Method for the study of gaseous oxidants for the oxidation of mercury gas

The oxidation rate constants of mercury gas (Hg ⁰) are difficult to determine because Hg ⁰ easily adsorbs on reactor walls and the reactions can be catalyzed by the solid surface. Also, the UV light which is commonly used to monitor the Hg ⁰ concentration can accelerate the reaction. We have demonstrated a procedure that allows the determination of gas-phase, surface-catalyzed, and photoinduced contributions in the kinetic study of the reaction of Hg ⁰ with chlorine gas. By experimenting with several reactors having different surface-to-volume ratios, the relative contributions of gas-phase and surface-catalyzed reactions were determined. The pressure-dependent study revealed that the gas-phase oxidation was by means of a three-body collision process. The third-order rate constant was determined to be 7.5 (±0.2) × 10 ^-39 cm ^6.molecules ^-2.s ^-1 with N ₂ as the third body at 297 ± 1°K. The surface catalyzed reaction on a quartz window was second order, and the rate constant was 2.7 × 10 ^-17 mL ^2.molecules ^-1.cm ^-2 sec. The photoinduced contribution was obtained by comparing the Hg ⁰ decay rate between continuous and intermittent irradiations. The utilization efficiency of 253.7 nm photons for Hg ⁰ oxidations at a chlorine concentration of 6.5 × 10 ¹⁵ molecules·mL ^-1 was <6.7 × 10 ^-4 molecules·photon ^-1.