Oxygen vacancy engineering in Fe doped akhtenskite-type MnO2 for low-temperature toluene oxidation

To clarify the discrepancies between oxygen vacancies (VO) types and concentrations, we designed two types of VO by substituting Mn4+ in akhtenskite-type MnO2 with low-valence manganese (e.g. Mn3+) or iron (e.g. Fe3+), denoted as VO (MM) and VO (FM), respectively. The concentration of VO was adjustable by the varied sample calcination temperature. The catalysts with more VO (FM) achieved higher toluene specific reaction rate and better stability than those of Mn3+ doped MnO2, ascribing to the chemical and reactive difference of their different Vo types. Toluene specific reaction rate have been quantitatively calculated to estimate the contribution of VO on catalytic performance and followed the order of F1M4 (2.73 mmol/(g h)) > F2M13 (2.29 mmol/(g h)) > F1M14 (1.52 mmol/(g h)) > Mn (0.91 mmol/(g h)) > F1M2 (0.89 mmol/(g h)) at 200 °C (Mn, F1M14, F2M13, F1M4 and F1M2 represented pure MnO2 and Fe doped MnO2 with Fe/Mn = 1/14, 2/13, 1/4, and 1/2, respectively). Additionally, the VO concentrations of as-prepared catalysts decreased with the increasing calcination temperature, which then led to the declining activity for toluene oxidation. Furthermore, in situ DRIFTS spectra demonstrated that the higher concentrations of VO (FM) sites accelerated the transformation of intermediates from benzaldehyde to benzoate, which was the intermediate to be further oxidized into CO2 and H2O.