V. Keller

Catalytic activity and XPS surface determination of tungsten carbide for hydrocarbon reforming. Influence of the oxygen

AbstractThe influence of oxygen on the catalytic properties of tungsten carbide has been studied for the reforming reaction of 2-methylpentane in a great excess of hydrogenn

Bulk Tungsten Carbide as Catalyst in Hydrocarbon Reactions: Association of Selectivity Differences with Surface Composition as Compared to the Selectivity of Pt Series Metals

left( {P_{H_2 } /P_{HC} = 150} right)

Correlation between “in situ” XPS characterizations and catalytic activity of MoO3/α-Al2O3 for hexenes and hexanes isomerization. Mechanistic approach using probe molecules

n. The oxygen surface concentration has been measured by the ratio of the signal area of the O1s and the W4f levels as determined by X-ray photoelectron spectroscopy. Two kinds of processes are observed: cracking and isomerization reactions. The introduction of oxygen at T=350°C leads to a deep decrease of the cracking reaction kinetics; about two orders of magnitude. The tungsten carbide, strongly selective for cracking reactions in absence of oxygen, becomes very selective for isomer production in the presence of oxygen.

Catalytic activity of bulk tungsten carbides for alkane reforming: I. Characterization and catalytic activity for reforming of hexane isomers in the absence of oxygen

Abstract Methylcyclopentane and 2-methylpentane reactions have been studied on several tungsten carbide surfaces: model surfaces prepared from tungsten ribbon for one part, and bulk carbide powders in another part. The presence of oxygen under oxidic form WOx, has a decisive influence on the reaction mechanisms. On the carbide surfaces without oxidic species, the catalytic reactions give mainly acyclic cracking and isomerisation. When oxidic species are present on the surface, the main reaction is the methylcyclopentane ring enlargement, giving cyclohexane and benzene. The results could be interpreted by a bifunctional mechanism, taking into account the metallic and carbidic sites on one hand, and the oxidic acid sites on the other hand.

Catalytic activity of bulk tungsten carbides for alkane reforming. III. Reaction mechanisms and the kinetic model

Summary In this paper, results of a research work conducted in four laboratories in the frame of the ECC Research Science Program are presented (Contract ST 2J 0467C(TT)). The preparation of bulk tungsten carbide by reaction of WO 3 with a CH 4 /H 2 mixture is studied by TPR. After passivation, the material is characterized by ESCA, X-Ray diffraction, electron microscopy and electron diffraction, physical adsorption (BET using N 2 and Kr) and chemical adsorption of H 2 and CO, TPR under H 2 , O 2 , and inert gaz. These last studies allowed to evidence the importance of the protection of the material by surface free carbon, in opposition with the high sensitivity of the material to oxygen when “liberated” of its carbon protection. The test reactions used are reforming reactions of hexanes and alkylcyclopentanes. Labelled hexanes with 13 C are used for the determination of the reaction mechanisms and comparison with the Pt series metals. Surface free carbon resulting namely from the preparation procedure inhibits all catalytic activity. In situ activation under H 2 at 1073K produces clean hexagonal WC, characterized by a catalytic activity essentially for cracking. This last catalyst is highly sensitive to traces of O 2 , that induces important modifications in selectivity. The oxycarbidic type catalyst exhibits selectivities similar to traditional bifunctional catalysts like Pt on zeolites.

Catalytic Activity of Bulk Tungsten Carbides for Alkane Reforming. II. Catalytic Activity of Tungsten Carbides Modified by Oxygen

The n reactions of labelled 2- and 3-methylpentanes were carried out on three different oxygen modified bulk tungsten carbides varying by their oxygen treatment temperature. In all cases, isomerization took place ia bond n shift mechanisms; no cyclic mechanism was involved in contrast to metals like Pt, Pd or Ir where such mechanism n occurs. Alkoxy and σ-alkyl intermediates were put forward as adsorbed species responsible for isomerization n for bulk tungsten carbides treated by O2 at moderate temperature (350°C) over a short period of time (5 min) n and at high temperature (700°C) for 4 h, respectively. These intermediate species are respectively correlated n to an acidic and a metallic behavior of the catalytic surfaces. The increase of the O2 treatment time at 350°C n results in the presence of both kinds of these mechanisms for isomerization, a bifunctional behavior takes place n where metallic and acidic characters are present. To explain the presence of terminal-13C on acidic catalysts, n a “protonated cyclopropane alkoxy species” intermediate responsible for isomerization was proposed. n The driving force for the formation of such species being the presence of stable alkoxy species, general n reaction pathways n are discussed for the different kinds of catalysts.

Catalytic Activity of Reduced MoO3/α-Al2O3 for Hexanes Reforming: I. Preparation, Characterization, and X-Ray Photoelectron Spectroscopy Studies

Abstract MoO 3 /α-Al 2 O 3 catalysts mainly exhibit bifunctional isomerization properties with dehydrogenation/hydrogenation on Mo(IV) “metallic sites” and isomerization on Mo(V) acidic sites. The proximity of these two kinds of active sites can act as dual sites with adsorption of the hydrocarbon on both metallic and acidic sites.