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

天然气的燃烧(主要由甲烷组成，即CH4)提供了我国能源需求的主要部分。尽管甲烷是一种相对“清洁”的化石燃料，但它的燃烧会产生二氧化碳(CO2)，而二氧化碳是温室气体(GHG)排放的主要成分。甲烷还可以与水蒸气(H2O)反应生成一氧化碳(CO)和氢气(H2)，这一过程被称为甲烷水蒸气重整。产物CO和H2气体进一步反应，产生各种燃料和化学品。该项目研究了另一种方法--甲烷“干法”重整(DRM)，该方法利用捕获的二氧化碳而不是水蒸气来产生CO和H2，从而减少温室气体总库存。无论采用何种技术，甲烷重整都是一个能源密集型过程。催化剂被用来降低操作温度，提高工艺效率，并推动反应得到所需的产品。干法重整比水蒸气重整更具挑战性，因此需要进行研究，以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。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技术的经济学，并纳入教育和推广活动，让高中生和本科生接触对燃料、化学品和环境行业如此重要的化学工程领域。DRM催化剂必须在高温下运行，这可能会破坏精心设计的合成结构或促进二次反应(例如，逆水煤气变换反应(RWGS)和结焦)，从而导致产品价值较低。与初级和次级过程相关的一个机制是，一些催化剂在循环的不同部分储存和释放氧气的能力。其他催化剂可以避免这种以氧为中心的路线，但代价是更高的活化能。这项工作开发了使用可还原和不可还原氧化物的混合催化剂，以结合两者的最佳性能，生成可在工业相关条件下操作的高度稳定和化学选择性的甲烷重整催化剂。在CeO_2催化剂上甲烷重整反应和RWGS反应是通过可移动的氧物种进行的。不可还原的催化剂覆盖层有可能限制活性金属位置的氢溢出，防止不必要的二次反应，稳定精心设计的催化剂结构，而不限制氧气在甲烷重整中的作用。利用模拟(密度泛函理论)和实验工作相结合的方法，该项目将开发高活性和结构稳定的催化剂，同时限制不需要的RWGS，这将降低H2：CO比。然而，后续的化学反应需要比标准干法重整条件下更高的H2/CO比率。因此，优化的分级催化剂将在低浓度水存在的恶劣条件下进行测试(即，蒸汽/二氧化碳“双重整”过程)，以进一步提高H2-CO比率。该奖项反映了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。

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.