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
Summary of Previous Results
Reference
Gans et al.
Gans et al.
Park et al.
Mordaunt
et al.
Heck et
al.
Brownswo Wang et al.
rd 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
Photofragmen
t imaging
Determination of H
and molecular
products
Trajectory
calculations
Date
λ/nm
2011
118.2
2011
121.6
2008
121.6
1993
121.6
1996
121.6 nm H
atom
105-115 nm
H2
Photolysis and
H atom
detection
(vuvLIF) at
Lyman α
1997
121.6
2000
118.2 and 121.6
2009
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) 0.17 ± 0.05
+ H2
CH2 (X 3B1) 0.48 ± 0.06
+ 2H
CH + H + H2 0.09
0.48 ± 0.05
0.69
0
0.22
-
0.59 ± 0.10
0.50 ± 0.06
0.03 ± 0.08
-
0
-
-
0.066 ± 0.012
0.10 ± 0.02
0.07
-
0.51
0.11
-
0.068 ± 0.013
0.02 ± 0.01
Total H
Total H2
0.55 ± 0.17
0.55 ± 0.05
0.31 ± 0.05
0.69
1.0 ± 0.5
0.51
1.31 ± 0.13
0.26 ± 0.05
0.47 ± 0.11 0.47 ± 0.10
0.65 ± 0.10
0.60 ± 0.10
0.51 ± 0.06
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
1CH
2
+ rare gas
1CH
2
+ RG → 3CH2 + RG
Gannon et al.
JCP 132 2010
Temperature Dependence of 1CH2 removal
by C2H2
5
Blitz et al
This work
Hayes et al
Hack et al
A(T/298 K)^n
4.5
3.5
-1
10 k 1/cm molecule s
-1
4
3
Gannon et al.
JPCA 114 2010
10
3
2.5
2
1.5
1
0.5
0
100
Monitor removal of 1CH2 by LIF
1CH + C H → C H + H
2
2 2
3 3
1CH + C H + M → C H + M
2
2 2
3 4
1CH + C H → 3CH + C H
2
2 2
2
2 2
Monitor calibrated production of H by LIF
200
300
400
500
Temperature/K
600
700
800
Product Temperature Dependence
k overall
k
reaction
k relaxation
relaxation
Temperature
H Atom Yields
ΓH
1CH
2
+
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 abstraction
Reactions
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 + H2O
Shannon 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
Total fluorescence signal/ arbitrary units
0.030
1CH
2
kr = 374000
78000 s-1
0.025
0.020
H
0.015
0.010
0.005
kd = 351000
19000 s-1
0.000
-5
0
5
10
15
Time / s
20
25
30
Experimental
• Generate 1CH2 by pulsed photolysis of ketene
• Monitor removal of 1CH2 by LIF
1CH + C H → C H + H
2
2 2
3 3
1CH + C H + M → C H + M
2
2 2
3 4
1CH + C H → 3CH + C H
2
2 2
2
2 2
• Monitor calibrated production of H by LIF
Master Equation Calculations
MESMER (Master Equation Solver for Multi Energy-well Reactions)
•K(E)’s calculated from RRKM theory.
W (E)
k (E) 
h ( E )
source
term
kji
ni(E)
A+B
kRi
nj(E)
kPj
kij
•Energy transfer calculated an
exponential down model
Ed
~150 - 450cm-1
Products
(infinite sink)
 n (E)
i
E
 n (E)
j
E
Master Equation Results
Experimental Pressure
1.2
1
150 K
200 K
H atom yield
0.8
250 K
300 K
0.6
0.4
0.2
0
1
10
100
1000
10000
100000
Pressure/Torr
Modelling shows no stabilization below 50 Torr
Balance of reaction is relaxation
Experimental
James Lockhart
Rotary Pump
Exhaust
Line /
Needle
valve
Reaction Cell
Probe Laser Pulse
282 nm
0.10
Fluorescence Signal / Arbitrary Units
Photodiode
Flash Photolysis LIF Detection
Rhodamine 6G
Dye Laser
PMT
Nd: YAG
Laser
0.08
Gas mixture flows in towards the cell
0.06
Photolysis
laser pulse
248 nm
0.04
Gas mixing
manifold
0.02
Excimer
Laser
0.00
0
1000
2000
Time / s
Boxcar Averager
3000
4000
5000
MFC
C
N2
MFC
C2H2
MFC
(CH3)3COOH
MFC
O2
II - OH + MEA (monoethanolamine)
4
5x10
4
kobs / s
-1
4x10
4
3x10
4
2x10
4
1x10
0
0.0
OH
?
PM
14
2.0x10
14
14
4.0x10
[MEA] / molecule cm
6.0x10
-3
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 - Recycling OH with Excess Oxygen
100% OH Yield
Fluorescence Signal / Arbitrary Units
1.0
0.8
0.6
0.4
Experimental OH Yield
0.2
OH Decay
Zero
OH Yield
in N2
0.0
0
500
1000
1500
2000
Time / s
2500
3000
3500
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.