(602f) Probing the Atomic-Scale Effects of Surface Oxidation on Methanol Oxidation Reactivity By Cu Oxide Surfaces: A Combined Density Functional Theory and Surface Science Study

Cu-based catalysts are used in several industrial processes, including water-gas shift, methanol formation, and alcohol oxidation. Specifically for alcohol oxidation, mounting evidence suggests that partially oxidized Cu sites are most active, but little is known about the structure of such active sites. Using a combination of theory and surface science experiments, we examine the methanol oxidation reaction on Cu(111) surfaces under varying degrees of oxidation with the goal of providing atomic-scale insight into the optimum geometric and electronic structure for the active sites. Over a well-defined, copper surface oxide, methanol oxidation proceeds according to Figure 1A: (1) two methanol molecules adsorb at adjacent sites; (2) O‒H scission occurs, producing adjacent CH₃O* species and an oxygen vacancy; (3) C‒H scission follows, forming H₂―which immediately desorbs―and CH₂O*; (4) CH₂O desorbs. The effect of surface oxidation on methanol oxidation activity was probed via sequential methanol temperature programmed desorption runs, combined with low-energy electron diffraction measurements, performed until all the oxygen was titrated from the surface (Figure 1B). This systematically scans over a wide range of oxygen coverages. The primary products observed are CH₂O and H₂, with maximum productivity occurring around an oxygen coverage of 50% of saturation (Figure 1B). Reaction energies of key steps were examined via density functional theory on Cu oxide surfaces with a range of oxygen coverages. The results show that O‒H and C‒H scission experience the greatest variation in reaction energy with oxygen coverage, with an increase in oxygen coverage simultaneously promoting C‒H scission and impeding O‒H scission (Figure 1C). When combined with electronic analyses, these results demonstrate that high methanol partial oxidation reactivity occurs due to the rapid oscillation in Cu oxidation state from Cu⁰, which promotes O‒H scission by stabilizing CH₃O*, to Cu⁺, which accelerates C‒H scission by simultaneous destabilization of CH₃O* and stabilization of CH₂O*.