PRASEODYMIUM-PALLADIUM BINARY OXIDE, CATALYST COMPOSITIONS CONTAINING THE SAME, AND METHODS OF USE

Abstract: A novel praseodymium-palladium oxide compound has the formula Pr4PdO7. The compound has utility as a catalyst for oxidation of combustion mixtures comprising oxygen (e.g., air) and gaseous carbonaceous fuels such as methane and displays excellent regeneration characteristics after exposure to decomposition-causing temperatures. The compound may be used in a catalyst composition by being coated as a washcoat on a suitable carrier. Catalytic combustion processes are carried out by contacting such combustion mixture with the catalyst under combustion conditions, including catalytically supported thermal combustion. Regeneration of over-temperatured catalyst is also provided for.

Description: PRASEODYMHJM-PALLADIUM BINARY OXIDE, CATALYST COMPOSITIONS CONTAININC- I.HPI)S._Of T1SF. BACKSROϋND OP THE INVENTION Field of the Invention The present invention relates to a novel binary oxide of praseodymium and palladium which has utility as a cata¬ lyst suitable for the combustion of gaseous carbonaceous fuels such as natural gas or methane, to a catalyst compos1- l Λ the same , aπc. to a process for catalytic combustion of such carbonaceous fuels using the novel com¬ pound as a catalyst for the combustion. Related Art Complexes of palladium oxide and rare earth metal ses- quloxldeβ are known, as shown by an article by C.L. McDanlel et al, "Phase Relations Between Palladium Oxide and the Rare Earth Sesquloxldes In Air", Journal of Research of the Natu¬ ral Bureau of Standards - A. Physics and Chemistry, Vol . 72A, No. 1, January-February, 1968. Pages 27-37 describe complexes of PdO and rare earth metal oxides. Specifically, the paper describes the study of equilibrium phase relations lπ an air environment between PdO and each of the following sesquloxl es: dgO-, Sm 2 0 g , L 2 0-}, EU j O-, GdgO,, °y2°3» Ho-,0,, V 2°3» E gO,- T- Q 2°3' ϊb 2°3 and Lu 2°3* The experimen ¬ tal procedure employed is described In section 3 at page 28 and describes mixing various combinations of PdO and the rare earth metal oxides and subjecting the mixture to pre¬ liminary heat treatments for a minimum of 18 hours at 770 β C and then at 78θ°C. After each heat treatment the materials were examined by x-ray diffraction techniques and following the preliminary heat treatments portions of each batch were fired at various temperatures, typically lyi g etween 1000°C and 3000°C as set forth In TABLE I, pages 29-33- Among other findings, the paper notes (In the summary of section II.3 at page 30 the dissociation temperature of PdO In air at atmospheric pressure to be 800 β C + 5°C, and that palladium oxide reacts with a number of the rare earth metal oxides to form binary compounds. What is described as the pseudobinary system Nd p O, * PdO is said to exemplify the typi¬ cal type of reaction and three binary oxide compounds of, respectively, 2:1, 1:1 and 1:2 molar ratios of Nd 2 0-,:PdO are disclosed, viz, 2Nd 2 0-, * PdO . Nd 2 0-, * PdO and Nd 2 0 3 '2P O. (The 5 compound 2Nd O-,'PdO may of course be written as N ^ O-,.) Analog compounds are noted for the Sm 2 0- j * PdO, Eu 2 0g" 0 and La-,0-, * PdO systems, with only the 2:1 and 1:2 compounds oc¬ curring in the latter system. However, it was noted that other rare earth oxide-palladium oxide combinations did not 0 react in the solid state. There were combinations of PdO with, respectively, Ho 2 0-, Y 2 ° Er °v Tm 2° ϊb 2° and LU p O,. (See the Abstract at page 27, and the last sentence on page 35.) Another article, by A. Kato et al, "Lanthanide B-Alumi- 5 na Supports For Catalytic Combustion Above 1000°C", Success¬ ful 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 com¬ bustion catalysts. The article states that endurance tests 0 on methane combustion performed at 1200°C proved that a Pd catalyst supported on lanthanum B-alumina has good resis¬ tance to thermal sintering (page 32). U.S. Patent 4,893,465 issued to Robert J. Farrauto et al describes a process for the catalytic combustion of car- 5 bonaceous materials, such as natural gas or methane, using a palladium oxide containing catalyst. In the process, the palladium oxide catalyst for the catalytic combustion is subjected to temperatures in excess of the decomposition temperature of palladium oxide to metallic palladium, the Q latter being inactive for catalysis of the combustion reac¬ tion. At atmospheric pressure the decomposition temperature of PdO is at least about 800°C. The stated improvement in the process of the Patent comprises restoring catalytic ac¬ tivity by lowering the temperature of the catalyst to a re- 5 generating temperature, i.e., a temperature at which Pd is oxidized to PdO, which in air at atmospheric pressure is from about 530°C to about 650°C, and maintaining the temper- ature within that range until desired catalytic activity is achieved by reoxidation of inactive Pd to PdO. The examples of the Patent utilize PdO/Al p O- as the catalyst. SUMMARY OF THE INVENTION In accordance with the present invention there is pro¬ vided a compound of the formula Pr^PdO-,. One aspect of the present invention provides a catalyst composition comprising a refractory carrier on which is dis- posed a catalytic material comprising a mixture of a refrac¬ tory inorganic binder, e.g., one or more of silica, alumina, titania and zirconia, and a catalytically effective amount of a binary oxide having the formula P ^PdOy In another aspect of the invention the binary oxide compound is contained in a reaction product obtained by heating a mixture of praseodymium oxide and palladium oxide to an elevated temperature and for a time sufficient to re¬ act the oxides to form the compound. Yet another aspect of the invention provides that the catalytic material comprises a mixture of a refractory inor¬ ganic binder and a catalytically effective amount of a re¬ action product obtained by heating a mixture of praseodymium oxide and palladium oxide to an elevated temperature for a time sufficient to react the oxides to form a binary oxide of praseodymium and palladium. A process aspect of the invention provides a process for the catalytically supported combustion of a gaseous car¬ bonaceous fuel. The process comprises the steps of forming a gaseous combustion mixture comprising the fuel and oxygen, for example, a mixture of natural gas or methane and air, and contacting the combustion mixture in a catalyst zone with a catalyst composition. The catalyst composition com¬ prises a refractory carrier on which is disposed a catalytic material comprising a mixture of a refractory inorganic binder and a catalytically effective amount of a binary ox¬ ide of praseodymium and palladium. The contacting is car¬ ried out under conditions, e.g., a temperature of from about 925°C to 1650°C and a pressure of from about 1 to 20 atmo¬ spheres in the catalyst zone, which are suitable for cata¬ lyzed combustion of the combustion mixture. Sustained com¬ bustion of at least a portion of the fuel in the mixture is thereby effected without substantial formation of oxides of nitrogen. A narrower range of suitable combustion condi¬ tions may be maintained in the catalyst zone, for example, a temperature of from about 1000°C to 1500°C and a pressure of from about 1 to 15 atmospheres. In one aspect of the invention, the catalyzed combus¬ tion is carried out under substantially adiabatic conditions at a reaction rate exceeding the mass transfer rate of gase¬ ous fuel and oxygen to the catalyst to attain catalytically supported thermal combustion of at least a portion of the fuel in the combustion mixture. Yet another aspect of the present invention provides a process for regenerating a catalyst composition comprising a refractory carrier on which is disposed a catalytic material comprising a mixture of (i) a refractory inorganic binder and (ii) a catalytically effective amount of a binary oxide of praseodymium and palladium having the formula Pr^PdO-, and which has sustained deactivation caused by being heated to a temperature above its deactivation temperature. The regen¬ eration comprises heating the catalyst composition in the presence of an oxygen-containing gas, e.g., air, at a regen¬ eration temperature below about 1200°C, e.g., at a tempera¬ ture in the range of from about 600°C to 1200°C. The regen¬ eration process, which may be a low temperature combustion process, may be conducted in conjunction with and subsequent to the normal higher temperature range combustion process. Other aspects of the invention are described below. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 through 7 are x-ray diffraction spectra ("XRD spectra") of the following materials: Figures 1, 2 and 3 show, respectively, the XRD spectra of reaction products containing the novel compound of the present invention obtained by using a dry-mixed oxide method to react, respectively, 1:1, 4:1 and 2:1 atomic ratios of Pr to Pd as mixtures of mixed praseodymium oxides and palladium oxide; Figure 4 shows as a reference the XRD spectra of the aforesaid mixed praseodymium oxide powder starting material; Figures 5 and 6 show the XRD spectra of reaction pro¬ ducts containing the novel compounds of the invention ob¬ tained by a solution-drying method; Figure 7 compares the XRD spectra of the compounds the binary praseodymium-pal¬ ladium oxide compound of the present invention; Figure 8 is a plot of a thermogravimetric analysis ("TGA") in air of a sample of a reaction product containing the novel compound of the invention showing temperature ver¬ sus sample weight, on which is superimposed a differential thermal analysis ("DTA") profile of the heat liberated or absorbed during chemical and phase transformations of the sample; and Figures 9 and 10 are plots corresponding to Figure 8 for, respectively, the known compounds La^PdO-, and NduPdO-,. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF The compound of the present invention finds utility as a catalyst for combustion of gaseous carbonaceous fuels such as natural gas, methane or other such fuels. Other suitable fuels include ethane, propane, butane, other hydrocarbons, alcohols, other carbonaceous materials and mixtures thereof. The compound of the present invention, like other binary ox¬ ides of palladium and rare earth metals, decomposes at ele¬ vated temperatures to form catalytically inactive species. However, the praseodymium-palladium oxide of the present in¬ vention demonstrates a striking ability to be regenerated into the catalytically active species, by exposure to an ox¬ ygen-containing gas at temperatures just a few hundred de¬ grees centigrade below its inactivation temperature. This stands in stark constrast to the very limited regenerative characteristics of the analogous lanthanum-palladium and neody iurn-palladi m binary oxides. The latter two compounds show catalytic activity for catalytic combustion of gaseous carbonaceous fuels comparable to that of the praseodymium- palladium oxide, but no significant regenerative character¬ istics as compared to the praseodymium-palladium compound. The compound of the invention may be prepared by either a dry mixed-oxide method or a solution-drying method. In the dry mixed-oxide method a mixture of a praseodymium oxide powder containing Pr ? 0-, and PdO powder was mixed in Pr to Pd atomic ratios of, for example, 1:1, 4:1 and 2:1. The mixed powders were mechanically ground together to a size range of about 50 to 100 micron diameter particles. The grinding was followed by calcination at about 1100°C in air for about 18 hours to provide a reaction mixture containing the compound of the invention. In the solution-drying method, suitable amounts of praseodymium nitrate hexahydrate, Pr(N0 ) 2 * 6H 2 0, and palladium nitrate, Pd(N0-) 2 * 2H 2 0, were mixed in an aque- ous solution at temperatures of between about 6θ°C and 90°C, heated to evaporate to dryness, and the resulting residue was calcined in air at about 1100°C for about 18 hours to provide a reaction mixture containing the compound of the invention. The presence in the reaction mixture of the bi- nary praseodymium-palladium oxide compound of the present invention having the formula PruPdO-,, was indicated by x-ray diffraction analysis. The following illustrative Examples 1 and 2 demonstrate the preparation of the compound of the present invention by the dry mixed-oxide method. Example 1 A mixture of 2.5 grams ("g") of palladium oxide and 3.4g of praseodymium oxide was ground in a mortar and pes- tie, to give a ground mixture having an atomic ratio of 2 Pr to 1 Pd. The palladium oxide powder was a commercially available material available from Engelhard Corporation and comprised about 86 weight percent Pd, balance oxygen. The praseodymium oxide powder was a commercially available mate¬ rial available from Aldrich Chemical Company who reports a composition of 82.7 weight percent Pr, balance oxygen. The praseodymium oxide is a mixture of various oxides such as PrO p and P^gO... The ground material was calcined in air at 78θ°C for 17 hours, reground and calcined in air for an ad¬ ditional 65 hours at 1100°C. The resulting material was subjected to x-ray diffraction analysis as described below, showing the presence of a compound of the formula Pr^PdOy as well as traces of PrO-,, r °n> Pd0 n<3 Pd « Example 2 A. The procedure of Example 1 was repeated except that lg of palladium oxide and 5.4g of the praseodymium oxide starting powder were used, giving an atomic ratio of 4 Pr to 1 Pd in the mixture. The resulting material was subjected to x-ray diffraction analysis as described below, showing the presence of a compound of the formula Pr j -PdO-, as well as traces of Pr0 2 , P^O,,, PdO and Pd. B. The same procedure as used in Part A of this Exam¬ ple 2 was used to prepare a sample except that the ratio of palladium oxide and praseodymium oxide was altered to give an atomic ratio of 2 Pr to 1 Pd. The resulting sample was tested to generate the XRD spectra of Figure 3. The following illustrative Examples 3 and 4 demonstrate preparation of the compound of the present invention by the solution-drying method. Example 3 2 moles of Pr(N0_)_ * 6H 2 0 crystals (obtained from Al¬ drich Chemical Company) were added to an aqueous solution in which there was dissolved 1 mole of Pd(N0,) 2 "2H-.0 (provided by Engelhard Corporation) and thoroughly mixed until all the praseodymium nitrate crystals dissolved. The reaction mix¬ ture had an atomic ratio of 2 Pr to 1 Pd. The solution was heated at 90°C to dryness and the dried residue was calcined in air at 1100°C for 65 hours. The resulting material was subjected to x-ray diffraction analysis as described below, showing the presence of a compound of the formula Pr^PdO-, as well as traces of Pr0 2 , P^gO.,, PdO and Pd. . Example 4 The procedure of Example 3 was repeated except that 4 moles of Pr(N0 ),"6H 2 0 crystals were added to an aqueous so- lution in which 1 mole of Pd(N0,) 2 * 2H 2 0 was dissolved. The reaction mixture had an atomic ratio of 4 Pr to 1 Pd. The reaction products obtained in Examples 1-4 were subjected to x-ray diffraction analysis. The x-ray diffrac- tion ("XRD") spectra were generated with Cu K-alpha radia¬ tion by standard powder procedures using a Phillips APD 3720 Diffractometer System at a step scan of 0.02 degrees 2 theta and a count time of one second per step to examine the reac¬ tion products of, respectively, Examples 1, 2, 3 and 4. Ac- cording to the XRD spectra results, the compound Pr^PdOy was formed by the dry mixed-oxide method employed in Examples 1 and 2 and the solution-drying method of Examples 3 and 4. The XRD analysis shows the presence of the new compound in addition to small amounts of the residual starting materi- als, i.e., Pr0 2 , ^gO.. (equivalent to PrO. g~), PdO and Pd. The formation of the new compound is found to be independent of the mixing ratios between the palladium oxide and praseo¬ dymium oxide powders. Figures 1 to 3 show the XRD spectra of the calcined reaction product made by the dry mixed-oxide method of Examples 1 and 2 and based on, respectively, 1:1, 4:1 and 2:1 atomic ratios of Pr to Pd. As a reference, the XRD spectra of the praseodymium oxide starting material pow¬ der is shown in Figure 4, from which it is clear that the starting praseodymium oxide powder contains both PrO p and In Figure 1 the XRD spectra peaks of the reaction pro¬ duct obtained by calcining the mixed, ground praseodymium oxide/palladium oxide 4 Pr to 1 Pd atomic ratio powder show that the Pr0 ? species was consumed completely during the solid state reactions, and converted into the new compound s P ecies ore reac¬ tive with PdO than is the Pr^O-^. As the atomic ratio of Pr to Pd decreases to 1:1 (see Figure 1), the amount of the un- reacted Pr_b-0,1.1 decreases while the amount of the metallic Pd (which originated from the decomposition of the PdO at high temperatures) increases. The peaks characteristic of the compound Pr^PdO-, are marked in Figures 1-3 by asterisks. Preparation of the Pr^PdO-, compound can also be achieved by the solution-drying method. Figures 5 and 6 show the XRD spectra of the calcined samples which were produced by the solution-drying method of, respectively, Examples 3 and 4, the peaks characteristic of Pr^PdO-, being marked by asterisks. Although the crystalline structure of the new compound is yet to be fully investigated, it is believed that the new compound has a crystal structure similar to the known com¬ pounds N ^PdOy and La^PdO-,. Figure 7 shows a comparison of the XRD spectra of the compounds Pr^PdO-,, La^PdO-, and Nd jj PdO-,. The major x-ray peaks of the compound Pr^PdO-, are seen to be basically aligned with those of the corresponding known La and Nd compounds. Thermogravimetric analyses indicated that the new com¬ pound is thermally stable in air up to about 1220°C and that, if heated to temperatures high enough to decompose the catalytically active binary oxide compound Pr^PdO-, (which results in formation of catalytically inactive species), will re-oxidize in air at temperatures less than 400°C below the decomposition temperature, to regenerate the compound. This thermal stability, as well as the regenerative charac¬ teristics, and catalytic activity of the new compound for oxidation of carbonaceous gaseous fuels, makes the new com¬ pound and reaction mixtures containing it well suited for practical use in catalytic combustion processes for carbon- aceous gaseous fuels. The thermogravimetric procedure used to develop the data represented in Figures 8, 9 and 10 and partially sum¬ marized below in TABLE I was as follows. Thermogravimetric analysis refers to measuring small weight changes in a sam¬ ple at different temperatures, such as the weight changes due to various chemical reactions undergone by the samples. A carefully weighed sample of 20 milligram ("mg") to 50mg size is placed in a quartz pan which is suspended from a weight measuring device manufactured by Thermal Sciences, Model STA 1500. Air at approximately 20 cπr/min is passed over the sample. A furnace gradually heats the sample at a rate of about 10°C/min to attain the temperatures shown on the abscissa of the graphs of Figures 8-10. The weight changes occasioned by the heating are plotted as the per¬ centage of the starting weight of the sample on the left- hand ordinate of the Figures to show the weight losses which result from decomposition of the compound and consequent loss of oxygen. The heating.was continued until a tempera- ture of 1400°C was attained and that temperature was held for about one minute. In addition to the weight changes, temperature changes caused by the heat of reaction (differ¬ ential thermal analysis) of the sample as decomposition and then oxidation take place were monitored during the heating period. The heating was then discontinued to allow the heated sample to cool in air, and changes in weight and tem¬ perature changes caused by heats of reaction due to chemical and/or phase changes were monitored during the cooling per¬ iod. Referring now to Figure 8, during the heating period of the thermogravimetric analysis program, identified on the plots as TGA with the heating period being indicated by the arrowheads directed rightwardly as viewed in Figure 8 (and in Figures 9 and 10), a gradual loss in weight was recorded starting at about 350°C with a small, sharp drop in weight at about 975°C, and a precipitous drop in weight from about 1225°C to 1325°C. This shows a substantial decomposition of -li¬ the Pr^PdO-. compound occurring between 1225°C and 1325°C with no further significant loss in weight upon heating to 1400°C. During the cooling period, which is indicated by leftwardly pointing arrowheads as viewed in Figure 8 (and in Figures 9 and 10), the weight of the sample is seen to re¬ main substantially constant in cooling from about 1400°C to about 1050°C with a rapid gain in weight as cooling contin¬ ues from about 1050°C to about 900°C. This gain in weight shows a nearly complete re-oxidation of the material has taken place by the time the sample has cooled to about 900°C. Although palladium oxides show catalytic activity for the thermal combustion reaction, palladium metal and praseodymium metal are substantially inactive for the reac¬ tion. Therefore, the nearly complete regeneration of the oxide species, and consequently, of catalytic activity, as indicated by the weight gain demonstrated in the cooling region from about 1038°C to 900°C, especially 1030°C to 1000°C, shows that the novel compound of the invention pos¬ sesses excellent regeneration properties. Further cooling below 900°C to about 650°C showed only a slight weight gain, but a further more modest weight gain is noted in cooling down to about 350°C. In contrast, the corresponding lanthanum and neodymium compounds., when subjected to the same heating and cooling cycle, showed very little regeneration ability. Thus, Fig¬ ure 9 shows that the binary lanthanum-palladium oxide under¬ went a precipitous loss in weight between about 325°C to 500°C, at which point the weight remained more or less sta¬ ble until a temperature of about 1250°C was attained at which an additional signficant weight loss was noted due to decomposition of the La^PdO-, compound, with the weight more or less stabilizing when heated between 1350°C to 1400°C. However, upon cooling to about 440°C, the weight of the com¬ pound remained substantially the same, showing no signifi- cant re-oxidation. Similarly, Figure 9 shows a significant weight loss of the corresponding binary neodymium-palladium oxide occurring in the 300°C to 450°C range, substantially stable weight until about 1200°C, with a further substantial weight loss due to dissociation of Nd^PdO,, in the tempera¬ ture range of 1200°C to 1300°C, after which the weight sta¬ bilized upon heating up to 1400°C. However, upon cooling to about 400°C, the weight remained substantially the same, showing very little re-oxidation to re-form the binary oxide compound. Superimposed on the TGA plots are differential thermal analysis temperature profiles, identified as DTA on the plots and measured in microvolts per milligram by a Pt/Rh thermocouple showing the heats of reaction of the sample. The thermocouple readings are shown on the righthand ordi- nate of the plots of Figures 8, 9 and 10, with rightwardly directed arrowheads showing the temperature recorded during the heating period and leftwardly directed arrowheads show ing the temperature recorded during the cooling period. The upwardly pointing peaks as shown in the plots are exotherms indicating an oxidation reaction and the downwardly pointing peaks are endotherms indicating decomposition of the oxide to the metals. It will be noted that these peaks coincide with the sample weight changes thereby confirming that the weight gains and losses are caused by, respectively, oxida¬ tion and decomposition of the sample. The following TABLE shows selected data from Figures 8, 9 and 10. TABLE I Thermal Properties of Pr^PdO-,, Nd^PdO-, and La^PdO-. Primary Active Species in the Reaction Product Pr j PdO-, Nd^PdO-. La^PdO-, Td = decomposition temperature, i.e., temperature at which the binary oxide decomposes to the metals. Tr = regeneration temperature, i.e., the temperature at which the metals re-oxidize to the binary oxide. TABLE I shows selected data from Figures 8-10 comparing the decomposition temperatures of the indicated palladium- containing compounds, the regeneration (re-oxidation) of the compounds during the cooling cycle, and the approximate weight recoveries during regeneration. As noted above, all three compounds have somewhat similar decomposi ion tempera- ture ranges. However, only the new compound Pr^PdO y shows a recovery of about 90-95% of its weight during cooling, com¬ pared to only about a 5% recovery for the corresponding Nd and La compounds. Furthermore, the large weight gain for the new compound occurs during cooling in air between 1038°C and 1000°C while the extremely limited regeneration of the other two compounds does not take place until cooling be¬ tween about 767°C and 730°C. For all three compounds, the regeneration of the oxide signaled by the regaining of weight, provides the active catalytic material. It is seen that about 90% by weight of the catalytically active praseo¬ dymium compound of the invention is regenerated whereas only about 5% by weight of the catalytically active lanthanum and neodymium compounds are regenerated. Further, the regenera¬ tion of the praseodymium binary compound occurs at a more desirable significantly higher temperature than the very limited regeneration of the other two compounds. This phen¬ omenon is believed due to more rapid exchange of 0-, in for¬ mation of the new compound. The striking regenerative capacity of the binary oxide Pr j ,Pd0 7 of the invention provides a great advantage over other binary oxides such as the corresponding La and Nd com¬ pounds. For example, upset conditions often occur in cata¬ lytic combustion operations resulting in an over-temperature condition existing in the catalyst zone for a time suffi- cient to decompose all or some of the palladium-rare earth metal catalyst and thereby inactivate, or reduce the activi¬ ty of, the catalyst. Even if the entire catalyst is not over-temperatured to deactivation, local hot-spots can occur which cause deactivation of parts of the catalyst, resulting in lowered catalytic activity. If such deactivation were to occur with a catalyst composition whose catalytically active component was La^PdO-,, Nd^PdO-, or some other binary palladi¬ um-rare earth metal oxide whose regenerative capacity was similarly severely limited, the catalyst would have to be replaced, thereby incurring considerable cost for the re¬ placement catalyst, labor and downtime of the combustion equipment. In contrast, the catalyst of the invention could be regenerated to a high level of catalytic activity simply by operating the combustion process at a lower temperature, e.g., about 600°C to 1050°C. The combustion mixture, con¬ taining as it does 95 to 99 volume percent air, provides ample oxygen to regenerate the catalyst during a relatively low temperature "heat soak" operation. Thus low-temperature combustion or any other suitable technique to contact the catalyst with a gas containing oxygen (usually air) at a temperature within the regeneration temperature range of the catalyst may be used to provide efficient and low-cost in situ regeneration of the catalyst of the present invention. Catalytic activity of the Pr^PdOy compound for methane combustion is shown in the following Example 5. Example 5 A gas mixture comprised of 1% by volume CH^ and the balance air was passed over a catalyst bed comprised of O.Oδg of the binary oxide catalysts of TABLE II in powder form mixed with 2.94g of a diluent alpha alumina powder. The catalyst and diluent powders had a particle size range between 53 and 150 microns in diameter. The gas mixture flow rate through the bed was 1.5 liters/min. A thermocou¬ ple was positioned in the bed to record its temperature. A furnace externally heated the bed and a Beckmann Industrial Model 400A Hydrocarbon Analyzer monitored the conversion of CH j . as a function of temperature. The temperature at which 30% conversion of CH j , was attained is given is in TABLE II below for a number of Pd-containing compounds used as the catalyst. TABLE II Primary Active Species in the Temperature (°C) For Reaction Product 30% Conversion of CH. j Pr^PdO-, 610 Nd^PdO-. 640 La^PdO-, 630 The behavior of the Pr ^ PdO-, binary oxide catalyst is quite similar to the corresponding Nd and La binary oxide catalysts insofar as catalytic activity is concerned, as in- dicated by the temperature required by each for 30% conver¬ sion of CH,. Thus, the methane combustion activity of the Pr^PdO-,-containing reaction mixture of the invention is com¬ parable to those of the Nd PdO-, and La^PdO-, compounds. The catalytically active reaction product listed in TABLE II as primarily containing Pr^PdO-, as the catalytically active species contains other materials including small amounts of other oxides such as PrO p , P^gO., and P d0 « However, the re¬ ported catalytic activity is overwhelmingly if not entirely due to the Pr^PdO-, compound. The compounds PrOp and rgO-. have no catalytic activity for the combustion of carbona¬ ceous fuels. As to PdO, although it is an excellent cata¬ lyst for that reaction, it is active at much lower tempera¬ tures (as low as about 271°C; for example, PdO shows signi¬ ficant catalytic activity for combustion of gaseous carbona- ceous fuels at, e.g., 300°C) . Inasmuch as TABLE II shows that the reaction product containing the Pr^PdO-, compound does not demonstrate significant catalytic activity for the combustion reaction (30% activity) until a temperature of about 610°C is attained, it is clear that very little PdO is present. Based on the weight losses observed in the thermo¬ gravimetric analysis of the samples, it is estimated that the Pr^PdO-, compound comprises about 20 weight percent of the solid reaction products obtained in the illustrated pre¬ parations of the compound of the present invention. In view of the good thermal stability and catalytic properties for, e.g., natural gas or methane combustion, the new compound can be used as a high temperature catalyst for catalytic thermal combustion applications. Further, the vastly supe¬ rior regenerative properties of the Pr^PdO-, compound of the invention as compared to corresponding La and Nd binary ox¬ ides provide a significant advantage in case of accidental over-temperature operation, which often occurs in practice during catalytic combustion operations. Example 6 The catalytic activity for methane combustion of the catalyst of Example 2 was measured. The reaction product obtained in Example 2 (comprising Pr^PdO ™ and other oxides as described) was admixed with gamma alumina to give a mix¬ ture comprising 7 weight percent of the reaction product obtained from Example 2 and 93 weight percent alumina. A slurry was made of 50 weight percent of the above material in water with an amount of acetic acid equal to 5% of the solids content. A cordierite honeycomb with 400 cells (gas flow passages) per square inch and measuring one inch (2.54 cm) in diameter and 1/2 inch (1.27 cm) in length was dipped into the slurry. An air knife was used to clear the chan¬ nels of excess slurry. The washcoated honeycomb was dried 16 hours at 110°C and then calcined at 500°C in air, for 2 hours to give the finished catalyst which exhibited a dry weight increase of about 1.5g per cubic inch of honeycomb catalyst. This weight increase is the weight of the wash- coat of catalyst plus alumina binder-diluent. This is a relatively small content of catalytically active material which, however, proved to be quite active. The resulting catalyst body was fixed in place in a reactor tube in order to test the catalyst to determine its activity. The test was run to determine the temperature required to sustain 20% 5 combustion of a 1 volume percent CH j , in air gas stream fed at 20 liters per minute at about ambient pressure (<_ 15 psi) through the catalyst. This catalyst required a temperature of 535°C to attain 20% combustion while an otherwise identi¬ cal honeycomb carrier without the catalytic material wash- 0 coat needed a temperature of 7l6°C to achieve the same level of methane conversion. Thus, the compounds of this invention may be used to catalyze combustion of a combustion mixture of oxygen, e.g., air, and a gaseous carbonaceous fuel, including fuels con- 5 taining methane, without significant formation of NOx. Such combustion 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 issued December 30, 1975 to William C. Pfefferle. This Patent discloses such a method 0 and apparatus for carrying out the method. Figure 4 of the Pfefferle Patent and the description at column 10, lines 29-49, disclose a system in which a fuel-air combustion mix¬ ture is introduced into a catalyst zone 34 which may be sized to provide for combustion of only a minor portion of 5 the fuel therein, with the major portion of the fuel being combusted by thermal combustion in a combustion zone 37 lo¬ cated downstream of the catalyst 34 and of larger volume than the catalyst 34. In this way combustion is initiated within a catalyst but, the majority of the combustion takes Q place as thermal combustion in a zone downstream of the cat¬ alyst. As explained in the aforesaid Pfefferle Patent, conven¬ tional, i.e., noncatalytic, thermal combustion systems of the type used for engines and power plants, such as gas tur- 5 bines, operate at combustion temperatures which are high enough to form nitrogen oxides ("NOx") including NO. This is because spark-ignited flammable mixtures of fuels such as natural gas and methane combust at temperatures of about 3300°F or higher, which results in the formation of substan¬ tial amounts of NOx from atmospheric nitrogen. Catalytic combustion has the advantage of occurring at lower tempera- tures in which the formation of NOx is avoided or greatly reduced. However, because of limitations on the rate of mass transfer of the fuel and oxygen to the catalyst sur¬ face, either a prohibitively large surface area of catalyst must be provided or the mass transfer rate must be increased to such an extent that an excessively high pressure drop across the catalyst will be sustained. The Pfefferle Patent overcomes these difficulties by employing catalytically sup¬ ported thermal combustion, based on the finding that if the operating temperature of the catalyst is increased substan- tially into the mass transfer limited region of operation the reaction rate begins to increase exponentially. Pfef¬ ferle theorized that the phenomenon may be explained by the fact that the catalyst surface and the gas layer near the catalyst surface are above a.temperature at which thermal combustion occurs at a rate higher than the catalytic rate, and the temperature of the catalyst surface is above the instantaneous auto-ignition temperature of the fuel-air ad¬ mixture. As a result, the fuel molecules entering this high temperature layer spontaneously burn without necessity of their being transported to the catalyst surface. As the combustion process proceeds, this high temperature gas layer becomes deeper until substantially the entire gas flow stream of the combustion mixture is ultimately raised to a temperature at which thermal combustion reactions occur. Consequently, the thermal combustion takes place throughout the entire gas stream, not merely adjacent the catalyst lay¬ er. The "instantaneous auto-ignition temperature" as de¬ fined by Pfefferle means that temperature at which the ig¬ nition lag of the fuel-air mixture entering the catalyst is negligible relative to the residence time in the combustion zone of the mixture undergoing combustion. The Patent literature shows numerous further develop- ments and modifications of the basic system disclosed by Pfefferle, including William C. Pfefferle U.S. Patents 3,940,923, 3,846,979, 3,975,900, and 4,094,142. In such a method, an intimate mixture of the fuel and air is formed and contacted in a combustion zone with a catalyst composi¬ tion comprising the novel compound of this invention. Com¬ bustion of at least a portion of the fuel is thereby at¬ tained under essentially adiabatic conditions at a rate sur¬ mounting the mass transfer limitation to form an effluent of high thermal energy. The combustion zone may be maintained at a temperature of from about 925°C to about 1650°C and the combustion is generally carried out at a pressure of from 1 to 20 atmospheres. Configuration of the catalyst to employ the catalyst of the present invention in a zone which, dur- ing normal operation is exposed to temperatures which do not exceed about 950°C will preclude significant deactivation of the catalyst during normal operation. Operating upsets which raise the temperature to levels, e.g., over 1050°C, at which the Pr^PdO-, compounds decompose, thereby inactivating the catalyst, may be rectified by cooling the catalyst to below about 950°C, e.g., to a temperature within the range 800°C to 900°C, thereby re-oxidizing the compound and regen¬ erating the catalyst. The combustion catalyst of this invention may be used in a segmented catalyst bed such as described in, for exam¬ ple, U.S. Patent 4,089,654. Dividing the catalyst configu¬ ration into upstream and downstream segments is beneficial not only from an operational standpoint, but also in terms of monitoring the performance of various sections of the bed. The catalyst composition may comprise a carrier or sub¬ strate such as a unitary structure comprising a ceramic body having a plurality of fine, parallel gas flow passages ex¬ tending therethrough, from an inlet face to an outlet face thereof, the thin walls of the passages being coated with a "washcoat" comprised of a catalytic material containing a binary praseodymium-palladium oxide in accordance with the present invention. The carrier or substrate may be cylin¬ drical in configuration, and usually is of circular or oval cross section with the inlet and outlet faces disposed per¬ pendicularly to gas flow passages and the longitudinal axis of the cylinder. Such carriers are sometimes referred to as honeycomb or monolith carriers and may be made- of any suit¬ able refractory material such as cordierite, mullite or oth¬ er ceramic-like materials which are extruded into the honey¬ comb body. Alternatively, a metal such as alternate layers of flat and corrugated stainless steel may be rolled into a honeycomb structure or the honeycomb may be made from cer¬ amic precursor fibers wound upon a mandrel and fired to pro¬ vide the honeycomb body. The flow channels in the resultant honeycomb structures may be of any desired cross section such as square, rectangular, triangular or hexagonal. The number of channels per square inch varies depending upon the particular application, and monolithic honeycombs are com¬ mercially available having anywhere from about 9 to 600 channels per square inch. The substrate or carrier portion of the honeycomb desirably is porous, but may be nonporous as in the case of metal substrates, and is catalytically relatively or completely inert to the oxidation reaction. The binary praseodymium-palladium oxide compound of this invention, alone or mixed with other materials (such as by using as the catalytic material the reaction product of Examples 1, 2, 3 or 4) may be mixed with a suitable refrac¬ tory metal oxide binder to provide a catalytic material washcoat which firmly adheres to the walls of the gas flow passages of a honeycomb type or other carrier. A suitable binder material may be, e.g., alumina or silica. The binder also serves as a diluent for the catalytic material. Typi¬ cally, a washcoat may contain from about 80 to 99 weight percent of a diluent-binder and from about 20 to 1 weight percent of catalyst comprising the binary oxide of the in- vention. The catalyst itself may comprise the novel Pr j ,Pd0 7 binary oxide compound alone, or it may comprise a mixture of materials containing, e.g., praseodymium oxides, palladium oxides, palladium and praseodymium metal and/or other binary praseodymium-palladium oxides. Preferably, at least about 20 weight percent of the catalyst itself comprises Pr j ,Pd0 7 . As those skilled in the art will appreciate, the decom- position and regeneration temperature ranges of the palladi¬ um-praseodymium binary oxide are dependent on the partial pressure of oxygen, and at higher pressures,, as for example might be encountered in conjunction with generation of com¬ bustion effluent useful for operation of gas turbines, the decomposition temperature at which the binary oxide will de¬ compose into catalytically inactive species will increase, as will the regeneration temperature at which the binary oxide will re-form. References to these decomposition and regeneration temperatures are all at atmospheric pressures, it being understood that at enhanced partial pressure of oxygen the decomposition and regenerating temperatures will shift upwardly. The determination of such increased temper¬ atures at higher oxygen partial pressures will be a matter well known to those skilled in the art. The demonstrated catalytic activity of the catalyst of the present invention, coupled with its thermal stability and good regenerative properties make it a good candidate for high temperature zones of catalytic combustors of the type described in the above-mentioned Pfefferle U.S. Patent 3,928,961 and Farrauto U.S. Patent 4,893,465.

Claims: THE CLAIMS What is claimed is: 1. A compound of the formula Pr^PdO-,. 2. A catalyst composition comprising a refractory car¬ rier on which is disposed a catalytic material comprising a mixture of a refractory inorganic binder and a catalytically effective amount of a binary oxide having the formula Pr^PdO-,. 3. The catalyst composition of claim 2 wherein the bi¬ nary oxide compound is contained in a reaction product ob¬ tained by heating a mixture of praseodymium oxide and pal¬ ladium oxide to an elevated temperature for a time suffi¬ cient to react the oxides to form said compound. 4. A catalyst composition comprising a refractory car¬ rier on which is disposed a catalytic material comprising a mixture of a refractory inorganic binder and a catalytically effective amount of a reaction product obtained by heating a mixture of praseodymium oxide and palladium oxide to an ele¬ vated temperature and for a time sufficient to react the ox¬ ides to form a binary oxide of praseodymium and palladium. 5. The catalyst composition of claim 4 wherein the bi¬ nary oxide comprises Pr^PdOγ. 6. The catalyst composition of claim 5 wherein the re¬ action product further comprises Pr0 2 , P^ O-,-, and PdO. 7. The catalyst composition of any one of claims 2, 3, 4 or 5 wherein the binder is selected from the group consis¬ ting of one or more of silica, alumina, titan!a and zirco¬ nia. 8. The catalyst composition of any one of claims 2, 3, 4 or 5 wherein the binder comprises alumina. 9. The catalyst composition of claim 8 wherein said binary oxide comprises from about 1 to 20 weight percent of said catalytic material and said binder comprises from about 99 to 80 weight of said catalytic material. 10. The catalyst composition of any one of claims 2, 3, 4 or 5 in which the carrier is a body having a plurality of parallel gas flow passages extending therethrough, the pas¬ sages being defined by walls on which the catalytic material is deposited as a washcoat. 11. A process for the catalytically supported combus¬ tion of a gaseous carbonaceous fuel which comprises (a) forming a gaseous combustion mixture comprising the fuel and oxygen, (b) contacting the combustion mixture in a catalyst zone with a catalyst composition comprising a refractory carrier on which is disposed a catalytic material comprising a mixture of (i) a refractory inorganic binder and (ii) a catalytically effective amount of a binary oxide of praseo¬ dymium and palladium, under conditions suitable for cata¬ lyzed combustion of the combustion mixture, thereby effect¬ ing sustained combustion of at least a portion of the fuel in said combustion mixture without substantial formation of oxides of nitrogen. 12. The process of claim 11 including conducting the catalyzed combustion under substantially adiabatic condi¬ tions at a reaction rate exceeding the mass transfer rate of gaseous fuel and oxygen to the catalyst to attain catalyti¬ cally supported thermal combustion of at least a portion of the fuel in said combustion mixture. 13. The process of claim 11 or claim 12 wherein the bi¬ nary oxide compound comprises Pr^PdO-,. 14. The process of claim 11 or claim 12 including main¬ taining a temperature of from about 925°C to 1650°C and a pressure of from about 1 to 20 atmospheres in the catalyst zone. 15. The process of claim 11 or claim 12 including main¬ taining a temperature of from about 1000°C to 1500°C and a pressure of from about 1 to 15 atmospheres in the catalyst zone. 16. The process of claim 11 or claim 12 wherein the fuel comprises methane and the oxygen is supplied by air. 17. The process of claim 16 wherein the combustion mix¬ ture comprises about 95 to 99 volume percent air as a source of said oxygen. 18. The process of claim 11 or claim 12 including regenerating the catalyst composition after deactivation thereof caused by heating the catalyst composition to a tem¬ perature above its decomposition temperature, by heating the catalyst composition in the presence of an oxygen-containing gas at a regeneration temperature of less than about 1200°C. 19. The process of claim 18 wherein the regeneration temperature is from about 600°C to 1200°C. 20. The process of claim 18 wherein the oxygen-contain¬ ing gas comprises air. 21. A process for regenerating a catalyst composition comprising a refractory carrier on which is disposed a cata¬ lytic material comprising a mixture of (i) a refractory in¬ organic binder and (ii) a catalytically effective amount of a binary oxide of praseodymium and palladium having the for¬ mula Pr^PdO-, and which has sustained deactivation caused by being heated to a temperature above its deactivation temper- ature, comprising heating the catalyst composition in the presence of an oxygen-containing gas at a regeneration tem¬ perature below about 1200°C. 22. The process of claim 21 including heating the cata¬ lyst composition at a temperature in the range, of from about 600°C to 1200°C. 23. The process of claim 21 or claim 22 wherein the oxygen-containing gas comprises air.

Inventors: CHOU TING C, FARRAUTO R J, KENNELLY TERESA, CHOU, TING, C., FARRAUTO, R., J., KENNELLY, TERESA
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1992
Main classification: B01J23
IPC codes: B01J23, B01J38, C01G55, F23C13