Hierarchical pore architecture of centric and pennate diatom biosilica influences TiO2 dispersion and photooxidation performance

Diatoms build hierarchically porous silica frustules with species-specific pore architectures, offering a library of biogenic scaffolds that synthetic mesoporous silicas cannot easily reproduce. How this frustule geometry influences the deposition of guest nanoparticles, and whether different geometries can serve as a design variable for catalysis performance, remains largely unresolved, as TiO 2 /biosilica studies have typically examined only a single species and been applied mostly to degradative reactions such as pollutant removal. Here, we directly compare the biosilica of two tropical marine diatoms with contrasting morphologies, the centric Cyclotella striata TBI (CS) and the pennate Navicula salinicola NLA (NS), as mesoporous scaffolds for TiO 2 immobilization by wet impregnation under identical conditions. Nitrogen physisorption confirmed mesoporosity, with pennate NS offering a higher surface area (206 m 2 /g) than centric CS (131 m 2 /g). Electron microscopy showed that the radially symmetric pore network of CS templated the uniform deposition of small TiO 2 nanoparticles (average 9 nm), whereas the anisotropic slit-like channels of NS produced larger, aggregated particles (average 19 nm) despite a higher overall loading (57.30 vs. 53.08 wt% TiO 2 ). In the photooxidation of benzyl alcohol to benzaldehyde, HPLC-DAD quantification (n = 3) showed that TiO 2 /Biosilica(CS) reached a markedly higher benzaldehyde yield (36.7 ± 1.6%) than TiO 2 /Biosilica(NS) (21.9 ± 1.2%) at comparable conversion (33.0 ± 3.4% vs. 29.7 ± 2.5%). CS also delivered higher intrinsic efficiency, reaching a turnover frequency of 4.11 × 10 −7 s −1 and an apparent quantum yield of 1.43 × 10 −2 % versus 2.28 × 10 −7 s −1 and 8.52 × 10 −3 % for NS, respectively. Radical-trapping experiments further revealed that the two morphotypes oxidized through different dominant reactive species, with hydroxyl radicals (•OH) prevailing on the centric CS scaffold and superoxide radical anions (O 2 •⁻) on the pennate NS scaffold. These results establish frustule geometry as a tunable structural handle over TiO 2 dispersion, with the centric morphology favoring higher yield, and position species-specific diatom biosilica as a biogenic, noble-metal-free platform for photocatalytic alcohol oxidation.