Development and Application of a ReaxFF Reactive Force Field for Cerium Oxide/Water Interfaces

Abstract

Ceria (CeO 2 ) is a well-known catalytic oxide with many environmental, energy production, and industrial applications, most of them involving water as a reactant, byproduct, solvent, or simple spectator. In this work, we parameterized a Ce/O/H ReaxFF for the study of ceria and ceria/water interfaces. The parameters were fitted to an ab initio training set obtained at the DFT/PBE0 level, including the structures, cohesive energies, and elastic properties of the crystalline phases Ce, CeO 2 , and Ce 2 O 3 ; the O-defective structures and energies of vacancy formation on CeO 2 bulk and CeO 2 (111) surface, as well as the absorption and reaction energies of H 2 and H 2 O molecules on CeO 2 (111). The new potential reproduced reasonably well all the fitted properties as well as the relative stabilities of the different ceria surfaces, the oxygen vacancies formation, and the energies and structures of associative and dissociative water molecules on them. Molecular dynamics simulations of the liquid water on the CeO 2 (111) and CeO 2 (100) surfaces were carried out to study the coverage and the mechanism of water dissociation. After equilibration, on average, 35% of surface sites of CeO 2 (111) are hydroxylated whereas 15% of them are saturated with molecular water associatively adsorbed. As for the CeO 2 (100) surface, we observed that water preferentially dissociates covering 90% of the available surface sites in excellent agreement with recent experimental findings.

Toward a Consistent Prediction of Defect Chemistry in CeO2

“…Several hybrid functionals have been used to study the bulk and defect properties of CeO 2 based on periodic models. 30 , 31 , 35 − 37 Notably, the oxygen vacancy formation energies predicted by hybrid functionals are much closer to experiment than the DFT+U predictions: 3.84 eV using PBE0 38 and 3.63–4.09 eV using HSE06. 7 , 30 , 31 Such improvements from hybrid functionals were also seen in other metal oxide systems with accurately predicted defect processes.…”

“… a Present work. b Reference . c Reference . d Reference . e Reference . f Reference . g Reference . h Room-temperature measurements. − i Low-temperature measurement at 100 K j Extrapolated to 0 K from thermal expansion measurements. ,− k Spectroscopic ellipsometry measurement on nanocrystalline or single-crystal CeO 2– x films. , l Steady-state and ultrafast transient absorption spectra measurement on stoichiometric CeO 2 thin film.…”

“… e Plane-wave PBE+U (U Ce 4 f = 5.0 eV) and HSE06 (25% HF) calculations using the 96-atom supercell f Plane-wave PBE0 (25% HF) calculations using the 96-atom supercell …”

“…Several hybrid functionals have been used to study the bulk and defect properties of CeO 2 based on periodic models. 30,31,35−37 Notably, the oxygen vacancy formation energies predicted by hybrid functionals are much closer to experiment than the DFT+U predictions: 3.84 eV using PBE0 38 and 3.63−4.09 eV using HSE06. 7,30,31 Such improvements from hybrid functionals were also seen in other metal oxide systems with accurately predicted defect processes.…”

“…Brugnoli et al 38 reported 3.84 eV for the formation energy of the NN1-NN2 configuration based on the periodic model using PBE0, compared with 4.04 eV from our QM/MM model using the same functional. Burow et al 142 conducted the periodic electrostatic embedded-cluster method (PEECM) using PBE0 but obtained a much lower formation energy (3.0 eV) than our QM/MM predictions and periodic calculations from Brugnoli et al 38 Our predictions at the PBE0 level of theory are also in line with the experimental heat of reduction of ceria, which is ca. 4.0 eV.…”