Metal oxide/activated carbon composites for the reactive adsorption and catalytic oxidation of formaldehyde and toluene in air

The synergistic effect of ‘reactive adsorption and catalytic oxidation (RACO)’ has been investigated to develop an efficient method to remove volatile organic compounds (VOCs: formaldehyde (FA) + toluene) using an amorphous-manganese dioxide-activated carbon (Am-MnO2-AC) composite. The RACO of FA and toluene onto Am-MnO2-AC was examined individually and as a mixture at multiple-isothermal conditions from room temperature (RT) to 300 °C. At RT, the values for 10% adsorption breakthrough volume (BTV10%) of single FA and toluene by Am-MnO2-AC were 45.9 and 152 L g−1, respectively, approx. 1.3-fold higher than those for their mixture. The BTV10% values for FA measured in single and mixture were 270–280 times higher than their pristine AC counterparts. The RACO performance of Am-MnO2-AC was investigated more intensively against a dual component system in reference to its crystalline forms (e.g., δ-MnO2) and two other metal-doped forms (i.e., cobalt and silver). Accordingly, Am-MnO2-AC exhibited the highest catalytic reactivity so as to achieve the complete oxidation of FA and toluene (as individual components) into carbon dioxide (CO2) at 100 and 275 °C, respectively. The thermal catalytic oxidation of FA and toluene onto Am-MnO2-AC was suggested to follow the Langmuir-Hinshelwood mechanism. The enhanced RACO performance of Am-MnO2-AC was ascribed to its higher surface porosity and higher concentration of oxygen vacancies (e.g., Mn3+/Mn4+ of 0.707 and adsorbed oxygen (OA)/lattice oxygen (OL) of 1.79). The present study is expected to provide deep insights into the practical application of metal oxide/AC composites for cleaner indoor air quality.