Mechanistic Insights into Electro-Oxidation of Solution CO on the Polycrystalline Gold Surface as Seen by in Situ IR Spectroscopy

Carbon monoxide (CO) adsorption and electro-oxidation on a gold (Au) ultrathin film deposited onto a silicon prism infrared (IR) window in a CO-saturated (≈1 mM) 0.1 M HClO 4 supporting electrolyte were investigated by in situ electrochemical attenuated-total-reflection (ATR) surface-enhanced IR reflection absorption spectroscopy (SEIRAS). By varying the reaction environment with sequential (CO and N 2 ) purging of the supporting electrolyte and the Au surface morphology with CO annealing, we were able to assign adsorbed CO to terrace-like and step-like sites to deconvolute the corresponding time- and potential-dependent IR spectra. The results of these spectral deconvolutions suggest strongly that in the CO-saturated supporting electrolyte the weakly bound CO interacted mainly with the strongly adsorbed CO on the step-like sites and likely formed a dipolar-coupled weak interacting pair with the latter. Model ab initio density functional theory (DFT) calculations confirm the existence of the weakly bound CO only over a CO monolayer adsorbed on the step-like Au sites. The weakly bound CO was also identified as the active reaction intermediate for CO oxidation reaction (COR) in the CO-saturated acidic supporting electrolyte and therefore was proposed to be largely responsible for the high COR activity frequently observed on Au electrodes.