Methanol synthesis via CO2 hydrogenation over a Au/ZnO catalyst: an isotope labelling study on the role of CO in the reaction process

Abstract

Methanol synthesis for chemical energy storage, via hydrogenation of CO₂ with H₂ produced by renewable energies, is usually accompanied by the undesired formation of CO via the reverse water–gas shift reaction. Aiming at a better mechanistic understanding of methanol formation from CO₂/H₂ on highly selective supported Au/ZnO catalysts we have investigated the role of CO in the reaction process using isotope labelling experiments. Using ¹³C-labelled CO₂, we found for reaction at 5 bar and 240 °C that (i) the methanol formation rate is significantly higher in CO₂-containing gas mixtures than in a CO₂-free mixture and (ii) in mixtures containing both CO₂ and CO methanol formation from CO increases with the CO content up to 1% CO, and then remains at 20% of the total methanol formation up to a CO₂/CO ratio of 1/1, making CO₂ the preferred carbon source in these mixtures. A shift in the preferred carbon source for MeOH from CO₂ towards CO is observed with increasing reaction temperatures between 240 °C and 300 °C. At even higher temperatures CO is expected to become the dominant carbon source. The consequences of these findings for the application of Au/ZnO catalysts for chemical storage of renewable energies are discussed.