Adsorption Of Diatomic Molecules Research Articles

Adsorption of diatomic molecules on nitrogenated holey graphene: Theoretical insights

Physical adsorption of three diatomic molecules (H2, CO, and HF) on graphene and recently synthesized N-doped holey graphene (4N-graphene) has been studied by using a set of theoretical methods to reveal peculiarities of their non-covalent interactions. Adsorption of hydrogen molecules shows approximately doubled interaction energies (Eint) for 4N-graphene compared with pristine one. At the same time, the CO molecule also shows considerable enhancements in Eint values for physisorption on 4N-graphene. The most significant advantage can be observed for the HF molecule. We connect the nearly ten times enhancement with the occurrence of the tetrafurcated bond. Ab initio molecular dynamics simulations witness that at T=77 K, CO and HF molecules remain at the position nearly the pore during the whole course of simulation. It confirms the potential usage of 4N-graphene as a candidate for gas storage. The present work, which observes the prospective model system, can propose a simple way of describing the complicated events that occur in doped pore regions of graphene-based adsorbents.

Adsorption of diatomic molecules on graphene, h-BN and their BNC heterostructures: DFT study

Physical adsorption of three diatomic molecules (H2, CO, and HCl) on graphene, hexagonal boron nitride (h-BN), and two heterostructures, mBNC (h-BN ring is surrounded by carbon atoms), and pBNC (carbon ring is surrounded by boron and nitrogen atoms), was studied by using density functional theory (DFT) and independent gradient model (IGM) calculations to reveal peculiarities of their non-covalent interactions. Adsorption of hydrogen molecules shows approximately equal adsorption energies (Ea) of ca.−1 kcal/mol for both graphene and heterostructures, whereas adsorption on h-BN is less favorable. On the other hand, the CO molecule when adsorbed on pBNC exhibits much higher Ea (−3.43 kcal/mol) in comparison with other studied adsorbents, and the excess reaches 26% for the case of h-BN. The pBNC heterostructure also shows the increased Ea of −5.87 kcal/mol for the HCl molecule. Compared with mBNC, the excess achieves 33%. The present work, which observes representative model systems, can propose a simple way of describing the complicated events that occur in interfacial regions of BNC adsorbents.

Molecular simulation of size-selective gas adsorption in idealised carbon nanotubes

Molecular simulations were used to examine the adsorption of diatomic molecules (nitrogen and oxygen) and similarly sized gases (argon and methane) in pores with van der Waals diameters similar in size to the gas diameters. Idealised carbon nanotubes were used to model generic pores, to better understand the effect of pore diameter on guest adsorption in the absence of defects, specific adsorption sites, or variations in pore diameter that often complicate studies of gas adsorption in other porous materials. Molecular dynamics simulations of open nanotubes show that argon and methane are able to enter tubes whose diameters are slightly smaller than the gas diameters. Diatomic gases are able to enter tubes that are significantly smaller than their kinetic diameters with the molecular axis aligned parallel to the nanotube. The results indicate that size-selective adsorption of these gases is theoretically possible, although differences in pore diameters of only a few tenths of an Angstrom are required. Grand canonical Monte Carlo simulations of a 3.38 Å nanotube indicate significant uptake by argon and oxygen, but not nitrogen or methane. The adsorption of nitrogen and methane gradually increases as the nanotube diameter approaches 4.07 Å, and all gases fully saturate a 4.54 Å nanotube. Of the nanotubes studied, the largest adsorption enthalpy for any gas corresponds to the 4.54 Å nanotube, with significantly lower enthalpies seen in the 5.07 Å nanotube. These results suggest an ideal pore diameter for each gas based on the gas–pore van der Waals interaction energies. Trends in the ideal diameter correlate with the minimum tube diameter accessible to each gas.

Trends in sticking and adsorption of diatomic molecules on the Al(111) surface

To deepen our understanding of sticking and chemisorption behaviors, a trend study of static and dynamic aspects of the interaction between diatomic molecules, including ${\mathrm{F}}_{2}$, ${\mathrm{O}}_{2}$, NO, CO, and ${\mathrm{N}}_{2}$, and the Al(111) surface is performed. General features of free-electron-like metals are extracted and ramifications to other metals are indicated. With a slight generalization of the common form of density-functional theory, potential-energy surfaces (PES's) are calculated for both the adiabatic ground state and some excited states, where one or several electrons have been transferred from the adsorbate to the substrate. One trend applies to chemisorption (from dissociative, over molecular, to absent), and another one to the sticking probability (from unity, over incidence-energy-dependent, to zero). Ground- and excited-state PES's, local densities of states, and estimated electron-transfer probabilities are utilized. Electron transfer and energy dissipation to electron-hole pairs are identified as key processes in the dissociative adsorption. We apply a simple but general sticking model, where the competition between the electron-tunneling and the nuclear-motion time scales plays a central role [Surf. Sci. 532-535, 126 (2003)], now with a first-principles calculation of excited-state PES's. Measured trends in sticking and chemisorption behaviors are accounted for. Trends can be understood qualitatively in terms of electronegativity, kinetic Pauli-repulsion ranges, bond orders, and asymmetries of the molecules.

Cellular automata approach to dissociative adsorption of diatomic molecules on a substrate with reconstruction

Cellular automata approach to dissociative adsorption of diatomic molecules on a substrate with reconstruction

Cellular automata approach to dissociative adsorption of diatomic molecules on a substrate with reconstruction

A cellular automaton model is applied to the chemisorption of a diatomic molecule on a substrate consisting of active and inert component sites and performing a surface reconstruction. In the absence of surface migration or desorption of the adatoms the simulation shows a sharp transition between the unreconstructed surface phase and the induced surface reconstruction (first-order transition). The effect of desorption and migration is to soften this transition, becoming second order. When the presence of immobile impurities is taken into account the results show that the saturation coverage decreases almost linearly in the interval 0 < c < 0.9, where c is the impurity fraction. The effect of the statistical isolation of active sites is to make the relationship nonlinear for high values of the impurities fraction. The results compare well with the predictions of the approximation scheme used by Nord, R. S., and Evans, J. W., 1985, J. chem. Phys., 82, 2795.

A method for predicting the adsorption energetics of diatomic molecules on metal surfaces

A model for determining the energy variations during adsorption of diatomic molecules on metal surfaces is presented. The procedure allows the total bond order to vary based on the probabilities of finding electron pairs in the forming and breaking bonds. Also, the relationship between bond energy and bond order for the gas-phase reactant is described by a Morse potential that is modified to account for experimental bond dissociation energies. With the model, the reaction trajectory and the structure of the corresponding transition state are determined directly from the energy surface. This yields the heat of molecular adsorption and the activation energy for dissociative adsorption and it indicates the conditions that favor dissociative rather than molecular adsorption. The results for adsorption of O 2, N 2, and H 2 on several different metal surfaces show reasonable agreement with available experimental data.

Hindered rotation of adsorbed diatomic molecules

We consider the Henry law adsorption of diatomic molecules on a uniform surface. Effects of hindered rotation are specifically included in the adsorption isotherm equation. Methods of applying thes...

The mechanism of chemisorption on metals

The available experimental data on the adsorption of diatomic molecules on a bare metal surface are analysed to establish the kinetic processes leading to chemisorption. It is shown that, with the possible exception of hydrogen, molecules physically adsorbed on the surface form a reservoir from which chemisorption can occur. Kinetic mechanisms involving such a physically adsorbed precursor are developed, both for reaction on a uniform surface as well as for reaction at surface singularities, such as grain boundaries or lattice steps. Analysis of the rate of adsorption of nitrogen on tungsten reveals that during the initial stages the rate of chemisorption is diffusion controlled, and occurs preferentially at heterogeneities of atomic dimensions, which are tentatively identified with lattice steps on the surface.

The heat of adsorption of diatomic molecules

The heat of adsorption of particles which occupy two sites on a surface is examined by determining the statistical equilibrium. In particular, equations which are appropriate to immobile films are obtained. The heats of adsorption calculated from these equations are compared with the experimental values obtained for the heat of adsorption of hydrogen on a tungsten surface, and with the values which were obtained using an empirical method.

Some properties of mobile and immobile adsorbed films

The essential difference between mobile and immobile adsorbed films of gases on solids when there is repulsive interaction between adsorbed particles is discussed. Ordinary statistical methods can be applied to the equilibrium configurations of the particles in a mobile film but not to immobile films. Films formed by the adsorption of diatomic molecules with dissociation are then considered. It is pointed out that for a mobile (equilibrium) film there are differences between the true heat of adsorption and the apparent heat obtained from the temperature variation of the evaporation rate. A detailed comparison is made by obtaining a formula for the apparent heat of evaporation and comparing it with that for the true heat obtained earlier by Wang, and the physical reasons for the differences are discussed. A model suitable for studying the pseudo-equilibrium states in immobile films is described and formulae and diagrams in connexion with the kinetics of the formation of such films and the variation of heat of adsorption with fraction of surface covered are given.

The kinetics of adsorption with long-range interaction between adsorbed particles

The rates of condensation and evaporation of adsorbed particles when they interact with long-range forces are deduced from kinetic considerations, including also the case of adsorption of diatomic molecules with dissociation. The rate of evaporation is compared with experiment, and the variation of the dipole moment with θ required to make the theory agree with experiment is discussed.Finally the formulae for the rates of condensation and evaporation are applied to the problem of the diffusion of gases through metals, and it is found that for the processes considered the effect of interaction does not alter the conclusion arrived at in an earlier paper concerning the diffusion equation at small p and at large p.