ABSTRACT

Bethany J. Auten, Christina J. Crump, Anil R. Singh and Bert D. Chandler

Department of Chemistry, Trinity University, 1 Trinity Place, San Antonio, TX, 78212-7200

bert.chandler@trinity.edu

Abstract We are developing a new method for preparing heterogeneous catalysts utilizing polyamidoamine (PAMAM) dendrimers to template metal nanoparticles.(1) In this study, generation 4 PAMAM dendrimers were used to template Pt or Au Dendrimer Encapsulated Nanoparticles (DENs) in solution. For Au nanoparticles prepared by this route, particle sizes and distributions are particularly small and narrow, with average sizes of 1.3 ± 0.3 nm.(2) For Pt DENs, particle sizes were around 2 nm.(3) The DENs were deposited onto silica and Degussa P-25 titania, and conditions for dendrimer removal were examined. The focus of these studies has been on identifying mild activation conditions to prevent nanoparticle agglomeration. Infrared spectroscopy indicated that titania plays an active role in dendrimer adsorption and decomposition; in contrast, adsorption of DENs on silica is dominated by metal-support interactions. Relatively mild (150° C) activation conditions were identified and optimized for Pt and Au catalysts. Comparable conditions yield clean nanoparticles that are active CO oxidation catalysts. Supported Pt catalysts are also active in toluene hydrogenation test reactions. Introduction Industrial heterogeneous catalysts and laboratory-scale model catalysts are commonly prepared by first impregnating a support with simple transition metal complexes. Catalytically active metal nanoparticles (NPs) are subsequently prepared through a series of high temperature calcination and / or reduction steps. These methods are relatively inexpensive and can be readily applied to numerous metals and supports; however, the NPs are prepared in-situ on the support via processes that are not necessarily well understood. These inherent problems with standard catalyst preparation techniques are considerable drawbacks to studying and understanding complex organic reaction mechanisms over supported catalysts.(4)

We are developing a new route for preparing model catalysts that uses Polyamidoamine (PAMAM) Dendrimer Encapsulated Nanoparticles (DENs) as NPs templates and stabilizers. PAMAM dendrimers, which bind transition metal cations in defined stoichiometries, can be used to template mono-and bimetallic NPs in solution. DENs have been used as homogeneous catalysts for a variety of organic reactions, including hydrogenations (particularly size-selective hydrogenations), and Suzuki, Heck, and Stille coupling reactions.(5) Bimetallic DENs are also active hydrogenation catalysts in solution, and their catalytic activity can be tuned by controlling NP composition.(5) The potential to ultimately control NP properties makes DENs extremely attractive as precursors to supported catalysts. The possibility of controlling particle size and composition makes DENs uniquely suited to exploring the relative importance of these effects on catalytic reactions. Because the nanoparticles are prepared ex situ and can be deposited onto almost any substrate or support, DENs offer the opportunity to examine tailored NPs using materials comparable to those employed as industrial catalysts. However, before DENs can be utilized as heterogeneous catalyst precursors, appropriate methods must be developed to remove the organic template. If activation conditions are too harsh, particle agglomeration may suppress the potential advantages of the dendrimer method. If activation conditions are too mild, incomplete removal of the dendrimer may leave organic residues on the particles and poison the catalyst. Background Previous work has focused largely on dendrimer removal from Pt-DENs supported on silica. Our studies have shown the amide bonds that comprise the dendrimer backbone are relatively unstable (they begin decomposing at mildly elevated temperatures, ca. 100 °C) and that the Pt nanoparticles help to catalyzed the dendrimer decomposition.(3,6,7) However, extended higher temperature oxidation and/or reduction treatments (several hours at 300 °C) are required to completely remove organic material from Pt DENs. For Pt/SiO2 catalysts, dendrimer oxidation appears to lead to the formation of surface carboxylates, which partially poison catalytic activity.(6,8) A recent surface science study by Chen and coworkers supports these general findings and provides convincing evidence that Pt plays an important role in catalyzing dendrimer decomposition.(9) Crooks and coworkers, who studied Pd and Au DENs immobilized in sol-gel titania, similarly reported the onset of dendrimer mass loss at relatively low temperatures (ca. 150 °C). Pd helped to catalyze dendrimer decomposition in their system, as well. Temperatures of 500 °C or greater were required to completely remove organic residues from their materials.(10) This treatment resulted in

substantial particle agglomeration, particularly for Au-based materials, although pore templating by the dendrimer mitigated the particle growth. Using Pt-DENs/SiO2, we showed that activation temperatures could be reduced to as low as 150 °C by using CO oxidation reaction conditions. Supported Pt catalysts pretreated in 1% CO & 25% O2 at 150 °C for 16 hours had essentially the same CO oxidation activity as catalysts oxidized at 300 °C for 16 hours. In this activation protocol, CO essentially acts as a protecting group: strong CO adsorption prevents Pt from participating in dendrimer oxidation and prevents dendrimer fragments from fouling the nanoparticle catalyst. Results and Discussion Based on our previous work developing activation conditions for supported Pt DENs, we began investigating dendrimer removal from supported Au DENs under CO oxidation catalysis conditions (1% CO, 25% O2). Au based catalysts are extremely sensitive to preparation techniques and sintering, so minimizing activation temperatures is in important consideration for this system. Additionally, in spite of the potential for high CO oxidation activity, it is unclear whether Au NPs will be capable of participating in dendrimer oxidation. Consequently, we began our study by directly monitoring CO oxidation activity during catalyst pretreatment. In these experiments, shown in Figure 1, the CO oxidation reactor system was charged with supported, in tact Au-DENs/TiO2, and CO oxidation activity was monitored as a function of time on stream at various temperatures (100, 125, and 150 ºC). The time required to reach maximum activity varied from 8 hours at 150 °C to 24 hours at 100 °C. We sought to keep activation temperatures as low as possible to minimize sintering, so higher temperatures were not investigated. Additionally, activation under CO oxidation conditions relies on adsorbed CO to “protect” the metal particle surface from poisoning by dendrimer decomposition products. The “protective” properties of CO diminish as adsorption equilibira favor free CO at higher temperatures.