David Lennon

The application of infrared spectroscopy to probe the surface morphology of alumina-supported palladium catalysts

Five alumina-supported palladium catalysts have been prepared from a range of precursor compounds [palladium(II) nitrate, palladium(II) chloride, palladium(II) acetylacetonate, and tetraamminepalladium(II) tetraazidopalladate(II)] and at different metal loadings (1-7.3 wt %). Collectively, this series of catalysts provides a range of metal particle sizes (1.2-8.5 nm) that emphasize different morphological aspects of the palladium crystallites. The infrared spectra of chemisorbed CO applied under pulse-flow conditions reveal distinct groupings between metal crystallites dominated by low index planes and those that feature predominantly corner/edge atoms. Temperature-programmed infrared spectroscopy establishes that the linear CO band can be resolved into contributions from corner atoms and a combination of (111)(111) and (111)(100) particle edges. Propene hydrogenation has been used as a preliminary assessment of catalytic performance for the 1 wt % loaded catalysts, with the relative inactivity of the catalyst prepared from palladium(II) chloride attributed to a diminished hydrogen supply due to decoration of edge sites by chlorine originating from the preparative process. It is anticipated that refinements linking the vibrational spectrum of a probe molecule with surface structure and accessible adsorption sites for such a versatile catalytic substrate provide a platform against which structure/reactivity relationships can be usefully developed.

Vibrational Spectroscopy with Neutrons: A Review of New Directions

Inelastic neutron scattering spectroscopy (INS) has enabled vibrational spectra to be measured for over 50 years. Most studies have used a type of spectrometer that is straightforward to build and use and that provides spectra that are not dissimilar to infrared and Raman spectra. In this Focal Point Review Article we show the advantages of a type of neutron vibrational spectrometer that has been largely unknown to the spectroscopy community. These instruments are able to access regions of low momentum transfer at relatively large energy transfer. This means that the C–H, N–H, and O–H stretch regions can be exploited by INS spectroscopy for the first time. The instruments generally have very large detector area, which means that they are significantly more sensitive than the more commonly used instruments. They also allow the energy transfer as a function of momentum transfer to be examined. After briefly outlining the basics of INS spectroscopy, we describe the operational principles of the instruments and show how flux and resolution can be traded. We then review how the advantages of the instruments can be used to gain understanding of molecular systems in areas as diverse as hydrogen storage, hydrogen bonding, and fullerenes. The instruments are starting to have a significant impact in studies of catalysts and this is illustrated with recent studies of hydrogen on fuel cell catalysts, methyl chloride synthesis, the deactivation of methane reforming catalysts, and a model carbon monoxide oxidation catalyst.

Catalysis in Application

Although catalysts are responsible for the manufacture and processing of a number of products in daily use, the subject of catalysis is still very much in its infancy, and the complexity of the processes still present major challenges. Catalysis in Application presents a snapshot of the most up-to-date developments in the field of applied catalysis. Coverage is principally in the areas of hydrogenation, dehydrogenation, chiral catalysis, environmental catalysis and catalyst deactivation, combining a unique mix of chemistry and chemical engineering. With its wide-ranging coverage, this book will be a welcome addition to the shelves of every practitioner in catalysis, both in industry and academia.

The effect of hydrogen concentration on propyne hydrogenation over a carbon supported palladium catalyst studied under continuous flow conditions

The rate of propyne hydrogenation at 298K is shown to be critically dependent on hydrogen concentration, with a discontinuity apparent when the hydrogen: propyne concentration exceeds 6:1. This step change is attributed to hydrogenation of some of the hydrocarbonaceous overlayer present on the working catalyst that exposes metal sites, which are then active for dissociative adsorption of hydrogen.

A NIXSW and NEXAFS investigation of thiophene on Cu(111)

The local registry and geometry of thiophene at sub-monolayer coverages on Cu(111) has been probed with normal incidence X-ray standing wavefield absorption (NIXSW) and near edge X-ray absorption fine structure (NEXAFS). Thiophene was found to adopt a flat geometry on the surface with the sulphur atom in an atop position. The relatively short data collection times required for the NIXSW technique were found to be beneficial when looking at thiophene overlayers which are sensitive to X-ray induced damage.

Experimental arrangements suitable for the acquisition of inelastic neutron scattering spectra of heterogeneous catalysts

Inelastic neutron scattering (INS) is increasingly being used for the characterization of heterogeneous catalysts. As the technique is uniquely sensitive to hydrogen atoms, vibrational spectra can be obtained that emphasize a hydrogenous component or hydrogen-containing moieties adsorbed on to an inorganic support. However, due to sensitivity constraints, the technique typically requires large sample masses (∼10 g catalyst). A reaction system is hereby described that enables suitable quantities of heterogeneous catalysts to be appropriately activated and operated under steady-state conditions for extended periods of time prior to acquisition of the INS spectrum. In addition to ex situ studies, a cell is described which negates the need for a sample transfer stage between reaction testing and INS measurement. This cell can operate up to temperatures of 823 K and pressures up to 20 bar. The apparatus is also amenable to adsorption experiments at the gas-solid interface.

ENHANCED PHOTODESORPTION OF NO ON ROUGHENED SILVER SURFACES

Abstract The photodesorption of NO at 458 nm on roughened silver is contrasted with that on Ag(111). The data for Ag(111) are discussed in terms of recent studies of the complex adsorption behaviour of this system; it is concluded that the most likely mechanism involves a substrate mediated dissociative electron attachment to the NO dimer followed by monomer NO desorption. A five-fold enhancement of the photodesorption cross section on the roughened surface is found on irradiation at 458 nm. It is suggested that this effect arises from an enhancement of the substrate-mediated process. Specifically it is proposed that surface plasmon excitation on the rough surface results in an enhanced hot electron yield, with the consequence of an enhanced cross section for photodesorption.

The application of inelastic neutron scattering to investigate the ‘dry’ reforming of methane over an alumina-supported nickel catalyst operating under conditions where filamentous carbon formation is prevalent

The use of CO2 in reforming methane to produce the industrial feedstock syngas is an economically and environmentally attractive reaction. An alumina-supported nickel catalyst active for this reaction additionally forms filamentous carbon. The catalyst is investigated by inelastic neutron scattering as well as elemental analysis, temperature-programmed oxidation, temperature-programmed hydrogenation, X-ray diffraction, transmission electron microscopy and Raman scattering. Isotopic substitution experiments, using 13CO2 for 12CO2, show the oxidant to contribute to the carbon retention evident with this sample. At steady-state operation, a carbon mass balance of 95% is observed. A kinetic scheme is proposed to account for the trends observed.

Deactivation and selectivity: The effect of hydrogen concentration in propyne hydrogenation over a silica-supported palladium catalyst

Propyne hydrogenation over a silica-supported palladium catalyst at 293 K has been investigated. Catalytic activity and selectivity is shown to be critically dependent on the hydrogen concentration, with higher hydrogen concentrations significantly reducing the deactivation rate. An equimolar C3H4:H2 mixture was 100% selective to propene whereas an excess hydrogen mixture produced both propene and propane in comparable amounts (propene selectivity = 55%). Correlations in carbon mass balance data and variations in the product distribution for the excess hydrogen mixture are interpreted as defining distinct regions of the catalyst lifetime, as the catalyst approaches steady state operation.

Butane dehydrogenation over Pt/alumina: activation, deactivation and the generation of selectivity

The dehydrogenation of butane over a Pt/alumina catalyst has been studied using pulse-flow techniques. The selectivity to butenes is generated as carbonaceous material is deposited. The final catalyst is sensitive to whether reduction has taken place in hydrogen or butane, with the more effective dehydrogenation catalyst being generated from a hydrogen reduction.