Alfred Frennet

Transient kinetics in heterogeneous catalysis by metals

Abstract This paper tries firstly to remember the origin of kinetic models in heterogeneous catalysis. The change of catalysis from an art to science induced by the introduction of the concepts of Langmuir is presented in Section 1 . In Section 2 , deficiencies contained in these concepts as applied, where one site is associated to one chemisorbed radical, are developed. Namely the curious contradictions obtained in the analysis of the hydrogen inhibiting term on the rate of hydrogenolysis reactions are analysed. In Section 3 , attention is drawn on information obtained by use of labelled molecules. These studies indicated a similar H2 inhibiting term on the only adsorption step. The interpretation of that effect results in the development of a multisite adsorption model. Section 4 introduces the interest and advantages of the use of the chemical transient method. Simultaneously, its technical limitations are briefly presented. 5 The C , 6 Chemical transient studies lead to a new insight in the mechanism of syngas reaction are devoted to the application of the transient method to two very different but important catalytic reactions. The first of these examples makes evident the interest in the combination of labelled molecules with chemical transient effects, to determine the rate, direct and inverse, of the successive elementary steps of the model reaction that is ethane hydrogenolysis. The second example, the CO–H2 reaction, is much more complex as the catalytically working surface is built during the transient phase, thus dramatically changing the surface properties. If it does not allow, as in the previous example, to obtain quantitative determinations of the rate of the elementary steps, it anyway provides unique information leading one to propose a new reaction scheme. One of the aims of this paper is to support the still large importance of kinetic studies in heterogeneous catalysis, a topic dear to Michel Boudart with whom one of us (A. Frennet) had the honour and pleasure to work.

Study of the preparation of bulk tungsten carbide catalysts with C2H6/H2 and C2H4/H2 carburizing mixtures

The influence of the preparation procedure of tungsten carbide on the mechanism of carburization is discussed. This work is focused on the reduction and the carburization of tungsten trioxide by a mixture of hydrocarbon and H2 to form WC. Temperature-programmed reaction spectra obtained with CH4, C2H6 and C2H4 have been measured. In presence of the CH4-H2 mixture, H2 is the reducing agent and the hydrocarbon is consumed for the carburization whereas C2H6 or C2H4 participates in the reduction of the tungsten oxide. The temperatures of reduction and carburization are lower by about 150 K using C2H6 or C2H4 instead of CH4. Such a decrease of the temperature of reduction of tungsten oxide is needed to avoid the formation of poorly reducible compounds that can occur during the preparation of supported tungsten carbide. Furthermore, the surface area of the resulting carbide is 25 m2/g with C2H6 and C2H4 and 10 m2/g with CH4. During the carburization, the deposit of excess carbon on the WC surface is larger with the C2 hydrocarbons than with CH4, but it protects the carbide and can be removed by hydrogen treatment.

Organometallic route to dimolybdenum carbide via a low-temperature pyrolysis of a dimolybdenum alkyne complex

Pyrolytic transformation of the complex Cp2Mo2(CO)4(dmad) (Cp=cyclopentadienyl, dmad=dimethylacetylenedicarboxylate) under hydrogen at 550 °C gives, after a passivation step, Mo2C with an excess of carbon and oxygen. These impurities can be withdrawn with an appropriate reductive post-treatment. Based on thermogravimetric analyses and pyrolysis mass spectroscopy performed on the precursor, a preliminary decomposition scheme has been proposed.

Pore structure of bulk tungsten carbide powder catalysts

Bulk tungsten carbide catalysts are prepared by direct carburization/reduction of tungsten trioxide in methane-hydrogen mixtures. The catalytic properties of such catalysts have been studied by several authors. The porous structure of these catalysts is studied by adsorption of N2, Kr, CF4 and neohexane. Adsorption isotherms and hysteresis loops for the catalysts suggest the presence of a microporous structure made of parallel plates distant approximately by 20 Å. These results are compared to those obtained using such catalysts for hydrogen oxidation and where condensation in the porous structure was observed.

Adsorption site of alkanes on metals and associated hydrogen pressure effects

Abstract A series of arguments, based on experimental results concerning mainly methane and ethane, in favour of an associative mechanism of the hydrocarbon with an adsorbed H atom for the adsorption step of alkanes on metal surfaces are reviewed. The reaction scheme for the adsorption step is written: As w has a smaller value than z, a coverage function in hydrogen, of the type (1-θH)Z if the adsorption step is rate determining, or (1-θH)Z-W if the adsorption step is near to equilibrium, contributes to the kinetic equation of the catalytic reaction. Due to the large value of z (8 or more), this function contains a hydrogen pressure and a temperature dependence of the same type as measured on the rate of the catalytic reaction. It also contains a demanding character.

In situ measurement of the surface area during catalyst preparation: development of a new method

Abstract The monolayer adsorption capacity of catalysts is usually determined ex situ, i.e. by physical adsorption of N 2 at 77xa0K via the BET method in a set-up separate from the reactor used for characterization or catalytic tests. We present here a new volumetric method working in the dynamic regime and using the physisorption of Ar at 77xa0K in the same microreactor where other characterization experiments are performed. The method allows the in situ measurement of the total surface area via a three-component gas system. The use of a third gas, as opposed to the two-component system used in commercial devices, allows us to take into account the considerable changes of the total volumetric flow rate associated with temperature variations. A mass spectrometer is employed as analytical device. The method is validated by comparing the monolayer capacity for Ar and N 2 on the same sample of Aerosil 200; the derived value of the Ar coverage cross-section is 19.2xa0A 2 . Using the same method, we measure the surface areas of Co-Cu-based catalysts during the various temperature-programmed pretreatments of their oxalate precursors. Results show that the surface area can change by a factor as large as 100 depending on the pretreatment.

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

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.

Long-chain alcohols from syngas

CO hydrogenation to long-chain alcohols was studied over CoCu based catalysts containing either Mg or Mn as dispersant. Catalysts were prepared by coprecipitation as metal oxalates. This method ensured the formation of Co-Cu mixed phases which were characterized by temperature programmed decomposition (TPDec) in hydrogen. Co-Cu mixed phases showed up as distinct features in TPDec allowing for an optimization of the catalyst composition with respect to selective alcohol formation. High pressure reaction studies (60 bar, H 2 :CO=2:1) in a fixed-bed flow reactor demonstrated Mn containing catalysts to perform best. Generally, long-chain products (alcohols, alkanes and alkenes) obeyed Anderson-Schulz-Flory distribution characteristics. For a Mn-Co-Cu catalyst with 1-1-1 relative composition an alcohol selectivity of 34% (CO 2 excluded, ∼4%) was found along with a chain growth probability of 0.61.

Thermodynamic parameters of H2 adsorption as criteria to characterize silica-supported palladium catalyst

The thermodynamics of hydrogen adsorption on palladium catalyst supported on silica is determined by isosteric analysis of series of isochores measured by the volumetric method. The variations of this thermodynamics (ΔH, ΔS) with adsorbed amount allows the discrimination between the different hydrogen species present on such a catalyst (hydrogen strongly and weakly chemisorbed on the palladium, absorbed in the palladium particles and molecularly adsorbed on the support). Namely, the variations of only the heat of adsorption with adsorbed amount allow the determination of hydride and weakly adsorbed hydrogen species. It is shown that these amounts can even be determined from one single isochore.