Title

Preparation of supported ruthenium and platinum nanoparticles by supercritical incorporation methods

Date of Completion

January 2006

Keywords

Engineering, Chemical|Engineering, Materials Science

Degree

Ph.D.

Abstract

Nanoscale composite materials have attracted a great deal of interest as components for catalytic, optical and magnetic systems. Supported platinum or ruthenium nanoparticles are particularly interesting, since they are employed as heterogeneous catalysts in hydrogenation/oxidation reactions and show high activity in mild conditions. In preparation of such materials, the control over either particle dimensions, including the particle size and distribution, or the metal concentration are the major problems for most of the conventional techniques. ^ In this study, we demonstrated that supercritical incorporation is an effective way to prepare supported nanoparticles with controllable particle size and uniform size distribution. This process involves the dissolution of a metallic precursor in a supercritical fluid and the exposure of a substrate to the solution. Subsequent treatments of the precursor/substrate composites result in supported nanoparticles. In this study, we focused on the investigation of the fundamental aspects of the supercritical incorporation process by conducting systematic kinetic and thermodynamic studies to understand the factors that influence the final properties of the materials. The interaction between the precursor and the substrate is an important component of the supercritical incorporation process. This interaction was explored using two different ruthenium precursors, bis(2,2,6,6-tetramethyl-3,5-heptanedionato)(1,5-cyclooctadiene) ruthenium (II) (Ru(cod)(tmhd)2), ruthenium acetylacetonate (Ru(acac) 3), and one platinum precursor, dimethyl(1,5-cyclooctadiene)platinum (II) (PtMe2COD) with a wide variety of substrates including carbon aerogels (CAs), carbon black (CB), silica (SiO2), γ-alumina (γ-Al2O3) and Nafion®. The thermodynamics of adsorption of organometallic precursors on the substrates was investigated. Specifically, five adsorption isotherms of Ru(cod)(tmhd) 2 on CAs were measured using a static method at different temperature and pressure. The isotherms could be represented by the modified Langmuir model. Meanwhile, we investigated the dynamics of the impregnation process by determining the uptake of the substrate as a function of time in a batch system and developed a dynamic model for predicting concentration profiles in the fluid phase. The model predicted the experimental data fairly well with one adjustable parameter, the tortuosity factor. The metal particles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and hydrogen chemisorption. The obtained metal particle dimaters on different supports ranged from 1.2 to 6.4 nm with very narrow size distribution. The results indicated that volatile precursors and high temperature led to large particle size. Pt was impregnated into Nafion® membrane. Fourier transform infrared spectrum (FTIR) indicated that the structure of Naffon® film did not change significantly during the SCFI processing. The Pd impregnated Nafion® membrane was used for DMFC. The methanol cross over was reduced effectively compared with the non-treated Naffon® membrane. ^