Decoding the Role of Adsorbates Entropy in the Reactivity of Single-Atom Catalysts

Single-atom catalysts (SACs) are rapidly gaining attention as a versatile class of materials that combine the advantages of both homogeneous and heterogeneous catalysis. A growing number of studies aim to identify potential new SACs or to describe their structure and reactivity through ab initio quantum chemical simulations. While many computational studies primarily address reactions involving small molecules, such as water splitting or CO2 reduction, the application scope of SACs is rapidly broadening to include the production of fine chemicals and the conversion of biomass-derived platform molecules, processes that involve larger, more complex reactants. Using density-functional theory (DFT) simulations, we demonstrate that, while an approximate treatment of entropy is acceptable for molecules with up to three atoms, it introduces substantial errors in reactions involving more complex molecules. Our results reveal a linear correlation between the entropy of adsorbed molecules and that of the corresponding isolated species, mirroring trends observed on extended catalytic surfaces. For the largest systems investigated in this study, the entropy of the free molecule is reduced by approximately 10-20% upon adsorption; for small molecules, this reduction can range from 50 to 70%. This disparity arises because, on SACs, the translational entropy is effectively zero, the rotational entropy is minimal, and the vibrational entropy increases with the size of the molecule. Moreover, the entropy of adsorbates scales linearly with the number of atoms in the molecule, allowing for the prediction of entropic contributions of adsorbates on SACs without additional computational cost. Using propyne hydrogenation as a test, we demonstrate that the reaction energy profile computed with current approximate approaches for estimating the entropy of adsorbates differs significantly from the profile where entropy is explicitly included. These findings highlight the importance of considering adsorbate entropy for accurately predicting the catalytic activity of SACs, particularly for reactions involving complex molecules.