Mechanisms of hydrolysis in a transverse jet zinc aerosol reactor

Abstract

The mechanisms of hydrolysis of zinc particles formed and entrained in a reacting gas flow were explored. Experiments were carried out in a transverse jet reactor designed for nucleation of Zn nanoparticles via a rapid quench followed by reaction at temperatures from 418 to 713K. The aerosol particles were collected on a filter and characterized via X-ray diffraction and scanning electron microscopy. Even for quench rates as high as 104K/s, nucleation of Zn vapor was incomplete. With a quench of Ar at temperatures less than the saturation temperature of Zn, 1 to 3μm hour-glass shaped hexagonal Zn particles were formed, consistent with formation via 2-D layer by layer by condensation and subsequent evaporation. Above the saturation temperature, Zn nanowires were formed from the vapor phase on the filter surface. With a steam quench, particles remained hexagonal but were generally smaller, from 300nm to 1μm across the hexagonal face. Above 573K, a shell of ZnO impeded evaporation of particles. Overall conversion of Zn to ZnO was dominated by the heterogeneous hydrolysis of Zn vapor, not hydrolysis of Zn particles. These results shed light on the mechanisms of particle growth and hydrolysis in Zn aerosol reactors. In particular, the finding that hydrolysis is dominated by a heterogeneous reaction between Zn(v) and steam is of critical importance to developing better approaches to react the Zn with steam, supporting prior work that shows that the heterogeneous vapor phase reaction is kinetically favored over the diffusion limited conversion of solid or liquid Zn particles.