Preparation of alkaline-earth metal silicates from gels and their NOx-adsorption behavior

Processing of Ca and Ba silicates from gels was investigated aiming at the application to a NOx absorbent. The gels were prepared from RnSi(OC2H5)4−n, HSi(OC2H5)3 and alkaline-earth metals. Although alkaline-earth alkoxides readily react with water to give a precipitate, alkaline-earth metal was reacted with an alcohol solution of partially hydrolyzed RnSi(OC2H5)4−n to give a stable mixture of alkaline-earth alkoxide and RnSi(OC2H5)4–n in alcohol solvent. HSi(OC2H5)3, which was used as a gelation acceleration agent, was added to the mixture. The solution was further hydrolyzed and allowed to gel at room temperature. The gels were transparent with no precipitation. The gels heat-treated at 600 °C were exposed to 1000 ppm NO diluted by Ar. FT-IR spectra of the Ca and Ba silicates showed the peaks corresponding to adsorbed NO species at the range of 1300–1600 cm−1, whereas non-Ca, Ba silicate showed no FT-IR peaks corresponding to adsorbed NO species. Since more than one peak appeared in FT-IR spectrum of the NO-treated samples, it is likely that some types of NOx adsorbate species exist on the surface of alkaline-earth silicates. It was known that the FT-IR peaks of adsorbed NO species disappeared at the heating temperature of 700 °C.