Entropies of weakly adsorbed molecules beyond the harmonic approximation
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Physics (MPPHS), MSc
Publicerad
2023
Författare
Cronquist, Olof
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
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.
Beskrivning
Ämne/nyckelord
entropy , free energy , adsorption , anharmonicity , van der Waals , zeolites , Complete Potential Energy Sampling , Thermodynamic Integration