Bulk Photodriven CO 2 Conversion through TiO 2 @Si(HIPE) Monolithic Macrocellular Foams

Abstract

Operating photo-induced reactions exclusively on catalyst surfaces while not exploiting the full catalyst volume generates a major footprint penalty for the photocatalytic reactor and leads to an inefficient use of the catalytic material. Photonic investigations clearly show that the solid foams have a strongly multidiffusive character, with photons being significantly trapped within the sample cores while addressing a photon mean free path l t = 20.1 ± 1.3 µm. This 3D process both greatly limits back-reactions and promotes outstanding selectivity toward methane (around 80%) generation, and even ethane (around 18%) through C-C coupling reaction, with residual carbon monoxide and dihydrogen contents (around 2%). Silica–titania TiO 2 @Si(HIPE) self-standing macrocellular catalysts lead to optimal efficient thicknesses up to 20 times those of powders, thereby enhancing the way for real 3D-photodriven catalytic processes above the millimeter scale and up to a 6 mm thickness. A rather simple Langmuir–Hinshelwood based kinetic model is proposed which highlights the strong dependence of photocatalytic reaction rates on light scattering and the crucial role on oxidation back-reactions. In addition, a strong correlation between light attenuation coefficient and photon mean free path and median pore aperture diameter is demonstrated, offering thus a tool for photocatalytic behavior prediction.