Surfacing techniques

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

Depth sensitivity is an equally important consideration in the analysis of surfaces. Techniques based on the detection of electrons or ions derive their surface sensitivity from the fact that these species cannot travel long distances in soflds without undergoing interactions which cause energy loss. If electrons are used as the basis of an analysis, the depth resolution will be relatively shallow and depend on both the energy of the incident and detected electrons and on characteristics of the material. In contrast, techniques based on high energy photons such as x-rays will sample a much greater depth due... 

The use of hard-surfacing techniques by welding and by inserts has contributed greatly to better maintenance and lower downtime [Lutes and Reid, Chem. Eng., 63(6), 243 (1956)]. 

Instruments combining XPS with one or more additional surface techniques are not uncommon. Such combinations use up relatively litde extra space but cost... 

P. R. Griffiths and J. A. de Haseth. Fourier Transfrsrm Infrared Spectrometry John Wiley Sons, New York, 1986. Chapters 1—8 review FTIR equipment in considerable detail. Chapters 9-19 describe applications, including surface techniques (Chapter 17). 

SALI is a reladvely new surface technique that delivers a quantitative and sensitive measure of the chemical composition of solid surfaces. Its major advantage, compared to its parent technique SIMS, is that quantitative elemental and molecular informadon can be obtained. SPI offers exciting possibilities for the analytical characterization of the surfaces of polymers and biomaterials in which chemical differ-endation could be based solely on the characteristic SALE spectra. 

Although SSMS cannot be considered a surface technique due to the 1-5 rm penetration of the spark in most materials, few other techniques can provide a trace elemental survey analysis of surfaces consisting of films or having depths of interest... 

The SSMS point-to-plane surface technique has been shown to be particularly useful in the survey analysis of epitaxial films, heavy metal implant contamination, diflRision furnace contamination, and deposited metal layers. 

After XPS, AES is the next most widely used surface-analytical technique. As an accepted surface technique AES actually predates XPS by two to three years, because the potential of XPS as a surface-specific technique was not recognized immediately by the surface-science community. Pioneering work was performed by Harris [2.125] and by Weber and Peria [2.126], but the technique as it is known today is basically the same as that established by Palmberg et al. [2.127]. 

Because of the inherently destructive nature of ion bombardment, the use of SSIMS alone in the study of the reactions of surfaces with gases and vapor must be viewed with caution, but in combination with other surface techniques it can provide valuable additional information. The parallel techniques are most often XPS,TDS, and LEED, and the complementary information required from SSIMS normally refers to the nature of molecules on surfaces and with which other atoms, if any, they are combined. 

Besides synthesis, current basic research on conducting polymers is concentrated on structural analysis. Structural parameters — e.g. regularity and homogeneity of chain structures, but also chain length — play an important role in our understanding of the properties of such materials. Research on electropolymerized polymers has concentrated on polypyrrole and polythiophene in particular and, more recently, on polyaniline as well, while of the chemically produced materials polyacetylene stih attracts greatest interest. Spectroscopic methods have proved particularly suitable for characterizing structural properties These comprise surface techniques such as XPS, AES or ATR, on the one hand, and the usual methods of structural analysis, such as NMR, ESR and X-ray diffraction techniques, on the other hand. 

P Messner, UB Sleytr. In IC Hancock, I Poxton, eds. Bacterial Cell Surface Techniques. Chichester, UK Wileys, 1988, pp 97-104. 

Secondary ion mass spectrometry (SIMS) is by far the most sensitive surface technique, but also the most difficult to quantify. When a surface is exposed to a beam of ions (Ar", 0.5-5 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection (desorption) of atoms, ions, and multi-atomic clusters. In SIMS, positive or negative secondary ions are detected directly with a quadrupole mass spectrometer. 

The examples of the model studies presented show how the meshing of modern surface techniques with reaction kinetics can provide valuable Insights Into the mechanisms of surface reactions and serve as a useful complement to the more traditional techniques. Close correlations between these two areas holds great promise for a better understanding of the many subtleties of heterogeneous catalysis. 

HREELS and TFD have played a unique role In characterizing the surface chemistry of systems which contain hydrogen since many surface techniques are not sensitive to hydrogen. We have used these techniques to characterize H2S adsorption and decomposition on the clean and (2x2)-S covered Ft(111) surface (5). Complete dissociation of H,S was observed on the clean Ft(lll) surface even at IlOK to yield a mixed overlayer of H, S, SH and H2S. Decomposition Is primarily limited by the availability of hydrogen adsorption sites on the surface. However on the (2x2)-S modified Ft(lll) surface no complete dissociation occurs at IlOK, Instead a monolayer of adsorbed SH Intermediate Is formed (5) ... 

Beck, L., S. Bosonnet, S. Reveillon, D. Eliot, and F. Pilon (2004), Silver surface enrichment of silver-copper alloys A limitation for the analysis of ancient silver coins by surface techniques, Nuclear Instrum. Meth. (B) 226(1-2), 153-162. 

Mark A. Barteau is Robert L. Pigford Professor and Chair of the Department of Chemical Engineering at the University of Delaware. He received his B.S. degree from Washington University in 1976 and his M.S. (1977) and Ph.D. (1981) from Stanford University. His research area is chemical engineering with specialized interests in application of surface techniques to reactions on nonmetals, hydrocarbon and oxygenate chemistry on metals and metal oxides, scanning probe microscopies, and catalysis by metal oxides. 

Comparison of STM data with those derived from other surface techniques is essential... 

Techniques based on the interaction of ions with solids, such as SIMS and LEIS, have undoubtedly been accepted in catalyst characterization, but are by no means as widely applied as, for example, XPS or XRD. Nevertheless, SIMS, with its unsurpassed sensitivity for many elements, may yield unique information on whether or not elements on a surface are in contact with each other. LEIS is a surface technique with true outer layer sensitivity and is highly useful for determining to what extent a support is covered by the catalytic material. RBS is less suitable for studying catalysts but is indispensable for determining concentrations in model systems, where the catalytically active material is present in monolayer-like quantities on the surface of a flat model support. 

SIMS is by far the most sensitive surface technique, but also the most difficult one to quantify. SIMS is very popular in materials research for making concentration depth profiles and chemical maps of the surface. The principle of SIMS is conceptually simple A primary ion beam (Ar+, 0.5-5 keV) is used to sputter atoms, ions and molecular fragments from the surface which are consequently analyzed with a mass spectrometer. It is as if one scratches some material from the surface and puts it in a mass spectrometer to see what elements are present. However, the theory behind SIMS is far from simple. In particular the formation of ions upon sputtering in or near the surface is hardly understood. The interested reader will find a wealth of information on SIMS in the books by Benninghoven et al. [2J and Vickerman el al. [4], while many applications have been described by Briggs et al. [5]. 

In principle, reactions can be carried out in UHV similarly to the proton formation reaction. By performing experiments in UHV rather than in situ one can manipulate the interface with a high level of precision, and the interface can be probed using the surface techniques mentioned in 4.3, which cannot be used in situ. In practice, the low temperature requirement of UHV and the lack of bulk solution are obstacles not easily overcome.50... 

With the work still in the infant stages, there is no accepted method of modeling electrode reactions with DFT. A few recent studies have attempted to include both electrostatic and solvent effects in DFT models of electrochemical reactions using different approaches.84-89 However, the lack of surface techniques available for in situ study of electrochemical cells hinders validation of models by experimental data. Results can only offer qualitative information at best. Despite the challenges, DFT modeling of electrochemical reactions offers promise as a method for providing insights into the electrochemical interface in cases where experiments are difficult. 

Applications of Surface Techniques to Chemical Bonding Studies of Minerals... 

During the last several years, a number of new instrumental surface techniques have been developed that are quite effective in detecting changes in the surfaces of minerals that have undergone chemically induced or natural geologic alteration. These techniques are quite sensitive (approximately 0.1-0.5% atomic concentration for x-ray photoelectron and Auger spectroscopy, for example), and they make it possible to monitor very small amounts of elements that may be present in the near surface material. Any change in the surface with respect to chemical composition may readily be measured qualitatively... 

Even though the vacuum-oriented surface techniques yield much useful information about the chemistry of a surface, their use is not totally without problems. Hydrated surfaces, for example, are susceptible to dehydration due to the vacuum and localized sample heating induced by x-ray and electron beams. Still, successful studies have been conducted on aquated inorganic salts (3), water on metals (3), and hydrated iron oxide minerals (4). Even aqueous solutions themselves have been studied by x-ray photoelectron spectroscopy (j>). The reader should also remember that even dry samples can sometimes undergo deterioration under the proper circumstances. In most cases, however, alterations in the sample surface can be detected by monitoring the spectra as a function of time of x-ray or electron beam exposure and by a careful, visual inspection of the sample. 

X-ray photoelectron spectroscopy (also called electron spectroscopy for chemical analysis, or ESCA) is a surface technique that can be used to detect elements qualitatively (and quantitatively, in some cases) in the surface layers of solids, as well as the chemical states (species) of the elements. The basic experimental apparatus for performing XPS studies includes an x-ray source (most commonly,... 

X-ray photoelectron spectroscopy can be performed on any sample that is vacuum compatible, i.e., does not decompose under vacuum in the range 10 - 10 10 torr. In most cases, the technique does little or no damage to the sample surface, although x-ray-induced chemical reactions such as those discussed above may occur. As a surface technique, it gives a signal that mirrors approximately the top 15 to 40A of the surface (18). 

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