THE INFLUENCE OF CATION SIZE UPON THE INFRARED-SPECTRUM OF CARBON-MONOXIDE ADSORBED ON PLATINUM-ELECTRODES

The infrared spectrum of carbon monoxide adsorbed on polycrystalline platinum electrodes in the presence of acetonitrile solutions is examined. Acetonitrile solutions provide a wide double layer region (approximately 4 V) in which to investigate the observed potential dependence of the infrared peak position. The wider window allows for better comparisons of experimental data with theoretical predictions than those which are available from measurements conducted in aqueous solutions. It is found that (i) near-monolayer surface coverage can be obtained from solutions saturated with CO, (ii) the coverage stays constant over roughly the entire double layer region, (iii) the CO adsorbs to the Pt surface exclusively in the linear bonded site, (iv) the position of the CO infrared band is linearly dependent upon the applied electrode potential, and (v) the rate of change of the peak position versus potential decreases with increasing size of the electrolyte cation. These observations indicate that the cation size alters the position of the outer Helmholtz plane to the extent that the double layer electric field strength is changed, consistent with the Gouy-Chapman-Stern model of the double layer and the electrochemical Stark effect description of the infrared behavior. For higher homologues of the tetra-n-alkylammonium cations (n > 6), the rate of change of the infrared peak position with potential does not continue the observed trend. This result is explained by the various conformations that the longer alkyl chains can assume to minimize the hydrodynamic radius of the cation. These compact conformations are thought to result in no appreciable change in hydrodynamic size of the cation over shorter chain lengths; hence, no change in the position of the outer Helmholtz plane.