Kinetics Of Oxidation Reaction Research Articles

Defective Photocathode: Fundamentals, Construction, and Catalytic Energy Conversion

AbstractThe development of a long‐term and sustainable energy economy is one of the most significant technological challenges facing humanity. Photoelectrochemical (PEC) technology is considered as the most attractive route for converting solar energy into chemical energy. However, the slow reaction kinetics of PEC oxidation and reduction greatly hinder its practical application. To address this issue, engineering photoelectrodes with various defects can significantly improve their catalytic performance, which can not only regulate catalyst electronic structure but also promote charge transfer/separation by serving as an active/adsorption/energy storage site. Herein, the defect engineering strategies for photoelectrodes are systematically summarized, focusing on the latest progress in defective photocathode for energy conversion. First, an overview of defect types, basic principles of photocathode, and the positive role of defects in the photocathode are provided. Second, the construction strategies and characterization methods of defective photocathode are summarized. Then, the progress of typical energy conversion applications, including hydrogen production, CO2 reduction, and nitrogen reduction over defective photocathode, is reviewed, highlighting the crucial role of defects in high catalytic performance. Finally, the challenges and future prospects of defective photocathode are discussed, aiming to bring new opportunities for the development of photocathode through defect engineering.

Short communication: The effects of ageing and storage environment on the oxidation response of uranium nitride (UN) powders

The oxidation behaviour of uranium nitride (UN) powder has been examined while allowed to age over a period of 129 days with both oxidation in synthesised air and high temperature water observed. The UN powder was stored in air and in an argon filled glove box (<3 ppm O2) to determine if the storage environment would affect the oxidation. Within this dataset, no significant change to the oxidation onset temperature or reaction kinetics was observed outside of measurement uncertainty with respect to the age of the powder.

Experiments and modeling of fluidized bed steam-biomass char gasification with a red-mud oxygen carrier at particle scale for chemical looping

Chemical Looping Combustion (CLC) of biomass provides a low-cost way for CO2 negative emission. It creates complex multi-component flows including solids of oxygen carrier and fuel, and gasses of steam and/or CO2 as well as other gasification products. Experiments and modeling of steam-gasification of biomass char were investigated using a low-cost red mud as oxygen carrier materials. Effects of temperature, steam concentration, H2 inhibition on chemical reaction kinetics and mass transfer were experimentally investigated and modeled. A self-designed macro-TGA was used to detect its oxidization kinetics under isothermal conditions. The red mud oxygen carrier reduced inhibitory effect of H2 on carbon reaction, thus enhancing char conversion remarkably. With the neglect of the external mass limitation on char conversion, the reaction kinetics during reduction process was calculated by an experimentally-fitted function, expressing the char conversion. Pore structure evolution during the char conversion on kinetics was considered and its function was obtained based on the experimental results. The radial gas profiles under the bed of red mud particles by the solution of interparticle and external mass transfer coupled with the kinetics of char gasification were obtained, revealing the whole process of char conversion in CLC process. A semi-empirical kinetic model, including both of chemical and diffusion processes, provided a good insight of oxidation reaction kinetics, which was proven to be better than the classic models. The experimental data and modeling results got high agreement in both reduction and oxidation reactions.

Modifying charge transfer between rhodium and ceria for boosted hydrogen oxidation reaction in alkaline electrolyte

Sluggish kinetics of hydrogen oxidation reaction (HOR) in alkaline solution has restricted the rapid development of hydrogen economy. Constructing catalyst with metal-oxide heterostructures can enhance HOR performance; however, little studies concentrate on charge transfer between them, and the corresponding effects on reactions remain unclear. Herein, we report charge-transfer-adjustable CeO2/Rh interfaces uniformly dispersed on multiwalled carbon nanotube (CNT), which exhibit excellent alkaline HOR performance. Results confirm that the charge transfer from Rh to CeO2 could be conveniently tuned via thermal treatment. Consequently, the adsorption free energies of H* in Rh sites and OH* adsorption strength in CeO2 could be adjusted, as corroborated by density functional theory study. The optimized CeO2/Rh interfaces exhibit an exchange current density and a mass-specific kinetic current of 0.53 mA cmPGM−2 and 830 A gPGM−1 at an overpotential of 50 mV, respectively, which surpasses most of the advanced noble-metal-based electrocatalysts. This work provides a new insight of harnessing charge transfer of heterostructure to enhance catalytic activities.

Activation of Bulk Li2S as Cathode Material for Lithium‐Sulfur Batteries through Organochalcogenide‐Based Redox Mediation Chemistry

AbstractLithium sulfide (Li2S) is considered as a promising cathode material for sulfur‐based batteries. However, its activation remains to be one of the key challenges against its commercialization. The extraction of Li+ from bulk Li2S has a high activation energy (Ea) barrier, which is fundamentally responsible for the initial large overpotential. Herein, a systematic investigation of accelerated bulk Li2S oxidation reaction kinetics was studied by using organochalcogenide‐based redox mediators, in which phenyl ditelluride (PDTe) can significantly reduce the Ea of Li2S and lower the initial charge potential. Simultaneously, it can alleviate the polysulfides shuttling effect by covalently anchoring the soluble polysulfides and converting them into insoluble lithium phenyl tellusulfides (PhTe‐SxLi, x&gt;1). This alters the redox pathway and accelerates the reaction kinetics of Li2S cathode. Consequently, the Li||Li2S‐PDTe cell shows excellent rate capability and enhanced cycling stability. The Si||Li2S‐PDTe full cell delivers a considerable capacity of 953.5 mAh g−1 at 0.2 C.

Activation of Bulk Li2S as Cathode Material for Lithium-Sulfur Batteries through Organochalcogenide-Based Redox Mediation Chemistry.

Lithium sulfide (Li2S) is considered as a promising cathode material for sulfur-based batteries. However, its activation remains to be one of the key challenges against its commercialization. The extraction of Li+ from bulk Li2S has a high activation energy (Ea) barrier, which is fundamentally responsible for the initial large overpotential. Herein, a systematic investigation of accelerated bulk Li2S oxidation reaction kinetics was studied by using organochalcogenide-based redox mediators, in which phenyl ditelluride (PDTe) can significantly reduce the Ea of Li2S and lower the initial charge potential. Simultaneously, it can alleviate the polysulfides shuttling effect by covalently anchoring the soluble polysulfides and converting them into insoluble lithium phenyl tellusulfides (PhTe-SxLi, x > 1). This alters the redox pathway and accelerates the reaction kinetics of Li2S cathode. Consequently, the Li||Li2S-PDTe cell shows excellent rate capability and enhanced cycling stability. The Si||Li2S-PDTe full cell delivers a considerable capacity of 953.5 mAh g-1 at 0.2 C.

Control of charge transfer direction by spatial engineering of redox active centers for boosting photocatalytic CO2 reduction

Photocatalytic CO2 reduction with H2O to value-added fuels is an advanced strategy to solve the energy crisis and realize carbon neutralization. Herein, the spatial engineering of redox-active sites on different facets of TiO2 was achieved by stepwise selective photo-deposition, which controls the transfer direction of photogenerated electrons and holes. The 2Cu/TiO2/2Co3O4 sample exhibited high photocatalytic CO2 reduction performance with the production of CO, CH4, CH3CH3, and CH2CH2 reached 9.41, 43.52, 5.39, and 1.41 μmol∙gcat-1∙h-1, respectively. Notably, the C2 products was generated over the 2Cu/TiO2/2Co3O4 composite in comparison with pure TiO2 under same reaction condition. The in-situ DRIFTS spectra identify the formation of critical intermediates (*CHO, *CH4, and CHO*CHO*) and reveal the reaction kinetics of H2O oxidation. Moreover, the DFT calculations combined with the in-situ spectral technique are applied to investigate the reaction pathways of photocatalytic CO2 reduction and C–C coupling. This work demonstrated that the spatial engineering of Cu and Co3O4 on TiO2 provides appropriate redox reaction sites to synergistically boost the photocatalytic CO2 reduction and C–C coupling.

Oxidation reaction kinetics of HTPB-boron carbide/polytetrafluoroethylene formulations as a solid fuel

A solid-fuel ramjet engine powered by boron carbide (B4C) can generate nearly the same amount of energy as boron. However, the inherent oxide layer on the B4C surface restricts the energy released during its oxidation. In this study, we examine the effect of polytetrafluoroethylene (PTFE) on the oxidation of B4C and the removal of the oxide layer during the oxidation reaction. The B4C/PTFE binary composite powder was prepared using a ball milling process at three different concentrations (5:20, 10:20, and 15:20). Hydroxyl-terminated polybutadiene (HTPB) loaded with binary composite powder was manufactured by vacuum casting technique. The pure HTPB and HTPB/PTFE fuels were manufactured as reference formulations. The B4C/PTFE binary composite powder was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), and high-resolution scanning electron microscope (HRSEM). The thermal oxidation characteristics of prepared fuel formulations were studied using the thermogravimetric technique. The kinetics of the oxidation reaction was studied using both isothermal and non-isothermal methods. The thermogravimetric curves showed that the oxidation reaction of HTPB-based fuel occurred in four steps. When B4C/PTFE binary composite powder is added to HTPB, it decomposes faster and at a higher rate. The decomposed fluorine species of PTFE significantly improved the oxidation reaction of composite powder and increased the energy release rate of B4C. The kinetic studies of oxidation suggested that the addition of B4C/PTFE binary composite powder into HTPB lowered the activation energy (Ea: S1 > S2 > S3 > S4 > S5) required to prompt the oxidation reaction. Based on thermal and kinetic results, a potential oxidation promotion mechanism of B4C/PTFE composite powder was proposed. The initial stages of B4C oxidation resulted in a glassy layer of B2O3 oxide forming at the interface of B4C and PTFE powder. As a result of the oxidation reaction between B4C/PTFE and O2, CF4, Trifluoroboron (BF3) and CO2 were generated. At high temperatures, the oxide shell ruptured, allowing the O2 to diffuse into the core of the B4C particle and releasing a large amount of heat energy.

Suppression of Undesired Losses in Organometal Halide Perovskite‐Based Photoanodes for Efficient Photoelectrochemical Water Splitting

Organometal halide perovskites (OHPs) have become potential candidates for high‐efficiency photoelectrodes for use in photoelectrochemical (PEC) water splitting. However, undesired losses, such as the non‐radiative recombination of photogenerated carriers and sluggish reaction kinetics of PEC water splitting, are the main limitations to achieving maximum efficiency for OHP‐based photoelectrodes. Herein, high‐efficiency OHP‐based photoanodes with a rational design that suppresses the undesired losses is reported. As a rational design for OHP‐based photoanodes, the defect‐passivated electron transport layers effectively suppress the undesired recombination of photogenerated carriers from the OHP layers. In addition, Fe‐doped Ni3S2 with a high catalytic activity promotes the reaction kinetics of PEC water oxidation, thereby suppressing the undesired losses at the interface between the OHP photoanodes and electrolytes. The fabricated Fe‐doped Ni3S2/Ni foil/OHP photoanodes exhibit a remarkable applied bias photon‐to‐current efficiency of 12.79%, which is the highest of the previously reported OHP‐based photoanodes by suppressing undesired losses. The strategies for achieving high‐efficiency OHP‐based photoanodes provide insights into the rational design of photoelectrodes based on OHPs.

Experimental Study on the Influence of Pore Structure and Group Evolution on Spontaneous Combustion Characteristics of Coal Samples of Different Sizes During Immersion.

To study the influence of water immersion on the evolution of the groups and spontaneous combustion characteristics of coal samples with different sizes, raw coal from the Fengshuigou Coal Mine operated by Pingzhuang Coal Company in Inner Mongolia was studied. The infrared structural parameters, combustion characteristic parameters, and oxidation reaction kinetics parameters of D1-D5 water immersion coal samples were tested, and the mechanism of spontaneous combustion during the oxidation of submerged crushed coal was investigated. The results were as follows. The water immersion process promoted the re-development of coal pore structure, and the micropore volume and average pore diameter were 1.87-2.58 and 1.02-1.13 times those of raw coal, respectively. The smaller the coal sample sizes, the more significant the change. At the same time, the water immersion process increased the contact point between the active group and oxygen in the coal, and the C=O, C-O, and -CH3/-CH2- groups in coal were further promoted to react with oxygen to generate -OH functional groups and improve the reactivity of coal. The characteristic temperature of water immersion coal was affected by the temperature rise rate, coal sample size, coal voidage, and other factors. Compared with the raw coal, the average activation energy of the water immersion coal with different sizes decreased by 12.4-19.7%, and the apparent activation energy of the coal sample with a size of 60-120 mesh was the lowest on the whole. In addition, the apparent activation energy in the low-temperature oxidation stage was significantly different.

Electronic Structure of Orthorhombic Bi4V2O11 and Role of Gap States in Photoelectrochemical Water Splitting

A group of bismuth vanadium oxides shows promise as an excellent material for photoanodes in water splitting. However, the efficiency of Bi4V2O11 in water splitting is not satisfactory compared to BiVO4. In this study, we investigated the electronic structure of orthorhombic Bi4V2O11 using density functional theory (DFT) and synchrotron soft X-ray spectroscopy techniques to clarify the causes of inefficiency. Our DFT calculations of the two-fold superlattice, as confirmed by near-edge X-ray absorption fine structure measurements, revealed its indirect-band-gap nature. An angle-resolved photoelectron spectroscopy study identified the states within the band gap, and the resonant photoemission technique and DFT simulation clarified that the origin of the gap states was anisotropic localization of electrons around V–O tetrahedra. Water absorption enhances the gap states, which dramatically impedes the kinetics of water oxidation reactions. Our results reveal the role of gap states in water oxidation reactions and offer new insights into manipulating gap states in 3d transition metal oxides.

PH-Responsive Flexible Copper Electrode with Switchable Wettability for Electrocatalytic Hydrogen/Oxygen Evolution Reactions and Urea Oxidation Reaction.

The development of a pH-responsive electrode with switchable wettability is of great importance in water electrolysis. Here, we designed a pH-responsive copper mesh/copolymer electrode to eliminate the adhesion of hydrogen/oxygen bubbles under high-speed water electrolysis by adjusting the wettability of the electrode surface. Furthermore, the kinetics of water oxidation and urea oxidation reactions on the as-prepared copper mesh/copolymer electrode were explored. Most noteworthy, the flexible water electrolysis performance of the as-prepared pH-responsive electrode was investigated for the first time. The results indicate that the copper mesh/copolymer electrode accelerates the hydrogen evolution reaction, oxygen evolution reaction, and urea oxidation reaction under better surface wettability, while it inhibits these reactions under poor surface wettability. The results provide insight into the development of unusual water electrolyzers with different pH electrolytes and the design of water electrolysis electrodes.

Boosting the Methanol Oxidation Reaction Activity of Pt-Ru Clusters via Resonance Energy Transfer.

The sluggish kinetics of the methanol oxidation reaction (MOR) with PtRu electrocatalyst severely hinder the commercialization of direct methanol fuel cells (DMFCs). The electronic structure of Pt is of significant importance for its catalytic activity. Herein, it is reported that low-cost fluorescent carbon dots (CDs) can regulate the behavior of the D-band center of Pt in PtRu clusters through resonance energy transfer (RET), resulting in a significant increase in the catalytic activity of the catalyst participating in methanol electrooxidation. For the first time, the bifunction of RET is used to provide unique strategy for fabrication of PtRu electrocatalysts, not only tunes the electronic structure of metals, but also provides an important role in anchoring metal clusters. Density functional theory calculations further prove that charge transfer between CDs and Pt promotes the dehydrogenation of methanol on PtRu catalysts and reduces the free energy barrier of the reaction associated with the oxidation of CO* to CO2 . This helps to improve the catalytic activity of the systems participating in MOR. The performance of the best sample is 2.76 times higher than that of commercial PtRu/C (213.0vs 76.99 ). The fabricated system can be potentially used for the efficient fabrication of DMFCs.

In situ topologically induced metastable phase Ni/r-Ni(OH)2@C heterostructures with abundant oxygen vacancies as efficient bifunctional electrocatalysts for energy-saving hydrogen production

To simultaneously accelerate both sluggish hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) kinetics, strongly coupling of transition metal hydroxides of high UOR activity with the transition metals of low hydrogen adsorption barrier for highly active HER is a promising strategy. Herein, a tractable hydrothermal protocol and a partial in-situ topological reduction strategy are developed to synthesize hcp Ni/r-Ni(OH)2 nanosheets containing abundant oxygen defects with an ultrathin carbon layer, which are utilized as the new bifunctional catalysts for both HER and UOR. As for HER, the surface electronic structure of Ni is optimized by introducing the oxygen defects coupling with hcp Ni with excellent electronic structure, which greatly enhances the dissociation ability of H2O and reduces the Ni-H bond strength, thus acquiring the optimized H* Gibbs free energy (∆GH*). As for UOR, abundant interfaces and the electron transfer between transition metal hydroxides/transition metal are favorable to regulate the urea adsorption. As expected, the hcp Ni/r-Ni(OH)2/C electrocatalyst shows good activity for HER (70.7 mV @ 10 mA cm−2), UOR (1.36 V @ 10 mA cm−2), and urea electrolysis (1.45 V @ 10 mA cm−2). This facile strategy provides a novel way to design high-performance electrocatalysts towards urea splitting, which can utilize to generate hydrogen as well as eliminate urea in water.

Dual modification of BiVO4 photoanode for synergistically boosting photoelectrochemical water splitting

Bismuth vanadate (BiVO4) is a promising nanomaterial for photoelectrochemical (PEC) water oxidation. However, the serious charge recombination and sluggish water oxidation kinetics limit its performance. Herein, an integrated photoanode was successfully constructed by modifying BiVO4 (BV) with In2O3 (In) layer and further decorating amorphous FeNi hydroxides (FeNi). The BV/In/FeNi photoanode exhibited a remarkable photocurrent density of 4.0 mA cm−2 at 1.23 VRHE, which is approximately 3.6 times larger than that of pure BV. And the water oxidation reaction kinetics has an over 200% increased. This improvement was mainly because the formation of BV/In heterojunction inhibited charge recombination, and the decoration of cocatalyst FeNi facilitated the water oxidation reaction kinetics and accelerated hole transfer to electrolyte. Our work provides another possible route to develop high-efficiency photoanodes for practical applications in solar conversion.

Copper and nickel-based oxides delicately tailored via galvanic reaction as electrocatalyst for solid-state direct urea fuel cell

The sluggish kinetics of urea oxidation reaction (UOR) critically restrict the prevalence of direct urea fuel cell (DUFC), utilizing electrolyte containing urea/urine. In this work, the copper and nickel-based composite oxides are prepared by meticulously controlling the galvanic reaction of copper ion to metallic nickel and the high-temperature oxidation process. Based on a systematic electrochemical characterization of the catalyst, the as-fabricated nano-architectures are optimized toward UOR performance, showing a low potential of 1.39 V vs. RHE to drive 100 mA cm−2 in alkaline solution. The button-like DUFCs configuration of as-developed catalysts exhibits a maximum power density of 1.19 mW cm−2. Furthermore, the DUFCs assembled with bio-urine, including men, dogs, cats, and rats as electrolytes without further purification, show a similar voltage. These findings open up a direction to convert bio-waste into treasure.

Enhancement of charge separation and hole utilization in a Ni2P2O7-Nd-BiVO4 photoanode for efficient photoelectrochemical water oxidation

It is necessary for photoelectrochemical (PEC) water splitting to reduce the electron-hole recombination rate and enhance the water oxidation reaction kinetics. Here, we prepared Ni2P2O7-Nd-BiVO4 composite photoanodes by coupling Ni2P2O7 co-catalysts to neodymium (Nd)-doped BiVO4 surfaces through photo-assisted electrodeposition. The Ni2P2O7-Nd-BiVO4 photoanode exhibits a high photocurrent density of 3.6 mA cm−2 at 1.23 V vs reversible hydrogen electrode (RHE), which is three times higher than that of the bare BiVO4 (1.2 mA cm−2). Detailed characterizations demonstrate that Nd doping reduces the band gap, significantly increases the carrier density and effectively reduces the charge transfer resistance. More importantly, the Ni2P2O7 co-catalyst has multiple roles. Specifically, it can act as a hole extraction layer to accelerate hole migration and inhibit hole-electron recombination. At the same time, it significantly improves the water oxidation reaction kinetics. In addition, it also provides more water oxidation active sites. This work provides ideas for the design and study of efficient BiVO4-based photoanodes.

Ultrafine PtRh-Co3O4 ternary alloy nanoparticles with enhanced electrocatalytic activity and long-term stability for alcohol electro-oxidation

To date, the development of structure-sensitive electrocatalysts is crucial, consisting of oxophilic metals and a conductive support with Pt-rich surfaces. In this study, PtRh-Co3O4 ternary alloy nanoparticles (∼2–3 nm) were uniformly distributed on carbon (PtRh-Co3O4/C) via a co-chemical reduction method. The chemical inertness and oxophilicity of Rh, along with the abundant oxygen defects of Co3O4, contributed to improving the kinetics of the methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) in PtRh-Co3O4/C by promoting the scission of C-C and C-H bonds. PtRh-Co3O4/C displayed high intrinsic activity for both MOR (6.8 mA/cm2Pt) and EOR (3.17 mA/cm2Pt) due to the strong electronic, ligand, and bifunctional effects. Even after 7000 potential cycles, it retained 81% (MOR) and 84% (EOR) of its initial value, indicating extended stability. Compared to other Pt-based benchmark catalysts, PtRh-Co3O4/C exhibited higher CO tolerance, extended activity, and stability, making it a promising electrocatalyst with competitive performance for alcohol electro-oxidation.

Coordination-driven self-assembly of MOF-based heterostructures for electrocatalytic methanol oxidation

Direct methanol fuel cells (DMFCs) are considered one of the most promising energy conversion devices due to their environmentally friendly and high-energy density features. However, the sluggish anode kinetics of the methanol oxidation reaction (MOR) acts as a bottleneck in the DMFCs system. Herein, a series of MOF-based heterogeneous electrocatalysts (NiWO4/MOF-74-x, Ni3V2O8/MOF-74-x) is constructed via a coordination-driven self-assembly strategy to enhance the catalytic property of the MOR. The heterogeneous interface between MOF-74 and Ni compounds is built by the Ni–O coordination interaction, which provides fast charge transfer and mass diffusion efficiency. Therefore, the optimal catalyst NiWO4/MOF-74 (3) exhibits excellent MOR activity and operation durability with a peak current density of 28.58 ​mA·cm-2. This work is expected to inspire the design of more efficient MOF-based heterogeneous electrocatalysts for MOR and other energy conversion applications.

Theoretical study on the reaction kinetics of CO oxidation by nitrogen-doped graphene catalysts with different ligand structures

Harmful carbon monoxide (CO) gas from vehicle exhaust and fossil fuel combustion seriously affects the ecological environment and human health, although this issue can be effectively solved by low-temperature catalytic combustion. Herein, an Fe and N codoped double vacancy graphene catalyst (Fe-CxNy) for the low-temperature catalytic combustion of CO is proposed, and the mechanism of the CO catalytic combustion reaction over the catalyst is systematically investigated by using density function theory (DFT). Four oxidation mechanisms, including Eley–Rideal (ER1 and ER2), Langmuir-Hinshelwood (LH) and Termolecular Eley–Rideal (TER), were compared in terms of oxidation pathways and energy distribution. All the results indicate that Fe-CxNy catalysts have efficient catalytic activity. The Fe-CN3 catalyst with the predominant LH reaction mechanism showed the best performance for the catalytic combustion of CO with only 0.20 eV, which was comparable to that of noble metals. The rate constant calculations further demonstrate that Fe-CN3 exhibits excellent catalytic performance at low temperatures. This work not only provides a theoretical basis for the design and development of low-temperature catalytic combustion CO catalysts, but also provides data support for the kinetic model of CO combustion reaction.