III International Conference

PP-II-161crystallization state of the support. This behavior can be due to the modification of theelectronic states or the structure of the Pd metallic particles depending on the nature of thesilicon nitride. The decrease of the catalytic activity after heating for 3h at 650 °C in presenceof the reagent mixture was observed for Pd/SiN-am and Pd/SiN-annl.The catalytic activity of Pd/α-Si 3 N 4 is strongly affected by the preparation procedure.Impregnation of silicon nitride using toluene solution of Pd acetate was the most effective [].The total conversion of CH 4 in the experimental conditions was achieved already at 400 °C.Conversely, the Pd/α-Si 3 N 4 catalyst obtained by aqueous impregnation is less active and lessstable. Total conversion of methane was never obtained. In conclusion, the catalytic behavioris very different despite similar average size (4.3 – 4.9 nm) and dispersion of the active phaseobserved by TEM for both catalysts. To evaluate the relations between the catalytic activityand the oxi-reduction properties of the catalysts, TPD of oxygen experiments were carriedout. The TPD profiles of oxygen from Pd significantly changed depending on the preparation.The desorption observed for both samples near 700 0 С is generally attributed to stabilizedoxide species (PdO x ). High temperature shift of the desorption peaks was observed for Pd/α-Si 3 N 4 –tol. For this active catalyst, a strong interaction between Pd and oxygen is thusevidenced. We can thus suppose that if water is used as an impregnation solvent, the surfaceacid-based properties of Si 3 N 4 support and/or of the Pd active phase are irreversibly damaged.In conclusion, Pd supported on α-Si 3 N 4 , prepared by impregnation of the Pd organiccompounds in toluene solutions are found to be the most active and stable under reactionconditions.References1. I. Kurzina, F.J. Cadete Santos Aires, G. Bergeret, J.C. Bertolini. Chem. Eng. J. 107 (2005) 45-53.2. F.J. Cadete Santos Aires, I. A. Kurzina, G.Garcia Cervantes, J.C. Bertolini. Catalysis Today. 117,4 (2006) 518-524.463