Cooperative effect induced by the mixing of Na-ZSM-5 and Pd/H3PW12O40/SiO2 in the selective catalytic reduction of NO with aromatic hydrocarbons

Pd was loaded on the dispersed H3PW12O40 (HPW) over the SiO2 surface, and the catalyst was applied to the selective reduction of NO with aromatic hydrocarbons. The catalyst exhibited high activity in the NO reduction when branched aromatic hydrocarbons, such as toluene and xylene, were used as reductants. The catalytic activity of Pd/HPW/SiO2 was improved remarkably by physically mixing it with Na-ZSM-5. From the temperature programmed desorption (TPD) of toluene and the analysis of the products, it was inferred that the activity was enhanced when Pd/HPW/SiO2 and Na-ZSM-5 were mixed. In other words, aromatic hydrocarbons were partially oxidized to yield oxygenated hydrocarbons, e.g., benzaldehyde and phthalic anhydride, over Pd/Na-ZSM-5; in this reaction, a part of Pd migrated from Pd/HPW/SiO2 to Na-ZSM-5 during the course of the physical mixing procedure. Subsequently, the oxygenated hydrocarbons reacted with NO entrapped with HPW over Pd to yield N2.