Genesis of V4+ in heteropoly compounds CsxH4-xPVMo11O40 during thermal treatment, rehydration and oxidation of methanol studied by EPR spectroscopy

Abstract

4). of the bulky caesium counter-cations prevents the removal of vanadium from the Keggin unit to the secondary structure and thus the formation of vanadyl bridged polymers. 2. Experimental Materials and treatment Detailed preparation procedures for the acid form and the caesium salts of the heteropoly compounds

Influence of dehydration effects on the optical spectra of H4PVMo11O40 in the visible and near infrared range: intra- and intercenter optical transitions in the V–Mo cluster

“…The quantitative determination of the vanadium (IV) content by EPR shows [13] that in the untreated heteropoly compounds vanadium is present as V(IV), and its amount is about 0.5-2 mol-%. In addition, the EPR spectra evidence proves that the removal of water by a He stream at room temperature for 24 hours does not change the degree of reduction.…”

UV/Vis/near-IR spectroscopic characteristics of H4–xCsxPVMo11O40 (x = 0, 2) catalyst under different temperatures and gas atmospheres

“…At this initial stage there is no reduction, the HPA is partly in the hydrated phase, the protons are not localized and reside on the bridging water moieties H 5 O 2 + . However, the quantitative determination of the V 4+ content by EPR [10] shows that even on this stage vanadium is present as V 4+ , its amount is about 0.5-2 mol %. At the same time a small amount of Mo 5+ is also detected.…”

“…Recently, the caesium containing compound with x = 2 proved to be the best catalyst among the different title compounds, for instance for the partial oxidation of ethene and propene by molecular oxygen [14][15][16]. An important role in the structural dynamics of this type of compound is played by the reducibility of the cationic centres, which was investigated by EPR spectroscopy [10]. Changes in the redox state of heteropoly compounds can also be investigated by the UV/Vis/near-IR spectroscopy.…”

“…Unfortunately, however, the title compound in particular with x = 0, is thermally unstable, which makes it especially difficult to determine the catalytically active center. Many efforts have been made to investigate the thermally unstable state, for example, by XRD [2,4], TG-DTA coupled with IMR-MS and DSC [2,5], IR [4,[6][7][8], EPR [9,10] and Raman [11][12][13]. To improve the stability and catalytic activity of such compounds, cations were introduced in the secondary structure [14,15].…”

Structural Evolution under Reaction Conditions of Supported (NH4)3HPMo11VO40 Catalysts for the Selective Oxidation of Isobutane

“…Thus not only vanadium was eliminated from the primary structure, but it was also reduced at the same time. In the case of CsxH4−xPVMo11O40, the reduction was explained by an auto-reduction process within the catalyst, the O 2− lattice oxygen being simultaneously oxidized to O2 [32]. However, in our case, the explanation lies in the elimination of ammonium, which is oxidized to ammonia by means of lattice oxygen, with concomitant reduction of V and/or Mo.…”