Insight into the Adsorption of Water on the Clean CeO2(111) Surface with van der Waals and Hybrid Density Functionals

Understanding the interaction between water and ceria surfaces is crucial in many catalytic applications. For the clean CeO2(111) surface, density functional theory (DFT) calculations using different generalized gradient approximations (GGAs) to the exchange-correlation functional and the DFT(GGA)+U method have found that the most stable configuration is on top of a surface cerium atom. However, they disagree on the nature of the adsorption state, with water molecularly adsorbed with one or two Os−H hydrogen bonds (Os indicates a surface oxygen atom) or as a hydroxyl pair (OsHads−OHads), with only one recent report suggesting that these two structures are very close in energy. In this work, we studied the adsorption of water on CeO2(111) employing different approximations to exchange and correlation within DFT, namely, the Perdew−Burke−Ernzerhof (PBE) GGA, DFT(PBE)+U, the Heyd−Scuseria−Ernzerhof (HSE06) hybrid functional, and van der Waals (vdW) density functionals [DFT(vdW-DF/vdWDF2)+U] with optimized exchange functionals (for vdW-DF, optB86b, revPBE, and optPBE; for vdW-DF2, rPW86). All of these methods predict close energies (10−100 meV range) for the two lowest-energy structures, the molecular structure with one Os−H bond and the hydroxyl pair. Our calculations show that these two species should be distinguishable by their infrared (IR) spectra. In particular, a rocking libration at 850 cm−1 could be used as an IR fingerprint to reveal the presence of the molecular structure. We found that the inclusion of vdW interactions increases binding energies by ∼0.18 eV, bringing them closer to the available experimental values.