Characterizing Structures and Intra-/Intermolecular Forces in Molecular CO2 Reduction Catalysts and Reaction Intermediates by Infrared Spectroscopy of Cryogenic Ions

碳中性、可持续能源经济的产生已被许多人认为是在不久的将来开发的最重要的新技术之一。将二氧化碳(CO2)转化为可化学使用的燃料是实现这一目标的一种有希望的方法。这样的过程很难实施，它们需要催化剂的辅助，即通过分子或材料来降低将二氧化碳转化为燃料所需的能量。为了开发经济高效的催化剂，我们需要了解二氧化碳是如何与催化剂分子相互作用的。在这项由化学系化学结构动力学和机理(CSDM-A)计划资助的项目中，科罗拉多大学博尔德分校的J.Mathias Weber教授正在使用最先进的质谱学和激光光谱分析技术来研究二氧化碳与催化剂分子的相互作用。感兴趣的催化剂是包含钴(Co)和Re(Re)等金属的分子，周围环绕着包含其他元素(例如碳(C)、氧(O)、氮(N))的结构。周围的结构影响二氧化碳与金属原子的结合方式(关键的催化过程起源于二氧化碳与金属原子的相互作用)。这些金属催化剂“络合物”是离子的(它们带有电荷)，这意味着它们可以用质谱仪进行分类和浓缩。所需的离子催化剂络合物被冷却到非常低的温度(低至5开尔文(K)或零下450华氏度)。在这些低温下，二氧化碳分子会结合到催化剂复合体上。一旦二氧化碳-金属络合物形成，红外激光就被用来测量这些络合物的振动。从振动中可以推断出络合物的结构。这种方法能够探测它们反应中的关键步骤，允许组装催化剂如何发挥作用的详细机制。这项工作的更广泛影响包括开发对环境影响小的新化学燃料来源的潜在社会效益，以及对研究生研究人员进行高级实验和计算技术培训。此外，韦伯小组正在开发一种基于网络的二氧化碳转化模拟，将实验室研究转移到课堂上，目的是增强学生对催化的理解。在这个项目中，Weber小组表征了用于二氧化碳转化的分子催化剂的红外光谱，以及与涉及此类催化剂的催化循环相关的其他物种。研究中的催化剂是由电喷雾电离产生的带电分子(离子)。它们与二氧化碳、质子供体和溶剂的络合物在一系列温度控制的离子陷阱中制备，并在飞行时间质谱仪中分离。用来自可调谐红外光源的脉冲光照射目标分子。它们在光子吸收时发生碎片，并在第二质量分析步骤中检测碎片。研究中的络合物的振动光谱是通过监测碎片并调节红外波长来测量的。光谱提供了控制催化剂功能的结构和分子间力的信息。结合量子化学计算，这些光谱提供了对二氧化碳电化学转化为其他更有价值的分子过程中所起作用的化学作用的机械洞察。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The generation of a carbon-neutral, sustainable energy economy has been recognized by many as one of the most important new technologies to be developed in the near future. The conversion of carbon dioxide (CO2) into chemically useable fuels is a promising approach to attain this goal. Such processes are difficult to implement, and they need to be assisted by catalysts, i.e., by molecules or materials that lower the energy required to convert CO2 into fuel. In order to develop cost-effective catalysts, we need to understand how CO2 interacts with catalyst molecules. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor J. Mathias Weber of the University of Colorado at Boulder is using a state-of-the-art combination of mass spectrometric and laser spectroscopy techniques to study the interactions of CO2 with catalyst molecules. The catalysts of interest are molecules that contain metals such as cobalt (Co) and rhenium (Re) surrounded by structures containing other elements (for example carbon (C) , oxygen (O), nitrogen (N)). The surrounding structures influence how CO2 binds to the metal atom (key catalytic processes originate in the CO2-metal atom interaction). These metal catalyst "complexes" are ionic (they possess electric charge), which means they can be sorted and concentrated using a mass spectrometer. The desired ionic catalyst complexes are cooled to very low temperatures (as low as 5 degrees Kelvin (K), or minus 450 degrees Fahrenheit). At these low temperatures, CO2 molecules bind to the catalyst complexes. Once the CO2-metal complexes are formed, infrared laser light is used to measure the vibrations of these complexes. From the vibrations, the structures of the complexes can be inferred. This approach enables probing key steps in their reactions, allowing the assembly of detailed mechanisms of how the catalysts function. The broader impacts of this work include potential societal benefits towards the development of new sources of chemical fuels with low environmental impact, as well as the training of graduate student researchers in advanced experimental and computational techniques. Moreover, the Weber group is developing a web-based simulation of carbon dioxide conversion, transporting the laboratory research into the classroom, with the aim to enhance student understanding of catalysis. In this project, the Weber group characterizes the infrared spectra of molecular catalysts for the conversion of carbon dioxide, as well as other species relevant for the catalytic cycle involving such catalysts. The catalysts under study are charged molecules (ions), generated by electrospray ionization. Their complexes with carbon dioxide, proton donors, and solvents are prepared in a series of temperature-controlled ion traps, and isolated in a time-of-flight mass spectrometer. The target molecules are irradiated with pulsed light from a tunable infrared light source. They fragment upon photon absorption, and the fragments are detected in a second mass analysis step. The vibrational spectra of the complexes under study are measured by monitoring fragments while tuning the infrared wavelength. The spectra yield information on the structures and intermolecular forces governing the function of the catalysts. Together with quantum chemical calculations, the spectra provide mechanistic insight into the chemistry at play in electrochemical conversion of carbon dioxide into other, more valuable molecules.This 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.