J. Phys. Chem. B 2005, 109, 7637-7642
7637
FTIR Studies of Zeolite Matrix Effects on the Properties of Palladium Clusters Confined in the Supercages of NaX and NaY Yan-Xia Jiang, Wei-Zheng Weng, Di Si, and Shi-Gang Sun* State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Institute of Physical Chemistry, Xiamen UniVersity, Xiamen 361005, China ReceiVed: May 10, 2004; In Final Form: February 21, 2005
Palladium clusters have been synthesized by the “ship-in-a-bottle” approach in the supercages of NaX and NaY faujasite zeolites. In comparison with CO adsorbed on a bulk Pd electrode, the same molecule adsorbed on the Pd clusters electrodes evoked an enhanced IR absorption (EIRA). The enhancement factors have been determined to be about 38 and 51 in NaX and NaY, respectively. IR band centers of linear-bonded CO, bridge-bonded CO, and multi-bonded CO in NaX are measured, respectively, 12, 14, and 11 cm-1 lower than those of the corresponding adsorption modes in NaY. The adsorption of CO and the oxidation of adsorbed CO in NaX matrix are faster than that in NaY matrix. These results suggest that part of the Pd2+ ions in NaX are located in sites III and III′ that are near the 12-ring window of the supercage of zeolite, which lead to the formation of small Pd clusters. The present study is of significant importance in exploring the dependence of catalyst properties on structures, as well as in understanding and predicting the locations and properties of metal clusters in zeolites.
Introduction Supported metal catalysts are important heterogeneous catalysts for industrial-scale processes, and much interest in designing and preparing tailor catalysts with well-defined structure has been generated.1,2 Both the size and the size-distribution of particles are the major parameters for supported metal catalysts. With the “ship-in-a-bottle” approach3-5 and using molecular sieve as “template”, the size of metal particle formed in “template” can possibly be controlled, and a sharp distribution in a narrow size region can be achieved. The materials thus formed possess the regular nanostructured arrangements and offer particular physical and/or chemical properties that could not be obtained on corresponding bulk materials.6,7 Gallezot et al.8 studied the hydrogenation of R,β-unsaturated aldehydes over transition metals supported on NaY and KY zeolites and found that both geometric and electronic effects influenced the selectivity to the unsaturated alcohol. Different methods, such as FTIR, XRD, NMR, HRTEM, and EXAFS, were used to investigate the transition metals clusters supported on zeolite. Boudart et al.9 probed the average number of Pt atoms per cluster supported on zeolite Y by 129Xe NMR. Sachtler et al.10-13 studied Pd carbonyl clusters synthesized in NaY and NaA zeolites by EXAFS, FTIR, and XRD and found that Pd13(CO)x clusters are formed and fit perfectly into a supercage of NaY,10-12 whereas Pd6(CO)y clusters come into being in the R-cage of NaA under similar conditions.13 Thus far, however, the structure and composition of Pd carbonyl clusters in NaX have not been given clearly. Because both of the NaX and NaY zeolites possess the same faujasite-type structure, Drozdova et al.14 have deduced that the Pd13(CO)x could be formed in NaX as it is formed in NaY. As a matter of fact, NaX and NaY have different Si/Al ratios (X, 1-1.5, and Y, >1.5) and electric field strengths, which will affect the * Corresponding author. Fax: + 86 592 2183047. E-mail: sgsun@ xmu.edu.cn.
location of charge-balancing cations and thus drastically impact the properties of catalysts or supported catalysts in these two materials. Many papers15-23 have reported the distribution of cations in faujasite zeolite experimentally15-21 and theoretically.22,23 In NaX and NaY zeolites, the cation may occupy one or more of the extraframework sites presented in Figure 1.22,24 Site I is located at the center of a hexagonal prism connecting sodalite cages. Site I′ is inside the sodalite cage facing site I. Site II is in front of the six rings inside the supercage. Site III with low symmetry is also in the supercage, near the four rings of the sodalite cage. Besides I, I′, II, and III, some cations in NaX may occupy the edges of the 4-rings at the 12-ring window, called site III′.22,23 Site III and III′ are believed to be of higher potential energy than sites I, I′, and II.23 The distribution of potential cations in NaX and NaY depends on the silicon-toaluminum ratio in the faujasite framework. Savitz et al.24 have investigated the adsorption of hydrofluorocarbons HFC-134 and HFC-134A on NaX and NaY zeolites and attributed the high selectivity on NaX zeolite to the presence of cations in sites III. The structures of catalysts have a significant influence on their properties. Sun et al.25 have discovered that CO and other spices, such as SCN-, poly-o-phenylenediamine (POPD), etc., adsorbed on nanometer-scale thin film materials of platinum group metals or alloys evoke abnormal IR effects (AIREs), which is characterized by the inversion of IR band direction (anomalous IR absorption), the enhancement of IR absorption, and the increase of IR bandwidth. CO adsorbed on Pd clusters confined in NaY6 and NaA26 produces an enhanced IR absorption (EIRA), which is characterized by the enhancement of IR absorption and the increase of IR bandwidth, as well as the absence of the inversion of IR band direction. Further studies have discovered that dispersed Pt and Pd nanoparticles can yield EIRA and their agglomerates give the AIREs.27,28 To further investigate the relationship between catalyst properties and
10.1021/jp0480089 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/26/2005
7638 J. Phys. Chem. B, Vol. 109, No. 16, 2005
Jiang et al. potential cyclic scans were carried out between 0.00 and -0.40 V for 60 min to ensure a saturation adsorption of CO on Pd clusters. CO species in solutions were then removed thoroughly by bubbling pure N2 gas into the solution for 15 min at -0.40 V to prevent the oxidation of adsorbed CO. The Pd clusters film electrode was pushed against the IR window to form a thin layer of solution during FTIR measurement. The spectral resolution was 8 cm-1, and 100 interferograms in total were collected and co-added into each singlebeam spectrum. The resulting spectrum is the potential difference spectrum of two single-beam spectra collected respectively at the sample potential and reference potential, which represents the relative change in reflectivity and is calculated using the following equation:
Figure 1. Representation of the extraframework cation sites SI, SI′, SII, SIII, and SIII′ in NaX and NaY zeolites. The size of the 12-ring window and that of the supercage of zeolite are 0.74 and 1.18 nm, respectively.
structures, and compare the effects of zeolite matrix on the properties of Pd clusters, NaX and NaY zeolites were used as the “nanometer-size-vessel” to synthesis Pd clusters. The special properties of Pd clusters and zeolite matrix effects have been studied employing in situ FTIR reflection and transmission spectroscopy. Experimental Section Materials. NaX (Si/Al ratio nearly one) and NaY (Si/Al ratio nearly five) were used as templates. The method to prepare Pd clusters confined in the supercages of NaX is the same as that of NaY described in a previous paper.6 For clarity, it is described briefly as follows. First, ion-exchanging was carried out by adding PdCl2 aqueous solution (1.0 × 10-3 M) to a slurry of NaX, which was stirred for an additional 24 h. The Cl- ions and surface adhered salts were completely removed by washing with chloride acid of pH ) 3 and ultrapure water during the filtration. The sample was calcined at 120 °C for 10 h. The Pd2+-loading NaX thus formed was denoted as Pd2+-NaX. Second, they were dispersed to a solution of 0.3 mL of dechlorethane containing 3 mg of poly(vinyl chloride) (PVC) to form a suspension. A defined quantity of the suspension was applied to a clean surface of glassy carbon (GC) substrate. A thin film around 10 µm in thickness was formed and noted as Pd2+-NaX/GC after drying in air for about 30 min. Third, the Pd2+ ions in Pd2+-NaX/GC were reduced to small primary Pd atoms, noted as Pd0EC-NaX/GC, by potential cyclic scans between 0.00 and -0.40 V. Fourth, the primary Pd atoms were assembled to form Pd clusters by admission and release of CO,10 noted as Pd0n(EC)-NaX/GC. An EG&G potentiosat/galvanosat (model 263A) was employed in electrochemical studies. A saturated calomel electrode (SCE) served as the reference electrode. All experiments were carried out at room temperature around 20 °C. In Situ FTIR Reflection Spectroscopy (in Situ FTIRS). In situ FTIRS measurements were carried out on a Nexus 870 FTIR spectrometer (Nicolet) equipped with an EverGlo IR source and a liquid nitrogen cooled MCT-A detector. An electrochemical IR cell with a thin layer configuration29 was employed in the in situ FTIR studies. A CaF2 disk was used as the IR window. The adsorption of CO was carried out by introducing pure CO gas into the electrochemical IR cell containing 0.1 M Na2SO4 solution (pH ) 3.3), at the same time
R(ES) - R(ER) ∆R (ES) ) R R(ER)
(1)
∆R/R(ES) is the resulting spectrum at sample potential ES. R(ES) is the single-beam spectrum collected at ES, in which CO species adsorb on the surface of the electrode. R(ER) is the single-beam spectrum recorded at the reference potential ER, in which the adsorbed CO species (COad) is oxidized completely. As a result, only COad and CO2 in the thin layer derived from COad oxidation at ER were subjected to in situ FTIR spectroscopic investigations. According to eq 1, downward-going bands can be ascribed to IR absorption at ES, and upward-going bands will be attributed to IR absorption at ER in the resulting spectrum. In Situ FTIR Transmission Spectroscopy. Pd2+-NaX, Pd2+-NaY, and Pd2+-NaA were respectively pressed into a self-supported disk of 10-mm in diameter (total weight of the samples is about 20 mg), and then put in a homemade transmission IR gas cell that is designed to treat the sample in situ. The cell was fitted into a furnace, gas inlet, and outlet. A CaF2 disk was used as the IR windows. The sample in the IR cell was evacuated (10-3 Torr) for 30 min at 50 °C and then was exposed to 105 Pa of H2 at 300 °C for 60 min, followed by evacuation at the same temperature. The materials thus formed were noted as Pd0C-NaX, Pd0C-NaY, and Pd0C-NaA. After being cooled to 50 °C under vacuum, the background spectrum (RB) was recorded on a Perkin-Elmer Spectrum 2000 system equipped with a liquid nitrogen cooled MCT B detector. To study the adsorption of CO, 105 Pa of CO was introduced in a batch mode to the IR cell at 50 °C. A series of sample spectra (RS) were collected successively along with the increase of exposure time of CO. The resulting spectra are reported as a conventional absorption spectrum, which is calculated as,
A ) -log(RS/RB)
(2)
The recording is a continuous ratio with the background spectrum RB according to eq 2. The spectral resolution was 2 cm-1, and four scans were collected and co-added for each spectrum. Results and Discussion Powder X-ray diffraction (XRD) of Pd2+-NaX yielded patterns that are identical to those of pure NaX, indicating that the crystallinity is maintained in Pd2+-NaX. 1. In Situ FTIRS Studies. To investigate IR optical properties of nanoscale materials, in situ FTIRS of CO adsorbed on bulk Pd electrode is examined first. The dashed line of Figure 2 depicts in situ FTIRS of CO adsorbed on bulk Pd. The ES was set at -0.45 V, where COad species are stable.6 The ER was set
FTIR Studies of Zeolite Matrix Effects
J. Phys. Chem. B, Vol. 109, No. 16, 2005 7639
Figure 2. In situ FTIR spectra for COad on bulk Pd and Pd0n(EC)NaX/GC electrodes in 0.1 M Na2SO4 (pH ) 3.3). ES ) -0.45 V, ER ) 1.00 V. The polarization time τ at ER increased from (a) to (j).
Figure 3. In situ FTIR spectra for oxidation of COad on Pd0n(EC)NaX/GC. R(ES) was continuously collected with the increase of polarization time τ at 1.00 V from 0 to 9 min, and R(ER) was taken at τ′, while COad was oxidized completely.
at 1.00 V, at which COad was oxidized immediately into CO2 species. From the spectrum of the bulk Pd, we can observe a downward-going band near 1940 cm-1 assigned to IR absorption of bridge-bonded CO (COB) at ES, and an upward-going band around 2345 cm-1 ascribed to IR absorption of CO2 at ER. The directions of the COB and CO2 IR bands are in accord with the prediction of eq 1. As indicated in the Experimental Section, only COad on the electrode surface is subjected to FTIRS investigation in the solution void of CO. The CO2 species were thus derived uniquely from the oxidation of COad at ER when the electrode potential was stepped from ES to ER. The CO2 species can all be retained in the thin layer between electrode and IR window in a relatively short time, because the diffusion from the thin layer to bulk solution is very slow. The enhancement of the IR absorption of COad was determined by direct comparison of the intensity of the COad IR band to that of CO2 IR band. Because the CO2 is in solution, and hence undergoes no enhancement, the intensity of the CO2 IR band (ACO2) is directly proportional to the quantity of COad that had been adsorbed on the electrode. The normalized intensity of COad bands (AN,COad) may be defined as the ratio of the intensity of COad (ACOad) to that of CO2 (ACO2). The AN,COad measured on the bulk Pd electrode is the ratio between ACOB and ACO2 and was calculated to be 0.16. From the kinetics point of view, the COad oxidation rates are very slow on Pd clusters confined in NaY, NaA, and SBA-15 molecular sieves as we have discussed in previous papers.6,26,30 Actually, COad on Pd clusters confined in zeolite matrix cannot be completely oxidized within a short time. Therefore, COad is oxidized partly to CO2 when the electrode potential was stepped from ES to ER, which resulted in the residual COad species on Pd clusters at ER. As a consequence, it may be barely possible to obtain quantificational information about COad from the resulting spectrum. It is known from eq 1 that when COad is oxidized completely into CO2 at reference potential ER, the intensity of the COad IR band can be obtained from the resulting spectrum. So, after the single-beam spectrum R(ES) at ES has been collected, the electrode potential was stepped to ER and then a series of single-beam spectra R(ER,τ) were collected at ER along with increasing the polarization time τ. A series of resulting spectra for CO adsorbed on Pd0n(EC)-NaX/GC electrode in 0.1 M Na2SO4 (pH ) 3.3) were calculated using eq 1 and are displayed in Figure 2. In addition to COB and CO2 bands, we can observe another downward-going band around
2060 cm-1 assigned to the IR absorption of linear-bonded CO (COL). What deserves more attention is that the intensities of the COB and COL bands increased gradually from spectrum a (τ ) 0.45 min) to spectrum j (τ ) 9.17 min), indicating that the quantities of COB and COL decrease gradually at ER with the increase of polarization time τ. The intensity of the COB and COL bands in the resulting spectrum remains constant starting from τ′, which indicates that the COad has been oxidized completely at ER,τ′. Therefore, the single-beam spectrum at τ′ was employed as the reference single-beam spectrum R(ER,τ′); the resulting spectrum at -0.45 V is shown by the dashed line of Figure 3. In this spectrum, the downward-going COL and COB bands at 2060 and 1936 cm-1 are observed, and a weak shoulder at 1903 cm-1 assigned to multi-bonded CO (COM) is also seen. The centers of the IR bands of COL, COB, and COM on Pd0n(EC)-NaX/GC are lower than those on Pd013(EC)-NaY/GC.6 In the latter case, the COL and COB bands as well as COM shoulder appear near 2072, 1950, and 1914 cm-1 in the spectra. Due to the fact that COad can be oxidized completely at a relatively longer time on a zeolite matrix, part of the CO2 species may have been diffused from the thin layer to the bulk solution. Consequently, the calculation of the normalized intensity of COad bands (AN,COad) is modified and expressed as the ratio of intensity between the oxidized COad (ACOad) and produced CO2 (ACO2), which has been described in a previous paper.6 The AN,COad was measured to be 6.13 on Pd0n(EC)-NaX/GC. The enhancement factor (∆IR) of IR absorption of COad on Pd0n(EC)NaX/GC is defined as the ratio of AN,COad of COad bands on Pd0n(EC)-NaX/GC to that on bulk Pd, and it has been measured to be 38. This value is much smaller than the value (51) obtained on Pd013(EC)-NaY/GC,6 and it is similar to the value (36) on Pd06(EC)-NaA/GC.26 IR band positions and IR enhancement factors of COad on electrodes of Pd clusters confined in different zeolite matrix are listed in Table 1, and the results have illustrated that the zeolite matrix has a significant effect on the properties of Pd clusters formed in it. The directions of IR bands on Pd0n(EC)-NaX/GC are the same as they are observed on bulk Pd, which is in accord with the prediction of eq 1. The fact indicates that the Pd clusters in NaX evoke EIRA. Our previous papers reported that the Pd clusters in NaY,6 NaA,26 and SBA-1530 could solely produce EIRA. However, other materials, such as the nanometer-scale thin film materials of platinum group metals and alloys,25 as
7640 J. Phys. Chem. B, Vol. 109, No. 16, 2005
Jiang et al.
TABLE 1: IR Band Positions and IR Enhancement Factors of COad on Pd Clusters on Different Zeolite Matrixes materials Pd013(EC)-NaY/GC Pd0n(EC)-NaX/GC Pd06(EC)-NaA/GC26 Pd013(C)-NaY Pd0n(C)-NaX Pd06(C)-NaA
COB COM enhancement COL factor band/cm-1 band/cm-1 band/cm-1 2072 2060 2040 2096 2089 2081
1950 1936 1930 1979 1974 1955
1914 1903 1898 1930 1916 1893
51 38 36
SCHEME 1
well as agglomerates of Pt nanoparticles27 and Pd nanoparticles,28 exhibit AIREs. This is due to the fact that Pd clusters in zeolites are well separated from each other, where the interactions between Pd clusters are negligible, whereas the interactions between nanoparticles are strong on the nanometerscale thin film materials of platinum group metals and alloys,25 as well as agglomerates of Pt nanoparticles27 and Pd nanoparticles.28 We have consequently suggested that the strong interaction between nanoparticles inside the thin films or the agglomerates may cause the AIREs. To study zeolite matrix effects on the properties of Pd clusters, it is necessary to track the process of COad oxidation on Pd clusters confined in the supercage of zeolite under conditions of thin layer configuration. We designed therefore an IR experimental procedure to monitor the process of oxidation of COad in situ, as shown in Scheme 1. The single-beam spectra collected at -0.45 and 1.00 V with successively increasing time τ until τ′ were used as sample single-beam spectra RS and RSτi, and the RRτ′ was used as a reference single-beam spectrum. The resulting spectra calculated using eq 1 are displayed in Figure 3. The intensity of the COB and COL bands can be measured directly from each resulting spectrum because COad has been removed completely at τ′ and the RRτ′ does not contain IR absorption of COad. It can be observed from Figure 3 that the COL band disappears within 2 min, and the intensity of the COB band decreases rapidly with the increase of the polarization time at 1.00 V. The variation of ACOB with τ at 1.00 V is shown in Figure 5 (O). The ACOB first decreases sharply with a slope (A/t) of -24.86 within 3 min, and then decreases slowly between 3 and 6 min, and finally tends to a constant value above 6 min. The kinetic process of oxidation of COad on Pd013(EC)NaY/GC was monitored also by Scheme 1, and in situ FTIR spectra of COad oxidation on Pd013(EC)-NaY/GC are displayed in Figure 4. The spectra exhibit some differences from those of Pd0n(EC)-NaX/GC: the intensity of the COL band decreases gradually and tends to disappear within 9 min, while the intensity of the COB band decreases steadily with the augmentation of polarization time. The variation of ACOB with τ at 1.00 V is depicted in Figure 5 (b). The ACOB decreases slowly and evenly with a slope (A/t) of -2.30, which indicates obviously that the rate of COad oxidation on Pd0n(EC)-NaX/GC is faster than that on Pd013(EC)-NaY/GC. Sachtler et al.31 interpreted that COL ligands would point outward through the supercages and COB ligands would perfectly fit inside of the supercage of NaY. This conclusion is in agreement with our experimental results; that is, the oxidation of COL is easy, and that of COB is difficult on Pd013(EC)-NaY/GC. These results imply also that the locations
Figure 4. In situ FTIR spectra for oxidation of COad on Pd013(EC)NaY/GC. Other conditions are the same as those in Figure 3.
Figure 5. The relationship between ACOB and polarization time τ at 1.00 V on Pd0n(EC)-NaX/GC (O) and Pd013(EC)-NaY/GC (b).
of Pd clusters in the supercages of NaY differ from that in the supercages of NaX. We find furthermore that COad oxidation on Pd013(EC)-NaY/GC in the thin layer IR cell is slower than that in the electrochemical cell, which can be attributed to the low concentration of cations in the thin layer IR cell. These cations provide the ionic carriers required to maintain charge balance when any species undergo an electron-transferring process. The positions near the 12-ring window of the supercage are easy at which to perform the electron transferring under the condition of thin layer configuration. Therefore, it is suggested that part of Pd2+ ions prefer to locate at sites III and III′ on NaX, and then Pd clusters are formed near the 12-ring window of the supercage in NaX matrix. 2. In Situ Transmission FTIRS Studies. Our previous investigation has shown that Pd clusters synthesized by electrochemical or chemical method exhibit similar IR optical properties.33 To investigate zeolite matrix effects on IR optical properties of Pd clusters, CO adsorption on Pd clusters synthesized by chemical method was studied further using in situ FTIR transmission spectroscopy. In the study, Pd0(C)-NaX, Pd0(C)-NaY, or Pd0(C)-NaA samples were exposed to 105 Pa of CO at 50 °C, and the adsorption of CO was monitored by in situ time-resolved FTIR transmission spectroscopy as described in the Experimental Section. Figure 6 displays the successively collected FTIR spectra of CO adsorbed on Pd0n(C)-NaX. After the sample had been exposed to CO for 0.05 min, we observed an IR band centered at 2144 cm-1 with P and R lines assigned to IR absorption of gaseous CO (COg), and three other bands around 2089, 1974, and 1916 cm-1, which are attributed to IR absorption of COL, COB, and COM species. Further exposure of the sample to CO leads to the increase of the intensities of these bands. The intensities of these bands remain constant from
FTIR Studies of Zeolite Matrix Effects
Figure 6. FTIR spectra of CO adsorption on Pd0n(C)-NaX after the different exposure times to CO at 50 °C; the dotted line is the spectrum with evacuation for 15 min.
Figure 7. FTIR spectra of CO adsorbed on Pd013(C)-NaY. Other conditions are the same as those in Figure 6.
0.35 min and thereafter. The ratio of ACOad at 0.05 min to that at 0.35 min is determined to be 80.24%. COad IR bands show a higher frequency shift at solid/gas interfaces in comparison with the same bands at solid/liquid interfaces; the latter may involve interactions between CO and H2O that are coadsorbed in the aqueous interfaces.32 COg species in the gas cell can be easily removed by simple evacuation (dashed line in Figure 6); in this process, the COg band disappears and all of the COL, COB, and COM bands are red-shifted due to the decrease of COad coverage. Figure 7 depicts CO adsorbed on Pd013(C)-NaY in exactly the same experiment process as CO adsorbed on Pd013(C)-NaX. However, the results are significantly different. Besides the COg band that is centered at 2144 cm-1, the COL, COB bands and the COM weak shoulder are observed, respectively, at 2096, 1979, and 1933 cm-1 in the spectra. The intensities of the COad bands increase with the increase of exposure time. The ratio of ACOad at 0.05 min to that at 0.35 min is 25.64%. After having removed gaseous CO species from the cell, the IR bands centers of COL, COB, and COM shifted also to low wavenumbers due to the decrease of COad coverage. In comparison with the adsorption of CO on Pd013(C)-NaY, the adsorption of CO on Pd0n(C)-NaX is relatively easy, indicating that the size and the positions of Pd clusters confined in NaX are different from that in NaY. To confirm this presumption, NaA zeolite was chosen for studying comparatively the matrix effects. NaA zeolite has a similar Si/Al ratio of NaX, and a size of the R-cage similar to
J. Phys. Chem. B, Vol. 109, No. 16, 2005 7641
Figure 8. The comparative spectra of CO adsorbed on Pd06(C)-NaA, Pd0n(C)-NaX, and Pd013(C)-NaY after exposing CO for 0.35 min. Other conditions are the same as those in Figure 6.
the size of the supercage in NaX. So Pd0(C)-NaA is exposed to CO for 0.35 min, and the spectra are compared in Figure 8. We observe from the spectrum of CO adsorbed on Pd06(C)NaA, besides the COg band, three IR bands centered at 2081, 1955, and 1893 cm-1 assigned respectively to COL, COB, and COM. As is evidenced by Figure 8 and Table 1, only a slight difference between the spectrum of Pd06(C)-NaA and that of Pd0n(C)-NaX appears, while the difference between the spectrum of Pd013(C)-NaY and that of Pd0n(C)-NaX is quite obvious. Besides, the centers of IR bands of COad show a red-shift from Y to X, and then to A matrix. Experiments and calculations34 performed for Pt particles supported on silica show that small particles will produce COad IR bands at lower wavenumbers than do large particles. Summarizing all of our experimental results and the literature data, we may conclude that (1) the size of Pd clusters is decreased from Pd013(C)-NaY to Pd0n(C)-NaX, then to Pd06(C)-NaA; (2) the IR absorption patterns of COad on Pd0n(C)-NaX and those on Pd06(C)-NaA are similar; and (3) the size of Pd clusters formed in the NaX matrix may be similar to those in the NaA matrix. Conclusions In the current study, zeolite was used as “nanometer-sizevessels” to synthesize Pd clusters by the method of “ship-ina-bottle”. The infrared optical properties of Pd clusters and zeolite matrix effects on the properties of Pd clusters confined in the supercages of NaX and NaY were studied comparatively using in situ FTIR reflection spectroscopy and in situ FTIR transmission spectroscopy with a CO molecule probe. In comparison with CO adsorbed on a bulk Pd electrode, CO adsorbed on Pd clusters confined in the supercage of NaY and NaX, as well as in R-cage of NaA, exhibits enhanced infrared absorption (EIRA), and the enhancement factors are determined to be 51,6 38, and 36,26 respectively. These new observations clarify a previous deduction: the isolated or well-dispersed nanoparticles exhibit the EIRA, whereas nanometer scale thin films of platinum group metals and alloys, as well as aggregative nanoparticles, display the AIREs.25,27 The distinct difference is attributed to the interaction between nanoparticles. Furthermore, the IR band center of COad on Pd0n-NaX shows a red-shift in comparison with COad on Pd013-NaY. Both adsorption of CO and oxidation of COad are faster on Pd0n-NaX than that on Pd013-NaY. These characteristics of the NaX matrix may be due to that part of the Pd2+ ions occupied primarily sites III and III′, and then Pd0n clusters were formed near the 12-ring
7642 J. Phys. Chem. B, Vol. 109, No. 16, 2005 window of the supercage of NaX. This facilitates the species (CO, Na+, etc.) diffusion and electron transferring and activates Pd clusters in NaX. Therefore, the circumstance of zeolite matrix, that is, the Si/Al ratio, quantities and sites of chargecompensating cations, as well as the size of the supercage or channel, etc., have a significant effect on the growth of Pd clusters as well as on the sizes of Pd clusters, which further invoke the differences in enhanced IR absorption and other properties of Pd clusters. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20173045, 20021002, 20433040) and the National key basic research and development program (2002CB211804). References and Notes (1) Beck, A.; Horvath, A.; Szucs, A.; Schay, Z.; Horvath, Z. E.; Zsoldos, Z.; Dekany, I.; Guczi, L. Catal. Lett. 2000, 65, 33. (2) Sordelli, L.; Martra, G.; Psaro, R.; Dossi, C.; Coluccia, S. Top. Catal. 1999, 8, 237. (3) Carmen, S.; Holderich, W. F. Catal. Today 2000, 60, 193. (4) Weitkamp, J. Solid State Ionics 2000, 131, 175. (5) Ichikawa, M. Platinum Met. ReV. 2000, 44, 3. (6) Jiang, Y. X.; Sun, S. G.; Ding, N. Chem. Phys. Lett. 2001, 344, 463. (7) Rioux, R. M.; Song, H.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. J. Phys. Chem. B 2005, 109, 2192. (8) Blackmond, D. G.; Swid, K. P.; Davis, M. E.; Gallezot, P. J. Catal. 1991, 131, 401. (9) Boudart, M.; Ryoo, R.; Valenca, G. P.; Grieken, R. V. Catal. Lett. 1993, 17, 273. (10) Zhang, Z.; Chen, H.; Sachtler, W. M. H. J. Chem. Soc., Faraday Trans. 1991, 87, 1413. (11) Zhang, Z.; Chen, H.; Shen, L.; Sachtler, W. M. H. J. Catal. 1991, 127, 213. (12) Vogel, W.; Knozinger, H.; Carvill, B.; Sachtler, W. M. H.; Zhang, Z. J. Phys. Chem. B 1998, 102, 1750.
Jiang et al. (13) Zhang, Z.; Cavalcanti, F.; Sachtler, W. M. H. Catal. Lett. 1992, 12, 157. (14) Drozdova, L.; Novakova, J.; Schulz-Ekloff, G.; Jaeger, N. Microporous Mesoporous Mater. 1999, 28, 395. (15) Vitale, G.; Mellot, C. F.; Bull, L. M.; Cheetham, A. K. J. Phys. Chem. B 1997, 101, 4559. (16) Jaramillo, E.; Auerbach, S. M. J. Phys. Chem. B 1999, 103, 9589. (17) Zhu, L.; Seff, K. J. J. Phys. Chem. B 1999, 103, 9512. (18) Eulenberger, G. R.; Shoemaker, D. P.; Keil, J. G. J. Phys. Chem. B 1967, 71, 1812. (19) Marra, G. L.; Fitch, A. N.; Zecchina, A.; Ricchiardi, G.; Salalaggio, M.; Bordera, S.; Lamberti, C. J. Phys. Chem. B 1997, 101, 10653. (20) Olson, K. H. Zeolites 1995, 15, 439. (21) Plevert, J.; Menorval, L.; Renzo, F.; Fajula, F. J. Phys. Chem. B 1998, 102, 3412. (22) Guliants, V. V.; Mullhaupt, J. T.; Newsam, J. M.; Gorman, A. M.; Freeman, C. M. Catal. Today 1999, 50, 661. (23) Buttefey, S.; Boutin, A.; Mellot-Draznieks, C.; Fuchs, A. H. J. Phys. Chem. B 2001, 105, 9569. (24) Savitz, S.; Siperstein, F. R.; Huber, R.; Tieri, S. M.; Gorte, R. J. J. Phys. Chem. B 1999, 103, 8283. (25) Lu, G. Q.; Sun, S. G.; Cai, L. R.; Chen, S. P.; Tian, Z. W.; Shiu, K. K. Langmuir 2000, 16, 778. (26) Jiang, Y. X.; Ding, N.; Sun, S. G. Proceedings of the International Sump-osium on Solid State Chemistry in China. In Frontiers of Solid State Chemistry; Feng, S. H., Chen, J. S., Eds.; World Scientific Publishing Co. Pte. Ltd.: River Edge, NJ, 2002; p 571. (27) Chen, W.; Zhou, Z. Y.; Chen, S. P.; Sun, S. G. J. Phys. Chem. B 2003, 107, 9808. (28) Jiang, Y. X.; Chen, W.; Liao, H. G.; Jin, L. Y.; Sun, S. G. Chin. Sci. Bull. 2004, 49, 1581. (29) Lin, W. F.; Sun, S. G. Electrochim. Acta 1996, 41, 803. (30) Jiang, Y. X.; Ding, N.; Sun, S. G. J. Electroanal. Chem. 2004, 563, 15. (31) Sheu, L. L.; Knozinger, H.; Sachtler, W. M. H. J. Am. Chem. Soc. 1989, 111, 8125. (32) Zou, S. Z.; Gomez, R.; Weaver, M. J. Surf. Sci. 1998, 399, 270. (33) Jiang, Y. X.; Sun, S. G.; Chen, S. P.; Ding, N. Chem. J. Chin. UniV. 2001, 11, 1860. (34) Sarah, L. H.; Ian, A. O’N.; David, J. S. J. Phys. Chem. B 2001, 105, 941.
