氨是世界上生产的最重要的化学品之一,，每年生产总量仅次于硫酸。它主要用于制造肥料,，这对于养活世界的人口至关重要。然而，其生产占世界能源消耗的 2%的 和温室气体排放量, 的约 1.5%，因此人们一直在寻找更可持续地生产氨的方法。
Ammonia is one of the most important chemicals produced in the world, ranking second only to sulfuric acid in the total volume produced each year. It is used mostly to make fertilizer, which is essential to feeding the world的 population. Yet its production accounts for up to 2 percent of the world的 energy consumption and about 1.5 percent of greenhouse gas emissions, so the search has been underway for ways to produce ammonia more sustainably.
生产氨, 的传统方法已经使用了一个多世纪，占, 产量的绝大多数，是哈伯-博世工艺,，该方法依靠化石燃料提供所需的热量。该过程中使用的氢气也主要由化石燃料生产。
The traditional way of making ammonia, in use for more than a century and accounting for the vast majority of production, is the Haber-Bosch process, which relies on fossil fuels to provide the needed heat. Hydrogen used in the process is also largely produced from fossil fuels.
还有另一种方法, 使用电化学代替热和压力,，但到目前为止，这种方法在所需规模上还没有任何经济竞争力。
There is another way, using electrochemistry instead of heat and pressure, but so far this method has not been anywhere near economically competitive at the scales needed.
现在，麻省理工学院的, 研究人员已经开发出一种方法来预测哪些材料最有希望作为电化学氨生产中的催化剂。催化剂有助于驱动化学反应,，它们的特性决定了这些反应进行的效率。新方法无需使用反复试验来测试数百万种可能的合金—中的每种可能的组合，这可能需要数年的时间—，而是可以大大加快材料的搜索速度，使这种低排放方法与哈伯-博世工艺具有竞争力。
Now, researchers at MIT have developed a way to predict which materials could be most promising as catalysts in electrochemical ammonia production. Catalysts help drive chemical reactions, and their properties determine how efficiently those reactions proceed. Rather than using trial and error to test each possible combination out of the millions of possible alloys — which can take years — the new approach could greatly speed up the search for materials that could make this low-emissions method competitive with the Haber-Bosch process.
“我们的方法确定了驱动氨生产中催化活性的关键物理特性,” 核科学与工程和材料科学与工程系的 Breen M. Kerr 教授 Bilge Yildiz, (DMSE) 说。研究结果可以指导寻找新的、更有效的催化剂化合物。
“Our approach identifies the key physical properties that drive catalytic activity in ammonia production,” says Bilge Yildiz, the Breen M. Kerr Professor in the departments of Nuclear Science and Engineering and Materials Science and Engineering (DMSE). The results can guide the search for new and more effective catalyst compounds.
随着世界’人口的增长, Athanitis说, “我们’将需要越来越多的食物,，而我们’能够养活这么多人的唯一原因是肥料。” 但是肥料所需的氨90%以上仍然是由能源密集型产品制成的他说，哈伯-博世工艺,，�%9自一个多世纪前首次问世以来就已经进行了高度优化,”。
As the world的 population grows, Athanitis says, “we’re just going to need more and more food, and the only reason why we’re able to sustain so many people is because of fertilizer.” But more than 90 percent of the ammonia needed for fertilizer is still made by that energy-intensive Haber-Bosch process, which “has been hyper-optimized since it first came out more than a century ago,” he says.
“如果我们’试图与社会保持一致’可持续发展和能源目标以及气候变化目标,我们确实需要想出另一种替代方案,”他解释道。目前，世界每年使用约 2 亿吨氨, “，因此理想情况下，我们希望能够找到一种方法来生产相同数量的氨, 甚至更多,，但以更节能的方式，并且二氧化碳排放量更低,” 他说。
“If we’re trying to keep in line with society的 sustainability and energy targets and climate change targets, we really need to come up with another alternative,” he explains. The world currently uses about 200 million metric tons of ammonia each year, “so ideally we want to be able to find a way to produce the same amount of ammonia, or even more, but in a more energy-efficient way and also with lower CO2 emissions,” he says.
用电生产氨并不是一个新想法。 “It的 实际上只是质子电子对和氮气之间的电化学反应。他说，这些技术存在,”。该方法使用与电解槽, 相同的基本原理，电解槽使用电力驱动设备中的化学反应。
Using electricity to produce ammonia is not a new idea. “It的 really just the electrochemical reaction between proton-electron pairs and nitrogen gas. And these technologies exist,” he says. The approach uses the same basic principles as electrolyzers, which use electricity to drive chemical reactions in devices.
但是，虽然该过程有效,但的对于工业规模生产来说效率不够。 “生产率和产量仍然太低,” Athanitis 说。 “即使一项技术可能对世界或气候更好,公司和资本主义会赢’t真的允许它，除非它’具有成本竞争力。”
But while the process works, it的 not efficient enough for industrial-scale production. “Production rates and yields are still too low,” Athanitis says. “Even though a technology might be better for the world or for the climate, companies and capitalism won’t really allow it unless it的 cost competitive.”
如何使其更具竞争力? 电化学过程中的关键成分是金属催化剂,，其特性决定其表面发生的反应。 “如果我们能以某种方式找到一种催化剂，可以减少所需的能量，并且对氨生产更具选择性,” Athanitis 说, “那么我们基本上就中大奖了。” 更具选择性的催化剂将产生更多的氨，同时减少不需要的副反应。
How to make it more competitive? The key ingredient in the electrochemical process is a metallic catalyst, whose properties govern the reaction that takes place on its surface. “If we can somehow find a catalyst that reduces the energy needed and is more selective for ammonia production,” Athanitis says, “then we could essentially hit the jackpot.” A more selective catalyst would produce more ammonia while reducing unwanted side reactions.
但找到理想的催化剂并不只是找到一种完美的材料那么简单。不同的材料可以改善反应,的不同部分，研究人员正在寻找可以使氨生产高效,、经济实惠,并大规模实用的组合。
But finding that ideal catalyst is not simply a matter of identifying one perfect material. Different materials can improve different parts of the reaction, and researchers are seeking combinations that can make ammonia production efficient, affordable, and practical at large scale.
“金属氮化物化合物为该反应以及识别电子,化学,和决定氮还原和氨电合成反应性的结构特性提供了理想的材料系统,” Yildiz说。
“Metal nitride compounds make an ideal material system for this reaction and for identifying the electronic, chemical, and structural properties that determine reactivity in nitrogen reduction and ammonia electrosynthesis,” Yildiz says.
为此目的，过渡金属可以形成有前景的氮化物合金,，历史上, “材料研究几乎是经过反复试验,” Athanitis 说。
Transition metals could form promising nitride alloys for this purpose, and historically, “materials research has been pretty much trial and error,” Athanitis says.
他说，通常的过程是采用一些现有材料并以某种方式“对其进行调整,”。 “It的 都在一定程度上受到科学和化学直觉的指导。”
The usual process is to take some existing material and “tweak it in some way,” he says. “It的 all somewhat guided by scientific and chemical intuition.”
现在，, 越来越多的, 计算工具被用来模拟物理相互作用并预测结果。一种称为密度泛函理论的方法使用量子力学来模拟材料,的特性和行为，使研究人员能够在实验室制造它们之前预测不同的原子排列可能如何表现。我们不是随机搜索每种可能的合金组合, Yildiz 说, “ 我们首先评估了材料的哪些微观特性使其适合氮还原。”
Now, increasingly, computational tools are being used to model the physical interactions and predict outcomes. A method called density functional theory uses quantum mechanics to simulate the properties and behavior of materials, allowing researchers to predict how different atomic arrangements may perform before making them in the lab. Rather than searching randomly through every possible alloy combination, Yildiz says, “we first assessed what microscopic properties of the material make them tick for nitrogen reduction.”
对于产氨催化剂,“we’正在研究过渡金属氮化物,” Athanitis 说, 因为已发现它们在这些电化学氮反应中有效。它们特别有效，因为“催化剂本身固有的氮成为反应的一部分。”
For ammonia-producing catalysts, “we’re looking at transition metal nitrides,” Athanitis says, because they have been found to be effective in these electrochemical nitrogen reactions. They are especially effective because “the nitrogen inherent to the catalyst itself becomes part of the reaction.”
这会产生一系列化学步骤，其中一个步骤提供驱动下一个,所需的部分能量，从而减少所需的输入能量。这有助于解决氮还原反应:中的主要瓶颈之一，即打破氮分子中的强键所需的高能量,，他说。
This produces a series of chemical steps in which one step provides part of the energy needed to drive the next, reducing the amount of input energy needed. This helps solve one of the major bottlenecks in the nitrogen reduction reaction: the high energy required to break the strong bonds in nitrogen molecules, he says.
但这个过程远非完美, Athanitis 说。 “ 仍然受到整个反应途径, 中某些步骤的限制，包括氮解离和氢转移。” 该研究试图识别这些瓶颈，, 借助机器学习, 确定这些金属的哪些合金可以克服这些瓶颈。
But the process is far from perfect, Athanitis says. It is “still limited by certain steps throughout the reaction pathway, including nitrogen dissociation and hydrogen transfer.” The study attempted to identify those bottlenecks and, with the help of machine learning, determine which alloys of these metals might overcome them.
了解,“it 可以为我们提供见解，并为我们如何调整这些材料以创造下一代更好的氮化物催化剂提供潜在策略,” Athanitis 说。
With that understanding, “it can give us insights and open up potential strategies for how we can tune these materials to create next-generation better nitride catalysts,” Athanitis says.
该方法是 “ 令人兴奋的工作” 可以帮助为设计用于氨生产的新型催化剂奠定基础, 未参与这项研究的威斯康星大学工程教授 Dane Morgan, 说。
The approach is “exciting work” that could help develop a foundation for designing new catalysts for ammonia production, says Dane Morgan, a professor of engineering at the University of Wisconsin who was not involved in this study.
“ 这项工作有助于阐明材料的基本电子特性与其作为制造氨的催化剂的作用之间的关系,” Morgan 说。 “这种理解可以通过更好的定性理解和加速计算筛选来帮助指导研究人员设计新催化剂,。”
“This work helps clarify how fundamental electronic properties of a material relate to its role as a catalyst in making ammonia,” Morgan says. “Such understanding can help guide researchers in designing new catalysts, both through better qualitative understanding and by accelerating computational screening.”
到目前为止, 研究纯粹是理论上的: 研究人员已经使用计算机模型来识别有前途的合金, 但这些材料仍然需要制造和测试。摩根指出，“将这些计算转化为实际的催化剂将需要许多额外的步骤,，因此距离真正产生有意义的影响可能还有一段距离。”
So far, the study is purely theoretical: The researchers have used computer models to identify promising alloys, but those materials still need to be made and tested. Morgan notes that “translating these calculations into practical catalysts will require many additional steps, so meaningful real-world impact is likely still some distance away.”
“他补充说，’可能,”的前沿总是存在着推动。 “我们认为我们’已经将候选材料的边界推到了之前,的范围之外，希望我们’已经接近了。但即使我们’还没有达到目标,，我们’仍然在朝着正确的方向前进。”
“There have always been pushes at the frontiers of what的 possible,” he adds. “We like to think we’ve pushed the boundary of candidate materials here beyond what was thought of before, and hopefully we’re almost there. But even if we’re not almost there, we’re still pushing in the right direction.”