Ashleigh E. Baber

Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations.

Tuning Hydrogen Adsorption Heterogeneous metal catalysts for hydrogenating unsaturated organic compounds need to bind molecular hydrogen strongly enough that it dissociates and forms adsorbed hydrogen atoms, but must not bind these atoms too strongly, or the transfer to the organic molecule will be impeded. Kyriakou et al. (p. 1209) examined surface alloy catalysts created when palladium (Pd) atoms are adsorbed on a copper (Cu) surface using scanning tunneling microscopy and desorption techniques under ultrahigh vacuum conditions. The Pd atoms could bind hydrogen dissociatively—which, under these conditions, the Cu surfaces could not—allowing the Cu surface to take up adsorbed hydrogen atoms. These weakly bound hydrogen atoms were able to selectively hydrogenate styrene and acetylene. Palladium atoms adsorbed on a copper surface activate hydrogen adsorption for subsequent hydrogenation reactions. 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.

Highly active copper-ceria and copper-ceria-titania catalysts for methanol synthesis from CO2

Converting CO2 into methanol by catalysis By hydrogenating CO2, scientists can transform a greenhouse gas into methanol, a desirable fuel. Graciani et al. cast copper in the role of the highly active catalyst for this reaction by putting copper particles on cerium oxide. The interface between the cerium oxide and the copper enables the reverse water-gas shift reaction that converts CO2 into CO, which reacts more readily with hydrogen to make methanol. This result takes a step forward in innovating catalysts for this environmentally friendly process. Science, this issue p. 546 Synergy at a metal-oxide interface generates highly active catalysts for carbon dioxide hydrogenation to methanol. The transformation of CO2 into alcohols or other hydrocarbon compounds is challenging because of the difficulties associated with the chemical activation of CO2 by heterogeneous catalysts. Pure metals and bimetallic systems used for this task usually have low catalytic activity. Here we present experimental and theoretical evidence for a completely different type of site for CO2 activation: a copper-ceria interface that is highly efficient for the synthesis of methanol. The combination of metal and oxide sites in the copper-ceria interface affords complementary chemical properties that lead to special reaction pathways for the CO2→CH3OH conversion.

Importance of the Metal–Oxide Interface in Catalysis: In Situ Studies of the Water–Gas Shift Reaction by Ambient-Pressure X-ray Photoelectron Spectroscopy†

The traditional approach to the optimization of metal/oxide catalysts has focused on the properties of the metal and the selection of the proper oxide for its dispersion. The importance of metal–oxide interfaces has long been recognized, [1] but the molecular determination of their properties and role is only now emerging. [2] Atoms with properties ranging from metallic to ionic are available at the interface and create unique reaction sites. We show herein how sites associated with a metal–ceria interface can dramatically change the reaction mechanism of the water–gas shift reaction (WGSR; CO + H2O!H2 + CO2). The WGSR is critical in the production of hydrogen. Multiple reaction mechanisms have been proposed. [3] In the redox mechanism, CO reacts with oxygen derived from the dissociation of H2O. In the associative process, the formation of a carbonaceous COxHy intermediate must precede the production of H2 and CO2. In situ studies are essential for the detection of surface species and active phases only present under the reaction conditions. [4] We present a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAP XPS), infrared reflection absorption spectroscopy (IRRAS), and density functional theory (DFT) calculations used to study the WGSR on CeOx nanoparticles deposited on Cu(111) and Au(111). Under WGSR conditions, adsorbed bent carboxylate (CO2 d� ) species were identified over both CeOx/Cu(111) and CeOx/ Au(111), with the ceria in a highly reduced state. By combining in situ experimental results with calculations, we

Experimental demonstration of a single-molecule electric motor

For molecules to be used as components in molecular machines, methods that couple individual molecules to external energy sources and that selectively excite motion in a given direction are required. Significant progress has been made in the construction of molecular motors powered by light and by chemical reactions, but electrically driven motors have not yet been built, despite several theoretical proposals for such motors. Here we report that a butyl methyl sulphide molecule adsorbed on a copper surface can be operated as a single-molecule electric motor. Electrons from a scanning tunnelling microscope are used to drive the directional motion of the molecule in a two-terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular scale in real time. The direction and rate of the rotation are related to the chiralities of both the molecule and the tip of the microscope (which serves as the electrode), illustrating the importance of the symmetry of the metal contacts in atomic-scale electrical devices.

A Quantitative Single-Molecule Study of Thioether Molecular Rotors

This paper describes a fundamental, single-molecule study of the motion of a set of thioethers supported on Au surfaces. Thioethers constitute a simple, robust system with which molecular rotation can be actuated both thermally and mechanically. Low-temperature scanning tunneling microscopy allowed the measurement of the rotation of individual molecules as a function of temperature and the quantification of both the energetic barrier and pre-exponential factor of the motion. The results suggest that movement of the second CH(2) group from the S atom over the surface is responsible for the barrier. Through a series of single-molecule manipulation experiments, we have switched the rotation on and off reversibly by moving the molecules toward or away from one another. Arrhenius plots for individual dibutyl sulfide molecules reveal that the torsional barrier to rotation is approximately 1.2 kJ/mol, in good agreement with the temperature at which the molecules appearance changes from a linear to a hexagonal shape in the STM images. The thioether backbone constitutes an excellent test bed for studying the details of molecular rotation at the single-molecule level.

Atomic-Scale Geometry and Electronic Structure of Catalytically Important Pd/Au Alloys

Pd/Au bimetallic alloys catalyze many important reactions ranging from the synthesis of vinyl acetate and hydrogen peroxide to the oxidation of carbon monoxide and trimerization of acetylene. It is known that the atomic-scale geometry of these alloys can dramatically affect both their reactivity and selectivity. However, there is a distinct lack of experimental characterization and quantification of ligand and ensemble effects in this system. Low-temperature, ultrahigh vacuum scanning tunneling microscopy is used to investigate the atomic-scale geometry of Pd/Au111 near-surface alloys and to spectroscopically probe their local electronic structure. The results reveal that the herringbone reconstruction of Au111 provides entry sites for the incorporation of Pd atoms in the Au surface and that the degree of mixing is dictated by the surface temperature. At catalytically relevant temperatures the distribution of low coverages of Pd in the alloy is random, except for a lack of nearest neighbor pairs in both the surface and subsurface sites. Scanning tunneling spectroscopy is used to examine the electronic structure of the individual Pd atoms in surface and subsurface sites. This work reveals that in both surface and subsurface locations, Pd atoms display a very similar electronic structure to the surrounding Au atoms. However, individual surface and subsurface Pd atoms are depleted of charge in a very narrow region at the band edge of the Au surface state. dI/dV images of the phenomena demonstrate the spatial extent of this electronic perturbation.

In Situ Imaging of Cu2O under Reducing Conditions: Formation of Metallic Fronts by Mass Transfer

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of in situ microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalysts chemical state and morphology are dynamically modified. The reduction of Cu2O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution in situ imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces.

An Atomic‐Scale View of Palladium Alloys and their Ability to Dissociate Molecular Hydrogen

Palladium and its alloys play a central role in a wide variety of industrially important applications such as hydrogen reactions, separations, storage devices, and fuel‐cell components. Alloy compositions are complex and often heterogeneous at the atomic‐scale and the exact mechanisms by which many of these processes operate have yet to be discovered. Herein, scanning tunneling microscopy (STM) has been used to investigate the atomic‐scale structure of Pd–Au and Pd–Cu bimetallics created by depositing Pd on both Au(111) and Cu(111) single crystals at a variety of surface temperatures. We demonstrated that individual, isolated Pd atoms in an inert Cu matrix are active for the dissociation of hydrogen and subsequent spillover onto Cu sites. Our results indicated that H spillover was facile on Pd–Cu at 420 K but that no H was found under the same H2 flux on a Pd–Au sample with identical atomic composition and geometry. In the case of Au, significant H uptake was only observed when larger ensembles of Pd were present in the form of nanoparticles. We report experimental evidence for hydrogen’s ability to reverse the tendency of Pd to segregate into the Au surface at catalytically relevant temperatures and our STM images reveal a novel H‐induced striped structure in which Pd atoms aggregated on top of the surface in regularly spaced rows. These results demonstrate the powerful influence an inert substrate has on the catalytic activity of Pd atoms supported in or on its surface and reveal how the atomic‐scale geometry of Pd–Au alloys is greatly affected by the presence of hydrogen.

Stabilization of Catalytically Active Cu+ Surface Sites on Titanium–Copper Mixed‐Oxide Films

The oxidation of CO is the archetypal heterogeneous catalytic reaction and plays a central role in the advancement of fundamental studies, the control of automobile emissions, and industrial oxidation reactions. Copper-based catalysts were the first catalysts that were reported to enable the oxidation of CO at room temperature, but a lack of stability at the elevated reaction temperatures that are used in automobile catalytic converters, in particular the loss of the most reactive Cu(+) cations, leads to their deactivation. Using a combined experimental and theoretical approach, it is shown how the incorporation of titanium cations in a Cu2O film leads to the formation of a stable mixed-metal oxide with a Cu(+) terminated surface that is highly active for CO oxidation.

Time-resolved studies of individual molecular rotors.

Thioether molecular rotors show great promise as nanoscale models for exploring the fundamental limits of thermally and electrically driven molecular rotation. By using time-resolved measurements which increase the time resolution of the scanning tunneling microscope we were able to record the dynamics of individual thioether molecular rotors as a function of surface structure, rotor chemistry, thermal energy and electrical excitation. Our results demonstrate that the local surface structure can have a dramatic influence on the energy landscape that the molecular rotors experience. In terms of rotor structure, altering the length of the rotors alkyl tails allowed the origin of the barrier to rotation to be more fully understood. Finally, time-resolved measurement of electrically excited rotation revealed that vibrational excitation of a C-H bond in the rotors alkyl tail is an efficient channel with which to excite rotation, and that the excitation is a one-electron process.