Molecular Adsorption Kinetics: Nonlinear Entropy–Enthalpy Loss Quantified by Constrained AIMD and Insights into the Adsorption-Site Determination on Metal Oxides

Quantifying the molecular adsorption/desorption rate is vital in ascertaining the catalytic activity/mechanism but falls far short of expectations, limited by the explicit description of the elusive entropy effect. Herein, we quantitatively unravel the free energy barriers of adsorption/desorption steps at specific temperatures by the state-of-the-art constrained molecular dynamic simulations, with the origin traced to the delicate entropy–enthalpy interplay; the augmented loss of entropy caused by minor enthalpy drops leads to an adsorption barrier, while the desorption barrier is largely determined by the enthalpy cost. As an example, the long-term adsorption-site puzzle of a CO molecule on metal oxide is resolved—the Co3+ on Co3O4(110) is the adsorption site instead of the lattice O2– that is found to be a reacting site. Notably, it is proved that the obtained barriers on Co3+ differ with any traditional estimation method and show an interesting linear temperature dependence. Generally, we explore the CO adsorption barriers on typical metal surfaces (Cu, Rh, Pd, Ag, Pt, and Au) and find a good scaling with the adsorption energies. We believe that these insights could deepen the fundamental understanding of adsorption/desorption kinetics in heterogeneous catalysis and inspire studies for accurate determination of adsorption processes.