Transition metal oxides, spectroscopic

We have summarized below recent results concerning spectroscopic / flow reactor investigations of hydrocarbons partial and total oxidation on different transition metal oxide catalysts. The aim of this study is to have more information on the mechanisms of the catalytic activity of transition metal oxides, to better establish selective and total oxidation ways at the catalyst surface, and to search for partial oxidation products from light alkane conversion. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

The ZSA phase diagram and its variants provide a satisfactory description of the overall electronic structure of stoichiometric and ordered transition-metal compounds. Within the above description, the electronic properties of transition-metal oxides are primarily determined by the values of A, and t. There have been several electron spectroscopic (photoemission) investigations in order to estimate the interaction strengths. Valence-band as well as core-level spectra have been analysed for a large number of transition-metal and rare-earth compounds. Calculations of the spectra have been performed at different levels of complexity, but generally within an Anderson impurity Hamiltonian. In the case of metallic systems, the situation is complicated by the presence of a continuum of low-energy electron-hole excitations across the Fermi level. These play an important role in the case of the rare earths and their intermetallics. This effect is particularly important for the valence-band spectra. 

Thermal desorption studies of preadsorbed oxygen on a number of transition metal oxides have been carried out by Iwamoto (170) and others who observed several desorption peaks for each oxide. In this type of work, reliable assignment of the desorption peaks to particular oxygen species is only possible when parallel spectroscopic studies are carried out. This is difficult for many of these oxides where only material of low specific surface area is available. 

There are many computational investigations of transition metal oxides, see, e.g., the recent review by Harrison [7]. Some studies have included the whole sequence of 3d metal oxides. In one of these studies, Bauschlicher and Maitre [17], employed different high-level ab initio methods. It was found that ScO - MnO and CuO were well described by single-reference based techniques and that the CCSD(T) method gave spectroscopic constants (re, uje and D0) in good agreement with experiments. For FeO - NiO multi-reference based techniques (CASSCF/ICACPF) were necessary to get good results. 

Transition metal oxides are the systems which make a challenge to any quantum chemical theory. Thus their theoretical investigation constitute an excellent benchmark for Density Functional Theory in both aspects methodological and practical one. Two transition metal oxide molecules are considered here in detail, VO and MoO, with emphasis put on their electronic structure, spectroscopic properties and metal - oxygen bonding features. Applicability of DFT to various electronic states is discussed and the quality of results within various computational schemes is examined. 

An important feature in the spectroscopic behavior is that of fluorescence or luminescence of certain lanthanide ions, notably Y and Eu, when used as activators in lanthanide oxide, silicate, or transition-metal oxide lattices. Oxide phosphors are used in color television tubes. Certain of the +2 ions trapped in CaF2 lattices, as well as organic cation salts of complex anions such as [Eu /J-diket4] , show laser activity. 

Many industrially important selective oxidation reactions are catalyzed by transition metal oxides. The activity of such catalysts is related to the reducibility of the transition metal ion, which enables the bulk oxide lattice to participate actively in the redox processes present in the Mars van Krevelen mechanism. Unfortunately, NMR spectroscopic investigations are severely limited by the occurrence of paramagnetic oxidation states. As a general rule, NMR signals from atoms bearing unpaired electron spins cannot be detected by conventional methtxls, and the spectra of atoms nearby are often severely broadened. For this reason, most of the work published in this area has dealt with diamagnetic vanadium(V) oxide-based catalysts. 

Zeolite catalysts incorporated or encapsulated with transition metal cations such as Mo, or Ti into the frameworks or cavities of various microporous and mesoporous molecular sieves were synthesized by a hydrothermal synthesis method. A combination of various spectroscopic techniques and analyses of the photocatalytic reaction products has revealed that these transition metal cations constitute highly dispersed tetrahedrally coordinated oxide species which enable the zeolite catalysts to act as efficient and effective photocatalysts for the various reactions such as the decomposition of NO into N2 and O2 and the reduction of CO2 with H2O into CH3OH and CH4. Investigations on the photochemical reactivities of these oxide species with reactant molecules such as NOx, hydrocarbonds, CO2 and H2O showed that the charge transfer excited triplet state of the oxides, i.e., (Mo - O ), - O ), and (Ti - O ), plays a significant role in the photocatalytic reactions. Thus, the present results have clearly demonstrated the unique and high photocatalytic reactivities of various microporous and mesoporous zeolitic materials incorporated with Mo, V, or Ti oxide species as well as the close relationship between the local structures of these transition metal oxide species and their photocatalytic reactivities. 

In this chapter, the local structures of the transition metal oxides, Ti, V, Mo, and Cr oxide single-site species, incorporated within zeolites or mesoporous sUica framework structures as well as the local structures of transition metal ions such as Ag" " exchanged into zeohte cavities were discussed, based on the results of various in situ spectroscopic investigations such as ESR, UV-Vis, photoluminescence and XAFS (XANES and EXAFS). 

Triflic acid-functionalized Zr-TMS (zirconium oxide with a mesostructured framework TMS, transition metal oxide mesoporous molecular sieves) have been extensively studied by Chidambaram et alP by the use of a variety of spectroscopic methods including DD/MAS NMR. The observed chemical shift of ca. 119 ppm and a Jcf coupling of ca 310 Hz showed that the triflic acid remained intact on the catalyst framework. 

Formation mechanism of SEI layers on cathodes in Li-ion batteries, their thermal and electrochemical stabihty, and their roles in affecting the cycle life and safety characteristics are well documented by many researchers [43 6]. Here we present some recent data on identifying the surface layer generation and their composition on transition metal oxide cathodes like spinel and layered materials by various spectroscopic techniques. The structural changes and the reaction at the surface during the first delithiation process in Li-rich layered material are explained. The effects of additives and coatings on electrode materials to their electrochemical performance are also discussed at the end. 

The second class of sol-gels contains redox-active metal oxides, such as tungsten oxide, vanadium pentoxide, manganese oxide, and other transition metal oxides. Moreover, many n-type semiconductors such as zinc oxide, barium titanate, and titanium dioxide can be used in this class (113). The structures of these gels are sensitive to the pH and oxidation state of the precursors. Many redox-active sol-gels exhibit electrochromism (different oxidation states exhibit different colors, allowing spectroscopic determination of redox states). These gels can also accommodate the reductive insertion of lithium and other moieties. 

Lenglet, M. Spectroscopic study of the chemical bond in 3d transition metal oxides. Correlation with the ionic-covalent parameter. Trends Chem. Phys. 1997, 6, 121-543. 

Unlike nitric oxide, NO, the monomeric radical sulfur nitride, NS, is only known as a short-lived intermediate in the gas phase. Nevertheless the properties of this important diatomic molecule have been thoroughly investigated by a variety of spectroscopic and other physical techniques (Section 5.2.1). The NS molecule is stabilized by coordination to a transition metal and a large number of complexes, primarily with metals from Groups 6, 7, 8 and 9, are known. Several detailed reviews of the topic have been published. ... 

Warwel, S., Sojka, M., and Rusch, M. Synthesis of Dicarboxylic Acids by Transition-Metal Catalyzed Oxidative Cleavage of Terminal-Unsaturated Fatty Acids. 164, 79-98 (1993). Wexle.r, D., Zink, J. I., and Reber, C. Spectroscopic Manifestations of Potential Surface Coupling Along Normal Coordinates in Transition Metal Complexes. 171,173-204 (1994). Willett, P., see Artymiuk, P. J. 174, 73-104 (1995). 

Spectroscopic techniques such as electron spin resonance (ESR) offer the possibility to "probe" the chemical environment of the interlayer regions. With the ESR technique, an appropriate paramagnetic ion or molecule is allowed to penetrate the interlayer, and chemical information is deduced from the ESR spectrum. Transition metal ions, such as Cu2+, and nitroxide radical cations, such as TEMPAMINE (4-amino-2,2,6,6-tetramethylpiperidine N-oxide) have been used as probes in this manner (6-14). Since ESR is a sensitive and non-destructive method, investigations of small quantities of cations on layer silicate clays at various stages... 

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