Constructing La 2 B 2 O 7 (B = Ti, Zr, Ce) Compounds with Three Typical Crystalline Phases for the Oxidative Coupling of Methane: The Effect of Phase Structures, Superoxide Anions, and Alkalinity on the Reactivity

作者：Junwei Xu, Yan Zhang, Xianglan Xu, Xiuzhong Fang, Rong Xi, Yameng Liu, Renyang Zheng, Xiang Wang · 发表于：ACS Catalysis · 年份：2019 · DOI：10.1021/acscatal.9b00022 · 被引用次数：224 · 研究领域：Nuclear materials and radiation effects、Catalysis and Oxidation Reactions、Advanced Condensed Matter Physics

To probe the phase structure–reactivity relationship of A 2 B 2 O 7 catalysts for the oxidative coupling of methane (OCM), three model La 2 B 2 O 7 compounds with Ti 4+, Zr 4+, or Ce 4+ at the B-site have been purposely designed. By decreasing the r A / r B ratios in the order of La 2 Ti 2 O 7 > La 2 Zr 2 O 7 > La 2 Ce 2 O 7, typical monoclinic layered perovskite, cubic ordered pyrochlore, and disordered defective cubic fluorite phase are formed, respectively. The reaction performance of the catalysts based on CH 4 conversion and C 2 product yield follow the order of La 2 Ce 2 O 7 > La 2 Zr 2 O 7 > La 2 Ti 2 O 7 . It has been discovered that superoxide O 2 – is the active oxygen species detected on all the catalysts and is responsible for the OCM reaction, whose amount follows also the sequence of La 2 Ce 2 O 7 > La 2 Zr 2 O 7 > La 2 Ti 2 O 7 . Moreover, the surface alkalinity related to the superoxide anions observes the same order. This testifies that the amount of surface superoxide O 2 – determines the OCM reaction performance over the La 2 B 2 O 7 compounds. On the basis of the characterization results, the formation of active O 2 – species could follow two pathways. For La 2 Zr 2 O 7 and La 2 Ce 2 O 7 possessing intrinsic 8a oxygen vacancies, O 2 – anions are formed by activating the oxygen species entering into the vacancies in the bulk and then migrating to the catalyst surface. For La 2 Ti 2 O 7 possessing no oxygen vacancies, they are formed directly by transforming...