Ammonia synthesis, nitride effect

There is very different behavior for the two-type adsorption states. For instance, H-type nitrogen has no effect on CO adsorption, while L-type nitrogen inhibits CO adsorption. L-type is more active for ammonia synthesis than H-type. If iron nitride was hydrogenated to ammonia, the formation rate of ammonia was also proportional to P, indicating that the H-tjrpe adsorption state of nitrogen is similar to that on iron nitride, i.e., one nitrogen atom is coordinated with several iron atoms,... 

The ammonia synthesis rates over various nitrides were measured. The experimental values of the rates are lower than the values that were estimated from isotope exchange rates after neglecting hydrogen effect. If the nitrides of Ba, Sr, Ca and Li are replaced by their hydrides, the activity will decrease. In the presence... 

Norskov proposed a more scientific and effective theory, i.e., interpolation in the periodic table, for designing bimetallic catalysts. According to this theory, a reasonable assumption is to combine the most active and less active elements for activation of nitrogen together to construct an active surface to achieve the best performance. The idea was proved by an efficient non-ruthenium catalyst on the basis of cobalt-molybdenum nitride, which can be regarded as the current climax of a systematically but empirically justified development based on the theoretical prediction of the ammonia synthesis catalyst research up to now. 

Cr has been found to be structural promoter [198, 199, 260, 282, 283], while others have found that Cr has no effect on the catalytic activity [284] or acts to reduce the catalytic activity [285]. The large reactivity of Cr toward N2 [286] and the considerable stability of nitrides of Cr [287] are complications in the deduction of the effect of small amounts of Cr added to the catalyst. Undoubtedly, this complication is the cause of much confusion in the study of the effects of trace amounts of a number of transition metals on the activity of ammonia synthesis catalysts. 

The rates of ammonia synthesis have been measured on several nitrides, and the results are shown on Table 3.14 [87]. If there is no hydrogen effect, ammonia synthesis rate can be estimated from the rate data of isotopic equilibration (see Table 3.14 foot note). The experimental values are mostly lower than the calculated values. In case of Ba, Sr, Ca, and Li, the nitride is changed to the hydride and the activity decreases. Sr, the most active equilibration element. 

There are a limited number of techniques used for the synthesis of ternary and quaternary nitrides. In fact, the vast majority of these nitrides have been synthesized using only two methods (1) the reaction of a metal nitride with a metal or another metal nitride and (2) the reaction of two metal powders with nitrogen gas or ammonia. The first method has been the most common approach (Table 8.1, Reaction 1). Since these reactions usually require high temperatures (1073-2073 K), the inductive effect is Used to effect product formation. Li3N is a favored starting material due to its stability and relatively low melting temperature (mp = 1086 K), which... 

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