SANS investigation of nitrogen sorption in porous silica

The mechanism of nitrogen sorption in porous silica was investigated by sma ll-angle neutron scattering (SANS). Two samples of porous silica were studi ed containing mesopores (pore sizes 5.5 and 9.5 nm, respectively) and addit ional micropores of irregular shape and statistical distribution. SANS curv es were recorded at a temperature of 78 K at various relative pressures pip e during adsorption, The experiment is based on contrast matching between s ilica and condensed nitrogen with regard to neutron scattering. The sorptio n process was characterized by the evaluation of the chord-length distribut ions extracted from SANS data for each pipe. In addition, a general approac h was developed to relate the SANS pattern during capillary condensation to the size distribution and the morphology of ordered mesopores. On the basi s of these evaluation methods, various uptake mechanisms could be described , which are micropore filling, the formation of nitrogen layers, and capill ary condensation. The analysis of the SANS data shows that the mean size of the remaining empty mesopores formally increases, and their size distribut ion becomes narrower during capillary condensation, which is in agreement w ith the predictions of the Kelvin equation. Furthermore, our study indicate s a significant degree of additional microporosity, the origin of which is discussed. For comparison, the experiment and the data evaluation were also applied to a disordered porous silica with a broad pore size distribution. The combination of SANS and nitrogen sorption turned out to be a powerful technique to investigate both the mechanisms of sorption and the structure of porous silicas in one experiment.