Vanadium phosphate

Vanadium phosphate catalysts were prepared by heating V2O4, phosphorus acid, either H3PO4 or H4P2O7, and water together in an autoclave at 145°C for 72 hours. Afterwards, the solid produced was recovered, washed with distilled water and dried in air at 120°C for 16 hours. Detailed preparation procedure is described in [79]. Such prepared precursors were activated in n-butane/air at 400°C to form the final catalysts. TEM and EELS are used to study the catalysts in Philips CM200 PEG microscope. 

MAS NMR has now been used to study LiCo02-derived layered materials - as well as a wide range of alternative cathode materials including manganates, vanadates, and iron and vanadium phosphates. We will now discuss the application of NMR to some of these materials to illustrate the type of information that has been (and can be) obtained by using this method. 

More recently, lithium vanadium phosphates (LisV2-(P04)s and Li3FeV(P04)3, with open NASICON framework structures, have also been studied. Reversible electrochemical lithium deintercalation/re-intercalation at a higher potential (in comparison to the couples seen for the oxides) of between 3... 

It is now considered, by most groups working in this area, that vanadyl pyrophosphate (VO)2P207 is the central phase of the Vanadium Phosphate system for butane oxidation to maleic anhydride (7 ). However the local structure of the catalytic sites is still a subject of discussion since, up to now, it has not been possible to study the characteristics of the catalyst under reaction conditions. Correlations have been attempted between catalytic performances obtained at variable temperature (380-430 C) in steady state conditions and physicochemical characterization obtained at room temperature after the catalytic test, sometimes after some deactivation of the catalyst. As a consequence, this has led to some confusion as to the nature of the active phase and of the effective sites. (VO)2P207, V (IV) is mainly detected by X-Ray Diffraction. 

Research related to the use of vanadium phosphates or V2O5 as oxidants of gases such as CO and SO2 in commercial processes shows that solid vanadyl sulfate can serve as a gas-permeable solid-phase electrolyte [102]. Two reversible redox features are observable at slow scan rates (20-150 mV s ) by CV in a gas-tight two-electrode cell packed with powdered VOSO4 3H2O between a 10-mm carbon disk and a 3-mm glassy carbon electrode. The V(IV/V) couple was observed at 0.55 V versus C, and the V(IV/III) couple was observed at —0.97 V. Unlike in aqueous solution where vanadyl sulfate is reduced to [V(H20)6] ", the V=0 bond in the solid remains intact. The oxidation of CO(g) can be observed when it is introduced into this cell. 

Vanadium is present as V " in stoichiometric VPP however, the latter can host V " or V species as defects, without undergoing substantial structural changes (5,6). Therefore, the role of the different V species in the catalytic behavior of VPP in n-butane oxidation has been the object of debate for many years (7-9). Moreover, the catalyst may contain crystalline and amorphous vanadium phosphates other than (VO)2P207 (10) for instance, outer surface layers of V phosphates may develop in the reaction environment, and play active roles in the catalytic cycle. This is particularly true in the case of the fresh catalyst, while the equilibrated system (that one which has been kept under reaction conditions for 100 hours at least) contains only minor amounts of compounds with V species other than V. ... 

However, their work was complicated by the fact that they used a porous platinum current collector underneath the oxide. If the gaseous atmosphere were able to come in contact with this porous platinum layer then, under reaction conditions, the platinum could exhibit a different oxygen activity from the vanadium phosphate making the e.m.f. of the cell extremely difficult to interpret. 

Herve et al. (57) investigated the thermal changes of structures by means of XRD and TG-DTA for Keggin-type heteropolyacids and proposed Scheme 2. Infrared spectroscopy of H4PMo, VO40 showed the release of vanadium atoms to form H3PM012O40 and vanadium phosphate species (55). Exposure to water vapor induces the decomposition of the latter (indicated by the disappearance of a band at ca. 1037-1030 cm -1) (58). 

The set of reactions defines the materials largely contained within this set of scientific studies. It deals mostly with vanadium oxides and vanadium phosphates followed by complex MMO phases and HPA. Figure 1.1 shows some relevant trends from the ISI database statistics. 

This loop is, however, affected by the availability of the reactant oxygen, which in surplus destroys the precursor VPO. Further, oxygen is positively needed to activate and re-oxidize the VxOy sites but leads also to more water formation that in turn hydrothermally deactivates the active mass. Likewise, water is needed to separate, via hydrolysis, the vanadium phosphate into VxOy and mobile phosphate. The multiplicity of the feedback loops is at first puzzling but explains the apparent stable steady state that can be reached with a catalyst undergoing so many chemical and microstructural transformations the multiplicity of controls prevents one single factor becoming dominant and thus potentially destabilizing the whole process. 

We shall summarize here fundamental results which point to newly discovered mechanisms which permit a control of ageing processes in catalysts. These mechanisms involve the acdon of surface mobile species, so-called spillover. The spillover species can stabilize catalysts against harmful solid-state reactions, in particular prevent reduction to less selective phases. Such reactions occur very frequently in selective oxidation catalysts, and constitute a major cause of deactivation. A typical example is constituted by vanadium phosphate catalysts used in the selective oxidation of butane to maleic ahydride. A few years ago, for example, many such catalysts lost a large part of their selectivity in a few months this selectivity dropped from the modest initial molar value of 55-60% to 45% or less. 

Another example is the activation of vanadium phosphate (VPO) catalysts in the oxidation of butane to maleic anhydride the fixed-bed VPO catalyst has to be contacted for several days with air-butane mixture before reaching normal activity. This case and that of Raney nickel (both unsupported) fall outside the scope of the present discussion... 

Hutchings GJ. Vanadium phosphate a new look at the active components of catalysts for the oxidation of butane to maleic anhydride. J. Mater. Chem. 2004 14(23) 3385-3395. 

Uranium mines are primarily the open pit type, but there is significant production from deep mines as well as from solution mining. Sometimes uranium is produced as a byproduct of mining operations for vanadium, phosphate, and gold. 

Stizza et al. (73,274) have investigated amorphous vanadium phosphates, which are also of interest in relation to a XAS study of the butane-maleic anhydride (V, P)0 catalysts (99a). From the V K edge useful information is obtained about the distortions in the vanadium coordination sphere [molecular cage effect on the pre-edge intensity (312)] and on the vanadium oxidation state. Notably, V4+ is silent to most spectroscopic methods. A mixed V4+-V5+ valence state can be measured from the energy shift of the sharp core exciton at the absorption threshold of the Is level of vanadium due to Is -f 3d derived molecular orbitals localized within the first coordination shell of vanadium ions. 

Vanadium Phosphate Materials as Selective Oxidation Catalysts... 

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. 

Ballarini et al. (8) posed the question of whether vanadium phosphate catalysts for n-butane oxidation offer the scope for further improvements. They concluded that as a consequence of the complexity of the dynamic surface species present on the catalyst, optimization of such material will not be forthcoming without further fundamental investigations. Previous investigations have involved probing of a number of catalyst parameters, including the V P ratio, the content of metal ion dopants, and the method of preparation. These and related topics are evaluated in detail below. 

Many well-characterized, crystalline vanadium phosphate phases have been identified, and their structures and catalytic properties have been well documented. Some of the most widely investigated are the V " ... 

FIGURE 1 The selective oxidation of n-butane to maleic anhydride catalyzed by vanadium phosphate. 

---