Activation of Molecular O₂ on CoFe₂O₄ (001) Surfaces: An Embedded Cluster Study

Abstract

Dioxygen activation pathways on the (001) surfaces of cobalt ferrite, CoFe2O4, were investigated computationally using density functional theory and the hybrid Perdew‐Burke‐Ernzerhof exchange‐correlation functional (PBE0) within the periodic electrostatic embedded cluster model. We considered two terminations: the A‐layer exposing Fe2+ and Co2+ metal sites in tetrahedral and octahedral positions, respectively, and the B‐layer exposing octahedrally coordinated Co3+. On the A‐layer, molecular oxygen is chemisorbed as a superoxide on the Fe monocenter or bridging a Fe−Co cation pair, whereas on the B‐layer it is adsorbed at the most stable anionic vacancy. Activation is promoted by transfer of electrons provided by the d metal centers onto the adsorbed oxygen. The subsequent dissociation of dioxygen into monoatomic species and surface reoxidation have been identified as the most critical steps that may limit the rate of the oxidation processes. Of the reactive metal‐O species, [FeIII−O]2+ is thermodynamically most stable, while the oxygen of the Co−O species may easily migrate across the A‐layer with barriers smaller than the associative desorption.

Interactions of water and short-chain alcohols with CoFe2O4(001) surfaces at low coverages

“…To theoretically model the (001) CFO surfaces, we used finite sized clusters which were validated in convergence studies of adsorption potentials in a previous study. 23 The quantum mechanically treated clusters have the following stoichiometry: Co 10 Fe 23 O 42 , Co 18 Fe 23 O 52 , Co 22 Fe 33 O 80 , and their structures close to a hemispherical shape are presented in Fig. 1.…”

“…1, modelling the different active sites at the (001)-A and B surfaces of CFO, were treated quantum mechanically in a setup for the embedding which has been evaluated in a previous study. 23 In this embedding setup, the two-dimensional periodic array of point charges was constructed by combining bulk and relaxed surface structures taken from periodic DFT(PBE)+U calculations. 24 For the electrostatic embedding we used full formal ionic charges: 2+ for surface Fe/Co at the A-layer, 3+ for surface Co at the B-layer, 2À for oxygen, and 2+/3+ for bulk Co/Fe.…”

Cyclohexane Oxidative Dehydrogenation on Graphene-Oxide-Supported Cobalt Ferrite Nanohybrids: Effect of Dynamic Nature of Active Sites on Reaction Selectivity

“…Furthermore, almost similar rates were observed for unsupported and supported catalysts (Figures 3a and 4a) and in all wt % loadings (Figure 4a, 7 nm) in oxygenated conditions confirm the sole kinetic relevance of these oxygen species in C−H bond abstraction. Such oxygenderived species could involve dioxygen cation (O 2 + ), superoxide ion (O 2 − ), peroxide ion (O 2 2− ), 55 and monoatomic adoxygen species 11,30,55,56 The selectivities, on the other hand, are significantly different for these samples under oxygenated conditions (Figure 3b). The higher selectivity toward CO 2 observed on unsupported particles in oxygenated conditions and no detection of cyclohexene reflect that the cyclohexane derived species must remain strongly bound and efficiently undergo C−C bond cleavage and O-insertion reactions to form CO 2 or undergo subsequent C−H bond cleavage to form full dehydrogenated product benzene.…”

“…The dotted lines are the best fits to the eq 2. (b) Corresponding selectivities toward CO 2 (black), benzene (red), and cyclohexene (blue) and % cyclohexane conversion (green dots) on 7 nm samples (13,30 S6). Surprisingly, the selectivities toward cyclohexene (∼2 to 5%) and benzene (75−78%) were found to be insensitive to O 2 /Cyclohexane ratios (in the range of 1− 25) (See Figure S6).…”

“…29 Cobalt ferrite in its most stable phase has an inverse spinel structure, (Fe 3+ )[Co 2+ Fe 3+ ]O 4 , in which tetrahedral sites (T d ) are occupied by Fe 3+ and one-half each of the octahedral sites (O h ) are occupied by Co 2+ and Fe 3+ , respectively, but an alternative normal spinel structure, (Co 2+ )(Fe 3+ ) 2 O 4 , is also relatively stable, in which tetrahedral sites (T d ) are occupied by Co 2+ and octahedral sites (O h ) are occupied by Fe 3+ , respectively. The rich oxidation states and variable coordination geometries make CoFe 2 O 4 an attractive candidate for O 2 activation, 30,31 which is a necessary step for alkane/alkanol oxidation and reduction reactions. 11,32 Based on the spectroscopic (XPEEM and XPS) and kinetic evidence, we propose that the Fe 3+ cations at tetrahedral locations (Fe 3+ /T d ) could be responsible for full cyclohexane dehydrogenation producing benzene while kinetics show that the oxygen-derived species (O*) are responsible for partial dehydrogenation producing cyclohexene.…”