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Dioxygen Activation Pathways Over Cobalt Spinel Nanocubes – From Molecular Mechanism Into Ab Initio Thermodynamics and O/ O Exchange Microkinetics 16
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Filip Zasada, Witold Piskorz, Janusz Janas, Eko Budiyanto, and Zbigniew Sojka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09597 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017
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Dioxygen Activation Pathways over Cobalt Spinel Nanocubes – from Molecular Mechanism into ab initio Thermodynamics and 16O2/18O2 Exchange Microkinetics
Filip Zasada*, Witold Piskorz, Janusz Janas, Eko Budiyanto, Zbigniew Sojka Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland *Corresponding Author: Filip Zasada, e-mail:
[email protected], phone/fax number: +48 12 663 2073
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Abstract: A unified molecular, thermodynamic and kinetic picture of dioxygen activation, surface diffusion and reactivity over stoichiometric and defected cobalt spinel (100) termination was provided by combination of GGA–DFT+U modeling and experimental isotopic exchange investigations using Co3O4 nanocubes. Various diatomic (CoO5c–O2––CoO5c superoxo, CoO5c–O22––CoT2c peroxo, (O–Osurf.)2– peroxo) and monoatomic (O–CoT2c and O– CoO5c metal-oxo) reactive oxygen species were described in detail regarding their electronic and magnetic structure. The band alignment diagrams between the pDOS of dioxygen and the exposed cobalt cations were constructed, and used to rationalize the revealed pronounced speciation of the surface oxygen, depending on the adsorption geometry (monodentate η1, bidentate η2, bridging µ), various extent of oxygen reduction (1, 2, 4 electrons), and the entailed complex spin relaxation. It was shown that surface cobalt cations work in tandem constituting dual CoO5c–CoO5c and CoO5c–CoT2c sites for O2 activation. The metal-oxo species were formulated in terms of the O– moieties ferromagnetically coupled to Co ions, and orbital overlap type (σ for O–CoT2c or π for O–CoO5c), is mainly responsible for the observed differences in their stabilities and mobilities. A 3-dimensional plot of the O/Co ratio as a function of T and pO2 provided a suitable contextual thermodynamic background for understanding the dioxygen/surface interactions, and was used to support the kinetic data of the isotopic
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O2/18O2 exchange reaction. The elaborated molecular mechanism of the
dioxygen interaction with the cobalt spinel (100) surface was applied for ab initio microkinetic modeling of the isotopic
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O2/18O2 exchange reaction in the reactant lean and
rich conditions, providing a theoretical account for TAP and TPSR experiments. Three stages of the evolution were distinguished: a latent (surface accumulation of the dissociated 18O* and 16
O* adspecies), transient (gradual development of the
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O18O isotopomer in the gas phase)
and an equilibrium (equilibration of the isotopic composition). It was also shown that CoT2c2+– 2 ACS Paragon Plus Environment
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O– acts as rigid (Ediff = 1.35 eV) spectator species, and only the labile CoO5c3+–O– (Ediff = 0.68 eV) are directly involved in the isotopic exchange. The simulated TPSR curves were confronted with the
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O2/18O2 exchange experiments performed on a cobalt spinel nanocube
catalyst, synthetized by hydrothermal method and characterized by XRD, RS and HR-TEM techniques. An excellent quantitative agreement between experiment and theory substantiates the developed molecular mechanism, selection of the kinetically relevant steps, and calculation of their energetic and entropic barriers.
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1.INTRODUCTION Open-shell transition metal oxides of spinel structure containing easily accessible electrons and oxygen ions constitute a wide group of materials with the potential application as oxidation catalysts,1,2,3 photo- and electrocatalysts4,5,6 and supercapacitor materials.7 An important feature of those oxides is their ability to adsorb and activate dioxygen with its eventual incorporation into the bulk.8,9 These molecular events are triggered by changes in the oxidation states of the active sites, yet without alteration of the overall crystal spinel structure. The rate of oxygen adsorption/desorption is governed by the efficiency of redox processes involving parent dioxygen admolecules and their derivatives (O2–, O22–, O–), dynamics of anion vacancy formation and annihilation, as well as by the electron/hole generation and transport.10 For instance, the molecular events of O2 adsorption, electron transfer activation, dissociation and surface diffusion constitute elementary steps of the suprafacial (LangmuirHinshelwood, Eley-Rideal) and the intrafacial (Mars-van Krevelen)11,12 mechanisms in heterogeneous catalysis.13,14,15 In the case of spinels, perovskites and akin materials, direct activation of dioxygen on the surface, via electron transfer provided by the dn>0 active centers, gives rise to a number of possible surface reactive oxygen species, such as superoxo O2–, peroxo O22– or metal-oxo O–.16,17 As a result, the dioxygen activation pathways become quite complex depending on the surface topology and the number of electrons that are transferred.18 In the same way, multistep dioxygen reduction/dissociation processes that are crucial for solid oxide fuel cells (SOFC) operation, may entail several possible pathways of oxygen activation, with or without oxygen vacancy being explicitly involved.19 For comprehensive studies of the dioxygen/oxide surface chemistry, we choose cobalt spinel (Co3O4) as a model system, taking into account its well-defined structure,20 easily controllable morphology and ability for facile redox-tuning.21,22,23 High catalytic performance of Co3O4 in redox processes is usually attributed to presence of reactive oxygen species
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(ROS) and to surface oxygen vacancies that can be produced even at mild conditions.24,25 The ROS adspecies may be stabilized on the truncated tetrahedral (CoT) and octahedral (CoO) cationic sites or on the anionic (O2–) surface centers.26 Recently, stability of these active oxygen species on the Co3O4 surface was preliminarily examined by DFT calculations corroborated by thermoprogramed measurements and isotopic experiments.27,28,29 Many computational studies on cobalt spinel surface activity have been carried out,30,31,32,33,34 however, despite the relevance of the dioxygen activation, a comprehensive molecular level description of the reaction mechanism including the effect of surface topology, energy levels alignment, thermodynamic stability of the resultant ROS entities at various T and pO2 conditions, and their recombination kinetics is still lacking. Herein, we describe for the first time the intimate mechanism for complete set of possible O2 activation pathways (adsorption/dissociation/diffusion) on the (100) surface exposed by the cobalt spinel nanocubes. We also provide a detailed, systematic characterization of the electronic and magnetic structure of all involved ROS adspecies, as well as full description of the energetic profiles for reaction each step. The influence of temperature and oxygen pressure on the ROS stability was assessed within the first principles thermodynamics. The obtained results were used for microkinetic ab initio modeling of the isotopic
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O2/18O2
exchange, and compared with the experimental TPSR studies.
2.COMPUTATIONAL SCHEMES AND EXPERIMENTAL METHODS 2.1.Molecular Modeling: DFT+U periodic calculations with use of projector augmented plane wave method (PAW) together with the PW9135 exchange-functional were employed by means of VASP code.36 To account for strong on-site Coulomb repulsion among the localized 3d electrons, the DFT+U level of theory was chosen,37 with the Hubbard U parameter set to U = 3.5 eV.38,39 Systematic validation of the applied calculation scheme against the experimental
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data is presented in Supporting Information, SI, Section 1). The standard Monkhorst-Pack grid40 (5×5×5 sampling mesh for bulk calculations and 5×5×1 for slab calculations), and the cutoff energy of 500 eV were employed. The slab model (see Figure S1 in SI section for details) exposing (1×1) element of the (100) surface was cleaved starting from the bulk cobalt spinel unit cell containing 56 ions. The nudged elastic band method (NEB)41,42 was used to calculate transition states (TS) of the investigated molecular events. In all calculations, for each TS five to nine NEB images (including the initial and final ones) were used. 2.2.Atomistic thermodynamics: To obtain 3-diemensional redox state diagram of the cobalt spinel surface, which translate the T,pO2 conditions into surface stoichiometry, we employed atomistic thermodynamic modeling following the scheme proposed elsewhere.43,44 Within this approach, in the case of cobalt oxide, the free energy, γ, of the surface containing NO oxygen atoms and NCo cobalt atoms relative to the bulk Co3O4, can be calculated as:
, γ, = , , −
"
,
− ! − # $% , &
(eq. 1)
/0-1 In (eq. 1) G,-./ ()*%+ and G() % represent the free energy of the overall slab and the free
energy of the bulk formula unit, respectively. Since the vibrational contributions to the Gibbs free energies of the bulk and the slab cancel to the large extent, they can be approximated by corresponding DFT electronic energies.45 The 2A term denotes the area of the surface exposed by the slab model (bottom and top), and $% , the chemical potential of dioxygen, factored as
;
7 $ , = 244 + $6 , + kln ! ;
