Effect of Na-addition on catalytic activity of Pt-ZSM-5 for low-temperature NO-H_2-O_2 reactions

摘要

The effect of additives on Pt-ZSM-5 catalysts was studied for the selective NO reduction by H_2 in the presence of excess O_2(NO-H_2-O_2 reaction)at 100 deg C.The reaction of NO in a stream of 0.08% NO,0.28% H_2,10% O_2,and He balance yielded N_2 with less than 10% selectivity,which could not be increased by changing Pt loading or H_2 concentration in the gas feed.Co-impregnation of NaHCO_3 and Pt onto ZSM-5 decreased the BET surface area and the Pt dispersion.Nevertheless,the Na-loaded catalyst(Na-Pt-ZSM-5)exhibited the higher NO_x conversion(>90%)and the N2 selectivity(ca.50%).Such a high catalytic activity even at high Na loadings(>10 wt.%)is completely contrast to other Na-added Pt catalyst systems reported so far.Further improvement of N_2 selectivity was attained by the post-impregnation of NaHCO_3 onto Pt-ZSM-5.In situ DRIFT measurements suggested that the addition of Na promotes the adsorption of NO as NO_2~--type species,which would play a role of an intermediate to yield N_2.The introduction of Lewis base to the acidic supports including ZSM-5 would be applied to the catalyst design for selective NO-H_2-O_2 reaction at low temperatures.