Preparation and Application of Perovskite Catalysts for Diesel Soot Emissions Control: An Overview

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.

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.

Review of Particle Filters for Internal Combustion Engines

Diesel engines have gradually become one of the main forces in the human transportation industry because of their high efficiency, good durability, and stable operation. However, compared with gasoline vehicles, the high emission of diesel vehicles forces manufacturers to introduce new pollutant control technologies. Although the particulate matter emissions of gasoline vehicles are lower than that of diesel vehicles, with the popularity of gasoline vehicles and the continuous rise of power, the impact of these particles on the environment cannot be ignored. Therefore, diesel particulate filters and gasoline particulate filters have been invented to collect the fine particles in the exhaust gas to protect the environment and meet increasingly stringent emission regulations. This paper summarizes the research progress on diesel particulate filters and gasoline particulate filters at present and comprehensively introduces the diesel particulate filter and gasoline particulate filter from the mechanism, composition, and operation processes. Additionally, the laws and regulations of various countries and the impact of gas waste particulates on the human body are described. In addition, the mechanisms of the diesel particulate filter, gasoline particulate filter, and regeneration were studied. Finally, the prospects and future directions for the development of particle filters for internal combustion engines are presented.

The decisive factor of hollow spherical network morphology of Nd1-xCexCo1-yCuyO3±δ perovskites towards soot oxidation

The perovskites Nd1-xCexCo1-yCuyO3 (x = 0–0.05, y = 0–0.1) have been synthesized using PVP-assisted sol–gel method and applied for soot oxidation reactions. XRD technique reveals the formation of orthorhombic phase with crystal volume of around ~ 214 Å3 and crystal size of ~ 25–40 nm. The interconnected nanoparticles with hollow spherical network morphology of particles are observed for the samples NdCoO3 (NC1) and Nd0.98Ce0.02Co0.95Cu0.05O3 (NC2) with particle sizes of around 300–500 nm. The samples experienced a charge transfer from ligand (O2−) to cobalt cation in UV region (210–260 nm) and also observed broad absorption bands in the visible region (380–600 nm). In addition, the bandgap energy of NC2 showed the lowest value (4.21 eV); as well as surface morphological advantage promoted the transport of surface-chemisorbed oxygen species in the inner and outer surface of catalysts surface due to the reducibility of the catalyst with the soot $$\left( {\frac{{{\text{O}}_{2}^{{{\text{x}} - }} }}{{{\text{O}}_{2}^{{{\text{x}} - }} + {\text{O}}^{2 - } }} = 0.90} \right)$$ O 2 x - O 2 x - + O 2 - = 0.90 . Furthermore, XPS results evidenced the higher content of Co2+ cation upon substitution of Ce/Cu into NC1, which successively formed more amount of Oβ-oxygen species. Remarkably, the perovskite NC2 showed the lowest soot oxidation temperature (T50% = 434 °C) among the investigated perovskites. Besides, the spherically networked morphology of NC2/NC1 samples also decided the soot oxidation process.

Engineered magnetic oxides nanoparticles as efficient sorbents for wastewater remediation: a review

The rapid urbanization and industrialization is causing worldwide water pollution, calling for advanced cleaning methods. For instance, pollutant adsorption on magnetic oxides is efficient and very practical due to the easy separation from solutions by an magnetic field. Here we review the synthesis and performance of magnetic oxides such as iron oxides, spinel ferrites, and perovskite oxides for water remediation. We present structural, optical, and magnetic properties. Magnetic oxides are also promising photocatalysts for the degradation of organic pollutants. Antimicrobial activities and adsorption of heavy metals and radionucleides are also discussed.

Catalytic Materials for Gasoline Particulate Filters Soot Oxidation

The energy efficiency of Gasoline Direct Injection (GDI) engines is leading to a continuous increase in GDI engine vehicle population. Consequently, their particulate matter (soot) emissions are also becoming a matter of concern. As required for diesel engines, to meet the limits set by regulations, catalyzed particulate filters are considered as an effective solution through which soot could be trapped and burnt out. However, in contrast to diesel application, the regeneration of gasoline particulate filters (GPF) is critical, as it occurs with almost an absence of NOx and under oxygen deficiency. Therefore, in the recent years it was of scientific interest to develop efficient soot oxidation catalysts that fit such particular gasoline operating conditions. Among them ceria- and perovskite-based formulations are emerging as the most promising materials. This overview summarizes the very recent academic contributions focusing on soot oxidation materials for GDI, in order to point out the most promising directions in this research area.

The Formation of Perovskite during the Combustion of an Energy-Rich Glycine-Nitrate Precursor

The effect of different regimes of combustion of glycine–nitrate precursors on the formation of perovskite phases (LaMnO₃ and LaCrO₃) without additional heat treatment was studied. The following three combustion regimes were compared: the traditional solution combustion synthesis (SCS), volume combustion synthesis (VCS) using a powdered precursor, and self-propagating high-temperature synthesis (SHS) using a precursor pellet. The products of combustion were studied using a series of physicochemical methods (attenuated total reflection infrared spectroscopy (ATR FTIR), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and thermal analysis). SHS was found to be the most productive regime for the formation of perovskite because of its ability to develop high temperatures in the reaction zone, which led to a reduced content of the thermally stable lanthanum carbonate impurities and to an increased yield and crystallite size of the perovskite phase. The reasons for the better crystallinity and purity of LaCrO₃ as compared with LaMnO₃ is also discussed, namely the low temperatures of the onset of the thermolysis, the fast rate of combustion, and the favorable thermodynamics for the achievement of high temperatures in the reaction zone.

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.

Most Recent Advances in Diesel Engine Catalytic Soot Abatement: Structured Catalysts and Alternative Approaches

Diesel engine emissions are typically composed of several hundred chemical compounds, partly present in the gas phase and partly in solid phase as particles, the so-called particulate matter or soot. The morphology of the catalyst is an important characteristic of soot particles’ abatement, since a good contact between catalyst and soot is mandatory. For practical purposes, the active species should be supported as a film on the structured carrier, in order to allow simultaneous soot filtration and combustion. This review focuses on the most recent advances in the development of structured catalysts for diesel engine catalytic soot combustion, characterized by different active species and supports, as well as by different geometric configurations (monoliths, foams, ceramic papers, or wire mesh); the most important peculiar properties are highlighted and summarized. Moreover, a critical review of the most recent advances in modeling studies is also presented in this paper. In addition, some highlights on some of the most recent alternative approaches proposed for limiting the soot emissions from diesel engines have been given, delineating feasible alternatives to the classical strategies nowadays used.

CoOx-decorated CeO2 heterostructures: effects of morphology on their catalytic properties in diesel soot combustion

The effect of the morphology, which exposes different crystal planes, on the physicochemical properties and catalytic activity in diesel carbon soot oxidation was studied using CoOx-decorated CeO2 (CoCeO2) heterostructured catalysts, such as nanorods (NRs), nanocubes (NCs), and nanoparticles (NPs). The CoOx/CeO2 nanorods (CoCeO2-NR) showed superior carbon soot combustion activity at lower temperatures to CoCeO2-NCs and CoCeO2-NPs under both tight and loose contact modes with soot combustion temperatures (T50) of 321 and 494 °C, respectively. A comprehensive analysis by means of X-ray diffraction, Raman spectroscopy, high-angle annular dark-field scanning transmission electron microscopy, in situ X-ray absorption fine structure, temperature-programmed reduction, oxygen storage/release measurements, and density functional theory calculations revealed that the improved activity of CoCeO2-NRs is mainly ascribed to the high oxygen release rate and strong redox capability of the supported Co species, with complete reversibility. This originates from the high reactivity of oxygen atoms on (110) surfaces, compared to (100) and (111) surfaces over CeO2. Additionally, CoCeO2-NRs displayed durability and recyclability without any significant loss of catalytic activity or structural change. These insights will aid in the rational design of practical catalysts for the purification of diesel exhaust and other important transformations.

Perovskite Structure Associated with Precious Metals: Influence on Heterogenous Catalytic Process

The use of perovskite-based materials and their derivatives can have an important role in the heterogeneous catalytic field based on photochemical processes. Photochemical reactions have a great potential to solve environmental damage issues. The presence of precious metals in the perovskite structure (i.e., Ag, Au, or Pt) may improve its efficiency significantly. The precious metal may comprise the perovskite lattice as well as form a heterostructure with it. The efficiency of catalytic materials is directly related to processing conditions. Based on this, this review will address the use of perovskite materials combined with precious metal as well as their processing methods for the use in catalyzed reactions.

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.

The effect of the structure and oxygen defects on the simultaneous removal of NOx and soot by La2−xBaxCuO4

La_2−xBa_xCuO₄(x = 0–0.6) perovskite-like oxides were prepared by the citric acid sol–gel method.

Gaseous Heterogeneous Catalytic Reactions over Mn-Based Oxides for Environmental Applications: A Critical Review

Manganese oxide has been recognized as one of the most promising gaseous heterogeneous catalysts due to its low cost, environmental friendliness, and high catalytic oxidation performance. Mn-based oxides can be classified into four types: (1) single manganese oxide (MnOx), (2) supported manganese oxide (MnOx/support), (3) composite manganese oxides (MnOx-X), and (4) special crystalline manganese oxides (S-MnOx). These Mn-based oxides have been widely used as catalysts for the elimination of gaseous pollutants. This review aims to describe the environmental applications of these manganese oxides and provide perspectives. It gives detailed descriptions of environmental applications of the selective catalytic reduction of NOx with NH₃, the catalytic combustion of volatile organic compounds, Hg⁰ oxidation and adsorption, and soot oxidation, in addition to some other environmental applications. Furthermore, this review mainly focuses on the effects of structure, morphology, and modified elements and on the role of catalyst supports in gaseous heterogeneous catalytic reactions. Finally, future research directions for developing manganese oxide catalysts are proposed.

Constructing a three-dimensionally ordered macroporous LaCrOδ composite oxide via cerium substitution for enhanced soot abatement

The addition of Ce into Cr-based perovskite restrained the growth of the crystal size and delayed the transformation from LaCrO₄ to LaCrO₃, and thus, the 3DOM structure was maintained even after calcination at 800 °C.

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.

KNO3 supported on three-dimensionally ordered macroporous La0.8Ce0.2Mn1−xFexO3 for soot removal

The KNO₃ supported on 3DOM La_0.8Ce_0.2Mn_1−xFe_xO₃ perovskites have been prepared and used for soot removal. The presence of potassium nitrate can accelerate the soot combustion at both low and high temperature. The catalysts exhibit high activities for soot combustion.

Facile synthesis of three-dimensionally ordered macroporous silicon-doped La0.8K0.2CoO3 perovskite catalysts for soot combustion

Three-dimensionally ordered macroporous (3DOM) silicon-doped La_0.8K_0.2CoO₃ perovskite catalysts were successfully prepared by a colloidal crystal templating method. The catalysts showed a well-ordered macroporous structure and exhibited high activity for soot removal.

Promising ceria–samaria-based nano-oxides for low temperature soot oxidation: a combined study of structure–activity properties

Nanostructured Ce–Sm mixed oxides are promising catalysts for diesel soot oxidation through facile active oxygen participation.