G. Kremenic

Chemisorption and catalysis on LaMO3 oxides

The adsorption of oxygen and isobutene and the catalytic activity for propene and isobutene oxidation have been studied on a series of LaMO3(M = Cr, Mn, Fe, Co and Ni) perovskite oxides. Coadsorption results point to the non-competitive adsorption of oxygen and isobutene; i.e. these molecules adsorb on different centres. Oxygen adsorption underwent a remarkable increase after isobutene had been preadsorbed on these oxides (enhanced adsorption). Activation energies for complete oxidation ranged between 16 kcal mol–1(LaMnO3, LaCoO3 and LaNiO3) and 31 kcal mol–1(LaFeO3). LaCrO3 showed some activity for methacrolein formation. Adsorption and catalytic-activity profiles showed maxima for LaMnO3 and LaCoO3. These results are discussed within the framework of the ideas of Dowden and Wells on the local symmetry of surface cations and its influence on chemisorption and catalysis and show the importance of localized interactions in the processes studied.

Surface properties of Co-precipitated VTiO catalysts and their relation to the selectiveoxidation of isobutene

Abstract The interaction between V 2 O 5 and TiO 2 (anatase) in a series of co-precipitated V Ti O catalysts was studied by X-ray diffraction, X-ray photoelectron spectroscopy, temperature-programmed reduction, laser-Raman spetroscopy, and catalytic performance for isobutene oxidation. It was found that the catalytic activity of these preparations depended on the V/(V+Ti) ratio, reaching a maximum for V/(V+Ti) = 0.11. For this catalyst the X-ray photoelectron spectra showed surface enrichment in a vanadium oxide phase, whereas the temperature-programmed reduction profiles revealed the lowest onset temperature for hydrogen reduction. Moreover, while X-ray diffraction data and Raman spectra showed that the oxidic vanadium species were highly dispersed over TiO 2 in V/(V+Ti)≤0.11 catalyst, the appearance of Raman bands at 998 and 697 cm −1 in vanadium-rich [V/(V+Ti)=0.2 and 0.5] catalysts were clear proof of the presence of tridimensional V 2 O 5 crystallites. The same conclusion was also reached by line broadening analysis of the most intense diffraction V 2 O 5 peaks in the X-ray diffraction patterns of the latter preparations, for which an average crystal size of 30.2 nm was calculated. From these data it seems that activity is directly related to a highly dispersed oxidic vanadium phase.

Selective oxidation of propene over supported vanadium oxide catalysts

Abstract This paper reports on selective oxidation of propene with oxygen in the presence of steam on Al 2 O 3 -, SiO 2 - and TiO 2 (anatase)-supported vanadium oxide catalysts. It was found that V 2 O 5 /TiO 2 preparations were more highly active catalysts than the V 2 O 5 /SiO 2 or V 2 O 5 /Al 2 O 3 preparations. Selectivity for acetic acid production increased with decreasing reducibility of the catalysts, which, in turn, was a function of the active phase-support interactions. Surface characterization of the catalysts was carried out by X-ray photoelectron (XPS) and laser Raman (LRS) spectroscopic techniques. The Raman spectra revealed that V 2 O 5 crystallites formation occurs at vanadium loadings of about 80% (V 2 O 5 /TiO 2 ) or 60% (V 2 O 5 /Al 2 O 3 ) of a vanadium oxide monolayer completely covering the surface of the support. On the V 2 O 5 /SiO 2 catalyst series, V 2 O 5 crystallites are present at extremely low vanadia coverages. The ratio of V 2 p 3/2 to M 2 p (M = Al, Si and Ti) obtained by XPS was taken as a measure of the relative dispersion of V 2 O 5 on different supports.

Acidity and oxidation activity of MoO3−UO3/SiO2 catalysts

The effect of UO3 on the acidity of MoO3−UO3/SiO2 catalysts has been studied by means of infrared spectroscopy of adsorbed pyridine. The surface acidity exhibited a maximum for the same U/(U+Mo) atomic ratio (=0.11) that yielded a maximum in total conversion for isobutene oxidation. The catalytic properties for oxidation are discussed in terms of the acidic properties of the samples.AbstractВлияние UO3 на кислотность катализаторов MoO3−UO3/SiO2 исследовали с помощью ИК спектроскопии адсорбированного пиридина. Поверхностная кислотность достигает максимума при том же значении атомного отношения U/(U+Mo)=0,11, при котором наблюдалось максимальное общее превращение в окислении изобутилена. Каталитические свойства окисления обсуждаются, исходя из кислотных свойств образцов.

A study of the role of bismuth in molybdenum-praseodymium-based catalysts

Abstract The effect of bismuth content in Mo-Pr-Bi-O catalysts used for propene oxidation has been studied. The catalytic activity for the overall oxidation process goes through a maximum for a Bi:Mo atomic ratio of 0.075. Selectivity for acrolein undergoes a remarkable increase and selectivities for other partial oxidation products decrease with increasing bismuth content. The catalytic activity of oxidation and selectivity for acrolein are much higher than those recorded on Mo-Pr-O catalysts. The surface acidity exhibited a maximum at the same Bi:Mo atomic ratio which yielded the maximum catalytic activity. X-ray photoelectron spectroscopy data indicated that bismuth is uniformly distributed in the catalysts up to a Bi:Mo ratio of 0.075, but a surface enrichment of bismuth occurred at higher bismuth contents. The catalytic properties for oxidation are discussed in terms of the acidic properties of the samples.

Isobutene oxidation on an MoPrBiOSiO2 catalyst

Abstract Isobutene oxidation on an MoPrBiO catalyst has been studied by means of gas adsorption and kinetic experiments. The adsorption results point to a non-competitive adsorption of oxygen and isobutene, the interaction of the former molecule with the catalyst surface being remarkably stronger than that of the latter. The apparent activation energy for isobutene oxidation (91 kJ mol −1 ) is lower than that found for propene oxidation. The rate data for methacrolein formation were analysed according to some kinetic models used for catalytic oxidation. The Mars-van Krevelen redox mechanism is the model that best describes the experimental results, although some participation of adsorbed oxygen in the oxidation reaction should not be ruled out.

Selective Oxidation of Propylene Over Rare Earth-Molybdate Catalysts

Catalytic activity for the selective oxidation of propylene over Mo-RE-O (RE=Pr, Sm, Tb, Yb) catalyst series, with Mo/(Mo+RE) atomic ratios ranging between 0 and 1, has been studied. For all catalyst series, both activity and selectivity to partial oxidation products exhibited a relative maximum in the Mo-rich compositions region. These data are interpreted in terms of surface and bulk characteristic of the catalysts as revealed by X-ray diffraction, temperature-programmed reduction, laser-Raman and X-ray photoelectron spectroscopic methods.

Structure and catalytic properties of silica-supported Mo-Pr oxide catalysts for propene selective oxidation

A study relating catalytic properties and structural characteristics of Mo-Pr oxide catalysts supported on silica, with Mo/(Mo + Pr) atomic ratios between 0 and 1, has been carried out. Catalytic activity for propene oxidation and selectivities to partial oxidation products were maximal in the atomic ratio range 0.8 to 1.0. The infra-red study of pyridine adsorption showed a maximum of surface acidity for the most active and selective catalysts. X-ray diffraction showed the amorphous character of phases present in the supported catalysts, despite the high active phase loading (20 wt %). FT-IR and Raman spectra showed the presence of surface polymolybdates of both octahedral and tetrahedral types of symmetry, as well as finelydivided ‘free MoO3’. The high activity and selectivity are associated with the presence of highly-dispersed polymolybdates with acidic characteristics, while praseodymium oxide is thought to have a role in oxygen activation on basic centres.

Mössbauer study of supported multicomponent catalysts based in the Mo−Fe system

Mössbauer spectroscopy was used to study the effect of K, Sb, and Bi concentration on the state of the iron atoms, and on the structure and ratio of iron phases in mixed Mo−Fe−Co−Te oxide catalysts supported on silica. The effect of Bi and Sb ions concerns mainly the distortion degree of the local symmetry in Fe octahedral environment. The iron reduction ability in the catalysts decreases with increasing concentration of Bi, K, and Sb.