Kinetic and mechanistic analysis for the photodegradation of gaseous formaldehyde by core-shell CeO2@LDHs

While the removal of gaseous formaldehyde in breathing air via photocatalytic oxidation technology (POT) has shown great promise for pratical applications, the search of highly efficient and stable catalysts and mechanistic interpretations are highly desirable. We synthesized a core-shell structured composite of CeO2 and layered double hydroxides (CeO2@LDHs). The degradation of gaseous formaldehyde was realized with high efficiency under visible light irradiation using a home-built continuous flow reaction device at an industrial reaction scale. It was confirmed that the catalytic activity of LDHs after CeO2 complexation was significantly improved. Under optimum reaction conditions, a formaldehyde conversion of 86.9 % was achieved by CeO2@LDHs after 300 min equilibrium (3.73 mu g g(-1) h(-1)). Kinetic study showed that the flow rate of gaseous formaldehyde had the largest influence on the degradation rate of formaldehyde, followed by the relative humidity and the light intensity with a descending order. The rate equation and activation energy of photocatalytic degradation of formaldehyde were also obtained and discussed. The catalytically active radicals and the reaction intermediates were detected on CeO2@LDHs, shedding light on the intepretation of the reaction mechanism. Under the oxidation of hydroxyl and superoxide radicals and hot holes, gaseous formaldehyde was eventually degraded into carbon dioxide and water, as a result of the generation of formic acid, carbon hydrogen oxygen radicals ((CHO)-C-center dot) and (CO2)-C-center dot- radicals. The present study demonstrates a systematic logical flow from design, preparation and mechanistic understanding of efficient LDHs-based photocatalyst for the degradataion of gaseous formaldehyde under practical conditions.