High catalytic activity and stability of Pd doped hexaaluminate catalysts for the CH 4 catalytic combustion A. Baylet * , S. Royer, P. Mare ´cot, J.M. Tatiboue ¨t, D. Duprez LACCO, UMR CNRS 6503, Universite ´ de Poitiers, 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France Received 8 May 2007; received in revised form 23 July 2007; accepted 27 July 2007 Available online 1 August 2007 Abstract In this work, different procedures, namely carbonate coprecipitation and modified solid–solid diffusion, were used to prepare hexaaluminate samples, unsupported or supported onto u-Al 2 O 3 . These samples were used as catalyst for the methane total oxidation as synthesized or after impregnation of 1 wt% Pd. It was observed that the modified solid–solid diffusion procedure is an efficient method to obtain the hexaaluminate structure. At a theoretical ratio x of hexaaluminate onto Al 2 O 3 less than 0.6 (xLa 0.2 Sr 0.3 Ba 0.5 MnAl 11 O 19 + (1x)Al 2 O 3 , with x = 0.25, 0.60), samples with high specific surface area and u-Al 2 O 3 structure are then obtained. Large differences in catalytic activity can be observed among the series of sample synthesized. All the pure oxide samples (i.e. without palladium) present low catalytic activity for methane total oxidation compared to a reference Pd/Al 2 O 3 catalyst. The highest activity was obtained for the samples presenting a u-Al 2 O 3 structure (with x = 0.60) and a high surface area. Impregnation of 1 wt% palladium resulted in an increase in catalytic activity, for all the solids synthesized in this work. Even if the lowest light-off temperature was obtained on the reference sample, similar methane conversions at high temperature (700 8C) were obtained on the stabilized u-Al 2 O 3 solids (x = 0.25, 0.60). Moreover, the reference sample is found to strongly deactivate with reaction time at the temperature of test (700 8C), due to a progressive reduction of the PdO x active phase into the less active Pd8 phase, whereas excellent stabilities in reaction were obtained on the pure and palladium-doped hexaaluminate and supported u-Al 2 O 3 samples. This clearly showed the beneficial effect of the support for the stabilization of the PdO x active phase at high reaction temperature. These properties are discussed in term of oxygen transfer from the support to the palladium particle. Oxygen transfer is directly related to the Mn 3+ /Mn 2+ redox properties (in the case of the hexaaluminate and stabilized u-Al 2 O 3 samples), that allows a fast reoxidation of the metal palladium sites since palladium sites reoxidation cannot occur directly by gaseous dioxygen adsorption and dissociation on the surface. # 2007 Elsevier B.V. All rights reserved. Keywords: Methane combustion; Hexaaluminate; Noble metals; Activity; Thermal stability 1. Introduction Due to more and more drastic environmental regulations, especially concerning the noxious emissions of nitrogen oxides, methane catalytic combustion appears since the beginning of the 90th [1] as an alternative to conventional thermal combustion for energy production. In the case of the conventional thermal combustion, some parts of the flame (hot spot) can reach temperatures as high as 1300 8C [1,2], leading to the production of a large amount of nitrogen oxides. A catalyst can then be used to avoid these hot spots by controlling the methane total oxidation at a much lower temperature at which production of nitrous oxides remain small [2]. Never- theless, as mentioned by McCarty [2], the stability of the catalysts at the temperature of work is the main encountered problem. Methane oxidation reaction was then extensively studied during the past few years over a wide variety of catalysts [3] where the most studied systems are noble metal (Pd and/or Pt) supported catalysts [3–6]. The highest catalytic activities at low and intermediate temperature (<500 8C) have been found over Pd based catalysts, oxidized Pd catalysts generally presenting the best performances [4–6]. Fujimoto et al. [7] then concluded that the oxidation mechanism on these kinds of catalysts is a Mars and van-Krevelen type redox mechanism, in which carbon atoms can react in the gas phase or in the adsorbed form with coordinatively unsaturated palladium sites located on the surface of palladium oxide crystallites. Nevertheless, these catalysts suffer from drastic www.elsevier.com/locate/apcatb Applied Catalysis B: Environmental 77 (2008) 237–247 * Corresponding author. Tel.: +33 5 49 45 35 22; fax: +33 5 49 45 34 99. E-mail address: alexandre.baylet@etu.univ-poitiers.fr (A. Baylet). 0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2007.07.031