Performance of Selective Catalytic Reduction of NOx by C3H8 over Co/ZSM-5 Catalysts under Excess Oxygen

Under the simulated conditions of light-duty diesel vehicles exhaust, the effect of the dispersion state and surface property of supported cobalt on the selective catalytic reduction of NOx by C3H8 (C3H8-SCR) over Co/ZSM-5 and Co-Cu/ZSM-5 catalysts was studied by various experimental techniques. The cobalt existed and dispersed on the catalyst external surface in the form of Co3+ and Co2+ species, and located in the zeolite channels in the form of Co2+ cations. The C3H8-SCR activity of the Co/ZSM-5 catalysts was related to the reducibility, NO adsorption-desorption performance of the cobalt ions in external CoOx, species dispersed on the catalyst surface. When Co loading was below 4.0wt%, the increase of Co loading would enhance the reducibility of the cobalt ions of CoOx species and NOx low-temperature conversion; when Co loading was above 4.0wt%, further increase of the Co loading led to the decrease of the ratio of exposed active centers on the surface of catalysts and the decrease of NO adsorption performance per cobalt ion, thus resulting in a decrease of NOx conversion and specific reaction rate at higher temperature. The optimum Co loading for the Co/ZSM-5 catalyst was about 4.0wt%. The formation of external Co3O4 bulk crystallites generated the strong NO adsorption on the Co/ZSM-5 catalyst and promoted C3H8 oxidizing combustion, and reduced the C3H8-SCR activity at high reaction temperature.