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Abstract

To investigate the influence mechanism of the light-off characteristics and secondary pollutants (N₂O and NH₃) of the Pd/Rh catalytic converter for natural gas engines, a global model of the catalytic converter and a more comprehensive catalytic reaction mechanism were first established. Further, the model and the catalytic reaction mechanism were validated by using experimental data. Then, the effects of exhaust temperature and exhaust composition (H₂O, O₂, and H₂ concentrations) on the catalytic converters were investigated. The results showed that a higher temperature reduced the light-off time t50 of NO and CO. The production of N₂O decreased to 0 at high temperatures due to the reduction reactions of R14 (N₂O+H₂ = H₂O+N₂) and R16 (N₂O+CO = CO₂ + N₂). NH₃ would form at the temperature above 825 K, and the required H₂ originated from SR and WGS reactions. Increasing the H₂ concentration promoted the conversion of NO and CO. When the temperature exceeds 600 K, increasing the H₂ concentration decreases the production of N₂O. H₂O has the opposite effect on the production of N₂O and NH₃ at high temperatures. Increasing the content of H₂O increased the conversion of NO and inhibited the conversion of CO. O₂ concentration at 4000 × 10⁻⁶ is beneficial to reduce secondary pollutants of N₂O and NH₃. At the condition of 730–820 K and 4000 × 10⁻⁶ O₂, the productions of N₂O and NH₃ were reduced to 0. This study contributes to the efficient application of Pd/Rh catalytic converters and further enhances the emission reduction of natural gas engines.

Keywords

Light-off characteristics Pd/Rh catalytic converter natural gas engine

Introduction

To alleviate energy and environmental problems, adopting natural gas as a vehicle fuel has become one of the effective ways to adjust the energy consumption structure of vehicles and reduce emissions.¹ The advantages of natural gas are lower greenhouse gas emissions and almost no particulate formation.² The natural gas engines with three-way catalytic converters (TWC) have become the dominant technology.³ However, pollutants such as CO and NOx would generate greenhouse gas N₂O and toxic corrosive gas NH₃ in the catalytic converter, which has received more attention in the latest emission standards.⁴

The core of the TWC is a multi-channel ceramic or metal honeycomb structure with a layer of precious metals (Pt, Pd, Rh) coated on the inner walls of the channels.⁵ Precious metals are active sites and act as catalysts for oxidation and reduction.⁶ The precious metal Pd is the most widely used in the after-treatment system of the natural gas engine, because of its good CH₄ oxidation capacity at low temperatures.⁷ However, compared to the Rh catalyst, the self-poisoning of hydrocarbon molecules leads to poor NOx reduction of the Pd catalyst.⁸ Furthermore, Rh plays an important role in enhancing NOx reduction and contributes to the suppression of NH₃ emissions.⁹

The performance of catalytic converter for CO oxidation and NO reduction is correlated with exhaust temperature, active metal composition and oxygen storage capacity (OSC),¹⁰ as demonstrated in the existing studies. Sadokhina et al.¹¹ found that NO promoted the oxidation of CH₄ in the presence of H₂O. H₂O can produce H₂ and form HNO₂ through an H₂-NO reaction at high temperatures, the HNO₂ is easy to react with CH₄. Gärtner et al.¹² found that H₂O inhibited CH₄ oxidation. The steam resistance activity can be obtained when added Pt to the Pd catalyst. Wang et al.¹³ found that NH₃ was generated by the hydrogen drop reaction of hydroxyl molecules. The hydroxyl-induced WGS reaction led to the formation of NH₃ on the Rh-CeO₂ catalyst. The hydroxylation induced by H₂O could promote the formation of NH₃. Wang et al.¹⁴ inferred that NH₃ was mainly generated in the 250°C–550°C range via ion molecular reaction mass spectrometry. The decrease of λ and OSC leads to the increase of NH₃ generation selectivity. The key of reducing NH₃ emission is to improve the OSC of TWC and control λ close to 1.0 under rich conditions. Adams et al.¹⁵ investigated the influence of temperature and cerium content on NH₃ formation through static and transient flow reactor experiments. The results showed that NH₃ formation is slightly delayed in the samples when the temperature is higher than 350°C, and N₂O is formed earlier than NH₃ when the temperature is lower than 175°C.

From the above studies, it can be noticed that most of the existing studies are based on single-metal catalytic converters and rarely deal with the reaction mechanism of Pd/Rh combination catalytic converters. Furthermore, natural gas engines have high exhaust temperatures and large H₂O content in the exhaust.¹⁶ The current studies seldom focus on the contribution pattern of H₂O, O₂ and H₂ to N2O and NH₃ generation on Pd/Rh catalytic converters at high temperatures. To investigate the light-off performance of NO and CO and the influence mechanism of N₂O and NH₃ production in Pd/Rh catalytic converters at high temperatures, a global model of the catalytic converter and a more comprehensive catalytic reaction mechanism suitable for natural gas engines were first established. Further, the model and the catalytic reaction mechanism were validated by using experimental data. Then, the influence of exhaust temperature and composition (H₂O, O₂ and H₂ concentrations) on Pd/Rh catalytic converters were investigated. This study contributes to the efficient application of Pd/Rh catalytic converters and further enhances the emission reduction of natural gas engines.

Methods and mechanism

Reaction kinetics equation and mechanism

The reaction kinetics equation is shown in equations (1) and (2) of the Supplemental material. As shown in the reaction kinetics equations, the reaction mechanism of the different catalysts is different. To ensure the accuracy of the calculation, the chemical reaction kinetics analysis is required. Furthermore, the parameters of $A_{i} and E_{i}$ related to the chemical reaction kinetic parameters should be modified and optimized in the simulation.

Combined with the reaction mechanism of the Pd/Rh catalyst in the software database and literature, the mechanism used in this study was established, which mainly included 10 gas-phase species and 23 reactions. Table 1 shows the global reaction mechanism over Pd/Rh catalyst, which covers the chemical reactions such as CH₄ oxidation and NO reduction. The mechanism of NO reduction considered the formation of NH₃, N₂, and N₂O. Defoort et al.¹⁷ noted that NO₂ was not found in the catalytic converter, thus the reactions involving NO₂ emission were not evaluated in this study. It is believed that both NH₃ production and oxidation reactions would occur in the catalytic converter. Figure 1 shows the reaction network of ammonia selective oxidation, which includes three detailed oxidation routes: (1) Direct catalytic oxidation method,¹⁸ (2) Indirect catalytic oxidation method,¹⁹ and (3) Internal selective catalytic reduction method.²⁰ From the three oxidation pathways of NH₃, it can be concluded that NH₃ will eventually be converted to N₂ and H₂O, as shown in Table 1 R18: 4NH₃ + 3O₂ = 2N₂ + 6H₂O. Therefore, the reaction is used as the mechanism of NH3 oxidation in this study.

Table 1.

Global reaction mechanism over Pd/Rh.²¹

No.	Reactant	Product	A_i/ $(s^{- 1})$	E_i/ $(kJ \cdot mo l^{- 1}$ )
R1	CO + 0.5O₂	CO₂	1.8 × 10²⁰	105.00
R2	H₂ + 0.5O₂	H₂O	5.4 × 10²⁰	85.00
R3	C₃H₈ + 5O₂	3CO₂ + 4H₂O	4.9 × 10²⁰	112.00
R4	C₃H₆ + 4.5O₂	3CO₂ + 3H₂O	1.3 × 10²⁰	105.00
R5	CH₄ + 2O₂	CO₂ + 2H₂O	9.0 × 10¹⁶	121.00
R6	CO + H₂O	CO₂ + H₂	5.6 × 10¹³	67.55
R7	C₃H₈ + 3H₂O	3CO + 7H₂	6.7 × 10¹⁶	136.00
R8	C₃H₆ + 3H₂O	3CO + 6H₂	6.0 × 10¹⁷	116.00
R9	CH₄ + H₂O	CO + 3H₂	1.2 × 10¹⁶	136.00
R10	CO + NO	CO₂ + 0.5N₂	1.6 × 10¹⁷	80.00
R11	H₂ + NO	H₂O + 0.5N₂	6.0 × 10¹⁵	71.00
R12	C₃H₆ + 9NO	3CO₂ + 3H₂O	1.7 × 10¹⁵	80.00
R13	H₂ + 2NO	H₂O + N₂O	5.5 × 10¹⁷	71.35
R14	N₂O + H₂	H₂O + N₂	1.8 × 10¹⁸	80.00
R15	CO + 2NO	CO₂ + N₂O	4.1 × 10¹⁶	80.00
R16	N₂O + CO	CO₂ + N₂	1.0 × 10¹⁶	68.79
R17	5H₂ + 2NO	2H₂O + 2NH₃	2.4 × 10¹⁷	69.00
R18	4NH₃ + 3O₂	6H₂O + 2N₂	6.1 × 10¹⁷	95.00
R19	HO₂ + 2CeO₂	H₂O + Ce₂O₃	3.1 × 10⁷	142.30
R20	H₂O + 2Ce₂O₃	H₂ + 2CeO₂	37.0	103.09
R21	CO + 2CeO₂	CO₂ + Ce₂O₃	1.7 × 10⁸	142.3
R22	CO₂ + Ce₂O₃	CO + 2CeO₂	43,000	139.20
R23	0.5O₂ + Ce₂O₃	2CeO₂	0.12	0.0

Figure 1.

Selective oxidation of ammonia.

TWC model

Figure 2 illustrates the test model and the structure of the catalytic converter, which is built by GT-Suite software. The test model consists of the reaction gas input and output modules, and the catalytic converter parameters module, as shown in Figure 2(a). The catalytic converter without considering the heat transfer losses is shown in Figure 2(b). Furthermore, the catalytic converter temperature was considered only along the axis variation, neglecting the variation along the radial direction. The diffusion effect of the model in the coating was not considered. The catalytic converter followed the three material conservation equations. In addition, the temperature of the catalytic converter varied in the axial direction, but not in the radial direction, and the diffusion effect within the coating was not considered. The structure parameters of the catalyst converter are shown in Table 2.

Figure 2.

TWC test model (a) and catalytic converter structure (b).

Table 2.

Parameters of the catalyst converter.

Parameters	Value
Precious metal loading	15.0 g/ft³
Precious metal dispersion	30%
Substrate length	0.0254 m
Substrate frontal diameter	0.0145 m
Substrate cell density	600 cpsi
Substrate wall thickness	0.0762E−03 m
Substrate density	2.5E+03 kg/m³
GPSV	100,000 h⁻¹
Washcoat loading	3.5 g/in³
Washcoat thickness	0.03E−3 m
Pressure	1.0 bar

The temperature of the catalytic converter varied in the axial direction, but not in the radial direction, and the diffusion effect within the coating is not considered.

Validation

Based on the established test model and catalytic converter parameters, preliminary calculations were developed. To verify the reliability of the test model, the calculated N₂O and NH₃ formation and NO and CO conversion were compared with the experimental data of Kang et al.²² as shown in Figure 3. The composition of the exhaust was as follows: 0.47% CO, 0.14% H₂, 0.25% NO, 9.25% CO₂, 18% H₂O, and balanced N₂. The selectivity of N₂O and NH₃ formation and the conversion of reactants were shown in equations (3) and (4) of the Supplemental Material. Notably, the trends of the simulated and experimental data were in general agreement, and the maximum relative error of the result was less than 5%. This is because the catalytic converter model only considers the main reaction processes and ignores the reactions that have little effect. In addition, it is feasible to carry out subsequent studies on the Pd/Rh catalytic converter model.

Figure 3.

Comparison of the formation of N2O and NH3 (a) and the conversion of NO and CO (b) experimentally and numerically.

Results and discussions

Effect of the temperature on light-off characteristics and secondary pollutants

The light-off characteristics of NO and CO

Figure 4 shows the conversion efficiency of NO and CO under different initial temperatures. The t₅₀ denotes the light-off time, and the t₉₀ signifies the complete conversion time. As shown, the t₅₀ and t₉₀ of NO and CO decreased with the increase of initial temperature. Compared with that of 373.15 K, the light-off time for NO and CO at 473.15 K advanced about 600 s. As we know, the higher initial temperature was conducive to the catalytic reaction. Notably, the conversion efficiency of NO first rose and then declined. This is because the NO reduction reaction is weakened due to the reactions of CO-O₂ (R1: CO + 0.5O₂ = CO₂) and H₂-O₂ (R2: H₂ + 0.5O₂ = H₂O) at this temperature. As the temperature increased, CH₄ generated CO and H₂ through the SR reaction, leading to a decrease in CO conversion. However, the generated CO and H₂ participated in the NO reduction reaction, resulting in another increase in NO conversion.

Figure 4.

Conversion efficiency of NO (a) and CO (b) at different initial temperatures.

The selectivity of N₂O and NH₃

Figure 5 shows the formation of N₂O and NH₃ at different inlet temperatures. As shown, the H₂-NO reaction proceeded in preference. This is because its activation energy is lower than that of the CO-NO reaction. At the temperature of 450 K, the concentrations of H₂ and NO decreased, which demonstrated that the H₂-NO reaction started at the stage. This results in the production of N₂O and NH₃. As the temperature rose, the reaction of CO-NO started at the temperature of 500 K, which also increased N₂O and NH₃ components. The production of NH₃ first increased and then decreased, which reached a peak of 268 × 10⁻⁶ at 546 K. There was almost no NH₃ formation when the temperature reached 620 K. The N₂O concentration increased slowly, reaching a peak concentration of 215 × 10⁻⁶ at 586 K. There was almost no N₂O and NH₃ formation at the temperature above 740 K. The reason is the occurrence of N₂O reduction and NH₃ oxidation reactions. Further, H₂ reacted with O₂ at high temperatures, thus CO served as the main reducing agent when the temperature reaches 550 K. It can be deduced that in the light-off period, the CO-NO reaction produces N2O through the reaction of R15: CO + 2NO = CO₂ + N₂O. Besides, CO and H₂ concentrations increased at 745 K, which might be attributed to the reaction of R9: CH₄ + H₂O = CO + 3H₂ and R6: CO + H₂O = CO₂ + H₂. H₂ reacted with NO, which generated NH₃ again at 845 K. This increased the formation of NH₃, which finally stabilized at 135 × 10⁻⁶. However, there was no N₂O at this stage. This is because, at high temperatures, N₂O is readily transformed into N₂ through reduction reactions. This was similar to the results of Huai et al.²³ who stated that more than 95% of N₂O emissions would be formed between 523 and 723 K.

Figure 5.

Variation of the concentrations of H₂, CO, NO and O₂ and the formation of N₂O and NH₃ at different inlet temperatures.

Figure 6 shows the variation of the production and selectivity of N₂O and NH₃ at different inlet temperatures. As shown, the selectivity of N₂O was always higher than that of NH₃ in the range of 350–740 K. As the temperature increased, the selectivity of N₂O and NH₃ declined to 0 in 740–825 K. This is because the production of N₂O and NH₃ is inhibited. Furthermore, the NH₃ selectivity increased slightly at the temperature of 825 K, and its concentration reached 135 × 10⁻⁶ when was above 980 K. At this stage, the selectivity of NH₃ stabilized at about 5%, while the N₂O selectivity was 0. The presence of NH₃ in 720 and 900 K was reported by Heeb et al.²⁴ who also pointed out that H₂ was the key factor in the production of N₂O and NH₃.

Figure 6.

Variation of the formation and selectivity of N₂O and NH₃ at different inlet temperatures.

In a word, in the range of 450–740 K, N₂O is primarily formed by the reduction reaction of NO-CO. NH₃ is mainly formed via NO-H₂ reduction reaction in the range of 450–620 K. Further, NH₃ can be formed at a temperature above 825 K, and the required H₂ in the stage originates from SR and WGS reactions. Besides, there is almost no N₂O formation at high temperatures. This is because N₂O is easily converted into N₂ again by the reactions of R14: N₂O + H₂ ₌ H₂O + N₂ and R16: N₂O + CO = CO₂ + N₂.

Analysis of N₂O and NH₃ reaction pathways

The formation of N₂O and NH₃

Figure 7 shows the variation of the production of N₂O, H₂, and NH₃ and the conversion of NO, CO, and H₂. The inlet flow of dry full feed signifies 0.25% NO, 0.14% H₂, and 0.47% CO, while the wet full feed is 0.25% NO, 0.14% H₂, 0.47% CO, and 18% H₂O. As illustrated in Figure 7(a), the production of N₂O increased at 395 K and reached the peak of 220 × 10⁻⁶ at 553 K under dry full feed. In this stage, H₂ concentration gradually declined to 0, but the concentration of CO changed less. This means the start time of the NO-H₂ reaction is earlier than the CO-NO reaction. NH3 was first found at 420 K and then reached a peak of 406 × 10⁻⁶ at 526 K under dry full feed. The NH₃ concentration declined to 270 × 10⁻⁶ at 624 K. This means that NH₃ is the mainly side-product at high temperatures, and the production of N₂O decreases to 0. As shown in Figure 7(b), the production of N₂O reached the peak of 212 × 10⁻⁶ at 540 K, while it was lower than that without H₂O addition. Further, the time corresponding to the peak was earlier than that without H₂O addition. As the temperature increased, the production of N₂O sharply declined to 0. This means adding H₂O can reduce the formation of N₂O. As the temperature increased, the production of NH₃ increased, which reached the peak of 970 × 10⁻⁶ at 700 K. Compared with that without H₂O addition, the peak increased significantly at high temperatures. Furthermore, the formation of NH₃ decreased to 880 × 10⁻⁶. This is because NH₃ is formed into N₂ via decomposition or oxidation reaction (R18) at high temperatures. Therefore, the productions of N₂O and NH₃ are closely related to H₂ conversion for Pd/Rh catalysts.

Figure 7.

Variation of the formation of N₂O, H₂, and NH₃ (a) and the conversion of NO, CO, and H₂ (b).

As for the conversion of NO, CO and H₂, a lower CO conversion was observed at the temperature above 550 K in Figure 7(a), which remained at 32%. A higher H₂ conversion was also observed at the temperature above 550 K, which remained at 100%. The reason is that without H₂O addition, WGS and SR reactions cannot occur. As illustrated in Figure 7(b), the production of H₂ rose sharply at 550 K. As the temperature increased, H₂ production decreased and then slowly increased. The conversion of CO increased sharply and finally stabilized at 100%. This means WGS and SR reactions occur at high temperatures. Therefore, the formation of NH₃ increases sharply with adding H₂O. The simulation results of CO and H₂ in this paper are consistent with the experimental data. For example, in the range of 473–573 K, OH et al.²⁵ found the conversion efficiency of CO and H₂ remained almost unchanged. However, a higher conversion was observed with H₂O addition and higher temperature.

Steam reforming and water gas shift reactions

A detailed simulation of SR and WGS reactions is performed on Pd/Rh TWC, and the results are shown in Figure 8. As shown, the WGS and SR reaction started at about 540 and 650 K, respectively. For the WGS reaction, H₂ was mainly produced at low temperatures. SR reaction was the main source of H₂ at high temperatures. This means that at high temperatures, the Pd/Rh catalyst would generate H₂. As presented in Figure 5, the production of N₂O and NH₃ increased sharply at 540 K because of the reaction of WGS. At 650 K, the production of H₂ and the conversion of CH₄ increased in the SR reaction. The amount of H₂ and the conversion of CH₄ reached a maximum at 840 K, which promoted a series of reactions such as H₂-NO and CO-NO. This is consistent with the result that NH₃ generates in large quantities when the temperature reaches 845 K. It indicates that the reactions of SR and WGS for the Pd/Rh catalyst are the main sources of H₂. A similar result was also noted by Nevalainen et al.²⁶

Figure 8.

Production of H₂ and conversion of CO and CH₄ for the reaction of WGS and SR: (a) WGS reaction and (b) SR reaction.

Effect of exhaust composition on light-off characteristics and secondary pollutants

Effect of H₂O content on light-off characteristics of NO and CO

Many studies have found that H₂O (up to 10%) in exhaust gas plays an important role as an oxidant for unburned hydrocarbons. Stotz et al.²⁷ investigated the effect of H₂O on catalyst activity and micro-kinetic mechanism, but the influence of H₂O on NH₃ and N₂O formation was still not well understood. Figure 9 shows the conversion efficiency of NO and CO at different H₂O contents. As shown in Figure 9(a), at a temperature below 620 K, the content of H₂O showed less effect on the NO conversion. The reason is that the temperature does not reach the reaction conditions. As the temperature increased, the NO conversion decreased. In the presence of H₂O, the NO conversion rate first dropped and then rose to 100%. The conversion of NO increased with the increase of H₂O content. The reason is that H₂O can diffuse, absorb and dissociate to the catalyst surface, which forms O(s). The O(s) may react with CH₄ and produce CO and H₂. As shown in Figure 9(b), at a temperature below 620 K, the content of H₂O has less effect on the CO conversion. As the temperature increased, the CO conversion of 9% H₂O increased and reached 100% at 626 K. However, in the cases of 18% and 27% H₂O, CO conversion efficiency declined and then rose. This means increasing H₂O content inhibits CO conversion within a certain range. The excess H₂O will react with CO through WGS reaction, which inhibits the conversion of CO at higher H₂O content.

Figure 9.

Conversion efficiency of NO (a) and CO (b) at different H₂O contents.

Effect of H₂O content on the formation of N₂O and NH₃

Figure 10 shows the variation of the production of N₂O and NH₃ at different H₂O contents. To further investigate the relationship between H₂O content and the production of N₂O and NH₃, H₂ was not set in the inlet fraction. Furthermore, the catalyst deactivation effect caused by H₂O was also ignored. As shown in Figure 10(a), the formation of N₂O was not affected by H₂O content in the range of 375–620 K. The N₂O concentration decreased at 620 K, and it eventually dropped to 0 in the presence of H₂O. The formation of N₂O declined to 0at 773 K in the case of 27% H₂O, while it existed at high temperatures in the case of 0% H₂O. This means H₂O could inhibit the formation of N₂O. The reason is that the addition of H₂O induces SR and WGS reactions, and these reactions produce CO and H₂ at high temperatures. The CO and H₂ reduce with N₂O (R14: N₂O + H₂ ₌ H₂O + N₂&R16: N₂O + CO = CO₂ + N₂), which results in a decrease in the N₂O formation. As illustrated in Figure 10(b), the production of NH₃ was 0 in the case of 0% H₂O. As the H₂O content increased, the peak of NH₃ concentration increased. For example, the peak of NH₃ concentration of 27% H₂O increased by 30 × 10⁻⁶ compared with that of 9% H₂O. The reason is that H₂O generates H₂ through SR and WGS reactions, which is beneficial to produce NH₃ through the H₂-NO reaction (R17: 5H₂ + 2NO = 2H₂O + 2NH₃). However, NH₃ is easily oxidized to N₂ under high temperatures and oxygen-rich conditions. This results that NH₃ concentration dropping to 0 at 630 K for all cases.

Figure 10.

Variation of the formation of N₂O (a) and NH₃ (b) at different H₂O contents.

Effect of O₂ concentration on light-off characteristics of NO and CO

Figure 11 shows the conversion efficiency of NO and CO at different O₂ concentrations. As shown in Figure 11(a), the conversion of NO was not affected by O₂ concentration at a temperature below 600 K. As the O₂ concentration increased, the NO conversion efficiency decreased at a temperature above 600 K. It means a higher O₂ concentration may result in a great decrease in NO conversion efficiency as the temperature rises, the NO conversion efficiency decreases more significantly at higher O₂ concentrations. Thus, the reduction reaction NO is inhibited by O₂ concentrations. The literature²⁸ believed that too much O₂ occupied the vacant sites on the catalyst, resulting in less N-O on the adsorption sites. This decreased the NO reaction rate. As shown in Figure 11(b), the T₅₀ and T₉₀ of CO decreased with the increase in O₂ concentration. This means the conversion of CO is promoted. At a temperature above 673 K, the conversion efficiency of CO decreased with the decrease of O₂ concentration. This means the conversion efficiency of CO is also inhibited under high temperatures and anoxic conditions.

Figure 11.

Conversion efficiency of NO (a) and CO (b) at different O₂ concentrations.

Effect of O₂ concentration on the formation of N₂O and NH₃

Figure 12 shows the variation of the production of N₂O and NH₃ at different O₂ concentrations. As illustrated in Figure 12(a), the production of N₂O increased and then declined. At the temperature below 593 K, the formation of N₂O was not affected by O₂ concentration for all cases. At the temperature above 593 K, the production of N₂O declined to 0 in the cases of O₂ concentration lowering 4000 × 10⁻⁶. Notably, in the cases of 8000 × 10⁻⁶ O₂, there was still N₂O formation at temperatures above 730 K. This means the consumption of H₂ could not affect N₂O formation under high O₂ concentrations and high temperatures. Therefore, CO plays a major role in N₂O formation at high temperatures, and N₂O is generated through R15. As shown in Figure 12(b), at a temperature below 583 K, the formation of NH₃ was not affected by O₂ concentration for all cases. At the temperature above 583 K, the curves of NH₃ concentration and O₂ concentration showed an overall downward trend. As the temperature increased, the production of NH₃ declined to 0 in the case of 8000 × 10⁻⁶ O_2. However, there was NH₃ formation at a temperature above 820 K in other cases. This means the formation of NH₃ is inhibited under high-temperature and oxygen-enriched conditions. Notably, at high temperatures, there was an amount of NH₃ formation in the cases of 0 × 10⁻⁶ O₂ and 1000 × 10⁻⁶ O₂. This is because, at high temperatures, the H₂ produced by the SR and WGS reactions cannot be consumed under low O₂ conditions, which generates NH₃ through NO reduction reaction. NH₃ oxidation reduces at low O₂ concentrations. The decomposition degree of NH₃ is low at high temperatures under rich steam conditions. This result was also reported in the study by Fujimoto et al.²⁸ The results showed that the production of NH₃ was hindered under enrichment conditions. Therefore, increasing the O₂ concentration is effective in reducing the production of N₂O and NH₃. In this study, the production of N₂O and NH₃ was reduced to 0 in the range of 700–800 K under the condition of 4000 × 10⁻⁶ O₂.

Figure 12.

Variation of the formation of N₂O (a) and NH₂ (b) at different O₂ concentrations.

Effect of H₂ concentration on light-off characteristics of NO and CO

H₂O in the exhaust gas induces SR and WGS reactions, and the reaction product H₂ significantly affects the conversion efficiency of NO and CO and the production of N₂O and NH₃. To visually analyze the effect of H₂ concentration, the effect of H₂ concentration on N₂O and NH₃ production was investigated. Figure 13 shows the conversion efficiency of NO and CO at different H₂ concentrations. As shown in Figure 13(a), as the H₂ concentration increased, the conversion efficiency of NO increased. As the temperature increased, the conversion of NO first increased and then decreased, which reached the peak at 613 K. This means rising H₂ concentration can increase the conversion efficiency of NO at high temperatures. As shown in Figure 13(b), for all cases, the conversion efficiency of CO was not affected by H₂ concentration with the increases of temperature.

Figure 13.

Conversion efficiency of NO and CO at different H₂ concentrations.

Effect of H₂ concentration on the formation of N₂O and NH₃

Figure 14 demonstrates the variation of the production of N₂O and NH₃ at different H₂ concentrations. As shown in Figure 14(a), for all cases, the formation of N₂O first increased and then declined. As the increase of H₂ concentration, the production of N₂O increases at the temperature below 600 K. In the case of 2100 × 10⁻⁶ H₂, the formation of N₂O reached the peak at 590 K, which was lower than 35 K of 0 × 10⁻⁶ H₂. This means that H₂ can promote the production of N₂O at low temperatures. due to the participation of H₂ as a reducing agent in the reaction, which participates in the N₂O generation reaction. Furthermore, as the H₂ concentration increased, the production of N₂O decreased at the temperature above 600 K. The reason is that excessive H₂ can react with N₂O, which generates N₂ at high temperatures. As demonstrated in Figure 14(b), there was no NH₃ formation in the absence of H_2. This means NH₃ is mainly generated by the H₂-NO reaction. As the H₂ concentration increased, the production of NH₃ increased. Compared with700 × 10⁻⁶ H₂, the peak of NH₃ formation increased by about 245 × 10⁻⁶ in the case of 2100 × 10⁻⁶ H₂. This means that H2 is beneficial in promoting the production of NH3. The production of NH₃ decreased to 0 at the temperature above 620 K. This is because NH₃ generates N₂ through the decomposition or oxidation (R18: 4NH₃ + 3O₂ ₌ 6H₂O + 2N₂) at high temperatures.

Figure 14.

Variation of the formation of N₂O (a) and NH₃ (b) at different H₂ concentrations.

Conclusion

In this study, a global model of the catalytic converter and a more comprehensive catalytic reaction mechanism suitable for natural gas engines were first established. Further, the model and the catalytic reaction mechanism were validated by using experimental data. Then, the effects of exhaust gas temperature and exhaust gas composition (H₂O, O₂, and H₂ concentrations) on the catalytic converters were investigated. The study concluded the following:

(1) The light-off characteristics and secondary pollutants are closely related to the temperature. A higher temperature reduced the light-off time t50 of NO and CO. N₂O was mainly generated through the CO reduction of NO reaction at temperatures of 450–740 K, while at high temperatures, the production of N₂O decreased to 0 due to the reduction reactions of R14: N₂O + H₂ = H₂O + N₂ and R16: N₂O + CO = CO₂ + N₂. NH₃ would form at the temperature above 825 K, and the required H₂ originated from SR and WGS reactions.

(2) H₂O has an opposite effect on the production of N₂O and NH3 at high temperatures. When the temperature was above 600 K, increasing the content of H₂O reduced the production of N₂O and increased the production of NH_3. Increasing the content of H₂O increased the conversion of NO and inhibited the conversion of CO.

(3) O₂ concentration at 4000 × 10⁻⁶ is beneficial to reduce secondary pollutants of N₂O and NH₃. At the condition of 730–820 K and 4000 × 10⁻⁶ O₂, the productions of N₂O and NH₃ were reduced to 0. When the temperature was above 600 K, increasing O₂ concentration decreased the NO conversion efficiency and increased the CO conversion efficiency.

(4) Increasing the H₂ concentration promoted the conversion of NO and CO. When the temperature exceeds 600 K, only N₂O is produced, and increasing H₂ concentration decreases the production of N₂O. This provides further insight into light-off characteristics and secondary pollutants production of natural gas engines.

(5) For future research directions, the ratios of Pd/Rh catalysts are worth exploring. Based on the current findings, it is instructive to perform the loading and proportioning of noble metals on the catalytic converter light-off characteristics and secondary pollutant production.

Supplemental Material

sj-docx-1-jer-10.1177_14680874231197855 – Supplemental material for Investigation of the light-off characteristics and secondary pollutants production mechanism in Pd/Rh catalyst for natural gas engines

Supplemental material, sj-docx-1-jer-10.1177_14680874231197855 for Investigation of the light-off characteristics and secondary pollutants production mechanism in Pd/Rh catalyst for natural gas engines by Yejian Qian, Xiaofei Wei, Fei Tang and Shun Meng in International Journal of Engine Research

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The work in this study received financial support from Science and Technology Major Project of Anhui (China) (Grant No.202003a05020023), National Natural Science Foundation of China (Grant No.51676062), Major R&D projects of key technologies of Hefei (J2020G33).

ORCID iD

Shun Meng

Supplemental material

Supplemental material for this article is available online.

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Supplementary Material

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Investigation of the light-off characteristics and secondary pollutants production mechanism in Pd/Rh catalyst for natural gas engines

Abstract

Keywords

Introduction

Methods and mechanism

Reaction kinetics equation and mechanism

TWC model

Validation

Results and discussions

Effect of the temperature on light-off characteristics and secondary pollutants

The light-off characteristics of NO and CO

The selectivity of N2O and NH3

Analysis of N2O and NH3 reaction pathways

The formation of N2O and NH3

Steam reforming and water gas shift reactions

Effect of exhaust composition on light-off characteristics and secondary pollutants

Effect of H2O content on light-off characteristics of NO and CO

Effect of H2O content on the formation of N2O and NH3

Effect of O2 concentration on light-off characteristics of NO and CO

Effect of O2 concentration on the formation of N2O and NH3

Effect of H2 concentration on light-off characteristics of NO and CO

Effect of H2 concentration on the formation of N2O and NH3

Conclusion

Supplemental Material

sj-docx-1-jer-10.1177_14680874231197855 – Supplemental material for Investigation of the light-off characteristics and secondary pollutants production mechanism in Pd/Rh catalyst for natural gas engines

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

Supplemental material

References

Supplementary Material

The selectivity of N₂O and NH₃

Analysis of N₂O and NH₃ reaction pathways

The formation of N₂O and NH₃

Effect of H₂O content on light-off characteristics of NO and CO

Effect of H₂O content on the formation of N₂O and NH₃

Effect of O₂ concentration on light-off characteristics of NO and CO

Effect of O₂ concentration on the formation of N₂O and NH₃

Effect of H₂ concentration on light-off characteristics of NO and CO

Effect of H₂ concentration on the formation of N₂O and NH₃