The role of zinc oxide in Cu ZnO catalysts for methanol synthesis and the water-gas shift reaction

All commercial catalysts for methanol synthesis and for the water-gas shift reaction in the low temperature region contain zinc oxide in addition to t he main active component, copper. The varied benefits of zinc oxide are ana lysed here. The formation of zincian malachite and other copper/zinc hydrox y carbonates is essential in the production of small, stable copper crystal lites in the final catalyst. Further, the regular distribution of copper cr ystallites on the zinc oxide phase ensures long catalyst life. Zinc oxide a lso increases catalyst life in the water-gas shift process by absorbing sul phur poisons but it is not effective against chloride poisons. In methanol synthesis, zinc oxide (as a base) removes acidic sites on the alumina phase which would otherwise convert methanol to dimethyl ether. Although bulk re duction of zinc oxide to metallic zinc does not take place, reduction to co pper-zinc alloy (brass) can occur, sometimes as a surface phase only. A new interpretation of conflicting measurements of adsorbed oxygen on the coppe r surfaces of methanol synthesis catalysts is based on the formation of Cu- O-Zn sites, in addition to oxygen adsorbed on copper alone. The possible ro le of zinc oxide as well as copper in the mechanisms of methanol synthesis is still the subject of controversy. It is proposed that, only under condit ions of deficiency of adsorbed hydrogen on the copper phase, hydrogen disso ciation on zinc oxide, followed by hydrogen spillover to copper, is signifi cant.