Thermodynamics, of ammonia synthesis

Due to the equilibrium ammonia concentration changes with H2/N2 according to thermodynamics of ammonia synthesis reaction. Therefore, outlet ammonia concentration cannot directly reflect the influence of H2/N2 on catal3dic activity. Hence, the catalytic efficiency is defined as the ratio of outlet ammonia concentration to equilibrium ammonia concentration at the same conditions. The effect of H2/N2 on catalytic efficiency for Ba-Ru-K/AC catalyst is shown in Table 6.45. It can be... 

When building up the Institute in Dahlem, Haber made limited use of the advantages offered by the Berlin physical chemistry community. Van t Hoff died the year Haber arrived in Berlin, while Haber and Nemst had previously engaged in a heated dispute concerning the thermodynamics of ammonia synthesis and were not close personally. Moreover, Haber brought most of the initial Institute staff with him from Karlsruhe. 

This structure is very similar to that of the (002) plane of (bulk) Fe4N and hence no reconstruction and the assignment of surface nitride. (It should be noted that the known bulk iron nitrides will never be thermodynamically stable under the conditions of ammonia synthesis but may be readily formed in NH3/H2 mixtures providing high virtual N2 pressures.)... 

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. 

Interaction of ammonia with iron surfaces at elevated temperatures (typically 670 K) leads to the formation of y- (Fe4N) and even e-nitrides/ These compounds, however, have no relevance under the conditions of ammonia synthesis for thermodynamic reasons. 

In 1908, Haber, a professor at the University of Karlsruhe, Germany, started a collaboration with the chemist Carl Bosch of the Badische Anilin- und Soda-Fabrik (BASF) at Ludwigshafen. Their 1909 experiment in which 8% of ammonia formed at 550°C and 175 atm confirmed the predictions from chemical thermodynamics and convinced them of the feasibility of ammonia synthesis at an industrial scale. 

From a thermodynamic model [396] of the chemisorption of H2 we have found that under an equilibrium pressure of 1 atmosphere, the coverage by H decreases from 1 at 500 K to 0.15 at 1000 K. By calculating the outcome of a volumetric chemisorption experiment from a model of the kinetics of ammonia synthesis [396], it is concluded that H is by far too weakly adsorbed to be used in a titration of the number of active sites on the catalyst surface. 

By construction the model reproduces the thermodynamics of adsorption for the intermediates and the kinetics of adsorption ad desorption for N . As no measured data for the catalytic activity have been used in the determination of the input parameters, the model may be tested by a comparison between the calculated and experimental rates of ammonia synthesis. The test is made by calculating the exit ammonia concentration from the input composition and operating conditions for the reactor. 

When I started to work in catalysis more than 50 years ago, first as a student in the Institute of Physical Chemistry with the late Prof. J. N. Bronsted and subsequently in the laboratories of Dr. Haldor Topsoe, the use of catalytic processes by industry and the literature on catalytic studies was still somewhat limited and allowed a single person to reasonably acquaint himself with the field. Today, catalysis is a step in the manufacture of most chemical products and most refinery streams undergo catalytic reactions. The number of physical tools applied to the study of catalysts is impressive and the literature on catalysis overpowering. A reasonably comprehensive volume on the topic of ammonia synthesis had to be a team effort. The first chapter entitled, Thermodynamic properties in Ammonia Synthesis " is written by Dr. Lars J. Christiansen. It should be emphasized that this chapter does not contain the complete thermodynamics on ammonia, but concentrates on the thermodynamic properties used for the design and operation of ammonia synthesis units. 

The production of ammonia is of historical interest because it represents the first important application of thermodynamics to an industrial process. Considering the synthesis reaction of ammonia from its elements, the calculated reaction heat (AH) and free energy change (AG) at room temperature are approximately -46 and -16.5 KJ/mol, respectively. Although the calculated equilibrium constant = 3.6 X 108 at room temperature is substantially high, no reaction occurs under these conditions, and the rate is practically zero. The ammonia synthesis reaction could be represented as follows ... 

Amino-derivatives.—Thermodynamic aspects of the synthesis of phospham from ammonia and red phosphorus ... 

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