Abstract
Isomorphic substitution of Rh at varying levels on the B site of lanthanum zirconate pyrochlore (La2Zr2O7; designated LZ) resulted in the formation of thermally stable catalysts suitable for fuel reforming reactions operating at 900 °C. Three specific catalysts are reported here: (a) unsubstituted lanthanum zirconate (LZ), (b) LZ with 2 wt% substituted Rh (L2RhZ), and (c) LZ with 5 wt% substituted Rh (L5RhZ). These catalysts were characterized by XRD, XPS, and H2-TPR. XRD of the fresh, calcined catalysts showed the formation of the pyrochlore phase (La2Zr2O7) in all three materials. In L5RhZ, the relatively high level of Rh substitution led to the formation of LaRhO3 perovskite phase which was not observed in the L2RhZ and LZ pyrochlores. TPR results show that the L5RhZ consumed 1.57 mg H2/gcat, which is much greater than the 0.508 H2/gcat and 0.155 mg H2/gcat for L2RhZ and LZ, respectively, suggesting that the reducibility of the pyrochlore structure increases with increasing Rh-substitution. DRM was studied on these three catalysts at three different temperatures of 550, 575, and 600 °C. The results showed that CH4 and CO2 conversion was significantly greater for L5RhZ compared to L2RhZ and no activity was observed for LZ, suggesting that the surface Rh sites are required for the DRM reaction. Temperature programmed surface reaction showed that L5RhZ had light-off temperature 80 °C lower than L2RhZ. The spent catalysts after runs at each temperature were characterized by temperature programmed oxidation (TPO) followed by temperature programmed reduction and XRD. The TPO results showed that the amount of carbon formed over L5RhZ is almost half of that formed on L2RhZ.
Highlights
Pyrochlores are a class of ternary metal oxides based on the fluorite structure with a cubic unit cell with a general formula of A2B2O7
Haynes et al [19] used L2RhZ pyrochlores for the partial oxidation of n-tetradecane, and their X-ray diffraction (XRD) pattern for pyrochlores match those in Fig. 1 for lanthanum zirconate (LZ) and L2RhZ in this study
The similar peak observed at 32° in the XRD pattern for L5RhZ pyrochlore (Fig. 1) suggests that 5 wt% Rh substitution resulted in the formation of a separate LaRhO3 perovskite phase
Summary
Pyrochlores are a class of ternary metal oxides based on the fluorite structure with a cubic unit cell with a general formula of A2B2O7. An important property of these materials is that catalytically active noble metals can be substituted isomorphically on the B site to form a crystalline catalyst. These materials consist of vacancies at the A and O sites, which facilitate oxygen ion migration within the structure [1]. For the pyrochlore structure to be stable it is necessary that the ionic radius ratio of A and B site cations be between 1.46 and 1.78 [1]. The ratio of the ionic radii for La2Zr2O7 is 1.61 [3]. If the ratio of the ionic radii is greater than 1.78, a perovskite phase can be formed.
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