Transmission and fluorescence X-ray absorption spectroscopy cell/flow reactor for powder samples under vacuum or in reactive atmospheres.

X-ray absorption spectroscopy is an element-specific technique for probing the local atomic-scale environment around an absorber atom. It is widely used to investigate the structures of liquids and solids, being especially valuable for characterization of solid-supported catalysts. Reported cell designs are limited in capabilities-to fluorescence or transmission and to static or flowing atmospheres, or to vacuum. Our goal was to design a robust and widely applicable cell for catalyst characterizations under all these conditions-to allow tracking of changes during genesis and during operation, both under vacuum and in reactive atmospheres. Herein, we report the design of such a cell and a demonstration of its operation both with a sample under dynamic vacuum and in the presence of gases flowing at temperatures up to 300 °C, showing data obtained with both fluorescence and transmission detection. The cell allows more flexibility in catalyst characterization than any reported.

Zeolite-supported rhodium complexes and clusters: switching catalytic selectivity by controlling structures of essentially molecular species.

Precise synthesis and characterization of site-isolated rhodium complexes and extremely small rhodium clusters supported on zeolite HY allow control of the catalyst selectivity in the conversion of ethene to n-butene or ethane, respectively, as a result of tuning the structure of the active sites at a molecular level.

Adsorption and reaction of Rh(CO)2(acac) on Al2O3/Ni3Al(111).

The Al(2)O(3)/Ni(3)Al(111) surface has been used as a template for the nucleation and growth of rhodium clusters using an organometallic precursor: Rh(CO)(2)(acac). When Rh(CO)(2)(acac) is deposited on the Al(2)O(3)/Ni(3)Al(111) surface, the molecule is observed to bind preferentially to specific sites associated with the film superstructure (known as the dot structure) and appears to be stable at temperatures up to 473 K at which point some sintering and aggregation processes begin. Annealing the sample to 673 K results in further sintering of the metal deposits as well as an apparent loss in the coverage of rhodium species possibly due to a combination of desorption and deligation. After annealing to 873 K the coverage of rhodium species decreases by about 50% with respect to the initial deposited coverage. Our results suggest that using an organometallic precursor rather than metal atoms to form deposited metal particles on oxide substrates may result in increased resistance to sintering processes.

Validation and generalization of a method for precise size measurements of metal nanoclusters on supports.

We recently described a data analysis method for precise (approximately 0.1 A random error in the mean for a 200 kV instrument with a 3A FWHM probe size) size measurements of small clusters of heavy metal atoms on supports as imaged in a scanning transmission electron microscope, including an experimental demonstration using clusters that were primarily triosmium or decaosmium. The method is intended for low signal-to-noise ratio images of radiation-sensitive samples. We now present a detailed analysis, including a generalization to address issues of particle anisotropy and biased orientation distributions. In the future, this analysis should enable extraction of shape as well as size information, up to the noise-defined limit of information present in the image. We also present results from an extensive series of simulations designed to determine the method's range of applicability and expected performance in realistic situations. The simulations reproduce the experiments quite accurately, enabling a correction of systematic errors so that only the approximately 0.1A random error remains. The results are very stable over a wide range of parameters. We introduce a variation on the method with improved precision and stability relative to the original version, while also showing how simple diagnostics can test whether the results are reliable in any particular instance.

Catalysis by oxide-supported clusters of iridium and rhodium: hydrogenation of ethene, propene, and toluene.

The hydrogenation reactions of ethene, propene, and toluene were used as probes of the catalytic properties of small clusters of rhodium (Rh6) and of iridium (Ir4 and Ir6) (as well as of larger aggregates of these metals) on oxide supports (gamma-Al2O3, MgO, and La2O3). The catalysts were characterized in the working state by extended X-ray absorption fine structure (EXAFS) spectroscopy, providing evidence of the cluster structures and cluster-support interactions; by infrared spectroscopy, providing evidence of hydrocarbon adsorbates and possible reaction intermediates on the clusters; and by kinetics of the hydrogenation reactions. The EXAFS data indicate that the metal clusters, while remaining intact and maintaining their bonding to the support during catalysis, underwent slight rearrangements to accommodate reactive intermediates. As the concentrations of reactive intermediates such as pi-bonded alkenes and alkyls on the clusters increased, the cluster frames swelled, and the clusters flexed away from the support. The data indicate self-inhibition of reaction by adsorbed hydrocarbons and differences between ethene hydrogenation and propene hydrogenation that may arise primarily from different adsorbate-adsorbate interactions.

Nearly uniform MgO-supported pentaosmium cluster catalysts.

[Os5C(CO)14]2- was synthesized on the surface of MgO by reductive carbonylation of adsorbed Os3(CO)12 at 548 K and 1 bar. The supported species were characterized by infrared (IR), 13C NMR, and extended X-ray absorption fine structure (EXAFS) spectroscopies. The IR and EXAFS data are consistent with the presence of [Os5C(CO)14]2-, formed in a yield of about 65%, along with smaller osmium carbonyl clusters. As the supported clusters were decarbonylated in flowing He or H2, they were characterized by IR and EXAFS spectroscopies, which indicate that the decarbonylation was complete after each treatment at 573 K. The EXAFS data characterizing the sample treated in He determine an Os-Os first-shell coordination number of 3.4, matching that of [Os5C(CO)14]2- and indicating that the Os5C frame was retained after decarbonylation in He. Treatment of MgO-supported [Os5C(CO)14]2- in H2 at 573 K resulted in the formation of aggregated osmium clusters larger than Os5C. The catalytic activity of Os5C for toluene hydrogenation was found to be an order of magnitude less than that of the aggregated osmium clusters, which are metallic in character.

Observation of ligand effects during alkene hydrogenation catalysed by supported metal clusters.

Homogeneous organometallic catalysts and many enzymes activate reactants through coordination to metal atoms; that is, the reactants are turned into ligands and their reactivity controlled through other ligands in the metal's coordination sphere. In the case of supported metal clusters, catalytic performance is influenced by the support and by adsorbed reactants, intermediates or products. The adsorbates are usually treated as ligands, whereas the influence of the supports is usually ascribed to electronic interactions, even though metal clusters supported on oxides and zeolites form chemical bonds to support oxygen atoms. Here we report direct observations of the structure of supported metal clusters consisting of four iridium atoms, and the identification of hydrocarbon ligands bound to them during propene hydrogenation. We find that propene and molecular hydrogen form propylidyne and hydride ligands, respectively, whereas simultaneous exposure of the reactants to the supported iridium cluster yields ligands that are reactive intermediates during the catalytic propane-formation reaction. These intermediates weaken the bonding within the tetrahedral iridium cluster and the interactions between the cluster and the support, while replacement of the MgO support with gamma-Al2O3 boosts the catalytic activity tenfold, by affecting the bonding between the reactant-derived ligands and the cluster and therefore also the abundance of individual ligands. This interplay between the support and the reactant-derived ligands, whereby each influences the interaction of the metal cluster with the other, shows that the catalytic properties of supported metal catalysts can be tuned by careful choice of their supports.

Reactivity of site-isolated metal clusters: propylidyne on gamma-Al2O3-supported Ir4.

To contrast the reactivity of supported metal clusters with that of extended metal surfaces, we investigated the reactions of tetrairidium clusters supported on porous gamma-Al2O3 (Ir4/gamma-Al2O3) with propene and with H2. Infrared, 13C NMR, and extended X-ray absorption fine-structure spectroscopy were used to characterize the ligands formed on the clusters. Propene adsorption onto Ir4/gamma-Al2O3 at 298 K gave stable, cluster-bound mu3-propylidyne. Propene adsorbed onto Ir4/gamma-Al2O3 at 138 K reacted at approximately 219 K to form a stable, highly dehydrogenated, cluster-bound hydrocarbon species approximated as CxHy (such as, for example, C3H2 or C2H). H2 reacted with Ir4/gamma-Al2O3 at 298 K, forming ligands (likely hydrides), which prevented subsequent reaction of the clusters with propene to form propylidyne. Propylidyne on Ir4 was stable in helium or H2 as the sample was heated to 523 K, whereupon it reacted with oxygen of the support to give CO. Propylidyne on Ir4 did not undergo isotopic exchange in the presence of D2 at 298 K. In contrast, the literature shows that propylidyne chemisorbed on extended metal surfaces is hydrogenated in the presence of H2 (or D2) and exchanges hydrogen with gaseous D2 at room temperature; in the absence of H2, it decomposes thermally to give hydrocarbon fragments at temperatures much less than 523 K. The striking difference in reactivities of propylidyne on clusters and propylidyne on extended metal surfaces implies the requirement of ensembles of more than the three metal surface atoms bonded to propylidyne in the surface reactions. The results highlight the unique reactivity of small site-isolated metal clusters.