Abstract
Understanding heteroatom doping effects on the interaction between H2O and cerium oxide (ceria, CeO2) surfaces is crucially important for elucidating heterogeneous catalytic reactions of CeO2-based oxides. Surfaces of CeO2 (111) doped with quadrivalent (Ti, Zr), trivalent (Al, Ga, Sc, Y, La), or divalent (Ca, Sr, Ba) cations are investigated using density functional theory (DFT) calculations modified for onsite Coulomb interactions (DFT + U). Trivalent (except for Al) and divalent cation doping induces the formation of intrinsic oxygen vacancy (Ovac), which is backfilled easily by H2O. Partially OH-terminated surfaces are formed. Furthermore, dissociative adsorption of H2O is simulated on the OH terminated surfaces (for trivalent or divalent cation doped models) and pure surfaces (for Al and quadrivalent cation doped surfaces). The ionic radius is crucially important. In fact, H2O dissociates spontaneously on the small cations. Although a slight change is induced by doping as for the H2O adsorption energy at Ce sites, the H2O dissociative adsorption at Ce sites is well-assisted by dopants with a smaller ionic radius. In terms of the amount of promoted Ce sites, the arrangement of dopant sites is also fundamentally important.
Highlights
This study examines the effects of divalent, trivalent, and quadrivalent dopants in CeO2 on H2O adsorption and dissociation
We consider the lattice contraction of the CeO2 surface along the doping as one reason because contraction occurs more with small cations
Doping of heteroatoms onto a CeO2 (111) surface plays an important role in H2O dissociative adsorption
Summary
Cerium oxide (ceria, CeO2) is a key material for many catalytic reactions such as three-way catalysts (TWCs),[1] water gas shift reactions,[2,3,4] and steam reforming.[5,6] proton conduction mediated by adsorbed H2O (surface protonics) onto a CeO2 surface is drawing remarkable attention for its application in electrolyte and novel low-temperature catalysis in an electric field.[7,8,9,10,11,12,13,14,15,16] For these studies, knowledge of H2O and CeO2 interaction is necessary. Many studies have been undertaken to elucidate the atomistic scale mechanism.[17–27]. Doping another element is widely known to be effective in tuning the CeO2 performance. The modification of CeO2 by doping is important for the reactions or conductions prompted by adsorbed H2O.10–12,49–51. This study examines the effects of divalent, trivalent, and quadrivalent dopants in CeO2 on H2O adsorption and dissociation. Using the DFT calculations, the ionic radius and the dispersion of dopants have been demonstrated as necessary for H2O–CeO2 interactions. We observed a longrange effect when we dope cations with small radius and high dispersion, which indicates that the reaction at Ce around the dopants is affected by slight cation doping
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