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.
Highlights
Nowadays gasoline and diesel engines are still the most used propulsion systems for both trucks and passenger cars
Many advances have been made in the synthesis of nanostructured ceria-based catalysts, whose reactivity depends on the particle size and on the crystalline planes exposed on the surface, as pointed out in several studies[11,16,17]
The synthesized catalysts were first characterized by complementary techniques in order to investigate their morphology, crystalline structure, surface area and surface species; some of the results obtained were previously reported elsewhere[31], but are briefly resumed here to present a complete description of the investigated materials
Summary
Nowadays gasoline and diesel engines are still the most used propulsion systems for both trucks and passenger cars. Nowadays Diesel vehicles, which are appreciated for their great efficiency and fuel economy, are equipped with a particulate filter that can entrap fine dusts This filter must be periodically regenerated, and 600 °C should be reached to cause the combustion of carbon particles, while the exhaust gas temperature of a Diesel engine is usually around 200–500 °C5,6. Depend on its intrinsic activity; since the reaction involves two solid phases, the efficiency of contact between the catalyst and the particulate plays a fundamental role[14,15] For this reason, many advances have been made in the synthesis of nanostructured ceria-based catalysts, whose reactivity depends on the particle size and on the crystalline planes exposed on the surface, as pointed out in several studies[11,16,17]. Monitoring the defect sites throughout the soot oxidation reaction presents several complexities[29] and only few observations of this process have been reported[30]
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