Iron-ammonia catalyst adsorption-desorption

Fastrup, B. (1994) Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis. Top. Catal., 1, 273. 

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. 

Figure 1. Adsorption isotherms for a pure iron synthetic ammonia catalyst for various gases near their boiling points. Curve lA is for physical plus chemical adsorption of CO. Curve IB is for physical adsorption occurring at -1830C after the evacuation of the samples at -78 C for an hour. The solid symbols are for desorption. (Reproduced from Ref. 22. Copyright 1937, American Chemical Society.)...

Fig. 8. Mossbauer spectra of adsorption and desorption of ammonia on an iron-on-silica gel catalyst at 25°C. [From M. C. Hobson, Nature (London) 214, 79 (1967).]...

J.J. Scholten, P. Zwietering, J.A. Konvalinka, and J.H. de Boer. Chemisorption of Nitrogen on Iron Catalysts in Connection with Ammonia Synthesis. Part 1. The Kinetics of the Adsorption and Desorption of Nitrogen. Trans. Faradav Soc. 55 2166 (1959). 

V is the rate of nitrogen adsorption and W is the rate of desorption. By assuming that the slow step in the decomposition of ammonia is the rate of nitrogen desorption and that the slow step in the synthesis is the rate of adsorption of nitrogen, one then has a basis for predicting the kinetic expressions for synthesis and decomposition of ammonia over the iron catalysts. 

The dissociation adsorption of N2 is supported by many experimental results (i) Only NH and N were detected after N2 and H2 were adsorbed on dual-promoter iron catalyst at reaction temperature and 101.3kPa and vacuumed at 200°C (ii) N2 adsorption state has never been found by XPS detection. Heat desorption data provided by Toyashima and Xiamen University showed that N2 is not the main adsorption species under the conditions of ammonia S3mthesis at 400°C-450°C (iii) ErtP° provides a powerful support for the dissociation adsorption of N2 by the following results from energy spectroscopy ... 

Based on the principle of equilibrium state approximation, only one elementary step is the determining step for the three kinetic models listed in Table 2.5. The overall rate of a reaction is determined by the rate of the rate determining step (RDS) with slowest rate, while other steps are approximately in chemical or adsorption equilibrium. There are three opinions or possibilities for the rate determining step of ammonia synthesis reaction on fused iron catalyst (i) RDS is the dissociation of adsorbed dinitrogen N2(ad) 2N (ad) (ii) the surface reaction of adsorbed species N (ad) + H (ad) NH (ad) (iii) the desorption of adsorbed ammonia NH3(ad) NHs(g). 

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]. 

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