Article pubs.acs.org/JPCA
High Temperature Shock Tube Studies on the Thermal Decomposition of O3 and the Reaction of Dimethyl Carbonate with O‑Atoms S. L. Peukert, R. Sivaramakrishnan,* and J. V. Michael* Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: The shock tube technique was used to study the thermal decomposition of ozone, O3, with a view to using this as a thermal precursor of O-atoms at high temperatures. The formation of O-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range, 819 K ≤ T ≤ 1166 K, at pressures 0.13 bar ≤ P ≤ 0.6 bar. Unimolecular rate theory provides an excellent representation of the falloff characteristics from the present and literature data on ozone decomposition at high temperatures. The present decomposition study on ozone permits its usage as a thermal source for Oatoms allowing measurements for, O + CH3OC(O)OCH3 → OH + CH3OC(O)OCH2 [A]. Reflected shock tube experiments monitoring the formation and decay of O-atoms were performed on reaction A using mixtures of O3 and CH3OC(O)OCH3, (DMC), in Kr bath gas over the T-range, 862 K ≤ T ≤ 1167 K, and pressure range, 0.15 bar ≤ P ≤ 0.33 bar. A detailed model was used to fit the O-atom temporal profile to obtain experimental rate constants for reaction A. Rate constants from the present experiments for O + DMC can be represented by the Arrhenius expression: kA(T) = 2.70 × 10−11 exp(−2725 K/T) cm3 molecule−1 s−1 (862−1167 K). Transition state theory calculations employing CCSD(T)/cc-pv∞z//M06-2X/cc-pvtz energetics and molecular properties for reaction A are in good agreement with the experimental rate constants. The theoretical rate constants can be well represented (to within ±10%) over the 500−2000 K temperature range by: kA(T) = 1.87 × 10−20T2.924 exp(−2338 K/T) cm3 molecule−1 s−1. The present study represents the first experimental measurement and theoretical study on this bimolecular reaction which is of relevance to the high temperature oxidation of DMC.
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INTRODUCTION Oxygenated species, e.g., methyl esters, alcohols, acetates, and carbonates, can be used as additives to diesel fuels. Incorporating these additives in the fuel increases the overall oxygen content and increases the possibility for complete diesel combustion, thereby reducing emissions of unburnt hydrocarbons, soot particles, and CO.1 In recent years, dimethyl carbonate (CH3OC(O)OCH3; DMC) has attracted interest as a potential oxygenated additive to diesel fuels because of its high oxygen content (∼53 wt %).2,3 Diesel engine studies3,4 have shown that smoke and soot emission can be reduced by adding DMC as an oxygenated agent. At the present time, limited production capacity is the reason for curtailed use as a fuel additive. With improvements and further developments in homogeneous and heterogeneous catalytic systems5,6 (e.g., zirconia complexes can catalyze the formation of DMC from methanol and CO2), it may become economically possible to produce DMC for extensive biofuel use. In a previous paper from this laboratory, the thermal decomposition of DMC and its reaction with D-atoms has been studied.7 Kinetics investigations, on compounds being used as model fuels or fuel additives, can contribute to a detailed understanding of ignition. For ignition, a quantitative © 2013 American Chemical Society
description of the formation and consumption of radicals such as H, O, OH, and CH3 is essential. These radicals can be produced from initiating reactions such as thermal dissociations and/or bimolecular reactions with O2. A small amount of radicals produced through these initiations is often sufficient to rapidly multiply radical populations by chain branching steps such as H + O2 → OH + O, or alternately bimolecular abstractions, RH + H/OH/O → R• + H2/H2O/OH. Abstractions are often the dominant fuel destruction processes at high temperatures. The present work on the bimolecular reaction O + DMC → CH3OC(O)OCH 2 + OH
(1)
is a continuation of our elementary kinetics studies on the initiation and propagation steps in DMC combustion. Reflected shock tube experiments on the O + DMC reaction (862 K ≤ T ≤ 1167 K; 0.15 bar ≤ P ≤ 0.33 bar) were performed using the sensitive O-atom atomic resonance absorption spectrometry (ARAS) method. Ozone (O3) was Received: January 18, 2013 Revised: February 22, 2013 Published: March 19, 2013 3729
dx.doi.org/10.1021/jp400613p | J. Phys. Chem. A 2013, 117, 3729−3738
The Journal of Physical Chemistry A
Article
Gases. High purity He (99.995%), used as the driver gas, was from AGA Gases. Research grade Kr (99.999%), the diluent gas in reactant mixtures, was from Praxair, Inc. The ∼10 ppm impurities (N2 < 5 ppm, O2 < 2 ppm, Ar < 1 ppm, CO2 < 0.5 ppm, H2 < 1 ppm, H2O < 3 ppm, Xe < 2 ppm, and THC < 0.2 ppm) are all either inert or in sufficiently low concentration so as to not perturb O-atom profiles. Ozone was synthesized using an electrical discharge from a tesla coil through a slow flow of 240 torr O2. O3 was collected on silica gel at 77 K. Since some O2 was also absorbed on silica gel at 77 K, the O3/O2 sample was outgassed removing most of the O2. However, it was not possible to completely remove O2. Therefore, in gas mixture preparations, [O3] was released together with residual [O2] by warming the trap. Gas mixtures of ([O3] + [O2]) in Kr diluent gas for the decomposition experiments, or [DMC] and ([O3] + [O2]) in Kr for the O + DMC experiments, were accurately prepared from pressure measurements using a Baratron capacitance manometer in an all glass high-purity vacuum line.
used as the source molecule for O-atoms. In the reflected shock wave regime O3 rapidly decomposes giving O2 and O
O3 → O2 + O
(2)
The thermal dissociation of O3 was initially characterized before its subsequent use as an O-atom precursor. Ozone thermal decomposition experiments spanned the T-range, 819 K ≤ T ≤ 1166 K, at pressures 0.13 bar ≤ P ≤ 0.6 bar. An RRKM model was used to rationalize the pressure and temperature dependence of the experimentally determined decomposition rate constants. In order to further analyze the bimolecular rate constants for O + DMC, k1, conventional transition state theory (TST) calculations have been applied using ab initio electronic structure methods for the energetics and molecular parameters. The theoretically determined rate constant predictions are then compared to the present experimental results and to a rate constant estimate from Glaude et al.8 To the best of our knowledge this is the first experimental kinetics study of the O + DMC reaction at combustion temperatures.
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THEORY O3 Dissociation. Jones and Davidson16 and Michael17 studied O3 dissociation behind reflected shock waves at different pressures and temperatures. Jones and Davidson applied HRRK-theory (Hinshelwood, Rice, Ramsperger, and Kassel) in order to obtain theoretical estimates for O3 dissociation rate constants, k2. Applying HRRK-theory resulted in a very good fit to the data of Davidson and Jones. Michael performed shock tube experiments at higher temperatures than Davidson and Jones. Experimentally determined second-order rate constants by Michael deviated substantially from the rate constant predictions provided by an approximate version of HRRKM-theory (Hinshelwood, Rice, Ramsperger, Kassel, Marcus). With increasing temperature, the theoretical rate constants overpredicted the experimental rate constants by more than a factor of 4. Michael concluded that this observation might be an indication for non-RRKM behavior in O3 dissociation at temperatures above 1000 K. In this case, the rate of the population of vibrational levels would be the rate determining step for reaction 2. In the present work the above interpretation will be examined and corrected in lieu of modern unimolecular rate theory. By considering the different pressures behind reflected shock waves, experimentally determined second-order rate constants from Jones and Davidson and Michael have been transformed into first-order rate constants. These first-order rate constants can now be described very well with modern theory. O + DMC. While abstraction, reaction 1, is expected to be the dominant process in the bimolecular reaction of O with DMC, addition across the CO was also considered. The two addition channels theoretically characterized are
EXPERIMENTAL SECTION
The present experiments, in Kr diluent, were performed with the reflected shock tube technique using O-atom ARAS detection. The method and the apparatus currently being used have been previously described.9,10 The shock-tube was constructed entirely from a 7-m (10.2 cm o.d.) 304 stainless steel tube with the cylindrical section being separated from the He driver chamber by a 4 mil unscored 1100-H18 aluminum diaphragm. The tube was routinely pumped between experiments to less than 1.3 × 10−11 bar by an Edwards Vacuum Products model CR100P packaged pumping system. Shock-wave velocities were measured with eight equally spaced pressure transducers (PCB Piezotronics, Inc., model 113A21) mounted along the downstream part of the test section and recorded with a 4094C Nicolet digital oscilloscope. Temperature and density in the reflected shock-wave regime were calculated from this velocity. This procedure has been given previously, and corrections for boundary layer perturbations have been applied.11−13 The oscilloscope was triggered by a pulse derived from the last velocity gauge signal. The photometer system was radially located at 6 cm from the end plate. O-Atom ARAS Detection. O-atom ARAS detection was used to follow [O]t quantitatively. With the O-atom photometer, a dry N2 filter and a CaF2 window (transmitting λ > 125 nm) are placed in front of the Hamamatsu R8487 solar blind photomultiplier tube to isolate the triplet resonance lines of Oatoms at 130 nm. An atomic filter section for the O-atom resonance lines is located between the lamp and the shock tube. The atomic filter section uses a fast flow of low pressure O2 gas. Before each run the O2 in the filter is discharged to produce a high concentration of O-atoms thereby removing all resonance lines from the optical path. This allows for a determination of the fraction of nonresonance light coming from the lamp. During an experiment the O-atom filter is turned off. In the present work the lamp was operated at 50 W microwave power (giving an effective lamp temperature of 490 K14) and with an O2 mole fraction of 1 × 10−3 in 2.0 mbar Helium (research grade) This photometer system has already been fully described.15 Signals were recorded with a LeCroy model LC334A oscilloscope.
O + CH3OC(O)OCH3 → (CH3O)2 C(O)O
(3)
O + CH3OC(O)OCH3 → (CH3O)2 COO
(4)
The rovibrational properties of the reactants, saddle points for the transition states, and products for the bimolecular reactions, 1, 3, and 4, were determined at the B3LYP/6-311+ +G(d,p), MP2/cc-pvtz, and M06-2X/cc-pvtz levels of theory. Higher level energy estimates for these stationary points were obtained using the CCSD(T)/cc-pV∞Z method, where the infinite basis set limits are estimated from an extrapolation of 3730
dx.doi.org/10.1021/jp400613p | J. Phys. Chem. A 2013, 117, 3729−3738
The Journal of Physical Chemistry A
Article
Table 1. Stationary Point Energies for the Reactions of DMC with Oa stationary point
CC//B3b
CC//MP2c
CC//M06d
O + CH3OC(O)OCH3 → CH3OC(O)OCH2 + H2 (TS) CH3OC(O)OCH2 + OH O + CH3OC(O)OCH3 → (CH3O)2COO (TS) CH3OCO(O)OCH3 O + CH3OC(O)OCH3 → (CH3O)2C(OO) (TS) CH3OC(OO)OCH3
8.33 −8.63 19.42 16.05 59.77 −0.55
8.23 −8.24 18.46 15.64 63.86 −0.92
7.93 −8.44 18.72 15.13 60.15 −0.39
a Values are in kcal/mol and include zero-point corrections at 0 K. These values are relative to corresponding reactants, O + CH3OC(O)OCH3. The rows incorporating reactions and marked by TS refer to barrier heights and the rows incorporating products refer to reaction exo- or endothermicities. bPresent CCSD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) calculations. cPresent CCSD(T)/cc-pV∞Z//MP2/cc-pvtz calculations. d Present CCSD(T)/cc-pV∞Z//M06-2X/cc-pvtz calculations.
results obtained from sequences of cc-pVnZ where n = (T,Q) basis sets.18,19 The energetics for the saddle points and reaction enthalpies at the CCSD(T)/cc-pV∞Z level of theory, using either the MP2 or DFT geometries, differ by no more than 0.4 kcal/mol for the abstraction channel. The T1 diagnostic,20 which is a measure of the multireference character, was
