氨分解Fe催化剂氮化机理及助剂设计的密度泛函理论与实验研究

Owing to the inexpensive and earth-abundant nature, Fe based catalysts possess vast potential for commercial application of ammonia decomposition. However, the lower catalytic activity of Fe catalyst inhibits its wide application. Consequently, investigating the nitridation mechanism, active phase, and active sites will be beneficial to rational design of Fe based catalysts for ammonia decomposition. In this proposal, by combining density functional theory and experimental study, the nitridation mechanism, active phase, and active sites of Fe based catalysts will be proposed. The structure-activity relationship of Fe-N binary catalysts is built. On this premise, the influences of the promoters (i.e., alkali metal, alkaline earth metal and rare earth metal) on the reaction mechanism as well as the nitridation process over Fe catalysts will be investigated. The activation energy of key step and d band center of Fe from promoter/Fe catalysts will be correlated. By combining the results of characterization and catalytic testing, an optimized promoter is screened. The effect of amount of the optimized promoter on the microstructure and reaction mechanism over Fe catalysts will be further studied, to obtain an optimal doping amount. This research is valuable not only for rational design of Fe-based catalysts with excellent performances, but also for in-depth understanding of the role of promoters in structural and morphological manipulation of metal catalysts.

Fe储量高、成本低，是潜在的工业氨分解催化剂，然而其低活性限制了广泛应用。因此，研究Fe基催化剂的氮化过程、活性相、活性位等有助于高效Fe基催化剂的理性设计。本项目将采用密度泛函理论和实验相结合的方法，在揭示氨分解Fe基催化剂氮化机制、明确其活性相和活性位、建立Fe-N二元体系结构与其活性之间构-效关系的前提下，考察助剂（碱金属、碱土金属、稀土金属）对Fe催化剂上氨分解反应历程及其氮化过程的影响机制，关联关键反应步骤活化能与Fe基催化剂中Fe的d带中心之间的关系，结合表征结果及氨分解催化性能总结同类助剂共性规律，筛选较优助剂。考察助剂掺杂量对Fe催化剂微观结构、氨分解反应历程的影响，结合实验研究建立Fe基催化剂结构与氨分解活性间的构-效关系。本项目结果不仅可用于指导高效Fe基催化剂的理性设计，还有助于深入了解助剂对金属催化剂结构和形貌等的调控机制。

助剂效应是过渡金属纳米催化领域的热点课题之一。目前，国内外对纳米催化剂助剂的选择主要采用“组分调控法”，通过形成合金或复合物等调变催化剂的“总包”价电子结构，很少采用“表面结构调控法”通过改变催化剂的表面结构进而获得高效催化剂。本项目提出“理性调变助剂来达到设计高性能氨分解Fe催化剂”的思路，并从实验和理论计算上证实了其可行性，同时探究了引入助剂方法的普适性，发现采用CCVD法可有效将C助剂引入到特定形貌的Fe和Ni基催化剂中，系统研究了不同C助剂对Fe基催化剂的微观结构、形貌、电子结构、氮化的影响机制，揭示了间隙C是Fe基催化剂高抗氮化性能的主要原因；比较研究了C助剂掺杂的Fe和Ni基催化剂与传统方法制备的Fe和Ni催化剂的结构、氨分解活性、反应机理等，发现C助剂掺杂的Fe催化剂活性和高温稳定性明显高于传统的催化剂，同时两种助剂掺杂的催化剂的活性趋势相反，结合DFT计算阐明了C助剂对Fe和Ni不同晶面原子电子结构调控机制及活性差异的根源；进一步研究了氮化有利Co-Mo双金属催化剂的制备及其氨分解活性和高温稳定性，提出Co3Mo3N是Co-Mo双金属催化剂氨分解反应的活性相。最终，建立了明确的C助剂掺杂Fe基催化剂结构和氨分解性能之间的构-效关系，阐明了氮化在氨分解反应中的特定影响机制（不同金属的氮化对活性影响差别巨大）。