New Insights into Low-Temperature Oxidation of Propane from

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New Insights into Low-Temperature Oxidation of Propane from Synchrotron Photoionization Mass Spectrometry and Multi-Scale Informatics Modeling Oliver Welz, Michael P. Burke, Ivan O. Antonov, C. Franklin Goldsmith, John David Savee, David L. Osborn, Craig Allen Taatjes, Stephen J Klippenstein, and Leonid Sheps J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b01008 • Publication Date (Web): 10 Apr 2015 Downloaded from http://pubs.acs.org on April 28, 2015

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The Journal of Physical Chemistry

New Insights into Low-Temperature Oxidation of Propane from Synchrotron Photoionization Mass Spectrometry and Multi-Scale Informatics Modeling Oliver Welz,1,*, # Michael P. Burke,2,3 Ivan O. Antonov,1 C. Franklin Goldsmith,2,& John D. Savee,1 David L. Osborn,1 Craig A. Taatjes,1 Stephen J. Klippenstein,2 Leonid Sheps1,* 1 2

Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551, USA Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60493, USA

3

Department of Mechanical Engineering, Department of Chemical Engineering and Data Sciences Institute, Columbia University, New York, NY 10027, USA

Present address: # Institute for Combustion and Gas Dynamics, University of Duisburg-Essen, Duisburg, Germany & School of Engineering, Brown University, Providence, RI 02912, USA

*: corresponding author information: Oliver Welz: email: [email protected], phone: +49(203)379-1888 Leonid Sheps: email: [email protected], phone: +1(925)294-2927

1 ACS Paragon Plus Environment


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Abstract Low-temperature propane oxidation was studied at P = 4 Torr and T = 530, 600, and 670 K by time-resolved multiplexed photoionization mass spectrometry (MPIMS), which probes the reactants, intermediates and products with isomeric selectivity using tunable synchrotron vacuum UV ionizing radiation. The oxidation is initiated by pulsed laser photolysis of oxalyl chloride, (COCl)2, at 248 nm, which rapidly generates a ~1:1 mixture of 1-propyl (n-propyl) and 2-propyl (i-propyl) radicals via the fast Cl + propane reaction. At all three temperatures, the major stable product species is propene, formed in the propyl + O2 reactions by direct HO2-elimination from both n- and i-propyl peroxy radicals. The experimentally derived propene yields relative to the initial concentration of Cl atoms are (20 ± 4)% at 530 K, (55 ± 11)% at 600 K, and (86 ± 17)% at 670 K at a reaction time of 20 ms. The lower yield of propene at low temperature reflects substantial formation of propyl peroxy radicals, which do not completely decompose on the experimental time scale. In addition, C3H6O isomers methyloxirane, oxetane, acetone and propanal are detected as minor products. Our measured yields of oxetane and methyloxirane, which are coproducts of OH radicals, suggest a revision of the OH formation pathways in models of lowtemperature propane oxidation. The experimental results are modeled and interpreted using a multi-scale informatics approach, presented in detail in a separate publication (Burke, M. P.; Goldsmith, C. F.; Klippenstein, S. J.; Welz, O.; Huang H.; Antonov I. O.; Savee J. D.; Osborn D. L.; Zádor, J.; Taatjes, C. A.; Sheps, L., Multi-Scale Informatics for Low-Temperature Propane Oxidation: Further Complexities in Studies of Complex Rections, submitted, 2015). The model predicts the time profiles and yields of the experimentally observed primary products well, and shows satisfactory agreement for products formed mostly via secondary radical-radical reactions. Keywords: Low-temperature autoignition, propane oxidation, photoionization mass spectrometry, hydrocarbon oxidation, peroxy radicals, kinetics modeling


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Introduction Low-temperature (T < ~900 K) oxidation chemistry plays an important role in the autoignition processes that, e.g., govern performance of internal combustion engines.1 The chain branching and heat release at low temperature influences when the reacting system is driven into hot ignition.2 Autoignition is unwanted in spark-ignition engines because it is associated with engine knock. In contrast, advanced engine designs (such as homogeneous charge compression ignition) rely on autoignition and are exquisitely sensitive to fuel-chemistry effects in lowtemperature oxidation reactions.3 For comprehensive combustion models to reliably predict autoignition, the underlying complex chemical kinetics must be understood in detail. A key part of low-temperature oxidation of alkanes (RH) starts with the reaction of initial fuel radicals R with O2 to form chemically activated alkyl peroxy (RO2) species: R + O2

RO2

(R1)

RO2 → HO2 + R-H RO2

(R2)

QOOH

(R3)

QOOH → OH + oxygenated species

(R4)

QOOH + O2

(R5)

OOQOOH

OOQOOH → OH + ketohydroperoxide

(R6)

ketohydroperoxide → OH + alkoxy radical

(R7)

At low temperature and sufficiently high pressure RO2 radicals are efficiently stabilized by molecular collisions. As temperature increases, unimolecular RO2 decomposition channels become available, with the major product pathway typically being direct elimination of HO2 via a 5-



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membered cyclic transition state, producing stable alkene co-products.4-5 Such pathways are essentially radical chain-terminating in the low-temperature regime, where HO2 is relatively unreactive. Alternatively, RO2 can isomerize to carbon-centered hydroperoxyalkyl radicals (QOOH), which subsequently decompose or react with O2 to yield hydroperoxyalkyl peroxy radicals (OOQOOH). QOOH decomposition forms primarily OH + cyclic ethers (a chain-propagating step1), whereas second O2 addition to QOOH can eventually lead to chain-branching production of 2 OH + alkoxy radical co-product (reactions R5 – R7).1, 6 A more comprehensive discussion of product channels from OOQOOH and from ketohydroperoxide specifically for propane as the fuel has been recently published.7-8 The kinetics of alkyl + O2 reactions involves a competition of sequential pathways, which proceed by transitions between adjacent potential wells, and “formally direct” pathways, which pass multiple transition states without stabilization in the intermediate wells. For example, one possible formally direct pathway is R + O2 → OH + oxygenated species

(R8)

which involves a chemically activated sequence R + O2 → RO2 → QOOH → OH + oxygenated species without stabilization of either RO2 or QOOH intermediates. Establishing accurate kinetics models that capture the temperature and pressure dependence of this complex network of reaction pathways is crucial to building rigorous combustion mechanisms. The propyl + O2 reaction is a prototype for larger alkane oxidation because it exhibits many of the mechanistic features typical for the reactions of larger alkyl radicals with O2. At the same time this system is small enough to allow both a high-level theoretical treatment and a rigorous interpretation of the experimental results. Consequently, it has been extensively studied both experimentally9-23 and computationally.7, 19-31 Previous experimental studies of the propyl + 4 ACS Paragon Plus Environment

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O2 reaction included quantification of stable species in steady-state reactors9, 15-17, 23 as well as time-resolved measurements of propyl radicals, propene,12-14 OH,18, 21, 30 and HO2.17, 19, 20, 23 The earliest steady-state experiments by Baker et al.9 showed that propene is the major stable product in propyl oxidation, using gas-chromatographic (GC) analysis of propane added to a slowly reacting mixture of H2/O2 at P = 500 Torr, T = 753 K. Four C3H6O isomers were identified: oxetane, propanal, and methyloxirane were assigned as primary oxidation products, but acetone was thought to arise mostly from secondary chemistry. More recently, Cord et al.23 quantified products of low-temperature propane oxidation in a jet-stirred reactor at P = 795 Torr and T = 530 – 730 K using both GC and synchrotron photoionization mass spectrometry. Their model successfully reproduced the main oxidation products, but showed larger discrepancies with measurements of minor and secondary products. Slagle et al. investigated the kinetics of the n-C3H7 + O213 and i-C3H7 + O214 reactions at low pressures (~0.5 – 7 Torr) using laser photolysis in a flow reactor and time-resolved photoionization mass spectrometry with fixed-wavelength discharge lamps for detection of propyl radicals and C3H6. For the n-C3H7 + O2 reaction, Slagle et al. reported rising propene yields from ~0.14 at 550 K to >0.8 at 635 K, evidence of the increased role of thermal decomposition of propyl peroxy radicals. In addition, time-resolved detection provided the first hints of the competition between rapid formally direct and slower thermal propene formation.13 Taatjes and co-workers used laser-spectroscopic methods to probe the time-resolved formation of OH19, 22, 32 or HO218, 20-21, 24 in propyl oxidation at pressures up to 60 Torr and temperatures up to 700 K, revealing the competition between prompt and thermal product channels. The experimental OH and HO2 time traces were modeled with rate coefficients derived from ab initio-based time-dependent multiple-well master-equation calculations on the n-C3H7 + O2 and i5 ACS Paragon Plus Environment


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C3H7 + O2 potential energy surfaces (PESs). Adjustments of the energies of key minima and first-order saddle points within the uncertainties of the quantum-chemical methods allowed for satisfactory agreement between the experimental and modeled time traces. In the present study, we investigate propyl radical oxidation between 530 and 670 K at 4 Torr using pulsed-photolytic Cl atom initiation24 and time-resolved multiplexed synchrotron photoionization mass spectrometry (MPIMS). This approach provides the advantage of simultaneous direct time-resolved detection of multiple stable products as well as transient intermediates with isomeric selectivity. Crucially, we extend the earlier time-resolved studies of OH and HO2 formation by detecting the stable co-products associated with these channels. Formation of OH in the propyl + O2 reaction can occur via several pathways associated with different oxygenated coproducts (methyloxirane, oxetane, acetone and propanal), and their detection allows us to probe the individual OH-forming channels directly. The experimental results are modeled and interpreted based on the multi-scale informatics (MSI) approach introduced by Burke et al.,33 applied to the propyl + O2 system. The present paper focuses on the analysis and mechanistic interpretation of the experimental results aided by the MSI model. Details about the modeling methodology are presented in a second paper.34

Experimental methods MPIMS experiments employed synchrotron-generated vacuum-ultraviolet (VUV) ionization radiation from the Chemical Dynamics Beamline at the Advanced Light Source of Lawrence Berkeley National Laboratory. Details of the setup can be found in previous publications,35-37 and only a brief summary is given here. The reaction occurred in a slow-flow heatable (300 – 1000 K) low-pressure (1 – 10 Torr) quartz tube with an inner diameter of 1.05 cm and a length of 6 ACS Paragon Plus Environment

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62 cm. Metered flows of propane (C3H8), O2, and the Cl atom precursor oxalyl chloride (COCl)2, all diluted in helium bath gas, were introduced by calibrated mass flow controllers and mixed in situ at the entrance of the reactor. An excimer laser operating at 248 nm with a repetition rate of either 4 or 10 Hz fired along the reactor axis and created a uniform Cl atom profile in both axial and radial dimensions of the reactor. Typical laser fluences were ~85 mJ cm-2. The pressure inside the reactor was maintained at 4 Torr using a butterfly valve in the exhaust line with active feedback control, and experiments were done at temperatures of 530, 600, and 670 K. The initial Cl atom concentration [Cl]0 from photolysis of (COCl)2 was calculated based on the concentration of (COCl)2 in the reactor and its measured depletion after photolysis, using a quantum yield Φ = 2 for Cl atom formation at 248 nm.38 The experimental conditions for the main dataset, consisting of photoionization energy scans, are given in Table 1. Additional survey experiments, designed to identify which mass peaks arise from radical-radical reactions, were done at selected fixed photon energies, either increasing [O2] (by up to a factor of 5.4) or reducing [Cl]0 (by a factor of 3) while maintaining a total pressure of 4 Torr. The reacting mixture was continuously sampled through a 650 µm-diameter pinhole in the reactor sidewall into a vacuum chamber, skimmed, and crossed with tunable VUV synchrotron ionizing radiation. The resulting cations were collimated and detected as a function of their mass-to-charge (m/z) ratio in an orthogonal extraction time-of-flight mass spectrometer. The mass resolution of the spectrometer, m/∆m ≈ 1500, was sufficient to distinguish products with the same nominal mass, but different elemental composition, such as C2H4O (m/z = 44.03) and C3H8 (m/z = 44.06). At a given photoionization energy E, full mass spectra were acquired in intervals of 20 µs from reaction times t = -20 ms to 130 ms (4 Hz repetition rate) or t = -10 ms to 40 ms (10 Hz repetition rate) relative to photolysis, resulting in time-resolved mass spectra I(m/z, 7 ACS Paragon Plus Environment


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t). Additionally, the energy of the ionizing photons was scanned in steps of 25 meV over the range 9.2 – 11.2 eV, leading to a three-dimensional dataset I(m/z, t, E). All ion signals were normalized to the photon flux of the ionizing radiation using a NIST-calibrated photodiode. By scanning the energy of the ionizing radiation, species can be identified and quantified at an isomer-resolved level. The analysis of the experimental data is in large part analogous to the analysis presented in Ref. 39. Briefly, the ion intensity at any m/z value is a sum of signals from all isomers of molecular mass m: ,

/ ,

In eq. (1), /

= / ∙ Λ ∙ ∑[ ∙ , ]

(1)

is the ion signal, integrated over kinetic times (t,t+∆t) and normal,

ized to the photon flux at ionization energy E,

is the concentration of isomer i in the

reactor, and σi(E) is its partial photoionization cross section at a given m/z. The quantity αm/z is an isomer-independent mass discrimination factor, and Λ is a mass- and photon energy-independent instrument sensitivity factor.37 We measured αm/z directly using calibration gas mixtures and determined the parameterization αm/z = mi0.60 for parent ions under our experimental conditions. Referencing all ion signals to propane, whose absolute concentration is known, allows the determination of Λ and the quantification of any other species with a known ionization crosssection. In the current work, we determined the absolute photoionization cross-sections of propane, acetone, oxetane, and methyloxirane employing the procedure described in detail in Ref. 40. Published cross-section data were taken from previous measurements for methyl,37 ethene,39 formaldehyde,41 hydrogen peroxide,42 propene,40 acetaldehyde,40 and propanal.43 Time-resolved absolute species concentrations were derived using the procedure outlined in Ref. 39. For all products quantified except the C3H6O isomers, parent ion signals consisted 8 ACS Paragon Plus Environment

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only of a single isomer, so that their concentration profiles were readily calculated from timeresolved ion traces. The C3H6O product ion signal is more complex, because four isomers – oxetane (ionization energy (IE): 9.65 eV),44 acetone (IE: 9.70 eV),44 propanal (IE: 9.96 eV),44 and methyloxirane (IE: 10.22 eV)44 – contribute to the overall C3H6O time profile. To extract isomerresolved time traces, we performed experiments at selected photon energies: 9.85 eV, where only acetone and oxetane contribute, 10.05 eV, which includes additional contribution from propanal, and 11 eV, where all four isomers contribute. The similar IEs of acetone and oxetane made timeresolved measurements of oxetane alone impractical, since they required long averaging of very weak signals at the ionization threshold of oxetane (below the IE of acetone, 9.70 eV). Therefore, in further analysis we treat the time profiles of acetone and oxetane as identical. Photoionization spectra integrated over a range of reaction times (see below) have sufficient signal-tonoise ratio to show that the total time-integrated yield (up to t = 30 ms) of oxetane is negligible at 530 K, and is

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