Surface characterization spectroscopic techniques

Most of the published promotional kinetic studies have been performed on well defined (single crystal) surfaces. In many cases atmospheric or higher pressure reactors have been combined with a separate UHV analysis chamber for promoter dosing on the catalyst surface and for application of surface sensitive spectroscopic techniques (XPS, UPS, SIMS, STM etc.) for catalyst characterization. This attempts to bridge the pressure gap between UHV and real operating conditions. 

Another technologically important reaction is the Fischer-Tropsch synthesis, with iron oxide being one of the components of some catalysts. A detailed understanding of the complex mechanism of this reaction can be obtained by studying the chemisorption of simple molecules on well-characterized surfaces by means of advanced surface-sensitive spectroscopic techniques. A few investigations of the interaction of small molecules (such as CO, CO2, H2O, O2, H2, and NO) (520-522) and organic molecules on iron oxide surfaces (523-527) have been carried out. 

Due to the surface sensitivity surface enhanced fluorescence has become particularly popular in the characterisation of thin molecular films, such as Langmuir-Blodgett films and self-assembled biomembranes. Two surface enhanced spectroscopic techniques (surface enhanced IR absorption, SEIRA, and surface enhanced fluorescence, SEF) were recently applied to the study of biomembrane systems by the group of Reiner Salzer [323]. With SEIRA, specific fingerprints of biomolecules could be obtained with a tenfold IR intensity enhancement With SEF signal enhancement factors greater than 100 were obtained. The enhancement factor was very dependent on the properties and structure of the metal clusters used. With the two techniques biomembranes formed from vesicles with embedded nicotinic acetylchoHne receptors were spectroscopically characterized. 

Characterization of ionic Liquid Surfaces by Spectroscopic Techniques... 

A beautiful and elegant example of the intricacies of surface science is the formation of transparent, thermodynamically stable microemulsions. Discovered about 50 years ago by Winsor [76] and characterized by Schulman [77, 78], microemulsions display a variety of useful and interesting properties that have generated much interest in the past decade. Early formulations, still under study today, involve the use of a long-chain alcohol as a cosurfactant to stabilize oil droplets 10-50 nm in diameter. Although transparent to the naked eye, microemulsions are readily characterized by a variety of scattering, microscopic, and spectroscopic techniques, described below. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

The studies discussed above deal with highly dispersed and therefore well-defined rhodium particles with which fundamental questions on particle shape, chemisorption and metal-support interactions can be addressed. Practical rhodium catalysts, for example those used in the three-way catalyst for reduction of NO by CO, have significantly larger particle sizes, however. In fact, large rhodium particles with diameters above 10 nm are much more active for the NO+CO reaction than the particles we discussed here, because of the large ensembles of Rh surface atoms needed for this reaction [28]. Such particles have also been extensively characterized with spectroscopic techniques and electron microscopy we mention in particular the work of Wong and McCabe [29] and Burkhardt and Schmidt [30], These studies deal with the materials science of rhodium catalysts that are closer to the ones used in practice, which is of great interest from an industrial point of view. 

Sorption processes are influenced not just by the natures of the absorbate ion(s) and the mineral surface, but also by the solution pH and the concentrations of the various components in the solution. Even apparently simple absorption reactions may involve a series of chemical equilibria, especially in natural systems. Thus in only a comparatively small number of cases has an understanding been achieved of either the precise chemical form(s) of the adsorbed species or of the exact nature of the adsorption sites. The difficulties of such characterization arise from (i) the number of sites for adsorption on the mineral surface that are present because of the isomorphous substitutions and structural defects that commonly occur in aluminosilicate minerals, and (ii) the difference in the chemistry of solutions in contact with a solid surface as compound to bulk solution. Much of our present understanding is derived from experiments using spectroscopic techniques which are able to produce information at the molecular level. Although individual methods may often be applicable to only special situations, significant advances in our knowledge have been made... 

While in situ techniques encompass all characterization/spectroscopic methods that can be used to probe the surface chemistry of an operating practical catalyst, its entirety is too large to cover in any real detail. Therefore the methods covered in this review were chosen based on their prevalence and use in the field and the potential for significant observations during reaction cycles. Some methods, such as ATR, TAP and catalytic shock tube, were chosen based on the potential of these methods and the likelihood that they will become more widely used as they are integrated with evolving spectroscopic techniques. 

It is imperative to monitor the details of chemical reactions at the molecular level, which operando data can contribute to immensely through the use of spectroscopy. Spectroscopic techniques have been used in the past mostly to characterize fresh or used catalysts, obtaining structural information relating to the bulk and surface of the solids. In addition, on-line gas analysis of... 

Spectroscopic techniques are extremely useful for the characterization of filler surfaces treated with surfactants or coupling agents in order to modify interactions in composites. Such an analysis makes possible the study of the chemical composition of the interlayer, the determination of surface coverage and possible coupling of the filler and the polymer. This is especially important in the case of reactive coupling, since, for example, the application of organofunctional silanes may lead to a complicated polysiloxane interlayer of chemically and physically bonded molecules [65]. The description of the principles of the techniques can be found elsewhere [15,66-68], only their application possibilities are discussed here. 

In the last three decades, we have designed and successfully prepared various supported metal complexes on oxide surfaces that exhibit unique catalytic activities and selectivities that are different from those of their homogeneous analogues [3,4,9, 12-15]. With the aid of several sophisticated spectroscopic techniques, the structures and roles of catalytically active species on surfaces have been characterized and identified [3, 4,9,12-25]. Chemical interactions between metal complexes and oxide surfaces can provide new reactivity of metal species by the construction of a spatially controlled reaction environment and the formation of unsaturated active metal species, leading to high catalytic activity, selectivity and durability [21-25]. 

In the next sections we introduce some of the most important experimental techniques of surface characterization. For the interested reader, a broad range of books on this topic is available (e.g. Refs. [346,347]). We start by discussing microscopy, continue with diffraction, and finally focus on spectroscopic methods. 

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