A series of LaNiO3 perovskite-type oxides were synthesized and the influence of the preparation methodology and partial substitution of Ni by Rh on the performance of these solids in the dry reforming of methane was studied. The precursor perovskite-type oxides were synthesized by the citrate sol-gel method and by co-precipitation using K2CO3 as precipitating agent. An additional LaNiO3 sample used as reference was prepared by impregnation of La2O3 with a nickel nitrate solution. Characterization of the synthesized perovskite-type oxides was followed by X-ray diffraction (ex situ and in situ), nitrogen adsorption, temperature-programmed reduction and photoelectron spectroscopy. In addition, structural and surface changes produced during reaction were followed by techniques such as thermogravimetric analyses, electron microscopy and photoelectron spectroscopy. Highly homogeneous and crystalline oxides were obtained with various particle sizes depending on the synthesis method. Results show influence of the synthesis procedure on the catalytic performance of these solids by formation of different lanthanum phases during reaction. It is observed that at low reaction temperatures (400-500 degrees C), all samples presented similar activity while at higher temperatures (550-600 degrees C) the solid prepared by impregnation was more active. Partial substitution of nickel by rhodium (5% mol) produces an increase in the catalytic activity. No deactivation of all synthesized solids was observed after 24 h on-stream. (C) 2008 Published by Elsevier B.V.

Two series of supported Pd catalysts were synthesized on new mesoporous-macroporous supports (ZrO2, TiO2) labelled M (Zr and Ti). The deposition of palladium was carried out by wet impregnation on the calcined TiO2 and ZrO2 supports at 400 degrees C (Pd/Zr-4, PD/Ti-4) and 600 degrees C (Pd/Zr-6, Pd/Ti-6) and followed by a calcination at 400 degrees C for 4 h. The pre-reduced Pd/MX catalysts were investigated for the chlorobenzene total oxidation and their catalytic properties where compared to those of a reference catalyst Pd/Ti-Ref (TiO2 from Huntsman Tioxide recalcined at 500 degrees C) and of a palladium supported on the fresh mesoporous-macroporous TiO2 (Pd/Ti). Based on the activity determined by T-50, the Pd/Ti and Pd/Ti-4 catalysts have been found to be more active than the reference one. Moreover activity decreased owing to the sequence: Pd/TiX >> Pd/ZrX and in each series when the temperature of Calcination of the support was raised. The overall results clearly showed that the activity was dependant on the nature of the support. The better activity of Pd/TiX compared to Pd/ZrX was likely due to a better reducibility of the TiO2 support (Ti4+ into Ti3+) leading to an enhancement of the oxygen mobility. Production of polychlorinated benzenes PhClx (x = 2-6) and of Cl-2 was also observed. Nevertheless at 500 degrees C the selectivity in HCl was higher than 90% for the best catalysts. (C) 2008 Elsevier B.V. All rights reserved.

The catalytic performances of pre-reduced palladium catalysts supported on lanthanum based perovskites LaBO3 (B=Co, Mn, Fe, Ni) were investigated for the total oxidation of chlorobenzene (PhCl; 1000ppmv) in air. The catalysts were prepared using a wet impregnation technique and Pd-nitrate was used as a palladium precursor. The catalytic performances were compared to those of a reference palladium catalyst supported on a conventional support, namely γ-Al2O3. Easiness of chlorobenzene destruction was found to follow the sequence based on the T50 values (temperature at which 50% of chlorobenzene was converted into products): Pd/LaMnO3+δ (243°C)>Pd/LaFeO3 (270°C)>Pd/Al2O3 (348°C)>Pd/LaCoO3 (360°C)>Pd/LaNiO3 (408°C). Complete conversion of chlorobenzene is reached at ca. 320–500°C, but at those temperatures substantial amounts of polychlorinated benzenes are formed. Quasi in situ XPS studies were monitored on Pd/LaCoO3 and Pd/LaFeO3 after each stage of the global process, namely after calcination, reduction and exposure to the flowing reactive mixture (1000ppmv PhCl in air) from room temperature to 230 and 310°C (Pd/LaFeO3) and to 280°C (Pd/LaCoO3). It was shown that the calcination treatment leads to a palladium which a BE higher than that of PdO and to a (B/La)XPS<1 which attests of a lanthanum enrichment at the XPS surface. After H2 treatment it was shown that palladium is totally reduced while the B cation is either unreduced (Fe3+) or reduced (Co3+ into Co2+ and Co0). In the reactive atmosphere, Pd0 is progressively (oxi)chlorined while the perovskite network is reconstructed with productions of LaOCl and Co3O4. The pre-reduced Pd/LaBO3 are more active than the perovskite alone for PhCl transformation but substantially increase the chlorination rate of PhCl. Among the different catalysts Pd/LaFeO3 shows the best compromise between PhCl oxidation and chlorination rates.

This study reports an extensive kinetic investigation of the CO + N2O reaction which is a major reaction involved with automotive exhaust catalysts during the overall reduction of NO, particularly during the start of a cold engine. Steady-state rate measurements were performed between 217 and 250 °C on Pt−Rh/γ-Al2O3−CeO2 with partial pressures in the range (4.5−12.0) × 10-3 and (2.0−7.4) × 10-3 atm respectively for CO and N2O. Particular attention has been paid toward the influence of ceria on the catalytic performances of noble metals. A bifunctional mechanism is proposed involving the redox properties of ceria which correctly explains the beneficial effect of ceria on the reduction of N2O by CO in the absence of NO. However, subsequent comparisons with previous optimized kinetic and thermodynamic constants for the CO + NO reaction on the same catalyst suggest that the promotional effect of ceria is suppressed. When N2O and NO coexist in the gas phase, only noble metals are active near the light-off temp...

Hierarchical bimodal macro-mesoporous zirconia oxide has been synthesized by a simple method in the presence of CTMABr surfactant. The synthesized zirconia having uniform macropores of 300–600nm in diameter with wormhole-like mesoporous walls and high surface area was calcined at 400 and 600°C and impregnated with 0.5wt.% of palladium and compared with classical 0.5wt.% Pd/ZrO2 catalyst for toluene oxidation. The highest activity of 0.5wt.%/macro-mesoporous zirconia calcined at 600°C was mainly explained by a rather high Pd dispersion and by H2-TPR measurements showing a higher quantity of PdO species easily reducible at 0°C.

0.5 wt% palladium supported on exchanged BEA and FAU zeolites were prepared, characterized and tested in the total oxidation of volatile organic compounds (VOCs). The BEA and FAU zeolites were exchanged with different cations to study the influence of alkali metal cations (Na+, Cs+) and H+ in Pd-based catalysts on propene and toluene total oxidation. The exchange with different cations (Na+, Cs+) and H+ led to a decrease of the surface area and the micropore volume. All Pd/BEA and Pd/FAU zeolites were found to be powerful catalysts for the total oxidation of VOCs. They were active at low temperature and totally selective for CO2 and H2O. However, their activity depends significantly on the type of zeolite and on the nature of the charge-compensating cation. The activity order for propene and toluene oxidation on FAU catalysts, Pd/CsFAU > Pd/NaFAU > Pd/HFAU, is the reverse of the activity order on BEA catalysts: Pd/HBEA > Pd/NaBEA > Pd/CsBEA. The catalytic activities can be rationalized in terms of the influence of the electronegativity of the charge-compensating cation on the Pd particles, the Pd dispersion, the PdO reducibility and the adsorption energies for VOCs.

Calcined and reduced catalysts Pd/LaBO3 (B = Co, Fe, Mn, Ni) were used for the total oxidation of toluene. Easiness of toluene destruction was found to follow the sequence based on the T50 values (temperature at which 50% of toluene is converted): Pd/LaFeO3 > Pd/LaMnO3+δ > Pd/LaCoO3 > Pd/LaNiO3. In order to investigate the activation process (calcination and reduction) in detail, the reducibility of the samples was evaluated by H2-TPR on the calcined catalysts. Additionally, characterization of the Pd/LaBO3 (B = Co, Fe) surface was carried out by X-ray photoelectron spectroscopy (XPS) at each stage of the global process, namely after calcination, reduction and under catalytic reaction at either 150 or 200 °C for Pd/LaFeO3 and either 200 or 250 °C for LaCoO3. The different results showed that palladium oxidized entities were totally reduced after pre-reduction at 200 °C for 2 h (2 L/h, 1 °C/min). As LaFeO3 was unaffected by such a treatment, for the other perovskites, the cations B are partially reduced as B3+ (B = Mn) or B2+ even to B0 (B = Co, Ni). In the reactive stream (0.1% toluene in air), Pd0 reoxidized partially, more rapidly over Co than Fe based catalysts, to give a Pd2+/Pd4+ and Pd0/Pd2+/Pd4+ surface redox states, respectively. Noticeably, reduced cobalt species are progressively oxidized on stream into Co3+ in a distorted environment. By contrast, only the lines characteristic of the initial perovskite lattice were detected by XRD studies on the used catalysts. The higher activity performance of Pd/LaFeO3 for the total oxidation of toluene was attributed here to a low temperature of calcination and to a remarkable high stability of the perovskite lattice whatever the nature of the stream which allowed to keep a same palladium dispersion at the different stages of the process and to resist to the oxidizing experimental conditions. On the contrary, phase transformations for the other perovskite lattices along the process were believed to increase the palladium particle size responsible of a lower activity.

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.

An investigation of the performances in Fischer-Tropsch reaction of 1 wt% M/WC(X) (M = Co, Ru; X=A, B), where A is a tungsten carbide protected by free carbon and B is a clean tungsten carbide, was carried out. Supported catalysts performances were compared to those of the parent tungsten carbides at 473K and 20 bar. It was found that WC(A) produces mainly hydrocarbons but also 20–40% alcohols, whereas WC(B) activity is only towards linear alkanes. Before catalytic test, a reduction in pure hydrogen allows obtaining Co0 and Ru0 dispersed on layers of free carbon covering the WC core for the WC(A), and on a surface free of oxygen for WC(B). Co as Ru dispersions are improved on WC(B) compared to WC(A). A direct consequence is that Co/WC(B) has a better activity than Co/WC(A). Ru–W alloy formation could be responsible of the inobservance of a better activity for Ru/WC(B). On contrary, addition of Ru on WC(A) highly increases the activity and the production of heavy hydrocarbons. This beneficial effect, not observed with cobalt, could be attributed to a better dispersion of ruthenium on a carbon polymeric surface of WC.

0.5wt% palladium supported on exchanged BEA and FAU zeolites have been characterised for volatile organic compounds (VOCs) catalytic oxidation. BEA and FAU zeolites have been exchanged with different cations to study the influence of alkali metal cations (Na+, Cs+) and H+ in Pd based catalysts on propene total oxidation. Oxidation of propene depends significantly on the type of zeolites and on the compensating cation of the zeolite. According to sample, the catalytic activity is explained by the surface areas of the catalysts, the adsorption energies for VOC, the influence of the electronegativity of the compensating cation on the Pd particles, the Pd dispersion and the PdO reducibility.

The paper deals with the effect of modification with cobalt of a series of LaFe1-xCoxO3 mixed-oxide perovskites as catalyst precursors in the combined reforming of methane with CO2 and O-2 for the production of syngas. The perovskite type oxides were synthesized by fine chemical using the citrate sol-gel method and characterized by techniques such as FT-IR, BET surface area, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), TPR and inductively coupled plasma emission spectroscopy (ICP), under the condition of as-synthesized, reduced and used samples. The results showed that through this synthesis it is possible to obtain highly crystalline, homogeneous and pure solids, with well-defined structures. After reduction, the stabilization by the presence of iron of highly dispersed Co metallic particles from the cation on position B over the corresponding oxides was evidenced, decreasing deactivation by avoiding the sintering of metals and coke formation on the surface of the catalysts.The existence of strong Fe-Co interaction in the LaFe1-xCoxO3 with x = 0.4-0.6 produces a synergetic effect improving methane conversion and H-2 production. (c) 2005 Elsevier B.V. All rights reserved.

The catalytic potential of a red mud, a waste from alumina production, was evaluated for the oxidation of VOC, in particular toluene. When dried, it consists mainly of Al(OH)(3) and Fe2O3. It was found that red mud calcination under an air stream at 500 degrees C leads to the formation of Al2O3 and Fe2O3, two oxides that are active and CO2 selective in the toluene oxidation reaction. The red mud activity can be correlated directly to the amount of iron in the sampling. Indeed, the chemical reaction specific rate (by gram of Fe2O3) in presence of red mud is similar to that observed in presence of a pure Fe2O3 iron oxide.

AbstractLe potentiel catalytique d'une boue rouge, déchet de la fabrication de l'alumine, a été évalué dans l'oxydation des COV et notamment du toluène. Séchée, elle contient essentiellement Al(OH)3 et Fe2O3. Il s'est avéré qu'une calcination de la boue rouge sous flux d'air à 500 °C conduit à la formation de Al2O3 et Fe2O3, oxydes qui sont actifs et sélectifs en CO2 dans la réaction d'oxydation du toluène. L'activité de la boue rouge peut être directement corrélée à la quantité de fer présente dans l'échantillon. En effet, la vitesse spécifique (par gramme de Fe2O3) de la réaction chimique en présence de la boue rouge est similaire à celle observée en présence d'un oxyde de fer Fe2O3 pur.

Abatement processes for the reduction of N2O emissions from acid nitric plants can be implemented in different positions. Among the different possibilities, a catalytic process set up between the ammonia converter and the absorber could be a practicable solution. In those running conditions, at high temperature, in the presence of NO, O2 and water, the catalytic decomposition of N2O (in the absence of a reducing agent) can take place. However, catalysts usually suffer from a strong deactivation owing to the occurrence of thermal sintering which significantly lowers their specific surface area. Catalytic testing performed at laboratory scale showed that zirconia based catalysts stabilised by yttrium incorporation could be of potential interest. However, the mode of yttrium incorporation seems to be a key factor. According to the preparation procedure, surface yttrium enrichment may occur and then strongly inhibit the catalytic decomposition of N2O. Co-precipitation methods can be profitably used for the preparation of modified-ZrO2 catalysts in order to obtain yttrium homogeneously distributed in the bulk material. According to this preparation method, a synergy effect on the catalytic activity and also on the stability has been observed on ZrO2 containing low amount of yttrium whereas an inhibiting effect prevails on highly loaded yttrium based catalysts irrespective of the mode of yttrium incorporation.

This paper deals with the kinetics of the reduction of nitric oxides by CO, which is a major reaction involved in automotive exhaust three-way catalysts. Spectroscopic and steady state kinetic measurements have been compared in order to explain the different selectivity behavior of Pt- and Rh-based catalysts, particularly during the cold start. It was found that in situ spectroscopy can be a useful tool either for stating on the nature of intermediates or checking the validity of a rate equation. Regarding the kinetics, subsequent calculations and comparisons of thermodynamic and kinetic parameters provide relevant information for modeling purposes. Also, it was found that our kinetic results can be correlated to surface modifications in the particular case of supported bimetallic Pt–Rh catalysts.

The subsequent reduction of N2O by CO is an important subreaction in the overall CO+NO reaction. This study reports a kinetic investigation of the single CO+N2O reaction on Rh/Al2O3, and on Pt–Rh/Al2O3, between 264 and 310°C with inlet partial pressures in the range 3.4–13.8×10−3 and 1.5–5.9×10−3 atm respectively for CO and N2O. Our mechanism proposal differs from those proposed earlier in the literature mainly by the dissociation step of chemisorbed N2O species involving a nearest neighbor vacant site. The difference in the selectivity behavior toward the transformation of NO into N2O on Pt and Rh has been related to competitive adsorptions between NO and N2O. A comparison of the values of the equilibrium constant for the adsorption of N2O with those earlier calculated for NO from the kinetic study of the CO+NO reaction on similar catalysts [J. Catal. 175 (1998) 194] shows that, on Pt, this competition is in favor of N2O, which can readsorb and dissociate at low temperatures and low NO conversions. In contrast, the strongest NO adsorption on Rh inhibits the subsequent CO+N2O reaction. In the second part of this study, the selected mechanism has been tentatively validated in a wider range of temperatures and conversions. The adsorbate coverage dependencies of the strength of the metal–adsorbate bond has been accounted for to model temperature-programmed conversion curves.

Perovskites type oxides La1−xCaxRu1−xNixO3 were synthesized by the citrate sol–gel method. Characterization was carried out underlying the modification of metal catalysts by promoters by total or partial substitution of A- and B-site cations. The influence of Ru partial substitution by Ni in the LaRuO3 structure and of La by Ca on LaRu0.8Ni0.2O3 was also investigated in the dry and combine reforming of methane. All perovskites series, showed a well-defined perovskite structure with surface areas between 2 and 17 m2/g. After reduction Ru(Ni) crystallites size in the order of 10–15 nm were produced. An oxygen deficiency was shown by most perovskites. Substitution of La by Ca of smaller ionic radium decreases the stability of the perovskites and lowers their reduction temperature. Among the calcium series, La0.8Ca0.2Ru0.8Ni0.2O3, proved to be the most active precursor catalyst with the highest selectivity to syngas. Correlation between the effect of partial or total substitution of La by Ca as A-site cation of the precursor perovskite, the catalytic activity and stability of in situ formed Ru(Ni) particles was established.

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.