Theoretical analysis of the adsorption of phosphoric acid and model phosphate monoesters on CeO₂ (111)

Ceria has been shown to catalyze the dephosphorylation of phosphate monoesters at ambient temperature. As a step toward understanding the reaction mechanism on ceria, first-principles density functional theory calculations in the GGA-PW91+U approach have been carried out to study the adsorption of phosphoric acid and two model phosphate monoesters, methyl phosphate and para-nitrophenyl phosphate, on CeO2 (111). Consistent patterns are found in terms of energetic, structural, electronic, and vibrational properties upon adsorption. For the parent molecules, bonding of the phosphorous atom directly to a surface lattice oxygen atom (O-latt) is the only stable molecular adsorption state. Common entities that reduce Ce4+ to Ce3+ (e.g., co-adsorbed H atom, surface and subsurface oxygen vacancy) mildly perturb the P-O-latt bond, but Ce3+ by itself has only a negligible effect. Deprotonation is exothermic for all three phosphate species on CeO2 (111), much more so if they are adsorbed in an oxygen vacancy via a phosphoryl O than on a stoichiometric surface. Infrared spectra have been simulated for the adsorbed states of the model phosphates for comparison with future experiments.