Catalytic Probe of the Surface Statistics of Palladium Crystallites

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Catalytic Probe of the Surface Statistics of Palladium Crystallites Deposited on Montmorillonite Bernadett Veisz,† Zolta´n Kira´ly,*,† Lajos To´th,‡ and Be´la Pe´cz‡ Department of Colloid Chemistry, University of Szeged, Aradi Vt. 1, H-6720 Szeged, Hungary, and Research Institute for Technical Physics and Material Science, P.O. Box 49, H-1525 Budapest, Hungary Received October 11, 2001. Revised Manuscript Received March 21, 2002

A viable colloid chemical preparation technique provided a series of low-loaded palladiummontmorillonite catalysts with cubooctahedral Pd particles ranging in size from 1.5 to 6.2 nm in mean diameter. These catalysts were used to test the structure sensitivity of the liquid-phase hydrogenation of styrene to ethylbenzene under mild conditions. An experimental correlation was sought between the specific rate of hydrogenation and the dispersion of the metal. The fractions of high-coordination terrace sites (face atoms) and low-coordination defect sites (edge and corner atoms) were calculated as a function of the crystallite size. The good correlation between the turnover frequencies and the defect site densities suggested that hydrogenation occurs on these defect sites and that terrace sites have only minimal catalytic activity or are inactive.

Introduction The mode of preparation has a significant effect on the morphology, the size and size distribution, the surface atom statistics, and the surface cleanliness of metallic crystallites, which in turn have a strong influence on the adsorptive and catalytic behavior of these crystallites. The (initial) turnover frequency of a catalytic reaction often displays a pronounced dependence on the dispersion of the metal, even if the catalyst samples originate from similar preparation methodologies. For instance, the liquid-phase hydrogenations of a variety of unsaturated hydrocarbons by supported Pd catalysts have proved to be structure-sensitive.1-4 This may occur if the catalytic reaction proceeds at characteristic surface ensembles but the method used to determine the degree of dispersion cannot differentiate between structurally different active centers.5 The catalytic probe of the surface of metallic particles therefore requires adequate surface statistics. The percentages of the three major crystallographic planes of face-centered cubic (fcc) Pd particles supported on alumina were calculated from a combination of H2, O2, and CO chemisorption measurements by Corma et al.6 Augustine and co-workers developed the single-turnover (STO) hydrogenation method, which can distinguish * Corresponding author. E-mail: [email protected]. † University of Szeged. ‡ Research Institute for Technical Physics and Material Science. (1) Carturan, G.; Facchin, G.; Cocco, G.; Enzo, S.; Navazio, G. J. Catal. 1982, 76, 405-417. (2) Gubitosa, G.; Berton, A.; Camia, M.; Pernicone, N. Stud. Surf. Sci. Catal. 1983, 16, 431-438. (3) Deganello, G.; Duca, D.; Martorana, A.; Fagherazzi, G.; Benedetti, A. J. Catal. 1994, 150, 127-134. (4) Jackson, S. D.; Kelly, G. J.; Watson, S. R.; Gulickx, R. Appl. Catal., A 1999, 187, 161-168. (5) Christmann, K. Introduction to Surface Physical Chemistry; Steinkopff-Verlag: Darmstadt, Germany, 1991; pp 193-256. (6) Corma, A.; Martı´n, M. A.; Pe´rez, J. Chem. Commun. 1983, 1512-1513.

between isomerization, direct saturation, and two-step saturation sites of metallic crystallites.7,8 Application of the STO method in alkene hydrogenations over supported Pd, Pt, and Rh catalysts suggested that it is the corner, edge, and adatoms on the surface of the metal particles that play the decisive role in hydrogenation reactions and that face atoms do not participate in these reactions.7-11 Smith et al. arrived at similar conclusions from the stepwise poisoning of supported Pd and Pt catalysts with CS2 until hydrogenation ceased; the poisoned sites correlated with the edges.12 An extension of the STO method to the heterogeneously catalyzed Heck coupling reaction also revealed that arylation takes place on coordinately unsaturated Pd surface sites.13 Le Bars et al. recently proposed a simple, but otherwise sensitive, method for the catalytic probe of the surface of Pd particles by using the Heck reaction.14 They used a series of polymer-stabilized colloidal Pd particles with different dispersions and found a good correlation between the rate of Heck coupling and the number of low-coordination surface sites, in agreement with the results of the related STO experiments.13 In the present study, the test method of Le Bars et al. is applied for the liquid-phase hydrogenation of (7) Augustine, R. L.; Warner, R. W. J. Catal. 1983, 80, 358-368. (8) Augustine, R. L.; Thompson, M. M.; Doran, M. A. Chem. Commun. 1987, 1173-1174. (9) Augustine, R. L.; Baum, D. R.; High, K. G.; Szivos, L. S.; O’Leary, S. T. J. Catal. 1991, 127, 675-697. (10) Mastalir, A Ä .; Notheisz, F.; Barto´k, M. J. Phys. Chem. Solids 1996, 57, 899-901. (11) Mastalir, A Ä .; Notheisz, F.; Barto´k, M. Catal. Lett. 1995, 35, 119-123. (12) Smith, G. V.; Notheisz, F.; Zsigmond, A Ä . G.; Ostgard, D.; Nishizawa, T.; Barto´k, M. In Proceedings of the 9th International Congress on Catalysis, Calgary 1988; Phillips, M. J.; Ternan, M., Eds.; Chem. Inst. Canada: Ottawa, Canada, 1988; Vol. 4., pp 1066-1073. (13) Augustine, R. L.; O’Leary, S. T. J. Mol. Catal. A: Chem. 1995, 95, 277-285. (14) Le Bars, J.; Specht, U.; Bradley, J. S.; Blackmond, D. G. Langmuir 1999, 15, 7621-7625.

10.1021/cm0112542 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/08/2002

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Table 1. Preparation and Characterization of Pd-MM Catalysts catalyst code

K2PdCl4 a [mM]

MTABrb [mM]

palladium hydrosolc [mL]

dPd(TEM) [nm]

r0[mol styrene/ mol Pd min]d

TOFTSe [s-1]

TOFTDSe [s-1]

TOFDSe [s-1]

Pd-MM15 Pd-MM21 Pd-MM26 Pd-MM30 Pd-MM39 Pd-MM46 Pd-MM62

0.40 0.28 0.40 0.80 1.40 2.75 2.75

39.0 19.5 39.0 78.0 117.0 78.0 117.0

72.0 105.0 72.0 36.0 21.0 10.5 10.5

1.5 ( 0.3 2.1 ( 0.5 2.6 ( 0.7 3.0 ( 0.9 3.9 ( 1.2 4.6 ( 1.7 6.2 ( 1.3

824.2 530.4 388.8 310.1 185.2 127.7 71.4

72.8 37.3 27.3 22.4 14.9 11.3 7.7

30.6 22.2 18.2 15.9 11.6 9.1 6.6

36.9 41.9 42.7 44.2 43.0 40.6 39.9

a Reductant: 20-fold excess of NaBH for Pd-MM15; 100-fold excess of NH NH for Pd-MM21, ..., Pd-MM62. b The critical micelle 4 2 2 concentration of MTABr in water at 298 K is 3.9 mM. c Each palladium hydrosol was added to 120 mL of 1 w/w% Na+MM-. d Initial rate of hydrogenation. Solvent: toluene, 1 mL; substrate: styrene, 0.1 mL; S/C ) 2000; 298 K; 230 kPa H2. e Turnover frequencies probed for the ensembles of terrace sites (TS), defect sites (DS), and terrace plus defect sites (TDS).

styrene by montmorillonite-supported Pd catalysts. Methods for the in situ generation of Pt, Ru, Pd, and Cu nanoparticles on montmorillonite were reported earlier,15-21 but a systematic study of the control over the size of the particles and the metal content of the catalysts was not performed. We recently developed an ex situ preparation method22-25 in which ultrafine Pd particles were synthesized prior to the formation of the palladium-clay catalyst, allowing for good control over the size of the preformed Pd particles.26 Application of this novel colloidal preparation technique provided a series of low-loaded palladium-montmorillonite catalysts with Pd particles ranging in size from 1.5 to 6.2 nm. In this way, the relative concentrations of face atoms and edge atoms were systematically altered with the aim of finding a correlation between the specific rate of hydrogenation and the numbers of surface sites of these kinds. Experimental Section Materials. Sodium montmorillonite, Na+MM- (fine fraction: d < 2 µm; cation-exchange capacity: CEC ) 1.13 mequiv g-1), was provided by Su¨d-Chemie AG. Potassium tetrachloropalladate, K2PdCl4 (98%, Aldrich), myristyltrimethylammonium bromide, MTA+Br- (99%, Sigma), hydrazine (5 M in water, Fluka), and NaBH4 (99%, Aldrich) were used as received. Toluene and styrene (99%, Aldrich Chemicals) were freshly distilled before use. Synthesis Procedure of Palladium-Montmorillonite Catalysts. Palladium-montmorillonite (Pd-MM) catalysts were prepared as follows. K2PdCl4 was dissolved in an aqueous solution of MTA+Br-. The formation of surfactant-stabilized metallic Pd particles was induced upon the addition of the reducing agent to the micellar solution. The palladium hydro(15) Harrison, J. B.; Berkheiser, V. E.; Erdos, G. W. J. Catal. 1988, 112, 126-134. (16) Giannelis, E. P.; Rightor, E. G.; Pinnavaia, T. J. J. Am. Chem. Soc. 1988, 110, 3880-3885. (17) Crocker, M.; Buglass, J. G.; Herold, R. H. M. Chem. Mater. 1993, 5, 105-109. (18) Crocker, M.; Herold, R. H. M.; Buglass, J. G.; Companje, P. J. Catal. 1993, 141, 700-712. (19) Kira´ly, Z.; De´ka´ny, I.; Mastalir, A Ä .; Barto´k, M. J. Catal. 1996, 161, 401-408. (20) Malla, P. B.; Ravindranathan, P.; Komarneni, S.; Roy, R. Nature 1991, 351, 555-557. (21) Malla, P. B.; Ravindranathan, P.; Komarneni, S.; Breval, E.; Roy, R. J. Mater. Chem. 1992, 2, 559-565. (22) Kira´ly, Z.; Veisz, B.; Mastalir, A Ä .; Ra´zga, Zs.; De´ka´ny, I. Chem. Commun. 1999, 1925-1926. (23) Mastalir, A Ä .; Kira´ly, Z.; Szo¨llo¨si, Gy.; Barto´k, M. J. Catal. 2000, 194, 146-152. (24) Mastalir, A Ä .; Kira´ly, Z.; Szo¨llo¨si, Gy.; Barto´k, M. Appl. Catal. 2001, 213, 133. (25) Kira´ly, Z.; Veisz, B.; Mastalir, A Ä .; Ko˜farago´, Gy. Langmuir 2001, 17, 5381-5387. (26) Veisz, B.; Kira´ly, Z. Unpublished work.

sol was then added to an aqueous suspension of Na+MMunder vigorous stirring to produce the Pd-MM catalyst. This material was purified in ethanol by repeated centrifugation and redispersion until the excess amphiphiles were removed. Finally, the catalyst was dried in the oven under vacuum. Seven catalyst samples were prepared (coded as Pd-MM15, ..., Pd-MM62) with mean particle diameters ranging from 1.5 to 6.2 nm. The Pd content of each catalyst sample was carefully set to 0.15 w/w%. The concentration conditions of the preparations are listed in Table 1. Measurements. Particle size analysis was performed with the use of a Philips CM-10 transmission electron microscope (TEM) operated at 100 kV. The sample for TEM analysis was obtained by placing a drop of Pd-MM dispersion in toluene onto a standard copper grid covered by a thin Formvar layer, and the solvent was then evaporated off in air at room temperature. The microscope was equipped with a Megaview II digital camera. The size distribution of the particles was determined by using AnalySIS 3.1 software. The average particle diameters with the standard deviations are given in Table 1. High-resolution transmission electron microscope (HRTEM) images were taken with a JEOL-3010 equipment operating at 300 kV and with a nominal resolution of 0.17 nm. A droplet of the parent palladium hydrosol was placed onto a Formvar-covered copper grid and measurements were made after solvent evaporation. The synthesis conditions were designed to produce 0.15 w/w% palladium/organoclay catalysts; this composition was achieved with an accuracy of (4%, as confirmed by inductively coupled plasma atomic emission spectroscopy at 229.7 and 324.3 nm (ICP-AES, Jobin Yvon 24). X-ray diffraction (XRD) measurements were performed by using a Philips PW diffractometer (40 kV, 35 mA, Cu KR radiation). The low-loaded Pd-MM catalysts were not suitable for XRD analysis of the metal. The Pd particles were therefore first concentrated via precipitation by the addition of 2-propanol to the parent palladium hydrosol. Washing of the precipitate with ethanol produced a strongly pyrophoric material. This is an indication of the extremely high sensitivity of the Pd clusters to atmospheric oxygen and of the retention of the nanoscopic character of the primary particles in the precipitate. Measurements on the Pd samples (hkl indices) and the Pd-MM catalysts (basal spacings) were made in the range 35° e 2θ e 90° and 1° e 2θ e 10°, respectively. Catalytic test reactions were carried out in an automated vibration microreactor attached to a constant-pressure hydrogenator.25 Measurements were made at 298 K and under a relative hydrogen pressure of 130 kPa. For each reaction, 1 mL solvent (toluene), 0.1 mL substrate (styrene), and a substrate/Pd molar ratio of 2000:1 were applied. Product analysis was effected by using an SRI 8610 A gas chromatograph. The kinetic measurements were reproducible to within 4%.

Results and Discussion Synthesis of Pd-MM Catalysts. The ultrasonic irradiation of [PdCl4]2- has been proposed for the preparation of Pd particles on an alumina support, an

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efficient catalyst in liquid-phase hydrogenation reactions.27 The precursor [PdCl4]2- was reduced to produce finely divided Pd particles in the presence of a neutral polymer,28 an anionic surfactant,29 a cationic surfactant,30 and sodium citrate.31 These stabilizing agents provided a protective adsorption layer around the nascent particles, thereby preventing interparticle aggregation. Besides the key role in stabilization, cationic surfactants may interact with Pd(II) precursor species before the reduction process is induced. The formation of palladium(II)-surfactant “adducts” between Pd(II) and tetraalkylammonium halides dissolved in organic solvents was recognized earlier.32-34 The existence of stoichiometric laurylammonium35a and cetylammonium salts35b of [PdCl4]2- has also been reported. Our spectroscopic investigations (1H NMR, IR, Raman, and UVvis) likewise indicated that K2[PdCl4] is not the real precursor of the Pd particles.26 In the presence of a large excess of MTA+Br-, [PdCl4]2- underwent a fast ligandexchange reaction with free Br-.35c The square-planar complex anion [PdBr4]2- coupled stoichiometrically with MTA+ cations to produce (MTA)2[PdBr4]. This product precipitated from dilute surfactant solutions (premicellar region) and was then isolated in pure form for analysis. In concentrated surfactant solutions (postmicellar region), however, this compound embedded in, or attached to, free MTABr micelles (analogous to solubilization) with a strong color intensification and a pronounced blueshift (from 291 to 251 nm) in the UVvis spectra. This finding is similar to that reported recently for positively charged Pd(II) complexes bound to anionic surfactant micelles, both electrostatic and hydrophobic interactions being operative in the binding process.36-38 For (MTA)2[PdBr4] bound to MTABr micelles, the high local concentration of the stabilizing agent at the reduction center facilitated the generation of ultrafine Pd particles. The mechanism of particle formation and a systematic study on the control over the particle size with precursor concentration, surfactant concentration, the length of the surfactant tail, and the strength of the reducing agent will be reported shortly.26 Some preliminary results are presented here in Figure 1. As expected, the size of the Pd particles increases with increasing K2[PdCl4] concentration. In contrast, the increase in (27) Okitsu, K., Yue, A., Tanabe, S., Matsumoto, H. Chem. Mater. 2000, 12, 3006-33011. (28) Teranishi, T.; Miyake, M. Chem. Mater. 1998, 10, 594-600. (29) Okitsu, K.; Bandow, H.; Maeda, Y.; Nagata, Y. Chem. Mater. 1996, 8, 315-317. (30) Berkovich, Y.; Garti, N. Colloids Surf. 1997, 128, 91-99. (31) Henglein, A. J. Phys. Chem. B 2000, 104, 6683-6685. (32) Bo¨nnemann, H.; Brijoux, W.; Brinkmann, R.; Fretzen, R.; Joussen, T.; Ko¨ppler, B.; Korall, P.; Neitler, P.; Richter, J. J. Mol. Catal. 1994, 86, 129-177. (33) Arul Dhas, N.; Gedanken, A. J. Mater. Chem. 1998, 8, 445450. (34) Bouquillon, S.; du Moulinet d’Hardemare, A.; AverbuchPouchot, M.-Th.; He´nin, F.; Muzart, J. Polyhedron 1999, 18, 35113516. (35) Gmelin Handbook of Inorganic Chemistry, Supplement Volume B 2: Palladium; Griffith, W.; Swars, K., Eds.; Springer-Verlag: Berlin, Germany, 1989 (a) p 124, (b) p 127, (c) p 204. (36) Cavasino, F. P.; Sbriziolo, C.; Turco Liveri, M. L. J. Phys. Chem. B 1998, 102, 5050-5054. (37) Cavasino, F. P., Sbriziolo, C., Cusumano, M., Gianetto, A. J. Chem. Soc., Faraday Trans. 1 1989, 85, 4237-4246. (38) Calvaruso, G.; Cavasino, F. P., Sbriziolo, C. J. Chem. Soc., Faraday Trans. 1996, 92, 2263-2268.

Veisz et al.

Figure 1. Mean size of Pd particles as functions of precursor concentration (K2PdCl4) and surfactant concentration (MTABr).

particle size with increasing MTABr concentration is surprising. Nevertheless, a similar observation was made for the Stokes radii of nanoscopic Pt clusters stabilized by alkyltrimethylammonium bromide surfactants.39 The background of this trend is, at present, not fully understood. One possible reason is that the rate of adsorption of the surfactant molecules on the nascent particles decreases with increase of the surfactant concentration in the postmicellar region. This explanation assumes that the particles are stabilized by free surfactant molecules and that the kinetics of adsorption is retarded by the micelles. As the palladium hydrosol stabilized by MTA+Brwas added to an aqueous suspension of Na+MM-, the cation-exchange reaction between Na+ and MTA+ rendered the montmorillonite surface hydrophobic. Simultaneously, the released Pd particles were restabilized by adhesion to the silicate layers.22-25 In this way, a hydrophobic Pd-MM material was formed which, after purification and drying, is readily dispersible in organic solvents such as toluene, THF, ethanol, and so forth to produce a stable colloidal suspension. Further, Pd-MM undergoes disaggregation and swelling in organic liquids, thereby facilitating the accessibility of the Pd particles for the catalytic probe molecules in the slurry.22-25 It should be stressed that the hydrophobic Pd-MM catalyst contains surfactant molecules only in the cation-exchange positions of the clay and that these MTA+ species, like the surface-bound Pd particles, are not leached out upon storage or catalytic applications. XRD, TEM, and HRTEM Measurements. XRD measurements suggested that the Pd particles are not regularly intercalated in the montmorillonite host. In a blank experiment, the peak reflecting the basal spacing of the Pd-free organoclay was positioned at 2θ ) 5.88°. For the Pd-loaded samples, this peak position shifted to slightly lower angles (below 4.6°) with a significant broadening of the half-height peak-width as compared to that for the unloaded organoclay. These observations suggest that the particles are predominantly situated on the external surface sites of the clay lamellae. One example of the XRD patterns of the Pd crystallites, corresponding to Pd-MM15, is given in Figure 2. The peaks at the diffraction angles of 40.1°, (39) Yonezawa, T.; Tominaga, T.; Toshima, N. Langmuir 1995, 11, 4601-4604.

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Chem. Mater., Vol. 14, No. 7, 2002 2885

Figure 2. XRD pattern of highly dispersed fcc Pd particles (sample Pd-MM15). Vertical lines indicate the locations of the corresponding Bragg reflections of the bulk metal.40

46.6°, 68.0°, 82.0°, and 86.5° correspond to the {111}, {200}, {220}, {311}, and {222} crystallographic planes of an fcc structure. The broadening of the peaks is related to the small size of the Pd clusters. The locations and the relative intensities of the Bragg reflections are otherwise in accordance with those reported for the bulk state.40 Representative electronmicrographs of the seven PdMM catalysts and the corresponding size distributions of the Pd particles on these catalysts are given in Figure 3. The particles are randomly distributed on the clay surfaces without any appreciable agglomeration. The particles are fairly monodisperse and, in general, the smaller the particles, the narrower the size distribution. Such Pd-MM materials display high catalytic performances in alkene hydrogenations22,25 and in alkyne semihydrogenations.23-25 The similar selectivities experienced for Pd-MM and Pd-HT (hydrotalcite, an anionic clay) suggested that the metal-support interaction does not play an important role in catalytic applications.25 HRTEM measurements provided further information on the structure and the dominant morphology of the Pd particles. Figure 4 depicts a particle (corresponding to Pd-MM46) lying in a favorable projection which is decisive as concerns its shape. A characteristic hexagonal periphery is observed on this image. From the atomic lattice structure, the crystallographic orientation is uniquely determined as [110]. When it is considered that, of the possible low index projections of the few plausible geometries, only the cubooctahedron possesses a hexagonal outer shape in this crystallographic orientation, the shape can be identified as cubooctahedral. Pd particles with such geometry have also been synthesized in other laboratories.28,41-44 (40) Swanson, T. Natl. Bur. Stand. (US) 1953, 539 I, 21. (41) Schmid, G. In Clusters and Colloids: From Theory to Applications; Schmid, G., Ed.; VCH: New York, 1994; pp 183-188. (42) Vargaftik, N. M.; Moiseev, I. I.; Kochubey, D. I.; Zamaraev, K. I. Faraday Discuss. Chem. Soc. 1991, 92, 13-29. (43) Schmid, G. Polyhedron 1988, 7, 2321-2329. (44) Schmid, G.; Harms, M.; Malm, J. O.; Bovin, J. O.; van Ruitenbeck, J.; Zandbergen, H. W.; Fu, W. T. J. Am. Chem. Soc. 1993, 115, 2046-2048. (45) Van Hardeveld, R.; Hartog, F. Surface Sci. 1969, 15, 189-230. (46) Van Hardeveld, R.; Hartog, F. Adv. Catal. 1972, 22, 75-113. (47) Mullin, J. W. Crystallisation; Butterworth: London, U.K., 1972; pp 1-27.

Figure 3. Transmission electronmicrographs and particle size distributions of Pd-MM catalysts.

Surface Statistics of Cubooctahedral Pd Clusters. The selected size interval of 1.5 to 6.2 nm is ideally suited to test the structure sensitivity stemming from the surface geometry because the most dramatic changes in the composition of the surface may be ascribed to this region.45,46 The total number of atoms constituting a perfect crystal is equal to the number of bulk atoms plus (48) Holden, A.; Singer, P. Crystals and Crystal Growing; Heinemann: London, U.K., 1964; pp 121-126.

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Veisz et al.

Figure 4. High-resolution transmission electron micrograph showing a cubooctahedral Pd particle in the [110] projection on an amorphous Formvar layer (sample Pd-MM46).

Figure 5. Combination forms of cube and octahedron. Insertion in the middle: the atomic shell structure of a 3-shell cubooctahedral particle (m ) 4, NT ) 147).

the number of surface atoms

NT ) NB + NS

(1)

velocities. If the growth rates were the same in all directions, the crystal would grow into a sphere. One consequence of the differences in the growth rates of different sorts of faces is the principle that a crystal is surrounded by its slowest-growing faces.48 For a truncated octahedron, the combination of a cube with an octahedron exhibits an octahedral habit, because the size of the octahedron is larger than the size of the cube. Such crystals are bordered by {100} square and {111} hexagonal faces. If the octahedron and the cube are equally developed, the combination form is termed a cubooctahedron, which is bordered by six {100} square and eight {111} triangular faces. The atomic shell structure of cubooctahedral particles from 1-shell (m ) 2) to 5-shell (m ) 6) implies the magic numbers NT ) 13, 55, 147, 309, 561 and, in general,

NT )

1 [10m3 - 15m2 + 11m - 3] 3

(4)

NB )

1 [10m3 - 45m2 + 71m - 39] 3

(5)

NS ) 10m2 - 20m + 12

(6)

The structure of a 3-shell particle is inserted in Figure 5. With the terminology used by Le Bars et al.,14 the surface sites of perfect crystals may be divided into highcoordination terrace sites, TS (face atoms only), and lowcoordination defect sites, DS (edge and corner atoms), as compared to the smooth, coordinately saturated surface of a sphere. For the surface statistics of cubooctahedra, we obtained

where the surface atoms are composed of face, edge, and corner atoms

NDS ) NE + NC ) 24(m - 2) + 12

(7)

N S ) NF + NE + NC

NTS ) 6(m - 2)2 + 4(m - 3)(m - 2)

(8)

(2)

For fcc crystals (as for the present Pd crystallites)45,46

dsph ) 1.105datNT1/3

(3)

where dsph is the diameter of a sphere with a volume NT times the volume occupied by one atom in the unit cell of the crystal, and dat is the atom diameter. In practice, dsph may be taken as the mean diameter of the Pd crystallites obtained from TEM analysis, and dat ) 0.274 nm for Pd; hence, NT can be calculated from eq 3. If m denotes the number of atoms lying on an equivalent edge, including corner atoms, NT and NB are given by a polynomial of the third degree in m, while NS is given by a polynomial of the second order in m. Van Hardeveld and Hartog reported such polynomials for a variety of fcc geometries,45 including the Archimedean truncated octahedron displayed in Figure 5. This kind of geometry was used by Le Bars et al. to probe the surface statistics of Pd particles in the Heck arylation reaction.14 Figure 5 further depicts the passage from {100} cubic (or hexahedral) to {111} octahedral geometry, and vice versa, by a progressive and symmetrical chopping-off of the corners.47 The intermediate semiregular solid forms (truncated cube, cubooctahedron, and truncated octahedron) are combinations of the regular cube and the regular octahedron. In terms of crystal growth, the shape of the crystal is conditioned by the relative growth

where the first term and the second term on the righthand side of eq 7 account for the edge and corner atoms, respectively. The right-hand side of eq 8 is the sum of the numbers of atoms sitting on the square and triangular faces. In view of these equations, the dispersion D of the Pd particles may be expressed in terms of one of the alternative relations

DTS )

NTS NDS NTS + NDS ; DDS ) ; DTDS ) (9) NT NT NT

where DTS, DDS, and DTDS are the dispersions expressed in terms of terrace sites, defect sites, and total (terrace plus defect) exposed sites, respectively. The dispersion values calculated from this set of equations are plotted against the crystallite size in Figure 6. For each kind of surface ensembles (X), the changes of the dispersion (DX ) NX/NT) with crystallite size are greatest for particle diameters of about d < 5 nm. The decay of the DX versus d plot becomes rather insensitive to X for d > 5 nm, owing to the small variations of the relative occurrence of the various types of surface atoms in this range. Catalytic Probe. In the kinetic experiments, the hydrogenation of styrene led ultimately to the formation of ethylbenzene. The kinetic curves for the seven catalysts are displayed in Figure 7. In every case, the

Palladium Crystallites Deposited on Montmorillonite

Figure 6. Surface statistics of cubooctahedral Pd particles plotted against mean particle diameter. The dispersions (D) are expressed in terms of the characteristic surface ensembles: terrace sites (TS), defect sites (DS), and terrace plus defect sites (TDS).

Figure 7. Kinetic curves for the conversion of styrene to ethylbenzene on Pd-MM15, ..., Pd-MM62 catalysts. Solvent: 1 mL toluene; substrate: 0.1 mL styrene; S/C ) 2000; 298 K; 230 kPa H2.

conversion versus time plot is linear over a large range of conversion. This trend implies zeroth-order kinetics with respect to styrene (i.e., the reaction rate is independent of the concentration of the substrate). In fact, this behavior is typical of the liquid-phase hydrogenation of olefins with Pd catalysts under constant hydrogen pressure.1 For each Pd-MM sample, the initial reaction rate, r0, was calculated from the slope of the straight line to give the activity of the catalyst in terms of (mol styrene)/(mol Pd min). The results are given in Table 1. Then, r0 was converted to initial turnover frequencies by using the dispersion data in Figure 6: TOF0 ) r0/D. The TOF values are the numbers of styrene molecules which undergo catalytic transformation per unit time per surface Pd atom, probed for the ensembles of TS, DS, and TDS. It should be noted that the dispersion values actually used in the calculations were slightly different from those presented in Figure 6. Terrace sites and defect sites in contact with the support are not accessible to reactant molecules, and a correction was therefore made in which the numbers of these contact sites were subtracted from the number of total surface sites. To this end, it was assumed that the mode of orientation of the adsorbed Pd crystallites is proportional to the number of square and triangular faces of the cubooctahedra (6:8). This correction proce-

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Figure 8. Normalized (or relative) turnover frequencies in catalytic hydrogenation of styrene to ethylbenzene, plotted against the Pd particle size ascribed to the Pd-MM15, ..., PdMM62 catalysts. The TOF values were calculated from the initial rates of hydrogenation (Figure 7) and the dispersion values (Figure 6) as TOF0 ) r0/D and TOFR ) TOF0(d)/TOF0(dmax).

dure for the exposed surface sites, however, had only a minor effect on the final results. The TOF0 values computed in this way for the different surface sites, TS, DS, and TDS, are listed in Table 1. These data were then normalized with respect to the corresponding TOF0 values obtained for the Pd-MM62 catalyst (possessing the largest Pd particle diameter). The normalized (or relative) turnover frequencies, TOFR, obtained for the seven catalyst samples are displayed in Figure 8. Because it is the number of sites active for the process under investigation, rather than the total number of surface atoms, that is of paramount importance, it may be expected that TOF0 should be constant (alternatively, TOFR should be unity) when the correct ensemble of surface sites is selected for the calculations. Figure 6 reveals that, no matter whether the total surface sites or the terrace sites are regarded as the catalytically active centers for the hydrogenation of styrene to ethylbenzene, TOFR decreases monotonically with increase of the particle size. In contrast, TOFR to a good approximation has a value of unity (alternatively, TOF0 ) 41.2 ( 1.7 s-1), independently of the particle size, if the sites of hydrogenation are the defect sites only. Besides the experimental error in the particle size analysis and the kinetic experiments, the scatter around unity originates from two major sources. First, the Pd particles are not monodisperse. Second, it is unlikely that the Pd particles are perfect crystallites, and thus different kinds of edge atoms, corner atoms, terrace vacancies, and adatoms, all differing in degree of coordinate unsaturation, may be exposed to the surface. Despite the uncertainties attributed to unresolved surface imperfections, the present test method proved conclusive. It is the low-coordination surface sites which take part in the hydrogenation of styrene, and the highcoordination terrace sites do not display any significant contribution to this reaction. Since r0 is independent of the styrene concentration, but is dependent on the hydrogen pressure applied,1 mechanistic considerations suggest that the rate-determining step of the reaction is the dissociative adsorption of hydrogen on coordinately unsaturated surface sites. In fact, the spectra of

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thermal desorption of hydrogen from Pt crystals49a and hydrogen beam reactive scattering studies49b provided evidence that defect sites are far more active than are terrace atoms for the cleavage of the H-H bond. The low solubility of hydrogen in liquid media, including toluene,50 implies a hydrogen-poor environment during liquid-phase hydrogenation and, hence, a low ratio of hydrogen to styrene at the surface. If the reaction of styrene with the Pd-H surface species and the desorption of the hydrogenated product (giving rise to the dissociative adsorption of further hydrogen molecules on edges) are fast, unreacted H atoms cannot migrate from defect sites to terrace sites before addition to the olefinic bond, and accordingly only defect sites are involved in the overall hydrogenation process. This is in accordance with the mechanistic picture of structure sensitivity and insensitivity generally observed for liquid-phase and gas-phase catalytic hydrogenations, respectively.12 Conclusions The controlled colloid synthesis described in the present work provided a series of well-defined Pd-MM (49) Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, NY, 1981 (a) pp 284-295, (b) pp 484-493. (50) Landolt-Bo¨ rnstein, The Properties of Matter in Aggregated States, Part 2, Volume 26, Solution Equilibria; Scha¨fer, K.; Lax, E., Eds.; Springer-Verlag: Berlin, Germany, 1962; pp 1/69-1/73.

Veisz et al.

catalysts. While the metal content was set to 0.15 w/w% for each catalyst, the mean diameter of the Pd particles was varied from 1.5 to 6.2 nm. The surface site densities (face, edge, and corner atoms) of the cubooctahedral particles were calculated as functions of the crystallite size. The test method of Le Bars et al.14 was adapted and applied to test the structure sensitivity of the liquidphase hydrogenation of styrene over these catalysts. The good correlation for the specific rate of hydrogenation involving edges suggested that only low-coordination surface sites are active in this reaction and that highcoordination face atoms are inactive. We propose that the structure sensitivity originates from the low ratio of hydrogen to styrene at the surface. The results of the present catalytic probe are in good agreement with the results of closely related STO and poisoning experiments reported earlier for the liquid-phase hydrogenation of alkenes over supported Pd catalysts. Further, the present work demonstrates the potential of colloid chemical methods for the preparation of shape- and sizecontrolled metal nanoparticles on solid supports ideally suited for surface-chemical studies in heterogeneous catalysis. Acknowledgment. This work was supported by the Hungarian Science Foundation through Grant OTKA T025002. CM0112542