CAS-Climate: Atomically Resolved Single-Molecule Microscopy of Catalytic Intermediates in CO2 Reduction

在化学系化学测量与成像(CMI)项目的支持下，耶鲁大学Udo Schwarz教授和Eric Altman教授领导的一个研究小组结合扫描探针显微镜(SPM)方法和理论，获得了一氧化碳(CO)和单分子催化剂钴(COPC)之间相互作用的详细图像。该项目将研究如何使用新的成像能力来探索表面相互作用和COPC催化剂上取代基的作用，以调整CO的吸附强度，CO是二氧化碳还原和转化为甲醇的关键中间体。这项研究旨在通过促进建立基于二氧化碳催化转化为甲醇的可靠的碳中性能源供应来解决与气候变化相关的问题。甲醇是一种易于储存和运输的商品化学品，可以直接用作燃料，也可以转化为其他液体燃料，如柴油、汽油或航空煤油。耶鲁大学最近的研究发现，固定化的COPC分子是一个有希望的平台，可以促进二氧化碳选择性转化为甲醇，但人们对如何优化这种潜在催化剂的活性、选择性和稳定性知之甚少。因此，研究小组正在开发一种化学成像方法，以显示单个COPC分子如何与支撑表面以及二氧化碳还原过程中的关键中间物种(包括CO)相互作用。这项工作有可能通过调节COPC催化剂上的取代基来提供优化这些相互作用的新方法。从这一努力中出现的详细的分子水平的理解可能会导致电催化性能的改善。除了与减缓气候变化相关的更广泛的影响外，该项目还将提供最先进的成像方法方面的高级学生培训机会，并支持面向公众的外展活动。二氧化碳的减少涉及产生作为中间体的一氧化碳，因此高效的甲醇生产需要一氧化碳与钴原子的结合既不太强也不太弱。虽然可以通过改变催化剂结构来微调CO的结合，但目前的光谱方法不足以在单分子的基础上了解CO的结合强度。为了缓解这一缺点，研究小组使用先进的扫描探针显微镜方法来局部测量作为催化剂结构和载体函数的CO吸附强度，目的是实现合理的催化剂优化。该项目的关键是扫描探针显微镜的最新进展，这些进展提供了成像分子结构、区分键级和测量分子结构中微小扭曲的能力，因为电子被注入到分子中时，使用带有CO官能化尖端的非接触原子力显微镜(NC-AFM)。为了补充信息，NC-AFM成像的结果与开尔文探针显微镜获得的局部功函数变化的测量和通过隧道光谱获得的电子结构相结合，从而可以绘制最高占据和最低未占据分子轨道的位置、总电荷密度以及进出载体的电子转移。作为这项工作的第三个主要内容，基于扫描隧道显微镜的作用光谱学应该能够在单个分子的基础上量化载体、吸附位置和几何结构以及取代基团如何在二氧化碳转化为液体燃料的过程中影响活性中间体的吸附。

With support from the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry, a research team led by Professors Udo Schwarz and Eric Altman at Yale University is using a combination of scanning probe microscopy (SPM) methods and theory to obtain a detailed picture of the interaction between carbon monoxide (CO) and the single-molecule catalyst cobalt phthalocyanine (CoPc). The project will examine how new imaging capabilities can be used to probe the roles of surface interactions and of substituent groups on the CoPc catalysts in order to adjust the adsorption strength of CO, a key intermediate in the reduction and conversion of CO2 to methanol. This research is designed to address issues related to climate change by contributing to the creation of a reliable, carbon-neutral energy supply based on the catalytic conversion of CO2 into methanol. Methanol is a commodity chemical that can readily be stored and transported and either used as fuel directly or converted into other liquid fuels such as diesel, gasoline, or aviation kerosene. Recent work at Yale has identified immobilized CoPc molecules as a promising platform for promoting selective CO2 conversion to methanol, but little is known about how to optimize the activity, selectivity, and stability of this potential catalyst. Therefore, the research team is developing a chemical imaging approach to show how individual CoPc molecules interact with the supporting surface and with key intermediate species in the CO2 reduction process, including CO. The work has the potential to provide new methods for optimizing these interactions by tuning substituent groups on the CoPc catalyst. The detailed molecular-level understanding that emerges from this effort could lead to improvements in electrocatalytic performance. In addition to broader impacts related to the mitigation of climate change, the project will provide advanced student training opportunities in state-of-the-art imaging methods and supports outreach activities for the general public.The reduction of CO2 involves generating CO as an intermediate, therefore efficient methanol production requires that CO binding to the cobalt atom in CoPc is neither too strong nor too weak. Although CO binding can likely be fine-tuned by changing the catalyst structure, current spectroscopic methods are inadequate for understanding CO binding strength on a single-molecule basis. To alleviate this shortcoming, the research team uses advanced scanning probe microscopy methods to locally measure the CO adsorption strength as a function of catalyst structure and support, with the goal of enabling rational catalyst optimization. Key to the project are recent advances in SPM that provide the ability to image molecular structures, distinguish bond orders, and measure small distortions in molecular structures as electrons are injected into molecules using non-contact atomic force microscopy (NC-AFM) with CO-functionalized tips. For complementary information, results from NC-AFM imaging are combined with the measurement of local work function variations obtained by Kelvin probe microscopy and the electronic structure obtained through tunneling spectroscopy, which allow mapping of the positions of the highest occupied and lowest unoccupied molecular orbitals, total charge densities, and electron transfer to and from the support. As the third major element of this effort, scanning tunneling microscopy-based action spectroscopy should enable quantification on an individual molecule basis of how supports, adsorption sites and geometries, and substituent groups influence the adsorption of reactive intermediates in the conversion of CO2 to liquid fuelsThis award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.