NH3 Synthesis Reaction

For the methanation and NH3 synthesis reaction the metals must be still more active in bond breaking the CO and N2 molecule both must dissociate under reaction conditions and the reaction then proceeds by hydrogenation of C, N and O on the surface. Additional factors that determine the performance of the metal in these reactions are the nature and bond strength of C, N and O on the surface. 

NH, NHj, NH3, and H species are together larger than the free-site fraction so that Langmuir-Hinshelwood conditions, with only one significant chemisorbed intermediate, do not obtain. In fact, quite early work had already indicated 54) that, in technical catalysis for NH3 synthesis, it is the bonding of Nj (as N) to the catalyst surface which determines the overall rate of the reaction. Correspondingly (55), at moderate temperatures at W, NH3 decomposes giving imide and nitride species on the surface. The rate of decomposition of the nitride species (chemisorbed N) as an intermediate in the NH3 synthesis reaction at Fe was shown by Mittasch et al. (5(5) to be equal to that of NH3 production. 

Catalytic reactions can be classified into structure-insensitive (hydrogenation, dehydrogenation, isomerization, hydrogen exchange) or structure-sensitive (hydrogenolysis, CO/H2, NH3 synthesis) reactions depending on the extent to which the rate of the reaction and its selectivity are affected by the size and morphology of the metal particles. 

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]. 

There appear to be considerable steric demands involved in formation of the Si-N bond in reaction (28). Support for the importance of steric constraints in dehydrocoupling comes from studies on the synthesis of oligosilazanes from PhSiHj and NH3. When reaction (29) is run at 60°C, NMR and elemental analysis... 

For Example 14-16, all for NH3 synthesis, the authors implied that a surface reaction is the rate-determining step. For given steps in these three cases the logL values are similar. Either Step 8 or Step 12 could be rate determining but the reacting surface species are probably not N2 and H2, and therefore these steps can probably be ruled out. Almost certainly none of the steps in Table VII are rate determining in NH3 synthesis. 

Thus in this example, the prod t indicated is fed into the next stage, where it is reacted with other species (H2O, N2, O2, toluene). Different temperatures and pressures are usually used in each stage to attain optimum performance of that reactor. For example, NH3 synthesis requires very high pressure (200 atm) and low temperature (250°C) because it is an exothermic reversible reaction, while NH3 oxidation operates at lower pressurse (-10 atm) and the reaction spontaneously heats the reactor to -800°C because it is strongly exothermic but irreversible. Formation of hquid HNO3 requires a temperature and pressure where liquid is stable. 

In fact, we usually want to operate exothemic reactions nonisothermally to take advantage of the heat release in the reaction to heat the reactor to a temperature where the rates are higher and reactor volumes can be smaller. However, if the temperature is too high, equilibrium limitations can limit the conversion, as we saw previously for NH3 and CH3OH synthesis reactions. 

Si02 nanoparticles AOT/decane/benzyl alcohol (BA)/water/ammonia (R = 6.8, BA/AOT molar ratio = 0-2.5) TEOS/H20 + NH3 (13.9 wt%) [TEOS] = 0.044 M h = [H20]/[TE0S] = 18.5 Microemulsions with BA/AOT >1.5 became unstable during synthesis reaction nearly spherical nanoparticles maximum in particle size at BA/AOT = 1.5 (32)... 

The dissociation of NH3 on Ru(0001) is the competing back reaction to NH3 synthesis on Ru(0001). In addition, NH3 has been proposed as a viable hydrogen storage/transport medium in a hydrogen economy [345] so that its dissociation liberating N2 and H2 are also important. 

To analytically determine such volcano curves for the simple model reaction, we need to make some further assumptions (the assumptions are realistic at least for the case of NH3 synthesis) ... 

Process waste-heat boilers then cool the reformed gas to about 371°C while generating high-pressure steam. The cooled gas-stream mixture enters a two-stage shift converter. The purpose of shift conversion is to convert CO to C02 and produce an equivalent amount of H2 by the reaction CO+H20 C02 + H2. Since the reaction rate in the shift converter is favored by high temperatures, but equilibrium is favored by low temperatures, two conversion stages, each with a different catalyst provide the optimum conditions for maximum CO shift. Gas from the shift converter is rhe raw synthesis gas, which, after purification, becomes the feed to the NH3 synthesis section. 

Four mechanisms have been advanced for the prebiotic formation of amino acids. The first involves a cyanohydrin (reaction 2) and a related route (reaction 3) can be invoked to account for the presence of hydroxy acids. These particular reactions have been studied in considerable detail both kinetically and in terms of thermodynamic quantities.347 An alternative route (4) involves the hydrolysis of a-aminonitriles, which are themselves formed directly in anhydrous CH4/NH3 mixtures.344 Cyanoacetylene, formed in CH4/N2 irradiations,349 yields significant amounts of asparagine and aspartic acids (reaction 5). Finally, a number of workers336,350"354 have proposed that HCN oligomers, especially the trimer aminoacetonitrile and the tetramer diaminomaleonitrile, could have been important precursors for amino acid synthesis. Reaction mixtures involving such species have yielded up to 12 amino acids. Table 11 indicates the range of amino acids produced in these kinds of sparking syntheses. Of some interest is the fact that close parallels between these kinds of experiments and amino acid contents of carbonaceous chondrite meteorites exist.331,355,356... 

Haber, on the other hand, was confident of his results and not overly pleased with Nernst s comments. Although he did have doubts about the feasibility of applying the ammonia synthesis reaction on an industrial scale, much of his concern was whether an apparatus could be constructedk to synthesize NH3 at high temperatures and pressures on a large scale. 

Figure 6.2. Reaction rate for NH3 synthesis. Dependence on the ammonia concentration at various pressures.

Purely adiabatic fixed-bed reactors are used mainly for reactions with a small heat of reaction. Such reactions are primarily involved in gas purification, in which small amounts of noxious components are converted. The chambers used to remove NO, from power station flue gases, with a catalyst volume of more than 1000 m3, are the largest industrial adiabatic reactors, and the exhaust catalyst for internal combustion engines, with a catalyst volume of ca. 1 L, the smallest. Typical applications in the chemical industry include the methanation of traces of CO and CO2 in NH3 synthesis gas, as well as the hydrogenation of small amounts of unsaturated compounds in hydrocarbon streams. The latter case requires accurate monitoring and regulation when hydrogen is in excess, in order to prevent complete methanation due to an uncontrolled temperature runaway. 

Fig. 7.3. Activation energy diagram for NH3 synthesis (1A) chemisorbed N2 dissociation (rate-limiting step) (1B) overall reaction for NH3 synthesis.

Fig 22.10. Reaction rate for NH3 synthesis. Dependence on the ammonia concentration at various pressures. (Courtesy of Wiley-VCH. Bakemeier, H., Huberich,T, et. al. "Ammonia" in Ullmann s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A 2, VCH Verlagsgesellschaft, Weinheim 1985, pp. 143-242. 

In 1905 Haber reported a successful experiment in which he succeeded in producing NH3 catalytically. However, under the conditions he used (1293 K) he only found minor amounts of NH3. He extrapolated his value to lower temperatures (at 1 bar) and concluded that a temperature of 520 K was the maximum temperature for a commercial process. This was the first application of chemical thermodynamics to catalysis, and precise thermodynamic data were not then known. At that time Haber regarded the development of a commercial process for ammonia synthesis as hopeless and he stopped his work. Meanwhile, Nernst had also investigated the ammonia synthesis reaction and concluded that the thermodynamic data Haber used were not correct. He arrived at different values and this led Haber to continue his work at higher pressures. Haber tried many catalysts and found that a particular sample of osmium was the most active one. This osmium was a very fine amorphous powder. He approached BASF and they decided to start a large program in which Bosch also became involved. 

The thermodynamic equilibrium is most favourable at high pressure and low temperature. The methanol synthesis process was developed at the same time as NH3 synthesis. In the development of a commercial process for NH3 synthesis it was observed that, depending on the catalyst and reaction conditions, oxygenated products were formed as well. Compared with ammonia synthesis, catalyst development for methanol synthesis was more difficult because selectivity is crucial besides activity. In the CO hydrogenation other products can be formed, such as higher alcohols and hydrocarbons that are thermodynamically favoured. Figure 2.19 illustrates this. 

The law of mass action is widely applicable. It correctly describes the equilibrium behavior of all chemical reaction systems whether they occur in solution or in the gas phase. Although, as we will see later, corrections for nonideal behavior must be applied in certain cases, such as for concentrated aqueous solutions and for gases at high pressures, the law of mass action provides a remarkably accurate description of all types of chemical equilibria. For example, consider again the ammonia synthesis reaction. At 500°C the value of K for this reaction is 6.0 X 10 2 F2/mol2. Whenever N2, H2, and NH3 are mixed together at this temperature, the system will always come to an equilibrium position such that... 

At 450°C, Kp = 6.5 X 10-3 atm-2 for the ammonia synthesis reaction. Assume that a reaction vessel with a movable piston initially contains 3.0 mol H2(g) and 1.0 mol N2(g). Make a plot to show how the partial pressure of NH3(g) present at equilibrium varies for the total pressures of 1.0 atm, 10.0 atm, 100. atm, and 1000. atm (assuming that Kp remains constant). (Note Assume these total pressures represent the initial total pressure of H2(g) plus N2(g), where PNHl = 0.)... 

After the homogenization process, a 2ml of TEOS (99.9%, Aldrich Chemical Co., USA) was added and mixed into the catalyst-included mixed solvents. As reaction time goes on after ftie TEOS addition, the hydrolysis and condensation reactions initiated, so that the transparent solution became white and white. The reaction rate highly depended on the reaction conditions such as the volume ratio of H2O to EtOH and the addition amount of NH3. The synftiesis temperature and time were room temperature and 4hrs, respectively. After the synthesis reaction had proceeded for 4 hours, the synthesized silica gel was washed with water three times by repeated centrifuging and dispersion in water, and then dried at 110°C for 72hrs. All the chemicals used in the present study were used without any furthermore purification. 

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