Individual Reaction Pathways Research Articles

The direct synthesis of H2O2 and in situ oxidation of methane: An investigation into the role of the support

The selective oxidation of methane to methanol, using H2O2 generated in situ from H2 and O2 has been investigated using bimetallic gold-palladium catalysts, prepared via an industrially relevant wet co-impregnation protocol on a range of zeolite and metal oxide supports. The choice of catalyst support was found to drastically influence catalyst performance, through control of both nanoparticle dispersion and Pd speciation. Notably in the case of those formulations prepared on metal oxides a direct correlation between catalytic performance towards H2O2 synthesis and methane valorisation exists, whereas in the case of the zeolitic-based analogues, no clear correlation could be drawn between activity towards individual reaction pathways.

Laser induced fluorescence and computational studies on the tropospheric photooxidation reactions of methyl secondary butyl ether initiated by OH radicals.

The kinetics of the reaction of methyl secondary butyl ether with OH radicals was investigated experimentally using the pulsed laser photolysis-laser induced fluorescence technique (PLP-LIF) over temperatures ranging from 268 to 363K. The rate coefficient value at 298K was measured to be (1.09 ± 0.02) × 10-11 cm3 molecule-1s-1 and the deduced Arrhenius expression is [Formula: see text]= (2.21 ± 0.29) × 10-12 exp ((471.71 ± 38.50)/T) cm3 molecule-1s-1. To complement the experimental data, the kinetic study of the title reaction was performed computationally at CCSD(T)/cc-pVTZ//M06-2X/6-311 + G(d,p) level of theory with the incorporation of tunnelling correction from 200 to 400K. The end products formed were qualitatively analyzed by using gas chromatography equipped with mass spectrometry (GC-MS) as detection technique and the mechanism for degradation was proposed. Thermochemical parameters were evaluated to determine the feasibility of individual reaction pathways. Atmospheric implications were evaluated and discussed in this manuscript.

Effect of Diffusion Constraints and ZnOx Speciation on Nonoxidative Dehydrogenation of Propane and Isobutane over ZnO-Containing Catalysts

Heterogeneously catalyzed gas–solid-phase reactions generally suffered from diffusion limitations in large-scale processes or in academic studies when zeolites were used as catalysts or supports. Here, we elucidated the effects of diffusion of reactants/products in nonoxidative propane (PDH) and isobutane dehydrogenation (iBDH) reactions on the performance of catalysts possessing differently structured ZnOx species on (S-1), dealuminated beta (deAl beta), and ZrO2. The catalysts were prepared through physically mixing ZnO and the support. Force-field molecular dynamics simulations revealed that the effectiveness factor η is larger than 0.99 in the PDH reaction over all catalysts and in the iBDH reaction over the ZnO-deAl beta catalyst, thus suggesting that mass transport limitations do not play any significant role. However, the iBDH reaction over S-1-based catalysts suffers from some diffusion limitations (0.35 < η < 0.9). Such conditions are favorable for cracking reactions responsible for isobutene selectivity loss. To compare intrinsic catalyst activity in the PDH and iBDH reactions over the ZnOx/S-1 catalyst, molecular-level insights into individual reaction pathways were derived from density functional theory calculations. The nature of active ZnOx sites was investigated by X-ray absorption spectroscopy and was established to depend on the kind of support material. Binuclear ZnOx species are formed inside small S-1 pores or on the surface of ZrO2, while three-dimensional multinuclear ZnOx clusters are generated in the β zeolite with larger pores. The latter show higher Zn-related activity in the PDH reaction under conditions free of any diffusion constraints. The developed ZnO–deAl beta showed the space–time yield of propene or isobutene formation of 2 kgC3H6 kgcat–1 h–1 or 6.3 kgi-C4H8 kgcat–1 h–1 at 550 °C and about 70–80% equilibrium alkane conversion with an olefin selectivity of about 90%. The activity values are higher than those reported for the state-of-the-art non-noble metal oxide catalysts tested at the same or even higher temperatures.

Research advances in chlorinated benzene-containing compound oxidation catalyzed by metal oxides: activity-enhanced strategies and reaction-facilitated mechanisms.

Chlorinated benzene-containing compounds (CBCs) refer to volatile organic compounds which simultaneously contain benzene rings and Cl atoms. It has been widely believed to cause serious harm to human health and the natural environment due to high toxicity, high persistence, and refractory degradation, thus, it is urgent to develop CBC abatement technology. In this review, several CBCs control techniques are compared, and the catalytic oxidation technology stands out for its good low-temperature activity and chlorine resistance of metal oxide catalysts. Then, the common and individual reaction pathways and water impact mechanisms of CBC catalytic oxidation on transition metal catalysts are concluded. Subsequently, three typical metal oxides (namely, VOx, MnOx, and CeO2-based catalysts) are introduced in the catalytic degradation of CBCs, whose catalytic activity influence factors are also proposed on active components, support properties, surface acidity, and nanostructure (crystal, morphology, etc.). Furthermore, the effective strategies to enhance the REDOX cycle and surface acidic sites are summarized by the doping of metals, the modification of support or/and acidic groups, and the construction of nanostructures. Finally, the key points for efficient catalyst design are speculated. This review may provide ideas for the breakthroughs of activity-enhanced strategies, the design of efficient catalysts, and research on reaction-promoted mechanisms.

Development of Chemical Kinetics Models from Atomistic Reactive Molecular Dynamics Simulations: Application to Iso-octane Combustion and Rubber Ablative Degradation.

Modeling the complex chemical phenomena resulting from multiple active species and long-chain polymers is limited by uncertainties in the reaction rate parameters, which increase rapidly with the number of active species and/or reaction pathways. Reactive molecular dynamics simulations have the potential to help obtain in-depth information on the chemical reactions that occur between the polymer (e.g., ablative material) and the multiple active species in an aggressive environment. In this work, we demonstrate that molecular dynamics (MD) simulations using the ReaxFF interatomic potential can be used to obtain the reaction kinetics of complex reaction pathways at high temperatures. We report two recently developed tools, namely, MolfrACT and KinACT, designed to extract chemical kinetic pathways by postprocessing reactive MD simulation data. The pathway extraction is based on a new algorithm, Consistent Reaction Stoichiometry via an Iterative Scheme (CReSIS), for the automated extraction of reactions and kinetics from MD trajectories. As a validation of the methodology, we first report the kinetic analysis and mechanisms for the high-temperature combustion of iso-octane. The observed reaction pathways are consistent with experimental models. In addition, we compare the activation energies of select iso-octane combustion pathways with experimental data and show that nanosecond timescale molecular dynamics simulations are sufficient to obtain realistic estimates of activation energies for different fuel consumption reaction pathways at high temperatures. The framework developed here can potentially be combined with time-series forecasting and machine learning methods to further reduce the computational complexity of transient molecular dynamics simulations, yet yielding realistic chemical kinetics information. Subsequently, the CReSIS scheme applied to ethylene-propylene-diene-monomer (EPDM) rubber ablative reveals that H2O, C2H4, and HCHO are the major products during the initial stages of the polymer degradation in high-temperature oxidative environments. While prior work involving ReaxFF simulations is restricted to overall rates of formation of any species, we extract kinetic information for individual reaction pathways. In this paper, we present several reaction pathways observed during the EPDM rubber degradation into the dominant products and report the pathway-specific reaction rates. Arrhenius analysis reveals that the dominant reaction pathway activation energies for the formation of water, ethylene, and formaldehyde are 34.42, 27.26, and 6.37 kcal/mol, respectively. In contrast, the activation energies for the overall formation (across all reaction pathways) of these products are in the 40-50 kcal/mol range.

Multiphase Oxidation of Sulfur Dioxide in Aerosol Particles: Implications for Sulfate Formation in Polluted Environments

Atmospheric oxidation of sulfur dioxide (SO2) forms sulfate-containing aerosol particles that impact air quality, climate, and human and ecosystem health. It is well-known that in-cloud oxidation of SO2 frequently dominates over gas-phase oxidation on regional and global scales. Multiphase oxidation involving aerosol particles, fog, and cloud droplets has been generally thought to scale with liquid water content (LWC) so multiphase oxidation would be negligible for aerosol particles due to their low aerosol LWC. However, recent field evidence, particularly from East Asia, shows that fast sulfate formation prevails in cloud-free environments that are characterized by high aerosol loadings. By assuming that the kinetics of cloud water chemistry prevails for aerosol particles, most atmospheric models do not capture this phenomenon. Therefore, the field of aerosol SO2 multiphase chemistry has blossomed in the past decade, with many oxidation processes proposed to bridge the difference between modeled and observed sulfate mass loadings. This review summarizes recent advances in the fundamental understanding of the aerosol multiphase oxidation of SO2, with a focus on environmental conditions that affect the oxidation rate, experimental challenges, mechanisms and kinetics results for individual reaction pathways, and future research directions. Compared to dilute cloud water conditions, this paper highlights the differences that arise at the molecular level with the extremely high solute strengths present in aerosol particles.

Visualization of Single Molecules Building a Viral Capsid Protein Lattice through Stochastic Pathways.

Direct visualization of pathways followed by single molecules while they spontaneously self-assemble into supramolecular biological machines may provide fundamental knowledge to guide molecular therapeutics and the bottom-up design of nanomaterials and nanodevices. Here, high-speed atomic force microscopy is used to visualize self-assembly of the bidimensional lattice of protein molecules that constitutes the framework of the mature human immunodeficiency virus capsid. By real-time imaging of the assembly reaction, individual transient intermediates and reaction pathways followed by single molecules could be revealed. As when assembling a jigsaw puzzle, the capsid protein lattice is randomly built. Lattice patches grow independently from separate nucleation events whereby individual molecules follow different paths. Protein subunits can be added individually, while others form oligomers before joining a lattice or are occasionally removed from the latter. Direct real-time imaging of supramolecular self-assembly has revealed a complex, chaotic process involving multiple routes followed by individual molecules that are inaccessible to bulk (averaging) techniques.

Unsupervised Idealization of Nano-Electronic Sensors Recordings with Concept Drifts: An Information Theory Approach for Non-Stationary Single-Molecule Data Analysis

Unsupervised Idealization of Nano-Electronic Sensors Recordings with Concept Drifts: An Information Theory Approach for Non-Stationary Single-Molecule Data Analysis

Kinetics of formic acid decomposition in subcritical and supercritical water – a Raman spectroscopic study

The decomposition of formic acid is studied in a continuous sub- or supercritical water reactor at temperatures between 300 and 430 °C, a pressure of 25 MPa, residence times between 4 and 65 s, and a feedstock concentration of 3.6 wt%. In situ Raman spectroscopy is used to produce real-time data and accurately quantify decomposition product yields of H2, CO2, and CO. Collected spectra are used to determine global decomposition rates and kinetic rates for individual reaction pathways. First-order global Arrhenius parameters are determined as log A (s−1) = 1.6 ± 0.20 and EA = 9.5 ± 0.55 kcal/mol for subcritical decomposition, and log A (s−1) = 12.56 ± 1.96 and EA = 41.90 ± 6.08 kcal/mol for supercritical decomposition. Subcritical and supercritical Arrhenius parameters for individual pathways are proposed. The variance in rate parameters is likely due to changing thermophysical properties of water across the critical point. There is strong evidence for a surface catalyzed free-radical mechanism responsible for rapid decomposition above the critical point, facilitated by low density at supercritical conditions.

Thermodynamic analysis of diesel hydrotreating reactions

Thermodynamic analysis of diesel hydrotreating reactions—Hydrodesulfurization (HDS), Hydrodenitrogenation (HDN) and Hydrodearomatization (HDA)—were studied by Gibbs free energy minimization to understand the effect of process variables such as temperature (200–600 °C), pressure (10–80 bar(a)) and H2/oil ratio (stoichiometric ratio – 5 times of stoichiometric ratio) on the product distribution. Dibenzothiophene (DBT), naphthalene and pyridine were considered as representative compounds to study the HDS, HDA and HDN reactions respectively and the reactions were studied in both gas phase and two phase (gas–liquid). It was found that the HDN reaction has no thermodynamic limitation while HDA reaction has significant thermodynamic limitation at low pressures and high temperatures. Higher H2 concentration favors the HDA equilibria but not very significantly. Overall HDS reaction has no thermodynamic limitation but for refractory sulfur compounds like DBT and alkyl substituted DBTs, the preference for individual reaction pathways—Direct Desulphurization (DDS) and Hydrogenation (HYD)—changes with change in process variables. It was shown that while the selection of operating temperature and H2 concentration for a commercial diesel hydrotreating reactor are governed by the reaction kinetics, the reactor pressure is mostly governed by the HDA reaction equilibria.

Derivatization Technique To Identify Specifically Carbonyl Groups by Infrared Spectroscopy: Characterization of Photooxidative Aging Products in Terpenes and Terpeneous Resins

Analysis of bioorganic materials by infrared spectroscopy (FT-IR) is frequently limited due to overlapping of diagnostic bands from the various components, which poses a fundamental problem to this analytical technique. The distinction of oxidized di- and triterpenes, for example, is hindered by the superposition of similar absorption bands of carbonyl functional groups summing up to a broad, nondistinctive signal. This study presents a technique for selective fluorination of various carboxylic acids by exposure to gaseous sulfur tetrafluoride. The derivatization treatment leads to characteristic band shifts, allowing the separation of otherwise overlapping bands. Accordingly, the IR bands of primary acids, α,β-unsaturated acids, tertiary acids, peroxy acids, esters, ketones, and α,β-unsaturated ketones are split into distinct absorption bands. The capability of this method is demonstrated on the example of natural resins and their ingredients, which are commonly known to be susceptible to oxidation at ambient conditions. The derivatization method enables one to identify various carbonyl containing functional groups by infrared spectroscopy, even in complex mixtures of terpenes. It unveils previously hidden degradation reactions running in terpenes and natural resins exposed to artificial aging by irradiation with light. New insight is presented on the individual reaction pathways of the terpenes hydroxydammarenone and abietic acid as well as of natural resin varnishes made from dammar and colophony.

Steady-State and Transient Kinetic Studies of the Acetoxylation of Toluene over Pd–Sb/TiO2

A combination of steady-state catalytic tests, transient studies with isotopic tracers, and kinetic modeling was used to derive detailed insights into the individual reaction pathways in the course of toluene acetoxylation over a Pd–Sb/TiO2 catalyst. This reaction can be considered as an environmentally friendly route for the production of benzyl alcohol. Benzyl acetate and benzaldehyde are the only products formed from toluene, while acetic acid gives CO2 in addition to benzyl acetate. The Arrhenius plots revealed apparent activation energies for formation of benzyl acetate and benzaldehyde of 24.9 and 27.5 kJ mol–1, respectively, thus, indicating that these products originate from the same surface intermediate, i.e. benzyl cation. The corresponding value for CO2 formation was 152.9 kJ mol–1. Transient isotopic studies and their kinetic evaluation demonstrated the participation of lattice oxygen and adsorbed oxygen species in activation of acetic acid, with the latter species favoring oxidation of the ac...

SEEDING THE PREGENETIC EARTH: METEORITIC ABUNDANCES OF NUCLEOBASES AND POTENTIAL REACTION PATHWAYS

Carbonaceous chondrites are a class of meteorite known for having a high content of water and organics. In this study, abundances of the nucleobases, i.e., the building blocks of RNA and DNA, found in carbonaceous chondrites are collated from a variety of published data and compared across various meteorite classes. An extensive review of abiotic chemical reactions producing nucleobases is then performed. These reactions are then reduced to a list of 15 individual reaction pathways that could potentially occur within meteorite parent bodies. The nucleobases guanine, adenine and uracil are found in carbonaceous chondrites in the amounts of 1$-$500 ppb. It is currently unknown which reaction is responsible for their synthesis within the meteorite parent bodies. One class of carbonaceous meteorites dominate the abundances of both amino acids and nucleobases$-$the so-called CM2 (e.g. Murchison meteorite). CR2 meteorites (e.g. Graves Nunataks) also dominate the abundances of amino acids, but are the least abundant in nucleobases. The abundances of total nucleobases in these two classes are $330 \pm250$ ppb and $16 \pm13$ ppb respectively. Guanine most often has the greatest abundances in carbonaceous chondrites with respect to the other nucleobases, but is 1$-$2 orders of magnitude less abundant in CM2 meteorites than glycine (the most abundant amino acid). Our survey of the reaction mechanisms for nucleobase formation suggests that Fischer-Tropsch synthesis (i.e. CO, H$_2$ and NH$_3$ gases reacting in the presence of a catalyst such as alumina or silica) is the most likely for conditions that characterize early states of planetesimals.

Acid Strength Controlled Reaction Pathways for the Catalytic Cracking of 1-Pentene to Propene over ZSM-5

The influence of acid strength was evaluated toward the selectivity to propene on conversion of 1-pentene. For the catalytic cracking of 1-pentene, the main reaction pathways and the molar ratio of propene to ethene (P/E ratio) were controlled by acid strength with the appropriate amount of total acid sites. The results showed that the P/E ratio increased with decreasing amounts of strong acid sites, since the activation energies of individual reaction pathways were influenced by acid strength to a different extent. Strong acid sites could promote pathway I′ (C52– → C22– + C32–) and pathway II′-1 (C62– → C22– + C42–), while weak acid sites preferred pathway II′ (2C52– → C10+ → C42– + C62–) and pathway II′-2 (C62– → 2C32–), since pathways II′ and II′-2 underwent some energetically favorable routes (tertiary–secondary, secondary–secondary) of carbenium ion intermediates. By manipulation of the acid strength distribution on ZSM-5, the P/E ratio and selectivity of propene could be significantly improved, suggesting that this can provide an important guideline for improving such a process. In addition, we also designed a coupled process combing butene and pentene coconversion, as pentene and butene could be produced during C42– and C52– catalytic cracking. The coupled process could offer a promising solution to gain high selectivity of propene from C4 and C5 olefin cracking.

Selective control over fragmentation reactions in polyatomic molecules using impulsive laser alignment.

We investigate the possibility of using molecular alignment for controlling the relative probability of individual reaction pathways in polyatomic molecules initiated by electronic processes on the few-femtosecond time scale. Using acetylene as an example, it is shown that aligning the molecular axis with respect to the polarization direction of the ionizing laser pulse does not only allow us to enhance or suppress the overall fragmentation yield of a certain fragmentation channel but, more importantly, to determine the relative probability of individual reaction pathways starting from the same parent molecular ion. We show that the achieved control over dissociation or isomerization pathways along specific nuclear degrees of freedom is based on a controlled population of associated excited dissociative electronic states in the molecular ion due to relatively enhanced ionization contributions from inner valence orbitals.

Pore-scale simulation of microbial growth using a genome-scale metabolic model: Implications for Darcy-scale reactive transport

Recent advances in microbiology have enabled the quantitative simulation of microbial metabolism and growth based on genome-scale characterization of metabolic pathways and fluxes. We have incorporated a genome-scale metabolic model of the iron-reducing bacteria Geobacter sulfurreducens into a pore-scale simulation of microbial growth based on coupling of iron reduction to oxidation of a soluble electron donor (acetate). In our model, fluid flow and solute transport is governed by a combination of the Navier–Stokes and advection–diffusion–reaction equations. Microbial growth occurs only on the surface of soil grains where solid-phase mineral iron oxides are available. Mass fluxes of chemical species associated with microbial growth are described by the genome-scale microbial model, implemented using a constraint-based metabolic model, and provide the Robin-type boundary condition for the advection–diffusion equation at soil grain surfaces.Conventional models of microbially-mediated subsurface reactions use a lumped reaction model that does not consider individual microbial reaction pathways, and describe reactions rates using empirically-derived rate formulations such as the Monod-type kinetics. We have used our pore-scale model to explore the relationship between genome-scale metabolic models and Monod-type formulations, and to assess the manifestation of pore-scale variability (microenvironments) in terms of apparent Darcy-scale microbial reaction rates. The genome-scale model predicted lower biomass yield, and different stoichiometry for iron consumption, in comparison to prior Monod formulations based on energetics considerations. We were able to fit an equivalent Monod model, by modifying the reaction stoichiometry and biomass yield coefficient, that could effectively match results of the genome-scale simulation of microbial behaviors under excess nutrient conditions, but predictions of the fitted Monod model deviated from those of the genome-scale model under conditions in which one or more nutrients were limiting.The fitted Monod kinetic model was also applied at the Darcy scale; that is, to simulate average reaction processes at the scale of the entire pore-scale model domain. As we expected, even under excess nutrient conditions for which the Monod and genome-scale models predicted equal reaction rates at the pore scale, the Monod model over-predicted the rates of biomass growth and iron and acetate utilization when applied at the Darcy scale. This discrepancy is caused by an inherent assumption of perfect mixing over the Darcy-scale domain, which is clearly violated in the pore-scale models. These results help to explain the need to modify the flux constraint parameters in order to match observations in previous applications of the genome-scale model at larger scales. These results also motivate further investigation of quantitative multi-scale relationships between fundamental behavior at the pore scale (where genome-scale models are appropriately applied) and observed behavior at larger scales (where predictions of reactive transport phenomena are needed).

Integration of Catalysis and Analysis is the Key: Rapid and Precise Investigation of the Catalytic Asymmetric Gosteli–Claisen Rearrangement

The kinetics of the Cu(II)(bisoxazoline)-catalyzed diastereo- and enantioselective Gosteli-Claisen rearrangement of 2-alkoxycarbonyl-substituted allyl vinyl ethers has been investigated by enantioselective on-column reaction gas chromatography (ocRGC). Enantioselective ocRGC integrates (stereoselective) catalysis and enantioselective chromatography in a single microcapillary, which is installed in a GC-MS for direct analysis of conversion and selectivity. Thus, this technique allows direct differentiation of thermal and stereoselectively catalyzed reaction pathways and determination of activation parameters and selectivities of the individual reaction pathways starting from stereoisomeric reactants with high precision. Two modes of operation of enantioselective ocRGC are presented to investigate noncatalyzed, i.e., conversion of isopropyl-2-(allyloxy)but-2Z-enoate 1 to isopropyl-3R,S-methyl-2-oxy-hex-5-enoate (±)-2 and the [Cu{(R,R)-Ph-box}](SbF(6))(2)-catalyzed Gosteli-Claisen rearrangement, i.e., conversion of isopropyl-2-(but-2'E-en-1-yloxy)but-2Z-enoate (E,Z)-3 to isopropyl-3S,4S-dimethyl-2-oxy-hex-5-enoate 4b. Eyring activation parameters have been determined by temperature-dependent measurements: Uncatalyzed rearrangement of 1 to (±)-2 gives ΔG(‡) (298 K) = 114.1 ± 0.2 kJ·mol(-1), ΔH(‡) = 101.1 ± 1.9 kJ·mol(-1), and ΔS(‡) = -44 ± 5 J·(K·mol)(-1), and catalyzed rearrangement of (E,Z)-3 to 4b gives ΔG(‡)(298 K) = 101.1 ± 0.3 kJ·mol(-1), ΔH(‡) = 106.1 ± 6.6 kJ·mol(-1), and ΔS(‡) = 17 ± 19 J·(K·mol)(-1).

Methanol synthesis on ZnO(000$\mathbf {\overline{1}}$1¯). III. Free energy landscapes, reaction pathways, and mechanistic insights

The interplay of physical and chemical processes in the heterogeneous catalytic synthesis of methanol on the ZnO(0001) surface with oxygen vacancies is expected to give rise to a complex free energy landscape. A manifold of intermediate species and reaction pathways has been proposed over the years for the reduction of CO on this catalyst at high temperature and pressure conditions as required in the industrial process. In the present study, the underlying complex reaction network from CO to methanol is generated in the first place by using ab initio metadynamics for computational heterogeneous catalysis. After having "synthesized" the previously discussed intermediates in addition to finding novel species, mechanistic insights into this network of surface chemical reactions are obtained based on exploring the global free energy landscape, which is refined by investigating individual reaction pathways. Furthermore, the impact of homolytic adsorption and desorption of hydrogen at the required reducing gas phase conditions is probed by studying such processes using different charge states of the F-center.

Contrasting the Individual Reactive Pathways in Protein Unfolding and Disulfide Bond Reduction Observed within a Single Protein

Identifying the dynamics of individual molecules along their reactive pathways remains a major goal of modern chemistry. For simple chemical reactions, the transition state position is thought to be highly localized. Conversely, in the case of more complex reactions involving proteins, the potential energy surfaces become rougher, resulting in heterogeneous reaction pathways with multiple transition state structures. Force-clamp spectroscopy experimentally probes the individual reaction pathways sampled by a single protein under the effect of a constant stretching force. Herein, we examine the distribution of conformations that populate the transition state of two different reactions; the unfolding of a single protein and the reduction of a single disulfide bond, both occurring within the same single protein. By applying the recently developed static disorder theory, we quantify the variance of the barrier heights, σ(2), governing each distinct reaction. We demonstrate that the unfolding of the I27 protein follows a nonexponential kinetics, consistent with a high value of σ(2) ∼ 18 (pN nm)(2). Interestingly, shortening of the protein upon introduction of a rigid disulfide bond significantly modulates the disorder degree, spanning from σ(2) ∼ 8 to ∼21 (pN nm)(2). These results are in sharp contrast with the exponential distribution of times measured for an S(N)2 chemical reaction, implying the absence of static disorder σ(2) ∼ 0 (pN nm)(2). Our results demonstrate the high sensitivity of the force-clamp technique to capture the signatures of disorder in the individual pathways that define two distinct force-induced reactions, occurring within the core of a single protein.

Flame structure studies of premixed ammonia/hydrogen/oxygen/argon flames: Experimental and numerical investigation

Structures of diluted ammonia–hydrogen–oxygen–argon flame have been investigated at low pressure by molecular beam mass spectrometry to study the effect of the initial hydrogen content, the equivalence ratio and the working pressure on ammonia combustion. Excluding a slight effect on the NO formation, the initial hydrogen content does not have a significant impact on the composition of the burned gases. Around stoichiometric conditions, a decrease of the equivalence ratio strongly increases the NO formation. Furthermore, at high temperature, the NO formation is favoured as it has been noticed in flames where, due to different heat transfer, the maximum temperature increases with a decrease of pressure.The best agreement of the numerical simulation with the experimental results has been obtained by using Konnov’s mechanism excepted for NH2 and N2O. The impact of the main individual reaction pathways is very similar in all investigated flames.