Bk. Hodnett

Influence of P/v Ratio On the Phase-composition and Catalytic Activity of Vanadium Phosphate Based Catalysts

Abstract The influence of the bulk P/V ratio of V-P-O catalysts on their phase composition, surface P/V ratio and catalytic activity for n-butane partial oxidation is reported. Catalysts were prepared by reduction of V2O5 in lactic acid followed by addition of O-H3PO4. Excess lactic acid was evaporated off and the solid precursor calcined in air at 773 K. The bulk P/V ratio was varied in the range 0.94 to 1.07. This range was sufficient to bring about large variations in the nature of the final catalyst obtained. Catalysts with tow P/V ratios formed as βVPO5 with vanadium in the +5 oxidation state. Those with high P/V ratios formed in a phase which we label β in which vanadium was in the +4 oxidation state. Scanning electron microscopy indicated that the catalysts increased in crystallinity as the P/V ratio decreased and EPMA indicated that the P/V ratio was homogeneous throughout each catalyst. Overall conversion of n-butane was at a maximum for P/V close to unity whereas selectivity was highest for the highest P/V ratio studied. The XPS study indicated that the surface P/V ratio did not change in the range of bulk compositions in which selectivity increased from ca. 10 to 50%. Arguments are presented which suggest that it is the solid state properties of these catalysts which largely determine selectivity and that the primary interaction of the hydrocarbon occurs with the aid of a V+5 related surface phase.

Influence of Metal-support Interactions On the Dispersion, Distribution, Reducibility and Catalytic Activity of Ni/sio2 Catalysts

Abstract The influence of metal-support interactions on the dispersion, distribution, reducibility and catalytic activity for benzene hydrogenation of Ni/Si0 2 catalysts, prepared by impregnation or deposition-precipitation, was studied. Weak interact-ions were observed for the former; the latter technique gave rise to strong inter-actions. Nickel contents were from 6–9 wt%. The impregnated precursor featured nickel evenly distributed between the internal (pores) and external surface of the support. Direct reduction did not noticeably change the distribution and a Ni/SiO 2 catalyst formed in which the active phase was finely divided and evenly distributed. Calcination of this precursor caused migration of the nickel to the external surface with the formation there of large crystallites of NiO. The pre-cursor prepared by deposition-precipitation, consisted of nickel hydro-silicate deposited uniquely on the external surface and in the pore mouths. Calcination or reduction of this material did not noticeably change the distribution, but high dispersion was observed in both cases. The stability of catalysts prepared by this technique is discussed in terms of interaction between nickel and silica.

Influence of the Phosphorus Vanadium Ratio On the Solid-state Chemistry and Redox Properties of Vanadium Phosphate-based Catalysts

The influence of the PV atomic ratio on the redox properties of two series of V-P-O catalysts was investigated. The catalysts were calcined at 773 or 923 K and the PV ratio was varied in the range 0.94 to 1.07. The 4+ oxidation state of vanadium was highly stabilized during calcination at 773 K by a slight excess of phosphorus. This stabilization was less pronounced at the higher calcination temperature. The influence of the PV ratio on the reactivity toward hydrogen of the catalysts calcined at 773 K was difficult to establish because the phase composition of these samples varied over a large range. It was established for the series calcined at 923 K that excess phosphorus, far from activating reduction of vanadium, tended to inhibit this process. Excess phosphorus also inhibited reoxidation of the bulk of these catalysts but tended to have little effect on the reoxidation of near surface layers. The catalytic activity in n-butane partial oxidation to maleic anhydride was compared with the redox behavior and a model for selective oxidation is presented which envisages excess phosphorus as regulating the flow of oxygen anions from the bulk to the surface. The primary interaction is postulated as being between the hydrocarbon and oxygen anions associated with a V5+ related phase.

Influence of Calcination Conditions On the Phase-composition of Vanadium-phosphorus Oxide Catalysts

Abstract The influence of calcination time and temperature on the formation of phases related to VPO catalysts with P/V ratios in the range 0.90 to 1.10 has been studied. In the preparation, lactic acid was used for the reduction of V2O5 and the reduced vanadium complex thus formed was reacted with o-H3PO4. The evolution of phases was followed by XRD. For calcinations at 773 K, low P/V ratios favour the formation of βVPO5; high ratios (>1) favour the formation of a reduced phase labelled β* (d = 4.67, 4.07, 3.14 and 2.59A). For all P/V ratios studied, this phase was not stable in air at 773 K but transformed slowly into βVPO5. The β*/βVPO5 ratio did not influence the surface area. The β* phase was not observed after calcination above 873 K. For calcination of catalysts with a P/V ratio of 1.10, carried out at 923 K, a reduced phase with principal d spacings at 3.90, 3.13 and 2.99Aand variously labelled B, β and (VO)2P2O7 in the literature, was observed.

Factors influencing the activity and selectivity of vanadium-phosphorus oxide catalysts for n-butane oxidation to maleic anhydride

Abstract The object of this study was to characterise the activity and selectivity of several series of vanadium-phosphorus oxide catalysts for n-butane conversion to maleic anhydride. Catalysts with P:V ratios in the range 0.94–1.10:1 were prepared. The first series was calcined in air at 773 K and was characterised by average oxidation states of vanadium between +4 and +5. For the second series calcined at 923 K this parameter was always close to +5 and was changed to +4 by reduction in hydrogen at 723 K. These catalysts were tested in conditions where changes in bulk composition were minimal. The results suggest high activity and good selectivity was obtained from catalysts which were composed of oxidized surface layers built upon a reduced core.

Influence of Cobalt On the Textural, Redox and Catalytic Properties of Stoichiometric Vanadium Phosphate

The influence of coprecipitated cobalt on the catalytic, redox and textural properties of βVPO5 is reported. Four catalysts were prepared in which the Co/V ratio was varied between 0 and 0.05 and the P/V ratio was held at unity. Cobalt strongly reduced the surface area of βVPO5 and diminished reactivity toward hydrogen was observed as a monotonous feature as the cobalt content increased. By contrast this material showed complex kinetic behaviour during reoxidation from reduced states. When reoxidation was confined to surface layers a maximum in rate was observed for Co/V ratios of ca. 0.02. Bulk reoxidation was strongly sensitive to small amounts of cobalt but it was independent of cobalt content above Co/V = 0.01. Results of catalytic testing for n-butane partial oxidation to maleic anhydride indicate that conversion was at a maximum for the undoped material but selectivity showed an optimum value for Co/V of ca. 0.02. Above this value a sharp drop in selectivity was observed. A strong increase in areal reoxidisability was observed for Co/V above 0.02. X.P.S. data indicated that some surface segregation of cobalt had occured for Co/V = 0.05. Two domains of influence by cobalt on the catalytic activity of βVPO5 were identified; one in which it influenced the solid state properties and increased selectivity, the other in which a surface enrichment in cobalt provoked total oxidation of n-butane.

Influence of coprecipitated cobalt on the reducibility of vanadium phosphate

The reducibility of β-vanadium phosphate doped with cobalt has been measured and correlated with changes in the textural properties brought about by this dopant.