J. Stoch

Effect of alkaline promoters on catalytic activity of V2O5/TiO2 and MoO3/TiO2 catalysts in oxidative dehydrogenation of propane and in isopropanol decomposition

Oxidative dehydrogenation of propane has been studied on V2O5/TiO2 and MoO3/TiO2 catalysts, consisting of 1 and 5 monolayers of the deposited oxides, promoted with Li, K and Rb cations. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS) and by isopropanol decomposition, a probe for acid-base properties. For both types of catalysts, irrespective of the vanadium or molybdenum content, the total activity in oxidative dehydrogenation of propane decreased for the promoted catalysts in the sequence: non-promoted ≥ Li > K > Rb-promoted catalyst. The propene yields and selectivities at equal conversion increased in the same order. The same sequence was also observed for the increase in the rate of acetone formation and the decrease in the rate of propene formation in the isopropanol decomposition on the promoted catalysts. It is proposed that the alkaline promoters decrease the acidity and increase the basicity of the catalysts, and hence facilitate desorption of propene from the catalyst surface, preventing it from further oxidation to carbon oxides. The XPS data suggest that the alkaline promoters increase the dispersion of the vanadia or molybdena phase in the high vanadium or molybdenum content catalysts.

Effect of Mg and Mn oxide additions on structural and adsorptive properties of Cu/ZnO/ZrO2 catalysts for the methanol synthesis from CO2

Effect of addition of the Mg and Mn promoters on the activity of the Cu/ZnO/ZrO2 catalysts in the reaction of the synthesis of methanol from CO2 and H2, and the steam reforming of methanol, as well as on the catalysts’ adsorptive properties with respect to reactants (CO, CO2, H2O, methanol) was studied. Using the X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) techniques as well as investigating the reactive N2O adsorption, it was revealed that addition of the promoters leads to an increase of the copper dispersion in the reduced catalysts. The surface layers are depleted of copper and enriched in zinc and zirconium. The promoters introduced are accumulated preferentially on the catalysts’ surface. Correlation between the adsorptive properties and the catalytic activity was established. The overall factor combining the adsorptive properties favoring the synthesis of methanol (RAF) and the catalytic activity increase in the series CuZnZr

Structure and catalytic activity of double oxide system: Cu–Cr–O supported on MgF2

Abstract Two series of chromium–copper catalysts were prepared: co-impregnated and consecutively impregnated. On the basis of XPS, IR, EPR, X-ray investigations, and catalytic tests, a conclusion was drawn, that on the surface of each sample, an interaction between copper and chromium ions occurred, leading to an electron transfer from chromium to copper. As a result, Cr 6+ and Cu 1+ centres were formed. These sites were responsible for the catalytic activity of studied samples. The order of active phase deposition determined the properties of consecutively impregnated catalysts. The best performance was found for consecutively impregnated copper–chromium catalysts, which showed the highest influence of copper–chromium interaction on the properties of such systems. The surface structures of particular catalysts were proposed.

Mechanochemistry : the activation method of VPO catalysts for n-butane partial oxidation

Abstract The paper describes the influence of the introduction of alkali and alkaline-earth metals into the VPO precursor and the effect of bismuth additive introduced into the VPO precursor by the mechanochemical treatment. XRD, XPS, DTA, TPD of NH3 and determination of catalytic activity and selectivity in n-butane oxidation have been carried out. The preparations studied after mechanochemical treatment in a planetary mill were: (a) VPO, composed of VOHPO4 · 0.5H2O, (b) VPBiO, containing the bismuth additive introduced during synthesis, (c) VPO/Bi2O3 and VPO/BiPO4, mechanical mixtures of the VPO precursor and Bi2O3 or BiPO4, respectively. Introduction of alkali and alkaline-earth metals additives increases the basicity of surface oxygen in vanadium pyrophosphate, which entails increase in the rate of n-butane oxidation. The mechanochemical treatment of the preparations changed their morphology, favouring the formation of the vanadyl (001) plane in the phosphate precursor. The XRD peaks of bismuth phosphate appeared instead of those of bismuth oxide and the +0.8 eV chemical shift in the XPS Bi4f band was observed. The change in the chemical state of VPO/Bi2O3 are an evidence of the occurrence of chemical reaction. Mechanochemical treatment of VPO/BiPO4 and VPO/Bi2O3 catalysts increases their activity in n-butane oxidation and the selectivity to maleic anhydride. Thus, mechanochemistry may be a promising method for introduction of promoting additives into the basic VPO composition.

PROPERTIES OF COBALT-PROMOTED (VO)2P2O7 IN THE OXIDATION OF BUTANE

Abstract This preliminary study focuses on the influence of cobalt additives on the composition of the vanadium-containing catalyst, and on cobalts other properties which are important for the production of industrial V-P-O catalysts. The V-P-Co-O catalysts were prepared by (A) coprecipitation and (B) impregnation. ‘Fresh’ catalysts were composed of VOHPO4·5H2O phase independent of their method of preparation and cobalt content. After reaction the catalysts contained (VO)2P2O7. Cobalt was uniformly distributed in the pellets. Its presence increases the content of phosphorus at the surface, which modifies the surface acidity and in turn improves the selectivity for n-butane oxidation. No changes of the profile of phosphorus with depth were observed, even after 500 h on stream, the surface composition of the catalyst remaining unchanged. Cobalt stabilizes the catalyst performance by forming cobalt phosphate which reduces phosphorus losses, improves its catalytic properties and prolongs its lifetime.

Mechanochemistry as activation method of the VPO catalysts for n-butane partial oxidation

Abstract The mechanochemical treatment of the V P O catalysts caused a substantial increase of both the catalytic activity in n-butane oxidation and the selectivity towards maleic anhydride. It was accompanied by a relative growth of the contribution of vanadyl plane (001) of VOHPO 4 ·0.5H 2 O which transforms into the topologically similar (100) plane of (VO) 2 P 2 O 7 . The mechanochemical treatment of the V P O catalysts containing bismuth changes the sample morphology, favouring the formation of the vanadyl plane and provoking surface chemical reaction. The data obtained for V P O/BiPO 4 and V P O/Bi 2 O 3 samples demonstrate that mechanochemistry is a new promising method for the introduction of promoting additives into the basic V P O composition.

Influence of the mechanochemical treatment on the reactivity of V-containing oxide systems

Abstract Mechanical treatment of V 2 O 5 or V-P-O catalysts causes a substantial increase of both catalytic activity in n -butane oxidation and the selectivity to maleic anyhydride. Changes in specific surface area and anisotropic deformation take place. Mechanochemical treatment of the initial reagent impairs such properties that they remarkably influence the catalytic properties of the final catalyst.

Silica-supported rhodium complexes. relation between catalyst structure and activity

Abstract A series of heterogenized catalysts has been prepared by the reaction of chlorobis(ethylene)rhodium(I) dimer and phosphinated silica. The structures of these catalysts have been varied by changing the length of the phosphine-alkylene chain, -(CH2)nPPh2, used to couple the rhodium center to the silica surface. The dependence of the activity of the silica-attached catalysts on the structure has been studied in the hydrosilylation of hexene-1. It was found that the activity was ten times higher for the complex linked to silica through a short chain, containing one methylene group, than for the other rhodium complexes linked via alkylene chains containing two to six carbon atoms. Variatons in the activity have been attributed to the differences in the nature of the catalytic species formed on the surface. A binuclear structure for the catalysts of lower activity and a mononuclear one for the catalyst of higher activity have been postulated and confirmed by e.s.c.a. and i.r. measurements.

Reduction kinetics of CuO in CuO/ZnO/ZrO2 systems

Reduction of copper oxide in the CuO/ZnO/ZrO2 system, which is an oxide precursor of the catalysts active in the synthesis of methanol from CO2 and H2, was investigated. Preparations with varying CuO contents were obtained by co-precipitation or complexing the components with citric acid; also modifying additives (MgO, MnO) were introduced. The prepared materials were characterised by determining their surface area (BET), the active surface of Cu using the method of reactive adsorption of N2O, the size of the CuO crystallites by XRD line broadening, the surface composition by XPS, and the kinetics of reduction using TPR, XRD and gravimetric methods. It was found that the reduction of CuO was an autocatalytic consecutive reaction and that the intermediate product was amorphous Cu2O. The autocatalytic effect is due to facilitated dissociation of H2 on the metallic copper formed. Water formed during the reaction hinders the reduction by blocking the hydrogen adsorption centres. Changes in preparation methods, CuO content, as well as introduction of additives, affect the rate of the CuO reduction mainly by morphological changes: content of the amorphous CuO phase, size of the CuO crystallites and its surface segregation.

n-Butane oxidation on VPO catalysts. Influence of alkali and alkaline-earth metal ions as additions

Abstract The mechanism by which the introduction of some elements modifies principal properties of the VPO system in oxidation of n-butane to maleic anhydride has been studied. The incorporation of Li, Na, K, Cs, Be, Mg, Ca, Ba at different concentrations which can easily donate electrons to the framework of vanadyl phosphate with P V ratio = 1.07 and 1.20, leads to an increase of the effective negative charge on oxygen atom and of the rate of butane oxidation. The presence of additives causes an increase of the surface P V ratio and corresponding changes of acidic properties of the catalysts. Selectivity towards maleic anhydride passes through a maximum when plotted as a function of the amount of acidic centres at the surface. The preparation of a catalyst characterised by high activity in butane oxidation and high selectivity to maleic anhydride requires a fine tuning of the basicity of surface oxygen atoms to accelerate the activation of butane and of the acidity of the surface to secure the appropriate residence time of the reaction intermediates.