Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. II. Diffusion limited kinetics in amorphous solid water

Abstract

The adsorption, desorption, and diffusion kinetics of N2 on thick (up to approximately 9 microm) porous films of amorphous solid water (ASW) films were studied using molecular beam techniques and temperature programmed desorption. Porous ASW films were grown on Pt(111) at low temperature (<30 K) from a collimated H2O beam at glancing incident angles. In thin films (<1 microm), the desorption kinetics are well described by a model that assumes rapid and uniform N2 distribution throughout the film. In thicker films (>1 microm), N2 adsorption at 27 K results in a nonuniform distribution, where most of N2 is trapped in the outer region of the film. Redistribution of N2 can be induced by thermal annealing. The apparent activation energy for this process is approximately 7 kJ/mol, which is approximately half of the desorption activation energy at the corresponding coverage. Preadsorption of Kr preferentially adsorbs onto the highest energy binding sites, thereby preventing N2 from trapping in the outer region of the film which facilitates N2 transport deeper into the porous film. Despite the onset of limited diffusion, the adsorption kinetics are efficient, precursor mediated, and independent of film thickness. An adsorption mechanism is proposed, in which a high-coverage N2 front propagates into a pore by the rapid transport of physisorbed second layer N2 species on top of the first surface bound layer.

The Effective Surface Area of Amorphous Solid Water Measured by the Infrared Absorption of Carbon Monoxide

“…This indicates that the ratio between 3and 4-coordinated water molecules is more or less a constant. As the properties of ASW surface is determined by the relative ratio between 2-, 3-, and 4-coordinated water molecules, this would indicate that other than the differing in the total pore surface area, the properties of the pore surface remain the same between 60 and 140 K. Previously, Zubkov et al (2007) performed TPD measurements of N 2 adsorbed on both compact and porous ASW, and found that if a correction for surface area is taken into account, the desorption energy distribution on porous ASW is nearly identical to that of compact ASW. Our results based on a different method agrees with their conclusion.…”

“…This happens at a higher CO dose than the full covering of the dOH bonds, likely because CO molecules preferentially occupy the dOH sites than non-dOH sites. In a prior study by Zubkov et al (2007), it is reported that the full coverage of pore surface by nitrogen adsorption happens simultaneously with the saturation of the shifted dangling bond intensity. They suggested that N 2 does not preferentially bind to dangling OH groups.…”

Irradiation-enhanced adsorption and trapping ofO2on nanoporous water ice

“…21 The coefficients s i and D i , are functions of and T. In our study with methane we analyzed the equilibrium isotherms with the condensation approximation, 22 where D = 0 for sites with binding energy above a value 0 ͑P , T͒, and infinity for sites with Ͻ 0 ͑P , T͒. The adsorbed molecules migrate rapidly by surface diffusion 5,23 and stick to the strongest available binding sites, and one can define N M , a maximum value of N below which nearly all the occupied sites have binding energies higher than 0 ͑P , T͒ ͑hence D =0͒. Therefore, the desorption term can be dropped from Eq.…”

Electron-induced chemistry of methyl chloride caged within amorphous solid water

“…As a typical porous and structurally dynamic material, ASW is very sensitive to the substrates' temperature. 18,39 It plays a crucial role since the packing of the water molecules is rather sensitive to the temperature, causing defects diffusion by water molecules reorientation 20, 40 at ≈105 K, followed by more packed ASW films at 135 K ± 15 K, i.e., tighter structure of the frozen water film, up to full crystalline ice formation at 155 K. The effect of ASW layer annealing temperature prior to MeCl adsorption on its overall sticking probability and subsequent desorption uptake is demonstrated in Fig. 1(b).…”