The spatial structure of compressed carbon monoxide adlayers on Pt(111) in aqueous acidic solution has been explored by means of in-situ scanning tunneling microscopy (STM) along with infrared reflection–absorption spectroscopy (IRAS). Besides offering a detailed structural picture of this electrochemical interface in comparison with the well-studied Pt(111)/CO system in ultrahigh vacuum (uhv) environments, the real-space structural information provided by STM allows an assessment of the obfuscating influence of dynamic dipole coupling upon IRAS binding-site assignments. In turn, the latter data provide an important crosscheck on the validity of binding-site assignments deduced from the STM images. Emphasis is placed on the structures formed from near-saturated CO solutions, encouraged by the electrode potential-induced adlayer phase transition at ca. 0 V vs SCE observed previously under these conditions by IRAS. At potentials below 0 V, a hexagonal close-packed (2×2)–3CO adlayer is observed, with a CO coverage, θCO, of 0.75. The z-corrugation pattern evident in the STM images indicates the presence of two threefold hollow and one atop CO per unit cell. This binding-site assignment is supported by the corresponding IRAS data which yield C–O vibrational bands at ca. 2065 and 1775 cm−1. The relative intensities of these two νCO bands, ca. 2:1, differs markedly from the 1:2 binding site occupancy deduced from STM. This apparent disparity, however, can be accounted for by dynamic dipole coupling effects between the atop and multifold CO oscillators. At potentials above 0 V (up to the onset of CO electrooxidation at ca. 0.25 V), a markedly different adlayer arrangement is formed, having a (√19×√19)R23.4°–13CO unit cell, with θCO=13/19. This hexagonal structure features CO binding in predominantly asymmetric sites inbetween atop and bridging geometries. A distinction between several alternate adlayer arrangements sharing (√19×√19) symmetry was achieved on the basis of the z-corrugation pattern along with the corresponding IRAS data upon consideration of dipole-coupling effects. Another CO adlayer structure, having a (√7×√7)R19.1°-4CO unit cell (θCO=4/7), was commonly observed at potentials below 0.2 V after the removal of solution-phase CO. These adlayer arrangements are distinctly different to the compressed Pt(111)/CO structures found in uhv. The increased accommodation of CO in multifold sites observed for the former can be understood chiefly from the markedly (ca. 1 V) lower surface potentials (and excess electronic surface charges) characterizing the electrochemical interface.

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.

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Abstract:The reaction entropies lSo,, of a number of transition metal redox couples of the form M(III)/( 11) in aqueous solution have bcen determined using nonisothermal electrochemical cells in order to explore the effect of varying the ligand structure upon the nature of the ion-solvent interactions. Examination of six aquo couples of the form M ( O H Z ) , ,~+ '~+ with varying metal M yielded ASo,, values in the range 36-49 eu. In order to scrutinize the effect of replacing aquo with ammine and simple anionic ligands, R u ( l I I ) / ( I I ) couples were employed since the relativc substitution inertness of both oxidation states allowed AS',, to be determined using cyclic voltammetry. The stepwise replacement of aquo by ammine ligands results in substantial reductions in ASo,, which are attributed to the smaller extent of ligand-solvent hydrogen bonding for ammine compared with aquo ligands. Substitution of both aquo and ammine by anionic ligands also resulis in substantial reductions in ASo,,. A number of M(III)/(II) couples containing chelating ligands were also examined. Sizable differcnces in ASo,, were found between Co(lll)/(ll) couples and the corresponding R u ( l l l ) / ( l l ) and Fe(lll)/(ll) couples. Suggested explanations are differences in ligand conformation and electron delocalization effects. The possible contribution of outer-sphere solvent structuring effects to the large reorganization energies observed for electron exchange of aquo complexes is noted. The validity of the assumptions required for the estimation of ASo,, from nonisothermal cell measurements is discussed.

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