(732a) Accurate Adsorbate Free Energies from First-Principles

In first-principles-based microkinetic models used in catalysis, free energies of adsorption and reaction are typically obtained by combining density functional theory (DFT) energies with standard approximate models, such as the harmonic oscillator, the hindered translator, or the two-dimensional ideal gas. While there has been an increasing focus on developing approaches to benchmark the reliability of the 0 K DFT energies, the performance of models used to treat the finite temperature contributions to the free energies has been analyzed to a lesser extent. I will discuss our approach (illustrated in Figure 1) to calculate accurate free energies using quantum mechanical solutions for adsorbate translational energy states extracted directly from first-principles potential energy surfaces. Through a series of case studies of adsorbates on metal surfaces, we show that the no one free energy model performs satisfactorily in all cases. Moreover, even combinations of different approximations sometimes deviate significantly from the free energies calculated by our first principles approach. Using observations from these case studies, we discuss how a full quantum mechanical approach may be extended to calculate accurate free energies for arbitrary adsorbate potential energy surfaces at computational cost similar to standard models.

Figure 1: Protocol for calculation of adsorbate free energy