M. Turco

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.

Layered zirconium-tin phosphates. II, Catalytic properties in the oxydehydrogenation of ethylbenzene to styrene

Abstract Zirconium-tin mixed phosphates have been studied as catalysts for the oxydehydrogenation of ethyl-benzene to styrene at T =400–500°C and atmospheric pressure. It has been found that these mixed compounds are much more active than pure zirconium and tin phosphates. Ethylbenzene conversion of ca. 50% with selectivity to styrene up to 90% has been obtained, the by-products being mainly CO x with lower amounts of benzaldehyde. The effect of operating parameters, such as contact time, reaction temperature and feeding ratio, has been the subject of a preliminary investigation. A reaction scheme is proposed in order to interpret these results. The influence of surface acidity on the reaction through the formation of a catalytically active coke is discussed.

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.

Tg/dta, Xrd and NH3-TPD Characterization of Layered VOPO4·2H2O and its Fe3+-Substituted Compound

Iron(III)-substituted vanadyl phosphate, [Fe(H2O)]0.20VO0.80PO4·2.25H2O (FeVOP), has been prepared and characterized by XRD and TG/DTA analyses. The new compound is isomorphous with layered tetragonal VOPO4·2H2O (VOP), but it possesses a lower interlayer distance. Information on the reactivity and surface acidity of both VOP and FeVOP has been obtained by NH3-TPD experiments. The hydrated materials adsorb high amounts of NH3 (up to 2 mmol g-1). Different ammonia-containing phases are formed, characterized by lower interlayer distances in comparison with the NH3-free parent compounds. NH3 is intercalated between the layers without displacement of water. The materials dehydrated by heat treatment at 450°C retain the layered structure but adsorb NH3 only on the external surface. A wide variety of acid sites, from weak to strong, was observed. A mechanism is proposed for the NH3- acid sites interaction. SEM micrographs of VOP and FeVOP are shown.

Studies on Water and Ammonia Programmed Thermodesorption of Mixed M(III)-vanadyl Phosphates

Hydrated M(III)-vanadyl phosphates (M (III)=Mn, Fe, Ga, Al) have been prepared and studied for water and ammonia adsorption properties by TG/DTA, NH3 TPD, FTIR and XRD techniques. The compounds have the same tetragonal layered structure of VOPO4 ⋅2H2 O, but shorter interlayer distances. Ammonia adsorption leads to intercalation of large amounts (0.19–0.39 mol/mol) of base between the layers of the materials, without displacement of water. The ammoniated phases obtained from these compounds have interlayer distances shorter than that of the corresponding precursors. In this connection an interaction mechanism NH3 -host is proposed. Treated at 450°C the materials adsorb ammonia only on the external surface because of the large decrease of the interlayer distance that prevents NH3 from entering the interlayer space. All M(III)-vanadyl phosphates present a wide distribution of strength of ammonia adsorbing sites.

Tin-Germanium Phosphates As Selective Catalysts For The Oxidative Dehydrogenation of Ethylbenzene To Styrene

Tin-Germanium phosphates, with formula Sn x Ge 1-x (HPO 4 ) 2 ˙H 2 O (O≤x≤ 1 ) were synthesized, characterized and tested as catalysts for the oxydehydrogenation of ethylbenzene to styrene. X-ray analysis showed that mixed compounds form solid solutions, in agreement with thermal analysis. Surface acid sites concentration increased with Ge content. Mixed compounds were more active than pure phosphates and were highly selective to ST (up to 97%), giving mainly CO x as byproducts. The role of acidity is discussed.

Layered zirconium-tin phosphates : I. Chemical and physical characterization

Abstract Crystalline zirconium-tin phosphates of formula Zr x Sn 1− x (HPO 4 ) 2 · H20 (0 ≤ x ≤1) were synthesized and characterized in terms of their physical and chemical properties. From X-ray and thermal analysis it was found that solid solutions are formed for every composition. The tin content in mixed phases has a marked influence on thermal behaviour, speeding up the kinetics of transformation to layered pyrophosphates (L-Py). Mixed phosphates possess surface areas which are markedly higher than pure phosphates. The method of ammonia thermodesorption (TPD) was employed for the acidity measurements. This method has allowed the evaluation of the concentration and strength of surface acid sites present on the external surface of L-Py or mixed hydrogen-L-Py phases obtained after a variety of thermal treatments. The acid strength of L-Py phases increases with increasing tin content.

Iron modified vanadyl phosphate as oxidation catalyst

Iron modified vanadyl phosphate with formula [Fe(H 2 O)] 0.2 (VO) 0.8 PO 4 2.25H 2 O has been characterized by XRD analysis and NH 3 TPD technique. Catalytic oxidation properties were tested by NH 3 oxidation reaction. XRD analysis showed that the compound is isomorphous with layered tetragonal VOPO 4 .2H 2 O and is stable up to 600°C. NH 3 TPD measurements gave evidence of a wide distribution of adsorbing sites, the strongest ones being able to oxidize NH 3 mainly to N 2 , and to N 2 O and NO in lower extent. NH 3 adsorbing properties were affected by the presence of water vapour. Catalytic tests showed appreciable activity, unlike the iron free compound that was found inactive. Catalytic activity was inhibited by water vapour. A reaction mechanism involving cooperation of Fe and VO groups was proposed.

NH3 TPD study and thermal behaviour of vanadium-exchanged titanium phosphates as catalysts : Reduction of NO with NH3

The ion exchange technique was employed for the preparation of VO2+ modified titanium phosphates as catalysts for the selective reduction of NO with NH3. The samples were prepared by contacting with a vanadyl sulphate solution different precursor materials, amorphous, crystalline or sodium half exchanged titanium phosphate. The vanadium contents of modified phosphates were in the range 0.08–2.3 wt%. XRD and thermal analysis TG/DTA showed that vanadium loading does not cause structural modification in hydrogen titanium phosphate. A vanadyl containing phase was obtained when half sodium titanium phosphate was employed. The NH3 TPD measurements indicated the presence of a wide distribution of NH3 adsorbing sites with medium-high strength. Catalytic activity measurements were performed under dilute conditions. It was found that the presence of vanadium even in low amounts strongly promote the catalytic activity.

Catalytic combustion of methane over LaMnO3 perovskite supported on La2O3 stabilized alumina. A comparative study with Mn3O4, Mn3O4-Al2O3 spinel oxides

Ten and 20 wt % LaMnO 3 perovskites supported on La 2 O 3 -stabilized γ-Al 2 O 3 were studied for catalytic combustion of methane. A comparison with Mn 3 O 4 and Mn 3 O 4 -Al 2 O 3 spinel oxides was also drawn. The catalysts were characterized by microanalysis, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and O 2 temperature-programmed desorption (TPD) techniques. Catalytic activity tests were carried out in a fixed-bed reactor at T=300–800°C, space velocity = 40000 h −1 , CH 4 concentration = 0.4% v/v, O 2 concentration = 10% v/v. Both XRD and microanalysis indicated a uniform dispersion of the perovskite phase. The structure of γ-alumina was retained after the treatment at 800°C, the treatment at 1100°C led to the transition to the θ and β phases. TPR measurements suggested the presence of a fraction of Mn 4+ in supported perovskites. The possible interaction of manganese with alumina, which stabilizes Mn 2+ , led to the reduction of the initial average oxidation state of manganese with the perovskite content and the temperature of treatment. O 2 desorption in TPD measurements was significant from spinel oxides, whereas negligible from supported perovskites. Supported perovskites gave complete CH 4 conversion within 650°C with 100% selectivity to CO 2 . The activation energy value, evaluated from a methane first-order rate equation, suggested the occurrence of the same reaction mechanism of unsupported LaMnO 3 . The preexponential factors of the catalysts treated at 800 °C were proportional to the perovskite content, in agreement with a monolayer model. Samples treated at 1100°C showed the same activity not depending on the perovskite content, suggesting that only a fraction of manganese in the 20 wt % LaMnO 3 is available for the reaction. This was related to the stabilization of a fraction of Mn 2+ , probably not involved in the reaction. Spinel oxides catalyze the reaction at lower temperature, giving complete conversion within 600°C with 100% selectivity to CO 2 . The activation energy was lower than that of supported perovskites. A correlation with the ability to desorb O 2 was hypothesized.