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 kd values of 0.095 and 0.051 h-1 compared to Ni-SiO2 (kd = 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 (kd = 0.025 h-1) and Ni-Sm-SiO2 (kd = 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.