Estimating the Thermochemistry of Adsorbates Based Upon Gas-Phase Properties

A method for estimating the enthalpy, entropy, and heat capacity for adsorbates on metals is presented. The only input parameters are the binding energy of the adsorbate and the geometric properties of the gas-phase precursor. The method assumes that the vibrational frequencies of the metal lattice and the gas-phase precursor are conserved upon adsorption. Six ??rules of thumb?? are presented to estimate the new vibrational frequencies that correspond to the loss of external translation and rotation upon adsorption. The method is tested against density functional theory calculations for 17 species and 36 reversible reactions for methane steam reforming on Ni(111)[3]. The heats of adsorption and heats of reaction at 800?°C are correctly predicted to within 1?kcal/mol. The entropy of reaction is less accurate, with an average deviation of 3.1?cal/mol/K, but in the context of rapid development for thermodynamically consistent mechanisms and computational catalyst screening for high-temperature applications, this error may be tolerable.