R. Grabowski

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

The role of different MoO3 crystal faces in elementary steps of propene oxidation

Abstract Activity and selectivity of MoO 3 crystallites of different properties in the oxidation of allyl compounds were compared with those observed in the oxidation of propene in order to elucidate the role of different crystal faces in the elementary steps of oxidation. The yield of acrolein from allyl compounds was linearly dependent on the surface area of the basal (010) face, indicating that this face is responsible for insertion of oxygen into the activated hydrocarbon molecule. Product distribution from propene may be explained by assuming that its activation takes place at the side (100) and (101) faces, and the total oxidation at all faces.

Oxidative dehydrogenation of isobutane on supported chromia catalysts

Abstract Oxidative dehydrogenation, ODH, of isobutane has been performed on chromia supported on SiO 2 , Al 2 O 3 , TiO 2 , ZrO 2 and MgO. The potassium promoted (K/Cr= 0.1) catalysts have also been examined. The supported chromia has been found active in the ODH of isobutane at relatively low temperatures 200–400°C, the total activity and selectivity to isobutene depending on the nature of the support and the potassium presence. The highest selectivities to isobutene (ca. 70% at 5% conversion) and the isobutene yields of ca. 9% have been obtained for Cr/O x TiO 2 and K-promoted CrO x /Al 2 O 3 preparations.

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.

Effect of doping of TiO2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia-titania catalysts

Vanadia phase (one monolayer) was deposited on TiO2 anatase doped with Ca2+, Al3+, Fe3+ and W6+ ions and the catalysts thus obtained (VMeTi) were characterized by XPS, work function technique, decomposition of isopropanol (a probe reaction for acido–basic properties) and tested in oxidative dehydrogenation of propane. The doping of the TiO2 support modifies physicochemical and catalytic properties of the active vanadia phase with respect to the undoped TiO2. The specific activity in the propane oxydehydrogenation decreases in the order: VFeTi>VWTi>VTi>VAlTi>VCaTi (3), whereas the selectivity to propene follows the sequence: VWTi VTi>VFeTi>VAlTi>VCaTi. This implies that the lower is the surface energy barrier for transfer of electrons from the catalyst to the reacting molecules the higher is the selectivity to the partial oxidation product. It is argued that owing to the decrease in this energy barrier the reoxidation step in the catalytic reaction, involving such a transfer: O2+4e→2O2− is fast, thus, preventing the presence of intermediate non-selective electrophilic oxygen species on the surface.

Catalytic activity of V2O5−TiO2 systems in the oxidation of o-xylene

The catalytic activity of V−Ti−O catalysts in the oxidation of o-xylene was studied with the pulse method for samples of different V/Ti ratios. The selectivity to C8 products of oxidation changes with the V2O5 content, the maximum selectivity being observed at low V2O5 contents (2–10 mol%).AbstractКаталитическая активность катализаторов V−Ti−O при окислении o-ксилола была изучена импульсньм методом в случае различных отношений V/Ti. Селективность по отношению к продуктам окисления C8 изменяется с изменением содержания V2O5; максимальная селективность наблюдалась при низком содержании V2O5 (2–10 мол. %).

Effect of alkali metals additives to V2O5/TiO2 catalyst on physicochemical properties and catalytic performance in oxidative dehydrogenation of propane

The effect of alkali metal additives Li, K, and Rb to V2O5/TiO2 catalyst on the rate of catalyst reduction with propane and reoxidation with oxygen, sorption of propene, and the electron work function has been examined. The results have been correlated with the catalytic performance in oxidative dehydrogenation, ODH, of propane. It has been found that the rates of reduction, reoxidation and the ODH of propane decrease in the order: VTi>LiVTi>KVTi>RbVTi. The activation energies of the reduction and reoxidation are not, however, affected by the presence of the alkali metals. The same sequence has been observed for the work function values of the catalysts. It is argued that alkali metal poisons the centres of the hydrocarbon activation. The yield and selectivity to propene in the ODH of propane increase, however, for the promoted catalysts, following the above sequence. This effect is ascribed to the decrease in the heat of the propene adsorption, which is due to the increase in the basicity and decrease in acidity on the promoted catalysts.

Oxidation of C2C4 alkanes on chromium oxide/alumina and on Cr2O3: catalytic and TPD studies

Abstract Oxidation of ethane, propane and n-butane on chromium oxide supported on alumina and on unsupported chromia yields carbon oxides as the main reaction products. The selectivity to olefins, the oxidative dehydrogenation products does not exceed about 8% at 10% conversion for propane and n-butane and 1% for the ethane reaction. Oxidative dehydrogenation is the main reaction for the isobutane reaction, the latter hydrocarbon showing, moreover, the highest overall activity. The TPD experiments show that the adsorbed alkane species desorb in the form of CO2 and H2O, only traces of olefins being observed.

Kinetics of oxidative dehydrogenation of propane over V2O5/TiO2 catalyst

Application of the pseudo-homogenous and steady-state adsorption model (SSAM) was analyzed for the description of the oxidative dehydrogenation (ODH) of propane over vanadia-titania catalyst, using an extensive set of the experimental data. It has been shown that the SSAM model adequately describes the kinetics of the studied reaction for a broad range of feed mixture, propane conversion and temperature. The pseudo-homogenous models must be consider as a rough approximation of the experimental results. It is possible to solve analytically the set of differential equations describing the SSAM model and to present the selectivities to propene and to the products of total oxidation (CO, CO2) as a function of the propane conversion and the temperature. Numerical methods (the Runge–Kutta method combined with the Levenberg–Marquardt method) were applied to solve the systems of differential equations which describe the kinetic of the ODH of propane as a function of contact time and to fit the solutions to the experimental data. The calculations allowed us to determine the rate constants (activation energies and pre-exponential factors) for all reaction steps considered.