Effects of Co3O4 Cocatalyst on InTaO4 for Photocatalytic Reduction of CO2 to CH3OH under Visible Light Irradiation

InTaO4 was synthesized by a solid-state reaction method using metal oxide as the starting materials. Co was added by incipient-wetness impregnation. The sample was pretreated by H2 (200 Torr) reduction at 500?C for 2 h and subsequent O2 (100 Torr) oxidation at 200?C for 1 h. The core-shell structure of metallic Co and Co3O4 was formed by this reduction-oxidation procedure. The catalysts were characterized by powder X-ray diffraction, scanning electron microscope, and ultraviolet-visible spectroscope. The photocatalytic reduction was carried out in a Pyrex reactor with KHCO3 or NaOH aqueous solution bubbled with ultra pure CO2 gas under visible light illumination. SEM micrographs show many small Co3O4 particles on the surface of InTaO4. The band gap of Co3O4-InTaO4 was 2.7 eV, confirming that these catalysts have the ability to reduce CO2 to methanol. The methanol yield increased with the amount of Co3O4 cocatalysts. The catalyst had a higher activity in KHCO3 aqueous solution than in NaOH solution. The InTaO4 catalyst with 1 wt% Co3O4 cocatalyst had the highest activity among all catalysts. Co3O4 was incorporate into the surface structure of InTaO4 to form CoxInTaO4-x. It resulted in more defect sites on the surface of InTaO4 and changed the valence band structure. It formed a Schottky barrier to suppress the electron-hole recombination.