HETEROGENEOUS CHEMICAL-KINETICS OF NO2 ON AMORPHOUS-CARBON AT AMBIENT-TEMPERATURE

A low-pressure reactor was used to study the heterogeneous chemistry of NO2 interacting with three types of amorphous carbon with widely differing physicochemical properties at ambient temperature. The uptake kinetics and the resulting products were measured using molecular beam sampled electron-impact mass spectrometry and in situ laser-induced fluorescence. The only major product resulting from the heterogeneous interaction of NO2 with all three carbon samples was NO with the oxidation product of carbon apparently remaining on the surface at ambient temperature. Both pulsed valve dosing and steady-state experiments were performed and revealed a complex reaction mechanism for both the uptake as well as product formation. The initial uptake coefficient gamma(0) for NO2 was (6.4 +/- 2.0) x 10(-2) and proved to be identical for all three types of amorphous carbon. The initial NO2 uptake rate was mass independent and scaled with the external surface. Applying a simple chemical kinetic model to both pulsed dosing and steady state experiments resulted in an effective surface for uptake on amorphous carbon ranging between factors of 2.8 and 8.4 larger than the geometrical surface area but inversely proportional to the measured BET surface area of the three types of amorphous carbon. The chemical kinetic model included competing processes between Langmuir-type adsorption and inhibition controlling the adsorption kinetics of NO2 and revealed that the NO generation rate differed greatly between the three carbon samples examined. All samples showed saturation effects of differing degree that were partially reversible through prolonged pumping at 10(-4) Torr and/or heating. Virgin amorphous carbon samples did not take up H2O vapor at 20 mTorr, and no HONO and/or HNO3 was detected in simultaneous NO2/H2O exposure experiments; CO and CO2 were detected when previously dosed amorphous carbon was heated by an incandescent lamp. Upon heating a sample previously exposed to NO2, a MS signal m/e 62 originating from NO3 and/or N2O5 was detected.