III International Conference

PP-II-162case of Pt-doped carbons. However, for direct synthesised materials the noble metalcompound is to some extent embedded in the carbon matrix, whereas, for impregnatedmaterials Pt is located inside the carbon support pore system only. In contrast, Au-dopedcarbons exhibit significant differences in their surface noble metal contents in respect ofthe synthesis route applied. Au-impregnated catalysts showed rather small noble metalparticles, predominantly occurring outside the pores of the carbon support. Differently,gold aggregates to very large particles in case of the direct synthesis route (results notshown here).However, in case of impregnated catalysts a significant influence of the metal particle size onthe specific catalytic activity was not established for Pt-particles > 4 nm, whereas a clearimpact of support pore widths on the specific catalytic activity was found (Fig. 1). Catalystswith Pt-particles < 2 nm reveal the highest specific catalytic activity. Attributed to masstransfer limitations catalysts prepared from MCM-48 are less active than those catalysts madefrom silica gel. However, analogous Au-samples are 2-3 times more active than Pt (not shownhere).Differently, specific catalytic activity of directly synthesised Pt-carbons depends onPt-particle sizes, whereas larger particles were found less potent oxidising glucose thansmaller particles are. Significant effect of support pore widths on the specific catalyticactivity could not be identified because of synthesis procedure used formation of bothcarbon pore system and metal particle occurs simultaneously. However, analogousAu-carbon materials tend to be inactive, due to their very large metal particle sizes (notshown here).AcknowledgmentThis work was supported by the DFG. Contract No: PA194/13-1 and KL 1202/3-1.References1. E. Auer, A. Freund, J. Pietsch, T. Tacke, Appl. Catal. A: Gen. 173 (1998) 259.2. R. Ryoo, S.H. Joo, S. J Jun., Phys. Chem. B, 103 (1999), 7743.3. Z. Ma, T. Kyotani, A. Tomita, Chem. Comm. 23 (2000) 2365.4. A.-H. Lu, W.-C. Li, W. Schmidt, W. Kiefer, F. Schüth, Carbon 42 (2004) 2939.466