Mechanocatalytic Ammonia Synthesis over Transition Metal Nitrides

合成氨生产是生产足够的肥料来养活世界的关键步骤。然而，占主导地位的Haber-Bosch工业过程需要高温和高压，这只有在非常大规模的集中式工厂中才具有商业可行性。 一个更可扩展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害。该项目研究了一种相对较新的催化类型（称为机械催化）在小规模氨（NH3）生产中的适用性。 更广泛的影响包括可持续化学的教育和培训课程以及在线讲座档案。机械催化涉及球磨机或类似设备中的碰撞，以激活固体催化剂并创造独特的反应环境，寿命有限。 因此，NH3制造可以在标称环境条件下操作。 N2分子中稳定的三键的活化是可能的，并且氨可以在催化剂返回到其非活化状态的同时形成。 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的性质和耐久性的影响，仍然存在许多问题。 该项目的重点是利用过渡金属氮化物催化剂的机械催化氨制造的驱动力。热点和高催化活性亚稳位点的形成是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位点往往会衰减，因此将开发新的表征方法来淬灭或维持它们，以进行有效的表征。 将使用落球实验、热导率测量和机械化学N2活化速率的组合来推导单个研磨碰撞事件和随后的热点衰减的瞬态能量平衡。得到的瞬态温度将是氨合成过程动力学模型的基础。 此外，还将确定与各种氮化物催化剂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知识价值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.