PARAMAGNETIC CENTERS IN UNIRRADIATED BORON-DOPED SILICA-GEL

Unirradiated alkoxide-derived silica gels containing 2.5 mol% B2O3, heated at 500-degrees-C for 0.5 h, exhibited an absorption peak at 218 nm which decreased in intensity with increase in temperature (800-degrees-C, 0.5 h) or duration of heating (1-5 h). This suggests that these effects are due to the formation and decay of paramagnetic defects in the gel structure. An electron spin resonance spectral signal (g = 2.002) confirmed the generation of such defects in a sample heated at 500-degrees-C/0.5 h. The signal decreased in intensity or vanished with increase in temperature or duration of heating.