Owain P. H. Vaughan

Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55-atom clusters.

Supported gold nanoparticles have excited much interest owing to their unusual and somewhat unexpected catalytic properties, but the origin of the catalytic activity is still not fully understood. Experimental work on gold particles supported on a titanium dioxide (110) single-crystal surface has established a striking size threshold effect associated with a metal-to-insulator transition, with gold particles catalytically active only if their diameters fall below ∼3.5 nm. However, the remarkable catalytic behaviour might also in part arise from strong electronic interaction between the gold and the titanium dioxide support. In the case of industrially important selective oxidation reactions, explanation of the effectiveness of gold nanoparticle catalysts is complicated by the need for additives to drive the reaction, and/or the presence of strong support interactions and incomplete understanding of their possible catalytic role. Here we show that very small gold entities (∼1.4 nm) derived from 55-atom gold clusters and supported on inert materials are efficient and robust catalysts for the selective oxidation of styrene by dioxygen. We find a sharp size threshold in catalytic activity, in that particles with diameters of ∼2 nm and above are completely inactive. Our observations suggest that catalytic activity arises from the altered electronic structure intrinsic to small gold nanoparticles, and that the use of 55-atom gold clusters may prove a viable route to the synthesis of robust gold catalysts suited to practical application.

Deprotection, Tethering, and Activation of a Catalytically Active Metalloporphyrin to a Chemically Active Metal Surface: [SAc](4)P-Mn(III)Cl on Ag(100)

The adsorption and subsequent thermal chemistry of the acetyl-protected manganese porphyrin, [SAc](4)P-Mn(III)Cl on Ag(100) have been studied by high resolution XPS and temperature-programmed desorption. The deprotection event, leading to formation of the covalently bound thioporphyrin, has been characterized and the conditions necessary for removal of the axial chlorine ligand have been determined, thus establishing a methodology for creating tethered activated species that could serve as catalytic sites for delicate oxidation reactions. Surface-mediated acetyl deprotection occurs at 298 K, at which temperature porphyrin diffusion is limited. At temperatures above approximately 425 K porphyrin desorption, diffusion and deprotection occur and at >470 K the axial chlorine is removed.

Dipole amplification: a principle for the self-assembly of asymmetric monomers on metal surfaces.

Fabrication of structures with nanometer-size dimensions is a major research theme, and scanning probe microscopes have played a pivotal role in the discovery, characterization and manipulation of such structures. Maneuvering of individual atoms, molecules or bonds—one at a time—by means of the STM tip has been used to create a variety of structures on solid surfaces. Potentially more useful, however, is the versatile and frequently explored approach which involves the spontaneous formation of an architecture by association of its molecular constituents as a result of noncovalent interactions—self-assembly. Inspired by solid-state and solution-based supramolecular chemistry, self-assembled surface nanostructures have been realized by exploiting noncovalent interactions between adsorbed molecules, thereby relying mainly on hydrogenbonding or metal–ligand interactions. However, the symmetry and electronic structure of the underlying substrate atomic lattice may also play a significant role, for example, leading to the formation of extended periodic structures associated with charge-density waves. Here, we report the discovery of self-assembled, localized nanostructures of styrene, a-methylstyrene (a-MS), and trans-methylstyrene (TMS) (Figure 1) on theAg(100) surface. There are no possibilities for hydrogen-bonding or metal–ligand-type interactions in these three cases.

Partial oxidations with NO2 catalyzed by large gold particles

Large gold particles catalyze alkene epoxidation by NO(2) under mild conditions, oxygen adatoms being the likely active species.

Adsorption geometry and core excitation spectra of three phenylpropene isomers on Cu(111)

Theoretical C 1s near edge x-ray absorption fine structure (NEXAFS) spectra for the C(9)H(10) isomers trans-methylstyrene, alpha-methylstyrene, and allylbenzene in gas phase and adsorbed at Cu(111) surfaces have been obtained from density functional theory calculations where adsorbate geometries were determined by corresponding total energy optimizations. The three species show characteristic differences in widths and peak shapes of the lowest C 1s-->pi(*) transitions which are explained by different coupling of the pi-electron system of the C(6) ring with that of the side chain in the molecules as well as by the existence of nonequivalent carbon centers. The adsorbed molecules bind only weakly with the substrate which makes the use of theoretical NEXAFS spectra of the oriented free molecules meaningful for an interpretation of experimental angle-resolved NEXAFS spectra of the adsorbate systems obtained in this work. However, a detailed quantitative account of relative peak intensities requires theoretical angle-resolved NEXAFS spectra of the complete adsorbate systems which have been evaluated within the surface cluster approach. The comparison with experiment yields almost perfect agreement and confirms the reliability of the calculated equilibrium geometries of the adsorbates. This can help to explain observed differences in the catalytic epoxidation of the three molecules on Cu(111) based on purely geometric considerations.

First observation of capping/uncapping by a ligand of a Zn porphyrin adsorbed on Ag(100)

A significant first step towards creation of catalytically active porphyrin-functionalised metal surfaces has been achieved.

Adsorbate conformation determines catalytic chemoselectivity: crotonaldehyde on the Pt(111) surface

Molecular orientation, which depends on surface coverage, determines whether or not catalytic hydrogenation is chemoselective.