An investigation of the oxygen pathways in the oxidative coupling of methane over MgO based catalysts

The oxidative coupling of methane to ethane and ethene has been investigated by admitting pulses of pure methane, pure oxygen, and mixtures of methane and oxygen to MgO, Li/MgO, and Sn/Li/MgO at temperatures ranging from 923 to 1073 K in a Temporal Analysis of Products (TAP) set-up. Moreover, pulses of oxygen followed by pulses of either methane, ethane, ethene, or carbon monoxide were applied to study the role of both adsorbed oxygen and surface lattice oxygen in the reaction mechanism. Two types of reversibly adsorbed oxygen are present on Sn/Li/MgO. The first type is strongly adsorbed oxygen, which desorbs from the surface on a time scale of 3 min at 973 K. This type of oxygen does not seem to be reactive toward methane. The second type of oxygen consists of weakly adsorbed oxygen species with a time scale of desorption amounting to 4 s at 973 K. The weakly adsorbed oxygen species are involved in the direct conversion of methane to carbon dioxide. Surface lattice oxygen is also interacting with the admitted reductants. The percentage of surface lattice oxygen reactive in the methane conversion is less than 0.1% of a theoretical monolayer on MgO at 1023 K. This value amounts to 27% for Li/MgO and 44% for Sn/Li/MgO at the same temperature. On Li/MgO and Sn/Li/MgO two different types of surface lattice oxygen are present. The first is active in methyl radical formation, while the second is involved in the direct conversion of methane to carbon dioxide. Weakly adsorbed oxygen and the second type of surface lattice oxygen are also involved in the nonselective reaction paths of ethane and ethene as well as in the consecutive oxidation of carbon monoxide. Strongly adsorbed oxygen is not involved in these reactions. The observations are consistent with the Lunsford mechanism [Ito, T., Wang, J.-X., Lin, C.-H., and Lunsford, J. H.,J. Am. Chem. Soc.107, 5062 (1985)] for the generation of methyl radicals over MgO-based catalysts. The increasing activity toward methane due to the addition of lithium and moreover tin to MgO can be explained by an increase in the amount of reactive surface lattice oxygen

Promoter effects in the Pt-catalysed oxidation of propylene glycol

Oxidation of aqueous solutions of propylene glycol has been performed at 333 K, pH = 8 using Pb-, Bi- and Sn-promoted Pt/graphite catalysts. Oxidation of propylene glycol towards pyruvic acid proceeds via two pathways, one via hydroxyacetone, the second via lactic acid. Experiments, performed under oxygen mass transfer limitations to avoid de-activation by over-oxidation showed significant effects of the promoters on both activity and selectivity. The addition of Pb and Bi enhances oxidation of lactic acid resulting in higher yields of pyruvic acid. The addition of Sn leads to high selectivities for both hydroxyacetone and pyruvic acid. The effect of Sn can be explained by the formation of a Sn–diol complex, where Sn is in the Sn(IV) state

Modelling of the deposition of molybdenum on silicon from molybdenum hexafluoride and hydrogen

The deposition of molybdenum on silicon from MoF6 and H2 is studied using a microbalance setup. The deposition rate is time dependent, which is explained by the significant contribution of the reduction of MoF6 by Si. A model taking into account both deposition routes and in particular diffusion of Si through the growing layer allows to describe the observations quantitatively. The relative importance of two routes was assessed and the kinetics of the reduction by H2 could be distinguished from the overall growth kinetics. A partial reaction order of 1.4 in hydrogen was found for the reduction of MoF6 by H2. The order in MoF6 is negative

The oxidative coupling of methane over MgO-based catalysts : a steady-state isotope transient kinetic analysis

To study the heterogeneous steps of the oxidative coupling of methane to ethane and ethene over MgO, Li/MgO and Sn/Li/MgO, oxygen and carbon dioxide isotope step experiments were carried out in the absence of reaction, and oxygen and methane isotope step experiments were carried out in a tubular reactor at 1023 K, atmospheric pressure, an inlet molar ratio of CH4/O2 equal to 4, a methane conversion of 24%, and an oxygen conversion of 85%. The steady-state axial total concentration profiles of the reactants, intermediates, and products have a significant influence on the shapes of the transient isotope responses under these conditions. Oxygen interacts strongly with all catalysts used by dissociative reversible adsorption, except for lined-out Li/MgO. Both surface and bulk lattice oxygen participate in the reaction. The promotion with lithium and even more with tin increases the mobility of oxygen in the bulk of the catalyst and the amount of exchangeable oxygen per unit BET surface area. Carbon in methane can either react to C2 products, without any significant interaction with the catalyst, or show a weak reversible interaction with the catalyst, which does not lead to C2 products. In the absence of reaction, carbon dioxide interacts with the catalyst only in the presence of lithium. Under reaction conditions, the experiments can be described satisfactorily by postulating a methoxy species as the only carbon-containing intermediate on the catalyst leading to carbon dioxide