Dominant Reaction Pathway Research Articles

A quantum chemical study on the reactivity of four licorice flavonoids scavenging ·OOCl3C

Isoliquiritigenin, liquiritigen, uralenol, and neouralenol are four natural licorice flavonoid compounds extracted from licorice, which have the ability of scavenging free radicals. In this paper, the density functional theory (DFT) was used to study the microscopic reaction mechanism of the four kinds of licorice flavonoids respectively scavenging ·OOCl3C in vivo. At the level of M06-2X/6-311+G(d,p), the geometries of all stationary points for hydrogen atom transfer (HAT) and radical adduct formation (RAF) pathways were optimized. The thermodynamic parameters and kinetic parameters of each reaction pathway were obtained, and the potential energy surface information of each reaction pathway was obtained too. Using the continuum solvation model based on solute electron density (SMD), the influence of the solvation effect on the reaction was calculated. The main mechanisms and reactive sites for the capture of ·OOCl3C by four licorice flavonoids, liquiritigen, isoliquiritigenin, uralenol, and neouralenol, were determined. Research indicates liquiritigen captures ·OOCl3C by HAT mechanism. The HAT pathway on the B′ ring of liquiritigen is the main reactive site. Isoliquiritigenin captures ·OOCl3C in the body through HAT and RAF mechanism. The RAF pathway is the dominant reaction pathway. The C1 site on the carbon–carbon double bond linking the two benzene rings is the main reactive site. Uralenol and neouralenol capture ·OOCl3C by HAT mechanism. The main reactive site of uralenol is B6 site, and the main reactive site of neouralenol is B′ 2 site. By this study, the potential energy surface information difficult to capture by experimental research is obtained, which provides a reliable theoretical basis for the further screening of highly active natural free radical scavengers of flavonoids in licorice and provides the support for the improvement of the development and application technology of licorice.

Dissection of the multichannel reaction of acetylene with atomic oxygen: from the global potential energy surface to rate coefficients and branching dynamics.

The O(3P) + C2H2 reaction is the first step in acetylene oxidation. The accurate kinetic data and the understanding of the reaction dynamics is of great importance. To this end, a full-dimensional global potential energy surface (PES) for the ground triplet state of the O(3P) + C2H2 reaction is constructed based on approximately 85 000 ab initio points calculated at the level of explicitly correlated unrestricted coupled cluster single, double, and perturbative triple excitations with the explicitly correlated polarized valence triple zeta basis set (UCCSD(T)-F12b/VTZ-F12). The PES is fit using the permutation invariant polynomial-neural network (PIP-NN) approach with a total root mean square error of 0.21 kcal mol-1. The key topographic features of the PES, including multiple potential wells and saddle points along different reaction pathways, are well represented by this fit PES. The kinetics and dynamics of the O(3P) + C2H2 reaction are investigated using the quasi-classical trajectory (QCT) method. The calculated rate coefficients are in good agreement with experimental data over a wide temperature range, especially when the temperature is lower than 1500 K. The product branch ratio has also been determined, which indicates the H + HCCO channel as the dominant reaction pathway at 298-3000 K, accounting for 80-90% of the overall rate coefficient, in agreement with experimental observations. The dynamics of the reaction is analyzed in detail.

Mechanism Deduction from Noisy Chemical Reaction Networks.

We introduce KiNetX, a fully automated meta-algorithm for the kinetic analysis of complex chemical reaction networks derived from semiaccurate but efficient electronic structure calculations. It is designed to (i) accelerate the automated exploration of such networks and (ii) cope with model-inherent errors in electronic structure calculations on elementary reaction steps. We developed and implemented KiNetX to possess three features. First, KiNetX evaluates the kinetic relevance of every species in a (yet incomplete) reaction network to confine the search for new elementary reaction steps only to those species that are considered possibly relevant. Second, KiNetX identifies and eliminates all kinetically irrelevant species and elementary reactions to reduce a complex network graph to a comprehensible mechanism. Third, KiNetX estimates the sensitivity of species concentrations toward changes in individual rate constants (derived from relative free energies), which allows us to systematically select the most efficient electronic structure model for each elementary reaction given a predefined accuracy. The novelty of KiNetX consists in the rigorous propagation of correlated free-energy uncertainty through all steps of our kinetic analyis. To examine the performance of KiNetX, we developed AutoNetGen. It semirandomly generates chemistry-mimicking reaction networks by encoding chemical logic into their underlying graph structure. AutoNetGen allows us to consider a vast number of distinct chemistry-like scenarios and, hence, to discuss the importance of rigorous uncertainty propagation in a statistical context. Our results reveal that KiNetX reliably supports the deduction of product ratios, dominant reaction pathways, and possibly other network properties from semiaccurate electronic structure data.

A comprehensive DFT study of CO2 catalytic conversion by H2 over Pt-doped Ni catalysts

PtNi bimetallic catalysts show superior performance for CO2 catalytic conversion by hydrogen, but the underlying mechanism and the key elementary steps in controlling the activity and selectivity of CO2 hydrogenation remain unclear. In present work, the complete reaction network for CO2 hydrogenation has been investigated systematically over Pt/Ni (111) surface based on periodic density functional theory, and active sites and reaction mechanism have been determined. It is found that HCOOH is mainly produced by undergoing the HCOO pathways while synthesis of CH3OH and CH4 via RWGS+CO hydrogenation is the dominant reaction pathway, and their selectivity are determined by the competitive reaction between hydrogenation and CO bond scission of H2COH species. The dissociation of COOH is regarded as the rate-determining step as it has the highest barrier (2.07 eV) in RWGS+CO hydrogenation. Moreover, it is observed that the doping of Pt on Ni surface can promote the transformation of CO2 into chemisorbed CO2δ− and reduce the barrier in H2 dissociation, which further facilitate the activation and hydrogenation of CO2. More importantly, the doped Pt atom could promote HxCO hydrogenation to HxCOH, meanwhile, suppress HxCOH dissociation into CHx. Especially, the activation barrier and reaction energy for C formation is markedly enhanced, and the ability for C hydrogenation is promoted over Pt/Ni (111) surface, which could lower the possibility of coke formation. These results provide helpful information in understanding the process of CO2 hydrogenation at atomic scale, and could benefit for the synthesis of Ni-based bimetallic catalysts.

PTR-MS studies of the reactions of H3O+ with a number of deuterated volatile organic compounds and the subsequent sequential reactions of the primary product ions with water under normal and humid drift tube conditions: Implications for use of deuterated compounds for breath analysis

Product ion distributions resulting from the primary reactions of H3O+ with nine D-labeled volatile organic compounds and the subsequent sequential reactions with H2O have been determined using a Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-TOF 8000 (IONICON Analytik GmbH)) at various reduced electric field (E/N) values ranging from 80 up to 150 Td and for two different absolute humidity levels of air sample < 0.1% and 5%. The specific D-labeled compounds used in this study are acetone-d6, toluene-d8, benzene-d6, ethanol-d (C2H5OD), ethanol-d2 (CH3CD2OH), ethanol-d6, 2-propanol-d8, 2-propanol-d3 (CD3CH(OH)CH3), and isoprene-d5 (CH2CHC(CD2)CD3). With the exception of the two 2-propanol compounds, non-dissociative proton transfer is the dominant primary reaction pathway. For 2-propanol-d8 and 2-propanol-d3 the major primary reaction channel involved is dissociative proton transfer. However, unlike their undeuterated counterparts, the primary product ions undergo subsequent deuterium/hydrogen isotope exchange reactions with the ever present water in the drift tube, the extent of which of course depends on the humidity within that tube. This exchange leads to the generation of various isotopologue product ions, the product ion branching percentages of which are also dependent on the humidity in the drift tube. This results in complex mass spectra and the distribution of product ions leads to issues of reduced sensitivity and accuracy. However, the effect of D/H exchange considerably varies between the compounds under study. In the case of acetone-d6 it is very weak (< 1%), because the exchange process is not facile when the deuterium is in the methyl functional group. In comparison, the H3O+/ benzene-d6 (C6D6) reaction and sequential reactions with water result in the production of the isotopologue ions C6Dn(H7-n)+ (where n = 0–6). Changing the value of E/N and/or the humidity in the drift tube considerably affects the amount of the isotope exchange reactions and hence the resulting sequential product ion distributions. An important conclusion of the findings from this work is that care must be taken in the choice of an exogenous deuterated compound for use in breath pharmacokinetic studies using proton transfer reaction mass spectrometry; otherwise the resulting D/H exchange processes impose interpretative problems.

Studying Chemistry in Micro-compartments by Separating Droplet Generation from Ionization.

Recent studies show that reactions inside micron-sized compartments (e.g., droplets, emulsions) can proceed at significantly accelerated rates and with different mechanisms compared to the same reactions in a macroscopic container. Many of these studies use electrospray ionization (ESI) to both generate droplets and to quantify, via mass spectrometry (MS), droplet reaction kinetics. The highly charged and rapidly evaporating droplets produced in ESI make it difficult to examine precisely the underlying cause for droplet-induced rate enhancements. Additionally, interpretation of the spectra from ESI-MS can be complicated by gas-phase ion-molecule and clustering reactions. Here, we use an approach where droplet generation is separated from ionization, in order to decouple the multiple possible sources of acceleration and to examine more closely the potential role of gas-phase chemistry. The production of sugar phosphates from the reaction of phosphoric acid with simple sugars (a reaction that does not occur in bulk solution but has recently been reported to occur in droplets) is measured using this approach to compare reactivity in droplets (i.e., with compartments) with that in the gas phase (i.e., without compartments). The same product ions that have been previously assigned to in droplet reactions are observed with and without compartmentalization. These results suggest that in some cases, gas-phase processes in the ionization region can potentially complicate the quantification and interpretation of accelerated reactions in droplets using ESI-MS (or one of its variants). In such cases, contributions from in-droplet chemistry cannot be ruled out, but we demonstrate that gas-phase processes can be a significant (and possibly dominant) reaction pathway. We suggest that future studies of rate acceleration in droplets be modified to better assess the potential for non-droplet-related processes. Graphical Abstract ᅟ.

A mechanistic study on partial oxidation of methane to methanol with hydrogen peroxide vapor in atmospheric dielectric barrier discharge

In this paper, a detailed fluid model is developed for the conversion of methane to methanol with hydrogen peroxide vapor in an atmospheric dielectric barrier discharge. The two-dimensional axisymmetric fluid model is constructed, in which 107 plasma chemical reactions and 28 different species are considered. Our attention is focused on physicochemical mechanism of methanol formation during the complicated chemical reaction processes of reactant molecules dissociation by electron impacting and neutral radical recombination. First, spatial and temporal characteristics of main radicals and ions, such as H, CH3, OH, CH3OH, CH3O, CH2OH, CH4+, CH3+, H2O2+, and H2O+, are presented. It is found that the streamer discharge is sustained by the direct electron-impact ionization of methane molecules. The dominant positive ion flux on to the dielectric surface is methane ion, and its peak value is located at axis. Then, the dominant chemical routes governing the production and loss of CH3OH and OH are discussed in detail. Finally, a schematic overview of dominant plasma reaction pathways for partial oxidation of methane to methanol with hydrogen peroxide vapor in atmospheric dielectric barrier discharge is summarized.

Adiabatic and constant volume decomposition process of condensed phase δ-1,3,5,7-tetranitro-1,3,5,7-tetrazocane at high temperatures: Quantum molecular dynamics simulations

We performed quantum molecular dynamics simulations to investigate the initiation chemistry of condensed phase δ-HMX at high temperatures by maintaining constant energy and volume to model adiabatic initiation process. The decomposition of HMX began by the C-N bond breaking in one molecule and by the C-H bond cleavage in other HMX molecule at 2400 K. At 2700 K, HMX is triggered by only one path that the C-N bond broke and the ring opened. At 3000 K, the decomposition of HMX is triggered by the C-H bond and N-O bond fission in the branch chains. There are seven decomposition channels observed during the whole decomposition stage. The N-O bond cleavage is a dominant reaction pathway. The boat configuration of the HMX molecule caused a new reaction channel to be happened by forming a new N-N bond. Another new reaction channel took place to form a new N-C bond due to intermolecular effects.

Toward polymers with oxazolidin-2-one building blocks through tetra-n-butyl-ammonium halides (Cl, Br, I) catalyzed coupling of epoxides with isocyanates

The selectivity of the reaction of 4,4-methylene diphenyl diisocyanate MDI with o-cresyl glycidyl ether OCGE yielding oxazolidin-2-one and isocyanurates, and applying tetra-n-butyl ammonium halide as catalysts is higher in the order chloride, bromide, iodide. The dominant reaction pathways were inferred from the reactivity of the system with various ratios of MDI to OCGE, and using IR spectroscopy to monitor the progress. The reaction proceeds in several steps with initial nucleophilic attack of the halide on the epoxide yielding an alkoxide intermediate. The alkoxide reacts with isocyanate to yield a carbamato intermediate that may either ring close to form oxazolidinone or catalytically form isocyanurates by addition of more isocyanate and backbiting. The formation of isocyanurate is the reaction with the lowest activation barrier. Isocyanurate acts also as an isocyanate precursor as it is ring opened by the alkoxide intermediates in a slower consecutive process. Oxazolidinone modified epoxy prepolymers could be prepared from bisphenol A diglycidyl ether and MDI.

Monitoring the methanol conversion process in H-ZSM-5 using synchrotron X-ray powder diffraction-mass spectrometry

Using synchrotron X-ray powder diffraction-mass spectrometry in combination with theoretical calculations, we probed the structural changes of adsorbed methanol species in H-ZSM-5 for the first time during catalytic conversion of methanol. Preliminary experimental and computational findings suggested that associative mechanism was the dominant reaction pathway in the early stage of the reaction at 200 °C. Also, we observed a minor contribution from dissociative mechanism of the adsorbed methoxy intermediate species, CH3-Oz (where Oz is a framework oxygen atom), at Brønsted acid sites. It led to methoxy accumulation during the late stage of the reaction, which was postulated to be the initial step that produced ‘hydrocarbon pool’ and ‘coke’ through the formation of CC bonds.

Comprehensive Hg/Br reaction chemistry over Fe2O3 surface during coal combustion

A combination of experiments, density functional theory (DFT) and kinetic calculations was used to systematically understand the detailed chemistry of heterogeneous mercury reaction with HBr over Fe2O3 surface. Fe2O3 shows catalytic activity for mercury reaction with HBr. The chemisorption mechanism is responsible for the adsorption of mercury species (Hg0, HgBr and HgBr2) on Fe2O3 surface. Heterogeneous mercury reaction with HBr over Fe2O3 surface follows Langmuir–Hinshelwood mechanism in which adsorbed Hg0 reacts with active surface bromine species derived from HBr decomposition. On the basis of the experimental and DFT calculation results, a new comprehensive heterogeneous reaction kinetic model was established to describe the detailed reaction process of Hg/Br over Fe2O3 surface. This heterogeneous model includes 17 elementary reactions governing the elimination and formation of mercury species on Fe2O3 surface. This kinetic model was validated against the experimental data. The model predictions were found to be in good agreement with the experimental data. X-ray photoelectron spectroscopy (XPS) results, DFT calculations and sensitivity analysis indicate that the dominant reaction pathway of Hg/Br over Fe2O3 surface is a four-step process Hg0 → Hg(s) → HgBr(s) → HgBr2(s) → HgBr2, in which gaseous Hg0 is first adsorbed on Fe2O3 surface and subsequently reacts with brominated iron site to form HgBr(s), HgBr(s) can be further converted to HgBr2(s) and released into flue gas. The proposed dimensionless temperature coefficient can be used to better understand the temperature-dependent relationship between heterogeneous Hg/Br chemistry and mercury transformation.

Ignition delay times of decalin over low-to-intermediate temperature ranges: Rapid compression machine measurement and modeling study

Autoignition characteristics of decalin, a bicyclic alkane, were investigated in a heated rapid compression machine (RCM) over a wide range of conditions. Ignition delay time (IDT) was measured at compressed pressures of 10, 15 and 20 bar, for equivalence ratios of 0.5, 1.0, 1.5 and 2.0, and at temperatures in the range 631−930 K. Negative temperature coefficient (NTC) behavior of decalin ignition delay time was observed within the temperatures of 750−860 K, in which the ignition delay time increases with rising temperature. The dependence of ignition delay time on compressed pressure, equivalence ratio, and oxygen concentration was systemically studied. A reasonable modification was made to a literature mechanism. The simulation results using the tuned mechanism are found to well capture the dependence of the measured ignition delay time on temperautre, pressure, equivalence ratio, and oxygen concentration over the entire temperature range. Correlation formulas of the simulated and measured ignition delay times were proposed to quantitatively reveal the ignition delay time dependence and to evaluate the mechanism performance. A reaction pathway analysis was carried out at low temperature (700 K), NTC temperature (850 K), and high temperature (1000 K), respectively, to identify the dominant reaction pathways consuming decalin and intermediate species. A sensitivity analysis was also performed at different temperatures and equivalence ratios to find out the important reactions that promote and inhibit decalin autoignition.

Comprehensive evolution mechanism of SOx formation during pyrite oxidation

Pyrite (FeS2) oxidation during coal combustion is one of the main sources of harmful SO2 emission from coal-fired power plants. Density functional theory (DFT) study was performed to uncover the evolution mechanism of SOx formation during pyrite oxidation. The results show that chemisorption mechanism is responsible for O2, SO2 and SO3 adsorption on FeS2 surface. The presence of formed oxidation layer (Fe2O3) weakens the interaction between O2 molecule and FeS2 surface. The adsorbed O2 molecule easily dissociates into active surface O atom for SOx formation. The dissociation reaction of O2 is activated by 77.38 kJ/mol, and exothermic by 138.46 kJ/mol. Compared to the further oxidation of SO2 into SO3, SO2 prefers to desorb from FeS2 surface. The dominant reaction pathway of SO2 formation from the oxidation of the outermost FeS2 surface layer is a three-step process: surface lattice S oxidation, SO2 desorption and replenishment of S vacancy by activated surface O atom. The elementary reaction of surface lattice S oxidation has an activation energy barrier of 197.96 kJ/mol, and is identified as the rate-limiting step. SO2 formation from the further oxidation of bulk FeS2 layer is controlled by a four-step process: bulk lattice S migration, lattice S oxidation, SO2 desorption and surface O atom deposition. Migration of lattice S from bulk position to the outermost surface shows a high activation energy barrier of 175.83 kJ/mol. The deposition process of surface O atom is a relatively easy step, and is activated by 21.05 kJ/mol.

Oxidation of 2-methylfuran and 2-methylfuran/n-heptane blends: An experimental and modeling study

There have been significant advances in understanding ignition behavior of oxygenated biofuels (mainly alcohols) and their blends with conventional fuel components. However, the oxidation behavior of lignocellulosic derived furanic compounds blended with hydrocarbons has received little attention. The present work is an experimental and numerical investigation of 2-methylfuran (2-MF) combustion and its blend with n-heptane. These results are compared with pure n-heptane results to better understand 2-MF reactivity. Ignition delay times of pure 2-MF and the 2-MF/n-heptane (50/50 2-MF/n-heptane molar %) blend in air were measured in three different facilities; a rapid compression machine and two different shock tubes. Experiments were performed in the temperature range of 861–913 K at a pressure of 20 bar for stoichiometric pure 2-MF. The ignition delay times of 2-MF/n-heptane blends were measured in the temperature range of 672–1207 K, at pressures of 10 and 20 bar, and at equivalence ratios of 0.5, 1.0, and 1.5. A comprehensive chemical kinetic model containing low- to high-temperature chemistry of 2-MF and n-heptane was formulated based on a combination of available 2-MF and n-heptane mechanisms and available theoretical studies on 2-MF form literature. The developed detailed kinetic model was validated against the ignition delay data measured in this work as well as against high-temperature shock tube ignition delay, flame speed, and flame species data from literature to ensure the competence of the model. The proposed mechanism predicts the measured and literature data to a reasonable extent. To elucidate fuel specific oxidation pathways, reaction path analyses were performed at various conditions. Furthermore, sensitivity analyses on the ignition delay times were conducted and the dominant reaction pathways in the oxidation of pure and binary mixtures at high, intermediate, and low temperatures were identified. It is found that the competition between n-heptane and 2-MF for ȮH radicals inhibits the consumption of n-heptane and promotes the consumption of 2-MF. This work provides the first insight into the global low-temperature oxidation behavior of a second generation furanic blended with a hydrocarbon.

Structures of the phosphinidene germylenoid HP=GeLiF and its cycloaddition reaction with ethylene

The structures of phosphinidene germylenoid HP=GeLiF were studied for the first time by using DFT (density functional theory) calculations. The geometries were optimized at the B3LYP/6-311+G (d, p) level at first and then the single-point energies were calculated at QCISD/6-311++G (d, p) level. Theoretical calculations predicted that HP=GeLiF has two equilibrium structures, the p-complex (1) and the three-membered-ring (2) structures, in which structure 1 has the lower energy and is more stable than 2. To exploit the reactivity of HP=GeLiF, the cycloaddition reaction of 1 and ethylene was investigated at the same level of theory. From the potential energy profile, we predicted that the cycloaddition reaction has one dominant reaction pathway. The calculated result shows that the dominant reaction pathway is a [2 + 2] cycloaddition reaction which is the interaction of two π bonds in HP=GeLiF and ethylene molecule, and a four-membered-ring P-heterocyclic germylene is formed. Since sp3 hybridization of Ge atom in this four-membered-ring germylene, it may further react with another ethylene and finally forming a spiro-Ge-heterocyclic compound involving phosphorus. This means that this reaction involves a [2 + 2] cycloaddition as the initial step, and then a [2 + 1] cycloaddition carried out.

Shape selectivity in acidic zeolite catalyzed 2-pentene skeletal isomerization from first principles

• The “dimethylcyclopropane” pathway is dominant for 2-pentene isomerization. • The ring-opening step is the rate-determining step for the isomerization reaction. • Van der Waals interactions would change the scaling relations in zeolite catalysis. Periodic density functional theory (DFT) calculations have been performed to examine the monomolecular reaction mechanism for 2-pentene skeletal isomerization catalyzed by acidic zeolite Beta, Mordenite, and ZSM-5. The use of periodic models allows consideration and analysis of zeolite steric constraints that occur within zeolite microspores. Three different reaction pathways, the “dimethylcyclopropane” (DMCP), methyl shift, and ethyl shift mechanisms, have been considered. The DMCP mechanism which includes rearrangement of protonated cyclopropane is proposed as the dominant reaction pathway, and the ring-opening step determines the overall reaction rate. Comparison of the effective energy barriers for 2-pentene isomerization over the three different zeolites reveals that large 12-ring channels of Beta and MOR have little steric hindrance effect on 2-pentene isomerization while small 10-ring of ZSM-5 significantly increases the reaction activation energy. Calculated results also indicate that the adsorption heat of ammonia is a good descriptor for the reactivity of zeolites without taking into account long-range dispersive forces. In contrast, linear scaling relations cannot be obtained by using the BEEF-vdW functional because van der Waals interactions between zeolites framework and guest molecules affects the energetics of the isomerization reaction and changes the transition state energy scaling relation in zeolite catalysis.

Three-dimensional interconnected nitrogen-doped mesoporous carbons as active electrode materials for application in electrocatalytic oxygen reduction and supercapacitors

In this paper, a series of nitrogen-doped mesoporous carbons (NMCs) with three-dimensional (3D) interconnected mesopores have been prepared using flour as carbon source, dicyanamide as nitrogen source and colloidal silica as hard template. The optimized material (NMC-4) prepared with the colloidal silica/flour mass ratio of 4 has a high nitrogen doping level of 5.69 at.% and large specific surface area of 995 m2 g−1 as well as 3D interconnected mesopores (12.9 nm). As the oxygen reduction reaction (ORR) electrocatalyst among various NMCs, NMC-4 exhibits the superior performance and much better stability and methanol crossover with a four-electron dominant reaction pathway compared to commercial 20 wt% Pt/C. Furthermore, as a supercapacitor (SC) electrode material, NMC-4 exhibits a high specific capacitance of 178.5 F g−1 at a current density of 0.5 A g−1 and long cycle life (94.5% capacity retention after 5000 cycles). It also shows a good rate capacity as 76.1% of original specific capacitance remains when the current density increases from 0.5 to 20 A g−1. The high-performance of NMCs results from the synergetic effects of 3D interconnected mesopores, large surface area, and high N-doping level, enabling fast mass transport and electron transfer during the electrochemical process. This work provides a facile and efficient strategy to heteroatom-doped carbons from extensively available biomass, showing great potentials in electrocatalysis, energy storage, and other applications.

Efficient removal of benzene in air at atmospheric pressure using a side-on type 172nm Xe2 excimer lamp.

The photochemical removal of benzene was studied in air at atmospheric pressure using a side-on type 172nm Xe2 excimer lamp with a wide irradiation area. After 1.5min photoirradiation, C6H6 (1000ppm) in air was completely converted to HCOOH, CO, and CO2 at a total flow rate of 1000mL/min. The initial decomposition rate of C6H6 was determined to be 1.18min-1. By using a flow system, C6H6 (200ppm) was completely removed at a total flow rate of 250mL/min. The conversion of C6H6 and the energy efficiency in the removal of C6H6 changed in the 31-100% and 0.48-1.2g/kWh range, respectively, depending on the flow rate, the O2 concentration, and the chamber volume. On the basis of kinetic model simulation, dominant reaction pathways were discussed. Results show that the O(3P) + C6H6 reaction plays a significant role in the initial stage of the C6H6 decomposition. Important experimental parameters required for further improvement of the C6H6 removal apparatus using a 172 excimer lamp were discussed based on model calculations.

Computational singular perturbation analysis of super-knock in SI engines

Pre-ignition engine cycles leading to super-knock were simulated with a 48 species skeletal iso-octane mechanism to identify the dominant reaction pathways that are present in super-knock. To mimic pre-ignition, a deflagration front was generated via a hot spot that is placed over the piston at close proximity to the end-wall. Computational singular perturbation (CSP) was used to analyze the chemical dynamics at various in-cylinder locations: a point at the center of the cylinder where the deflagration front consumes the air/fuel mixture and two points located at 3 mm from the end-wall where super-knock and mild knock occur. The CSP analysis of the point at the center of the cylinder reveals weak two-stage ignition-like dynamics with a short second stage. At the other points, a pronounced two-stage ignition is displayed with a long second stage. A distinct contribution of formaldehyde (CH2O) at the second stage of ignition that adds to fast explosive modes in the super-knock points is not observed in the point at the center. A comparison between knock and super-knock analysis indicates that a similar set of reactions is responsible for the abnormal behavior but the fast explosive time scales are comparatively slower for knock, indicating lower reactivity, which results in the reduced intensity of knock. The analyzed results decoded important reactions responsible for the occurrence of super-knock.

A skeletal reaction scheme for selective catalytic reduction of NOx with NH3 over CeO2/TiO2 catalyst

CeO2/TiO2 has been regarded as the promising catalyst used for the selective catalytic reduction (SCR) of NOx with NH3. The complete NH3-SCR reaction mechanism of NO over CeO2/TiO2 catalyst was systemically investigated using density functional theory (DFT) calculations. Based on the DFT-derived energetics, a skeletal reaction scheme including 20 elementary reaction steps was proposed for NH3-SCR reaction of NO over CeO2/TiO2 catalyst. The results indicate that NH3, NO, O2, and NO2 adsorption on CeO2/TiO2(110) surface is dominated by the chemisorption mechanism. The successive dehydrogenation process of NH3 is cleaved by NH2 dehydrogenation step. NH2 derived from NH3 dehydrogenation step can stably exist on CeO2/TiO2 catalyst surface and serve as a key intermediate in NH3-SCR reaction. The activation energy barrier of N2O formation is higher than that of N2 formation, which is responsible for the excellent N2 selectivity of CeO2/TiO2 catalyst. NO2 formation under the typical low-temperature SCR conditions can make a contribution to NO reduction over CeO2/TiO2 catalyst. Reaction pathway analysis illustrates that the dominant reaction pathway of NO reduction by NH3 over CeO2/TiO2 catalyst is a three-step process: (1) NH2 formation from NH3 dehydrogenation, (2) N-N coupling reaction, and (3) NH2NO decomposition. During NO reduction, NH2 formation from NH3 dehydrogenation is the rate-determining step of N2 formation.