Spectroscopy absorption, particle-specific

As with other diffraction techniques (X-ray and electron), neutron diffraction is a nondestructive technique that can be used to determine the positions of atoms in crystalline materials. Other uses are phase identification and quantitation, residual stress measurements, and average particle-size estimations for crystalline materials. Since neutrons possess a magnetic moment, neutron diffraction is sensitive to the ordering of magnetically active atoms. It differs from many site-specific analyses, such as nuclear magnetic resonance, vibrational, and X-ray absorption spectroscopies, in that neutron diffraction provides detailed structural information averaged over thousands of A. It will be seen that the major differences between neutron diffraction and other diffiaction techniques, namely the extraordinarily... 

Modern atomic theory received a shot in the arm when it was recognized that the individual atom has light absorption and emission spectra occurring at narrow lines of the spectrum at specific wavelengths, as opposed to the broad bands typical of the polyatomic molecules and compounds. Since the line spectrum of each element is characteristic of that element, atomic spectroscopy can be used for precise elementary analysis of many types of chemically simple and complex materials. These studies make use of the wave character of light, as well as light s particle character. 

The other approach is to study real catalysts by using in-situ techniques such as infrared and Mossbauer spectroscopy, extended X-ray absorption fine structure (EXAFS) and XRD, either under reaction conditions, or - as occurs more often -under a controlled environment after quenching the reaction. These in-situ techniques, however, are usually not sufficiently specific to yield the desired atom-byatom characterization of the surface, and often they determine the overall properties of the particles. The situation is represented schematically in Figure 1.8. 

These experiments demonstrate that tunable diode laser absorption spectroscopy is well suited for in situ measurements of species concentrations in combustion flows, when a line-of-sight technique is appropriate, and for accurate measurements of spectroscopic parameters needed to characterize high-temperature absorption lines. The technique is sensitive, species specific and applicable to a large number of important combustion species including reactive intermediates, and hence it should prove to be a useful tool in future studies of combustion chemistry. The potential of tunable laser absorption spectroscopy in particleladen flows should also be noted (12), in that modulation of the laser wavelength on and off an absorption line allows simple discrimination against continuum extinction by particles. 

For many years there was no sufficiently specific method for the identification of characteristic GSRs. One could not see metallic particles because of their small size (5-50 pm) and their presence was ascertained indirectly by means of colouring chemical reactions or such instrumental methods as atomic absorption spectroscopy (AAS), neutron activation analysis (NAA) or XRF. These methods, however, are... 

Metal-free phthalocyanine (H2PC) exists in the a, P, r, and X polymorphic forms. The a,P, and X forms have been characterized by electrochemistry [36a], fluorescence spectroscopy [36b], and optical absorption [36c] and a,p, and t forms by C-NMR spectroscopy [36d], optical absorption, and X-ray powder diffraction [36e]. Figure 19 shows the optical absorption spectra of the four forms. The t form has been further characterized as to particle shape (Types I and II) [36f]. The as-synthesized y9-H2Pc is converted to a-H2Pc by acid pasting and this can be converted to the X or T forms with mechanical milling under specific conditions. The X and r forms convert to the P with thermal, solvent, or extended mechanical treatment. 

The interfacial aqueous coordination chemistry of natural particles, in particular their surface complexation reactions, owes much of its development to the research of Werner Stumm. Beginning with the tentative interpretation of specific adsorption processes in terms of chemical reactions to form inner-sphere surface complexes, his seminal questions spawned a generation of research on the detection and quantitation of these surface species. The application of noninvasive spectroscopy in this research is exemplified by electron spin resonance and extended X-ray absorption fine structure studies. These studies, in turn, indicate the existence of a rich variety of surface species that transcend the isolated surface complex in both structure and reactivity, thereby stimulating future research in molecular conceptualizations of the particle-water interface. 

For colloids with a physically adsorbed surfactant or cca, the adsorption isotherm is important. The adsorbant concentration on the particle surface can be measured by infrared spectroscopy using diffuse reflectance and by ESCA. Absolute concentrations are difficult to determine with ESCA on "rough" surfaces, and a calibration point is required with other techniques. The change of the concentration of adsorbant in solution after adsorption on the colloid surfaces can be detected by elemental analysis of supernatant with plasma emission or atomic absorption if adsorbant contains specific element(s). When colloids are sterically stabilized, the effectiveness of the stabilization can be evaluated with solvent-nonsolvent techniques and with temperature studies ( 25,26). 

Even more spectacular results in terms of the increasing importance of nanocatalysis for bulk industrial processes have recently been reported by Kuipers and de Jong [32, 33]. By dispersing metallic cobalt nanoparticles of specific sizes on inert carbon nanofibers the authors were able to prepare a new nano-type Fischer-Tropsch catalyst. A combination of X-ray absorption spectroscopy, electron microscopy, and other methods has revealed that zerovalent cobalt particles are the true active centers which convert CO and H2 into hydrocarbons and water. Further, a profound size effect on activity, selectivity, and durability was observed. Via careful pressure-size correlations, Kuipers and de Jong have found that or cobalt particles of 6 or 8nm are the optimum size for Fischer-Tropsch catalysis. The Fischer-Tropsch process (invented in 1925 at the Kaiser-Wilhelm-Institute for... 

The use of Fourier-transformed infrared spectroscopy (FTIR) in particles produced by spray chilling mainly predicts the detection of possible interactions between the active ingredient and the lipid carrier. Normally, the analysis individually studies the components (matrix and active compound, and other components, when available) that will form the capsules. Specific absorption bands will be observed in determined regions or in a specific wavelength. After that, the analysis of the microparticles that are already formed is carried out. The discussion involves changes in the absorption bands of ingredients and formed microparticles, with the comparison of the peaks observed in both cases. 

In order to get answers to these questions, the ability to better characterize catalysts and electrocatalysts in situ under actual reactor or cell operating conditions (i.e., operando conditions) with element specificity and surface sensitivity is crucial. However, there are very few techniques that lend themselves to the rigorous requirements in electrochemical and in particular fuel cell studies (Fig. 1). With respect to structure, in-situ X-ray diffraction (XRD) could be the method of choice, but it has severe limitations for very small particles. Fourier transform infra red (FTTR), " and optical sum frequency generation (SFG) directly reveal the adsorption sites of such probe molecules as CO," but cannot provide much information on the adsorption of 0 and OH. To follow both structure and adsorbates at once (i.e., with extended X-ray absorption fine stmcture (EXAFS) and X-ray absorption near edge stmc-ture (XANES), respectively), only X-ray absorption spectroscopy (XAS) has proven to be an appropriate technique. This statement is supported by the comparatively large number of in situ XAS studies that have been published during the last decade. 16,17,18,19,20,21,22,23,24,25 highly Versatile, since in situ measme-... 

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