Electrocatalytic Reduction of CO_2 on Cu and Au/W Electrode Surfaces: Empirical (DEMS) Confirmation of Computational (DFT) Predictions

This work describes the employment of differential electrochemical mass spectrometry (DEMS) as a supplementary experimental approach to theory in the study of the reaction mechanism of the Cu-catalyzed electrochemical reduction of CO_2 by investigating the reduction of reactants and (postulated) intermediates. The empirical inferences: (i) CO is one of the first products of CO_2 reduction, as well as the first intermediate in the formation of more reduced products. (ii) Formaldehyde is not a precursor for C=C bond formation but is an intermediate for the production of methane and ethanol. (iii) Both methane and ethanol can be generated from CO_2 through the protonation of CO and through the HCHO intermediate. (iv) The generation of CH_4 and CH_3CH_2OH from CO and CO_2 has a much higher activation barrier than from HCHO; not unexpected since the formaldehyde intermediate is formed after the (computationally determined) rate-limiting CO-protonation step. In this work, DEMS was also used to test the theoretical prediction suggesting the viability of a bimetallic near-surface alloy (NSA) consisting of Au and W as a CO_2-reduction electrocatalyst selective towards the formation of methanol as a product, as opposed to methane, ethylene or ethanol. At an overlayer NSA that consisted of n monolayers (ML) of Au on a polycrystalline W electrode, W(pc)-n[(1×1)-Au], no methane, methanol, ethylene or ethanol was detected when the coverage of Au was at submonolayer (n = 0.5) or multilayer (n ≥ 2) coverages. However, when the NSA contained only 1 ML of Au, methanol was generated exclusively