Experimental and modelling studies of the adsorption of acetone on ice surfaces at temperatures around 200 K
The thermodynamics and kinetics of the adsorption of acetone on ice films as well as their dependence on ice film thickness and ice age have been studied in the temperature range T= 190 – 220 K using a coated wall flow tube reactor (CWFT) coupled with QMS detection. A kinetic model has been developed in order to describe the adsorption and desorption kinetics of acetone on ice surfaces in tubular flow reactors. The rate coefficients for adsorption and desorption as well as adsorption isotherms have been derived by applying the model to the temporal profiles of the gas phase concentration at the exit of the flow reactor. It was found that acetone adsorption is entirely reversible and that the adsorption capacity depends on temperature and film thickness and decreases with the age of the ice film. The time constant of the ageing effect depends on the ice mass and is most pronounced at low acetone gas phase concentrations ( 2.0 x 1011 molecules/cm3) and at low temperatures. It is suggested that under these conditions acetone is initially adsorbed with a high rate and high surface coverage on cubic ice (Ic) adsorption sites. Upon ageing cubic ice is converted into hexagonal ice (Ih) for which the rate of adsorption and the surface coverage are lower. Using two-site dynamic modelling the rate coefficients for adsorption (kads) and desorption (kdes) as well as the Langmuir constant (KL) and the maximum number of adsorption sites (cs,max) as obtained for the adsorption of acetone on each of these ice phases in the respective temperature range are kads(Ic) = 3.8 x 10-14 T0.5 cm3s-1, kdes(Ic) = 4.0x1011 exp(-5773/T) s-1, KL(Ic) = 6.3x10-25 exp(5893/T) cm3, cs,max(Ic) 1014 cm-2 and kads(Ih) = 2.9 x 10-15 T0.5 cm3s-1, kdes(Ih) = 1.5x107 exp(-3488/T) s-1, KL(Ih) = 5.0x10-22 exp(3849/T) cm3, cs,max(Ih) = 6.0x1014 cm-2, respectively. The cs,max (Ih) and cs,max (Ic) were found to be dependent on ice thickness, however cs,max (Ic) is relative lab time and temperature dependent as well.
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