Mark Swihart,  UB Distinguished Professor and Department Chair, Chemical and Biological Engineering, University at Buffalo (SUNY)

Flame-based Aerosol Synthesis of Metal Nanoparticles and Supported-Metal Nanostructures

Abstract: This talk will introduce a nanomaterial synthesis method that we call the High-Temperature Reducing Jet (HTRJ) process, describe some of the materials generated by this process, and discuss their applications. Flame-based processes are the dominant commercial approach to the production of large-volume nanomaterials such as carbon black, fumed silica, and titania nanoparticles. However, flame-based synthesis of non-noble metal nanoparticles is not usually possible, due to the presence of an oxidizing region in the flame and coupling between flame chemistry and particle formation chemistry. Our HTRJ process overcomes these limitations and also allows synthesis at lower temperature than in conventional flame reactors. In our HTRJ reactor system, the hot combustion products of a fuel-rich hydrogen flame pass through a converging-diverging nozzle to accelerate them to sonic or supersonic velocity. An aqueous precursor solution injected at the throat section of the nozzle is atomized by the high velocity gas stream, providing exceptionally rapid heating and mixing of the hot gas and liquid droplets. For many metals, the droplets evaporate, and the precursor decomposes, initiating nucleation of particles in a reducing environment containing excess H2. After the reaction zone, particles are cooled immediately to prevent further particle growth and coalescence. This approach separates combustion chemistry from particle formation chemistry, allowing use of low-cost aqueous solutions of metal nitrates and other salts as precursors. Metals that can be reduced by hydrogen in the presence of water are generated as metallic nanoparticles. Complete conversion of precursors to particles allows precise composition control of alloys and multi-component mixtures. Metals that cannot be reduced by hydrogen in the presence of water form as oxides in this system, and very often follow a droplet-to-particle route, rather than a gas-to-particle route, producing dense or hollow micro- or nano-spheres. Our recent and current efforts include single-step synthesis of metal alloy particles supported on metal oxides or reduced graphene oxide (RGO), integrated synthesis and surface passivation, production of mesoporous silica and other mesoporous ceramics by droplet-to-particle conversion, and applications of these materials in conductive inks, membranes for hydrogen/CO2 separation, and catalysis.