R. Spinicci

Synthesis and characterization of non-stoichiometric LaFeO3 perovskite

Abstract La (1− e ) FeO (3−1.5 e ) perovskite catalysts ( e =0,xa00.1,xa00.2,xa00.3) are synthetized by low-temperature thermal decomposition of La and Fe nitrates and further heating to 873 K in air. They are characterized by DTA, XRD, BET, SEM, Mossbauer spectroscopy and tested for the catalytic combustion of methane. All oxides are monophasic; for e =0 the structure is orthorhombic, whereas for 0 e ≤0.3 a pseudocubic symmetry is evident. The Mossbauer spectra are characterized by antiferromagnetic sextuplets; for e =0.3 a doublet is also present. The catalytic activity toward CH 4 oxidation appears to be strongly enhanced in the samples with e >0; the most active composition corresponds to e =0.1, while for e =0.2 and 0.3 the catalytic activity is slightly decreased. The results are explained assuming that the excess of iron oxide in the e >0 samples corresponds to the presence of surface of layers of FeO 5 polyhedra, coordinatively unsaturated, according to proposed structural model.

Catalytic properties of stoichiometric and non-stoichiometric LaFeO3 perovskite for total oxidation of methane

Abstract Perovskite LaFeO 3 and derived materials La (1− ϵ ) FeO (3−1.5 ϵ ) with ϵ =0.1, 0.2, 0.3 have been prepared as polycrystalline powders by thermal low-temperature decomposition of La and Fe nitrates, a method which ensures a high surface area. They have been used in catalytic methane combustion in the range 300–450xa0°C, showing a 100% selectivity to carbon dioxide. The reaction is indeed characterised by two ranges with different dependence of the reaction rate on temperature, as shown by the Arrhenius plot, whose slope decreases markedly above about 375xa0°C. Temperature programmed desorption experiments of oxygen allowed us to ascertain the presence of two surface oxygen species, whose desorption occurs, respectively before and after the above indicated temperature on this basis, we propose here an hypothesis on the way of working of the catalysts. The characterisation of the catalysts has been then completed by means of temperature programmed reduction, whose results contribute to strengthen the hypothesis that the catalyst reduction operated by methane involves a smaller number of lattice planes than reduction of hydrogen.

Catalytic behaviour of stoichiometric and non-stoichiometric LaMnO3 perovskite towards methane combustion

Abstract Three kinds of Mn-containing perovskites, LaMnO 3 , La 0.9 MnO 3 and LaMn 0.9 O 3 , have been investigated for methane combustion in the temperature range 475–600xa0°C. It has been possible to evidence that LaMn 0.9 O 3 shows the greatest catalytic activity, probably because of its surface area, and also because its defect structure is favourable for development of active sites. The conversion observed as a function of temperature shows two ranges of well-defined slopes in the Arrhenius diagram and an explanation of this phenomenon is then formulated on the basis of the occurrence of diffusional limitations. The catalysts have been then characterised by means of temperature programmed desorption and reduction. TPD experiments have revealed two forms of surface oxygen species and in the higher temperature range investigated in activity experiments it is possible to argue a mobility between the two related types of sites: an interconversion between the two types of active species is therefore possible and can be decisive, as in the case of La 0.9 MnO 3 .

Characterization of the catalytic activity of a thermally activated hydrotalcite-like compound in propylene oligomerization.

Abstract A hydrotalcite-like compound, Ni 4 Al 2 (OH) 12 CO 3 · nH 2 O, was precipitated on silica in order to obtain a precursor frequently used as a catalyst in propylene oligomerization, after thermal treatment up to 773 K. The effect of the decomposition temperature of Ni 4 Al 2 (OH) 12 CO 3 ·nH 2 O was studied and the temperature programmed reduction, made in order to establish the optimum pretreatment conditions. As revealed by ESR the chosen treatment yelded Ni(I) sites, responsible for the reaction. The significance of these Ni(I) sites was confirmed both by the minor role played in the reaction by the strongest Bronsted acid sites and by the experiments with adsorbed carbon monoxide , which revealed that, once the reduced metal ion sites are blocked, the catalytic activity became very low . Temperature programmed desorption runs with adsorbed propylene were also carried out in order to characterize the interactions between propylene and the catalyst.

Analysis of the properties of lithium-yttrium catalysts for methane coupling

The activity and the selectivity of two catalysts for methane coupling, namely 2% Li2O/Y2O3 and LiYO2, have been studied and analysed under both steady state and non-steady state conditions in the temperature range 670–760 °C. The role of the surface reaction and the characterisation of the surface species active in the catalytic reaction were the main goals of the investigation. With this aim, the investigation was centred on the analysis of some important effects on catalyst activity and selectivity: the effect of the ratio of methane to oxygen; that of the temperature; that of the contact time. With the same aim, a study was carried out under transient conditions at the very beginning of the reaction and an analysis of the apparent activation energy was made. The whole set of experiments was completed by a study of the reactivity of ethane, and its ability to undergo dehydrogenation and formation of ethylene under the same conditions of methane coupling. It was possible therefore to detect the strong adsorption of CO2 even at such high temperatures and its importance in reducing selectivity to C2-hydrocarbons; moreover the surface plays an important role in the overall reaction up to about 700 °C for Li2O/Y2O3 and up to about 730 °C forLiYO2. This result is confirmed also by the evidence that the apparent activation energy is higher at the lowest temperatures for both catalysts.

A study of the propylene oligomerization on silica-alumina supported nickel oxide

Abstract The oligomerization of propylene has been investigated on catalysts containing nickel oxide: it has been possible to check that NiO/Al2O3-SiO2 shows good levels of activity and selectivity, while NiO/SiO2 appears to be inactive. The presence of alumina appears therefore necessary to ensure these properties to the catalyst, but an important role is played by the Ni(I) species, which have been shown to exist on a catalyst surface when alumina is present. A correct reducing pretreatment (at high temperature or at low temperature under UV irradiation) is thus decisive in obtaining interesting activity and selectivity, thanks to the formation of surface Ni(I) species. Subsequently a kinetic analysis of the reaction by means of the temperature programmed reaction technique allowed us to conclude that the reaction itself appears to be governed by a bimolecular mechanism reinforcing the hypothesis that two types of sites are necessary for the progress of the reaction: the low valent nickel sites and the acidic sites of alumina.

Characterization by means of temperature programmed reduction and desorption of propylene metathesis catalyst

Abstract Temperature programmed desorption runs were performed on a 14% Re207/Al203 catalyst, prepared by using commercial amorphous alumina as support. By changing the adsorption temperature, some conclusions have been drawn about the adsorptive properties of this catalyst towards propylene, and subsequentely these properties have been compared with those towards ethylene and 2-trans- and 2-cis-butene. Finally, temperature programmed reduction runs allowed us to relate these features to the redox properties of the catalyst.

A study of the properties of catalysts for propylene metathesis by means of the temperature programmed reaction method

Abstract A previous study of propylene metathesis [1] has been extended to active phases formed either by Re2O7 plus MoO3 in various percentages or only by MoO3, always supported on Al2O3. Firstly it has been possible to verify that at 80 C a phase formed by equal weight amounts of MoO3 and Re2O7 gives a higher propylene yield, even if by lowering the selectivity. Temperature programmed desorption and reduction experiments highlighted the result that MoO3 (less reducible than Re2O7) allows the existence of a form of propylene more weakly chemisorbed on the surface, provided with a greater mobility and reactivity. The effect of temperature has been subsequently investigated by using the temperature programmed reaction method; the use of this technique allowed us to point out the variation of the catalytic conversion of propylene with temperature, in accordance with the different content of MoO3 and Re2O7 in the active phase. The kinetic analysis of the results of these experiments allowed us to check that the presence of Re2O7 evidences the existence of two temperature ranges, corresponding to different values of the activation energy: an appropriate procedure allowed us to obtain a value of the heat of adsorption of propylene on Re2O7 from these results. A correlation has been subsequently carried out between these results and those achieved from the temperature programmed desorption experiments, in order to express hypotheses on the type of the adsorbed propylene involved in the metathesis reaction on the investigated catalysts.