CO2 sorption of a thin silica layer determined by spectroscopic ellipsometry

Optical properties of a thin amorphous silica membrane and the supported ga mma -alumina layer on which it was coated were obtained from spectroscopic ellipsometry. The thicknesses of gamma -alumina and silica layers from elli psometric spectra were 1.654 mum and 73 nm, respectively. The porosity of t he gamma -alumina layer was 51%. The porosity of the silica layer (15 - 25% ), appeared to be smaller than that of unsupported si[ica material prepared by a similar method. Determination of the porosity of the silica layer; ho wever; was quite inaccurate, because optical properties of die pure materia l were not exactly known. Ellipsometry was also used to determine the sorpt ion behavior of CO2 in the gamma -alumina and silica layers. For both layer s the observed sorption behavior could be described by a Langmuir isotherm (c(CO2 . max) = 0.84 and 2.8 - 3.0 mmol.g(-1), respectively), with Arrheniu s-type temperature dependence (sorption heat 21.6 +/- 1.0 and 27.0 +/- 1.3 kJ.mol(-1), respectively). The adsorption behavior of supported and unsuppo rted gamma -alumina appear-ed to be similar The heat of sorption was larger for supported thin silica layers than for unsupported bulk silica, suggest ing smaller pores in the thin layer.