On the origin of the spontaneous formation of nanocavities in hexagonal bronzes (W,V)O₃

Abstract

Vanadium free WO3 and vanadium-containing W0.92V0.08O3-x oxides with the HTB structure have been prepared by hydrothermal synthesis. The addition of vanadium rises up the stability range of the hexagonal phase which became a stable polymorph up to 600º C. The structural characterization performed revealed the presence of a dense population of polyhedral nanocavities on the crystals surface distributed along particular crystallographic directions.Nanocavities are formed during the annealing process leading to catalyst activation and the process has been followed by TG/DTA-MS, HRTEM and

Structure–Reactivity Correlations in Vanadium‐Containing Catalysts for One‐Pot Glycerol Oxidehydration to Acrylic Acid

“…However, the diffraction peaks of these two samples are slightly broader than those of hexagonal tungsten/vanadium oxide,W V ( Figure 1, trace c). As previously demonstrated by high-resolutiont ransmission electronm icroscopy (HRTEM) with selected area electron diffraction (SAED), as well as morphological and spectroscopic analyses, [18,28,29] there is remarkable evidencet hat provest hat the majority of vanadium in VO-WO x is incorporated as extra-framework species.O nly if another transition element (e.g.,V ,N b, Mo, Ti)e nters the hexagonal tungsten oxide framework is the HTB structure stable at temperatures significantly higher than those bearable by the materialb uilt up by only Wa toms (WO x ). Ar ole might also be played by the lower heat-treatmentt emperatureu sed for WO x and VO-WO x relative to that used for WV (i.e.,4 50 vs. 600 8C);i ndeed, if heateda tt emperatures > 450 8C, the former two structures evolve into monoclinic forms (i.e.,W O 3 and V-doped WO 3 ).…”

“…Despite the fact that the content of vanadium in the ion-exchangeds ample was lower than that in WV (VO-WO x 0.15 vs. 0.21, see Ta ble S1), the distinct catalytic behavior cannot be attributed merely to the different compositions;i ndeed, previously reported V-containing HTBs with inframework Vs peciesa nd as imilar V/W ratio to that in VO-WO x showeda crylic acid selectivity > 20 %u nder identical reaction conditions (see Ref. Overall, although the absolutev alues of the product selectivities might also depend on the real accessibility of the Vs ites on the different samples (e.g.,a crolein and other C 3 moleculesc an enter the micropores of zeolites, [42] whereas HTBs have only external surface area availablef or catalysis), [29] the remarkable differences displayed by the catalysts reported herein as af unction of temperature and feed molarr atios open important questions. Moreover,d ifferently from WV,V O-WO x did not show any significant change in catalytic performance upon exposure to higher partial pressures of the reactants (see Figure S4 B).…”

Promoted Hexagonal Tungsten Bronzes as Selective Catalysts in the Aerobic Transformation of Alcohols: Glycerol and Methanol

“…The higher surface area of h-WOx can be related to the lower heat-treatment temperature (see sample WV-0 in ref. [19]). …”

“…Thus, tungsten bronzes crystals have been proposed as supports [20], catalysts [15][16][17][18][19] and photocatalysts [14a]. One interesting application is the use of multicomponent tungsten-based bronzes as selective catalysts in the one-pot oxidative transformation of glycerol to acrylic acid [15][16][17][18], in which both redox and acid sites are required for selective catalysts [16,18].…”

“…They can be ternary compounds of the type AxMyOz in which M is a transition metal (Mo, W, V, Nb, Ta, Ti, ..) and A is an alkali, alkaline earth or NH4 + ion [5,6], and seem to be of interest as catalysts and photocatalysts [7][8][9][10][11][12][13][14][15][16][17][18][19][20].…”