FT-IRRAS

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

Figure 1. FT-IRRAS double modulation spectrum of six monolayers of Cadmium Arachidate on silver. The inset shows the background water vapor and carbon dioxide present in the measurement. IBM Instruments IR/90 series.

To achieve surface selectivity in our studies of low area metal catalyzed reactions, we have developed FT-IRRAS techniques for the in situ study of surface adsorbates. With these methods, we are making progress toward examination of complex reactions. 

Preliminary results using fixed polarizer FT-IRRAS to study the melting and hydration of cadmium arachidate on low area metals show that both irreversible and reversible conformational changes can be... 

Kunimatsu K, Golden WG, Seki H, Philpott MR. 1985a. Carbon monoxide adsorption on a platinum electrode studied by polarization modulated FT-IRRAS. 1. Co Adsorbed in the double-layer potential region and its oxidation in acids. Langmuir 1 245 -250. 

FIG. 4 FT-IRRAS spectrum in midfrequency region for gold substrate treated with 5 -TsTsTsTsTsTCTCATACATG-3. The i-polarized radiation was introduced on the sample at 85° off the surface normal and data were collected at a spectral resolution of 4 cm with 256 scans. 

Figure 2.4 FT-IRRAS spectrum of the electrode/electrolyte interface for CO adsorbed qn a smooth platinum electrode in 0.5 M H2SOA at 0.4 V vs. NHE. From W.G. Golden, K. Kunimatsu and H. Seki, J. Phys. Chem., 88 (1984) 1275. Copyright 1984 American Chemical Society.

Figure2.21 FT-IRRAS spectraforthe H2O band 1648cm (PAH-Os(ll))i5(PSS)i5. Numbers indicate the oxidation-reduction cycle number. Taken from Ref. [153].

Polyvinylimidazoles are effective anti-oxidants for copper at elevated temperatures. Below 250° C, there is no major degradation of the coated polyvinylimidazole films on copper. Furthermore, degraded polyvinylimidazole films can suppress oxidation even at 400 C. Finally, polyvinylimidazoles are more effective anti-oxidants than benzotriazole and imidazoles at elevated temperatures. It is also demonstrated that FT-IRRAS is a useful technique to study degradation of polymeric coatings and corrosion of metal simultaneously. 

FT-IRRAS Fourier transform infrared reflection-absorption sepctroscopy... 

Fig. 32. FT-IRRAS spectra for adsorbed CN on polycrystalline Ag (a) and Au (b) electrodes in 0.1 M KCN+0.5M K2SO4. Adsorption potentials as indicated. (Taken from [111]). Reprinted by permission of Elsevier Science...

Fig. 33. FT-IRRAS spectra of the adsorbed CN on a polycrystalline Cu electrode at different adsorption potentials. Experimental conditions as indicated in the figure. (After [116]). Reprinted by permission of the Electrochemical Society.

The adsorption of cyanide on Pd electrodes was studied by using a combination of polarization and potential modulation (FT-IRRAS and SNIFTIRS) [124]. The reason for this combination is to enhance the surface signal, since two FT-IRRAS spectra taken at two different potentials were ratioed to obtain the SNIFTIRS spectrum. Despite this effort, a solution band at 2135 cm" (also observed with s-polar-ized light) persists in the spectrum (Fig. 36). The spectrum in this Figure taken with p-polarized radiation presents, however, a strong band at 1980 cm", which was attributed to a bridged-bonded cyanide ion, and a weak band at 2065 cm" assigned to linearly adsorbed C-down CN". 

Fig. 40. FT-IRRAS of adsorbed thiocyanate ions on Ag at 0.0 V vs. Ag/Ag+ in solutions of various eoncentrations as indieated. (Taken from [133]). Reprinted by permission of the American Chemical Society.

Fig. 50. Block diagram of instrumentation used for FT-IRRAS experiment.

Fig. 51. FT-IRRAS spectrum of the electrode/solution interface for CO adsorption on a smooth platinum electrode in IN H2S04 at 0.4 V (NHE).

Fig. 53. Change of the FT-IRRAS spectrum with potential of the Ag/O.lN KCN solution interface. All the spectra are referred to the CN -free 0.5 M Kj,S04 solution.

SNIFTRS Subtractively Interfacial Fourier Transform Infra-Red Spectroscopy PM-FT-IRRAS Polarization Modulated Fourier Transform Infra-Red Reflection Absorption Spectroscopy... 

The basic principle of the PM-IRRAS method (Fig. 4.1) is to combine the FT-IRRAS experimental conditions with a fast modulation of the polarization state of the incident electric field (ideally between p and s hnear states) and to extract from the detected intensity (using electronic filtering and demodulation) the two signals (Rp-Rg) and (Rp-t-RJ in order finally to compute the differential reflectivity spectrum AR/R [18]. 

The controlled deposition of ruthenium on well-defined surfaces, such as Pt(hkl) [95-103] and Au hkl) [38-40], has been characterized by electrochemical measurements, Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS), XPS and STM measurements. The interest in these studies is mainly concentrated on the ruthenium modification of a platinum surface because of its extreme importance in electrocatalysis. It has been demonstrated that a ruthenium-deposited Pt( 111) substrate showed an extremely high activity in methanol oxidation compared to ruthenium-deposited Pt(hkl) electrodes with other crystallographic orientations [98, 99]. 

Mateo-Marti and Pradier investigated the chemical modifications occurring within a tripeptide molecule (Glu-Pro-Glu IGF), when exposed to the influence of UV irradiation. The authors described the successful inclusion of the IGF molecule in an argon matrix, followed by the in situ UV irradiation of the sample and subsequent characterization of the compound photo-reactivity. The studies have been performed by combining, Fourier-transform reflexion absorption spectroscopy (FT-IRRAS) and X-ray photoelectron spectroscopy (XPS). The comparison of the IR spectra of the isolated oligopeptide in an argon matrix obtained before and after UV irradiation, revealed significant differences that could be associated either to a partial deprotonation of the molecule or to a tautomeric conversion of amide bonds to imide ones. XPS analyses undoubtedly confirmed the second hypothesis (amido-imido tautomerism Fig. 22). 

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