In-Situ QXAFS Studies on the Dynamic Coalescence and Dispersion

Sep 12, 2007 - At the same time, Pd-Osurface1 bonds appeared at 0.213 ... formed up to 573 K, where long Pd-Osurface2 bonds appeared at 0.288 nm...
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J. Phys. Chem. C 2007, 111, 14426-14432

In-Situ QXAFS Studies on the Dynamic Coalescence and Dispersion Processes of Pd in the USY Zeolite Kazu Okumura,* Kazuo Kato, Takashi Sanada, and Miki Niwa Department of Materials Science, Faculty of Engineering, Tottori UniVersity, Koyama-cho, Tottori 680-8552, Japan ReceiVed: May 19, 2007; In Final Form: July 18, 2007

Dynamic coalescence and dispersion processes of Pd in a USY zeolite were successfully followed by an in situ quick XAFS method involving cycles of temperature-programmed reduction (TPR) and temperatureprogrammed oxidation (TPO), which were conducted repeatedly one after another. In the first TPR process, the agglomeration of Pd progressed up to 673 K to yield Pd clusters with Pd-Pd coordination number (CN) ) 7.5, probably in the supercage of USY. On further increasing the temperature, the Pd particles dispersed onto acid sites to form Pd clusters with CN ) 4.3. At the same time, Pd-Osurface1 bonds appeared at 0.213 nm owing to the strong interaction between the Pd clusters and the framework of the USY zeolite. After cooling to room temperature, the flowing gas was switched to O2 and the TPO experiment (first run) was carried out. In the TPO, the spontaneous dispersion of PdO onto the acid sites took place through the oxidation of Pd clusters via the agglomeration of Pd. In the following TPR (second run), stable Pd4 clusters were formed up to 573 K, where long Pd-Osurface2 bonds appeared at 0.288 nm. The formation of the stable Pd4 clusters was repeatedly observed in the third TPR. From these findings, we could demonstrate the high potential of the QXAFS technique to follow the complex behavior of Pd, that is, coalescence, migration, and dispersion, during TPR and TPO cycles. Moreover, the results presented here indicated the possibility of regulating the location and size of Pd clusters by making use of the strong metal-support interaction with the USY support.

1. Introduction Metal clusters encapsulated in zeolite pores have attained significant interest from the viewpoint of their catalytic characteristics. The introduction of metals in zeolites has been achieved through various techniques such as chemical vapor deposition, the ship-in-bottle method, and the decomposition of carbonyl or organometallic clusters in zeolite pores.1-5 In particular, attention has been paid to palladium clusters occluded in Y-type zeolites because Pd exhibits a high catalytic activity in versatile reactions, that is, the selective reduction of NO, total combustion of hydrocarbons, and organic reactions.6-9 Previous studies using XRD, temperature-programmed reduction (TPR) and desorption, UV-vis diffuse reflectance, and extended X-ray absorption fine structure (EXAFS) have shown that the location of Pd can be controlled by the calcination temperature.10,11 Recently, we found that Pd6 clusters were obtained through the introduction of Pd in the pores of H-ZSM-5 and H-mordenite and a successive reduction in an H2 stream.12 The formation of the Pd6 clusters was reversibly observed upon repeated treatments with O2 and H2. Such regeneration of clusters was promoted through the spontaneous migration of molecular PdO onto the acid sites of H-type zeolites under an O2 atmosphere, which was evident from the EXAFS analysis. Resasco et al. also reported that the morphology of the oxidized Pd species strongly depended on the acidity of support.13 The Pd clusters generated in the supercage of an H-Y zeolite were active and reusable in Heck reactions.14 From these findings, it was postulated that the acid sites played a key role in the generation of metal Pd clusters inside zeolites. In fact, in contrast to the * Corresponding author. E-mail: [email protected].

H-type zeolites, the severe aggregation of Pd progressed over the Na-type zeolite without the formation of stable metal clusters. The present study is focused on the clustering process of Pd in the pore of a USY zeolite. The Pd or bimetallic Pd-Pt supported on the USY zeolite has been found to exhibit high sulfur tolerance in the hydrogenation of aromatics and hydrodesulfurization activity.15,16 For this purpose, the quick XAFS (QXAFS) technique was first applied to detect the detailed structural change of Pd. This technique is a powerful tool to study the dynamic structural changes of materials. Unlike the usual step-scan method, the monochromator is continuously moved in the QXAFS technique. Using this technique, it is possible to collect data in a short time and detect the changes in the structure of metals such as those occurring during coalescence, migration, and dispersion under in situ conditions. In general, the formation of the metal cluster has been analyzed after quenching the treated samples. However, it seems to be important to directly follow the continuous structural changes of Pd using an in situ cell because it enables the observation of the generation of stable metal clusters in the pores of zeolites. The QXAFS technique is particularly suitable for the detection of Pd species occluded inside the pores of zeolites, which is otherwise difficult to study. In our study, a Pd loading less than 0.4 wt % was required to obtain uniform Pd clusters. In fact, a Pd loading greater than 0.8 wt % led to the aggregation of Pd as observed in the H-Y zeolite.14 In addition, X-ray absorption in zeolites is small in comparison to that of Pd at the energy region of Pd K-edge (24.3 keV). This fact is beneficial for collecting high-quality data even at high temperatures despite a low Pd loading.

10.1021/jp073874h CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007

Coalescence and Dispersion of Pd in the USY Zeolite

Figure 1. TPR and TPO cycles adopted for in situ QXAFS measurements.

J. Phys. Chem. C, Vol. 111, No. 39, 2007 14427 %). For the EXAFS analysis, the oscillations were extracted by a spline smoothing method. The oscillation was normalized by an edge height around 50 eV above the threshold. The Fourier transformation of the k3-weighted EXAFS oscillation from k space to r space was performed over a range of 30-120 nm-1 to obtain a radial distribution function. The inversely Fourier filtered data were analyzed using a curve-fitting method in the k range between 30 and 120 nm-1.20 In particular, the Fourier transform and curve-fitting analyses of the Pd K-edge EXAFS data collected at room temperature were carried out over the k range of 30-150 nm-1. The Debye-Waller factors of Pd-O and nearest-neighboring Pd-Pd were extracted from the spectra of PdO and Pd powder, respectively, that were measured at the same temperatures for practical samples. Curve-fitting analysis was initiated by employing the extracted Debye-Waller factors. The inversely Fourier filtered data were analyzed by the usual curve-fitting method based on eq 1:

χ(k) )

∑NjFj(k) exp(-2σj2kj2) sin(2krj + φj(k))/krj2 kj ) (k2 - 2m∆E0j/p2)1/2

Figure 2. Pd K-edge XAFS spectra for 0.4 wt %-Pd loaded on USY measured in an 8% H2 flow (the first TPR run). Temperature ramping rate, 5 K min-1.

2. Experimental USY (Si/Al2 ) 7.51) zeolites were supplied by Shokubai Kasei Co. Pd was loaded on the USY by an ion-exchange method using Pd(NH3)4Cl2 solution. The Pd loading was 0.4 wt %. Prior to the measurements, the samples were calcined in an N2 flow and successively oxidized under an O2 flow at 773 K for 4 h. Synchrotron radiation experiments were conducted with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2007A1417). The storage ring of SPring-8 was operated at 8 GeV. The Si(111) monochromator was continuously moved from 4.8 to 4.4° in 1 min. The energy of the X-ray was calibrated using a Pd foil as the reference. With the in situ measurement, a wafer form of the sample was placed in a quartz cell. The thickness of the sample was selected to be 1.5 cm in order to provide an edge jump of 0.3. In practice, eight pieces of the sample disk (60 mg per disk; 7 mm diameter) were placed in a quartz holder. The sample placed in the cell was heated from room temperature to 773 K with a ramp rate of 5 K min-1 in 8% H2 flow (TPR, total flow rate: 90 mL min-1) or 8% O2 flow (temperature-programmed oxidation (TPO), flow rate: 90 mL min-1) at atmospheric pressure. Helium was used as a balance gas. The cycles of TPR and TPO were repeated, as shown in Figure 1. The Pd K-edge XAFS data were recorded every 10 min during TPR and TPO processes. Examples of raw spectra for Pd/USY measured in the first TPR are given in Figure 2. It is observed that fairly good spectra were obtained despite the low Pd loading (0.4 wt

(1)

where Nj, rj, σj, and ∆E0j represent the coordination number (CN), bond distance, Debye-Waller factor, and difference in the threshold energy between the reference and the sample, respectively. The degree of error bars in the present curve-fitting analysis for ∆E0j and σj were estimated to be 4 eV and 0.002 nm, respectively. Fj(k) and φj(k) represent the amplitude and phase shift functions, respectively. For the curve-fitting analysis, the empirical phase shift and amplitude functions for Pd-O and Pd-Pd were extracted from the data for PdO and Pd foil, respectively, that were measured at the same temperature for the analysis of samples. The errors in the analysis were estimated by the R factor (Rf) determined by eq 2:

Rf )

∫|k3χobs(k) - k3χcalc(k)|2 dk/∫|k3χobs(k)|2 dk

(2)

The analysis of EXAFS data was performed using the “REX2000 ver. 2.0.4” program produced by RIGAKU ltd. 3. Results 3.1. First Temperature-Programmed Reduction (TPR) and Temperature-Programmed Oxidation (TPO). As mentioned in the experimental section, the TPR and TPO measurements were repeated one after another (Figure 1); first, the changes in the structure of Pd loaded on the USY zeolite were measured by using the QXAFS technique in an atmosphere of hydrogen (first TPR). The Fourier transforms of the k3χ(k) EXAFS collected after every 10 K are given in Figure 3. Initially, the single Pd-O peak could be seen at 0.16 nm, which was directly assigned to the Pd-O bond characteristic of PdO. However, the Pd-Pd bonds that are characteristics of bulk PdO were not seen in the spectrum; this implies the formation of highly dispersed PdO in the initial stage. The intensity of the Pd-O peak gradually decreased. This was accompanied by an increase in the temperature, and the new peak attributable to the Pd-Pd bond of Pd metal appeared at 0.24 nm as a result of the reduction of the dispersed PdO. On further increasing the temperature above 673 K, a new peak appeared at 0.18 nm (phase shift uncorrected), which corresponded to a longer bond in comparison to the covalent Pd-O bond of PdO. The peak could probably be assigned to the oxygen in the framework of the USY zeolite (denoted as Pd-Osurface1) by considering that Pd was already reduced to Pd0. The CNs of these bonds were

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Figure 3. Pd K-edge EXAFS Fourier transforms for Pd/USY measured in the TPRs. Fourier transforms range, 30-120 nm-1.

Figure 4. Coordination numbers determined by curve-fitting analysis plotted as a function of temperature measured in TPRs. b: Pd-Pd (metal), 4: Pd-O, 0: Pd-Osurface1.

determined by curve-fitting analysis, and the data are summarized in Figure 4. In the analysis, the phase shift and background amplitude for the Pd-O and Pd-Pd bonds were extracted from those of the bulk PdO and Pd foil, respectively, that were measured at the same temperatures. In the initial step of the first TPR, the CN of the Pd-O bond decreased up to 523 K, which was alternated with an increase in the CN of the Pd-Pd bond up to 7.5 at 673 K. The CN of the Pd-Pd bond decreased on a further increase in the temperature, indicating the dispersion of previously agglomerated Pd metal at elevated temperatures. At the same time, the CN of Pd-Osurface1 bond increased, which was accompanied by an increase in the temperature. This fact suggested that a strong interaction between the framework of the USY zeolite and Pd led to the dispersion of the Pd metal. This phenomenon appears to be interesting, taking into account that the heating at a high temperature usually results in the severe sintering of the metal. The sample was cooled to room temperature, and the first TPO experiment was subsequently carried out after switching the flowing gas with an 8% O2 flow. The EXAFS Fourier transforms and the CNs determined from the spectra are given in Figures 5 and 6, respectively. The CN of Pd-Osurface1 decreased up to 500 K. On the other hand, the CN of the Pd metal reached 7.1 at 513 K. This change implied that the removal of the Pd metal clusters from the framework of the zeolite and the agglomeration took place in the initial step. On further increasing the temperature, the CNs of Pd-Pd decreased and, in turn, the CN of covalent Pd-O increased. In spite of the disappearance of the Pd-Pd bond attributable to the Pd metal at 773 K, the Pd-Pd bond of PdO did not appear, suggesting the formation of highly dispersed PdO. 3.2. Second TPR, Second TPO, and Third TPR. After the first TPO experiment, the sample was cooled to room temperature and the second TPR experiment was carried out after

Figure 5. Pd K-edge EXAFS Fourier transforms for Pd/USY measured in the TPOs. Fourier transforms range, 30-120 nm-1. The spectrum of bulk PdO was collected at room temperature.

switching the flowing gas with an 8% H2 flow. The Fourier transforms and the CNs determined by the curve-fitting analysis are shown in Figures 3 and 4, respectively. As can be observed in the Fourier transforms, the Pd-Pd bond of Pd metal was already observed at 313 K. Accompanied by an increase in the temperature, the covalent Pd-O decreased and finally disappeared at 500 K to yield Pd0. The CN of Pd metal obtained at 500-573 K was as low as 2.7, indicating the formation of extremely small metal Pd clusters having about 4 atoms. Subsequently, the CN(Pd-Pd) of the Pd clusters started to increase to a temperature above 600 K. At the same time, the Pd-Osurface1 bond appeared. The appearance of the Pd-Osurface1 bond suggested a strong interaction between the Pd clusters and the zeolite wall, as observed in the first TPR. This change

Coalescence and Dispersion of Pd in the USY Zeolite

J. Phys. Chem. C, Vol. 111, No. 39, 2007 14429

Figure 6. Coordination numbers determined by curve-fitting analysis plotted as a function of temperature measured in TPOs. b: Pd-Pd (metal), 4: Pd-O, 0: Pd-Osurface1.

Figure 8. Curve-fitting analysis of the k3-weighted Pd K-edge EXAFS measured at room temperature (300 K) for the sample treated after the third TPR. Solid lines: experimental; dotted lines: simulated.

Figure 7. k3-weighted Pd K-edge EXAFS (a) and their Fourier transforms (b) of Pd/USY measured in the course of third TPR. The spectra were collected after cooling to room temperature (300 K). Fourier transforms range, 30-150 nm-1.

implied that the Pd4 clusters agglomerated and migrated onto the wall of the USY zeolite. The Fourier transforms and the change in the CNs in the second TPO experiment are summarized in Figures 5 and 6, respectively. The profile of the second TPO was different from that of the first TPO in that the oxidation of Pd clusters progressed without an increase in the CN(Pd-Pd). As a result, the removal of the Pd clusters from the zeolite framework and their oxidation took place simultaneously in the second TPO. The Fourier transforms and the data for curve-fitting analysis of the EXAFS data obtained in the third TPR run are included in Figures 3 and 4, respectively. The changes in the CNs were nearly similar to those of the second run. In other words, the formation of Pd4 clusters was observed after the disappearance of the Pd-O bond; this was followed by an increase in the CN(Pd-Pd) and the appearance of the Pd-Osurface1 bond at an elevated temperature. This fact implied that the formation of the Pd4 clusters was reversible upon repeated oxidation and reduction treatments. 3.3. EXAFS Measured at Room Temperature during the Third TPR. The Pd K-edge EXAFS data were collected after the sample was cooled to room temperature during the third TPR experiment after the temperature reached 473, 573, 673, and 773 K in order to achieve the precise analysis of the Pd clusters. Figure 7 shows the raw k3χ(k) and their Fourier transforms of the data obtained at room temperature. A fairly good agreement was confirmed between the experimental data and the fitted data in the curve-fitting analysis, as given in Figure 8. The data for the determined parameters are listed in Table 1. The EXAFS spectrum of the Pd/USY treated at 473 K could be fitted with two waves (the covalent bond of PdO + PdPd), which implied that the reduction of Pd was incomplete at this temperature. Although we attempted to analyze the EXAFS data of Pd/USY reduced at 573 K using a Pd-Pd bond, the

fitting was insufficient, as shown in Table 1 and Figure 8 (Rf ) 1.2%). However, the addition of a long Pd-Osurface2 bond with CN ) 0.8 remarkably improved the fitting accuracy (Rf ) 0.02%). It should be noted that the correction of CN(PdOsurface2) was made according to the following formula, where λ ) 0.6 nm (eq 3):

NEXAFS ) N exp[-2(REXAFS - RREF)/λ]

(3)

This was necessary to avoid the low CN that was induced by a difference greater than 0.05 nm in the distance between the reference point and the sample.17 The improvement in the fitting accuracy achieved by the addition of Pd-Osurface2 implied the existence of a contribution of the zeolite framework coordinated to the Pd clusters that was different from PdOsurface1. The determined CN(Pd-Pd) obtained by two-wave fitting was calculated to be 2.7. It should be noted that the CN value agreed well with those obtained from the EXAFS data collected during the third TPR, as shown in Figure 4. The distance between the Pd-Pd metal bonds was calculated to be 0.276 nm, which was slightly longer than the nearest-neighboring Pd-Pd distance of the Pd foil (0.274 nm). With a further increase in the temperature up to 773 K, the CN of the Pd-Pd bond increased up to 5.6, indicating the coalescence of Pd4 clusters to yield larger clusters. At the same time, a new peak appeared at 0.213 nm with CN ) 2.5 at 773 K, which was attributable to the Pd-Osurface1, as already mentioned above (third TPR). This change implied that the coalescence of Pd was accompanied by its attachment onto the zeolite wall. In this case, the addition of the long Pd-Osurface2 wave did not improve the fitting accuracy. 3.4. XANES Data Analysis. In order to determine the relative concentrations of metal Pd0 and Pd oxides, the Pd K-edge XANES spectra were fitted by the combination of the spectra for Pd/USY collected before and after the change in the oxidation state using the least-square method. In other words, the spectrum obtained before the H2 flow and that obtained at 573 K were employed for the calculation of the spectra for TPRs. On the other hand, the spectrum obtained before the O2 flow and that obtained at 773 K were employed for the calculation of the spectra in TPOs. Figure 9 displays an example of the fitted data of Pd/USY treated with 8% H2 at 493 K in the first TPR step. It can be seen that the quantitative fitting of

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TABLE 1: Curve-Fitting Analysis of Pd K-edge EXAFS Data Measured at Room Temperature for 0.4 wt %-Pd/USY after Treatments with 8% H2 in the Third TPR temp.a 473 573 (1 wave) 573 (2 waves) 673 773 (PdO)g (Pd foil)g

scatter

CNb

Rc (nm)

∆E0d (eV)

DWe (nm)

Rf f (%)

O Pd Pd Osurface2 Pd Osurface1 Pd Osurface1 Pd O Pd

1.0 ( 0.4 2.4 ( 0.5 2.8 ( 0.5 0.8 ( 0.5 2.7 ( 0.4 1.2 ( 0.6 4.3 ( 0.5 2.5 ( 0.3 5.6 ( 0.2 4 12

0.201 ( 0.003 0.276 ( 0.001 0.276 ( 0.001 0.288 ( 0.004 0.276 ( 0.001 0.212 ( 0.003 0.274 ( 0.001 0.213 ( 0.002 0.273 ( 0.001 0.202 0.274

8 2 8 13 7 14 6 17 6

0.0085 0.0076 0.0077 0.0054 0.0078 0.0091 0.0087 0.0095 0.0084

0.5 1.2 0.02 0.4 0.6

a Treated temperatures. b Coordination number. c Bond distance. d Difference in the origin of photoelectron energy between the reference and the sample. e Debye-Waller factor. f Residual factor. g Data of X-ray crystallography; Fourier filtering range: 0.12-0.30 nm.

Figure 9. XANES spectrum of Pd/USY at 493 K measured in the first TPR and the linear combination of weighted XANES spectra before TPR and that treated at 573 K in an 8% H2 flow (dotted line).

the achieved spectrum is sufficient to calculate the relative concentrations of Pd2+ and Pd0. The calculated ratio of Pd0 and the total amount of Pd in the TPRs and TPOs are summarized in Figure 10. It can be observed that the change in the valence of Pd in the second and third TPRs completely overlapped, which was different from that of the first TPR. In these cases, 40% of Pd was already transformed to Pd0 in the initial stage of the TPR experiment. This fact was consistent with the appearance of Pd-Pd bonds of Pd0, as observed in the Fourier transforms of Figure 3. In the first TPO processes, the complete transformation of Pd0 to Pd2+ took place at 700 K, while it was delayed to 763 K in the second TPO. The transformation of Pd0 to Pd2+ progressed monotonically in the second TPO. 3.5. EXAFS Measured for the TPR of Pd Loaded on NaY. In order to reveal the effect of the acidity of the USY zeolite, the reducing process of Pd/Na-Y was measured; for this measurement, the acid sites of FAU-type zeolite were substituted with Na+ ions (Catalysts & Chemicals Ind. Co.; Si/Al2 ) 5.1). Figure 11 shows the Fourier transforms and CNs plotted as a function of the temperature for 0.4 wt % Pd/Na-Y. It can be seen from the Fourier transforms that the intensity of the PdPd bond is larger than that of Pd/USY from the beginning of the TPR. The Pd-Pd bond is slightly shifted by a longer distance up to 353 K. This shift may be caused by the expansion of the interatomic distance owing to the absorption of H2 into the interstitial spaces of Pd particles. The CN of Pd-Pd of Pd0 continued to increase without the appearance of a plateau, as observed in the second and third TPR processes of Pd/USY.

Figure 10. Pd0/(Pd2+ + Pd0) ratio determined from XANES spectra of Pd/USY in the (a) TPRs and (b) TPOs.

The ultimate CN at 773 K was 11.7, which was close to that of a close-packed structure with fcc, and it was far larger than that observed for the USY support. The change in the CN indicated that the growth of Pd metal continuously progressed without the formation of stable Pd clusters. From the changes in the growing manner of the Pd metal, it could be noted that the acid sites played a key role in keeping the Pd clusters inside the pores of the USY zeolite. 4. Discussion In this study, the in situ QXAFS technique was effectively applied to follow the clustering and dispersion processes of Pd in zeolites, and we found that the TPR and TPO cycles had a significant influence on the genesis of Pd in the USY support.

Coalescence and Dispersion of Pd in the USY Zeolite

Figure 11. (a) Pd K-edge EXAFS Fourier transforms for Pd/Na-Y measured in the TPR; Fourier transforms range, 30-120 nm-1. (b) Coordination numbers determined by curve-fitting analysis plotted as a function of temperature. b: Pd-Pd (metal), 4: Pd-O.

Figure 12. Proposed model structures of (a) Pd4 and (b) Pd13 clusters generated in the sodalite cage of USY zeolite in the second and third TPRs.

During the first TPR process, the CN(Pd-Pd) progressively increased even after the complete disappearance of the Pd-O bond and the CN(Pd-Pd) reached 7.5 at 673 K. The maximum CN was close to that of the Pd55 clusters (CN ) 7.8), corresponding to the two-shell structure of the cuboctahedron (13 + 42 atoms). The size of this cluster was estimated to be 1.0 nm, which was close to the diameter of the supercage of the FAU-type zeolite. Therefore, it was inferred that the Pd agglomerated into the large clusters that occupied the supercage due to the treatment up to a temperature of 673 K. Further, CN(Pd-Pd) started to decrease when the temperature was increased. In general, metal atoms sinter to produce aggregated particles at elevated temperatures. Therefore, the change observed above 673 K seems to be unique in the sense that previously agglomerated Pd is disrupted to produce a dispersed Pd. Simultaneously, the Pd-Osurface1 bond appeared when the temperature was increased. This change suggested a strong interaction between the dispersed Pd clusters and the zeolite framework that worked as an anchor for the dispersed Pd clusters. One possibility for such a change is that the strong Brønsted acid sites located in the sodalite cage induced the dispersion of Pd metal. In agreement with this assumption, the strength of Brønsted acid sites in the sodalite cage (∆H ) 122 kJ mol-1) was found to be greater than that in the supercage (∆H ) 116 kJ mol-1).18 The bond distance of the Pd-Osurface1 bond was calculated to be 0.213 nm, which was longer than that of the covalent Pd-O bond (0.202 nm), but it was much shorter than that of the Pd-Osurface2 bond (0.288 nm). In turn,

J. Phys. Chem. C, Vol. 111, No. 39, 2007 14431 the bond distance of the Pd-Osurface1 bond was close to those of Ir-, Pt-, and Rh-loaded catalysts, which were typically found at 0.21-0.22 nm.19 This value indicated a close approach of the Pd clusters to the zeolite framework. In the first TPO experiment, CN(Pd-Pd) increased instead of the disappearance of the Pd-Osurface1 bond. This implied that the previously anchored Pd clusters detached from the framework of the USY zeolite to the agglomerated one. Then, the CN(Pd-Pd) decreased and the CN of the covalent Pd-O bond increased; this was accompanied by the oxidation of the Pd metal without the formation of the Pd-Pd bond characteristics of bulk PdO. Such a change indicated that the previously agglomerated Pd clusters had undergone oxidation to produce highly dispersed PdO. The spontaneous dispersion of PdO was probably induced by the strong interaction between the acid sites of the USY zeolite and PdO that exhibited the basic characteristics, as observed in the Pd loaded on H-ZSM-5 or H-mordenite.20 In the subsequent second TPR, the formation of very small clusters with CN ) 2.7 was observed at 573 K. No other long bonds are attributable to Pd-Pd bonds other than the nearest-neighboring bonds, as shown in Figure 7; this implies the formation of extremely small Pd clusters. The detailed curve-fitting analysis revealed the contribution of a new Pd-O bond with R ) 0.288 nm and CN ) 0.8. The distance of the Pd-Osurface2 bond was considerably longer than that observed in the first TPR process (R ) 0.213 nm) and the covalent Pd-O bond (R ) 0.202 nm) of PdO. A long Pd-O bond similar to this was reported in the PdY zeolite and the PdNaX zeolite. For instance, Bergeret et al. assigned a long bond distance of 0.274 nm to Pd in SI′* positions, which were slightly shifted from SI′ along the C3V axis.10 Moller et al. proposed an analogous model structure.17 The Pd-Pd bond distance of the Pd4 clusters was calculated to be 0.276 nm, which was longer than that of the Pd foil and Pd clusters obtained at 673-773 K by 0.02 nm. The possibility of the expansion of the Pd-Pd bond owing to the adsorption of H2 could be ruled out since the H2 flow was turned off before cooling to room temperature. The strong interaction between Pd4 clusters and the walls of the USY zeolite was probably responsible for the elongation of the Pd-Pd bond, taking into account the appearance of the Pd-Osurface2 bond. Of particular interest is the formation of the Pd4 clusters that was repeatedly observed in the second and third TPR. A close similarity was also observed in the change in the oxidation state of Pd, as shown in Figure 10. The formation of highly dispersed PdO in the second TPO was probably responsible for the repeated formation of Pd4 clusters. Moreover, the presence of acid sites may play an important role in the generation of stable Pd clusters, considering that a simple aggregation was observed in the Na-Y zeolite that had no acid sites. Similar to our observation, Sachtler et al. proposed that the interaction between metal atoms and zeolitic protons lead to electron-deficient [PdnH]+ adducts.21 Guillemot et al. also reported that Pd clusters are better dispersed on the H-Y zeolite than on the Na-Y zeolite.22 The total acid amount of the USY sample employed here was calculated to be 0.96 mol kg-1 as measured by NH3 TPD,23 whereas the loaded amount of Pd was 0.038 mol kg-1. Therefore, the acid amount was much larger than the amount of Pd loaded on USY. The stoichiometry was calculated to be O2/Pd ) 0.5 and H2/Pd ) 1.0 in the TPOs and TPRs as measured by a gas chromatograph equipped with a TCD detector, respectively. The fact meant that the formation of metal Pd clusters and molecular-like PdO took place reversibly in the cycles of TPOs and TPRs. On the basis of the above arguments, the model structures realized at 573 and 773 K in the second and third TPR were proposed in

14432 J. Phys. Chem. C, Vol. 111, No. 39, 2007 Figure 12. The geometry was optimized DMol3 produced by Accelrys Inc. with LDA level. When the Pd/USY was reduced at 573 K in the second and third TPR, each palladium atom was located in front of one hexagonal window in SI′ positions within the sodalite unit, analogous to the structural model given by Moller et al. as displayed in Figure 12a.17 On the other hand, the formation of Pd13 clusters was proposed at 773 K in the second and third TPR, where the space of the sodalite cage was fully accommodated with Pd13 clusters (Figure 12b). Therefore, it was supposed that the close proximity of Pd13 clusters and the wall of the sodalite cage give rise to the appearance of the short Pd-Osurface1 bond at 0.213 nm with CN ) 2.5. 5. Conclusions The structural changes in Pd loaded (0.4 wt %) on the USY zeolite was successfully followed using an in situ QXAFS technique. The most striking result obtained was that the dynamic behavior of Pd atoms interacting with the zeolite framework could be directly observed by QXAFS. In other words, we could observe the coalescence and dispersion behavior of Pd depending on the temperature-programmed oxidation and reduction cycles. In the first TPR, Pd agglomeration progressed at 673 K, and this was followed by the dispersion through the interaction with the framework of the USY zeolite. The structural change in Pd in the second and third TPR processes was different from that in the first one in that the formation of stable Pd4 clusters was observed around 573 K. The present results point to the possibility of regulating the location and size of Pd by choosing an appropriate gas, a suitable temperature, and an adequate number of TPR-TPO cycles. References and Notes (1) Weber, W. A.; Gates, B. C. J. Catal. 1998, 180, 207. (2) Brabec, L.; Nova´kova´, J. J. Mol. Catal. A 2001, 166, 28.

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