Insights into the role of Zn and Ga in the hydrogenation of CO2 to methanol over Pd

Abstract

The hydrogenation of CO₂ to methanol is a viable alternative for reducing greenhouse gases net emissions as well as a route for hydrogen storage and transportation. In this context, the synthesis of active and selective catalysts is a relevant objective. In this work, we study the promotion of Pd with Ga and Zn in the hydrogenation of CO₂ to methanol at 800 kPa and 220–280 °C. Mono and intermetallic catalysts (Pd/SiO₂, PdGa/SiO₂ and PdZn/SiO₂) were synthesized by incipient wetness impregnation with the aid of triethanolamine as an organic additive, obtaining similar average metal particle sizes (between 9 and 12 nm). Kinetic analysis reveals that the addition of Ga and Zn increases the turnover frequency for methanol formation by an order of magnitude without significant changes in the reaction rate of the reverse water-gas shift (r-WGS) which is a parallel undesired reaction. The selectivity to methanol (at 220 °C) thus increases from 3% for Pd/SiO₂ to 12% for PdGa/SiO₂ and 30% for PdZn/SiO₂. XPS studies, Infrared analysis of CO adsorption, and XRD analyses show the presence of intermetallic phases Pd₂Ga and PdZn on the surface. The results suggest that Ga and Zn promote Pd, increasing its activity towards the synthesis of methanol, by creating more active sites for this reaction. These sites are likely formed by intermetallic compounds such as Pd₂Ga and PdZn.