Iron oxide catalysts hematite

Figure 4.21 shows the XPS core level spectra of the Fe 2p and Zr 3d electrons measured for the stable active catalyst. The outer surface is covered with iron oxide (in hematite-like forms) and zirconia exists as non-stoichio-metric Zr02-x. In the iron 2p spectrum, a contribution of metallic iron is visible indicating that the surface oxide film is thin within the information depth of XPS (ca. 2.5 nm). It has been suggested that the surface oxide stabilizes the iron... 

The synthesis of iron oxide catalysts leads to the hematite (Fe203) phase after calcination. Before the WGS reaction the hematite phase is reduced to magnetite phase (Fe304), since magnetite is the active phase for the WGS reaction. Usually, the reduction is taking place in the presence of process gas between 350 and 450 °C. Process gas is a mixture of CO, CO2, H2 and water vapour. The representative reactions are shown as follows ... 

An increasing intensity of the diffraction peaks of hematite is observed when comparing the dried and calcined catalyst as shown in Fig. 2(a), indicating that hematite forms at M er temperatures. No obvious diffraction peaks to lithium such as lithium iron oxide (LiFcsOg) could probably be ascribed to the small fraction of lithium or overlapped peaks betwem hematite and lithium iron oxide. The diffraction peak intensity of magnetite in tested catalysts increases significantly. 

The principal iron oxides used in catalysis of industrial reactions are magnetite and hematite. Both are semiconductors and can catalyse oxidation/reduction reactions. Owing to their amphoteric properties, they can also be used as acid/base catalysts. The catalysts are used as finely divided powders or as porous solids with a high ratio of surface area to volume. Such catalysts must be durable with a life expectancy of some years. To achieve these requirements, the iron oxide is most frequently dis-... 

It is known that Py adsorbed on hematite reacts with the surface at elevated temperature. The appearance of new IR bands after treatment above 423K indicates the formation of a new surface complex. It is believed that the new bands stem from surface-bound 2,2 -bipyridyl or o( -pyridone (refs. 18,19). Concomittantly with Py oxidation iron oxide becomes reduced and is losing its transmission. Whatever is the reaction product Gef -Fe Og catalysts start to oxidize Py at higher temperature and bind much less newly formed surface complex than ot-Fe Oj (Figs. 3 and 4). 

Carbothermic reactions are sohd-sohd reactions with carbon that apparently take place through intermediate CO and CO2. The reduction of iron oxides has the mechanism Fe Oy-H CO =>xFe-t C02, C02 + C=>2C0. The reduction of hematite by graphite at 907 to 1007°C in the presence of hthium oxide catalyst was correlated by the equation 1 — (1 — x) = kt. The reaction of sohd ihnenite ore and carbon has the mechanism FeH03-l- CO=>Fe -I-Ti02 + C02, CO2 + C = 2CO. A similar case is the preparation of metal carbides from metal and carbon, C -I- 2H2 CH4, Me -I- CH4 MeC -I- 2H2. 

Iron modified zeolites and ordered mesoporous oxides have been studied as catalysts for the sulfur dioxide oxidation in sulfur rich gases. Both zeolitic materials and mesoporous oxides show very good activity in this reaction. Other than solid state or incipient wetness loaded MCM-41 materials, the zeolites do not show an initial loss of activity. However, they loose activity upon prolonged exposure to reaction conditions around 700°C. The zeolitic samples were analyzed via X-ray absorption spectroscopy, and the deactivation could be related to removal of iron from framework sites to result in the formation of hematite-like species. If the iron can be stabilized in the framework, these materials could be an interesting alternative to other iron based catalysts for the commercial application in sulfur rich gases. 

Iron oxide is a well reducible phase, the comparison of reduction profiles of supported Fe-catalysts may give information on the dispersion state of iron. In Fig. 5.10, two TPR spectra of two dispersed FeOx catalysts on zirconia prepared by conventional impregnation and adsorption methods are shown [17]. Reduction of pure hematite (Fe203) by hydrogen is a complex event that can proceed in different steps... 

The Shell catalysts were made by dry mixing powdered iron oxide, chromic acid, and potassium carbonate, mixing with water, extrading before drying, and then calcining at 800°-950°C. The high-temperature calcinations was critical and later work indicated that it optimized surface area and pore size as the iron oxide was converted to a-hematite. 

Fig. 28.1. Results (symbols) and simulations (lines) of an experiment at 25 °C by Liger et al. (1999 their Fig. 6) in which uranyl was oxidized by ferrous iron in the presence of nanoparticulate hematite, which served as a catalyst. Vertical axis is amount of NaHCCE-extractable uranyl, which includes uranyl present in solution as well as that sorbed to the nanoparticles in the experiment, nearly all the uranyl was sorbed. Broken line shows results of a simulation assuming uranyl forms a single surface complex, >Fe0U020H, which is catalytically active solid line shows simulation in which a non-catalytic site of this stoichiometry is also present. Inset is an expanded view of the first few hours of reaction.

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