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J. Phys. Chem. 1996, 100, 16310-16317
Support and Temperature Effects in Platinum Clusters. 1. Spatial Structure Boyan I. Boyanov*,† and Timothy I. Morrison Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616 ReceiVed: May 14, 1996; In Final Form: June 26, 1996X
The effects of Brønsted acidity on the spatial structure and electronic properties of platinum clusters supported on zeolite Y have been examined with X-ray absorption (XANES and EXAFS) and X-ray photoelectron spectroscopy. The clusters contain 10-25 Pt atoms on average, with a nearest-neighbor distance of 2.70 ( 0.01 Å. Static disorder in the atomic distribution-while certainly present in the supported metal-is shown to be symmetric on average. Increasing Brønsted acidity of the zeolite support has no measurable systematic effect on the spatial structure of the clusters but results in reproducible 5-10% enhancement of near-edge features in the L2,3 X-ray absorption spectra, as well as 0.2-0.3 eV shifts in Pt 4f and 4d core-level binding energies and valence-band thresholds. Evidence is presented that the interaction between the electronic levels of the cluster and the support is not dominated by charge-transfer effects but results in the creation of unoccupied antibonding states above the Fermi level. Structure of atomic origin isolated from the EXAFS data exhibits no dependence on the measurement temperature and the acidity of the zeolite support. The implications of these results for current XANES-based methods for estimation of valence-band charge count are discussed.
I. Introduction Nanodispersed supported metal clusters have been widely used over the past 20-30 years as heterogeneous catalysts. Such clusters are known to catalyze commercially important reactions.1-3 Various metal species and supports have been used, with Pt supported on zeolites, alumina, and silica being some of the more popular choices, especially in the petrochemical industry. This is due to the relative ease of preparation of such systems and their ability to catalyze relevant hydrocarbon transformation reactions. Besides being commercially important, supported metal clusters are interesting from a fundamental point of view. Because of their small size and easily modifiable environment, they are well-suited for the study of metal-adsorbate interactions.2 Their properties can be expected to be intermediate between those of metals and isolated atoms and may be used to study effects such as the onset of metallic behavior. For example, in comparison to bulk metals, clusters with fewer than 100 atoms have lower melting points, shorter near-neighbor distances, and possibly a different crystal structure.6 In addition, recent work has suggested that such clusters may undergo a metal-insulator transition at temperatures below 80 K7 and may oxidize under conditions in which the bulk metal is completely inert.8 With ever-increasing capabilities of electronic computers, the properties of clusters with 5-50 atoms are amenable to detailed theoretical calculations9 and may be used as a testing ground for new theories of chemical effects and electronelectron interactions.10 Zeolites have been particularly attractive as cluster supports in practical applications. Their well-defined pore structure provides spatial constraints that prevent cluster agglomeration at elevated temperatures and the decline in catalytic activity that accompanies such agglomeration. Even though substantial effort has been devoted to the elucidation of the morphology and electronic properties of zeolite-supported Pt clusters,2 several issues are still unresolved. * To whom correspondence should be addressed. † Present address: Department of Physics, North Carolina State University, Box 7518, Raleigh, NC 27695-7518. X Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01400-1 CCC: $12.00
It is necessary to elucidate further the spatial structure of supported clusters of diameter less than 10-15 Å. Larger clusters (diameter >15-20 Å) have been modeled numerically12,13 and have been characterized with direct probes, such as high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). These clusters have been shown to possess the usual face-centered cubic structure and bond lengths.6,14-17 However, smaller clusters (diameter of 10-15 Å or less) are not suitable for HRTEM and XRD studies. Other probes, e.g., extended X-ray absorption fine structure (EXAFS), wide angle X-ray scattering (WAXS), nuclear magnetic resonance (NMR), and chemisorption, have been used in such cases.3-5 Typical estimates on the number of atoms in the cluster vary from 6-10 to 30-40.2,4-6 This is often the case even for clusters prepared under similar, if not identical, experimental conditions. The origin of the apparent bond contractions in small clusters is uncertain. Near-neighbor distances determined from EXAFS and WAXS measurements of “bare” (i.e., without adsorbed gases) clusters show contractions of approximately 0.07 Å (2.5%) relative to the bond length in bulk Pt (2.77 Å). When not accounted for explicitly in the modeling of EXAFS data, asymmetric disorder in the atomic distribution will cause contractions in the apparent near-neighbor distances and erroneous results for the coordination numbers.19,20 Available experimental data for supported Pt clusters suggest that static disorder and anharmonic potentials may make a nonnegligible contribution to the measured signal. For example, the EXAFS Debye-Waller (DW) factor, which is a measure of the rootmean-square variation in the bond length in the cluster, is known to be anomalously high for supported Pt clusters, has a stronger temperature dependence than bulk Pt, and has nonnegligible extrapolated zero-temperature value.18,21 Studies of supported Pt clusters at elevated temperatures (300-400 °C)18 have detected the characteristic signature of asymmetric disordersk3 terms in the phase of EXAFS data.22 Similar results have been obtained for Rh and Ru clusters on various supports.21 Amplitude and phase mismatches in published fits3 also raise questions about possible disorder contributions to the apparent contractions. In addition, the need to quantify any static disorder © 1996 American Chemical Society
Support and Temperature Effects in Pt Clusters. 1 in the atomic distribution is reinforced by the fact that such disorder is known to have a dramatic effect on the electronic structure of noble metals.23,24 The question of unoccupied d-character states and charge transfer in supported metals has been the subject of some controversy. Early X-ray absorption near-edge structure (XANES) results indicated variations in the white line area of the supported metal as large as 50% when the acidity of the zeolite lattice was varied through proton and/or multivalent ion exchange.25 XANES modifications were correlated with enhancements in the catalytic activity of the clusters. It was suggested that charge transfer from the cluster to the support, which can explain the variations in the XANES, is also responsible for the enhanced catalytic activity.25,26 However, XANES results obtained by other researchers demonstrated that the variations in the white line area are much smaller than initially thought and generally do not exceed a few percent.27,28 The discrepancy with earlier results was attributed to “thickness effects”.29 In addition, a recent communication by one of the original authors of the charge transfer model found no systematic changes in the X-ray absorption white line of the supported metal upon multivalent ion exchange.3 The primary supporting evidence for the charge-transfer model has been the systematic trend observed in XANES white lines and XPS binding energies (BE) of the supported metal, e.g., as a function of the type and quantity of secondary exchange ions in the zeolite. In the case of XANES studies this interpretation is based on the assumption that the shape of the absorption spectrum in the XANES region is dominated by the electronic structure and the empty partial DOS (of appropriate symmetry) of the absorbing atom. However, exceptions to this “rule” are known even for gross features in the spectrum,31,32 which casts some doubt on the common assumption that the small variations (≈5-10% change27) typically observed in supported clusters are due solely to charge-transfer effects. Detailed investigations of the anomalous temperature dependence of the white line of supported Pt clusters28 suggest that scattering and other effects may make a nonnegligible contribution to the absorption spectrum in the XANES region. The fact that the magnitude of effects in the L2 XANES spectrum of supported Pt is at least twice as large as for effects in the L3 spectrum33,34 also poses difficulties for the chargetransfer model, especially when the substantial spin-orbit splitting of the Pt 5d levels (1.5-2.5 eV) and the predominant d5/2 character of 5d holes and occupied states are taken into account.9,35 For example, recent studies of Au overlayers on In show that for Au coverage of as high as six monolayers the Au 5d3/2 line remains virtually unperturbed, while the 5d5/2 line broadens and shifts to lower BE.36 It has also been suggested that at least part of the increase in the line width of spectral features observed for clusters at low coverage, in both XPS and XANES spectra, is a result of the existence of an inhomogeneous distribution of adsorption sites for small clusters.37 This raises questions about the limits of applicability of currently existing methods for estimating d-band occupancy from XANES difference spectra.33 The origin and interpretation of core-level XPS BE shifts are also uncertain. Early XPS studies of supported platinum clusters reported BE shifts that were in apparent agreement with the charge-transfer model.38,39 However, it is still unclear whether the shifts are due to initial- or final-state effects, with authors favoring both the former37,40,41 and the latter.42,43 Recent calculations have demonstrated that the correlation between BE shifts and charge transfer in adlayer systems is unreliable.44 In particular, it was shown that, unlike bulk alloys, core-level BE
J. Phys. Chem., Vol. 100, No. 40, 1996 16311 shifts in adlayer systems are often dominated by effects other than charge transfer and may have the same sign in both the adlayer and the substrate. Such behavior has been documented even for bulk zeolite systems without supported metals.45 Since the electrostatic (Madelung) potential and dielectric response of the zeolite framework are strongly affected by exchange ions,46 relaxation (screening) effects are also likely to play an important role in determining the overall shape and position of core-level lines.47 The purpose of this work is to investigate the above questions with the methods of X-ray absorption and photoelectron spectroscopy and attempt to answer as many of them as possible. In particular, the authors will show the following: (i) Zeolite-supported Pt clusters prepared and measured under “standard” conditions (see section [IIA) contain 10-25 atoms on average, in agreement with recent measurements by other researchers.3-6 (ii) Static disorder, while certainly present in supported clusters, is symmetric on average. Through the use of ab initio numerical modeling it will be demonstrated that data processing artifacts cannot affect this conclusion. Thus, up to a small correction for the large DW factor, the measured contraction in the cluster bond length is intrinsic. (iii) Increasing zeolite framework acidity results in small (510%) systematic modifications of the X-ray absorption white line of the supported metal. The observed modifications can be explained in terms electronic effects, though not necessarily charge transfer to the support. Evidence will be presented that the increase in the unoccupied DOS is due to antibonding states created by the interaction of the cluster with the support. (iv) Previously reported temperature effects in the XANES spectrum of supported Pt28 are confirmed. However, the details of redistribution of spectral weight as a function of temperature suggest that the temperature dependence of the XANES spectrum is not caused by charge transfer between the cluster and the support. This and other evidence pointing to the presence of measurable XANES effects not attributable to charge transfer suggests that presently used XANES-based methods for the quantitative estimation of d-charge count may not be applicable to supported metals. (v) Structure of atomic origin, AXAFS,10,48 is clearly present in absorption data from supported Pt clusters, and it is also possible to identify double-electron excitation thresholds. The atomic origin of this effect is confirmed by the lack of temperature dependence in the observed structure over an extended energy range. However, AXAFS appears to be insensitive to small changes in the electronic properties, at least within the resolution of this work. This paper is divided in two parts. Part 1 describes the general sample preparation and characterization procedures and presents an analysis of the spatial structure of the supported metal. Part 2 is devoted to the elucidation of the effects of zeolite acidity on the electronic structure of the supported metal.49 II. Experiment and Data Reduction A. Sample Preparation. All samples used in this study were prepared from commercial Na-exchanged zeolite Y (UOP Molecular Sieves). The zeolite was washed in NaCl solution (500 mg of NaCl per gram of zeolite) prior to ion exchange to remove any potential Na deficiency found in some commercial zeolites.50 In order to investigate the effects of zeolite acidity on the electronic structure of supported Pt clusters, some of the samples were first exchanged with NH4Cl. The NH4Cl exchange was
16312 J. Phys. Chem., Vol. 100, No. 40, 1996
Boyanov and Morrison
TABLE 1: Chemical Composition of the Zeolite Samplesa loading (wt %) sample
Pt
Nab
unit cell formula
Pt4H3 Pt4H7 Pt4H11 Pt4H20
5.6 ( 0.2 5.9 ( 0.2 6.0 ( 0.2 5.8 ( 0.2
7.6 6.9 6.3 4.9
Pt4H3Na45(AlO2)56(SiO2)136 Pt4H7Na41(AlO2)56(SiO2)136 Pt4H11Na37(AlO2)56(SiO2)136 Pt4H20Na28(AlO2)56(SiO2)136
a The samples are labeled according to the convention Pt H , where m n m and n are respectively the number of Pt atoms and protons per unit cell. b See text for a discussion on error estimates for Na concentrations.
carried out in water solution for 24 h at room temperature at concentrations of 25, 50, and 75 mg of salt per gram of zeolite. This corresponds to nominal loadings of 8, 16, and 24 [NH4]+ ions per unit cell, which is well below the known maximum exchange levels for NaY zeolite51 (38 [NH4]+ ions per unit cell). The samples were then washed free of Cl- ions, dried in air overnight, and converted to protonic form according to procedures described below. Platinum loading was achieved through room-temperature ion exchange in water solution with 85 mg of [Pt(NH3)4]Cl2 (Alfa) per gram of zeolite for 24 h, corresponding to a nominal loading of approximately four [Pt(NH3)4]2+ ions per unit cell. Following the exchange, the zeolite was washed free of Cl- ions and dried in air overnight. The chemical compositions of the precursor and the final products were determined by Galbraith Labs (Knoxville, TN). Vacuum drying at 110 °C (2 h) prior to analysis was requested for all samples. The unit-cell formula of the precursor is Na56(AlO2)56(SiO2)136, corresponding to Si/Al ratio of 2.43. The compositions of the final products are given in Table 1, where they are labeled according to the convention PtmHn, where m and n are respectively the actual number of Pt2+ and H+ ions per unit cell. The uncertainty in the Pt loading is at most (1 and is limited primarily by the reproducibility of the Galbraith measurements (typically (0.2 wt %). The uncertainty in the proton loading, which was determined from the difference in +2 }. This Na loading of the precursor and the samples, is { -4 includes contributions from the uncertainty in the hydration state of the zeolite at the time of the measurement and the possibility of Na washing.52 All NH4Cl-exchanged samples (Pt4H7, Pt4H11, and Pt4H20) were converted to protonic form according to well-established procedures53,54 prior to Pt exchange. Small batches of zeolite (
