In situ spectroscopic methods

The application of eUipsometry for investigating optical properties and film thickness has been known for a long time. This method is based on the measurement of changes of the state of polarization during reflection from the film covered metal surface. The method has become more widely applicable through the development of computer, because the evaluation of data (thickness, refraction, and absorption index) from the polarization parameters (intensity and phase changes) is tedious and complicated. An example for the application of this method will be shown in Chapter 10. 

Spectroscopic reflectance methods are UV/vis reflectance spectroscopy and infrared reflection absorption spectroscopy (IRRAS) with several variations. For the application of these methods a mirror-Uke electrode surface is needed. This can be avoided if the scattered 

A wide range of spectroelectroehemical techniques has been developed to study solution-phase and surface electrochemistry. UV/visible spectroscopy can be performed either in transmission mode if the substrate electrode is transparent or in reflectance mode if the substrate is opaque and sufficiently reflective. Changes in the composition of the surface or solution that are a consequence of electrode reactions can be followed by monitoring the absorption or reflectance. Diode array spectro- 

The introduction of in-situ infrared spectroscopy to electrochemistry has revolutionised the study of metal/electrolyte interfaces. Modnlation or sampling techniques are applied in order to enhance sensitivity and to separate snrface species from volume species. Methods such as EMIRS (electrochemicaUy modulated IR spectroscopy) and SNIFTIRS (subtractively normalised interfacial Fonrier Transform infrared spectroscopy) have been employed to study electrocatalytic electrodes, for example. There have been surprisingly few studies of the semiconductor/electrolyte interface by infrared spectroscopy. This because up to now little emphasis has been placed on the molecnlar electrochemistry of electrode reactions at semiconductors because the description of charge transfer at semiconductor/electrolyte interfaces is derived from solid-state physics. However, the evident need to identify the chemical identity of snrface species should lead to an increase in the application of in-situ FTIR. 

FTIR has been used to study the sihcon/electrolyte interface. The formation of porous silicon on n-Si during photoetching in fluoride media can readily be followed since the hydrogen-terminated surface is identified by the Si-H stretch bands centred around 2100 cm (Peter et ah, 1989 Peter et ah, 1990a). Similarly, the transition from a hydrogen-terminated to an oxide-covered surface during electropolishing has been followed by in-situ infrared spectroscopy (da Fonseca et al, 1996 and 1997). 

In principle, IR methods can be used to monitor photogenerated carriers in the same way as microwave reflectance methods. However, PMC can detect changes in carrier density as low as 10 cm, whereas the sensitivity of IR is typically six orders of magnitude worse. 

In-situ luminescence measurements have been used to study the semiconductor/ electrolyte interface for many years (e.g. Petermann et al., 1972). Luminescence may result from optical excitation of electron/hole pairs that subsequently combine with the emission of light (photoluminescence). Alternatively, minority carriers injected from redox species in the electrolyte can recombine with majority carriers and give rise to electroluminescence. The review by Kelly et al. (1999) summarises the main features of photoluminescence (PL) and electroluminescence (EL) at semiconductor electrodes. The experimental arrangements for luminescence measurements are relatively straightforward. Suitable detectors include a silicon photodiode placed close to the sample, a conventional photomultiplier or a cooled charge-coupled silicon detector (CCD). The CCD system is used with a grating spectrograph to obtain luminescence spectra. 

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The reaction cycle discussed is generally accepted for unmodified cobalt and unmodified rhodium catalysts. But it has to be stressed here that to date no one has been able to prove the single steps conclusively it is still a subject of research, with modern techniques like in situ spectroscopic methods and molecular modeling in conjunction with kinetic investigations. 

A better understanding of the theoretical background is another key for progress. A basic requirement for a more rational design of catalyst systems is to get more insight into the complex mechanism. The tools are in situ spectroscopic methods under reaction conditions. 

N.K. Mehta, J. Goenaga-Polo, S.P Hemandez-Rivera, D. Hernandez, M.A. Thomson and P.J. MeUing, Development of an in situ spectroscopic method for cleaning validation using mid-IR fiber optics. Spectroscopy, 18(4), 14 (2003). 

In situ spectroscopic measurements of a catalytic system provide a considerable opportunity to determine the chemical species present under reactive conditions. FTIR and NMR have been the two most frequently used in situ spectroscopic methods (see Chapters 2 and 3). They have been successfully used to identify labile, non-isolatable transient species believed to be involved in the catalytic product formation. Furthermore, efforts have been made to use this information in order to obtain more detailed kinetics, by decoupling induction, product formation, and deactivation. Thus, in situ spectroscopic techniques have the potential for considerably advancing mechanistic studies in homogeneous catalysis. 

With a basic mechanism at hand, a rational approach for designing catalysts with desired properties becomes possible. However, despite progress in the direct observation of surface intermediates using high pressure, realistic in situ spectroscopic methods and deeper insight into basic reaction processes, the capability of rationally designing an electrocatalytic surface with a set of desired properties has not yet fully been achieved. 

For these researchers, transients are not merely helpful but essential. Because each method has limitations, it is desirable to use two and even three transient methods for one reaction. Rotating disk and microelectrode techniques and the steady-state methods, summarized in Table 7., may be added to the armory. In the background are the developing in situ spectroscopic methods, which, if their time of operation can be made short enough,15 may eventually do some of the things the transient methods purport to achieve. For reactions with intermediates, spectroscopic methods may eventually offer more information than do transients, even though some of these are oriented to give information on intermediates. 

Recent advances in the development of non-invasive, in situ spectroscopic scanned-probe and microscopy techniques have been applied successfully to study mineral particles in aqueous suspension (Hawthorne, 1988 Hochella and White, 1990). In situ spectroscopic methods often utilise molecular probes that have diagnostic properties sensitive to changes in short-range molecular environments. At the particle-solution interface, the molecular environment around a probe species is perturbed, and the diagnostic properties of the probe, which can be either optical or magnetic, then report back on surface molecular structure. Examples of in situ probe approaches that have been used fruitfully include electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin-probe studies perturbed vibrational probe (Raman and Fourier-transform IR) studies and X-ray absorption (Hawthorne, 1988 Hochella and White, 1990 Charletand Manceau, 1993 Johnston et al., 1993). 

The activation of catalysts has been studied frequently, but mainly empirically and less systematically. Very often, in particular in academic research, the activation is performed under less defined conditions with respect to temperature control and gas-phase environment. Instead of parameter variation based on trial and error, the application of powerful standard techniques, like thermal analysis or temperature-programmed reaction, and the complementary use of in situ spectroscopic methods will contribute to a deeper understanding of activation processes. 

For many oxidation reactions, the presence of ionic liquids appears to have a beneficial effect in that increased reaction rates are observed and stabilisation of catalytically active intermediates in these solvents has been demonstrated by means of in situ spectroscopic methods in several cases. And while the oxygen solubility is very poor in common ionic liquids solid oxidants are often dissolved more efficiently, resulting in increased reaction rates, ever under biphasic conditions. 

In order to improve the fuel utilization in a Direct Alcohol Fuel Cell (DAFC) it is important to investigate the reaction mechanism and to develop active electrocatalysts able to activate each reaction path. The elncidation of the reaction mechanism, thus, needs to combine pnre electrochemical methods (cyclic voltammetry, rotating disc electrodes, etc.) with other physicochemical methods, such as in situ spectroscopic methods (infrared and UV-VIS" reflectance spectroscopy, or mass spectroscopy such as EQCM, DEMS " ), or radiochemical methods to monitor the adsorbed intermediates and on line chromatographic techniques"" to analyze qnantitatively the reaction products and by-products. 

H-beta zeolite proved to be an active and selective catalyst for alkylation of benzene with propene. In situ spectroscopic methods were applied to follow the formation and the evolution of surface intermediates and products,. It was found that when benzene is taken alone on the zeolite surface, its adsorption is reversible up to 473 K. On the contrary propene undergoes to several transformations even at 295 K. Isopropylbenzene behaves as propene, giving the same intermediates and products by decomposition at higher temperatures. Isopropyl cations formed upon chemisorption of propene on Broensted acid sites are the key intermediates for the alkylation reaction and are responsible for the faster deactivation via unsaturated caibenium ions formation. 

Cu electrodes electrochemically produced CH4, CgH and alcohols from COg and CO in aqueous media at high current densities, and Ni electrodes at lower current densities.[3-7] Fe, not active in COj reduction, can reduce CO to hydrocarbons.[8] It is thus interesting to reveai intermediate species on these eiectrodes in the electrochemical reduction of CO2 and CO by in-situ spectroscopic method. 

A key requirement for in-situ spectroscopic methods in these systems is surface specificity. At Uquid/Uquid junctions, separating interfacial signals from the overwhelmingly large bulk responses in linear spectroscopy is not a trivial issue. On the other hand, non-Unear spectroscopy is a powerful tool for investigating the properties of adsorbed species, but the success of this approach is closely linked to the choice of appropriate probe molecules (besides the remarkably sensitivity of sum frequency generation on vibrational modes of water at interfaces). This chapter presents an overview of linear and non-linear optical methods recently employed in the study of electrified liquid/liquid interfaces. Most of the discussion will be concentrated on the junctions between two bulk liquids under potentio-static control, although many of these approaches are commonly employed to study liquid/air, phospholipid bilayers, and molecular soft interfaces. 

DUA Duarte, A.R.C., Anderson, L.E., Duarte, C.M.M., and Kazarian, S.G., A comparison between gravimetric and in situ spectroscopic methods to measitre the sorption of CO2 in a biocompatible polymer, J. Supercrit. Fluids, 36, 160, 2005. 

The development of in situ spectroscopic methods for electrochemical applications was a major step to extend the experimental arsenal of electrochemistry. A description and comparison of the different methods can be found in the literature. ... 

With this brief introduction to this technique, we now look into some examples where these in situ spectroscopic methods have been used effectively to elucidate the mechanism of heterogeneously catalyzed selective aerobic oxidation of benzyl alcohol to benzaldehyde, and in the identification of the active sites of supported metal catalysts. 

The active site responsible for the aerobic oxidation of alcohols over Pd/AljO, catalysts has long been debated [96-lOOj. Many reports claim that the active site for this catalyst material is the metallic palladium based on electrochemical studies of these catalysts [100, 101]. On the contrary, there are reports that claim that palladium oxide is the active site for the oxidation reaction and the metalhc palladium has a lesser catalytic activity [96,97). In this section, we present examples on how in situ XAS combined with other analytical techniques such as ATR-IR, DRIFTS, and mass spectroscopic methods have been used to study the nature of the actual active site for the supported palladium catalysts for the selective aerobic oxidation of benzylic alcohols. Initially, we present examples that claim that palladium in its metallic state is the active site for this selective aerobic oxidation, followed by some recent examples where researchers have reported that ojddic palladium is the active site for this reaction. Examples where in situ spectroscopic methods have been utilized to arrive at the conclusion are presented here. For this purpose, a spectroscopic reaction cell, acting as a continuous flow reactor, has been equipped with X-ray transparent windows and then charged with the catalyst material. A liquid pump is used to feed the reactants and solvent mixture into the reaction cell, which can be heated by an oven. The reaction was monitored by a transmission flow-through IR cell. A detailed description of the experimental setup and procedure can be found elsewhere [100]. Figure 12.10 shows the obtained XAS results as well as the online product analysis by FTIR for a Pd/AljOj catalyst during the aerobic oxidation of benzyl alcohol. 

Microspectroscopy. In situ spectroscopic methods are essential to a fundamental understanding of catalytic reactions, thanks to their ability to unravel useful structure-function relationships. However, most of these techniques average information over the whole catalyst sample, whereas in many cases it is of utmost importance to probe distinct areas of catalyst particles or grains to reveal how the structural features render into catalytic function. For this purpose, microscopic methods may be of assistance. The following section lists the examples of application of in situ microspectroscopic methods in catalysis. 

Many combined setups have been developed in the past decades to study catalyst synthesis and reaction processes, many of which employ synchrotron radiation. Perhaps the first example of a successful combination of two techniques is X-ray absorption spectroscopy (XAFS) and diffraction, which was soon followed by the combination of SAXS and WAXS. Other examples in which two techniques have been combined to study systems under reaction include XAFS/FTIR, XRD/Raman, XAFS/UV-Vis, and a number of setups that use non-X-ray based radiation, such as UV-Vis/Raman, FTIR/UV-Vis, NMR/UV-Vis, and EPR/UV-Vis. A number of reports have recently appeared in which the number of combined techniques has been increased to three, including SAXS/WAXS/XAFS, UV-Vis/Raman/XAFS (148), and EPR/UV-Vis/Raman (218), and SAXS/WAXS/XAFS (219). In what follows, we illustrate the power three-in-one in situ spectroscopic methods to unravel chemistry of catalytic solids. 

The main characteristics of in situ spectroscopic methods are given in Table 1.1. Each spectroscopic technique has its own strengths and weaknesses, which determine its utility for studying additives directly in the polymeric matrix. The applicability depends on the identity of the particular additive and polymer matrix, on concentration and amount of sample available, analysis time desired, and need for quantitation. Polymers for which no solvent can be found present analytical difficulties, especially if appreciable amounts of fillers or additives are present. In favourable cases, rapid additive analyses can be carried out without extensive pretreatment steps, i.e. without extraction by UV spectrometry [la], NMR [2] or UV desorption/mass... 

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