Compensation effect. A DFT study of the activation of N2O over M-CHA (M = Fe2+, Co2+, RuO2+, RuO+)

The compensation effect is investigated using DFT calculations of the activation of the N2O molecule over mononuclear cations (Fe2+, Co2+) and cationic oxo-particles (RuO2+, RuO+), identified as perspective active sites. Constrained MD simulations are used to calculate Helmholtz free energies of activation at 300 K, 420 K, and 700 K. Reaction rates calculated according to the transition state theory are used to construct Arrhenius plots (AP). Activation energies, derived from the AP, are 60.9 kJ/mol, 77.2 kJ/mol, 99.4 kJ/mol, and 105.1 kJ/mol for RuO+, Fe2+, Co2+, and RuO2+, respectively. A linear dependence between enthalpy of activation ΔH‡ and entropy of activation ΔS‡ is observed only when the reaction exhibits the isokinetic behavior. The change of entropy of activation is approximately three orders smaller than the change of enthalpy. For the activation of N2O, a rare case of the anticompensation is observed. The reaction rates computed for the temperature range between 300 K and 700 K increase in order: RuO+ > Fe2+ > Co2+ > RuO2+.

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► Ab initio MD simulation of N2O activation over Fe2+, Co2+, Ruo2+, Ruo+ in zeolite using DFT.
► Path-integrated free energies are used to calculate reaction rates.
► Arrhenius plot is constructed from first principles.
► Compensation effect and enthalpy–entropy dependence show the effect of anticompensation.
► Electronic reasons behind the anticompensation remain unclear.