In Situ Intermediates Determination and Cytotoxicological Assessment in Catalytic Oxidation of Formaldehyde: Implications for Catalyst Design and Selectivity Enhancement under Ambient Conditions.

Formation and decay of formaldehyde oxides (CHOO) affect the complete oxidation of formaldehyde. However, the speciation and reactivity of CHOO are poorly understood because of its extremely fast kinetics and indirect measurements. Herein, three isomers of CHOO (i.e., main formic acid, small dioxirane, and minor CHOO Criegee) were in situ determined and confirmed as primary intermediates of the room-temperature catalytic oxidation of formaldehyde with two reference catalysts, that is, TiO/MnO -CeO and Pt/MnO -CeO. CHOO Criegee is quite reactive, whereas formic acid and dioxirane have longer lifetimes. The production, stabilization, and removal of the three intermediates are preferentially performed at high humidity, matching well with the decay rate of CHOO at approximately 6.6 × 10 s in humid feed gas faster than 4.0 × 10 s in dry feed. By contrast, given that a thinner water/TiO interface was well-defined in TiO/MnO -CeO, fewer reductions in the active sites and catalytic activity were found when humidity was decreased. Furthermore, lethal intermediates mostly captured at the TiO/MnO -CeO surface suppressed the toxic off-gas emissions. This study provides practical insights into the rational design and selectivity enhancement of a reliable catalytic process for indoor air purification under unfavorable ambient conditions.