Richard B. Moyes

Recent applications of microwave heating in catalysis

Abstract Comparative results are presented for the use of conventional heating and microwave heating in the areas of catalyst preparation, catalyst characterisation and catalytic reactions. In all cases the application of microwave heating appears to have beneficial effects.

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.

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.

The influence on selectivity of the environment of catalyst sites: II. Modification of cobalt and nickel hydrogenation catalysts by the action of sulfur and other nonmetals

The effect of the presence of sulfur on the activity and selectivity of nickel and cobalt powders has been investigated by examination of 1,3-butadiene hydrogenation at 100 °C. Nickel and cobalt surfaces devoid of sulfur give preferential 1-butene formation by 1:2 addition (type A behavior), whereas surfaces contaminated by sulfur, in sufficient quantity and suitably incorporated, give preferential 2-butene formation with a high trans:cis ratio (type B behavior). Sulfur-contaminated catalysts have been prepared by the reduction of (CoO + CoS) and analogous nickel mixtures, by the adsorption of thiophen and of hydrogen sulfide on nominally clean cobalt powders, and by transfer of sulfur from a contaminated alumina support. The electronic effects on surface sites of sulfur present in an ad-layer and of sulfur incorporated in the surface are distinguishable. Contamination by phosphorus, arsenic, selenium, bromine, and chlorine (but not oxygen) gives type B catalysts similar to those obtained by contamination with sulfur. The effect was not permanent in the case of halogen contamination. This work (i) interprets our previous reports of the existence of “two forms” of nickel and of cobalt, (ii) established 1,3-butadiene hydrogenation as a test reaction for the determination of the extent of contamination of nickel or cobalt catalysts by sulfur and some other nonmetals, and (iii) provides a method for the modification of catalyst selectivity in that the balance of 1:2 and 1:4 addition in 1,3-alkadiene hydrogenation can be controlled.

Hydrogen sorption by molybdenum sulphide catalysts

This paper reports the measurement of the vibration spectra of hydrogen sorbed by molybdenum sulphide and alumina supported molybdenum sulphide. The intensities of the spectra support the idea that the hydrogen is bonded to one or more sulphur atoms rather than metal atoms as is the case for 2H–H0.1TaS2. Preliminary neutron diffraction data support this conclusion.

The adsorption of hydrogen and hydrogen sulphide on tungsten sulphide; isotherm and neutron scattering studies

Abstract Adsorption isotherms for hydrogen and hydrogen sulphide on a WS2 surface have been measured at 400°C at pressures up to 1 at. Inelastic neutron scattering spectra have been recorded for the materials produced. Both hydrogen and hydrogen sulphide adsorb dissociatively to produce S-H bonds. Hydrogen sulphide adsorption is smaller than that of hydrogen and it is suggested that this is because dissociation leads also to sulphidation of the surface vacancies at which the adsorption occurs. In both cases the hydrogen atoms produced as a consequence of the dissociation are able to diffuse over the surface to form bonds to available sulphur atoms.

Electronic effects in butadiene hydrogenation catalysed by the transition metals

Abstract Buta-1,3-diene hydrogenation to but-1-ene, trans -but-2-ene and cis -but-2-ene has been catalysed by evaporated films of the majority of the elements of Groups 3–11 of the Periodic Table. The effects on product composition of H-absorption into the metals of Groups 3–5 and of S-adsorption onto the metals of Groups 6–10 are also described. For the pure metals of each transition series, the but-1-ene yield increases with increasing Pauling electronegativity indicating that an electronic effect governs the overall extent of 1:2-addition. The effect of H-absorption into V, Zr, Nb, and Hf films and the general effect of S-adsorption onto films of the later transition elements is to increase the trans : cis ratio in the but-2-ene which indicates the stabilisation of π-allylic half-hydrogenated states at positively polarised metal atom sites, M δ + . The hydrided and sulphided systems differ in that 1:2-addition by simple hydrogenation of one double bond in the reactant is retained in the former but poisoned in the latter.

Chemisorption and catalysis by metal clusters: III. Hydrogenation of ethene, carbon monoxide, and carbon dioxide, and hydrogenolysis of ethane catalyzed by supported osmium clusters derived from Os3(CO)12 and from Os6(CO)18

Properties are described for catalysts containing high nuclearity metal clusters (nuclearity ~12) derived from Os3(CO)12 and Os6(CO)18 and supported on silica, alumina, titania, or ceria. Ethene hydrogenation (325–535 K), ethane hydrogenolysis (395–665 K), CO hydrogenation (455–665 K), and CO2 hydrogenation (455–715 K) have been examined in pulsed-flow and static reactors. The high nuclearity osmium clusters, protected against sintering by retained ligand-CO, ligand-C, and a support-cluster interaction, are stable under these conditions and provide highly reproducible activity. Freshly prepared catalysts each exhibit an initial non-steady state, during which hydrocarbon is progressively retained and activity rises, passes through a maximum, and declines to a steady state value. Catalysts in the steady state continue to retain hydrocarbon which is probably branched in structure and unsaturated in character. Such retained hydrocarbon species mediate hydrogen atom transfer to reacting adsorbed species. Their concentrations, which have been determined by infrared spectroscopy, 14C-tracer studies, and material balances, are compared with the known site concentrations associated with fresh cluster-derived catalysts. Catalysts in the steady state exhibit activities the magnitudes of which diminish with increasing support-cluster interaction, viz., silica-supported clusters > titania-supported clusters > alumina-supported clusters. Preliminary measurements using a ceria-supported catalyst suggest that activity versus the strength of the support-cluster interaction exhibits a “volcano” relationship. Adsorption of ethene, ethane, and CO occurs at osmium atom sites on the high nuclearity osmium clusters, and the reaction intermediates are also adsorbed at these sites. CO2, however, is adsorbed and reacts at ligand-C sites. Detailed mechanisms are presented for ethene, CO, and CO2 hydrogenations, of which some aspects have been investigated by use of 14C as an isotopic tracer. Most cluster-derived catalysts show exceptional activity for ethane hydrogenolysis, some apparent turnover numbers being 2 orders of magnitude higher than for supported metallic osmium. The osmium clusters adsorb reactants less strongly than metallic osmium, because of their commitment to bonding with the protective CO-ligands, and this weaker reactant adsorption may provide superior catalytic properties.

Support effects in the ruthenium-catalyzed hydrogenation of carbon monoxide: Ethene and propene addition

Abstract CO hydrogenation catalyzed at about 500 K by 2.6% Ru/silica (I), 1.5% Ru/13X-zeolite (II), 17% Ru/titania (III), and 5% Ru/magnesia (IV) gave methane and 1-alkenes as primary products. 1-Alkene isomerization and hydrogenation gave internal alkenes and alkanes as secondary products. Specific activity varied in the sequence III ⪢ II > I > IV whereas selectivity for methane formation, as opposed to higher hydrocarbon formation, varied in the sequence I > II > III > IV. Comparison of one catalyst with another showed that when the methane yield was high the fraction of higher hydrocarbon appearing as alkane at moderate conversions was also high, and vice versa. Ethene addition to CO hydrogenation over (I) and (II) at low conversions (2 to 15%) markedly increased the rate of higher hydrocarbon formation without greatly influencing the methanation rate, whereas ethene addition over (III) and (IV) enhanced the rate of higher hydrocarbon formation by a factor of less than 2 and reduced the methanation rate. Propene addition to CO hydrogenation over (I) increased the rates both of higher hydrocarbon formation and of C2-hydrocarbon formation, again without markedly affecting the methanation rate. The single most important factor in the determination of the total product distribution is the availability of adsorbed hydrogen which varies from catalyst to catalyst in the sequence I > II > III > IV. The activity sequence is ascribed to various metal-support effects.

Measurement of temperature during microwave heating (chemical reactions enhancement)

A miniature gas thermometer, using a pressure transducer, has been shown to be an effective means of indicating the temperature of samples being heated by microwave radiation without the unwanted effects of more traditional temperature-measuring probes. Such a thermometer can also form the basis of a straightforward safety device to prevent the accumulation of flammable vapours when organic molecules are exposed to microwaves.