Mechanism of CO2 Conversion to Methanol on a Highly Representative Model Cu/ZnO Interface

Abstract

The mechanism of CO₂ hydrogenation to methanol is modelled using plane-wave DFT applied to a representative model Cu₈-ZnO catalyst system (CZ), obtained via unbiased Monte Carlo exploration of Cu cluster growth over a reconstructed polar ZnO surface. Enhanced CO₂ adsorption and activation is found at the active Cu/ZnO interfacial site – resembling a V_O vacancy – compared to sites on other Cu-based systems. Three competing methanol formation mechanisms (the formate, carboxyl and CO hydrogenation pathways) are investigated; the least energy-demanding pathway followed the formate mechanism: CO₂* → HCOO* → H₂COO* → H₂COOH* → H₂CO* → H₃CO* → H₃COH. We report the coexistence of several formate adsorbates, some of which being highly stable spectators that were observed spectroscopically. Only one higher energy interfacial Cu/ZnO formate species is a true intermediate relevant for catalysis, undergoing subsequent hydrogenation to methanol. The methoxy intermediate is also highly stable, in agreement with its spectroscopic observation. The most energy-demanding elementary process is hydrogenation of methoxy to methanol (E_a = 1.20 eV). Furthermore, the calculations indicate the possible role of CO and H₂CO* in scavenging surface O* by forming CO₂* or H₂COO*, thus preventing the poisoning of active sites. Finally, water is expected to form from O* on a pure Cu site only, but not the Cu/ZnO interfacial site relevant for MeOH production. The calculations presented provide valuable new insights that allow a more complete rationalisation of experimental observations. They suggest the key steps to enhance catalysis involves destabilizing the long-lived H₃CO* favouriting its hydrogenation and fast desorption or stabilizing competing intermediates such as H₂COH*.