Nanoparticles Driven by Reduction and Sulfidation Processes

Feb 18, 2009 - 9500, Bairro Agronomia, CP 15051, CEP 91501-970, Porto Alegre, RS, Brazil, Instituto de Quımica,. UniVersidade Federal do Rio Grande d...
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J. Phys. Chem. C 2009, 113, 3909–3916

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Monitoring Atomic Rearrangement in PtxPd1-x (x ) 1, 0.7, or 0.5) Nanoparticles Driven by Reduction and Sulfidation Processes Fabiano Bernardi,† Maria C. M. Alves,‡ Agne`s Traverse,§ Dagoberto O. Silva,‡ Carla W. Scheeren,‡ Jairton Dupont,‡ and Jonder Morais*,† Instituto de Fı´sica, UniVersidade Federal do Rio Grande do Sul (UFRGS), AVenida Bento Gonc¸alVes, 9500, Bairro Agronomia, CP 15051, CEP 91501-970, Porto Alegre, RS, Brazil, Instituto de Quı´mica, UniVersidade Federal do Rio Grande do Sul (UFRGS), AVenida Bento Gonc¸alVes, 9500, Bairro Agronomia, CP 15003, CEP 91501-970, Porto Alegre, RS, Brazil, Laboratoire de Chimie-Physique, UMR8000, UniVersite´ de Paris-Sud, Baˆt. 349, 91405 Orsay Cedex, France ReceiVed: June 20, 2008; ReVised Manuscript ReceiVed: NoVember 12, 2008

PtxPd1-x (x ) 1, 0.7, or 0.5) nanoparticles submitted to hydrogen reduction and posterior H2S sulfidation at 150 or 300 °C were characterized by in situ X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The in situ XAS measurements allowed monitoring of short-range order changes around the Pt atoms induced by the thermal processes. The surface sensitivity and atom specific characteristics of XPS provided additional information about the chemical state of the atoms present in the outermost layers of the nanoparticles. Our experiments also indicate a Pd migration toward the surface of the nanoparticles driven by the thermal processes. We observed that the reduction process is necessary prior to the occurrence of any sulfur reaction and that the number of chemisorbed sulfur atoms is directly proportional to the quantity of Pd atoms. 1. Introduction Increasingly intense research in the area of nanotechnology is driven by the numerous potential applications of nanostructures in several fields, such as catalysis,1 optoelectronics,2 and information storage.3 Since catalytic processes are highly dependent on surface properties,4 the high surface-to-volume ratio of nanoparticles makes them especially attractive for this use. The catalytic properties of metallic nanoparticles have been used in a variety of industrial processes. Moreover, forming bimetallic systems through the addition of a second metal provides a method for tailoring catalytic activity and selectivity through cooperative effects.5,6 Concerning environmental and clean-fuel legislation,7,8 recently, considerable attention has been paid to developing catalysts with high hydrogenation of aromatics and hydrodesulfurization activities.9,10 These studies indicate that particulate emissions in diesel exhaust gases can be reduced by decreasing the fuel’s sulfur content. A system highly active in the reduction of aromatics has already been obtained, though it was very susceptible to sulfur poisoning.11 Thus, the use of these catalysts is still limited by severe pretreatment requirements until sulfur tolerance can be greatly improved. Since the mechanism of metal poisoning by sulfur compounds involves strong chemisorption of the S-containing molecule on the metal sites followed by its hydrogenolysis, as represented by Me0 + H2S S Me-S + H2, the resulting H2S may lead to the formation of a stable and inactive Me-S species on the catalyst’s surface.9 However, this equilibrium may shift to the left-hand side when hydrogen pressure is high, when the * To whom correspondence should be addressed. Phone: +55 51 33086525. Fax: +55 51 33086510. E-mail: [email protected]. † Instituto de Fı´sica, UFRGS. ‡ Instituto de Quı´mica, UFRGS. § Universite´ de Paris-Sud.

physicochemical characteristics of the metal atoms are modified, or when both occur. In comparison with other bimetallic systems, the combination of platinum and palladium is particularly advantageous in catalysts used for hydroisomerization, hydrocracking, hydrogenation, and hydrotreatment. For example, a bimetallic catalyst containing platinum and palladium supported on alumina has been demonstrated to improve sulfur resistance in a diesel hydrotreating process more than a platinum monometallic catalyst.12 This improved sulfur resistance is generally proposed to result from decreased electron density of the platinum in the presence of palladium.13 Studies on the sulfidation of Pt and Pt-Pd catalysts have mainly been done on supported catalysts, such as Pt/Al2O3 or SiO2 and Pt-Pd/Al2O3 or SiO2.14-23 Few studies have addressed nonsupported nanoparticles.24-26 The majority of studies on the structural properties of Pt-Pd nanoparticles address systems in as-prepared and reduced conditions.12,23,26-30 To our knowledge, detailed studies concerning the atomic structures and distribution induced by sulfidation of nonsupported PtPd nanoparticles are nonexistent. The goal of this paper is to characterize PtxPd1-x nanoparticles with x ) 0.5, 0.7, and 1, thereby achieving a detailed description of the Pt neighboring changes and the atomic arrangement inside the nanoparticles as they undergo thermal treatments. The nanoparticles were submitted to two types of thermal processes: hydrogen reduction (named reduction); or hydrogen reduction followed by H2S sulfidation (named sulfidation). The results were achieved with in situ X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation excitation. XAS is widely employed to characterize the structural and electronic properties of catalysts and nanoparticles.31 One advantage of XAS over other techniques is the possibility of carrying out in situ measurements. In this work, in situ XAS analysis allowed us to obtain the short-range order and structure

10.1021/jp805465x CCC: $40.75  2009 American Chemical Society Published on Web 02/18/2009

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Figure 1. (i) EXAFS signals at the Pt L3 edge for the PtxPd1-x nanoparticles for different x values and (ii) the corresponding Fourier transform for the (a) as prepared, (b) reduced at 300 °C, and (c) sulfided at 300 °C. The black points represent the experimental data, and the gray line, the fitting.

around the nanoparticles’ Pt atoms as they were submitted to heating in a reactive gas environment (reduction or sulfidation). We used a reactor specifically designed for these experiments, ensuring that the induced structural changes could be monitored. The in situ measurements were carried out both in serial mode at steady state conditions (constant temperature and gas flow) and in the energy dispersive mode at a constant temperature, which collected the time-dependent spectra after the addition of a reactive gas. Extended X-ray absorption fine structure (EXAFS) and dispersive X-ray absorption spectroscopy (DXAS) measurements established the structural behavior of the nanoparticles. Once we were confident that the thermal processes had changed the structure of the nanoparticles, as observed by XAS, we obtained complementary information by performing XPS on the post-treated samples. XPS provides the electronic structure and chemical states of all elements present in the nanoparticles. The surface sensitivity and element-specific characteristics of XPS were also exploited to provide information about the chemical state of the atoms present in the nanoparticles’ outermost layers. We have also associated the XPS with argon sputtering to investigate if the nanoparticles’ cores have a different composition from their shells. By comparing the XPS results obtained for the as-prepared, reduced, and sulfided samples, we were able to explore the influence of the thermal treatments on their composition, chemical state, and atomic rearrangement. Our main results show (i) nanoparticle composition (i.e., the x value) is a key factor in determining the amount of sulfur that will be adsorbed (or chemisorbed) by the catalyst, provided that the nanoparticle has been reduced prior to the H2S reaction; (ii) PtS, PtS2, PdS, and PdS2 are all identifiable after the sulfidation process; and (iii) there is an atomic rearrangement of Pt and Pd atoms inside the nanoparticles, leading to a core-shell structure induced by the thermal processes under controlled gas atmosphere. 2. Experimental Methods 2.1. Preparation of the Nanoparticles. PtxPd1-x nanoparticles were prepared by dissolution of the Pt and Pd precursors, Pt2(dba)3 and Pd(acac)2, in 1 mL of 1-n-buthyl-3-methylimida-

TABLE 1: Results Obtained from the Quantitative Analysis of the EXAFS Data for the Coordination Shell Yielding the Coordination Number (N), Distance (R), and the Debye Waller Factor (σ2) for the As-Prepared, Reduced (T ) 300 °C), and Sulfided (T ) 300 °C) Samples pair

0.5

Pt-Pt Pt-Pd Pt-Pt Pt-Pd Pt-Pt

As-Prepared 5.8 ( 0.3 2.748 ( 0.005 4.0 ( 0.2 2.748 ( 0.005 8.6 ( 0.3 2.735 ( 0.002 2.2 ( 0.2 2.735 ( 0.002 10.8 ( 0.3 2.750 ( 0.002

0.46 ( 0.06 0.86 ( 0.03 0.50 ( 0.03 0.53 ( 0.07 0.44 ( 0.02

Pt-Pt Pt-Pd Pt-Pt Pt-Pd Pt-Pt

5.9 ( 0.3 4.3 ( 0.3 7.3 ( 0.4 2.6 ( 0.3 11.6 ( 0.3

Reduced 2.714 ( 0.004 2.714 ( 0.004 2.734 ( 0.002 2.734 ( 0.002 2.751 ( 0.002

0.63 ( 0.05 0.83 ( 0.05 0.57 ( 0.04 0.72 ( 0.08 0.89 ( 0.03

Pt-S Pt-Pt Pt-Pd Pt-S Pt-Pt Pt-Pd Pt-S Pt-Pt

Sulfided 2.5 ( 0.1 2.262 ( 0.004 4.1 ( 0.6 2.756 ( 0.008 1.1 ( 0.1 2.756 ( 0.008 0.63 ( 0.07 2.26 ( 0.02 8.4 ( 0.4 2.760 ( 0.003 2.0 ( 0.2 2.760 ( 0.003 0.5 ( 0.2 2.21 ( 0.04 11.3 ( 0.3 2.752 ( 0.002

0.18 ( 0.04 0.8 ( 0.1 0.23 ( 0.08 0.3 ( 0.1 0.92 ( 0.04 0.43 ( 0.05 2.2 ( 0.9 0.91 ( 0.06

0.7 1 0.5 0.7 1 0.5 0.7 1

N

R (Å)

σ2 (10-2 Å2)

x

zolium hexafluorophosphate (BMIPF6). The resulting solution was reacted with molecular hydrogen at 75 °C and 4 atm for 5 min.32 The nanoparticles were isolated by centrifugation of the black solution formed by this reaction. TEM and XRD studies32 showed that this procedure leads to the formation of 3.8-, 4.5-, and 4.8-nm-average-diameter PtxPd1-x nanoparticles, for which x ) 1, 0.7, and 0.5, respectively. 2.2. In Situ XAS Measurements. For the XAS experiments, about 10 mg of the nanoparticle powder was compacted to produce 5-mm-diameter pellets. The pellet was introduced to the previously described reactor,24 which allows for controlled thermal treatment of the sample under controlled gas flow. Transmission mode measurements were performed at the LNLS (Brazilian Synchroton Light Laboratory) at the XAFS1 beamline.33 The spectra were collected at the Pt L3 edge using a

Monitoring Atomic Rearrangement in PtxPd1-x

Figure 2. Comparison of EXAFS signals at the Pt L3 edge after sulfidation process at 150 and 300 °C. The fitting is represented by the gray line.

channel-cut Si (111) crystal and three argon-filled ionization chambers. A standard Pt foil was used to calibrate the monochromator.Thespectrawereacquiredintherangeof11 440-12 200 eV with 2 eV steps and 2s/point. Two to four scans were acquired to improve the signal-to-noise ratio. Experiments at Pd K edge (24 350 eV) were not performed due to low photon flux for this energy range at LNLS. The XAS spectra of the PtxPd1-x as-prepared samples were measured at room temperature. The samples were then submitted to three processes: (i) the temperature was increased at a rate of 15 °C/min to the selected temperature (150 or 300 °C) under a He flux of 13 cm3/min. This temperature was maintained under He flow for 20 min. (ii) After this, the reduction process was begun under a gas mixture of 78% He (≈13 cm3/min) + 22% H2 (≈ 4 cm3/min), for 20 min. (iii) After reduction, sulfidation was accomplished by flowing 75% He (≈13 cm3/min) + 21% H2 (≈ 4 cm3/min) + 4% H2S (≈0.7 cm3/min) for 30 min. XAS spectra were acquired at the end of each step. The total gas pressure at the sample was kept at approximately 35 psi. 2.3. In Situ DXAS Measurements. Dispersive XAS measurements were performed in the same reactor used for the XAS experiments at the LNLS DXAS beamline.34 The monochromator consists of a curved Si(111) crystal (dispersive polychromator) that focuses the beam in the horizontal plane down to about 200 µm and in the vertical plane to about 500 µm. The detector was a position-sensitive CCD camera. The reactor was placed in the beamline, taking care to place the pellet at the X-ray focal point. The measurements were performed at the Pt L3 edge during sulfidation of the Pt0.5Pd0.5 nanoparticles at 300 °C using the same gas fluxes as in the XAS measurements. The time resolution was about 100 ms, and a spectrum was collected every 18 s, with an accumulation time of 3 s. 2.4. XPS Measurements. After each process, upon completion of the XAS measurements, the samples were introduced into the SXS beamline35 endstation (also at LNLS) where XPS measurements are taken. The spectra were collected using an InSb (111) double crystal monochromator at a fixed photon energy of 1840 eV. The hemispherical electron analyzer (Physical Electronics model 10-360) was set at a pass energy of 23.5 eV, and the energy step was 0.1 eV, with an acquisition time of 500 ms/point. The use of synchrotron radiation excitation provided spectra with intense signal and an excellent overall

J. Phys. Chem. C, Vol. 113, No. 10, 2009 3911 resolution of about 0.13 eV. The base pressure used inside the chamber was about 1.3 × 10-9 mbar. The monochromator photon energy calibration was done at the Si K edge (1839 eV). An additional calibration of the analyzer’s energy was performed after every sample change using a standard Ag foil (Ag 3d5/2 peak at 368.3 eV). We also considered the C 1s peak value of 284.5 eV as a reference to verify possible charging effects. The samples were placed on carbon tape, and the XPS measurements were obtained at a 45° takeoff angle at room temperature. The samples were studied before and after ion sputtering using an Ar+ ion beam at 3 keV for 10 min at a pressure of 4.0 × 10-6 mbar and impinging at a grazing incidence of 10° with respect to the sample surface. 2.5. Data Analysis. The EXAFS data were analyzed in accordance with the standard procedure of data reduction,31 using IFEFFIT.36 FEFF was used to obtain the phase shift and amplitudes.37 The EXAFS signal χ(k) was extracted then Fourier-transformed using a Kaiser-Bessel window with ∆k range of 8.1 Å-1. Single and multiple scattering events were considered in the fitting procedure. The S02 was fixed at 0.84 for all samples, and in some cases, it was necessary to apply cumulant expansion (c3 and c4 values around 10-4). XPSPeak version 4.1 was used to fit the XPS results. All peaks were adjusted using a Shirley type background and an asymmetric Gaussian-Lorentzian sum function (25% Lorentzian contribution). The fwhm of the XPS components were allowed to vary 0.6 eV around their typical values. 3. Results 3.1. In Situ XAS Measurements. The EXAFS spectra at the Pt L3 edge for bimetallic nanoparticles (PtxPd1-x x ) 0.5 and 0.7) as well as monometallic Pt nanoparticles were collected for the as-prepared, reduced at 300 °C and sulfided at 300 °C samples. The EXAFS signals χ(k) and the Fourier transforms (FT) as well as the best fit of the signal (gray line) are presented in Figure 1. The EXAFS signals for the as-prepared PtxPd1-x nanoparticles for x ) 0.5, 0.7 and 1 (Figure 1a) are generally similar to those of bulk Pt, where the characteristic signature of the fcc structure can be found in the nanoparticles. For x ) 1 the overall shape is identical to bulk Pt with a small dumping of the oscillations. Pd replacement of part of the Pt atoms in the bimetallic systems induces stronger signal damping as the Pd content increases, which is clearly seen for k > 5 Å-1 in Figure 1a. The damping could be due to particle structural disorder and (or) size reduction. Since TEM indicates a slight diameter increase when x goes from 1 to 0.5, structural disorder is assumed to be the cause of the oscillation damping. The first peak in the FT of standard Pt corresponds to platinum atoms in the position of nearest neighbors. The peak is split due to variation of the backscattering amplitude with k for heavy atoms.31 The FT of PtxPd1-x nanoparticles (x ) 0.5 and x ) 0.7) has two peaks associated with Pt-Pt and Pt-Pd bonds in the coordination shell. The absence of contributions at greater distances indicates only short-range order. For x ) 1, only Pt-Pt bonds are observed, and there is a reduction in the contribution of higher shells compared to Pt metal. After reduction at 300 °C (Figure 1b), the Pt0.5Pd0.5 presents a shift in the Pt-Pt and Pt-Pd distances for lower values. No significant changes for the x ) 1 and 0.7 systems were observed. It is interesting to note how the two bimetallic systems (x ) 0.5 and 0.7) behaved differently during the sulfidation at 300 °C (Figure 1c). The system with the higher Pd content is strongly sulfided, as can be observed by the strong change in the EXAFS

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Figure 3. (a) Evolution of the XANES measurements at the Pt L3 edge for the Pt0.5Pd0.5 nanoparticles submitted to the sulfidation process at 300 °C. The time axis corresponds to the elapsed minutes from the beginning of the sulfidation process. (b) Comparison of selected XANES curves (solid lines), where the dotted line is used to guide the eyes. (c) Plot of the first (black line) and the last (red line) spectra for better comparison of the intensities in the near edge structure.

pattern that yields a strong Pt-S contribution at about 1.8 Å in the FT. For x ) 0.7, only a small asymmetry in the FT first peak is observed that can be associated with the formation of a Pt-S bond. We observed that sulfidation occurs only when the samples have been previously reduced under the conditions described above. Typical R-factors in the quantitative analysis (Table 1), provided by the IFEFFIT code, were between 0.008 and 0.023, illustrating good fit quality. For the bimetallic systems, the number of Pt-Pt or Pt-Pd neighbors does not follow the sample composition; that is, the NPt-Pd/NPt-Pt ratio is different from (1 - x)/x. For all the as-prepared samples, the metal-metal distances (Pt-Pt ) 2.77 Å and Pd-Pd ) 2.73 Å) are close to those of a fcc lattice and match, within the acceptable range of uncertainty, that of pure fcc Pt. For the reduced samples, there is significant bond contraction, from 2.748 to 2.714 Å, for the bimetallic system x ) 0.5. The N values from the EXAFS results for the as-prepared samples are in agreement with the TEM findings.32 Indeed, since the coordination number is related to the average nanoparticle diameter,38 it increases with the addition of Pd in the alloy. After the reduction process, the coordination number of Pt increases for all particles. For x ) 1, it goes to 11.6 ( 0.3 (almost corresponding to bulk Pt), whereas the Pt coordination numbers for the alloy samples add up to only 9.9 ( 0.3 (x ) 0.7) and 10.2 ( 0.3 (x ) 0.5). In addition, the reduced samples present a decrease in the coordination number with the addition of Pd. These results indicate that a sintering has occurred for the pure Pt nanoparticle, and the presence of Pd inhibits the sintering of a dispersed Pt phase in the alloy samples. This role of Pd is in agreement with previous results found in the literature.12,39 Therefore, the atomic rearrangement with formation of a core-shell structure in the Pt-Pd nanoparticles may also work as a mechanism to restrain the sintering process. We have also compared the sulfidation of Pt0.5Pd0.5 nanoparticles at 150 and 300 °C (Figure 2). The sulfidation at a lower temperature shows a less pronounced Pt-S peak, indicating that

fewer S atoms were bonded to the nanoparticles. The fitting results indicated that at 300 °C, the N value was (2.5 ( 0.1), but at 150 °C, N ) (0.8 ( 0.2). 3.2. In Situ DXAS Measurements. The DXAS experiment was used to get preliminary information on the kinetics of sulfidation of the Pt0.5Pd0.5 nanoparticles at 300 °C, and the results are shown in Figure 3. The nanoparticles were previously reduced using the same procedure described for the XAS measurements. The zero on the time scale corresponds to the introduction of the H2S. There is an increase in the intensity of the absorption edge that corresponds to a 2p f 5d transition. This is due to the oxidation of Pt, which yields more empty electronic states in the 5d band. A change in the position of the first oscillation maximum beyond the main edge is also noticeable. The position of this feature is stabilized after 7.5 min of sulfidation and remains unchanged (Figure 3b). Thus, one may consider sulfidation a slow process, when compared to the DXAS measurement but fast in comparison to the acquisition time of a XAS spectrum, which is ∼40 min. For this reason, the XAS measurements presented in this paper were performed on samples that had already reached a steady state. 3.3. XPS Measurements. Figure 4 shows the Pt 4f and Pd 3d photoemission spectra for the as-prepared, reduced at 300 °C and sulfided at 300 °C PtxPd1-x nanoparticles (x ) 0.5, 0.7, and 1). For comparison purposes, all the spectra are presented with the same x and y scale, together with the fitting curves. Sulfur 2p photoemission spectra were also collected (not shown here) and were observed only in the sulfided samples. The peak positions and the chemical components obtained from the XPS fitting results are presented in Table 2 for the sulfided nanoparticles. For the as-prepared samples (Figure 4a), the Pt 4f region displays the Pt 4f7/2 and Pt 4f5/2 doublet at 70.8 and 74.1 eV, respectively. These energies correspond to Pt-Pt bonds (Pt0). The Pt 4f spectra obtained for all as-prepared nanoparticles are quite similar, independent of their composition. The Pd 3d region displays the Pd 3d5/2 (335.2 eV) and Pd 3d3/2 (340.5 eV) doublet as well as the Pt 4d3/2 (332.0 eV) peak.40 The binding energy

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Figure 4. XPS measurements at Pt 4f and Pd 3d regions for different x values for (a) as prepared, (b) reduced at 300 °C, and (c) sulfided at 300 °C. The vertical lines indicate the binding energies of the observed chemical components.

values for the Pd 3d peaks correspond to Pd-Pd bonds (Pd0).41 The Pd0 component has a shakeup satellite, as described previously.42 The satellite is located at 7.2 eV below the Pd 3d5/2 peak, and it was considered in the Pd0 component fitting procedure. The peak at 336.4 eV43 corresponds to the palladium oxide (PdO) component. After reduction (Figure 4b), the observed Pt and Pd chemical components remain the same. The PdO component is still present, which is probably the result of brief air exposure prior to the XPS measurements. After sulfidation (Figure 4c), the Pt0 component is no longer observed in the Pt 4f region for all samples. A new component appears (Pt 4f7/2 at 72.0 eV), which was assigned to the Pt-S bonds.20 For the pure Pt nanoparticles (x ) 1), the PtS2 component is observed (Pt 4f7/2 at 73.4 eV), in addition to the PtS at 72.0 eV. The Pd0 components for the bimetallic samples are also no longer observable and are replaced by PdS for x ) 0.7 and by PdS2 for x ) 0.5.

A closer analysis of the Pd 3d region allows us to obtain information about both the Pd and the Pt atoms, since it displays photoemission lines at similar kinetic energies and, consequently, comparable electron mean free paths of about 2 nm. Therefore, each Pd 3d region spectrum of the bimetallic samples allows us to monitor the relative intensity of the Pt and Pd peaks within the same probed range. Hence, the evolution of the Pt 4d3/2/Pd 3d peak area ratio after the reduction and sulfidation processes can be compared, and its variations could be related to different Pt and Pd depth distribution. After the reduction process (Figure 4b), there is a decrease in the Pt 4d3/2/Pd 3d peak ratio as compared to the as-prepared samples (Figure 4a). This implies that upon reduction, Pd content increases at the probed depth of 2 nm. The Pt 4d 3/2 (in the Pd region) is no longer observed in the sulfided samples (Figure 4c), indicating the enrichment of Pd atoms at the probed depth. Since the Pt 4f photoelectrons have higher kinetic energy than the Pt 4d3/2 ones, the Pt 4f must originate from deeper atoms;

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TABLE 2: XPS Results Obtained from the Fitting of the Experimental Curves with Identification of the Different Chemical Components for the Sulfided Nanoparticles at 300 °C before sputtering x

peak

Eb (eV)

chem comp

0.5

Pt 4f7/2

72.0

PtS

Pd 3d5/2

337.0

PdS2

S 2p3/2 Pt 4f7/2

162.2 163.3 72.0

PtS PdS2 PtS

Pd 3d5/2

336.3

PdS

S 2p3/2

161.8 162.2 72.0 73.4 169.2 162.2 161.8

PdS PtS PtS PtS2 S+6 PtS PtS2

0.7

1

Pt 4f7/2 S 2p3/2

after sputtering Eb (eV) 70.8 72.0 335.2 336.3 162.2 161.8 70.8 72.0 335.2 336.3 161.8 162.2 70.8 72.0 169.2 162.2

chem comp Pt0 PtS Pd0 PdS PtS PdS Pt0 PtS Pd0 PdS PdS PtS Pt0 PtS S+6 PtS

TABLE 3: Pt 4d 3/2/Pd 3d Peak Area Ratio Obtained from the XPS Fitting for the As-Prepared, Reduced (300 °C), and Sulfided (300 °C) PtxPd1-x Nanoparticles (x ) 0.5 and 0.7) Pt 4d3/2/ Pd 3d peak area ratio x 0.5 0.7

sample as-prepared reduced 300 sulfided 300 as-prepared reduced 300 sulfided 300

before sputtering °C °C °C °C

0.28 ( 0.03 0.20 ( 0.01 0.00 ( 0.01 0.76 ( 0.02 0.52 ( 0.01 0.054 ( 0.02

after sputtering 0.33 ( 0.02 0.31 ( 0.02 0.13 ( 0.01 0.77 ( 0.03 0.76 ( 0.02 0.47 ( 0.01

its signal is clearly detectable after sulfidation. The significant decrease in the Pt 4f signal when comparing the as-prepared sample to the reduced one and to the sulfided one also reflects the rearrangement of the Pt atoms toward the nanoparticle core. To confirm the observation of Pd surface enrichment, we submitted the samples to Ar+ sputtering to eliminate their outermost atoms. Figure 5 compares the XPS measurements for the Pt0.5Pd0.5 nanoparticles before (i) and after (ii) sputtering. The as-prepared sample shows no significant changes after sputtering, which proves that sputtering does not induce atomic rearrangement. The Pt 4f region results (Figure 5a) demonstrate a clear increase in intensity after the sputtering for the reduced and sulfided nanoparticles with a simultaneous decrease in the Pd peak intensity (Figure 5b). Moreover, the Pt 4d3/2 peak, which is not visible in the sulfided samples, reappears after sputtering. Thus, one can deduce from these observations that there is a modification of the atomic arrangement inside the nanoparticles after either the reduction or sulfidation processes, resulting in a Pd-enriched surface and a Pt-enriched core. Table 3 shows the Pt 4d3/2/Pd 3d peak area ratio for the as-prepared, reduced at 300 °C, and sulfided at 300 °C PtxPd1-x nanoparticles (x ) 0.5 and 0.7) before and after sputtering. This table provides a quantitative argument for the observed Pd surface enrichment. This rearrangement is accompanied by a modification of the metal-sulfur bonds. The Pt0 component is observed again after sputtering (Figure 5a), indicating that PtS is present at the surface. Simultaneously, the presence of PdS2 is no longer observed after sputtering and is replaced by the Pd0 and PdS components. We observed analogous behavior for the Pt0.7Pd0.3 nanoparticles in the XPS analysis.

TABLE 4: Comparison of the Results Obtained from XPS Measurements for the Sulfided Pt0.5Pd0.5 Submitted to 150 and 300 °C before sputtering T (°C) 150

peak

chem comp

72.0 73.4 337.0

PtS PtS2 PdS2

Pt 4f7/2

162.2 163.3 72.0

PtS PdS2 PtS

Pd 3d5/2

337.0

PdS2

S 2p3/2

162.2 163.3

PtS PdS2

Pt 4f7/2 Pd 3d5/2 S 2p3/2

300

Eb (eV)

after sputtering Eb (eV)

chem comp

70.8 72.0 335.2 337.0 162.2

Pt0 PtS Pd0 PdS2 PtS

70.8 72.0 335.2 336.3 161.8 162.2

Pt0 PtS Pd0 PdS PdS PtS

Table 4 shows the XPS results for Pt0.5Pd0.5 nanoparticles sulfided at two temperatures, 150 and 300 °C. The overall XPS results (Tables 2 and 4) indicate that sulfur is present in different chemical states: PtS, PdS, PtS2, and PdS2. In addition, the S 2p region indicates the presence of S6+ and SO4-2 compounds only for Pt nanoparticles sulfided at 300 and 150 °C, respectively. Discussion We may highlight two main results of the study presented here. The first one concerns the dependence of the sulfidation process on the sample’s chemical state (either oxidized or reduced), sample composition (x value), temperature, and duration of the thermal processes. The second one involves the observation of the atomic rearrangement inside the nanoparticles. Regarding the sulfidation, it takes place only after PtxPd1-x reduction under the conditions described here. For Pt0.5Pd0.5 nanoparticles, the sulfidation is achieved in 7.5 min, as seen by in situ DXAS measurements. The observed sulfidation process is considerably faster than similar results found in the literature. For example, Bando et al.15 have obtained a sulfidation process at 280 °C that is accomplished after ∼100 min for the PtPd/ Al2O3 system. Once reduction is achieved, sulfur incorporation increases proportionally with the amount of Pd. The presence of platinum sulfide observed for all sulfided samples was also obtained for sulfided-supported Pt catalysts.19-21 As established previously,21 PtS is responsible for a bonding effect in some reactions instead of the structural effect caused by sulfate compounds. The XPS results from the S 2p region shows the presence of SO42- and S6+ compounds with energies of 168.2 and 169.2 eV, as reported previously.19-21 We believe that the sulfate components are due to the samples’ exposure to air between the XAS and XPS measurements. The sulfidation of Pt (x ) 1) nanoparticles at 300 °C produces PtS and PtS2, and the latter has not been found in similar investigated systems.19-21 In contrast, only PtS was observed for Pt0.7Pd0.3 and Pt0.5Pd0.5 nanoparticles sulfided at the same temperature, in addition to PdS and PdS2, respectively. Therefore, PtS2 is not formed for alloy samples, which have created a Pd protecting shell. We believe that the interaction between Pt and Pd atoms may promote an electron deficient character of the Pt atoms in the interface between the core and shell regions, as previously reported.9,12,13,15,22,39 In contrast, PtS2 and PdS2 are present in Pt0.5Pd0.5 nanoparticles sulfided at lower temperature (150 °C), implying a less pronounced electron deficient character of Pt. It is noteworthy that an electron deficient character of Pd has also been observed.44 Hence, these

Monitoring Atomic Rearrangement in PtxPd1-x

J. Phys. Chem. C, Vol. 113, No. 10, 2009 3915

Figure 5. (a) Pt 4f and (b) Pd 3d XPS regions before (i) and after (ii) sputtering with Ar+ for Pt0.5Pd0.5 nanoparticles. The vertical lines indicate the binding energies of the observed chemical components.

Figure 6. Atomic rearrangement model suggested for PtxPd1-x (x ) 0.7 or 0.5) samples for the (i) as prepared, (ii) reduced, and (iii) sulfided conditions.

results suggest that the temperature is also an important parameter that influences the final sulfidation products. Concerning the second main result, we clearly observed the atomic rearrangement within the PtxPd1-x nanoparticles, which is induced by the thermal processes under hydrogen or sulfur atmosphere. Using the surface sensitivity of the XPS technique and monitoring photoemission peaks at a similar probe depth, we are able to monitor the chemical composition near the surface region of the nanoparticles. On the basis of our findings, we propose that initially the Pt and Pd atoms have no preferential site inside the nanoparticles. The reduction process promotes a migration of Pd atoms to the nanoparticles’ surface. A postsulfidation process increases the tendency of atomic rearrangement and promotes the formation of metal-sulfur bonds at the nanoparticle surface. Thus, both thermal processes induce the formation of a core-shell structure in the bimetallic nanoparticles. In addition, XPS signals of metal-metal bonds reappear on sulfided samples submitted to sputtering. For example, after sputtering of the sulfided Pt0.5Pd0.5 sample, one observes a change in chemical states from PtS to (PtS + Pt0) and from PdS2 to (PdS + Pd0). It can be understood as a gradient of PtSz or PdSz compounds (z ) 0, 1, or 2) with an increase in z from

the inner to the outer region. A scheme of the atomic organization inside the nanoparticles is proposed in Figure 6. The core-shell structure suggested here has been noticed by other authors in previous papers.23,26,12,29,30 In ref 12, the authors found this same structure by XPS measurements performed before and after argon bombardment for a sample with a Pt/Pd ratio of 3/1 after reduction at 300 °C, starting from a random distribution for the as-prepared samples. Guillon et al.22 have found that a PtPd/Al2O3 system under exposure to sulfur has a core-shell structure with a PdS shell, but without Pt-S bonds. In contrast, we have found an important contribution of the Pt-S bond in our samples from both XPS and EXAFS analysis. We do not attribute the difference to the support of Al2O3 used22 because Pt-S bond has been found previously15 for a PtPd/ Al2O3 system. We believe that this difference may be due to the higher temperatures employed here (300 °C), as opposed to 100 °C in ref 22. In fact, for the PtPd nanoparticles sulfided at 150 °C (see Figure 2), we found a weak contribution of Pt-S from XAS, and the core-shell structure was also observed by XPS. Hwang et al.,45 in a theoretical work, deduced an equation to predict the atomic distribution of nanoparticles on the basis of

3916 J. Phys. Chem. C, Vol. 113, No. 10, 2009 EXAFS analysis. In this case, the equation takes into account values obtained from measurements at both Pt and Pd edges to calculate the J factor, as named by the authors. Therefore, by calculating just the J factors for the Pt L3 edge, we have found values that agree with the core-shell structure proposed in our work formed after the thermal processes. Coming back to the role of Pd in sulfidation, we have noticed that the bonding enthalpies of a Pd-S and Pt-S formation are practically the same under thermodynamical equilibrium. Nevertheless, the sulfidation process for our nanoparticles is improved by the presence of Pd atoms. Taking into account the atomic arrangement proposed in Figure 6, in which Pd atoms are localized preferentially at the nanoparticle surface after a reduction process, we suggest that Pd atoms display an increased probability to bond with sulfur in the case of low-dimensionality configuration. Conclusions In summary, in situ XAS measurements of the PtxPd1-x (x ) 1, 0.7 or 0.5) nanoparticles were performed during reduction and sulfidation processes at 150 and 300 °C. The vicinity of Pt atoms was modified by the elimination of lighter atoms after reduction and by the incorporation of S during sulfidation. The in situ DXAS measurements showed a fast process of sulfur incorporation in the nanoparticles. The presence of S atoms increases with the amount of Pd, which improves the capability of the nanoparticles to form metal-sulfur bonds. The XPS measurements supported the XAS observations and identified the different chemical states of the elements present in the nanoparticles. In particular, they showed that the reduction and sulfidation processes induce an atomic rearrangement with a formation of a core-shell structure, with a Pd-enriched shell and a Pt-rich core. We conclude that the migration of Pd atoms toward the surface and the number of Pd atoms in the nanoparticles are key factors in the sulfur trapping mechanism from the gas phase. We plan to perform future experiments on similar systems but containing higher Pd concentrations and to monitor the atomic local order around the Pd atoms, as well. Acknowledgment. We thank the support given by the LNLS staff. Work funded by CNPq, CT-PETRO, CT-ENERG, and LNLS (DXAS 3384, DXAS 4556, DXAS 5305, XAFS1 5269, XAFS1 5695, and SXS 5734 proposals). F.B. thanks CNPq for his Ph.D. fellowship. References and Notes (1) He, J.; Ichinose, I.; Kunitake, T.; Nako, A.; Shirashi, Y.; Toshima, N. J. Am. Chem. Soc. 2003, 125, 11034. (2) Kamat, P. V.; Huehn, R.; Nicolaescu, R. J. Phys. Chem. B 2002, 106, 788. (3) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (4) Frenkel, A. I.; Hills, C. W.; Nuzzo, R. G. J. Phys. Chem. B 2001, 105 (51), 12689. (5) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (6) Sinfelt, J. H. J. Catal. 1973, 29, 308. (7) Directive of the European Parliament and of the Council on the Quality of Petrol and Diesel Fuels, Brussels, COM 241 final, 2001. (8) Cooper, B. H.; Donnis, B. B. L. Appl. Catal., A 1996, 137, 203.

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