Mechanistic insights into the role of zinc oxide, zirconia and ceria supports in Cu-based catalysts for CO 2 hydrogenation to methanol

Abstract

Copper-based catalysts enable the hydrogenation of CO 2 to methanol (MeOH). Selective MeOH synthesis requires the interaction of copper with a metal oxide support to facilitate CO 2 adsorption and hydrogenation. Here, we systematically appraise the role of ZnO, ZrO 2 and CeO 2 in Cu-based catalysts for low-pressure (1 bar) MeOH synthesis. During temperature-programmed desorption (TPD) of CO 2 , the bare ZnO, ZrO 2 , and CeO 2 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-ZrO 2 , leading to a 1.5-fold improvement as compared to Cu-ZnO. Upon Cu-CeO 2 , CO 2 was prone to undergo reduction to CO and desorb, resulting in a MeOH yield 9 times lower than over Cu-ZrO 2 . Experiments aiming at intermittent operation with successive start-ups and shut-downs showed that the performance of Cu-ZrO 2 catalyst remained stable, whilst Cu-ZnO deteriorated monotonically, ascribed to sintering driven by OH and H 2 O 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-ZrO 2 . Unlike Cu-ZnO, the formation of bicarbonate over Cu-ZrO 2 allows surface OH species to participate in MeOH synthesis, which we link to the superior performance and stability of Cu-ZrO 2 .