Optimized preparation of highly catalytically active Co<sub>3</sub>O<sub>4</sub> and photothermal degradation of VOCs

<p indent="0mm">Volatile organic compounds (VOCs) are a class of organic compounds that are easily volatilized at room temperature, causing serious damage to the human body, soil, and water bodies, and affecting the balance of the ecosystem. Cobalt tetraoxide (Co₃O₄) has a very high catalytic degradation ability for VOCs and is considered to be one of the metal oxides with the greatest potential to replace precious metal catalysts. Relevant studies have shown that Co₃O₄ catalysts prepared under different conditions have excellent photocatalytic and thermocatalytic activities. This paper discusses the effects of precursors, precipitants, reaction temperatures, and subsequent heat treatment temperatures on the catalytic degradation activity of benzene by the prepared Co₃O₄ catalysts. The activity differences of the samples were determined by the degradation rate of 15 μL benzene and the CO₂ generation rate of the Co₃O₄ catalysts prepared under different conditions at 290°C, and the optimized preparation scheme of the Co₃O₄ catalyst with high catalytic activity was obtained. The study found that the Co₃O₄ catalyst prepared with cobalt acetate as a precursor, urea as a precipitant, 90°C reaction, and subsequent 300°C calcination showed the best activity in photothermocatalytic degradation of VOCs. The optimal Co₃O₄ catalyst had a catalytic degradation rate of 95.3% for <sc>15 μL</sc> benzene in only <sc>5 min,</sc> and maintained a catalytic degradation rate of more than 95% for <sc>15 μL</sc> benzene after 10 photothermocatalytic degradation stability tests, proving its good catalytic stability. Combined with the test characterization results of XRD, SEM, UV-Vis-IR, TG, etc., the reasons for the difference in catalytic degradation efficiency of Co₃O₄ catalysts were explored: Co₃O₄ catalysts prepared under different conditions have different absorption capacities for ultraviolet-visible-infrared light, and calcination at a too high temperature will increase the surface area of the Co₃O₄ catalyst, resulting in a reduction in its active attachment sites. The optimal Co₃O₄ catalyst was further analyzed for photocatalysis at room temperature, thermocatalysis at different temperatures, and photothermocatalytic activity. It was found that its photothermal synergistic catalytic efficiency at 200, 240, and 290°C was always greater than the simple sum of photocatalysis and thermocatalysis. The activation energy of the photothermal catalytic reaction is lower than that of the thermal catalytic reaction. Due to the reduction of activation energy, photothermal synergistic catalysis can significantly accelerate the reaction rate. The electron or energy excitation caused by the participation of light can trigger more reactant molecules to cross the energy barrier and accelerate the reaction. The photogenerated holes generated by photocatalysis can promote the release of lattice oxygen, thereby accelerating the formation of oxygen vacancies. Photogenerated electrons can reduce the reduction energy barrier of oxygen molecules during the reaction, accelerate the adsorption and dissociation of oxygen, allow oxygen vacancies to be quickly refilled, and restore the activity of the catalyst. Therefore, the Co₃O₄ catalyst has a photothermal synergistic effect in the catalytic degradation of VOCs, that is, the active species O²⁻, ·OH and C₆H₆⁺ produced by photocatalysis are more active than O₂, H₂O and C₆H₆ participating in the reaction in traditional thermocatalysis, which accelerates the thermocatalytic redox of Co₃O₄.