Silica-supported Iron Oxide Catalysts Research Articles

Spatially and Size Selective Synthesis of Fe-Based Nanoparticles on Ordered Mesoporous Supports as Highly Active and Stable Catalysts for Ammonia Decomposition

Uniform and highly dispersed γ-Fe(2)O(3) nanoparticles with a diameter of ∼6 nm supported on CMK-5 carbons and C/SBA-15 composites were prepared via simple impregnation and thermal treatment. The nanostructures of these materials were characterized by XRD, Mössbauer spectroscopy, XPS, SEM, TEM, and nitrogen sorption. Due to the confinement effect of the mesoporous ordered matrices, γ-Fe(2)O(3) nanoparticles were fully immobilized within the channels of the supports. Even at high Fe-loadings (up to about 12 wt %) on CMK-5 carbon no iron species were detected on the external surface of the carbon support by XPS analysis and electron microscopy. Fe(2)O(3)/CMK-5 showed the highest ammonia decomposition activity of all previously described Fe-based catalysts in this reaction. Complete ammonia decomposition was achieved at 700 °C and space velocities as high as 60,000 cm(3) g(cat)(-1) h(-1). At a space velocity of 7500 cm(3) g(cat)(-1) h(-1), complete ammonia conversion was maintained at 600 °C for 20 h. After the reaction, the immobilized γ-Fe(2)O(3) nanoparticles were found to be converted to much smaller nanoparticles (γ-Fe(2)O(3) and a small fraction of nitride), which were still embedded within the carbon matrix. The Fe(2)O(3)/CMK-5 catalyst is much more active than the benchmark NiO/Al(2)O(3) catalyst at high space velocity, due to its highly developed mesoporosity. γ-Fe(2)O(3) nanoparticles supported on carbon-silica composites are structurally much more stable over extended periods of time but less active than those supported on carbon. TEM observation reveals that iron-based nanoparticles penetrate through the carbon layer and then are anchored on the silica walls, thus preventing them from moving and sintering. In this way, the stability of the carbon-silica catalyst is improved. Comparison with the silica supported iron oxide catalyst reveals that the presence of a thin layer of carbon is essential for increased catalytic activity.

A Silica-Supported Iron Oxide Catalyst Capable of Activating Hydrogen Peroxide at Neutral pH Values

Iron oxides catalyze the conversion of hydrogen peroxide (H(2)O(2)) into oxidants capable of transforming recalcitrant contaminants. Unfortunately, the process is relatively inefficient at circumneutral pH values because of competing reactions that decompose H(2)O(2) without producing oxidants. Silica- and alumina-containing iron oxides prepared by sol-gel processing of aqueous solutions containing Fe(ClO(4))(3), AlCl(3), and tetraethyl orthosilicate efficiently catalyzed the decomposition of H(2)O(2) into oxidants capable of transforming phenol at circumneutral pH values. Relative to hematite, goethite, and amorphous FeOOH, the silica-iron oxide catalyst exhibited a stoichiometric efficiency, defined as the number of moles of phenol transformed per mole of H(2)O(2) consumed, which was 10-40 times higher than that of the iron oxides. The silica-alumina-iron oxide catalyst had a stoichiometric efficiency that was 50-80 times higher than that of the iron oxides. The significant enhancement in oxidant production is attributable to the interaction of Fe with Al and Si in the mixed oxides, which alters the surface redox processes, favoring the production of strong oxidants during H(2)O(2) decomposition.

Epoxidation of propylene in the gas phase

The gas-phase epoxidation of propylene using N2O, air and air-ammonia mixture as an oxidants was studied. Propylene can be epoxidized by nitrous oxide with a yield as high as 13.3% over silica supported iron oxide catalysts modified by amines. The iron oxide dispersion, the acidity of the support and the nitrogen-containing modifiers are the key factors determining the catalytic performance. We suggest a reaction pathway involving two concurrent mechanisms: the radical oxidation of propylene to acroleine, hexanediene, etc., and a non-radical oxidation leading to epoxide. Propylene is epoxidized with air over silica-supported iron oxide catalysts at a conversion of about 0.2%. Using air as an oxidizing agent, the presence of gaseous ammonia improves the propylene conversion by 10-fold preserving the considerable selectivity (up to 60%). This observation suggests a reaction mechanism involving the oxidation of ammonia to nitrous oxide in the first step, which subsequently produces active oxygen species, which selectively oxidize propylene to propylene oxide (PO).

Direct gas-phase epoxidation of propene with nitrous oxide over modified silica supported [formula omitted] catalysts

Vapour phase epoxidation of propene with nitrous oxide over alkaline containing silica supported iron oxide catalysts was experimentally investigated. A reaction network is deduced from the trends of product selectivities as function of propene conversion using different catalysts. Important side reactions are parallel propanal formation and consecutive isomerisations of propylene oxide (PO) to propanal, acetone and allyl alcohol. Rates of these reactions were affected by catalyst composition, which influenced acid-base properties, number of silanol groups and surface area. A PO selectivity of ∼ 75 % and PO space–time yields of 0.02– 0.03 g PO g cat - 1 h - 1 were obtained with a catalyst containing ironoxo-species and caesium ions/-oxides. These results can only be obtained at a high surplus of N 2 O versus propene concentration in the reactor feed. Likely reasons for that is an inhibition of propylene oxide's formation rate by propene and propylene oxide itself.

Gas Phase Epoxidation of Propene by Nitrous Oxide over Silica-Supported Iron Oxide Catalysts

Catalysts consisting of sodium-promoted silica gel-supported iron oxide were found to catalyze the gas phase epoxidation of propene by nitrous oxide. Selectivities to propene oxide of 40–60% at propene conversions in the range of 6–12% were observed. The influence of different parameters on the yield to propene oxide is discussed: the amount and the dispersity of iron oxide, the procedure of alkali promotion, the morphology of the support, and the reaction parameters.

Reduction of nitrogen oxides by carbon monoxide over an iron oxide catalyst under dynamic conditions

The reduction of NO and N 2O by CO over a silica-supported iron oxide catalyst was investigated by the transient response method, with different initial oxidation states of the catalyst, i.e. completely reduced (Fe 3O 4), or oxidised (Fe 2O 3). The influence of CO pre-adsorption was also studied. From the material balance on the gas phase species, it was shown that the composition of the catalyst changes during relaxation to steady-state. The degree of reduction of the catalyst at steady-state could thus be estimated. During the transient period, CO was shown to inhibit N 2O as well as NO reductions by adsorption on reduced sites. The activity of the reduced catalyst was found to be substantially higher as compared to the oxidised catalyst for both reactions. On this basis, it was attempted to keep the catalyst in a reduced state by periodically reducing it with CO. As a result, a significant increase in the performance of the reactor with respect to steady-state operation could be achieved for N 2O reduction by CO. Finally, the dynamic behaviour of the N 2O–CO and NO–CO reactions made it possible to evidence reaction steps, the occurrence of which could not be shown during our previous investigations on the separate interactions of the reactants with the catalyst.

Surface Coordinate Geometry of Iron Catalysts: Formation of Tetrahedral/Octahedral Site on Silica Surface with/without Sodium Promoter

Sodium-modified, silica-supported iron oxide catalysts (Na–Fe/SiO2) calcined at 300, 500, 650, and 800°C have been compared with unmodified Fe/SiO2treated in the same conditions by using FeK-edge XANES, EXAFS, XRD, and Mössbauer techniques. Each iron on Fe/SiO2is found to have 5.0 nearest oxygen neighbors at an average distance of 1.94 Å; there is no change after calcination at 800°C. However, remarkable changes have been observed for Na–Fe/SiO2calcined at temperatures higher than 500°C. After calcination at 800°C, only tetrahedrally coordinated Fe3+is observed. Accordingly, the coordination number of the nearest oxygens decreases to 4.1 with a significantly shortened Fe–O bond length of 1.89 Å. Supporting irons on sodium-pretreated silica (Fe/Na–SiO2) and then calcinating the catalyst obtained at 800°C results in a similar phenomenon.

Characterization of iron oxide in [formula omitted] catalyst

Silica-supported iron oxide catalyst was prepared by hydrolysis of a mixed solution of ethyl silicate and iron(III) nitrate dissolved in ethylene glycol. A gel obtained by the hydrolysis was dried and calcined at various temperatures. The iron oxide particles thus prepared were maghemite (γ-Fe 2O 3), deduced by EXAFS (extended X-ray absorption fine structure) measurements and the sample color. The maghemite particles were classified into two groups: one is small but detectable by transmission electron microscopy (TEM) and the other is too small to be detected by TEM. The former was well controlled to a consistent size, which depended on the calcination temperature, and exhibited ferrimagnetic properties. The latter showed superparamagnetic properties and was estimated to be smaller than 10 Å in particle size by magnetic and Mössbauer measurements at low temperature. For comparison, the catalyst was also prepared by a conventional impregnation method using silica powder and an aqueous solution of iron(III) nitrate. Iron oxide thus prepared was hematite (α-Fe 2O 3) and the particle sizes were broadly distributed.

Metal oxide-support interactions in silica-supported iron oxide catalysts probed by nitric oxide adsorption

Metal oxide-support interactions in silica-supported iron oxide catalysts probed by nitric oxide adsorption