CO2 Reforming of CH4 over Ru-Substituted Pyrochlore Catalysts: Effects of Temperature and Reactant Feed Ratio

Dry reforming of methane (DRM) was performed on a 1% Ru-substituted lanthanum strontium zirconate pyrochlore, La1.97Sr0.03Ru0.05Zr1.95O7 (LSRuZ), and results were compared to a commercially available 0.5% Ru/Al2O3 catalyst at different bed temperatures and reactant feed ratios. X-ray diffraction of the fresh LSRuZ confirmed the formation of the La2Zr2O7 pyrochlore face-centered crystal lattice using a modified Pechini synthesis procedure. Results from X-ray photoelectron spectroscopy (XPS) on the fresh calcined pyrochlore showed the presence of RuO2, which was also present along with both RuO3 and RuO4 in the 0.5% Ru/Al2O3 catalyst. Temperature-programmed reduction (TPR) showed that Ru was reducible in both catalysts, but the primary TPR peak for Ru reduction was 140 degrees C higher for the LSRuZ pyrochlore than that for 0.5% Ru/Al2O3, consistent with the incorporation of Ru within the pyrochlore. The effect of the CH4/CO2 feed ratio on the activity was studied by varying the feed ratio as 1:1, 2:1, and 1:2 at a constant temperature of 785 degrees C. At 635 degrees C, 0.5% Ru/Al2O3 showed higher apparent CO2 conversion (X-CO2) and a lower H-2/CO ratio in the product gas than the LSRuZ pyrochlore, suggesting that the extent of the reverse water gas shift (RWGS) reaction was greater on 0.5% Ru/Al2O3. At 835 degrees C, equilibrium conversions of CH4 and CO2 were reached on both catalysts and remained constant with time over 7 h. There were no significant differences in X-CO2 and X-CH4 between the two catalysts with time on stream at 735 and 835 degrees C. The effects of the CH4/CO2 inlet feed ratio on products was also studied at a constant space velocity for both catalysts. Results showed that, for CH4/CO2 = 1:2, X-CO2 for 0.5% Ru/Al2O3 was 70% and a H-2/CO ratio of about 0.60 compared to 62% and 0.63 for the pyrochlore. The higher X-CO2 and lower H-2/CO ratio for 0.5% Ru/Al2O3 suggest that the RWGS reaction on this catalyst is kinetically faster than on LSRuZ. For CH4/CO2 = 2:1, X-CH4 of 0.5% Ru/Al2O3 was 62% and that for pyrochlore was 58%. This small but statistically significant difference indicates slightly higher CH4 decomposition over 0.5% Ru/Al2O3 than pyrochlore at this feed ratio. Temperature-programmed oxidation (TPO) of the post-run catalysts showed that the LSRuZ catalyst formed less oxidizable carbon per gram of reducible Ru (g(carbon)/g(Ru)) compared to 0.5% Ru/Al2O3, likely because of the greater oxygen ion mobility in the pyrochlore.