Catalytic Ozonation in Arrayed Zinc Oxide Nanotubes as Highly Efficient Mini-Column Catalyst Reactors (MCRs): Augmentation of Hydroxyl Radical Exposure.

Abstract

Reactor design is significant to catalytic ozonation for an efficient mass transfer and exposure of the powerful but short-lived hydroxyl radicals (HO•). Herein, five groups of zinc oxide nanotube arrays with pore sizes from 168 to 10 nm were produced as mini-column catalyst reactors (MCRs) for internal catalytic ozonation, whose performance was comparatively studied on the kinetics of ozone transfer, consumption, and radical probe interaction. Using an RCT value describing HO• exposure, all the MCRs with sufficient ozone transfer featured an RCT level of at least 3.2 × 10-6, which is substantially higher than most values in referenced works (10-9∼10-6) and that for microscale reactors in our work (∼10-8). Furthermore, the HO• exposure dramatically increased with diminishing pore size, causing an elevated RCT up to 8.0 × 10-5 for the smallest MCR with 10 nm pore. The interphase formed in this flow-through system might have enriched HO• radicals produced via surface, and for a smaller MCR, the effect would be greater with a more confined microfluidic region. Investigations on electron paramagnetic resonance and the treatment of ozone-recalcitrant organics corroborated the nanoscale effect of MCR on augmentation of HO• exposure. This study offers a new way to design nanotube reactors for internal HO•-based heterogeneous catalysis.