Effect of Na-addition on catalytic activity of Pt-ZSM-5 for low-temperature NO-H2-O2 reactions

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 100degreesC. 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 NaHCO3 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 NOx conversion (>90%) and the N-2 selectivity (ca. 50%). Such a high catalytic activity even at high Na loadings (greater than or equal to 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 NaHCO3 onto Pt-ZSM-5. In situ DRIFT measurements suggested that the addition of Na promotes the adsorption of NO as NO2--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. (C) 2004 Elsevier B.V. All rights reserved.