Mechanistic insights into the role of zinc oxide, zirconia and ceria supports in Cu-based catalysts for CO[formula omitted] hydrogenation to methanol

Abstract

Copper-based catalysts enable the hydrogenation of CO2 to methanol (MeOH). Selective MeOH synthesis requires the interaction of copper with a metal oxide support to facilitate CO2 adsorption and hydrogenation. Here, we systematically appraise the role of ZnO, ZrO2 and CeO2 in Cu-based catalysts for low-pressure (1 bar) MeOH synthesis. During temperature-programmed desorption (TPD) of CO2, the bare ZnO, ZrO2, and CeO2 exhibited desorption events at both low (70–100 °C) and high (400–700 °C) temperatures, also observed when combining each support with copper. Investigations in a packed bed reactor showed suppressed CO by-production over Cu-ZrO2, leading to a 1.5-fold improvement as compared to Cu-ZnO. Upon Cu-CeO2, CO2 was prone to undergo reduction to CO and desorb, resulting in a MeOH yield 9 times lower than over Cu-ZrO2. Experiments aiming at intermittent operation with successive start-ups and shut-downs showed that the performance of Cu-ZrO2 catalyst remained stable, whilst Cu-ZnO deteriorated monotonically, ascribed to sintering driven by OH and H2O species. In situ infrared spectroscopy demonstrated that methanol synthesis over Cu-ZnO progresses via a single pathway with formate species, whilst parallel formate and bicarbonate pathways were demonstrated over Cu-ZrO2. Unlike Cu-ZnO, the formation of bicarbonate over Cu-ZrO2 allows surface OH species to participate in MeOH synthesis, which we link to the superior performance and stability of Cu-ZrO2.