Showing papers on "Reaction rate published in 2018"

A physical catalyst for the electrolysis of nitrogen to ammonia.

Abstract: Ammonia synthesis consumes 3 to 5% of the world's natural gas, making it a significant contributor to greenhouse gas emissions. Strategies for synthesizing ammonia that are not dependent on the energy-intensive and methane-based Haber-Bosch process are critically important for reducing global energy consumption and minimizing climate change. Motivated by a need to investigate novel nitrogen fixation mechanisms, we herein describe a highly textured physical catalyst, composed of N-doped carbon nanospikes, that electrochemically reduces dissolved N2 gas to ammonia in an aqueous electrolyte under ambient conditions. The Faradaic efficiency (FE) achieves 11.56 ± 0.85% at -1.19 V versus the reversible hydrogen electrode, and the maximum production rate is 97.18 ± 7.13 μg hour-1 cm-2. The catalyst contains no noble or rare metals but rather has a surface composed of sharp spikes, which concentrates the electric field at the tips, thereby promoting the electroreduction of dissolved N2 molecules near the electrode. The choice of electrolyte is also critically important because the reaction rate is dependent on the counterion type, suggesting a role in enhancing the electric field at the sharp spikes and increasing N2 concentration within the Stern layer. The energy efficiency of the reaction is estimated to be 5.25% at the current FE of 11.56%.

Degradation Chemistry and Stabilization of Exfoliated Few-Layer Black Phosphorus in Water

Abstract: Exfoliated black phosphorus (BP), as a monolayer or few-layer material, has attracted tremendous attention owing to its unique physical properties for applications ranging from optoelectronics to photocatalytic hydrogen production. Approaching intrinsic properties has been, however, challenged by chemical reactions and structure degradation of BP under ambient conditions. Surface passivation by capping agents has been proposed to extend the processing time window, yet contamination or structure damage rise challenges for BP applications. Here, we report experiments combined with first-principle calculations that address the degradation chemistry of BP. Our results show that BP reacts with oxygen in water even without light illumination. The reaction follows a pseudo-first-order parallel reaction kinetics, produces PO23–, PO33–, and PO43– with reaction rate constants of 0.019, 0.034, and 0.023 per day, respectively, and occurs preferentially from the P atoms locating at BP edges, which yields structural de...

Plasmon-Enhanced Catalysis: Distinguishing Thermal and Nonthermal Effects

Abstract: In plasmon-enhanced heterogeneous catalysis, illumination accelerates reaction rates by generating hot carriers and hot surfaces in the constituent nanostructured metals In order to understand how photogenerated carriers enhance the nonthermal reaction rate, the effects of photothermal heating and thermal gradients in the catalyst bed must be confidently and quantitatively characterized This is a challenging task considering the conflating effects of light absorption, heat transport, and reaction energetics Here, we introduce a methodology to distinguish the thermal and nonthermal contributions from plasmon-enhanced catalysts, demonstrated by illuminated rhodium nanoparticles on oxide supports to catalyze the CO2 methanation reaction By simultaneously measuring the total reaction rate and the temperature gradient of the catalyst bed, the effective thermal reaction rate may be extracted The residual nonthermal rate of the plasmon-enhanced reaction is found to grow with a superlinear dependence on illu

Efficient Destruction of Pollutants in Water by a Dual-Reaction-Center Fenton-like Process over Carbon Nitride Compounds-Complexed Cu(II)-CuAlO2

Abstract: Carbon nitride compounds (CN) complexed with the in-situ-produced Cu(II) on the surface of CuAlO2 substrate (CN-Cu(II)-CuAlO2) is prepared via a surface growth process for the first time and exhibits exceptionally high activity and efficiency for the degradation of the refractory pollutants in water through a Fenton-like process in a wide pH range. The reaction rate for bisphenol A removal is ∼25 times higher than that of the CuAlO2. According to the characterization, Cu(II) generation on the surface of CuAlO2 during the surface growth process results in the marked decrease of the surface oxygen vacancies and the formation of the C–O–Cu bridges between CN and Cu(II)-CuAlO2 in the catalyst. The electron paramagnetic resonance (EPR) analysis and density functional theory (DFT) calculations demonstrate that the dual reaction centers are produced around the Cu and C sites due to the cation−π interactions through the C–O–Cu bridges in CN-Cu(II)-CuAlO2. During the Fenton-like reactions, the electron-rich center...

An Aluminum-Sulfur Battery with a Fast Kinetic Response.

Abstract: The electrochemical performance of the aluminum-sulfur (Al-S) battery has very poor reversibility and a low charge/discharge current density owing to slow kinetic processes determined by an inevitable dissociation reaction from Al2 Cl7- to free Al3+ . Al2 Cl6 Br- was used instead of Al2 Cl7- as the dissociation reaction reagent. A 15-fold faster reaction rate of Al2 Cl6 Br- dissociation than that of Al2 Cl7- was confirmed by density function theory calculations and the Arrhenius equation. This accelerated dissociation reaction was experimentally verified by the increase of exchange current density during Al electro-deposition. Using Al2 Cl6 Br- instead of Al2 Cl7- , a kinetically accelerated Al-S battery has a sulfur utilization of more than 80 %, with at least four times the sulfur content and five times the current density than that of previous work.

Selective CO2 reduction to C3 and C4 oxyhydrocarbons on nickel phosphides at overpotentials as low as 10 mV

Abstract: We introduce five nickel phosphide compounds as electro-catalysts for the reduction of carbon dioxide in aqueous solution, that achieve unprecedented selectivity to C3 and C4 products (the first such report). Three products: formic acid (C1), methylglyoxal (C3), and 2,3-furandiol (C4), are observed at potentials as low as +50 mV vs. RHE, and at the highest half-reaction energy efficiencies reported to date for any >C1 product (99%). The maximum selectivity for 2,3-furandiol is 71% (faradaic efficiency) at 0.00 V vs. RHE on Ni2P, which is equivalent to an overpotential of 10 mV, with the balance forming methylglyoxal, the proposed reaction intermediate. P content in the series correlates closely with both the total C products and product selectivity, establishing definitive structure–function relationships. We propose a reaction mechanism for the formation of multi-carbon products, involving hydride transfer as the potential-determining step to oxygen-bound intermediates. This unlocks a new and more energy-efficient reduction route that has only been previously observed in nickel-based enzymes. This performance contrasts with simple metallic catalysts that have poor selectivity between multi-carbon products, and which require high overpotentials (>700 mV) to achieve comparable reaction rates.

Atmospheric autoxidation is increasingly important in urban and suburban North America

Abstract: Gas-phase autoxidation—regenerative peroxy radical formation following intramolecular hydrogen shifts—is known to be important in the combustion of organic materials. The relevance of this chemistry in the oxidation of organics in the atmosphere has received less attention due, in part, to the lack of kinetic data at relevant temperatures. Here, we combine computational and experimental approaches to investigate the rate of autoxidation for organic peroxy radicals (RO_2) produced in the oxidation of a prototypical atmospheric pollutant, n-hexane. We find that the reaction rate depends critically on the molecular configuration of the RO_2 radical undergoing hydrogen transfer (H-shift). RO_2 H-shift rate coefficients via transition states involving six- and seven-membered rings (1,5 and 1,6 H-shifts, respectively) of α-OH hydrogens (HOC-H) formed in this system are of order 0.1 s^(−1) at 296 K, while the 1,4 H-shift is calculated to be orders of magnitude slower. Consistent with H-shift reactions over a substantial energetic barrier, we find that the rate coefficients of these reactions increase rapidly with temperature and exhibit a large, primary, kinetic isotope effect. The observed H-shift rate coefficients are sufficiently fast that, as a result of ongoing NO_x emission reductions, autoxidation is now competing with bimolecular chemistry even in the most polluted North American cities, particularly during summer afternoons when NO levels are low and temperatures are elevated.

Complete electron economy by pairing electrolysis with hydrogenation

Abstract: Electrosynthesis provides a method of driving organic reaction chemistry under ambient conditions with electricity. Pairing two reactions together enables the synthesis of two valuable chemicals with no waste product. Here we report the paired electrolysis of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde with the concomitant formation of 1-hexene from 1-hexyne in an electrochemical cell. These reaction chambers are separated by a dense palladium membrane that reduces protons formed at the anode to hydrogen atoms that can permeate through the palladium foil to hydrogenate 1-hexyne. The palladium membrane enables two reactions to be performed in distinct reaction conditions: hydrogenation in organic solvents and electrochemical oxidation in aqueous electrolyte. The starting materials in both chambers react quantitatively over 5 hours of electrolysis, and selectivities ≥95% can be achieved for 4-methoxybenzaldehyde and 1-hexene with control of reaction conditions. Exquisite control of the reaction kinetics and selectivities of each of the individual reactions is demonstrated. Electrolysis uses clean electricity to form chemical products but typical water electrolysis produces hydrogen which is hard to store oxygen which is a waste gas. Here, paired electrolysis is performed with an palladium membrane reactor to carry out two organic reactions simultaneously. The dense palladium membrane enables the two reactions to proceed in different solvents and the reaction rates and selectivities can be independently controlled.

The sensitizing effects of NO2 and NO on methane low temperature oxidation in a jet stirred reactor

Abstract: The oxidation of neat methane (CH 4 ) and CH 4 doped with NO 2 or NO in argon has been investigated in a jet-stirred reactor at 107 kPa, temperatures between 650 and 1200 K, with a fixed residence time of 1.5 s, and for different equivalence ratios (Φ), ranging from fuel-lean to fuel-rich conditions. Four different diagnostics have been used: gas chromatography (GC), chemiluminescence NO x analyzer, continuous wave cavity ring-down spectroscopy (cw-CRDS) and Fourier transform infrared spectroscopy (FTIR). In the case of the oxidation of neat methane, the onset temperature for CH 4 oxidation was above 1025 K, while it is shifted to 825 K with the addition of NO 2 or NO, independently of equivalence ratio, indicating that the addition of NO 2 or NO highly promotes CH 4 oxidation. The consumption rate of CH 4 exhibits a similar trend with the presence of both NO 2 and NO. The amount of produced HCN has been quantified and a search for HONO and CH 3 NO 2 species has been attempted. A detailed kinetic mechanism, derived from POLIMI kinetic framework, has been used to interpret the experimental data with a good agreement between experimental data and model predictions. Reaction rate and sensitivity analysis have been conducted to illustrate the kinetic regimes. The fact that the addition of NO or NO 2 seems to have similar effects on promoting CH 4 oxidation can be explained by the fact that both species are involved in a reaction cycle interchanging them and whose result is 2CH 3 + O 2 = 2CH 2 O + 2H. Additionally, the direct participation of NO 2 in the NO 2 + CH 2 O = HONO + HCO reaction has a notable accelerating effect on methane oxidation.

Velocity-resolved kinetics of site-specific carbon monoxide oxidation on platinum surfaces.

Abstract: Catalysts are widely used to increase reaction rates. They function by stabilizing the transition state of the reaction at their active site, where the atomic arrangement ensures favourable interactions 1 . However, mechanistic understanding is often limited when catalysts possess multiple active sites-such as sites associated with either the step edges or the close-packed terraces of inorganic nanoparticles2-4-with distinct activities that cannot be measured simultaneously. An example is the oxidation of carbon monoxide over platinum surfaces, one of the oldest and best studied heterogeneous reactions. In 1824, this reaction was recognized to be crucial for the function of the Davy safety lamp, and today it is used to optimize combustion, hydrogen production and fuel-cell operation5,6. The carbon dioxide products are formed in a bimodal kinetic energy distribution7-13; however, despite extensive study 5 , it remains unclear whether this reflects the involvement of more than one reaction mechanism occurring at multiple active sites12,13. Here we show that the reaction rates at different active sites can be measured simultaneously, using molecular beams to controllably introduce reactants and slice ion imaging14,15 to map the velocity vectors of the product molecules, which reflect the symmetry and the orientation of the active site 16 . We use this velocity-resolved kinetics approach to map the oxidation rates of carbon monoxide at step edges and terrace sites on platinum surfaces, and find that the reaction proceeds through two distinct channels11-13: it is dominated at low temperatures by the more active step sites, and at high temperatures by the more abundant terrace sites. We expect our approach to be applicable to a wide range of heterogeneous reactions and to provide improved mechanistic understanding of the contribution of different active sites, which should be useful in the design of improved catalysts.

Catalytic oxidation of 1,2-dichloroethane over three-dimensional ordered meso-macroporous Co 3 O 4 /La 0.7 Sr 0.3 Fe 0.5 Co 0.5 O 3 : destruction route and mechanism

Abstract: Three-dimensional ordered meso-macroporous La0.7Sr0.3Fe0.5Co0.5O3 (3DOM LSFCO)-supported Co3O4 catalysts were designed and prepared via a PMMA-templating strategy for the total oxidation of 1,2-dichloroethane (1,2-DCE). The physicochemical properties of all synthesized samples were characterized by XRD, FE-SEM, TEM, HAADF-STEM, low-temperature N2 sorption, XPS, H2-TPR, and in situ FT-IR. The introduction of Co3O4 increases the generation rate of oxygen vacancy, playing a crucial role in adsorption and activation of oxygen species. The special 3DOM structure of perovskite-type oxide promotes 1,2-DCE molecules to effectively and intimately contact with the surface adsorbed oxygen over supported catalysts and further accelerates the redox process. Compared with pure LSFCO, all the Co3O4 supported catalysts show superior catalytic performance with reaction rate increases from 5.53 × 10−12 to 2.29 × 10−11 mol g−1 s−1 and Ea decreases from 74.7 to 22.6 KJ mol−1. Amongst, the 10Co3O4/3DOM LSFCO catalyst exhibits the best catalytic activity, highest resistance to chlorine poisoning and lowest by-products concentration because of the largest amount of surface adsorbed oxygen. CO2, CO, HCl, and Cl2 are the main oxidation productions, while some typical reaction intermediates such as vinyl chloride, 1,1,2-trichloroethane and trichloroethylene are also observed, especially over the 3DOM LSFCO sample. Furthermore, the reaction mechanism of 1,2-DCE oxidation over obtained catalysts was proposed based on the results of gas chromatography, in situ FT-IR, and on-line MS. It is believed that the Co3O4/3DOM LSFCO are promising catalysts for the total removal of chlorinated volatile organic compounds.

Effects of Electrolyte Buffer Capacity on Surface Reactant Species and the Reaction Rate of CO2 in Electrochemical CO2 Reduction

Abstract: In the aqueous electrochemical reduction of CO2, the choice of electrolyte is responsible for the catalytic activity and selectivity, although there remains a need for more in-depth understanding of electrolyte effects and mechanisms. In this study, using both experimental and simulation approaches, we report how the buffer capacity of the electrolytes affects the kinetics and equilibrium of surface reactant species and the resulting reaction rate of CO2 with varying partial CO2 pressure. Electrolytes investigated include KCl (nonbuffered), KHCO3 (buffered by bicarbonate), and phosphate-buffered electrolytes. Assuming 100% methane production, the simulation successfully explains the experimental trends in maximum CO2 flux in KCl and KHCO3 and also highlights the difference between KHCO3 and phosphate in terms of pKa as well as the impact of the buffer capacity. To examine the electrolyte impact on selectivity, the model is run with a constant total current density. Using this model, several factors are el...

Reaction rate for carbon burning in massive stars

Abstract: Carbon burning is a critical phase for nucleosynthesis in massive stars. The conditions for igniting this burning stage, and the subsequent isotope composition of the resulting ashes, depend strongly on the reaction rate for C12+C12 fusion at very low energies. Results for the cross sections for this reaction are influenced by various backgrounds encountered in measurements at such energies. In this paper, we report on a new measurement of C12+C12 fusion cross sections where these backgrounds have been minimized. It is found that the astrophysical S factor exhibits a maximum around Ecm=3.5-4.0 MeV, which leads to a reduction of the previously predicted astrophysical reaction rate.

Fe3S4/Fe7S8-promoted degradation of phenol via heterogeneous, catalytic H2O2 scission mediated by S-modified surface Fe2+ species

Abstract: Enhancing OH productivity via heterogeneous, catalytic H2O2 activation is a long-standing conundrum in H2O purification and thus requires the renovation of conventional reaction systems. The initial step in realizing advanced H2O2 decomposition via heterogeneous catalytic manner is the exploration of the solid capable of efficiently cleaving O O bond inherent to H2O2 and minimizing the loss of catalytic species during vigorous reaction dynamics. While using phenol as a model compound for recalcitrants, this paper highlights the use of Fe3S4/Fe7S8 as a catalyst to enhance OH productivity and thus promote phenol degradation via electro-Fenton reaction over conventional Fe2O3, Fe3O4, and other sulfide analogue (FeS2). Materials’ characterizations and kinetic interpretation of reaction runs under controlled environments served to substantiate the benefits which were provided by Fe3S4/Fe7S8 during the reaction. Fe3S4/Fe7S8 incorporated greater amount of S-modified, surface-exposed Fe2+ sites to cleave H2O2 than FeS2. This improved catalytic consequence of Fe3S4/Fe7S8 (i.e., phenol conversion and initial reaction rate), as also evidenced by control runs detailing H2O2 decomposition in conjunction with tert-butyl alcohol-driven OH scavenging. Filtration control runs as well as recycle runs were also used to verify that Fe3S4/Fe7S8 could heterogeneously catalyze H2O2 scission under the mild, adequate reaction environments, which were realized by the use of low electrical powers and the catalyst immobilized on a cathode.

Unimolecular reaction of acetone oxide and its reaction with water in the atmosphere.

Abstract: Criegee intermediates (i.e., carbonyl oxides with two radical sites) are known to be important atmospheric reagents; however, our knowledge of their reaction kinetics is still limited. Although experimental methods have been developed to directly measure the reaction rate constants of stabilized Criegee intermediates, the experimental results cover limited temperature ranges and do not completely agree well with one another. Here we investigate the unimolecular reaction of acetone oxide [(CH3)2COO] and its bimolecular reaction with H2O to obtain rate constants with quantitative accuracy comparable to experimental accuracy. We do this by using CCSDT(Q)/CBS//CCSD(T)-F12a/DZ-F12 benchmark results to select and validate exchange-correlation functionals, which are then used for direct dynamics calculations by variational transition state theory with small-curvature tunneling and torsional and high-frequency anharmonicity. We find that tunneling is very significant in the unimolecular reaction of (CH3)2COO and its bimolecular reaction with H2O. We show that the atmospheric lifetimes of (CH3)2COO depend on temperature and that the unimolecular reaction of (CH3)2COO is the dominant decay mode above 240 K, while the (CH3)2COO + SO2 reaction can compete with the corresponding unimolecular reaction below 240 K when the SO2 concentration is 9 × 1010 molecules per cubic centimeter. We also find that experimental results may not be sufficiently accurate for the unimolecular reaction of (CH3)2COO above 310 K. Not only does the present investigation provide insights into the decay of (CH3)2COO in the atmosphere, but it also provides an illustration of how to use theoretical methods to predict quantitative rate constants of medium-sized Criegee intermediates.

Entrapped Single Tungstate Site in Zeolite for Cooperative Catalysis of Olefin Metathesis with Brønsted Acid Site

Abstract: Industrial olefin metathesis catalysts generally suffer from low reaction rates and require harsh reaction conditions for moderate activities. This is due to their inability to prevent metathesis active sites (MASs) from aggregation and their intrinsic poor adsorption and activation of olefin molecules. Here, isolated tungstate species as single molecular MASs are immobilized inside zeolite pores by Bronsted acid sites (BASs) on the inner surface. It is demonstrated that unoccupied BASs in atomic proximity to MASs enhance olefin adsorption and facilitate the formation of metallocycle intermediates in a stereospecific manner. Thus, effective cooperative catalysis takes place over the BAS-MAS pair inside the zeolite cavity. In consequence, for the cross-metathesis of ethene and trans-2-butene to propene, under mild reaction conditions, the propene production rate over WO x/USY is ca. 7300 times that over the industrial WO3/SiO2-based catalyst. A propene yield up to 79% (80% selectivity) without observable deactivation was obtained over WO x/USY for a wide range of reaction conditions.

Pushing the Limits on Metal-Organic Frameworks as a Catalyst Support: NU-1000 Supported Tungsten Catalysts for o-Xylene Isomerization and Disproportionation.

Abstract: Acid-catalyzed skeletal C–C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, o-xylene isomerization/disproportionation is a probe reaction for strong Bronsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal–organic framework (MOF), as a catalyst for o-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), 31P NMR, N2 physisorption, and X-ray diffraction (XRD) show that the catalyst is structurally stable with time-on-stream and that WOx clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000...

Chemical-Reaction-Controlled Phase Separated Drops: Formation, Size Selection, and Coarsening

Abstract: Phase separation under nonequilibrium conditions is exploited by biological cells to organize their cytoplasm but remains poorly understood as a physical phenomenon. Here, we study a ternary fluid model in which phase-separating molecules can be converted into soluble molecules, and vice versa, via chemical reactions. We elucidate using analytical and simulation methods how drop size, formation, and coarsening can be controlled by the chemical reaction rates, and categorize the qualitative behavior of the system into distinct regimes. Ostwald ripening arrest occurs above critical reaction rates, demonstrating that this transition belongs entirely to the nonequilibrium regime. Our model is a minimal representation of the cell cytoplasm.