Molecule Reactions Research Articles

Recommendations for improving rigor and reproducibility in site specific characterization

Heterogeneous catalysis is driven by the interaction of reactant molecules and the catalyst surface. The locus of this interaction as well as the surrounding ensemble of atoms is referred to as the catalyst active site. Active site characterization attempts to distinguish active catalytic sites from inactive surface sites, to elucidate the structural and chemical nature of active sites, and to quantify active site concentration. Numerous techniques have been demonstrated to provide compositional and structural information about the active sites within a catalyst. However, each technique has its own limitations and experimental pitfalls that can lead to data misinterpretation or irreproducible results. This work aims to provide an overview of the types of data that can be collected, to outline common experimental challenges and how to avoid them, and to assemble relevant references for the most used active site characterization techniques. More broadly, we aim to outline best practices for researchers to collect, interpret, and report active site characterization data in a way that provides the most benefit to the broader catalysis community. Increasing the rigor and reproducibility of active site characterization offers a strategy to better link properties with catalytic performance and to enable the community to develop consensus concerning these relationships.

Comparison of Measurements for Enhanced Diffusion Problem in Chemical Reaction Systems

Comprehensive SummaryThe problem of molecular diffusion in the soup of chemical reactions attracts mounting interest across fields ranging from chemistry to biophysics to material science. Chemical reactions involve bond breakup and formation, whose time scale is typically on the orders of fs to ps, while molecular diffusion occurs at time scales of μs to ms. The two processes are often considered orthogonal, given the vastly different scales. The serial results show that the enzyme's diffusion is enhanced in a substrate‐dependent manner, which was further extended to small molecule reaction systems, challenging this classical paradigm. However, the results from different groups using different techniques do not quantitatively agree, and a general mechanism is yet to be understood. We summarize experimental studies on diffusion problems and seek to reconcile the interpretation with understanding the limits of measurement tools and the chemical nature of reaction systems. Understanding molecular diffusion in chemical reactions will provide fresh thoughts in designing chemical systems such as molecular machines that harvest work at the nanoscale in a controllable manner.

Effects of Hypoxia and Reoxygenation on Metabolic Profiles of Cardiomyocytes.

In vitro cellular models provide valuable insights into the adaptive biochemical mechanisms triggered by cells to cope with the stress situation induced by hypoxia and reoxygenation cycles. The first biological data generated in studies based on this micrometric life-scale has the potential to provide us a global overview about the main biochemical phenomena presented in some reported preconditioning therapies in life-scale of higher dimensions. Thus, in this study, a cell incubator was designed and manufactured to produce a cellular model of heart hypoxia followed by reoxygenation (HfR) through consecutive repetitions of hypoxia-normoxia gas exchange. Samples of cellular extracts and culture media were obtained from non-proliferative cardiomyocytes (CMs) cultivated under challenging HfR (stressed CMs) and regular cultivation (unstressed CMs) in rounds of four days for each case. Metabolomic based on proton magnetic resonance spectroscopy (1H-MRS) was used as an analytical approach to identify and quantify the metabolomes of these samples, the endo- and exo-metabolome. Despite the stressed CMs presented over 90% higher cellular death rate compared to the unstressed CMs, the metabolic profiles indicates that the surviving cells up-regulate their amino acid metabolism either by active protein degradation or by the consumption of culture media components to increase coenzyme A-dependent metabolic pathways. This cell auto-regulation mechanism could be well characterized in the first two days when the difference smears off under once the metabolomes become similar. The metabolic adaptations of stressed CMs identified the relevance of the cyclic oxidation/reduction reactions of nicotinamide adenine dinucleotide phosphate molecules, NADP+/NADPH, and the increased tricarboxylic acid cycle activity in an environment overloaded with such a powerful antioxidant agent to survive an extreme HfR challenge. Thus, the combination of cellular models based on CMs, investigative methods, such as metabolomic and 1H-MRS, and the instrumental development of hypoxia incubator shown in this work were able to provide the first biochemical evidences behind therapies of gaseous exchanges paving the way to future assays.

Decoupled Crystallization and Particle Growth of BiVO4 via Rapid Thermal Process for Enhanced Charge Separation

AbstractBiVO4 is one of the most promising candidates for photoelectrochemical water oxidation. However, the poor crystallinity and short hole diffusion length limit its charge separation. One bottleneck stems from the contradiction between high crystallinity and small particles via conventional furnace heating processes. This paper describes the design and fabrication of BiVO4 photoanodes via rapid thermal process (RTP), rather than furnace heating, to decouple the constraints between nucleation, crystallization, and growth processes of BiVO4. The higher heating ramp rate of RTP compared with furnace heating promotes the fast diffusion of reactant molecules, which elevates the nucleation rate above the particle growth rate of BiVO4, leading to small particles with high crystallinity. Moreover, the ultra‐high heating temperature makes it possible to crystallize the small BiVO4 particle within a short time. Thus, a high crystallinity can be obtained for the RTP‐treated BiVO4 while maintaining small particle size, achieving a charge separation efficiency of up to 82%, 30% higher than that of furnace‐treated BiVO4 photoanode.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives.

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Hierarchical molecular sieve-based Ce-Ru oxide for enhanced catalytic oxidation of chlorobenzene: Insight into the synergistic effect of Ce and Ru, the role of molecular sieve

In this work, a series of hierarchical molecular sieve were synthesized via adjusting the ratio of Si/Al. Then, Ce and Ru were supported on the molecular sieve and the loading amount of Ru were optimized. Catalytic evaluation showed that 5Ce0.5Ru/HZ-3 with the value Si/Al ratio of 80 performed the best catalytic activity of chlorobenzene oxidation with T50 and T90 values of 238 ℃ and 278 ℃, respectively and no chlorobenzene conversion can be seen on the 5Ce/HZ-3 and HZ-3, indicating that Ru was the active sites on 5Ce0.5Ru/HZ-3 and Ce played a promoter role in chlorobenzene oxidation. Catalytic stability showed that 5Ce0.5Ru/HZ-3 can maintain the complete conversion of chlorobenzene oxidation at 300°C for 190 h and the CO2 and HCl selectivity kept around 80% and 27%, respectively, while 5Ce0.5Ru performed worse catalytic stability and no HCl was detected. Subsequently, characterization results implied that HZ-3 with the Si/Al ratio of 80 had the biggest mesoporous volume of 0.5193 cm3·g−1 on the basis of micropore volume around 0.1236 cm3·g−1, which was beneficial to the diffusion of reactant molecule; Raman spectra implied that there is a strong interaction between Ce and Ru. H2-TPR and O2-TPD showed that 5Ce0.5Ru/HZ-3 possessed the biggest consumption of hydrogen (0.486 mmol·g−1), surface adsorbed oxygen (0.683 mmol·g−1) and surface lattice oxygen (0.626 mmol·g−1), which was conducive to the adsorption and dissociation of reactant molecule; XPS showed 5Ce0.5Ru/HZ-3 had the biggest molar percentage of Ru4+ species (79.50%) and Ce3+ species (44.14%), which can induce the generation of much more oxygen vacancies. Overall, the introduction of Ce can result in the improvement of physical-chemical properties of 5Ce0.5Ru/HZ-3. Finally, the possible reaction route was given based on the in-situ DRIFTS and TD/GC-MS result.

Ethanolamine gas sensors based on NdFeO3 modified hexagonal pyramid shaped ZnO nanocrystals

It is well known that two metal oxides can form depletion layers at the interface by constructing heterojunctions due to the different Fermi energy levels of each metal oxide, which can improve the carrier mobility and provide significant advantages for gas sensing. In this work, NdFeO3 nanoparticles were grown on ZnO with hexagonal cone structure by secondary hydrothermal method to successfully construct n-ZnO/p-NdFeO3 heterojunctions, and the crystal structures, oxygen vacancies, microstructures, optical properties, and gas-sensing properties of the ZnO/NdFeO3 composites were investigated. The ZnO/NdFeO3 sensors showed good ethanolamine (EA) The response characteristics of the ZnO/NdFeO3 sensor to ethanolamine (EA) with a concentration of 100 ppm EA at 240 °C (Rg/Ra) is about 25, which improves the response by a factor of 1.6 and 1.32 compared with that of NdFeO3 and ZnO. Meanwhile, it has excellent selectivity, good cyclic reproducibility, and durable long-term stability with response/recovery times of 19 s/49 s. The improved EA sensing performance of NdFeO3-modified ZnO, in addition to its large specific surface area due to its unique rough inhomogeneous structure, is also related to the higher concentration of oxygen vacancies at the ZnO/NdFeO3 interface after the composite. In addition, the construction of ZnO/NdFeO3 heterostructure brings more efficient carrier migration, which is favorable to the surface reaction and diffusion of EA molecules, and lowers the ZnO operating temperature, thus reducing the energy consumption. The ZnO/NdFeO3 composite sensor based on n-p heterojunction provides a valuable strategy for the detection of volatile organic compounds.

Single‐Carbon Atom Doping Reactions Using Atomic Carbon and Its Equivalents

AbstractAtomic carbon, a zerovalent atom with four non‐bonded electrons, has potential utility in organic synthesis as platforms for the formation of four new covalent bonds at the carbon center. Single‐carbon atom doping (SCAD) is the process of inserting an atomic carbon into a reactant molecule with no loss of atoms in the reactant. In this Concept, we overview SCAD reactions, which can be classified by physical or chemical methods.

Introduced gas regulating fast and highly exothermic reactions with in situ gas generation in microreactors

AbstractThe oxidation of cyclohexanol/cyclohexanone with nitric acid as a fast and highly exothermic process was hard to control at low molar ratios of oxidant to organic substrate even in microreactors. Process of runaway gas generation was observed due to the local hot spots, which decreased the selectivity of adipic acid. Additional gas was introduced into microreactors before the reaction, changing the previous wave annular flow to the uniform slug flow. Through the microscopic high‐speed photography and CFD simulation, it was found that the internal convection made the reactant molecules in the aqueous slug distribute evenly instead of gathering near the interface before, thus eliminating the local hot spots and stabilizing the hydrodynamic characteristics. The reaction performance was evaluated under different conditions, and a higher adipic acid selectivity of 93.6% was achieved at a molar ratio of 6.7 with the combination of microreactors and conventional reactors.

Tuning the interfacial interactions between alumina support and pseudo single atom platinum-tin catalytic sites for heavy naphtha reforming

Heavy naphtha reforming, a crucial downstream process in oil refineries for improving the octane number of the gasoline feedstock, requires expensive Pt metal catalysts and it accounts for 60% of the catalyst cost. These conventional Pt nanoparticle sizes range between 1 – 2 nm and thus, only ≈ 80% of the Pt atoms of the particle are available for the catalytic reactions. In this work, we hypothesized that it might be possible to achieve 100% atom utilization if the Pt atoms are distributed as single or pseudo-single atom clusters, where all the atoms are exposed to the reactant molecules and by tuning their Interfacial Interactions. We aimed to significantly reduce the Pt contents by dispersing the active Pt sites as pseudo-single atoms on industrially relevant γ-Al2O3 support. Interfacial Interactions and stability of Pt sites were then tuned by using tin (Sn) as a promoter. These catalysts were then evaluated for heavy naphtha reforming in a fixed-bed reactor. The catalyst with Sn/Pt mole ratio of 1.68 showed excellent performance with aromatics of 83 wt% and research octane number (RON) of ≈ 103. These results indicated our catalyst preparation methodologies and interfacial interaction tuning successfully disperse Pt on the spherical alumina support leading to better atom economy. With 30–50% less Pt content when compared to the industrially used catalyst, made the heavy naphtha reforming process economical and sustainable.

Surface control of Ni-Al2O3 dry reforming of methane catalyst by composition segregation

Catalyst surface control is of great importance considering that a catalytic reaction initially starts with surface atoms’ interaction with reactant molecules. In the present study, we synthesized the Ni-Al2O3 dry reforming of methane (DRM) catalyst via the spray-pyrolysis-assisted evaporation-induced self-assembly (EISA) method in order to systematically investigate catalyst surface change as controlled by composition segregation. The present results showed that segregation in the Ni-Al2O3 catalyst had successfully occurred by the post-annealing process and that the segregated Ni nanoparticles were located on the catalyst surface with simultaneous reduction to metal. The size of the surface Ni nanoparticles was highly dependent on the post-annealing temperature, whereas the electronic properties did not consistently align with the trend of particle-size growth, indicating that the particle-growth mechanism underwent alterations from segregation to sintering as the temperature was increased. In other words, when the reduction temperature for metal segregation is excessively high, particle-growth on the external catalyst surface, as induced by post-annealing, is primarily attributable to particle sintering rather than to metal segregation. This phenomenon directly results in decreased Ni surface density. The catalytic DRM reaction revealed that due to sintering, the catalytic performance did not align with the trend of Ni particle-size growth, and the conversion of CH4/CO2 was closely associated with Ni surface density. This information will offer valuable insight to future research focused on development of Ni-based catalysts for DRM reactions.

Reaction of NO molecule at 4H-SiC/SiO2 interface and its orientation dependence: a first-principles study

The reactions of NO molecules at the 4H-SiC/SiO2 interface on various plane orientations are theoretically investigated using density functional calculations to clarify the microscopic mechanisms of the NO post-oxidation annealing (POA) process. We find that the energy barriers for nitrogen incorporation reaction are smaller than those for nitrogen desorption irrespective of the plane orientation, indicating that N atoms are preferentially incorporated. However, on the Si-face we a find possible NO reaction process without CO molecule formation and the reaction with NCO molecule formation when the interface includes pre-incorporated N atoms. Owing to the reaction resulting in the formation of an NCO molecule, the incorporation of N atoms can be suppressed, which could be one of the possible origins for the N density saturation in the NO-POA. The calculated results suggest that not only the structural stability of the interface but also reaction kinetics are crucial for the incorporation of N atoms during the NO-POA.

Synergistic impacts of radiative flow of Maxwell fluid past a rotating disk with reactive conditions: An Arrhenius model analysis

This study presents an analysis of the hydromagnetic radiative flow of a Maxwell fluid past a rotating disk around its own axis with an angular velocity Ω0 which has enormous applications in metal casting (help to optimize the solidification and cooling of molten metals), plastic molding and aerospace engineering (cooling system of aircraft engines). The flow is constrained by boundaries that induce convection and chemical reactions described by the Arrhenius model. By applying similarity transformations, the non-linear partial differential equations, which describe the flow phenomenon, are metamorphosed into the non-dimensional system of ordinary differential equations (ODEs). The obtained ODEs are then solved numerically using ‘Keller box method’, an efficient computational technique, by implementing the finite-difference approach. The influence of different parameters, namely magnetic field strength, radiation parameter, reaction rate constant, and convective heat transfer coefficient, on the flow and heat/mass transfer characteristics are investigated. The findings suggest that the higher activation energy leads to a slower rate of reaction. As the reaction rate decreases, the consumption of reactant molecules and the reduction in their concentration also slow down. The influence of the convective heat transfer coefficient is also examined in terms of its effect on the thermal and concentration boundary layers. As the stretching parameter increases, the Maxwell fluid undergoes greater deformation, leading to greater strain and a consequent slowing of the fluid's radial velocity. Heat transfer is enhanced by thermal radiation and convective condition at the surface of rotating disk. The findings of this study provide valuable insights into the industries dealing with Maxwell fluids, enabling them to enhance process efficiency, optimize resource utilization, and develop sustainable and environmentally friendly solutions.

Structural design and particle size examination on NiO-CeO2 catalysts supported on 3D-printed carbon monoliths for CO2 methanation

3D-printed high-surface carbon monoliths have been fabricated and tested as catalyst supports of CO2 methanation active phases (NiO-CeO2, 12 wt% Ni). The carbon carriers show a developed microporosity and good adherence to the catalytic phases of NiO-CeO2, showing great stability and cyclability. Two monolith designs were used: a conventional parallel-channeled structure (honeycomb) and a complex 3D network of non-linear channels built upon interconnected circular sections (circles), where flow turbulences along the reactant gas path are spurred. The effect of the active phases particle size on the catalyst distribution and the overall performance has been assessed by comparing NiO-CeO2 nanoparticles of 7 nm average (Np), with a reference counterpart of uncontrolled structure (Ref). The improved radial gases diffusion in the circles monolith design is confirmed, and nanoparticles show enhanced CO2 methanation activity than the uncontrolled-size active phase at low temperatures (< 300 ºC). On the contrary, the Ref catalysts achieve higher CH4 production at higher temperatures, where the reaction kinetics is controlled by mass transfer limitations (T > 300 ºC). SEM and Hg porosimetry evidence that nanoparticles are deposited at deeper penetration through the narrow micropores of the carbon matrix of the monolithic supports, which tend to accumulate on the channels surface remaining more accessible to the reactant molecules. Altogether, this study examines the impact of the channel tortuosity and the active phase sizing on the CO2 methanation activity, serving as ground knowledge for the further rational and scalable fabrication of carbon monolith for catalytic applications.

Coupling and electronic synergistic effects of Fe/CeO2 composite to achieve high efficiency and selectivity for RWGS reaction

Transition metal iron, because of its high oxygen mobility, excellent thermal stability and low price, is often used in the study of RWGS reaction. However, the bottleneck problem of iron-based catalysts is the poor CO2 conversion rate. Herein, the Fe/CeO2 composite catalyst with Fe-O-Ce was successfully constructed, which has both high CO2 conversion and high CO selectivity for the low-temperature RWGS reaction. Furthermore, the influence of Fe0 contents is studied, and the result indicated that the CO2 conversion rate and CO selectivity are up to 50.05% and 100% at 500 ºC by the use of 20Fe/CeO2. Moreover, CeO2 is the best catalyst holder than the others. The physical and chemical properties of the catalysts are investigated by in-situ XPS, XRD, low-temperature N2 absorption-desorption (BET) and CO2-TPD. Nano zero-valent iron (Fe0) is the main active center for the RWGS reaction, which easily releases the outer electrons to activate reactant molecules. The introduction of the CeO2 is conducive to the high dispersion of Fe0 nanoparticles and the formation of abundant active centers. With the increase of Fe loading, the electron-rich Fe0 induces a strong electronic effect on the electron-deficient CeO2. The regularity of catalyst are changed by a large amount of zero-valent iron entering into the CeO2 lattice, then formed a lot of Fe-O-Ce solid solution. A large amount of zero-valent iron enter into the CeO2 lattice, inhibiting the grain growth and aggregation of monometallic species, and promoting nano Fe species highly dispersed on CeO2 carriers, leading more active centers exposure. Meanwhile, the abundant oxygen vacancy on xFe/CeO2 surface is advantage for the adsorption and activation of H2 and CO2 molecules. Therefore, the coupling and electronic synergistic effects of the active component Fe0 and the carrier CeO2 make xFe/CeO2 becoming a promising candidate catalyst for RWGS reaction.

Hollow Porous Structures for Improving the Sensitivity of Colorimetric Array Gas Sensors

AbstractDetecting volatile organic compounds (VOCs) in exhaled breath is a promising research area for disease diagnosis. Mass transfer behavior plays a crucial role in determining the detection performance of VOCs. However, the high mobility of exhaled breath poses a substantial challenge for the mass transfer process. Here, the distinct mass transfer behavior of mobile gaseous molecules is discovered by employing the schlieren imaging technique to visualize gas flow. The results demonstrate that hollow porous structures exhibit a higher gas flow flux than solid film and microporous structures, thereby generating a sufficient probability of contact reactions for mobile gaseous molecules. The proposed colorimetric array sensors offer enhanced sensitivity and stability for detecting mobile gaseous molecules. The discovery will serve as a foundational milestone is anticipated in elucidating the behavior of gas mass transfer and promoting its potential application in disease detection through the analysis of human exhaled breath.

Modeling of tubular membrane contactors for ozonation of water reveals reduced bromate formation with static mixers

Bromate formation and insufficient micropollutant degradation are two major problems in wastewater ozonation. The conventional process using bubble columns has the drawback of an inhomogeneous ozone distribution, causing increased bromate formation. Membrane contactors are a promising alternative as they can dissolve ozone bubble-free. This work presents a simulation of a tubular membrane contactor in COMSOL Multiphysics that includes flow, mass transfer, ozone decay, and reactions of organic molecules and bromide with ozone and hydroxyl radicals. Gaseous ozone and water flow on the lumen and shell side of the membrane, respectively. Static mixers are placed in the water channel, coaxial between the membrane and housing. The simulations show a 75% increase in ozone flux through the membrane, a homogenized ozone distribution, and 13 times higher degradation of a medium-oxidizable micropollutant in the presence of additional static mixers. Furthermore, static mixers enable the use of a lower gaseous ozone concentration, resulting in 31% lower bromate formation at the same ozone exposure without affecting micropollutant degradation. Overall, we demonstrate that membrane contactors with static mixers can be used as a strategy to reduce bromate formation.

Plasmon-Switched Kinetics for Formic Acid Dehydrogenation: Selective Adsorption Driven by Local Field and Hot Carriers.

Understanding illumination-mediated kinetics is essential for catalyst design in plasmon catalysis. Here we prepare Pd-based plasmonic catalysts with tunable electronic structures to reveal the underlying illumination-enhanced kinetic mechanisms for formic acid (HCOOH) dehydrogenation. We demonstrate a kinetic switch from a competitive Langmuir-Hinshelwood adsorption mode in dark to a non-competitive type under irradiation triggered by local field and hot carriers. Specifically, the electromagnetic field induces a spatial-temporal separation of dehydrogenation-favorable configurations of reactant molecule HCOOH and HCOO- due to their natural different polarities. Meanwhile, the generated energetic carriers can serve as active sites for selective molecular adsorption. The hot electrons act as adsorption sites for HCOOH, while holes prefer to adsorb HCOO- . Such unique non-competitive adsorption kinetics induced by plasmon effects serves as another typical characteristic of plasmonic catalysis that remarkably differs from thermocatalysis. This work unravels unique adsorption transformations and a kinetic switching driven by plasmon nonthermal effects.

Single-Molecule Catalysis in the Palladium Cross-Coupling Reaction Cycle.

Single-molecule (SM) methods are applied to study various types of catalytic processes in chemical and biochemical reactions. In this study, the Suzuki-Miyaura cross-coupling reaction forming a fluorescent product is investigated within the SM approximation. Stochastic analysis of emission intermittency in selected nanoscopic spots allows us to determine the single-molecule turnover frequency (SM-TOF) of the Pd catalyst in a specific probe reaction. We generate and analyze simulated intermittency time traces of a single catalyst surrounded by reactant molecules to assess the reliability of the method applied to real intermittency time trace data from hundreds of nanoscopic fluorescence spots. The results demonstrate that the proposed method can be used to evaluate the average SM-TOF of Pd in a cross-coupling reaction.

Photooxidation of organic compounds by mesoporous silica functionalized with rose bengal

AbstractThe reuse of waste and the use of renewable materials are of great environmental and economic interest. With that in mind, the synthesis of mesoporous silica can be performed using rice husk ash, which is an industrial waste generated in large quantities. This mesoporous material can be functionalized with photoactive molecules, enabling application in photocatalysis. In this work, we used mesoporous silica nanoparticles functionalized with rose bengal dye (RB) for photooxidation reactions of organic molecules, such as the antibiotic cefuroxime 1‐acetoxyethyl ester (cefuroxime axetil). The characterization results showed the formation of one‐dimensional channels with uniform size and two‐dimensional hexagonal arrangement, with surface area and pore diameter of 230 m2 g−1 and 5 nm, respectively. The infrared and UV‐Vis spectroscopy indicated the functionalization of silica with photoactive molecules. Finally, SBA‐15/RB was applied for the photooxidation of ascorbic acid and antibiotic cefuroxime axetil under irradiation with green light, proving to be efficient for the degradation of cefuroxime axetil. Investigation of the reactive oxygen species involved in photodegradation showed that the photooxidation mechanism mainly involves singlet oxygen.