Dry (CO2) reforming of methane (DRM) is a well-studied reaction that is of both scientific and industrial importance. This reaction produces syngas that can be used to produce a wide range of products, such as higher alkanes and oxygenates by means of Fischer-Tropsch synthesis. DRM is inevitably accompanied by deactivation due to carbon deposition. DRM is also a highly endothermic reaction and requires operating temperatures of 800-1000 °C to attain high equilibrium conversion of CH4 and CO2 to H2 and CO and to minimize the thermodynamic driving force for carbon deposition. The most widely used catalysts for DRM are based on Ni. However, many of these catalysts undergo severe deactivation due to carbon deposition. Noble metals have also been studied and are typically found to be much more resistant to carbon deposition than Ni catalysts, but are generally uneconomical. Noble metals can also be used to promote the Ni catalysts in order to increase their resistance to deactivation. In order to design catalysts that minimize deactivation, it is necessary to understand the elementary steps involved in the activation and conversion of CH4 and CO2. This review will cover DRM literature for catalysts based on Rh, Ru, Pt, and Pd metals. This includes the effect of these noble metals on the kinetics, mechanism and deactivation of these catalysts.

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Isomorphic substitution of Rh at varying levels on the B site of lanthanum zirconate pyrochlore (La 2 Zr 2 O 7 ; 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 H 2-TPR. XRD of the fresh, calcined catalysts showed the formation of the pyrochlore phase (La 2 Zr 2 O 7) in all three materials. In L5RhZ, the relatively high level of Rh substitution led to the formation of LaRhO 3 perovskite phase which was not observed in the L2RhZ and LZ pyrochlores. TPR results show that the L5RhZ consumed 1.57 mg H 2 /g cat , which is much greater than the 0.508 H 2 /g cat and 0.155 mg H 2 /g cat 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 CH 4 and CO 2 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.

Increases in worldwide methane production from biological and fossil sources have led to an increased level of interest in the dry reforming of methane (DRM) to produce syngas. Experimental efforts have shown that select pyrochlore materials, such as the Rh-substituted lanthanum zirconate pyrochlore (LRhZ), are catalytically active for DRM, exhibit long-term thermal stability, and resist deactivation. This work seeks to allow further catalyst improvements by elucidating surface reaction kinetics via steady-state isotopic transient kinetic analysis (SSITKA) of dry reforming on the LRhZ pyrochlore. Isotopically labeled CH4 and CO2 were used in multiple SSITKA experiments to elucidate the migration of carbon atoms to product species. Short surface residence times at 650 and 800 °C (<0.6 s) were observed for DRM intermediates involved in reversible reactions, and the participation of all surface metal atoms as active sites for DRM, not only Rh, is suggested. Isotopic responses and kinetic isotope effects are explained using reaction mechanism details derived from density functional theory studies of the surface reactions.

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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.

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