Palladium oxide supported on alumina decomposes in two distinct steps in air at one ... 500°C) activity of a supported palladium containing catalyst is due to the ...
Applied Catalysis A: General, 81 (1992) 227-237 Elsevier Science Publishers B.V., Amsterdam
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APCAT 2209
Catalytic chemistry of supported palladium for combustion of methane R.J. Farrauto*, M.C. Hobson, T. Kennelly, E.M. Waterman Engelhard Corporation, Research and Development, (USA), tel. (+ l-908)2055306, fax. (+ l-908)2055300
101 Wood Avenue, Iselin, NJ 08830-0770
(Received 2 August 1991, revisedmanuscriptreceived 11 November 1991)
Abatract The high-temperaturecatalytic chemistry of supported palladium for methane oxidation has been studied. Palladium oxide supported on alumina decomposes in two distinct steps in air at one atmosphere.The first step occurs between750 and 666” C and is believedto be a decomposition of palladiumoxygen species dispersedon bulk palladiummetal designated (PdOJPd). The second decomposition is between660 and 650°C and behaves like crystallinepalladium oxide designated (PdO). To reform the oxide, the temperaturemust be decreased well below 650°C. Thus, there is a significant hysteresis between decomposition to palladium and re-formation of the oxide. Above 5CWC, methane oxidation occurs readily when the catalyst contains PdO. However, when only palladium metal is present no oxygen adsorption occurs and no methane activity exists. One may conclude that the high temperature ( > 500°C) activity of a supportedpalladium containing catalyst is due to the ability of palladiumoxide to chemisorb oxygen. Palladium,as a metal, does not chemisorb oxygen above 650°C and thus, is completely inactive toward methane oxidation. Keywords: combustion, methane oxidation, palladiumoxide, thermogravimetricanalysis.
INTRODUCTION
The superiority of palladium as a catalyst for the complete oxidation of methane to carbon dioxide and water has been well known for years [ 11.The nature of the active sites at temperatures below 500°C where catalytic oxidation is initiated, has also been the subject of numerous studies [ 2-41. As important at this temperature range is, certain combustion processes require that the catalyst continue functioning after experiencing temperatures well in excess of 1000°C. For example, to drive a gas turbine for power generation not only must the catalyst be sufficiently active to lightoff the gas feed at low temperatures, but it must maintain activity after experiencing temperatures well above 1000°C [5]. However, depending on oxygen partial pressure, PdO decomposes in a small temperature range above 800 oC. Thus, the interactions of 9926-669X/92/$05.96
0 1992 Elsevier Science Publishers B.V. All rights reserved.
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oxygen and palladium in the catalytic oxidation of methane over a temperature range between 500 and 1300” C is of importance in this process. The commercial interest in high temperature combustion of methane (the major component in natural gas) is derived from a catalytic combustion process which can utilize a catalyst to initiate combustion and bring the gas-phase temperature sufficiently high to initiate thermal reactions [ 6-111. The process permits extremely high combustion efficiencies at temperatures between 1300 and 1400°C with emissions of carbon monoxide, unburned hydrocarbons and NO, not exceeding 10 ppm. The thermal chemistry of oxygen and palladium has been reported in many studies [ 12-171 all of which conclude that the interactions are complicated by different types of reactions, some of which involve solid solution of oxygen in palladium [11,12] with slow conversion to the crystalline palladium oxide, [ 151 while others suggest direct formation of PdO [ 16,171. Lieske and Volter [ 161 suggest two types of PdO on A1203 exist, the most stable of which results from an interaction of PdO with A1203. It is the nature of the palladium species required to sustain methane combustion between 500 and 1000” C which is the subject of this paper. EXPERIMENTAL
Catalyst preparation Commercial y-alumina (condea) was calcined at 950°C for two hours and screened to 53-150 p. The BET surface area was 120 m”/g. Ten grams of this support was impregnated with a 10% Pd (NO,), solution (Engelhard) by the incipient wetness method to give approximately 4 wt.-% Pd on the finished catalyst. The palladium salt was then fixed on the A1203 by reduction with aqueous hydrazine. The catalyst was dried at 120°C overnight and calcined at 500 oC for two hours to give what is designated as the fresh catalyst. Test procedure Fresh catalyst (0.06 g) was mixed with a-alumina diluent (2.94 g) screened to 53-150 pm. The catalyst/diluent mixture was held on a porous quartz frit in a quartz reactor tube with a 2 cm I.D. The tube was positioned vertically in a programmable tube furnace. A thermocouple ,was positioned axially and extended about half way into the catalyst bed. The flow of 1% methane in zero grade air ( < 1 ppm other hydrocarbons) was established by a mass flow controller and set at 1.5 l/min at atmospheric pressure. The gas exiting the reactor was analyzed by a Beckman Industrial model 400A Hydrocarbon Analyzer. The analyzer was zeroed on air and spanned to 100% on the fuel mixture at ambient condition. The test was initiated by ramping the furnace to a selected
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maximum temperature, holding for a limited time and then cooling the reactor. A multi-channel strip chart simultaneously recorded the catalyst bed temperature and the concentration of hydrocarbon in the gas stream. A DuPont 951 Thermal Analyzer was used for all TGA profiles. Zero grade air was passed through the sample chamber at 40 cm3/min at atmospheric pressure. Heating and cooling rates were ZO”C/min unless otherwise noted. Between 20 and 30 mg of catalyst sample was placed in a platinum sample pan and the initial weight recorded at the beginning of each run. In the Model 951 the sample pan is suspended from the horizontal balance arm and the measuring thermocouple is located just above and to one side of the sample. RESULTS
Thermogravimetric analysis PdO on alumina The TGA profile of Fig. 1 was generated by heating fresh PdO on A1203 catalyst in air at 20”C/min. There is a typical weight loss due to volatilization of adsorbed water on the Al2O3 support that continues from room temperature to about 300’ C then slowly tapers to 1000 ’ C. Above 800’ C PdO decomposition to palladium metal occurs and is complete by about 850” C. The observed weight loss of 0.63% agrees well with the calculated value of 0.616% for the amount of PdO present. Following decomposition, heating was continued to 1100” C and held for 30 min. The temperature program was then reversed allowing the catalyst to cool in air. No weight gain due to re-oxidation of the palladium metal was observed until about 650°C where a sharp increase ending at 530” C was observed. The 96.00
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Fig. 1. Thermal decomposition of freshly prepared 4% PdO/A1203 in air; (-) cooling.
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weight increase was about l/3 of the weight loss observed during the decomposition. The difference in temperature between decomposition and re-oxidation represents a hysteresis of 150 -+25’ C. After cooling, the temperature ramp was then reversed to observe the weight changes associated with reheating, Fig. 2. At about 530” C, a small steady weight gain was initiated and continued to 730°C. Further heating resulted in two distinct weight losses. Clearly discernable from the derivative of the weight losses shown in the insert, Fig. 2. The first occurs between 750 and 800” C with a magnitude which was slightly greater than that of the weight gain between 530” C and 730°C. Continued heating resulted in a second weight loss between 800 and 85O”C, the value of which is slightly smaller than that regained in the cool down of the first cycle. At 950’ C the temperature ramp was again reversed similar to the cooling portion of Fig. 1. Re-oxidation of palladium metal was again initiated at 650 and complete at 530°C. Its magnitude was comparable to the first cycle re-oxidation. A third cycle produced a profile essentially identical to cycle 2. The reversibility of the first step was studied by observing re-oxidation after a partial decomposition of the oxide. The experiment was repeated, but in the second cycle the temperature ramp was reversed after the first weight loss was complete at about 785”C, Fig. 3. A distinct weight gain upon cooling was observed around 740’ C, the magnitude of which essentially agrees with that lost in the heating portion of the cycle. Thus, this reoxidation or adsorption of oxygen upon cool-down shows only a small hysteresis of about 45’ C compared to 150°C following complete decomposition. To measure the reversible nature of the second decomposition, this sample was cycled again and heated to 950’ C, 100.30
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Fig. 2. Second cycle thermal decomposition of 4% PdO/A1203 in air; (-) heating, (- - -) cooling. The inset is the fit derivative of the heating curve data showing the two decomposition steps between 750 and 850 “C.
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Fig. 3. Second cycle partial decomposition ( - - - ) cooling.
of 4% PdO/A1203 at 785°C in air; (--_)
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then cooled to 680°C. During a 30-min hold at this temperature, no weight gain was observed. Subsequent cooling again showed the weight gain commencing at about 650 oC. Unsupported PdO Fig. 4 was generated using PdO powder prepared by using the same procedure as PdO on A1,03. Only one weight loss between 800°C and 84O”C, in which the crystalline PdO decomposes to palladium metal is observed. The weight loss observed, 13.1%, agrees with the theoretical 13.07% for decomposition of PdO to Pd. The temperature was increased to 1000°C then reversed to allow generation of a cool-down curve. A small weight gain was observed at about 740°C. The temperature was again reversed for a second cycle shown in Fig. 5. A slow weight gain of 0.15% during heating was observed starting at about 650’ C and peaking at about 770’ C followed by a sharp, smooth single process weight loss ending at about 800’ C. On cooling, a small weight gain of about 0.07% was observed starting at about 740°C. It was complete at about 650°C. Note the expansion of the ordinate scale in Fig. 5 to show these weight changes that are nearly indiscernible in the cooling curve in Fig. 4. Therefore, for the second cycle, only one weight loss corresponding to the decomposition of PdO, between 770 and 800’ C is observed for the unsupported material. Crystalline PdO which decomposes above 800 oC apparently has not formed. Catalytic oxidation of methane on PdO supported on Al,O, Samples of PdO/A1203 were evaluated for oxidation activity as a function of temperature as described in the Experimental section. A methane conversion
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Fig. 4. Thermal decomposition of unsupported PdO in air; (-)
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Fig. 5. Second cycle decomposition of unsupported PdO in air; (-)
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versus temperature plot is shown in Fig. 6. During heat-up conversion shown by the solid line is first noted at about 34O”C, rising to 30% by 430°C and nearing completion at 650’ C. The reactor system including the y-alumina alone has a small but significant catalytic activity towards the oxidation of methane as shown by the dotted line in Fig. 6. The furnace ramp continues to increase the catalyst temperature up to lOOO”C, well beyond the decomposition temperature of PdO to palladium metal. The temperature is then reversed and the sample cooled in CHJair. At 700’ C, conversion, indicated by the dashed line, follows the conversion due to A1203 only (dotted line) indicating no contribution from the palladium. If the activity due to the baseline (dotted line) is
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Fig. 6. Methane conversion by 4% PdO/A1203 as a function of temperature from 300 to 900°C; (-) heating, (- - - ) cooling, ( - - * * ) baseline. Fig. 7. Methane conversion by 4% PdO/A1203 as a function of temperature not exceeding 770°C; (--) heating, (- - -) cooling, (.*.*) baseline.
subtracted from the total, the conversion falls to virtually zero. In other words, the activity would qualitatively follow the TGA profile of Fig. 2. As the Pd/A1203 continues to cool, there is suddenly a restoration of activity beginning at about 680’ C with a maximum of 80% near 600°C. Upon continued cooling the conversion curve effectively overlaps that generated during heat-up. If the catalyst is held above 680’ C no increase in activity is observed. Fig. 7 shows the same conversion versus temperature profile as in Fig. 6 except the catalyst (second cycle) was only brought to 785’ C, corresponding to the first weight loss (decomposition of PdO,) of Fig. 2. With cooling, the total conversion falls slowly to a minimum of only 75% at 680°C followed by an increase to 80% at 600°C. The regain in activity parallels the weight gain observed in the TGA cooling curve, Fig. 3. DISCUSSION
Thermal analysis The weight losses observed for supported and unsupported PdO, Figs. 1 and 4, agree well with each other and with the theoretical values. The decomposition temperature ranges observed between 800 and 850’ C also agree well with
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published data [ 12,18,19]. All references report a single process decomposition for the unsupported PdO similar to our observation. The TGA second cycle heat-up profiles, displayed in Figs. 2 and 5, clearly show gradual weight increases followed by a significant increase in the rate of oxygen pick-up commencing in the range of 570 to 600°C and continuing to about 750°C. The general shape of the weight gain profiles and the temperature ranges are quite similar to those reported for palladium metal [ 121. This similarity suggests that palladium metal at low temperature chemisorbs oxygen and forms some oxygen-palladium species which promotes chemisorption of oxygen as the temperature is increased. This species is designated as (PdOJ Pd). Once a skin of this species is formed, subsequent oxygen pick-up above about 570°C results in dissolution and/or reaction of oxygen into the underlying palladium metal. The amount of oxygen pick-up far exceeds that of a monolayer based on chemisorption studies. These arguments agree with those of Turner and Maple [ 131 who report an accelerated rate of oxygen pick-up on palladium at temperatures above 400 “C. It also agrees with the oxygen penetration studies of Campbell et al. [ 141 who propose that oxygen forms a solid solution with bulk palladium metal through an oxygen saturated surface. The oxygen saturated surface may slowly convert to a crystalline PdO structure as reported by Guiot (15). One might also invoke the mechanism of Ruckenstein and Chen [ 171, who observed pitting and cavity formation due to oxidation of the underlying palladium metal exposed by migration of PdO on to the alumina surface. A restructuring of the PdO crystallites has also been proposed by Baldwin and Burch [ 51 who, along with Briot and Primet [ 201, found that aged catalysts are more active for methane oxidation at low temperatures than fresh. The lack of reversibility of re-oxidation on cooling, Figs. 2 and 5, has not been previously reported. It might be anticipated since complete oxidation of the metal to the oxide is known to be a slow process. However, the large hysteresis of approximately 150’ C for the supported metal is surprising. The standard deviation of the re-oxidation onset temperature was 25’ C for six runs at four different cooling rates between 5 and 40’ C/min. There appeared to be no correlation between onset temperature and cooling rate, although a trend may have been obscured by experimental error in the small sampling. An induction mechanism can be ruled out based on one run at a cooling rate of 40”C/min and no weight gain during a 30-min hold at 680°C. It would be difficult to explain a mechanism by which an induction period greater than 30 min vanished on decreasing the sample temperature by 30’ C. Thus, re-oxidation is not simply a time-temperature rate process, but requires a specific temperature. The onset temperature for re-oxidation of the unsupported palladium, Fig. 5, is much higher, 74O”C, than for the supported systems, but still is about 40’ C below the onset temperature for decomposition. The support must have some role in this difference. We have no explanation for it at this time. Given the results generated thus far, it is not surprising that at least two
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different oxygen loss profiles should be observed. The first loss of oxygen occurs at a lower temperature than bulk crystalline palladium oxide decomposition, is close to reversible, Fig. 3, and is comparable in magnitude to the weight pick-up during heat-up. Furthermore, it only occurs when palladium metal is present, suggesting it is a non-crystalline palladium oxide species dispersed on bulk palladium metal. These results are consistent with literature reports of two forms of palladium oxide on alumina. Lieske and Volter [ 161 suggested the existence of bulk PdO crystallites and a PdO surface complex on alumina based on TPR experiments. Indirect evidence of two oxide species were obtained by Hicks et al. [ 211. They reported methane oxidation activities with widely different turnover numbers which were dependent on the PdO support. Much higher turnover numbers were found for PdO on palladium metal crystallites as compared to PdO dispersed on A1203. Vass et al. [22] also observed decreases in O/Pd ratio and activity for methane oxidation as the temperature approached that required to decompose PdO. Bulk palladium oxide seems to be well established in each case. The nature of the second species is unresolved. The kinetics of these decompositions are currently under investigation in the author’s laboratory. Activity for methane oxidation Palladium containing catalysts are reported to be the most active for methane oxidation [ 151. Cullis and Willat [2] report that reduced palladium on alumina is more active at 400°C than the oxidized form. In contrast, Takahashi et al. [3] reported that palladium on mordenite was more active for methane oxidation at 400°C when oxidized. They argue that PdO enhances oxygen adsorption, lowering the activation energy for methane oxidation. This apparent conflict can be resolved within the framework of our findings. We are reporting that at higher temperatures, when PdO is not present on the surface, there is no oxygen chemisorption and the activity for methane oxidation is essentially zero. Once the temperature falls below 65O”C, oxygen chemisorbs, dissociates and reacts with palladium metal forming PdO, some of which is possibly redispersed according to the mechanism of Chen and Ruckenstein [ 231. A regain in activity is then observed. When the catalyst temperature is raised to 785°C and the oxide is only partially decomposed, activity recovers rapidly as the temperature is lowered into a region where the PdOJPd phase is stable. The results of Cullis and Willatt can be explained as the formation of the PdO,/Pd species under their pretreatment and reaction conditions. The Takahashi et al. observation may be the result of migration of ion exchanged palladium ions to the surface of mordenite particles and the formation of additional PdO. A proposed model for the thermal chemistry in air is presented in Fig. 8. Well dispersed PdO (open circles) decomposes and then agglomerates to pal-
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Fresh
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