Effects of Electronic Structure Modifications on the Adsorption of

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17052

J. Phys. Chem. C 2007, 111, 17052-17060

Effects of Electronic Structure Modifications on the Adsorption of Oxygen Reduction Reaction Intermediates on Model Pt(111)-Alloy Surfaces Matthew P. Hyman and J. Will Medlin* Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: June 30, 2007; In Final Form: August 29, 2007

In this work, the relationship between electronic structure and adsorption energies of oxygen reduction reaction (ORR) intermediates (O, OH, OOH, O2, and H2O) is investigated for modified Pt surfaces. Model surfaces were constructed to examine lattice strain and electronic ligand effects. Compressive strain, which broadens the metal d band, was found to destabilize adsorption of all of the intermediates. Whereas binding energy shifts due to strain correlate well for all of the intermediates examined, shifts in O adsorption energy resulting from ligand contributions were found not to correlate with the other intermediates. Additionally, the adsorption energy of oxygenate intermediates was found not to depend solely on the d-band center of the surface. Although the d-band center is important, adsorption is also dependent on the electron density near the Fermi level.

1. Introduction Fuel cells enable the efficient production of electricity from hydrogen and oxygen.1 However, numerous challenges exist that prevent the viability of fuel cells. One area of intense research is the development of new cathode materials for catalyzing the oxygen reduction reaction (ORR).2-13 Since the electrocatalytic ORR began to receive significant attention in the 1960s, it has been known that platinum exhibits the best oxygen reduction kinetics among the noble metals.14,15 However, ORR rates on pure Pt are several orders of magnitude less than rates of the hydrogen oxidation reaction, the complementary reaction that takes place at fuel cell anodes.16 Additionally, cathodes require significantly higher Pt loadings than anodes, due to the decreasing ORR rates with increasing specific surface area.7 The scarcity and cost of platinum have motivated the search for more efficient ORR catalysts. Pt-alloys, such as PtNi, PtCr, PtFe, and PtCo exhibit improved ORR rates, especially at higher voltages.7 It has long been recognized that the accumulation of oxygen species, either O or OH, decreases ORR rates via a reduction in the availability of surface sites and a repulsion of the ORR intermediates.14,17,18 This is evident in the transition from Langmuir adsorption kinetics at low potentials to Temkin adsorption kinetics at high potentials. In the Temkin regime, the electrode surface is covered with oxygenate species that increase the activation barrier for the ORR in addition to decreasing the availability of active sites. Teliska et al. used XANES to identify the oxygenate species as adsorbed on top sites at 0.84 V, which corresponds to OH rather than O.19 The authors observed a correlation between OH coverage on Pt and Pt-alloy particles and ORR rate. Mayrhofer et al. also demonstrated a relationship between OH coverage and ORR rate on Pt particles of varying size using cyclovoltammetry.20 Murthi et al. studied the ORR on Pt, PtCo, and PtFe using 1 and 6 M trifluoromethyl sulfonic acid (TFMSA) to evaluate the effect of water in the electrolyte.21 XAS analysis revealed that the formation of OH on all three electrodes in 6 M TFMSA * To whom correspondence should be addressed. E-mail address: [email protected]..

was negligible, while voltammetry experiments revealed no Temkin governed kinetics for any of the surfaces. This suggests that water dissociation is mainly responsible for OH accumulation at high potentials, rather than OH primarily accumulating during the forward ORR. Additionally, the authors found the alloys to be equally active (PtCo) to or more active (PtFe) than Pt using 1 M TFMSA, which has a higher water content than 6 M TFMSA. This observation implies that the alloys inhibit water dissociation and OH accumulation, which increases the availability of active sites. Compared to other transition metals, Pt adsorbs oxygen with intermediate bond strength. Ru, for instance, adsorbs oxygen so strongly that the surface fully oxidizes, while Au adsorbs oxygen so weakly that molecular oxygen desorption is more favorable than dissociation.22,23 Pt adsorbs oxygen strongly enough so that dissociation is favorable but not so strongly to oxidize the surface. Several studies have applied the understanding of this phenomenon to Pt alloys, attempting to use the adsorption energy of atomic oxygen as a metric for characterizing surfaces of Pt-alloys.24,25 Volcano plots show that peak ORR rates occur at intermediate O adsorption energies, which confirms that weak adsorption prevents oxygen from being reduced, while strong adsorption stabilizes OH adsorption. This is evident in the voltammetry experiments in TFMSA.21 In 1 M TFMSA, the ORR current density at 0.9 V is lower on Pt than on PtCo or PtFe due to formation of OH on the surface. Using 6 M TFMSA, which contains significantly less water, prevents OH from accumulating to high levels on Pt at 0.9 V. As a consequence, the current density is higher on Pt than on the alloys. The OH inhibition of the alloys is accompanied by a lesser activity toward oxygen reduction. Close examination of the data of Zhang et al.24 and Stamenkovic et al.25 reveal that the relationship between O adsorption energy and ORR activity is more qualitative than quantitative, which suggests that O adsorption alone is not sufficient to predict the ORR activity of a catalyst. In designing new ORR catalysts, one must understand the factors in addition to O adsorption that are important and exploit them to optimize both OH inhibition and the intrinsic ability to reduce oxygen.

10.1021/jp075108g CCC: $37.00 © 2007 American Chemical Society Published on Web 10/18/2007

Effects of Electronic Structure Modifications

J. Phys. Chem. C, Vol. 111, No. 45, 2007 17053

TABLE 1: Geometric and Electronic Properties of Surfaces surface Pt(111) Pt3.83 Pt3.91 Pt4.07 Pt4.15 PtNiPt PtCuPt PtFePt PtCoPt PtRuPt PtRhPt PtPdPt PtAgPt PtOsPt PtIrPt PtAuPt PtRh PtRu PtIr PtPd PtOs PtAg PtAu Pt/Pt3Co Pt/Pt3Ni Pt/Pt3Fe

alloy site

Pb Mb Pb Mb Pb Mb

Pt-Pt distance (Å)

d-band center (eV)

DOS at EFa

2.82 2.71 2.76 2.88 2.93 2.82 2.82 2.82 2.82 2.82 2.82 2.82 2.82 2.82 2.82 2.82 2.72 2.74 2.74 2.81 2.84 2.94 2.95 2.76 2.76 2.76 2.76 2.77 2.77

-2.36 -2.49 -2.44 -2.31 -2.22 -2.57 -2.27 -2.77 -2.67 -2.77 -2.57 -2.28 -1.79 -2.91 -2.74 -1.88 -2.76 -2.83 -2.85 -2.28 -2.65 -1.59 -1.60 -2.34 -2.63 -2.35 -2.57 -2.29 -2.59

1 0.66 0.76 1.02 1.06 0.62 0.40 0.57 0.66 0.80 0.93 0.93 0.39 0.74 0.84 0.40 0.64 0.50 0.88 0.89 0.82 0.50 0.69 0.61 0.66 0.71 0.71 0.60 0.58

Cu added to Pt shrink the bulk lattice, compressing the surface and increasing ORR rates. Excessive compression may actually inhibit the ORR rate, as Mukerjee et al. found the rate to decrease after reaching a maximum at approximately 2% compression.13 The enhancement of PtM surfaces without Pt overlayers, on the other hand, likely results from the inhibition of OH accumulation on Pt sites via repulsion of OH adsorbed on M sites. Zhang et al. demonstrated this using Pt0.8M0.2 monolayers deposited on Pd(111).28 Surfaces that bound OH strongly on the M sites exhibited enhanced ORR activity with a maximum enhancement at 20% M. Given the difficulty in controlling the catalyst surface properties experimentally, new insights into the ORR activity enhancement of PtM alloys may be gained through the use of theoretical techniques such as density functional theory (DFT). DFT has previously been used to explore the effects of alloys on oxygen adsorption and dissociation, as well as water and hydroxyl adsorption.29,30 While the ORR pathway has been explored using DFT on Pt single crystals, there has been no study that examines the complete pathway on modified Pt surfaces such as alloys, pseudomorphic overlayers, or PtMPt sandwiches. One reason for this is the uncertainty of the reaction mechanism. However, we previously reported the following pathway as being most favorable on Pt(111):31

O2 gas f O2 bridge O2 bridge + H+ + e- f OOHbridge OOHbridge f Ofcc + OHtop

a Density of states at the Fermi level normalized to that of Pt(111). Label P refers to Pt atom coordinated only with other Pt atoms. Label M refers to Pt atom coordinated with non-Pt ligand.

b

Obtaining fundamental information about the roles of alloys on the ORR kinetics is difficult, however, because of the difficulty in controlling surface properties. Properties that influence ORR activity include the Pt-Pt distance, particle size, and alloy distribution, all of which are affected by preparation conditions.10,26,27 To complicate matters, these properties can change after the catalyst has been used. Toda et al. found that after performing ORR experiments on PtNi, PtCo, and PtFe films of varying compositions, the surfaces became free of the alloying metal, indicating that leaching had occurred.9 Enhancement in the ORR activity of these alloys resulted from electronic structure changes to the Pt overlayer formed during the leaching. Pt overlayers can also form after annealing at high temperatures, which induces the platinum to segregate to the surface. Stamenkovic et al. found the activities of Pt overlayers to be dependent on the method of formation.4 In that study, the annealed PtM samples were found to be more active than the leached samples despite both being surface enriched in Pt. The difference in alloy composition in the subsurface region may explain the difference in activities. Somewhat surprisingly, Teliska et al. found the presence of a Pt “skin” on PtCr and PtFe after ORR operation but not on PtCo or PtNi.19 In the PtCr and PtFe electrodes, the alloying metals migrated to the bulk, leaving the surface enriched in pure Pt. However, the distribution of atoms in PtCo and PtNi did not change during ORR operation. Both the alloys were evenly distributed throughout the metal. That some alloys form Pt overlayers while others do not suggests two modes of activity enhancement. The ORR enhancement on Pt overlayers appears to be electronic in nature resulting from lattice strain. Several studies have found a linear dependence of the Pt-Pt spacing on the ORR activity, regardless of the alloying metal.11,12 Metals such as Cr, Co, Fe, Ni, and

Ofcc + H+ + e- f OHbridge OHbridge f OHtop OHtop + H+ + e- f H2Otop In the above mechanism, the subscripts refer to the adsorption sites that the species bind to on the Pt(111) surface. Although earlier work has suggested that the ORR activity depends on the atomic oxygen adsorption energy, rational catalyst design may benefit from an understanding of how alloys alter the adsorption of other species in the oxygen reduction and water dissociation reactions. This involves identifying which adsorbates are most important in determining ORR activity and determining if the activity can be predicted from the amount of strain and the identity of the alloying metal. In this work, we report DFT calculations exploring the effects of alloys via three different modes. The electronic ligand effect can be isolated by substituting the second layer of a Pt(111) slab with 3d, 4d, and 5d transition metals, creating a PtMPt sandwich. This approach has been used previously to study O and H adsorption.32 Likewise, the strain effect induced in alloys through altering the lattice constant can be isolated by changing the lattice parameter of the Pt(111) slab from its most stable value.33-36 Both effects can be combined in a pseudomorphic overlayer model in which a Pt layer is placed over a transition metal substrate.24,37 We also combine both effects by performing calculations on Pt3Co(111), Pt3Ni(111), and Pt3Fe(111) substrates covered by a Pt overlayer. These represent more moderately strained alloys with smaller ligand contributions than either the corresponding overlayers or PtMPt sandwiches. 2. Methods Density functional theory (DFT) calculations were performed with the Vienna Ab-Initio Simulation Package (VASP)38,39 using

17054 J. Phys. Chem. C, Vol. 111, No. 45, 2007

Hyman and Medlin TABLE 2: Adsorption Energies (kJ/mol) of Intermediates on Pt(111)a H2O -271.7

OH -163.4

O -116.0

O2 -68.2

OOH -119.5

a Energies referenced to O and H in the gas phase. H O adsorption 2 2 2 referenced to gas-phase H2O is -22.7 kJ/mol.

Figure 1. Energy change of adsorbates on strained surfaces relative to Pt(111): O (b), O2 (O), OH ([), OOH (0), and H2O (9).

plane-wave basis sets based on the projector-augmented wave (PAW) method.40 Basis sets were made finite using a cutoff energy of 400 eV. Bulk lattice constants were optimized using an 11 × 11 × 11 Monkhorst-Pack k-point mesh. Surface calculations were performed with fcc(111) or hcp(0001) surfaces modeled by 2 × 2 unit cells of four substrate layers and vacuum space equivalent to six metal layers. The top two metal layers were allowed to relax, while the bottom two were fixed in their bulk positions. PtMPt sandwiches consisted of a Pt top layer, a substituted second layer, and Pt in the third and fourth layers. Pt pseudomorphic overlayer slabs consisted of a Pt layer on top of three layers of the substrate. Sandwich and pseudomorphic overlayer calculations were performed with the 3d, 4d, and 5d metals in groups 8-11 (Fe to Cu). Excluded from the pseudomorphic overlayer calculations are the 3d metals, which severely strain the Pt surface. To combine both ligand and strain effects for the 3d metals (except for Cu), Pt overlayers were deposited epitaxially on Pt3M(111) substrates. These calculations, as well as PtMPt calculations with Fe, Co, and Ni, were performed with spin-polarization. On pure Pt surfaces, strain was induced by using slabs with lattice constants of 3.83, 3.91, 4.07, and 4.15 Å, compared to the optimized value of 3.99 Å. The strained Pt(111) surfaces will be referred to herein as Pta, where a is the lattice constant in Å. Adsorption of H2O, O, OH, O2, and OOH was examined on the surfaces listed in the preceding paragraph at 0.25 monolayer (ML) coverage. Atomic oxygen adsorbed preferentially in fcc hollow sites for all calculations; thus, only this adsorption energy is reported, while water adsorbed preferentially on the top site. Site preferences for hydroxyl adsorption were dependent on the surface, with adsorption most favorable on either the top site or bridge site. On most surfaces, the difference in energy between the two states is small ( O2 > OH, indicating that only a small compression is likely to be desirable. Mukerjee et al. found ORR rates to decrease for greater than 2% compression.13 When strain and ligand effects are combined, the ligand contribution is somewhat unpredictable. Figure 11 displays the adsorption energy changes for the Pt/Pt3M surfaces and Pt3.91. It should be noted that the lattice constants of Pt/ Pt3Co and Pt/Pt3Fe are 3.90 and 3.92 Å, respectively, but it may be assumed the small lattice constant variation will not significantly skew the comparison between surfaces. Both Pt/ Pt3Co and Pt/Pt3Fe bind OH 0.11 eV more weakly than Pt(111), a significant reduction in adsorption energy and equal to that of Pt3.91. This is not surprising since OH adsorbs preferably on top sites with only Pt nearest neighbors. The destabilization of OH on Pt/Pt3Ni is smaller but still significant. Oxygen adsorption, both atomic and molecular, is destabilized on Pt/ Pt3Co and Pt/Pt3Fe compared to Pt3.91. Oxygen adsorption on Pt/Pt3Ni is comparable to adsorption on Pt3.91. ORR studies in both fuel cell MEAs and in 0.1 M HClO4 have observed increases in ORR rates on alloy surfaces in the order PtNi < PtCo < PtFe.9,13 Figure 11 suggests that this may be because OH destabilization is smaller on PtNi, while O destabilization is too large on PtCo. However, another study in 0.1 M HClO4 observed that the activity increases Pt3Ni < Pt3Fe < Pt3Co for annealed surfaces and Pt3Fe < Pt3Ni < Pt3Co surfaces in which the 3d metal had been leached out, although the differences in activity for the leached surfaces are smaller.4 Therefore, comparison between calculations in this paper and experimental results is clearly hindered by the imprecise understanding of the structure of the surfaces examined experimentally. Nonetheless, the results of the Pt/Pt3M alloy calculations, along with experimental studies,11,12 indicate that straining the surface is necessary to inhibit the accumulation of OH but that the ligand effect is also important, albeit to a lesser degree. Exploratory calculations found that increasing the first subsurface layer of Pt/Pt3Co to 50% Co has only a small influence on the adsorption energies. Atomic oxygen adsorption is further destabilized by 0.05 eV and OH by 0.01 eV. Molecular oxygen is actually stabilized by 0.01 eV by increasing the subsurface Co concentration. The small effect on subsurface concentration underscores the strong dependence on Pt-Pt distance. 5. Conclusion DFT calculations were performed on altered Pt(111) surfaces to understand how lattice strain and electronic ligand effects influence the adsorption of intermediates of the electrocatalytic oxygen reduction reaction. Lattice strain was induced by compressing and expanding the Pt(111) lattice, while ligand effects were introduced by substituting 3d, 4d, and 5d transition metals into the subsurface. In addition, the two effects were combined by using Pt pseudomorphic overlayers on Pt3M alloy surfaces enriched in Pt. While OH, O2, OOH, and H2O adsorption energies correlated very well with each other, correlation with O adsorption energies was found to be poor. The adsorption energy of OH is likely the most important factor in a surface’s resistance to inhibition of the oxygen reduction

17060 J. Phys. Chem. C, Vol. 111, No. 45, 2007 reaction, while the O and O2 adsorption energies help determine the kinetically limited rates. Through weakened OH adsorption, the catalyst surface may become more resistant to OH accumulation, eliminating the repulsion of ORR intermediates. Compressive strain, due to a decrease in the Pt-Pt bond distance, was found to be the primary mechanism of OH destabilization on Pt/Pt3M surfaces but was also found to destabilize O and O2 adsorption. Thus, overly strained surfaces are likely to be unsuitable ORR catalysts. With the exception of strained Pt(111), the d-band center by itself was found to not be predictive of adsorption energy trends due to the uneven redistribution of the d band induced by ligand effects. Additionally, adsorption on surfaces with both strain and ligand effects cannot be predicted based on the effects separately. However, on surfaces with weak ligand effects, such as Pt/Pt3M surfaces, adsorption energy shifts can be approximated based on the Pt-Pt bond distance. Acknowledgment. M.P.H. thanks the Department of Education’s Graduate Assistance in Areas of National Need (GAANN) program for financial support. This research was supported in part by the National Science Foundation through the San Diego Supercomputer Center under Grant Number COB227 and utilized the Datastar system. References and Notes (1) Fuel Cell Technology Handbook; Hoogers, G., Ed.; CRC Press: Boca Raton, FL, 2003, p 311. (2) Ioroi, T.; Yasuda, K. J. Electrochem. Soc. 2005, 152, A1917. (3) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493. (4) Stamenkovic, V. R.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2006, 128, 8813. (5) Savadogo, O.; Lee, K.; Oishi, K.; Mitsushima, S.; Kamiya, N.; Ota, K. I. Electrochem. Commun. 2004, 6, 105. (6) Paulus, U. A.; Wokaun, A.; Scherer, G. G.; Schmidt, T. J.; Stamenkovic, V.; Radmilovic, V.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. B 2002, 106, 4181. (7) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal., B 2005, 56, 9. (8) Zhang, L.; Lee, K.; Zhang, J. J. Electrochim. Acta 2007, 52, 3088. (9) Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. J. Electrochem. Soc. 1999, 146, 3750. (10) Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J. J. Phys. Chem. 1995, 99, 4577. (11) Jalan, V.; Taylor, E. J. J. Electrochem. Soc. 1983, 130, 2299. (12) Min, M. K.; Cho, J. H.; Cho, K. W.; Kim, H. Electrochim. Acta 2000, 45, 4211. (13) Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J. J. Electrochem. Soc. 1995, 142, 1409. (14) Damjanovic, A. Progress in the Studies of Oxygen Reduction during the Last Thirty Years. In Electrochemistry in Transition; Murphy, O. J., Srinivasan, S., Conway, B. E., Eds.; Plenum Press: New York, 1992; p 107.

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