Une étude critique des méthodes de préparation de catalyseurs monométalliques permet de voir que les techniques impliquant un échange (ionique ou chimique) conduisent à des catalyseurs à forte dispersion avec des moyens relativement simples. Au niveau de la caractérisation des catalyseurs, il ne semble pas exister de méthodes exhaustives permettant d'étudier les phases métalliques contenant des cristallites de taille inférieure à 1. 0 nm. Les techniques de microscopie électronique à haute résolution, la spectroscopie XPS et l'EXAFS semblent par contre les plus prometteuses. L'influence du support joue à plusieurs niveaux, en particulier sur la morphologie, la structure et sur les propriétés électroniques de la phase métallique. L'intérêt particulier d'étudier les effets de support résulte dans la création de phase métallique plus active et sélective. The current aim of catalyst research is to develop more and more active and selective dispersed metallic phases for a given reaction as well as to improve their stability. Catalytic properties depend directly on the physicochemical characteristics of the metallic phase (size of crystallites, composition, electron structure, interactions with the support). This is why improving such catalysts requires mastering synthesis routes and the characterizing of small supported metallic particles. The aim of this critical study is to highlight both the preparation methods leading to well-controlled monometallic phases and the physical techniques suited for characterizing them. By considering the metal-support entity as a veritable catalytic space, we have examined the direct influence of the nature of the support on the end characteristics of the metallic particles (morphology, structure, electron properties). We have found that impregnation techniques with an interaction (physical or chemical) can lead to high-dispersion metallic phases associated with a narrow dispersion of crystallite size. Among other techniques used, note will be taken of ion implantation, vapodeposition and radiolytic synthesis. Among overall and controlled-atmosphere characterization techniques, XPS and EXAFS spectroscopy were used to characterize, respectively, the electron properties and the crystalline structure (chemical environment) of metallic particles. Small metallic particles (less than 1. 0 nm in size) are more difficult to characterize because of the limitations of the physical techniques normally used. High-resolution electron microscopy was used for the local characterization of these small particles. The present results mainly concern the size of these small aggregates, but this technique also gives information on their morphology (aggregates of atoms, individual atoms in the aggregate) and on their crystalline structure (microdiffraction). Likewise, there is an entire domain of indirectcharacterization that is widely used, in which especially the accessibility and degree of oxidation reduction are measured of the metals by means of reagents or probe molecules (H2, O2, CO). However, this very precious information in the field of catalysis must always be compared with the result of direct characterization (size, electron properties). Concerning the influence of the support, this review shows that the metal-support pair is a full-fledged entity and that it is almost impossible to characterize it from the individual properties of the metal and of the support. At present, the principal results have to do with semiempirical classifications concerning the stability of metallic phases during heat treatments and with the influence of the metal-support interaction force on the morphology and electron properties of particles (load transfer, modifications of the electron structure of the transition metal).

Two series of chlorided-alumina-supported Sn catalysts were synthesized with different precursors, SnCl2 or SnBu4, and with various contents of Sn. The acidity of the catalysts including 0.2 wt% Sn was characterized by FTIR adsorption-desorption of 2,6-dimethylpyridine (Bronsted acidity) and pyridine (Lewis acidity) and compared with that of Al2O3-Cl. The catalytic activity of the synthesized materials was investigated for the transformation of n-propylbenzene under reforming conditions. The results show that the incorporation of Sn into Al2O3-Cl is not harmless. Neither the nature of the Sn precursor nor the content of Sn evenly affects the acidity and the distribution of the isomerized and cracked products. The correlation between the product distribution obtained for the transformation of n-propylbenzene and the acidity of the Al2O3-Cl and the 0.2Sn(SnCl2)/Al2O3-Cl catalysts supports the formation of isopropylbenzene and benzene catalyzed by Bronsted acid sites via carbenium ion chemistry. In contrast, the production of toluene and ethylbenzene occurs via radical chemistry. The formation of these products is assumed to be catalyzed by Lewis acid sites with different strengths. The stability of the proposed radical intermediate species is consistent with the involvement of stronger Lewis acid sites in the production of toluene compared with those involved in that of ethylbenzene. The catalytic cycles responsible for the formation of toluene and ethylbenzene via radical pathways over an Al-O pair are reported. Finally, it is worth noting that benzene is always the major product of the cracked compounds. (c) 2004 Elsevier Inc. All rights reserved.

A new preparation technique is presented allowing the controlled variation of the average size of platinum particles in small steps of 0.2–0.5nm in the size range between 1 and 2nm. The particle growth is achieved by refilling, i.e. by increasing the metal loading of a parent catalyst in a separate preparation step. The technique makes use of the selective decomposition of a platinum complex, platinum bis-acetylacetonate, Pt(acac)2, on reduced platinum particles. Anchoring of the complex on the surface of the support and thus appearance of new small particles is completely avoided by appropriate surface treatments developed on the basis of a study of the reaction mechanisms between Pt(acac)2 and the support. Three types of surface sites could be identified in this study by their specific reactivity towards Pt(acac)2. A model is presented describing the nature of these sites, the occurrence of which is correlated with the degree of dehydroxylation of the alumina surface. The efficiency of the new technique in increasing the particle size while reducing to a minimum any broadening of the initial narrow size distribution is demonstrated. The results are discussed in terms of particle shapes and two- and three-dimensional growth mechanisms.

Alumina-supported Sn and PtSn particles are studied by 119Sn Mössbauer spectroscopy after oxidation and reduction under various conditions. The observed species of Sn(IV), Sn(II) and Sn(0) are grouped in several categories, each being characterised by distinct structural properties. Tin phases in contact with the support or with platinum are identified. The results are used to establish a model describing the phase transformations occurring in PtSn particles under oxidising or reducing conditions. Particular attention is paid to the reduction/reoxidation mechanisms governing H2/O2 double titrations. An increased reactivity of tin towards oxygen, induced by the contact with platinum, is demonstrated. It is shown that tin contributes to the oxygen uptake VO1 of a first titration cycle by platinum-catalysed transformation of Sn(II) into an oxometallic phase Pt(x)Sn(O). The oxygen titre VO2 of a second cycle is due to O2 chemisorption on platinum only.

The anchoring and decomposition mechanisms of platinum(II) bis-acetylacetonate on alumina surfaces are studied by X-ray absorption spectroscopy at the platinum LIII edge. A distinction is made between highly reactive surfaces which are partially dehydroxylated and exhibit coordinatively unsaturated surface sites, and deactivated surfaces which are covered by a monolayer of OH groups. The samples are studied after three stages of a wet impregnation synthesis: after drying at room temperature, drying at 120 °C and calcination at 350 °C. The XANES signal and the filtered EXAFS signals of the first and second coordination shells around platinum were analyzed. Two different mechanisms are discussed for the two types of supports.

The effect of moderate sintering of the platinum particles of a precoked Pt/SiO2 catalyst on the transformation of n-PB was investigated under reforming conditions (773 K, 5 bar, and H2/HC = 5). The results showed that the larger the platinum particles, the greater the formation of cyclic compounds. Furthermore, a concentration balance between the ethylbenzene and the cyclic compounds was revealed, suggesting a common reactive adsorbate for both reaction products. According to a previously proposed reactive adsorbate for ethylbenzene and the results of studies of Pt/SiO2 and PtSn/SiO2 catalysts, a reactive adsorbate was proposed for the formation of the cyclic compounds. This latter reactive adsorbate, which results from the reactive adsorbate leading to ethylbenzene, is consistent with either a decrease in the size of the Pt ensembles or an increase in the electron density of the platinum particles of the Pt/SiO2 catalyst sample with the smaller particles after coke deposition. It is still uncertain as to whether the concentration balance phenomenon is due to a geometric ensemble effect, a change in the electronic density of the platinum clusters, or both. The studies with a tin-promoted catalyst were discussed in relation to this new finding and a detailed reaction network for the transformation of n-PB over metallic sites is proposed.

Kinetics of n-propylbenzene hydrodealkylation (HDA) was investigated over model catalysts under reforming operating conditions (773 K, 5 bars, and H2/HC=5). Silica-supported platinum catalysts (Pt/SiO2 and PtSn/SiO2) and alumina materials (Al2O3 and 1 wt% Cl–Al2O3) were chosen to evaluate the influence of the metallic and acidic phases on the HDA reactions, respectively. The kinetic study showed (i) that the HDA processes occurred to a significant extent over both catalytic phases under identical operating conditions and (ii) that hydrodealkylated products (benzene, toluene, and ethylbenzene) were formed through concurrent pathways whatever the nature of the catalytic phase. In addition, product intermediates were identified and mechanisms were suggested to explain the HDA reactions over either different patches of metallic sites or different acid sites (Brønsted and Lewis). Finally, as compared to toluene and ethylbenzene, the formation rate of benzene was the highest over the acidic function and the lowest over Pt/SiO2.

Addition of tin by the organometallic route modifies strongly the stability and regenerability of pure Pt/SiO2 in the reaction of isobutane dehydrogenation to isobutene. Two routes were used: in the first one, tetra n-butyl reacted on the surface of silica supported platinum particles, leading to surface PtSn alloys of different composition. In the second route, tetra n-butyl tin reacted first on the silica surface, to form (=SiO)(2)-Sn-II species; then, platinum particles were generated on the modified silica surface. The stability of these two kinds of bimetallic catalysts has been studied during the oxidation-reduction processes used to regenerate the catalysts. The modification of the structure and texture of the solids was followed by electron microscopy (CTEM and EDAX), or oxygen and hydrogen chemisorption.

Selective hydrogenolysis of Sn(n-C4H9)(4) on a Pt/SiO2 catalyst has been carried out at various temperatures and coverages of the metallic surface to prepare via surface organometallic chemistry a well-defined class of bimetallic catalysts. The stoichiometry and kinetics of the reaction was followed by the careful analysis of reagents and products, including extraction of unreacted reagents, and elemental analysis of the samples. The various surface species formed were characterized by electron microscopy (CTEM and TEM EDAX) and EXAFS analysis. Possible structures of the surface organometallic fragments were considered using molecular modeling. At 50 degrees C, the hydrogenolysis reaction occurs selectively on the platinum surface with exclusive evolution of n-butane. There is first formation of a Sn(n-C4H9)(3) fragment grafted on the platinum particle which undergoes a stepwise cleavage of two tin-carbon sigma-bonds to form a stable Pt-Sn(n-C4H9) fragment. Regardless of the reaction time, surface coverage, or loading, the number of grafted butyl fragments per platinum is never greater than unity, that is to say that when Sn(n-C4H9)(3) is formed the platinum coverage by tin is 0.3 whereas when Sn(n-C4H9) is formed the platinum coverage is closer to 1. It is therefore suggested that the surface composition is governed by the bulkiness of the alkyl chains which are "close packed" on the surface. At 100 degrees C, the reaction takes place both on the platinum and the silica surface. On the platinum surface, the same fragments (namely Sn(n-C4H9)(3), Sn(n-C4H9)(2), and Sn(n-C4H9)) were identified, but simultaneously on the silica surface, the well-described =SiOSn(n-C4H9)(3) species was also formed. Thermal treatment under hydrogen of Pt-8-Sn(n-C4H9) lead to alkyl-free tin atoms which are located at the periphery of the particle as evidenced by Sn K edge EXAFS (Pt-Sn distance of 2.75 Angstrom with a coordination number of ca. 4). Even if the organotin fragments are grafted with a coverage of unity, after their complete hydrogenolysis at 300 degrees C, about 40% of the platinum is still accessible to H-2 chemisorption. This could be explained by the increase of the particle diameter (+0.5 Angstrom) which prevents a close packing of the tin atoms around the particle and leaves some platinum atoms still accessible to the hydrogen. After treatment of the catalyst at higher temperatures, typically 500 degrees C, the structure of the catalyst is slightly changed since the tin atoms migrate into the first monolayer of the particle, as evidenced by a significant increase of the tin coordination number (ca. 4.4-5.6) as determined by EXAFS. Hypothetical surface structures have been proposed on the basis of molecular modeling of platinum particles covered by various surface organotin fragments.

The selective dehydrogenation of isobutane into isobutene was studied on silica-supported bimetallic Pt-Sn. Several bimetallic catalysts were carefully prepared by selective hydrogenolysis of Sn(n-C4H9)4on Pt. Previous EXAFS studies have shown that this hydrogenolysis is a stepwise transformation of a Pt-Sn(n-C4H9)3fragment into a surface alloy. It was shown that after hydrogen treatment at 550°C, tin and platinum are in reduced form (zero-valent oxidation state) and that the tin atoms are located on the surfrace of the metallic particles. The presence of tin on platinum caused a decrease in hydrogen or carbon monoxide chemisorption, but an increase of the oxygen consumption. The decrease of H2and CO chemisorption is explained by the decrease of the number of accessible platinum atoms due to the increased number of surface tin atoms. The increase in the O2chemisorption was explained by the following reaction which represents a phase segregation: PtsSnx/SiO2+1/2(y+xy′)O2→(PtOy)s(SnOy′)x/SiO2. The values of y and y′ was about 1 and 2 at respectively 25°C and 300°C. Thermodesorption of adsorbed CO on several reduced PtSn catalysts showed no shift of the ν (CO) frequency, suggesting negligible electronic effect of tin atoms on the platinum atoms when both are reduced. At 550°C under atmospheric pressure of hydrogen and isobutane, the presence of tin increases drastically, both the selectivity and the activity of the isobutane conversion into isobutene (for Sn/Pts=0.85, the selectivity is higher than 99% and the TOF, based on total platinum atoms, is greater than 6 s−1). The increase in selectivity could be explained by the “site isolation effect” and the increase in activity could be due to the inhibition of the coke formation (which poisons the active surface). A mechanism of dehydrogenation and hydrogenolysis of isobutane based on elementary steps of organometallic chemistry has been proposed which accounts both for the high selectivity and activity of the bimetallic catalysts as compared to pure Pt/SiO2.

The kinetics of the CO2/H2 reaction over ZnAl2O4 and Cu–ZnAl2O4 catalysts at 250°C up to 0.3 MPa have been followed by in situ FT-IR spectroscopy. Both methanol and carbon monoxide formation were enhanced in presence of copper. They were also produced by independent routes through different adsorbed species. Formate (type I, I′ and II), methoxy and carbonate species were identified on the support and, in addition, copper formate and copper carbonyl species when copper was present. The hydrogenation of carbonate species to copper formate species was found rate determining in methanol synthesis over the Cu–ZnAl2O4 catalyst whereas type I formate species were shown to be the active intermediate for this reaction over the ZnAl2O4 support. Carbon monoxide resulted from the water gas shift reaction probably through the same species as methanol formed over the Cu–ZnAl2O4 catalyst whereas it seemed to stem from formate species of type II in the case of the ZnAl2O4 support. Type II formate species were shown inactive in presence of copper whereas the methoxy species adsorbed on the support were found inactive in presence and in absence of copper. The comparison of these results with those previously obtained with the CO/H2 mixture showed that the nature and the role of the detected species strongly depended on the reactive atmosphere and on the presence or not of copper in the catalyst composition.

The CO/H2 reaction over ZnAl2O4 (support) and CuZnAl2O4 (catalyst) performed at 250°C up to 0.3 MPa, was studied by combining in-situ FT-IR experiments and kinetic investigations. The obtained results showed the existence of two reaction mechanisms for methanol formation involving formate species (mainly on ZnAl2O4) and carbonyl species (on CuZnAl2O4), as key reaction intermediates. Inactive formate species were also evidenced. Carbon dioxide, which is the initial major product formed on the ZnAl2O4 support, is produced through the partial reduction of the latter.

Cu-Zn-Al methanol synthesis catalysts have been studied by thermodesorption experiments in order to establish a correlation between catalytic activity and the concentration of active intermediates on the surface. It has been found that the methanol Synthesis activity not only depends on the concentration of formate species adsorbed on copper, but also on the amount of one particular type of hydrogen species favoured by ZnO and/or Cu-ZnO interaction. Thus the catalytic activity is related both to the copper metal surface area, and to the ability of the ZnO-Al2O3 oxide phase to generate and store active hydrogen species. This may explain the promoting or synergetic effect of ZnO on copper in methanol synthesis.

The stoichiometry and the kinetics of the hydrogenolysis of tetra n-butyl tin on silica and alumina supported rhodium catalysts has been determined at 298 K in n-heptane. Various Rh/SiO2 and Rh/Al2O3 catalysts have been prepared by exchange from [RhCl(NH3)(5)] (OH)(2) and by impregnation from RhCl3 respectively. After reduction, metallic dispersions were found to vary from 0.45 to 0.9 as determined by hydrogen chemisorption measurements. Concerning the hydrogenolysis of the organotin complex, the amount of grafted tin was found to depend on the amount of tetra n-butyl tin introduced: since this amount is lower than the value corresponding to a ratio Sn/Rh-s of 0.7, all the tin complex is grafted and the amount of grafted tin is directly proportional to the number of surface rhodium atoms; on average, each surface rhodium atom is able to graft 0.8 Sn(n-C4H9)(x) fragment. Above this ratio, the amount of grafted tin is almost independent of the amount of complex introduced. These results strongly support the hypothesis that the reaction occurs selectively on the metallic surface. The stoichiometry of the hydrogenolysis, determined by the amount of butane evolved per mole of grafted tin, depends on the coverage of the rhodium surface: for a low coverage, all the butyl groups are hydrogenolyzed, showing the formation of 'naked' tin atoms. For a full coverage, some butyl groups remain fixed on the surface showing the presence of grafted organotin species on the rhodium surface. The stoichiometry of the hydrogenolysis also depends on the reaction time for a given concentration of tetra n-butyl tin: at short reaction times which correspond to low coverages of the metallic particles, all the butyl groups are removed whereas this phenomenon is not observed at longer reaction times for which alkyl groups remain on the metallic surface. Kinetics of the hydrogenolysis indicates that the reaction is first order in 'free surface adsorption site' and zero order in tetra n-butyl tin.

Alteration of the dehydrogenation activity of α-alumina-supported platinum particles induced by a high-temperature (723 K) treatment in argon atmosphere has been investigated. The methylcyclohexane dehydrogenation rate over reduced platinum particles is highly dependent on the argon treatment applied at 723 K after hydrogen treatment at the same temperature. With an increasing exposure time to the argon stream, the reaction rate increases sharply to a maximum and gradually diminishes afterwards to a stable value for a prolonged exposure time. This change is the result of two modifying actions induced by inert gas: (1) the desorption of tightly bound hydrogen species, responsible for the low initial activity, and (2) the reconstruction of the platinum crystallite surface toward a more stable active surface. The presence of "high temperature hydrogen" is systematically observed on small particles after hydrogen treatment at 723 K and seems to be related to the specific electronic properties of small particles and not to their surface structure. The reconstruction of the surface of the particles is by contrast very important for obtaining stable catalytic activity. Highly dispersed (80-90%), restructured catalysts were found to be similar in stability to mediumly dispersed (45%) catalysts.

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Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to study the structural evolution of monometallic (Pt/Al2O3) and bimetallic (Pt-Re/Al2O3) catalysts with metal loadings representative of those used industrially under conditions close to those seen during preparation (calcination and reduction), during catalysis (under a hydrogen-hydrocarbon mixture), and during regeneration (simulated by cycles of reduction and oxidation). During the catalytic reforming of n-heptane, the formation of bonds to carbon species is observed directly by EXAFS. Despite the fact that only platinum-carbon bonds are observed, bimetallic systems show significant differences compared with monometallic systems with, in the case of Pt-Re, a reduction in the temperature range over which carbon bonding is observed, and structural modifications of the metallic particles. Under the same conditions, no carbon bonding is observed for the Pt-Sn system. In the case of Pt-Sn/Al2O3, the EXAFS results, confirmed by transmission electron microscopy, demonstrate the high resistance to sintering of this bimetallic system compared to the monometallic system.

The effects of the catalytic reforming of n-heptane on the metallic phase of Pt/Al2O3 and PtRe/Al2O3 catalysts have been studied by EXAFS spectroscopy. In both catalysts, a new platinum carbon bond is formed during the catalytic reaction, coming from the deposition of carbonaceous residues over the metallic phase. However, in the case of the PtRe/Al2O3 catalyst this bond is formed only at lower temperature and is accompanied by important changes in the structure of the metallic particles. All these results are consistent with the existence of a PtRe intermetallic phase in the bimetallic catalyst, which intercepts more efficiently the coke precursors.

The state of platinum in a PtSn/Al2O3 catalyst prepared from organometallic precursors was studied in situ by EXAFS and XANES spectroscopies. While the XANES spectra show that the platinum is completely reduced after treatment in H-2 at 773 K, the results obtained by EXAFS indicate that, besides platinum and tin, oxygen atoms are present in the first coordination shell of platinum, but with a Pt-O distance longer than for platinum oxide (2.24 vs 2.02 angstrom). These findings can be rationalized assuming the formation of a highly dispersed bimetallic platinum-tin phase, which is stabilized on the alumina surface through Pt-O-Sn2+ bonds.