G. Leclercq

Catalytic properties of transition metal carbides: II. Activity of bulk mixed carbides of molybdenum and tungsten in hydrocarbon conversion

The dehydrogenation of cyclohexane and the hydrogenolysis of butane were studied on bulk tungsten-molybdenum mixed carbides as a function of their composition. Some analogies with noble metals have been confirmed for selectivity in hydrogenolysis. Tungsten carbide gives only single hydrogenolysis like platinum, while molybdenum carbide leads to multiple hydrogenolysis like ruthenium. When the activities in cyclohexane dehydrogenation and butane hydrogenolysis are plotted versus the relative surface composition Mo(Mo + W), curves with a minimum are obtained. This change of the activities of the mixed carbide catalysts correlates with the molybdenum surface enrichment deduced from XPS results.

An attempt at modelling the activity of Pt-Rh/Al2O3 three-way catalysts in the CO+NO reaction

Abstract This study deals with the kinetic behaviour of a Pt-Rh/Al 2 O 3 three-way catalyst (TWC) in the CO+NO reaction close to the actual conditions. Practically, it consists in validating a previous rate equation, established at 300°C from a selected mechanism [P. Granger, J.J. Lecomte, L. Leclercq, G. Leclercq, J. Catal . 175 (1998) 194], in a wider range of temperature particularly near 100% NO conversion. Preliminary calculations using a non linear least square method lead to an estimation of pre-exponential factors (for k , the rate constant of the rate limiting step and for λ CO and λ NO , the adsorption equilibrium constants of CO and NO), the activation energy for the dissociation of adsorbed NO molecules, and the enthalpies of CO and NO adsorption (Δ H ads,CO and Δ H ads,NO ). The temperature-programmed conversion and selectivity curves obtained in differential conditions using a recycle fixed bed flow reactor have been modelled using these adjusted parameters. Based on CO chemisorption observations it was found that a better fit is obtained by accounting for the coverage dependency of Δ H ads,CO and Δ H ads,NO . Finally, we have attempted to quantify such effects.

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 These impurities can be withdrawn with an appropriate reductive post-treatment.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.

Hydrogenolysis of saturated hydrocarbons: IV. Kinetics of the hydrogenolysis of ethane, propane, butane, and isobutane over nickel

Abstract Kinetics of catalytic hydrogenolysis of ethane, propane, butane, and isobutane over 5 wt% Ni on dealuminated silica-alumina have been investigated in a flow reactor at atmospheric pressure. These hydrocarbons can be classified according to their increasing rate of hydrogenolysis at 200 °C in the sequence ethane, isobutane, propane, and butane, but their differences of reactivity over nickel are much lower than on platinum. The kinetic orders and apparent activation energies have been determined. All the results fit the rate equation derived from the kinetic scheme proposed by A. Cimino, M. Boudart and H. S. Taylor ( J. Phys. Chem . 58 , 796 (1954)). When possible, the rate constant of the rupture of the CC bond and a value proportional to the equilibrium constant of adsorption have been calculated. The low differences in reactivity observed for the hydrocarbons come from the rate constant differences of the hydrogenolysis step while the equilibrium constants remain nearly the same. These results are in complete opposition to what had been found on platinum. The much higher activity of nickel compared to that of platinum in hydrogenolysis is due to a huge increase in the reactivity of the adsorbed species reflected by an increase of the CC bond scission rate constant.

Investigation of the Intrinsic Activity of ZrxCe1−xO2 Mixed Oxides in the CO + NO Reactions: Influence of Pd Incorporation

The intrinsic activity of various ZrxCe1−xO2 mixed oxides and after a Pd deposition has been investigated in the CO + NO reactions from temperature-programmed experiments performed under stoichiometric conditions. It has been found that the activity of ZrxCe1−xO2 depends on either the specific surface area or the number of Ce cations and their intrinsic activity, Zr0.5Ce0.5O2 being the most active support. The addition of palladium strongly enhances the catalytic activity of the supports probably due to a synergistic effect between CeO2 and the metal since the initial activity of palladium-based catalysts is directly related to their Ce content. Such a catalytic enhancement has been explained by a “bifunctional” mechanism involving active sites probably composed of Pd and ceria. A strong deactivation operates leading to the disappearance of the beneficial effect of ceria. Such a deactivation seems to be dependent on the support composition, Pd supported Zr0.25Ce0.75O2 being the most resistant to deactivation.

Methane as alternative in the selective reduction of NO over supported palladium catalysts in lean conditions: role of redox properties of support materials

The reduction of CH4 by NO has been investigated in the presence of oxygen on palladium supported on alumina, ceria–zirconia mixed oxides and perovskite materials, mainly LaCoO3. The activation procedure, under oxygen or hydrogen, drastically influences the catalytic performances of both catalysts. The stabilisation of a metallic or oxidic Pd phase leads to poor activity in the conversion of NO in the absence of oxygen. On the other hand, oxygen enhances the activity, particularly on the reduced Pd/LaCoO3, in the CH4 + NO reaction. Such results have been explained by different interactions between palladium and the support.

AN EPR INVESTIGATION ON THE REACTIVITY OF OXYGEN FROM CERIA MODIFIED BIMETALLIC PT-RH/AL2O3 CATALYSTS IN THE CO + NO REACTION

Abstract The CO+NO reaction has been studied on Pt-Rh/Al2O3-CeO2 at PNO=5.6×10−3 atm and PCO=5×10−3 atm between 25 and 500°C. EPR technique was used to investigate the mechanism of this reaction. O2− species were detected after reaction and it seems that their formation is closely related to changes in catalytic performances of Pt-Rh/Al2O3-CeO2.

Surface Raman spectroscopic study of NO transformation over Pd-based catalysts

Correlations between in situ Raman spectroscopic and catalytic measurements on palladium based three-way catalysts during the reduction of NO by CO have been tentatively established. Particular attention to the selectivity towards the transformation of NO into N2O has been paid in order to explain why N2O is the main N-containing product during the engine cold start. A comparative study of the selectivity from temperature-programmed and steady-state experiments on bulk and supported palladium catalysts shows that bulk Pd is significantly more selective towards the production of N2 than Pd/Al2O3 at low conversion and low temperature. In addition, the subsequent reduction of N2O by CO occurs more readily on bulk palladium. In parallel to this catalytic information, Raman spectra recorded in comparable experimental conditions reveal different spectral features relative to the nature of chemisorbed species, and to the development of surface PdO islands mainly on Pd/Al2O3. Both differences have been compared and discussed in the light of a previous mechanism proposed earlier in the literature.

Optimization of Ni and Ru catalysts supported on LaMnO3 for the carbon dioxide reforming of methane

Abstract In this work a series of Ni and Ru supported on LnMnO 3 perovskite-like oxide has been studied as catalyst precursors for the carbon dioxide reforming of methane to syngas. The LaMnO 3 perovskite was synthesized by the citrate sol-gel method. The effect of parameters such as reaction temperature, amount of supported metal, space velocity, reactant partial pressure, and time on stream were investigated, and optimized to higher yields of syngas. Different techniques were applied to characterize the structural features of the LaMnO 3 perovskite support and of the catalysts. It was observed that the citrate method produced a homogenous solid with a cubic crystal structure. The catalytic activity, the CO yields and selectivity for the best Ni (10%) and Ru (1%) catalysts was similar when compared under isoconversion conditions. CH 4 and CO 2 conversions were approximately 39% and 54% respectively, while the CO selectivity reached a value of 78%. No reactivity was observed for the pure perovskite. A significant decrease in coke deposition for Ni catalysts was obtained which constitutes an advantage for future developments of commercial reforming catalysts.

Synthesis of ditungsten carbide by controlled decomposition of Cp2W2(CO)4 (dmad) under a hydrogen atmosphere

Controlled decomposition studies of the bis(cyclopentadienyl)ditungsten(tetracarbonyl)-dimethylacetylenedicarboxylate (Cp2W2(CO)4(dmad)) under flowing hydrogen show that it totally decomposes at 600 °C for 2 h to give a pure bulk W2C as revealed by X-ray diffraction. Excess oxygen and carbon at the surface are detected by X-ray photoelectron spectroscopy. An in situ temperature-programmed X-ray diffraction experiment performed on Cp2W2(CO)4 (dmad) shows the detection temperature of W2C to be 600 °C, the sample being amorphous or microcrystalline below that temperature. Based on previous results obtained for the decomposition of Cp2Mo2(CO)4(dmad) on the one hand, and thermogravimetric and chromatographic analyses performed on Cp2W2(CO)4 (dmad), on the other, a decomposition scheme of the latter under hydrogen has been proposed.