Kinetics of NH3 Synthesis

The kinetics of ammonia synthesis is treated in detail in chapter a. The experimental results on the kinetics of ammonia synthesis and decomposition will be included in the present chapter to the extent that these results illustrate aspects of the mechanism. 

Calculations using a numerical model for the kinetics and mechanism of ammonia synthesis show that the reaction orders and activation energy in this model are not constant but show some dependence on the reaction conditions for the catalyst [396], 

---

While the number of parameters may be large, only a few of the parameters are usually significant. The significant parameters are easily determined by calculation of the sensitivity of the calculated rate at typical conditions to a small variation in the value of each parameter. For the NH3 synthesis, for instance, the rate of dissociation of N2 and the binding energy for N are the most significant parameters and the kinetics of desorption of N2 in a TPD experiment is rather closely related to the kinetics of NH3-synthesis. 

The numerical models of the kinetics of NH3 synthesis will be discussed in Sect. 2.7.3 All contain a model of the kinetics of N2 adsorption... 

While it is fairly obvious that Al and Ca are structural promoters and increase the activity of the catalyst by increasing the specific area, it is also obvious that K does increase the catalytic activity, but does not do so by increasing the specific area. While the effects of K on the kinetics of NH3 synthesis may fairly easily be observed in a high pressure reaction experiment, the cause of the effects are very hard to deduce from the observed kinetics. 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

The kinetics of the synthesis of ammonia in the presence of an industrial catalyst have been investigated.25 The study was performed in a flow system at various working temperatures. The kinetics and mechanisms of several reactions of NH3 have been studied,26 including ... 

Ammonia Synthesis over Non-Iron Catalysts and Related Phenomena 125 Table 3.8. Kinetic parameters of NH3 synthesis according to the temkin-phzhev equation... 

It is known that chlorine acts as severe poison for NH3 synthesis [20,21]. Hence recent kinetic studies used chlorine-free Ru precursors like Ru3(CO)i2 [8,22] or Ru(N0)(N03)3 [7]. In addition to chlorine, the presence of sulphur was found to poison Ru catalysts. Fig. 2A demonstrates that both poisons may originate from the Ru precursor. The binding energies for the Cl 2p peak and of the S 2p peak observed for Ru prepared form RUO3 are typical for chloride and sulfide anions, respectively [23]. Ru prepared from Rus(CO)i2 was found to have a significantly higher purity. As shown in fig. 2B, sulphur and chlorine impurities can also originate from the support. The XPS data of MgO with a purity of 98 % reveal the presence... 

Studying the kinetics of the interaction of N2 with the Ru catalysts revealed that the Cs promoter enhances both the rate of dissociative chemisorption and the rate of recombinative desorption. Ru catalysts were found to be rather inactive for NH3 synthesis without alkali... 

Bidentate sites may be available in DNA itself. In the binding of Hg2+ and Ag+ we surmise that the binding is to amino-groups of cytidine and adenine and possibly the N(7) of guanine as all these positions are accessable in DNA. The introduction of a more kinetically permanent group such at [Pt(NH3)2]2+ attached to two such groups, e.g. two amino groups could lead to the formation of an inter-strand cross-link and an inhibition of DNA synthesis (90). 

The preparation and characterization of K3[0s(CN)5(NH3)] 2H20 have been described. This complex is a useful starting material for the synthesis of other [Os (CN)5L] species (e.g., L = py, pyz) and [0s(CN)5(H20)] can be obtained by the controlled aquation of [Os(Cf 5(NH3)f The kinetics of these ligand displacements have been investigated and mechanisms for the processes have been discussed. The UV-vis spectrum of each complex in the series [Os"(CN)5L] in which L is py, pyz, Mepz, or derivatives thereof, exhibits an intense, asymmetric MLCT absorption, split by spin-orbit coupling, and the effects of the electronic properties of L on the spectra have been examined. Redox properties of the complexes and the kinetics of the dissociation of pyz from [Os(CN)s(pyz)] have also been reported. ... 

The kinetic isotope effect observed for the FTT synthesis by Lancet and Anders71) on C02, CH4, C2 + (which means ethane and heavier hydrocarbons) and a wax fraction recovered from the catalyst, is not in disagreement with the experimental values (comparison between Fig. 2,1 and II A). The same group 1 have pointed out that in the case of FTT in the presence of NH3, the amino acid data (813C as high as +44%0) would not be inconsistent with the experimental values either (see Fig. 1). 

Stoichiometric number— The concept of stoichiometric number, introduced by Horiuti and Ikusima in 1939, was initially applied to reactions associated with the synthesis of NH3. In electrochemistry, stoichiometric number was used in the kinetics of hydrogen and oxygen evolution. Generally, stoichiometric number of a substance B is the z/p coefficient in the stoichiometric equation of the multistep -> chemical reaction... 

DNA synthesis that occurred during the experiment. These latter authors claimed that, when a correction factor was applied, the overall binding to DNA increased in a similar fashion for both isomers. These experiments were complicated because the platinum compounds were dissolved in the coordinating solvent Mc2SO, which is known to react with trans-DDF with a half-life of 8 min (123). The compound rrans-[Pt(NH3)2(Me2SO)Cl]Cl alters both the kinetics of binding to DNA as well as the adduct spectrum (123). Curiously, attempts to control for this effect in phosphate-buffered saline were performed under different conditions than those described in Ref. 23. The later of the only two time points tested does not appear to fit the trend of continuously increasing tran -DDP-DNA adducts, even after correction for DNA synthesis (107). 

Ruthenium(II) [Ru(NH3)50H2] can be most efficiently prepared by zinc amalgam reduction of [RuCl(NH3)5]Cl2 in aqueous solution. More recently, an alternative route avoiding Zn + and Cl" ion has been developed based on the aquation of electrochemically reduced [Ru(03SCF3)(NH3)5] " . Alternative routes include the photolysis and acid-catalysed hydrolysis of [Ru(NH3)g] and the reduction of [Ru(NH3)5(OH2)]. The lability of the aqua ligand in these systems makes [Ru(NH3)jOH2] an excellent starting material for the synthesis of substituted pentaammine complexes, and for the study of their kinetics of formation. 

For more complex reversible reactions, such as the oxidation of SO2 or the synthesis of NH3, numerical calculations are needed to determine rj, because the kinetics are complex and the diffusivities of reactants and products are different. 

As examined in Section 4.5.4, the effectiveness factor is only constant and independent of the degree of conversion if we have an irreversible first-order reaction (Tab. 4.5.5, Example 4.5.6). This is not the case for NH3 synthesis, which is with regard to the kinetics much more complicated as we have a reversible nth order reaction according to the Eqs. (6.1.9) and (6.1.10). Hence, the effectiveness factor depends on the reactant (H2 and N2) and product (NH3) concentrations and thus on the axial position in a fixed bed reactor. This leads to a decrease of the intrinsic rate along the bed (at almost constant effective diffusion coefficients), and in return to an increase of the effectiveness factor from the reactor inlet to the outlet as shown in Figure 6.1.7 for an isothermal and therefore hypothetical ammonia reactor apart from the assumption of isothermality, the parameters used for the calculations correspond to an industrial reactor. 

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). 

---