EAGER: Low-temperature Coupling of Methane Surrogates over Single Atom Catalysts: Elucidation of Elementary Reactions for C-C Bond Formation

甲烷是一种强效温室气体，也是天然气的主要成分。目前，大多数天然气燃烧产生的二氧化碳（CO2）也是一种温室气体。该项目研究了一种利用甲烷的替代方法，即甲烷的非氧化偶联反应（NOCM）。NOCM通过将甲烷主要转化为乙烯和芳香族化合物（广泛的化学产品的重要前体）来避免二氧化碳的形成。虽然该反应并不新鲜，但由于对复杂反应化学的不完全了解，限制了其潜在的应用，这反过来又使设计有效的NOCM催化剂变得复杂。为了克服这些限制，这个早期概念探索性研究资助（EAGER）项目研究了NOCM过程中发生的基本反应，这种反应是一种新型催化剂，称为单原子催化剂（SACs），使用烷基卤化物甲烷替代分子。NOCM从化石来源和生物甲烷资源中生产烯烃和芳烃的潜力代表了完全碳中和方法制造化学品和燃料的权宜之计。NOCM是一个高度吸热的反应，需要使用高温，这导致气相化学对观察到的表面催化反应的产物分布有重大影响。该项目将研究甲烷的烷基卤化物分子替代物的反应。替代物允许低温反应性，从而消除了与气相化学贡献相关的并发症。该项目考察了SAC的结构和组成对形成碳-碳键以产生不同大小的烯烃和芳烃的能力的影响。具体来说，将在中等温度（200至400摄氏度）下研究SACs上C-C键形成化学的三个方面，其中表面反应主要是：(i) -CH3（ads）和=CH2（ads）表面片段之间的偶联；（ii） -CH3（ads）表面片段与C2物质（-C2H5（ads）或C2H4(g)， NOCM的产物）之间的偶联；（iii）通过环三聚化化学在表面催化形成芳香。三个实验任务将集中在定量的反应速率，选择性和机制细节的碳-碳耦合使用同位素标记的底物。第四个目标将集中在基于密度泛函理论的C-C键形成SAC的发现，以及有前途的SAC候选物的微动力学模型的发展。催化剂将在反应前后通过互补技术进行表征，以评估催化剂的单原子性质和NOCM化学对催化剂结构的影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Methane is a potent greenhouse gas and the chief component of natural gas. Presently, most natural gas is combusted resulting in emissions of carbon dioxide (CO2) – also a greenhouse gas. The project investigates an alternative approach to methane utilization employing a reaction known as non-oxidative coupling of methane (NOCM). NOCM avoids the formation of CO2 by converting the methane primarily to ethylene and aromatic compounds – important precursors to a wide range of chemical products. While the reaction is not new, its potential application has been limited by incomplete understanding of the complex reaction chemistry which, in turn, complicates the design effective NOCM catalysts. To overcome those limitations, this EArly-concept Grant for Exploratory Research (EAGER) project examines the elementary reactions that occur during NOCM over a novel class of catalysts known as single-atom catalysts (SACs) using alkyl halide methane surrogate molecules. The potential of NOCM to produce alkenes and aromatics from fossil-derived and biological methane resources represents a stop-gap solution to fully carbon-neutral approaches to the manufacture of chemicals and fuels. NOCM is a highly endothermic reaction, necessitating the use of high temperatures, which leads to a significant impact of gas-phase chemistry on the observed product distribution of the surface-catalyzed reaction. The project will investigate reactions of alkyl halide molecular surrogates of methane. The surrogates allow for low temperature reactivity, thus eliminating complications associated with gas-phase chemistry contributions. The project examines the impact of SAC structure and composition on the ability to form carbon-carbon bonds to produce alkenes of varying size and aromatics. Specifically, three aspects of C-C bond formation chemistry on SACs will be investigated at moderate temperatures (200 to 400 degrees Celsius) where surface reactions prevail: (i) coupling between -CH3(ads) and =CH2(ads) surface fragments; (ii) coupling between -CH3(ads) surface fragments and C2 species (either -C2H5(ads) or C2H4(g), a product of NOCM) and (iii) surface-catalyzed aromatic formation through cyclotrimerization chemistry. The three experimental tasks will focus on quantifying reaction rates, selectivity, and mechanistic details of C-C coupling using isotopically labeled substrates. A fourth aim will focus on density functional theory-based discovery of SACs for C-C bond formation, and the development of microkinetic models for promising SAC candidates. The catalysts will be characterized before and after reaction by complementary techniques to assess the single atom nature of the catalysts and the impact of NOCM chemistry on the catalyst structure.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.