Catalysts, ammonia photosynthesis

Examination of the thermodynamics and kinetics of proposed chemical reactions for the artificial photosynthesis of ammonia presents several problems, not the least of which is the logical and rhetorical one of discussing mechanisms for a process while suggesting that it does not occur. The latter problem can be largely resolved by understanding beforehand that the purpose of the present section is to call into question and to dispute the proposition that ammonia is synthesized photocatalytically by irradiation of N2 and H20 by the artificial methods described already. We can examine the application in reported ammonia photosynthesis reactions of both categories of catalyst discussed in Section IV.A. 

Molecules cannot move fast enough to keep the reaction as reported at equilibrium. For example, a recent paper on ammonia photosynthesis over oxide promoters by Augugliaro et al. [67] reports that ammonia was formed at 0.04 imol h-) in their system, and so this sets the minimal reverse rate at equilibrium. Nitrogen flowed into their system at 0.4 cm3 s-1 and so we can ask ourselves how fast ammonia molecules would have to move toward the surface of the catalyst in order to maintain the reverse rate. It transpires that the ammonia molecules would have to travel fast enough to cross the known universe (i.e., 2 x 1010 light years) about 1021 times in one second. This absurd result is intended only as another way of demonstrating that reaction 2, if it occurred, could not come to equilibrium. 

Two years later Taqui Khan et al. [108] published another paper which described similar experiments in more detail. The rate of ammonia production was given as 6.8 moles NH3 per mole of catalyst per hour. It appears that the term the catalyst referred to the ruthenium complex 1 rather than to CdS. The maximum yield of ammonia and the initial rate of ammonia production varied directly with the initial concentration of 1. Ammonia was determined in the reactor and in HC1 traps by Nessler s method. Yields were shown on a graph, with a maximum yield of approximately 0.065 mol. We note that this is an exceptionally large amount of ammonia from photosynthesis and that photon efficiencies of several percent are implied. 

Despite many studies there is available, at this time, no unequivocal report of artificial photocatalytic synthesis of ammonia from nitrogen and water on heterogeneous catalysts. The chemical reaction 1, shown at the beginning of this chapter, remains speculative 17 years after it was initially reported. No rational start has yet been made on determining a mechanism for the reported nitrogen fixation process, despite the fact that a clear demonstration and understanding of any such process would be of great importance. Despite 17 years of research, the reported yields of ammonia from artificial photosynthesis remain at or near the limits of detection by routine analytical methods. As we stated earlier in this chapter, the concept of a major discovery that remains forever on the borderline of detectability is internally contradictory. The current state of the field is one where serious scientific skepticism is appropriate. 

The Question of Artificial Photosynthesis of Ammonia on Heterogeneous Catalysts 235... 

Photosynthesis and plasmasynthesis of ammonia could be important in the future [7-13]. There are many good reviews on ammonia synthesis, mainly based on an iron catalyst [1, 14-24], as well as other various catalysts [22, 24-26]. 

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