Construction of substrate-dependent 3D structured MnO ₂ catalysts for diesel soot elimination

Guided by oxidation-reduction reactions, we in-situ grew MnO ₂ coating on three novel 3D structured foams, Ni, Fe and Fe ₇₅ Ni ₂₅ (written as MN, MF and MFN) for diesel soot elimination. Systematic characterizations including SEM, HRSTEM-EDS, XRD, Raman spectroscopy, FT-IR, XPS, H ₂ -TPR and soot-TPR revealed that the substrates affected greatly the morphology, content, structure, surface and redox ability of MnO ₂ coatings. MnO ₂ of MN was thin nanoflake, but became thicker on MF and partly turned into nanowire on MFN. The contents of K stabilizer in MnO ₂ coatings are in proportion to the concentrations of Fe ³⁺ and Ni ³⁺ dopants. MF showed the best pristine activity owing to abundant surface oxygen species, bulk K ⁺ , and high-valence manganese. But MF deactivated seriously in 6% H ₂ O (T ₅₀ increased by ~30 °C) because of excessive hydrophily and K ⁺ loss. MN and MFN displayed splendid reusability; especially 90% of soot loaded on MFN could be removed within 1500s at 420 °C in a simulated diesel exhaust. Ni ³⁺ could effectively stabilize K ⁺ and tune surface hydrophily. The used catalysts displayed good adherence against exfoliation. Further, the excellent heat conductivity of metal foams and low coating weights would eliminate hot spots from soot combustion.