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J. Phys. Chem. C 2007, 111, 11150-11156
Relevance of Spinodal Decomposition for Support Formation and Metal Dispersion in Cogelled Pd/SiO2 Catalysts Ce´ dric J. Gommes,*,† Bart Goderis,‡ and Jean-Paul Pirard† Department of Chemical Engineering, UniVersite´ de Lie` ge, Alle´ e du 6 aouˆ t, 3, B6a, B-4000 Lie` ge, Belgium, and Molecular and Nanomaterials, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F, B-3001 HeVerlee, Belgium ReceiVed: January 25, 2007; In Final Form: April 5, 2007
The structure formation of cogelled Pd/SiO2 catalysts is followed in situ at the nanometer scale using timeresolved small-angle X-ray scattering (SAXS). The SAXS patterns are analyzed in terms of a reaction-induced spinodal decomposition that is responsible for the formation of the silica support and most likely for the metal dispersion as well. The results are discussed in the light of recent electron tomography characterizations of the catalysts. The latter technique reveals regularity in the metal dispersion, the origin of which is likely to be the very occurrence of a spinodal phase separation.
1. Introduction Cogelation is a powerful sol-gel method to prepare highly dispersed mono- and bimetallic catalysts supported on silica.1-8 It consists in copolymerizing a main silica precursor, typically tetraethoxysilane (TEOS), with a modified silicon alkoxide able to form a chelate with a metal cation such as Pd2+, Ni2+, Ag+, Cu2+, Fe3+, etc. Cogelation enables the building up of a cagelike silica structure around active metal nanoparticles to protect them from sintering by a migration and coalescence mechanism during the catalyst operation.9 Indeed, previous studies showed that cogelled catalysts contain small (2-3 nm) metal particles buried inside a microporous skeleton with monodispersed micropores about 1 nm across. Because they are larger than the micropores of the silica skeleton in which they are buried, the highly dispersed metal crystallites are caged10,11 and still completely accessible.12 In addition to being completely accessible, a mass transfer study shows moreover that metal particles are easily accessible through the very particular funnel structure of the support provided that the diffusing molecules are not too large.13 In the case of bimetallic catalysts, cogelation has proven efficient for the preparation of alloy nanoparticles.4,7,8 With the aim of tuning the properties of cogelled catalysts to match new applications in catalysis as well as in other fields such as life sciences (e.g., magnetic resonance medical diagnosis), many issues are still to be addressed. For instance, the uncontrolled appearance of large metal particles on the outer surface of silica when the metal loading increases,12 in addition to the highly dispersed ones, should be limited as much as possible. Furthermore, up to now and to the best of our knowledge, no means has been found to tailor the size of the micropores in the silica matrix encapsulating the active particles. This would enable extending the use of cogelled catalysts for the catalytic processing of molecules that are too large to reach easily the active sites. This would also offer the prospect of applying the concept of shape selectivity, common to zeolites.14 * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +32 (0)4366 3558. Fax: +32 (0)4366 3545. † Universite ´ de Lie`ge. ‡ Katholieke Universiteit Leuven.
Figure 1. Transmission electron tomogram of a fragment of sample Pd1.1, obtained as described in ref 10. The surface of the silica is represented as translucent, which makes the Pd particles visible as black dots inside the silica.
Another challenge is to disperse and anchor the active species in the mesoporous structure outside the silica. In the case of bi- or multimetallic catalysts, depending on the nature and on the number of reactions to be catalyzed in the reactor, it could also be useful to obtain either homogeneous alloy particles or, on the contrary, various particle families with different compositions and, thus, different functions. These achievements are almost impossible by a trial and error procedure. They require a deep understanding of the physicochemical mechanisms that control the structure of the cogelled catalysts obtained up to now. This paper deals with cogelled Pd/SiO2 gels, of which the microstructure and the catalytic activity have been thoroughly analyzed in previous studies.5,10 Figure 1 shows an electron tomogram that illustrates the 3D morphology at the nanometer scale of a Pd/SiO2 cogelled sample after drying and calcination, according to an imaging technique described elsewhere.10 The nanostructure is that of a mesoporous skeleton, with Pd nanoparticles buried inside the silica, as described above. The metallic Pd particles are about 3 nm across; in Figure 1 the surface of the silica is translucent which makes the Pd particles visible. The formation of that structure is analyzed in situ in
10.1021/jp070648z CCC: $37.00 © 2007 American Chemical Society Published on Web 07/07/2007
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J. Phys. Chem. C, Vol. 111, No. 30, 2007 11151
Figure 2. Time-resolved SAXS patterns measured on the gelling solutions for samples (a) Pd1.1, (b) Pd3.1, and (c) Pd4.5. The maximum is superimposed with a circle, when it is visible. The thick solid line is the pattern corresponding to the gel point.
TABLE 1: Synthesis of the Pd/SiO2 Xerogel Catalysts catal
Pd(acac)2 (g)
EDAS (cm3)
TEOS (cm3)
0.18 M NH3 (cm3)
ethanol (cm3)
gel time (min)
Pd1.1 Pd3.1 Pd4.5
0.097 0.206 0.433
0.14 0.30 0.62
12.3 12.1 11.8
5.0 5.0 5.0
32.6 32.6 32.6
60 76 104
this study using time-resolved small-angle X-ray scattering (SAXS). The SAXS patterns and their time evolution are typical of a reaction-induced spinodal phase separation of the silica precursors from the solvent. This process would therefore be responsible for the nanometer-scale structure of the catalyst and most likely for the metal dispersion itself. 2. Experimental Methods 2.1. Synthesis of the Samples. The general method for preparing the Pd/SiO2 xerogel catalysts analyzed in this paper has been described elsewhere.5 Palladium acetylacetonate powder (Pd(acac)2) and 3-((2-aminoethyl)amino)propyltrimethoxysilane (EDAS) are mixed together in ethanol. The slurry is then stirred at room temperature for about 0.5 h, until a clear yellow solution is obtained. After addition of tetraethoxysilane (TEOS), a solution of aqueous 0.18 M NH3 in ethanol is added to the mixture. Three samplesslabeled Pd1.1, Pd3.1, and Pd4.5sare analyzed in this paper, the compositions of which are reported in Table 1. The numbers correspond to the actual metal loading of the samples (in wt %) determined on the final material, after drying and calcining.5 The gel times are also reported in Table 1; they are defined as the moment when the solution no longer flows when the synthesis flask is tilted. 2.2. SAXS Measurements. Small-angle X-ray scattering (SAXS) measurements were performed at the Dutch-Flemish SRG beam line (DUBBLE, BM26B) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Immediately after its preparation, a small fraction of the reacting solution is extracted from the flask and placed in a 1.5 mm thick cell with parallel mica windows. Consecutive in situ pinhole SAXS patterns are recorded over time spans of 10 s on a quadrant detector placed at 4.25 m from the sample. A correction is made for the detector response, and the data are normalized to the intensity of the primary beam measured by an ionization chamber placed upstream from the sample. A second ionization chamber placed downstream from the sample enables the absorption of X-rays by the sample to be determined. The sample holder motors enable movement in the two directions perpendicular to the beam. The SAXS intensity is expressed as a function of the scattering vector modulus q ) 4π/λ sin(θ/2), λ being the wavelength (set to 1 Å) and θ the scattering angle. The intensity scattered by the empty sample holder is measured and subtracted from the scattering patterns.
For all patterns, the lowest measured angle corresponds to q ≈ 0.01 Å-1, and the highest angle, to q ≈ 0.2 Å-1. Therefore, in the used configuration, the SAXS probes structures that are smaller than 2π/q ≈ 60 nm and larger than 2π/q ≈ 3 nm. The characteristic sizes of both silica support and of metal dispersion fall within that range, as the typical thickness of the silica struts and the distance between neighboring metallic particles are about 10-15 nm.10 3. Results 3.1. Time-Resolved SAXS Measurements of the Gelling Solutions. Figure 2 plots the scattered intensity I(q,t) as a function of both scattering vector modulus q and reaction time t. For all samples, a maximum in the patterns appears early during the reaction. Its intensity progressively increases, and its position shifts toward smaller scattering angles with time, as emphasized in each pattern by a circle. The structural evolution of the gels at the nanometer scale levels off toward the end of the experiment. The thick black line corresponds to the gel time (reported in Table 1). For all samples, the moment when the evolution of the SAXS patterns stops roughly coincides with the macroscopic gel point. Toward the end of the runs and at very low angles q < 0.01 Å-1, the scattered intensity increases with decreasing q. This means that a large-scale structure has formed with a size beyond the upper resolution limit of the SAXS. The latter structure undoubtedly corresponds to macropores (with a width larger than 50 nm) that are indeed present in the final xerogel catalysts.5 Figure 3 plots the SAXS patterns on double logarithmic scales. This figure shows that the scattering does not obey a power law at large scattering angle. If the data at large angles are fitted to a power law anyway (not shown), the found exponent increases steadily with reaction time, from values close to 0 until a value close to 3 toward the end of the runs. In particular, the Porod law I ∼ q-4, characteristic of the scattering by a structure with clear-cut interfaces,15 is not observed for any sample even at the end of the runs. The time evolution of the position qm and of the intensity Im of the maximum in the SAXS pattern are plotted in Figure 4a,b. There is a time range during which the evolution of qm obeys a power law qm ∼ t-β, with exponent β between 1/3 and 1. The
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Figure 3. Time-resolved SAXS patterns on double logarithmic scales measured on the gelling solutions for samples (a) Pd1.1, (b) Pd3.1, and (c) Pd4.5. The time span between each plotted pattern is 15 min. The solid line in each graph is the Porod law I ∼ q-4 that would be expected for the scattering by a structure with clear-cut interfaces.
Figure 4. Time evolution of (a) the position qm and (b) the intensity Im of the maximum in the SAXS scattering patterns and (c) relation between Im and qm, for samples Pd1.1 (×), Pd3.1 (4), and Pd4.5 (O) In the inset of (b), the curves are arbitrarily shifted vertically, and they are shifted horizontally in (c).
Figure 5. 2D scan in the x and y directions of the region of sample Pd4.5 irradiated by X-rays during the reaction: (a) total scattered intensity measured by the detector; (b) X-ray absorption by the sample. During the reaction, the X-ray beam moved regularly from y ) 0 mm (t ) 0 min) to y ) 4 mm (t ) 120 min), along the black line indicated in subfigure a.
maximum of the scattered intensity Im seems to follow a power law increase toward the end of the run (Figure 4b) and an exponential growth at early reaction times (inset of Figure 4b). Figure 4c plots Im against qm over the entire reaction time. All samples seem to converge asymptotically to a power law of the type Im ∼ qm-3. 3.2. Photoreduction of Pd by X-rays. At the end of the measurements, all samples have a thin black line crossing their entire thickness at the exact spot where they were crossed by the X-ray beam. This results from the photoreduction of palladium under X-ray irradiation, as often reported for other metals (e.g., refs 16 and 17). For all samples, the onset of the photoreduction phenomenon is found to coincide with the gel point. This was evidenced by moving slowly the sample holder containing the irradiated
reacting solution, at a constant speed of 15 µm/min. When the sample is examined at the end of the run, it is translucent where it was hit by the X-rays while it was still liquid, but it is black where it was hit by the X-rays while it was already a gel. The blackening is accompanied by an increase of the scattered intensity and of the X-ray absorption by the sample. The portion of sample Pd4.5 that was irradiated during the gel formation was examined again after the end of the run. Figure 5 shows a 2D scan with the X-ray beam of that portion, in the x and y directions. During the gel formation, the sample was moved in the y direction at a rate of 15 µm/min (see previous paragraph). Therefore, the 4 mm range of the y-axis, from 0 mm to +4 mm, corresponds to approximately 2 h of reaction time. The position of the black zone in the sample coincides with the
Cogelled Pd/SiO2 Catalysts
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11153 The Pd is present in the gels under the form of an organometallic complex with EDAS; the metallic nanoparticles in the final catalysts (see Figure 1) form during the calcining and reduction. This is notably supported by the work of Sacco et al.18 that shows that uncalcined Pd/SiO2 xerogels exhibit the same catalytic activity for the cyclopropanation of olefins than the Pd complex in homogeneous solution, which proves that Pd is under the same state in both cases. In Table 2, the electron density of the Pd(EDAS)2 organometallic complex is estimated by assuming that the molar volume of the complex is the sum of the volume of the metal and of the two complexant molecules required by the stoichiometry of the complexing reaction.3,4 To estimate roughly the contribution of each phase to the scattering, it is useful to estimate theoretically what the total intensity scattered by the sample, defined by15
Figure 6. Intensity scattered at a given angles as a function of time (sample Pd3.1). The inset shows the same data plotted on a logarithmic scale. At all angles, the scattered intensity increases at the gel point (about 76 min).
increase of the scattered intensity and of the absorption in Figure 5; it occurs at the same time as the gel point. The fact that the photoreduction of Pd occurs at the gel point is further confirmed by the observation that the scattered intensity increases at the gel time. This is illustrated in Figure 6 in the case of sample Pd3.1. For sample Pd1.1, the scattered intensity also increases at the gel time, although with a lower amplitude. For sample Pd4.5 a slight increase is observed at the gel point, immediately followed by a somewhat irregular lowering. This observation presumably points to a damage of the gel’s structure by the X-rays after the gel time, for the sample with the highest metal loading. 4. Discussion 4.1. General Analysis of the SAXS Data. A practical difficulty in analyzing the SAXS patterns comes from the fact that the nanostructure of the samples is triphasic. It comprises (i) the silica, (ii) the pore space filled with the gel’s mother liquor (mostly ethanol), and (iii) the palladium complexes or the metallic particles. It is therefore a priori not obvious how the SAXS data have to be analyzed and to which phase any particular feature of the SAXS patterns has to be assigned. From the point of view of X-ray scattering, what characterizes a phase is its electron density. Two phases with the same electron density are indistinguishable by SAXS.15 It is therefore useful to estimate the electron densities of all three phases. The electron densitysexpressed in Faradays, F, per cm3sof the molecules involved in the sol-gel reaction can be roughly estimated as Ne/VM, where Ne is the number of moles of electrons in 1 mol of neutral molecules and VM is its molar volume in its pure state (see Table 2).
Q)
∫0∞ q2I(q) dq
(1)
would be if the system were strictly polyphasic, with clear-cut interfaces. For a triphasic system, one has19
Q ) C[φ1φ2(F1 - F2)2 + φ1φ3(F1 - F3)2 + φ2φ3(F2 - F3)2] (2) where φi and Fi are the volume fractions and electron densities of the phases and C is a constant that shall be set equal to 1 in the following. Let us estimate Q from the data in Tables 1 and 2, in the following three situations, namely (i) there is no metal in the samples and the only phases are the silica and the liquor, (ii) the metal is under the form of Pd(EDAS)2, and (iii) the Pd is present under a metallic form. Let phase 1 be the silica, phase 2 be the pore-filling liquor, and phase 3 be the phase to which the Pd belongs (either the Pd(EDAS)2 complex or a metallic phase). As a Porod scattering (with exponent 4) is not observed, the silica-rich phase in the gels is presumably not dense silica; its physical properties are likely intermediate between those of dense silica and of unpolymerized TEOS. From Table 2, reasonable estimates are F1 ) 0.75 F/cm3 and VM ) 100 cm3/ mol. Using this value of VM and the data in Table 1, the volume fraction of the silica-rich phase is estimated to be φ1 ) 0.11. As for the liquor, it contains mostly ethanol and water and probably some unpolymerized TEOS as well; it is reasonable to assume F2 ) 0.5 F/cm3. Neglecting the presence of metal in the gels is equivalent to assuming φ3 ) 0, which leads to φ2 ) 1 - φ1 ) 0.89. Using these values of φ and of F in eq 2, one finds Q ) 6.1 × 10-3 (F/cm3)2. For sample Pd4.5, which has the largest metal loading, if metal is present under the form of Pd(EDAS)2, one estimates from Tables 1 and 2 that φ3 ) 0.013 and F3 ) 0.65 F/cm3. With the same value of F1, F2, and φ2 as previously and φ1 ) 1 - φ2 - φ3, one finds from eq 2 that Q ) 5.7 × 10-3 (F/ cm3)2. This value is just about 10% lower than the value
TABLE 2: Physical Characteristics of the Molecules in Their Pure Statea molecule
Fm (g/cm3)
M (g/mol)
Ne (F/mol)
VM (cm3/mol)
F (F/cm3)
water ethanol TEOS SiO2 Pd EDAS Pd(EDAS)2
1 0.79 0.93 2 12 1.02
18 46 208 60 103 222
10 26 114 30 46 122 290
18 58 223 30 8.6 218 444.6
0.55 0.45 0.51 1 5.3 0.56 0.65
a Key: Fm, density; M, molar mass; Ne, number of electrons in the neutral molecule expressed in Faraday/mol; VM, molar volume; F, electron density.
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Figure 8. (a) Cahn’s growth rate R(q) of the concentration fluctuations of samples Pd1.1 and Pd4.5. (b) Example of initial exponential growth rate of the scattered intensity at a few representative angles (sample Pd4.5), from which R(q) was determined.
Figure 7. Schematic view of the main events of spinodal phase separation: (a) to (b) amplification of the statistical fluctuations of silica precursors’ concentration with a given characteristic length; (c) the silica and solvent phases created and (d) coarsen to reduce their interfacial area. The darkness in the figures symbolizes the silica precursors’ concentration.
calculated previously by assuming that no metal was present in the samples. Given the roughness of the estimations, this means that the presence of Pd(EDAS)2 complexes is not expected to modify significantly the scattering patterns. In the case where Pd is metallic with F3 ) 5.3 F/cm3, one estimates from Tables 1 and 2 that φ3 ) 2.4 × 10-4. In this case, using the same values of F1, F2, and φ2 as previously and φ1 ) 1 - φ2 - φ3, eq 2 predicts Q ) 11.6 × 10-3 (F/cm3).2 This value is almost the double of the value of Q estimated if Pd were not present in the gels. Therefore, unlike Pd(EDAS)2 complexes, metallic Pd is expected to increase significantly the scattered intensity. The blackening phenomenon reported in section 3.2 is related to the appearance of metallic Pd through the photoreduction of Pd cations under X-ray irradiation. As discussed above, this process is expected to be accompanied by a significant increase of the scattered intensity, which is actually observed (see Figure 6). In summary, before the gel time, the SAXS patterns can be analyzed as if they resulted from the scattering of a biphasic system made of (i) the mother liquor phase and (ii) a single silica phase containing the Pd complex. This is no longer the case after the gel point, when metallic Pd is present in the samples. 4.2. Mechanism of Gel Formation. The very presence of a maximum in SAXS patterns hints at spinodal structures of the type illustrated in Figure 7.20,21 The electron density variation along any line drawn randomly through such as structure is almost periodic with periodicity 2π/qm, which is the physical origin of a maximum in the scattering curves. Spinodal structures can be obtained naturally through phase separation processes.21 In particular, a reaction-induced phase separation process22 has recently been proposed to explain the structuring of metal-free gels synthesized from TEOS and EDAS alone23,24 or from TEOS and another organically substituted trialkoxysilane.25 Figure 7 sketches the sequence of events likely to be responsible for the formation of the gel’s nanostructure, on the basis of the SAXS data of Figures 2 and 3. Any solution is
subject to statistical concentration fluctuations (Figure 7a). During the polymerization of the silica precursors, the average molar mass increases which lowers the solubility of the silica in the solvent.22 Accordingly, the concentration fluctuations amplify (Figure 7b), which eventually leads to the formation of two distinct phases (Figure 7c).21 The initial size of the phase separated domains results from a balance between (i) the tendency of unlike species to segregate, which favors the formation of large domains, and (ii) the limited mobility of the silica macromolecules that favors the formation of small domains. Once the phases are created, they coarsen to lower their interfacial area (Figure 7d). The evolution stops when the polymerization has proceeded to such an extent that the morphology becomes frozen in. The various stages of spinodal decomposition (SD) can be followed in the evolution of the SAXS patterns (Figures 2-4).20 At the beginning of the reaction (t < 20 min) the maximum scattered intensity Im increases exponentially with time (inset of Figure 4b) while the position of the maximum remains almost unchanged (Figure 4a). The scatter in the points of Figure 4a at early times results from the fact that the maximum in the SAXS patterns is very broad, as visible in Figure 3. The mentioned evolution of the SAXS patterns is typical of the amplification of the concentration fluctuations with a given characteristic length (Figure 7a to Figure 7b), which is generally referred to as early stage of SD.20 The early stage of SD is conveniently analyzed within Cahn’s linear theory,21 according to which the intensity scattered at any angle increases exponentially with time as
I(q,t) ) I(q,0) exp[2R(q)t]
(3)
where R(q) is the exponential growth rate of the concentration fluctuations. Figure 8a plots the values of R(q) of gelling solutions Pd4.5 and Pd1.1, estimated from linear fits of ln(I) vs time, as illustrated in Figure 8b. For each solution, R(q) has a maximum, the position of which qm0 corresponds to the most unstable concentration fluctuation and which sets the angle at which the maximum appears in the SAXS patterns. The shape of the curves R(q) is not, however, in agreement with Cahn’s linear theory.21 The latter notably predicts that fluctuations with a wavevector larger than x2qm0 are stable and are therefore characterized by negative values of R(q). In the present case, an initial increase of the SAXS is observed at all angles, which means that all the fluctuations are unstable. It has to be stressed that deviations from Cahn’s theory are common for polymer blends or solutions undergoing spinodal decomposition.26
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J. Phys. Chem. C, Vol. 111, No. 30, 2007 11155
The intermediate stage of SD is characterized by the differentiation of the phases (Figure 7b to Figure 7c).20 From the point of view of SAXS, one expects the exponent of the asymptotic scattering to increase during the intermediate stage, eventually reaching the value 4, characteristic of a structure with clear-cut interfaces.15 Although no clear power-law scattering is observed, the SAXS patterns become steeper and steeper at large angles at intermediate reaction times (Figure 3), which is likely to result from the phase differentiation. At the same moment, the maximum in the SAXS patterns moves toward smaller scattering angles, which reflects a coarsening process (Figure 7d). For all samples, there is a time range during which the coarsening obeys qm ∼ t-β, with β slightly lower than 1 (Figure 4a). The particular value β )1 is typical of a hydrodynamic limited process, driven by the surface tension of the interface between the two phases.27 The value β ) 1/3, also plotted in Figure 4a, is typical of a diffusion-limited process;27 this value would notably be observed if the coarsening of the structure proceeded through the DLA-like aggregation of smaller building blocks.28 The experimental observation of an exponent close to 1 can therefore be taken as an additional argument in favor of a spinodal decomposition mechanism over the building block aggregation process that occurs in many sol-gel systems.29 The coarsening process is the only one that continues during the late stage of SD.20 During the late stage, the phases are completely differentiated with a clear-cut interface, which for the SAXS means a Porod scattering with exponent 4 at large angles. Furthermore, in many instances, the SAXS patterns obey the following dynamical scaling during the late stage of SD (e.g., ref 30):
I(q,t) ) L(t)3F(qL(t))
(4)
Here F is a scaling function and L(t) is a time-dependent length scale. Equation 4 implies that the SAXS patterns all fall on the same curve when they are properly rescaled. This is not possible in the present case as the SAXS patterns at different times have different shapes, which is clear from Figure 3. However, from eq 4 the position of the scattering maximum should scale as qm ∼ 1/L and its intensity as Im ∼ L3, from which one should have Im ∼ qm-3. The latter power law is roughly followed by the SAXS patterns toward the end of the runs (Figure 4c). The evolution of the morphology at the nanometer scale, as assessed by the SAXS, stops at the gel time (Figure 2). This observation is a priori not obvious, as gelation is a macroscopic event that is generally related only to the appearance of a percolating network and it is not expected to coincide with any microscopic event. In many gelling systems, there is evidence from dynamic light scattering that the systems are still quite mobile at the microscopic scale, even after the gel point (e.g., ref 29). This is not the case for the present systems that seem to freeze at all scales simultaneously. This is also indirectly supported from the observation that the photoreduction of Pd begins at the gel point, which could result from an immobilization of the Pd cations at that same moment. The overall scenario that is compatible with the SAXS data is therefore that the structuring of the gels occurs via reactioninduced spinodal decomposition (SD). There is evidence of first stage SD, with notably an exponential growth of the scattered intensity. There is also evidence of second stage SD, with the SAXS patterns becoming steeper and steeper, characteristic of phase differentiation. During the same time the coarsening of the phases occurs, as testified by the displacement of the maximum toward smaller angles. Pure late stage SD, however,
is not observed as the ongoing polycondensation reactions freeze the morphology of the gels in a state characteristic of intermediate stage of SD. 4.3. Metal Dispersion. After drying and calcining of the samples, the metal is present under the form of nanometer-sized metallic particles regularly dispersed in the middle of the struts that form the silica support (see Figure 1).10 A pending question with cogelled catalysts is that of the mechanisms that govern the metal dispersion, related in fine to the catalytic properties of the materials. The SAXS data reported here give some insight into this issue. It first has to be noted that, in absence of photoreduction effects, the metallic particles are not present in the gels themselves where the metal is present under the form of organometallic complexes. The metal particles are formed during the drying and calcining of the samples, as discussed in section 4.1. One possibility is that the organometallic complexes are initially statistically distributed throughout the gel’s solid phase. The metallic particles would then form by the coalescence of the metal atoms, after the destruction of the complexes during the calcining and reduction. If this were the case, one would expect a random statistical distribution of the metal particles at the end of the process and most notably (i) a significant fraction of the metal outside of the silica and (ii) a broad log-normallike size distribution of the metal particles.31 As this is not the case for the samples with a low metal loading (e.g., Pd1.15,10), the simplest hypothesis is that the organometallic complexes in the gels are positioned where the metallic particles are in the xerogel catalysts; i.e., that they are regularly dispersed in the middle of the struts that form the silica skeleton of the gel.10 The very occurrence of a spinodal phase separation, with a well-defined characteristic length scale,20,21 is reminiscent of the regularity of the metal dispersion in the xerogel catalysts. It is therefore likely that the phase separation process itself controls not only the structuring of the silica skeleton (as discussed in section 4.2) but also the dispersion of the metal that is anyway linked chemically to the silica molecules. In the discussion so far it has been implicitly assumed that the distribution of the molecular weight of the polymerizing silica is narrow. If this is not the case, the phase separation process is progressive. The largest macromolecules, being the less soluble, segregate early; the species that have a lower molecular weight at that moment segregate later. Clearly, the EDAS moleculessto which the Pd atoms are linkedsare much more prone to hydrolysis and to subsequent condensation than TEOS in the same conditions (e.g., ref 32). Therefore, at any stage of the polymerization, the Pd(EDAS)2 complexes are undoubtedly incorporated in the species with the largest molecular weight. As the latter segregate earlier than the rest of the silica precursors, it is not surprising to find them at the end of the process buried deep inside the silica skeleton. Their regular dispersion is not surprising either as the spinodal decomposition process in which the entire nanostructure of the gel originates leads to an almost periodic structure, as testified by the presence of a maximum in the SAXS patterns (see Figure 2). 5. Conclusions It has been shown in this paper that the nanostructure of Pd/ SiO2 cogelled catalysts forms via reaction-induced spinodal decomposition. During the polymerization reaction, the solubility of the silica molecules in the solvent decreases because their molecular weight increases; this triggers the precipitation of the silica into a spinodal-like structure with nanometer-sized
11156 J. Phys. Chem. C, Vol. 111, No. 30, 2007 domains. This process is evidenced in the time-resolved SAXS patterns measured during the formation of the gels. A maximum appears in the scattering curves early during the process, with an intensity that increases with time and a position that shifts toward smaller scattering angles. The latter evolutions reflect the appearance of structures with a well-defined characteristic length and their subsequent coarsening, as expected for spinodal decomposition. The evolution stops abruptly at the gel time when the structure of the gels is frozen in a state characteristic of the intermediate stage of spinodal decomposition. The freezing of the structure at all length scale is also indirectly supported by the fact that the Pd atoms undergo a photoreduction by the X-rays at the gel time. It is particularly interesting to note that a previous electron tomography analysis revealed a regularity in the metal dispersion,10 the origin of which can be found in the very occurrence of a spinodal decomposition that naturally leads to a structure with a well-defined length scale. To the best of our knowledge, this is the first demonstration that phase separation can be exploited to disperse effectively a metal for the synthesis of supported catalysts. Acknowledgment. C.J.G. is grateful to the Belgian National Fund for Scientific Research (FNRS) for a postdoctoral research fellow position; B.G. is a postdoctoral fellow of the Fund for Scientific Research Flanders (FWO-Vlaanderen). We thank Dr. Wim Bras and Dr. Igor Dolbnya (DUBBLE-CRG/ESRF) as well as Dr. Christelle Alie´ and Dr. Benoit Heinrichs (University of Lie`ge) for their assistance in acquiring the in situ SAXS patterns and Dr. Silvia Blacher (University of Lie`ge) for her help in analyzing the SAXS data. This work was supported by the National Funds for Scientific Research, Belgium, the Re´gion Wallonne, Direction Ge´ne´rale des Technologies de la Recherche et de l’EÄ nergie, and the Ministe`re de la Communaute´ Franc¸ aise, Direction de la Recherche Scientifique. References and Notes (1) Breitscheidel, B.; Zieder, J.; Schubert, U. Chem. Mater. 1991, 3, 559. (2) Schubert, U. New J. Chem. 1994, 18, 1049. (3) Heinrichs, B.; Noville, F.; Pirard, J.-P. J. Catal. 1997, 170, 366. (4) Heinrichs, B.; Delhez, P.; Schoebrechts, J.-P.; Pirard, J.-P. J. Catal. 1997, 172, 322.
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