Catalytic ozonation by palladium–manganese for the decomposition of natural organic matter

Theoretical and experimental investigation of the mechanism of the catalytic ozonation process by using a manganese-based catalyst

The mechanisms of heterogeneous catalytic ozonation using a manganese catalyst have been studied in this work. The effects of catalyst, pH and the radical scavengers on the degradation of a model pollutant (i.e. phenol) were evaluated. The contribution of hydroxyl radicals and molecular ozone on the oxidation of phenol was studied for both homogeneous and heterogeneous systems. It was observed that the contribution of hydroxyl radial was 11-44% at pH 3-9. The heterogeneous catalyst may pose a rather complex mechanism as compared to homogenous catalyst. In presence of manganese catalyst, the decomposition ozone is very fast and generates more hydroxyl radicals. This helps the rapid oxidation of phenol at a wide range of pH 3-9. The surface of the catalyst undergoes multiple oxidation reaction and plays the major role in generation of the hydroxyl radicals. A considerable amount of phenol degradation was observed even at low catalyst dose (i.e. 0.5 g L^-1). The effect of catalyst on the ozone mass transfer in water was also studied in this work. It was observed that, the overall performance of the phenol degradation is dependent on both chemical reaction due to catalytic action and gas-liquid mass transfer rate.

Green Synthesized Palladium Coated Titanium Nanotube Arrays for Simultaneous Azo-Dye Degradation and Hydrogen Production

In this study, electrodes of titanium dioxide nanotube arrays (TNAs) were successfully synthesized by applying the anodic oxidation etching method, as well as the use of green synthetic technology to add reducing agents of tea or coffee to reduce metal palladium from palladium chloride. Synthesis of palladium modified TNAs (Pd/TNAs) was conducted by the microwave hydrothermal method after the metal palladium was reduced. In order to identify the surface structure, light absorption and elemental composition, TNAs and Pd/TNAs were characterized by X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Furthermore, to test the photocurrent density, electron resistance, and hydroxyl radicals by I-t plot, electrochemistry impedance spectroscopy (EIS), and electron paramagnetic resonance (EPR) were investigated. The photocurrent (4.0 mA/cm2) of Pd/TNAs-C (using coffee as the reducing agent) at +1.0 V (vs. Ag/AgCl) was higher than that of the pure TNAs (1.5 mA/cm2), illustrating that Pd/TNAs-C can effectively separate photogenerated electrons and holes. Pd/TNAs is a favorable material as a photoanode for the photoelectrochemical (PEC) removal of organic pollutants in wastewater.

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.