Study of coke formation in selective catalytic reduction of nitric oxide with propylene on MFI catalyst

Abstract

In order to find the mechanism of SCR (Selective Catalytic Reduction) of NO by propylene on the MFI catalyst (ZSM-5), coke was used as a probe molecule by based on the idea that coke deposited on the active sites decreased the activity of the catalyst. The catalysts, both fresh and spent catalysts, were characterized by TPO (Temperature Programmed Oxidation), TPR (Temperature Programmed Reduction), BET surface area and pore size distribution measurement, Cu sites measurement by the N₂O adsorption technique, and acid site measurement by pyridine adsoption method. It was found that the formation of coke affected only hydrocarbon conversion but did not affect the NO to N₂ conversion. Moreover, coke mainly adsorbed on the acid sites of the catalyst and preferred to adsorbed on the Lewis acid sites. Oxygen adsorbed on the Cu sites of the catalyst may play some important role in the conversion of NO to N₂. In addition, it was suggested that O₂ could spilled-over from the Cu sites to the acid sites of the zeolite in the Cu/Na-ZSM-5 system. From experiment and literature, the probably mechanism of SCR on Cu/Na-ZSM-5 was proposed, NO reacted with O₂ to form NO₂ on the Cu sites and NO₂ reacted with hydrocarbon on the acid sites of the zeolite to produce nitrogen.