Acidic and Electrosurface Properties of Binary TiO2-SiO2 Xerogels Using EPR of pH-Sensitive Nitroxides.

Denis O Antonov,Andrey B Shishmakov,Daria P Tambasova,Elena G Kovaleva,Igor A Kirilyuk

doi:10.3390/gels7030119

Denis O Antonov, Andrey B Shishmakov + Show 3 more

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https://doi.org/10.3390/gels7030119

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Abstract

The binary xerogels TiO2-SiO2 are widely used as catalysts and their carriers in organic synthesis. Characterization and adjustment of the electrostatic properties of the surface and the local acidity inside the pores, are necessary for the further development of TiO2-SiO2 xerogels applications. This research investigates acid–base equilibria in the pores, and the surface electrostatic potential (SEP) of binary TiO2-SiO2 xerogels, by the EPR of stable pH-sensitive nitroxide radicals. These radicals are small enough to penetrate directly into the pores, and to be adsorbed onto the surface of the material under study. This makes it possible to obtain valuable information on the acidic and electrosurface properties of the studied system. The highest negative surface electrical charge associated with surface electrical potential (SEP) was equal to −196 ± 6 mV. It was induced by the surface of the sample with a 7% TiO2 content. The local acidity inside the pores of this sample was found to be higher, by approximately 1.49 pH units, as compared to that in the external bulk solution.

Highlights

The binary SiO2-TiO2 xerogels have been applied in a variety of fields of science and technology
The infrared absorTphtieoninbfraanredd, wssppieethccttrtruhumemmoofaftxthihmeeSaSiOiaOt2 21x1exr5eo5rgoaegnled(lF(i1Fg0iu6gr2uerc2ema2)−ah1)a,hdaanadndaiannnteinanbtsesenocsroepmtciopomlnexpb-lasenhxda-spwheadiptheda mabasxoimrputimonabta7n9d8, cwmit−h1thcoermreasxpiomnadaint g11t5o5satrnedtc1h0i6n2gcvmib−1r,aatniodnasnoafbSsio–rOp–tiSoinbboanndds.with a maximum at 798 cm−1 corresponding to stretching vibrations of Si–O–Si bonds
continuous wave (CW) X-band electron paramagnetic resonance (EPR) spectroscopy of a pH-sensitive nitroxide radical, used as a freemotioned or adsorbed spin probe, was applied to study acid–base equilibria in the pores and surfaces of TiO2-SiO2 binary xerogels that were prepared by a sol-gel technique in the wet ammonia vapors atmosphere

Summary

Introduction

The binary SiO2-TiO2 xerogels have been applied in a variety of fields of science and technology These materials are widely used in optical waveguides and sensors [1], hydrophilic coatings, selective sorbents for purification of air [2,3], and aqueous media [4] made from organic and inorganic compounds [5,6,7], and in self-purifying materials, based on glass [8] or cotton [9]. These are the reactions of the hydrolysis and gelation of alkoxides or other reactive compounds in an alcohol solution, with the addition of acids and alkalis as catalysts

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