Hagg carbide

The fixed-bed catalyst is a siUca-based extmdate containing precipitated iron oxide promoted with potassium and copper. The catalyst is activated by hydrogen reduction of most of the iron cataly2ed by small amounts of copper. As the catalyst is used, additional reduction occurs and Hagg carbide [12127 5-6] Fe C2, is formed. 

To verify that steady state catalytic activity had been achieved, the catalyst was allowed to operate uninterrupted for approximately 8 hours. The catalyst was then removed from the reactor and the surface investigated by XPS. The results are shown in Figure 2c. The two major changes in the XPS spectrun were a shift in the iron 2p line to 706.9 eV and a new carbon Is line centered at 283.3 eV. This combination of iron and carbon lines indicates the formation of an iron carbide phase within the XPS sampling volume.(J) In fact after extended operation, XRD of the iron sample indicated that the bulk had been converted to FecC2 commonly referred to as the Hagg carbide.(2) It appears that the bulk and surface are fully carbided under differential reaction conditions. 

Path Parameters Generated by FEFF (Single Scattering) for the Hagg Carbide... 

When reduced Fe/Ti02 is used as a catalyst for the reaction between CO and H2 to form hydrocarbons (the Fischer-Tropsch synthesis) the spectrum changes entirely. All metallic iron has been converted into a new phase. The spectrum is that of a crystallographically well-defined iron carbide, namely the Hagg carbide, or %-Fe5C2. Apparently the strongly reducing atmosphere has affected the unreduced iron as well all ions are now present as Fe2+. 

Hagg carbide was called iron percarbide" by Jack. 

Reaction (a) is measurable at about 275°C. The rate of decomposition of Hagg carbide, reaction (b), depends upon the amount of carbidic carbon present. For a completely carbided synthetic-ammonia-type catalyst the reaction, which proceeds approximately according to Equation (2),... 

Braude and Bruns (16) found that the rate of carburization of iron was greater with synthesis gas (H2 + CO) than with pure carbon monoxide. Similarly, Hall (15) found that the carburization of a reduced iron catalyst at 275°C. was slightly faster with 1H2 + 4CO gas than with pure carbon monoxide. In 48 hours, C values of 0.44 and 0.42 were obtained with 1H2 + 4CO and carbon monoxide, respectively, and fairly pure Hagg carbide was formed in both cases. 

Podgurski, Kummer, DeWitt, and Emmett (14) carburized reduced fused catalysts with propane, butane, and pentane at 325°C. Although x-ray patterns of Hagg carbide were found, the carbon content of these preparations approached 7.5 weight-% as a limit rather than 9.1% which was obtained upon carburization with carbon monoxide. Hall (15) found that during carburization of a reduced fused catalyst (Bureau of Mines number D3001) in n-butane at 300°C., C increased only to 0.22, and the carbide phase was cementite rather than Hagg carbide. Methane is too stable thermodynamically to carburize iron rapidly at low temperatures however, at 500°C. relatively pure cementite may be prepared with methane (15). 

When nitrides are treated with carbon monoxide at moderate temperatures, two reactions occur. The first may be termed a completion reaction in which carbon enters the interstitial phase until C + N increases to about 0.5. This reaction is more rapid than the second, the substitution reaction in which carbon replaces nitrogen. Similar reactions are observed when carbides of iron (Hagg carbide or cementite) are... 

Hagg carbide was the final phase up to 500°C., whereas cementite was formed at 700°C. Hagg carbide appeared when N had been decreased below 0.18, i.e., when about 64% of the nitrogen corresponding to the upper limit of the e-phase (Fe N) had been replaced by carbon. According to Jack s phase diagram (Fig. 2), the upper limit of the ("-carbonitride phase may be as high as C + N = 0.56. 

The role of carbides in the synthesis of hydrocarbons has been widely considered ever since the carbide theory was first postulated by Fischer and Tropsch in 1926 (20). Although recent experimental studies indicate that the carbide theory is largely incorrect, that is, that bulk-phase carbides are not intermediates in the formation of higher hydrocarbons, iron catalysts converted to Hagg carbide or cementite are usually more active than similar raw or reduced catalysts (21). (For a review of the carbide theory up to 1950, see p. 571 of reference 22.) The selectivity of carbided iron catalysts is essentially the same as that of corresponding reduced catalysts. Nitrides of iron are usually more active than reduced or carbided catalysts, and the catalyst selectivity is significantly different. 

After six weeks of test X273A, the catalyst was treated with hydrogen at 300°C. for 9 hours. This treatment removed the nitrogen but left a sizable amount of carbon in the catalyst (C = 0.24). X-ray analysis indicated the presence of -Fe, Cu, and possibly Hagg carbide. The activity of this reduced, used catalyst was not significantly different from that of the nitride however, the selectivity was greatly changed as shown in Fig. 6. 

With catalysts converted to fairly pure Hagg carbide or cementite, the rates of oxidation and elemental carbon deposition were very slow at 7.8 atm. (21). However, at 21.4 atm. these carbides oxidized at least as rapidly as metallic iron, the carbidic carbon being transformed to elemental carbon (21). 

Characterization of the used catalysts ( 15% Fe) has shown the presence of x Fe5c2 (Hagg carbide) and Fe30 for all materials investigated. For an Fe/ZSM-5 catalyst, it is estimated that 80% of the iron is in the metallic form after the reduction step (12). Following the carbiding step, X and c-carbides are detected in addition to Fe30. ... 

Hofer and Cohn (87) Thermomagnetic determination of Hagg carbide in used Fischer-Tropsch catalysts. 

Carbon buildup at 598 K became clearly evident when the bulk composition attained Fe2C, mostly Hagg carbide. Similar behavior was observed by Storch et al. (1), who reported that the amount of carbidic carbon became constant after carburizing in 0.1 atm CO at 598 K for three hours and by Hofer et al. (13) during carburization in CO at lower temperatures. Turkdogen and Vinter (14) studied carburization of iron in CO and H2/CO mixtures, and they reported that the rate of carbon deposition increased with temperature in the range 673-1073 K and that the carbon deposition ceased when the iron was converted to cementite. 

Curve II shows the same sample after having been heated to 800° in a stream of nitrogen. The Curie point is no less well defined, but has fallen to about 205° which is near the Curie point for cementite. The Hagg carbide has clearly undergone a reaction owing to its instability at high temperatures. 

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