Role of axial versus radial pore orientation in mesoporous silica particles, on its effect in photocatalysis via impregnated TiO<inf>2</inf> nanoparticles in pores

Heterogeneous catalyst design in environmental systems involve optimization of both the support and the active (nanoparticle-catalyst), impregnated within the support, in order to drive the reaction rate to be maximum for faster pollutant removal; of which, rhodamine B dye degradation has been used to illustrate the outcome of this work. In this regard, to first compare the role of pore-orientation in mesoporous silica support, we synthesized: (i) radial pore containing nano-spherical silica (RPNS) and (ii) axial pore containing cylindrical silica particles (of SBA-15). RPNS was found to give higher degradation rate, with simultaneously optimized 36.7 wt.% of TiO2 nano-catalyst in RPNS; thereby also addressing optimization of the loading amount of the active. This system achieved, as much as, 58% increased photocatalytic rate than SBA-15, with the same TiO2 content; with RPNS being also better than the commercial TiO2 catalyst (P25 Degussa). This is attributed to number of pores in RPNS being 4.6 times more than that in SBA-15, with all RPNS pores being aligned with the incident photon due to its radial orientation, compared to SBA-15, where in contrast, most part of the pores are hidden from incident photons. UV–-Vis powder spectroscopy ruled out the alternative possibility that, transmission of incident light in RPNS could be higher compared to light-absorption, which could have possibly favoured higher available photon-flux in RPNS. Therefore, it is the difference in pore orientation (namely radial pores being better than axial), which causes better photocatalysis by TiO2 containing RPNS. Thus, the right pore-orientation in substrate, coupled with optimum catalyst loading, can significantly improve performance in any diffusion, adsorption, reaction-driven system, which would play out similarly in heterogenous catalysis of any pollutant.