NH3 synthesis

Steam reforming of CH4 CH4 + H2O = CO + 3H2 NH3 synthesis from the elements Hydrogenation of CO and CO2 to form hydrocarbons (Fischer-Tropsch syndresis)... 

Most studies of the effect of alkalis on the adsorption of gases on catalyst surfaces refer to CO, NO, C02, 02, H2 and N2, due to the importance of these adsorbates for numerous industrial catalytic processes (e.g. N2 adsorption in NH3 synthesis, NO reduction by CO). Thus emphasis will be given on the interaction of these molecules with alkali-modified surfaces, especially transition metal surfaces, aiming to the identification of common characteristics and general trends. 

A recent electrochemical promotion study ofNH3 synthesis43 utilized a commercial fully promoted Fe-based NH3 synthesis catalyst (BASF S6 -10RED) deposited on CaZro.9lno.i03 a, a proton conductor. 

Figure 9.34. (a) Effect of inlet H2/N2 ratio on the rate of NH3 synthesis over promoted Fe/CaZr0,9ln0,03 under open-circuit (O) and for UWR=-1.0V ( ) (b) Corresponding p (r/r0) ( ) and A (=ArH/(-I/F)) ( ) values.43 Reprinted with permission from the American Chemical Society. 

A related approach is to interface an industrial promoted catalyst with a solid electrolyte (Fig. 12.2). In this case the bulk of the commercial catalyst must be conductive. This concept has been already demonstrated for the case of NH3 synthesis on Fe-based promoted commercial catalysts (BASF S6-10 RED)16 and for the case of SO2 oxidation on V2O5-K2S2O7 based catalysts (Haldor-Topsoe VK-58).17... 

This study has been already discussed in section 9.2.4.16 A commercial NH3 synthesis catalyst (BASF S6-10 RED) was milled and deposited via a... 

M. Stoukides The name of M. Stoukides is associated with the first electrochemical promotion studies and publications in 1981 (Chapter 1) when he as a graduate student of C. Vayenas at MIT was investigating ethylene epoxidation on Ag/YSZ. In recent years the group of Professor M. Stoukides in Thessaloniki has made interesting electrochemical promotion studies ofH2S decomposition and C2H4 and NH3 synthesis at elevated temperatures near the border of electrochemical promotion and electrocatalysis. 

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]. 

As expected, 7-AI2O3 (BET area 110 m /g) turned out to be the more stable support with a higher surface area than MgO (BET area 52 m /g). The BET area of RU/AI2O3 was found to be 104 m /g after NH3 synthesis at 773 K which decreased significantly to 70 m /g as a result of cesium impregnation. After NH3 synthesis at 773 K, the specific area of Ru/MgO was observed to be 25 m /g compared with 52 m /g found for the MgO support. Cesium impregnation caused a further decrease in specific area to 23 m /g. 

Results of the H2 chemisorption measurements after NH3 synthesis based on H/Ru = 1/1. NHs synthesis was run at 773 K with Ru/MgO and RU/AI2OS, and at 673 K with all alkali-promoted catalysts. The mean particle size was calculated assunung spherical particles. 

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... 

The rate constants in table 4 for Ru/AlaOs should be considered as initial rate constants since it was not possible to achieve a higher coverage of N— than 0.25. Furthennorc, it was not possible to detect TPA peaks for Ru/AlaOs within the experimental detection limit of about 20 ppm. Ru/MgO is a heterogeneous system with respect to the adsorption and desorption of Na due to the presence of promoted active sites which dominate under NH3 synthesis conditions. The rate constant of desorption given in table 4 for Ru/MgO refers to the unpromoted sites [19]. The Na TPD, Na TPA and lER results thus demonstrate the enhancing influence of the alkali promoter on the rate of N3 dissociation and recombination as expected based on the principle of microscopic reversibility. Adding alkali renders the Ru metal surfaces more uniform towards the interaction with Na. 

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... 

N2-H2 mixture (with small amounts of Ar and CH4) [33]. The amount of air added to the secondary reformer is adjusted to give the desirable H2/N2 ratio (which is close to 3 for the NH3 synthesis). The secondary reformer is similar to the autothermal reformer described in the previous section. The pressure at the outlet of the secondary reformer is in the range 2.5-3.5 MPa. The outlet temperatures from the primary and secondary reformers are 750-850°C and 950-1050°C, respectively. 

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. 

The values of in the preceding table indicate the conditions for which NH3 production is possible. It is seen that one must use either low temperatures or very high pressures to attain a favorable equihbrium. At 25°C, where bacteria operate, the equihbrium constant is very large and the conversion is very high, while at higher temperatures Xjq and Pnhj fall rapidly. The best catalysts that have been developed for NH3 synthesis use Fe or Ru with promoters, but they attain adequate rates only above -300°C where the equihbrium conversion is -3% at 1 atm. 

Modem ammonia synthesis reactors operate at -200 atm at -350°C and produce nearly the equihhrirrm conversion ( 70%) in each pass. The NH3 is separated from unreacted H2 and N2, which are recycled back to the reactor, such that the overaU process of a tubular reactor plus separation and recycle produces essentially 100% NH3 conversion The NH3 synthesis reactor is fairly small, and the largest components (and the most expensive)... 

This process has many similarities to NH3 synthesis. The pressure is not as high for acceptable conversions, and modem methanol plants operate at -250°C at 30-100 atm and produce nearly equilibrium conversions using Cu/ZnO catalysts with unreacted CO and H2 recycled back into the reactor. 

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. 

A typical NH3 synthesis catalyst (10) contains iron oxide plus 1% K2O, 1-2% AI2O3, and may contain v l% CaO on the surface. After fusion and reduction the surface is largely metallic iron plus reduced promoters concentrated on the surface ( ). Sze and Wang (11) have shown that a catalyst washed with Ce(N03)3 and subsequently reduced is much more active than the conventional catalyst, Table II, Mischmetal salts may be substituted for the cerium salt. 

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 NH3 synthesis rate is proportional to 0 . Since the calculated synthesis rate is in reasonable agreement with independent measurements, it is unlikely that the low value of 0 should be significantly in error. 

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. 

In tlie steam-hydrocarbon reforming process, steam at temperatures up to 850°C and pressures up to 30 atmospheres reacts with the desulfurized hydrocarbon feed, in the presence of a nickel catalyst, to produce H2. CO, ( G CH4, and some undecomposed steam. In a second process stage, these product gases are further reformed. Air also is added at this stage to introduce nitrogen into the gas mixture. The exit gases from this stage are further puntied to provide the desired 3 parts H. to 1 part Nj which is the correct empirical ratio for NH3 synthesis. See also Ammonia. 

Haber-Bosch NH3 synthesis magnetite (Fe) h2, n2 nh3 fertilizer, gunpowder, explosives... 

Fastrup, B. (1994) Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis. Top. Catal., 1, 273. 

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