Adrianus J. van Dillen

Iron oxide dehydrogenation catalysts supported on magnesium oxide. Part 2.—Reduction behaviour

The reduction behaviour of iron oxide supported on magnesium oxide has been studied using temperature-programmed reduction (TPR), X-ray diffraction (XRD), and high-field magnetic measurements. The catalysts, containing 1–6 wt.% Fe, were prepared by incipient-wetness impregnation of a preshaped magnesium oxide support with organometallic complexes, such as, ammonium iron(III) citrate and ammonium iron(III) EDTA. In the fresh calcined catalyst the iron oxide is present as magnesium ferrite, MgFe2O4. With TPR two broad reduction peaks, reflecting the sequential reduction of Fe3+via Fe2+ to Fe0, were observed. High-field magnetic measurements enabled us to describe the reduction behaviour in detail. In the first reduction step, proceeding at ca. 630 K, MgFe2O4 is reduced to FeO partly via Fe3O4. A second reduction step at 750 K results in the formation of metallic iron. Part of the metallic iron has been found to be present in the form of superparamagnetic particles within the MgO support.

Coordination of palladium on carbon fibrils as determined by XAFS spectroscopy

To investigate the anchoring of precious metals onto carbon fibrils Pd/C-fibril samples were investigated in both the precursor and the reduced state by X-ray absorption fine structure (XAFS) spectroscopy. Analysis of the XAFS data of the freshly prepared catalyst showed a palladium–tetraamine complex in interaction with carbon. The proposed model involves [Pd(NH3)4]2+ coordinated to the carbon fibril surface, most probably stabilised by the carboxylic groups (through O–H bridging and charge balance) and the π-electron system of the support. After insitu reduction, analysis pointed to small palladium particles of ca. 15 A present on the fibrils with a significant Pd–C interaction.

Iron oxide dehydrogenation catalysts supported on magnesium oxide. Part 1.—Preparation and characterization

Iron oxide catalysts supported on magnesium oxide have been prepared by impregnation of a preshaped magnesium oxide support to incipient wetness with poorly crystallizing organometallic complexes. As organometallic complexes ammonium iron(III) citrate and ammonium iron(III) EDTA have been used. After impregnation the iron has appeared to be homogeneously distributed throughout the magnesium oxide support bodies. Upon calcination in air, the precursors decompose to form magnesium ferrite at temperatures as low as ca. 800 K. Calcination in air at 973 K results in the formation of small MgFe2O4 crystallites with a typical size of ca. 200 A, as determined by X-ray diffraction and electron microscopy.

Iron oxide dehydrogenation catalysts supported on magnesium oxide. Part 3.—But-1-ene dehydrogenation activity

The catalytic behaviour of iron oxide catalysts supported on magnesium oxide in the non-oxidative dehydrogenation of but-1-ene to buta-1,3-diene has been studied. Under dehydrogenation conditions the magnesium ferrite phase of the fresh catalyst was found to be reduced to FeO in the bulk of the ferrite particles, where FeO is strongly stabilized by the formation of a solid solution with MgO. At the surface of the particles magnesium ferrite is reduced to Fe3O4. In the reduced state a catalyst containing ca. 3 wt.% Fe shows a but-1-ene conversion of 26% combined with a buta-1,3-diene selectivity of 65% at 873 K. The activation energy for dehydrogenation was found to be 47 kcal mol–1.The (unpromoted) catalyst gradually deactivates owing to carbon deposition. Under dehydrogenation conditions the catalyst surface is completely covered by a layer of amorphous carbon within 20 h.

Surface structure of untreated parallel and fishbone carbon nanofibres: an infrared study

KEYWORDS:carbon ¥carbon nanofibres ¥IR spectroscopy ¥nanostructures¥surface analysisCarbon nanofibres (CNFs) that are obtained by catalytic decom-position of carbon-containing gases over small metal particlesare a promising catalyst support material for liquid-phasereactions. The fibres are mechanically strong and can withstandthe forces executed on them by stirring the reaction medium.Furthermore, the skeins of fibres possess a mesoporous macro-structure, decreasing the chance of encountering diffusionlimitation during catalytic reactions in the liquid phase. Thestructure of the CNFs can be tuned by changing the growthconditions and their hydrophobicity can be altered by surfaceoxidation. Moreover, carbon nanofibres are very pure. No othertypes of carbon, such as carbon onions, fullerenes or amorphouscarbon, are formed and no heteroatoms such as sulfur areincorporated during synthesis. They are chemically inert and canbe used in strongly acidic or basic environments. Finally, whengrown in a fluidised bed reactor, carbon nanofibres can beobtained at low cost, making an application as catalyst supportmaterial possible.

Magnesium oxide as a support material for dehydrogenation catalysts

Two commercial preshaped magnesium oxides, viz. pellet-shaped magnesium oxide (Englhard, Mg-0601) and spherical magnesium oxide (Sud-Chemie, T-4403), have been evaluated for use as carrier material in supported dehydrogenation catalysts. The influences of temperature and steam atmosphere on the textures of the support materials have been studied.The texture of the pellet-shaped support has been found to be well controlled by thermal treatment in air. Upon thermal treatment the B.E.T. surface area of this magnesium oxide decreases, while the average pore radius increases. Typically, a B.E.T. surface area of 6 m2 g–1 and an average pore radius of 1000 A is obtained upon treatment at 1173 K. Spherical magnesium oxide (Sud-Chemie, T-4403) has a very low surface area of 0.18 m2 g–1 and an average pore radius of 40 000 A. Calcination in air at temperatures up to 1473 K does not affect the texture. In 4.8 bar of steam at 423 K both magnesium oxides react completely to magnesium hydroxide. This is accompanied by a drastic increase of the B.E.T. surface area and a decrease of the mechanical strength. At 973 K and a steam pressure of 1 bar no reaction with water is observed.

The interaction of oxygen with isolated silver particles of ca. 70 nm supported on α-alumina. Part 1.—Oxygen sorption and temperature-programmed desorption measurements

The interaction of oxygen with a silver catalyst containing isolated silver particles of ca. 70 nm supported on α- Al2O3 has been studied using (combined) volumetric adsorption and temperature-programmed desorption measurements after various pretreatments. The amount of oxygen taken up is greatly influenced by the pretreatment. Considerable amounts of oxygen are able to penetrate into the silver lattice. Impurities present in or on the silver particles strongly affect the amount of oxygen taken up at 443 K. It is argued that penetrated oxygen can facilitate the reorganization of the silver surface giving rise to low-index facets.

The interaction of oxygen with isolated silver particles of ca. 70 nm supported on α-alumina. Part 2.—CO oxidation measurements

The oxidation of carbon monoxide with molecular oxygen over a catalyst containing isolated silver particles of ca. 70 nm, supported on α-Al2O3 has been studied after various pretreatment. Both the extent of oxygen sorption and the activity were greatly affected by the pretreatment. A rise in activity was observed after reduction and consecutive cooling in hydrogen, whereas a drop followed an oxidizing treatment. A severe deactivation of the catalyst occurred after reduction at 673 K followed by cooling in nitrogen. The differences in activity due to the various pretreatments are discussed in terms of changes in the surface structure of the silver particles.

Influence of pretreatment on the properties of Ag/α-Al2O3 catalysts containing large (± 1 µm) pure and Cs-promoted silver particles. Part 2.—CO oxidation measurements

The catalytic activity for the oxidation of CO with molecular oxygen of silver catalysts containing large silver particles (ca. 1 µm) has been studied. Both pure and caesium-promoted silver catalyst were investigated. The various pretreatments to which the catalysts were subjected, i.e. reduction or oxidation at different temperatures, influenced their activity. The promoted catalyst proved to be somewhat less susceptible to the various pretreatments.Arrhenius plots of the catalysts often exhibited a sharp break, giving rise to two sets of kinetic parameters. An apparent activation energy of 40 kJ mol–1 was found at low temperatures, whereas an activation energy of 60 kJ mol–1 was observed at high temperatures. It is argued that the activation energy of 40 kJ mol–1 can be ascribed to the reaction of CO with oxygen atoms in the neighbourhood of defects and the activation energy of 60 kJ mol– to the reaction of CO with oxygen penetrated into the bulk at the same sites. The variation in activity could be attributed to changes in the pre-exponential factor.A separate discussion is devoted to a comparison of the present results with the findings on silver catalysts with a smaller mean particle size we reported on previously.

Influence of pretreatment on the properties of Ag/α–Al2O3 catalysts containing large(± 1 µm) pure and Cs-promoted silver particles. Part 1.—Extent of oxygen and hydrogen sorption and TPD studies

In order to elucidate the influence of the pretreatment on the (catalytic) properties of silver catalysts containing large silver particles (± 1 µ m), the interaction of oxygen and hydrogen with reduced and pre-oxidized samples has been investigated. The effect of the presence of caesium on the silver surface has also been investigated. Volumetric (ad)sorption of oxygen and hydrogen, TPD experiments, combined with chemisorption measurements, and electron microscopy were used as characterization techniques.The combined results indicate that the penetration of oxygen into these large silver particles does not occur to any significant extent. When the catalysts are saturated with oxygen, only ca. 40% of the monolayer coverage of oxygen atoms is situated in the subsurface layers.This type of pretreatment exhibited small, but distinct, effects on the uptake of oxygen and hydrogen at 443 K. Higher uptake of oxygen was observed after reduction of the catalyst at more elevated temperatures. The presence of caesium lessens considerably the variation in the uptake.