J. Phys. Chem. C 2008, 112, 16463–16469
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Hydrogen Reduction of Adams’ Catalyst in Ionic Liquids: Formation and Stabilization of Pt(0) Nanoparticles Carla W. Scheeren,† Josiel B. Domingos,§,* Giovanna Machado,†,‡ and Jairton Dupont†,* Laboratory of Molecular Catalysis-Institute of Chemistry-UFRGS-AVenida Bento Gonc¸alVes, 9500 Porto Alegre 91501-970 RS Brazil; UniVersidade de Caxias do Sul, Departamento de Engenharia Quı´mica, Rua Francisco Getu´lio Vargas, 1130, Caxias do Sul 95070-560, RS-Brazil, and Laboratory of Biomimetic Catalysis, Chemistry Department, UFSC-Santa Catarina South Carolina 88040-900, Brazil ReceiVed: June 2, 2008
The kinetic data on cyclohexene hydrogenation with PtO2 (Adams’ catalyst) dispersed in 1-n-butyl-3methylimidazolium tetrafluoroborate (BMI.BF4) and hexafluorophosphate (BMI.PF6) ionic liquids at room temperature and 75 °C, with substrate/catalyst ratios of 1/1000 and 1/4000, suggest the formation of large agglomerates of bulk metal catalysts. The results of this study show that the kinetic curves, which show induction periods in some cases, only have a good fit with the double autocatalytic mechanism, indicating the formation of bulk metal and its probable participation in the catalytic activity of the system, in particular at high precursor concentrations. However, it was not possible to observe these large agglomerates in the ionic liquids with the naked eye. In fact, in situ TEM analysis of several samples, including those obtained at the end of the induction period and at the end of the reaction, showed that only platinum nanoparticles with a mean diameter of around 3.0 nm were formed. Moreover, similar nanoparticle sizes were indicated by XRD analysis of isolated metal nanoparticles, suggesting that no bulk metal was formed. These results show that although the kinetic curves are consistent with a double autocatalytic mechanism, they do not seem to represent truly formation of large agglomerates of bulk metal. The direct application of such a mechanism is not appropriate for the formation of Pt(0) nanoparticles via salt metal reduction in imidazolium ionic liquids. Therefore, the nanoclusters of zerovalent platinum in imidazolium ionic liquids resulting from the reduction of the Adams’ catalyst are mainly responsible for the high catalytic activity in the hydrogenation of olefins. 1. Introduction Various types of high surface area forms of platinum nanoparticles are among the most popular and used metal-based hydrogenation catalysts.1 Numerous methods, mostly based on colloidal techniques, have been developed in recent years to produce these platinum catalysts. Pt(IV) oxide (known as Adams’ catalyst) is one of the most used metal precursors because, on reduction with hydrogen, it readily provides a highly dispersed active form of platinum, which can be obtained in situ immediately prior to the introduction of the unsaturated compounds. The generally accepted concept is that synthesis of stable Pt(0) nanoparticles is routinely accomplished without the formation of bulk metal. However, it was recently demonstrated2-4 that Pt(0) nanoparticles formed from platinum salt precursors in organic solvents self-assemble into bulk metal, under reductive conditions, following a novel double autocatalytic mechanism (Scheme 1). Moreover, under these specific conditions, the agglomerated bulk metal (C) is the dominant substrate (cyclohexene) hydrogenation catalyst. It has been recently reported that various types of nanomaterials can be easily prepared in ionic liquids.5-13 For example, stable and redispersible transition metal nanoparticles of the type [M(0)]n are easily accessible through simple reduction with molecular hydrogen of transition-metal compounds dissolved * To whom correspondence should be addressed. E-mail:dupont@ iq.ufrgs.br. † Laboratory of Molecular Catalysis-Institute of Chemistry-UFRGS ‡ Universidade de Caxias do Sul, Departamento de Engenharia Quı´mica § Laboratory of Biomimetic Catalysis, Chemistry Department-UFSC
SCHEME 1: Four-Step Mechanism for Transition-Metal Nanocluster Nucleation, Growth and Agglomeration4a
a The four equations correspond to: (a) slow nucleation of catalyst precursor A to a nanocluster B, (b) autocatalytic surface growth, (c) agglomeration step leading to the formation of bulk metal C, and (d) autocatalytic agglomeration of smaller nanoparticles with larger bulk metal particles.
in imidazolium ionic liquids.5,14,15 It has been suggested that the combination of an intrinsic high charge and the steric bulk of these salts, which can be described as polymeric supramolecules with weak interactions,16-20 can create an electrostatic and steric colloid-type stabilization of transition-metal nanoparticles, similar to the model proposed for the stabilization of nanoclusters by polyoxo-anions or by tetralkylammonium salts.21-24 In particular, the controlled decomposition of [Pt2(dba)3] (dba) dibenzylidene acetone) dispersed in 1-n-butyl3-methylimidazolium tetrafluoroborate (BMI.BF4) and hexafluorophosphate (BMI.PF6) ionic liquids, in the presence of cyclohexene, by molecular hydrogen, produces Pt(0) nanoparticles. The formation of these nanoparticles follows the twostep [A f B, A + B f 2B (k1, k2)] autocatalytic mechanism.
10.1021/jp804870j CCC: $40.75 2008 American Chemical Society Published on Web 09/27/2008
16464 J. Phys. Chem. C, Vol. 112, No. 42, 2008 SCHEME 2: Basic Reaction Paths Involved in the Hydrogenation of Cyclohexene by PtO2 Dissolved in Ionic Liquids with the Formation of Nanoparticles
Scheeren et al. TABLE 1: Kinetic Constants, k1, k2, k3, and k4, Obtained in the Hydrogenation of Cyclohexene by the Catalyst Precursor PtO2 Dispersed in BMI.PF6 Ionic Liquid equations used in the PtO2/ kinetic cyclohexene k1 k2 k3 k4 modela (h-1)b (M-1h-1)b (M-1h-1)b (M-1h-1)b molar ratio 1/4000 1/4000 1/1000 1/1000 1/1000 a
a, b a, b, c a, b a, b, c a, b, c, d
From Scheme 1. stoichiometry.
It is therefore worth verifying whether, under reductive conditions, for platinum salts, the formation of particles in ionic liquids follows the same mechanism as that in organic solvents or that observed for the decomposition of Pt(0) organometallic precursors. Herein, we report the results of an investigation of the formation of Pt(0) nanoparticles25-30 under reductive conditions, using two imidazolium ionic liquids as the reaction medium and metal nanoparticle stabilizer. 2. Results and Discussion Pt(0) Nanoparticle Formation Kinetics. The system consists of dispersing PtO2, which under hydrogen reduction produces only water as a byproduct, in the hydrophobic ionic liquid BMI.PF6 and the hydrophilic analogue BMI.BF431 and performing the cyclohexene hydrogenation reaction using molecular hydrogen. Because cyclohexene hydrogenation is a fast reaction it can serve as a reporter reaction to follow the formation of Pt(0) nanoclusters (Scheme 2), which is expected to be the true catalyst of the reaction. A hydrogen pressure of 6 atm was used to avoid the mass transfer problems encountered in hydrogenation reactions performed in imidazolium ionic liquids.32 Figure 1 (left) shows the kinetic curve of cyclohexene consumption (H2 uptake, see experimental) in the presence of PtO2 under 6 atm of constant hydrogen pressure at 75 °C. The sigmoidal shape of this curve has previously been observed in
0.143 0.184 0.094 0.086 0.677 b
204.00 329.88 58.10 78.72 904.39
15.96 7.26 9.40
146.17
Rate constants corrected by the reaction
several hydrogenation reactions of olefins and it is characteristic of transition-metal nanoparticle formation under reductive conditions (e.g., iridium nanoparticles stabilized by poly oxoanions).33 The kinetic curve in Figure 1 (left) can be fitted to a two-step mechanism of metal nanoparticle formation: A f B, a slow nucleation step (rate constant k1), followed by an A + B f 2B autocatalytic surface growth step (rate constant k2), where A is the catalyst precursor and B is the Pt(0) nanoparticles (equations a and b, Scheme 1). However, with increasing PtO2 concentration, the hydrogenation curve exhibits a short induction period of about 0.4 h (Figure 1, right) and attempts to fit the curve with the two-step (equations a and b, Scheme 1) or threestep (equations a, b, and c, Scheme 1) mechanisms were unsuccessful. Such an induction period has been observed by Finke2,3 in systems with the formation of large agglomerates of bulk metal, as in the [PtCl2(1,5-COD)] reduction under hydrogen with N(n-Bu)3 and a proton sponge (1,8-bis(dimethylamino)naphthalene). The kinetic curves could only be fitted by including a new autocatalytic agglomeration step for the agglomeration of small particles (B) and larger bulk metal-like particles (C) (step d, Scheme 1). When this four-step mechanism is applied to the kinetic curve of part B of Figure 1, an excellent fit to the observed data is observed (straight line), indicating the formation of bulk metal
Figure 1. Cyclohexene hydrogenation curves with the catalyst precursor PtO2 dispersed in BMI.PF6 ionic liquid under 6 atm constant hydrogen pressure at 75 °C. (Left) 1/4000 PtO2/cyclohexene molar ratio, the curve fit is to the two-step mechanism (k1 ) 14.3 × 10-2 h-1 and k2 ) 204 M-1 h-1), there was no significant difference in the rate constants when fitting the curve to the three-step mechanism; (Right) 1/1000 PtO2/cyclohexene molar ratio, the curve fits are to the two-step (k1 ) 9.4 × 10-2 h-1, k2 ) 58.1 M-1 h-1) and four-step (k1 ∼ 67.7 × 10-2 h-1 and k2 ≈ 904.4 M-1 h-1, k3 ≈ 9.4 M-1 h-1 and k4 ≈ 146.2 M-1 h-1) mechanisms; all rate constants were corrected by the reaction stoichiometry (Experimental Details).
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Figure 2. Parts of the transmission electron micrographs (left) of platinum nanoparticles in BMI.PF6 at 75 °C and PtO2/cyclohexene molar ratios of: (A) 1/1000, at the end of induction period; (B) 1/1000, at the end of the reaction; and (C) 1/4000, at the end of the reaction. The histograms (right) show that the particle size distribution (300 particle counts) can be reasonably well fitted by a Gaussian curve.
Figure 3. Cyclohexene hydrogenation curves with the catalyst precursor PtO2 dispersed in BMI.PF6 ionic liquid under 6 atm constant hydrogen pressure at 30 °C and a 1/1000 PtO2/cyclohexene molar ratio, the curve fit is to the two-step mechanism (k1 ) 31.3 × 10-3 h-1 and k2 ) 11.5 × 10-3 M-1 h-1).
and its probable participation in the catalytic activity of the system at high precursor concentrations. The increase in the formation of large agglomerates of bulk metal with increasing
metal salt precursor concentration is predicted by Finke’s novel double autocatalytic mechanism. The rate constants are summarized in Table 1. Although all of the kinetic data for the cyclohexene hydrogenation with PtO2 dispersed in BMI.PF6 at 75 °C, with a 1/1000 substrate/catalyst ratio, are consistent with the formation of large agglomerates of the bulk metal catalyst, it was not possible to see these large agglomerates in solution with the naked eye. Moreover, we verified by TEM analysis that at the end of the induction period (part A of Figure 2) and at the end of the reaction (part B of Figure 2), only platinum nanoparticles with a mean diameter of around 3 nm were formed. These results are similar to those obtained in a more diluted substrate system (C/S 1/4000), where there was formation of platinum nanoparticles with a mean diameter of around 2.5 nm (part C of Figure 2). It should also be noted that the effect of decreasing temperature on the disappearance of the induction period was also not consistent with this novel mechanism. As reported by Finke and co-workers, changing the temperature at which the reduction takes place should have no effect on the number of steps in the mechanism. However, as shown in Figure 3, when the temperature of the PtO2 reduction in BMI.PF6 was set at 30 °C (1/ 1000 catalyst/substrate ratio), the induction period was not
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Figure 4. Parts of the transmission electron micrographs (left) of platinum nanoparticles in BMI.PF6 with a 1/1000 PtO2/cyclohexene molar ratio at the end of the reaction at 30 °C. The histograms (right) show that the particle size distribution (300 particles counts) can be reasonably well fitted by a Gaussian curve.
Figure 5. Cyclohexene hydrogenation curves with the catalyst precursor PtO2 dispersed in BMI.BF4 ionic liquid under 6 atm constant hydrogen pressure at 75 °C and (A) 1/1000 PtO2/cyclohexene molar ratio, the curve fit is to the two-step mechanism (k1 ) 39.7 × 10-2 h-1, k2 ) 1.0 × 10-3 M-1 h-1); (B) 1/4000 PtO2/cyclohexene molar ratio, the curve fit is to the two-step mechanism (k1 ) 13.5 × 10-2 h-1 and k2 ) 25.5 × 10-3 M-1 h-1).
observed and the kinetic curve could be fitted to the simplest two-step mechanism (steps a and b, Scheme 1). Moreover, the TEM analysis showed only formation of platinum nanoparticles with a mean diameter of around 2.3 nm (Figure 4). To check whether the induction period could be observed in a different ionic liquid, we performed the cyclohexene hydrogenation with PtO2 dispersed in BMI.BF4 at 75 °C and at the same concentrations of PtO2/cyclohexene used in the previous experiments. As shown in Figure 5, no induction period was observed for this ionic liquid and the TEM analysis indicated only the formation of nanoparticles with a mean diameter of 3.2 nm (Figure 6). These results show that although the kinetic curves, which show induction periods, only have a good fit with the double autocatalytic mechanism, they misrepresent the formation of large agglomerates of bulk metal. Therefore, we could not verify that such a mechanism is involved in the formation of Pt(0) nanoparticles via metal reduction in ionic liquids. In fact, our results strongly indicate that the nanoclusters of zerovalent platinum are mainly responsible for the high catalytic activity in the hydrogenation of olefins, under reductive conditions, in the presence of ionic liquids. To discard the hypothesis of the formation of bulk platinum agglomerates, which were not detected by in situ TEM analysis,
the size of the nanoparticles isolated from the ionic liquids were analyzed by X-ray diffraction (XRD). The PtO2 dissolved in the ionic liquids 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF6) or 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF4) form a gray solution. When this solution was submitted to 4 bar of hydrogen for 0.5 h it afforded a black suspension. Centrifugation of this mixture afforded a black solid, which was washed with dichloromethane and dried under reduced pressure. The phase structure of these isolated materials was analyzed by XRD (Figure 7). The XRD pattern (Figure 7) of the isolated material confirmed the presence of crystalline Pt(0) and the mean diameter could be estimated from the XRD diffraction pattern by means of the Debye-Scherrer equation calculated from the full width at halfmaximum (fwhm) of the (111),(200), (220), (311), and (222) planes obtained with Rietveld’s refinements. The most representative reflections of Pt(0) were indexed as face-centered cubic (fcc) with the unit cell parameter a ) 3.9231Å. The simulations of Bragg reflections and Rietveld’s refinements were performed with a pseudo-Voigt function. It is worth noting that the use of the fwhm of a peak to estimate the size of the crystalline grains by means of the Scherrer equation has serious limitations because it does not take into account the existence of the distribution of sizes and the presence of defects in the crystalline lattice. Therefore, the calculation of average grain diameter from the fwhm of the peak can overestimate the real value because the larger grains give a strong contribution to the intensity, whereas the smaller grains only enlarge the base of the peak. Moreover, the presence of a significant amount of defects causes an additional enlargement of the diffraction line. Considering this enlargement, the size determined can be smaller than the actual size of the grains. This drawback can be minimized by the use of Rietveld’s refinement method. Indeed, these discrepancies were confirmed by the values obtained for the average diameters of the nanoparticles of BMI.PF6 and BMI.BF4 without the structural refinement (4.0 and 5.3 nm), which significantly differ from those obtained after applying Rietveld’s refinement (3.3 and 3.9 nm) for the samples BMI.PF6 and BMI.BF4, respectively, with a 1/1000 PtO2/cyclohexene molar ratio. The mean diameters of the Pt(0) nanoparticles, calculated using Rietveld’s refinement, for a 1/1000 PtO2/cyclohexene molar ratio, were 3.3 and 3.6 nm for BMI.PF6 and BMI.BF4, respectively, and for a 1/4000 PtO2/cyclohexene molar ratio the values were 3.9 and 3.4 nm for BMI.PF6 and BMI.BF4, respectively. For the reaction with BMI.PF6 at 30 °C, the mean diameters calculated for the Pt(0) nanoparticles were 3.6 and 3.5 nm for molar ratios of 1/1000 and 1/4000 PtO2/cyclohexene,
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Figure 6. Parts of the transmission electron micrographs of platinum nanoparticles in BMI.BF4 at the end of the reaction, at 75 °C and with (A) 1/1000 PtO2/cyclohexene molar ratio, (B) 1/4000 PtO2/cyclohexene molar ratio, and the respective histograms showing the particle size distribution (300 particles counts) that can be reasonably well fitted by a Gaussian curve.
Figure 7. X-ray diffraction pattern of the Pt(0) nanoparticles prepared through PtO2 reduction in BMI.PF6 (left) and in BMI.BF4 (right), at 75 °C, PtO2/S ) 1000.
respectively. These latter values are much closer to those determined by TEM (Table 2). In summary, although all of the kinetic curves that present induction periods show a good fit with the double autocatalytic mechanism, they do not appear to represent truly formation of large agglomerates of bulk metal in imidazolium ionic liquids. In situ TEM analysis of various samples at the end of the induction period and at the end of the hydrogenation reaction
showed that only platinum nanoparticles with a mean diameter of around 3.0 nm were formed. Moreover, similar nanoparticle sizes were estimated by the XRD analysis of isolated metal nanoparticles, suggesting that no bulk metal was formed. The direct application of such a mechanism is not appropriate for the formation of Pt(0) nanoparticles via salt metal reduction in imidazolium ionic liquids. The zerovalent platinum nanoparticles in imidazolium ionic liquids-resulting from the reduction of
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TABLE 2: Mean Diameter (nm) of Platinum Nanoparticles Determined by TEM and XRD (Rietveld’s Refinement), Prepared from PtO2 Reduction in BMI.PF6 and BMI.BF4 sample
a
BMI.PF6 (75 °C)
BMI.PF6 (30 °C)
BMI.BF4 (75 °C)
Pt(0)
TEM (nm)
XRD (nm)a
TEM (nm)
XRD (nm)
TEM (nm)
XRD (nm)a
C/S ) /1000 C/S ) 1/4000
2.3 ( 0.3 2.7 ( 0.4
3.3 ( 0.4 3.2 ( 0.4
2.3 ( 0.3 2.5 ( 0.3
3.6 ( 0.4 3.5 ( 0.4
3.0 ( 0.4 2.7 ( 0.3
3.9 ( 0.4 3.4 ( 0.4
Mean diameter obtained with Debye-Scherrer equation method and in situ.
Figure 8. EDS analysis of the Pt(0) nanoparticles prepared in BMI.PF6 at 75 °C, PtO2/S ) 1000.
Adams’ catalyst-are mainly responsible for the high catalytic activity in the hydrogenation of olefins. Experimental Details General. All reactions were performed under argon atmosphere using Schlenk techniques. The substrates (Acros or Aldrich) were obtained from commercial sources and used as received. The ionic liquids were prepared according to known procedures,34,35 dried over molecular sieves (4 Å), and their purity was checked by the AgNO3 test, 1H and 13C NMR, and cyclic voltammetry. The water36 (
