Nitrogen Activation in a Mars–van Krevelen Mechanism for Ammonia Synthesis on Co3Mo3N

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• 作者：Constantinos D. Zeinalipour-Yazdi ; Justin S. J. Hargreaves ; C. Richard A. Catlow
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：December 24, 2015
• 年：2015
• 卷：119
• 期：51
• 页码：28368-28376
• 全文大小：601K
• ISSN：1932-7455

文摘

Co₃Mo₃N is one of the most active catalysts for ammonia synthesis; however, little is known about the atomistic details of N₂ adsorption and activation. Here we examine whether N₂ can adsorb and activate at nitrogen surface vacancies. We have identified the most favorable sites for surface nitrogen vacancy formation and have calculated vacancy formation free energies (and concentrations) taking into account vacancy configurational entropy and the entropy of N₂ at temperature and pressure conditions relevant to ammonia synthesis (380–550 °C, 100 atm) via a semiempirical approach. We show that 3-fold hollow bound nitrogen-containing (111)-surfaces have surprisingly high concentrations (1.6 × 10¹⁶ to 3.7 × 10¹⁶ cm^–2) of nitrogen vacancies in the temperature range for ammonia synthesis. It is shown that these vacancy sites can adsorb and activate N₂ demonstrating the potential of a Mars–van Krevelen type mechanism on Co₃Mo₃N. The catalytically active surface is one where 3f-hollow-nitrogens are bound to the molybdenum framework with a hexagonal array of embedded Co₈ cobalt nanoclusters. We find that the vacancy-formation energy (VFE) combined with the adsorption energy can be used as a descriptor in the screening of materials that activate doubly and triply bonded molecules that are bound end-on at surface vacancies.