Multiple oxides

Oxidation Catalysis. The multiple oxidation states available in molybdenum oxide species make these exceUent catalysts in oxidation reactions. The oxidation of methanol (qv) to formaldehyde (qv) is generally carried out commercially on mixed ferric molybdate—molybdenum trioxide catalysts. The oxidation of propylene (qv) to acrolein (77) and the ammoxidation of propylene to acrylonitrile (qv) (78) are each carried out over bismuth—molybdenum oxide catalyst systems. The latter (Sohio) process produces in excess of 3.6 x 10 t/yr of acrylonitrile, which finds use in the production of fibers (qv), elastomers (qv), and water-soluble polymers. 

Analysis of CEELS line shapes often show chemical shifts that have been used to study FeB alloys after recrystallization, C-H bonding in diamondlike films and multiple oxidation states. 

Because of their multiple oxidation states, the chalcogens, particularly sulfur, can engage in numerous redox couples participating in acid-base, oxidation-reduction, precipitation, and complexation equilibria. 

In most uranium ores the element is present in several, usually many diverse minerals. Some of these dissolve in sulfuric acid solutions under mild conditions, while others may require more aggressive conditions. Thus, while it may be comfortable to recover 90-95% of the uranium present, it may be tough or impractical to win the balance amount of a few percent economically. Some of the most difficult uranium minerals to leach are those of the multiple oxide variety, most commonly brannerite and davidite. These usually have U(IV) as well as U(VI), together with a number of other elements such as titanium, iron, vanadium, thorium, and rare earths. To extract uranium from these sources is not as easy as other relatively simpler commonly occurring sources. 

With few exceptions, they exhibit multiple oxidation states. 

Cyclic chain termination by antioxidants. Oxidation of some substances, such as alcohols or aliphatic amines, gives rise to peroxyl radicals of multiple (oxidative and reductive) activity (see Chapters 7 and 9). In the systems containing such substances, antioxidants are regenerated in the reactions of chain termination. In other words, chain termination occurs as a catalytic cyclic process. The number of chain termination events depends on the proportion between the rates of inhibitor consumption and regeneration reactions. Multiple chain termination may take place, for instance, in polymers. Inhibitors of multiple chain termination are aromatic amines, nitroxyl radicals, and variable-valence metal compounds. 

The conversion of hydroperoxide/peroxide to superoxide is a one-electron redox reaction and requires the presence of transition metals having accessible multiple oxidation states as in biological iron or manganese clusters (e.g., Fe(II, III, IV) clusters of monooxygenase or the Mn(II, HI, IV) clusters of photosystems). Ti is usually not reduced at ambient temperatures. The various possibilities that could facilitate the transformation of hydroperoxo/peroxo to superoxo species are as follows ... 

While color is often discussed as being closely related to iron and its oxidation state, it is also related to all other soil components. When soil color indicates oxidizing conditions, all multiple oxidative state capable cations are... 

Examples of cations that are present in significantly lower concentrations than the simple cations are iron, manganese, zinc, copper, nickel, and cobalt. Except for cobalt, these have multiple oxidations states in soil as shown in Table 6.1. Because of their multiple oxidation states, they may be present as many more species than the simple cations. Typically, the higher oxidation states predominate under oxidizing conditions, while the lower oxidation states predominate under reducing conditions. However, it is common to find both or all oxidation states existing at the same time in either aerobic or anaerobic soil [7,8],... 

Manganese in soil has many characteristics that are similar to iron for instance, it exists in multiple oxidation states Mn2+, Mn3+, and Mn4+. Although manganese can exist in the laboratory in other oxidations states, from -3 to +7, the +2 to +4 species are the ones commonly found in soil. Manganese forms various oxide and hydroxide species and chelates with many soil components. Its low oxidation state (i.e., Mn2+) is more soluble and more available than its high oxidation state (i.e., Mn4+). 

It should be recalled that one of the principal properties of transition metals is their aptitude to accede to multiple oxidation states. Thus, the main scope of an electrochemical study is to ascertain whether a metal complex, prepared in a certain oxidation state, is stable also in different oxidation states, or whether the lifetimes of these oxidation states are too short to observe stable products. Whenever stable oxidation states are identified, the chemist might be able to prepare and fully characterize these new complexes. 

Chapter 12, Section 2). It is generally accepted that the site responsible for the oxidation of dihydrogen and the reduction of protons is the nickel-containing assembly, possibly exploiting its capacity to shuttle between the multiple oxidation states Ni(III)/Ni(II)/Ni(I)/Ni(0). 

Other examples of oxidant-iron(III) adducts as intermediates in iron porphyrin-catalyzed reactions have been published as listed in references 54a-k. Competitive alkene epoxidation experiments catalyzed by iron porphyrins with peroxy acids, RC(0)00F1, or idosylarenes as oxidants have been proposed to have various intermediates such as [(porphyrin)Fe (0-0-C(0)R] or [(porphyrin)Fe (0-I-Ar)]. Alkane hydroxylation experiments catalyzed by iron porphyrins with oxidant 3-chloroperoxybenzoic acid, m-CPBA, have been proposed to operate through the [(porphyrin)Fe (0-0-C(0)R] intermediate. J. P. CoUman and co-workers postulated multiple oxidizing species, [(TPFPP )Fe =0] and/or [(TPFPP)Fe (0-I-Ar)] in alkane hydroxylations carried out with various iodosylarenes in the presence of Fe(TPFPP)Cl, where TPFPP is the dianion of me50-tetrakis(pentafluorophenyl)porphyrin. ... 

Selenium has a complex chemistry in the environment because of its multiple oxidation states and variable surface adsorption properties. Qualitatively it is analogous to sulfur occurring in the oxidation states +6 (selenate, Se04 ), +4 (selenite, SeOs "), 0 (elemental selenium) and —2 (Se, selenide) The Se anion closely resembles S (radii 0.20 and 0.185nm, respectively) and is often associated with sulfide minerals. Also, like S, Se is subject to volatilization through biological methylation. 

The results actually showed a deracemization of the racemic hydroxyester 10 as opposed to enantioselective hydrolysis with formation of optically pure (R)-hydroxyester 10 and only 20 % loss in mass balance. Small quantities of ethyl 3-oxobutanoate 9 (<5%) were also detected throughout the reaction, leading the authors to suggest a multiple oxidation-reduction system with one dehydrogenase enzyme (DH-2) catalysing the irreversible reduction to the (R)-hydroxy-ester (Scheme 5). 

Two classes of catalysts account for most contemporary research. The first class includes transition-metal nanoparticles (e.g., Pd, Pt), their oxides (e.g., RUO2), and bimetallic materials (e.g., Pt/Ni, Pt/Ru) [104,132-134]. The second class, usually referred to as molecular catalysts, includes all transition-metal complexes, such as metalloporphyrins, in which the metal centers can assume multiple oxidation states [ 135 -137]. Previous studies have not only yielded a wealth of information about the preparation and catalytic properties of these materials, but they have also revealed shortcomings where further research is needed. Here we summarize the main barriers to progress in the field of metal-particle-based catalysis and discuss how dendrimer-encapsulated metal nanoparticles might provide a means for addressing some of the problems. 

For cubic structures with more than one interionic distance—as, for instance, spinels (multiple oxides of type AB2O4 with A and B cations in tetrahedral and octahedral coordination with oxygen, respectively)—it is stiU possible to use equation 1.109, but the partial derivatives must be operated on the cell edge, which is, in turn, a function of the various interionic distances (OttoneUo, 1986). 

Figure 8.23A shows a simplified Eh-pH diagram for the Mn-O-H system. Within the stability field of water, manganese occurs in three valence states (2+, 3 +, and 4+). Figure 8.23A shows the condensed phases relative to the three valence states as the hydroxide pyrochroite Mn(OH)2 (2+), multiple oxide haus-mannite Mu304 (2+, 3 + ), sesquioxide Mu203 (3 + ), and oxide pyrolusite Mn02 (4+). 

Ottonello G. (1986). Energetics of multiple oxides with spinel structure. Phys. Chem. Minerals., 13 79-90. 

We now consider the abstract presented in excerpt 8G, which reverses the emphases observed in excerpts 8C-8F. In excerpt 8G, moves 1 and 2 comprise the bulk of the abstract, and move 3 is just a single sentence. Much of the abstract is used to elucidate important gaps in the field that (1) a size-based standard for regulating atmospheric particulate matter is inadequate because it does not take into account chemical composition, and (2) compositional analyses are difficult, especially for metals, because they often exist in multiple oxidation states. These gaps help to establish the relevance of the authors work. Only in the last sentence do the authors mention their work specifically (move 3). Here, they identify their method (micro-XANES) and tell readers what results will be presented at the conference. 

For the ILD process, it is less feasible to continue measuring over field oxide, because multiple oxide layers are present (also see Fig. 6). The true thickness and thickness uniformity of the particular level of an ILD proeess can be confounded by the presence of the underlying previous layers. Thus, after the first metal process, it is preferable to measure the oxide thickness directly on a metal line. 

The Re- Os method was first applied to extraterrestrial samples in the early 1960s when Hirt et al. (1963) reported a whole-rock isochron for 14 iron meteorites that gave an age of 4 Ga. Further development of this system was hindered by several technical difficulties. Rhenium and osmium each exist in multiple oxidation states and can form a variety of chemical species, so complete digestion of the samples, which is required to chemically separate rhenium and osmium for mass spectrometry, is difficult. In addition, accurate determination of rhenium abundance and osmium isotopic composition requires spiking the samples with isotopically labeled rhenium and osmium, and equilibration of spikes and samples is challenging. A third problem is that osmium and, particularly, rhenium are very difficult to ionize as positive ions for mass spectrometry. These problems were only gradually overcome. 

---