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

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 NO_x. It was found that the addition of Zn could effectually heighten the selectivity of methane to NO_x. 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 NO_x, 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.