The role of ceria-based catalysts in oxidation reactions

Nowadays, environmental pollution is a widely discussed problem which is reaching alarming levels worldwide. The catalytic oxidation has proved to be one of the most performing techniques for the pollutants abatement coming from both mobile and stationary sources. The engineering of several synthesis methods and the evolution of characterization procedures are fundamental to the choice of the correct catalytic system for any applications. Among these, ceria-based systems play an essential role in the oxidation reactions due to their unique and peculiar redox properties. They can store and release oxygen in a quick and easy way due to the presence of the Ce4+/Ce3+ pair while maintaining good structural stability even if operating in complex systems such as the soot abatement. Newer preparation techniques allow the synthesis of nanostructured ceria particles with careful control of the nucleation and growth rates during the formation of the crystals. The performance of nanostructured catalysts can increase or decrease according to the preferential exposition of different crystalline planes. Concerning the cerium oxide, the large presence of (110) and (100) planes makes nanorods and nanocubes more active than nano-octahedra, which are characterized by (111) facets. Modified-ceria catalysts own different energetical and structure features compared to the pure ceria. The redox properties and the oxygen mobility are indeed improved with the insertion of metallic dopants in the lattice structure. The advanced characterization techniques and the different catalytic test prove that the nanostructured equimolar ceria-praseodymia show the highest activity in the soot oxidation reaction compared to the other compositions. The further inclusion of zirconium in the Ce-Pr oxides improves both the thermal and the structural stability of the system. Platinum-based catalysts are instead the best samples for the NO oxidation; comparable performance is achieved using the equimolar Ce-Pr oxides. The CO abatement is effective on ceria nanocubes which, as a result of the calcination thermal treatment, are characterized by a rounded structure due to the edges and the corners truncation. Copper-ceria systems synthesised via the SCS method also exhibit good activity for the CO oxidation. Concerning the VOCs, the SCS-ceria is a great material for the naphthalene conversion, while the SCS-Ce0.6Cu0.4 sample is active for the ethene destruction. The innovative and engineered preparation methods of Pd-CeO2 catalysts (e.g. core-shell structures and the ball-milling procedure) ensure the CH4 oxidation in a not too high-temperature range, although the active phase determination of this reaction is still an unsolved point. On ceria catalysts, the PdO seems particularly active at a higher temperature, while at low temperature the presence of different palladium oxidation states such as Pd0, Pd2+ e Pd4+ appears important. Additionally, microfluidic devices based on multi-inlet reactors are considered for the production of ceria-based catalysts with reproducible features.