CATALYTIC COMBUSTION PROCESS USING SUPPORTED PALLADIUM OXIDE CATALYSTS

Abstract: A process for operating a palladium oxide-containing catalytic combustor is useful, e.g., for powering a gas turbine. The palladium oxide is supported on a metal oxide such as alumina, lanthanide metal oxide-modified alumina, ceria, titania or tantalum oxide. The method involves maintaining control of the temperature within the combustor in such a manner as to insure the presence of palladium oxide. By maintaining the temperature below the decomposition onset temperature of palladium oxide (which is catalytically active for catalytic combustion) into metallic palladium (which is catalytically inactive) deactivation of the catalyst is avoided and high catalytic activity is retained. Regeneration of catalyst following inactivation resulting from an over-temperature is accomplished by using a heat soak in a regeneration temperature range which varies depending on the particular metal oxide used to support the palladium oxide.

Description: CATALYTIC COMBUSTION PROCESS USING SUPPORTED PALLADIUM OXIDE CATALYSTS Cross-Reference to Related Applications This application is a continuation-in-part of United States Patent Application No. 07/465,678 filed January 16, 1990, which is a continuation-in-part of United States Pa¬ tent Application No. 07/234,660 filed August 22, 1988, now United States Patent 4,893,465. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a particularly ad- vantageous process for the catalytically supported combus¬ tion of carbonaceous materials, including natural gas and methane. In a more specific aspect, this invention re¬ lates to a process for catalytically-supported combustion of natural gas or methane using a supported palladium ox- ide catalyst, without the formation of substantial amounts of nitrogen oxides. Burning of carbonaceous fuels is associated with for¬ mation of air pollutants, among the most troublesome of which are nitrogen oxides (NOx). Nitrogen oxides form whenever air-supported combustion takes place at open flame temperatures. One approach to eliminating nitrogen oxides involves chemically modifying the oxides after their formation. This approach has drawbacks, including the high cost associated with attempting to eliminate 100% of a once-formed pollutant. A more direct method of elim¬ inating nitrogen oxides is to operate the combustion pro¬ cess at a lower temperature so that no formation of nitro¬ gen oxide occurs. Such low temperature combustion can take place in the presence of catalysts, and it is to such a low temperature combustion process that this invention is directed. In general, conventional adiabatic, thermal combus¬ tion systems (e.g., gas turbine engines) operate at such high temperatures in the combustion zone that undesirable nitrogen oxides, especially NO, are formed. A thermal combustion system operates by contacting fuel and air in flammable proportions with an ignition source, e.g., a spark, to ignite the mixture which will then continue to burn. Flammable mixtures of most fuels burn at relatively high temperatures, i.e., about 3300°F and above, which in¬ herently results in the formation of substantial amounts of NOx. In the case of gas turbine combustors, the forma- tion of NOx can be reduced by limiting the residence time of the combustion products in the combustion zone. How¬ ever, due to the large quantities of gases being handled, undesirable quantities of NOx are nonetheless produced. It has long been realized that little or no NOx is formed in a system which catalytically burns a fuel at relatively low temperatures as compared to uncatalyzed thermal combustion. Typically, such catalytic combustion of natural gas or methane, for example, utilizes a pre- burner or thermal combustor which employs flame combustion to preheat combustion air to a temperature of 700°C or higher. Once the catalyst is sufficiently hot to sustain catalysis, the preburner is shut down and all the fuel and air are directed to the catalyst. Preheat is then only due to compressor discharge. Such a catalytic combustor, if operated at temperatures below about 1300°C-1400 o C, avoids the nitrogen oxide formation which occurs at the higher temperatures which are characteristic of the flame combustion. A description of such a catalytic combustion process and apparatus is found, for example, in U.S. Pa- tent 3,928,961. See also U.S. Patents 4,065,917 and 4,019,316. Such catalytic combustion as described above which will function effectively at a high space velocity has, however, heretofore been generally regarded as commercial- ly unattractive. A primary reason for this lack of com¬ mercial attractiveness has been the absence of an economi¬ cally competitive method for catalytic combustion of natu¬ ral gas. Description of Related Art Catalytically supported combustion processes have been described in the prior art. See, e.g., Pfefferle, U.S. Patent 3,928,961. The use of natural gas or methane in catalytic combustion has been taught in the art, as has the use of a palladium catalyst to promote such combus¬ tion/oxidation. See Cohn, U.S. Patent 3,056,646 wherein the use of palladium catalyst to promote methane oxidation is generically disclosed, as is an operable temperature range, 271°C to 900°C (see column 2, lines 19-25). Note also that this Patent states "the higher the operating temperature, the shorter will be the catalyst life and the more difficult will be subsequent ignition after catalyst cooling". Other patents directed to the use of platinum group metals as catalysts for methane oxidation at temper¬ atures above 900°C include U.S. Patents 3,928,961; 4,008,037; and 4,065,917. The literature also describes the thermal decomposition of PdO to Pd metal at tempera¬ tures of 800°C in air at atmospheric pressure. See Kirk Othmer Encyclopedia of Chemical Technology, Vol. 18, p. 248 which states that palladium acquires a coating of ox¬ ide when heated in air from 350°C to 790°C but that above this temperature the oxide decomposes and leaves the bright metal. The present invention finds particular utility in a process for the start-up of catalytically supported com¬ bustion. Prior art references directly related to such start-up are Pfefferle, U.S. Patent 4,019,316 and Pfef¬ ferle, U.S. Patent 4,065,917. CL. McDaniel et al, "Phase Relations Between Palla¬ dium Oxide and the Rare Earth Sesquioxides in Air," Jour¬ nal of Research of the Natural Bureau of Standards - A. Physics and Chemistry, Vol. 72A, No. 1, January-February, 1968, pages 27-37, describe complexes of PdO and other rare earth oxides. Specifically, the paper describes PdO in combination with each of the following sesquioxides La 2 0 2 , Eu 2 0 3 , Gd 2 0 3 , Dy 2 0 3 , Ho 2 0 3 , Y 2 0 3 , Er 2 0 3 , Tm 2 0 3 , Yb 2 0 3 and Lu 2 0 3 . A. Kato et al, "Lanthanide B-Alumina Supports For Catalytic Combustion Above 1000°C," Successful Design of Catalysts, 1988, Elsevier Science Publishers, pages 27-32, describes the preparation of support materials consisting of lanthanide oxides and alumina for use as combustion catalysts. The preparation comprises preparing a mixed solution of a lanthanide element nitrate (e.g., a nitrate of Y, La, Ce, Pr, Nd, Sm, etc.) and A1 2 (N0 3 ) 3 , neutraliz¬ ing the solution by adding dilute aqueous ammonia to form a precipitate, and washing, drying and calcining the pre¬ cipitate at 500°C. The powder, with 1% added graphite, was formed into cylindrical tablets and calcined at 700°C. The resultant support was impregnated with a solution of Pd(N0 3 ) 2 to provide 1% by weight Pd, then calcined at 500°C, then at 1200°C. The article states that the use of La, Pr and Nd as the lanthanide element gave rise to B-alumina (page 28) and that endurance tests on methane combustion performed at 1200°C demonstrated that a Pd cat¬ alyst supported on lanthanum B-alumina has good durability and resistance to thermal sintering (pages 31 and 32). SUMMARY OF THE INVENTION Generally, one aspect of the present invention is di¬ rected to a method for operating a catalytic combustor using a palladium-containing catalyst and using a novel set of unexpectedly effective operating parameters which permits high catalytic activity, and results in on-going retention and regeneration of such activity. Another general aspect of the present invention pro- vides a process for catalytic combustion which involves the discovery that the temperatures of palladium oxide de¬ composition and recombination may be varied depending on the metal oxide support used for the palladium oxide, and the present invention is directed to utilizing this varia- tion to optimize catalytic combustion processes. More specifically, in accordance with the present in¬ vention there is provided a process for starting a combus¬ tion system to catalytically combust a gaseous carbona- ceous fuel (for example, a gas comprising r..ethane, e.g., natural gas or some other methane-rich cas) with air in a combustor in the presence of a palladium oxide-containing catalyst. The process comprises the following steps. A decomposition onset temperature at which the palladium oxide-containing catalyst decomposes at an oxygen partial pressure equal to that found in the combustor is predeter¬ mined. A reformation onset temperature at which the pal¬ ladium oxide-containing catalyst will, at the same oxygen partial pressure found in the combustor, reform into pal¬ ladium oxide after being subjected to the decomposition temperature is also predetermined. A flow of hot gases from a preburner is utilized to heat the catalyst to a temperature high enough to initiate combustion of the fuel with air upon contact thereof with the catalyst. Thereaf¬ ter, the flow of hot gases from the preburner is reduced while supplying air and the fuel for combustion to the combustor downstream of the preheater. Upon overheating of the catalyst (whether by the preburner hot gases or otherwise, e.g., during combustion operation) to a first temperature in excess of the decomposition onset tempera¬ ture of the catalyst, whereby the catalyst sustains a dim¬ inution of catalytic activity, catalytic activity is thereafter restored by lowering the temperature of the catalyst to a temperature not greater than the reformation onset temperature and maintaining the temperature at or below the reformation onset temperature until a desired degree of catalytic activity of the catalyst is achieved, and then maintaining the catalyst below the aforesaid de- composition onset temperature. In one aspect of the present invention, the palladium oxide is supported on a metal oxide selected from the group consisting of ceria, titania, tantalum oxide, lan¬ thanide metal oxide-modified alumina and mixtures of two or more thereof. The lanthanide metal oxide-modified alu¬ mina may be, for example, a lanthanum oxide-modified alu¬ mina, a cerium oxide-modified alumina or a praseodymium oxide-modified alumina, or mixtures of two or more there- of. Another aspect of the present invention provides a process for starting a combustion system to catalytically combust a carbonaceous fuel with air in a combustor in the presence of a palladium oxide supported on a metal oxide support. The process comprises utilizing a flow of hot gases from a preburner to heat the catalyst to a tempera¬ ture high enough to initiate combustion of the fuel with air upon contact thereof with the catalyst, and thereafter reducing the flow of hot gases from the preburner while supplying air and fuel for combustion to the combustor downstream of the preheater. Upon heating of the catalyst to a first temperature in excess of at least about 775°C (whether by the preheater or otherwise, e.g., during com- bustion operation) , at which first temperature catalyst deactivation occurs, catalytic activity is thereafter re¬ stored by lowering the temperature of the catalyst to a catalyst reactivation temperature which is lower than about 735°C, and maintaining the temperature at or below the catalyst reactivation temperature until desired cata¬ lytic activity is achieved. The temperature of the cata¬ lyst is then maintained below about 735°C. Yet another aspect of the present invention provides for a process for catalytic combustion of a mixture of a gaseous carbonaceous fuel and air by contacting the mix¬ ture with a metal oxide-supported palladium oxide cata¬ lyst, wherein the catalyst for the catalytic combustion has been subjected to a temperature in excess of the tem¬ perature at which deactivation of the catalyst occurs, which temperature is at least about 775°C at atmospheric pressure. The present invention provides an improvement comprising restoring catalytic activity of the catalyst by lowering the temperature of the catalyst into a regenerat¬ ing temperature range at least about 44°C below the deac- tivation temperature, and maintaining the temperature within that range for a time sufficient to restore cata¬ lytic activity to said catalyst. As described below, dif¬ ferent catalyst deactivation temperatures, different cata- lyst reactivation onset temperatures, and different tem¬ perature ranges below the deactivation temperature may be employed depending on the particular metal oxide support employed in the catalyst. Another aspect of the present invention provides for employing the combustion effluent discharged from the com¬ bustor to run a gas turbine. The present invention also provides a process for the catalytically supported combustion of a gaseous carbona- ceous fuel which comprises the following steps. A mixture of the fuel and oxygen is formed to provide a combustion mixture, and the combustion mixture is contacted under conditions suitable for catalyzed combustion thereof with a catalyst composition comprising a catalytic material consisting essentially of a catalytically effective amount of palladium oxide dispersed on a metal oxide support se¬ lected from the group consisting of ceria, titania, tanta¬ lum oxide and lanthanide oxide-modified alumina. Other aspects of the invention, including selecting specific metal oxide supports for the palladium oxide cat¬ alyst to establish specified decomposition and reformation temperatures, are set forth below in the Detailed Descrip¬ tion of the Invention and Preferred Embodiments Thereof. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a partial schematic breakaway view of a preburner/catalytic combustor system which is operable in accordance with one embodiment of the present invention; and Figure 2 is a thermogravimetric analysis (TGA) plot of temperature plotted on the abscissa versus percentage change in sample weight in air plotted on the right-hand ordinate. Superimposed on the TGA plot is a plot of per¬ cent conversion of 1% methane in air (an index of activi- ty) on the left-hand ordinate versus the temperature on the abscissa. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF At atmospheric pressure palladium-containing cata¬ lysts are known to lose activity when subjected to temper- atures in excess of about 800°C, at which temperatures palladium oxide decomposes into palladium metal. The in¬ teraction of palladium oxide with reducing agents exacer¬ bates such decomposition into palladium metal. One aspect of the present invention is concerned with compensating for an over-temperature event (or a continuing series of such over-temperature events) which causes catalyst deac¬ tivation. In the event of such over-temperature, the present invention utilizes procedures for regeneration of the catalyst, in situ. For example, using a typical pal- ladium on alumina catalyst, when start-up or operation of the catalytic combustor results in exposing the ignition catalyst to a temperature in excess of about 800°C at at¬ mospheric pressure, resulting in loss of catalyst activi¬ ty, the over-temperature is, according to the present in- vention, followed by an atmospheric pressure regenerating temperature soak between about preferably 530°C to 650°C and more preferably 560°C to 650°C, which oxidizes the palladium on alumina to active palladium oxide. Even if the entire catalytic combustor does not reach a catalyst inactivating over-temperature, isolated hot spots within the catalytic combustor may be subjected to an over-tem¬ perature, and the heat soak of the present invention will provide a catalyst regenerating benefit. Thus, a regener¬ ating temperature soak according to the present invention unexpectedly regenerates the activity lost due to an over-temperature in all or part of the combustor. As those skilled in the art will appreciate, the above-stated temperature ranges are dependent on the par¬ tial pressure of oxygen. At higher pressures, as for ex- ample might be encountered in conjunction with generation of combustion effluent useful for operation of gas tur¬ bines, the decomposition temperature at which palladium oxide will decompose into metallic palladium will in- crease, as will the regeneration temperature at which pal¬ ladium oxide will reform. References hereinafter to these temperatures are all at atmospheric pressures, it being understood that at enhanced partial pressure of oxygen the decomposition and regenerating temperatures will shift up¬ wardly, and that the determination of such increased tem¬ peratures at higher oxygen partial pressures will be a matter well known to those skilled in the art. In a method of the present invention for operating a palladium oxide-containing catalytic combustor useful, e.g., for powering a gas turbine, control of the tempera¬ ture is maintained within the catalytic combustor in such a manner as to insure the presence of palladium oxide, which is catalytically active for the catalytic combustion reaction. By maintaining the temperature below about 800°C, decomposition into metallic palladium of palladium oxide supported on an unmodified alumina support is avoided and high catalytic activity is maintained. How¬ ever, in the event of an over-temperature, or reduction of palladium oxide as a result of chemical interaction with a reducing agent, such as an excess of fuel, regeneration following inactivation due to loss of PdO can be accom¬ plished by bringing a deactivated catalyst comprising pal¬ ladium on an alumina support to a temperature within the regenerating temperature range of about preferably 530°C to 650°C, and more preferably 560°C to 650°C, where reoxi- dation occurs at a reasonable rate. Further, according to the present invention, the tem¬ peratures of palladium oxide decomposition, and the tem- peratures of palladium oxide reformation are varied by changing or modifying the metal oxide support used for the palladium oxide. The temperature ranges stated above are those which are effective for palladium on an unmodified alumina support. However, the temperature for reformation of palladium oxide is, to an extent, dependent on the met¬ al oxide used to support the palladium, and other suitable metal oxide support materials, such as ceria, titania and tantalum oxide, and modified alumina supports, such as alumina modified with cerium oxide, lanthanum oxide and praseodymium oxide, have characteristic temperatures at which palladium oxide thereon will decompose and recom- bine. These characteristic temperatures, which can be de- termined by those skilled in the art by means such as, for example, thermogravimetric analysis, permit the selection of appropriate metal oxide support materials, and thus provide control over palladium oxide decomposition/refor¬ mation temperature ranges. Figure 1 schemati ally depicts apparatus for carrying out catalytic combustion using a combustor having a pre- combustion chamber 20 fed via line 15 with air supplied from a compressor 25, and supplied with fuel from a nozzle 13 connected to fuel line 14. The fuel and air together pass through a mixer 17 prior to entering the precombus- tion chamber 20. Feeding into the precombustion chamber via injector line 18 is a preburner 12, also connected to the air line 15 and fuel line 14. Preburner 12 sprays hot combustion gases into chamber 20 from injector line 18. The catalyst is positioned on a supporting monolith 10 from which the hot combustion gases move downstream to drive turbine 30. Example 1 The procedure used to obtain the data graphed in Fig¬ ure 2 was as follows. First, a sample of a conventional palladium on aluminum oxide catalyst was prepared accord¬ ing to a standard procedure, viz., gamma alumina was cal¬ cined at 950°C for 2 hours and then screened to particle sizes between 53 and 150 microns. This gamma alumina was used as a catalyst carrier. The use of gamma alumina as a catalyst carrier in this example was, as those skilled in the art will readily appreciate, simply a matter of choice. Other suitable carriers include, for example, modified alumina (i.e., aluminas which contain surface area stabilizers such as silica, barium oxide, lanthanum oxide and cerium oxide) silica, zeolites, titania, zirco- nia and ceria as well as mixtures of the foregoing. As described below, certain of these modified aluminas as well as other supports such as ceria, titania and tantalum oxide enable adjustment of the palladium oxide decomposi¬ tion/reformation temperature ranges to desired levels. In any case, ten grams of the described (unmodified) alumina carrier was impregnated with a Pd(N0 3 ) 2 '2H 2 0 solution by the incipient wetness method to give approximately 4 wt% Pd on the finished catalyst. The Pd was then fixed on the catalyst by a conventional reduction with an aqueous hy- drazine solution. The reduced catalyst was dried at 120°C overnight and calcined at 500°C for 2 hours to give what will hereafter be designated as "fresh catalyst". The TGA profile of Figure 2 was generated by heating this fresh PdO on Al 2 0 3 catalyst in air at 20°C/min. The heating portion of the graph depicts a weight loss above about 800°C where decomposition of PdO to Pd metal occurs. Following decomposition, heating continued to 1100°C where it was held for 30 minutes. The temperature program was then reversed allowing the catalyst to cool in air. Unexpectedly, no weight in¬ crease due to re-oxidation of the Pd metal was observed until about 650°C, below which a sharp increase was ob¬ served which plateaus at about 560°C to 530°C. Upon con¬ tinued cooling below 530°C there was a small but steady weight increase down to room temperature. Repeated heat¬ ing and cooling cycles of the same sample demonstrates the same temperature-dependent weight changes. Referring to other data graphed on Figure 2, the per¬ cent conversion plot as read on the left ordinate of Fig- ure 2 is a measure of catalytic activity. The procedure used to obtain the graphed data on cat¬ alytic activity was as follows: a 0.06 gram ("g") sample of catalyst, prepared as described above, was mixed with 2.94g of a diluent (alpha-alumina) which had been screened to a particle size range of from 53 to 150 microns. The resultant 3g catalyst charge was supported on a porous quartz frit in a 1" diameter quartz reactor tube. The tube was then positioned vertically in a programmable tube furnace. A thermocouple was positioned axially in the catalyst bed for continuous monitoring and connections to a gas (fuel) stream secured. A fuel mixture of 1% methane in zerograde air (air containing less than 5 parts per million by weight H 2 0 and less than 1 part per million by weight hydrocarbon calculated as CH 4 ) metered by a mass flow controller was flowed through the system at a rate of 3 liters per minute. The use of methane as a fuel was, as those skilled in the art will readily appreciate, simply a matter of choice. Other suitable fuels would include, for example, natural gas, ethane, propane, butane, other hy¬ drocarbons, alcohols, other carbonaceous materials, and mixtures thereof. The term "carbonaceous materials" or "carbonaceous fuels" includes each of the foregoing. The gas exiting the reactor was analyzed by a Beckman Indus¬ trial Model 400A Hydrocarbon Analyzer. The analyzer was zeroed on air and spanned to 100% on the fuel mixture at ambient conditions. The procedure was initiated by ramp¬ ing the furnace to a selected maximum temperature. This temperature was held for a limited time and then the fur¬ nace was shut off and the reactor permitted to cool. A multi-channel strip chart simultaneously recorded the cat¬ alyst bed temperature and the concentration of hydrocarbon in the exit gas stream. This data thus provided a profile of the temperature dependence of methane oxidation/combus¬ tion. The activity of the catalyst, as determined by the percent conversion of the methane fuel, was measured at various increasingly higher temperatures and the results were plotted as the dashed line in Figure 2. Figure 2 shows that at progressively higher temperatures the per¬ cent conversion of the methane becomes greater, until at approximately 800°C the conversion becomes essentially 100%. At this temperature, the reaction in effect became a thermal reaction as opposed to a catalytic reaction. The activity data in Figure 2 also demonstrates that the continuous, rapid increase in percent conversion with an increase in temperature is followed by a rapid decrease in percent conversion with a reduction in temperature. The decrease in percent conversion (or activity) undergoes a reversal below about 700°C during a cooling cycle, at which point percent conversion (activity) begins to in¬ crease until a temperature of about 600°C is obtained. At that point, the catalyst again demonstrated the same ac¬ tivity as the catalyst had initially demonstrated (during the heating cycle) at that temperature. This observation was made for all repeated cycles. Example 2 Further samples of PdO on A1 2 0 3 were pre-calcined in air for 17 hours to 1100°C followed by cooling in air to room temperature. TGA profiles of these samples were qualitatively identical to second cycles of fresh samples. Thus, in both cases the PdO decomposes to Pd metal during heat-up, and PdO forms below about 650°C during cool down. Example 3 PdO powder was prepared using the identical procedure as for PdO on Al 2 0 3 . Heating of this sample clearly showed only one weight loss process between 810°C and 840°C in which the PdO decomposes to Pd metal. The weight loss observed, approximately 13%, is consistent with de- composition of PdO to Pd. Example 4 Samples of PdO/Al 2 0 3 were calcined to 1100°C in air and evaluated for activity as a function of temperature as described above. During heat-up, conversion was first noted at about 340°C and slowly rose to 30% at about 430°C after which percent conversion rapidly increased with tem¬ perature up to 90% at about 650°C. Above this temperature the thermal process became significant. The furnace ramp continued to increase catalyst temperature up to 1000°C, well beyond the temperature of decomposition of PdO to Pd metal. The temperature was then reduced and the sample cooled in CH 4 /air. At about 720°C the thermal process began to extinguish and the conversion fell far below the conversion observed during heat-up, demonstrating that the catalyst had lost activity. The catalyst activity at this point became virtually zero. As the Pd/Al 2 0 3 continued to cool and the conversion due to the thermal component decreased to about 50%, there was a sudden unexpected increase in activity at about 680°C and a maximum activity of about 70% at 650°C. The conversion curve upon continued cooling effectively over- laps that generated during heat-up. The TGA profile on a sample of the same catalyst, calcined to 1100°C in air for 17 hours clearly showed de¬ composition of PdO to Pd metal during heating. Upon cool¬ ing the large hysteresis in re-oxidation is observed to occur around 650°C and is complete at 575°C, closely tracking the activity, performance. Example 5 A sample of fresh PdO on Al 2 0 3 was heated in air to 950°C, well beyond the range where any weight loss oc¬ curred. The sample was then cooled to 680°C and held at that temperature for 30 minutes. No weight gain occurred. The sample was then cooled to 650°C at which temperature weight gain commenced. This example thus demonstrates that the hysteresis depicted in Figure 2 is a temperature dependent process, not a rate process. Example 6 A sample of fresh PdO on Al 2 0 3 catalyst was heated in air to 950°C, and then cooled to 680°C and held at that temperature for 30 minutes as in Example 5. The activity of the catalyst as indicated by its ability to catalyze the combustion of 1% methane in air was then measured. The catalyst was then cooled to 650°C and its activity again measured. The activity at 650°C was much greater than at 680°C, again demonstrating that the hysteresis de¬ picted in Figure 2 is a temperature dependent process, not the result of a rate process. Example 7 The dependence of palladium oxide decomposition tem¬ perature and reformation temperature on the metal oxide support was established by preparing samples of palladium on alumina, on tantalum oxide, on titania, on ceria and on zirconia and measuring in air decomposition and reforma¬ tion temperatures using thermogravimetric analysis. The method of preparation for the five samples shown below in TABLE I was as follows: A. 4wt% Pd/Alumina Alumina sold under the trademark CATAPAL SB by Vista Chemical Company was calcined at 950°C for 2 hours and then sieved to 53 to 150 micron particle size; 9.61g of the alumina was impregnated with an aqueous solution of palladium nitrate using the incipient wetness technique. The palladium was then reduced using aqueous hydrazine. This material was dried at 110°C overnight and then cal¬ cined at 500°C for 2 hours in air to produce the finished catalyst. B. 4wt% Pd/Ceria 5g of SKK cerium oxide (Ce0 2 ) was impregnated with palladium nitrate as was done for the previous sample, ad- justing the total volume of the impregnating solution to the incipient wetness of the support. The sample was then reduced, dried, and calcined at 500°C for 2 hours in air as was done for the Pd on alumina sample. C. 4wt% Pd/Zirconia A 5g sample of commercially available zirconia (Mag¬ nesium Elecktron SC101 Grade) was impregnated with palla¬ dium and handled just as was the Pd/ceria sample. D. 4wt% Pd/Titania A sample of commercially available titania was cal¬ cined at 950°C for 2 hours and 8.2g was then impregnated with palladium and handled just as was the Pd/ceria sam- ple. E. 4wt% Pd/Tantalum Oxide A 5g sample of commercially available tantalum oxide (Ta 2 0 5 ) (Morton Thiokol) was impregnated with palladium just as was the Pd/ceria sample. The low incipient wet¬ ness of this material required a two-step impregnation with a drying step in between. The rest of the prepara¬ tion was the same as for the Pd/ceria. The TGA profile of the catalysts was generated as de¬ scribed above with respect to the TGA profile of Figure 2, that is, by heating the fresh catalyst samples in air at a rate of 20°C per minute. The results attached are set forth in TABLE I. TABLE I Decomposition and Reformation Temperatures for Palladium on Various Metal Oxide Supports De rees Centi rade TABLE I lists the temperature (T D ) for onset of PdO decomposition to Pd, the temperature (T R ) for onset of re¬ formation of PdO and the hysteresis equal to the differ¬ ences (T D -T R ) , all at atmospheric pressure in air for pal¬ ladium oxide supported on five different metal oxides. TABLE I shows that palladium oxide on alumina, tantalum oxide, titania, and ceria supports exhibits little varia¬ tion in decomposition temperature. However, the choice of metal oxide does result in a pronounced effect on the re- formation temperature. The differences between decomposi¬ tion onset and reformation onset temperatures (T D -T R ) vary from 210°C for Al 2 0 3 to 44°C for the Ce0 2 supported palla¬ dium. Typically, the smaller this difference (and the higher the reformation onset temperature), the easier it is to regenerate activity in a gas turbine. Accordingly, for catalyst compositions containing one of the catalysts of TABLE I which are over-temperatured so as to sustain deactivation, the catalytic activity may be restored by lowering the temperature of the catalyst into a reforma- tion onset temperature range which is lower than T R for the metal oxide support employed, and thereafter maintain¬ ing the temperature of the catalyst below about T D for the metal oxide support employed. The last metal oxide support listed in TABLE I is Zr0 2 . As seen from TABLE I, zirconia promotes premature decomposition of PdO to Pd at 682°C and inhibits reforma¬ tion to a low temperature of 470°C. This catalyst, there¬ fore, has a large range and a relatively low temperature at which Pd metal is stable in an oxidizing environment. This is not a desirable property for methane oxidation. These Examples 7A-7E demonstrate that activity of a palladium oxide-containing catalyst, as measured by its ability to promote the oxidation of methane, can be pre¬ served by utilizing the catalyst at temperatures below the palladium oxide decomposition temperature which is the temperature at which catalyst deactivation will occur; and that, if activity is lost through over-temperature, activ¬ ity can be restored by subjecting the deactivated catalyst to a heat soak at an effective temperature which depends on the metal oxide support being used with the palladium, and which effective temperature is below that at which on¬ set of reformation of PdO occurs. This applies as well to modified alumina-supported catalysts, which are prepared by impregnating alumina with a suitable, e.g., nitrate, form of the rare earth metal. The alumina supports em¬ ployed to prepare the supported catalysts comprised pri¬ marily gamma-alumina but calcination during catalyst prep- aration caused the formation of other phases, such as the beta, kappa, delta and theta forms of alumina, which, to¬ gether with the gamma form, were present in the finished supports. A fixed weight of the alumina is impregnated with, e.g., a lanthanum nitrate, cerium nitrate or praseo- dymium nitrate, or mixtures thereof, by mixing the solu¬ tion of the nitrate with the alumina and then adding pal¬ ladium to the composite after calcination. After addition of the rare earth metal nitrate solu-- tion to the alumina, the sample is calcined in air, for example, at temperatures in excess of about 950°C for a time period of at least 2 hours. Palladium is then added by the incipient wetness technique using a palladium ni¬ trate solution. The sample is then reduced with aqueous hydrazine, dried and then calcined in air at temperatures in excess of about 500°C for a time period of at least 2 hours. If a high palladium concentration is desired in the catalyst composition, the impregnation step with pal¬ ladium nitrate is repeated. The catalyst composition of this invention may also be prepared by impregnating with a suitable solution of a palladium salt a rare earth oxide-modified alumina. Such modified alumina is one which has been previously impreg¬ nated with a solution of a rare earth metal compound and then calcined according to methods known in the art, usu- ally at temperatures in excess of 500°C, to provide a rare earth oxide-modified alumina. The atomic ratio of palla¬ dium to the rare earth metal used to modify the alumina is generally from about 1:2 to about 4:1; preferably it is from about 1:2 to about 1:1 for lanthanum-modified alumi- na; from about 1:1 to about 4:1 for cerium-modified alumi¬ na; and from about 1:2 to about 2:1 for praseodymium-modi¬ fied alumina. Generally, when modified alumina is em¬ ployed as the metal oxide support for the palladium oxide the decomposition temperature of palladium oxide which, at atmospheric pressure, is about 800°C for palladium oxide on unmodified alumina as discussed above, is shifted to a temperature range of about 920°C to 950°C. Palladium ox- ide supported on modified alumina in accordance with this aspect of the invention shows good activity for catalyzing the combustion of carbonaceous gaseous fuels and stability of the catalyst at operating temperatures which may safely be set at, for example, 900°C. The following examples illustrate the use of modified alumina supports for the palladium oxide catalyst. Example 8 A. 1.74 grams of Ce(N0 3 ) 3 '6H 2 0 was dissolved in 3 milliliters of deionized water and the resulting solution was added to 10.01 grams of gamma alumina powder sold un¬ der the trademark CATAPAL by Vista Chemical Company. The wetted alumina powder was dried overnight at 110°C and then calcined in air at 950°C for two hours to provide a ceria-modified alumina. A quantity of 3.43 grams of pal¬ ladium nitrate solution (10 weight percent Pd) was diluted with 1.7 grams of deionized water and then added to the ceria-modified alumina. Aqueous hydrazine was then added to reduce the palladium on the support. The mixture was then dried at 110°C for 17 hours and then calcined in air at 500°C for 2 hours to provide the sample of TABLE II containing 0.004 moles of each of Pd and Ce, i.e., Pd and Ce in a 1:1 molar ratio. B. The procedure of Part A was repeated with differ- ent appropriate amounts of cerium nitrate and palladium nitrate impregnation to provide the other ceria-modified alumina supported catalysts of TABLE II containing the in¬ dicated molar amounts of Ce and Pd. Example 9 The procedure of Example 8 was exactly repeated ex¬ cept that La(N0 3 ) 3 '6H 2 0 in appropriate amounts was used in place of the Ce(N0 3 ) 3 * 6H 2 0 to provide the lanthana-modi- fied alumina samples of TABLE II containing the indicated molar amounts of La and Pd. Example 10 The procedure of Example 8 was exactly repeated ex¬ cept that Pr(N0 3 ) 3 " 6H 2 0 in appropriate amounts was used in place of the Ce(N0 3 ) 3 * 6H 2 0 to provide the praseodymium- modified alumina samples of TABLE II containing the indi¬ cated molar amounts of Pr and Pd. Example 11 The activities of the catalysts prepared according to Examples 8-10 were measured in a quartz tube reactor. In each case a quantity of 0.06 grams of the catalyst was di- luted in 2.94 grams of alpha-alumina and supported on a quartz frit. The reactant gas stream contained 1% methane in air. The reactor was heated in an electric tube fur¬ nace so that the catalyst bed ranged in temperature from room to about 1000°C. The gas stream was monitored con- tinuously for hydrocarbon content. The activity is de¬ fined as the catalyst bed temperature at which 30% of methane is combusted. The results are shown in TABLE II, which also shows thermal measurements made on an Omnitherm Atvantage II TGA951 instrument. The samples were heated at 20°C/minute in air. The decomposition temperatures (T D ) in the TABLE are those temperatures at which 80% of the weight loss sustained at temperatures greater than 700°C has been completed. REO(1) Pd(2) (Moles) (Moles) 0 008 002 004 008 Ce 002 004 ,004 008 ,002 008 ,004 ,008 Pr .002 004 .004 .008 .002 ,008 .004 .008 (1, "REO" is the rare earth metal content of the samples in moles of the metal per ten grams of fresh alumina. (2>l, Pd" is the palladium metal content of the sample in moles of Pd per ten grams of fresh alumina. (3) T ft = Activity Temperature, the temperature (in degrees Centigrade) at which combustion of 30% (vol.) of the CH 4 present in a 1% (vol.) CH 4 in air mixture takes place at a 1.5 liters per minute flow rate through a sample of the catalyst. (4) T D80 = Decomposition Onset Temperature, the temperature (in degrees Centigrade) at which 80% of the weight loss attributed to PdO decomposition to Pd is attained. (5) T R = Regeneration Onset Temperature, the temperature (in degrees Centigrade) at which regeneration of the cata- lyst by oxidation of Pd to PdO commences. (6) T D -T R represents the hysteresis discussed above. The data of TABLE II show that although the inclusion of the lanthanide (rare earth) metal oxides in the alumina generally decreased the activity of the catalyst as indi¬ cated by the activity temperature with increasing addition of rare earth oxide, T D80 , the temperature at which 80% of the weight loss attributed to decomposition of the palla¬ dium oxide catalyst is attained, was increased by the presence of the rare earth oxide modifier. The catalyst attained by utilizing a lanthanide metal-modified alumina as the metal oxide support is more resistant to high tem¬ peratures and therefore would find use in the higher tem¬ perature zones of a catalytic combustion catalyst where its somewhat reduced activity would be more than offset by the increased temperature. It will be noted that different definitions of Decom¬ position Onset Temperature, T D , as defined in the footnote to TABLE I, and T D80 as defined in footnote (4) of TABLE ii are employed for, respectively, the unmodified (single compound) and modified (more than a single compound) metal oxide supports. This is because whereas the unmodified metal oxide supports such as those listed in TABLE I above exhibit sharp and definite Decomposition Onset Tempera- ture, the modified metal oxide supports of the type illus¬ trated in TABLE II exhibit decomposition over a broad tem¬ perature range, for example, palladium oxide on cerium- modified alumina supports exhibit decomposition tempera¬ ture ranges of from about 80 to 131 degrees Centrigrade, depending on the palladium oxide loading and the atomic ratio of Ce to Pd. Accordingly, for modified metal oxide supports, the point at which 80% by weight of the total decomposition weight loss occurs was arbitrarily selected as the Decomposition Onset Temperature. In the process of this invention, a carbonaceous fuel containing methane may be combusted with air in the pres¬ ence of a catalyst composition containing palladium depos- ited as palladium oxide on a metal oxide support without any significant formation of NOx. Such catalytic combus¬ tion of the gaseous carbonaceous fuel is carried out by methods known in the prior art as illustrated in, for ex¬ ample, U.S. Patent 3,928,961. In such a method, an inti- mate mixture of the fuel and air is formed, and at least a portion of this combustion mixture is contacted in a com¬ bustion zone with the catalyst composition of this inven¬ tion, thereby effecting substantial combustion of at least a portion of the fuel. Conditions may be controlled to carry out the catalytic combustion under essentially adia- batic conditions at a rate surmounting the mass transfer limitation to form an effluent of high thermal energy. The combustion zone is at a temperature of from about 1700°F to about 3000°F and the combustion is generally carried out at a pressure of from 1 to 20 atmospheres. The combustion catalyst of this invention may be used in a segmented catalyst bed such as described in, for ex¬ ample, U.S. Patent 4,089,654. Dividing the catalyst con¬ figuration into segments is beneficial not only from an operational standpoint, but also in terms of monitoring the performance of various sections of the bed. The cata¬ lyst system comprises a catalyst configuration consisting of a downstream catalyst portion and an upstream catalyst portion protected therefrom. Generally, the catalyst compositions used in the pro¬ cess of the invention may comprise a monolithic or unitary refractory steel alloy or ceramic substrate, such as a honeycomb-type substrate having a plurality of parallel, fine gas flow channels extending therethrough, the walls of which are coated with a palladium-containing catalyst composition, specifically, palladium oxide dispersed on a refractory metal oxide support as described above. Gener¬ ally, the amount of palladium oxide in the catalyst will depend on the anticipated conditions of use. Typically, the palladium oxide content of the catalyst will be at least about 4 percent by weight of the total weight of palladium oxide and refractory metal oxide support (wash- coat), calculated as palladium metal. The flow channels in the honeycomb substrate are usually parallel and may be of any desired cross section such as rectangular, triangu¬ lar or hexagonal shape cross section. The number of chan¬ nels per square inch may vary depending upon the particu- lar applications, and monolithic honeycombs are commer¬ cially available having anywhere from about 9 to 600 chan¬ nels per square inch. The substrate or carrier portion of the honeycomb desirably is a porous, ceramic-like materi¬ al, e.g., cordierite, silica-alumina-magnesia, mullite, etc. but may be nonporous, and may be catalytically rela¬ tively inert. While the invention has been described in detail with respect to specific preferred embodiments thereof, • it will be appreciated by those skilled in the art that numerous variations thereto may be made which nonetheless lie with¬ in the spirit and scope of the invention and the appended claims.

Claims: THE CLAIMS What is claimed is: 1. A process for starting a combustion system to catalytically combust a gaseous carbonaceous fuel with air in a combustor in the presence of a palladium oxide-con¬ taining catalyst, which comprises: (a) predetermining a decomposition onset tempera¬ ture at which the palladium oxide-containing catalyst de¬ composes at an oxygen partial pressure equal to that found in the combustor; (b) predetermining a reformation onset tempera¬ ture at which the palladium oxide-containing catalyst will, at said same oxygen partial pressure found in the combustor, reform into palladium oxide after being sub¬ jected to the decomposition temperature; (c) utilizing a flow of hot gases from a preburn¬ er to heat said catalyst to a temperature high enough to initiate combustion of said fuel with air upon contact with said catalyst; (d) thereafter reducing the flow of hot gases from the preburner while supplying air and said fuel for combustion to the combustor downstream of said preheater; and (e) upon overheating of the catalyst to a first temperature in excess of the decomposition onset tempera¬ ture of the catalyst, whereby the catalyst sustains a dim¬ inution of catalytic activity, thereafter restoring cata¬ lytic activity by lowering the temperature of the catalyst to a temperature not greater than the reformation onset temperature and maintaining the temperature at or below the reformation onset temperature until a desired degree of catalytic activity of the catalyst is achieved, and then maintaining the catalyst below the aforesaid decompo¬ sition onset temperature. 2. The process of claim 1 wherein the carbonaceous fuel comprises methane. 3. The process of claim 1 wherein combustion efflu¬ ent discharged from the combustor is employed to run a gas turbine. 4. The process of claim 1 wherein the palladium ox¬ ide is supported on a metal oxide selected from the group consisting of ceria, titania, tantalum oxide and lantha¬ nide metal oxide-modified alumina. 5. The process of claim 4 wherein the metal oxide comprises ceria and the reformation onset temperature at atmospheric pressure is about 730°C. 6. The process of claim 4 wherein the metal oxide comprises titania and the reformation onset temperature at atmospheric pressure is about 734°C. 7. The process of claim 4 wherein the metal oxide comprises tantalum oxide and the reformation onset temper¬ ature at atmospheric pressure is about 650°C. 8. The process of claim 4 wherein the metal oxide comprises a lanthanum oxide-modified alumina and the re¬ formation onset temperature at atmospheric pressure is about 735°C. 9. The process of claim 4 wherein the metal oxide comprises a cerium oxide-modified alumina and the reforma¬ tion onset temperature at atmospheric pressure is about 743°C. 10. The process of claim 4 wherein the metal oxide comprises a praseodymium oxide-modified alumina and the reformation onset temperature at atmospheric pressure is about 719°C. 11. A process for starting a combustion system to catalytically combust a carbonaceous fuel with air in a combustor in the presence of palladium oxide supported on a metal oxide support, which comprises utilizing a flow of hot gases from a preburner to heat said catalyst to a tem¬ perature high enough to initiate combustion of said fuel with air upon contact with said catalyst, thereafter re¬ ducing the flow of hot gases from the preburner while sup¬ plying air and fuel for combustion to the combustor down¬ stream of said preheater, and, upon overheating of the catalyst to a first temperature in excess of at least about 775°C, at which first temperature catalyst deactiva¬ tion occurs, thereafter restoring catalytic activity by lowering the temperature of the catalyst to a catalyst re¬ activation temperature which is lower than about 735°C and maintaining the temperature at or below the catalyst reac¬ tivation temperature until desired catalytic activity is achieved, and thereafter maintaining the temperature of the catalyst below about 775°C. 12. The process of claim 11 including carrying out the process at atmospheric pressure. 13. The process of claim 11 or claim 12 wherein the carbonaceous fuel comprises methane. 14. The.process of claim 12 wherein catalytic activi¬ ty is restored by lowering the temperature of the catalyst into a reactivation temperature range which at atmospheric pressure is from about 600°C to about 650°C. 15. The process of claim 12 wherein catalytic activi¬ ty is restored by lowering the temperature of the catalyst into a reactivation temperature range which at atmospheric, pressure is from about 650°C to about 700°C. 16. The process of claim 14 or claim 15 wherein the metal oxide support is selected from the group consisting of ceria, titania and tantalum oxide. 17. The process of claim 11 wherein catalytic activi¬ ty is restored by lowering the temperature of the catalyst into a reactivation temperature range which at atmospheric pressure is from about 675°C to about 734°C. 18. The process of claim 11 wherein catalytic activi¬ ty is restored by lowering the temperature of the catalyst into a reactivation temperature range which at atmospheric pressure is lower than about 744°C, and after the desired catalytic activity is achieved maintaining the temperature of the catalyst below about 775°C. 19. The process of claim 11 or claim 18 wherein the metal oxide support is selected from the group consisting of lanthanum oxide-modified alumina, cerium oxide-modified alumina and praseodymium oxide-modified alumina. 20. The process of claim 11 wherein combustion efflu¬ ent discharged from the combustor is employed to run a gas turbine. 21. In a process for catalytic combustion of a mix¬ ture of a gaseous carbonaceous fuel and air by contacting the mixture with a catalyst comprising palladium oxide supported on a metal oxide support, wherein the catalyst for said catalytic combustion has been subjected to a tem¬ perature in excess of the temperature at which deactiva¬ tion of the catalyst occurs, which temperature is at least about 775°C at atmospheric pressure, the improvement com¬ prising restoring catalytic activity by lowering the tem¬ perature of the catalyst into a regenerating temperature range at least about 44°C below the deactivation tempera¬ ture and maintaining the temperature within that range for a time sufficient to restore catalytic activity to said catalyst. 22. The process of claim 21 wherein the metal oxide support is selected from the group consisting of ceria, unmodified alumina, tantalum oxide and titanium oxide and in which the temperature at which deactivation of the cat¬ alyst occurs is at least about 775°C when the metal oxide support comprises ceria, at least about 810°C when the metal oxide support comprises alumina, at least about 810°C when the metal oxide support comprises tantalum ox¬ ide, and at least about 814°C when the metal oxide support is titanium oxide, and in which catalytic activity is re¬ stored by lowering the temperature of the catalyst into a regenerating temperature range which is below the tempera¬ ture at which deactivation of the catalyst occurs by at least about 44°C when the metal oxide support comprises ceria, by at least about 210°C when the metal oxide sup¬ port comprises unmodified alumina, by at least about 160°C when the metal oxide support comprises tantalum oxide and by at least about 80°C when the metal oxide support com¬ prises titanium oxide. 23. The process of claim 21 or claim 22 wherein the carbonaceous fuel comprises methane. 24. The process of claim 21 or claim 22 wherein the combustion effluent discharged from the combustor is em¬ ployed to run a gas turbine. 25. The process of claim 21 or claim 22 wherein the temperature in excess of the decomposition temperature is reached during start-up. 26. The process of claim 21 wherein the metal oxide support is selected from the group consisting of ceria, titania, tantalum oxide and lanthanide metal oxide-modi¬ fied alumina. 27. The process of claim 26 wherein the lanthanide metal oxide is selected from the group consisting of ceri- um oxide, lanthanum oxide, praseodymium oxide and mixtures thereof. 28. The process of claim 21 wherein the metal oxide comprises ceria and restored catalytic activity is achieved by lowering the temperature of the catalyst into a reactivation temperature range which at atmospheric pressure is from about 700°C to 730°C. 29. The process of claim 21 wherein the metal oxide comprises titania and restored catalytic activity is achieved by lowering the temperature of the catalyst into a reactivation temperature range which at atmospheric pressure is from about 660°C to 734°C. 30. The process of claim 21 wherein the metal oxide comprises tantalum oxide and restored catalytic activity is achieved by lowering the temperature of the catalyst into a reactivation temperature range which at atmospheric pressure is from about 570°C to 650°C. 31. The process of claim 21 wherein the metal oxide comprises a cerium oxide-modified alumina and restored catalytic activity is achieved by lowering the temperature of the catalyst into a reactivation temperature range which at atmospheric pressure is from about 516°C to 743°C. 32. The process of claim 21 wherein the metal oxide of the catalytic material comprises a praseodymium oxide- modified alumina and restored catalytic activity is achieved by lowering the temperature of the catalyst into a reactivation temperature range which at atmospheric pressure is from about 470°C to 767°C. 33. A process for the catalytically supported combus¬ tion of a gaseous carbonaceous fuel which comprises (a) forming a mixture of said fuel and air to provide a com- bustion mixture, (b) contacting said combustion mixture under conditions suitable for catalyzed combustion thereof with a catalyst composition comprising a catalytic materi¬ al consisting essentially of a catalytically effective amount of palladium oxide dispersed on a metal oxide sup¬ port selected from the group consisting of ceria, titania, tantalum oxide, cerium-modified alumina, lanthanum-modi¬ fied alumina and praseodymium-modified alumina.

Inventors: FARRAUTO ROBERT J, HOBSON MELVIN C JR, KENNELLY TERESA, WATERMAN EARL M, FARRAUTO, ROBERT, J., HOBSON, MELVIN, C., JR., KENNELLY, TERESA, WATERMAN, EARL, M.
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1993
Main classification: F23C11
IPC codes: F23C13, F23C99, F23R3