The Progress in Understanding the Mechanisms of Methanol and Formic Acid Electrooxidation on Platinum Group Metals (a Review)

The reactions of electrooxidation of methanol and formic acid pertain to the most important model electrocatalytic processes and are used in direct low-temperature fuel cells. The electrooxidation mechanisms of these substances were actively studied for many decades. Considerable progress in this field was achieved due to the combined use of electrochemical techniques, in situ IR spectroscopy, differential electrochemical mass spectrometry, isotope-kinetic method, ab initio calculations in terms of the density functional theory, and comparison with the results of gas-phase investigations. The fundamental role in understanding the mechanism of processes was played by measurements on single-crystal faces and surfaces with the known ratio of terraces, steps, and kinks. This allowed the information accumulated for catalysts formed by metal nanoparticles to be interpreted and the role of the structure and size factors in electrocatalysis to be revealed. Attention is focused on the nature of adsorbates and intermediates, the detailed reaction routes, the mechanism of possible slow stages, the pH effects, the roles of the nature of anions in acidic solutions and of the nature cations in alkaline solutions. The effect of the catalyst loading and the multistage character of electrooxidation processes on the efficiency of real fuel cells is noted. The mechanisms of Langmuir-Hinshelwood and Eley-Rideal are analyzed as applied to electrooxidation processes as well as certain peculiarities of CO adsorbate electrooxidation. The results on the mechanism of interaction between adsorbed oxygen and C1-compounds are discussed. The specific features of processes on bimetallic surfaces and the strategy for designing catalysts based on the views on the mechanism of processes, the control over the structure/ composition of the surface and its specific decoration with metal adatoms are considered. Certain topical research directions are formulated aimed at deeper understanding of the mechanisms of electrooxidation of C1-compounds.