Role of axial versus radial pore orientation in mesoporous silica particles, on its effect in photocatalysis via impregnated TiO2 nanoparticles in pores

Abstract 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.