Hematite catalysts

Influence of the sodium-based precipitants on the properties of aluminum-doped hematite catalysts for ethylbenzene dehydrogenation... 

Some results of the reduction of hematite by graphite at 907 to 1,007°C in the presence of lithium oxide catalyst were correlated by the equation 1 — (1 — xY = kt. The reaction of solids ilmenite and carbon has the mechanism... 

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. 

From Fig.2 (a), A solid phase transformation fiom hematite, Fc203 to magnetite, Fe304, is observed, indicating that the active sites of the catalj are related to Fc304. Suzuki et. al also found that Fe304 plays an important role in the formation of active centers by a redox mechanism [6]. It is also observed that the hematite itself relates to the formation of benzene at the initial periods, but no obvious iron carbide peaks are found on the tested Li-Fe/CNF, formation of which is considered as one of the itsisons for catalyst deactivation [3,6]. 

It seems that Au ions of Au(OH) Cl4 complex, formed by the first aging at room temperature, are reduced to Au particles by electron transfer from the coordinated OH ions on the surface of hematite as a catalyst of the electron transfer. As a consequence, the essential reducing agent is water. The optimum pH to yield the maximum quantity of Au particles was ca. pH 5.9, as measured at room temperature, corresponding to the pH of the above standard system. Au ions are reduced to metallic Au by electron transfer from coordinated OH ions on the surfaces of hematite particles through their catalytic action. 

The liquid-phase reduction method was applied to the preparation of the supported catalyst [27]. Virtually, Muramatsu et al. reported the controlled formation of ultrafine Ni particles on hematite particles with different shapes. The Ni particles were selectively deposited on these hematite particles by the liquid-phase reduction with NaBFl4. For the concrete manner, see the following process. Nickel acetylacetonate (Ni(AA)2) and zinc acetylacetonate (Zn(AA)2) were codissolved in 40 ml of 2-propanol with a Zn/Ni ratio of 0-1.0, where the concentration of Ni was 5.0 X lO mol/dm. 0.125 g of Ti02... 

The catalyst, composite of crystalline hematite particles embedded into a mesopor-ous silica SBA-15 matrix was used. They accounted the following reactions for ultrasound coupled with Fenton-like reagent ... 

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.

Figure 6.6 In situ XRD of an alumina-supported iron catalyst during reduction in H2 at 675 K reveals the transition of a-Fe202 (hematite) via Fe(04 (magnetite) to metallic iron as a function of time. The graph shows the degree of reduction of supported and unsupported oc-Fe203 as determined from the XRD measurements (from Jung and Thomson f 14]).

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-... 

Fig. 19.1. X-ray diffractograms of two lots of an Fe oxide catalyst (Nanocat ) consisting of 2-line ferrihydrite (a) and a mixture of this with hematite (b).

The airbags in automobiles contain hematite which serves as a catalyst for the rapid release of N2 which inflates the bags (G. Buxbaum, pers. comm.). 

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). 

When iron catalysts are exposed to FT synthesis reaction environments, the catalysts first transform from hematite into magnetite. The transformation into magnetite is rapid and occurs pseudomorphically where the shape of the hematite crystals is retained including their swiss-cheese morphology. The transformation from magnetite to carbide is slow and is affected... 

In the absence of excess sulfur, the sulfated hematite did not show any activity. Post-reaction analysis of this catalyst showed that it was converted primarily into a-Fe. Addition of sulfur to the reaction mixture increased the activity of this Fe-based catalyst for all the reactions studied. XRD analysis of the spent catalyst indicates that it is transformed to a pyrrhotite (Fe1 A.S) phase. The results shown in Figure 27.6 and the values listed in Table 27.3 for the sulfated hematite, are those obtained with added sulfur. 

With the exception of quinoline HDN, all catalysts are active for the reactions studied. All three catalysts are found to attain complete sulfur removal in the form of H2S (100% HDS) from benzothiophene after 1 h of reaction time (Figure 27.6(a)). The desulfurization pathway is found to go through the rapid hydrogenation of benzothiophene to dihydrobenzo-thiophene, followed by cleavage of the C-S bond in dihydrobenzo-thiophene, to yield ethyl benzene as the main product of the reaction. As seen in Table 27.3, the oxynitride and the oxycarbide are almost twice as active for HDS than the sulfated hematite. In terms of the turnover frequency, the oxycarbide is more active than the oxynitride for all reactions tested. 

Less activity than that observed in the HDS reaction is seen for the hydrogenation of naphthalene (Figure 27.6(b)). All three catalysts exhibit comparable hydrogenation activity as evidence by the rates in Table 27.3. Tetralin is found to be the main product of the reaction with little decalin produced. Even though the sulfated hematite shows the highest percentage of hydrogenation, the oxynitride and the oxycarbide are more active per unit area of catalyst (Table 27.3). 

In summary, Mo2N Ov and Mo2CAOy are found to be more active per unit area than the sulfated hematite for all the reactions studied, despite the fact that the particle surface is heavily oxidized. In terms of turnover rate, the oxycarbide is found to be 2.4 to 3 times more active than the oxynitride. The hydrogen present in the reaction medium may be activating the oxycarbide and oxynitride catalysts, by partially removing surface oxygen. This is consistent with results from TG-MS experiments.22 The order of activity for the oxynitride and oxycarbide catalysts is the following HDS > HYD > HDO HDN. 

The catalytic activity of Fe carbides, molybdenum oxynitride and oxycarbide has been evaluated for coal liquefaction and heteroatom removal of model compounds related to coal. Preliminary results show that the LP nanoparticles are active catalysts for coal liquefaction. In fact, they are more active for heteroatom removal than a molybdenum promoted sulfated hematite, even though surface characterization indicates that as introduced into the reactor they exhibit surface oxidation. 

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