We report on electrochemical analysis and stress measurements of hydrogen and CO₂ reactions on Au and Cu surfaces in aqueous solutions with different pHs. Interestingly, surface activation and catalysis of hydrogen and CO₂ on transition metal surfaces show preferential reactivity on select surfaces under controlled solution conditions. Here, we explore the reaction mechanism and early stages of catalysts activation during hydrogen and CO₂ reactions on well-defined Au surfaces under controlled solution conditions, i.e. pH and anion species.
Well-defined Au surfaces are prepared using evaporation and atomic layer electrochemical deposition. Electrochemical analysis shows changes in surface reactivity toward HER and CO₂ reduction as the surface composition and structure are altered. Our stress analysis shows that hydrogen activation on surfaces causes compressive stress, that can be used as a signature for activity toward hydrogen reactions. Interestingly, in-situ stress measurements of Au surfaces do not show surface activation of hydrogen even at low pHs. However, our analysis reveals stress responses corresponding to CO₂ interaction with Au surface. The CO₂ stress responses are evident at ca. 0.4-0 V vs. RHE. The stress response is different from what is observed for hydrogen activation on surfaces. We also use rate dependence of stress response to identify the surface charge and flux effects of primary activation steps. We particularly show that CO₂ related features are flux dependent and are most dominant at pH=7. Our studies show that at higher pHs water is involved in the hydrogen reactions even on Pt surfaces. Features that are likely due to water interactions are also observed on Au surfaces, however, in the presence of CO₂, features that evolve with scan rate and cycling appear in stress analysis. We will discuss the origins of CO₂ related features on Au surfaces. These studies will provide insight into activity and selectivity descriptors for CO₂ reduction reactions on transition metal surfaces in aqueous media.

Kim Hong Kue

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