Adsorption of ortho and para H2 on NaCl(001)

Abstract

Fourier transform infrared spectroscopy (FTIR) experiments together with interaction potential calculations are performed to interpret the adsorption–desorption properties of para and ortho H2 molecules physisorbed on the (001) faces of NaCl crystallites. Isotherms of the adsorption process show preferential adsorption of para-H2 from gaseous normal-H2 at surface temperature around 10 K. Isobars of the desorption process show preferential desorption of para-H2 from a mixed ortho-H2 and para-H2 adsorbate. A quantum description of the molecular motions corroborates preferential adsorption of p- and (M=±1) o-H2 on the cation sites, as proposed in previous classical studies. It is shown that the adsorption energy is smaller for the para than for the (M=±1) ortho species in agreement with experimental measurement of desorption energy, while (M=0) o-H2 is only slightly bound to the substrate. This indicates that the ionic surface can discriminate all the H2 species. The determination of the bar spectrum for a set of independently adsorbed H2 species can interpret most of the infrared signals assigned to adsorption on the terrace sites. More specially the experimental behavior, with temperature and coverage, of the frequencies and relative intensities of the pure vibrational peaks and of the translation–vibration combination bands is very well explained in terms of hindered external motions of the admolecule.

A Monte Carlo simulation study of H2 layers on NaCl(001)

“…Therefore, the surface potential, described above, was fit to a one-dimensional harmonic oscillator model, and the zero point energy was estimated to be 0.177 kcal/mol (z-motion frequency) which yields a binding energy of À0.69 kcal/mol. This binding energy matches the estimate value of Vidali and Karimi [14] but lies at the lower end of the experimental binding energy range of values (À0.646 to À1.038 kcal/mol [2][3][4].…”

“…Note that J is the rotational quantum number of the molecule and m J is the rotational magnetic quantum number that specifies the orientation of the axis of rotation. Subsequently, Heidberg et al [3,5] studied this system at T = 11 AE 1 K using the same technique. They found that a monolayer of H 2 molecules forms a (1 Â 1) structure and proposed that only the Na + sites are occupied.…”

“…The H 2 adsorption on a NaCl surface has been studied using infrared (IR) spectroscopy and helium atom scattering (HAS) [1][2][3][4][5] techniques. Ewing's group [2] studied the adlayer structures and observed a commensurate (1 Â 1) structure at a temperature of 5.2 K. They proposed a monolayer structure consisting of two adsorption sites with p-H 2 (J = 0) molecules located on top of Na + sites and o-H 2 (J = 1) above Cl À sites.…”

Density functional theory study of flat and stepped NaCl(001)

“…Indeed, this absence of insight into clean NaCl͑001͒ persists despite several recent theoretical studies of atomic and molecular adsorption on NaCl͑001͒. [11][12][13][14] As part of our ongoing work into the chemical and physical properties of salt surfaces, 15 we aim here to elucidate some of the key structural and energetic properties of flat and stepped NaCl͑001͒ with density functional theory ͑DFT͒. The particular interesting and unresolved issues that we intend to shed light on are ͑i͒ the surface energy and atomic structure of NaCl͑001͒; ͑ii͒ the step formation energy of the most commonly observed monoatomic ͑100͒-like steps on NaCl͑001͒; ͑iii͒ the relative stabilities of Na and Cl terminated ͑111͒-like steps, their stabilities compared to the neutral ͑100͒-like steps, and insight into what conditions of temperature and pressure ͑if any͒ such steps will become stable; and ͑iv͒ the electronic structure of Na and Cl atoms at NaCl͑001͒ terraces and steps, aiming, in particular, to understand how these atoms differ from atoms inside the bulk of a NaCl crystal.…”

Tailoring band gaps of insulators by adsorption at surface defects: Benzoic acids on NaCl surfaces

“…Indeed only a few theoretical adsorption studies on NaCl have been made within the last decade and most of them were dedicated to some simple adsorbates such as water and H 2 . 14,[17][18][19][20][21] Consequently the microscopic picture of the interaction between these organic molecules and NaCl surface is not clear. Neither is the origin of contact charging between NaCl and KCl.…”