White X-ray Si (hkk) 300 - 500 mm mirror 300 - 800 mm sample absorber polychromator slit slit PSD 2 θ θ DXAFS Studies on the Spontaneous Dispersion of PdO Interacted with Zeolites Fig. 2. Pd K -edge XAFS spectra for Pd (0.4 wt%) loaded on H-ZSM-5 (Si/Al 2 = 23.8) measured in an O 2 flow. Metal-support interaction in supported metal catalysts is important in understanding the structure and catalysis of supported metal. Previous studies concerning palladium catalysis, such as selective reduction of NO and total oxidation of methane, revealed that the catalysis of the Pd center was significantly affected by the acid property of the support. One of the reasons for these effects could be attributed to the strong interaction between PdO and acid sites of supports. Indeed, we have observed spontaneous migration of agglomerated metal Pd into the highly dispersed PdO on acid sites under O 2 atmosphere at elevated temperature. In addition it was found that the spontaneous dispersion of PdO upon repetition of reduction and oxidation treatment was reversible. The phenomena implied that the interaction between PdO and acid sites of zeolites plays an important role in the determination of dynamic behavior and structure of PdO. In this study, we tried to directly observe the dispersion process of PdO onto acid sites of zeolites. Although previous studies were primarily carried out under static conditions, the dynamic structural change of Pd could be directly observed using in situ and q uic k measurement of Pd structure. F or this purpose, energy-dispersive EX A FS (DX A FS) techni q ue newly e q uipped in S Pring- 8 was applied for the measurement of the local structure of Pd during Pd oxidation. Pd K -edge DX A FS was measured at beamline BL28B2 . An S i polychromator crystal was used in a L aue configuration with (4 22 ) net plane to obtain an X -ray beam with a dispersed energy region. The schematic configuration of DX A FS measurement is shown in F ig. 1 . A waferli k e sample was placed in a q uartz in situ cell. The sample was heated from room temperature to 773 K with ramping rate of 5 K min - 1 in an O 2 flow at atmospheric pressure. The spectra measured for 0 . 3 sec were accumulated 10 times in every 10 K . An example of raw spectra for Pd /H - ZS M- 5 (S i / Al 2 = 2 3 . 8) is shown in F ig. 2. It was confirmed that data with a high S/ N ratio could be obtained despite the low concentration of Pd (0 . 4 wt %) . F ourier transforms of the k 3 χ (k) EX A FS for Pd loaded Na- and H - ZS M- 5 were shown in F ig. 3 . The coordination numbers of Pd-O, Pd-Pd ( metal ) and Pd- ( O ) -Pd ( oxide ) calculated based on the curve fitting analysis were given in F ig. 4 . Initially, the formation of metal Pd was observed from the appearance of an intense nearest-neighboring Pd-Pd pea k situated at 0 .2 5 nm on both Pd /H - ZS M- 5 and Na- ZS M- 5 . F rom the coordination number of the Pd-Pd pea k , the Pd particle was supposed to locate on the external surface of zeolites. In the case of Pd / Na- ZS M- 5 (F ig. 3( a )) , the Pd-Pd ( metal ) gradually reduced accompanied by increasing the temperature in an oxygen flow. Alternatively, new pea k s appeared at 0 . 15 and 0 . 3 2 nm. These pea k s could be assigned to the Pd-O and Pd- ( O ) -Pd characteristic of bul k PdO that is apparent from the comparison with the Fig. 1. Schematic view of DXAFS spectrometer with Laue configuration. 24000 24500 25000 Photon Energy (eV) μ t 0.1 773 K 723 K 673 K 623 K 573 K 523 K 61 Kazu Okumura a and Kazuo Kato b (a) Faculty of Engineering, Tottori University (b) SPring-8 / JASRI E-mail: okmr@chem.tottori-u.ac.jp spectrum of bulk PdO. The generation of the Pd-(O)-Pd indicated that the metal Pd was simply transformed into the agglomerated PdO on the external surface of Na-ZSM-5 at elevated temperature. On the other hand, the metal Pd-Pd peak steeply disappeared at 643 K in the spectra of Pd/H-ZSM-5 (Fig. 3(b)). At the same time, the intensity of the Pd-O peak increased, indicating that the oxidation of metal Pd progressed with increasing the temperature. However, in contrast to the Na form of zeolites, the Pd-(O)-Pd peak due to the agglomerated PdO did not appear. The fact indicated that the agglomerated metal Pd was migrated o n t h e a c i d s i t e s o f z e o l i t e s t o g e n e r a t e h i g h l y d i s p e r s e d P d O , because the intensity of Pd-(O)-Pd shell reflected the size of PdO. In addition, the data suggested that the migration of Pd occurred immediately after the oxidation of the metal Pd particle since the oxidation and the dispersion of Pd took place at the same time, which was confirmed from the data given in Fig. 4(b). Another d i s p e r s i o n p r o c e s s o f P d O w a s revealed from the comparison with H-ZSM-5 and Na-ZSM-5. That is to say, the transformation of the metal Pd-Pd into Pd-O was steep and occurred at lower temperature over H-ZSM-5 with respect to Na-ZSM-5. Taking this into consideration, it could be assumed t h a t , i n a d d i t i o n t o s p o n t a n e o u s dispersion of PdO, the oxidation of metal Pd was promoted through the strong metal-support interaction between acid sites of H-ZSM-5 and PdO which possessed basic character. Fig. 3. Pd K -edge EXAFS Fourier transforms for Pd loaded on (a) Na- ZSM-5 (Si/Al 2 = 23.8) and (b) H-ZSM-5 (Si/Al 2 = 23.8) measured in an O 2 flow; Pd loading, 0.4 wt%; temperature ramping rate, 5 K min -1 . The spectra of PdO and Pd foil were collected at room temperature. 0 2 3 4 5 1 6 Pd-O 773 K 723 K 673 K 623 K 573 K 523 K (b) (a) Pd-(O)-Pd PdO Pd-Pd (Pd 0 ) × 0.5 Pd foil Amplitude (arb. units) 20 40 60 Distance (0.1 nm) 0 1 2 3 4 5 6 Fig. 4. Relationship between the coordination numbers of Pd loaded on (a) Na-ZSM-5 and (b) H-ZSM-5 and the temperature measured in an O 2 flow. References [1] K. Okumura, R. Yoshimoto, T. Uruga, H. Tanida, K. Kato, S. Yokota and M. Niwa: J. Phys. Chem. B 108 (2004) 6250. [2] K. Okumura et al. : Phys. Scripta - in press. [3] K. Okumura et al. : Chem. L ett. 32 (2003) 636. [4] K. Okumura and M. Niwa: Catal. Surv. Jpn. 5 (2002) 121. 550 600 650 700 750 Temperature (K) Coordination Number 0 2 4 6 8 10 550 600 650 700 750 Pd-Pd (metal) Pd-O Pd-(O)-Pd (oxide) (a) ( b) 62
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