Collaborative Research: The role of oxide overlayers on adsorbate migration and metal sintering in reactions of CO2

天然气（主要由甲烷组成，即，CH 4）提供了我国能源需求的主要部分。 虽然甲烷是一种相对“清洁”的化石燃料，但其燃烧产生的二氧化碳（CO2）构成温室气体（GHG）排放的主要成分。 甲烷也可以与蒸汽（H2O）反应，以在称为甲烷蒸汽重整的过程中产生一氧化碳（CO）和氢气（H2）。产物CO和H2气体进一步反应以产生广泛的燃料和化学品。 该项目研究了一种替代方法-甲烷“干”重整（DRM）-利用捕获的CO2而不是蒸汽产生CO和H2，从而减少温室气体总量。 甲烷重整，通过任何技术，是一个能源密集型的过程。 催化剂用于降低操作温度，提高工艺效率，并驱动反应得到所需产物。 干重整比蒸汽重整更具挑战性，因此需要进行旨在确定在高温反应条件下稳定的更具活性和选择性的催化剂的研究。 该项目通过结合理论，计算和实验方法来确定有效的DRM催化剂来满足这些需求。此外，该项目将调查DRM技术的经济性，并将教育和推广活动纳入高中和本科生接触化学工程领域-对燃料，化学品和环境工业非常重要。 DRM催化剂必须在高温下操作，这可能破坏精心设计的合成结构或促进次级反应（例如，逆水煤气变换反应（RWGS）和焦炭形成），导致较低价值的产品。 与一级和二级过程相关的一种机制是某些催化剂在循环的不同部分期间储存和释放氧的能力。其他催化剂可以以较高的活化能为代价避免这种以氧为中心的路线。这项工作开发了混合催化剂，使用可还原和不可还原的氧化物，以联合收割机在产生高度稳定和化学选择性的甲烷重整催化剂，可用于在工业相关条件下操作两者的最佳性能。同时甲烷重整和RWGS反应在二氧化铈催化剂上发生通过移动的氧物种。 不可还原的催化剂覆盖层具有限制氢从活性金属位点溢出的潜力，防止不希望的二次反应并稳定精心设计的催化剂结构，而不限制氧在甲烷重整中的作用。该项目将模拟（密度泛函理论）和实验工作相结合，开发高活性和结构稳定的催化剂，同时限制不需要的RWGS，从而降低H2：CO比例。 然而，制备化学品的后续反应需要比在标准干重整条件下可能的更高的H2与CO比率。 因此，优化的分层催化剂将在低浓度水存在的恶劣条件下进行测试（即，该奖项反映了NSF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。

Combustion of natural gas (chiefly comprised of methane, i.e., CH4) provides a major portion of our nation’s energy needs. Although methane is a relatively “clean” fossil fuel, its combustion produces carbon dioxide (CO2) which constitutes the major component of greenhouse gas (GHG) emissions. Methane can also be reacted with steam (H2O) to produce carbon monoxide (CO) and hydrogen (H2) in a process known as methane steam reforming. The product CO and H2 gases are further reacted to produce a wide range of fuels and chemicals. The project investigates an alternative approach – “dry” reforming of methane (DRM) - which utilizes captured CO2, rather than steam, to generate CO and H2, thus decreasing the overall GHG inventory. Methane reforming, via any technology, is an energy intensive process. Catalysts are utilized to reduce operating temperatures, improve process efficiency, and drive the reactions to desired products. Dry reforming is even more challenging than steam reforming, thus creating a need for research aimed at identifying more active and selective catalysts that are stable under high-temperature reaction conditions. The project addresses those needs by combining theoretical, computational, and experimental methods to identify effective DRM catalysts. In addition, the project will investigate economics of DRM technology, and incorporate educational and outreach activities exposing high-school and undergraduate students to the field of chemical engineering – so important to the fuels, chemicals, and environmental industries. DRM catalysts must operate at high temperatures, which can destroy carefully designed synthetic structures or promote secondary reactions (e.g., reverse water-gas shift reaction (RWGS) and coke formation) that result in lower value products. One mechanism associated with both the primary and secondary processes is the ability of some catalysts to store and release oxygen during different parts of the cycle. Other catalysts can avoid this oxygen-centric route at the expense of higher activation energies. This work develops hybrid catalysts, using both reducible and non-reducible oxides, to combine the best properties of both in generating highly stable and chemically selective methane reforming catalysts which can be used to operate at industrially relevant conditions. The simultaneous methane reforming and RWGS reactions over ceria catalysts occur through mobile oxygen species. Non-reducible catalyst overlayers have the potential to limit hydrogen spillover from the active metal sites, preventing the unwanted secondary reaction and stabilizing the carefully designed catalyst structure without limiting the role of oxygen in the methane reforming. Using a combination of simulation (density functional theory) and experimental work, the project will develop highly active and structurally stable catalysts while limiting the undesired RWGS, which decreases the H2:CO ratio. However, subsequent reactions to make chemicals require higher H2-to-CO ratios than are possible under standard dry reforming conditions. As such, the optimized hierarchical catalysts will be tested under harsh conditions in the presence of low concentrations of water (i.e., a steam/CO2 “bi-reforming” process) to further increase the H2-to-CO ratio.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.