Backward Reaction Research Articles

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal.

Smart viscoelastic materials that respond to specific stimuli are one of the most attractive classes of materials important to future technologies, such as on-demand switchable adhesion technologies, actuators, molecular clutches, and nano-/microscopic mass transporters. Recently it was found that through a special solid-liquid transition, rheological properties can exhibit significant changes, thus providing suitable smart viscoelastic materials. However, designing materials with such a property is complex, and forward and backward switching times are usually long. Therefore, it is important to explore new working mechanisms to realize solid-liquid transitions, shorten the switching time, and enhance the contrast of rheological properties during switching. Here, a light-induced crystal-liquid phase transition is observed, which is characterized by means of polarizing light microscopy (POM), photorheometry, photo-differential scanning calorimetry (photo-DSC), and X-ray diffraction (XRD). The light-induced crystal-liquid phase transition presents key features such as (1) fast switching of crystal-liquid phases for both forward and backward reactions and (2) a high contrast ratio of viscoelasticity. In the characterization, POM is advantageous in offering information on the spatial distribution of LC molecule orientations, determining the type of liquid crystalline phases appearing in the material, and studying the orientation of LCs. Photorheometry allows measurement of a material's rheological properties under light stimuli and can reveal the photorheological switching properties of materials. Photo-DSC is a technique to investigate thermodynamic information of materials in darkness and under light irradiation. Lastly, XRD allows studying of microscopic structures of materials. The goal of this article is to clearly present how to prepare and measure the discussed properties of a photorheological material.

Sustainable synthesis of AgNPs/strontium-titanate-perovskite-like catalysts for the photocatalytic production of hydrogen

Abstract SrTiO3-cubic-like photocatalysts with average size of 35–40 nm were synthesized in 2 h without further calcination treatment. Structural defects or contamination were negligible according to the DRX and FTIR analyzes. The H2 generation rates were 386 and 463 μmol h−1 g−1 using an ethanol-water scavenger suspension with the SrTiO3 and Ag/SrTiO3 oxides, respectively. In comparison with other works reported in the literature, the hydrothermal synthesis of SrTiO3 and the UV-pen lamp radiation used for the H2 production may open a new strategy for the sustainable synthesis of 3-dimensional photoactive nanomaterials. The functionalization of nanocubes with highly monodispersed silver or rhodium nanoparticles provided an additional increase in the hydrogen photoproduction rate. In this sense, the silver nanoparticles (AgNPs) act as a sink for the photogenerated electrons through a Schottky barrier in the metal-perovskite contact region, boosting up the separation of the charge carriers. The identification of the surface species by XPS as well as the Rhodamine B photo-oxidation were performed to better understand the surface backward reaction in the photocatalytic production of hydrogen. The AgNPs worked as co-catalysts, preserving the stability of the most photoactive catalysts for 32 h.

Acyl-CoA:lysophosphatidylcholine acyltransferases (LPCATs) of Camelina sativa seeds: biochemical properties and function

Main conclusionThe transfer of polyunsaturated fatty acids from phosphatidylcholine to other lipids involves several enzymes. In Camelina sativa seeds, acyl-CoA:lysophosphatidylcholine acyltransferases could be one of the most important players in this process.The transfer of polyunsaturated fatty acids from the location of their synthesis (phosphatidylcholine) to other lipids, e.g., triacylglycerol, remains insufficiently understood. Several enzymes could be involved in this process. One of these enzymes is acyl-CoA:lysophosphatidylcholine acyltransferases (LPCATs). In Camelina sativa seeds, LPCATs could be one of the most important players in this process. Our data clearly indicate that the CsLPCATs present in developing seeds have the potential to transfer almost all polyunsaturated fatty acids synthesised on phosphatidylcholine to the acyl-CoA pool. CsLPCAT activity is the highest at 30 °C, and the enzymes operate well at a pH of 7.0–11.0, with the best activity at a pH of 9.0. The activity of CsLPCATs was inhibited by calcium and magnesium ions at a concentration of 0.05–2 mM. In the forward reaction, CsLPCATs preferentially utilise 18:2-CoA; however, other C18 unsaturated fatty acids are also well accepted. In the backward reactions, there is no clear discrimination between the C18 unsaturated fatty acids utilised by the enzymes for phosphatidylcholine remodelling. The activity of CsLPCATs does not differ much between the stages of seed development.

Integrating Pt@Ni(OH)2nanowire and Pt nanoparticle on C3N4with fast surface kinetics and charge transfer towards highly efficient photocatalytic water splitting

Overall water splitting is vital in solar-hydrogen conversion. In addition to charge separation, the regulation of surface kinetics and suppression of backward reaction become particularly crucial. Herein, an all-in-one Pt·Ni(OH)2/C3N4 photocatalyst is proposed by integrating Pt@Ni(OH)2 composited nanowires and isolated Pt clusters on C3N4. In this heterostructure, Pt@Ni(OH)2 with rich coordinatively unsaturated sites effectively boost O2 evolution, and Pt-O-Ni interaction retards OO bond cleavage thus inhibiting backward H2O regeneration. Meanwhile isolated Pt forms an Schottky junction with C3N4 for electron transfer and proton reduction. Consequently, Pt·Ni(OH)2/C3N4 achieves stoichiometric water splitting with H2/O2 evolution of 1330/632 μmol g−1 h−1, and an outperforming AQE of 4.2% at 420 nm. Our work manifests that accelerating charge migration, inhibiting backward reaction and tuning surface kinetics are dominant in water splitting, for which a rational design of robust redox pathways is necessary.

Alkali-metal-oxides coated ultrasmall Pt sub-nanoparticles loading on intercalated carbon nitride: Enhanced charge interlayer transportation and suppressed backwark reaction for overall water splitting

Photocatalytic overall water splitting by using layered polymeric materials has been considered a promising way for clean fuel H2 production, however the catalytic performance is still unsatisfactory due to the sluggish charge separation as well as serious backward reaction of water splitting. We herein demonstrate the clever design and construction of alkali-metal-oxides membrane coated ultrasmall Pt sub-nanoparticle that is deposited on alkali-metal-atoms intercalated carbon nitride for efficient overall water splitting. We verify that the intercalated alkali-metal-atoms (K, Na, Li) would bridge the adjacent layers of carbon nitride for efficient charge interlayer transportation, boosting charge carrier separation; while the formation of mixed alkali-metal-oxides shell on the ultrasmall Pt sub-nanopartile would function as a membrane that is H2 permeable but O2 impermeable. Such a selective permeable membrane is found to block H2/O2 recombination to form H2O. Benefited from these advantages, the as-prepared photocatalyst exhibits excellent photocatalytic H2 evolution performance (vacuum condition: AQY = 37%, air condition: AQY = 4.9%) as well as overall water splitting activity in a stoichiometric ratio of 2:1 with suppressed H2/O2 recombination. The innovative design demonstrated here provides new ideas for designing advanced photocatalysts with high charge separation for efficient overall water splitting with no H2/O2 recombination.

Role of Bicarbonate Ions in Aqueous Solution as a Carbon Source for Photocatalytic Conversion of CO2 into CO

Ag/SrNb2O6 nanorods exhibited a high CO formation rate and high selectivity toward CO evolution in aqueous solutions containing bicarbonate ions even without CO2 bubbling. Notably, the formation rate of CO reached as high as 287 μmol h–1 with selectivity toward CO evolution of higher than 94.1% when NH4HCO3 was used as an additive. Both the formation rate of CO and selectivity toward CO evolution increased with the concentration of HCO3–. According to the experimental results and analytical chemistry calculations, it was concluded that CO2(aq) obtained by the dissociation of HCO3– is the actual reactant in the photocatalytic conversion of CO2. In contrast, the HCO3– species in the aqueous solution is beneficial for improving the photocatalytic activity and selectivity toward CO evolution by increasing the adsorption of carbon-related species on the photocatalyst surface and/or suppressing the backward reaction for the photocatalytic conversion of CO2.

Assembly of large-area reduced graphene oxide films for the construction of Z-scheme over single-crystal porous Bi5O7I nanosheets

In this work, we have firstly achieved the construction of bismuth oxyiodide (Bi5O7I)/reduced graphene oxide (rGO)/ZnO Z-scheme photoelectrochemical (PEC) system without backward reactions through loading large-area ZnO quantum dots (QDs)/rGO films on Bi5O7I nanosheets. Single-crystal porous Bi5O7I nanosheets with numerous oxygen vacancies (OVs) were firstly fabricated through the calcination of BiOI in reductive glycols. The single-crystal facilitates charge transport, nanoporous structure promotes light absorption and OVs improves charge separation efficiency. As a result, single-crystal porous Bi5O7I nanosheets with OVs exhibited higher PEC performance than other morphologies reported before. Moreover, The PEC activity of Bi5O7I can be further enhanced through loading large-area ZnO QDs/rGO films to construct a pure Z-scheme charge transfer system, which not only achieves efficient separation of electron-hole pairs but also retains its excellent redox ability. To the best of our knowledge, the photocurrent density of Bi5O7I/rGO/ZnO heterostructures is the highest among Bi5O7I-based samples. For comparison, an opposite Z-scheme model has also been built up by replacing Bi5O7I with WO3, in which the photocurrent density decreased conversely. Therefore, it can be known that a pure Z-scheme system without backward reactions can be successfully prepared through loading ZnO QDs/rGO films on photoelectrodes.

Single-Electron-Trapped Oxygen Vacancy on Ultrathin WO3·0.33H2O {100} Facets Suppressing Backward Reaction for Promoted H2 Evolution in Pure Water Splitting.

Solar water splitting to produce hydrogen is a promising solution for global energy issues. One of the main bottlenecks in this technology is the spontaneous fast backward reaction (2H2 + O2 → H2O, Δ G < 0), limiting the solar energy conversion efficiency. How to suppress backward reaction is vitally important but rarely reported. Here we found that single-electron-trapped oxygen vacancy (Vo·) can suppress spontaneous backward reaction in pure water splitting. Taking WO3·0.33H2O catalyst as an example, ultrathin WO3·0.33H2O {100} facets with large amount of surface Vo· realized a continuous H2 evolution from pure water splitting with a productivity of 9.9 μmol/g·h without the assistance of any sacrifice agent and noble metal cocatalyst. Quantum chemical calculations revealed that the backward-reaction suppression ability of Vo· is attributed to the high concentration of localized electrons around Vo·, stimulating unidirectional simultaneous water dissociation into H and OH under light irradiation.

(Invited) Effect of Particle Size on Photocatalytic Reaction Kinetics Including Multielectron-Transfer Processes

Oxidation of water into molecular oxygen (O2 + 4H+ + 4e– → 2H2O) must be the largest-scale chemical reaction occurring on the earth as a part of photosynthesis by plants, and the corresponding backward half-cell reaction, reduction of oxygen into water (O2 + 4H+ + 4e– → 2H2O) might proceed in the same scale by the activity of plants and animals to get energy by combustion of chemicals keeping the concentration of oxygen on the earth. It is well known that the forward and backward reactions are counter oxidation and reduction reactions, respectively, of photocatalytic water splitting/carbon-dioxide fixation and photocatalytic decomposition of organic compounds by semiconductor particles. Standard electrode potential (SEP) of this four electron-transfer redox (O2 + 4H+ + 4e– = 2H2O) is 1.23 V (versus SHE = standard hydrogen electrode potential (0 V) in definition). Almost all the papers reporting oxygen evolution use this SEP, while those of photocatalytic organics decomposition tend to use SEP for one-electron transfer, O2 + e– = O2•– (–0.28 V), for discussion. One of the possible reasons for such difference is that it is convenient to presume the production of superoxide anion radical which may produce "reactive oxygen species (ROS)" to decompose organic compounds. Anyway, it can be stated that those proposed number of transferred electrons seems speculative without any experimental evidence. We have been studying light-intensity dependence (LID) of photocatalytic reaction rates using highly intense UV-LEDs with intensity maximally ca. 3 W cm-2. For oxygen evolution from aqueous titania suspensions in the presence of electron acceptors such as iodate (IO3 –) or iron(III) ions under deaerated conditions, small anatase (8 nm) and rutile (13 nm) particles show bimodal, at first second and then first order, LID in the lower and middle intensity range while large anatase and rutile particles show simple first-order LID in the whole intensity range. This is attributable to two-electron (positive hole) transfer process in the initial step. An interesting feature is that forth-order LID was observed with small anatase particles at high (2.5 W cm-2) intensity to suggest shift of the kinetics to four-electron transfer process [1]. For oxygen reduction, LID analyses for the rate of oxidative decomposition of acetic acid by a bismuth tungstate photocatalyst suggested that two-electron oxygen reduction predominantly proceeds [2]. On the basis of these results, "effective particle size" defined as size of particle (or particle aggregates) in which multiple electrons (positive holes) are accumulated to induce multielectron processes is proposed. This "effective particle size" may govern the effect of size on photocatalytic reaction rates. [1] Takeuchi, S.; Takashima, M.; Takase, M.; Ohtani, B. Chem. Lett. 2018, 47, 373-376. [2] Hori, H.; Takashima, M.; Takase, M.; Ohtani, B. Catal. Today 2018, 313, 218-223.

(Invited) Modification By an Ag–Cr Core–Shell Cocatalyst for Highly Selective Photocatalytic Conversion of CO2 by H2o

A core−shell structure Ag−Cr dual cocatalyst loaded Ga2O3 significantly improved the formation rate of CO and selectivity toward CO evolution, compared with bare Ga2O3, Ag/Ga2O3, Cr/Ga2O3, and other Ag−metals/Ga2O3. The backward reaction tests, which produced CO2 from CO and O2 in H2O, indicated that the modification of Cr on the surface of Ag cocatalyst drastically suppressed the backward reaction for the photocatalytic conversion of CO2. This Ag−Cr dual cocatalyst modification strategy offers a facile approach for remarkably enhancing the photocatalytic efficiency for the conversion of CO2 by H2O.

Fast‐and‐Giant Photorheological Effect in a Liquid Crystal Dimer

AbstractMechanically responsive organic materials can change their viscoelastic properties in response to external stimuli. However, materials that exhibit highly contrasting viscoelastic properties coupled with fast and reversible switching between the states involved have remained elusive. Here, it is shown that a nonsymmetric photoresponsive liquid crystal dimer exhibits photoswitching of its viscoelastic properties (shear viscosity, storage, and loss moduli) with remarkable contrast of up to 106 while transitioning between crystal and nematic phases. This switching is reversible and takes less than 100 s for both forward (trans‐cis photoisomerization) and backward (cis‐trans photoisomerization) reactions due to the coexistence of two allotropes containing two types of stereoisomers. This combination of highly contrasting viscoelastic behavior with fast and reversible switching establishes a whole new performance level for mechanically responsive organic materials and offers very considerable application potential in such diverse areas as photoswitchable adhesives, in vibration control, and as novel brakes.

Analysis of catalytic heat transfer for a multi-species gas mixture

Stagnation point catalytic heat transfer model for a multi-species gas mixture has been developed in this work. The analysis extends the existing tertiary gas mixture catalytic model that assumes thermochemical equilibrium at the edge of a chemically frozen boundary layer. The catalytic heat transfer is computed numerically by solving the governing equations with matrix diagonalization and shooting methods. The present model is applied to compute the heat transfer for both tertiary gas mixture O-O2-Ar and quinary gas mixture O-O2-N-N2-Ar. For the case of the tertiary gas mixture, the present model predicts in the finite-catalytic region, the stagnation-point heat transfer due to recombination is similar with that of the existing models of Goulard and Park. In the highly-catalytic region, however, noticeable differences are observed owing to how the species mass fraction at the wall is computed. Unlike the Goulard approach treating the oxygen and nitrogen as a single recombination efficiency, in the case of the quinary gas mixture, the oxygen recombination efficiency needs to be decoupled with that of nitrogen. The present global model for the tertiary gas mixture is also compared with the Computational Fluid Dynamics (CFD) approach. The results suggest that the present model predicts a similar diffusive stagnation heat transfer to that of the CFD approach, within some acceptable range, when a zero backward reaction rate constant is assumed in the chemical reaction modeling.

Evolution of Catalysis: 3′‐Phospho adenosine 5′‐phosphosulfate synthase (PAPSS) and Methionine gamma Lyase deaminase (Mgld) as an Experimental and Computational Model System

Binding of substrate or ligand to a protein is the first common step between a receptor protein and an enzyme. However, the receptor protein usually bear low Kd, that is higher affinity towards the ligand. Whereas, many of the enzymes bear higher Km, that is relatively lower affinity. What is the criteria behind this distinction? The product, is the transformed substrate and it would have to leave the active site so that it can perform its function outside the binding pocket or it can be transformed further by a separate set (s) enzyme(s). Between this inter play, in a bifunctional enzyme, a product from one active site pocket is a substrate for the neighboring domain. Therefore, the product of domain [(1), ATP sulfurylase] is a substrate for the domain [(2) APS kinase], the new active site. This mandates that the active site 2 must have higher affinity (low Km) than that of the domain 1, so that it stays bound tighter and gets transformed into product 2. This would avoid futile cycle of substrate of domain 2 leaving back to be bound to domain 1 to become the substrate for the domain 1 backward reaction, or to occlude/inhibit. My data, from ATP sulfurylase (domain 1) and APS kinase (domain 2) of PAPSS kinetics proves the fundamentally perfected evolutionary concept. The computational data supports this concept as well. Afterall, nature rarely fails to achieve perfection and sophistication, especially after millions of years, as part of the evolutionary process. Certain enzymes like tryptophan synthetases have developed channeling as a means to perfect its overall efficiency of ultimate product formation. PAPSS is a good model system to address the binding energies, Km, Kcat. PAPSS is a standalone enzyme that binds to five different nucleotides ATP, AMP, ADP, APS, and PAPS. In addition, this enzyme binds to two different, a phospho and a sulfo nucleotides. Methionine gammalyase deaminase (Mgld) is a different example that use PLP as a coenzyme to assist catalysis. Mgld cleaves C‐S bond of methionine to form methylthiol and the PLP bound 2‐aminobutyrate is deaminated to form alpha‐keto butyrate. Here the active site is constrained and very little flexibility is observed. Thus, the evolution of enzyme active site catalysis will be discussed using PAPSS and Mgld systems.Support or Funding InformationN/AThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Blocking backward reaction on hydrogen evolution cocatalyst in a photosystem II hybrid Z-scheme water splitting system

Photocatalytic Z-scheme water splitting is considered as a promising approach to produce solar hydrogen. However, the forward hydrogen production reaction is often impeded by backward reactions. In the present study, in a photosystem II-integrated hybrid Z-scheme water splitting system, the backward hydrogen oxidation reaction was significantly suppressed by loading a PtCrOx cocatalyst on a ZrO2/TaON photocatalyst. Due to the weak chemisorption and activation of molecular hydrogen on PtCrOx, where Pt is stabilized in the oxidized forms, PtII and PtIV, hydrogen oxidation is inhibited. However, it is remarkably well-catalyzed by the metallic Pt cocatalyst, thereby rapidly consuming the produced hydrogen. This work describes an approach to inhibit the backward reaction in the photosystem II-integrated hybrid Z-scheme water splitting system using Fe(CN)63−/Fe(CN)64− redox couple as an electron shuttle.

Chemical kinetics of a two-step transesterification of dyacrodes edulis seed oil using acid-alkali catalyst

A chemical kinetic model for acid-alkali catalyzed methanolysis of high free fatty acid (FFA) underutilized African pear seed oil (APSO) is presented for the first time. A two-step: esterification followed by transesterification in a stirred batch reactor was carried out over 100 min, at 55–65 °C temperature range. Physico-chemical properties of the methyl ester was determined and discussed in accordance with relevant international methods and specifications. Composition of the reaction mixtures were determined using GC-FID and Karl Fischer titration methods. Integrated Gauss-Jordan elimination method on 4.03 GNU Octave software, 2016 version was efficient, simple and a fast approach in determining the rate constants. Rate constants of the forward and backward reactions of esterification were 0.732, 1.255, 2.031 and 0.549, 0.845, 1.421 L/g min respectively while those of the forward and backward reactions of the triglycerides, diglycerides and monoglycerides hydrolysis were 0.442, 0.0476 and 0.0493; 0.0476, 0.0851, and 0.1347; 0.1049, 0.2010 and 0.3319 and 0.0241, 0.0430 and 0.0597; 0.0113, 0.2172 and 0.2540; 0.0517, 0.1011 and 0.1570 L/g min respectively. Activation energy of the forward and backward esterification and transesterification reactions respectively were higher than the reverse reactions which indicates that the formation of free fatty acid (FFA), triglycerides and diglycerides were faster. Rates of reactions were found to increase with temperature and mixing rate (200, 400 and 800 rpm). The hydrolysis of monoglycerides to diglycerides, diglycerides to triglycerides and triglycerides to diglycerides at 55 °C, 60 °C and 65 °C respectively were ascertained as the rate determining steps.

Analysis of Experimental Data on CO and N2O Interaction with CO2 Production Based on the Results of DFT Calculations

The exchange process between carbon monoxide and nitrous oxide CO + N2O $$ \rightleftarrows $$ СO2 + N2 resulting in СO2 formation is studied. The methods of quantum mechanics are used to calculate transition states, vibration frequencies and reaction pathways. The direct process of oxygen atom transfer and a multiple-step process via several intermediate complexes are considered. In the framework of the transition state theory, the constants of the forward and backward reactions are calculated. A critical analysis of experimental data available in the literature is carried out based on the comparison with the results of calculations The recommended rate constants are presented in the general Arrhenius form for a wide range of temperatures: 300–2500 K.

Preparation and study of electrocatalytic activity of Ni-Pd(OH)2/C nanocomposite for hydrogen evolution reaction in alkaline solution

A new catalyst (Ni-Pd(OH)2/C) for hydrogen evolution reaction (HER) was prepared by coelectrodeposition of Pd(OH)2/C nanoparticles and Ni on a Cu substrate in two steps. Furthermore, the effect of Mo ions in alkaline solution (1 M NaOH) on the electrocatalytic activity of Ni-Pd(OH)2/C nanocomposite was studied as an in-situ activator for the HER. The various electrochemical methods were employed to study the HER activity of the investigated new catalyst, including linear sweep voltammetry (LSV), the steady-state polarization Tafel curves, electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA). The electrochemical measurements showed that the Ni-Pd(OH)2/C nanocomposite as a catalyst for the HER has an excellent catalytic activity with good stability in alkaline solution. Furthermore, the rate constants of the forward and backward reactions of Volmer and Heyrovský steps were estimated using Tafel-impedance data and revealed that the proton discharge electrosorption or Volmer reaction (k1= (6.8 ± 0.7) × 10−8 mol cm−2 s−1) was the rate determining step (RDS) of the HER on the surface of Ni-Pd(OH)2/C nanocomposite. Also, it was observed that the presence of Mo ions in alkaline solution could significantly increase the HER activity of Ni-Pd(OH)2/C nanocomposite. The comparison of RDS rate constant value with surface roughness (Rf) of Ni-Pd(OH)2/C catalyst showed that its high activity toward the HER originated from both increase in the surface roughness (∼20%) and increase in synergistic effect (∼80%).

The alkaline transition of cytochrome c revisited: Effects of electrostatic interactions and tyrosine nitration on the reaction dynamics

Here we investigated the effect of electrostatic interactions and of protein tyrosine nitration of mammalian cytochrome c on the dynamics of the so-called alkaline transition, a pH- and redox-triggered conformational change that implies replacement of the axial ligand Met80 by a Lys residue. Using a combination of electrochemical, time-resolved SERR spectroelectrochemical experiments and molecular dynamics simulations we showed that in all cases the reaction can be described in terms of a two steps minimal reaction mechanism consisting of deprotonation of a triggering group followed by ligand exchange. The pKaalk values of the transition are strongly modulated by these perturbations, with a drastic downshift upon nitration and an important upshift upon establishing electrostatic interactions with a negatively charged model surface. The value of pKaalk is determined by the interplay between the acidity of a triggering group and the kinetic constants for the forward and backward ligand exchange processes. Nitration of Tyr74 results in a change of the triggering group from Lys73 in WT Cyt to Tyr74 in the nitrated protein, which dominates the pKaalk downshift towards physiological values. Electrostatic interactions, on the other hand, result in strong acceleration of the backward ligand exchange reaction, which dominates the pKaalk upshift. The different physicochemical conditions found here to influence pKaalk are expected to vary depending on cellular conditions and subcellular localization of the protein, thus determining the existence of alternative conformations of Cyt in vivo.

Numerical simulation of a CO2, CO, O2, O, C mixture: Validation through comparisons with results obtained in a ground-based facility and thermochemical effects

Various issues of numerical simulation of high-enthalpy non-equilibrium flows of a gas mixture of CO2, CO, O2, O, and C are discussed. Navier–Stokes equations are considered within the framework of one-, two-, and three-temperature approximations with allowance for chemical reactions. In the one-temperature approximation, chemical reactions are modelled by the Arrhenius law. In the multi-temperature approximations, simulations are performed with the use of the Treanor–Marrone model for carbon dioxide dissociation and the Park model for dissociation of diatomic gas molecules, which take into account vibrational non-equilibrium of molecules. The Arrhenius law is also used for calculating recombination reactions and also forward and backward exchange reactions. Translational-vibrational and vibrational-vibrational energy exchange is simulated by a simplified Landau–Teller model. Some examples of flows with the use of the above-mentioned approximations are calculated. It is demonstrated that the one-temperature approach ensures an adequate reproduction of experimental data on the shock standoff distance, heat flux, and pressure coefficient on the cone, which are obtained in a high-enthalpy wind tunnel. The calculation of the flow around a cylinder under Martian re-entry conditions reveals a significant effect of thermochemical non-equilibrium.

Reversibility of enzymatic reactions might limit biotransformation of organic micropollutants

Biotransformation of many organic micropollutants (OMPs) in sewage treatment plants is incomplete leading to their release into the environment. Recent findings suggest that thermodynamic aspects of the reaction as chemical equilibrium limit biotransformation, while kinetic parameters have a lower influence. Reversibility of enzymatic reactions might result in a chemical equilibrium between the OMP and the transformation product, thus impeding a total removal of the compound. To the best of our knowledge, no study has focused on proving the reversible action of enzymes towards OMPs so far. Therefore, we aimed at demonstrating this hypothesis through in vitro assays with bisphenol A (BPA) in the presence of kinase enzymes, namely acetate kinase and hexokinase, which are key enzymes in anaerobic processes. Results suggest that BPA is phosphorylated by acetate kinase and hexokinase in the presence of ATP (adenosine 5-triphosphate), but when the concentration of this co-substrate decreases and the enzymes loss their activity, the backward reaction occurs, revealing a reversible biotransformation mechanism. This information is particularly relevant to address new removal strategies, which up to now were mainly focused on modifying the kinetic parameters of the reaction.