Production of CH₄ and CO on Cu_xO and Ni_xO_y coatings through CO₂ photoreduction

CO₂ capture and photocatalytic reduction to hydrocarbons is an interesting yet challenging area that requires photocatalysts with the capability to capture and photoconvert CO₂ simultaneously. Furthermore, earth-abundant photocatalysts with high efficiency and product selectivity are essential for commercialization. Thus, two earth-abundant photocatalysts based on copper and nickel oxides were selected to produce solar fuels from CO₂ photoreduction. The photocatalysts were immobilized on commercial glass fibers substrates by a facile one-step microwave-hydrothermal method. Cu_xO (x = 1, 2) and Ni_xO_y (x = 1, 2 and y = 1, 3) coatings on glass fiber were evaluated as photocatalysts in two different reactors to investigate the selectivity in a continuous reactor and a batch system. Two different light sources were employed: a heterochromatic lamp to simulate part of the solar light in the continuous reactor and a LED visible light in the batch reactor. CuO/Cu₂O photocatalysts exhibited a selective production of CH₄ (95 µmol g⁻¹ h⁻¹) and CH₃OH (177 µmol g⁻¹ h⁻¹) from CO₂ photoreduction in the continuous and batch continuous systems, respectively. The superior performance was attributed to the unique rod-shape morphology, the presence of oxygen vacancies, and efficient charge transfer in the CuO/Cu₂O heterostructure with high affinity towards CO₂, resulting the formation of mono- and bidentate carbonate species during the CO₂ photoreduction reaction. Ni_xO_y coating with 2D cubic shape produced CO (103 µmol g⁻¹ h⁻¹) and HCOOH (4245 µmol g⁻¹ h⁻¹), associating with the low CO₂ affinity and less efficient charge separation compared to CuO/Cu₂O heterostructure.