S. David Jackson

The Roles of Subsurface Carbon and Hydrogen in Palladium-Catalyzed Alkyne Hydrogenation

Alkynes can be selectively hydrogenated into alkenes on solid palladium catalysts. This process requires a strong modification of the near-surface region of palladium, in which carbon (from fragmented feed molecules) occupies interstitial lattice sites. In situ x-ray photoelectron spectroscopic measurements under reaction conditions indicated that much less carbon was dissolved in palladium during unselective, total hydrogenation. Additional studies of hydrogen content using in situ prompt gamma activation analysis, which allowed us to follow the hydrogen content of palladium during catalysis, indicated that unselective hydrogenation proceeds on hydrogen-saturated β-hydride, whereas selective hydrogenation was only possible after decoupling bulk properties from the surface events. Thus, the population of subsurface sites of palladium, by either hydrogen or carbon, governs the hydrogenation events on the surface.

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.

Metal Oxide Catalysis

With its two-volume structure, this handbook and ready reference allows for comprehensive coverage of both characterization and applications, while uniform editing throughout ensures that the structure remains consistent. The result is an up-to-date review of metal oxides in catalysis. The first volume covers a range of techniques that are used to characterize oxides, with each chapter written by an expert in the field. Volume 2 goes on to cover the use of metal oxides in catalytic reactions. For all chemists and engineers working in the field of heterogeneous catalysis.

Understanding Palladium Hydrogenation Catalysts: When the Nature of the Reactive Molecule Controls the Nature of the Catalyst Active Phase

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany, Institute of Isotopes, Hungarian Academy of Sciences, Post Office Box 77, Budapest H-1525, Hungary WestCHEM, Department of Chemistry, University of Glasgow, Glasgow G128 QQ, Scotland, UK Universite de Lyon, Institut de Chimie, Laboratoire de Chimie, Ecole Normale Superieure de Lyon and CNRS 46 Allee d’Italie, 69364 Lyon, Cedex 07 (France)

Catalytic depolymerisation of isolated lignins to fine chemicals using a Pt/alumina catalyst: part 1—impact of the lignin structure

Four lignin preparations with different contents of alkyl–aryl ether bonds were depolymerised using an alumina supported platinum catalyst. The results showed that the proportion of β-O-4 linkages is the crucial factor for both the yield and the nature of the monomeric products. Highly condensed lignin generated mainly non-alkylated phenolic products while uncondensed lignin generated mainly phenolic products retaining the 3-carbon side-chain. These phenolic products with the 3-carbon chain still attached were considerably less abundant than the maximum potential yield calculated from selective cleavage of alkyl–aryl ether bonds by thioacidolysis, demonstrating that the scope for improved yield remains. Although the catalytic conversion yield rose with an increasing content of labile ether linkages in the lignin structure, optimisation of the catalytic depolymerisation was increasingly required to minimize side reactions. Gel permeation chromatography showed that the products converged towards the same molecular weight distribution regardless of the starting material. The full potential of the highly uncondensed lignin was reached only after the minimisation of condensation reactions during the catalytic conversion.

The liquid-phase hydrogenation of phenyl acetylene and styrene on a palladium/carbon catalyst

Abstract The hydrogenation of phenyl acetylene and styrene have been studied over a palladium/carbon catalyst. The kinetics of the reactions were investigated and activation energies of 26±2 kJ mol −1 and 41±8 kJ mol −1 were obtained for phenyl acetylene hydrogenation and styrene hydrogenation, respectively. Both reactions were found to be zero order concerning the alkyne or alkene. However the order in phenyl acetylene changed from zero order to first order at approximately 60% conversion. This change was due to the effect of styrene co-adsorption and not the concentration of phenyl acetylene. Competitive hydrogenation between the alkene and alkyne resulted in a dramatically reduced rate of hydrogenation for both species. This reduced rate was explained by a reduction in the amount of surface hydrogen as well as the altered bonding of the phenyl acetylene.

A study of nitrobenzene hydrogenation over palladium/carbon catalysts

The hydrogenation of nitrobenzene has been studied over three palladium/carbon catalysts using methanol and isopropanol as solvents. A solvent and palladium particle size effect have both been observed, with the nature of the particle size effect dependent upon the solvent. This may be related to a change in the rate-determining step.

Chemisorption and catalysis by metal clusters: I. Characterisation of materials obtained by impregnation of Os3(CO)12 and Os6(CO)18 onto silica, alumina, and titania

Abstract Os 3 (CO) 12 and Os 6 (CO) 18 were impregnated onto silica, alumina, and titania and characterised in the freshly impregnated state and in states achieved by subjecting the freshly impregnated material to (i) washing, (ii) heating to 523K (temperature-programmed decomposition), and (iii) storage at room temperature. The original clusters interact with the support surfaces and are converted to a family of species A of empirical formula Os n (CO) xn C yn , where the most likely value of n is 12, 2.0 ≤ x ≤ 3.0, and 0.0 ≤ y ≤ 0.4. Retention of osmium-osmium bonding in species A is demonstrated by ultraviolet/visible reflectance spectroscopy and the upper limit of n is suggested by electron microscopy. Infrared spectra of species A contain three bands and indicate the presence of carbonyl ligands bonded to osmium atoms in formal zero, partial negative, and partial positive oxidation states. Species A chemisorbs carbon monoxide and oxygen at 293K, the extent of oxygen chemisorption being the same as that of strong CO chemisorption. A bridged structure for adsorbed-CO is proposed. [ 18 O]CO adsorbed onto species A does not equilibrate, even at high temperatures, with linearly bonded [ 16 O]CO-ligands, confirming that adsorbed-CO and ligand-CO are different states of bound CO. CO 2 is formed, probably by a Boudouard reaction, during temperature-programmed decomposition of all freshly impregnated materials, and hence species A prepared in this way may contain ligand-C. Speculations as to likely cluster structures for species A are presented. Chemisorption and catalytic properties will be described in later papers.

Effect of carbon deposits on reactivity of supported Pd model catalysts

Alumina-supported Pd model catalysts were prepared by Pd evaporation onto a thin alumina film grown on a NiAl(110) substrate. Adsorption and co-adsorption of ethene, CO and hydrogen on Pd/Al2O3/NiAl(110) covered by carbon species, formed by ethene dehydrogenation at ∼550 K, was studied by temperature programmed desorption (TPD). TPD results show that carbon deposits do not prevent adsorption but inhibit dehydrogenation of di-σ bonded ethene. Carbon species suppress CO adsorption in the highly coordinated sites and also suppress the formation of hydrogen ad-atoms on the surface. The ethene hydrogenation reaction performed by co-adsorption of hydrogen and ethene is inhibited by the presence of carbon deposits. The inhibition is independent of particle size studied (1-3 nm). The effects are rationalized in terms of a site-blocking behavior of carbon species occupying highly coordinated sites on the Pd surface.

Competitive reactions in alkyne hydrogenation

Abstract Competitive hydrogenation reactions between alkynes have been studied over a palladium/carbon catalyst. The competitive reactions of 1-pentyne, phenyl acetylene, 2-pentyne, and 1-phenyl-1-propyne have been examined. The results show that in general, the competitive reaction results in a reduction of the hydrogenation rate for both alkynes. However, the 1-pentyne/2-pentyne couple revealed a rate enhancement for both alkynes. This is thought to be due to enhanced hydrogen transfer on the surface. Terminal alkynes reduced the rate of hydrogenation of a competing alkyne more effectively than internal alkynes. Phenyl acetylene was very sensitive to the presence of the second alkyne with its hydrogenation rate being reduced proportionately more than the aliphatic alkyne. This may be related to a disruption of its π–π stacking ability. Competitive hydrogenation increased selectivity to the respective alkenes but had no effect on the cis : trans ratio.