Oxidation of nitric oxide to nitrogen dioxide at ppm levels by lead dioxide

A lead dioxide tube for oxidation of nitric oxide was prepared by anodic oxidation of a lead electrode of 30 mm i.d. and 300 mm in length. Electrolysis was carried out in 5% sulufuric acid with a platinum wire as a cathod by a current density of 10 mA/cm² for 30 h, and the deposited lead dioxide was washed with water and dried at (70±5)°C for 3 h. The prepared lead dioxide tube completely oxidized 1 ppm nitric oxide in air stream at a relative humidity higher than 30%. The gas flow rate was 300 ml/mn. It could be increased up to 600 ml/mn without change of oxidation efficiency. The oxidizing power was proved to last for a long time by a continuous test of several handred hours. Influence of relative humidity of the air stream on the oxidizing power was studied. The oxidizing power depends on the amount of adsorbed on the surface of the oxide than on the relative humidity of the gas stream. The oxidizing power remained unchanged at a temperature range of 10 to 40°C, showing almost 100% oxidation of nitric oxide to nitrogen dioxide. The lead dioxide tube thus prepared was set in an automatic continuous nitrogen oxide analyzer, instead of the ordinary potassium permanganate-sulfuric acid solution oxidizer. This apparatus worked satisfactorily for one month without any readjustment. This dioxide tube can be recovered without tedious procedure. It may be possible to reproduce the dioxide automatically with a simple equipment attached to the analyzer, which provides a good removal technique of nitrogen oxides.