Microkinetic models of heterogeneously catalyzed reactions require accurate kinetic and thermophysical properties. It is generally assumed that rate constants of elementary steps are more important. However, the Gibbs free energy of adsorbates is also crucial since it defines the stability of adsorbed intermediates and the thermodynamic equilibrium. The Gibbs free energy is composed of an enthalpic and entropic contribution. Entropy is a species-specific property and depends only on the partition functions, whereas enthalpy is a species-interrelated quantity that is only meaningful with a reference. The gas-phase community has established standards to derive enthalpies, which enables the combination of electronic structure and experimental data into thermochemical networks with highly accurate enthalpies of formation. The catalysis community has not adopted similar standards. Therefore, each group uses its own conventions, which leads to very different values of the enthalpies and substantial variations in the predictions of microkinetic models.
I will introduce a standardized linear algebra framework of all approaches to derive enthalpies of adsorbates. The standardized framework provides a tool for integrating DFT, gas-phase thermochemistry, and experimental surface science data. Combining data from various sources with different fidelity in isodesmic reference reactions that maximize error cancellation makes it possible to move beyond the limitations of the DFT accuracy. These error cancellation reactions lead to considerably more accurate enthalpies from DFT without additional computational costs. Microkinetic modeling of the oxidative dehydrogenation of n-alkanes over Pt(111) with the thermophysical properties determined through different methods reveals large variations in the concentration profiles and the rate-controlling steps. Therefore, accurate enthalpies of adsorbates are urgently needed for predictive chemical kinetic modeling. By demonstrating the impact of adsorbate thermochemistry, this study aims to spark a discussion in the catalysis community to develop clear standards to facilitate the re-use and sharing of data.
