NaY

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J. Phys. Chem. 1996, 100, 5070-5077

Characterization by EXAFS, NMR, and Other Techniques of Pt/NaY Zeolite at Industrially Relevant Low Concentration of Platinum K. I. Pandya,† S. M. Heald,‡ J. A. Hriljac, L. Petrakis,* and J. Fraissard§ Department of Applied Science, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: October 24, 1995; In Final Form: December 26, 1995X

In-situ extended X-ray absorption fine structure (EXAFS), 1H and Xe nuclear magnetic resonance (NMR), H2 chemisorption, X-ray powder diffraction (XPD), and high-resolution electron microscopy (HREM) techniques were used to understand, as precisely as possible, carefully prepared Pt/Y zeolite samples at industrially relevant low levels of Pt loadings. These techniques were used to determine the local structure, size, and location of the metal particles for a series of 0.8 wt % Pt/NaY zeolite catalysts which have been reduced at 300, 500, and 650 °C. The EXAFS and NMR results show that the metal particles are smallest, as expected, for the sample reduced at 300 °C, with an average size of 11.3 Å and containing 30 atoms, assuming a spherical shape. These particles are located primarily inside the supercages. For the samples reduced at 500 and 650 °C, the size and location of the Pt particles are distinctly different from those for the 300 °C sample; i.e., the particles are larger and increasingly located outside the supercages. The sizes of the Pt particles obtained from the first shell analysis for the samples reduced at 500 and 650 °C are 23 and 45 Å, respectively. The Pt particles for the sample reduced at 650 °C are completely outside the zeolite crystallite, while for the 500 °C sample 23% of the particles are outside. The XPD results show that the full-widthhalf-maximum (fwhm) of the zeolite X-ray diffraction peaks decreases as the reduction temperature increases, indicating enhanced crystallinity of the framework and repair of the possible damage from the agglomeration and movement of the Pt particles. Comparison of EXAFS results of samples prepared with different techniques shows that the average metal particle size is 10 ( 2 Å for reduction temperatures up to 360 °C, and above this a rapid growth of metal particles is seen.

Introduction Heterogeneous catalysts consisting of small metal particles supported on high surface area materials play a key role in reforming processes.1 A typical industrial grade catalyst has a metal concentration < 1 wt %, and the average size of the metal particles is 10-15 Å. Since their activity is known to be mainly a surface property, considerable research has been carried out to develop preparative procedures that maximize the metal dispersion. A quantitative characterization of the metal dispersion is extremely important to understand the reaction mechanisms and to optimize the preparative procedures. The study of such catalytic systems has received much attention both from a fundamental standpoint as well as for practical purposes. Quantitative information about the metal dispersion and particle size can be obtained by several techniques such as chemisorption, high-resolution electron microscopy, photoelectron spectroscopy, X-ray powder diffraction (XPD), 129Xe NMR, and X-ray absorption fine-structure (XAFS) spectroscopy.2-5 Each of these techniques has some advantages and limitations, and it is necessary to use several techniques to obtain consistent results. The EXAFS technique provides accurate determination of the coordination numbers, bond distances, and relative disorder (root mean square deviation in bond distances). The information about the coordination numbers and bond distances * To whom inquiries should be sent: Department of Applied Science, Brookhaven National Laboratory, P.O. Box 5000, Upton, NY 11973-5000. † North Carolina State University, Physics Department, Raleigh, NC 27695. ‡ Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60515. § Laboratoire de Chimie des Surface Asscocie au CNRS UA 870, Universite Pierre et Marie Curie, 4 Place Jussieu 75252 Paris Dedex 05, France. X Abstract published in AdVance ACS Abstracts, February 15, 1996.

0022-3654/96/20100-5070$12.00/0

can further be used to derive the size and shape of the metal particles.6,7 For small metal particles, EXAFS provides valuable information about the metal-support bonding.8 We had carried out 129Xe NMR measurements to determine the average particle size for a series of 0.8 wt % Pt/NaY catalysts which were reduced at 300, 500, and 650 °C, respectively.9 The 129Xe NMR technique uses information derived from the 129Xe NMR spectra before and after dosing with hydrogen to calculate the number of Pt particles present. As the amount of Pt in the sample is known, the average number of Pt atoms per particle can be obtained. The sample reduced at 300 °C showed the smallest particles, while the catalysts reduced at 500 and 650 °C showed larger metal particles. The metal dispersion determined from chemisorption and electron microscopy showed the same trend in particle size. In order to extend that study and understand the details of these industrially relevant systems, we have carried out in-situ EXAFS experiments to determine the local structure and the particle size. In addition, we have characterized the systems by 1H and 129Xe NMR, H2 chemisorption, electron microscopy, and high-resolution synchrotron XPD measurements in order to quantitatively understand the nature of the platinum particles in the Y zeolite. The structure and dispersion of small Pt particles in various zeolites have been studied extensively with EXAFS. Boudart and co-workers have studied Pt particles in Y zeolite by various techniques.10,11 Their study included samples prepared by the procedure of Dalla Betta and Boudart12 and reduced at 217 and 400 °C, respectively. The EXAFS analysis of the first coordination shell showed that the average particle size was 9 Å for the sample reduced at 217 °C, and 15 Å for the sample reduced at 400 °C. Tzou and co-workers have studied the effects of calcination and reduction temperatures on the size and location of the Pt particles in Y zeolite.13 Their study © 1996 American Chemical Society

Characterization of Pt/NaY Zeolite included Pt/NaY catalysts that were calcined and reduced at 360 and 550 °C. Both the size and location of the Pt particles were strongly influenced by the calcination temperature. The average particle size was 5-10 Å for the sample calcined and reduced at 360 °C. In another study, Tzou and co-workers studied a Pt/NaY catalyst that was prepared by the ion-exchange technique, calcined at 300 °C, and reduced at 500 °C.14 This catalyst showed very small particles with a diameter of approximately 7-8 Å. Our work contributes to the findings from these earlier studies because of its approach regarding metal loading, synthesis techniques, EXAFS measurements, data analysis, and the combination of characterization techniques. We have employed low metal loading in combination with an impregnation method of synthesis, which is expected to influence the metal dispersion achieved. Our EXAFS experiments were carried out at low temperature, which enhances the contribution of the higher shells and provides a wider data range to carry out a more detailed analysis. We have extended the analysis up to the third-nearestneighbor shell, in our attempt to determine the shape of the metal particles and to check for bimodal size distribution. Experimental Section Sample Preparation. The Pt/NaY catalysts were prepared, as described earlier, by incipient wetness impregnation with an aqueous solution of Pt(NH3)4(NO3)2 to provide 0.8 wt % of Pt, reflecting the industrial catalysts.9 Each sample was dried overnight in a vacuum oven at 100 °C, heated at 5 °C/min from room temperature to 150 °C in a flow of dry air, heated at 5 °C/min from 150 to 290 °C in a flow mixture of 75% air and 25% steam, held at 290 °C in the flowing air-steam mixture for 2 h, and cooled to room temperature in dry air. Three different components of the resulting Pt/NaY catalyst were treated at different temperatures in hydrogen as follows: (a) Catalyst Pt/NaY300 was flushed with N2 at room temperature and then heated at 10 °C/min to 300 °C in a flow of 5 L of H2 per gram of catalyst per hour. After 4 h at 300 °C, the catalyst was cooled to room temperature in flowing H2. The H2 was flushed out with N2 and then with 1% air in N2 for 30 min. (b) Catalyst Pt/NaY500 was treated the same as catalyst Pt/NaY300 except that, after 4 h at 300 °C, it was heated at 10 °C/min to 500 °C and held at that temperature for 100 h. (c) Catalyst Pt/NaY650 was treated the same as catalyst Pt/NaY500 except that Pt/NaY650 was heated to 650 °C and held at that temperature for 24 h. EXAFS Experiments. The Pt/L3 edge EXAFS measurements were carried out at beamline X-11A of the National Synchrotron Light Source using a standard transmission setup and a Si(111) double crystal monochromator. The monochromator was calibrated at the Pt/L3 edge (11564 eV) by measuring an EXAFS spectrum for a 5 µm thick platinum foil. The intensities of the incident and the transmitted X-ray beam were monitored by ionization chambers. The gas mixtures in the ionization chambers were selected to absorb 20% of the intensity of the incident beam and 80% of the intensity of the transmitted beam at the Pt/L3 edge. The monochromator was detuned by 50% to reduce the higher harmonic radiation present in the beam. The resolution of the monochromator at this energy is ∼ 2 eV. For EXAFS measurements, approximately 140 mg of a catalyst sample was ground to a fine powder and pressed into a self-supporting wafer which yielded 0.8 mg/cm2 of Pt and a total absorption (µx) of about 2.5 at the Pt/L3 edge. The sample was placed in a controlled environment cell which allowed insitu EXAFS measurements at low temperature.14 The samples

J. Phys. Chem., Vol. 100, No. 12, 1996 5071 were reduced in the cell at 300 °C (heating rate 5 °C/min) for 1 h in flowing hydrogen (99.999 purity). The rate of heating was reduced to 2 °C/min in the temperature range 80-100 °C to avoid sintering caused by the removal of water. The sample was cooled to room temperature in flowing hydrogen. The EXAFS measurements were carried out at ∼-127 °C in hydrogen at atmospheric pressure. Six scans were measured for each sample, and the data analysis was carried out on the averaged data. H2 Chemisorption, Xenon Adsorption, and NMR. Before any hydrogen chemisorption the sample was evacuated while the temperature was raised slowly from 25 to 300 °C under 10-5 Torr. Then the sample was treated with 300 Torr of O2 at 300 °C for 1 h, followed by desorption under 10-5 Torr, reduction with H2 at 300 °C for 1 h, and finally evacuation at 400 °C for 12 h. A first adsorption isotherm of H2 corresponds to the total adsorbed hydrogen. After evacuation at 26 °C under 10-5 Torr for about 20 min, the “reversible” physisorbed hydrogen is eliminated. A second isotherm is obtained using the same sample. The difference between the two isotherms thus plotted is roughly a horizontal straight line. The number of molecules irreversibly adsorbed at room temperature (Nirr) is obtained by extrapolating this difference to zero pressure. Nirr corresponds to a monolayer of hydrogen atoms on the platinum surface. Xenon was coadsorbed on these samples, and 129Xe NMR was observed at room temperature, at a frequency of 24.9 MHz, in order to determine the distribution of chemisorbed hydrogen in the narrow NMR tube and the location of Pt particles inside or outside the zeolite crystallites. Electron Microscopy. In order to determine the size of the larger particles, ex-situ HREM measurements were carried out. The TEM specimens were prepared by ultramicrotomy. The support material was dissolved away, leaving behind thin slices of the samples disposed on a holey-carbon TEM grid. Images of the thin slices were free of the grainy contrast. The thickest part of the fragments of this section that were examined was about 50 nm.9 X-ray Powder Diffraction Experiments. The ex-situ highresolution X-ray powder diffraction data were collected at beamline X7A at the National Synchrotron Light Source.16 A wavelength of 1.1506 Å was chosen using a Ge(111) channelcut monochromator and the exact value determined by calibration with a silicon standard. A Ge(220) perfect crystal analyzer and a scintillation counter detector combination was used. The catalysts were exposed to air prior to the XPD measurements. This would cause oxidation of the Pt surface particles. EXAFS of the air-exposed particles confirmed this with the extend of oxidation being higher for the Pt/NaY300 sample. For each catalyst, approximately 10 mg was sealed in a 1 mm glass capillary and spun during the data collection to enhance sample averaging. The diffraction data were collected over several selected regions of the X-ray powder diffraction pattern. These were chosen such that they covered six reasonably intense zeolite diffraction peaks over a fairly large range of 2θ values, as well as those regions where peaks due to platinum metal would be expected. Results EXAFS. The EXAFS data for the reduced catalysts were analyzed using a standard procedure described elsewhere.3 A platinum foil EXAFS spectrum measured at -173 °C was used to provide the reference phase shifts and amplitude functions for the first three Pt-Pt coordination shells. The raw (unfiltered) EXAFS functions for the reduced catalysts are shown in Figure

5072 J. Phys. Chem., Vol. 100, No. 12, 1996

Pandya et al. TABLE 2: Fourier Transformation Parameters Used for Isolating the EXAFS Contributions of the First, Second, and Third Coordination Shells sample

shell

∆k, Å-1

∆r, Å

∆k, Å-1 fit range

PTY300

1 2 3 1 2 3 1 2 3

2.6-16.2 2.6-16.2 2.6-16.2 2.6-16.2 2.6-16.2 2.6-16.2 2.6-16.2 2.6-16.2 2.6-16.2

1.5-3.3 3.6-4.2 4.1-5.1 1.5-3.3 3.6-4.2 4.1-5.1 1.5-3.3 3.6-4.2 4.1-5.1

4-15 6-15 6-15 4-15 6-15 6-15 4-15 6-15 6-15

PTY500 PTY650

TABLE 3: Structural Parameters Obtained from EXAFS Analysis with Pt Foil as the Reference Compounda sample

shell

Nb

R,c Å

∆σ,d Å2

E0,e eV

Pt/NaY300

1 2 3 1 2 3 1 2 3

7.5 2.7 5.0 9.7 3.3 12.5 11.0 3.8 18.5

2.75 3.90 4.78 2.75 3.92 4.81 2.75 3.92 4.78

0.0041 0.006 0.003 0.0036 0.004 0.006 0.0043 0.005 0.0065

1.0 0.0 2.0 1.0 2.0 5.2 2.0 2.0 7.0

Pt/NaY500 Pt/NaY650

Figure 1. In-situ EXAFS functions for samples reduced at (a) 300, (b) 500, and (c) 650 °C.

Figure 2. Radial structure functions for the reduced catalysts: dotted line, Pt/NaY300; dashed line, Pt/NaY500; solid line Pt/NaY650.

TABLE 1: Fourier Transformation Parameters Used To Prepare Reference Phase and Amplitude Functions from the EXAFS Data of the Pt Foil shell

∆k, Å-1

∆r, Å

Nref

Rref (Å)

1 2 3

2.6-17.3 2.6-17.3 2.6-17.3

1.5-3.3 3.6-4.2 4.1-5.1

12 6 24

2.77 3.92 4.80

1, where oscillations extending up to k ) 16 Å-1 are seen. The corresponding radial structure functions (RSFs, k3 weighted, ∆k ) 2.6-15.6 Å-1) are shown in Figure 2. The major peak in the RSF corresponds to the first Pt coordination shell. The second and third coordination shells are also seen. Although the EXAFS contribution of the second and third coordination shells is significantly less than that of the first, the excellent data quality allows detailed analysis of the higher ones. The Fourier transformation parameters and the crystallographic data used to derive the reference phase and amplitude functions from the EXAFS spectrum of Pt foil are given in Table 1. The Fourier transformation parameters used to isolate various

a Accuracy: (10%, shell 1; (20%, shells 2 and 3. b Accuracy: (1%. c Accuracy: (10%, shell 1; (30%, shells 2 and 3. d Accuracy: (20%.

EXAFS contributions for the catalyst samples are given in Table 2. The contribution of the first coordination shells was analyzed utilizing the full data range employing the k1-k3 fitting technique.6 The analysis of the higher shells is more complicated. For a fcc structure, the EXAFS contribution of the noncollinear multiple scattering processes occurring within the first coordination shell is significant and it interferes with the single-scattering contribution of the second and the third coordination shells.17 It is necessary to model the contribution of the noncollinear multiple scattering processes in order to determine the structural parameters beyond the first coordination shell. Theoretical calculations carried out using the FEFF 5.03 code showed that the noncollinear multiple scattering processes primarily affect the low region, and its intensity decays rapidly as compared to that of the single-scattering EXAFS. Hence in order to avoid/minimize the interference from the noncollinear multiple scattering processes, the low k region (k < 6 Å-1) was not included in the analysis of the second and third coordination shells. The choice of lower limit (kmin ) 6 Å-1) was made to avoid the low k region as much as possible and at the same time have adequate range in the k-space to carry out a four parameter fit.3 The quality of fit was checked both in k- and r-space. The entire data analysis procedure was repeated using theoretical phase and amplitude functions calculated from the FEFF code.18 A good agreement in both sets of results (experimental and theoretical phase-amplitude functions) was obtained. The structural parameters obtained from EXAFS analysis are listed in Table 3. The structural parameters of the higher shells are less accurate as compared to those of the first shell primarily due to the overlap with neighboring shells (e.g. the first and the third shells overlap with the second shell) and the multiple scattering processes. The coordination numbers and bond distances obtained from the EXAFS analysis were then used to determine the average size and shape of the metal particles.6,7 Table 3 shows the average number of atoms in a particle for each sample assuming fcc structure and spherical shape. The average particle size as determined from the first shell analysis is 12.4 Å for the sample

Characterization of Pt/NaY Zeolite

J. Phys. Chem., Vol. 100, No. 12, 1996 5073

TABLE 4: Particle Size Determined from Coordination Numbers sample

shell

Natomsa

Dparticle, Å

Pt/NaY300

1 2 3 1 2 3 1 2 3

40 (26-81) 30 (25-43) 20 (15-30) 321 (110-1865) 80 (43-100) 100 (43-250) 2915 (455-∞) 100 (90-225) 1055 (90-large)

12.4 11.0 10.6 22.8 15.2 16.4 46.7 16.5 33.4

Pt/NaY500 Pt/NaY650

a Average particle size determined from average coordination numbers. Assuming a spherical shape, the range of sizes corresponding to the error bars in the coordination numbers (Table 2) is given in the parentheses.

reduced at 300 °C. Such particles could be accommodated inside the zeolite supercages. The corresponding sizes for the Pt/NaY500 and Pt/NaY650 catalysts are 23 and 47 Å, respectively. Such large particles cannot reside inside the supercages without damaging the zeolite framework. The particle sizes listed in Table 4 are determined from the average coordination numbers, and the ranges are estimates based on the error bars of coordination numbers (Table 3). Those determined from the first shells are higher than those determined from a higher shell (second or third). This is especially the case for the Pt/NaY500 and Pt/NaY650 samples. This could be due to the fact that the particles are nonuniform (size and shape). If a significant fraction of the platinum is forming small particles ( 45-50%, hydrogen migrates easily from one particle to another. Then the coverage of particles increases more homogeneously and the chemical shift increases linearly with NH2. The end of the plateau (point B) corresponds to about 48% recoverage of all particles. (In reality, if we deduct the spillover concentration, this coverage must be somewhat less by 5-10%.) The abscissa of point B is NB ) 0.59 × 1019/g and 0.44 × 1019/g for Pt/NaY300 and Pt/NaY500, respectively. The position of this plateau depends on the particle size, as has been shown by de Menorval et al.22 and confirmed by Gay23 and Haul.24 According to these previous results, the position of AB for Pt/NaY300 and Pt/NaY500 at -31.2 and -18.7 ppm shows that the mean particle diameter is less than 10 Å and close to 18-20 Å, respectively. For Pt/NaY650 we could not obtain this plateau because it is impossible to detect the signal at low H2 concentrations on account of the big size of the particles. So it is not possible to determine the particle size of Pt/NaY650 by this method. 129Xe NMR. The spectrum of xenon adsorbed in NaY consists of a single line whose chemical shift, δ(NaY), increases linearly with pressure PXe. The spectrum of xenon adsorbed in Pt/NaY300 and Pt/NaY500 consists of two lines: The first, ∝ has a chemical shift equal to δ(NaY) at the same pressure, and it is characteristic of the supercages without platinum. The second line, a, more shifted than ∝, corresponds to a zone, “a”, in which there are supercages with Pt particles. When hydrogen is adsorbed with NH2 < NB, one detects a third signal b whose chemical shift δ(b) is between δ(NaY) and δ(a) and which is characteristic of zone “a” but where the Pt particles have adsorbed hydrogen. The H2 chemisorption creates two zones in the NMR tubes: one includes the particles bearing H2 (with a coverage θ ∼ 0.5), and the other bare particles.19 When NH2

Pandya et al. increases (