Soot Erased: Catalysts and Their Mechanistic Chemistry

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. The scientific framework of this research can be elucidated through the following key aspects: (i) This review situates itself within the broader context of pollution management, emphasizing the importance of effective soot oxidation in reducing emissions and mitigating environmental impacts. (ii) The central research question of this study pertains to the identification and evaluation of catalysts for soot oxidation, with a specific emphasis on ceria-based catalysts. The formulation of this research question arises from the need to enhance our understanding of catalytic mechanisms and their application in environmental remediation. This question serves as the guiding principle that directs the research methodology. (iii) This review seeks to investigate the catalytic mechanisms involved in soot oxidation. (iv) This review highlights the efficacy of ceria-based catalysts as well as other types of catalysts in soot oxidation and elucidate the underlying mechanistic strategies. The significance of these findings is discussed in the context of pollution management and environmental sustainability. This study contributes to the advancement of knowledge in the field of catalysis and provides valuable insights for the development of effective strategies to combat air pollution, ultimately promoting a cleaner and healthier environment.

Effect of Exhaust Gas Recirculation Combined with Selective Catalytic Reduction on NO x Emission Characteristics and Their Matching Optimization of a Heavy-Duty Diesel Engine

Exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) have become important technologies to reduce the NO _x emission of heavy-duty diesel engines and meet the increasingly stringent emission regulations. This paper studied the effect of EGR combined with SCR on the NO _x emission characteristics of a heavy-duty diesel engine based on the engine bench test. The results showed that the NO reduction rate of EGR-coupled SCR increased with the increase of engine load, and the effect was no longer significant when the NO reduction rate exceeded a certain limit under the same working conditions. EGR combined with SCR has little effect on NO₂ emission reduction, and the increase of engine speed can significantly improve the efficiency of the NO₂ reduction rate at 75 and 100% load. 25% opening of the EGR valve (OEV) and 50% OEV have very similar effects on the NO _x reduction rate when the engine speed is at a low level. Compared with low engine speeds, increased OEV or ammonia NO _x molar ratio (ANR) had a more obvious effect on the NO _x reduction rate at high engine speeds. SCR combined with low valve-opening EGR had a more significant effect on the NO _x reduction rate. The increase of OEV led to the increase of fuel consumption rate, but the effect on the fuel consumption rate decreased gradually with the increase of diesel engine speed. Meanwhile, this study optimized the matching relationship between OEV and ANR based on the data of the genetic algorithm, which provides a theoretical research method and application basis for diesel engine-matching of EGR and SCR.

Soot Oxidation in a Plasma-Catalytic Reactor: A Case Study of Zeolite-Supported Vanadium Catalysts

The plasma-catalytic oxidation of soot was studied over zeolite-supported vanadium catalysts, while four types of zeolites (MCM-41, mordenite, USY and 5A) were used as catalyst supports. The soot oxidation rate followed the order of V/MCM-41 > V/mordenite > V/USY > V/5A, while 100% soot oxidation was achieved at 54th min of reaction over V/MCM-41 and V/mordenite. The CO2 selectivity of the process follows the opposite order of oxidation rate over the V/M catalyst. A wide range of catalyst characterizations including N2 adsorption–desorption, XRD, XPS, H2-TPR and O2-TPD were performed to obtain insights regarding the reaction mechanisms of soot oxidation in plasma-catalytic systems. The redox properties were recognized to be crucial for the soot oxidation process. The effects of discharge power, gas flow rate and reaction temperature on soot oxidation were also investigated. The results showed that higher discharge power, higher gas flow rate and lower reaction temperature were beneficial for soot oxidation rate. However, these factors would impose a negative effect on CO2 selectivity. The proposed “plasma-catalysis” method possessed the unique advantages of quick response, mild operation conditions and system compactness. The method could be potentially applied for the regeneration of diesel particulate filters (DPF) at low temperatures and contribute to the the emission control of diesel engines.

Study of Ce, Ca, Fe, and Mn-Doped LaCoO3 Perovskite Oxide for the Four-Way Purification of PM, NOx, CO, and HC from Diesel Engine Exhaust

Perovskite-type catalysts were widely used in the field of automobile exhaust purification due to their inherent physicochemical properties and excellent doping characteristics. A series of La_1−xM_xCo_1−yN_yO₃ (M = Ce, Ca; N = Fe, Mn) perovskite-type catalyst samples were prepared by sol-gel method for the four-way purification of PM, NO_x, CO, and HC emitted by diesel exhaust. The activity of catalyst samples was tested by simulation experiments and hydrogen temperature-programmed reduction (H₂-TPR). Catalyst samples were characterized by means of XRD, FT-IR, SEM, BET, and XPS analysis. The results demonstrated that the perovskite-type catalyst samples with a particle pore size of 3–5 μm can be prepared by sol-gel method. When A-site of LaCoO₃ perovskite-type oxide was doped by cerium ions, the catalyst samples produced small distortion. The doping of cerium ions to A-site was more conducive to the four-way purification of diesel exhaust than calcium ions. La_0.8Ce_0.2CoO₃ perovskite-type samples showed the best purification efficiency, and the purification efficiencies of PM, NO_x, CO, and HC were 90%, 85%, 94%, and 100%, respectively. When the B-site of La_0.8Ce_0.2CoO₃ perovskite was doped with iron ions, the purification efficiency of catalyst samples for PM and NO_x can be further enhanced. When the B-site of La_0.8Ce_0.2CoO₃ perovskite was doped with manganese ions, the purification efficiency of the catalyst samples for PM can be further enhanced. It can be seen from the simulation experiments that La_0.8Ce_0.2Co_0.7Fe_0.3O₃ perovskite was the best doping amount, and the purification efficiencies of PM, NO_x, CO, and HC were 95%, 92%, 94%, and 100%, respectively.

Structure Sensitivity and Hydration Effects in Pt/TiO2 and Pt/TiO2–SiO2 Catalysts for NO and Propane Oxidation

The NO and propane oxidation activities of a series of 1%Pt/TiO2–SiO2 catalysts show different underlying trends as the support composition changes. Surface characterisation of the catalysts indicates that the trend for NO conversion is consistent with the oxidation rate being dependent on the degree of metallic character of the Pt nanoparticles, rather than their morphology. Although a similar correlation is expected for the total oxidation of propane, it is masked by the effects of adventitious ions originating during manufacture of the support materials. When residual chloride is present in the support, most of the exposed Pt is stabilised in its low-activity ionic form; while support materials containing W or oxidised-S ions give rise to catalysts with much higher activity than expected from their measured Pt0 content. When a Cl-containing, but SiO2-free, TiO2 support material is pre-treated hydrothermally, the propane-oxidation activity of the resultant Pt/TiO2 catalyst is substantially improved, so that it matches the performance of highly-metallic Pt supported on TiO2 containing 16 wt% SiO2. The hydrothermal pre-treatment removes residual chloride from the support material, but it also leaves the catalyst in a hydrated state. We show that, by controlling the metallic content of Pt nanoparticles, understanding the promoting and inhibiting effects of adventitious ions, and optimising the degree of catalyst hydration, the activity of 1%Pt/TiO2–SiO2 catalysts can be made to exceed that of a benchmark 2%Pt/γ-Al2O3 formulation for both NO and propane oxidation.

Mechanism of accelerating soot oxidation by NO2 from diesel engine exhaust

NO₂ oxidation of soot exhausted from engines is more efficient than O₂ under low-temperature conditions, and is crucial for diesel particulate filter to control soot pollution. To explore the principle behind accelerating soot oxidation by NO₂, this paper uses density functional theory to reveal soot oxidation process by NO₂. This study contributes to understanding rules of soot oxidation by NO₂ and perfecting soot oxidation models to develop soot emission control technologies. Results show that NO₂ oxidation of pyrene radical involves three steps. Firstly, NO₂ attacks the C∗ atom to form -C (NO₂) with reaction energy of 306.3 kJ/mol, which decomposes to produce a -C (O) compound. Secondly, another NO₂ molecule climbs over an energy barrier of 8.8 kJ/mol, and changes into a -C (ONO₂) intermediate on -C (O). Finally, the N or O atom of NO₂ attacks -C (O) for a second time to help open aromatic ring for releasing CO or CO₂. Further decomposition of -C (NO₂) and -C (ONO₂) requires activation energies of 81.6 kJ/mol, 75.7 kJ/mol, and 53.5 kJ/mol, respectively, on preferential pathways. Calculations prove that attacks of O atom from NO₂ on C∗ help open the aromatic ring more efficiently than N atom.

An Overview on the Catalytic Materials Proposed for the Simultaneous Removal of NOx and Soot

Vehicular pollution has become a major problem in urban areas due to the exponential increase in the number of automobiles. Typical exhaust emissions, which include nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO), soot, and particulate matter (PM), doubtless have important negative effects on the environment and human health, including cardiovascular effects such as cardiac arrhythmias and heart attacks, and respiratory effects such as asthma attacks and bronchitis. The mitigation measures comprise either the use of clean alternative fuels or the use of innovative technologies. Several existing emission control technologies have proven effective at controlling emissions individually, such as selective catalytic reduction (SCR) and lean NOx trap (LNT) to reduce NOx and diesel particulate filter (DPF) specifically for PM abatement. These after-treatment devices are the most profitable means to reduce exhaust emissions to acceptable limits (EURO VI norms) with very little or no impact on the engine performances. Additionally, the relative lack of physical space in which to install emissions-control equipment is a key challenge for cars, especially those of small size. For this reason, to reduce both volume and cost of the after-treatment devices integrated catalytic systems (e.g., a sort of a "single brick") have been proposed, reducing both NOx and PM simultaneously. This review will summarize the currently reported materials for the simultaneous removal of NOx and soot, with particular attention to their nature, properties, and performances.

Effect of A-site substitution on the simultaneous catalytic removal of NOx and soot by LaMnO3 perovskites

The simultaneous catalytic removal of NO_x and soot by La_1−xM_xMnO₃ (M = K, Sr, Pr; x = 0, 0.1, 0.2, 0.3) perovskites was investigated.

Perovskite-based catalysts for the control of nitrogen oxide emissions from diesel engines

Nitrogen oxides removal over a wide range of perovskite-based catalysts together with their property-activity relationships.

Simultaneous removal of PM and NOx over highly efficient 3DOM W/Ce0.8Zr0.2O2 catalysts

Herein, three-dimensional ordered macropore (3DOM) x% W/Ce_0.8Zr_0.2O₂ (x = 0.5, 0.8, 1, 3) catalysts were prepared and employed for the simultaneous removal of PM (particulate matter) and NO_x from diesel engine exhaust.

The existing states of potassium species in K-doped Co3O4 catalysts and their influence on the activities for NO and soot oxidation

The fresh and washed K-doped Co₃O₄ catalysts were compared with pure Co₃O₄ in order to investigate the existing states of K species and their influence on the activities for NO and soot oxidation.