Nickel-Supported on La2Sn2O7 and La2Zr2O7 Pyrochlores for Methane Steam Reforming: Insight into the Difference between Tin and Zirconium in the B Site of the Compound

La2Sn2O7 and La2Zr2O7, two pyrochlore compounds with different B-site cations, were prepared and used as supports for Ni in methane steam reforming. Compared with Ni/gamma-Al2O3, both Ni/La2Zr2O7 and Ni/La2Sn2O7 show very stable reaction performance. Whereas Ni/La2Zr2O7 also displays reasonably high activity for the reaction, the activity of Ni/La2Sn2O7 is extremely low. It was found that severe coking occurred with Ni/g-Al2O3, but no coke formation was observed on the two pyrochlore catalysts. On the reduced and spent Ni/La2Sn2O7 catalyst, the Ni3Sn2 and Ni3Sn alloys were detected; these alloys suppressed coke formation but also decreased the activity of the catalyst. In comparison, a large amount of La2O2CO3 was formed on the used Ni/La2Zr2O7 catalyst; these species reacted with the carbon deposits formed on the Ni particles and continuously restored the Ni sites. Thus, coking was effectively suppressed and the initial high activity of the catalyst was maintained. Thus, Ni/La2Zr2O7 is a superior catalyst having the potential for industrial use.