Effect of the Chemical States of Copper on Methanol Decomposition and Oxidation

The decomposition and oxidation reactions of CH₃OH over metallic Cu(100) and Cu₂O-covered Cu(100) surfaces are studied by using a combination of in situ ambient-pressure X-ray photoelectron spectroscopy, Auger electron spectroscopy, and density functional theory calculations. We identify the sequential chemical transformation pathways from bond cleavage to the formation of intermediates and final products under operational conditions. Accumulative surface adsorption of CH₃O species on metallic Cu(100) impedes the decomposition of CH₃OH. Co-dosing on metallic Cu(100) with low pressures of 1 × 10^-4 Torr CH₃OH + 1 × 10^-4 Torr O₂ results in partial oxidation of CH₃OH, where the chemisorbed O_ads reduces surface sites available for CH₃O adsorption, decreasing the surface activity for CH₃OH decomposition. In contrast, the Cu₂O overlayer formed under the elevated pressures of 0.33 Torr CH₃OH + 0.66 Torr O₂ promotes the total oxidation of CH₃OH into the final products of CO₂ and H₂O, arising from the active reaction between lattice O within Cu₂O and intermediates of CH₃O, CH₂O, HCOO, and CO. Despite the more favorable O-H bond scission, C-O bond scission also occurs to result in surface accumulation of CH_x on metallic Cu(100), blocking active sites for decomposition reactions of CH₃OH and CH₃O. By comparison, the CH_x species on the Cu₂O-covered Cu(100) undergo oxidation into CO₂ and H₂O with lattice O in the Cu₂O overlayer, thereby freeing active sites for the total oxidation of CH₃OH. These results highlight the distinct roles of metallic Cu and Cu₂O in the pathways of CH₃OH decomposition and oxidation reactions, offering practical insights for the design of Cu-based catalysts with tailored reactivity and selectivity.