Enhancing the sintering stability of NiCo/CeO2-Al2O3 catalyst in dry reforming of methane by shaping the Ostwald ripening diffusion path

Abstract

Dry reforming of methane (DRM) is attractive for producing value-added syngas by consuming the CH₄ and CO₂ greenhouse gases, but the catalysts are challenged by sintering and coking. A series of NiCo/CeO₂-Al₂O₃ catalysts with the nominal composition of (Ni_0.5Co_0.5)Al_xCe_10-xO_y (x = 0–10) were prepared by sol–gel method. This work demonstrates that the formation of (NiCo)Al_xO_y spinel (or spinel-like) phase enhanced the metal-support interaction (MSI) and plays a crucial role in the catalyst’s performance. Small (NiCo)Al_xO_y grains reside in the holes and along the grain boundaries of the CeO₂ matrix in the fresh catalysts. (NiCo)Al_xO_y becomes amorphous under DRM conditions and exhibits a high affinity to both NiCo NPs and the CeO₂ grains, keeping the MSI strong enough to direct the sintering of metal NPs to the Ostwald ripening pathway. We proposed that the sintering of NiCo NPs in the x ≥ 2 catalysts follows the Ostwald ripening pathway: (NiCo)Al_xO_y serves as the diffusion path for the metal atoms while CeO₂ serves as the spacer shaping the length and cross-section of the diffusion path. A proper Al₂O₃/CeO₂ ratio makes the inter-grain diffusion path long and narrow, which effectively hinders the sintering of NiCo NPs. The NiCo/CAO-2 catalyst with the stoichiometric Ni_0.5Co_0.5Al₂O₄ spinel composition exhibits the best DRM performance. The CH₄ and CO₂ conversions at 700 ℃ achieved 76 % and 84 %, respectively. The catalyst was stable during the 100-hour DRM test, and the carbon deposition rate was small. The present work provides a new strategy for developing anti-sintering catalysts for high-temperature reactions.