Ln2Zr2O7 compounds (Ln = La, Pr, Sm, Y) with varied rare earth A sites for low temperature oxidative coupling of methane

Abstract With the objective to develop catalysts having application potential for oxidative coupling of methane (OCM) at relatively lower temperature. A series of Ln 2 Zr 2 O 7 compounds with varied rare earth A sites have been prepared by a co-precipitation method. XRD and Raman have proved that pure Ln 2 Zr 2 O 7 compounds have been successfully prepared for all the catalysts. By decreasing the r A /r B ratio, their crystalline structure transform from an ordered pyrochlore (La 2 Zr 2 O 7 ) to a less ordered pyrochlore (Pr 2 Zr 2 O 7 and Sm 2 Zr 2 O 7 ) and eventually to a defective cubic fluorite phase (Y 2 Zr 2 O 7 ). H 2 -TPR, O 2 -TPD and XPS have testified that the amount of surface active O 2 − species follows the order of La 2 Zr 2 O 7 > Pr 2 Zr 2 O 7 > Sm 2 Zr 2 O 7 > Y 2 Zr 2 O 7 , which is well consistent with the reaction performance, indicating that the abundance of surface O 2 − sites is a critical factor influencing the reaction performance. CO 2 -TPD has demonstrated that a better catalyst generally possesses a larger amount of surface moderate alkaline sites, which is another factor to affect the reaction performance. It is concluded that the concerted interaction between the two types of surface active sites controls the reaction performance of the Ln 2 Zr 2 O 7 catalysts. In comparison with the state-of-the art Mn/Na 2 WO 4 /SiO 2 , La 2 Zr 2 O 7 , the best catalyst, exhibits much improved reaction performance below 750 °C