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

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 R CT value describing HO• exposure, all the MCRs with sufficient ozone transfer featured an R CT 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 R CT 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.