Determination of the Active Sites over Rh Substituted Pyrochlores for Dry Reforming of Methane

Dry reforming of methane (DRM) to syngas (H₂ and CO) has been well-studied over a number of catalysts. The high temperatures needed to produce syngas inevitably result in carbon formation. Although pyrochlores are known to be thermally stable at conditions required for dry reforming, there is very little literature on this reaction for these catalysts. Catalytically active metals like Rh and Ru can be isomorphically substituted into the crystalline structure of the pryrochlores to produce stable catalyst. These materials also resist carbon formation due to their inherent oxygen conductivity at conditions required for DRM. Here, we report the use of Rh-substituted- lanthanum zirconate (La₂Zr₂O₇) pyrochlores for DRM. Specifically three catalysts (a) unsubstituted lanthanum zirconate (LZ) for baseline comparison, (b) 2% and (c) 5% (by wt) Rh-substituted L2RhZ and L5RhZ pyrochlores were characterized using CH₄ and H₂ temperature programmed reduction (TPR) and their activity was studied at different temperatures of 550 ºC, 575 ºC, and 600 ºC. H₂ TPR showed that the reducibility of the pyrochlore increases with an increase in the metal substitution. CH₄ TPR clearly shows that the lattice oxygen in pyrochlores is reactive and helps limit carbon formation. Activity results showed that the CH₄ and CO₂ conversion increase with an increase in active metal substitution into the pyrochlore lattice.