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-博施（Haber–Bosch, HB）工艺通过将氮气（N₂）与氢气（H₂）合成为氨气（NH₃），在粮食生产用肥料及诸多其他大宗商品的合成中发挥着至关重要的作用。不过，HB工艺能耗极高，占全球能源总产量的2%，且严苛的高压高温工况使得氨气生产装置的建造成本极为高昂。近年来，针对HB工艺催化剂的改进进展缓慢且仅为渐进式小幅优化。为加速优化型HB工艺催化剂的研发进程，我们基于最新提出的完整反应机理，开发了一种分层高通量催化剂筛选（hierarchical high-throughput catalyst screening, HHTCS）方法。该方法可从共计18种候选元素中筛选出非过渡金属（NTM）元素，这类元素可通过表面与次表面掺杂，显著提升活性最高的纯铁基表面——体心立方铁(111)晶面（Fe-bcc(111)）的催化效率。令人意外的是，我们发现了一种此前从未被报道或提出的极具应用潜力的次表面掺杂剂——硅（Si），这一发现凸显了次表面铁原子在氮气还原反应中的关键作用。随后，我们通过量子力学（QM）模拟，推导了硅掺杂Fe-bcc(111)晶面的HB工艺完整反应路径，并将该路径与动力学蒙特卡洛（kMC）模拟相结合，预测得到：相较于纯铁催化剂，硅掺杂铁催化剂的转换频率（TOF）在典型极端HB工艺条件下（反应物压力200 atm、温度500 ℃）提升约13倍，在理想HB工艺条件下（反应物压力20 atm、温度400 ℃）提升约43倍。尤为关键的是，硅掺杂铁催化剂可在更为温和的工况下，达到纯铁催化剂在200 atm/500 ℃时的转换频率水平。例如，在500 ℃、反应物压力降至20 atm的条件下，或是分别将温度与反应物压力降至400 ℃与60 atm的工况下，均可实现这一目标。采用此类新型催化剂、可在更温和工况下运行的生产装置，其建造成本将大幅降低，从而能够在有肥料需求的本地站点直接开展生产。