FT-IR Investigation of the Thermal Decomposition of Poly(amidoamine

Feb 24, 2007 - D. Samuel Deutsch,† Attilio Siani,† Paul T. Fanson,‡ Hirohito Hirata,§ Shinichi Matsumoto,|. Christopher T. Williams,† and Mic...
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J. Phys. Chem. C 2007, 111, 4246-4255

FT-IR Investigation of the Thermal Decomposition of Poly(amidoamine) Dendrimers and Dendrimer-Metal Nanocomposites Supported on Al2O3 and ZrO2 D. Samuel Deutsch,† Attilio Siani,† Paul T. Fanson,‡ Hirohito Hirata,§ Shinichi Matsumoto,| Christopher T. Williams,† and Michael D. Amiridis*,† Department of Chemical Engineering, UniVersity of South Carolina, Columbia, South Carolina 29208, Research and Materials DiVision, Toyota Technical Center, Ann Arbor, Michigan 48105, Material Engineering DiV. III, Toyota Motor Corporation, Shizuoka, 410-1193 Japan, and Material Engineering DiV. I, Toyota Motor Corporation, Aichi, 471-8571 Japan ReceiVed: September 7, 2006; In Final Form: December 24, 2006

The decomposition and removal of poly(amidoamine) (PAMAM) dendrimers from inorganic metal oxide surfaces frequently used as catalyst supports was investigated by the use of FT-IR spectroscopy. Spectra of fourth-generation hydroxyl-terminated PAMAM dendrimers (G4OH) on γ-Al2O3 were collected first at room temperature and were subsequently analyzed with all bands assigned to the vibrational frequencies of dendrimer functional groups. Bands corresponding to amide and ethylenic groups decrease in intensity upon heating at 150 °C, while new bands corresponding to surface carboxylate species appear in their stead. Thus, the process of dendrimer removal occurs in two stages: dendrimer decomposition to form adsorbed carboxylates followed by the removal of these carboxylates from the surface. The dendrimer generation (i.e., G3OH vs G4OH) does not affect the rate of this process. However, the temperature required for completion of the first stage rises with increasing G4OH weight loading. Other factors that influence the rate of overall dendrimer removal were found to include the type of gas-phase environment used and the presence or absence of metal species within the dendrimer. Specifically, an oxidizing environment, or the presence of either platinum or rhodium, facilitates complete dendrimer removal at lower temperatures. Finally, although the rate of dendrimer removal is very similar on both alumina and zirconia, the conformations of the adsorbed dendrimers on these supports are different.

1. Introduction Traditional methods for synthesizing heterogeneous catalysts often involve the deposition of an inorganic salt precursor onto a porous support via incipient wetness impregnation or coprecipitation techniques, followed by rigorous drying, calcination, and reduction steps. Attempts to control the size of the resulting metal particles are often made through the optimization of precursor solution conditions and subsequent thermal treatments under different time/temperature protocols. However, such preparation methods often result in poor control over metal particle size, geometry, and dispersion.1 Heterogeneous catalyst synthesis through the use of dendrimer-metal nanocomposites (DMNs) is a more creative avenue that embodies an “atom-up” design for size and compositional control of supported metal nanoparticles. A number of research groups,2-5 including our own,6,7 have been investigating the properties of poly(amido)amine (PAMAM) dendrimers, a type of hyper-branched, spherical polymer, for use as metal nanoparticle stabilizers. Crooks et al.8 demonstrated that metal cation precursors can be complexed with functional groups in the branches of PAMAM dendrimers and suggested that subsequent treatment with sodium borohydride in solution can yield zerovalent metal nanoparticles “encapsulated” in the dendrimer structure. The number of sites for metal/dendrimer * Corresponding author. Phone: (803) 777-7356. Fax: (803) 777-8265. E-mail: [email protected]. † University of South Carolina. ‡ Toyota Technical Center. § Material Engineering Div. III, Toyota Motor Corporation. | Material Engineering Div. I, Toyota Motor Corporation.

complexation varies by polymer generation, thus making it possible to tune the number of metal atoms complexed within each dendrimer, and thus, the size of the resulting nanoparticles within. These unique properties of PAMAM dendrimers, in principle, can allow for the synthesis of DMNs with narrow metal particle size distributions for a range of different sizes (e.g., 1-4 nm).9,10 The use of DMNs for liquid-phase catalytic reactions has been reported by different groups.11-15 In these examples, the presence of the dendrimer does not isolate the active metal sites from the liquid-phase reactants. Compounds such as cyclooctadiene11,13 and various unsaturated alcohols12 can readily permeate the dendrimer, reach the catalytically active sites, undergo hydrogenation, and move to the periphery of the polymer without substantial difficulty. In fact, the presence of the dendrimer may prevent nanoparticle aggregation under reaction conditions or facilitate separation of the catalyst from the reactants and products after the completion of the reaction. DMNs deposited on inorganic oxide supports may also be used for liquid-phase reactions. Such heterogeneous systems do not differ significantly from the previous ones, since the active metal sites again appear to be readily accessible for reaction. Liu et al.2 have conducted a proof-of-concept experiment with a Pt40G4OH/Al2O3 catalyst (1 wt % Pt) that exhibited activity for the oxidation of CO in a flowing liquid stream. However, as we have shown previously, a similar catalyst (i.e., 1 wt % Pt40G4OH/SiO2) dried at room-temperature conditions and exposed to a flowing gaseous mixture of 1% CO in He at 100 °C does not exhibit any substantial adsorption of carbon monoxide.6 This result was attributed to the collapse of the

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Decomposition of PAMAM Dendrimers dendrimer on the inorganic oxide surface following the removal of the solvent, which leads to blockage of the active metal sites. Goddard and co-workers16 have performed molecular simulations with G4 through G6 PAMAM dendrimers investigating the effect of solvent on dendrimer structure and have shown that the presence of solvents leads to substantial swelling of the dendrimer. Furthermore, Ploehn and co-workers17 have shown through atomic force microscopy measurements that dried G4OH and Pt40G4OH samples exhibit hemispherical, and not spherical, features on mica surfaces, in accord with the assumption of dendrimer collapse. Thus, the synthesis of dendrimer-derived supported metal catalysts for gas-phase reaction systems requires the removal of the dendrimer precursor prior to the use of such catalysts. Such a removal step may result in coke formation, accumulation of organic species on the surface, and/or metal sintering, depending on the conditions used. In all cases, the activity of the resulting catalysts will be negatively affected. The goal of this work is to fully characterize the decomposition and removal of PAMAM dendrimers from the surfaces of inorganic oxide catalyst supports through the use of FT-IR spectroscopy. Empty dendrimers, as well as platinum and rhodium DMNs, were deposited on γ-Al2O3 and ZrO2 supports and were subjected to thermal treatments at successively and gradually increasing temperatures (in most cases up to 450 °C) in either oxidizing or reducing environments. The choice of metals and supports was based on the importance of these materials for a variety of catalytic applications, including ones in hydrogen production, selective CO oxidation, and NO reduction. The dendrimer decomposition and removal processes were monitored in situ in order to discern the identity and stability of different surface species formed at the various stages of polymer decomposition. The insight provided by this study is important for the pretreatment and activation of catalysts prepared from dendrimer-metal nanocomposite precursors. 2. Experimental Section 2.1. Materials. Hydroxyl-terminated third-(G3OH) and fourth(G4OH) generation PAMAM dendrimers were obtained in a methanol solution from Aldrich. Prior to use, the methanol was removed under N2 flow at room temperature, and the resulting viscous polymer was diluted with deionized water (18 MΩ‚cm Milli-Q). H2PtCl6‚6H2O (99.95% purity, Alfa Aesar), and RhCl3‚ xH2O (Rh 38.5%-45.5%, Alfa-Aesar) were used as received. A commercially available γ-Al2O3 (Alfa Aesar) and a proprietary ZrO2 were used as the supports. Both materials were calcined at 500 °C in air for 4 h prior to their use. 2.2. Synthesis. The synthesis of Pt DMNs has been reported previously.8 Briefly, an appropriate amount of a 2.5 × 10-2 M aqueous solution of H2PtCl6‚6H2O was added under nitrogen flow to a 0.17 M aqueous solution of G4OH dendrimer in order to obtain the desired molar ratio of metal to G4OH dendrimer. The mixture was stirred under nitrogen flow for 3 days to allow for the complexation of the Pt cations with the functional groups of the dendrimer. The dendrimer-Pt complexes were subsequently mixed with a freshly prepared NaBH4 solution (molar ratio of BH4- to metal cation of 8 to 1) to facilitate the reduction of the incorporated Pt cations. Following this step, the solution containing the Pt DMNs was added to a beaker containing previously calcined γ-Al2O3. The slurry was mixed via a magnetic stir bar at ambient conditions until the water was evaporated, and the contents of the beaker were dried. The resulting powder was caked onto the bottom of the beaker and was recovered with a stainless steel spatula.

J. Phys. Chem. C, Vol. 111, No. 11, 2007 4247 The same approach was used for the synthesis of Rhdendrimer nanocomposites. However, instead of a 3-day metaldendrimer complexation period, the Rh samples required only 1 day as indicated by in situ UV-vis measurements. The rhodium DMNs were deposited onto a commercial ZrO2 support. The nominal metal loading of both supported metal catalysts used in this study was 1 wt %. Empty G3OH and G4OH PAMAM dendrimer samples with different dendrimer loadings were also prepared on both the γ-Al2O3 and ZrO2 supports following the same standard wet impregnation and drying steps used for the preparation of supported DMNs. 2.3. Characterization. FT-IR spectra of the different samples were collected in the single beam absorbance mode with a resolution of 2 cm-1, using a Nicolet Nexus 470 spectrometer equipped with a MCT-B detector. A 6.5 cm-long cylindrical stainless steel flow cell was used. The two ends of the cell were capped with water-cooled NaCl windows. Samples were pressed into 12 mm wafers with a “thickness” of approximately 20 mg/cm2. The cell was mounted on an adjustable screw to allow for acquisition of spectra from two wafers aligned side by side in a parallel configuration. While these wafers were treated simultaneously in the cell, the infrared beam was only incident on one wafer at a time. The use of two wafers in the same experiment allowed for a direct reference of the sample spectra to the spectra of the bare support under identical conditions. Consequently, difference spectra thus obtained and utilized herein, contain bands corresponding to the vibrational modes of the supported material remaining on the surface at a given temperature. A heating element wrapped around the cell allowed for collection of in situ spectra at different temperatures. The cell temperature was monitored by a thermocouple placed in close proximity to the catalyst sample. The temperature was raised in steps of 50 °C from room temperature to 450 °C. Several spectra were collected at a particular temperature until changes in the spectra were negligible, (i.e., a steady state was reached). Only then, was the next temperature increase applied. During the dendrimer decomposition studies, in situ spectra were collected either in flowing H2 or a 10% O2-90% He mixture. Total gas flowrates were maintained at approximately 70 mL/ min. 3. Results and Discussion 3.1. Empty G4OH PAMAM Dendrimers on γ-Al2O3. Spectra of empty G4OH dendrimers supported on γ-Al2O3 are shown in Figure 1. The dendrimer loading in this case is 1.8 wt % G4OH, and corresponds to the amount of dendrimer present in a Pt40G4OH/γ-Al2O3 catalyst containing 1 wt % Pt. All of the different bands observed at room temperature can be assigned to the vibrational modes of the dendrimer functional groups.18-22 The strong peaks centered at 1647 and 1548 cm-1 can be assigned to the characteristic amide I (CdO stretching) and amide II (C-N stretching and C-N-H bending) vibrations of the dendrimer, respectively. Similarly, the band at 3080 cm-1 can be assigned to a resonance-enhanced overtone of the amide II band, whereas the relatively weak band at 1245 cm-1 can be assigned to the opening of the C-N-H group (amide III).23 Previous results suggest that the amide I & II bands may decrease in width and increase in sharpness with greater degrees of polymer hydration, a phenomenon that is most likely due to an increased level of inter- and intramolecular hydrogen bonding, creating more uniformly oriented amide carbonyls and N-H moieties.24 In the case of the dried and supported

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Figure 1. FT-IR spectra of 1.8 wt % G4OH PAMAM dendrimer supported on γ-Al2O3 obtained at different temperatures in flowing H2. (a) 1100-1800 cm-1 range; (b) 2500-4000 cm-1 range.

PAMAM dendrimers, the bands in the amide I and II region (Figure 1a) exhibit substantial breadth (i.e., full widths near halfmaximum intensities of 75 and 90 cm-1 for the amide I and amide II bands, respectively) suggesting that not all of the amide functional groups are interacting with their neighbors to the same extent. This hypothesis is further underscored by the breadth of the hydrogen-bonded N-H and O-H stretching bands of

the amide and dendrimer terminal groups centered at approximately 3300 and 3500 cm-1, respectively (Figure 1b).23 The additional bands observed in the room-temperature spectrum shown in Figure 1 correspond to C-H deformations and C-C backbone skeletal vibrations. Specifically, the asymmetric and symmetric stretching vibrations of methylene C-H bonds are observed at 2936 and 2825 cm-1, respectively.23

Decomposition of PAMAM Dendrimers

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TABLE 1: FT-IR Band Assignments for γ-Al2O3-Supported G4OH PAMAM Dendrimers band (cm-1)

functional group assignment

3500 3300 3080 2972 2936 2825 1647 1548 1464 1435 1350 1245 1157, 1128

O-H stretching23 amide N-H stretching23 overtone of amide II18-22 asymmetric C-H methyl stretching23 asymmetric C-H methylene stretching23 symmetric C-H methylene stretching23 amide CdO stretching (amide I)18-22 amide C-N stretching & C-N-H bending/closing (amide II)18-22 H-C-H Scissoring25 H-C-H asymmetric deformations25 H-C-H rocking, wagging, and twisting23 amide C-N-H opening amide III23 CsC skeletal stretching23

Second generationa hydroxyl-terminated PAMAM dendrimer a

The structure of a G2OH PAMAM dendrimer, and not that of a G4OH one, is shown for simplicity.

Dendrimer reconfiguration or bond scission on the support may explain the shoulder observed at 2972 cm-1, which can be assigned to asymmetric methyl C-H stretching. The corresponding symmetric methyl C-H stretch (ca. 2880 cm-1) is known to have a lower intensity and is probably overlapped by the predominant methylene C-H stretching bands in the same area. At lower wavenumbers (Figure 1a), a weak band corresponding to methylene scissoring is observed at 1464 cm-1, whereas another weak band at 1435 cm-1 can be assigned to asymmetric methyl deformations.25 The broad band at 14001300 cm-1 centered approximately at 1350 cm-1 is more difficult to assign. Although not always readily discernible with infrared spectroscopy, it is usually attributed to a combination of methylene rocking, wagging, and twisting deformations.23 Finally, the weak doublet at approximately 1157 and 1128 cm-1 can be assigned to skeletal C-C stretching.23 Worthy of mention is the absence of any tertiary amine vibrations in the room-temperature spectrum of the supported G4OH PAMAM dendrimers (see Figure 1). Such vibrations are often overlapped by the stronger bands induced by other functionalities. For example, the N-C3 stretching vibration is weak in infrared spectroscopy and coincides with the dendrimer’s C-C deformation modes.26 In addition, the C-H stretches of the amine N-CH2 groups are almost identical to those of the dendrimer’s methylene groups (C-CH2) observed at 2825 cm-1.23 A summary of the FT-IR room-temperature band

assignments corresponding to the different functional groups of G4OH PAMAM dendrimers on the γ-Al2O3 support is shown in Table 1. The remaining spectra in Figure 1 were recorded at elevated temperatures in order to explore the conditions necessary to decompose the dendrimer and remove its fragments from the support. The characteristic vibrational modes of the supported dendrimer remained unchanged from room-temperature up to approximately 100 °C. However, at 150 °C, both the amide I and II bands decreased in intensity, while a new band appeared in the 1575 cm-1 region. The broad band corresponding to methylene vibrations located at approximately 1400-1300 cm-1 decreased in intensity as well, suggesting the outset of dendrimer decomposition at this temperature. The results further suggest that, in the temperature range between 150-200 °C, the polymeric structure of the PAMAM dendrimers is destroyed and an assemblage of smaller adsorbed fragments is formed. For example, with the exception of a small band corresponding to the amide I vibration, all of the characteristic vibrational modes of the supported dendrimer that were readily identifiable at room-temperature were no longer observable at 200 °C. Instead, a new set of bands emerged in the same region. Just as PAMAM dendrimers are synthesized via Michael addition of ethylene diamine and methylacrylate, it has been proposed that PAMAM dendrimers in solution decompose via a reverse Michael addition process.27 In our case,

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Figure 2. FT-IR spectra of 1.8 wt % G3OH PAMAM dendrimer supported on γ-Al2O3 obtained at different temperatures in flowing H2.

the thermal decomposition of supported PAMAM dendrimers in flowing hydrogen appears to proceed through a reverse amidation process, resulting in the formation of carboxyl and carboxylic groups. The most prominent feature located at approximately 1575 cm-1 is the combination of at least two bands centered at 1585 and 1567 cm-1 and can be assigned to the antisymmetric stretches of COO- groups of adsorbed formate and acetate species, respectively.28-30 The symmetric stretch of the adsorbed formate is observed at 1374 cm-1, whereas that of the acetate can be found at 1451 cm-1.31-33 Finally, one possible assignment for the broad band centered at 1305 cm-1 is to an adsorbed acetaldehyde species.33 Upon further temperature increases, the bands corresponding to formate vibrations decrease in intensity until they finally disappear at approximately 350 °C. In contrast, the more stable acetates remain present on the alumina surface even after treatment at 400 °C. Treatment at 550 °C finally lowers the intensity of the acetate bands suggesting that such a high temperature is necessary for the removal of these last fragments from the support. Previous studies by Busca and Lorenzelli31,33 investigated formaldehyde adsorption, as well as C-C and C-H bond disruption, over various metal oxide surfaces. The aldehydes, linear and branched alkanes, alkenes, aromatics, and ketones used in these studies all formed surface carboxylates on oxide supports. Thus, it is not unlikely that a highly organized polymer containing carbonyls and C3 chains could do so as well. In fact, the desorption temperatures of the decomposed dendrimer residues coincide with those of adsorbed formates and acetates, as confirmed by previous literature reports,33 and further validate the band assignments of these species. In light of these observations, the overall process of the removal of supported dendrimers from γ-Al2O3 may be considered to take place in two stages: a dendrimer decomposition stage, followed by the decomposition and/or desorption of the products formed in the first stage. Under the conditions examined so far, the first stage begins at temperatures below

150 °C and is completed by 200 °C. The second stage occurs over a broad temperature range and is completed by 550 °C. 3.2. Empty G3OH PAMAM Dendrimers on γ-Al2O3. A 1.8 wt % G3OH/γ-Al2O3 sample was also examined in order to determine the effect of dendrimer generation on the dendrimer decomposition and removal process. Spectra of γ-Al2O3supported G3OH dendrimers were obtained under conditions identical to the ones described in the previous section and are shown in Figure 2. Although some of the characteristic bands of the G3OH dendrimer are not as intense as those in the spectra of the supported G4OH dendrimers, the process of dendrimer decomposition appears to be the same. Once again, the formation of adsorbed carboxylates (formates and acetates) can be observed at 200 °C. The formate species formed are removed from the γ-Al2O3 surface at approximately 400 °C, whereas the acetate species remain bound to the alumina at higher temperatures. These results are not surprising since identical dendrimer weight loadings were used for the two samples. Consequently, the number of terminal hydroxyl groups and tertiary amines were similar, and the only difference was the size of the supported molecules and, thus, the degree of separation between internal and peripheral functional groups of the dendrimers. Based on the FT-IR results shown in Figures 1 and 2, this parameter, and thus, the dendrimer generation, has a negligible effect on dendrimer decomposition and the removal of the decomposition products. 3.3. Effect of Weight Loading on G4OH Decomposition. Spectra of G4OH/γ-Al2O3 with dendrimer loadings of 1.8, 3.7, and 7.3 wt % (corresponding to 1, 2, and 4 wt % Pt in Pt40G4OH/γ-Al2O3 catalysts, respectively) were also collected before and during dendrimer decomposition in flowing hydrogen under conditions identical to those described in the previous sections. These spectra are similar to the ones shown in Figures 1 and 2 and are not shown for the sake of brevity. All samples exhibit similar behavior at room-temperature suggesting that adhesion of the dendrimer to the γ-Al2O3 support, as well as

Decomposition of PAMAM Dendrimers

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Figure 3. FT-IR spectra of 6.9 wt % G4OH PAMAM dendrimer supported on ZrO2 obtained at different temperatures in flowing H2.

the conformation of the supported dendrimers, do not vary with G4OH loading. Results obtained at temperatures greater than 25 °C, however, suggest differences in the rate and temperature of decomposition with dendrimer weight loading. Specifically, decomposition of the G4OH dendrimers took place at lower temperatures when lower dendrimer loadings were used. This temperature difference of approximately 50 °C between the 1.8 and 7.3 wt % loadings can be rationalized in terms of the coverage of the dendrimer on the γ-Al2O3 support. Theoretical monolayer coverages of 0.15, 0.30, and 0.60 monolayers are calculated respectively for 1.8, 3.7, and 7.3 wt % loadings of G4 PAMAM dendrimer on the γ-Al2O3 support used.16 However, since the dendrimer solution becomes very concentrated during the solvent evaporation stage of the wet impregnation procedure,34 we cannot exclude the possibility that cohesive dendrimer-dendrimer interactions may be have led to the formation of localized multilayers of the dendrimer on the γ-Al2O3 surface. This may explain the higher temperatures required for dendrimer decomposition at higher PAMAM weight loading, since there is a strong likelihood that this process is taking place at the dendrimer-alumina interface. It is also possible that, although the intact dendrimers may not form a complete monolayer, saturation of the support may be achieved by the carboxylates that remain on the surface upon completion of the first phase of dendrimer decomposition, especially in the case of the highest initial dendrimer loading. Additionally, as the weight loading of G4OH increased, an increase was observed in the intensity of the broad band in the 1700-1600 cm-1 range in the spectra recorded at 200-300 °C (i.e., upon completion of the first stage of dendrimer decomposition). This broad band can be assigned to carbonyl species with a variety of coordinations with nitrogen and carbon atoms, which are no longer part of the organized polymeric structure. Thus, the rise of this band with increasing dendrimer weight loading can be rationalized by the decomposition of greater

amounts of dendrimer at increased weight loadings, which results in the presence of greater quantities of organic material (including carbonyl groups from the decomposing amide functionalities) on the surface, ultimately yielding a variety of carbonyl compounds. Finally, all three samples exhibited the formation of mixed carboxylates on the surface at higher temperatures. In each case, the bands assigned to acetate vibrations remained present in the spectra, even at 450 °C. 3.4. Empty G4OH PAMAM Dendrimers on ZrO2. Spectra of ZrO2-supported G4OH PAMAM dendrimers obtained under conditions identical to the ones described in the previous sections are shown in Figure 3. At room-temperature, the main difference between the spectra of the ZrO2- and γ-Al2O3-supported materials is the presence of a new band in the spectrum of the G4OH/ZrO2 sample at 1735 cm-1, well outside the wavenumber range corresponding to the amide carbonyl, which was not previously observed with G4OH/γ-Al2O3. This band has been assigned to the carbonyl stretching vibration of carboxylic groups.18 The fact that this band is only present in the spectra of the ZrO2-supported sample suggests that some differences of dendrimer conformation can be observed on different supports. Upon increasing the temperature, the band at 1735 cm-1 steadily decreased in intensity until it fully disappeared at 250 °C (Figure 3). The disappearance of this band coincides with the growth of a new band in the 1570-1540 cm-1 range, which is partially overlapped by the amide II bands. This new band can be assigned to the vibrational modes of formates and acetates bound to the zirconia support, since these bands appear at lower frequencies on ZrO2 than on γ-Al2O3.31 Thus, as the temperature increases from room-temperature to 250 °C, the carboxylic groups present appear to react to form carboxylates bound to the zirconia support. At the same time, the skeletal structure of the dendrimers also decomposes, contributing to carboxylate formation. The G4OH dendrimers on both supports decompose

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Figure 4. FT-IR spectra of 1.8 wt % G3OH PAMAM dendrimer supported on γ-Al2O3 obtained at different temperatures in a flowing mixture of 10% O2 in He.

to form the residual organic species at approximately 250 °C. Since the dendrimer weight loadings of both samples are similar, this result suggests that dendrimer loading, rather than the nature of the support, appears to be the most important factor. Finally, the spectra obtained at higher temperatures indicate that the adsorbed carboxylates remain bound to both supports even after treatment at 450 °C. 3.5. Effect of Gaseous Environment on Dendrimer Decomposition. Supported empty dendrimers were also treated under oxidizing conditions, and the results obtained with 1.8 wt % G3OH/γ-Al2O3 are shown in Figure 4. The spectra collected at room temperature under reducing (Figure 2) and oxidizing (Figure 4) conditions are identical, indicating that the gas-phase environment does not induce any substantial restructuring of the dendrimer on the surface under these conditions. In both cases, the dendrimer decomposition takes place at similar temperatures (i.e., 200-250 °C). However, a difference is observed during the removal of the carboxylates formed following dendrimer decomposition. With the sample treated under oxidizing conditions, there is a higher rate of acetate removal at lower temperatures. This is expected, since gas-phase oxygen can react with the adsorbed carboxylates toward CO and CO2. With regard to the treatment of supported DMNs, this result is quite significant, as it has been shown that the adsorbed decomposition products of the dendrimer can impede the accessibility of potential reactants to the metal active sites.34 Nevertheless, since some metals (e.g., Ru) may sinter rapidly under oxidizing conditions,7 it is important to have a full understanding of the dendrimer removal process under both reducing and oxidizing environments. 3.6. Effect of Metal Presence on Dendrimer Decomposition. Spectra of Pt40G4OH/γ-Al2O3 (1 wt % Pt; 1.8 wt % G4OH) collected under flowing hydrogen at different temperatures are shown in Figure 5. Upon comparison of the spectrum collected at room temperature with the corresponding spectrum

of the nonmetal-containing 1.8 wt % G4OH/γ-Al2O3 material (Figure 1a), it becomes apparent that there are significant differences in the region corresponding to amide vibrations (1700-1500 cm-1). Specifically, a decrease in the intensity of the amide II band is observed in the presence of Pt. Metal cation precursors interact with the internal nitrogen atoms of amides and amines during the synthesis of DMNs.35,36 Thus, it is likely that the interaction of platinum with the internal nitrogen atoms results in the lower intensity of the N-H stretching band (1554 cm-1) observed. Additionally, a weak shoulder at approximately 1610 cm-1 is observed at room temperature. Although it is difficult to unambiguously assign this band, it is observed in every sample in which metal cations are present within the dendrimer. Together, the decrease in intensity of the amide II band and the new shoulder at 1610 cm-1 suggest that the presence of platinum results in an alteration of dendrimer configuration on the alumina support at room temperature and, possibly, initiation of the dendrimer decomposition process, even at room temperature. Upon heat treatment, the dendrimer component decomposes to form adsorbed carboxylate species by 200 °C in both the presence and absence of Pt. However, upon further treatment to 450 °C, the bands corresponding to acetate vibrations (ca. 1560 cm-1) disappear faster in the spectra of the Pt-containing sample, indicating that Pt catalyzes the second phase of overall dendrimer removal (i.e., the further reaction and desorption of surface carboxylates). Dendrimer decomposition and removal from the same sample under oxidizing conditions (Figure 6) follows the same steps. However, the rate of carboxylate removal from the surface is higher in this case and takes place at lower temperatures. These results are in agreement with our own previous studies of DMNs,6 as well as thermogravimetric analysis and temperature programmed desorption results reported by Chen and coworkers.37

Decomposition of PAMAM Dendrimers

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Figure 5. FT-IR spectra of Pt40G4OH (1 wt % Pt; 1.8 wt % G4OH) supported on γ-Al2O3 obtained at different temperatures in flowing H2.

Figure 6. FT-IR spectra of Pt40G4OH (1 wt % Pt; 1.8 wt % G4OH) supported on γ-Al2O3 obtained at different temperatures in a flowing mixture of 10% O2 in He.

This behavior is further enhanced in the case of Rh. Spectra of the G4OH/ZrO2 (6.9 wt % G4OH) and Rh20G4OH/ZrO2 (1 wt % Rh; 6.9 wt % G4OH) samples are shown in Figure 7, panels a and b, respectively. Just as in the case of the Pt system discussed above, the shapes and relative intensities of the roomtemperature amide bands are different in the presence and

absence of Rh, indicating differences in dendrimer configuration in the presence of Rh. Furthermore, at room temperature, the band at 1735 cm-1, assigned to carboxylic carbonyls is almost eliminated from the spectrum in the presence of Rh. Again, a shoulder on the amide I band located at approximately 1617 cm-1 is clearly observed at room temperature, similar to the

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Figure 7. FT-IR spectra of a) G4OH (6.9 wt %) and b) Rh20G4OH (1 wt % Rh; 6.9 wt % G4OH) supported on ZrO2 obtained at different temperatures in flowing mixtures of 10% O2 in He.

case of the Pt40G4OH/γ-Al2O3 sample. These results suggest that, when rhodium is present in the system, once again, the supported dendrimer adopts a different configuration on the support in addition to undergoing moderate decomposition, even at room temperature. Significant differences were also observed between the spectra of G4OH/ZrO2 and Rh20G4OH/ZrO2 (Figure 7, panels a and b,

respectively) at higher temperatures. Whereas a temperature of 250 °C is necessary to substantially decrease the intensities of the amide I and II bands in the absence of Rh, this decrease starts at approximately 100 °C in the presence of Rh, and the bands are completely eliminated at 200 °C. In addition, upon further heat treatment, the bands corresponding to adsorbed carboxylates decline faster in the presence of Rh. These results

Decomposition of PAMAM Dendrimers suggest, once again, that rhodium catalyzes the process of dendrimer decomposition and removal from ZrO2 similarly to platinum in the Pt-DMN/γ-Al2O3 system. 4. Conclusions Dendrimer removal is an important and necessary step for the synthesis of supported catalysts prepared from DMN precursors for use in heterogeneous catalytic applications with gas-phase reactants. The goal of this work was to characterize this critical step in order to facilitate the optimization of pretreatment conditions for these materials. The analysis of FTIR spectra obtained at different temperatures indicates that the overall dendrimer removal occurs in a two stage process. The first stage involves the decomposition of the dendrimer skeleton and functional groups to form mixed carboxylates on the catalyst surfaces. The second stage is the removal of these dendrimer decomposition products at elevated temperatures. There were no appreciable differences between decomposition of supported G3OH and G4OH PAMAM dendrimers, whereas increasing the weight loading of the G4OH dendrimer resulted in higher temperatures needed for the breakdown of the organized supported polymer. Although both alumina- and zirconiasupported G4OH dendrimers undergo decomposition and carboxylate removal at similar conditions, the dendrimer configurations on these supports are different. The gas-phase environment also plays an important role in dendrimer removal, since the adsorbed fragments of decomposed dendrimer are removed faster in an oxidizing environment rather than in a reducing one. Most pertinent to catalyst activation is the effect of the presence of a metal on the removal of the dendrimer component from supported DMNs. The presence of either Pt or Rh alters the configuration of the supported dendrimer and leads to the decomposition of the dendrimer functional groups upon introduction to the support, even at room temperature. Further heat treatment results in dendrimer decomposition and removal at temperatures 50-100 °C lower than those observed for supported empty dendrimers, suggesting that the metals catalyze each stage of the overall dendrimer decomposition process. Acknowledgment. The authors acknowledge the Toyota Technical Center and Toyota Motor Corporation for the financial support and permission to publish this work. Authors at the University of South Carolina further acknowledge partial financial support by the National Science Foundation (NSF Award CTS-0103135). Finally, D.S.D. and M.D.A. acknowledge the Graduate Fellowship support from the United States Environmental Protection Agency. Although the research described in this article has been funded in part by the United States Environmental Protection Agency, it has not been subjected to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred.

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