Mechanocatalytic Ammonia Synthesis over Transition Metal Nitrides

制造氨是生产足够养活世界的化肥的关键一步。然而，占主导地位的Haber-Bosch工业过程需要高温和压力，而这些温度和压力只有在非常大规模的集中式工厂中才能在商业上可行。一个更具可扩充性的过程将允许在偏远或农村地区分散化肥生产，从而降低运输成本和相关危险。该项目调查了一种被称为机械催化的相对新型的催化剂是否适用于小规模的氨(NH3)制造。更广泛的影响包括关于可持续化学的教育和培训课程以及在线讲座档案。机械催化涉及在球磨机或类似设备中进行碰撞，以激活固体催化剂，并在有限的寿命内创造独特的反应环境。因此，NH3制造可以在名义上的环境条件下运行。氮气分子中稳定的三键可能被激活，当催化剂返回到其未活化状态时，可以形成氨。然而，关于反应环境的性质、不同反应步骤的动力学以及对催化剂性能和耐久性的影响，仍然存在许多问题。该项目的重点是使用过渡金属氮化物催化剂的机械催化制氨的驱动力。热点和高催化活性亚稳中心的形成是对观察到的机械催化活性的最可能的解释。因此，针对每一种现象都有具体的目标。由于机械催化过程中的高活性中心往往会衰变，因此将开发新的表征方法来抑制或维持它们，以实现有效的表征。结合落球实验、热导率测量和机械力化学氮气活化速率，将得出单个研磨碰撞事件和随后热点衰减的瞬时能量平衡。由此得到的瞬时温度将成为氨合成过程动力学模型的基础。此外，将确定与各种氮化物催化剂的性能相关的化学和机械描述符。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Ammonia manufacturing is the key step to producing sufficient fertilizer to feed the world. However, the dominant Haber-Bosch industrial process requires high temperatures and pressures that are only commercially viable in centralized plants at very large scale. A more scalable process would allow for decentralized fertilizer production in remote or rural areas, thus reducing transportation costs and associated hazards. The project investigates the suitability of a relatively new type of catalysis, known as mechanocatalysis, for small-scale, ammonia (NH3) manufacturing. Broader impacts include educational and training classes on sustainable chemistry and an online lecture archive.Mechanocatalysis involves collisions in a ball mill or similar device to activate solid catalysts and create unique reactive environments with limited lifetime. NH3 manufacturing can thus be operated at nominally ambient conditions. Activation of the stable triple bond in the N2 molecule is possible, and ammonia can be formed while the catalyst returns to its non-activated state. However, many questions remain regarding the nature of the reactive environments, kinetics of different reaction steps, and the impact on properties and durability of the catalysts. The project focuses on the driving forces for mechanocatalytic ammonia manufacturing utilizing transition metal nitride catalysts. The formation of hot spots and highly catalytically active metastable sites are the most probable explanations for observed mechanocatalytic activity. Consequently, specific aims target each of these phenomena. Since highly active sites in mechanocatalytic processes tend to decay, novel characterization approaches will be developed to quench or sustain them to allow for effective characterization. A combination of ball drop experiments, measurements of thermal conductivity, and the rate of mechanochemical N2 activation will be used to derive a transient energy balance for an individual milling collision event and the subsequent decay of the hot spot. The resulting transient temperature will be the basis for kinetic models of the ammonia synthesis process. In addition, chemical and mechanical descriptors will be identified correlating to the performance of the various nitride catalysts.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.