NEXAFS technique

Obviously, it is important to extend the fundamental information, obtained on the single crystal model surfaces, to the more realistic powder materials. In principle, the comparison of the electronic properties of these two systems can be achieved by using the near-edge X-ray absorption fine structure (NEXAFS) technique, which can be applied to investigate the properties of both single crystals and powder materials. The catalytic properties of the model and powder carbide materials can also be compared if the relevant reactions are carried out for both systems. 

In this paper, we will report the electronic and catalytic reactivities of the model VC/V(110) surface, and our attempt to extend them to VC powder catalysts. By using high-resolution electron energy loss spectroscopy (HREELS) and NEXAFS techniques, we observed that the surface properties of V(110) could be significantly modified by the formation of vanadium carbide some of the experimental results on these model surfaces were published previously.3-5 We will discuss the selective activation of the C-H bond of isobutane and the C=C bond of isobutene on V(110) and on VC/V(110) model systems. These results will be compared to the catalytic performances of vanadium and vanadium carbide powder materials in the dehydrogenation of isobutane. 

The metal-polymer studies were done in collaboration with C.A. Kovac, M.J. Goldberg, J.F. Morar and R.A. Poliak. The polymer-metal studies were done in collaboration with S.P. Kowalczyk. I wish to thank J. Stohr and A. Hitchcock for many enlightening discussions on NEXAFS techniques and interpretation and for timely access to their data throughout the course of these studies. Research was carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. 

For further structural characterization, mono- and multilayer films of the various mixtures were transferred onto hydrophobic and hydrophilic (oxidized) platinum substrates. It was determined that higher surface pressures, (about 20mN/m) and higher ratio mixtures of 3HDP resulted In the most stable monolayers, which could be transferred facilely onto the platinum substrates. Near Edge X-Ray Absorption Fine Structure (NEXAFS) technique using synchrotron radiation, were used,to study the... 

FTIR measurements can be seen to complement the NEXAFS technique, which has unequivocally proved to be extremely sensitive and effective in estimating molecular order. 

In Chapter 7, the Raman and near-edge X-ray absorption fine structure (NEXAFS) techniques have been used by Sandi to investigate the electronic and structural properties of carbonaceous materials and those of electrodes made from the synthesized carbons. The electrochemical performance of the carbon anodes is compared and related to the electronic and structural features of the SEI layer. 

By combining the near-edge X-ray absorption fine structure (NEXAFS) technique and first principles calculations, Kunisu etal. [143]first studied thelocal environment of Mn dopant in ZnO. In polycrystalline bulk ZnO NEXAFS at Mn K-edge was found to remain in the same shape as shown in Figure 5.23 up to 5%. The experimental spectra is reproduced well by first principles calculations of (Zn,Mn)0 sohd solution, which confirmed that Mn substituted at the Zn sites. For Mn concentration higher than 5%, Mn304 precipitates started to appear. 

The NEXAFS-technique is mainly used for deriving two different types of information. First, the spectroscopical data are useful in determining the electronic structure of an adsorbate. In the case of acetylene (C2H2), for example, adsorption on a Cu-surface leads to a splitting of the two tr-levels, which are degenerated in the gas-phase. The technique can thus be used to obtain information about the electronic structure and is complementary to ultraviolet photoelectron spectroscopy (UPS) in the sense that unoccupied states are observed. The main advantage over UPS stems from the fact that in NEXAFS... 

The NEXAFS technique is element specific and represents interactions with the nearest neighbor atoms. This property can be used to determine the orientation and electronic properties of individual moieties in a multicomponent system. For example, the sulfur L edge spectra in polythiophene systems or the nitrogen K edge spectra in polypyrrole... 

It is expected that future studies wiU probe molecular adsorption with an increased level of sophistication and further appHcation to non-UHV systems, such as high-pressure gas phase and the soUd-Hquid environments, using the fluorescence yield detection method are anticipated. It has been shown that the NEXAFS technique is particularly powerful when combined with other experimental probes as it provides information that can be difficult to extract using crystallographic methods that require a degree of long-range order in the system under study. 

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