Desorption trends of small alcohols and the disruption of intermolecular interactions at defect sites on Au(111).

Gold-based catalysts have received tremendous attention as supports and nanoparticles for heterogeneous catalysis, in part due to the ability of nanoscale Au to catalyze reactions at low temperatures in oxidative environments. Surface defects are known active sites for low temperature Au chemistry, so a full understanding of the interplay between intermolecular interactions and surface morphology is essential to an advanced understanding of catalytic behavior and efficiency. In a systematic study to better understand the adsorption and intermolecular behavior of small alcohols (C1-C4) on Au(111) defect sites, coverage studies of methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, and isobutanol have been conducted on Au(111) using ultrahigh vacuum temperature programmed desorption (UHV-TPD). These small alcohols molecularly adsorb on the Au(111) surface and high resolution experiments reveal distinct terrace, step edge, and kink adsorption features for each molecule. The hydrogen-bonded (H-bonded) networks of small alcohols on Au(111), except for 1-butanol and isobutanol, have been previously imaged on the molecular level at low temperatures by scanning tunneling microscopy. Primary C1-C3 alcohols exhibit planar H-bonded long extended zigzag chain networks while 2-butanol arranges in tetramer clusters of H-bonded molecules due to steric hindrance inhibiting the proximity of molecules on Au(111). Herein, the desorption energy of small primary alcohols was shown to trend linearly with increasing C1-C4 carbon chain length, indicating that the H-bonded molecular packing of 1-butanol resembles that of methanol, ethanol, and 1-propanol, while isobutanol and 2-butanol deviate from the trend. Butanol isomer studies allow the prediction of isobutanol long extended chains in contrast to tetramers. The distinction between the desorption of butanol isomers highlights the role of intermolecular interactions due to the difference in molecular packing structures on Au(111). Furthermore, by studying the energetics of terrace H-bonded networks in comparison with molecular adsorption at undercoordinated step edge and kink defect sites, it is shown that the contribution of stabilizing van der Waals forces to the overall adsorption energy is less for small alcohols adsorbed at kink sites (3.1 kJ mol-1 per CH2) and similar for those adsorbed at step edge (4.8 kJ mol-1 per CH2) and Au terrace sites (4.9 kJ mol-1 per CH2).

Chirality at two-dimensional surfaces: A perspective from small molecule alcohol assembly on Au(111).

The delicate balance between hydrogen bonding and van der Waals interactions determines the stability, structure, and chirality of many molecular and supramolecular aggregates weakly adsorbed on solid surfaces. Yet the inherent complexity of these systems makes their experimental study at the molecular level very challenging. In this quest, small alcohols adsorbed on metal surfaces have become a useful model system to gain fundamental insight into the interplay of such molecule-surface and molecule-molecule interactions. Here, through a combination of scanning tunneling microscopy and density functional theory, we compare and contrast the adsorption and self-assembly of a range of small alcohols from methanol to butanol on Au(111). We find that longer chained alcohols prefer to form zigzag chains held together by extended hydrogen bonded networks between adjacent molecules. When alcohols bind to a metal surface datively via one of the two lone electron pairs of the oxygen atom, they become chiral. Therefore, the chain structures are formed by a hydrogen-bonded network between adjacent molecules with alternating adsorbed chirality. These chain structures accommodate longer alkyl tails through larger unit cells, while the position of the hydroxyl group within the alcohol molecule can produce denser unit cells that maximize intermolecular interactions. Interestingly, when intrinsic chirality is introduced into the molecule as in the case of 2-butanol, the assembly changes completely and square packing structures with chiral pockets are observed. This is rationalized by the fact that the intrinsic chirality of the molecule directs the chirality of the adsorbed hydroxyl group meaning that heterochiral chain structures cannot form. Overall this study provides a general framework for understanding the effect of simple alcohol molecular adstructures on hydrogen bonded aggregates and paves the way for rationalizing 2D chiral supramolecular assembly.

Impact of branching on the supramolecular assembly of thioethers on Au(111).

Alkanethiolate monolayers are one of the most comprehensively studied self-assembled systems due to their ease of preparation, their ability to be functionalized, and the opportunity to control their thickness perpendicular to the surface. However, these systems suffer from degradation due to oxidation and defects caused by surface etching and adsorbate rotational boundaries. Thioethers offer a potential alternative to thiols that overcome some of these issues and allow dimensional control of self-assembly parallel to the surface. Thioethers have found uses in surface modification of nanoparticles, and chiral thioethers tethered to catalytically active surfaces have been shown to enable enantioselective hydrogenation. However, the effect of structural, chemical, and chiral modifications of the alkyl chains of thioethers on their self-assembly has remained largely unstudied. To elucidate how molecular structure, particularly alkyl branching and chirality, affects molecular self-assembly, we compare four related thioethers, including two pairs of structural isomers. The self-assembly of structural isomers N-butyl methyl sulfide and tert-butyl methyl sulfide was studied with high resolution scanning tunneling microscopy (STM); our results indicate that both molecules form highly ordered arrays despite the bulky tert-butyl group. We also investigated the effect of intrinsic chirality in the alkyl tails on the adsorption and self-assembly of butyl sec-butyl sulfide (BSBS) with STM and density functional theory and contrast our results to its structural isomer, dibutyl sulfide. Calculations provide the relative stability of the four stereoisomers of BSBS and STM imaging reveals two prominent monomer forms. Interestingly, the racemic mixture of BSBS is the only thioether we have examined to date that does not form highly ordered arrays; we postulate that this is due to weak enantiospecific intermolecular interactions that lead to the formation of energetically similar but structurally different assemblies. Furthermore, we studied all of the molecules in their monomeric molecular rotor form, and the surface-adsorbed chirality of the three asymmetric thioethers is distinguishable in STM images.

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

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.

Structure and energetics of hydrogen-bonded networks of methanol on close packed transition metal surfaces.

Methanol is a versatile chemical feedstock, fuel source, and energy storage material. Many reactions involving methanol are catalyzed by transition metal surfaces, on which hydrogen-bonded methanol overlayers form. As with water, the structure of these overlayers is expected to depend on a delicate balance of hydrogen bonding and adsorbate-substrate bonding. In contrast to water, however, relatively little is known about the structures methanol overlayers form and how these vary from one substrate to another. To address this issue, herein we analyze the hydrogen bonded networks that methanol forms as a function of coverage on three catalytically important surfaces, Au(111), Cu(111), and Pt(111), using a combination of scanning tunneling microscopy and density functional theory. We investigate the effect of intermolecular interactions, surface coverage, and adsorption energies on molecular assembly and compare the results to more widely studied water networks on the same surfaces. Two main factors are shown to direct the structure of methanol on the surfaces studied: the surface coverage and the competition between the methanol-methanol and methanol-surface interactions. Additionally, we report a new chiral form of buckled hexamer formed by surface bound methanol that maximizes the interactions between methanol monomers by sacrificing interactions with the surface. These results serve as a direct comparison of interaction strength, assembly, and chirality of methanol networks on Au(111), Cu(111), and Pt(111) which are catalytically relevant for methanol oxidation, steam reforming, and direct methanol fuel cells.

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.

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 catalyst's 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.

Probing adsorption sites for CO on ceria.

Ceria based catalysts show remarkable activity for CO conversion reactions such as CO oxidation and the water-gas shift reaction. The identification of adsorption sites on the catalyst surfaces is essential to understand the reaction mechanisms of these reactions, but the complexity of heterogeneous powder catalysts and the propensity of ceria to easily change oxidation states in the presence of small concentrations of either oxidizing or reducing agents make the process difficult. In this study, the adsorption of CO on CuOx/Cu(111) and CeOx/Cu(111) systems has been studied using infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. IR peaks for the adsorbed CO on O/Cu(111) with only chemisorbed oxygen, well-ordered Cu2O/Cu(111) and disordered copper oxide [CuOx/Cu(111)] were observed at 2070-2072, 2097-2098 and 2101-2111 cm(-1), respectively. On CeOx/Cu(111) systems CO chemisorbs at 90 K only on Cu sites under ultra-high vacuum (UHV) conditions, whereas at elevated CO pressures and low temperatures adsorption of CO on Ce(3+) is observed, with a corresponding IR peak at 2162 cm(-1). These experimental results are further supported by DFT calculations, and help to unequivocally distinguish the presence of Ce(3+) cations on catalyst samples by using CO as a probe molecule.

Assisted deprotonation of formic acid on Cu(111) and self-assembly of 1D chains.

Formic acid (HCOOH) deprotonates on the open surfaces of Cu(110) and Cu(100) when exposed at 300 K. However, this does not occur on the close-packed surface of clean Cu(111). In this study, we show that the deprotonation of formic acid on atomically flat Cu(111) surfaces can be induced by pre-adsorbing polymeric formic acid clusters at low temperatures, and then annealing the system to break the acidic O-H bond of HCOOH adsorbed on the edges of the polymeric clusters. The thermal activation of HCOOH to bidentate formate was studied using a combination of infrared reflection absorption spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy, and near edge X-ray absorption fine structure spectroscopy. Extended 1D formate structures self-assemble due to a templating effect introduced by the formation of long α-polymeric formic acid chains commensurate with the substrate.

Viewing and inducing symmetry breaking at the single-molecule limit.

Symmetry breaking by photons, electrons, and molecular interactions lies at the heart of many important problems as varied as the origin of homochiral life to enantioselective drug production. Herein we report a system in which symmetry breaking can be induced and measured in situ at the single-molecule level using scanning tunneling microscopy. We demonstrate that electrical excitation of a prochiral molecule on an achiral surface produces large enantiomeric excesses in the chiral adsorbed state of up to 39%. The degree of symmetry breaking was monitored as a function of scanning probe tip state, and the results revealed that enantiomeric excesses are correlated with the intrinsic chirality in scanning probe tips themselves, as evidenced by height differences between single molecule enantiomers. While this work has consequences for the study of two-dimensional chirality, more importantly, it offers a new method for interrogating the coupling of photons, electrons, and combinations of physical fields to achiral starting systems in a reproducible manner. This will allow the mechanism of chirality transfer to be studied in a system in which enantiomeric excesses are quantified accurately by counting individual molecules.

Hydrogen-bonded assembly of methanol on Cu(111).

Investigation of methanol's surface chemistry on metals is a crucial step towards understanding the reactivity of this important chemical feedstock. Cu is a relevant metal for methanol synthesis and reforming, but due to the weak interaction of methanol with Cu, an atomic scale view of methanol's coverage-dependent ordering and self-assembly on Cu(111), the most abundant facet of most nanoparticles, has not yet been possible. Low and variable temperature scanning tunneling microscopy coupled with density functional theory reveal a coverage-dependent range of highly ordered structures stabilized by two hydrogen bonds per molecule. While extended chains that resemble the hydrogen-bonded zigzag structures reported for solid methanol are an efficient way to pack methanol at higher coverages, lower surface coverages yield isolated hexamer units. These hexamers form the same number of hydrogen bonds as the chains but appear to repel one another on the surface. Annealing treatments lead to the desorption of methanol with almost no decomposition. This data serves as a useful guide to both the preferred adsorption geometries and energies of a variety of methanol structures on Cu(111) surfaces as a function of surface coverage.

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

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.

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.

Visualization of hydrogen bonding and associated chirality in methanol hexamers.

Using a combination of scanning tunneling microscopy (STM) and density functional theory the hydrogen bond directionality and associated chirality of enantiopure clusters is visualized and controlled. This is demonstrated with methanol hexamers adsorbed on Au(111), which depending on their chirality, adopt two distinct molecular footprints on the surface. Controlled STM tip manipulations were used to interconvert the chirality of entire clusters and to break up metastable chain structures into hexamers.

Dynamics of molecular adsorption and rotation on nonequilibrium sites.

It is generally accepted that important events on surfaces such as diffusion and reactions can be adsorption site dependent. However, due to their short lifetime and low concentration in most systems, adsorbates on nonequilibrium adsorption sites remain largely understudied. Using low-temperature scanning tunneling microscopy, site-dependent adsorption is shown for the molecule butyl methyl sulfide, which is trapped in multiple metastable adsorption sites upon deposition onto a Au(111) surface at 5 K. As this molecule does not have enough energy to diffuse to its preferred adsorption site on the surface, it is possible to study the behavior of individual molecules in a variety of nonequilibrium sites. Here we present atomic-scale data of the same chemical species in three independent, metastable adsorption sites and equilibration to a single equilibrium site as a function of either electrical or thermal excitation. Butyl methyl sulfide exhibits distinctly different physical properties at all four adsorption sites, including rotational dynamics and appearance in scanning tunneling microscopy (STM) images. An energy profile is proposed for the adsorption and equilibration of these species, and a correlation is drawn between rotational barrier and adsorption energy.

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.

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 rotor's 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 rotor's alkyl tail is an efficient channel with which to excite rotation, and that the excitation is a one-electron process.

Hydrogen dissociation and spillover on individual isolated palladium atoms.

Using a combination of low-temperature scanning tunneling microscopy and density functional theory it is demonstrated how the nature of an inert host metal of an alloy can affect the thermodynamics and kinetics of a reaction pathway in a much more profound way than simply a dilution, electronic, or geometric effect. This study reveals that individual, isolated Pd atoms can promote H2 dissociation and spillover onto a Cu(111) surface, but that the same mechanism is not observed for an identical array of Pd atoms in Au(111).