Constructing a core–shell rare earth-manganese-zirconium composite compound to improve catalytic activity of NO oxidation

NO catalytic oxidation is the key performance of the diesel oxidation catalyst (DOC). We present a facile deposition method for the core–shell rare-earth manganese-zirconium composite oxide that shows the Mn mullite phase uniform loading on the surface of zirconium-based composite (YMO/CYZO), which demonstrates a superior NO oxidation catalytic performance in simulated diesel combustion conditions and better thermal stability than mullite phase YMn₂O₅ oxide. The NO oxidation at 250 °C over YMO/CYZO-a approaches 25.2% in contrast to 13.52% over YMn₂O₅-a. Then the catalytic performance of YMO/CYZO, YMO and commercial 1 wt% Pt/Al₂O₃ in a NO + O₂ atmosphere was compared. The maximum conversion rate of YMO/CYZO to NO oxidation is 89.6% at 274 °C with a GHSV of 50000 h^–1, and the performance is superior to that of YMO (82.8% at 293 °C) and 1 wt% Pt/Al₂O₃ (68.6%, 335 °C). The NO-temperature programmed desorption (NO-TPD) and diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) results reveal that YMO/CYZO has multiple NO adsorption sites and high storage capacity. Furthermore, density functional theory (DFT) calculation indicates that YMO/CYZO has lower oxygen vacancy formation energies (E_v = 0.93 eV) and favorable NO adsorption energies (E_ads = −2.1 eV). Moreover, in situ X-ray photoelectron spectroscopy (XPS) characterization shows that the core–shell structure of YMO/CYZO has the potential to transmit active oxygen species to help realize Mn³⁺ to Mn⁴⁺ during the reaction process to enhance the conversion of NO∗ molecules, while NO oxidation reactions follow the MvK mechanism.