Effect of silica wall microporosity on the state and performance of TiO₂ nanocrystals in SBA-15 matrix

TiO₂ guest phase was incorporated by internal hydrolysis (IH) method inside SBA-15 mesostructured silica matrices with high and low microporosity (14.2% and 4.7% of microporous pore volume, respectively). TiO₂ phase was located inside the SBA-15 pores in form of small crystals with anatase structure without blocking the mesopores over wide range of loadings (8-50 wt%) (N₂ sorption, HRTEM and XRD). In the highly microporous SBA-15 (SBA-15-HM), the crystallization of titania anatase phase was detected at 150 °C due to initiation of the crystallization process in the micropores. This is supported by the fact that the crystallization was significantly delayed and started at 350 °C inside the SBA-15 with low microporosity (SBA-15-LM). Therefore, it was proposed that the formation of nanocrystalline titania in SBA-15 micropores initiates the nucleation stage thus enhancing the crystallization process of titania in the mesopores. Furthermore, micropores enhanced the dispersion of TiO₂ phase. As a result, TiO₂/SBA-15-HM adsorbed more vanadia than TiO₂/SBA-15-LM. The catalytic activity in selective catalytic reduction (SCR) of NO with ammonia was proportional to the vanadia content. Thus, V₂O₅-TiO₂/SBA-15-HM catalysts were more active than V₂O₅-TiO₂/SBA-15-LM at all TiO₂ loadings due to the higher vanadia content in TiO₂/SBA-15-HM than in TiO₂/SBA-15-LM. These results show that SBA-15 wall microporosity strongly affects the crystallization, state and performance of the guest phase confined in mesoporous channels of silica matrix.