The interaction between water and metal oxide surfaces is an important process in many atmospheric, environmental, and catalytic chemical systems. Ambient-pressure X-ray photoelectron spectroscopy (APXPS) enables the investigation of solid/liquid and solid/vapor interfaces at relevant relative humidity conditions, providing information about chemical speciation of both the outermost atomic layers of the solid surface as well as surface adsorbates. Here, we present work demonstrating the application of APXPS to study the adsorption and dissociation of water vapor on polycrystalline oxides of copper and molybdenum. We find that copper oxide surfaces hydroxylate readily at low relative humidities, with the extent of hydroxide phase formation being dependent on the copper oxidation state. Adventitious environmental carbon species also adsorb readily to these CuOx surfaces and influence the extent of surface hydroxylation. Surface defects on the molybdenum oxides are shown to be important sites for facilitating water dissociation and thus enabling molecular water adsorption. For both of these metal oxide systems, and as had been shown previously for many other metal oxides, surface hydroxylation precedes the adsorption of molecular water at low relative humidities. The generality of this phenomenon suggests that under typical “real-world” conditions, most metal oxide surfaces are covered at least in part by hydroxyl groups and molecular water.

Investigating water adsorption on surfaces using ambient-pressure XPS: Effects of humidity on copper and molybdenum oxides

253rd National Meeting (2017)

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