Investigation of Engine Exhaust Conversion and N₂O/NH₃ Generation on Pd-Based Catalyst

Abstract

Natural gas (NG) engine catalysts face unique challenges in emission control due to their distinct raw emission characteristics. This study investigates the exhaust conversion and by-product generation of a Palladium-based catalyst of an NG engine through small-sample catalyst experiments, mainly focusing on the effect of feed gas composition on the conversion efficiency and N₂O/NH₃ emissions. Results show that N₂O is generated via NO reduction by H₂ (80~275 °C) and CO (275~400 °C) in the temperature range of 80~400 °C. NH₃ generation occurs at 175~550 °C, mainly via NO reduction by H₂ (supplied from the water–gas shift (WGS) reaction) and CO below 425 °C and exclusively by H₂ (supplied from the steam reforming (SR) reaction) above 425 °C. An increase (0.9705~1.0176) in lambda enhances CO and CH₄ conversion while reducing N₂O and NH₃ emissions, but it inhibits NO conversion and promotes NO₂ formation. A lambda of 0.9941 achieves high conversion efficiency (≥90%) for CO, CH₄, and NO, with reduced N₂O and zero NH₃ emissions. An increase in H₂O (8~16%) accelerates the WGS and SR reactions, improving pollutant conversion. However, it aggravates N₂O and NH₃ emissions, with peak levels rising by 54% and 31%, respectively. Increased H₂ (500~1500 ppm) preferentially consumes NO and reversely shifts the equilibrium of the WGS and SR reactions, reducing CO and CH₄ conversion while improving NO conversion. And it promotes N₂O selectivity at high temperature and NH₃ selectivity at low temperature and peak emissions, with peak concentrations increasing by 58% and 15%, respectively. These findings reveal the by-product formation mechanism in the catalyst, providing valuable insights for the emission control of NG-fueled engines.

1. Introduction

The goals of carbon peaking and carbon neutrality introduce new emission limits across various sectors relying on carbon-based fuels. Motor vehicles, as a significant carbon-based fuel consumer and major source of emissions, face stringent challenges [1,2]. Traditional vehicular engines primarily rely on gasoline (C₄–C₁₂) and diesel (C₁₀–C₂₂) as fuels. Both of them produce substantial carbon emissions after fuel combustion or after-treatment of exhaust gas [3]. In comparison, natural gas (NG) emits relatively lower levels of carbon dioxide due to its high hydrogen-to-carbon ratio [4]. Additionally, the gaseous state of NG facilitates the formation of a more homogeneous air/fuel mixture, potentially reducing the production of incomplete oxidation products (such as carbon monoxide, total hydrocarbons, and particulate matter) [5]. Most importantly, the straightforward manufacturing process of NG results in a relatively low cost [6], making it more economically viable. Consequently, NG is a promising engine fuel that can support carbon neutrality goals in the short term, particularly in countries and regions with abundant NG reserves [7].

Past NG engines partially adopted selective catalytic reduction as the after-treatment system, which exposed a risk that the NOx emission could become uncontrolled under real driving conditions [8]. Therefore, the current China-VI emission regulation mandates stoichiometric combustion and a three-way catalyst (TWC) as the exclusive technical route for controlling NG engine exhaust emissions [9]. A TWC achieves an efficient conversion for regulated pollutants (including CO, HC, and NOx), with NOx serving as an oxidant and CO and HC as reductants [10]. However, its performance is influenced by the type of noble metal applied within the honeycomb carrier [11,12,13]. Among the platinum group metal (including Pt, Pd, Rh)-based catalysts, Pd stands out due to its relatively low price [14], excellent thermal stability, wide operating window, and efficient catalytic performance nearly equivalent to that of other metal-based catalysts [15,16]. This makes Pd-based catalysts one of the best choices for NG engines [17,18].

However, the operation of TWCs inevitably involves some side reactions, leading to the post-catalyst emission of secondary pollutants, including N₂O and NH₃ [19,20]. N₂O, a potent greenhouse gas, has a global warming potential approximately 298 times as high as that of CO₂ [21]. NH₃, as a precursor of the ammonia ions, can react with sulfuric acid and nitric acid to generate secondary particulates, further aggravating urban secondary aerosols [22]. Additionally, the deposition of ammonium salts can lead to water eutrophication and soil acidification, severely affecting the ecological environment [23]. Emission inventory data from the Beijing–Tianjin–Hebei region from 2008 to 2020 revealed that traffic-related ammonia emissions are continuously increasing, while emissions from other sources are decreasing [24]. Similarly, ammonia emissions from motor vehicles have surpassed those from other sources in some densely populated areas in the United States [25]. Current emission regulations have established limits for N₂O emissions from light-duty vehicles and NH₃ emissions from heavy-duty vehicles [26], which highlights the necessity of studying post-catalyst emissions of N₂O and NH₃ from NG engines.

Emissions of N₂O and NH₃ from NG engines have been partially documented in previous studies. Zhang et al. [27] investigated the NH₃ emission from NG engines under different operating conditions. They found that NH₃ emissions under steady-state cycles were primarily influenced by raw NOx and CO emissions, whereas under transit cycles, they were affected by the O₂ induced during the motoring process. Zhu et al. [28] evaluated the emission characteristics of NG engines across various test cycles and observed that N₂O and NH₃ emissions were significantly influenced by the cold-start fraction of the test cycle, with concentrations being higher during cold-start tests compared to hot-start tests. Li et al. [29] further concluded that the peak emission of N₂O mainly appears at a low-temperature stage of the cycle, while the peak emission of NH₃ mainly appears at a relatively high-temperature stage. These studies have basically revealed the macro-level emission characteristics of N₂O and NH₃ from NG engines.

To explore the formation characteristics of N₂O and NH₃ in TWCs, several researchers have conducted detailed studies based on the catalyst experiments. Zheng et al. [30] experimentally investigated the generation of N₂O and NH₃ in different noble metal (Pd, Rh, and Pd/Rh)-based catalysts used in gasoline engines. Their findings revealed that the threshold temperatures for N₂O and NH₃ formation are closely correlated with the light-off temperature of NOx. Among the tested catalysts, Rh-based catalysts exhibited the highest N₂O emissions, while Pd-based catalysts showed the highest NH₃ emissions. Building on this, Wang et al. [31] explored the generation pathways of N₂O and NH₃ in Pd/Rh-based catalysts for gasoline engines. They found that N₂O was predominantly formed through NO reduction by H₂ or CO at low temperatures (below 240 °C), whereas NH₃ formation was mainly driven by NO reduction by H₂ at higher temperatures (250~550 °C). Wang et al. [32] further examined the sources of H₂ required for NH₃ formation. They identified two pathways: (1) the reaction between CO and the terminal hydroxyl or bridging hydroxyl on the catalyst surface at low temperatures and (2) high-temperature reactions, including the water–gas shift (WGS) reaction between CO and H₂O and the steam reforming (SR) of HC and H₂O. These studies have partially revealed the formation mechanism of by-products of TWCs, but all of them have only reflected the catalyst performance of gasoline engines. Because the simulated exhaust gas in these past studies mainly adopted C₃H₆ and C₃H₈ as THCs and the test temperature was relatively low, these studies could not accurately represent the emission characteristics of NG engines. Compared to gasoline engines, NG engines typically operate at higher exhaust temperatures and exhibit distinct exhaust compositions, including elevated concentrations of CH₄ and H₂O. The variation in exhaust temperature could affect the light-off characteristics of catalysts, while CH₄ is always considered a challenge for NG engine exhaust emission control due to its stable characteristics.

Some researchers studied the formation characteristics of by-products of TWCs in NG engine exhaust atmosphere. Qian et al. [16] used simulations to analyze the generation pathways of N₂O and NH₃ in Pd-based catalysts for NG engines. They found that N₂O and NH₃ were generated at nearly the same temperatures, which is inconsistent with the emission characteristics of an NG engine under the transient cycle [29]. Nevalainen et al. [33] studied the effect of the simulated NG engine exhaust gas composition on N₂O and NH₃ formation under several constant high-temperature points (440~530 °C). Results demonstrated that reducing NOₓ concentrations effectively decreased both N₂O and NH₃ emissions, while reducing CH₄ concentrations specifically lowered NH₃ emissions. Although these studies provide partial insights into the formation mechanisms of N₂O and NH₃ from NG engine catalysts, it can be found that a study based on simulations cannot comprehensively replicate the working process of a TWC, leading to a remarkable discrepancy between simulation results and actual emissions. And a limited number of experimental studies are only conducted under a partial constant temperature point, which cannot reflect the TWC performance and by-product formation characteristics under the whole operating conditions of an NG engine.

Therefore, this study conducted a series of experiments to evaluate the catalytic performance of TWCs and investigate the generation characteristics of by-products, with a focus on a high CH₄ concentration in feed gas and an extensive working temperature window for the catalyst, more closely resembling realistic NG engine exhaust compared to past studies. And a new-type quantum cascade laser was used to record the real-time exhaust pollutants; it had a more rapid response and higher specified component selectivity and sensitivity compared to the traditional FTIR and GC used in past studies. All of these features of this study’s design were selected with the aim of understanding the catalytic characteristics of NG TWCs more accurately.

2. Methodology

This study analyzed the catalytic performance of a TWC and the generation characteristics of by-product pollutants (N₂O and NH₃) through a series of small-sample catalyst experiments. Firstly, the conversion efficiency of a TWC for three regulated pollutants (including CO, CH₄, and NO) was measured on the self-built small-sample catalyst experiment bench, and the effects of catalyst temperature and feed gas composition on TWC performance were further analyzed. Subsequently, following the curves of reactant conversion and by-product generation under the specified conditions, the generation pathways of N₂O and NH₃ were analyzed. Finally, the concentrations of N₂O and NH₃ under different catalyst temperatures and feed gas compositions were measured to analyze the effect of the temperature and feed gas composition on the generation of by-products.

2.1. Small-Sample Catalyst

The catalyst used for this study was obtained from Kunming Sino-Platinum Metals Co. Ltd., Kunming, China; its detailed specifications are shown in

Table 1. Detailed specifications of TWC.

Parameter	Value	Unit
Diameter × length	118.4 × 100	mm
Mesh number	750	/
Precious metal	Pd	/
Washcoat	γ-Al2O3, Ce0.5Zr0.5O2	/
Support	Cordierite	/

. A small-sample catalyst, with a length of 30 mm and a diameter of 15 mm, was cut from the full-size honeycomb TWC and used for the subsequent specific experimental test.

2.2. Feed Gas

The feed gas was designed to simulate NG engine exhaust gas. The major distinction between NG engine and gasoline engine exhaust gas lies in the CH₄ concentration in THC emission, which accounts for about 90% of THCs in NG engine exhaust, compared to only 40% in gasoline exhaust [5]. Secondly, NG engine exhaust typically contains a lower carbon oxide concentration and a higher H₂O concentration compared to gasoline exhaust. Therefore, this study adopted CH₄ as the exclusive HC component in the feed gas to characterize the exhaust characteristics of an NG engine. The other gas composition and the corresponding concentrations were determined by partially referring to values reported in the literature [16,33] as well as the detection range of the measurement equipment, with the final feed gas of CO₂ 8%, CO 4500 ppm, CH₄ 1500 ppm, NO 1000 ppm, O₂ 0~7000 ppm, H₂ 0~1500 ppm, H₂O 0%~16%.

2.3. Design of Experiment

The measurements of composition concentration during the TWC working process were conducted on a self-built small-sample catalyst experimental bench, with the detailed test arrangement shown in Figure 1.

As can be seen, the experimental bench mainly includes 1, feed gas; 2, volume flowmeter; 3, liquid water; 4, water pump; 5, premix tank; 6, heating furnace; 7, quartz tube; 8, thermocouple; 9, small-sample catalyst; 10, sampling probe; 11, emission measurement system; 12, host computer; 13, ECU; 14, heating belt. Both the gas flowmeters connecting with seven gases and the water pump connecting with the liquid water are connected to the premix tank (which is heated to 200 °C), which can achieve vaporization of the liquid water and the premix between the gas compositions and water vapor, further forming the feed gas with the specified composition, concentration, and space velocity (30,000 1/h), with the lambda of the feed gas being defined using Formula (1) [5,15]. The premix tank is connected to the heating furnace (which can be maximally heated to 950 °C). And the pipe between the premix tank and heating furnace is covered by the heating belt (which maintains a temperature of 150 °C), aiming to avoid the condensation of water vapor. The target temperatures of the premix tank, heating furnace, and heating belt are set by the ECU. A thermocouple is installed on the heating furnace to detect the real-time temperature of the furnace. A quartz tube, as the reactor of the catalyst, is embedded in the furnace. The small-sample catalyst is placed at the middle of the quartz tube, with the gap between the catalyst and the quartz tube filled with quartz cotton. Another thermocouple is installed 2 cm from the catalyst to detect the real-time temperature of the catalyst. A quantum cascade laser (QCL) detector based on the Lambert–Beer law is used to measure the real-time composition concentration. For the QCL, the newest portable emission measurement technology of Horiba (Kyoto, Japan), a tuned laser is adopted as the light source, which provides high radiation energy and an accurate wavelength corresponding to the featured absorption of specific composition. In addition, a laser light source also reduces the interference associated with vibration, favoring reliable measurement at low concentrations [26]. The measured data is recorded in the host computer. The detailed specifications of all measuring equipment are listed in Table S1, which can be found in the Supplementary Materials.

$$\lambda ={\displaystyle \frac{{2O}_2+2{CO}_2+CO+NO+H_{2}O}{{2CO}_2+2CO+{4CH}_4+H_{2}O}}$$

(1)

where λ is the excess air ratio of the feed gas.

Before the measurement, the fresh small-sample catalyst needed to be preheated in a N₂ atmosphere at 500 °C for 30 min, with the aim of activating the catalyst. And the QCL detector needed to be calibrated, mainly including the calibration of the span and the checking of linearity; the calibrated results are shown in Figure S1, which can be found in the Supplementary Materials.

During the measurement, the premix tank, heating belt, and heating furnace were first heated to 200 °C, 150 °C, and 80 °C, respectively. The flowmeters and water pump were adjusted to form the specified feed gas, which flowed into the quartz tube. Subsequently, the feed gas was sampled at the inlet of the quartz tube, and the corresponding composition concentration was measured using the QCL detector. When the catalyst temperature reached the target value, the feed gas was sampled at the outlet of the quartz tube, and the corresponding concentration was measured using the QCL detector.

It is noted that each measurement was made for a specified gas composition and initiated when the catalyst temperature reached 80 °C, followed by a subsequent measurement at 100 °C, and thereafter, measurements were conducted in 25 °C increments up to 800 °C, with data being recorded at each temperature point. And the data recording did not begin until all composition concentrations reached a relatively steady state, with the recording process lasting at least 60 s and the corresponding average value being the final measured result.

2.4. Estimation of Catalytic Performance

The catalytic performance of the catalyst in this study was characterized by the conversion efficiency of three regulated pollutants (including CO, CH₄, and NO), which can be calculated using Formula (2).

$$\eta ={\displaystyle \frac{c_{in}-c_{out}}{c_{in}}}\times 100\%$$

(2)

where $\eta$ denotes the conversion efficiency of the pollutant; $c_{in}$ is the pollutant concentration at the inlet of the quartz tube, ppm; and $c_{out}$ is the pollutant concentration at the outlet of the quartz tube, ppm.

3. Results and Discussion

This section first studies the catalytic performance of a TWC under different temperatures and feed gas compositions. Subsequently, the generation pathways of N₂O and NH₃ are explored, and the effects of temperature and feed gas composition on the generation of N₂O and NH₃ are analyzed.

3.1. Conversion Efficiency of Regulated Pollutant

The conversion of regulated pollutants (including CO, CH₄, and NO) not only characterizes the catalytic performance of a TWC, but also affects the generation of by-products (N₂O and NH₃). Therefore, the conversion efficiency of three regulated pollutants under different feed gas compositions is studied before by-products are studied.

3.1.1. Effect of O₂ Concentration on Conversion Efficiency

NG engines with stochiometric combustion generally adopt the PID method to control the real air amount in the range near the theoretical value, which means there is always a certain proportion of O₂ included in the exhaust gas due to the fluctuation of the control signal. Theoretically speaking, a suitable amount of O₂ benefits the conversion of CO and CH₄, but excessive O₂ could inordinately consume reductants, further disturbing the original reaction pathway, which is unfavorable for NO conversion and possibly results in an increase in by-products. Therefore, the conversion efficiency of three regulated pollutants (including CO, CH₄, and NO) is studied under feed gas with different lambda values (from 0.9705 to 1.0176, with detailed composition and concentration being shown in

Table 2. Feed gas under different O₂ concentrations.

Lambda	O2 (ppm)	CO2 (%)	CO (ppm)	CH4 (ppm)	NO (ppm)	H2O (%)	H2 (ppm)
0.9705	1000	8	4500	1500	1000	8	-
0.9863	3000	8	4500	1500	1000	8	-
0.9941	4000	8	4500	1500	1000	8	-
1.0020	5000	8	4500	1500	1000	8	-
1.0176	7000	8	4500	1500	1000	8	-

), with the results being shown in Figure 2, Figure 3 and Figure 4.

As shown in Figure 2, CO conversion begins at 125~150 °C, but the efficiency remains low (≤15%) at temperatures under 200 °C due to the limited number of activated molecules, which results in a slow reaction rate. Interestingly, increasing the lambda of the feed gas slightly enhances CO conversion below 200 °C. As shown in Figure 4, NO conversion is also observed in this temperature range, suggesting that both O₂ and NO participate in the CO oxidation at low temperature. When the catalyst temperature exceeds 200 °C, the CO conversion efficiency sharply increases, reaching 100% at peak performance. The earliest peak is observed at 250 °C, while the latest occurs at 350 °C, depending on the lambda value. The sharp increase is attributed to the higher proportion of activated molecules at elevated temperatures, which accelerates the reaction rate. Additionally, high temperature likely triggers more reaction pathways, such as the WGS reaction, further consuming CO. The promotion effect of an increased lambda becomes more pronounced in this range. For instance, as the lambda increases from 0.9705 to 1.0176, the light-off temperature (where the conversion efficiency reaches 50%) decreases from 252 °C to 210 °C, and the T₉₀ (temperature for 90% conversion) decreases from 275 °C to 223 °C. At temperatures above 350 °C, CO conversion efficiency begins to decline under certain lambda conditions, coinciding with the onset of CH₄ conversion (as shown in Figure 3). The oxidation of CH₄ competes with CO for available O₂, reducing CO conversion efficiency at high temperatures. However, increasing the lambda mitigates this effect by ensuring sufficient O₂ supply for both CO and CH₄ oxidation. For example, when the lambda is increased to 0.9941, CO conversion efficiency remains above 90% even at high temperature due to the abundance of O₂ available for oxidation. These findings provide key insights for optimizing three-way catalyst (TWC) performance in real-world applications. In low-temperature scenarios, such as cold-start conditions, a slight increase in lambda can modestly enhance CO oxidation, potentially reducing cold-start emissions, which are a major contributor to overall vehicle emissions. Furthermore, lowering the light-off temperature and T₉₀ through precise lambda adjustments reduces the energy demand for catalyst activation, contributing to improved fuel efficiency and lower CO₂ emissions. Lastly, at high temperatures, maintaining an optimal lambda ensures efficient oxidation of both CO and CH₄.

As shown in Figure 3, the conversion temperature of CH₄ is significantly higher than that of CO, starting at approximately 350 °C. This is because there is a stable structure of a symmetric tetrahedron and carbon–hydrogen covalent bonds with high bond energy in the CH₄ molecule [34], resulting in a stable chemical property. Below 400 °C, CH₄ conversion efficiency remains low (≤10%), and a simultaneous decline in NO conversion efficiency is observed across most lambdas. This indicates that the CH₄ oxidation at this temperature primarily relies on O₂. When the catalyst temperature exceeds 425 °C, the CH₄ conversion efficiency sharply increases, reaching its peak. Under the partial lambda conditions (0.9863 and 0.9941), the rapid CH₄ oxidation is accompanied by a marked increase in NO conversion, suggesting that NO begins to participate in CH₄ oxidation. The effect of lambda on CH₄ conversion mainly reflects two aspects. At temperatures below 575 °C, the CH₄ conversion efficiency does not monotonously increase with lambda according to the T₅₀ and T₉₀ of CH₄ conversion, with the optimum conversion being achieved at a lambda of 0.9863, which means that excessive O₂ is not beneficial for CH₄ conversion under low temperature. This is because excessive O₂ could possibly inhibit CH₄ absorption by competitively absorbing on the active site of the catalyst and weaken the activation of CH₄ by oxidizing Pd to PdO. When the temperature reaches above 575 °C, a lower lambda value (0.9705) results in decreased CH₄ conversion efficiency. This phenomenon is likely due to the oxidation of NH₃, a by-product of the TWC process, at high temperatures. The oxidation of NH₃ competes with CH₄ for available O₂, thereby weakening CH₄ conversion. As lambda increases, this effect is progressively mitigated. This is because the additional O₂ supply becomes sufficient to simultaneously oxidize both NH₃ and CH₄, maintaining a consistently high CH₄ conversion efficiency (≥90%) at elevated temperatures. These findings imply that CH₄ oxidation has different requirements for lambda control at different temperatures.

As shown in Figure 4, NO conversion begins at approximately 125~150 °C, and the efficiency remains low below 200 °C. Given the absence of NO₂ generation within this temperature range (as shown in Figure 5), it can be concluded that NO primarily acts as an oxidant, reacting with CO below 200 °C. However, the presence of O₂ limits the oxidation effect of NO, and this effect is further weakened as lambda increases. With the increase in temperature, the NO conversion efficiency manifestly increases, reaching a peak at approximately 250 °C. A remarkable discrepancy observed between different lambdas is that the peak NO conversion efficiency reaches 100% under the lambda of 0.9705, whereas it does not exceed 20% under higher lambdas. Additionally, NO₂ generation is evident at higher lambda values, indicating that NO reacts with both CO and O₂ in this temperature range, serving primarily as an oxidant for CO under fuel-rich conditions and being directly oxidized by O₂ under lean combustion. Beyond 250 °C, NO conversion efficiency begins to decline, while NO₂ generation continues to increase, reaching its peak at approximately 325 °C. This phenomenon suggests that although NO can be directly oxidized to NO₂ under lean combustion, the reduction of NO through CO or other reductive substances (such as H₂) remains the dominant pathway for NO consumption. According to the CH₄ conversion characteristics presented in Figure 3, NO participates in CH₄ oxidation at temperatures above 400 °C, resulting in a secondary increase in NO conversion efficiency at higher temperatures. Furthermore, Figure 5 reveals that NO₂ concentration is positively correlated with lambda, while Figure 4 shows that the light-off temperature and peak NO conversion efficiency are negatively correlated with lambda at high temperatures. This indicates that NO conversion at elevated temperatures is still primarily driven by reactions with CH₄, CO, and other reductive substances, with a minor contribution from direct reactions between NO and O₂. These findings highlight the importance of balancing lambda values to optimize NO reduction efficiency while minimizing NO₂ emissions, particularly under high-temperature conditions.

3.1.2. Effect of H₂O Concentration on Conversion Efficiency

H₂O not only triggers the WGS reaction of CO and the SR reaction of CH₄, directly affecting the conversions of CO and CH₄, but also indirectly affects NO conversion through the H₂ generated from these two reactions. Given that there is usually a high H₂O concentration in NG engine exhaust, the effect of H₂O concentrations (including 0, 8%, and 16%, with detailed composition and concentration being shown in

Table 3. Feed gas under different H₂O concentrations.

H2O (%)	CO2 (%)	CO (ppm)	CH4 (ppm)	NO (ppm)	O2 (ppm)	H2 (ppm)
0	8	4500	1500	1000	-	-
8	8	4500	1500	1000	-	-
16	8	4500	1500	1000	-	-

) on the TWC conversion efficiency is studied under O₂-absent conditions, with the results being shown in Figure 6, Figure 7 and Figure 8.

As depicted in Figure 6, the maximum CO conversion efficiency under the H₂O-absent condition is only 32%, which is attributed to the limitation of the proportion of CO and NO in the feed gas. In comparison, the peak conversion efficiency of CO exceeds 90% under the condition with H₂O, indicating that the involvement of H₂O could substantially promote CO conversion through the WGS reaction, and the WGS reaction contributes a remarkably larger proportion of the total CO conversion under O₂-absent conditions, compared to the reaction between CO and NO. Furthermore, it seems that a further increase in H₂O concentration has a positive effect on CO conversion. When the H₂O concentration increases from 0 to 8%, the promotion effect of H₂O on CO conversion can be initially observed after 200 °C. When the H₂O concentration is further increased to 16%, the promotion effect of H₂O on CO conversion can be immediately observed above 150 °C, which means that the trigger temperature of the WGS reaction is possibly related to the H₂O concentration, with a value as low as 150 °C. With an increase in temperature, the CO conversion efficiency starts to decrease beyond 425 °C under conditions with H₂O, which means that the active WGS reaction mainly happens below 425 °C, with the corresponding reaction rate being inhibited by the reverse shift of the reaction equilibrium beyond 425 °C. It is noted that the CO conversion efficiency with 16% H₂O is higher than that with 8% H₂O in most of the temperature range. This is because the increased H₂O, as the reactant, could promote the forward shift of the WGS reaction equilibrium at low temperature (below 425 °C). And the increased H₂O, as the product, can inhibit the reverse shift of the WGS reaction equilibrium at high temperature (above 425 °C). These findings indicate that the H₂O included in the exhaust gas has a positive effect on CO conversion, and a high H₂O concentration is beneficial for the enlargement of this effect.

As shown in Figure 7, the peak conversion efficiency of CH₄ is no more than 80% under H₂O-absent conditions but reaches 100% under conditions with H₂O, indicating that the involvement of H₂O could promote CH₄ conversion by triggering the SR reaction. When the H₂O concentration is increased from 0 to 8%, the promotion effect of H₂O concentration initially appears at approximately 400 °C. When the H₂O concentration is further increased to 16%, the promotion effect of H₂O concentration is observed early, between 325 and 350 °C, indicating that the increased H₂O is beneficial for a decrease in the trigger temperature of the SR reaction. Furthermore, the CH₄ conversion efficiency with 16% H₂O remains higher than that with 8% H₂O below 600 °C, meaning that the increased H₂O could accelerate the SR reaction rate. However, there is no remarkable discrepancy for the peak conversion efficiency under conditions with H₂O, indicating that both 8% and 16% H₂O could meet the requirement of an optimum reaction equilibrium for the SR reaction. These findings indicate that the H₂O included in the exhaust gas has a positive effect on CH₄ conversion. The increased H₂O concentration only slightly accelerates the SR reaction rate but does not affect the final reaction equilibrium.

As shown in Figure 8, there is no significant impact of H₂O concentration on NO conversion efficiency below 150 °C. There is slow conversion for NO under H₂O-absent conditions, with the peak conversion efficiency being attained at 350 °C. Above 150 °C, the involvement of H₂O remarkably accelerates NO conversion, with the peak conversion efficiency being achieved at 275 °C under conditions with H₂O, implying that H₂O promotes NO conversion by the WGS reaction. This is because H₂O can generate H₂ by the WGS reaction, which provides an additional reductant for NO conversion. When the H₂O concentration is increased from 8% to 16%, NO conversion is further accelerated. This is because the increased H₂O can accelerate the WGS reaction rate, further increasing the H₂ generation.

3.1.3. Effect of H₂ Concentration on Conversion Efficiency

NG combustion is accompanied by high temperature and the generation of high-concentration H₂O, inevitably triggering the WGS and SR reactions in the catalyst, further leading to a certain proportion of H₂ existing in the engine exhaust gas. H₂, as a kind of potent reductant, possibly competitively consumes some oxidants, further affecting the conversion of reductive pollutants (including CO and CH₄). Therefore, the effect of H₂ concentration (from 0 ppm to 1500 ppm, with detailed composition and concentration being shown in

Table 4. Feed gas under different H₂ concentrations.

H2 (ppm)	H2O (%)	CO2 (%)	CO (ppm)	CH4 (ppm)	NO (ppm)	O2 (ppm)
0	8	8	4500	1500	1000	-
500	8	8	4500	1500	1000	-
1000	8	8	4500	1500	1000	-
1500	8	8	4500	1500	1000	-

) on the conversion efficiency of CO, CH₄, and NO is studied under O₂-absent conditions, with the results being shown in Figure 9, Figure 10 and Figure 11.

As shown in Figure 9, increasing H₂ concentration significantly enhances NO conversion, with this effect being first observed at 125~150 °C and sustained until NO is fully converted. When the H₂ concentration increases from 500 ppm to 1500 ppm, the T₉₀ of NO decreases by 110 °C. This is because H₂ directly reacts with NO through multiple pathways, accelerating the NO consumption.

In O₂-absent conditions, NO serves as the primary oxidant for reductive substances (e.g., H₂, CO). Once NO is preferentially consumed by H₂, it may theoretically reduce CO conversion efficiency. However, as shown in Figure 10, H₂ concentration has no significant effect on CO conversion efficiency below 225 °C, which can be attributed to the slow reaction rate between CO and NO at low temperatures, where the small amount of NO required for CO conversion is fully supplied by the remaining NO that has not reacted with H₂, regardless of the H₂ concentration. As the temperature increases, H₂ shows a remarkable inhibition effect on CO conversion within the range of 225~500 °C. When the H₂ concentration increases from 500 ppm to 1500 ppm, the peak CO conversion efficiency decreases from 86% to 78%. Two factors contribute to this phenomenon: (1) At higher temperatures, the reaction rate between CO and NO increases, leading to greater NO consumption for CO oxidation. However, the increased H₂ competes with CO for NO, reducing CO conversion efficiency. (2) As aforementioned, the WGS reaction is the main pathway for CO conversion under O₂-absent conditions. Higher H₂ concentrations may reversely shift the WGS reaction equilibrium [35], further suppressing CO conversion.

Similarly, as shown in Figure 11, an increase in H₂ concentration exhibits a slight inhibitory effect on CH₄ conversion at high temperatures. NO, which is initially used to oxidize CH₄, is preferentially consumed by the increased H₂, reducing CH₄ conversion efficiency. Additionally, elevated H₂ concentrations may inhibit the reversible SR reaction [36], another critical pathway for CH₄ conversion, further decreasing CH₄ conversion efficiency.

3.2. Generation of N₂O and NH₃

3.2.1. Generation Pathway

To determine the generation path of by-products, both the production of N₂O and NH₃ are comparatively analyzed under the oxygen-absent condition (with hydrogen and without hydrogen, with detailed composition and concentration being shown in

Table 5. Specifications of feed gas.

H2 (ppm)	H2O (%)	CO2 (%)	CO (ppm)	CH4 (ppm)	NO (ppm)
0	8	8	4500	1500	1000
1500	8	8	4500	1500	1000

), following the conversion curve of potential reactants (including CO, CH₄, and NO), with the results being shown in Figure 12 and Figure 13.

2NO + H₂→N₂O + H₂O

(3)

2NO + CO→N₂O + CO₂

(4)

5H₂ + 2NO→2H₂O + 2NH₃

(5)

3H₂ + 2NO + 2CO→2NH₃ + 2CO₂

(6)

CH₄ + 2H₂O→4H₂ + CO₂

(7)

CO + H₂O→H₂ + CO₂

(8)

As shown in Figure 12, when H₂ is present in the feed gas, NO begins to be converted at 80~100 °C, accompanied by the generation of N₂O. This indicates that NO is reduced to N₂O under these conditions. Generally, NO can be converted to N₂O through reactions (3) and (4) at relatively low temperatures. Since no N₂O is generated at the same temperature under H₂-absent conditions (as shown in Figure 13), the N₂O generation can be attributed to reaction (3) (namely H₂ reducing NO to N₂O). Additionally, the absence of CO conversion in this temperature range further supports the conclusion that reaction (4) does not occur. This can be explained by the higher activation energy of reaction (4) (95 kJ/mol) compared to reaction (3) (77.6 kJ/mol). When the catalyst temperature reaches 125–150 °C, CO begins to be converted under H₂-absent conditions, accompanied by NO conversion and N₂O generation. This suggests that the activation of reaction (4) is triggered by the increased temperature and cooperates with reaction (3) to accelerate both NO reduction and N₂O production. At 200 °C, the N₂O concentration reaches its peak before subsequently decreasing. This decline in N₂O concentration is likely due to enhanced reductant and catalyst activation at higher temperatures, which favor direct NO reduction to N₂ or NH₃, bypassing N₂O as an intermediate.

Interestingly, NH₃ production begins at approximately 175 °C in the presence of H₂, primarily due to reactions (5) and (6). Under H₂-absent conditions, NH₃ production is delayed until the temperature exceeds 200 °C (as shown in Figure 13). This delay can be attributed to the reliance on the WGS reaction of CO to generate H₂ in the absence of H₂ in the feed gas. While the WGS reaction can theoretically generate H₂ between 150 and 200 °C, the quantity of H₂ generated at lower temperatures is insufficient to reduce NO to NH₃, owing to the temperature-dependent reaction rate of the WGS reaction. As the temperature increases, the WGS reaction is accelerated, producing sufficient H₂ to facilitate NH₃ production while improving CO and NO conversion.

Notably, even after CO conversion efficiency starts to decline beyond 400–425 °C, NH₃ concentration continues to increase, reaching its peak at 550–575 °C. This suggests that reaction (5) dominates NH₃ production beyond 425 °C. Additionally, CH₄ conversion, which begins at 350–375 °C, likely contributes to NH₃ production by providing additional H₂ through the SR reaction (reaction (7)) of CH₄. When the catalyst temperature exceeds 575 °C, NH₃ concentration begins to decline. This decrease can be attributed to two factors: (1) NH₃ decomposition under high temperatures and (2) the catalyst temperature exceeding the effective working window of the SR reaction of CH₄. As the SR reaction weakens and eventually terminates, H₂ generation decreases, further reducing NH₃ production.

3.2.2. Effect of Feed Gas Composition on N₂O and NH₃

Given the effect of the feed gas composition on the conversion efficiency, all these compositions could act as a reductant or an oxidant to participate in the reactions in a TWC, inevitably aggravating the emission of by-products. Therefore, this section analyzes the effect of feed gas composition (including O₂, H₂O, and H₂) on the generation of by-products (N₂O and NH₃).

Effect of O₂ Concentration on N₂O and NH₃

Figure 14 and Figure 15 show the generation curve of N₂O and NH₃ under different lambdas. As shown in Figure 14, there is a generally identical variation trend of N₂O concentration under different lambdas; namely, the N₂O concentration always initially increases to the peak and then decreases as the temperature rises. However, both the peak concentration and the corresponding temperature threshold decrease significantly with increasing lambda. This phenomenon can be reasonably explained from two perspectives. First, the increased O₂, as a more potent oxidant compared to NO, preferentially consumes more reductive substances (as shown in Figure 2), slowing down or even inhibiting reactions (3) and (4), thereby reducing the initial production of N₂O. Second, as shown in Figure 5 and Figure 14, NO begins to be directly oxidized to NO₂ at 225 °C, while the N₂O concentration starts to decline above this temperature. This suggests that the oxidation of NO to NO₂ is another factor contributing to the reduction in the peak N₂O concentration. The conversion of NO to NO₂ decreases the availability of NO, further weakening reactions (3) and (4). Quantitatively, when the lambda of the feed gas increases from 0.9705 to 0.9941, the peak concentration of N₂O decreases from 103 ppm to 29 ppm, representing a 72% reduction. Additionally, the temperature corresponding to the peak concentration decreases by 25 °C. As lambda further increases to 1.0176, the peak N₂O concentration decreases by up to 87%, with the corresponding temperature threshold being reduced by 50 °C. These results indicate that a rich air/fuel mixture is a major contributor to the high N₂O emissions observed in three-way catalysts (TWCs).

Figure 15 illustrates NH₃ generation under different lambdas. NH₃ generation is observed only under rich fuel conditions, specifically at lambdas of 0.9705 and 0.9863. As the catalyst temperature increases from 80 °C to 700 °C, the NH₃ concentration always first increases to the peak and subsequently decreases under different lambdas. It is noted that the temperature threshold at which NH₃ starts to be generated gradually increases with the lambda of the feed gas. According to Figure 6, NH₃ generation starts at 225 °C under H₂- and O₂-absent conditions, with H₂ primarily being supplied by the WGS reaction of CO at low temperatures and the SR reaction of CH₄ at higher temperatures. However, when O₂ is included in the feed gas, NH₃ generation is delayed to 250 °C and 450 °C for lambdas of 0.9705 and 0.9863, respectively. This delay occurs because the increased O₂ preferentially reacts with CO, CH₄, and H₂ from the WGS and SR reactions, leaving insufficient reductants for NH₃ formation at relatively low temperatures. Furthermore, the peak NH₃ concentration decreases as lambda increases. When the lambda value increases from 0.9705 to 0.9863, the peak NH₃ concentration decreases from 447 ppm to 121 ppm, a 73% reduction. This can be attributed to two factors: (1) Increased O₂ preferentially consumes CO, CH₄, and H₂, weakening reactions (5)–(8) and reducing initial NH₃ formation. (2) At higher temperatures, the additional O₂ oxidizes NH₃ directly, further lowering final NH₃ emissions. When the lambda increases to 0.9941, NH₃ production ceases entirely. Despite this, the NO conversion efficiency remains at 100% (as shown in Figure 4), indicating that all NH₃ generated is completely oxidized by O₂ under rich fuel conditions. When the lambda value increases to lean fuel conditions (e.g., 1.0020 and 1.0176), NH₃ generation is absent, and NO conversion efficiency drops significantly (as shown in Figure 4). This suggests that excess O₂ consumes all available reductants, directly suppressing NH₃ formation. These findings underscore the critical role of lambda optimization in reducing N₂O and NH₃ emissions, indicating that operating engines closer to stoichiometric conditions minimizes reductant-driven by-products like NH₃ and N₂O while maintaining high NO conversion rates.

Effect of H₂O Concentration on N₂O and NH₃

Figure 16 and Figure 17 show the generation curve of N₂O and NH₃ under different H₂O concentrations. As shown in Figure 16, the N₂O generation is very low under H₂O-absent conditions below 275 °C, while the existence of H₂O could substantially promote N₂O generation. And this promotive effect is enlarged by the increase in H₂O concentration. When the H₂O concentration is increased from 8% to 16%, the peak concentration of N₂O is increased by 54%. This is because H₂O can provide H₂ through the WGS reaction, further generating NH₃ through the reaction between NO and H₂. This phenomenon means that N₂O generation at low temperatures (below 275 °C) is mainly dominated by the reaction between NO and H₂, with a relatively small contribution of the reaction between NO and CO. Above 275 °C, N₂O starts to decrease under conditions with H₂O, while N₂O generation still continuously increases until 400 °C under conditions without H₂O. This phenomenon means that the N₂O generation at relatively high temperatures is mainly dominated by the reaction of CO and NO, and the involvement of H₂O could reduce the selectivity for the conversion of NO to N₂O.

As shown in Figure 17, although the increased H₂O concentration starts to promote the WGS reaction generating H₂ at 150 °C, its effect on NH₃ generation remains minimal below 250 °C. This is because the generated H₂ primarily reacts with NO to form N₂O. With the increase in temperature, the activation of the catalyst is improved, leading to the reaction between H₂ and NO shifting toward NH₃ formation, with a decrease in N₂O. Therefore, it can be found that the involvement of H₂O remarkably promotes NH₃ generation above 250 °C. When the H₂O concentration is increased from 0 to 8%, NH₃ generation is remarkably increased. This is because H₂O provides a large amount of H₂ through both the WGS reaction and SR reaction. When the H₂O concentration increases from 8% to 16%, H₂ generation is further accelerated. The peak NH₃ generation rises from 494 ppm to 648 ppm, marking a 31% increase. This result not only reveals the effect of H₂O concentration on the NH₃ generation of an NG engine TWC, but also implies that an NG engine has a higher risk of post-catalyst by-product emissions due to the high H₂O concentration included in the exhaust gas, compared to gasoline engines.

Effect of H₂ Concentration on N₂O and NH₃

Figure 18 and Figure 19 show the generation curves of N₂O and NH₃ under different H₂ concentrations. As shown in Figure 18, an increase in H₂ concentration significantly aggravates N₂O generation. When the H₂ concentration increases from 500 ppm to 1500 ppm, the peak N₂O concentration rises from 177 ppm to 280 ppm, an increase of 58%. This is because the increase in H₂ promotes reaction (3), leading to greater initial N₂O generation. Furthermore, the presence of H₂ extends the working temperature window for N₂O generation. Under H₂-absent conditions, N₂O generation terminates at 400 °C, as the available H₂ is fully consumed in NH₃ formation beyond this temperature. However, when 500 ppm H₂ is included in the feed gas, the temperature threshold for N₂O termination shifts to 450 °C. This threshold is further delayed with an increase in H₂ concentration, indicating that excess H₂ could enhance the selectivity for the conversion of NO to N₂O at higher catalyst temperatures.

As shown in Figure 19, higher H₂ concentrations significantly decrease the temperature threshold at which NH₃ generation begins. When only 500 ppm H₂ is included in the feed gas, NH₃ cannot be generated until the catalyst temperature reaches 250 °C. This is because the initial H₂ concentration is too low, and the initial H₂ is competitively consumed by the N₂O generation reaction, being insufficient to trigger reactions (5) and (6) at relatively low temperatures. When the H₂ concentration increases to 1500 ppm, NH₃ generation can begin at 175 °C, meaning that excess H₂ enhances the selectivity for the conversion of NO to NH₃ at low temperature. The availability of excess H₂ enables the reduction of NO to NH₃ through reactions (5) and (6) at these lower temperatures. Additionally, the NH₃ concentration increases with the H₂ concentration. When H₂ concentration increases from 500 to 1500 ppm, the peak NH₃ concentration increases from 634 ppm to 732 ppm, an increase of 15%, driven by the enhanced activity of reactions (5) and (6). However, the temperature at which NH₃ concentration peaks remains unchanged, with NH₃ levels beginning to decline at 550–575 °C under all tested H₂ concentrations. This indicates that H₂ concentration does not influence NH₃ decomposition at higher temperatures.

4. Conclusions

This study focuses on the catalytic performance of a TWC used for an NG engine. Small-sample catalyst experiments were conducted to investigate the exhaust conversion efficiency and by-product (N₂O and NH₃) generation characteristics, with particular emphasis on the influence of feed gas composition. The main conclusions are as follows:

(1) N₂O is mainly generated through the reduction of NO by H₂ or CO at 80~400 °C. The former mainly occurs at 80~275 °C, and the latter mainly occurs at 275~400 °C. NH₃ generation occurs at 175~550 °C, mainly via NO reduction by H₂ (supplied from the WGS reaction) and CO below 425 °C and exclusively by H₂ (supplied from the SR reaction) above 425 °C.

(2) Increasing lambda enhances the conversion of CO and CH₄ while reducing N₂O and NH₃ emissions, but it inhibits NO conversion and promotes NO₂ formation. A lambda of 0.9941 achieves high conversion efficiency (≥90%) for CO, CH₄, and NO, with reduced N₂O and zero NH₃ emissions. These results suggest that the lambda should be slightly maintained in the fuel-rich region to optimize TWC performance and minimize by-product emissions.

(3) Increasing H₂O concentration accelerates the WGS and SR reactions, improving the conversion of three regulated pollutants. However, it significantly increases N₂O and NH₃ emissions, with peak levels rising by 54% and 31%, respectively, as H₂O concentration increases from 8% to 16%. These findings highlight the trade-off for H₂O between enhanced pollutant conversion and increased by-product emissions, also revealing the higher post-catalyst emission risks in NG engines due to their higher H₂O content compared to gasoline engines.

(4) An increase in H₂ concentration preferentially consumes NO and reversely shifts the equilibrium of the WGS and SR reactions, slightly reducing CO and CH₄ conversion while improving NO conversion. However, higher H₂ levels significantly promote N₂O selectivity at high temperature and NH₃ selectivity at low temperature and peak emissions, with peak concentrations increasing by 58% and 15%, respectively, when H₂ rises from 500 to 1500 ppm.

Overall, the results demonstrate that under optimized λ control, the TWC achieves significant co-conversion of CO, CH₄, and NO in natural gas engine exhaust, while careful management of H₂O and H₂ concentrations is necessary to minimize secondary emissions of N₂O and NH₃.