Iron catalysts doubly promoted

The theory of Kobozev is open to criticism of the character recently stated by Kummer and Emmett (184), who observed that there was a very rapid exchange between the isotopes of nitrogen in the presence of singly and doubly promoted iron synthetic ammonia catalyst. Kobozev (167) had concluded earlier that iron synthetic ammonia catalysts consisted of an ensemble of iron atoms to which were attached the promoter molecules. Each ensemble was capable of adsorbing only one nitrogen molecule further, in accordance with his theory, these ensembles, separated from one another by geometrical barriers, would make the... 

It is useful, however, to recall an example which does not conform to this condition, the decomposition of ammonia on doubly-promoted iron synthetic ammonia catalysts as studied by Love and EmmettT They found a kinetic equation,... 

There exists comparatively little information on the activation energy of the chemisorption of hydrogen on other metallic surfaces. The only additional data reported are those for doubly promoted iron catalysts by Emmett and Harkness (25). They obtained 10.4 +1.0 kcal./mole for Ae by means of equation (3) for the initial 2 cm. uptake of hydrogen in the temperature range from —78.5° to — 96.5°C. 

Brunauer and Emmett (70) concluded from an adsorption study of carbon monoxide, carbon dioxide, and nitrogen on doubly promoted iron catalysts that 1 wt. % of potassium oxide contained in the catalyst covers more than 50% of its total surface. Additional observations in agreement with these findings were made by Matsui (71). 

According to Brunauer and Emmett (73), the surface of the doubly promoted iron catalyst contains two types of sites corresponding, respectively, to Type A and Type B of hydrogen adsorption (first and... 

The various factors that can contribute to deactivation of iron Fischer-Tropsch (FT) catalysts include transformation of the active phase into an inactive constituent, poisoning by carbonaceous species and heteroatoms, and loss of active phase surface area. Progress in elucidating the causes of deactivation is hampered by the inability to conclusively identify the active phase in iron FT catalysts. In recent work involving doubly promoted, unsupported iron catalysts, the sequence of phase transformations shown in Figure 1 that take the catalyst from its as-prepared hematite phase to iron carbide [1,2] was postulated. 

For a doubly promoted magnetic iron oxide catalyst, working on a nitrogen hydrogen mole ratio of 1 3 at a pressure of 100 atm and 450°C, and a space velocity of 5,000(hr ), approximately 13-15% ammonia could be expected [10]. Much lower conversions are obtained from singly promoted or nonpro-moted iron. 

Ozaki et al. (33) compared the rate of ammonia synthesis on a doubly promoted iron catalyst with that of deuteroammonia, and found that deuterium reacts markedly faster than hydrogen imder the same reaction condition. From the kinetic data, as well as the isotope effect, they reached the conclusion that the rate-determining step of the overall reaction is the chemisorption of nitrogen on a surface mainly covered with NH radicals, and that the isotope effect is due to the fact that NH is adsorbed more strongly than ND. 

These results are consistent with earlier literature [63, 64] in which the effects of potassium on doubly promoted (aluminum oxide and potassium) catalysts were studied. It was shown that the turnover number for ammonia synthesis is roughly the same over singly (aluminum oxide) and double promoted iron when 1 atm reactant... 

Kummer and Emmett utilized this technique with a doubly promoted iron catalyst. Thus, radioactive and nonradioactive samples of CO were added in succession as two separate fractions to the reduced catalyst. The chemisorbed CO layer was removed by pumping and analyzed for CO. The results showed that the second fraction of added CO tended to desorb first. However, the results also showed that a partial rapid exchange occurred between the two fractions equivalent to about 50 percent of the iron surface even when adsorption was carried out at -196 °C. 

Figure 15 Influence of pressure In ethanol tracer runs over doubly promoted Iron catalysts, D3001 A, H7, 1 atm, 245-275 C B, HI9,7.5 atm, 241 °C C, HI 5, 21 atm, 242 °C (redrawn from Reference 11).

The above reaction is reversible so that primary alcohols may be dehydrogenated to an aldehyde which could decarbonylate to produce CO. It has been shown that the 1-alcohol and corresponding aldehyde are at or near an equilibrium composition when using a doubly promoted iron catalyst at 7 atm. The CO produced by the above reaction could produce CO2 through the WGS reaction ... 

As can be seen by the data in Figure 4, the CO2 produced when [l- C]-l-pentanol was added to the synthesis gas fed to a C-73 doubly promoted iron catalyst had a much higher radioactivity/mole than did the CO. It is not possible to produce CO2 with a higher C/mole than the CO that it is derived from in the WGS reaction. It was therefore proposed that the CO2 is formed directly from the added alcohol and not from the reverse of the alkene carbonylation reaction. 

With doubly-promoted iron synthetic ammonia catalysts the same type of observations are repeated. Data on desorption and readsorption are shown in Fig. 7, while in Fig. 8 are the values obtained on raising and lowering the temperature. It will be noted that the latter curves do not show the double maxima at —78 and 110 C. earlier found by Brunauer and Emmett (10) and examined by them in detail as types A and B adsorption. 

Fia. 6. Comparison of the isotherms for total carbon monoxide adsorption at —183° and for the physical adsorption on about 45 g. of pure iron synthetic ammonia catalyst (No. 973) and on a similar quantity of a doubly promoted iron catalyst (No. 931) (39). 

C. carbon monoxide is quickly chemically adsorbed by the surface of a pure iron catalyst in amounts sufficient to cover the entire surface (4). Accordingly, it seemed reasonable to conclude that whenever the volume of CO chemisorption on an iron catalyst promoted with AljOa and KjO was smaller than the volume of nitrogen required to form a monolayer over the entire catalyst, one had an indication that part of the surface was being covered up by promoter molecules that were concentrating preferentially in the surface layer. For the doubly promoted catalyst, such measurements... 

In summary then, the group at the Fixed Nitrogen Research Laboratory by 1925 had shown that doubly promoted iron catalysts containing about 3% aluminum oxide and one percent potassium oxide were entirely satisfactory for commercial use and would, if operated on pure gas, have a very long life. Actually, many similar commercial catalysts are said to retain their activity for more than 5 years. 

Figure 4. Comparison of the total and the van der Waals adsorption of carbon monoxide for doubly promoted catalyst 931 and pure iron catalyst 973. (Reproduced from Ref. 36. Copyright 1937, American Chemical Society.)...

Figure 5. Comparison of the total and van der Waals a sorp ion o carbon dioxide on a doubly promoted and on a pure iron catalyst (42).

German chemist in catalysis. He obtained a doctorate in chemical philosophy at Leipzig University nnder the supervision of Bodenstein after twists and turns. In 1904, he joined BASF and studied on ammonia synthesis as an assistant of Bosch. He noticed in study of iron nitrides that trace components changed catalytic performance dramatically and conducted studies of multi-component catalysts. He is a genius who found the doubly promoted iron catalysts only in more year. Haber and Bosch were awarded Nobel Prize for their contribntion to the technology of ammonia synthesis. It should be noted that the contribntion of Mittasch was no less than those of them. 

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