XPS and DFT Studies on the Autoxidation Process of Cu Sheet at Room Temperature

Abstract

To better understand the autoxidation mechanism of Cu-based catalysts, we studied the oxidation of Cu sheet exposed to ultrahigh vacuum and air at ambient temperature using X-ray photoelectron spectroscopy (XPS) and density functional theory. Six main stages of Cu autoxidation are revealed: (1) dissociative adsorption of O2, (2) coexistence of nondissociative and dissociative H2O adsorption, (3) diffusion of O and OH into Cu bulk, (4) formation of metastable Cu2O layer, (5) further oxidation and formation of metastable Cu(OH)2 and CuO layer, and (6) transformation phase of the metastable Cu(OH)2 to CuO. The formation of Cu(OH)2 depends on the relative humidity of air and the concentration of adsorbed OH of the Cu sheet. On the basis of these results, we propose that the preservation time of the Cu-based catalysts should be decreased or the catalysts should be preserved in a high vacuum and at low relative humidity. Considering the time of the sample preparation before ex situ XPS analysis and other surfac...