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Energy & Fuels 1998, 12, 1130-1134
A Comparative Study of Sulfur Poisoning and Regeneration of Precious-Metal Catalysts N. Shawal Nasri, Jenny M. Jones,* Valerie A. Dupont, and Alan Williams Department of Fuel & Energy, University of Leeds, Leeds, LS2 9JT, U.K. Received May 5, 1998. Revised Manuscript Received August 17, 1998
This paper investigates sulfur poisoning and catalyst regeneration for alumina-supported platinum, palladium, and rhodium catalysts. Reduced catalysts and regenerated sulfur-poisoned catalysts were studied for their activity in the catalytic combustion of methane in a flow microreactor. CO adsorption, in conjunction with FTIR spectroscopy, was used to probe the metal sites on the catalysts. Spectroscopic investigations indicate that the nature of the metal species on the catalysts is different for the freshly reduced and regenerated catalysts. In the case of Pd/Al2O3, successful regeneration of the metal was evident, while for Pt/Al2O3, an increase in the high-frequency CO signal at 2075 cm-1 was observed, possibly indicative of metal-sulfur interactions which are not removed by regeneration or an increase in the metal particle size upon regeneration. Finally, in the case of Rh/Al2O3, the regenerated catalyst has an increased Rh+:Rh0 ratio compared to the freshly reduced catalyst. The reaction studies showed the order of the activity for the freshly reduced catalysts to be Pd > Rh > Pt. After exposure to H2S, it was possible to regenerate the catalysts by reduction. The regenerated Pt/Al2O3 and Rh/Al2O3 catalysts were more active in the combustion of methane than their freshly reduced analogues, while the Pd/Al2O3 catalyst was slightly less active. The success of regeneration of the active catalyst by reduction follows the order Rh > Pt > Pd.
Introduction Precious-metal oxidation catalysts find application in three-way catalytic converters used on diesel automotive exhausts, catalytic burners fitted in gas turbines, and domestic water heaters. In all cases, the catalysts are exposed to variable concentrations of sulfur species in the gas streams. For example, the odorants in domestic gas supplies consist of alkylsulfides, mercaptans, and hydrogen sulfide, totalling a concentration of approximately 5 ppm. The adverse impact of sulfur compounds on catalytic performance is well-known and is the subject of much current research, particularly in the case of reforming and three-way catalysts.1-4 Several factors have been identified which contribute to catalyst poisoning by sulfur compounds. First, it is apparent that sulfur species bond very strongly to the active sites of the catalysts, usually the metal atoms, forming stable surface metal sulfides and thereby prevent the reactants (oxygen, fuel, nitric oxide, depending on the catalyst selectivity) from adsorbing at the surface.2,4 A second factor that may contribute to sulfur poisoning is sulfating of the support. For example, sulfonation of CeO2 or Al2O3 alters the crystalline structure and nature of the support.5 This is expected to have an impact on the metal-support (1) Borgna, A.; Garetto, T. F.; Monzon,. J. Chem. Soc., Faraday Trans. 1997, 93, 2445-2450. (2) Chang, J.-R.; Chang, S.-L.; Lin, T.-B. J. Catal. 1997, 169, 38346. (3) Engler, B. H.; Lindner, D.; Lox, E. S.; Schafersindlinger, A.; Ostgathe, K. Stud. Surf. Sci. Catal. 1995, 96, 441-460. (4) Paal, Z.; Matusek, K.; Muhler, M. Appl. Catal. A: Gen. 1997, 149, 113-132.
interaction, and in addition, the ability to reverse this process varies with the catalyst. In contrast to these findings, an increase in the activity of Pt/Al2O3 catalysts in dehydrogenation has been reported, which was attributed to different sulfur-metal interactions depending upon the platinum salt used in the catalyst preparation.6 A similar enhancement in the activity of Pt/ Al2O3 by H2S in the catalytic combustion of methane has also been reported.7 Clearly, there are at least three factors which influence not only the degree of sulfur poisoning of preciousmetal catalysts, but also the ease of their regeneration and even a promotional effect: The nature of the metal and the sulfur-metal and sulfur-support interactions, which in turn may be influenced by catalyst preparation. The aim of the present work is to study the ease of regeneration of precious-metal catalysts after their exposure to sulfur compounds (H2S) and the effect of the regeneration process on the metal species present on the catalyst. The nature of the metal species before and after regeneration was probed by CO adsorption, in conjunction with FTIR spectroscopy. The freshly reduced and regenerated catalysts were also compared in their activity in the catalytic combustion of methane. (5) Waqif, M.; Baxin, P.; Saur, O.; Lavalley, J. C.; Blanchrd, G.; Touret, O. Appl. Catal. B: Environ. 1997, 11, 193-205. (6) Reyes, P.; Pecchi, G.; Oportus, M.; Fierro, J. L. G. Bull. Soc. Chilena Quim. 1996, 41, 173-179. (7) Lee, J. H., Trimm, D. L.; Cant, N. C. Third International Workshop on Catalytic Combustion, Amsterdam, Sept 23-25, 1996. Proceedings to be published in special issue of Catal. Today, Ed. Geus.
10.1021/ef980104j CCC: $15.00 © 1998 American Chemical Society Published on Web 09/30/1998
Study of Sulfur Poisoning and Regeneration
Experimental Section Catalyst Preparation. Three different catalysts of 2 wt % Pt, Pd, or Rh supported on γ-alumina were prepared using wet impregnation from solutions of Pt(II) acetylacetonate (in toluene), Pd(NO3)3‚H2O (aq), and Rh(NO3)3‚2H2O (aq). Calcination was achieved by heating at 4 °C/min to 475 °C for 240 min. The surface areas of the catalysts, determined by the BET method, are 114 (2 wt % Pt/Al2O3), 124 (2 wt % Pd/Al2O3), 123 (2 wt % Rh/Al2O3), and 134 m2/g (γ-alumina support). FTIR Spectroscopic Studies. A 30-35 mg amount of sieved catalysts, Rh > Pt > blank support (on a weight basis) for both freshly reduced and regenerated catalysts. In addition, CO2 was the major product (Figure 3) and no CO formation was detected. Inspection of Figure 2 indicates that for Rh/Al2O3 and Pt/Al2O3, the regenerated catalysts are actually slightly more active than the freshly reduced catalysts while the regeneration of Pd/Al2O3 results in a slightly less-active catalyst. The effect of H2S is clearly demonstrated for Pd/Al2O3, where the activity decreases markedly after exposure to H2S. Evidence for sulfur interactions with the metal on H2S-exposed Pd/Al2O3 and Rh/Al2O3 catalysts has also been observed by XPS.12 The blank support was also studied for its activity in the combustion of methane. This possessed little activity in the temperature range studied. It is interesting to note that exposure to H2S followed by “regeneration” poisoned γ-alumina toward this reaction. This inhibition is thought to be due to HS and its oxidation products, SO and SO2, reacting with the chain-branching species, H, OH, and O.16 The kinetics of the catalytic combustion of methane over precious-metal and other metal catalysts have been established previously.17-20 For alumina-supported Pd and Rh catalysts, the rate is almost independent of oxygen.19,20 In the case of alumina-supported Pt catalysts, the reaction is zero order in oxygen when the CH4: O2 ratio is below stoichiometric ( Pd > Pt, but the difference in the preexponential terms means that the rate at 623 K follows the order observed experimentally Pd > Rh > Pt. Upon exposure to H2S, the activation energies for the combustion of methane increases for Pd/Al2O3 from 84 to 112.6 kJ mol-1. This increase in activation energy is compensated partially by an increase in the preexponential term, but even so, the rate of conversion at
623 K, km623, is decreased from 0.27 to 0.052 cm3 g-1 s-1, i.e, the catalyst is severely poisoned by sulfur. However, regeneration restores the catalyst activity to close to its original value; km623 increases to 0.20 cm3 g-1 s-1. Regeneration also restores the activity of the Rh/Al2O3 and Pt/Al2O3 catalysts sin fact, the reduction regeneration step results in more active forms of both of these catalysts. For Rh/Al2O3, km623 increases from 0.034 to 0.072 cm3 g-1 s-1 after regeneration, while for Pt/Al2O3, km623 increases from 0.017 to 0.031 cm3 g-1 s-1. The success of the regeneration of the catalysts, thus, follows the order Rh > Pt > Pd. In the case of Pd/Al2O3, the almost complete regeneration is in agreement with the CO adsorption studies, where there was no evidence for sulfur poisoning. The implied change in particle size from these studies may account for the slight decrease in the reaction rate in the catalytic combustion of methane. The change in activity after regeneration of Pt/Al2O3 indicates that the catalyst was modified by H2S. This also concurs with the evidence for Pt-S interactions from XPS spectroscopy12 and possibly in the CO adsorption spectrum on the regenerated catalyst. These results also corroborate the finding of Hicks et al.,10 that the high-frequency band in the CO adsorption spectrum may correlate with the high reaction rate in the catalytic reaction of methane. The difficulty of completely removing sulfur from Pt/Al2O3 compared to Pd/Al2O3 has also been reported for the dehydrogenation of cyclohexane reaction21 and is thought to be associated with the difference in metal-sulfur bond strengths. The modified ratio of Rh/Rh+ species present on the catalyst as a result of sulfur exposure and regeneration increases the rate of combustion of methane. Conclusions The precious-metal catalysts studied showed different activities in the combustion of methane and were affected differently by exposure to H2S. The kinetic parameters associated with the catalytic combustion of methane over the freshly reduced and regenerated catalysts are reported. In the case of Pd/Al2O3, comparison of the freshly reduced and analogous regenerated catalyst indicates that the latter catalyst was only slightly less active in the catalytic combustion of methane. In the case of Rh/Al2O3 and Pt/Al2O3, the regenerated catalysts were slightly more active than the analagous freshly reduced catalysts. The ease of regeneration of the catalysts in terms of catalytic activity followed the order Rh/Al2O3 was easier than Pt/Al2O3 was easier than Pd/Al2O3, and this can be correlated with the interpretation of the changes in the adsorption spectra of CO on the freshly reduced and regenerated catalysts. These studies indicate that the type of metal species present on the catalyst is altered by exposure to sulfur, and this has an impact on the ease of catalytic combustion of methane over the catalysts. Acknowledgment. The authors thank The University Tecnologi, Malaysia, for financial support for Mr. S. Nasri and also Dr. R. Brydson and Mr K. Dickinson for obtaining XPS spectra of some of the catalysts. EF980104J (21) Hoyos, L. J.; Primet, M.; Praliaud, H. J. Chem. Soc., Faraday Trans. 1992, 88, 113-119.
