This work describes the employment of differential electrochemical mass spectrometry (DEMS) to investigate selectively pre-adsorbed reactants and (postulated) intermediates as a supplementary experimental approach in the study of the reaction mechanism of the Cu-catalyzed electrochemical reduction of CO₂. The results show the following empirical inferences: (i) CO is one of the first products of CO₂ reduction, as well as the first intermediate in more advanced reactions that include formation of hydrocarbons and oxygenates; this is in conformity with the (almost) unanimously held view. (ii) Formaldehyde, HCHO, is not a precursor for C=C double-bond formation. (iii) HCHO is an intermediate for the production of methane and ethanol. (iv) Methane and ethanol can be generated from adsorbed CO via two ways: one requires a theoretically postulated surface species, CO protonated on the C atom, and the other involves adsorbed HCHO, constituted after the rate-limiting CO-protonation step. (v) The generation of CH₄ and CH₃CH₂OH from CO has a much higher activation barrier than from HCHO; not unexpected since the formaldehyde intermediate is formed after the slow Cu–OCH formation and, consequently, is not highly activated.

This work also presents a study based on DEMS that tested the theoretical prediction suggesting the viability of a bimetallic near-surface alloy (NSA) made up of Au and W as a CO₂-reduction electrocatalyst selective towards the formation of methanol as a product, away from methane, ethylene or ethanol typically produced using Cu as the catalyst. 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 were detected when the coverage of Au was at submonolayer (n = 0.5) or multilayer (n ≥ 2) coverages. 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 (qualitatively) 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 production of CO and formates that cannot be studied by DEMS.