Multimetallic CO2 Reduction Catalysts as Artificial Cofactors

地球上的二氧化碳(CO2)水平已经达到了历史最高水平，减缓人为气候变化是我们这个时代的决定性挑战。然而，全球经济和我们的社会严重依赖化石燃料，它满足了世界80%的能源需求，占我们日常生活中所依赖的所有化学商品的95%。一个例子是乙烯，它是一种两碳分子，目前是在一种高耗能和高污染的石化过程中从化石燃料中生产出来的。乙烯被称为平台化学品，因为它用于合成广泛的其他化学品，是许多常见塑料中的关键单体。一项新兴技术是能够将二氧化碳排放直接转化为碳基化学品，从而大大减少其生产对环境的破坏。大自然将这种“废气”用作生产生物质的主要单碳材料，化学工业开始意识到二氧化碳作为生产乙烯等重要化学物质的廉价、可再生原料的潜力。二氧化碳的转化是具有挑战性的，因为分子非常稳定和无反应，需要大量的能量输入才能使其发生反应。需要催化剂来降低这一能源需求，人们对用于二氧化碳活化的新型过渡金属催化剂的兴趣也越来越大。目前，催化剂效率低，对特定含碳产品的选择性低，阻碍了该技术的大规模工业应用。虽然已经开发了催化剂来从二氧化碳生成一碳产物，但很少有例子能形成多碳(C2+)产物。这是一个需要克服的关键障碍：像乙烯这样的C2+化合物代表着高经济价值和降低全球变暖潜力之间的最佳权衡，如果它们可以利用可再生电力从二氧化碳中产生的话。这是一个对化工行业大规模脱碳创新具有巨大潜在影响的机会，因为每年产生的乙烯比任何其他有机化合物都多，其年产量约释放2亿吨二氧化碳。在现有的催化剂中，没有一种对C2+产物具有足够的活性或选择性。这部分是因为对C-C键形成的要求缺乏基本的了解；这一理论基础需要做出合理的步骤来设计改进的催化剂。为了应对这些挑战，这项研究的灵感来自于能够以良好的活性和选择性将二氧化碳还原为C2+烃的酶催化剂-但不能规模化。模型酶是自然界中用于将二氧化碳还原为CO的一氧化碳脱氢酶和用于将氮还原为氨的固氮酶。这些酶在其活性部位含有多个过渡金属，位于可能发生两个二氧化碳单元耦合的地方。在研究人员实验室最近的发现引发的一项合作中，将开发结合合成催化剂和酶的优点的混合催化剂。将测试它们在合成二氧化碳转化中的使用，利用受限催化剂位置的原理来促进多个金属位置之间的C-C键形成。从二氧化碳生产化学品需要能量输入，而能源必须来自脱碳来源以减少排放。我们的催化剂将使用电子来驱动反应，因为可再生电能正变得越来越廉价。对反应机理的动态研究将有助于设计更有效的催化剂，最终目的是实现将二氧化碳转化为任何含碳分子的催化方法。借用酶的一两个诀窍，这项研究将推动化学工业更接近成为真正的、无废物的循环经济的一部分，并帮助实现产生负二氧化碳排放的目标。

Carbon dioxide (CO2) levels on Earth have reached an all-time high, and mitigating man-made climate change is a defining challenge of our era. However the global economy and our society are critically dependent on fossil fuels, which meet 80% of the worlds energy demands and account for the production of 95% of all chemical commodities we rely on in our everyday lives. One example, ethylene, is a two-carbon molecule that is currently produced from fossil fuels in a highly energy-consuming and polluting petrochemical process. Ethylene is called a platform chemical as it used to synthesise a wide range of other chemicals, and is a crucial monomer in many common plastics. One emerging technology is to enable the direct conversion of CO2 emissions into carbon-based chemicals, thereby much reducing the environmental damage caused by their production. Nature uses this 'waste' gas as its primary one-carbon building-block for biomass, and the chemical industry is beginning to realise the potential of CO2 as a cheap, renewable feedstock for producing of vital chemicals such as ethylene. Conversion of CO2 is challenging as the molecule is very stable and unreactive, and a vast energy input is required to make it react. Catalysts are needed to lower this energy requirement and interest is growing in new transition metal catalysts for CO2 activation. The scalable industrial application of this technology is currently held back by poor catalyst efficiency and low selectivity for a particular carbon-containing product. Although catalysts have been developed to generate one-carbon products from CO2, there are very few examples in which multi-carbon (C2+) products are formed. This is a key barrier to be overcome: C2+ compounds like ethylene represent the best trade-off between high economic value and a reduction in global warming potential, if they could be produced from CO2 using renewable electrical energy. This is an opportunity with massive potential impact for decarbonising innovation at scale in the chemicals industry, since more ethylene is produced each year than any other organic compound, and its annual production releases around 200 million tons of CO2. Of the existing catalysts, none are sufficiently active or selective for C2+ products. This is partly due to a lack of fundamental understanding about the requirements for C-C bond forming; this theoretical underpinning is needed to make rational steps to design improved catalysts.To address these challenges, this research takes inspiration from enzyme catalysts which are able to reduce CO2 to C2+ hydrocarbons with good activity and selectivity - but cannot be scaled. The model enzymes are the carbon monoxide dehydrogenases for CO2 reduction to CO in Nature, and nitrogenases for reduction of nitrogen to ammonia. These enzymes harbour multiple transition metals in their active sites, positioned where coupling of two CO2 units can occur. In a collaboration initiated by recent discoveries in the researchers' laboratories, hybrid catalysts that combine the benefits of synthetic catalysts and enzymes will be developed. Their use in synthetic CO2 conversion will be tested, taking this principle of confined catalyst sites to promote C-C bond formations between multiple metal sites.Producing chemicals from CO2 requires an energy input, and energy must come from a decarbonised source to reduce emissions. Our catalysts will use electrons to drive the reaction, since renewable electrical energy is becoming increasingly available at low cost. Studying the reaction mechanisms 'on-the-fly' will inform the design of more efficient catalysts, with the ultimate aim of realising a catalytic method for converting CO2 into any carbon-containing molecule. Borrowing a trick or two from enzymes, this research will move the chemical industry a step closer to becoming part of a true, waste-free, circular economy, as well as helping to make the goal of generating negative CO2 emissions a reality.