David A. Whan

Promotion of methanol synthesis and the water-gas shift reactions by adsorbed oxygen on supported copper catalysts

The surfaces of the copper metal crystallites of working Cu/ZuO/Al2O3 and other copper catalysts are partially oxidised in reaction mixtures for methanol synthesis and the water-gas shift reaction. Work with unsupported polycrystalline copper has confirmed earlier results that copper metal is the active phase in supported copper catalysts. The coverage of adsorbed oxygen, O(a), up to half-monolayer, was determined by reaction with N2O and it was found to be controlled by the overall reaction CO2(g)= CO(g)+ O(a). The free energy of formation of O(a) was calculated to be –240 kJ mol–1 at 513 K. The induction period found in methanol synthesis from CO–CO2–H2 mixtures is consistent with the calculated rate of formation of O(a). The role of O(a) in the methanol synthesis and water-gas shift reactions is both as promoter and reaction intermediate. The dissociative chemisorption of hydrogen on copper is promoted by O(a) but this is not necessary for the reactions. Experiments with unsupported polycrystalline copper have shown that O(a) both increases the extent of physisorption of CO2 and creates new chemisorbed states of CO2, with desorption energies of 109, 113 and 125 kJ mol–1. O(a) is also essential for the dissociative chemisorption of water on copper. A regenerative mechanism for the water-gas shift reaction on copper [involving the formation and reaction of O(a)] has been established by observation of the separate stages. The adsorbed formate intermediate, pivotal in methanol synthesis from carbon dioxide, is irrelevant to the water-gas shift reaction.

Temperature programmed reduction of alumina-supported iron, cobalt and nickel bimetallic catalysts

Temperature programmed reduction studies of coprecipitated iron-cobalt-alumina and iron-nickel-alumina catalysts have shown that the presence of cobalt or nickel improves the reducibility of iron based catalysts and increases the Fe2+/Fe3+ ratio in the mixed oxide catalyst precursors. Results for a commercial ammonia synthesis catalyst, and for a prereduced and stabilized material, are also briefly reported.

Hydrogenation of acetylene in excess ethylene on an alumina-supported palladium catalyst at atmospheric pressure in a spinning basket reactor

The hydrogenation of acetylene in the presence of ethylene and carbon monoxide has been investigated in a spinning-basket reactor on a commercial supported palladium catalyst. The rate of disappearance of acetylene was found to be controlled by pore diffusion at low acetylene concentrations and was independent of acetylene at high acetylene pressures. In the non-diffusion-controlled region the reaction of acetylene was first order in hydrogen, and the rate of ethane formation was independent of both the acetylene and ethylene pressures. This latter observation is consistent with the presence of sites which can hydrogenate ethylene even in the presence of acetylene. The nature of the C4 products has been shown to be dependent on the packing of adsorbed acetylene molecules and on the availability of surface hydrogen. The analysis of a heavy involatile product is also reported.

The activity of supported molybdenum hexacarbonyl catalysts for the disproportionation of propene

Abstract The disproportionation of propene has been investigated over catalysts prepared from molybdenum hexacarbonyl supported on silica, alumina, silica-alumina and magnesium oxide. The activity of these catalysts is found to depend markedly on the temperature at which the support is outgassed before the molybdenum hexacarbonyl is added and on the temperature of activation after addition of the hexacarbonyl. These variations in activity are discussed in terms of the sites on the supports which are available for interaction with the molybdenum hexacarbonyl, and the possible state of the molybdenum in active catalysts.

Hydrogenation of acetylene in excess ethylene on an alumina supported palladium catalyst in a static system

Studies have been made of the kinetics of the hydrogenation of mixtures containing 2 % of acetylene in ethylene on an alumina supported palladium catalyst in a static system. Information has been obtained about the nature of the selectivity of the reaction of acetylene. Detailed analyses of the kinetics have been made as the ratio of the pressure of acetylene to ethylene decreases and subsequently as the remaining hydrogen is used up.The poisoning action of carbon monoxide, which selectively inhibits the hydrogenation of ethylene, has also been studied.Tracer studies using carbon-13 labelled acetylene, light ethylene and hydrogen with analysis by combined gas chromatography-mass spectrometry, have shown unambiguously that ethane produced from the hydrogenation of a mixture of acetylene in ethylene comes predominantly from the ethylene.The reaction between acetylene, ethylene and deuterium gives a greater understanding of the surface processes involved and produces ethane which is mainly [2H2]ethane.It is postulated that two types of site exist on the surface, type X which hydrogenates both acetylene and ethylene and on which acetylene is adsorbed ∼2200 times more strongly than ethylene at 293 K and type Y which is easily poisoned by carbon monoxide and can hydrogenate ethylene even in the presence of acetylene.

Comparison of the shape selective properties of ferrierite, ZSM-5 and ZSM-11

Characterization of the pore structures of ferrierite, ZSM—5 and ZSM—11 was performed by measuring the rates and extents of adsorption of a series of molecules of different kinetic diameters. The results were consistent with the reported pore dimensions of the three zeolites. Comparison of activities and selectivities in methanol conversion revealed a direct correlation between the nature of the intracrystalline pore structure and the aromatic product distribution. In addition, a marked enhancement in p-xylene selectivity was observed for mixed ferrierite/ZSM—5 catalysts. Results obtained during but-1-ene isomerization were consistent with an acid-catalysed mechanism involving the secondary butyl carbenium ion as reaction intermediate. A significant feature of but-1-ene isomerization over ferrierite was the marked preferential formation of cis-but-2-ene as reaction product. This was attributed to restricted rotation of the secondary butyl carbenium ion within the constrained environment of the ferrierite pore structure (that is, as a consequence of transition-state selectivity).

The exchange of methane, ethane, and propane with deuterium on silica-supported nickel catalyst

Abstract The rates and product patterns for the exchange of methane, ethane, and propane on nickel/silica catalysts are reported. The principal products were CH 3 D and CH 4 for methane, C 2 H 4 D 2 , C 2 H 2 D 4 , and C 2 D 6 for ethane, and C 3 H 7 D and C 3 H 6 D 2 for propane. The results are interpreted in terms of competition between deuterium and hydrocarbon molecules for adsorption on different specific surface sites.

57Iron transmission and conversion electron mössbauer spectroscopic characterization of iron-cobalt ammonia synthesis catalysts

Abstract The results of a 57 Fe transmission and conversion electron Mossbauer spectroscopic (CEMS) study of a series of Fe-Co ammonia synthesis catalysts are reported. The catalysts were prepared by conventional coprecipitation techniques, followed by calcining, reducing and stabilisation steps. The calcined samples are multiphasic and are composed mainly of iron oxides and/or cobalt spinels. The reduced and stabilised samples are shown to consist of a Fe-Co alloy core encased in an oxide shell in which the Co/Fe ratio exceeds the Co/Fe ratio of the alloy core. It is likely that the iron-cobalt distribution in the alloy core is inhomogeneous.

Ammonia synthesis and related reactions over iron-cobalt and iron-nickel alloy catalysts. Part I. Catalysts reduced at 853 K

Abstract Studies have been undertaken of ammonia synthesis, nitrogen chemisorption and nitrogen isotope exchange over a series of iron-cobalt and iron-nickel alloy catalysts containing 3% of alumina as a structural stabiliser. Catalysts were reduced for 18 h at 853 K before use and metal surface areas were measured by adsorption of carbon monoxide. While nitrogen chemisorption could be observed below 673 K the rate decreased with coverage so markedly that temperatures of 773 K or more were required to attain adsorption equilibrium within a reasonable period. Ammonia synthesis was observed from 550 K but at 700 K or over the rate of synthesis was affected by the back reaction becoming significant. Higher temperatures in the range from 773 K to 853 K were required to give measurable rates of nitrogen isotope exchange. Thus the temperatures for ammonia synthesis correspond to the start of nitrogen chemisorption and those for exchange to the higher temperatures needed for reversible adsorption and the establishment of the adsorption equilibrium. The pattern of activity with composition for the cobalt alloys showed a small maximum at a composition of iron (95%) cobalt (5%) for all three processes (adsorption, synthesis and exchange). A similar but less well-defined maximum was found with catalysts containing from 5% to 15% of nickel. Possible reasons for the enhanced activity of these alloys are discussed.

Hydrogenolysis of saturated hydrocarbons on evaporated platinum films

The reactions of ethane, propane, n-butane, isobutane and neopentane have been examined in the presence of deuterium on platinum films. Details of the products of exchange, hydrogenolysis and, where possible, isomerization are reported. The kinetic behaviour of the hydrogenolysis reaction showed a trend in that reactions of ethane were acceleratory, those of propane maintained a constant rate and those of molecules with four or more carbon atoms decelerated with time. The retardation in rates was much more marked with branched chain than with linear hydrocarbons.The results are discussed in terms of the nature of the adsorbed species accumulating on the surface, species possessing some carbonium ion character being likely precursors to the material responsible for the observed poisoning. Little additional mechanistic information was obtained from an analysis of the deuterium contents of the various molecules as exchange was fast compared to hydrogenolysis or isomerization, leading to rapid isotopic equilibration.A fragmentation correction scheme taking into account the influence of molecular composition on bond rupture has also been developed for analysis of the mass spectral data.