Catalysts for Removal of Soot: History
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Zareen Zuhra
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Shuo Li
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Guanqun Xie
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Xiaoxia Wang

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. 

  • soot
  • impact
  • oxidation
  • catalysts

1. Soot Formation and Methodology for Soot Oxidation

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]. 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]. 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]. The overall composition of diesel exhausts and their threats to human life and the environment are described in Table 1 [9][10][11][12]. 

Table 1. Threats of exhaust components to human life [9][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
For these components, the major concern is the emission of soot [13]. Soot is a type of fine, black powder that is composed of carbon along with various other chemical compounds [14]. It is produced when organic matter or hydrocarbons, such as fossil fuels or wood, undergo incomplete combustion. Incomplete combustion occurs when there is not enough oxygen available for the fuel to burn completely. Soot particles are very small and can vary in size, with some being fine enough to become suspended in the air as particulate matter [15][16]. This fine particulate matter can be harmful to human health when inhaled, as it can penetrate deep into the lungs and potentially cause respiratory problems [17][18]. Moreover, soot is a common byproduct of various combustion processes, including those in car engines, industrial furnaces, and the burning of wood or other fuels in household stoves and fireplaces [19][20]. It is also a major component of air pollution and can contribute to smog formation and other environmental issues [21]. Top of Form Diesel particulates exhibit a bimodal distribution of nuclei mode particles and accumulation mixture mode particles, as shown in Figure 1 [2][22]. Nuclei mode particles are normally defined as particles having a size of less than 50 nm, and these particles easily evaporate. The accumulation of crucial carbon components where gases are adsorbed on this surface, namely, long-chain hydrocarbons, results in the formation of accumulation mode particles, which have sizes ranging from 50 to 1000 nm [23].
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Figure 1. SEM image of soot aggregates in diesel exhaust (a), other minor structures of a soot particle (b), major structures of diesel soot materials (c1) and HR-TEM image of soot collected from a combustion chamber (c2) [2][22].
These particles are initially commonly produced during the preliminary steps of soot formation and can originate from two mechanistic approaches: (1) collision coalescence, which is a physical route; and (2) surface development, which is a chemical route that occurs during the diffusion part of incineration [24]. Later, the development of the prime bits results in collisions and the subsequent formation of an agglomeration of clusters of these tiny particles; consequently, soot is formed [25]. It is well reported that the fuel undergoes two oxidation procedures that occur (1) at the fuel-rich premixed zone and (2) at the flame of diffusion on the outside boundary. PAHs are considered the focal soot pioneers and are generated in the premixed zone along with fuel-rich mixtures that develop in the middle because a portion of the surface is actually oxidized in the flame of diffusion [26][27]. The process of soot formation involves the generation of the first aromatic ring structure, the decomposition of the fuel molecule, and subsequently the growth of the polycyclic aromatic hydrocarbon (PAH), which is a molecular pioneer and particle that is used in the nucleation stages [28]. The major portion of the soot product is produced through surface development procedures that normally occur after nucleation. This phenomenon includes the connecting of gas-stage moieties, for example, these hydrocarbons, to the seeming of the elements, which allow the aggregation of the total mass while not increasing the number of soot bits [29]. These fundamental bits act as nuclei; however, weighty hydrocarbons could physically or chemically interact during the exhaust stroke or diffusion ignition. After that, the next stage is particle coagulation, wherein collisions among the particles result in an increase in the average particle size while the number of particles decreases; thus, an increase in the overall mass of the soot does not occur [30]. A scanning electron microscopic (SEM) image of various forms of black carbon particles is shown in Figure 2 [31].
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Figure 2. SEM images of various forms of particles [31].
Nevertheless, as a result of the pyrolytic conditions that occur in the post-flame zone, graphitic carbon material is fabricated from the amorphous soot material, as revealed, resulting in a minute decrease in the mass of the particle without a change in the number of particles [17][32]. Meanwhile, PAH oxidation and soot materials are restricted as they are associated with the generation of these specific moieties [30]. A possible pathway of soot formation from the gas stage to the formation of agglomerated solid particles is illustrated in Figure 3 [2][31].
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Figure 3. A schematic pathway for the formation of soot particles [2][31].

2. Effect on Health and the Environment

Persistent uptake of diesel output materials could lead to various chronic breathing illnesses. Also, diesel exhaust is complex mixture; research indicates it has a harmful influence on human health and damages the green atmosphere [33]. The capital city of China (Beijing) is facing enormous problems with its air quality, with smog occurring very frequently [34]. Among all the contaminants that cause smog, a considerable part originates from locomotive dissipation.
Soot particles in the atmosphere can absorb sunlight and heat, contributing to global warming [35]. This is particularly concerning in regions with snow and ice, as the presence of soot on these surfaces can cause them to absorb more heat, accelerating the melting process [36]. Soot can settle on surfaces, including vegetation and bodies of water. This can harm ecosystems and aquatic life. When soot settles on ice or snow, it reduces their reflectivity (albedo), causing more heat absorption and faster melting [37]. Exposure to soot particles has been linked to a range of health problems, including cardiovascular diseases, lung cancer, and premature death. Children, the elderly, and individuals with pre-existing health conditions are particularly vulnerable [38].
One of the most efficient techniques for removing soot particles is trapping them through filters, followed by oxidation. Meanwhile, the oxidation of diesel exhaust occurs at elevated temperatures (600 °C) with uncatalyzed soot filters [39]. Thus, a catalyst coating of the filters is needed to lower the soot oxidation temperatures to values within the temperature range of diesel exhausts (200–350 °C) [40]. Additionally, a catalyst is obligatory for soot removal; platinum-based catalysts are considered highly efficient, wherein NO reacts with oxygen to form NO2 in the dissipated gas. This has a substantial consequence for soot oxidation. However, platinum has the disadvantage of its high cost; hence, an inexpensive alternative is desirable.

3. Diversity in Catalysts for Removal of Soot

Recent research efforts have focused on developing platinum group metal-free catalysts for soot oxidation. Moreover, innovative catalyst materials, such as transition metal oxides and non-noble metal catalysts, have shown promising results in catalyzing soot oxidation while addressing cost and sustainability concerns [41]. Similarly, nanostructured catalyst materials have emerged as a prominent trend in soot oxidation research [42]. Nanostructured catalysts exhibit increased surface area and enhanced catalytic activity, enabling more efficient soot oxidation at lower temperatures. Research has explored various nanoarchitectures, including nanoparticles, nanowires, and nanotubes, to optimize catalytic performance [43][44]. Advances in catalyst support materials have further improved the durability and effectiveness of catalysts in soot oxidation [45][46]. Innovative support materials, such as perovskites, zeolites, and mesoporous materials, have been investigated for their potential to enhance catalyst stability and longevity under harsh operating conditions [47][48][49]. However, the identification of ceria-based catalysts as highly effective materials for soot oxidation has only recently been reported [50][51]. Ceria-based catalysts, known for their oxygen storage and release capabilities, have played a prominent role in reducing the temperature required for soot oxidation and enhancing catalytic activity.
PGM (platinum group metal) catalysts are extremely dynamic [52][53]. PGMs, including platinum, palladium, and rhodium, are commonly used as catalysts for the oxidation of soot in various applications, particularly automotive exhaust systems. The surface environments of the costly metals and catalysts, their surface area, and other surface parameters significantly clean the emissions gas during automobile catalytic conversion. Normally, Rh, Ru, Au, and Pt particles adhere to the base material. However, they are costly, and they are susceptible to greater price increases with increasing demands due to their low richness [54][55].
Perovskite catalysts have gained significant attention in recent years due to their potential applications in various catalytic processes, including soot oxidation [56]. They are normally represented using the formula ABO3, wherein the A and B parts represent two cations of dissimilar dimensions, while O is an anion bridge that interacts with cations [57][58]. Part A generally belongs to elements of alkaline/alkali earth metals (Sr, Cs, Ca, Ba, Ra) [59][60][61][62][63] and/or rare earth (La, Ce, Nd, etc.) [64][65][66] with a larger radius of approximately 0.90 Å compared with the transition metal of part B (Ag [67], Fe [68], Co, Zn, Cu, Ni, Mn, Cr, Ru, Al [65]) with an approximate radius of 0.51 Å [69]. In a cubic cell, the constituent A-atom occupies the dice corner positions (0, 0, 0), B conquers the position of body center (1/2, 1/2, 1/2), and O occupies the position of face center (1/2, 1/2, 0). In addition, part A, which is normally coordinated to 12 oxygen molecules, forms a dodecahedral site. Part B is occupied by six O-atoms in octahedral coordination [70][71]. 
Layered double hydroxides (LDHs) are a class of materials that have been explored for various catalytic applications, including soot oxidation. LDHs are also known as hydrotalcite-like compounds or anionic clays. They are made of positively charged metal hydroxide layers and charge-balancing anions in the interlayer regions [72][73].
LDHs exhibit anion exchange characteristics due to the weakly carbonate-bonded anions in their interlayer region. Moreover, selection for the anions can be done at the initial precipitation during its preparation. It is important to mention here that during calcination, metal-oxide products can be produced from these LDHs, which are further used for various applications [74][75]. In addition, the resultant metal oxides possess higher surface areas, which is normally beneficial for catalysis applications. 
Mixed metal oxides (MMOs) of inner and outer transition metals, alkaline, rare earth, and alkali group metals have potential for various catalytic applications [76]. Different reactions have been used for MMO synthesis. For example, alkylation, the Mannich reaction, oxidation, reduction, multicomponent, condensation, cycloaddition, deprotection, hydroxylation, and other reactions can be done successfully in different reaction conditions [77]. The mixed metal oxides use an interesting mechanistic approach to convert the NO and soot into their respective components, such as NO2 and CO2 (Figure 4) [75].
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Figure 4. A graphical explanation of the role of mixed metal oxides materials with NO in soot oxidation [75].

4. Ceria-Based Mixed Metal Oxides

Ceria (CeO2) is of most significant importance as a component of three-way catalysts (TWCs) given its storage capacity (OSC) for oxygen [78]. It has attained a significant rank among the metal oxides that have been extensively studied to date [79][80]. 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]. 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]. 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]. 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].

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]. 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].

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]. 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]. 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].

This entry is adapted from the peer-reviewed paper 10.3390/molecules28196884

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