Infrared band intensity-coverage

Some characteristics of, and comparisons between, surface-enhanced Raman spectroscopy (SERS) and infrared reflection-absorption spectroscopy (IRRAS) for examining reactive as well as stable electrochemical adsorbates are illustrated by means of selected recent results from our laboratory. The differences in vibrational selection rules for surface Raman and infrared spectroscopy are discussed for the case of azide adsorbed on silver, and used to distinguish between "flat" and "end-on" surface orientations. Vibrational band intensity-coverage relationships are briefly considered for some other systems that are unlikely to involve coverage-induced reorientation. 

Comparisons Between Corresponding Surface Raman and Infrared Spectra Band Intensity-Coverage Relationships... 

The most recent fairly comprehensive review Of the vibrational spectra of transition metal carbonyls is contained in the book by Braterman1. This provides a literature coverage up to the end of 1971 and so the subject of the present article is the literature from 1972 through to the end of 1975. Inevitably, some considerable selectivity has been necessary. For instance, a considerable number of largely preparative papers are not included in the present article. Tables A-E provide a general view of the work reported in the period. Table A covers spectral reports and papers for which topics related purely to vibrational analysis are not the main objective. Papers with the latter more in view are covered in Table C. Evidently, the division between the two is somewhat arbitrary. Other tables are devoted to papers primarily concerned with the spectra of crystalline samples — Table B — to reports of infrared and Raman band intensities — Table D and sundry experimental techniques or observations - Table E. Papers on matrix isolated species, which are covered elsewhere in this volume, are excluded. 

Other workers in this field were critical of the concept put forward by A. Kiselev and Zhdanov. Sidorov (Leningrad State University) (151, 153), in infrared spectroscopic studies of water adsorption at a low degree of coverage, had observed that the intensity of the absorption band due to the free surface OH groups (voh = 3750 cm-1) remained unchanged. Similar experiments were carried out by other researchers. It was concluded by Sidorov and by V. Kiselev and co-workers (Moscow State University, Physics Department) (164-166) that the initially free H2O molecules are adsorbed not on OH group sites but on more active centers that they called centers of the Il-type. 

In a practical sense the effects described in the foregoing discussion place a severe limitation on the applicability of spectral studies of adsorbed molecules to the detailed elucidation of the adsorption process and of the stereochemistry involved in surface catalysis. Since the absorption intensity may be either enhanced or decreased as a result of adsorption on a surface, and may either increase or decrease with variation in surface coverage, it becomes very difficult indeed to use spectral data as a measure of the surface concentration of adsorbed species. This is of particular importance when more than one species occupies the surface e.g., physisorbed and chemisorbed species. In this case the absolute concentration of either species on the surface cannot be measured directly nor can it be reliably inferred from a comparison of the intensity of the bands corresponding to these two species. Moreover, in the identification of an adsorbed species the relative intensities of two or more bands characteristic of that species e.g., the CH stretching and the CH deformation frequencies for adsorbed hydrocarbons, cannot be used as evidence for the structure of the adsorbed species since the absorption coefficients of the individual bands may change in opposite directions as a function of surface coverage. Thus the relative intensities of such bands cannot be compared to the relative intensities of the same bands observed in solution or in the gas phase. A similar difficulty arises when attempts are made to use the electronic spectra of adsorbed molecules to complement the infrared spectra for identification purposes. 

Fourier transform infrared spectroscopy (FTIR) is a powerful technique to probe real-time adsorbed surface species (reactants, intermediates, products) and solution constituents due to selected molecular dipole bond vibrations induced by tuned incident radiation [100]. FTIR has been used to study the formic acid electrooxidation reaction mechanism in situ by stepping or scanning the potential where species of interest are generated, from either high potentials where the intermediate species are completely oxidized (a clean surface, >1 V vs. RHE) or low potentials where the intermediate species approaches the coverage limit (blocked surface, <0.05 V vs. RHE) [100]. The three observed reaction intermediates for formic acid electrooxidation are linearly bonded COl, bridge-bonded COb, and bridge-bonded formate (HCOOad) with vibrational bands at 2,052-2,080 cm 1,810-1,850 cm , and 1,320 cm , respectively [27, 98]. The vibration frequencies of the adsorbates are influenced by the electronic characteristics and electrochemical potential of the electrode surface. Additional peaks of lesser intensity are observed for the water adlayer and sulfate/bisulfate at the electrode interface [27, 98]. 

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