P. Wehrer

The multi-surface structure and catalytic properties of partially reduced WO3, WO2 and WC + O2 or W + O2 as characterized by XPS

The XPS at different experimental conditions of WO3, WO2, W(0), WC,W(0) + O2 and WC + O2 are reported. Extensive hydrogenolysis reactions of 2-methylpentane yielding methane a major product were observed on relatively pure W(0) and WC surfaces. Isomerization reactions for the same reactant were observed on WO2 or partially oxydized W(0) and WC surfaces, while bulk WO3 does not show any catalytic reactivity at the beginning of the reaction. The catalytic activity of WO2 is attributed to the presence of a certain density of states of the 5d, 6s electrons present at the Fermi-level on the W4+ cation and observed in the XPS of the valence band energy region of WO2. Such reduction by H2 and the hydrocarbon reactant of bulk WO3 or that present on W(0) or WC surfaces results in the presence of WO2 and to a lesser extent WO and W(0), which explains the isomerization reactions carried on these modified surfaces.

Surface electronic structure and isomerization reactions of alkanes on some transition metal oxides

XP spectra of some reduced transition metal oxides are presented. Different number of free nd,(n+1)s valence electrons in each case could be observed by the presence of a certain density of states (DOS) at the Fermi-level in the valence band (VB) energy region of the XP spectrum. Catalytic isomerization reactions of 2-methylpentane yielding 3-methylpentane and n-hexane at 350°C have been observed on these reduced valence surface states. The bifunctionel mechanism in terms of metallic and acidic sites required for such reactions is proposed by considering the metallic properties of the rutile deformed structure through the C-axis in the case of MoO 2 and WO 2 , while the oxygen atom(s) in the lattice structure exhibit Bronsted acidic properties. On the other hand, highly reduced or clean surfaces of these transition metals yield hydrogenolysis catalytic reactions for the same reactant with methane as the major product. In all cases, the exposure of the lower valence oxidation states of bulk transition metal oxides to air results in the surface partial oxidation to the stable oxides such as MoO 3 , WO 3 , V 2 O 5 and Nb 2 O 5 .

Obtaining tungsten carbides from tungsten bipyridine complexes via low temperature thermal treatment

Tungsten carbides were prepared by thermal treatment under nitrogen of a tungsten bipyridine complex. The catalysts obtained have been characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), temperature programmed reduction (TPR) and elemental analysis. The reaction with 2-methylpentane was used as a chemical probe for the formation of tungsten carbide.

The multi-structure of oxidized-reduced tungsten carbide surface(s)

The XPS of bulk tungsten carbide, partially oxidized WC surfaces at 373 and 573 K as well as tungsten trioxide have been reported. Bulk WC has been prepared from WO3 as a starting material in a mixture of CH4 (20%) and H2 (80%) at 1150 K for 4 h, while partially oxidized WC surfaces were prepared by oxygen chemisorption on a clean WC surface at 200 K, then the temperatures were raised to 373 and 573 K respectively. The XPS of a freshly prepared WC reveals the presence of a small amount of WO3 on the surface and a slightly higher concentration in the bulk. The oxygen-exposed fresh WC surfaces and surfaces treated at temperatures higher than 373 K show the presence of WO3 in a considerable quantity depending on the length and the treatment temperature. Ar+ bombardment of this partially oxidized surface reduces WO3 to WO2 and W(0), while WC is partially reduced to W(0). Isomerization reactions of alkanes on oxygen-exposed WC surface occurs in reality on a composite surface structure containing WC, WO3, WO2 and elemental W(0).

Surface characterization and catalytic properties of supported tungsten and platinum-tungsten carbide and oxycarbide

Bulk tungsten carbide has been prepared from ammonium paratungstate. Oxidic oxygen in relatively small amount is always present on the WC surface regardless of the method of carburization. Tungsten(VI) oxide in Al2(WO4)3 can be reduced directly and in apparently one single step to the elemental W(0) state by Ar+ bombardment. Under H2 the reduction starts at 720 K. In 12 and 28% W03 supported on γ-Al2O3 and in Al2(WO4)3, almost 90% of the tungsten could be converted to the carbide form. Extensive hydrogenolysis products for n-hexane reactions on freshly prepared WC were observed while selectivity to isomerization associated with decrease in activity occurs upon exposure of the WC to oxygen at 620 K. Supported tungsten carbide(s) and oxycarbide(s) on γ-Al2O3 have comparable catalytic behaviour to those obtained on the bulk systems. The presence of Pt in these supported systems did not improve the catalytic performances and even did not show the catalytic properties of Pt. This was attributed to the severe conditions of catalyst preparations. Although the catalytic activity of tungsten carbide(s) obtained from the carburization of Al2(WO4)3 is very low, the selectivity suggests the presence of tungsten carbide plus some WO, species.

XPS studies of supported tungsten carbide(s)

The XPS of carburized Al2(WO4)3, 12% WO3, 28% WO3 and (3.9% Pt + 26% WO3) supported on γ-Al2O3 has been reported. Argon ion bombardment or hydrogen at 1073K enabled to reduce more than 50% of the W(VI) to W(0) in apparent one single step process. Severe conditions of reduction by H2 and carburization using CH4/H2 mixture were required in order to transform 90% of the W in Al2(WO4)3 to tungsten carbide. Lesser amount of WC were obtained in the case of supported systems due to a very strong metal-support interaction. Argon ion bombardment employed as reducing agent enabled to optimize the reduction conditions of these tungsten oxide systems. It has also been observed that WC could be reduced to the elemental W(0) upon Ar+ bombardment. Although indications about the presence of oxycarbide species have been observed, no conclusive evidence about its exact structure could be formulated.

Etude de la reaction du gaz carbonique avec le carbone aux temperatures elevees et sous de basses pressions

Abstract In order to complete and to determine more precisely previous data a new investigation on the kinetics of the C + CO 2 reaction has been carried out over a broader range of temperature (800–2000°C) and pressure (between 10 −4 and 10 −1 Torr) with an improved apparatus and using more varied carbon samples. All types of carbons exhibit the same peculiarities in kinetics related to changes in the intrinsic reactivity of the surface. The general features of the kinetics are more similar than hitherto believed to those of the C + O 2 reaction. Indeed, the two reactions differ only in their absolute rates. This difference is discussed on the basis of the mechanism previously proposed by Duval.

About the stability of alkane isomerizing catalysts made up of molybdenum oxides

Abstract Mixtures of molybdenum metal and suboxides, resulting from a partial reduction of MoO3 by hydrogen or by a mixture of hydrogen and alkane in the range 300–400°C, are known for their ability to readily isomerize alkanes in the presence of hydrogen. In this work we found that this property is transitive when very high space velocities values of the gaseous flow are attained, cracking of the alkane becoming the main reaction after a few hours. This occurred because in the presence of the reducing flow, molybdenum oxides — and in particular the previously proposed isomerizing species as molybdenum dioxide and molybdenum oxycarbide — tend inevitably to the metallic state. The opposite behaviour reported in the literature — high and stable isomerization activity with time — results only from the molydenum oxides reduction rate slow down, owing to a strong inhibition by the water vapour. This banal auto-inhibition effect — but so far ignored in the isomerization studies — becomes predominant when the space velocity value of the gaseous flow is too low.

Reactivity of hexanes (2MP, MCP and CH) on W, W2C and WC powders. Part II. Approach to the reaction mechanisms using concepts of organometallic chemistry

Abstract The reactivity of 2-methylpentane (2MP), methylcyclopentane (MCP) and cyclohexane (CH) on reproducible and well-characterized surfaces of W, W2C and WC has been studied. A great deal of effort was put on the characterizations by physisorption, chemisorption and photoemission spectroscopy. The results obtained with these reference materials can be used for comparison with those appearing in the literature and sometimes debated because of the inconstancy of the active states due to non-stoichiometric compositions (excess of carbon, decarburization, oxidation by oxygen impurities). In agreement with the work of Boudart et al. [F.H. Ribeiro, R.A. Dalla Betta, M. Boudart, J. Baumgartner, E. Iglesia, J. Catal. 130 (1991) 86; F.H. Ribeiro, M. Boudart, R.A. Dalla Betta, E. Iglesia, J. Catal. 130 (1991) 498; E. Iglesia, J.E. Baumgartner, F.H. Ribeiro, M. Boudart, J. Catal. 131 (1991) 523; E. Iglesia, F.H. Ribeiro, M. Boudart, J.E. Baumgartner, Catal. Today 15 (1992) 307.] we confirmed that reforming reactions do not take place in the temperature range 80–400°C and for our experimental conditions. The dehydrogenation of cyclohexane appeared only when a strong poisoning by carbonaceous residues occurred. For 2-methylpentane and methylcyclopentane the extensive hydrogenolysis character of WC is higher than for W2C and WC surfaces. The various hydrogenolysis mechanisms of 2MP, MCP and CH on W, W2C and WC surfaces are interpreted by different possible reaction intermediates deduced from concepts of organometallic chemistry.

Décomposition de l'acétylène, de l'éthylène et du benzène sur le carbone aux très hautes températures et sous de basses pressions

Abstract At high temperatures (1000–2000°C) and low pressures (10−5−10−2 Torr) ethylene, acetylene and benzene decompose helerogeneously on pyrolytic carbon giving mainly hydrogen and deposited carbon, with collision yields of the order of 10−4. The kinetics of these carbon deposition reactions show some striking similarities with carbon removal reactions by oxygen or oxygenated compounds. The true reaction order of these decomposition reactions is one above 1400°C, but becomes smaller at lower temperatures. This behaviour, common in gas-solid reactions, is generally interpreted as an inhibition due to chemisorption of some intermediate or reaction product. Evidence is also obtained that decomposition of the hydrocarbon molecules only occurs on peculiar sites of the carbon surface, i.e. the decomposition is not a purely thermal process, but involves a specific chemical interaction with the surface. Moreover, the behaviour of the pyrocarbon surface in carbon deposition reactions is similar to that observed in gasification reactions, i.e. the reactivity of the surface accommodates itself to the temperature and pressure conditions, as revealed by the observation of “transitory” and “stationary rates”. Transitory rates show that the surface deactivates with increasing temperatures (Figs. 4 and 5) [from which a maximum in the stationary rate results (Figs. 1–3)] and decreasing pressures (Figs. 7 and 8). The interpretation assumes that reaction sites are continuously created as an effect of carbon atoms deposition, but also deactivated by a thermal healing process. A main difference between carbon deposition reactions from hydrocarbons and carbon gasification reactions concerns the temperature range where reactivity is temperature dependent: in carbon deposition reactions, deactivation of the pyrocarbon surface is still effective up to much higher temperatures (Fig. 12).