Distribution Of Reaction Products Research Articles

Influence of Support Acidity and Ir Content on the Selective Ring Opening of Decalin over Ir/SiO2–Al2O3

The influence of the addition of HCl and Ir (1 wt %) to different SiO2–Al2O3 supports of varying silica content was studied in the reaction of selective ring opening of decalin. The addition of HCl to silica–alumina supports containing 70 and 80 wt % SiO2 was found to have little influence in the distribution of reaction products compared to the calcined supports. The incorporation of Ir to the silica–alumina catalysts has a beneficial effect, increasing the decalin conversion, being this effect more noticeable in the low acidity supports, i.e., those containing 30–40 wt % SiO2. The iridium-containing materials display the highest yield of cracking products, ring opening products, and ring contraction products. Increasing the reaction temperature promotes cracking and dehydrogenation but markedly decreases the selectivity to ring contraction products. At 350 °C a slight decrease in selectivity to ring opening products occurs, though the overall increase in conversion results in an increased yield of these...

Photocatalytic conversion of acetate into molecular hydrogen and hydrocarbons over Pt/TiO2: pH dependent formation of Kolbe and Hofer-Moest products

The photocatalytic generation of H2 and hydrocarbons from aqueous acetic acid has been studied employing TiO2 P25-based photocatalyst particles. The effect of pH on the distribution of reaction products and their formation rates during the photochemical as well as the photoelectrochemical oxidation of acetic acid has been investigated. The photocatalytic formation rates for H2 and hydrocarbons were found to be higher for Pt-loaded TiO2 than for bare TiO2. The major reaction products resulting from the photocatalytic decomposition of aqueous acetic acid, as determined quantitatively in the gas phase, were found to be H2, CO2, and CH4. Furthermore, traces of C2H6, C3H8, CO, CH3CHO, HCHO, CH3OH, C2H5OH, and HCOOH were also detected. After 15h of illumination, the average formation rates of H2, CO2, CH4, and C2H6 evolved at pH 2 were found to be 22, 65, 35, and 2µmol/h, respectively. The ratio of H2 to hydrocarbons strongly depends on the pH values, i.e., at acidic pH the ratio of H2 to CH4 formation was found to be 0.6. On the contrary, at neutral and basic pH values negligible amounts of hydrocarbons were formed and H2 was found to be the main product with formation rates of 12 and 5µmol/h at pH 7 and 11, respectively. It is therefore assumed that the hydroxide ion has a significant effect on the reaction pathways. Due to the fact that methanol and ethanol are formed as reaction products, water or hydroxide ions are apparently required for the formation of the major oxidizing agent, that is the hydroxyl radical. Herein, a detailed mechanism for the photocatalytic decomposition of acetic acid at different pH values is presented.

The Role of Molecular Modeling in TiO₂ Photocatalysis.

Molecular Modeling methods play a very important role in TiO2 photocatalysis. Recent advances in TiO2 photocatalysis have produced a number of interesting surface phenomena, reaction products, and various novel visible light active photocatalysts with improved properties. Quantum mechanical calculations appear promising as a means of describing the mechanisms and the product distributions of the photocatalytic degradation reactions of organic pollutants in both gas and aqueous phases. Since quantum mechanical methods utilize the principles of particle physics, their use may be extended to the design of new photocatalysts. This review introduces molecular modeling methods briefly and emphasizes the use of these methods in TiO2 photocatalysis. The methods used for obtaining information about the degradabilities of the pollutant molecules, predicting reaction mechanisms, and evaluating the roles of the dopants and surface modifiers are explained.

Effects of Substituents on the SN2 Free Energy of Activation for α-O-4 Lignin Model Compounds

Current methods for processing lignocellulosic biomass fail to take full advantage of the phenolic chemicals available in lignin, the third most common polymer on the planet. The complexity of the lignin polymer structure, the lack of knowledge of the mechanisms by which it breaks down, and the difficulties in predicting and controlling the reaction product distribution make improvement in processing methods challenging. The use of model compounds, which contain only a few key features of the native polymer, allow the reactivity of the overall polymer to be investigated more simply by isolating key elements. In our previous work, we examined the mechanism of acidolysis for two α-O-4 lignin model compounds, benzylphenyl ether (BPE) and 1-(phenoxyethyl)benzene (PEB). In the present work, we examine the rate-limiting step of this mechanism, a nucleophilic attack on the α-carbon commensurate with protonation of the ether oxygen, for other α-O-4 model compounds that contain features more reminiscent of native lignin, including compounds based on native model dimers. The effects of individual substituents and combinations of substituents are also examined in order to determine their contributions to the reactivity of native lignin dimers. Simple relationships are examined between ground-state properties and reactivity. The results of these calculations show that the effects of substituents on the reactivity of BPE-based compounds are quite different from those of PEB-based compounds. While reasonable correlations can be found for BPE reactivity and properties across substituents, PEB shows much less predictable reactivity except among small subsets of substituents. Overall, the trends observed here provide useful information about the reactivity of α-O-4 bonds in lignin during acidolysis as a function of the local chemical environment.

Mass Transfer, Micromixing and Chemical Reactions Carried out in the Rotor-Stator Mixer

Two pairs of fast competitive complex test reactions: neutralization competing with ethyl chloroacetate hydrolysis in the first case, and neutralization competing with 2,2-dimetoxypropane (DMP) hydrolysis in the second case, have been applied do study drop breakup, mass transfer and micromixing in two types of the rotor-stator mixers: Silverson 088/150 MS and T 50 Ultra-Turrax® - IKA. In experiments effects of process conditions on the product distribution of chemical test reactions and the drop size distribution were determined. The product distribution of complex test reactions was evaluated based on results of high- performance liquid and gas chromatography measurements. The drop size distribution was measured with the Malvern MasterSizer just after the process. The multifractal model of intermittent turbulence as well as mass transfer and micromixing models were applied to interpret and predict the course of the processes of drop breakage, mass transfer, mixing and complex chemical reactions. The population balance modeling was applied to integrate effects of different mixing and chemical reaction effects. Based on experimental data and model predictions the energetic efficiencies of drop breakage and mass transfer in the rotor-stator mixers were identified and discussed. The models of mass transfer in liquid-liquid systems are discussed in detail. The first models based on physics of mass transfer phenomena were proposed by Levich and Batchelor in sixties and seventies years of the 20th century, and were further developed by other researchers. Here we apply two models, the first one was proposed by Polyanin and the second one by Favelukis and Lavrenteva. Both models are based on the original concept of Levich, the second one includes effects on mass transfer of drop deformation to the shape of prolate ellipsoid. Limitations of both models are presented and possibilities of improvement of their performance are discussed.

Investigation of intermediates in non-oxidative coupling of methane by non-thermal RF plasma

Non-oxidative coupling of methane by low pressure RF plasma was investigated with real time monitoring tools such as Langmuir probe, and ion mass and energy spectrometer. Process parameters were systematically changed to generate various plasma conditions and thus to identify key factors affecting the methane activation. From real time analysis, it was found that effects of electron energy and ion energy on methane conversion were minimal when the electron energy of plasma is greater than methane activation energy. On the other hand, electron concentration and the residence time in plasma were strongly correlated with the degree of methane conversion. Moreover, it should be noted that the density of methanium (CH5+) which is a byproduct of ion-molecule reaction pathway of methyl radical formation (CH3), was reversely related to the extent of methane conversion due to active reactions among the methanium and C2 product molecules. Furthermore, for tested experimental conditions in this study, the product distribution of the methane plasma reaction was more closely related with the methane conversion level than with other plasma parameters. This real time analysis results can be used to derive meaningful insights to help designing an effective synergistic plasma-catalyst system.

Finite-Rate Oxidation Model for Carbon Surfaces from Molecular Beam Experiments

An oxidation model for carbon surfaces has been developed where the gas–surface reaction mechanisms and corresponding rate parameters are based solely on observations from recent molecular beam experiments. In the experiments, a high-energy molecular beam containing atomic and molecular oxygen (93% atoms and 7% molecules) was directed at a high-temperature carbon surface. The measurements revealed that carbon monoxide (CO) is the dominant reaction product and that its formation required a high surface coverage of oxygen atoms. As the temperature of the carbon sample was increased during the experiment, the surface coverage reduced and the production of CO was diminished. Most importantly, the measured time-of-flight distributions of surface reaction products indicated that CO and carbon dioxide () are predominately formed through thermal reaction mechanisms as opposed to direct abstraction mechanisms. These observations have enabled the formulation of a finite-rate oxidation model that includes surface-coverage dependence, similar to existing finite-rate models used in computational fluid dynamics simulations. However, each reaction mechanism and all rate parameters of the new model are determined individually based on the molecular beam data. The new model is compared to existing models using both zero-dimensional gas–surface simulations and computational fluid dynamics simulations of hypersonic flow over an ablating surface. The new model predicts similar overall mass loss rates compared to existing models; however, the individual species production rates are completely different. The most notable difference is that the new model (based on molecular beam data) predicts CO as the oxidation reaction product with virtually no production, whereas existing models predict the exact opposite trend.

The influence of salt additives on the macrocyclic product distributions in double-amidation reactions

The influence of salt additives on the macrocyclic product distributions in double-amidation reactions

Theoretical study of fragmentation pathways and product distribution of deprotonated aspartic acid

In this study, the fragmentation processes of deprotonated aspartic acid to eliminate CO2, NH3, and H2O were investigated by a quantum-mechanism computation at the B3LYP/6-311++G(2df,2pd) and MP2/6-31+G(d,p) levels of theory. By constructing the fragmentation reaction potential energy profile using the Gibbs free energies and enthalpies of the located stationary points, we explored both the preferred dissociation pathways and the product distribution. Furthermore, the thermal energy correction to key stationary points was computed to evaluate the temperature dependences of the dominant reaction channels and product distribution. Additionally, the similarities and differences between the present theoretical investigation and available experiment were discussed in detail.

Mass transfer and chemical test reactions in the continuous flow rotor-stator mixer

This work is on mass transfer in liquid-liquid two-phase systems. To study mass transfer in the continuous flow in-line rotor-stator mixer, a method based on experimental determination of the product distribution of a set of complex chemical reactions has been applied together with drop breakage experiments. Experiments were carried out using the high-shear rotor-stator Silverson mixer for both drop dispersion and intensification of mass transfer. Volume fraction of organic phase was in the range between 0.01 and 0.015. The product distribution of complex reactions was determined based on high-performance liquid chromatography measurements. The drop size distribution was measured with the Malvern MasterSizer just after the process. Results of modeling of mass transfer with chemical reaction were used to characterise energetic efficiency of mass transfer.

Reaction of nitrous oxide with methane to synthesis gas: A thermodynamic and catalytic study

The aim of the present study is to explore the coherence of thermodynamic equilibrium predictions with the actual catalytic reaction of CH4 with N2O, particularly at higher CH4 conversions. For this purpose, key process variables, such as temperature (300°C–550°C) and a molar feed ratio (N2O/CH4= 1, 3, and 5), were altered to establish the conditions for maximized H2 yield. The experimental study was conducted over the Co-ZSM-5 catalyst in a fixed bed tubular reactor and then compared with the thermodynamic equilibrium compositions, where the equilibrium composition was calculated via total Gibbs free energy minimization method.The results suggest that molar feed ratio plays an important role in the overall reaction products distribution. Generally for N2O conversions, and irrespective of N2O/CH4 feed ratio, the thermodynamic predictions coincide with experimental data obtained at approximately 475°C–550°C, indicating that the reactions are kinetically limited at lower range of temperatures. For example, theoretical calculations show that the H2 yield is zero in presence of excess N2O (N2O/CH4= 5). However over a Co-ZSM-5 catalyst, and with a same molar feed ratio (N2O/CH4) of 5, the H2 yield is initially 10% at 425°C, while above 450°C it drops to zero. Furthermore, H2 yield steadily increases with temperature and with the level of CH4 conversion for reactions limited by N2O concentration in a reactant feed. The maximum attainable (from thermodynamic calculations and at a feed ratio of N2O/CH4= 3) H2 yield at 550°C is 38%, whereas at same temperature and over Co-ZSM-5, the experimentally observed yield is about 19%.Carbon deposition on Co-ZSM-5 at lower temperatures and CH4 conversion (less than 50%) was also observed. At higher temperatures and levels of CH4 conversion (above 90%), the deposited carbon is suggested to react with N2O to form CO2.

A Deep Insight into the Details of the Interisomerization and Decomposition Mechanism of o-Quinolyl and o-Isoquinolyl Radicals. Quantum Chemical Calculations and Computer Modeling.

The isomerization of o-quinolyl ↔ o-isoquinolyl radicals and their thermal decomposition were studied by quantum chemical methods, where potential energy surfaces of the reaction channels and their kinetics rate parameters were determined. A detailed kinetics scheme containing 40 elementary steps was constructed. Computer simulations were carried out to determine the isomerization mechanism and the distribution of reaction products in the decomposition. The calculated mole percent of the stable products was compared to the experimental values that were obtained in this laboratory in the past, using the single pulse shock tube. The agreement between the experimental and the calculated mole percents was very good. A map of the figures containing the mole percent's of eight stable products of the decomposition plotted vs T are presented. The fast isomerization of o-quinolyl → o-isoquinolyl radicals via the intermediate indene imine radical and the attainment of fast equilibrium between these two radicals is the reason for the identical product distribution regardless whether the reactant radical is o-quinolyl or o-isoquinolyl. Three of the main decomposition products of o-quinolyl radical, are those containing the benzene ring, namely, phenyl, benzonitrile, and phenylacetylene radicals. They undergo further decomposition mainly at high temperatures via two types of reactions: (1) Opening of the benzene ring in the radicals, followed by splitting into fragments. (2) Dissociative attachment of benzonitrile and phenyl acetylene by hydrogen atoms to form hydrogen cyanide and acetylene.

One-step conversion of xylose to furfuryl alcohol on sulfated zirconia-supported Pt catalyst—Balance between acid and metal sites

Pt/ZrO2-SO4 catalysts bearing different acid/metal site ratios were prepared and the balance between those two sorts of surface centers was investigated on liquid-phase chemoconversion of xylose. Direct conversion of xylose to furfuryl alcohol was successfully reported for the very first time on a single multifunctional catalyst and it was found to proceed via intermediate formation of the ketopentose isomer. The impact of the disposal of acid and metal sites on catalyst surface was assessed by comparing the synthesized single multifunctional catalysts with dual catalytic systems at which active sites are placed on two different solid surfaces. Reaction network and product distribution were shown to depend on such arrangement. It was suggested that furfuryl alcohol is formed through dehydration-hydrogenation cascade reactions on independent acid and metal sites on dual catalytic systems while one-step conversion occurs on vicinal acid-metal pair sites created on single multifunctional catalysts. The creation of isolated metal centers on multifunctional catalyst surface was seen to allow the formation of xylitol while the existence of sole acid sites led to the production of furfural.

In operando x-ray tomography for next-generation batteries: a systematic approach to monitor reaction product distribution and transport processes

Computed tomography with x-rays is a powerful tool to analyze the complex reaction and transport processes that occur inside electrochemical storage devices. To this day, a better insight into the occurring processes is needed and will yield improvements in energy density and cycling stability of next-generation batteries. Herein we present general considerations for the use of x-ray tomography of batteries to gain a detailed insight during operation. Furthermore, we present examples for the tomography of zinc–oxygen batteries, sodium–oxygen batteries and metal–sulfur batteries, elucidating performance limiting degradation processes such as dendrite formation and loss of liquid electrolyte. With the method applied, we aim to establish an effective link between the battery and x-ray community by offering a guideline on how to apply x-ray tomography to propel research on battery materials and entire batteries.

A New Anion Receptor for Improved Interface Between the Lithium- and Manganese-Rich Layered Oxide Cathode and the Electrolyte

Significant progress has been made in lithium-ion batteries for applications in electric vehicles and grid energy storage. The deep market penetration of the LIB technology would be further accelerated if methods to further increase energy density, stability and cycle life can be identified and developed. Although intensively studied in the last several decades, cathode materials are still considered a major limiting factor in LIB performance. In the past few years, remarkable efforts have been dedicated to studying a family of lithium- and manganese-rich layered oxides, with a general formula of Li1+x M1-x O2 (M=Mn, Ni, Co etc). [1] Depending on the composition, the oxides can deliver capacities exceeding 250 mAh/g at an average potential of about 4.6 V vs. Li+/Li. While they have the potential to drastically improve the energy density of lithium-ion batteries, there are several critical problems, including a large first-cycle irreversible capacity loss associated with the activation process, subsequent-cycle voltage decay characterized by a continuous decrease in average discharge voltage during cycling, transition-metal dissolution into the electrolyte, sluggish kinetics, and a high impedance at low state of charge (SOC), prevent their application in commercial batteries. In addition, the high charging potential of these materials along with the catalytic properties of the high-valent transition metals in the charged cathode oxidize carbonate solvents and decompose the electrolyte. [2] These reactive interactions between the electrode and the electrolyte, or side reactions, not only leave surface deposits onto the cathode, but also modify the intrinsic properties, including elemental, chemical and structural, of the cathode surface. While the stability of resulted cathode-electrolyte interface (CEI) layer, like solid electrolyte interface of anode, is largely influenced by the chemical nature (both organic and inorganic) and the distribution of the side reaction products, studies have shown that a build-up in CEI can increase the charge transfer resistance and deteriorate electrochemical performance. Previous research has demonstrated the important role of electrolyte additives in influencing the CEI and battery performance.[3] For example, boron-based anion receptors that can dissolve electrolyte decomposition byproducts, such as LiF, Li2O and Li2O2, are capable of enhancing long-term cycling of LIBs. To date, however, most studies have focused on the compositional engineering of additives. The general knowledge of how electrolyte additives influence the changes on the cathode particle surface and interfere with the electrolyte/electrode side reactions is nearly nonexistent. Such studies would have a profound impact on additive development that not only benefits the entire electrolyte community but also the cathode community. Herein, We report a new electrolyte additive, tris (2, 2, 2-trifluoroethyl) borate (TTFEB), to improve the discharge capacity, coulombic efficiency and cycle life of cells with a lithium- and manganese-rich layered cathode (in Fig1). TTFEB suppressed the growth of insulating inorganic byproducts at the cathode/electrolyte interface, inhibited the growth of surface film resistance, and alleviated the cell polarization during cycling. Furthermore, surface transition-metal (mostly Mn) reduction and structural transformation in cathode were mitigated when the electrode was cycled in a TTFEB containing electrolyte. The work highlights the importance of electrolyte additives in preserving the structure of cathode materials and helps us understand the underlying mechanism of the improved electrochemical performance enabled by electrolyte additives.

Reactive force field simulation studies on the combustion behavior of n-octanol

n-Octanol is a new promising fuel which is considered as an alternative to conventional diesel. In order to understand the combustion characteristics of n-octanol, reactive molecular dynamics (ReaxFF MD) simulation of n-octanol has been carried out at temperatures from 2000 to 4000K with equivalence ratios ranging from 0.5 to 2.0. Ethylene and formaldehyde have been found to be major intermediates in the oxidation of n-octanol which is in agreement with the available experimental data. Reaction mechanisms have been proposed for the formation of major intermediates (C2H4 and CH2O) and final products (CO and CO2) from where it is revealed that intermediates are mainly consumed by the build-up of hydroxyl and hydroperoxyl radicals. It was observed that the CO and CO2 formation from formaldehyde and ethylene starts with the attack of molecular oxygen or hydroxyl radical or hemolytic OH bond cleavage. Overall, a good qualitative agreement with the available experimental data in terms of product distribution has been found. The results indicate that ReaxFF MD simulations can give a detailed description of the reaction mechanism and product distribution for the combustion for oxygenated fuel such as octanol.

A study of the low-temperature oxidation of a long chain aldehyde: n-hexanal

Large aldehydes are important species among the oxygenated products formed during the oxidation of fuels (e.g. alkanes or alkenes) and biofuels (e.g. long chain alcohols), but their oxidation chemistry has been scarcely studied up to now. In this work, a study of the oxidation of hexanal has been performed in a jet-stirred reactor over the temperature range 475–1100K, at a residence time of 2s, a pressure of 106.7kPa, an inlet fuel mole fraction of 0.005 and at three equivalence ratios (0.25, 1 and 2). Reaction products were quantified using two analytical methods: gas chromatography and cw-cavity ring-down spectroscopy. In addition to species usually measured during oxidation study (such as CO, CO2, water, H2O2 and C1-C3 olefins, aldehydes and ketones), specific low temperature oxidation species were also observed: C6 species, such as δ- and γ-caprolactones and hexanoic acid, but also C5 species, such as 2-methyl-tetrahydrofuran and pentanal which are typical reaction products detected during the oxidation of n-pentane. A detailed kinetic model has been developed using an automatic generation software. This model was used to highlight the specificities of the oxidation chemistry of long chain aldehydes and to understand the formation routes of the C5 and C6 low-temperature oxidation products. Simulations indicate that the low energy of the aldehyde CH bond has a significant effect on the fuel reactivity and on the distribution of reaction products.

Comparison of Chain-Length Preferences and Glucan Specificities of Isoamylase-Type α-Glucan Debranching Enzymes from Rice, Cyanobacteria, and Bacteria.

It has been believed that isoamylase (ISA)-type α-glucan debranching enzymes (DBEs) play crucial roles not only in α-glucan degradation but also in the biosynthesis by affecting the structure of glucans, although molecular basis on distinct roles of the individual DBEs has not fully understood. In an attempt to relate the roles of DBEs to their chain-length specificities, we analyzed the chain-length distribution of DBE enzymatic reaction products by using purified DBEs from various sources including rice, cyanobacteria, and bacteria. When DBEs were incubated with phytoglycogen, their chain-length specificities were divided into three groups. First, rice endosperm ISA3 (OsISA3) and Eschericia coli GlgX (EcoGlgX) almost exclusively debranched chains having degree of polymerization (DP) of 3 and 4. Second, OsISA1, Pseudomonas amyloderamosa ISA (PsaISA), and rice pullulanase (OsPUL) could debranch a wide range of chains of DP≧3. Third, both cyanobacteria ISAs, Cyanothece ATCC 51142 ISA (CytISA) and Synechococcus elongatus PCC7942 ISA (ScoISA), showed the intermediate chain-length preference, because they removed chains of mainly DP3-4 and DP3-6, respectively, while they could also react to chains of DP5-10 and 7–13 to some extent, respectively. In contrast, all these ISAs were reactive to various chains when incubated with amylopectin. In addition to a great variation in chain-length preferences among various ISAs, their activities greatly differed depending on a variety of glucans. Most strikingly, cyannobacteria ISAs could attack branch points of pullulan to a lesser extent although no such activity was found in OsISA1, OsISA3, EcoGlgX, and PsaISA. Thus, the present study shows the high possibility that varied chain-length specificities of ISA-type DBEs among sources and isozymes are responsible for their distinct functions in glucan metabolism.

CO2 Electrochemical Reduction on Copper-Based Nanocatalysts: Investigation of Selectivity for Ethylene and Methane Via on-Line Mass Spectrometry

Introduction The increase of the world demand for energy and the depletion of fossil fuels in conjunction with the global warming crisis, has forced many research aiming at obtaining a new source of energy free of environmental damage, sustainable and with low cost. Researchers have studied the electrochemical reduction of the CO2 molecule into carbon-containing useful fuels, for different application, including fuel cells [1,2]. Copper-based electrocatalysts has shown to catalyze the CO2 electroreduction into the hydrocarbons, such as methane (CH4) and ethylene (C2H4), in aqueous media. Copper show quiteunique properties because it is the only metal capable of producing hydrocarbons from CO2, with appropriate Faradaic efficiencies (FE), in aqueous solutions, and at mild conditions of temperature and pressure [3]. However, on-line investigations of the reaction products during the CO2 electrocatalytic reduction using nanoparticles are very scarce in the literature. In the present study, the CO2 electroreduction, catalyzed by carbon-supported Cu nanoparticles (Cu/C) was investigated by using rotating disk electrodes (RDE) on-line coupled to a Differential Electrochemical Mass Spectrometry (DEMS) equipment. Experimental The Cu/C electrocatalysts, with 33 wt.% and 50 wt.% of metal copper on Vulcan carbon (XC-72R), were prepared according to a previously published method, using sodium citrate/citric acid as stabilizer and buffering media, and borohydride as the reducer [4]. The structural features were investigated by X-ray diffraction (XRD) measurements. The electrochemical experiments were conducted using an Autolab potentiostat (PGSTAT 30). The working electrode was prepared depositing 300 μL of a suspension composed by 10 mg of catalyst, 2.5 mL of isopropanol and 250 μL of Nafion (5% of water), on an disk-shaped Toray carbon paper disk (TGPH-030 E-TEK, 30wt.%, 1.3 cm diameter). This was placed onto the top of a regular RDE, with an Au disk, and the electric contact was made by using an Au wire. A ring-shaped Au foil was used as the counter Ag/AgCl/Cl-(sat.) as the reference electrode. The electrolyte was N2 or CO2-saturated 0.1 mol L-1 KHCO3 solution, depending on the experiment (CO2 bubbling was maintained during the measurements). For the product distribution analysis, the RDE was positioned close to probe (ca.1 mm), which was composed by a Teflon membrane (Gore-Tex -pore size 0.02 µm) placed onto the top a glass frit, inserted onto a glass tube [5]. This was connected to the DEMS equipment using a below tube. Results and Discussion Fig. 1 presents the ionic currents of the mass signals m/z = 2 (H2), 15 (methane), 26 (ethylene) and 44 (CO2) obtained during DEMS measurements of potential step from OCP to -2.5 V and linear sweep experiments for the CO2 electroreduction catalyzed by 50 wt.% Cu/C (a) and by a Cu RDE rod (b) (included for comparison). One can be observed the signals for CH4, C2H4 and H2 for the Cu rod (bulk Cu) during both experiment protocols. However, and interestingly, for the 50 wt.% Cu/C electrocatalyt, only the signals for C2H4 and H2 are evident (similar result was obtained for 33 wt.t% Cu/C, but with lower ionic current intensities due its lower total surface area – not shown for brevity). These results revealed that C2H4 is selectively formed in relation to CH4. This may indicate that the Cu/C nanoparticle active sites stabilize a reaction intermediate for the C2H4 formation. Accordingly, from bulk to nanosized Cu particles, the (100) facets may increase an this may stabilized the C-C coupling or the 2-carbon atoms intermediate that will receive additional proton and electron transfer, producing ethylene [6]. Additional experiments will be addressed to investigate the effect of the Cu/C nanoparticle size, and of the additional of ad-atoms, on the reaction product distribution, and this will be included in the present study. Acknowledgements The authors thank FAPESP (2014/26699-3 and 2013/16930-7) for financial support.

Control of the Position of Oxygen Delivery in Soybean Lipoxygenase-1 by Amino Acid Side Chains within a Gas Migration Channel

Understanding gas migration pathways is critical to unraveling structure-function relationships in enzymes that process gaseous substrates such as O2, H2, and N2 This work investigates the role of a defined pathway for O2 in regulating the peroxidation of linoleic acid by soybean lipoxygenase 1. Computational and mutagenesis studies provide strong support for a dominant delivery channel that shuttles molecular oxygen to a specific region of the active site, thereby ensuring the regio- and stereospecificity of product. Analysis of reaction kinetics and product distribution in channel mutants also reveals a plasticity to the gas migration pathway. The findings show that a single site mutation (I553W) limits oxygen accessibility to the active site, greatly increasing the fraction of substrate that reacts with oxygen free in solution. They also show how a neighboring site mutation (L496W) can result in a redirection of oxygen toward an alternate position of the substrate, changing the regio- and stereospecificity of peroxidation. The present data indicate that modest changes in a protein scaffold may modulate the access of small gaseous molecules to enzyme-bound substrates.