Ni/Ln2Zr2O7 (Ln = La, Pr, Sm and Y) catalysts for methane steam reforming: the effects of A site replacement
A series of Ln2Zr2O7 supports (Ln = La, Pr, Sm and Y) with different “A” sites were prepared by a glycine–nitrate combustion method and used to support Ni to prepare catalysts for methane steam reforming for hydrogen production. It is revealed by XRD and Raman techniques that with the decrease of the rA/rB ratio in the sequence La, Pr, Sm and Y, the structures of the compounds become less ordered with the transformation of the bulk phase from ordered pyrochlore (La2Zr2O7) to less ordered pyrochlore (Pr2Zr2O7 and Sm2Zr2O7) and subsequently to defective fluorite (Y2Zr2O7). XPS demonstrated that the oxygen vacancies and mobility of the compounds also improve with the sequence. As supports for Ni, those possessing more mobile oxygen species display evidently enhanced coke resistance. In addition, as evidenced by H2-TPR, the supported Ni active sites have a stronger interaction with those supports having a higher degree of disorder, which improves both the Ni dispersion and the thermal stability of the prepared Ni/Ln2Zr2O7. Y2Zr2O7 support with a defective fluorite structure has the highest amount of mobile oxygen species. Therefore, the Ni active species has a stronger interaction with it than with the other three supports, which results in the smallest Ni grains with the highest metallic active surface area. As a consequence, Ni/Y2Zr2O7 exhibits the highest activity, stability and coke resistance among all of the catalysts. It is concluded that A site replacement by rare earth cations with different radii influences the structures of Ln2Zr2O7 significantly, which ultimately affects the reaction performance of the prepared Ni/Ln2Zr2O7 catalysts for methane steam reforming.