Ammonia Synthesis Catalyst under Working Conditions

Ammonia Synthesis Catalyst under Working Conditions 

For ammonia synthesis, we still need to determine the coverages of the intermediates and the fraction of unoccupied sites. This requires a detailed knowledge of the individual equilibrium constants. Again, some of these may be accessible via experiments, while the others will have to be determined from their respective partition functions. In doing so, several partition functions will again cancel in the expressions for the coverage of intermediates. 

100 bar, and approaching 68% of the equilibrium ammonia concentration. [Adapted from P. Stoltze, Phys. Scr. 36 (1987) 824.] 

Overall, catalytic processes in industry are more commonly described by simple power rate law kinetics, as discussed in Chapter 2. However, power rate laws are simply a parameterization of experimental data and provide little insight into the underlying processes. A micro-kinetic model may be less accurate as a description, but it enables the researcher to focus on those steps in the reaction that are critical for process optimization. 

---

Ammonia Synthesis Catalyst under Working Conditions... 

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. 

It has been claimed that carbon-supported ruthenium-based catalysts for ammonia synthesis show some important drawbacks, such as high catalyst cost and methanation of the carbon snpport under industrial reaction conditions. This has stimnlated the research for alternative catalysts, although the use of carbon snpports is a common feature. One example of these new catalysts is provided by the work of Hagen et al. [61], who reported very high levels of activity with barinm-promoted cobalt catalysts snpported on Vulcan XC-72. It was demonstrated that althongh cobalt had received little attention as a catalyst for ammonia synthesis, promotion with barium and the nse of a carbon support resulted in very active catalysts with very low NH3 inhibition. 

Suppose that you worked at the U.S. Patent Office and a patent application came across your desk claiming that a newly developed catalyst was much superior to the Haber catalyst for ammonia synthesis because the catalyst led to much greater equilibrium conversion of N2 and H2 into NH3 than the Haber catalyst under the same conditions. What would be your response ... 

Calculations relating adsorption properties measured on clean single-crystal surfaces at (very) low pressure with actual rates of ammonia synthesis at high pressures described later in this chapter require some measurement of the free metallic iron surface area of the reduced synthesis catalysts. The free metallic iron surface area is generally calculated from the extent of chemisorption of carbon monoxide. In view of the importance of the free metallic iron surface area in work dealing with the mechanism of ammonia synthesis, it is important to review the adsorption of carbon monoxide on different iron surfaces reduced under different conditions. 

In addition to the processes mentioned above, there are also ongoing efforts to synthesize formamide direcdy from carbon dioxide [124-38-9J, hydrogen [1333-74-0] and ammonia [7664-41-7] (29—32). Catalysts that have been proposed are Group VIII transition-metal coordination compounds. Under moderate reaction conditions, ie, 100—180°C, 1—10 MPa (10—100 bar), turnovers of up to 1000 mole formamide per mole catalyst have been achieved. However, since expensive noble metal catalysts are needed, further work is required prior to the technical realization of an industrial process for formamide synthesis based on carbon dioxide. 

---