The Design and Use of a Photochemical Flow Reactor: A Laboratory Study of the Atmospheric Chemistry of Cyanoacetylene on Titan

Abstract

The laboratory investigation of the atmospheric photochemistry of planets and satellites is mainly carried out in static systems. These studies are often poor models of chemical processes in atmospheres because: (1) much higher mixing ratios of minor constituents must be used to accurately determine the amount of reactant consumed and to obtain sufficient products for analysis, (2) secondary photolysis of the initial photoproducts often occurs, (3) wall reactions occur, and (4) most of the starting material is converted to products to obtain enough for spectroscopic analysis. The use of a photochemical flow reactor either circumvents or minimizes these problems by using gas mixtures and photolysis conditions more representative of a planetary atmosphere. A gas mixture, composed of a small amount of a reactant gas diluted in a much larger amount of carrier gas, is flowed past a UV lamp for an extended period of time. Unconsumed reactants and products are collected in traps downstream until amounts sufficient for spectral analysis are collected. FTIR and NMR analysis provides structural information and quantitative data on their rates of formation. The feasibility of this approach for the investigation of planetary atmospheres has been demonstrated by the photolysis of mixing ratios of 10 −3–10 −6 of cyanoacetylene, (2-propynenitrile, HC 3N) in nitrogen gas. Hydrogen cyanide (HCN), acetonitrile (CH 3CN), acrylonitrile (CH 2CHCN), and a polymer have been identified as reaction products. The quantum yields for reactant loss and product formation have been determined. Aspects of polymer structure have been determined by FTIR. Its empirical formula has been determined on the basis of the reaction products produced, and its morphology has been examined by scanning electron microscopy. It is concluded from the high quantum yields for HCN and CH 3CN formation that the C/N ratio of the polymer is high. This was confirmed by infrared analysis of the polymer where it was observed that the intensity of the C≡N stretching frequency decreases as the HC 3N mixing ratio is lowered to a mixing ratio closer to that of Titan.