Vanadate

About: Vanadate is a research topic. Over the lifetime, 4497 publications have been published within this topic receiving 120109 citations. The topic is also known as: vanadate.

V2O5-WO3/TiO2 catalysts under thermal stress: Responses of structure and catalytic behavior in the selective catalytic reduction of NO by NH3

Abstract: V2O5-WO3/TiO2 catalysts of compositions typically used in SCR applications were subjected to calcinations in air at temperatures between 873 K and 1023 K for different durations, starting from a mildly calcined state in which species structure originating from wet preparation had been fixed by calcination at 623 K for 1 h. After calcination, samples were examined with respect to their SCR activity and to structural changes by nitrogen physisorption, XRD, Raman spectroscopy, EPR, temperature-programmed reduction, XPS and Low-Energy Ion Scattering (LEIS). Driven by loss of BET surface area, samples exhibited a complex behavior with up to three well-separated maxima of SCR activity before final deactivation. While the first of these maxima depended on the calcination rate (temperature), the other two maxima occurred at specific ranges of the BET surface area. During calcination, tungstate was segregated from the support surface forming WO3 under more severe conditions. Observations in the EPR spectra are in conflict with assignments of observed SCR activities to isolated vanadate sites. Assuming major contributions of dimeric vanadate to the catalytic performance, a model was proposed, which explains the first two activity maxima while the origin of the third one remains to be elucidated. In this model, segregation of tungstate results in formation of active V-O-V structures from less active isolated vanadate species previously separated by excessive amounts of tungstate. The main role of tungstate is to provide optimum sizes of vanadate ensembles, but a direct favorable influence of tungstate on vanadate is not excluded.

Characterization of in vitro proton pumping by microsomal vesicles isolated from corn coleoptiles.

Abstract: Corn (Zea mays L. cv Golden Cross Bantam) coleoptile microsomal vesicles have been isolated which are capable of ATP-driven H+-transport as measured by [14C]methylamine accumulation and quinacrine fluorescence quenching. Formation of the pH gradient in vitro shows a high specificity for ATP·Mg, is temperature-sensitive, exhibits a pH optimum at 7.5, and is inhibited by carbonyl cyanide-m-chlorophenylhydrazone. Of the divalent cations tested, Mn2+ is almost as effective as Mg2+, while Ca2+ is ineffective. Excess divalent cations, particularly Ca2+, reduces the pH gradient. H+ transport is strongly promoted by anions, especially chloride, while potassium does not affect pump activity. Studies with 36Cl− indicate that ATP-driven H+ transport into the vesicles is associated with chloride uptake. Both carbonyl cyanide-m-chlorophenylhydrazone and the anion transport inhibitor, 4,4′-diisothiocyano-2,2′-disulfonic acid stilbene, inhibit methylamine accumulation and 36Cl− uptake. Proton pumping is also blocked by diethyl stilbestrol and N,N′-dicyclohexylcarbodiimide, but is insensitive to oligomycin and vanadate. These properties of the pump are inconsistent with either a mitochondrial or plasma membrane origin.

Mechanistic Studies of Methanol Oxidation to Formaldehyde on Isolated Vanadate Sites Supported on High Surface Area Anatase

Abstract: The mechanism for methanol oxidation on both TiO2 and V/TiO2 was investigated using temperature-programmed experiments with in-situ infrared spectroscopy. Infrared and Raman spectroscopy, along with XANES, show that the V/TiO2 sample consists predominantly of isolated VO4 units after calcination. Methanol was found to adsorb on the catalyst in three ways at 323 K: (1) molecularly, (2) across Ti−O−Ti bonds to form Ti−OCH3/Ti−OH pairs, and (3) across V−O−Ti bonds to form V−OCH3/Ti−OH pairs. Upon heating, two desorption peaks for CH3OH and H2O were observed on all samples below 500 K. Although TiO2 produced small amounts of CH2O, the addition of vanadium greatly enhanced the rate of formaldehyde formation. Also, on the V/TiO2 samples, it was noticed that the Ti−OCH3 groups disappear much more rapidly than on TiO2 alone. This is likely due to the reverse spillover of methoxide species from Ti to V, with the reaction occurring at lower temperatures at the vanadium center. Formate species were also detected du...