P.J. Gellings

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 VzO in V2O5, only appears when more than a monolayer of VOx is present. 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.

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.

Selective gas phase oxidation of toluene by vanadium oxide/TiO2 catalysts

Several supported metal oxide catalysts were studied qualitatively in the selective gas phase oxidation of toluene. Of these catalysts the combination of vanadium oxide and TiO2 was most effective and was therefore studied more thoroughly. Two types of TiO2 were used as support. For catalysts supported on Degussa TiO2 the catalytic properties improved with increasing vanadium content up to monolayer coverage becoming constant at higher loadings, indicating that the vanadium oxide present as multilayers and/or crystallites does not affect the catalytic reaction. The activity and selectivity of catalysts supported on Tioxide TiO2 were much lower and increased continuously with increasing vanadium content (up to 4 x monolayer coverage). This different catalytic behaviour is attributed to the presence of impurities (phosphorus and potassium) in the Tioxide material, which had a large, negative effect on the catalytic properties.

Preparation of supported vanadium and molybdenum oxide catalysts using metal acetylacetonate complexes

Supported vanadium and molybdenum oxide catalysts were prepared by reaction of the corresponding acetylacetonate complex in a non-aqueous solution with the surface hydroxyl groups of the carrier. Continuous or batch adsorption of the metal acetylacetonate from toluene, as well as wet impregnation from ethanol, resulted in a uniform coverage of the support. The applied metal oxide was probably present on the surface as a monomolecular dispersion. When readsorption or reimpregnation from toluene was carried out with TiO2 as support, metal oxide crystallites were formed, which could readily be detected with laser Raman spectroscopy. Reimpregnation from ethanol led to a complete occupancy of the surface hydroxyl groups of the carrier without the formation of metal oxide multilayers or crystallites.

Factors influencing the temperature-programmed reduction profiles of vanadium pentoxide

The temperature-programmed reduction (t.p.r.) of bulk V2O5 has been examined as part of a study of the reducibility of V2O5-containing catalysts. T.p.r. profiles have been studied as a function of flow rate, heating rate and sample weight. From experiments at different flow rates it is concluded that the order of the reduction rate in hydrogen is low or even zero. A rule of thumb has been derived to provide an easy check on possible exhaustion of hydrogen in the feed. The influence of sample weight and heating rate is explained in terms of the formation of water in the sample during reduction. The reduction of bulk V2O5 to V2O3 proceeds in several steps; intermediate species include V6O13 and VO2. The apparent activation energy of ca. 200 kJ mol–1 indicates that solid-state diffusion influences the reduction process of V2O5.

Infrared study of the selective oxidation of toluene and o-xylene on vanadium oxide/TiO2

Infrared spectroscopy was used to obtain information on the mechanism of the selective oxidation of toluene and o-xylene over vanadium oxide catalysts. The interaction of these aromatic hydrocarbons and the products benzaldehyde and o-tolualdehyde with the surface of a V2O5/TiO2 monolayer catalyst was investigated at different temperatures under conditions comparable to those of the catalytic reaction. The infrared results obtained for each of these compounds showed a great resemblance, indicating that their oxidation proceeds along the same reaction path. Coordinatively adsorbed aldehydes, carboxylate-like structures, and benzoate species could be identified as intermediates on the surface of the catalysts. On the basis of the spectroscopic observations a possible reaction mechanism has been proposed.

Influence of phosphorus and potassium impurities on the properties of vanadium oxide supported on TiO2

The catalytic properties of vanadium oxide catalysts supported on TiO2 from Tioxide were strongly affected by phosphorus and potassium, present as impurities in the TiO2 support. The effects observed were stronaly dependent on the type of hydrocarbon oxidised. In the oxidation of toluene to benzoic acid the impurities had a large negative influence on the activity and maximum yield. For the oxidation of o-xylene to ohthalic anhydride this negative effect was only observed at relat- ively low vanadium contents. At higher contents (above monolayer coverage) improved catalytic properties were obtained for catalysts supported on the contaminated TiO2 support. When the phosphorus and potassium impurities were both largely removed by extraction with water optimum catalytic behaviour was achieved at much lower vanadium contents in both oxidation reactions. The effect of each of the two impurities separately was also investigated using vanadium oxide catalysts deliberately contaminated with various amounts of either phosphorus or potassium. From the results of the catalytic oxidation experiments it was concluded that the addition of phosphorus resulted in an increase of the surface acidity of vanadium oxide/TiO2 catalysts. The effect of potassium was much larger and was attributed to an alternation of the nature of the reactive sites, possibly because of the formation of amorphous bronzes.

Vanadium Oxide Monolayer Catalysts

The oxidation of methanol over vanadium oxide, unsupported and applied as a monolayer on yAlsOa, CeOz , TiO, , and ZrOa , 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 COr 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 CO*. 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.

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.

The role of potassium as a promoter in iron catalysts for ammonia synthesis

Five ammonia synthesis catalysts, mainly differing in potassium content, were prepared from a commercial doubly promoted iron catalyst. The activities of these catalysts were measured at 350–450 °C and 5–200 atm. The experimental reaction rates were fitted to the modified Temkin rate equation. Increasing the potassium content from 0.1 to 3.8 wt% results in increasing the order in H2 from 0.7 to 1.5. The change from singly to doubly promoted behavior is gradual. The nature of catalysts with a relatively high K-content is changed, preventing the formation of NH (or NH2) groups so that N-atoms become the main species on the surface, which explains the higher order in H2.