Transition metal oxides cation valence states

The cations in transition metal oxides often occur in more than one oxidation state. Molybdenum oxide is a good example, as the Mo cation may be in the 6-r, 5-r, and 4+ oxidation states. Oxide surfaces with the cation in the lower oxidation state are usually more reactive than those in the highest oxidation state. Such ions can engage in reactions that involve changes in valence state. 

Hydroperoxides are decomposed readily by multivalent metal ions, i.e., Cu, Co, Fe, V, Mn. Sn, Pb, etc., by an oxidation-reduction or electron-transfer process. Depending on the metal and its valence state, metallic cations either donate or accept electrons when reacting with hydroperoxides. Either one or two electrons may be transferred depending on die metal. With most transition metals, e.g., Cu, Co, and Mn, both valence states react with hydroperoxides via one electron transfer. Thus, a small amount of transition-metal ion can decompose a large amount of hydroperoxide and, consequently, inadvertent contamination... 

Oxides of the lanthanide rare earth elements share some of the properties of transition-metal oxides, at least for cations that can have two stable valence states. (None of the lanthanide rare earth cations have more than two ionic valence states.) Oxides of those elements that can only have a single ionic valence are subject to the limitations imposed on similar non-transition-metal oxides. One actinide rare-earth oxide, UO2, has understandably received quite a bit of attention from surface scientists [1]. Since U can exist in four non-zero valence states, UO2 behaves more like the transition-metal oxides. The electronic properties of rare-earth oxides differ from those of transition-metal oxides, however, because of the presence of partially filled f-electron shells, where the f-electrons are spatially more highly localized than are d-electrons. 

It is clear that much work remains to be done to extend our understanding to polax surfaces of transition metal oxides in which the cations have partially filled d orbitals. An especially challenging issue is related to mixed valence metal oxides, such as Fe304, in which the cations exist under two oxidation states. In addition, considering the rapid development of ultra-thin film synthesis and characterization, a simultaneous effort should be performed on the theoretical side to settle the conditions of stability of polar films. More generally, on the experimental side, it seems that one of the present bottlenecks is in a quantitative determination of the surface stoichiometry, an information of prominent interest to interpret the presence or absence of reconstruction. 

Transition metal oxides exhibit a number of properties that are conducive to catalytic applications, including thermal and mechanical stability needed to survive severe reaction conditions. More importantly, transition metal cations can typically exist in several different valence states. Titanium dioxide has a bulk band gap energy of about 3.2 eV, but electrons can be placed in (3d) gap states... 

The phenomenon of electrochromism can be defined as the change of the optical properties of a material due to the action of an electric field. The field reversal allows the return to the original state. In practice, when the material is polarized in an electrochemical cell, the change of colour is conelated to the insertion/extrac-tion of small ions H" ", Li" ". This insertion/extraction is monitored by the passage from cathodic to anodic polarization which allows to go from bleached (or coloured) state to coloured (or bleached) one. This property belongs to all (or almost all) transition metal oxides it corresponds to the change of valency of the cation Ni "> Ni " O . .. accompanied... 

The surfaces of metal oxides and their H2 chemisorption characteristics have been far less studied than the surfaces of elemental metals and semiconductors [113,133]. Cation surface states are formed on ideal oxide surfaces at about 2 eV below the bottom ofthe conduction band. The charge of the surface ions is found to be reduced compared with that of the bulk ions and this leads to an enhanced co valency at the surface. The reduction amounts to less than 10 % for oxides of simple metals such as MgO and to 20-30% for transition metal oxides. Cluster and slab calculations reveal that special surface state bands with metallic character can be formed on polar surfaces by charge compensation effects. To what extent the metallic band accounts for special catalytic activity is not yet known [114]. 

The second chemical contribution to the total interaction energy is present if an ionic or a covalent chemical bond between the adsorbed molecule and the surface can be formed. Since covalent bonds also depend on the overlap between the wave functions of the subsystems, their distance dependence is exponential, see Table 1, as is that of the Pauli repulsion. In general, covalent bonds are only possible if at least one of the two partners possesses partially occupied valence orbitals. In contrast to the adsorption at metal or semiconductor surfaces, such a situation is rarely encountered at insulator and in particular at oxide surfaces. In most cases, the ions at the surface of an insulator try to adopt a closed shell electronic structure as they do in the bulk, as for instance the Na+ and Ck ions in NaCl or the Mg + and ions in MgO. Counterexamples are transition metal oxides in which the metal cations possess partially occupied d-shells which might form chemical bonds with the adsorbed molecule. One famous example is the interaction between NO and the NiO(lOO) surface where both the Ni + cations (d configuration with a A2g ground state) and the NO radical ( 11 ground state) have partially filled valence shells (see below). 

Cation Valence States of Transitional Metal Oxides Analyzed by Electron Energy-Loss Spectroscopy... 

Characteristic of transition metal oxides is that the cation may have more than one oxidation state. Iron, for example, has three stable oxides, FeO, Fe304, and Fe203, and several oxyhydroxides, such as FeOOH in a number of different structures, as well. Surfaces of oxides with the cations in lower oxidation state are generally more reactive than those with the cation in its highest oxidation state. In addition, these ions can participate in reactions that involve changes in valence state. 

When a d-metal atom loses electrons to form a cation, it first loses its outer s-electrons. However, most transition metals form ions with different oxidation Variable valence is discussed further states, because the ( -electrons have similar energies and a variable number can be... 

Peroxyl radicals with a strong oxidative effect along with ROOH are continuously generated in oxidized organic compounds. They rapidly react with ion-reducing agents such as transition metal cations. Hydroxyl radicals react with transition metal ions in an aqueous solution extremely rapidly. Alkyl radicals are oxidized by transition metal ions in the higher valence state. The rate constants of these reactions are collected in Table 10.5. 

Clay Minerals as Lewis Acids. Lewis acid sites in a clay mineral are exchangeable (2) or structural ( 0) transition metal cations in the higher valence state, such as Fe + and Cu +, and octahedrally coordinated aluminum exposed at the crystal edges (38). Reduction of both exchanged and structural (octahedral) transition metal cations in the upper oxidation state is a reversible process (12,... 

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