Silica-supported Iron Catalysts Research Articles

Iron-57 and iridium-193 Mössbauer spectroscopic studies of supported iron-iridium catalysts

57Fe and193Ir Mossbauer spectroscopy shows that silica- and alumina-supported iron-iridium catalysts formed by calcination in air contain mixtures of small particle iron (III) oxide and iridium(IV) oxide. The iridium dioxide in both supported catalysts is reduced in hydrogen to metallic iridium. The α-Fe2O3 in the silica supported materials is predominantly reduced in hydrogen to an iron-iridium alloy whilst in the alumina-supported catalyst the iron is stabilised by treatment in hydrogen as iron(II). Treatment of a hydrogen-reduced silica-supported iron catalyst in hydrogen and carbon monoxide is accompanied by the formation of iron carbides. Carbide formation is not observed when the iron-iridium catalysts are treated in similar atmospheres. The results from the bimetallic catalysts are discussed in terms of the hydrogenation of associatively adsorbed carbon monoxide and the selectivity of supported iron-iridium catalysts to methanol formation.

Infrared study of ammonia–carbon monoxide reactions on silica-supported iron catalysts

Infrared spectroscopy has been used to study the reactions of carbon monoxide and ammonia over Fe/SiO2 catalysts at ca. 298–723 K. Adatoms of nitrogen resulting from the dissociative adsorption of ammonia reacted with CO to form surface isocyanate on iron at 298–523 K. At high temperatures spillover of isocyanate to the silica surface occurred. An accompanying decomposition reaction led to the appearance of intense infrared bands at 2120 and 2055 cm–1, which are ascribed to surface cyano complexes of iron. Hydrogen cyanide was adsorbed on iron at ca. 298 K as FeCNH. At 573 K complete oxidation of bulk iron occurred, giving bulk cyanide and surface isocyanate groups on the silica support. Subsequent standing at ca. 293 K in vacuum led to the formation of HCN polymerisation products.

Infrared study of ammonia and nitric oxide adsorption on silica-supported iron catalysts

Infrared spectra are reported of ammonia, nitrogen and nitric oxide adsorbed on Fe/SiO2 with and without the addition of K2O promoter. Ammonia is coordinatively bound to the iron surface at ambient temperature and at high temperature decomposes to give adsorbed dinitrogen with an infrared band at 2180 cm–1. Dinitrogen was more strongly adsorbed on Fe/SiO2 containing K2O than on Fe/SiO2 alone. Nitric oxide adsorbs at Fe0 and cationic Fen+ sites on iron the latter being generated by the dissociative adsorption of NO. Hydrogen reacts with adsorbed NO to form ammonia and water and promotes Fen+—NO surface species at the expense of Fen+—NO. NO displaces CO from iron, giving CO2(g) and NCO(ads) as products. The effects of K2O on the reactions between NO and H2 or CO have been studied. The kinetics of ammonia decomposition over Fe/SiO2 at 683–823 K are briefly reported.

Volumetric and infrared studies of carbon monoxide adsorption on silica-supported iron catalysts characterised by thermogravimetric analysis and X-ray diffraction

The reduction of iron(III) nitrate dispersed on silica has been investigated as a function of reduction temperature by t.g.a., X.r.d, total CO uptakes and infrared spectra of adsorbed CO. There was no evidence for iron–silica interactions. Increased extent of reduction favoured enhanced dissociative and associative adsorption of CO but raising the reduction temperature above ca. 650 K favoured subsequent dissociative adsorption of CO at the expense of adsorbed CO detectable by infrared spectroscopy. Well reduced iron particles were susceptible to rapid contamination by traces of oxygen and complete poisoning (for CO adsorption) by excess oxygen. Enhanced uptake of CO by K2O-promoted Fe–SiO2 exhibited a maximum at 0.5% K2O loading. Addition of Al2O3 also promoted CO adsorption but the effects of K2O and Al2O3 in doubly promoted catalyst were not additive.

An investigation of metal-support interactions in some iron, ruthenium and ironȁruthenium catalysts by in situ iron-57 Mössbauer spectroscopy

The reduction by hydrogen of alumina- and silica-supported iron catalysts has been studied in situ by iron-57 Mo˝ssbauer spectroscopy. The results from catalysts with loadings of 5% iron show that the stronger metal support interaction between iron and alumina gives rise to small particle iron oxide which is partially reduced in hydrogen to iron(II) whilst silica-supported iron, in which the metal-support interaction is weaker, forms large particle magnetically ordered α-Fe2O3 which is reduced to metallic iron and iron(II). Treatment of these and some alumina- and silica-supported ironȁruthenium catalysts of similar total metal loading in atmospheres of carbon monoxide and hydrogen give Mo˝ssbauer spectra which show that the reducibility of iron depends on both the ruthenium content of the bimetallic catalyst and the character of the support. Such treatment gives rise to the formation of an iron carbide phase in the silica-supported iron catalysts.Alumina- and silica-supported catalysts containing 10% iron, or 5% ruthenium, or 5% iron-5% ruthenium are more active towards carbon monoxide hydrogenation than those with lower iron loadings. The different catalytic properties of these materials, together with the changes induced in the activity and selectivity of alumina- and silica-supported ruthenium by the incorporation of iron, reflect the variation in metal-support interaction as the nature of the metallic phase is altered.

Mössbauer spectroscopy observations of local temperature rises in a silica-supported iron catalyst

Mössbauer spectroscopy has been used to observe the local (or microscopic) temperature of a silica-supported iron catalyst used in the Fischer-Tropsch synthesis reaction. The internal magnetic field of the iron carbide crystallites, measured while the catalyst was situated in an in-situ cell, was used to measure temperature of the crystallites under reaction conditions. At nominal bulk temperatures of 275 and 300 °C, temperature rises of 13 and 19 °C were recorded when the gas flow to the catalyst was changed from helium to the 3 H 2 CO reaction mixture. These local temperature rises of the supported crystallites were in contrast to observed decreases in the bulk catalyst temperature when the reaction mixture was introduced. The iron carbide crystallites transformed from the ε′ phase to the more stable χ-carbide after prolonged treatment in helium.

Silica-supported iron nitride in Fischer-Tropsch reactions: II. Comparison of the promotion effects of potassium and nitrogen on activity and selectivity

The performance of silica-supported iron catalysts promoted by potassium and nitrogen has been studied for the conversion of CO to hydrocarbons. Reactions were investigated at 250 °C and at 1, 7.8, and 14 atm in a differential conversion range of less than 6%. Significant differences exist in the behavior of these supported catalysts in comparison to prior studies of fused or precipitated materials. It was found that an unpromoted Fe-carbide catalyst was more active at lower pressures and less active at higher pressures than the nitrided catalyst. Moreover, it was found that potassium-promoted catalysts were less active than catalysts without potassium. Both Fe-nitride and potassium-promoted catalysts showed higher olefin selectivities, higher water-gas shift activity, lower alcohol yields, and larger proportions of higher molecular weight products than their unpromoted counterparts. The product distribution parameters are presented for all catalysts at different pressures and conversion levels. The significance of the incorporation of alcohols into the distribution plot is discussed and a mechanism for alcohol formation is proposed. The effect of nitrogen on the catalytic reaction is discussed in terms of a donor model theory in which nitrogen atoms donate electrons to the iron d band. From the product distribution of the supported Fe-nitride and potassium-promoted catalysts, it is concluded that nitrogen and potassium have a similar promoting function on the Fischer-Tropsch reactions and further that an additive effect is observed for Fe-K-nitride catalysts.

Orientation towards alcohols in CO, H2 reactions on some iron catalysts

Adding rare earths like dysprosium or lanthanum to silica-supported iron or iron-copper catalysts enhances the formation of C1 to C5 alcohols and the total conversion in CO−H2 reactions. Copper rises the alcohol orientation and also the reactivity.

Allene–methylacetylene isomerization over silica-supported cobalt and iron catalysts

Equilibrium between allene and methylacetylene was studied over iron catalyst in the temperature range of 165 to 200 °C, with allene or methylacetylene as the initial reactant. In both cases similar equilibrium product-distribution (allene 16% and methylacetylene 84%), fairly close to the calculated value, is observed.Kinetics of allene-methylacetylene isomerization was studied between 70 and 165 °C. The order of reaction was always one and temperature independent. Both activity and calculated energy of activation (11.8 and 11.2 kcal/mol over iron and cobalt respectively) were similar for the two catalysts, suggesting that active sites for the reaction were also similar. This similarity, as well as the first order kinetics, are fully explained by a reaction mechanism based on adsorbate migration.

Infrared studies of carbon monoxide and hydrogen adsorbed on silica-supported iron and cobalt catalysts

A temperature-programmed infrared study of carbon monoxide and carbon monoxide-hydrogen mixtures adsorbed on silica-supported iron and cobalt catalysts was made. Bands observed at 2181 and 2169 cm −1 for carbon monoxide on cobalt and iron, respectively, were attributed to physical adsorption on oxide impurities. For cobalt a band observed in the range 2020 to 2060 cm −1 increased in frequency within this range as the CO pressure was increased. This change in frequency was pressure reversible. This has been interpreted in terms of the number of CO ligands bound to one cobalt atom. In the presence of iron a mixture of carbon monoxide and hydrogen gives rise to methane as well as to a small amount of ethane plus traces of propane. Preadsorption of hydrogen leads to formation of methane only. In the presence of cobalt, mixtures of carbon monoxide and hydrogen only give methane. A band attributed to linearly held carbon monoxide on cobalt has been correlated with methane formation. An intermediate complex formed between adsorbed carbon monoxide on cobalt and hydrogen, stable within the temperature range 125 to 195 °C, has been inferred from the limited methane formation within this temperature range.