Comments on the paper by M.-S. Zheng and S.-G. Sun entitled ‘In situ FTIR spectroscopic studies of CO adsorption on electrodes with nanometer-scale thin films of ruthenium in sulfuric acid solutions’ [J. Electroanal. Chem. 500 (2001) 223

Abstract

It has been claimed that so-called abnormal infrared effects (AIREs) occur in the infrared reflectance spectra of particulate metals deposited on both moderately reflecting (e.g. glassy carbon) and highly reflecting (e.g. smooth Pt) substrates upon chemisorption on the metal particles of IR-absorbing molecules, typically CO. These AIREs would be characterized by: (i) increase, instead of decrease, of the IR reflectance at the absorption frequency of the chemisorbed molecule, producing anti-absorption bands (band inversion) (ii) enhancement of the intensity of the IR bands of the chemisorbed molecule (iii) increase of the full width at half-maximum (FWHM) of the absorption band of the chemisorbed molecule. It is proved here that these results need not be assigned to any new abnormal effects, but, on the contrary, are well-known phenomena. (i) Band inversion is perfectly explained by Fresnel's reflectance equations. (ii) The intensity increase of the IR bands is due to SEIRA (surface enhanced infrared absorption), a phenomenon discovered in 1980, and due to the large electric field increase produced between metal particles dispersed in a matrix. (iii) The somewhat higher FWHM of CO adsorbed on particulate as compared with massive metals can well be accounted for by the sensitivity of this parameter to changes in adsorbent structure. In conclusion, no new abnormal effects have to be invoked in order to explain the reported behaviour of CO and other molecules adsorbed on particulate metals deposited on moderately reflecting substrates, a category which also includes highly reflecting metals covered by a suitable layer of metal particles.