Effect of washcoat modification with metal oxides on the activity of a monolithic Pd-based catalyst for methane combustion

The deposition of Ni, Co, Ce or Fe oxides onto the washcoat surface in the 0.5%Pd/Al2O3 catalyst enhances conversion of CH4. Catalytic activity of the Pd-catalysts containing cobalt oxide depends on the incorporated amount of cobalt oxide and the method of incorporation. The highest activities were those of the 0.5%Pd/0.3%Co/Al2O3 and 1%Pd/0.3%Co/Al2O3 catalysts (cobalt oxide deposited onto the surface of Al2O3) and the 0.5%Pd/5%Co3O4–Al2O3 catalyst (mixed washcoat). Total SSA, Pd dispersion and Pd crystallite size in the x%Pd/y%Co/Al2O3 catalysts depend on the incorporated amount of PdO and cobalt oxide. Pd dispersion in the 1%Pd/Al2O3 catalyst increases from 4% to 20% upon deposition of 14 wt.% Co3O4 (by mass Al2O3) onto the Al2O3 surface (1%Pd/0.3%Co/Al2O3). This increase in Pd dispersion influence the increase in the activity of the 1%Pd/Al2O3 catalyst. On the surface of the 0.5%Pd/5%Co3O4–Al2O3 catalyst Pd occurs mainly in the form of PdO and displays considerable mobility under conditions of temperature variations—cyclically undergoing reduction and oxidation. At 500 °C, in vacuo, the reduction was irreversible and parallelled by the agglomeration of metallic Pd crystallites. At room temperature, cobalt occurred on the catalyst surface in the form of Co+2 ions (CoAl2O4) and was reduced to Co0 at 500 °C (in vacuo). Up to 500 °C, the reduction of Co was reversible.