G. Webb

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.

The hydroisomerization of n-butenes: I. The reaction of 1-butene over alumina- and silica-supported rhodium catalysts

Abstract The reaction of 1-butene with hydrogen has been studied using alumina- and silica-supported rhodium catalysts in the temperature range −20 ° to 80 °C. The kinetics and apparent activation energies for hydrogenation and isomerization are reported. The initial (cis/trans) ratio in 2-butene is greater than the thermodynamic equilibrium value; this ratio decreases as the temperature is increased. The support appears to have little effect. Possible mechanisms for hydrogenation and isomerization are discussed. Isomerization is thought to proceed via a 1-methyl-π-allyl intermediate and the implications of this postulate are discussed.

The hydroisomerization of n-butenes: 2. The reaction of 1-butene over mercury-poisoned rhodium-silica catalysts

Abstract The influence of adsorbed mercury upon the rates of hydrogenation, isomerization and olefin exchange over silica-supported rhodium catalysts has been studied in the temperature range 0–69 °, using deuterium and radioactive tracer techniques. The results show that whereas the rates of hydrogenation and 1-butene exchange decrease uniformly with increasing mercury coverage, the rate of isomerization is virtually independent of mercury coverage up to θ Hg > 80%. The results are interpreted in terms of a model in which hydrogenation and olefin exchange occur directly on the metal, while isomerization involves the migration of adsorbed 1-butene from the metal to the support followed by isomerization on the silica.

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.

Supported nickel catalysts: Hydrogenolysis of ethane, propane, n-butane and iso-butane over alumina-, molybdena-, and silica-supported nickel catalysts

The catalytic properties of three supported nickel catalysts, 0.97 wt.% Ni/SiO2, 0.95 wt.% Ni/Al2O3 and 0.54 wt.% Ni/MoO3, are reported for the hydrogenolysis of ethane, propane, n-butane and iso-butane. The reactions were carried out in a flow reactor at atmospheric pressure. The three nickel catalysts had the following order of hydrogenolysis activity: Ni/SiO2>Ni/Al2O3>Ni/MoO3. The active site for the hydrogenolysis reactions over the three nickel catalysts is formed insitu and is likely to contain a carbonaceous component. Any carbonaceous component that is formed will not necessarily be the same in each catalyst as the laydown will be a function of the characteristics of the fresh catalysts (nickel dispersion, support etc.). The Ni/SiO2 catalyst showed the highest activity for the hydrogenolysis reactions of the alkanes tested. The higher specific rate of hydrogenolysis of the Ni/SiO2 catalyst is likely to be an effect not only of the small particle size of the nickel but also the manner in which carbonaceous matter builds up on these particles. The Ni/MoO3 catalyst had a lower activity than would be expected from its nickel dispersion. The suppression activity on the Ni/MoO3 sample may be related to a strong interaction between the metal and the support. The selectivity behaviour shown during hydrogenolysis by the Ni/SiO2 and Ni/Al2O3 catalysts was typical of that expected for nickel catalysts (multiple hydrogenolysis, demethylation, low isomerisation). These selectivity features can be accounted for by a reverse Fischer–Tropsch synthesis mechanism that involves 1,1,2 adsorbed alkylidene intermediates on the catalyst surface. The Ni/MoO3 catalyst demonstrated uncharacteristic isomerization activity during the hydrogenolysis of n- and iso-butane. This can be accounted for by a bifunctional mechanism involving acid sites on the MoO3 support.

Supported nickel catalysts: Preparation and characterisation of alumina-, molybdena-, and silica-supported nickel, and the identification of reactive oxygen on these catalysts by exchange with isotopically labelled carbon dioxide

Nickel catalysts, supported on alumina, silica, and molybdena, have been prepared by impregnation and co-crystallization. In the precursor state the catalysts were characterised by UV–visible spectroscopy, thermogravimetric analysis/differential thermal analysis (TGA/DTA), and X-ray photoelectron spectroscopy (XPS). The nickel was principally in the 2+ oxidation state with an octahedral coordination. However, the ligand sphere surrounding the nickel ion was sensitive to the support, indicating that the species on the different supports were not identical thus suggesting a metal complex–support interaction. Reduction was followed by temperature programmed reduction (TPR) and TGA, the results of which indicated that reduction and decomposition of nickel nitrate occurred simultaneously. X-ray diffraction (XRD) analysis revealed that, with the Ni/MoO3 sample, no hydrogen bronze was formed on reduction. The reduced catalysts were characterised by carbon monoxide chemisorption, carbon dioxide chemisorption, and by reaction of buta-1,3-diene with dihydrogen. In the absence of a dihydrogen stream it was found that the catalysts adsorbed no carbon monoxide due the presence of sub-monolayer quantities of surface oxygen. The extent of the oxygen was quantified by exchange with isotopically labelled carbon dioxide. Differences in the electronic nature of the nickel between the Ni/MoO3 sample and the other catalysts wererevealedbytheirbehaviourtowardsbuta-1,3- diene hydrogenation.

Deactivation and regeneration of alkane dehydrogenation catalysts

A study of the deactivation and regeneration of a Pt/alumina catalyst used for propane dehydrogenation has shown that initially the deposit is made up of C-1 fragments, which age to form polycyclic aromatics. Both surface hydrogen and carbon can exchange with the gas phase; however, some of the carbon cannot be removed from the metal function by treatment in oxygen at 873 K. As the catalyst ages it becomes increasingly selective to propene, indicating that the carbonaceous deposit has a major role in defining the selectivity and the activity of the catalyst.

Structural studies of (Pt-Ge)/γ-Al2O3 reforming catalysts by transmission electron microscopy and microanalysis

Abstract Reforming catalysts (Pt-Ge)/γ-Al2O3 were investigated by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDXS). Several phases including single-crystal and twin-crystal platinum, PtGe and Pt3Ge2 alloys, and a superlattice structure Pt3Ge, were found in coexistence in the catalysts. The value of the parallel combination of TEM and EDXS to the structure determination of nanometre-sized alloy particles has been evaluated.

Preparation and Properties of a Pt/Silica and its Comparison with Europt-1

Summary A 0.76% Pt/silica has been prepared by conventional impregnation from aqueous solution using hexachloroplatinum(IV) ions as the Pt source. Its adsorption characteristics and reactivity are compared with those of the standard reference catalyst EUROPT-1 (a 6.3 wt% silica-supported Pt) which was prepared by ion exchange using platinum(II)tetrammine ions as the Pt source. Platinum dispersion by HRTEM and particle morphology by EXAFS are reported. The catalysts both showed high metal dispersion and comparable behaviour in oxygen chemisorption and butadiene hydrogenation, whereas they differed with respect to mean Pt particle size and showed different behaviours in carbon monoxide chemisorption, cyclopropane hydrogenolysis, and enantioselective methyl pyruvate hydrogenation. Clearly the choice of comparators is of importance in catalyst evaluation.

An isotope labelling study of the deactivation of a Pt/alumina catalyst used for propane dehydrogenation.

Publisher Summary This chapter investigates the form and reactivity of the carbonaceous deposit using 13 C and 2 H transient isotopic labeling techniques in a study of propane dehydrogenation. The activity and selectivity of the Pt/alumina catalyst for propane dehydrogenation is critically dependent on the extent of formation of a carbonaceous deposit, which contains both carbon and hydrogen, on the catalyst. Within the carbonaceous deposit, there is mixing of both the carbon and the hydrogen as revealed by isotopic tracer studies. Deactivation of the catalyst has been followed and the amounts of carbon deposited determined. Only a relatively small fraction of the carbon laydown on the surface can be removed by high temperature dioxygen treatment. After regeneration, carbon continues to build up on the catalyst surface in subsequent propane dehydrogenation reactions. Pretreatment of the catalyst with carbon monoxide or toluene at the reaction temperature results in carbon laydown on the catalyst, which dramatically reduces the amount of carbon deposition and increases the selectivity in subsequent propane dehydrogenation reactions. However, the carbon deposited during the pretreatment is different from that formed during propane dehydrogenation. Regeneration of the catalysts has also been examined and the efficiency of the regeneration method and its effect on the catalyst established. The effects of pre-poisoning of the catalyst, using either carbon monoxide or toluene as a specific carbon species, on the deactivation and selectivity of the catalyst for propene formation has also been investigated.