Integrated catalytic systems for simultaneous NOx and PM reduction: A comprehensive evaluation of synergistic performance and combustion waste energy utilization

Integrated catalytic systems for simultaneous NOx and PM reduction: A comprehensive evaluation of synergistic performance and combustion waste energy utilization | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Integrated catalytic systems for simultaneous NOx and PM reduction: A comprehensive evaluation of synergistic performance and combustion waste energy utilization Dikra Bakhchin, Rajesh Ravi, Oumaima Douadi, Mustapha Faqir, Elhachmi Essadiqi This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4187531/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 6 You are reading this latest preprint version Abstract The global transition towards sustainable automotive vehicles has driven the demand for energy-efficient internal combustion engines with advanced aftertreatment systems capable of reducing nitrogen oxides (NOx) and particulate matter (PM) emissions. This comprehensive review explores the latest advancements in aftertreatment technologies, focusing on the synergistic integration of in-cylinder combustion strategies, such as low-temperature combustion (LTC), with post-combustion purification systems. Selective catalytic reduction (SCR), lean NOx traps (LNT), and diesel particulate filters (DPF) are critically examined, highlighting novel catalyst formulations and system configurations that enhance low-temperature performance and durability. The review also investigates the potential of energy conversion and recovery techniques, including thermoelectric generators and organic Rankine cycles, to harness waste heat from the exhaust and improve overall system efficiency. By analyzing the complex interactions between engine operating parameters, combustion kinetics, and emission formation, this study provides valuable insights into the optimization of integrated LTC-aftertreatment systems. Furthermore, the review emphasizes the importance of considering real-world driving conditions and transient operation in the development and evaluation of these technologies. The findings presented in this article lay the foundation for future research efforts aimed at overcoming the limitations of current aftertreatment systems and achieving superior emission reduction performance in advanced combustion engines, ultimately contributing to the development of sustainable and efficient automotive technologies. GHG alleviation Low-temperature combustion NOx & PM mitigation NH3-SCR heterogenous Catalysis technologies Combined catalysts systems Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Highlights This article reviews recent innovations in effective aftertreatment systems for the purification of diesel engine emissions, especially nitrogen oxides and particulate matter. This review depicts various methodologies, including selective catalytic reduction (NH3-SCR) to reduce NOx emissions and diesel particulate filters to minimize soot and particulate matter. This article evaluates various novel materials and methodologies, including LNT-SCR, to reduce NOx emissions and diesel particulate trapping and oxidation to minimize ash, soot, and PM. This review proposes a novel combined catalytic system with high efficiency to significantly enhance the overall thermal and catalytic efficiency of aftertreatment systems. This article explores the electrification of emissions aftertreatment modules, the use of thermoelectric oxides for boosting catalyst performance at start-up, and the cooling of exhaust for energy extraction and conversion purposes as potential engineering solutions for emission reduction. 1. Introduction The global transition towards sustainable automotive vehicles has created a widespread demand for energy-efficient internal combustion engines with lower emissions (Ying Huang et al. 2023 ). Specifically, advanced aftertreatment systems, combined with in-cylinder innovations such as low-temperature combustion (LTC), can dramatically reduce particulate matter and nitrogen oxide (NOx) emissions, which are byproducts of the combustion process (Douadi O et al. 2022 ; Seongsu Kim et al. 2023). The development and optimization of these technologies are crucial for meeting increasingly stringent emission regulations while maintaining high engine performance and efficiency (André Chun et al. 2023; Tamilvanan A et al. 2023 ; Hamed Kazemi et al. 2019). This comprehensive review aims to provide an in-depth overview of the latest research advancements in aftertreatment methodologies, including selective catalytic reduction (SCR), lean NOx traps (LNT), and diesel particulate filters (DPF) (Gao J et al. 2019 ). The primary objective is to explore novel approaches for energy conversion and recovery that can enhance emission reduction capabilities and overall system efficiency. By examining the synergistic effects of pre-combustion and post-combustion purification technologies, this review seeks to identify strategies for effectively mitigating emissions while optimizing engine performance (Boretti A, 2020 ; Leach F et al. 2020 ). Low-temperature combustion strategies, such as reactivity-controlled compression ignition (RCCI) and partially premixed compression ignition (PPCI), have emerged as promising techniques for overcoming the traditional NOx/soot trade-off inherent in diesel combustion (Venugopal I P et al, 2021; Suraj C et al. 2022 ). These advanced combustion modes operate at lower temperatures, avoiding the formation of both NOx and soot, regardless of the local equivalence ratio (Jiang Z et al. 2020 ). LTC has the potential to significantly reduce engine-out emissions, thereby relaxing the demands on aftertreatment systems. However, the implementation of LTC poses unique challenges for aftertreatment systems, particularly SCR, in low-temperature and cold-start conditions (Marzouk Osama, 2023; Zhang Y et al. 2021 ). SCR catalysts, which rely on the injection of a reducing agent (typically ammonia derived from urea) to convert NOx into nitrogen and water, face limitations in terms of catalytic activity and ammonia slip at low exhaust temperatures (Huang J et al. 2023 ). This issue is especially pronounced during cold starts when a significant portion of NOx is emitted. The light-off temperature required for efficient NOx reduction is often too high for real engine operating conditions, leading to unabated NOx emissions (Zheng J et al. 2023 ). Additionally, side reactions can lead to the formation of nitrous oxide (N 2 O), a potent greenhouse gas with a global warming potential 298 times higher than carbon dioxide. These challenges necessitate the development of advanced SCR catalysts with improved low-temperature activity, sulfur tolerance, and thermal stability (Wardana MKA et al. 2023). Lean NOx traps, which store NOx under lean conditions and reduce it to nitrogen under rich conditions, also face challenges in terms of storage capacity, regeneration efficiency, and durability. The integration of LNT and SCR systems has shown promise in enhancing NOx reduction performance, but further research is needed to optimize these hybrid configurations for LTC applications (Senthil R et al. 2023). Diesel particulate filters, designed to capture and oxidize soot particles, must contend with the altered particulate matter characteristics resulting from LTC (Sun C et al. 2020 ). The lower exhaust temperatures associated with LTC can hinder passive regeneration, necessitating the development of advanced regeneration strategies and catalytic coatings to maintain DPF efficiency and durability (Wong SF et al. 2023 ) This review critically examines the current state of aftertreatment technologies and their integration with LTC strategies. It explores novel catalyst formulations, such as zeolite-based materials, perovskites, and mixed metal oxides, which have shown promise in enhancing low-temperature performance and durability. The review also investigates the potential of energy conversion and recovery techniques, such as thermoelectric generators and organic Rankine cycles, to harness waste heat from the exhaust and improve the overall efficiency of the aftertreatment system. Furthermore, this review delves into the complex interactions between engine operating parameters, combustion kinetics, and emission formation in LTC engines. By understanding the trade-offs between combustion efficiency, engine performance, and emissions, researchers can develop targeted strategies for optimizing both in-cylinder and aftertreatment processes. Advanced modeling techniques, such as computational thermal studies and kinetic simulations, are mediated as powerful tools for guiding the design and optimization of integrated LTC-aftertreatment systems. The review also highlights the importance of considering real-world driving conditions and transient operation in the development and evaluation of aftertreatment systems for LTC engines. Figure 1 . shows a Schematic diagram of a typical LTC diesel engine setup. Cold starts, low-load operation, and frequent transients pose significant challenges for emission control, requiring adaptive and robust control strategies. The integration of advanced sensors, diagnostics, and control algorithms is explored as a means to ensure optimal performance and compliance with emission regulations under diverse operating conditions. By providing a comprehensive analysis of the advancements and challenges in aftertreatment systems for LTC engines, this review aims to contribute to the development of sustainable and efficient automotive technologies. The insights gained from this study can guide future research efforts towards overcoming the limitations of current aftertreatment technologies in low-temperature conditions and achieving superior emission reduction performance in advanced combustion engines (Bakhchin D et al. 2023 ; Manjunath S P et al. 2023). The comprehensive nature of this review, covering the latest advancements in aftertreatment technologies, their integration with LTC strategies, and the consideration of real-world driving conditions, makes it a valuable resource for researchers, engineers, and policymakers working towards the development of clean and efficient automotive technologies. By providing a critical analysis of the state-of-the-art and identifying promising research avenues, this review aims to accelerate the progress towards sustainable transportation solutions that meet the growing demand for energy-efficient and environmentally friendly vehicles. 2. NOx and PM Emission Trade-offs 2.1. Trade-offs between NOx and PM emissions in low temperature combustion Diesel engines have high fuel efficiency and energy density, but face challenges with soot and NOx emissions due to combustion conditions (Zhang K et al. 2023 ). ocal equivalence ratio and combustion temperature affect NOx and soot emissions. Standard diesel combustion at high temperatures generates NOx and soot, but lower temperatures can prevent their formation (Riyadi T et al. 2023; Panda S R et al. 2022). Figure 2. depicts (a) Optimised local equivalence ration of low temperature PCCI and HCCI, (b) The effect of lean prmixed combustion on NOx attenuation. Low-temperature premixed combustion (LTC) research aims to address the NOx/soot trade-off by achieving more homogeneous air-fuel mixtures (Teoh Y et al. 2022 ). LTC strategies like RCCI and PPCI target lower local equivalence ratios and temperatures to reduce NOx and soot. LTC can effectively reduce NOx and soot formation through careful control of fuel reactivity and injection timing. However, implementing LTC comes with challenges such as increased HC and CO emissions, combustion instability, and a limited operating range (Gürbüz Hüseyin, 2020 ) . 2.2. Strategies for simultaneous reduction of NOx and PM In order to utilize LTC for reducing NOx and PM emissions, a comprehensive approach involving in-cylinder and aftertreatment strategies is needed. In-cylinder methods optimize combustion, while aftertreatment systems address remaining pollutants. EGR is a promising in-cylinder strategy that lowers combustion temperature and suppresses NOx formation. Careful optimization of EGR rate is crucial to balance NOx and soot reduction. Manipulating fuel injection parameters can also help control heat release and reduce NOx and soot formation. Post-injections can aid in soot oxidation and reducing PM emissions. Fuel formulation, such as biodiesel, alcohols, and natural gas, can affect NOx and soot trade-off. The impact on NOx emissions varies with different fuel properties and engine conditions. Advanced fuel blends are being researched to optimize NOx and soot reduction (Lu, Mingming et al.2023). Despite effective in-cylinder strategies, aftertreatment systems are still necessary for meeting emission regulations. SCR is commonly used for NOx reduction in diesel engines. SCR performance depends on exhaust temperature, with optimal efficiency in a narrow range (Velmurugan, A., et al. 2024 ). Low-temperature SCR catalysts are being developed to achieve high NOx conversion at temperatures below 200°C. Zeolite-based catalysts, like Cu-zeolites and Fe-zeolites, show promise due to high surface area, thermal stability, and ammonia storage at low temperatures (Laguna O et al. 2021 ). Novel catalyst support materials such as ceria-zirconia and titanium dioxide are explored to enhance low-temperature activity and sulfur resistance. Lean NOx traps (LNTs) store NOx under lean conditions and reduce it under rich conditions. LNTs oxidize NOx to NO2 and store it as nitrates, releasing and reducing it under rich conditions. Figure 3 . depicts the exhaust gas conversion process through oxidation catalysts. LNTs don't need an external reducing agent but are sensitive to sulfur poisoning and require precise lean-rich cycling control (Naik G G et al. 2023). The combination of LNT and SCR systems, known as LNT-SCR or NSR-SCR, is a promising solution for efficient NOx reduction in LTC engines. LNT functions as a NOx storage device and ammonia generator, while downstream SCR catalyst uses the generated ammonia for further NOx reduction (Bhagat Ram Narayan et al. 2023). This synergistic approach maximizes NOx conversion efficiency over a wide temperature range. Optimizing the LNT-SCR system, including catalyst formulation, sizing, and regeneration strategy, is crucial for performance and fuel penalty minimization (Jung S et al. 2020). DPFs are commonly used for PM reduction by trapping soot particles. Soot accumulation in DPFs requires periodic regeneration, often involving high-temperature oxidation. Low-temperature combustion poses challenges for DPF regeneration due to decreased exhaust temperatures (Khdary N H et al. 2022). Catalyzed DPFs (CDPFs) are developed to enhance soot oxidation at lower temperatures. Incorporating catalytic materials onto DPFs can improve soot ignition and regeneration efficiency. Fuel-borne catalysts are explored to enhance soot oxidation kinetics in DPFs. Active regeneration strategies increase exhaust temperature but may impact fuel consumption. SCR-on-DPF integration is a cost-effective solution for NOx and PM reduction (Ahmed N N R et al. 2022). Optimization of catalyst coating and regeneration strategies is crucial for SCR-on-DPF systems. Waste heat recovery technologies can enhance LTC engine efficiency and aftertreatment systems. LTC engine exhaust gas contains thermal energy that can be converted into useful work. Figure 4 . illustrates ICE thermal energy conversion through a cooling system using WHR. TEGs convert temperature gradient into electrical energy using the Seebeck effect. ORC systems use exhaust heat to generate mechanical or electrical power via a turbine (Zhang Y et al. 2023 ). The combination of ORC with LTC engines enhances thermal efficiency and boosts power for aftertreatment. Turbocompounding uses an extra turbine in the exhaust to recover energy from high-pressure gases. The turbine's mechanical energy can aid the engine or power a generator. Turbocompounding enhances LTC engine fuel efficiency and provides extra power for aftertreatment. Integrating waste heat recovery with LTC engines and aftertreatment needs a comprehensive approach. Design optimization and control strategies are crucial for maximizing system efficiency. Coordination with engine and aftertreatment operation is key for optimal performance (Lisi L et al. 2020 ; Kok Sin Woon et al. 2023 ). 3. The requirements of SCR technology development 3.1. Principles and types of SCR catalysts Selective Catalytic Reduction (SCR) is a widely used technique for NOx reduction in diesel vehicles. Urea-SCR involves the injection of an aqueous urea solution, which decomposes to form NH 3 via thermal hydrolysis. The NH 3 then reacts with NOx over a catalyst to form nitrogen and water [50]. However, SCR faces challenges such as NH 3 slip at low temperatures (< 250°C) and limited catalytic activity during cold-start conditions. HC-SCR, using onboard diesel fuel as a reductant, emerged as an alternative approach to overcome these issues, but it suffers from high light-off temperatures (> 300°C) and the formation of N 2 O (Zhao L et al. 2020 ; Martinović, F et al. 2021 ). Various SCR catalysts like Cu-zeolite, Fe-zeolite, and V2O 5 -WO 3 /TiO 2 have been studied. Cu-zeolite and Fe-zeolite catalysts are promising due to high surface area, thermal stability, and NH3 storage abilities. V2O5-WO3/TiO2 catalysts resist sulfur well but have lower NOx conversion efficiency than zeolite-based catalysts (Kim B et al. 2020 ; Sittichompoo S et al. 2022 ). NH 3 -SCR is an efficient technology for NOx control, with V 2 O 5 -WO 3 /TiO 2 being a widely used commercial SCR catalyst for high de-NOx efficiency at 300–400°C ( Awad O I et al. 2022). A summary of commercially used catalyst for SCR are reviewed in the table.1. Table.1 The topmost used catalyst for SCR reaction and their properties Vanadia Catalysts V2O5-WO3/TiO2 Preferred in areas with high-sulfur fuels, vanadia catalysts have greater resistance to sulfation but suffer from lower NOx conversion rates and stability issues at temperatures above ~ 500°C. Copper–Zeolite Catalysts Cu-ZSM Copper–zeolite, particularly with small-pore zeolites like CHA chabazite, has been the leading choice due to its high conversion rate over a broad temperature range and good hydrothermal stability. Ongoing development focuses on enhancing low-temperature conversion, durability, and sulfur tolerance. Sulfur poisoning significantly impacts the performance of Cu–ZSM catalysts. However, thermal treatment can recover much of the lost activity, but challenges remain in fully restoring performance after sulfur exposure. Fe–Zeolite Catalysts Fe-ZSM Iron–zeolite catalysts differ from copper variants by offering better performance at higher temperatures and reduced ammonia oxidation, making them preferable for applications requiring high sulfur resistance and lower desulfation temperatures. Manganese-Based Catalysts MnXO Manganese-based catalysts, such as Ce-Mn/TiO2 and MnO2/ZrO2, are under investigation for their potential to enhance low-temperature activity and NOx conversion (> 90% NOx conversion in the 140–260 ◦C range) while also offering high sulfur resistance and reducing N2O formation. 3.2. Advancements in catalytic materials and performance of LNT for SCR reaction Lean NOx Traps (LNTs), also known as NOx Storage and Reduction (NSR) catalysts, operate by storing NOx under lean conditions and reducing it to nitrogen under rich conditions (Muhammad Farhan et al. 2023 ). LNTs have been combined with SCR catalysts to enhance NOx reduction efficiency over a wide temperature range. Recent advancements in LNT materials include the development of perovskite-based catalysts, such as BaCoO 3 and SrCoO 3 , which exhibit high NO oxidation capacity (Alcantara APMP et al. 2023 ; Yu YS et al. 2021 ). Platinum group metals (PGMs) like Pt, Rh, and Pd have also been incorporated into LNT catalysts to improve low-temperature performance and reduce N 2 O formation, all of which is shown in Fig. 5 . 3.3. Challenges and solutions for low-temperature SCR The role of hydrogen in SCR has been extensively studied. It promotes the formation and decomposition of organo-NOx species at lower temperatures, increases the availability of hydrogen, and has an effectivness of 95% on thermal and prompt NOx removal (Ravi R et al. 2018; Appavu Prabhu et al. 2019; Zhang Ahiging et al. 2023). One of the main challenges in SCR is the low catalytic activity at temperatures below 200°C, which is prevalent during cold-start conditions. To address this issue, researchers have focused on developing low-temperature SCR catalysts (Cao, Dao Nam, et al. 2020). Some solutions include: Zeolite-based catalysts (Cu-zeolite and Fe-zeolite) with high surface area and thermal stability. Novel catalyst support materials, such as ceria-zirconia and titanium dioxide, thermo electric promoters using TEPOC to enhance low-temperature activity and sulfur resistance. The use of hydrogen, carbon dioxide to similtaneousely improve low-temperature NOx reduction efficiency. 3.4 Integration of LNT with SCR systems The combination of LNT and SCR systems, known as LNT-SCR or NSR-SCR, has emerged as a promising solution for efficient NOx reduction in diesel engines (Rahman, SM Ashrafur, et al. 2021). In this configuration, the LNT serves as a NOx storage device and an NH 3 generator, while the downstream SCR catalyst utilizes the generated NH3 for further NOx reduction. This synergistic approach takes advantage of the strengths of both technologies, enabling high NOx conversion efficiency over a wide temperature range. However, the optimization of the LNT-SCR system, including catalyst formulation, sizing, and regeneration strategy, is crucial for maximizing performance and minimizing fuel penalty (Kozina Ante et al. 2020). 3.5 Advancements in LNT catalyst materials and performance Recent advancements in LNT catalyst materials have focused on improving low-temperature performance, sulfur resistance, and thermal stability. Perovskite-based materials, such as BaCoO 3 and SrCoO 3 , have shown high NO oxidation capacity and have been used as LNT catalysts. Platinum group metals (PGMs), including Pt, Rh, and Pd, have been incorporated into LNT catalysts to enhance low-temperature NOx storage and reduction (Rajesh Ravi et al. 2020; Castoldi Lidia, 2020). Additionally, the optimization of catalyst support materials, such as Al2O 3 , CeO 2 , and ZrO 2 , has been investigated to improve the dispersion and stability of active components. Figure 6 illustrates the SCR and LNT technologies in diesel engine. The development of Pt/Rh-BaO bimetallic LNT catalysts has also shown promise in improving NOx storage and reduction efficiency under lean-burn conditions at low temperatures. The addition of Rh to the Pt/BaO system has been found to accelerate NOx release and increase NOx reduction efficiency. Furthermore, the optimization of the physicochemical properties of Pt in Pt-BaO/Al2O 3 LNT catalysts, such as surface area, oxygen storage capacity, and particle size, has been studied to better understand NOx sorption and storage kinetics (Rajesh, A. Vembathu, et al.2020; Kim Hyung Jun et al. 2022).The following equations illustrates the mechanism of reaction occuring in the NOx reduction process: • Urea thermal hydrolysis : (1) CO(NH 2 ) 2 + H 2 O → 2NH 3 + CO 2 • Standard SCR reaction: (2) 4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O • Fast SCR reaction: (3) NO + NO 2 + 2NH 3 → 2N 2 + 3H 2 O • NO oxidation (LNT): (4) NO + 0.5O 2 → NO 2 • NOx storage (LNT): (5) BaO + 2NO 2 + 0.5O 2 → Ba(NO 3 ) 2 • NOx reduction (LNT): (6) Ba(NO 3 ) 2 + 3H 2 → BaO + 2NO + 3H2O Ba(NO 3 ) 2 + 3CO → BaO + 2NO + 3CO 2 4. Advanced integrated thermal strategies and energy storage 4.1 Pyroelectric and thermoelectric materials potential usage in thermal energy recycling and promotion of LNT-SCR reactions Pyroelectric and thermoelectric materials have shown significant potential in thermal energy recycling and the promotion of LNT-SCR reactions in automotive exhaust aftertreatment systems. These materials can convert waste heat from the exhaust gas into electrical energy, which can be used to power various components of the aftertreatment system or to enhance the catalytic reactions. Pyroelectric materials generate an electrical potential when subjected to a temperature change (Saikia, Navarun, et al. 2023; Peng Qingguo et al. 2024; Joshi Ameya, 2022). This phenomenon, known as the pyroelectric effect, can be exploited to harvest waste heat from the exhaust gas. The generated electrical energy can be used to power electrical heaters or other components in the aftertreatment system, reducing the load on the vehicle's alternator and improving overall energy efficiency (Kurzydym, Damian, et al.2022; Zhang, Xuewen, et al. 2023; Rajesh Ravi et al. 2023 ). This electricity can be used to power various sensors, actuators, or control units in the aftertreatment system, or to directly heat the catalysts to their optimal operating temperatures. Figure 7 . depicts after-treatment thermal management through exhaust heat recirculation . The integration of pyroelectric and thermoelectric materials into LNT-SCR systems can provide several benefits. First, the electrical energy generated by these materials can be used to heat the LNT catalyst during cold starts or low-temperature operation, improving its NOx storage capacity and NH 3 generation efficiency. Second, the electrical energy can be used to power the urea dosing system in the SCR catalyst, reducing the need for an external power source and improving the system's response time (Tan, Dongli, et al. 2023 ). Furthermore, the waste heat recovery achieved through pyroelectric and thermoelectric materials can help to maintain the optimal operating temperature of the LNT-SCR system, even under varying engine load conditions. This can lead to improved NOx and PM reduction efficiency, as well as increas ed catalyst durability and longevity (Ni Peiyong et al. 2020). The efficiency of pyroelectric and thermoelectric materials in waste heat recovery can be described by the following equations (Deng Jiaojun et al. 2021; Feng Renhua et al. 2023) The pyroelectric energy conversion: P = p × A × dT/dt Where: P = generated electrical power (W) p = pyroelectric coefficient (C/m 2 ·K) A = surface area of the pyroelectric material (m 2 ) dT/dt = rate of temperature change (K/s) Thermoelectric energy conversion: V = α × ΔT where: V = generated voltage (V) α = Seebeck coefficient (V/K) ΔT = temperature difference across the thermoelectric material (K) The pyroelectric coefficient (p) and Seebeck coefficient (α) are intrinsic properties of the materials and depend on their composition and structure. Higher values of these coefficients indicate better performance in waste heat recovery (Iskra Z. Koleva et al. 2023). To maximize the benefits of pyroelectric and thermoelectric materials in LNT-SCR systems, researchers are focusing on developing advanced materials with higher energy conversion efficiencies, improved thermal stability, and better compatibility with the harsh exhaust environment. Additionally, optimizing the system design and control strategies to effectively integrate these materials into the aftertreatment system is crucial for achieving the best performance and energy savings (Deng Y et al. 2019 ). 5. Combined Catalytic Systems for efficient thermal management 5.1 Synergies between LNT, SCR, and DPF technologies The combination of LNT, SCR, and DPF technologies has shown great potential for simultaneously reducing NOx and PM emissions in diesel engines. LNT catalysts store NOx under lean conditions and release it under rich conditions, along with NH 3 generation. The downstream SCR catalyst then utilizes the generated NH 3 for further NOx reduction. DPFs, on the other hand, physically trap soot particles and periodically regenerate through oxidation (Q Xue et al. 2022). The synergies between these technologies can be exploited by integrating them into a single aftertreatment system. For example, the LNT-SCR configuration allows for efficient NOx reduction over a wide temperature range, as the LNT provides NH 3 for the SCR reaction even at low temperatures (Pereda-Ayo B et al. 2013). Additionally, the DPF can be coated with SCR or LNT catalysts (SCR-on-DPF or LNT-on-DPF) to achieve simultaneous PM and NOx reduction in a compact system, all of which is depicts in Fig. 8. 5.2. Novel combined catalytic system configurations Several novel combined catalytic system configurations have been proposed to maximize the synergies between LNT, SCR, and DPF technologies (Mei, X et al. 2021)., Fig. 9 shows a thermally efficient new engineered after treatment combination. Some notable configurations include: LNT-SCR-DPF: In this configuration, the LNT is placed upstream of the SCR and DPF. The LNT stores NOx during lean operation and releases it as NH 3 during rich regeneration, which is then utilized by the SCR catalyst. The DPF downstream captures and oxidizes the PM. SCR-on-DPF: This configuration integrates the SCR catalyst onto the DPF substrate, allowing for simultaneous NOx and PM reduction in a single component. The compact design saves space and reduces system complexity. LNT-on-DPF with downstream SCR: This system combines an LNT-coated DPF with a downstream SCR catalyst. The LNT-on-DPF stores NOx and PM, while the downstream SCR further reduces the NOx using the NH3 generated during LNT regeneration. 5.3 Performance evaluation and optimization of combined systems The performance of combined catalytic systems depends on various factors, such as catalyst formulation, system configuration, and operating conditions. Researchers have focused on optimizing these parameters to maximize NOx and PM reduction efficiency while minimizing fuel consumption and system complexity (Rongrong Gui et al. 2022). One key aspect is the optimization of the catalyst formulation for each component. For example, the LNT catalyst should have high NOx storage capacity and NH 3 selectivity during regeneration, while the SCR catalyst should have high NOx conversion efficiency and low NH 3 slip. The DPF substrate and coating should be designed to minimize pressure drop and facilitate efficient soot oxidation (Choi, Dongho, et al. 2021). Another important factor is the control of the regeneration strategy for the LNT and DPF components. The frequency and duration of the rich regeneration events should be optimized to maximize NOx and PM reduction while minimizing fuel consumption. This can be achieved through advanced control algorithms that take into account the real-time engine operating conditions and aftertreatment system state (Fayyazbakhsh, Ahmad, et al. 2022). The performance of combined catalytic systems can be evaluated using various metrics, such as NOx and PM conversion efficiency, NH 3 slip, fuel consumption, and system durability. Experimental studies and numerical simulations are used to assess the performance under different operating conditions and to identify potential areas for improvement (Palani, Yogesh, et al. 2022). Equations: (a) NOx storage in LNT (lean phase): BaO + 2NO 2 + 0.5O 2 → Ba(NO 3 ) 2 (b) NOx release and NH 3 generation in LNT (rich phase): Ba(NO 3 ) 2 + 3H 2 → BaO + 2NH 3 + 3H 2 O (c) SCR reactions: Standard SCR: 4NH 3 + 4NO + O 2 → 4N 2 + 6H 2 O (d) Fast SCR: 2NH 3 + NO + NO 2 → 2N 2 + 3H 2 O (e) Soot oxidation in DPF: C (soot) + O 2 → CO 2 C (soot) + NO 2 → CO 2 + NO In these equations, the LNT catalyst (e.g., BaO) stores NOx as nitrates (Ba(NO 3 ) 2 ) during lean operation and releases it as NH 3 during rich regeneration. The generated NH3 then participates in the standard and fast SCR reactions over the SCR catalyst, reducing NOx to N 2 (Manigandan, S., et al. 2020). In the DPF, soot (C) is oxidized by O 2 and NO 2 to form CO 2 , with NO 2 being more effective at lower temperatures compared to O 2 . 6. Future Perspectives and Challenges 6.1. Emerging catalyst materials and technologies The development of advanced catalyst materials and technologies is crucial for meeting the emission regulations and improving the efficiency of aftertreatment systems (Mourad, M et al. 2021 ). Some emerging catalyst materials include: Perovskite-based catalysts : Perovskite oxides, such as LaCoO 3 and LaMnO 3 , have shown promise as alternatives to traditional platinum group metals (PGMs) in LNT and SCR catalysts. These materials exhibit high thermal stability, excellent redox properties, and good sulfur tolerance, making them attractive candidates for high-temperature applications. Ceria-based catalysts : Ceria (CeO 2 ) and its derivatives have gained attention as catalyst supports and promoters. Ceria-based materials can enhance the low-temperature activity and stability of LNT and SCR catalysts, as well as promote soot oxidation in DPFs. Metal-organic frameworks (MOFs) : MOFs are highly porous materials with tunable chemical and physical properties. They have been explored as catalyst supports and adsorbents in aftertreatment systems, offering high surface area, controllable pore size, and good thermal stability. MOFs can be functionalized with active metal sites to achieve desired catalytic properties. In terms of emerging technologies, the integration of non-thermal plasma (NTP) with catalytic aftertreatment systems has shown potential for improving low-temperature performance and reducing the dependence on precious metals (Norouzi, Armin, et al. 2023). NTP can generate highly reactive species, such as oxidizing radicals and excited molecules, which can enhance the catalytic reactions at lower temperatures. The combination of NTP with LNT, SCR, or DPF catalysts can lead to improved NOx and PM reduction efficiency, as well as faster catalyst light-off during cold starts. 6.2. Trends in emission regulations and their impact on aftertreatment systems The global trend in emission regulations is towards more stringent limits on NOx and PM emissions from diesel engines. For example, the Euro 7 standards, expected to come into effect in the mid-2020s, are likely to impose even lower NOx and PM limits than the current Euro 6 regulations. Similarly, the CARB has proposed the Heavy-Duty Low NOx Omnibus Regulation, which aims to reduce NOx emissions from heavy-duty trucks by up to 90% below current standards. These trends in emission regulations will have a significant impact on the design and development of aftertreatment systems (Lyu Liqun, et al. 2023; Xiang, Pan, et al. 2024). To meet the lower emission limits, aftertreatment systems will need to achieve higher NOx and PM conversion efficiencies, especially at low temperatures and under real-world driving conditions. This will require the development of advanced catalyst materials, improved system architectures, and more sophisticated control strategies (Zhang, Size, et al. 2024). Furthermore, the focus on real-world driving emissions (RDE) testing will necessitate the optimization of aftertreatment systems for a wider range of operating conditions, including low-load and low-temperature operation. This will require a better understanding of the complex interactions between engine, aftertreatment, and control systems, as well as the development of adaptive and predictive control algorithms (Vedagiri Praveena, et al. 2020; Apicella Barbara, et al. 2020). 6.3. Research directions and opportunities for further improvement To address the challenges posed by the future emission regulations and the need for more efficient and cost-effective aftertreatment systems, several research directions and opportunities have been identified: Development of multi-functional catalyst materials : Researchers are exploring the development of catalyst materials that can perform multiple functions, such as simultaneous NOx and PM reduction, or combined LNT and SCR functionality. These multi-functional materials can help to simplify the aftertreatment system architecture, reduce precious metal loading, and improve overall system efficiency. Advanced catalyst characterization techniques : The use of advanced characterization techniques, such as in-situ X-ray absorption spectroscopy (XAS), environmental transmission electron microscopy (ETEM), and operando Raman spectroscopy, can provide valuable insights into the structure-activity relationships of catalyst materials under realistic operating conditions. These techniques can help to guide the rational design and optimization of novel catalyst formulations. Predictive modeling and simulation : The development of predictive models and simulation tools can assist in the design and optimization of aftertreatment systems, reducing the need for extensive experimental testing. These models can incorporate detailed kinetic mechanisms, fluid dynamics, and heat transfer effects to predict the performance of catalytic reactors under various operating conditions. Machine learning algorithms can also be employed to identify optimal catalyst formulations and operating strategies. Integration with advanced combustion technologies : The integration of advanced combustion technologies, such as low-temperature combustion (LTC) and reactivity-controlled compression ignition (RCCI), with optimized aftertreatment systems can lead to significant reductions in both NOx and PM emissions. The development of integrated engine-aftertreatment control strategies can help to maximize the benefits of these technologies and ensure compliance with future emission regulations. By addressing these research directions and opportunities, the scientific community and automotive industry can work towards the development of more efficient, cost-effective, and environmentally sustainable aftertreatment systems for diesel engines. The successful implementation of these technologies will play a crucial role in meeting the challenges posed by the future emission regulations and contribute to the overall reduction of the environmental impact of transportation. 7. Conclusions The development of efficient and sustainable after-treatment systems for diesel engines is crucial in the global transition towards energy-efficient and environmentally friendly vehicles. This comprehensive review has explored the latest advancements in after-treatment methodologies, focusing on the synergistic effects of in-cylinder combustion strategies and post-combustion purification technologies to effectively mitigate nitrogen oxides (NOx) and particulate matter (PM) emissions. Low-temperature combustion (LTC) strategies, such as reactivity-controlled compression ignition (RCCI) and partially premixed compression ignition (PPCI), have emerged as promising techniques for overcoming the traditional NOx/soot trade-off inherent in diesel combustion. By achieving a more homogeneous air-fuel mixture and operating at lower temperatures, LTC has the potential to significantly reduce engine-out emissions. However, the implementation of LTC poses unique challenges for after-treatment systems, particularly in low-temperature and cold-start conditions. SCR catalysts, LNT, and DPF have been extensively investigated to address the limitations of LTC after-treatment. Novel catalyst formulations, such as zeolite-based materials, perovskites, and mixed metal oxides, have shown promise in enhancing low-temperature performance and durability. The integration of LNT and SCR systems (LNT-SCR) has demonstrated superior NOx reduction efficiency over a wide temperature range, while SCR-on-DPF and LNT-on-DPF configurations offer compact solutions for simultaneous PM and NOx reduction. This review has also explored the potential of energy conversion and recovery techniques, such as thermoelectric generators and organic Rankine cycles, to harness waste heat from the exhaust and improve the overall efficiency of the after-treatment system. The complex interactions between engine operating parameters, combustion kinetics, and emission formation in LTC engines have been highlighted, emphasizing the importance of a comprehensive approach to optimize in-cylinder and after-treatment processes. The insights gained from this study can guide future research efforts towards overcoming the limitations of current after-treatment technologies in low-temperature conditions and achieving superior emission reduction performance in advanced combustion engines. By providing a critical analysis of the state-of-the-art and identifying promising research avenues, this review contributes to the development of sustainable and efficient automotive technologies that meet the growing demand for energy-efficient and environmentally friendly vehicles. As the global community continues to strive for cleaner transportation solutions, the synergistic integration of advanced combustion strategies and innovative after-treatment systems will be crucial in achieving the ambitious emission reduction targets set by regulatory bodies worldwide. The findings presented in this review lay the foundation for further advancements in this field, paving the way for a greener and more sustainable future in the automotive industry. Abbreviations LTC Low Temperature Combustion ICE Internal Combustion Engines SCR Selective Catalytic Reduction PGM Platinum Group Material DOC Diesel Oxidation Catalyst DPF Diesel Particulate Filter LD & HD Light Duty & Heavy Duty ATS After Treatment Systems NSC NOx Storage Catalyst OSM Oxygen Storage Material PM Particulate Matter NOx Nitrogen Oxides (NO, NO2) LNT Lean NOx Traps RCCI Reactivity-Controlled Compression Ignition PPCI Partially Premixed Compression Ignition NH3 Ammonia N2O Nitrous Oxide HC Hydrocarbons CO Carbon Monoxide EGR Exhaust Gas Recirculation TEG Thermoelectric Generator ORC Organic Rankine Cycle XAS X-ray Absorption Spectroscopy ETEM Environmental Transmission Electron Microscopy RDE Real-world Driving Emissions MOF Metal-Organic Framework NTP Non-Thermal Plasma CARB California Air Resources Board DMB Diesel/Methanol/n-Butanol PCCI Premixed Charge Compression Ignition HCCI Homogeneous Charge Compression Ignition PEMS Portable Emissions Measurement System SEMS Smart Emissions Measurement System SDPF SCR-coated Diesel Particulate Filter WHR Waste Heat Recovery CRDI Common Rail Direct Injection TEPOC Thermoelectric Power Generator & Oxidation Catalyst Declarations Acknowledgements: The authors would like to thank LERMA Labs, Rabat International University, for ensuring the availability of facilities. Author Contributions: All the authors contributed to the study conception and design. Dikra Bakhchin, Oumaima Douadi, and Rajesh Ravi performed the material preparation, data collection and analysis. Faqir Mustapha and Elhachmi Essadiqi provided supervision and facilities in the LERMA laboratory. Dikra Bakhchin wrote the first draft of the manuscript, and all the authors commented on previous versions of the manuscript. All the authors have read and approved the final manuscript. Declaration of competing interest : The authors declare the following financial interests /relationships which may be considered as potential competing interests: Dikra Bakhchin reports financial support was provided by National Center for Scientific and Technical Research. Funding This research work was fully funded by The National Center of Scientific and Technical research in Morocco and Foundation UM6P under the APRD project “Design and development of exhaust heat recovery system and emission reduction technology for multi cylinder CRDI engine”. The authors would like to thank LERMA Labs, Rabat International University, for ensuring the availability of facilities. Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References Ahmad, N. N. R., Mohammad, A. W., Mahmoudi, E., Ang, W. L., Leo, C., & Teow, Y. H. ( 2022 ). An overview of the modification strategies in developing antifouling nanofiltration membranes. 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Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major Revision 11 Jun, 2024 Reviewers agreed at journal 16 May, 2024 Reviewers invited by journal 15 May, 2024 Editor invited by journal 14 May, 2024 Editor assigned by journal 10 Apr, 2024 First submitted to journal 08 Apr, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4187531","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":302651418,"identity":"9847a840-ca62-417a-99ab-9162bc3171f2","order_by":0,"name":"Dikra Bakhchin","email":"","orcid":"","institution":"International University of Rabat: Universite Internationale de Rabat","correspondingAuthor":false,"prefix":"","firstName":"Dikra","middleName":"","lastName":"Bakhchin","suffix":""},{"id":302651419,"identity":"86607e77-8271-4de5-b85d-ec88b822a54d","order_by":1,"name":"Rajesh Ravi","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7UlEQVRIiWNgGAWjYHCCBAbGBhDN2PiAwYBELc0GxGphgGphYJMgSrV5+4GHD3/uYMjnn5HcVs1TcI+Bv/0A64YfeLTInElINuY9w2A540Zi220eg2IGiTMJbDd78GiRYEhIk2ZsA3rizEGQlgQGhhsMbDd48Gnhf5D+8ydQizxQSzFIizxQy80/+LRIJKQx8AK1GBxvbGMGaTEAarmN1xaJB8nSvG0SBobHG5sl5xgk8BieAXpKBq/DchI//myzMZA7zP7ww5s/CXJyxw8fu/kGjxYGBp4EkE4EFx5NuAH7AQIKRsEoGAWjYMQDAO2gSRt2lPXlAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0001-5359-8761","institution":"International University of Rabat: Universite Internationale de Rabat","correspondingAuthor":true,"prefix":"","firstName":"Rajesh","middleName":"","lastName":"Ravi","suffix":""},{"id":302651420,"identity":"45918e82-cdfa-42ba-88f0-a67f3491f3fd","order_by":2,"name":"Oumaima Douadi","email":"","orcid":"","institution":"International University of Rabat: Universite Internationale de Rabat","correspondingAuthor":false,"prefix":"","firstName":"Oumaima","middleName":"","lastName":"Douadi","suffix":""},{"id":302651421,"identity":"f7dd37e6-c574-4259-a5d4-066e0130bcac","order_by":3,"name":"Mustapha Faqir","email":"","orcid":"","institution":"International University of Rabat: Universite Internationale de Rabat","correspondingAuthor":false,"prefix":"","firstName":"Mustapha","middleName":"","lastName":"Faqir","suffix":""},{"id":302651422,"identity":"c7956c54-4bda-4263-b48a-7782ee22aa21","order_by":4,"name":"Elhachmi Essadiqi","email":"","orcid":"","institution":"International University of Rabat: Universite Internationale de Rabat","correspondingAuthor":false,"prefix":"","firstName":"Elhachmi","middleName":"","lastName":"Essadiqi","suffix":""}],"badges":[],"createdAt":"2024-03-29 11:06:56","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4187531/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4187531/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":57031404,"identity":"141c106d-5121-48d7-a678-cfa39c0aafa6","added_by":"auto","created_at":"2024-05-23 17:10:48","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":74035,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of a typical LTC diesel engine setup (Latha H. S., et al. 2019)\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4187531/v1/934bd2db7317a12b3e894dbf.jpg"},{"id":57031402,"identity":"fc7b7094-d9bf-4836-a7ae-9f3a929e7c8a","added_by":"auto","created_at":"2024-05-23 17:10:48","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":73949,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Optimised local equivalence ration of low temperature PCCI and HCCI (Li, Xinzhe et al. 2023).\u003c/p\u003e\n\u003cp\u003e(b) The effect of lean prmixed combustion on NOx attenuation. (Mohd Nurazzi N et al . 2021)\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4187531/v1/95e7b0e569c35a13fdcf944f.jpg"},{"id":57031411,"identity":"60c19415-5ae0-4673-b030-e38ba6359cfc","added_by":"auto","created_at":"2024-05-23 17:10:50","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":42532,"visible":true,"origin":"","legend":"\u003cp\u003eExhaust gas conversion process through oxidation catalysts (Besssagnet B et al. 2021)\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4187531/v1/9e4c4d9dc2205e44ae591d50.jpg"},{"id":57031695,"identity":"25da96d4-cb87-4519-845d-574c15cb61fb","added_by":"auto","created_at":"2024-05-23 17:18:49","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":69865,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of ICE thermal energy conversion through a cooling system using WHR (Shukla, M. K et al. 2023)\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4187531/v1/995d6d65ad7dac223948f901.jpg"},{"id":57031405,"identity":"3b533950-4c96-4e8e-8283-5b1d8a966f38","added_by":"auto","created_at":"2024-05-23 17:10:49","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":63579,"visible":true,"origin":"","legend":"\u003cp\u003eThe mechanism of NOx reduction in lean Nox trap catalyst during SCR reaction (Doppalapudi, A. T et al. 2023)\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4187531/v1/b692aeb4844a5bed99409424.jpg"},{"id":57031407,"identity":"af9e2882-05dc-4f0b-ae82-cf9a83f34da2","added_by":"auto","created_at":"2024-05-23 17:10:49","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":71634,"visible":true,"origin":"","legend":"\u003cp\u003eIllustration of SCR and LNT technologies in diesel engine (Sonawane Utkarsha et al. 2021)\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4187531/v1/8c49fae6f2373b52ed7786af.jpg"},{"id":57031403,"identity":"a96433b1-4a33-4ca5-89be-4b5bcf277418","added_by":"auto","created_at":"2024-05-23 17:10:48","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":40807,"visible":true,"origin":"","legend":"\u003cp\u003eAfter-treatment thermal management through exhaust heat recirculation \u0026nbsp;(Lion Simone et al. 2020)\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4187531/v1/bf26f9e7952c4aca6f06c5b2.jpg"},{"id":57031409,"identity":"69e532fe-3985-4406-abec-7c7741258997","added_by":"auto","created_at":"2024-05-23 17:10:49","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":72522,"visible":true,"origin":"","legend":"\u003cp\u003eSynergistic arrangement of modern after treatment systems (You, R et al. 2019)\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4187531/v1/5339ba35e01b36141b7e6ff8.jpg"},{"id":57031696,"identity":"127389bb-6adc-46f1-a697-e9e7858d67eb","added_by":"auto","created_at":"2024-05-23 17:18:49","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":73398,"visible":true,"origin":"","legend":"\u003cp\u003eThermally efficient new engineered after treatment combination (Yakoumis I, 2021)\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4187531/v1/bb8ccfa4e6c08890da943a1f.jpg"},{"id":57031922,"identity":"2ab64d52-e260-4469-9c92-75f7119d9bf5","added_by":"auto","created_at":"2024-05-23 17:26:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1640880,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4187531/v1/540eba55-0b43-4349-a9f3-8b84d696dc77.pdf"}],"financialInterests":"","formattedTitle":"Integrated catalytic systems for simultaneous NOx and PM reduction: A comprehensive evaluation of synergistic performance and combustion waste energy utilization","fulltext":[{"header":"Highlights","content":"\u003cul\u003e\n \u003cli\u003eThis article reviews recent innovations in effective aftertreatment systems for the purification of diesel engine emissions, especially nitrogen oxides and particulate matter.\u003c/li\u003e\n \u003cli\u003eThis review depicts various methodologies, including selective catalytic reduction (NH3-SCR) to reduce NOx emissions and diesel particulate filters to minimize soot and particulate matter.\u003c/li\u003e\n \u003cli\u003eThis article evaluates various novel materials and methodologies, including LNT-SCR, to reduce NOx emissions and diesel particulate trapping and oxidation to minimize ash, soot, and PM.\u003c/li\u003e\n \u003cli\u003eThis review proposes a novel combined catalytic system with high efficiency to significantly enhance the overall thermal and catalytic efficiency of aftertreatment systems.\u003c/li\u003e\n \u003cli\u003eThis article explores the electrification of emissions aftertreatment modules, the use of thermoelectric oxides for boosting catalyst performance at start-up, and the cooling of exhaust for energy extraction and conversion purposes as potential engineering solutions for emission reduction.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eThe global transition towards sustainable automotive vehicles has created a widespread demand for energy-efficient internal combustion engines with lower emissions (Ying Huang et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Specifically, advanced aftertreatment systems, combined with in-cylinder innovations such as low-temperature combustion (LTC), can dramatically reduce particulate matter and nitrogen oxide (NOx) emissions, which are byproducts of the combustion process (Douadi O et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Seongsu Kim et al. 2023). The development and optimization of these technologies are crucial for meeting increasingly stringent emission regulations while maintaining high engine performance and efficiency (Andr\u0026eacute; Chun et al. 2023; Tamilvanan A et al. \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Hamed Kazemi et al. 2019). This comprehensive review aims to provide an in-depth overview of the latest research advancements in aftertreatment methodologies, including selective catalytic reduction (SCR), lean NOx traps (LNT), and diesel particulate filters (DPF) (Gao J et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The primary objective is to explore novel approaches for energy conversion and recovery that can enhance emission reduction capabilities and overall system efficiency. By examining the synergistic effects of pre-combustion and post-combustion purification technologies, this review seeks to identify strategies for effectively mitigating emissions while optimizing engine performance (Boretti A, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Leach F et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eLow-temperature combustion strategies, such as reactivity-controlled compression ignition (RCCI) and partially premixed compression ignition (PPCI), have emerged as promising techniques for overcoming the traditional NOx/soot trade-off inherent in diesel combustion (Venugopal I P et al, 2021; Suraj C et al. \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). These advanced combustion modes operate at lower temperatures, avoiding the formation of both NOx and soot, regardless of the local equivalence ratio (Jiang Z et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). LTC has the potential to significantly reduce engine-out emissions, thereby relaxing the demands on aftertreatment systems. However, the implementation of LTC poses unique challenges for aftertreatment systems, particularly SCR, in low-temperature and cold-start conditions (Marzouk Osama, 2023; Zhang Y et al. \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). SCR catalysts, which rely on the injection of a reducing agent (typically ammonia derived from urea) to convert NOx into nitrogen and water, face limitations in terms of catalytic activity and ammonia slip at low exhaust temperatures (Huang J et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). This issue is especially pronounced during cold starts when a significant portion of NOx is emitted. The light-off temperature required for efficient NOx reduction is often too high for real engine operating conditions, leading to unabated NOx emissions (Zheng J et al. \u003cspan citationid=\"CR92\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Additionally, side reactions can lead to the formation of nitrous oxide (N\u003csub\u003e2\u003c/sub\u003eO), a potent greenhouse gas with a global warming potential 298 times higher than carbon dioxide. These challenges necessitate the development of advanced SCR catalysts with improved low-temperature activity, sulfur tolerance, and thermal stability (Wardana MKA et al. 2023). Lean NOx traps, which store NOx under lean conditions and reduce it to nitrogen under rich conditions, also face challenges in terms of storage capacity, regeneration efficiency, and durability. The integration of LNT and SCR systems has shown promise in enhancing NOx reduction performance, but further research is needed to optimize these hybrid configurations for LTC applications (Senthil R et al. 2023).\u003c/p\u003e \u003cp\u003eDiesel particulate filters, designed to capture and oxidize soot particles, must contend with the altered particulate matter characteristics resulting from LTC (Sun C et al. \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The lower exhaust temperatures associated with LTC can hinder passive regeneration, necessitating the development of advanced regeneration strategies and catalytic coatings to maintain DPF efficiency and durability (Wong SF et al. \u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) This review critically examines the current state of aftertreatment technologies and their integration with LTC strategies. It explores novel catalyst formulations, such as zeolite-based materials, perovskites, and mixed metal oxides, which have shown promise in enhancing low-temperature performance and durability. The review also investigates the potential of energy conversion and recovery techniques, such as thermoelectric generators and organic Rankine cycles, to harness waste heat from the exhaust and improve the overall efficiency of the aftertreatment system. Furthermore, this review delves into the complex interactions between engine operating parameters, combustion kinetics, and emission formation in LTC engines. By understanding the trade-offs between combustion efficiency, engine performance, and emissions, researchers can develop targeted strategies for optimizing both in-cylinder and aftertreatment processes. Advanced modeling techniques, such as computational thermal studies and kinetic simulations, are mediated as powerful tools for guiding the design and optimization of integrated LTC-aftertreatment systems. The review also highlights the importance of considering real-world driving conditions and transient operation in the development and evaluation of aftertreatment systems for LTC engines. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. shows a Schematic diagram of a typical LTC diesel engine setup. Cold starts, low-load operation, and frequent transients pose significant challenges for emission control, requiring adaptive and robust control strategies. The integration of advanced sensors, diagnostics, and control algorithms is explored as a means to ensure optimal performance and compliance with emission regulations under diverse operating conditions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eBy providing a comprehensive analysis of the advancements and challenges in aftertreatment systems for LTC engines, this review aims to contribute to the development of sustainable and efficient automotive technologies. The insights gained from this study can guide future research efforts towards overcoming the limitations of current aftertreatment technologies in low-temperature conditions and achieving superior emission reduction performance in advanced combustion engines (Bakhchin D et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Manjunath S P et al. 2023). The comprehensive nature of this review, covering the latest advancements in aftertreatment technologies, their integration with LTC strategies, and the consideration of real-world driving conditions, makes it a valuable resource for researchers, engineers, and policymakers working towards the development of clean and efficient automotive technologies. By providing a critical analysis of the state-of-the-art and identifying promising research avenues, this review aims to accelerate the progress towards sustainable transportation solutions that meet the growing demand for energy-efficient and environmentally friendly vehicles.\u003c/p\u003e"},{"header":"2. NOx and PM Emission Trade-offs","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1. Trade-offs between NOx and PM emissions in low temperature combustion\u003c/h2\u003e\n \u003cp\u003eDiesel engines have high fuel efficiency and energy density, but face challenges with soot and NOx emissions due to combustion conditions (Zhang K et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e). ocal equivalence ratio and combustion temperature affect NOx and soot emissions. Standard diesel combustion at high temperatures generates NOx and soot, but lower temperatures can prevent their formation (Riyadi T et al. 2023; Panda S R et al. 2022). Figure 2. depicts (a) Optimised local equivalence ration of low temperature PCCI and HCCI, (b) The effect of lean prmixed combustion on NOx attenuation. Low-temperature premixed combustion (LTC) research aims to address the NOx/soot trade-off by achieving more homogeneous air-fuel mixtures (Teoh Y et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). LTC strategies like RCCI and PPCI target lower local equivalence ratios and temperatures to reduce NOx and soot. LTC can effectively reduce NOx and soot formation through careful control of fuel reactivity and injection timing. However, implementing LTC comes with challenges such as increased HC and CO emissions, combustion instability, and a limited operating range (G\u0026uuml;rb\u0026uuml;z H\u0026uuml;seyin, \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e) .\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2. Strategies for simultaneous reduction of NOx and PM\u003c/h2\u003e\n \u003cp\u003eIn order to utilize LTC for reducing NOx and PM emissions, a comprehensive approach involving in-cylinder and aftertreatment strategies is needed. In-cylinder methods optimize combustion, while aftertreatment systems address remaining pollutants. EGR is a promising in-cylinder strategy that lowers combustion temperature and suppresses NOx formation. Careful optimization of EGR rate is crucial to balance NOx and soot reduction. Manipulating fuel injection parameters can also help control heat release and reduce NOx and soot formation. Post-injections can aid in soot oxidation and reducing PM emissions. Fuel formulation, such as biodiesel, alcohols, and natural gas, can affect NOx and soot trade-off. The impact on NOx emissions varies with different fuel properties and engine conditions. Advanced fuel blends are being researched to optimize NOx and soot reduction (Lu, Mingming et al.2023).\u003c/p\u003e\n \u003cp\u003eDespite effective in-cylinder strategies, aftertreatment systems are still necessary for meeting emission regulations. SCR is commonly used for NOx reduction in diesel engines. SCR performance depends on exhaust temperature, with optimal efficiency in a narrow range (Velmurugan, A., et al. \u003cspan class=\"CitationRef\"\u003e2024\u003c/span\u003e). Low-temperature SCR catalysts are being developed to achieve high NOx conversion at temperatures below 200\u0026deg;C. Zeolite-based catalysts, like Cu-zeolites and Fe-zeolites, show promise due to high surface area, thermal stability, and ammonia storage at low temperatures (Laguna O et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). Novel catalyst support materials such as ceria-zirconia and titanium dioxide are explored to enhance low-temperature activity and sulfur resistance. Lean NOx traps (LNTs) store NOx under lean conditions and reduce it under rich conditions. LNTs oxidize NOx to NO2 and store it as nitrates, releasing and reducing it under rich conditions. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. depicts the exhaust gas conversion process through oxidation catalysts. LNTs don\u0026apos;t need an external reducing agent but are sensitive to sulfur poisoning and require precise lean-rich cycling control (Naik G G et al. 2023). The combination of LNT and SCR systems, known as LNT-SCR or NSR-SCR, is a promising solution for efficient NOx reduction in LTC engines. LNT functions as a NOx storage device and ammonia generator, while downstream SCR catalyst uses the generated ammonia for further NOx reduction (Bhagat Ram Narayan et al. 2023). This synergistic approach maximizes NOx conversion efficiency over a wide temperature range. Optimizing the LNT-SCR system, including catalyst formulation, sizing, and regeneration strategy, is crucial for performance and fuel penalty minimization (Jung S et al. 2020).\u003c/p\u003e\n \u003cp\u003eDPFs are commonly used for PM reduction by trapping soot particles. Soot accumulation in DPFs requires periodic regeneration, often involving high-temperature oxidation. Low-temperature combustion poses challenges for DPF regeneration due to decreased exhaust temperatures (Khdary N H et al. 2022). Catalyzed DPFs (CDPFs) are developed to enhance soot oxidation at lower temperatures. Incorporating catalytic materials onto DPFs can improve soot ignition and regeneration efficiency. Fuel-borne catalysts are explored to enhance soot oxidation kinetics in DPFs. Active regeneration strategies increase exhaust temperature but may impact fuel consumption. SCR-on-DPF integration is a cost-effective solution for NOx and PM reduction (Ahmed N N R et al. 2022). Optimization of catalyst coating and regeneration strategies is crucial for SCR-on-DPF systems. Waste heat recovery technologies can enhance LTC engine efficiency and aftertreatment systems. LTC engine exhaust gas contains thermal energy that can be converted into useful work. Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. illustrates ICE thermal energy conversion through a cooling system using WHR. TEGs convert temperature gradient into electrical energy using the Seebeck effect. ORC systems use exhaust heat to generate mechanical or electrical power via a turbine (Zhang Y et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e). The combination of ORC with LTC engines enhances thermal efficiency and boosts power for aftertreatment. Turbocompounding uses an extra turbine in the exhaust to recover energy from high-pressure gases. The turbine\u0026apos;s mechanical energy can aid the engine or power a generator. Turbocompounding enhances LTC engine fuel efficiency and provides extra power for aftertreatment. Integrating waste heat recovery with LTC engines and aftertreatment needs a comprehensive approach. Design optimization and control strategies are crucial for maximizing system efficiency. Coordination with engine and aftertreatment operation is key for optimal performance (Lisi L et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e; Kok Sin Woon et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. The requirements of SCR technology development","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1. Principles and types of SCR catalysts\u003c/h2\u003e\n \u003cp\u003eSelective Catalytic Reduction (SCR) is a widely used technique for NOx reduction in diesel vehicles. Urea-SCR involves the injection of an aqueous urea solution, which decomposes to form NH\u003csub\u003e3\u003c/sub\u003e via thermal hydrolysis. The NH\u003csub\u003e3\u003c/sub\u003e then reacts with NOx over a catalyst to form nitrogen and water [50]. However, SCR faces challenges such as NH\u003csub\u003e3\u003c/sub\u003e slip at low temperatures (\u0026lt;\u0026thinsp;250\u0026deg;C) and limited catalytic activity during cold-start conditions. HC-SCR, using onboard diesel fuel as a reductant, emerged as an alternative approach to overcome these issues, but it suffers from high light-off temperatures (\u0026gt;\u0026thinsp;300\u0026deg;C) and the formation of N\u003csub\u003e2\u003c/sub\u003eO (Zhao L et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e; Martinović, F et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eVarious SCR catalysts like Cu-zeolite, Fe-zeolite, and V2O\u003csub\u003e5\u003c/sub\u003e-WO\u003csub\u003e3\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e have been studied. Cu-zeolite and Fe-zeolite catalysts are promising due to high surface area, thermal stability, and NH3 storage abilities. V2O5-WO3/TiO2 catalysts resist sulfur well but have lower NOx conversion efficiency than zeolite-based catalysts (Kim B et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e; Sittichompoo S et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). NH\u003csub\u003e3\u003c/sub\u003e-SCR is an efficient technology for NOx control, with V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e-WO\u003csub\u003e3\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e being a widely used commercial SCR catalyst for high de-NOx efficiency at 300\u0026ndash;400\u0026deg;C ( Awad O I et al. 2022). A summary of commercially used catalyst for SCR are reviewed in the \u003cstrong\u003etable.1.\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eTable.1\u003c/strong\u003e The topmost used catalyst for SCR reaction and their properties\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tabb\" border=\"1\"\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003eVanadia Catalysts\u003cbr\u003eV2O5-WO3/TiO2\u003cbr\u003e\u003c/th\u003e\n \u003cth align=\"left\"\u003ePreferred in areas with high-sulfur fuels, vanadia catalysts have greater resistance to sulfation but suffer from lower NOx conversion rates and stability issues at temperatures above ~\u0026thinsp;500\u0026deg;C.\u003cbr\u003e\u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003eCopper\u0026ndash;Zeolite\u003cbr\u003e\u003c/td\u003e\n \u003ctd align=\"left\"\u003eCatalysts Cu-ZSM Copper\u0026ndash;zeolite, particularly with small-pore zeolites like CHA chabazite, has been the leading choice due to its high conversion rate over a broad temperature range and good hydrothermal stability. Ongoing development focuses on enhancing low-temperature conversion, durability, and sulfur tolerance. Sulfur poisoning significantly impacts the performance of Cu\u0026ndash;ZSM catalysts. However, thermal treatment can recover much of the lost activity, but challenges remain in fully restoring performance after sulfur exposure.\u003cbr\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003eFe\u0026ndash;Zeolite Catalysts\u003cbr\u003eFe-ZSM\u003cbr\u003e\u003c/td\u003e\n \u003ctd align=\"left\"\u003eIron\u0026ndash;zeolite catalysts differ from copper variants by offering better performance at higher temperatures and reduced ammonia oxidation, making them preferable for applications requiring high sulfur resistance and lower desulfation temperatures.\u003cbr\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003eManganese-Based\u003cbr\u003eCatalysts MnXO\u003cbr\u003e\u003c/td\u003e\n \u003ctd align=\"left\"\u003eManganese-based catalysts, such as Ce-Mn/TiO2 and MnO2/ZrO2, are under investigation for their potential to enhance low-temperature activity and NOx conversion (\u0026gt;\u0026thinsp;90% NOx conversion in the 140\u0026ndash;260 ◦C range) while also offering high sulfur resistance and reducing N2O formation.\u003cbr\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\u003cbr\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2. Advancements in catalytic materials and performance of LNT for SCR reaction\u003c/h2\u003e\n \u003cp\u003eLean NOx Traps (LNTs), also known as NOx Storage and Reduction (NSR) catalysts, operate by storing NOx under lean conditions and reducing it to nitrogen under rich conditions (Muhammad Farhan et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e). LNTs have been combined with SCR catalysts to enhance NOx reduction efficiency over a wide temperature range. Recent advancements in LNT materials include the development of perovskite-based catalysts, such as BaCoO\u003csub\u003e3\u003c/sub\u003e and SrCoO\u003csub\u003e3\u003c/sub\u003e, which exhibit high NO oxidation capacity (Alcantara APMP et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e; Yu YS et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). Platinum group metals (PGMs) like Pt, Rh, and Pd have also been incorporated into LNT catalysts to improve low-temperature performance and reduce N\u003csub\u003e2\u003c/sub\u003eO formation, all of which is shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3. Challenges and solutions for low-temperature SCR\u003c/h2\u003e\n \u003cp\u003eThe role of hydrogen in SCR has been extensively studied. It promotes the formation and decomposition of organo-NOx species at lower temperatures, increases the availability of hydrogen, and has an effectivness of 95% on thermal and prompt NOx removal (Ravi R et al. 2018; Appavu Prabhu et al. 2019; Zhang Ahiging et al. 2023). One of the main challenges in SCR is the low catalytic activity at temperatures below 200\u0026deg;C, which is prevalent during cold-start conditions. To address this issue, researchers have focused on developing low-temperature SCR catalysts (Cao, Dao Nam, et al. 2020). Some solutions include:\u003c/p\u003e\u003cspan\u003eZeolite-based catalysts (Cu-zeolite and Fe-zeolite) with high surface area and thermal stability.\u003cbr\u003eNovel catalyst support materials, such as ceria-zirconia and titanium dioxide, thermo electric promoters using TEPOC to enhance low-temperature activity and sulfur resistance.\u003cbr\u003eThe use of hydrogen, carbon dioxide to similtaneousely improve low-temperature NOx reduction efficiency.\u003cbr\u003e\u003c/span\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4 Integration of LNT with SCR systems\u003c/h2\u003e\n \u003cp\u003eThe combination of LNT and SCR systems, known as LNT-SCR or NSR-SCR, has emerged as a promising solution for efficient NOx reduction in diesel engines (Rahman, SM Ashrafur, et al. 2021). In this configuration, the LNT serves as a NOx storage device and an NH\u003csub\u003e3\u003c/sub\u003e generator, while the downstream SCR catalyst utilizes the generated NH3 for further NOx reduction. This synergistic approach takes advantage of the strengths of both technologies, enabling high NOx conversion efficiency over a wide temperature range. However, the optimization of the LNT-SCR system, including catalyst formulation, sizing, and regeneration strategy, is crucial for maximizing performance and minimizing fuel penalty (Kozina Ante et al. 2020).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.5 Advancements in LNT catalyst materials and performance\u003c/h2\u003e\n \u003cp\u003eRecent advancements in LNT catalyst materials have focused on improving low-temperature performance, sulfur resistance, and thermal stability. Perovskite-based materials, such as BaCoO\u003csub\u003e3\u003c/sub\u003e and SrCoO\u003csub\u003e3\u003c/sub\u003e, have shown high NO oxidation capacity and have been used as LNT catalysts. Platinum group metals (PGMs), including Pt, Rh, and Pd, have been incorporated into LNT catalysts to enhance low-temperature NOx storage and reduction (Rajesh Ravi et al. 2020; Castoldi Lidia, 2020). Additionally, the optimization of catalyst support materials, such as Al2O\u003csub\u003e3\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e, and ZrO\u003csub\u003e2\u003c/sub\u003e, has been investigated to improve the dispersion and stability of active components. Figure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e illustrates the SCR and LNT technologies in diesel engine.\u003c/p\u003e\n \u003cp\u003eThe development of Pt/Rh-BaO bimetallic LNT catalysts has also shown promise in improving NOx storage and reduction efficiency under lean-burn conditions at low temperatures. The addition of Rh to the Pt/BaO system has been found to accelerate NOx release and increase NOx reduction efficiency. Furthermore, the optimization of the physicochemical properties of Pt in Pt-BaO/Al2O\u003csub\u003e3\u003c/sub\u003e LNT catalysts, such as surface area, oxygen storage capacity, and particle size, has been studied to better understand NOx sorption and storage kinetics (Rajesh, A. Vembathu, et al.2020; Kim Hyung Jun et al. 2022).The following equations illustrates the mechanism of reaction occuring in the NOx reduction process:\u003c/p\u003e\n \u003cp\u003e\u003cem\u003e\u0026bull; Urea thermal hydrolysis\u003c/em\u003e:\u003c/p\u003e\n \u003cp\u003e(1) \u0026nbsp;CO(NH\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e + H\u003csub\u003e2\u003c/sub\u003eO \u0026rarr; 2NH\u003csub\u003e3\u003c/sub\u003e + CO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e\u003cem\u003e\u0026bull;\u0026nbsp;\u003c/em\u003eStandard SCR reaction:\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e(2) 4NO + 4NH\u003csub\u003e3\u003c/sub\u003e + O\u003csub\u003e2\u003c/sub\u003e \u0026rarr; 4N\u003csub\u003e2\u003c/sub\u003e + 6H\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\n \u003cp\u003e\u003cem\u003e\u003cem\u003e\u0026bull;\u0026nbsp;\u003c/em\u003eFast SCR reaction:\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003cp\u003e(3) NO + NO\u003csub\u003e2\u003c/sub\u003e + 2NH\u003csub\u003e3\u003c/sub\u003e \u0026rarr; 2N\u003csub\u003e2\u003c/sub\u003e + 3H\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\n \u003cp\u003e\u003cem\u003e\u0026bull;\u0026nbsp;\u003c/em\u003eNO oxidation (LNT):\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e(4) NO + 0.5O\u003csub\u003e2\u003c/sub\u003e \u0026rarr; NO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e\u003cem\u003e\u0026bull;\u0026nbsp;\u003c/em\u003eNOx storage (LNT):\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e(5) BaO + 2NO\u003csub\u003e2\u003c/sub\u003e + 0.5O\u003csub\u003e2\u0026nbsp;\u003c/sub\u003e\u0026rarr; Ba(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e\u003cem\u003e\u0026bull;\u0026nbsp;\u003c/em\u003eNOx reduction (LNT):\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;(6) Ba(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e + 3H\u003csub\u003e2\u003c/sub\u003e\u0026rarr; BaO + 2NO + 3H2O\u0026nbsp;Ba(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e + 3CO \u0026rarr; BaO + 2NO + 3CO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Advanced integrated thermal strategies and energy storage","content":"\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Pyroelectric and thermoelectric materials potential usage in thermal energy recycling and promotion of LNT-SCR reactions\u003c/h2\u003e \u003cp\u003ePyroelectric and thermoelectric materials have shown significant potential in thermal energy recycling and the promotion of LNT-SCR reactions in automotive exhaust aftertreatment systems. These materials can convert waste heat from the exhaust gas into electrical energy, which can be used to power various components of the aftertreatment system or to enhance the catalytic reactions. Pyroelectric materials generate an electrical potential when subjected to a temperature change (Saikia, Navarun, et al. 2023; Peng Qingguo et al. 2024; Joshi Ameya, 2022). This phenomenon, known as the pyroelectric effect, can be exploited to harvest waste heat from the exhaust gas. The generated electrical energy can be used to power electrical heaters or other components in the aftertreatment system, reducing the load on the vehicle's alternator and improving overall energy efficiency (Kurzydym, Damian, et al.2022; Zhang, Xuewen, et al. 2023; Rajesh Ravi et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). This electricity can be used to power various sensors, actuators, or control units in the aftertreatment system, or to directly heat the catalysts to their optimal operating temperatures. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003e. depicts after-treatment thermal management through exhaust heat recirculation .\u003c/p\u003e \u003cp\u003eThe integration of pyroelectric and thermoelectric materials into LNT-SCR systems can provide several benefits. First, the electrical energy generated by these materials can be used to heat the LNT catalyst during cold starts or low-temperature operation, improving its NOx storage capacity and NH\u003csub\u003e3\u003c/sub\u003e generation efficiency. Second, the electrical energy can be used to power the urea dosing system in the SCR catalyst, reducing the need for an external power source and improving the system's response time (Tan, Dongli, et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Furthermore, the waste heat recovery achieved through pyroelectric and thermoelectric materials can help to maintain the optimal operating temperature of the LNT-SCR system, even under varying engine load conditions. This can lead to improved NOx and PM reduction efficiency, as well as increas ed catalyst durability and longevity (Ni Peiyong et al. 2020).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe efficiency of pyroelectric and thermoelectric materials in waste heat recovery can be described by the following equations (Deng Jiaojun et al. 2021; Feng Renhua et al. 2023)\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eThe pyroelectric energy conversion:\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eP\u0026thinsp;=\u0026thinsp;p \u0026times; A \u0026times; dT/dt\u003c/p\u003e \u003cp\u003eWhere:\u003c/p\u003e \u003cp\u003eP\u0026thinsp;=\u0026thinsp;generated electrical power (W)\u003c/p\u003e \u003cp\u003ep\u0026thinsp;=\u0026thinsp;pyroelectric coefficient (C/m\u003csup\u003e2\u003c/sup\u003e\u0026middot;K)\u003c/p\u003e \u003cp\u003eA\u0026thinsp;=\u0026thinsp;surface area of the pyroelectric material (m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003cp\u003edT/dt\u0026thinsp;=\u0026thinsp;rate of temperature change (K/s)\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eThermoelectric energy conversion:\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eV\u0026thinsp;=\u0026thinsp;α\u0026thinsp;\u0026times;\u0026thinsp;ΔT\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003ewhere:\u003c/p\u003e \u003cp\u003eV\u0026thinsp;=\u0026thinsp;generated voltage (V)\u003c/p\u003e \u003cp\u003eα\u0026thinsp;=\u0026thinsp;Seebeck coefficient (V/K)\u003c/p\u003e \u003cp\u003eΔT\u0026thinsp;=\u0026thinsp;temperature difference across the thermoelectric material (K)\u003c/p\u003e \u003cp\u003eThe pyroelectric coefficient (p) and Seebeck coefficient (α) are intrinsic properties of the materials and depend on their composition and structure. Higher values of these coefficients indicate better performance in waste heat recovery (Iskra Z. Koleva et al. 2023). To maximize the benefits of pyroelectric and thermoelectric materials in LNT-SCR systems, researchers are focusing on developing advanced materials with higher energy conversion efficiencies, improved thermal stability, and better compatibility with the harsh exhaust environment. Additionally, optimizing the system design and control strategies to effectively integrate these materials into the aftertreatment system is crucial for achieving the best performance and energy savings (Deng Y et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Combined Catalytic Systems for efficient thermal management","content":"\u003cdiv id=\"Sec14\"\u003e\n \u003ch2\u003e5.1 Synergies between LNT, SCR, and DPF technologies\u003c/h2\u003e\n \u003cp\u003eThe combination of LNT, SCR, and DPF technologies has shown great potential for simultaneously reducing NOx and PM emissions in diesel engines. LNT catalysts store NOx under lean conditions and release it under rich conditions, along with NH\u003csub\u003e3\u003c/sub\u003e generation. The downstream SCR catalyst then utilizes the generated NH\u003csub\u003e3\u003c/sub\u003e for further NOx reduction. DPFs, on the other hand, physically trap soot particles and periodically regenerate through oxidation (Q Xue et al. 2022).\u003c/p\u003e\n \u003cp\u003eThe synergies between these technologies can be exploited by integrating them into a single aftertreatment system. For example, the LNT-SCR configuration allows for efficient NOx reduction over a wide temperature range, as the LNT provides NH\u003csub\u003e3\u003c/sub\u003e for the SCR reaction even at low temperatures (Pereda-Ayo B et al. 2013). Additionally, the DPF can be coated with SCR or LNT catalysts (SCR-on-DPF or LNT-on-DPF) to achieve simultaneous PM and NOx reduction in a compact system, all of which is depicts in Fig.\u0026nbsp;8.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec15\"\u003e\n \u003ch2\u003e5.2. Novel combined catalytic system configurations\u003c/h2\u003e\n \u003cp\u003eSeveral novel combined catalytic system configurations have been proposed to maximize the synergies between LNT, SCR, and DPF technologies (Mei, X et al. 2021)., Fig.\u0026nbsp;9 shows a thermally efficient new engineered after treatment combination. Some notable configurations include:\u003c/p\u003e\n \u003cul\u003e\n \u003cli\u003e\n \u003cp\u003eLNT-SCR-DPF: In this configuration, the LNT is placed upstream of the SCR and DPF. The LNT stores NOx during lean operation and releases it as NH\u003csub\u003e3\u003c/sub\u003e during rich regeneration, which is then utilized by the SCR catalyst. The DPF downstream captures and oxidizes the PM.\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eSCR-on-DPF: This configuration integrates the SCR catalyst onto the DPF substrate, allowing for simultaneous NOx and PM reduction in a single component. The compact design saves space and reduces system complexity.\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eLNT-on-DPF with downstream SCR: This system combines an LNT-coated DPF with a downstream SCR catalyst. The LNT-on-DPF stores NOx and PM, while the downstream SCR further reduces the NOx using the NH3 generated during LNT regeneration.\u003c/p\u003e\n \u003c/li\u003e\n \u003c/ul\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\"\u003e\n \u003ch2\u003e5.3 Performance evaluation and optimization of combined systems\u003c/h2\u003e\n \u003cp\u003eThe performance of combined catalytic systems depends on various factors, such as catalyst formulation, system configuration, and operating conditions. Researchers have focused on optimizing these parameters to maximize NOx and PM reduction efficiency while minimizing fuel consumption and system complexity (Rongrong Gui et al. 2022). One key aspect is the optimization of the catalyst formulation for each component. For example, the LNT catalyst should have high NOx storage capacity and NH\u003csub\u003e3\u003c/sub\u003e selectivity during regeneration, while the SCR catalyst should have high NOx conversion efficiency and low NH\u003csub\u003e3\u003c/sub\u003e slip. The DPF substrate and coating should be designed to minimize pressure drop and facilitate efficient soot oxidation (Choi, Dongho, et al. 2021).\u003c/p\u003e\n \u003cp\u003eAnother important factor is the control of the regeneration strategy for the LNT and DPF components. The frequency and duration of the rich regeneration events should be optimized to maximize NOx and PM reduction while minimizing fuel consumption. This can be achieved through advanced control algorithms that take into account the real-time engine operating conditions and aftertreatment system state (Fayyazbakhsh, Ahmad, et al. 2022). The performance of combined catalytic systems can be evaluated using various metrics, such as NOx and PM conversion efficiency, NH\u003csub\u003e3\u003c/sub\u003e slip, fuel consumption, and system durability. Experimental studies and numerical simulations are used to assess the performance under different operating conditions and to identify potential areas for improvement (Palani, Yogesh, et al. 2022).\u003c/p\u003e\n \u003cp\u003eEquations:\u003c/p\u003e\n \u003cp\u003e(a) NOx storage in LNT (lean phase): BaO\u0026thinsp;+\u0026thinsp;2NO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;0.5O\u003csub\u003e2\u003c/sub\u003e \u0026rarr; Ba(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e(b) NOx release and NH\u003csub\u003e3\u003c/sub\u003e generation in LNT (rich phase): Ba(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e + 3H\u003csub\u003e2\u003c/sub\u003e \u0026rarr; BaO\u0026thinsp;+\u0026thinsp;2NH\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;3H\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\n \u003cp\u003e(c) SCR reactions: Standard SCR: 4NH\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;4NO\u0026thinsp;+\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e \u0026rarr; 4N\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;6H\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\n \u003cp\u003e(d) Fast SCR: 2NH\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;NO\u0026thinsp;+\u0026thinsp;NO\u003csub\u003e2\u003c/sub\u003e \u0026rarr; 2N\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;3H\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\n \u003cp\u003e(e) Soot oxidation in DPF: C (soot)\u0026thinsp;+\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e \u0026rarr; CO\u003csub\u003e2\u003c/sub\u003e C (soot)\u0026thinsp;+\u0026thinsp;NO\u003csub\u003e2\u003c/sub\u003e \u0026rarr; CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;NO\u003c/p\u003e\n \u003cp\u003eIn these equations, the LNT catalyst (e.g., BaO) stores NOx as nitrates (Ba(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e) during lean operation and releases it as NH\u003csub\u003e3\u003c/sub\u003e during rich regeneration. The generated NH3 then participates in the standard and fast SCR reactions over the SCR catalyst, reducing NOx to N\u003csub\u003e2\u003c/sub\u003e (Manigandan, S., et al. 2020). In the DPF, soot (C) is oxidized by O\u003csub\u003e2\u003c/sub\u003e and NO\u003csub\u003e2\u003c/sub\u003e to form CO\u003csub\u003e2\u003c/sub\u003e, with NO\u003csub\u003e2\u003c/sub\u003e being more effective at lower temperatures compared to O\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"6. Future Perspectives and Challenges","content":"\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003e6.1. Emerging catalyst materials and technologies\u003c/h2\u003e \u003cp\u003eThe development of advanced catalyst materials and technologies is crucial for meeting the emission regulations and improving the efficiency of aftertreatment systems (Mourad, M et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Some emerging catalyst materials include:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003e \u003cem\u003ePerovskite-based catalysts\u003c/em\u003e: Perovskite oxides, such as LaCoO\u003csub\u003e3\u003c/sub\u003e and LaMnO\u003csub\u003e3\u003c/sub\u003e, have shown promise as alternatives to traditional platinum group metals (PGMs) in LNT and SCR catalysts. These materials exhibit high thermal stability, excellent redox properties, and good sulfur tolerance, making them attractive candidates for high-temperature applications.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cem\u003eCeria-based catalysts\u003c/em\u003e: Ceria (CeO\u003csub\u003e2\u003c/sub\u003e) and its derivatives have gained attention as catalyst supports and promoters. Ceria-based materials can enhance the low-temperature activity and stability of LNT and SCR catalysts, as well as promote soot oxidation in DPFs.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cem\u003eMetal-organic frameworks (MOFs)\u003c/em\u003e: MOFs are highly porous materials with tunable chemical and physical properties. They have been explored as catalyst supports and adsorbents in aftertreatment systems, offering high surface area, controllable pore size, and good thermal stability. MOFs can be functionalized with active metal sites to achieve desired catalytic properties.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eIn terms of emerging technologies, the integration of non-thermal plasma (NTP) with catalytic aftertreatment systems has shown potential for improving low-temperature performance and reducing the dependence on precious metals (Norouzi, Armin, et al. 2023). NTP can generate highly reactive species, such as oxidizing radicals and excited molecules, which can enhance the catalytic reactions at lower temperatures. The combination of NTP with LNT, SCR, or DPF catalysts can lead to improved NOx and PM reduction efficiency, as well as faster catalyst light-off during cold starts.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003e6.2. Trends in emission regulations and their impact on aftertreatment systems\u003c/h2\u003e \u003cp\u003eThe global trend in emission regulations is towards more stringent limits on NOx and PM emissions from diesel engines. For example, the Euro 7 standards, expected to come into effect in the mid-2020s, are likely to impose even lower NOx and PM limits than the current Euro 6 regulations. Similarly, the CARB has proposed the Heavy-Duty Low NOx Omnibus Regulation, which aims to reduce NOx emissions from heavy-duty trucks by up to 90% below current standards. These trends in emission regulations will have a significant impact on the design and development of aftertreatment systems (Lyu Liqun, et al. 2023; Xiang, Pan, et al. 2024). To meet the lower emission limits, aftertreatment systems will need to achieve higher NOx and PM conversion efficiencies, especially at low temperatures and under real-world driving conditions. This will require the development of advanced catalyst materials, improved system architectures, and more sophisticated control strategies (Zhang, Size, et al. 2024). Furthermore, the focus on real-world driving emissions (RDE) testing will necessitate the optimization of aftertreatment systems for a wider range of operating conditions, including low-load and low-temperature operation. This will require a better understanding of the complex interactions between engine, aftertreatment, and control systems, as well as the development of adaptive and predictive control algorithms (Vedagiri Praveena, et al. 2020; Apicella Barbara, et al. 2020).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003e6.3. Research directions and opportunities for further improvement\u003c/h2\u003e \u003cp\u003eTo address the challenges posed by the future emission regulations and the need for more efficient and cost-effective aftertreatment systems, several research directions and opportunities have been identified:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003e \u003cem\u003eDevelopment of multi-functional catalyst materials\u003c/em\u003e: Researchers are exploring the development of catalyst materials that can perform multiple functions, such as simultaneous NOx and PM reduction, or combined LNT and SCR functionality. These multi-functional materials can help to simplify the aftertreatment system architecture, reduce precious metal loading, and improve overall system efficiency.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cem\u003eAdvanced catalyst characterization techniques\u003c/em\u003e: The use of advanced characterization techniques, such as in-situ X-ray absorption spectroscopy (XAS), environmental transmission electron microscopy (ETEM), and operando Raman spectroscopy, can provide valuable insights into the structure-activity relationships of catalyst materials under realistic operating conditions. These techniques can help to guide the rational design and optimization of novel catalyst formulations.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cem\u003ePredictive modeling and simulation\u003c/em\u003e: The development of predictive models and simulation tools can assist in the design and optimization of aftertreatment systems, reducing the need for extensive experimental testing. These models can incorporate detailed kinetic mechanisms, fluid dynamics, and heat transfer effects to predict the performance of catalytic reactors under various operating conditions. Machine learning algorithms can also be employed to identify optimal catalyst formulations and operating strategies.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cem\u003eIntegration with advanced combustion technologies\u003c/em\u003e: The integration of advanced combustion technologies, such as low-temperature combustion (LTC) and reactivity-controlled compression ignition (RCCI), with optimized aftertreatment systems can lead to significant reductions in both NOx and PM emissions. The development of integrated engine-aftertreatment control strategies can help to maximize the benefits of these technologies and ensure compliance with future emission regulations.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eBy addressing these research directions and opportunities, the scientific community and automotive industry can work towards the development of more efficient, cost-effective, and environmentally sustainable aftertreatment systems for diesel engines. The successful implementation of these technologies will play a crucial role in meeting the challenges posed by the future emission regulations and contribute to the overall reduction of the environmental impact of transportation.\u003c/p\u003e \u003c/div\u003e"},{"header":"7. Conclusions","content":"\u003cp\u003eThe development of efficient and sustainable after-treatment systems for diesel engines is crucial in the global transition towards energy-efficient and environmentally friendly vehicles. This comprehensive review has explored the latest advancements in after-treatment methodologies, focusing on the synergistic effects of in-cylinder combustion strategies and post-combustion purification technologies to effectively mitigate nitrogen oxides (NOx) and particulate matter (PM) emissions. Low-temperature combustion (LTC) strategies, such as reactivity-controlled compression ignition (RCCI) and partially premixed compression ignition (PPCI), have emerged as promising techniques for overcoming the traditional NOx/soot trade-off inherent in diesel combustion. By achieving a more homogeneous air-fuel mixture and operating at lower temperatures, LTC has the potential to significantly reduce engine-out emissions. However, the implementation of LTC poses unique challenges for after-treatment systems, particularly in low-temperature and cold-start conditions.\u003c/p\u003e \u003cp\u003eSCR catalysts, LNT, and DPF have been extensively investigated to address the limitations of LTC after-treatment. Novel catalyst formulations, such as zeolite-based materials, perovskites, and mixed metal oxides, have shown promise in enhancing low-temperature performance and durability. The integration of LNT and SCR systems (LNT-SCR) has demonstrated superior NOx reduction efficiency over a wide temperature range, while SCR-on-DPF and LNT-on-DPF configurations offer compact solutions for simultaneous PM and NOx reduction. This review has also explored the potential of energy conversion and recovery techniques, such as thermoelectric generators and organic Rankine cycles, to harness waste heat from the exhaust and improve the overall efficiency of the after-treatment system. The complex interactions between engine operating parameters, combustion kinetics, and emission formation in LTC engines have been highlighted, emphasizing the importance of a comprehensive approach to optimize in-cylinder and after-treatment processes. The insights gained from this study can guide future research efforts towards overcoming the limitations of current after-treatment technologies in low-temperature conditions and achieving superior emission reduction performance in advanced combustion engines. By providing a critical analysis of the state-of-the-art and identifying promising research avenues, this review contributes to the development of sustainable and efficient automotive technologies that meet the growing demand for energy-efficient and environmentally friendly vehicles. As the global community continues to strive for cleaner transportation solutions, the synergistic integration of advanced combustion strategies and innovative after-treatment systems will be crucial in achieving the ambitious emission reduction targets set by regulatory bodies worldwide. The findings presented in this review lay the foundation for further advancements in this field, paving the way for a greener and more sustainable future in the automotive industry.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"602\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eLTC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eLow Temperature Combustion\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eICE\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eInternal Combustion Engines\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eSCR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eSelective Catalytic Reduction\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003ePGM\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003ePlatinum Group Material\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eDOC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eDiesel Oxidation Catalyst\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eDPF\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eDiesel Particulate Filter\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eLD \u0026amp; HD\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eLight Duty \u0026amp; Heavy Duty\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eATS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eAfter Treatment Systems\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eNSC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eNOx Storage Catalyst\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eOSM\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eOxygen Storage Material\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003ePM\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eParticulate Matter\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eNOx\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eNitrogen Oxides (NO, NO2)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eLNT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eLean NOx Traps\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eRCCI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eReactivity-Controlled Compression Ignition\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003ePPCI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003ePartially Premixed Compression Ignition\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eNH3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eAmmonia\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eN2O\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eNitrous Oxide\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eHC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eHydrocarbons\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eCO\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCarbon Monoxide\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eEGR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eExhaust Gas Recirculation\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eTEG\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eThermoelectric Generator\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eORC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eOrganic Rankine Cycle\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eXAS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eX-ray Absorption Spectroscopy\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eETEM\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eEnvironmental Transmission Electron Microscopy\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eRDE\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eReal-world Driving Emissions\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eMOF\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eMetal-Organic Framework\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eNTP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eNon-Thermal Plasma\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eCARB\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCalifornia Air Resources Board\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eDMB\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eDiesel/Methanol/n-Butanol\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003ePCCI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003ePremixed Charge Compression Ignition\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eHCCI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eHomogeneous Charge Compression Ignition\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003ePEMS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003ePortable Emissions Measurement System\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eSEMS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eSmart Emissions Measurement System\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eSDPF\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eSCR-coated Diesel Particulate Filter\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eWHR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eWaste Heat Recovery\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eCRDI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCommon Rail Direct Injection\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eTEPOC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eThermoelectric Power Generator \u0026amp; Oxidation Catalyst\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to thank LERMA Labs, Rabat International University, for ensuring the availability of facilities.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the authors contributed to the study conception and design. Dikra Bakhchin, Oumaima Douadi, and Rajesh Ravi performed the material preparation, data collection and analysis. Faqir Mustapha and Elhachmi Essadiqi provided supervision and facilities in the LERMA laboratory. Dikra Bakhchin wrote the first draft of the manuscript, and all the authors commented on previous versions of the manuscript. All the authors have read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare the following financial interests /relationships which may be considered as potential competing interests: Dikra Bakhchin reports financial support was provided by National Center for Scientific and Technical Research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research work was fully funded by The National Center of Scientific and Technical research in Morocco and Foundation UM6P under the APRD project “Design and development of exhaust heat recovery system and emission reduction technology for multi cylinder CRDI engine”. The authors would like to thank LERMA Labs, Rabat International University, for ensuring the availability of facilities.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAhmad, N. N. R., Mohammad, A. W., Mahmoudi, E., Ang, W. L., Leo, C., \u0026amp; Teow, Y. H. (\u003cstrong\u003e2022\u003c/strong\u003e). An overview of the modification strategies in developing antifouling nanofiltration membranes. 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Chemosphere, 317, 137765\u003c/li\u003e\n\u003cli\u003eXiang, Pan, et al. \u0026quot;Experimental investigation on gas emission characteristics of ammonia/diesel dual-fuel engine equipped with DOC+ SCR aftertreatment.\u0026quot; \u003cem\u003eFuel\u003c/em\u003e 359 (\u003cstrong\u003e2024\u003c/strong\u003e): 130496.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"GHG alleviation, Low-temperature combustion, NOx \u0026 PM mitigation, NH3-SCR, heterogenous Catalysis technologies, Combined catalysts systems","lastPublishedDoi":"10.21203/rs.3.rs-4187531/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4187531/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe global transition towards sustainable automotive vehicles has driven the demand for energy-efficient internal combustion engines with advanced aftertreatment systems capable of reducing nitrogen oxides (NOx) and particulate matter (PM) emissions. This comprehensive review explores the latest advancements in aftertreatment technologies, focusing on the synergistic integration of in-cylinder combustion strategies, such as low-temperature combustion (LTC), with post-combustion purification systems. Selective catalytic reduction (SCR), lean NOx traps (LNT), and diesel particulate filters (DPF) are critically examined, highlighting novel catalyst formulations and system configurations that enhance low-temperature performance and durability. The review also investigates the potential of energy conversion and recovery techniques, including thermoelectric generators and organic Rankine cycles, to harness waste heat from the exhaust and improve overall system efficiency. By analyzing the complex interactions between engine operating parameters, combustion kinetics, and emission formation, this study provides valuable insights into the optimization of integrated LTC-aftertreatment systems. Furthermore, the review emphasizes the importance of considering real-world driving conditions and transient operation in the development and evaluation of these technologies. The findings presented in this article lay the foundation for future research efforts aimed at overcoming the limitations of current aftertreatment systems and achieving superior emission reduction performance in advanced combustion engines, ultimately contributing to the development of sustainable and efficient automotive technologies.\u003c/p\u003e","manuscriptTitle":"Integrated catalytic systems for simultaneous NOx and PM reduction: A comprehensive evaluation of synergistic performance and combustion waste energy utilization","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-23 17:10:44","doi":"10.21203/rs.3.rs-4187531/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revision","date":"2024-06-12T00:43:26+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2024-05-16T23:52:11+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-05-15T07:16:16+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Environmental Science and Pollution Research","date":"2024-05-14T09:59:11+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-04-10T05:07:31+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2024-04-08T06:57:30+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"fd6418b2-4c79-41a9-a42c-07fef1c45601","owner":[],"postedDate":"May 23rd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2024-07-03T06:31:15+00:00","versionOfRecord":[],"versionCreatedAt":"2024-05-23 17:10:44","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4187531","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4187531","identity":"rs-4187531","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}