Study of CO oxidation on polycrystalline Pt electrodes in acidic solution by ATR-SEIRAS

Adsorption and electro-oxidation of CO on a polycrystalline Pt electrode in acidic solutions were systematically revisited by in situ attenuated-total- reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) in conjunction with related Gram-Schmidt response analysis. CO was either adsorbed in the double-layer region, i.e., 0.45 V (RHE) (denoted as CO_@DL) or in the hydrogen underpotential deposition region, i.e., 0.1 V (RHE) (denoted as CO_@UPD). The results indicate that the CO_@UPD and H ₂O_free coexisted structure (or simply costructure) forms only at a sufficiently high global CO coverage (H₂O_free denotes hydrogen-bonding-broken water); In contrast, the CO_@DL and H₂O_free costructure forms in an earlier adsorption phase, less dependent on the global CO coverage. The partial oxidation of CO from solution and weakly adsorbed CO_L at the active sites is suggested to yield a prepeak that occurs with the relaxation of the CO_ad-H ₂O_free costructure and the disorganization of the outer water net layers. In the main oxidation process, the oxidation of CO _@UPD tends to proceed via the "mean-field approximation" kinetics due to the high CO_ad mobility resulting from the oxidation prepeak. The oxidation process of CO_@DL is, however, likely via the "nucleation and growth" kinetics due to the good stability of the local CO_@DL and H₂O_free costructured islands. The H₂O_free can be better assigned to the "probe" of the local CO_ad coverage rather than the main oxygenated species for CO_ad oxidation according to the spectral results for both CO_@UPD and CO_@DL.