CO2 Electroreduction Over Extremely Small Nanoparticles: Opening New Reaction Pathways

电化学二氧化碳还原反应(CO2RR)是将二氧化碳转化为增值化学品、遏制全球变暖的一种很有前途的解决方案。在过去的十年里，人们致力于研究几种催化剂上的反应机理--主要是金属基材料--以推进从CO2RR生产不同化学原料的前景。所取得的了解使一氧化碳和甲酸盐等产品的反应速度和选择性都很高。然而，甲烷和乙烯等用途和市场规模巨大的碳氢化合物，仍有待电催化剂设计的突破，才能实现大规模、经济可行的生产。要设计出高性能的电催化剂，对反应机理的深入了解是至关重要的，尤其是对于具有多个复杂反应步骤的碳氢化合物。反应机理的理论描述是基于密度泛函理论(DFT)的计算，并且主要是基于晶态金属。纳米颗粒，通常为25纳米或更大，也因其高表面积而被用于实验。然而，对于尺寸在0.5~2 nm的超小纳米粒子，还没有系统的研究来证明其构效关系。我们对纳米铜颗粒的初步研究表明，当颗粒尺寸从200 nm减小到0.5 nm时，反应路径发生了显著的变化。在大颗粒上，乙烯是主要产物，而在极小颗粒上，甲烷是主要产物。在Cu0.5 nm上的初步结果表明，该电极对甲烷的高电流密度为1.6A/cm2，选择性大于85%。在这个项目中，一个由DFT计算、纳米颗粒合成、高级表征和电子显微镜方面的专家团队致力于系统地合成尺寸可控在0.5-2 nm的纳米颗粒，并将DFT与原位光谱相结合来了解铜、银、锡及其二元和三元体系的反应机理。本项目获得的新认识将开辟新的反应途径，并将改变催化剂设计的范式。

Electrochemical CO2 reduction reaction (CO2RR) is a promising solution to convert carbon dioxide into value-added chemicals and curb the global warming. During the last decade, much effort has been devoted to investigating the reaction mechanism on several catalysts-mostly metal-based materials-to advance the prospects of producing different chemical feedstocks from CO2RR. The attained understanding has enabled achieving high reaction rate and selectivity for products such as carbon monoxide and formate. However, hydrocarbons with huge applications and market size, such as methane and ethylene, still wait for breakthroughs in electrocatalyst design to realize their large-scale and economically feasible production. To design a high performing electrocatalyst, a deep understanding of the reaction mechanism is crucial, especially for hydrocarbons with several complicated reaction steps. Theoretical descriptions of the reaction mechanism are based on the density functional theory (DFT) computations and mostly on crystalline metals. Nanoparticles, usually 25 nm or larger, are also used in experiments for their high surface area. However, there is no systematic study to demonstrate the structure-activity relationship for extremely small nanoparticles with 0.5 to 2 nm sizes. Our preliminary studies on Cu nanoparticles has shown that when the particle size decreases from 200 nm to 0.5 nm, the reaction pathway significantly changes. On large particles, ethylene is the dominant product, while on extremely small particles methane is the main product. The preliminary results on Cu 0.5 nm showed a record performance of high current density of 1.6 A/cm2 and selectivity greater than 85% for methane. In this project, a team of experts in DFT computations, nanoparticles synthesis, advanced characterization, and electron microscopy, aim to systematically synthesize nanoparticles with controlled sizes of 0.5 to 2 nm, and combine DFT with in-situ spectroscopy to understand the reaction mechanism on Cu, Ag, and Sn, and their binary and ternary systems. The novel understanding obtained in this project will open up new reaction pathways and will change the paradigm for catalyst design.