In Situ Hard X-ray Photoelectron Study of O2 and H2O Adsorption on

May 10, 2016 - In this study, H2O and O2 adsorption and dissociation as the first step of the reduction process were investigated by in situ hard X-ra...
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In Situ Hard X‑ray Photoelectron Study of O2 and H2O Adsorption on Pt Nanoparticles Yitao Cui,† Yoshihisa Harada,†,‡ Eiji Ikenaga,§ Rui Li,∥ Naoki Nakamura,⊥ Tatsuya Hatanaka,# Masaki Ando,⊥ Toshihiko Yoshida,⊥ Guo-Ling Li,▽ and Masaharu Oshima*,† †

Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan Institute for Solid State Physics, The University of Tokyo, 1-1-1 Kouto, Sayo-cho, Hyogo 679-5198, Japan § JASRI/SPring-8, 1-1-1 Kouto, Sayo-cho, Hyogo 679-5198, Japan ∥ Department of Chemistry, Liaocheng University, Liaocheng 252059, China ⊥ Toyota Motor Corp., 1200 Mishuku, Susono, Shizuoka 410-1193, Japan # Toyota Central R&D Laboratories., Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan ▽ School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471023, China ‡

S Supporting Information *

ABSTRACT: To improve the efficiency of Pt-based cathode catalysts in polymer electrolyte fuel cells, understanding of the oxygen reduction process at surfaces and interfaces in the molecular level is essential. In this study, H2O and O2 adsorption and dissociation as the first step of the reduction process were investigated by in situ hard X-ray photoelectron spectroscopy (HAXPES). Pt 5d valence band and Pt 3d, Pt 4f core HAXPES spectra of Pt nanoparticles upon H2O and O2 adsorption revealed that H2O adsorption has a negligible effect on the electronic structure of Pt, while O2 adsorption has a significant effect, reflecting the weak and strong chemisorption of H2O and O2 on the Pt nanoparticle, respectively. Combined with ab initio theoretical calculations, it is concluded that Pt 5d states responsible for Pt−O2 bonding reside within 2 eV from the Fermi level.

1. INTRODUCTION

understanding of the process in the molecular level, such as H2O and O2 adsorption and dissociation, is quite important. From the viewpoint of electronic states, interaction between Pt 5d projected density of states (d-pDOSs) and valence states of the intermediate species is crucial to understand the adsorption/desorption process. Upon chemical adsorption of a molecule on a Pt surface, hybridization with the valence states of the adsorbate splits the Pt d-pDOS into bonding and antibonding states. One of the most frequently used methods to monitor the splitting is Pt L-edge X-ray absorption fine structure (XAFS),6,7 which probes the antibonding fraction of the split Pt 5d states. To discuss the Pt 5d orbitals responsible

To reduce the energy impact on the environment, polymer electrolyte fuel cells (PEFCs) have attracted much attention as environmental power sources in this decade.1,2 Because oxygen reduction reaction (ORR) on the cathode is much slower than the hydrogen oxidation reaction on the anode, a large number of Pt or Pt alloy catalysts is required on the cathode. To obtain higher performance of PEFCs, it is strongly requested to reduce the high over potential in the four-electron reduction process of oxygen.3 One of possible mechanisms for the high over potential is the interaction between Pt catalysts and remaining intermediates such as OH species.4 The more strongly the intermediates bond to Pt atoms, the more effectively they are dissociated5 and hardly desorbed, which might hinder the successive ORR, resulting in the degraded cell performance. Therefore, to improve the efficiency of the Pt-based catalysts, © XXXX American Chemical Society

Received: March 7, 2016 Revised: May 9, 2016

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DOI: 10.1021/acs.jpcc.6b02402 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C for the adsorption, however, it is more straightforward to probe changes in the “occupied” Pt 5d orbital before and after the adsorption. According to the d-band center theory, the centroid of the “occupied” Pt d-pDOS is used as a good measure of the bond strength with adsorbates.8,9 While the conventional X-ray photoelectron spectroscopy (XPS) and vacuum ultraviolet photoelectron spectroscopy (UPS) provide information about the mixed Pt 5d and O 2p “occupied” electronic states, hard Xray photoelectron spectroscopy (HAXPES) can directly probe Pt 5d electronic states in the valence band up to 10 eV, owing to the very small photoionization cross sections for the O 2p and Pt 6p orbitals against the excitation energy around 8 keV as well as the low contribution of Pt 6s, 6p orbitals to the total DOSs compared with the Pt 5d states. (Explanations for this dominant selection of Pt 5d states are described in Section 1 of Supporting Information.) In this work, we will present a systematic use of the HAXPES to evaluate the interactions of H2O and O2 adsorbed on Pt nanoparticle catalysts at in situ conditions to be a prototypical work for probing intermediates by monitoring Pt 5d valence electronic states.

total surface area) in the equilibrium morphology of commercially available Pt nanoparticles according to Wulff construction method16 based on surface energy considerations,17 the active domain of the Pt nanoparticles was modeled by a Pt (111) surface using a three-layer periodic (2 × 2) slab and experimental equilibrium lattice constant of 3.92 Å. The vacuum gap between the two successive Pt metal slabs was set to 15 Å, and it was large enough so that the two successive slabs did not interact significantly. All calculations used spin polarization. The cutoff for the wave functions and charge density was set to 30 and 300 Ry, respectively. The first Brillouin zone was sampled using 4 × 4 × 1 Monkhorst−Pack k-points mesh. Methfessel−Paxton method18 was used for smearing with a broadening parameter of 0.02 Ry. Broyden− Fletcher−Goldfarb−Shanno (BFGS) method was used for optimization of geometric structure ensuring residual forces, and change in total energy was below10−3 Ry/au and 10−4 Ry, respectively, between the BFGS steps. All calculations were completed with Quantum-Espresso package.19 More details can be found elsewhere.20

2. EXPERIMENTS The Pt/C nanoparticle sample (average particle size around 2 to 3 nm and Pt loading around 46.6 wt %) is commercially available catalyst from Tanaka Kikinzoku Kogyo (TKK), Japan. The preparation condition of samples for in situ HAXPES measurement is presented in Section 2 of the Supporting Information. The in situ HAXPES measurements were carried out with a VG Scienta R4000 hemispherical electron analyzer at undulator beamline BL47XU of SPring-8.10 The incident X-rays was monochromatized with Si (111) double crystal and Si (444) channel-cut monochromators. To remove the nondipole effects, we fixed the analyzer at the parallel direction of X-ray polarization. The ex situ reduced sample was fixed to Cu holders by carbon-conducting tapes and then was mounted on the in situ cell, as described in Section 3 of the Supporting Information. The takeoff-angle (TOA) was fixed to 89°. The energy position of the Au 4f7/2 peak was used for energy calibration. A total energy resolution of 230 meV was evaluated by Au Fermi edge at room temperature. The inelastic mean free path (IMFP λ) of Pt was ∼6 nm (with the electron kinetic energy around Fermi level, 7939 eV) calculated from the TPP-2 M equation,11 generating an effective information depth of approximately 14−18 nm (3λ, from Pt 3d core level to valence band) below the surface, much larger than the diameter of the Pt nanoparticles (2 to 3 nm). Hence, the electronic structures of whole Pt nanoparticles can be detected by HAXPES. Moreover, hard-X-ray-induced photoelectrons keep relatively high kinetic energy, easily passing a certain gas layer and a 15 nm thick Si3N4 membrane window with less energy loss than that of soft X-rays. All core-level spectra were fitted with CasaXPS12 by subtracting a linear background for the O 1s core level and a Shirley background for the Pt 3d and 4f core levels, respectively.

4. RESULTS AND DISCUSSION To monitor the amount of O2 and H2O gas in the in situ HAXPES experiments, we measured O 1s core-level spectra for different experimental conditions. As shown in Figure S3 of the Supporting Information, contribution of gas phase O221,22 and H2O was confirmed as the double-peak structure at 538−540 eV21,22 and the small but sharp peak appearing around 535.5 eV,23,24 respectively. The Pt 3d and 4f core levels show little changes for the ex situ reduction and H2O adsorption conditions compared with those for the in situ reduction, while a large oxidized component appears for O2 adsorption, as shown in Figure 1. For the ex situ reduction, possible O2 chemisorption to the Pt surface during sample preparation would be easily removed in the high vacuum, which was confirmed by Zhu et al. in their high-pressure scanning tunneling microscopy (HP-STM) and XPS measurements.25 The negligible change in the Pt 3d and 4f

3. CALCULATIONS The first-principles DFT calculations were performed with generalized gradient approximation (GGA)13 of Perdew− Burke−Ernzerhof (PBE) exchange-correlation functional14 and a plane-wave basis set. Vanderbilt ultrasoft pseudopotential15 was used to describe the ionic cores with scalar-relativistic calculations. Because the (111) facets are dominated (∼83% in

Figure 1. Pt 3d5/2 (a) and 4f (b) core-level spectra of Pt nanoparticles under in situ/ex situ reductions and 100 Pa O2 and H2O (N2 as carrier gas, RH100%) adsorption conditions. B

DOI: 10.1021/acs.jpcc.6b02402 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. (a) Valence band spectra under in situ/ex situ reductions, 100 Pa O2 ,or H2O (N2 as carrier gas, RH100%) adsorption conditions. (b) Calculated projected Pt d-pDOSs of bare Pt (111) surface with different adsorbates, (c) experimental difference spectra of ex situ reduced, H2O, and O2 adsorbed conditions (solid lines) obtained by subtracting the in situ reduced spectrum, together with calculated difference spectra of H2O and O2 adsorption (dashed lines) to bare surface obtained by subtracting the in situ reduced spectrum.

On the contrary, for chemisorption of O2 (denoted as O2Che.) the short Pt−O bond length (1.98 to 2.08 Å) and the strong Pt−O bonding (calculated adsorption energy of −0.78 eV) favor the strong overlap of O 2p and Pt 5d orbitals, and the resulting bonding and antibonding states are broadened, especially significantly for the O 2p fraction compared with that of isolated O2 molecule. The O 2p bonding states around 8 eV interact weakly with Pt 5d states, as shown in Figure S6f, while the O 2p antibonding states around the Fermi level are strongly hybridized with Pt 5d states and part of Pt 5d electronic states are pushed out above the Fermi level, which is responsible for the drops of Pt 5d DOSs around the Fermi level, as shown in Figures S6f and S5b. With the electron transfer from Pt 5d to unfilled O 2p antibonding orbitals of the O2 molecule, O−O bonds are effectively weakened, which results in a notable bond length change from 1.24 Å of isolated O2 molecule to 1.36 Å of chemisorbed O2 on the Pt surface. Therefore, the significant decrease in the DOS around the Fermi level in the experiment can be clearly explained by charge transfer from the Pt 5d states around the Fermi level to the O 2p states, which smears out the fine DOS structures of the Pt (111) surface. Therefore, it is reasonable that the difference spectra of both calculated and experimental results show closely related structures at 0−2 eV and >5 eV. All of the above results clearly demonstrate that O2 molecules should be chemisorbed on the Pt surface in our experiments. Both physisorption (not shown) and chemisorption of an H2O molecule on the Pt (111) surface have negligible effect on the Pt 5d DOSs of the Pt (111) surface. As discussed above, it is reasonable that H2O physisorption has negligible effect on the Pt 5d DOSs compared with that of O2 physisorption. For the chemisorption of H2O on Pt surface, due to the large Pt−O bond length (2.58 Å) and the strong H−O bonding (almost identical to that of isolated H2O molecule), the centroid of the O 2pz DOS, which is responsible for the Pt−O bonding, is localized at a relatively low binding energy around 4 to 5 eV, which indicates a very weak Pt−O bonding (calculated adsorption energy of −0.19 eV) and leaves negligible bonding and antibonding states for H2O chemisorption, as shown in Figure S6e. These calculations well explain our experimental results by H2O adsorption. The O2 coverage in our experiments can be estimated from quantitative analysis of the spectral changes. Owing to the large

levels for the H2O adsorption indicates small binding energy of H2O chemisorption to the Pt surface. Figure 2a shows Pt 5d contribution to the valence band spectra under ex situ reduction (in vacuum), after in situ reduction (in vacuum), after H2O (N2 as carrier gases, with pressure of 100 Pa and 100% relative humidity (RH) at room temperature) adsorption, and after O2 (with pressure of 100 Pa) adsorption conditions. As shown as the difference spectra of the ex situ reduced H2O and O2 adsorption conditions relative to that of in situ reduced condition in Figure 2c, only for the O2 adsorption does the valence band show clear intensity drop from the in situ reduction up to 2 eV from the Fermi level, while for the ex situ reduction a very small drop was found and for H2O adsorption almost no change was observed, which is in accordance with the observed changes in the Pt core levels. Both the core and valence band spectra indicate the weak binding of H2O to the Pt surface, and slight charge transfer from Pt to H2O is expected even for the chemisorption. It is well known that adsorption of a molecule starts from physisorption with a slight lowering of the core level. When the molecule moves closer toward the Pt surface, stronger hybridization between the molecular orbitals of the adsorbed molecule and the Pt surface occurs, resulting in charge transfer from Pt to the adsorbate, which is called chemisorption. Therefore, one can distinguish physisorption and chemisorption by first-principles DFT calculations for the Pt 5d partial DOSs on the adsorption sites. For the reasons above, and to find the different mechanisms of chemisorbed O2 and H2O on the Pt surface, we performed first-principles DFT calculations on the Pt (111) surface. The calculated projected DOSs (Pt 5d, O2s, 2p, H 1s) for bare Pt (111) surface, O2 chemisorption, as well as for H2O chemisorption on the Pt (111) surface are presented in Figure S6d−f, respectively. Pt 5d DOSs for bare Pt (111) surface, O2 physisorption/chemisorption, as well as H2O chemisorption on Pt (111) surface are presented in Figure 2b with the corresponding difference spectra relative to that of the bare (reduced) Pt (111) surface in Figure 2c. As shown in Figure 2b, physisorption of O2 (denoted as O2Phy.) does not change the Pt 5d DOS from that of the pure Pt (111) surface, which means the absence of charge transfer from Pt 5d to the O2 molecule. C

DOI: 10.1021/acs.jpcc.6b02402 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C probing depth of HAXPES, even Pt atoms inside the nanoparticles can be probed. For the particle size of 2.6 nm with the surface ratio of ∼50% taking into account the cross section and assuming the half coverage of O2 adsorption as expected from the calculation for saturated O2 condition, the ratio of the oxidized Pt atoms should be ∼25%, which is nearly three times larger than the observed oxidized component in the Pt 3d (9.15%) and 4f (8.33%) peak fittings. This suggests that the gas pressure of 100 Pa is not sufficient to provide a saturated condition for O2 adsorption and much higher gas pressure is necessary to realize the full coverage. At the full coverage, other processes such as deep oxidization of Pt to PtOx, O2 dissociation, radical hydration, and so on may occur and should affect the core-level and valence band spectra further. In our experiments for unsaturated condition, high valency Pt species due to deep oxidation were negligibly small. The dissociation of O2 molecules into two atomic O species may occur in our experiment at room temperature because it is a thermally activated process with an energy barrier of only 0.29 eV.26 Pang et al.27 calculated the band structure of atomic O adsorbed on the three-fold hollow site of the Pt (111) surface. The calculated Pt 5d band structures for the dissociated species are quite similar to the chemisorbed O2 in Figure 2b, which leaves the possibility of the coexistence of chemisorbed O2 and low coverage atomic O in our experiment.

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02402. Additional information on the enhancement in Pt 5d DOSs by HAXPES, sample fabrication for HAXPES, and the other confirmation of the samples by XAFS are presented. (PDF)

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We acknowledge S. Yamamoto, H. Niwa, J. Miyawaki, and H. Kiuchi for fruitful discussions, K. Yamazoe for strong support of the XAFS and in situ HAXPES experiments, and H. Oji, and S. Yasuno for strong support of the ex situ HAXPES experiments. The synchrotron radiation experiments were performed at BL14B2, BL46XU, and BL47XU of SPring-8 with the approval of JASRI (proposal nos. 2014A1774, 2014B1657, 2015A1554, 2015A1681, and 2015A1691).

5. CONCLUSIONS In situ hard X-ray photoelectron spectroscopy was carried out to reveal the adsorption effects of H2O and O2 molecules on Pt 5d electronic states of Pt nanoparticles. Combined with ab initio theoretical calculations, it is confirmed that the chemisorption of H2O does not affect the 5d electronic states of Pt due to the weak bonding to Pt, while the chemisorption of O2 causes a drastic decrease in Pt electronic states up to 2 eV from the Fermi level owing to strong hybridization of Pt 5d and O 2p orbitals. From quantitative analysis of the spectral change we have found that only around one-third of the full coverage of O2 adsorption occurs at 100 Pa, which may regulate other processes than O2 dissociation, such as deep oxidization of Pt or radical hydration.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-3-5841-7191. Fax: +81-3-5841-8744. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.jpcc.6b02402 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b02402 J. Phys. Chem. C XXXX, XXX, XXX−XXX