Analysis of the Kinetics and Yields of OH Radical Production from the CH₃OCH₂ + O₂ Reaction in the Temperature Range 195–650 K: An Experimental and Computational study

Abstract

ReuseUnless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. have been measured over the temperature and pressure ranges of 195 -650 K and 5 -500 Torr by detecting the hydroxyl radical using laser induced fluorescence following the excimer laser photolysis (248 nm) of CH3OCH2Br. The reaction proceeds via the formation of an energised CH3OCH2O2 adduct, which either dissociates to OH + 2 H2CO or is collisionally stabilised by the buffer gas. At temperatures above 550 K, a secondary source of OH was observed consistent with thermal decomposition of stabilized CH3OCH2O2 radicals. In order to quantify OH production from the CH3OCH2 + O2 reaction, extensive relative and absolute OH yield measurements were performed over the same (T, P) conditions as the kinetic experiments. The reaction was studied at sufficiently low radical concentrations (~10 11 cm -3 ) that secondary (radical + radical) reactions were unimportant and the rate coefficients could be extracted from simple bi-or tri-exponential analysis. Ab initio (CBS-GB3) / master equation calculations (using the program MESMER) of the CH3OCH2 + O2 system were also performed to better understand this combustion-related reaction as well as be able to extrapolate experimental results to higher temperatures and pressures. To obtain agreement with experimental results (both kinetics and yield data), energies of the key transition states were substantially reduced (by 20 -40 kJ mol -1 ) from their ab initio values and the effect of hindered rotations in the CH3OCH2 and CH3OCH2OO intermediates were taken into account.The optimised master equation model was used to generate a set of pressure and temperature dependent rate coefficients for the component nine phenomenological reactions that describe the CH3OCH2 + O2 system, including four well-skipping reactions. The rate coefficients were fitted to Chebyshev polynomials over the temperature and density ranges 200 to 1000 K and 1×10 17 to 1×10 23 molecule cm -3respectively for both an N2 and He bath gas. Comparisons with an existing autoignition mechanism show that the well-skipping reactions are important at a pressure of 1 bar but are not significant at 10 bar. The 3 main differences derive from the calculated rate coefficient for the CH3OCH2OO CH2OCH2OOH reaction, wh...

Global Uncertainty Propagation and Sensitivity Analysis in the CH3OCH2 + O2 System: Combining Experiment and Theory To Constrain Key Rate Coefficients in DME Combustion

“…Given the complicated electronic structure in this region of the potential energy surface it is also possible that other channels exist which were not discovered in the calculations performed by Eskola et al 6 In view of these observations, larger uncertainty ranges were assigned to the transition state energies.…”

“…7 This reaction system is a key step in the combustion of dimethyl ether (DME) [8][9][10][11][12][13] and, with interest in the use of DME as a possible fuel 14 , the CH 3 OCH 2 + O 2 reaction has attracted substantial attention. [15][16][17][18][19] The experiments 6 probed the reaction system by observation of OH, formed by dissociation of CH 2 OCH 2 OOH. Both the time dependence of OH formation and its yield were observed.…”

“…Modes corresponding to internal rotations or torsions were modelled as hindered rotations and these modes were projected out of the Hessian matrix according to the method of Sharma et al 28 The CBS-QB3//MPW1K/aug-cc-pVTZ frequencies, rotational constants and MPW1K/6-31+G hindrance potentials for internal rotations from Eskola et al 6 were used in the current study.…”

“…Transition states can be more problematic, and when discussing the ab initio calculations in the work of Eskola et al 6 the authors pointed out that the transition state TS2 in particular exhibited a large T1 diagnostic of 0.036 which is indicative of multireference effects.…”

“…In the current work, the fits were carried out using the same experimental data as in the work by Eskola et al 6 , but the additional parameters, E(RO 2 ), E(QOOH) and ILT A were also allowed to vary. In the work of Eskola et al 6 a detailed comparison was made of both a hindered rotor model and harmonic oscillator only model for the system. Of the two models, the hindered one is the more physically realistic and the differences such a model makes to the fitted parameters has already been discussed in detail.…”