The Role of Surface Sites on the Oscillatory Oxidation of Methanol on Stepped Pt[n(111) × (110)] Electrodes

Reaction rates and mechanisms of most electrocatalytic reactions are known to critically depend on the structure of the electrode surface. Examples of structure sensitive electrocatalytic systems include the reduction of oxygen and the oxidation of small organic molecules on platinum, for example. Even more intricate is the effect of the interfacial structure on the oscillatory dynamics commonly observed in some electrocatalytic systems. This is somewhat expected because several adsorption and reaction steps are simultaneously active during self-organized potential or current oscillations. Herein we present results of the effect of surface structure on the oscillatory electro-oxidation of methanol in acidic media on Pt(111), Pt(110), and stepped surfaces Pt(776), Pt(554), Pt(775), and Pt(332). The system was investigated at two methanol concentrations and under voltammetric and galvanostatic regimes. The voltammetric activity toward the electro-oxidation of methanol on stepped surfaces followed this sequence: Pt(776) < Pt(554) < Pt(775) < Pt(332), at high methanol concentration. The reaction rates increase with the density of (110) sites, but small (111) terraces were also found to contribute to the overall process. In terms of potential oscillations, we found specificities that were unambiguously assigned to the surface structure. In particular, the following features were found according to the specific surface studied: period-adding sequences of mixed-mode oscillations; a new type of mixed-mode oscillation; and a particular separation between two types of sequential oscillations. The understanding of the relationship between the surface structure and the underlying dynamics of the surface chemistry during oscillations is a key challenge and our findings in this direction are discussed.G.T.F. and H.V. (Grant 2013/16930-7) acknowledge São Paulo Research Foundation (FAPESP) for financial support. H.V. (Grant 306060/2017-5) and GTF (Grant 305273/2017-5) acknowledges Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support. This study was partially financed by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001