One-Step Flame-Synthesis of Carbon-Embedded and -Supported Platinum Clusters

Abstract

Carbon-embedded or -supported platinum clusters (1−12 wt % Pt) were made rapidly by a scalable, single-step flame spray pyrolysis (FSP) process. Pt-containing precursors dissolved in xylene were sprayed and combusted in a controlled oxidation atmosphere resulting in nanostructured, carbon-embedded Pt clusters. Reversing the order of particle formation by combusting xylene alone and the Pt precursor downstream onto the freshly made carbon particles led to carbon-supported Pt clusters. Both carbon-embedded and -supported Pt clusters possessed the self-preserving size distribution of aerosols grown by coagulation in the free-molecular regime. This indicates a homogeneous gas-phase formation pathway rather than the heterogeneous one typically observed in flame synthesis of noble metal catalysts on ceramic supports. These Pt/C composites were tested as catalysts and characterized by scanning and high-resolution transmission electron microscopy, Raman spectroscopy, nitrogen adsorption, X-ray diffraction, and CO chemisorption. Specific surface areas ranged from 25 to 200 m2/g and Pt clusters were well-dispersed. Carbon-embedded Pt clusters (2–5 nm) were not accessible for CO chemisorption and inactive as catalysts for hydrogenation of cyclohexene indicating hermetic carbon coating of platinum clusters. In contrast, carbon-supported Pt clusters (5–15 nm) chemisorbed CO and were active hydrogenation catalysts demonstrating the accessibility of their Pt surface.