What Are the Mechanisms of Heterogeneous Ozone Catalytic Oxidation Reactions?

As an advanced technology for treating organic pollutants, heterogeneous ozone catalytic oxidation works mainly through the following three mechanisms:

I. Free radical reaction mechanism

The core of the radical mechanism is that ozone adsorbed on the catalyst's active sites decomposes into the more oxidizing hydroxyl radical (·OH). See Figure 1.

In studies of TiO2-catalyzed ozonation of nitrobenzene, adding tert-butanol reduced removal efficiency, indicating oxidation by hydroxyl radicals. Electron paramagnetic resonance confirmed that the presence of TiO2 accelerated hydroxyl radical production in the ozonation system.

Figure 1 Mechanism of hydroxyl radical production from ozone molecule decomposition

II. Surface coordination complexation mechanism

Organics are adsorbed on the catalyst surface/interface through coordination complexation and then oxidized and decomposed by ozone molecules. Early on, Andreozzi et al. found that when the solution pH is above the pHpzc of MnO2, MnO2 shows low catalytic activity, while at pH below pHpzc it exhibits good catalytic activity, and oxalic acid oxidation follows a coordination-complexation degradation mechanism. Studies of sulfosalicylic acid degradation also confirmed that the organics first adsorb on the MnO2 catalyst surface/interface and are then attacked by ozone molecules, generating catalytic activity. Reaction pathways for metal-loaded catalysts fall into two classes: (1) organics are chemisorbed on the catalyst surface/interface, playing only a coordination-complexation role; (2) both organics and ozone are adsorbed on the catalyst surface/interface, undergoing redox reactions via electron transfer. See Figure 2.

Figure 2 Ozone catalytic oxidation mechanism of metal-loaded catalysts

III. Synergistic mechanism

In the third mechanism, the catalyst interacts with both ozone and organics, and the reaction can occur on the catalyst surface/interface and in the bulk liquid. Under acidic conditions the ozone molecule reaction is significant; under alkaline conditions ozone molecules and hydroxyl radicals jointly remove organic pollutants. Using carbon-based multi-walled carbon nanotubes to degrade phenol, phenol conversion reached 100% after 120 minutes; for refractory oxalic acid, H2O2 is produced during the reaction and radical inhibitors had little effect on oxalic acid degradation, showing interaction between the catalyst, ozone and organics. See Figure 3.

Figure 3 Proposed reaction mechanism for catalytic oxidation of oxalic acid by multi-walled carbon nanotubes (MWCNT) (R = pollutant)

• Technical innovation: Based on in-depth research into the mechanisms of heterogeneous ozone catalytic oxidation, SINOKLE Co., Ltd. has developed a highly efficient heterogeneous ozone catalyst. Using proprietary technology, it integrates multiple active metal oxides (including precious metals) with high-porosity microporous forming technology, hydrophilic anti-fouling modification, anti-clogging technology and more.

• This heterogeneous ozone catalyst has the following technical innovations:

(1) Multiple transition metal oxides (including precious metals) are used as catalytic components, with optimized ratios to suit different wastewaters and provide high catalytic activity.

(2) Multi-stage precision temperature-controlled sintering is adopted, ensuring stable active components, reducing production energy consumption and catalyst loss during use, and lowering secondary pollution risk.

(3) A high-activity material serves as the catalyst carrier; its porous structure provides high specific surface area, and non-leaching catalytic components are doped in to enhance stability. The carrier has high mechanical strength and long service life.

(4) The catalyst effectively lowers the reaction activation energy to achieve deep oxidation, maximizing removal of organic pollutants.

(5) It accelerates the self-decomposition of ozone in water, raises the ·OH concentration and enhances ozone oxidation, with oxidation efficiency 2~5 times that of ozone oxidation alone.

(6) Atomic layer deposition modifies the catalyst, giving the carrier super-hydrophilicity, reducing fouling, scaling and clogging risks and enabling long-period stable operation.

• Performance parameters of the high-efficiency heterogeneous ozone catalyst: