Rhodium Nanoparticle Anchoring on AlPO4 for Efficient

Sep 21, 2014 - ABSTRACT: Rhodium catalysts exhibited higher dispersion with tridymite-type AlPO4 supports than with Al2O3 during thermal aging at 900 ...
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Rhodium Nanoparticle Anchoring on AlPO4 for Efficient Catalyst Sintering Suppression Masato Machida,*,†,‡ Saki Minami,† Keita Ikeue,†,‡ Satoshi Hinokuma,†,‡ Yuki Nagao,§ Takahiro Sato,§ and Yunosuke Nakahara§ †

Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan ‡ Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Kyoto Daigaku Katsura, Saikyo, Kyoto 615-8520, Japan § Catalysts Strategic Division, Engineered Materials Sector, Mitsui Mining & Smelting Co., Ltd., 1013-1 Ageoshimo, Ageo, Saitama 362-0025, Japan S Supporting Information *

ABSTRACT: Rhodium catalysts exhibited higher dispersion with tridymite-type AlPO4 supports than with Al2O3 during thermal aging at 900 °C under an oxidizing atmosphere. The local structural analysis via X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray absorption fine structure, and infrared spectroscopy suggested that the sintering of AlPO4supported Rh nanoparticles was significantly suppressed because of anchoring via a Rh−O−P linkage at the interface between the metal and support. Most of the AlPO4 surface was terminated by phosphate P−OH groups, which were converted into a Rh−O−P linkage when Rh oxide (RhOx) was loaded. This interaction enables the thin planar RhOx nanoparticles to establish close and stable contact with the AlPO4 surface. It differs from Rh−O−Al bonding in the oxide-supported catalyst Rh/Al2O3, which causes undesired solid reactions that yield deactivated phases. The Rh−O−P interfacial linkage was preserved under oxidizing and reducing atmospheres, which contrasts with conventional metal oxide supports that only present the anchoring effect under an oxidizing atmosphere. These experimental results agree with a density functional theory optimized coherent interface RhOx/ AlPO4 model.

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conditions causes Pt redispersion.6−9,15 The Pt−O−Ce bond formation was found to provide an anchor for Pt and act as a driving force that promotes the redispersion of Pt oxide from the metallic particles grown under a reducing atmosphere. Rhodium was also stabilized by an Nd2O3-enriched ZrO2 surface, which formed Rh−O−Nd interfacial bonding.11 Notably, these previous studies utilize the metal−support interactions that occur under an oxidizing atmosphere, where Rh is thermodynamically stable in the oxide form, RhOx, and capable of electrostatic interactions with oxide supports. Tridymite-type aluminum orthophosphate (AlPO4) was proven to be a robust support material that produces optimum interactions with Rh-based species, which facilitates high dispersion and thermal stabilization.16,17 Moreover, AlPO4 was found to minimize the Rh loading threshold unlike conventional support oxides such as MgO, Al2O3, TiO2, ZrO2, and CeO2. Further investigations on local structure are necessary to elucidate the relations among the metal−support

INTRODUCTION The demands for Rh, Pt, and Pd as three-way catalysts (TWCs) used in gasoline-fueled automobiles have increased to exceed 50% of the total demands in the last two decades.1 These precious metals exist as highly dispersed nanoparticles in fresh catalysts, but gradually, as a result of thermal sintering, they agglomerate into large particles that exhibit low surface area. Extensive research was performed to overcome this problem and reduce precious metal use in TWCs. Several state-of-the-art catalysts that utilize various metal−support interactions have been proposed to suppress sintering or enable metal redispersion.2−14 Practical uses for some of these catalysts have already been found. A research group at Daihatsu proposed a self-regeneration mechanism of a Pd catalyst in Pd-substituted LaFeO 3 perovskite catalysts under an oscillating redox atmosphere.2,3 While Pd2+ cations occupy the perovskite-type oxide structure under an oxidizing atmosphere, metallic Pd nanoparticles deposit on the surface under a reducing atmosphere. This dynamic change of the local structure enhances its sintering tolerance. Research groups at Toyota have reported that the interaction between Pt and CeO2 at 800 °C under oxidizing © 2014 American Chemical Society

Received: August 20, 2014 Revised: September 18, 2014 Published: September 21, 2014 5799

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The XAFS data were processed using the REX 2000 program (Rigaku). EXAFS oscillations were extracted by the fitting of a cubic spline function through the post edge region. The k3-weighted EXAFS oscillation in the 30−158 nm−1 region was subjected to Fourier transformation. For the curve-fitting analysis, phase shift and backscattering amplitude functions of the Rh−Rh and Rh−O−Rh shells were extracted from the EXAFS data obtained for Rh foil and Rh2O3. The curve-fitting analysis of the Rh−O−P and Rh−O−Al shells was performed using theoretical parameters. DFT Calculations. Spin-polarized generalized gradient approximation (GGA) electronic structure calculations were conducted using the Vienna ab initio simulation package (VASP).18 The calculations were performed using projector-augmented wave potentials for Al, P, Rh, and O atoms. A plane wave basis set with a cutoff of 520 eV was combined with the Perdew−Burke−Ernzerhof exchange−correlation functional.19 The occupied electronic states were summed using a 5 × 1 × 1 Monkhorst−Pack k-point mesh20 for the (001) interface model of Rh2O3(AlPO4)4. The unit cell parameters and atomic coordinates of AlPO4 and Rh2O3 were optimized using a convergence criterion of 0.02 eV Å−1. The initial crystallographic parameters used in the optimization were taken from previous reports.21,22 The AlPO4 (001) slab was modeled using a (1 × 1 × 2) supercell (AlPO4)4 that comprised 24 atomic layers. The Rh2O3 (001) monolayer was placed on one side of the AlPO4 slab with a 30 Å vacuum separation to construct an (001) interface consisting of 29 atomic layers (Rh2Al4P4O19). To calculate the lattice relaxation of this interface, the top 11 atomic layers were allowed to relax until all forces on the atoms were less than 0.02 eV Å−1. The polarity was compensated by introducing dipolar corrections along the axis perpendicular to the (001) surface.

interaction, catalytic activity, and thermal stability of the Rh/ AlPO4 catalyst. Here, a combined experimental and theoretical approach was developed to study the local structure of Rh/ AlPO4 in oxidized and reduced states. To explore the origin of stable Rh-based species formed on the AlPO4 surface, the experimental characterization was conducted by transmission electron microscopy (TEM), X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), and Fouriertransform infrared spectroscopy (FT-IR). Density functional theory (DFT) calculations were performed to generate a structural model of the Rh/AlPO4 interface. These results led to a novel anchoring method on phosphate surfaces to stabilize Rh nanoparticles that are useful for automotive TWCs.

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EXPERIMENTAL SECTION

Catalyst Preparation. Tridymite-type AlPO4 was prepared from Al(NO3)3 (Wako Pure Chemicals Ind., 99.9%) and H3PO4 (Wako Pure Chemicals Ind., 85%). A H3PO4 solution (0.05 mol) in deionized water (50 mL) was added dropwise to an Al(NO3)3 solution (0.05 mol) of deionized water (50 mL) with vigorous stirring. Next, the pH of the supernatant was adjusted to 4.5 by the dropwise addition of aqueous ammonia (25%). The resulting white gel was recovered by centrifugation, washed several times with deionized water, and dried in air at 100 °C. The solid product was calcined in air at 1000 °C for 5 h to give tridymite-type AlPO4. The calcined material exhibited a Brunauer−Emmett−Teller (BET) surface area (SBET) of 120 m2 g−1. The AlPO4-supported Rh (0.4 wt % as Rh metal) was prepared by the equilibrium adsorption of Rh(NH3)63+ in an aqueous solution (Tanaka Kikinzoku Kogyo Co.), followed by being washed with pure water, airdried at 100 °C, and under air calcination at 600 °C for 3 h. The γAl2O3-supported Rh catalyst (SBET = 126 m2 g−1) was prepared by the impregnation of an aqueous solution of Rh(NO3)3 followed by airdrying at 100 °C and air calcination at 600 °C for 3 h. As-prepared catalysts were thermally aged at 900 °C in a 10% H2O air stream. Characterization. Powder X-ray diffraction measurements were performed using monochromatic Cu Kα radiation (30 kV, 20 mA, Multiflex, Rigaku), and the Rh content was determined by energydispersive X-ray fluorescence (EDXL300, Rigaku). The TEM micrographs were acquired using a FEI TECNAI F20 transmission electron microscope operating at 200 kV. The XPS spectra were recorded using a VG Sigmaprobe spectrometer under Mg Kα radiation (15 kV, 20 mA). Binding energy calculations were validated using the peak position of C 1s as an internal reference. The charging effect was ruled out in this measurement because the binding energy for C 1s was identical for all samples. The normal operating pressure in the analysis chamber was maintained below 10−6 Pa during the measurement. SBET was calculated using N2 adsorption isotherms measured at 77 K (Belsorp-mini, Bel Japan). The Rh dispersion was determined by pulsed CO chemisorption at 50 °C (Belcat, Bel Japan). Before the measurements, the catalyst was reduced in the presence of H2 at 200 °C. The metal dispersion is expressed in terms of the molar ratio of chemisorbed CO per loaded Rh (CO/Rh). In situ FT-IR spectra were recorded in the OH stretching frequency region using a Nicolet 6700 spectrometer (Thermo) equipped with a temperature-controllable diffuse reflectance reaction cell. This cell was connected to a gas-flow system that facilitated measurements under controlled gas environments. The powder samples were finely ground and placed in a crucible inside the cell before they were heated at increasing temperatures under He flow, which removed any adsorbed water. Rh/AlPO4 samples that displayed different Rh loadings were pretreated using He flow at elevated temperatures for 1 h before the measurements were taken. The Rh K-edge EXAFS was recorded on an NW10A station at the Photon Factory Advanced Ring, High Energy Accelerator Research Organization (KEK) using a Si(311) double-crystal monochromator. The spectra were recorded at room temperature in a transmission mode. Incident and transmitted X-rays were monitored in 17 cm- and 31 cm-long ionization chambers filled with Ar and Kr, respectively.

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RESULTS AND DISCUSSION Rhodium Sintering Suppression by AlPO4. Because supported Rh catalysts in TWCs are deactivated by thermal aging in an oxidizing atmosphere,23−25 catalytic tests were conducted after the catalysts were aged in a 10 vol % H2O/air stream. In contrast to Rh/Al2O3, which displays a dramatic deactivation, Rh/AlPO4 retains a high catalytic activity for NO, CO, C3H6, and O2 in a simulated TWC exhaust even after thermal aging at 900 °C for 500 h.16,17 To rationalize this activity difference, the surface area and metal dispersion before and after thermal aging were compared for Rh/AlPO4 and Rh/ Al2O3 (Table 1). The CO/Rh value of Rh/Al2O3 was almost Table 1. BET Surface Area (SBET) and Rh Metal Dispersion (CO/Rh) before and after Thermal Aging SBET (m2 g−1) catalyst

a

Rh/AlPO4 Rh/Al2O3

none 115 121

b

CO/Rh (%)

25 hc

500 hc

noneb

25 hc

500 hc

65 71

46 54

43 65

22 nulld

15 nulld

a 0.4 wt % Rh loading. bAfter air calcination at 600 °C for 3 h (asprepared). cAfter aging at 900 °C in a 10% H2O/air flow for 25 or 500 h. dAmounts of CO adsorption were negligibly small.

negligible after the initial thermal aging for 25 h and showed a greater decrease than expected when its SBET value decreased from 121 to 71 m2 g−1. In contrast, AlPO4 maintained higher CO/Rh values despite its smaller SBET value (65 m2 g−1). The TEM micrographs demonstrated that a large part of the Rh nanoparticles smaller than 10 nm that were present in Rh/ AlPO4 remained during the 500 h of aging (Supporting Information). On the other hand, Rh/Al2O3 comprised numerous larger agglomerates (>100 nm) containing Rh and Al after 25 h of aging. The thermal deactivation mechanism of Rh/Al2O3 was extensively studied.26−30 Under high-temper5800

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ature aging (>600 °C) in the presence of O2, several deactivation routes for Rh/Al2O3 that involve rhodium oxide (RhOx) dissolution into Al2O3, rhodium aluminate (RhAlOx) formation, or Rh encapsulation by Al2O3 were proposed. These routes are consistent with the appearance of large agglomerates observed by TEM. Rhodium particle morphologies on AlPO4 are also important. As revealed previously,16 supported Rh nanoparticles form a thin planar structure that spreads over the AlPO4 surface, which is indicative of strong interfacial interactions (Supporting Information). Therefore, the interaction between Rh and AlPO4 generates an efficient anchoring effect that stabilizes the Rh nanoparticles without inducing deactivating solid-state reactions. The oxidation states of the Rh species anchored onto AlPO4 were evaluated using the Rh 3d XPS spectra before and after thermal aging (Figure 1 and Table 2). The as-prepared Rh/

thermal aging, the intensity of each peaks is not a simple measure of the metal dispersion of these supported catalysts. Unlike Rh/Al2O3, the Rh species retained their single oxide form on the AlPO4 surface, which suggests that the anchoring effect may stem from interfacial interactions between Rh2O3 and AlPO4. Local Structure Analysis of Oxidized Rh by EXAFS. The local coordination environment of Rh was investigated by XAFS to elucidate the origin of the anchoring effect that significantly enhances the thermal stability of Rh on AlPO4. Figure 2 shows the Fourier transforms of the Rh K-edge

Figure 2. Fourier transformed Rh K-edge EXAFS for (a) Rh/AlPO4 and (b) Rh/Al2O3 after calcination at 600 °C for 3 h in air. (c) Rh2O3 and (d) Rh metal foil were used as references.

EXAFS for the as-prepared Rh/AlPO4 and Rh/Al2O3 without phase shift corrections. The peaks in Figure 2 are therefore shifted to shorter values from true atomic distances. Both catalysts showed an intense peak at approximately 0.2 nm, which was attributed to a Rh−O shell, but their second shell peaks were different from those of Rh2O3. The second shell was carefully analyzed by curve-fitting that used seven types of possible shell combinations for Rh/AlPO 4 (Supporting Information). The best fit was achieved when the contribution of an Rh−O−P shell was taken into consideration with Rh−Rh and Rh−O−Rh shells (Table 3). This Rh−O−P linkage formation suggests that Rh species bind to the AlPO4 phosphate unit, which is in agreement with the thin, planar RhOx particles spread over the AlPO4 surface (Supporting Information). On the other hand, the contribution of Rh−O− Rh (CN = 0.19) is very weak considering the fact that Rh exists as the oxide. According to the high-resolution TEM images (Supporting Information), the RhOx nanoparticles on AlPO4 have a highly disordered amorphous structure due to a metal− support interface interaction via the Rh−O−P linkage, which will generate unequal Rh−O−Rh coordination environments with weak EXAFS intensities. This is in accordance with a large Debye−Waller factor for Rh−O−Rh (Table 3). Upon thermal aging at 900 °C, Rh/AlPO4 displayed two strong peaks that corresponded to Rh−O−Rh shells because RhOx partly grew as Rh2O3 (Supporting Information). The presence of these Rh2O3 particles should therefore influence EXAFS and make the contribution of Rh−O−P bonding difficult to detect. As shown in Figure 2 and Table 3, Rh−Rh and Rh−O−Al shells contributed to the second coordination for Rh/Al2O3. The coordination number for Rh−O−Al increased from 1.9 to

Figure 1. Rh 3d XPS spectra for Rh/AlPO4 and Rh/Al2O3 (a) after air calcination at 600 °C for 3 h and after aging in a 10% H2O/air stream at 900 °C (b) for 25 h and (c) for 500 h.

Table 2. Peak Fitting Results of Rh 3d5/2 XPS Spectra before and after Thermal Aging binding energy (eV) catalyst a

Rh/AlPO4 Rh/AlPO4(25 h)b Rh/AlPO4(500 h)b Rh/Al2O3a Rh/Al2O3(25 h)b Rh/Al2O3(500 h)b

Rh0

Rh3+

306.4

309.5 309.2 309.5 309.6

306.4 306.4 306.5

Rh4+

(Rh3+ + Rh4+)/Rh (%)

309.9 310.1

81 100 100 86 81 69

After air calcination at 600 °C for 3 h (as-prepared). bAfter aging at 900 °C in a 10% H2O/air flow for 25 h.

a

AlPO4 showed Rh 3d5/2 peaks at 306.4 and 309.5 eV attributable to Rh0 and Rh3+, respectively.31−33 Peak intensities suggested that more than 80% Rh existed as Rh2O3 (Rh3+). After aging, the peak ascribable to Rh0 disappeared, which indicates that all Rh species were Rh3+. The as-prepared Rh/ Al2O3 also showed similar spectra, but thermal aging caused the Rh 3d5/2 peak to shift from 309.6 to approximately 310 eV. According to previous studies on the thermal aging of Rh/ Al2O3, this unusual peak corresponds to unreducible Rh4+ formed by a solid-state reaction between RhOx and γAl2O3.33−35 Because Rh4+ became a dominant species after 5801

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Table 3. Curve-Fitting Results of Rh K-edge EXAFS of Rh/AlPO4 and Rh/Al2O3 Catalystsa catalyst Rh/AlPO4

b

Rh/Al2O3b

shell

CNc (±0.2)

rd(nm, ±0.02)

σ2e/10−4 (nm2, ±0.02)

Rh−O Rh−Rh Rh−O−P Rh−O−Rh Rh−O Rh−Rh Rh−O−Al

4.3 0.32 1.4 0.19 5.8 0.41 1.9

0.203 0.269 0.309 0.354 0.203 0.265 0.313

0.15 0.42 0.36 0.64 0.13 0.42 0.49

a Intervals of k-space to r-space of FT are 30−138 nm−1 for Rh/AlPO4 and 30−158 nm−1 for Rh/Al2O3. bAfter air calcination at 600 °C for 3 h (asprepared). cCoordination number. dAtomic distance. eDebye−Waller factor.

cm−1. These bands were assigned to the P−OH and Al−OH stretching modes, respectively,37 on the AlPO4 surface and were not visible in the presence of adsorbed water because of the extensive hydrogen bonding. The peak for P−OH stretching was considerably more intense than was the peak for Al−OH stretching, which indicates that more than 80% of the AlPO4 surface may be terminated by phosphate units that bear P−OH groups. Figure 4 shows the FT-IR spectra of Rh/AlPO4 in the OH stretching region for different Rh loadings. When dehydrated

3.9 upon aging for 500 h, which suggests that the solid-state reactions between RhOx and Al2O3 formed increased product yields with aging (Supporting Information). These reactions produced very large agglomerates as described above (Supporting Information). In contrast, these reactions did not occur between RhOx and AlPO4. Interestingly, this discrepancy is associated with the nature of bonding in AlPO4. The tridymite AlPO4 phase used here is a tetrahedral SiO 4 framework analogue that comprises P−O and Al−O bonds, which differ in character. More specifically, the P−O bonds are covalent, whereas the Al−O bonds are ionic; therefore, tridymite AlPO4 consists of [PO4]3− tetrahedra isolated by Al3+ cations and does not present any oxygen bridges between Al and P atoms, which is confirmed by Raman spectroscopy.36 This structure may explain why bonding occurs between PO4 and RhOx at the RhOx/AlPO4 interface. Because EXAFS suggested the existence of anchoring effects via Rh−O−P interface bonding in Rh/AlPO4, this metal− support interaction was investigated using FT-IR. Figure 3

Figure 4. FT-IR spectra of Rh/AlPO4 for different Rh loadings after dehydration at 500 °C under a He flow.

under a He flow at 500 °C, the catalysts exhibited similar spectra to that of unloaded AlPO4 (Figure 3); however, the P− OH band intensity decreased monotonically with increased Rh loading. Considering the Rh−O−P interface bonding suggested by EXAFS, this decrease in intensity clearly indicates the conversion of P−OH to P−O−Rh on the AlPO4 surface. Although the Al−OH band intensity also decreased with increased Rh loading, its contribution to the interfacial bonding was expected to be small because of its low concentration on the AlPO4 surface. Local Structure Analysis of Reduced Rh. The Rh/AlPO4 catalysts were treated with a 5% H2/He flow at 200 °C to study the interactions between reduced Rh and AlPO4. Changes in the oxidation state of Rh were studied by XPS and EXAFS. For Rh/AlPO4, the Rh 3d5/2 peak shifted from 309.5 to 308.0 eV upon reduction by H2 at 200 °C, and the binding energy remained almost constant upon the further increase in

Figure 3. FT-IR spectra of AlPO4 after dehydration at elevated temperatures under a He flow.

shows the spectra of unloaded AlPO4 in the OH stretching region after being heated at elevated temperatures under a He flow. At room temperature, AlPO4 showed a very broad and strong absorption band centered at approximately 3600 cm−1 because its highly hydrophilic surface was covered with physisorbed water, which corresponded to 11 wt % of the sample kept under ambient atmosphere at 25 °C and 50% relative humidity. Once this water was desorbed by being heated above 100 °C, two sharp bands, which intensified with increased heat-induced dehydration, appeared at 3677 and 3793 5802

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Rh−O−P, and Rh−Rh shells is consistent with the anchoring of metallic Rh particles to the AlPO4 surface via Rh−O−P bonding. Figure 7 shows the FT-IR spectra of Rh/AlPO4 for different Rh loadings after H2 reduction at 200 °C. Both sharp OH

reduction temperature (Figure 5). The observed binding energy surpassed the reported value for Rh metal (306.4

Figure 5. Rh 3d XPS spectra of Rh/AlPO4 (a) after air calcination at 600 °C for 3 h and after reduction in a 5% H2/He stream at (b) 200 °C, (c) 300 °C, and (d) 600 °C.

eV),31−33 which may stem from the electron-deficient character of Rh that arose from the interaction with acidic AlPO4. Figure 6 shows the Fourier transforms for Rh K-edge EXAFS of Rh/

Figure 7. FT-IR spectra of Rh/AlPO4 for different Rh loadings after dehydration at 500 °C in a He flow and subsequent reduction at 200 °C in a 5% H2/He flow.

stretching bands decreased in intensity with increased Rh loading, similar to that of oxidized Rh/AlPO4 (Figure 4). This result demonstrates that the interfacial Rh−O−P bonding forms in the presence of RhOx as well as metallic Rh species. This differs dramatically from what occurs in conventional supported precious metal catalysts, which only exhibit anchoring effects under an oxidizing atmosphere, because interfacial bonding typically relies on electrostatic interactions between support and precious metal oxides.5,7,12,38 When precious metals are reduced to their metallic states, electrostatic interactions become very weak.39 On the other hand, these interactions remained significant in the reduced Rh/AlPO4, which suggests that covalent P−O bonds in the PO4 unit influence the interfacial bonding, although the detailed mechanism is unclear at this stage. DFT Calculation of the Rh2O3/AlPO4 Interface. Several transition metal oxides containing V5+, Cr3+, Mn2+, Fe2+, and Co2+ cations have shown interactions with P−OH groups on AlPO4 surfaces;40 however, their local structure and nature of bonding with the PO4 unit are uncertain. The present experimental results suggest for the first time that the thermal stabilization of Rh nanoparticles on AlPO4 is associated with the Rh−O−P interfacial bonding. Therefore, an interface model of oxidized Rh/AlPO4 was proposed and analyzed using

Figure 6. Fourier transformed Rh K-edge EXAFS of Rh/AlPO4 (a) after air calcination at 600 °C for 3 h and (b) after reduction in a 5% H2/He stream at 200 °C. (c) Rh foil was used as a reference.

AlPO4 after H2 reduction at 200 °C. Unlike the as-prepared catalyst, the reduced catalysts yielded an intense peak attributable to an Rh−Rh shell and a lower intensity peak for an Rh−O shell. Furthermore, the best fit indicates that the Rh− O−P shell contribution was preserved upon reduction (Table 4). Similar results were obtained when Rh/AlPO4 was reduced at temperatures exceeding 400 °C. The coexistence of Rh−O,

Table 4. Curve-Fitting Results of Rh K-edge EXAFS of Catalysts after H2 Reductiona catalyst

shell

CNc (±0.2)

rd(nm, ±0.03)

σ2e/10−4 (nm2, ±0.02)

Rh/AlPO4b

Rh−O Rh−Rh Rh−O−P

1.6 1.2 1.8

0.203 0.269 0.309

0.15 0.42 0.36

Interval of k-space to r-space of FT is 30−138 nm−1. bAfter air calcination at 600 °C for 3 h (as-prepared) and subsequent reduction in 5% H2/He at 200 °C. cCoordination number. dAtomic distance. eDebye−Waller factor. a

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periodic DFT calculations. Compared to four other AlPO4 polymorphs, the tridymite phase exhibits the highest anchoring effect for Rh nanoparticles.17 Figure 8 shows the crystal

Table 5. Comparison of Atomic Distances Obtained by DFT and EXAFS EXAFSa

DFT

a

structures of Rh2O3 and AlPO4 that were used as input geometries for the calculations. Although tridymite AlPO4 is classified as a hexagonal lattice of the space group P63 mc (a = 0.50976 nm, c = 0.83441 nm), it is only stable at high temperatures (≥598 K).41 Upon being cooled from this hexagonal configuration, it successively turns into distorted forms, such as the monoclinic phase (P1121, a = 0.50800 nm, b = 0.50748 nm, c = 0.83009 nm, γ = 119.625) at 473 K.21 Rh2O3 crystallizes in a rhombohedral structure (R3cH, a = 0.5127 nm, c = 1.3853 nm).22 The (001) planes are very similar in their lattice dimensions and symmetries for AlPO4 and Rh2O3, which provides a coherent Rh2O3/AlPO4 interface for modeling. As expected from the crystal structures (Figure 8), several coherent interface models are possible between the (001) planes of Rh2O3 and AlPO4. Among the seven combinations, the combination that enables Rh−O−P bonding with a monodentate configuration at the interface was selected because the surface terminated by the P−O groups presents a lower surface energy than do those terminated by P(−O)2 and P(−O)3 and is considered more stable. The optimized interface structure (Figure 9) incorporated two types of Rh (Rh1 and Rh2) in the surface layer. Table 5 compares the computed atomic distances for these Rh sites with those observed by EXAFS (Table 3). Seven of the calculated Rh−O distances in the 0.1830−0.2046 nm range corresponded to the strongest EXAFS peak at 0.203 nm. The Rh1−Rh2 distance (0.2703 nm) almost equaled the EXAFS value of 0.269 nm. A similar

b

shell

r (nm)

shell

r (nm)

Rh1−O Rh1−O Rh1−O Rh1−O Rh1−Rh2 Rh1−Rh2 Rh1−O Rh1−P

0.1904 0.1925 0.1947 0.2046 0.2703 0.3112 0.3347 0.3358

Rh2−O Rh2−O Rh2−O

0.1830 0.1849 0.1854

Rh2−Rh1 Rh2−Rh1 Rh2−O

0.2703 0.3112 0.3360

Rh2−O Rh2−Rh1 Rh2−O

0.3381 0.3409 0.3649

Rh1−Rh2 Rh1−O

Figure 8. Crystal structures of hexagonal Rh2O3 and monoclinic tridymite AlPO4 projected on their (001) planes.

b

0.3409 0.3545

shell

rb(nm)

Rh−O Rh−Rh

0.203 0.269

Rh−P

0.309

Rh−Rh

0.354

EXAFS data are from Table 3. bAtomic distance.

agreement was obtained for the third shell of Rh1−Rh2 for a distance of 0.3409 nm. The optimized interface structure showed a relatively longer Rh−P distance (0.336 nm) than did EXAFS (0.309 nm, Table 3). As shown in Figure 9, the Rh− O−P bond was bent to 138.8°, which determines the Rh−P distance. Similar bent Rh−O−P bonds were observed in the crystal structure of RhPO4,42 where monodentate and bidentate PO4 coordinations to Rh yield Rh−P distances of 0.321 and 0.273 nm, respectively. The optimized structure for an Rh−P distance of 0.336 nm, which is about 5% more than for monodentate coordination, may therefore be more consistent with the EXAFS results. The optimized structure also contains information about the bond character of Rh−O−P; the Rh−O portion of Rh−O−P bonding is ionic in contrast to the highly covalent P−O portion (Supporting Information). However, no significant difference was found in Rh−O between Rh−O−P and Rh−O−Rh. Consequently, the corresponding DFT and EXAFS results demonstrate that interfacial Rh−O−P bonding via a PO4 unit plays a key role in the anchoring effect on the supported Rh catalyst, which enables efficient sintering suppression. The DFT calculation of the reduced Rh metal/ AlPO4 interface is more difficult because of the absence of coherent interface models. Cluster models, where several Rh atoms are adsorbed on the AlPO4 surface, are therefore currently under investigation. In addition, the electronic effect of the interfacial Rh−O−P bonding on the catalytic property of Rh nanoparticles is of great interest, although the beneficial feature is the sintering suppression for long-term stability. Details on the relationship between the electronic effect and catalytic property will be discussed in our forthcoming paper.

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CONCLUSIONS This study demonstrated that Rh nanoparticles could be thermally stabilized by an anchoring effect via Rh−O−P bonding to a phosphate unit on the tridymite AlPO4 surface. The Rh−O−P bonding proved that the thin planar RhOx particles intimately contacted the AlPO4 surface, which stabilized the Rh nanoparticles against sintering during hightemperature aging. In contrast, the corresponding oxide supported catalyst, Rh/Al2O3, incurred serious deactivation because of sintering, and the solid reaction products presented low activity. Another important feature of the interfacial interaction is its insensitivity to redox treatment. In contrast to precious metal catalysts deposited on conventional oxide supports, this interaction is valid under oxidizing and reducing

Figure 9. Optimized (001) Rh2O3/(AlPO4)4 interface model. 5804

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Chemistry of Materials

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atmospheres. This feature is expected to be quite useful for sintering suppression under an oscillating reduction−oxidation atmosphere, such as an automotive TWC.

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ASSOCIATED CONTENT

S Supporting Information *

TEM micrographs, curve-fitting results on the Rh K-edge EXAFS, and the electron localization function (ELF) of Rh/ AlPO4. 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]. Phone: +81-96-3423651. Fax: +81-96-342-3651. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Elements Science and Technology Project from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT). This work was partly performed under the management of the “Elements Strategy Initiative for Catalysts & Batteries (ESICB)” supported by MEXT. The XAFS experiments were conducted on the NW10A at Photon Factory, High Energy Accelerator Research Organization (KEK) (Proposal No. 2009G574).

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REFERENCES

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dx.doi.org/10.1021/cm503061g | Chem. Mater. 2014, 26, 5799−5805