Citrate-mediated structure and enthalpy-entropy trade-off in Ni-CeO2 catalysts for low-temperature CO2 methanation

Abstract

Catalytic activity in heterogeneous reactions is often interpreted in terms of activation enthalpy, although reaction rates are governed by the activation free energy, which includes both enthalpic (Δ H ‡ ) and entropic (Δ S ‡ ) contributions. This simplification is particularly limiting in CO 2 methanation under non-stoichiometric conditions, where performance is highly sensitive to hydrogen availability. Here, we investigate how the Δ H ‡ -Δ S ‡ balance determines the apparent activation free energy governing catalytic performance in Ni-CeO 2 catalysts. A series of catalysts was synthesized by varying the citrate-to-metal ratio as a single preparation parameter, enabling modification of metal-support interactions, defect density, and interfacial structure. Kinetic analysis based on transition state theory reveals a trade-off between the apparent activation enthalpy and entropy across the catalyst series. The most active catalyst (NiCeO 2 –1.37Cit) exhibits the highest activation enthalpy (100 kJ·mol −1 ) but a more favorable activation entropy, resulting in a lower apparent activation free energy and enhanced activity at low-temperature. This behavior is maintained under varying H 2 /CO 2 feed ratios. Under lean conditions (H 2 /CO 2 = 1:1) and 275 °C, NiCeO 2 –1.37Cit achieves 95% CH 4 selectivity, corresponding to a methane yield of 7.5%, compared to 3.5 and 4.3% for the compared catalysts under identical conditions. Structural characterization correlates this performance with increased Ce 3+ concentration, enhanced reducibility, lattice distortion, and a more developed Ni-CeO 2 interface, consistent with a catalytic surface associated with a more favorable entropic contribution to activation. The enthalpy-entropy balance provides a framework to rationalize the relationship between catalyst structure, kinetic response, and catalytic performance, offering insight into catalyst design through control of the enthalpy-entropy balance.