Wet Chemical Growth and Thermocatalytic Activity of Cu-Based Nanoparticles Supported on TiO2 Nanoparticles/HOPG: In Situ Ambient Pressure XPS Study of the CO2 Hydrogenation Reaction

The present study examines the synthesis of unique Cu nanostructured model catalysts and their catalytic activity toward CO2 hydrogenation under moderate temperature and pressure reaction conditions. Cu-based nanoparticles (NPs) were synthesized by two chemical deposition methods: (1) 5 nm spherical Cu(OH)2 NPs deposited on highly oriented pyrolytic graphite (HOPG) by exposing the HOPG substrate to a colloidal solution of copper, and (2) photocatalytic reduction of [Cu(H2O)6]2+ onto a high density of 15 nm TiO2 NPs grown on HOPG by physical vapor deposition. This photocatalytic reduction results in the deposition of mixed Cu(OH)2 and Cu2O films, while few-nm sized Cu-based NPs are formed on the TiO2 NPs upon subsequent reduction. The chemistry, structure, and morphology of the resulting samples were characterized using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The thermocatalytic activity for the CO2 reduction reaction (CO2RR) under H2 was evaluated with synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and temperature-programmed desorption (TPD) experiments. Several intermediates, including CO2δ−, HCOO, O–CH3, CO32–, CHx, and CO, were observed using AP-XPS. The TiO2 NPs show activity toward the formation of methanol (CH3OH) that occurs mainly through an O–CH3 intermediate. The TiO2 NPs-core–carbon-shell (TiO2@C NPs) shows a clear selectivity toward methane (CH4). The Cu/TiO2 NPs show, however, an activity toward CO, CH4, and CH3OH that depends strongly on the percentage of oxygen present on the Cu NPs surface. This study particularly shows the importance played by the TiO2 NPs for CO2 adsorption and activation and the Cu NPs for H2 and CO2 dissociation. The CO2RR mechanisms are discussed on the basis of the intermediate formation and the surface structure and composition.