Catalytic Sonochemistry for Clean Hydrogen from Ammonia

英国计划经历一场“绿色工业革命”，以减缓全球变暖，并在2050年前实现净零排放。改用氢，一种前景看好的零碳燃料，是这一计划的一部分，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，限制了其用途。在氨中化学储存氢气是可取的，因为它更安全，更容易储存和运输，并从已建立的供应链中受益。然而，将氨裂解回氢气需要催化剂微妙地平衡抑制反应的两个限速步骤：1)在高温下迅速从催化剂上解吸氨；2)在低温下无法将氢和氮从与催化剂结合的氨中转化出来。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，只有在稀有和昂贵的元素在高温下操作时才能实现，从而挑战了氨作为氢库的效用。有趣的是，静态催化的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，这涉及到在冷(&lt；150摄氏度)和热(&gt；500摄氏度)温度之间每秒转换1000到100万次，这在操作上是困难的。声化学利用声音产生气泡，这些气泡膨胀和收缩以加强化学反应，并可能提供一种独特的快速振荡温度的方法。当气泡在其初始尺寸以上膨胀和收缩时，其温度保持与氨吸附到催化剂上的环境温度相同。低于初始大小的气泡可能会迅速收缩，压缩气体，使其升温至500摄氏度以上。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解。压缩后，气泡膨胀回到原来的大小，冷却回到环境温度，并再次开始循环。这种声化学方法需要在催化剂周围控制气泡的现场运动。然而，目前的声化学过程并不能控制气泡的动力学。我们最近已经证明，纳米结构催化剂也可以作为气泡的成核位置，极大地提高反应速度。然而，这项工作使用了更简单的化学作为概念验证，并没有充分开发在解决更具挑战性的多相催化反应方面的潜力。本项目试图推进我们的声化学方法来实现氨裂解。我们假设，快速的冷热循环可以通过纳米结构催化剂成核的气泡来实现，并将克服与氨裂解相关的传统动力学限制。我们首先开发新型催化空化器，并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，并证明氨气裂解是可能的。这些结果随后将被用于技术经济模型，以评估潜在的工业影响。我们的主要创新是将空化试剂与催化剂相结合来增强声化学过程，这一点尚未完成，是声化学的范式转变。因此，壳牌、埃克森美孚、NPL和SENFI UK Ltd.都支持我们的研究愿景、提出的项目，并希望实现一条可持续的清洁氢气生产路线。

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.