Dynamic photocatalytic reduction of CO₂ to CO in a honeycomb monolith reactor loaded with Cu and N doped TiO₂ nanocatalysts

Highlights: • Cu-N/TiO₂ catalysts and cordierite honeycomb monolith tested for CO₂ reduction. • Cu/TiO₂ yielded 14 times higher CO than the N/TiO₂ and 64 times than TiO₂. • N/TiO₂ was suitable for trapping holes and favored dynamic CH₄ formation. • Cu/TiO₂ gave negative effect on CH₄ production, but enhanced dynamic CO yield. • Cordierite honeycomb support enhanced CO₂ reduction with improved selectivity. - Abstract: Cordierite honeycomb monoliths loaded with N/TiO₂ and Cu/TiO₂ nanocatalysts for dynamic photocatalytic CO₂ reduction with H₂ to CO in a continuous photoreactor illuminated with UV-light irradiations have been investigated. The nanocatalysts, loaded over the monoliths channels using sol-gel dip-coating method, were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), N₂ adsorption-desorption, X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance and photoluminescence (PL) analysis. Crystalline and anatase TiO₂ structure with nanoparticles evenly supported over the cordierite monolith channels were observed. Cu and N presented over TiO₂, shifted band gap energy towards visible region and hindered charges recombination rate. Loading Cu and N greatly improved TiO₂ photoactivity for dynamic CO₂ reduction to CO. Due to high photoactivity and selectivity, Cu/TiO₂ assisted system yielded 14 times higher CO than the N/TiO₂ and 64 times the amount of copper observed over pure TiO₂ in a continuous operation of photoreactor. This significant improvement in Cu/TiO₂ activity was noticeable due to efficient trapping and transport of electrons by Cu-metal. With unique properties, N/TiO₂ showed good activity for continuous CO₂ reduction to CH₄. In addition, a photocatalytic reaction mechanism is proposed to understand the experimental results over Cu and N modified TiO₂ catalysts in a continuous operation of photoreactor.