Richard M. Lambert

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.

A single crystal study of the silver-catalysed selective oxidation and total oxidation of ethylene

Abstract Ethylene oxidation has been investigated on a well-characterised Ag(111) single crystal surface at pressures of up to 50 Torr. In the absence of promoters and moderators, chemisorbed atomic oxygen reacts with adsorbed ethylene to yield both ethylene oxide and (CO 2 + H 2 O). Chemisorbed dioxygen, though present, appears to play no direct role in either of these reactions; the presence of subsurface oxygen is necessary for selective oxidation but not for total oxidation. Batch reactor studies yield rate parameters for both partial and total oxidation which are consistent with the values reported for conventional supported catalysts; selectivity decreases with increasing temperature, pressure, and ethylene coverage. Acetaldehyde, acetic acid, and oxalic acid are identified as reaction intermediates in the pathway to CO 2 formation. Results for the oxidation of C 2 D 4 confirm these observations, and the observed kinetic isotope effect indicates that H-transfer rather than CC cleavage is rate-determining in the combustion of both ethylene and ethylene oxide. Possible reaction pathways and mechanisms are examined.

Chemisorption and Reactivity on Supported Clusters and Thin Films

Preface. Introduction to Heterogeneous Catalysis R.M. Lambert. Thin Films as Model Catalysts D.R. Rainer, D.W. Goodman. Metal Deposits on Thin Well Ordered Oxide Films: Morphology, Adsorption and Reactivity M. Baumer, et al. The Growth and Stability of Ultrathin Films on Metal and Oxide Surfaces T.E. Madey. Size Effects in Heterogeneous Catalysis: A Surface Science Approach C. Henry, et al. Supported Clusters, Structure, Reactivity and Microscopic Processes in Catalysis M. Ichikawa. Quantitative Determination of Molecular Adsorbate Structures D.P. Woodruff. The Structure and Reactivity of TiO2(110) Supported Palladium and Rhodium B.E. Hayden. Angle-Scanned Photoelectron Diffraction: A Structural Probe for Near-Surface: Atomic Layers G. Granozzi, M. Sambi. Co-Adsorption on Metal-Oxide Crystal Surfaces: Cases of CO/Cu/ZnO(0001) and CO2/Na/TiO2(110) P.J. Moller. Theory of Adsorption and Surface Reactions B. Hammer, J.K. Norskov. Density Functional Cluster Calculations on Metal Deposition at Oxide Surfaces N. Rosch, G. Pacchioni. Theory of Heterogeneous Catalytic Reactivity Using the Cluster Approximation R.A. van Santen. Cluster Modelling of Oxide Surfaces: Structure, Adsorption and Reactivity G. Pacchioni. Theoretical Modelling of Chemisorption and Reactions on Metal-Oxide Surfaces L.G.M. Pettersson, et al. Stability of Polar Oxide Surfaces: Oxygen Vacancies and Non-Stoichiometric Reconstructions C. Noguera, et al. Computer Simulation of Structural, Defect and Surface Properties of Solids C.R.A. Catlow, et al. Index.

Adsorption-desorption properties and surface structural chemistry of chlorine on Cu(111) and Ag(111)

Abstract The adsorption, desorption, and structural properties of chlorine adlayers on Cu(111) and Ag(111) have been studied by LEED, Auger, Δϑ, and thermal desorption measurements. Ancillary experiments were also carried out on cuprous chloride for purposes of comparison with the Cu(111)-Cl data. Chlorine adsorption is rapid on both metals and follows precursor kinetics, the absolute initial sticking probabilities being ~1.0 (Cu) and ~0.5 (Ag). Δϑ results suggest that significant depolarisation of the chemisorption bond occurs at high coverages, the maximum values being + 1.2 eV (Cu) and + 1.8 eV (Ag). On Cu(111), adsorption leads to the formation of a sequence of well-ordered phases; in order of increasing coverage, these are as follows: (√3 × √3)R30°, (12√3 × 12√3)R30°, (4√7 × 4√7)R19.2°, and (6√3 × 6√3)R30°. On Ag(111) (√3 × √3)R30°, and (10 × 10) structures are observed. All six structures are susceptible to a straightforward interpretation in terms of coincidence lattices resulting from the progressive uniform compression of a hexagonal layer of Cl atoms. This interpretation is consistent with all the experimental results, and gives values for the nearest-neighbour ClCl spacing on both Cu(111) and Ag(111) which are in good agreement with other work on other surfaces. Chlorine desorbs exclusively as atoms from both metals with first-order desorption kinetics, and apparent desorption energies of 236 (Cu) and 209 (Ag) kJ mol −1 . These values, which depend on an assumed pre-exponential factor of 10 13 s −1 , are shown to be inconsistent with the thermochemical constraints on the system necessitated by the complete absence of Cl 2 desorption. Lower limits for the pre-exponential factors are then deduced, and the values are found to be consistent with the differences between the CuCl and AgCl systems.

Basic studies of the oxygen surface chemistry of silver: Chemisorbed atomic and molecular species on pure Ag(111)

Abstract The interaction of oxygen with Ag(111) has been studied over the pressure range 10 −2 −1.0 Torr. Thermal desorption measurements using isotopically labelled molecules unambiguously establish the presence of a stable chemisorbed dioxygen species which co-exists with adsorbed atomic oxygen. Dissolved oxygen undergoes exchange with the latter species but not with the former. The maximum dioxygen population is found to be markedly sensitive to gas dosing pressure; a model is proposed which accounts for these observations and for related observations on alkali-doped Ag. XP and UP spectral features can be correlated with the two types of oxygen species; angle-resolved XP and Auger spectra indicate that O 2 (a) resides on the metal surface whereas O(a) is located within the surface. The XP spectra also suggest that in the case of O 2 (a) the molecular axis may lie perpendicular to the surface.

Chemisorption studies on cobalt single crystal surfaces: I. Carbon monoxide on Co(0001)

Abstract The chemisorption of CO on Co(0001) and on a polycrystalline specimen has been studied by LEED, Auger spectroscopy, and thermal desorption measurements. Annealing of the polycrystal was found to result in a surface dominated by crystallites of (0001) orientation in the surface plane, along with a few (1012) oriented crystallites. CO adsorbs on the clean surface at 300 K with an initial sticking probability of 0.9 and the system follows precursor state kinetics. The saturation coverage under UHV conditions corresponds to a well-ordered (√3 × √3)R30° structure; with PCO>5 × 10-9 a uniform compression of the adlayer takes place and a (√7 × √7)R19.2° structure begins to form. Models are proposed for these two ordered phases which are in agreement with the observed relative coverage data and the appearance of the corresponding desorption spectra. The desorption enthalpy of CO at low coverages is 103 ± 8 kJmol-1, and a fairly sharp fall in this enthalpy occurs for coverages > 1 3 . In many respects, the systems behaviour closely resembles that of Ni(111)-CO. Oxygen contamination leads to the appearance of a strongly adsorbed CO state with a desorption enthalpy of ~170 kJmol-1. This is reminiscent of a strongly adsorbed non-dissociated state of CO on Ru(1011) which occurs under similar conditions.

Photoelectron spectroscopy and heterogeneous catalysis: Benzene and ethylene from acetylene on palladium (111)

The kinetic, structural and reactive properties of acetylene overlayers on Pd(111) have been investigated between 155 and 900 K by ARUPS, XPS, LEED, Δφ, programmed desorption and molecular beam measurements. At low temperatures ( < 220 K) a flat-lying acetylenic surface species predominates; at higher temperatures the dominant species appears to contain olefinic bonds and has its C-C axis perpendicular to the surface. Reaction studies indicate that the latter species may act as the intermediate to partial hydrogenation ( → C2H4); the former behaves as the precursor for a remarkable low temperature trimerisation to benzene.

The oxidation of CO by no on Pt(111) and Pt(110)

Abstract The competitive coadsorption at 300K of CO and NO on Pt(111) and Pt(110) has been studied, as well as the Langmuir-Hinshelwood reaction between these molecules which occurs when the substrate is heated. The reaction products consisted mainly of N 2 and CO 2 with smaller amounts of N 2 O and O 2 : no detectable NO 2 was observed. A variety of experimental methods were used, though the bulk of the results presented here were obtained by thermal desorption. Some experiments were carried out using 13 CO and 15 NO in order to resolve certain ambiguities due to mass interference which arise when the reactants contain the common isotopes of C and N. A reaction mechanism is proposed which accounts for the relative amounts of individual products, and certain other striking features of the system. These include the variation of product distribution with the composition of the reaction mixture, the correlation between CO 2 yield and the presence of certain CO binding states, and the complete unreactivity of the α NO state. Gaseous CO was found both to displace β NO and to cause a β NO → α NO transformation on both Pt(111) and Pt(110). The kinetics of these processes were studied, numerical values were obtained for the reaction probabilities, and possible interaction pathways are discussed.

The adsorption of nitric oxide on Pt(111) and Pt(110) surfaces

Abstract The thermal and electro impact behaviour of NO adsorbed on Pt(111) and Pt(110) have been studied by LEED, Auger spectroscopy, and thermal desorption. NO was found to adsorb non-dissociatively and with very similar low coverage adsorption enthalpies on the two surfaces at 300 K. In both cases, heating the adlayer resulted in partial dissociation and led to the appearance of N2 and O2 in the desorption spectra. The (111) surface was found to be significantly more active in inducing the thermal dissociation of NO, and on this surface the molecule was also rapidly desorbed and dissociated under electron impact. Cross sections for these processes were obtained, together with the desorption cross section for atomically bound N formed by dissociation of adsorbed NO. Electron impact effects were found to be much less important on the (110) surface. The results are considered in relation to those already obtained by Ertl et al. for NO adsorption on Ni(111) and Pd(111), and in particular, the unusual desorption kinetics of N2 production are considered explicitly. Where appropriate, comparisons are made with the behaviour of CO on Pt(111) and Pt(110), and the adsorption kinetics of NO on the (110) surface have been examined.

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.