Comparative Study on the Effect of La₂O₃, CeO₂, Nd₂O₃, and Sm₂O₃ on Ni-SiO₂ for Dry Reforming of Methane

Four lanthanide-oxide-modified Ni-SiO₂ catalysts were prepared via the β-cyclodextrin-assisted impregnation method and were comparatively evaluated for dry reforming of methane (DRM) under the severe conditions of 750 °C, CH₄/CO₂ molar ratio of 1, and GHSV of 84 000 mL g^-1 h^-1. The addition of lanthanide oxide improves the Ni dispersion, the promotional effect of which is related to the type of lanthanide oxide used. The lanthanide oxide enhances the anti-sintering capacity of the modified catalysts, attributable to the confinement effect of the formed Ni-lanthanide oxide interface, which is dependent on the lanthanide oxide dispersion. All of the modified catalysts display a higher CH₄ activity than that of Ni-SiO₂ (145.1-248.7 mmol min^-1 g^-1 of Ni vs 61.6 mmol min^-1 g^-1 of Ni). However, Ni-La-SiO₂ and Ni-Ce-SiO₂ encounter rapid deactivation with k_d values of 0.095 and 0.051 h^-1 compared to Ni-SiO₂ (k_d = 0.058 h^-1), which is assigned to extreme loss of active Ni sites caused by serious sintering and/or an extensive decrease in the surface area. The catalytic stability of Ni-Nd-SiO₂ (k_d = 0.025 h^-1) and Ni-Sm-SiO₂ (k_d = 0.004 h^-1) is significantly improved compared to Ni-La-SiO₂, Ni-Ce-SiO₂, and Ni-SiO₂, and the best stability with a TOS of 70 h is achieved over Ni-Sm-SiO₂, which can be well-explained as the enhanced anti-sintering and anti-coking capacities as well as the stable textural property. The consecutive CH₄/CO₂-TPSR results vigorously prove that the addition of Sm₂O₃ simultaneously enhances the CH₄ decomposition and CO₂ dissociation while reducing the temperature difference of the two reactions, which results in a better balance between carbon generation and total gasification processes, and hence, the high activity and superior anti-coking capacity are achieved over Ni-Sm-SiO₂. The above understanding provides an important guidance for modulating the CH₄ and CO₂ activation to achieve the balance between carbon generation and gasification processes.