Size distribution of Pt nanoparticles supported on alumina and silica substrates. A differential resonant SAXS study

S. J. A. FIGUEROA; J.M. RAMALLO-LÓPEZ; F.G. REQUEJO; G. KELLERMANN; A.F. CRAIEVICH; Y. CHIN; E. IGLESIA

Campinas, Brasil

Congreso; 18ª Reunião Anual de Usuários (RAU).; 2008

Laboratorio Nacional de Luz Sincrotron

Supported metal clusters are widely applied to catalyze many chemical reactions, such as methanation, reforming, and partial oxidation [1-3]. The size of the metallic clusters strongly influences the turnover rates and selectivity for structure-sensitive catalytic reactions [4]. For CO2-CH4 and H2O-C H4 reforming and CH4 decomposition reactions. CH4 activation turnover rates increase for decreasing size of noble metal nanoclusters [5], which is consistent with experimental and theoretical evidences that demonstrate the higher reactivity of unsaturated coordination of surface atoms on single crystals [6]. Pt surfaces exhibit a reactivity for activation of C-H bonds in CH4 higher than other noble metals (Rh, Ru, Ir) for an equivalent cluster size [5]. We have studied two samples containing 2% wt of Pt nanoparticles, one of them being supported on SiO2 and the other on Al2O3 porous and amorphous substrates. Differential resonant (or anomalous) small angle X-ray scattering (ASAXS) measurements performed at the XRD2 LNLS synchrotron beam line, employing several photon energies, allowed us to quantitatively determine the size distribution of Pt nanoparticles for both samples. The use of the differential ASAXS technique enabled us to identify the SAXS intensity produced by metallic clusters, which in the studied type of materials is usually much weaker than the strong contribution from the nanoporous substrate. By assuming a modeled structure consisting of a dilute set of spherical and homogeneous metal clusters embedded in a two-electron density nanoporous substrate, differences in the volume weighted radius distributions determined for each sample were clearly established. Details of the rather complex ASAXS technique when applied to dilute systems and special care that was required along our experiments is described and discussed. [1] B. Jenewein, M. Fuchs, K. Hayek, Surf. Sci. 532 (2003) 364. [2] M. C. J. Bradford, M. A. Vannice, Appl. Catal. A 142 (1996) 97. [3] S. Arrii, F. Mor¯n, A. J. Renouprez, J. L. Rousset, J. Am. Chem. Soc. 126 (2004)1199. [4] M. Boudart, Adv. Catal. 20 (1969) 153. [5] J. Wei, E. Iglesia, J. Phys. Chem. 108 (2004) 4094. [6] D. Bazin, Topics Catal. 18, 79 (2002).