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Sorption of SO2 over a commercial SOx adsorber was studied at a laboratory scale using a fixed bed reactor. The adsorbent material contains mainly alumina, barium oxide and precious metals. Sulphates formed by SO2 sorption are reduced by carbon monoxide or hydrogen in the presence of water. The main species emitted during the reduction was H2S. The species COS was emitted at a very low level when carbon monoxide was used as a reducing agent. The total removal of sulphur was not possible in the absence of water, the sulphur remaining bonded to platinum (or palladium) sites. The role of water is to hydrolyse these sulphides, leading to H2S or COS. The species SO2 was also emitted when an aged catalyst was used, without a loss of the catalytic reduction efficiency. After a first sequence adsorption–regeneration, the adsorption capacity was not affected and the second regeneration was more efficient. Such a SOx adsorber can be placed upstream a NOx adsorber to avoid poisoning of this latter by SO2.

It has been found that the photocatalytic activity of classical TiO2 for the purification of polluted effluents containing volatile organic compounds (VOCs) can be greatly improved using the effect of the coupling of semiconductors and oxides. Thus, a new mixed photocatalyst has been prepared leading to a composite ternary system, WO3/SiC–TiO2. The results indicate that this combination can result in an enhancement in the photocatalytic activity toward methylethylketone (MEK) oxidation at room temperature by a factor greater than four with a total mineralization into CO2 and H2O, without creating toxic by-products. The efficiency of the composite system strongly depends on the relative amounts of WO3 and SiC in the various samples because an optimum in photo-activity is observed when the weight ratios WO3 and SiC are about 3.5 and 20%, respectively. The increase and synergy in photo-oxidation activity, in comparison to TiO2 and to the binary TiO2-based systems, WO3/TiO2 and SiC–TiO2, are obtained as the simultaneous effect of WO3 and SiC. Addition of WO3 mainly results in the retarding of electron-hole recombination process by an electron trapping effect and in the modification of surface acidity compared to the case of pure TiO2. On the other hand, the coupled system of the two illuminated semiconductors, TiO2 and SiC, leads to a simultaneous electron transfer from SiC to TiO2 and hole transfer from TiO2 to SiC, increasing spatial charge separation and thus preventing charge recombinations, detrimental to photocatalytic efficiency.

This study has an important impact on gasoline engine-pollution control working under lean conditions. While NOx trap systems can remove NOx under an oxidative atmosphere, they are poisoned by SOx present in the exhaust gases. In order to protect NOx traps, an upstream SOx trap has to be used. SO2 adsorption was studied in the presence of water and oxygen. Model and commercial catalysts were tested between 300 and 700 °C. In order to assign the TPD peaks, the decomposition of commercial sulphates was studied versus the temperature. Adsorption capacity is not sensitive to oxygen and SO2 concentrations but is strongly related to barium content. Cerium content is not a key parameter for SO2 adsorption capacity in the presence of oxygen. XPS analysis allowed us to differentiate between all the species formed during the adsorption process. When the catalysts are aged, specific surface area decreases as well as adsorption capacity.

The photocatalytic degradation of butyl acetate in the gas phase was investigated in a fixed-bed continuous annular reactor using a titanium dioxide semiconductor, Pt/TiO2 and WO3/TiO2 catalysts. Platinum was deposited on the titanium by adsorption of PtCl62− anions, and WO3/TiO2 was prepared by a conventional impregnation method using an aqueous solution of ammonium paratungstate. Different samples, with different nominal weight loadings in Pt and WO3, were tested. For each catalyst sample studied, air containing butyl acetate and water vapors in various molar ratios was fed at 200 cm3/min to the photoreactor. The roles of the reaction temperature and of H2O in the photocatalytic oxidation of butyl acetate and the influence of the content of TiO2, Pt, and WO3 were investigated. Although an initial decay in photocatalytic activity was observed, partly issued from an initial deactivation, total mineralization was achieved under all experimental conditions. For relative water humidity between 50 and 75% and for adequate TiO2 covering ratios, the photocatalytic activity of TiO2 toward the photocatalytic degradation of butyl acetate can be greatly improved by the addition of WO3, mainly due to a prevention of the initial deactivation and of photogenerated charge recombination. This optimized photocatalyst is very stable on stream and no regeneration treatments are needed. The influence of Pt and WO3 on photocatalytic activity are discussed with regard to chemical and electronic modifications of TiO2.

Activity and selectivity of a series of bifunctional sulphated zirconium-pillared montmorillonite (Zr-MS), aluminium-pillared acid-activated montmorillonite (Al-MH), aluminium-pillared sodium montmorillonite (Al-MNa), and γ-alumina catalysts supported palladium were compared in hexane hydroconversion reactions. Hydrogen and hydrocarbon partial pressures as well as reaction temperatures were changed. The product distributions were analysed and kinetic data were collected. Sulphated Zr-pillared montmorillonite supported palladium catalyst is the more active catalyst and acts as an excellent hexane hydroisomerisation catalyst giving very high isomer selectivity around 98%. At the opposite, 1 wt.% Pt/γ-alumina is the less active and less selective catalyst. In skeletal isomerisation of hexanes and in methylcylopentane reactions we pointed out that both acid strength and metallic behaviour are important. The latter one prevents any blocking of the active sites by formation of (hydro)carbon residues thanks to its hydrogenation power. Activation energy values, enthalpy and entropy of activation have been determined. In all the reactions studied and with all the catalysts used, a compensation effect is observed. To explain the results, a metal–proton adduct is proposed as active site. The relative strength of the acid function versus the metallic one explains the results.

The relation between the electronic structure of platinum metallic aggregates and their selectivity is discussed. A correlation is underlined between the energy of the d -band center, e d , and the catalytic reactions such as hydrocarbon isomerization. Similar behavior appears when CO chemisorption on a platinum surface is compared with activity in alkane isomerization. This relationship can be used to interpret the changes in the mechanisms observed in acid catalysis, with bimetallic systems, and with catalysts treated by microwaves.

The asymmetric hydrogenation of trans-2-methyl-2-pentenoic acid in hexane over two well defined E-40692 and E-5220 Engelhard Pd/Al2O3 catalysts modified with cinchonidine was re-examined, the literature results have been reproduced and it was found that the E-40692 catalyst lead to the best conversions and %e.e. Deuteration experiments have been conducted under identical (solvent, catalyst, constituents ratios and temperature) conditions: percentage of H/D exchange (in the starting substrate and in the saturated product) as well as percentage of deuteration at all carbons have been determined by 1 H NMR and indicated that double bond migration and cis/trans isomerization occurred during hydrogenation/deuteration but that these processes contributed less at higher pressure (40 bar) and in the presence of cinchonidine.

Potentiometric titration of poly(acrylic acid) and hydroquinone-functionalized poly(acrylic acid) was conducted in the presence of copper (II). The effects of hydroquinone functionalizing and copper (II) complexing on the potentiometric titration of poly(acrylic acid) were studied in an ionic environment and in its absence. Henderson-Hasselbalch equation was applied to assess its validity for this titration. Coordination number and the stability constants of the copper- (II-)complexed polymers were determined, and results showed the formation of mostly monodentate and bidentate copper- (II-)polymer complexes.

Isomerization and cracking reactions of n-heptane on 0.2 wt.% platinum supported on sulfated zirconia (Pt/SZ) are investigated in such a way to get maximum kinetic data. The experimental conditions were varied as follows: (i) two ranges of hydrogen pressure, low pressure (LP) 190–760 Torr and high pressure (HP) 900–3000 Torr were studied; (ii) reaction temperatures of 150, 200 and 250 °C were investigated; and (iii) hydrocarbon partial pressures from 5 to 40 Torr were analyzed. Reaction orders versus hydrogen pressure and hydrocarbon pressure, and apparent activation energy values have been determined. The Pt/SZ catalyst displays an original catalytic behavior: (i) at LP, the hydrogen reaction order is negative, while it is positive at HP, and in both cases, these orders tend towards zero when the reaction temperature increases; (ii) the apparent activation energy values are lower at HP than at LP; and (iii) the reaction orders versus hydrocarbon are equal to 1 whatever the experimental conditions are. The changes in the experimental conditions involve modifications of the kinetic parameters. Such results are explained by the fact that the relative proportions of metallic sites or acid sites are influenced by the experimental conditions. The metal–proton adduct site [H(Mm)(H+)x]x+ explains these experimental results. We interpret that at hydrogen pressures higher than 760 Torr, the excess of hydrogen provokes a shift in the reactivity from the metallic part of the adduct to the acid one. In fact, at HP and at high reaction temperature, the reactivity is controlled by the acid sites, while at LP and low temperature, it is managed by the metallic and the acid sites.

There is not yet a straightforward answer concerning the mechanism(s) of selective catalytic reduction of NOx, by hydrocarbons. In this study, a systematic approach of this reaction was undertaken over 0.5 wt.% Pd/Al2O3. Successively, oxygen, NO and C3H6, which is the reductant, were either suppressed or substituted by NO2, acetone, propanal, 1- or 2-nitropropane. A “memory effect”, i.e. N2 formation in (NO+O2) after (NO+O2+C3H6) reaction, was observed. Moreover, a concurrence exists between the overshoot in the CO2 formation and N2 formation peaks in (NO+O2+C3H6) reaction under transient conditions, continuously raising the temperature. The more the reactants were adsorbed, the higher the nitrogen production amount was. To interpret these results, strong adsorption phenomena have to be invoked. From the results obtained with the use of carbonyl and nitro-compounds, it seems that both species may be involved in the DeNOx reaction mechanisms.

In a multicenter study, potassium dichromate-preserved stools from patients infected with Cryptosporidium parvum ( n = 20), C. hominis ( n = 20), and other Cryptosporidium species ( n = 10) and 60 controls were examined using four immunochromatographic assays. Assay sensitivity ranged between 50.1% and 86.7% for C. parvum and C. hominis but was

International audience; The influence of the presence of H2O on the contact between carbon, used as model soot, and a model four-way catalyst (1% Pt-10% BaO/Al2O3) was investigated. NOx adsorption/TPD cycles at 300 degrees C together with XRD, XPS and DRIFTS characterizations showed that only surface nitrate species are destabilized by the carbon present in the catalytic bed, leading to a decrease of the NOx storage capacity and carbonate species formation. In another way, injection of water in the reactive gas flow decreases also the NOx storage capacity of the catalyst, but promotes the formation of stable nitrate species. A non-cumulative effect of carbon and water was observed. It was proposed that a competition between the destabilization, by carbon, of weakly bonded surface nitrate species and the enhancement of bulk nitrate species formation in the presence of water occurs.

An organic–inorganic hybrid mesoporous silica was synthesized via post-grafting of KIT-6 with 4-(triethoxysilyl)butyronitrile. All samples were characterized using their N2 adsorption–desorption isotherms, XRD, FT–IR, TEM, SEM, and PT. The adsorption potential of this material for removing Pb(II) from aqueous solutions was investigated via the batch technique, and the effects of pH and contact time were studied. Experimental data showed that the maximum Pb(II) adsorption, 76%, occurred in the pH range around 6. The adsorption equilibrium was reached within 40 min for 10 wt.%COOH/KIT-6. The adsorption data were fitted using the Langmuir and Freundlich isotherms, and the obtained modeling equilibrium adsorption data suggested that the 10 wt.%COOH/KIT-6 sample contained homogeneous adsorption sites that fit the Langmuir adsorption model well. The pseudo-second-order model described well the 10 wt.%COOH/KIT-6 adsorption process. The desorption and regeneration experiments indicated that ≈95% of the metal desorbed and the adsorbent could be regenerated via an acid treatment without altering its properties.

The mechanism of NOx decomposition is reviewed. This reaction is first studied on metal or oxide single crystals, in order to get solid basis from catalysis on well-defined surface structures. If we understand few things about this reaction, at least the catalytic surface is known. This statement is not 100% correct, as the surface is reconstructed during the catalytic reaction; and this point is considered in the manuscript. Then we analyse this reaction on metallic catalysts supported on metal oxides. The crystallites are more or less dispersed, and the support can play the role of a modifier or of a promoter. A general consensus emerges on (i) the chemisorption of the reactant(s) which causes adsorbate-induced restructuring of the surfaces, (ii) the substrate, which is influenced by the redox properties of the support and by the particle sizes of the metallic aggregates, (iii) the fact that low-coordinated surface atoms are the more active in NO bond breaking, and (iv) the general pathways of the reaction, where redox mechanisms involving nitrosyl species, as intermediates, moving to nitrite-nitrosyl or dinitrosyl species are proposed. All these points are discussed.

Stable and active hydroisomerisation catalysts have been prepared by impregnating acid pillared clay minerals with hydrogenation–dehydrogenation functions due to palladium. Contact reactions of hexanes on 1 wt.% Pd/alumina pillared acid-activated montmorillonite and 1 wt.% Pd/sulphated zirconium pillared sodium montmorillonite exhibit much better activities and isomerisation selectivities compared to those obtained on conventional pillared montmorillonite. The sulphated zirconium pillared sodium montmorillonite catalyst is two to three times more active than the other catalysts. The use of C6 cycoalkanes as reactants is a good indicator to quantify and to determine which parameters governs the reaction pathway either, the metal sites or the acid sites located on the support. Thus, it permits us to determine which mechanism will dominate. We propose for the adsorption step an associative mechanism between the hydrocarbon and the adsorbed hydrogen which leads on metal, on acid or on both metal plus acid catalysts to the formation of [CnH2n+3] as intermediate.

Face to the difficulty to find an efficient catalyst in the reduction of NO by hydrocarbon in diesel exhaust gases, we have examined the interaction of some concerned reactant gases, NO, C3H6 and O2, with a catalyst constituted of nanometer scale platinum particles deposited on alumina. Indeed, in order to understand the catalytic process, it seems essential to have information on the adsorption process and on the evolution of the structural parameters of the particles when submitted to such gases. This work has been performed on a 1% Pt/Al2O3 catalyst by using in situ EXAFS measurements. We have concentrated our study, on the one hand, on the influence of NO, C3H6 in the presence or not of an excess of oxygen and, on the other hand, on the influence of the total mixture. The most striking result is the growth of the platinum particles under NO. Moreover, when the catalyst is submitted either to a mixture of NO + O2 or C3H6 + O2, the particles are not fully oxidised. With the total mixture, the particles both grow and are oxidised.

To better understand N2 formation in the selective catalytic reduction of NOx by propene under oxidative atmosphere, we studied the catalytic behavior of bimetallics. Surface segregation phenomena may provoke geometric and electronic modifications of the catalytic active sites. To explain our results we invoked: (i) the facile way to oxidize the bimetallic aggregates, (ii) the presence of atoms with low coordination number able to be oxidized first and (iii) an eventual formation of like “vic-diNOx species” on such bimetallics leading to N2 formation.

Isomerization reactions of n-heptane, n-octane and n-nonane are studied on sulphated-zirconia-supported 0.2 wt% Pt, Pd or Ir catalysts. Evolutions of isomer selectivity versus total conversion and reaction temperatures are analysed. When total conversion (αT) is increased, isomer selectivity (%Sisom) is decreased and the slope of the curve %Sisom=f(αT) is more pronounced when the carbon number in the alkane is more important. At isoconversion, around 20%, below 473 K, cracking is favoured over isomerization reaction, and above 473 K it is the reverse. Moreover, with n-heptane, when the catalytic reaction occurred at 423 K and at low conversion, αT≤20%, we observed a large decrease in the isomer selectivity percentages on Pd/SZ and Ir/SZ compared to Pt/SZ. What is remarkable is that, at this low temperature, both metals are inactive in the carbon–carbon bond rupture. To explain these results the following points are raised: (i) an associative mechanism is proposed for the adsorption step of the alkane involving an agostic intermediate species where the carbon–hydrogen bonds act as ligands to the transition metal centres forming covalent C–H⋅⋅⋅M systems, and (ii) a metal–proton adduct site, which gathers metallic and acidic sites is suggested. This approach seems to better explain our results than the “traditional” bifunctional mechanism.

Cet article presente une nouvelle cellule reactive dediee a l'etude in-situ par spectroscopie d'absorption X de catalyseurs de depollution pour moteurs Diesel. Cet appareillage a ete developpe pour collecter des spectres EXAFS pendant la reaction chimique, l'echantillon etant place sous un flux gazeux constitue des principaux gaz reactifs constitutifs des echappements de moteurs Diesel. Une attention particuliere a ete portee sur la forme et le materiau du porte-echantillon. En effet, une geometrie speciale a ete elaboree pour etudier le catalyseur directement dans sa forme finale, c'est-a-dire tel qu'il est dans le pot d'echappement en tenant compte des conditions reactionnelles (temperature et vitesse volumique horaire elevee).

Face aux difficultes pour trouver un catalyseur efficace dans la reaction de reduction de NO par les hydrocarbures en milieu oxydant dans le cadre du traitement post-combustion des moteurs Diesel, on a etudie l'interaction de certains des gaz concernes (NO, C 3 H 6 et O2) avec un catalyseur constitue de particules de platine a l'echelle nanometrique depose sur un support alumine. Il est important en effet de comprendre le processus catalytique associe a cette reaction de reduction, notamment l'evolution structurale et electronique des particules pendant l'acte catalytique. On a ainsi pu mettre en evidence que la presence de NO seul entrainait une augmentation importante de la taille moyenne des particules. D'autre part, l'influence de NO ou de C 3 H 6 en presence d'un exces d'oxygene est sensible au niveau de l'oxydation totale des particules: celle-ci est en effet moins importante. En presence d'un melange complet regroupant ces trois gaz, il s'avere que les particules s'agregent et s'oxydent.

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

An in situ EXAFS study has been done on a pre-reduced and pre-oxidised 1.16% Pt/γ-Al2O3 catalyst in order to determine the effect of 1% NO (balance N2) versus temperature at atmospheric pressure on the platinum particles. Changes were observed in the crystallite morphologies from 200°C due to sintering processes on the pre-reduced catalyst. On the pre-oxidised one, the phenomenon is much less pronounced. This study specifies the reactivity of platinum clusters deposited on γ-alumina versus NO.

The acidity–basicity effect on the catalytic behavior of exchanged Pt-β zeolites in hexane reforming reactions has shown: (i) from X-ray photoelectron spectroscopy, that the platinum binding energy decreases when the basicity increases (therefore there is an electronic shift from zeolite to Pt particles which increases with basicity); (ii) at low hydrogen pressure ( P H 2=226 or 38 Torr), the apparent activation energy for the 2MP reaction increases when the basicity of the zeolite increases; (iii) the ring enlargement reaction of methylcyclopentane to cyclohexane and benzene is increased when Bronsted acidity is increased; (iv) with acyclic molecules as 2-methylpentane and 4-methyl 1-pentene and n -hexane, the isomerization selectivity is high when the acidity is high but decreases when the basicity increases; (v) from the use of 13 C-labeled molecules, no cyclic mechanism was detected but only bond shifts, for which the ethyl bond shift predominates on the acidic catalyst and the methyl bond shifts on the basic catalyst. The combination of these experimental points with kinetic data suggests that the hydrocarbon rearranges at the metal–support interface and that electron donor or electron acceptor effects due to the acidity–basicity of the zeolite lead to changes of the reaction mechanism pathway.

Reduction of nitrogen oxides under oxidative atmosphere is a very extensively studied reaction, but it is still very difficult to understand and to follow the various pathways of the reaction. Two alumina supported 0.2 wt% platinum catalysts, reduced by hydrogen in an oven heated either by microwave irradiations or by Joule effect, with different metal dispersion of 60% and 90%, respectively, were studied. By the use of labelled compounds we were able to show the presence of an exchange reaction between 15 N 16 O and 15 N 18 O which occurred on both catalysts. It means that [ 15 N 16 O 18 O ] is the intermediate species. Such product, 15 N 18 O , is less formed on the microwave catalyst “MW” than on the classical one “CT”. Experiments were performed at 22 and 550 Torr, between 150°C and 250°C. Near atmospheric pressure, “MW” catalyst gives higher initial rates for 15 N 2 formation than the “CT” catalysts. At low temperature, the nitrogen selectivity is higher on “MW” catalyst than on the other catalyst. From the apparent activation energy values, one may deduce that several mechanisms are responsible for the 15 N 2 formation depending on the reaction temperature and the catalyst used. On the 0.2% Pt/Al2O3 “CT” catalyst, an additive process between propene and 15 NO takes place at low temperature giving nitroso and oxime intermediate species. At high temperature, a partial oxidation of propene occurs, giving a ketone, before the 15 NO reduction to 15 N 2 . With this catalyst only two sites with different activity are involved. On the 0.2% Pt/Al2O3 “MW” catalyst the reactants are seated on three sites with different activity. This particularity reinforce the proposals concerning the “MW” catalyst which may exhibit particular shapes for the aggregates having different crystallographic orientations. What is surprising, for this “MW” catalyst, is the fact that we already observed a specific reactivity under reductive atmosphere in reforming reactions [J.M. Thiebaut, G. Roussy, M.S. Medjram, F. Garin, L. Seyfried, G. Maire, Catal. Lett. 21 (1993) 133; L. Seyfried, F. Garin, G. Maire, J.M. Thiebaut, G. Roussy, J. Catal. 148 (1994) 281] and now its behaviour is also particular under oxidative atmosphere.

Our study is focussed on the 15 NO reduction with alkane (propane C3H8) and alkene (propene C3H6) in the presence of a large excess of 18 O 2 . Alumina supported platinum catalysts are used as well as platinum–zinc bimetallic catalysts. The catalytic experiments were performed in a recirculation set up coupled to a magnetic mass spectrometer. Since 150°C, C 18 O 2 , C 18 O 16 O as well as 15 N 18 O are formed which shows that 18 O 2 is dissociated on these catalysts, that nitrogen–oxygen bond is broken and oxygen exchange reactions take place. The participation of 16 O-adsorbed from the support cannot be excluded. About the possible mechanisms involved we noticed that on platinum catalyst, propene and 15 NO disappearance reactions have the same activation energy (Ea) values at low reaction temperatures and propene and 18-oxygen have the same activation energy values at high reaction temperatures. On the other hand, on Pt–Zn catalyst two groups of similar activation energy values are obtained for propene and 18-oxygen consumption reactions at low and high reaction temperatures, respectively. These observations lead us to propose that, on platinum catalyst at a low temperature, an additive compound with propene and 15 NO may be involved, and on Pt–Zn catalyst a partially oxidised hydrocarbon may participate in the catalytic reaction as well as on platinum catalyst at a high reaction temperature. Only one type of sites seems to be involved on platinum and platinum–zinc catalysts, but at high temperature, on the latter catalyst, at least two types of sites are involved with different activity. This deviation may explain why Pt–Zn catalysts have a higher activity than Pt catalysts.

Kinetic studies of 2-methylpentane skeletal transformations over Pt/Na-β zeolite have been carried out under steady state conditions. Negative reaction orders versus hydrogen, and positive reaction orders versus 2-methylpentane were obtained. In order to explain these kinetic data several mechanistic models based on ideas of multiple adsorption for the organic reactant and competition in adsorption were tested against experimental data. The best description for 2-methylpentane consumption rate was achieved applying a reactive (associative) chemisorption mechanism. Selectivity towards the main reaction product 3-methylpentane was seen to be dependent on reaction pressure. In the mechanistic model we propose an involvement of at least two different types of mechanism with different modes of adsorbed 2-methylpentane, more precisely with different number of adsorption sites. Comparison between experimental and calculated values was performed through statistical data fitting. Good description has been achieved.

La complementarite entre deux techniques specifiques au rayonnement synchrotron, la spectroscopie d'absorption X et la diffraction anomale est illustree a travers l'etude d'entites de taille nanometrique deposees sur un oxyde leger. En premier lieu, une caracterisation d'agregats metalliques Pt-Co deposes sur une zeolite montre tous les avantages d'une telle approche. Citons parmi eux, la determination de la taille de la particule grâce a la connaissance du nombre de premier voisin metallique et de la nature du reseau cristallographique de la particule. Pour les oxydes divises, cette demarche permet une mesure precise de l'etat electronique du zinc, du taux d'occupation des sites tetraedriques ainsi que de la taille du cristallite de l'oxyde etudie.

This paper describes the effect of microwaves on EuroPt-3 and EuroPt-4 catalysts which have been studied in a number of European laboratories affiliated with the council of Europe's research group on catalysis and are now well known standard catalysts. It presents the study of 2-methylpentane isomerization on these catalysts as a function of the heating mode. Microwaves change some of the catalytic properties. In the case of EuroPt-3 catalyst, a standard containing 0.95% chlorine, the isomer selectivity is increased by a factor 2 when the reaction is performed under an electromagnetic field. On EuroPt-4, a standard bimetallic catalyst, the distributions of the hexane products formed in the isomerization of 2-methylpentane are different when the reactions occur under the two heating modes.

The catalytic transformations of 2-methylpentane (2MP), and methylcyclopentane (MCP), over Co NaY , PtCo NaY , PtCo Al 2 O 3 and NaY samples have been investigated. Co NaY was found to be an excellent isomerization catalyst, in contrast to Co Al 2 O 3 on which the main route was observed to be cracking. This is attributed to the difference in cobalt reducibility over the two supports. The use of 13C-labelled 2-methyl pentanes made the elucidation of the mechanism of isomerization on these samples possible. A cyclic mechanism (CM) operates only on metal particles, while for both metal particles and support, a bond shift mechanism (BS) is the main route for isomerisation. On PtCo NaY samples, the contribution of the cyclic mechanism to isomerisation relative to isomerisation by bond shift route is significantly enhanced on the catalysts with cobalt loading higher than 22 at.%. The results are discussed in terms of small metal particles inserted into NaY supercage and the enhanced reduction of cobalt ions.

This paper describes in close molecular detail the complex reactions which occur when a variety of hydrocarbons are adsorbed on fresh or oxygen-modified bulk tungsten carbide catalysts. On fresh tungsten carbide without oxygen, cracking reactions are the only reaction processes occurring; the results have been interpreted with a kinetic treatment which assumes successive carbon–carbon bond ruptures. The conditions under which the bulk tungsten carbide powders are treated with oxygen have a determinantal influence on their catalytic properties. If air is introduced at moderate temperature (350°C), an acidic character of the catalyst is observed which leads to a bifunctional mechanism. Cracking reactions are the most important processes, and corresponding mechanisms have been interpreted using the presence of OH surface groups leading to alkoxy species as intermediates. If air is introduced in flowing hydrogen at high temperature (700°C), the metallic character of the material is observed, leading to cracking and isomerization reactions explained via classical metallacyclobutane and σ-alkyl species, respectively.

The influence of oxygen on the reforming activity of bulk tungsten carbide (WC) has been studied for the reaction of pentanes, hexanes, heptanes, and two olefins (2-methyl-2-pentene and 4-methyl-1-pentene). Depending on the air treatment, at low (−78°C), moderate (350°C), or high (700°C) temperature, these alkanes lead to different reaction products as a result of different reaction mechanisms. Whatever the oxygen treatment, heptanes react faster than hexanes, which are more reactive than pentanes. Furthermore, cyclanes (methylcyclopentane or ethylcyclopentane) are less reactive than linear alkanes (n-pentane,n-hexane, orn-heptane), which react more slowly than the branched ones (isopentane, 2-methylpentane, 3-methylhexane). Whatever the oxygen treatment, no cyclic mechanism is involved and isomerization proceeds only through two kinds of bond-shift mechanisms. In order to obtain more information about the possible mechanisms, i.e., a bifunctional mechanism with dehydrogenation/hydrogenation on metallic sites and carbenium ion rearrangement on acidic sites, two unsaturated reactants (2-methyl-2-pentene and 4-methyl-1-pentene) have been tested. The reaction mechanisms and a kinetic model are discussed in detail in a forthcoming paper.

The adaptation of the catalytic surface to the gas phase may induce modifications of the atomic arrangement. The interaction between the gas phase composition and three-way catalyst type were investigated both by in situ EXAFS experiments and in a laboratory reactor. In both cases, experimental conditions simulating the exhaust from a catalytic converter were reproduced. Activity measurements in an oscillating feed of CO, NO, C3H5 and O2 revealed that application of a traditional kinetic study was not possible owing to the transformation of the active site during the course of the reaction. The occurrence of a redox process involving rhodium when using Pt—Rh/Al2O3 was observed as well as that of Pt—Rh alloy formation.

A highly flexible method for direct and quantitative determination of surface Brønsted acidity of solids in terms of number, type and strength of the acid sites based on quantitative H/D exchange kinetics between the acid solid and gaseous D2O has been developed and applied to materials covering the whole range of acidity.

The chemical and catalytic properties of supported alloyed metal nanoparticles are reviewed. After describing the theoretical methods commonly used to predict the electronic structure of nanoalloys and the experimental methods used to characterize their geometrical structure, we focus on the catalytic properties of these systems and the opportunities that alloying provide to modulate them. Prototypical processes of key industrial relevance are used as test examples, including catalytic pollution control as three-way catalysts and NO x decomposition, the Fischer–Tropsch process, reforming of alkanes, electrocatalysis, and new energy resources as fuel cells and hydrogen storage. A section discussing how to bridge the gap between model studies employing surface science methods and technological heterogeneous catalytic systems concludes the chapter.

Polyacrylamide was synthesized and chemically modified (PAAm) with 1,2,4,5-tetrahydroxybenzene (THB) or benzene-1,2,4,5-tetrol by applying the reaction conditions of Minisci. A degree of modification as high as 42% was obtained under the following conditions: temperature of 70–80 °C, reaction time of 3 hours and [2,5-dihydroxy-1,4-benzoquinone]-to-[AAm] ratio of 2.5. The synthesized PAAm and its modified form (THB–PAAm) were characterized by UV–Vis, Fourier transform infrared spectroscopy, 13 C nuclear magnetic resonance, differential scanning calorimetry and thermogravimetric analysis. A severe degradation of the modified PAAm was observed, as confirmed by its relatively low molecular weight. Yet, the glass-transition temperature of THB–PAAm was found to be drastically lower than that of pristine PAAm, 63.1 versus 161.19 °C. A study of copper(II) adsorption by the cross-linked PAAm and THB–PAAm resins was undertaken by varying the following parameters: pH, time, temperature, ionic strength, sorbent mass and initial copper(II) concentration. Adsorption of Pb(II) and Cd(II) by the resins and their corresponding desorption extents were estimated. A new way of crosslinking polyacrylamide and its modified form through an oxidative decarboxylation process is applied.

La reduction sous flux d'hydrogene d'un catalyseur, 25% Co/SiO 2 , pour la synthese d'hydrocarbures a ete suivie par spectroscopie d'absorption des rayons X (EXAFS) in situ. Les analyses numeriques EXAFS ont permis d'identifier la phase spinelle Co 3 O 4 du catalyseur avant traitement. La reduction de Co 3 O 4 en deux etapes. CoO puis Co°, observee en EXAFS, confirme ainsi les etudes menees en XPS (surface) et en TPR (bulk). Le nombre de coordination du cobalt a ete determine a chaque etape de reduction.

l'absorption X est une technique puissante pour l'etude de catalyseurs trois voies a tres faibles teneurs en metaux precieux (Pt, Rh). Il a ete montre l'existence de distances interatomiques d PtO et d Rho contractees par comparaison aux oxydes massiques PtO 2 et Rh 2 O 3 , attribuees a un effet de taille de particule a temperature inferieure a 800°C, alors qu'a 1600°C les distances d PtO , d RhO sont anormalement grandes dues a une tres forte interaction des metaux avec le support Al 2 O 3 . La mise en evidence de phases alliages, a T > 800°C conforte nos resultats anterieurs. L'allure des transformees de Fourier (T.F.) est alors tres particuliere, confirmee par une modelisation des modules de la T.F. pour Pt x Rh 1-x (0 < x

La connaissance de l'environnement du platine pour des catalyseurs bimetalliques Pt - Sn/Al 2 O 3 - Cl oxydes ou reduits, peut permettre d'expliquer les proprietes catalytiques de la phase metallique. Les caracteristiques de l'environnement du platine peuvent etre fournies par une analyse EXAFS du catalyseur, meme si la teneur massique en platine est inferieure a 0,5%. Dans ce travail, nous avons etudie un catalyseur de reformage Pt - Sn/Al 2 O 3 - Cl a l'etat oxyde et reduit par analyse EXAFS in situ. Sur le catalyseur oxyde, nous avons mis en evidence la formation d'oxydes du type [PtO x Cl y ] et la proximite des oxydes de platine et d'etain. L'analyse EXAFS realisee in situ sur le catalyseur reduit montre que 20% du platine forme un alliage du type PtSn avec l'etain.

International audience; The chemical and catalytic properties of supported alloyed metal nanoparticles are reviewed. After describing the theoretical methods commonly used to predict the electronic structure of nanoalloys and the experimental methods used to characterize their geometrical structure, we focus on the catalytic properties of these systems and the opportunities that alloying provide to modulate them. Prototypical processes of key industrial relevance are used as test examples, including catalytic pollution control as three-way catalysts and NOx decomposition, the Fischer–Tropsch process, reforming of alkanes, electrocatalysis, and new energy resources as fuel cells and hydrogen storage. A section discussing how to bridge the gap between model studies employing surface science methods and technological heterogeneous catalytic systems concludes the chapter.

Hydrogenolysis of 2-methylpentane, n-hexane and methylcyclopentane has been studied on tungsten carbide WC between 150 and 350°C. The experiments have been performed in a classical flow reactor under a great excess of hydrogen (pH2/pHC = 150) flowing at atmospheric pressure. The catalyst samples have been prepared in situ in the flow reactor system by reduction and carburization of WO3 powder by a mixture of CH4(20%)/H2(80%) at T = 850°C for 4 h. The BET surface area is around 12 m2/g. The reactions of these three hydrocarbons give mainly cracking, leading to molecules with less and less carbon atoms as temperature increases. At T = 350°C, only methane is observed. Isomerization products, due to skeletal rearrangement, are mainly observed at low temperature, but they represent only a few percent of the total reaction process. The results show that tungsten carbide exhibits catalytic properties close to those of metals like ruthenium, iridium, and nickel in the field of cracking reactions.

Formation of Coo phases with different surface structure over 10 wt% Co/Al2O3 and their catalytic properties were induced by pretreatments in H2 at 570 K for 1 h or 20 h. Electronic behaviour of the Coo phase, which consists of small (after 1 h reduction) or large bulk-like particles (after 20 h reduction), did not change during the CO hydrogenation after 5 h on stream as was determined by XPS. On the basis of the measured C2+ hydrocarbon selectivities the CO molecules are suggested to dissociate on small Co particles to a larger extent than on large cobalt particles. The slight decrease in the catalytic activity with increasing time on stream obtained for the long-term reduced sample is explained by the change in the surface Coo content detected by XPS. The increase in the catalytic activity along with the change in olefin selectivity, measured for the sample reduced for 1 h, is interpreted by the change of a reaction path involving the Coo-support interface during the initial period of the reaction.

The reactions of n -butane and isobutane have been studied in a flow system in the presence of hydrogen at 250°C using sulfuric acid treated zirconia as catalyst. The conversion of the butanes occurs with high selectivity in isomerization. The branched isomer reacts much faster than the linear one. The use of 13 C labelled starting material and analysis of the isotope distribution in the products show that the rearrangement is of intramolecular nature in accord with the superacidic properties of the catalyst.

The effects of microwave irradiation of an alumina-supported 0.2 wt.% platinum catalyst are studied. In the case of the isomerization of hexanes, the selectivity in isomers is increased (by a factor 1.3 to 2) when the thermal treatment of the catalyst and/or the reaction are performed under the electromagnetic field. The isomer selectivity remains high even when such catalysts are subsequently tested under classical heating conditions. These permanent and profitable effects of the microwaves could be interpreted in terms of modifications of the geometric and/or electronic properties of the platinum particles. It is pointed out that the microwave irradiation could be a new and promising way for activating catalysts.

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Highly ordered W-MCM-48 mesoporous materials containing isolated W atoms in tetrahedral framework positions were successfully synthesized following the S+I- pathway, up to a Si/W of 40. When tungsten content was increased up to a Si/W of 20, the ordered cubic structure was only partially maintained, and for a Si/W of 10 an amorphous phase was obtained. Highly isolated tetrahedral framework tungsten atoms in the W-MCM-48 with a Si/W of 40, have been identified by UV-vis band at 225 nm, IR-TF band at 970cm(-1) and XRD. The W 4f XPS results suggest that the tungsten atoms exist in two oxidation states, W4+ and W5+. The morphology of the samples varies as a function of tungsten content. The W-MCM-48 samples with a Si/W ratio of 40 existed as crystals with a unique crystalline morphology consisting of cubes truncated rhombic dodecahedrons belonging to the cubic Ia3d space group, while the samples with a Si/W ratio of 20 exhibited a different morphology consisting of spheres and cubes truncated by rhombic dodecahedrons. A comparison of samples with Si/W of infinity, Si/W of 40 and Si/W of 20 was performed using the conversion of MCP carried out at 450 degrees C under H-2. (C) 2010 Elsevier B. V. All rights reserved.

The catalytic behaviour of silica and alumina supported Pt-Mo catalysts has been studied in the isomerization of hydrogenolysis of hexane hydrocarbons. XPS analyses were performed to follow the different oxidation states of molybdenum alone, or with platinum under hydrogen or under argon bombardment. The XPS analysis showed that molybdenum interacts more strongly with alumina than with silica. As a result, two classes of catalysts were identified, depending on the presence of the reduced relatively mobile, state of molybdenum on the catalyst surface. At low molybdenum content, the catalytic behaviour is similar to the platinum catalysts. But when the molybdenum concentration is increased another type of behaviour emerges, linked to a dual activity between platinum and molybdenum. It has been observed that hydrocarbon structure has a strong influence on the catalyst selectivity. When no tertiary carbon atoms are present in the hydrocarbon, the cracking reactions occur very deeply. Only secondary and primary carbon atoms are involved in the degradation process. The isomerization reaction is only a bond shift reaction.

Ceria, platinum, and platinum-ceria catalysts supported on [gamma]-alumina have been characterized by temperature-programmed reduction (TPR) using CO as reducing agent. In all cases, CO disproportionation was found to be negligible. Nevertheless, a small CO[sub 2] TPR signal was detected on the [gamma]-alumina support at temperatures higher than 673 K, and has been attributed to the water-gas shift reaction between gaseous CO and the hydroxyl groups of the alumina surface. The comparison between the CO-TPR spectra of CeO[sub 2]/Al[sub 2]O[sub 3] and Pt-CeO[sub 2]/Al[sub 2]O[sub 3] has revealed that platinum induces a downscale shift on the temperature of the ceria reduction peak. Two possible interpretations of this platinum-ceria interaction are proposed: (i) a C-O bond weakening when CO is absorbed on platinum particles and (ii) a decrease of the Ce-O bond strength for ceria localized near platinum. 18 refs., 6 figs., 1 tab.

Pt-WO 3 /Al 2 O 3 (Pt:W = 1:1) catalysts were prepared by different methods and characterized by chemisorption, TPD, TPR, and TEM techniques. The various catalysts do not show important differences in the catalytic reactions studied. Pt-WO 3 /Al 2 O 3 systems catalyse hexane isomerisation with low selectivity. Isotopic tracer measurements show that the reaction proceeds via classical cyclic and bond-shift mechanisms. Inside the general reactian pathways of the isoalkanes, methyl migration is favoured and involves the participation of the tertiary carbon atom both in the carboncarbon bond rupture and in the carboncarbon bond reformation. On these catalysts no correlation between the relative contribution of the cyclic mechanism and the nonselective hydrogenolysis of the methylcyclopentane was found. Moreover, 2-methyl-2-pentene follows an exactly similar reaction pathway to 2-methylpentane. The results are explained by the suggestion that the "Pt-WO x " interface is responsible for the catalytic behavior of these systems. A mechanism for hydrocracking and isomerisation is proposed.

The isomerization of 2-methylpentane over a 0.2 wt% platinum dispersed on γ-alumina catalyst is studied when the catalyst is heated either by microwave irradiation or by a classical oven. The selectivity for isomerization is considerably increased, from 45 to 80%, when the catalyst has been previously treated by microwaves or when the catalytic reaction is performed under the electromagnetic field. The isomerization power of such an irradiated catalyst remains high even when it is subsequently tested according to classical conditions. It is consequently suggested that the microwaves induced durable and profitable modifications of the catalytic behaviour of alumina-supported platinum.