S. De Rossi

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.

Structural characterization of malachite-like coprecipitated precursors of binary CuOZnO catalysts

Copper and zinc hydroxycarbonates with Cu/Zn atomic ratios of 100/0, 92/8, 85/15, 77/23, 67/33, and 50/50 were prepared by the coprecipitation method. The samples with the ratios 100/0, 92/8 and 85/15 were monophasic with the monoclinic P2/sub 1//a malachite structure, whereas the richer zinc samples contained in addition to the malachite phase the orthorhombic B22/sub 1/2 aurichalcite, whose concentration was very low in the 77/23 specimen but increased continuously with zinc content. The amount of aurichalcite in the biphasic powders has been estimated by means of quantitative X-ray diffraction and differential thermal analysis. With only those X-ray reflections belonging to the malachite structure taken into account, the unit cell constants for all the solid solution hydroxycarbonates have been determined, showing that as a whole the cell volume of malachite decreases continuously with increasing zinc content. This result is related, as shown also by DTA measurements and thermochemical literature data, to an increasing stability of compounds richer in zinc, to increasing covalent effects, and/or to less octahedral site distortion around the metal atoms when zinc substitutes for Cu/sup 2 +/ ions in the malachite lattice. Magnetic susceptibility and thermogravimetric measurements together with diffuse reflectance spectra are also presented and discussed.

Studies on chromia/zirconia catalysts I. Preparation and characterization of the system

Abstract The preparation and characterization (chemical, textural, DTA, XRD, XPS) of chromium oxide/zirconia, of interest as hydrogenation catalysts, are reported. The support (obtained via ZrOCl2 hydrolysis) can be tailored in surface area from high (360 m2g−1) to low (about 20 m2g−1) values, and in texture from microporous to mesoporous and macroporous according to the treatment temperature (from 383 to 923 K) of the starting “hydrous zirconia.” By contacting the support with Cr(VI) solution, chromium-loaded specimens, ZC, are prepared. The Cr uptake is roughly constant (1.5 to 1.9 Cr atoms nm−2) for zirconia previously heated at T ≥ 573 to 923 K. Higher loadings can be reached on the hydrous zirconia. Supported Cr oxide is an effective antisintering agent for zirconia, and it also opposes the tetragonal → monoclinic transition. By subjecting the ZC specimens to various heat and redox treatments, the average oxidation number of Cr, n changes. From an initial value of +6, n decreases to +5.5 after oxygen treatment at 773 K, to +2.5 after CO treatment at 623 K, and can be restored to +5.5 if the sample is reoxidized in oxygen at 773 K. Treatment in H2O vapor at 723 K of a reduced ( n = 2.5 ) specimen brings n to 3.0. The existence of Cr(VI), Cr(V), Cr(III), and Cr(II) is inferred and is supported by XPS analysis. A separate paper presents an ESR investigation and discusses the nature of the surface Cr species.

Catalytic behavior and surface chemistry of copper/alumina catalysts for isopropanol decomposition

Abstract Supported copper on γ-Al 2 O 3 catalysts (1, 3, and 4 Cu atoms per 100 Al atoms) have been pretreated in a controlled way in oxygen and in hydrogen in order to have a known surface concentration of Cu(II), Cu(I), and Cu(0). These samples were tested as catalysts for the decomposition of 2-propanol. Below 443 K the samples showed only dehydrogenating activity which increased with the extent of catalyst reduction. A linear relationship was found between the kinetic constant and the surface area of copper metal. It is hence proposed that surface Cu(0) species are responsible for the dehydrogenation. The effect of the copper concentration on the sample stability under reaction conditions is also discussed.

Studies on chromia/zirconia catalysts. II, ESR of chromium species

Abstract The characterization of CrO x /ZrO 2 samples (Cr content 0.05 to 6 wt%) by means of ESR spectroscopy is reported. On samples heated in O 2 at increasing temperatures up to 1173 K, the presence of Cr(V) (γ-signal, g ‖ = 1.960 and g ‖ = 1.979) is detected by ESR. Its concentration (Cr(V) ions nm −2 ) is found to increase with temperature, remaining about constant above 773 K. Experiments with the 53 Cr isotope allow assignment of the species to a surface mononuclear chromyl-complex in a square pyramidal configuration. At higher temperatures (generally at T ⩾ 973 K, depending also on textural features of the ZrO 2 support and Cr content) the ESR signals of (i) a chromia-like phase (β′-signal, g = 1.98 and Δ H pp = 1500–1800 G) and (ii) α-Cr 2 O 3 (g = 1.98, Δ H pp = 480–500 G, spectra recorded at T ⩾ 308 K) are observed in addition to Cr(V). The particle size of the β− is too small (⩽7 nm) to show strong antiferromagnetic interactions. On samples reduced with CO, the γ-signal sharply decreases with increasing temperature of the reduction, and disappears at 623 K. In the more concentrated samples and after extensive reduction only, an ESR signal from Cr(III) is observed (δ-species, σ ≈ 2.2 with a broad maximum at g in the range 3.8 to 5.0), and assigned to weakly interacting Cr(III) ions exposed on the surface of ZrO 2 . If reduced samples are treated with H2O at increasing temperatures up to 1073 K, the selective oxidation of Cr(II) to Cr(III)β species (g = 1.98, ΔH pp = 1500–1600 G) is observed. Species Cr(III)-β and Cr(IIl)-β′ differ from each other by cluster size only, as indicated by their different redox behavior. Reoxidation with O 2 at room temperature only minimally restores the γ-signal, and hardly affects the 8-signal. Full reversibility is achieved upon heating in 0, at 773 K. ESR results and average oxidation numbers from redox cycles allow the identification of two distinct redox couples on the ZrO 2 surface: Cr(III)/Cr(V) and Cr(II)/Cr(VI). The stabilization effect of the ZrO 2 matrix on the various chromium species is discussed.

Catalytic behavior and surface chemistry of the ZnOAl203 system for the decomposition of 2-propanol

Samples of zinc oxide supported on γ-Al2O3 (5, 15, 30, and 36 Zn atoms per 100 Al atoms) have been calcined at different temperatures and subjected to oxidation-reduction treatments in order to investigate the type and the extent of the metal-support interaction and the defect state of the surface. The samples were then tested as catalysts in the decomposition of 2-propanol. They were found to give both dehydrogenation and dehydration. The dehydrogenation reaction is due to “free” zinc oxide present on the surface of the support, whereas the dehydration is due to the support. The effect of the calcination temperature on the formation of a “surface spinel” and of the redox pretreatments on the activity and selectivity are also discussed.

Characterization of well dispersed copper species on the surface of ZnO by x-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy was employed to characterize Cu(II) species on the surface of calcined CuO-ZnO systems with Cu/Zn atomic ratios ⪯ 0.10. Such solids are of interest as precursors of Cu-ZnO catalysts. It is shown that well dispersed Cu(II) species on the surface of ZnO have unique spectroscopic properties. For the Cu(2p3/2) transition the intensity of the satellite structure, typical of bulk Cu(II) compounds, is strongly reduced or even absent in the more dilute samples. To explain this result, it is suggested that a stronger Cu-O covalent bond than in bulk CuO is formed at the interface between the surface oxygen ions of ZnO and the supported copper species. The strong interaction between Cu(II) species and the support leads on reduction to the stabilization of Cu(I) species at the periphery of metallic copper particles which could play a role in the reaction mechanisms of CO hydrogenation at the surface of Cu-ZnO catalysts.

Comparison of carbon monoxide and nitrous oxide adsorption on copper/zinc oxide catalysts

Abstract The determination of metal specific surface areas by selective chemisorption on a supported metal offers the advantage of monitoring the surface exposed to the catalytic partners, and it is therefore directly related to the catalytic processes. Among supported metals, particular attention has been devoted to copper containing systems such as Cu/ZnO and Cu/ZnO/Al2O3 methanol synthesis catalysts [1–4]. In recent years many papers have appeared concerning both the physico-chemical and technical aspects of methods based mainly on the great affinity of copper towards oxygen, derived either from dioxygen species or from nitrous oxide. One inherent difficulty was the stringent requirement that bulk diffusion be excluded in order to avoid apparently large O/Cu ratios. By comparison with irreversible oxygen adsorption at 78 K. Parris and Klier [3] showed that reversible carbon monoxide adsorption at 293 K can also be used to titrate copper surface areas in Cu/ZnO catalysts, as demonstrated by the good linearity found between the two capacities. On the other hand, nitrous oxide dissociative adsorption was considered not to be reliable because of bulk oxidation and metal particle size dependence, as previously reported by Scholten and Konvalinka [5]. Therefore, it seemed of interest to compare the room temperature dissociative adsorption of nitrous oxide, which leaves on the surface oxygen atoms that possibly penetrate into the bulk, with the room temperature carbon monoxide adsorption, which correlates only with surface sites. In this paper we present the results of a systematic study of the correlation between carbon monoxide and nitrous oxide room temperature chemisorption capacities of several Cu/ZnO catalysts of wide compositional range and prepared by different procedures.

Cell for the in situ study of heterogeneous catalysts by transmission and fluorescence XAS spectroscopy

A simple cell, assembled with commercial parts, suitable for in situ X-ray-absorption spectroscopy measurements of heterogeneous catalysts, has been designed. The cell, light and easy to handle, allows thermal treatments of the sample under investigation up to 823 K in a reducing or oxidizing atmosphere and measurements at both high and liquid-nitrogen temperature. The cell was tested by studying the decomposition, in an oxygen flow, of ammonium metatungstate to WO3. Extended X-ray absorption fine-structure measurements of the ammonium metatungstate before and after a thermal treatment at 773 K are reported.

Structural, surface, and catalytic properties of bismuth molybdovanadates containing foreign atoms: IV. Surface characterization and redox behaviour of iron-containing bismuth molybdovanadate catalysts by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy has been applied to the study of the surface composition and the redox behaviour of multicomponent oxide catalysts having the scheelite structure and general formula Bi 1−x 3 □ x 3−y Me y (V 1−x Mo x−y Fe y )O 4 (Me = Fe or Bi, □ = cation vacancies), before and after exposure to hydrogen, propene, and a propene + oxygen mixture. The results point to a different reactivity toward the various reactants. By interaction with hydrogen, an extensive reduction is obtained with considerable change of the catalyst surface composition. The reduction behaviour depends on the sample composition, the iron-containing catalysts being reduced at low temperature (140 °C). As regards propene treatment, a preferential reactivity of bismuth with propene has been found, regardless of the sample composition. Bismuth is reduced at low temperature (175 °C) with ensuing migration to the surface. Under conditions leading to the catalytic oxidation of propene, a high surface stability is exhibited by these compounds. The catalysts remain oxidized and the surface composition reflects that of the bulk. A correlation with the catalytic properties has also been made on the basis of the different mobility of the oxygen ions among the samples examined.