Skip to main content
European Commission logo print header
Content archived on 2022-12-23

Are the HCFC/HFC compounds environmentally acceptable alternatives ?

Objective

- Elucidation of primary reaction mechanisms of atmospheric reactions of metastable oxygen atoms (O1D) and hydroxyl radicals (OH) with halogenated hydrocarbons (HCFCs, HFCs, HBCs);
- Investigation of the primary processes in the UV and VUV photodissociation of halogenated hydrocarbons (HCFCs, HFCs, HBCs);
- A more detailed understanding of the actual molecular reaction and dissociation mechanisms. Detailed information about the reaction/dissociation rates and pathways that will allow the environmental impact of the degradation of halogenated hydrocarbons in the atmosphere to be assessed.
Expected Outcome

- The measured quantum state distributions will give insight into the underlying reaction/photodissociation mechanism in the initial step of the atmospheric degradation of the HFCs/HCFCs/HBCs. The thermal rate coefficients and the absolute reactive cross section will allow to determine the rate of removal of these compounds by O(1D)/OH radicals in the atmosphere.


Results so far

- The kinetics of the reaction of O(1D) with CHF2C1 (HCFC-22) have been studied using the laser photolysis-laser induced fluorescence (LP/LIF) pump-and-probe technique. Thermal rates for the gas-phase reaction were measured in a flow system. O(1D) atoms were generated via laser photodissociation of N20. Using a calibration method where the photodissociation of H202 was employed to produce well-defined OH radical concentrations the branching ratio for the OH producing channel was determined. In addition, reaction dynamics studies were carried out under low pressure conditions where translationally excited O(1D) atoms with a well-defined collision energy distribution, were allowed to react with CHF2C1 and the absolute reaction cross section as well as the population distribution of the fine structure quantum states of the OH product radicals were measured under single-collision conditions using LIF. The OH product radical spin-orbit states were found to be equally populated. Also the OH Lambda-doublet distributions showed no preference for either one of the two components. A comparison of the nascent OH internal product quantum state distributions with the results of RRKM calculations, in which it was assumed that the reaction proceeds via a CF2C1-OH transition state, clearly showed the non-statistical character of the measured OH vibrational and rotational distributions. The analysis of the measured rotational state distributions suggests that the reaction O(1D) + CHF2C1 actually proceeds via two different mechanisms : one in which the oxygen atom inserts into the C-H bond, leading to CF2C1OH-complex formation followed by an OH producing unimolecular decomposition and a second one in which the oxygen atom directly abstracts a H atom during its approach to the CHF2C1 reagent. Experiments in which H atom product formation is investigated are currently under way in order to quantify the relative contribution of the complex formation pathway in more detail. UV and VUV photodissociation dynamics studies have so far been carried out for chlorinated methanes (CHnC14-n, n= 1-4) and CHF2C1 (HCFC-22) and CF2C1CH3 (HCFC-142b) after excitation at 193.3 nm.
- Carrying out laboratory studies to investigate the gas-phase reaction dynamics and kinetics of the reactions of metastable oxygen atoms (O1D) and hydroxyl radicals (OH) with selected halogenated hydrocarbons. Measurements of product quantum state distributions and reactive cross sections, reaction rates and branching ratios in order to obtain information about the underlying reaction mechanisms;
- Laser-based laboratory studies on the UV and VUV photodissociation dynamics of halogenated hydrocarbons. Measurements of primary product distributions and branching ratios at different wavelengths. Extension of the phtolysis wavelength ranges down to the atmospherically important Lyman-alpha spectral line. Development of an experimental method for the measurement of absolute halogen and hydrogen atom photolysis quantum yields;
- Comparison of the experimental results with calculations based on ab initio molecular structure date and/or statistical models for different mechanisms for reaction (complex forming versus abstraction) and dissociation (two-body versus three-body).

Topic(s)

Data not available

Call for proposal

Data not available

Coordinator

Ruprecht-Karls-Universität Heidelberg
EU contribution
No data
Address
Im Neuenheimer Feld 253
69120 Heidelberg
Germany

See on map

Total cost
No data