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 HClO4 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 N2) 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.