Ni/Y_2B_2O_7 (B=Ti, Sn, Zr and Ce) catalysts for methane steam reforming: On the effects of B site replacement

摘要

In this study, a series of Y2B2O7 compounds with a fixed Yttrium cation A site but with different B Sn, Zr and Ce) sites have been synthesized and used to support Ni for methane reforming for hydrogen production. By replacing the B site with Ti, Sn, Zr and Ce cations in sequence, the rA/rB ratios of the resulted Y2B2O7 compounds become smaller. As a consequence, the crystalline structures of the compounds become less ordered with the transformation of the bulk phase from well-ordered pyrochlore (Y2Ti2O7) to less ordered pyrochlore (Y2Sn2O7) and subsequently to defective fluorite (Y2Zr2O7 and Y2Ce2O7). XPS results have revealed that on the surfaces of Ni/Y2Ti2O7 and Ni/Y2Ce2O7, higher O/(Y+B) atomic ratios can be achieved than on the other two catalysts, indicating the presence of more abundant oxygen species, which is beneficial to remove the carbon deposits. In comparison with Y2Zr2O7 and Y2Ce2O7, the supported Ni or Ni3Sn2 active sites have stronger interaction with Y2Ti2O7 and Y2Sn2O7 supports, which anchors the active sites tighter on the supports and suppresses its aggregation effectively, thus obtaining catalysts with larger active metallic surface areas and better thermal stability. As a result, the stability and coking resistance of the catalysts can be enhanced. For the reduced Ni/Y2Sn2O7, Ni3Sn2 alloy has formed, which improves the coking resistance of the catalyst but degrades its activity significantly. On Ni/Y2Ti2O7 catalyst, which possesses the largest amount of active surface oxygen species, the strongest Ni interaction with the support can also be obtained, therefore, it exhibits the highest activity, stability and strongest coking resistance among all of the catalysts. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.