Elucidating the crystal-facet dependent catalytic performance of coupled flake-CuO/Cu-zeolite hybrid catalysts for coal-gas-SCR

Abstract

Multi-active sites were usually molded to achieve better catalytic performance for those reactions involving multiple reactants. In this work, CuO/Cu-ZSM-5 (CuO/Cu-Z5) hybrid catalysts with structurally well-defined CuO nanoparticles and site-isolated Cu ions were synthesized and applied for selective catalytic reduction by coal-gas (coal-gas-SCR). The activity evaluation reveals that participation of CuO could significantly improve NOx conversion of Cu-Z5, and Flake-CuO nanoparticle exhibits the best catalytic performance. Several comprehensive characterizations manifest that Flake-CuO/Cu-Z5 contains the highest density of oxygen vacancies, on which the richly exposed CuO (200) facet shows high oxygen mobility and low oxygen vacancy formation energy that verified by DFT calculation. Additionally, Flake-CuO provides superior redox ability and highly active sites for NO adsorption and oxidation to NO2, which can remarkably accelerate subsequent SCR rates. In situ DRIFTS demonstrates that more adsorbed NO2 and bidentate nitrate species generated on Flake-CuO/Cu-Z5, and the amount of other vital intermediate species (such as NHx and CH3NO) were also largely formed over the sample. Interestingly, after injecting reductants, the bidentate nitrate on Rod-CuO/Cu-Z5, Sheet-CuO/Cu-Z5, and Cu-Z5 is partially transformed to monodentate nitrate, whereas Flake-CuO/Cu-Z5 could prohibit this process due to those substantial chemisorbed oxygen and appropriate adsorption ability of intermediates species over CuO (200) facet. The mass migration between CuO and Cu2+ ions revealed that CuO (200) facet not only catalyzes the SCR reaction but also could transfer those formed nitrates and activated reactants to Cu2+ ions in zeolite framework nearby to accomplish the following reaction, fulfilling the acceleration of SCR process.