Si-Doped Fe Catalyst for Ammonia Synthesis at Dramatically Decreased Pressures and Temperatures

The Haber–Bosch
(HB) process combining nitrogen (N2) and hydrogen (H2) into ammonia (NH3) gas
plays an essential role in the synthesis of fertilizers for food production
and many other commodities. However, HB requires enormous energy resources
(2% of world energy production), and the high pressures and temperatures
make NH3 production facilities very expensive. Recent advances
in improving HB catalysts have been incremental and slow. To accelerate
the development of improved HB catalysts, we developed a hierarchical
high-throughput catalyst screening (HHTCS) approach based on the recently
developed complete reaction mechanism to identify non-transition-metal
(NTM) elements from a total set of 18 candidates that can significantly
improve the efficiency of the most active Fe surface, Fe-bcc(111),
through surface and subsurface doping. Surprisingly, we found a very
promising subsurface dopant, Si, that had not been identified or suggested
previously, showing the importance of the subsurface Fe atoms in N2 reduction reactions. Then we derived the full reaction path
of the HB process for the Si doped Fe-bcc(111) from QM simulations,
which we combined with kinetic Monte Carlo (kMC) simulations to predict
a ∼13-fold increase in turnover frequency (TOF) under typical
extreme HB conditions (200 atm reactant pressure and 500 °C)
and a ∼43-fold increase in TOF under ideal HB conditions (20
atm reactant pressure and 400 °C) for the Si-doped Fe catalyst,
in comparison to pure Fe catalyst. Importantly, the Si-doped Fe catalyst
can achieve the same TOF of pure Fe at 200 atm/500 °C under much
milder conditions, e.g. at a much decreased reactant pressure of 20
atm at 500 °C, or alternatively at temperature and reactant pressure
decreased to 400 °C and 60 atm, respectively. Production plants
using the new catalysts that operate under such milder conditions
could be much less expensive, allowing production at local sites needing
fertilizer.

哈伯-博世（HB）过程将氮气（N₂）与氢气（H₂）结合生成氨气（NH₃），在化肥合成以促进食品生产以及众多其他商品的生产中扮演着至关重要的角色。然而，HB过程需消耗巨大的能源资源（约占世界能源生产的2%），且高压高温的条件使得氨气生产设施成本极高。近期在改进HB催化剂方面的进展虽有所取得，但增量有限且进展缓慢。为加速优质HB催化剂的开发，我们基于近期研发的完整反应机制，开发了一种分级高吞吐量催化剂筛选（HHTCS）方法，旨在从18个候选元素中识别出非过渡金属（NTM）元素，从而显著提升最活跃的铁表面，Fe-bcc(111)的效率，通过表面及次表面掺杂实现。令人惊讶的是，我们发现了一种具有极大潜力的次表面掺杂剂Si，该元素此前尚未被识别或建议使用，凸显了次表面铁原子在氮气还原反应中的重要性。随后，我们通过量子力学（QM）模拟推导出Si掺杂Fe-bcc(111)的HB过程全反应路径，并将其与动力学蒙特卡洛（kMC）模拟相结合，预测在典型的极端HB条件下（200 atm的反应物压力和500 °C的温度），Si掺杂Fe催化剂的转换频率（TOF）将提高约13倍，在理想HB条件下（20 atm的反应物压力和400 °C的温度），TOF将提高约43倍，相较于纯铁催化剂。更重要的是，Si掺杂Fe催化剂在200 atm/500 °C的条件下，能够在更为温和的条件下实现与纯铁催化剂相同的TOF，例如在反应物压力降低至20 atm且温度保持在500 °C，或者将温度和反应物压力分别降低至400 °C和60 atm。采用新催化剂且在上述温和条件下运行的工厂，其成本将大幅降低，使得在需要化肥的本地进行生产成为可能。