Bulk characterization

Surface and bulk characterization were carried out using electron spectroscopy for chemical analysis (ESCA or XPS) and x-ray diffraction (XRB). The results will be discussed In relation to methanatlon activity. 

Surface and Bulk Characterization of Ternary Alloys (ThNl Fe )... 

Pt and Tl, surface chemistry, 577-78 ternary, surface and bulk characterization, 310-12 Alumina... 

Bulk characterization of calcined precursors and reduced catalysts was carried out by X-ray diffractometry using Cu K radiation. Reduced catalysts were first passivated by exposure to N2O as described above. Line-broadening analysis was carried out on the Fe(llO) reflection to obtain the iron particle size. Overlap with the MgO(200) reflection limited its usefulness to the more highly-loaded catalysts. 

The substantial literature on the bulk characterization of porous materials using conventional techniques provides a useful foundation as a starting point for overlapping the basic physics of traditional materials analysis with the parameters that can be measured using NMR. 

For a precipitated iron catalyst, several authors propose that the WGS reaction occurs on an iron oxide (magnetite) surface,1213 and there are also some reports that the FT reaction occurs on a carbide surface.14 There seems to be a general consensus that the FT and WGS reactions occur on different active sites,13 and some strong evidence indicates that iron carbide is active for the FT reaction and that an iron oxide is active for the WGS reaction,15 and this is the process we propose in this report. The most widely accepted mechanism for the FT reaction is surface polymerization on a carbide surface by CH2 insertion.16 The most widely accepted mechanism for the WGS reaction is the direct oxidation of CO with surface 0 (from water dissociation).17 Analysis done on a precipitated iron catalyst using bulk characterization techniques always shows iron oxides and iron carbides, and the question of whether there can be a sensible correlation made between the bulk composition and activity or selectivity is still a contentious issue.18... 

Composite components, both fiber (or adherend) and matrix, have chemical, morphological and structural variability that can be operative at or near surfaces to form an interphase. Bulk characterization of these materials ignores these components because of the dilution of their effect in the bulk. However, when composites are fabricated, fiber to fiber distances are on the order of tenths of microns. Interphase structures are also on the order of tenths of microns and can be a significant proportion of the structure of the material between fibers. 

Section 5.3 considered NMR spectroscopic approaches to the bulk characterization of oxides and oxidation catalysts. Gatalytic activity is, however, intrinsically linked with the nature of the catalyst surface and hence a number of techniques have been developed in order to probe this. As discussed in Section 5.1, two of the most significant parameters impacting on catalyst activity are the acid-base characteristics of a surface and the redox properties of the material, and NMR techniques exist to probe both of these characteristics. One of the most common techniques to probe surface structure is GP-MAS NMR, in particular CP from hydrogen to the nucleus of interest-either the metal or the oxygen of the metal oxide. Historically, the source of surface H species has often been those naturally present on the catalyst surface, as chemisorbed hydroxyls or physisorbed water. As such, much of the work in this area involves the study of supports such as Si02. Applications of CP-MAS and other spectroscopic approaches to the study of oxide surfaces are outlined in the following sections. 

Limitations of bulk characterization techniques when applied to study catalytic reactions occurring at the surface. 

Schreyer D, Chatelain A (1985) Lattice contraction in small particles of SrCl2 Gd explicit size effect in EPR and Raman spectroscopy. Surface Sci 156 712-719 Schultz, JM (1982) Diffraction for Materials Scientists. Prentice-Hall, Englewood Cliffs, NJ Schwartz LH, Cohen JB (1987) Diffraction from Materials. Springer-Verlag, Berlin Scolan E, Magnenet C, Massiot D, Sanchez C (1999) Surface and bulk characterization of titanium-oxo clusters and nanosized titania particles through solid state NMR. J Mater Chem 9 2467-2474... 

Characterization of complex organic matter like humic substances (HS) is a formidable task (7). A variety of destructive and non-destructive methods have been applied. Among the non-destructive methods, spectroscopic methods, such as NMR and FT-IR have proven to be very useful in providing information about the structure of these materials as a whole. Of these, NMR has proven to be the best method for bulk characterization, especially solid-state I R, where the relative contribution of specific carbon types can be made (2). Different structural parameters (aromaticity,... 

The adsorption of microcystins has been shown to be strongly affected by NOM [78]. In this work the effect, for four microcystin variants, was shown to be a function of the DOC concentration. This is prohahly a result of the direct competitive effect, where the competitive NOM, those compounds approximately the same molecular weight as the microcystins, makes up the hulk of the NOM. Therefore the bulk characterization parameter, DOC, gives an indication of the concentration of competing compounds, where for MIB and geosmin it could not [27, 63, 69]. 

Asami et al. have reported a methane conversion of 13% and a Cj selectivity of 71% at 800 C using a 20 wt% Pb-Mg-0 catalyst. Surface and bulk characterization studies of catalysts suggest that at low loadings the Pb is indeed highly dispersed on the surface of the catalyst. ... 

Complete characterization includes determination of both the bulk and surface properties of nanomaterials, since both can influence impacts on the environment and biological systems. Bulk characterization consists of studying size, shape, phase, electronic stmcture, and crystallinity, while surface characterization looks at surface area, atomic structure, surface composition, and functionality. Specific examples of bulk and surface characterization methods are described below. 

X-ray diffraction (XRD) is a bulk characterization method used to determine the phase and crystallinity of nanomaterials. The size of a parliele ean affect the thermodynamic stability of its phase because the surface eontribution to total See energy, G, becomes increasingly significant as particle size decreases ... 

Bulk characterization yields information on the macroscopic properties of the biomaterial such as thermal, mechanical, solubility, optical, and dielectric properties. Surface characterization yields morphological information that is critical for interfacing the implant or drug delivery device with the host tissue. This could be achieved by microscopic and spectfoscopic methods. Next in the hierarchy is the characterization of processes such as biodegradation mechanism and kinetics under biomimetic in vitro conditions. Cases of implanted device failure need to be assessed by systematic interrogation of explanted medical devices. After knowing the basic characteristics of the biomaterial, real-time investigation of in vivo processes plays a major role in the successful journey of an implant. 

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