VUV Photolysis of Methane: Laboratory Studies and Reaction Exploration

1. Laboratory Studies of VUV CH4 Photolysis and Reactions of the Resulting Radicals Robin Shannon, Mark Blitz, Mike Pilling, Dwayne Heard, Paul Seakins University of Leeds, UK

2. Background to Leeds • Leeds has long background in Laboratory Reaction Kinetics with applications to: • Combustion • Pyrolysis • Atmospheric Chemistry • Additionally field work on OH and HO2 detection (spectroscopic) and hydrocarbons (chromatography) • Development of large models (MCM) • Theory on pressure dependent reactions • New STFC grant on methane photolysis and benzene formation on Titan

3. Outline • Methane Photolysis • Previous work • Possible approaches • Reactions of 1CH2 • Rare gas collisions • Reaction vs relaxation • Reactions of CH • Recent studies with Laval expansion system (Heard)

4. 1. Methane Photolysis Gans et al. PCCP Front cover

5. CH4Photolysis – Background Product Channels: • CH3 + H • 1CH2 + H2 • 3CH2 + 2H • CH + H + H2 Smith and Nash, Icarus, 2006

6. CH4Photolysis – Previous Work • C • Gans et al. PCCP 2011

7. CH4Photolysis – Previous Work Summary of Previous Results

8. CH4Photolysis – Possible approaches • Repeat of Gans et al. approach (synchrotron photolysis source?) • Direct detection of CH via laser induced fluorescence • Enhanced end product analysis studies • Excimer lamps (e.g. 126 nm) as strong sources (>50 mW cm-2) • Chemical conversion (3CH2 particularly difficult to detect via optical spectroscopy) • Use of PTR-MS for sensitive end-product analysis, H3O+ + RH → RH+ + H2O (soft ionization)

9. 2. 1CH2Reactions – Temperature Dependence

10. Importance of 1CH2 reactions Wilson and Atreya, JGR 108, E2 5014, 2003

11. 1CH2 + rare gas 1CH2 + RG →3CH2 + RG Gannon et al. JCP 132 2010

12. Temperature Dependence of 1CH2 removal by C2H2 Gannon et al. JPCA 114 2010 Monitor removal of 1CH2 by LIF 1CH2+ C2H2→ C3H3 + H 1CH2+ C2H2 + M → C3H4 + M 1CH2+ C2H2→3CH2 + C2H2 Monitor calibrated production of H by LIF

13. Product Temperature Dependence k k overall reaction k relaxation relaxation Temperature

14. H Atom Yields • Relaxation increases with decreasing temperature • Opposite of rare gas behaviour • Relaxation will be more important for planetary • atmospheres – more focus on 3CH2 chemistry ?

15. PES showing surface crossing Crossing is below entrance channel Gannon et al. Faraday Discussions 147 2010 (Glowacki and Harvey, Bristol)

16. 3. CH Reactions

17. CH Chemistry • Reactivity very high – capable of reacting with N2 • Important intermediate for increasing carbon number CH + CH4→H + C2H4 • Single channel so useful calibration reaction • More usually several open channels CH + CH3OH → HCHO + CH3 CH + CH3OH → H + CH3CHO

18. 4. Product Studies from Laval Reactor (Blitz, Shannon and Heard)

19. Low temperature kinetics of abstraction Reactions OH + CH3COCH3→ H2O + CH2COCH3 Barrier, so activated process – what is happening at low T? Shannon et al. PCCP 16 2014

20. Product Formation OH + CH3OH →CH3O + H2O Shannon et al. Nature Chem. 5 2013

21. 5. Summary • CH4 photolysis yields are important • Currently uncertainty on CH4 photochemistry • New experiments to be undertaken as part of STFC project building on expertise in atmospheric and combustion studies • 1CH2 chemistry shows interesting T dependence, not always taken into account in models. More focus on 3CH2? • Acceleration in loss rates at low temperatures associated with chemical reaction. Furtherexperiments in Laval systems in progress

22. Reagent and product time profiles 1CH2 H

23. Experimental • Generate 1CH2 by pulsed photolysis of ketene • Monitor removal of 1CH2 by LIF 1CH2 + C2H2 → C3H3 + H 1CH2 + C2H2 + M → C3H4 + M 1CH2 + C2H2 → 3CH2 + C2H2 • Monitor calibrated production of H by LIF

24. Master Equation Calculations å n ( E ) i E å n ( E ) j E MESMER (Master Equation Solver for Multi Energy-well Reactions) • K(E)’s calculated from RRKM theory. source k term ji k Pj n (E ) n (E ) i j k A + B k Ri ij • Energy transfer calculated an exponential down model ~150 - 450cm-1 Products (infinite sink)

25. Master Equation Results Experimental Pressure Modelling shows no stabilization below 50 Torr Balance of reaction is relaxation

26. Experimental James Lockhart

27. Flash Photolysis LIF Detection Rotary Pump Exhaust Line / Needle valve Reaction Cell Probe Laser Pulse 282 nm Rhodamine 6G Dye Laser Nd: YAG Laser PMT Photodiode Gas mixture flows in towards the cell Photolysis laser pulse 248 nm MFC N2 C MFC C2H2 Gas mixing manifold Excimer Laser MFC (CH3)3COOH MFC O2 Boxcar Averager

28. II - OH + MEA (monoethanolamine) PM ? OH uptake k = (7.59 ± 0.31) 10-11 s-1 cm-3 Gas phase oxidation will compete with aerosol uptake Onel, L; Blitz, M. A; Seakins, P. W J.Phys.Chem.Lett2012, 3, 853−856

29. II - Recycling OH with Excess Oxygen 100% OH Yield Experimental OH Yield OH Decay in N2 Zero OH Yield

30. MESMER • Master Equation Solver for Multi Energy-well Reactions • MESMER 3.0 Released 24th Feb 2014. Contact Robin Shannon (R.Shannon@leeds.ac.uk) for more information.