Karen Wilson

A spectroscopic study of the chemistry and reactivity of SO2 on Pt{111}: Reactions with O2, CO and C3H6

The chemisorption and reactivity of SO2 on Pt{111} have been studied by HREELS, XPS, NEXAFS and temperature-programmed desorption. At 160 K SO2 adsorbs intact at high coverages, with η2 SO coordination to the surface. On annealing to 270 K, NEXAFS indicates the SO2 molecular plane essentially perpendicular to the surface. Preadsorbed Oa reacts with SO2 to yield adsorbed SO4, identified as the key surface species responsible for SO2-promoted catalytic alkane oxidation. Coadsorbed CO or propene efficiently reduce SO2 overlayers to deposit Sa, and the implications of this for catalytic systems are discussed.

Growth morphology and electronic properties of ultrathin Al films on Pt(111)

The growth morphology and electronic properties of A1 films on Pt( 111 ) have been investigated by STM, XPS and AES at 160 K and 300 K. Film growth is kinetically controlled at both temperatures, with formation of 2D A1 dendrites during the initial stages of deposition. At 300 K, completion of the first monolayer is followed by 3D island formation, while at 160 K layer-by-layer growth persists for A1 coverages _> 5 monolayers. Pt core-level and valence-band X-ray photoelectron spectra show growth of a high binding energy interfacial Pt state during 300 K deposition, indicative of Pt/A1 surface alloying. No significant alloying occurs during 160 K growth. At 300 K, deposition of second-layer A1 signals the onset of adatom place exchange with the underlying Pt, reflecting a rather delicate balance between surface energy and free energy of alloy formation. The limiting Pt/AI surface alloy stoichiometry corresponds to PtzAls, and the associated Pt core level shift is in quantitative accord with an interesting linear correlation which is found between the composition and associated core-level shifts of other Al-transition metal alloys.

Heterogeneous catalysts for clean technology:spectroscopy, design, and monitoring

Reactive, but not a reactant. Heterogeneous catalysts play an unseen role in many of todays processes and products. With the increasing emphasis on sustainability in both products and processes, this handbook is the first to combine the hot topics of heterogeneous catalysis and clean technology. It focuses on the development of heterogeneous catalysts for use in clean chemical synthesis, dealing with how modern spectroscopic techniques can aid the design of catalysts for use in liquid phase reactions, their application in industrially important chemistries - including selective oxidation, hydrogenation, solid acid- and base-catalyzed processes - as well as the role of process intensification and use of renewable resources in improving the sustainability of chemical processes. With its emphasis on applications, this book is of high interest to those working in the industry.

Structure and stability of the platinum/aluminium interface: alloying and substrate vacancy formation on Pt{111}/Al

The structure and stability of Al overgrowths on platinum have been studied by LEED, STM, Auger spectroscopy and photoemission. Annealing dendritic Al islands on Pt{111} to 500 K promotes intralayer Al adatom diffusion across the substrate terraces. Evaporation from smaller Al dendrites results in island densification and coalescence, accompanied by Al/Pt intermixing at ascending steps. Simultaneously, triangular vacancy islands appear that decorate the step edges of the platinum surface. This process plays an integral part in the intermixing of Pt and Al at step sites. Metastable compact islands of the Al overgrowth persist up to 800 K, above which temperature interlayer transport commences, resulting in the formation of an ordered (2 × 2) PtAl surface alloy. Thus, a low-temperature quasi-one-dimensional alloying mechanism is succeeded by a place exchange mechanism at higher temperatures. The (2 × 2) alloy, formed for Al coverages above 0.5 ML, remains stable up to approx. 1000 K, at which point, complete Al dissolution occurs, though some Pt vacancy islands persist at the surface.

Applications of XPS to the study of inorganic compounds

The surface behaviour of materials is crucial to our everyday lives. Studies of the corrosive, reactive, optical and electronic properties of surfaces are thus of great importance to a wide range of industries including the chemical and electronics sectors. The surface properties of polymers can also be tuned for use in packaging, non stick coatings or for use in medical applications. Methods to characterise surface composition and reactivity are thus critical to the development of next generation materials. This report will outline the basic principles of X-ray photoelectron spectroscopy and how it can be applied to analyse the surfaces of inorganic materials. The role of XPS in understanding the nature of the active site in heterogeneous catalysts will also be discussed.

SO2-promoted propane oxidation over Pt/Al2O3 catalysts

The origin of SO 2 -promoted propane oxidation over Pt/alumina catalysts has been elucidated utilising spectroscopic and kinetic measurements over dispersed Pt/Al 2 O 3 and model γ-Al 2 O 3 /Pt(111) systems. HREELS, NEXAFS, TPRS and XPS studies show that under oxidising conditions SO 2 promotes the dissociative chemisorption of C 3 H 8 at both Pt and support sites via surface sulphation. Interfacial sulphoxy species play a key role in activating propane for subsequent combustion at metal sites via a bifunctional spillover mechanism. The loading dependence of the promotional effect is rationalised in the light of X-ray and TEM measurements that reveal catalyst sulphation is accompanined by the reduction and concomitant sintering of small ( 2 particles to form large, active Pt metal clusters.

In situ observation of a surface catalysed chemical reaction by fast X-ray photoelectron spectroscopy

The first application of time-resolved fast X-ray photoelectron spectroscopy as a chemically specific, quantitative probe for surface catalysed organic synthesis is described. The trimerisation of ethyne to benzene over a catalytically active Pd(111) surface has been followed in situ by C 1s core-level XPS. Ethyne reacts to form flat-lying (η6) benzene above ≈150 K with a low activation barrier of≈5±1 kJ mol−1. At 280 K a tilted (η1) benzene moeity is formed on the surface, some of which desorbs, the remainder interconverting to flat-lying benzene above 330 K. Unreacted ethyne is not present above 330 K. Flat-lying benzene begins desorbing≈500 K, in competition with C-H bond cleavage to form a tilted (η1 or η2) phenyl species. The latter may disproportionate yielding gaseous benzene and carbon.