Iron catalysts sintered

Catalyst composition also depends on the type of reactor used. Fixed-bed iron catalysts are prepared by precipitation and have a high surface area. A silica support is commonly used with added alumina to prevent sintering. Catalysts for fluidized-bed application must be more attrition-resistant. Iron catalysts produced by fusion best satisfy this requirement. The resulting catalyst has a low specific surface area, requiring higher operating temperature. Copper, another additive used in the preparation of precipitated iron catalysts, does not affect product selectivity, but enhances the reducibility of iron. Lower reduction temperature is beneficial in that it causes less sintering. 

Structural promotion A highly dispersed support can provide and (or) stabilize a high surface area of the catalyst supported by it. A typical example is ammonia synthesis where the thermal sintering of the iron catalyst is inhibited by alumina (although the phase configuration is different). 

Table 5.1 shows an application of XPS to the study of the promoted iron catalyst used in the Haber synthesis of ammonia. The sizes of the various electron intensity peaks allows a modest level of quantitative analysis. This catalyst is prepared by sintering an iron oxide, such as magnetite (Fe304) with small amounts of potassium nitrate, calcium carbonate, aluminium oxide and other trace elements at about 1900 K. The unreduced solid produced on cooling is a mixture of oxides. On exposure to the nitrogen-hydrogen reactant gas mixture in the Haber process, the catalyst is converted to its operative, reduced form containing metallic iron. As shown in Table 5.1, the elemental components of the catalyst exhibit surface enrichment or depletion, and the extent of this differs between unreduced and reduced forms. 

The deactivation of a Fischer-Tropsch precipitated iron catalyst has been investigated by means of a novel reactor study. After use of the catalyst in a single or dual pilot plant reactor, sections of the catalyst were transferred to microreactors for further activity studies. Microreactor activity studies revealed maximum activity for catalyst fractions removed from the region situated 20 - 30% from the top of the pilot plant reactor. Catalyst characterization by means of elemental analyses, XRD, surface area and pore size measurements revealed that (1 deactivation of the catalyst in the top 25% of the catalyst bed was mainly due to sulphur poisoning (2) deactivation of the catalyst in the middle and lower portions of the catalyst bed was due to catalyst sintering and conversion of the iron to Fe304, Both these latter phenomena were due to the action of water produced in the Fischer-Tropsch reaction. 

Nitrided iron catalysts are remarkably stable toward oxidation and deposition of elemental carbon in the synthesis at both 7.8 and 21.4 atm. Shultz, Seligman, Lecky, and Anderson (23) described composition changes in nitrided fused, sintered, and precipitated catalysts in the synthesis. In a test (X218) with 1H2 + ICO synthesis gas at 7.8 atm., the catalyst was sampled frequently for analysis. X-ray diffraction patterns showed only the e-nitride or carbonitride phase, except for weak... 

The sintered iron catalysts have been developed by Lurgi/Rubrchemic to the pilot plant stage (3]. 

In the ARGE fixed bed process Idgh boiling fractions and waxes are mainly obtained [4J, Under the same curidiliuns, sintered iron catalysts yield mainly hydrocarbons in the gasoline and diesel range. This can be explained by their lower specific surface area, pore volume and specific activity. Of the three catalyst types, (he fused iron catalyst Is characterized by the lowest specific surface area, pore volume and activity and is thus operated at higher temperatures. 

Due to the lower operating temperature (< 250 C). prccipitalcd iron catalysts show little sensitivity towards carbon deposition. However, these catalysts are deactivated by sintering which lead to a reduction of the surface area from about 200 m /g for a freshly-conditioned catalyst to about 50 m /g for a used catalyst. 

The hot gas recycle process made it necessary to use iron catalysts of adequate mechanical strength. Sintered catalysts showed better resistance against the erosive influence of fast moving gases than highly active precipitation catalysts. 

Catalyst Sintered Fe Fe oxide powder reduced Fused iron (reduced)... 

More recent studies have shown that a magnetic method may reveal the distribution of particle sizes in supported nickel catalysts. The method appears to be effective down to near-atomic dimensions, and it permits independent determination of rates and activation energies for the reduction process as contrasted with the sintering, or particle-growth, process. The structural relationship of impurities or promoters, such as copper, in the nickel is readily determined, and extension of the method to cobalt and iron catalysts seems possible. 

The deactivation of bulk iron oxide during methane combustion has been studied. The observed deactivation behaviour has been explained as the result of two simultaneous deactivation mechanisms. In the initial phase of reaction both are in operation and the activity drops rapidly as a consequence of both catalyst sintering and of the depletion of lattice oxygen in the outer layers, due to a partial reduction of the catalytic surface. In later stages, catalyst deactivation is almost exclusively due to sintering imder reaction conditions. A kinetic model of deactivation is presented, together with the physicochemical characterization of fresh and partially deactivated catalysts. 

We now encounter a semantic problem of considerable size. It has been recognised for a very long time that the activity of metal catalysts can be helped by the presence of quite small amounts of substances that of themselves have no or little activity. This concept first achieved prominence in the development of iron catalysts for ammonia catalysts, and of iron and cobalt catalysts for Fischer-Tropsch synthesis, and the term promoter was applied to these substances. They were of two kinds (i) structural promoters such as alumina, which acted as grain stabilisers and prevented metal particle sintering and (ii) electronic promoters such as potassium that entered the metallic phase and actually enhanced its activity. In these cases the metal is the major component, so that the catalyst is a promoted metal rather than a supported metal. 

Wyckoff and Crittenden (34) used x-ray line broadening to measure the crystal size of a reduced iron catalyst before and after heating it to 600 C. They shovjed that the promoted catalysts were stable to this temperature whereas the pure iron catalysts or those promoted only with K2 0 sintered badly and grew large crystals. This work was done before the development of a method for measuring surface area. It predicted properly, however, the stabilizing effect of both aluminum oxide and two promoters, aluminum oxide and potassium oxide. 

Another example of a structure promoter is AI2O3 in ammonia synthesis. It was long assumed diat AI2O3 hinders the sintering of the iron following reduction of the catalyst, but it is now believed that AI2O3 favors the formation of highly catalytically active (111) surfaces of the iron catalyst. 

G. A. Khger, L. S. Glebov, R. A. Fridman, E. I. Bogolepova, and A. N. Bashkirov [Kinet. Catal., 19, 489 (1978)] studied the kinetics of the hydroamination of 2-octanone and other aliphatic ketones over a sintered iron catalyst ... 

Pure iron(iii) oxide performs rather poorly as a WGS catalyst, due to rapid catalyst deactivation by sintering. Traditional iron catalysts typically consist of iron(iii) oxide (80-90% by mass), chromium(iii) oxide (8-10% by mass) and small amounts of other stabilisers and promoters such as copper(ii) oxide, aluminium oxide, alkali metals, zinc oxide and magnesium oxide. The small fraction of chromium(iii) oxide acts to prevent catalyst sintering, and also promotes the catalytic activity of iron. Catalyst deactivation is typically caused by poisons in the feedstock gases and by deposition of solids on the catalyst surface. 

Synthesis of ammonia. The synthesis reaction is dependent on the conditions of equilibrium and the kinetics of the reaction. The latter is dictated by the efficacy of the catalyst, which in turn is chosen because of its cheapness and activity. Iron is the only realistic catalyst, but its activity can be greatly increased by the use of suitable promoters. It is prepared by melting iron oxide, refractory oxides such as potassium and aluminium oxides. A solid sheet forms on cooling, and is broken down into 5-10 mm lumps. The whole is then reduced in the ammonia synthesizer, where the oxide is converted to iron crystallites separated by the refractory oxides and covered in part by KOH as a promoter. The KOH can enhance the reactivity twofold. This catalyst must be used within the temperature range 400°-540 °C. Below this the catalyst becomes uneconomically inactive above, it sinters and loses surface area. An improved iron catalyst of higher activity and longer life is a feature of the AMV process. It is important to note that much of the reason for improved and continued activity is due to the careful removal of poisons such as CO, CO2, and H2S. 

Most of the early work on aluminum oxide, in relation to ammonia synthesis, suggests that the role of aluminum oxide in the ammonia synthesis catalyst was simply to increase the surface area of the iron catalyst and to inhibit sintering which usually occurs with high surface area metallic particles. " This belief is supported by BET measurements, which showed that the surface of the industrial... 

Structural promotion can take two main forms, although both are concerned with maintaining the effective surface area of the catalyst active component. The use of alumina to generate the pore structure of iron catalysts has been investigated extensively and is discussed fully in an earlier chapter. In the absence of alumina, iron sinters on reduction, giving low surface areas. In the presence of the optimum level of alumina (approximately 2%) surface areas as high as 25 m g" can be obtained although, in the presence of potassium, this is reduced to 10-15 g ... 

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