Detection of Adsorbed Species

The creation of two types of chemisorbed oxygen on Pt and Ag surfaces subject to NEMCA conditions is clearly shown by cyclic voltammetry (Fig. 5.27)319,50,52 or by the similar Potential-Programmed-Reduction (PPR) 

Note that upon increasing the time of 02 desorption, denoted tHe, the peak A, corresponding to chemisorbed O, decreases much faster than the back-spillover oxygen peak B. Similarly upon increasing the holding time, tH, of positive potential application (UWr=0.2 V) peak A reaches saturation first, followed by a gradual approach to saturation of the backspillover oxygen peak B. 

The thickness 51 of a cyclic voltammogram at a fixed UWR value also conveys useful information. It is related to the scan rate u and to the capacitance Cd of the electrode-electrolyte interface via  

One can thus use the voltammograms of Figs. 5.27 and 5.28 to estimate Cd values of the order of200 pF/cm2 of solid electrolyte. 

If only the three-phase-boundaries (tpb) were electrocatalytically active one would expect Cd values of the order of 10 pF/cm2. The thus measured high Cd values also provide evidence that the charge transfer zone is extended over the entire gas-exposed electrode surface, i.e. that an effective double layer is formed over the entire gas exposed electrode surface. 

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A strength of ATR spectroscopy in heterogeneous catalysis is the opportunity it affords for detection of adsorbed species while the catalyst is working. Detection is... 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

Characteristic Reactions and the Detection of Adsorbed Species by a Radiotracer Method... 

Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

By the total internal reflection condition at the liquid-liquid interface, one can observe interfacial reaction in the evanescent layer, a very thin layer of a ca. 100 nm thickness. Fluorometry is an effective method for a sensitive detection of interfacial species and their dynamics [10]. Time-resolved laser spectrofluorometry is a powerful tool for the elucidation of rapid dynamic phenomena at the interface [11]. Time-resolved total reflection fluorometry can be used for the evaluation of rotational relaxation time and the viscosity of the interface [12]. Laser excitation can produce excited states of adsorbed compound. Thus, the triplet-triplet absorption of interfacial species was observed at the interface [13]. 

Haneda et al. [134,135] studied the formation and reaction of adsorbed species in NO reduction by propene over Ga203-Al203. IR transient reaction technique was employed to examine the reactivity and dynamic behaviour of surface species. The catalyst was first exposed to either C3H6/02/Ar or NO/Oz/Ar at 623 K for a long time to form and accumulate the surface species. The catalyst was further purged with pure Ar and the reaction gas then switched to various gas mixtures. Changes in the intensity of IR bands were measured with time on stream. The main surface species detected by IR during... 

Sensitivity how to detect the tiny absorptions from a monolayer of adsorbed species when conventional detectors were noisy and conventional IR sources weak. 

The coordination chemist may be interested in the electrosynthesis of compounds, the generation and detection of unstable species in unusual oxidation states and the study of their mechanisms of decay or their spectroscopic properties, or in simply obtaining thermodynamic data. These, and related topics such as using electrogenerated metallo intermediates to catalyze the transformation of inert molecules, modifying the properties of an electrode surface by adsorbing or otherwise binding a coordination compound to it, or fundamental aspects of electron-transfer kinetics, are readily studied by the application of modem electrochemical techniques. 

The initial role of vibrational spectroscopy is to suggest the types of functional groups that are present. In favorable cases, this can lead on to the detection of particular surface species through the recognition of their more complete spectral patterns. Subsequently, when structural calibration has been achieved or confirmed by diffraction methods, vibrational spectra provide much the most efficient means of exploring the incidence of the various types of adsorbed species on a wide range of surfaces. 

The experimental detection and quantitation of surface species on soil particles and other natural colloids is a difficult area of research because of sample heterogeneity, low surface concentrations and the need to investigate solid adsorbents in the presence of liquid water. Unambiguous information about the molecular structure and stability of adsorbed species can be obtained only with in situ surface spectroscopy (see also Chapter 3). Invasive spectroscopic methods that require sample desiccation or high-vacuum techniques (e.g. electron microscopy, X-ray... 

Through the analysis of adsorption isotherms and 129Xe NMR results of the co-adsorbed xenon, we have shown that the dispersal of benzene molecules depends on not only the cation distribution but also the amount of benzene adsorbate within the supercage of zeolite adsorbents. We have also demonstrated for the first time that this well known indirect technique has the capability not only to probe the macroscopic distribution of adsorbate molecule in zeolite cavities but also to provide dynamic information about the adsorbate at the microscopic level. Conventional H and 13C NMR which directly detect the adsorbate species, although providing complimentary results, are relatively less sensitive. 

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