Iron catalyst, amorphous activity

First attempts to check this hypothesis [23] revealed a superior catalytic activity of iron in amorphous iron-zirconium alloys in ammonia synthesis compared to the same iron surface exposed in crystalline conventional catalysts. A detailed analysis of the effect subsequently revealed that the alloy, under catalytic conditions, was not amorphous but crystallized into platelets of metastable epsilon-iron supported on Zr-oxide [24, 25]. 

Source of Activity in other Siliceous Catalysts.—Although various oxides can be combined with silica to give amorphous, acidic catalysts, the replacement of aluminium in zeolites (specially non-faujasitic zeolites) has proved to be very difficult with any element other than gallium. Materials of ZSM-5 structure with iron or boron in place of aluminium have been claimed recently, but it is not yet certain that either iron or boron is part of the zeolite lattice or that the catalytic activity observed is not due to residual lattice aluminium. 

CFCs were decomposed to HCl, HF, and CO2 at 150 °C to 350 °C by the reaction of H2O over amorphous alloy catalysts consisting of at least one element selected from the group of Ni and Co, at least one element selected from the group Nb, Ta, Ti, and Zr, and at least one element selected from the group Ru, Rh, Pd, Ir, and Pt. The alloys were activated by immersion in HF [105]. CFCs are decomposed by the reaction of water vapor at temperatures above 300 °C in the presence of iron oxide supported on activated carbon [106]. They are also decomposed by steam in... 

Rayment T. made a meaningful test that the powder catalyst was reduced with sky-high space velocity at 450°C and atmospheric pressure, so that the concentration of water vapor was reduced to the level of difficult to measure by conventional methods. It is found from X-ray diffraction result that the catalyst after reduction mainly is not a-Fe, but amorphous iron with very high activity. Naturally, this method cannot be applied in industry. 

It should be pointed out here that areas of amorphous or glassy material can also be found in reduced catalysts (see later in Section 2.6). This material, which is also rich in promoter oxides, remains therefore unreduced in the activation process but is, in contrast to the large inclusions in the catalyst precursor, very finely divided between the iron crystallites. Using selected area electron diffraction, it can be identified as a mixture of a highly defective spinel phase with a truly amorphous (ring pattern) phase. The nature of this material, which seems to be a common ingredient in all types of iron catalyst, is difficult to assess because of the notorious analytical difficulties presented by amorphous minor phases. It may, however, play an important role for the operation of the catalyst in acting as an inert spacer material between active iron particles. This role will be further discussed in the final section of the chapter. 

Arata and Hino found that better catalysts could be obtained by calcining Fe(OH)3 at 573 — 873 K. The hydroxide was prepared by hydrolyzing FeCls or Fe(N03)3 9H20. The alkylation reactions were carried out at room temperature with 50 cm of toluene solution (0.5 mol 1 ) of benzyl chloride, t-butyl chloride or acetyl chloride and 0.1 g (for benzylation or t-butylation) or 0.5g (for acetylation) of catalyst. Benzylation and t-butylation was completed within 2 min and 10 min, respectively. For acetylation with acetyl chloride, the reaction was slow, the conversion being 28% after 6 h of reaction. The reaction with acetyl bromide is slighdy faster conversion of 30% was obtained after 4 h.The isomer distribution of alkyltoluenes was 42% ortho, 6% meta and 52% para for benzylation and 3% meta and 97% para for butylation with /-butyl chloride. It was presumed that iron chloride formed on the surface of amorphous iron oxide by its reaction with hydrogen chloride is a catalytically active species for alkylation. 

Iron catalysts supported on silica were synthesized by Suslick et al. [20]. The Fe / Si02 catalysts amorphous nature and particle size in the range 3-8 nm and were submitted to catalytic tests Fischer-Tropsch reactions, and their activities were compared with those of a catalyst Fe / Si02 prepared by dry impregnation of the... 

Introduction of a support (i.e. Ti02) to these systems can induce positive effects on the active phase (FeMo), as to avoid an excessive sintering of the particles during the thermal treatment and/or modification of the reduction capacity and the acid properties. The synergy of iron molybdate and the support is therefore another way for improving the catalytic performance of these solids. Such benefitial effects have been detected in bismuth-molybdenum-titania mixed oxides prepared via sol-gel, in addition these solids resulted to be amorphous materials with a unique morphology and extraordinary dispersion of the active phase [12]. These results encouraged us to extend this field to iron molybdenum oxide catalysts. 

The mimic is prepared by sequential ion-exchanges with iron(ll) and Pd(ll) tetrammine cations followed by calcinations and reduction of the Pd(ll) to Pd(0) as previously described(14). A material with 2wt% Fe(ll) and 1wt% Pd(0) is used by immersing the dry zeolite solid in neat substrate alkane and then pressuring the reaction vessel with a 3 1 mixture of oxygemhydrogen. After shaking this mixture at room temperature for 4 hours the products are analyzed by capillary GC. As a control to assess the intrinsic selectivity of such a Pd/Fe system in the absence of steric effects of the zeolite, catalysts prepared with amorphous silico-aluminate supports were run for comparison. In these cases all reactions must take place at the particle surface since there is no interior pore structure available. In addition, comparison of reaction selectivities of this catalysts with our zeolite materials allows us to ascertain that the Fe active sites must be actually inside (and not on the exterior surface) of the zeolite crystallites. 

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