Xiaowa Nie

Selectivity of CO2 Reduction on Copper Electrodes: The Role of the Kinetics of Elementary Steps†

On the right path: Based on DFT calculations (incorporating the role of water solvation) of the activation barriers of elementary steps, a new path that leads to methane and ethylene for CO(2) electroreduction on Cu(111) was identified. Methane formation proceeds through reduction of CO to COH (path II, see picture), which leads to CH(x) species that can produce both methane and ethylene, as observed experimentally.

Theoretical insight on reactivity trends in CO2 electroreduction across transition metals

Density Functional Theory (DFT) based models have been widely applied towards investigating and correlating the reaction mechanism of CO2 electroreduction (ER) to the activity and selectivity of potential electrocatalysts. Herein, we examine the implications of the theoretical choices used in DFT models that impact the stability of the reaction intermediates and the limiting potential (UL) of the activity/selectivity determining steps in CO2 ER across transition metals. Three theoretical choices are considered: (i) the type of exchange-correlation (XC) functional, (ii) the surface facet of the metal electrocatalyst, and (iii) the effect of solvation. The impact of the theoretical choices is also studied in the context of deriving scaling relationships for electrocatalyst screening. The analyses reveal that the choice of XC functional (PBE versus RPBE) can alter binding energies of CO2 ER intermediates by 0.30 eV, but have little impact on surface reaction energetics. Surface termination has greater impact, as OH*-terminated adsorbates bind weaker on average by 0.26 eV on stepped facets. Including explicit local solvation stabilizes the OH*-terminated adsorbates, preferentially decreasing the UL for CO* → COH* reduction. Trends in CO2 ER selectivity across metals predicted using scaling correlations differ signficantly from explicitly calculated values due to deviations from the linear binding energy correlations. The difference is most pronounced when the effect of explicit solvation is considered.

DFT insight into the effect of potassium on the adsorption, activation and dissociation of CO2 over Fe-based catalysts

Catalytic conversion of CO2 including hydrogenation has attracted great attention as a method for chemical fixation of CO2 in combination with other techniques such as CO2 capture and storage. Potassium is a well-known promotor for many industrial catalytic processes such as in Fischer-Tropsch synthesis. In this work, we performed density functional theory (DFT) calculations to investigate the effect of potassium on the adsorption, activation, and dissociation of CO2 over Fe(100), Fe5C2(510) and Fe3O4(111) surfaces. The function of K was analyzed in terms of electronic interactions between co-adsorbed CO2 and K-surfaces which showed conspicuous promotion in the presence of K of the adsorption and activation of CO2. The adsorption strength of CO2 on these surfaces ranks as oct2-Fe3O4(111) > Fe(100) > Fe5C2(510). Generally, we observed a direct proportional correlation between the adsorption strength and the charges on the adsorbates. Adding K on the catalyst surface also reduces the kinetic barrier for CO2 dissociation. CO2 dissociation is more facile to occur on Fe(100) and Fe5C2(510) in the presence of K whereas the Fe3O4(111) surfaces impede CO2 dissociation regardless of the existence of K. Instead, a stable CO3- species is formed upon CO2 adsorption on Fe3O4(111) which will be directly hydrogenated when sufficient H* are available on the surface. Our results highlight the origin of the promotion effect of potassium and provide insight for the future design of K-promoted Fe-based catalysts for CO2 hydrogenation.