Study of nitrogen-hydroxyl interactions on mesoporous silicas by adsorption microcalorimetry at 77 K

DEBORA MAIA; KARIM SAPAG; REDOUANE MELOUKI; JEAN ROUQUEROL; PHILIP L. LLEWELLYN

Baltimore

Congreso; FOA 11, 11th International Conference on the Fundamentals of Adsorption; 2013

The surface chemistry plays an important role on the adsorption properties of a porous solid. In the case of silica, this surface chemistry is dominated by hydroxyl groups. During physisorption at 77 K these hydroxyl groups are expected to affect the orientation of the nitrogen molecule at the surface and, subsequently, the BET surface area when calculated with the conventional, constant, cross-sectional area of 0.162 nm2 [1, 2]. In a previous work on two silica gels it was shown, by varying the surface concentration of hydroxyl groups (using increasing outgassing temperatures) that the enthalpies of adsorption of nitrogen varied indeed with this surface concentration whereas the enthalpies of adsorption of argon did not [1]. Our aim here is to study in more detail the probable correlation between the N2 -OH energy of interaction and the orientation of the nitrogen molecule. For that purpose, we selected two well-defined mesoporous silicas, namely MCM-41 and SBA-15, in view of making easier the modelling and simulation needed to understand the calorimetric data. The latter were collected during the first step of the work and are presented here. The samples were outgassed at increasing temperatures. The differential enthalpies of adsorption have been directly measured at 77 K with a specially designed calorimeter immersed in a liquid nitrogen bath. The gas is introduced to the sample using a quasi- equilibrium method which allows for a continuous recording of the adsorption energies and a high-resolution measurement of the adsorption isotherm. The results obtained for the two materials are compared with those obtained with the silica gels and equally with a porous carbon.