A<sup>+1</sup>Nb<sup>5+</sup>O<sub>3</sub> (A = Li, Na, K) Perovskites with Different Fine Structures for Oxidative Coupling of Methane: Tracing the Crystalline Phase Effect on the Surface Active Sites
Junwei Xu, Ying Gong, Zhixuan Zhang, Qiyao Xiao, Xianglan Xu, Xiuzhong Fang, Xiang Wang
Abstract
Targeting to unravel the catalytic sites of some specific perovskites for OCM reaction, and elucidate the influences of crystal phase changes on the active sites, three A+1Nb5+O3 (A = Li, Na, K) complex oxides were synthesized in the pure phase. The reaction performance is ranked as NaNbO3 > KNbO3 > LiNbO3 for the catalysts. It is revealed that LiNbO3 possesses a hexagonal ilmenite-type phase, and NaNbO3 and KNbO3 possess orthorhombic and tetragonal perovskite phases, respectively. Due to the fine structure difference, NaNbO3 and KNbO3 have higher oxygen mobility than LiNbO3, as testified by the electric conductivity measurements. Moreover, the Nb–O bond strength of NaNbO3 and KNbO3 is much weaker than that of LiNbO3, which promotes also the oxygen mobility and phase transformation of the two samples, especially in the high temperature OCM process. As a result, more oxygen vacancies/defects are generated on NaNbO3 and KNbO3, which facilitates the efficient formation of OCM active and selective surface O2– species and basic sites. Subsequently, the OCM performance of NaNbO3 and KNbO3 can be evidently improved. Combining the above advantages, NaNbO3 exhibits the optimal OCM performance in all the samples.