Ammonia synthesis elementary reactions

The first step in constructing a micro-kinetic model is to identify all the elementary reaction steps that may be involved in the catalytic process we want to describe, in this case the synthesis of ammonia. The overall reaction is... 

Guideline 6. The great majority of known elementary steps are bimolecular, the remainder being unimolecular or termolecular. Any reaction where the stoichiometric coefficients of the reactants add up to four or more must involve a multiplicity of steps. The ammonia synthesis reaction is known to occur by a number of steps rather than as... 

One or more steps may form a dead end in the form of an intermediate formed through an elementary reaction and consumed exclusively by the reverse of this step. Although the dead-end will not contribute to the overall reaction rate, the step may affect the kinetics if the intermediate is strongly adsorbed on the surface. The poisonous effect of H2O in ammonia synthesis is an example. 

In many cases, however, the sequence of kinetically relevant elementary steps can be reduced to two steps (M. Boudart and G. Djega-Mariadassou, Kinetics of Heterogeneous Catalytic Reactions, Princeton University Press, Princeton, 1984, p. 90). For example, the sequence given above for ammonia synthesis can be greatly simplified by assuming step 1 is rate-determining and all other steps are nearly equilibrated. The two relevant steps are now ... 

The ammonia synthesis reaction is one of the most studied and best understood reactions in heterogeneous catalysis, but it has been the subject of few papers involving transient methods. SSITKA experiments have been performed at 350-500°C and 204-513 kPa using a commercial Haldor-Tops0e KMIR catalyst, with iron triply promoted by AI2O3, K2O, and CaO (262). Similar studies using K-promoted Ru/Si02 have also been reported 263). The promoted Ru catalyst is much more active than Ru alone, and new, very active sites are detected on the promoted catalyst. It seems that the analysis of this type of experiment would benefit from the elementary-step approach, as exemphfied by Kao et al. 107) two kinds of sites can be included in such a model. 

For instance, if we write down the ammonia synthesis reaction from the elements as N2 + 3 H2 = 2 NHs, we can particularize the elementary reactions involved in this process to the stoichiometric numbers M = 1, 2, and 3, respectively, as ... 

The equilibrium constant of the overall heterogeneous catalytic reaction can be expressed by the multiplication of the equilibrium constants of the elementary steps, which constitute the overall reaction. In the particular case of ammonia synthesis (eq.7.97), the equilibrium constant is given as... 

In the kinetic modelling of catalytic reactions, one typically takes into account the presence of many different surface species and many reaction steps. Their relative importance will depend on reaction conditions (conversion, temperature, pressure, etc.) and as a result, it is generally desirable to introduce complete kinetic fundamental descriptions using, for example, the microkinetic treatment [1]. In many cases, such models can be based on detailed molecular information about the elementary steps obtained from, for example, surface science or in situ studies. Such kinetic models may be used as an important tool in catalyst and process development. In recent years, this field has attracted much attention and, for example, we have in our laboratories found the microkinetic treatment very useful for modelling such reactions as ammonia synthesis [2-4], water gas shift and methanol synthesis [5,6,7,8], methane decomposition [9], CO methanation [10,11], and SCR deNO [12,13]. 

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-... 

We have made a distinction between an overall reaction and its elementary steps in discussing the law of mass action and the Arrhenius equation. Similarly, the basic kinetic laws treated in this section can be thought of as applying primarily to elementary steps. What relationships exist between these elementary steps and the overall reaction In Table 1.1 we gave as illustrations the rate laws that have been established on the basis of experimental observations for several typical reactions. A close look, for example, at the ammonia synthesis result is enough to convince one that there may be real difficulties with mass action law correlations. This situation can extend even to those cases in which there is apparent agreement with the mass action correlation but other factors, such as unreasonable values of the activation energy, appear. Let us consider another example from Table 1.1, the decomposition of diethyl ether ... 

Consider the sequence of elementary reactions involved in the ammonia synthesis... 

In 1993, almost 20 years later, another symposium was held in the honor of H. Topsoe and A. Nielsen, two of the leaders in industrial ammonia synthesis, and there, a general agreement was reached that the essential aspects of the kinetics and mechanism of this process are now more or less understood (20). This progress has essentially to be attributed to the large number of experimental studies applying surface physical techniques to well-defined, single-crystal surfaces (the surface science approach) that in combination with quantum-chemical calculations and theoretical treatment of the kinetics of the various elementary reactions (microkinetics) provided a rather closed picture. 

According to Gerhard Ertl (born 1936, Nobel Prize in Chemistry in 2007), the reaction scheme of ammonia synthesis may be formulated by the following sequence of elementary steps ... 

Mechanism of Catalytic Ammonia Synthesis Reaction 2.3.1 Elementary reactions... 

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). 

Obviously, in order to consume the intermediates formed in former steps, some steps must be carried out more than once. The stoichiometric numbers of every elementary step for ammonia synthesis reactions, as shown above, are 1, 3, 2, 2 and 2, respectively. If there is only one rate determining step and different stoichiometric number a for each elementary step, it is possible to use stoichiometric number to decide which step are RDS. Once the stoichiometric number of RDS in reactions is known, the rate equation of reverse reaction can be calculated from forward reaction rate and thermodynamic equilibrium relationships. 

The initial method for the measurement of a was to determine the rate at approaching equilibrium. Taking ammonia synthesis over iron catalyst as an example,tracer is N. Assuming elementary step sequence of the reaction as follows ... 

This is a continuous process. In addition to questions about the elementary steps of the reaction and the importance of the real structure and subnitrides for the catalyst efficiency, as well as the wide-open question about new catalyst materials, there are also new challenges for new applications. The ammonia synthesis industry still requires higher efficient catalyst. 

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... 

TABLE 5.1. Energetics of the Elementary Reactions Derived from the Potential Energy Diagram (Fig. 5.1) of Ammonia Synthesis"... 

Therefore the energetics listed in Table 5.1 are the exact re-expression, in elementary reaction terms, of the potential energy diagram published by Ertf for ammonia synthesis. 

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