Methanol formation by CO2 hydrogenation on Au/ZnO catalysts – Effect of total pressure and influence of CO on the reaction characteristics

• Au/ZnO is similarly active, more selective for MeOH than Cu/ZnO/Al2O3.
• Kinetically controlled activity/MeOH selectivity increase with increasing pressure.
• CO2 is directly hydrogenated to MeOH, with no intermediate CO formation.
• Competitive CO2 and CO hydrogenation in CO2/CO/H2 mixtures on Au/ZnO.
• MeOH formation is faster from CO2 hydrogenation than from CO hydrogenation.

As part of concepts for chemical energy storage of excess electrical energy produced from renewable sources, we have investigated the performance of a Au/ZnO catalyst for the formation of methanol from CO2 and H2 at pressures between 5 and 50 bar and at 240 °C, focusing on the effect of total pressure and on the influence of CO, and compared it to the activity and selectivity of commercial Cu/ZnO/Al2O3 catalysts. Kinetic measurements reveal that Au/ZnO catalysts have similar methanol formation rates and superior selectivity toward methanol compared to Cu/ZnO/Al2O3 over the whole range of pressure. CO, which can be formed during CO2 hydrogenation via the reverse water–gas shift (RWGS) reaction, has little effect on the methanol formation for low CO concentrations up to 1%, while higher CO concentrations significantly diminish the methanol formation rate, at constant CO2 and H2 partial pressures. From these kinetic data and from additional in-situ infrared (IR) spectroscopy measurements, performed during CO2 hydrogenation as well as during CO hydrogenation at 5 bar, we conclude that methanol formation from CO2 and CO proceeds via different, independent reaction pathways and therefore CO is not an intermediate in the hydrogenation of CO2. Consequences of these results on possible applications of Au/ZnO in renewable energy concepts will be discussed.

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