C Catalysts Investigated by Physical

DD2137 Venezia, Italy. Received November 9, 1999. In Final Form: February 14, 2000. In this work we have determined, on a series of 0.5% Pd/C catalyst...
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Nanostructural Features of Pd/C Catalysts Investigated by Physical Methods: A Reference for Chemisorption Analysis Giuliano Fagherazzi,*,† Patrizia Canton,† Pietro Riello,† Nicola Pernicone,‡ Francesco Pinna,§ and Marino Battagliarin† Dipartimento di Chimica Fisica, Universita` di Venezia, DD2137 Venezia, Italy, Consultant, Via Pansa 7/c, 28100 Novara, Italy, and Dipartimento di Chimica, Universita` di Venezia, DD2137 Venezia, Italy Received November 9, 1999. In Final Form: February 14, 2000 In this work we have determined, on a series of 0.5% Pd/C catalysts, the palladium particle sizes by the following physical techniques: (i) X-ray diffraction (XRD) line broadening (LB) method, associated with the Rietveld method, (ii) small-angle X-ray scattering, and (iii) transmission electron microscopy. The catalysts, suitably aged at different temperatures (673, 773, 873, and 973 K), had significantly different metal dispersions. Since the XRD-LB technique is not able to measure directly very small metal particles or clusters (roughly e25 Å in size), because they give diffuse X-ray scattering spreading out into the background, we have tackled this problem by means of a suitably tailored Rietveld quantitative analysis. This analysis allowed determination of the Pd fraction “visible” in the Voigtian XRD peaks and its average crystallite size using the LB method. As to the nanoparticle size of the undetectable fraction, an average value of 20 Å was assumed, corresponding to the size of a cubooctahedral perfect cluster of the fourth order. Combining all these data, real effective Pd average particle sizes could be calculated and compared with the corresponding values found by CO chemisorption. It was found that a surface Pd/CO stoichiometry of 2 must be assumed, irrespective of Pd dispersion, to get correct values of the average Pd particle size.

1. Introduction The measurement of metal particle size in supported metal catalysts, related to metal dispersion and to metal surface area, is of the utmost importance to understand catalyst performances, not only for fundamental studies but also in industrial applications. Chemisorption of suitable probe molecules is the most widely used technique, especially in industrial laboratories due to its inherent simplicity and low expensiveness, but it suffers the following main limitations: cannot be used for most spent catalysts, due to surface contamination; strong metal support interaction (SMSI) phenomena (mainly for low content metals on oxides) can drastically modify chemisorption; is problematic or impossible in the presence of alloyed particles; more in general, as chemisorption gives, after having assumed an adsorption stoichiometry, nothing but the number of exposed atoms able to interact with the probe molecules in the chosen experimental conditions, in practice a strict connection may not exist with real metal particle size. X-ray diffraction (XRD) techniques provide an important alternative1,2 to chemisorption, even if they suffer some limitations too. The aim of this paper is to measure, on a systematic series of 0.5% Pd/C catalysts, laboratory aged at different temperatures, the Pd particle sizes by the physical techniques X-ray diffraction line broadening (LB) method, associated with the Rietveld method, small-angle X-ray scattering (SAXS), and transmission electron microscopy †

Dipartimento di Chimica Fisica, Universita` di Venezia. Consultant. § Dipartimento di Chimica, Universita ` di Venezia. ‡

(1) Gallezot, P. In Catalysis, Science and Technology; Anderson, J. A., Boudart, M., Eds.; Springer: Berlin, 1984; Vol. 5, Chapter 4. (2) Matyi, R. J.; Schwartz, L. H.; Butt, J. B. Catal. Rev. Sci. Eng. 1987, 29, 41.

(TEM), this latter from a qualitative point of view, and then to use this information to optimize the procedure for the calculation of metal particle size from chemisorption data. Recently some of us3 have investigated, on this type of catalysts, the relationship between the Pd particle size, as determined by chemisorption, and the catalytic activity in the purification of terephthalic acid. As we decided to use the X-ray techniques as a reference to check the validity of chemisorption data, it is important to assess the limitations of the XRD-LB method and to try to overcome them by suitable procedures. The main point is that with this method it is practically impossible to detect directly very small metal crystallites or clusters roughly less than 25 Å in size, because these objects give diffuse X-ray scattering spreading out into the background.3-6 Due to this phenomenon, the average metal particle size, as determined from the Voigtian XRD peakprofile broadening in highly dispersed metal-supported catalysts,6 can be strongly overestimated because a large fraction of very small particles cannot be taken into account. On the other hand, in catalysts with relatively large metal particle size (some hundreds of angstroms), present for instance in very aged catalysts, the average crystallite size, as measured by the LB method, can be smaller than the true average particle size, as determined, for example, by SAXS, or observed by TEM. This behavior can be ascribed to the formation of multidomain particles, and, as well-known, whereas the LB method measures the (3) Pernicone, N.; Cerboni, M.; Prelazzi, G.; Pinna, F.; Fagherazzi, G. Catal. Today 1998, 44, 129. (4) Riello, P.; Canton, P.; Benedetti, A. Langmuir 1998, 14, 6617. (5) Riello, P.; Canton. P.; Benedetti, A.; Fagherazzi, G. In Advances in X-ray Analysis: Proceedings of Denver Conferences; ICDD: Newton Square, PA, 1999; Vol. 42. (6) Fagherazzi, G.; Canton, P.; Riello, P.; Pinna, F.; Pernicone, N. Catal. Lett. 2000, 64, 119.

10.1021/la991463p CCC: $19.00 © 2000 American Chemical Society Published on Web 04/14/2000

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ordered domain (or crystallite) size, the SAXS technique instead measures the whole particle size.7 Unfortunately, due to the strong interference effects between the scattering of very fine metal particles and that of pores, occurring with supports of very high microporosity, the conventional SAXS technique cannot be satisfactorily employed in highly dispersed 0.5% Pd supported on active carbon, without using the “poremaskant” technique8 (which is, however, a problematic experimental technique indeed). It is reasonable to think that only for poorly dispersed catalysts, having large enough metal particle size, these interference effects may be negligible. Recently, some of us9 have used the ASAXS (anomalous small angle X-ray scattering) technique, for the nanostructural investigation of Au/C catalysts. In such a way the contribution due to the support could be totally eliminated and the differential signal could be attributed to the metal phase only. As a matter of fact, in a 0.2% Au/C fresh catalyst a population of ultrafine metal particles (clusters) with 17 Å average size was determined by ASAXS, while broad haloes below the XRD strongest Au peaks were correspondingly observed.4 This size agrees with a diameter of about 19 Å, which can be calculated for a perfect Au cubo-octahedron cluster of the third order, containing 147 atoms. Since the ASAXS technique cannot be used as a routine technique and since it gives more or less reliable results depending on the element investigated (for example, for palladium it is much less sensible than for gold), it is important to find an X-ray laboratory method able to evaluate quantitatively the scattering due to metal clusters, which do not contribute to the “visible” Voigtian XRD peaks (due to metal crystallites). We have shown4-6 that, using a suitably tailored X-ray Rietveld analysis,10 which is able to separate the different contributions to the background scattering, it is possible to obtain the metal phase amount due to the clusters undetectable by the usual LB method. As a matter of fact, the apparently “hidden” scattering, due to Pd clusters with a size smaller than about 25 Å, could be evaluated with this modified Rietveld method, also in catalysts with a very low metal load.5-6 Moreover, it is possible to assume a reasonable average size for these “hidden” palladium ultrafine particles on the basis of the previous ASAXS result. In fact, this latter result can be considered independent enough of the peculiar metal investigated. Therefore, on the basis of this reasonable hypothesis and taking into account both the metal fraction of the “hidden” clusters (as determined by Rietveld analysis) and the average crystallite size of the “visible” Pd crystallites (as measured by the LB method), an effective average size for the whole Pd particles population (clusters plus crystallites), present in Pd/C catalysts, can be calculated and used to check the corresponding values obtained with CO chemisorption. TEM images have been taken on some samples in order to qualitatively support the results obtained with the other techniques. 2. Theory and Methodology 2.1. Modified X-ray Rietveld Analysis. After separating the experimental intensity contributions Yl(2θ) of (7) Guinier, A. In Small-Angle Scattering of X-rays; J. Wiley & Sons: New York, 1955. (8) White, T. E., Jr.; Kirkin, P. W.; Gould, R. W.; Heinemann, H. J. Catal. 1972, 25, 407. (9) Benedetti, A.; Polizzi, S.; Riello, P.; Pinna, F.; Goerick, G. J. Catal. 1997, 171, 345. (10) Riello, P.; Fagherazzi, G.; Clemente, D.; Canton, P. J. Appl. Crystallogr. 1995, 28, 115.

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each phase from the global X-ray scattering pattern of a multisystem of M phases,10 the fractional weight amount, Wl, of the lth phase, when the chemical compositions of these phases are known, becomes11 nl

Nl Wl )

wil ∑ i)1

(1)

nl

M

Nl ∑wil ∑ l)1 i)1 with

CNl ) limsp f ∞ CNl(sp) ) p

limsp f ∞

Yl

∫0s AP(s)s2 ds nl

(2)

2 ∫0 [|fi0|2 + Iinc ∑ i ]s ds i)1 ∞

where Nl is the number of composition units (unit cells in the case of crystalline phases) of the lth phase constituted of nl atoms; wil is the atomic weight of the ith atom of the lth composition unit; A and P are the absorption and polarization factors, respectively; fi0 is the tabulated atomic scattering factor; Iinc i is the incoherent scattering, which can be evaluated using the analytical expression and the relevant parameters published by Smith, Thakkar, and Chapman12 and must be corrected for the Breit-Dirac factor, for specific absorption effects and for the bandpass function of the monochromator if it is used on the diffracted beam;13,14 s ) 2 sin(θ)/λ, with 2θ the Bragg angle and λ the radiation wavelength; sp is the upper experimental limit of measurement. The proportionality constant, C, only depends on the measurement setup when particle microabsorption effects can be ignored and, therefore, disappears in eq 1. For mixed powders of two components having very different X-ray linear µ absorption coefficients such as those presented by Pd and C (mass absorption coefficient of 206 and 4.6, respectively), this occurs when the product: (µPd - 〈µ〉)d, where d is the average Pd particle size, is less than 0.01.15,16 The microabsorption behavior of our samples will be discussed, where the XRD results are reported. The value of Wl can be estimated by assessing the asymptotic mean value of the ratio CNl(sp) in the range of sp > 1.0 Å-1. To separate the palladium from the carbon scattering, the air-corrected17 diffraction pattern of the catalysts was fitted by using the palladium face-centered cubic (fcc) structure and the experimental diffraction pattern of the active carbon employed as catalyst support (biphase model). For this purpose we have used a suitable equation based on our modified Rietveld approach (eqs 3 and 3a of ref 11). (11) Riello, P.; Canton, P.; Fagherazzi, G. J. Appl. Crystallogr. 1998, 31, 78. (12) Smith, V. H., Jr.; Thakkar, A. J.; Chapman, D. C. Acta Crystallogr. 1975, A31, 391. (13) Ruland, W. Br. J. Appl. Phys. 1964, 15, 1301. (14) Riello, P.; Canton, P.; Fagherazzi, G. Powder Diffr. 1997, 12, 160. (15) Brindley, G. W. Philos. Mag. 1945, 36, 347. (16) Taylor, J. C.; Matulis, C. E. J. Appl. Crystallogr. 1991, 24, 14. (17) Ottani, S.; Riello, P.; Polizzi S. Powder Diffr. 1993, 8, 149.

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Since the integrated global intensities are independent of the atomic spatial ordering,18 this approach is valid for both crystalline and amorphous phases. In the case of crystalline phases, the integrated corrected intensity of the lth phase is related to the scale factor Kl, refined in the Rietveld procedure, as follows

∫

∞ corr I 0 l

2

l

4πs ds ) K

16π2Vcl 180λ3

CNl )

nl

2 ∫0 [|fi0|2 + Iinc ∑ i ]4πs ds i)1 ∞

16π2 1 l K Vcl 180 λ3

(4)

where the particle form factor is

sin(hr) - hr cos(hr)

(5)

(hr)3

V(r) is the particle volume and h ) 2πs. The average particle size can be obtained by fitting the suitably corrected experimental scattering intensities using a particularly flexible function describing the particle size distribution. For this purpose we have chosen the Schultz function defined as

P(r) )

( )

z+1 1 Γ(z + 1) 〈r〉

(z+1)

∫-ttI(h2 + t2)1/2 dt

(

rz exp -

z+1 r 〈r〉

)

(6)

1 kVmCm VgSav NAFm

(9)

where Sav is the average chemisorption stoichiometry (average number of metal atoms linked to one probe molecule), Vm is the molar volume (22414 cm3/mole), Cm is the surface density of metal atoms, NA is Avogadro’s number (6.02 × 1023 atoms/mole), Fm is the metal density (g/cm3), and k is the constant depending on metal particle shape and contact with the support surface. Introducing in (9) the values pertaining to Pd, FPd ) 12.02 g/cm3 and CPd ) 1.27 × 1015 atoms/cm2 (ref 22), and taking k ) 5 (cubic model, which considers a reasonable part of the Pd particle surface in contact with the support, also if it is equiaxic or spheroidal), one obtains

Φav(A) ) 1970

1 VgSav

(10)

if Vg is expressed as cm3/gPd and Φav is connected with metal dispersion and with the metal surface area through well-known equations.23 However, we will restrict our treatment to Φav, because it represents the physical property directly measurable by XRD techniques. From eq 10 it can be seen that Φav, when measured by chemisorption techniques, depends heavily on the value chosen for Sav, which, until known, was arbitrarily assumed either 1 or 2 for CO chemisorption.23 The present work will allow, through the use of advanced XRD techniques, the determination of Φav on the basis of a sound theoretical approach, and therefore the ability to choose the most reliable value of Sav from experimental data. It is worth noticing that Φav is a surface-weighted mean.24 3. Experimental Section

z > -1 with z (>-1) a parameter related to the shape of P(r); for z f ∞ P(r) f δ(r - 〈r〉), where 〈r〉 is the numerical mean radius of the particles. A surface-weighted 〈d〉SAXS particle size can be determined from P(r) using the equation

∫0∞P(r)r3 dr 〈d〉SAXS ) ∞ ∫0 P(r)r2 dr

(8)

where 2t is the slit length and the I(h2 + t2)1/2 intensities, as theoretically calculated in the pinhole geometry and defined by eq 4, can be analytically resolved.20,21 2.3. Chemisorption Analysis. For a supported metal catalyst the average metal particle size, Φav, is related to the volume of gas chemisorbed per gram of metal Vg by the following equation

Φav )

∫ P(r) V2(r) S2(h,r) dr

S(h,r) ) 3

J(h) ∝

(3)

where nl is the number of atoms in the unit cell whose volume is Vcl. It is worth noticing that eq 1 can be considered a generalization of the equation reported by Hill and Howard,19 found by them using a completely different theoretical approach, when all phases are crystalline. The air-corrected carbon pattern used in the refinement was first fitted using a polynomial function of the 5th order and 6 pseudo-Voigt. This particular choice is not important: in fact we only wanted to smooth the function used in the Rietveld analysis. 2.2. Small-Angle X-ray Scattering Analysis. The small-angle intensity profile, I(h), scattered (pinhole geometry) by a dilute system with a continuous normalized distribution P(r) of spherical particles with radius r, is given by the following Fourier transform

I(h) ∝

support has to be suitably subtracted) as follows

(7)

To fit the experimental small-angle intensities, J(h), we have taken into account the slit smearing effect, due to the Kratky camera geometry, so that the numerical fitting procedure involves indirectly the smeared experimental intensities J(h) (from which the scattering of the (18) Ruland, W. Acta Crystallogr. 1961, 14, 1180. (19) Hill, R. J.; Howard, C. J. J. Appl. Crystallogr. 1987, 20, 467.

The “as received” catalyst (sample C1) is a commercial catalyst (registered as D3065 by Chimet Co., Arezzo, Italy) used for the industrial purification of terephthalic acid and is formed by a nominal 0.5 wt % of palladium, supported on active carbon. Effectively, the Pd concentration of this catalyst, as measured by the atomic absorption technique, has been found to be 0.51 ( 0.02 wt %. Five other samples were prepared by thermally treating sample C1 at the temperatures of 573 K (sample C2), 673 K (sample C3), 773 K (sample C4), 873 (sample C5), and 973 K (sample C6) for 24 h, in a controlled atmosphere of (H2/H2O) ) 1, which allows avoiding Pd contamination by carbon atoms from any source. XRD patterns were recorded in air at 295 K, with a step size of 0.05°, on a 10-140° 2θ range. The intensities were collected (20) Aragon, S.; Pecora, R. J. Chem. Phys. 1976, 64, 2395. (21) Riello, P.; Benedetti, A. J. Chem. Phys. 1997, 106, 8660. (22) Anderson, J. B.; Pratt, K. C. In Introduction to Characterization and Testing of Catalysts; Academic Press: Sydney, 1985; p 7. (23) Le Maitre, J. L.; Menon, P. G.; Delannay, F. In Characterization of Heterogeneous Catalysts; Delannay, F., Ed.; Dekker: New York, 1984; p 299. (24) Borodzinski, A.; Bonarowska, M. Langmuir 1997, 13, 5613.

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Figure 1. The XRD pattern of C1, fitted using our Rietveld procedure. In the inset the global diffuse scattering is evidenced, which represents the sum of Compton scattering, thermal diffuse scattering from the Pd phase, and smoothed experimental carbon scattering, suitably scaled. The weighted residuals, defined28 as ∆Yi(weighted) ) (Yoi - Yci)/(Yoi)1/2, are reported at the bottom. The goodness of fit S is 1.1 and the refined Pd unit cell edge, a, is 3.895(2) Å. For reasons of graphics, the air-corrected experimental intensities are indicated by circles spaced at 0.15° in 2θ. in the preset-time mode in accordance with a specific program, which employed higher collection times in the angular ranges where the Pd peaks are defined, and lower times outside these ranges. The Rietveld program, which is an implemented version of the DBWS-9600 code written by Sakthivel and Young,25 takes into account these different collection times when calculating the estimated standard deviation of the optimized parameters. A Philips X’Pert system (PW3020 vertical goniometer and PW3710 MPD control unit), equipped with a focusing graphite monochromator on the diffracted beam and with a proportional counter (PW1711/90) with an electronic pulse height discrimination, was used. Moreover, a divergence slit of 0.5°, a receiving slit of 0.2 mm., an antiscatter slit of 0.5°, and Ni-filtered Cu KR radiation (30 mA, 40 kV), were employed. Air scattering Yair was collected (10 s step-1 and step-size of 0.5°) on the same 2θ range, fitted with a suitable function (two pseudo-Voigt functions and a straight line), and corrected in accordance with the method used by Ottani, Riello, and Polizzi17 taking into account the presence of the sample. The scattering of the same active carbon, used for the manufacture of the “as received” Pd/C catalyst, was also corrected for air scattering before applying it in the refinement. No change in the X-ray pattern of the carbon was observed, even when thermally treated up to 973 K. SAXS measurements were made using a Kratky (PAAR) camera with an entrance slit of 25 µm. Nickel-filtered Cu KR radiation, a pulse-height discriminator, and a proportional counter were used in the preset counting mode (1.6 × 104 counts per angular step) of data collection. Air scattering and absorption effects were taken into account, as usual. CO chemisorption measurements were performed at 25 °C, using a homemade pulse flow system. Prior to measurements, each sample was subjected to a pretreatment involving exposure to hydrogen at 25 °C for 1 h, followed by helium purge at the same temperature for 2 h. TEM images were taken with a JEOL electron microscope (model JEM 200CX) operating at 200 kV. Small aliquots of the powder samples were dispersed in laboratory grade octane (RPE Carlo Erba) by immersion in an ultrasonic cleaner for 30 s, which also helped disrupt possible particle aggregates. Droplets of the suspension were deposited onto carbon-coated, 200 mesh copper grids (TAAB Laboratories Equipment Ltd.) and allowed to dry overnight in a Petri dish prior to introduction in the column. (25) Sakthivel, A.; Young, R. A. In User’s Guide to Programs DBWS9006PC; Georgia Institute of Technology: Atlanta, GA, 1990.

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Figure 2. As in Figure 1, but for C2 sample. S and a are 1.5 and 3.897(1) Å, respectively.

Figure 3. As in Figure 1, but for C5 sample. S and a are 1.7 and 3.895(1) Å, respectively. Table 1. Microstructural Parameters Obtained by XRD Techniques on the Palladium Phase Supported on Active Carbona Rietveld analysis C1 (as recvd) C2 (573 K) C3 (673 K) C4 (773 K) C5 (873 K) C6 (973 K)

WPd (wt %)

xb

0.12 0.30 0.28 0.34 0.50 0.47

0.24 0.59 0.55 0.67 0.98c 0.92c

average particle sizes (Å) 3/2〈d〉s 〈d〉seff1 〈d〉seff2 〈d〉SAXS 73 85 108 144 165 246

21 32 32 42 165 246

24 36 36 47 165 246

n.d. n.d. n.d. n.d. 157 320

a The Rietveld results have a percentage error of about (5%, while the other ones have a percentage error of about (10%. b These figures are obtained with reference to the Pd content of 0.51 ( 0.02 wt %, measured using atomic absorption technique. c For both these catalysts x was rounded to 1.00 (no presence of nanoclusters).

4. Results and Discussion Figures 1, 2, and 3 show, as examples, the Rietveld refined diffraction patterns of C1, C3, and C6. Table 1 reports the palladium weight percentage, WPd (as calculated by eq 1), and the Pd fraction, x, which gives rise to the “visible” XRD peaks, as determined by our approach. The peak intensities progressively increase, as the aging temperature increases. Owing to microadsorption effects, the weight percentage WPd of the most sintered (C6) sample, for which TEM measurements showed a significant number of Pd particles with sizes higher than 500 Å, appeared slightly underestimated. As a matter of fact, WPd apparently decreases from 0.50% of sample C5, to

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Figure 4. TEM image of the “as received” catalyst (C1). The bar corresponds to 250 Å.

Figure 5. TEM image of sample C2 (thermally treated at 673 K). The bar corresponds to 250 Å.

0.47% of sample C6. For both these very sintered samples x was rounded to 1.00, as no Pd crystallite with size less than 25 Å is likely to be present. On the contrary, in the “as received” catalyst (sample C1), only 24% of Pd atoms scatter giving the measurable XRD peaks, as determined by Rietveld analysis. Evidently, the remaining fraction of atoms forms clusters so small that their scattering is diffused into the background. Samples C3 and C4, aged at temperatures up to 723 K, show an intermediate behavior. In fact, an appreciable

fraction of “XRD-invisible” Pd is still present but much lower than that in the “as received” catalyst. The fourth column of Table 1 shows the surface-averaged crystallite diameters, 3/2〈d〉s, as determined with the LB Fourier method,2 by fitting26,27 the 111 Pd peaks with pseudo-Voigt functions. Taking into account the instru(26) Enzo, S.; Fagherazzi, G.; Benedetti, A.; Polizzi, S. J. Appl. Crystallogr. 1988, 21, 536. (27) Polizzi, S.; Benedetti, A.; Fagherazzi G.; Franceschin, S.; Goatin, C.; Toniolo, L. J. Catal. 1987, 106, 483.

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Figure 6. TEM image of the support (active carbon) in a region practically free of Pd particles, as it appears by observing the “as received” C1 catalyst. The bar corresponds to 250 Å.

ment broadening using a suitable reference standard, an analytical Fourier transform, V(s), of the analytical XRD corrected peak profile could be obtained. As indicated by Warren,28 the average surface-weighted size, 〈d〉s, was determined from the intercept, on the s axis, of the tangent of V(s) at s ) 0. It is worth noting that the correction factor of 3/2 transforms the chord mean of a spherical particle into the corresponding diameter.2 Line broadening was entirely ascribed to crystallite size effects. In fact, an accurate analysis of lattice disorder, which is another possible cause of line broadening, cannot be performed on this kind of low-loaded catalysts for which only the 111 peak profile was intense enough to be fitted correctly. It is possible to note in Table 1 that (3/2)〈d〉s systematically increases, as sintering temperature increases. The fifth column of Table 1 reports an effective average Pd particle size, 〈d〉eff s , surface-weighted too, as obtained by eq 11. This equation can easily enough be derived from the general definition of a surface-weighted particle size (see Appendix 1), when Rietveld analysis data and LB analysis data are suitably combined

〈d〉eff s )

1 x 1-x + 20 (3/2)〈d〉s

(11)

where 20 Å corresponds to the diameter of a perfect cubooctahedral cluster of the fourth order, i.e., formed by four atomic shells. It is worth noting that different assumptions on the average size of “hidden” nanoclusters give relatively (see Table 1, where the fifth small changes of 〈d〉eff s , refers to 17 Å cubo-octahedral clusters of column, 〈d〉eff1 s the third order, while the sixth column, 〈d〉eff2 s , refers to 20 Å cubo-octahedral clusters of the fourth order). As a matter of fact, it is possible to see in the TEM micrographs of C1 and C3 catalysts (Figures 4 and 5) a lot of very fine (28) Warren, B. E. X-ray Diffraction; Addison-Wesley Publishing Co.: Reading, MA, 1969; Chapter 13.

Figure 7. A highly magnified TEM image of C1.

particles. These particles appear much darker than the very fine microstructure shown by the support (see Figure 6). Figure 7 shows an higher magnification image, obtained with another transmission electron microscope (Philips CM300, operating at 300 kV), of catalyst C1, where very small Pd particles (about 20-30 Å), clearly appear. Figure 8 shows a TEM image of the most sintered catalyst (sample C6), where, instead, large sintered Pd particles are visible. On the whole, the assumption of 20 Å for the average size of Pd nanoclusters seems reasonable. Table 1 records the average surface-weighted Pd particle sizes, 〈d〉SAXS, as determined by SAXS, for samples C5 and C6 only. In fact, since, for the less sintered C1, C2, C3, and C4 catalysts, we have not used the “pore-maskant”

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Figure 8. TEM image of C5 (thermally treated at 973 K). The bar corresponds to 250 Å. Table 2. Comparison between Average Particle Sizes (Å) from XRD and CO Chemisorption XRD techniques C1 (as recvd) C2 (573 K) C3 (673 K) C4 (773 K) C5 (873 K) C6 (973 K)

Figure 9. SAXS experimental curves of thermally treated C4 and C5 samples, compared with the corresponding one of the support, normalized to the same scale.

technique,7,27 the interference effects between the Pd particles scattering and the pores scattering was too high to allow a correct SAXS analysis. Only for the catalysts containing larger Pd particle sizes, after the carbon scattering subtraction, was it possible to correctly interpret the resulting curve and obtain an average Pd particle size on the basis of the modelistic approach described in paragraph 2.2. Figure 9 shows the corresponding SAXS experimental curves. The suitably normalized experimental curve of the carbon scattering is also shown below the catalysts SAXS curves. As indirect proof of the presence of clusters, it is worth noting that, when Rietveld refinement is performed using very flexible profile functions as the split hyperLorentzian ones (which, however, are not correct for the crystallite scattering description),

CO chemisorption

〈d〉seff2

〈d〉SAXS

Pd/CO ) 1

Pd/CO ) 2

24 36 36 47 165 246

n.d. n.d. n.d. n.d. 157 320

46 70 76 116 300 644

23 35 38 58 150 322

it automatically attributes the XRD peak tails of the most dispersed catalysts, even if very diffuse and asymmetric, to the palladium phase. As a result, the Rietveld quantitative analysis gave, for all the six samples investigated, WPd values close to 0.50%, in such way not distinguishing the contributions of crystallites from those of nanoclusters. values corresponding to an In Table 2 the 〈d〉eff2 s average Pd nanocluster size of 20 Å are compared with those obtained from CO chemisorption measurements when stoichiometries Pd/CO ) 1 and Pd/CO ) 2 are used. It is evident that a very good agreement exists for Pd/CO ) 2, while the assumption Pd/CO ) 1 is clearly unacceptable. In Table 2 the 〈d〉SAXS values of the two most sintered catalysts (C5 and C6) are also reported. These values are in very good agreement with those obtained from the CO chemisorption measurements, but only if the Pd/CO ) 2 stoichiometry is assumed. Recent papers30,31 definitely show that for Pd supported on oxides bridge-bonded CO is the most abundant species, even if for very small particles an increasing strength of the Pd-CO linear bond was found.31 As it was very recently demonstrated32 that bridge-bonded CO species increase when the support acidity decreases, on carbon-supported (29) Young R. A. In The Rietveld Method; Young, R. A., Ed.; Oxford University Press: Oxford, England, 1993; Chapter 1. (30) Voogt, E. H.; Coulier, L.; Gijzeman, O. L. S.; Geus, J. W. J. Catal. 1997, 169, 359. (31) Dropsch, H.; Baerns, M. Appl. Catal. A 1997, 158, 163. (32) Mojet, B. L.; Miller, J. T.; Ramaker, D. F.; Koningsberger, D. C. J. Catal. 1999, 186, 373.

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Pd practically the whole chemisorbed CO should be bridgebonded, thus fully supporting our experimental data.

the University of Cagliari for fruitful suggestions regarding TEM measurements carried out in her laboratory.

5. Conclusions

Appendix 1

From the data obtained by XRD-LB, SAXS, and TEM techniques (Table 1) it is possible to conclude that for highly or fairly highly dispersed catalysts (samples C1, C2, C3, and C4), the Pd crystallite size, as directly obtained by the XRD-LB method only, is strongly overestimated, as shown by the Rietveld procedure (compare 3/2〈d〉s with 〈d〉seff). Only if the Rietveld method is used to determine the Pd fraction present as small nanoclusters is it possible to suitably correct the crystallite sizes, as determined by the XRD-LB method. For these highly dispersed catalysts the SAXS technique fails, unless the very cumbersome “pore-maskant” technique is employed. On the contrary, for very poorly dispersed catalysts (sample C6), the XRDLB method underestimates the palladium particle size, since the particle is very likely constituted by some smaller coherent domains, which are the objects measured by XRDLB (compare 3/2〈d〉s with 〈d〉SAXS). In this case the SAXS technique, which measures the whole polydomain particle size, gives more correct results. For intermediate dispersions (sample C5) the crystallite size, as directly determined by the XRD-LB method is close to the particle size determined by SAXS. As concerns the chemisorption techniques, it has been demonstrated that for Pd/C catalysts, when using pulse flow CO chemisorption at room temperature, the chemisorption stoichiometry to be used for the calculation of the average Pd particle size (or Pd dispersion, or Pd surface area) is Pd/CO ) 2, irrespective of the value of Pd dispersion.

Equation 11 can be obtained from the definition of the average 〈d〉tot surface-weighted particle diameter due to two numerical distributions of large, P1(r), and small, P2(r), particle radii:

Acknowledgment. We are grateful to Ing. M. Cerboni of Chimet Co. (Arezzo, Italy) who supplied us with the “as received” catalyst and with the active carbon used as support. Many thanks are due to Professor A. Musinu of

∫Ptot(r)r3 dr ∫P1(r)r3 dr + ∫P2(r)r3 dr )2 〈d〉tot ) 2 ∫Ptot(r)r2 dr ∫P1(r)r2 dr + ∫P2(r)r2 dr

(12)

Taking into account that x is the unit volume fraction of the large particles and (1 - x) that of the small particles, it is possible to write, under normalization conditions

∫P1(r)r3 dr + ∫P2(r)r3 dr ) x + (1 - x) ) 1 and eq 11 becomes

〈d〉tot )

2

∫ ∫P1(r)r3 dr 2

x P1(r)r dr

+

)

∫

(1 - x) P2(r)r2 dr

∫P2(r)r3 dr 1 (13) x 1-x + 〈d〉1 〈d〉2

where 〈d〉1 and 〈d〉2 are the surface-weighted particle diameters of the large and small particles, respectively. Equation 13 is equivalent to eq 11. LA991463P