Photo- and Thermocatalytic CO₂ Methanation: A Comparison of Ni/Al₂O₃ and Ni-Ce Hydrotalcite-Derived Materials under UV and Visible Light

Catalysts derived from Ni/Al/Mg/Ce hydrotalcite were prepared via a co-precipitation method, varying the Ce/Al atomic ratio. All of the catalytic systems thus prepared were tested for CO₂ methanation under dark and photocatalytic conditions (visible and ultraviolet) under continuous flow with the light intensity set to 2.4 W cm^-2. The substitution of Al by Ce formed a solid solution, generating oxygen vacancies and Ce³⁺/Ce⁴⁺ ions that helped shift the dissociation of CO₂ towards the production of CH₄, thus enhancing the activity of methanation, especially at lower temperatures (<523 K) and with visible light at temperatures where other catalysts were inactive. Additionally, for comparison purposes, Ni/Al₂O₃-based catalysts prepared via wetness impregnation were synthesized with different Ni loadings. Analytical techniques were used for the characterization of the systems. The best results in terms of activity were as follows: Hydrotalcite with Ce promoter > Hydrotalcite without Ce promoter > 25Ni/Al₂O₃ > 13Ni/Al₂O₃. Hydrotalcite, with a Ce/Al atomic ratio of 0.22 and a Ni content of 23 wt%, produced 7.74 mmol CH₄ min^-1·g_cat at 473 K under visible light. Moreover, this catalyst exhibited stable photocatalytic activity during a 24 h reaction time with a CO₂ conversion rate of 65% and CH₄ selectivity of >98% at 523 K. This photocatalytic Sabatier enhancement achieved activity at lower temperatures than those reported in previous publications.