Reversible Sulfidation of Pt0.3Pd0.7 Nanoparticles Investigated by in

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Reversible Sulfidation of Pt0.3Pd0.7 Nanoparticles Investigated by in Situ Time-Resolved XAS Jocenir Boita,† Fabiano Bernardi,† Marcus Vinícius Castegnaro,† Lucas Nicolao,† Maria C. M. Alves,‡ and Jonder Morais*,† †

Laboratório de Espectroscopia de Elétrons (LEe−), Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Avenida Bento Gonçalves, 9500, Bairro Agronomia, CP 15051, CEP 91501-970, Porto Alegre, RS, Brazil ‡ Instituto de Química, Universidade Federal do Rio Grande do Sul (UFRGS), Avenida Bento Gonçalves, 9500, Bairro Agronomia, CP 15003, CEP 91501-970, Porto Alegre, RS, Brazil ABSTRACT: In the present study, in situ X-ray absorption spectroscopy (XAS) was used to monitor the structural evolution of isolated Pt0.3Pd0.7 nanoparticles (NPs) subjected to different gaseous atmospheres. Time-resolved XAS measurements were performed at the Pt L3 edge in the dispersive mode, during which X-ray absorption near edge spectra were sequentially collected. The NPs were initially activated under a reducing atmosphere at 300 °C (H2 + He) and subsequently exposed to a sulfidation process at the same temperature (H2 + He + H2S at 300 °C). Then, the sulfided NPs were thermally treated under a reducing atmosphere, and the reversibility of the sulfidation process was successfully accomplished, for the first time in these unsupported and well-characterized nanoscale systems. The atomic local order in the vicinity of the Pt atoms was investigated by extended X-ray absorption fine structure spectroscopy throughout all of these thermal treatments, monitoring the chemical stability of the metal−sulfur bonds and allowing kinetic modeling, from which the activation energies for the sulfidation process were estimated.

1. INTRODUCTION Recently, considerable attention has been directed toward the development of catalysts with higher catalytic activity for the hydrogenation of aromatics (HYD) and hydrodesulfurization (HDS) reactions. These studies indicated that even noble metal-based catalysts are susceptible to sulfur poisoning.1 Hence, the development of catalytic materials less susceptible to sulfur contamination is an important issue. Comparatively, bimetallic catalysts are quite an exciting system to investigate since their behavior is distinct from monometallic cases.2,3 In the case of Pt−Pd, thermally induced atomic redistribution has been shown within nanoparticles (NPs), forming a Pd shell and a Pt-rich core. In addition, the degree of sulfidation depends on the concentration of Pd in the NPs. The presence of a second metal modifies the distribution of the active sites, causing changes in the reaction paths. At the atomic level, a charge transfer process from the Pd to Pt atoms triggers an electron deficient character in Pd, which facilitates the interaction between the NPs and the sulfur compounds.4,5 Therefore, improvements to the catalytic activity, selectivity, and poisoning resistance depend on the chosen experimental parameters and conditions that may be adjusted for each synthesis, such as temperature, composition, size of the active NPs, and the interaction between the support and the bimetallic alloy.6−9 As previously described,4 in situ X-ray absorption spectroscopy (XAS) allows following the structural and electronic properties evolution of bimetallic NPs under reactive conditions. The use of an appropriate experimental setup10 makes it possible to monitor, at the atomic level, NPs during the activation process at 300 °C under an H2 atmosphere. This © 2014 American Chemical Society

is an important step in order to eliminate impurities adsorbed at the NP surface and is a fundamental procedure for the following step, the sulfidation process with H2S at 300 °C.5,11 The XAS variation called in situ dispersive X-ray absorption spectroscopy (DXAS) provides the momentum to get accurate information during a reaction process.12 It allows time-resolved measurements of the electronic structure, providing more details on the structural evolution mechanisms. In the present work, we extended our prior studies to investigate the removal of Pt−S bonds formed on sulfided Pt0.3Pd0.7 NPs, aiming to shed light on the chemical stability of metal−S bonds. The processes responsible for the formation and dissociation of such bonds were monitored by timeresolved in situ DXAS measurements, unveiling the details of these thermally driven reactions and estimating their activation energies. Even though these energies are accessible in a moderate temperature range, once triggered, these solid-state reactions are irreversible, as a consequence of the inherent collective nature. This aspect has positive consequences on both sides of the sulfidation/reduction phase transformation, as our results show: sulfidation of Pt0.3 Pd0.7 nanoparticles was achieved under a reactive H2S atmosphere at 300 °C and was stable; i.e., it could not be reversed even under reductive environment, unless the sample was heated up to 400 °C, at which point sulfur poisoning was completely reversed. These results were supported by structural parameters obtained from Received: October 12, 2013 Revised: January 31, 2014 Published: February 20, 2014 5538

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Figure 1. Normalized Pt L3 edge XANES spectra for the Pt0.3Pd0.7 NPs collected during the (a) activation process. (b) The first and the last curves are compared to the XANES spectra of a Pt reference foil and PtS2.

the fitting procedures of the extended X-ray absorption fine structure (EXAFS) spectra, searching for a correlation between the sulfur reactivity and the atomic arrangement after the removal of S. Therefore, this work foresees a process for the possible recovery and recycling of poisoned noble metal-based catalysts.

collected by a position-sensitive CCD camera. The reactor was installed at the beamline, taking care to place the pellet at the X-ray focal point. 2.3. XAS Measurements. The steady mode in situ XAS experiments were performed at the LNLS XAFS1 beamline15 after each thermal process, under the same reaction conditions as used for the in situ DXAS measurements. The XAS spectra were collected at the Pt L3 edge using a channel-cut Si(111) crystal and three argon-filled ionization chambers. A standard Pt foil was used to calibrate the monochromator. The spectra were acquired in the range of 11440−12200 eV with a 2 eV step and 2 s/point; two to four scans were collected in order to improve the signal-to-noise ratio. In addition, ex situ X-ray absorption near edge spectra (XANES) spectra at the Pd K edge (24350 eV) for the asprepared, activated, sulfided, and reduced samples were collected using the LNLS XDS beamline. The spectra were acquired at room temperature in transmission mode with three ionization chambers using a Si(311) double-crystal monochromator and a toroidal focusing mirror. A standard Pd foil was used for energy calibration. Six to nine spectra were collected in order to improve the signal-to-noise ratio. Each spectrum was acquired in the range of 24200−24600 eV with a 2 eV step and 2 s/point. 2.4. Data Analysis. The time-resolved DXAS results acquired during the sulfidation and reduction processes were analyzed to extract the kinetics of the Pt0 fraction present in the NPs in each case. We chose the initial and final XANES curves of the sulfidation kinetics as the standard patterns to evaluate the different platinum fraction contributions. The following equation was used for this linear combination: μobs = C1μS + C2μ0, where μobs is the measured absorption coefficient, while μ0 and μS are the two representative XANES curves, for activated and fully sulfided samples, respectively. The corresponding coefficients were normalized (C1 + C2 = 1) in order to quantify the relative fraction of platinum. The ATHENA 16 program was used for the deconvolution procedures. The in situ XAS spectra were analyzed in accordance with the standard procedure of data reduction,17 using the IFEFFIT package.18 The FEFF program was used to obtain the phase shift and amplitudes.19 The EXAFS signal χ(k) was extracted and then Fourier transformed (FT) using a Kaiser−Bessel

2. EXPERIMENTAL METHODS 2.1. Nanoparticle Synthesis. The Pt0.3Pd0.7 nanoparticles were prepared by the dissolution of the Pt and Pd precursors (Pt2(dba)3 and Pd(acac)2, respectively) in 1 mL of 1-n-butyl-3methylimidazolium hexafluorophosphate (BMIPF 6) ionic liquid. The resulting solution was allowed to react with molecular hydrogen at 75 °C and 4 atm for 5 min. At the end of the process, a black solution was formed, and the nanoparticles were isolated by centrifugation.13 The size of the NPs was 4.2 ± 0.8 nm, as determined by TEM.11 2.2. DXAS Measurements. In situ dispersive XAS (DXAS) experiments were used to monitor Pt L3 edge evolution during different thermal processes. Approximately 10 mg of the Pt0.3Pd0.7 nanoparticle powder was compacted to produce pellets with a diameter of 5 mm. For the in situ measurements, one pellet at a time was introduced to a specially designed reactor for XAS experiments10 that allowed the controlled thermal treatment of a sample under controlled gas flow. Each sample was submitted to activation, sulfidation, and reduction reactions under gaseous environments. For these experiments, the nanoparticles were activated at 300 °C for 2 h 20 min under 95% He + 5% H2 flux and then exposed to the sulfidation process by inserting a flux of 95% He + 5% H2S during 2 h and 20 min. After the sulfidation process, while keeping the same temperature of 300 °C, the nanoparticles were submitted to a reducing atmosphere (95% He + 5% H2) for 15 min, revealing no sign of an ongoing reduction process. After heating at 400 °C for 1 h under the same conditions, reduction was finally achieved. The total gas pressure on the sample was kept at approximately 35 psi during all processes. These experiments were conducted at the DXAS beamline of the LNLS (Brazilian Synchrotron Light Laboratory).14 The monochromator consisted of a curved Si(111) crystal (dispersive polychromator) that focuses the beam on the horizontal plane down to about 200 μm and on the vertical plane to about 500 μm. The time-resolved spectra were 5539

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Figure 2. Normalized Pt L3 edge XANES spectra for the Pt0.3Pd0.7 NPs collected during the (a) sulfidation process. (b) The first and the last spectra of this process are compared to the XANES of Pt0 and PtS2.

window with Δk range of 7.7 Å for the Pt L3 edge. To obtain the phase shift and amplitudes, a cluster of atoms with 10 Å radius was built considering the face-centered cubic (fcc) crystal structure of Pt. To simulate the Pt0.3Pd0.7 nanoparticles, Pt atoms were randomly replaced by Pd atoms respecting the composition of the Pt0.3Pd0.7 NPs. Single and multiple scattering events were considered in the fitting procedure. All the presented fittings of the absorption data were k2-weighted. During the EXAFS fitting, the number of free parameters (used as variables in the fitting procedure) was always lower than the number of independent parameters. The amplitude reduction term (S02) value was fixed at (0.89) (Pt absorber) for all samples, which was obtained from the fitting of standard Pt foil spectra, and the use of cumulant expansion was necessary (c3 and c4 values around 10−4 and 10−5, respectively).

3. RESULTS AND DISCUSSION 3.1. Activation and Sulfidation Processes at 300 °C. The time-resolved DXAS results obtained during the activation process of the Pt0.3Pd0.7 NPs are shown in Figure 1a. A comparison of the first 40.1 min with the spectra of the standard Pt foil in Figure 1b suggests that the as-prepared NPs were already in the metallic state. The results obtained during the following sulfidation reaction are shown in Figure 2a. Figure 2b displays a comparison between the XANES spectra collected at 0 min (end of activation) and 46 min (end of sulfidation) with the standard compounds (Pt foil and PtS2). The sulfidation reached complete stabilization after 45.9 min, as indicated by the observed changes in the intensity of the 2p → 5d transition (Pt L3 edge), which are consistent with an increase in the number of empty electronic states of 5d symmetry, suggesting the formation of Pt−S bonds. 3.2. Reduction Process. The reversibility of sulfidation was studied by submitting the sample to a reducing atmosphere, right after the sulfidation process. In the first attempt, the use of the same conditions applied during the activation process (95% He + 5% H2 at 300 °C) was not efficient to remove the Pt−S bonds, as shown in Figure 3. The increase in temperature to 400 °C under the same atmosphere used in the activation process enabled the removal of sulfur bonds, and the Pt atoms returned to the metallic state (Figure 4a and b).

Figure 3. Pt L3 edge in situ XANES spectra collected before and after attempting S atom removal at 300 °C. After 15 min, no significant changes in the XANES spectra were observed.

3.3. Activation Energies. A typical deconvolution of a XANES spectrum that provides the fraction of μS (green curve) and μ0 (blue curve) is shown in Figure 5a. The resulting fractions of metallic platinum (C2 values) during the sulfidation at 300 °C and reduction at 400 °C are displayed in Figure 5b. Each point corresponds to one deconvolution of a selected XANES spectrum. The observation of sulfur removal from the NPs only at a higher temperature (400 °C) suggests that this process is thermally activated. In the previous session, we established the gradual decrease in metallic platinum in the NPs during the sulfidation reaction as a consequence of their sulfur poisoning. On general grounds, we expect both these processes of phase transformation of the nanoparticles to be thermally driven. Thermally activated phenomena translate into an Arrhenius temperature dependence of the solid-state reaction rates, regardless of the underlying driving mechanism (diffusionlimited or nucleation-limited); that is, the time scales to attain these reactions have the form τ = τ0 exp(EA/kBT),20 where EA is the activation energy and τ0 is an unknown characteristic time 5540

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Figure 4. Normalized XANES spectra of the Pt0.3Pd0.7 NPs collected during the (a) reducing process at 400 °C. (b) The first and the last spectra of this process are compared to the XANES of Pt0 and PtS2.

Figure 5. (a) Typical deconvolution of the XANES curve measured at t = 96 s of the sulfidation process using a linear combination of the first (μ0) to the last (μS) spectra collected. (b) Kinetics of the Pt0 fraction present in the NPs during the sulfidation and reduction processes.

for the second step in Pt0.5Pd0.5 NPs cannot discriminate the exponential from quadratic time evolutions. We argue that, under similar reactive environments, the characteristic time scales τ0 controlling the mechanisms for the onset of sulf idation of the Pt atoms are similar and can be considered to be the same, especially when confronted with the strong temperature dependence evidenced in the Arrhenius form. The best fit to an exponential decay (A exp(−t/τ)) of the Pt0 fraction gives

scale that depends on the specific constituents and the microscopic processes. In order to support our analysis of the sulfidation process, we gathered data from previous and comparable sulfidation experiments of bimetallic Pt0.5Pd0.5 NPs performed at 150 and 300 °C.4 The kinetic fraction of metallic platinum taken from these data is shown using a log−linear scale in Figure 6b and a, respectively. These results were compared with the data displayed in Figure 5b but now shown in a log−linear scale in Figure 6c and d for the sulfidation and reduction processes, respectively. The sulfidation of Pt atoms in these bimetallic NPs depends on the temperature and relative concentration of both metals but also shares common features. In the low-temperature case of Pt0.5Pd0.5 NPs at 150 °C, there was an initial inactive period of time, required for sulfur atoms to react with Pd-rich shell atoms and reach the Pt-rich core. Then sulfidation started as a very slow process and, within the experiment time scale, it had barely started. Apart from this case, higher temperature sulfidation at 300 °C starts immediately and consists of two steps, for both concentrations: a first (faster) exponential one and a second (slower) quadratic oneeven though the best fits

Pt 0.5Pd 0.5 at 150 °C: τ = 5988 ± 276 s Pt 0.5Pd 0.5 at 300°C: τ = 250 ± 6s Pt 0.3Pd 0.7 at 300°C: τ = 120 ± 4s

Comparing the Pt0.5Pd0.5 at 150 and 300 °C, we obtained an estimation of the activation energy for the onset of sulfidation of EA(Pt0.5Pd0.5) = 0.442(5) eV. The same activation energy for Pt0.3Pd0.7 could be obtained comparing the kinetics at 300 °C for the two different concentrations: EA(Pt0.3Pd0.7) = 0.406(5) eV. These values are in accordance with the increasing sulfur reactivity observed with an increased amount of palladium present in the bimetallic nanoparticles. 5541

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Figure 6. Log−linear plots for the time evolution of the sulfidation (a−c) and reduction (d) processes represented in terms of the percentage of metallic platinum (% Pt0) present in the NPs. Solid lines are best fits to A exp(−t/τ) (green) and (t/τ2 + B)−2 (blue).

Figure 7. (i) EXAFS signals at the Pt L3 edge and (ii) corresponding Fourier transforms for Pt0.3Pd0.7 NPs after each process, designated activated, sulfided, and reduced, as well as the results for a standard Pt0 sample. The dots represent the experimental measurements, and the line represents the best fit theoretical curves.

EXAFS analysis, evaluating the atomic arrangement by the detailed analysis of the coordination number variation around the Pt atoms. Figure 7 shows the EXAFS χ(k) signals and the corresponding Fourier transforms (FT) after each process, designated “activated”, “sulfided”, and “reduced”, as well as the results for a standard Pt0 sample. The best fit results are also presented in Figure 7. The EXAFS curve χ(k) of the activated and the reduced samples displays smothered oscillations due to the short-range order of the NPs. The FT has a double peak in

Unlike the sulfidation process, we could not distinguish two different steps in the reduction kinetics, which followed an exponential increase of the Pt0 fraction at 400 °C under an H2 atmosphere, similar to what is expected in nucleation processes. The characteristic time for this reaction was τ = 248 ± 4 s. 3.4. EXAFS Analysis. As the next step, in situ XAS measurements were performed in order to obtain EXAFS signals after each step (activation at 300 °C, sulfidation at 300 °C, and reduction at 400 °C). Therefore, we were able to understand the structural evolution of the Pt0.3Pd0.7 NPs via the 5542

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between 2 and 3 Å associated with Pt−Pt and Pt−Pd scattering in the coordination sphere. The observed changes in the EXAFS signal frequency for the sulfided sample correspond to the formation of shorter distance bonds, i.e., Pt−S bonds. The quantitative results for the coordination shell are displayed in Table 1. The corresponding R-factor values were Table 1. Results from the Quantitative Analysis of the EXAFS Data for the Coordination Sphere Providing the Coordination Number (N), Distance (R), and Debye− Waller Factor (σ2) for the Activated, Sulfided (T = 300 °C), and Reduced (T = 400°C) Nanoparticles sample activated (T = 300 °C) sulfided (T = 300 °C) reduced (T = 400 °C)

N

pair Pt−Pt Pt−Pd Pt−S Pt−Pt Pt−Pd Pt−Pt Pt−Pd

6.1 4.3 2.0 5.6 3.2 8.2 3.1

± ± ± ± ± ± ±

R (Å) 0.5 0.3 0.1 1.1 0.2 1.1 0.4

2.67 2.67 2.31 2.74 2.74 2.68 2.68

± ± ± ± ± ± ±

0.02 0.02 0.02 0.02 0.02 0.02 0.02

σ2 (10−2 Å2) (0.92 (0.92 (1.31 (1.84 (1.84 (1.46 (1.46

± ± ± ± ± ± ±

0.04) 0.04) 0.04) 0.05) 0.05) 0.05) 0.05)

Figure 8. Normalized Pd K edge XANES spectra for the as-prepared, activated, sulfided, and reduced Pt0.3Pd0.7 NPs. The XANES spectra of a standard Pd and PdS are also shown.

between 0.007 and 0.03, demonstrating the good quality of the adjustments. The variation observed for N, the coordination number, was not in agreement with the nominal NPs composition (Pt0.3Pd0.7). The vicinities of Pt contained rather more Pt than Pd atoms. This behavior was already observed in a previous study11 and is associated with an atomic rearrangement with a core−shell structure, composed of a Pt-rich core and a Pd-rich shell. The interatomic distances for the activated and reduced samples are the values expected for bulk alloy. The sulfidation process was demonstrated by the Pt−S component at 2.3 Å followed by the increase in the Pt−Pt and Pt−Pd distances. Changes were also noticeable in the coordination numbers, while maintaining the core−shell structure. After reduction at 400 °C, the coordination numbers were within the uncertainty similar to those obtained for the activated sample with similar Debye−Waller factors, suggesting that the sample returned to the original state. Supplementary ex situ XAS measurements at the Pd K edge were also performed in order to probe the local environment of the Pd atoms in the NPs. Figure 8 displays the XANES spectra for the as-prepared, activated, sulfided, and reduced Pt0.3Pd0.7 NPs in comparison with those of standard Pd and PdS. The results confirm that the as-prepared and activated NPs are in the metallic state. The curve corresponding to the fully sulfided NPs shows similar features observed for the PdS reference sample, which suggests the formation of Pd−S bonds. As observed for the Pt L3 edge, after the 400 °C reduction process, the Pd K edge XANES curve is identical to that of the activated Pt0.3Pd0.7 NPs, which confirms that nanoparticles were completely regenerated.

Here, we successfully applied time-resolved in situ XAS at the Pt L3 edge to investigate the sulfidation kinetics in Pt0.3Pd0.7 NPs, as well as its reversibility using a reduction process. Unambiguously, the sulfur removal took place in the vicinities of both Pt and Pd atoms. The EXAFS results presented here strongly indicate that, even after the removal of sulfur, the core−shell structure remains, as the coordination numbers after reduction are similar to those observed after the activation process at 300 °C. This demonstrates the efficient removal of sulfur compounds absorbed on NPs using H2 at 400 °C without damaging the core−shell structure formed in the activation step, making the nanoparticles catalytically active for further reactions. Simple assumptions for the absorption/desorption kinetics of the NP Pt0 fraction variation during the sulfidation and reduction processes allowed us to demonstrate the nature of these solid-state reactions: the early stage exponential kinetics are consistent with independent nucleation events in the Pt-rich core boundaries21,22 with subsequent growth of the metal sulfide phase into the metallic phase, in the case of the sulfidation process, and conversely for the reduction process. In this scenario, the late stage of the sulfidation kinetics can be pictured as a slower interface advance of the sulfide phase, due to reduced mobility at boundaries between different sulfide phases.23

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AUTHOR INFORMATION

Corresponding Author

*Phone number: +55 51 33086525. E-mail: [email protected].

4. CONCLUSIONS In a previous publication, it was shown that thermal treatments induce an atomic rearrangement in Pt0.3Pd0.7 NPs, resulting in a core−shell structure with a Pt-rich core and a shell rich in Pd. It was also observed that the both Pt and Pd atoms interact with the gaseous phase during the activation and sulfidation processes. Additionally, it has been demonstrated that the degree of sulfidation (amount of metal−S bonds) increases proportionally with the concentration of Pd in the NPs.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support provided by the CNPq, CAPES, FAPERGS, and LNLS (proposals DXAS-8150, XAFS1-8766, and XAFS1-15318). We are also grateful to Professor J. Dupont for providing the samples. J.B. and M.V.C. thank CNPq and 5543

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(20) Bamford, C. H.; Tipper, C. F. H. Comprehensive Chemical Kinetics: Reactions in the Solid State; Elsevier: Amsterdam, 1980; Vol. 22. (21) Gunton, J. D.; M., S. M.; Sahni, P. S. The Dynamics of First-Order Phase Transitions, in Phase Transitions and Critical Phenomena; Academic Press: London, 1983; Vol. 8. (22) Sear, R. P. Nucleation: Theory and Applications to Protein Solutions and Colloidal Suspensions. J. Phys.: Condens. Matter 2007, 19, 033101(28pp). (23) Kelton, K.; Greer, A. L. Nucleation in Condensed Matter: Applications in Materials and Biology; Elsevier: Amesterdam, 2010; Vol. 15.

CAPES for their Ph.D. fellowships. L.N. thanks CAPES for his postdoctoral fellowship.

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REFERENCES

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