O₂ Activation on Ceria Catalysts—The Importance of Substrate Crystallographic Orientation

Abstract

An atomic-level understanding of dioxygen activation on metal oxides remains one of the major challenges in heterogeneous catalysis. By performing a thorough surface-science study of all three low-index single-crystal surfaces of ceria, probably the most important redox catalysts, we provide a direct spectroscopic characterization of reactive dioxygen species at defect sites on the reduced ceria (110) and (100) surfaces. Surprisingly, neither of these superoxo and peroxo species was found on ceria (111), the thermodynamically most stable surface of this oxide. Applying density functional theory, we could relate these apparently inconsistent findings to a sub-surface diffusion of O vacancies on (111) substrates, but not on the less-closely packed surfaces. These observations resolve a long standing debate concerning the location of O vacancies on ceria surfaces and the activation of O on ceria powders.

Metal Cluster Models for Heterogeneous Catalysis: A Matrix‐Isolation Perspective

“…[131] The carbenec omplexs pontaneously reacts with CH 4 to yield the complex with two methyl groups (CH 3 ) 2 Ti(H)(OH), whichi sa lso formed directly by UV irradiation of (CH 3 )Ti(H)(O)(CH 4 ). For CH 3 Cl, insertion into the CÀ Hb ond as well as the CÀCl bond is observed. ScO reacts with CH 3 Cl to yield at first the ScO(CH 3 Cl) complex, and after irradiation both CH 3 OScCl and CH 2 ClScOH.…”

“…Ga 2 H 2 .H owever,t he detailedq uantum chemical study by Kçhn et al [39] showed that the reaction is able to proceed via asmall barrier by an intersystem crossing to the singlet potential energy surface. At first, due to the reduced symmetry of the system during the approacho ft he reactants, am ixingb etween the original 3 ). Hence, resultsshow that two low-lying excited electronic states of Ga 2 are important during the courseo ft he reaction.…”

“…Another challenge is the fact that the catalyst particles offer to the substrates surfaces with different packing schemes of the surface atoms (Miller indices). For example, very recent work showed that O 2 activation by CeO 2 catalysts is very different for CeO 2 (1 1 0) or CeO 2 (1 0 0) surfaces on the one hand (where superoxo and peroxo complexes are formed) and the more densely packed CeO 2 (1 1 1) surfaces on the other hand (where no superoxo or peroxo complexes are formed) . The mechanism of the Haber–Bosch process was disclosed because it was proven that only the speed changes with the pressure of both gaseous reactants, or with the packing motifs of the surfaces, but not the elementary steps .…”

“…For example, very recent work showed that O 2 activation by CeO 2 catalysts is very different for CeO 2 (110)o rC eO 2 (1 00)s urfaces on the one hand (wheresuperoxo and peroxocomplexes are formed) and the more denselyp acked CeO 2 (111)s urfaces on the other hand (where no superoxo or peroxo complexes are formed). [3] The mechanism of the Haber-Bosch process was disclosedb ecause it was proven that only the speed changes with the pressure of both gaseous reactants, or with the packing motifs of the surfaces, but not the elementary steps. [4] Further complicationsm ight arise from an (often underestimated) involvement of the oxide support in substrate activation for catalysts in which metal nanoparticles are supported by highsurface-area oxide solids.…”

Direct Phenolysis Reactions of Unactivated Amides into Phenolic Esters Promoted by a Heterogeneous CeO2 Catalyst

“…Herein, we report a simple, additive‐free and reusable heterogenous catalytic system based on CeO 2 for the direct phenolysis of unactivated amides into the corresponding phenolic esters. A systematic mechanistic study revealed that the unique combination of acid/base and redox properties of CeO 2 promotes this esterification reaction. This catalytic system not only offers a facile means to cleave the amide C−N bond, but also significantly promotes the use of amides and phenols as important building blocks for the synthesis of valuable esters.…”