L. Azancot, P. Tarifa, F. Cazaña, E. Romeo, A. Monzón
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.