Reactive adsorption and catalytic oxidation of gaseous formaldehyde at room temperature by a synergistic copper-magnesium bimetal oxide biochar composite

The interactive hybridization of reactive adsorption and catalytic oxidation (RACO) was investigated for the removal of formaldehyde (FA) by fine-tuning the relative compositions of bimetallic oxides between copper (Cu) and magnesium (Mg) on biochar (RH550). The best performer against 100 ppm FA was 8%-Cu-8%-Mg/RH550 when assessed in terms of 10% breakthrough volume (3 L atm g(-1)). The corresponding values for reference materials (e.g., RH550, 16%-Cu/RH550, and 16%-Mg/RH550) were 0.4, 1.1, and 0.5 L atm g(-1), respectively. Adsorption kinetic models confirmed the synergistic roles of physicochemical interactions between the target (FA molecules) and composite material (both organic (RH550) and inorganic (metal oxides) phases). The characterization of the spent Cu-Mg/RH550 suggests that the adsorbed FA molecules were partially oxidized into dioxymethylene and formate. Remarkably, Cu-Mg/RH550 led to the complete room-temperature mineralization of FA into carbon dioxide (CO2) for up to 50 h time-on-stream (artificially set maximum test duration) without any deactivation (gas hourly space velocity of 2,229 h(-1)). The oxidation reaction of FA is expedited by the partial reduction of copper(II) oxide (in Cu-Mg/RH550) to copper(I) oxide. According to the density functional theory based simulations, the metal oxides make the oxidation reaction thermodynamically favorable by hosting hydrogen abstracted from the adsorbed FA molecules. The Cu-Mg synergy also enhances the capture and activation of molecular oxygen (O-2) to boost FA oxidation while sustaining the catalytic process. Overall, this study offers new insights into the synergistic role of RACO in the effective removal of FA under ambient conditions.