Letter pubs.acs.org/JPCL
Investigation of the Redispersion of Pt Nanoparticles on Polyhedral Ceria Nanoparticles Tianxiao Wu,†,‡ Xiqiang Pan,† Yibo Zhang,† Zhenzhen Miao,†,‡ Bin Zhang,†,‡ Jingwei Li,†,‡ and Xiangguang Yang*,† †
State Key Laboratory of Rare Earth Resource Utilization, Green Chemistry and Process Laboratory, Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences, Changchun, Jilin 130022, China ‡ University of Chinese Academy of Sciences, Beijing 100039, China S Supporting Information *
ABSTRACT: Redispersion of platinum nanoparticles (Pt NPs) on ceria is an important route for catalyst regeneration and antisintering. Here, we investigate the redispersion of Pt on ceria nanoparticles with defined surface planes including cubes ({100}) and octahedra ({111}). It is observed that Pt redispersion takes place only on ceria cubes in an alternating oxidation and reduction atmosphere. A quicker alternation rate is beneficial for such redispersion. On the basis of our experimental results and understandings toward this process, we proposed that the redispersion takes place at the moment of alternation of oxidation and reduction.
SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
N
exhibits different properties, for instance, surface stability,14,21 oxygen vacancy formation energy,17,22 oxygen storage capacity (OSC),14,23 and interaction with surface molecules.16,24 Herein, we investigated the thermal stability of Pt NPs on cubic and octahedral ceria in different atmospheres including an oxidative atmosphere (20% O2/N2), reductive atmosphere (5% H2/Ar), or alternating atmosphere of 20% O2/N2 and 5% H2/Ar with diverse frequencies at 600 °C. Our results show that Pt NPs could redisperse only on cubic ceria NPs covered with a {100} plane treated in an alternating atmosphere but not in single oxidative or reductive atmospheres. Such redispersion of Pt NPs is hindered for Pt supported on octahedral ceria NPs covered with a {111} plane no matter what kind of atmosphere is employed. Cubic and octahedral CeO2 NPs were hydrothermally synthesized using reported methods.14,15 The face-centered cubic structure of CeO2 (fluorite, JCPDS 34-0394) for these two polyhedral NPs was confirmed from their powder X-ray diffraction (XRD) patterns (Figure S1, Supporting Information). The morphologies of cubic and octahedral ceria NPs could be clearly seen from their scanning electron microscope (SEM) pictures (Figure S2, Supporting Information). The length ranges of the two regular morphological ceria are 50− 400 nm for cubes and 110−150 nm for octahedra. Pt NPs with
oble metallic nanoparticles (NPs) such as platinum (Pt), rhodium (Rh), and palladium (Pd) supported on inorganic oxides like Al2O3 and CeO2 have been widely used as catalysts in heterogeneous catalysis.1,2 By reducing the particle size of these noble metallic NPs, their catalytic activities could be enhanced, and the loading of noble metal and cost of making the catalyst would be reduced.3 However, smaller noble metallic NPs tend to coarsen during catalytic reaction as a consequence of their high surface energy.4−6 To surmount this obstacle, tremendous efforts have been devoted to improve the stability of supported catalysts.7,8 Furthermore, disintegration of noble metal NPs could be achieved in different atmospheres. For example, Rh NPs could disrupt into isolated RhI(CO)2 species in a CO-containing atmosphere,9 while Pd NPs could be converted into epitaxial PdO thin films on a MgO substrate in O2.10 Y. Nagai et al. reported that the particle size of Pt on ceria or a substrate containing ceria could be reduced after treatment in air or in an alternating oxidative and reductive atmosphere at high temperature, for example, 600−800 °C. They proposed that oxygen adsorbed on the surface of large Pt particles favors the mobile Pt oxide species forming, migrating, and being trapped on the support surface through a relatively strong PtO−support interaction.11,12 Recently, ceria NPs with controlled surface planes such as nanorods ({111}), nanocubes ({100}), and nano-octahedra ({111}) have been successfully obtained and widely used as catalysts in carbon monoxide oxidation, hydrogen oxidation, water−gas shift reaction, and so on.13−20 Experiments and theoretical simulations suggested that a different crystal plane © XXXX American Chemical Society
Received: April 29, 2014 Accepted: July 2, 2014
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a narrow size distribution of (3.5 ± 1) nm (Figure S2c, Supporting Information) were prepared following an ethylene glycol reduction method.25 The samples of 1 wt % Pt NPs on the two polyhedral ceria NPs were achieved via a conventional wet impregnation method. The particle size of ceria NPs, surface area of ceria, atomic percentage of Pt, and surface Pt atomic percentage of the two Pt loaded ceria samples are summarized in Table S1 (Supporting Information). TEM images of the two loaded samples shown in Figure 1a and b
activities (Figure 1e,f) agree well with the TEM data. The T100 of Pt/cubes decreases from 180 to 160 °C, while a reversal trend occurs for Pt/octahedra. The increase of particle size of Pt NPs was observed for Pt/ cubes and Pt/octahedra when treated in single 20% O2/N2 or 5% H2/Ar for 30 min. The average sizes increased to 6.0 and 5.8 nm for Pt/cubes and Pt/octahedra in an oxidation atmosphere. In a reduction atmosphere, the average sizes became 4.6 and 7.0 nm for Pt/cubes and Pt/octahedra (Figures 2 and S4, Supporting Information). Because the particle size of
Figure 2. TEM images and Pt particle size distribution of samples treated in single 20% O2/N2 or single 5% H2/Ar. (a) Pt/cubesoxidized, (b) Pt/octahedra-oxidized, (c) Pt/cubes-reduced, and (d) Pt/octahedra-reduced.
Pt/octahedra grew larger in all three atmospheres, it is believed that the Ostwald ripening process dominated in the heat treatment.5,6 The Pt 4f XPS spectra (Figure 3) show that most of the Pt on Pt/octahedra is Pt0 in both oxidation and reduction atmospheres, while part of the Pt on Pt/cubes is positively charged in both atmospheres. This suggests that Pt/cubes have a stronger metal−support interaction than Pt/octahedra.26,27 H2-TPR is used to test the reducibility of samples. Figure 4 shows that Pt/cubes has a low-temperature peak at 126 °C, which could be attributed to the surface O of CeO2 in the neighborhood of Pt NPs that could react with Pt.26,28 The corresponding peak for Pt/octahedra is at 285 °C. These results indicate a weaker reducibility of Pt/octahedra that probably resulted from the weaker metal−support interaction. From the results in Figures 1c and 2a and c, Pt NPs on Pt/ cubes redispersed only in an alternating atmosphere while growing in the single reduction or oxidation atmospheres. We can say that sintering or growing of Pt NPs is an inevitable process throughout the heat treatment, while redispersion happens at the moment of alternation of oxidation and reduction. The final size of Pt NPs is controlled by the two opposite effects. The data of Pt/cubes treated in an alternating atmosphere of diverse frequencies are shown in Figures 5 and
Figure 1. TEM images and Pt particle size distribution (a−d) and CO oxidation activity (e, f) of samples as-received and treated. N indicates the number of particles included in each particle size distribution. (a) Pt/cubes-as-received (the top right corner is the HRTEM of cubes), (b) Pt/octahedra-as-received (the top right corner is the HRTEM of octahedra), (c) Pt/cubes-O20H30, (d) Pt/octahedra-O20H30, (e) CO oxidation activity of Pt/cubes in (a) and (c), and (f) CO oxidation activity of Pt/octahedra in (b) and (d).
show that the Pt NPs remained nearly unchanged. Two lattice fringes of 0.27 and 0.32 nm could be seen from HRTEM results (insets of Figure 1a,b) of the two substrates, which are consistent with the d spacing values of the {100} and {111} planes of ceria and verify the exposed {100} and {111} crystal planes of cubic and octahedral ceria.14,17,18 After heat treatment in an alternating atmosphere of 20% O2/N2 for 20 s and 5% H2/Ar for 30 s for a total of 30 min (-O20H30), the average size of Pt NPs on cubic ceria decreased from 3.7 to 2.4 nm but increased from 3.9 to 5.9 nm on octahedral ceria (Figure 1c and d). The numbers of Pt NPs were counted from their corresponding panorama views in Figure S3 (Supporting Information). Their CO oxidation 2480
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min in an alternating atmosphere of 20% O2/N2 for n seconds and 5% H2/Ar gases for m seconds, and HF represents a high flow of gases for 100 mL/min). In addition, more larger Pt NPs were found on Pt/cubes-O60H60 and Pt/cubes-O60H30, and fewer larger Pt NPs were found on Pt/cubes-HF-O5H10. This implies that redispersion is favored in the alternation atmosphere with a quicker rate. The inevitable sintering of Pt NPs on Pt/cubes, typically with O20H60 and O5H10 (Figure S5b, S5c, Supporting Information) could be ascribed to H2O generated by 2H2 + O2 = 2H2O (the dead volume between the valve and sample and the sample itself would lead to mixing of H2 and O2, especially in low flow). In our process, we also noticed the appearance of water on the tube wall in O5H10. To prove this, we treated Pt/cubes with 10% water vapor, and heavier sintering was observed (Figure S5d, Supporting Information). The excellent redispersion effect of HF-O5H10 may come from the quicker frequency and the less mixing of the two atmospheres. Even though, Pt/octahedra treated with HF-O5H10 still sintered (Figure S7, Supporting Information). The immediate result of alternation of the reduction and oxidation atmospheres is the formation and disappearance of surface oxygen vacancies of ceria. Experimental results and theoretical calculations22,28 indicated that the relative stability of the surfaces and the formation energy of oxygen vacancies followed the order {111} > {100}, while the surface relaxations followed a reversal order. The presence and absence of zigzagged edges on Pt/cubes and Pt/octahedra, respectively, from our TEM characterizations on reduced samples (Figure 2c,d) further support the observations and explanations in the literature. The results of H2-TPR are consistent with a much higher OSC of cubic ceria than that of octahedral ceria.14,18 The surface oxygen vacancies favor the redispersion of Pt NPs on Pt/cubes. Such surface oxygen vacancies that dictated antigrowth of noble metallic NPs have been also observed in the cases of Pt NPs on MgO{100}29 and Au on CeO2.30 On the basis of the above analysis, an indication could be made that the redispersion is related to the formation and disappearance of surface oxygen vacancies and related defects of ceria. A possible mechanism of Pt NPs redispersion on CeO2 in an alternating atmosphere is proposed as follows: (i) Upon introducing an oxidative atmosphere, oxygen vacancies on reduced Pt/CeO2−x disappear. Pt−O−Ce or possible Pt−O−O−Ce forms in the interface, which would increase the Pt/CeO2 interface energy and flatten Pt NPs to some extent.7,31 Some Pt atoms on NPs would be pulled to the interface. (ii) Upon introducing a reductive atmosphere, PtOx is reduced to Pt metal. Subsequently, hydrogen spillover from Pt to Ce occurs. Pt−O−Ce and Pt−O−O−Ce bonds break. Meanwhile, oxygen vacancies and related defects form. Finally, Pt atoms are trapped at oxygen vacancies and defects. Pt/cubes has a higher probability for process (i) than Pt/ octahedra for the stronger Pt−cube interaction. For process (ii), cubes ({100}) must have a stronger trap effect for the more reducible surface and higher density of defects.17 There may be a more energy-favorable Pt/CeO2−x structure for the intense surface reconstruction of the reduced {100} surface. In summary, Pt NPs supported on CeO2 cubes ({100}) and octahedra ({111}) show different thermal stability when treated in various atmospheres at 600 °C. It is observed that Pt NPs could redisperse only on cubic ceria NPs covered with the {100} plane treated in an alternating atmosphere but not in
Figure 3. Pt 4f XPS spectra of samples after oxidation (-O) and reduction (-H).
Figure 4. H2-TPR of samples oxidized at 600 °C for 10 min.
Figure 5. TEM images and Pt particle size distribution (measurements from the full TEM images given here and in Figure S6, Supporting Information) of Pt/cubes treated in an alternating atmosphere of diverse frequencies. (a) Pt/cubes-O60H60 and (b) Pt/cubes-HFO5H10.
S5 (Supporting Information). It shows that redispersion takes place on Pt/cubes treated in the frequencies of O60H60 (Figure 5a, with the average particle size of 2.97 nm), O60H30 (Figure S5a, Supporting Information), O20H30 (Figure 1c), and HF-O5H10 (Figure 5b, with the average particle size of 1.45 nm) (OnHm represents samples treated for a total of 30 2481
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(5) Ouyang, R. H.; Liu, J. X.; Li, W. X. Atomistic Theory of Ostwald Ripening and Disintegration of Supported Metal Particles under Reaction Conditions. J. Am. Chem. Soc. 2013, 135, 1760−1771. (6) Simonsen, S. B.; Chorkendorff, I.; Dahl, S.; Skoglundh, M.; Sehested, J.; Helveg, S. Direct Observations of Oxygen-Induced Platinum Nanoparticle Ripening Studied by In Situ TEM. J. Am. Chem. Soc. 2010, 132, 7968−7975. (7) Newton, M. A. Dynamic Adsorbate/Reaction Induced Structural Change of Supported Metal Nanoparticles: Heterogeneous Catalysis and Beyond. Chem. Soc. Rev. 2008, 37, 2644−2657. (8) Mei, D.; Kwak, J. H.; Hu, J.; Cho, S. J.; Szanyi, J.; Allard, L. F.; Peden, C. H. F. Unique Role of Anchoring Penta-Coordinated Al3+ Sites in the Sintering of γ-Al2O3-Supported Pt Catalysts. J. Phys. Chem. Lett. 2010, 1, 2688−2691. (9) Suzuki, A.; Inada, Y.; Yamaguchi, A.; Chihara, T.; Yuasa, M.; Nomura, M.; Iwasawa, Y. Time Scale and Elementary Steps of COInduced Disintegration of Surface Rhodium Clusters. Angew. Chem., Int. Ed. 2003, 42, 4795−4799. (10) Nolte, P.; Stierle, A.; Kasper, N.; Jin-Phillipp, N.; Reichert, H.; Rühm, A.; Okasinski, J.; Dosch, H.; Schöder, S. Combinatorial HighEnergy X-ray Microbeam Study of the Size-Dependent Oxidation of Pd Nanoparticles on MgO(100). Phys. Rev. B 2008, 77, 115444/1− 115444/7. (11) Nagai, Y.; Dohmae, K.; Ikeda, Y.; Takagi, N.; Tanabe, T.; Hara, N.; Guilera, G.; Pascarelli, S.; Newton, M. A.; Kuno, O.; et al. In Situ Redispersion of Platinum Autoexhaust Catalysts: An On-Line Approach to Increasing Catalyst Lifetimes? Angew. Chem., Int. Ed. 2008, 47, 9303−9306. (12) Nagai, Y.; Hirabayashi, T.; Dohmae, K.; Takagi, N.; Minami, T.; Shinjoh, H.; Matsumoto, S. Sintering Inhibition Mechanism of Platinum Supported on Ceria-Based Oxide and Pt-Oxide-Support Interaction. J. Catal. 2006, 242, 103−109. (13) Wu, Z.; Schwartz, V.; Li, M.; Rondinone, A. J.; Overbury, S. H. Support Shape Effect in Metal Oxide Catalysis: Ceria-NanoshapeSupported Vanadia Catalysts for Oxidative Dehydrogenation of Isobutane. J. Phys. Chem. Lett. 2012, 3, 1517−1522. (14) Mai, H.-X.; Sun, L.-D.; Zhang, Y.-W.; Si, R.; Feng, W.; Zhang, H.-P.; Liu, H.-C.; Yan, C.-H. Shape-Selective Synthesis and Oxygen Storage Behavior of Ceria Nanopolyhedra, Nanorods, and Nanocubes. J. Phys. Chem. B 2005, 109, 24380−24385. (15) Yan, L.; Yu, R.; Chen, J.; Xing, X. Template-Free Hydrothermal Synthesis of CeO2 Nano-octahedrons and Nanorods: Investigation of the Morphology Evolution. Cryst. Growth Des. 2008, 8, 1474−1477. (16) Wu, Z.; Li, M.; Overbury, S. H. On the Structure Dependence of CO Oxidation over CeO2 Nanocrystals with Well-Defined Surface Planes. J. Catal. 2012, 285, 61−73. (17) Wu, Z.; Li, M.; Howe, J.; Meyer, H. M.; Overbury, S. H. Probing Defect Sites on CeO2 Nanocrystals with Well-Defined Surface Planes by Raman Spectroscopy and O2 Adsorption. Langmuir 2010, 26, 16595−16606. (18) Dai, Q.; Huang, H.; Zhu, Y.; Deng, W.; Bai, S.; Wang, X.; Lu, G. Catalysis Oxidation of 1,2-Dichloroethane and Ethyl Acetate over Ceria Nanocrystals with Well-Defined Crystal Planes. Appl. Catal., B 2012, 117, 360−368. (19) Pan, C.; Zhang, D.; Shi, L.; Fang, J. Template-Free Synthesis, Controlled Conversion, and CO Oxidation Properties of CeO2 Nanorods, Nanotubes, Nanowires, and Nanocubes. Eur. J. Inorg. Chem. 2008, 2008, 2429−2436. (20) Agarwal, S.; Lefferts, L.; Mojet, B. L.; Ligthart, D. A. J. M.; Hensen, E. J. M.; Mitchell, D. R. G.; Erasmus, W. J.; Anderson, B. G.; Olivier, E. J.; Neethling, J. H.; et al. Exposed Surfaces on ShapeControlled Ceria Nanoparticles Revealed through AC-TEM and Water−Gas Shift Reactivity. ChemSusChem 2013, 6, 1898−1906. (21) Nörenberg, H.; Harding, J. H. The Surface Structure of CeO2 Single Crystals Studied by Elevated Temperature STM. Surf. Sci. 2001, 477, 17−24. (22) Nolan, M.; Parker, S. C.; Watson, G. W. The Electronic Structure of Oxygen Vacancy Defects at the Low Index Surfaces of Ceria. Surf. Sci. 2005, 595, 223−232.
single oxidative or reductive atmospheres. Redispersion is favored in an alternation atmosphere with a quicker rate. Such redispersion of Pt NPs is hindered for Pt supported on octahedral ceria NPs covered with a {111} plane no matter what kind of atmosphere is employed. Water vapor is produced under alternation and would inhibit redispersion. It is concluded that redispersion takes place at the moment of alternation of oxidation and reduction, which may be related to the continuous formation and disappearance of surface oxygen vacancies and defects of ceria, while sintering exists throughout the whole treatments. Pt NPs are more easily redispersed on cubes for the stronger metal−support interaction. Finally, a possible mechanism for redispersion is proposed, which would be helpful in understanding the migration of surface atoms of supported catalysts.
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EXPERIMENTAL METHODS The experimental procedure is shown in Supporting Information. Samples treated in various atmospheres were examined in a customer-designed system with a switch valve and temperature-controlling system, as depicted in Figure S8 in the Supporting Information. Throughout the redispersion experiment, 25 mg samples were heated to 600 °C with a heating rate of 10 °C/min in 20% O2/N2, 5% H2/Ar, or an alternating atmosphere of 20% O2/N2 and 5% H2/Ar of diverse frequencies at 600 °C with the flow of 30 mL/min (100 mL/ min for Pt/CeO2−HF) and maintained for 30 min and then cooled down to room temperature under the same atmosphere. The 10% H2O vapor was produced by Ar bubbles through water at 46 °C with a 0.1 MPa saturated vapor pressure.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental procedure and XRD, SEM, and TEM data. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (21273221). We thank Prof. Cheng Wang, Zhixiang Zhang, Haifeng Li, and Baiqi Shao for fruitful discussion and help in drawing and English. We thank Dr. Yunchun Zhou for TEM measurements.
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