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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
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
Outline
1. Methane Photolysis– Previous work– Possible approaches
2. Reactions of 1CH2
– Rare gas collisions– Reaction vs relaxation
3. Reactions of CH 4. Recent studies with Laval expansion system
(Heard)
1. Methane Photolysis
Gans et al.PCCP Front cover
CH4 Photolysis – Background
Product Channels:• CH3 + H
• 1CH2 + H2
• 3CH2 + 2H
• CH + H + H2
Smith and Nash, Icarus, 2006
CH4 Photolysis – Previous Work
• C
• Gans et al. PCCP 2011
CH4 Photolysis – Previous Work
Reference Gans et al. Gans et al. Park et al. Mordaunt et al.
Heck et al.
Brownsword et al.
Wang et al. Lodriguito et al.
Method Direct determination of CH2 and CH3
Direct determination of CH2 and CH3
Simultaneous photolysis and detection of H atoms by LIF
ToF H atom kinetic energy spectroscopy
Photofragment imaging
Photolysis and H atom detection (vuvLIF) at Lyman α
Determination of H and molecular products
Trajectory calculations
Date 2011 2011 2008 1993 1996 1997 2000 2009λ/nm 118.2 121.6 121.6 121.6 121.6 nm H
atom105-115 nm H2
121.6 118.2 and 121.6 121.6
CH3 + H 0.26 ±0.04 0.42 ± 0.05 0.31 ± 0.05 0.49 0.66 - 0.29 ± 0.07 0.39 ± 0.03
CH2 (a 1A1) + H2
0.17 ± 0.05 0.48 ± 0.05 0.69 0 0.22 - 0.59 ± 0.10 0.50 ± 0.06
CH2 (X 3B1) + 2H
0.48 ± 0.06 0.03 ± 0.08 - 0 - - 0.066 ± 0.012 0.10 ± 0.02
CH + H + H2 0.09 0.07 - 0.51 0.11 - 0.068 ± 0.013 0.02 ± 0.01
Total H 1.31 ± 0.13 0.55 ± 0.17 0.31 ± 0.05 1.0 ± 0.5 0.47 ± 0.11 0.47 ± 0.10 0.60 ± 0.10Total H2 0.26 ± 0.05 0.55 ± 0.05 0.69 0.51 0.65 ± 0.10 0.51 ± 0.06
Summary of Previous Results
CH4 Photolysis – 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)
2. 1CH2 Reactions – Temperature Dependence
Importance of 1CH2 reactions
Wilson and Atreya, JGR 108, E2 5014, 2003
1CH2 + rare gas
1CH2 + RG → 3CH2 + RG
Gannon et al.JCP 132 2010
Temperature Dependence of 1CH2 removal by C2H2
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
100 200 300 400 500 600 700 800
Temperature/K
101
0 k1/
cm3 m
ole
cule
-1 s
-1
Blitz et alThis workHayes et alHack et alA(T/298 K)^n
Gannon et al.JPCA 114 2010
Monitor removal of 1CH2 by LIF1CH2 + C2H2 → C3H3 + H1CH2 + C2H2 + M → C3H4 + M1CH2 + C2H2 → 3CH2 + C2H2
Monitor calibrated production of H by LIF
Product Temperature Dependence
Temperature
k k overall
k relaxation
reaction
relaxation
H Atom Yields
1CH2 +ΓH
195 K 250 K 298 K 398 K 498 K
C2H2 0.28 ± 0.11 0.53 ± 0.15 0.88 ± 0.09 1.1 ± 0.16 1.1 ± 0.42
C2H4 0.35 ± 0.09 0.51 ± 0.13 0.71 ± 0.08 0.86 ± 0.16 1.08 ± 0.19
• Relaxation increases with decreasing temperature• Opposite of rare gas behaviour• Relaxation will be more important for planetary atmospheres – more focus on 3CH2 chemistry ?
PES showing surface crossing
Crossing is below entrance channel
Gannon et al.Faraday Discussions 147 2010(Glowacki and Harvey, Bristol)
3. CH Reactions
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
4. Product Studies from Laval Reactor (Blitz, Shannon and Heard)
Low temperature kinetics of abstractionReactions
OH + CH3COCH3 → H2O + CH2COCH3
Barrier, so activated process – what is happening at low T?Shannon et al. PCCP 16 2014
Product Formation
OH + CH3OH → CH3O + H2OShannon et al. Nature Chem. 5 2013
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. Further
experiments in Laval systems in progress
Reagent and product time profiles
-5 0 5 10 15 20 25 30
0.000
0.005
0.010
0.015
0.020
0.025
0.030
To
tal f
luo
resc
en
ce s
ign
al/
arb
itra
ry u
nits
Time / s
kr = 374000 � 78000 s-1
kd = 351000 � 19000 s-1
1CH2
H
Experimental
• Generate 1CH2 by pulsed photolysis of ketene
• Monitor removal of 1CH2 by LIF1CH2 + C2H2 → C3H3 + H1CH2 + C2H2 + M → C3H4 + M 1CH2 + C2H2 → 3CH2 + C2H2
• Monitor calibrated production of H by LIF
Master Equation Calculations
MESMER (Master Equation Solver for Multi Energy-well Reactions)
A + B kRi
kji
kij
kPj
sourceterm
nj(E)ni(E)
Products(infinite sink)å
Ei En )(
åE
j En )(
•K(E)’s calculated from RRKM theory.
)(
)()(
Eh
EWEk
•Energy transfer calculated an exponential down model
dE ~150 - 450cm-1
Master Equation Results
0
0.2
0.4
0.6
0.8
1
1.2
1 10 100 1000 10000 100000
H at
omyi
eld
Pressure/Torr
150 K
200 K
250 K
300 K
Modelling shows no stabilization below 50 TorrBalance of reaction is relaxation
Experimental Pressure
Experimental
James Lockhart
Flash Photolysis LIF Detection
C
Gas mixing manifold
MFC
MFC
MFC
MFC
N2
C2H2
(CH3)3COOH
O2
Reaction CellExhaust Line / Needle valve
Rotary Pump
Gas mixture flows in towards the cell
Photolysis laser pulse 248 nm
Rhodamine 6G Dye Laser
Nd: YAG Laser
Probe Laser Pulse 282 nm
Photodiode
PMT
Boxcar Averager
Excimer Laser
0 1000 2000 3000 4000 5000
0.00
0.02
0.04
0.06
0.08
0.10
Flu
ore
sce
nce
Sig
na
l / A
rbitr
ary
Un
its
Time / s
0.0 2.0x1014 4.0x1014 6.0x10140
1x104
2x104
3x104
4x104
5x104
[MEA] / molecule cm-3
k obs / s
-1
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.Lett 2012, 3, 853−856
II - OH + MEA (monoethanolamine)
OHuptake? PM
II - Recycling OH with Excess Oxygen
0 500 1000 1500 2000 2500 3000 3500
0.0
0.2
0.4
0.6
0.8
1.0F
luo
resc
en
ce S
ign
al /
Arb
itra
ry U
nits
Time / s
OH Decay in N2
0 500 1000 1500 2000 2500 3000 3500
0.0
0.2
0.4
0.6
0.8
1.0F
luo
resc
en
ce S
ign
al /
Arb
itra
ry U
nits
Time / s
Zero OH Yield
0 500 1000 1500 2000 2500 3000 3500
0.0
0.2
0.4
0.6
0.8
1.0F
luo
resc
en
ce S
ign
al /
Arb
itra
ry U
nits
Time / s
100% OH Yield
0 500 1000 1500 2000 2500 3000 3500
0.0
0.2
0.4
0.6
0.8
1.0F
luo
resc
en
ce S
ign
al /
Arb
itra
ry U
nits
Time / s
Experimental OH Yield
MESMER
• Master Equation Solver for Multi Energy-well Reactions
• MESMER 3.0 Released 24th Feb 2014. Contact Robin Shannon ([email protected]) for more information.