Georgios Kyriakou

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.

Sonogashira coupling on an extended gold surface in vacuo: reaction of phenylacetylene with iodobenzene on Au(111).

Temperature-programmed reaction measurements supported by scanning tunneling microscopy have shown that phenylacetylene and iodobenzene react on smooth Au(111) under vacuum conditions to yield biphenyl and diphenyldiacetylene, the result of homocoupling of the reactant molecules. They also produce diphenylacetylene, the result of Sonogashira cross-coupling, prototypical of a class of reactions that are of paramount importance in synthetic organic chemistry and whose mechanism remains controversial. Roughened Au(111) is completely inert toward all three reactions, indicating that the availability of crystallographically well-defined adsorption sites is crucially important. High-resolution X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy show that the reactants are initially present as intact, essentially flat-lying molecules and that the temperature threshold for Sonogashira coupling coincides with that for C-I bond scission in the iodobenzene reactant. The fractional-order kinetics and low temperature associated with desorption of the Sonogashira product suggest that the reaction occurs at the boundaries of islands of adsorbed reactants and that its appearance in the gas phase is rate-limited by the surface reaction. These findings demonstrate unambiguously and for the first time that this heterogeneous cross-coupling chemistry is an intrinsic property of extended, metallic pure gold surfaces: no other species, including solvent molecules, basic or charged (ionic) species are necessary to mediate the process.

Identity of the Active Site in Gold Nanoparticle-Catalyzed Sonogashira Coupling of Phenylacetylene and Iodobenzene

XPS, TEM, and reaction studies were used to examine the catalytic behavior of gold species deposited on lanthana toward the cross-coupling of phenylacetylene and iodobenzene. Atomically dispersed Au(I) and Au(III) were catalytically inert, whereas metallic Au(0) nanoparticles were both active and very selective. Thus it is metallic gold and not ionic gold that provides the catalytically active sites. Au(0) nanoparticles supported on silica, gamma-alumina, and BaO were active but relatively unselective; however, as with lanthana, ceria-supported Au(0) nanoparticles showed high selectivity. This strong promoting effect of the lanthanide oxide supports on Sonogashira selectivity cannot be accounted for in terms of acid/base, redox, or SMSI effects; it may be tentatively ascribed to metal --> support hydrogen spillover.

Sonogashira coupling catalyzed by gold nanoparticles: Does homogeneous or heterogeneous catalysis dominate?

A variety of measurements indicates that Au nanoparticle‐ catalyzed Sonogashira coupling of iodobenzene and phenylacetylene is predominantly a heterogeneous process. Large gold particles are much more selective than small ones, which is consistent with this view. Substantial leaching of Au into the solution phase occurs during the reaction, but the resulting supernatant liquid exhibits immeasurably low catalytic activity; TONs for the nanoparticles are orders of magnitude higher than those for the leached Au, once more pointing to the primacy of heterogeneous chemistry. These properties are independent of the support material, implying that they are intrinsic to metallic Au nanoparticles. Reaction data and quantitative analysis of the solid and solution phases by XPS and ICP‐MS, respectively, showed that catalytic activity ceased when all the metallic Au had dissolved. Conversely, when starting with a soluble Au complex, a long inactive induction phase is followed by the sharp onset of reaction and steadily increasing catalytic activity, consistent with the eventual nucleation and growth of gold particles. Again, the implication is that, for the nanoparticle‐catalyzed reaction, heterogeneous catalysis is by far the most important process.

Aspects of heterogeneous enantioselective catalysis by metals

Some aspects of metal-catalyzed heterogeneous enantioselective reactions are reviewed with specific reference to four different systems where the phenomena that control enantioselection appear to be very different. In the case of glucose electro-oxidation, it is clear that any intrinsic chirality present at the metal surface plays a vital role. With α-keto hydrogenation, achiral surfaces modified by the adsorption of chiral agents become effective enantioselective catalysts and the formation of extended arrays of chiral species appears not to be of importance: instead a 1:1 docking interaction controlled by hydrogen bonding between the adsorbed chiral modifier and the prochiral reactant determines the outcome. Hydrogen bonding also plays a central role in β-ketoester hydrogenation, but here fundamental studies indicate that the formation of ordered arrays involving the reactant and chiral ligand is of importance. Asymmetric C═C hydrogenation, though relatively little studied, has the potential for major impact in synthetic organic chemistry both on the laboratory scale and in the manufacture of fine chemicals and pharmaceuticals. The structural attributes that determine whether a given chiral ligand is effective have been identified; the ability to form strong covalent bonds with the metal surface while also resisting hydrogenation and displacement by the strongly adsorbing reactant under reaction conditions is an essential necessary condition. Beyond this, ligand rigidity in the vicinity of the chirality center coupled with resistance to SAM formation is a critically important factor whose absence results in racemic chemistry.

Molecular-Scale Perspective of Water-Catalyzed Methanol Dehydrogenation to Formaldehyde

Methanol steam reforming is a promising reaction for on-demand hydrogen production. Copper catalysts have excellent activity and selectivity for methanol conversion to hydrogen and carbon dioxide. This product balance is dictated by the formation and weak binding of formaldehyde, the key reaction intermediate. It is widely accepted that oxygen adatoms or oxidized copper are required to activate methanol. However, we show herein by studying a well-defined metallic copper surface that water alone is capable of catalyzing the conversion of methanol to formaldehyde. Our results indicate that six or more water molecules act in concert to deprotonate methanol to methoxy. Isolated palladium atoms in the copper surface further promote this reaction. This work reveals an unexpected role of water, which is typically considered a bystander in this key chemical transformation.

Heterogeneously Catalyzed Asymmetric Hydrogenation of C═C Bonds Directed by Surface-Tethered Chiral Modifiers

Asymmetric hydrogenation of C=C bonds is of the highest importance in organic synthesis, and such reactions are currently carried out with organometallic homogeneous catalysts. Achieving heterogeneous metal-catalyzed hydrogenation, a highly desirable goal, necessitates forcing the crucial enantiodifferentiating step to take place at the metal surface. By synthesis and application of six chiral sulfide ligands that anchor robustly to Pd nanoparticles and resist displacement, we have for the first time accomplished heterogeneous enantioselective catalytic hydrogenation of isophorone. High resolution XPS data established that ligand adsorption from solution occurred exclusively on the Pd nanoparticles and not on the carbon support. All ligands contained a pyrrolidine nitrogen to enable their interaction with the isophorone substrate while the sulfide functionality provided the required interaction with the Pd surface. Enantioselective turnover numbers of up to approximately 100 product molecules per ligand molecule were found with a very large variation in asymmetric induction between ligands: observed enantiomeric excesses increased with increasing size of the alkyl group in the sulfide. This likely reflects varying degrees of ligand dispersion on the surface: bulky substituent groups hinder close approach of ligand molecules to each other, inhibiting close-packed island formation, favoring dispersion as separate molecules, and leading to effective asymmetric induction. Conversely, small substituents favor island formation leading to very low asymmetric induction. Enantioselective reaction most likely involves initial formation of an enamine or iminium species, confirmed by use of an analogous tertiary amine, which leads to racemic product. Ligand rigidity and resistance to self-assembled monolayer formation are important attributes that should be designed into improved chiral modifiers.

Sonogashira cross-coupling and homocoupling on a silver surface:chlorobenzene and phenylacetylene on Ag(100)

Scanning tunneling microscopy, temperature-programmed reaction, near-edge X-ray absorption fine structure spectroscopy, and density functional theory calculations were used to study the adsorption and reactions of phenylacetylene and chlorobenzene on Ag(100). In the absence of solvent molecules and additives, these molecules underwent homocoupling and Sonogashira cross-coupling in an unambiguously heterogeneous mode. Of particular interest is the use of silver, previously unexplored, and chlorobenzene-normally regarded as relatively inert in such reactions. Both molecules adopt an essentially flat-lying conformation for which the observed and calculated adsorption energies are in reasonable agreement. Their magnitudes indicate that in both cases adsorption is predominantly due to dispersion forces for which interaction nevertheless leads to chemical activation and reaction. Both adsorbates exhibited pronounced island formation, thought to limit chemical activity under the conditions used and posited to occur at island boundaries, as was indeed observed in the case of phenylacetylene. The implications of these findings for the development of practical catalytic systems are considered.

Molecular-scale surface chemistry of a common metal nanoparticle capping agent: triphenylphosphine on Au(111).

Phosphine-stabilized Au clusters have been extensively studied and are used in various applications due to their unique structural, catalytic, and electronic properties. Triphenylphosphine (PPh(3)) is a key stabilizing ligand in the synthesis of Au nanoclusters. Despite its intense use in nanoparticle synthesis protocols, little is known regarding its surface chemistry, monolayer structure, density, and packing arrangement, all of which are important descriptors of functionality. Here, in contrast to sparse earlier investigations, we report that PPh(3) forms very ordered structures on Au(111). Atomic-scale imaging reveals that monolayer formation is accompanied by a partial lifting of the Au(111) surface reconstruction and ejection of extra Au atoms in the surface layer. Interestingly, these atoms are trapped and stabilized as two-dimensional Au nanoislands within the molecular layer. This behavior is in contrast to thiols, also common capping agents, which tend to remove Au atoms beyond those extra atoms present in the native reconstruction and form vacancy islands on the surface. Our data illustrate PPh(3)s milder reactivity and reveal a new picture of its packing structure. These results shed new light on the surface chemistry of this important ligand for organic, organometallic, and nanoparticle synthesis.

Quantum Tunneling Enabled Self-Assembly of Hydrogen Atoms on Cu(111)

Atomic and molecular self-assembly are key phenomena that underpin many important technologies. Typically, thermally enabled diffusion allows a system to sample many areas of configurational space, and ordered assemblies evolve that optimize interactions between species. Herein we describe a system in which the diffusion is quantum tunneling in nature and report the self-assembly of H atoms on a Cu(111) surface into complex arrays based on local clustering followed by larger scale islanding of these clusters. By scanning tunneling microscope tip-induced scrambling of H atom assemblies, we are able to watch the atomic scale details of H atom self-assembly in real time. The ordered arrangements we observe are complex and very different from those formed by H on other metals that occur in much simpler geometries. We contrast the diffusion and assembly of H with D, which has a much slower tunneling rate and is not able to form the large islands observed with H over equivalent time scales. Using density functional theory, we examine the interaction of H atoms on Cu(111) by calculating the differential binding energy as a function of H coverage. At the temperature of the experiments (5 K), H(D) diffusion by quantum tunneling dominates. The quantum-tunneling-enabled H and D diffusion is studied using a semiclassically corrected transition state theory coupled with density functional theory. This system constitutes the first example of quantum-tunneling-enabled self-assembly, while simultaneously demonstrating the complex ordering of H on Cu(111), a catalytically relevant surface.