Adsorption of Acetate on Au(111): An in‐situ Scanning Tunnelling Microscopy Study and Implications on Formic Acid Electrooxidation

The adsorption of acetate on an Au(111) electrode surface in contact with acetic acid at pH 2.7 was imaged in‐situ using scanning tunnelling microscopy (STM). Two different ordered structures were imaged for acetate adsorbed in the bidentate configuration on the unreconstructed 1×1 surface at 0.95 V (vs. the saturated calomel electrode, SCE). The first structure,(19×19)R23.45∘ , is metastable and transforms at constant potential within 20 minutes to a (2×2) structure, which is thermodynamically more favourable. The (2×2) acetate adlayer starts to form at step edges and propagates via nucleation and growth onto terraces. The findings from in‐situ STM are in agreement with the electrochemical behaviour of acetate on Au(111) characterized by voltammetry. A comparison is made with formate adsorption on Au(111). While acetate is not reactive, in contrast to formate, it can act as a spectator species in formic acid electrooxidation. The phase transition within an acetate adlayer on Au(111) is studied by in‐situ scanning tunnelling microscopy. First, a (√19×√19)R23.45° structure is formed, which subsequently transforms into a (2×2) structure. Formic acid oxidation is enabled in the presence of mixed acetate+formate adlayers. Unreactive acetate and reactive formate compete for catalytic sites.