Core−Shell Phase Separation and Structural Transformation of Pt3Sn

ARTICLE pubs.acs.org/JPCC

Core-Shell Phase Separation and Structural Transformation of Pt3Sn Alloy Nanoparticles Supported on γ-Al2O3 in the Reduction and Oxidation Processes Characterized by In Situ Time-Resolved XAFS Yohei Uemura,† Yasuhiro Inada,‡ Kyoko K. Bando,§ Takehiko Sasaki,|| Naoto Kamiuchi,^ Koichi Eguchi,^ Akira Yagishita,# Masaharu Nomura,# Mizuki Tada,3 and Yasuhiro Iwasawa*,O †

Department of Chemistry, Graduated School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Department of Applied Chemistry, College of Life Science, Ritsumeikan University, Nojihigashi Kusatsu, Shiga 525-8577, Japan § Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan ^ Department of Energy and Hydrocarbon Chemistry, Graduated School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan # Photon Factory, Institute of Materials Structure Science, High-Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan 3 Institute for Molecular Science, Nishigo-Naka, Myodaiji, Okazaki, Aichi 444-8585, Japan O Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan

)

‡

ABSTRACT: Unique catalytic capabilities of supported bimetallic nanoparticles with synergistic functions mark them as a significant advancement in catalytic technologies. The dynamic behavior and kinetics of structural change of catalysts in the reduction and oxidation processes are fundamental issues to understand their catalytic properties and performances as well as to regulate the structure and composition in alloy nanoparticles. The core-shell phase separation and structural transformation of Pt3Sn alloy nanoparticles on γ-Al2O3 during the reduction and oxidation processes were characterized by in situ time-resolved energy-dispersive XAFS (DXAFS) and quick XAFS (QXAFS) techniques. The time-resolved XAFS techniques provided the kinetics of the change in structures and oxidation states of the bimetallic nanoparticle catalyst. The oxidation of Pt3Sn nanoparticles on γ-Al2O3 with O2 at 673 K proceeded by three successive steps via two intermediates to form PtO core nanoparticles with SnO2 shells, whereas the reduction of the oxidized nanoparticles with H2 at 673 K proceeded as a single process with similar rate constants at Pt and Sn sites. The kinetic parameters and mechanisms for the reduction and oxidation of the Pt3Sn/ γ-Al2O3 catalyst were determined by the time-resolved XAFS techniques.

’ INTRODUCTION Supported Pt nanoparticles have been utilized as active catalysts in a variety of catalytic reactions, such as automobile exhaust gas cleaning, reforming of hydrocarbons, electrode reactions in fuel cells, and so on. These performances have been much improved and regulated by alloy formation with other metals because of electronic (ligand) effect, geometric (ensemble) effect, strain effect, isolation effect, synergistic effect, and so on.1 For example, Sn promotes the Pt catalysis for selective hydrogenation and dehydrogenation,2-16 oxidative hydrocarbon reforming,6,10,16-32 fuel cell operation,33-64 and so on,65-69 but the active alloy phases easily collapse into each metal oxide under oxygen ambient, resulting in a decrease in the catalytic performances. In the case of electrodes, the alloyed nanoparticles are dissolved out, and their r 2011 American Chemical Society

efficiency of power generation steeply drops. Understanding the formation and phase segregation mechanisms of bimetallic nanoparticles on supports with high surface areas is important to improve and regulate the catalytic performances, to explore new catalytic functions and electrode materials, and to find optimum compositions of bifunctional phases with efficient synergistic effects. Dynamic behaviors and reaction kinetics of reactants, products, and intermediate molecules in chemical reactions and catalysis have been extensively studied thus far, whereas kinetics Received: November 27, 2010 Revised: February 7, 2011 Published: March 04, 2011 5823

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The Journal of Physical Chemistry C and dynamics of the structural change in supported catalysts under the working conditions are still in infancy despite progress in surface science of well-defined single crystal planes.70,71 The dynamic behavior of catalysts is relevant directly to the genesis and origin of heterogeneous catalysis, and hence in situ timeresolved characterization of catalysts provides new pieces of information on the active structure and development of novel catalytic materials with synergistic functions. X-ray absorption fine structure (XAFS) is a unique technique, which can determine structural parameters of supported bimetallic nanoparticle catalysts under any ambients and reaction conditions in chemical processes.6,72,73 In particular, in situ time-resolved energy dispersive XAFS (DXAFS) and quick XAFS (QXAFS) are powerful tools to study the kinetics/dynamics of the change in structures and electronic states of supported catalysts.74-107 For example, elementary steps and their rate constants for the structural transformation of a Pt/C fuel cell catalyst under the operating conditions were determined by the time-resolved QXAFS and DXAFS.75 The kinetics of the structural change in an active Re10 cluster/ZSM-5 catalyst during direct phenol synthesis from benzene with O2 was also characterized by the time-resolved DXAFS.74,76 These pieces of information may be hard to obtain from other physical techniques. In this Article, the reductive formation and oxidative phase separation of Pt3Sn alloy nanoparticles supported on γ-Al2O3 in the reduction and oxidation processes at 673 K, respectively, were studied by the in situ time-resolved DXAFS and QXAFS techniques at Pt LIII-edge and Sn K-edge. The mechanisms of the alloy formation and core-shell phase separation are discussed on the basis of the structural kinetics and structural parameters of the Pt3Sn bimetallic nanoparticles on γ-Al2O3.

’ EXPERIMENTAL SECTION Preparation of Pt3Sn Nanoparticles. Pt3Sn alloy nanoparticles supported on γ-Al2O3 were prepared by a conventional impregnation method using H2PtCl6 and SnCl2. H2PtCl6 3 6H2O (0.14 g) was dissolved in 10 mL of 0.4 M HCl aq., to which solution 0.06 g of SnCl2 3 2H2O was added at RT. Then, 1 g of γAl2O3 was gradually added to the solution with stirring. After 1 h of stirring at RT, the solution was kept at 348 K under stirring overnight, followed by evaporation to remove the solvent. The obtained solid powder with pink color was calcined at 673 K for 4 h in air, followed by reduction with H2 at 673 K. The loadings of Pt and Sn were 3 and 1.8 wt %, respectively. The reduced Pt3Sn/ γ-Al2O3 sample was oxidized with O2 at 673 K, and the obtained oxidized sample was reduced with H2 at 673 K to form the reduced Pt3Sn/γ-Al2O3 sample again. In Situ XAFS Measurements. XAFS spectra at the Pt LIII-edge (E0 = 11 562 eV) were measured at BL-9C station of KEK-PF using a Si(111) monochromator in a transmission mode, and those at the Sn K edge (E0 = 29 194.7 eV) were measured at the NW10A station of KEK-PF-AR equipped with a Si(311) monochromator in a transmission mode. A batch-type in situ XAFS cell was used in all XAFS measurements. In situ quick XAFS measurements were conducted at 673 K under H2 or O2 atmosphere, and the acquisition time was 30 s to obtain a whole XAFS spectrum at Pt LIII-edge. Dispersive XAFS Experiments. Time-resolved energy-dispersive XAFS (DXAFS) measurements were performed at NW2A station of KEK-PF-AR. A Bragg-type Si(311) curved crystal (R = 1.5 m) was used as a polychromator for XAFS

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Figure 1. XRD of Pt3Sn/γ-Al2O. (A) Oxidized sample. (B) Reduced sample. 2, Pt3Sn; O, γ-Al2O3.

measurements at Pt LIII-edge, and a Laue-type Si(511) curved crystal (R = 0.9 m) was used for XAFS measurements at Sn K-edge. For Pt LIII-edge measurements, Rh-coated double flat mirrors were used to eliminate high-order harmonics. An in situ batch-type XAFS cell was placed on their focus points of X-rays via the polychromators. A 1024-channel photodiode array (PDA, S3904-1024F, Hamamatsu Photonics) was used as a positionsensitive detector. We performed the energy calibrations by measuring EXAFS spectra of Pt and Sn foils. A series of timeresolved spectra was measured every 0.1 to 1 s at 673 K. XAFS Analysis. The background of all XAFS spectra were removed with REX2000 (ver. 2.5, RIGAKU). Victoreen function was employed for background, and the spline smoothing method with Cook and Sayers criteria was used as the μ0 method. The extracted EXAFS oscillation was k3-weighted and Fourier-transformed to r-space over k = 30-120 nm-1. The fittings of k3weighted EXAFS oscillation in r-space were conduced with Artemis. Phase shift and amplitude functions for Pt-O, Pt-Pt, Pt-Sn, Sn-O, Sn-Pt, and Sn-Sn were obtained from FEFF (ver 8.4).108 TEM and XRD Measurements. High-resolution TEM photographs were taken using a Philips CM200 FEG field emissiontransmission electron microscope (FE-TEM) with a CCD camera (Gatan). XRD measurements were conducted using the multipurpose X-ray diffraction system (RIGAKU) with Cu KR radiation.

’ RESULTS AND DISCUSSION XRD of Pt3Sn/γ-Al2O3. Figure 1 shows XRD patterns of Pt3Sn/γ-Al2O3 measured by Cu KR X-ray as probe. The XRD patterns of the reduced Pt3Sn/γ-Al2O3 prepared by the reduction with H2 at 673 K showed two peaks around 2θ = 39 and 79 in addition to four peaks for γ-Al2O3. The former peak corresponds to Pt3Sn(111) (2θ = 38.786), and the latter corresponds to Pt3Sn(311) (θ = 78.9628). The XRD patterns in Figure 1 reveal that most of the Pt-Sn alloy nanoparticles on γ-Al2O3 possess the composition of Pt3Sn in an fcc structure, as expected. The XRD patterns of the oxidized sample exhibited four peaks, which all are attributed to γ-Al2O3 (2θ = 37.06, 45.66, 60.66, 66.70) and no characteristic peaks for Pt-Sn mixed oxides on γ-Al2O3. XAFS Spectra of Pt3Sn/γ-Al2O3 at Pt LIII-Edge. Figure 2(left) shows XANES spectra at Pt LIII-edge for the reduced and oxidized Pt3Sn/γ-Al2O3, a reduced Pt/γ-Al2O3, and reference samples. The XANES spectra for the reduced Pt3Sn/γ-Al2O3 and the reduced Pt/γ-Al2O3 were measured under a nitrogen atmosphere. The white line intensity for the reduced Pt3Sn/γ-Al2O3 5824

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The Journal of Physical Chemistry C and the reduced Pt/γ-Al2O3 was less than that for Pt foil. The white lines of Pt/γ-Al2O3 and Pt foil were 11 564.8 eV and 11 565.0 eV, respectively, which are almost the same, whereas the white line of the reduced Pt3Sn/γ-Al2O3 shifted to a slightly higher energy (11,565.9 eV). The intensity of the white line reflects the vacancy population of 5d orbital of Pt. The position of the white line reflects the energy difference between the initial state and the final state of the excited electrons. The energy difference becomes bigger if Pt receives electrons from the surrounding atoms because the lowest unoccupied orbital energy of Pt increases by the electron donation. The degree of 5d orbital vacancy can be ordered as follows; the reduced Pt3Sn/γ-Al2O3 < the reduced Pt/γ-Al2O3 < Pt foil. Ramallo-L
opez et al.109 reported the similar results on PtSn/SiO2. The decrease and shift of the white line are caused by the electron donation from Sn to Pt. Kim et al.46 also reported that the white line shifts to higher energy when the ratio of Sn to Pt in PtSn/C increases. Therefore, the situation of the surroundings of Pt in the reduced Pt3Sn/ γ-Al2O3 is different from that in the reduced Pt/γ-Al2O3. Figure 2 (right) shows Fourier transforms (FTs) of k3weighted EXAFS oscillation at the Pt LIII-edge for the reduced and oxidized Pt3Sn/γ-Al2O3, the reduced Pt/γ-Al2O3, and references. The intensity of the FT for the reduced Pt3Sn/γAl2O3 was small, which makes it difficult to obtain well-fitted results. The similar aspects were reported on PtSn/γ-Al2O3110 and PtSn/SiO2.111 The interference of scattered electrons by Pt and Sn causes the weak and splitted feature in the FT for the

Figure 2. (left) XANES spectra of the samples and references at Pt LIIIedge and (right) the Fourier transforms (FT) of k3-weighted EXAFS oscillations χ(k). (1) Reduced Pt3Sn/γ-Al2O3, (2) oxidized Pt3Sn/γAl2O3, (3) reduced Pt/γ-Al2O3, (4) Pt foil, (5) PtCl2, and (6) PtO2. The XANES spectra for 1-3 were measured in situ at 673 K. The XANES spectra for 4-6 were measured at RT.

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reduced Pt3Sn/γ-Al2O3. Nevertheless, the curve fitting analysis of the EXAFS data for the reduced Pt3Sn/γ-Al2O3 catalyst was successfully conducted, as shown in Figure 3. The curve-fitting for the EXAFS data of the difficult Pt-Sn system in Figure 3 had relatively larger R factors (Table 1), but the discussion on the local structures around Pt atoms could be done with safety. The fitting results are listed in Table 1. The reduced Pt3Sn nanoparticles supported on γ-Al2O3 (A) exhibited Pt-Pt, Pt-Sn, and Pt-Sn bonds, and there was no Pt-O bonding. The EXAFS data were well-fitted by assuming three shells, Pt-Pt þ Pt-Sn þ PtSn, and the EXAFS data could not satisfactorily be fitted by one shell of Pt-Pt and two shells Pt-Pt þ Pt-Sn. The total coordination number value was low, but the ratio of the coordination number of Pt-Pt bonds to the coordination number of Pt-Sn bonds was 2.5, which is similar to the expected value of 3 for Pt3Sn. The EXAFS analysis is in agreement with the result of XRD in Figure 1. In the Pt3Sn/γ-Al2O3 system, the interference of the Pt and Sn scattering prevents from determining exact values of the coordination numbers. The FT for the oxidized Pt3Sn/γ-Al2O3 in Figure 2 (right) shows a main peak of ∼0.16 nm, which is straightforwardly attributed to Pt-O bond. The coordination number of Pt-O bonds in the oxidized Pt3Sn/γ-Al2O3 was determined by the curve fitting to be as small as 3.1, as shown in Table 1. The white line intensity of the oxidized sample was larger than that of PtCl2 but much smaller than that of PtO2, as shown in Figure 2 (left). From the results of XANES and EXAFS, it is suggested that the coordination environment of Pt in the oxidized Pt3Sn/γ-Al2O3 is similar to that in PtO nanoparticles rather than PtO2 nanoparticles. The formation of PtO nanoparticles on γ-Al2O3 was confirmed by high-resolution TEM images described hereinafter. XAFS Spectra of Pt3Sn/γ-Al2O3 at Sn K-Edge. Figure 4 shows XANES spectra of the reduced and oxidized Pt3Sn/γ-Al2O3 samples and references at Sn K-edge. Compared with the white line intensities of Sn foil and SnO, Sn in the reduced Pt3Sn/γAl2O3 is suggested to be in the metallic state but positively charged because of electron transfer from Sn to Pt by alloying. The white line intensity of the oxidized Pt3Sn/γ-Al2O3 was similar to that for SnO2. It is evident that Sn was transformed to 4þ valency by oxidation of the metallic Pt3Sn/γ-Al2O3 with O2 at 673 K. In contrast with the EXAFS data at Pt LIII-edge, the EXAFS data at the Sn K-edge for the reduced and oxidized Pt3Sn/γ-Al2O3 samples were not useful in understanding the structures of the Pt3Sn/γ-Al2O3 samples. In particular, in the reduced Pt3Sn/γ-Al2O3, the FT peaks due to the scatterings from Sn and Pt atoms located around Sn atom should appear at 0.2 to 0.3 nm, but any definite peaks were not observed in the 0.2 to 0.3 nm region (not shown here) despite the Pt3Sn alloy crystal structure (XRD in Figure 1). The reason is due to

Figure 3. Curve-fitting results of the EXAFS data for (a) the reduced Pt3Sn/γ-Al2O3, (b) the 25 s oxidized Pt3Sn/γ-Al2O3, and (c) the 15 m oxidized Pt3Sn/γ-Al2O3. The in situ XAFS spectra of the reduced Pt3Sn/γ-Al2O3 and the 25 s oxidized Pt3Sn/γ-Al2O3 were measured by the quick XAFS technique, and that of the 10 m oxidized Pt3Sn/γ-Al2O3 was measured by the DXAFS technique. The XAFS measurements were conducted at 673 K. 5825

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Table 1. Structural Parameters Determined by the Curve Fitting Analysis of in Situ EXAFS Spectra at Pt LIII-Edge for the Reduced Pt3Sn/γ-Al2O3 (A), the 25 s Oxidized Pt3Sn/γ-Al2O3 (C), and the 10 m Oxidized Pt3Sn/γ-Al2O3 (D) A

C

D

coordination number (N), bond Pt-Pt

Pt-Sn

Pt-Sn

Pt-O

bond distance (r), Debye-Waller factor (σ)

reduction at 673 K

oxidation at 673 K for 25 s

N

5(1)

9(2)

ΔE/eV r/10-1 nm

0(3) 2.76(2)

0(5) 2.69(5)

σ2/10-2 nm2

0.015(2)

0.022

N

1(1)

ΔE/eV

6(2)

r/10-1 nm

2.76(1)

σ2/10-2 nm2

0.020(6)

N

1(1)

ΔE/eV r/10-1 nm

6(2) 2.61(1)

σ2/10-2 nm2

0.020(6)

oxidation at 673 K for 10 min

N

1.2(3)

ΔE/eV

2(7)

5(5)

r/10-1 nm

1.97(4)

1.98(3)

σ2/10-2 nm2

0.006

0.003(3)

14.0% 3 e k e 12

9.5% 3 e k e 12

R factor k range/101 nm-1

6.0% 3 e k e 12

3.1(5)

r range/10-1 nm

1 e r e 3.4

1 e r e 3.4

1 e r e 3.4

fitting space

r space

r space

r space

Figure 4. XANES spectra for the samples and references at Sn K edge. (1) Reduced Pt3Sn/γ-Al2O3, (2) the oxidized Pt3Sn/γ-Al2O3, (3) Sn foil, (4) SnO, and (5) SnO2. The XANES spectra for the reduced and oxidized Pt3Sn/γ-Al2O3 were measured in situ at 673 K. The XANES spectra for Sn foil, SnO, and SnO2 were measured at RT.

interference of the scattered electrons by Pt and Sn atoms with each other in the reduced Pt3Sn/γ-Al2O3, which diminishes the total EXAFS oscillation at Sn K-edge. Kinetics of the Change in the Structure and Oxidation State of the Pt3Sn/γ-Al2O3 Catalyst in the Reduction Process with H2 by Time-Resolved DXAFS. Figure 5a shows a series of DXANES spectra at Pt LIII-edge in the reduction process of the oxidized Pt3Sn/γ-Al2O3 catalyst with PtO þ SnO2 (PH2 = 23.6 kPa,

Figure 5. In situ time-resolved DXAFS measurements at Pt LIII edge in the reduction process of the oxidized Pt3Sn/γ-Al2O3 catalyst at 673 K. (a) Series of DXANES spectra measured every 0.1 s under PH2 = 23.6 kPa. (b) Time profile of the change in the white line intensity of panel a. (A dashed line is a fitting result with a linear function.) (See the text.) (c) Change of the white line intensity over time at different H2 pressures. (1) 3.0, (2) 7.0, (3) 23.6, and (4) 39.2 kPa. (d) Dependency of the rate constants in the reduction at 673 K on the PH2. The error of k0 Pt was estimated to be 10% considering the arbitrary property of fitting range.

T = 673 K), which were measured every 0.1 s. The white line intensity decreased after H2 admission to the in situ XAFS 5826

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cell, and its decay against the reduction time was plotted in Figure 5b. The change in the white line intensity was nearly completed in 2 s. Because the variation of the white line with reduction time is equivalent to the variation of the amount of PtO in the catalyst, one can decide the dynamic behavior and structural kinetics of the PtO nanoparticles in the reduction process. The change in the white line intensity in the reduction of the catalyst was analyzed by fitting with a linear function of time τ as follows. The white line intensity at a time τ during the reduction process is expressed by a linear function of time τ (eq 1) μtðτÞ ¼ μti - k0Pt τ=ðμti - μtf Þ

ð1Þ

where μt(τ) is the white line intensity (absorbance) at a time τ, μti is the absorbance in the initial stage (τ = 0 s), μtf is the absorbance in the final stage, and k0 Pt is the observed reaction rate constant. The rate equation is described by eq 2 dXPt =dτ ¼ k0Pt

ð2Þ

where XPt is defined as the ratio [PtO]/[PtO]0, where [PtO] is the amount of PtO at τ (>0 s) and [PtO]0 is the amount of PtO at τ = 0 s. Because k0 Pt should depend on the pressure of H2, timeresolved DXAFS measurements at different pressures of H2 were conducted every 0.1 s, as shown in Figure 5c. It took longer time to reach μtf when the pressure of H2 became lower. The μt variations at PH2 = 39.2, 23.6, and 7.0 kPa were analyzed as a single-step process, and the rate constants were calculated and plotted against PH2 in Figure 5d, which shows a linear relationship. At a very low H2 pressure like PH2 = 3.0 kPa, the μt varied with time by a two-step process, where the first reduction step was followed by a slower second step. The second step may be due to an alloying process of Pt with Sn, but it is not clear at the moment. In the case of PH2 = 3.0 kPa, the k0 Pt in the first step was plotted in Figure 5d. The first and second steps were not definitely discriminated with each other in the μt versus τ plots in Figure 5c. The k0 Pt is expressed as k0 Pt = kPtPH2 from the linear relationship in Figure 5d, where the rate constant kPt was estimated to be 0.035 s-1 kPa-1. Figure 6a shows a series of DXANES spectra at Sn K-edge in the reduction process at PH2 = 20.3 kPa and T = 673 K. The spectra were measured every 0.2 s. An isosbestic point was observed at 29 220 eV, which indicates that the XANES spectra in the series can be described by a linear combination of μti and μtf. In this context, the transformation of SnO2 to Sn in the reduction of the oxidized Pt3Sn/γ-Al2O3 sample is regarded as a single-step process without definite intermediates. The decrease in the white line intensity with time is shown in Figure 6b. The time profile of the white line intensity was fitted by an exponential function as μt = μti exp(-k0 Snτ), where k0 Sn is the rate constant for the reduction. The rate equation of the reduction of the catalyst is written by eq 3 .

d½SnO2 =dτ ¼ k0Sn ½SnO2 

ð3Þ

The reduction of SnO2 in the oxidized Pt3Sn/γ-Al2O3 is of the first order with respect SnO2. In situ DXANES measurements were conducted at various H2 pressures to examine the pressure dependence of k0 Sn (Figure 6c). The kSn values were determined by the exponential-curve fitting in the similar manner to the analysis of k0 Pt and plotted against PH2 in Figure 6d. The k0 Sn is approximately expressed as k0 Sn = kSnPH2 from Figure 6d, where the rate constant kSn was estimated to be 0.039 s-1 kPa-1. The time-profile of the reduction of SnO2/γ-Al2O3 without Pt was also measured by DXAFS under the identical condition to the

Figure 6. In situ time-resolved DXAFS measurements at Sn K-edge in the reduction process of the oxidized Pt3Sn/γ-Al2O3 at 673 K. (a) Series of DXANES spectra as PH2 = 20.3 kPa. (b) Change in the intensity of white line over time. (A dashed line is a fitting result with a single exponential function.) (c) Time profile of the white line intensity at different pressures of H2. (1) 7.5, (2) 17.2, and (3) 20.3 kPa. (d) Rate constants for the reduction process at 673 K obtained by the exponential-curve fitting. The error of k0 Sn was estimated to be 10% considering the arbitrariness of fitting range.

case of the reduction of the oxidized Pt3Sn/γ-Al2O3. In contrast with the reduction of SnO2 in the oxidized Pt3Sn/γ-Al2O3, Sn in the SnO2/γ-Al2O3 was hardly reduced. This indicates that the existence of Pt can promote the reduction of SnO2, where a possible reducing species for SnO2 is H atoms spilt over from Pt. The similar Pt-assisted reduction of Sn species in PtSn/γ-Al2O3 was proposed by several groups.112-115 DelAngel et al.115 reported the consumption of H2 in TPR of the SnO2/γ-Al2O3 due to a partial reduction of Sn from 4þ to 2þ, whereas the amount of H2 consumption of PtSn/γ-Al2O3 was larger than the amount of H2  consumption for Sn4þ to Sn2þ. Barias et al.116 also reported the similar larger H2 consumption in TPR and the shift of TPR peak to a lower temperature due to the coexistence of Pt. The apparent activation energy for the reduction of PtO nanoparticles in the oxidized Pt3Sn/γ-Al2O3 calculated from temperature dependency of kPt determined by the time-resolved DXANES analysis was estimated to be 0 ( 5 kJ mol-1. The value involves adsorption energy of H2 on the catalyst surface. Passos et al.117 reported the adsorption enthalpy of H2 on Pt/γ-Al2O3 and PtSn/γ-Al2O3 to be 56 and 49 kJ mol-1, respectively. By using the value, the activation energy for the reduction of PtO in the oxidized Pt3Sn/γ-Al2O3 sample is estimated to be ∼50 kJ mol-1. As for the SnO2 phase in the oxidized Pt3Sn/γ-Al2O3 sample, the apparent activation energy was calculated to be 5 ( 5 kJ mol-1, and the activation energy was also estimated to be ∼50 kJ mol-1. It is to be noted that the time-resolved DXAFS technique provides direct information on the structural kinetics of individual metal sites in a catalyst. Kinetics of the Change in the Structure and Oxidation State of the Pt3Sn/γ-Al2O3 Catalyst in the Oxidation Process with O2 by Time-Resolved DXAFS. The oxidation process of the reduced Pt3Sn/γ-Al2O3 with the metallic Pt3Sn alloy 5827

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which indicates that the oxidation process proceeds by two successive processes via an intermediate state. Indeed, the time profile of the white line intensity could not be fitted with a single exponential function, but the change was fitted well by two exponential functions below μt ¼ μtf þ C 3 expð - k01, Pt τÞ þ D 3 expð - k02, Pt τÞ

ð4Þ

where μt and μtf are the absorbance at time τ and the absorbance of the final state, respectively. C and D are constant coefficients, where μtf þ C þ D = μti at τ = 0 s, where μti is the absorbance of the initial state. Figure 7c,d shows the variations of the white line intensity at 11 566 eV and the absorbance at 11 571 eV, respectively, with time in the oxidation at 673 K at different O2 pressures. The rate constant k0 1,Pt for the first step in the oxidation of the Pt3Sn/γ-Al2O3 catalyst was determined from the variation of the absorbance with time (dashed curve (2)) in Figure 7b. The rate constant k0 2,Pt for the following slower oxidation step was estimated by fitting the variation of the white line intensity with oxidation time (solid curve (1)) in Figure 7b using the two exponential functions in eq 4. In this fitting treatment, the value of k0 1,Pt obtained by fitting the dashed curve (2) was used. The k0 1,Pt depended on PO2, whereas the k0 2,Pt was independent of PO2, as shown in Figure 7e. We assumed the Langmuir type adsorption of O2 on the catalyst as follows k01, Pt ¼ k01, Pt KPO2 =ð1 þ KPO2 Þ

Figure 7. In situ time-resolved DXAFS measurements at Pt LIII-edge in the oxidation process of the Pt3Sn/γ-Al2O3 catalyst at 673 K. (a) Series of DXANES spectra at PO2 = 21.0 kPa. (b) Time profiles and fitting curves of the changes in the white line intensity at 11 566 eV and the absorbance at 11 571 eV. Solid curve (1): a fitting result with double exponential functions; dashed curve (2): a fitting result with single exponential function. (c) Time profile of the white line intensity at different O2 pressures. (1) 39.6, (2) 21.0, (3) 7.6, and (4) 3.2 kPa. (d) Time profile of the absorbance at 11 566 eV at different O2 pressures. (1) 39.6, (2) 21.0, (3): 7.6, and (4) 3.2 kPa. (e) Rate constants k0 1,Pt (]) and k0 2,Pt (4) for the oxidation of the Pt3Sn/γ-Al2O3 at 673 K, determined by the DXANES analysis. The error bars of k0 1,Pt and k0 2,Pt were estimated to be 10%, considering the arbitrariness of fitting range.

structure was measured by time-resolved DXAFS at both Pt LIIIedge and Sn K-edge. Figure 7a shows a series of the DXANES spectra at Pt LIII-edge during the oxidation reaction for 25 s at PO2 = 21.0 kPa and T = 673 K. The DXAFS spectra were measured every 0.1 s after oxygen was admitted in the in situ XAFS cell at τ = 0 s. The white line at arrow (1) developed with time, whereas the intensity around 11566 eV decreased. There were two isosbestic points at 11 568 eV before 5 s and 11 571 eV (arrow (2)) after 5 s, as shown in Figure 7a. The absorbance at 11 571 eV (arrow (2)) was plotted against the oxidation time in Figure 7b, which decreased at first and then remained unchanged after 5 s, whereas the absorbances at the white line and around 11 568 eV still changed after 5 s, proving the presence of the second isosbestic point at 11 571 eV. The change of the absorbance (arrow (2) in Figure 7a) was fitted by a single exponential function, as shown by a dashed line in Figure 7b. Figure 7b also shows the variation of the white line intensity. The white line intensity increased soon after O2 admission into the in situ XAFS cell, and the intensity further grew up after 5 s,

ð5Þ

where k1,Pt is the rate constant independent of PO2 and K is the adsorption equilibrium constant. By fitting the k0 1,Pt with eq 5, the values k1,Pt and K were calculated to be 4 ( 1 s-1 and 0.017 ( 0.009 kPa-1, respectively. The k0 2,Pt was also estimated to be 0.15 s-1. Figure 8a shows a series of the DXANES spectra at Sn K-edge during the oxidation reaction for 25 s at PO2 = 21.8 kPa and T = 673 K. An isosbestic point was observed at 29 220 eV, which indicates that the oxidation of Sn in the Pt3Sn/γ-Al2O3 catalyst proceeds as a single-step similar to the case of the reduction of SnO2 in the oxidized Pt3Sn/γ-Al2O3 sample. The white line intensity at Sn K-edge was plotted as a function of reaction time in Figure 8b. The white line intensity (absorbance μt) increased immediately after O2 was admitted to the in situ XAFS cell. The absorbance at time τ can be expressed as μt = μtf þ Δμt exp(-k0 Snτ). This time profile of the oxidation of Sn is different from the time profile of the oxidation of Pt in the Pt3Sn/γ-Al2O3 catalyst. Figure 8c shows the change of the white line intensity at different PO2 pressures and Figure 8d shows the dependence of the rate constant k0 Sn on PO2. Assuming eq 5 also for k0 Sn, kSn was calculated to be 4 s-1 using K = 0.017 obtained from the fitting in Figure 7e. The rate constant kSn for the Sn oxidation was the same as that kPt for the Pt oxidation. Thus, it is concluded that the first oxidation process of Pt and the oxidation of Sn in the Pt3Sn/ γ-Al2O3 catalyst occur simultaneously. The apparent activation energy for the first step of the Pt oxidation in the Pt3Sn/γ-Al2O3 catalyst was determined to be 0 kJ mol-1 by the Arrhenius plot of the rate constant k0 1,Pt. The apparent activation energy for the Sn oxidation in the Pt3Sn/γAl2O3 catalyst was also 0 kJ mol-1 by the Arrhenius plot of the rate constant k0 Sn. These results reflect the rapid oxidation event, as seen in Figures 7b and 8b. The apparent activation energy for the second Pt oxidation step with the rate constant k0 2,Pt was calculated to be 23 kJ mol-1. 5828

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Figure 9. Time profile of the white line intensity at Pt LIII-edge in the oxidation of Pt3Sn/γ-Al2O3 with O2 (PO2 = 20 kPa) at 673 K. (A) Reduced Pt3Sn/γ-Al2O3, (B) catalyst after 10 s of oxidation, (C) catalyst after 25 s of oxidation, and (D) catalyst after 510 s of oxidation. The error bars of the third rate constant were estimated to be 10%, considering the arbitrariness of the fitting.

Figure 8. In situ DXAFS measurements at Sn K-edge in the oxidation process of the Pt3Sn/γ-Al2O3 catalyst at 673 K. (a) Successive DXANES spectra at PO2 = 21.8 kPa. (b) Time profile of the white line intensity. (A dashed line is a fitting result with single exponential function.) (c) Time profile of the white line intensity at different O2 pressures at 673 K. (1) 21.8, (2) 10.7, and (3) 5.9 kPa. (d) Dependence of the rate constant k0 Sn on PO2 at 673 K. The error bars of k0 Sn were estimated to be 10%, considering the arbitrariness of fitting range.

The kinetics of the catalyst itself determined by time-resolved DXAFS reveals that the oxidation of Pt in the Pt3Sn alloy nanoparticles on γ-Al2O3 proceeds as the two-step processes, whereas the oxidation of Sn to SnO2 is the single-step process. Pt is further oxidized with the rate constant k2,Pt after the oxidation of Sn is completed. How does the second process of Pt oxidation proceed? Kamiuchi et al.118 observed PtSn alloy nanoparticles supported on SnO2 after oxidation with O2 at 673 K for 30 min by high-resolution TEM. They measured the formation of Pt nanoparticles of 1 to 2 nm surrounded by amorphous SnO2, which assembled to form larger particles. A similar SnO2 shell phase may also be produced in the 25 s oxidation of the Pt3Sn alloy nanoparticles on γ-Al2O3. Long Period Pt Oxidation in Pt3Sn/γ-Al2O3. Figure 9 shows the change of the white line intensity at Pt LIII-edge up to 510 s in the oxidation process at 673 K. Figure 7b revealed that the Pt oxidation in the first 25 s period proceeded by a two-step regime. After 25 s, Pt was further oxidized with O2 at 673 K, and the oxidation saturated after 510 s. As a consequence, the overall time profile of the white line intensity at the Pt LIII-edge was fitted with three exponential functions, indicating the three successive oxidation processes. The rate constant of the third oxidation process was estimated to be 0.008 s-1 at T = 673 K and PO2 = 20 kPa from the fitting result in Figure 9. TEM Images of Pt3Sn/γ-Al2O3. Figure 10 are typical highresolution TEM images of the Pt3Sn/γ-Al2O3 samples (Figure 9A,B,D). Figure 10a shows a Pt3Sn nanoparticle on γ-Al2O3 (A) after reduction with H2 at 673 K. The nanoparticle size was estimated to be 11-13 nm. There are clear stripe patterns in the particle, which are lines of a lattice plane. The separation of the two adjacent lattice planes was estimated to be 0.2019 nm, which is attributed to d(200) for Pt3Sn (0.2009 nm),

confirming the formation of Pt3Sn alloy nanoparticles on γ-Al2O3 in agreement with the XRD data in Figure 1. The Pt3Sn nanoparticle looks to be surrounded by an amorphous phase in Figure 10a, but we suppose that this is due to inevitable oxidation of the nanoparticle surface by exposing the sample to air for a short while at room temperature before putting the sample in a TEM chamber. Figure 10b shows the TEM image of a nanoparticle after 10 s of oxidation of (A) at 673 K, whose size was estimated to be 11 nm. The lattice pattern of the nanoparticle (B) was not seen as clearly as (A). The nanoparticle after the 10 s of exposure to O2 at 673 K may not have a rigid crystal structure. However, there were some places that showed a lattice image with 0.1923 nm separation. This value corresponds to 0.1962 nm of d(111) for Pt fcc crystal. In addition to that, there existed amorphous phases surrounding the particle. The oxidation of Sn to SnO2 was completed in a few seconds, as shown in Figure 8, whereas the fraction of oxidized Pt in the oxidation of Pt3Sn/γ-Al2O3 for 10 s was small, as shown in Figure 9. Therefore, the surrounding amorphous phase may be SnO2 shell, whereas the core of the nanoparticle is composed of disordered Pt phase. Figure 10c shows a nanoparticle after oxidation of Pt3Sn/γ-Al2O3 for 10 min at 673 K. The lattice image was clearly observed. The separation of the lattice planes (0.2115 nm) was almost the same as that (0.2177 nm) for PtO(110). The shading near the particle edge was darker than that of the central part of the nanoparticle, as seen in Figure 10c. Because the shading of TEM images reflects transmittance of electrons, the transmission of electrons in the edge region may be more difficult. The TEM images together with the EXAFS data (Table 1) indicate that the central region of the nanoparticle may be thinner than the edge region. PtO nanoparticles can interact with γ-Al2O3 and SnO2 more strongly than metallic Pt nanoparticles, and the surface energy of PtO nanoparticles would be smaller than that of γ-Al2O3, resulting in restructuring of the nanoparticles by oxidation at 673 K for 10 min. In this study, the size of the nanoparticles on γ-Al2O3 did not change significantly during the redox cycles. This indicates that the particle size around 10 nm should be stable. This is contrast with the case of much smaller Pt-Sn nanoparticles around 1 to 2 nm, which grew after repeated redox cycles.6 Phase Separation and Structural Transformation of Pt3Sn/ γ-Al2O3. Figure 11 shows a plausible mechanism for the phase separation and structural transformation of Pt3Sn/γ-Al2O3 in the reduction and oxidation processes at 673 K. Considering the 5829

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Figure 10. High-resolution TEM images of the Pt3Sn/γ-Al2O3 catalyst (a) after reduction with H2 at 673 K, (b) after 10 s of oxidation with O2 at 673 K, and (c) after 10 min oxidation with O2 at 673 K, which correspond to Figure 9A,B,D, respectively.

Figure 11. Proposed mechanism for the oxidation and reduction of Pt-Sn nanoparticles on γ-Al2O3. The kinetic parameters were determined by the time-resolved XAFS.

TEM images (Figure 10) and the time profile of white line intensities (Figure 9), there are at least four structures with different oxidation states of Pt and Sn in the redox processes. By reduction of the Pt-Sn catalyst with H2 at 673 K metallic Pt3Sn alloy nanoparticles are produced on γ-Al2O3 surface as evidenced by XRD (Figure 1), TEM (Figure 10) and EXAFS (Table 1), which is shown as A in Figure 11. The Pt3Sn alloy nanoparticles on γ-Al2O3 readily decompose by exposing to PO2 = 21.8 kPa for a few second at 673 K, when all Sn atoms in the Pt3Sn nanoparticles with the dimension of 11-13 nm are oxidized to form SnO2 (Figure 8), resulting in phase separation that occurs to form a core-shell structure with Pt core and SnO2 shell in a few seconds. The surface of the Pt nanoparticles is also oxidized, as suggested by the increase in the white line intensity in Figure 7, where the oxidation rate is proportional to the adsorbed amount of oxygen on the catalyst surface because the rate is given by eq 5 (Figure 7e). The catalyst in this stage is illustrated as B in Figure 11. After the completion of Sn oxidation to form SnO2, the oxidation of Pt nanoparticles with PO2 = 21.0 kPa at 673 K further proceeds over 25 s, as shown in Figure 7b. The rate constant in this slower oxidation of Pt nanoparticles is independent of PO2 (Figure 7e). This implies that the amount of adsorbed oxygen on the PtO surface layer may be independent of PO2 in the wide range of PO2 3.3-39.6 kPa. Alternatively, it is more plausible that the further oxidation from the PtO surface layer into the bulk may be proportional to the oxygen concentration of the PtO surface layer, which remains unchanged at PO2 = 3.3-39.6 kPa. The coordination number of Pt-O bonds in the 25 s oxidized catalyst is 1.2 on average around Pt atoms, as shown in Table 1, which suggests the formation of several PtO layers on the Pt nanoparticle surface. The coordination number of Pt-Pt bonds is estimated to be 9 (Table 1), which is smaller than that expected

from spherical Pt nanoparticles, but the amorphous/disordered part of the Pt nanoparticles suggested by the high-resolution TEM (Figure 10b) would decrease the Pt-Pt bond number in the EXAFS analysis. We propose that the Pt nanoparticles are deformed toward thin particles by the further oxidation for 10 min at 673 K (Figure 11D), as suggested by the TEM (Figure 10c) and EXAFS data (Table 1). More favorable interaction of PtO nanoparticles with γ-Al2O3 and SnO2 than metallic Pt nanoparticles and the smaller surface energy of PtO nanoparticles than γ-Al2O3 are regarded as the driving force of the restructuring of the particle shape. As a consequence, there are three successive steps in the oxidation of the Pt3An/γ-Al2O3 catalyst via the two intermediate structures (B and C), as suggested by the time-resolved XAFS data at both Pt LIII-edge and Sn K-edge. In the reduction process, the PtO nanoparticles are reduced with H2 at 673 K directly to Pt metallic nanoparticles, as suggested by the presence of an isosbestic point in the XANES spectra at Pt LIII-edge of Figure 5. The SnO2 layers on the PtO nanoparticles are also reduced with H2 at 673 K directly to metallic Sn, as suggested by the presence of an isosbestic point in the XANES spectra at Sn K-edge of Figure 5. The rates of the reduction of PtO and SnO2 to Pt and Sn, respectively, on γ-Al2O3 are similar to each other, and the reduction process can not be discriminated from the following Pt3Sn alloy formation process under the present conditions as shown in Figures 5b and 6b. The time-resolved XAFS reveals that the reduction of the oxidized Pt3Sn/γ-Al2O3 catalyst with H2 at 673 K rapidly proceeds with the similar rates for PtO and SnO2, and it is completed in a few seconds even in the case of nanoparticles of >10 nm dimension, whereas the change in the structure and oxidation state of the reduced Pt3Sn/γ-Al2O3 catalyst in the oxidation with O2 at 673 K is 5830

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’ CONCLUSIONS The structural transformation and core-shell phase separation of Pt3Sn nanoparticles on γ-Al2O3 were characterized by in situ time-resolved XAFS techniques. The mechanisms of the oxidation processes of Pt and Sn in the Pt3Sn alloy nanoparticles on γ-Al2O3 were different from each other because of their much different oxophilicities. The first step of the oxidation at 673 K was rapid and completed in a few seconds with the rate constants k1,Pt = 4 s-1 and kSn = 4 s-1, where the Pt3Sn alloy nanoparticles transformed to Pt cores and SnO2 shells, as indicated by DXANES and HR-TEM. Then, the surface of the Pt core nanoparticles was oxidized into the bulk to form PtO surface layers with a Pt-O bond distance of 0.197 nm. This second oxidation step was independent of O2 pressure, and the rate constant and activation energy were k2,Pt = 0.15 s-1 and ΔEa,Pt = 23 kJ mol-1, respectively. Finally, the remaining Pt cores were fully converted to PtO nanoparticles with surrounding SnO2 shells accompanied with restructuring of the particle shape, as suggested by QXAFS and HR-TEM. The rate constant of the final oxidation was 0.008 s-1, which was much smaller than those for the first and second oxidation steps. The reduction of the oxidized catalyst back to the Pt3Sn/γ-Al2O3 catalyst proceeded as a single step with the similar rate constants ∼0.037 s-1 kPa-1 for both Pt and Sn oxidations. The structural kinetics of catalysts themselves under the working conditions is a fundamental issue to understand the origin and genesis of dynamic catalysis and to design efficient alloy nanoparticle catalysts, which can be directly characterized by in situ time-resolved XAFS. ’ AUTHOR INFORMATION Corresponding Author

*Tel: þ81-42-443-5921. Fax: þ81-42-443-5483. E-mail: iwasawa@ pc.uec.ac.jp.

’ ACKNOWLEDGMENT We thank Dr. Tatsuo Kadono (AIST) and Dr. Tetsushi Kodaira (AIST) for their help in XRD measurements. This work was financially supported by a grant-in-aid of JSPS and by a XAFS fuel cell project of NEDO. All XAFS and DXAFS measurements were conducted in Photon Factory, KEK as PAC nos. 2007G568 and 2009G211. ’ REFERENCES (1) Ponec, V.; Bond, G. Catalysis by Metals and Alloys; Elsevier: New York, 1995; Vol. 95. (2) Aksoylu, A. E.; Freitas, M. M. A.; Figueiredo, J. L. Catal. Today 2000, 62, 337–346. (3) Aksoylu, A. E.; Freitas, M. M. A.; Figueiredo, J. L. Appl. Catal., A 2000, 192, 29–42. (4) Alcala, R.; Shabaker, J. W.; Huber, G. W.; Sanchez-Castillo, M. A.; Dumesic, J. A. J. Phys. Chem. B 2005, 109, 2074–2085. (5) Coloma, F.; Llorca, J.; Homs, N.; Ramírez-de-la-Piscina, P.; Rodríguez-Reinoso, F.; Sep
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