Comparative studies on promotional effect of Pr6O11, Nd2O3 and Sm2O3 on Ni–SiO2 for pressurized carbon dioxide reforming of methane

The 7 wt% rare earth metal oxide promoted Ni–SiO2 catalysts of Ni–7Pr6O11–SiO2, Ni–7Nd2O3–SiO2, and Ni–7Sm2O3–SiO2 were prepared by the complex-decomposition method, and were comparatively evaluated for pressurized carbon dioxide reforming of methane (CRM) under severe conditions of 750 °C, 1.0 MPa, CH4/CO2 = 1, and gas hourly space velocity of 53200 mL/(g·h). The addition of rare earth metal oxide does not affect the Ni dispersion, and all of the catalysts show similarly high Ni dispersion of 16.0% ± 0.2%. As a result, all of the catalysts are highly active for pressurized CRM, the initial CH4 conversions of which approach the thermodynamic equilibrium (47.0% ± 0.2%). In contrast, a clearly favorable effect of the added rare earth metal oxide on the stability of Ni–SiO2 was revealed from the CRM results for a time-on-stream of 50 h, and the highest stability without an observable decrease in the conversions of CH4 and CO2 was obtained over Ni–7Sm2O3–SiO2. Based on the characterization results of thermogravimetric differential scanning calorimetry (TG-DSC) and transmission electron microscopy (TEM), the improved stability of Ni–7Pr6O11–SiO2, Ni–7Nd2O3–SiO2, and Ni–7Sm2O3–SiO2 for pressurized CRM was manifested mainly as the suppressed formation of carbon nanotubes over the catalyst surface, the extent of which is dependent on the specific rare earth metal oxide. Moreover, the consecutive temperature programmed surface reaction of CH4, CO2, and O2 over Ni–7Sm2O3–SiO2 vigorously reveals that the addition of Sm2O3 into Ni–SiO2 inhibits the CH4 decomposition but enhances the oxidization of the carbon species by CO2, leading to the well-balanced rates for forming and removing the coke over Ni–7Sm2O3–SiO2. These findings are not only beneficial to deeply understanding the promotional effect of rare earth metal oxides on Ni-based catalysts for CRM, but also important for extending the application of the less studied rare earth metal oxides as promoters for the metal-supported catalysts.