Dissociative Adsorption Of Hydrogen Molecule Research Articles

Dissociative Adsorption of Hydrogen Molecules at Al2O3 Inclusions in Steels and Its Implications for Gaseous Hydrogen Embrittlement of Pipelines

Hydrogen embrittlement (HE) of steel pipelines in high-pressure gaseous environments is a potential threat to the pipeline integrity. The occurrence of gaseous HE is subjected to associative adsorption of hydrogen molecules (H2) at specific “active sites”, such as grain boundaries and dislocations on the steel surface, to generate hydrogen atoms (H). Non-metallic inclusions are another type of metallurgical defect potentially serving as “active sites” to cause the dissociative adsorption of H2. Al2O3 is a common inclusion contained in pipeline steels. In this work, the dissociative adsorption of hydrogen at the α-Al2O3(0001)/α-Fe(111) interface on the Fe011¯ plane was studied by density functional theory calculations. The impact of gas components of O2 and CH4 on the dissociative adsorption of hydrogen was determined. The occurrence of dissociative adsorption of hydrogen at the Al2O3 inclusion/Fe interface is favored under conditions relevant to pipeline operation. Thermodynamic feasibility was observed for Fe and O atoms, but not for Al atoms. H atoms can form more stable adsorption configurations on the Fe side of the interface, while it is less likely for H atoms to adsorb on the Al2O3 side. There is a greater tendency for the occurrence of dissociative adsorption of O2 and CH4 than of H2, due to the more favorable energetics of the former. In particular, the dissociative adsorption of O2 is preferential over that of CH4. The Al-terminated interface exhibits a higher H binding energy compared to the O-terminated interface, indicating a preference for hydrogen accumulation at the Al-terminated interface.

Dissociative adsorption of hydrogen and methane molecules at high-angle grain boundaries of pipeline steel studied by density functional theory modeling

Pipelines provide an economic and efficient means for hydrogen transport, contributing to accelerated realization of a full-scale hydrogen economy. Dissociative adsorption of hydrogen molecules (H2) occurring on pipe steels generates hydrogen atoms (H), potentially resulting in hydrogen embrittlement of the pipelines. This is particularly important for existing pipelines transporting hydrogen in blended form with methane (CH4). In this work, a density functional theory model was developed to investigate the dissociative adsorption of H2 and CH4 at high-angle grain boundaries (HAGB), a typical type of hydrogen traps contained in steels, and the stable adsorption configurations. Results demonstrate that the dissociative adsorption of both H2 and CH4 at the HAGB is thermodynamically feasible under pipeline operating conditions. Compared with crystalline lattice sites, the HAGB possesses the most negative free energy change, a lower energy barrier and the lowest H-adsorption energy, making the HAGB, especially the quasi three-fold site, become the most stable site for hydrogen adsorption. The saturation coverage of hydrogen at HAGB is calculated to be 1.33. The iron-H bonds are formed at the HAGB by charge consumption at Fe atoms and electron accumulation at H atoms, following a so-called electron hybridization mechanism. The CH4 adsorption at HAGB affects the H2 adsorption. Without pre-adsorption of CH4, the hydrogen adsorption at the HAGB is more stable. Although an elevated CH4 partial pressure decreases the thermodynamic tendency for H2 adsorption, it cannot hinder occurrence of the H2 dissociative adsorption.

Thermodynamics of spontaneous dissociation and dissociative adsorption of hydrogen molecules and hydrogen atom adsorption and absorption on steel under pipelining conditions

While hydrogen pipelines have attracted increased attention, safety of the pipelines has been a concern in terms of hydrogen embrittlement (HE) occurring upon hydrogen atom (H) generation and permeation in the steels. In this work, thermodynamic analyses regarding H generation and adsorption on pipeline steels by two potential mechanisms, i.e., spontaneous dissociation and dissociative adsorption, were conducted through theoretical calculations based on Gibbs free energy change of the H generation reactions. Moreover, H adsorption free energy and configurations were determined based on density functional theory (DFT) calculations. Effects of H adsorption site, H coverage and hydrostatic stress on H adsorption and absorption were discussed. Spontaneous dissociation of hydrogen gas molecules to generate hydrogen atoms is thermodynamically impossible. Dissociative adsorption is thermodynamically feasible at wide temperature and pressure ranges. Particularly, an increased hydrogen gas partial pressure and elevated temperature favor the dissociative adsorption of hydrogen. Hydrogen atoms generated by dissociative adsorption mechanism can adsorb stably at On-Top (OT) and 2-fold (2F) Cross-Bridge sites of Fe (100), while hydrogen adsorption at 2F site is more stable due to a higher electron density and a stronger electronic hybridization between Fe and H. The influence of H atom coverage on dissociative adsorption occurs at low coverages only, i.e., 0.25–1.00 ML as determined in this work. External stresses make dissociative adsorption more difficult to occur compared with a fully relaxed steel. Both tetrahedral sites (TS) and octahedral sites (OS) can potentially host absorbed H atoms at subsurface of the steel. Absorbed H atoms will be predominantly trapped at TS due to a low energy path and exothermic feature. Diffusion of H atoms from steel surface to the subsurface is more difficult compared with the dissociative adsorption.

Prevention of Hydrogen Damage Using MoS₂ Coating on Iron Surface.

The prevention of hydrogen penetration into steels can effectively protect steels from hydrogen damage. In this study, we investigated the effect of a monolayer MoS2 coating on hydrogen prevention using first-principles calculations. We found that monolayer MoS2 can effectively inhibit the dissociative adsorption of hydrogen molecules on an Fe(111) surface by forming a S–H bond. MoS2 coating acts as an energy barrier, interrupting hydrogen penetration. Furthermore, compared with the H-adsorbed Fe(111) film, the work function of the MoS2-coated film significantly increases under both equilibrium and strained conditions, indicating that the strained Fe(111) film with the MoS2 coating also becomes more corrosion resistant. The results reveal that MoS2 film is an effective coating to prevent hydrogen damage in steels.

Characteristics of Hydrogen Sensors Based on Thin Tin Dioxide Films Modified with Platinum, Palladium, Silver, and Yttrium

The dependence of the characteristics of hydrogen sensors based on thin Pt/Pd/SnO2:Sb, Ag, Y and Ag/SnO2:Sb, Ag, Y films obtained by the method of magnetron sputtering at a direct current on the humidity level of the gas mixture during operation in thermal cyclic mode is investigated. The analysis of the experimental data was carried out using the expressions obtained on the basis of the dissociative adsorption model of hydrogen and water molecules on the SnO2 surface, taking into account the presence of three types of adsorbed particles: O–, OH, OН–. The use of complex modifiers Pt/Pd, Ag on the surface and Ag + Y in the bulk of tin dioxide films allows to reduce the influence of humidity on the characteristics of sensors. The role of these modifying additives in the process of dissociative adsorption of hydrogen molecules on the surface of SnO2 films in humid environment is established.

Dissociative Adsorption of Hydrogen Molecule on Aluminum Clusters: Effect of Charge and Doping

The dissociative chemisorption of molecular hydrogen on charged and neutral aluminum clusters Al12X (X = Mg, Al, Si) was investigated using DFT and a modified G3(MP2)-RAD procedure. Reaction barriers and enthalpies were determined for both neutral and singly charged clusters. The lowest barrier for dissociative adsorption of H2 on a neutral cluster was found for the Al12Mg cluster, whereas the highest barrier was found to be on the closed-shell Al12Si. The interaction of H2 with Al13(+) is found to proceed via an association complex that is 0.07 eV lower in energy than the isolated species and from which the barrier to H2 dissociative adsorption is only 0.16 eV. The most exothermic reaction of H2 with Al12X occurs for the Al13(+)/H2 system. In comparison, reactions with the closed-shell Al13(-) and Al12Si clusters are found to be endothermic. The barriers for H2 desorption from the dihydrogenated clusters are generally quite substantial.

Strong Interaction at the Palladium/Alumina Interface of Membrane during Hydrogen Permeation at Elevated Temperature

Abstract Hydrogen flux through palladium membrane supported on α-alumina significantly decreased during permeation testing at elevated temperatures (650–850 °C). Strong interaction between palladium and α-alumina, i.e., formation of less hydrogen permeable palladium–aluminum alloy was suggested via reduction of alumina under hydrogen atmosphere at high temperature. Migration of aluminum to the membrane top surface might obstruct the dissociative adsorption of hydrogen molecules.

Antiferromagnetic coupling between two adjacent dangling bonds on Si(001): Total-energy and force calculations

Scanning tunneling microscopy experiments reported that desorption from the hydrogen- and halogen-terminated Si(001) surfaces exhibits frequently the two types of dangling-bond (DB) configurations. One is the intradimer configuration, where two DBs are within a single Si dimer, and the other is the interdimer configuration, where two DBs are on one side of two adjacent Si dimers. Our spin-polarized density-functional-theory calculations show that the intradimer configuration is nonmagnetic with a buckled-dimer geometry, while the interdimer configuration is antiferromagnetic with two adjacent symmetric dimers. In addition, we show that when the dissociative adsorption of hydrogen molecule occurs across the ends of two adjacent dimers on a clean Si(001) surface, such an antiferromagnetic coupling between two adjacent DBs still exists, thereby giving rise to a structural transformation from buckled to symmetric dimers.

Functionalization of carbon nanotubes by covalently bonded graphite nanoplatelets: a theoretical study

the obvious way for an enhancement ofthe hydrogen adsorption capacity of carbon-based materials isthe creation of new tubular nanostructures with greater surfaceareas, as compared with ‘classical’ cylinder-like carbon nano-tubes (among other offered approaches, for example, a decora-tion of carbon nanotubes with titanium atoms that is favourablefor dissociative adsorption of hydrogen molecules

Ab initiostudy of hydrogen interaction with pure and nitrogen-doped carbon nanotubes

Detailed studies of mechanisms for hydrogen dissociative adsorption and diffusion on pure and nitrogen-doped (8, 0) carbon nanotubes are carried out using the first-principles density functional theory method. (1) For pure carbon nanotubes, we have identified the energetically most favorable dissociative pathway for hydrogen adsorption, with a barrier height of $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We also found that the adsorbed hydrogen atoms can act as an autocatalyst for further dissociative adsorption of hydrogen molecules. (2) It is found that on pure carbon nanotubes the diffusion of hydrogen atoms is constrained by interaction with neighboring adsorbed hydrogen atoms. The diffusion barrier is around $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for an isolated hydrogen atom, but becomes substantially higher at around $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the presence of adsorbed hydrogen in neighboring positions. (3) Doping the nanotube with nitrogen considerably alters the catalytic effects of the carbon nanotube for hydrogen dissociative adsorption. The dissociative adsorption of hydrogen on the carbon nanotube is greatly enhanced, with the barrier substantially reduced to ca. $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The differences in the barrier heights are explained through analysis of the electronic structure changes of the nanotube.

Shape selective and hydrogen spillover approach in the design of sulfur-tolerant hydrogenation catalysts

This paper presents a novel approach to preparing sulfur-tolerant noble metal catalysts and the proof-of-concept experimental results. Our approach combines the concepts of shape selectivity and hydrogen spillover to the design of sulfur-tolerant catalysts. Platinum particles were encapsulated in the NaA-zeolite cages during zeolite synthesis. The pore opening was subsequently reduced by potassium ion exchange and chemical vapor deposition of tetraethyl orthosilicate (TEOS), allowing hydrogen molecules to enter but preventing hydrogen sulfide molecules from contacting the platinum particles. The success of platinum encapsulation in KA-zeolite was confirmed by XRD, H 2 and CO chemisorption, and TOF-SIMS techniques. A sulfur-tolerant catalyst was prepared by mixing TEOS-treated Pt/KA-zeolite with HY-zeolite and γ-alumina, and tested in the hydrogenation of naphthalene. Hydrogen species produced via dissociative adsorption of hydrogen molecules on platinum particles, situated inside KA-zeolite cages, migrated through surface diffusion to HY-zeolite and γ-alumina. Our experimental data demonstrated that the encapsulated platinum particles continuously generated hydrogen surface species that were used to hydrogenate naphthalene adsorbed on HY-zeolite and γ-alumina. These platinum particles showed resistance to sulfur poisoning when pure hydrogen was replaced by 3% H 2S in hydrogen in the naphthalene hydrogenation reaction.

Scattering and dissociative adsorption of H2 on the armchair and zigzag edges of graphite

We performed quantum dynamics calculations on the scattering and dissociative adsorption of hydrogen molecules incident on the armchair and zigzag edges of graphite layers, using relevant potential-energy surfaces (PESs) recently obtained by Diño et al. [e-J. Surf. Sci. Nanotech. 2, 77 (2003), and references therein]. By employing the coupled channel method to determine the reflection and sticking probabilities, we compared the hydrogen scattering and dissociative adsorption dynamics on the two graphite surfaces. Our findings show the different scattering behaviors of H2 for the armchair edge and for the zigzag edge, which enable the identification of an unknown graphite edge from its interaction with H2. The scattering on the zigzag edge is due to the highly curved region of the PES reaction path for H2 interacting with the zigzag edge, whereas the scattering for the armchair edge is caused by a potential barrier. The reflection probability initially decreases with increasing the kinetic energy in both cases but gradually increases for the zigzag edge. Our findings also indicate that the zigzag edge can adsorb hydrogen better than the armchair edge, mainly due to the absence (presence) of an activation barrier in the zigzag (armchair) edge. There is a very weak dependence of the sticking probability on the initial vibrational state of H2 for both graphite edges. The difference in the vibrational effect is due to the relative position of the curved region with respect to the potential barrier (well).

Monte Carlo simulation of hydrogen absorption in palladium and palladium–silver alloys

A modified Monte Carlo (MC) simulation was performed to investigate the hydrogen absorption behavior in Pd and Pd–Ag alloys of the composition Pd x Ag 1− x ( x=0.7–0.8) under H 2 pressure (0.1 MPa) at different temperatures. The present method employed can consider the dissociative adsorption of hydrogen molecule and the subsequent absorption of hydrogen atom by formalizing the relationship between the pressure of hydrogen molecule and hydrogen atom. The potential parameters were determined to reproduce the solution enthalpy of hydrogen in pure metals. The results are in good agreement with experimental findings as well as previous theoretical studies. We confirmed that our method is useful to simulate the absorption of hydrogen in metals and alloys.

Metal and oxide thin film MO CVD as a base for nanostructure and superlattice formation

Mechanism and kinetics of Ni, Cu and Y 2 O 3 thin film deposition are considered. It is shown that the decomposition temperature of chelate compounds decreases in hydrogen atmosphere. The film deposition temperature decreasing is defined by catalytic reaction of dissociative adsorption of hydrogen molecules. Then hydrogen atoms react with precursor molecules. The temperature of Y 2 O 3 film deposition from Y(III) β-diketonate reaches 500°C because the CVD process is produced in inert atmosphere. Use of H 2 O vapor addition to above mentioned CVD process permits to increase the precursor adsorption on the substrate surface and provides films with good electrical properties. All these principles can be used for nanostructure and superlattice formation by an adsorption CVD method.

Highly reactive dissociative adsorption of hydrogen molecules on partially H-covered Si(001) surfaces: A density-functional study

Despite intensive research the atomistic reaction path leading to the dissociative adsorption of hydrogen molecules on a Si(001) substrate is still debated. In this paper density-functional calculations employing a generalized gradient approximation for the exchange-correlation functional will be presented for the interdimer reaction paths on both clean and H-precovered Si(001) surfaces. It turns out that silicon surface dimers with one of their two dangling bonds saturated with a hydrogen atom result in very reactive ${\mathrm{H}}_{2}$-adsorption sites. The high reactivity is explained to be due to dangling-bond states close to the Fermi level, which can very efficiently hybridize with the antibonding $1s{\ensuremath{\Sigma}}_{\mathrm{u}}^{*}$ state of the ${\mathrm{H}}_{2}$ molecule. This is analogous to the high reactivity of step sites on Si(001) vicinal surfaces discussed by Kratzer et al. On the clean surface the calculations produce an adsorption-energy barrier for the interdimer reaction path which is even slightly smaller than the respective barrier for the common intradimer reaction path. Thus, provided its prefactor is not too small, the interdimer reaction path might be relevant for the sticking of ${\mathrm{H}}_{2}$ on Si(001).

Hydrogen adsorption and desorption on silicon revisited

Recent molecular beam data on the energy dependence of sticking coefficients for dissociative adsorption of hydrogen molecules on silicon are analyzed in terms of the five dimensional (5D) quantum reaction dynamics used before to describe data on sticking as a function of surface temperature and on state resolved desorption. The sticking coefficient of the 5D model depended strongly on the energy of surface phonons but weakly on the kinetic energy of the incoming H2 molecules. In a recent molecular beam experiment the latter dependence was measured directly and found to be substantially stronger than concluded from the earlier data. These results have important consequences for the energy and angular distribution of desorbing particles: If the parameters of the 5D model are adjusted so as to describe the new data, it predicts a mean normal kinetic energy of desorbing particles which is now about twice the thermal value. This prediction is rather model independent and can be derived almost directly from the measured data by invoking “detailed balance.” This new increased normal mean energy then is closer to ab initio generalized gradient approximation (GGA) slab calculations predicting about three times the thermal value. The increased kinetic energy also leads to a more forward-peaked angular distribution ∝(cos θ)11 to 12 instead of ∝(cos θ)3 to 4.

H2dissociative adsorption on Pd(111)

The dissociative adsorption of hydrogen molecules on the (111) surface of palladium is studied by total-energy calculations using the density-functional theory within the framework of the generalized gradient approximation. A variety of dissociation pathways is investigated. Both activated and nonactivated pathways are found. We carry out a detailed analysis of the electronic structure along the different pathways to understand the microscopic mechanism of bond breaking and bond formation. This analysis allows us to account for the energetic ordering of the transition states. Precursor states are also identified for the system considered here.

REACTION PATH FOR THE DISSOCIATIVE ADSORPTION OF HYDROGEN ON THE (100) SURFACES OF FACE-CENTERED-CUBIC TRANSITION METALS

Detailed ab-initio local-density-functional studies of the potential-energy surfaces for the dissociative adsorption of hydrogen molecules on the (100) surfaces of face-centered-cubic transition and noble metals are presented. We show that the energetically most favorable reaction path is determined by quantum-mechanical steering effects arising from the formation and modification of covalent metal–hydrogen bonds. Variations of the local chemical reactivity with the filling of the d-band are discussed at the example of rhodium, palladium and silver surfaces.

H2 DISSOCIATIVE ADSORPTION ON SURFACES OF Ni, Pd and Cu

The dissociative adsorption of hydrogen molecule on (100), (110) and (111) surfaces of Ni, Pd and Cu are investigated using the embeded-atom method (EAM). First, the EAM potentials for H-metal surfaces are obtained by fitting the adsorption energies and the length of adsorption bond for single H atom on different surfaces of Ni, Pd and Cu. Then, the activation barriers Ea, adsorption heat q2d and corresponding H-metal bond length R for dissociation of H2 through various pathways are calculated. The potential energy contours for H2 dissociation on (110) surfaces of different metals are presented. The calculated results show that the dissociative adsorption of a hydrogen molecule depend on the species of substrate, the surface orientation and the dissociative pathways. The activation barrier for H2 dissociation on Nisurface is very low, while it is higher for H2 dissociation on Cu surface. The dissociative adsorption of H2 is easier on open (110) surface than that on close-packed(111) surfaces. Among the discussed dissociation pathways, it appears that the most favourable pathway for H2 is that from bridge site to the adjacent centre sites.

Dissociative Adsorption of Hydrogen Molecule on MgO Surfaces Studied by the Density Functional Method

Dissociative Adsorption of Hydrogen Molecule on MgO Surfaces Studied by the Density Functional Method