Marco Faticanti

Catalytic activity of LaMnO3 and LaCoO3 perovskites towards VOCs combustion

The catalytic combustion of some volatile organic compounds (VOCs) has been investigated over LaMnO3 and LaCoO3 perovskite-type oxides. Redox titration has shown that cobalt is present in LaCoO3 exclusively in the 3+ oxidation state whereas LaMnO3 contains considerable amount of Mn4+ (35%) in addition to Mn3+. VOCs reactivity towards total oxidation follows the trend: acetone > isopropanol > benzene. The oxidation of isopropanol occurs through the formation of acetone in the homogeneous reaction. The increase of the oxygen partial pressure is beneficial for total oxidation of acetone. The adsorptive properties of the catalysts towards the VOCs and H2 have been examined by means of temperature programmed desorption. The LaMnO3 surface is the most reactive to the adsorption of VOCs and H2. The role of the adsorbed oxygen has been studied by examining the variations of the electrical conductivity of the catalysts during the processes of oxygen adsorption–desorption.

Anderson-type heteropolyoxomolybdates in catalysis: 2. EXAFS study on γ-Al2O3-supported Mo, Co and Ni sulfided phases as HDS catalysts

Abstract The nature of the Co (or Ni) and Mo sulfided species obtained starting from γ-Al 2 O 3 -supported Anderson heteropolyoxomolybdates (CoMo 6 and NiMo 6 ) has been analyzed by means of the extended X-ray absorption fine structure (EXAFS) technique with the aid of temperature-programmed reduction (TPR) and spectroscopic measurements. Results are discussed in relation to the adsorption isotherms and to the catalytic activity for the thiophene hydrodesulfurization (HDS) reaction. A comparison with traditional catalysts has been made.

XAS characterization and CO oxidation on δ-alumina supported La, Mn, Co and Fe oxides

δ-Al2O3 supported La, Mn, Co and Fe containing catalysts were prepared by impregnation of δ-Al2O3 with citrate-type precursors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray absorption spectroscopy (XAS), and BET specific surface area determination. XRD revealed the presence of δ-Al2O3 in all cases and, at 30 wt.% of metal loading, of other single (α-Mn2O3 and α-Fe2O3) or mixed (LaAlO3, LaMnO3, CoAl2O4 and LaFeO3) oxide phases. XAS suggested the formation of some oxide phases also at lower loading. In particular, all Mn and 10 wt.% La–Mn containing samples revealed the formation of α-Mn2O3, while the 30 wt.% La–Mn bimetallic sample showed the formation of LaMnO3 perovskite. All Co containing samples revealed the presence of CoAl2O4 spinel. Fe containing samples showed the formation of α-Fe2O3, while La–Fe containing ones, that of LaFeO3 perovskite. Catalytic tests of CO oxidation were performed in the temperature range 300–800 K. The sample containing 30 wt.% of La and Mn in the form of LaMnO3 perovskite dispersed on δ-Al2O3 was found the most active among all the examined catalysts. Most of the Co containing catalysts were found active at RT too, but they deactivated rapidly. None of the Fe-based samples was active at RT and these catalysts were found, on average, to be substantially less active than the Mn- and Co-based ones.

LaAl1-XMnXO3 perovskite-type oxide solid solutions: structural, magnetic and electronic properties

The structural, magnetic and electronic properties were investigated for LaAl1−xMnxO3 (x = 0.0, 0.005, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) perovskites prepared by the citrate method and calcined at 1073 K. The materials were characterised by XRD, BET surface area, diffuse reflectance spectroscopy (DRS), magnetic susceptibility measurements and electron paramagnetic resonance (EPR). XRD analysis showed that all the LaAl1−xMnxO3 samples are single phase perovskite-type oxides. A non-linear increase of the unit cell lattice parameters with x was observed. Particle sizes and surface areas were in the range of 270–1200 A and 4–33 m2 g−1, respectively. The oxidation state of manganese in the higher Mn-containing samples was checked by chemical titration, the following Mn4+/Mntot ratios being found: 0.45 for x = 0.4, 0.43 for x = 0.6, 0.49 for x = 0.8, and 0.35 for LaMnO3. Optical spectra revealed some bands attributable to both Mn4+ and Mn3+ species. A magnetic susceptibility study showed ferromagnetic behavior that decreases with the decrease of x, the high Al-containing materials exhibiting a paramagnetic behavior. Depending on the Mn-content, EPR analysis revealed the occurrence of several Mnn+ species: Mn4+ exchange-coupled by ferromagnetic interaction to nearest neighbor Mn3+, Mn4+ exchange-coupled by antiferromagnetic interaction to near-nearest neighbor Mn3+, isolated Mn4+, isolated Mn2+.

LaAl1−xCrxO3 perovskite-type solid solutions: Structural, electronic, magnetic properties and catalytic activity towards CO oxidation

Perovskite-type LaAl1−xCrxO3 samples were prepared from citrate precursors and calcined at several temperatures up to 1073 K. The materials were characterised by XRD, diffuse reflectance spectroscopy (DRS), EPR and magnetic susceptibility measurements. At 773 K all samples are amorphous, at 923 K the perovskite structure starts to form in addition to other La–Cr oxides such as LaCrO4 and La2CrO6, whereas at 1073 K all samples are single phase LaAl1−xCrxO3 perovskite-type solid solutions. EPR analysis confirmed the presence of both Cr3+ and Cr5+ in the materials fired at 923 K. DRS patterns, magnetic susceptibility measurements and EPR analysis confirmed the presence of Cr3+ alone in all the perovskite-type solid solutions obtained at 1073 K. For these samples: (i) the particle sizes and surface areas were found to be in the 610 to >1000 A and 3–7 m2 g−1 ranges, respectively; (ii) a linear increase of the unit cell volume, V, with x was observed, in agreement with expectations based on the ionic size of interchanging Al3+ and Cr3+ ions. The perovskite samples calcined at 1073 K were tested for CO oxidation. All Cr-containing materials are much more active than LaAlO3 pointing to the essential role of the transition metal ion in developing highly active catalysts. The activity trend and the variation of the activation energy with the increase in chromium content suggest a change in the active site from mononuclear to polynuclear.