(621dg) Optimization of Pyrochlore Catalysts for Dry Reforming of Methane: A Computationally Guided Experimental Work

The conversion of methane into syngas is of growing importance given recent increases in methane production world-wide. Furthermore, using CO₂ as the co-feed offers many environmental advantages; therefore, dry reforming of methane (DRM) has long been considered an attractive method for converting methane from geological or biological sources into syngas. In recent years, experimentalists have shown that Rh-substituted lanthanum zirconate pyrochlore (LRhZ) catalysts are active and stable at the high temperatures needed for the dry reforming of methane (DRM), so that this reaction has received renewed attention.     

To enable further improvements to these catalysts, the reaction mechanism for DRM on Rh-doped lanthanum zirconate (LRhZ) was attained using density functional theory (DFT). Following the identification of favored reaction sites for all elementary reactions, reaction and activation energies were calculated and used to discern the primary reaction pathway. Simulations show that inclusion of Rh decreases activation barriers, including the barrier for the rate limiting CHO dehydrogenation step, which makes the plane (111) catalytically active for DRM. Results also show that the limiting reaction step is on the CH₄ dehydrogenation path, which agrees with experimental observations. Therefore, this computational study has set the ground to optimize pyrochlore catalyst towards syngas production.

Computational analysis of the rate determining step and atomic carbon adsorption on the surface suggested Pd as an effective co-dopant to enhance H₂ to CO ratios in the syngas mixture. This bimettalic doped Rh-Pd lanthanum zirconate pyrochlore (Rh-Pd-LZ) was synthesized, characterized and tested. It exhibited greater H₂ to CO ratios along with high CO₂ and CH4 conversions. These catalyst performance data show how the rate limiting step, i.e. the CHO dehydrogenation was successfully targeted and its activation barrier was reduced. Hence, the present work successfully optimized a pyrochlore catalyst for DRM starting from ab initio understanding of the reactive system.