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 NPscore−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.

Investigations of catalysts for electrochemical CO2 reduction have mainly focused on improving their activity and selectivity, while studies on the stability of these catalysts are less scrutinized and generally lacking. In this study, we investigate the stability of a model catalyst system consisting of CuO x nanoparticles selectively deposited on TiO2 nanoparticles on a highly oriented pyrolytic graphite (HOPG) substrate under electrochemical CO2 reduction conditions. X-ray photoelectron spectroscopy (XPS) was used to study changes in the chemical composition and approximate amount of the nanoparticles after electrochemical reduction. Scanning electron microscopy (SEM) was used to track the structure and movement of specific particles after electrochemical reduction, and atomic force microscopy (AFM) was used to monitor morphological changes on the substrate surface. Herein, we show that sufficiently reducing potentials lead to mobility of some (approximately 30%) of the TiO2 nanoparticles. The TiO2 nanoparticle mobility results in some agglomeration and vertical growth of the nanoparticles, as the mobile nanoparticles tend to attach to the top of the stationary particles. It is also shown that, upon agglomeration, the mobile TiO2 nanoparticles undergo a reduction in diameter, while the diameter of the stationary particles remains unchanged. These results highlight the importance of stability studies in order to understand the degradation mechanisms of model electrochemical catalysts in an effort to mitigate catalyst deactivation and maintain activity and selectivity over long periods of time.

Liquid jet X-ray photoelectron spectroscopy was used to investigate changes in the local electronic structure of acetic acid in the bulk of aqueous solutions induced by solvation effects. These effects manifest themselves as shifts in the difference in the carbon 1s binding energy (ΔBE) between the methyl and carboxyl carbons of acetic acid. Furthermore, molecular dynamics simulations, coupled with correlated electronic structure calculations of the first solvation sphere, provide insight into the number of water molecules directly interacting with the carboxyl group that are required to match the ΔBE from the photoelectron spectroscopy experiments. This comparison shows that a single water molecule in the first solvation shell describes the photoelectron ΔBE of acetic acid while at least 20 water molecules are required for the conjugate base, acetate, in aqueous solutions.