This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H2 over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H2 reaction Pt/Al2O3>Pd/Al2O3>Rh/Al2O3 and for the NO+CO reaction Rh/Al2O3>Pd/Al2O3> Pt/Al2O3. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H2 reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al2O3 during the NO+H2 reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N2O) irrespective of the nature of the reducing agent (CO or H2). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N2 via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H2+O2 reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.

One weight percent Co or Ru dispersed over Mo2C have been investigated as catalysts for Fischer-Tropsch reaction and compared to bulk Mo2C. It was found that Mo2C gives light hydrocarbons, alcohols and CO2. Noteworthy activity increases following the sequence: Mo2C < Ru/Mo2C < Co/Mo2C. Compared with Mo2C selectivity, the addition of Ru decreases the alcohol production whereas Co increases formation of heavy hydrocarbons. The carbon number distributions of hydrocarbons are consistent with a Schulz-Flory equation excepted for the cobalt based catalyst. (C) 2003 Elsevier B.V. All rights reserved.

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.

The interaction between gaseous oxygen and a catalyst Pt-Rh supported on gamma-Al2O3 (specific surface area: 100 m(2) g(-1)) was investigated by means of a differential desorption technique called intermittent temperature-programmed desorption (ITPD). Experiments were carried out under conventional secondary vacuum (P approximate to 10(-4) Pa). Essentially, three desorption steps alpha, beta(1) and beta(2) were observed, occurring around 350, 650, and 750 K, respectively. Steps beta(1) and beta(2) stem from the desorption of oxygen strongly and dissociatively adsorbed on the metallic particles. Desorption step alpha is better observed after oxygen adsorption at ambient temperature; it corresponds to weakly bonded oxygen. The high values of the frequency factors strongly suggest that no readsorption occurred when oxygen desorbed through steps beta(1) and beta(2). Desorption activation energies for steps beta(1) and beta(2) were estimated at 219 and 305 kJ/mol, respectively.

A series of perovskite-like oxide in which the A-site cation of the precursor perovskite, LaRu0.8M0.2O3, was partially or totally substituted by calcium, samarium and neodymium have been used to produce in situ nanoparticles of Ru(Ni) well dispersed on a stable support for the carbon dioxide reforming of methane. Perovskites of the type Ln(x)Ca(1 - x)Ru(0.8)Ni(0.2)O(3) (Ln = La3+, Sm3+, Nd3+) were synthesized as catalysts precursors. The reduced solids of nominal composition (Ru,Ni)/CaO and/or La2O3, Sm2O3, Nd2O3, were used as catalysts. The La1 - xCaxRu0.8Ni0.2O3 series showed a well-defined perovskite structure with surface areas between 3 and 10 m(2)/g. However, when lanthanum was replaced by samarium and neodymium, the presence of pyrochlore structures, together with the perovskites, were obtained. After reduction Ru(Ni) crystallites size between 9 and 17 nm were produced. The substitution of La by cations of smaller ionic radii (Ca, Nd, Sm) decrease the stability of the perovskites and lower their reduction temperature. Among the calcium series, La0.8Ca0.2Ru0.8Ni0.2O3 and La0.5Ca0.5Ru0.8Ni0.2O3, proved to be the most active catalysts with the highest selectivity to CO. While samarium-containing perovskite was the best among the lanthanide series. Correlations between the effect of partial or total substitution of A-site cations of the precursor perovskite and the catalytic activity and stability of in situ formed nickel and ruthenium particles were established. (C) 2003 Published by Elsevier B.V.

Hydrodechlorination of carbon tetrachloride (CCl4) was investigated on group VI metal carbides (WC, W2C, Mo2C) prepared by temperature programmed carburization of WO3 and MoO3 in 10–20% CH4/H2 mixtures. Their catalytic performances have been compared with those of 1wt.% Pt/γ-Al2O3 catalyst. Experiments were carried out at atmospheric pressure in a fixed bed flow reactor at 120 and 250°C with a molar ratio H2/CCl4=12. WC shows the best selectivity towards chloroform (CHCl3), whatever the temperature and particularly at 120°C (selectivity above 99%) while W2C and Mo2C produce tetrachloroethylene in substantial amount. Mo2C and W2C deactivate rapidly with time irrespective of the temperature, whereas WC exhibits a good stability at 120°C. Surface characterization indicates that the deactivation on M2C (M=Mo, W) could be correlated to the formation of a chlorocarbonaceous polymer.

The adoption of restrictive standards for NOx and COV emissions from mobile (automotive exhaust gases) or stationary sources (industrial plants) leads to continuous improvements on the existing processes or the development of alternative technologies. Among the different strategies already applied, the use of catalytic processes seems of great interest with regard to their flexibility and cost efficiency.

The adsorption of NO and its reaction with CO have been investigated on Pt/Al2O3 and Rh/Al2O3 by in situ infrared spectroscopy. Different spectroscopic features have been observed on Rh and Pt after NO exposure at various temperatures, mainly related to the build up of positively, neutral and negatively charged NO species on Rh, with neutral NO species predominating on Pt. The formation of Rh‐NO+ and gem‐dicarbonyl species RhI(CO)2 during the adsorption of NO and the CO + NO reaction has been associated mainly with surface Rh oxidation at low temperature. On the other hand, the reductive properties of CO prevail above 250°C leading to the disappearance of the gem‐dicarbonyl structure. These infrared observations have been discussed on the basis of previous kinetic observations. Copyright © 2002 John Wiley & Sons, Ltd.

Unsupported β-Mo2C have been synthesized by temperature programmed reaction in CH4/H2 mixtures and characterized by XRD, XPS and BET measurements. These solid compounds have been tested for the hydrogenation of toluene at 3.0MPa with or without thiophene in a fixed bed reactor. Their activities have been compared to those of a reference catalyst 1wt.% Pt/γ-Al2O3. β-Mo2C hydrogenates toluene and shows a deactivation during the first stages of the reaction at 150°C. With 50ppm thiophene the 1% Pt/γ-Al2O3 deactivates rapidly and totally after exposure of six sulfur atoms on each Pt. β-Mo2C shows a high activity at 200°C which reflects a good tolerance to sulfur in the first stages of the reaction but then deactivates quite linearly with time. It seems that one sulfur atom titrates one surface molybdenum atom. Characterization of the spent catalysts show that the bulk structure is retained. XPS results reveal that the surface is sulfided (as a carbosulfide and/or MoS2). This surface transformation is believed to be responsible for the deactivation of the catalysts as these phase sites are inactive for toluene hydrogenation.

The influence of ceria on the kinetic behavior of noble metals in the reduction of NO by CO has been investigated at 120 and 300°C on a freshly prepared Pt–Rh/Al2O3–CeO2 catalyst, and on an aged catalyst after reaction for 16 h at 500°C, within pressure ranges of 2.3–9×10−3 atm for CO and 1.5–8×10−3 atm for NO, using a differential fixed-bed flow reactor. These two temperatures have been selected from temperature-programmed experiments because they correspond to two very different regimes of activity of noble metals with ceria. It has been found that a rate equation derived from a bifunctional mechanism involving reaction paths either on metal or on ceria can correctly fit rate measurements performed at 120°C. In contrast a conventional mechanism earlier proposed for modeling the CO+NO reaction on Pt–Rh/Al2O3 at 300°C, where only noble metals are involved, enables modeling of rate measurements at 300°C on Pt–Rh/Al2O3–CeO2, which suggests that the interaction between ceria and noble metals is suppressed at that temperature. The temperature dependency of the rate constants and equilibrium constants for NO and CO adsorption has been quantified in order to explain such changes in the kinetic behavior of Pt–Rh/Al2O3–CeO2.

Unsupported β-Mo2C have been synthesized by temperature programmed reaction in CH4/H2 mixtures and characterized by XRD, XPS and BET measurements. These solid compounds have been tested for the hydrogenation of toluene at 3.0 MPa with or without thiophene in a fixed bed reactor. Their activities have been compared to those of a reference catalyst 1 wt.% Pt/γ-Al2O3. β-Mo2C hydrogenates toluene and shows a deactivation during the first stages of the reaction at 150 °C. With 50 ppm thiophene the 1% Pt/γ-Al2O3 deactivates rapidly and totally after exposure of six sulfur atoms on each Pt. β-Mo2C shows a high activity at 200 °C which reflects a good tolerance to sulfur in the first stages of the reaction but then deactivates quite linearly with time. It seems that one sulfur atom titrates one surface molybdenum atom. Characterization of the spent catalysts show that the bulk structure is retained. XPS results reveal that the surface is sulfided (as a carbosulfide and/or MoS2). This surface transformation is believed to be responsible for the deactivation of the catalysts as these phase sites are inactive for toluene hydrogenation.

Tungsten carbides were prepared by temperature-programmed reaction of WO3 with methane/hydrogen mixture and activated according to several thermal treatments by varying the temperature or the feed composition. Modification of these parameters was expected to change the surface composition (C/W). Such surface modifications have been related to acid–base properties observed in the decomposition of isopropanol. Finally, it was found that such acidic properties are probably involved in the dehydrohalogenation of chloropentafluoroethane responsible mainly for the formation of unsaturated compounds.

We have attempted to model the rate of NO transformation on a sintered Pt–Rh/Al2O3 three-way catalyst (TWC) in various temperature and conversion conditions close to the actual ones in TWCs. For this purpose a rate expression, previously established from kinetic measurements performed at a single temperature of 300°C, was used. The temperature dependency of the kinetic and thermodynamic parameters, using a non-linear optimisation, has been previously determined. Then, temperature-programmed experiments, performed in a fixed-bed flow reactor at atmospheric pressure under differential conditions, have been modelled using such parameters. A similar procedure has been achieved for modelling the TP experiments on Rh/Al2O3. Both results have been compared and discussed in the light of previous surface characterisations.

This study deals with the kinetic behaviour of a Pt-Rh/Al2O3 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 λCO and λNO, 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 (ΔHads,CO and ΔHads,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 ΔHads,CO and ΔHads,NO. Finally, we have attempted to quantify such effects.

The kinetics of the reaction between CO and O2 over bimetallic Pt-Rh/Al2O3 catalysts was studied with a differential fixed bed flow reactor running under reducing conditions (CO/O2>5). Temperature, CO and O2 partial pressures were in the range 25–500°C, 5×10−3 to 9×10−3 and 0.25×10−3 to 2×10−3 atm, respectively. Kinetic results have been interpreted in the light of various mechanisms proposed in the literature. The best agreement is obtained with a mechanism scheme involving a bimolecular reaction between adsorbed CO and adsorbed O2 molecules as determining step. A rate expression has been derived as a function of adsorption equilibrium constants of O2 and CO, λO and λCO, and of the rate constant of the limiting step k respectively. These parameters have been estimated at 215°C, then the influence of temperature on λi and k has been investigated. Finally, the effect of NO addition to the feed upon the reaction rate has been studied. It is shown that CO is adsorbed much more strongly than O2 under our experimental conditions.

Perovskites type oxides LaMO3 (M = Ru, Ni, Mn) were synthesized by the citrate Sol-Gel method and tested as catalysts for the CO2 reforming of methane. The influence of Ru partial substitution for Ni in the LaRuO3 structure on the activity and selectivity performance was also investigated. The results were compared with those obtained with catalyst samples prepared by wet impregnation. The effects of parameters such as reaction temperature, space velocity, CH4/CO2 ratio and time on stream were investigated and optimized to higher yields of syngas. XRD, BET surface area, TEM-EDX, IR, XPS, TPR and H2 chemisorptions characterized all the solids. Among all the solids investigated, the LaRu0.8Ni0.2O3 precursor was the most active and selective catalyst, reaching values of 89% and 90% in methane conversion and CO selectivity respectively even after 150 hours on stream. A significant decrease in coke deposition for all the catalysts was obtained which constitutes an advantage for future developments of commercial reforming catalysts.

The intrinsic activity of various Zr x Ce1−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 Zr x Ce1−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.

The synthesis of transition metal carbides of tungsten and molybdenum has been carried out via temperature programmed reactions (TPRs) of metal oxides or passivated nitrides. Their specific chloropentafluoroethane conversion rates were at best one order of magnitude less than that of a reference Pd based catalyst. The intrinsic rates range from 4.7 to 14.7nmolm−2s−1 and decrease as follows: Mo2C>WC>W2C≈WC1−x>MoC1−x. The group VI carbide samples catalyse hydrodehalogenation and dehydrofluorination. WC appears to be as selective towards pentafluoroethane (HFC-125) as the Pd based catalyst. Then the selectivity decreases in the following sequence: W2C>Mo2C>WC1−x>MoC1−x. All the carbide catalysts deactivate at the early stages of the reaction. Based on the XPS results and the product distribution of the reaction, the deactivation has been mainly attributed to a site blocking phenomenon due to a strong deposit of polymeric carbon and of hydrofluorocarbon polymers. Polymerisation of detected unsaturated compounds take place on acidic sites probably generated by fluoride and/or chloride in the course of the reaction.

The influence of the composition of a carburizing CH4–H2 gas mixture on the process of reduction–carburization over WO3 has been studied. Bulk tungsten carbide synthesis has been carried out from WO3 in different CH4–H2 mixtures (CH4–H2=1/1–3/1; CH4–N2=1/1; pure CH4) at atmospheric pressure by temperature-programmed reduction–carburization (TPRC). The composition of the reaction products has been monitored and quantified by gas chromatography analysis (GCA) and the results have been compared to those obtained for a reference sample WC20 (CH4–H2=1/4). The solids have been characterized by elemental analysis, XRD, XPS, and BET surface area measurements. The overall process is complex. Considering first the reduction, both H2 and CH4 act as oxides reducing agents and are converted respectively into H2O, CO, and to a less extent CO2. If the reduction steps follow the same sequence observed under pure H2, WO3→W20O58→WO2→W, with the strong difference that W metal is detected only at the surface to be rapidly carburized, the overall reduction process can be accomplished under CH4–H2 mixtures at temperatures all the lower than PCH4/PH2 increases. Prereduction of WO3 into bulk WO2 allows an easier reduction in practically pure CH4 (95% (v/v) CH4–H2) as reduction with CH4 increases the rate of the WO2→W transformation. Studies of the carburization suggest that CH4 decomposes on a metallic surface into C (or CHx) species before bulk WO2 reduction followed by surface carburization. Then carbon diffuses into the bulk of the solid to give first α-W2C whose formation occurs rapidly. α-W2C transformation into WC is slower and seems to be very much influenced by the ratio PCH4/PH2 which controls the rate of carbon deposit at the surface of the solid. The best surface area carbide of 27 m2.g−1 consisting of a core of α-W2C covered with α-WC has been obtained by using WO2 as starting material.