Effect of cobalt oxide on performance of Pd catalysts for lean-burn natural gas vehicles in the presence and absence of water vapor

Pd-based catalysts modified by cobalt were prepared by co-impregnation and sequential impregnation methods, and characterized by X-ray powder diffraction (XRD), N_2 adsorption/desorption (Brunauer-Emmet-Teller method), CO-chemisorption and X-ray photoelectron spec-troscopy (XPS). The activity of Pd catalysts was tested in the simulated exhaust gas from lean-burn natural gas vehicles. The effect of Co on the performance of water poisoning resistance for Pd catalysts was estimated in the simulated exhaust gas with and without the presence of water vapor. It was found that the effect of Co significantly depended on the preparation process. PdCo/La-Al_2O_3 catalyst prepared by co-impregnation exhibited better water-resistant performance. The results of XPS indicated that both CoAl_2O_4 and Co_3O_4 were present in the Pd catalysts modified by Co. For the catalyst prepared by sequential impregnation method, the ratio of CoAl_2O_4/Co_3O_4 was higher than that of the catalyst prepared by co-impregnation method. It could be concluded that Co_3O_4 played an important role in improving water-resistant performance.