State of Supported Rhodium Nanoparticles for Methane Catalytic Partial Oxidation (CPO): FT-IR Studies Elisabetta Finocchio,* ,² Guido Busca, ² Pio Forzatti, ‡ Gianpiero Groppi, ‡ and Alessandra Beretta ‡ Dipartimento di Ingegneria Chimica e di Processo, UniVersita ` di GenoVa, P.le Kennedy 1, 16129 GenoVa, Italy, and Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milano, Italy ReceiVed May 18, 2007. In Final Form: June 28, 2007 The effect of pretreatments as well as of rhodium precursor and of the support over the morphology of Rh nanoparticles were investigated by Fourier transform infrared (FT-IR) spectroscopy of adsorbed CO. Over a Rh/alumina catalyst, both metallic Rh particles, characterized by IR bands in the range 2070-2060 cm -1 and 1820-1850 cm -1 , and highly dispersed rhodium species, characterized by symmetric and asymmetric stretching bands of Rh I (CO) 2 gem-dicarbonyl species, are present. Their relative amount changes following pretreatments with gaseous mixtures, representative of the catalytic partial oxidation (CPO) reaction process. The Rh metal particle fraction decreases with respect to the Rh highly dispersed fraction in the order CO ≈ CO/H 2 > CH 4 /H 2 O, CH 4 /O 2 > CH 4 > H 2 . The metal particle dimensions decrease in the order CH 4 /O 2 > H 2 > CH 4 /H 2 O > CO > CO/H 2 . Grafting from a carbonyl rhodium complex also increases the amount and the dimensions of Rh 0 particles at the catalyst surface. Increasing the ratio (extended rhodium metal particles/highly dispersed Rh species) allows a shorter conditioning process. The surface reconstruction phenomena going on during catalytic activity are related to this effect. 1. Introduction Catalytic partial oxidation (CPO) of methane is an exothermic process allowing the efficient production of hydrogen for application, for example, in fuel cell technologies. 1 The performances of Rh/alumina and Rh/zirconia catalysts for methane CPO have been previously studied by Beretta et al. in a structured wall reactor. 2 Results clearly pointed out that the final performance of the supported Rh catalysts are strongly influenced by the conditioning procedure. In a dedicated investigation on the conditioning of highly dispersed Rh/Al 2 O 3 catalysts, it was shown that outstanding performances were achieved after repeated CH 4 partial oxidation runs, wherein the reaction temperature was stepwise increased from 573 to 1123 K. Run after run, the conversion of methane and the selectivity of synthesis gas increased until a close approach to equilibrium value was reached even at millisecond contact times. Since the process of methane partial oxidation involves a complex network of structure sensitive reaction steps (e.g., CH 4 and CO dissociative adsorptions), it was proposed that the observed evolution of the yield was chemical evidence of a reconstruction of the Rh particles, driven by the high reaction temperatures and the interaction of the surface with the reacting mixture. 3 On the other hand, other preparation methods such as the prepara- tion of embedded rhodium nanoparticles in an alumina matrix may produce catalysts which have a better initial performance in methane CPO, not needing repeated activation steps, although the activity reached at a stationary state can be comparable. 4 To understand the chemistry of the phenomena involved in catalyst stabilization, an in-depth study of the surface species formed in different conditions is needed. Fourier transform infrared (FT-IR) spectroscopy of adsorbed carbon monoxide 5 is a well-known technique for the charac- terization of metal surfaces in the cases of both bulk and supported metal catalysts. The spectroscopy of the surface carbonyl species formed upon CO adsorption allows us to have information on the state and the nature of the adsorbing metal species. This technique has been widely applied to supported Rh catalysts. 6 CO adsorption on reduced rhodium frequently gives rise to the detection of a typical couple of strong bands due to asymmetric and symmetric stretching modes of Rh I (CO) 2 gem-dicarbonyl complexes (D bands). These species are quite stable, as a result of a strong covalent σ-bond and π-back bond. A well-known mechanism 7-9 involving the reaction of CO with the support OH groups causes the disruption of the very small Rh particles only, or the oxidation of isolated Rh atoms, with the formation of such gem-dicarbonyl complexes: Thus, the detection of gem-dicarbonyl complexes is considered significant for the existence of highly dispersed Rh metal particles on the catalyst surface. On the other hand, CO adsorption on extended Rh metal particles gives rise, as usual, to linear (L) and bridging (B) Rh 0 - CO species. These Rh 0 CO complexes are less stable, possibly * To whom correspondence should be addressed. Telephone: +39-010- 3536027. Fax: +39-010-3536028. E-mail: elisabetta.finocchio@unige.it. ² Universita ` di Genova. ‡ Politecnico di Milano. (1) Rostrup-Nielsen, J. R.; Sehested, J.; Nørskov, J. K. AdV. Catal. 2002, 47, 65. (2) Bruno, T.; Beretta, A.; Groppi, G.; Roderi, M.; Forzatti, P. Catal. Today 2005, 99, 89. (3) Beretta, A.; Bruno, T.; Groppi, G.; Tavazzi, I.; Forzatti, P. Appl. Catal., B 2007, 70, 515. (4) Montini, T.; Condo `, A. M.; Hickey, N.; Lovey, F. C.; De Rogatis, L.; Fornasiero, P.; Graziani, M. Appl. Catal., B 2007, 73, 84. (5) Lear, T.; Marshall, R.; Lopez-Sanchez, J. A.; Jackson, D. D.; Klapotke, T. M.; Baumer, M.; Rupprechter, G.; Freund, H.-J.; Lennon, D. J. Chem. Phys. 2005, 123, 174706. (6) Hadjiivanov, K. I.; Vayssilov, G. N.; AdV. Catal. 2002, 47, 308. (7) Paul, D. K.; Marten, C. D.; Yates, J. T., Jr. Langmuir 1999, 15, 4508. (8) Yates, J. T., Jr.; Duncan, T. M.; Vaughan, R. W. J. Chem. Phys. 1979, 71, 3908. (9) Basu, P.; Panayotov, D.; Yates, J. T., Jr. J. Am. Chem. Soc. 1988, 110, 2074. (1/x)Rh x 0 + 2CO + - OH f O 2- + Rh I (CO) 2 + 1 / 2 H 2 10419 Langmuir 2007, 23, 10419-10428 10.1021/la7014622 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/24/2007