Ammonia synthesis dissociative chemisorption

The heat of chemisorption is, of course, the energy difference between the chemical bonds formed and those broken. One of the strongest bonds to be broken in dissociative chemisorption on metals is the N-N bond of N2. This chemisorption is known to be rate limiting in ammonia synthesis. Brill et al. reported in 1967 field emission results indicating that N2 adsorption on Fe is strongest on the (111) face." Then-suspicion that this might be the initial step in ammonia synthesis over Fe catalysts... 

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. 

The alkali metals have a role to play in ammonia synthesis. The K20 promoter in the conventional NH3 synthesis catalyst enhances the chemisorption of nitrogen and causes a hydrogen-promoted dissociation of the N2... 

Alkali metals are often used as additives during catalytic reactions. They are bonding modifiers that is, they influence the bonding and thus the reactivity of the coadsorbed molecules. Potassium is a promoter in CO hydrogenation reactions where CO dissociation is desired and is one of the elementary reaction steps. The alkali metal also reduces the hydrogen chemisorption capacity of the transition metal. Potassium is a promoter in ammonia synthesis for the opposite reason, because it weakens the NH3 product molecule bonding to the metal, thereby reducing its sur-... 

Surface Science of Ammonia Synthesis Structure Sensitivity of Ammonia Synthesis Kinetics of Dissociative Nitrogen Adsorption Effects of Aluminum Oxide in Restructuring Iron Single-Crystal Surfaces for Ammonia Synthesis Characterization of the Restructured Surfaces Effect of Potassium on the Dissociative Chemisorption of Nitrogen on Iron Single-Crystal Surfaces in UHV... 

Under the assumption that dissociative nitrogen chemisorption is the ratedetermining step and that the heat of adsorption varies linearly with coverage, Temkin and Pyzhev (18) derived their famous and widely used rate equation for the kinetics of ammonia synthesis ... 

Results of measurement of structure sensitivity are shown in Table 1 from which it can be seen that the (111) surface of iron catalyses the synthesis of ammonia from hydrogen and nitrogen at least 418 times faster than the ordered (110) surface and 16 times faster than the ordered (100) surface. Our result is consistent with the low pressure chemisorption studies of Ertl and co-workers where it was reported that the rate of dissociative chemisorption of nitrogen is greatest on the (111) plane of iron... 

The actual activity ratio of Fe(lll) Fe(lOO) Fe(llO) at 798 K was found to be 418 25 1 for catalysis of ammonia synthesis, ammonia being formed at a rate of 4.6 X 10 moles cm sec on the Fe(lll) face at this temperature. This is in broad agreement with the ratio obtained (3, 7, 8) for nitrogen dissociative chemisorption (60 3 1), the discrepancy probably being due to slight differences in crystal orientation (which would have a great effect in the case of the least active (110) crystal), temperature differences (the chemisorption results were obtained at 550 K, whereas the... 

Hinrichsen, O., Rosowski, F., Homung, A., Muhler, M. and Ertl, G. (1997). The kinetics of ammonia synthesis over Ru-based catalysts. Part I. The dissociative chemisorption and associative desorption of N2, J. Catal. 165, 33-44. 

To model a catalytic reaction, some knowledge of the elementary reaction steps must be assumed. For ammonia synthesis it is usually accepted that the dissociative chemisorption of nitrogen is the rate-limiting step, a process which requires two adjacent open sites on the catalyst surface. " Using Langmuir-Hinshelwood kinetics the rate of ammonia synthesis (denoted by r) can be written as... 

It was proposed by Brill that the rate-determining step in ammonia synthesis was molecular rather than dissociative nitrogen chemisorption. On this basis, the following equation was derived for the kinetics of ammonia synthesis ... 

The chemisorption of CO is interesting for the study of ammonia synthesis catalysts since this reaction provides a way of titrating the number of active sites on the surface. The reaction is complicated by the dissociation of CO. 

If the chemisorption or dissociation of dinitrogen is rate-determining in the ammonia synthesis as has been demonstrated, the isotopic equilibration of dinitrogen... 

The rate-determining step of ammonia synthesis and decomposition on Ni powder was studied using tracer isotopes (determination of the stoichiometric number) at 743 K. Dissociative adsorption and desorption were the slowest steps [225]. Dissociative adsorption was studied in few cases. The slight extent of chemisorption of N2 at 523 K to 573 K was speculated to be atomic species on Ni powder [226]. A spectroscopic study of surface nitride on polycrystalline Ni was carried out under a N2 pressure of 200 Torr [227]. 

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