Ni Supported On Pyrochlores For CH₄ Reforming For H₂ Production: On The Effects Of A And B Site Replacement

With the rapid development of global economy,non-renewable fossil fuels,such as coal and oil,are consuming faster and will be exhausted eventually.On the other hand,the use of fossil fuels in un-clean ways leads to increasingly environmental pollutions,resulting in deteriorated human living conditions.Methane,as the main component of naturnal and shale gas,is cleaner and with more abundant sources compared with coal and oil.Methane reforming is one of the main ways to realize high value-added methane transformation,among which methane steam reforming is one of the major means of industrial hydrogen production.Currently,about 50%industrially used hydrogen is produced by this method.Nevertheless,it is well-known that Ni-based catalysts suffer from severe deactivation due to the sintering of the Ni active sites and coking under high temperature reforming reaction condition.Therefore,with the purpose to modulate the structure of supports and control the Ni particle size based on the understanding of the methane reforming reaction mechanism,in this study,A₂B₂O₇ composite oxides with different A site or B site cations have been prepared and used as supports for Ni to prepare catalysts for the reaction.The main contents and results summarized as follows:Part one:Ln₂Zr₂O₇ composite oxides with Zr as B site cation and different A site cations(La³⁺,Pr³⁺,Sm³⁺and Y³⁺)have been prepared and used as supports for Ni to prepare catalysts for methane steam reforming for hydrogen production.It is revealed by XRD and Raman techniques that with the decreasing of the r _A3+/r _B4+ratio in the sequence of La³⁺,Pr³⁺,Sm³⁺and Y³⁺,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?leading to the improvement of oxygen mobility.In addition,as evidenced by H₂-TPR,the supported Ni active sites have stronger interaction with those supports having higher degree of disorder,which improves both of the Ni dispersion and thermal stability of the prepared Ni/Ln₂Zr₂O₇.Y₂Zr₂O₇support with a defective fluorite structure owns the most amount of mobile oxygen species.As a consequence,Ni/Y₂Zr₂O₇ exhibits the highest activity,stability and coke resistance among all of the catalysts.Part two:a series of Y₂B₂O₇ composite oxides with Y as A site cation and different B sites cations(B=Ti⁴⁺,Sn⁴⁺,Zr⁴⁺and Ce⁴⁺)were prepared 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 decreasing of the r _A3+/r _B4+ratio in the sequence of Ti⁴⁺,Sn⁴⁺,Zr⁴⁺and Ce⁴⁺,the structures of the compounds become less ordered with the transformation of the bulk phase from ordered pyrochlore?Y₂Ti₂O₇?to less ordered pyrochlore?Y₂Sn₂O₇?and subsequently to defective fluorite?Y₂Zr₂O₇ and Y₂Ce₂O₇?.XPS demonstrated that on the surfaces of Ni/Y₂Ti₂O₇ and Ni/Y₂Sn₂O₇,there are more mobile oxygen species than the surfaces of Ni/Y₂Zr₂O₇ and Ni/Y₂Ce₂O₇.In comparison with Ni/Y₂Zr₂O₇ and Ni/Y₂Ce₂O₇,as evidenced by H₂-TPR,the supported Ni or Ni₂Sn₃ active sites have apparently stronger interaction with Ni/Y₂Ti₂O₇ and Ni/Y₂Sn₂O₇ supports,which anchors the active sites tighter on the support surfaces and suppresses its aggregation effectively,thus obtaining catalysts with larger active metallic surface areas and better thermal stability.For the reduced Ni/Y₂Sn₂O₇,Ni₂Sn₃ alloy has formed,which also improves the coke resistance of the catalyst but degrades its activity significantly,which limits its industrial application prospect.As a consequence,Ni/Y₂Ti₂O₇ exhibits the highest activity,stability and coke resistance among all of the catalysts.