Infrared spectral comparison of electrochemical carbon monoxide adlayers formed by direct chemisorption and methanol dissociation on carbon-supported platinum nanoparticles

A comparison is made between electrochemical infrared reflection-absorption spectra for carbon monoxide adlayers formed on carbon-supported platinum nanoparticle films by direct chemisorption from solution CO and via methanol dissociation. In addition to the importance of the C/Pt materials as electrocatalysts in methanol-based fuel cells, clarifying the nature of the extent of CO formation from the latter solute is motivated by the use of methanol as a source of chemisorbed CO at these and related interfaces. As in previous studies, commercial C/Pt materials were employed, having metal loadings from 10 to 60%, corresponding to nanoparticle diameters from ca. 2 to 9 nm. Ultrathin C/Pt films with excellent infrared as well as voltammetric characteristics were prepared by physical deposition onto gold. Absolute C-O stretching (nu(CO)) bands as a function of electrode potential, CO coverage, and nanoparticle size were obtained upon solution CO dosing, as usual, by subtracting a reference spectrum measured following CO electrooxidation. Such "absolute" absorbance spectra, however, could not readily be obtained in the presence of methanol solute, obliging the utilization instead of "bipolar" potential-difference infrared (PDIR) spectra where the reference spectrum is also acquired within the potential region where the adlayer is stable. The interpretation of such bipolar PDIR data is complicated for the present systems by the broad asymmetric shape of the component nu(CO) bands, exacerbated for thicker films by their anomalous optical properties. Nevertheless, a detailed understanding of the methanol PDIR spectra, including interpretation of unexpected potential-dependent band intensities and complex bipolar band shapes, was achieved by comparison with corresponding absolute as well as bipolar spectra from solution CO dosing. This analysis, along with cyclic voltammetry, also indicates that the methanol dissociation yields only intermediate-coverage CO adlayers. These spectral findings invite a reassessment of some conclusions from an earlier infrared study (Rice, C.; Tong, Y. Y.; Oldfield, E.; Wieckowski, A.; Hahn, F.; Gloaguen, F.; Leger, J.-M.; Lamy, C. J. Phys. Chem. B 2000, 104, 5803), including a comparison of C-13 NMR data for the C/Pt-methanol systems in relation to the infrared properties.