Selective Catalytic Reduction of Nitric Oxide in Diesel Engine Exhaust over Monolithic

Selective catalytic reduction (SCR) of nitric oxide (NO) in diesel engine exhaust over Cu-Zn/ZSM-5 washcoated ceramic monolithic catalysts is reported. The washcoat component was prepared by ion-exchanging ZSM-5 (Si/Al=40) with zinc while copper was incorporated through impregnation. The dispersed washcoat component was then incorporated into 400 cpsi ceramic monolith through a dipping process with the final loadings between 19.6 wt. % and 31.4 wt. %. The SCR process was studied with a feed comprising of 900 ppm NO, 2,000 ppm iso butane and 3 % oxygen at gas hourly space velocities (GHSV) between 5,000 and 13,000 h-1. NO conversion increased until a loading of 23.6 wt. % to give a conversion of 88 % at 400 °C. The activity dropped at higher loadings due to the partial blockage of cell openings and diffusion limitations while unstable washcoating adherence was also demonstrated. After an initial deactivation of about 10 % in the first 48 h, this catalyst showed stable residual activity. Between 325 and 375 °C, minimal effect on the activity was detected when the space time was reduced from 0.94 s to 0.24 s, suggesting the absence of external mass transfer limitations for up to a GHSV of 16,000 h-1