Opposite Effects of Co and Cu Dopants on the Catalytic Activities of Birnessite MnO₂ Catalyst for Low-Temperature Formaldehyde Oxidation

Abstract

Defect engineering is an effective strategy to enhance the activity of catalysts for various applications. Herein, it was demonstrated that in addition to enhancing surface properties via doping, the influence of dopants on the surface–intermediate interaction is a critical parameter that impacts the catalytic activity of doped catalysts for low-temperature formaldehyde (HCHO) oxidation. The incorporation of Co into the lattice structure of δ-MnO2 led to the generation of oxygen vacancies, which promoted the formation of surface active oxygen species, reduced activation energy, and enhanced catalytic activity for low-temperature oxidation of HCHO. On the contrary, Cu doping led to a drastic suppression of the catalytic activity of δ-MnO2, despite its enhanced redox properties and slight increase in the surface concentration of active oxygen species, compared to pristine δ-MnO2. Diffuse reflectance infrared Fourier transform analysis revealed that in the presence of Cu, carbonate intermediate species accumulate on the surface of the catalysts, leading to partial blockage of active sites and suppression of catalytic activity.

In Situ Fabrication of Highly Dispersed Co–Fe-Doped-δ-MnO2 Catalyst by a Facile Redox-Driving MOFs-Derived Method for Low-Temperature Oxidation of Toluene

“…It can be seen that the Co/Mn and Fe/Mn ratio of the Co–Fe-δ-MnO 2 catalyst exhibited an upward trend as the amount of Co–Fe PBA added increased from 0 to 20 wt %, while the K/Mn ratio remains roughly constant with that of the δ-MnO 2 catalyst. These results indirectly evidence that the Co and Fe ions were incorporated into the δ-MnO 2 framework rather than substituting K ions in the interlayer structure of birnessite …”

“…As depicted in Figure b, the asymmetric O 1s spectra of the obtained catalysts can be segmented into two peaks at 529.9 and 531.4 eV ascribed to the lattice oxygen (O latt ) and surface adsorption oxygen species (O ads ), respectively . It is commonly recognized that the content of O ads reflects the amount of oxygen vacancy on the catalysts since O 2 can be adsorbed at oxygen vacancies and transformed into O ads on the catalyst’s surface. , Thus, the molar ratio of O ads to O latt increased from 0.255 for δ-MnO 2 to 0.657 for Co–Fe-δ-MnO 2 -25% as the amount of Co–Fe PBA added increased (Table ), suggesting that the amount of surface oxygen vacancies were gradually increased. , These results are consistent with the Mn 2p 3/2 and EPR analysis above. Furthermore, the binding energy of O 1s over Co–Fe-δ-MnO 2 shifts to a lower value compared to that of δ-MnO 2 , indicating weaker interaction between the Mn and O atoms. , As a result, the mobility of lattice oxygen could be improved so that the cycling of the emergence-annihilation of reactive oxygen species could be easily realized during the reaction process, thereby enhancing the catalysis performance …”

“…14,17,22 Meanwhile, a considerable number of obscure or distorted lattice fringes (highlighted by white ovals, Figure 2h) were identified in Co−Fe-δ-MnO 2 -20%, implying the distortion of [MnO 6 ] octahedra due to the Co and Fe species dispersing in the lattice of δ-MnO 2 . 39,42 Moreover, elemental mapping images for the Co− Fe-δ-MnO 2 -20% depicted in Figure 2k exhibited that K, Mn, O, Co, and Fe elements are homogeneously distributed on the catalysts. Together with the XRD results that no distinguishable peaks correspond to Co and Fe species (Figure 1), it was demonstrated that the redox-driving MOFs-derived method is an effective way to realize the homogeneous incorporation of Co and Fe ions into δ-MnO 2 .…”

“…These results indirectly evidence that the Co and Fe ions were incorporated into the δ-MnO 2 framework rather than substituting K ions in the interlayer structure of birnessite. 39 Furthermore, SEM characterizations were carried out to investigate the morphology of various samples. It is revealed that the Co−Fe-δ-MnO 2 -20% has a cubic nanoflower-like shape (Figure 2a), which appears to combine the cubic structure of Co−Fe PBA (Figure 2b) with the nanoflower-like morphology of δ-MnO 2 (Figure 2c).…”

“…Together with the XRD results that no distinguishable peaks correspond to Co and Fe species (Figure 1), it was demonstrated that the redox-driving MOFs-derived method is an effective way to realize the homogeneous incorporation of Co and Fe ions into δ-MnO 2 . 22,39 These phenomena are probably attributed to the highly ordered metal cluster in Co− Fe PBA working perfectly as dopant sources. Based on the analysis above, it can be concluded that highly dispersed Co− Fe-δ-MnO 2 catalysts with PBA-like nanostructure were successfully prepared through the redox-driving route.…”