Reduction of CO2 on Cu and Au/W electrode surfaces: A study by differential electrochemical mass spectrometry

Abstract

This work describes results from an attempt to employ differential electrochem. mass spectrometry (DEMS) of selectively pre-adsorbed reactants and (postulated) intermediates as a supplementary exptl. approach in the study of the reaction mechanism of the Cu-catalyzed electrochem. redn. of CO2. The results prompt the following empirical inferences: (i) CO is the first product of CO2 redn., as well as the first intermediate in more advanced reactions that include formation of pure and oxygenated hydrocarbons; this is in conformity with the (almost) unanimously held view. (ii) HCHO is not a precursor for C=C double-bond formation. (iii) HCHO is an intermediate for the prodn. of methane and ethanol. (iv) The generation of CH4 and CH3CH2OH from adsorbed CO occurs via two pathways: one requires a theor. postulated surface species, CO protonated on the C atom, and the other involves adsorbed HCHO, constituted after the rate-limiting protonation step. (v) The generation of CH4 and CH3CH2OH from CO has a much higher activation barrier than conversions from HCHO; not unexpected since the reactions transpire after the slow Cu-OCH+ formation and, consequently, are not highly activated. This work also presents results from an exptl. study based on DEMS that tested the theor. prediction that suggested the viability of a bimetallic near-surface alloy (NSA) electrode made up of Au and W as a CO2-redn. electrocatalyst selective towards the formation of CH3OH as a product, away from methane, ethylene or ethanol. At an overlayer NSA that consisted of n monolayers (ML) of Au on a polycryst. W electrode, W(pc)-n[(1×1)-Au], no methane, ethylene or ethanol were detected, when the coverage of Au was at submonolayer (n = 0.5) or multilayer (n ≥ 2) coverage. However, when the overlayer contained only 1 ML of Au, methanol was generated exclusively. The anticipated CH3OH-product- selectivity of the W(pc)-(1×1)-Au NSA has thus been (qual.) confirmed. The CH3OH-selective activity was 52 μA cm-2 for a Faradaic efficiency of 0.50 %; the bulk of the current was expended towards H2 evolution and, since the topmost layer was Au, most likely in the prodn. of CO and formates that are undetectable by DEMS.

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