C14H10 polycyclic aromatic hydrocarbon formation by acetylene addition to naphthalenyl radicals observed.

The formation of polycyclic aromatic hydrocarbons (PAHs) during combustion has a substantial impact on environmental pollution and public health. The hydrogen-abstraction-acetylene-addition (HACA) mechanism is expected to be a significant source of larger PAHs containing more than two rings. In this study, the reactions of 1-naphthalenyl and 2-naphthalenyl radicals with acetylene (C2H2) are investigated using VUV photoionization time-of-flight mass spectrometry at 500 to 800 K, 15 to 50 torr, and reaction times up to 10 ms. Our experimental conditions allow us to probe the Bittner-Howard and modified Frenklach HACA routes, but not routes that require multiple radicals to drive the chemistry. The kinetic measurements are compared to a temperature-dependent kinetic model constructed using quantum chemistry calculations and accounting for chemical-activation and fall-off effects. We measure significant quantities of C14H10 (likely phenanthrene and anthracene), as well as 2-ethynylnaphthalene (C12H8), from the reaction of the 2-naphthalenyl radical with C2H2; these results are consistent with the predictions of the kinetic model and the HACA mechanism, but contradict a previous experimental study that indicated no C14H10 formation in the 2-naphthalenyl + C2H2 reaction. In the 1-naphthalenyl radical + C2H2 reaction system, the primary product measured is C12H8, consistent with the predicted formation of acenaphthylene via HACA. The present work provides direct experimental evidence that single-radical HACA can be an important mechanism for the formation of PAHs larger than naphthalene, validating a common assumption in combustion models.

Direct Kinetics and Product Measurement of Phenyl Radical + Ethylene.

The phenyl + ethylene (C6H5 + C2H4) reaction network was explored experimentally and theoretically to understand the temperature dependence of the reaction kinetics and product distribution under various temperature and pressure conditions. The flash photolysis apparatus combining laser absorbance spectroscopy (LAS) and time-resolved molecular beam mass spectrometry (MBMS) was used to study reactions on the C8H9 potential energy surface (PES). In LAS experiments, 505.3 nm laser light selectively probed C6H5 decay, and we measured the total C6H5 consumption rate coefficients in the intermediate temperature region (400-800 K), which connects previous experiments performed in high-temperature (pyrolysis) and low-temperature (cavity-ring-down methods) regions. From the quantum chemistry calculations by Tokmakov and Lin using the G2M(RCC5)//B3LYP method, we constructed a kinetic model and estimated phenomenological pressure-dependent rate coefficients, k(T, P), with the Arkane package in the reaction mechanism generator. The MBMS experiments, performed at 600-800 K and 10-50 Torr, revealed three major product peaks: m/z = 105 (adducts, mostly 2-phenylethyl radical, but also 1-phenylethyl radical, ortho-ethyl phenyl radical, and a spiro-fused ring radical), 104 (styrene, co-product with a H atom), and 78 (benzene, co-product with C2H3 radical). Product branching ratios were predicted by the model and validated by experiments for the first time. At 600 K and 10 Torr, the yield ratio of the H-abstraction reaction (forming benzene + C2H3) is measured to be 1.1% and the H-loss channel (styrene + H) has a 2.5% yield ratio. The model predicts 1.0% for H-abstraction and 2.3% for H-loss, which is within the experimental error bars. The branching ratio and formation of styrene increase at high temperature due to the favored formally direct channel (1.0% at 600 K and 10 Torr, 5.8% at 800 K and 10 Torr in the model prediction) and the faster ß-scission reactions of C8H9 isomers. The importance of pressure dependence in kinetics is verified by the increase in the yield of the stabilized adduct from radical addition from 80.2% (800 K, 10 Torr) to 88.9% (800 K, 50 Torr), at the expense of styrene + H. The pressure-dependent model developed in this work is well validated by the LAS and MBMS measurements and gives a complete picture of the C6H5 + C2H4 reaction.

Direct Measurement of Radical-Catalyzed C6H6 Formation from Acetylene and Validation of Theoretical Rate Coefficients for C2H3 + C2H2 and C4H5 + C2H2 Reactions.

The addition of vinylic radicals to acetylene is an important step contributing to the formation of polycyclic aromatic hydrocarbons in combustion. The overall reaction 3C2H2 → C6H6 could result in large benzene yields, but without accurate rate parameters validated by experiment, the extent of aromatic ring formation from this pathway is uncertain. The addition of vinyl radicals to acetylene was investigated using time-resolved photoionization time-of-flight mass spectrometry at 500 and 700 K and 5-50 Torr. The formation of C6H6 was observed at all conditions, attributed to sequential addition to acetylene followed by cyclization. Vinylacetylene (C4H4) was observed with increasing yield from 500 to 700 K, attributed to the ß-scission of the thermalized 1,3-butadien-1-yl radical and the chemically activated reaction C2H3 + C2H2 → C4H4 + H. The measured kinetics and product distributions are consistent with a kinetic model constructed using pressure- and temperature-dependent reaction rate coefficients computed from previously reported ab initio calculations. The experiments provide direct measurements of the hypothesized C4H5 intermediates and validate predictions of pressure-dependent addition reactions of vinylic radicals to C2H2, which are thought to play a key role in soot formation.

From benzene to naphthalene: direct measurement of reactions and intermediates of phenyl radicals and acetylene.

Hydrogen-abstraction-C2H2-addition (HACA) is one of the most important pathways leading to the formation of naphthalene, the simplest two-ring polycyclic aromatic hydrocarbon (PAH). The major reaction channels for naphthalene formation have previously been calculated by Mebel et al., but few experiments exist to validate the theoretical predictions. In this work, time-resolved molecular beam mass spectrometry (MBMS) was used to investigate the time-dependent product formation in the reaction of a phenyl radical with C2H2 for the first time, at temperatures of 600 and 700 K and pressures of 10 and 50 Torr. A pressure-dependent model was developed with rate parameters derived from Mebel et al.'s calculations and from newly calculated pathways on the C8H7 PES at the G3(MP2,CC)//B3LYP/6-311G** level of theory. The model prediction is consistent with the MBMS product profiles at a mass-to-charge ratio (m/z) of 102 (corresponding to the H-loss product from C8H7, phenylacetylene), 103 (the initial C8H7 adduct and its isomers plus the 13C isotopologue of phenylacetylene), 128 (naphthalene), and 129 (C10H9 isomers plus the 13C isotopologue of naphthalene). An additional C8H7 isomer, bicyclo[4.2.0]octa-1,3,5-trien-7-yl, not considered by Mebel et al.'s calculations, contributes significantly to the signal at m/z 103 due to its stable energy and low reactivity. At high C2H2 concentrations, bimolecular reactions dominated the observed chemistry, and the m/z 128 and m/z 102 MBMS signal ratio was measured to directly determine the product branching ratio. At 600 K and 10 Torr, branching to the H-loss product (phenylacetylene) on the C8H7 PES accounted for 7.9% of phenyl radical consumption, increasing to 15.9% at 700 K and 10 Torr. At 50 Torr, the branching was measured to be 2.8% at 600 K and 6.2% at 700 K. Adduct stabilization is favored at higher pressure and lower temperature, which hinders the formation of the H-loss product. The pressure-dependent model predicted the observed branching ratios within the experimental uncertainty, indicating that the rate parameters reported here can be used in combustion mechanisms to provide insights into phenyl HACA reactions and PAH formation.

Modeling of aromatics formation in fuel-rich methane oxy-combustion with an automatically generated pressure-dependent mechanism.

With the rise in production of natural gas, there is increased interest in homogeneous partial oxidation (POX) to convert methane to syngas (CO + H2), ethene (C2H4) and acetylene (C2H2). In POX, polycyclic aromatic hydrocarbons (PAH) are important undesired byproducts. To improve the productivity of such POX processes, it is necessary to have an accurate chemical mechanism for methane-rich combustion including PAH. A new mechanism was created to capture the chemistry from C0 to C12, incorporating new information derived from recent quantum chemistry calculations, with help from the Reaction Mechanism Generator (RMG) software. For better estimation of kinetics and thermochemistry of aromatic species, including reactions through carbene intermediates, new reaction families and additional data from quantum chemistry calculations were added to RMG-database. Many of the rate coefficients in the new mechanism are significantly pressure-dependent at POX conditions. The new mechanism was validated against electron-ionization molecular beam mass spectrometry (EI-MBMS) data from a high-temperature flow reactor reported by Kohler et al. In this work quantification of additional species from those experiments is reported including phenylacetylene (C8H6), indene (C9H8), naphthalene (C10H8) and acenaphthylene (C12H8) at many temperatures for several feed compositions. Comparison of the experimental species concentration data and the new kinetic model is satisfactory; the new mechanism is generally more accurate than other published mechanisms. Moreover, because the new mechanism is composed of elementary chemical reaction steps instead of global fitted kinetics, pathway analysis of species could be investigated step-by-step to understand PAH formation. For methane-rich combustion, the most important routes to key aromatics are propargyl recombination for benzene, reactions of the propargyl radical with the phenyl radical for indene, and hydrogen abstraction acetylene addition (HACA) for naphthalene.

A combined photoionization time-of-flight mass spectrometry and laser absorption spectrometry flash photolysis apparatus for simultaneous determination of reaction rates and product branching.

In recent years, predictions of product branching for reactions of consequence to both combustion and atmospheric chemistry have outpaced validating experiments. An apparatus is described that aims to fill this void by combining several well-known experimental techniques into one: flash photolysis for radical generation, multiple-pass laser absorption spectrometry (LAS) for overall kinetics measurements, and time-resolved photoionization time-of-flight mass spectrometry (PI TOF-MS) for product branching quantification. The sensitivity of both the LAS and PI TOF-MS detection techniques is shown to be suitable for experiments with initial photolytically generated radical concentrations of ∼1 × 1012 molecules cm-3. As it is fast (µs time resolution) and non-intrusive, LAS is preferred for accurate kinetics (time-dependence) measurements. By contrast, PI TOF-MS is preferred for product quantification because it provides a near-complete picture of the reactor composition in a single mass spectrum. The value of simultaneous LAS and PI TOF-MS detection is demonstrated for the chemically interesting phenyl radical + propene system.

Phenyl radical + propene: a prototypical reaction surface for aromatic-catalyzed 1,2-hydrogen-migration and subsequent resonance-stabilized radical formation.

The C9H11 potential energy surface (PES) was experimentally and theoretically explored because it is a relatively simple, prototypical alkylaromatic radical system. Although the C9H11 PES has already been extensively studied both experimentally (under single-collision and thermal conditions) and theoretically, new insights were made in this work by taking a new experimental approach: flash photolysis combined with time-resolved molecular beam mass spectrometry (MBMS) and visible laser absorbance. The C9H11 PES was experimentally accessed by photolytic generation of the phenyl radical and subsequent reaction with excess propene (C6H5 + C3H6). The overall kinetics of C6H5 + C3H6 was measured using laser absorbance with high time-resolution from 300 to 700 K and was found to be in agreement with earlier measurements over a lower temperature range. Five major product channels of C6H5 + C3H6 were observed with MBMS at 600 and 700 K, four of which were expected: hydrogen (H)-abstraction (measured by the stable benzene, C6H6, product), methyl radical (CH3)-loss (styrene detected), H-loss (phenylpropene isomers detected) and radical adduct stabilization. The fifth, unexpected product observed was the benzyl radical, which was rationalized by the inclusion of a previously unreported pathway on the C9H11 PES: aromatic-catalysed 1,2-H-migration and subsequent resonance stabilized radical (RSR, benzyl radical in this case) formation. The current theoretical understanding of the C9H11 PES was supported (including the aromatic-catalyzed pathway) by quantitative comparisons between modelled and experimental MBMS results. At 700 K, the branching to styrene + CH3 was 2-4 times greater than that of any other product channel, while benzyl radical + C2H4 from the aromatic-catalyzed pathway accounted for ∼10% of the branching. Single-collision conditions were also simulated on the updated PES to explain why previous crossed molecular beam experiments did not see evidence of the aromatic-catalyzed pathway. This experimentally validated knowledge of the C9H11 PES was added to the database of the open-source Reaction Mechanism Generator (RMG), which was then used to generalize the findings on the C9H11 PES to a slightly more complicated alkylaromatic system.

Anthracene/phenothiazine π-conjugated sensitizers for dye-sensitized solar cells using redox mediator in organic and water-based solvents.

Metal-free dyes (MD1 to MD5) containing an anthracene/phenothiazine unit in the spacer have been synthesized. The conversion efficiency (7.13 %) of the dye-sensitized solar cell using MD3 as the sensitizer reached approximately 85 % of the N719-based standard cell (8.47 %). The cell efficiency (8.42 %) of MD3-based dye-sensitized solar cells (DSSCs) with addition of chenodeoxycholic acid is comparable with that of N719-based standard cell. The MD3 water-based DSSCs using a dual-TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl)/iodide electrolyte exhibited very promising cell performance of 4.96 % with an excellent Voc of 0.77 V.

Phenothiazinedioxide-conjugated sensitizers and a dual-TEMPO/iodide redox mediator for dye-sensitized solar cells.

Metal-free dyes containing a phenothiazinedioxide entity in the spacer were synthesized. The best conversion efficiency (7.47%) of the dye-sensitized solar cell (DSSC) by using new sensitizers with chenodeoxycholic acid as a co-adsorbent and the I(-) /I3 (-) electrolyte reached over 90% of that of the standard N719-based cell (8.10%). A new type of ionic liquid containing the nitroxide radical (N-O(.) ) and iodide was successfully synthesized and applied to the DSSCs. If the I(-) /I3 (-) electrolyte was replaced with a dual redox electrolyte, that is, a TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) derivative with a dangling imidazolium iodide entity, the cell exhibited a high open-circuit voltage of 0.85 V and a cell efficiency of 8.36%.

Ionic liquid with a dual-redox couple for efficient dye-sensitized solar cells.

A new type of ionic liquid that contains a nitroxide radical (N-O(.)) and iodide as two redox couples, JC-IL, has been successfully synthesized for high-performance dye-sensitized solar cells (DSSCs). Both of the redox couples exhibit distinct redox potentials and attractive electrochemical characteristics. The UV/Vis absorption spectra of JC-IL shows a low-intensity peak compared to the strong absorption of I2 in the wavelength region of 350-500 nm. The high open-circuit voltage of DSSCs with JC-IL is over 850 mV, which is approximately 150 mV higher than that of the DSSCs with a standard iodide electrolyte. The dramatic increase in the standard heterogeneous electron-transfer rate constant leads to an increase in the short-circuit current for JC-IL compared to that of 2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO). DSSCs with the JC-IL electrolyte show promising cell efficiencies if coupled with dyes CR147 (8.12%) or D149 (6.76%). The efficiencies of the DSSCs based on the JC-IL electrolyte are higher than those of DSSCs based on either TEMPO electrolyte or standard iodide electrolyte alone.

Benzothiadiazole-containing donor-acceptor-acceptor type organic sensitizers for solar cells with ZnO photoanodes.

Dye-sensitized solar cells using nanocrystalline ZnO as the photoanode and a metal-free sensitizer with a benzothiadiazole entity directly connected to the 2-cyanoacrylic acid acceptor exhibited an efficiency (5.18%) higher than those using the TiO(2) photoanode. Use of a hierarchical ZnO back scattering layer further improved the efficiency to 5.82%.