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Single-atom catalysts (SACs) are promising atom-efficient materials, with potentially superior performances with respect to their nanoparticulate counterparts. Because of its practical importance a...

International audience; Photocatalytic hydrogen production from water or oxygenates is foreseen as a sustainable energy production route. In spite of recent performance achievements through Pt particle downsizing, the knowledge on the prototypical Pt/TiO 2 photocatalyst operation mechanism, in particular the electronic state and the stability of the Pt phase under reaction conditions, remains limited. We have investigated atomically dispersed Pt/TiO 2 catalysts by pre/post-reaction scanning transmission electron microscopy and operando X-ray absorption spectroscopy-mass spectrometry in gas-phase ethanol dehydrogenation conditions under ultraviolet-visible light irradiation. The catalysts were prepared by a simple impregnation method on two types of commercial titania. While the general effect of the reaction on initially fully oxidized (Pt IV) single-atom catalysts (SACs) is Pt reduction and aggregation, this combined phenomenon can be inhibited in two manners. First, when supported on high-surface-area anatase, the Pt SAC retains an intermediate oxidation state, and its clustering is limited to Pt dimers and trimers. Second, with the same support, a mild reducing pretreatment generates, through the formation of Pt-Ti bonds, near-neutral and ultradispersed Pt species (from single atoms to nanometric clusters) with the highest stability and activity in photocatalytic hydrogen evolution.

The oxidative dehydrogenation of allyl alcohol to acrolein over bulk alloyed and supported silver catalysts has been studied, with the aim of designing optimal Ag-based catalysts for this specific reaction. All of the tested catalysts were highly active and selective to acrolein at temperatures ranging between 340 and 380 °C. The results show that the alloying of Ag with Au or Cu leads to alloys with upper surface compositions comprising approximately between 35 and 65% silver. Although these elements improve the activity of Ag catalysts, they decrease their selectivity, leading to lower acrolein yields. Various different Ag catalyst supports were prepared (LaPO4, Carbon nanotubes CNT, TiO2, Al2O3 and SiO2) using the impregnation method, and subsequently tested. This study reveals that the catalyst with the best performance is Ag/SiO2 with a yield in acrolein of 87.2% at 360 °C. The results obtained with the tested catalysts are discussed, and the reasons of performance improvement or degradation caused by the presence of Au or Cu on the active silver sites, or by the presence of the supports, are raised.

The deposition and sintering of nanoparticles (NPs) containing both Cu and Mn could be an economically and technically interesting alternative to current processes for the metallisation of through Si vias (TSVs). We propose to use ionic liquids (ILs) to (i) synthesise suspensions of suitable NPs and (ii) spread and anneal those suspensions directly on technological substrates. Indeed, ILs readily stabilise NPs and possess good thermal stability. Suspensions of metallic Cu and MnO are formed upon decomposition of solutions of selected Cu and Mn precursors in an IL, C1C4ImNTf2. Upon decomposition of mixed solutions of both precursors, single populations of NPs are obtained, a good indication that both elements are incorporated in these NPs. Finally, upon spreading and annealing these suspensions on samples coated with SiO2, typical of TSV technology, a partial reduction of SiO2 is observed, in agreement with the formation of a MnSiO3 phase typical of self-formed barriers.

MICROSCOPIE+ECI2D:ING+FMO:JRO:MAO:PAF:LPI; International audience; Atomically dispersed metals promise the ultimate catalytic efficiency, but their stabilization onto suitable supports remains challenging owing to their aggregation tendency. Focusing on the industrially-relevant Pt/-Al2O3 catalyst, in situ X-ray absorption spectroscopy and environmental scanning transmission electron microscopy allow us to monitor the stabilization of Pt single atoms under O2 atmosphere, as well as their aggregation into mobile reduced subnanometric clusters under H2. Density functional theory calculations reveal that oxygen from the gas phase directly contributes to metal-support adhesion, maximal for single Pt atoms, whereas hydrogen only adsorbs on Pt, and thereby leads to Pt clustering. Finally, Pt cluster mobility is shown to be activated at low temperature and high H2 pressure. Our results highlight the crucial importance of the reactive atmosphere on the stability of single-atom versus cluster catalysts.

Atomically dispersed metals promise the ultimate catalytic efficiency, but their stabilization onto suitable supports remains challenging owing to their aggregation tendency. Focusing on the industrially-relevant Pt/γ-Al2O3 catalyst, in situ X-ray absorption spectroscopy and environmental scanning transmission electron microscopy allow us to monitor the stabilization of Pt single atoms under O2 atmosphere, as well as their aggregation into mobile reduced subnanometric clusters under H2. Density functional theory calculations reveal that oxygen from the gas phase directly contributes to metal-support adhesion, maximal for single Pt atoms, whereas hydrogen only adsorbs on Pt, and thereby leads to Pt clustering. Finally, Pt cluster mobility is shown to be activated at low temperature and high H2 pressure. Our results highlight the crucial importance of the reactive atmosphere on the stability of single-atom versus cluster catalysts.

SSCI-VIDE+ECI2D:ING+FMO:JRO:LPI; International audience; Pt-CeO2 catalysts were prepared by solution combustion synthesis (SCS), a simple and fast one-pot method, using glycine or oxalyl dihydrazide (ODH) as the fuel and Pt chloride,or nitrate as the metal precursor. The samples were characterized by ICP-OES, N-2 volumetry, XRD, XPS, aberration-corrected (S)TEM, CO-DRIFTS and microRaman spectroscopy. The SCS catalysts were evaluated in CO oxidation and preferential oxidation (PROX) of CO in H-2 excess, and compared to PtiCeO(2) and Pt/Al2O3 catalysts prepared by incipient wetness impregnation. The glycine fuel yields better results than ODH in terms of Pt dispersion and catalytic performance. The fresh Cl-containing catalysts consist of PtO nanopartides poorly active in CO oxidation, while the catalysts synthesized from Pt nitrate contain highly active Pt delta+ (0 < delta < 2) species in interaction with ceria. Consistently, the most efficient catalyst for H-2-free CO oxidation is the one synthesized from glycine and Pt nitrate. For PROX, the selectivity to CO2 is 100% at low temperature and decreases in a similar way for all the Pt-CeO2 catalysts above ca. 100 degrees C. With respect to H-2-free conditions, the CO PROX activity of the Pt-CeO2 catalysts is considerably enhanced (activity multiplied by up to 60 at 110 degrees C), while the activity gairvis comparatively minor for Pt/Al2O3. We conclude that hydrogen, by increasing the mobility of oxygen species at the surface of ceria, promotes the support-assisted pathway of CO oxidation. (C) 2016 Elsevier B.V. All rights reserved.

The metal–support interaction plays an important role in gold catalysis. We employ here crystalline cubic (α-) and hexagonal (β-) phases of heterometallic fluoride NaYF4 nanoparticles (NPs), obtained by the decomposition of a single source precursor [NaY(TFA)4(diglyme)] (TFA = trifluoroacetate), as nonoxide supports for gold catalysts. Using an isostructural gadolinium analogue, we also obtained doped α-NaYF4:Gd3+ and β-NaYF4:Gd3+ NPs. A successful deposition of ∼1% by weight gold NPs of average size 5–6.5 nm on these doped and undoped metal fluorides using HAuCl4·3H2O afforded Au/NaYF4 catalysts which were thoroughly characterized by using several physicochemical techniques such as X-ray diffraction, Brunauer–Emmett–Teller analysis, high-resolution transmission electron microscopy, energy-dispersive X-ray spectrometry, and X-ray photoelectron spectroscopy. A comparative study of the above catalysts for different oxidation reactions show that while for the aerobic oxidation of trans-stilbene in solution phase, they are either better (in terms of stilbene conversion) or at par (in terms of trans-stilbene oxide yield) in comparison to the reference catalyst Au/TiO2 of the World Gold Council, their activity toward CO oxidation reactions in gas phase remains much less than that of gold catalysts supported on metal oxides.

Highly dispersed Au, Ag and Au-Ag alloy nanoparticles were homogeneously embedded in a chemically inert matrix made of silicalite-1 multi-hollow single crystals. The particles were fully accessible to small reactant molecules such as CO. The Au nanoparticles were as active as the Ag-Au material for CO oxidation at room temperature. Operando diffuse reflectance FT-IR spectroscopy revealed the presence of both metals at the surface of the particles, silver replacing a large fraction of the Au atoms otherwise located at low coordination sites. This study shows that CO adsorption followed by IR is a useful technique to estimate the surface composition of nanoalloy particles buried in a zeolite matrix.

The metal-support interaction plays an important role in gold catalysis. We employ here crystalline cubic (α-) and hexagonal (β-) phases of heterometallic fluoride NaYF4 nanoparticles (NPs), obtained by the decomposition of a single source precursor [NaY(TFA)4(diglyme)] (TFA = trifluoroacetate), as nonoxide supports for gold catalysts. Using an isostructural gadolinium analogue, we also obtained doped α-NaYF4:Gd3+ and β-NaYF4:Gd3+ NPs. A successful deposition of ∼1% by weight gold NPs of average size 5-6.5 nm on these doped and undoped metal fluorides using HAuCl4·3H2O afforded Au/NaYF4 catalysts which were thoroughly characterized by using several physicochemical techniques such as X-ray diffraction, Brunauer-Emmett-Teller analysis, high-resolution transmission electron microscopy, energy-dispersive X-ray spectrometry, and X-ray photoelectron spectroscopy. A comparative study of the above catalysts for different oxidation reactions show that while for the aerobic oxidation of trans-stilbene in solution phase, they are either better (in terms of stilbene conversion) or at par (in terms of trans-stilbene oxide yield) in comparison to the reference catalyst Au/TiO2 of the World Gold Council, their activity toward CO oxidation reactions in gas phase remains much less than that of gold catalysts supported on metal oxides.

Palladium platinum (PdPt) has been intensively studied these last decades due to high conversion rate in hydrogen oxidation at room temperature with significant exothermic effects. These remarkable properties have been studied by measuring the temperature variations of alumina (Al2O3) supported nanosized PdPt nanoparticles exposed to different hydrogen concentrations in dry air. This catalyst is expected to be used as a sensing material for stable and reversible ultrasensitive hydrogen sensors working at room temperature (low power consumption). Structural and gas sensing characterisations and catalytic activity of PdPt / Al2O3 systems synthesised by co-impregnation will be presented. Catalytic characterisations show that the system is already active at room temperature and that this activity sharply increases with rise in temperature. Moreover, the increase of the PdPt proportion in the co-impregnation process improves the activity, and very high conversion can be reached even at room temperature. The thermal response (about 3 °C) of only 1 mg of PdPt / Al2O3 is reversible, and the time response is about 5 s. The integration of PdPt / Al2O3 powder on a flat substrate has been realised by the deposition onto the powder of a thin porous hydrophobic layer of parylene. The possibility of using PdPt in gas sensors will be discussed.

We use a colloidal deposition method to prepare gold nanoparticles with similar size distributions centered at 3 nm over various anatase titania supports. All UV100, PC500 and AK350 titanias are loaded with similar amount of gold (1.0 ± 0.2 wt.%) which is in similar electronic and optical environments, as shown by X-ray photoelectron spectroscopy (XPS) and UV–vis. This allows us to assess the effect of the titania crystallization, morphology and chemical composition on the catalytic properties of gold in the aerobic epoxidation of trans-stilbene. We find that Au/UV100 is more active than Au/PC500 and Au/AK350 but that selectivities are similar on all materials. Epoxide yields on the other hand critically depend on the support functionalization and surface composition. TG–DTA characterization of the bare titania powders reveals indeed that AK350, which leads to the least active catalyst, is slightly less hydroxylated than PC500 and UV100. This indicates that surface titanol groups might be involved in the epoxidation of trans-stilbene. The presence of boron oxide on Au/UV100 (XPS), due to reaction of UV100 with the NaBH4 reductant during the synthesis, is also thought to promote the epoxide-forming mechanism. This chemical promotion effect appears to compensate for the specific and beneficial gold–P25 interaction. As a result, Au/UV100 is more efficient than the reference Au/P25 catalyst for this reaction.

MICROSCOPIE+ING+SLI:ATU:JRO:FMO:MAO:LBU:FRM:DFA; International audience; Standard chemical and physical methods to control the size of supported metallic nanoparticles are generally not compatible with the harsh conditions of applications (high temperature, presence of steam). Sintering of nanoparticles usually occurs, leading to particle growth and eventually to a degradation of physical properties. Here we describe a generic process for synthesizing nanoalloys of controlled size (2-10 nm) and composition encapsulated in zeolite nanoboxes. The originality of the synthesis design is that the nanoboxes act both as nanoreactors in which the bimetallic particles are formed and as protective ultra-microporous shells, which prevent sintering by coalescence even under harsh conditions. Moreover, the present process allows for control over the size and composition of the NPs, which are factors that ultimately determine the catalytic properties. A series of mono-(Au, Pd, Pt) or bi- (AuAg, PdAg, PtAg and PdPt) metallic nanoparticles individually encapsulated in hollow silicalite-1 single crystals have been prepared and characterized. The simultaneous presence of Ag and Au at the surface of Au161Ag22 NPs has been demonstrated by results of the catalytic activity for CO oxidation near room temperature.

International audience; This article addresses the main results of the PEPS (PEllet Photonic Sensor) project, whcich aims at developing a new gas sensing transducer via a technological breakthrough: the combination of photonics (insensitive to external electromagnetic disturbances) and catalysis (reversibility, limited energy consumption). Indeed, catalytic reactions are often exothermic and this heat can modify the properties of optical devices. The experimental studies performed during the PEPS project highlighted the rapid and reversible response at room temperature of catalytic powders towards different concentrations of H2 in air. The thermal and optical properties of the materials used for the integrated photonic component have also been studied. These results have been exploited for the design of the photonic transducers. The feasibility of the physical transduction principle has been demonstrated by developing different prototypes based on Bragg gratings and Multimode Interference Components.

The model catalysts Au/Al 2 O 3 , Au/ZrO 2 , and Au/TiO 2 were produced by laser vaporization of a metallic gold rod followed by deposition of the formed clusters onto the support powders. This technique allows to obtain a narrow size distribution of highly dispersed gold particles on the support and, most importantly, similar sizes whatever the support. This makes it possible to accurately study of the influence of the support identity on the catalytic reaction. A detailed investigation of the preferential oxidation of CO in the presence of H 2 was undertaken. Catalytic performances in the PROX reaction were compared with those in the oxidation of CO in the temperature range of 25–420 °C. A boost in the conversion of CO was observed in the presence of H 2 at low temperature; the extent of this boost is dependent on the support identity. Hence the reactivity order found for CO oxidation (Au/Al 2 O 3 ≪ Au/ZrO 2 2 ) was changed. In fact, in the presence of H 2 , the reaction rates for the oxidation of CO become rather similar on all three systems.

Gamma-Al2O3, ZrO2, and TiO2 gold supported model catalysts have been synthesized by laser vaporization. Structural characterization using Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy experiments have shown that the gold clusters deposited on the different supports have similar distribution of size centered around 3 nm and are in the metallic state. However, X-ray photoemission measurements also indicate lower binding energies than the usual Au 4f(7/2) at 84.0 eV for both alumina and titania supported catalysts, indicating a modification of the electronic structure of the metal. One has taken benefit of these features to study the influence of the nature of the support toward CO oxidation activities without being hindered by particle size or gold oxidic species effects. By comparing the activities of the different catalysts, it is concluded that the nature of the support directly affects the activity of gold. The following tendency is observed: titania and zirconia are superior to alumina as supports, titania being slightly better than zirconia. From XPS and activity results we can conclude that the existence of negatively charged clusters is not the key point to explain the high activity observed for Au/ZrO2 and Au/TiO2 catalysts and also that metallic Au is the major catalytically active phase. Hence, due to their very nature, titania and to a less extent zirconia should participate to the catalytic process.

International audience; The optical properties of mixed (Ni0.5Ag0.5)n and Pt/Ag clusters are investigated in the size range 2-5 nm. Low Energy Ion Spectroscopy (LEIS) experiments show that the cluster surface is entirely covered by silver atoms for the two systems. The optical spectra of Ni/Ag clusters exhibit a large Surface Plasmon Resonance (SPR), damped and widened when the cluster size decreases, in agreement with a classical model assuming a core-shell geometry and including the reduction of the conduction electron mean-free path in the silver shell. For Pt/Ag clusters, no SPR emerges in the size range 2-5 nm, although it is predicted within a classical model, a pronounced SPR appearing only for clusters larger than 10 nm in diameter.

Since the ability to provide lattice oxygen should be an important factor dictating the catalytic activity of supported gold catalysts, we synthesized, simply by oxidation of the primary AuCe, AuZr, AuCeZr alloys, intimate mixtures of nanostructured Au with CeO 2 , ZrO 2 or Ce–Zr mixed oxides which have unique redox properties and high mobility of lattice oxygen. This makes it possible to accurately study the influence of the support identity on the catalytic activity. A detailed investigation of the preferential oxidation of CO (PrOx) in the presence of H 2 was undertaken also with the oxidation of CO and the oxidation of H 2 . For CO oxidation, it immediately appears that the nature of the oxide affects significantly the activity of gold, ceria being the best whereas for H 2 oxidation no or only slight support effect has been observed. Catalytic performances in the PROX reaction were compared with those in the oxidation of CO. A change in the conversion of CO was observed in the presence of H 2 at low temperature for all system, the effect depending on the support identity. A boost is clearly observed on Au/ZrO 2 whereas the samples containing ceria present an opposite effect: when H 2 is introduced in the reactive mixture, the CO conversion rate is drastically decreased. According to a Mars–van Krevelen mechanism, implying lattice oxygens of the cerium oxide it was suggested that the presence of H atoms in the vicinity of gold should reduce considerably the oxygen spillover activity. At higher temperature, hydrogen can desorb from ceria and the opposite trend is observed. Whereas the rate of CO’s oxidation decreases in a regular and fast way with the temperature for AuZr, it decreases more slowly when ceria is present. This could explain the better selectivity obtained with ceria containing gold catalysts as compared to gold–zirconia material in the high temperature range. Indeed when H 2 is no longer present on the support, oxygen spillover activity should be recovered and CO oxidation favored. In any case we showed that Au/CeO 2 , in our conditions, is not active in the WGS reaction.

MICROSCOPIE:RX+ECI2D:ING+FMO:MAO:FBS:JRO:LPI; International audience; PGM-CeO2 (PGM = Pt, Pd, Ir, Rh, Ru) catalysts were prepared by one-step solution combustion synthesis (SCS) and characterized by elemental analysis, N-2 volumetry, aberration-corrected HRTEM, X-ray diffraction, and CO-DRIFTS. The samples, consisting of 2-6 nm metal nanoparticles supported on mesoporous ceria, were tested in CO oxidation in absence and presence of hydrogen (PROX). To our knowledge, this work presents the first comparison of all the platinum-group metals (except Os) for these reactions in the same conditions. The as-prepared SCS catalysts are active in CO oxidation and a reducing treatment has no significant effect on their performances. While the best catalyst in H-2-free CO oxidation is Rh-CeO2, the addition of a high hydrogen excess decreases the Rh catalyst activity but enhances the CO oxidation rate on all other systems, including alumina-supported metals employed as reference catalysts. The resulting PROX turnover frequencies (Pt > Pd > Rh > Ir > Ru) follow the trends predicted by published density-functional-theory calculations considering the COad + O-ad elementary reaction as the rate-determining step. Pt-CeO2 is not only the most active but also the most selective catalyst, reaching near 100% CeO2 at low temperature (ca. 100 degrees C). The alumina-supported catalysts appeared less active than their ceria-supported counterparts in both reactions. The effect of heating/cooling cycles on the reaction kinetics was also investigated. Whereas the Pt, Pd and Ir ceria-supported catalysts were stable throughout PROX cycles, Rh and Ru ones exhibited apparently chaotic behaviors above ca. 200 degrees C, which are proposed to be induced by favorable CO dissociation and methanation pathways and/or variable metal oxidation states. (C) 2015 Elsevier B.V. All rights reserved.

Synthesis of extremely pure nanoporous system was used to investigate the intrinsic catalytic properties of gold toward oxidation reactions. The poor activity observed contrasts with the high activity reported up to now on nearly identical materials except that they contain superficial impurities resulting from the manufacturing process. This low activity shows that gold alone cannot activate the O−O bond without some influence from a substrate such as charge-transfer processes or specific interfacial sites.

MATERIAUX:SURFACES+TDR:FMO:JRO; Au-Ag nanoporous systems were prepared by a recently developed method. Contrary to the electrochemical dealloying, the methodology applied in this work proved that it allowed to prepare bimetallic AuAg NP systems with controlled chemical compositions. Compared with monometallic Au or Ag catalysts, the alloy catalysts exhibited high activity toward CO and/or H-2 oxidation and exceptionally high selectivity at low temperature for CO oxidation in the presence of H-2 showing a synergistic effect between Au and Ag. Among the different characterizations, Low-Energy Ion-Scattering Experiments provided composition data about the topmost atomic layer, i.e., where catalytic reactions occur and allowed us to establish a clear correlation between top layer surface concentration and reactivity. A pronounced dependence of the reaction rate on the silver concentration has been observed only for H-2 oxidation. This strong dependence has been correlated with the number of surface sites constituted of adjacent Ag atoms required for dissociative oxygen adsorption. The high selectivity of the Au-Ag alloys with respect to pure gold has been ascribed to a large segregation of silver, especially on low coordination sites, inhibiting the H-2 adsorption. (C) 2013 Elsevier Inc. All rights reserved.

Optical properties of mixed clusters (AuxAg 1 - x ) n and (NixAg 1 - x ) n , produced by laser vaporization and embedded in an alumina matrix, are reported. The size effects are investigated for different concentrations (x = 0.25, 0.5 and 0.75) in the diameter range 2-4 nm. For alloyed clusters (AuxAg 1 - x ) n of a given size an almost linear evolution of the surface plasmon frequency ω s with the concentration is observed (between those of pure gold and pure silver clusters). Moreover the blue-shift and the damping of the resonance with decreasing size is all the more important as the gold concentration in the particles increases. Such results are in agreement with theoretical calculations carried out in the frame of the time-dependent local-density-approximation (TDLDA) including an inner skin of ineffective screening and the porosity of the matrix. The optical response of (NixAg 1 - x ) n clusters exhibits a surface plasmon resonance in the same spectral range as the one observed for pure silver clusters, but considerably damped and broadened. For a given mean cluster size 3.0 nm, a blue-shift of the resonance is observed when increasing the nickel concentration (between x = 0.25 and x = 0.75). The results are in good qualitative agreement with classical predictions in the dipolar approximation, assuming a core-shell geometry.

γ-Al2O3-supported Pd, Pt, and bimetallic Pd–Pt catalysts have been produced by laser vaporization of the respective bulk metals and alloys. Transmission electron microscopy and energy-dispersive X-ray analysis experiments show that these bimetallic clusters have a perfectly well defined stoichioemetry. Moreover, pure or bimetallic clusters have similar size distributions. One has taken benefit of these features to study in detail the resistance to sulfur poisoning as a function of the composition and to verify to what extent what is claimed in the literature, i.e., an improved resistance to sulfur poisoning of Pd–Pt catalysts as compared to pure metals, is verified. The catalytic properties of Pd, Pt, Pd17Pt83, and Pd65Pt35 in the hydrogenation of tetralin in the presence of varied amounts of H2S have been compared. Contrary to what is claimed, the coexistence of Pd with Pt in the same particle has no influence on the hydrogenation activity nor on the sulfur tolerance of the catalysts. Hence, it has been shown that Pt has no electronic influence on the reactivity of Pd atoms.

To explain the beneficial effect of Mn addition to Pd catalysts in CO–NO conversion, a detailed study of the surface structure and composition of the catalytic sites was undertaken. The combination of analytical microscopy and EXAFS confirms the presence of bimetallic particles in which manganese is partly alloyed to palladium, as described in Part 1 (A. J. Renouprez, J. F. Trillat, B. Moraweck, J. Massardier, and G. Bergeret, J. Catal.179, 390 (1998)). Monte-Carlo modeling coupled with a simple energetic model and low-energy ion scattering (LEIS) shows that Mn has a larger concentration at the surface of the particles than the mean value measured by chemical analysis. This is attributed to a segregation of this element at the surface. To establish a correlation between the structure of these particles and their reactivity, infrared experiments of CO, NO adsorptions and coadsorption were carried out at both 300 and 573 K. The measured frequencies were interpreted on the basis of density functional theory (DFT) quantum chemical calculations. At 300 K, on pure Pd, CO and NO are located on the same sites and are displaced by each other. On the alloys, both molecules remain partly on the surface on adsorption of the other. DFT calculations show that the Mn atom at the surface induces a significant shift of the NO stretch frequency to lower values, especially if a Mn atom is present in the surface site. On mixed Pd–Mn sites, the chemisorbed NO molecule is found in a geometry quasi-parallel to the surface, with a long N–O bond and a low stretch frequency. This structure is stable on Pd3Mn, but not on pure Pd. This lying down geometry is a precursor for the dissociation. A dual-functional mechanism for the NO reduction, involving Mn oxides with oxygen vacancies and this form of adsorbed NO, is proposed.

Pure Pd, Pt, and bimetallic Pd–Pt clusters produced by laser vaporization of bulk metals or alloys have been deposited on a high surface area alumina (γ-Al 2 O 3 ). The bimetallic clusters have a perfectly well-defined stoichiometry, as demonstrated by energy-dispersive X-ray analysis experiments. Transmission electron microscopy shows that pure or bimetallic clusters have similar size distributions. The catalytic properties of Pd, Pt, Pd 17 Pt 83 , and Pd 65 Pt 35 supported clusters have been investigated in the vapor-phase toluene hydrogenation over a wide range of reaction conditions. The activation energies are 44 ± 6 kJ/mol. The pressure dependence on toluene varies from 0 at 333 K to 0.7 at 500 K whereas this dependence on H 2 increases from approximately 0.5 at 333 K up to 2.2 at 500 K. The results obtained with the pure metals corroborate those reported in the literature for catalysts prepared by chemical methods. The activities of the bimetallic catalysts do not show any synergy between Pd and Pt but rather a perfect additivity of their individual catalytic properties. Pt has been shown to have no electronic influence on the reactivity of Pd atoms. The nonlinear variation of the turnover frequency as a function of the Pd surface concentration is interpreted on the basis of an active site composed of a pair of atoms.

Amorphous silicon carbide films have been prepared by low-energy cluster beam deposition. Transmission electron microscopy and tapping mode atomic force microscopy reveal the structure of the film which is formed by a random distribution of SiC nanoparticles. The size of the particles was about 8–10 nm and it is greater than the size of the incident free particles (less than 1 nm). However, the inner structure of the supported particles does not reveal long-range order. Both Raman spectroscopy and Fourier-transformed infrared spectroscopy show clearly limited chemical ordering and phase separation evidenced by the presence of silicon-rich, carbon-rich and SiC-rich areas.

Pd-Pt nanoclusters are obtained by the focusing of an Nd:YAG laser onto rods of alloys. The aggregates, which are produced by plasma cooling via short helium bursts synchronized with the laser pulses, are collected on amorphous carbon or silicon substrates, in a IIFIV chamber. Transmission electron microscopy (TEM) experiments show that the diameters of the clusters range between 1,5 and 4.5 nm, and analytical microscopy indicates that they have the sanie composition as the vaporized rods. Low-energy ion scattering (LEIS) also shows that the surface of the obtained clusters is Pd enriched: the Pd concentration in the first atomic layer is found to be equal to 38% for a Pd17Pt83 rod composition and 87% for the Pd65 Pt35 alloy. The catalytic activity of these clusters in the hydrogenation of 1,3-butadiene to butenes and butane is measured in static mode, with mass spectrometry detection. The reactivity of the bimetallic clusters is explained by the atomic local order and low-coordination sites considered as “hot sites”.

We have investigated SiC films, obtained by the low-energy cluster beam deposition (LECBD) technique, and using AFM Raman and XPS spectroscopies. We produce these films at room temperature and in an ultrahigh vacuum environment to protect them from the pollution. The inner morphology of the clusters is close to an amorphous-like structure. However, most of the theoretical models predict an sp 3 hybridization, in disagreement with our experimental results. We find that the films are composed mainly by free components formed by rich silicon, rich carbon, and rich SiC regions, respectively, In our case, the mean hybridization of the rich carbon region is mainly sp n -like with 2 < n < 3.

The preparation and bulk characterization of a series ofsilica-supported Pd–Ni catalysts obtained either by coexchangeor by organometallic chemistry have been described in Part I(J. F. Faudon, F. Senocq, G. Bergeret, B. Moraweck, G. Clugnet, C.Nicot, and A. Renouprez,J. Catal.144, 460 (1993)). The present work describes their catalytic activity in thehydrogenation of 1,3-butadiene, in which palladium is one orderof magnitude more active than nickel, in relation with theirsurface composition measured by LEIS, a powerful physicalmethod, still seldom applied to study the surface of real catalysts. Both types of Pd–Nicatalysts exhibit an activity increasing monotonically with thepalladium concentration,without any synergy effect. A more rapidincrease of initial activity with the concentration is, however,observed for the catalysts with organometallic precursors thanfor those prepared by exchange. This difference is explained bythe migration of palladium from the bulk to the surface of theparticles, shown by XPS and LEIS measurements, which is morepronounced on the large particles of the catalysts prepared byorganometallic chemistry. Indeed, the theoretical predictionsfor bulk alloys, based on thermodynamic calculations butcorrected for the effect of particle size, are in agreement withthe LEIS results and do conclude to more enrichment on largeparticles. Compared to nickel, the Pd–Ni alloys undergo lessdeactivation. A larger proportion of 1-butene andtrans-2-butene is formed on the alloys than on pure nickel.This variation of selectivity is explained by the presence onnickel of astrongly adsorbeddi-σ metallocyclobuteneintermediate coexisting with the di-π adsorbed form of the dienenormally present on palladium. This strongly adsorbed speciescan also explain the low activity of nickel and its large deactivation.

MATERIAUX:INGENIERIE:SURFACES+LBU:CAQ:ATU:FMO:JRO:DFA; We report an original and scalable synthesis pathway to produce encapsulated gold nanoparticles. Precise control of the gold particles is achieved in the range of 1-10 nm through the impregnation of silicalite-1 with a controlled concentration of gold solution, followed by dissolution-recrystallization of the zeolite.

Supported bimetallic Pd-Pt clusters with a well-defined stoichiometry are produced using a laser-vaporization source. Free clusters are also studied by time-of-flight mass spectrometry and photofragmentation. The supported clusters are characterized by energy dispersive x-ray analysis. The low binding energy of palladium atoms compared to that of platinum is clearly demonstrated in both free and supported clusters. The reactivity is briefly discussed.

We report a new synthesis of size-controlled bimetallic Ru–Cu nanoparticles (NPs) in ionic liquids using two organometallic precursors. Interestingly enough, upon mixing ruthenium and copper precursors, smaller bimetallic NPs (1.9–2.8 nm) are formed as compared to single metals (Cu: 5 nm; Ru: 4 nm) in a large range of copper molar fractions (χCu = 0.005–0.91). Surface and microscopy techniques evidence that NPs have metallic copper on the surface and crystalline hexagonal close-packed metallic ruthenium core. This structure is further assessed by the catalytic activity of the bimetallic NPs, showing that conversion of benzene in cyclohexane is reduced with increasing χCu. A new and versatile synthesis of size-controlled bimetallic NPs under 10 nm using different decomposition rates of organometallic precursors in ionic liquids is described.

We study PdPt bimetallic clusters in both free and supported phases. These clusters have been produced with a laser vaporization source. Free clusters directly produced by the source are studied by time of flight mass spectrometry and photofragmentation technique. We observed a sequential evaporation of Pd atoms in the mixed clusters consistent with a palladium segregation process. This tendency has been also observed on supported particles from which the structure and the composition are determined by high resolution transmission electron microscopy and energy dispersive x‐ray analysis. A main result is that each particle has the composition of the massic rod vaporized in the source. The supported particles are well crystallized and exhibit truncated octahedron shapes. Experimental observations are well explained using a modified tight binding model. Indeed, within this model, we found that the equilibrium shape is strongly related to the variation of the cohesive energy with atomic coordination number. Also, some preliminary results on the specific reactivity of these bimetallic clusters are presented.

H2-free CO oxidation and preferential CO oxidation in the presence of H2 (PROX) have been studied on Ir100–xPdx nanoalloys (x = 0, 11, 45, 89, and 100) supported on alumina or amorphous silica–alumina. The catalysts were prepared by impregnation of acetylacetonates and characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. It is shown that, depending on the temperature conditions, Ir-rich catalysts undergo oxidation or reduction, leading to complex reaction kinetics upon heating or cooling. The surface acidity of the support does not significantly affect the CO oxidation performances, but decreases the CO PROX activity due to the probable promoting role of basic OH groups in this reaction. In addition, although the H2-free CO oxidation and the PROX turnover frequencies increase with the Pd content, the CO2 selectivity of Ir100–xPdx catalysts (x = 11 and 45) is found superior to those of Ir catalysts, Pd catalysts, and an Ir + Pd mecha...

The combined use of low-energy ion-scattering and angular X-ray photoemission spectroscopy experiments permitted determination of the surface concentration and the concentration depth profile of Pd1Ni99 and Pd5Ni95 alloys equilibrated at 870 K. There was evidence that the outermost surface is largely enriched in Pd: the Pd surface compositions of Pd1Ni99 and Pd5Ni95 were found to be as large as 20 and 50 at.%, respectively. Such results fit well with the predictions based on a recently developed thermodynamic model. Reactivity of both alloys towards 1,3-butadiene hydrogenation was also measured (at 300 K and under an excess of hydrogen pressure). The activity of Pd1Ni99 (5 × 1015 molecules cm−2 s−1) was found to be of the same order of magnitude as for (111) and (110) Pd single-crystal faces. Furthermore, a strong exaltation was observed for Pd5Ni95 (30 × 1015 molecules cm−2 s−1), the selectivity in butenes being always about unity. The results are discussed in terms of geometric andior electronic effects. A definition of the "active site" is proposed: the active site may involve a group of Pd atoms (probably two or a little more), but not mixed sites constituted of Pd and Ni atoms. These Pd atoms are found to be electronically modified by surrounding Ni atoms.

Silicon segregates to a large extent to the surface of a Cu 3 Si alloy stabilized at 700 K. The top layer contains as much as 70 at% Si, the second and third layers are also significantly enriched in silicon. Such a phenomenon could largely account for the renewal of the surface layers of the silicide as the silicon is consumed during the direct synthesis (of methylchlorosilane by reaction with methyl chloride). As demonstrated by calculations of the segragation free enthalpy based on the broken-bond model, the difference in surface tension between Cu and Si is the main driving force involved in the observed segregation.

The catalytic activity of Pd aggregates vapor-deposited on graphite has been measured in the 1,3-butadiene hydrogenation. It is generally decreasing with particle size. But, such a decrease is largely depending upon the hydrogen and the hydrocarbon pressure. Indeed, with low hydrocarbon pressure (about 1 Torr) and in presence of large excess of hydrogen (Ph2/Pc4h6=100) the activity of the smaller aggregates is comparable to that of larger particles. The different behaviour is consequently attributed to a size dependent deactivation. Such a deactivation could be due to carbon deposits generated by dehydrogenation of the diene admolecules. This behaviour is tentatively related with marked differences observed by XPS on the smaller aggregates, both with respect to the core level energies and to the density of states into the valence band.

Roze, Emmanuel Gravejat, Paul Quinet, Elodie Rousset, Jean Luc Bianchi, Daniel; The adsorption of CO at 300 K on a reduced (H-2, 713 K) 1% Au/Al2O3 solid leads to the formation of a linear CO species adsorbed on the Au particles (denoted L1) characterized by an IR band at 2098 cm(-1) that slightly shifts to higher wavenumbers with the decrease in the coverage. For adsorption pressures P-a 愦灭;gt; 5 kPa, it is shown that there is a progressive reconstruction of the Au surface with the increase in the duration of the adsorption t(a) leading to the disappearance of the Au sites associated with the L1 CO species and to the formation of new Au sites forming a linear CO species (denoted L2) characterized by an IR band at 2070 cm(-1). The heats of adsorption of the two adsorbed species are determined as the function of their respective coverages by using the adsorption equilibrium infrared spectroscopy procedure previously developed. It is shown that they linearly decrease with the increase in their coverages from 62 to 43 kJ/mol for the L1 and from 100 to 50 kJ/mol for the L2 CO species at coverages 0 and 1, respectively. The values for the L1 CO species are consistent with a previous study on Au/TiO2 and with literature data on model Au particles. The higher heat of adsorption of the L2 CO species as compared to that of L1 can be a driving force for the reconstruction of the Au particles. Moreover, it is shown that the L2 CO species presents a property similar to that of adsorbed CO species on reduced supported metal particles: the position of its IR band shifts to lower wavenurnbers with the decrease in its coverage (the singleton is situated at 2025 cm(-1)). The sintering of the gold particles as a function of the number of pretreatment/reconstruction cycles leads to the detection of a third linear CO species characterized by an IR band at 2048 cm(-1) at 300 K with heats of adsorption of 80 and 60 kJ/mol at low and high coverages, respectively.

Quinet, Elodie Piccolo, Laurent Morfin, Franck Avenier, Priscilla Diehl, Fabrice Caps, Valerie Rousset, Jean-Luc; The kinetics of CO oxidation, H-2 oxidation and preferential CO oxidation (PrOx) over Au/Al2O3 catalysts have been investigated. The catalysts with the smallest particles (similar to 2 nm) are the most active for all three reactions. As previously observed, the presence of H-2 greatly promotes CO oxidation, which becomes faster than CO-free H-2 oxidation at low temperature. From these results and on the basis of previous works, we propose a complete PrOx mechanism. The reaction involves Au-OOH, Au-OH and Au-H intermediates, also involved in H-2 oxidation, and benefits from the presence of low-coordination sites. (c) 2009 Elsevier Inc. All rights reserved.

Bimetallic clusters, all containing gold, have been produced by laser vaporisation of bulk alloys followed by deposition of the formed clusters onto Al2O3 and TiO2 powders or flat silica supports. This technique allows a narrow size distribution of highly dispersed gold-based nanoparticles on powders and nanocrystalline structured thin films on 2D supports to be obtained. The catalytic performances of the as-obtained AuFe, AuNi, AuTi powdery catalysts have been studied in the PROX reaction and compared with those obtained in the oxidation of CO in the temperature range 25-300 degrees C. By comparing the activities of the different catalysts, it is concluded that the nature of the gold partner directly affects the activity of gold. The following tendency is observed: AuFe and AuNi have rather similar activities, significantly lower than that of AuTi. In this paper, we also present a first attempt to study reactivity of original self-supported systems. We show that significant CO oxidation reactivity can be obtained over unsupported nanoporous AuTi and PdAu thin films. By completely excluding the support effect, unsupported catalysts could provide a way of understanding the relevant catalytic mechanisms more easily.

Quinet, Elodie Morfin, Franck Diehl, Fabrice Avenier, Priscilla Caps, Valerie Rousset, Jean-Luc; Although alumina-supported gold nanoparticles are poor catalysts for the oxidation of carbon monoxide, they have turned out to be promising candidates for the preferential oxidation of CO in hydrogen-rich streams (PrOx), as hydrogen apparently enhances the CO oxidation rate. The mechanism of this promotion effect is unclear. In this study, we carry out kinetic measurements on the PrOx reaction catalyzed by a 0.9% Au/Al2O3 catalyst, which is prepared by direct anionic exchange. We show that the apparent activation energy of the oxidation of CO is lower than that of the oxidation of H-2, whatever the hydrogen content in the feed. On the other hand, the hydrogen partial reaction order is higher in the oxidation of H-2 than in the oxidation of CO. Thus, the CO oxidation rate is significantly increased at low temperature by the introduction of only a small amount of hydrogen in the reactant mixture. At higher temperatures, the selectivity to CO2 decreases due to competition with the oxidation of H-2. Higher hydrogen concentrations cause the competition between CO and H-2 oxidations to start at lower temperatures. It is proposed that hydrogen reacts with oxygen to yield highly oxidizing intermediates that selectively react with CO as long as the energetic barrier to produce water from these intermediates is not crossed. (C) 2007 Elsevier B.V. All rights reserved.

Quinet, Elodie Piccolo, Laurent Daly, Helen Meunier, Frederic C. Morfin, Franck Valcarcel, Ana Diehl, Fabrice Avenier, Priscilla Caps, Valerie Rousset, Jean-Luc; The kinetics and mechanism of the preferential oxidation of carbon monoxide in the presence of hydrogen (PrOx) over an unsupported gold powder (mean particle size similar to 20 nm and free of silver) have been investigated using How fixed bed catalytic testing and diffuse reflectance infrared Fourier transform spectroscopy coupled to mass spectrometry (operando DRIFTS or DRIFTS-MS). It is shown that the presence of H-2 has a favourable effect on the oxidation of CO, either by strongly accelerating the reaction or by preventing the catalyst deactivation, depending on the conditions used. Variation of the hydrogen partial pressure has allowed us to determine partial reaction orders for both CO oxidation and H-2 oxidation under PrOx conditions. An infrared band at similar to 2113 cm(-1), corresponding to on-top CO adsorption on metallic gold, has been observed below 150 degrees C. In addition, adsorbed hydroxyl groups gradually develop simultaneously to gas-phase water in the course of the reaction at increasing temperatures. The promotional effect of hydrogen is ascribed to highly oxidative HxOy intermediates formed from the interaction between H-2 and O-2 on the gold surface. (C) 2008 Elsevier B.V. All rights reserved.

Lignier, Pascal Mangematin, Stephane Morfin, Franck Rousset, Jean-Luc Caps, Valerie; Molecular oxygen can be used as the main oxidant to selectively epoxidize trans-stilbene over Au/TiO2 catalysts, in a free-radical process. However, the nature of the radical initiator has a critical influence on the reaction selectivity. tert-Butylhydroperoxide (TBHP, catalytic amount) leads to high yields of epoxide; on the other hand, hydrogen peroxide and di-tert-butylperoxide merely cause degradation of trans-stilbene. The choice of the solvent is also critical. Amongst the selected solvents. only alkyl-substituted cyclohexanes lead to high yields of epoxide, despite the poor dispersion of the catalytic powder. Other solvents. including the more polar ones and cyclohexane, are significantly less efficient, both in terms of total activity and epoxide production. The latter does not go beyond the yield expected from the potential stoichiometric reaction between TBHP and trans-stilbene (5%). On the basis of these results, an aerobic epoxidation mechanism is proposed in which molecular oxygen is activated by a substituted cyclohexyl radical produced by abstraction of a tertiary hydrogen atom from the solvent molecule by a tert-butylperoxy radical. (C) 2008 Elsevier B.V. All rights reserved.

International audience; To study the role of strain effects on catalytic properties in the case of supported particles, core–shell PdNi bimetallic nanoparticles were prepared by chemical synthesis and studied in hydrogenation of buta-1,3-diene. Multitechnique characterisation (EDS, TEM, EXAFS) results indicate good homogeneity of the bimetallic particles in terms of size and composition. The core–shell structure of NiPd bimetallic particles and the compressive stress induced on Pd atoms in the top layer due to its larger atomic radius compared with Ni, have been evidenced by EXAFS analyses. According to the literature, Pd activity for buta-1,3-diene hydrogenation is amplified when Pd is strained at the surface of Ni single crystal faces, mainly in the case of (110) planes. This compressive stress is supposed to produce surface reconstructions with undulated Pd row structures. In our case, no amplification of the Pd activity for buta-1,3-diene hydrogenation performed in liquid-phase conditions was observed for a monolayer of Pd strained on 5 nm Ni particles. Consequently, the compressive strain on the Pd atoms at the surface of the nanoparticles seems not to play a major role in the amplification of the activity for buta-1,3-diene hydrogenation. Surface reconstructions observed on extended surfaces to relax the surface stress seems to be the key point for Pd activity amplification. For supported particles, we propose that this relaxation phenomenon does not occur for such small three-dimensional particles.

International audience; The gold reference catalyst Au/TiO2 exhibits high activity in the stereoselective epoxidation of trans-stilbene in methylcyclohexane in the presence of 5 mol% TBHP, by taking part in a chain reaction involving the activation of molecular oxygen by a radical produced from methylcyclohexane