CO Average Chemisorption Stoichiometry in Highly Dispersed

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Langmuir 2002, 18, 6530-6535

Pd/CO Average Chemisorption Stoichiometry in Highly Dispersed Supported Pd/γ-Al2O3 Catalysts Patrizia Canton,† Giuliano Fagherazzi,*,† Marino Battagliarin,† Federica Menegazzo,‡ Francesco Pinna,‡ and Nicola Pernicone§ Dipartimento di Chimica Fisica, Universita` di Venezia, Via Torino 155b, I-30170 Venezia-Mestre, Italy; Dipartimento di Chimica, Universita` di Venezia, DD2137, I-30123 Venezia, Italy; and Consultant, Via Pansa 7/c, I-28100 Novara, Italy Received October 25, 2001. In Final Form: January 29, 2002 Following our previous papers regarding Pd/C and Pd/SiO2 catalysts, the determination of the Pd/CO average chemisorption stoichiometry from experimental data has been extended to the Pd/alumina system. To this purpose, the Pd crystallite size determined by the well-established line-broadening (LB) method, using X-ray powder diffraction (XRPD) associated with the Fourier analysis of suitable best-fitted peak profiles, and the chemisorbed CO volume, determined by the pulse flow method, were employed. HRTEM was also used to check the structural features of the Pd particles, which proved to be monodomains of cubooctahedral shape. A high-dispersion 5 wt % Pd/alumina catalyst was prepared by impregnation and thermally treated at different temperatures up to 1073 K. Even at this high temperature a good Pd dispersion was retained. A suitable subtraction of the X-ray scattering, due to the support, was done to obtain the real profile of the supported metal, from which the surface-weighted average particle diameters were calculated. Using these values and the corresponding CO chemisorbed volumes, a Pd/CO average chemisorption stoichiometry close to 2 was obtained. On the basis also of our previous work regarding Pd/C and Pd/SiO2 catalysts, it can be concluded that for all supported Pd catalysts such stoichiometry is close to 2, independent of support nature and Pd dispersion, when chemisorbed volumes are measured by the routinely used pulse flow method.

1. Introduction Chemisorption with CO, as the probe molecule, is a widely used technique, especially in industrial laboratories, for measuring metal particle size in supported Pd catalysts, due to its inherent simplicity. However, to use this method, the exact average chemisorption stoichiometry, Sav, namely the ratio between the total number of surface metal atoms and the total number of CO molecules chemisorbed in the chosen experimental conditions, must be known. This parameter is present in the equation that gives the average metal particle size as a function of the volume of gas chemisorbed per gram of metal.1,2 It is worth noting that IR and NMR spectroscopies clearly show3-6 that CO chemisorbs on supported Pd as both linear and bridge-bonded species, whose ratio may depend both on the nature of the support and on the metal dispersion. It should be also taken into account that a fraction of metal atoms may remain unoccupied, due to the steep decrease in the strength of the chemisorption bond when the surface † Dipartimento di Chimica Fisica, Universita ` Ca’ Foscari di Venezia. ‡ Dipartimento di Chimica, Universita ` Ca’ Foscari di Venezia. § Consultant. * To whom correspondence should be addressed: Phone: +39041-2348545. Fax: +39-041-2346747. E-mail: [email protected].

(1) Le Maitre, J. L.; Menon, P. G.; Delannay, F. In Characterization of Heterogeneous Catalysts; Delannay, F., Ed.; Dekker: New York, 1984; p 299. (2) Fagherazzi, G.; Canton, P.; Riello, P.; Pernicone, N.; Pinna, F.; Battagliarin, M. Langmuir 2000, 16, 4539. (3) Gubitosa, G.; Berton, A.; Camia, M.; Pernicone, N. In Preparation of Catalysts III; Poncelet, G., et al., Eds.; Elsevier: Amsterdam, The Netherlands, 1983; p 431. (4) Sheu, L. L.; Karpinski, Z.; Sachtler, W. M. H. J. Phys. Chem. 1989, 93, 4890. (5) Zilm, K.; Bonneviot, L.; Hamilton, D. M.; Webb, G. G.; Haller, G. L. J. Phys. Chem. 1990, 94, 1463. (6) Zilm, K.; Bonneviot, L.; Haller, G. L.; Han, O. H.; Kermarec, M. J. Phys. Chem. 1990, 94, 8495.

coverage increases.7 Moreover, “3-fold” bonded CO molecules could be present.8 As a consequence of this unfavorable, but often unrecognized, situation, only an average chemisorption stoichiometry can be defined, on the basis of the values of the average metal particle size obtained with well-established physical techniques. On the contrary, in most papers on supported Pd catalysts, when CO chemisorption is used (hydrogen chemisorption gives many problems, due to the formation of Pd hydride and of subsurface hydrogen), Sav is arbitrarily assumed, mostly as unity. In the case of Pd/SiO2 and Pd/C supported catalysts, we have shown 2,9 that a fairly good agreement is reached between particle sizes obtained with the linebroadening (LB) method in the frame, the X-ray powder diffraction (XRPD) technique, and/or the small-angle X-ray scattering (SAXS) technique and those obtained by CO chemisorption only by assuming a value of 2 for Sav. This means that in most previous papers the Pd dispersion was largely underestimated (in some cases by 100%) and correspondingly the particle size overestimated. The error may become still higher when a static volumetric double-isotherm procedure is used, which frequently underestimates the chemisorbed volume. In fact, as the chemisorbed volume cannot be directly correlated to the number of surface atoms, using a complex chemisorption procedure is meaningless. We have used the pulse flow method, which is simpler and less expensive. The aim of the present paper is to determine, on an experimental basis, the Sav value on highly dispersed Pd particles supported on γ-Al2O3, to complete the series of the carriers usual in the industrial practice. In fact, Pd (7) Conrad, H.; Ertl, G.; Koch, S.; Latta, E. E. Surf. Sci. 1974, 43, 462. (8) Gillet, E.; Channakhone, S.; Matolin, V.; Gillet, M. Surf. Sci. 1985, 152/153, 603. (9) Fagherazzi, G.; Benedetti, A.; Polizzi, S.; Di Mario, A.; Pinna, F.; Signoretto, M.; Pernicone, N. Catal. Lett. 1995, 32, 293.

10.1021/la015650a CCC: $22.00 © 2002 American Chemical Society Published on Web 07/23/2002

Pd/γ-Al2O3 Catalysts

on alumina represents the most widely used Pd catalyst in the chemical industry, and its properties have been extensively studied. In this way, we can provide to catalytic researchers a well-established procedure for the experimental measurement of metal dispersion in Pd catalysts supported on different carriers. The investigation was carried out on highly dispersed 5% Pd catalysts supported on γ-Al2O3. These catalysts, prepared by using impregnation techniques, were recently studied by our group in the hydrogenation of benzaldehyde as well as for their behavior in poisoning by sulfur.10 In the present nanostructural investigation of Pd/γAl2O3 catalysts, high-resolution transmission electron microscopy (HRTEM) was used as a powerful complementary physical technique in order to check the structural and morphological features of the Pd particles. As it is known, the LB-XRPD technique can give correct results, provided that the particles are monodomains, i.e., single crystallites. 2. Experimental Section The 5% Pd/Al2O3 catalyst (sample C1) was prepared by using incipient wetness impregnation, with a H2PdCl4 solution, of γ-Al2O3 (Condea, 257 m2/g surface area) previously impregnated with a NaOH solution (NaOH/Pd ) 1.3 molar) and dried at 383 K. The catalyst was then treated at 773 K in air for 4 h. The Pd concentration of this catalyst, as measured with the atomic absorption technique, was found to be 5.3 wt %. Three other samples were prepared by thermally treating sample C1, in air, at the temperatures of 873 K (sample C2), 973 K (sample C3), and 1073 K (sample C4) for 4 h. Prior to XRPD measurements, all the samples were reduced with H2 at 423 K for 1 h. CO chemisorption measurements were taken at 298 K using a homemade pulse flow system. Prior to measurements, samples were subjected to a pretreatment involving exposure to hydrogen flow for 1 h at 423 K, followed by He purge for 2 h at the same temperature. XRPD patterns were recorded in air at 295 K, with a step size of 0.05° in a 2θ scattering angle. The intensities were collected with a fairly high statistics in three runs at the preset time mode of 10 s per angular step each and then averaged. A Philips X’Pert system was used, equipped with a focusing graphite monochromator on the diffracted beam and a proportional counter with an electronic pulse height discrimination. 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. High-resolution transmission electron microscopy (HRTEM) micrographs were taken with a JEM 3010 (JEOL) electron microscope operating at 300 kV. A few milligrams of the powder samples were ground in an agata mortar with high-purity isopropyl alcohol: the suspension was sonicated for 5 min in order to further disrupt possible agglomerates. A 5 µL droplet of suspension was transferred onto an amorphous carbon film, coating a 200 mesh copper grid (TAAB Laboratories Equipment Ltd.), and dried in a desiccator.

3. Methodology 3.1. X-ray Powder Diffraction (XRPD). We showed that for Pd catalysts supported on active carbons, which have very high apparent surface areas (in the range 10001500 m2/g) and therefore large amounts of micropores, the subtraction of the X-ray scattering due to the support material from the global XRPD pattern is not always reliable because of interference effects.2,11 For these composite systems, the two-phase model is not appropriate because the LB-XRPD method cannot determine crys(10) Pinna, F.; Menegazzo, F.; Signoretto, M.; Canton, P.; Fagherazzi, G.; Pernicone, N. Appl. Catal. A 2001, 219, 195. (11) Fagherazzi, G.; Canton, P.; Riello, P.; Pinna, F.; Pernicone, N. Catal. Lett. 2000, 64, 119.

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Figure 1. XRPD pattern of C1 (dots). The continuous line represents the scattering of the support (γ-Al2O3) which has to be subtracted from the total scattering due to the catalyst C1.

tallite (or nanocluster) sizes smaller than about 20 Å. Consequently, measurements of the crystallite sizes, performed only on the visible XRPD peaks, obviously overestimate the real average crystallite sizes. We showed2,11 that, by using a suitably tailored quantitative Rietveld analysis, it was possible to accurately determine the metal fraction responsible for the visible Pd peaks of the Pd/C catalysts. This approach, together with suitable assumptions about the shape and size of the undetectable Pd nanoclusters, gave surface-weighted crystallite means close enough to those obtained by chemisorption when the Pd/CO average chemisorption stoichiometry was assumed equal to 2. Fortunately, in the present case, the structured γ-Al2O3 XRPD pattern (owing to its crystalline cubic, spinel-like, disordered structure) could be correctly subtracted from the global pattern of the catalyst. In this way, the XRPD pattern due to pure Pd could be obtained and analyzed. On the contrary, a Rietveld analysis appeared to be problematic to this system and was not carried out. However, the amount of micropores is negligible in our γ-Al2O3, as shown by the nitrogen adsorption isotherm. This can explain why interference effects are negligible for Pd/γ-Al2O3 catalysts so that a two-phase model can be correctly applied in the case of very small Pd particles as well. The subtraction of the X-ray scattering of the support from the XRPD patterns of the Pd/γ-Al2O3 catalysts was carried out scaling appropriately the support pattern in such way that it was possible to correctly superimpose, in different and suitably large ranges, the support pattern to that of the catalyst. In fact, in these angular regions, only the X-ray scattering contribution of the support was present. This procedure was favored by the low number of Pd XRPD peaks. The two different examples in Figures 1 and 2 show the XRPD patterns of catalysts C1 and C4 where the pattern of the support (the continuous line) is traced. It is possible to see that the above-mentioned conditions for a correct subtraction are well satisfied. For catalyst C1, due to the very small Pd crystallite size, only the first two Pd peaks (111 and 200) were evident, and the chosen angular diffraction range from 30° to 60° in 2θ was sufficient for the line broadening analysis (see Figure 1). For catalyst C4, as well as for the other two catalysts C2 and C3, the angular range from 25° to 90° in 2θ was used, where a higher number of peaks were suitable for the investigation. While only a single-peak (111) profile Fourier analysis could be carried out for the most dispersed catalyst (C1)

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Figure 2. XRPD pattern of C4 (dots). The continuous line represents the scattering of the support (γ-Al2O3) which has to be subtracted from the total scattering due to the catalyst C4.

since the other peaks were too weak in the difference pattern, for all the other catalysts the couple of reflections 111-222 could be investigated by applying the WarrenAverbach method.12 In this way, the possible presence of lattice microstrains could be investigated as well. The Fourier analysis of the fitted analytical profiles, obtained in the difference patterns, was performed by using a published procedure.13 Two suitably constrained pseudo-Voigtian (pV) functions were employed to describe each peak profile formed by the KR1-KR2 doublet, while the residual background was described by a straight line. By using an analytical Fourier transform, the so-obtained profiles were suitably desmeared from the instrumental broadening experimentally derived from a reference standard sample (R-quartz). For the most dispersed catalyst, the so-corrected Fourier transform, AS(L), was obtained for the best-fitted XRPD 111 profile, where L represents a distance in the real space. In this case, the line broadening was entirely ascribed to effects in the Pd crystallite size. For the other three less dispersed catalysts, the more complete and precise Warren-Averbach analysis12,14 could be applied by using the 111-222 couple of Pd reflections. In accordance with this method, the following equation can be written for the instrument-corrected Fourier whole transform, A(L), of the peak profile:

(

ln A L,

1

)

dhkl2

) ln[AS(L)] -

2π〈2(L)〉 dhkl2

L2

(1)

where hkl are the Laue indices of the chosen reflections, dhkl is the interplanar spacing of the peak under consideration (111 or 222, in this case), AS(L) is the Fourier transform due only to size effects, and 〈2(L)〉 is the meansquare lattice microstrain distribution function. Thus, by using the Fourier transforms, A(L), of at least two different peaks (belonging to the same family of crystallographic planes), it was possible to determine the AS(L), as well as rms microstrains, 〈2(L)〉1/2, conventionally determined at half crystallite size. As indicated by Warren,14 Smith,15 and Matyi, Schwartz, and Butt,16 surface-weighted average sizes (defined as (12) Warren, B. E.; Averbach, B. L. J. Appl. Phys. 1952, 23, 497. (13) Enzo, S.; Fagherazzi, G.; Benedetti, A.; Polizzi, S. J. Appl. Crystallogr. 1988, 21, 536. (14) Warren, B. E. X-ray Diffraction; Addison-Wesley Publ. Co.: Reading, MA, 1969; p 274. (15) Smith, W. L. J. Appl. Crystallogr. 1972, 5, 127. (16) Matyi, R. J.; Schwartz, L. H.; Butt, J. B. Catal. Rev.sSci. Eng. 1987, 29, 41.

columns or chords in the case of spherical crystallites) perpendicular to the (hkl) planes, 〈d〉s, can be determined from the intercepts on the L-axis of the tangents to the AS(L) transformed profiles at L ) 0. By multiplying this size by a factor of 3/2, Smith15 and Matyi et al.16 showed that the corresponding diameter can be obtained for a single spherical particle. We proposed2 that this factor holds for distributions of differently sized spherical particles as well. Therefore, it is possible to transform the mean of the chords for a population of spherical crystallites into the corresponding surface-weighted diameter mean by multiplying by the factor 3/2. It is worth noting that the same reasoning holds in the definition of the so-called Porod diameter of SAXS theory.17 Another, independent approach was used here in order to obtain the average diameters of the Pd crystallites, considered to be spherically shaped as justified in the observed TEM images. Since the XRPD data are considered in this research to be the basis on which the Pd/CO average chemisorption stoichiometry is determined, we think that it is convenient to use two independent methods in order to compare and check the accuracy and coherence of the particle diameters sought for. As shown by Ciccariello, Fagherazzi, and Benedetti,18 a number distribution function, Π(L), can be obtained for the diameters of spherically shaped crystallites by using the following equation:

d2AS(L) Π(L) ) -C

d dL

dL2 L

(2)

where C is a normalization constant. Therefore, a surface, can be determined by the averaged diameter, 〈d〉diam s equation

〈d〉

diam s

∫0∞Π(L)L3 dL ) ∞ ∫0 Π(L)L2 dL

(3)

Since the C constant in eq 3 can be simplified, the Π(L) function can be obtained at an arbitrary scale. It is worth noting that a similar equation is reported by Gallezot19 for determining the average particle diameter using SAXS theory, where the AS(L) function is replaced by the characteristic function of the particle γ(r). 3.2. Chemisorption Analysis. For a supported metal catalyst, the average chemisorption stoichiometry Sav (average ratio between Pd surface atoms and chemisorbed CO molecules) is related to the volume of gas chemisorbed per gram of metal, Vg, by the classical equation:

Sav )

1 kVmCm VgΦav NAFm

(4)

where Φav(Å) is the average metal particle size, Vm is the molar volume (22 414 cm3/mol), Cm is the surface density of metal atoms, NA is Avogadro’s number (6.02 × 1023 atoms/mol), Fm is the metal density (g/cm3), and k is a factor depending on the metal particle shape and the extent of surface contact with the support. (17) Guinier, A.; Fournet, G. Small-Angle Scattering of X-Rays; J. Wiley & Sons: New York, 1955; p 158. (18) Ciccariello, S.; Fagherazzi, G.; Benedetti, A. Acta Crystallogr. 1990, A46, 187. (19) Gallezot, P. In Catalysis, Science and Technology; Anderson, J. A., Boudart, M., Eds.; Springer: Berlin, 1984; Vol. 5, Chapter 4.

Pd/γ-Al2O3 Catalysts

Figure 3. XRPD difference pattern of C1, fitted with pseudoVoigt functions, due to Pd crystallites.

According to Anderson,20 Cm is equal to 1.27 × 1015 atoms/cm2 for Pd in the case of “spheroidal” particles with crystallite surface areas “ideally” composed of equal amounts of (111), (100), and (110) planes. In the case of cubooctahedral crystallites we have calculated for Cm a value of 1.42 × 1015 atoms/cm2 with crystallite surfaces composed of equal amounts of (111) and (100) planes.21 For a cubic shape, when one face is in contact with the support, k ) 5, while for an ideal spherical shape, for which no contact surface exist, k ) 6. In the case of fcc cubooctahedra, k ranges from 5 to 6. Evidently, an high rugosity of the support can favor the occurrence of different contact areas between Pd particles and the support itself, along different orientations in the space. Also, a particle entrapped within a pore can offer a larger contact surface area. Therefore, a value of k ) 5 can be reasonably assumed in order to take into account the above-discussed aspects. From eq 4 it can be seen that by taking Φav, which is a surface-weighted mean,21 as the correct average metal particle size value obtained from advanced X-ray diffraction techniques, the average chemisorption stoichiometry Sav can be easily determined from the experimental Vg values. It is clear that, as the average metal particle size inversely depends on Sav, a correct choice of the latter is mandatory. With the present approach, exemplified here for Pd/γ-Al2O3, it is possible to calculate from experimental data the most reliable value for the Pd/CO average chemisorption stoichiometry, to be used for the determination of the Pd particle size (or metal dispersion or surface area) from chemisorption measurements.

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Figure 4. XRPD difference pattern of C4, fitted with pseudoVoigt functions, due to Pd crystallites. It is possible to see some very weak spurious peaks, indicated with an asterisk, caused by a residual background noise produced by the mathematical operation of difference carried out. Also, these spurious peaks were suitably fitted with pV functions.

4. Results and Discussion Figures 3 and 4 show, as examples, the difference patterns of samples C1 and C4 and the relevant, bestfitted, continuous line obtained by the methodology described above. As expected, all difference patterns show a meaningfully high noise, but this noise has not prevented us from obtaining fairly good fits with weighted pattern factors,22 Rwp, ranging from 0.20 to 0.09, when the sintering temperature of the catalyst increases from 773 to 1073 K. It is possible to note in Figure 4 some very weak, spurious peaks (indicated with an asterisk) due to a residual background noise produced by the mathematical operation of difference. (20) Anderson, J. B. Structure of Metallic Catalysts; Academic Press: London, 1975; p 296. (21) Borodzinski, A.; Bonarowska, M. Langmuir 1997, 13, 5613. (22) Young, R. A. The Rietveld Method; I.U.Cr. Oxford Sci. Publ.: Oxford, 1993; p 22.

Figure 5. AS(L) Fourier transforms, due to crystallite size broadening only. The intercept, with L axis, of the tangent at the origin gives the searched after surface-weighted average crystallite size 〈d〉s.

Figure 5 shows the AS(L) Fourier transforms, due to crystallite size broadening only, which are plotted in function of L for the four Pd/γ-Al2O3 catalysts. The tangent at the origin is shown, from the intercept of which with the L axis the surface-weighted average crystallite size 〈d〉s is obtained. These 〈d〉s values, multiplied by the factor 3/ , are reported in Table 1. They represent the surface2 averaged diameters of the crystallites, considered as spherically shaped. The average crystallite diameters, 〈d〉diam , obtained by eq 3, are reported in the same table. s

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Table 1. Pd/CO Average Chemisorption Stoichiometry Obtained from the Surface-Weighted Average Particle Diameters As Determined by LB-XRPD on Pd/γ-Al2O3 Catalysts av particle diameter from LB-XRD techniques from diameters distribution 〈d〉diam (Å)a s

Pd/CO av stoich

sample

Fourier tangent method 3/ 〈d〉 (Å)a 2 s

C1 (treated at 773 K) C2 (treated at 873 K) C3 (treated at 973 K) C4 (treated at 1073 K)

14 17 30 60

13 15 24 55

2.0 2.1 2.1 1.9

a

k)5 Sava

The percentage error of these figures can be estimated as (10%.

The values of the rsm 〈2(L)〉1/2 microstrains (conventionally defined at half crystallite size), found on the C2, C3, and C4 catalysts using the Warren-Averbach method, were very small (less than 0.0001). This result is justified by the thermal treatment at relatively high temperatures (773-1073 K) to which the catalysts were subjected. From Table 1, it is possible to see that both 3/2〈d〉s and 〈d〉diam values are in good agreement with each other. The s last column of Table 1 reports the average Sav chemisorption stoichiometries, calculated from eq 4, using (i) for Φav, as averaged values between 3/2〈d〉s and 〈d〉diam s obtained by LB-XRPD; (ii) k ) 5; and (iii) Cm ) 1.42 × 1015 atoms/cm2. It is possible to see in Table 1 that the Pd/CO average chemisorption stoichiometry is close to 2 for all investigated catalysts. However, we stress that these results do not mean that almost all the CO-adsorbed molecules are bridge-bonded to Pd atoms since a fraction of exposed Pd atoms could be unoccupied and/or some CO molecules could be “3-fold”-bonded to Pd atoms,8 thus increasing the Pd/CO apparent average stoichiometry. In other words, a significant fraction of chemisorbed CO could well be linearly bonded to Pd. For instance, if the number of linearly bonded Pd atoms is equal to the number of the unoccupied Pd atoms, Sav results always equal to 2, if remaining Pd atoms are involved in bridge bonds. The fraction of linearly bonded Pd atoms could also increase as particle size decreases, as proposed by some researchers.3-4,20 In particular, in the paper by Sheu, Karpinski, and Sachtler,4 a plot is reported in which, for very high dispersions corresponding to particle sizes of about 15 Å, the average Pd/CO stoichiometry decreases to about 1.65, starting from values of about 1.9 for sizes of about 50 Å. On the other hand, some researchers claim that there is no particle effect with the adsorption of CO on SiO2/Si(100)-supported Pd particles.23 In any case, many researchers have recently shown, using such diverse techniques as microcalorimetry and TPD,24 ellipsometry,23 IR, and other spectroscopic investigations in function of the support alkalinity,25 that bridge-bonded CO is always the dominant CO(ads) species. It is worth noting that Beck et al.26 and, more recently, Horvath et al.27 used an average Pd/CO chemisorption stoichiometry of 1.5 in the case of silica-supported Pd nanoparticles in order to attain an (23) Voogt, E. H.; Coulier, L.; Gijzeman, O. L. S.; Geus, J. W. J. Catal. 1997, 169, 359. (24) Dropsch, H.; Baerns, M. Appl. Catal., A 1997, 158, 163. (25) Mojet, B. L.; Miller, J. T.; Ramaker, D. E.; Koningsberger, D. C. J. Catal. 1999, 186, 373. (26) Beck, A.; Horva´th A.; Szu¨cs, A.; Schay, Z.; Horva´th, Z. E.; Zsoldos, Z.; De´ka´ny, I.; Guczi, L. Catal. Lett. 2000, 65, 33. (27) Horva´th, A.; Beck, A.; Sa´rka´ny, A.; Koppa´ny, Zs.; Szu¨cs, A.; De´ka´ny, I.; Horva´th, Z. E.; Guczi, L. Solid State Ionics 2001, 141-142, 147.

Figure 6. TEM micrograph of C2 catalyst, where several Pd particles are shown, all showing roughly circular projections.

Figure 7. HRTEM image of sample C1. Monodomain particles of palladium are evidenced, within which interference fringes, due to (111) lattice planes, can be observed.

acceptable agreement between particle sizes obtained with TEM and those obtained with chemisorption. In this respect it should be recalled that different experimental conditions in chemisorption measurements may give different Vg values. The average Pd particle diameters we have determined by LB-XRD techniques are qualitatively confirmed by TEM. Figure 6 shows, as an example, a TEM image of C2 catalyst, where darker Pd particles are well visible. Since, in this magnification scale, all particles, which can assume all possible space orientation, appear roughly circular, the assumption made in the LB-XRPD analysis that the Pd particles are spherical is well acceptable. If a larger magnification scale is used, as shown in Figures 7-9, where HRTEM images of some Pd particles of C1, C3, and C4 catalysts are recorded, the “spheroidal” shape appears to be cubooctahedral. It is interesting to note that in these micrographs, as well as in many others of the four samples investigated (not recorded here for the sake of brevity), single crystallites appear where interference fringes due to (111) lattice planes are shown. This is direct proof of the fact that the LB-XRPD analysis employed can determine, in

Pd/γ-Al2O3 Catalysts

Figure 8. HRTEM image of sample C3. A monodomain particle of palladium is evidenced, within which interference fringes, due to (111) lattice planes, can be observed. A cubooctahedral shape can be inferred.

Figure 9. HRTEM image of sample C4. A monodomain particle of palladium is evidenced, within which interference fringes, due to (111) lattice planes, can be observed. A cubooctahedral shape can be inferred.

the present case of highly dispersed Pd/γ-Al2O3 catalysts, a crystallite size which really represents the size of the entire particle. This is a necessary condition for a correct

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use of the LB-XRPD technique and, at the end of the investigation, confirms that the correct Pd/CO average chemisorption stoichiometry has been obtained. This does not always occur. For example, in the case of Pd/C catalysts, we recently showed2 that the Pd particles, when greater than 150 Å in size, become polydomain particles; therefore, the LB-XRPD analysis, which measures the domain size, is unsuitable for obtaining reasonable particle sizes to be used for the determination of the average Pd/ CO chemisorption stoichiometry. In the latter case only SAXS (in the area of XRD techniques) can give,2,9 if interference effects between the metal phase and the carrier are not present, accurate surface-weighted average particle sizes, since this technique is not biased by the presence of domains inside the particles.28 Moreover, it is important to remember that, for a correct comparison with chemisorption data, the crystallite sizes must be determined, when LB-XRPD techniques are used, as surface-weighted means. If volume-weighted crystallite sizes are determined, larger crystallite sizes are obtained in comparison with the corresponding surface-weighted means. This happens, for example, when using the wellknown classical Scherrer formula, which gives volumeweighted crystallite sizes. Unfortunately, no LB-XRPD simplified method is able to give surface-weighted crystallite sizes, and therefore, Fourier analysis is mandatory for obtaining these parameters that are so important in heterogeneous catalysis. 5. Conclusions From LB-XRPD and HRTEM investigations, it is possible to conclude that, for highly dispersed Pd/γ-Al2O3 catalysts (average particle sizes from 13 to 14 to 55-60 Å), the Pd crystallites are monodomain particles. In these conditions, the LB-XRPD physical method allows the calculation, from experimental data, of a Pd/CO average chemisorption stoichiometry of about 2, if surface-weighted values of particle sizes are correctly determined by using Fourier methods. This agrees well with our previous results obtained on Pd catalysts, supported on active carbon and on silica, showing different metal dispersions,2,9 where an Sav value of 2 was assumed in order to obtain an acceptable agreement between XRD and chemisorption data, thus showing that for supported Pd catalysts there is no appreciable dependence of Pd/CO average chemisorption stoichiometry on support nature and Pd dispersion. LA015650A (28) Guinier, A.; Fournet, G. Small-Angle Scattering of X-Rays; J. Wiley & Sons: New York, 1955; p 165.