as described above) is combined with a rare earth oxide the combined system is particularly effective as an oxidation and combustion catalyst.

The present invention therefore provides a catalytic composition, said composition comprising a catalytic metal selected from palladium, nickel, platinum, rhodium, silver, ruthenium, cobalt, iron, moiibdene and tungsten in combination with a material exhibiting electron donor properties or a precursor of an electron donor (eg, a transition metal or transition metal oxide, or aluminum or zinc, or the oxides thereof) and additionally comprises an oxide exhibiting varying stable oxidation states, for example oxides of rare earths.

Examples of suitable rare earth oxides, therefore, include oxides of La, Ce, Pr, Nd, Pn, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Desirably the rare earth metal should have at least two stable valences and mention should be made of the following metals in this aspect: Ce, Pr, and Gd. In particular, the oxides of Pr are preferred as an additive in the catalytic composition of the present invention giving a catalytic capacity of oxidized and improved combustion.

In a particularly preferred example, the catalytic composition of the present invention comprises a basic palladium / zinc oxide catalyst with a PrOx additive. The shutdown temperature of such a system has been found to be in the region of 100-120 * C.

The present invention therefore provides a catalytic system for use in motor technology, said catalytic system comprising catalysts as defined above together with an oxide with stable variable valence states, for example an oxide of a rare earth metal.

During experimental investigations the exothermic reaction produced when the catalyst of the present invention reaches its quench temperature caused an increase in temperature of about 80 * C. This exothermic reaction can be usefully coupled to produce a sensor, the temperature change due to the exothermic reaction in the 'off' being detected by a thermoelectric interaction, for example by observing the change in the resistance of the sensor itself or in the connectors to the sensor following the increase in temperature.

Accordingly, the present invention provides a sensor for detecting the presence of oxidizable or combustible material, said sensor comprising a catalyst or a catalyst system as defined hereinbefore.

In one embodiment the sensor arrangement comprises a "wheatstone" bridge circuit having a sensor covered with the catalyst or the catalytic composition balanced against a reference load.Preferably the reference load is identical in each aspect to the covered sensor apart from the absence of the catalyst in the coating At the beginning of the exothermic reaction a change in the resistance of the sensor is an indicator that the reaction takes place The change in the resistance of the sensor is detected in itself, for example in a bridge circuit. Wheatstone '.

In a further aspect of the present invention there is provided the use of the catalyst or catalyst composition as defined above in a sensor for detecting oxidizable or combustible matter.

In one embodiment the sensor of the present invention can be used as a "sniffer" to provide information and a mechanism for controlled feedback on the efficiency of the removal of harmful gases, for example from the exhaust fumes of a car and other equipment. of gas combustion. The high exotherm released from the catalytic system can be usefully employed in fuel cell technologies.

The present invention also provides a method for forming the catalyst of the present invention. The method involves placing the required amounts of the components in a crucible together with a quantity of deionized water and then 'boiling' the mud into a cake. The cake is broken into particulate form and then calcined in oxygen. For example, calcination can occur at 250 * C for approximately 8 hours. The product is cooled to about 50 * C and hydrogen is passed over the product at such a temperature for about 1 hour. The temperature is then increased slowly and incrementally, for example the temperature is increased by approximately 50 * C every hour, with a continuous passage of hydrogen over the product. Once the temperature reaches approximately 237 * C the product can then be used as described above.

Thus the present invention provides a method for forming a catalyst as described above, characterized in that after calcination the temperature is lowered, hydrogen is passed over the reaction mixture and the temperature is then incrementally increased in the presence of hydrogen.

The process as described above is especially beneficial where the starting materials are a palladium compound and a zinc compound. The final product formed following the process described above is essentially a palladium-zinc composition with a strong metallic character. Palladium acetate and zinc acetate are particularly suitable as starting materials. Instead of an acetate, nitrate or any other compound (eg, halogen) can also be used instead. The palladium acetate and the zinc acetate are intimately mixed, thermally degraded and then reduced as described above.

In any system of the present invention as described above, it may be beneficial to present the catalyst on a support. Any suitable support known in the art may suffice but particular mention may be made of zirconia or? - alumina. Desirably the? -used alumina has a low content of -OH, for example '3? -aluminium Degussa.

Preferably zirconia is used as support. In a particularly preferred embodiment zirconia can be used as a support for a palladium / zinc oxide catalyst.

The conditions used in the experimental work carried out to date include a flow reactor, a flow of 4000 hours 1, gas chromatography in line for analysis and thermal increase of the temperature until the shutdown is achieved.

Example 1 Synergistic effect on the palladium / zinc oxide catalyst.

Studies of the catalytic properties of the Pd / ZnO /? -alumin system on the conversion to carbon monoxide and combustion of hydrocarbons in the presence of clean dioxygen have indicated that palladium and zinc appear to have a synergetic interrelation, being stabilized even at high temperatures.

A palladium-zinc catalyst on glass wool was heated at 140-180 ° C in a reactor. The melting of the glass wool was obtained, indicating that the effective temperature on the combustion surface of the catalyst was 550-600 * C, the catalyst was not affected. It is known that palladium forms palladium oxide at 550 * C. The catalytic composition therefore stabilized palladium and zinc.

Example 2 Stabilization of zinc oxide by cerium.

Zinc oxide stabilized by cerium having a surface area at 78 m2 / gram was heated at 1000 * C for two hours in air. The surface area after heating was 74 m2 / gram, thus indicating that the ceria had stabilized the zinc oxide.

Example 3 Elemental analysis of catalysts based on palladium / zinc oxide.

The following table indicates the positions' of the elemental energy of the catalysts as well as their respective relationships. ESCA measurements were carried out using a Multilab VG HB100 system and monochromatic X-rays AIKa (1486 eV) were used to generate the spectra 1) 12 m2 / g ZnO, a) 107 m2 / g ZnO, 2) Ratio Pd: Zn0 of 1: 2, a) Ratio Pd: ZnO of 1: 10 Catalyst 4 was subjected to burning combustion conditions and catalyst 5 was worked by burning it in propane and oxygen which are fiery burning conditions.

Catalysts 1 and 2 acted as controls. The zinc component of the catalytic system of sample 2, (a fresh ZnO /? - AI2O3 catalyst) appears at 1024.4 eV. This value indicates that zinc is in oxidized form, since metallic zinc ionizes at 1020.8 eV. For a Pd / ZnO /? Catalyst - Freshly prepared AI2O3 (sample 3) the peak position of the zinc line changes to 1022.8 eV thereby indicating the presence of metallic zinc component in the catalytic formulation.

The ESCA results show that a Pd / ZnO /? catalyst -Al203 freshly reduced gives the palladium function in an electron-rich condition as evidenced by the ionization peak at 335.2 eV (sample 3). This is in contrast to the fresh catalyst sample Pd /? - AI2? 3 which gives an increase to a peak Pd 3d5 / 2 at 335.5 eV. This state of slight improvement of the electron density associated with the Pd environment can be attributed to the presence of a zinc metal component in the catalytic formulation. The ionization energy obtained for the Zn2p 3 peak is at 1022.8 eV and confirms the high metallic character associated with the surface of the Zn component.

As soon as the catalyst sample 3 is worked under CO and 02 the results generated from the ESCA analysis show that the ionization energy of the metal function of the Pd moves towards that of the metal position Pd found in the Pd /? - AI2? 3 fresh (sample 1) to 335.4 eV. The Pd / Zn ratio has also increased from 0.0289 to 0.0356 confirming an enrichment of Pd on the catalyst surface. These results are consistent with an aggregation of Pd on the surface of the catalyst.

Working the catalyst Pd / ZnO /? - AI20 for the combustion of propane in dioxygen results in an improved enrichment of the Pd at the surface relative to the combustion of carbon monoxide in dioxygen. The zinc component of the worked system Pd / ZnO /? - AI? Oa for the propane combustion process is in an oxidized form relative to that of the fresh catalysts and worked with CO, respectively.

The reaction of the catalyst under severe oxidizing conditions during the hydrogenolysis of 1,1,2-trichlorotrifluoroethane results in a surface where the metal functions Pd and Zn are segregated and the Pd is in an oxidized state as is evident by the ionization energy of 337.0 eV. Hence, the oxidation and reduction catalytic reactions on the Pd / ZnO /? - AI2? 3 system result in a Pd surface enrichment separation process at the expense of global Pd depletion. These results are consistent with the peritectic phase diagram for a Pd / Zn alloy, where an alloy composed of 3% Pd precipitates the phase? + Zn.

The metal palladium function has been found to be evenly distributed over the surface of? -AI? Os, with the zinc component in close proximity.

The catalytic operation results in an enrichment of the surface in the metal Pd and a segregation of the Pd and Zn components and indicates that the segregation process is a precursor for the deactivation of the catalyst.

Example 4 Preparation of a monolith for its incorporation into a vehicle exhaust system.

A 10% by weight solution containing 26.39 g of d-alumina (Detussa) was impregnated with 2.22 g of palladium nitrate, 3.66 g of zinc nitrate and 4.19 g of praesidium nitrate sic (prasodymium).

The solution was digested at 110 ° C for a period of 8 hours (to allow the ions to impregnate the alumina) before immersing the dry monolith in the solution.

The monolith was immersed in the solution, drained and baked at 120 * C for 8 hours before being incorporated into a vehicle exhaust system.

Example 5 The palladium-based vehicle exhaust formulation has been studied using a purposely constructed flow line that is capable of controlling the flow of carbon monoxide (CO), dioxygen (02), nitrous oxide (NO), a hydrocarbon sample of propane (RH) and dinitrogen as the carrier gas.

These gases can be reacted as required, either as an individual reagent or as part of a mixture of gases at various predetermined partial pressures. The flow line is adjusted with built-in gas sampling facilities linked to in-situ gas chromatographic analysis.

A conventional monolith sample of platinum / rhodium / ceria /? -alumina vehicle exhaust (Pt / Rh / CeO * /? -AI203) is used as a standard against which the activity of various catalyst formulations are measured. The spatial hourly velocities of the gas were maintained at 4000 h-1 to offer a residence time of c / rca 1 second.

The initial work required profiling the conversions of the respective gases on the conventional catalyst monolith. The results of the conversion of CO into O2 are shown in the attached figure 1. The results show that the commercial catalyst exhibits a slow increase to the reaction ('off') over the temperature range of 60-130 * C before the complete 'tearing' to the catalytic reaction where the conversion Total CO is achieved at 140 * C. For this reaction to take place the catalyst system has to absorb the CO in the environment of the metal Pt and the 02 in the function CO * and hence the catalytic system has to operate efficiently in both of these adsorption processes before combustion and desorption of carbon dioxide (CO2) from the surface. The results for a Pd /? -AI203 system are shown in figure 1. The results show that the combustion of CO on this catalytic surface is less efficient than the conventional catalyst because of the relatively high temperature (190 * C) required to initiate the combustion process. Nevertheless, the results show that the combustion process 'starts' at this temperature, a profile that is dissimilar to that of the commercial system. We must also bear in mind that the Pd /? - AI2O3 system does not have an oxygen reorganizing component in the system at this stage.

The addition of zinc oxide to the catalyst formulation reduces the "off" temperature for the Pd / ZnO /? - AI2O3 system by circa 35"C to 160 * C confirming that the modified catalyst is already showing higher efficiency for the Conversion of CO to the pure Pd /? - AI2O3 catalyst The results confirm that the surface has been modified to allow the reaction to proceed with a lower activation energy.A X-ray photoelectron spectroscopy (XPS) analysis confirms that the surface of the catalyst is an alloy of palladium and zinc with about 14% of the surface composition being Pd. Again the results show that the system is able to 'start' the reaction in a different way to the observed profile of reaction temperature of the commercial catalyst. This catalyst system was able to operate over a period of 72 h using a sample load containing 20 mg of Pd, demonstrating the high catalytic activity of the material The sample of Pd / ZnO /? -AI2O3 does not contain any oxygen mobilizer, although the reduced metal component will show an affinity for oxygen. XPS analysis of the worked catalyst shows no zinc metal elution of the catalyst over the duration of the reaction, confirming the thermal and chemical stability invoked on the catalyst by the combination of those two metal components in the catalyst formula.

The addition of praseodymium oxide (PrOx) as an oxygen mobilizer to the formula Pd / ZnO /? - AI2O3 has a marked effect on the "shutdown" for combustion (figure 1). The "off" temperature was measured at 120 ° C, and the reaction profile shows that the catalytic system "starts" at that temperature, to offer complete CO combustion.

This palladium-based formula operates at a lower 'off' temperature than the commercial vehicle exhaust catalyst. The catalyst sample exhibits this high activity over a 72 h period without reduction in CO conversion. These results confirm the stability of the catalyst during the reaction with little or no deactivation due to the restructuring of the surface of the catalyst, or to the deposit of carbonaceous residues on the surface of the catalyst, or to the formation of inert metal environments due to ia oxidation of the metal function. The PrOx was selected as the oxygen storage component due to the redox characteristics similar to CeOx and avoiding the claims of the prior art of the CeOx system.

Each of these four catalytic systems were monitored for their efficiency in converting RH, for the commercial catalyst was around 6% at an "off" temperature of 185 ßC. Although the Pd /? - AI2O3 system acted poorly during this reaction, the Pd / ZnO /? - A Oa and Pd / ZnO / PrOx /? - AI2O3 systems gave conversions from 55 in RH to 140 * C a comparable conversion with the system commercial Pt / Rh / CeOx /? - AI203 but at a lower temperature 45 * C. The lower temperature for catalyst activation is an important environmental feature, as there is pressure on car manufacturers to reduce vehicle exhaust emissions. Since this catalyst formula 'boots' at low temperatures, the catalyst will therefore operate more rapidly reducing emissions relative to the current commercial catalyst formulation. This feature is very important since many road users use their cars for short trips in which the conventional catalyst has not reached the temperature required to begin the conversion process. The operating temperature of the catalytic bed during the combustion of hydrocarbons was > 550"C as exemplified by the melting of the glass wool plug that supports the catalyst bed in the reactor, to a glass bubble This indicates that the catalytic formulation is exhibiting good thermal stability The Pd / ZnO / PrOx system /? - AI2O3 performed well under poor burn conditions, and in cases where the system was moved to rich burn, catalyst deactivation was rapid XPS analysis shows that deactivation of the catalyst under rich burn conditions is due to deposition of carbon in the metal and PrOx functions respectively.

In favor of exemplifying the role of zirconia in the Pd / ZnO system in the vehicle exhaust sensor technology, MEL (Magnesium Electron and Light) supplied zirconia marked with La for the tests. A catalyst with 5% weight of Pd / ZnO was prepared using Melcat zirconia 680/01 (Zr? 2) as a support material. The flow reactor was charged with 0.5 g of catalyst (containing 25 mg Pd). The reactor was operated under constant WHSV conditions of 4000h-1. Variations in the partial pressures of the reactant gases were compensated using an OFN balance.

The Pd / Zn catalyst supported in zirconia exhibits activity towards NO conversion in the presence of CO. At a temperature of 125 ßC the CO is converted into C02 (a temperature similar to the "off" of CO in 02) At a reaction temperature of 150 * C both the CO and the NO are converted to CO? and N2 respectively. The second time through the "off" offered the temperature for reaction of 190 * C. The results show that the Pd / ZnO / La.ZrOx system closely mimics the pre-off characteristics shown by the conventional catalyst until the reactor temperature reaches 125 * C, where the catalyst "starts" to total combustion of the fraction of CO. This temperature is about 35 * C lower than that obtained by the conventional system.This is considered to be the lowest theoretical temperature required for a "shutdown", particularly for hydrocarbons, since water is generated as a product, and the catalyst system has to operate at temperatures high enough to remove any water produced as steam.

The combustion of propane under poor burning conditions starts at 140 ° C with a low conversion of around 5%. Under the reactor conditions used, the conversion factor was identical to that obtained by the conventional catalyst, however, the 'off' temperature was 45 * C lower.After about 4 h in line the conversion was reduced to 4% of efficiency, mixing CO with propane, dioxygen and dinitrogen on the catalyst, gave an "off" temperature of 135 * C. Conversion was around 33% for all gases, when the temperature increased to> 250 * C the conversion fell, until 300"C the conversion of all gases was recorded at 23%. Sequential phases through I "off" temperature give the results: Corrida Temp. of 'off' 'C Conversion% 2 160 18 3 175 27 4 185 23 followed by a constant conversion of 23% at 300"C.

Reducing the CO flow by 0.5 and increasing the propane flow x2 increases the "off" temperature to 175 ° C. The conversion reduces to 8% of the gases, as a result that is consistent with the combustion of CO dominating the catalysis.

By combining 5% by weight of Rd / ZnO /? - alumina catalyst with 7% by weight of Pd / ZrOx catalyst and operating under conditions identical to those described above for the gas mixture, a conversion of 15% is obtained at 175 * C, showing that the addition of Rh / ZrO "to the system results in the system being less efficient than the Pd / ZnO / ZrOx system.

The most surprising results come from studies involving NO conversions. Using the Pd / ZnO / La.ZrOx catalyst with a CO / NO feed, the conversion of CO to CO was observed? a 125 * C. When the reactor temperature was increased to 150 * C all the CO and NO were converted to C02 and N2 respectively. When the CO flow was stopped the conversion of NO to N2 was completed and reintegrated with the supply of CO feed to the reactor. These results confirm that the Pd / ZnO / La.ZrO * catalyst system does not require the use of the expensive rhodium component in the catalyst formulation and that the Pd / ZnO / ZrO system is an alternative candidate catalyst system as a single-phase catalyst , low cost for three-way catalysts in vehicle exhaust catalysis.

Claims (1)

1. R E I V I N D I C A C I O N S A catalyst for oxidation or combustion reactions comprising a catalytic metal selected from palladium, nickel, platinum, rhodium, silver, ruthenium, cobalt, iron, molybdenum and tungsten in combination with a material that is an electron donor or a precursor of a electron donor A catalyst as claimed in claim 1 wherein the material is a metal or metal oxide. A catalyst as claimed in claim 1 wherein the material is an organic material such as a ligand or polymer with electron donating groups. A catalyst as claimed in claims 1 or 2 comprising a metal plus zinc or zinc oxide. A catalyst as claimed in claims 1 or 2 comprising palladium plus zinc or zinc oxide. A catalyst as claimed in any of the preceding claims wherein the ratio of metal to material is approximately 1: 2. A catalyst as claimed in any of the preceding claims wherein the components are in the form of a close mixture or an alloy. A catalytic exhaust system comprising a catalyst as claimed in any of the preceding claims. A catalyst as claimed in any of the preceding claims further comprising an oxide exhibiting variable stable oxidation states. A catalyst as claimed in claim 9 wherein the oxide is a rare earth metal oxide selected from the oxides of La, Ce, Pr, Nd, Pn, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. A catalyst as claimed in any of claims 1 to 10 comprising a zirconia or d-alumina support. A catalyst for oxidation and combustion reactions comprising palladium and zinc oxide on a zirconia support. A catalyst as claimed in claim 12 further comprising a rare earth metal oxide. A process for the production of a catalyst for oxidation or combustion reactions, said process comprising the steps of mixing palladium and zinc salts, thermally degrading and reducing the reaction mixture. A process as claimed in claim 14 wherein the thermally degraded reaction mixture is reduced by the passage of hydrogen over the reaction mixture and the temperature is incrementally increased in the presence of hydrogen The use of a catalyst as claimed in any of the preceding claims in the catalysis of combustion of combustible hydrocarbons. R E S U E N The invention relates to an oxidation and combustion catalyst such as the type comprising palladium and zinc oxide, the catalyst can be supported on an alumina or zirconium support and can further comprise a metal oxide such as cerium or praseodymium oxide . The catalyst exhibits a lower "off" temperature than conventional combustion catalysts and does not require the presence of rhodium.