Ammonia synthesis on iron catalyst

Studies of ammonia synthesis on iron catalyst suggest that the reaction occurs through surface imine radicals. 

The state of iron ammonia catalysts is dealt with in the following chapters, and x-ray, magnetic, and electric data will be discussed together with adsorption measurements. Information about the catalysts combined with kinetic experiments has led to a fairly good qualitative understanding of ammonia synthesis on iron catalysts, but owing to the extremely complicated nature of the catalyst surface during reaction, a quantitative treatment based on data of catalyst and reactants will not be attained in the near future. 

It is found in many reactions that a particular surface is favored. For example the (111) surface is particularly active in fee and hep metals. A strong dependence is foimd for ammonia synthesis on iron catalysts (Table 5-13) [T35]. Ammonia synthesis is one of the most structurally sensitive reactions. The opposite order was foimd for the decomposition of ammonia on copper, i.e., (111)>(100). In the decomposition of formic acid, the (111) surface is three times more active than (110) or (100). 

FIGURE 6.8. Microkinelics of ammonia synthesis on iron catalysts comparison of the yields calculated on the basis of the mechanism presented in Fig. 6.7 (y-axis) with experimental data from industrial plants (x-axis). The straight line... 

In other words, the stoichiometric number determined from the transfer of a tracer is also equal to that of the overall reaction and of the rate determining step. In the example of ammonia synthesis on iron catalyst, Ords can be equal to one or two. Experimental studies have reported both values. Horiuti et foyjjcl CTrds = 2 for an iron catalyst at near equilibrium. Tanaka ... 

The first application of Temkin theory in history was on cataljdic reaction for ammonia synthesis on iron catalyst. The famous reation rate equation of Temkin-Pyzhev was obtained, corresponding to the overall reaction ... 

In favor of a reasonably low temperature and thus high-equilibrium concentration, in technical ammonia synthesis the rate of the reaction is supported by the use of a solid catalyst. A complete description of the reaction kinetics therefore has to include the kinetics of the reactants adsorption and desorption on the catalyst surface. A first successful, integrated kinetic model of the ammonia synthesis on iron catalysts has been developed by Temkin and Pyzhev. An improved model is available from Brunauer, Love, and Keenan. A comprehensive surv of the catalysis of ammonia synthesis is given in Ref. [7]. 

The effect of alkali additives on N2 chemisorption has important implications for ammonia synthesis on iron, where alkali promoters (in the form of K or K20) are used in order to increase the activity of the iron catalyst. 

An experimental study of kinetics of ammonia synthesis on iron (101), cobalt, and nickel (96) catalysts, at ammonia concentrations much lower than that at equilibrium, showed that at pressures of the order of 1 atm the second of these possibilities is realized.7 When far from equilibrium, the... 

It is well established that ultrasmall metal clusters on supports have catalytic properties distinct from those properties of large bulk-like particles, as illustrated by the selective oxidation of propylene to propylene oxide by gold, alkene and arene hydrogenation catalysis,and CO oxidation. In these examples, the catalytic properties improve as the clusters become smaller. On the other hand, a reduction in size of the metal cluster can lead to less desirable catalytic properties as seen for ammonia synthesis on iron. Various explanations have been offered to account for the unique properties of nanoscaled metal catalysts, however, much remains to be understood. Clearly, experimental and theoretical studies will be required to develop an in-depth under-... 

The idea of C7 sites being the most active site in ammonia synthesis on iron has been suggested in the past. Dumesic et al. [42] found that the turnover number for ammonia synthesis was lower on small iron particles than on larger ones. Pretreatment of an Fe/MgO catalyst with ammonia enhanced the turnover number over small iron particles, but did not affect the larger particles. This result was explained by noting that the concentration of C7 sites would be expected to be higher on the smaller iron particles and that restructuring induced by ammonia enhanced the number of these sites on the catalyst. 

The experimental work of the past 50 years leads to the conclusion that the rate-determining step in ammonia synthesis over iron catalysts is the chemisorption of nitrogen. The question as to whether the nitrogen species involved on the surface is molecular or atomic is still not conclusively resolved, though, in my opinion, the direct participation of nitrogen in an atomic form seems more likely than that in molecular form. 

It was found during studies of ammonia synthesis on iron that the incorporation of a condenser downstream of the sample valve in the external circulation loop of the HPLP apparatus (Fig. 7), enabled the system to be run as a flow rather than a batch reactor. This is true for any reaction system where the reactants are more volatile than the products, since the condenser temperature can be adjusted to trap the products almost exclusively, allowing a nearly pure stream of reactants to impinge on the catalyst. In the case of ammonia synthesis, (where, next to the product, nitrogen at a partial pressure of 5 atm was the most condensable species) a slurry of isopentane (— 159.9 °C) was found to be the ideal condenser medium. During a study of rhenium-catalyzed ammonia synthesis the isopentane condenser was switched in periodically to reduce the ammonia partial pressure to below that at which it appeared to poison the catalyst. In this way, the rhenium was able to produce ammonia in excess of the amount usually leading to poisoning. 

The direct comparison of the catalytic activity and selectivity of surfaces with different orientations provides information abont the influence of the atomic structure. This has been well described (for example, see [15]). It is well established that catalytic reactions may depend on the atomic structure of the surface (i.e., they are structure sensitive). A classic example of a catalytic reaction that is sensitive to the atomic sUucture of the catalyst s surface is ammonia synthesis on iron surfaces [15], The (111) and (211) surfaces of iron exhibit a significantly higher reaction rate than the (100), (210), and (110) faces. This structural effect has been ascribed to C7 sites (i.e., Fe atoms with a coordination number of 7, or number of nearest neighbors), which exist only on the (111) and (211) surfaces. Now, what if the structure of the catalytic surface during the reaction differs substantially from the initially pure, well-defined crystalline metal surface For example, depending on the gas pressure (i.e., the chemical potential) new structures may become stable (see Sect. 8.2.2). Or what if only a small percentage of uncontrolled or varying defects and steps completely dominate the activity In the remainder of this chapter, these questions will be addressed. 

The first attempt to use the simultaneous solution approach for ammonia synthesis over iron catalysts gave estimated reaction rates several orders of magnitude lower than those found in practice. This is discussed in the chapter on iron kinetics, but can be attributed to inadequacies in the underlying database, in particular the use of low-coverage adsorption data. The closest approach to a mechanistically acceptable model has come from studies into ammonia decomposition on platinum. " The characteristics of this reaction over many transition metals are ... 

The second chapter Structure and Surface Chemistry of Industrial Ammonia Synthesis Catalysts is written by Dr. Per Stoltze. This chapter deals with the structure and surface chemistry of iron-based ammonia synthesis catalysts of the type used by industry. Certain studies of single crystal surfaces are included to the extent that they serve to add information to the main topic. This chapter includes a presentation of the unreduced catalyst the reduction process and the bulk and surface structure of a reduced catalyst. A thorough discussion is given of the different states of sorption of nitrogen and of chemisorption of hydrogen, carbon oxides, ammonia and oxygen. The last part of the chapter gives a detailed account of the mechanism of ammonia synthesis on iron. 

Equation (305) describes the ammonia synthesis rate not only on iron catalysts, but also over molybdenum catalyst (105), tungsten (106), cobalt (95), nickel (96), and other metals (107). Equation (300) describes ammonia decomposition on various metals (provided that there is enough H2 in the gas phase). 

It should be noted that the results for the formic acid decomposition donor reaction have no bearing for ammonia synthesis. On the contrary, if that synthesis is indeed governed by nitrogen chemisorption forming a nitride anion, it should behave like an acceptor reaction. Consistent with this view, the apparent activation energy is increased from 10 kcal/mole for the simply promoted catalyst (iron on alumina) to 13-15 kcal/mole by addition of K20. Despite the fact that it retards the reaction, potassium is added to stabilize industrial synthesis catalysts. It has been shown that potassium addition stabilizes the disorder equilibrium of alumina and thus retards its self-diffusion. This, in turn, increases the resistance of the iron/alumina catalyst system to sintering and loss of active surface during use. 

Let us now consider an example of a non-linear mechanism, including a reaction that involves two molecules of some intermediates. The probable reaction mechanism for ammonia synthesis on an iron catalyst can be represented as... 

Thus, here we have two independent routes. For a linear mechanism of ammonia synthesis on an iron catalyst we will have... 

Sulfur, Phosphorus, and Arsenic Compounds. Sulfur, occasionally present in synthesis gases from coal or heavy fuel oil, is more tightly bound on iron catalysts than oxygen. For example, catalysts partially poisoned with hydrogen sulfide cannot be regenerated under the conditions of industrial ammonia synthesis. Compounds of phosphorus and arsenic are poisons but are not generally present in industrial synthesis gas. There are... 

Another test of validity is to check the performance of the model against experimental rate data obtained far from equilibrium. The microkinetic model presented in Table 7.3.1 predicts within a factor of 5 the turnover frequency of ammonia synthesis on magnesia-supported iron particles at 678 K and an ammonia concentration equal to 20 percent of the equilibrium value. This level of agreement is reasonable considering that the catalyst did not contain promoters and that the site density may have been overestimated. The model in Table 7.3.1 also predicts within a factor of 5 the rate of ammonia synthesis over an Fe(lll) single crystal at 20 bar and 748 K at ammonia concentrations less than 1.5 percent of the equilibrium value. 

A microkinetic analysis of ammonia synthesis over transition metals is presented in Section 7.3. Use the results of that analysis to explain how adsorbed nitrogen atoms (N ) can be the most abundant reaction intermediate on iron catalysts even though dissociative chemisorption of N2 is considered the rate-determining step. 

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. 

Scholten, J. J. F., Chemisorption of nitrogen on iron catalysts in connection with ammonia synthesis. Thesis, Delft, 1959. 

Tennison S R (1991), Alternative N on iron catalysts in Catalytic Ammonia Synthesis Fundamentals and Practice, p. 303, (Ed.) Jennings J R, Plenum Press. 

Similarly to the concept of the biographical nonuniform surfaces mechanism (7.97) can be used to derive a rate expression in supposition of lateral interactions. Assuming that surface species other than chemisorbed nitrogen are present on the surface in inferior quantities, which is backed by experimental evidences showing that nitrogen adsorption on iron catalysts proceeds at a rate approximately equal to that of ammonia synthesis, the equilibrium constant of step 2 in eq. (7.97) can be expressed, following the general treatment, as... 

The early development of catalysts for ammonia synthesis was based on iron catalysts prepared by fusion of magnetite with small amounts of promoters. However, Ozaki et al. [52] showed several years ago that carbon-supported alkali metal-promoted ruthenium catalysts exhibited a 10-fold increase in catalytic activity over conventional iron catalysts under the same conditions. In this way, great effort has been devoted during recent years to the development of a commercially suitable ruthenium-based catalyst, for which carbon support seems to be most promising. The characteristics of the carbon surface, the type of carbon material, and the presence of promoters are the variables that have been studied most extensively. 

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