CO2 hydrogenation to methanol promoted by Cu and metastable tetragonal CexZryOz interface

Abstract

Designing effective catalyst to improve the activity of CO2 hydrogenation to methanol is a potential avenue to realize the utilization of CO2 resources. Herein we construct three kinds of Cu/CexZryOz (CCZ) catalysts with different crystal phases of CexZryOz solid solutions, which demonstrate distinct activity and methanol selectivity in the order of metastable tetragonal-CCZ (CCZ-t″, parts of oxygen in CexZryOz were replaced by tetragonal phase from cubic fluorite phase) > tetragonal-CCZ (CCZ-t) > cubic-CCZ (CCZ-c) for CO2 hydrogenation to methanol. Structural analysis reveals that oxygen vacancies, surface hydroxyls and unsaturated Cu species of CCZ all follow the same sequence as that of activity and methanol selectivity, indicating that the above features are beneficial to improve the catalytic reaction performance. Temperature programmed experiments and mechanism studies show that the interface between Cu and tetragonal (t and t″) CexZryOz can promote CO2 adsorption, and the adsorbed CO2 is more reactive and can generate active bidentate carbonate species, which can be hydrogenated to form active monodentate and bidentate formate species under CO2 and H2 atmosphere. These intermediates should be crucial to the formation of methanol product. CCZ-t″ has stronger H2 activation ability than CCZ-t, which makes the former catalyst have more intermediates and higher methanol selectivity. In contrast, CO2 mainly adsorbs on cubic CexZryOz support of CCZ-c, but its H2 spillover ability is low, which hinders the reaction process. In addition, the strong adsorption of surface intermediates on CCZ-c is also not conducive to methanol formation. Results here demonstrate that constructing active Cu-support interfaces may be an important approach to design effective catalyst for CO2 hydrogenation.