Dissociation Of H2 Molecules Research Articles

Novel Graphene-Based Reversible Physisorption Materials for High Hydrogen Uptake

Carbon-based nanomaterials (graphene, carbon nanotube, microporous carbon, graphitic carbon etc.) are excellent H2 storage materials due to high molecular H2 uptake at the micropores and large H2 adsorption at the graphene network via the formation of dipoles in H2 molecules through London dispersion forces. On the other hand, incorporation of transition metal nanoparticles within the nanocarbon network facilitates dissociation of H2 molecules into constituents H-atoms over the metal nanoparticles, followed by diffusion into the carbon nano-matrix via ‘spill-over’ effect for polarization/hybridization of metal-hydrogen to create high H2 binding energy for high hydrogen adsorption. Therefore, a nanocomposite of graphene-microporous carbon-metal nanoparticle network is expected to deliver excellent H2 uptake capacity and acts as a novel reversible physisorption material for hydrogen storage applications, which is very important for sustainable renewable energy integration.A simple, cost effective sputtering method is used to fabricate metal incorporated graphitic microporous carbon film and differential resistance measurement showed high H2 uptake. Average number of graphene layer formation is dependent on sputtering parameters, which manifest bi-to-multilayer graphene. With increase in graphene layers, H2 adsorption also increases due to a fourfold effect: higher molecular hydrogen uptake at the graphene layers, more active sites into the micropores, promotion of molecular dissociation into atomic hydrogen by the metal nanoparticles, and the formation of hydrogenated carbon via destruction of the π-bonds.

Regulating the d-band center of Cu nanoparticles for efficient photo-driven catalytic CO2 reduction

The conversion of CO2 into solar fuels via photo-driven catalytic reduction presents a promising avenue toward achieving carbon neutrality. Herein, copper-based catalysts are prepared and explored for photo-driven photothermal CO2 reduction reaction. The optimized catalyst exhibits a consistent CO production rate of 165.9 mmol g−1 h−1 and 100% CO selectivity at ambient pressure. We unveil the intricate adsorption dynamics at play: H2 molecules predominantly interact with and activate at Cu sites, while CO2 molecules preferentially adsorb and activate at interfacial sites within the composites. Furthermore, we elucidate how the positive shift of the d-band center (εd) for Cu 3d orbitals significantly enhances H2 adsorption and activation. Crucially, the subsequent dissociation of H2 molecules at Cu sites drives the efficient conversion of adsorbed CO2 molecules at interfacial sites into CO. Overall, our findings not only advance the theoretical understanding but also offer practical insights for realizing photothermal catalytic CO2 reduction reactions.

Unlocking the Potential of Cu/Ti3C2Tx MXene Catalyst in Plasma Catalytic CO2 Hydrogenation

Plasma catalytic CO2 hydrogenation has emerged as a promising approach for converting CO2 into valuable chemicals such as CO, offering a potential solution to mitigate greenhouse gas emissions. In this study, we investigate the synergistic effect between plasma and a Cu/Ti3C2Tx MXene catalyst for CO2 hydrogenation via the reverse water gas shift (RWGS) reaction in a dielectric barrier discharge (DBD) reactor. Our findings reveal a significant enhancement in CO2 conversion (33.1%) and CO yield (31.9%) when using the Cu/Ti3C2Tx catalyst compared to plasma alone or with a Ti3C2Tx foam. Plasma treatment creates unsaturated Ti sites on the Ti3C2Tx surface, dramatically improving CO2 adsorption and activation, while Cu nanoparticles significantly enhances the activation and dissociation of H2 molecules on the catalyst surface. These surface adsorbed hydrogen species readily hydrogenate adsorbed CO2 molecules through the Langmuir-Hinshelwood mechanism. Furthermore, these surface hydrogen species can also participate in the conversion of gas-phase CO2 molecules through the Eley-Rideal pathway. This work sheds light on the synergistic plasma catalytic mechanism and provide insights for optimizing CO2 hydrogenation processes using MXene catalysts.

Constructing fully exposed Pt atomically dispersed catalysts for enhanced multifunctional selective hydrogenation reactions

Constructing highly efficient and low-cost catalysts is essential for the selective hydrogenation reaction. Herein, we developed fully exposed Pt atomically dispersed catalysts (PtAD/C-Al2O3) by a simple carbon modified strategy. Importantly, the co-existence of Pt-O1C2 single atoms and Pt nanoclusters in the PtAD/C-Al2O3 system could exhibit excellent catalytic activity in the selective hydrogenation of quinoline and p-nitrophenol. Notably, the quinoline conversion and the corresponding selectivity of 1,2,3,4-tetrahydroquinoline is 99.6 % and 99.3 %, respectively. Meanwhile, the turnover frequency of PtAD/C-Al2O3 (1081 h−1) is 3.8 and 5.7 times higher than that of Pt single-atom catalysts (PtSA/C-Al2O3) and Pt nanoparticles (PtNPs/C-Al2O3), respectively, during quinoline hydrogenation. Besides, the rate constant of PtAD/C-Al2O3 could reach 192.5 min-1mgPt-1 for p-nitrophenol hydrogenation, which is much better than most of the reported catalysts. Mechanism studies revealed the synergistic effect between Pt-O1C2 single atoms and Pt nanoclusters acts as the main role in enhancing the hydrogenation activity. Specifically, Pt single atoms sites mainly adsorb and activate quinoline molecules, while Pt nanoclusters boost the dissociation of H2 molecules to H atoms. This work paves an origin carbon modified method to construct Pt atomically dispersed catalysts for efficient multifunctional hydrogenation.

Structural Heredity in Catalysis: CO2 Self-Selective CeO2 Nanocrystals for Efficient Photothermal CO2 Hydrogenation to Methane.

The chemical inertness of CO2 molecules makes their adsorption and activation on a catalyst surface one of the key challenges in recycling CO2 into chemical fuels. However, the traditional template synthesis and chemical modification strategies used to tackle this problem face severe structural collapse and modifier deactivation issues during the often-needed post-processing procedure. Herein, a CO2 self-selective hydrothermal growth strategy is proposed for the synthesis of CeO2 octahedral nanocrystals that participate in strong physicochemical interactions with CO2 molecules. The intense affinity for CO2 molecules persists during successive high-temperature treatments required for Ni deposition. This demonstrates the excellent structural heredity of the CO2 self-selective CeO2 nanocrystals, which leads to an outstanding photothermal CH4 productivity exceeding 9mmol h-1 mcat -2 and an impressive selectivity of >99%. The excellent performance is correlated with the abundant oxygen vacancies and hydroxyl species on the CeO2 surface, which create many frustrated Lewis-pair active sites, and the strong interaction between Ni and CeO2 that promotes the dissociation of H2 molecules and the spillover of H atoms, thereby greatly benefitting the photothermal CO2 methanation reaction. This self-selective hydrothermal growth strategy represents a new pathway for the development of effective catalysts for targeted chemical reactions.

The birth and early evolution of a low-mass protostar

Context. Understanding the collapse of dense molecular cloud cores to stellar densities and the subsequent evolution of the protostar is of importance to model the feedback effects such an object has on its surrounding environment, as well as describing the conditions with which it enters the stellar evolutionary track. This process is fundamentally multi-scale, both in density and in spatial extent, and requires the inclusion of complex physical processes such as self-gravity, turbulence, radiative transfer, and magnetic fields. As such, it necessitates the use of robust numerical simulations. Aims. We aim to model the birth and early evolution of a low-mass protostar. We also seek to describe the interior structure of the protostar and the radiative behavior of its accretion shock front. Methods. We carried out a high resolution numerical simulation of the collapse of a gravitationally unstable 1 M⊙ dense molecular cloud core to stellar densities using 3D radiation hydrodynamics under the gray flux-limited diffusion approximation. We followed the initial isothermal phase, the first adiabatic contraction, the second gravitational collapse triggered by the dissociation of H2 molecules, and ≈247 days of the subsequent main accretion phase. Results. We find that the subcritical radiative behavior of the protostar’s shock front causes it to swell as it accretes matter. We also find that the protostar is turbulent from the moment of its inception despite its radiative stability. This turbulence causes significant entropy mixing inside the protostar, which regulates the swelling. Furthermore, we find that the protostar is not fully ionized at birth, but the relative amount of ionized material within it increases as it accretes matter from its surroundings. Finally, we report in the appendix the results of the first 3D calculations involving a frequency-dependent treatment of radiative transfer, which has not produced any major differences with its gray counterpart.

Fundamentals of Green Steel Production: On the Role of Gas Pressure During Hydrogen Reduction of Iron Ores

Out of the multitude of researched processing routes for sustainable ironmaking, hydrogen-based direct reduction and hydrogen plasma smelting reduction (HyPSR) are currently the most promising candidates for a successful industrial application. Both processes operate under gaseous atmospheres, which turn the partial and absolute pressure of hydrogen into a relevant process parameter. Here, we present first insights into the influence of total pressure and concentration of hydrogen on the reduction of hematite, focusing on the more pressure-sensitive route (HyPSR). The effect of pressure on the dissociation of H2 molecules into metastable H atoms or H+ ions,- and the overall hydrogen utilization is discussed using a thermodynamic approach. Validation experiments were conducted to testify the practical feasibility of adjusting these parameters. A decrease in the total pressure of the system from 900 mbar to 450 mbar resulted in an improved hydrogen utilization, thus suggesting that a larger population of H atoms can exist in the plasma arcs ignited under 450 mbar. An increase in the hydrogen concentration to 20 vol.% lead to undesired evaporation, likely because of a parallel increase in plasma temperature. Possibilities and challenges for exploiting these pressure-related phenomena for the industrial production of green steel are outlined and discussed.

Effect of low concentration of impurity gases on the hydrogen absorption performance of uranium

The performance of uranium as a hydrogen storage material has attracted much attention. Herein, the hydrogen absorption properties of depleted uranium in impure hydrogen containing He, Ar, CH4, N2, CO, CO2 and O2 were studied by PVT method and XPS analysis. When these impurity gases were mixed in H2 at low concentrations (0.1%∼1.5%), their behavior can be classified into three categories. The He, CH4 and Ar were chemically inert to the activated uranium powder under room condition. These three gases inhibited the absorption kinetics during the whole stage by hindering the diffusion of H2 molecules, showing a blanketing effect. The N2 and O2 did not affect the absorption kinetics but reduced the capacity by forming nitrides and oxides. The poisoning effect of N2 was weaker than that of O2. The CO and CO2 not only affected the hydrogen absorption capacity, but also strongly inhibited the absorption kinetics. These two gases are chemically adsorbed on the uranium surface to form passivation layers, thus inhibiting the adsorption and dissociation of H2 molecules and the diffusion of H atoms. The poisoning and retardation effect of CO2 were much stronger than that of CO. The above conclusions are important to further study the reactivity of mixed gas with uranium, and can also be used as a reference for other hydrogen storage systems.

Adsorption and dissociation of H2 molecule over first-row transition metal doped C24 nanocage as remarkable SACs: A comparative study

Transition metal doped carbon materials such as carbon nanotubes and fullerenes have been extensively investigated for hydrogen adsorption and storage applications. However, the strength of these hydrogen storage material is mainly dependent on the metal-support and hydrogen-metal interaction energies. In this work, we have designed, and explored transition metal doped C24 (TM@C24) complexes as single atom catalysts for hydrogen dissociation. Adsorption energy results for all studied TM@C24 complexes reveal the high thermodynamic stability of designed TM@C24 catalysts. Among all considered TMs@C24 catalysts, the highest adsorption energy (−6.22 eV) is calculated for Mn@C24 catalyst. Moreover, H2 dissociation mechanism is evaluated for both gas phase and in aqueous media to get insight into the solvent effect. In both gas phase and in aqueous media, the best catalytic activity for hydrogen dissociation is computed for Mn@C24 catalyst with the lowest energy barrier of 0.04 eV and 0.30 eV, respectively. NCI analysis is carried out to confirm the shared shell interactions (covalent interactions) between adsorbed hydrogen and TM@C24 complexes. Furthermore, natural bond orbital and electron density differences analysis are also performed to explore the activation and dissociation of H2 molecule. Overall results reveal that Mn@C24 complex can at as a promising low cost, highly abundant and noble metal free single atom catalyst to effectively catalyze hydrogen dissociation reaction.

FePO4 supported Rh subnano clusters with dual active sites for efficient hydrogenation of quinoline under mild conditions.

Chemoselective hydrogenation of quinoline and its derivatives under mild reaction conditions still remains a challenging topic, which requires a suitable interaction between reactants and a catalyst to achieve high performance and stability. Herein, FePO4-supported Rh single atoms, subnano clusters and nanoparticle catalysts were synthesized and evaluated in the chemoselective hydrogenation of quinoline. The results show that the Rh subnano cluster catalyst with a size of ∼1 nm gives a specific reaction rate of 353 molquinoline molRh-1 h-1 and a selectivity of >99% for 1,2,3,4-tetrahydroquinoline under mild conditions of 50 °C and 5 bar H2, presenting better performance compared with the Rh single atoms and nanoparticle counterparts. Moreover, the Rh subnano cluster catalyst exhibits good stability and substrate universality for the hydrogenation of various functionalized quinolines. A series of characterization studies demonstrate that the acidic properties of the FePO4 support favors the adsorption of quinoline while the Rh subnano clusters promote the dissociation of H2 molecules, and then contribute to the enhanced hydrogenation performance. This work provides an important implication to design efficient Rh-based catalysts for chemoselective hydrogenation under mild conditions.

A density functional theory study of adsorption and dissociation of H2 molecule on small PdnAgm (n + m ≤ 4) metal clusters

The adsorption and dissociation of hydrogen molecule on small PdnAgm (n + m ≤ 4) clusters is studied employing density functional theory (DFT) calculations. The lowest energy structures of the PdnAgm clusters are initially presented. Based on the obtained structures, various geometries of H2 adsorption and dissociation with these clusters are examined. The cluster size and composition of the PdnAgm clusters are found to control the nature of the adsorption process. The results suggest that molecular adsorption is more favorable on the Pd atom and mixed clusters, while hydrogen dissociation is more favorable on the Pdn (n = 2–4) clusters. The Pd dimer and trimer are found to be the most reactive clusters for hydrogen adsorption, while single Ag atom is the least reactive one. The natural bond orbital (NBO) population analysis indicates that hydrogen atoms tend to draw electrons from the metal clusters in the dissociative form and donate electrons to the metal clusters in the molecular adsorption form. The density of states (DOS) of the PdnAgm trimers are also presented and discussed. The interaction and stabilities of cluster on the graphene support has also been performed.

Theoretical understanding on all-solid frustrated Lewis pair sites of C2N anchored by single metal atom.

The design of all-solid heterogeneous catalysts with frustrated Lewis pairs (FLPs) has attracted much attention recently because of their appealing low dissociation energy for H2 molecules due to which a promotion of hydrogenation reaction is expected. The sterically encumbered Lewis acid (metal site) and base (nitrogen site) in the cavity of single transition metal atom-doped M/C2N sheets make them potential candidates for the design of catalysts with FLPs, while a comprehensive understanding of their intrinsic property and reactivity is still lacking. Calculations show that the complete dissociation of the H2molecule into two H* states at the N sites requires two steps: heterolytic cleavage of the H2molecule and the transfer of H* from the metal site to the N site, which are strongly related to the acidity of the metal site. Ni/C2N and Pd/C2N, which outperform the other eight transition metal atom (M) anchored M/C2N candidates, possess low energy barriers for the complete dissociation of H2 molecules, with values of only 0.30 and 0.20eV, respectively. Furthermore, both Ni/C2N and Pd/C2N catalysts can achieve semi-hydrogenation of C2H2 into C2H4, with overall barriers of 0.81 and 0.75eV, respectively, which are lower than those reported for many other catalysts. It is speculated that M/C2N catalysts with intrinsic FLPs may also find applications in other important hydrogenation reactions.

First principles DFT analysis on the diffusion kinetics of hydrogen isotopes through bcc iron (Fe): Role of temperature and surface coverage

The diffusion of interstitial H, D, and T in Fe metal at different temperature are evaluated using ab-initio density functional theory and transition state theory. The thermal expansion coefficient, Helmholtz free energy of activation, jump factor of diffusion was determined by use of activation energy and phonon calculations. The calculated diffusion coefficient was well described by a constant activation energy, (D = D0exp (-Ea)/kT)) with Ea = 0.016, 0.041, and 0.050 eV and D0 = 1.042 × 10−7, 0.736 × 10−7 and 0.572x10−7 m2.s−1for H, D, and T, respectively using harmonic transition state theory (hTST) with temperature correction. The calculated permeability and solubility also followed the Arrhenius relation.The earlier experimentally reported higher diffusivity of H atom at lower temperatures than that at higher temperature (200–600 K) was well explained by Wigner + hTST model with temperature correction and semi-classical transition state theory (SC-TST). The present computed results followed the similar trend of experimental findings. Further, the ideal fracture energy was evaluated using the Born-Haber thermodynamic cycle for varied coverage of H, D and T to account for decohesion-based embrittlement. The lighter H atom was seen to cause more decohesion-based embrittlement compared to D and T due to reduced cohesion. Additionally, the effect of temperature on ideal fracture energy was evaluated for H, D and T in Fe. The decohesion-based embrittlement was increased with increase in the temperature for H, D and T up to certain temperature (for H lower than 500 K, for D around 500 K and for T above 500 K) but with further increase in the temperature, the ideal fracture energy was reduced due to decrease in the adsorption energy and increase in the solution enthalpy of H isotopes. Further, the path and kinetics of dissociation and reassociation of H2 molecule were established on Fe (100) surface at 0.5 and 2.0 ML coverage. At lower concentration (0.5 ML), dissociation of H2 molecule on Fe (100) surface was favored, whereas at high concentration (2.0 ML), reassociation of H2 molecule was favored.

In Situ Formation of Mg2Ni on Magnesium Surface via Hydrogen Activation for Improving Hydrogen Sorption Performance

To improve the hydrogen sorption rate and lower the dehydrogenation temperature, the Mg2Ni was formed in situ on the Mg surface by the hydrogen activation from Mg–NiO and Mg–NiCl2 composites. Among these two, Mg–5NiO absorbs 6.19 wt % H2 in 40 min at 320 °C, considerably higher than Mg–5NiCl2 (5.52 wt % H2) or pure milled Mg (4.09 wt % H2). When compared with MgH2, the decomposition peak temperature and apparent activation energy of MgH2–5NiO are reduced by, respectively, 24.6 °C and 49.7 kJ/mol H2 while, respectively, 40.5 °C and 24 kJ/mol H2 for MgH2–5NiCl2. Micro-structural analysis indicates that the in situ formed Mg2Ni on the Mg surface is of critical importance for improving hydrogen storage performance of the material. The in situ formed Mg2Ni facilitates the dissociation of H2 molecules during the hydrogenation process and benefits the cleavage of the Mg–H bond during dehydrogenation for H2 formation. The strategy of in situ formation from precursors can inspire the design of solid material for hydrogen storage.

Studies of Ni-Mg catalyst for stable high efficiency hydrogen storage

Doping engineering is an effective strategy to improve the hydrogen storage capacity of magnesium-based materials. Here, we report a magnesium composite containing trace amounts of nickel catalyst (Ni-Mg), in which Ni and Mg layers form a new structural phase. The test shows that Ni-Mg composite has a hydrogen storage capacity of 7.5 wt%, with rapid absorption/desorption kinetics and stable reversible absorption/desorption performance on cycling. Ni-Mg exhibits a superior hydrogen storage performance as compared to the MgNi alloys and Mg-based materials previously reported in the literature. We conclude that a new MgNi phase and MgNi2 intermetallic compound are formed during the H2 adsorption process. Based on the theoretical calculations, we show that Ni facilitates hydrogenation by accelerating the dissociation of H2 molecules into the H atoms, while the MgNi phase formed during the hydrogenation is conducive to the subsequent dehydrogenation, promoting H atoms diffusion between the subsurface and the surface. This work provides a guidance for the development of the efficient hydrogen storage materials.

A Density Functional Theory Study of Adsorption and Dissociation of H2 Molecule on Small Pdnagm (N + M ≤ 4) Metal Clusters

A Density Functional Theory Study of Adsorption and Dissociation of H2 Molecule on Small Pdnagm (N + M ≤ 4) Metal Clusters

A Density Functional Theory Study of Adsorption and Dissociation of H2 Molecule on Small Pdnagm (N + M ≤ 4) Metal Clusters

A Density Functional Theory Study of Adsorption and Dissociation of H2 Molecule on Small Pdnagm (N + M ≤ 4) Metal Clusters

Influence of the Fe-doping on hydrogen behavior on the ZrCo surface

Ab initio calculations have been carried out to investigate the adsorption, dissociation, and diffusion of atomic and molecular hydrogen on the Fe-doped ZrCo (110) surface. It is found that the adsorption of H2 on doped surface seems thermodynamically more stable with more negative adsorption energy than that on the pure surface, and the dissociation energy of H2 on doped surface is much bigger therefore. However, compared with the pure system, there are fewer adsorption sites for spontaneous dissociation. After dissociation, the higher hydrogen adsorption strength sites would promote the H atom diffusion towards them where they can permeate into the bulk further. Furthermore, the ZrCo (110) surface possesses much higher hydrogen permeability and lower hydrogen diffusivity than its corresponding ZrCo bulk. Moreover, further comparison of the present results to analogous calculations for pure surface reveals that the Fe dopant facilitates the H2 molecule dissociation. Unfortunately, this does not improve the hydrogen storage performance of ZrCo alloy due to the H atom diffusion on the surface and into bulk are prevented with higher reaction energetic barriers by doping Fe. Consequently, ZrCo (110) surface modified with Fe atoms should not be preferred as a result of its terrible hydrogen permeability. A clear and deep comprehending of the inhibiting effect of Fe dopant on the hydrogen storage of ZrCo materials from the perspective of the surface adsorption of hydrogen are obtained from the present results.

Two-step model for reduction reaction of ultrathin nickel oxide by hydrogen

Nickel (Ni) is used as a catalyst for nitric oxide decomposition and ammonia production but it is easily oxidized and deactivated. Clarification of the reduction process of oxidized Ni is essential to promote more efficient use of Ni catalysts. In this study, the reduction processes of ultrathin oxide films formed on Ni(111) surfaces by thermal oxidation under vacuum and a hydrogen atmosphere were investigated by in situ time-resolved photoelectron spectroscopy. On the basis of these results, we propose a reaction model for the reduction of Ni oxide films. Our results show that the reduction of Ni oxide films on heating under vacuum does not yield a clean Ni(111) surface owing to formation of a residual stable suboxide structure on the Ni(111) surface. Conversely, in a hydrogen atmosphere of 1 × 10−5 Pa, the Ni oxide was completely reduced and a clean Ni(111) surface was obtained, even when heating below 300 °C. The reduction in a hydrogen atmosphere was best described by a two-step reaction model. The rate of the first step depends on the reduction temperature, and the rate of the second step depends on the H2 pressure. The rate-limiting process for the first step is surface precipitation of O atoms and that of the second step is dissociation of H2 molecules.

Enhancement of hydrogen storage properties of Li12+xMg3-xSi4-ySny (x=y=0.48) phase by modification with LixZnO/La2O3-CNT composites

Two kinds of LixZnO-CNT and La2O3-CNT composite additives (CA) are employed to improve the hydrogen absorption/desorption processes of Li12+xMg3-xSi4-ySny phase. The effect of the addition of composite additives on the improvement of hydrogen sorption kinetics of intermetallic phase is due to the following factors: CA are more stable than Li contain intermetallic; CA provides diffusion pathways to most of the grains inside the material, where the dissociation of H2 molecules into H atoms takes place; CA surface has a relatively higher reactivity to H2 dissociation. The substituted (Si atoms by Sn) and doped by La2O3-CNT composite Li12+xMg3-xSi4-ySny alloy have the highest uptake and release capability (9.8 wt%). LixZnO-CNT give 9.3 wt% hydrogen uptake and release, which are still much better than undoped Li12+xMg3-xSi4-ySny (8.9 wt%). Li12+xMg3-xSi4-ySny is a maximally disordered intermetallic phase (MD-IP) with a cubic structure (I-43d, a = 10.7409(8) Å) related to Cu15Si4-type.