Mintcho S. Tikhov

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.

The surface chemistry of acetic acid on Pd{111}

XPS, temperature-programmed reaction and HREELS have been used to study the adsorption and reactions of acetic acid on Pd{111}. At 170 K the adsorbed monolayer contains intact and dissociated acetic acid molecules, the latter consisting of a mixture of bidentate acetate and another species tentatively identified as monodentate acetate. The monodentate acetate appears to resemble closely the acetate species observed under reaction conditions at the surface of a pure palladium vinyl acetate synthesis catalyst. Thermal decomposition of the adsorbate yields CO2, H2O, CO, H2 and carbon. The associated processes may be rationalised in terms of two reaction channels, one due to the monodentate and the other due to the bidentate acetate.

Efficient visible light-active N-doped TiO2 photocatalysts by a reproducible and controllable synthetic route.

A reproducible and controllable method allows the synthesis of practical quantities of efficient, visible light active TiO(2)(N) photocatalysts in which the nitrogen content may be varied to achieve optimum performance.

Propene Epoxidation over K-Promoted Ag/CaCO3 Catalysts: The Effect of Metal Particle Size

With increasing loading of potassium promoter, Ag/CaCO3 catalysts exhibit a clear maximum in selectivity towards propene epoxide formation. This behavior correlates with changes in silver particle size distribution as revealed by HREM and XPS. Maximum selectivity and activity are achieved when the catalyst contains a large proportion of Ag particles whose size lies in the intermediate range 20-40 nm. Ag particles that are either much smaller or much larger than this are less selective towards epoxidation. The mechanistic implications of these findings are discussed and comparison is made with the corresponding properties of Ag/α-alumina catalysts normally used for ethene epoxidation.

Nucleation and growth of catalytically active Pd islands on Au(111)-22 × studied by scanning tunnelling microscopy

Abstract The nucleation and growth of Pd films on Au(111)-(22 × substrates from 0.07 ML coverage has been characterized using an ultra high vacuum scanning tunnelling microscope. Initially, polygonal islands nucleate and grow at sites near the surface edge dislocations in the elbows of the herringbone reconstruction. At low coverage, the herringbone reconstruction remains relatively undisturbed and most Pd islands are found on fcc regions of the Au(111) substrate. Increasing coverage leads to distortion of the underlying reconstruction which in turn affects the surface morphology of the Pd islands. Atomic resolution images of Pd island surfaces show that they are well ordered and close packed. Second-layer growth is also in the form of polygonal islands on the underlying layers. The morphological evolution of the system with increasing Pd coverage provides a good explanation for its catalytic behaviour.

Structural and electronic properties of Sn overlayers and PdSn surface alloys on Pd(111)

Abstract The first two layers of Sn deposited on Pd(111) at 300 K grow in layer-by-layer fashion after which crystallite formation commences. The electronic properties of these overlayers are dependent on the size of the 3D Sn islands. The occurrence of Sn→Pd valence charge-transfer is inferred, due allowance being made for initial and final state effects in the photoemission data. Evidence is presented for a significant Pd surface core-level shift enhancement by Sn of ≈0.7 eV. Depending on the initial Sn loading, heating generates stable monolayer (Pd2Sn) or multilayer (Pd3Sn) surface alloys exhibiting √3 and (2 × 2) periodicities, respectively. The very different CO adsorption capacity of these two phases indicates that on Pd Sn alloy surfaces, only pure Pd threefold hollow-sites are capable of strongly chemisorbing CO.

Chemoselective Catalytic Hydrogenation of Acrolein on Ag(111): Effect of Molecular Orientation on Reaction Selectivity

The adsorption and hydrogenation of acrolein on the Ag(111) surface has been investigated by high resolution synchrotron XPS, NEXAFS, and temperature programmed reaction. The molecule adsorbs intact at all coverages and its adsorption geometry is critically important in determining chemoselectivity toward the formation of allyl alcohol, the desired but thermodynamically disfavored product. In the absence of hydrogen adatoms (H(a)), acrolein lies almost parallel to the metal surface; high coverages force the C=C bond to tilt markedly, likely rendering it less vulnerable toward reaction with hydrogen adatoms. Reaction with coadsorbed H(a) yields allyl alcohol, propionaldehyde, and propanol, consistent with the behavior of practical dispersed Ag catalysts operated at atmospheric pressure: formation of all three hydrogenation products is surface reaction rate limited. Overall chemoselectivity is strongly influenced by secondary reactions of allyl alcohol. At low H(a) coverages, the C=C bond in the newly formed allyl alcohol molecule is strongly tilted with respect to the surface, rendering it immune to attack by H(a) and leading to desorption of the unsaturated alcohol. In contrast with this, at high H(a) coverages, the C=C bond in allyl alcohol lies almost parallel to the surface, undergoes hydrogenation by H(a), and the saturated alcohol (propanol) desorbs.

Electrochemical promotion of NO reduction by CO and by propene

Electrochemical promotion (EP) provides an efficacious means of catalyst promotion. The effects are reversible and the phenomenon provides a uniquely effective and controllable means for in situ tuning of the working catalytic system. EP studies of the catalytic chemistry of NO reduction by CO and by propene over Pt films supported on β″-alumina (a sodium ion conductor) demonstrate that major enhancements in activity are possible when Na is electrochemically pumped to the catalyst surface. Both reactions exhibit strong electrochemical promotion under appropiate conditions of temperature, gas composition and catalyst potential. The data indicate that Na increases the strength of NO chemisorption relative to CO or propene, a process that is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, thought to be the reaction initiating step. The overall kinetic behaviour and the selectivity towards N 2 formation on catalyst potential are in agreement with this hypothesis. XP spectroscopy data confirm that the mode of operation of the electrochemically promoted Pt film does indeed involve reversible pumping of Na to or from the solid electrolyte.

A versatile new method for synthesis and deposition of doped, visible light-activated TiO2 thin films

A flexible and widely applicable method allows the deposition of carbon-doped visible light-activated photocatalytic TiO2 thin films on a variety of substrates.

Electrochemical promotion of environmentally important catalytic reactions

The performance of conventional heterogeneous metal catalysts may be enhanced by the addition of so-called promoter species that are used to modify the intrinsic metal surface chemistry with respect to activity and/or selectivity. Electrochemical methods provide an alternative, radically different and uniquely efficacious method of catalyst promotion. Substantial and reversible changes in catalyst perfomance can be induced by back-spillover ions pumped from a solid electrolyte to the surface of a catalytically active electrode: one hasin situ control of the working catalyst.Studies of the electrochemical promotion of NO reduction over Pt films supported on β″-alumina (a sodium ion conductor) demonstrate that major enhancements in activity are possible when Na is pumped to the catalyst surface. We have examined the NO+CO reaction and the reaction of NO with propene. Both reactions are relevant to control of automotive and other emissions, and both exhibit strong electrochemical promotion. By simulating lean-burn engine conditions, we have also demonstrated that EP of a Pt catalyst very substantially enhances the ability of NO to oxidise propene in an oxygen-rich atmosphere. Reaction kinetic data obtained as a function of catalyst potential, temperature and gas composition indicate that Na increases the strength of NO chemisorption relative to CO or propene, a process that is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which is the critical reaction-initiating step. XP spectroscopy under the appropriate conditions of temperature and catalyst potential confirms that the mode of operation of the elctrochemically promoted Pt film does indeed involve reversible pumping of Na to or from the solid electrolyte.