Transient responses of the local electronic and geometric structures of vanado-molybdo-phoshate catalysts H3+nPVnMo12−nO40 in selective oxidation

The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

“…In-situ UV-Vis-near-IR spectroscopy A description of the modified UV-Vis-near-IR diffuse reflectance spectrometer, the home-made flow-through microreactor and other experimental details can be found elsewhere [10,12]. All spectroscopic measurements were carried out with Spectralon ® (Labsphere) as reference material.…”

“…The widths of the distribution of desorption temperatures and their starting points depend on sample composition (structure) and on experimental conditions (kinetics). After the removal of crystal and constitutional water further oxygen evolution takes place and the systems undergo internal redox reaction [10]. This complex reactivity that is partly reversible with temperature calls for an in-depth structural study in order to identify the true nature of the catalytically active material.…”

“…This can be studied in situ using optical spectroscopy. An essential advance in the studies of optical spectra of catalysts of different stages of their transformation was achieved by applying the in situ diffuse reflectance UV/Vis/near-IR spectroscopy (DRS) [10,11], which proved to be a suitable technique for probing both d-d and charge transfer transitions at reaction temperature and under realistic gas compositions [12]. A number of groups have reported exsitu optical spectra [13,14,15,16,17].…”

“…Mixed phases of partially salified HPA have been proposed forming a core-shell system of the Cs 3 A-salt and the free acid under catalytic conditions [25]. Furthermore, it has been suggested that the molybdenum cations [28] as well as the vanadium cations [29,30] migrate from the Keggin anion to cationic positions in the structure [31,32] and that the heteropoly acids become partially reduced under reaction conditions [33,34]. This migration of cations from the Keggin anion has been speculated to be responsible for the catalytic activity of the heteropoly oxomolybdates with Keggin structure [25,29,30,33,35,36,37].…”

“…Again, no Lewis sites were found. However, as HPA are thought to activate oxygen through a Mars-van-Krevelen mechanism [10,42], the presence of oxygen vacancies and, thus, Lewis acid sites, i.e. coordinatively unsaturated (cus) cations, appears plausible.…”