L. Lisi

Oxidative dehydrogenation of propane over vanadium and niobium oxides supported catalysts

Publisher Summary This chapter discusses the activity and selectivity of catalysts based on niobium and vanadium oxides supported on high surface area anatase TiO2 in ethane oxidative dehydrogenation (ODH). Specifically, the influence of the cooperation of vanadium and niobium oxides supported phases as components, inducing redox and acid properties, respectively, together with the effect of the preparation conditions on the catalytic performances have been studied. The vanadia–itania catalysts are very active, but with low selectivity, because of their high reducibility. Catalytic performances of VOx/TiO2 systems in ethane ODH are improved by the addition of niobium. When TiO2 is coimpregnated by vanadium and niobium oxides, the presence of niobium enhances the selectivity to ethylene at low vanadium content, whereas it slightly depresses the activity without enhancing the selectivity at high vanadium content. This should be because of the effect of niobium on vanadium reducibility, especially affected at low vanadium content. By changing the order of addition of vanadia and niobia to the support, catalysts with slightly different redox and acid properties are obtained. At low vanadium loading, supporting the two oxides at the same time results in the best catalytic performances, while at high loading a two steps impregnation gives the best results.

TiO2 supported vanadyl phosphate as catalyst for oxidative dehydrogenation of ethane to ethylene

Abstract Bulk and TiO 2 supported VOPO 4 has been investigated for the oxidative dehydrogenation of ethane. XRD, SEM, TG analyses and BET surface area measurements indicated that vanadyl phosphate is highly dispersed on the support up to mono-layer coverage. A fraction of vanadium is present as V(IV) in the calcined samples as evaluated by EPR and TPR techniques. Both reducibility and acidity of vanadium phosphate is strongly enhanced by deposition on TiO 2 with respect to the bulk phase, as shown by TPR and NH 3 TPD technique, respectively. The supported catalysts are active and selective in the oxidative dehydrogenation of ethane to ethylene in the temperature range 450–550°C, the mono-layer catalyst giving the best performances. Ethylene selectivity decreases with the contact time but increases with the temperature. The former effect indicates that ethylene is further oxidized to CO x at high contact times. The effect of the temperature was attributed to the formation of V(IV), favoured at increasing temperature. This hypothesis was supported by TPR experiments carried out after catalytic tests at 550°C that indicated a significant increase of the fraction of V(IV) after the reaction.

Effect of water on the kinetics of nitric oxide reduction over a high-surface-area V2O5/TiO2 catalyst

Abstract The influence of water on the kinetics of nitric oxide reduction with ammonia over a V 2 O 5 /TiO 2 catalyst has been investigated at different water partial pressures (0–300 Pa). An integral reactor operating at space velocity (sv) of 900 000 h −1 was employed. Nitric oxide and ammonia initial partial pressures were 10–200 Pa, oxygen 2700 Pa; the temperature ranged from 250 to 350°C. It has been found that the presence of water in the feed affects the reaction rate, inhibiting it. Two types of kinetic models based on a reaction mechanism involving the formation of a nitrosamidic intermediate compound were developed by assuming the adsorption of water as a reversible or irreversible step. Langmuir, Freundlich and Temkin equations for water adsorption were introduced in the kinetic models. It has been found that a kinetic model based on the irreversible adsorption of water following a Temkin isotherm allows the kinetics of nitric oxide reduction to be described for the whole range of water partial pressures. These results were interpreted according to a poisoning effect of the ammonia adsorbing sites by water.

Vanadium oxide catalysts supported on laser-synthesized titania powders: Characterization and catalytic activity in the selective reduction of nitric oxide

Abstract Laser-activated pyrolysis was applied to the synthesis of ultrafine TiO2, producing a non porous monocrystalline material in anatase phase with high specific surface area and uniform particle size. This powder was used as support for the preparation of vanadia catalysts with different compositions by wet impregnation. The pure TiO2 and the vanadia catalysts were characterized by X-ray diffraction, transmission electron microscopy, Fourier transform IR, X-ray photoelectron spectroscopy, ammonia temperature-programmed desorption and were tested in the reaction of nitric oxide reduction by ammonia. It has been proposed that two different VOx species formed in the submonolayer region: a VIV containing species which prevails up to 6 wt.-% V2O5 and a VV containing species which develops from 6 wt.-% V2O5 up to monolayer completion. The structure of these species has been hypothesized. A reaction scheme has been proposed to interpret the catalytic activity for the selective catalytic reduction (SCR) of NOx. The selectivity of catalysts for nitrogen is related to the prevalence of the VIV containing species.

Catalytic activity of powder and monolith perovskites in methane combustion

Abstract Honeycomb monolith perovskite catalysts were prepared from ultradispersed powders of mixed oxides of rare-earth metals (La–Ce or Dy–Y) and transition metals (Ni, Fe, Mn) by mechanochemical methods. A plasmochemical method was used to obtain La–Ni containing monoliths. The catalytic activity of powders and monoliths was compared in the catalytic combustion of methane. The intrinsic catalytic properties of the active components (apparent kinetic constant and energy of activation) were not significantly affected by the manufacturing procedure of monoliths in a large range of temperatures. Best performance was exhibited by La–Ni oxides containing monoliths which possess the highest pore volume and fraction of macropores.

COMPARISON OF BEHAVIOUR OF RARE EARTH CONTAINING CATALYSTS IN THE OXIDATIVE DEHYDROGENATION OF ETHANE

Abstract Catalyst promotion by addition of either La and Sm to MgO or Na aluminate to Sm2O3 and La2O3 has been investigated for the oxidative dehydrogenation of ethane in the temperature range 550–700°C. With all unpromoted and promoted catalysts, the selectivity to ethylene is strongly enhanced by the temperature, the highest values being obtained at 700°C. Sm2O3 is the most active among the bulk oxides, while samarium addition to MgO results in higher surface area, but does not enhance the catalytic activity. Ethylene productivity on La2O3 promoted MgO samples is higher than with pure La2O3, Sm2O3 and MgO, not only due to the stabilising effect of La on MgO surface area, but also due to a higher intrinsic activity. With both bulk oxides and rare earth promoted MgO, the selectivity to ethylene strongly increases by decreasing the O2/C2H6 feed ratio, while it is quite unaffected by ethane conversion and catalyst composition, in agreement with the hypothesis that the main role of catalyst in the experimental conditions investigated is to produce ethyl radicals which are converted in the gas phase to CO and C2H4. When La2O3 is modified by the addition of sodium aluminate the catalytic behaviour significantly changes, likely due to a different, mostly heterogeneous reaction mechanism. On aluminate promoted lanthana, ethane is converted to ethylene with higher yields which do not depend on the feed ratio. Moreover, only CO2 is produced as by-product, the formation of CO being quite negligible.

Enhancement of hydrothermal stability of Cu-ZSM5 catalyst for NO decomposition

RECu-ZSM5 samples (RE = La, Sm or Ce) have been prepared by ion-exchange of H-ZSM5 with rare earth and copper ions in an aqueous solution both simultaneously and consecutively (RE before copper), and their physico-chemical and catalytic properties in NO decomposition have been compared with those of a Cu-ZSM5 with a similar copper content. The catalysts have been characterized by N2 adsorption at 77 K (BET) and XRD before and after aging cycles under wet (2–2.5 vol % H2O) conditions at 450–500°C. NO decomposition tests have been carried out in a fixed bed reactor at 450°C under dry or wet conditions on both fresh and aged catalysts. NO adsorption tests have been performed at 120°C on the same samples. The addition of RE does not affect the crystalline structure, surface area, and porosity but strongly enhances the catalytic activity and the hydrothermal stability of Cu-ZSM5, which, on the contrary, is completely deactivated under wet conditions. The two-step exchanged catalysts provide better performances likely due to the larger RE exchange level obtained with this method. A unique linear correlation between the amount of N2O produced in the NO adsorption tests, related to the reoxidation of prereduced Cu+ sites, and the TOF, estimated from the catalytic activity tests, has been found for both fresh and aged catalysts.

Physico-chemical study of selective catalytic reduction vanadia-titania catalysts prepared by the equilibrium adsorption method

Abstract Sub-monolayer V 2 O 5 /TiO 2 catalysts were prepared by the equilibrium adsorption method using a high surface area TiO 2 anatase powder. They were characterized by the following techniques: X-ray diffraction, BET surface area measurements, electrical conductivity, electron paramagnetic resonance, UV-VIS, laser-Raman spectroscopy and 51 V solid state nuclear magnetic resonance (NMR). The physico-chemical characterization shows that vanadium is present in the 5+ oxidation state and that the vanadium oxide phase is well dispersed on the surface of the support. The conditions of preparation markedly influence the nature of the surface species: alkaline pH promotes the formation of isolated tetrahedral species, whereas with acidic pH more condensed vanadium oxide species, containing octahedrally coordinated vanadium, are formed. The calcination gives rise to a further aggregation of the surface vanadium oxide phase. The activity of the catalysts in the selective reduction of NO by NH 3 in the presence of O 2 was determined. The analysis of kinetic data combined with the results of physico-chemical characterization leads to a good relationship between the reaction rate and the amount of octahedrally coordinated vanadium, as evaluated by NMR spectra under ambient conditions.

Catalytic combustion of methane over transition metal oxides.

Simple and mixed metal oxides containing Co, Mn, Cr and Fe have been investigated as catalysts for the combustion of methane in the temperature range 300-600°C under diluted conditions. The effect of the catalyst composition on the catalytic performances and on the redox properties has been evaluated. Single metal oxides containing Cr, Co and Mn show comparable activities and were found more active than Fe 2 O 3 . Mixing of Co with Cr oxide and Fe oxide with Mg and Zn to give spinels, improves the catalytic activity with respect to pure compounds. Temperature programmed reduction (TPR) shows that redox properties are strongly dependent on the catalyst composition. Fe based mixed oxides are more hardly reducible than the other catalysts, this effect being related to the dilution of Fe with bivalent cations. The comparison of the kinetic parameters, evaluated on the base of a first order rate equation, gave evidence of a correlation between the activation energy values and the ease of the reduction showing that the oxides reducible at lower temperature give rise to a reaction mechanism with a lower activation energy.

Comparative study of catalytic behaviour of bulk-like and highly dispersed supported vanadyl orthophosphate catalysts in the oxidative dehydrogenation of ethane

The catalytic behaviour in the oxidative dehydrogenation of ethane of TiO2- and SiO2-supported catalysts containing bulk-like VOPO4 particles has been compared to that of TiO2-supported highly dispersed VOPO4. The catalysts have been characterised by XRD analysis and BET measurements. Redox properties have been studied by TPR experiments. A correlation of catalytic activity and C2H4 selectivity with redox properties has been proposed.