O₂ Activation over Ag-Decorated CeO₂(111) and TiO₂(110) Surfaces: A Theoretical Comparative Investigation

Abstract

Periodic spin-polarized hybrid density functional theory calculations have been performed to investigate the reactivity of pristine, O-defective, and Ag-decorated CeO 2 (111) and TiO 2 (110) surfaces with a small Ag 10 cluster toward O 2 . The adsorption of O 2 and its subsequent dissociation have been studied in order to provide a better understanding of the role of the oxide, the metallic nanoparticle, and their interaction in the reactivity of composite metal/metal oxide materials toward O 2 , as potential catalysts to this reaction. Structural, energetic, electronic, and vibrational properties of all species involved in the different reaction paths considered have been fully characterized. On the stoichiometric surfaces, Ag 10 is oxidized and reduces surface Ce 4+ / Ti 4+ ions, while on the O-defective surfaces, the adhesion of silver is promoted only on CeO 2 but unfavored on TiO 2 . On the other hand, on the silver-free supports, O 2 strongly adsorbs at vacancies and preferentially reduces to peroxide. When no O vacancies are considered on the Ag 10 -decorated supports, the net positive charge on Ag 10 actually prevents the adsorption and reduction of O 2 . Instead, when O vacancies are included, two reaction pathways are observed; oxygen molecules can weakly absorb on the silver cluster as a superoxide moiety or strongly adsorb at the vacancy as peroxide. The dissociation of the O−O bond of the peroxide is favored both from the thermodynamic and kinetic points of view in silver-decorated surfaces, in contrast with the silver-free cases. In addition, Ag 10 /CeO 2 shows higher activity toward the O 2 adsorption and dissociation than Ag 10 /TiO 2 , which can be related both to the higher ionicity and superior electron storage/release ability of ceria with respect to titania, thus leading to the weakening of the O−O bond and providing lower activation barriers for oxygen reduction. These results deepen the current understanding of the reactivity of metal/metal oxide composites toward O 2 , especially elucidating how the surface stoichiometry affects the charge state of the metal clusters, and hence the reactivity of these interfaces toward O 2 , with potential important consequences when such composites are considered for catalytic applications.

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

“…Dispersion interactions were not included in the training set since their inclusion through the Grimme’s dispersion correction D2 was shown to have a negligible effect on the adsorption energy, conformation geometries, and relative stabilities of water on ceria surfaces . Following to our previous works, two distinct basis sets were used. ,, The less extensive BS1 basis set was used for geometry optimization, except for the bulk CeO 2 and Ce 2 O 3 models to which the BS2 basis set was used. The more extensive BS2 basis set was employed to compute energies of the models optimized with the BS1 basis set and elastic properties of bulk cerium oxides.…”

“…31 Following to our previous works, two distinct basis sets were used. 59,62,63 The less extensive BS1 basis set was used for geometry optimization, except for the bulk CeO 2 and Ce 2 O 3 models to which the BS2 basis set was used. The more extensive BS2 basis set was employed to compute energies of the models optimized with the BS1 basis set and elastic properties of bulk cerium oxides.…”

Single-Atom High-Temperature Catalysis on a Rh1O5 Cluster for Production of Syngas from Methane

“…Notably, this insertion step is kinetically barrierless and thermodynamically exothermic (panel 1 in Figure ), suggesting a different path for activating molecular O 2 on Rh 1 /TiO 2 in contrast to dissociation of OO on the oxygen vacancy on the transition metal oxides. Different from the barrierless kinetics of molecular O 2 on this single-atom catalyst, Rh 1 /TiO 2 , the kinetic barrier for activation of O 2 on Co 3 O 4 , TiO 2 , and CeO 2 is in the range of about 0.5–2.0 eV, which varies at different faces and vacancies. Compared to the activation of O 2 on an oxygen vacancy of transition metal oxides, this distinctly different barrierless activation mechanism of O 2 on the site of Rh 1 /TiO 2 demonstrated the significance of Rh 1 in the high activity of Rh 1 /TiO 2 for POM.…”

O2 Activation with a Sterically Encumbered, Oxygen-Deficient Polyoxovanadate-Alkoxide Cluster

“…Although the MvK mechanism is well-accepted in RMO catalysis, limitations of in situ spectroscopic analyses of these heterogeneous systems have inhibited progress in the direct experimental observation of the proposed reactivity at the surface of metal oxides. − This is often complicated by the fact that reoxidation of the metal-oxide surface (passivation of O atom defects) is kinetically rapid, rendering the formation of isurface activation intermediates challenging to observe. As a result, researchers have turned to theoretical investigations to probe the energy landscape of the reactivity of O 2 with O atom vacancies in these materials. − An alternative and increasingly popular approach uses homogeneous molecular models. These soluble, metal oxide assemblies can provide valuable, atomistic insight into the surface reactivity of bulk solids.…”