Reaction kinetics of ozone at low concentrations with n-hexane soot

The reaction of ozone in low concentrations (2.6-41.2 ppm) with n-hexane soot has been studied by interfacing Fourier transform-infrared spectroscopy with a long optical path cell and microgravimetry. The rates and rate laws for ozone loss, soot mass gain, and CO2 and H2O formation have been determined at 22 degrees C. The rates and the rate laws associated with the increase of soot mass, due primarily to the formation of surface carboxylics, show differences between freshly-prepared and aged soot. Part of this difference has its origin in the adsorption of molecular oxygen, which participates in the ozone-soot reaction. Material balance calculations reveal that while O-3 decomposition exceeds O-containing product formation (CO2, H2O, carboxylics), one stage of the reaction shows excess product apparently due to the involvement of O-2. A complex reaction sequence is seen to involve essentially three reaction stages: a rapid initial catalytic O-3 decomposition, followed by rapid mass increase and CO2 and H2O evolution, continuing with a long period of oxidation during which the rate of ozone loss is second order in ozone. The latter observation implies the involvement of atomic oxygen in the oxidation process.