Stability of Cu/TiO₂ Nanoparticle Model Catalysts under Electrochemical CO₂ Reduction Conditions

Abstract

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 CuOx 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.