The cooperative effect of Zn on Co/HZSM-5 catalyst for NO reduction with methane

Abstract

The cooperation of Zn and Co in the Zn-Co/HZSM-5 catalyst was investigated. NO was selectively reduced by CH4 to N2 in the presence of excess O2, and the catalytic activity depended on both the activation of CH4 and the adsorption properties of NOx. It was found that the addition of Zn could effectually heighten the selectivity of methane to NOx. The results of H2-TPR, NH3-TPD and XPS proved that addition of Zn into Co/HZSM-5 could inhibit the formation of bulk Co3O4 on the outer surface of the catalyst. Reducing the bulk Co3O4 would restrain the combustion of methane and improve the selectivity of methane to NOx, which was very consistent with the experimental results. MS-TPD results showed that Zn contributed the form of NO2 and strengthened its adsorption on the Co/HZSM-5 catalyst. So the reaction mechanism is proposed to occur via two successive elementary steps. First NO is oxidized to NO2 on the dispersed CoOx sites or Co2+ active sites; then NO2 is adsorbed on Zn2+ sites, and further reacts with methane on proton acid sites. The key step is the adsorption of NO2. Zn directly participates in the reaction by adsorption of NO2.