Electrooxidation of Methanol at SnOx–Pt Interface - American

Letter pubs.acs.org/JPCL

Electrooxidation of Methanol at SnOx−Pt Interface: A Tunable Activity of Tin Oxide Nanoparticles Wei-Ping Zhou,*,† Wei An,† Dong Su,‡ Robert Palomino,§ Ping Liu,† Michael G. White,†,§ and Radoslav R. Adzic† †

Chemistry Department and ‡Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States § Department of Chemistry, Stony Brook University, Stony Brook New York 11974, United States S Supporting Information *

ABSTRACT: Tin oxide nanoparticles supported on polycrystalline Pt exhibit a sizedependent promoting effect for the methanol oxidation reaction (MOR). We find that the deposition of 2 nm SnO2 nanoparticles on Pt electrode surfaces results in an activity increase for the MOR up to 40 times over bare Pt electrodes. Increasing the size of the SnO2 nanoparticles reduces the MOR activity enhancement, and at a size ∼20 nm, the SnO2 NPs show a negligible effect on the activity of bare Pt surfaces. Density functional theory calculations suggest that the catalytic activity of SnO2/Pt surfaces is strongly affected by the binding energy of adsorbed OH species on the SnO2 nanoparticles. In particular, a weaker OH−Sn interaction on the small, Pt-supported SnO2 NPs favors the release of adsorbed OH species for effectively oxidizing COads, the blocking intermediate in the MOR on Pt, making this combination an excellent catalyst. These results illustrate the importance of size-dependent chemical properties of nanostructured tin oxides in the catalytic performance of tin-oxide-promoted Pt electrocatalysts. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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affect the thermodynamic properties (heat capacity, vibrational entropy, etc.) of the adsorbed water layers.22 Subbaraman et al.13 reported that the interaction between the formed OH species and metal oxide (CoOx, NiOx, etc.) nanostructures at Pt−metal oxide interface plays an important role in electrocatalysis. Despite extensive efforts, however, there is no a clear picture about the influence of OH−tin oxide interactions on the MOR. Moreover, little is known about how these reaction steps are influenced by the chemical and structural properties of nanostructured SnOx and Pt-SnOx interactions.16−19 The lack of the knowledge hampers the further development and optimization of Pt-tin oxide binary and ternary catalysts.16−19 Here we report an investigation of the MOR over polycrystalline Pt (pc-Pt)-supported tin oxides NPs with diameters ranging from 2 to 20 nm. The deposition of SnO2 NPs on bulk Pt surfaces allows us to investigate in detail the synergetic effects of Pt−tin-oxide interface sites and the influence of size-dependent chemical properties of tin oxide NPs in the kinetics of the MOR.14,20,21 We find that the catalytic performance of SnO2 NPs on pc-Pt electrodes exhibits a strong dependence on the SnO2 particle size as well as the available SnOx−Pt interface sites. Density functional theory (DFT) calculations suggest that the binding energy (BE) of

lectro-oxidation of methanol to CO2, the anode reaction for direct methanol fuel cells (DMFCs), is among of the most important and well-studied reactions in electrocatalysis.1−3 Oxidative removal of chemisorbed CO via a Langmuir− Hinshelwood reaction (COad + OHad → CO2 + H+) is one of the key steps influencing the overpotentials of the methanol oxidation reaction (MOR) on Pt electrodes, the benchmark catalyst used in DMFCs.1−6 Therefore, extensive efforts have been undertaken to identify and understand cocatalysts that can facilitate this reaction step on Pt surfaces.1−10 Metal oxides are usually considered to be effective catalysts for the dissociation of H2O,11−16 a catalytic property that is crucial to promote the Pt for the MOR. For example, the tinoxide-promoted Pt nanocatalysts have been shown to have high performance for the MOR.17−19 The high activity of Pt/SnOx catalysts has been attributed to a bifunctional mechanism, whereas the formed OH species on tin oxides at low potentials facilitates the CO oxidation during the MOR.16−19 The formation of OH species from dissociative adsorption of water is believed not to be difficult on tin oxides; recent studies suggest that water dissociation can spontaneously occur even on a well-ordered SnO2(110) surface at room temperature.15,16 However, the dissociation of water on bulk tin oxides relies heavily on tin oxide surface preparation and structural properties.11,15 Moreover, our previous studies also indicate that SnOx nanostructures at the SnOx/Pt interface have chemical properties different from bulk tin oxides.20,21 The size of the tin oxide nanoparticles has also been suggested to © 2012 American Chemical Society

Received: October 5, 2012 Accepted: October 25, 2012 Published: October 25, 2012 3286

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adsorbed OH species, rather than the energetics of water dissociation process, is strongly affected by the size of the SnO2 NPs. Compared with large bulk-like SnO2 NPs, a weaker OH− Sn interaction on the small, Pt-supported SnO2 NPs favors the release of OH species for effectively oxidizing chemisorbed CO at Pt sites and thereby enhances the overall MOR activity. The SnO2 (NPs)/pc-Pt electrodes were prepared in a twostep process, which includes the ethylene glycol synthesis of SnO2 NPs at 190 °C in air,23 followed by their immobilization on a pc-Pt electrode (see the Supporting Information for experimental details).24 Representative high-resolution transmission electron microscopy (HRTEM) images of the SnO2 NPs are presented in Figure 1. Histograms of the particle size

NPs with sizes of 2 and 20 nm as well as a bare pc-Pt electrode in 0.5 M CH3OH in 0.1 M HClO4 solution at room temperature. Deposition of 2 nm SnO2 nanoparticles (at an approximate coverage of 50% for Figure 2a) on the pc-Pt electrode results in an active surface for the MOR that exhibits a large negative shift in the onset potential (∼0.17 V) and a significantly higher current as compared with the bare pc-Pt electrode. In addition, the MOR activity also shows a strong dependence on the surface coverage of the 2 nm SnO2 NPs (Figure 2b). The catalytic activity of the SnO2(2 nm)/pc-Pt surface reaches a maximum at a ∼ 50% coverage of pc-Pt surface and then decreases at higher SnO2 coverage. These observations are consistent with the bifunctional mechanism.3,7,10 By contrast, adding 20 nm SnO2 NPs on Pt at a coverage between ∼20 and 60% shows very little effect on MOR activity because the onset potential and current density of the SnO2(20 nm)/Pt electrode are approximately the same as that of bare pc-Pt electrodes. Chronoamperometric measurements shown in Figure 2c confirm the significant enhancement in catalytic performance and stability for the SnO2(2 nm)/pc-Pt electrodes. After running the reaction for 3600s at a potential of 0.25 V at room temperature, the SnO2(2 nm)/Pt electrode with a ∼50% coverage still exhibits high activity, with a measured current density of ∼20 μA/cm2. This is ∼10 times higher than that of the SnO2(5 nm)/Pt electrode (2.4 μA/cm2) and roughly 40 times greater than that of the bare pc-Pt (0.5 μA/cm2) (Figure 2d). Moreover, the SnO2(20 nm)/Pt electrode generates essentially the same current density (0.8 μA/cm2) as that of the bare pc-Pt electrode, again showing that the 20 nm SnO2 NPs do not promote the reaction. These results clearly demonstrate that depositing small SnO2 NPs on a pc-Pt electrode leads to significantly enhanced activity for the MOR, and this promoting effect quickly decreases as the SnO2 particle size increases to 20 nm. It is also noteworthy here that the MOR activity of SnO2(2 nm)/Pt surface is comparable to that previously reported for Ru/Pt catalysts, which are currently the most effective electrocatalysts known for the MOR.1,2,8,10 To investigate the chemical state of the tin oxide NPs before and after electrochemical reaction, we mounted the SnO2(NPs)/pc-Pt electrodes in a UHV chamber with XPS instrumentation and an in vacuo transfer setup, which allows transfer to an electrochemical cell without air exposure.24 Figure 3 shows a comparison of Sn 3d core-level XPS spectra obtained from a pc-Pt electrode covered with 2 and 20 nm SnO2 NPs before and after electrochemical testing. Prior to electrochemical measurements, both the 2 and 20 nm SnO2 NPs show a line shape with a dominant Sn4+ oxidation state.20,21,25 Surprisingly, electrochemical testing causes a significant change in the Sn 3d line shape for the 2 nm SnO2 NPs but has a negligible effect for the 20 nm NPs. In particular, the peak intensity for Sn(IV) species in the 2 nm SnO2 NPs decreases considerably after electrochemical measurements, and the feature for reduced Sn(II) species at lower BE becomes the dominant peak. The latter is most likely an oxidation form of Sn that interacts strongly with the Pt substrate that is known to have a 3d BE less than bulk SnO.20,25,26 The corresponding O 1s spectra (Figure s4 of the Supporting Information) also indicate that hydroxyl-species are the dominant oxygen species adsorbed on 2 nm NPs before and after electrochemical testing. By contrast, the Sn 3d spectra for the 20 nm NPs shows a negligible amount of Sn(IV) reduction after electrochemical

Figure 1. Comparison of voltammetric curves for pc-Pt-supported SnO2 NPs at size of 2 (solid red line) and 20 nm (dashed black line) in 0.1 M HClO4 solution. Scan rate: 50 mV s−1. Currents in the present work are normalized to the bare Pt surface area (Hupd charge after double-layer correction), prior to SnO2 deposition, with assuming a relationship of 210 μC/cm2pt. Typical HRTEM images of SnO2 nanoparticles at 2 (a) and 20 nm (b) are highlighted.

distribution (Figure s1 of the Supporting Information) indicate that the average particle sizes for three SnO2 NPs samples are: 2 ± 1, 5 ± 2, and 20 ± 10 nm. For the remainder of the letter, these three samples will be identified by their average size value, that is, 2, 5, and 20 nm. The selected area electron diffraction pattern (Figure s2 of the Supporting Information) shows that the continuous rings from the 2 and 20 nm SnO2 NPs can be indexed to the same crystal planes in a rutile SnO2. The immobilization method led to a relatively good dispersion of SnO2 nanoparticles on Pt with some agglomeration (Figure s3 of the Supporting Information). Figure 1 shows the cyclic voltammetry (CV) of the pc-Pt electrode partially covered by the 2 and 20 nm SnO 2 nanoparticles. The salient CV feature on the SnO2(2 nm)/ pc-Pt electrodes is the oxidation/reduction processes between 0.2 and 0.5 V that are not observed on the SnO2(20 nm)/pc-Pt electrodes. The change of the hydrogen adsorption charge (Hads) due to the addition of SnO2 nanoparticles on Pt has been used to determine the SnO2 surface coverage. All potentials are referenced to a silver−silver chloride (sat. NaCl) electrode. Figure 2a compares polarization curves for methanol oxidation over a pc-Pt electrode partially covered with SnO2 3287

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Figure 2. (a) Comparison of current−potential polarization curves for a pc-Pt-supported SnO2 nanoparticles at size of 2 (red line) and 20 nm (green line) and a bare Pt electrode (black line) in 0.5 M CH3OH and in 0.1 M HClO4 solution. Sweep rate is 10 mV/s. (b) Current density for the MOR on pc-Pt electrodes partially covered with 2 nm SnO2 NPs at 3600s and at 0.25 V. The fraction of Pt sites is determined from the change of Hupd before and after SnO2 deposition. Uncertainties of activity were determined from at least two measurements. (c) Comparison of current−time plots for the MOR on pc-Pt electrode partially covered by SnO2 NPs with sizes of 2 (red), 5 (blue), and 20 nm (green) at ∼50% coverage and a bare pc-Pt electrode (black) in a methanol solution for 3600s reaction time at 0.25 V at room temperature. (d) Current density from the corresponding surfaces of panel c at 3600 s at 0.25 V.

describe larger (≥20 nm) SnO2 particles, whereas a Sn7O14 cluster with an adopted bulk-like structure was used to model the small NPs (1 to 2 nm) on Pt(111) (Figure 4, see SI for computational details). We focused our calculations on two key elementary steps that significantly influence the MOR activity on Pt electrocatalysts, that is, H2O dissociation (H2O*→ H* + *OH) and CO oxidation (CO* + *OH → H* + CO2), to describe the promoting effects of SnO2 on Pt.6,8 The Pt−CO interaction in calculation is treated to be independent of the SnO2 particle size.20 In accordance with our previous study,16 water dissociation on a SnO2(110) surface is an energetically favored process with an OH BE of −1.60 eV, whereas it is a highly activated process on a clean Pt(111) surface.29 Our calculation shows that dissociative adsorption takes place at the coordinatively unsaturated Sn sites (SnCUS) with the resulting H-atoms attached to nearby bridging oxygen (OBR, Figure 4). On small supported NPs, dissociative adsorption of water occurs on similar SnCUS-like sites at the Sn7O14/Pt(111) interface. (See Figure 4.) Moreover, the calculations predict that water dissociation should be facile at the Sn7O14/Pt(111) interface, with a very small activation barrier (0.12 eV, Figure s5 of the Supporting Information). Given the small difference on the energetics of water dissociation between SnO2(110) and Sn7O14/Pt(111) surfaces, the calculations suggest that the size

Figure 3. XPS spectra of Sn 3d for pc-Pt supported SnO2 NPs at a size of 2 (a) and 20 nm (b) before and after electrochemical measurements. See refs 20 and 21 for the assignment of the surface Sn species indicated by the dashed lines.

testing, but the O 1s spectra indicates an increase in adsorbed hydroxyl-species (Figure s4 of the Supporting Information). To better understand the apparent size effects of Ptsupported SnO2 NPs toward the MOR, we carried out periodic DFT calculations using Vienna ab initio simulation package (VASP).27,28 The extended SnO2(110) surface was used to 3288

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Figure 5. Side view of the 3D charge density difference isosurfaces. (a) Bare SnO2/Pt(111) and (b) OH-adsorbed SnO2/Pt(111). Color scheme: deep blue, deep gray, red, and black balls represent Pt, Sn, O, and H atoms, respectively. Yellow and light-blue isosurfaces represent charge accumulation (i.e., gain of electron density) and depletion (i.e., loss of electron density) in the space with respect to isolated adsorbate and clean slab. The isovalue chosen to plot the isosurfaces is 0.02 e/Å3. Charge density difference is defined as Δρ = ρ(slab+ads) − ρslab − ρads.

Figure 4. Calculated PDOS of sp state of one Sn atom (see arrow), at which H2O dissociation occurs. (a) Bare SnO2(110) versus bare Sn7O14/Pt(111) and (b) OH-adsorbed SnO2(110) versus OHadsorbed Sn7O14/Pt(111). Color scheme: deep blue, deep gray, red, and black balls represent Pt, Sn, O, and H atoms, respectively. The solid and dashed lines represent SnO2(110) and Sn7O14/Pt(111), respectively.

In summary, a combination of experimental (CV, CA, XPS, TEM) and theoretical (DFT) approaches was used to investigate the MOR on pc-Pt supported SnO2 nanoparticles. The catalytic performance of SnO2 NPs/pc-Pt electrocatalysts for the MOR shows a strong dependence on the size of SnO2 NPs as well as the available Pt-SnOx interface sites. We find that the deposition of 2 nm SnO2 NPs on Pt electrode surfaces at ∼50% coverage results in a ∼40 times activity increase over bare Pt electrodes, whereas adding 20 nm SnO2 NPs leads to a negligible activity enhancement. Furthermore, XPS has identified the formation of reduced Sn(II)O, a chemical state of Sn believed to be essential for surface CO oxidation process,20,21 on Pt-supported 2 nm SnO2 NPs. Our DFT study suggests that the unique chemical properties of Pt-supported small SnO2 NPs, which is attributed to the Pt−Sn interaction and the structural flexibility of small NPs, results in a weaker binding of adsorbed OH species than that on the bulk-like large NPs. The easy release of OH species on small, Pt-supported NPs leads to more effective oxidation of chemisorbed CO at Pt interface sites, making this surface an excellent catalyst for the MOR. Such activity dependency on the chemical properties of tin oxide nanoparticles provides a fundamental basis for the future design of highly active tin oxide−Pt nanocatalysts for DMFCs applications.

of the SnO2 NPs has a very small effect for O−H bond cleavage. However, the BE of adsorbed OH species at the Sn7O14/Pt(111) interface (BE = −1.32 eV) is significantly lower than that on the SnO2(110) surface (BE = −1.60 eV). This leads to a smaller positive Gibbs free energy (ΔGrxn, 298 K) for the subsequent CO oxidation step on Sn7O14/Pt(111) (ΔGrxn = 1.12 eV) compared with that on SnO2(110)/Pt(111) (ΔGrxn = 1.40 eV). Overall, these calculations suggest that the lower BE of OH species on the Sn7O14/Pt(111) surface allows them to react more readily with chemisorbed CO at Pt interface sites, making a better catalyst for the MOR. The unique chemical properties of small, Pt-supported SnO2 NPs are attributed to the interaction between SnO2 and Pt(111) and the flexibility of its structure. The latter has been noted for small metallic nanoparticles and can play an important role in catalysis.30,31 As shown in the projected density-of-states calculations (PDOS) in Figure 4a, the sp state of the Sn atom on the bare Sn7O14/Pt(111) surface (red) is slightly more populated from −4 eV up to Fermi level than that of SnO2(110) (black), suggesting a partial reduction of small SnO2 NPs when interacting with Pt(111), which agrees with a previous study of SnOx nanostructures on Pt(111).20,21 This is also confirmed by the charge density difference isosurface (Figure 5a) showing the charge depletion from Pt (blue) and the charge accumulation (yellow) via the SnCUS−Pt bond (bond length, 2.61 Å). When interacting with OH, the SnCUS site on SnO2(110) stays mostly intact (left top, Figure 4) and binds strongly the OH species with a SnCUS → OH electron transfer (Figure 4b). By contrast, the adsorbed OH on Sn7O14/ Pt(111) surface pulls the SnCUS of the small NP out of the Pt surface. This results in a large bond separation between the SnCUS and the Pt (bond length, 4.04 Å) and a distorted local structure near SnCUS with a broken Sn−Sn bond (left bottom, Figures 4 and 5b). Consequently, the charge accumulation (yellow) along the SnCUS−Pt bond disappears (Figure 5b) and the charge density around the SnCUS sites decreases. This adsorbates-induced structural change in the Pt-supported Sn7O14 NP leads to a weaker SnCUS−OH interaction and, as noted above, improves the overall energetics for the subsequent oxidation of chemisorbed CO at Pt interface sites.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the experimental procedures and equipment descriptions, computation methods, and additional TEM and SEM images, XPS spectra, and computational results. This material is available free of charge via the Internet http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences under contract no. DE-AC0298CH10886. TEM measurements were carried out at the Center for Functional Nanomaterials (CFN) at Brookhaven 3289

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(20) Axnanda, S.; Zhou, W.-P.; White, M. G. CO Oxidation on Nanostructured SnOx/Pt(111) Surfaces: Unique Properties of Reduced SnOx. Phys. Chem. Chem. Phys. 2012, 14, 10207−10214. (21) Zhou, W.-P.; Axnanda, S.; White, M. G.; Adzic, R. R.; Hrbek, J. Enhancement in Ethanol Electrooxidation by SnOx Nanoislands Grown on Pt(111): Effect of Metal Oxide-Metal Interface Sites. J. Phys. Chem. C 2011, 115, 16467−16473. (22) Spencer, E. C.; Ross, N. L.; Parker, S. F.; Kolesnikov, A. I.; Woodfield, B. F.; Woodfield, K.; Rytting, M.; Boerio-Goates, J.; Nayrotksy, A. Influence of Particle Size and Water Coverage on the Thermodynamic Properties of Water Confined on the Surface of SnO2 Cassiterite Nanoparticles. J Phys Chem C 2011, 115, 21105−21112. (23) Jiang, L.; Sun, G.; Zhou, Z.; Sun, S.; Wang, Q.; Yan, S.; Li, H.; Tian, J.; Guo, J.; Zhou, B.; Xin, Q. Size-Controllable Synthesis of Monodispersed SnO2 Nanoparticles and Application in Electrocatalysts. J. Phys. Chem. B 2005, 109, 8774−8778. (24) Lewera, A.; Zhou, W. P.; Vericat, C.; Chung, J. H.; Haasch, R.; Wieckowski, A.; Bagus, P. S. XPS and Reactivity Study of Bimetallic Nanoparticles Containing Ru and Pt Supported on a Gold Disk. Electrochim. Acta 2006, 51, 3950−3956. (25) Rotermund, H. H.; Penka, V.; Louise, L. A. D.; Brundle, C. R. Oxygen Interaction with Pd3Sn: X-ray Photoelectron Spectroscopy and Secondary Ion Mass Spectrometry. J. Vac. Sci. Technol., A 1987, 5, 1198−1202. (26) Mukerjee, S.; McBreen, J. An In Situ X-Ray Absorption Spectroscopy Investigation of the Effect of Sn Additions to CarbonSupported Pt Electrocatalysts: Part I. J. Electrochem. Soc. 1999, 146, 600−606. (27) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558−561. (28) Kresse, G.; Furthmaller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (29) Rossmeisl, J.; Norskov, J. K.; Taylor, C. D.; Janik, M. J.; Neurock, M. Calculated Phase Diagrams for the Electrochemical Oxidation and Reduction of Water over Pt(111). J. Phys. Chem. B 2006, 110, 21833−21839. (30) Barrio, L.; Liu, P.; Rodriguez, J. A.; Campos-Martin, J. M.; Fierro, J. L. G. A Density Functional Theory Study of the Dissociation of H2 on Gold Clusters: Importance of Fluxionality and Ensemble Effects. J. Chem. Phys. 2006, 125, 164715. (31) Tao, F.; Grass, M. E.; Zhang, Y.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. Reaction-Driven Restructuring of Rh-Pd and Pt-Pd Core-Shell Nanoparticles. Science 2008, 322, 932−934.

National Laboratory (BNL). The computing work was performed at CFN Cluster at BNL and at the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the U.S. DOE under Contract No. DE-AC02-05CH11231. R.P. was supported by the NSF GRFP.

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