Characterization oxidation state analysis

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

As shown in Table 1, a remarkable variety of alkene complexes bearing metal centers in a low oxidation state have been isolated and their structures have been determined by X-ray analysis. All the C-C bond distances in olefins coordinated to early transition metals at low oxidation states are more or less elongated compared to free ethylene. These structural data, together with those from NMR studies [14], indicate a major contribution of the metallacyclo-propane structure (2), a fact which is also supported by calculation studies [15]. In the case of ethylene bridging two metal centers such as [ Cp2ZrX 2(iu-f/-C2H4)] (3), the M-C bond could be characterized as a er-bond and there is a little contribution from the / -ethylene canonical structure [16-18]. 

Evidently, the most interesting materials are those in a fractional oxidation state, with general formula (cation)[M(dmit)2] (n > 1), since they can exhibit both electrical and magnetic properties. Only eight such complexes have been reported so far. All of them but (BDTA)[Ni(dmit)2]2 [89] have been obtained as powders. They have in general been poorly characterized, and their stoichiometries have been determined from elemental analysis. Among these powdered compounds, the... 

XAS has been successfully employed in the characterization of a number of catalysts used in low temperature fuel cells. Analysis of the XANES region has enabled determination of the oxidation state of metal atoms in the catalyst or, in the case of Pt, the d band vacancy per atom, while analysis of the EXAFS has proved to be a valuable structural tool. However, the principal advantage of XAS is that it can be used in situ, in a flooded half-cell or true fuel cell environment. While the number of publications has been limited thus far, the increased availability of synchrotron radiation sources, improvements in beam lines brought about by the development of third generation sources, and the development of more readily used analysis software should increase the accessibility of the method. It is hoped that this review will enable the nonexpert to understand both the power and limitations of XAS in characterizing fuel cell electrocatalysts. 

An empirical method for correlating the oxidation state of a metal ion with the coordination geometry and the bond lengths Bond valence sum analysis has been used in characterizing the structural features of vanadium-dependent haloperoxidases . ... 

This section considers aspects and examples of the dissolution behaviour of individual iron oxides. Additional data are listed in Table 12.3 for a range of experimental conditions. As yet, characteristic dissolution rates carmot be assigned to the various iron oxides (Blesa Maroto, 1986). There are, however, some consistent differences between oxides with considerable stability differences, hence a comparison of the oxides is included here. In addition, the reactivity of any particular oxide may vary from sample to sample, depending on its source (natural or synthetic) and the conditions under which it formed. To illustrate this. Table 12.4 summarizes conditions and results from dissolution experiments in which a range of samples of the same oxide was compared. How the properties of the sample influence its dissolution behaviour is still not fully understood. A thorough characterization of the samples by solid state analysis, e. g. by EXAFS, to provide a basis for understanding the dissolution behaviour is, therefore, desirable. 

A diamagnetic compound whose analysis corresponds to Li[Nb(bipy)3]-3.5THF was reported to form by reduction of NbCl5 with Li2bipy. As bipy can behave as bipy7 and in the absence of any spectroscopic data, the metal s oxidation state may still be questioned.723,724 Na[Ta(dmpe)2(CO)2] was characterized by its derivatives only (Scheme 10). 

The tools available for surface composition characterization are electron spectroscopy for chemical analysis (ESCA), Auger spectroscopy (AES), ion scattering spectroscopy (ISS), and secondary ion mass spectroscopy (SIMS). ESCA spectroscopy is used more widely than the others for studying the surface composition and oxidation states of industrial catalysts, and thus its application will be discussed in limited detail. 

Many complexes that appeared to be An" based on their empirical formulae are really mixed oxidation state Au An complexes. A number of well-characterized examples are given in Table 1. Electron spectroscopy for chemical analysis (ESCA), Au Mossbauer spectroscopy, and single-crystal X-ray structure analyses have been used to distinguish these mixed oxidation state complexes from true Au complexes. The compound AuCb was shown to be a mixed Au -Au tetrameric cyclic complex [AU4CI8] (12) by X-ray structure analysis. 

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