Shape Changes of PVP-capped Pt Nanoparticles Induced by Deposition on Mesoporous Silica (SBA-15) Monitored by TEM, SAXS and EXAFS

L. J. GIOVANETTI; J.M. RAMALLO-LÓPEZ; M. FOXE; L. JONES; M.M. KOEBEL; G.A. SOMORJAI; A. F. CRAIEVICH; M. SALMERON; F. G. REQUEJO

Pretoria

Congreso; International Conference on Frontiers of Polymers and Advanced Materials (11th ICFPAM); 2011

University of Pretoria

Supported nanoparticles (NPs) are used as catalysts in many reactions. Traditional synthesis methods usually lead to a wide size distribution of randomly shaped NPs. The reaction rates, activity and selectivity may be different on  each individual catalyst particle, depending on morphology, size, crystal structure and composition.  We investigated the existence of ventual changes in shape of initially spherical PVP capped Pt NPs, about 2 nm in diameter, homogenously dispersed in a colloidal suspension, after their deposition onto a SBA-15 nanoporous silica substrate. Several experimental techniques were used, namely transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS).  We found that Pt NPs in liquid suspension exhibit morphological changes. After being impregnated in mesoporous SBA-15, the Pt NPs which were  spherical before impregnation transform their shape, becoming flat and platelet-like. We attribute this change of shape to the existence of a strong interaction between the atoms of Pt NPs and SBA-15 pore surfaces.  The results of the fitting procedure of EXAFS spectra corresponding to Pt NPs in suspension and impregnated in the SBA-15 substrate indicate a contraction in the average Pt first-neighbour distance relative to bulk Pt (0.2771 nm); the contraction is higher than for NPs in suspension. This effect is the expected consequence of the higher number of atoms located at the surface of flattened NPs. These results indicate that, even at room temperature, the interaction with the SBA-15 support strongly affects the structure and morphology of PVP-capped Pt NPs. It is concluded that, in the analysis of the reactivity and other physico-chemical properties of supported NPs, the possibility of strong changes in their structure and shape should not be discarded a priori.