Timothy J. Lawton

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.

Single atom alloy surface analogs in Pd0.18Cu15 nanoparticles for selective hydrogenation reactions

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.

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.

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.

The interplay of covalency, hydrogen bonding, and dispersion leads to a long range chiral network: The example of 2-butanol

The assembly of complex structures in nature is driven by an interplay between several intermolecular interactions, from strong covalent bonds to weaker dispersion forces. Understanding and ultimately controlling the self-assembly of materials requires extensive study of how these forces drive local nanoscale interactions and how larger structures evolve. Surface-based self-assembly is particularly amenable to modeling and measuring these interactions in well-defined systems. This study focuses on 2-butanol, the simplest aliphatic chiral alcohol. 2-butanol has recently been shown to have interesting properties as a chiral modifier of surface chemistry; however, its mode of action is not fully understood and a microscopic understanding of the role non-covalent interactions play in its adsorption and assembly on surfaces is lacking. In order to probe its surface properties, we employed high-resolution scanning tunneling microscopy and density functional theory (DFT) simulations. We found a surprisingly rich degree of enantiospecific adsorption, association, chiral cluster growth and ultimately long range, highly ordered chiral templating. Firstly, the chiral molecules acquire a second chiral center when adsorbed to the surface via dative bonding of one of the oxygen atom lone pairs. This interaction is controlled via the molecules intrinsic chiral center leading to monomers of like chirality, at both chiral centers, adsorbed on the surface. The monomers then associate into tetramers via a cyclical network of hydrogen bonds with an opposite chirality at the oxygen atom. The evolution of these square units is surprising given that the underlying surface has a hexagonal symmetry. Our DFT calculations, however, reveal that the tetramers are stable entities that are able to associate with each other by weaker van der Waals interactions and tessellate in an extended square network. This network of homochiral square pores grows to cover the whole Au(111) surface. Our data reveal that the chirality of a simple alcohol can be transferred to its surface binding geometry, drive the directionality of hydrogen-bonded networks and ultimately extended structure. Furthermore, this study provides the first microscopic insight into the surface properties of this important chiral modifier and provides a well-defined system for studying the networks enantioselective interaction with other molecules.

An Atomic Scale View of Methanol Reactivity at the Cu(1 1 1)/CuOx Interface

The interaction and reaction of MeOH with Cu is of foremost importance in a range of heterogeneously catalyzed processes. A critical intermediate in the molecular transformation of MeOH on Cu‐based catalysts is the methoxy species, the formation of which comprises the first elementary step in the MeOH steam reforming reaction. The interface length between metallic Cu(1 1 1) and copper oxide dictates the conversion of MeOH to formaldehyde (H2CO), although the microscopic details of MeOH’s adsorption, diffusion, and reaction at this complex interface are not clear. Here, STM was used to study the reaction of MeOH with the morphologically complex Cu(1 1 1)/copper oxide [Cu(1 1 1)/CuOx] surface. STM is the only technique that can simultaneously characterize various complex oxide surface structures and MeOH’s adsorption and reaction sites at the atomic scale. Variable‐temperature STM measurements enabled the spatial distribution of both intact and dissociated MeOH to be tracked as the reaction proceeded and revealed that diffusion and spillover are key processes in MeOH decomposition. The presence of extended metallic Cu areas interconnected with disordered oxide structures, coupled with the weak binding of MeOH on Cu, provides the driving force for MeOH to migrate to interfacial sites at which deprotonation occurs. At elevated temperatures, methoxy buildup at the Cu(1 1 1)/CuOx interface is observed along with evidence consistent with spillover. These data provide the first clear atomic scale picture of the interaction and reaction of MeOH with oxidized Cu(1 1 1), the most common facet of Cu in nanoparticle catalysts.

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.