First-principles Assessment of the Role of Water in the Reduction Half Cycle of Low-Temperature NH3-SCR over Cu-CHA

Dispersion corrected density functional theory calculations show that the presence of H2O in the Reduction Half-Cycle (RHC) of NH3-SCR affects the free energy of the kinetically-relevant transition state (TS) leading to a reduction in the rate and activation energy with respect to dry conditions. In particular, H2O enthalpically stabilizes the kinetically-relevant TS by 20 kJ mol(-1) with respect to the dry counterpart. Such enthalpic stabilization vanishes when van der Waals (vdW) interactions are excluded from the calculations, thus showing the preeminent role of non-specific dispersion forces in the reduction of the activation enthalpy. At the same time, the enthalpic stabilization is more than compensated by the additional entropy losses of the TS brought forth by the presence of H2O in the CHA cage. Calculated enthalpy and entropy changes with respect to the dry case agree quantitatively with the experimental measurements and reflect the modified reacting environment in the presence of H2O. As a result, this study provides theoretical underpinnings on the mechanistic role of H2O in the RHC and, on a more general basis, highlights the importance of the molecular scale description of the reaction environment in voids of molecular dimensions.

Automated summary

Dispersion corrected density functional theory calculations reveal that the presence of H2O in the Reduction Half-Cycle (RHC) of NH3-SCR decreases the rate and activation energy by enthalpic stabilization of the kinetically-relevant transition state (TS). Non-specific dispersion forces play a crucial role in reducing the activation enthalpy. However, the enthalpic stabilization is counteracted by additional entropy losses caused by the presence of H2O. The calculated enthalpy and entropy changes agree well with experimental measurements, emphasizing the importance of molecular scale description of reaction environments.