Highly selective and stable nickel catalysts supported on ceria promoted with Sm₂O₃, Pr₂O₃ and MgO for the CO₂ methanation reaction

The present work reports on the investigation of the catalytic performance for the methanation of CO₂ over Ni catalysts based on CeO₂, and for the first time, of Ni catalysts supported on binary CeO₂-based oxides, namely, Sm₂O₃-CeO₂, Pr₂O₃-CeO₂ and MgO-CeO₂. The supports were obtained using the microwave assisted sol-gel method under reflux, while the catalysts were prepared by the wet impregnation method. For the investigation of the morphological, textural, structural and other intrinsic properties of the catalytic materials a variety of characterization techniques were used, i.e., Raman spectroscopy, XRD, N₂ physisorption-desorption, CO₂-TPD, H₂-TPR, H₂-TPD, XPS and TEM. Carbon deposition and sintering were investigated using TEM. It was shown that the addition of Sm³⁺ or Pr³⁺, incorporated into the lattice of CeO₂, generated oxygen vacancies, but the Ni/Pr-Ce catalyst was found to possess more surface oxygen vacancies (e.g. Ce⁴⁺-Ov-Pr³⁺ entities). Moreover, modification of CeO₂ using Sm³⁺ or Pr³⁺ restricted the agglomeration of nickel active sites and led to the genesis of Lewis basic positions. These characteristics improved the hydrogenation reaction at lower temperature. On the other hand, the addition of Mg²⁺ resulted at strong metal support interactions reinforcing the resistance of the Ni/Mg-Ce catalyst against sintering. Furthermore, the addition of Sm³⁺, Pr³⁺ and Mg²⁺ cations increased the overall basicity and the moderate adsorption sites and led to the formation of smaller Ni nano particles; these physico-chemical properties enhanced the CO₂ methanation reaction. Finally, the activity experiments (WGHSV = 25,000 mL g⁻¹ h⁻¹, H₂/CO₂ = 4:1, T =350 °C) showed that at lower reaction temperature the Ni/Pr-Ce had the highest catalytic performance in terms of CO₂ conversion (54.5%) and CH₄ yield (54.5%) and selectivity (100%). The TOF values were found to follow the order Ni/Pr-Ce >> Ni/Mg-Ce > Ni/Sm-Ce > Ni/Ce.