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

The rise in atmospheric CO2 concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO2 as a feedstock. Prime among these is the hydrogenation of CO2 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 CO2 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 ß-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 m2g-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 CO2 conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO2 hydrogenation and will aid future catalyst discovery.

Solvent Free Synthesis of PdZn/TiO2 Catalysts for the Hydrogenation of CO2 to Methanol.

Catalytic upgrading of CO2 to value-added chemicals is an important challenge within the chemical sciences. Of particular interest are catalysts which are both active and selective for the hydrogenation of CO2 to methanol. PdZn alloy nanoparticles supported on TiO2 via a solvent-free chemical vapour impregnation method are shown to be effective for this reaction. This synthesis technique is shown to minimise surface contaminants, which are detrimental to catalyst activity. The effect of reductive heat treatments on both structural properties of PdZn/TiO2 catalysts and rates of catalytic CO2 hydrogenation are investigated. PdZn nanoparticles formed upon reduction showed high stability towards particle sintering at high reduction temperature with average diameter of 3-6 nm to give 1710 mmol kg-1 h of methanol. Reductive treatment at high temperature results in the formation of ZnTiO3 as well as PdZn, and gives the highest methanol yield.