Vanadium oxides, bulk structure

In the bulk form, vanadium oxides display different oxidation states and V—O coordination spheres and exhibit a broad variety of electronic, magnetic, and structural properties [96, 97], which make these materials attractive for many industrial applications. Prominent examples range from the area of catalysis, where V-oxides are used as components of important industrial catalysts for oxidation reactions [98] and environment pollution control [99], to optoelectronics, for the construction of light-induced electrical switching devices [100] and smart thermo-chromic windows. In view of the importance of vanadium oxides in different technological applications, the fabrication of this material in nanostructured form is a particularly attractive goal. 

The growth of vanadium oxide overlayers on Rh(l 11) converges after a number of intermediate stages to the formation of a three-dimensional bulk-like epitaxial V203 film [90], which is oriented with the (0 0 01) plane of its corundum structure parallel to the Rh(l 1 1) substrate surface. The V203 phase is the thermodynamically stable... 

Self-assembled nanorods of vanadium oxide bundles were synthesized by treating bulk V2O5 with high intensity ultrasound [34]. By prolonging the duration of ultrasound irradiation, uniform, well defined shapes and surface structures and smaller size of nanorod vanadium oxide bundles were obtained. Three steps which occur in sequence have been proposed for the self-assembly of nanorods into bundles (1) Formation of V2O5 nuclei due to the ultrasound induced dissolution and a further oriented attachment causes the formation of nanorods (2) Side-by-side attachment of individual nanorods to assemble into nanorods (3) Instability of the self-assembled V2O5 nanorod bundles lead to the formation of V2O5 primary nanoparticles. It is also believed that such nanorods are more active for n-butane oxidation. 

XRD patterns of the prepared samples V-Mo-Zeolite are similar to that of zeolites which suggests that the metal species (i.e. oxide, cations,...) are well dispersed through the zeolites structure and the absence of bulk phases in the XRD patterns implies that for these samples the molybdenum and vanadium oxides are present in either a nanocrystalline state or as a small crystallites which measured less than 4 nm in diameter. Furthermore, XRD and FTIR (1500-400 cm 1) showed no significant damage of the zeolite host structure after exchange and thermal treatment except for the sample V2MoMor. 

One of the most significant results from the advent of these surface science studies on oxides relevant for the present catalytic applications is the fact that oxides can be multiply terminated and that they are not terminated [154, 180, 186-190] in cuts through the bulk structure. This is not unexpected in general [98,156,179] but it is of great value to know this in attempts to understand the mechanisms that activate oxides for catalysis. These rigorous studies must be differentiated from more empirical studies carried out on termination issue with qualitative methods and without predictive power but with the still invaluable advantage that they can be applied [97,191-193] to complex MMO catalyst systems. Such studies can be used to probe the surface reactivity, to address the issue of segregation of, for example, vanadium out of an MMO system and to compare different qualities of the nominally same material with speculative assumptions about the influence of defects. 

The spectroscopic study of the nature of vanadium oxide [VOx] supported on a high surface area Ti02 (anatase) indicated the formation of three different VOx structures [48] a) isolated V4+ ions, part of which was coordinatively unsaturated, strongly bonded to the surface hydroxy groups of the support b) bidimensional clusters of VOx with mainly V5+ after calcinations, reducible under mild conditions to V4+ and also to V3+ to some extent (these species weakly interact with the support surface) c) V205 appeared when cove-rage was about the monolayer and was presented as bulk multiplayer structures. The authors observed the existence of at least two different isolated surface V4+ species, which caused splitting of the low-field hfs lines in parallel orientation. 

Beyond providing bulk structural information about 3-D metal oxide phases, Raman spectroscopy can also provide information about the terminating (and thus 2-D) surface layers of bulk metal oxides. For example, surface Nb=O, V = O, and Mo=O functionalities were detected by Raman spectroscopy for bulk Nb2Os, and for vanadium-niobium, molybdenum-vanadium, molybdenum-niobium, and vanadium-antimony mixed oxide phases (Guerrero-Perez and Banares, 2004 Jehng and Wachs, 1991 Zhao et al., 2003). 

Recent studies of supported vanadium oxide catalysts have revealed that the vanadium oxide component is present as a two-dimensional metal oxide overlayer on oxide supports (1). These surface vanadium oxide species are more selective than bulk, crystalline V2O5 for the partial oxidation of hydrocarbons (2). The molecular structures of the surface vanadium oxide species, however, have not been resolved (1,3,4). A characterization technique that has provided important information and insight into the molecular structures of surface metal oxide species is Raman spectroscopy (2,5). The molecular structures of metal oxides can be determined from Raman spectroscopy through the use of group theory, polarization data, and comparison of the... 

It is well established [1-3] that vanadyl pyrophosphate (VO)2P207 is an essential component of the most selective VPO catalysts. For example, structural and chemical characterization studies of "reactor equilibrated" VPO catalysts indicate that the predominate crystalline phase is vanadyl pyrophosphate (VO)2P207 [1-3], that the bulk P/V ratio is close to 1.0, and that the average vanadium oxidation state is close to -1-4.0 [3-5]. A number of studies [2,5] have indicated that alkane oxidation primarily involves oxygen adspecies adsorbed at vanadium surface sites, and relatively little bulk lattice oxygen. 

Literature descriptions of active sites on oxide catalysts are often speculative and very often just generate a picture of the surface active site by extrapolation of the bulk structure. In general they envisage approach of the starting material to the active site in a preferred orientation without any indication of how the preferred orientation is established. In addition, the description of the active site is usually restricted to a small number of molecules. For example, the vanadium phosphorus oxide catalysts used for n-butane oxidation to maleic anhydride is based on the vanadyl pyrophosphate structure and an active site architecture is... 

A review of First Principles simulation of oxide surhices is presented, focussing on the interplay between atomic-scale structure and reactivity. Practical aspects of the First Principles method are outlined choice of functional, role of pseudopotential, size of basis, estimation of bulk and surface energies and inclusion of the chemical potential of an ambient. The suitability of various surface models is discussed in terms of planarity, polarity, lateral reconstruction and vertical thickness. These density functional calculations can aid in the interpretation of STM images, as the simulated images for the rutile (110) surface illustrate. Non-stoichiometric reconstructions of this titanium oxide surface are discussed, as well as those of ruthenium oxide, vanadium oxide, silver oxide and alumina (corundum). This demonstrates the link between structure and reactivity in vacuum versus an oxygen-rich atmosphere. This link is also evident for interaction with water, where a survey of relevant ab initio computational work on the reactivity of oxide surfaces is presented. 

One strategy to avoid bias is the use of simulated annealing to g erate novel, but reasonable, surface structures, such as the 50 or more V Oy (Vs x Vs) and (2 X 2) reconstructions of Ref. [30]. The authors have confidence in this set of models based on their imderstanc g of the vanadium oxide chemistry, from calculating the various bulk phases, more complex unsujqjorted thin films and finally the complete system of oxide layers on a metal support. 

The link between structure and reactivity is again demonstrated by the complicated succession of vanadium oxide surface phases predicted by FP [77]. At certain O2 partial pressures, the metal substrate is computed to stabilise thin film phases that are not known in equivalent bulk form. The impliaation is that STM studies of thin insulator fihm on conducting substrates may have to contend with the complex, and sometimes novel, chemistry of thin films [2]. A phase diagram of non-stoichiometric surfaces is also generated by FP in Ref. [53], this time for silver oxidation. The aim is to bri(%e the pressure gap between ultra-high-vacuum research and the industrial reality of high-pressure reactors. 

The molecular structures of the dehydrated, isolated, and polymeric surface vanadia species are depicted in Figure 1.3. The surface vanadia structural chemistry under dehydrated conditions reflects the known bulk vanadium oxide inorganic chemistry, in that isolated and polymeric VO4 units form with only monoxo terminal V=0 bonds. 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

The electronic structure of bulk VO has been calculated by different band structure methods [110-114] and using correlated electron procedures [115]. This Mott-Hubbard metal, which forms a rocksadt type lattice, is the simplest of all single valence oxides of vanadium and has been treated theoretically already a long time ago. As an example, Neckel et al. [114] have published results from self-consistent APW calculations for the experimentally known lattice geometry... 

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