Facile dissociation of reactants and weak binding of intermediates are key requirements for efficient and selective catalysis. However, these two variables are intimately linked in a way that does not generally allow the optimization of both properties simultaneously. By using desorption measurements in combination with high-resolution scanning tunneling microscopy, we show that individual, isolated Pd atoms in a Cu surface substantially lower the energy barrier to both hydrogen uptake on and subsequent desorption from the Cu metal surface. This facile hydrogen dissociation at Pd atom sites and weak binding to Cu allow for very selective hydrogenation of styrene and acetylene as compared with pure Cu or Pd metal alone.

We report a novel synthesis of nanoparticle Pd-Cu catalysts, containing only trace amounts of Pd, for selective hydrogenation reactions. Pd-Cu nanoparticles were designed based on model single atom alloy (SAA) surfaces, in which individual, isolated Pd atoms act as sites for hydrogen uptake, dissociation, and spillover onto the surrounding Cu surface. Pd-Cu nanoparticles were prepared by addition of trace amounts of Pd (0.18 atomic (at)%) to Cu nanoparticles supported on Al2O3 by galvanic replacement (GR). The catalytic performance of the resulting materials for the partial hydrogenation of phenylacetylene was investigated at ambient temperature in a batch reactor under a head pressure of hydrogen (6.9 bar). The bimetallic Pd-Cu nanoparticles have over an order of magnitude higher activity for phenylacetylene hydrogenation when compared to their monometallic Cu counterpart, while maintaining a high selectivity to styrene over many hours at high conversion. Greater than 94% selectivity to styrene is observed at all times, which is a marked improvement when compared to monometallic Pd catalysts with the same Pd loading, at the same total conversion. X-ray photoelectron spectroscopy and UV-visible spectroscopy measurements confirm the complete uptake and alloying of Pd with Cu by GR. Scanning tunneling microscopy and thermal desorption spectroscopy of model SAA surfaces confirmed the feasibility of hydrogen spillover onto an otherwise inert Cu surface. These model studies addressed a wide range of Pd concentrations related to the bimetallic nanoparticles.

Spillover of reactants from one active site to another is important in heterogeneous catalysis and has recently been shown to enhance hydrogen storage in a variety of materials [1][2][3][4][5][6][7] . The spillover of hydrogen is notoriously hard to detect or control 1,2,4-6 . We report herein that the hydrogen spillover pathway on a Pd/Cu alloy can be controlled by reversible adsorption of a spectator molecule. Pd atoms in the Cu surface serve as hydrogen dissociation sites from which H atoms can spillover onto surrounding Cu regions. Selective adsorption of CO at these atomic Pd sites is shown to either prevent the uptake of hydrogen on, or inhibit its desorption from, the surface. In this way, the hydrogen coverage on the whole surface can be controlled by molecular adsorption at a minority site, which we term a "molecular cork" effect. We show that the molecular cork effect is present during a surface catalyzed hydrogenation reaction and illustrate how it can be used as a method for controlling uptake and release of hydrogen in a model storage system 1,2,[4][5][6]8 .Hydrogen activation, uptake, and reaction are important phenomena in heterogeneous catalysis, fuel cells, hydrogen storage devices, materials processing and sensing [1][2][3][4][5][6][7][8][9][10][11] . Much attention has been devoted to materials that exhibit facile activation and weak binding of hydrogen, as these properties lead to the best energy landscape for storage or chemical reactivity [12][13][14] . Spillover is a common method by which a reagent can be activated at one location and then reacted at another, and it is commonly invoked to explain the synergistic relationship between metals in an alloy or metal/metal oxide mixtures 1,[3][4][5][6][7]12,15 . For example, in heterogeneous catalysis hydrogen spillover from metal particles to reducible oxide supports is implicated as an important step in a variety of reactions including hydrogenations, hydroisomerizations, and methanol synthesis 1,3,6 . Hydrogen spillover has also been shown to significantly enhance the performance of hydrogen storage materials such as metal organic frameworks, zeolites and many carbon-based nanostructures 2,4-6 . In these cases, the addition of small metal particles, typically Pt or Pd, promotes uptake by activating molecular H 2 and facilitating spillover of hydrogen atoms (H a ) onto the support. Despite these advances, the mechanism of spillover in most systems remains poorly understood, and with the exception of hydrogen bridges 16 in storage systems, methods for mediating the spillover pathway do not exist. In this paper we describe how the hydrogen spillover pathway on the Pd/Cu alloy system can be controlled via the reversible adsorption of a spectator molecule (CO) at minority Pd atom sites. The use of a model system amenable to study by scanning tunnelling microscopy (STM) was critical in order to monitor the detailed distribution of Pd atoms, H a and CO molecules, all of which are distributed heterogeneously at the atomic-scale. This information ca...

We report the activity of shape-controlled metal oxide (CeO(2), ZnO and Fe(3)O(4)) supported gold catalysts for the steam reforming of methanol (SRM) and the water gas shift (WGS) reactions. Metal oxide nanoshapes, prepared by controlled hydrolysis and thermolysis methods, expose different crystal surfaces, and consequently disperse and stabilize gold differently. We observe that similar to gold supported on CeO(2) shapes exposing the {110} and {111} surfaces, gold supported on the oxygen-rich ZnO {0001} and Fe(3)O(4) {111} surfaces shows higher activity for the SRM and WGS reactions. While the reaction rates vary among the Au-CeO(2), Au-ZnO and Au-Fe(3)O(4) shapes, the apparent activation energies are similar, indicating a common active site. TPR data further indicate that the reaction lightoff coincides with the activation of Au-O-M species on the surface of all three oxide supports evaluated here. Different shapes contain a different number of binding sites for the gold, imparting different overall activity.

The continuous-flow preparation of biodiesel using a commercially available scientific microwave apparatus offers a fast, easy route to this valuable biofuel. The methodology allows for the reaction to be run under atmospheric conditions and performed at flow rates of up to 7.2 L/min using a 4 L reaction vessel. It can be utilized with new or used vegetable oil with methanol and a 1:6 molar ratio of oil/alcohol. Energy consumption calculations suggest that the continuous-flow microwave methodology for the transesterification reaction is more energy-efficient than using a conventional heated apparatus.