Origin of the infrared reflectance increase produced by the adsorption of CO on particulate metals deposited on moderately reflecting substrates

Potential-difference IR spectra of CO chemisorbed on noble metal electrodes have, if the two potentials are within the stability range of chemisorbed CO, a bipolar shape, since the frequency of the absorption maximum of the CO stretching band increases with increasing electrode potential. Therefore, in an IR spectrum with the reference at the lower potential the higher frequency lobe of the bipolar band is negative. An anomalous bipolar band with the opposite shape was first reported in 1994 for CO chemisorbed on Pt particles deposited on a basal plane graphite disc, and the anomaly was attributed to a reversible potential-induced migration of the chemisorbed CO molecules from Pt terraces to edge or kink sites. Anomalous bipolar bands were later found for other metals deposited on glassy carbon, and it was observed that the anomalous shape was due to the fact that the adsorbed CO produced an increase, instead of a decrease, of the IR reflectance at the frequency of the absorption maximum. On the basis of our observation of the band also with s-polarized radiation, which shows that the metals surface selection rule cannot be applied to glassy carbon, it is shown that this reflectance increase is due simply to the use of a glassy carbon substrate, which absorbs moderately, and therefore reflects IR radiation only moderately. Consequently, adsorption of a strong IR absorber, such as CO, will increase the reflectivity of glassy carbon. (C) 1999 Elsevier Science S.A. All rights reserved.