Cation valence states, of transitional metal oxides

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

In the previous chapter it was shown how measurements of polarized absorption spectra in the visible to near-infrared region can provide information on such crystal chemical problems as oxidation states of transition metal ions, coordination site symmetries and distortions, cation ordering and the origins of colour and pleochroism of minerals. Much attention was focused in chapter 4 on energies of intervalence charge transfer transitions appearing in electronic absorption spectra of mixed-valence minerals. 

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. 

The NASICON structure was chosen because it can be readily synthesized, is thermally very stable, and can accommodate a large fraction of vacancies and cation substitutions [9-12], In addition, this structure possesses two features which should be important for the catalyst design as envisioned above. First, it is a phosphate and hence expected, owing to its acidic nature, to stabilize the lower oxidation states of transition metals, e.g., V second, owing to its structure, layered octahedral metal centers with variable valence are separated from each other by redox inactive tetrahedral phosphate groups, i.e., the structure provides for isolation of descrete layers. 

Since 1970 perovskite-type oxides (ABO3) have been suggested as substitutes for noble metals in automotive exhaust catalysts [1]. These oxides are efficient for oxidation reactions when for reduction the results obtained from the literature are dissimilar [2], mainly due to huge differences in the experimental conditions. The properties of perovskite-based catalysts are a flmction of the spin and the valence state of the metal in the B site cation, which is surrounded octahedrally by oxygen. The A site cation is located in the cavity made by these octahedra. For some perovskite-type oxides, their electronic structures have been pointed out to be similar to those of transition metals on the basis of theoretical... 

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. 

In a series of papers Palenik and his coworkers (Palenik 1997a,6,c Kanowitz and Palenik 1998 Wood and Palenik 1998, 1999a,6 Wood et al. 2000) have determined bond valence parameters for transition metals. Some of these have been chosen to be independent of oxidation state in an attempt to provide values of Rq that can be used when the oxidation state of the cation is not known. While these parameters are not as accurate as those that take the oxidation state into account, they can be used to make an approximate determination of the oxidation state, after which the correct value of Rq can be substituted. 

In recent studies See et al. (1998) and Shields et al. (2000) suggest that Rq sometimes depends on factors other than the oxidation states of the cation and anion. To obtain correct bond valence sums around transition metals with nitrogen ligands, it is necessary to use different values of Rq depending on the coordination number of as discussed in Section 9.2. 

Naskar et al. (1997) were interested in using bond valences to determine oxidation states around transition-metal cations, particularly those with negative or zero formal oxidation states. Since these numbers cannot, in principle, be reached by the standard equations, they proposed to create a fictional positive oxidation state by arbitrarily adding 4.0 to the actual oxidation state. They proposed to write the valence sum rule in the form of eqn (A 1.10) ... 

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... 

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... 

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