Study on the Effect of Precious Metal Loading and Pt/Pd Ratio on Gaseous Pollutant Emissions from Diesel Engines

Abstract

This study systematically investigated the influence of catalyst formulation parameters (precious metal loading and Pt/Pd ratio) in diesel oxidation catalysts (DOCs)+catalyzed diesel particulate filter (CDPF)+selective catalytic reduction (SCR) on gaseous pollutant emissions from diesel engines. Results indicate that under varying conditions, the impact of catalyst formulation on DOC system performance—such as temperature rise characteristics, pressure drop, and brake specific fuel consumption (BSFC)—remains limited. Notably, exhaust temperature exerts a decisive influence on carbon monoxide (CO) and hydrocarbon (HC) conversion efficiency, significantly outweighing the impact of exhaust flow rate. Increasing precious metal loading and Pt proportion markedly optimizes CO and HC ignition characteristics by lowering ignition temperatures. However, under high-load conditions, conversion efficiencies across different catalyst formulations tend to converge. Specifically, under low-load conditions, a competitive adsorption mechanism between CO and HC causes HC conversion efficiency to exhibit an inverse trend relative to CO. Furthermore, higher precious metal loading and Pt content significantly enhance the catalyst’s NO₂ formation capacity at equilibrium temperatures, while higher Pd content contributes to improved thermal stability. Higher precious metal loading and Pt content increase nitrogen oxides (NO_x) conversion efficiency. CDPF possesses the ability to further oxidize NO.

1. Introduction

As global environmental regulations become increasingly stringent, diesel engine emission control technology has emerged as a research hotspot. Diesel engines, serving as highly efficient power sources, are widely used in transportation, construction machinery, and other fields. However, the issue of gaseous pollutant emissions—including NO_x, HC, and CO—remains an urgent challenge requiring resolution [1]. With the implementation of China’s National VI and other diesel vehicle emission regulations, unprecedentedly stringent requirements have been imposed on the control of gaseous pollutant emissions. These regulations not only limit instantaneous pollutant concentrations but also set explicit requirements for the average conversion efficiency during actual on-road vehicle operation. This regulatory trend compels the industry to seek the optimal balance between catalytic performance, cost, and durability within complex aftertreatment systems. However, the existing literature lacks a systematic correlation between catalyst formulations and their full-life-cycle economic benefits, directly impacting the precise design and cost control of catalyst technologies in industrial applications.

Catalyst formulation is the core factor determining DOC performance, encompassing the selection and ratio of active components, support materials, and co-catalysts. Different formulation designs directly impact the catalyst’s activity, selectivity, and stability, thereby influencing the purification efficiency of gaseous pollutants from diesel engines [2]. Precious metals constitute the core cost component of catalysts, and increasing their loading directly leads to near-linear growth in material costs. In industrial practice, the optimal formulation does not pursue the highest-performance loading, but rather the minimum effective loading that meets emission regulations while maintaining necessary safety margins. From a durability perspective, while higher precious metal loading provides a greater reserve of active sites to compensate for gradual deactivation during long-term operation, its core challenge lies in thermal aging stability. Notably, highly active formulations often exhibit thermal stability disadvantages, potentially causing their initial performance advantages to rapidly diminish after high-temperature aging. Consequently, final catalyst formulation selection represents a classic multi-objective optimization process. It requires integrating systematic aging experiments with total cost of ownership analysis to identify solutions that achieve the optimal balance between comprehensive performance and full lifecycle costs for specific application scenarios. In recent years, researchers have continuously enhanced DOC catalytic performance through optimizing precious metal loading, improving support structures, and adding rare earth elements [3].

Zou investigated the effects of DOC formulations on diesel engine emission systems. Key findings include: DOC formulations do not affect fuel economy. Increasing precious metal loading and Pd proportion enhances CO and HC conversion rates at low temperatures. NO conversion rates exhibit greater sensitivity to Pt content. High NO₂ concentrations at low temperatures favor SCR efficiency, while SCR efficiency consistently exceeds 90% at high temperatures. DOC formulations have no significant impact on CDPF pressure drop [4]. Zhang evaluated the impact of the DOC+CDPF+SCR system on diesel engine emissions. The combination of DOC and CDPF significantly reduced emissions of CO, HC, and particulate matter. SCR effectively reduced NO_x but increased N₂O emissions and NH₃ slip. Reducing the SCR size by one-third maintained high NO_x conversion efficiency, though NH₃ slip increased. ASC coating eliminates NH₃ slip. Overall, a miniaturized SCR system with DOC+CDPF and Ammonia Slip Catalyst (ASC) coating represents the most cost-effective option [5]. Zhang investigated the combined effects of DOC and CDPF on emissions from non-road diesel engines. Key findings: The aftertreatment system did not affect engine power or fuel economy. DOC’s CO and HC reduction efficiency increased with load, showing even better results when combined with CDPF. Increasing the catalyst loading yielded a more significant reduction in HC. The DOC increased the NO₂ proportion, and its combination with the CDPF had varying effects on NO₂ at different temperatures. The DOC and CDPF exhibited opposite effects on reducing different types of particulate matter and significantly influenced the particle size distribution [6]. Hu evaluated the application effectiveness of an integrated DOC+CDPF+SCR aftertreatment system on non-road diesel engines. The system minimally impacted engine performance while significantly reducing emissions of HC, CO, NO_x, particulate matter (PM), and particulate number (PN), with all metrics falling below China’s Non-Road Stage IV emission standards. DOC and CDPF synergistically achieved efficient removal of HC and CO. Although the NO/NO₂ ratio fluctuated under transient conditions, the overall system demonstrated excellent stability and high efficiency [7]. He’s team designed and synthesized a Pt/Bi-doped YMnO composite catalytic material for the catalytic conversion of carbon dioxide. Under complex conditions involving multiple diesel exhaust components, this catalytic system demonstrated outstanding low-temperature catalytic performance, enabling complete conversion of carbon monoxide at a low temperature of 210 °C. The study revealed that the incorporation of bismuth promotes the formation of a unique Pt-O-Bi structure. This structure facilitates unidirectional electron flow, significantly accelerating the oxidation of carbon monoxide at low temperatures while effectively mitigating competitive adsorption phenomena among carbon monoxide, hydrocarbons, and nitrogen oxides. This research provides an innovative solution to the challenge of competitive oxidation reactions at key active sites within multi-component exhaust systems [8]. Zhang investigated the critical role of DOC in diesel engine aftertreatment systems, covering its operating principles, preparation methods, development trends, and strategies for countering SO₂ poisoning. DOC not only effectively oxidizes HC and CO pollutants but also converts NO to NO₂, thereby enhancing CDPF regeneration efficiency and NO_x conversion efficiency in SCR systems. The paper specifically highlights the detrimental effects of SO₂ on DOC performance and proposes methods to mitigate poisoning and develop SO₂-resistant catalysts. These research findings provide important references for optimizing future diesel engine emission control technologies and the rational selection of DOCs [9]. Li et al. systematically studied the impact of DOC on particulate matter microstructure via engine bench tests and advanced analytical techniques. The results demonstrated that DOC treatment triggered a series of distinct microstructural alterations: decreased average particle size, thinner interfacial layers, reduced fractal dimension, and increased specific surface area [10].

Addressing two major limitations in prior research—the lack of systematic investigation across a wide range of Pt/Pd ratios and loading combinations, and insufficient understanding of catalyst dynamic response patterns under full operating conditions—this study constructed a series of catalysts with precise compositions. By conducting full-condition performance evaluations within an integrated aftertreatment system it uncovered key reaction mechanisms and provides scientific justification for achieving cost-optimal catalyst design in industrial applications.

This study systematically investigated the influence of DOC formulations on gaseous pollutant emissions from diesel engines. The literature indicates that the selection and ratio of active components, the characteristics of the support material, and the addition of co-catalysts play a decisive role in catalyst performance. The optimized catalyst formulation significantly enhances conversion efficiencies for HC, CO, and NO_x while simultaneously improving low-temperature activity and high-temperature stability. The findings not only reveal intrinsic correlations between catalyst formulations and pollutant emissions, but also provide crucial references for developing highly efficient and economical DOC technologies. This research holds positive implications for advancing diesel engine emission control technologies.

2. Results and Discussion

2.1. Effects on DOC Temperature Change, DOC Pressure Change, and BSFC

Figure 1 illustrates the influence of the characteristics of catalyst composition on DOC temperature difference, pressure drop, and BSFC under different operating conditions at a constant rotational speed of 1500 r·min⁻¹. As shown in Figure 1a, the DOC temperature difference exhibits a gradual upward trend with increasing load. However, the DOC temperature difference for various catalyst formulations remained largely stable around 8 °C under different operating conditions. Furthermore, variations in precious metal loading and Pt:Pd ratio did not exhibit a clear pattern in influencing the DOC temperature difference. Shown in Figure 1b, the DOC pressure drop increases with engine load, peaking at approximately 3 kPa under full load. Notably, the variation among different catalyst formulations was minimal, consistently within a 1.5 kPa range. The results indicate that the DOC pressure drop is predominantly governed by engine operating conditions, with no significant correlation to catalyst composition [11]. Figure 1c reveals that the trend of BSFC with increasing load exhibits an inverse relationship to the patterns of DOC temperature difference and pressure drop. The most significant decrease in BSFC occurs during the load increase from 0% to 10%. When the load reaches 20%, BSFC drops below 220 g/(kW·h) and remains relatively stable thereafter. Additionally, variations in precious metal loading and Pt:Pd ratio exhibit no statistically significant effect on BSFC [12]. It is worth noting that the “load” in the figure refers to the “engine load”.

Figure 2 shows the effects of catalyst formulations on DOC temperature difference, pressure difference, and BSFC under external characteristic conditions. As shown in Figure 2a, with increasing load, the DOC temperature difference first rises and then decreases. The DOC temperature difference is not significantly affected by the precious metal loading or the Pt/Pd ratio. As shown in Figure 2b, as the load increases, the pressure difference across the DOC shows a gradual increase, with the increase generally within 1 kPa. As shown in Figure 2c, with increasing load, the BSFC differences among the formulations are negligible. In summary, under external characteristic conditions, it can be observed that the precious metal loading and Pt/Pd ratio have no significant effect on the DOC temperature difference, DOC pressure, or BSFC.

2.2. Effects on CO and HC Emissions

Figure 3 illustrates the relationship between exhaust temperature and the reduction rates of CO and HC pollutants. The research findings indicate that the reduction rates of CO and HC are primarily governed by the parameter of exhaust temperature. Specifically, when the exhaust temperature reaches approximately 160 °C, the CO concentration significantly decreases to near-zero levels; whereas when the temperature rises to 240 °C, the HC concentration substantially drops to around 10 ppm. This phenomenon can be attributed to a competitive adsorption mechanism between CO and HC on the catalyst surface under low-temperature conditions, where CO gains dominance due to its stronger reducing properties [13,14]. Subsequent research will focus on investigating the influence of patterns of catalyst composition variations on CO and HC conversion performance under different operating conditions.

Figure 4 illustrates the effect of catalyst formulations on CO conversion efficiency under different loading conditions. It can be observed that CO conversion efficiency increases with rising loading. At 0% loading, catalysts with different DOC formulations all exhibit varying degrees of improvement in CO conversion efficiency, generally ranging from 25% to 45%. As the precious metal loading increases, the low-temperature CO conversion efficiency of DOC shows a marked enhancement. Notably, catalysts with a platinum-palladium ratio of 8:1 and pure platinum (1:0) exhibit high CO conversion efficiencies, with the pure platinum catalyst achieving the highest conversion rate. This indicates that the pure platinum catalyst demonstrates optimal performance for low-temperature CO reduction [15]. At a 10% loading condition, since the exhaust temperature exceeded the CO ignition temperature across all DOCs, all catalyst formulations achieved CO conversion rates exceeding 95%.

Figure 5 illustrates the impact of catalyst formulations on HC reduction efficiency under varying loading conditions. Comparative analysis of Figure 3 and Figure 4 reveals that at lower exhaust temperatures, HC conversion efficiency is significantly influenced by CO reduction performance. Precious metal loading levels and composition schemes that effectively lower CO ignition temperatures and enhance low-temperature reduction performance show limited improvement in HC emissions during idle conditions [16]. However, once exhaust temperatures exceed CO ignition temperatures, these optimized schemes exhibit HC reduction effects comparable to their CO reduction performance. This phenomenon can be attributed to competitive adsorption between CO and HC on the catalyst surface at low exhaust temperatures, coupled with CO’s stronger reducing properties, causing fluctuations in HC conversion efficiency under low-load conditions. Comprehensive studies indicate that higher precious metal loading and platinum content ratios are more conducive to promoting the oxidation decomposition reactions of both CO and HC during low-load operation [17]. Under low-temperature conditions, CO molecules preferentially occupy Pt active sites, forming a surface coverage layer that significantly inhibits the HC oxidation reaction pathway. While pure Pt catalysts exhibit outstanding CO oxidation activity, their strong adsorption characteristics readily lead to reduced HC conversion efficiency in real mixed exhaust gas environments. Introducing Pd components to construct Pt-Pd bimetallic systems effectively modulates the adsorption properties of active sites, alleviating site blocking caused by competitive adsorption and thereby broadening the catalyst’s effective operating window [18]. Research findings indicate that achieving precise control over surface adsorption behavior through balanced Pt/Pd ratios and synergistic design of promoters and supports is crucial for developing high-performance DOCs adapted to complex operating conditions. This study provides important theoretical foundations and design directions for optimizing catalyst formulations for practical applications.

2.3. Effect on NO_x Emissions

Figure 6 illustrates the influence of DOC composition on the NO₂/NO_x ratio under different operating conditions. Figure 6a shows the effect of different loads on the NO₂ proportion at 1500 r·min⁻¹. It can be observed that the NO₂ proportion initially increases with load and then decreases, reaching a maximum at 50% load. This phenomenon can be attributed to the DOC inlet temperature being approximately 350 °C at 50% load, which is close to the thermodynamic equilibrium point for NO oxidation to NO₂ [19]. Continued increases in exhaust temperature, however, inhibit the conversion of NO to NO₂. The study also found that NO conversion efficiency significantly improves with increased precious metal loading and a Higher proportion of Pt. Figure 6b shows the effect of different exhaust flow rates on the NO₂ proportion at 1500 r·min⁻¹. It can be seen that the NO₂ proportion reaches its peak at 800 r·min⁻¹. This is primarily because the DOC inlet temperature remains around 350 °C under this operating condition, and the space velocity is significantly lower than other operating conditions, creating a favorable environment for NO₂ formation [20]. As the rotational speed increases, the NO₂ proportion fluctuates under the combined influence of exhaust temperature and space velocity. Across the entire rotational speed range, the DOC sample with a precious metal loading of 1.75 kg/m³ demonstrated optimal NO conversion performance. When the Pt:Pd ratio was 1:0, the DOC achieved its best NO conversion capability, with an average post-DOC NO₂ proportion of 50%. Although the pure Pt catalyst DOC demonstrated outstanding NO oxidation activity, its NO oxidation performance at the highest exhaust temperature operating points was lower than that of the 4:1 ratio catalyst. The performance gap with the 8:1 ratio catalyst also narrowed significantly, indicating that the introduction of the Pd component may enhance the thermal stability of the NO oxidation process [21].

Figure 7 illustrates the NO₂ Proportion After DOC and CDPF for Different Schemes at 1500 r·min⁻¹ and 80% Load. As exhaust gases pass through the CDPF, the proportion of NO₂ consistently shows a further increasing trend. At the CDPF outlet, the differences in NO₂ proportions produced by different catalyst formulations are relatively minor. This phenomenon highlights the critical role of the CDPF within the entire aftertreatment system. Comparing NO₂ concentration data at the CDPF inlet and outlet under different formulations reveals that while higher DOC, precious metal loading, or platinum content effectively increases the initial NO₂ concentration at the CDPF inlet, the overall NO₂ generation efficiency of the system does not significantly improve [22]. This confirms that the catalytic coating within the CDPF possesses a significant capacity for sustained NO oxidation, enabling it to synergize with the DOC in maintaining NO₂ concentrations at the SCR system inlet at an optimal level [23]. Therefore, in practical engineering applications, there is no need to excessively increase the precious metal loading in the DOC section to pursue a higher NO₂ proportion, providing clear justification for cost optimization in the aftertreatment system [24].

Figure 8 illustrates the impact of various DOC formulations on NO_x conversion efficiency following SCR. It can be observed that as the precious metal loading increases, the number of active sites within the DOC rises, enhancing the efficiency of NO oxidation to NO₂. The formation of NO₂ promotes subsequent SCR reactions or passive regeneration processes, thereby reducing net NO_x emissions. Higher precious metal loading enhances the catalyst’s oxygen storage capacity, which helps maintain redox reaction equilibrium and optimizes NO_x conversion conditions. Once loading reaches a certain threshold, active sites tend toward saturation, and further increases in loading yield diminishing returns for NO_x conversion. At this point, NO_x emissions stabilize and may even slightly increase due to reduced specific surface area caused by precious metal particle agglomeration. In DOCs, the Pt:Pd ratio significantly influences NO_x conversion efficiency. The Pt:Pd ratio of 4:1 typically exhibits optimal NO_x conversion performance due to its combination of Pt’s high NO oxidation activity and Pd’s synergistic promotion effect. In contrast, a Pt:Pd ratio of 8:1, despite its higher Pt content, may yield limited performance improvements due to insufficient Pd co-catalytic effects. Pure Pt catalysts (1:0) typically exhibit the lowest NO_x conversion efficiency, owing to the lack of Pd’s thermal stability and synergistic effects [25]. Therefore, a rational Pt:Pd ratio is crucial for optimizing DOC catalytic performance.

3. Experimental Design

3.1. Equipment Parameters

The test utilized a heavy-duty electronically controlled high-pressure common rail turbocharged diesel engine, with its basic parameters listed in

Table 1. Key Technical Parameters.

Property	Numerical Value
Displacement	8/L
Rating speed	2400 r·min−1
Maximum torque speed	2000 r·min−1
Rated Power	187 kW
Maximum torque	1345 N·m
Compression ratio	18.5

. An AVLG44 dynamometer(AVL List GmbH, Graz, Austria) was employed for testing, while a HORIBA-MEXA7500D gas analyzer(HORIBA Group, Kyoto City, Japan) conducted real-time measurements of gaseous pollutants. A K-type thermocouple(Jiangsu Huajiang Automation Instrument Co., Ltd., Huai’an City, China) was used to measure the DOC inlet temperature. Three sampling points were established within the aftertreatment system: pre-DOC (Point 1), after-DOC (Point 2), after-CDPF (Point 3)and after-SCR (Point 4). The test equipment and measurement point layout are shown in Figure 9. The post-treatment system under testing consists of DOC + CDPF + SCR. Temperature, backpressure, and pollutant emissions were measured at each sampling point. Key technical parameters of the diesel engine are detailed in Table 1.

3.2. Experimental Design

This study was designed to create five experimental groups, systematically investigating the effects of noble metal ratios and loading on DOC performance. The diesel fuel used complies with China VI standards, with sulfur content below 10 ppm (mass fraction). The primary focus was on Pt:Pd ratios of 4:1, 8:1 (No.4), and 1:0 (No.5), evaluating their impact on low-temperature emission reduction, thermal stability, and durability. Additionally, three loading levels—0.88 (No.1), 1.32 (No.2), and 1.75 kg/m³ (No.3) (with Pt:Pd = 4:1 maintained constant)—were employed to focus on low-temperature emission reduction performance. The overall objective is to optimize the noble metal ratio and loading to enhance the comprehensive performance of the DOC. For the convenience of subsequent discussion, the five catalyst formulations are ranked as indicated in parentheses. Test conditions are as follows: 0%, 10%, 20%, 40%, 60%, 80%, and 100% load conditions at 1500 r·min⁻¹ were used to study the effects of different formulations at varying exhaust temperatures. An 80% load condition between 800 and 2400 r·min⁻¹ was employed to investigate the effects of different formulations at varying exhaust flow rates. All reported steady-state operating point data represent the average of at least three consecutive measurements taken after the engine reached stable operating conditions. Repeatability verification tests were conducted for critical operating points (e.g., ignition characteristics curves at different load rates). Results demonstrated a relative standard deviation of less than 5% for measured values, confirming the excellent repeatability of experimental data.

Table 2. Specifications of DOC.

Property	Numerical Value
Cell density	400 cpsi
Carrier diameter	304 mm
Carrier length	101 mm
Wall thickness	4 mm
Pore diameter	1~10 µm
Catalytic washcoat	γ-Al2O3
Carrier material	Cordierite

lists the key technical parameters of DOC.

Table 3. Specifications of CDPF.

Property	Numerical Value
Cell density	300 cpsi
Carrier material	Cordierite
Wall thickness	3 mm
Precious Metal Loading	0.65 kg/m3
Pt:Pd	4:1

lists the key technical parameters of CDPF.

Table 4. Specifications of SCR.

Property	Numerical Value
Cell density	500 cpsi
Active Component	Cu-SSZ-13
Wall thickness	3 mm
Washcoat	γ-Al2O3
Carrier material	Cordierite

lists the key technical parameters of SCR.

Table 5. Table 3 summarizes the resolution and uncertainty of the main instruments used in this study.

Property	Resolution	Uncertainty
Dynamometer (speed measurement)	1 rpm	±0.3%
Dynamometer (torque measurement)	0.01 N m	±0.2%
Flow meter sensor	0.01 g	±0.3%
Pressure transducer	0.01 MPa	±0.3%
Gas analyzer
CO measurement	0.01%	<0.2%
HC measurement	2 ppm	<0.2%
NOx measurement	1 ppm	<0.2%

summarizes the resolution and uncertainty of the main instruments used in this study.

4. Conclusions

This study investigates the effects of five different catalyst formulations on gaseous pollutants. Based on the experimental analysis, the following conclusions are drawn:

(1) Under different operating conditions, The precious metal loading and Pt content in the DOC formulation have a minor impact on the DOC temperature difference, DOC pressure difference, and BSFC.

(2) Increasing the precious metal loading and Pt proportion effectively lowers the ignition temperatures for CO and HC, by approximately 160 °C and 240° C, respectively. As loading increases, further raising precious metal loading and Pt proportion has negligible effects on CO and HC conversion rates. Due to competitive adsorption between HC and CO at low exhaust temperatures, the patterns exhibited by precious metal loading levels and ratios for less reducible HC differ from those observed for CO.

(3) Increasing precious metal loading and Pt proportion promotes NO₂ formation. At higher exhaust temperatures, increasing Pd proportion enhances catalyst thermal stability, thereby increasing NO₂ proportion. Higher precious metal loading and Pt proportion improve NO_x conversion efficiency.

From an industrial application perspective, the findings of this study provide direct guidance for cost optimization in catalyst formulations. While increasing Pt content and total precious metal loading enhances low-temperature activity, the market price of Pt is typically several times that of Pd. Therefore, a formulation strategy solely focused on catalytic performance would significantly elevate catalyst costs. The phenomenon identified in this study—that CDPFs possess sustained NO oxidation capability—is particularly significant. It implies that extreme NO₂ concentrations need not be pursued by excessively increasing Pt loading in the DOC section, providing scientific justification for reducing overall system costs.

This study employed ultra-low sulfur diesel (S < 10 ppm) to minimize the acute poisoning effects of sulfur on catalyst performance. However, it must be noted that during long-term durability testing, even very low sulfur levels may gradually accumulate on the catalyst surface. This accumulation could lead to physical coverage or chemical poisoning of active sites by sulfate species, particularly under low-temperature conditions. Such effects may reduce oxidation activity toward CO and HC, potentially impacting the ability to oxidize NO to NO₂. This study focuses on characterizing the performance of fresh catalysts. Future work will include evaluating performance after sulfur-containing aging to examine catalyst durability comprehensively. During high-load operation or regeneration of diesel engines, elevated temperatures cause sintering of precious metal particles and a sharp decline in specific surface area, resulting in irreversible loss of catalytic activity. Additionally, phase transitions in the support material may occur, further compromising its stability and catalytic performance. This study primarily focuses on the performance of fresh catalysts, with their behavior after long-term thermal aging remaining unclear. Subsequent studies will conduct accelerated aging tests on catalyst formulations to quantify performance degradation patterns under severe conditions. Concurrently, advanced characterization techniques (e.g., XRD, BET) will be used to analyze the physicochemical properties of aged catalysts, revealing deactivation mechanisms at atomic and molecular scales.