Activating Inert Alkali‐Metal Ions by Electron Transfer from Manganese Oxide for Formaldehyde Abatement

Abstract

Alkali-metal ions often act as promoters rather than active components due to their stable outermost electronic configurations and their inert properties in heterogeneous catalysis. Herein, inert alkali-metal ions, such as K and Rb , are activated by electron transfer from a Hollandite-type manganese oxide (HMO) support for HCHO oxidation. Results from synchrotron X-ray diffraction, absorption, and photoelectron spectroscopies demonstrate that the electronic density of states of single alkali-metal adatoms is much higher than that of K or Rb , because electrons transfer from manganese to the alkali-metal adatoms through bridging lattice oxygen atoms. Electron transfer originates from the interactions of alkali metal d-sp frontier orbitals with lattice oxygen sp orbitals occupied by lone-pair electrons. Reaction kinetics data of HCHO oxidation reveal that the high electronic density of states of single alkali-metal adatoms is favorable for the activation of molecular oxygen. Mn L -edge and O K-edge soft-X-ray absorption spectra demonstrate that lattice oxygen partially gains electrons from the Mn e orbitals, which leads to the upshift in energy of lattice oxygen orbitals. Therefore, the facile activation of molecular oxygen by the electron-abundant alkali-metal adatoms and active lattice oxygen are responsible for the high catalytic activity in complete oxidation of HCHO. This work could assist the design of efficient and cheap catalysts by tuning the electronic states of active components.

The Promotional Effect of Copper in Catalytic Oxidation by Cu-Doped α-MnO2 Nanorods

“…The effect of the doped Cu ions on electron transfer was elaborated by soft-X-ray adsorption first. Two peaks in the Mn L 3 -edge X-ray absorption spectra of α-MnO 2 and Cu 1 /MnO 2 were assigned to the 2p 3/2 → t 2g and 2p 3/2 → e g transitions (Figure a) . In comparison with those of α-MnO 2 , the e g orbitals of the Mn ions in Cu 1 /MnO 2 shifted down by 0.2 eV in energy.…”

Noble Metal Single‐Atom Catalysts for the Catalytic Oxidation of Volatile Organic Compounds

“…Previous works showed that noble metal nanoparticles (Pt, Pd, Rh, and Au) [33] catalyzed HCHO oxidation at low temperatures [34] . Comparably, most Ag‐, [23,32a] Na‐, [32b] Rb‐, [32e] K‐, [32c] Au‐ [32d,h] and Pt‐ [32f,g,35] based SACs catalyzed the formaldehyde at the temperature of >50 °C in HCHO oxidation, while several metal SACs showed the oxidation of formaldehyde at room temperature (Table 2). [32f,g,35] For example, A 9.7 wt% Ag 1 /HMO−A SAC prepared using the anti‐Ostwald Ripening (AOR; Figure 5a) completely converted formaldehyde at 110 °C [23,32a] .…”

“…The stronger the EMSI, the higher depletion of the 4d electronic state of the Ag active site, which resulted in a higher activation ability towards O 2 , further leading to higher catalytic activity in the low‐temperature oxidation of HCHO. Similarly, alkali‐metal SACs supported on the HMO prepared by the AOR method (Na, [32b] K, [32c] and Rb [32e] ) completely oxidized HCHO at 100–120 °C [23,32a–c,e] . The high electron density of the alkali‐metal single atoms and high activity of lattice oxygen were beneficial to the activation of molecular oxygen, contributing to the high catalytic activity of the alkali‐metal SACs in HCHO oxidation.…”

“…Moreover, a Ti‐decorated Ti 3 C 2 O 2 [32j] monolayer proceeded the reaction with the Eley–Rideal (E−R) mechanism rather than the L−H mechanism. HCHO oxidation reaction over M/HMO (M=Ag, Na, K, Rb), [23,32a–c,e] Au 1 /α‐MnO 2 , [32d] Au 1 /CeO 2 [32h] and Pt 1 ‐Mn/TiO 2 [32f] SACs were reported to follow the MvK mechanism, as shown below: (1) the dissociation of molecular oxygen to form O s at the interface between active metal and support, and (2) the oxidation of HCHO or adsorbed formate (the stable intermediate in the HCHO oxidation reaction) by O s . Therefore, the mobility and reducibility of the O s were essential.…”

“…Therefore, the mobility and reducibility of the O s were essential. It was suggested that the high activity of M 1 /HMO (M=Ag, Na, K, Rb) SAC was attributed to the O s of M 1 /HMO having the O 2p orbitals with the high electronic density from the interactions of O with metal [23,32a–c,e] . Then, the reducibility of the O s was improved, enhancing the catalytic activity.…”