Pd Bimetallic Catalysts

730 195Pt

J. Phys. Chem. 1996, 100, 730-733

NMR of Polymer-Protected Pt/Pd Bimetallic Catalysts Y. Y. Tong,†,‡ Tetsu Yonezawa,§,| Naoki Toshima,§ and J. J. van der Klink*,‡ Institut de Physique Expe´ rimentale, Ecole Polytechnique Fe´ de´ rale de Lausanne, CH-1015 Lausanne, Switzerland, Department of Applied Chemistry, Faculty of Engineering, UniVersity of Tokyo, 113 Tokyo, Japan, and Institut de Chimie Physique Ecole Polytechnique Fe´ de´ rale de Lausanne, CH-1015 Lausanne, Switzerland ReceiVed: June 15, 1995; In Final Form: September 5, 1995X

Colloids of Pt, Pt0.8Pd0.2, and Pt0.2Pd0.8 particles of approximately 2.5 nm diameter with poly (N-vinyl-2pyrrolidone) as protecting agent were dried, and the 195Pt NMR spectrum and nuclear spin-lattice relaxation rates were measured. The spectra indicate that the particles are indeed bimetallic and that the composition of their interior corresponds well to the nominal value. A strong variation of catalytic activity of these systems with composition around Pt0.2Pd0.8 has been reported in the literature. We suggest that this effect is related to corresponding variations in the local densities of electronic states at the Fermi energy on the surface sites of the catalytic metal.

Introduction The shift of the NMR frequency of a given nucleus in a paramagnetic metal (the Knight shift K) is determined by the electron spin magnetization at the nuclear site and forms a local probe of the electron spin susceptibility. For platinum and palladium, the total shift is the sum of a positive part Ks and a negative part Kd, determined by the densities of states at the Fermi energy Ds of s-like and Dd of d-like electronic states. In a perfect crystal of pure platinum, all nuclei experience exactly the same electron spin magnetization and therefore the NMR line is sharp. In a small particle, or in a random bulk alloy, the atomic sites are no longer all equivalent, Ds and Dd vary from site to site, and the NMR line broadens. Under certain assumptions, the values of Ds and Dd at a given atomic site can be deduced1 from measured values of the Knight shift K and of the spin-lattice relaxation rate T1-1 of the nucleus at that site: Ds and Dd now represent the local densities of states (s-LDOS and d-LDOS) at that site. The LDOS values on surface sites play a role in certain theories of chemisorption : in agreement with theoretical predictions, it has, for example, been found that “promotion” of the catalyst with alkali salts increases the surface LDOS,2 whereas hydrogen chemisorption decreases it.3 The 195Pt NMR spectra of oxide-supported platinum catalysts have been explained4,5 by assuming that (mainly) Dd is strongly site-dependent and has a lower value on surface sites than on sites deep in the interior of the particles; the value is supposed to vary exponentially with distance from the surface, the characteristic length being called the healing length λ. The lowfield region of the NMR spectrum is due to surface platinums and the high-field region to atoms in the interior of the particles. The NMR spectrum is fitted to the particle size distribution (obtained from transmission electron microscopy, TEM, micrographs) using the “NMR layer model”.5 In this model, one starts by representing the TEM data as a layer distribution of the atoms: the fractions of atoms in the surface sites (equal to the dispersion of the catalyst); in the †Present

address: Institut de Recherches sur la Catalyse, CNRS, 2, Av. A. Einstein, F-69626 Villeurbanne Cedex, France. ‡ Institut de Physique Expe ´ rimentale. § Department of Applied Chemistry. | Institut de Chimie Physique. X Abstract published in AdVance ACS Abstracts, December 15, 1995.

0022-3654/96/20100-0730$12.00/0

subsurface sites; in the sub-subsurface sites and so on. Site statistics for cubooctahedral particles are used to make the conversion. The NMR spectrum is fitted by assuming that the atoms in the nth layer of the particle (n ) 0 representing the surface, n ) 1 the subsurface layer, and so on) give rise to a symmetric NMR line centered at Knight shift Kn given by5

Kn ) K∞ - (K∞ - K0) exp(-n/m)

(1)

where K∞ is the shift in the corresponding bulk, K0 the shift at the surface, and m the healing factor (to transform m into λ, multiply by approximately 230 pm, the thickness of a layer). The parameters of the fit (K0, m, and the line width for each layer) are obtained by considering spectra for samples with different size distributions. The catalytic activity of polymer-protected Pt1-xPdx bimetallic clusters has been found to vary strongly with composition around x ) 0.8.6 EXAFS data for such systems have been modeled considering 55-atom clusters with a nonrandom distribution of the two components.7 In terms of layer statistics, such clusters have an atom fraction of 1/55 in layer two and of zero in higher-numbered layers. In medium-dispersion Pt/oxide catalysts, the healing length for the NMR frequency is 1.35 atomic layers.5 Roughly speaking, this means that the resonance frequency of a given Pt nucleus is determined by the “compositions” of the nearest and next-nearest atomic layers (the atoms in a monometallic catalyst have either Pt atoms or voids as neighbors). EXAFS on the other hand gives an average composition of the nearest atomic layer (in monometallic catalysts: the average coordination number). Here we present results on bimetallic platinum-palladium systems, hoping that they will contribute to the understanding of the variations in catalytic activity as a function of composition. As a preliminary, we will discuss literature data for bulk Pt1-xPdx alloys, emphasizing the intrinsically local character of the NMR experiment. Analysis of Data for Bulk Alloys In the bulk, platinum and palladium form random alloys Pt1-xPdx at all compositions, and their magnetic susceptibility χ(x)8 and 195Pt NMR parameters (Knight shift K(x), spin-lattice relaxation T1-1(x), and 195Pt-195Pt spin-spin coupling constant © 1996 American Chemical Society

195Pt

NMR of Polymer-Protected Pt/Pd Catalysts

J. Phys. Chem., Vol. 100, No. 2, 1996 731

Figure 1. (a) Variation of s-like, d-like, and total densities of state on “average” Pt sites in bulk Pt1-xPdx with x, as deduced from NMR data. (b) Variation of local susceptibility on Pt sites and on Pd sites, as deduced from the curves in (a) and from experimental data on alloy susceptibilities (ref 8).

J(x))9,10 have been studied quite some time ago. Due to unfavorable nuclear parameters, no 93Pd NMR data are available. To simplify the discussion, we will neglect the (rather considerable) broadening of the NMR line and assume that the “average” values of the LDOS on platinum sites can be deduced from NMR parameters measured at the center of gravity9 or at the maximum10 of the NMR line. As a first step, we separate the experimentally determined value of the Knight shift, Kexp, into two parts: Ks, due to the density of states of s-like electrons Ds, and Kd, due to Dd. (We neglect the orbital contribution to the shift.) To make this separation, we use the experimentally determined value of the spin-spin coupling constant Jexp. It is believed10 that the spin-spin coupling constant J(x) is mainly determined by Ds(x), therefore we assume that Ks is proportional to Jexp:

Kd(x) ) Kexp(x) - Ks(x)

(2)

Ks(x)/Ks(0) ) Jexp(x)/J(0)

(3)

with Ks (0) ) 0.78% and J(0) ) 4.0 kHz. From data for Kexp (x)9 and for Jexp(x),10 and with the equations for Ks and Kd given in ref 1, we deduce the local densities of state Ds(x) and Dd(x) on the “average” platinum site in a bulk Pt1-xPdx alloy given in Figure 1a. To test the consistency, we calculate1 from these densities of state the spin-lattice relaxation times, the Knight shifts, and (using eq 3) the spin-spin couplings shown in Figure 2. The experimentally determined value8 of the susceptibility (as function of composition) χexp(x) is considered to be the sum of a contribution χPt(x) of “average” Pt sites and a contribution χPd(x) of “average” Pd sites:

χexp(x) ) (1 - x)χPt(x) + xχPd(x) calculated1

(4)

χPt(x) can be from the LDOS values in Figure 1a, and χPd, shown in Figure 1b, is found from eq 4. It is seen from Figure 1b that the Fermi energy densities of states on both the platinum and the palladium sites of these alloys vary rather rapidly when x decreases from 1 to 0.8 and more slowly thereafter. We will come back to this result in the Conclusion section.

Figure 2. NMR parameters T1T, K, and J on “average” Pt sites in bulk Pt1-xPdx alloys. The experimental points are from refs 9 and 10; the continuous lines are calculated from the local densities of states in Figure 1a.

Experimental Section For the preparation of colloidal polymer-protected Pt and Pt/ Pd bimetallic clusters, an alcohol reduction method was used.6 Palladium(II) chloride was dissolved in ethanol (1.32 mmol dm-3) and stirred for 1 day, resulting in a clear yellow solution. Hexachloroplatinic(IV) acid (H2PtCl6‚6H2O) was dissolved in pure water (1.32 mmol dm-3); a clear yellow solution formed. The water was purified by a Millipore Milli-Q ultrapure system. Both solutions were mixed at designated mole ratios with pure water and ethanol to form 1 dm3 of water-ethanol (1:1, v/v) solutions having a total concentration of metal ions of 0.66 mmol dm-3. An amount of PVP (poly(N-vinyl-2-pyrrolidone), K-30, MW ) 40 000; Fluka) measuring 300 mg (2.7 mmol in monomeric units, 4.1 times the total amount of metal ions in moles) was added to each solution as a protective reagent. The mixture, in a 2 dm3 flask, was heated to 90-95 °C and refluxed for 2 h in air. The color of the solution changed to transparent dark brown, indicating complete reduction of metal ions. To prepare the NMR samples, the solvent was first evaporated under vacuum at less than 90 °C. The concentrated colloidal cluster solution was then dried under vacuum at room temperature for 1 day, resulting in flake form black polymer films containing metal clusters. Their size distributions are given in Figure 3 a-c. Repeated redissolving and redrying verified that the size distributions are not significantly affected by this procedure. For the NMR experiments, the metal/polymer films were loaded in glass tubes closed with simple stoppers. NMR of Pt1-x Pdx Particles The NMR spectrum and spin-lattice relaxation times of the pure platinum polymer-protected particles are practically the same as those in clean-surface oxide-supported catalysts of similar dispersion (as long as strong metal-support interactions, SMSI, are avoided; for a metal NMR study of the latter, see ref 11). This is a very remarkable result, since it implies that the interaction of the polymer with the surface platinums is weak and/or restricted to a small number of sites. The spectrum predicted using the layer distribution from Figure 4, and all other

732 J. Phys. Chem., Vol. 100, No. 2, 1996

Tong et al.

Figure 3. Particle size histograms for the three polymer-protected catalyst samples used in this work. The number of particles counted is 376 in (a), 346 in (b), and 491 in (c).

Figure 5. 195Pt NMR spectra of polymer-protected catalysts, obtained by the point-by-point spin echo method. The rightmost arrows indicate the resonance position K∞ in bulk samples; the leftmost arrows indicate the position KG of the center of gravity of the spectra. The dashed line in (a) is the spectrum predicted from the site statistics in Figure 4 and parameters of the NMR layer model for Pt/TiO2 catalysts in ref 5.

Figure 4. Site statistics derived from Figure 3 using the method of ref 5 and assuming the lattice parameter of bulk Pt. Fraction of atoms in layer n; n ) 0 gives the fraction of atoms in the surface of the particles, n ) 1 in the subsurface layer, etc. The line is calculated for a monodisperse sample with particle diameter 2.35 nm.

parameters as fitted to spectra of Pt/TiO2 catalysts in ref 5 shows qualitative agreement with the observed spectrum (Figure 5). No NMR data are available for pure palladium catalysts. From magnetic susceptibility studies12 it has been found that the susceptibility of surface atoms is less than that of the bulk. A similar conclusion has been reached for Pd cluster molecules.13 In terms of densities of state D, this means that in Pd particles D is lower on surface sites than on bulk sites, just as in Pt. Therefore we assume that the 195Pt NMR spectra of Pt0.2Pd0.8 and Pt0.8 Pd0.2 particles can also be interpreted with an exponential-healing model, eq 1. In 195Pt NMR spectra of catalysts, the nuclei in a bulk-Ptlike environment (those that have roughly two layers of platinum atoms around them) resonate in the region 1.12-1.14 G/kHz in Figure 5. It is seen that the fraction of such nuclei (the integral of the spectrum over that region; the three spectra are normalized to the same total area) decreases with increasing Pd content. As far as 195Pt NMR goes, the size distributions of our samples are completely characterized by the data in Figure 4. Since the site statistics of layers 0 to 4 are virtually identical, and the absolute values for the deeper layers are very small, the effective size distributions are all the same (and very close to that for a monodisperse sample of 2.35 nm particles). Therefore the differences in the NMR spectra cannot be due to

differences in size distribution: they must be an effect of the alloying. The rightmost arrows in Figure 5 give the (average) resonance position in the corresponding bulk materials.9, 10 It is seen that the high-field edges of the spectra follow these positions very well, as expected from eq 1 for layers with n g 2m. Furthermore, the nuclear spin-lattice relaxation rates at the high-field edges tend toward the corresponding bulk values, see Figure 6. This shows that on the scale of 1-2 healing lengths, the composition of the interior of the particles is to a good approximation that of the overall formula (and therefore the same holds for the surface composition). As explained in the Introduction, the values of K0 and m in eq 1 must be determined by comparing spectra for samples with different size distributions. For the bimetallic compositions these are not (yet) available, and only a range of m values can be found, corresponding to the possible range of values for K0(x), by considering the position KG of the center of gravity of the spectrum. According to eq 1, KG is given by

KG ) K∞ - (K∞ - K0) ∑fn exp(-n/m)

(5)

n

where fn is the fraction of atoms in layer n, given in Figure 4. The experimental values of KG are indicated by the leftmost arrows on the spectra in Figure 5. Allowing K0(x) in eq 5 to be anywhere between 1.090 G/kHz and the maximum of the spectra in Figure 5, we find m ) 0.58-1.52 for x ) 0, m ) 1.49-4.1 for x ) 0.2, and m ) 2.24-9.8 for x ) 0.8. The trend is to increasing healing length (on the Pt-sites) with increasing x. The Korringa products T1T (Figure 6) are essentially temperature-independent in all points of the spectra for x ) 0 and x ) 0.2. In the surface region of the spectrum for x ) 0.8, the T1 is comparatively shorter at low temperature, indicating an increase of the effective density of states. The order of

195Pt

NMR of Polymer-Protected Pt/Pd Catalysts

J. Phys. Chem., Vol. 100, No. 2, 1996 733 EXAFS data for PtxPd1-x samples with mean diameters of 1.51.6 nm.7 Since both the typical length scale and particle diameters are different in the two experiments, the discrepancy may only be apparent. Here we want to point out another possible reason for the rapid variation of catalytic activity with composition around x ) 0.8. As mentioned in the Introduction, some theories of chemisorption focus on the local densities of states at the Fermi energy on the surface sites (surface-LDOS) of the catalytic metal. The present NMR data for bimetallic catalysts are not sufficiently complete to deduce the values of the surfaceLDOS, in the way it has been done for pure platinum catalysts with different surface treatments.2,3 However, we demonstrate that on the interior sites of the bimetallic particles the LDOS varies strongly with composition, just as in the bulk alloys (see Figure 1). It is likely, although not yet proven, that a similar variation occurs in the surface-LDOS, thereby affecting the chemisorption and catalytic properties of the bimetallic surface. Acknowledgment. One of us (T.Y.) thanks the EPFL for a scholarship and another (Y.Y.T.) thanks the Institut de Catalyse for hospitality while this manuscript was completed. This research was partly supported by the Swiss National Science Foundation (Grant 21-31127.91).

Figure 6. The Korringa product (spin-lattice relaxation time multiplied by temperature) as a function of position in the spectra of Figure 5. The solid symbols are for bulk samples (ref 10), and the curve represents the values in Pt/TiO2 catalysts.

magnitude of T1T in the surface region of all three spectra is compatible with the existence of atoms in a metallic environment. Conclusion The 195Pt NMR of polymer-protected particles of Pt1-xPdx with average diameters approximately 2.35 nm shows that there is little interaction between the polymer and the platinum atoms in the surface, that the particles are indeed bimetallic, and that on the NMR length scale (the “healing length”) the average chemical composition of the particles corresponds well to the nominal formula. On this length scale (typically two atomic layers) we do not observe the deviations from random coordination found (on the one-layer length scale) from an analysis of

References and Notes (1) Bucher, J. P.; van der Klink, J. J. Phys. ReV. B 1988, 38, 11038. (2) Tong, Y. Y.; Martin, G. A.; van der Klink, J. J. J. Phys.: Condens. Matter 1994, 6, L 533. (3) Tong, Y. Y.; van der Klink, J. J. J. Phys. Chem. 1994, 98, 11011. (4) Makowka, C. D.; Slichter, C. P.; Sinfelt, J. H. Phys. ReV. B 1985, 31, 5663. (5) Bucher, J. P.; Buttet, J.; van der Klink, J. J.; Graetzel, M. Surf. Sci. 1989, 214, 347. (6) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc., Faraday Trans. 1993, 89, 2537. (7) Toshima, N.; Harada, M.; Yonezawa, T.; Kushihashi, K.; Asakura, K. J. Phys. Chem. 1991, 95, 7448. (8) Treutmann, W. Z. Angew. Physik. 1970, 30, 5. (9) Kobayashi, S.; Launois, H.; Lederer, P.; Froidevaux, C.; Treutmann, W.; Vogt, E. Solid State Commun. 1968, 6, 265. (10) Narath, A.; Weaver, H. T. Solid State Commun. 1968, 6, 413. (11) Bucher, J. P.; van der Klink, J. J.; Graetzel, M. J. Phys. Chem. 1990, 94, 1209. (12) Ladas, S.; Dalla Betta, R. A.; Boudart, M. J. Catal. 1978, 53, 356. (13) van Leeuwen, D. A.; van Ruitenbeek, J. M.; Schmid, G.; de Jongh, L. J.; Phys. Lett. A 1992, 170, 325.

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