CO₂Reforming of CH₄over 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, La 1.97 Sr 0.03 Ru 0.05 Zr 1.95 O 7 (LSRuZ), and results were compared to a commercially available 0.5% Ru/Al 2 O 3 catalyst at different bed temperatures and reactant feed ratios. X-ray diffraction of the fresh LSRuZ confirmed the formation of the La 2 Zr 2 O 7 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 RuO 2, which was also present along with both RuO 3 and RuO 4 in the 0.5% Ru/Al 2 O 3 catalyst. Temperature-programmed reduction (TPR) showed that Ru was reducible in both catalysts, but the primary TPR peak for Ru reduction was 140 °C higher for the LSRuZ pyrochlore than that for 0.5% Ru/Al 2 O 3, consistent with the incorporation of Ru within the pyrochlore. The effect of the CH 4 /CO 2 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 °C. At 635 °C, 0.5% Ru/Al 2 O 3 showed higher apparent CO 2 conversion ( X CO 2 ) 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/Al 2 O 3 . At 835 °C, equilibrium conversions of CH 4 and CO 2 were reached on both catalysts and remained constant with time over 7 h. There were no significant differences in X CO 2 and X CH 4 between the two catalysts with time on stream at 735 and 835 °C. The effects of the CH 4 /CO 2 inlet feed ratio on products was also studied at a constant space velocity for both catalysts. Results showed that, for CH 4 /CO 2 = 1:2, X CO 2 for 0.5% Ru/Al 2 O 3 was 70% and a H 2 /CO ratio of about 0.60 compared to 62% and 0.63 for the pyrochlore. The higher X CO 2 and lower H 2 /CO ratio for 0.5% Ru/Al 2 O 3 suggest that the RWGS reaction on this catalyst is kinetically faster than on LSRuZ. For CH 4 /CO 2 = 2:1, X CH 4 of 0.5% Ru/Al 2 O 3 was 62% and that for pyrochlore was 58%. This small but statistically significant difference indicates slightly higher CH 4 decomposition over 0.5% Ru/Al 2 O 3 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/Al 2 O 3, likely because of the greater oxygen ion mobility in the pyrochlore.