Coupling X-ray Scattering and Nitrogen Adsorption: An Interesting Approach for the Characterization of Ordered Mesoporous Materials. Application to Hexagonal Silica

The structural evolution of a 2D hexagonal mesoporous silica powder during nitrogen adsorption at low temperature is followed by X-ray diffraction. The values for the pore radius and wall microporosity obtained with this technique compare well to the estimations drawn from volumetric measurements. It is further shown that capillary condensation is not accompanied by material shrinkage; on the contrary, a slight increase in the unit cell parameter is observed. A comparison of the diffracted intensity before gas adsorption and after capillary condensation allows the silica skeleton density to be estimated precisely. In the intermediate situation, intensity calculations are performed in the frame of a conventional model of a nitrogen film condensed on the mesopore walls; estimates of the film thickness are in good agreement with predictions. It is further possible to separate the evolution of the nitrogen content of the micro- and mesopores.