Synthesis and Characterization of Dendrimer-Derived Supported

J. Phys. Chem. C 2008, 112, 13837–13845

13837

ARTICLES Synthesis and Characterization of Dendrimer-Derived Supported Iridium Catalysts Yaritza M. Lo´pez-De Jesu´s,† Aure´lie Vicente,‡ Gwendoline Lafaye,‡ Patrice Marećot,‡ and Christopher T. Williams*,† Department of Chemical Engineering, UniVersity of South Carolina, Columbia, South Carolina 29208, and Laboratoire de Catalyse en Chimie Organique, UniVersite´ de Poitiers, F-86022 Poitiers Cedex, France ReceiVed: January 8, 2008; ReVised Manuscript ReceiVed: July 3, 2008

The synthesis of Ir/γ-Al2O3 using the dendrimer metal nanocomposites (DMN) approach is reported. Fourth generation hydroxyl-terminated polyamidoamine dendrimer was complexed with Ir3+ in aqueous solution and the process monitored using ultraviolet-visible and X-ray absorption spectroscopy. No discernible reduction of Ir3+ to form zerovalent nanoparticles was observed after bubbling hydrogen or adding NaBH4 into the complex solution. Standard wet impregnation of the DMN precursors were used to prepare Ir/γ-Al2O3, which were compared with conventionally prepared samples. In situ transmission Fourier transform infrared spectroscopy during dendrimer thermal decomposition in different atmospheres and CO adsorption allowed for identification of catalyst activation treatments that expose the maximum metal surface area. The particle size distributions of these catalysts were investigated using high resolution transmission electron microscopy, revealing that all of the catalysts have small particle sizes (0.4-3 nm) with narrow distributions. An optimized oxidation/reduction treatment produced a DMN-derived supported catalyst with higher metallic dispersion. DMN-derived catalysts were tested for liquid-phase hydrogenation of benzonitrile, and show an increase in TOF with increasing dispersion. The selectivity toward dibenzylamine is affected by the catalyst preparation method, with the oxidation/reduction treatment resulting in lower selectivity. 1.0. Introduction Transition metal-based heterogeneous catalysts are frequently employed in numerous industrial reactions, as well as environmental applications such as automotive catalytic converters and fuel cell technologies. Among transition metals, noble metals are widely used since they possess unique properties that enhance activity and selectivity in chemical reactions. Supported iridium catalysts are outstanding candidates for a variety of catalytic reactions due to their stability, activity and selectivity under reaction conditions.1-6 For example, Ir catalysts possess unique characteristics for effective and selective synthesis of substituted N-aryl-hydroxylamines, chloro-substituted anilines and both symmetric and asymmetric azoxybenzenes.6 Furthermore, supported Ir has been successfully employed for hydrogenation of R,β-unsaturated aldehydes to selectively produce unsaturated alcohols.4,5,7-11 Supported Ir catalysts are commonly prepared via wet impregnation4,5,12-16 and incipient wetness impregnation.11,17-20 In supported metal catalysts, it is often desired to minimize the amount of noble metal needed (i.e., cost) by maximizing the metal surface area per unit volume. Consequently, the ability to produce small and highly dispersed metal nanoparticles is of great interest. However, conventional synthetic techniques (e.g., impregnation, ion-exchange) followed by calcination/reduction steps often result in wide particle size distributions. In addition * Corresponding author: E-mail: [email protected]. Telephone: 1 803 777 0143. Fax: 1 803 777 8265. † Department of Chemical Engineering, University of South Carolina. ‡ Laboratoire de Catalyse en Chimie Organique, Universite ´ de Poitiers.

to less than optimal performance, such nonuniform materials can also be difficult to characterize.21 Indeed, the nonuniformity, wide size distributions and unknown compositions (in the case of multimetallic catalysts) are considered drawbacks to studying and understanding reaction mechanisms.22 Several synthetic techniques have been developed over years to stabilize nanoparticles in solution. The resulting electrostatic or steric protection can allow improved control over heterogeneous catalyst synthesis. These new approaches include the use of polymers, surfactants, well defined organometallic cluster complexes, size-selected metal clusters, and ionic liquids. One recent method that has received considerable attention is the use of dendrimer-metal nanocomposites (DMN) as precursors. Dendrimers are monodisperse, hyperbranched spherical polymers that emanate from a central core with repetitive branching units, allowing for controllable size. For example, hydroxylterminated poly(amidoamine) (PAMAM-OH) dendrimers have been used extensively to chelate metal ions via their interior tertiary amine and secondary amide functional groups. The interior void spaces are then used for stabilization and creation of metal cluster or nanoparticles upon treatment with reducing agent.23-45 Such dendrimer metal nanocomposites have been successfully employed as precursors for supported metal nanoparticles on high surface area oxide supports. Narrow metal particle size distributions have been observed for different transition metals.25,29,30,34,35,40,41,45 For example monometallic Pt,25-31,45 Pd,26,27 Au,26,27,40,43 Ru,34,35 and Cu23,24,26 and bimetallic Pd-Pt,27 Pt-Au,40 Pd-Au,27 and Ag-Au27 catalysts have been prepared using different activation procedures. In some cases, it has been shown that dendrimer-derived catalysts have

10.1021/jp800152f CCC: $40.75  2008 American Chemical Society Published on Web 08/15/2008

13838 J. Phys. Chem. C, Vol. 112, No. 36, 2008 produced narrower particle size distribution and smaller particle sizes than conventionally prepared catalysts.34 Perhaps more important, it has been shown that DMN allow control of particle size by varying the dendrimer generation and metal to dendrimer molar ratios.26,35,39,43,46 However, there is as of yet no information about supported Ir catalyst synthesis using the DMN approach. This paper presents a study of the synthesis of Ir/γ-Al2O3 catalysts via the DMN precursor approach using fourth generation hydroxyl-terminated (G4OH) PAMAM dendrimer. Iridiumdendrimer complexation and reduction treatment in the precursor solution were examined with ultraviolet-visible (UV-vis) and X-ray absorption near edge (XANES) spectroscopies. An appropriate activation protocol was obtained via thermal dendrimer removal and CO adsorption studies using in situ transmission Fourier transform infrared (FTIR) spectroscopy. The amount of the residual carbon in the catalyst surface was investigated using temperature programmed oxidation (TPO). The resulting average particle sizes and particle size distributions were determined through analysis of high resolution transmission electron microscopy (HRTEM) micrographs. The DMN approach was found to produce well dispersed catalysts with different particle size depending on the treatment conditions, in contrast to the materials prepared by simple wet impregnation. The effects of using DMN precursor are more pronounced in catalyst with higher weight loadings. In addition, dendrimerderived catalysts show similar activity and selectivity for liquidphase benzonitrile hydrogenation when compared conventionally prepared catalyst. 2.0. Experimental Methods 2.1. Synthesis of Ir40G4OH. The G4OH-PAMAM (Aldrich) dendrimer was obtained as 10 wt % solution in methanol which, was removed by stripping with N2 (UHP) atmosphere at room temperature. Subsequently, 0.17 mM dendrimer solution was prepared using deionized 18 MΩ cm (Milli-Q) water. Then, 6.50 mL of 8.0 mM of IrCl3 · 3H2O (AlfaAesar) aqueous solution was mixed with 7.58 mL of the G4OH-PAMAM dendrimer. The resulting mixture was diluted with deionized water for a total volume of 40 mL, to generate Ir to dendrimer mole ratio of 40:1 (Ir40G4OH). This ratio was chosen to allow for equilibrium dispersion of the ion precursor throughout the dendrimers, since fourth generation hydroxyl-terminated PAMAM dendrimer has up to 62 amines available sites for metal ion coordination. The final solution was a yellow color, with concentration of 1.3 mM Ir and 0.0325 mM G4OH dendrimer. This solution was stirred under N2 atmosphere until the UV-vis complexation spectra among Ir and dendrimer did not show further changes (see Results and Discussion). The formation of Ir nanoparticles in solution was attempted either by bubbling H2 (UHP) for 2 h through the solution or by adding a 10-fold excess solution of NaBH4 (Aldrich) at room temperature. 2.2. Synthesis of Ir Catalysts. Dendrimer-derived (DD) and conventionally derived (CD) Ir/γ-Al2O3 catalysts were prepared by standard wet impregnation of the Ir40G4OH complex solution or aqueous IrCl3 · 3H2O, respectively, onto powdered γ-alumina (AlfaAesar, 45 m2/g). The alumina was calcined for 4 h at 500 °C in air prior to impregnation. In each case, a slurry solution was formed and the water was evaporated by stirring for 2 days in air at room temperature. Catalysts with weight loadings of 1% and 2.5% were synthesized. 2.3. Characterization. The complexation process was monitored at room temperature via UV-vis spectroscopy using a Shimadzu UV-2101PC scanning spectrophotometer with quartz

Lo´pez-De Jesu´s et al. cells of 1 cm path length. A background spectrum from an identical cell filled with deionized water was subtracted. XANES spectroscopy experiments were performed at X-ray beamline 2-3 of the Stanford Synchrotron Radiation Laboratory (SSRL) at the Stanford Linear Accelerator Center, Menlo Park, CA. The storage ring energy and current were 3 GeV and 50-100 mA, respectively. Double-crystal monochromator Si(220) was used to select a single energy detuned by 20% to minimize higher harmonic effects in the X-ray beam. DMN solutions were studied in fluorescence mode using a 13 element germanium detector. The data were collected at room temperature using an in situ X-ray absorption spectroscopy (XAS) cell designed to handle corrosive liquid samples without air exposure47,48 positioned at 45° from the incident X-ray beam and detector. Normalized X-ray absorption spectra were obtained by dividing the absorption intensity by the height of the absorption edge. These steps and further analysis of the reduced data were performed utilizing XDAP software.49 In situ FTIR spectra were recorded using a Nicolet Nexus 470 spectrometer equipped with mercury-cadmium-telluride B (MCT-B) detector cooled by liquid nitrogen. FTIR spectra were collected in single beam absorbance mode with a resolution of 4 cm-1. Catalyst samples of approximately 50 mg were prepared as self-supporting pellets with a diameter of 12 mm. These samples were placed in a variable temperature gas flow transmission cell made of stainless steel. The cell has a length of 10 cm with two IR-transparent NaCl windows cooled with flowing water. The temperature of the cell was monitored by a thermocouple placed near to the catalyst sample. The heating was achieved using a heating element wrapped around the cell and an Omega CN76000 temperature controller. The dendrimer decomposition studies were carried out in three different atmospheres: helium (UHP), hydrogen (UHP), and 10% O2/He (UHP). The temperature was increased in stepwise increments from room temperature to 425 at 5 °C/min. At each selected temperature several spectra were collected until steady state was achieved. The background for these experiments was taken using a pellet of previously calcined alumina. The background spectra were recorded each time just before the sample spectrum was taken. Therefore, the FTIR peaks shown can be attributed clearly to vibrations of functional groups of G4OH-PAMAM dendrimer. To determine treatments that expose the largest number of Ir surface atoms, the molecular adsorption of CO onto the catalyst was examined after various treatment steps. These treatments include both an oxidation/reduction procedure and a single reduction procedure. Catalyst pellets were placed inside the FTIR cell and first exposed to He flowing gas (∼70 mL/min) in other to remove any impurities. Subsequently, the catalysts were exposed to the desired gas and heated at 5 °C/min from room temperature to a selected temperature where it was maintained at that temperature for the necessary time. After cooling to room temperature, the system was purged with He for 15 min and a background spectrum was recorded. A 1% CO/He mixture was then flowed through the catalyst for 15 min, followed by He for an additional 15 min to purge CO gas and remove weakly bonded CO species. Once the CO adsorption spectrum was taken, the procedure was repeated at the next higher temperature. These FTIR spectra were deconvoluted using Galactic PeakSolve peak-fitting software. The software allows for the use different fitting models (e.g., Gaussian, Lorentzian, log-normal) to obtain the peak position, width, height, and area of the overlapping peaks.

Dendrimer-Derived Supported Ir Catalysts

Figure 1. Time-dependent UV-vis spectra for Ir40G4OH complex solution.

TPO of the catalysts was done using a 1% O2/He gas mixture, with a gas flow rate of 12 mL/min. The TPO experiments were performed in a fixed-bed quartz reactor with 50 mg of sample. The temperature range was 25-550 °C with a ramp of 5 °C/ min. The carbon dioxide production was monitored by a thermal conductivity detector. The purpose of the TPO experiments was to quantify the remaining dendrimer fragments on the catalyst surface after different thermal treatments. HRTEM imaging of Ir particles were carried out on a Philips CM 120 microscope operating at 120 kV. Supported Ir catalysts were ultrasonically dispersed in ethanol prior to their deposition onto a copper grid covered by carbon support film. The experimental volume-surface mean diameter and particle size distributions were obtained by measurement of a minimum of 300 randomly selected particles from at least three images of each sample. 2.4. Benzonitrile Hydrogenation. Liquid-phase benzonitrile hydrogenation experiments were carried out using a 100 mL high-pressure autoclave batch reactor. The reactor was loaded with 80 mL of ethanol, 1 mL of diglyme as an internal indicator, and 0.5 g of treated catalyst. Then, it was sealed and leak tested with 300 psi of H2, followed by a five pressure/vent cycles in order to remove any trace oxygen. Before introducing benzonitrile to the reactor, the temperature was increased to 100 °C at 300 psi H2, while the content of the reactor was stirred at 1000 rpm. When, the desired conditions were attained, 2 g of benzonitrile was introduced into the reactor using a high-pressure pump. Several liquid samples were taken at different reaction times and analyzed by gas chromatography using a HewlettPackard 5890 Series II gas chromatograph equipped with an HP-5 capillary column and a flame ionization detector. 3.0. Results and Discussion 3.1. Ir40G4OH in Solution. As shown in Figure 1, two main UV-vis bands at ca. 330 and 390 nm were detected for 1.3 mM IrCl3 · 3H2O aqueous solution. Similar peak maxima have been observed before in kinetics studies of aquation of Ir(III) complexes.50,51 The absorption band in the ultraviolet region (240 - 350 nm) can be assigned to the spin allowed π f π* transitions from water ligands.52-54 The peak at ca. 390 nm is a recognized spin allowed metal-to-ligand charge transfer (1MLCT)52-54 i.e., Ir to chlorine ligands. The spectrum of the 0.0325 mM G4OH-PAMAM dendrimer solution (Figure 1) shows a broad shoulder at 270 nm which corresponds to π f π* transitions of the amides groups.55,56 Time-dependent spectra

J. Phys. Chem. C, Vol. 112, No. 36, 2008 13839

Figure 2. Ir LIII-edge XANES spectra for (Ir3+)40G4OH complex and IrCl3 aqueous solutions before and after reduction with H2.

for G4OH-PAMAM dendrimer/IrCl3 · 3H2O solution mixture shows gradual changes occurring over seven days. The transition peaks observed for the aqueous solution of IrCl3 decrease gradually, suggesting Ir complexation with the dendrimer. In addition, the ∼270 nm band shifts to higher wavelength with time. This shift is attributed to the interaction between Ir and the dendrimer functional groups.57-60 Further changes were not observed after 7 days of complexation, suggesting that maximum Ir-dendrimer complexation is achieved in this time period. The resulting (Ir3+)40G4OH solution with a pH of 4.5 was subsequently saturated with H2 or combined with a 10-fold excess of NaBH4 at room temperature and atmospheric pressure with a goal of reducing the Ir3+ to Ir0 clusters. Such treatment has been used previously in studies of Au, Pd, Cu, Pt and Ru DMNs 23-45 and Me´vellec et al.61 used NaBH4 to reduce Ir3+ in presence of N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium chloride salt as surfactant to obtain Ir nanoparticles. However, no significant changes in the UV-vis spectrum were observed after (Ir3+)40G4OH solution was saturated with these reducing agents. Furthermore, the solution was yellowish before and after H2 and NaBH4 treatments, with no precipitation observed. The complexation and reduction treatments steps were further examined using liquid-phase XANES measurements. Figure 2 shows normalized XANES spectra for IrCl3 · H2O and (Ir3+)40G4OH aqueous solutions before and after H2 exposure. The band structure curves were integrated numerically to determine the average white line area, which is associated with the transition probability of 2p2/3 electrons to d empty states and is often correlated with the electron density on metal atoms.62,63 The average white line areas are 12.4 and 10.6 for (Ir3+)40G4OH and IrCl3 aqueous solutions, respectively, before and after H2 bubbling. In the case of NaBH4, the average white line area decreased only by around 10% for both solutions (see Supporting Information, Figure S1). These are further indications that the attempted reduction of Ir3+ can not be achieved at room temperature and atmospheric pressure using these reducing agents. The XANES spectra for the Ir-dendrimer complex solution and the Ir trichloride aqueous solution are different. The average white line area is around 15% higher for the former solution. Such differences reveal the variety of Ir-ligands bonding interactions. A more detailed XANES and EXAFS study of IrCl3-G4OH interactions will be the subject of a forthcoming report. Although Ir nanoparticles are not formed in solution, the DMN precursor was utilized to prepare supported Ir catalysts

13840 J. Phys. Chem. C, Vol. 112, No. 36, 2008

Figure 3. FTIR spectra for G4OH-PAMAM dendrimer thermal decomposition in (a) He, (b) O2, and (c) H2 atmosphere.

to investigate the role of dendrimer in localizing Ir3+ ions together on the catalyst support. It was decided to use the DMN precursor treated with H2 to avoid having unwanted Na+ in the resulting catalysts. 3.2. Ir40G4OH/γ-Al2O3: Dendrimer Decomposition. Previous research on DMN-derived catalysts has demonstrated that it is necessary to remove the dendrimer by thermal treatment in order to expose the metal on the support surface and induce catalytic activity.28,30,34 Therefore, in situ FTIR spectroscopy was used to study the dendrimer thermal decomposition. Parts a-c of Figure 3 show spectra for (Ir3+)40G4OH/γ-Al2O3 during thermal treatment under pure He, 10% O2/He, and pure H2, respectively. In all three cases two main peaks related to the presence of G4OH-PAMAM dendrimer on γ-Al2O3 support are evident at room temperature (top spectra).28,31,34 The highest peak centered at 1644 cm-1 is assigned to the CdO stretching vibration of the amide group (i.e., amide I).28,30-32,34,36 The peak

Lo´pez-De Jesu´s et al. at 1556 cm-1 arises from the associated C-N stretching and C-N-H bending vibrations (i.e., amide II).28,30-32,34,36 The broad feature at 1350 - 1460 cm-1 that is present in all three environments at room temperature has been found previously when Pt and Ru are present.31,34 This feature is mostly composed of three peaks at ca. 1460, 1430, and 1394 cm-1, which correspond to methylene (CH2) scissoring vibrations,28,30,31,34,36 asymmetric methyl deformations and a combination of methylene rocking, wagging and twisting deformations, respectively.31 Independent of the gaseous environment, the two main peaks start decreasing slightly as low as 50 °C. Dendrimer decomposition studies performed under H2 for empty dendrimers suggest that dendrimer remains intact up to 100 °C.31 Thus, the presence of Ir in the supported catalyst accelerates the dendrimer thermal decomposition as has been found for other transition metals.30,31,36 Significant dendrimer decomposition occurs at 150 °C under reducing environment,11,31,36 while in oxidative and inert environments large decreases in the amide I and II vibrations occurred at 200 °C. A new band is also observable at ca. 1585 cm-1 at 100 °C for oxidative and reducing environments, while for inert atmosphere it is evident at 150 °C. This peak is assigned to the antisymmetric stretches CO2- groups of adsorbed formate and acetate species on the support.31 The symmetric adsorption of CO2- groups occurs around 1440-1335 cm-1, thus coinciding with methylene vibrations and making it very difficult to distinguish between them. Nevertheless, the small band at ca. 1377 cm-1, which appears at 150 °C for H2 and O2 and at 200 °C for He, is likely from the symmetric stretch of adsorbed formate.31 In H2, after 250 °C the 1460-1390 cm-1 peaks show a blue shift and an increase in intensity up to 350 °C, suggesting a build up of carboxylates on the surface. These peaks are largely removed by 425 °C. In H2 the amide I peak is completely removed from surface at 425 °C, while in O2 and He a small portion is still present at 425 °C. During the H2 treatment a small peak at 1305 cm-1 appears above 150 °C, grows to a maximum at 350 °C, and then is removed at 425 °C. This peak corresponds to acetaldehyde species.31 In He and O2 the methylene vibrations remain largely in the same position from room temperature up to 425 °C. In contrast, these peaks are much less pronounced in H2 at 425 °C. This suggests that the removal of dendrimer is more extensive under reducing atmosphere at elevated temperature. For all three environments, some aliphatic residue (∼1460 cm-1) remains after (Ir3+)40G4OHPAMAM dendrimer thermal decomposition at 425 °C.34,36 In general, there is more residue evident after He exposure than there is for the oxidizing and reducing environments. In addition, peaks between 1990 and 2150 cm-1 emerge for the three different environments. They appear at 100 °C in H2, 150 °C in O2, and 200 °C in He. These features are attributed to a combination of amine chloride (C-NH3+Cl-) species or carbonyl groups adsorbed on the exposed Ir species.64-66 The evolution of these peaks is dependent on the flowing gas. For He three peaks are present even up to 425 °C, while in O2 there are only two peaks remaining. In H2 a broad peak with two shoulders appears, but decreases dramatically at 400 °C and is removed at 425 °C. Therefore, G4OH-PAMAM dendrimer and its decomposition products can be removed most effectively from dendrimer-derived 1 wt % Ir/γ-Al2O3 by using hydrogen treatment. 3.3. Catalyst Activation. Given that dendrimer thermal decomposition under oxygen and hydrogen was more complete than in He, only oxidation and/or reduction protocols to expose Ir particles on the catalysts surface were examined further. Room

Dendrimer-Derived Supported Ir Catalysts temperature FTIR spectra were obtained for CO adsorption after catalyst treatments at various temperatures and exposure durations. Figure 4a shows the effect on the CO adsorption after the catalyst was exposed to 10% O2/He at 250, 350, and 425 °C for 30 min at each. While temperatures below 250 °C were studied as well, much less CO was adsorbed. Clearly, the highest adsorption of CO occurred when the catalyst was treated under the oxidative mixture at 350 °C. A main peak at ca. 2080-90 cm-1 is clearly observed after the oxidation treatment at 250 °C, and grows substantially by 350 °C before diminishing at 425 °C. Since, the maximum CO uptake (as indicated by the envelope intensity) was obtained after treatment with O2 at 350 °C, the exposure time was varied at this temperature and a CO adsorption spectrum was recorded after each exposure. As shown in Figure 4b, the highest CO adsorption was obtained after exposure to oxygen at 350 °C for 30 min. The main peak blue-shifted as the oxygen exposure length increased, and peaks at higher frequencies were noticed as well. In addition, the negative peak at 1890 cm-1 (Figure 4b) indicates that the CO contact at room temperature helps to remove dendrimer decomposition byproduct originated under O2 at 350 °C. Since a reduced catalyst is desired, the subsequent reduction in H2 was examined in a similar way. Figure 4c shows that treatment in H2 at 400 °C was required to maximize CO uptake. Again, temperatures below 350 °C were studied (not shown) and exhibited much less adsorbed CO. Different CO adsorption peaks are observed after the reduction by H2. A broad envelope is present at 350 °C, where four overlapping peaks are detected at around 2070 cm-1, 2060 cm-1, 2045 cm-1, and 2010 cm-1. The peak at 2045 cm-1 is less evident when the reduction temperature is increased and the weak band at around 1820 cm-1 is shifted to higher frequency. It is notable that no negative going peaks were present upon CO adsorption after the H2 treatments. This suggests that the H2 is effective in removing dendrimer fragments that remain from the O2 treatments. As above, spectra were collected to determine the optimal reduction length by exposing the catalyst to H2 at 400 °C for different times. As shown in Figure 4d, the maximum CO uptake was obtained after reduction at 400 °C for 1 h. Therefore, one effective dendrimer decomposition/metal activation treatment consists of oxidation at 350 °C for 30 min followed by reduction at 400 °C for 1 h. As mentioned above, CO adsorption studies were also performed following activation and dendrimer removal in only H2. Figure S2a (see Supporting Information), shows the effect of varying reduction temperature from 350 to 425 °C on the adsorption of CO. Again, treatment below 350 °C yielded significantly less CO absorption. The reduction treatment at 400 °C results in the largest band envelope consisting of a main peak at 2075 cm-1, a shoulder at around 2015 cm-1, and a weak band ∼1830 cm-1. Time-dependent spectra taken after reductions at 400 °C are shown in Figure S2b, revealing that a treatment of 2 h is sufficient to maximize CO uptake. Therefore, a second favorable treatment for the dendrimer-derived Ir catalyst involves direct reduction in H2 at 400 °C for 2 h. As described earlier in this manuscript and reported in the literature,27,29 some carbon species, as coke or aliphatic residue, can remain on the catalyst surface after dendrimer thermal decomposition. In order to quantify the remaining carbon on the catalyst surface, the dendrimer-derived 1 wt % Ir/γ-Al2O3 was characterized by TPO. Prior to the TPO, the same batch of catalyst was split out in samples, and each of them underwent a different thermal treatment: (i) a single oxidation at 350 °C for 30 min; (ii) an oxidation at 350 °C for 30 min followed by


Figure 4. CO adsorption after (a) O2 at different temperatures, (b) O2 at 350 °C for different times, (c) H2 at different temperatures, and (d) H2 at 400 °C for different times.


Lo´pez-De Jesu´s et al.

Figure 5. TPO profiles of 1 wt % Ir40G4OH/γ-Al2O3: (s) dried (before any thermal treatment); (-4-) after oxidation at 350 °C for 30 min; (...) after oxidation at 350 °C for 30 min followed by reduction at 400 °C for 1 h; (-0-) after reduction at 400 °C for 2 h.

a reduction at 400 °C for 1 h; (iii) a single reduction at 400 °C for 2 h. The catalysts were then cooled down to room temperature and flushed with nitrogen. For comparison, TPO was also performed on a dried sample i.e., before any thermal treatment. These results are summarized in Figure 5. The 1 wt % Ir/γ-Al2O3 catalyst was prepared with iridium to dendrimer molar ratio of 40:1, leading to a dendrimer weight percent of 1.9. Since G4OH-PAMAM dendrimer is composed of 52.3% carbon, the catalyst contains 1 wt % carbon before any treatment. Indeed, for the dried sample, CO2 is produced from 250 to 550 °C. Two main peaks, centered at 370 and 430 °C, are discernible (Figure 5). The assignment of these different peaks cannot be determined on the basis of current data. Further study would be required to complete the identification. The total CO2 formation is 834 µmol/gcatalyst, which implies the presence of 1 wt % of carbon on this sample, which is in accordance with the theoretical value. All the carbon was removed from the catalyst surface in the course of the TPO experiment. Figure 5 show that the thermally treated dendrimer-derived 1 wt % Ir/γ-Al2O3 exhibit drastically decreased CO2 production. After the single oxidation treatment, the CO2 formation starts only at 290 °C and is complete by 475 °C. Contrary to the dried sample, there is only one broad peak centered at 395 °C. Therefore, after an oxidation at 350 °C for 30 min, only one type of carbon species remains on the catalyst. Moreover, the total CO2 formation is 166 µmol/gcatalyst instead of 834 µmol/ gcatalyst for the dried sample. Thus, the oxidation treatment allows removal of 80% of the carbon present (i.e., 0.2 wt % of carbon remains on the surface). When the oxidation procedure is followed by a reduction at 400 °C for 1 h (Figure 5, oxidation/ reduction), the broad peak disappears and only a small amount of CO2 is formed in the 250 - 475 °C temperature range. This treatment procedure therefore leaves only 0.05 wt % residual carbon on the surface. However, the single reduction treatment procedure is more effective to remove the dendrimer since 99% of the carbon is removed. This is on accordance with the in situ FTIR dendrimer decomposition studies discussed above. With each of these two activation treatment protocols established, the room temperature CO adsorption spectra for dendrimer-derived (DD) 1 wt % Ir/γ-Al2O3 was further examined by peak deconvolution using spectral curve fitting. For comparison, FTIR spectra were also obtained for the 1 wt % Ir/γ-Al2O3 prepared by simple wet impregnation. Similar peak maxima were found after both activation protocols for DD and conventionally prepared catalysts. For example, Figure 6 shows typical deconvolution of spectra obtained for DD and conven-

Figure 6. CO adsorption deconvoluted spectrum for (a) dendrimerderived and (b) conventional Ir/γ-Al2O3 after oxidation at 350 °C followed by reduction at 400 °C.

tionally derived (CD) Ir/γ-Al2O3 after the O2/H2 treatment. However, the intensities of the peaks varied depending on the catalysts and treatment. Peak assignments based on the available (and often conflicting) literature are summarized in Table 1.3,67-76 Assignment of the peaks between 2000 and 2090 cm-1 is complicated by the varying assignments found in literature. Two types of species have been generally observed on supported catalysts: linear CO (LCO) and gem dicarbonyls (GCO). Each of these is now considered in turn. Lynds observed a main peak at 2070 cm-1 for CO adsorbed on 0.5 and 1.83 wt % Ir/γ-Al2O3 catalysts, which was assigned to LCO.71 The band shifted around 30 cm-1 lower as the CO coverage decreased, presumably due to decreased dipole coupling. Howe et al.70 and Solymosi et al.73 examined CO adsorption on 2.5 wt % Ir/Al2O3 and 5 wt % Ir/Al2O3, respectively. They ascribed peaks observed between 2060 and 2080 cm-1 to be LCO bonded to larger Ir nanoparticles (i.e., with mean crystallite sizes in the range of 3.0-7.0 nm). A peak at ca. 2020 cm-1 was recognized as LCO adsorbed on smaller Ir clusters or support-bound Ir atoms. Howe et al. observed the same peaks for 2.5 wt % Ir/SiO2.70 However, McVicker et al. specifically assigned a peak around 2020 cm-1 to be LCO adsorbed on larger Ir clusters on alumina support.77 Erdo˜he´lyi et al.,76 Nawadali et al.,3 and Bourane et al.67 examined the CO adsorption on 1 wt % Ir/Al2O3 observing the LCO peak at ca. 2060 cm-1. In addition, Bourane et al. observed a strong peak at 2044 cm-1 that was assigned to LCO species on fully reduced Ir sites.67 Finally, Toolenar et al. found two peaks at 2050 and 2080 cm-1, which were designated to CO adsorbed on low coordinated Ir sites (i.e., edges and corners)

Dendrimer-Derived Supported Ir Catalysts


TABLE 1: CO Average Adsorption Wavenumber (cm-1) of Ir/γ-Al2O3 characteristic peaks assigments (Ir0 activation treatment

catalyst

O2 at 350 °C 30 min. and H2 at 400 °C 1 h H2 at 400 °C 2 h a

a

DD CDa DD CD

)2 > CO

Ir < (CO)2 dicarbonyl +

Ir0-CO

Ir+-CO

bridge

antisymmetric

symmetric

linear

linear

1804 1806 1832 1824

2014 2008 2013 2010

2078 2078 2084 2083

2042, 2059 2038, 2057 2048, 2068 2046, 2065

2101 2101 2103 2102

DD ) dendrimer-derived; CD ) conventionally derived.

TABLE 2: Structural and Catalytic Properties of DD and CD Supported Ir Catalysts catalyst 1 wt % Ir/γ-Al2O3

2.5 wt % Ir/γ-Al2O3

fractional R′BN,I synthetic activation mean diameter volume to surface (nm) mean diameter (nm)b dispersionc (mol · L-1 · molIr-1 · min-1) methoda treatment DD CD DD CD DD CD

H2 H2 O2/H2 O2/H2 O2/H2 O2/H2

1.2 1.1 1.0 1.1 1.2 1.4

1.5 1.3 1.2 1.4 1.4 1.7

0.73 0.84 0.91 0.78 0.78 0.64

10.5 ( 3.4 6.6 ( 0.6 29.4 ( 3.4 14.2 ( 1.5 17.4 ( 1.4 16.8 ( 0.1

TOFd (min-1)

selectivity (%)e DBA

1.1 ( 0.3 0.6 ( 0.1 2.6 ( 0.3 1.5 ( 0.2 1.7 ( 0.2 1.9 ( 0.1

78 ( 9 86 ( 3 57 ( 1 52 ( 3 36 ( 1 41 ( 2

a j ) ∑in niDp,i3/∑in niDp,i2, where ni is the number of particles DD ) dendrimer-derived; CD ) conventionally derived. b Calculated using D j , where am is the effective average area and Dp,i is the measured diameter.81 c Metallic dispersion (DM) is calculated by DM ) 6(VM/am)/D occupied by a metal atom in the surface (7.69 × 10-20 m2/atom), VM is the volume per metal atom in the bulk given by MW/FN0, MW is the atomic weight, F is the density, and N0 is Avogadro’s number.81 d Based on metallic dispersion obtained from HRTEM data. e At 20% conversion of benzonitrile.

and CO adsorbed on high coordinated Ir sites (i.e., single crystal planes), respectively.74 Several investigators have also noted the presence of the bands consistent with GCO. For example, Bourane et al. studied the CO species adsorbed on to 1 wt % Ir/γ-Al2O3 and found a pair of bands at ca. 2080 and 2000-2013 cm-1.67 On the basis of their behavior, these bands were recognized as arising from Ir-(CO)2.67,72 More specifically, the band at around 2080 cm-1 is due to the symmetric dicarbonyl vibration and the ca. 2010 cm-1 band is due to by the antisymmetric carbonyl vibration.69,76 Generally these bands are noticed in highly dispersed Ir catalysts, where the GCO species adsorbs on small clusters or isolated Ir atoms that can accommodate the simultaneous adsorption of two CO molecules.72 On the basis of these studies and the high dispersion of the catalysts examined here (see below), the following assignments are made. The two peaks between 2040 and 2070 cm-1 are attributed to LCO adsorbed on low coordinated Ir sites. The pair of peaks at ca. 2080 and 2010 cm-1 is assigned to GCO species. The small band just above 2100 cm-1 is assigned to linear CO adsorbed on Ir ions.68 It has been proposed that such species arise from oxidative disruption of Ir crystallites upon CO adsorption.72 Bridge CO (BCO) adsorbed on supported Ir catalysts is not observed regularly. Nevertheless, Guerra et al. observed that BCO adsorption yield peaks from 1890 to 1910 cm-1.69 In the present study, the BCO band appears between 1805 and 1830 cm-1 depending on the treatment. This low frequency is consistent with very low coverage. 3.4. Particle Size. HRTEM micrographs were analyzed for DD and CD catalysts after activation using both treatment protocols (see Supporting Information, Figure S3). It was noticed that after each treatment there were spherical Ir nanoparticles supported on alumina with narrow particle sizes distribution. The theoretical Ir particle diameter is estimated assuming cubic closed-packed (ccp) crystal structure and considering the smallest sphere that circumscribes 40 Ir atoms. The equation used to calculate Ir nanoparticle diameter (Dp) can be written as

3

3Fn 4π

Dp ) 2

(1)

where F is the unit density in nm3 atom-1 and n is the number of atoms. Given the Ir ccp unit cell parameter of 0.3839 nm, the theoretical particle diameter for 40 atoms of Ir is calculated as 1.03 nm. The experimental mean diameters, volume-tosurface mean diameters (VSMD) and metal dispersions are summarized in Table 2. It is noticeable that the VSMD is around 20% higher than the mean diameters for all the catalysts investigated. Therefore, the majority of the Ir atoms are contained within particles that are larger than the mean diameter. For dendrimer-derived catalysts, the Ir nanoparticles exhibit a smaller average diameter and higher dispersion for the oxidation/reduction (OR) treatment than for the reduction (R) treatment. This indicates that the thermal removal of the dendrimer and catalyst activation under oxygen treatment induce less agglomeration or sintering. Cunha et al. also reported an increase in particle size when reduction was used alone for Ir/ γ-Al2O3 catalysts prepared via incipient wetness.78 In contrast, here the conventionally prepared catalysts showed relatively little variation with treatment. When the OR treatment is employed to remove the dendrimer the mean particle diameter is equivalent to the theoretical value. Unlike what has been observed previously for other metals such as ruthenium and rhodium,34,79 there is relatively little effect on particle sizes when dendrimer precursors are used to prepare 1 wt % Ir/γ-Al2O3. However, analysis of histograms obtained from images of DD and CD 2.5 wt % Ir/γ-Al2O3 reveal some more significant differences. As shown in Figure 7, the DD catalyst exhibits smaller particle size and narrower particle size distribution than the CD catalyst. Therefore, DMN precursors do appear to play at least a modest role controlling the sintering/ agglomeration processes of Ir nanoparticles over alumina. Although Ir supported on alumina exhibits strong resistance to sintering due to strong metal support interaction,19 slightly smaller particle sizes could be achieved when oxidation/ reduction treatment and DMNs were employed.


Lo´pez-De Jesu´s et al. wt % Ir DD catalyst treated with O2/H2 has a higher TOF by than all of the other 1 wt % catalysts examined. The TOF is found to increase with decreasing particle size in the case of the two O2/H2-treated DD catalysts. In contrast, the O2/H2treated CD catalysts exhibit the opposite trend. In general, the TOF of O2/H2-treated catalysts are roughly two and a half-times more active than their H2-treated counterparts, suggesting that this is a preferred treatment method to maximize activity. The selectivity was strongly dependent on the treatment conditions, with the O2/H2 method resulting in (on average) a 20-30% lower selectivity toward the secondary amine. One possible cause of these differences in selectivity could be differences in the oxidation state of Ir after the various treatments. Such effects are currently being investigated using X-ray photoelectron spectroscopy (XPS) and, along with more detailed kinetic investigations, will be addressed in a future publication. Nevertheless, the preliminary results obtained here suggest that the DMN-derived materials are effective for benzonitrile hydrogenation catalysts and that they exhibit differences in performance when compared with those prepared by more conventional methods. 4.0. Conclusions

Figure 7. HRTEM micrographs and particle size distribution histograms of (a) 2.5 wt % Ir40G4OH/γ-Al2O3 and (b) 2.5 wt % Ir/γ-Al2O3 treated with oxidation/reduction treatment.

3.5. Benzonitrile Hydrogenation. Liquid-phase benzonitrile hydrogenation was used as a probe reaction, to examine the catalytic properties of dendrimer-derived and conventionalderived supported Ir catalysts. Nitrile hydrogenation reactions are mostly employed to produce primary, secondary and tertiary amines.9 In our case, the main products for this reaction are benzylamine (i.e., primary amine), dibenzylamine (secondary amine) and toluene. The catalysts were treated ex situ with the oxidation/reduction and reduction treatments discussed above. The initial rate of reaction, initial turnover frequency (TOF), and dibenzylamine (DBA) selectivity at 20% benzonitrile conversion are reported in Table 2. Figure S4 (see Supporting Information) provides an example of a typical benzonitrile concentration versus time profile, and the procedure by which the initial rate of reaction and selectivity were calculated. The TOF reported here is calculated using the theoretical metallic dispersion calculated from HRTEM data. The reactor system was found to be free of both external and internal mass transfer limitations. The former was confirmed by measurements made at various stirring speeds, which showed that the reaction rate was invariable with this parameter above 500 rpm. The latter was confirmed by employing the Madon-Boudart experimental criterion,80 which revealed that the TOF was relatively unaffected for catalysts with similar dispersion and different Ir weight loadings. As shown in Table 2, all DD and CD supported Ir catalysts were found to be active for benzonitrile hydrogenation. The 1

The complexation between IrCl3 salt and G4OH PAMAM dendrimer in water under inert atmosphere at room temperature is accomplished during seven days. Ir nanoparticles are not formed in with the exposure to H2 or NaBH4 saturated solutions. Two optimal dendrimer removal treatments were found through monitoring the dendrimer decomposition and CO adsorption on Ir via FTIR spectroscopy. HRTEM results show spherical monodispersed Ir nanoparticles on γ-Al2O3. The final properties of the DD 1 wt % Ir/γ-Al2O3 catalysts are different depending on the dendrimer removal and activation treatment protocol. In contrast, the CD catalyst particle size distribution was invariant with treatment. The use of the DMN precursors reduces the growth in particle size for catalysts with large Ir loadings when the oxidation/reduction protocol is employed. Finally, DMNderived catalysts were shown to be effective benzonitrile hydrogenation catalysts, with catalytic properties than were different than those prepared by conventional means. Acknowledgment. We gratefully acknowledge the National Science Foundation (NSF NIRT Award CTS-0103135), Alfred P. Sloan Foundation Program from National Action Council for Minorities in Engineering (NACME), Inc., and Southeastern Alliance for Graduate Education and the Professoriate (SEAGEP) for their financial support. In addition, we thank to the Stanford Synchrotron Light Source (SSRL), a national user facility operated by Stanford University in California on behalf of the U.S. Department of Energy for access to perform XANES. Supporting Information Available: Figures showing Ir LIIIedge XANES spectra for (Ir3+)40G4OH complex and IrCl3 aqueous solutions before and after reduction with NaBH4, FTIR spectra for CO adsorption on DD 1 wt % Ir/γ-Al2O3 catalysts after H2 exposure at different temperatures and times, HRTEM micrograph and particle size distribution histograms for DD and CD 1 wt % Ir/γ-Al2O3 catalysts, and a typical benzonitrile concentration-time profile obtained during catalytic studies. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R.; Dupont, J. Chem. Eur. J. 2003, 9, 3263.

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JP800152F

Synthesis and Characterization of Dendrimer-Derived Supported

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