Giovanni Bagnasco

Decomposition of Hydrogen Peroxide on MnO2/TiO2 Catalysts

Catalysts containing 2-10 wt% MnO 2 supported on TiO 2 were examined for H 2 O 2 decomposition. Catalysts were prepared by impregnation of a high TiO 2 surface area. X-ray defraction patterns, scanning electron micrographs, and surface area measurements gave evidence of a uniform distribution of the active phase on the support surface and of the absence of segregated manganese oxides phases. Temperature programmed reduction measurements showed the presence, beside MnO 2 , of different Mn oxides species formed by interaction between the active phase and the support surface. The MnO 2 reducibility increased, whereas the mean oxidation state of manganese decreased with increasing manganese content. Catalytic tests were performed in a batch reactor with 50 or 70% concentration H 2 O 2 solutions. Catalytic activity was very high at the beginning of the tests and decreased with time, reaching a final constant value that increased with manganese content. Kinetic constants, evaluated assuming a first-order reaction rate, were comparable or higher than those of similar manganese-based catalysts. An optimal MnO 2 content was found, corresponding to the quite complete surface monolayer coverage. A reaction mechanism involving an Mn 4 + -Mn 3 + redox couple has been proposed.

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.

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.

TPD study of NH3 adsorbed by different phases of zirconium phosphate

Abstract Thermodesorption of NH 3 has been used to measure the acidity of α-zirconium hydrogen phosphate and different phases obtained from this material by thermal treatments. It was found that samples treated at temperatures lower than 300 °C, consisting of hydrated or anhydrous α-phases, adsorbed an amount of ammonia corresponding almost to neutralization of all acidic -POH groups and formation of a well-characterized diammonium phase. Samples treated at t ≥ 400 °C, in which partial or total condensation of interlayer -POH groups occurred, showed a strongly reduced capacity for adsorption of NH 3 , because of the formation of P-O-P bridges between layers which hindered diffusion of NH 3 . After pretreatment of Zr hydrogen phosphate at 600 °C, the TPD spectrum of ammonia adsorbed at room temperature showed only the signal of NH 3 adsorbed by surface -POH sites, indicating that its interaction with internal sites was now precluded. From the shape of the TPD curves from these samples information on the strength of surface acidic sites was deduced.

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.

Determination of the surface acidity of layered metal phosphates by NH3 temperature-programmed desorption.

Abstract The acid properties of layered Me phosphates (MeTi, Ge, Zr, Sn) were determined by NH3 temperature-programmed desorption. Careful establishment of the preheating conditions suppressed interference from NH3 intercalation and allowed the surface acidity to be measured. A scale of acidity strength in line with the catalytic behaviour of these phosphates in acid-catalysed reactions was obtained.

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.