CO Management for Hydrogen Processes Through a Catalytic Oxidation Mechanism on Dual-Doped Perovskites with Tuned Co and Ni Ratios

Abstract

In hydrogen processes, managing CO emissions and removal by catalytic oxidation is crucial during H₂ production, storage/transportation, and use, ensuring the efficiency and safety of hydrogen systems and contributing to more sustainable energy solutions. Perovskite-structured transition metal oxide catalysts have been widely studied in various energy and environmental applications due to their extensive compositional modifications and electronic adjustments, facilitating catalytic behavior. Here, Ce-based perovskite catalysts with dual active metal doping at varying Co and Ni ratios are investigated to understand their structural and redox properties in CO oxidation. The reaction mechanism involves CO adsorption, oxygen activation, and redox cycling, confirming catalytic turnover. In situ DRIFTS analysis reveals real-time surface transformations with catalytic activity, which vary with Co and Ni doping ratio. Relatively, CO adsorption on Co³⁺ dominates the low-temperature activity, whereas Ni contributes to the efficiency at elevated temperatures. LCCNTxy (La_0.7Ce_0.1Co_xNi_yTi_0.6O₃) with x = 0.3 and y = 0.1 exhibits the highest performance, achieving T₁₀ above 40 °C and the fastest T₉₀ at 230 °C. This study highlights the compositional tuning in dual-doped perovskites and complementary roles of Co and Ni in CO oxidation for developing efficient industrial catalysts.

1. Introduction

The transition to a hydrogen-based energy system demands the development of sustainable technologies that uphold environmental protection and energy efficiency. However, the widespread adoption of hydrogen faces the critical challenge of carbon monoxide (CO) management throughout the processes of production, storage, transportation, and utilization for environmental friendliness. Firstly, the current limitations in hydrogen production and storage technology result in the introduction of impurities into hydrogen fuel, which can cause damage to catalytic systems such as fuel cells that rely on high-purity hydrogen [1,2]. Secondly, the existing hydrogen transport infrastructure heavily depends on fossil fuel-based technologies, resulting in indirect CO emissions that compromise its environmental advantages. These emissions are especially significant in energy-intensive applications, such as large-scale shipping, where substantial CO can be indirectly generated throughout the logistics process [3,4]. To address these issues, decarbonization technologies with effective CO management strategies are essential.

CO oxidation provides a direct solution to these challenges and is a pivotal reaction in the environmental clean-up and hydrogen purification processes. Generally, catalysts are used to facilitate the conversion of CO to carbon dioxide (CO₂), addressing both the toxicological and operational risks associated with CO contamination [5,6,7]. In particular, transition metal oxides (TMOs) have established themselves as pivotal materials in heterogeneous catalysis, offering unrivalled versatility and efficiency in addressing important environmental issues [8,9]. TMOs excel in this area due to their ability to stabilize multiple oxidation states, such as Mn⁺/M⁽ⁿ⁻¹⁾⁺ (where M is Ni, Co, Fe, etc.), which enables dynamic redox processes essential for catalytic activity and stability [10,11].

Perovskite oxides (ABO₃) stand out among TMOs for their highly tunable cubic lattice framework, with larger A-site cations (alkali, alkaline earth, or rare-earth metals) at the unit cell corners and smaller B-site transition metals at the center, surrounded by oxygen anions. This structure enables compositional modifications and doping, inducing oxygen vacancies, adjusting the electronic structure, and enhancing redox activity for optimized catalytic behavior [12,13,14,15]. Previous studies on Ce-doped perovskites have highlighted the role of Ce’s redox flexibility (Ce⁴⁺/Ce³⁺) in enhancing oxygen mobility and providing unique oxygen storage capacity (OSC), which are critical for catalytic activity [16,17]. Building on this foundation, our research team developed a novel framework using La and Ce, leveraging Ce’s smaller ionic radius to enable higher Ni doping levels compared to prior studies [18]. This work focuses on the La_0.7Ce_0.1Co_xNi_0.4−xTi_0.6O₃ perovskite system, employing a fixed 7:1 La-to-Ce ratio to maintain structural stability. The Co-to-Ni ratio at the B-site was systematically varied to investigate their complementary roles in redox processes. This precise approach isolates the contributions of Co and Ni to catalytic performance while ensuring the structural integrity of the perovskite framework, paving the way for advanced catalytic materials with optimized redox.

In doped perovskite catalysts, the CO oxidation mechanism is intricately tied to the oxidation states of the transition metals inside the perovskite framework [19,20]. Ni-based perovskites leverage the complementary catalytic roles of Ni⁴⁺ and Ni²⁺ species. Ni⁴⁺ facilitates the robust CO adsorption by enhancing interactions with active sites, while Ni²⁺ drives subsequent oxidation reactions, ensuring efficient redox cycles [21,22]. Conversely, in Co-based perovskites, CO molecules adsorb onto Co³⁺ cations, initiating lattice oxygen abstraction. This lattice oxygen reacts with adsorbed CO, forming CO₂ and regenerating the catalytic site. Such processes underline the critical role of transition metal redox dynamics in sustaining catalytic activity [23,24].

Dual-doped TMOs could significantly enhance the redox capabilities, resulting in improved catalytic performance. A study by Song et al. (2023) demonstrated that asymmetric Ni-O-Co active sites in Ni-doped Co-based spinel structures increased the mobility and activity of lattice oxygen, thereby lowering intermediate conversion energy and enhancing reaction efficiency [25]. Additionally, Ni and Co co-doped systems have been reported to promote the reaction between adsorbed CO and lattice oxygen, suggesting that co-doping maximizes the synergistic effects of the two metals, leading to superior catalytic performance in CO oxidation. Controlling the Ni-to-Co ratio further optimizes the balance between lattice oxygen activation and surface oxygen vacancy generation. This balance enhances the redox cycle’s stability, providing a pathway for tuning catalytic activity and durability [26]. These findings underscore the potential of co-doping as a strategic approach to designing effective CO oxidation catalysts, but there is still a need for investigation into the synergetic reaction mechanism at the perovskite structured system.

The Langmuir–Hinshelwood (L-H) mechanism dominates CO oxidation at low temperatures, where surface interactions prevail. In this pathway, CO and oxygen adsorb onto neighboring active sites, such as Co³⁺ and Ni⁴⁺, react to form CO₂, and desorb, regenerating active sites for continuous catalysis [27,28,29]. Co³⁺ facilitates strong CO adsorption and redox transitions, while Ni⁴⁺ enhances high-temperature activity through multivalent redox flexibility. At higher temperatures, the Mars–van-Krevelen (MvK) mechanism becomes significant, involving lattice oxygen in the catalytic cycle. CO reacts with lattice oxygen to form CO₂, creating oxygen vacancies that are replenished by gas-phase oxygen through lattice oxygen mobility [30]. This study emphasizes the interplay between these mechanisms, focusing on the L-H pathway modulated by Co and Ni ratios while acknowledging the MvK mechanism’s crucial role in sustaining catalytic activity at elevated temperatures through lattice oxygen involvement.

Here, we investigated the balancing roles of Co and Ni in CO oxidation via compositional tuning in the perovskite lattice to develop efficient reaction pathways. The ability to modulate the Co³⁺/Co²⁺ and Ni⁴⁺/Ni²⁺ ratios provides a powerful tool for tuning catalytic sites, impacting CO adsorption strength and interactions with oxygen species. This tunability is critical for optimizing catalytic efficiency and adapting to specific environmental conditions. To probe the intricate relationship between oxidation states and catalytic performance, operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to monitor real-time surface transformations during CO oxidation reactions. This approach allowed for direct observation of product evolution and the dynamic interplay between transition metal oxidation states and adsorbed species. By elucidating the role of oxidation state variability and contributions of the L-H mechanism, this study provides a way for the rational design of efficient TMOs catalysts to address pressing environmental challenges.

2. Results and Discussion

2.1. Crystallographic Analysis of LCCNT Catalysts

To elucidate the structural characteristics of the catalysts, X-ray diffraction (XRD) analysis was conducted on all samples calcined at 1400 °C. Figure 1a displays the XRD patterns of the LCCNTxy (La_0.7Ce_0.1Co_xNi_yTi_0.6O₃) catalysts, revealing that the original LCCNT predominantly adopts a cubic Pm-3m structure, indicating the formation of a highly crystalline and pure single phase. Notably, no impurity peaks were detected in the XRD patterns, demonstrating that a homogeneous phase was achieved even with the inclusion of more than 13 wt% transition metal dopants (Co and Ni) within the perovskite lattice. This single-phase stability is attributed to the lattice expansion induced by the incorporation of large Ce ions, which effectively accommodates transition metal doping without phase segregation [31].

Further analysis of the crystalline phase characteristics is provided in Figure 1b, which presents a magnified view of the XRD peaks in the (110) plane. These detailed peaks highlight the structural integrity and crystallinity of the doped perovskite phase.

Table 1. Crystalline characterization from XRD results.

Catalysts	Abbreviation	(110) Peak (°)	d-Spacing (Å)	Cell Volume (Å3) a	Lattice Parameter (Å) a	Space Group
La0.7Ce0.1Co0.3Ni0.1Ti0.6O3	LCCNT31	32.28	2.772	60.13	3.198	Pm-3m
La0.7Ce0.1Co0.2Ni0.2Ti0.6O3	LCCNT22	32.32	2.768	59.97	3.194	Pm-3m
La0.7Ce0.1Co0.1Ni0.3Ti0.6O3	LCCNT13	32.34	2.766	59.86	3.192	Pm-3m

^a Rietveld refinement results of XPert HighScore Plus.

summarizes the calculated lattice parameters, including d-values and cell volumes, underscoring the subtle but notable changes in the crystal lattice in response to TMOs ratio. Specifically, the main peak d-value increased from 2.766 Å to 2.772 Å, accompanied by an expansion in the cell volume from 59.86 ų to 60.13 ų. This trend reflects a structural relaxation of the lattice as changing the Co and Ni ratios, which facilitates increased spacing between lattice planes.

The observed XRD peak shift can be attributed to structural strain within the perovskite lattice induced by the incorporation of doped transition metals. This strain arises from the difference in ionic radii between Co³⁺ (0.54 Å), Co²⁺ (0.74 Å), Ni⁴⁺ (0.50 Å), and Ni²⁺ (0.70 Å), as well as the host B-site cations, leading to localized lattice distortions [32]. These distortions result in the expansion of the unit cell, as evidenced by the shift to lower 2θ values, reflecting the structural relaxation required to accommodate the dopants. The expansion of the unit cell volume and the shift to lower 2θ values can be directly associated with the increase in Co³⁺ and Co²⁺ species and the decrease in Ni⁴⁺ and Ni²⁺ species.

The observed unit cell volume expansion and the resultant lattice modifications highlight the structural adaptability of the Ce-based perovskite framework, which can incorporate high levels of transition metal dopants of Co and Ni while maintaining phase purity and crystallinity [33]. This structural expansion is especially crucial in enhancing the material’s catalytic properties, as it potentially increases the availability of active sites and improves oxygen mobility within the lattice [34]. This tunability is critical for catalytic applications, where lattice flexibility can impact catalytic activity, particularly in redox processes like CO oxidation.

2.2. Structural and Elemental Analysis of LCCNT Catalysts

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of the LCCNT catalysts, shown in Figure 2a,e,i, confirmed the homogeneity as a single phase, and energy dispersive spectroscopy (EDS) mapping was also combined to analyze the elemental distribution.

Lattice spacing analysis in LCCNT31, LCCNT22, and LCCNT13 (Figure 2b,f,j) revealed values of 0.278 nm, 0.276 nm, and 0.276 nm, respectively, for the (110) plane. The slight increase in lattice spacing suggests interatomic distance expansion within the Co and Ni lattice, leading to structural relaxation and unit cell expansion, as also confirmed from the previous XRD analysis. Fast Fourier Transformation (FFT) patterns (inserts in Figure 2b,e,h) confirm the well-preserved cubic symmetry of the perovskite structure, with zone axes indexed to [110] [35]. These patterns highlight the crystallographic integrity of the catalysts, which remains intact despite high-temperature calcination and metal doping. This high degree of crystallinity is generally crucial for the catalysts’ stability and durability during redox cycling, ensuring their long-term performance in catalytic applications [36].

The elemental analysis in Figure 2c,g,k shows a homogeneous dispersion of La, Ce, Ti, Co, Ni, and O across the catalyst matrix, indicating a uniform incorporation of these elements into the perovskite structure. The weight percent (wt%) data obtained from TEM analysis reflects calculations based solely on Co and Ni content, excluding contributions from La, Ce, Ti, and O (Figure 2d,h,l). According to this calculation method, the wt% ratios for Co and Ni closely correspond to the intended stoichiometric ratios of 78.2:21.8, 56.3:43.7, and 32.2:67.8 for the LCCNT31, LCCNT22, and LCCNT13 samples, respectively. These values represent Co-to-Ni ratios of approximately 3:1, 1:1, and 1:3. This uniform composition ensures an even distribution of active sites, which is crucial for maintaining catalytic activity and stability during reactions. The double-doped perovskite system, with its well-dispersed and stoichiometrically balanced elements, demonstrates the ability to perform efficiently in catalytic applications, leveraging its stable and homogeneously integrated active sites.

2.3. Surface Chemistry and Oxidation States of LCCNT Catalysts

X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface chemical properties and oxidation states of the LCCNT catalysts. Figure 3 shows the XPS spectra for O 1s, Ni 3p, and Co 2p regions with a summary of the binding energies and peak areas in

Table 2. Binding energies and peak areas for O 1s, Ni 3p, and Co 2p XPS spectra.

Catalysts	OL		OD		Ni2+		Ni4+		Co2+		Co3+
	BE a (eV)	Area b (%)	BE a (eV)	Area b (%)	BE a (eV)	Area b (%)	BE a (eV)	Area b (%)	BE a (eV)	Area b (%)	BE a (eV)	Area b (%)
LCCNT31	529.2	57	531.1	43	67.1	68	68.9	32	779.3 797.0	36	781.0 795.4	64
LCCNT22	529.2	55	531.1	45	67.1	49	68.9	51	779.3 791.1	40	781.0 795.4	60
LCCNT13	529.2	51	531.1	49	67.1	42	68.9	58	779.3 797.0	48	781.0 795.4	52

^a Measurement binding energy (eV) and ^b relative percentages (%).

, providing insights into the oxygen species and oxidation states of the doped metals.

The O 1s spectrum (Figure 3a) reveals two peaks at 531.5 eV (oxygen deficiency, O_D) [37] and 529.2 eV (surface lattice oxygen, O_L) [38]. O_D corresponds to oxygen vacancies, while O_L represents lattice-bound oxygen. Ce doping in A-site-deficient perovskites increases oxygen vacancies due to Ce’s high oxygen mobility, which enhances catalytic activity [19]. Surface oxygen structure plays a key role, directly participating in the CO oxidation process. LCCNT31 had the highest proportion of lattice oxygen, while LCCNT13 exhibited a relatively high oxygen vacancy concentration (Table 2). The incorporation of Ni into the Co-based perovskite structure has been shown in other O₂-TPD studies to significantly increase oxygen deficiency compared to Co-only perovskites [23]. This suggests that while Co enhances the stability of lattice oxygen, Ni promotes the formation of bulk oxygen vacancies, enhancing redox activity critical for catalytic performance.

The high lattice oxygen concentration in LCCNT31 due to Co doping may enhance the mobility and availability of the lattice oxygen in the bulk, which promotes the CO oxidation reaction [39]. On the other hand, the high oxygen vacancy concentration observed in LCCNT13 can increase the generation of absorbed oxygen species, which aids catalytic activity at high temperatures [40].

The oxidation states and distribution of the doped transition metals in the LCCNT catalysts were analyzed using XPS, focusing on the Ni 3p and Co 2p regions. These insights provide a deeper understanding of how metal content influences catalytic performance. The Ni 3p spectrum (Figure 3b) displayed peaks at 69 eV (Ni⁴⁺) and 67 eV (Ni²⁺) [41,42], with no change in position across catalysts but increasing intensity with Ni content. This suggests that Ni maintains a stable electronic environment during doping. The Ni⁴⁺ concentration increased from 32% in LCCNT31 to 51% in LCCNT22 and 58% in LCCNT13, indicating that changes in the oxidation state of Ni are closely related to the adjustment in the B-site electronic state to preserve charge neutrality in the perovskite lattice [43]. The Co 2p spectrum (Figure 3c) showed peaks for Co²⁺ (798 eV and 782 eV) [44] and Co³⁺ (794 eV and 778 eV) [45], with additional satellite peaks at 800 eV and 790 eV. Co³⁺ concentration increased from 52% in LCCNT13 to 64% in LCCNT31, reflecting more active sites for CO oxidation, as Co³⁺ is the main active site. Higher concentrations of Co³⁺ and lattice oxygen in LCCNT31 can enhance the oxygen mobility, redox cycling, and active site density, enabling effective CO oxidation performance at low temperatures. Meanwhile, LCCNT13, with its higher oxygen vacancies and surface-stabilized Ni⁴⁺, enhances catalytic activity at high temperatures. The high-temperature synthesis (1400 °C) and Ce⁴⁺/Ce³⁺ redox environment facilitate the transformation of Ni²⁺ into Ni⁴⁺, compensating for oxygen vacancies and promoting efficient redox cycling. Post-synthesis analysis reveals that Ni⁴⁺ and Ni²⁺ are the predominant oxidation states within the perovskite structure. The dynamic redox transitions of Ni during the CO oxidation process, including intermediate states such as Ni⁺ and Ni³⁺, enhance catalytic activity through its multivalent redox flexibility. Furthermore, based on other H₂-TPR studies of Co- and Ni-doped perovskites, the reduction temperature of Ni is lower than that of Co, indicating its higher reducibility and stronger contribution to the redox process [46,47]. These findings underscore that the relative ratios of Co and Ni directly impact their oxidation states, influencing catalytic behavior and performance.

2.4. Catalytic Activities of CO Oxidation

The influence of Co and Ni content on the catalytic performance of perovskite-based catalysts for CO oxidation was systematically studied. As shown in Figure 4a, the catalytic activities of LCCNT13, LCCNT22, and LCCNT31 were evaluated based on the temperatures corresponding to CO conversions of 10% (T₁₀), 50% (T₅₀), and 90% (T₉₀). Additionally, activation energies for specific conversion ranges were calculated to provide a comprehensive understanding of the catalytic efficiency at different stages of the reaction (Figure 4b).

Compared to the tested catalysts, LCCNT31 demonstrated the most rapid reaction initiation, achieving T₁₀ approximately 20 °C lower than LCCNT22 and 40 °C lower than LCCNT13. This highlights its superior low-temperature activity, which can be attributed to its higher Co metal doping and Co³⁺ content. Furthermore, LCCNT31 achieved the fastest T₉₀ at 230 °C, highlighting its ability to sustain high catalytic performance across the reaction range. The catalytic efficiency of the catalysts followed the sequence LCCNT31 > LCCNT22 > LCCNT13, emphasizing the positive influence of Co on the overall activity.

To explore the influence of Co and Ni content on catalytic efficiency in detail, activation energies were calculated for two temperature ranges: T₁₀–T₅₀ and T₅₀–T₉₀. This analysis distinguished between low- and high-temperature performance, respectively. LCCNT31 showed the lowest activation energy up to T₅₀, requiring only 31.5 kJ/mol, approximately 40% lower than that of LCCNT13. This reduction in activation energy underscores the ability of higher Co content to promote rapid reaction initiation at low temperatures. Beyond T₅₀, LCCNT31 maintained its efficiency, exhibiting the lowest activation energy up to T₉₀ (39.5 kJ/mol), which confirms the effectiveness of Co in sustaining catalytic activity across the temperature range. However, the difference in activation energies became less pronounced compared to LCCNT22 (42.7 kJ/mol) and LCCNT13 (44.0 kJ/mol) at high temperatures, which underscores the critical role of Co and Ni ratios in determining catalytic behavior. A higher Co content effectively lowers the energy barrier for initial activation, facilitating earlier reaction onset. Conversely, a greater Ni enhances sustained conversion at elevated temperatures, supporting efficient high-temperature performance. This delicate balance between Co and Ni compositions enables the fine-tuning of catalytic activity to optimize efficiency across varying temperature regimes. In particular, the rapid CO oxidation observed at low temperatures, facilitated by the Co³⁺ active sites, plays a crucial role in accelerating the overall reaction rate. Meanwhile, Ni ensures sustained catalytic activity by providing high-temperature stability. The findings emphasize the importance of compositional tuning in perovskite-based catalysts, demonstrating that an optimal Co-to-Ni ratio enhances both low- and high-temperature performance. These insights provide valuable guidance for the rational design of high-performance catalysts tailored for environmental applications, such as CO oxidation.

To evaluate the stability of the LCCNT catalysts, repeated CO oxidation testing was conducted using LCCNT31, the catalyst exhibiting the highest performance. As shown in Figure 4c, the reused sample exhibited similar catalytic behavior to the pristine and no significant degradation of catalytic performance was observed. XRD analysis of the repeated CO oxidation reaction catalysts confirmed that no structural collapse occurred (Figure 4d). This result suggests that the high crystallinity of LCCNT31 contributes to its structural stability, which is closely related to its excellent durability and sustained catalytic performance.

2.5. Adsorption Behavior in CO Oxidation

The L-H mechanism has been widely employed to describe CO oxidation reactions, with a particular focus on the influence of doped Co and Ni ratios at the perovskite surfaces. To understand the adsorption behavior of CO on Co and Ni sites under CO oxidation conditions, DRIFTS was conducted in Figure 5. The study revealed overlapping adsorption peaks for Co-CO and Ni-CO species, as well as interference from gas-phase CO peaks. To disentangle the overlaps, the CO flow was stopped while monitoring peak evolution. Peak fitting was then performed to determine the specific adsorption states and strengths on each metal site (

Table 3. Assignment of the bands observed during in situ DRIFTS: Co^δ+ groups (1 ≤ α < 3), Ni^β+ groups (1 < β < 3), and Ni⁰.

Catalysts	Elements	50 °C		150 °C		250 °C
		Wavenumber (cm−1)	Area (%)	Wavenumber (cm−1)	Area (%)	Wavenumber (cm−1)	Area (%)
LCCNT31	Coα+	2062, 2112, 2182	67.31	2063, 2115, 2183	64.13	2064, 2116, 2184	61.08
	Niβ+	2089, 2149, 2209	32.69	2091, 2152, 2212	35.87	2091, 2152, 2215	38.92
	Ni0	-	-	-	-	-	-
LCCNT22	Coα+	2068, 2115, 2180	57.86	2068, 2117, 2180	59.47	2070, 2117, 2182	59.65
	Niβ+	2096, 2150, 2210	42.14	2096, 2151, 2210	40.53	2092, 2151, 2212	40.35
	Ni0	-	-	-	-	-	-
LCCNT13	Coα+	2066, 2119, 2181	46.84	2066, 2121, 2182	47.34	2063, 2123, 2185	44.66
	Niβ+	2097, 2152, 2207	53.16	2097, 2152, 2213	50.92	2097, 2153, 2213	52.96
	Ni0	-	-	2034	1.74	2033	2.38

).

When CO adsorbs onto vacancy sites in the catalysts, it forms CO* species that interact with Co in different oxidation states. DRIFTS analysis showed that CO* strongly adsorbs onto Co³⁺ and Co²⁺ sites, forming Co²⁺-CO (2180–2185 cm⁻¹) and Co⁺-CO (2112–2123 cm⁻¹, 2062–2070 cm⁻¹) complexes, respectively [48,49]. These peaks indicate that Co³⁺ plays a critical role in stabilizing CO on the catalyst surface, facilitating effective redox cycling and promoting catalytic turnover. The strong adsorption observed at these sites is crucial for initiating the CO oxidation process, especially at lower temperatures. In Ni-rich catalysts, CO* exhibits weaker adsorption across a broader range of oxidation states, from Ni⁺ to Ni⁴⁺. Distinct adsorption peaks were observed, including Ni⁰-CO (2033–2034 cm⁻¹), Ni⁺-CO (2089–2097 cm⁻¹ and 2149–2153 cm⁻¹), and Ni²⁺-CO (2207–2215 cm⁻¹), respectively [50,51,52]. In particular, the formation of CO peaks adsorbed on Ni⁰ at high temperatures indicates that CO oxidation is actively promoted.

The DRIFTS analyses revealed distinct CO adsorption behavior on Co and Ni sites in the dual-doped perovskite catalyst surfaces, influenced by the relative proportions of Co and Ni. As shown in Table 3, LCCNT31 exhibited the highest CO adsorption on Co sites, highlighting the critical role of Co³⁺ in facilitating strong CO interactions. In LCCNT22, CO adsorption on Co sites remained significant despite the increased Ni content, indicating that Co continues to play a key role in the catalytic mechanism. In LCCNT13, even with a higher Ni content, the CO adsorption on Co sites remained prominent, underscoring the dominant role of Co in CO adsorption across all compositions. At elevated temperatures, the CO adsorption contributions of Co and Ni converged, suggesting a more pronounced role for Ni in sustaining catalytic activity at higher temperatures. This behavior is attributed to Ni’s ability to adopt multiple oxidation states (Ni⁰ to Ni⁴⁺), enabling greater redox flexibility and activity in Ni-rich catalysts such as LCCNT13. The catalytic mechanism also involves oxygen dissociation, driven by Ce⁴⁺ ions, which generate reactive oxygen species (O*) and Ce³⁺-O complexes. These active species react with adsorbed CO to produce CO₂, completing the catalytic cycle. The results reaffirm the importance of Co³⁺ and Ni³⁺ in facilitating CO adsorption and redox cycling, underscoring their pivotal roles in the CO oxidation reaction.

As a result, LCCNT31, which has higher Co content, exhibited a smaller decrease in CO adsorption peaks after the CO flow was stopped compared to the Ni-rich LCCNT13 catalyst. This behavior suggests that Co forms stronger bonds with CO, enabling greater CO adsorption and desorption capacity [53]. The higher adsorption stability of CO on Co³⁺ in LCCNT31 explains its superior catalytic performance, particularly at low temperatures. In contrast, LCCNT22 and LCCNT13 demonstrated more diverse adsorption states due to Ni’s ability to accommodate various oxidation states, which supports enhanced catalytic activity at higher temperatures.

2.6. Mechanism of CO Oxidation on LCCNT Catalysts

The CO oxidation reaction on LCCNT catalysts was estimated using the L-H mechanism, as illustrated in Figure 6. This mechanism evaluates CO oxidation in Ce-based dual-doped (Co and Ni) perovskites through CO*-assisted O₂ dissociation and the redox behavior of transition metals [54]. The reaction pathways were analyzed based on the specific oxidation states of Co and Ni, which govern the adsorption and activation of CO during the catalytic cycle.

Essentially, our proposed CO oxidation process at the Ce-based perovskite catalysts follows a CO*-assisted O₂ dissociation pathway, as outlined below:

CO + * ↔ CO* (* is vacancy site)

(1)

O₂ + Ce⁴⁺ ↔ O* + Ce³⁺ − O

(2)

In this mechanism, CO adsorbs onto vacancy sites to form CO*, while molecular oxygen dissociates on Ce⁴⁺ sites, generating reactive oxygen species (O*) and Ce³⁺-O complexes. This process underscores the role of cerium in facilitating oxygen activation and maintaining redox cycling essential for sustained catalytic activity.

Based on this oxygen reaction, the CO oxidation route in Co-rich Ce-based dual-doped perovskites is expected by the following steps:

Co³⁺ + CO* → Co²⁺ − CO

(3)

Co²⁺ + CO* → Co⁺ − CO

(4)

The high redox activity of Co enables efficient transitions between oxidation states. Co³⁺ readily reacts with CO*, facilitating its reduction to Co²⁺, while Co²⁺ subsequently interacts with CO* to form Co⁺ species. This cycle is maintained as Co continuously regenerates its 3+ state due to its strong redox properties, ensuring steady catalytic turnover. The dominant role of Co³⁺ in CO adsorption and activation highlights its importance in the low-temperature initiation of the reaction.

The reaction pathway in Ni-rich Ce-based dual-doped perovskites involves multiple oxidation states of Ni, as outlined below:

Ni⁴⁺ + CO* → Ni³⁺ − CO

(5)

Ni³⁺ + CO* → Ni²⁺ − CO

(6)

Ni²⁺ + CO* → Ni⁺ − CO

(7)

Ni⁺ + CO* → Ni⁰ − CO

(8)

Ni’s reduction to multiple oxidation states, including Ni⁴⁺, Ni²⁺, Ni⁺, and Ni⁰, enhances catalytic activity at elevated temperatures. DRIFTS analysis confirms active CO adsorption on Ni sites, with transitions from Ni⁴⁺ to Ni⁰ providing diverse active sites for CO activation and conversion to CO₂. This redox flexibility supports continuous oxygen activation, complementing Co’s low-temperature activity. At higher temperatures, Ni’s multivalent redox transitions play a key role in maintaining catalytic efficiency, as lattice oxygen becomes more involved in the reaction. In Ni-rich catalysts like LCCNT13, this behavior ensures efficient catalytic turnover, demonstrating the synergistic effects of Co and Ni in optimizing performance across temperature ranges.

In the synergistic pathway of CO oxidation reactions within Ce-based dual-doped (Co and Ni) perovskites, the combined use of Co and Ni enhances catalytic performance, with their complementary roles creating a synergistic effect. Notably, the primary reaction mechanism varies depending on the specific Co-to-Ni ratio, highlighting the importance of tuning their proportions to optimize the catalytic pathway. The catalytic routes reveal that highly Co-doped perovskites excel in maintaining high redox activity between Co²⁺ and Co³⁺, driving low-temperature CO oxidation through efficient adsorption and activation. In contrast, highly Ni-doped perovskites leverage the multi-valence redox transitions of Ni, including metallic states, to enhance oxygen activation and catalytic turnover at higher temperatures. This complementary behavior of Co and Ni highlights the importance of compositional tuning in Ce-based perovskites to optimize catalytic activity across different temperature regimes. These findings provide a mechanistic basis for designing advanced perovskite catalysts tailored for specific applications in CO oxidation and other related processes.

3. Materials and Methods

3.1. Synthesis of Co and Ni Co-Doped Perovskite Oxides

Lanthanum (III) oxide (La₂O₃, ≥99.9%%, Sigma-Aldrich, St. Louis, MO, USA), cerium (IV) oxide (CeO₂, 99.9%, Sigma-Aldrich, St. Louis, MO, USA), titanium (IV) oxide (TiO₂, anatase, 99.8%, Sigma-Aldrich, St. Louis, MO, USA), cobalt (II, III) oxide (Co₃O₄, 99.5%, Sigma-Aldrich, St. Louis, MO, USA), and nickel (II) oxide (NiO, 99.99%, Sigma-Aldrich, St. Louis, MO, USA) were used as precursors. La_0.7Ce_0.1Co_XNi_{0.4 − X}Ti_0.6O₃ (X = 0.1, 0.2, 0.3) were prepared by solid state synthesis. The precursors were quantitatively added to a beaker and mixed with acetone and 1 wt.% Hypermer KD1 dispersant. The acetone was evaporated at room temperature for half a day with constant stirring. The obtained powder was transferred to an alumina combustion boat and calcined at 1000 °C for 12 h. To increase the uniformity and crystallinity of the perovskite structure, the calcined powder was pressed into the pellets and calcined at 1400 °C for 12 h.

3.2. Characterizations

The X-ray diffraction (XRD) patterns of the samples were conducted at room temperature using a SmartLab SE (Rigaku, Tokyo, Japan) with Cu-Kα1 radiation (1.54056 Å) and scanned at 2θ angles from 20 to 80°. The transmission electron microscope (TEM) and energy-dispersive spectroscopy (EDS) measurement carried out on JEM-ARM 200F NEOARM (JEOL, Tokyo, Japan). The chemical bonding structures were investigated using X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, Waltham, MA, USA) with an Al X-ray source (1486.6 eV). All the binding energies were referenced to the C 1s peak at 284.8 eV.

3.3. CO Oxidation Performance

The catalyst reaction of CO oxidation conducted in a fixed-bed quartz tube (9.5 nm inside diameter) reactor under atmospheric pressure. First, 100 mg of the samples was introduced in the reactor and pre-treated in situ with a 5% H₂/N₂ atmosphere at 350 °C for 2 h. After cooling to 50 °C, the gas mixture of 20,000 ppm CO and 10.0 vol.% O₂ from the air (21% O₂ and 79% N₂) at N₂ balance was used, with a total gas flow rate of 100 mL∙min⁻¹ for a gas volumetric flow rate (GHSV) value of 60,000 mL∙g_cat⁻¹∙h⁻¹. During the light-off experiment, the reactor heated the sample temperature at a rate of 5 °C ∙min⁻¹ and held it for 15 min (step size = 20 °C, temperature range = 50–310 °C). The repeated CO oxidation test for LCCNT31 was conducted immediately after completing the initial experiment. Following the first cycle, the catalyst was cooled to 50 °C under an N₂ atmosphere. Subsequently, CO and air were injected under the same conditions as the initial run, and the CO oxidation experiment was repeated by heating the system to 310 °C. The composition of the reactor effluent gas was analyzed by online gas chromatography (Agilent GC 8890, Agilent Technologies, Santa Clara, CA, USA) with a thermal conductivity detector (TCD) with a molecular sieve 5A packed column. The CO conversion (%) was calculated by the following equation:

$$\mathrm{C}\mathrm{O} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\%\right)={\displaystyle \frac{{\left[\mathrm{C}\mathrm{O}\right]}_{\mathrm{i}\mathrm{n}}-{\left[\mathrm{C}\mathrm{O}\right]}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\left[\mathrm{C}\mathrm{O}\right]}_{\mathrm{i}\mathrm{n}}}}\times 100$$

The activation energy (Ea) (kJ∙mol⁻¹) for CO oxidation was calculated at T₁₀ to T₅₀ and T₅₀ to T₉₀, respectively. The CO conversion reaction rate (r_CO (mol∙g¹∙s⁻¹)) equation of CO oxidation is as follows:

$${\mathrm{r}}_{\mathrm{c}\mathrm{o}}={\displaystyle \frac{{\left[\mathrm{C}\mathrm{O}\right]}_{\mathrm{i}\mathrm{n}}\times {\mathrm{X}}_{\mathrm{C}\mathrm{O}}\times \mathrm{F}}{{\mathrm{m}}_{\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}}}}$$

where [CO]_in is the CO concentration in the feed gas, X_CO is the CO conversion, F is the total flow rate (mL∙min⁻¹), and m_catalyst is the catalyst mass (g). The activation energy equation is as follows:

$$\mathrm{l}\mathrm{n}{\mathrm{r}}_{\mathrm{c}\mathrm{o}}=-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}\mathrm{T}}}+\mathrm{l}\mathrm{n}\mathrm{A}$$

where A is pre-finger factor (s⁻¹).

3.4. In Situ DRIFTS

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed using a Nicolet iS50 (Thermo Fisher Scientific, Waltham, MA, USA) with a mercury cadmium telluride (MCT) detector and 32 scans with resolution of 8 cm⁻¹. To measure CO gaseous peaks, a background spectrum was collected with potassium bromide (KBr) treated under an N₂ flow at 50 °C for 2 h. The catalyst was then mixed with KBr in a 1:1 weight ratio. CO adsorption experiments were programmed with a 1 vol.% CO and 10 vol.% O₂ from the air (21% O₂ and 79% N₂) at N₂ balance with a total gas flow rate of 50 mL∙min⁻¹ for 30 min, and CO adsorption spectra were collected. A subsequent CO desorption program completely removed the CO gas under an N₂ flow for 10 min and collected CO desorption spectra. The adsorption/desorption cycle temperature range was from 50 °C to 250 °C.

4. Conclusions

This study investigated Ce-based perovskite catalysts with dual doping of varying Co and Ni contents for CO oxidation, demonstrating that oxidation states and metal ratios significantly influence the catalytic mechanism. Structural analysis confirmed the perovskite lattice’s adaptability, with strain-induced peak shifts from doped Co³⁺, Co²⁺, Ni⁴⁺, and Ni²⁺ species leading to unit cell expansion, enhanced oxygen mobility, and redox activity. Co³⁺ and Ni⁴⁺ were identified as key active sites, with Co³⁺ dominating at low-temperature activity and Ni⁴⁺ enhancing high-temperature reactions.

LCCNT31 demonstrated the highest catalytic efficiency, achieving the lowest activation energy (31.5 kJ/mol up to T50) and the fastest T90 at 230 °C. Its superior performance was attributed to the synergistic interaction between Co and Ni, with Co facilitating strong CO adsorption and redox transitions for low-temperature activity while Ni supported multivalent redox transitions for high-temperature efficiency. The high concentration of surface oxygen and the well-distributed Co³⁺ and Ni⁴⁺ active sites further enhanced oxygen mobility, thereby contributing to the overall catalytic performance.

Therefore, our work emphasizes the importance of compositional tuning in perovskite-based catalysts to achieve optimal redox properties and oxygen mobility. By carefully balancing Co and Ni content, the catalysts demonstrated distinct reaction mechanisms across a wide temperature range, establishing a robust foundation for designing efficient transition metal oxide catalysts for energy and environmental applications, such as CO management in hydrogen-based energy systems that uphold environmental protection and energy efficiency.