STUDIES WITH CIONO2 - THERMAL-DISSOCIATION RATE AND CATALYTIC CONVERSION TO NO USING AN NO/O3 CHEMI-LUMINESCENCE DETECTOR

A NO/O3 chemiluminescence detector equipped with a gold catalyst is adapted to provide a measure of the thermal decomposition rate of ClONO2 in an N2/O2 gas mixture and, in a related way, provide the absolute concentration of ClONO2 in a flowing gas stream. The approach is to add ClONO2, in the parts per million by volume (ppmv) range, to the flow stream of the detector in the presence of excess NO. As the sample is heated, ClONO2 is thermally dissociated and the subsequent scavenging reaction of ClO with NO produces Cl and NO2. Cl goes on to react with ClONO2 to form NO3 which, in turn, reacts with NO to produce NO2. The loss of NO from the flow is precisely monitored downstream in the detector by the change in the chemiluminescence produced in the reaction of NO with reagent O3. If the reaction rates with NO are given, the NO loss at a fixed temperature can be modeled to yield a dissociation rate constant for ClONO2. Results were obtained for temperatures between 353 and 413 K and for pressures in the range of 66-160 Torr (8.8-21.3 kPa). The data is best fit by the expression 10-6.16 exp(-90.7 kJ mol-1/RT) cm3 s-1 molecule-1, which is in good agreement with earlier results. When combined with the rate constant for the association reaction of ClO and NO2, these results yield a larger equilibrium constant for the reaction than indicated in previous direct measurements. A value for ΔHf°289.15 for ClONO2 of 22.9 kJ mol-1 is obtained from a third-law thermochemical analysis of the data. The initial ClONO2 concentration in the sample is assumed to equal the absolute loss of NO measured when the dissociation and scavenging reactions have gone to completion. This affords the opportunity to calibrate the efficiency of other methods for the detection of ClONO2. Results are presented for the conversion efficiency of ClONO2 to NO found for a gold catalyst at 573 K with CO present as a reducing agent. © 1990 American Chemical Society.