The influence of surface acid density on the freezing behavior of water confined in mesoporous MCM-41 solids (Journal article)

Sklari, S./ Rahiala, H./ Stathopoulos, V./ Rosenholm, J./ Pomonis, P.

The freezing behavior of water confined in the pores of five mesoporous MCM-41 materials of controlled acidity has been studied using H-1-NMR. The surface acidity of Si, Si/Al and Si/Ti-based MCM-41 solids was determined using NH3 temperature programmed desorption experiments. The Full Width at Half Maximum of the H-1-NMR signal in the temperature range 300 K > T > 200 K, corresponding to the spin-spin re:laxation time 1/T-2 was recorded at 5 degrees intervals. The 1/T-2 values for all samples show a smooth increase as temperature drops down to T approximate to 225-230 K. At this region the core water in the pores freezes leaving a layer of water between the pore wall and the core ice unfreezable. This shell of unfreezable water undergoes a continuous transition as temperature drops, Below T < 225-230 K the slope d(1/T-2)/dT depends on the surface acid density. An energy relationship (1/T-2) = A exp(Q/RT) was used to calculate the Q and A values which show a compensation effect. The energy parameter Q assumes values between 14-19 kJ mol(-1) and depends on the acid-acid site distance. A model of water molecules bridging the acid surface sites is proposed. The number N of water molecules bridging these sites is suggested to be related to Q via a charge - hopping mechanism. The Q value becomes zero when the distance between the acid sites is similar to5.5 Angstrom which corresponds to the unfreezable shell water layer in the pores. (C) 2001 Elsevier Science B.V. All rights reserved.
Institution and School/Department of submitter: Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
Keywords: mesoporous,water,freezing,h-1-nmr,surface acidity,nuclear-magnetic-resonance,neutron-scattering,porous silica,size,pore,nmr,diffraction,dynamics,h-1-nmr,glass
ISSN: 1387-1811
Link: <Go to ISI>://000172090500001
Publisher: Elsevier
Appears in Collections:Άρθρα σε επιστημονικά περιοδικά ( Ανοικτά)

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