Vanadium activity detection

Fig. 3. compares the ammonia conversion for nanostructured vanadia/TiOa catalysts pretreated with O2 and 100 ppm O3/O2 gases. The reactions were conducted at 348 K for 3 h. No N2O and NO byproducts were detected in the reactor outlet. It is clear from the figure that higher vanadium content is beneficial to the reaction and ozone pretreatment yields a more active catalyst. Unlike the current catalysts, which require a reaction temperature of at least 473 K, the new catalyst is able to perform at much lower temperature. Also, unlike these catalysts, complete conversion to nitrogen was achieved with the new catalysts. Table 2 shows that the reaction rate of the new catalysts compared favorably with the established catalysts. 

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. 

The change in the oxidation state of the vanadium ion has also been observed in the ESR spectra of the soluble V(acac)3/A1(C2H5)2C1 catalyst at various temperatures. At temperatures below —40 °C no ESR signal could be detected, which suggests that the vanadium ions exsist in the trivalent state. A broad ESR signal (AH 20 mT) apperared at g — 1.98 at temperatures above —30 °C, and its intensity increased with temperature to reach a constant value at 20 °C. Thus, these spectral data indicate that the vanadium species active for the living polymerization of propylene are in the trivalent state. 

In 2005, a selection of pre-catalysts were employed in the vanadium-catalyzed oxygenation of 3,5-ditert-butylcatechol. ESI-MS experiments that were conducted on the post-reaction solutions led to the detection of two common negative ions [V0(DTBC)2]- (Eig. 2C) and [V(DTBC)3]- (DTBC = 3,5-di-tert-butylcatecholate dianion) [41]. Through kinetic experiments the species corresponding to [V(DTBC)3] was ruled out as the catalytically active species and the neutral species that was shown to correspond to [VO(DTBC)2] , namely (VO(DBSQ)(DTBC))2 (DBSQ=3,5-di-tert-butylsemiquinone anion), was reported as a common catalyst for this reaction. While this study describes an adventitiously-charged system (the proposed active catalyst is in fact neutral), it is described here along with the other vanadium-catalyzed systems for cohesion. 

In addition to the structure in the dehydrated state, the structure of supported vanadia catalysts under redox reaction conditions is directly related to the catalytic performance. Vanadia catalysts are usually reduced to some extent during a redox reaction, and the reduced vanadia species have been proposed as the active sites [4, 19-24]. Therefore, information on the valence state and molecular structure of the reduced vanadia catalysts is of great interest. A number of techniques have been applied to investigate the reduction of supported vanadia catalysts, such as temperature programmed reduction (TPR) [25-27], X-ray photoelectron spectroscopy (XPS) [21], electron spin resonance (ESR) [22], UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) [18, 28-32], X-ray absorption fine structure spectroscopy (XAFS) [11] and Raman spectroscopy [5, 26, 33-41]. Most of these techniques give information only on the oxidation state of vanadium species. Although Raman spectroscopy is a powerful tool for characterization of the molecular structure of supported vanadia [4, 29, 42], it has been very difficult to detect reduced supported... 

Currentiy, trace metal pollution is not a problem in the Amazon River, with the exception perhaps of localized small-scale occurrences. However, the basin includes several sites of elevated metal releases linked to gold and manganese ore mining and industrial discharges from the large cities. Seyler and Boaventura discuss the likelihood of contamination from these sources. Moreover, they come to the provocative conclusion that elevated fluxes of manganese, copper, vanadium, arsenic, and nickel may already be detectable at the river s mouth due to anthropogenic activities in the basin. Possible metal contamination should be taken seriously in the Amazon, as even small amounts of the more toxic metals may lead to widespread adverse effects in the river s aquatic systems. 

A novel class of haloperoxidases, in which a heme prosthetic group was absent, was detected in brown algae (Phaeophyceae) by Vilter and coworkers (13-15). These publications escaped the attention of most biochemists involved in peroxidase research. Only when some of this work was published in the journal Phytochemistry (16) was there as increasing awareness of these findings. A clue for the involvement of vanadium was also published (16). It was shown that the bromoperoxidase could be inactivated at low pH and reactivated by vanadate. These results were subsequently confirmed (17, 18) when it was shown that vanadium was present in a number of bromoperoxidases from different sources and was essential for enzymatic activity. To date, these sources include the enzymes from the brown seaweed Ascophyllum nodosum... 

Interparticle mobility is proven by electron microprobe scans of cyclic metal impregnated (CMI)[6] Residcat 767Z4+ which incorporates RV4+ technology. Since the catalyst and the RV4+ were simultaneously exposed to the metals during the CMI procedure, the rate of deposition of vanadium on the catalyst and trap surfaces should be similar. However, the catalyst particles, contain virtually no detectable vanadium. In contrast, the RV4+ particles containing the Active Trap Component are high in vanadium. This is another indication of particle to particle vanadium mobility[6]. ... 

Epitaxy may reasonably be mentioned for the VOx/anatase system. But there are other cases where epitaxy is doubtful. There is no detectable evidence of Sb204 decorating epitaxially FeSb204. If the picture of oppositely oriented flat pyramids as active sites for butane oxidation is correct, the hypothesis attributing a crucial role to an epitaxy between various vanadium phosphates may lose credibility. 

Vanadium phosphates (VPO) of different structure are suitable precursors of veiy active and selective catalysts for the oxidation of C4-hydrocarbons to maleic anhydride [e.g. 4] as well as for the above mentioned reaction [5,6]. Normally, VOHPO4 Va H2O is transformed into (V0)2P207 applied as the n-butane oxidation catalyst. Otherwise, if VOHPO4 V2 H2O is heated in the presence of ammonia, air and water vapour a-(NH4)2(V0)3(P207)2 as XRD-detectable phase is formed [7], which is isostructural to a-K2(V0)3(P207)2. Caused by the stoichiometry of the transformation reaction (V/P = 1 V/P = 0.75) (Eq. 2) and the determination of the vanadium oxidation state of the transformation product ( 4.11 [7]) a second, mixed-valent (V 7v ) vanadium-rich phase must be formed. 

Vanadium heteropolymolybdate-t-BuOOH remain unaltered like that of hydrogen peroxide system. The active species in the case of t-BuOOH system could not be detected. 

Many industrially important selective oxidation reactions are catalyzed by transition metal oxides. The activity of such catalysts is related to the reducibility of the transition metal ion, which enables the bulk oxide lattice to participate actively in the redox processes present in the Mars van Krevelen mechanism. Unfortunately, NMR spectroscopic investigations are severely limited by the occurrence of paramagnetic oxidation states. As a general rule, NMR signals from atoms bearing unpaired electron spins cannot be detected by conventional methtxls, and the spectra of atoms nearby are often severely broadened. For this reason, most of the work published in this area has dealt with diamagnetic vanadium(V) oxide-based catalysts. 

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