| Summary: | To avoid resource waste and environmental pollution, a chain of ErO<sub>x</sub>-boosted MnO<sub>x</sub>-modified biochars derived from rice straw and sewage sludge (Er<sub>y</sub>Mn<sub>1-y</sub>/BACs, where biochars derived from rice straw and sewage sludge were defined as BACs) were manufactured for formaldehyde (HCHO) elimination. The optimal 15%Er<sub>0.5</sub>Mn<sub>0.5</sub>/BAC achieved a 97.2% HCHO removal efficiency at 220 °C and exhibited favorable E<sub>HCHO</sub> and thermal stability in a wide temperature window between 180 and 380 °C. The curbed influences of H<sub>2</sub>O and SO<sub>2</sub> offset the boosting effect of O<sub>2</sub> in a certain range. Er–Mn bimetallic-modified BACs offered a superior HCHO removal performance compared with that of BACs boosted using Er or Mn separately, owing to the synergistic effect of ErO<sub>x</sub> and MnO<sub>x</sub> conducive to improving the samples’ total pore volume and surface area, surface active oxygen species, promoting redox ability, and inhibiting the crystallization of MnO<sub>x</sub>. Moreover, the support’s hierarchical porous structure not only expedited the diffusion and mass transfer of reactants and their products but also elevated the approachability of adsorption and catalytic sites. Notably, these prominent features were partly responsible for the outstanding performance and excellent tolerance to H<sub>2</sub>O and SO<sub>2</sub>. Using in situ DRIFTS characterization analysis, it could be inferred that the removal process of HCHO was HCHO<sub>ad</sub> → dioxymethylene (DOM) → formate species → CO<sub>2</sub> + H<sub>2</sub>O, further enhanced with reactive oxygen species. The DFT calculation once again proved the removal process of HCHO and the strengthening effect of Er doping. Furthermore, the optimal catalytic performance of 15%Er<sub>0.5</sub>Mn<sub>0.5</sub>/BAC demonstrated its vast potential for practical applications.
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