Article pubs.acs.org/JPCC
Thermodynamics and Hysteresis of Oxide Formation and Removal on Platinum (111) Surfaces Edward F. Holby,† Jeff Greeley,‡ and Dane Morgan*,† †
Materials Science Program, University of Wisconsin-Madison, 1509 University Ave., Madison, Wisconsin 53706, United States Center for Nanoscale Materials, Argonne National Lab, 9700 South Cass Ave., Argonne, Illinois 60439, United States
‡
S Supporting Information *
ABSTRACT: Oxygen adsorption on Pt(111) has many implications for a wide range of technologies including PEM fuel cells. Using DFT, we have calculated the stable phases for oxygen on Pt(111) surfaces up to one monolayer of oxygen coverage. Our predicted stable phases are consistent with electrochemical measurements and are in agreement with Temkin/Frumkin isotherm conditions. We predict a new phase at one monolayer that suggests a simple mechanism for oxygen place-exchange to subsurface positions. Analysis of the phase diagram provides a possible explanation for the hysteresis observed in Pt cyclic voltammograms in aqueous environments.
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on other oxidized transition metal surfaces.15−17 Furthermore, the work of Gu and Balbuena with DFT confirms that, at coverages greater than 4/5 ML, partial subsurface oxygen occupation becomes more stable than all surface FCC hollow site occupation.18 Despite its widespread acceptance, attempts to identify the mechanisms and structures associated with place-exchange reveal that a significant revision of the simple place-exchange picture is needed. First, it is not clear what path oxygen can take to move subsurface. Gu and Balbuena18 suggest a placeexchange mechanism in which oxygen diffuses from an FCC hollow site onto an HCP hollow site and then into the tetra-II subsurface site. However, there is a kinetic barrier for this process in excess of 1.2 eV, which would make observation of such a place-exchanged phase at room temperature very unlikely over typical cyclic voltammogram (CV) time scales. Although additional paths may exist to move oxygen subsurface, it is generally not clear how O can penetrate the Pt surface at a reasonable rate so as to move into a subsurface position during experimental time scales. A second issue with the placeexchange model is that several studies have recently shown stable structures at higher oxygen coverages that include movement of Pt atoms from their surface positions quite different from that envisioned in place-exchange.13,19−21 In particular, Devarajan et al.21 and Hawkins et al.20 have proposed a buckling mechanism in which, instead of oxygen occupying the Pt subsurface, Pt surface atoms are buckled into the oxygen layer (forming a PtO2 stripe). These shifted Pt play
INTRODUCTION Oxygen adsorption on Pt can affect both the catalytic activity of Pt surfaces1−4 and the long-term durability of Pt catalysts.5−7 The work of Jerkiewicz et al.8 has established that oxygen is the dominant adsorbate at voltages greater than 0.8 V in aqueous environments suggesting that oxygen will play a critical role in Pt catalysis at many practical potentials. At coverages above ∼1/2 monolayer (ML) the oxide on Pt goes through a phase transition of some kind, leading to so-called irreversible oxide, which creates a dramatic hysteresis in the Pt voltammogram. Despite its importance, a molecular level understanding of the structure and formation/removal mechanisms of the irreversible oxide on Pt are still lacking and remain a matter of significant research interest.2,6 This work focuses on oxygen on the Pt(111) surface (Pt(111)/O) as Pt(111) is the most stable Pt surface and known to be a dominant facet in Pt catalyst nanoparticles. We use ab initio methods to identify the stable and metastable surface phases for Pt(111)/O and how these govern the formation/removal kinetics of irreversible oxide. The most well established theory to explain the formation of irreversible oxide is known as place-exchange and was developed by Conway, Jerkiewicz, and co-workers.8−11 In the place-exchange model, at coverage greater than ∼1/2 ML oxygen begins to shift from its surface sites (Pt FCC hollow sites) and enters the Pt subsurface. This place-exchange process reduces the interaction energy between surface oxygen and stabilizes the oxidized surface. In the original model the interactions were assumed to be dipole−dipole coupling,9 although we demonstrate in Section 2 of the Supporting Information that a simpler electrostatic interaction dominates. Experimental studies do seem to confirm place-exchange behavior8,12−14 as well as the presence of subsurface oxygen © 2012 American Chemical Society
Received: November 9, 2011 Revised: March 2, 2012 Published: April 14, 2012 9942
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Figure 1. Calculated formation energies versus oxygen coverage with convex hull. Hollow boxes indicate phases on the convex hull and hollow circles represent phases very near to the convex hull. Circle markers represent phases with oxygen arranged on FCC sites (with possibly some minor surface Pt displacement). Square markers represent phases that contain buckled PtO2-like stripes. Triangle markers represent place-exchanged phases where O resides below Pt. One ML phases are labeled as all FCC (total surface occupation without buckling), place exchanged with a single subsurface O, PtO2-like following Hawkins et al.,20 and two hybrid structures that have buckling and place exchanged subunits (Figure 2). The other stable phases at 1/9 ML, 1/4 ML, and 3/4 ML are also labeled. All energies are given in the Supporting Information.
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a similar role to oxygen movement in place exchange in that they act to screen oxygen−oxygen interactions, thus stabilizing the surface. A comprehensive surface phase diagram, that combines both buckled Pt and subsurface oxygen, is needed to fully understand the true nature of irreversible oxide on Pt and is the focus of this work.
RESULTS AND DISCUSSION
Calculated formation energies and the stable convex hull are plotted in Figure 1. We find stable phases to occur at 1/9 ML, 1 /4 ML, 3/4 ML, and 1 ML. Phases at 1/16 ML, 2/9 ML, 1/3 ML, and 1/2 ML are considered nearly degenerate with the convex hull as they are within 10 meV/Pt site of the convex hull line. In low coverage phases where the oxygen can be moved relatively far apart, the surface occupation of FCC hollow sites on the surface are preferential to buckled or place-exchange phases. However, the surface FCC phase and the buckled phase at 1/2 ML are nearly degenerate. 1/2 ML coverage is therefore high enough that the oxygen−oxygen interaction of the FCC occupying oxygen atoms offsets the energy cost of PtO2 stripe formation. As the PtO2 stripes become more dense at higher oxygen coverage, the screening affect of the stripes in their roughly horizontal configuration becomes lessened. Unscreened oxygen atoms of neighboring stripes begin to interact and we find that at 1 ML, it is more favorable to form a place-exchanged phase. This new 1 ML phase is discussed in detail below. Overall the energy to put an oxygen on the surface increases with increasing coverage due to oxygen−oxygen interactions. Fitting a single effective interaction to the DFT data turns out to match well values fit to experimental CV data (Section 2 in Supporting Information) further supporting the validity of the DFT data. Only three candidates for 1 ML phases are represented in the DFT literature: full FCC hollow site occupation,30 simple place exchange structure reported by Gu with one subsurface oxygen,18,22 and an α-PtO2 like surface phase.23,31 Using DFT, this work predicts a new, more stable phase at 1 ML (Figure 2). This phase is roughly 230 meV per oxygen more stable than the α-PtO2 like surface phase, the most stable phase previously reported. This new phase combines aspects of both
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COMPUTATIONAL METHODS To establish the stable surface phases for Pt(111)/O we assemble the DFT literature data for oxygen binding energies as a function of coverage for the Pt(111)/O system18,20,22−28 and, using a common reference state, determine candidate stable phases. These phases are then calculated using the same code and reference state for direct comparison along with several other high coverage phase candidates. Formation energy (FE), binding energy per oxygen (BE), and surface free energy as a function of applied voltage, (γ(V)) are calculated from DFT using the Vienna Ab-initio Simulation Package (VASP).29 All results are strictly valid only at zero Kelvin but are assumed to be applicable to interpret data at room temperature. Computational details are given in Section 1 in the Supporting Information. To establish the stable phases four critical types of configurations were considered: FCC occupation phases (with some minor Pt surface reconstruction), buckled phases (PtO2-like stripes formed on the surface), place-exchanged phases (phases that contain O below Pt and may include PtO2 stripes), and hybrid phases that contain both buckled and place exchanged subunits. These structures represent the critical stable phases that are likely to influence system properties at room temperature and particularly at oxygen coverages above 1 /2 ML. Unfortunately, because of the complexity of the different Pt(111)/O phases, a comprehensive search of stable oxygen configurations has not been performed is still an open challenge for future methods. 9943
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/2 of the top Pt displaced, we predict the mixed phases would have 35% of their Pt displaced. Thus, with respect to displaced Pt, our structures are in excellent agreement with the experimental results of Nagy and You, which further supports our model of the phase stability in this coverage region. To interpret electrochemical data it is convenient to represent the phase stability as a function of applied electrochemical potential. Part a of Figure 3 shows the area
Figure 2. Hybrid place-exchange/buckled phase found to be most stable 1 ML coverage phase.
Pt buckling and oxygen place-exchange, and so we refer to it here as the hybrid structure. Two types of linear, O−Pt−O stripes (PtO2 stripes) appear in this structure. The first type of PtO2 stripe is a buckling stripe parallel to but above the Pt surface, with raised Pt, that is similar to the stripes proposed in refs.20,21 The second type of PtO2 stripe is a place-exchange-like stripe that is a rotation of the buckling chain, almost perpendicular to the surface, with oxygen under Pt. The Pt in the place-exchanged PtO2 stripe is displaced from the original Pt surface by 2.8 Å in the direction normal to the surface. This hybrid structure allows both for Pt screening of oxygen−oxygen interactions as well as an increased distance between oxygen not in the same PtO2 stripe. This structure also accounts for the experimental observations of subsurface oxygen, which cannot be explained by just buckling. Finally, the hybrid structure resolves the problem of how oxygen can move subsurface despite the large barriers associated with directly hopping oxygen through the Pt surface.18 The hybrid structure allows place-exchange through a simple rotation of PtO2 stripes. This rotation is likely to have a low barrier and readily occur at room temperature on short time scales. The specific charges on the displaced Pt and associated oxygen in the hybrid phase are compared to other relevant phases in Section 4 of the Supporting Information. It is useful to compare our structures to what other researchers have proposed at high oxygen coverage. Imai, et al.32 examined Pt surface oxidation on nanoparticles at 1.4 V and found formation of α- and then β-PtO2. Unfortunately, these structures are somewhat higher in oxygen composition than examined in this work so direct comparison is difficult. However, a more direct comparison is possible to the work of Nagy and You.13 Our 0.75 ML and 1 ML structures, whereas not identical to the high coverage phases proposed by Nagy and You13 (their Structures 5 and 6), have similar features and fraction of displaced surface Pt suggesting our phases are consistent with their experimental data and its implied Pt displacements. In fact, on the basis of the changes in electron density perpendicular to the surface in their Figure 6, we extract from Nagy and You13 that at a coverage of ∼0.85 ML (1.7 e-/ Pt in their nomenclature), about 35% (37.5% based on their proposed Structure 6 of their Figure 9) of the top Pt surface layer is perturbed relative to the uncovered surface. Applying the lever rule to our convex hull (Figure 1) yields 60% 0.75 ML and 40% 1 ML phases at 0.85 ML coverage. As the 0.75 ML phase has 1/4 of the top Pt displaced and the 1 ML phase has
Figure 3. (a) Surface energy curves as a function of applied voltage for different oxygen coverage phases at pH 2. Boxes indicate onset of a new stable surface phases at 1/9, 1/4, 3/4, and 1 ML. (b) Experimental oxide coverage observed during 20 mV/s CV sweep in 10 mM HF (from ref 34). Letters indicate key transitions and are discussed in the text.
normalized surface energy vs voltage (vs SHE) and the most stable phase at a given voltage is the one with the lowest surface energy at that voltage. Where the lines cross, relative stability shifts from one phase to another. The voltage regions where each phase is stable is given in Table 1. Details on how to Table 1. Predicted Stable Phases and Their Predicted Stable Voltage Ranges at pH 2 phase
voltage
0 ML 1 /9 ML 1 /4 ML 3 /4 ML 1 ML
V ≤ 0.71 V 0.71 V < V ≤ 0.76 V 0.76 V < V ≤0.99 V 0.99 V < V ≤ 1.23 V V > 1.23 V
construct part a of Figure 3 are given in Section 1 of the Supporting Information. These findings are consistent with previous studies26,33 although here we have added the buckled and hybrid phases. Because of stability of Pt buckling at higher oxygen coverages, the 3/4 ML buckled phase is predicted to be the next stable phase after 1/4 ML. At higher voltages, the hybrid buckled/place-exchanged structure becomes the most stable surface phase. We can now apply our new understanding of the Pt(111)/O equilibrium phases versus potential to interpret the Pt aqueous CV. The following interpretation contains significant speculation, but provides a plausible picture of which phases are 9944
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we associated with buckling, begin to occur at the upper end of the anodic sweep, with some remaining down to ∼0.59 V in the cathodic sweep. The above view of oxide formation and removal based on DFT analysis and comparison to experiment explains the hysteretic behavior in the Pt(111) CV. The buckled and hybrid structures are the irreversible oxide phases that lead to hysteresis as their onset, and removal, are kinetically inhibited.
involved in each stage of the CV curve. We compare our results to the recent CVs of Wakisaka et al.,34 who measured output current density of a Pt(111) crystal in 10 mM HF at a sweep rate of 20 mV/s. To facilitate the interpretation we represent the CV as oxygen coverage versus potential (part b of Figure 3). This transformation of the CV is done by integrating the current (removing a constant double layer current, taken to be the values at 0.49 V) and assuming a 1 ML surface charge density of 440 μC/cm2. From thermodynamic considerations, we expect plateaus to represent stable phases and steps to represent the transition between them. On the anodic sweep we associate the A to B region with the 1 /4 ML phase as it has a clear plateau at about 1/4 ML and extends over a very similar voltage range to that predicted for stability of the 1/4 ML structure (Table 1). The step from B to C represents the onset of a new phase and the partial leveling off at C near 1/2 ML suggests it is a 1/2 ML phase that is forming. The 1/2 ML phase is nearly stable in our calculations and the competing stable phase, the buckled 3/4 ML, is likely to be kinetically inhibited due to the buckling processes that must take place. We therefore believe that the region B to C represents the onset of a metastable 1/2 ML phase, probably with oxygen occupying the kinetically easy to access FCC sites. If the 3/4 ML phase is assumed to be inhibited the 1/2 ML FCC phase is predicted to form at 1.05 V, close to the observed onset at point B. From C to D, there is steady oxide growth but no clear features, consistent with the system being out of equilibrium and undergoing slow formation of the 3/4 ML buckled and 1 ML hybrid phases. The presence of kinetic inhibition is consistent with these higher coverage phases requiring significant distortion of the Pt surface. This voltage region is therefore expected to lead to irreversible oxide formation, consistent with experiments.8,9,11 By point D we expect the oxide consists of an out of equilibrium mixture of 1/2, 3/4, and 1 ML oxide phases. On the cathodic sweep we see that oxide continues to grow from D to E, consistent with this region being above the 1.23 V potential for forming 1 ML and the system not yet having achieved a full 1 ML coverage. At E the oxide growth stops as the 1 ML phase is no longer stable and the system is only stable with up to 3/4 ML. Thus, from E to F the oxide begins to be removed from the Pt surface and the system approaches the 3/4 ML coverage. Below the voltage of F the 3/4 ML phase is no longer stable and we expect a transition to 1/4 ML. Consistent with the instability of the 3/4 ML phase the coverage starts to decrease below 3/4 ML for voltages below 0.99 V. However, the transition to 1/4 ML does not occur rapidly and from F to G the oxygen removal is slow. This slow removal is presumably due to the kinetic inhibitions associated with decomposing the buckled 3/4 ML phase. By point G, the driving force for oxide removal has become extremely strong (the surface has far more oxide than the 1/4 ML expected at equilibrium) and the kinetic inhibitions associated with decomposing the buckled 3/4 ML phase are finally overcome. The oxide is then removed relatively rapidly, finally beginning to level out near point H. This leveling occurs somewhat below the stability of even the 1/4 ML phase and well into the lower voltage ranges where a more complete model including OH formation would be required to interpret the trends. We find that an approximately 1/6 coverage remains on the Pt surface at 0.5 V on the cathodic sweep. The interpretation of 3/4 ML buckled phases causing the hysteresis is consistent with the STM findings of Wakisaka et al. in which bumpy spots, which
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CONCLUSIONS In summary, the predicted Pt(111)/O stable surface phase diagram shows that up to 1 ML, there are four stable phases (at concentrations 1/9, 1/4, 3/4, and 1 ML). The stable 1 ML structure has not previously been identified and is a hybrid that contains elements of both buckled and place exchanged systems. This phase provides a molecular level understanding of what a place exchanged system may look like and suggests a low-barrier pathway by which place exchange and bulk PtO formation can occur. Detailed analysis shows that the features in the Pt(111)/O CV up to 1.3 V can be explained in terms of stable and metastable phases identified by the DFT, including a clear association of hysteresis with the buckled and hybrid structures.
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ASSOCIATED CONTENT
S Supporting Information *
Phase diagram formalism, analysis/discussion of oxygen− oxygen interaction, VASP energies and Bader charge analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Materials Science and Engineering Department, University of Wisconsin-Madison, 1509 University Ave., Madison, WI 53706, USA. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Department of Energy Basic Energy Science Hydrogen Fuel Initiative award number DEFG02-05ER15728 for financial support and NSF National Center for Supercomputing Applications award NCSADMR060007 for computational support. Jeff Greeley acknowledges support from the Department of Energy, Office of Science, Office of Basic Energy Sciences, under the Early Career Program. Also, thank you to Y. Shao-Horn, S. Kocha, J. Meyers, D. Myers, W. Schneider, and I. Szlufarska for thoughtful discussions on oxide surfaces.
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