Low-temperature oxidative removal of gaseous formaldehyde by an eggshell waste supported silver-manganese dioxide bimetallic catalyst with ultralow noble metal content

Abstract

Under dark/low temperature (DLT) conditions, the oxidative removal of gaseous formaldehyde (FA) was studied using eggshell waste supported silver (Ag)-manganese dioxide (MnO₂) bimetallic catalysts. To assess the synergistic effects between the two different metals, 0.03%-Ag-(0.5–5%)-MnO₂/Eggshell catalysts were prepared and employed for DLT-oxidation of FA. The steady-state FA oxidation reaction rate (mmol g^-1 h^-1), when measured using 100 ppm FA at 80 °C (gas hourly space velocity (GHSV) of 5308 h^-1), varied as follows: Ag-1.5%-MnO₂/Eggshell-R (9.4) > Ag-3%-MnO₂/Eggshell-R (8.1) > Ag-1.5%-MnO₂/Eggshell (7.5) > Ag-5%-MnO₂/Eggshell-R (7.2) > Ag-1.5%-MnO₂/CaCO₃-R (6.8) > MnO₂-R (6) > Ag-0.5%-MnO₂/Eggshell-R (3.2) > Ag/Eggshell-R (2.6). (Here, ‘R’ denotes hydrogen-based thermochemical reduction pretreatment.) The temperature required for 90% FA conversion (T₉₀) at the same GHSV exhibited a contrary ordering: Ag/Eggshell-R (175 °C) > Ag-0.5%-MnO₂/Eggshell-R (123 °C) > Ag-5%-MnO₂/Eggshell-R (113 °C) > MnO₂-R (99 °C) > Ag-1.5%-MnO₂/Eggshell (96 °C) > Ag-3%-MnO₂/Eggshell-R (93 °C) > Ag-1.5%-MnO₂/Eggshell-R (77 °C). The eggshell catalyst outperformed the ones made of commercial calcium carbonate due to the presence of defects in the former. The MnO₂ co-catalyst enhances the catalytic activities through the capture and activation of atmospheric oxygen (O₂) with rapid catalytic regeneration. Also, MnO₂ favorably captures the hydrogen of the adsorbed FA molecules to make the oxidation pathway thermodynamically more favorable.