Next Article in Journal
Implementing the Sustainable Development Goals in the Curricula of University Degrees: Initial Steps
Previous Article in Journal
Dual-Purpose Utilization of Sri Lankan Apatite for Rare Earth Recovery Integrated into Sustainable Nitrophosphate Fertilizer Manufacturing
Previous Article in Special Issue
Collaborative Multi-Objective Optimization of Combustion and Emissions in Circulating Fluidized Bed Boilers Using the Bidirectional Temporal Convolutional Network and Hybrid Dung Beetle Optimizer
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Sustainability
Volume 17
Issue 14
10.3390/su17146354
sustainability-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

DOC Study on the Effects of Catalyst Active Component Loading and Carrier Properties on the Catalytic Conversion Efficiency of Key Gaseous Pollutants

by
Yantao Zou
and
Liguang Xiao
*
School of Materials Science and Engineering, Jilin Architecture University, Changchun 130052, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(14), 6354; https://doi.org/10.3390/su17146354
Submission received: 1 May 2025 / Revised: 26 June 2025 / Accepted: 9 July 2025 / Published: 11 July 2025
(This article belongs to the Special Issue Technology Applications in Sustainable Energy and Power Engineering)
Browse Figures
Versions Notes

Abstract

Based on engine bench testing, this study investigated the effect of diesel oxidation catalytic converter (DOC) formulations on the gaseous emissions performance of diesel engines equipped with a DOC+ catalyzed diesel particulate filter (CDPF)+selective catalytic reduction (SCR) system after the treatment system. The experimental results indicate that changes in DOC formulations have no significant effect on engine fuel economy. As the precious metal loading increases and the Pt/Pd ratio decreases, the T50 for CO and HC decreases, and the low-temperature conversion rates (<300 °C) for CO and HC increase. However, as the temperature continues to rise, the beneficial effect of increased precious metal loading or Pd on CO and HC conversion rates gradually weakens. The average conversion rates in the high-temperature range (≥300 °C) show little difference. The NO conversion rate increases with increasing precious metal loading. The NO conversion rate is more sensitive to Pt content, with higher Pt content formulations promoting NO oxidation, contrary to the trends observed for CO and HC conversion rates. When the SCR inlet temperature is low, high NO2 concentrations are beneficial for improving the SCR’s NOx conversion efficiency. When the SCR inlet temperature is high, the SCR’s NOx conversion efficiency exceeds 90% with no significant differences. No significant impact of DOC formulation changes on CDPF pressure drop under external conditions was observed.

1. Introduction

With the increasingly severe global environmental problems, sustainable development has become a core issue of common concern for the international community. As one of the major sources of greenhouse gas and air pollutant emissions, the innovation of emission reduction technologies in the transportation industry is crucial to realizing a low-carbon economy and an environmentally friendly society [1]. Diesel engines are widely used in light-duty vehicles due to their high thermal efficiency and power performance. However, the gaseous pollutants they emit pose a serious threat to air quality and human health. Therefore, the development of high-efficiency and low-emission diesel aftertreatment technologies, especially the optimization of the formulation of diesel DOC, has become an important direction of current research.
Diesel engines are widely used in transportation, agricultural machinery, construction machinery, and other fields, but their pollution of the atmosphere can not be ignored. Relying only on diesel engine purification technology means being unable to meet the increasingly stringent emission regulations and diesel exhaust after-treatment technology must be used at the same time [2]. Diesel exhaust after-treatment technologies include the DOC [3], the diesel particulate filter (DPF), the CDPF [4], SCR, and the continuous regeneration particulate trap (DOC+CDPF) [5]. Among them, the DOC has a very obvious emission reduction effect on conventional gaseous substances, can oxidize CO, HC, etc., and can also oxidize NO into NO2 to assist the passive regeneration of the CDPF and the rapid response of the SCR [6]; the DPF can effectively capture particulate matter and achieve the highest capture efficiency of more than 90% [7]; the CDPF is a coating of precious metals and rare earth elements in the interior of the DPF so that it has the ability to capture particulate matter and be passively regenerated [8], and it is coated with precious metals and rare earth elements inside the DPF so that it has the ability to capture particles and passive regeneration [9]; and SCR reduces NOx by injecting urea and utilizing the urea decomposition product NH3 [8]. In the face of increasingly stringent multi-pollutant synergistic control requirements, traditional single after-treatment technologies have shown their limitations. Based on this, integrated after-treatment systems (DOC+CDPF+SCR combination) have become a research hotspot because of their synergistic purification efficacy [10].
However, in the coupling process of multiple post-processors, different structural parameters, catalytic agent formulations, coating processes, and integration methods will have a significant impact on the integrated emission reduction efficiency. The catalyst formulation, coating process, and integration method will have different impacts on the comprehensive emission reduction effect. Diesel engines compliant with the latest emissions regulations must be equipped with an exhaust aftertreatment system (DOC+DPF+SCR). Before integrating the system with the engine, it is necessary to complete the catalytic converter’s structural optimization and extensive bench calibration tests; however, this process is both costly and time-consuming. Tan [11] proposes a design and optimization scheme for heavy-duty diesel engine after-treatment systems: First, establish one-dimensional and three-dimensional computational models considering structural parameters; then, based on the calibrated models, analyze the effects of structural parameters on pollutant conversion rates, temperature, and pressure drop; and finally, determine the optimal design scheme and use three-dimensional models to study substrate temperature distribution and pressure distribution. This method provides effective guidance for optimizing diesel engine aftertreatment systems [11]. Low-temperature combustion diesel engines are more fuel-efficient than traditional diesel engines. However, they have lower exhaust temperatures and emit more CO and HC. To meet emissions regulations, DOC must be optimized to oxidize CO and HC more efficiently at low temperatures. Hazlett investigated the effect of water on the oxidation of CO and C3H6 by Pt, Pd, and Pt-Pd/Al2O3 catalysts in the exhaust treatment of low-temperature combustion diesel engines. The study found that water promotes CO oxidation by Pd and Pt-Pd catalysts but inhibits Pt catalysts, while in C3H6 oxidation, water is detrimental to Pd but beneficial to Pt. The study, using DRIFTS analysis, demonstrated that water alters the adsorption characteristics of different catalysts toward CO and slightly affects the selectivity of certain oxidation products, providing a key theoretical basis for optimizing low-temperature diesel engine oxidation catalysts [12]. He studied and developed a Pt/Bi-doped YMnO composite catalyst for DOC. The catalyst exhibited excellent low-temperature activity in a competitive atmosphere of multicomponent diesel exhaust gas, with a complete conversion temperature of CO of only 210 °C. The Bi doping induced the formation of a Pt-O-Bi structure, which facilitated unidirectional electron transfer, accelerated the oxidation of CO in the low-temperature region, and efficiently suppressed the competitive adsorption among CO/HC/NO. This study provides a new idea to solve the problem of competitive oxidation of key active sites in multi-component exhaust gases [13]. Zhang [14] investigated the working principle, preparation method, and development trend of DOC, and focused on the influence of sulfur dioxide (SO2) on DOC and the coping strategy. The research in this paper provides a theoretical basis for the design and application of DOC in the future, which promotes the clean emission of diesel engines and realizes the goal of sustainable development [14]. Li’s study [15] delved into the effect of DOC on the nanostructural properties of particulate matter through engine bench tests and advanced characterization techniques. The results showed that DOC treatment significantly altered the nanostructure of particulate matter, including decreasing the particle mean radius, interfacial layer thickness, and fractal dimension, while increasing the specific surface area [15].
To improve fuel economy, enhancing the low-temperature performance of diesel emission control is crucial. Liang investigated the effect of different MnO2/Mn2O3 phase ratios on the low-temperature performance of platinum-based diesel oxidation catalysts. Approximately equal ratios of MnOx phases can form a three-phase (platinum, MnO2, and Mn2O3) interface structure, resulting in smaller platinum particle sizes and interface rates (1.6–11.1 times higher) compared to other platinum–indium monomanganese oxides. Additionally, higher oxygen mobility and more active oxygen species may also result from the positive effects of the platinum/manganese oxide interface, which can activate reactants and significantly increase the TOF value (1.4–20.8 times). These results indicate that modifying the multi-phase metal/oxide interface has the potential to disperse platinum, thereby significantly enhancing catalytic efficiency [16]. Luy studied the key changes in the characteristics of diesel engine particles before and after DOC treatment. Using various characterization methods such as atomic force microscopy and thermogravimetric analysis, he focused on analyzing the adsorption characteristics (attractiveness, adhesion force, and adhesion energy) and physical and chemical properties (particle size, graphitization degree, and organic matter content) of the particles. The findings reveal that post-injection conditions significantly enhance the adsorption performance of particles, but DOC treatment effectively reduces these properties. Particle adsorption force primarily depends on particle size and graphitization degree, while adhesion force is directly related to soluble organic matter content. These findings provide important insights into understanding particle deposition behavior and optimizing post-treatment systems [17]. Nie [18] investigated the effect of processing and dosage of intestinal algae on the removal of diesel vehicle exhaust pollutants as a peroxide catalyst dopant. The intestinal algae were pretreated by grinding and high-temperature pyrolysis, and the doped La0.8Ce0.2Co0.4Mn0.6O3 catalyst was prepared by solgel method. The results showed that the doping of intestinal algae improved the performance of the catalyst, and the high-temperature pyrolysis treatment was more effective, with the NO purification efficiency exceeding 60%. The best performance of the perlite-doped catalyst was achieved when the additional amount of intestinal algae was 0.2 g [18]. Meng [19] explored the effect of catalyst reverse diffusion on the oxidation characteristics of the particle layer during the regeneration of CDPF. It was found that the pressure drop of CDPF varied in a three-stage process with increasing thickness of the particulate layer: At a thickness of less than 80 μm, reverse diffusion significantly promoted oxidation and the pressure drop rate was higher than that of the conventional DPF; at a thickness of more than 80 μm, the diameter of the effective airflow micropore decreased, and the pressure drop rate of the CDPF was lower than that of the DPF, but it lasted for longer time with a smaller value of the total pressure drop. In addition, there was a time delay in the pressure and thickness reduction of CDPF, which was particularly pronounced at thicknesses greater than 193 um [19]. Precious metal catalysts, due to their excellent catalytic activity, are widely used in the catalytic oxidation reactions of various gaseous pollutants (CO, CH4). However, in actual industrial exhaust gases or vehicle exhaust, they are easily affected by SO2 and become deactivated. Therefore, Shan system [20] summarized the research on the catalytic oxidation of gaseous pollutants by precious metal catalysts in the presence of SO2. First, the influence mechanism of SO2 and its interaction with catalysts were discussed. Based on sulfur poisoning, design strategies for sulfur-resistant catalysts were proposed, such as inhibiting SO2 adsorption/sulfate formation, promoting its desorption/decomposition, and constructing sacrificial sites, along with methods to achieve these strategies (e.g., core–shell structures, doping, and structural regulation). Additionally, catalyst regeneration methods are explored. Finally, the research challenges and prospects in this field are highlighted, aiming to deepen the understanding of sulfur poisoning and guide the design and regeneration of sulfur-resistant catalysts [20]. SO2 and H2O cause severe deactivation of cerium oxide-titanium dioxide (CeO2-TiO2) catalysts in the ammonia selective catalytic reduction reaction. Transition metal doping is an effective method to enhance the resistance of CeO2-TiO2 catalysts to SO2 and H2O. SO2 and H2O cause CeO2-TiO2 catalysts to deactivate in the NH3-SCR reaction. Yang [21] evaluated the effects of five transition metal (W, Nb, Mo, Fe, and Mn) dopants on the resistance and activity of CeO2-TiO2 catalysts using density functional theory calculations. W and Fe dopants enhanced reaction activity but had poor resistance, while Nb and Mn dopants exhibited good resistance but limited activity. The CeMoTi catalyst doped with Mo performed the best, combining excellent resistance to poisoning and reaction performance. Mo reduced SO2 adsorption and oxidation, decreased sulfate deposition, promoted NH3 adsorption and competition, inhibited the formation of inert nitrates, and enhanced the SCR reaction [21].
The optimization of the performance of DOC is of great significance in reducing gaseous pollutant emissions from light-duty diesel vehicles, and it is also one of the key technologies for promoting the green transformation and sustainable development of the transportation industry. In the context of the global practice of sustainable development strategy, reducing mobile source emissions and improving energy efficiency have become important ways to realize a low-carbon economy and combat climate change. This study focuses on the formulation optimization of DOC catalysts, and by precisely regulating the precious metal active components, carrier materials, and other key factors, we are committed to exploring highly efficient and economical catalytic solutions to meet the increasingly stringent emission regulations and take into account the sustainable use of resources. In this paper, the specific effects of DOC catalyst formulation on the gaseous pollutant emission reduction of light-duty diesel vehicles when DOC+CDPF+SCR is integrated into practical applications are thoroughly investigated. Specifically, this study systematically investigated the variation in the emission characteristics of diesel engines with different precious metal loadings and Pt/Pd ratios, and the related tests were conducted based on standard engine bench tests to ensure the accuracy and reliability of the data. This study aims to provide a solid scientific basis for the development of cleaner and more efficient diesel engine after-treatment technologies, which will, in turn, promote the sustainable development of the transportation sector.

2. Test System

To investigate the effect of DOC formulation on the emission reduction performance of an after-treatment system under real diesel engine exhaust conditions, a series-coupled configuration of DOC, CDPF, and SCR was evaluated in a bench test. The test platform consisted of a turbocharged inter-cooled diesel engine with a displacement of 1.96 L. The engine parameters are detailed in Table 1.
The main emission test instruments include the MEXA-1600D gaseous analyzer(Horiba, Shizuoka Prefecture, Japan), EEPS-3090 particle size analyzer (TSI, Shoreview, MN, USA), and Dekati DI-2000 jet diluter(Dekati Ltd., Kangasala, Finland). The test system has four measurement points, collecting data before the DOC, before the CDPF, after the CDPF and after SCR. The test is carried out under the steady-state condition of the diesel engine. The prototype was tested under two operating conditions: (1) external characteristic curve testing at speed intervals of 1000–3000 r/min with measurements taken at 500 r/min increments; and (2) load characteristic testing at 2000 r/min (maximum torque speed) under 10%, 25%, 50%, 75%, and 100% load conditions, with each load point tested three times and the results averaged. Between test groups, the DOC was replaced for subsequent testing. To maintain consistent test conditions, the initial DOC temperature from the first test group served as the reference. Each subsequent test was conducted only after stabilizing the DOC temperature within ±5 °C of this reference value.
In this study, the design of the DOC catalyst followed a systematic approach, aiming to investigate the effects of precious metal loading and Pt/Pd ratio on the removal efficiency of gaseous pollutants and downstream SCR functionality. The design process comprehensively considered the catalytic oxidation mechanisms of target pollutants (CO, HC, etc.), the differences in the activity characteristics of precious metals (Pt and Pd), and cost-effectiveness. Through literature review and preliminary screening, five representative combinations of precious metal loading levels (covering high, medium, and low levels) and Pt/Pd ratios (fixed and variable ratios) were selected. The catalysts were prepared using the standard impregnation method on commercial cordierite supports, with strict control of preparation conditions to ensure comparability. This design strategy aims to reveal the key role of precious metal parameters in the performance of integrated after-treatment systems. The test design consisted of five kinds of DOC, with precious metal formulations of 2150 (Pt/Pd, 5:1), 1400 (5:1), 900 (5:1), 900 (7:1), and 900 (10:1) g/m3, labeled as 1,2,3,4,5 DOC, in addition to different catalyst formulations. The rest of the parameters are consistent (see Table 2).

3. Results and Discussion

3.1. Load Characterization

The ignition temperature characterization evaluates the low-temperature activity of the catalyst, which mainly depends on the catalyst components. Figure 1 shows the CO, HC, and NO conversion rates of each DOC formulation at different loads of 2000 r/min, and the DOC inlet temperature corresponds to the load.
As shown in Figure 1a, with the increase in loading, the DOC front discharge temperature increases, the CO in situ emission decreases, and the CO conversion efficiency increases; on one hand, this is due to the increase in catalytic activity by the increase in temperature, and on the other hand, it is due to the decrease in the in situ emission that increases the proportion of CO attached to the active site [22]. The CO light-off temperatures (T50) of DOCs from No. 1 to No. 5 are about 158 °C, 170 °C, 175 °C, 212 °C, and 234 °C, and the T50 of CO decreases with the increase in the loading and the decrease in the Pt/Pd ratio of the noble metal. The effect of catalyst formulations on CO conversion was primarily observed under low-temperature conditions (<300 °C). DOC samples 1–5 exhibited average CO conversion rates of 64.7%, 56.1%, 49.6%, 37.8%, and 31.7%, respectively. The data indicate that CO conversion efficiency in the low-temperature region improves with both increasing precious metal loading and higher Pd content in the formulation. In the high-temperature section (≥300 °C), the average CO conversion rates of DOCs 1–5 were 94.1%, 92.9%, 88.3%, 80.5%, and 85.4%, and the gap between the emission reduction performances of the formulations was narrowed, with an average conversion rate of 88.2%. With the increase in load, the HC emission decreases, and the change rule of its conversion rate is similar to that of CO, as shown in Figure 1b. The HC conversion rate increases with the increase in precious metal loading as well as Pd content, and the T50 is approximate to 209 °C, 215 °C, 223 °C, 240 °C, and 273 °C. In the high-temperature section, the average HC conversion rates of DOC 1~5 were 78.5%, 70.5%, 73.6%, 75.8%, and 74.2%, and the patterns of change were the same as that of CO.
The precious metal content and the ratio of Pd were positively correlated with the ignition characteristics of the two; the T50 of CO was lower than that of HC for all formulations, and the low-temperature conversion rate was significantly higher than that of HC. On one hand, the reactive potentials of CO and HC were different in the catalysts, and the C–H bonding energy of alkanes was larger than the C–O bonding energy, so a higher reaction temperature was required [23]; on the other hand, the presence of CO will increase the ignition temperature of Pd and Pt to HC because there is a competitive adsorption relationship between CO and HC in the active site, and the adsorbed CO needs to be desorbed to vacate the active site for HC adsorption [21]. As the load increases, NO emissions from the original engine increase, and the NO conversion rate first rises and then decreases, as shown in Figure 1c. The NO conversion rates of all formulations reach their peak at 75% load and decrease slightly at 100% load. This phenomenon is mainly governed by the thermodynamic equilibrium characteristics of the NO oxidation reaction [24]. Unlike the CO and HC transition rates, the NO transition rate is more sensitive to the Pt content, and formulations with high Pt content can promote the oxidation of NO.

3.2. External Characterization

Figure 2a–c show the variation rules of the fuel consumption rate, pressure drop of the DOC and CDPF, and temperature difference before and after the DOC when the engine is operated according to external characteristics. It can be seen that there is no significant difference in fuel consumption rate between No. 1 to No. 5 programs, indicating that the change in formula will not have a significant impact on the engine economy. If exhaust backpressure is too high, it will affect the engine’s normal air exchange process, so this paper is on the DOC and CDPF before and after the pressure change statistics. Overall 1~5 DOC pressure drops are increased with the speed, but there are some differences among the formulations. The largest average pressure difference is observed with the 2150 (5:1) g/m3 formulation, which is about 2.4 kPa; the smallest is observed with the 900 g/m3 formulation, and the pressure difference varies very little among the three different Pt/Pd ratios of 900 g/m3. The average pressure drop of DOC No. 3~5 is 1.7 kPa, 1.7 kPa, and 1.6 kPa, from which, it can be seen that the increased coating thickness leads to a marginal rise in DOC pressure drop, whereas variations in Pt/Pd ratios show minimal effect. The CDPF pressure drop displays minor fluctuations across different schemes, with average values for DOC No. 1–5 measuring 4.6 kPa, 4.9 kPa, 4.8 kPa, 4.6 kPa, and 4.8 kPa, respectively, representing a narrow variation range of 4.1%. These findings indicate that modifications to DOC catalyst formulations exert a negligible influence on CDPF pressure drop characteristics [25].
The DOC is placed upstream of the whole set of post-processors, which oxidizes CO and HC, on one hand, and NO at the same time to provide the required NO2 for the downstream post-processor. On the other hand, it also reduces the loss of discharge temperature and, thus, improves the purification efficiency of the downstream post-processor. So, in this paper, the temperature changes before and after the DOC are compared, as shown in Figure 2d. It can be found that the temperature after the DOC is slightly lower than that before the DOC in all working conditions. The observed phenomenon primarily stems from the uninsulated catalyst shell, which leads to significant heat dissipation in the catalyst body section. The exothermic heat generated by DOC catalytic oxidation proves insufficient to compensate for this thermal loss [26]. Temperature differentials exhibit fluctuations without demonstrating a clear correlation with rotational speed variations. However, formulation-dependent temperature differences are evident, showing a general trend of increasing temperature differentials with higher catalyst loading amounts.
When the engine exhaust is discharged, it will flow through the DOC, the CDPF, and SCR, in turn, in order to study the influence of the DOC formulation on the whole after-treatment device. This paper analyzes the emission characteristics after the CDPF and after SCR when the engine is operated by the external characteristics, and the emission characteristics of CO, THC, and NO2/NOx ratios after the CDPF are shown in Figure 3.
The variation rules of CO and HC were similar, with higher emissions from the original machine at low rotational speeds, while the emissions from the 900 (10:1) g/m3 formulation were significantly higher than those from the other formulations, suggesting that an increase in the ratio of Pt/Pd is detrimental to the low-temperature conversion rate, which is consistent with the analysis in Section 3.1. The average conversion rates of the DOC+CDPF to CO for the five formulations were 96.8%, 95.7%, 94.7%, 96.1%, and 95.2%, and to THC, they were 88.1%, 81.8%, 72.9%, 87.4%, and 83.5%, respectively, which shows that the CO and HC emissions of formulations No. 1~3 were reduced in the order of No. 3~5. The CO and HC emissions of No. 3 were the highest, followed by No. 5, and No. 4 was the lowest. The differences in CO conversion rates among the formulations after the DOC+CDPF were not large, which were all above 94%, but the HC conversion rates varied slightly with different loadings and Pd ratios, and the 900 (7:1) and 2150 (5:1) g/m3 formulations had the best emission reduction effect.
As shown in Figure 3c, the NO2/NOx ratio of the original exhaust is very low, close to 0. After the DOC+CDPF, the NO2/NOx ratio increases dramatically, with the DOC oxidizing a portion of NO to NO2, and the gas mixture containing both NO and NO2 enters the downstream CDPF, where NO continues to be oxidized by the catalyst, while a portion of NO2 oxidized particulate matter is reduced. The NO2/NOx ratio remains generally high up to 2000 r/min and then decreases significantly to a minimum of 3000 r/min. The reasons for the decrease in the NO2/NOx ratio have been explained in the previous section and will not be repeated. Between the formulations, the NO2/NOx ratios increased sequentially with the increase in precious metal content [27].
The DOC, positioned upstream in the exhaust after-treatment system, serves dual functions: oxidizing CO and HC emissions while simultaneously generating NO2 for downstream treatment components [28]. Additionally, it helps minimize exhaust temperature losses, thereby enhancing the purification efficiency of subsequent aftertreatment devices. This study specifically examines temperature variations across the DOC, revealing that outlet temperatures consistently remain slightly lower than inlet temperatures under all operating conditions [29].

3.3. Analysis of the Impact of DOC Formulations on Downstream Post-Processors

The CDPF relies on the extremely high oxidizing power of NO2 to achieve continuous passive regeneration. SCR has different reaction speeds under different NOx compositions; an appropriate increase in NO2 helps to accelerate the reaction speed, and the ideal NO: NO2 is 1:1 [30]. Therefore, the NO2 content is very important for the performance of the downstream CDPF and SCR, and the average NO2/NOx ratios of post-DOC and post-CDPF were calculated in Figure 4 in order to compare the effects of different formulations of the DOC on the after-treatment system. In this study, the effects of different DOC formulations on the exhaust after-treatment system were analyzed by comparing and analyzing the patterns, as shown in figure. The distribution characteristics of the NO2/NOx ratio at the outlet of the DOC and CDPF were analyzed. The experimental data showed that the average NO2/NOx ratios after the CDPF are higher than those after the DOC, and the difference in the average NO2/NOx ratios between the DOC and CDPF is not the same in different DOC formulations; it is smaller in the case of a larger precious metal loading and in the case of a smaller precious metal loading [31]. It decreases with increasing Pt/Pd ratio. In the deep-bed structure of the CDPF, the noble metal (Pt/Pd) catalytic coating area significantly contributes to the NO oxidation reaction, resulting in a localized NO2 concentration-enriched zone. The cake layer portion of NO2 reacts with carbon fumes, which is a region of low NO2 concentration, and the difference in the concentration of NO2 drives the diffusion of NO2 for multiple reactions of passive regeneration. The concentration of NO2 increases after the CDPF if the consumption of NO2 used for particulate oxidation is less than the production of NO2, and the NO2 concentration downstream of the CDPF will increase [32]. When the DOC is formulated with a high precious metal loading or a high Pt/Pd ratio, the NO2 concentration in the CDPF is high. On one hand, due to the diffusion of NO2, it may adhere to the active sites of the precious metal, which forms a certain inhibition of NO adsorption. On the other hand, for the NO + O2 = NO2 reaction, the increase in NO concentration is favorable to the increase in the chemical reaction rate. This phenomenon suggests that although high NO2 concentration after the DOC is beneficial to the oxidation of particulate matter in the CDPF, the precious metals in the CDPF can also continue to oxidizing NO, so it is not necessary to excessively increase the amount of precious metals coated in the DOC in order to obtain high NO2 [33].
The conversion rate of SCR to NOx for each scheme is shown in Figure 5, which shows that when the SCR inlet temperature is low, the high NO2 concentration is conducive to improving the conversion efficiency of SCR to NOx [34]; when the SCR inlet temperature is high, the conversion efficiency of SCR to NOx reaches more than 90%, and there is no obvious difference.

3.4. Characterization of Different Positions in the Integrated Reprocessing System

In order to study the change rule of temperature, pressure, and NO2 ratio in the downstream direction when the DOC, the CDPF, and SCR are used in tandem, the average values of each program at each measurement point at 2000 r/min and 75% load are statistically analyzed, and Figure 6 shows the statistical graphs of the relative pressures, temperatures, and NO2 ratios before the DOC, after the DOC, after the CDPF, and after SCR. The relative pressure at each measurement point decreases gradually in the direction of the airflow. After passing through the DOC, the average pressure drop for all schemes is 1.5 kPa, and after passing through the CDPF, the average pressure drop is 4.2 kPa. The exhaust gas temperature at each measurement point decreases gradually in the direction of airflow. The temperature drop is greatest in the SCR, followed by the DOC, with an average of approximately 30.8 °C. The heat generated by oxidation is less than the heat dissipated [35]. The temperature drop in the CDPF is relatively small, approximately 5.2 °C. The NO2/NOx ratio increases significantly after passing through the DOC, with an average increase of approximately 23.8%, and increases slightly after passing through the CDPF, with an average increase of approximately 1.8%.

4. Conclusions

This study optimized the DOC catalyst formulation to enhance the emission control efficiency of diesel engines, taking into account the economy and stability of the exhaust system, which provides a scientific basis for the advancement of diesel engine emission control technology and actively contributes to the achievement of the goals of sustainable development and environmental protection. The results show that through technological innovation and optimized design, pollutant emissions from diesel engines can be effectively reduced, air quality improved, and contributions made to the construction of a clean, low-carbon, and sustainable transportation system.
(1)
With the increase in precious metal loading and the decrease in the Pt/Pd ratio, the T50 of CO and HC decreases, and the conversion rates of CO and HC in the low-temperature section (<300 °C) increase; however, with the continuous increase in temperature, increasing precious metal or palladium content has a diminishing effect on CO and HC conversion, and the difference between the average conversion rates of the high-temperature section (≥300 °C) is relatively small.
(2)
With the increase in load, the NO conversion rate first increased and then decreased, and the NO conversion rate of all formulations reached the peak at 75% load and slightly decreased at 100% load. Before the rotational speed of 2000 r/min, the NO2/NOx ratio stayed at a high level, after which there was a decreasing trend, and it reached the lowest value at 3000 r/min. The NO conversion rate increased with the increase in the load of precious metal. The NO conversion rate increases with the increase in precious metal loading. The NO conversion rate is more sensitive to the Pt content, and the formulation with high Pt content can promote the oxidation of NO, contrary to the change rule of the CO and HC conversion rate.
(3)
There is no obvious difference in the fuel consumption rate between different DOC formulations. The increase in precious metal coating will slightly increase the DOC pressure drop. The change in the DOC catalyst formulation does not have much effect on the CDPF pressure drop. The temperature after the DOC is slightly lower than that before the DOC, and there are fluctuations in the temperature difference. Therefore, the change in formulation will not have a significant effect on the engine economy.
(4)
The average NO2/NOx ratios after the CDPF are higher than those after the DOC, and NO is further catalytically oxidized in the CDPF. When the inlet temperature of SCR is low, the high NO2 concentration is conducive to the improvement in the conversion efficiency of SCR to Nox. When the inlet temperature of SCR is high, the conversion efficiency of SCR to NOx reaches more than 90% without any obvious difference.

Author Contributions

Y.Z.: Methodology, Software, Writing—Original Manuscript, Conceptualization, Writing—Reviewing and Editing. L.X.: Visualization. All authors commented on the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research work was supported by the 13th Five-Year National Key Research and Development Program (2018YFD1101000).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

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

  1. Caliskan, H.; Ergun, Y.; Karali, I.H.; Caglayan, H.; Hong, H.; Kale, U.; Matijošius, J. Investigating the diesel engine emission performances with various novel emission filters. Energy 2025, 318, 134775. [Google Scholar] [CrossRef]
  2. Liu, W.; Gao, Y.; You, Y.; Jiang, C.; Hua, T.; Xia, B. Nonlinear model predictive control(NMPC) of diesel oxidation catalyst (DOC) outlet temperature for active regeneration of diesel particulate filter (DPF) in diesel engine. Energy 2024, 293, 130658. [Google Scholar] [CrossRef]
  3. Zhang, Y.; Zhang, Y.; Lin, Y.; Fang, L.; Lou, D. Particle filter performance of soot-loaded diesel particulate filter the effect of its regeneration on the particle number size distribution. J. Clean. Prod. 2024, 461, 142651. [Google Scholar] [CrossRef]
  4. Zhang, B.; Li, X.; Tang, S.; Wan, Q.; Jia, G.; Liu, B.; Li, S. Effects analysis on hydrocarbon removal performance of an adsorptive catalytic gasoline particulate filter in the gasoline engine during cold start. Energy 2023, 283, 128527. [Google Scholar] [CrossRef]
  5. Chen, J.Y.; Tan, Q.P.; Liu, Y.; Wang, K.; Wang, J.X.; Yang, M.X.; Lou, M.D.; Hu, Y.Z.; Yang, M.W. Interaction between NOx reduction soot oxidation for selective catalytic oxidation-selective catalytic reduction catalyst coated on diesel particulate filter: An experimental numerical study. Chem. Eng. J. 2025, 506, 160055. [Google Scholar] [CrossRef]
  6. Caliskan, H.; Mori, K. Environmental, enviroeconomic and enhanced thermodynamic analyses of a diesel engine with diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) after treatment systems. Energy 2017, 128, 128–144. [Google Scholar] [CrossRef]
  7. Huang, H.; Chen, R.; Tao, S.; Wang, Y.; Zhang, L. Study of soot dynamic behavior catalytic regeneration in diesel particulate filters. Chem. Eng. J. 2024, 489, 151498. [Google Scholar] [CrossRef]
  8. Fan, L.; Shi, W.; Jing, J.; Dong, Z.; Yuan, J.; Qu, L. An artificial intelligence strategy for multi-objective optimization of Urea-SCR for vehicle diesel engine by RSM-VIKOR. Energy 2025, 317, 134667. [Google Scholar] [CrossRef]
  9. Lou, D.; Chen, Y.; Zhang, Y.; Jue, K.; Tan, P.; Hu, Z.; Fang, L. Analysis of temperature and pressure characteristics in catalyzed diesel particulate filter operation for heavy-duty diesel engine. Fuel 2022, 328, 125248. [Google Scholar] [CrossRef]
  10. Huo, Y.; Liu, J.; Wu, D.; Shao, Y.; Song, X.; Guo, Z.; Liu, A.; Li, Q.; Chen, J. Contribution of Aftertreatment Technologies to Alleviating SOA and Toxicity Generation from Typical Diesel Engine-Emitted I/SVOCs. Environ. Sci. Technol. 2024, 58, 22722–22732. [Google Scholar] [CrossRef]
  11. Tan, P.; Yao, C.; Wang, D.; Zhu, L.; Hu, Z.; Lou, D. Design and optimization of exhaust gas aftertreatment system for a heavy-duty diesel engine. J. Cent. South. Univ. 2022, 29, 2127–2141. [Google Scholar] [CrossRef]
  12. Hazlett, M.J.; Epling, W.S. Mechanistic effects of Water on Carbon monoxide and propylene oxidation on platinum and palladium bimetallic catalysts. Catal. Today 2021, 360, 401–410. [Google Scholar] [CrossRef]
  13. He, D.; Chen, Y.; Li, S.; Liu, Y.; Zhang, H.; Jiao, Y.; Jiao, Y.; Zhao, M.; Wang, J.; Chen, Y. Unidirectional electron transfer on bismuth-doped Pt/YMn2O5 for efficient CO oxidation as diesel oxidation catalysts. ACS Catal. 2024, 14, 7353–7368. [Google Scholar] [CrossRef]
  14. Zhang, Z.; Tian, J.; Li, J.; Cao, C.; Wang, S.; Lv, J.; Zheng, W.; Tan, D. The development of diesel oxidation catalysts and the effect of sulfur dioxide on catalysts of metal-based diesel oxidation catalysts: A review. Fuel Process. Technol. 2022, 233, 107317. [Google Scholar] [CrossRef]
  15. Li, R.; Yang, D.; Liu, F.; Zhu, J.; Hu, Q. Study on the nano-structure characteristics of particle before and after diesel oxidation catalyst for diesel engines. Energy Sources Part A 2024, 46, 275–293. [Google Scholar] [CrossRef]
  16. Liang, Y.; He, D.; Ding, X.; Wang, J.; Zhao, M.; Chen, Y. Effect of MnOx phase on Pt-based catalyst for enhancing CO/C3H6/NO oxidation performance. Int. J. Hydrogen Energy 2022, 47, 30722–30731. [Google Scholar] [CrossRef]
  17. Lyu, X.; Wang, K.; Liang, X.; Cui, L.; Wang, Y. Analysis of the correlation between mechanical and physicochemical properties of particles based on a diesel oxidation catalytic system. Sci. Total Environ. 2024, 926, 171898. [Google Scholar] [CrossRef]
  18. Nie, W.; Liu, C.; Luo, C.; Hua, Y.; Yan, X.; Lian, J. Effect of pretreatment methods and doping amount of Enteromorpha on the exhaust purification performance of perovskite catalysts used in underground diesel vehicles. J. Clean. Prod. 2024, 445, 141191. [Google Scholar] [CrossRef]
  19. Meng, Z.; Deng, M.; Wu, D.; Ou, J.; Liu, X.; Bao, Z. The back diffusion of catalysts on the oxidation characteristics of particle layer during the CDPF active regeneration process. Fuel 2024, 365, 131155. [Google Scholar] [CrossRef]
  20. Shan, C.; Hou, X.; Han, R.; Jia, Q.; Hou, N.; Wang, Y.; Liu, C.; Liu, Q. Recent Advances of Gaseous Pollutant Catalytic Oxidation over Precious Metal Catalysts with SO2 Exposure. Environ. Sci. Technol. 2025, 59, 2348–2367. [Google Scholar] [CrossRef]
  21. Yang, Z.; Song, J.; Zhang, G.; Sun, X.; Cheng, S.; Zhang, X.; Jiang, Y. Unraveling the Promotion for SO2 and H2O Resistance of Transition Metal-doped CeO2-TiO2 Catalysts in NH3-SCR Reaction: A DFT Study. J. Hazard. Mater. 2025, 489, 137563. [Google Scholar] [CrossRef] [PubMed]
  22. Resitoglu, A.I.; Altinisik, K.; Keskin, A.; Ocakoglu, K. The effects of Fe2O3 based DOC and SCR catalyst on the exhaust emissions of diesel engines. Fuel 2020, 262, 116501. [Google Scholar] [CrossRef]
  23. Orihuela, M.P.; Miceli, P.; Ramirez-Rico, J.; Fino, D.; Chacartegui, R. Ceria-based catalytic coatings on biomorphic silicon carbide: A system for soot oxidation with enhanced properties. Chem. Eng. J. 2021, 415, 128959. [Google Scholar] [CrossRef]
  24. Epling, W.S.; Campbell, L.E.; Yezerets, A.; Currier, N.W.; Parks, J.E. Overview of the fundamental reactions and degradation mechanisms of NOx storage/reduction catalysts. Catal. Rev. 2004, 46, 163–245. [Google Scholar] [CrossRef]
  25. Promhuad, P.; Sawatmongkhon, B.; Theinnoi, K.; Wongchang, T.; Chollacoop, N.; Sukjit, E.; Tunmee, S.; Tsolakis, A. Effect of Metal Oxides (CeO2, ZnO, TiO2, and Al2O3) as the Support for Silver-Supported Catalysts on the Catalytic Oxidation of Diesel Particulate Matter. ACS Omega 2024, 9, 19282–19294. [Google Scholar] [CrossRef] [PubMed]
  26. Xu, H.; Zhang, J.; Zhang, D.; Guo, Y.; Wu, F. Catalytic oxidation desulfurization of silica-gel-supported ionic liquid [Bmim] CoCl3 coupling oxone. Fuel 2021, 288, 119655. [Google Scholar] [CrossRef]
  27. Wang, P.; Jin, M.; Yu, D.; Bai, S.; Lei, L. Evolution Mechanism of N2O for the Selective Catalytic Reduction of NOx by NH3 Over Cu-SSZ-13 Assisted Fe-BEA Catalysts. Catal. Lett. 2021, 151, 3381–3395. [Google Scholar] [CrossRef]
  28. Papeta, O.P.; Zubkov, I.N.; Kataria, Y.V.; Saliev, A.N.; Svetogorov, R.D.; Agliullin, M.R.; Chemes, A.A.; Savost’yanov, A.P.; Yakovenko, R.E. Deactivation of Hybrid Cobalt Catalyst Based on Hierarchical Porous Zeolite in Fischer–Tropsch Synthesis. Energy Fuels 2025, 28, 134–156. [Google Scholar] [CrossRef]
  29. Chen, H.; Wang, H.; Chen, Z.; Zhao, H.; Geng, L.; Gao, N.; Yi, C.; Xu, H. Research progress on the spray, combustion and emission of polyoxymethylene dimethyl ethers as a diesel blend fuel: A review. Fuel 2022, 324, 124731. [Google Scholar] [CrossRef]
  30. Lang, Q.; Zhang, B.; Liu, Z.; Chen, Z.; Xia, Y.; Li, D.; Ma, J.; Gai, C. Cohydrothermal Carbonization of Corn Stalk and Swine Manure: Combustion Behavior of Hydrochar by Thermogravimetric Analysis. Bioresour. Technol. 2018, 271, 75–83. [Google Scholar] [CrossRef]
  31. Wang, J.; Ren, D.; Zhang, N.; Lang, J.; Du, Y.; He, W.; Norinaga, K.; Huo, Z. Boosting in-situ hydrodeoxygenation of fatty acids over a fine and oxygen-vacancy-rich NiAl catalyst. Renew. Energy 2023, 202, 952–960. [Google Scholar] [CrossRef]
  32. Zuo, Q.; Tang, Y.; Zhu, G.; Wei, K.; Guan, Q.; Zhang, B.; Shen, Z. Investigations on the soot combustion performance enhancement of a catalytic gasoline particulate filter in equilibrium state for reducing the BSFC of gasoline direct injection engine. Fuel 2021, 284, 119032. [Google Scholar] [CrossRef]
  33. Huang, B.; Hong, W.; Shao, K.; Wu, H. Sensitivity Analysis Study of Engine Control Parameters on Sustainable Engine Performance. Sustainability 2024, 16, 11107. [Google Scholar] [CrossRef]
  34. Ferreira, J.; Andrade, D.I.; Fuziki, M.E.K.; de Almeida, L.N.; Colpini, L.M.; Lenzi, G.G.; Tusset, A.M. Catalytic Systems in the Reduction of Nitrogen Oxide Emissions in Diesel-Powered Trucks. Sustainability 2022, 14, 6662. [Google Scholar] [CrossRef]
  35. AAtzl, B.; Pupp, M.; Rupprich, M. The Use of Photocatalysis and Titanium Dioxide on Diesel Exhaust Fumes for NOx Reduction. Sustainability 2018, 10, 4031. [Google Scholar] [CrossRef]
Sustainability 17 06354 g001
Figure 1. Original emissions and conversion of CO, HC, and NO.
Figure 1. Original emissions and conversion of CO, HC, and NO.
Sustainability 17 06354 g001
Sustainability 17 06354 g002
Figure 2. Fuel consumption rate, DOC and CDPF pressure drop, and DOC temperature change.
Figure 2. Fuel consumption rate, DOC and CDPF pressure drop, and DOC temperature change.
Sustainability 17 06354 g002
Sustainability 17 06354 g003
Figure 3. Original emissions and reduction ratio of CO, HC, and NO2/NOx.
Figure 3. Original emissions and reduction ratio of CO, HC, and NO2/NOx.
Sustainability 17 06354 g003
Sustainability 17 06354 g004
Figure 4. NO2/NOx ratio after DOC and CDPF.
Figure 4. NO2/NOx ratio after DOC and CDPF.
Sustainability 17 06354 g004
Sustainability 17 06354 g005
Figure 5. Reduction ratio of NOx after SCR.
Figure 5. Reduction ratio of NOx after SCR.
Sustainability 17 06354 g005
Sustainability 17 06354 g006
Figure 6. Variation trend of temperature, pressure, and NO2/NOx of 2000 r·min–1 with 75% load.
Figure 6. Variation trend of temperature, pressure, and NO2/NOx of 2000 r·min–1 with 75% load.
Sustainability 17 06354 g006
Table 1. Specifications of test engine.
Table 1. Specifications of test engine.
PropertyNumerical Value
Rating power77 kW
Rating speed3500 r·min−1
Maximum torque speed2000 r·min−1
Bore × Stroke80 × 98 mm
Maximum torque260 N·m
Compression ratio18
Table 2. Specifications of DOC.
Table 2. Specifications of DOC.
PropertyNumerical Value
Cell density300 cpsi
Carrier diameter145 mm
Carrier length120 mm
Wall thickness4 mm
Pore diameter1~10 um
Catalytic washcoatγ-Al2O3
Carrier materialCordierite
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Zou, Y.; Xiao, L. DOC Study on the Effects of Catalyst Active Component Loading and Carrier Properties on the Catalytic Conversion Efficiency of Key Gaseous Pollutants. Sustainability 2025, 17, 6354. https://doi.org/10.3390/su17146354

AMA Style

Zou Y, Xiao L. DOC Study on the Effects of Catalyst Active Component Loading and Carrier Properties on the Catalytic Conversion Efficiency of Key Gaseous Pollutants. Sustainability. 2025; 17(14):6354. https://doi.org/10.3390/su17146354

Chicago/Turabian Style

Zou, Yantao, and Liguang Xiao. 2025. "DOC Study on the Effects of Catalyst Active Component Loading and Carrier Properties on the Catalytic Conversion Efficiency of Key Gaseous Pollutants" Sustainability 17, no. 14: 6354. https://doi.org/10.3390/su17146354

APA Style

Zou, Y., & Xiao, L. (2025). DOC Study on the Effects of Catalyst Active Component Loading and Carrier Properties on the Catalytic Conversion Efficiency of Key Gaseous Pollutants. Sustainability, 17(14), 6354. https://doi.org/10.3390/su17146354

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop