In situ Raman analyses of the soot oxidation reaction over nanostructured ceria-based catalysts

Abstract

To reduce the emissions of internal combustion engines, ceria-based catalysts have been widely investigated as possible alternatives to the more expensive noble metals. In the present work, a set of four different ceria-based materials was prepared via hydrothermal synthesis, studying the effect of Cu and Mn as dopants both in binary and ternary oxides. In situ Raman analyses were carried out to monitor the behaviour of defect sites throughout thermal cycles and during the soot oxidation reaction. Despite ceria doped with 5% of Cu featured the highest specific surface area, reducibility and amount of intrinsic and extrinsic defects, a poor soot oxidation activity was observed through the standard activity tests. This result was confirmed by the calculation of soot conversion curves obtained through a newly proposed procedure, starting from the Raman spectra collected during the in situ tests. Moreover, Raman analyses highlighted that new defectiveness was produced on the Cu-doped catalyst at high temperature, especially after soot conversion, while a slight increase of the defect band and a total reversibility were observed in case of the ternary oxide and pure/Mn-doped ceria, respectively. The major increment was related to the extrinsic defects component; tests carried out in different atmospheres suggested the assignment of this feature to vacancy-free sites containing oxidized doping cations. Its increase at the end of the tests can be an evidence of peroxides and superoxides deactivation on catalysts presenting excessive oxygen vacancy concentrations. Instead, ceria doped with 5% of Mn exhibited the best soot oxidation activity, thanks to an intermediate density of oxygen vacancies and to its well-defined morphology.

Enhancing Electrochemical CO2 Reduction using Ce(Mn,Fe)O2 with La(Sr)Cr(Mn)O3 Cathode for High‐Temperature Solid Oxide Electrolysis Cells

“…This component is usually assigned to oxygen vacancies coupled with the presence of Ce 3+ or other aliovalent cations. [34][35][36] In addition, the bands at 320, 375, and 660 cm -1 identified in the spectra of manganese-and iron-doped ceria are presumably related to Mn 3 O 4 , as shown in the Raman spectrum of Mn-doped CeO 2 in Figure S1 in the Supporting Information. Therefore, it is reasonable to infer that the sites with oxygen vacancy defects at the triple-phase boundary (TPB) might affect the chemi-absorption steps with enhancing oxygen supply.…”

Nanostructured Equimolar Ceria-Praseodymia for Total Oxidations in Low-O2 Conditions

“…Ceria properties can be further tuned by inserting certain dopant ions into its crystal structure, in order to improve the activity and stability of the catalytic system. The addition of a foreign element promotes the formation of different kinds of defects in ceria lattice, e.g., oxygen vacancies, which have been recognized to play an active role during catalysis [16][17][18][19][20]. In fact, the oxidation reactions are believed to occur via a Mars-van Krevelen-type mechanism over ceria-based catalysts, involving the formation and refilling of oxygen vacancies [11,21].…”

New Approach to Improve Ionic Conductivity at Low Temperature by the Decomposition of KHCO3 in the Nanocomposite Electrolyte @

“…LCDC spectrum presents two weak peaks located at 247 cm −1 and 547 cm −1 . These two peaks, ascribed by many researchers, highlight the formation of reduced Ce 3+ ions conjugated with compensating population of intrinsic oxygen lacunae after calcium and lanthanum integration into the ceria skeleton [24,25] . The mechanism in the form of oxygen vacancies can be predicted by the following defect equation [26] …”