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The synthesis and analysis of inorganic material combinatorial libraries by the split-pool bead method were demonstrated at the proof-of-concept level. Millimeter-size spherical beads of porous γ-alumina, a commonly used support material for heterogeneous catalysts, were modified with Al13O4(OH)24(H2O)12⁷⁺ cations in order to promote irreversible adsorption of the anionic fluorescent dyes Cascade Blue, Lucifer Yellow, and Sulforhodamine 101. The compositions of individual beads were easily determined through three split-pool cycles using a conventional fluorescence plate reader. Small split-pool material libraries were made by adsorbing noble metal salts (H2PtCl6, H2IrCl6, and RhCl3) into the beads. Analysis of these beads by micro-X-ray fluorescence showed that quantitative adsorption of metal salts without cross-contamination of beads could be achieved at levels (0.3 wt % metal loading) relevant to heterogeneous catalysis. The method offers the potential for synthesis of rather large libraries of inorganic materials through relatively simple benchtop split-pool chemistry.

In-situ X-ray absorption near-edge structure (XANES) experiments were performed on a working reformate-air fuel cell to study the structure of carbon supported Pt−Ru anode electro-catalyst. The fuel cell was operated in a normal mode without the use of supplemental electrolytes. A fresh membrane electrode assembly (MEA) and a conditioned MEA were studied at different operating conditions of the fuel cell and different feed (pure H2 or H2/CO [100 ppm CO]) at the anode. The in-situ Pt LIII-edge and Ru K-edge XANES of the fuel cell MEAs showed metallic characteristics under all operating conditions. These results demonstrate that, under the reducing conditions of normal fuel cell operation, the Pt−Ru catalyst exists as a metallic phase(s).

AbstractTomographic energy-dispersive diffraction imaging (TEOOI) is a technique, which exploits a well-defined energetic white X-ray beam from a synchrotron to gain diffraction information from volume elements within a bulk sample. Most tomographic methods rely on spectroscopic absorption or fluorescence signals to provide elementaUcompositional contrast; by comparison TEDDI provides structural contrast, through diffraction, irrespective of whether there is any elemental variation, At the heart of this technique is energy-dispersive diffraction (EDD), which is a mode of X-ray diffraction less-familiar to most researchers and is therefore briefly outlined prior to a basic description of the TEDDI technique; this technique is then illustrated by studies carried out on white beam instruments at the SRS (station 16.4) and ESRF (beamlines ID9 and 1030) synchrotron sources, with examples taken from materials/engineering science involving both static and dynamic systems.

Dispersed and bilayered V2O5/TiO2/SiO2 catalysts were successfully synthesized by the incipient wetness impregnation method, and their surface structures under various conditions were extensively investigated by combined in situ Raman, UV−vis−NIR diffuse reflectance and X-ray absorption near-edge spectroscopies, as well as X-ray photoelectron spectroscopy. Temperature-programmed reduction and methanol oxidation were employed as chemical probe reactions to examine the reducibility and reactivity/selectivity of these catalysts. The spectroscopic results revealed that both vanadium oxide and titanium oxide on SiO2 are dispersed as two-dimensional metal oxide overlayers. The surface vanadium oxide species on the dispersed TiO2/SiO2 are predominantly isolated VO4 units [O&dbd;V(O−support)3] in the dehydrated state and become polymerized VO5/VO6 units upon hydration. The surface vanadium oxide species preferentially interact with the surface titanium oxide species on silica, and the V(V) cations possess oxygenated ligands of Si(IV)−O^- and Ti(IV)−O^- in the O&dbd;V(O−support)3 unit with varying ratios from 3:0 to 0:3, which depends on the vanadia and titania loadings. The varying ratio of the Si(IV)−O^- and Ti(IV)−O^- oxygenated ligands significantly affects the chemical properties of the isolated VO4 units. Consequently, the reducibility of the surface vanadium oxide species is altered and the reduction occurs over a wider temperature range. In addition, the methanol oxidation turn-over frequency of the surface VO4 species on TiO2/SiO2 increases an order of magnitude relative to V2O5/SiO2. Thus, the oxygenated ligands around the V cations play a critical role in determining the reactivity of the surface vanadium oxide species on the oxide supports.

The molecularly dispersed V2O5/SiO2 supported oxides were prepared by the incipient wetness impregnation of 2-propanol solutions of V-isopropoxide. The experimental maximum dispersion of surface vanadium oxide species on SiO2 was achieved at ~12 wt % V2O5 (~2.6 V atoms/nm²). The surface structures of the molecularly dispersed V2O5/SiO2 samples under various conditions were extensively investigated by in situ Raman, UV−vis−NIR DRS and XANES spectroscopies. The combined characterization techniques revealed that in the dehydrated state only isolated VO4 species are present on the silica surface up to monolayer coverage. Interestingly, the three-member siloxane rings on the silica surface appear to be the most favorable sites for anchoring the isolated, three-legged (SiO)3V&dbd;O species. Hydration dramatically changes the molecular structure of the surface vanadium oxide species. The specific structure of the hydrated surface vanadium oxide species is dependent on the degree of hydration. The molecular structure of the fully hydrated vanadium oxide species closely resembles V2O5·nH2O gels, rather than V2O5 crystallites. The fully hydrated surface vanadium oxide species are proposed to be chain and/or two-dimensional polymers with highly distorted square-pyramidal VO5 connected by V−OH−V bridges, which are stabilized on the silica surface by the sixth neighbor of Si−OH hydroxyls via Si−OH···V hydrogen bonds. In analogy to the hydration process, alcoholysis occurs during methanol chemisorption, and similar molecular structures are proposed to interpret the interaction between methanol molecules and the surface vanadium oxide species on silica.

The evolution of the dispersed surface molybdate phase on high-surface-area magnesium oxide is studied as a function of weight loading of MoO3 and calcination temperature using in situ Mo L3-edge X-ray absorption near edge spectroscopy, in situ laser Raman spectroscopy, and thermogravimetry−mass spectroscopy. Under ambient, hydrated conditions, the magnesium support is present as magnesium hydroxide, rather than MgO, and the molybdate species is present on the surface as an isolated tetrahedral species for weight loadings of 5−20 wt % MoO3. Under these hydrated conditions the form of the molybdate species is controlled by the net surface pH at the point of zero charge and is the same as that observed in aqueous solution. As the catalyst is calcined in air, the molybdate species transforms from a tetrahedral species, MoO4, to an octahedral one, MoO6. This transformation occurs at the same temperature, ~450 °C, as that at which the support changes from hydroxide to oxide. This transformation temperature is independent of the MoO3 loading. It is suggested that it is this support-induced structural change that drives the change in the form of the surface molybdate species and not the desorption of adsorbed water.

The bonding and reactivity of allyl groups (CH2CHCH2) on a Cu(100) surface are studied by temperature-programmed desorption and near edge X-ray absorption fine structure measurements. Surface allyl groups are generated by dissociative adsorption of allyl chloride at 110 K and/or a coupling reaction of surface methylene (CH2) and vinyl (C2H5) groups. Allyl groups on a Cu(100) surface coordinate as a nearly flat-lying π-allyl species. Allyl radical ejection is the dominant reaction pathway upon heating the Cu(100) surface, which is drastically different from the behavior of allyl on Ag(111) and Al(100) surfaces, where allyl groups couple to form 1,5-hexadiene and disproportionate to produce propylene and an unidentified polymeric species, respectively. On a Cu(100) surface precovered with hydrogen atoms, allyl groups are scavenged by hydrogen atoms to produce propylene at ~ 250 K.

SiO2-supported VPO catalysts with different P/V ratios were prepared, used in selective oxidation of butane, and characterized by X-ray diffraction (XRD), laser Raman spectroscopy (LRS), near-edge X-ray absorption spectroscopy (XANES), and ³¹P magic angle spinning NMR. Similar to the SiO2-supported catalysts of lower metal loadings studied previously, the maleic anhydride selectivity increased with increasing P/V ratio, to greater than 50 mol % for P/V = 2. XRD and LRS detected predominantly α1-VOPO4 or a phase that is structurally very similar for both P/V = 1 and P/V = 2 samples. However, within the detection limit of the XANES experiment, the P/V = 1 catalyst contained mostly V⁵⁺ under reaction conditions, whereas the P/V = 2 catalyst contained 100% V⁴⁺.

Dechlorination and hydrodechlorination of multiply-chlorinated ethanes and propanes on a clean Cu(100) surface have been studied by Auger electron spectroscopy, temperature-programmed desorption, and chemical displacement measurements. The rate-limiting step in degradation is dissociation of the first C−Cl bond in the molecule, and this process is more facile in CCl2 groups than in CCl groups. The activation energies for C−Cl bond scission on Cu(100) are 12−20% of the gas phase bond dissociation energies, and the extent of dissociation by the physisorbed molecules is a sensitive function of the relative rates of C−Cl bond scission and molecular desorption. The chlorinated hydrocarbon fragments generated on the surface by C−Cl bond cleavage undergo facile α- or β-chlorine elimination while all C−H bonds remain intact. β-Chlorine elimination dominates in cases where chlorine is present at both the α- and β-positions, and as a result of β-chlorine elimination, alkenes are generated and evolved to the gas phase. In cases where no β-Cl is present, α-chlorine elimination yields surface carbene intermediates, which readily couple to form longer chain alkenes. Surface hydrogen atoms readily scavenge these carbene intermediates to form alkyl groups. Upon thermal activation, these alkyls are converted to alkenes via β-hydride elimination or to alkanes via coupling with surface hydrogen. All processes subsequent to the initial dissociative adsorption of the chlorinated hydrocarbon occur with 100% selectivity, and all hydrocarbon products are evolved from the surface below 300 K. No carbon is detected on the surface after reaction, but Cl remains adsorbed up to 750 K where it is evolved as CuCl.

The molecularly dispersed TiO2/SiO2 supported oxides were prepared by the incipient wetness impregnation of 2-propanol solutions of titanium isopropoxide. Experimental monolayer dispersion of surface titanium oxide species on SiO2 was reached at ~4 Ti atoms/nm² with a two-step impregnation procedure. The surface structures of the molecularly dispersed TiO2/SiO2 under various environments were extensively investigated by in-situ spectroscopic techniques (e.g., Raman, UV−vis−NIR DRS, and XANES) as well as XPS. The combined characterization techniques revealed the consumption of surface Si−OH groups and the formation of Ti−O−Si bridging bonds. In the dehydrated state, the surface Ti atoms in the 1% TiO2/SiO2 sample (0.24 Ti atoms/nm²) are predominantly found to be isolated TiO4 units, whereas at maximum surface coverage (~4 Ti atoms/nm²), two-dimensional polymerized TiO5 units are dominant on the silica surface. The in-situ spectroscopic studies demonstrated that the coordination and ligands of the surface Ti cations change upon hydration/dehydration as well as during methanol oxidation. Methanol oxidation showed that the molecularly dispersed surface titanium oxide species exhibit completely different catalytic behavior (predominantly redox products) compared to bulk titanium oxide (predominantly dehydration products). Furthermore, the TOF of the surface titanium oxide species is strongly dependent on their local structures and varies by 1 order of magnitude (isolated TiO4 ≫ polymerized TiO5). These new results provide fundamental insights about molecular structure−reactivity/selectivity relationships of the molecularly dispersed TiO2/SiO2 supported oxides.

The sensitivity of Pt L‐edge XANES to local geometric and electronic structure in various Ptnclusters is investigated using the ab initioself‐consistent FEFF8 code. Calculations based on FEFF8 are found to be in good agreement with experiment. For pure Pt clusters the XANES can distinguish between 2‐ and 3‐dimensional clusters. Self‐consistency is important in determining the variation of XANES with cluster size. The effect of a support is also studied. In Pt‐Clxclusters the presence of a Cl‐Pt bond leads to a `hybridization peak', i.e., a peak in the Cl d‐density of states (d‐DOS) mixed with Pt d‐states. For Pt‐H clusters hydrogen addition is well correlated with the growth of a broad shoulder on the white line. This change is attributed largely to AXAFS, i.e.to a corresponding change in the atomic background absorption.

Treatment of the (110) and (100) surfaces of iron with 20 torr H2O vapor causes surface restructuring.