Monolayer- and crystal-type MoO3 catalysts: Their catalytic properties in relation to their surface structures

Various MoO3 catalysts have been prepared by means of adsorption of molybdenum on supports from molybdate solutions or from the gas phase. Complete monomolecular layers of Mo6+ oxide can be prepared on the carriers Al2O3, Cr2O3, TiO2, CeO2, and ZrO2, whereas on SiO2 crystallites of MoO3 are formed. Reduction experiments show that the higher valencies of Mo are stabilized in the case of a monomolecular layer. Alcohol dehydration, pentene hydrogenation, and poisoning of these reactions with pyridine reveal that MoO2 present as a monolayer is less acidic than crystalline MoO2. On the complete monolayer catalysts investigated, mostly more than 70% of the dehydration and hydrogenation activities can be correlated with sites showing a relatively high acidity which are equivalent to 10–20% of the Mo content. The CO oxidation rates on the oxidized catalysts are antiparallel to those of the reactions on the reduced ones mentioned above; relatively basic sites preferentially chemisorb CO. The conclusion is that the activity pattern of the catalysts is a function of the acidity of the supports. It is suggested that Mo5+ ions contribute to the formation of the active acid sites after reduction with hydrogen

Vanadium oxide monolayer catalysts : The vapor-phase oxidation of methanol

The oxidation of methanol over vanadium oxide, unsupported and applied as a monolayer on γ-Al2O3, CeO2, TiO2, and ZrO2, was studied between 100 and 400 °C in a continuous-flow reactor. At temperatures from 150 to about 250 °C two main reactions take place, (a) dehydration of methanol to dimethyl ether and (b) partial oxidation to formaldehyde. A very slight direct oxidation to CO2 proceeds simultaneously. At higher temperatures two further reactions take place, i.e., (c) consecutive oxidation of the ether and/or formaldehyde to CO and (d) consecutive oxidation of CO to CO2. Selectivity to formaldehyde increased with decreasing reducibility of the catalyst, which in turn was a function of the catalyst-support interactions. Since the reducibility of V(V) has been shown to be related to the charge/radius ratio of the cation of the carrier, the selectivity to formaldehyde is also determined by this ratio

Structure and reactivity of titania-supported oxides. Part 1: vanadium oxide on titania in the sub- and super-monolayer regions

Vanadium oxide has been deposited on TiO2 (washed anatase, 10 m2g−1; Degussa P-25, 55 ±3 m2g−1; Eurotitania, 46 m2g−1) by aqueous impregnation of (NH4)2[VO(C2O4)2] and by reaction with VOCl3, VO(OR)3 (R=iBu) and VO(acac)2 in organic solvents. Single applications of the last tree reagents form not more than a monolayer of vanadium oxide VOx, a monolayer being defined as 0.10 wt.% V2O5 per m2 of surface. When less than about four monolayers of VOx are present, there is in most cases only a single TPR peak: Tmax values, which increase with V2O5 content, are almost independent of the method used but vary slightly with the support (P-25 < Eurotitania < washed anatase). The 995 cm−1 band, characteristic of V&z.dbnd;O in V2O5, only appears when more than a monolayer of VOx is present.\ud \ud In the sub-monolayer region, VOx is best formulated as an oxohydroxy species bonded to two surface oxygens. As the V2O5 content is increased, layers of disordered V2O5 are formed on limited areas of the surface, but crystalline V2O5 only occurs, probably on top of the disordered V2O5, when the V2O5 content exceeds about four monolayers, and takes the form of acicular crystals exposing only planes perpendicular to the a and b axes

Solid state aspects of oxidation catalysis

The main subject of this review is the consideration of catalytic oxidation reactions, which are greatly influenced by solid state effects in the catalyst material. Emphasis is laid upon the correlation between the presence of mobile ionic defects, together with the associated ionic conductivity, and the catalytic performance. Both total and selective oxidation reactions and oxidative conversion reactions are considered. Well-known examples of such behaviour include oxidative methane conversion with lanthanide oxides, carbon monoxide oxidation on fluorite type catalysts, selective olefin oxidation using vanadia based catalysts, etc. Furthermore, because oxygen exchange between gas and solid is always part of the oxidation process, this is considered too.\ud \ud The discussion of the application of the solid oxides under consideration to practically important oxidation processes, together with the influence thereon of their solid state properties, forms a major part of this review. Computational modelling and simulation of catalyst structure and behaviour is also considered.\ud \ud Special attention is given to the potentialities offered by using ionic and mixed conducting oxides either as the electrode material in a solid electrolyte fuel cell (SOFC) or as a separating, dense membrane in a ceramic membrane reactor. The use of porous membranes in such reactors is also taken into consideration. On the one hand these may be used to study the above relationship between catalytic behaviour and solid state properties, on the other hand to obtain a reactor configuration allowing better use of reactants and/or catalysts. Besides the controlled supply of (or removal) of oxygen to (or from) the side where the catalyst and the reactants are located, a promising feature of both experimental approaches is that the oxygen flux may alter the relative presence of different oxygen species (O2,O,O2−,O22−,O3−,O−, etc.) on the catalyst surface. In this way species are provided having a strong influence on the selectivity for partial oxidation reactions and oxidative conversion reactions.\u

Electrical and catalytic properties of some oxides with the fluorite or pyrochlore structure. CO oxidation on some compounds derived from Gd2Zr2O7

The catalytic properties of some mixed zirconates with the pyrochlore or fluorite structure have been investigated using CO oxidation as the test reaction. The presence of terbium ions, leading to mixed conductivity, and the extent of pyrochlore ordering affect the kinetic behaviour and the catalytic activity of the investigated materials. Bismuth-containing compounds show an increased rate of reoxidation