Fuller Jon, Fortunelli Alessandro, Goddard William A, An Qi
Department of Chemical and Materials Engineering, University of Nevada - Reno, Nevada 89577, USA.
Materials and Procs Simulation Center (MSC), California Institute of Technology, Pasadena, California 91125, USA.
Phys Chem Chem Phys. 2019 Jun 7;21(21):11444-11454. doi: 10.1039/c9cp01611b. Epub 2019 May 21.
To provide guidelines to accelerate the Haber-Bosch (HB) process for synthesis of ammonia from hydrogen and nitrogen, we used Quantum Mechanics (QM) to determine the reaction mechanism and free energy reaction barriers under experimental reaction conditions (400 °C and 20 atm) for all 10 important surface reactions on the Fe(211) reconstructed (Fe(211)R) surface. These conditions were then used in full kMC modeling for 30 minutes to attain steady state. We find that the stable surface under Haber-Bosch conditions is the missing row 2 × 1 reconstructed surface (211)R and that the Turn Over Frequency (TOF) is 18.7 s per 2 × 2 surface site for 1.5 Torr NH pressure, but changes to 3.5 s for 1 atm, values close (within 6%) to the ones on Fe(111). The experimental ratio between (211) and (111) rates at low (undisclosed) NH pressure was reported to be 0.75. The excellent agreement with experiment on two very different surfaces and reaction mechanisms is a testament of the accuracy of QM modeling. In addition, our kinetic analysis indicates that Fe(211)R is more active than Fe(111) at high pressure, close to HB industrial conditions, and that (211)R is more abundant than (111) via a steady-state Wulff construction under HB conditions. Thus, at variance with common thinking, we advocate the Fe(211)R surface as the catalytically active phase of pure iron ammonia synthesis catalyst under HB industrial conditions.
为了提供指导方针以加速由氢气和氮气合成氨的哈伯-博施（HB）过程，我们运用量子力学（QM）来确定在实验反应条件（400°C和20个大气压）下，Fe(211)重构表面（Fe(211)R）上所有10个重要表面反应的反应机理和自由能反应势垒。然后将这些条件用于完整的动力学蒙特卡罗（kMC）建模30分钟以达到稳态。我们发现，在哈伯-博施条件下稳定的表面是缺失行2×1重构表面（211）R，对于1.5托的NH压力，每2×2表面位点的周转频率（TOF）为18.7每秒，但在1个大气压下变为3.5每秒，该值与Fe(111)上的值接近（在6%以内）。据报道，在低（未公开）NH压力下（211）和（111）速率的实验比值为0.75。在两个非常不同的表面和反应机理上与实验的出色吻合证明了QM建模的准确性。此外，我们的动力学分析表明，在接近HB工业条件的高压下，Fe(211)R比Fe(111)更具活性，并且在HB条件下通过稳态伍尔夫构造，（211）R比（111）更丰富。因此,与通常的观点不同,我们主张在HB工业条件下，Fe(211)R表面是纯铁氨合成催化剂的催化活性相。
