Article

Study of CO Oxidation on Polycrystalline Pt Electrodes in Acidic Solution by ATR-SEIRAS

The Journal of Physical Chemistry C (Impact Factor: 4.84). 07/2011; 115:16378-16388. DOI: 10.1021/jp104181y

ABSTRACT 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 H2Ofree coexisted structure (or simply costructure) forms only at a sufficiently high global CO coverage (H2Ofree denotes hydrogen-bonding-broken water); In contrast, the CO@DL and H2Ofree costructure forms in an earlier adsorption phase, less dependent on the global CO coverage. The partial oxidation of CO from solution and weakly adsorbed COL at the active sites is suggested to yield a prepeak that occurs with the relaxation of the COad-H2Ofree 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 COad 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 H2Ofree costructured islands. The H2Ofree can be better assigned to the “probe” of the local COad coverage rather than the main oxygenated species for COad oxidation according to the spectral results for both CO@UPD and CO@DL.

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    ABSTRACT: The effect of the electrode potential in the reactivity of platinum stepped single crystal electrodes with (111) terraces toward CO oxidation has been studied. It is found that the CO adlayer is significantly affected by the potential at which the adlayer is formed. The electrochemical and FTIR experiments show that the adsorbed CO layer formed in acidic solution at 0.03 V vs SHE is different from that formed at −0.67 V vs SHE in alkaline solutions. The major effect of the electrode potential is a change in the long-range structure of CO adlayer. The adlayer formed in alkaline media presents a higher number of defects. These differences affect the onset and peak potential for CO stripping experiments. The stripping voltammogram for the adlayer formed at −0.67 V vs SHE always shows a prewave and the peak potential is more negative than that observed for the adlayer formed at 0.03 V vs SHE. This means that the apparent higher activity for CO oxidation observed in alkaline media is a consequence of the different CO adlayer structure on the (111) terrace, and not a true catalytic effect. The different behavior is discussed in terms of the different mobility of CO observed depending on the electrode potential. Also, the FTIR frequencies are used to estimate the pzc (potential of zero charge) for the Pt(111) electrode covered with a CO adlayer.
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