Molecular-Scale Simulation of Water Adsorption and Chemisorption on Copper Oxide Surfaces

Understanding how water adsorbs on metal oxide surfaces is essential for describing interfacial processes relevant to atmospheric chemistry, heterogeneous catalysis, and aerosol-cloud interactions. While recent experiments have shown pronounced adsorption-desorption hysteresis for water on nonporous oxides such as copper(II) oxide (CuO), the molecular mechanisms underlying this behavior remain unclear, particularly in the presence of chemisorption and surface hydroxylation. Molecular simulations provide a unique route to directly resolve these processes at the atomic scale.

In this work, we investigate water adsorption on the CuO(111) surface using a combined grand-canonical Monte Carlo (GCMC) and reactive molecular dynamics (MD) approach. GCMC simulations were performed at fixed temperature and water chemical potential to obtain adsorption isotherms directly comparable to experiment. Interatomic interactions were described using a ReaxFF reactive force field, allowing spontaneous water dissociation, proton transfer, and dynamic surface restructuring. Adsorption isotherms were constructed over a wide range of chemical potentials and converted to relative humidity using the simulated condensation chemical potential. The simulations reveal a multistage adsorption mechanism. At low chemical potentials, water adsorbs primarily via dissociative chemisorption, leading to progressive hydroxylation of the CuO surface. As chemical potential increases, additional water accumulates non-uniformly as hydrogen bonded clusters rather than as a continuous film. Near saturation, these clusters coalesce and trigger rapid multilayer growth. Reactive MD simulations show that chemisorbed species remain mobile and influence cluster stability, growth pathways, and desorption behavior. Simulated adsorption isotherms are in good agreement with experimental measurements and capture key features associated with adsorption-desorption hysteresis. By extracting adsorption parameters directly from the simulations, we assess the applicability of Frenkel-Halsey-Hill type multilayer adsorption models to reactive oxide surfaces, demonstrating that chemisorption must be explicitly accounted for in molecularly based adsorption frameworks.