kinetics and oh yield measurements to constrain energy barriers in the ch 3 och 2 + o 2 reaction
DESCRIPTION
Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction . Arkke Eskola , Scott Carr, Robin Shannon, Mark Blitz, Mike Pilling, Struan Robertson, Paul Seakins and Baoshan Wang University of Leeds, UK. Introduction – DME as a potential fuel. - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/1.jpg)
Kinetics and OH yield measurements to constrain energy barriers in the
CH3OCH2 + O2 reaction
Arkke Eskola, Scott Carr, Robin Shannon, Mark Blitz, Mike Pilling, Struan Robertson,
Paul Seakins and Baoshan WangUniversity of Leeds, UK
![Page 2: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/2.jpg)
Introduction – DME as a potential fuel
• Dimethylether, CH3OCH3 has great potential as a fuel
• DME can be used as a neat fuel in compression ignition engines or additive to diesel
• Compatible with current engine technologies and can be distributed through LPG networks
• Potential for manufacture from methane or biomass
![Page 3: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/3.jpg)
Introduction – DME combustion
• DME is ideally suited to HCCI engines (homogeneous charge, compression ignition)
‘HCCI can be characterized as a controlled chemical auto-ignition process and an important feature is the unusually large role that fuel chemistry plays in
determining combustion characteristics when compared to diesel or SI engines’ Westbrook and Curran
• The relatively low temperatures of DME combustion minimise NOx production
• DME shows the classic negative temperature dependence, but the mechanism is different from alkanes
0.8 1.0 1.2 1.4 1.60.1
1
10
100 Pure DME, = 2.0Ig
nitio
n de
lay
time
/ ms
1000 K / T
RCM 7 atm ST 13 atm ST 30 atm
Poor agreement(delay time is log scale)
Data and modelling from Curran
![Page 4: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/4.jpg)
Introduction – Origin of negative temperature dependence
OH + CH3OCH3 H2O + CH3OCH2
CH3OCH2 + O2 + M CH3OCH2O2 + MCH3OCH2O2 CH2OCH2OOH
CH2OCH2OOH 2HCHO + OHCH2OCH2OOH + O2 chain branching precursor
• Competition between CH2OCH2OOH reactions determines NTC
• CH3OCH2 CH3 + HCHO can also play a role
![Page 5: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/5.jpg)
CH3OCH2 + O2 Potential Energy Surface
CH3OCH2 + O2
CH3OCH2O2
TS1
TS2
2HCHO + OH
CH2OCH2OOH
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
CH3OCH2 → HCHO + CH3
CH2OCH2O2H → OH + 2HCHOOH + HCHO → HCO + H2O
H + O2 → HO2
CH3OCH2 + O2 → CH3OCH2O2
OH + CH3OCH3 → H2O + CH3OCH2
CH2OCH2O2H + O2 → O2CH2OCH2O2H
CH3OCH2O2 → CH2OCH2O2H
Sensitivities to Ignition DelaysAt 850 K (Zhao et al. 2008)
![Page 6: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/6.jpg)
Objectives
• Study the kinetics of CH3OCH2 + O2 as a function of T, p monitoring OH production
• Quantify the fraction of OH production as a function of T, p
• Model kinetics and yields using Master Equation, based on ab initio PES
• Do measurements allow constraints on the barriers on PES and allow extrapolation beyond experimental conditions?
• Higher temperature measurements and studies of chain branching to follow
![Page 7: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/7.jpg)
Experimental
• Reactions carried out in conventional slow flow, laser flash photolysis system with OH detection by laser induced fluorescence
• CH3OCH2Br + h (248 nm) CH3OCH2 + Br• Eskola et al. Chem Phys Lett (2010)• OH detected by off-resonance fluorescence• Stainless steel cell heated for 298 - 450 K• Cooled by immersion for 195 - 298 K
![Page 8: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/8.jpg)
Results - Kinetics
• Reactions carried out under pseudo-first-order conditions ([O2] >> [CH3OCH2]). Fits to traces give k’
• Bimolecular rate coefficients obtained from a plot of k’ vs [O2]
• Stabilization of initially formed CH3OCH2O2* chemically activated adduct requires 3rd body and hence kinetics are pressure dependent
• Note, not the characteristic ‘Lindemann’ curve as chemically activated CH3OCH2O2* can decompose to 2HCHO + OH
![Page 9: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/9.jpg)
Results - Yields
• The height of the signal proportional to OH yield • The OH yield will increase with decreasing
pressure and should → 1
• The relative yield, β, is given by:
CH3OCH2 + O2
CH3OCH2O2
TS1
TS2
2HCHO + OH
CH2OCH2OOH
+ M
CH3OCH2O2* OH + 2H2COCH3OCH2 + O2
CH3OCH2O2
kC
kM[M]
(R2b)
(R2a)
Scheme 1.
CH2OCH2OOH*
]He)[/(1
]He)[/(1OCHCH
OCHCH
cHe
refcHe
ref023ref
023
kkkk
![Page 10: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/10.jpg)
Results – Yields (2)
• A plot of 1/β vs [He] should be a straight line• Make reference pressure close to zero (5 Torr)
so extrapolation is short. • Assumes no other channel other than OH
production at zero pressure
])He[1(1
c
Heref k
k
![Page 11: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/11.jpg)
Determination of yields via kinetics
• Monitor OH decays in the presence of DME and DME/O2. In latter case OH is regenerated
CH3OCH2 OH + 2H2COCH3OCH3 + OH
CH3OCH2O2
kR2b
O2, [M]
Scheme 2.
Initiationt-C4H9OOH + 248nm
CH3CO + O2Cl + CH3OCH3 + O2
k1
kR2a
O2
OH is recycled, if O2 present
![Page 12: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/12.jpg)
Determination of yields via kinetics (2)
331121 OCHCHO kkk 331332121 OCHCH1OCHCHOO kkk
1
21 O1
kk
![Page 13: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/13.jpg)
Calculations ab initio
• Potential energy calculated at CBS-QB//mpw1k/avtz level. Main channel shown:
CH3OCH2 + O2
CH3OCH2O2
TS1
TS2
2HCHO + OH
CH2OCH2OOH
-34.8kcal
-9.8
-25.0
-3.0
![Page 14: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/14.jpg)
Calculation – Master Equation
• Data (kinetics AND yields) simulated using MESMER
• RRHO approximation with treatment of hindered rotors in CH3OCH2O2
• Vibrational frequencies from ab initio calculations
• ILT used to generate microcanonical rate coefficients for reverse reaction, RO2 → R + O2
• Fitting kinetics and yields without hindered rotors gave inconsistent ∆Ed
![Page 15: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/15.jpg)
Fits to the experimental data
![Page 16: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/16.jpg)
6.4 5.5
4.6 3.7
2.8
1.91.5
1.5
1.9 2.8
3.7
4.6 5.5
1.2 1.1
-15.0 -14.8 -14.6 -14.4 -14.2 -14.0 -13.8 -13.6 -13.4 -13.2 -13.0-9.0
-8.8
-8.6
-8.4
-8.2
-8.0
-7.8
-7.6
-7.4
-7.2
-7.0TS
2
TS1
Parameters
Parameter Ab initio value MESMER value
CH3OCH2O2 -34.8 kcal -33.6 kcal
TS1 -9.8 -13.8
CH2OCH2OOH -25.0 -25.0
TS2 -3.0 -8.3
Ed 200 cm-1
![Page 17: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/17.jpg)
Discussion points
• Simultaneous fitting of yields and kinetics constrain parameters
• Significant difference between fitting and ab initio, but:
• Variation of energies with methods suggests spin contamination issues
• Use of hindered rotor removes the need for a temperature dependent Ed
G4//B3LYP G4//MP2 CBS-QB3 CBS//MP2 CBS//mpw1k APNO//mpw1k
TS1 -8.8 -13.3 -11.5 -16.0 -11.3 -10.4
TS2 -0.1 7.2 -3.6 9.4 -3.3 -1.8
![Page 18: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/18.jpg)
Conclusions (1)
Objectives• Study the kinetics and branching ratio of CH3OCH2 + O2
as a function of T, p monitoring OH productionDone 195 – 450 K. Higher temperature work to follow.
• Model kinetics and yields using Master Equation, based on ab initio PES.
• Do measurements allow constraints on the barriers on PES?Yes, but still uncertainties
• and allow extrapolation beyond experimental conditions?No, currently uncertainties on PES and density of states calculations too great
Done
![Page 19: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/19.jpg)
Conclusions and outlook
• Hindered rotor removes the need for temperature dependent Ed, but:– Requires calculation of potential for hindered rotation– Treatment of other low frequency modes?
• Uncertainties around potential energy surfaces preventing wider application
Outlook• At higher temperatures, thermal production from stabilized
CH3OCH2O2 becomes important
• Decomposition of CH3OCH2 will become important
• Uncertainties around mechanism of QOOH + O2
• Points to be addressed in current application with Klippenstein and Curran on DME chemistry
![Page 20: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/20.jpg)
Acknowledgments
Thanks to:EPSRC for research funding and studentship for
Scott CarrNERC for studentship for Robin Shannon
NCAS for supporting Dr Mark BlitzFinnish Government for partial support for Dr
Arkke Eskola
![Page 21: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/21.jpg)
0.8 1.0 1.2 1.4 1.60.1
1
10
100 Pure DME, = 2.0Ig
nitio
n de
lay
time
/ ms
1000 K / T
RCM 7 atm ST 13 atm ST 30 atm
Poor agreement(delay time is log scale)
Data and modelling from Curran
![Page 22: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/22.jpg)
CH3OCH2 + O2
CH3OCH2O2
TS1
TS2
2HCHO + OH
CH2OCH2OOH
![Page 23: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/23.jpg)
CH3OCH2 + O2
CH3OCH2O2
TS1
TS2
2HCHO + OH
CH2OCH2OOH
+ M
![Page 24: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/24.jpg)
CH3OCH2 + O2
CH3OCH2O2
TS1
TS2
2HCHO + OH
CH2OCH2OOH
-34.8kcal
-9.8
-25.0
-3.0
![Page 25: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/25.jpg)
TS1 -8.8 -13.3 -11.5 -16.0 -11.3 -10.4
TS2 -0.1 7.2 -3.6 9.4 -3.3 -1.8
TS3 1.0 1.1 0.4 0.5 0.20 1.6
TS4 2.3 5.1 1.4 3.0 0.0 0.6
TS5 - -6.0 - -5.3 -0.6 -0.1
TS6 -64.2 -64.1 -64.8 -64.9 -65.1 -63.3
G4//B3LYP G4//MP2 CBS-QB3 CBS//MP2 CBS//mpw1k APNO//mpw1k
G4//B3LYP G4//MP2 CBS-QB3 CBS//MP2 CBS//mpw1k APNO//mpw1k
TS1 -8.8 -13.3 -11.5 -16.0 -11.3 -10.4
TS2 -0.1 7.2 -3.6 9.4 -3.3 -1.8
![Page 26: Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction](https://reader035.vdocument.in/reader035/viewer/2022062501/56816847550346895dde29f1/html5/thumbnails/26.jpg)