Study Of Reaction Mechanism Research Articles

Theoretical insight into the promotion effect of potassium additive on the water-gas shift reaction over low-coordinated Au catalysts.

How to elucidate the effect of alkali metal promoters on gold-catalyzed water-gas shift reaction intrinsically remains a challenging, because that the complex synergy effects such as strong metal-support interactions, interfacial effects, and charge transfer of supported metal catalysts makes people difficulty in the understanding the alkali promotion phenomenon in nature. Herein, we report a systematically study of whole water-gas shift reaction mechanism on pure and the K-modified defected-Au(211) (i.e., by removing one surface Au atom from perfect Au(211) and make one model with the Au-Au coordination number is six) by using the microkinetic modeling based on first principles. Our results indicate that the presence of K can increase the adsorption ability of oxygen-containing species via the attractive coulomb interaction, has no significant effect on the adsorption of H species, but inhibits the adsorption of CO due to the steric effect. K promoter stabilizes the water adsorption by ~0.3 eV, which results in one order increasing of whole reaction rate. Interestingly, the strong promotion effect of the K can be assigned to the significant direct space interaction between K and the adsorbate H2O* through the inducted electric field, which can be further confirmed by the posed negative electric field on the unpromoted D-Au(211). Microkinetic modeling results revealed that the carboxyl mechanism is the most likely to occur, redox mechanism is the next one, and the formate mechanism is the least likely to occur. For different kinds of alkali metal additives, the adsorption strength of water molecules gradually weakens from Li to Cs, but Na shows the best promoter behavior at the low temperature. By considering the effect of K contents on the reactivity of water-gas shift reaction, we found that the K with the medium coverage (~0.2~0.3 ML) has the strongest promoting effect. It is expected that the conclusion of this work can be extended to other WGSR catalytic systems like Cu(or Pt). All calculations were performed by using the plane-wave based periodic method implemented in Vienna ab initio simulation package (VASP, version 5.4.4), where the ionic cores are described by the projector augmented wave (PAW) method. The exchange and correlation energies were computed using the Perdew, Burke and Ernzerhof functional with the vdw correction (PBE-D3). The transition states (TSs) were searched using the climbing image nudged elastic band (CI-NEB) method. Some electronic structure properties like work function was predicated by the DS-PAW software. Microkinetic simulation was carried out using MKMCXX software.

Reaction Mechanism and Kinetics Study of Wheat Polysaccharide via Modified Reductor Technique in Bead Making Process

The reaction mechanism and kinetics investigation of the substitution of titanium in wheat flour polysaccharide (referred to here as titaniumation) in aqua medium during bead making process was carried out. The use of the Modified Reductor Technique was implored to study the reaction kinetics. The study was carried out at varying temperatures between 25 – 70oC. The study showed that the titaniumation of the wheat polysaccharide molecules increased with time and temperature. Hence, at 25oC, the degree of titaniumation (DT) of the ring increased from 0.41 % to 0.49 % and at elevated temperature of 70oC, DT increased from 0.58 to 0.94 % as time progressed from 30 to 210 mins, respectively, while keeping both amount of wheat polysaccharide and TiO2 constant, and thus achieving >1.9-fold. This also implies that the titaniumation of wheat polysaccharide using TiO2 is favourable at elevated temperature. The study was able to reveal that the reaction mechanism of the titaniumation of the polysaccharide molecules is an electrophilic substitution reaction, favouring the ortho position, while the kinetics study showed that the titaniumation reaction is of first order reaction.

Reaction mechanism study and modeling of thermal runaway inside a high nickel-based lithium-ion battery through component combination analysis

To diagnose and elucidate thermal runaway accompanying gas evolution of a lithium-ion battery, it is essential to understand the thermal side reactions that lead to thermal runaway inside a lithium-ion battery. It is very useful to make a reliable model that represents these reactions to analyze thermal runaway processes in order to secure battery safety and overcome high costs of large-scale experiments. This study proposes the reaction mechanism and the reaction model through the design of experiments with the combination of battery components such as a cathode, an anode, an electrolyte, and a separator. To develop the reaction mechanism, the peak temperature and calorific value of each reaction are obtained by using a differential scanning calorimeter. The change of mass and produced gas from each reaction are identified by using an online thermogravimetry-mass spectrometer. Based on these measurements, the reaction model is developed by estimating kinetic parameters obtained from the Kissinger analysis. The reaction model exhibits root-mean-square-error of 1.91 mW, 21.79 mW, and 4.53 mW in the electrolyte, the cathode and the anode, respectively, as compared to differential scanning calorimeter results, confirming its high fidelity. The proposed model illustrates the variation of volume fractions of each phase inside a lithium-ion battery to simulate electrochemical performance degradation during thermal runaway stage. The change in internal pressure is also evaluated by using the change in mass and volume of each phase. Based on the mechanism and model derived from this study, it is possible to pinpoint the electrochemical performance degradation and heat generation characteristics during thermal runaway.

Study of the Coupling Effect of Multiple Factors on the Photoreaction Process Based on a Standard Photoreactor

Accurate, stable, and reproducible photocatalytic reaction systems remain a major challenge when performing photocatalytic mechanism studies. In this paper, we designed and built a standard photocatalytic reactor to realize the precise adjustment of each parameter related to the photoreaction to meet the demand for the accurate study of photochemical reaction mechanisms and can realize the personalized settings for different photochemical reactions. Based on this standard reactor, the influence of each independent parameter on the conversion rate of the reaction was studied based on the model reaction of 9,10-diphenylanthracene oxidation, and a mathematical model of the coupling influence of multiple factors to the photochemical reaction was obtained using the dimensionless data processing method. The mathematical model can help to carry out the coordinated selection of various parameters of photochemical reactions, reduce the reaction cost, and improve the reaction efficiency. Furthermore, we demonstrate the simplicity and accuracy of this reactor in measuring and calculating the quantum efficiency of photochemical reactions. This work is of great guidance to exploring the basic principles of photochemical reactions, the selection of reaction parameters, and the construction of photochemical reactors.

Studies of NO Reduction with Ammonia Over Copper Species Deposited on MCM‐41 Nanospheres ‐ Experimental Evidence of Reaction Mechanism

AbstractNanospheres of MCM‐41 were modified with copper by template ion‐exchange (TIE) and cocondensation methods. In the samples modified by TIE, dispersion of copper was significantly improved by their post‐treatment in ammonia solutions. Highly dispersed copper species were significantly more active in low‐temperature NH3‐SCR than CuO aggregates. Copper dispersed on silica MCM‐41 catalytically operated at lower temperatures than copper deposited on silica‐alumina MCM‐41. The studies of reaction mechanism showed that ammonia and NOx species compete for these same adsorption sites (copper cations) and NH3 can remove NOx from such sites. Dispersed copper species are more active in NO to NO2 oxidation than CuO aggregates. Accumulation of oxidised NOx species in the absence of ammonia was more privileged on CuO aggregates. It is postulated that the main reaction pathway in the low‐temperature range is reaction of ammonia bounded to copper with NOx from gas phase or loosely bounded to the catalyst surface.

Fabrication and reaction mechanism study of Co(OH)F@Al nanowire arrays: A functional fluorine-containing metastable intermolecular composite

Metastable intermolecular composites (MICs) comprising solid fuels and oxidizers at nano-scale exhibit high energy densities and fast reaction speeds. As most metal and metalloid fuels are passivated by the natural oxide layer on the surface which acts as a reaction barrier, fluorine-containing oxidizers are of particular interest to promote the reactivity through interfacial fluoridation. In this work, cobalt hydroxy fluoride (Co(OH)F) was introduced as a functional fluorine-containing oxidizer, and core-shell structured Co(OH)F@Al nanoarray was fabricated onto a glass substrate. The thermal behavior and reaction mechanism of pure Co(OH)F and Co(OH)F@Al composites were thoroughly investigated, and Co(OH)F@Al was compared with CoO@Al and Co3O4@Al with identical morphology. In the Co(OH)F@Al composite, a pre-ignition reaction was observed which was attributed to the etching of Al2O3 shell by HF and the fluoridation of Al core by HF and CoF2. With the assistance of pre-ignition reaction, Co(OH)F@Al can complete main exothermal reaction before Al melting in a solid-state mechanism, and its peak temperature is around 200 ℃ lower than those of MICs employing CoO and Co3O4 as oxidizers.

A Mechanism Study of Redox Reactions of the Ruthenium-oxo-polypyridyl Complex.

Over the years, RuIV(bpy)2(py)(O)2+([RuIVO]2+) has garnered considerable interest owing to its extensive use as a polypyridine mono-oxygen complex. However, as the active-site Ru=O bond changes during the oxidation process, [RuIVO]2+ can be used to simulate the reactions of various high-priced metallic oxides. In order to elucidate the hydrogen element transfer process between the Ruthenium-oxo-polypyridyl complex and organic hydride donor, the current study reports on the synthesis of [RuIVO]2+, a polypyridine mono-oxygen complex, in addition to 1H and 3H (organic hydride compounds) and 1H derivative: 2. Through 1H-NMR analysis and thermodynamics- and kinetics-based assessments, we collected data on [RuIVO]2+ and two organic hydride donors and their corresponding intermediates and established a thermodynamic platform. It was confirmed that a one-step hydride transfer reaction between [RuIVO]2+ and these organic hydride donors occurs, and here, the advantages and nature of the new mechanism approach are revealed. Accordingly, these findings can considerably contribute to the better application of the compound in theoretical research and organic synthesis.

Recent Advances in Metal-Organic Framework-Based Nanomaterials for Electrocatalytic Nitrogen Reduction.

The production of ammonia under moderate conditions is of environmental and sustainable importance. The electrochemical nitrogen reduction reaction (E-NRR) method has been intensively investigated in the recent decades. Nowadays, the further development of E-NRR is largely hindered by the lack of competent electrocatalysts. Metal-organic frameworks (MOFs) are considered as the next-generation catalysts for E-NRR, featuring their tailorable structures, abundant active sites and favorable porosity. To present a comprehensive review on both the fundamental and advanced development in MOFs catalyst-based E-NRR field, this paper first introduces the basic principles of E-NRR, including the reaction mechanism, major apparatus components, performance criteria, and ammonia detection protocols. Next, the synthesis and characterization methods for MOFs and their derivatives are discussed. In addition, a reaction mechanism study via density functional theory calculations is also presented. After that, the recent advancement of MOF-based catalysts in the E-NRR field as well as the modification approaches on MOFs for E-NRR optimization is elaborated. Finally, the current challenges and outlook of MOF catalyst-based E-NRR field are emphasized.

Theoretical study of the nitrosation reaction mechanism of N-nitrosation reaction catalyzed by the cupin domain of SznF enzyme toward the modified L-Arg with methyl nitrogen deprotonation

The SznF enzyme provides a novel approach to synthesize the N-nitrosourea pharmacophore. In this work, the modified L-Arg with methyl nitrogen deprotonation(L-DHMAd), a not yet studied substrate’s reaction mechanism of the critical N-nitrosation reaction catalyzed by the cupin domain of SznF enzyme is investigated, through the combined quantum mechanics/molecular mechanics method. Basis on our calculation results, the optimal N-nitrosation reaction pathway can be divided in to three reaction steps:1) superoxo addition with accompanying of Cε-Nω bond cleavage; 2) O1-O2 bond heterolytic cleavage; 3) NMe-Nω bond coupling. The first reaction step is the rate-determining step of the entire optimal reaction with an energy barrier of 20.6 Kcal/mol. Significantly, Cε-Nω bond could be homolytic cleaved spontaneously during the Cε-O2 bond formation in the first reaction step. This appearance mainly attributes to the Cε-Nω single bond of the L-DHMAd, which could be more feasible to cleave to maintain the Cε sp2 hybridization within the superoxo addition. The subsequent reaction step is the O1-O2 bond heterolytic cleavage, which involves a minimum energy crossing point to form the stable intermediate with Fe(IV) = O1 species at quintet state. Finally, NMe and Nω of L-DHMAd are very energetic favorable to coupling to form the N-nitroso product. This is consist with the experimental observations that two nitrogen atoms of the N-nitroso product are both from the same arginine substrate. Our work could contribute to the deeper understanding of the N-nitrosation reaction catalyzed by SznF enzyme, and might enlighten further studies of biomimetic chemistry of SznF enzyme.

Theoretical study on the degradation mechanism, kinetics and toxicity for aqueous ozonation reaction of furan derivatives

The compounds including furan-2,5-dicarboxylic acid (FDCA), 2-methyl-3-furoic acid (MFA), and 2-furoic acid (FA), containing Furan ring are considered to be possessing high ozone reactivity, although in depth studies of their ozonation processes have not been carried out yet. Hence, mechanism, kinetics and toxicity by quantum chemical, and their structure activity relationship are being investigated in this study. Studies of reaction mechanisms revealed that during the ozonolysis of three furan derivatives containing C=C double bond, furan ring opening occurs. At temperature (298 K) and pressure of 1 atm, the degradations rates of 2.22 × 103 M−1 s−1 (FDCA), 5.81 × 106 M−1 s−1 (MFA) and 1.22 × 105 M−1 s−1 (FA) suggested that the reactivity order is: MFA > FA > FDCA. In the presence of water, oxygen and ozone, the Criegee intermediates (CIs) as the primary products of ozonation would produce lower molecule weight of aldehydes and carboxylic acids by undergoing degradation pathways. The aquatic toxicity reveals that three furan derivatives play green chemicals roles. Significantly, most of the degradation products are least harmful to organisms residing in the hydrosphere. The mutagenicity and developmental toxicity of FDCA is minimum as compared to FA and MFA, which shows the applicability of FDCA in a wider and broader field. Results of this study reveal its importance in the industrial sector and degradation experiments.

Unraveling the role of Co3O4 facet for photothermal catalytic oxidation of methanol via operando spectroscopy and theoretical investigation

Tubular, pie- and bread-shaped forms of Co3O4 with exposed {110}, {112} and {111} facets were prepared and compared in their photothermal catalytic performance and reaction pathways during the oxidation of methanol. Among them, the Co3O4 with exposed {110} facet exhibited the best photothermal catalytic performance (95% methanol conversion, 93% CO2 yield) under solar irradiation, while also maintaining good stability and moisture resistance. Reaction mechanism studies showed that the {110} facets had a strong adsorption capacity for formaldehyde, which facilitated its conversion to formate. The transformation of formaldehyde to formate species was the key step. The key step on the {110} facet was conversion of formaldehyde to a mono-dentate formate species, while conversion on the {112} and {111} facets was mainly to bi-dentate formate species. This study demonstrated that the design of preferential exposed crystal facet can regulate the pathway of photothermal catalytic reaction and realize efficient solar energy utilization.

In-depth study of kinetics, thermodynamics, and reaction mechanism of catalytic pyrolysis of disposable face mask using spent adsorbent based catalysts

Millions of face mask has been converted to waste since the onset of COVID-19 virus. Hence, present study explores the feasibility of converting disposable face masks to energy through catalytic pyrolysis process using a low-cost waste (spent aluminum hydroxide/oxide nanoparticle adsorbent) derived catalyst. Thermogravimetric analysis of the non-catalytic and catalytic pyrolysis of disposable face mask was conducted at varied heating rates of 10 °C/min, 20 °C/min, 30 °C/min, 40 °C/min, and 50 °C/min, respectively. Iso-conversional methods, Kissinger Akahira Sunose (KAS) and Ozawa Flynn Wall (OFW) were used for the kinetic study. The reaction mechanism was analyzed using Criado's z-master plot (CZMP) method along with the determination of thermodynamic parameters of the process. Results found that the addition of a catalyst to the process benefits the overall efficacy of the process by reducing the activation energy (Ea) (without catalyst; OFW-Ea: 188.7 kJ/mol, KAS-Ea: 186.2 kJ/mol) as well as lowering the disordered state of the process. Metal doped catalyst (Ni/ γ-Al2O3) (OFW-Ea: 168.4 kJ/mol, KAS-Ea: 167.8 kJ/mol) shows a larger reduction in activation energy in comparison to bare alumina (γ-Al2O3) (OFW-Ea: 183.2 kJ/mol, KAS-Ea: 180.4 kJ/mol). The current study presented disposable face masks as reclaimable in terms of energy and waste-derived catalyst as a potent solution to be explored in place of high-cost commercial catalysts.

Study of Reaction Mechanisms in α + 69Ga reaction at ≈10 – 50 MeV

The excitation functions of 69Ga(α, n)72As, 69Ga(α, 2n)71As, 69Ga(α, 3n)70As, 69Ga(α, x)69Ge, 69Ga(α, x)68Ga and 69Ga(α, x)67Ga reactions produced in the interaction of α-projectile with 69Ga-target were studied at ≈ 10-50 MeV. The produced nuclei were different isotopes of As, Ge, and Ga, some of which have important medical applications. The theoretical model predictions were based on the statistical code COMPLETE, and the predicted results were compared and discussed with existing experimental data. Good agreement between the theoretical predictions and experimental results were obtained. Pearson's relational statistics showed moderate to strong positive correlations between the theoretically predicted and experimentally measured reaction cross-sections. Furthermore, the present investigation revealed significant pre-compound contributions in the studied energy range. Therefore, it is important to consider the admixture of pre-equilibrium and equilibrium modes of reactions when predicting the reaction cross-sections.

A novel mechanism of B4C-TiB2-graphite composites with high flexural strength by spark plasma sintering

The B4C-TiB2-graphite composites with a high flexural strength of 915.4 ± 41.7 MPa was prepared by spark plasma sintering (SPS) at 1900 °C under 60 MPa for 10 min. The duplex structure reinforcement was formed with lamellar graphite and submicron TiB2 grains inter-embedded in each other. Likewise, the flexural strength and fracture toughness of the composites were significantly strengthened. Synthesis reaction mechanism studies showed that the sintering densification is promoted by the fast thermal diffusion of boron and carbon in SPS. The effect of the amount of TiB2 and graphite on the flexural strength of the composites was investigated, and the B4C-TiB2-graphite composites exhibit excellent combined mechanical properties at room temperature, (flexural strength of 915.4 ± 41.7 MPa, fracture toughness of 5.18 ± 0.40 MPa·m1/2, Vickers hardness of 32.14 ± 1.72 GPa). In addition, the strengthening mechanism revealed that strong internal stress and excellent dissipation ability of crack propagation energy of TiB2-graphite duplex structure made the composite with high flexural strength and fracture toughness.

Low-temperature synthesis of colloidal few-layer WTe2 nanostructures for electrochemical hydrogen evolution

High-quality transition metal tellurides, especially for WTe2, have been demonstrated to be necessarily synthesized under close environments and high temperatures, which are restricted by the low formation Gibbs free energy, thus limiting the electrochemical reaction mechanism and application studies. Here, we report a low-temperature colloidal synthesis of few-layer WTe2 nanostructures with lateral sizes around hundreds of nanometers, which could be tuned the aggregation state to obtain the nanoflowers or nanosheets by using different surfactant agents. The crystal phase and chemical composition of WTe2 nanostructures were analyzed by combining the characterization of X-ray diffraction and high-resolution transmission electron microscopy imaging and elements mapping. The as-synthesized WTe2 nanostructures and its hybrid catalysts were found to show an excellent HER performance with low overpotential and small Tafel slope. The carbon-based WTe2–GO and WTe2–CNT hybrid catalysts also have been synthesized by the similar strategy to study the electrochemical interface. The energy diagram and microreactor devices have been used to reveal the interface contribution to electrochemical performance, which shows the identical performance results with as-synthesized WTe2–carbon hybrid catalysts. These results summarize the interface design principle for semimetallic or metallic catalysts and also confirm the possible electrochemical applications of two-dimensional transition metal tellurides.

Thermocatalytic Decomposition of Dimethyl Methylphosphonate Based on CeO2 Catalysts with Different Morphologies

The catalytic performances of the catalysts and decomposition mechanisms of dimethyl methylphosphonate (DMMP), a commonly used nerve agent simulant, are well understood based on previous studies. However, the effects of the morphology of the catalyst on DMMP decomposition performance and mechanisms remain unexplored. Thus, in this work, experimental studies were conducted on the thermocatalytic decomposition of DMMP on CeO2 nanomaterials with different morphologies, e.g., irregular nanoparticles, nanorods, and nanocubes. From the performance evaluation, CeO2 nanorods exhibited higher DMMP thermocatalytic decomposition performance as compared to irregular nanoparticles and nanocubes. The primary reaction pathways were the same on all three morphologies of materials, according to in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study, whereas side reaction paths showed variable behaviors. According to the catalytic reaction mechanism study, the surface lattice oxygen played a vital role in the thermocatalytic decomposition of DMMP and the accumulation of phosphates, carbonates, and formates were the main factors for deactivation of the catalyst. The behavior of CeO2 catalyst with different morphologies in the thermocatalytic decomposition of DMMP was revealed in this work, and this will be useful for the future design of high-performing catalysts for the efficient degradation of chemical toxicant.

The effect mechanism of HCl on chromium removal by CaO sorbent

A combination of experiments and density functional theory (DFT) calculations was conducted to explore the effect mechanism of HCl on Cr adsorption over CaO surface. The experimental results suggested that CaO showed good removal performance for Cr. The best Cr adsorption efficiency was 74.51% at 1000 °C in N2. HCl had an adverse effect on Cr removal due to the competition with Cr for active sites and the promotion effect on Cr volatilization by HCl. O2 can offset the inhibitory effect of HCl on Cr capture by CaO with the coexistence of HCl. The XPS results suggested that Cr capture by CaO contained adsorption and oxidation. The combined action of HCl and O2 can further promote the oxidation of chromium on CaO. DFT calculations demonstrated that HCl, CrCl3, and CrO2Cl2 adsorptions on CaO (001) surface were chemisorptions with the adsorption energies of −170.32, −288.25 and −125.56 kJ/mol, respectively. CrCl3 (Cr(III)) was formed on CaO surface via a five-step reaction pathway (Cr → Cr(ads) → CrCl(ads) → CrCl2(ads) → CrCl3(ads) → CrCl3). CrCl2 formation was the rate-determining step, and the energy barrier was 160.81 kJ/mol. CrO2Cl2 (Cr(VI)) preferred to form via chlorination-oxidation process with four steps (Cr → Cr(ads) → CrCl2(ads) → CrO2Cl2(ads) → CrO2Cl2). The rate-determining step of CrO2Cl2 formation on CaO surface in the pathway was CrCl2(ads) → CrO2Cl2(ads). The activation energy barrier of CrO2Cl2 formation on CaO surface was 238.60 kJ/mol. This study of reaction mechanism contributed to deeply understand the effect of HCl on the transformation of chromium species over CaO surface.

Photorelease Reaction Mechanism Study of a Diarylethene Caged Dual Functions Compound

Femtosecond transient absorption, nanosecond transient absorption, and nanosecond resonance Raman spectroscopy techniques coupled with density functional theory calculations were performed to unravel the photocyclization and photorelease mechanisms of a diarylethene based compound (1o) containing two caged groups (OMe and OAc). Since the ground state parallel (P) conformer of 1o with a large dipole moment is stable in DMSO, the fs-TA transformations observed for 1o in DMSO mainly have contributions from the P conformer, which undergoes an intersystem crossing to generate a corresponding triplet state species. In a less polar solvent like 1,4-dioxane, in addition to the P path behavior of 1o, an antiparallel (AP) conformer can also take place to give a photocyclization reaction from the Franck-Condon state and finally give a deprotection via this pathway. This work provides a deeper understanding for these reactions, which not only facilitate the applications of diarylethene compounds but also help for the future design of functionalized diarylethene derivatives for particular applications.

Accelerating the photocatalytic degradation of toluene by unidirectional electron transfer via FRET strategy

Humanity's need for clean air has stimulated their research interest in the remediation of volatile organic compounds (VOCs). As an environmentally friendly pollution control technology, photocatalysis can easily generate oxidative free radicals for the mineralization of VOCs. Herein, we report a Förster resonance energy transfer (FRET)-inspired Ag/SrSO4/TiO2 photocatalyst with 2.5-fold higher activity for toluene degradation than pristine TiO2. The study of the catalyst reaction mechanism shows that the interlayer SrSO4 between the plasmonic Ag and the photocatalyst TiO2 is the key factor to guide the unidirectional transfer of electrons. A proper thickness of the insulator interlayer (whether SrSO4 or BaSO4) can reduce the recombination efficiency of photo-generated electrons and holes, resulting in an excellent toluene degradation ability. This finding demonstrates the excellent versatility of the catalyst system, which provides a novel direction for the modification of typical photocatalysts.

Experimental Studies of Reaction Mechanisms in Organometallic Chemistry and Catalysis

Experimental Studies of Reaction Mechanisms in Organometallic Chemistry and Catalysis