Ammonia synthesis rates

Figure 9.33. Ammonia synthesis rate and current response to a step change in the catalyst potential Uwr of the promoted Fe/CaZro9lno, Oj.u catalyst,43 Reprinted with permission from the American Chemical Society.

These effects profoundly influence the ammonia, as shown in Fig. 8.28 where the ammonia synthesis rate is plotted for two basal planes of iron and the same iron surfaces modified with 0.1 Ml of potassium. 

Recently, independent publications by Kojima and Aika and workers from Topsoe have reported on the high activity and stability of Cs/ C03M03N based catalysts for ammonia synthesis. The activity has been found to be significantly higher than for the commercial iron based catalyst. Table 3 gives a comparison of the ammonia synthesis rates of Cs and K promoted catalysts at various promoter loadings (2, 5, 10 and 30 mol%) for low conversion at 400°C and atmospheric pressure. ... 

Equation (305) describes the ammonia synthesis rate not only on iron catalysts, but also over molybdenum catalyst (105), tungsten (106), cobalt (95), nickel (96), and other metals (107). Equation (300) describes ammonia decomposition on various metals (provided that there is enough H2 in the gas phase). 

Figure 6.4. Ammonia synthesis rate constant dependence on hydrogen-nitrogen ratio. (Reproduced by permission of Wiley-VCH)...

Table 19. ( values for the ammonia synthesis rate equation... 

This section describes in detail three topics in heterogeneous catalysis to which DFT calculations have recently been applied with great effect, the prediction of CO oxidation rates over RuO2(110), the prediction of ammonia synthesis rates by supported nanoparticles of Ru, and the DFT-based design of new selective catalysts for ethylene epoxidation. All three examples involve the careful application of DFT calculations and other appropriate theoretical methods to make quantitative predictions about the performance of heterogeneous catalysts under realistic operating conditions. 

Further work on modified Fe single crystals explored the role of promoters such as aluminium oxide and potassium [49, 50 and 51]. It was found that the simple addition of aluminium oxide to Fe single crystal surfaces decreased the ammonia synthesis rate proportionally to the amount of Fe surface covered, indicating no favourable interaction between Fe and aluminium oxide under those conditions. However, by exposing an aluminium-oxide-modified Fe surface to water vapour, the surface was oxidized, inducing a favourable interaction between Fe and the Al O. This interaction resulted in a 400-fold increase in ammonia synthesis activity for A1 0yFe( 110) as compared to Fe( 110) and an activity for A1 0 e( 110) comparable to that of Fe... 

In certain conditions the rate of the ammonia synthesis reaction in the reverse direction is very small and the overall reaction rate is determined by the ammonia synthesis rate in the forward direction. According to the conventional derivation based on the model of biographically nonuniform surfaces this rate in the forward direction is given by... 

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). 

It can be seen that from VA to VIII groups, the initial adsorption heat of nitrogen decreases and ammonia synthesis rate reaches a maximum value, and after that the rate goes down due to the reduction of surface coverage of adsorbed nitrogen. 

The ammonia synthesis rates over various nitrides were measured. The experimental values of the rates are lower than the values that were estimated from isotope exchange rates after neglecting hydrogen effect. If the nitrides of Ba, Sr, Ca and Li are replaced by their hydrides, the activity will decrease. In the presence... 

Temkin-Pyzhev equations mentioned above was in agreement with a number of kinetic measurement made on various catalysts such as Mo, W, Tc, Ru, Os and promoted Fe. One characteristic feature of ammonia synthesis rate is the retardation by the product ammonia, and reasonably explained by the Temkin theory. The assumption of rate determining step is also supported by the chemisorption of nitrogen. 

The catalysts for ammonia synthesis are porous particles with weenie and interlaced micro-pores. The active sites playing the role of surface catalysis are distributed on the internal surfaces formed by these micro-pores. The internal surface area of ammonia synthesis after reduction is about 10m -g -15m -g , and the external surface area is only 0.1 m g F So, the surface area playing the role of surface catalysis mainly is internal surface. The equivalent diameter of catalyst particles used in industrial ammonia reactor is between 1.5 mm and 13 mm, and the inhibition effect of diflfusion should be considered in real ammonia synthesis rates. When designing industrial reactor, the resistance of external diffusion can be neglected by increasing contact between gas flow and external sm-face of catalysts. The catalytic reaction processes for ammonia synthesis pertain to considerable internal diffusion process in most cases. 

Table 6.7 Influence of promoter on ammonia synthesis rate on 2% Ru/AC catalyst...

Table 6.42 Ammonia synthesis rates of Ru/C cateJysts with optimization of single or double promoter (p = 90 ben, T = 400° C, H2 N2 = 3 1, x= 10%)...

Number Catalysts Initial temperature of methanation/ C Activation energy for methanation/ (kJ/mol) Ammonia synthesis rate/ (mmol. g h ) ... 

The synthesis conditions are 400°C, 50 bar, gas composition (H2 N2) 3 1 containing 5% NH3. The numbers are obtained by combining a micro-kinetic model describing ammonia synthesis rates with the linear relation existing between the potential energy and the activation energy for N2 dissociation. The known entropy barrier for N2 dissociation and the effect of adding electropositive promoters such as K and Cs have been taken into account in the model. 

Table 10.4 Ammonia synthesis rates of rare earth metals intermetallic compounds (5 MPa and Sv = l,20,000h )...

The pressure of gas was 101.325 kPa. The ammonia synthesis rate was stable after passage of current for 2-6 min and this rate was at least three magnitudes higher than that of conventional catalytic reactor. The conversion of hydrogen was close to 100% which eliminated thermodynamic equilibrium limitation. The main problem of this method is that the conductivity of SCY ceramic is very poor at normal temperature. Even at 570°C, its conductivity is unsatisfied because the current density was smaller than 2 mA-cm and could not be further increased. This limited the efficiency of ammonia synthesis and theoretical research. The use of solid electrolyte with high proton conductivity at low temperature to replace the SCY ceramic may be favorable to decreasing the synthesis temperature and increasing the current density and production efficiency. 

The experimental results confirm that the amount of ammonia formed increases linearly with the content of supplied water vapor, indicating that the ammonia is formed by reaction of water vapor with nitrogen ion (Fig 10.11). The average ammonia synthesis rate is 0.72 mol h m. Although the current efficiency of ammonia is 23% only, it demonstrates the probability of ammonia sjmthesis by using water and nitrogen gas at atmospheric pressure. 

Figure 10.13(a) shows the ammonia synthesis rates (mol H-s ) at constant voltage and inlet Ph2/Pn2 =0.82. Reaction rate increases from 6.9 x 10 mol H-s to 1.35 X 10 mol H s as increasing of reaction time. p = r/vo is defined as enhancement coefficient in this figure and its value is 2. A = Ar/(—I/F) is defined as inductive efficiency, reflecting the influence of electrochemical proton transfer to catalyst. This parameter is used to differentiate the inductive and non-inductive efficiency. The A is 0.6 in Fig. 10.13(a), indicating that 60% of proton transfers to... 

Fig. 10.15 shows the influence of potential (Uwr) catalyst on ammonia synthesis rates. This reaction demonstrates a strong electrophilic effect, i.e. negative potential can promote reaction while positive potential can envenom reaction. The catalyst will deactivate after prolonged treatment at positive potential (3 h), suggesting the existence of hysteresis phenomena. The treatment of H2 can restore the activity of catalysts. 

The value of the desorption activation energy used by Stoltze and Norskov was 161 kJ mol" (cf. 226 kJ mol" of Table 5.2), from which a value of 7.9 x 10 for the desorption preexponential term can be derived. This is a lowering of the normal desorption A-factor by 4.9 x 10"", considerably less than the value of 10" found for the sticking probability. Nevertheless, using this combination of values, Stoltze and Norskov predict ammonia synthesis rates which are in exact agreement with those found experimentally. 

The ammonia synthesis rate per iron surface area was calculated in this way. The iron surface area of the commercial Topspe KMIR catalyst was determined by the carbon monoxide chemisorption method, assuming that each carbon monoxide molecule titrates two iron atoms (this method gives a rather higher iron surface area than usually accepted). After integration of the rate equation for a piston-flow reactor, values for the ammonia concentration in the exit gas were obtained in very good agreement with the experimental values as shown in Fig. 4.14. However, the authors themselves prefer not to stress such agreement, in view of the approximations introduced and because of the uncertainties in some of the input data. 

Figure 8.6. The relation between calculated and experimental NH3 concentration for a K-promoted catalyst operating at 7-20 MPa, 573-723 K, 0-25 ppm H2O. (a) The effect of HjO has been neglected, (b) The effect of HjO has been treated by including the HjO + 3 — 2H + O reaction in their ammonia synthesis rate expression. Data from Stoltze and Nprskov. By permission of the American Institute of Physics.

In the 1970s, a catalyst system promoted by metallic potassium [73, 74] was studied. The ammonia synthesis rates at 80 kPa and 588 K over transition metals supported on active carbon and promoted by metallic potassium are given in Fig. 3.2 [69]. The activity of isotopic equilibration of N2 over the same series of catalysts at 30 kPa of N2 and at 588 K are shown in Fig. 3.3 [75]. The same reaction over Raney metals are also shown in this figure [76]. In these cases ruthenium is the most active metal. There is a common belief that Fe, Ru and Os are the most active elements in ammonia synthesis, ammonia decompo-... 

The effect of added metallic potassium is similarly observed with respect to the isotopic equilibration of N2 on Ru/AC [97] as well as on pure ruthenium [50]. The addition of metallic potassium results in about a 500 fold increase in the activity of Ru at 673 K, while the increase in the ammonia synthesis rate is less extensive (25 fold), due to the inhibition by the presence of hydrogen (cf. Sections 3.3.2, 3.3.3.1) [98]. The inhibition by hydrogen is accounted for by the competitive adsorption of hydrogen as well as by a decreased adsorption constant for N2 [86, 98]. 

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