Effect of Anharmonicity on Adsorption Thermodynamics

Area of Science:

• Computational Chemistry
• Materials Science
• Chemical Thermodynamics

Background:

• Anharmonic corrections are crucial for accurate vibrational energy calculations in extended systems.
• Understanding adsorption thermodynamics is vital for catalysis and materials design.
• Previous methods struggled with accurately capturing anharmonic effects in complex systems.

Purpose of the Study:

• To develop and implement a scheme for calculating anharmonic corrections to vibrational energies.
• To investigate the thermodynamics of small molecule adsorption on catalytically relevant surfaces.
• To apply the method to methane adsorption on acidic chabazite (H-CHA) and MgO(001).

Main Methods:

• Generating 1D anharmonic model potentials from distorted equilibrium structures.
• Utilizing both rectilinear (Cartesian) and curvilinear (internal) representations for normal modes.
• Calculating ab initio enthalpies, entropies, and Gibbs free energies.

Main Results:

• Curvilinear representations effectively decouple modes at higher potential orders, yielding expected thermodynamic trends.
• Accurate adsorption enthalpies, entropies, and Gibbs free energies for methane on H-CHA and MgO(001) were computed.
• Calculated desorption temperature (44 K) and prefactor (4.26 × 10^12 s^-1) for methane on MgO(001) show excellent agreement with experiments.

Conclusions:

• The implemented scheme accurately accounts for anharmonic effects in vibrational energies.
• The method provides reliable thermodynamic data for adsorption processes, crucial for catalysis.
• Results demonstrate the method's validity and potential for broader applications in materials science.