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Hyper-Volcano Surface for Oxygen Reduction Reactions over Noble Metals Yasuharu Okamoto*,†,‡ and Osamu Sugino‡ The Nano Electronics Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki, 305-8501, Japan, and Institute for Solid State Physics, UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8581, Japan ReceiVed: September 11, 2009; ReVised Manuscript ReceiVed: January 28, 2010
A first-principles elucidation is provided for a reason why platinum shows excellent catalysis concerning the oxygen reduction reaction (ORR). The present research is based on an intensive simulation of the ORR pathways performed under realistic electric double layer conditions and on a multidimensional thermodynamic analysis that directly takes into account the multistep character of ORR. The ORR on four noble metals (Pt, Pd, Ru, and Au) can be characterized by three processes, O2 reduction to OOH, bond breakage between O atoms, and avoidance of OH and O poisons. We show that thermodynamic analysis of these processes yields a Sabatier volcano plot. Introduction Many electrocatalytic reactions that occur by way of an adsorbed intermediate show a volcano-type plot when the reaction rate is plotted against a descriptor of the adsorption properties, e.g., the adsorption bond strength, the d-band character,1 and so on.2 The two-dimensional analogue of the volcano plot is called the volcano surface.3 The volcano plot or surface illustrates a competition between adsorption of the reactants and desorption of the products known as the Sabatier principle,4 which describes a trade-off between the catalyst-adsorbate interactions so as to activate the adsorbate but avoid poisoning on the catalyst surface. The volcano plot explains phenomenologically why Pt efficiently electrocatalyzes various reactions compared with other elemental transition metals, and has been used as a handy but very useful guideline for catalyst design.5,6 The fact that the electrochemical reactions are characterized by a single descriptor so simply and generally is surprising, because the reactions occur at working electrodes that are considerably much more complex than a well-defined single crystalline surface under vacuum. In particular, the volcano plot applies to multistep processes, such as the oxygen reduction reaction (ORR) on noble metal alloys.7 This resembles a situation in heterogeneous catalysis where multistep surface processes involving a number of association reactions and desorption processes may be considered as hypothetical one-step desorption processes.8 The implication of the volcano plot has attracted renewed interest9 a few decades after the earlier studies.2,5,6 Performing first-principles calculations on a number of metal surfaces, Nørskov et al.10 showed that the adsorption energy of the intermediates, e.g., N2, CO, NO, and O2, is closely correlated to the activation energy of the bond-breaking reaction on the surface known as the Brønsted-Evans-Polanyi relation.11,12 Abild-Pedersen et al.13 investigated adsorbates, consisting of C, N, O, or S which is hydrogenated or non-hydrogenated, on a number of transition metal surfaces with and without steps, and * To whom correspondence should be addressed. E-mail: y-okamoto@ df.jp.nec.com. † NEC Corporation. ‡ University of Tokyo.
found their adsorption energies are linearly dependent to each other; this property is called the “scaling property”. Those theoretical works explained to some extent the volcano plot using a typical surface science approach and provided a basis for catalyst design in silico, while the solvation effects, which peculiarize electrocatalysis, have remained incompletely explained. Meanwhile, the solution side of the electrode/solution interface was recently incorporated into theoretical models by using the first-principles molecular dynamics (FPMD) simulation techniques. The structure of water was investigated at the Pt(111)/water interface and the effect of the bias voltage was discussed.14 The Volmer step of the hydrogen evolution reaction (HER), H+aq + e- f Hads, was investigated by applying overvoltage to the Pt(111)/water interface and the concerted exchange of the solvated proton and the surface excess electron was elucidated.15,16 The ORR was also investigated: Solvated protons were introduced into the water interfaced with ZrO217 or with N-doped graphite18 to follow the reaction dynamics, and the free energy of the reaction was computed in the latter case with a thermodynamic integration methodology: The reaction dynamics of O2 was investigated on a Pt(111) surface with three or four water molecules being coadsorbed.19,20 These theoretical works illustrated significant solvent effects albeit the amount was not always explicitly quantified. It is noteworthy, however, that the solvation effects were not perfectly incorporated into those models because, for example, the simulation time is typically less than 10-12 s, which is not long enough to sample fluctuations of the water or to advance the reaction near the equilibrium potential. Although these two approaches, the total-energy calculation and the FPMD simulation, have their own shortcomings difficult to overcome immediately, one should notice their complementary nature. If both are utilized effectively in a combined way, such a theoretical scheme would greatly advance our understanding of electrocatalysis. Before establishing such a combined scheme in the future, it will be important to understand in detail how the two approaches would provide similar, or different, results regarding the reaction pathways, rate, and so on. In this context the purpose
10.1021/jp9087805 2010 American Chemical Society Published on Web 02/18/2010
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Figure 1. Snapshots of FPMD simulation for ORR on Pt(111)/H2O. Tan, red, and white represent Pt, O, and H atoms, respectively. (a) O2 was initially 1.88 Å above the Pt(111) surface in a t-b-t configuration, (b) product of the first reduction at 1.27 ps, (c) product of the O-OH bondbreaking reaction at 1.54 ps, and (d) product of the second reduction at 7.45 ps.
of this study is to clarify the similarity/difference in the two approaches. The ORR pathways are examined as an example. In addition to the comparison of the approaches, we show that the multistep character of the ORR can be analyzed properly by the “hyper-volcano surface”, which can be referred to as an extension of a volcano surface with three adsorption energies of the intermediates Oads, OHads, and OOHads. Methods In our simulation of ORR on Pt, Pd, Ru, and Au(111), we added an H3O+ ion into the water while maintaining charge neutrality of the entire computational cell similar to that in refs 17 and 21 instead of controlling the excess surface electrons as was done in refs 14, 16, and 22 to model a biased surface interfaced with an acid solution. Herein, if a reaction did not occur in this “one-proton model” within several picoseconds, another H3O+ was added to promote the reaction. Most simulations required the “two-proton model”, except for the second ORR step on Pt, Pd, and Au surfaces where the oneproton model was sufficient. All first-principles calculations were performed with use of the Simulation Tool for the Atom TEchnology (STATE) program package.23 The atomic geometry and electronic structure were calculated in accordance with the DFT framework within a generalized gradient approximation (GGA) to the exchange-correlation functional formulated by Perdew, Burke, and Ernzerhof (PBE96).24 The ultrasoft pseudopotentials for Pt, Pd, Ru, Au, O, and H atoms have been generated by using the process described by Vanderbilt.25 The partial core correction scheme described by Louie et al.26 was used for all atoms except H atoms. Plane-wave basis sets with cutoff energies of 25 and 225 Ry were employed to expand wave functions and charge density, respectively. The first-principles molecular dynamics (FPMD) simulations were performed on a metal(111)/H2O interface under a periodic boundary condition: FCC Pt, Pd, Ru, and Au(111) 3-layer slabs with (3 × 3) periodicity composed of a total of 18 atoms and a water slab composed of 16 H2O molecules were used to describe the solvent surrounding the metal. The computational cells in the FPMD simulation were (8.37 × 4.83 × 20.04 Å3), (8.22 × 4.75 × 19.90 Å3), (8.06 × 4.65 × 19.80 Å3), and (8.79 × 5.07 × 20.30 Å3) for Pt, Pd, Ru,
and Au, respectively. The density of the water slab was ≈0.9-1.1 g/cm3 in the initial state (considering the volume of the metal slab and adsorbate). The mass of the hydrogen atoms was replaced by that of deuterium to increase the time step (∆t ) 1.21 fs) in the FPMD simulations. Velocity scaling was used to maintain the temperature at 363 K in the FPMD simulations. Brillouin zone integration was performed for the (2 × 4 × 1) k-points. All simulations began with O2 adsorbed on a bare metal(111) surface in order to compare ORRs on four different surfaces under the same conditions, although an oxide or hydroxyl surface may have been more plausible than a bare surface, especially for Ru(111). The adsorption energies of the ORR intermediates were examined by using the same metal slab employed in the FPMD simulations without solvent molecules. Spin-polarized GGA calculations were performed to determine the adsorption energy on metal/vacuum interfaces. On the other hand, spin nonpolarized GGA calculations were used in the FPMD simulations as the spin polarization should have a negligible affect on the prediction of the qualitative ORR pathways.21 Results and Discussion a. ORR at a Pt(111)/H2O Interface. First of all let us examine the ORR pathways at a Pt(111)/H2O interface. A model of the Pt(111)/H2O interface consisting of a (3 × 3) 3-layer Pt slab with an O2 molecule on it and a water slab of 16 H2O molecules to include the solvent effects was annealed at 363 K for 1.2 ps. One/two H atoms were then added to an H2O in the water slab to introduce one/two protons into the computational cell. Figure 1a shows the setup of the two-proton model for ORR in an acidic solution. In the one-proton model that considered only the lower H3O+ (Figure 1a), no reaction occurred during the first 3 ps. By contrast, in the two-proton model, the first reduction (O2ads + H3Oaq+ + e- f OOHads + H2Oliq) occurred at 1.27 ps (Figure 1b). As shown in Figure 1c, the O-OH bond spontaneously broke on the surface to give (OHads(top) + Oads(fcc)), which averted HOOH formation as the product. This indicates that the 4e- reduction is predominant, whereas the unfavorable 2e- reduction is suppressed. The upper proton in the water slab subsequently approached the surface through the proton-relay mechanism, and reacted with OHads as (OHads + H3Oaq+ + e- f 2H2Oliq) rather than with Oads
Oxygen Reduction Reactions over Noble Metals (Figure 1d). The preferential reduction of OHads can be explained by the relative stability of (H2O + Oads) compared to (OHads + OHads). A separate DFT calculation on the surface adsorption energies indicated that the estimated energy difference between the two states was 0.69 eV. It is noteworthy that the dynamical process leading to Oads via OHads + Oads is similar to that Wang and Balbuena19,20 found dominant by performing an FPMD simulation although they took significantly different modeling of the water side; they used a few water molecules adsorbed on a Pt(111) surface, where a proton was solvated. In this sense the obtained pathway is robust enough to be quite insensitive to the details of modeling. The total-energy calculation by Nørskov et al.27 showed that the pathway is dominated by O2ads f OOHads f Oads, which is similar to ours but is different in that OOHads appears as a stable intermediate although it appeared only transiently in the FPMD simulations. The difference may be ascribed to the solvent effect, but one should also be aware of different values used for the oxygen coverage and bias potential which prevent direct comparison. Nevertheless, the reason why these three simulations/calculations yielded a similar pathway, albeit difference exists, may be explained by lack of alternative pathways of equivalent activity in the simulation condition. When taking a time average of the total energy, i.e., sum of the kinetic energy of ions and the Kohn-Sham (KS) energy, in our simulation, the estimated decline of electric potential from the standard potential of an electrode reaction of O2 before the first ORR step was 1.10 V (see the Supporting Information), which corresponds to the overpotential in the FPMD simulation and is higher than the actual overpotential of fuel cells, that is, 0.4 V.28 This means that the electrode potential in the simulation is significantly low and it accelerates the reaction so as to occur within a few picoseconds in the simulation. We found that most of the H2O molecules near the Pt surface take the H-down configuration during the MD simulation. This seems to show that the electrode potential in the simulation is lower than the potential of zero charge of the Pt/H2O system. Note that, in studying the Volmer step of the HER,16 the reactant energy was increased by introducing excess electrons onto the Pt surface but in our case the increase in the energy was achieved by introducing multiple solvated protons. These two approaches may look quite different to each other but one can recognize the difference as arising from the finiteness of the cell in that the difference can be diminished arbitrarily in principle as the cell is enlarged. Having investigated the first half of ORR (Oads on Pt(111)/ H2O), we examined the latter half beginning from the product of the first half in the same manner. We confirmed the occurrences of the third reduction (Oads + H3Oaq+ + e- f OHads + H2Oliq) and fourth reduction (OHads + H3Oaq+ + e- f 2H2Oliq). Thus, these FPMD simulations suggest that the pathway for the 4e- reduction on Pt(111) is O2ads f OOHads f Oads + OHads f Oads + H2Oliq f OHads + H2Oliq f 2H2Oliq, which hereafter is referred to as path A. Note that the pathway for the latter half is equivalent to those obtained from the FPMD simulation of ref 19 and from the static total-energy calculation.27 Experimentally, a pathway via HOOHads was suggested in ref 29 while that without HOOHads was shown dominant in ref 30; experimental identification of the path is thus yet to be settled, while it appears to be settled regarding the first charge transfer process on Pt(111)31 and the redox processes between OHads and H2Oliq.32
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Figure 2. Positive correlation of the calculated adsorption energy of the intermediates on Pt, Pd, Ru, and Au(111): Eads(O) vs Eads(O2) (diamonds), Eads(OOH) (squares), and Eads(OH) (triangles) in eV. Lines (L1, L2, and L3) obtained by linear fitting are shown: L1, Eads(O2) ) 0.6789Eads(O) - 1.7938; L2, Eads(OOH) ) 0.7843Eads(O) - 1.9358; L3: Eads(OH) ) 0.3748Eads(O) + 0.7793.
b. ORR on Other Noble Metal Surfaces. Next the ORR pathways on Pd, Ru, and Au(111) surfaces were investigated. Before showing the simulation results, it is worth mentioning here that these surfaces bind O2, OOH, O, and OH intermediates in increasing order of strength as Au , Pt < Pd , Ru (Figure 2) and that the adsorption energy of O (Eads(O)) shows a clear positive correlation with the adsorption energy of other intermediates (Eads(X); X ) O2, OOH, and O); note that the latter relationship is a special case of the “scaling properties” mentioned in the Introduction,13 except for OOH. By performing the FPMD simulations we found that the ORR pathway on Pd(111) coincides with path A on Pt(111). However, the situation on Ru(111) was quite different. O2ads on Ru(111) spontaneously and immediately broke the O-O bond, which dissociated into two Oads’s. Each Oads was subsequently attacked by a proton, and adsorbed on a hollow site as OHads. Then the H of OHads was transferred to another OHads after being relayed by two water molecules to yield Oads and H2Oliq (Figure 3). The latter half of the ORR pathway on Ru(111) was confirmed to coincide with the one on Pt(111). Thus, the 4e- reduction pathway on Ru(111) can be written as O2ads f Oads + Oads f OHads + Oads f OHads + OHads f Oads + H2Oliq f OHads + H2Oliq f 2H2Oliq. Hereafter, this pathway is referred to as path B. Path B is similar to the “dissociative mechanism” (O2ads f Oads + Oads f Oads + OHads f Oads + H2Oliq f OHads + H2Oliq f 2H2Oliq) obtained from the total-energy calculation27 but is different in that (OHads + Oads) is reduced not by a direct hydrogenation of OHads but by a hydrogenation of Oads followed by the proton relay (OHads + OHads fOads + H2Oliq). The difference highlights the important role of solvents at the electrode/water interface. Consistent with the simulation, we find that (H2O + Oads) is more stable than (OHads + OHads) by 0.32 eV in a DFT calculation under a vacuum condition. The ORR pathway on the Au(111) surface significantly differed from the ones on the platinum group metals. Unlike on Pt and Pd where the O-OH bond on the surface broke, on Au(111), OOH formed by the first reduction moved away from the surface and was subsequently reduced in solution to form HOOH. Although the following reaction did not occur in the simulations, the reaction of HOOH f OHads + OHads is exothermic by 0.25 eV. Hence, the ORR pathway for the 4ereduction on Au(111) is O2ads f OOHads f HOOH f OHads + OHads f OHads + H2Oliq f 2H2Oliq. Hereafter, we refer to this as path C.
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Figure 3. Snapshots of isomerization on Ru(111)/H2O: OHads + OHads f H2Oliq + Oads. Green, red, and white represent Ru, O, and H atoms, respectively. (a) OHads + OHads at 1.84 ps, (b) H transfer through the proton-relay mechanism at 1.89 ps, and (c) H2Oliq + Oads at 1.90 ps.
Regarding the first reduction of path C, there is no equivalent pathway deduced on the basis of total-energy calculation as far as we know, but a similar one was shown by FPMD simulation on an N-doped carbon.18 Note that the different pathway of Au from that of other noble metals may be explained by the weak interaction between the surface and reactant or intermediates (Figure 2). Note also that path C is unfavorable for catalytic reactions because the formation of HOOH requires a low electric potential ( 3.546 (OOHads f Oads + OHads) (P2a) Eads(O) > 3.062
(O2 gas f 2Oads)
-e × η < 1.148 + Eads(OH) - Eads(O)
(Oads + H+ +
e- f OHads) -e × η < 1.916 - Eads(OH)
(P2b)
(P3a)
(OHads + H+ + e- f H2Oliq) (P3b)
where units are in eV. The allowed area for path A is given by the four inequalities P1, P2a, P3a, and P3b and is drawn in Figure 4a, which is bordered by the η-dependent planes of P1, P3a, and P3b and the η-independent plane of P2a. The allowed area of path B is similarly given by P2b, P3a, and P3b, which is drawn in Figure 4c; the borders do not depend on Eads(OOH) in this case and thus the area is two-dimensional. The area becomes smaller as η is reduced and when it is almost zero, or at the point of vanishing overpotential, converges to point A where (Eads(O), Eads(OH), Eads(OOH)) ) (3.06, 1.92, 1.43) in eV. It is interesting that point A is located on the P2a plane. Because the scaling properties best apply to Eads(O) vs Eads(OH) as Eads(OH) ) 0.3748Eads(O) + 0.7793 eV (Figure 2), it is advantageous to analyze the area on a plane ()cross section) defined by this equation, called the O-OH plane hereafter. Figure 4b shows the cross section of the allowed area of path A, on which noble metals are located according to their adsorption energies. Pt is located nearest to A and the corresponding value of η is 0.46 V (see Figure 4b), which agrees well with the experimentally observed one28 in spite of drastic approximations made in our approach. Pd is located slightly more away from A and has a larger value for η. Pd is nearer to the plane P3a than the plane P1, which means that the ORR is rate-limited more by the P3a process than by the P1 process.
Oxygen Reduction Reactions over Noble Metals
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4477 adsorption energies are much larger than that of OOHads naturally explains why the O-O bond broke directly without going through OOH and the reaction suffers from both O and OH poisons. Au is also far from A because its Eads(O) and Eads(OH) are too small, demonstrating the inert character of a Au surface. When the whole result is plotted against Eads(O), it shows a volcano curve as observed experimentally34 and theoretically.27 Now we analyze the allowed area using the pathways used in ref 27 Regarding path B, the pathway is different at the second reduction, OHads + Oads f OHads + OHads f Oads + H2Oliq in our case and OHads + Oads f Oads + H2Oliq in ref 27. When the inequalities are derived, however, both provide the same one, indicating no effect appears on η, while for path A, the difference is at the second reduction, OOHads f Oads + OHads f Oads + H2Oliq in our case and OOHads f Oads + H2Oliq in ref 27. In the latter case, by combining P2a and P3b, P2a is modified as
-e × η < Eads(O) - Eads(OOH) - 1.630 (OOHads + H+ + e- f Oads + H2Oliq)
Figure 4. (a) Hyper-volcano surface of (Eads(O), Eads(OH), Eads(OOH)) for path A at different η values (0.0, 0.25, 0.5, 0.75, and 1.0) calculated from the exothermic requirement of four processes P1, P2a, P3a, and P3b. (b) The same as panel a but the surface is cut by the O-OH plane (L3 in Figure 2) and adsorption energies of Pt, Pd, Ru, and Au are also plotted. (c) Volcano surface of (Eads(O), Eads(OH)) for path B calculated from the exothermic requirement of three processes P2b, P3a, and P3b.
On the contrary, the opposite is true for Pt, where OOH is more weakly bound. Although OOHads appeared in the FPMD simulation as a transient intermediate both on Pt and Pd, it might have appeared more transiently on Pt. Ru has larger Eads(O) and Eads(OH) and is further away from A, and hence η becomes much larger. The fact that these
(P2aa)
This change facilitates the reduction of OOHads affecting the value of η, but, in the region where the noble metals are located, the effect on η is negligibly small. These results led us to conclude that the modification in the pathways does not immediately lead to an appreciable change in η as long as the simple thermodynamic analysis is used. However, the dynamical and solvation effects are considered more important as Eads(OOH) is reduced because the transient nature of OOHads is thereby more apparent. In those cases, one should be aware of the possible invalidation of the static thermodynamic analysis. Still, we can explain how the overpotential appears for noble metals and can predict how that may be improved within our thermodynamic approach. The overpotential appears because existing elemental noble metals are not located at point A (Figure 4a), and as long as the scaling properties are valid, A is most closely approached near the point where Pt is located. The mechanism is clear; as Eads(O) drops from Ru to Pt, Oads and OHads are more easily desorbed but Eads(OOH) drops as well and OOH is less easy to be adsorbed to initiate the reaction. However, if the scaling property for Eads(OOH) vs Eads(O) is invalidated in some way, we can approach A further. This means that a catalyst that better electrocatalyzes O2 than Pt would have larger Eads(OOH) and smaller Eads(O). Note that this discussion does not go so beyond what has been given theoretically27 or experimentally,2,5,6 but we would like to point out that the guiding principle is more transparent when analyzed multidimensionally with all the adsorption energies directly taken into account. Acknowledgment. The authors thank M. Otani, Y. Morikawa, I. Hamada, N. Watari, and T. Ikeshoji for their useful discussions. Calculations were made using the Earth Simulator at the Earth Simulator Center and the Hitachi SR11000 in the supercomputing center at the University of Tokyo. Part of this research was performed by “Advanced Large-scale Computational Simulation Services” supported by Open Advanced Facilities Initiative for Innovation (Strategic Use by Industry) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. This work was partly supported financially by the New Energy and Industrial Technology Development Organization (NEDO).
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