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Monitoring of Pt Nanoparticle Formation by H2 Reduction of PtO2: An in Situ Dispersive X-ray Absorption Spectroscopy Study Fabiano Bernardi,† Maria C. M. Alves,‡ and Jonder Morais*,† Laborato´rio de Espectroscopia de Ele´trons (LEe-), 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, and 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 ReceiVed: July 2, 2010; ReVised Manuscript ReceiVed: October 29, 2010
In situ dispersive X-ray absorption spectroscopy (DXAS) measurements were used to monitor the Pt nanoparticle formation by hydrogen reduction of PtO2 at 150 °C. The kinetics of the reduction process, as well its dependence on the percentage of H2 in the reductive gas mixture, was investigated. The results reveal that a complete reduction of the PtO2 was obtained using a gas mixture with 55% of H2 in the time of 20 min, resulting in nanoparticles with an estimated mean diameter of 2.8 nm. In addition, the in situ DXAS investigation of the H2S sulfidation of the formed Pt nanoparticles at 150 °C revealed a low degree of sulfidation, in opposition to the Pt-Pd bimetallic systems. 1. Introduction The reduction process is commonly used on metal-based heterogeneous catalysts to bring them into an active form prior to a catalytic reaction. It is an important step since the final properties of the catalyst are strongly influenced by the reduction parameters. Generally, during reduction, the catalyst is heated under a reductive gaseous mixture containing H2. Catalysts based on precious metals, such as palladium, platinum, and rhodium, are used in a vast array of processes of economic and environmental importance.1-3 Concerning environmental and clean-fuel legislation,1 recently considerable attention has been paid to develop catalysts with high hydrogenation of aromatics (HYD) and hydrodesulfurization (HDS) activities.2,4 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.5 Thus, the use of these catalysts is still limited by the severe pretreatment requirements, until sulfur tolerance can be greatly improved. The mechanism of metal poisoning by sulfur compounds involves strong chemisorption of the S-containing molecule on the metal sites as represented by the equilibrium: Me0 + H2S S Me-S + H2. Various types of platinum nanoparticles are among the most popular and used metal-based hydrogenation catalysts.6 Numerous methods, mostly based on colloidal techniques, have been developed in recent years to produce these platinum catalysts. Pt(IV) oxide (known as Adams’ catalyst) is one of the most used metal precursors because, on reduction with hydrogen, it readily provides a highly dispersed active form of platinum, which can be obtained in situ immediately prior to the introduction of the unsaturated compounds. The formation of Pt nanoparticles by hydrogen reduction of PtO2 in ionic liquids has recently been reported.7 Ideally, more detailed chemical information on the reduction process can be obtained through * To whom correspondence should be addressed,
[email protected]. † Laborato´rio de Espectroscopia de Ele´trons, Instituto de Fı´sica. ‡ Instituto de Quı´mica.
in situ measurements. X-ray absorption spectroscopy (XAS) is widely employed to characterize the structural and electronic properties of catalysts and nanoparticles.8 One advantage of XAS over other techniques is the possibility of carrying out atomspecific in situ measurements. In recent work,9 we studied nonsupported PtxPd1-x (x ) 1, 0.7, or 0.5) nanoparticles submitted to hydrogen reduction and posterior sulfidation by in situ XAS. We observed that the reduction process is necessary prior to the occurrence of any sulfur reaction and that the amount of chemisorbed sulfur atoms is proportional to the quantity of Pd atoms. Since the reduction is a key process in order to observe sulfidation, we understand that monitoring the atom’s oxidation state during the reduction process is critical to successfully understand the reaction with sulfur. In situ XAS measurements of Pt-supported catalysts during the reduction process has been reported.10-20 To our knowledge, detailed DXAS studies concerning real-time monitoring of the reduction process of PtO2 leading to Pt nanoparticles are inexistent. In this work, in situ dispersive XAS (DXAS) measurements were applied to follow the kinetics of the reduction process of PtO2 leading to Pt nanoparticles. We observed the dependence of the reduction process on three distinct gas mixtures of H2 and He. Once the Pt nanoparticles were formed, they were submitted to interaction with sulfur while new DXAS measurements were collected. The results show that a low amount of sulfur is adsorbed (or chemisorbed) by the catalyst and that, for a complete reduction of Pt nanoparticles at 150 °C during 40 min, it is necessary to use a high amount of H2 in the reductive gas mixture. 2. Experimental Methods The samples were submitted to the reduction process at 150 °C using different gas mixtures in order to study the reduction process’ dependence on time at distinct percentages of H2. The gas mixtures utilized were (i) 22% H2 + 78% He, (ii) 55% H2 + 45% He, and (iii) 100% H2. For the sample submitted to step ii, sulfidation was accomplished by flowing 33% He + 39% H2 + 28% H2S. DXAS spectra were acquired during the reduction and sulfidation processes. The total gas flux was ≈17
10.1021/jp106134r 2010 American Chemical Society Published on Web 11/10/2010
Pt Nanoparticle Formation
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21435 considered in the fitting procedure. The amplitude reduction term (S02) value was fixed at 0.84, and obtained from the fitting of a Pt foil standard spectra measured at the same experimental conditions as the reduced Pt. 3. Results and Discussion
Figure 1. Evolution of the XANES measurements at the Pt L3 edge for the PtO2 sample submitted to the reduction process at 150 °C and with a gas mixture of 22% H2 + 78% He. The time axis corresponds to the elapsed minutes from the beginning of the reduction process.
cm3/min and the pressure at the sample was kept at approximately 35 psi. The time resolution was about 100 ms and a spectrum was collected every 18 s with an accumulation time of 3 s. For the DXAS experiments, about 10 mg of the PtO2 powder (Sigma) was compacted to produce a pellet with a 5 mm diameter. The sample pellet used was introduced in a previously described reactor,21 which allows controlled thermal treatment of the sample under controlled gas flow. The dispersive measurements were performed at the LNLS (Brazilian Synchrotron Light Laboratory) DXAS beamline.22 The monochromator consists 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 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. Extended X-ray absorption fine structure (EXAFS) data were analyzed in accordance with the standard procedure of data reduction,8 using IFEFFIT.23 FEFF was used to obtain the phase shift and amplitudes.24 The EXAFS signal χ(k) was extracted and then Fourier transformed using a Kaiser-Bessel window with ∆k range of 6.0 Å-1. Single scattering events were
The time-dependent in situ DXAS spectra measurements accomplished during the reduction process at 150 °C with a gas mixture of 22% H2 + 78% He at 150 °C is not enough to successfully reduce the PtO2 in the time of 40 min (Figure 1). The zero on the time scale corresponds to the introduction of the reductive gas. We have demonstrated in a previous study9 that this gas composition has successfully reduced oxidized Pt nanoparticles in similar conditions of flow and time, but at 300 °C. This result is in accordance with studies of supported Pt nanoparticles reported in the literature17,19 and points out the process’s strong dependence on the temperature. In ref 17, the reduction of a platinum complex ([Pt2+(NH3)4](NO3)2 supported on SiO2 was performed in a 100% H2 flow occurring at 175 °C. In the case of Pt/Al2O3,19 the reduction from Pt4+ to Pt0 in a 100% H2 flow was observed at 200 °C. Figure 2a shows in situ DXAS spectra collected during reduction of the PtO2 at 150 °C using the proportion of 55% H2 + 45% He in the gas mixture. Initially the samples spectrum is characteristic of Pt4+, as expected. The absorption edge corresponds to a 2p f 5d transition, and it is noticeable that its intensity decreases as the reaction progresses. This modification in the edge height is related to the decrease in the number of holes in the 5d band, in agreement with the transformation of Pt4+ f Pt0. The broad feature at 11625 eV is split during the reduction process, concomitant with the formation of metallic platinum. A comparison between the first and the last spectrum is presented in Figure 2b, which shows a better visualization of the overall changes observed in the spectra. When pure H2 is used, the reduction process is faster and, after approximately 10 min, no more changes in the XANES spectra are observed (Figure 3). The DXAS spectra for reduction under 100% H2 and 55% H2 + 45% He flows were analyzed by the linear combination of the XANES spectra to evaluate the fraction of Pt0 formed with time. The following equation was used: µobs ) C1µ+4 + C2µ0, where µobs, µ+4, and µ0 represent the observed curve and
Figure 2. (a) Evolution of the XANES measurements at the Pt L3 edge for the PtO2 sample submitted to the reduction process at 150 °C and with a gas mixture of 55% H2 + 45% He. The time axis corresponds to the elapsed minutes from the beginning of the reduction process. The arrow points out the split of the oscillation at 11625 eV. (b) Plot of the first (solid green line) and the last (short-dash red line) spectra for better comparison of the intensities in the near-edge structure.
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Figure 3. (a) Evolution of the XANES measurements at the Pt L3 edge for the PtO2 sample submitted to the reduction process at 150 °C and with a 100% H2 flow. The time axis corresponds to the elapsed minutes from the beginning of the reduction process. The arrow points out the split of the oscillation at 11625 eV. (b) Plot of the first (solid green line) and the last (short-dash red line) spectra for better comparison of the intensities in the near-edge structure.
Figure 4. Typical linear combination of the first (Pt4+) and the last (Pt0) XANES-measured spectra for the reduction process with a 100% H2 flow at t ) 288 s.
the first (Pt4+) and the last (Pt0) measured spectra, respectively. The addition between the constants C1 and C2 was constrained to 1 (C1 + C2 ) 1). The energy interval of the observed curve used to perform the linear combination was between -20 and +100 eV around the absorption edge (Figure 4). The fractions of Pt0 in the course of the reduction process are plotted as a function of time in Figure 5. The reduction processes performed with 55% and 100% of H2 have a kinetics governed by an exponential law, which accounts for the initially rapid changes in the Pt0 fraction µ0, followed by the relatively slow changes for a longer reduction time. It is an indication of a pseudo-first-order one-step reaction (on the time scale). The rate constant obtained from the exponential adjustment is 0.00038 and 0.00010 s-1 for 100% H2 and 55% H2 + 45% He flows, respectively. It shows that the case with a percentage of 100% H2 has a rate constant around 3.8 times higher than the case with 55% H2 + 45% He flow. This kind of behavior for the reduction process was already proposed in the literature.10 Figure 6a shows the time evolution of the Fourier transform (FT) obtained from the χ(k) signal of the in situ DXAS measurements during the reduction process at 150 °C with a 100% H2. At the beginning of the reduction process (t ) 0 s), one observes only one peak associated with the Pt-O bonds in
Figure 5. Kinetics of the Pt0 fraction formed during the reduction process at 150 °C with a 100% H2 flow (full points) and 55% H2 + 45% He flow (empty points). The points represent the values obtained from the linear combination utilized. The line corresponds to the exponential adjustment of the data.
Figure 6. (a) Time evolution of the Fourier transform (FT) obtained from the in situ XAS measurements of the PtO2 sample reduced at 150 °C with a 100% H2 flow. (b) Plot of the first (solid green line) and last (short-dash red line) FT during the reduction process.
the coordination shell. With the increase in time, this peak gradually disappears with concomitant appearance of the peaks associated to higher distances or Pt-Pt bonds. After 576 s, there are no observable changes in the FT, suggesting that the system is stabilized. Figure 6b presents a comparison between the first and the last FT obtained during the reduction process. The
Pt Nanoparticle Formation
Figure 7. Magnitude (straight lines) and real part (dashed lines) of the Fourier transform for the reduced sample (t ) 40 min). Comparison between the experimental data (black) and the fitting results (red).
change in the position of the first peak is clearly observed. The adjustment of the FT peaks for the reduced sample (t ) 40 min) provided the Pt coordination number of NPt-Pt ) 9.2 ( 1.0. In addition, the Pt atoms are located at a mean distance of RPt-Pt ≈ 2.75 ( 0.02 Å, close to the value for bulk Pt (RPt-Ptbulk ≈ 2.77 Å). The good quality of the fitting curves may be visualized when compared with the experimental data of the FT (magnitude and real part) as shown in Figure 7, which provided a low R-factor of 0.005. Using the coordination number of the coordination shell, it is possible to obtain the mean Pt nanoparticle diameter d.25 On the basis of our results, a value of NPt-Pt ) 9.2 would correspond to nanoparticles with d ) 2.8 nm. After 40 min of reduction of PtO2 and the formation of the Pt nanoparticles at 150 °C, the sulfidation process was started employing a gas mixture of 33% He + 39% H2 + 28% H2S, at the same temperature. The amount of H2S here is much higher than that used in a previous work (4%), which, after sulfidation under the same temperature, showed a weak sulfidation.9 Now, using a H2S concentration seven times higher (28%), we are
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21437 checking the Pt nanoparticles’ sulfur resistance under an extreme condition. The evolution of the spectra during the sulfidation reaction is presented in Figure 8a. The zero on the time scale corresponds to the introduction of the sulfur containing the gas mixture. The XANES spectra stabilize after approximately 20 min of sulfidation and remain unchanged. The changes observed after sulfidation are quite small; however, the slight increase in the intensity of the absorption edge and the shift in position of some of the XANES features can be associated to the oxidation of Pt0 (Figure 8b). No noticeable changes were observed in the EXAFS part of the spectra after sulfidation. In the previous study, where the Pt nanoparticles were submitted to the 78% He + 21% H2 + 4% H2S gas mixture at 300 °C, a low degree of sulfidation was obtained.9 When the sulfidation reaction is performed in a stronger oxidizing atmosphere at 150 °C (this work), the low degree of sulfidation is still observed. As already established, the reduction process is necessary prior to the occurrence of any sulfur reaction.9 Since the reduction was successfully achieved after 40 min with the 45% He + 55% H2 gas mixture, we cannot attribute the low sulfidation level reached to a nonmetallic state of the Pt nanoparticles formed. For bimetallic nanoparticles, the number of sulfur atoms increases with the amount of Pd in the PtxPd1-x nanoparticles.9 The results obtained in this work, together with the previous ones obtained for PtxPd1-x nanoparticles (x ) 1, 0.7, or 0.5),9 enabled us to correlate the low sulfidation level to the absence of Pd atoms. 4. Conclusions In summary, the H2-reduction process of PtO2 leading to Pt nanoparticle formation was investigated by in situ DXAS and as a function of the H2 percentage in the reductive gas at 150 °C. A complete reduction was achieved after 40 min, when the H2 content was necessarily 55%. We have observed that the kinetics of Pt0 fraction formation during the reduction process is governed by an exponential dependence law in time. The formed Pt nanoparticles have an estimated mean diameter of 2.8 nm and presented a resistance against sulfidation, in opposition to the Pt-Pd bimetallic systems. Acknowledgment. We thank D. O. Silva and Professor J. Dupont for providing the PtO2. We are grateful for the support
Figure 8. (a) Evolution of the XANES measurements at the Pt L3 edge for the Pt nanoparticles submitted to the sulfidation process at 150 °C and with a gas mixture of 33% He + 39% H2 + 28% H2S. The time axis corresponds to the elapsed minutes from the beginning of the sulfidation process. (b) Plot of the first (solid green line) and the last (short-dash blue line) spectra for better comparison of the intensities in the near-edge structure.
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given by the LNLS staff. Work funded by CNPq, CAPES, and LNLS (DXAS 4556, DXAS 5305 proposals). F.B. thanks CNPq for his Ph.D. fellowship. References and Notes (1) Cooper, B. H.; Donnis, B. B. L. Appl. Catal., A 1996, 137, 203– 223. (2) Navarro, R. M.; Pawelec, B.; Trejo, J. M.; Mariscal, R.; Fierro, J. L. G. J. Catal. 2000, 189, 184–194. (3) Newton, M. A.; Coldeira, C. B.; Arias, A. M.; Garcia, M. F. Nat. Mater. 2007, 6, 528–532. (4) Yasuda, H.; Yoshimura, Y. Catal. Lett. 1997, 46, 43–48. (5) Barbier, J.; Lamy-Pitarra, E.; Marecot, P.; Boitiaux, J. P.; Cosyns, J.; Verna, F. AdV. Catal. 1990, 37, 279–318. (6) Rampino, L. D.; Nord, F. F. J. Am. Chem. Soc. 1941, 63, 2745– 2749. (7) Scheeren, C. W.; Domingos, J. B.; Machado, G.; Dupont, J. J. Phys. Chem. C 2008, 112, 16463–16469. (8) X-ray Absorption: Principles, applications and techniques of EXAFS, SEXAFS and XANES in Chemical Analysis; Chemical Analysis; Koningsberger, D. C., Prins, R., Eds.; John Wiley & Sons: New York, 1988; Vol. 92. (9) Bernardi, F.; Alves, M. C. M.; Traverse, A.; Silva, D. O.; Scheeren, C. W.; Dupont, J.; Morais, J. J. Phys. Chem. C 2009, 113, 3909–3916. (10) Allen, P. G.; Conradson, S. D.; Wilson, M. S.; Gottesfeld, S.; Raistrick, I. D.; Valerio, J.; Lovato J. Electroanal. Chem. 1995, 384, 99– 103.
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