Catalytic behaviour of Cl-free and Cl-containing Pd/Al2O3 catalysts in thetotal oxidation of methane at low temperature

Pd/Al2O3 catalysts prepared by the impregnation of alumina free of chlorine with H2PdCl4 and Pd(NO3)(2) precursors were characterised and their cataly tic activity in methane oxidation was measured under oxygen-rich conditions . The presence of residual chlorine ions either originating from the precur sor or from impregnation with HCl was shown to strongly inhibit the convers ion of methane. Residual chlorine ions could be slowly removed under reacti on conditions in the form of HCl. This was observed by in situ measurement of HCl departure and catalytic activity. Experimental data were explained b y a simple model in which residual chlorine would block PdO surface active sites during the reaction. The resulting catalyst, free of chlorine ions, w as similar to the one prepared from the nitrate precursor in terms of dispe rsion and activity, Chlorine-free catalysts exhibited slow deactivation wit h time on stream, which was tentatively attributed to the slow conversion o f the active PdO phase into a less active Pd(OH)(2) phase. This interpretat ion was supported by the poisoning effect of water vapour observed upon add ition of water to the feed while CO2 had no influence. Regeneration of the catalyst could be achieved by purging in dry carrier above 500 degrees C, w hich could be ascribed to the decomposition of Pd(OH)(2) into PdO. (C) 2000 Elsevier Science B.V. All rights reserved.