Spectroscopic techniques, studies surface adsorption

Catalytic events such as adsorption, breaking, or bond formation are related to the surface composition of the catalyst. Understanding the surface composition is fundamental to the comprehension of the catalytic system. This section will develop this aspect by considering (1) chemisorption studies and use of molecular probes, for example, in connection with the use of Fourier transform IR spectroscopy, (2) the role of hydrogen and H2S, and (3) the use of spectroscopic techniques for surface characterization. 

Secondary Ion Mass Spectroscopic Studies of Adsorption and Reaction at Metal Surfaces Correlations with Other Surface-Sensitive Techniques... 

The adsorption of Intact molecules Is encountered In many areas of electrochemistry. A complete description of the adsorbed state In terms of the orientation of the molecule, the way In which It bonds to the surface, the perturbation of the molecular structure caused by this additional bonding and the Interaction between adjacent molecules Is the ultimate goal of spectroscopic techniques. As more systems are studied by the EMIRS and SNIFTIRS methods, ways are being found to assess more of this Information. 

Infrared spectroscopy can be considered as the first important modem spectroscopic technique that has found general acceptance in catalysis. The most common application of infrared spectroscopy in catalysis is to identify adsorbed species and to study the way in which these species are chemisorbed on the surface of the catalyst. In addition, the technique is useful in identifying phases that are present in precursor stages of the catalyst during its preparation. Sometimes the infrared spectra of adsorbed probe molecules such as CO and NO give valuable information on the adsorption sites that are present on a catalyst. 

Spectroscopic techniques may provide the least ambiguous methods for verification of actual sorption mechanisms. Zeltner et al. (Chapter 8) have applied FTIR (Fourier Transform Infrared) spectroscopy and microcalorimetric titrations in a study of the adsorption of salicylic acid by goethite these techniques provide new information on the structure of organic acid complexes formed at the goethite-water interface. Ambe et al. (Chapter 19) present the results of an emission Mossbauer spectroscopic study of sorbed Co(II) and Sb(V). Although Mossbauer spectroscopy can only be used for a few chemical elements, the technique provides detailed information about the molecular bonding of sorbed species and may be used to differentiate between adsorption and surface precipitation. 

In addition to the indirect experimental evidence coming from work function measurements, information about water orientation at metal surfaces is beginning to emerge from recent applications of a number of in situ vibrational spectroscopic techniques. Infrared reflection-absorption spectroscopy, surface-enhanced Raman scattering, and second harmonic generation have been used to investigate the structure of water at different metal surfaces, but the pictures emerging from all these studies are not always consistent, partially because of surface modification and chemical adsorption, which complicate the analysis. 

Some very important surface properties of solids can be properly characterized only by certain wet chemical techniques, some of which are currently under rapid improvement. Studies of adsorption from solution allow determination of the surface density of adsorbing sites, and the characterization of the surface forces involved (the energy of dispersion forces, the strength of acidic or basic sites and the surface density of coul-ombic charge). Adsorption studies can now be extended with some newer spectroscopic tools (Fourier-transform infra-red spectroscopy, laser Raman spectroscopy, and solid NMR spectroscopy), as well as convenient modern versions of older techniques (Doppler electrophoresis, flow microcalorimetry, and automated ellipsometry). 

The development of modem surface science techniques over the past decade has allowed for the investigation of numerous properties of adsorption at the atomic level. These techniques offer information about the electrode surface and the molecular identity of the species adsorbed at the electrode surface. These techniques have been used to study the electrode surface however, many require a gas phase or ex situ environment as summarized in Table 1. XAS including the two complimentary parts, x-ray absorption near edge stracture (XANES) and extended x-ray absorption fine stmc-ture (EXAFS) are in situ/operando spectroscopic techniques that can provide unique site-specific adsorbate bonding and particle size information. Further, these techniques do not suffer from the adsorption by the bulk electrolyte such as in the IR region. X-ray Diffraction (XRD) of course also provides detailed stractural... 

Study of the modification of solid surfaces requires, preferably, surface sensitive methods. Spectroscopic techniques, for example X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopy are excellent tools for gathering information on the chemical surface composition and the kind and number of functional surface groups. The fact that the carbon and nitrogen containing organic phase is only introduced during the adsorption procedure and locally fixed on the outside of the particles allows the use of established methods for polymer and solid-state characterization, particularly NMR and solid-state NMR spectroscopy (e.g. 13C CP MAS NMR). 

A second type of adsorption is called chemisorption. In this case, the adsorption energy is comparable to the chemical bond energies and adsorbate molecules have the tendency to be localized at particular sites even though surface diffusion or some molecular mobility may still occur. Due to the chemical nature of the interactions between the gas and the solid surface, the equilibrium gas pressure in the adsorption system can be extremely low. This enables one to study the adsorbent-adsorbate system under high vacuum using diffraction and spectroscopic techniques for the identification of the actual species presented on the surface and the determination of their packing and chemical state. 

Other mixed catalysts (79), indicating that the lowest exchange current density (and catalyst activity) is reached when the d band is filled. Currently, little information exists on the surface structure, metal interactions, and adsorption characteristics of bimetallic clusters in an electric field. Recently developed electron spectroscopic techniques and comparison with similar studies on conventional mixed catalysts (774, 775) could shed some light on the catalytic action of bimetallic electrodes. 

From the above discussion it becomes apparent that some conflicting experimental evidence exists on hydrocarbon adsorption and on surface intermediates. This arises primarily from the use of electrocatalysts of varying histories and pretreatments. It should be stressed that many adsorption studies were performed on anodically pretreated platinum. The removal of surfaces oxides from such electrodes may have not been always accomplished when the surface was cathodically reduced in some experiments, as outlined in Section IV,D. Obviously, different surface species could exist on bare or on oxygen-covered electrocatalysts. Characterization of surface structure and activity and of adsorbed species using modern spectroscopic techniques would provide useful information for fuel cell and selective electrocatalytic oxidations and reductions. 

Understanding of the structure of the adsorbed surfactant and polymer layers at a molecular level is helpful for improving various interfacial processes by manipulating the adsorbed layers for optimum configurational characteristics. Until recently, methods of surface characterization were limited to the measurement of macroscopic properties like adsorption density, zeta-potential and wettability. Such studies, while being helpful to provide an insight into the mechanisms, could not yield any direct information on the nanoscopic characteristics of the adsorbed species. Recently, a number of spectroscopic techniques such as fluorescence, electron spin resonance, infrared and Raman have been successfully applied to probe the microstructure of the adsorbed layers of surfactants and polymers at mineral-solution interfaces. 

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