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Unique Role of Anchoring Penta-Coordinated Al3þ Sites in the Sintering of γ-Al2O3-Supported Pt Catalysts Donghai Mei,*,† Ja Hun Kwak,*,† Jianzhi Hu,† Sung June Cho,‡ J anos Szanyi,† § ,† Lawrence F. Allard, and Charles H. F. Peden* †
Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, ‡Department of Applied Chemical Engineering, Chonnam National University, Korea, and §Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
ABSTRACT γ-Al2O3-supported Pt group catalysts are widely used in many industrially important catalytic processes. However, γ-Al2O3-supported Pt catalysts are prone to deactivation via metal sintering at high temperatures, in oxidative reaction environments, or both. Using a combination of experimental HRTEM and EXAFS measurements and theoretical DFT calculations, we find that pentacoordinated Al3þ sites (Alp) on the γ-Al2O3(100) surface can inhibit Pt sintering both thermodynamically and kinetically because of their strong interactions with atomic Pt or Pt oxide species. The present work suggests a promising approach for stabilizing the size and morphology of supported catalytically active phases. SECTION Surfaces, Interfaces, Catalysis
lthough γ-Al2O3-supported Pt group catalysts are important for a large number of industrial applications,1-3 they invariably suffer from deactivation at high temperatures due to sintering.4 Although significant efforts have been made to understand the interactions between Pt and γ-Al2O3 and their role in determining sintering rates,5-8 the underlying mechanisms that control the size and morphology of supported Pt clusters on γ-Al2O3 are still not clear. In particular, the interactions between Pt clusters and likely anchoring sites on γ-Al2O3 surfaces are poorly understood. The relationships between the morphology of the supported Pt phase and metal-support interactions in Pt/γ-Al2O3 catalysts have been studied using extended X-ray absorption fine structure (EXAFS) and high-resolution scanning transmission electron microscopy (HR-STEM).3,9-13 Vaarkamp et al. found that the initially formed 3D Pt clusters on the γ-Al2O3 surface converted to 2D Pt rafts when the sample was reduced at 450 °C.3 Nellis and Pennycook observed very highly dispersed (including single atom) Pt clusters on γ-Al2O3 using HR-STEM.13 All of these prior studies point to specific interactions between Pt and the γ-Al2O3 surface in determining the structural properties and stability of alumina-supported Pt catalysts. Combining experimental evidence with theoretical calculations, we recently showed that the coordinatively unsaturated pentacoordinated Al3þ sites (Alp) on the (100) facets of γ-Al2O3 are the primary anchoring sites for active catalytic phases.14-16 Pt atoms occupy Alp sites on γ-Al2O3 with 1 wt % Pt loading, whereas 2D Pt clusters (“rafts”) formed at higher loadings because of the specific interactions between Pt and Alp sites.15 Recent results have demonstrated that oxide-supported atomically dispersed Pt is also catalytically active.17-19 For example, Fu et al. found that the atomic Pt and Au species, embedded in the ceria support material, are active for the water-gas shift
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reaction, whereas supported Pt or Au nanoparticle actually do not participate in the reaction.17 In this Letter, we discuss the specific role of the Alp sites in Pt sintering over a γ-Al2O3 support on the basis of HR-STEM and EXAFS results combined with density functional theory (DFT) calculations. Figure 1 shows HR-STEM images of 1 and 10 wt % Pt/ γ-Al2O3 samples after calcination at 300 (A and D) and 600 °C (B, C, E and F). The HR-STEM image of the 300 °C calcined 1 wt % Pt/Al2O3 sample (A) clearly shows the almost exclusively atomic distribution of Pt on the γ-Al2O3 support, whereas 2D clusters with an average size of ∼1 nm are observed for the 10 wt % Pt/γ-Al2O3 sample (D). These results are consistent with previous observations in that atomic dispersions of Pt were present in Pt/γ-Al2O3 catalysts as the Pt loading was decreased below a certain threshold.7 It is well known that Pt/γ-Al2O3 catalysts sinter when calcined in air (O2) above 500 °C. However, Figure 1B,C still shows that after a 600 °C calcination in dry air the 1 wt % Pt/γ-Al2O3 sample shows monoatomically dispersed Pt coexisting with small (100 nm) Pt clusters (panel F) as well as a considerable amount of highly dispersed 1 to 2 nm Pt clusters with a 2D (raft-like) morphology are identified in an HR-STEM image (panel E) for the 10 wt % Pt/γ-Al2O3 sample after 600 °C calcination. This suggests that the extent of Pt sintering is sensitive to the Pt loading. Recently, an average Pt/Alp ratio of ∼7 was reported for a 10 wt % Pt/γ-Al2O3 sample calcined at 600 °C.10 This suggests Received Date: August 2, 2010 Accepted Date: August 27, 2010 Published on Web Date: August 31, 2010
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Figure 2. DFT calculated chemical potential differences (Δμ) for the sintering from Pt to Pt4 on the γ-Al2O3(100) surface. A positive value of Δμ indicates that the process is thermodynamically unfavorable.
Pt-Pt CN increased to ∼10.8. This is still lower than the CN of 12 expected for large, bulk-like particles with a size of ∼100 nm.20 If the portion (no less than 10%) of the highly dispersed Pt (CN of ∼1) on the γ-Al2O3-supported sample is taken into account, then the averaged CN becomes 11 for this bimodal particle size distribution, which is in accordance with the EXAFS-estimated number of 10.8. In view of this agreement, we suggest that the bimodal distribution of Pt clusters originates in the strong interaction between Pt and the Alp sites on γ-Al2O3 support and is responsible for the underestimation of CN in Pt/γ-Al2O3 catalysts rather than anharmonic vibrations, as previously suggested.21 To gain more insight into why highly dispersed (even monatomic) Pt clusters are present on γ-Al2O3 even at high Pt loadings after calcination at high temperatures, we performed DFT calculations to study the interactions of metallic Pt and PtO (the latter for the case of Pt/γ-Al2O3 catalysts under highly oxidative conditions) with the AlP sites. (See the Supporting Information for details of these calculations.) Because the unsaturated Alp sites are only present on the (100) facets of γ-Al2O3, a fully dehydrated γ-Al2O3(100) surface was adopted to represent the γ-Al2O3 support.22 The thermodynamic driving forces for the sintering (aggregation) of supported Pt atoms and PtO monomers are determined by Gibbs free energy changes. As shown in Figure 2, the formation of Pt dimers from isolated Pt singlets is thermodynamically unfavorable (the same trend was obtained for PtO to (PtO)2 result not shown in Figure 2). After Pt2 or (PtO)2 clusters form, sintering processes for both of these species become thermodynamically favorable. The strong interactions in Pt-Alp and PtO-Alp pairs, calculated as -3.50 and -2.20 eV, respectively, also impede migration of atomic Pt and PtO monomers. Indeed, high migration barriers (∼1.7 eV) are found for both PtO and monatomic Pt species. Clearly, this suggests that the aggregation of isolated single Pt atoms or PtO monomers to Pt2 or (PtO)2 clusters is not only thermodynamically but also kinetically disfavored by the strong interaction between Pt/PtO and the anchoring Alp sites of the γ-Al2O3 support.
Figure 1. HR-STEM and TEM images of Pt/γ-Al2O3 samples after calcinations in dry air at 300 and 600 °C. (A) 1 wt % at 300 °C; (B) 1 wt % at 600 °C; (C) 1 wt % at 600 °C; (D) 10 wt % at 300 °C; (E) 10 wt % at 600 °C; and (F) 10 wt % at 600 °C.
that over 25% of the Alp sites on the γ-Al2O3(100) surface still interact with Pt, even though significant Pt sintering has occurred. One might expect that the Alp sites would become Pt-free as large Pt clusters (several tens of nm) form on the γ-Al2O3 support. Our data, however, show that more than 10% of the originally loaded Pt still interacts with the Alp sites. In other words, 1 wt % of Pt out of the 10 wt % total loading remains strongly bonded to the Alp sites, even after high temperature calcination that causes significant Pt sintering. Therefore, the consequence of this strong interaction between Pt and Alp is the inhibition of Pt sintering on the γ-Al2O3 support. The above conclusion is further supported by our Pt L3 edge EXAFS data (see Supporting Information) obtained for Pt/γ-Al2O3 samples with different Pt loadings. Simulation of the experimental EXAFS data found no Pt-Pt bonding for a 1 wt % Pt/γ-Al2O3 sample calcined at 300 °C. This is consistent with the corresponding HR-TEM image (Figure 1A) where only a highly dispersed Pt phase in the form of single Pt atoms is observed on γ-Al2O3. Note that Pt-Al coordination is also present for this γ-Al2O3-supported catalyst at these low Pt coverages. The 10 wt % Pt/γ-Al2O3 sample has a similar coordination environment to that of the 1 wt % sample when it is calcined at 300 °C. Analysis of the EXAFS results for this highly Pt loaded sample gives a low Pt-Pt coordination number (CN) of 1.2. The averaged Pt-Pt bond distance is 0.308 nm, which corresponds to highly dispersed PtOx species. For the 10 wt % Pt/γ-Al2O3 sample calcined at 600 °C, the simulated EXAFS data show that the averaged
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This explains why a bimodal Pt distribution, observed in the HR-TEM and EXAFS experiments, exists for Pt/γ-Al2O3 catalysts, even after high-temperature calcination. The formation of highly dispersed (∼1 nm) PtO rafts is also related to the strong interaction between these 2D particles and the underlying Alp sites.15 Furthermore, similar strong interactions were suggested to also explain the role of oxide additives (e.g., La2O3 and BaO) in impeding high-temperature phase changes of γ-Al2O3.23,24 In agreement with experimental observations, DFT calculations indicate that the sintering process is both thermodynamically and kinetically hindered by strong interactions of atomic Pt and PtO with the Alp sites on the γ-Al2O3 support. Therefore, increasing the quantity of (100) facets of γ-Al2O3, on which the Alp sites reside, may provide a possible solution way to the sintering of γ-Al2O3-supported metal catalysts. In particular, high Pt dispersion may be more readily maintained by maximizing the number of Alp sites on the γ-Al2O3 support. The strong binding between Pt (PtO) and Alp should ensure stable morphologies and ultimately active site control at the atomic level.
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SUPPORTING INFORMATION AVAILABLE Materials prepara-
tion, experimental procedures (EXAFS), and the details of theoretical calculations. This material is free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: donghai.mei@ pnl.gov (D.M.);
[email protected] (J.H.K.);
[email protected] (C.H.F.P.).
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ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences and by an LDRD project at Pacific Northwest National Laboratory. Sample preparation and NMR experiments were performed in the Environmental Molecular Sciences Laboratory (EMSL). HR-STEM images were acquired at the High Temperature Materials Laboratory at ORNL. EXAFS data were collected using the National Synchrotron Light Source at Brookhaven National Laboratory under contract no. DE-AC0298CH10886. Computing time was granted by the National Energy Research Scientific Computing Center and EMSL (st30469).
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