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This work is mainly focused on investigating the effects of different doped metal cations on the formation of Ce20 M1 Ox (M=Zr, Cr, Mn, Fe, Co, Sn) composite oxides and their physicochemical and catalytic properties for NO reduction by CO as a model reaction. The obtained samples were characterized by using N2 physisorption, X-ray diffraction, laser Raman spectroscopy, UV/Vis diffuse reflectance spectroscopy, inductively coupled plasma atomic emission spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction by hydrogen and by oxygen (H2 -TPR and O2 -TPD), in situ diffuse reflectance infrared Fourier transform spectroscopy, and the NO+CO model reaction. The results imply that the introduction of M(x+) into the lattice of CeO2 increases the specific surface area and pore volume, especially for variable valence metal cations, and enhances the catalytic performance to a great extent. In this regard, increases in the oxygen vacancies, reduction properties, and chemisorbed O2 (-) (and/or O(-) ) species of these Ce20 M1 Ox composite oxides (M refers to variable valence metals) play significant roles in this reaction. Among the samples, Ce20 Cr1 Ox exhibited the best catalytic performance, mainly because it has the best reducibility and more chemisorbed oxygen, and significant reasons for these attributes may be closely related to favorable synergistic interactions of the vacancies and near-surface Ce(3+) and Cr(3+) . Finally, a possible reaction mechanism was tentatively proposed to understand the reactions.

The continuously updated after-treatment system of diesel engines drives the selective catalytic reduction of ammonia (NH 3 -SCR) catalysts to withstand hydrothermal stability at higher temperatures (>800 °C). To meet the tightened emission of nitrogen oxides (NO x ), yttrium ions (Y 3+ ) were employed to improve the hydrothermal stability of the Cu/SSZ-39 catalyst at 900 °C. X-ray diffraction, solid nuclear magnetic resonance, H 2 temperature-programmed reduction, NH 3 temperature-programmed desorption, and in situ diffuse reflectance infrared Fourier transform spectra verified the key role of Y 3+ on the stability of the structure of SSZ-39 zeolite and the isolated copper species. In detail, the optimized structural stability at 900 °C of the Y-modified Cu/SSZ-39 catalyst is achieved as a result of the spatial and electronic effects of Y 3+ at ion-exchanged sites. Furthermore, Y 3+ ions in the cages of SSZ-39 zeolite induce more isolated copper species in the double six-member rings of SSZ-39 zeolite. The modified catalyst by 1 wt % Y (Cu/Y 1.0 -SSZ-39) shows the best deNO x performance after hydrothermal aging at 900 °C for 10 h, including higher than 70% NO x conversion at 200−550 °C and the maximum NO x conversion of 94% at 300 °C, which is remarkably higher than the maximum NO x conversion of 38% over Cu/SSZ-39 after same hydrothermal aging. This work provides a vital research strategy for fabricating the extraordinary hydrothermally stabilized Cu-based zeolite catalysts for NH 3 -SCR reaction, which would meet the "near-zero" emission after hydrothermal aging at 900 °C in the future.

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