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 CO 2 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(H 2 O) 6 ] 2+ onto a high density of 15 nm TiO 2 NPs grown on HOPG by physical vapor deposition. This photocatalytic reduction results in the deposition of mixed Cu(OH) 2 and Cu 2 O films, while few-nm sized Cu-based NPs are formed on the TiO 2 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 CO 2 reduction reaction (CO 2 RR) under H 2 was evaluated with synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and temperature-programmed desorption (TPD) experiments. Several intermediates, including CO 2 δ−, HCOO, O–CH 3, CO 3 2–, CH x, and CO, were observed using AP-XPS. The TiO 2 NPs show activity toward the formation of methanol (CH 3 OH) that occurs mainly through an O–CH 3 intermediate. The TiO 2 NPs-core–carbon-shell (TiO 2 @C NPs) shows a clear selectivity toward methane (CH 4 ). The Cu/TiO 2 NPs show, however, an activity toward CO, CH 4, and CH 3 OH that depends strongly on the percentage of oxygen present on the Cu NPs surface. This study particularly shows the importance played by the TiO 2 NPs for CO 2 adsorption and activation and the Cu NPs for H 2 and CO 2 dissociation. The CO 2 RR mechanisms are discussed on the basis of the intermediate formation and the surface structure and composition.