The Critical Role of βPdZn Alloy in Pd/ZnO Catalysts for the Hydrogenation of Carbon Dioxide to Methanol

Abstract

The rise in atmospheric CO ₂ concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO ₂ as a feedstock. Prime among these is the hydrogenation of CO ₂ to make methanol, which is a key commodity chemical intermediate, a hydrogen storage molecule, and a possible future fuel for transport sectors that cannot be electrified. Pd/ZnO has been identified as an effective candidate as a catalyst for this reaction, yet there has been no attempt to gain a fundamental understanding of how this catalyst works and more importantly to establish specific design criteria for CO ₂ hydrogenation catalysts. Here, we show that Pd/ZnO catalysts have the same metal particle composition, irrespective of the different synthesis procedures and types of ZnO used here. We demonstrate that all of these Pd/ZnO catalysts exhibit the same activity trend. In all cases, the β-PdZn 1:1 alloy is produced and dictates the catalysis. This conclusion is further supported by the relationship between conversion and selectivity and their small variation with ZnO surface area in the range 6–80 m ²g ^–1. Without alloying with Zn, Pd is a reverse water-gas shift catalyst and when supported on alumina and silica is much less active for CO ₂ conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO ₂ hydrogenation and will aid future catalyst discovery.