Temperature Dependence of the Artificial Photosynthesis Reactions Catalyzed by Nanostructured Co/CoO

Carbon dioxide (CO₂) and water (H₂O) have been converted into hydrocarbons at temperature ranging from 58 to 242 °C through an artificial photosynthesis reaction catalyzed by nanostructured Co/CoO. The experimental results show that chain hydrocarbons (alkane hydrocarbons) (C H₂, where 3 ≤ n ≤ 16) mainly form at a temperature higher than about 60 °C, the production rate reaches a maximum at 130 °C, and abruptly decreases above 130 °C, and then gradually increases until 220 °C. While the temperature is higher than 220 °C, benzene (C₆H₆) and its derivatives such as toluene (C₇H₈), p-xylene (C₈H₁₀), and C₉H₁₂ form. The modeling of temperature dependence of the reaction rate reveals that the vaporization of the adsorbed water contributes to the sharp peak; the activation energy is estimated as about 1 eV, which is in agreement with the reaction of CO and H₂ to synthesize chain hydrocarbons. The experimental results support the mechanism that the chemisorbed CO₂ and physisorbed H₂O on the CoO surface are disassociated or excited with light, and the disassociated or excited molecules then synthesize hydrocarbons. When most of the water molecules leave from the CoO at temperature higher than 220 °C, the hydrogen source is of very low concentration while the carbon source remain the same because of the chemisorption, and thus benzene and its derivatives with low hydrogen atom number form.