Rare earth metal doped nickel catalysts supported on exfoliated graphitic carbon nitride for highly selective CO and CO2 methanation

Abstract

Rare earth metals like Ce and La oxides have emerged as potential promoter candidates for CO and CO2 methanation due to their unique properties. Herein, we report the synthesis by a facile impregnation method of nickel catalysts supported on exfoliated graphitic carbon nitride with cerium (Ni-Ce/eg-C3N4) and lanthanum (Ni-La/eg-C3N4) as promoters. Compared to Ni/eg-C3N4, the promoted catalysts showed an increased number of mesopores, a higher degree of dispersion of Ni nanoparticles, moderate metal-support interaction, and more surface basic sites. In terms of catalytic performance, Ni-Ce/eg-C3N4 catalyst exhibited the best CO methanation (XCO = 72%, SCH4 = 89%) and CO2 methanation (XCO2 = 83%, SCH4 > 99%) activities at ca. 279 °C and 297 °C, respectively. Considering the very high thermodynamic stability of the CO2 molecule, the temperature to achieve maximum conversion of CO2 was only 18 °C higher than CO, demonstrating the significant effect of the Ce promoter in the adsorption and activation of CO2 molecules. This can be attributed to the generation of sufficient active sites of surface defects or oxygen vacancies originating from the reduction of the CeO2 and Ni-Ce-O solid solution. Additionally, the creation of abundant mesopores in eg-C3N4 support increased the capacity of surface basic sites to promote the adsorption of CO2 molecules. The enhancement of CO2 adsorption and activation sites by the Ce promoter and eg-C3N4 support could be the key factor for the high catalytic activity of CO2 methanation at low temperatures.