NANO LETTERS
Nanoclusters in Nanocages: Platinum Clusters and Platinum Complexes in Zeolite LTL Probed by 129Xe NMR Spectroscopy
2002 Vol. 2, No. 11 1269-1271
Bryan A. Enderle,† Andrea Labouriau,‡ Kevin C. Ott,‡ and Bruce C. Gates*,† Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616, and Chemical Science and Technology DiVision, Mail Stop J514, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received July 11, 2002; Revised Manuscript Received August 17, 2002
ABSTRACT Platinum nanoclusters and mononuclear platinum complexes in zeolite KLTL were characterized by 129Xe NMR spectroscopy at temperatures from 100 to 296 K; the chemical shift increased with decreasing temperature, consistent with xenon’s increasingly strong interactions with the platinum. The room-temperature chemical shift of xenon characterizing the zeolite containing the platinum clusters was 148.7 ppm, but that of the zeolite containing the platinum complexes was only 98.3 ppm, slightly greater than the value representing the bare zeolite, 95.4 ppm. These data, combined with literature data characterizing similar samples, indicate that the 129Xe chemical shift increases with the size of the metal species in the zeolite, up to the point at which entry of the Xe atom into the space containing the metal is constrained by the geometry of the metal species and the zeolite cage; in the limiting case, the chemical shift is substantially reduced because a Xe atom interacts with an encaged nanocluster only through a cage window.
Metal nanoclusters in zeolite nanocages are important catalysts.1 Xenon atoms are sensitive probes of the nanoporous spaces of zeolites (as indicated by the 129Xe chemical shifts measured by NMR spectroscopy2), and they also probe species within the pores.3-14 The available data characterizing zeolite-encaged species are limited by the nonuniformity of these species in most of the reported samples.11 Platinum clusters in zeolite LTL are highly selective industrial catalysts for the conversion of straight-chain alkanes into aromatics,15-17 and the clusters in well-made catalysts of this type are extremely small (roughly 10 atoms each) and nearly uniform, as shown by extended X-ray absorption fine structure (EXAFS) spectroscopy and transmission electron microscopy.15 Our goal was to characterize these encaged nanoclusters (and oxidized species formed from them) by 129Xe NMR spectroscopy and to interpret the results in the context of the nanocluster/nanocage geometry. The samples have already been characterized by EXAFS spectroscopy and transmission electron microscopy.15,16,18 The zeolite, with space group P6/mmm,19-21 had a Si/Al atomic ratio of 2.6, as determined from the peak area ratios of Si(1Al), Si(2Al), etc. determined by 29Si NMR spectroscopy. Preparation and handling of the zeolite were carried * Corresponding author. † University of California. ‡ Los Alamos National Laboratory. 10.1021/nl025697w CCC: $22.00 Published on Web 09/12/2002
© 2002 American Chemical Society
out in the absence of air and moisture on a double-manifold Schlenk line and in a N2-filled glovebox (Braun MB-150M). The zeolite was treated in O2 (Matheson Extra Dry Grade) at 573 K for 4 h and evacuated for 12 h at 10-3 Torr prior to characterization by NMR spectroscopy or Xe uptake measurements. The preparation of the zeolite incorporating platinum clusters (1.2 wt % Pt) is described elsewhere;15,16,18 a fraction of this sample was oxidized by treatment in flowing O2 at 533 K and atmospheric pressure for 4 h followed by evacuation for 12 h at 10-3 Torr. Samples for 129Xe NMR spectroscopy were weighed and packed into 8-mm-diameter glass tubes (Wilmad) in an N2filled glovebox. Approximately 4 Xe atoms per unit cell of the zeolite were condensed into the tube, and approximately 18 Torr of He (99.995%) was added to increase the rate of heat transfer during measurement of the 129Xe NMR spectra, which were collected with a Varian Unity 400 spectrometer operating at 110.629 Hz with a homemade transmission line probe. A 10 µs π/2 width with a 3-s recycle delay was used. The sample in an Oxford model CF 1200 cryostat was cooled in steps from room temperature to 100 K with 20 to 30 min allowed for attainment of constant temperature. The 129Xe chemical shift was measured at each step; the number of transients varied from 200 to 512. Prior to each set of measurements, the chemical shift of an external standard sample of xenon gas at 2.0 atm and room temperature was
Figure 1. 129Xe NMR data representing bare zeolite KLTL and platinum clusters in zeolite KLTL.
measured. The chemical shifts were corrected to that at zero pressure.22,23 A room-temperature adsorption isotherm of xenon in KLTL zeolite (163 mg) was measured with a typical volumetric gas adsorption apparatus. Xenon gas (Matheson, 99.995%) was added to the system containing the evacuated sample, and the initial and final pressures were recorded, the latter attained within 5 min. The xenon was then removed by evacuation for 30 min and the experiment repeated at various initial pressures. The adsorption isotherm representing xenon in zeolite KLTL24 is nearly linear for pressures up to almost 500 Torr, in agreement with the expected Henry’s law behavior.3,5,6,12,25 The 129Xe chemical shift (δXe) is related as follows to the amount of xenon sorbed per unit mass of zeolite: δXe ) δ0 + δ1F + δ2F2 + δ3F3...
(1)
where δ0 accounts for Xe-zeolite interactions; δ1, δ2, and δ3 are virial coefficients of the resonance shift in density accounting for Xe-Xe interactions,3 and F is the mass of xenon sorbed per unit mass of zeolite.26,27 The value of F was calculated from our data to be 4.8 × 1020 atoms/cm3, corresponding to the four atoms of xenon per unit cell of the zeolite. The data analysis was simplified because the higher-order terms (δ2F2, δ3F3,...) are negligible when F is less than 2.7 × 1021 atoms/cm3.26 Only a single resonance was observed for each measurement, regardless of temperature. There was no observed signal corresponding to bulk xenon gas. The signal line width was narrow at room temperature and increased with decreasing temperature; the spectra are similar to those shown by Labouriau et al.10 Figure 1 shows the 129Xe chemical shift for the bare zeolite and that incorporating platinum nanoclusters over the temperature range of 100-296 K. The estimated error in δXe is (0.5 ppm at the highest temperature and approximately (1.5 ppm at the lowest. 1270
The room-temperature 129Xe chemical shifts characterizing the samples containing platinum clusters and platinum complexes formed from them (148.7 and 98.3 ppm, respectively) are greater than that observed for xenon in the bare zeolite (95.4 ppm), consistent with a significant influence of the platinum species on the chemical shift. The roomtemperature 129Xe chemical shift characterizing the encaged platinum clusters (with an average diameter of about 7 Å15) (148.7 ppm) is similar to a reported value of 137 ppm for the zeolite containing platinum clusters (2.0 wt %) less than about 10 Å in diameter.28 Ours is the first report of 129Xe chemical shifts characterizing zeolite KLTL and platinum clusters in it over a wide temperature range. The small difference between the 129Xe chemical shift characterizing the oxidized sample made from the supported platinum clusters and that representing the bare zeolite indicates that the oxidized platinum species occupied only little space in the zeolite, consistent with EXAFS data29 indicating mononuclear platinum complexes (the platinum atoms were coordinated to oxygen atoms in the zeolite framework).24 Similarly, some of the smallest metal clusters on zeolite supports are characterized by chemical shifts only slightly greater than those representing the support (e.g., Ir4 (or Ir6) in zeolite NaY, 72 ppm, compared to the value of 66 ppm characterizing the bare zeolite).9,10 The data indicate that the xenon atoms interacted much more strongly with the platinum clusters than with the mononuclear platinum complexes. The main pores of zeolite LTL are parallel, consisting of relatively wide channels with ellipsoidal cages connected by windows (7.1 Å in diameter, corresponding to a 12-ring).30 The average diameter of the platinum clusters determined by EXAFS spectroscopy (approximately 7 Å)15 implies that they reside in the ellipsoidal cages of the main pore channels, as they are too large to fit within the other pores. The nanoclusters in these cages are accessible to xenon atoms, but, as the maximum channel diameter is about 13 Å and the average cluster diameter about 7 Å, a limit is approached in which the xenon atoms (4.4 Å in diameter) barely fit through a main channel containing a platinum cluster (Figure 2). Thus, we explain the pattern of Figure 2 with a geometrical model, as follows: when the metal species are small, the 129 Xe chemical shift increases with increasing size of this species, indicating the stronger interaction of xenon with the larger metal species. A xenon atom in a cage with a metal species may reside there and interact with the metal for some time, accentuating the chemical shift. However, as the metal species becomes so large as to limit the entry of the xenon atom into the cage with it, the Xe-metal interaction becomes limited. Clusters that nearly fill a cage (those larger than about 8 Å in diameter) prevent the xenon atoms from fully entering the supercages; thus, xenon probes the encaged clusters only through the cage windows, which lessens the interaction and decreases the 129Xe chemical shift.10,11 This pattern explains the maximum in the curve of Figure 2. This geometric argument is similar to that invoked previously.10,11 Now, by plotting the data (including both our Nano Lett., Vol. 2, No. 11, 2002
Acknowledgment. This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences Contract FG02-87ER13790; the NEAT-IGERT project supported by the National Science Foundation (IGERT grant DGE-9972741); and by the U.S. Department of Energy contract No. W-7405-ENG-36 as part of the Los Alamos Catalysis Initiative. We thank J. T. Miller of B. P. Amoco for providing the sample of zeolite-supported platinum. References
Figure 2. Dependence of the 129Xe chemical shift on the volume fraction of the encaged species in the zeolite cage. The 129Xe chemical shift reported for the metal species is relative to that of the 129Xe chemical shift characterizing the bare zeolite. Legend: A, bare zeolite reference; B, mononuclear platinum complex in zeolite KLTL; C, Ir4 in zeolite NaY;10 D, H2Os(CO)4 or a related mononuclear osmium carbonyl in zeolite NaX;11 E, platinum clusters with an average diameter of 7 Å in zeolite KLTL (this work); F, Ir4(CO)12 in zeolite NaY;10 G, [Os5C(CO)14]2- in zeolite NaX;11 and H, Rh6(CO)16 in zeolite NaY.10
observations and those representing other nanoclusters in zeolite nanocages (including metal carbonyls), as shown in Figure 2, we have normalized them, subtracting the 129Xe chemical shift representing the contribution of the respective bare zeolite from that representing the supported metal species; these values are plotted vs the estimated volume fraction of the encaged species, where the diameters of the cages are estimated to be 13 Å for the main channel of zeolite KLTL and 12 Å for zeolites X and Y. We suggest that the pattern of Figure 2 is general enough to allow predictions of the approximate sizes of encaged clusters on the basis of 129 Xe NMR spectroscopy, but we emphasize that it requires nearly uniform clusters in the cages, and these are not easily made. Our interpretation neglects the influence of the composition of the encaged species, which is expected to have some effect on their interactions with xenon. In summary, KLTL zeolite-supported platinum clusters and mononuclear complexes formed from them by oxidation are clearly distinguished from each other by 129Xe NMR spectroscopy, being characterized by 129Xe chemical shifts of 148.7 and 98.3 ppm, respectively, at room temperature. Our data and others indicate a maximum chemical shift of xenon interacting with zeolite-encaged metal species about 8 Å in diameter, with the strength of the interaction increasing until the cluster/cage geometry restricts the entry of xenon atoms into the cage; in the limit, the xenon atoms interact with the encaged clusters only through cage windows.
Nano Lett., Vol. 2, No. 11, 2002
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