Nanocrystalline CeO2 supplies reactive oxygen in the form of surface eta1 superoxide species and peroxide adspecies at the one-electron defect site to the supported active species of gold for the oxidation of CO.

X-ray absorption near-edge spectra and temperature-programmed oxidation and reduction data demonstrate that Au(I) and Au(0) are both present in working MgO-supported gold catalysts for CO oxidation. EXAFS data indicate gold clusters with essentially the same average diameter (about 30 A) in each catalyst sample. Thus, the results provide no evidence of an effect of gold cluster size on the catalytic activity, but both the catalytic activity and the surface concentration of Au(I) were found to decrease with increasing CO partial pressure (as Au(0) was increasingly formed), demonstrating that the catalytic sites incorporate Au(I).

We report the encapsulation of platinum species in highly siliceous chabazite (CHA) crystallized in the presence of N,N,N-trimethyl-1-adamantammonium and a thiol-stabilized Pt complex. When compared to Pt/SiO or Pt-containing Al-rich zeolites, the materials in this work show enhanced stability toward metal sintering in a variety of industrial conditions, including H, O, and HO. Remarkably, temperatures in the range 650-750 °C can be reached without significant sintering of the noble metal. Detailed structural determinations by X-ray absorption spectroscopy and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy demonstrate subtle control of the supported metal structures from ∼1 nm nanoparticles to site-isolated single Pt atoms via reversible interconversion of one species into another in reducing and oxidizing atmospheres. The combined used of microscopy and spectroscopy is critical to understand these surface-mediated transformations. When tested in hydrogenation reactions, Pt/CHA converts ethylene (∼80%) but not propylene under identical conditions, in contrast to Pt/SiO, which converts both at similar rates. These differences are attributed to the negligible diffusivity of propylene through the small-pore zeolite and provide final evidence of the metal encapsulation.

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The discovery of new materials for separating ethylene from ethane by adsorption, instead of using cryogenic distillation, is a key milestone for molecular separations because of the multiple and widely extended uses of these molecules in industry. This technique has the potential to provide tremendous energy savings when compared with the currently used cryogenic distillation process for ethylene produced through steam cracking. Here we describe the synthesis and structural determination of a flexible pure silica zeolite (ITQ-55). This material can kinetically separate ethylene from ethane with an unprecedented selectivity of ~100, owing to its distinctive pore topology with large heart-shaped cages and framework flexibility. Control of such properties extends the boundaries for applicability of zeolites to challenging separations.

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Novel catalysts for environmental and energy-related conversions are increasingly emerging from rational design based on the understanding of relationships between structure at the molecular level and catalyst performance. Gold that is highly dispersed on metal oxides has surprisingly been found to be an active and selective catalyst for numerous reactions, [1,2] including CO oxidation.[3] Contradictory hypotheses have been advanced to account for the CO oxidation activity, [4][5][6][7][8][9][10][11][12][13] and the nature of the active sites and reactive oxygen intermediates remains elusive. [14,15] Herein we show by timeresolved spectroscopy of working catalysts consisting of gold nanoclusters on nanocrystalline CeO 2Àx that h 1 -superoxide and peroxide intermediates are formed at one-electron defect sites at the metal-support interface and oxidize adsorbed CO to CO 2 . The reactive oxygen species are not formed on conventionally prepared CeO 2 , and their formation on nanocrystalline CeO 2Àx is enhanced by the presence of the gold. This report is the first that unambiguously identifies and quantifies reactive oxygen species in low-temperature CO oxidation catalysis. The new concept advanced here is the representation of the catalytically active species as a composite that uniquely facilitates the formation of reactive oxygen species at the metal-support interface. The generality of the concept is exemplified by results showing that the nanocrystalline oxide can be either CeO 2Àx or Y 2 O 3 .[16] This concept opens new avenues to the design of novel materials with improved activities and selectivities for catalytic oxidation.We prepared high-surface-area (S BET = 180 m 2 g À1 ; BET refers to Brunauer, Emmett, and Teller) and thermally stable nanocrystalline CeO 2Àx with the fluorite structure by selfassembly in a liquid-crystal phase of individual CeO 2 nanoparticles with an average diameter of 5 nm.[17] Calcination of the nanocrystalline CeO 2Àx at 873 K either in the absence or in the presence of 20 wt % H 2 O produced minor changes in the BET surface area (S BET = 160 m 2 g À1 ). CeO 2 was also prepared by a conventional precipitation-calcination method to facilitate a comparison with the nanocrystalline material and to allow investigation of the influence of the surface structure.[12] Deposition-precipitation of gold on the two supports yielded samples with gold loadings in the range of 0.9-4.6 wt %. These samples were characterized by the following spectroscopic methods as they functioned as catalysts for CO oxidation at steady state in flow reactors: X-ray absorption near edge structure (XANES); extended Xray absorption fine structure (EXAFS); Raman; IR. Each of the samples catalyzed CO oxidation but gold supported on conventional CeO 2 was much less active than that on the nanocrystalline support. The spectra provide information that characterizes the oxidation state(s) and structure of the gold atoms as well as reactive oxygen species derived from the support and reactants.Highly active gold supported o...

Gold clusters on the surface of MgO powder (calcined at 673 K) were prepared from adsorbed [Au(CH 3 ) 2 (acac), where acac is C 5 H 7 O 2 ] and characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near edge spectroscopy (XANES). One sample initially contained gold predominantly in the form of clusters approximated as Au 6 on the basis of the EXAFS data showing first-and second-shell Au-Au coordination numbers of 4.0 ( 0.4 and 1.0 ( 0.1, respectively. The other sample initially contained larger clusters, with an average diameter of about 30 Å (containing about 100 atoms each, on average), as shown by the EXAFS first-and second-shell Au-Au coordination numbers of 9.4 ( 0.9 and 3.5 ( 0.4, respectively. The samples, in each of the three gases CO, O 2 , and He and in the presence of CO + O 2 during CO oxidation catalysis, were investigated by EXAFS spectroscopy and XANES in a cell that was also a flow reactor. Data obtained during steady-state CO oxidation indicate the presence of gold clusters with an average diameter of about 30 Å, regardless of the initial size of the supported clusters. The XANES results demonstrate the simultaneous presence of both zerovalent and cationic gold in these catalysts.