Review Catalytic combustion of methane at high temperatures: Cerium effect on PdO/Al 2 O 3 catalysts Lı ´lian M.T. Simplı ´cio, Soraia T. Branda ˜o *, Daniela Domingos, Franc ¸ois Bozon-Verduraz, Emerson A. Sales Universidade Federal da Bahia, Chemistry Institute, Campus de Ondina, Salvador, Bahia, CEP. 40170-290, Brazil Contents 1. Introduction ....................................................................................................... 2 2. Experimental ...................................................................................................... 3 2.1. Reagents .................................................................................................... 3 2.1.1. Preparation of the catalysts.............................................................................. 3 2.1.2. Characterization....................................................................................... 3 2.1.3. Temperature programmed catalytic activity tests ............................................................ 3 3. Results and discussion ............................................................................................... 3 4. Conclusions ....................................................................................................... 7 Acknowledgements ................................................................................................. 7 References ........................................................................................................ 7 1. Introduction In recent decades catalytic combustion has been widely investigated as an alternative to conventional combustion due to its many practical applications both for pollution abatement and power generation. Catalytic combustion offers the possibility of producing heat and energy at much lower temperatures than conventional thermal combustion, thus reducing the emission of pollutants such as CO, NO x and unburned hydrocarbons (UHCs) [1–9]. Noble metals display considerable activity in hydrocarbon oxidation and palladium has been widely reported in the literature [2–5,7,8,10–12] as the catalyst of choice in methane combustion. Some of the reasons for this include: first, palladium-based catalysts are extremely active in methane oxidation which guarantees ignition at low temperatures, below 400 8C; second, palladium species formed under reaction conditions (up to 800 8C) present low volatility; finally, these systems have the unique capability of temperature self-control associated with the rever- sible PdO (active)/PdO (inactive) transformation. Catalytic combustion at high temperatures (over 600 8C) requires thermally stable catalysts, resistant not only to support sintering but also resistant to decomposition and/or sintering of the active phase. There is general agreement in the literature [1– 12] that the active phase of palladium catalysts is palladium oxide, PdO. This, in turn, is only thermodynamically stable at tempera- tures below 600 8C. Above 600 8C PdO decomposes into metallic palladium which is thermodynamically stable in the high Applied Catalysis A: General 360 (2009) 2–7 ARTICLE INFO Article history: Received 19 October 2008 Received in revised form 1 March 2009 Accepted 4 March 2009 Available online 17 March 2009 Keywords: Methane combustion Palladium catalysts Cerium effect ABSTRACT PdO/Al 2 O 3 and PdO/CeO 2 /Al 2 O 3 catalysts were prepared in order to investigate the role of palladium precursors and cerium addition on the catalytic properties of these systems and on the thermal stability of PdO (active phase) in the catalytic combustion of methane at high temperatures (above 600 8C). The catalysts were obtained from different palladium precursors and presented distinct interactions with CeO 2 /Al 2 O 3 , thermal stability of PdO and catalytic activity. The use of cerium improved PdO thermal stability and the cerium effect was more pronounced on the catalyst prepared from acetylacetonate indicating that a PdO–CeO 2 interaction is more favorable in this system. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +55 71 32836882; fax: +55 71 32374117. E-mail address: brandao@ufba.br (S.T. Branda ˜o). Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.03.005