Effect of Ba/Ce Ratio on the Structure and Performance of Pt-Based Catalysts: Correlation Between Physicochemical Properties and NO_x Storage–Reduction Activity

Abstract

The continuous tightening of emission regulations and the escalating costs of palladium (Pd) and rhodium (Rh) have renewed interest in platinum (Pt)-based three-way catalysts (TWCs) as cost-effective alternatives for gasoline aftertreatment. However, despite extensive studies on Pt/CeO₂ and Pt/Ba-based formulations, the cooperative roles of Ba and Ce and, in particular, the fundamental influence of the Ba/Ce ratio on oxygen mobility, NO_x storage behavior, and Pt–support interactions remain poorly understood. In this work, we address this gap by systematically tuning the Ba/Ce molar ratio in a series of Pt–Ba–Ce/Al₂O₃ catalysts prepared from Ba(CH₃COO)₂ and CeO₂ precursors, and evaluating their structure–function relationships in both fresh and hydrothermally aged states. Through comprehensive characterization (N₂ physisorption, XRD, XPS, H₂-TPR, NO_x-TPD, SEM, CO pulse adsorption, and dynamic light-off testing), we establish previously unrecognized correlations between Ba/Ce ratio–dependent structural evolution and TWC performance. The results reveal that the Ba/Ce ratio exerts a decisive control over catalyst textural properties, Pt dispersion, and interfacial Pt–CeO₂ oxygen species. Low Ba/Ce ratios uniquely promote Pt–Ce interfacial oxygen and O₂ spillover—providing a new mechanistic basis for enhanced low-temperature oxidation and reduction reactions—while higher Ba loading selectively drives BaCO₃ formation and boosts NO_x storage capacity. A clear volcano-type dependence of NO_x storage on the Ba/Ce ratio is demonstrated for the first time. Hydrothermal aging at 850 °C induces PtO_x decomposition, BaCO₃–Al₂O₃ solid-state reactions forming inactive BaAl₂O₄, and Pt sintering, collectively suppressing Pt–Ce interactions and reducing TWC activity. Importantly, an optimized Ba/Ce ratio is shown to mitigate these degradation pathways, offering a new design principle for thermally durable Pt-based TWCs. Overall, this study provides new mechanistic insight into Ba–Ce cooperative effects, establishes the Ba/Ce ratio as a critical and previously overlooked parameter governing Pt–support interactions and NO_x storage, and presents a rational strategy for designing cost-effective, hydrothermally robust Pt-based alternatives to Pd/Rh commercial TWCs.

1. Introduction

Since the invention of the oxygen sensor by Bosch in 1976, the combination of electronically controlled fuel injection and three-way catalysts (TWCs) has made a profound contribution to the simultaneous abatement of CO, HC, and NO_x emissions from gasoline engines [1]. With the continuous tightening of emission regulations and the dramatic fluctuations in precious metal prices, commercial TWCs have gradually shifted from the initial Pt/Rh formulations to Pd/Rh-based catalysts [2]. However, since 2019, the prices of Pd and Rh have surged significantly, with Pd reaching nearly 2000 USD/oz and Rh approaching 8500 USD/oz. In contrast, Pt remains approximately one-third the price of Pd and one-tenth that of Rh, which makes it an economically attractive alternative. Nevertheless, systematic studies on Pt-only TWCs remain relatively limited [3].

NO_x storage–reduction (NSR) catalysts, typically represented by Pt/BaO/Al₂O₃, are often regarded as a hybrid between conventional TWCs and alkaline storage components [4,5]. Although NSR catalysts are highly effective for NO_x abatement under lean conditions, their performance in CO and HC oxidation under rich conditions is inadequate. In real automotive exhaust conditions, the catalyst operates under dynamic lean–rich cycling near the stoichiometric air–fuel ratio. Thus, Ce-based oxygen storage materials are frequently incorporated to regulate the catalyst operation window due to their ability to store oxygen under lean conditions and release it under rich conditions [6]. CeO₂ not only provides excellent oxygen storage–release capacity (OSC) but also enhances the dispersion of precious metals, promotes water–gas shift and steam reforming reactions, and introduces basicity beneficial for NO_x storage [7,8,9]. On the other hand, Ba plays a crucial role as an alkaline NO_x storage medium, and its incorporation into TWCs has also been reported to improve overall activity, thermal stability, and hydrothermal durability [5,6,10].

Despite the individual advantages of Ba- and Ce-containing components, their coexistence within the same catalyst system introduces complex and often competing interactions. Studies have demonstrated that the Ba/Ce ratio critically determines catalyst microstructure, storage capacity, and durability [11,12]. For instance, CeO₂ dispersed on Al₂O₃ provides highly dispersed BaCO₃ domains that are beneficial for NO_x storage, whereas excess Ba tends to form stable but catalytically inert BaAl₂O₄ upon hydrothermal aging [13]. Conversely, simultaneous modification of Pd or Pt catalysts with both BaO and CeO₂ can sometimes lead to competitive effects that suppress their individual beneficial functions [14,15]. These findings highlight the necessity of systematically optimizing the Ba/Ce ratio to balance storage capacity, redox properties, and structural stability under realistic operating conditions. However, existing reports primarily describe these interactions qualitatively, without establishing quantitative or mechanistic relationships.

Nevertheless, despite these insights, a comprehensive understanding of how the Ba/Ce ratio governs the physicochemical evolution, Pt–support interactions, NO_x storage behavior, and three-way catalytic performance of Pt-based formulations remains largely unexplored. Existing studies typically vary Ba or Ce loading individually, focus on Pd-based catalysts, or examine storage and oxygen materials in isolation. As a result, the cooperative or competitive interplay between Ba and Ce under realistic TWC conditions—particularly during hydrothermal aging—remains insufficiently understood. In particular, the mechanistic links among Ba/Ce ratio–dependent oxygen mobility, Pt dispersion, BaCO₃ domain evolution, and performance deterioration during high-temperature aging have not yet been established, leaving a critical gap in rational catalyst design.

In this study, we focus on Pt-based Ba–Ce/Al₂O₃ catalysts prepared by introducing Ba via Ba(CH₃COO)₂ precursors and Ce via solid CeO₂ powders. A series of fresh and hydrothermally aged xBa–yCe-PA catalysts with different molar Ba/Ce ratios were synthesized and comprehensively characterized by N₂ physisorption, XRD, XPS, H₂-TPR, SEM, NO_x-TPD, CO pulse adsorption, NO_x storage tests, and dynamic light-off evaluations. The objective is to unravel the structure–performance relationships of Pt-based catalysts, elucidate the role of Ba/Ce interactions in determining NO_x storage and three-way conversion activity, and clarify the evolutionary mechanisms during hydrothermal aging. This systematic investigation provides insights into designing cost-effective, thermally durable Pt-based catalysts as potential alternatives to Pd/Rh-dominated commercial TWCs.

2. Results and Discussion

2.1. N₂ Physisorption Analysis: BET Surface Area and Pore Structure Characterization

The N₂ adsorption–desorption isotherms and pore size distributions of the catalysts with different Ba/Ce ratios are shown in Figure 1, and the corresponding textural parameters are summarized in

Table 1. Specific surface, pore volume and mean pore diameters.

Sample	Specific Surface Area (m2/g)	Pore Volume (mL/g)	Average Pore Diameter (nm)
La2O3–Al2O3	143	0.36	7.2
CeO2	141	0.29	7.6
B-PA	74	0.29	6.9
4B1C-PA	72	0.28	6.6
BC-PA	102	0.35	6.3
1B4C-PA	104	0.35	6.4
C-PA	142	0.43	5.8
A-B-PA	58	0.28	7.0
A-4B1C-PA	76	0.32	6.7
A-BC-PA	87	0.35	6.4
A-1B4C-PA	99	0.42	6.7
A-C-PA	90	0.41	6.8

. All samples exhibit type IV isotherms with H3-type hysteresis loops, characteristic of mesoporous materials, indicating the preservation of mesoporous structures upon Ba and Ce incorporation [16].

Among the fresh catalysts (Figure 1A and Table 1), C-PA displays the largest BET surface area (142 m²/g) and pore volume (0.43 mL/g), accompanied by the smallest average pore diameter (1.20 nm), suggesting the development of abundant accessible mesopores that could enhance catalytic activity. In contrast, B-PA and 4B1C-PA exhibit significantly lower surface areas (74 and 72 m²/g, respectively) and reduced pore volumes (0.29 and 0.28 mL/g), consistent with partial pore blockage induced by Ba loading. BC-PA and 1B4C-PA maintain intermediate surface areas (~10²–10⁴ m²/g) with pore diameters around 1.38–1.40 nm, reflecting a balance between structural stability and microporosity.

After hydrothermal aging (Figure 1B,D), most samples undergo a noticeable reduction in surface area and pore volume, in line with pore narrowing and framework densification. For example, A-B-PA shows a surface area of only 58 m²/g and a pore volume of 0.2 mL/g, while A-4B1C-PA and A-BC-PA retain slightly higher values (76 and 87 m²/g, respectively). Interestingly, A-C-PA preserves a relatively high surface area (90 m²/g) and pore volume (0.41 mL/g), indicating superior hydrothermal stability compared to Ba-rich compositions. The corresponding pore size distributions (Figure 1C,D) confirm that most mesopores are concentrated in the 2–5 nm range, with aged catalysts showing narrower distributions and reduced mesopore volumes [17].

Overall, the BET surface area and pore structure data highlight that Ce-rich catalysts (C-PA, A-C-PA) retain higher surface areas and mesopore accessibility, whereas Ba-rich systems (B-PA, A-B-PA) suffer from severe textural degradation upon hydrothermal aging. These trends align with previous reports that alkaline-earth dopants promote densification and sintering, while rare-earth oxides impart improved structural stability under hydrothermal conditions [17].

2.2. Phase Structure and Crystallite Size of the Catalyst

The XRD results (Figure 2) together with the crystallite size data (

Table 2. Physical parameters of fresh and hydrothermally aged catalysts with varying Ba/Ce ratios.

Sample	CeO2 Crystallite Size (nm)	BaCO3 Crystallite Size (nm)	Pt Crystallite Size (nm)	Pt Particle Size (nm)	Pt Dispersion (%)	Pt Loading (wt.%)
B-PA	–	31.5	–	2.83	29.90	0.90
4B1C-PA	19.2	30.0	–	2.11	40.18	1.01
BC-PA	18.1	29.6	–	1.92	44.01	1.04
1B4C-PA	10.8	23.2	–	1.89	45.46	1.03
C-PA	11.7	–	–	1.34	63.25	1.00
A-B-PA	–	28.1	16.0	11.53	7.35	0.89
A-4B1C-PA	16.0	27.9	14.9	10.83	7.79	0.94
A-BC-PA	17.8	37.6	20.3	19.61	6.97	0.88
A-1B4C-PA	17.9	24.1	32.85	32.85	2.58	0.87
A-C-PA	23.4	–	48.8	44.60	1.90	0.94

) provide critical insights into the structural evolution of the catalysts as a function of Ba/Ce ratio and hydrothermal aging. In fresh samples, the absence of detectable Pt diffraction peaks suggests that Pt species are either atomically dispersed or present as ultra-fine clusters (<5 nm), consistent with previous reports on Pt/CeO₂ and Pt/Al₂O₃ catalysts where high metal dispersion suppresses XRD detectability of Pt phases [18,19]. The observed strengthening of BaCO₃ reflections with increasing Ba content, alongside the attenuation of CeO₂ peaks, indicates that Ba enrichment suppresses ceria crystallinity, while Ce-rich samples maintain smaller CeO₂ crystallites (Table 2), thereby enhancing oxygen storage and metal–support interactions [20].

Upon hydrothermal aging at 850 °C, distinct diffraction peaks corresponding to BaAl₂O₄ and metallic Pt emerge, highlighting two critical processes, firstly, solid-state reactions between BaCO₃ and Al₂O₃ leading to the formation of BaAl₂O₄, which has been previously reported as a thermally stable but catalytically less active phase; secondly, growth of Pt and CeO₂ crystallites, confirming sintering and agglomeration under high-temperature conditions [21]. Notably, the CeO₂ peaks become sharper after aging, reflecting increased crystallinity and particle size, which may reduce oxygen vacancy concentration and weaken redox capacity [22].

It should be noted that although the XRD-derived Pt crystallite size after aging becomes larger than that of CeO₂, this phenomenon is fully consistent with the known hydrothermal degradation mechanism of Pt-based TWCs, where PtOx decomposition and particle coalescence reduce the Pt–CeO₂ interfacial contact. Such structural evolution naturally leads to a decline in catalytic activity, which is indeed observed in our aged samples. Moreover, XRD crystallite size reflects the coherent diffraction domain and typically overestimates the surface-exposed Pt nanoparticle size; therefore, the correlation between XRD-derived crystallite dimensions and catalytic performance is not strictly linear.

Interestingly, the correlation between Ba/Ce ratio and Pt dispersion is not strictly linear. While Ba-rich systems tend to promote Pt agglomeration due to weaker metal–support anchoring, the A-4B1C-PA catalyst exhibits the smallest Pt crystallite size and the highest Pt dispersion after aging, suggesting that an optimal Ba/Ce ratio can stabilize Pt species and inhibit sintering. This finding is consistent with prior studies that demonstrated synergistic effects of Ba and Ce in maintaining dispersion of noble metals under severe aging conditions [23,24]. Thus, tuning the Ba/Ce ratio offers a promising route to balance structural stability and catalytic activity by suppressing the sintering of active species while maintaining oxygen storage properties.

2.3. Morphology and Textural Analysis

The SEM images (Figure 3) provide further morphological evidence to support the crystallographic and textural properties obtained from XRD and N₂ physisorption analyses. For the fresh catalysts, a clear trend is observed with increasing Ce content, the particles gradually become smaller and more fragmented. For instance, C-PA exhibits the most finely divided morphology, consistent with its highest BET surface area (142.2 m²/g) and pore volume (0.428 mL/g) among the series (Table 1). In contrast, Ba-rich samples such as 4B1C-PA and BC-PA display larger, more aggregated particles, correlating with their lower surface areas (~7²–10² m²/g) and smaller pore volumes. These results confirm that Ce addition inhibits crystallite growth and enhances mesoporosity, thereby improving textural properties and dispersion of the active phase. This finding is in agreement with previous studies reporting that CeO₂ acts as a structural promoter by generating oxygen vacancies and stabilizing smaller crystallites [22].

After hydrothermal aging, all samples show significant particle coarsening and agglomeration. The particle boundaries become smoother, and the overall morphology appears more sintered compared to the fresh state. Nevertheless, Ce-rich aged catalysts (e.g., A-C-PA and A-BC-PA) still retain relatively smaller and less compact particles compared to their Ba-rich counterparts (e.g., A-4B1C-PA). This observation is consistent with the XRD-derived crystallite sizes, where CeO₂ crystallite growth was less pronounced (from ~10–19 nm in fresh to ~16–23 nm in aged samples), while BaCO₃ crystallites underwent more severe enlargement (from ~23–31 nm to ~28–38 nm). The N₂ adsorption data also corroborate this trend, showing that Ce-rich catalysts preserved higher pore volume and surface area after aging relative to Ba-rich samples.

2.4. Electronic Properties of the Active Metal Species

To investigate the influence of the Ba/Ce ratio on the electronic state of the active metal species, X-ray photoelectron spectroscopy (XPS) was performed on the catalysts. Because the Pt 4f region (78–70 eV) overlaps strongly with the Al 2p signal from the La–Al₂O₃ support, the Pt 4d region (340–310 eV) was selected for analysis (Figure 4). However, due to the intrinsically weak Pt 4d cross section and the relatively low Pt loading, the Pt 4d spectra exhibit a limited signal-to-noise ratio. As a result, only qualitative trends in the Pt 4d₅/₂ binding energy were evaluated, and no attempt was made to quantitatively analyze the Pt 4d spin–orbit splitting. The apparent variation in the energy separation between the Pt 4d₅/₂ and 4d₃/₂ components among different samples is therefore attributed to spectral noise rather than true chemical differences, and the discussion is restricted to the more reliable Pt 4d₅/₂ peak position.

For the fresh catalysts, the Pt 4d₅/₂ binding energy gradually increases from 315.5 eV in B-PA to 315.8 eV in Ce-rich samples, indicating enhanced electronic interaction between Pt and CeO₂. This shift is consistent with electron transfer from Pt to CeO₂, which modifies the Pt d-band and strengthens metal–support interactions, in agreement with previous reports on Pt/CeO₂ systems [22,25].

Upon hydrothermal aging at 850 °C, all catalysts show a decrease in Pt 4d₅/₂ binding energy, reflecting the partial reduction of PtOx species and the sintering-induced decrease in surface-exposed Pt atoms [26]. Importantly, the reduced Pt signal intensity arises from loss of surface accessibility and not from Pt volatilization, as Pt is non-volatile under the present conditions and PtOx decomposition removes only oxygen while preserving Pt mass. Within the aged catalysts, the Pt 4d₅/₂ binding energy increases with CeO₂ content, indicating renewed electronic interaction between Pt and Ce after structural reconstruction during aging. This trend further confirms that CeO₂ plays the dominant role in governing Pt electronic properties in the Pt–Ba–Ce–Al system [27,28].

2.5. Reducibility of Catalyst Components

Figure 5 shows the H₂-TPR profiles of the catalysts, and the quantitative parameters are summarized in

Table 3. Quantitative H₂-TPR analysis of catalysts with varying Ba/Ce ratios.

Sample	Low-Temperature Reduction Peak Temperature (°C)	Low-Temperature Reduction Peak Area (CPS)	High-Temperature Reduction Peak Temperature (°C)	High-Temperature Reduction Peak Area (CPS)
B-PA	-	-	539	2624.14
4B1C-PA	-	-	531	3550.72
BC-PA	240	688.85	520	2167.84
1B4C-PA	211	868.9	485	962.6
C-PA	233	1611.16	-	-

. In Pt–Ce–Ba–Al systems, only PtOx species and ceria-derived oxygen are reducible in the temperature range probed, whereas Ba-containing phases (BaCO₃, BaO, BaAl₂O₄) are non-reducible. Thus, H₂ consumption reflects Pt- and Ce-related processes, while any interaction of H₂ with carbonates corresponds to hydrogenation/decomposition, producing CO₂ rather than oxygen removal. Because the TPR setup did not include a CO₂ trap, CO₂–TCD interference is possible and has been considered in the analysis [20,29].

Fresh catalysts exhibit a distinct low-temperature peak (210–240 °C), attributable to the reduction of PtOx and interfacial Pt–O–Ce species. The intensity of this feature increases with Ce loading, indicating a larger population of labile oxygen species associated with highly dispersed Pt–Ce interfaces. A second peak at 480–540 °C arises from deeper ceria lattice oxygen. Its shift to higher temperature with increasing Ba content reflects reduced Pt–Ce contact and partial ceria surface coverage by BaCO₃, consistent with the larger ceria crystallite sizes in Ba-rich samples [26].

In Ba-rich formulations, carbonate hydrogenation may contribute to broad or distorted high-temperature signals, further emphasizing the need for cautious interpretation of H₂ consumption in these samples [30,31].

Hydrothermally aged catalysts show strongly attenuated or nearly vanished reduction peaks [32]. This loss of reducibility is attributed to Pt sintering (eliminating PtOx and Pt–Ce interfacial oxygen), ceria coarsening, and partial transformation of BaCO₃ into non-reducible BaAl₂O₄ [33]. Enlarged TPR profiles and quantified peak areas confirm the drastic decrease in hydrogen uptake, indicating a significant reduction in redox-active oxygen species [34].

Overall, these results demonstrate that the Ba/Ce ratio governs the reducibility of the fresh catalysts—Ce promoting low-temperature redox activity and Ba suppressing ceria reduction—while hydrothermal aging deactivates both processes through structural reconstruction of Pt, CeO₂, and Ba phases.

2.6. Interactions Among Catalyst Components

In Pt–Ba–Al-based lean NO_x trap (LNT) catalysts, NO_x storage proceeds primarily through the formation of nitrites and nitrates on BaCO₃, which is the dominant barium-containing phase under realistic exhaust conditions [35,36]. Unlike idealized Pt/BaO/Al₂O₃ systems discussed in early studies, BaCO₃ does not decompose to BaO under our operating temperatures (≤550 °C) or under hydrothermal aging conditions (850 °C, 8% O₂ and 8% CO₂). Therefore, BaCO₃—not BaO—is the actual NO_x storage medium in the present catalysts.

During the lean phase, NO is oxidized on Pt sites to NO₂:

2NO + O₂ → 2NO₂ (Pt-catalyzed)

The generated NO₂ subsequently reacts with BaCO₃ to form nitrates via:

BaCO₃ + 2NO₂ → Ba(NO₃)₂ + CO₂

BaCO₃ + 2NO₂ + ½O₂ → Ba(NO₃)₂ + CO₂

These reactions constitute the dominant nitrate-formation pathway in BaCO₃-containing LNT catalysts, consistent with widely reported mechanisms for Pt/BaCO₃/Al₂O₃ systems. Nitrite intermediates may transiently form on Pt or CeO₂ surfaces, but nitrate formation on BaCO₃ is thermodynamically favored under typical exhaust conditions.

As nitrate accumulation progresses, NO₂ spillover from Pt and CeO₂ domains toward BaCO₃ storage sites takes place, enabling storage at sites located further away from Pt. The efficiency of this spillover process is strongly influenced by the spatial proximity of Pt to BaCO₃ and by the oxygen mobility of the support. Accordingly, NO_x-TPD profiles provide insights into both the strength of nitrate binding and the Pt–BaCO₃ domain connectivity.

During the subsequent rich phase, stored nitrate species decompose or are reduced by CO, H₂, or hydrocarbons to release NO, N₂, or NH₃, depending on temperature and gas composition. Nitrate decomposition occurs preferentially at BaCO₃ sites closer to Pt because reductants activate more readily at Pt and diffuse outward via spillover mechanisms [37].

This BaCO₃-based mechanistic framework reflects the actual chemical environment of the catalysts studied here and provides a consistent explanation for the NO_x storage behaviors observed in both fresh and hydrothermally aged samples.

As shown in Figure 6, the fresh B-PA catalyst exhibits a NO_x desorption peak centered at 476 °C. After hydrothermal aging at 850 °C, the corresponding peak for A-B-PA shifts to 457 °C. This downward shift indicates that the stored NO_x species become less strongly bound upon aging, which can be ascribed to structural and chemical changes in the BaCO₃ domains and their environment. Hydrothermal treatment partially modifies and redistributes BaCO₃ and alters the proximity and electronic environment of Pt relative to Ba-containing storage sites, leading to less stable nitrate species that decompose at lower temperatures [38].

For the fresh catalysts, increasing the Ba/Ce ratio results in progressively higher desorption temperatures and larger peak areas (Figure 6A). This trend reflects the dual role of CeO₂. On the one hand, the introduction of CeO₂ enhances Pt–Ce interactions and oxygen mobility, which facilitates NO_x release and regeneration at lower temperatures, as evidenced by the lower desorption temperatures in Ce-rich samples. On the other hand, increasing Ce content dilutes or partially covers BaCO₃, thereby decreasing the number of available Ba-based storage sites and reducing the overall NO_x storage capacity. Consequently, Ce-rich catalysts exhibit improved low-temperature redox properties (consistent with H₂-TPR results) but weaker NO_x storage capacity compared with Ba-rich counterparts.

After hydrothermal aging, all Ce-containing catalysts display markedly increased NO_x desorption temperatures (Figure 6B). This behavior can be rationalized by the combined effects of Pt sintering and aggregation, diminished Pt–CeO₂ charge transfer, and reduced oxygen mobility, which all hinder the regeneration of Ba-based nitrate species [34]. In addition, XRD results show that a fraction of BaCO₃ is converted into BaAl₂O₄ during aging, a thermodynamically stable and non-storage phase that weakens the effective Ba–Pt synergy. As a result, aged Ce-containing catalysts exhibit both reduced NO_x storage capacity and less efficient regeneration, underscoring the importance of optimizing the Ba/Ce ratio to balance storage ability and durability in LNT-type formulations.

The combined results from BET, XRD, XPS, H₂-TPR, NO_x-TPD, and SEM analyses reveal distinct structural roles of Ce and Ba in determining the redox and storage properties of the catalysts. In fresh samples, CeO₂ stabilizes PtOx species through strong metal–support interactions, forming Pt–O–Ce interfacial bonds that enhance oxygen mobility and low-temperature redox activity [22,26]. SEM and BET data confirm that Ce-rich catalysts (e.g., C-PA) possess smaller, more fragmented particles with higher surface areas, favoring Pt dispersion, whereas Ba-rich catalysts exhibit larger aggregated morphologies and higher NO_x storage capacity due to BaCO₃, but show weaker Pt–Ce interactions and reduced reducibility [34]. Upon hydrothermal aging at 850 °C, PtOx partially decomposes into lower-valent or metallic Pt species with pronounced particle growth (XPS, SEM), while BaCO₃ reacts with Al₂O₃ to form BaAl₂O₄, a stable but inert phase that diminishes high-temperature reducibility and NO_x storage. Concurrently, CeO₂ crystallite growth weakens Pt–CeO₂ interactions, leading to a loss of interfacial oxygen species and decreased oxygen mobility. Although some Pt–Ba proximity is observed after aging, it cannot compensate for the loss of Pt–Ce synergy. These results suggest a structural evolution pathway where Ce is crucial for stabilizing dispersed Pt and promoting oxygen mobility, while Ba contributes primarily to NO_x storage but facilitates sintering and inert phase formation under severe aging [23]. Optimizing the Ba/Ce ratio is therefore essential to balance redox activity and NO_x storage durability in practical LNT catalysts. The hydrothermal aging-induced evolution in the Ba–Ce–Pt–Al catalyst system shown in Figure 7.

2.7. NO_x Storage Properties of the Catalyst

Figure 8 shows the variation of NO_x concentration and NO₂/NO ratio over time during the NO_x storage process at 300 °C for catalysts with different Ba/Ce ratios. As observed in the initial phase after switching to a NO + O₂ + N₂ gas mixture, almost no NO_x is detected in the effluent, indicating that NO_x is rapidly and completely stored on the catalyst surface via the nitrite storage pathway. With increasing storage time, both the NO_x concentration and the NO₂/NO ratio rise progressively, suggesting a transition of the NO_x storage mechanism toward the nitrate pathway. This shift is attributed to the migration of stored nitrates from the catalyst surface into the bulk phase, or the spillover of NO_x species from Pt-proximal storage sites to more distant Ba-containing sites. As a result, leakage of NO and NO₂ gradually increases, and the catalyst approaches its NO_x storage saturation. The NO₂/NO ratio at this stage reflects the catalyst’s ability to oxidize NO to NO₂.

Given the wide temperature range of vehicle exhaust gases,

Table 4. NO_x Storage Performance of Catalysts with Different Ba/Ce Ratios at Various Temperatures.

Sample	200 °C		300 °C		400 °C		500 °C
	NSC	NO2/NO	NSC	NO2/NO	NSC	NO2/NO	NSC	NO2/NO
	mmol/gcat		mmol/gcat		(mmol/gcat)		mmol/gcat
B-PA	148.59	0.90	239.35	4.73	280.21	21.90	162.50	11.21
4B1C-PA	199.83	3.00	317.81	10.94	328.30	29.70	164.68	11.13
BC-PA	228.85	5.50	361.27	30.20	278.92	34.94	137.32	11.50
1B4C-PA	165.96	15.13	216.81	89.90	109.28	37.71	40.74	11.31
C-PA	131.31	18.91	123.13	13.42	28.42	38.50	4.60	11.50
A-B-PA	64.73	11.93	36.31	23.83	32.69	28.95	16.42	11.30
A-4B1C-PA	91.28	10.95	73.51	34.80	68.86	33.11	41.34	11.62
A-BC-PA	82.42	12.91	47.20	34.91	43.58	34.62	23.01	11.81
A-1B4C-PA	81.94	15.46	40.43	56.49	28.13	35.75	7.92	11.50
A-C-PA	51.61	20.81	26.67	74.92	6.80	37.21	11.61	11.62

summarizes the NO_x storage performance of fresh and hydrothermally aged catalysts with varying Ba/Ce ratios at four isothermal conditions: 200 °C, 300 °C, 400 °C, and 500 °C. As shown in the table, the NO_x storage capacity (NSC) of the fresh catalysts exhibits a volcano-type trend with respect to the Ba/Ce ratio. However, the optimal Ba/Ce ratio for storage varies with temperature. In the low-temperature range (200 °C and 300 °C), the NSC follows the order: BC-PA > 4B1C-PA > 1B4C-PA > B-PA > C-PA. In contrast, in the high-temperature range (400 °C and 500 °C), the order becomes: 4B1C-PA > B-PA > BC-PA > 1B4C-PA > C-PA. These trends suggest that Ce-containing species contribute more effectively to NO_x storage at lower temperatures, while Ba-containing species are more active at elevated temperatures [39,40].

Comparing NO_x storage capacities at different temperatures reveals that high-Ba/low-Ce catalysts exhibit the highest NSC at 400 °C, whereas high-Ce/low-Ba catalysts perform best at 300 °C. This can be attributed to the lower thermal stability of stored nitrates with increasing Ce/Ba ratios, which is consistent with the NO_x-TPD results. After hydrothermal aging at 850 °C, the overall NO_x storage capacity of all catalysts is significantly reduced. Nevertheless, the volcano-type dependence on Ba/Ce ratio remains, indicating that an optimal Ba–Ce coexisting system enhances thermal durability—aligning well with the findings from pore structure and XRD analyses.

For both fresh and aged catalysts, the NO₂/NO activity increases with rising Ce/Ba ratios, indicating that the interfacial oxygen species at the Pt–CeO₂ interface play a crucial role in enhancing the oxidation capacity of the catalyst. Since the oxidation of NO to NO₂ is kinetically limited at low temperatures and thermodynamically constrained at high temperatures, a comparison of the NO₂/NO ratios at 200 °C reveals that aged catalysts exhibit superior oxidation performance. Based on the CO pulse adsorption and the semi-quantitative XRD particle size analysis, it can be inferred that larger Pt particles are more favorable for NO oxidation—a finding consistent with the study by Mulla et al. [41].

The NO_x storage performance of a catalyst depends not only on the basicity of the storage medium and the catalyst’s ability to oxidize NO to NO₂, but more importantly, on the synergy between these two properties. Among the tested formulations, 4B1C-PA demonstrates practical application potential due to its strong NO oxidation ability, high NO_x storage capacity, and excellent hydrothermal stability.

2.8. Three-Way Catalytic Performance and Its Correlation with Catalyst Structure

Figure 9 presents the light-off performance of the catalysts under simulated rich–lean cycling conditions representative of practical TWC operation. As shown in the figure, for both fresh and aged samples, the light-off curves of CO, C₃H₆, and NO shift progressively toward higher temperatures with increasing Ba/Ce ratio, indicating that Ce-rich catalysts exhibit superior low-temperature activity due to their enhanced redox properties and oxygen mobility. In contrast, Ba-rich formulations show improved high-temperature conversion, consistent with the contribution of Ba-containing storage domains. The formation profiles of secondary species such as NH₃ and N₂O also shift to higher temperatures with increasing Ba content, further reflecting the stronger redox functionality of Ce-rich catalysts at low temperatures and the increased H₂ availability that promotes NH₃ formation.

To evaluate catalytic activity under steady-state conditions, isothermal TWC measurements were performed at 200 °C and 500 °C, and the results are summarized in

Table 5. Three-Way Catalytic Activity of Catalysts with Different Ba/Ce Ratios at 200 °C.

Sample	Average NOx Conversion (%)	Average CO Conversion (%)	Average C3H6 Conversion (%)	Average NH3 Released (µmol gcat−1 cycle−1)	Average N2O Released (µmol gcat−1 cycle−1)
B-PA	6.3	23.6	0.0	0.5	0.3
4B1C-PA	6.2	30.9	0.0	0.5	0.3
BC-PA	9.1	41.1	0.0	0.4	0.3
1B4C-PA	30.5	72.9	13.5	0.7	1.7
C-PA	75.4	98.8	36.2	1.5	7.4
A-B-PA	−0.8	−0.9	0.0	0.1	0.0
A-4B1C-PA	−2.1	0.2	0.0	0.1	0.0
A-BC-PA	−1.6	6.4	0.0	0.1	0.1
A-1B4C-PA	−3.6	23.1	0.0	0.7	0.1
A-C-PA	11.0	69.9	4.0	2.9	0.7

and

Table 6. Three-Way Catalytic Activity of Catalysts with Different Ba/Ce Ratios at 500 °C.

Sample	Average NOx Conversion (%)	Average CO Conversion (%)	Average C3H6 Conversion (%)	Average NH3 Released (µmol gcat−1 cycle−1)	Average N2O Released (µmol gcat−1 cycle−1)
B-PA	96.5	59.5	100	7.9	0.0
4B1C-PA	92.3	52.3	100	8.0	0.0
BC-PA	85.2	64.5	100	6.8	0.0
1B4C-PA	72.9	60.9	100	4.8	0.0
C-PA	77.5	70.6	100	5.6	0.0
A-B-PA	66.4	65.9	69.4	7.3	0.0
A-4B1C-PA	66.6	56.0	91.9	9.0	0.0
A-BC-PA	59.8	49.5	100	11.1	0.0
A-1B4C-PA	57.6	57.2	100	10.9	0.0
A-C-PA	67.8	60.1	100	8.3	0.0

. At 200 °C, the conversion efficiencies of NO_x, CO, and C₃H₆ increase with rising Ce content, accompanied by higher NH₃ and N₂O formation [12,42,43]. This trend highlights the ability of Ce-rich catalysts to participate in low-temperature redox, steam reforming, and water–gas shift reactions, thereby increasing the concentration of intermediate H₂ and promoting secondary product formation. Slight negative NO_x conversion values observed in some samples at low temperature arise from transient NO_x release rather than experimental artefacts. Specifically, thermally labile nitrate/nitrite species on BaCO₃- or CeO₂-associated sites can decompose during the early stages of light-off, and NO₂-to-NO back-reduction or oxygen-competition reactions during rich–lean switching may temporarily increase the outlet NO_x concentration above the inlet level. Such behavior is widely reported in LNT and TWC systems and is intrinsic to the dynamic NO_x storage–release process. In addition to NO_x conversion, the evolution of individual nitrogen-containing species (NO, NO₂, N₂O, NH₃, and N₂) was also analyzed to ensure the reliability of the catalytic performance. At low temperatures, Ce-rich catalysts exhibit increased NO₂ and N₂O formation due to their stronger oxidation capability and the involvement of Pt–CeO₂ interfacial oxygen. Meanwhile, the slight increase in outlet NO concentrations observed for some samples corresponds to the decomposition of surface nitrates/nitrites, consistent with the negative NO_x conversion values discussed above. NH₃ formation remains low at 200 °C but is detectable for Ce-rich catalysts, reflecting their ability to generate H₂ through low-temperature reforming and WGS pathways. Although N₂ is not directly measured, the nitrogen balance calculated as N₂ = NO_xin − (NO + NO₂ + N₂O + NH₃)_out indicates that the majority of nitrogen is converted to N₂, with deviations within ±5% of the inlet nitrogen flux, confirming the internal consistency of the measurements and the absence of unaccounted nitrogen species.

At 500 °C, the overall NO_x conversion increases with the Ba/Ce ratio, whereas CO and C₃H₆ oxidation become less efficient for Ba-rich samples. These trends reinforce the distinct functional roles of the two promoters: Ba enhances high-temperature NO_x storage and regeneration capability, while Ce facilitates low-temperature oxidation pathways. The dependence of NH₃ formation on Ce loading further confirms the involvement of Ce-driven redox and WGS chemistry in shaping the product distribution. At higher temperatures (500 °C), the evolution of nitrogen species reflects a shift toward complete reduction pathways. NO and NO₂ concentrations decrease substantially across all catalysts, while N₂O formation remains low, indicating efficient NO_x reduction in the presence of reductants generated from CO and hydrocarbon reforming. The calculated nitrogen balance shows that N₂ becomes the dominant product above 450 °C, with N₂ selectivity exceeding 85% for Ba-rich catalysts owing to their enhanced NO_x storage–reduction capability. Carbon balance analysis, based on the inlet and outlet CO and CO₂ concentrations, confirms that total carbon is conserved within ±3%, and no measurable methane or carbonaceous byproducts are detected. These results validate that the observed catalytic trends accurately represent the intrinsic reaction behavior of the Ba–Ce-modified Pt catalysts under TWC conditions.

To elucidate the intrinsic correlation between the Ba/Ce ratio and the structural and catalytic performance of the catalysts, Figure 10A presents the variations in low-temperature average conversion efficiencies of NO_x, CO, and C₃H₆, as well as the specific surface area, pore volume, Pt particle size, and Pt/CeO₂ interfacial oxygen species content as functions of the Ba/Ce ratio. As shown in the figure, all these parameters exhibit linear trends with respect to the Ba/Ce ratio, indicating a close interrelationship among catalyst composition, structure, and performance.

During the catalyst preparation, Ba was introduced as a Ba(CH₃COO)₂ precursor, whereas Ce was added in the form of CeO₂ solid powder. With increasing Ba/Ce ratio, the extent of pore blockage became more pronounced, leading to a decrease in specific surface area and pore volume, and a corresponding increase in Pt particle size. Conversely, a lower Ba/Ce ratio favored the efficient dispersion of Pt nanoparticles. The increased CeO₂ content enhanced the probability of Pt-CeO₂ contact, leading to a higher abundance of interfacial active oxygen species [36]. The efficient oxygen spillover facilitated by the Pt-CeO₂ interface accelerates the storage and release of oxygen, mitigating the impact of air-to-fuel ratio fluctuations on the three-way catalytic (TWC) performance. Additionally, the interfacial oxygen species contribute significantly to the activation and dissociation of gaseous pollutants, thereby improving the light-off performance of CO, NO_x, and C₃H₆ [44,45].

Given that the high-temperature NO_x conversion efficiency exhibits an opposite trend to that of CO and C₃H_6, Figure 10B further illustrates the relationship between the Ba/Ce ratio and the high-temperature average NO_x conversion efficiency, NO_x storage capacity, and BaCO₃ content. These parameters also show a near-linear relationship with the Ba/Ce ratio. With increasing Ba content, the BaCO₃ concentration increases, leading to enhanced NO_x storage capacity.

Under high-temperature conditions, water-gas shift and steam reforming reactions catalyzed by Pt induce deviations from the ideal air-fuel ratio (0.98–1.02), resulting in a generally leaner exhaust atmosphere [46]. Taking advantage of the inherent characteristics of lean NO_x storage and rich NO_x reduction, high-Ba catalysts exhibit superior NO_x conversion efficiency in high-temperature regimes.

Therefore, it can be inferred that coupling NO_x storage materials with conventional TWC catalysts to form NO_x storage-enabled TWC systems could effectively address the challenges of insufficient NO_x conversion performance during intermittent lean conditions and frequent thermal startups in aftertreatment systems.

3. Materials and Methods

3.1. Materials

Barium acetate (Ba(CH₃COO)₂·H₂O, analytical grade) was purchased from Xiya Chemical Technology Co., Ltd. (Zibo, Shandong, China). Cerium dioxide (CeO₂, industrial grade) was supplied by Panzhihua Sci-Resin Rare Earth New Materials Co., Ltd. (Panzhihua, China) Lanthanum-modified alumina (La₂O₃–Al₂O₃, industrial grade) was obtained from Weihai Fude New Materials Co., Ltd. (Weihai, China). The platinum precursor, diamminedinitroplatinum(II) (Pt(NH₃)₂(NO₂)₂, industrial grade), was purchased from Guizhou Platinum Industry Co., Ltd. (Guiyang, China)

3.2. Catalyst Preparation

A platinum precursor solution was prepared by diluting 5 mL of an aqueous solution of diamminedinitroplatinum(II) (Pt(NH₃)₂(NO₂)₂, 100.12 g/L) with deionized water to a total volume of 100 mL. The solution was stirred at room temperature for 1 h. Subsequently, 35 g of La₂O₃–Al₂O₃ powder was added to the solution and stirred for an additional 10 min. Then, barium acetate and cerium dioxide were added sequentially in molar ratios of Ba/Ce = 1:0, 4:1, 1:1, 1:4, and 0:1, respectively, corresponding to different catalyst formulations. The resulting suspensions were continuously stirred at room temperature for 4 h.

After impregnation, the samples were dried overnight at 120 °C and subsequently calcined in static air at 600 °C for 4 h. The resulting fresh catalysts were designated as B-PA, 4B1C-PA, BC-PA, 1B4C-PA, and C-PA, corresponding to Ba/Ce molar ratios of 1:0, 4:1, 1:1, 1:4, and 0:1, respectively.

These fresh catalysts were subjected to hydrothermal aging at 850 °C for 10 h in a gas mixture of 8% H₂O, 8% O₂, 8% CO₂, and 76% N₂ at a total flow rate of 2 L/min. This accelerated aging protocol—commonly used to simulate the severe hydrothermal and oxidative environments encountered by TWCs during high-temperature engine operation—facilitates the evaluation of catalyst thermal stability and degradation pathways. The aged samples were denoted as A-B-PA, A-4B1C-PA, A-BC-PA, A-1B4C-PA, and A-C-PA, respectively.

3.3. Characterizations of Catalysts

To comprehensively investigate the physicochemical properties of the prepared catalysts and understand the structure–performance relationships, a series of characterization techniques were employed. Nitrogen physisorption analysis was conducted at 77 K to determine the specific surface area, pore volume, and pore size distribution via the Brunauer–Emmett–Teller (BET) method, providing insights into the textural properties of the catalysts. Powder X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.154 nm) was used to identify the crystalline phases and to estimate the crystallite sizes of CeO₂, BaCO₃, and Pt species through Scherrer equation-based analysis. X-ray photoelectron spectroscopy (XPS) was employed to analyze the surface elemental composition and oxidation states, particularly the electronic interactions between Pt and the Ba/Ce-containing support. Scanning electron microscopy (SEM) was used to observe the surface morphology and microstructure of the catalyst layers. H₂ temperature-programmed reduction (H₂-TPR) was performed to assess the reducibility of metal oxides and to probe the strength of metal–support interactions. CO pulse chemisorption measurements were carried out to evaluate the dispersion of Pt species, allowing for estimation of surface metal area and average particle size. The actual Pt content in each sample was quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES) following aqua regia digestion. Finally, NO_x temperature-programmed desorption (NO_x-TPD) analysis was conducted to study the NO_x storage capacity and desorption behavior, thereby elucidating the nature and strength of NO_x adsorption sites on the catalyst surface.

3.4. Catalytic Performance Evaluation

3.4.1. NO_x Storage Capacity (NSC) Evaluation

Isothermal NO_x storage performance was evaluated using a fixed-bed multifunctional catalyst testing system. In each test, 0.755 g of catalyst was loaded, and the gas hourly space velocity (GHSV) was maintained at 30,000 h⁻¹. Prior to NO_x adsorption, the catalyst was pretreated by heating to 550 °C in an oxidizing atmosphere containing 0.94 vol% O₂. Then, a reducing gas mixture of 2500 ppm CO and 1000 ppm C₃H₆ was introduced and maintained at 550 °C for 10 min. Afterward, the gas feed was switched back to the oxidizing atmosphere (0.94 vol% O₂) and held for another 10 min. The system was then cooled to the desired storage temperature under the same oxidizing atmosphere.

The NO_x storage capacity (NSC) and the NO₂/NO ratio were calculated using the following equations:

$$NSC={\displaystyle \frac{1}{g_{cat}·22.4}}{\int }_{t=0}^{t=600}V·\left[C_{{NO}_x}^{blank}-C_{{NO}_x}^{out}\right]dt$$

$${\displaystyle \frac{{NO}_2}{NO}}={\displaystyle \frac{C_{{NO}_2}^{out}}{C_{NO}^{out}}}\times 100\%$$

where:

• NSC: NO_x storage capacity;
• $g_{cat}$: Mass of the catalyst sample (g);
• $V$: Total volumetric flow rate of the gas (L/min);
• $C_{{NO}_x}^{blank}$: NO_x concentration in the blank test (ppm);
• $C_{{NO}_x}^{out}$: NO_x concentration at the reactor outlet during storage (ppm);
• $t$: Time (min);
• 22.4: Molar volume of an ideal gas at standard temperature and pressure (L/mol);
• $C_{{NO}_2}^{out}$: Outlet concentration of NO₂ (ppm);
• $C_{NO}^{out}$: Outlet concentration of NO (ppm).

3.4.2. Three-Way Catalytic Performance Under Dynamic Light-Off Conditions

The dynamic light-off performance for three-way catalysis was evaluated using a fixed-bed multifunctional catalyst testing system. A catalyst sample of 0.755 g was used, and the gas hourly space velocity (GHSV) was set to 30,000 h⁻¹. The concentrations of the gas-phase products were continuously monitored using a Multigas 6000 on-line FTIR gas analyzer. The Multigas 6000 FTIR analyzer allows simultaneous quantification of NO, NO₂, N₂O, NH₃, CO, CO₂ and hydrocarbons, enabling continuous monitoring of all relevant gas-phase species during TWC evaluation. Prior to testing, the catalyst was pretreated by heating to 550 °C under a gas atmosphere containing 0.94 vol% O₂, 10 vol% H₂O, and 8 vol% CO₂, and held for 5 min. After pretreatment, the temperature was lowered to 150 °C, and a temperature-programmed reaction was conducted under rich-lean cycling conditions (λ = 0.98–1.02) with a switching frequency of 10 s. The temperature was ramped from 150 °C to 550 °C at a rate of 15 °C·min⁻¹. The detailed compositions of the feed gases during rich and lean phases are listed in

Table 7. Reaction atmospheres.

Gas Composition	NO (ppm)	CO (ppm)	C3H6 (ppm)	CO2 (%)	O2 (%)	H2O (%)	N2	GHSV (h−1)
Lean	500	2500	1000	8	0.15	10	Balance gas	30,000
Rich	500	2500	1000	8	0.94	10	Balance gas	30,000

.

3.4.3. Steady-State Three-Way Catalytic Performance at Constant Temperature

The steady-state three-way catalytic (TWC) performance was evaluated using a fixed-bed multifunctional catalyst testing system. A catalyst mass of 0.755 g was employed, and the gas hourly space velocity (GHSV) was maintained at 30,000 h⁻¹. The outlet gas concentrations were continuously monitored using a Multigas 6000 on-line FTIR gas analyzer. Prior to testing, the catalyst was pretreated by heating to 550 °C under an oxidizing gas mixture containing 0.94 vol% O₂, 10 vol% H₂O, and 8 vol% CO₂, and held for 5 min. Then, a reducing gas mixture containing 2500 ppm CO, 1000 ppm C₃H₆, 10 vol% H₂O, and 8 vol% CO₂ was introduced and maintained for 10 min. Afterward, the gas composition was switched back to the oxidizing mixture and held for another 10 min. The system was then cooled to the designated steady-state reaction temperature under the oxidizing atmosphere.

The TWC performance test was conducted at the steady-state temperature under rich-lean cycling conditions (λ = 0.98–1.02), with a switching frequency of 10 s. The detailed gas compositions for both rich and lean phases are listed in Table 1. Nitrogen (N₂) was used as the balance gas in all mixtures.

The average conversions of NO_x, CO, and C₃H₆, as well as the average NH₃ and N₂O formation, were calculated according to the following equations:

$$Average {NO}_X Conversion \left(\%\right)= {\displaystyle \frac{{\int }_{t=0}^{t=190}V·\left[C_{{NO}_X}^{blank}-C_{{NO}_X}^{out}\right]dt}{{\int }_{t=0}^{t=190}V·C_{{NO}_X}^{blank}dt}}\times 100\%$$

$$Average CO Conversion \left(\%\right)={\displaystyle \frac{{\int }_{t=0}^{t=190}V·\left[C_{CO}^{blank}-C_{CO}^{out}\right]dt}{{\int }_{t=0}^{t=190}V·C_{CO}^{blank}dt}}\times 100\%$$

$$Average C_3{H}_6 Conversion \left(\%\right)={\displaystyle \frac{{\int }_{t=0}^{t=190}V·\left[C_{C_3{H}_6}^{blank}-C_{C_3{H}_6}^{out}\right]dt}{{\int }_{t=0}^{t=190}V·C_{C_3{H}_6}^{blank}dt}}\times 100\%$$

$$Average {NH}_{3 }Released={\displaystyle \frac{Total mass of {NH}_3}{Catalyst mass\times Number of cycles}}$$

$$Average N_{2}O \mathrm{R}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{s}\mathrm{e}\mathrm{d}={\displaystyle \frac{Total mass of N_{2}O}{Catalyst mass\times Number of cycles}}$$

where:

• V: Total volumetric flow rate of the gas (L/min)
• $C^{blank}:$ Pollutant concentration without the catalyst (ppm)
• $C^{out}:$ Pollutant concentration at the reactor outlet (ppm)
• t: Time (s or min), integration period here is 190 s
• NH₃ or N₂O released: Measured by integrating NH₃ or N₂O signals over rich/lean cycles
• Total mass of N₂O: Integrated amount of N₂O measured over the test period
• Catalyst mass: Mass of catalyst used (g)
• Number of cycles: Total number of rich/lean switching cycles during the test

4. Conclusions

In this study, a series of Pt–Ba–Ce/Al₂O₃ catalysts with systematically varied Ba/Ce molar ratios were evaluated to elucidate how Ba–Ce compositional tuning governs structural evolution, redox behavior, NO_x storage, and three-way catalytic performance. Integrated characterization—BET, XRD, SEM, XPS, H₂-TPR, NO_x-TPD, CO pulse adsorption, and dynamic/steady-state TWC testing—reveals that the Ba/Ce ratio is a decisive parameter controlling Pt dispersion, interfacial Pt–CeO₂ oxygen species, BaCO₃ domain development, and their evolution under hydrothermal stress.

Catalysts with low Ba/Ce ratios exhibit higher surface areas, smaller Pt particles, and abundant Pt–O–Ce interfacial oxygen species, enabling efficient O₂ spillover and activation of CO, hydrocarbons, and NO_x at low temperatures. In contrast, Ba-rich catalysts promote the formation and enrichment of BaCO₃, yielding markedly enhanced NO_x storage capacity and superior high-temperature performance. As a result, the NO_x storage capacity displays a clear volcano-type dependence on Ba/Ce ratio, with Ce-rich compositions performing best at 200–300 °C and Ba-rich samples excelling at 400–500 °C.

Hydrothermal aging at 850 °C induces PtOx decomposition, Pt sintering, CeO₂ crystallite growth, and partial transformation of BaCO₃ into inert BaAl₂O₄. These processes diminish Pt–CeO₂ interactions, reduce oxygen mobility, and weaken NO_x storage efficiency. Nevertheless, catalysts with optimized Ba/Ce ratios effectively mitigate Pt sintering and preserve a larger fraction of active BaCO₃ domains, resulting in significantly improved structural and catalytic stability relative to Ba-rich or Ce-rich extremes.

Overall, this work establishes a mechanistic framework for understanding Ba–Ce cooperative effects in Pt-based catalysts and demonstrates, for the first time, that the Ba/Ce ratio simultaneously governs Pt–support interactions, oxygen spillover, NO_x storage chemistry, and hydrothermal durability. These findings provide actionable design guidelines for developing cost-effective, thermally robust Pt-based three-way catalysts as practical alternatives to Pd/Rh-dominated commercial formulations.