In situ FTIR spectroscopic studies of adsorption of CO, SCN-, and poly(o-phenylenediamine) on electrodes of nanometer thin films of Pt, Pd, and Rh: Abnormal infrared effects (AIREs)
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Pt, Pd, and Ph films of a few nanometers in thickness supported on glassy carbon (GC) and other substrates were prepared by electrochemical voltammetry. STM patterns illustrated that the prepared thin films are composed of crystallites of layer structure and exhibit a low surface roughness. Studies of in situ FTIR spectroscopy on chemisorption of CO and SCN- and formation of a polymer of o-phenylenediamine (POPD) on electrodes of nanometer thin films have been conducted to explore the abnormal infrared effects (AIREs), which consist of two main characteristics: (1) inversion of IR bands; (2) the enhancement of IR absorption of adsorbates. The results demonstrated that the AIREs depend mainly on the structure and the chemical nature of nanometer thin films. In all cases of chemisorption on thin films of platinum-group metals supported on GC or supported on polymer-covered GC, the direction of IR bands of adsorbates is inverted in comparison with the direction of IR bands of the same adsorbates on corresponding massive metal electrodes. The IR absorption of adsorbed CO species (COad) on nanometer thin films of Ph, Pt, and Pd supported on GC has been enhanced respectively by a factor of 11, 20, and 26. The fact that the IR absorption of adsorbed CO and SCN- has been enhanced but the IR absorption of POPD has not suggested that the IR absorption enhancement in AIREs is related to an effect of short;range domain of surface. The results in the present paper demonstrated also that the AIREs belong to a new phenomenon of IR reflection spectroscopy and relate to effects of material at the nanometer scale. The present study manifests remarkable advantages of AIREs for studying surface processes and may contribute considerably to fundamental studies of electrocatalysis and reflection spectroscopy.