Kinetics of ammonia synthesis

If all the experimental evidence presented in the preceding sections is put together, the reaction scheme for the catalytic synthesis of ammonia on iron-based catalysts can unequivocally be formulated in terms of the following steps  

Before turning to these quantitative aspects, it is first necessary to discuss the qualitative features of the presented mechanism and its energetics which provide conceptual insight into the progress of this reaction. The progress of the reaction 

The assumption that nitrogen adsorption was the rate-limiting step of ammonia synthesis had already been applied in the derivation of the Temkin rate equations, which have proved to be quite successful in describing the kinetics of industrial ammonia synthesis provided that the empirical parameters included were properly selected. This formalism also includes the nonuniformity of the surface, viz. the variation of the adsorption parameters with coverage. As was pointed out by Boudart, integration of the rates over this (assumed) distribution yields an expression for the overall turnover rate that is interestingly rather similar to the equation which results for a uniform surface (modeled by Langmuir kinetics) with 

Even more important than these quantitative data are the more qualitative features of this model, reproducing the trends on the rate during variation in temperature, gas flow, pressure, as well as the influence of potassium coverage 

A general conclusion from these models based on single-crystal data is that the most abundant surface species under practical synthesis conditions will be adsorbed atomic nitrogen ( 90%), despite the fact that its formation is the rate-limiting step of the overall reaction. 

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Kinetics of Ammonia Synthesis on Re Crystal Surfaces. The kinetics of the ammonia synthesis reaction on Re were studied on the Re(ll50) surface which is composed of atoms having seven-fold coordination. The rate of production at 70 K and 20 atm. ... 

Many works were devoted to the mechanism and kinetics of ammonia synthesis. In accordance with the outline of this article, only the main results of studies of ammonia synthesis by the author with co-workers will be presented here other works will be cited, but in connection with our studies. The discussions of kinetics and the mechanism of ammonia synthesis in the reviews published in this series (90) and in the contribution by Nielsen et al. (91) can serve as supplementary sources of information. The contribution by Malina (92) contains interesting historical data and an extensive list of references. 

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... 

The basic equation most widely accepted to express the kinetics of ammonia synthesis is that of Teml and Pyzbev (1940). It expresses the reaction rate as a function of the partial pressures of the reactants and products ... 

Contradictory data on the kinetics of ammonia synthesis, especially in the earlier literature, in some circumstances may reflect a lack of attention to the influence of impurities in the gas. If oxygen compounds are present in the synthesis gas, reversible poisoning of the adsorbing areas, in accordance with an equilibrium depending on the temperature and the water vapor-hydrogen partial pressure ratio, must be taken into account when developing rate equations (see also Section 3.6.1.5). 

Mathematical models [216] for calculating these effectiveness factors involve simultaneous differential equations, which on account of the complex kinetics of ammonia synthesis cannot be solved analytically. Exact numerical integration procedures, as adopted by various research groups [157], [217]-[219], are rather troublesome and time consuming even for a fast computer. A simplification [220] can be used which can be integrated analytically when the ammonia kinetics are approximated by a pseudo-first-order reaction [214], [215], [221], according to the Equation (21) ... 

Describe the main differences in the kinetics of ammonia synthesis over iron catalysts compared to ruthenium catalysts. 

Numerous studies of the kinetics of ammonia synthesis and decomposition have been made. With a few exceptions, work has tended to show that the slow step in the synthesis of ammonia is the chemisorption of nitrogen and the slow step for the decomposition is the desorption of nitrogen. Furthermore, it turns out that the decomposition and synthesis of ammonia usually involve in the rate expression a term where y/x is close to 1.5. In 1940, Temkin and Pyzhev derived an equation consistent with both of these observations [M. I. Temkin and V. Pyzhev, Acta Physiochim. U.R.S.S. 12, 327 (1940)]. It has formed the basis for most of the kinetic treatments of ammonia synthesis and decomposition in recent years. 

P. Stoltze and J.K. Norskov, An interpretation of the high-pressure kinetics of ammonia synthesis based on a microscopic model. /. Catal. 110,1 (1988). 

HansenJB (1995), Kinetics of ammonia synthesis and decomposition , p. 149 in Ammonia Catalysis and Manufacture (Ed.) Nielsen, A., Springer-Verlag, Heidelburg, Germany. 

M. Temkin and V. Pyzhev. Kinetics of ammonia synthesis, promoted iron catalysts. Acta Physicochimica U.R.S.S., l2 3) 327-356, 1940. 

However, Murakami et al. (2005) suggested that the ammonia synthesis rate may not depend on only electrolysis potential. Other factors such as catalytic activity of electrode material, partial pressure of gaseous reactants, and temperature are cmcial parameters for the kinetics of ammonia synthesis (Mamellos, Zisekas, Stoukides, 2000 Skodra Stoukides, 2009). 

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 ... 

This isotherm may be derived from kinetic considerations for intermediate surface coverages (0.2 < 0 < 0.8), but it does not lend itself to multicomponent adsorption and also fails to predict the limiting conditions of — 0 when Pa 0 and 9 — when 00. Even though it was used for correlating the kinetics of ammonia synthesis, the Temkin isotherm has not found much use in the kinetic analysis of solid-catalyzed gas-phase reactions. 

Until recently, studies of the kinetics of ammonia synthesis have been carried out... 

Temkin M.J., Pyzhev V. Kinetics of ammonia synthesis on promoted iron catalysts. Acta Physiochim., 1940 12 327- 56. 

Analysis of microreaction (intrinsic) kinetics of ammonia synthesis... 

The microkinetics analysis, as a useful and powerful tool to interpret, harmonize and consolidate the study of catal3dic phenomena, can describe various results obtained at wide experimental conditions. For ammonia synthesis reaction discussed in this section, the microkinetic models are evaluated from the experimental data such as the sticking coefficient of dissociated nitrogen adsorption, the spectrum of programmed-temperature desorption of adsorbed nitrogen as well as the kinetics of ammonia synthesis at industrial conditions and at laboratory conditions are far from equilibrium. 

This book comprises 10 chapters which can be classified into four parts. The first part deals with the catalyst itself, including the development (Chapter 1), chemical components and physical structure (Chapter 3) preparation and reduction (Chapters 4 5) and the performance evaluation of the catalysts (Chapter 7). Those of ruthenium catalysts are solely put in Chapter 6. The second part is about the reaction mechanism and kinetics of ammonia synthesis (Chapter 2). The third part is a combination of the above two, namely, is centered on the relationship between the performance of catalysts and reaction, which includes reaction condition, reactor, process and application condition and its impact on the economic benefit of... 

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. 

Studies on the microscopic dynamics of dissociative nitrogen adsorption will certainly continue in the future, but there is now general agreement about the nature of surface species involved and the kinetic parameters of this process on the Fe( 111) surface, such data being essential for the kinetics of ammonia synthesis. 

The potential energy diagram (Fig. 5.11), derived from the detailed kinetics of ammonia synthesis obtained from a considerable body of work by Ertl and co-workers " in combination with experimentally derived A-factors, predicts rates of ammonia synthesis within a factor of 10 of those found experimentally when the kinetics are extrapolated by calculation to pressures (107 bar) and temperatures (723 K) used in the industrial manufacture of ammonia. Furthermore, the overall activation energy predicted by the model (62kJmoP ) is also in good agreement with that (81 kJ moP found by Somorjai and co-workers " at 20 bar. 

KINETICS OF AMMONIA SYNTHESIS AND INFLUENCE ON CONVERTER DESIGN... 

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 ... 

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