The characterization of BaF2/Y2O3 catalysts for the OCM reaction

摘要

The reactivities of Y2O3 and BaF2/Y2O3 catalysts have been investigated for the oxidative coupling of methane (OCM) reactions. With CH4:O-2:N-2 = 2.47:1:11.4 and a total flow rate of 50 mt min(-1), after 4 h of reaction time at 750 degrees C, the CH4 conversion and C-2 selectivity over Y2O3 were 29.9 and 26.2%, respectively, giving a C-2 yield of 7.8%. When 30 mol% of BaF2 was added, the CH4 conversion, C-2 selectivity, and C-2 yield were enhanced to 35.3, 55.4, and 19.5%, respectively. With a 95 mol% BaF2/Y2O3 catalyst, we could achieve a 22.4% C-2 yield with CH4 conversion and C-2 selectivity, respectively, equal to 36.1 and 62.1%. X-ray diffraction (XRD) investigation of the BaF2/Y2O3 catalysts revealed that the cubic Y2O3 lattice had expanded slightly while the cubic BaF2 phase contracted. Based on the results of O-2 absorption as well as temperature-programmed reduction (TPR) studies, we know that the BaF2/Y2O3 catalysts have higher ability in O-2 activation than the undoped Y2O3 catalyst. We suggest that the ionic exchange which occurred between the BaF2 and Y2O3 phases caused the generation of active defects. We used the EPR technique to monitor the generation of trapped electrons in the 95 mol% BaF2/Y2O3 catalyst. The results indicated that there were reducible oxygen ions which existed largely in the bulk. After H-2 reduction between 500 and 700 degrees C, a kind of trapped electron with EPR signal centered at 2.0871 was formed. H-2 reduction above 700 degrees C could result in the generation of another type of trapped electron with EPR signal centered at 2.0087. The 2.0871 signal showed doublet superhyperfine structures while the 2.0087 one was symmetrical. We interpret the former as being due to trapped electrons interacting with the Y3+ ions, while the latter due to trapped electrons shared among the orbitals of the surrounding cations. The 2.0871 trapped electrons spread throughout the sample, while the 2.0087 ones existed only on the surface. The involvement of surface-trapped electrons in activating O-2 was observed at 25 degrees C, while above 500 degrees C, trapped electrons in the bulk were also involved. According to the results obtained, we have reasons to believe that in the OCM reaction, the BaF2/Y2O3 catalysts were reduced by hydrogen dissociated from methane, and trapped electrons were generated. These trapped electrons could serve as active sites for O-2 activation. (C) 1998 Academic Press. [References: 27]