Soot formation is an inevitable consequence of the combustion of carbonaceous fuels in environments rich in reducing agents. Efficient management of pollution in various contexts, such as industrial fires, vehicle engines, and similar applications, relies heavily on the subsequent oxidation of soot particles. Among the oxidizing agents employed for this purpose, oxygen, carbon dioxide, water vapor, and nitrogen dioxide have all demonstrated effectiveness.
A few years ago, diesel engines gained worldwide fame owing to their unique features, such as low fuel consumption, long range, and greater thermal efficiency, compared with other engines [1][2][1,2]. The other side of the picture is also quite harsh given diesel engine exhaust emissions, which create a great threat for us on this planet [3][4][5][3,4,5]. The composition of their exhaust emanations is a combination of different gases, vapors, particulate matter (soot), liquid aerosols, nitrogen, water, CO, NOx, SOx, and polycyclic aromatic hydrocarbons (PAHs), and they make our world alarmingly polluted with air pollution [6][7][8][6,7,8]. The overall composition of diesel exhausts and their threats to human life and the environment are described in Table 1 [9][10][11][12][9,10,11,12].
Pollutants | Concentration | Threats |
---|---|---|
Soot | 20–200 mg/m3 | Eyes problems, cancer, asthma, skin infections, lung damage, heart issues |
NOx | 30–1000 ppm | Chest pain, respiratory and lungs problems, cough |
SOx | Proportional to fuel S content | Acid rain, skin problems |
CO2 | 2–12 vol% | Green house effect, acid rain, lung disease |
CO | 100–1000 ppm | Hpertension, head pressure, lung disease |
HC | 50–500 ppm | Eyes irritation, lungs issues, respiratory problems |
PAH | 0.3 mg/mil | Kindney and liver damage |
Ceria (CeO2) is of most significant importance as a component of three-way catalysts (TWCs) given its storage capacity (OSC) for oxygen [78][125]. It has attained a significant rank among the metal oxides that have been extensively studied to date [79][80][126,127]. The research direction proposed by Trovarelli has opened a new door for ceria-based catalysts, indicating their potential in theoretical and practical applications as well as providing structural insights for their derived catalysts. Meanwhile, they function to support and boost the catalytic performances of metal catalysts [81][128]. The effects of the nanometric sizes and morphologies of ceria-based catalysts have been studied since the last decade, and various studies have reported on their synthesis pathways, chemical properties, geometries, characteristics, and catalytic performance in the oxidation of CO to date [82][129]. Recently, a correlation has been reported for redox properties between surface properties and the crystal morphology of ceria-based cubes, polyhedrons, and rods. Observations indicated that face reconstruction, size, and nanomorphology influence their performance, selectivity and stability [83][130]. In the ceria cubic structure, the fcc group, which is regarded as a stable surface plane, shows a lower coordination number compared to its bulk with divergent terminating structures on surfaces, including repetitive O-Ce-O interlayers, both elements Ce and O, and a O-Ce-O-Ce echoing unit. However, in thermally controlled systems, stable surfaces are normally generated during crystal growth and finally develop specific nanoshapes [84][131].
Remarkably, every stable plane displays various reduction features. The redox process of Ce4+ to Ce3+ produces vacancies for oxygen that play a vital role in oxygen packing and oxidation reactions. There is no theoretical basis; the growth plans follow the order of reactivity for oxygen vacancy defect formation, providing the basis for experimental work to assess the relationship between the catalytic performance and nanocrystal morphology of ceria [85][133]. The oxygen vacancies and surface chemistry strongly depend on the nanometric size of particles, and these factors are strongly enhanced when the particle size is less than 10 nm. Oxygen vacancy creation modeling investigations focused on size revealed that their energy is governed by the position of the oxygen atom lattice; for nanoparticles (NPs) with a size of 2–4 nm, its value approaches the minimum level [86][134].
The catalytic performance of the nanorods was observed to be associated with loosely bound oxygen. The nanorods’ performances were lower than those of nanowires, regardless of the fact that nanorods and nanowires exhibit predominantly reactive planes; this could be attributed to a higher concentration of surface-active planes [87][137]. Hierarchically, mesoporous ceria is prepared using diatom templates, which have greater Ce3+ content, a high specific surface area (SSA) (78 m2 g1), facile reducibility, a higher number of oxygen vacancies, and enhanced CO oxidation compared with bulk ceria. Moreover, ultrasound synthesis was reported to form nanoflowers, nanospheres, nanorods, and nanoribbons of ceria nanostructures (size ~5 nm) [88][138]. This synthesis was performed in a single step using various kinds of ionic liquids. The shape and structure of the final product depend on how it was heated. For example, under [C4mim][Tf2N], the ionothermal fabrication method produced flower and nanorod shapes, while the ultrasound method produced nanospheres. Nanoshape activity order followed the order of the SSAs; however, this order was not found to be proportional to them, indicating that oxygen vacancies as well as structural defects play crucial roles. Sonochemistry under [C4mim][Tf2N] generates nanospheres with the best performance. This is because the nanospheres have a large SSA, a mesoporous structure, a higher number of surface oxygen vacancies, and small particle size [89][139].