Catalytic performance of Pd-Ni-Co/Al 2 O 3 catalysts in methane combustion

Pd-based catalysts were prepared by successive wet impregnation of γ-Al 2 O 3 with aqueous solutions of Ni, Co and Pd nitrates: Ni and Co were first co-deposited from a mixed solution, followed by Pd introduction in a second impregnation step. They were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), temperature programmed oxidation (O 2 -TPD), X-ray fluorescence measurements (XRF), BET method. The variation of the nickel content during the synthesis of the PdNix0.5Co/Al 2 O 3 catalysts leads to the formation of different oxide phases on the surface. At nickel contents up to 1 wt%, the main phases are NiO weakly interacting with the support and NiAl 2 O 4 and CoAl 2 O 4 . As the nickel content increases, strongly non-stoichiometric oxide phases appear, consisting of NiO moderately interacting with Al 2 O 3 and mixed Ni–Al phases. Pd 0 , Pd 2+ and Pd 4+ species are all observed on the surface of the catalysts calcined at 450 °C, whereas calcination of the cobalt- and nickel-modified support at 800 °C results in the formation of aluminate phases on the surface and the stabilisation of palladium as PdO. The highest activity of the catalyst containing 5 wt% Ni is very likely a result of the good contact of PdO with nickel aluminate and NiO. High-temperature calcination at 800 °C enhanced the catalytic activity of the catalyst with the lowest nickel content, owing to the increased amount of nickel aluminate on the surface.