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 Pd-Zn/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 Pd-Ga/SiO₂ and 30% for Pd-Zn/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.

R. Manrique, R. Jiménez, J. Rodríguez-Pereira, V. G. Baldovino-Medrano, and A. Karelovic
International Journal of Hydrogen Energy, Volume 44, Issue 31, 21 June 2019, Pages 16526-16536

The promotion of Ga on SiO₂ supported Cu in the hydrogenation of CO₂ to methanol at 800 kPa and 200–280 °C was investigated. Cu/SiO₂ and CuGa/SiO₂ catalysts were prepared by a water-in-oil microemulsion technique resulting in Cu clusters of 4–6.5 nm. It was found that Ga addition increased the methanol formation rate by an order of magnitude without significantly changing that for reverse water gas shift (RWGS). This trend is also evidenced by the decrease in the apparent activation barrier for methanol formation from 78 (for Cu/SiO₂) to 26–39 kJ mol⁻¹ when Ga was added, but not for RWGS (107–132 kJ mol⁻¹). Kinetic and in situ DRIFTS analyses revealed that formate intermediates are adsorbed on both Cu and Ga₂O₃ and that methoxy hydrogenation could be the rate determining step of methanol synthesis. In the case of RWGS, a zero order of CO formation with respect to H₂ concentration was consistent with a redox mechanism and with the reaction occurring predominantly on Cu sites. The results suggest that Ga promotes Cu increasing methanol selectivity, likely by creating new active sites for methanol formation without modifying its oxidation state, which under reaction conditions remains mostly metallic.

J. C. Medina, M. Figueroa, R. Manrique, J. R. Pereira, P. D. Srinivasan, J. J. Bravo-Suárez, V. G. Baldovino Medrano, R. Jiméneza and A. Karelovic
Catal. Sci. Technol., 2017,7, 3375-3387