The synergy between Cu and ZnO in methanol synthesis catalysts

Abstract

The hydrogenation of CO₂ over physically-mixed Cu/SiO₂ and ZnO/SiO₂ was carried out to clarify the synergetic effect between Cu and ZnO in Cu/ZnO methanol synthesis catalysts. The activity of the physical mixtures significantly increased with increasing reduction temperature in the range of 573–723 K. TEM-EDX results definitely showed that ZnO_x moieties migrated from ZnO/SiO₂ particles onto the surface of Cu particles when the physical mixtures were reduced at high temperatures above 573 K. Upon the migration of the ZnO_x species, the oxygen coverage on the surface of Cu, measured after the hydrogenation of CO₂, increased with the reduction temperature. The results clearly showed that the synergetic effect of ZnO in the physical mixtures can be ascribed to the creation of active sites such as Cu⁺ which the ZnO_x moieties stabilize on the Cu surface. Further, XRD results showed that the migrated ZnO_x species partly dissolved into the Cu particles to form a Cu—Zn alloy.