ROLE OF ZNO IN PROMOTING METHANOL SYNTHESIS OVER A PHYSICALLY-MIXED CU/SIO2 AND ZNO/SIO2 CATALYST

Physical mixtures of Cu/SiO2 and ZnO/SiO2 were characterized in order to elucidate the role of ZnO in a Cu/ZnO catalyst for the hydrogenation of CO2. The specific activity of the physical mixture sufficiently increased with rising reduction temperature in the range of 573 to 723 K. The oxygen coverage of copper surface in the physical mixture during methanol synthesis increased from Q=0.08 to 0.15 with rising reduction temperature. The results indicate that the active species is not only Cu+ but also Cu-0. Characterization of the physical mixture by XRD, TEM,and EDX indicated that ZnOx moieties formed by the reduction at high temperature migrated on the surface of Cu particles and dissolved into the bulk of copper to form Cu-Zn alloy. It is concluded that ZnOx on the surface of copper particles stabilizes Cu+ which is pivotal catalytic species.