Peter B. Wells

Characterization of the standard platinum/silica catalyst europt-1. 4. chemisorption of hydrogen

Abstract Isotherms for hydrogen adsorption on reduced EUROPT-1 (253 to 771 K) and for hydrogen desorption from EUROPT-l (193 to 592 K) have been measured by the

Characterization of the standard platinum/silica catalyst europt-1.3. the size distribution of the platinum-containing particles

Abstract The size distribution of the platinum-containing particles in as-received EUROPT-1 has been determined by high resolution transmission electron microsc

Platinum group metals as heterogeneous enantioselective catalysts

Enantioselective hydrogenation has been reviewed from the standpoint of the chemistry of the metallic active phase. A comparative study is presented of the behaviour of the platinum group metals as catalysts for pyruvate ester hydrogenation, and this is extended to embrace the hydrogenation of other functions as far as the literature permits. Attention is drawn to the operation of thermodynamic and kinetic factors in the determination of enantioselectivity, to the molecular complexities of the adlayer in which enantioselection is achieved, and to approaches that might be made to advance our understanding of these systems.

Characterization of the standard platinum/silica catalyst europt-1 2. preparation, physical properties, and chemical composition

Abstract 6 kg EUROPT-l was prepared specially for this project by Johnson Matthey. Pt(NH 3 ) 4 2+ was impregnated onto silica by use of an ion exchange met

Characterization of the standard platinum/silica catalyst europt-1. 5.chemisorption of carbon monoxide and of oxygen

Abstract Isotherms for carbon monoxide adsorption and for oxygen adsorption on reduced EUROPT-1 at room temperature have been measured by the volumetric method, A summary of the main features of the characterization of EUROPT-1 is presented.

Hydrogenation of alkenes over supported gold

Gold has been supported upon SiO2 and γ-Al2O3 using an impregnation technique and HAuCl4 solutions. The severity of the conditions required to effect the reduction/decomposition of the acid to metallic Au upon the support decreased and the activity per unit weight of metal in the hydrogenation of pent-1-ene at 373 K increased, as the concentration of Au decreased from 5.0 to 0.01%. Au supported upon SiO2 at these concentrations could be atomically dispersed, but in lower dispersions when upon γ-Al2O3. An increase in Au dispersion as its concentration decreased, which would provide an explanation for the unusual activity of these catalysts, may have been detected only by electron spin resonance and not by more traditional methods of characterisation. Preliminary results for SiO2-supported Ag and Pt are compared with Au.

Patterns of behavior in catalysis by metals

Abstract The relative merits of “technical” and “clean” metal catalysts for the study of the hydrogenation of unsaturated hydrocarbons are discussed, and it is concluded that the former have substantial advantages provided that there are parameters which are essentially independent of the precise physical form of the catalyst. A survey of the literature shows that (a) the degree of selectivity shown in the hydrogenation of multiply unsaturated hydrocarbons and (b) the tendency to give olefin isomerization and exchange during hydrogenation constitute such parameters. The information in the literature is supplemented by new experimental work on the noble, Group VIII, metals (reported in outline); it is shown that the second row metals (Ru, Rh, Pd) all give substantial isomerization and exchange, whereas the third row metals (Os, Ir, Pt) give little. These results are used to explain the selectivity sequence observed in the hydrogenation of actylenes and diolefins.

Origin of enhanced rate in the platinum-catalysed enantioselective hydrogenation of methyl pyruvate

Abstract The remarkable 20-fold rate enhancement that accompanies the achievement of enantioselectivity in Pt-catalysed methyl pyruvate hydrogenation at 293 K and 10 bar pressure has been shown to be the consequence of three distinct roles played by the cinchona alkaloid modifier. The modifier promotes the rate (i) by its action as an alicyclic N-base, (ii) by its ability when adsorbed to facilitate the establishment of an enhanced surface coverage of adsorbed hydrogen, and (iii) by its effect as an aromatic adsorbate on the apparent specific activity of platinum. These effects have been demonstrated in experiments in which various N-bases have been used to promote the racemic rate of methyl pyruvate hydrogenation at 293 K, and in which H2/D2 exchange at 195 K has been used to estimate variations in the surface coverage of adsorbed hydrogen. The importance of the three effects is: (i) > (ii) ≈ (iii) and their combined effect accounts for the observed enhancement. It is proposed that the N-bases stabilise one of the half-hydrogenated states by H-bonding.

Support effects in ethene hydrogenation catalyzed by platinum

The reaction of ethene with deuterium has been catalyzed at 20 °C by platinum supported on titania, magnesia, alumina, silica, and silica-alumina. Product compositions, which have been analyzed by Kemballs method, show that the likelihood of ethene desorption from the various catalysts increased in the sequence Pt/silica ≈ Pt/alumina ≈ Pt/silica-alumina < Pt/magnesia < Pt/titania. The behavior of adsorbed ethyl groups at these platinum surfaces was little influenced by the nature of the support. Silica, alumina, and silica-alumina behave as inert supports under the conditions used, whereas titania and magnesia interact with platinum in a manner that appears to cause a reduction in the strength of ethene adsorption. For Pt/titania, this is attributed to the effects of partial reduction of the titania at elevated temperatures; for Pt/magnesia, the effect may result from the preferential development of low-index planes at the surface of the platinum crystallites.

The influence on selectivity of the environment of catalyst sites: III. The role of hydrogen occlusion in group VIII metals

The extent of hydrogen occlusion in powdered Ru, Rh, Os, Ir, Pt, and Au prepared by reduction of the chlorides, and in Co prepared by reduction of the oxide, has been measured by butene titration at 373 K and (except for Co and Ru) by exchange with deuterium. Hydrogen occlusion increased in the sequence Au (= zero)