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 (R.Shannon@leeds.ac.uk) for more information.