Article pubs.acs.org/JPCC
Water Oxidation on MnOx and IrOx: Why Similar Performance? Michael Busch,† Elisabet Ahlberg,† and Itai Panas*,‡ †
Department of Chemistry and Molecular Biology, Electrochemistry, University of Gothenburg, S-412 96 Gothenburg, Sweden Department of Chemistry and Biotechnology, Energy and Materials, Chalmers University of Technology, S-412 96 Gothenburg, Sweden
‡
S Supporting Information *
ABSTRACT: The critical steps in water oxidation at a binuclear Mn(II−IV) oxide site are revisited. Ideal stabilities of intermediates are confirmed by comparing to results for a binuclear Ir(III−V) system. The latter in turn is known to be an excellent water oxidation catalyst. The inefficiency of the binuclear Mn(II−IV) site is owing to the high activation energy for the chemical step whereby MnIVO double bonds on adjacent sites are broken prior to forming the MnIIIOOMnIII peroxy moiety. A rationale for Mn(II−IV)Mn(III−V) mixed oxidation state for water oxidation catalysis, analogous to mixed transition metal oxide systems, is offered. Possible virtues of the kinetic stability of the binuclear MnIVO moiety are discussed, utilizing its oxidizing power by sidestepping oxygen evolution.
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INTRODUCTION Efficient water splitting is a key requirement in any sustainable energy system that hopes to successfully utilize hydrogen as an energy carrier. The efficiency of the catalyst is controlled by the electrocatalytic abstraction of hydrogen from molecular water and the ability to regenerate the catalytic sites, i.e., by oxygen evolution. The hydrogen abstraction is associated with a surface redox process whereby the metal site is reduced and the adsorbed oxygen is oxidized. When oxygen evolution is employed to regenerate the catalyst, OO bond formation becomes a central reaction step. This step can follow either a mononuclear1−4 or a binuclear5,6 pathway. The former mechanism proceeds via an electrochemical OO bond formation between a transition metal TMO• intermediate and a bulk water molecule, resulting in the formation of a TMOOH intermediate. This reaction path is vigorously pursued by several groups.1−4 The binuclear mechanism, which is also found experimentally,7−10 assumes separation of the overall reaction into electrochemical and chemical steps. The former steps comprise the oxidation of water whereby surface TMO species are formed. The subsequent chemical steps correspond to an internal surface redox step whereby two neighboring TMO• groups produce a TMOOTM μperoxo bridged intermediate. This is followed by release of 3 O2.5 Support for such an interpretation is found in experiment.11 In the case of the mononuclear mechanism, the only parameter that can be optimized is the stability of the single TMO moiety to provide optimal reactivity for the attacking water molecule. On the basis of this mechanism, the optimal catalysts were found to be iridium oxide and ruthenium oxide.2,3,12 This result is in agreement with experiment.8,10,13−15 Although this simple understanding allows us to predict the reactivity of pure oxides correctly, it can be argued that its © 2012 American Chemical Society
simplicity may offer an artificial restriction when searching for improved catalysts. The slightly more complex binuclear reaction pathway in contrast opens the possibility to achieve improved catalytic performance by mixing transition metal oxides with opposite reactivities, e.g., the inactive Fe(II−IV) 6 with Co(II−IV), which oxidizes water at a large overpotential.6 The validity of this concept was demonstrated in previous work.6,16 In ref 16 the homonuclear Mn(II−IV) system was shown to exhibit nearly ideal intermediate stabilities. It was argued that it is the rigidity of heterogeneous catalysts17 in conjunction with a high activation barrier associated with the breaking of the MnO double bond16 prior to μ-peroxo bond formation that renders the recovery of the catalytic sites a slow process. This is in contrast to the Mn(III−V) system where the negligible activation barrier comes at the price of a more positive potential for the formation of the oxo intermediate. The objectives of the present study are 3-fold. First, the apparent ideal energetics of the intermediates associated with the homonuclear Mn(II−IV) site is revisited and compared to the corresponding binuclear Ir(III−V) system. The latter is known to be an excellent OER catalyst, as rationalized by the mononuclear paradigm.2,12 Here, the binuclear perspective is provided for completion. Second, an alternative electronic pathway for OER on the manganese system, inspired by the study on heteronuclear systems is demonstrated. It involves mixing Mn(II−IV) and Mn(III−V) moieties on the binuclear site. The fact that such flexibility cannot be articulated by the mononuclear paradigm lends it additional importance. Third, the inefficiency of the homonuclear Mn(II−IV) site for the OER is discussed in the context of cogeneration of energy and valuable chemicals7 where such a property is indeed preferred. Received: September 10, 2012 Revised: November 23, 2012 Published: December 10, 2012 288
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COMPUTATIONAL DETAILS The oxides are modeled employing the 1-dimensional 2monolayer MgOx(OH)y rig model shown in Figure 1a. This
algorithm. For the slab system the cell parameters and the lower most MgO layer were kept fix. All oxidation steps employ the tyrosine/tyrosyl radical TyrOH/TyrO• reference system as hydrogen sink. Similar to previous studies,5,6,16 the manganese oxide electronic structure was modeled assuming Hund’s rule and ferromagnetic coupling between the manganese atoms. In the case of iridium oxide an open shell low spin configuration was used, i.e., triplet for the dihydroxo structure, doublet for the oxo-hydroxo structure, and an open shell singlet for the dioxo intermediate.
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THE TEST RIG EMBEDDING Experimentally studied OER catalysts often comprise nanoparticles8,15,21 that generally show weak Madelung potential.22 Classical slab calculations in contrast describe a situation where transition metal ions are embedded in a system displaying a strong Madelung potential as well as chemical confinement effects. Owing to this difference, the present study resorts to a 1-dimensional embedding exhibiting the weak Madelung potential and flexibility, typical for nanocrystalline systems. The validity of the test rig for modeling molecular systems was confirmed in ref 6, showing the applicability of a slab system for modeling catalytic properties of sites on nanotextured surfaces. To quantify the differences between both models, the energy profiles of a Mn(II−IV) system in a three-layer slab consisting of two layers of MgO and one layer manganese oxide (Figure 1b) and a Mn(II−IV) dimer embedded into the test rig were studied. Indeed, major quantitative differences between both models were found. Though the oxidation of two MnIIIOH to two MnIVO is endothermic by 1.26 eV versus the TyrOH/TyrO• reference system for the slab system, the oxidation steps are approximately thermoneutral with respect to the TyrOH/ TyrO• system for the test rig embedding. The energetics of the oxidation steps are mirrored in the OO bond formation.
Figure 1. Test rig employed to model the reactivity of the transition metal oxides (a) and the 3 ML MgO slab (b) are depicted. (Color code: green, Mg; red, O; gray, H; violet, TM).
model has previously been shown to combine low computational costs with semiquantitative accuracy.6,10,16 The oxidation states are adjusted by adding/removing hydrogens to the trans standing oxo/hydroxo ligands. The slabs were divided by 7 Å of vacuum to avoid interactions. An increase of the vacuum to 15 Å did not result in any significant changes. All density functional theory (DFT) calculations were performed using the CASTEP plane wave code18 employing the PBE19 generalized gradient approximation (GGA) functional. A 400 eV cutoff together with a 1 × 1 × 1 k-point set was used. Both the k-point set and the cutoff energy were checked for convergence and no improvements are found upon increasing these parameters (Supporting Information). In the case of slab calculations the energy was taken from single point calculations employing a 2 × 2 × 1 k-point set using the geometry previously obtained by Γ-point calculations. The core electrons were modeled with ultrasoft pseudopotentials.20 Both the geometry and the length of the 1-dimensional rig were optimized using the BFGS
Figure 2. Energetics of the key reaction steps for pure Mn(III−V), Mn(II−IV), and Ir(III−V) are shown. The total energies for the dihydroxo, dioxo, and peroxo intermediates of the Mn(II−IV) are taken from ref 16. 289
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similar flexibilities of the support, such as the test rig model at hand,6,17 the energy contribution from the bending of the catalytic sites can be assumed constant. In this case the energy for the breaking of the TMO bond to form TMO• moieties can be taken as a descriptor for the relative activation barriers. However, when systems with very different supports are compared, i.e., the test rig and the slab model described above, this descriptor would be misleading due to the unequal contributions from the required bending. Thus, the comparison between the slab and the rig embedding is necessarily restricted to a purely thermodynamic perspective. On a rigid support, the Mn(II−IV) dimer was found inactive toward the OER due to the expected high costs of breaking the partial MnIVO double bond.16 A measure for the amount of partial double bonding character of the TMO bond is the amount of radical character on the oxygen. Indeed, the nature of the TMO bindings may be understood from the computed spin densities (Table 1). Thus, the IrVO dimer
Here, the recombination of two comparably unstable MnIVO moieties to form Mn III OOMn III results in an exothermicity of 0.76 eV for the slab whereas it is endothermic by 0.37 eV in the case of the test rig embedding. Considering these differences in the energetics, it is cautioned that the efficiency of the classical slab model in describing the energetics of nanostructured surfaces and nanoparticles cannot be taken for granted, as this analogy may be highly flawed.
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OER AT MANGANESE AND IRIDIUM OXIDES Assuming a binuclear mechanism, the key steps of the OER were found to comprise oxidation of TMOH to TMO and the subsequent OO bond formation.5 Employing these reaction steps as descriptors, the reactivities of the binuclear Mn(III−V) 6 and the Mn(II−IV) 16 systems were demonstrated in previous works.6,16 In the present set of calculations, the Mn(III−V) dimer exhibits endothermicities of 0.56 and 0.66 eV versus the employed TyrOH/TyrO• reference system for the oxidation of two MnIVOH moieties to form MnVO, respectively (Figure 2). The subsequent OO bond formation is exothermic by 0.80 eV. From the thermodynamics of water oxidation23 and the potential of the Mn(II−IV) redox couple,23 it is inferred that the OER could be sustained already at this oxidation state. Indeed recent DFT calculations showed that the Mn(II−IV) system has very promising energetics.16 The oxidation of the two hydroxo groups to MnIVO moieties is found to occur approximately at the TyrOH/TyrO• redox couple; i.e., the first step is exothermic by 0.33 eV, the second is endothermic by 0.35 eV, and the subsequent OO bond formation step is endothermic by 0.37 eV (Figure 2). The degree to which this energy profile mimics that of an ideal catalyst can be appreciated by comparing it to iridium oxide, a well-known and very active catalyst for water oxidation.10,13,14 Although the oxidation state responsible for the OO bond formation and oxygen evolution is still debated,24−26 there are good reasons to expect Ir(V) to be the active component.10 Thus, the energy profile of the Ir(III−V) dimer, shown in Figure 2, can be expected to reflect the energetics of the active catalyst rendering the predictions of the mononuclear and binuclear paradigms in agreement. Similar to the Mn(II−IV) dimer the oxidation steps from dihydroxo to dioxo moieties are found to be close to the TyrOH/TyrO• reference system; i.e., oxidation step 1 is exothermic by 0.09 eV and oxidation step 2 endothermic by 0.18 eV, followed by a slightly endothermic OO bond formation step. From the similarities between the energy profiles of Mn(II−IV) and Ir(III−V), Mn(II−IV) could be inferred to represent an excellent OER catalyst. However, in contrast to iridium oxide, which is experimentally found to catalyze the OER at a relatively small overpotential,10,13,14 the onset for water oxidation at manganese oxide is found at a substantially more positive potential than that of the Mn(II−IV) redox couple.27 The inactivity of MnIVO toward oxygen evolution was previously attributed to kinetic control of the OO bond formation step16 in that it may be understood as a two-step reaction sequence comprising the breaking of partial TMO double bonds to form TMO• species followed by the OO bond formation between two TMO • moieties.17 Previous studies6,17 indeed revealed that the sum of the energy required to break the TMO double bond and the energy contribution from the bending of the catalyst may be taken as an upper bound of the activation barrier. Thus, in systems with very
Table 1. Summary of All Relevant Mulliken Spin Densities of the Dioxo Intermediates system
spin
TM1
TM2
O1
O2
(IrVO)2 (MnIVO)2 (MnVO)2 Mn1VO Mn2IVO
singlet septet quintet sextet
−0.82 2.54 2.42 2.50
0.82 2.54 2.42 2.50
−0.96 0.52 −0.48 −0.30
0.96 0.52 −0.50 0.28
emerges as two antiferromagnetic coupled triplet states, i.e., one IrIVO• with two spin up electrons and one with two spindown. For each IrO moiety, the 4d(xz,yz) forms bonding and antibonding π states with oxygen 2p. The local triplet states imply that two electrons occupy degenerate π* orbitals on each IrO site. In principle, the ferromagnetically coupled quintet state would be the proper choice, and indeed, both the ferromagnetic quintet and the antiferromagnetic singlet state of the IrVO dimer were modeled. The difference in the total energies was found to be approximately 0.02 eV. Interestingly, in the case of the ferromagnetically coupled MnIVO dimer, the electron that delocalizes locally between MnIV and O on each MnIVO site occupies an antibonding Mn 3d(xz,yz)O 2p p-orbital. This is consistent with the fact that MnIVO has one valence electron less than Ir V O. When we go to ferromagnetically coupled MnVO dimer, the local antibonding π* states are vacant and the bonding π states become the highest occupied states. Antiferromagnetically coupled electrons between MnV and O emerge owing to the bonding. As a further manifestation to the locality the TMO bonding, it emerges from the spin population that the mixed MnIVO MnVO dimer has one electron in a delocalized π* orbital on the former site, while on the latter site said π* orbital is vacant. This is in agreement with the corresponding homonuclear MnIVO dimer and MnVO dimer, respectively. It is inferred from the Mulliken spins that the lowest energy barrier can be expected for the Ir(III−V) system, whose ground state already displays pure oxygen radical character. In the case of the Mn(III−V) and Mn(II−IV) systems, the formation of a pure radical MnO• dimer, as reflected in the transition from a septet MnIVO to a 11-tet MnIIIO•, is found at 2.3 eV above the ground state for two oxo moieties compared to 0.9 eV from the excitation of two MnVO to form MnIVO•. These energy costs and the endothermicity of the OO bond formation of the binuclear Mn(II−IV), which also adds to the effective barrier 290
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Figure 3. Energy profile of mixed Mn(II−IV)Mn(III−V) dimers embedded into a MgOx(OH)y rig for the dihydroxo−oxo-hydroxo−dioxo −μ-peroxo path is displayed.
for OO bond formation, imply that the Mn(II−IV) system is unreactive toward the OER. This finding is in agreement with experiment.27 The Mn(III−V) system on the other hand allows for OO bond formation at a comparably small activation barrier. A more extensive discussion of this approach to assess the activation energies for 2 TMO forming TMOO TM peroxy bonds can be found in ref 17. It is important to emphasize that the differences in the reactivity between iridium and manganese oxide are purely due to unfavorable kinetics. A possibility to take advantage of the inability of MnIVO to evolve oxygen is to aim for the regeneration of the catalyst by oxidizing a substrate molecule, thus avoiding the oxygen evolution step altogether. This possibility was demonstrated previously for RuO2.7 In that case the addition of Cl− was required to block the OER and instead achieve oxidation of ethylene to oxirane. However, in the case of manganese oxide it is anticipated that the MnIVO moieties are by themselves kinetically protected from undergoing oxygen evolution, and thus manganese oxide is expected to improve on the performance of RuO2. Having seen the differences between the energy profiles of Mn(III−V) and Mn(II−IV) and learned how Mn(II−IV) comes out inactive toward the OER, it is tempting to apply the concept of mixing transition metal oxides with opposite energy profile6,16 to improve the catalytic performance by allowing the system to disproportionate. Although in previous work6,16 complementary transition metal oxides were mixed to achieve such improved performance it is likely that the same effect can also be achieved by mixing Mn(III−V) and Mn(II−IV), suggesting that this is what is actually happening when a manganese oxide catalyst eventually evolves O2. Mixed Mn(II−IV)Mn(III−V) systems may be modeled either by removing a hydrogen from one of the trans standing hydroxo ligands or by removing a hydrogen from a μ-OH bridge. The energy profiles of both systems are shown in Figure 3. Beginning with the distinct Mn(II−IV)Mn(III−V) system, which uses the trans standing oxo and hydroxo groups to facilitate the split oxidation state catalyst, it is interesting to note that the oxidations of the hydroxo moieties to oxo are very similar to those found for the corresponding pure systems. The assumed first oxidation of MnIIIOH to MnIVO is found exothermic by 0.21 eV compared to 0.33 eV found for a pure Mn(II−IV) dimer. The following oxidation of MnIVOH to MnVO is found to be endothermic by 0.77 eV compared to 0.66 eV
found for a pure Mn(III−V) dimer. Owing to the endothermicity of the second oxidation step the OO bond formation is exothermic by only 0.16 eV. The obtained energetics are indeed analogous to what is found for the mixed transition metal oxide systems which show improved activity toward the OER.6,16 Thus, similar to the heterogeneous systems, the mixed oxidation state systems can also be expected to show an improved activity compared to the pure oxidation state systems. Besides the beneficial energetics the kinetics is also expected to be improved because only one expensive MnIVO partial double bond needs to be broken. Additionally, the OO bond formation is thermoneutral, which in turn lowers the effective activation barrier further compared to the pure Mn(II−IV) system. A mixed oxidation state system may also be formed by deprotonating one MnIVOH site while protonating an oxobridge, thus resulting in a MnIVO MnIVOH site. As can be seen from Figure 3, the energetics of such a system is comparable to what was found for the distinct Mn(II−IV) Mn(III−V) system. The oxidation of MnIIIOH to MnIVO is found to be exothermic by 0.17 eV followed by an endothermic oxidation of the MnIVOH moiety to MnVO at 0.55 eV. The slight lowering of the oxidation costs can be explained by a structural buffering effect of the doped (μ-O)(μOH) bridge. The subsequent OO bond formation is found at 0 energy difference. While the OO bond formation is aided by a lowering of the activation barrier upon mixing MnIVO and MnVO sites, the overall potential necessary to drive the OER is defined by the potential of the most expensive oxidation step. Thus, a mixing of transition metal oxides6,16 or oxidation states could be argued to only improve on the kinetics but not lower the required overpotential. This is certainly true assuming that (a) the OER would necessarily have to pass the dioxo intermediate and (b) the binuclear site was isolated. In the case of (a), it should, however, be kept in mind that this intermediate is merely used as a descriptor to extract differences in reactivities. Therefore, any catalytic pathway that employs such a structure constitutes a worst case. In a more realistic situation the lowering of the required overpotential by shortcutting the path from an oxo−hydroxo intermediate to a μ-peroxo intermediate via a concerted H+/e− oxidation step coupled with the OO bond formation may be expected. In the case of (b), utilization of redox processes between the oxidized binuclear site and 291
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surrounding transition metal ions would lower the onset potential for the OER.
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CONCLUSIONS In the present study, water oxidation is taken to produce transition metal oxy intermediates. The lowest energy path for oxygen evolution and catalyst regeneration proceeds by initial breaking of the double-bonds followed by nearest neighbor peroxy bond formation, and subsequent stepwise O 2 desorption.5,6 In the case of MnVO the instability of the double-bond is manifested in a corresponding highly exothermic MnIVOOMnIV peroxy bond formation step. In the case of MnIVO on the other hand, the stabilities of all intermediates are ideal for OER, i.e., minimal overpotential. However, the chemical rate limiting steps corresponding to the breaking of the MnIVO double bonds are significantly more difficult than those in MnVO. In the present study results are presented, which imply that the OER path with fastest conversion rate and smallest overpotential employs a binuclear site with two different oxidation states on adjacent metals, i.e., MnIVO and MnVO. Support for such an interpretation is found in recent experimental studies.28,29 This mixed oxidation state scenario for homobinuclear catalytic sites may be favored by employing systems with high defect concentrations. Indeed, experiments show an increase of activity at manganese oxides upon dissolution of manganese,29 which may result in a higher defect concentration. The effect of mixing oxidation states is understood in the context of recent studies predicting matching heterobinuclear sites.6,16 The latter in turn were taken to explain experimentally observed favorable mixed oxide electrocatalysts for OER. The complexity of the Mn system was contrasted by the OER on an Ir(III−V) binuclear site, where the IrVO intermediates formed at minimal overpotentials exhibit pronounced biradical character, thus readily transforming into the crucial IrIVOOIrIV intermediate owing to the weak double bonding character of IrVO. A possibility to exploit the kinetic hindrance to molecular oxygen desorption, replaces oxygen evolution by substrate (e.g., ethylene) oxidation.
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ASSOCIATED CONTENT
* Supporting Information S
Convergence tests for the k-point set and the cutoff energy. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the European Commission through the FP7 Initial Training Network “ELCAT” (Grant Agreement No. 214936-2).
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REFERENCES
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