Chemical and Biological Engineering Department, Center for Micro-Engineered Materials, University of New Mexico, Albuquerque, NM 87131, USA.
Phys Chem Chem Phys. 2018 May 30;20(21):14679-14687. doi: 10.1039/c8cp01643g.
We report herein a density functional theory study of the nitrogen electroreduction and hydrogen evolution reactions on cubic molybdenum carbide (MoC) in order to investigate the viability of using this material as an electro-catalyst for ammonia synthesis. Free energy diagrams for associative and dissociative Heyrovsky mechanisms showed that nitrogen reduction on cubic MoC(111) can proceed via an associative mechanism and that small negative potentials of -0.3 V vs. standard hydrogen electrode can onset the reduction of nitrogen to ammonia. Kinetic volcano plots for hydrogen evolution showed that the MoC[110] surface is expected to have a high rate for the hydrogen evolution reaction, which could compete with the reduction of nitrogen on cubic MoC. The comparison between the adsorption energies of H-adatoms and N-adatoms also shows that at low potentials adsorption of hydrogen atoms competes with nitrogen adsorption on all the MoC surfaces except the MoC(111) surface. The hydrogen evolution and accumulation of H-adatoms can be mitigated by introducing carbon vacancies i.e. increasing the ratio of metal to carbon atoms, which will significantly increase the affinity of the catalytic surface for both nitrogen molecules and N-adatoms.
本文通过密度泛函理论研究了立方碳化钼（MoC）上的氮电还原和析氢反应，以探究将该材料作为氨合成电催化剂的可行性。缔合和离解Heyrovsky 机制的自由能图表明，氮在立方 MoC(111)上的还原可以通过缔合机制进行，并且相对于标准氢电极的小负电势-0.3 V 可以引发氮气还原为氨。析氢的动力学火山图表明，MoC[110]表面预计具有高析氢反应速率，这可以与立方 MoC 上氮的还原相竞争。H 原子和 N 原子吸附能的比较还表明，在低电势下，除 MoC(111)表面外，所有 MoC 表面上 H 原子的吸附都与氮的吸附竞争。通过引入碳空位（即增加金属与碳原子的比例）可以缓解析氢和 H 原子的积累，这将显著增加催化表面对氮气分子和 N 原子的亲和力。
