Dissociative Chemisorption Of Hydrogen Research Articles

Zr-based Laves phases with nitride/hydride ions for ammonia synthesis

Early transition metal-based intermetallic compounds, such as ZrV2, ZrMn2, and ZrCr2, can reversibly absorb large amounts of hydrogen and are of great interest for hydrogen storage. The dissociative hydrogen chemisorption can proceed easily on the surface, allowing the rapid formation of intermetallic hydrides. In this study, we present the results of initial NH3 activity tests conducted on various Zr-based AB2-type intermetallic compounds, free of precious transition metals and rare earth metals. A dissociative hydrogen chemisorption together with nitrogen storage proceeds on AB2-type Laves phase intermetallic compounds accompanied by a lattice expansion. The formed intermetallic nitride hydride ZrCr2N0.4H1.4 exhibits stable catalytic activity as an ammonia synthesis catalyst.

Tuning the Catalytic Activity of Pd x Ni y (x + y = 6) Bimetallic Clusters for Hydrogen Dissociative Chemisorption and Desorption.

Density functional theory was used to study dissociative chemisorption and desorption on PdxNiy (x + y = 6) bimetallic clusters. The H2 dissociative chemisorption energies and the H desorption energies at full H saturation were computed. It was found that bimetallic clusters tend to have higher chemisorption energy than pure clusters, and the capacity of Pd3Ni3 and Pd2Ni4 clusters to adsorb H atoms is substantially higher than that of other clusters. The H desorption energies of Pd3Ni3 and Pd2Ni4 are also lower than that of the Pd6 cluster and comparable to that of the Ni6 cluster, indicating that it is easier to pull the H atom out of these bimetallic catalysts. This suggests that the catalytic efficiency for specific PdxNiy bimetallic clusters may be superior to bare Ni or Pd clusters and that it may be possible to tune bimetallic nanoparticles to obtain better catalytic performance.

Exploring enhanced hydrogen adsorption on Ti doped Al nanoclusters: A DFT study

The detailed mechanism of adsorption and dissociation of molecular hydrogen on bare and Ti doped Al6 clusters has been studied using Density Functional Theory (DFT). The study shows that the doping of Ti atom significantly changes the geometric and electronic structures of Al6 cluster. Noticeable electronic charge transfer takes place from Al to Ti atoms in TiAl5. The number of energy states near the Fermi level of bare Al6 cluster also increases upon Ti doping. Moreover, the weak molecular hydrogen physisorption on Al6 becomes relatively stronger in TiAl5 with binding energy value of −0.12 eV. Similarly, dissociative hydrogen chemisorption energy on TiAl5 (−2.80 eV) is higher than that on Al6 (−2.74 eV). The computed molecular hydrogen gravimetric content in TiAl5 cluster is 3.2 wt%. Further, dissociation of H2 in TiAl5 involves an activation barrier of 0.54 eV in contrast with barrier less dissociation in bare Al6 cluster.

Dissociative Chemisorption and Oxidation of H2 on the Stoichiometric IrO2(110) Surface

We investigated the dissociative chemisorption and oxidation of H2 and D2 on the stoichiometric IrO2(110) surface (“s-IrO2(110)”) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. We find that the dissociative chemisorption of hydrogen occurs efficiently on s-IrO2(110) during adsorption at 90 K, with ~ 90% of the dissociated H2 oxidizing to H2O during TPRS and evolving in a broad feature between 400 and 800 K. We also observe small quantities of H2 desorbing in TPRS peaks at 200 and 530 K, and show that these peaks arise from the desorption of molecularly-adsorbed H2 and the recombination of atomic hydrogen, respectively. Our results demonstrate that H2 dissociation on s-IrO2(110) occurs by a precursor-mediated mechanism wherein H2 molecules adsorb strongly on coordinatively-unsaturated Ir atoms (Ircus) and the resulting σ-complexes then serve as precursors for H2 bond cleavage. Our DFT calculations predict that H2 adsorbs strongly on an atop-Ircus site, and that the H2 complex can dissociate by a facile pathway involving H-transfer to a neighboring bridging O atom (Obr) to produce an H-Ircus/HObr pair. For this pathway, we predict that the energy barrier for dissociation is ~ 65 kJ/mol lower than the binding energy of the adsorbed H2 complex. We also find that the total hydrogen uptake on s-IrO2(110) saturates at an H2 coverage of ~ 0.65 ML during adsorption at 90 K, and present evidence that this limited uptake results from a strong influence of HObr groups on H2 σ-complex formation. Finally, we used DFT to examine pathways for H2O formation on s-IrO2(110) and find that steps leading directly to H2O formation are energetically demanding and likely determine the overall rate of H2 oxidation on s-IrO2(110).

Competitive Molecular and Dissociative Hydrogen Chemisorption on Size Selected Doubly Rhodium Doped Aluminum Clusters

The interaction of hydrogen with Al n Rh2 + (n = 10–13) clusters is studied by mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy. Comparing the IRMPD spectra with predictions obtained using density functional theory calculations allows for the identification of the hydrogen binding geometry. For n = 10 and 11, a single H2 molecule binds dissociatively, whereas for n = 12 and 13, it adsorbs molecularly. Upon adsorption of a second H2 to Al12Rh2 +, both hydrogen molecules dissociate. Theoretical calculations suggest that the molecular adsorption for n = 12 and 13 is not due to kinetic impediment of the hydrogenation reaction by an activation barrier, but due to a higher binding energy of the molecularly adsorbed hydrogen–cluster complex. Inspection of the highest occupied molecular orbitals shows that the hydrogen molecule initially forms a strongly bound Kubas complex with the Al11–13Rh2 + clusters, whereas it only binds weakly with Al10Rh2 +.

Isotopic H/D exchange on graphenes. A combined experimental and theoretical study

Adsorption of H2/D2 on graphene (G), graphene oxide (GO), single walled carbon nanotube (SWCNT), N-doped graphene [(N)G], and a sample of active carbon (C) has led to the detection of HD, indicating dissociative chemisorption of hydrogen on the surface of the material. The amount of HD detected follows the order G>SWCNT>GO∼(N)G∼C, G giving about five-fold higher H2/D2 adsorption and HD exchange level than SWCNT and about ten-fold larger values than that of the other samples. Quantum-chemistry calculations modeling a carbon atom vacancy on a G cluster estimates an activation barrier for H2 dissociation of ca. 84kJ/mol for a mechanism involving under coordinated carbon atoms at the defect site.

Hydrogen physisorption based on the dissociative hydrogen chemisorption at the sulphur vacancy of MoS2 surface

We provide a new insight that the sulphur-depleted MoS2 surface can store hydrogen gas at room temperature. Our findings reveal that the sulphur-vacancy defects preferentially serve as active sites for both hydrogen chemisorption and physisorption. Unexpectedly the sulphur vacancy instantly dissociates the H2 molecules and strongly binds the split hydrogen at the exposed Mo atoms. Thereon the additional H2 molecule is adsorbed with enabling more hydrogen physisorption on the top sites around the sulphur vacancy. Furthermore, the increase of the sulphur vacancy on the MoS2 surface further activates the dissociative hydrogen chemisorption than the H2 physisorption.

Steric and chemical effects on the hydrogen adsorption and dissociation on free and graphene–supported palladium clusters

Palladium doping enhances the hydrogen storage capacity of nanoporous carbons. The purpose of this work is to asses the effect of the carbonaceous support on the adsorption of hydrogen on Pd clusters. Hydrogen adsorbs on Pd clusters following two channels: molecular adsorption and dissociative chemisorption. These two adsorption channels are investigated on free Pd clusters and Pd clusters supported on pristine and defective (with vacancies) graphene using the Density Functional Formalism. Pd6 is taken as case study. Free Pd6 can adsorb twelve hydrogen molecules in the molecular form, a number higher than the nine and eight molecules that can be adsorbed on the cluster supported on pristine graphene and on a graphene vacancy, respectively. However the most stable adsorption channel is, in all cases, the dissociative chemisorption of hydrogen. As the cluster is being loaded with hydrogen, there is a competition between the two adsorption channels. Pd6 supported on a graphene vacancy is able to dissociate three hydrogen molecules, whereas the free cluster can dissociate up to seven molecules. In both cases, six additional molecules can be adsorbed in the molecular form. The higher saturation limit obtained for the free clusters is explained in terms of the steric and chemical effects of the supporting layer. These effects are of primarily importance to asses the role of the Pd dopant on the adsorption and storage of hydrogen on nanoporous carbons.

Hydrogen sensing characteristics from carbon nanotube field emissions.

An innovative hydrogen sensing concept is demonstrated based on the field emission from multi-walled carbon nanotubes, where the low emission currents rise in proportion to hydrogen partial pressures above 10(-9) Torr. Experimental and first principles studies reveal that the sensing mechanism is attributed to the effective work function reduction from dissociative hydrogen chemisorption. The embedded Ni catalyst would assist both the hydrogen dissociation and work function reduction. This technique is promising to build miniature low cost hydrogen sensors for multiple applications. This work is valuable for studies of nanocarbon-gas reaction mechanisms and the work function properties in adsorption related applications, including field emission, hydrogen storage, energy cells, and gas sensing.

Kinetic study of the selective hydrogenation of styrene over a Pd egg-shell composite catalyst

This is a study on the kinetics of the liquid-phase hydrogenation of styrene to ethylbenzene over a catalyst of palladium supported on an inorganic–organic composite. This support has a better mechanical resistance than other commercial supports, e.g. alumina, and yields catalysts with egg-shell structure and a very thin active Pd layer. Catalytic tests were carried out in a batch reactor by varying temperature, total pressure and styrene initial concentration between 353–393 K, 10–30 bar, and 0.26–0.60 mol L−1. Kinetic models were developed on the assumptions of dissociative hydrogen chemisorption and non-negligible adsorption of hydrogen and styrene. Final chemical reaction expressions useful for reactor design were obtained. The models that best fitted the experimental data were those ones that considered the surface reaction as the limiting step. In this sense, a two-step Horiuti–Polanyi working mechanism with half hydrogenation intermediates gave the best fit of the experimental data. The heats of adsorption of styrene and ethylbenzene were also estimated.

Quantum chemical simulation of the dissociative chemisorption of hydrogen on metal surfaces of nanoclusters

The mathematical and quantum chemical simulation of surfaces of Cd, Ba, Ca, Cu, Cs, Ti, Ni, Pd, W, and Pt metals was performed. The structural data were obtained for these surfaces, the behavior of a hydrogen molecule near these surfaces was studied. The calculations were performed in the framework of the density functional theory using the hybrid Becke—Lee—Yang—Parr functional in the LANL2DZ basis set. Mathematical modeling of the studied surfaces in the nanocluster approximation was performed by the Alexandrovich method. The results indicate that metals Ni, Pd, Pt, W, and Ti exhibit high catalytic activity in the dissociative chemisorption of hydrogen, while Ba, Cd, Cs, Ca, and Cu are lowly efficient because of the formation of hydrides.

Six-dimensional quantum dynamics of dissociative chemisorption of H2 on Co(0001) on an accurate global potential energy surface.

Cobalt is a widely used catalyst for many heterogeneous reactions, including the Fischer-Tropsch (FT) process, which converts syngas (H2 and CO) to higher hydrocarbons. As a result, a better understanding of the key chemical steps on the Co surface, such as the dissociative chemisorption of H2 as an initial step of the FT process, is of fundamental importance. Here, we report an accurate full-dimensional global potential energy surface for the dissociative chemisorption of H2 on the rigid Co(0001) surface constructed from more than 3000 density functional theory points. The high-fidelity potential energy surface was obtained using the permutation invariant polynomial-neural network method, which preserves both the permutation symmetry of H2 and translational symmetry of the Co(0001) surface. The reaction path features a very low barrier on the top site. Full-dimensional quantum dynamical calculations provide insights into the dissociation dynamics and influence of the initial vibrational, rotational, and orientational degrees of freedom.

Rice–Ramsperger–Kassel–Marcus Simulation of Hydrogen Dissociation on Cu(111): Addressing Dynamical Biases, Surface Temperature, and Tunneling

The effects of dynamics, surface temperature, and tunneling on the dissociative chemisorption of hydrogen on Cu(111) are explored using a dynamically biased precursor-mediated microcanonical trapping (d-PMMT) model. Transition state vibrational frequencies were taken from recent generalized gradient approximation density functional theory (GGA-DFT) electronic structure calculations, and the model’s few remaining parameters were fixed by optimizing simulations to a limited number of quantum-state-resolved associative desorption experiments. The d-PMMT model reproduces a diverse variety of dissociative chemisorption and associative desorption experimental results and, importantly, largely captures the surface temperature dependence of quantum-state-resolved dissociative sticking coefficients. Molecular translational energy parallel to the surface was treated as a spectator degree of freedom. The efficacy of molecular rotational energy to promote dissociation, relative to normal translational energy, varied ...

Sequential hydrogen dissociation from a charged Pt13H24 cluster modeled by ab initio molecular dynamics

AbstractPlatinum is one of the most valuable catalysts that have been used in the catalytic fields of hydrogenation, fuel‐cell technologies, and photocatalytic water splitting. In this work, the dissociative chemisorption of hydrogen in a molecular form on a Pt13H24 cluster with cuboctahedral symmetry (Oh), which has one center Pt atom and side 12 Pt atoms (each bonded by two hydrogen), is investigated by using first‐principles density functional theory calculation. After computing geometry optimization on the Pt13H24 cluster, the equilibrium distances of PtPt and PtH are inspected. Two nonequilibrium modifications, for example, addition of electrons and heat treatment were applied on the cluster, with the aim to simulate the experimental reaction of hydrogen on the Pt13 H24 cluster. An ab initio molecular dynamics (MD) is simulated after the cluster is charged and heat treated at high temperature to compute the trajectory for the positions of all the atoms. The computation results reveal that H2 desorption is observed during the MD simulation, and higher temperature is beneficial to the H2 desorption. An elementary hydrogen evolution mechanism on the charged Pt cluster can be established as 2Hads+ + 2e− + Ptads→ [PtadsHadsHads] → H2↑ + Pt. A rate (fs−1) of hydrogen desorption from different cluster models is determined, together with the hydrogen dissociative activation energy as a function of total charge. © 2012 Wiley Periodicals, Inc.

Hydrogen sequential dissociative chemisorption on Nin(n = 2~9,13) clusters: comparison with Pt and Pd

Hydrogen dissociative chemisorption and desorption on small lowest energy Ni(n) clusters up to n=13 as a function of H coverage was studied using density functional theory. H adsorption on the clusters was found to be preferentially at edge sites followed by 3-fold hollow sites and on-top sites. The minimum energy path calculations suggest that H(2) dissociative chemisorption is both thermodynamically and kinetically favorable and the H atoms on the clusters are mobile. Calculations on the sequential H(2) dissociative chemisorption on the clusters indicate that the edge sites are populated first and subsequently several on-top sites and hollow sites are also occupied upon full cluster saturation. In all cases, the average hydrogen capacity on Ni(n) clusters is similar to that of Pd(n) clusters but considerably smaller than that of Pt(n) clusters. Comparison of hydrogen dissociative chemisorption energies and H desorption energies at full H-coverage among the Ni family clusters was made.

Hydrogen sorption performance of MgH 2 doped with mesoporous nickel- and cobalt-based oxides

The effect of mesoporous Co 3O 4, NiCo 2O 4 and NiO on the hydrogen sorption performance of MgH 2 was investigated. These oxides were synthesized by multi-step nanocasting and introduced during the high-energy ball milling of MgH 2 powder to act as catalysts. Hydrogen desorption on the as-milled powders was assessed upon heating the samples from room temperature to 400 °C. In all cases, the onset temperature for desorption was lowered by taking advantage of the introduced additives. The NiO-doped sample displayed the best response, the desorption rate being 7 times faster than in pure MgH 2. Complementary kinetic studies on this particular sample revealed that the sorption activation energies were much lower (50 kJ/mol for absorption and 335 kJ/mol for desorption) than the corresponding ones for undoped MgH 2 (57 kJ/mol for absorption and 345 kJ/mol for desorption), thus proving the catalytic activity of the mesoporous NiO oxide. Significantly, the X-ray powder diffraction (XRPD) patterns taken on the NiO-doped sample after discharging/charging cycles revealed that Mg could fully hydrogenate at the end of the charging process, while Mg metal was still detected in the undoped (pure) sample. Favored conditions for dissociative chemisorption of hydrogen could be ascribed to the formation of metallic Ni arising from complete or partial reduction of NiO, as observed in the XRPD patterns.

EFFECT OF SCANDIUM ON HIDROGEN DISSOCIATION ENERGY AT MAGNESIUM SURFACE: AB INITIO DFT STUDY

The dissociative chemisorption of hydrogen on both pure and Sc-incorporated Mg(0001) surfaces have been studied by ab initio density functional theory (DFT) calculation. The calculated dissociation energy of hydrogen molecule on a pure Mg(0001) surface (1.200 eV) is in good agreement with comparable theoretical studies. For the Sc-incorporated Mg(0001) surface, the activated barrier decreases to 0.780 eV due to the strong interaction between the molecular orbital of hydrogen and the d metal state of Sc. This could explain the experimentally observed improvement in absorption kinetics of hydrogen when transition metals have been introduced into the magnesium materials. Keywords: Dissociation, Adsorption, Chemisorptions, DFT, Magnesium

Energetics and dynamics of hydrogen adsorption, desorption and migration on a carbon-supported palladium cluster

The functionality of metal-doped carbon materials in catalytic and hydrogen storage applications is governed by the characteristics of their interaction with hydrogen. The dynamics of hydrogen chemisorption, its migration and desorption are investigated by first-principles molecular dynamics simulations of a model Pd4 cluster supported on a coronene molecule. Longer time scale events are accelerated using the metadynamics technique. This article is a comprehensive report of the energetics and dynamics of (i) dissociative chemisorption of molecular hydrogen on a carbon-supported Pd cluster, (ii) transport of atomic hydrogen on the cluster, towards the carbon support and (iii) the associative desorption of atomic hydrogens in a molecular form. It is found that the dissociative chemisorption of hydrogen on the cluster involves a negligible energy barrier and takes place readily at room temperature. The migration of atomic hydrogen from the tip of the tetrahedral Pd4 cluster to its sides, on the other hand, involves an energy barrier of ∼6 KJ/mole. This energy barrier, however, is reduced when the cluster is partially saturated with pre-existing hydrogens adsorbed on its sides. The energy barrier for the subsequent migration of atomic hydrogen, from the sides of the cluster towards the carbon support, is however not affected significantly by the presence of pre-existing hydrogens. Overall, the transport of atomic hydrogen from the tip of the cluster towards the carbon support is energetically favourable and only involves low energy barriers. The associative desorption of two hydrogen atoms from the cluster to form molecular hydrogen is an endothermic process and also involves a small energy barrier. This work sheds light onto the mechanism of the hydrogen interaction with Pd-doped-carbon-supported catalytic materials and, more generally, onto the mechanism of hydrogen spillover on such materials.

A theoretical study of the dissociative chemisorption of hydrogen on carbon nanotubes

Potential profiles were obtained for the chemisorption of hydrogen on (n, n) and (n, 0) carbon nanotubes. The energy barriers and rate constants for hydrogen molecule sorption on and desorption from various nanotubes were determined. The constants for sorption and desorption were used to calculate sorption-desorption equilibrium constants. Sorption on outside nanotube surfaces was found to be more favorable energetically than sorption on inside surfaces.

Hydrogen dissociative chemisorption and desorption on saturated subnano palladium clusters (Pdn, n = 2–9)

H2 sequential dissociative chemisorption on small palladium clusters was studied using density functional theory. The chosen clusters Pdn (n = 2-9) are of the lowest energy structures for each n. H2 dissociative chemisorption and subsequent H atom migration on the bare Pd clusters were found to be nearly barrierless. The dissociative chemisorption energy of H2 and the desorption energy of H atom in general decrease with the coverage of H atoms and thus the catalytic efficiency decreases as the H loading increases. These energies at full cluster saturation were identified and found to vary in small energy ranges regardless of cluster size. As H loading increases, the clusters gradually change their bonding from metallic character to covalent character. For the selected Pd clusters, the capacity to adsorb H atoms increases almost proportionally with cluster size; however, it was found that the capacity of Pd clusters to adsorb H atoms is, on average, substantially smaller than that of small Pt clusters, suggesting that the catalytic efficiency of Pt nanoparticles is superior to Pd nanoparticles in catalyzing dissociative chemisorption of H2 molecules.