Showing papers on "Reaction rate constant published in 2016"
TL;DR: Using an operando x-ray microscopy platform that maps the dynamics of the Li composition and insertion rate in LixFePO4, it is found that nanoscale spatial variations in rate and in composition control the lithiation pathway at the subparticle length scale.
Abstract: The kinetics and uniformity of ion insertion reactions at the solid-liquid interface govern the rate capability and lifetime, respectively, of electrochemical devices such as Li-ion batteries. Using an operando x-ray microscopy platform that maps the dynamics of the Li composition and insertion rate in LixFePO4, we found that nanoscale spatial variations in rate and in composition control the lithiation pathway at the subparticle length scale. Specifically, spatial variations in the insertion rate constant lead to the formation of nonuniform domains, and the composition dependence of the rate constant amplifies nonuniformities during delithiation but suppresses them during lithiation, and moreover stabilizes the solid solution during lithiation. This coupling of lithium composition and surface reaction rates controls the kinetics and uniformity during electrochemical ion insertion.
369 citations
TL;DR: Scanning electrochemical microscopy was used to determine the HER kinetic data for both unstrained S vacancies and strained S vacancies on the basal plane of MoS2 monolayers, and the strained S vacancy has an electron-transfer rate 4 times higher than that of the un Strained S vacancy.
Abstract: Molybdenum disulfide (MoS2), with its active edge sites, is a proposed alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Recently, the inert basal plane of MoS2 was successfully activated and optimized with excellent intrinsic HER activity by creating and further straining sulfur (S) vacancies. Nevertheless, little is known about the HER kinetics of those S vacancies and the additional effects from elastic tensile strain. Herein, scanning electrochemical microscopy was used to determine the HER kinetic data for both unstrained S vacancies (formal potential Ev0 = −0.53 VAg/AgCl, electron-transfer coefficient αv = 0.4, electron-transfer rate constant kv0 = 2.3 × 10–4 cm/s) and strained S vacancies (Esv0= −0.53 VAg/AgCl, αsv = 0.4, ksv0 = 1.0 × 10–3 cm/s) on the basal plane of MoS2 monolayers, and the strained S vacancy has an electron-transfer rate 4 times higher than that of the unstrained S vacancy. This study provides a general platform for measuring the kinetics of two-dimens...
230 citations
TL;DR: The transformation efficiency and products of an odorous compound 2,4,6-trichloroanisole (TCA) at the wavelength of 254 nm in the presence of persulfate were investigated for the first time and a tentative pathway was proposed, where the initial one-electron oxidation of TCA by SO4(-) and further reactions of the formed cation intermediate TCA were involved.
198 citations
TL;DR: In this article, a detailed chemical kinetic model is presented for the reaction of the important intermediate H2NO with O2, and the rate constant for the intermediate was determined from ab initio calculations to be 2.994 exp(−9510 K/T)cm3mol−1/s−1.3
190 citations
TL;DR: In this article, the authors study the heat/mass transfer effects on rotating flow of Maxwell fluid due to unidirectional stretching surface and derive self-similar form of boundary layer equations which are solved numerically.
Abstract: Here we study the heat/mass transfer effects on revolving flow of Maxwell fluid due to unidirectional stretching surface. Mass transfer process is modeled in terms of binary chemical reaction and activation energy. Modified Arrhenius function for activation energy is invoked. Traditional boundary layer approximations are utilized to simplify the governing equations. Using similarity method, self-similar form of boundary layer equations are derived which are solved numerically. The solutions depend on dimensionless numbers such as the rotation parameter λ , the Deborah number β , the Prandtl number Pr , the Schmidt number Sc , activation energy E , fitted rate constant n and temperature difference parameter δ . We found that the solute concentration in binary mixture is proportional to both rotation parameter λ and activation energy E . The reaction rate σ and fitted rate n both provide reduction in the solute concentration. Thermal boundary layer becomes thicker and heat transfer rate diminishes when fluid is subjected to a larger rotation rate.
178 citations
TL;DR: The tunnel effect, for example, changes reaction paths and branching ratios, enables chemical reactions in an astrochemical environment that would be impossible by thermal transition, and influences biochemical processes.
Abstract: Quantum mechanical tunneling of atoms is increasingly found to play an important role in many chemical transformations. Experimentally, atom tunneling can be indirectly detected by temperature-independent rate constants at low temperature or by enhanced kinetic isotope effects. In contrast, the influence of tunneling on the reaction rates can be monitored directly through computational investigations. The tunnel effect, for example, changes reaction paths and branching ratios, enables chemical reactions in an astrochemical environment that would be impossible by thermal transition, and influences biochemical processes.
159 citations
TL;DR: Graphene-manganese ferrite (rG-MnFe2O4) was developed to degrade NH3 as mentioned in this paper, which showed a degradation of 92.0% was achieved through visible light irradiation of 50.0mg L−1 NH3-N solution at pH 10.5 in the presence of 1.0mmol/L H2O2 for 10h.
135 citations
TL;DR: In this article, a Ni-PY5 complex has been found to act as an electrocatalyst for oxidizing water to dioxygen in aqueous phosphate buffer solutions.
113 citations
TL;DR: In this article, the first systematic synthetic study of bismuth oxyiodide/graphitic carbon nitride (BiOxIy/g-C3N4) nanocomposite preparation using a controlled hydrothermal method is reported.
Abstract: The first systematic synthetic study of bismuth oxyiodide/graphitic carbon nitride (BiOxIy/g-C3N4) nanocomposite preparation using a controlled hydrothermal method is reported. The structure and morphology of BiOxIy/g-C3N4 photocatalysts are characterized by XRD, TEM, FT-IR, HR-XPS, FE-SEM-EDS, UV-vis-DRS and BET. The photodegradation activities are evaluated against the decolorization of crystal violet (CV) in aqueous solution under visible light illumination. In particular, the catalytic performance illustrates the best reaction rate constant, being 0.170 h−1 using Bi7O9I3/Bi5O7I/g-C3N4 composite as the photocatalyst; which is 5, 4, and 1.5 times higher than the reaction rate constant of BiOI, g-C3N4, and Bi7O9I3/Bi5O7I, as photocatalysts, respectively. From the quenching effects of different scavengers, the EPR results demonstrate that the reactive O2˙− plays the major role and h+ and ˙OH play minor roles in the CV degradation. The probable photodegradation mechanisms are proposed and discussed in this research. This work is useful for the synthesis of BiOxIy/g-C3N4 and the photocatalytic degradation of the CV for future applications in environmental pollution and control.
111 citations
TL;DR: It is suggested that the major products, highly oxygenated organic peroxides, are lost to the chamber walls and an IEPOX global production rate of more than 100 Tg C each year is estimated from this chemistry using a global 3D chemical transport model, similar to earlier estimates.
Abstract: The atmospheric oxidation of isoprene by the OH radical leads to the formation of several isomers of an unsaturated hydroxy hydroperoxide, ISOPOOH. Oxidation of ISOPOOH by OH produces epoxydiols, IEPOX, which have been shown to contribute mass to secondary organic aerosol (SOA). We present kinetic rate constant measurements for OH + ISOPOOH using synthetic standards of the two major isomers: (1,2)- and (4,3)-ISOPOOH. At 297 K, the total OH rate constant is 7.5 ± 1.2 × 10–11 cm3 molecule–1 s–1 for (1,2)-ISOPOOH and 1.18 ± 0.19 × 10–10 cm3 molecule–1 s–1 for (4,3)-ISOPOOH. Abstraction of the hydroperoxy hydrogen accounts for approximately 12% and 4% of the reactivity for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. The sum of all H-abstractions account for approximately 15% and 7% of the reactivity for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. The major product observed from both ISOPOOH isomers was IEPOX (cis-β and trans-β isomers), with a ∼ 2:1 preference for trans-β IEPOX and similar total yields ...
108 citations
TL;DR: In this article, the authors used low reactive volcanic ash for making geopolymer cement and found that the result was a mixture of poly(ferro-sialate-siloxo) and poly (ferro sialate disiloxo)-binder type with a formula close to [Ca,Na,K,Mg]
Abstract: This paper looks at the possibility of using low reactive volcanic ash for making geopolymer cement. The research is directed towards (a) alteration of the reactivity of volcanic ash by mechanical activation, and (b) use of mechanically activated volcanic ash for the synthesis of a geopolymer. The effect of mechanical activation was quite visible on particle size distribution and the degree of crystallinity. The disappearance of some anorthite peaks and appearance of quartz peaks in volcanic ashes milled for 120 min demonstrate the change in mineralogy. The appearance of an intense carbonate band with milling time could be related to sorption of atmospheric CO2 on the grains surface during mechanical activation. The manifestation of mechanical activation of volcanic ash was prominent on (a) the reaction kinetics, (b) microstructural development, and (c) physico-mechanical properties of the geopolymer product. The rate constant and extent of geopolymerization increased with milling time but decreased with curing temperature. This decrease is in non-conformity with other alumina-silicate materials used for geopolymerization such as metakaolin and fly ash. FEG-SEM and EDAX results revealed that the geopolymer gel obtained is mixture of poly(ferro-sialate-siloxo) and poly(ferro-sialate-disiloxo) binder type with a formula close to [Ca,Na,K,Mg]–[–Fe–O–]x–[Si–O–Al–O–]1−x–[–Si–O–]y. The physico-mechanical properties changed significantly. Setting time reduced by >95% in samples milled for 60 min or more. The compressive strength which was negligible for 0–30 min milled volcanic ash reached 29–54 MPa after 60–120 min of milling time. Heat curing influenced the early age (7 and 28 days) compressive strength but the 90 day compressive strength of both ambient and heat cured samples were comparable.
TL;DR: In this paper, the authors investigated the oxidation behavior of copper at temperatures below 300 °C and its mechanism, and proposed a methodology to slow down the oxidation rate based on the observed mechanism.
Abstract: In this study, we investigate the oxidation behavior of copper at temperatures below 300 °C and its mechanism. A methodology to slow down the oxidation rate is then proposed based on the observed mechanism. The oxides formed after oxidation at low temperatures have fine crystal sizes; the rate constants reach 2×10-15 m2/s and 6×10-14 m2/s at 200 °C and 300 °C, respectively. A passivation treatment at 600 °C in nitrogen produces a thin oxide layer composed of relatively large Cu2O crystals. The presence of such a layer slows down the oxidation rate constants by an order of magnitude. This study demonstrates that the oxidation of copper at low temperatures is controlled by the grain boundary diffusion. Increasing the crystal size in the surface oxide reduces the oxidation rate significantly.
TL;DR: In this paper, the degradation performance of 2,4-Di-tert-butylphenol (2,4D) by the UV/persulfate (UV/PS) process was evaluated for the first time.
TL;DR: In this paper, the authors showed that the reaction rates for the half-cells are of the same order of magnitude with their respective rate constants depending on the composition of the electrode material.
Abstract: Vanadium flow batteries are a promising system for stationary energy storage. One of their shortcomings is a low power density caused by slow kinetics of the redox reactions. To alleviate this drawback, many studies tried to catalyze the redox reactions. However, up to now, there is no consensus in the literature on which of the two half-cell reactions, the V2+/V3+ or the VO2+/VO2+reaction, features the slower electron transfer. The present study is the first showing that reaction rates for the half-cells are of the same order of magnitude with their respective rate constants depending on the composition of the electrode material. The surface functional groups hydroxyl, carbonyl, and carboxyl on carbon increase the wetted surface area, catalyze the V2+/V3+ redox reaction, but impede the VO2+/VO2+ redox reaction. This complex situation was unraveled by using a newly developed procedure based on electrochemical impedance spectroscopy. Reaction mechanisms based on these results are discussed.
TL;DR: In this paper, the acceleration of oxidation of p-Nitrophenol, a known soil pollutant, by thermally activated persulfate was studied for the application of soil remediation.
TL;DR: In this paper, the pyrolysis of seven lignin model compounds was investigated in a micropyrolyzer connected to GC-MS/FID, and the rate constants for the different reaction pathways were predicted from ab initio density functional theory calculations and preexponential literature values.
TL;DR: In this article, a novel La-Y-PbO 2 electrode was prepared by electrodeposition technique and the degradation mechanism for aspirin was explored, which had a more compact surface with a smaller grain size of crystal, accompanying with increased concentration of active oxygen species.
TL;DR: In this article, the authors identify a benchmark for the performance of a small-scale ammonia synthesis plant powered by wind energy and present an analysis of how the current production rate is controlled by three resistances: catalytic reaction, ammonia separation by condensation, and recycling of unreacted gas.
Abstract: This work identifies a benchmark for the performance of a small-scale ammonia synthesis plant powered by wind energy. The energy used is stranded, far from urban centers but near locations of fertilizer demand. The wind energy drives the pressure swing absorption of air to make nitrogen and the electrolysis of water to make hydrogen. These are combined in the small-scale continuous Haber process to synthesize ammonia. The analysis of runs of the small plant presented in this article permits an assessment of how the current production rate is controlled by three resistances: catalytic reaction, ammonia separation by condensation, and recycling of unreacted gas. The measured catalytic reaction rates are consistent with separate experiments on chemical kinetics and with published reaction mechanisms. The condensation rates predicted are comparable with literature correlations. These rate constants now supply a rigorous strategy for optimizing this scaled-down, distributed ammonia plant. Moreover, this method...
TL;DR: In this article, high-ordered TiO2 and WO3-TiO2 nanotubes were prepared by one-step electrochemical anodizing method and cobalt has been successfully deposited on these nano-graphs by photo-assisted deposition process.
TL;DR: In this article, an ultrasound and mixing assisted desulfurization of synthetic oil containing sulfur compounds of benzothiophene and dibenzothiopane was carried out using different polyoxometalate catalysts, H 2 O 2 oxidant and a phase transfer agent.
TL;DR: In this article, a new bimetallic catalyst supported by environmentally benign nanocrystalline ZSM-5 was developed to reduce nitrate completely and selectively to nitrogen gas without producing nitrite.
Abstract: A new bimetallic catalyst supported by environmentally benign nanocrystalline ZSM-5 (NZSM-5), was developed to reduce nitrate completely and selectively to nitrogen gas without producing nitrite. The catalyst was optimized by use under a variety of conditions (i.e., promoter metal type (Sn, Cu, Ag, Ni)), noble metal type (Pd, Pt, Au), promoter metal concentration (0–3.4 wt%), noble metal concentration (0–2.8 wt%), catalyst calcination temperature (0–550 °C), H2 flow rate (0–60 mL/min), and CO2 flow rate (0–60 mL/min). Complete nitrate removal with the highest nitrogen selectivity (91%) was achieved using 1%Sn-1.6%Pd-NZSM-5 catalyst under optimized conditions that included: initial nitrate concentration: 30 mg/L NO3-N; calcination temperature: 350 °C; H2 flow rate: 30 mL/min; and CO2 flow rate: 60 mL/min for 60 min. The estimated kinetic rate constant of the catalyst is 16.40 × 10−2 min−1, the catalyst-loading normalized rate constant is 65.60 × 10−2 min−1 gcat−1, while Pd-loading normalized rate constant is 410 × 10−2 L/min gPd−1. The catalyst showed remarkable nitrate removal (100%) and nitrogen selectivity (>88%) for up to five successive reactions with consistent kinetics. A 100% nitrate removal and >81% nitrogen selectivity was also achieved by the catalyst for five repeated cycles. However, the kinetics gradually slowed down to 4.36 × 10−2 min−1 over five repeated cycles, (still superior to fresh catalysts already reported in the literature). Characterization tests confirmed that the used catalyst was chemically stable, and that the decrease in its reactivity was due mainly to the sintering of metallic nano particles during the regeneration process.
TL;DR: The contribution of water to the zwitterion deprotonation seems to be more significant than that of LysK for the above-mentioned kinetic conditions, and LysK shows higher chemical reactivity toward CO2 than the industrial standard MEA and most of amino acid salts.
Abstract: Aqueous amino acid salts are considered as an attractive alternative to alkanolamine solvents (e.g., MEA) for carbon dioxide (CO2) absorption. The kinetics of CO2 into unloaded aqueous solutions of potassium lysinate (LysK) was studied using a wetted wall column at concentrations ranging from 0.25 to 2.0 M and temperatures from 298 to 333 K. Physicochemical properties of aqueous LysK solutions such as density, viscosity, and physical solubility of CO2 were measured to evaluate the reaction rate constants. The reaction pathway is described using zwitterion mechanism taking into account the effect of ionic strength on the reaction rate. Under the fast pseudo-first-order regime, the reaction rate parameters were obtained and correlated in a power-law reaction rate expression. LysK shows higher chemical reactivity toward CO2 than the industrial standard MEA and most of amino acid salts. Its reaction rate constants increase considerably with concentration and temperature. The reaction order is found to be an a...
TL;DR: This method is applied to a key reaction of an industrially relevant catalytic process, the methylation of small alkenes over zeolites, and yields results that agree with experiment within chemical accuracy limits.
Abstract: The ab initio prediction of reaction rate constants for systems with hundreds of atoms with an accuracy that is comparable to experiment is a challenge for computational quantum chemistry. We present a divide-and-conquer strategy that departs from the potential energy surfaces obtained by standard density functional theory with inclusion of dispersion. The energies of the reactant and transition structures are refined by wavefunction-type calculations for the reaction site. Thermal effects and entropies are calculated from vibrational partition functions, and the anharmonic frequencies are calculated separately for each vibrational mode. This method is applied to a key reaction of an industrially relevant catalytic process, the methylation of small alkenes over zeolites. The calculated reaction rate constants (free energies), pre-exponential factors (entropies), and enthalpy barriers show that our computational strategy yields results that agree with experiment within chemical accuracy limits (less than one order of magnitude).
TL;DR: This study developed methods to predict DBPs degradation within the UV/H2O2 AOP via UV direct photolysis and hydroxyl radical (•OH) reaction, so that DBPs most likely to pass through the AOP could be predicted.
Abstract: Previous research has demonstrated that the reverse osmosis and advanced oxidation processes (AOPs) used to purify municipal wastewater to potable quality have difficulty removing low molecular weight halogenated disinfection byproducts (DBPs) and industrial chemicals. Because of the wide range of chemical characteristics of these DBPs, this study developed methods to predict their degradation within the UV/H2O2 AOP via UV direct photolysis and hydroxyl radical (•OH) reaction, so that DBPs most likely to pass through the AOP could be predicted. Among 26 trihalomethanes, haloacetonitriles, haloacetaldehydes, halonitromethanes and haloacetamides, direct photolysis rate constants (254 nm) varied by ∼3 orders of magnitude, with rate constants increasing with Br and I substitution. Quantum yields varied little (0.12–0.59 mol/Einstein), such that rate constants were driven by the orders of magnitude variation in molar extinction coefficients. Quantum chemical calculations indicated a strong correlation between ...
TL;DR: In this article, rate constants for the reaction of 4-methylbenzoquinone (4MBQ) with proteins, thiol and amine compounds were determined under pseudo first-order conditions by UV-vis stopped-flow spectrophotometry.
TL;DR: In this paper, a highly efficient heterogeneous photo-Fenton system (Fe2O3/g-C3N4/H2O2/visible light) has been developed.
Abstract: In this study, a highly efficient heterogeneous photo-Fenton system (Fe2O3/g-C3N4/H2O2/visible light) has been developed. The heterogeneous catalyst Fe2O3/g-C3N4 in this system was successfully prepared by growing Fe2O3 nanoparticles on the surface of g-C3N4. The Fe2O3 nanoparticles could achieve high dispersion on the surface of g-C3N4 and form a heterojunction with g-C3N4 to improve the charge separation. In addition, the combination of the Fenton's reagent Fe2O3/H2O2 and the photocatalyst g-C3N4 greatly enhances the rate of the Fenton's reaction with the assistance of the photocatalytic process. The results showed that the Fe2O3/g-C3N4 catalyst had a superior catalytic activity as compared with the single component of Fe2O3 or g-C3N4 and the mechanical mixture of Fe2O3 and g-C3N4. The catalyst prepared with 3 mL of FeCl3 aqueous solution shows the best photo-Fenton photocatalytic efficiency with a reaction rate constant of 0.02461 mg L–1 min–1, which is about 45.4, 8.4 and 7.2 times larger than that of pure Fe2O3 (0.0005418 mg L–1 min–1), pure g-C3N4 (0.00294 mg L–1 min–1) and the mechanically mixed Fe2O3/g-C3N4 (0.0034 mg L–1 min–1), respectively. A possible mechanism for the visible-light-irradiated photo-Fenton photocatalysis is proposed, and the Fe2O3/g-C3N4 catalyst exhibited stable performance without obvious loss of catalytic activity after four successive runs, showing a good application prospect for the photo-oxidative degradation of organic contaminants in wastewater.
TL;DR: The results reveal that ruthenium(0) nanoparticles were successfully supported on ceria and the resulting Ru(0)/CeO2 is a highly active, reusable and long-lived catalyst for hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency value of 361 min(-1).
Abstract: Ruthenium(0) nanoparticles supported on ceria (Ru0/CeO2) were in situ generated from the reduction of ruthenium(III) ions impregnated on ceria during the hydrolysis of ammonia borane. Ru0/CeO2 was isolated from the reaction solution by centrifugation and characterized by ICP-OES, BET, XRD, TEM, SEM-EDS and XPS techniques. All the results reveal that ruthenium(0) nanoparticles were successfully supported on ceria and the resulting Ru0/CeO2 is a highly active, reusable and long-lived catalyst for hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency value of 361 min−1. The reusability tests reveal that Ru0/CeO2 is still active in the subsequent runs of hydrolysis of ammonia borane preserving 60% of the initial catalytic activity even after the fifth run. Ru0/CeO2 provides a superior catalytic lifetime (TTO = 135 100) in hydrogen generation from the hydrolysis of ammonia borane at 25.0 ± 0.1 °C before deactivation. The work reported here includes the formation kinetics of ruthenium(0) nanoparticles. The rate constants for the slow nucleation and autocatalytic surface growth of ruthenium(0) nanoparticles were obtained using hydrogen evolution as a reporter reaction. An evaluation of rate constants at various temperatures enabled the estimation of activation energies for both the reactions, Ea = 60 ± 7 kJ mol−1 for the nucleation and Ea = 47 ± 2 kJ mol−1 for the autocatalytic surface growth of ruthenium(0) nanoparticles, as well as the activation energy of Ea = 51 ± 2 kJ mol−1 for the catalytic hydrolysis of ammonia borane.
TL;DR: In this paper, a new type of heterojunction photocatalyst, Bi2SiO5/g-C3N4, was prepared using a controlled hydrothermal method.
Abstract: A new type of heterojunction photocatalyst, Bi2SiO5/g-C3N4, was prepared using a controlled hydrothermal method. The structure and morphology of the Bi2SiO5/g-C3N4 photocatalyst were characterized by XRD, HR-TEM, FE-SEM-EDS, HR-XPS, FT-IR, PL, BET, EPR, and UV-Vis-DRS. The obtained Bi2SiO5/g-C3N4 photocatalyst exhibits enhanced photocatalytic activity on the decolorization of crystal violet (CV) under visible-light irradiation. In particular, the catalytic performance illustrates the best reaction rate constant of 0.1257 h−1 using Bi2SiO5/g-C3N4 as the photocatalyst, which is 5 and 3 times higher than the reaction rate constants of Bi2SiO5 and g-C3N4 as photocatalysts, respectively. This study shows that Bi2SiO5/g-C3N4 can be used to suppress the recombination of photoinduced electron–hole pairs and contribute to the enhanced photocatalytic efficiency of semiconductors in the visible light-driven catalysis. The quenching effects of different scavengers, and EPR results demonstrate that the reactive O2˙− plays the major role and ˙OH, h+ and 1O2 play minor roles in CV degradation. The probable photodegradation mechanisms are proposed and discussed.
TL;DR: A mechanistic study of the copper-catalyzed oxidation of the methylene group of aryl(di)azinylmethanes was performed and showed promising results in the determination of the racemates for high-performance liquid chromatography.
Abstract: A mechanistic study of the copper-catalyzed oxidation of the methylene group of aryl(di)azinylmethanes was performed. Initial reaction rates were measured making use of in situ IR reaction monitoring and a kinetic analysis of the reaction was executed. The reaction proved to be first order in oxygen concentration. For substrate and acid concentration, saturation kinetics due to O2 mass transfer limitation were observed. The occurrence of mass transfer limitation was further confirmed by examining the effect of the stirring rate on the initial reaction rate. Interestingly, the effect of the concentration of the catalyst on the rate shows that higher loadings result in a maximal initial rate, followed initially by a steady decrease and subsequently a rate plateau when the concentration is increased further. Mass transfer limitation and increased concentration of dinuclear catalytically active species rationalizes this hitherto unprecedented rate behavior. Continuous-wave and pulsed electron paramagnetic resonance methods were used to characterize the catalytic species present in the solution during the reaction and confirmed the presence of both mono- and dinuclear copper species. Analysis of a diverse substrate scope points towards imine–enamine tautomerization as a crucial process in the oxidation reaction. DFT calculations of these equilibrium constants (pKeq) provided us with a qualitative tool to predict whether or not a substrate is viable for oxidation under the reaction conditions developed.
TL;DR: Investigating how formamide forms in the interstellar medium is a hot topic in astrochemistry, which can contribute to the understanding of the origin of life on Earth.
Abstract: Investigating how formamide forms in the interstellar medium is a hot topic in astrochemistry, which can contribute to our understanding of the origin of life on Earth. We have constructed a QM/MM model to simulate the hydrogenation of isocyanic acid on amorphous solid water surfaces to form formamide. The binding energy of HNCO on the ASW surface varies significantly between different binding sites, we found values between ∼0 and 100 kJ mol−1. The barrier for the hydrogenation reaction is almost independent of the binding energy, though. We calculated tunneling rate constants of H + HNCO → NH2CO at temperatures down to 103 K combining QM/MM with instanton theory. Tunneling dominates the reaction at such low temperatures. The tunneling reaction is hardly accelerated by the amorphous solid water surface compared to the gas phase for this system, even though the activation energy of the surface reaction is lower than the one of the gas-phase reaction. Both the height and width of the barrier affect the tunneling rate in practice. Strong kinetic isotope effects were observed by comparing to rate constants of D + HNCO → NHDCO. At 103 K we found a KIE of 231 on the surface and 146 in the gas phase. Furthermore, we investigated the gas-phase reaction NH2 + H2CO → NH2CHO + H and found it unlikely to occur at cryogenic temperatures. The data of our tunneling rate constants are expected to significantly influence astrochemical models.