Kinetics Of Hydrogenation Research Articles

Approaching Theoretical Performances of Electrocatalytic Hydrogen Peroxide Generation by Cobalt‐Nitrogen Moieties

AbstractElectrocatalytic oxygen reduction reaction (ORR) has been intensively studied for environmentally benign applications. However, insufficient understanding of ORR 2 e−‐pathway mechanism at the atomic level inhibits rational design of catalysts with both high activity and selectivity, causing concerns including catalyst degradation due to Fenton reaction or poor efficiency of H2O2 electrosynthesis. Herein we show that the generally accepted ORR electrocatalyst design based on a Sabatier volcano plot argument optimises activity but is unable to account for the 2 e−‐pathway selectivity. Through electrochemical and operando spectroscopic studies on a series of CoNx/carbon nanotube hybrids, a construction‐driven approach based on an extended “dynamic active site saturation” model that aims to create the maximum number of 2 e− ORR sites by directing the secondary ORR electron transfer towards the 2 e− intermediate is proven to be attainable by manipulating O2 hydrogenation kinetics.

Approaching Theoretical Performances of Electrocatalytic Hydrogen Peroxide Generation by Cobalt-Nitrogen Moieties.

Electrocatalytic oxygen reduction reaction (ORR) has been intensively studied for environmentally benign applications. However, insufficient understanding of ORR 2 e- -pathway mechanism at the atomic level inhibits rational design of catalysts with both high activity and selectivity, causing concerns including catalyst degradation due to Fenton reaction or poor efficiency of H2 O2 electrosynthesis. Herein we show that the generally accepted ORR electrocatalyst design based on a Sabatier volcano plot argument optimises activity but is unable to account for the 2 e- -pathway selectivity. Through electrochemical and operando spectroscopic studies on a series of CoNx /carbon nanotube hybrids, a construction-driven approach based on an extended "dynamic active site saturation" model that aims to create the maximum number of 2 e- ORR sites by directing the secondary ORR electron transfer towards the 2 e- intermediate is proven to be attainable by manipulating O2 hydrogenation kinetics.

Kinetics of hydrogenation of dimethyl 1,4‐cyclohexanedicarboxylate to 1,4‐cyclohexanedimethanol

AbstractThe kinetics of dimethyl 1,4‐cyclohexanedicarboxylate (DMCD) hydrogenation, via the half‐hydrogenated product methyl 4‐(hydroxymethyl) cyclohexanecarboxylate (MHMCC), to 1,4‐cyclohexanedimethanol (CHDM) in gas phase was established over CuMnAl catalyst. The intrinsic kinetic experiments were carried out on a fixed‐bed reactor under a wide range of reaction conditions with temperature varied from 493 to 523 K, pressure varied from 4 to 6 MPa, and weight hourly space velocity of DMCD varied from 0.948 to 3.792 h−1. Eight rival possible two‐site LHHW models were proposed to simulate the experimental results. The model involving dissociative adsorption of esters and surface reaction as the rate‐determining step was found to fit the experimental data best. The rather large activation energy values (138.4 and 121.4 kJ·mol−1) of two reactions (DMCD to MHMCC and MHMCC to CHDM) suggest that the reactions are both temperature sensitive. The coefficients of determination (R2) for DMCD conversion and CHDM selectivity were 0.989 and 0.983 respectively. Statistical and thermodynamic test further verified the reliability of the results.

Influence of Ni interlayer on hydrogen absorption and cyclicity in Gd thin films

The Pd/Gd bilayers and Pd/Ni/Gd trilayers were prepared onto glass substrates by magnetron sputtering. Detailed XPS in-situ studies of the Pd/Gd bilayers allowed to estimate the thickness of the mixed layer at about 4 nm. Further studies showed that the additional Ni interlayer can significantly reduce the interface mixing during the deposition process, leading to the formation of a rather narrow Ni–Gd interface. The structure of the obtained layers before and after the hydrogenation was examined by the standard X-ray diffraction method. The hydrogenation kinetics was studied in-situ at room temperature and pressure up to 1 bar using simultaneous measurements of resistivity and optical transmittance. Furthermore, hydrogen absorption by electrochemical method was monitored using optical transmittance. It was found that the use of an additional 4 nm Ni layer significantly improved the cyclicity of the hydrogen absorption/desorption from the electrolyte, while maintaining low switching time (10 s).

Effect of temperature on particle shape, size, and polycrystallinity of Nd-Fe-B powders obtained by hydrogen decrepitation

This work presents a detailed study of hydrogen decrepitation (HD) to obtain monocrystalline Nd-Fe-B powder. The effect of decrepitation temperature has been investigated to optimize both particle size and shape. Differential scanning calorimetry was applied to analyze the hydrogenation kinetics of Nd2Fe14B and Nd-rich phases in the range of 25 to 300 °C. Thermogravimetry and X-ray diffraction allowed determining the hydrogen absorption of the whole alloy and the matrix phase, respectively. While scanning electron microscopy (SEM) was used to visualize particle shape and size, dynamic image analysis was applied to evaluate quantitatively these properties. The high monocrystallinity of the powder was confirmed by electron backscattering diffraction. The partial pressure of hydrogen required to initiate the hydrogenation reactions decreases when the temperature increases. The hydrogen absorbed by the whole alloy and, in particular, by the Nd2Fe14B phase decreases with temperature. Below 150 °C, the hydrogen absorbed by the Nd2Fe14B phase produces a significant transgranular cracking that is undesirable for particle shape. At 300 °C, the fast and limited absorption of hydrogen by the Nd-rich phase causes insufficient intergranular fracture and, hence, polycrystallinity. Between 150 and 300 °C, the controlled fragmentation resulted in monocrystalline particles with a more equiaxial shape, which is a suitable precursor to develop anisotropic ultrafine powders by the hydrogenation, disproportionation, desorption, recombination (HDDR) process.

Theoretical Study on the Catalytic CO2 Hydrogenation over the MOF-808-Encapsulated Single-Atom Metal Catalysts

The search for new catalytic agents for reducing excess CO2 in the atmosphere is a challenging but essential task. Due to the well-defined porous structures and unique physicochemical properties, metal–organic frameworks (MOFs) have been regarded as one of the promising materials in the catalytic conversion of CO2 into valuable platform chemicals. In particular, introducing the second metal (M) atom to form the MII–O–Zr4+ single-atom metal sites on the Zr nodes of MOF-808 would further greatly improve the catalytic performance. Herein, CO2 hydrogenation reaction mechanisms and kinetics over a series of MOF-808-encapsulated single-atom metal catalysts, i.e., MII–MOF-808 (MII = CuII, FeII, PtII, NiII, and PdII), were systematically studied using density functional theory calculations. First, it has been found that the stability for the encapsulation of a divalent metal ion follows the trend of PtII > NiII > PdII > CuII > FeII, while they all possess moderate anchoring stability on the MOF-808 with the Gibbs replacement energies ranging from −233.7 to −310.3 kcal/mol. Two plausible CO2 hydrogenation pathways on CuII–MOF-808 catalysts, i.e., formate and carboxyl routes, were studied. The formate route is more favorable, in which the H2COOH*-to-H2CO* step is kinetically the most relevant step over CuII–MOF-808. Using the energetic span model, the relative turnover frequencies of CO2 hydrogenation to various C1 products over MII–MOF-808 were calculated. The CuII–MOF-808 catalyst is found to be the most active catalyst among five MII–MOF-808 catalysts.

Thermodynamics modeling of ZrCo-H systems with biaxial compressive strain during desorption

As known, inferior cycling performance and deteriorated de-/hydrogenation kinetics resulting from by hydrogen-induced disproportionation severely impedes engineering applications of zirconium-cobalt (ZrCo) alloys used in the industry of hydrogen isotope storage. In this work, the first principles calculations are conducted to investigate the thermodynamic behavior of ZrCo hydrides with and without biaxial compressive strain during desorption. Importantly, the whole dehydrogenation process is divided into two steps due to the presence of intermediate ZrCoHx (x = 2, 2.25, 2.50, and 2.75). During Step I, the hydrided phase ZrCoH3 is decomposed into ZrCoHx (ZrCoH3 → ZrCoHx), and then the phase transformation from ZrCoHx to ZrCo (ZrCoHx → ZrCo) occurs in Step II. Afterwards, the equilibrium pressures, enthalpy changes, and entropy changes involved in the reactions are determined through the thermodynamic relations. The results demonstrate that compared to the second step, there is a superiority of dehydrogenation performance for the first step, which is derived from higher equilibrium pressure. Moreover, the equilibrium pressure increases with the decrease of the H content in ZrCoHx because of the intensification of vibrational degrees of freedom of H atoms. Further study concerns on the influence of biaxial strain (-3% - 0%) along [010] and [001] on thermodynamics properties of the ZrCo-H systems, indicating that the strain deteriorates the lattice stability and improves the equilibrium pressure. By contrast, it is more favorable to enhancing the dehydrogenation performance when the compressive strain is beyond 1.5%.

Effect of interfacial mixed layer on hydrogen absorption in yttrium thin films

Pd/Y bilayers and Pd/Ti/Y trilayers were prepared in a UHV chamber by DC/RF magnetron sputtering. The characterization of the Pd-Y and Ti-Y interfaces was performed immediately after the deposition using in-situ X-ray photoelectron spectroscopy (XPS). The results indicated significant interface mixing for the Pd/Y bilayers. Further studies showed that the additional Ti layer deposited between the palladium and yttrium layers can significantly reduce the mixing of the Pd-Y layers during the deposition process at room temperature, leading to the formation of a sharp Ti-Y interface. The structure of the obtained layers before and after the hydrogenation was examined by the standard X-ray diffraction method. The hydrogenation kinetics was studied in-situ at room temperature and pressure up to 1 bar using simultaneous measurements of resistivity and optical transmittance. It was found that for a thickness of Ti equal to 5 nm, the shortest transition time to the YH3 phase was obtained while maintaining a short switching time.

Dual tuning of the de-/hydrogenation thermodynamics and kinetics of the Mg–Ni alloy by introducing the Ag–H bond: enhanced hydrogen storage properties at moderate temperatures

The onset dehydrogenation temperature of Mg85Ni14.8Ag0.2 reduces to 178 °C due to the enhanced hydrogen pumping effect of Mg2NiH4 by adjusting the Ni–H bond.

Kinetics of greenhouse gas CO2 hydrogenation over K-promoted Cu/ZnO/Cr2O3 catalyst towards sustainable aviation fuel production

In sustainable aviation fuel development, the catalytic approach for CO2 hydrogenation to methanol can introduce side reactions that can significantly influence the process efficiency and final product quality. For process intensification, modeling tools are crucial to predicate and understand the reaction pathways to improve the operating conditions or catalyst design. Herein, we propose a new microkinetic modeling approach that combines the unity bond index−quadratic exponential potential (UBI-QEP) method and the Langmuir-Hinshelwood–Hougen-Watson (LHHW) technique to examine the conversion of CO2 and H2 to methanol on a Cu(1 1 1)-dominated catalyst. For the mechanism study, the heat of adsorption values for the reaction intermediates and the activation barrier for all the elementary steps are calculated by the UBI-QEP method. The estimation of the pre-exponential parameter is done by fitting the model to the experimental data. This procedure decreases the computational time and enhances the model’s accuracy by bypassing complex theoretical calculations. To validate the model, the synthesis of methanol from CO2 and H2 is conducted over a K-promoted Cu/ZnO/Cr2O3 catalyst, using a lab-scale plug-flow fixed-bed reactor (PF-FBR). The computational results suggest that the synthesis of methanol over Cu(1 1 1)-facets preferentially follows the formate pathway, with the hydrogenation of formate acting as a rate-determining step (RDS). The validated model (R2 = 0.99) predicts an activation energy (Ea) of 63.70 kJ.mol−1 for CO2 conversion over the studied catalyst. The proposed microkinetic model can serve as a reliable tool for elucidating the role of active sites that may control the performance of a CO2 hydrogenation catalyst.

Optical Tunability and Characterization of Mg-Al, Mg-Ti, and Mg-Ni Alloy Hydrides for Dynamic Color Switching Devices.

Mg shows great potential as a metal hydride for switchable optical response and hydrogen detection due to its ability to stably incorporate significant amounts of hydrogen into its lattice. However, this thermodynamic stability makes hydrogen removal difficult. By alloying Mg with secondary elements, the hydrogenation kinetics can be increased. Here, we report the dynamic optical, loading, and stress properties of three Mg alloy systems (Mg-Al, Mg-Ti, and Mg-Ni) and present several novel phenomena and three distinct device designs that can be achieved with them. We find that these materials all have large deviations in refractive index when exposed to H2 gas, with a wide range of potential properties in the hydride state. The magnitude and sign of the optical property change for each of the alloys are similar, but the differences have dramatic effects on device design. We show that Mg-Ti alloys perform well as both switchable windows and broadband switchable light absorbers, where Mg0.87Ti0.13 and Mg0.85Ti0.15 can achieve a 40% transmission change as a switchable window and a 55% absorption change as a switchable solar absorber. We also show how different alloys can be used for dynamically tunable color filters, where both the reflected and transmitted colors depend on the hydrogenation state. We demonstrate how small changes in the alloy composition (e.g., with Mg-Ni) can lead to dramatically different color responses upon hydrogenation (red-shifting vs blue-shifting of the resonance). Our results establish the potential for these Mg alloys in a variety of applications relating to hydrogen storage, detection, and optical devices, which are necessary for a future hydrogen economy.

Catalytic hydrogenation of p-nitroanisole over Raney nickel for p-aminoanisole synthesis: Intrinsic kinetics studies

The understanding of intrinsic kinetics was crucial in guiding reactor design. However, the intrinsic kinetics of catalytic hydrogenation of p-nitroanisole (PNA) to p-aminoanisole remains obscure. In this work, the kinetic characteristics were experimentally and theoretically investigated under various experimental conditions. From the power law analysis of the initial reaction rate, the reaction order was found to be from 0.34 (at 343 K) to 0.53 (at 353 K) for hydrogen and 0.31∼ 0.34 for PNA (at concentrations less than 90 g·L−1), while the reaction was considered to be zero-order under higher PNA concentration (>90 g·L−1). The data was also used to fit Eley-Rideal and Langmuir-Hinshelwood-Hougen-Watson (LHHW) models. It was found that the rate-controlling step was the adsorption of dual-site dissociative H2 based on LHHW model. The accuracy of the screened model was verified by correlation, between the predicted data and experimental data. This study could provide significant guidance for the hydrogenation reaction and reactor.

High-loading, ultrafine Ni nanoparticles dispersed on porous hollow carbon nanospheres for fast (de)hydrogenation kinetics of MgH2

Magnesium hydride (MgH2) is one of the most promising hydrogen storage materials for practical application due to its favorable reversibility, low cost and environmental benign; however, it suffers from high dehydrogenation temperature and slow sorption kinetics. Exploring proper catalysts with high and sustainable activity is extremely desired for substantially improving the hydrogen storage properties of MgH2. In this work, a composite catalyst with high-loading of ultrafine Ni nanoparticles (NPs) uniformly dispersed on porous hollow carbon nanospheres is developed, which shows superior catalytic activity towards the de-/hydrogenation of MgH2. With an addition of 5 wt% of the composite, which contains 90 wt% Ni NPs, the onset and peak dehydrogenation temperatures of MgH2 are lowered to 190 and 242 °C, respectively. 6.2 wt% H2 is rapidly released within 30 min at 250 °C. The amount of H2 that the dehydrogenation product can absorb at a low temperature of 150 °C in only 250 s is very close to the initial dehydrogenation value. A dehydrogenation capacity of 6.4 wt% remains after 50 cycles at a moderate cyclic regime, corresponding to a capacity retention of 94.1%. The Ni NPs are highly active, reacting with MgH2 and forming nanosized Mg2Ni/Mg2NiH4. They act as catalysts during hydrogen sorption cycling, and maintain a high dispersibility with the help of the dispersive role of the carbon substrate, leading to sustainably catalytic activity. The present work provides new insight into designing stable and highly active catalysts for promoting the (de)hydrogenation kinetics of MgH2.

Pairing d-Band Center of Metal Sites with π-Orbital of Alkynes for Efficient Electrocatalytic Alkyne Semi-Hydrogenation.

Electrocatalytic alkyne semi-hydrogenation has attracted ever-growing attention as a promising alternative to traditional thermocatalytic hydrogenation. However, the correlation between the structure of active sites and electrocatalytic performance still remains elusive. Herein, the energy difference (∆ε) between the d-band center of metal sites and π orbital of alkynes as a key descriptor for correlating the intrinsic electrocatalytic activity is reported. With two-dimensional conductive metal organic frameworks as the model electrocatalysts, theoretical and experimental investigations reveal that the decreased ∆ε induces the strengthened d-π orbitals interaction, which thus enhances acetylene π-adsorption and accelerates subsequent hydrogenation kinetics. As a result, Cu3 (HITP)2 featuring the smallest ∆ε (0.10eV) delivers the highest turnover frequency of 0.36s-1 , which is about 124 times higher than 2.9×10-3 s-1 for Co3 (HITP)2 with the largest ∆ε of 2.71eV. Meanwhile, Cu3 (HITP)2 presents a high ethylene partial current density of -124mAcm-2 and a large ethylene Faradaic efficiency of 99.3% at -0.9V versus RHE. This work will spark the rapid exploration of high-activity alkyne semi-hydrogenation catalysts.

Impact of hydronium ions on the Pd-catalyzed furfural hydrogenation

In aqueous mediums, the chemical environment for catalytic reactions is not only comprised of water molecules but also of corresponding ionized species, i.e., hydronium ions, which can impact the mechanism and kinetics of a reaction. Here we show that in aqueous-phase hydrogenation of furfural on Pd/C, increasing the hydronium ion activities by five orders of magnitude (from pH 7 to pH 1.6) leads to an increase of less than one order of magnitude in the reaction rate. Instead of a proton-coupled electron transfer pathway, our results show that a Langmuir-Hinshelwood mechanism describes the rate-limiting hydrogen addition step, where hydrogen atom adsorbed on Pd is transferred to the carbonyl C atom of the reactant. As such, the strength of hydrogen binding on Pd, which decreases with increasing hydronium ion concentration (i.e., 2 kJ molH2−1 per unit pH), is a decisive factor in hydrogenation kinetics (rate constant +270%). In comparison, furfural adsorption on Pd is pH-independent, maintaining a tilted geometry that favors hydrogen attack at the carbonyl group over the furan ring.

Role of Ni in PtNi Alloy for Modulating the Proton–Electron Transfer of Electrocatalytic Hydrogenation Revealed by the In Situ Raman–Rotating Disk Electrode Method

Electrocatalytic hydrogenation of nitrobenzene to aniline is of industrial and environmental importance, but it remains a great challenge due to the high-toxicity intermediates and low hydrogenation kinetics. Here, we design a PtNi alloy via Ni-doping, which favors the low-toxicity and low free-energy pathway. Further, we perform surface-enhanced Raman spectroscopy coupled to a rotating disk electrode to obtain direct spectroscopic evidence of intermediates during the electron transfer. Combined with the kinetic isotope effect and pH dependence results, it has been shown that Ni-doping alters the structural properties to weaken the adsorption of nitrobenzene, resulting in preferential proton transfer using water as the hydrogen source. This process is revealed to be proton transfer followed by the electron transfer pathway that improves reaction kinetics and reduces undesirable intermediates. This work enables in situ detection and reveals the role of transition metal doping in modulating the proton–electron transfer in typical electrocatalytic hydrogenation reactions.

A new perspective on the initial hydrogenation of TiFe0.9M0.1 (M = V, Cr, Fe, Co, Ni) alloys gained from surface oxide analyses and nucleation energetics

Despite the promise of TiFe-based alloys as low-cost solid-state hydrogen storage materials with mild operating conditions and reasonable hydrogen capacity, their initial hydrogenation process is difficult, hindering broad utilization. The effect of alloying element on the initial hydrogenation kinetics of TiFe alloys, TiFe0.9M0.1 (M = V, Cr, Fe, Co and Ni), was evaluated by analyzing changes to the passivating surface oxide layer that inhibits hydrogen permeation, as well as the ease of initial-stage hydrogen absorption into the underlying matrix. X-ray photoelectron spectroscopy and atom probe tomography revealed key variations in surface oxide compositions and thinning of the passivating oxide layer compared to pure TiFe, which suggests suppressed oxide growth by alloying-induced elemental redistribution. At the same time, density functional theory calculations predicted exothermic formation of hydride nuclei when alloying with V or Cr, as well as a reduced nucleation barrier when alloying with Co or Ni. Overall, these results are consistent with the observed experimental trend of the activation kinetics. We propose that improvements in activation kinetics of TiFe with alloying arises from the combined effect of reduced passivating oxide thickness and easier hydride nucleation, offering a starting point for alloy design strategies towards further improvement.

The role of oxide in hydrogen absorption and desorption kinetics of MgH2-based material

MgH2-based hydrides have a very high affinity to oxygen, leading to the degradation of their hydrogen storage properties. Herein, we use MgO doping and air exposure methods to add oxygen to the MgH2-TiH2 materials, and the hydrogen storage properties of these materials are systematically investigated. The results show that MgO doping accelerates the ambient temperature absorption kinetics, impedes the Mg crystalline growth, and reduces the desorption temperature. In contrast, the air exposure significantly deteriorated the kinetics of MgH2-TiH2 under both ambient and high temperature. The hydrogenation kinetics of the air-exposed sample can be recovered under cycling. Our results indicate that the effect of MgO is highly dependent of the means to introduce MgO and the location of MgO in the matrix structure. Air-exposure is the main factor for the degradation of hydrogen storage properties, but the MgO doping positively affects the performance of the Mg-based hydride.

Improvement of Hydrogenation and Dehydrogenation Kinetics of As-Cast AZ91 Magnesium Alloy via Twin Parallel Channel Angular Extrusion Processing

In the current study, Twin Parallel Channel Angular Extrusion (TPCAE) as a developed SPD processing technique is used to improve the hydrogen storage properties of AZ91 cast alloy. The processing is conducted at different temperatures, ranging from 340 °C down to 200 °C. The hydrogen absorption and desorption tests are conducted kinetically at three different temperatures, using a Sievert-type apparatus. Remarkable improvement in the absorption kinetic is achieved as a result of the TPCAE processing. A maximum absorption capacity of 6.1 wt.% within a time span of 2000 s is achieved for the sample with three passes of processing complemented at 250 °C. Also, the kinetic of dehydrogenation is improved significantly and complete desorption at 350 °C is achieved for all the processed samples within a time span of maximum 2500 s. By calculating the activation energy of hydrogenation and evaluating the microstructure changes, it is found that implementing sufficient thermomechanical work level along with applying the last pass of the process at lower temperature results in a reduction of the activation energy and improvement of the hydrogenation kinetic.

Effect of novel La-based alloy modification on hydrogen storage performance of magnesium hydride: First-principles calculation and experimental investigation

LaNi4.5X0.5(X = Fe, Co, Al, Ni) alloys are prepared and doped into MgH2 by ball milling to enhance the hydrogen storage performance, respectively. Both hydrogenation & dehydrogenation kinetics are experimentally measured, and the results indicate that LaNi4.5Fe0.5 doped MgH2 exhibits the top H2 absorption/desorption performance compared with other samples. It is worth noting that the optimal doping amount of LaNi4.5Fe0.5 is selected as 15 wt.% combined with the consideration of working temperature and H2 storage capacity. More importantly, the as-prepared MgH2-15 wt.%LaNi4.5Fe0.5 sample may rapidly introduce the equivalent H2 within 1000 s at mild temperature and pressure of 200 °C and 1 MPa, and it is completely impossible for MgH2 to occur any hydrogenation under this operation conditions. Moreover, the activation energy barriers for hydrogenation & dehydrogenation decline by 62.90% & 20.75% respectively, suggesting the successful doping and property of La-based alloy inside MgH2 as expected. In addition, a variety of as-prepared alloys are experimentally compared and discussed, and the related the heterostructure interface, electronic structure and the formation energy of H vacancy are carried out based on the first-principles calculation, which confirm that LaNi4.5Fe0.5 owns the most favorable catalytic modification effect on Mg-based material.