Kinetics Of Hydrogenation Research Articles

H2 Plasma Reducing Ni Nanoparticles for Superior Catalysis on Hydrogen Sorption of MgH2

Highly fine, dispersive, and active catalysts are essential for lowering the operating temperature of MgH2, a promising high-capacity material for solid-state hydrogen storage. In this work, ultrafine Ni nanoparticles (2–6 nm) are synthesized from the precursor nickel acetylacetonate (C10H14NiO4) on the surface of MgH2 by H2 plasma reduction process, followed by further ball milling. The obtained composite could rapidly release more than 6.5 wt % H within 10 min at 275 °C. Even at a low temperature of 225 °C, up to 6 wt % H could be desorbed. The MgH2–Ni composite also exhibits excellent low-temperature hydrogenation kinetics and almost no capacity degradation over nine hydrogenation/dehydrogenation cycles. The significant improvement in the hydrogen-storage properties is attributed to the in situ formation of ultrafine and stable Mg2NiH0.3 nanocrystals during cycling. This work provides a convenient approach to synthesize ultrafine metal nanoparticles for catalytic applications in the field of high energy storage density hydride materials.

Mechanochemical assisted hydrogenation of Mg-CNTs-Ni:kinetics modeling and reaction mechanism

MgH2, as one of the most potential hydrogen storage materials, has received considerable attention. However, its development has been plagued by its slow hydriding kinetics. Mechanochemical milling is an effective and convenient method for producing MgH2@CNTs-Ni through a solid–gas reaction between Mg-CNTs-Ni and H2. However, the hydriding kinetics and the corresponding mechanism during the milling have not been mentioned. Thus, hydriding kinetics of Mg-CNTs-Ni was researched for the purpose of gaining insights into the reaction mechanism and obtaining the best milling parameters. Also, phase transition during the milling was analyzed using XRD and TEM. It is apparent that the Avrami-Erofeev (AE) model can well describe the mechanochemical process, which contains solid solution, phase transition and growth-controlled regime. Besides, kinetics of hydrogenation is limited by nucleation and growth of MgH2. By fitting with the Arrhenius equation, average apparent activation energy for the hydrogenation of Mg-CNTs-Ni is estimated to be 47.8 kJ mol−1.

Understanding Hydrogenation Chemistry at MgB2 Reactive Edges from Ab Initio Molecular Dynamics.

Solid-state hydrogen storage materials often operate via transient, multistep chemical reactions at complex interfaces that are difficult to capture. Here, we use direct ab initio molecular dynamics simulations at accelerated temperatures and hydrogen pressures to probe the hydrogenation chemistry of the candidate material MgB2 without a priori assumption of reaction pathways. Focusing on highly reactive (101̅0) edge planes where initial hydrogen attack is likely to occur, we track mechanistic steps toward the formation of hydrogen-saturated BH4- units and key chemical intermediates, involving H2 dissociation, generation of functionalities and molecular complexes containing BH2 and BH3 motifs, and B-B bond breaking. The genesis of higher-order boron clustering is also observed. Different charge states and chemical environments at the B-rich and Mg-rich edge planes are found to produce different chemical pathways and preferred speciation, with implications for overall hydrogenation kinetics. The reaction processes rely on B-H bond polarization and fluctuations between ionic and covalent character, which are critically enabled by the presence of Mg2+ cations in the nearby interphase region. Our results provide guidance for devising kinetic improvement strategies for MgB2-based hydrogen storage materials, while also providing a template for exploring chemical pathways in other solid-state energy storage reactions.

Role of Mn-substitution towards the enhanced hydrogen storage performance in FeTi

The role of Mn substitution in FeTi towards the hydrogenation kinetics and hydrogen storage capacity was investigated using a combination of experimental and theoretical tools. Pristine and Mn-substituted FeTi was produced by electro-deoxidation of oxide precursors, such as natural ilmenite, titania and manganese dioxide. The produced materials were evaluated for hydrogen storage capacity. Ab-initio density functional theory (DFT) calculations were performed to understand the thermodynamics and kinetics of hydrogen absorption in pristine and doped FeTi. DFT calculations demonstrate that although thermodynamic and kinetic parameters of hydrogen absorption with Mn substitution is similar to those in the pristine FeTi, oxygen affinity of Mn at the surface is higher than Fe or Ti. We conclude that Mn acts as a sacrificial oxidizing element and oxidizes more readily at the surface over Fe or Ti, resulting in easy activation of the FeTi alloy. We show superior cyclic-hydrogen absorption behavior in bulk in FeTi with Mn substitution. After 20 charge-discharge cycles, the measured hydrogen storage capacity of FeTi with Mn substitution was steady ∼120 mA h/g (0.8 wt %), which is noticeably higher than that of pristine FeTi. The experimental and theoretical results shows that in case of a practical hydrogen storage scenario, Mn substitution will benefit in reducing absorption fatigue in FeTi. Further, most likely it may not be possible to use pristine FeTi phase.

Study on hydrogen storage property of (ZrTiVFe) Al high-entropy alloys by modifying Al content

(ZrTiVFe)xAly high-entropy alloys are potential hydrogen storage materials because of their intermediate properties of high hydrogen uptake capacity and fast kinetics. In this study, equimolar and non-equimolar (ZrTiVFe)80Al20 and (ZrTiVFe)90Al10 alloys were prepared, and the effect of Al content on the microstructure, element distribution, and hydrogen storage properties of (ZrTiVFe)xAly alloys were investigated. The results show that both alloys are composed of C14 Laves phase, a small amount of tetragonal and HCP phases. With the increase of Al content, the content and the size of C14 Laves phase decrease, the V element content in C14 Laves phase also decreases, which is resulted from the contents of Zr, V, and Fe element constituting the C14 Laves phase decrease. The Ti element can combine with the excessive Al to form a tetragonal phase around the C14 phase, and the growth of the tetragonal phase causes the refinement of the C14 phase. The (ZrTiVFe)90Al10 alloy absorbed 1.3 wt. % H at room temperature, which indicates the better hydrogenation capacity and kinetics. The improvement of hydrogen storage property is resulted from the increased C14 Laves phase, more V element in C14 Laves phase and the severe lattice distortion of (ZrTiVFe)90Al10 alloy. It is found that the (ZrTiVFe)80Al20 alloy can be activated easily with only three cycles, which is caused by the refined Laves phase. After hydrogenated, H atoms are dissolved into the lattice space of C14 Laves phase for both alloys, and crystalline structure is not changed.

Kinetic modelling of the methanol synthesis from CO2 and H2 over a CuO/CeO2/ZrO2 catalyst: The role of CO2 and CO hydrogenation

This work addresses the kinetics of the CO2 hydrogenation to methanol over a Cu/CeO2/ZrO2 catalyst studied using single-site, dual-sites and three adsorption sites kinetic models. Physicochemical constraints and statistical indicators are used as tool for model discrimination. The best performing model is used to elucidate the reaction mechanism and the relative roles of the Cu-sites and oxygen vacancies. The results show that the dissociative adsorption of H2 occurs on the Cu0 sites, while CO2 is attracted to the oxygen vacancies created by the CeO2-ZrO2 solid solution. Then, the adsorbed H interacts preferentially with the carbon atom, favouring the so-called “formate” route. The CO formed via the r-WGS reaction could either desorb to the gas phase or react via hydrogenation to methanol. Analysis of the relative contributions of the CO2 and CO hydrogenation (i.e. direct and indirect pathways, respectively) to the methanol synthesis reveals that the latter is in fact preferential at high temperatures (i.e. about 100% of methanol is produced from CO at 260 ⁰C and 30 bar), and it shows an optimum vs the H2:CO2 ratio (c.a. 7 at 200 ⁰C and 30 bar), which corresponds to the saturation of the Cu0 sites with H2. Thus, this work provides an essential tool (i.e., kinetic model) for the design of reactors and processes based on novel catalysts, and importantly, it offers a deeper understanding of the reaction mechanism as basis for further catalyst development.

Hydrogenation kinetics of halogenated nitroaromatics over Pt/C in a continuous Micro-packed bed reactor

Halogenated aromatic anilines are series of crucial intermediates for the synthesis of pigments, medicines and pesticides in the fine chemistry. However, efficient inhibition of undesired dehalogenation is difficult to achieve in the commonly used batch reactors. Although employing continuous flow technology and modified catalysts could alleviate dehalogenation, obtaining high selectivity of halogenated aromatic anilines especially for brominated aromatic amines and iodine aromatic amines is still challenging. In this study, a continuous flow system based on micro-packed bed reactor was developed and a kinetic model of the hydrogenation of halogenated nitrobenzene was established. Kinetic parameters for the reductions of p-chloronitrobenzene, p-bromonitrobenzene and p-iodonitrobenzene were acquired and the kinetic model exhibited good fit for the transformation of each substance. Moreover, the accuracy of kinetic model was validated by the comparison with the values in literature. The obtained kinetic model offered helpful information for the process optimization of hydrogenation of halogenated nitroaromatics in continuous flow system.

De-hydrogenation/Rehydrogenation Properties and Reaction Mechanism of AmZn(NH2)n-2nLiH Systems (A = Li, K, Na, and Rb)

With the aim to find suitable hydrogen storage materials for stationary and mobile applications, multi-cation amide-based systems have attracted considerable attention, due to their unique hydrogenation kinetics. In this work, AmZn(NH2)n (with A = Li, K, Na, and Rb) were synthesized via an ammonothermal method. The synthesized phases were mixed via ball milling with LiH to form the systems AmZn(NH2)n-2nLiH (with m = 2, 4 and n = 4, 6), as well as Na2Zn(NH2)4∙0.5NH3-8LiH. The hydrogen storage properties of the obtained materials were investigated via a combination of calorimetric, spectroscopic, and diffraction methods. As a result of the performed analyses, Rb2Zn(NH2)4-8LiH appears as the most appealing system. This composite, after de-hydrogenation, can be fully rehydrogenated within 30 s at a temperature between 190 °C and 200 °C under a pressure of 50 bar of hydrogen.

Double-Win Properties Response of Mechanical and Hydrogen Absorption in Melt-Spun Ti-Zr-Ni-Cr Amorphous Metals

The effect of spinning rates on mechanical properties and hydrogen absorption/desorption properties of Ti47Zr31Ni14Cr8 amorphous ribbons have been investigated in the present work. A fully amorphous structure was confirmed by x-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis of the ribbons obtained from spinning rates of 30 m s−1 to 45 m s−1. The uniformity of amorphous ribbons and their mechanical properties were improved with the increase in the spinning rate. Scanning electron microscopy (SEM) revealed that the fracture surface of amorphous ribbons had a cleavage feature and vein-like pattern when the spinning rates were 30 m s−1 and 45 m s−1, respectively. Because of the influence of flow units on the kinetic process of hydrogen absorption, the hydrogenation kinetics and hydrogen desorption capacity of amorphous ribbons were enhanced with the increased spinning rates. After the amorphous ribbons absorbed a large amount of hydrogen, ZrH2, TiH2, and (ZrTi)2Ni7 crystalline phases were formed from the amorphous matrix. Hydrogen promotes amorphous phase decomposition and the crystallinity of the new phases led to deterioration of the mechanical properties.

Effect of hypo-stoichiometry on microstructure and hydrogenation behaviors of multiphase ZrTi0.1V2-x alloys

ZrTi0.1V2-x (x = 0, 0.2, 0.4) alloys are designed to investigate the effect of hypo-stoichiometry on hydrogenation properties of Laves phase dominated multiphase alloys. The samples are synthetized by arc melting and homogenizing annealing treatment. Phase abundances and lattice parameters are studied by XRD and Rietveld refinement. The morphology of each phase is confirmed by SEM and EDS, hydrogenation kinetics, pressure-composition-temperature characteristics and hydrides have been studied. Annealing increases the content of Laves phase and Zr3V3O, while decreases that of V-BCC and α-Zr. The decrease in atomic ratio B/A, i.e. V/(Zr + Ti), results in the increase of Zr3V3O, the decrease of Laves and V-BCC, and the lattice expansion for each phase. Alloys show easy activation and fast hydrogenation kinetics, which is controlled by a different process. Hydrogen absorption capacity and formation enthalpy increase while equilibrium pressure decreases with the decrease in B/A ratio, which could be related to the decrease of V-BCC phase and lattice expansion for each phase. Zr8V16H36.29 is the dominated hydride after saturated hydrogenation, the proportion of hydrides and their hydrogen mass fraction accord with the hydrogen absorption behavior. Among the samples, ZrTi0.1V1.6 shows the best overall properties for rapid and massive hydrogen absorption at low pressure.

Catalyzed LiBH4 Hydrogen Storage System with In Situ Introduced Li3BO3 and V for Enhanced Dehydrogenation and Hydrogenation Kinetics as Well as High Cycling Stability

Catalyzed LiBH₄ Hydrogen Storage System with <i>In Situ</i> Introduced Li₃BO₃ and V for Enhanced Dehydrogenation and Hydrogenation Kinetics as Well as High Cycling Stability

Effect of PdCl2 catalyst on the hydrogenation properties and sorption kinetics of Mg

Slow hydrogenation kinetics and high de/absorption temperature of Mg/MgH2 still a challenge in solid state hydrogen storage systems for industrial and automobile applications. The effect of PdCl2 on the hydrogenation properties including kinetics of Mg/MgH2 has been investigated in the present study. The nanocomposites of Mg and PdCl2 were prepared using Ball Mill method with different concentrations of PdCl2. The synthesized nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), pressure-composition-isotherm (PCI) and differential scanning calorimetry (DSC). The result indicates that the maximum hydrogen storage capacity were achieved 6.15 wt% for Mg-5 wt% PdCl2 nanocomposites, while milled Mg only absorb 3.417 wt% H2 at 300 °C. The desorption kinetics not so much improved with PdCl2 concentration. It is concluded that the onset temperature of Mg-PdCl2 nanocomposites shifted towards lower side than milled Mg using DSC analysis. The SEM and XRD patterns confirm the phase identity maintain during ball milling. The new phases of PdH0.778, MgCl2, Mg6Pd and various phases of MgH2 in MgH2-PdCl2 nanocomposites were confirmed on the basis of XRD analysis and reported data of the sample.

On the First Hydrogenation Kinetics and Mechanisms of a Tife0.85cr0.15 Alloy Produced by Gas Atomization

On the First Hydrogenation Kinetics and Mechanisms of a Tife0.85cr0.15 Alloy Produced by Gas Atomization

On the First Hydrogenation Kinetics and Mechanisms of a Tife0.85cr0.15 Alloy Produced by Gas Atomization

On the First Hydrogenation Kinetics and Mechanisms of a Tife0.85cr0.15 Alloy Produced by Gas Atomization

ВЛИЯНИЕ СОСТАВА РАСТВОРИТЕЛЯ НА КИНЕТИКУ ГИДРОГЕНИЗАЦИИ 4-НИТРО-2'-ГИДРОКСИ-5'-МЕТИЛАЗОБЕНЗОЛА НА СКЕЛЕТНОМ НИКЕЛЕ В ВОДНЫХ РАСТВОРАХ 2-ПРОПАНОЛА

The article is devoted to the analysis of the kinetics of hydrogenation of 4-nitro-2'-hydroxy-5'-methylazobenzene on skeletal nickel in aqueous solutions of 2-propanol without additives and with additions of acetic acid and hydroxide. Elucidation of the effect of the solvent composition on the kinetics of transformations of compounds containing several reactive groups is a theoretical and practically significant problem. According to the data obtained, during the hydrogenation of 4-nitro-2'-hydroxy-5'-methylazobenzene on skeletal nickel, the introduction of an acid or base into an aqueous solution of 2-propanol of azeotropic composition leads to a change in the ratio of directions in the parallel-sequential scheme of transformations of the starting compound. With the introduction of acetic acid, the contribution of the direction due to the transformation of the azo group noticeably increases, while the introduction of sodium hydroxide completely suppresses this process and the addition of hydrogen in the first phase of the reaction is carried out exclusively at the nitro group in the starting 4-nitro-2'-hydroxy -5'-methylazobenzene. The change in the contributions of the directions associated with the transformation of the azo or nitro group in 4-nitro-2′-hydroxy-5′-methylazobenzene cannot be caused by a change in the excess adsorption values of the hydrogenated compound. When the adsorption values of 4-nitro-2'-hydroxy-5'-methylazobenzene change within 30%, the rate constants of hydrogenation of the starting compound under the influence of the solvent change more than 8 times. One of the reasons for such a change in the observed rate constants for the hydrogenation of 4-nitro-2'-hydroxy-5'-methylazobenzene may be a change in the ratio of forms of hydrogen adsorbed on the catalyst surface under the influence of a solvent.

Importance of thermal conductivity and stress level during a phase (hydride) transformation in magnesium

There are many different systems of an autonomous energy storage including accumulators and storage devices for renewable energy. Systems based on reversible metal hydrogenation have recently been introduced. The selection of metals is based on considerations of temperature and pressure conditions of the hydrogenation/dehydrogenation cycle, as well as the desired storage hydrogen capacity. Magnesium is one of the main challenging metals with respect to these main conditions since having a hydrogen capacity up to 7.6 w.%. For Mg forming MgH2, it was soon established that the size of particles plays a critical role since the kinetics (rate) of hydride formation accelerates when the size of the particles decreases. The present study shows that the overall diameter of the particles is the main characteristic controlling the kinetics of hydride formation because of distinct issues. A distribution of the size entails a strong dispersion transferring the heat of reaction, which characterizes Mg to MgH2 phase transition. Moreover, the formation of MgH2, is accompanied by a great increase of the unit-cell volume, developing noticeable internal stresses within the surface layers of the particles, thus turning to a systematic flaking and a systematic decrease of sizes of the powder particles. The results of the numerical modeling comply with the experimental data. This makes it possible to predict the best size of the initial Mg powder able to achieve fast kinetics during hydrogenation. Furthermore, the present analysis demonstrates the best hydrogenation kinetics, not only when using fine powders, but also when the deviation from the average particle size is minimized.

Kinetics of liquid-phase phenol hydrogenation enhanced by membrane dispersion

Cyclohexanone is an important chemical raw material for the manufacture of nylon 6 and nylon 66. Developing green technologies to produce cyclohexanone is urgent. Herein, a fixed-bed reactor enhanced by membrane dispersion was structured for liquid-phase phenol hydrogenation over Pd/Al2O3 to efficiently prepare cyclohexanone, and a macroscopic kinetics model was established for the first time by introducing the membrane parameters including the membrane area and membrane pore size. The reaction rate of phenol and solubility of hydrogen can be calculated based on the developed model. The theoretical values are in line with the experimental data within relative error of ±10%. More importantly, the model can be used to determine the optimal membrane pore size, membrane area and operating conditions under the premise of achieving the expected conversion, which is beneficial for the fixed-bed reactor enhanced by membrane dispersion in a continuous catalysis.

Remarkable catalytic effect of Ni and ZrO2 nanoparticles on the hydrogen sorption properties of MgH2

In the present study, the catalytic effect of Ni and ZrO2 nanoparticles on the hydrogen absorption and desorption properties of MgH2 has been investigated. The MgH2 nanocomposites were prepared by high-energy ball-milling. The morphology, phase structure, thermal behavior, and hydrogen storage properties of the materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), and the pressure-composition temperature (PCT) methods. ZrO2 and Ni nanoparticles were homogenously dispersed into the MgH2 matrix. The calculated apparent activation energy for dehydrogenation was 63.4 kJ/mol, which was decreased by 80.1 kJ/mol compared to that of as-milled MgH2. As a result, MgH2+5 wt.%Ni+5 wt.%ZrO2 demonstrated improved dehydrogenation and hydrogenation kinetics at 310 °C. The MgH2+5 wt.%Ni+5 wt.%ZrO2 sample released about 6.83 wt.% and absorbed about 6.10 wt.% in less than 30 min. Therefore, the co-catalysis of Ni and ZrO2 significantly enhances the hydrogenation and dehydrogenation properties of MgH2.

A method to evaluate hydrogen donation ability of hydrogen-storage solvent in hydrogenation process by radical-precursor compounds

Hydrogen storage using organic solvent has attracted worldwide attention. Such solvents, generally named hydrogen storage solvent (HS), are expected to release H2 but can also be used directly in hydrogenation reactions. Hydrogen donation capacity (QHS-H) and initial hydrogen donation temperature (Ti) of HSs are important parameters in hydrogenation reaction. This work develops a simple method to determine these two parameters using radical-precursors (RP) containing weak covalent bonds. The test is carried out in an ambient-pressure glass microreactor system. The idea is estimating QHS-H by the amount of hydrogen obtained by RPs (QRP+H) and determining Ti by the first-order kinetics of RP hydrogenation reaction. Reliability of the method is verified by evaluating QHS-H of the structure–known HSs, 9,10-dihydrophenanthrene (DHP), 9,10-dihydroanthracene (DHA) and tetralin (THN) with benzyl phenyl ether (BPE) and bibenzyl (BBZ) as the RPs. Results show that QRP+H is close to QHS-H, and the p-value of F-test is higher than 0.05 at a confidence level of 95%. The Ti can be determined by the first-order kinetics of BPE hydrogenation.

Atomically Dispersed Zinc(I) Active Sites to Accelerate Nitrogen Reduction Kinetics for Ammonia Electrosynthesis.

Developing highly active and stable nitrogen reduction reaction (NRR) catalysts for NH3 electrosynthesis remains challenging. Herein, an unusual NRR electrocatalyst is reported with a single Zn(I) site supported on hollow porous N-doped carbon nanofibers (Zn1 N-C). The Zn1 N-C nanofibers exhibit an outstanding NRR activity with a high NH3 yield rate of ≈16.1µg NH3 h-1 mgcat -1 at -0.3V and Faradaic efficiency (FE) of 11.8% in alkaline media, surpassing other previously reported carbon-based NRR electrocatalysts with transition metals atomically dispersed and nitrogen coordinated (TM-Nx ) sites. 15 N2 isotope labeling experiments confirm that the feeding nitrogen gas is the only nitrogen source in the production of NH3 . Structural characterization reveals that atomically dispersed Zn(I) sites with Zn-N4 moieties are likely the active sites, and the nearby graphitic N site synergistically facilitates the NRR process. In situ attenuated total reflectance-Fourier transform infrared measurement and theoretical calculation elucidate that the formation of initial *NNH intermediate is the rate-limiting step during the NH3 production. The graphitic N atoms adjacent to the tetracoordinate Zn-N4 moieties could significantly lower the energy barrier for this step to accelerate hydrogenation kinetics duing the NRR.