An in Situ Time-Dependent Study of CO Oxidation on Pt(111) in Aqueous Solution by Voltammetry and Sum Frequency Generation

In situ time-dependent measurements using voltammetry and sum frequency generation (SFG) were carried out to study CO oxidation on a Pt(111) electrode. The scan-rate dependence of CO oxidation was measured by changing the scan rate from 50 to 0.5 mV/s. While the basic shape of the CO stripping curves was unaffected by sweep rate, the oxidation potential of CO was lowered by 0.1 V, from 0.84 to 0.74 V. The measurements of SFG spectra and CO stripping were carried out simultaneously to clarify the discrepancy between SFG and CO stripping measurements reported in earlier SFG studies. Our results showed that the SFG peak attributed to the CO stretch vanished immediately after the CO stripping current reached its maximum value. In the preignition potential region, the frequency shift of the CO stretch observed by SFG agrees with that observed by IR spectroscopy. Additionally, in the ignition potential region, a significant blue shift of CO stretching frequency was observed. For the first time, a 360% enhancement of SFG intensity coinciding with the red shift of stretching frequency was observed in the preignition potential region. While it was not observed by IR spectroscopy, it shows similar behavior to limited Raman spectroscopy measurements obtained from a roughened Pt surface.