J. Haber

The structure and redox properties of vanadium oxide surface compounds

The redox properties of VO layers supported on TiO2, A12O3, and SiO2 have been examined and discussed in terms of structural models. On this basis monodispersed VO4 units are postulated on the (001) plane of anatase (TiO2), located at the cationic sites with two terminal and two bridging oxygens, one of the latter being removed on reduction. Dimeric, pyrovanadate-type species seem to be predominantly formed on γ-Al2O3, whereas grains of poorly crystalline V2O5 are observed on SiO2. The structure of the vanadium-oxygen species determines the number of oxygens which can be removed per V atom.

ESCA studies of copper oxides and copper molybdates

Abstract Valence band, Cu 2p 3 2 , O 1s, Mo 3d , and Cu L 3 M 45 M 45 photoelectron and X-ray-induced Auger spectra were recorded for metallic copper, Cu 2 O, CuO, Cu 2 Mo 3 O 10 , Cu 6 Mo 4 O 15 , CuMoO 4 , Cu 3 Mo 2 O 9 , and Cu 3.85 Mo 3 O 12 . Cu 2p 3 2 binding energy is 0.9 eV lower for Cu + -containing molybdates than for Cu 2 O and 0.7 eV higher for Cu 2+ -containing molybdates with respect to that of CuO. Calculation of net chemical shift demonstrates the influence of Madelung potential on the binding energy of core electrons. On the basis of differences in binding energy it was possible to distinguish between various Cu-containing phases and to follow the surface redox processes of copper molybdates which, as it was seen, follow the same reactions as in the bulk processes. Auger spectra suggest the presence of a very thin layer of “surface phase” common for all five studied molybdates and independent of bulk structure and composition.

Vanadium pentoxide I. Structures and properties

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Structure and properties of V 2 O 5 are described and the role of three types of oxygen atoms present in the lattice: vanadyl oxygen atoms O(1) coordinated only to one vanadium atom, and bridging oxygen atoms O(2) and O(3) coordinated to two and three vanadium atoms, respectively, is discussed. Equilibration of gas phase oxygen with vanadium oxides results in the formation of the intrinsic defect structure of V 2 O 5 consisting of oxygen vacancies. Discussion of the properties of vacancies as deduced from measurements of electrical conductivity, EPR and IR spectra and the stability of different oxygen vacant sites as obtained from quantum-chemical calculations is given. Mechanism of the reduction of V 2 O 5 through crystallographic shear, resulting in the formation of V 6 O 13 or V 4 O 9 , is illustrated. It is shown that because of a pronounced anisotropy of V 2 O 5 crystal structure sensitivity of catalytic reactions appears. Two types of crystal planes are exposed. The (0 1 0) basal plane has all chemical bonds almost fully saturated. The non-bonding d-orbitals of V ions have the LUMO character and act as Lewis acid sites, whereas the lone electron pairs of bridging oxygen atoms have the HOMO character and behave as Lewis basic sites. On the (1 0 0) and (0 0 1) planes cleavage leaves coordinatively unsaturated vanadium and oxygen ions, which develop Bronsted acid-base interactions with reacting molecules, causing the heterolytic chemisorption. Oxygen vacancies in the lattice are replenished through oxidation by gas phase oxygen, which sometimes is considered as oxygen chemisorption.

X-ray photoelectron spectra of oxygen in oxides of Co, Ni, Fe and Zn

Abstract X-ray photoelectron spectra of Co-, Ni-, Fe- and Zn-oxides obtained by oxidation of metal films, as well as in powder form, were investigated. Two O (1

Interaction of allyl iodide with molybdate catalysts for the selective oxidation of hydrocarbons

Abstract The interaction of allyl iodide with MoO 3 , Bi 2 O 3 , and molybdates of Bi, Co, and Mg has been studied in the temperature range of 270–480 °C. Although practically inactive in the allylic oxidation of propylene, MoO 3 yielded 100% acrolein at 320 °C, when in contact with allyl iodide. On Bi 2 O 3 , only 1,5-hexadiene and benzene were formed, whereas bismuth molybdate produced both oxygenated and nonoxygenated products. This confirms the multicenter model of allylic oxydation, the low-valent cation being responsible for activation of the hydrocarbon molecule and Mo-O polyhedra for insertion of oxygen. Stoichiometric CoMoO 4 and MgMoO 4 were inactive in the formation of acrolein from allyl iodide because the acrolein-like intermediate formed on the surface was unable to desorb. This indicates that the third condition of an active catalyst is the ability to desorb the oxygenated product by releasing the oxygen ion of the lattice sufficiently readily. When an excess of MoO 3 is present in these molybdates, they become active in the formation of oxygenated products.

Physicochemical and catalytic properties of the system chromium oxides-oxygen-water

This paper describes investigations of various chemical, electrical, and catalytic properties of materials, obtained by calcination of chromic oxide gel. Samples of four different preparations of chromic oxide gel, whereof one contained additions of magnesium, were annealed for 5 hr in air at various temperatures between 100 ° and 700 °. As shown by differential thermal, thermogravimetric, and X-ray analyses, dehydration first results in the formation of X-ray-amorphous monohydrate, which is stable over a wide range of temperatures between 280–360 °. Simultaneously with dehydration the oxidation of the material takes place. The oxidation increases with increasing temperature of annealing. The OCr ratio, corresponding to maximal oxidation at 350 °, amounts to about 2. It has been shown by applying various methods of chemical analysis that the excess charges are present in such material in the form of hexavalent chromium, the proper formula being thus Cr2O3 · CrO3 and not CrO2. At 400 ° crystallization of Cr2O3 occurs, the oxide being at first considerably oxidized on the surface. This oxidation decreases on annealing at still higher temperatures. Addition of magnesium has no influence on the oxidation state of the amorphous material, but greatly increases the amount of excess charges in crystalline Cr2O3. Measurements of the electrical conductivity and thermoelectric power indicate that the conduction mechanism is similar in the amorphous material and in the crystalline oxide. Changes of electrical conductivity and of its activation energy are related to the changes of concentration of Cr6+ ions. The catalytic activity in H2O2 decomposition has been determined and specific rate constants, k, frequency factors k0, and activation energies, E, of the reaction have been computed. The reaction exhibits a compensation effect, which hints at the changes of energy states of active centers with the temperature of annealing. This is consistent with the results of electrical conductivity measurements, since changes of activation energy of electrical conductivity are parallel to changes of activation energy of the reaction. The experiments reveal a correlation between rate constants, k, and surface concentration of Cr6+ ions, whereas discussion of the reaction mechanism shows that frequency factors k0 rather than rate constants k should be related to this concentration.

Monolayer V2O5/TiO2 and MoO3/TiO2 catalysts prepared by different methods

Abstract Two series of supported V2O5/TiO2 and MoO3/TiO2 catalysts were synthesized using different preparation methods to determine the possible influence of the preparation procedures upon the final molecular structures of the catalysts. It has been found by means of ambient and in situ Raman spectroscopy that all the employed preparation methods lead to the same types of surface metal oxide species on the titania support. The hydrated surface metal oxide species of both series of the catalysts undergo, after heating in flowing dry air, reversible conversion into the dehydrated analogues. The preparation methods appear to have no influence on the final surface metal oxide structures, but can affect (delay) the process of their formation, as in the case of dry impregnation (thermal spreading) of the V2O5/TiO2 system, due to the thermodynamic and diffusion factors involved.

Catalytic properties of MoO3 revisited

Abstract One of the most striking features of molybdenum oxide is the versatility of its catalytic properties, which are determined by the valence state of molybdenum ions and their local environment. It may be anticipated that MoO3 surface must contain catalytic sites which are active in different types of elementary steps. Different probe catalytic reactions were thus applied to monitor the changes of the concentrations of various types of active sites in the course of the reduction of MoO3 to MoO2 and to unravel their role in the complex reaction network of such molecule as butene. Isomerization of 3,3-dimethylbutene-1 was used to characterize Bronsted acid sites, the ability of the MoO3 surface to generate allylic species was determined by the isomerization of 2,3-dimethylbutene-2. The presence of sites inserting oxygen into the hydrocarbon molecule was identified by the reaction of allyl iodide to form acrolein. Dehydrogenation of butene-1 to butadiene was applied as the test reaction to detect sites responsible for abstraction of β hydrogen. Total oxidation was taken as a measure of the concentration of sites adsorbing electrophilic oxygen. The nature of different sites is discussed.

Physical and chemical characterization of surface vanadium oxide supported on titania: influence of the titania phase (anatase, rutile, brookite and B)

Abstract Different phases of titania were prepared and used to support ca. 1 wt.-% V 2 O 5 . The different titania phases prepared were: anatase (A22), rutile (R28), brookite (BT110) and B-phase (B18). Physical characterization of the various vanadia-titania catalysts was performed using X-ray photoelectron spectroscopy (XPS), in situ Raman and 51 V solid state nuclear magnetic resonance (NMR) spectroscopy. The XPS results reveal that the all the catalysts contain various levels of impurities. In situ dehydration Raman shows, for all the samples, the stretching vibration of the terminal VzO bond at ca. 1030 cm −1 . Solid state 51 V NMR spectra of all the samples in the dehydrated state show basically the same powder pattern with a peak maximum around −660 to −670 ppm. The combined Raman and NMR results indicate that the same surface vanadium oxide species is present on all the titania supports irrespective of the crystal structure of the bulk titania phase. Partial oxidation of methanol show similar activity and selectivity for the various vanadia-titania catalysts. The reaction selectivity was primarily to formaldehyde and methyl formate (92–96%). The turnover number for methanol oxidation was essentially the same for all the vanadia-titania catalysts and ranged from 1.4 to 2.8 s −1 . These results indicate that the type of titania phase used as the support is not critical for partial oxidation over vanadia-titania catalysts as long as other parameters (e.g. surface impurities ) are similar. Thus, the structure-reactivity studies of the different vanadia-titania catalysts suggest that the specific titania phase is not a critical parameter in determining the physical or chemical nature of the surface vanadia phase.

Manual of Methods and Procedures for Catalyst Characterization

The manual provides details and recommendations concerning the experimental methods used in catalysis. The objective is to provide recommendations on methodology (rational approaches to preparation and measurements). It is not intended to provide specific methods of preparation or measurement, nor is it concerned with terminology, nomenclature, or standardization.