Thermal decomposition of ClOOCl

Area of Science:

• Atmospheric Chemistry
• Photochemistry
• Chemical Kinetics

Background:

• Chlorine oxides play a crucial role in stratospheric ozone depletion.
• The equilibrium between chlorine monoxide (ClO) and its dimer (ClOOCl) is a key process in polar ozone chemistry.
• Accurate kinetic data for ClOOCl formation and decomposition are essential for atmospheric modeling.

Purpose of the Study:

• To directly measure the unimolecular decomposition rate constants (k-1) of chlorine peroxide (ClOOCl).
• To determine the low-pressure limiting rate constant and the activation energy for ClOOCl decomposition.
• To compare experimental results with previous studies and assess inconsistencies in kinetic data.

Main Methods:

• In situ preparation of ClOOCl in a temperature-controlled photoreactor.
• Photolysis of OClO/N2 mixtures followed by addition of NO2.
• Simultaneous monitoring of IR and UV spectra to determine first-order decay of ClOOCl absorption.
• Spectral stripping to isolate ClOOCl absorption and derive rate constants.

Main Results:

• The low-pressure limiting rate constant for ClOOCl decomposition was determined: k-1,0 = [N2] x 7.6 x 10(-9) exp[(-53.6 +/- 6.0) kJ mol(-1)/RT] cm3 molecule(-1) s(-1).
• The derived activation energy for decomposition was E0 = 66.4 +/- 3.0 kJ mol(-1).
• Measured decomposition rate constants are 1.5 to 4.2 times higher than previously reported values, depending on temperature and pressure.

Conclusions:

• Direct measurements confirm higher ClOOCl decomposition rates than previously inferred.
• Significant inconsistencies exist between current dissociation and recombination rate constants for the ClO + ClO <=> ClOOCl system.
• An updated equilibrium constant (Kc) for the ClO + ClO <=> ClOOCl reaction was derived for the 200-300 K range.