Ni/Y2B2O7 (B=Ti, Sn, Zr and Ce) catalysts for methane steam reforming: On the effects of B site replacement

Abstract In this study, a series of Y 2 B 2 O 7 compounds with a fixed Yttrium cation A site but with different B (B Ti, 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 r A /r B ratios of the resulted Y 2 B 2 O 7 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 (Y 2 Ti 2 O 7 ) to less ordered pyrochlore (Y 2 Sn 2 O 7 ) and subsequently to defective fluorite (Y 2 Zr 2 O 7 and Y 2 Ce 2 O 7 ). XPS results have revealed that on the surfaces of Ni/Y 2 Ti 2 O 7 and Ni/Y 2 Ce 2 O 7 , 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 Y 2 Zr 2 O 7 and Y 2 Ce 2 O 7 , the supported Ni or Ni 3 Sn 2 active sites have stronger interaction with Y 2 Ti 2 O 7 and Y 2 Sn 2 O 7 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/Y 2 Sn 2 O 7 , Ni 3 Sn 2 alloy has formed, which improves the coking resistance of the catalyst but degrades its activity significantly. On Ni/Y 2 Ti 2 O 7 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.