Unraveling the mechanisms of room-temperature catalytic degradation of indoor formaldehyde and its biocompatibility on colloidal TiO2-supported MnOx–CeO2

Abstract

This work overcomes the limitations in room-temperature and moisture-dependent activity of transition metal oxide-based catalysts for sub-ppm formaldehyde removal. The active site exposure and self-assembly hydrophilicity were highlighted in MnO_x–CeO₂ (MCO) nanospheres after the loading of colloidal 2.1 wt% TiO₂ particles (TO–MCO). Approximately 57% (relative humidity = 72%) and 41% (dry air) recycling catalytic activities at 35 °C were achieved. Our results proved that surface electron transfer, which was previously weakened because of the loss of surface oxygen species and unsuitable defect-site depositions of low active ions, in the MCO catalyst was recovered via the dispersion of hydrophilic Ti–O groups. This electron transfer was also strongly correlated with the specific surface area, porosity, and oxidation states of transition metals. The greater active site exposure derived from the cyclic electron transfer eventually enhanced the HCHO chemisorption and participation of oxygen species on the surface of TO–MCO throughout the bimetallic (Mn–Ce) dismutation reactions. The abundant superoxide radicals that were activated by these oxygen species prompted a nucleophilic attack on carbonyl bonds. Direct photoionization mass spectrometry determined formic acid, dioxirane (minor), and HOCH₂OOH (little) as intermediates governing the HCHO selectivity to CO₂. The cytotoxicity of catalysts exposed to yeast cells was evaluated for their potential environmentally friendly application indoors.