Mechanism of the Surface Hydrogen Induced Conversion of CO2 to Methanol at Cu(111) Step Sites

Abstract

Cu/ZnO/Al2O3 is an industrially important heterogeneous catalyst for the conversion of CO2 to methanol, which is in worldwide demand, and for the solution of the activation mechanism of catalytically inactive CO2. Recent studies have achieved numerous improvements in active sites of catalysts for this process, which can be described as "active copper with step sites" decorated with ZnOx. In spite of these improvements, the mechanism of this process is still unknown, and even its initial stage remains unclear. In this study, we simplified the catalytic system to bare Cu(111) and Cu(775) surfaces in order to systematically determine the mechanistic effects of step sites. The reaction was conducted by using a CO2/H-2 gas mixture at 1 Torr at various temperatures and characterized with infrared reflection absorption spectroscopy (IRRAS). The initial activation of CO2 was found to occur only with the coadsorption of hydrogen; it cannot on its own be converted into other activated species. This coadsorbed hydrogen induces the dissociation of CO2 and converts it into CO2 surface oxygen (O*), and surface hydroxyl (HO*). These species are subsequently converted to carbonate (CO3*), bicarbonate (HCO3*), and formate (HCOO*). One significant observation is that the number of these formate species on step sites continuously decreases with increases in the number of CH2 species during stepwise heating. In addition, a continuous reaction is obtained from formate transfer from terrace to step. Also, an instantaneous feature of methoxy (CH3O*) was observed during the evacuation process. These phenomena strongly indicate that formate is an essential intermediate, especially on steps, for the conversion of CO2 to methanol and that the reduction in its level during this process is due to step-by-step hydrogenation.