Catalytic Sonochemistry for Clean Hydrogen from Ammonia

英国计划进行“绿色工业革命”，以减缓全球变暖，并在2050年前达到净零排放。改用氢是一种有前途的零碳燃料，是该计划的一部分，需要对当前的氢经济和相关技术进行大规模改进。然而，氢气很难储存和运输，限制了它的用途。人们希望将氢化学储存在氨中，因为它更安全，更容易容纳和运输，并受益于已建立的供应链。然而，将氨裂化回氢气需要精确平衡抑制反应的两个限速步骤的催化剂：1）在高温下氨从催化剂快速解吸和2）在低温下不能从结合到催化剂的氨改革氢气和氮气。在固定的操作条件下，这种平衡为催化剂活性创造了一个最佳温度，只有在高温下操作的稀有和昂贵的元素才能达到催化剂活性，因此挑战了氨作为氢存储器的效用。有趣的是，静态催化的这种理论最大值可以通过在有利于这些相反的限速步骤的操作条件之间快速切换来克服，即，动态催化对于氨裂解，这涉及到每秒在冷（< 150 ℃）和热（> 500 ℃）温度之间转换一千到一百万次，这在操作上是困难的。声化学利用声音产生膨胀和收缩的气泡，以增强化学反应，并可能提供快速振荡温度的独特手段。当气泡膨胀和收缩超过其初始尺寸时，其温度保持等于环境温度，由此氨将吸附到催化剂上。低于其初始尺寸，气泡可能会迅速收缩，压缩气体并使其升温至500 ℃以上。这些热压缩创造了一个局部高能微环境，非常适合氨的催化裂化。压缩后，气泡膨胀回原来的大小，冷却回环境温度，再次开始循环。这种声化学方法需要在催化剂周围控制气泡运动。然而，目前的声化学过程不控制气泡动力学。我们最近已经表明，纳米结构的催化剂，也作为气泡的成核点大大提高反应速率。然而，这项工作使用了更简单的化学作为概念验证，并没有充分利用潜力，在解决更具挑战性的多相催化反应。该项目旨在推进我们的方法声化学实现氨裂解。我们假设，快速冷热循环是可以实现的气泡成核的纳米结构的催化剂，并将克服传统的动力学限制与氨裂解。我们首先开发新的催化空化剂，并使用更简单的化学方法研究它们的声化学。之后，我们将推进空化计量，并证明氨裂解是可能的。这些结果将用于技术经济模型，以评估潜在的工业影响。我们的关键新奇是空化剂与催化剂的组合，以增强声化学过程，这还有待完成，是一个范式转变声化学。因此，壳牌、埃克森美孚、NPL和SENFI UK Ltd.都支持我们的研究愿景、拟议项目，并希望实现可持续的清洁制氢路线。

The UK plans to undergo a "green industrial revolution" to mitigate global warming and reach net-zero by 2050. Switching to hydrogen, a promising zero-carbon fuel, is part of this plan and requires a massive improvement on the current hydrogen economy and associated technologies. Hydrogen gas, however, is difficult to store and transport, limiting its utility. It is desirable to chemically store hydrogen in ammonia because it is safer and easier to contain and transport and benefits from an established supply chain. However, cracking ammonia back to hydrogen requires catalysts that delicately balances two rate-limiting steps that inhibit the reaction: 1) rapid desorption of ammonia from the catalyst at hot temperatures and 2) inability to reform hydrogen and nitrogen from ammonia bound to the catalyst at cold temperatures. Under fixed operating conditions, this balance creates an optimal temperature for catalyst activity only achieved with rare and expensive elements operating at high temperatures, thus challenging the utility of ammonia as a hydrogen store.Interestingly, this theoretical maximum for static catalysis may be overcome by rapidly switching between operating conditions that favour these opposing rate-limiting steps, i.e., dynamic catalysis. For ammonia cracking, this involves shifting between cold (< 150 C) and hot (> 500 C) temperatures a thousand to a million times per second, which is operationally difficult.Sonochemistry uses sound to create bubbles that expand and contract to enhance chemical reactions and may provide a unique means of rapidly oscillating temperature. As a bubble expands and contracts above its initial size, its temperature remains equal to the ambient temperature whereby ammonia will adsorb onto the catalyst. Below its initial size, the bubble may rapidly shrink, compressing the gas and causing it to heat up to temperatures above 500 C. These hot compressions create a local high-energy microenvironment ideal for catalytic cracking of ammonia. After compression, the bubble expands back to its original size, cooling back to ambient temperatures and starting the cycle again.This approach to sonochemistry requires site-controlled bubble motion around a catalyst. Yet current sonochemical processes do not control bubble dynamics. We have recently shown that nanostructured catalysts that also function as nucleation sites for bubbles vastly improve reaction rates. However, this work used simpler chemistry as a proof-of-concept and did not fully exploit the potential in addressing more challenging heterogenous catalytic reactions.This project seeks to advance our approach to sonochemistry to achieve ammonia cracking. We hypothesize that rapid hot-cold cycles are achievable with bubbles nucleated by nanostructured catalysts and will overcome the conventional kinetic limitations associated with ammonia cracking. We start the project by first developing novel catalytic cavitation agents and study their sonochemistry using simpler chemistries. After, we will advance cavitation metrology and demonstrate that ammonia cracking is possible. These results will then be used in technoeconomic models to assess the potential industrial impact. Our key novelty is the combination of cavitation agents with catalysts to enhance sonochemical processes, which has yet to be done and is a paradigm shift in sonochemistry. As such, Shell, ExxonMobil, NPL, and SENFI UK Ltd. all support our research vision, proposed project, and desire to achieve a sustainable route to clean hydrogen production.