Introduction:
Traditional CO oxidation and water gas shift (WGS) catalysts are typically noble metal nanoparticles dispersed on metal oxide supports. It has been shown that not all the material loaded on the support is catalytically active, and the support itself may participate in the reaction [1]. One aspect that has been difficult to study has been the nature of interactions between the noble metal and the oxide support. Thus it is important to study the catalytic nature of the noble metal nanoclusters in the absence of a metal oxide support. One must create gold nanoclusters analogous to those found on the supports in traditional catalysts on non oxide supports. These nanoclusters should be in the 1-4 nm size range. Previous attempts to form nano sized Au clusters did not result in clusters in the desired size range [2]. Dendrimers offer a new synthesis pathway for the formation of noble metal nanoclusters. Until the first work with Dendrimer Nanocluster Composites (DNCs) by Crooks et al.[4], unsupported noble metal nanoclusters comparable to those formed on metal oxide supports have not been readily available.
Materials and Methods:
Materials. Amine-terminated fifth-generation (G5.NH2) PAMAM dendrimers with ethylenediamine cores were obtained as 5% methanol solution (Sigma-Aldrich). HAuCl4 (Alfa Aesar), HCl, and NaBH4 (Sigma-Aldrich) were used without further purification. 18Ω-cm Milli-Q deionized water (Millipore, Bedford, MA) was used as solvent.
Preparation. The following procedure was followed to synthesize varying concentrations of G5.NH2 (Au55) [4,5]. Gold precursor solution was added to a dilute G5.NH2 dendrimer solution at a molar ratio of 55 Au equivalents per dendrimer. The resulting mixture was stirred at room temperature for 40 min, after which time the pH of the solution was adjusted to between 2 and 3 using HCl. Once the pH of the mixture became stable an amount of a 1.0 M solution of NaBH4 in 0.3 M aqueous NaOH equivalent to 10x molar excess of NaBH4 was added. Finally the pH of the mixture was adjusted to neutral if necessary.
Results and Discussion:
Poly(amidoamine) (PAMAM) dendrimers are used to prepare gold nanoclusters. These gold nanoclusters are more monodisperse than those seen in conventional gold nanoclusters prepared on transition metal oxide supports (Fig. 1). Similar results have been reported for Ru DNCs versus Ru supported on Al2O3 [6]. The efficiency of these DNC materials as CO oxidation catalysts will be reported. These DNCs can now serve as part of a model catalytic system to better understand the mechanism and active species in CO oxidation on supported noble metal catalysts.
Figure 1. TEM image of G5.NH2 (Au55) (should be noted that only Au nanoclusters show up in TEM image as dendrimers are not electron rich enough to be visible with out staining)
Figure 2. Histogram showing particle size distribution present in Fig. 1
Significance:
DNCs can be used as catalysts to determine the effect of the missing oxide support to better understand the mechanism and active species in CO oxidation on supported noble metal catalysts. To properly investigate the role of the interaction between metal and oxide it is necessary to have a baseline activity of nanoclusters of the noble metal without the support study has been possible synergistic interactions between the noble metal and the support.
While much work has been done using DNCs in catalyst preparation, the idea of using the free-standing dendrimer encapsulated nanocluster colloids as catalysts for CO oxidation is new.
References
1. Qi, F., Saltsburg, H. and Flytzani-Stephanopoulos, M. Science 301, 935 (2003).
2. Kronewitter, S. “Synthesis and Characterization of Gold Nanoparticles Supported by Polypeptides for Catalytic CO Oxidation” Tufts University, Medford, MA, 2005
3. Zhao, M., Sun, L., and Crooks, R.M. J. Am. Chem. Soc. 120, 4877 (1998)
4. Gröhn, F., Bauer, B.J., Akpalu, Y.A., Jackson, C.L., and Amis, E.J. Macromolecules 33, 6042 (2000).
5. Scott, R.W.J., Wilson, O.M., and Crooks, R.M. Chem. Mater. 16, 5682 (2004).
6. Lafaye, G., Williams, C.T., Amiridis, M.D. Catal. Lett. 96, 43 (2004)
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