Adsorption of carbon monoxide on a smooth palladium electrode: an in-situ infrared spectroscopic study

Adsorption of carbon monoxide on a smooth palladium electrode in 1 M HClO/sub 4/ saturated with CO was studied by two in-situ IR reflectance spectroscopic methods: EMIRS (electrochemically modulated infrared reflectance spectroscopy) and LPSIRS (linear potential sweep infrared reflectance spectroscopy). Two types of adsorbed CO, linear and bridged, were identified from the observed IR spectra, the latter being the predominant surface species. The C-O stretching frequency of the linear CO shifts to higher frequencies at more positive potentials with a slope of 48 cm/sup -1//V. The frequency of the bridged CO increases by 63 cm/sup -1/ between -0.5 and 0.9 V(NHE) and its integrated band intensity decreases continuously in the same potential region while the intensity of the linear CO is almost constant up to 0.1 V and then decreases gradually with increasing positive potential. The surface selection rule of the IR reflection absorption spectroscopy was tested for the present system by using the p- and s-polarized light. It was found that only p-polarized light gave the IR spectra of CO adsorbed on the palladium electrode thus proving the selection rule at the electrode/solution interface.