Entropies of weakly adsorbed molecules beyond the harmonic approximation

Abstract

Adsorption processes and reaction kinetics are described by the free energy. While the enthalpy contribution to the free energy can be evaluated with reasonable accuracy thanks to the developments of density functional theory (DFT), methods to accurately describe the entropy contributions are in many cases missing. One important example is adsorption in zeolites where some adsorbates are physisorbed, which is a state where the common harmonic approximation, as well as more sophisticated methods, can not accurately describe the entropy. Zeolites are crystalline microporous materials and have many applications, for example as catalysts. An important catalytic reaction over a zeolite material is selective catalytic reduction of NOx with ammonia as reducing agent (NH3-SCR). In this thesis, the entropy of species related to the NH3-SCR reaction is explored using two different methods, namely Complete Potential Energy Sampling (CPES) and Thermodynamic Integration (TI). Both methods rely on potential energy surfaces that are obtained with DFT calculations. In CPES, the full multidimensional potential energy surface is sampled by, for example, metadynamics. In TI, the starting point is the harmonic reference system to which calculates anharmonic corrections to the free energy are evaluated. Successful implementation of each method was demonstrated by reproducing results from previous works. When applicable, entropies were compared to experimental data, where only CPES showed to be in good agreement. However, free energy is the primary result of TI and our results are in good agreement with experimental data for the free energy and it is uncertain whether the entropy extraction, underlying method, or simulation setup is the culprit of TI. The results presented in the thesis increase the general understanding of entropies of weakly adsorbed molecules and, in particular, the understanding of entropy changes along the NH3-SCR reaction.