Pd-Loaded Mesoporous Silica

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Surface and Structural Features of Pt/Pd-Loaded Mesoporous Silica-Delaminated Zirconium Phosphate Systems B. Pawelec, S. Murcia-Mascaro´s, and J. L. G. Fierro* Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, Cantoblanco, 28049 Madrid, Spain Received March 5, 2002. In Final Form: June 21, 2002 Mixed ZrPO4‚O2P(OH)2‚2H2O-SiO2 (ZrPSi-x) mesoporous materials containing variable amounts of SiO2 (x ) 53.4-90.8 wt % SiO2) were prepared by the sol-gel method. The original γ-zirconium phosphate (γ-ZrP) material was exfoliated in a water-acetone mixture before mixing with the desired amounts of tetrapropylammonium hydroxide and tetraethyl silicate solution. The textural properties of these calcined solids were evaluated from the nitrogen adsorption-desorption isotherms. The values of the BET area increased with SiO2 loading and reached a maximum value for a SiO2 loading of 79.0%, after which they decreased strikingly at higher loadings. However, pore size distributions were quite similar (3.4-4.5 nm) and narrow in all the samples. Structural characterization of these materials by X-ray diffraction revealed that the positions of the peaks are virtually the same for all the substrates, thus confirming that the γ-ZrP layers remain unaltered during the exfoliation-flocculation and silica-deposition processes. Upon further impregnation of these materials with platinum and palladium salts, nanoparticles of Pt and Pd developed on the support surface. Pt/Pd-ZrPSi-x catalysts were characterized using chemical analysis, nitrogen adsorption-desorption isotherms at 77 K, transmission electron microscopy (TEM), temperatureprogrammed desorption of NH3 (TPD), X-ray diffraction, FTIR spectroscopy of adsorbed CO, volumetric CO chemisorption, and X-ray photoelectron spectroscopy measurements. The sample Pt/Pd-ZrPSi-64 exhibited only one class of small (5.2 nm determined by CO chemisorption) metal particles, its specific area was rather high (335 m2/g), and it exhibited acidic layers regularly distributed on the silica matrix and a fairly homogeneous metal dispersion. The other samples have metal phases deposited on the external surface, mostly on the SiO2 phase. For the Pt/Pd-ZrPSi-64 sample, the FTIR spectra of CO chemisorbed on the metal phases and photoelectron spectra of Pt 4f and Pd 3d core levels preclude the involvement of electronic effects in the supported Pt and Pd nanoparticles, suggesting that the Pd and Pt particles are separated on the support surface. Observation of the simultaneous hydrogenation of toluene and naphthalene in the presence of small amounts of an S-containing compound (dibenzothiophene) confirmed the impact of these structures on reactivity. It was observed that toluene hydrogenation, which is in turn a demanding hydrogenation reaction, is strongly favored in the Pt/Pd-ZrPSi-64 sample, in which small metal particles developed in the close neighborhood of acid sites located on the support. An explanation of the synergetic effect involving H2 dissociation on the dispersed metal particles followed by migration of H-atoms via spillover until the acid sites where toluene molecules are adsorbed is proposed.

1. Introduction Zirconium phosphates are inorganic ion-exchange materials with a layered structure.1 γ-Zirconium phosphate, with a general formula of ZrPO4‚O2P(OH)2‚2H2O (γ-ZrP), has received far less attention than its R-analogue Zr(HPO4)2‚H2O because, unlike the latter, its chemical formulation and structure have been elucidated only recently.2,3 In the single γ-layer of zirconium phosphate, Zr atoms are located on two different planes in a pseudohexagonal arrangement. In this structure, each Zr atom is connected by two types of phosphate groups (Figure 1): the PO4 tetrahedron bridges four different zirconium atoms, and the O2P(OH)2 groups are situated alternately above and below the Zr atom. Each phosphate group has two oxygen atoms bonded with zirconium atoms and two oxygen atoms bonded to an exchangeable proton.4 Thus, * To whom correspondence should be addressed. Fax: +34 91 585 4760. E-mail: [email protected]. (1) Alberti G. In Solid-state Supramolecular Chemistry: Two- and Three-dimensional Inorganic Networks; Alberti G., Bein T., Eds.; Comprehensive Supramolecular Chemistry Series; Lehn J. M., Ed.; Pergamon: Elsevier Science Ltd, 1996; Vol. 7, Chapter 5. (2) Christensen, A. N.; Krogh Anderson, E.; Krogh Anderson, I. G.; Alberti, G.; Nielsen, M.; Lehmann, M. S. Acta Chem. Scand. 1990, 44, 865. (3) Poojary, D. M.; Shpeizer, B.; Clearfield, A. J. Chem. Soc., Dalton Trans. 1995, 111.

from the catalytic point of view, γ-ZrP are interesting materials, since the protons of γ-ZrP can be exchanged with metal ions or polyoxocations5 and show a high degree of acidity arising from O2P(OH)2 groups. The major drawback in the catalytic applications of this layered material is the limited access of reactant molecules to the interlayer region. This problem can be solved by intercalation of polar organic molecules between the ZrP layers.6 Recently, several pillared layered metal(IV) phosphates have been obtained.7-13 Since such zeolitelike mesoporous materials have a large surface area and (4) Colo´n, J. L.; Thakur, D. S.; Yang, C.-Y.; Clearfield, A.; Martin, C. R. J. Catal. 1990, 124, 148. (5) Alberti, G.; Constantino, U.; Murcia-Mascaro´s, S.; Vivani, R. Supramol. Chem. 1995, 6, 513. (6) Olivera-Pastor, P.; Maireles-Torres, P.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A.; Cassagneau, D. J.; Jones, J.; Rozie´re, J. Chem. Mater. 1996, 8, 1758. (7) Perez-Reina, F. J.; Olivera-Pastor, P.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A.; Fierro, J. L. G. J. Solid State Chem. 1996, 122, 231. (8) Alberti, G.; Marmottini, F.; Murcia-Mascaro´s, S.; Vivani, R. Angew. Chem., Int. Ed. Engl. 1994, 33, 1594. (9) Sylvester, P.; Cahill, R.; Clearfield, A. Chem. Mater. 1994, 6, 1994. (10) Alberti, G.; Cavalaglio, S.; Marmottini, F.; Matusek, K.; Megyeri, J.; Szirtes, L. Appl. Catal., A 2001, 218, 219. (11) Wang, Jun; Li, Q.; Yao, J. Appl. Catal., A 1999, 184, 181. (12) Alberti, G.; Cavalaglio, S.; Dionigi, C.; Marmottini, F. Langmuir 2000, 16, 7663. (13) Alberti, G.; Dionigi, C.; Giontella, E.; Murcia-Mascaro´s, S.; Vivani, R. J. Colloid Interface Sci. 1998, 188, 27.

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acid zeolite catalysts, the hydrogenation reaction could take place on metal sites as well as on acid sites17,18 or in the metal/acid interfacial region.19-21Recently, this has been confirmed at our laboratory for Pt/Pd-β-zeolite.22 The dual site behavior was also observed for Ni and Mo supported on alumina-pillared R-zirconium phosphate catalysts,23which showed good performance in tetralin hydrogenation in the presence of 1000 ppm of DBT (45% yield) and, simultaneously, good activity in the ringopening reaction.23 Within the scope of the above, in this study, the bifunctionality of the hybrid Pt/Pd/γ-ZrP-SiO2 catalysts was examined in the simultaneous hydrogenation of toluene and naphthalene mixtures in the presence of dibenzothiophene in order to confirm the involvement of both metallic and acid sites in the hydrogenation reactions. 2. Experimental Section

Figure 1. Structure of γ-zirconium phosphate (ZrPO4‚O2P(OH)2‚2H2O).

high acidity, they have found potential use as supports of catalytic phases or even as actual catalysts.13,14 Nevertheless, the possibility of gaining access to the internal region of the galleries of such pillared materials recently led to the adoption of another elegant option to render the internal area accessible to reactant molecules. This basically consists of the exfoliation of the lamellar zirconium phosphate through intercalation of the wateracetone molecules between the sheets of the γ-ZrP. The process involves an initial swelling, which weakens the van der Waals forces between the sheets, and a final exfoliation under conditions of vigorous stirring. Such an approach was followed in the present work in order to prepare colloidal dispersed γ-ZrP.13 Care must be taken in order to avoid reorganization of the sheets in the original packed structure along the flocculation process. Accordingly, our next step was to perform flocculation in acidic media in the presence of some soluble silica precursors.12 This new class of hybrid γ-ZrP-SiO2 materials exhibits very high specific areas and surface acidities, which make them excellent candidates to support a highly dispersed metal phase. Our aim in the present work was to deposit platinum and palladium particles simultaneously on the surface of the hybrid γ-ZrP-SiO2 materials and characterize their morphology and the resulting surface structures in some detail. Assuming that the metal particles should be in the close neighborhood of the acid sites on the surface of the lamellae, we developed new bifunctional systems, which combine the acidity and hydrogenation/ dehydrogenation abilities of metal sites. As documented in several reports, the metal function is essential for the hydrogenation of aromatic compounds.15,16 However, some previous results have suggested that, over noble metal/ (14) Jacobson, A. In Comprehensive Supramolecular Chemistry; Alberti, G., Bein, T., Vol. Eds., Marie Lehn, J. Ed.; Solid State Supramolecular Chemistry: Two and Three Dimensional Inorganic Networks; Pergamon/Elsevier: Elmsford, NY/Amsterdam, 1996; Vol. 7, pp 315-336.

2.1. Catalyst Preparation. The methodology used for the preparation of γ-zirconium phosphate (hereafter γ-ZrP) was as previously described.13 Exfoliation of γ-ZrP (ZrPO4‚O2P(OH)2‚ 2H2O) was carried out as follows. A 1-g amount of γ-ZrP was suspended in 50 mL of a 1:1 water-acetone mixture and kept under reflux for 15 min.13 To obtain mixed γ-ZrP-SiO2 (hereafter ZrPSi-x, where x is wt % of SiO2) systems with compositions of ZrPSi-91, -84, -64, and -54, different amounts (127, 26.7, 15.5, and 8.15 mL) of tetrapropylammonium hydroxide (TPAH) and tetraethyl silicate (TEOS) solution (15% SiO2) were added dropwise to the exfoliated γ-ZrP dispersion. The final mixture was subsequently coagulated at room temperature by adding 7.5 mL of acetic acid. With the exception of the γ-ZrPSi-64 carrier, to which lithium acetate was added, other materials were prepared by adding KCl (up to a pH close to 5) to the final gel mixture. Finally, the gels obtained were dried at 353 K and then calcined in air at 873 K for 2 h. Pt/Pd-ZrPSi-x samples were prepared by different methods. Two Pt/Pd-ZrPSi-91 and Pt/Pd-ZrPSi-54si catalysts were prepared by simultaneous impregnation of the respective composite carriers (1 g) with 100 mL of water solution containing 0.04 g of PdCl2 and 0.032 g of H2PtCl6‚H2O. The subindex SI refers to simultaneous impregnation. After adsorption equilibrium had been reached, the excess water was removed in a rotary evaporator. Following this, the impregnate was dried at 383 K in air for 4 h and finally calcined in air at 573 K for 4 h (heating rate 4 K/min). Additionally, the two samples Pt/Pd-ZrPSi-54CI (the subindex CI refers to consecutive impregnation) and Pt/ Pd-ZrPSi-64 were prepared by impregnation of the respective supports with 0.04 g of PdCl2. After the above drying and calcination steps, the Pd catalysts were again impregnated with 0.320 mL of the 0.51 M H2PtCl6 solution and then dried and calcined under the same conditions. Only in the case of the Pt/ Pd-ZrPSi-64 sample was the lithium acetate solution added during the first impregnation with PdCl2. The preparation of the Pt/Pd-ZrPSi-84 catalyst differed from that described above for the other catalysts, since this sample was prepared according to the ion-exchange/organic extraction procedure and was used as dried gel. The organic template was eliminated by extraction in ethanol during the exchange reaction with the metals. Briefly, 1 g of 26 wt % of ZrP and 84 wt % of SiO2 were suspended in 100 mL of a solution containing 85 mL (15) Figueras, F.; Gomez, R.; Primet, M. Adv. Chem. Ser. 1973, 121, 480. (16) Basset, J. M.; Dalmai-Imelik, G.; Primet, M.; Mutin, R., J. Catal. 1975, 37, 22. (17) Wang, J.; Quanzi, L.; Yao, J. Appl. Catal., A 1999, 184, 181. (18) Liu, H.; Lei, G. D.; Sachtler, W. M. H. Appl. Catal., A 1996, 137, 167. (19) Chou, P.; Vannice, M. A. J. Catal. 1987, 107, 129. (20) Lin, S. D.; Vannice, M. A. J. Catal. 1993, 143, 539, 554, 563. (21) Rahaman, M. V.; Vannice, M. A. J. Catal. 1991, 127, 251. (22) Pawelec, B.; Mariscal, R.; Navarro, R. M.; Van Bokhorst, S.; Rojas, S.; Fierro, J. L. G. Appl. Catal., A 2002, 225 (1 and 2), 223. (23) Herna´ndez-Huesca, R.; Me´rida-Robles, J.; Maireles-Torres, P.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A. J. Catal. 2001, 203, 122.

Pt/Pd-Loaded Zirconium Phosphate-Silica Systems

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Table 1. Chemical Compositions of the Pt/Pd-γ-ZrPSi Catalystsa catalyst

ZrP

SiO2

Pt

Pd

K

Li

Pt/Pd-ZrPSi-91b Pt/Pd-ZrPSi-84c Pt/Pd-ZrPSi-64d Pt/Pd-ZrPSi-54CId Pt/Pd-ZrPSi-54SIb

5.08 15.90 25.56 40.73 40.29

90.80 79.03 64.13 53.55 53.41

1.14 2.01 2.88 2.29 1.89

1.99 1.87 1.73 1.15 1.66

0.99 1.19 0.01 2.28 2.75

1.08

a Chemical composition (wt %) as measured by chemical analysis. Simultaneous impregnation. c Extraction with Pt and Pd ethanol solution. d Consecutive impregnation (the calcined Pd samples were impregnated with Pt precursor).

b

of ethanol, 15 mL of water-acetone 1:1, 0.320 mL of a 0.51 M solution of H2PtCl6, and ∼0.04 g of PdCl2. The suspension was kept under stirring at 323 K for 24 h, after which the solid was separated by centrifugation and was washed with a wateracetone-ethanol mixture. The drying and calcination steps were the same as those described above for the other samples. The final composition of the Pt/Pd-ZrPSi-x catalysts obtained is reported in Table 1. 2.2. Catalyst Characterization. The chemical compositions of the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Perkin-Elmer Optima 3300DV instrument. The solid sample was treated with an aqueous solution of HF at 363 K. The textural properties of the catalysts were evaluated from the N2 (Air Liquide, 99.994%) adsorption-desorption isotherms obtained at 77 K over the whole range of relative pressures, using a Micromeritics ASAP-2000 apparatus, for samples previously outgassed at 413 K for 18 h. A value of 0.1620 nm2 was taken for the cross section of the physically adsorbed N2 molecule. BET specific areas were computed from these isotherms by applying the BET method over the 0.005-0.25 P/P° range. In all cases, correlation coefficients above 0.999 were obtained. The ZrPSi precursors as well as the fresh and used metalloaded samples were characterized by X-ray diffractometry, according to the step-scanning procedure (step size 0.02°) with a computerized Seifert 3000XRD diffractometer using Cu KR radiation, and a PW 2200 Bragg-Brentano θ/2θ goniometer equipped with a bent graphite monochromator and an automatic slit. Metallic palladium and platinum crystallite diameters (DPt, DPd) were estimated from application of the Scherrer equation to the XRD patterns of the catalysts. XRD line-broadening measurements were carried out using the Pt(111) peak (39.76 or 40.04 in 2θ) and the Pd(111) peak (40.79 or 40.119 in 2θ). The width at half-maximum (fwhm) of these peaks was corrected for instrumental broadening (b). For transmission electron microscopy (TEM) measurements, the reduced (H2, 573 K) and used samples were crushed and dispersed in acetone and then spread on a holey carbon-copper microgrid. X-ray energy dispersive spectroscopy (XEDS), selected area electron diffraction (SAED) patterns, and images were collected on a JEOL FX 200 electron microscope operating at 200 kV. Particle size distribution was evaluated from several micrographs taken from the same sample. Particle size was defined as the average diameter of the particles. The average particle size distribution was calculated using the equation d ) ∑nidi/∑ni, where ni is the number of particles with diameter di and ∑ni is the number of particles used to build the size distribution. Volumetric CO chemisorption isotherms at 303 K were obtained in order to estimate metal dispersion and also particle size. Before measurements, the sample was reduced under H2 at 573 K for 1 h and then was outgassed at 10-5 mbar. After cooling the sample to room temperature, CO was admitted and the adsorption isotherm was measured. The extrapolation to zeropressure of the linear part of the isotherm gives the amount of strongly chemisorbed CO. Surface particle size was deduced from the hypothesis that one platinum or palladium atom chemisorbs one CO molecule under the standard conditions used here. The infrared spectra of chemisorbed CO were recorded with a Nicolet 5ZDX Fourier transform spectrophotometer, working with a resolution of 4 cm-1 over the entire spectral range, and averaged over 100 scans. The samples, in the form of self-

supporting wafers (thickness ∼ 10 mg/cm2), were reduced in flowing hydrogen at 573 K for 1 h and then were outgassed under vacuum at the same temperature for 1 h. After admission of CO at room temperature (30 mbar), the fraction of physically adsorbed molecules was removed by outgassing at room temperature for 15 min. Net infrared spectra of chemisorbed CO were obtained after subtraction of the background spectrum of the solid. On calculating dispersion, we assumed the stoichiometry of CO for either Pd or Pt to be unity. Average metal particle size was estimated using the equation d ) 5/FS (d, metal particle size; F, density of the metal; S, surface area of the metal). The crosssectional areas of Pd and Pt were assumed to be 0.0787 and 0.0800 nm2, respectively.24 Temperature-programmed desorption (TPD) profiles of ammonia were obtained on a semiautomatic Micromeritics TPD/ TPR 2900 apparatus interfaced with a computer. Before TPD experiments, samples of 0.050 g were pretreated in an He (Air Liquide, 99.996%) stream at 383 K for 1 h. Because ammonia can be adsorbed by hydrogen bond or dipolar interactions, ammonia adsorption was determined at high temperature (400 K) in order to overcome NH3 physisorption. TPD experiments were run in a stream of 5% NH3/He (Air Liquide) flow (50 mL/ min) at 400 K for 1 h and then by heating the sample at a rate of 5 K/min up to 800 K. The water evolved was trapped in a KOH trap located just before the TCD. In the case of metal-layered phosphate materials, it is impossible to promote selective desorption, that is, with water, after the sample has been saturated with ammonia because water cannot displace the ammonia interacting even with weak acidic sites. Because of peak overlapping, a computer program was used to resolve the TPD curves and, on the basis of the outcome, the contribution of each peak was calculated. Semiquantitative comparison of catalyst acidities was accomplished using Gaussian treatment. The photoelectron spectra of the spent catalysts were recorded on a VG Escalab 200R electron spectrometer equipped with a hemispherical electron analyzer, using a Mg KR (hν ) 1253.6 eV; 1 eV ) 1.603 × 10-19 J) X-ray source. A PDP 11/04 computer (Digital Equipment) was used to record and analyze the spectra. The samples, previously protected under isooctane in order to avoid exposure to air, were outgassed at 10-5 mbar and then were transferred to the ion-pumped analysis chamber, where residual pressure was kept below 7 × 10-9 mbar during data acquisition. Pt 4f, Pd 3d, Zr 3d, P 2p, and Si 2p core-level spectra were recorded, and the corresponding binding energy (BE) was referenced to the C 1s line at 284.9 eV (accuracy within (0.1 eV). Each spectral region of the photoelectrons of interest was scanned a number of times to obtain a good signal-to-noise ratio. The intensities of the peaks were estimated by calculating the integral of each peak after subtraction of an S-shaped background and fitting the experimental peak to a combination of Lorentzian/ Gaussian lines of variable proportions. 2.3. Activity Measurements. Hydrogenation of aromatic compounds was performed in a continuous-down-flow catalytic reactor. The model feed was composed of the following: 0.007 wt % dibenzothiophene (≈113 ppm S), 2 wt % naphthalene (NP), and 20 wt % toluene (T) dissolved in the hexadecane. Activity tests were performed using 0.25 g of catalyst diluted with SiC. The procedure for catalyst activation involved heating to a reduction temperature of 573 K (heating rate 4 K/min) in an H2/N2 mixture (ratio 1:10 v/v, flow rate 55 mL/min) at atmospheric pressure, followed by isothermal reduction at this temperature for 2 h. Catalytic activities were measured at T ) 498 K, 50 bar of total pressure, weight hourly space velocity (WHSV) of 49.8 h-1, and an H2/feed ratio of 202 L(N)/L. Liquid samples were analyzed by GC with FID (Varian Model Star 3400 CX chromatograph) equipped with a 30 m × 0.53 mm DB-1 column (J&W Scientific). With the exception of unreacted model feed compounds (DBT, NP, T), biphenyl (BP), cyclohexylbenzene (CHB), tetralin, decalin, and methylcyclohexane (MCH) were the only products detected. Activity is described in terms of HDS of DBT and HYD of toluene and naphthalene using turnover frequency (TOF: molecules/atom‚s) data obtained on the basis of exposed metal atoms, calculated from dispersion values. (24) Anderson, J. R. In Structure of Metallic Catalysts; Academic Press: London, 1975; p 296.

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Table 2. Textural Property Data of the Calcined Pt/ Pd-γ-ZrPSi Catalystsa catalyst Pt/Pd-ZrPSi-91 Pt/Pd-ZrPSi-84 Pt/Pd-ZrPSi-64 Pt/Pd-ZrPSi-54CI Pt/Pd-ZrPSi-54SI

SBET (m2/g) Vmesop (cm3/g) d (nm) Vb (cm3/g) 115 639 335 277 282

0.09 0.53 0.44 0.30 0.31

3.4 3.5 4.5 3.5 3.5

30 (41) 180c 92 (187) 78 (101) 79 (101)

a BET area (S BET), mesopore volumes (Vmesop), and average pore diameter (d) from N2 adsorption-desorption isotherms at 77 K. Data for the support are in parentheses. b Volume of nitrogen adsorbed at 77 K for P/P° ) 0.2. c Corresponds to the metal-free substrate.

3. Results 3.1. Chemical and Textural Analyses. The chemical compositions of the Pt/Pd-ZrPSi-x samples are summarized in Table 1. The amount of silica varied over a broad range, while the Pt and Pd loading changed to a lesser extent. The textural properties of all catalysts were evaluated from nitrogen adsorption-desorption isotherms. The BET areas and meso- and micropore volumes together with the amounts of nitrogen adsorbed at a P/P° ) 0.2 are summarized in Table 2. The values of BET areas increased with SiO2 loading up to Pt/Pd-ZrPSi-84 and then decreased dramatically. The same trend was observed for the amounts of nitrogen adsorbed at P/P° ) 0.2 (relative pressure at which the nitrogen monolayer could be formed25). As expected, BET areas decreased upon Pt and Pd incorporation. The greater loss of N2at P/P° ) 0.2 for the Pt/Pd-ZrPSi-64 sample suggests that the metal components and/or Li species are plugging the pore openings.26-28 Figure 2 shows the representative N2 adsorptiondesorption isotherms of the Pt/Pd-loaded ZrPSi-91, ZrPSi84, ZrPSi-64, and ZrPSi-54CI samples. According to the IUPAC classification, the adsorption-desorption isotherms of these samples are of type II, V, IV, and I/II, respectively, whereas their hysteresis loops belong to type H4, H2, H1, and H3, respectively.25 Pore size distributions, which are illustrated in the inset of Figure 2, were derived from numerical analysis of the adsorption data using the Barret-Harkins-Jura (BHJ) method. The inset in Figure 2 indicates a much wider mesopore distribution for the Pt/Pd-ZrPSi-64 sample than those of other samples. From the adsorption branch of the hysteresis loop, slit-shaped pores were deduced for the Pt/Pd-ZrPSi-54CI sample (curve c). This type of pore is typical of layered materials (note that the Pt/Pd-ZrPSi-54CI sample has the greatest ZrP content). In the case of the Pt/Pd-ZrPSi-54CI sample, both meso- and macroporosity were seen to be present. As compared with the cases of other samples, Pt/Pd-ZrPSi91 (curve d) showed the lowest pore volume (0.09 cm3/g), which is consistent with the nonporous structure inferred from its hysteresis loop. From the hysteresis loop of the Pt/Pd-ZrPSi-84 sample (curve a), a typical corpuscular system with no defined pore shape may be tentatively suggested. In this case, the highest BET area (639 m2/g) comes from the porous structure of the silica component. By applying t-plot analysis, the micropore volume of the Pt/Pd-ZrPSi-84 sample was 0.02 cm3/g, whereas for the other samples it was negligible (