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)

Pt, Pd, and Ph films of a few nanometers in thickness supported on glassy c arbon (GC) and other substrates were prepared by electrochemical voltammetr y. STM patterns illustrated that the prepared thin films are composed of cr ystallites 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 f ilms have been conducted to explore the abnormal infrared effects (AIREs), which consist of two main characteristics: (1) inversion of IR bands; (2) t he enhancement of IR absorption of adsorbates. The results demonstrated tha t the AIREs depend mainly on the structure and the chemical nature of nanom eter thin films. In all cases of chemisorption on thin films of platinum-gr oup metals supported on GC or supported on polymer-covered GC, the directio n 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 d omain of surface. The results in the present paper demonstrated also that t he AIREs belong to a new phenomenon of IR reflection spectroscopy and relat e to effects of material at the nanometer scale. The present study manifest s remarkable advantages of AIREs for studying surface processes and may con tribute considerably to fundamental studies of electrocatalysis and reflect ion spectroscopy.