Ammonia reduction versus oxidation

An electroactive substance may be feduced to. a lower oxidation state at a certain potential and then be reduced to a stiU lower oxidation state when the potential reaches another more negative value. For example, copper(II) in ammonia solution is reduced at a graphite electrode to a stable Cu(I)-ammine complex at —0.2 V versus SCE, which is then reduced to the metal at —0.5 V, each a one-electron reduction step. In such cases, two successive voltammetric waves will be recorded as in curve (c) in Figure 15.2. The relative heights of the waves will be proportional to the number of electrons involved in the reduction or oxidation. In this case, the two waves would be of equal height. 

In Fig. 4.25, the rate of ammonia synthesis versus % free iron surface, as determined by carbon monoxide TPD (see experimental section), is shown graphically. The rate of ammonia synthesis decreases roughly in proportion to the amount of iron covered by the aluminum oxide and potassium. The only mechanism for this reduction in rate is site-blocking, which occurs during initial reaction conversions (Pnhj ranges from 0 torr to 3 torr during this measurement). 

The V(mes)3(THF) (mes = mesityl) complex displays a reversible oxidation at —0.25 V versus Cp2Fe/THF, and a reversible reduction at —2.50 V versus Cp2Fe/THF although the latter appears to have slow electrode kinetics [47]. If the atmosphere is switched from Ar to N2, new electrochemical features appear. CV and bulk electrolysis studies showed that the new electrode product was [(mes)3 V — N = N — V(mes)3] . This species can be oxidized to a monoanion at —2.25 V versus Cp2Fe/THF and reduced to a trianion at —2.81 V versus Cp2Fe/THF. Attempts to generate the trianion by bulk electrolysis result in decomposition, but both the anion and dianion yield ammonia and hydrazine upon protonolysis. The anion s... 

Two effects cause the low production capacity of large-grained catalyst. First, large grain size retards transport of the ammonia formed inside the catalyst into the bulk gas stream. This is because the ammonia transport proceeds by slow diffusion through the pore system. The second effect is a consequence of the fact that a single catalyst grain in the oxide state reduces from the outside to the interior of the particle. The water vapor produced inside the catalyst by reduction comes into contact with already reduced catalyst on its way to the outer surface of the catalyst. This induces a severe recrystallization. As an example, if the particle size increases from about 1 to 8 mm, the inner surface decreases from 11 to 16 m2/g to 3 to 8 m2/g74. Therefore the choice of catalyst requires the optimization of 1) catalyst size versus catalyst activity, 2) catalyst size versus pressure drop across the converter and 3) the impact of 1 and 2 on... 

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