Lin Liwu

An in situ Mössbauer investigation of the influence of metal–support and metal–metal interactions on the activity and selectivity of iron–ruthenium catalysts

The hydrogen reduction of a series of alumina- and silica-supported iron–ruthenium catalysts has been studied in situ by iron-57 Mossbauer spectroscopy. The data, complemented by results from temperature-programmed reduction experiments, show that ruthenium enhances the reducibility of iron and that the metal–support interaction in the silica-supported samples is weaker than in those supported on alumina.The performance of the iron–ruthenium catalysts for carbon monoxide hydrogenation has been evaluated in a fixed-bed microreactor, and the activity of the alumina-supported catalysts was found to be higher than those supported on silica. However, alumina-supported catalysts in which the iron content exceeded ca. 27 atom % showed a gradual decrease in activity. At ca. 65 atom % this began to approach the activities of the silica-supported materials, which were generally less dependent on the iron content of the bimetallic phase. Catalysts with low metal loadings were more active than their counterparts with high metal loadings. These variations in catalytic activity have been interpreted in terms of the stronger interactions between the metallic phase and alumina, as compared with silica, supports and the smaller metallic particle sizes which are obtained on alumina.Metal–support interactions and particle dispersions have also been found to influence selectivity. Hence ruthenium, which interacts strongly with alumina, showed high activity and tended to form higher-molecular-weight hydrocarbons, whilst silica-supported ruthenium was less active and favoured the formation of lower hydrocarbons. The addition of iron to alumina-supported ruthenium shifted the product distribution towards lower hydrocarbons but had little influence on the selectivity of catalysts supported on silica.The variations in metal–support interaction which result from changes in the iron concentration and which are reflected in the catalytic properties of the solids have been correlated with the Mossbauer data, which showed the capacity of iron to donate electrons to ruthenium.

Roles of oxygen and carbon dioxide on methane oxidative coupling over CaO and Sm2O3 catalysts

Abstract The effect of partial pressures of oxygen and carbon dioxide on the rate of methane conversion over CaO and Sm 2 O 3 catalysts has been investigated by measuring the concentration gradients of products and reactants along the catalyst bed. The results show that the oxidative reaction of methane mainly occurred in the reaction zone of the bed where the methane, oxygen and catalyst co-exist. In this zone, although the oxygen concentration decreases significantly, the rate of methane conversion remains constant over sm 2 O 3 catalyst at 873 K. No inhibition of the catalyst activity by carbon dioxide was observed over Sm 2 O 3 catalyst. By contrast, at the same reaction temperature of 873 K, CaO catalyst can be completely deactivated behind the reaction zone where the partial pressure of carbon dioxide produced was only 0.8 KPa. When the reaction temperature was elevated to 973 K and 1073 K, the rate of conversion of methane increased sharply to a maximum at the entrance of the bed and then slowly dropped with increasing bed height due to carbon dioxide formation. The curve of rate distribution displays a sharp drop at the point where the concentration of oxygen reaches zero. Based on the above results, a kinetic equation for the methane conversion rate has been suggested which is zero order in oxygen and first order in methane, and the isotherm for carbon dioxide adsorption can be described by a Langmuir equation.

An iron-57 Mössbauer spectroscopy and EXAFS study of the hydrogen pretreatment of titania-supported iron-ruthenium catalysts

The changes in composition and structure which are induced in a titania-supported iron-ruthernium catalyst following treatment in hydrogen have been investigated in situ by57Fe Mössbauer spectroscopy and by EXAFS. The results show that ruthenium dioxide is readily reduced at temperatures below ca. 500°C to ca. 20 Å clusters of metallic ruthenium whilst α-Fe2O3 is partially reduced at 130°C to Fe2+ and Fe0. The Fe3+ which is formed by the reoxidation of Fe2+ under the reducing conditions at 500°C segregates to the interface of the bimetallic phase and the titania support. It is suggested that continued treatment at 700°C produces a high dispersion of iron which is coordinated to oxygen atoms of the support. The ca. 20 Å clusters of metallic ruthenium may be envisaged as being anchored to the support via iron-ruthenium bonds

Studies on the TS-1 Zeolites Synthesized by Tetrapropylammonium Bromide as Template

the influence of the content of titanium, the variety of the alkali and the amounts of the templates in the gels on the synthesis of the ts-1 zeolites prepared by means of tetrapropylammonium bromide (tpabr) as a template were investigated by h-1-->c-13 cp/mas nmr, si-29 mas nmr, ir, xrd and element analysis techniques. the results showed that the structures of the resultant ts-1 zeolites were transferred from monoclinic symmetry to orthorhombic symmetry with the increase of the titanium content in the framework. the added alkali didnt act as a template in the synthesis of ts-1 zeolites, but the strong alkali conditions were of benefit to the incorporation of the titanium atoms into the lattices. when the molar ratios of tpabr/sio2 in the gels varied from 0.05 to 0.25, the ti content in the framework showed independence of the amounts of tpabr in gels.

Oxidation of iron in titania-supported iron–ruthenium under reducing conditions: in situ evidence from 57Fe Mössbauer spectroscopy

In situ studies of ruthenium-rich iron-ruthenium clusters supported on titania of differing surface areas by 57Fe Mossbauer spectroscopy show that the pre-reduced bimetallic phase undergoes partial oxidation when treated in atmospheres of hydrogen or hydrogen and carbon monoxide.