Article pubs.acs.org/JPCC
Water Oxidation Mechanism on Alkaline-Earth-Cation Containing Birnessite-Like Manganese Oxides Jingxiu Yang,†,‡ Hongyu An,†,‡ Xin Zhou,*,† and Can Li*,† †
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, 116023, Dalian, P. R. China ‡ University of Chinese Academy of Sciences, No. 19A Yuquan Road, 100049, Beijing, P. R. China S Supporting Information *
ABSTRACT: The CaMn4O5 complex in Photosystem II (PSII) is known for its high efficiency in catalyzing the water oxidation reaction. A layered birnessite-like manganese oxide with intercalated metal cations has been prepared and reported to mimic the complex. However, the water oxidation reaction on the Ca2+-containing birnessite is far less active than the oxygen evolution complex (OEC). To determine the reason, the water oxidation mechanism has been studied on Cabirnessite by the density functional theory. It is found that the OO bond formation between a μ-oxo and an OH− occurs after the neighboring Mn (III) is oxidized to Mn (IV). Compared to the CaMn4O5 in PSII, the μ-oxo on the surface of Ca-birnessite is less flexible and therefore less active. Additionally, the overpotential for the water oxidation reaction depends on the cation and increases in the order of Ca2+ < Sr2+< Mg2+. Therefore, flexible μ-oxo and appropriate cations are predicted to be important factors governing the catalytic activity of manganese oxide catalysts.
1. INTRODUCTION The water oxidation quantum yield in Photosystem II (PSII) is much higher than that in current artificial photosynthesis systems. Recent studies revealed that the atomic structure of the oxygen evolution complex (OEC) in PSII is an oxo-bridged CaMn4O5 cluster primarily containing Mn (III) and Mn (IV) ions.1−4 This special CaMn4O5 structure has been regarded as the core of efficient water oxidation. Confirmed by EPR,4,5 EXAFS,3,6 DFT,7,8 etc., it is widely recognized that Mn (III) is oxidized stepwise to Mn (IV) until the OO bond is formed. To improve artificial photocatalysts, one idea pursued by researchers was to mimic the OEC.9−15 Recently, a biomimetic manganese oxide catalyst was prepared and characterized as a layered birnessite containing Ca2+ and trace Na+ ions between neighboring layers by XRD and XAS techniques. 16,17 Manganese oxide in the birnessite phase has also been proven to be an effective catalytic structure for the electrolysis of water.18 On the basis of the known experimental evidence, it appears that Mn (III, IV) ions in the catalytic structure are pertinent to the water oxidation.16−18 However, there is a huge gap of at least 4 orders of magnitude between the activity of Cabirnessite and PSII in typical microalgae.16,19 Understanding why there is such a large difference between the catalysis of natural and artificial photosynthesis is highly desired. Another interesting phenomenon in birnessite is that its catalytic activity depends on the intercalated alkaline earth cations. The trend of the catalytic activity can be formulated as Ca2+> Sr2+ > Mg2+.16 In PSII, besides Ca2+, only Sr2+© XXXX American Chemical Society
reconstructed PSII could recover approximately 50% steady activity.20,21 Mg2+ is believed to be incapable of replacing Ca2+ because no effect is found with Mg2+.22 Whether or not Ca2+ functions in the same manner for the OEC and birnessite, understanding the role of the metal cation is helpful for the design of artificial photocatalysts for the water oxidation reaction. In this work, we have found that the oxidation of Mn (III) to Mn (IV) occurs before the OO bond formation. Cabirnessite is less active than the OEC because its overpotential is 0.26 V higher than that of the OEC. In addition, the replacement of Ca2+ with Mg2+ or Sr2+ results in a higher energy uphill in the fourth electron−proton transfer (EPT) step.
2. METHOD AND MODELS 2.1. Bulk and Surface Model. By careful comparison, we have found that the reported Mg2+-containing-birnessite catalyst16 is quite similar to a previously reported Mg-rich birnessite.23 The crystal structure of the synthetic Mg-rich birnessite has been revealed by electron diffraction and the Rietveld method.23 According to the literature,23 the unit formula is Mg0.27Mn1.87O4·1.6H2O, and the unit-cell parameters are a = 5.049 Å, b = 2.845 Å, c = 7.051 Å, and β = 96.65°. The Received: June 23, 2015 Revised: July 24, 2015
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DOI: 10.1021/acs.jpcc.5b05989 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Table 1. Optimized Lattice Parameters of Mg-birn, Ca-birn, and Sr-birn Mg-Birn (exp)a MgMn8O16·8H2O CaMn8O16·8H2O SrMn8O16·8H2O a
a (Å)
b (Å)
c (Å)
α
β
γ
10.10 10.19 10.25 10.24
5.69 5.75 5.73 5.76
7.05 7.06 7.12 7.15
90.00 89.87 91.75 91.69
96.63 97.25 96.64 97.45
90.00 89.42 89.71 89.95
The experimental data for Mg-birn is from ref 23.
structure for birnessites containing other cations, such as Cs+, K+, Ba2+, and Na+, are also similar.24,25 Therefore, the unit cell of MIIAMn8O16·8H2O (MIIA = Mg, Ca, and Sr) was built to simulate the three birnessites based on the reported crystal structure of Mg-rich birnessite. Different birnessite-like catalysts with Mg2+, Ca2+, and Sr2+ ions between the layers are denoted in this work as Mg-birn, Ca-birn, and Sr-birn, respectively. The calculation of the water oxidation reaction was performed on the (001) surface of Mg-birn, Ca-birn, and Srbirn, because birnessite is layered vertical to the (001) plane. The long distance between two layers (∼7 Å) has decreased the possibility of interlayer formation of OO bond, so we just focus on one layer constructed by MnO6 octahedrons, metal cations, and water molecules. Three water molecules are added on the surface to form six coordinated Ca2+. According to the reported XANES,16 the mean oxidation states of manganese for Mg-birn, Ca-birn, and Sr-birn are 3.5−3.6, lower than the mean Mn valence (3.75) in the bulk model. However, the stoichiometric ratio of MIIA to Mn in the bulk model is in accordance with the experimental value. This leads us to the deduction that there may be oxygen defects or additional protons on the surface. We considered two possible situations in our work and proposed the most stable configuration for the surface. 2.2. Computational Method. The water oxidation reaction (2H2O → O2 + 4H+ + 4e−) involves the transfer of four electrons accompanied by four protons. It is assumed that electrons are transferred stepwise, accompanied by a proton removal. In this work, a surface with positive or negative charge would give a different energy value depending on the depth of the vacuum layer. Therefore, the ΔG for electron transfer or proton transfer cannot be evaluated. All the free energy barriers in this work are thermodynamical barriers due to mentioned difficulty to determine the transition state. To address the EPT steps, a special thermodynamic scheme was adopted (for detail, please check Supporting Information (SI)), which has been used in the investigations of photocatalytic or electrocatalytic mechanisms.26−28 All the computations in this work were performed by VASP codes.29,30 The optimizations of the bulk and the surface model were performed with the generalized gradient approximation plus Hubbard-U to DFT (GGA+U). 31 The projector augmented wave (PAW) pseudopotentials and the Perdew− Burke−Ernzerhof (PBE) functional were used as implemented in VASP. It has been reported that Ueff = 3.9 is appropriate for manganese oxides,32,33 and therefore, this value was adopted in this work. As shown in Table S1, the energy correction of 1.11 eV per O2 was chosen to match the experimental formation free energy of water.34 The chosen chemical potential for oxygen also makes the calculated enthalpies match the experimental value of simple Li2O and Na2O.34 The unit cell of CaMn8O16 was fully relaxed and optimized using 3 × 7 × 5 Monkhorst− Pack type of k-points sampling and a cutoff energy of 400 eV
for the basis function. Ionic relaxation could only stop if the force was less than 0.01 eV/Å. On the basis of the optimized bulk structure, the layered surface (001) was presented as slab models repeated periodically with a vacuum region of 15 Å between repeated slabs. All the atoms in the slabs were allowed to move. The 3 × 7 × 1 Monkhorst−Pack type of k-points samplings and a cutoff energy of 400 eV were used for the surface.
3. RESULTS AND DISCUSSION 3.1. Geometry Structure. As displayed in Table 1, the difference between the calculated and experimental values for the lattice of Mg-birn is within 1%, indicating that the optimized model could describe the catalyst precisely. The c parameter for each model (7.06, 7.12, and 7.15 Å), as long as the interlayer distance, is also close to what is observed (7.0− 7.1 Å) from the XRD. With different intercalated cations, the lattice of the corresponding unit cell does not change much. Besides the lattice parameters, Table 2 compares the important distances in our model to the reported EXAFS Table 2. Important Distances of the Optimized Bulk and Surface Structures for Ca-birn and the Data from EXAFS MnCa MnMn (s)a MnMn(l)a MnOa
EXAFS (Å)b
bulk (Å)
surface (Å)
3.30 2.88/2.98 4.99 1.90/2.28
3.09 2.88/2.99 5.01 1.93/2.22
3.36 2.87/3.03 5.04 1.93/2.32
a
the value of the denoted distance for bulk and surface is the average value in the corresponding unit cell, respectively. bthe experimental EXAFS data are from ref 16.
data. The MnMn distance for the bulk and surface models is similar and basically matched to the experimental value. The MnO and MnCa distances for the surface are closer to the corresponding EXAFS values than those for the bulk. Therefore, the given surface model can generally represent the birnessite catalyst. The optimized structures of bulk Mg-birn, Ca-birn, and Srbirn are shown in Figure 1 (a), (b), and (c). Judging by the obvious Jahn−Teller distortion, the two Mn atoms in a row are Mn (III), as noted. This deduction is also supported by their relatively large magnetic moments of approximately 3.8 μB. (The magnetic moment of each Mn in the bulk model is listed in the SI, Table S2.) Comparing birnessites containing Mg2+, Ca2+, and Sr2+, the position of Mg2+ is biased from that of Ca2+ or Sr2+. Moreover, the number of water molecules coordinated to Mg2+, Ca2+ and Sr2+ is 3, 4, and 4, respectively. Mg2+ in birnessite is quite different from Ca2+ or Sr2+, as mentioned above. Three of the four water molecules around Ca2+ are strongly attracted to the Ca2+, while the other is weakly bound to the MnO6 octahedron, as shown in Figure 1(b). This is why we B
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Figure 1. Optimized unit cells of (a) Mg-birn, (b) Ca-birn, and (c) Sr-birn. The Mn (III) atoms have been noted by blue circles.
Figure 2. Cross view (a) and the top view (without water) (b) of the stable (001) surface of Ca-birn.
added three coordinated water molecules around Ca2+ in our surface model. Clearly, except for Ca2+, there is no other site for the water to which to coordinate. For the surface of Ca-birn, it has been found that the structure with two additional protons is more stable than that with an oxygen defect, energetically. After the search of the protonation sites, the surface structure shown in Figure 2 is proven to be the most stable one. On the Ca-birn surface, the four Mn atoms around Ca2+ are five-coordinated Mn (III), labeled as Mn1, Mn2, Mn3, and Mn4 in Figure 2, while the other four Mn ions are Mn (IV). Due to the lower average valence of Mn and the additional protons in the surface model, the MnO and MnCa distances for the surface are longer than those for the bulk. 3.2. Water Oxidation Mechanism. With the assumption of four EPT steps to achieve O2 evolution, the mechanism of the water oxidation reaction on the surface of Ca-birn is shown in Figure 3. With U = 1.23 V, the first, second, and third EPT steps are thermodynamically accessible, whereas, the fourth step is endothermic with a total overpotential of 0.41 V. The formation of the O22− state involved in the fourth EPT has to overcome an additional energy barrier of 0.30 eV, although the subsequent O2 formation (state 6) and release (state 1) are totally exothermic during the fourth step, as shown in inset of Figure 3. Therefore, the fourth step is the rate-determining step. If the potential applied to this system is greater than 1.94 V (η ≥ 0.71 V), all steps are thermodynamically accessible. In state 1, the lengths of the MnO bonds along the Jahn− Teller axis of Mn1 (III) are 2.266 and 2.411 Å as denoted, and the magnetic moment of Mn1 is about 3.8 μB. After the first EPT step, Mn1 is oxidized from Mn (III) (blue) to Mn (IV)
(purple), accompanied by a proton release (yellow) from the previously uncoordinated OH−. If another H+ instead of the yellow one is released, the valence of Mn1 will not change. Only with the denoted proton removed, the magnetic moment of Mn1 decreases from 3.8 to 3.2 μB, and the Mn1O bonds along the Jahn−Teller axis are also shortened to 2.051 and 1.923 Å. As the valence of Mn1 increases, the electrostatic interaction between the μ-oxo and Ca2+ decreases. Following the trend of Mn1, the second and third EPT occur for Mn3 and Mn2, as shown in state 2−4, respectively. When Mn3 is oxidized, there is no proton bound to the uncoordinated O as shown in state 2. After a series of trials, it is more favorable that the proton (yellow) which is H-bonded to the O is released. After the first three EPT steps, state 4 is achieved. The calculated ΔG for the oxidation from Mn (III) to Mn (IV) is on average 1.09 eV, which is close to the experimental standard value (1.01 eV). In state 4, the only Mn (III) is Mn4. The three CaO (μoxo) bonds are already longer than those in state 1. The oxidation of Mn4 does not help form the OO bond, but rather separates Ca2+ from the MnO6 layer. In addition, the μ4oxo bound to Mn1 (IV), Mn2 (IV), Mn3 (IV), and Ca2+ and OH− bound to Ca2+ are the most probable candidates for O O bond formation. The charge density on the μ4-oxo is slightly lower than that on the OH− in state 4. Supposing one electron is transferred from the μ4-oxo in the fourth EPT, nucleophilic attack might occur between the μ-oxo and the OH−. After the nucleophilic attack, the proton of the OH− would be removed to stabilize the structure. Unfortunately, the process of nucleophilic attack has not been performed because addressing the charged surface is beyond the scope of our method. C
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Figure 3. Scheme of the water oxidation on the surface of Ca-birn. The centered part is the Gibbs free energy profile of the four electron−proton transfer steps of water oxidation, and the red inset shows the detailed OO bond formation and the following processes in the fourth step. All the relevant states are shown as denoted. The purple atoms represent Mn (IV) and the blue atoms represent Mn (III). The yellow proton represents the proton to remove in the next step.
Table 3. Evaluated ΔG for the Electron-Proton Transfer Steps of the Water Oxidation Reaction in PSII ΔG vs P680+/P680a −1
S0 S1−1 S2−1 S3−1 S0−1
+ + + +
(H+ + e) 2(H+ + e) − H2O(l) 3(H+ + e) − H2O(l) 4(H+ + e) + O2(g) − 2H2O(l)
0 −0.61 −1.01 −1.48 −2.23
ΔG per (H+ + e) P680+ vs SHEb +1.788 +1.788 +1.788 +1.788 +1.788
× × × × ×
0 1 2 3 4
ΔG vs SHE 0 1.18 2.56 3.88 4.92
a ΔG values for different EPT steps in the second column are entirely from Siegbahn’s work.7 The structure for each intermediate could be found in Figure S3. bThe change in potential between P680+ and the SHE calculated based on the change in energy for the full catalytic cycle: (4.92 to −2.23 eV)/4e = 1.788 V per (H+ + e)
However, the ΔG of the OO bonding has been evaluated. The OO bond is formed, accompanied by an electron− proton release and the reduction of the adjacent Mn (IV). In state 5, an O22− species is formed (magnetic moment is approximately 0) with reduced Mn3 (III). The O22− species is further oxidized to form O2 with the reduction of Mn1 and Mn2 (state 6). In this state, at least one water molecule should coordinate to the surface either to compensate the surface defect caused by O2 formation or to be directly oxidized by the O22− species to form O2. After the O2 desorption, the surface would return to state 1 by adsorbing a H2O from the solution. The process of O2 desorption followed by H2O adsorption is endothermic (0.12 eV). The O2 formation and release process (5 → 6 → 1) is in general energetically downhill by 0.30 eV, which suggests that once the OO bond is formed, the O2 will be formed and subsequently released.
It is proposed that the oxidation of Mn (III) to Mn (IV) prefers to occur before the oxidation of the O2− ion. In this system, if the oxidation of the O2− ion occurs prior to the oxidation of the Mn ion, then the first step has to overcome the Gibbs free energy of 2.29 eV uphill as denoted by the red dashed line in Figure 3. The obtained structure has an O− ion on the surface as shown in Figure S2. Other possible oxidization sites are also tested and found to be less stable than the structure mentioned above. The energy required by the direct oxygen oxidation (2.29 eV) is much higher than the proposed first step (1.16 eV) and even higher than the highest energy barrier for state 5 (1.94 eV). Therefore, the scheme of the direct water oxidation is not appropriate for this system. 3.3. Comparison of Ca-birn and the OEC. Although the water oxidation mechanism in PSII is not conclusively determined, the atomic structures of the S1, S2, and S3 states have been confirmed by the X-ray structure1,2 and spectrosD
DOI: 10.1021/acs.jpcc.5b05989 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C copies such as EXAFS,3 EPR, and ENDOR techniques.4 Besides, it is quite confirmed that the OO bond is formed between a slow exchanged water and a fast exchanged water.35 However, little experimental information regarding the structure of the S4 state has been reported. With confirmation of the slow water as O5, the assignment of the fast water is still under debate.36 By DFT method, Siegbahn has already reported a complete water oxidation mechanism including each electron or proton transfer step.7 The scheme of the mechanism could be found in SI Figure S3. For each electron− proton transfer, the calculated free energy is defined by the potential of P680+ as shown in Table 3 Column 2. After adding the potential difference between P680+ and SHE, the evaluated ΔG of each EPT steps for PSII are given in Table 3 Column 4. To reveal the difference of Ca-birn and the OEC, we have compared the free energy profiles of the OEC and Ca-birn as shown in Figure 4. It could be found that the overpotential for
minimum potential of the second and third steps for PSII is higher than the corresponding potential for Ca-birn. Because the total ΔG for the reaction is 4.92 eV, different potentials for the second and third steps lead to the situation that the minimum ΔG for the formation and release of O2 is 1.04 and 1.64 eV (numbers are not shown in Figure 4) for PSII and Cabirn, respectively. Additionally, the active centers of Ca-birn and PSII are quite different, although CaMn3O4 cube could be found in both the OEC and Ca-birn, as shown in Figure 5(a) and (b). When the OO bond is formed, the previous MnO bond would have elongated or broken. Therefore, the flexible MnO bond is beneficial to reduce the energy barrier of OO bond formation. In the OEC (Figure 5(a)), O5 is bound to Mn3 and Mn4 by O 2p and Mn 3d eg orbitals, as denoted.37 For sixcoordinated Mn (III) ions, MnO bonds contributed by Mn 3d eg orbitals are naturally longer than those formed by Mn 3d t2g, and are therefore more flexible. Moreover, O5 is in the large space caused by the Jahn−Teller distortion between Mn1 (III) and Mn4 (III), which makes O5 even more reactive. Actually, O5 is proposed to be involved in the formation of the OO bond in three different mechanisms.4,7,36 According to Siegbahn’s work, it is proposed that the other candidate water molecule could bind to Mn1 and be situated between Mn1 and O5 so that the OO bond is formed through OO coupling. The higher potentials of the second and third EPT steps may be caused by the repulsion between O5 and the other water molecule. This makes the OO bond formation much easier because the OO repulsion has been overcome step by step. In contrast, no such active structure could be found in Ca-birn. The potentially active μ-oxo in Ca-birn (Figure 5 (b)) is bound to one eg orbital and two t2g orbitals. Strongly restricted by the two short MnO bonds contributed by the O 2p and Mn 3d t2g orbitals, the flexibility and the activity of the μ-oxo are largely decreased. Meanwhile, the Jahn−Teller distortion is not as strong as in the OEC. Consequently, Ca-birn is less active than the OEC as the water oxidation catalyst. On the basis of the discussion above, it is concluded that the activity of the μ-oxo is of great importance. Catalysts with surface μ-oxo bonding to more than one Mn 3d eg orbital and less t2g orbitals may increase the flexibility and activity for water oxidation.
Figure 4. Comparison of the free energy profiles of the water oxidation reaction in the OEC and Ca-birn (U = 1.23 V). The numbers 1−4 represent the numbers for the electron−proton steps.
the water oxidation reaction on Ca-birn (0.41 V) is much higher than that in PSII (0.15 V). In the first, second, and third EPT steps, three Mn (III) atoms are oxidized one by one for both PSII and Ca-birn, and the OO bond formation then occurs in the fourth step. (The detailed process for the reaction in PSII is shown in Figure S3.) The ΔG for the first EPT step in Ca-birn is approximately as large as that in PSII, but the
Figure 5. Structures of the active center for the PSII (a) and Ca-birn (b). The MnO bonds denoted by the blue arrows are contributed by Mn deg and O 2p orbitals. E
DOI: 10.1021/acs.jpcc.5b05989 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 6. Comparison of the Gibbs free energy profiles (U = 1.23 eV) of water oxidation on the surface of Mg-birn, Ca-birn, and Sr-birn. The inset shows the detailed OO bond formation and the following processes in the fourth step. The structures at the top and bottom are the relevant states as denoted.
Table 4. Calculated ΔG (eV) for Each Step of the Water Oxidation Reaction for Mg-birn, Ca-birn, and Sr-birn Mg-birn 1→2 + H+ + e 2→3+ H+ + e 3→4+ H+ + e 4→5+ H+ + e 5 + H2O(l) → 6 6+ H2O(l)→1 + O2(g)
Ca-birn
Sr-birn
U=0V
U = 1.23 V
U=0V
U = 1.23 V
U=0V
U = 1.23 V
1.33 0.89 0.47 2.72 −0.57 0.07
0.1 −0.34 −0.76 1.49 −0.57 0.07
1.16 1.01 1.11 1.94 −0.42 0.12
−0.07 −0.22 −0.12 0.71 −0.42 0.12
1.16 0.84 1.14 1.94 −0.31 0.14
−0.07 −0.39 −0.09 0.71 −0.31 0.14
3.4. Effect of Ca 2+ , Sr 2+ , and Mg 2+ on Water Oxidization. In birnessite, Ca2+ works as the water-binding site, as do other metal cations, such as Sr2+ and Mg2+. Although similar water oxidation mechanisms have been observed in the Ca-birn, Sr-birn, and Mg-birn, the alteration of the metal cation could influence the free energy profile. Figure 6 shows the ΔG profile of water oxidation for Mg-birn, Ca-birn, and Sr-birn with U = 1.23 V. The values of ΔG for each step in all three models are also listed in Table 4. First, compared to Ca-birn, the overpotential for Mg-birn in the first EPT step is higher by 0.1 V when U = 1.23 V. The following two EPT steps are exothermic. The overpotential of the fourth EPT step is 0.99 V, which is mainly contributed by the high energy barrier to OO formation in state 5. During the first, second, and third EPT steps (shown in SI Figure S4), as the interactions between O and Mn become stronger, the attraction of Mg2+ to the surface becomes weaker. Due to the short radius of Mg2+, its location shifts as the reaction proceeds. As a result, before (state 4) and after (state 5) the formation of O22−, the clear migration of Mg2+ and its binding OH− have
been observed. The perceivable change of the surface geometry may cause the much higher energy barrier in state 5 than that for Ca-birn thermodynamically. Because the energy barrier of OO formation for Mg-birn (1.49 eV) is much higher than that for Ca-birn (0.71 eV), it is deduced that the O2 evolution efficiency for Mg-birn should be lower than that for Ca-birn. In contrast, the minimum potential of almost every step for Sr-birn is quite close to the value for Ca-birn. The three EPT steps where Mn is oxidized are exothermic and thus are quick (U = 1.23 V). The potential surface of the third step for Sr-birn is lower than that for Ca-birn, suggesting the reaction is thermodynamically easier for Sr-birn before state 5. However, the overpotentials of the rate-determining EPT (fourth) step for Ca-birn and Sr-birn are 0.41 and 0.54 V, respectively, which may result in less effective O2 production for Sr-birn than for Ca-birn. Unlike Mg-birn, the higher overpotential for Sr-birn than for Ca-birn is caused by the process of O2 formation instead of O22− formation. As shown in the inset of Figure 6, the energy uphills at state 5 for both Sr-birn and Ca-birn have the same F
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value, whereas, from state 5 to state 6, the energy downhill for Ca-birn is more downward than that for Sr-birn. In state 6, MnIVO bonds are elongated or broken to oxidize O22− to O2. Upon careful comparison, the MnO bonds relevant to Mn1 and Mn2 become longer for Ca-birn than for Sr-birn when O2 is formed on the surface, as shown in the bottom of Figure 6. In other words, the different potential surface at state 6 is caused by the different extent to which the MnIVO bonds are broken. Combining the experimental and our calculated results, it could be deduced that the less dramatic energy downhill for O2 formation may be one of the reasons why Sr-birn is less active than Ca-birn.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05989. Thermodynamic scheme; the energy correction for O2; the magnetic moments; supporting figures; and additional references (PDF)
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REFERENCES
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4. CONCLUSIONS On the basis of the DFT investigation into the water oxidation reaction on the layered-birnessite-like manganese oxide, we have found both similarities and differences between the mechanisms for Ca-birn and for PSII. Three Mn (III) atoms are oxidized to Mn (IV) stepwise, and then, O22− is formed between an OH− and a μ-oxo and further oxidized to O2, accompanied by the reduction of Mn (IV). The calculated overpotential for Ca-birn is 0.26 V higher than that for PSII because of the less flexible μ-oxo on the surface of Ca-birn. Ca2+ works as the water-binding site. Substitution of Ca2+ with other cations, such as Mg2+ and Sr2+, can influence the minimum potential for the last EPT step. The replacement with Mg2+ could cause the energy barrier to increase for the OO bond formation, while substitution by Sr2+ could lead to a less dramatic energy downhill during the O2 formation process. Our results may, to some extent, explain why the water oxidation activity on birnessite is lower by orders of magnitude than that on the OEC and the impact of different alkaline earth metal cations on the catalytic activity. We expect that manganese oxides with more flexible μ-oxo and the corresponding appropriate cations could achieve better performance for the water oxidation reaction.
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Corresponding Authors
*Tel: 86-411-84379252; fax: 86-411-84694447; e-mail: xzhou@ dicp.ac.cn (X.Z.). *Tel: 86-411-84379070; fax: 86-411-84694447; e-mail: canli@ dicp.ac.cn (C.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We appreciated the help of Prof. Per E. M. Siegbahn with the discussion regarding the mechanism in PSII. This work is financially supported by National Natural Science Foundation of China under Grant 21473183 and 21361140346. We have also appreciated the financial support of the National Basic Research Program of China (973 program 2014CB239400). G
DOI: 10.1021/acs.jpcc.5b05989 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.5b05989 J. Phys. Chem. C XXXX, XXX, XXX−XXX
