Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK.
Phys Chem Chem Phys. 2018 Jul 11;20(27):18729-18736. doi: 10.1039/c8cp02381f.
Recently we reported an Eley-Rideal/Mars-van Krevelen mechanism for ammonia synthesis on cobalt molybdenum nitride (Co3Mo3N). In this mechanism hydrogenation of activated dinitrogen occurs directly from the gas phase in a low barrier step forming a hydrazinylidene intermediate [double bond, length as m-dash]NNH2. In this paper we study whether such a mechanism of ammonia synthesis could occur on the (111) surface of another metal nitride, Mn6N5+x (x = 1), as this would explain the low-T ammonia synthesis activity of Co3Mo3N. We find that although N2 adsorbs more strongly than H2 on the (111) surface, having also examined the (110) and the (100) surface, N2 is not significantly activated when adsorbed in an end-on configuration. The hydrogenation reactions via an Eley-Rideal mechanism are all high barrier processes (>182 kJ mol-1) and therefore an Eley-Rideal mechanism for ammonia synthesis is predicted to not occur on this material unless there are high temperatures. Our study indicates that the fact that an Eley-Rideal/Mars-van Krevelen mechanism occurs on Co3Mo3N is a result of the stronger activation of dinitrogen at nitrogen vacancies when dinitrogen is adsorbed in an end-on configuration.
最近，我们报道了在钴钼氮化物（Co3Mo3N）上合成氨的 Eley-Rideal/Mars-van Krevelen 机制。在该机制中，气相中的活化氮气直接加氢形成腙亚氨基中间体[双键，长度为 m-dash]NNH2，这是一个低势垒步骤。在本文中，我们研究了这种氨合成机制是否可能发生在另一种金属氮化物 Mn6N5+x（x = 1）的（111）表面上，因为这将解释 Co3Mo3N 在低温下合成氨的活性。我们发现，尽管 N2 在（111）表面上的吸附强度强于 H2，但也对（110）和（100）表面进行了研究，当 N2 以端到端构型吸附时，它并没有被显著活化。通过 Eley-Rideal 机制的加氢反应都是高势垒过程（>182 kJ mol-1），因此除非温度很高，否则预测氨合成的 Eley-Rideal 机制不会在这种材料上发生。我们的研究表明，Eley-Rideal/Mars-van Krevelen 机制在 Co3Mo3N 上发生是由于在端到端构型中吸附时，氮空位对二氮的更强活化。
