Single Reaction Channel Research Articles

Hydrogen- and oxygen-atom transfers in the thermal activation of benzene mediated by Cu2O2+ cations.

The mass-selected copper oxide cluster cations Cu2O2+ are successfully prepared by laser ablation and reacted with benzene in a linear ion trap reactor. The cluster reaction is characterized by reflectron mass spectrometry in conjunction with density functional theory calculations. Four types of reaction channels are observed: (1) Cu2OH+ + C6H5O˙, (2) Cu2C6H6+ + O2, (3) Cu(C6H6)2+ + Cu and (4) Cu2O2C6H6+, in which the first one is the major product. Observation of the products Cu2OH+ indicates that oxygen atom transfer (OAT) accompanying hydrogen atom transfer (HAT) occurs, and the oxygen-centered radical (O-˙) in Cu2O2+ is crucial to the cleavages of C-H and Cu-O bonds. It is interesting that HAT and OAT occur in a single reaction channel to form C6H5O˙, which has not been reported in the reactions of gas-phase ions with benzene. The calculated results are in agreement with the experimental observations, and no two-state reactivity scenario is present in the potential energy surface of channel 1. This work can provide useful insights into the nature of active sites over copper oxides and reaction mechanisms in the corresponding heterogeneous reactions.

High-Throughput Preparation of Monodispersed Layered Double Hydroxides via Microreaction Technology

A facile and reproducible method based on microreaction technology has been developed for the continuous preparation of layered double hydroxides (LDHs) with narrow particle size distribution. Mg–Al–CO3, Mg–Al–NO3, Mg–Al–Cl, and other different kinds of LDHs containing binary, ternary, and quadruple metal cations were successfully prepared. Due to the enhanced mixing, a high slurry throughput of 300 mL/min in a single reaction channel was obtained.

Evaluation of Combustion Mechanisms Using Global Uncertainty and Sensitivity Analyses: A Case Study for Low‐Temperature Dimethyl Ether Oxidation

ABSTRACTA global uncertainty analysis is performed for three current mechanisms describing the low‐temperature oxidation of dimethyl ether (Aramco Mech 1.3, Metcalfe et al., Int J Chem Kinet 2013, 45, 638–675; Zheng et al., Proc Combust Inst 2005, 30, 1101–1109; Liu et al., Combust Flame 2013, 160, 2654–2668) with application to simulations of species concentrations (CH2O, H2O2, CH3OCHO) corresponding to existing data from an atmospheric pressure flow reactor and high‐pressure ignition delays. When incorporating uncertainties in reaction rates within a global sampling approach, the distributions of predicted targets can span several orders of magnitude. The experimental profiles, however, fall within the predictive uncertainty limits. A variance‐based sensitivity analysis is then undertaken using high dimensional model representations. The main contributions to predictive uncertainties come from the CH3OCH2 + O2 system, with isomerization, propagation, chain‐branching, secondary OH formation, and peroxy–peroxy reactions all playing a role. The response surface describing the relationship between sampled reaction rates and predicted outputs is complex in all cases. Higher order interactions between parameters contribute significantly to output variance, and no single reaction channel dominates for any of the conditions studied. Sensitivity scatter plots illustrate that many different parameter combinations could lead to good agreement with specific sets of experimental data. The Aramco scheme is then updated based on data from a recent study by Eskola et al. (J Phys Chem A, in press), which presents quite different temperature and pressure dependencies for the rates of CH3OCH2O2 → CH2OCH2O2H and CH2OCH2O2H → OH+2CH2O compared with currently used values and includes well skipping channels. The updates from Eskola worsen the agreement with experiments when used in isolation. However, if the rate of the CH2OCH2O2H + O2 channel is subsequently reduced, very good agreement can be achieved. Owing to the complex nature of the response surface, the tuning of this channel remains speculative. Further detailed studies of the temperature and pressure dependence of the CH3OCH2O2 + O2, CH2OCH2O2H + O2 system are recommended to reduce uncertainties within current dimethyl ether mechanisms for low‐temperature conditions.

On the Coherent Description of Diffusion-Influenced Fluorescence Quenching Experiments II: Early Events

In a previous article we showed how to perform and analyze steady-state and nanosecond time-resolved experiments on fluorescence quenching by electron transfer in a coherent manner. Now, by making use of a superior time resolution, we explore the first stages of this kind of reaction. The novel information gained enables us to merge the results on the viscosity and the driving-force dependencies of the reaction rate. A unique set of parameters for a single reaction channel suffices to describe all the results in the frame of differential encounter theory for diffusion-influenced, bimolecular, remote electron-transfer reactions. The inclusion of the solvent structure is crucial for the understanding of the reaction kinetics. To the authors' best knowledge, this is the first time that such a comprehensive set of data has been successfully and jointly explained in the field, with physically sound parameters for electron-transfer reactions.

Modelling and simulation of microchannel catalytic WGS reactor for an automotive fuel processor

The water-gas shift (WGS) is one of the major steps for H2 production from gaseous, liquid and solid hydrocarbons. It is used to produce hydrogen for ammonia synthesis, to adjust the hydrogen-to-carbon monoxide ratio of synthesis gas, to detoxify gases. The WGS reactor is widely used as a part of fuel processors which produce hydrogen-rich stream from hydrocarbon-based fuels in a multi-step process. The WGS unit is placed downstream the fuel reformer in order to increase overall efficiency of hydrogen production and to lower CO content in reformate. Fuel processors stand for considerable option for fuelling PEM fuel cells for both portable and stationary applications. Micro-structured reactors are used with benefits of process miniaturization, intensification and higher heat and mass transfer rates compared with conventional reactors. Micro-structured reactor systems are essential for processes where potential for considerable heat transfer exists as well as for kinetic studies of highly exothermic reactions at near-isothermal conditions. Modelling and simulation of a microchannel reactor for the WGS reaction is presented. The mathematical models concern a single reaction channel with porous layer of catalyst deposited on the metallic wall of the microstructure unit. Simplified one-phase and more sophisticated two-phase models, with separate mass and energy balances for gas and solid phase at different levels of complexity, were developed. The models were implemented into gPROMS process modelling software. The models were used for an estimation of parameters in a kinetic expression using experimental data obtained with a new WGS catalyst. The simulations provide detailed information about the composition and temperature distribution in gas phase and solid catalyst inside the channel.

Binomial leap methods for simulating stochastic chemical kinetics

This paper discusses efficient simulation methods for stochastic chemical kinetics. Based on the tau-leap and midpoint tau-leap methods of Gillespie [D. T. Gillespie, J. Chem. Phys. 115, 1716 (2001)], binomial random variables are used in these leap methods rather than Poisson random variables. The motivation for this approach is to improve the efficiency of the Poisson leap methods by using larger stepsizes. Unlike Poisson random variables whose range of sample values is from zero to infinity, binomial random variables have a finite range of sample values. This probabilistic property has been used to restrict possible reaction numbers and to avoid negative molecular numbers in stochastic simulations when larger stepsize is used. In this approach a binomial random variable is defined for a single reaction channel in order to keep the reaction number of this channel below the numbers of molecules that undergo this reaction channel. A sampling technique is also designed for the total reaction number of a reactant species that undergoes two or more reaction channels. Samples for the total reaction number are not greater than the molecular number of this species. In addition, probability properties of the binomial random variables provide stepsize conditions for restricting reaction numbers in a chosen time interval. These stepsize conditions are important properties of robust leap control strategies. Numerical results indicate that the proposed binomial leap methods can be applied to a wide range of chemical reaction systems with very good accuracy and significant improvement on efficiency over existing approaches.

Ozone isotope enrichment: isotopomer-specific rate coefficients

Six rate coefficients of relative ozone formation contradict the role of molecular symmetry in the process that results in the enrichment of heavy ozone isotopomers. The results show that collisions between light atoms, such as 16O, and heavy molecules, such as 34O2 and 36O2, have a rate coefficient advantage of about 25 and 50 percent, respectively, over collisions involving heavy atoms and light molecules. These results suggest that the observed isotope effect for each isotopomer may be caused by the preponderance of a single reaction channel and not through molecular symmetry selection.

Kinetic isotope effects in solution Reactions of muonium atoms as H-isotopes

Muonium (Mu) and protium (1H) differ by only 0.43% in reduced mass but by a factor of 8.8 in atomic mass, so they constitute an excellent pair of atoms with which to examine kinetic isotope effects. When these atoms react with solutes in water, the ratio of observed rate constants (kM/kH) ranges from 103 to 10−2. Many of these effects can be accounted for by differences in diffusion, zero-point energies, quantum tunneling and steric hindrance when they involve a single reaction channel to form isotopomeric products. But there are also isotope effects which differ in kind not just in degree, where different reactions are involved for Mu and H leading to different reaction products. In general, Mu falls between H-atoms and hydrated electrons in its reactivity. Examples where Mu and H take different paths include Mu behaving as a nucleophilic addition reagent towards substituted benzenes, and where Mu adds to the carbonyl of a ketone while H abstracts a hydrogen from an adjacent alkyl group.This is certainly not an exhaustive review of Mu vs. H kinetic isotope effects in solution. Rather, it is a selective look at some 25 solutes in water—which includes most of those showing subtle or unusual effects, and those which have emerged as a result of the unique mass difference and the potential quantum character of these two H-isotopes.

Studies of infrared multiphoton dissociation rates of protonated aliphatic alcohol dimers

The rates of photodissociation induced by absorption of infrared radiation of protonated dimers of 2-propanol and its deuterated analogues are reported. Protonated alcohol dimer ions created in a quadrupole ion store (QUISTOR) were irradiated in isolation and as a function of parent gas pressure and added collision gas (N2). The relative weights and nature of ionic photoproducts were determined by mass spectrometry. The effect upon photolysis of collisions both prior to and during irradiation were examined. Photodissociation rates were defined by σD, which is the measured photodissociation rate constant normalized to the laser intensity. Nascent dimer ions which can possess up to 130 kJ mol−1 internal energy dissociated readily with σD of 15.8 × 10−2 cm2 extrapolated to collision-free conditions. Photodissociation products from nascent dimer ions were produced via two major reaction channels. As the number of collisions suffered by the nascent dimer ion prior to irradiation was increased, the value of σD diminished and a single reaction channel became more prevalent. After some 270 collisions the ions appear to be collisionally relaxed prior to photolysis. The value of σD of 13.1 × 10−21 cm2 extrapolated to collision-free conditions was obtained. The products from photodissociation of relaxed ions were obtained from a single reaction channel. An increase in σD values obtained with added nitrogen collision gas indicated a blockage or "bottleneck" in multiphoton absorption which is attributed to rotational population depletion. Three deuterated analogues of 2-propanol showed similar behaviour to that of 2-propanol with the exception that deuteron-bound dimers formed from 2-propanol (OD) and perdeutero-2-propanol failed to show rotational population depletion. The apparent lack of a "bottleneck" in multiphoton absorption by deuteron bound dimers is attributed to the diminished spacings between rotational levels of these species compared with the rotational level spacings for proton-bound dimers. Proton-bound ethanol dimers were selected for the examination of temperature effects upon photodissociation rates. While values for σD, extrapolated to collision-free conditions, of 15.6 × 10−21 cm2 at 321 K (in agreement with the 2-propanol results obtained solely at this temperature) and 8.55 × 10−21 cm2 at 293 K were obtained, the precise interpretation of such a marked temperature effect is not immediately obvious.

Reduction of the effective manifold of states in models of multiphoton laser-induced dissociation and chemistry

In the $N$-level atom (or molecule) model, a set of conditions are developed whereby (i) a band of reaction channels can be approximated by a single reaction channel, and (ii) this single reaction channel can be approximated by a population-loss term. The reactions are restricted to those which remove the system from the laser-excited ladder.