CO₂ sorption of a thin silica layer determined by spectroscopic ellipsometry

Optical properties of a thin amorphous silica membrane and the supported γ-alumina layer on which it was coated were obtained from spectroscopic ellipsometry. The thicknesses of γ-alumina and silica layers from ellipsometric spectra were 1.654 m and 73 nm, respectively. The porosity of the γ-alumina layer was 51%. The porosity of the silica layer (15-25%), appeared to be smaller than that of unsupported silica material prepared by a similar method. Determination of the porosity of the silica layer, however, was quite inaccurate, because optical properties of the pure material were not exactly known. Ellipsometry was also used to determine the sorption behavior of CO₂ in the γ-alumina and silica layers. For both layers the observed sorption behavior could be described by a Langmuir isotherm (CCO₂, max = 0.84 and 2.8-3.0 mmol·g^-1, respectively), with Arrhenius-type temperature dependence (sorption heat 24.6±1.0 and 27.0±1.3 kJ·mol^-1, respectively). The adsorption behavior of supported and unsupported -alumina appeared to be similar. The heat of sorption was larger for supported thin silica layers than for unsupported bulk silica, suggesting smaller pores in the thin layer.