Tribomechanical modification of Bi promoted vanadyl phosphate systems 1: An improved catalyst and insight into structure–function relationship

Abstract

Bi promoted VOHPO 4 ·0.5H 2 O (Bi-VHP) was mechanically treated in ethanol and air for several times and the morphology was studied by several characterisation techniques such as SEM, TEM, EELS, BET and XRD. The data were compared to those from the reference sample (Bi-VHP unmilled) and to those from a sample thermally treated in vacuum. The equilibrated product (VO) 2 P 2 O 7 (VPP), that was obtained from Bi promoted VOHPO 4 ·0.5H 2 O was included in the comparison. SEM shows that the reference sample consists mainly of flat smooth needles, that form a blossom secondary morphology. Under mechanical treatment the secondary morphology is lost and packages form with coin-shape particles. XRD revealed that milling in ethanol for 5 min reduced the particle size but did not change the phase. Milling in air for 28 min resulted in an amorphous phase, whereas treatment above 30 min led to crystalline VPP. Thermal activation induced a phase change to VPP with the initial material still being present as a minority phase. The ball mill method is effective for catalysts activation as it increases the specific surface area and hence activity and selectivity per volume. On the other hand ball milling destroys the crystallinity and reduces the conversion and yield per unit surface area.

Surface‐Charge‐Induced Reversible Phase Transitions of Bi Nanoparticles

“…The phenomena described below are, in general, observed for the Bi nanoparticles supported on all the three different substrates used in the present work (VPO (vanadium phosphorus oxide catalyst material promoted with Bi), [16] C-films, and Bi agglomerates) in the same electron beam intensity range. The study of the fluctuations is based on a large series of observations.…”

“…Bi supported on vanadium phosphorus oxide (VPO) was prepared using Bi-enriched VPO samples designed for butane oxidation [16] dispersed on carbon-coated Cu grids as starting material. The VPO samples are extremely sensitive to the electron beam in the TEM and lead to the formation of nanometersized Bi particles.…”

Some properties of milled vanadium phosphates

“…Pure and doped vanadium phosphates were prepared via precipitation from vanadium pentaoxide and phosphoric acid in presence of organic reducer (Ayub et al 2003;Hutchings 2004). The typical procedure of synthesis was following: corresponding amounts of V 2 O 5 , oxides of metals (ratio V:P:Me = 1:1.15:0.025 for Bi or 0.1 for Zr and Mo), n-butanol and oxalic acid were refluxed with excess H 3 PO 4 for 16 h at 117°C.…”

“…At the same time, mechanochemical treatment (MChT) allows to activate solid and, especially, increase its dispersity (specific surface area) and significantly change its surface characteristics (Heinike 1984), including vanadium phosphates (Ji et al 2002;Ayub et al 2003). Therefore, the study of interfacial processes solid/liquid for activated phosphates, particularly VHP, in aqueous solutions of electrolytes seems to be interesting and useful.…”

Mechanochemical synthesis of barium titanate and its photocatalytic properties

“…The latter fact can be explained in the following way. It is well known that milling results in appearance of structural defects in solids [12] which play a part of additional catalytically active centres [24] included in photocalytic processes [6,11,19,25]. Milling leads to titanium reduction and formation of oxygen vacancies at grain boundaries.…”