P. Delichère

Gold Nanoparticles Supported on Passivated Silica: Access to an Efficient Aerobic Epoxidation Catalyst and the Intrinsic Oxidation Activity of Gold

Well-defined and perfectly dispersed [( identical withSiO)Au(I)] surface species supported on silica have been obtained via surface organometallic chemistry and transformed upon mild reduction (H(2), 300 degrees C) into small (1.8 +/- 0.6 nm) Au particles supported on silica passivated with SiMe(3) functionalities. Improved performance in liquid-phase aerobic epoxidation has been achieved, and the intrinsic activity of gold in oxidation is revealed.

Alloying effect on the adsorption properties of Pd50Cu50{111} single crystal surface

The surface composition of the Pd50Cu50{111} single crystal as measured by LEIS corresponds to Pd45Cu55 in the very first layer. This small Cu segregation did not induce a special surface structure as observed by LEED. The classical (1 × 1) LEED pattern was observed both after annealing and during gas adsorption. The chemical properties of Pd-Cu alloy have been investigated by adsorption of CO, NO and H2 and also by XPS. The amount of CO adsorbed on the surface at 160 K is at least four times higher than for NO or H2. Furthermore, the adsorption energies decrease on Pd and increase significantly on Cu in alloy with respect to adsorption on pure metals. The significant shifts of core and valence bands observed by XPS and, moreover, the change in adsorption energies of CO and NO were attributed to a strong electronic interaction between Pd and Cu upon alloying. This Pd-Cu interaction has been interpreted in terms of bond formation between the almost full valence “sd” band of Pd and the resonant “dsp” band of Cu near the Fermi level. An explanation using back-donation capabilities of the Pd-Cu alloy surface to CO and NO is also given to support the formation of a hybridized molecular orbital.

Vanadyl pyrophosphate catalysts: Surface analysis by XPS and LEIS

Abstract Different (VO)2P2O7 catalysts have been prepared from the same VO(HPO4)·0.5H2O precursor. These catalysts, characterized by XRD, have been compared with respect to their catalytic performances in the n-butane oxidation to maleic anhydride. The surface oxidation state of vanadium has been measured by XPS thanks to the decomposition of the V2p3/2 peak or the difference of binding energy between the O1s and V2p3/2 levels. The relative percentage of V4+ and V5+ depends on the temperature of preparation and on the time of activation under the reaction mixture. The surface (P/V) atomic ratio, in the range 1.5–1.9 as measured by XPS, is not very sensitive to the method of preparation or to thermal treatments. This relatively high (P/V) ratio has been further substantiated by LEIS measurements using 3 He + / 4 He + isotopes for quantification. Besides a possible preferential deposit of carbonaceous species on the surface vanadium dimers, it has been proposed that the high (P/V) ratio measured on (VO)2P2O7 catalysts could be attributed, at least in parts, to the presence of vanadium vacancies within a few amorphous surface layers.

Pt10Ni90(111) single crystal alloy: Determination of the surface composition by AES, XPS and ISS

Abstract The surface composition of the Pt 10 Ni 90 (111) single crystal alloy has been determined from AES, XPS and ISS experiments. The clean surface is largely enriched with platinum: 30–40% in the top layer instead of 10% in the bulk. This enrichment concerns mainly the ultimate surface layer and appears to be only slightly dependent on the sample temperature.

Electronic properties of supported Pd aggregates in relation with their reactivity for 1,3-butadiene hydrogenation

The electronic and catalytic properties of supported Pd particles have been studied by measurements of XPS core level shifts and activity for 1,3-butadiene hydrogenation reaction respectively. For samples prepared by atom beam deposition on carbonaceous supports the TOF at steady state decreases rapidly with decreasing particle size in the range of particle diameter 2.8 nm-1.2 nm. For these small particles, the Pd 3d core level shows an upwards shift and a larger half width compared to the values for massive Pd samples. For the Pd/SiO2 particles of similar size, both the 3d core level energy and the TOF are comparable to these of the massive Pd systems.

Surface reactivity of platinum-nickel single crystal alloys: Carbon monoxide adsorption

Abstract Carbon monoxide adsorption has been studied on (111) faces of single-crystal alloys — Pt10Ni90, Pt50Ni50 and Pt78Ni22 — by vibrational EELS and TDS measurements. On the nickel rich sample, on which the outer layer is notably enriched in platinum with respect to the nominal bulk concentration, multi-bonded nickel sites and on-top platinum sites for CO chemisorption are evidenced. The multi-bonded sites are preferentially filled. In the platinum rich samples, which exhibit a quasi complete platinum surface layer, linearly bonded species are observed first; multi-bonded CO adspecies appear only at higher coverage. Whatever the considered alloy sample the metal-CO bond strengths are weaker than on pure metals, as demonstrated by the lower values of both the metal-CO stretching mode energy and the temperature of the thermodesorption mass-28 peak. That weakening is more pronounced on the Pt50Ni50 sample.

Post-synthesis introduction of transition metals in surfactant-containing MCM-41 materials

Abstract V and TiMCM-41 have been prepared by post-synthesis modification of an as-made pure silica material in the liquid phase. Relatively high metal contents can be introduced without affecting the long-range ordering of the parent material. Solids with increasing metal contents have been characterized by X-ray diffraction, UV–VIS, IR and X-ray photoelectron spectroscopies. The results indicate that the metal species are essentially atomically dispersed at low loading (Me/Si

Nature of active oxygen in the n-butane selective oxidation over well-defined VPO catalysts: an oxygen isotopic labelling study

Abstract Using gaseous 18 O 2 , we have investigated the respective role of lattice and adsorbed oxygen in the n -butane oxidation to maleic anhydride (MA) and CO x on well-crystallized (VO) 2 P 2 O 7 and on a long term activated VPO catalyst. The reaction was performed at 350°C in a glass reactor, with a dry-ice cold trap and a recirculation pump. The reactor was connected to a mass spectrometer for on-line gas analysis. It has been checked that oxygen isotopic heteroexchange did not occur at a significant rate. The changes with reaction time of the 18 O isotopic labelling shows that only lattice oxygen is active for the formation of MA and the other products. For MA, this conclusion has been further confirmed by comparing the experimental evolution of the isotopic distribution (amu 98, 100, 102 and 104) with calculated mass spectra assuming a progressive replacement of lattice 16 O by 18 O in agreement with a redox mechanism. When the steady-state 18 O labelling of products is achieved, the oxygen balance indicates the presence of 18 O within a few surface layers of the catalyst. This result has been further confirmed by post-reaction analysis using low energy ion spectroscopy, providing a direct evidence of both 18 O and 16 O on the surface.

The influence of potassium and the role of coadsorbed oxygen on the chemisorptive properties of Pt(100)

The adsorption of K on Pt(100) has been followed by thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES); carbon monoxide was used as a probe for the modification of the chemical properties of K promoted surfaces. The role of subsequent adsorption of oxygen on the K modified surfaces has also been measured. For low potassium coverage (θK = 0 to 0.35), the mass-28 TDS peak temperature of adsorbed CO increases continuously with the K coverage, indicating an increase of the adsorption energy of CO which has been explained by a substantial charge donation from K into the 2π∗ orbitals of CO via long range interactions through the platinum substrate. No oxygen uptake was detected after oxygen exposure at room temperature. For high potassium content (θK = 0.45 to 1), the mass-28 TDS peak temperature of coadsorbed CO is very narrow and remains constant at 680 K. We propose the formation of a COKPt surface complex which decomposes at 680 K, since K desorption is detected concomitantly to CO. On such K covered surfaces, the oxygen uptake is promoted, and it cancels the modifications of the surface properties induced by potassium.

Design of grafted copper complex in mesoporous silica in defined coordination, hydrophobicity and confinement states

A bio-inspired synthesis of a silica grafted polydentate copper(II) complex is developed following the structural concept of metalloproteins where well defined metal ion coordination state, hydrophobic environment and confined space are present. Mesostructured porous silica of MCM-41 type replaces the proteic matrix while the pore surface is engineered according to a molecular stencil patterning technique combining both partial hydrophobization and site isolation in order to mimic the enzymatic cavity. The overall five-step synthesis includes the sol–gel formation of the silica matrix followed by partial removal of the structure directing agent and, sequential surface chemical modifications. This new methodology is illustrated here using trimethylsilyl functions to dilute bromopropylsilyl tripod tethers that undergo, directly in the pores, in a subsequent step nucleophilic substitution by a tetradentate ligand N,N′-bis(2-pyridinylmethyl)ethane-1,2-diamine (L42). The metallation of the grafted ligand is obtained in the final step by merely contacting the solid with copper(II) chloride or triflate ethanolic solutions. Different techniques such as powder XRD, N2 adsorption–desorption, elemental analysis, IR, XPS, EPR and EXAFS were combined together with an emphasis on quantification to reach a quasi-molecular description at each functionalisation step of the internal surface of the materials.