Article pubs.acs.org/JPCC
Solvation Effects on OH Adsorbates on Stepped Pt Surfaces Ryosuke Jinnouchi, Akihiro Nagoya, Kensaku Kodama, and Yu Morimoto* Toyota Central R&D Laboratories, Inc., 41-1 Yokomichi Nagakute, Aichi 480-1192, Japan S Supporting Information *
ABSTRACT: Density functional theory calculations were applied to OH formations on stepped Pt electrodes of Pt[n(111) × (111)] (n = 3, 4, and 6) for examining solvation effects on the OH adsorbates. Results indicated that OH adsorbates at terrace sites are slightly destabilized by water molecules adsorbed at step sites forming 1-dimensional water chains whereas OH adsorbates at step sites are significantly destabilized by water molecules adsorbed at terrace sites forming 2-dimensional honeycomb structures. On stepped Pt surfaces with narrow terrace widths, water molecules cannot exist at terrace sites, and therefore, the solvation effects on OH adsorbates at step sites disappear. Hence, OH adsorbates are formed at step sites at a low potential region, ca. 0.3 V (standard hydrogen electrode (SHE)). When high-coverage CO adsorbates are present on the stepped Pt surfaces, water molecules cannot exist at the terrace sites either because strongly bound CO molecules exclude the water molecules. In such conditions, OH formation potentials decrease significantly, too. Thermodynamic stabilities of OH adsorbates are, therefore, controlled not only by the local surface morphology but also by long-ranged interfacial solvation environments. In other words, the stability and presence/absence of OH adsorbates should be considered to be totally different with water adsorbates (like in inert conditions) and without them (like in the CO oxidation).
1. INTRODUCTION
current peaks A and A′ generated by hydrogen desorptions and adsorptions at (111)-terraces,1,11,12,21
Electrocatalytic activities of Pt depend strongly on thermodynamic stabilities of reaction intermediates and spectators.1 Activities of hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR), for example, are dominated by the stability of hydrogen adsorbates,2,3 and activities of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are dominated by the stability of hydroxyl group and/or oxygen adsorbates.2,4−6 Activities of CO oxidation reaction (COR) and alcohol oxidations are also considered to be influenced strongly by the stability of hydroxyl adsorbates as well as CO adsorbates.7−10 To clarify mechanisms underlying the observed trends in catalytic activities, a correct understanding of adsorbate stabilities is essential. For obtaining information on adsorbate stabilities, cyclic voltammetry is a reliable experimental method. From peak potentials and their temperature dependences, enthalpies and entropies of adsorbates can be accurately obtained,11−13 and by using well-defined single-crystal surfaces, morphological and compositional dependences of thermodynamic stabilities can be systematically examined.14−20 Voltammograms, however, depend on surface morphologies and compositions in a highly complex manner in many cases, and reaction mechanisms are difficult to understand. The difficulty is exemplified by voltammograms on Pt[n(111) × (111)] surfaces shown in Figure 1(I) or in other literature,14,15 where Pt[n(111) × (111)] surfaces comprise (111)-terraces with n atomic rows inserted between (110)-oriented steps as shown in Figure 1(II). When the terrace width n is decreased, © XXXX American Chemical Society
H+(aq) + e− ↔ H(ad)
(R1)
are shifted toward lower potential regions, and current peaks B and B′ generated by hydroxyl formations and removals,11,13,18,22,23 H 2O(aq) ↔ OH(ad) + H+(aq) + e−
(R2)
are shifted toward higher potential regions, where (aq) and (ad) in reactions R1 and R2 indicate chemical species in aqueous and adsorbed phases, respectively. It should be noted also that peak shoulders C and C′ evolve rapidly when the terrace width n becomes
