Associative Desorption Of H2 Research Articles

Deposition of an ultrathin palladium (Pd) coating on SAPO-34 membranes for enhanced H2/N2 separation

Hydrogen energy has attracted great attention due to its properties of high energy transferring efficiency and zero pollution emission. Zeolite membranes are promising candidates for H2 separation because of their uniform, molecular-sized pores and high thermal and mechanical stabilities. However, thicker membranes or modification treatments are often necessary to eliminate the defects formed during synthesis and post calcination, leading to low gas permeance. Herein, we reported the deposition of an ultrathin palladium (Pd) coating on SAPO-34 membranes to improve H2 separation performance. H2/N2 selectivity was greatly increased by deposition of an ultrathin Pd coating on SAPO-34 membranes, while maintaining similar H2 permeance. This might be attributed to the dissociative adsorption and associative desorption of H2 on Pd, as well as fast diffusion of H2 through ultrathin Pd coating. We also noticed that excessive Pd deposition would lead to the formation of cracks on SAPO-34 membranes, leading to deteriorated membrane performance.

The Temperature-Programmed Desorption of H2 from Cu/ZrO2

The desorption kinetics of hydrogen from a polycrystalline Cu/ZrO2 catalyst was investigated under atmospheric pressure using temperature-programmed desorption (TPD) experiments in a microreactor set-up. Different heating rates were applied under equal conditions with a carefully reduced catalyst. The hydrogen TPD peaks were symmetric and centered slightly above 300 K indicating associative desorption of H2 from metallic Cu. Using heating rate variation, the kinetic parameters $$A_{des}$$ and $$E_{des}$$ were determined to be 1.24 × 109 s−1 and 68 kJ mol−1, respectively. As the modeling with constant values of $$A_{des}$$ and $$E_{des}$$ yielded signals which were too narrow, dependence of $$E_{des}$$ on coverage was introduced applying $$E_{des} - K (\Theta _H)^n$$ . By application of the “full-analysis” method an optimal fit to the experimental data was found. Setting n = 1 resulted in the best fit and a value of 61 kJ mol−1 − (6.25 kJ mol−1 × $$\Theta _H$$ ) for $$E_{des}$$ was determined.

Ab Initio Study of H2 Associative Desorption on Ad-Dimer Reconstructed Si(001) and Ge(001)-(2×1) Surfaces

We investigate the pathways of hydrogen migration and associative desorption of H2 on the Si(001) and Ge(001)-(2×1) reconstructed surfaces with adsorbed ad-dimers, using density functional theory methods. Although the trends obtained with common semilocal exchange–correlation functionals such as the generalized gradient approximation are correct, we show that the use of exact short-range Fock exchange in the calculations (by means of hybrid functionals) strongly affects the magnitude of the desorption barriers, especially for Ge surfaces, leading to a better estimate of the desorption temperatures. Our results for H2 desorption kinetic barriers are well-supported experimentally and can therefore help to elucidate the mechanisms driving atomic layer epitaxy growth processes.

Surface Coverage Effects on the Formation of Molecular Hydrogen on a Graphite Surface via an Eley−Rideal Mechanism

The associative desorption of H2(O) on a graphite(0001) surface via an Eley-Rideal mechanism has been studied using a time-dependent wave packet method. In particular, the importance of a coverage of other light H atoms on the surface was investigated. We find that the H2 molecules are formed less vibrationally excited than in previous studies with no surface coverage of H atoms. We also find that the presence of other (nonreactive) hydrogen atoms does have an effect on the formation probabilities of H2(O), especially at low translational energies of the incoming H atom and at resonances.

Time-Dependent Quantum Mechanical Calculations on the Formation of Molecular Hydrogen on a Graphite Surface via an Eley−Rideal Mechanism

The associative desorption of H2 (ν,j) on a graphite(0001) surface via an Eley−Rideal mechanism has been studied theoretically. In our calculations we used a time-dependent wave packet method treating three degrees of freedom quantum mechanically. A newly developed potential energy surface based on plane-wave density functional calculations was employed. In our 3D calculations we find less vibrational excitation for the product H2 molecules than in calculations that used only two degrees of freedom. However, the product H2 molecules are formed rotationally excited. This could have important implications for the chemistry of H2 in the interstellar medium and the interpretation of astronomical data.

The temperature-programmed desorption of hydrogen from copper surfaces

The desorption kinetics of H2 from a Cu/ZnO/Al2O3 catalyst for methanol synthesis were studied under atmospheric pressure in a microreactor set-up by performing temperature-programmed desorption (TPD) experiments after various pretreatments of the catalyst. Complete saturation with adsorbed atomic hydrogen was obtained by dosing highly purified H2 for 1 h at 240 K and at a pressure of 15 bar. The TPD spectra showed symmetric H2 peaks centered at around 300 K caused by associative desorption of H2 from Cu metal surface sites. H2 TPD experiments performed with different initial coverages resulted in peak maxima shifting to higher temperatures with lower initial coverages indicating that the desorption of H2 from Cu is of second order. The microkinetic analysis of the TPD traces obtained with different heating rates yielded an activation energy of desorption of 78 kJ mol−1 and a corresponding frequency factor of desorption of 3×1011 s−1> in good agreement with the kinetic parameters obtained with Cu(111) under UHV conditions.

Quantum-state-specific dynamics of the dissociative adsorption and associative desorption of H2 at a Cu(111) surface

We have determined the dependence of the dissociative adsorption probability in the zero coverage limit, S0, for H2 on Cu(111) as a function of translational energy, Ei, and incidence angle, θi, vibrational state, v, and rotational state, J. We have also obtained information on the effect of surface temperature, Ts, on this probability. These results have been obtained by combining the findings of two separate experiments. We have obtained the form of the dependence of S0 on Ei at Ts=925 K for a range of quantum states from desorption experiments via the principle of detailed balance. We have obtained absolute S0 values from direct molecular beam adsorption experiments, which reveal that S0 scales with the so-called ‘‘normal energy,’’ En=Ei cos2 θi. The desorption experiments provide detailed information for J=0 to 10 of H2(v=0) and for J=0 to 7 of H2(v=1). The beam experiments additionally provide information on the adsorption of H2(v=2), averaged over J. All measurements are consistent with adsorption functions with an s-shaped form, which can be described by S0=A(1+erf(x))/2, where x=(En−E0)/W. Values of W are ∼0.16 and 0.13 eV for v=0 and v=1, respectively, at Ts=925 K, falling by about 0.05 eV for Ts=120 K, and with only a slight dependence on J. Values of A are insensitive to v and J, with a value of ∼0.25. S(En,v,J) curves are thus similar for different v and J, but shifted in En. In contrast, we find that the values of E0, which determine the mid-point of the curves, have a strong dependence on v and J. Specifically, E0 for H2(v=0) molecules is about 0.6 eV, falling to 0.3 and 0.1 eV for H2(v=1) and H2(v=2), respectively. Translational energy is thus about twice as effective as vibrational energy in promoting dissociation. E0 rises with increasing J at low J, before falling at high J, indicating that rotational motion hinders adsorption for low rotational states (J<4), and enhances adsorption for high rotational states (J≳4). Results are compared with similar studies on the D2/Cu(111) system and with recent calculations. Finally, these results are used to predict the dependence of the rate of dissociation on temperature for a ‘‘bulb’’ experiment with ambient hydrogen gas in contact with a Cu(111) surface. This simulation yields an activation energy of 0.47 eV for temperatures close to 800 K, compared to a literature value of 0.4 eV from experiment. Analysis of the temperature dependence reveals that the dominant reason for the increase in rate at high temperature is the increase in population of the high energy tail of the translational energy distribution.

Reactive scattering using efficient time-dependent quantum mechanical wave packet methods on an L-shaped grid

Most quantum mechanical time-dependent wave packet methods represent the wave packet on Cartesian grids. For reactive systems these grids can contain many points at which the wave function amplitude is negligible throughout the calculation. A significant fraction of the computational resources in a calculation may be used in calculating and storing values of the wave function at these grid points. This work describes a method which uses the Fourier transform method to calculate the kinetic energy of a wave function on a grid which covers only reactant and product regions of the potential. For a two-dimensional system this corresponds to the use of an L-shaped grid. The time propagation is performed using the Chebyshev propagation method. The method is demonstrated for a model problem treating dissociative adsorption and associative desorption of H2 from a flat surface. The amount of computational resources required for these calculations is much less than would be needed if the full rectangular grid was used.

Dissociative adsorption and associative desorption of H2 on a flat surface

A quantum mechanical time-dependent method was used to study the dynamics of dissociative adsorption and associative desorption of H2 on a flat, static surface. We used a two-dimensional model in which the molecular axis was held parallel to the surface and the diatom internuclear separation and distance above the surface were the dynamic variables. A modified London–Eyring–Polanyi–Sato (LEPS) potential described the molecule–surface interactions. The wave function for the molecule was represented by its values on a spatial grid of points. The wave function was propagated by expanding the time evolution operator in a series of Chebyshev polynomials and using the properties of the Fourier transform to calculate the kinetic energy. The computational requirements of the problem were significantly reduced by using an L-shaped grid which deletes a large number of points where it is known a priori that the wave-function amplitude vanishes. State-to-state transition probabilities were calculated as a function of the initial translational and vibrational energy for potentials with early, late, and intermediate barriers. The location of the barrier has a strong effect on the energy threshold for reaction and on the distribution of energy between vibration and translation in the products.

Dynamics of interaction of H 2 and D 2 with Ni(110) and Ni(111) surfaces

Abstract Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H2 and D2 on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s0, irrespective of surface temperature Ts and mean kinetic energy normal to the surface 〈 E⊥ 〉. The desorbing molecules exhibit a cos θe angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s0 is only about 0.05 if both the gas and the surface are at room temperature. s0 is again independent of Ts, but increases continuously with 〈 E⊥ 〉 up to a value of ∼0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos5θe angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.

Dynamics of interaction of H2 and D2 with Ni(110) and Ni(111) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H2 and D2 on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s0, irrespective of surface temperature Ts and mean kinetic energy normal to the surface 〈 E⊥ 〉. The desorbing molecules exhibit a cos θe angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s0 is only about 0.05 if both the gas and the surface are at room temperature. s0 is again independent of Ts, but increases continuously with 〈 E⊥ 〉 up to a value of ∼0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos5θe angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.