Electrocatalytic pathways on carbon-supported platinum nanoparticles: Comparison of particle-size-dependent rates of methanol, formic acid, and formaldehyde electrooxidation

The voltammetric electrooxidation rates of formic acid, formaldehyde, and methanol in acidic electrolyte on carbon-supported platinum nanoparticle films with varying particle diameters (d) in the range of ca. 2-9 nm are examined with the objective of comparing the nanoparticle size sensitivity for these related yet distinct electrocatalytic processes. The reaction rates on the larger nanoparticles (d > 4 nm) are similar to those observed on polycrystalline Pt when normalized to the same microscopic Pt surface area. As noted previously, the rates of methanol electrooxidation decrease for Pt nanoparticle diameters below 4 nm. However, formic acid electrooxidation exhibits the opposite behavior, with rates increasing markedly for d < 4 nm, while formaldehyde electrooxidation displays little sensitivity to the Pt nanoparticle size. However, the extent of chemisorbed CO formation from all three reactants, as deduced from voltammetric and infrared spectral data, diminishes with decreasing d, the CO coverages for a given nanoparticle size being in the order methanol < formic acid < formaldehyde. These nanoparticle-size-dependent electrocatalytic and CO adsorptive findings are consistent with the occurrence of a Pt site "ensemble effect", where reactant dehydrogenation to form CO, and also in the case of formaldehyde and especially methanol to yield the reactive intermediate en route to CO2 production, is impeded by the sharply decreasing availability of contiguous Pt terrace sites for d < 4 nm. This structural model is consistent with infrared measurements using CO as a nanoparticle structural probe, which show a rapidly decreasing proportion of terrace relative to edge Pt sites for d < 4 nm, in harmony with atomic packing considerations. The markedly enhanced electrocatalyic rates for formic acid oxidation on the smaller nanoparticles are attributed to the lack of a "Pt site ensemble" requirement for this process, coupled with decreased CO poisoning: unlike the other two reactions, oxygen addition (from coadsorbed-OH) is not necessarily required in order to produce CO2 from formic acid.