Simple Approximation for the Ideal Reference State of Gases Adsorbed on Solid-State Surfaces

Area of Science:

• Thermodynamics
• Surface Science
• Computational Chemistry

Background:

• Reference states are crucial for thermodynamic calculations but less explored for adsorbed species compared to pure substances and solutes.
• Standard state conventions for adsorbed species require careful consideration for accurate thermodynamic data organization and tabulation.

Purpose of the Study:

• To compare seven distinct reference state models for calculating equilibrium constants and transition state theory rate constants for adsorbed species on flat surfaces.
• To introduce a new, simplified reference state model for adsorbed species that offers computational advantages and a physically meaningful high-temperature limit.

Main Methods:

• Evaluation of seven reference state models: ideal 2D harmonic oscillator, ideal rigid-molecule harmonic oscillator, ideal 2D harmonic oscillator with separable surface modes, 2D ideal gas, ideal 2D hindered translator, ideal 2D hindered translator with lowest-order barriers, and a simple ideal 2D hindered translator.
• Analysis of model applicability for localized, mobile, and freely translating adsorbates.
• Assessment of computational efficiency, particularly for models reducing the need to calculate diffusion barrier heights.

Main Results:

• Models 5-7 (hindered translators) consistently handle both mobile and localized adsorbates, unlike models 1-3 (localized) and model 4 (freely translating).
• Models 6 and 7 offer reduced computational cost by avoiding explicit calculation of diffusion barriers.
• The proposed simple ideal 2D hindered translator exhibits a physically realistic high-temperature limit and is applicable to non-flat surfaces.

Conclusions:

• The choice of reference state significantly impacts calculations of equilibrium and rate constants for adsorbed species.
• The simple ideal 2D hindered translator provides a practical, computationally efficient, and versatile option for modeling adsorbed species on various surfaces.
• Users should select reference states based on model appropriateness and computational feasibility for their specific research needs.