Sungsik Lee

Increased Silver Activity for Direct Propylene Epoxidation via Subnanometer Size Effects

Silver Cluster Catalysts for Propylene Oxide The formation of ethylene oxide—in which an oxygen atom bridges the double bond of ethylene—can be made directly and efficiently from ethylene and oxygen with the aid of silver catalysts (typically comprising a small silver cluster on aluminum oxide). Similar approaches are not so successful for making propylene oxide—an important starting material for polyurethane plastics, which are made from chlorinated intermediates. Lei et al. (p. 224) report that silver trimers, Ag3, deposited on alumina are active for direct propylene oxide formation at low temperatures with only a low level of formation of CO2 by-product, unlike larger particles that form from these clusters at higher temperatures. Density functional calculations suggest that the open-shell nature of the clusters accounts for the improved reactivity. Clusters of three silver atoms deposited on alumina are active for the low-temperature direct formation of propylene oxide. Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag3 clusters and ~3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.

Catalytically active Au-O(OH)x- species stabilized by alkali ions on zeolites and mesoporous oxides

We report that the addition of alkali ions (sodium or potassium) to gold on KLTL-zeolite and mesoporous MCM-41 silica stabilizes mononuclear gold in Au-O(OH)x-(Na or K) ensembles. This single-site gold species is active for the low-temperature (<200°C) water-gas shift (WGS) reaction. Unexpectedly, gold is thus similar to platinum in creating –O linkages with more than eight alkali ions and establishing an active site on various supports. The intrinsic activity of the single-site gold species is the same on irreducible supports as on reducible ceria, iron oxide, and titania supports, apparently all sharing a common, similarly structured gold active site. This finding paves the way for using earth-abundant supports to disperse and stabilize precious metal atoms with alkali additives for the WGS and potentially other fuel-processing reactions. Alkali atoms help disperse catalytically active gold on high–surface-area alumina and silica supports. Dispersing catalytic gold as widely as possible In order to maximize the activity of precious metals in catalysis, it is important to place the metal on some support with a high surface area (such as a zeolite) and to maintain the metal as small clusters or even atoms to expose as much metal as possible. The latter goal is more readily achieved with oxides of reducible metals such as cerium or titanium than with the aluminum and silicon oxides that make up most zeolites and mesoporous oxides. Yang et al. show that sodium and potassium can stabilize gold along with hydroxyl and oxo groups to create highly active catalysts for the water-gas shift reaction at low temperatures, a reaction that can be useful in applications such as fuel cells. Science, this issue p. 1498

Selective propene epoxidation on immobilized au(6-10) clusters: the effect of hydrogen and water on activity and selectivity.

Epoxidation made easy: Subnanometer gold clusters immobilized on amorphous alumina result in a highly active and selective catalyst for propene epoxidation. The highest selectivity is found for gas mixtures involving oxygen and water, thus avoiding the use of hydrogen. Ab initio DFT calculations are used to identify key reaction intermediates and reaction pathways. The results confirm the high catalyst activity owing to the formation of propene oxide metallacycles. Al green, Au yellow, O red, and C gray.

Tackling CO Poisoning with Single-Atom Alloy Catalysts

Platinum catalysts are extensively used in the chemical industry and as electrocatalysts in fuel cells. Pt is notorious for its sensitivity to poisoning by strong CO adsorption. Here we demonstrate that the single-atom alloy (SAA) strategy applied to Pt reduces the binding strength of CO while maintaining catalytic performance. By using surface sensitive studies, we determined the binding strength of CO to different Pt ensembles, and this in turn guided the preparation of PtCu alloy nanoparticles (NPs). The atomic ratio Pt:Cu = 1:125 yielded a SAA which exhibited excellent CO tolerance in H2 activation, the key elementary step for hydrogenation and hydrogen electro-oxidation. As a probe reaction, the selective hydrogenation of acetylene to ethene was performed under flow conditions on the SAA NPs supported on alumina without activity loss in the presence of CO. The ability to maintain reactivity in the presence of CO is vital to other industrial reaction systems, such as hydrocarbon oxidation, electrochemical methanol oxidation, and hydrogen fuel cells.

Growth of Metal Oxide Nanowires from Supercooled Liquid Nanodroplets

Nanometer-sized liquid droplets formed at temperatures below the bulk melting point become supercooled as they grow through Ostwald ripening or coalescence and can be exploited to grow nanowires without any catalyst. We used this simple approach to synthesize a number of highly crystalline metal oxide nanowires in a chemical or physical vapor deposition apparatus. Examples of nanowires made in this way include VO(2), V(2)O(5), RuO(2), MoO(2), MoO(3), and Fe(3)O(4), some of which have not been previously reported. Direct evidence of this new mechanism of nanowire growth is found from in situ 2-dimensional GISAXS (grazing incidence small angle X-ray scattering) measurements of VO(2) nanowire growth, which provides quantitative information on the shapes and sizes of growing nanowires as well as direct evidence of the presence of supercooled liquid droplets. We observe dramatic changes in nanowire growth by varying the choice of substrate, reflecting the influence of wetting forces on the supercooled nanodroplet shape and mobility as well as substrate-nanowire lattice matching on the definition of nanowire orientation. Surfaces with defects can also be used to pattern the growth of the nanowires. The simplicity of this synthesis concept suggests it may be rather general in its application.

Combined temperature-programmed reaction and in situ x-ray scattering studies of size-selected silver clusters under realistic reaction conditions in the epoxidation of propene

The catalytic activity and dynamical shape changes in size-selected nanoclusters at work are studied under realistic reaction conditions by using a combination of simultaneous temperature-programmed reaction with in situ grazing-incidence small angle x-ray scattering. This approach allows drawing a direct correlation between nanocatalyst size, composition, shape, and its function under realistic reaction conditions for the first time. The approach is illustrated in a chemical industry highly relevant selective partial oxidation of propene on a monodisperse silver nanocatalyst. The shape of the catalyst undergoes rapid change already at room temperature upon the exposure to the reactants, followed by a complex evolution of shape with increasing temperature. Acrolein formation is observed around 50 degrees C while the formation of the propylene oxide exhibits a sharp onset at 80 degrees C and is leveling off at 150 degrees C. At lower temperatures acrolein is produced preferentially to propylene oxide; at temperatures above 100 degrees C propylene oxide is favored.

Adsorbate-induced structural changes in 1-3 nm platinum nanoparticles.

We investigated changes in the Pt-Pt bond distance, particle size, crystallinity, and coordination of Pt nanoparticles as a function of particle size (1-3 nm) and adsorbate (H2, CO) using synchrotron radiation pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) measurements. The ∼1 nm Pt nanoparticles showed a Pt-Pt bond distance contraction of ∼1.4%. The adsorption of H2 and CO at room temperature relaxed the Pt-Pt bond distance contraction to a value close to that of bulk fcc Pt. The adsorption of H2 improved the crystallinity of the small Pt nanoparticles. However, CO adsorption generated a more disordered fcc structure for the 1-3 nm Pt nanoparticles compared to the H2 adsorption Pt nanoparticles. In situ XANES measurements revealed that this disorder results from the electron back-donation of the Pt nanoparticles to CO, leading to a higher degree of rehybridization of the metal orbitals in the Pt-adsorbate system.

Oxidative dehydrogenation of cyclohexene on size selected subnanometer cobalt clusters: improved catalytic performance via evolution of cluster-assembled nanostructures.

The catalytic activity of oxide-supported metal nanoclusters strongly depends on their size and support. In this study, the origin of morphology transformation and chemical state changes during the oxidative dehydrogenation of cyclohexene was investigated in terms of metal-support interactions. Model catalyst systems were prepared by deposition of size selected subnanometer Co(27±4) clusters on various metal oxide supports (Al(2)O(3), ZnO and TiO(2) and MgO). The oxidation state and reactivity of the supported cobalt clusters were investigated by temperature programmed reaction (TPRx) and in situ grazing incidence X-ray absorption (GIXAS) during oxidative dehydrogenation of cyclohexene, while the sintering resistance monitored with grazing incidence small angle X-ray scattering (GISAXS). The activity and selectivity of cobalt clusters shows strong dependence on the support. GIXAS reveals that metal-support interaction plays a key role in the reaction. The most pronounced support effect is observed for MgO, where during the course of the reaction in its activity, composition and size dynamically evolving nanoassembly is formed from subnanometer cobalt clusters.

Combined TPRx, in situ GISAXS and GIXAS studies of model semiconductor-supported platinum catalysts in the hydrogenation of ethene

The preparation, characterization and catalytic reactivity of a GaN supported Pt catalyst in the hydrogenation of ethene are presented in this feature article, highlighting the use of in situ characterization of the material properties during sample handling and catalysis by combining temperature programmed reaction with in situ grazing incidence small-angle X-ray scattering and X-ray absorption spectroscopy. The catalysts are found to be sintering resistant at elevated temperatures as well as during reduction and hydrogenation reactions. In contrast to Pt particles of approximately 7 nm diameter, smaller particles of 1.8 nm in size are found to dynamically adapt their shape and oxidation state to the changes in the reaction environment. These smaller Pt particles also showed an initial deactivation in ethene hydrogenation, which is paralleled by the change in the particle shape. The subtle temperature-dependent X-ray absorbance of the 1.8 nm sized Pt particles indicates that subtle variations in the electronic structure induced by the state of reduction by electron tunnelling over the Schottky barrier between the Pt particles and the GaN support can be monitored.

Oxidation and reduction of size-selected subnanometer Pd clusters on Al2O3 surface.

In this paper, we investigate uniformly dispersed size-selected Pd(n) clusters (n = 4, 10, and 17) on alumina supports. We study the changes of clustered Pd atoms under oxidizing and reducing (O2 and CO, respectively) conditions in situ using ambient pressure XPS. The behavior of Pd in the clusters is quite different from that of Pd foil under the same conditions. For all Pd clusters, we observe only one Pd peak. The binding energy of this Pd 3d peak is ~1-1.4 eV higher than that of metallic Pd species and changes slightly in CO and O2 environments. On the Pd foil however many different Pd species co-exist on the surface and change their oxidation states under different conditions. We find that the Pd atoms in direct contact with Al2O3 differ in oxidation state from the surface Pd atoms in a foil under reaction conditions. Compared to previous literature, we find that Pd 3d peak positions are greatly influenced by the different types of Al2O3 supports due to the combination of both initial and final state effects.