Dynamics Of Hydrogen Adsorption Research Articles

Adsorption dynamics of hydrogen and deuterium in a carbon molecular sieve bed at 77 K

The adsorption equilibrium and rate of H2 and D2 on a Carbon Molecular Sieve (CMS) with wide pore size distribution were measured at 77K using a volumetric method. The adsorption equilibrium or kinetic ratios for D2 to H2 decreased with increasing pressure. The adsorption uptake curve showed a slow adsorption rate with time after displaying a fast adsorption rate in the initial period. Adsorption rate constants of D2 were larger than those of H2 at low pressure, and the ratio of D2 to H2 decreased with an increase in pressure. However, in breakthrough experiments for dynamic separation efficiency of hydrogen isotope separation, the CMS showed higher breakthrough separation factors than the corresponding adsorption equilibrium or kinetic ratios. The breakthrough separation factor reached 1.53 at a total gas pressure of 400.0kPa and a flow rate of 129.8cm3/min in the adsorption bed length of 1.0m. In addition, the shape of D2 breakthrough curve was slightly affected by the change of flow rate, but that of H2 breakthrough curve showed almost constant pattern. And meso and macropores of the CMS gave a negative effect on the separation of H2 and D2. Therefore, the key factor in separating H2 and D2 from the mixture depended not only on their equilibrium selectivity but also on their dynamic diffusional difference in the CMS bed due to the quantum molecular sieving effect in space-limited pore at low temperature.

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.

The dynamics of hydrogen adsorption on polycrystalline uranium

The dynamics of dissociative hydrogen adsorption on clean polycrystalline uranium has been studied using temperature programmed desorption and supersonic molecular beams. The initial sticking probability was measured as a function of incident kinetic energy between 3 and 255 meV. Two adsorption channels were observed; a non-activated direct channel was shown to be active over the entire energy range studied and a low energy indirect channel that was characterised by a decrease in sticking probability with increasing beam energy, and an insensitivity to both surface temperature and a range of hydrogen coverages. Together these results suggest the existence of an unaccommodated molecular precursor that has sufficient lifetime and mobility to locate favourable sites and dissociatively adsorb.

Hydrogen desorption from polycrystalline platinum chemically modified by sulfur pre-coverage

Hydrogen desorption energetics from clean polycrystalline platinum (Pt(Poly)) and (Pt(Poly)) with sulfur (S) pre-coverage (S/Pt(Poly)) have been studied using temperature programmed desorption (TPD) over a wide range of hydrogen and S coverages. Hydrogen desorption energies have been calculated and tabulated. At saturation, hydrogen desorbs from clean Pt(Poly) in three states (384, 320, and 282 K). The 384 K state is attributed to hydrogen from defect sites. The 320 and 282 K states are both attributed to hydrogen from (1 1 1) sites, the latter state induced by repulsive lateral interaction between hydrogen in neighboring sites. On clean Pt(Poly), the desorption energy for hydrogen from defect sites is essentially fixed over the entire coverage range, dilute to saturation. In contrast, the desorption energy for hydrogen from (1 1 1) sites begins to decrease when about 50% of the surface is saturated. The decrease onset is attributed to significant accumulation of hydrogen in fcc three-fold hollows relative to hcp hollows. With increasing hydrogen and S coverages, the energies of desorption converge to 4.5 kcal/mol, the barrier height to hydrogen diffusion on (1 1 1) terraces of d-band transition metal surfaces via classical over-the-barrier hopping. Therefore, increasing S coverage has the same effect on the binding strength of Pt( Poly) for hydrogen as increasing hydrogen coverage (site blocking). But more interestingly, at high surface coverage, atomic hydrogen recombination immediately after hopping out of the adsorption well without surface diffusion is indicated. Though, S site blocking has been found as the primary cause of decreased hydrogen desorption temperatures, several trends in the data cannot be rationalized using a pure site blocking argument alone. Sulfur induced additional repulsive lateral interaction at high coverage is strongly suggested, through-space, short range electronic effects. Further, long range S effects (through space) are also clearly supported. The uniform percent decrease in hydrogen adsorption independent of exposure is not consistent with pure site blocking but can be readily explained by a decreased sticking coefficient on S/Pt(Poly) surfaces due to long range S effects. In particular, the strong inhibition of hydrogen adsorption on S/Pt(Poly) surfaces (the long range effect) is attributed mostly to electrostatic repulsion of approaching hydrogen caused by non-bonding electron pairs in hybrid valence sulfur orbitals normal to the surface. Insight has also been gained into the dynamics of hydrogen adsorption. The study of fixed hydrogen exposures as a function of S coverage shows zero order kinetics for the initial rate of desorption of the 320 K state, i.e. the leading edges are `pinned'. Thus, hydrogen adsorption on Pt(Poly) in patches nucleated at defects that attain similar density independent of coverage is implied. Key steps in the adsorption and desorption trajectories as indicated by this TPD work are illustrated in a schematic.

Dynamics of hydrogen adsorption on promoter-and inhibitor-modified nickel surfaces

This paper deals with changes in the adsorption dynamics for hydrogen on Ni(111) introduced by an inhibitor (potassium) and a promoter (oxygen). At low surface coverage each potassium atom generates an area of increased barrier height for hydrogen dissociation. Depending on the H 2-particle energy this inactive area can range from 7 to 100 adsorption sites. At high potassium coverage the adsorption behavior of H 2 on nickel becomes more similar to adsorption on a noble metal like copper, exhibiting a barrier height of about 0.3 eV. Oxygen as coadsorbate generates an inhibited area in its close surroundings, further away an area with reduced barrier height is established. A much smaller area compared topotassium is modified by each oxygen atom. For the case of oxygen and hydrogen a repulsive interaction of the coadsorbates is observed; in contrast, potassium and hydrogen exhibit an attractive interaction. The changes in adsorption dynamics for H 2 introduced by the coadsorbates are discussed in relation to the work functions observed.

Dynamics of hydrogen adsorption on clean and alkali-metal covered Cu(110)

The activated dissociative adsorption of H2 and D2 on a clean Cu(110) and a Cu(110)-K(1 × 2) reconstructed surface has been studied using supersonic molecular beams. Enhanced sticking of H2 over D2 at any particular normal energy in the pure beam experiment on Cu(110) is interpreted as an indication of the importance of the vibrational coordinate in the dissociation dynamics. The relative contributions of vibrational and translational energy in accessing the barrier is separated in vibrationally hot, translationally cold seeded beams. Results for hydrogen are presented which indicate a translational onset for the first vibrationally excited state H2(v= 1) at 130 meV. This result is consistent with a theoretically predicted barrier to dissociation of ca. 700 meV which lies somewhat in the exit channel of a two-dimensional potential-energy surface (2D-PES). The effect of alkali-metal adsorption on the (1 × 2) reconstructed surface is shown to enhance hydrogen adsorption, increase the activation energy to desorption and reduce the activation energy to adsorption. This result is consistent with a reduction in the height of a late barrier caused by a deeper atomic potential in the ‘product’ region of the 2D-PES.

Microfacets of the (1 × 2) reconstructed Pt(110) surface seen in the adsorption dynamics of hydrogen

Microfacets of the (1 × 2) reconstructed Pt(110) surface seen in the adsorption dynamics of hydrogen