Subith Vasu

Direct Kinetic Measurements of Criegee Intermediate (CH2OO) Formed by Reaction of CH2I with O2

Criegee Sighting Standard mechanistic models for the reaction of ozone with unsaturated hydrocarbons implicate a transient carbonyl oxide compound, termed the “Criegee intermediate,” which has largely eluded detection. Welz et al. (p. 204; see the Perspective by Marston) have now detected the compound by using mass spectrometry, following the low-pressure photolytic reaction of oxygen with diiodomethane, and measured its decay kinetics in the presence of nitric oxide, nitrogen dioxide, and sulfur dioxide. Reaction rates were higher than expected, suggesting that the intermediate may play a more prominent role in atmospheric chemistry than previously assumed. An elusive intermediate implicated in atmospheric oxidation chemistry has been identified in the laboratory. Ozonolysis is a major tropospheric removal mechanism for unsaturated hydrocarbons and proceeds via “Criegee intermediates”—carbonyl oxides—that play a key role in tropospheric oxidation models. However, until recently no gas-phase Criegee intermediate had been observed, and indirect determinations of their reaction kinetics gave derived rate coefficients spanning orders of magnitude. Here, we report direct photoionization mass spectrometric detection of formaldehyde oxide (CH2OO) as a product of the reaction of CH2I with O2. This reaction enabled direct laboratory determinations of CH2OO kinetics. Upper limits were extracted for reaction rate coefficients with NO and H2O. The CH2OO reactions with SO2 and NO2 proved unexpectedly rapid and imply a substantially greater role of carbonyl oxides in models of tropospheric sulfate and nitrate chemistry than previously assumed.

Experimental Study of the Rate of OH + HO2 → H2O + O2 at High Temperatures Using the Reverse Reaction

The rate constant of the reaction OH + HO(2) --> H(2)O + O(2) (1) can be inferred at high temperatures from measurements of the rate of its reverse reaction H(2)O + O(2) --> OH + HO(2) (-1). In this work, we used laser absorption of both H(2)O and OH to study the reverse reaction in shock-heated H(2)O/O(2)/Ar mixtures over the temperature range 1600-2200 K. Initial H(2)O concentrations were determined using tunable diode laser absorption near 2.5 microm, and OH concentration time-histories were measured using UV ring dye laser absorption near 306.7 nm. Detailed kinetic analysis of the OH time-history profiles yielded a value for the rate constant k(1) of (3.3 +/- 0.9) x 10(13) [cm(3) mol(-1) s(-1)] between 1600 and 2200 K. The results of this study agree well with those reported by Srinivasan et al. (Srinivasan, N.K.; Su, M.-C.; Sutherland, J.W.; Michael, J.V.; Ruscic, B. J. Phys. Chem. A 2006, 110, 6602-6607) in the temperature regime between 1200 and 1700 K. The combination of the two studies suggests only a weak temperature dependence of k(1) above 1200 K. Data from the current study and that of Keyser (Keyser, L.F. J. Phys. Chem. 1988, 92, 1193-1200) at lower temperatures can be described by the k(1) expression proposed by Baulch et al. (Baulch, D.L.; Cobos, C.J.; Cox, R.A.; Esser, C.; Frank, P.; Just, Th.; Kerr, J.A.; Pilling, M.J.; Troe, J.; Walker, R.W.; Warnatz, J. J. Phys. Chem. Ref. Data 1992, 21, 411), k(1) = 2.89 x 10(13) exp(252/T) [cm(3) mol(-1) s(-1)]. However, it should be noted that some previous studies suggest a k(1) minimum around 1250 K (Hippler, H.; Neunaber, H.; Troe, J. J. Chem. Phys. 1995, 103, 3510-3516) or 1000 K (Kappel, C.; Luther, K.; Troe, J. Phys. Chem. Chem. Phys. 2002, 4, 4392-4398).

Shock Tube/Laser Absorption Measurements of the Reaction Rates of OH with Ethylene and Propene

Reaction rates of hydroxyl (OH) radicals with ethylene (C₂H₄) and propene (C₃H₆) were studied behind reflected shock waves. OH + ethylene → products (rxn 1) rate measurements were conducted in the temperature range 973-1438 K, for pressures from 2 to 10 atm, and for initial concentrations of ethylene of 500, 751, and 1000 ppm. OH + propene → products (rxn 2) rate measurements spanned temperatures of 890-1366 K, pressures near 2.3 atm, and initial propene concentrations near 300 ppm. OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH₃)₃-CO-OH, and monitored by laser absorption near 306.7 nm. Rate constants for the reactions of OH with ethylene and propene were extracted by matching modeled and measured OH concentration time-histories in the reflected shock region. Current data are in excellent agreement with previous studies and extend the temperature range of OH + propene data. Transition state theory calculations using recent ab initio results give excellent agreement with our measurements and other data outside our temperature range. Fits (in units of cm³/mol/s) to the abstraction channels of OH + ethylene and OH + propene are k₁ = 2.23 × 10⁴ (T)(2.745) exp(-1115 K/T) for 600-2000 K and k₂ = 1.94 × 10⁶ (T)(2.229) exp(-540 K/T) for 700-1500 K, respectively. A rate constant determination for the reaction TBHP → products (rxn 3) was also obtained in the range 745-1014 K using OH data from behind both incident and reflected shock waves. These high-temperature measurements were fit with previous low-temperature data, and the following rate expression (0.6-2.6 atm), applicable over the temperature range 400-1050 K, was obtained: k₃ (1/s) = 8.13 × 10⁻¹² (T)(7.83) exp(-14598 K/T).

Reactions of OH with butene isomers: measurements of the overall rates and a theoretical study.

Reactions of hydroxyl (OH) radicals with 1-butene (k(1)), trans-2-butene (k(2)), and cis-2-butene (k(3)) were studied behind reflected shock waves over the temperature range 880-1341 K and at pressures near 2.2 atm. OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH(3))(3)-CO-OH, and monitored by narrow-line width ring dye laser absorption of the well-characterized R(1)(5) line of the OH A-X (0, 0) band near 306.7 nm. OH time histories were modeled using a comprehensive C(5) oxidation mechanism, and rate constants for the reaction of OH with butene isomers were extracted by matching modeled and measured OH concentration time histories. We present the first high-temperature measurement of OH + cis-2-butene and extend the temperature range of the only previous high-temperature study for both 1-butene and trans-2-butene. With the potential energy surface calculated using CCSD(T)/6-311++G(d,p)//QCISD/6-31G(d), the rate constants and branching fractions for the H-abstraction channels of the reaction of OH with 1-butene were calculated in the temperature range 300-1500 K. Corrections for variational and tunneling effects as well as hindered-rotation treatments were included. The calculations are in good agreement with current and previous experimental data and with a recent theoretical study.

High-Temperature Measurements and a Theoretical Study of the Reaction of OH with 1,3-Butadiene

The reaction of hydroxyl (OH) radicals with 1,3-butadiene (C(4)H(6)) was studied behind reflected shock waves over the temperature range 1011-1406 K and at pressures near 2.2 atm. OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH(3))(3)-CO-OH, and were monitored by narrow line width ring dye laser absorption of the well-characterized R(1)(5) line of the OH A-X (0,0) band near 306.7 nm. OH time histories were modeled using a comprehensive 1,3-butadiene oxidation mechanism, and rate constants for the reaction of OH with 1,3-butadiene were extracted by matching modeled and measured OH concentration time histories. Detailed error analyses yielded an uncertainty estimate of +/-13% at 1200 K for the rate coefficient of the target reaction. The current data extends the temperature range of the only previous high-temperature study for this reaction. The rate coefficient and the branching fractions for the H-abstraction channels of the target reaction were also calculated over the temperature range 250-2500 K using variational transition-state theory based on QCISD(T)/cc-pVinfinityZ//B3LYP/6-311++G(d,p) quantum chemistry. The calculations are in good agreement with the experimental results above 1200 K.

Chemical Reaction CO+OH(•) → CO2+H(•) Autocatalyzed by Carbon Dioxide: Quantum Chemical Study of the Potential Energy Surfaces.

The supercritical carbon dioxide medium, used to increase efficiency in oxy combustion fossil energy technology, may drastically alter both rates and mechanisms of chemical reactions. Here we investigate potential energy surface of the second most important combustion reaction with quantum chemistry methods. Two types of effects are reported: formation of the covalent intermediates and formation of van der Waals complexes by spectator CO2 molecule. While spectator molecule alter the activation barrier only slightly, the covalent bonding opens a new reaction pathway. The mechanism includes sequential covalent binding of CO2 to OH radical and CO molecule, hydrogen transfer from oxygen to carbon atoms, and CH bond dissociation. This reduces the activation barrier by 11 kcal/mol at the rate-determining step and is expected to accelerate the reaction rate. The finding of predicted catalytic effect is expected to play an important role not only in combustion but also in a broad array of chemical processes taking place in supercritical CO2 medium. It may open a new venue for controlling reaction rates for chemical manufacturing.

Laser Ignition and Flame Speed Measurements in Oxy-Methane Mixtures Diluted With CO2

Ignition and flame propagation in methane/O2 mixtures diluted with CO2 are studied. A laser ignition system and dynamic pressure transducer are utilized to ignite the mixture and to record the combustion pressure, respectively. The laminar burning velocities (LBVs) are obtained at room temperature and atmospheric pressure in a spherical combustion chamber. Flame initiation and propagation are recorded by using a high-speed camera in select experiments to visualize the effect of CO2 proportionality on the combustion behavior. The LBV is studied for a range of equivalence ratios (ϕ = 0.8–1.3, in steps of 0.1) and oxygen ratios, D = O2/(O2 + CO2) (26–38% by volume). It was found that the LBV decreases by increasing the CO2 proportionality. It was observed that the flame propagates toward the laser at a faster rate as the CO2 proportionality increases, where it was not possible to obtain LBV due to the deviation from spherical flame shape. Current LBV data are in very good agreement with existing literature data. The premixed flame model from chemkin pro (Reaction Design, 2011, CHEMKIN-PRO 15112, Reaction Design, San Diego, CA) software and two mechanisms (GRI-Mech 3.0 (Smith et al., 1999, “The GRI 3.0 Chemical Kinetic Mechanism,” http://www.me.berkeley.edu/gri_mech/) and ARAMCO Mech 1.3 (Metcalfe et al., 2013, “A Hierarchical and Comparative Kinetic Modeling Study of C1–C2 Hydrocarbon and Oxygenated Fuels,” Int. J. Chem. Kinetics, 45(10), pp. 638–675)) are used to simulate the current data. In general, simulations are in reasonable agreement with current data. Additionally, sensitivity analysis is carried out to understand the important reactions that influence the predicted flame speeds. Improvements to the GRI predictions are suggested after incorporating latest reaction rates from literature for key reactions.

Shock Tube Ignition Delay Times and Modeling of Jet Fuel Mixtures

Ignition delay times were measured for stoichiometric jet fuel mixtures (Jet-A and JP-8) in air behind reflected shock waves in a heated shock tube. Reflected shock conditions spanned the following ranges: temperature 875 to 1220 K, pressure 18 to 36 atm, and an equivalence ratio of 1.0. Ignition times were measured using sidewall-mounted piezo-electric transducers and confirmed at the same location with OH* emission measurements at 306 nm. The shock tube and mixing assembly were heated to 100 and 125 C, respectively, to prevent condensation of the lower vapor pressure/higher boiling point components of the fuel. The Jet-A/JP-8 results have very low scatter and are in excellent agreement with the limited previous shock tube measurements. To the best of our knowledge, we report the first shock tube ignition time data for JP-8. Although JP-8 composition is slightly different from Jet-A, their ignition times are in close agreement. The experimental results were compared with model calculations of ignition time using the models of Ranzi et al. (2005) and Zhang et al. (2005), applied to a jet fuel surrogate composition proposed by Violi et al. (2002).

Ignition and Flame Propagation in Oxy-Methane Mixtures Diluted With CO2

Ignition and flame propagation in methane/O2 mixtures diluted with CO2 are studied. A laser ignition system and dynamic pressure data are utilized to ignite the mixture and to record the combustion pressure, respectively. The laminar burning velocities (LBV) are obtained at room temperature and atmospheric pressure in a spherical combustion chamber. Flame initiation and propagation is recorded by using a high-speed camera in select experiments to visualize the effect of CO2 proportionality on the combustion behavior. The laminar burning velocity is studied for a range of equivalence ratios (ϕ =0.8–1.3, in steps of 0.1), and oxygen ratios, D=O2/(O2+CO2) (26–38% by volume). It was found that the LBV decreases by increasing the CO2 proportionality. It was observed that the flame propagates toward the laser at a faster rate as the CO2 proportionality increases. Current experiments are in very good agreement with existing literature data. The premixed flame model from CHEMKIN PRO [1] software and two mechanisms (GRI-Mech 3.0 [2] and ARAMCO Mech 1.3 [3]) are used to simulate the current data. In general, simulations are in reasonable agreement with current data though the mechanisms predict slower flame speeds. The LBV values obtained by the ARAMCO 1.3 mechanism are closer to the experimental values. Additionally, sensitivity analysis is carried out to understand the important reactions that influence the predicted flame speeds. Improvements to the GRI predictions are suggested after incorporating latest reaction rates from literature for key reactions.Copyright

High-pressure shock tube experiments and modeling of n-dodecane/air ignition

We have measured ignition delay times of n-dodecane/air mixtures over a range of conditions including pressures of 18-31 atm, temperatures of 943-1177 K, and equivalence ratios (φ) of 0.5-1.0, utilizing the heated, high pressure shock tube (HPST) at Stanford University. The shock tube and mixing assembly were heated to 120 and 170 C, respectively, to prevent condensation of n-dodecane fuel (because of its low vapor pressure). Ignition delay times behind reflected shocks were measured using side-wall pressure and OH* emission diagnostics. Shock tube ignition time measurements can provide excellent validation targets for refinement of jet fuel kinetic modeling, and n-dodecane is widely used as the principal representative for n-alkanes in jet fuel surrogates. To the best of our knowledge, we report the first gas-phase shock tube ignition time data for n-dodecane. We also provide comparisons with the ignition delay time predictions of two detailed JP-8 mechanisms that include n-dodecane as an important JP-8 surrogate component.