Removal of nitrogen oxides (NO
x) using an ozonization and catalysis hybrid process was investigated. The role of the ozonization in the hybrid process is to increase NO
2 content by oxidizing a part of NO, thereby leading to improving the performance of the catalytic reduction. It was found that the rate of the oxidation of NO to NO
2 in the ozonization chamber was very fast in a temperature range of 443 to 473 K, almost completed in a few tens of milliseconds. The decomposition of ozone into molecular oxygen was not significant, and one mole of ozone approximately reacted with one mole of NO. In addition, a kinetic study revealed that the ozonization method produces a negligible amount of byproducts such as NO
3 and N
2O
5. The effect of the content of NO
2 in NO
x on the catalytic reduction efficiency was examined by adjusting the degree of NO oxidation in the ozonization chamber. The selective reduction of NO
x to N
2 by a monolithic catalyst (V
2O
5-WO
3/TiO
2) was largely enhanced when the content of NO
2 was equal to that of NO, indicating that the mixture of NO and NO
2 reacts faster than NO.
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