The Influence of Inert Ions on the Reactivity of Manganese Oxides

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The Influence of Inert Ions on the Reactivity of Manganese Oxides Michael Busch, Richard Baochang Wang, Anders Hellman, Jan Rossmeisl, and Henrik Grönbeck J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10760 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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The Journal of Physical Chemistry

The Influence of Inert Ions on the Reactivity of Manganese Oxides Michael Busch,∗,† Richard Baochang Wang,† Anders Hellman,† Jan Rossmeisl,‡ and Henrik Grönbeck† †Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden ‡Department of Chemistry, Copenhagen University DK-2100 Copenhagen, Denmark E-mail: [email protected]

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Abstract Inert ion doping is a possible method to modify electrical conductivity and catalytic activity of transition-metal oxide electrocatalysts. Despite the importance of dopants, little is known about the underlying mechanisms for the change of the system properties. We have performed Density Functional Theory calculations to investigate the influence of different valent ions on the O2 evolution reaction activity of β-MnO2 and Mn2 O3 . While Mn2 O3 is unaffected by dopants, β-MnO2 is strongly affected by ions placed in a subsurface position. Doping does not affect the ion charge at the active site but instead it effects the bond character. This is concluded through an analysis of the Density Overlap Regions Indicator and the Density of States showing that the partially covalent nature of the bonds in β-MnO2 is responsible for the changes in the adsorbate binding energies. This mechanism is not active in the mostly ionic Mn2 O3 . These results highlight the need to explicitly consider the detailed bonding situation and to go beyond simple charge transfer considerations when describing doping of transition metal oxide catalysts.

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Introduction Oxidation of water to oxygen is the central anode reaction in electrochemical synthesis of hydrogen. At an industrial scale, the O2 evolution reaction (OER) is catalyzed by Dimensionally Stable Anodes (DSAs) which consist of a mixture of Ti, Ir and Ru oxides painted on titanium. 1,2 Dopants such as Sn or Zn may also be added to further improve the electrical conductivity or catalytic performance. 3–5 Although DSAs combine long-term stability with high efficiency, 1 a drawback is, the need of rare Ru and Ir oxides. An interesting alternative to DSAs are biomimetic catalysts. In enzymes, the OER is catalyzed by a manganese based catalyst and the active site consists of a CaMn4 Ox tetramer. 6,7 Similar to industrial electrocatalysts, Photosystem II (PS II) allows for water oxidation at low overpotentials. Unfortunately, however, the enzyme suffers from a comparably short lifetime. 8 In the quest for alternative OER catalysts based on abundant non-noble metals, a large number of biomimetic molecular 9–12 and solid state electrocatalysts 13–26 have been studied. Based on Density Functional Theory (DFT) calculations 13–17 and experiments, 18–24 manganese 13–15,22,23 and cobalt 13–15,19,24 oxides have been identified as promising candidates. Further improvements on the activity of pure Mn oxides have been observed upon formation of defects. 27 This beneficial effect has been attributed to the formation of bi-nuclear sites with manganese in two different oxidation states. 28 This improvement is, however, a result of the low stability of manganese oxides towards dissolution and therefore in contradiction with the need of a highly stable OER catalyst. A promising path to increased stability of alternative semiconductor materials is doping with inert ions or non-inert ions. 29 An inert ion is not directly affected by the surface chemistry, whereas a non-inert ion is. In addition to modifying the stability of the catalyst, this may also have significant influence on the activity of the material. For example, doping of TiO2 is known to alter the electrochemical properties significantly, converting the oxide into a fair electrocatalyst. 30,31 The changes in activity have been attributed to charge transfer from the dopant to the active site. 30 Similar effects have also been observed for other transition metal oxides 3–5,32–34 and are well known in 3



heterogeneous catalysis. 35 Moreover, experiments indicate, that manganese oxides 36,37 may benefit from doping with different valent inert ions. For example replacing Ca2+ with other alkaline earth metal ions quenches the OER in PS II. 38 Despite the importance of doping for tuning electrochemical properties of oxides, little is known under what circumstances the addition of different valent ions affects the electrocatalytic properties. In this paper, we explore the changes in the chemical properties of doped Mn2 O3 and β-MnO2 by means of first principles calculations. While Mn2 O3 is unaffected by dopants, β-MnO2 is strongly affected by ions placed in a subsurface position. The chemical properties are unaffected by dopants placed at the surface or separated from the active site. This striking difference is taken as a starting point to explore the mechanism behind the doping effect in Mn oxides.

Computational Details All structures were modeled at the Generalized Gradient Approximation (GGA) RPBE level of theory as implemented in the GPAW 39,40 DFT code (build: 0.9.0.8965). A finite difference grid spacing of 0.15 ˚ A was used. Additional HSE06 41,42 single point calculations at the converged RPBE structures were performed with VASP 5.4.1 43–45 in order to extract Density of States (DOS). A plane-wave cut-off of 650 eV was used. Valence electrons were treated explicitly, while the interactions between the valence shell and the core was treated with the Projector Augmented Wavefunctions (PAW) method. 46 For O, the considered valence shell consists of the 2s2 2p4 electrons. For the transition metals, a valence shell consisting of the (n − 1)s2 (n − 1)p6 (n − 1)dx ns2 electrons was treated explicitly and the (n − 1)d10 ns2 np3 electrons were considered in the valence shell for Sb and Bi. n corresponds to the main quantum number. Spin-polarized calculations were performed assuming a high-spin electronic configuration with ferromagnetic coupling between the manganese ions. A similar procedure has been employed successfully for a number of systems. 9,14,15,28 The stability of

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the results with respect to the spin state was checked by explicitly converging the spin for pure and Ta doped β-MnO2 . Equivalent to the calculations with fixed spin, a ferromagnetic high spin situation was obtained. Geometries were relaxed using the BFGS algorithm as implemented in ASE 3.6.0. 47 The structures were considered to be optimized when the largest force was below 0.05 eV/˚ A. Pure and doped Mn oxides were modeled with uncharged unit cells. All redox potentials were calculated following the procedure described by Rossmeisl et al. 48 and energy differences are reported versus the Normal Hydrogen Electrode (NHE) at pH 0. In this procedure, conversion from electronic energies to free energies is achieved by adding adsorbate dependent, zero-point energy and entropy contributions to the electronic binding energies. 48 The corrections are summarized in the Supplementary Information (SI). Bader charges were extracted from RPBE and HSE06 calculations using the Bader charge analysis program (version 1.0) developed by Henkelman et al. 49,50 β-MnO2 has a rutile structure and the calculated lattice parameters (a=4.45 ˚ A and c=2.96 ˚ A) are in good agreement with the experimental lattice parameters of 4.40 ˚ A and 2.88 ˚ A. 51 Since the purpose of this study is to obtain general insights on how defects affect the chemistry at Mn oxides no analysis of the most stable dopant positions was performed. Instead, only doping in cis and trans position was considered to model the influence of different configurations. In case of trans-doping, which corresponds to placing the dopant in a subsurface position, a p(2x1) surface cell was used and the doping was achieved by replacing a bulk Mn4+ with the ion of choice (see Figure 1). Doping in cis-position corresponds to replacing a surface Mn5+ ion. This was studied, employing a p(3x1) surface cell in order to reduce the interactions between the dopants. For both doping positions, a asymmetric two monolayer (ML) slab was used to model the surface (see Figure 1). The slabs were separated by 15 ˚ A of vacuum and terminated on the ”bulk” side by hydroxo moieties in order to preserve the bulk Mn4+ oxidation state. Following previous work, 52 the surface side of the slab was assumed to be fully oxidized and, thus, terminated by oxo moieties. All surface manganese ions are consequently in a formal +V oxidation state.

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α-Mn2 O3 has a highly complex bixbyite crystal structure. To reduce the computational costs for modeling the required large unit cell, we considered a simplified Mn2 O3 system where each unit cell is reduced to two Mn2 O3 units per cell. 52 This model system is constructed to preserve the most important features of Mn2 O3 , namely the octahedral coordination of Mn ions and the +III oxidation state of Mn in the bulk. The model has previously been used successfully to describe the OER activity of Mn2 O3 . 53 Following previous work, 52,53 the 110 surface was modeled with a 2 ML asymmetric slab and a 2x1 surface cell. The slab was terminated to have all ”bulk” manganese ions in a formal oxidation state of +III. The surface was assumed to be fully covered by Mn4+ −O. 52 Both surface and subsurface doping is considered. The slabs were also in this case separated by a vacuum of at least 15 ˚ A. For both oxides, 2x2x1 [p(2x1)] or 2x1x1 [p(3x1)] k-point sets were employed. Bulk Mn2 O3 and β-MnO2 were modeled with a 4x4x6 k-point set. Gas phase H2 and H2 O were calculated in a cube with side length 15 ˚ A using only the gamma point. Comparison with 3 ML and 5 ML slabs of β-MnO2 and Mn2 O3 were performed to test the convergence of the chosen model system. In both cases, good qualitative agreement of the binding energies between the slabs was observed, e.g. the ∗−O binding energies in the pure oxides vary by less than 0.1 eV (* corresponds to a surface site). Similar differences were observed for Ta5+ doped β-MnO2 and Ti4+ doped Mn2 O3 . Our results for the ∗−O binding energy at β-MnO2 and Mn2 O3 are in good agreement with previous calculations (within 0.2 eV). 13 This discrepancy may be a result of differences in the computational methodology. Upon replacing RPBE by HSE06 the ∗−O binding energies for pure and doped β-MnO2 and Mn2 O3 are destabilized by at least 1.3 eV. Despite these quantitative changes identical trends are observed for HSE06 and RPBE.

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Results and Discussion Pure Manganese Oxides The energy profiles of the OER reaction at pure β-MnO2 and Mn2 O3 are shown in Figure 2. Assuming a classical mono-nuclear mechanism, water oxidation proceeds through ∗−OH, ∗−O and ∗−OOH intermediates. 48,54 On β-MnO2 , the oxidation of water to hydroxide (*OH) and further to an oxo (*=O) intermediate at a coordinatively unsaturated site (cus) requires a redox potential of 1.65 eV and 1.44 eV, respectively. The subsequent electrochemical O-O bond formation resulting in a ∗−OOH intermediate, requires 1.73 eV, rendering this step potential determining. By subtracting the thermodynamic OER potential of 1.23 eV, a theoretical overpotential of 0.5 eV is obtained which is in excellent agreement with experiments. 55 Moving to Mn2 O3 , the intermediates are stabilized owing to the lower formal oxidation state of Mn. With the mononuclear mechanism, the oxidation of water to ∗−O via ∗−OH occurs at redox potentials of only 0.86 eV and 1.22 eV. A potential of 2.22 eV is required to form ∗−OOH. This corresponds to a theoretical overpotential of 0.99 eV which renders the possibility to oxidize water through Mn4+ −O unlikely. Indeed, oxidation of Mn4+ −O to Mn5+ −O is possible already at significantly lower potentials. Thus, in agreement with previous work, 28,53 oxidation to Mn5+ −O prior to the onset of water oxidation must be expected.

Doping in trans-Position As a starting point of the discussion of the influence of inert dopants, we considered doping in trans-position, e.g. the dopant was placed such, that it interacts with the O ligand standing opposite of the active site (Figure 1). The effect of doping is studied employing pure β-MnO2 and Mn2 O3 as references. The chosen dopants have 2 to 6 valence electrons. According to the electrochemical potentials of the metals, 56 an oxidation to the highest possible oxidation 7



states must be expected under OER conditions. Since none of the considered ions possess any redox chemistry in the potentials relevant for water oxidation, 56 they are expected to be inert, i.e. the oxidation state should not be affected by the OER surface chemistry. For the sake of simplicity we will refer to these ions by their highest formal oxidation state. As a direct participation of the dopants in the water oxidation mechanism is unlikely, the activity may only be influenced indirectly by geometric and ligand effects. 32,57 Ligand effects are a result of differences in the valency between dopant and the substituted manganese ion whereas geometric effects are introduced by differences in the ion sizes between dopants and manganese or a lattice mismatch between the support and the catalyst.

β-MnO2 The ligand effects can, based on the formal oxidation state of the dopant, be divided into two general situations. Dopants may either be of higher or lower valency compared to the substituted ion. Assuming subsurface doping, a bulk Mn4+ ion is replaced by metal ions with a formal oxidation state of +V and +VI to model the influence of a higher valent dopant. For the considered metals, the oxidation states are V5+ , Ta5+ , Sb5+ , Bi5+ , Mo6+ and W6+ , respectively. Doping with lower valent ions is achieved by replacing Mn4+ with Mg2+ and Sc3+ . Additionally substitution of Mn4+ by iso-valent +IV ions is considered in order to reveal any effects resulting from the pure substitution of manganese by an inert ion of similar ion size. For this purpose, Ti4+ and Hf4+ are considered. The results are summarized in Figure 3. The adsorption energies of the doped β-MnO2 oxides are ordered according to the oxidation state of the dopant. Very similar trends are observed for all three OER intermediates. Substitution of a Mn4+ by a same valent Hf4+ or Ti4+ ion results in only minor changes in the adsorption energies as compared to pure β-MnO2 (see Figure 3) and the activity towards water oxidation should remain virtually unchanged (see Figure 4). This is indeed expected considering the robustness of the redox properties of manganese oxides despite large differences in the embedding ranging from

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molecular catalysts 9 to MgOx (OH)y . 14,15,28 Furthermore, geometric effects introduced by minor differences in the ion size of Mn4+ , Ti4+ and Hf4+ seem to have minor influence on the electrocatalytic properties of β-MnO2 . This finding is in good agreement with experiments 37 that have compared the performance of Ca2+ doped birnessite, marokite and α-Mn2 O3 . Birnessite (or δ-MnO2 ) has a layered structure which is equivalent to brucite with octahedrally coordinated Mn4+ ions. Marokite has a spinel structure with a CaMn2 O4 stoichiometry. Thus, the Ca2+ ions are placed in +II positions. Experiments comparing the activity of the three oxides towards OER showed that only in the case of birnessites improvements in the activity are obtained whereas marokite and α-Mn2 O3 showed an equally low activity. 37 This is in line with our results showing that the sole presences of a different valent ion does not affect the activity of Mn oxides. A significant destabilization of all intermediates compared to pure β-MnO2 is predicted upon doping with with Sc3+ and Mg2+ (Figure 3). For the Mg2+ doped system, the ∗−OH and ∗−O intermediates are shifted from 1.59 eV and 3.09 eV for the pure oxide to 1.98 eV and 3.59 eV, respectively. A smaller destabilization is observed when doping with Sc3+ . Interestingly, the OOH intermediate is not as destabilized by lower valent dopants as one could expect based on the robust linear scaling relationships between OH and OOH. 13,58 Instead, the calculated adsorption energies are only 4.98 eV for the Mg2+ and 4.89 eV for the Sc3+ doped systems, which is clearly smaller than the 5.28 eV (Mg2+ ) and 5.12 eV (Sc3+ ) predicted by linear scaling relationships. 13,58 These findings indicate a change of the electronic structure of OOH from a peroxide to a weaker bound protonated superoxide. This interpretation is supported by an increase of the ∗−OOH bond length from 1.99 ˚ A for the undoped system to 2.04 ˚ A (Sc3+ ) and 2.09 ˚ A (Mg2+ ), respectively. Doping by higher valent ions is achieved by using V5+ , Ta5+ , Sb5+ and Bi5+ as +V as well as Mo6+ and W6+ as +VI substituents. The influence of higher valent ions is studied employing Ta5+ and W6+ as test cases before generalizing the obtained results for the other considered +V and +VI dopants. In contrast to doping with lower valent ions, the interme-

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diates are stabilized upon replacement of Mn4+ with higher valent ions (see Figure 3). For Ta5+ doped β-MnO2 , the ∗−OH intermediate is stabilized by approximately 0.3 eV compared to the pure oxide. An additional stabilization by 0.16 eV is obtained for the W6+ doped system. The oxo-adsorption energy is reduced from 3.09 eV for the pure system to 2.64 eV for Ta5+ and further to 2.5 eV for W6+ . For the OOH intermediate, the adsorption energies decrease from 4.82 eV to 4.5 eV in both cases. Similar to the +IV doped systems, the activity is expected to remain constant, provided the formal oxidation state of the cation remains the same. This is indeed found for all +V ions. Typically, the overpotentials are in the order of 0.6 eV for a mono-nuclear water oxidation mechanism. An interesting deviation is observed for Mo. In contrast to the other dopants, it is no longer completely inert but affected by the changes of the chemical environment induced by different adsorbates. For example, the ∗−OH hydroxo and ∗−OOH peroxo intermediates, display adsorption energies, similar to the +V doped systems, of 1.33 eV and 4.50 eV, respectively. Upon oxidizing OH to a ∗−O moiety, this changes and the Mo doped system becomes similar to a +VI ion doped β-MnO2 . At this stage, the adsorption energy is 2.51 eV, which compares well to the 2.5 eV found for W6+ but is significantly lower than the 2.64 eV to 2.73 eV found for typical +V ions. The OER activity of the pure and doped Mn oxides can be compared in a volcano plot (see Figure 4). Typically, β-MnO2 is found close to the top of the volcano with an almost ideal overpotential of 0.5 eV. Ti4+ and Hf4+ appear close to pure β-MnO2 . The destabilization upon doping with lower valent Mg2+ and Sc3+ ions shifts β-MnO2 to the weak binding side of the volcano, rendering the oxidation of water to ∗−O potential determining. The overpotentials are increased to 0.75 eV upon doping with Mg2+ and 0.59 eV when inserting Sc3+ . Similarly, no improvements in terms of a lower overpotential are observed upon doping with +V and +VI ions due to the stabilization of the already too stable intermediates. Instead, the overpotential increases to 0.62 eV and 0.60 eV in Ta5+ and W6+ doped β-MnO2 , respectively. It is noteworthy, that the reduction in activity is found only, when assuming a

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mono-nuclear mechanism. Considering a bi-functional 53,59,60 or bi-nuclear mechanism, 9,15,61 the top of the volcano is shifted to an O binding energy of 2.46 eV for an optimal O donor or H acceptor group. Under these circumstances, +V and +VI doped system are positioned almost ideally while +II and +III doped β-MnO2 is not affected by the change in mechanism. Mn2 O3 In Mn2 O3 , the OER proceeds through Mn ions in formal oxidation states varying between Mn2+ −OH2 and Mn4+ −O. While Mn4+ −O is too stable to facilitate O-O bond formation at reasonable overpotentials or rates, 13,28,52 it might benefit from a destabilization through doping with lower valent ions. The influence of different valent ions is summarized in Figure 3. The choice of dopants is restricted to one candidate for each different valent ion as generally only the formal oxidation state of the dopant is of relevance, as shown above. Interestingly, the strong dopant effect observed for β-MnO2 is absent in Mn2 O3 and the binding energies remain virtually unchanged. This behavior is independent of the choice of dopant and is observed for all considered adsorbates. As a result, all considered Mn2 O3 systems are placed far from the top of the volcano at the strong binding side (Figure 4).

Analysis of the Dopant Effect β-MnO2 The very different behavior of the two Mn oxides offers a framework to explore the mechanism behind the doping effect in detail. Previous studies on TiO2 indicated that the changes in binding energies upon doping with different valent cations originate from a charge transfer to the active site. 30 A similar effect may also be the origin of the doping effect in β-MnO2 as substitution of an Mn ion in β-MnO2 by a lower valent ion formally results in the removal of 1 (Sc3+ ) or 2 (Mg2+ ) electrons from the valence band. To test the possibility of a charge transfer mechanism in β-MnO2 , we analyzed the changes in the Bader charges at the active Mn site. In a simple charge transfer picture, lower valent ion doping should result in an 11



increase of the formal Mn charge in order to compensate for the lower valent dopant, thus destabilizing the adsorbates. Analysis of the Bader charges at the active Mn site, however, indicates only minor changes (see Figure 5). Similarly, the changes in Bader charges upon doping with higher valent ions are too small to support a charge transfer to the active site. Although hybrid functionals have been shown to favor charge separation, 62 similar trends were observed with the HSE06 functional. Accordingly, a different mechanism must be the origin of the ligand effect in β-MnO2 . Assuming a more covalent bonding situation, changes in the adsorbate Mn−O bond induced by the dopant may be the origin of ligand effects. To explore this mechanism, we analyzed the Partial Density of States (PDOS) and the Density Overlap Regions Indicator (DORI) 63 for the *=O adsorbate case. Originally, DORI was developed to distinguish between covalent and non-covalent intermolecular interactions. 63 A similar discrimination in bonding patterns also exists in solids, as solids may display covalent, metallic or ionic bonds. Ionic and metallic bonds are, similar to non-directed intermolecular interactions. A proof of principle comparing purely ionic and metallic bonding with covalent bonding situation is shown in the SI. Whereas a DORI analysis does not distinguish metallic from ionic bonds, a clear difference to a covalent bonds is present. In the DORI, non-covalent metallic and ionic bonds appear as a fully smeared out overlap indicator while a covalent bond is indicated by a clearly localized overlap indicator. The differences in the bonding pattern can be tracked by considering qualitative changes in the DORI between the bulk O and the surface Mn ions (see Figure 6a). Comparing pure and Sc3+ doped β-MnO2 reveals significant differences in this bond. For the pure system, an intermediate situation with a somewhat localized DORI is observed. Upon doping with Sc3+ , the bonding region between Mn and O is localized further. Similar changes are also observed in the PDOS of the active Mn and the trans-standing O (Figure 6b). In pure β-MnO2 , most of the O 2p states are distributed in the energy range between -2 eV to 0 eV. Upon doping with Sc, most of the O 2p states are found between -7 eV to -5 eV. Additionally, the antibonding π ∗ orbital formed between O 2p and Mn dxz/yz

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is shifted above the Fermi level. This corresponds to the stabilization of bonding states between O 2p and Mn 3d, which in turn correlates to a stronger Mn−O bond. The opposite is observed when doping with Ta5+ . Here, Mn t2g π ∗ antibonding states become occupied and are located just below the Fermi level. This in turn corresponds to the purely ionic bonding observed in DORI and to a weakening of the Mn−O bond. These changes in the chemical bonding are accompanied by an elongation of the subsurface Mn/Sc-O bond from 1.89 ˚ A for pure β-MnO2 to 2.11 ˚ A for the Sc3+ doped oxide. This indicates a weakening of the Sc−O bond as compared to the pure oxide. Similarly, the bond length between the bulk-O and the surface-Mn ion is reduced from 2.29 ˚ A to 2.12 ˚ A when doping with a +III ion. Upon stabilizing Mn−O through doping of β-MnO2 with higher valent ions, DORI indicates opposite changes in the bonding situation. Considering the Ta5+ doped case, the Ta−O bond is shifted towards a more covalent bond indicated by a stronger localization of the overlap region. This strengthening of the Mn-O bond is accompanied by a weakening of the Ta-O bond indicated by an increase of the bond length by 0.1 ˚ A to 2.44 ˚ A. The opposite behavior is observed in the bonding between active Mn and adsorbate O. Assuming the simple Angular Overlap Model 64 (AOM), the d-orbitals are split into a set of e∗g orbitals, which are σ ∗ antibonding, and non-bonding t2g orbitals. In the presence of π-donor or acceptor ligands, the t2g orbitals may split into π bonding and antibonding contributions. According to the PDOS shown in Figure 7b, the Mn−O bond in pure β-MnO2 is mainly characterized by the overlapping of dxz and dyz orbitals with O 2p states in the valence band region from -2.5 eV to -1.7 eV and between the Fermi level and 2 eV in the conduction band region. In the AOM, this corresponds to a splitting of dxz and dyz into π bonding and π ∗ antibonding contributions, occupied by one electron. The remaining electron of the formal Mn5+ species is in this case placed in the nonbonding dxy orbital. The antibonding σ ∗ orbital is placed in the conduction band region between 2 eV and 4 eV. In light of the AOM analysis, this corresponds to a weak double bond formed between Mn and O with significant contributions from the Mn O radical mesomeric structure.

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Upon doping β-MnO2 with Sc3+ , the bonding patterns change significantly (Figure 7a). Under these conditions, a hole is created which, according to an analysis of the differential charge densities, is localized at the surface bridging O (see SI). This hole strongly affects the electron density between the surface Mn and the cus-O by generating an increased density of O 2p states at the Fermi level together with a decreased overlap between the O 2p states and the Mn dxz orbitals. This corresponds to a hole placed at the cus-O, or chemically speaking, the splitting of the t2g orbitals collapses and the dxz and dyz become mostly nonbonding. Thus, an unstable singly bonded Mn O radical with negligible contributions from the Mn−O double bond mesomeric structure is formed. In the Ta5+ doped case, the excess electron introduced through a +V ion appears to be delocalized between Mn, the cus-O and the trans-standing O as indicated by the differential charge density shown in the supplementary information. From the PDOS (Figure 7c), it is clear that the dxz orbitals overlap with O 2p states. These π ∗ antibonding contributions to the resulting Mn−O double bond are placed just below the Fermi level. The dyz /py contributions to the double bond are similar to pure β-MnO2 placed far above the Fermi level. Accordingly, there is no apparent charge transfer from Ta5+ into the Mn t2g orbitals which shows that the changes in binding energies should not be attributed to a charge transfer. Instead, the observed charge transfer pattern may indicate a strengthening of the Mn−O double bond through an increase of the π-donor strength of the O adsorbate. Mn2 O3 In contrast to β-MnO2 , no changes in the binding energies of the considered adsorbates are observed for Mn2 O3 . This very different behavior correlates with significant changes in the bonding situation. Our DORI analysis indicates a purely ionic bonding in the bulk as shown by a completely delocalized overlap region indicator between the surface Mn and the transstanding O (see Figure 6c). No changes in the DORI for the bulk Mn−O bond are observed upon doping with different valent ions. This corresponds well with what is observed in the

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PDOS of the bulk O and active Mn ions shown in Figure 6d. In the PDOS O 2p states appear from -8 eV to the Fermi level. Comparing the PDOS obtained for Ti4+ and Mg2+ doped Mn2 O3 , identical bonding patterns are observed. This is perfectly in line with what is observed in DORI and the lack of dopant influence observed in the energetics. Only when comparing pure and doped Mn2 O3 additional Mn t2g states are shifted below the Fermi level. Since these states do not overlap with the O 2p band, they can be considered non-bonding with respect to the bond between the trans standing O and Mn. Accordingly, this difference does, in agreement with the energetics and the DORI analysis, not influence the overall bond strength. Nevertheless, significant changes are observed for the Mn−Obulk bond lengths. Substitution of Mn3+ with Ti4+ results in an increase of this bond length by 0.11 ˚ A to 2.58 ˚ A while it decreases by 0.24 ˚ A to 2.23 ˚ A upon doping with Mg2+ . However, due to the lack of covalent bonding, these changes do not affect the chemistry at the surface Mn site. Thus, a trans-influence as found for β-MnO2 can be excluded, which is in line with the lack of variations in the binding energies. Instead, a charge transfer of the excess hole or electron is induced by the different valent dopant ion. In contrast to TiO2 , 30 this charge transfer does not affect the binding energies. Indeed, the excess electron appears to be delocalized in the Ti4+ doping case between the Mn ions in the second layer within the xy plane (see supplementary information). In the PDOS shown in Figure 8, this corresponds to the occupation of states just above the Fermi-level. These states are delocalized between the subsurface O, the active Mn and the adsorbate. Accordingly, there is no variation in the binding energies. A hole, placed in the adsorbate Mn−O bond, appears when doping with lower valent ions. The PDOS indicates that the hole is placed in the dxz/yz /px/y states. According to the AOM analysis, these states are π ∗ antibonding. Furthermore, comparing the PDOS of the Mg2+ doped system with that of pure Mn2 O3 indicates an additional downwards shift of the remaining occupied dxz/yz /px/y antibonding π ∗ states. Similarly, a destabilization of the higher lying e∗g , which are σ ∗ antibonding, is observed. This shift indicates a decreased band

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dispersion, which is a general sign of a weaker bond, thus, compensating the depopulation of the antibonding π ∗ states. Accordingly the binding energies do not change.

Validating the trans-Influence So far the only indications for the chemical bonding origin of the dopant induced changes in β-MnO2 is the qualitative DORI and PDOS analysis. A possibility to verify our hypothesis chemically, is doping β-MnO2 in cis-position. Indeed, the origin of the differences in the adsorbate binding energies may have significant impact on the circumstances under which doping with different valent ions can affect the adsorbate binding energies. The facile changes in the bonding between the trans-standing O and Mn, resulting in very different adsorbate binding energies, have also been observed in complex chemistry and are known as ”trans-influence”. 65–68 The trans-influence is a qualitative concept describing the competition between the adsorbate and a trans-standing ligand for the same set of orbitals. Correspondingly, strengthening the covalent bond between the trans-standing ligand and the central ion results in a weakening of the bond between the adsorbate and the central ion and vice versa (see Figure 9). In complex chemistry, this effect is generally induced by exchanging ligands. In the present case, the oxo-ligand remains unchanged whereas the bond strength between the trans-standing O and the active site varies due to differences in the interactions with the dopant. No comparable changes occur if the manipulation is performed at a cis-standing ligand. Thus, considering doping in cis-position offers an ideal test to verify the origin of the doping effect observed in β-MnO2 . In Figure 10, the influence of dopants placed in cis-position is summarized. In contrast to the trans-doped situation, only minor changes in the binding energies are observed. Similarly, no changes in the adsorption strength is apparent for a Ta5+ or Sc3+ placed in cis position in a 2x1 unit cell or placed subsurface in a 4x1 unit cell with no direct connection to the active site. This confirms that the differences in the binding energy are induced by subtle changes in a mostly covalent Mn-O bond as only under these conditions the position of the dopant is of

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relevance. As shown for TiO2 , 30,31 doping effects owing to charge transfer are independent of the position. Similar to β-MnO2 , no changes in the adsorbate binding energies are observed upon doping Mn2 O3 in cis-position. This is not surprising as the charge transfer is again buffered by the surrounding redox active Mn ions.

Conclusions In summary, we have investigated the influence of inert dopants on the stability of the OER intermediates at β-MnO2 and Mn2 O3 . Significant changes in the adsorbate binding energies were observed upon doping β-MnO2 opposite to the active site. Higher valent ions stabilize the considered adsorbates whereas lower valent ions result in a destabilization. Dopants with the same valency as Mn have only a minor effect on the binding energies. Adsorption on Mn2 O3 is independent from doping. A detailed analysis of the charge distribution in doped Mn oxides reveals, that opposite to TiO2 , 30 a charge transfer is not the origin of the doping effect. Instead, the surrounding Mn ions act as a ”redox buffer”, mitigating the effect of the additional holes or electrons introduced through doping. The detailed analysis of the electronic structure of β-MnO2 with DORI and PDOS reveals changes in the chemical bond between the bulk O and the active Mn site upon doping. These changes affect the strength of the bond between Mn and the adsorbate and is the origin of the changes in the adsorbate binding energies. Mn2 O3 lacks comparable variations in the Mn−Obulk bond, which explains the absence of a dopant effect. A direct consequence of this ”transinfluence” 65–68 is the fact that only dopants placed opposite to the adsorbate can influence the adsorption energies in β-MnO2 . These results highlight the need to go beyond simple charge transfer considerations when describing the changes in binding energies through inert ion doping. A promising possibility to take advantage of the trans-influence in β-MnO2 could be the use of a supporting oxide that could steer the placement of the ”dopant” into the trans-position.

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Supporting Information Available The supporting information contains the adsorption energies of the relevant OER intermediates, a summary of Bader charges and Density of States. Additionally, a comparison of DORIs obtained for systems display typical ionic, metallic and covalent bonds is presented.

Acknowledgement We acknowledge financial support from the Swedish Research Council through the Röntgen˚ Angström project In-situ High Energy X-ray Diffraction from Electrochemical Interfaces. The calculations were performed at C3SE (Göteborg) through a SNIC grant.

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Figure 1: The employed β-MnO2 and Mn2 O3 model systems. Atomic color-code: violet (Mn); green (trans standing dopant); blue (cis standing dopant); red (O); white (H).

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Figure 2: The energy profiles of pure β-MnO2 (solid black line) and Mn2 O3 (dashed-dotted green line). A mono-nuclear OER mechanism at cus is assumed. The ideal case (dotted line) corresponds to a situation without overpotential.

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Figure 3: The influence of inert ions on the binding energies. All systems assume doping in trans-position. a) ∗−OH/β-MnO2 , b) ∗−O/β-MnO2 , c) ∗−OOH/β-MnO2 , d)∗−OH/Mn2 O3 , e) ∗−O/Mn2 O3 and f) ∗−OOH/Mn2 O3 .

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Figure 4: OER volcano plot summarizing the activity of doped Mn oxides. Red - Mn2 O3 ; black - β-MnO2

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Figure 5: Bader charges at the Mn adsorption site in β-MnO2 . All Bader charges were obtained at the RPBE level of theory. The straight line is added as a guide for the eye.

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Figure 6: Comparison of Density Overlap Regions Indicators (DORIs) and PDOS for the Mnsurface −Otrans bonds for pure and doped β-MnO2 and Mn2 O3 . a) DORI for β-MnO2 b) PDOS for β-MnO2 c) DORI for Mn2 O3 d) PDOS for Mn2 O3

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Figure 7: Partial Density of States (PDOS) for Sc3+ (a), pure (b) and Ta5+ (c) doped β-MnO2 assuming an O-covered surface. The anticipated ligand field splittings are shown.

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Figure 8: Partial Density of States (PDOS) for Mg2+ (a), pure (b) and Ti4+ (c) doped Mn2 O3 assuming an O-covered surface. The anticipated ligand field splittings are shown.

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Figure 9: Schematic representation of the trans-influence.

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Figure 10: The influence of cis-doping on the ∗−O binding energies at β-MnO2 (a) and Mn2 O3 (b).

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TOC Graphic

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The Influence of Inert Ions on the Reactivity of Manganese Oxides

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