Progress in Metal Oxide Catalysts for Carbon Monoxide Oxidation

Abstract

As one of the most intensively researched reaction systems in the field of multiphase catalysis, the technological breakthrough of CO oxidation is of great significance in addressing the health hazards and environmental pollution caused by high CO concentrations. Catalytic oxidation technology has become a core means of eliminating CO pollution due to its high efficiency and energy saving. This paper provides an in-depth analysis of various types of metal-catalyzed CO oxidation reactions and explores in detail the characteristics of the action of different mechanisms. On this basis, optimization strategies are proposed, including metal doping and controllable oxygen vacancy construction to enhance reaction selectivity and stability. In addition, the influence mechanisms of common gas components such as H₂O and SO₂ on the catalytic reaction of CO are analyzed, and targeted optimization strategies are proposed. The reaction mechanisms of CO on the metal catalyst surface are then examined. Finally, the development direction of catalysts in the field of CO deep purification is prospectively discussed, which provides theoretical support and technical routes for the construction of an efficient and stable industrialized CO oxidation system.

1. Introduction

Carbon monoxide is a colorless, odorless, flammable and toxic gas that can quickly combine with hemoglobin to form carboxyhemoglobin, thus causing hemoglobin to lose its oxygen-carrying capacity and resulting in hypoxia in human tissues, which is regarded as the “silent killer” [1]. Due to its abundance in the atmosphere, carbon monoxide is listed as one of the six main monitoring factors in the Air Quality Index in China [2]. Specifically, CO emissions primarily originate from two major sources: firstly, the transport sector, particularly motor vehicle exhaust emissions, which constitute the most direct and significant source of CO in urban environments; secondly, industrial production processes, including steelmaking, ironmaking, coke oven operations, and solid waste incineration [3]. Currently, common removal techniques for CO packages are adsorption [4], catalytic [5] and electrochemical [6]. It is widely recognized that thermal catalytic oxidation of CO is among the most effective methods of CO purification. This technology achieves effective CO removal by converting CO to CO₂ under relatively mild reaction conditions using a catalyst [7].

There has been a significant increase in research and development focused on catalysts for the catalytic oxidation of CO. The most prevalent catalysts for CO oxidation are noble metals, such as Au [8], Pd [9], Pt [10], Ru [11], and Ag [12], as well as non-precious metals, such as Mn [13], Co [14], Fe [15], Cu [16], Zr [17] and Ce [18], etc. Among them, the noble metal catalysts showed higher adsorption capacity for CO, in contrast to the non-precious metal catalysts, which were limited by smaller atomic size and weaker electron trapping ability, and their surface electronic structures were more inclined to catalytic activation reactions with O₂ molecules. Nonetheless, it is important to acknowledge that, in spite of the substantial advancements witnessed in this domain, there persists a dearth of systematic comparative studies that explore the conformational relationships exhibited by disparate metal systems. Firstly, the mechanism of the CO-catalyzed reaction remains to be fully elucidated. Secondly, the modulation of the catalytic properties is influenced by a multitude of parameters, including the size effect on the nanoparticles, oxygen vacancies (OV), strong metal–support interactions (SMSI), and reducibility, which are still subjects of debate. The design of CO catalysts necessitates the exploration of intrinsic and extrinsic characteristics, as well as dynamic synergistic interactions among catalyst components.

Based on an extensive review of the literature, this paper provides a comprehensive analysis of noble metal catalysts—highlighting their exceptional low-temperature activity, size-dependent effects, and support-mediated synergies—as well as non-noble metal systems, emphasizing their cost efficiency, redox properties, and regulatory roles when used as supports. It systematically evaluates the dual impacts of H₂O and SO₂ on CO oxidation performance and elaborates the underlying reaction mechanisms. In response to key challenges in the field, such as the scarcity of precious metals, their susceptibility to poisoning, the limited activity of non-noble metals, and the complexity of reaction pathways, this study proposes forward-looking strategies. These include the development of low-cost single-atom and cluster catalysts, the optimization of non-noble metal composite systems, and the design of bifunctional hydrophobic and sulfur-resistant protective layers to isolate toxic species. By integrating theoretical insight with practical design, this work lays a solid foundation for constructing highly efficient, stable, and environmentally benign industrial CO oxidation systems.

2. Typical Precious Metal Catalysts

In the process of CO oxidation, noble metal catalysts have been shown to possess exceptional CO adsorption capacity. This phenomenon can be attributed to the unique d-electron orbital properties of the catalysts, which serve to significantly reduce the activation energy barrier of the CO*→OOCO* conversion process. This reduction in energy barrier is achieved through the formation of stable carbonyl intermediates with the reactant molecules. The result of this process is a significant enhancement in catalytic activity and an improvement in reaction stability [19]. However, its high cost and limited resources have hindered its large-scale industrial application. At present, platinum group metals (Pt, Pd, Ru) and other noble metals (Au, Ag)-based loaded catalysts predominate in the industrial system. Their surface electronic structure tunability and anti-sintering properties provide important regulatory dimensions for optimizing catalytic performance.

2.1. Pt-Based Catalysts

Platinum, recognized for its extensive research in the domain of precious metal catalysts, has garnered considerable scholarly attention in the context of CO oxidation reactions. The catalytic activity is affected by factors such as nanoparticle size and carrier type. Boubnov et al. [20] prepared a series of Pt/Al₂O₃ catalysts with Pt particle sizes ranging from 1 to 10 nm and found that those with sizes of 2–3 nm exhibited the highest activity. The Pt particles of these catalysts possessed a significant number of low-coordinated, highly active surface Pt atoms, which resulted in a substantial enhancement in the efficiency of the CO oxidation reaction. This finding corroborates Kim et al.’s [21] study in the Pt/TiO₂ system, where small-sized Pt particles provide more active sites, thus increasing the probability of adsorption and reaction of the reactants and improving the reaction rate. As shown in Figure 1a. Jia et al. [22] prepared different morphologies of Pt species on ND@G composite carriers, including single atoms (Pt₁), fully exposed clusters (Ptn) and nanoparticles (Ptp). It was found that the fully exposed Pt cluster catalyst (0.5 wt% Ptn/ND@G) exhibited excellent activity in low-temperature CO oxidation and that the fully exposed Pt clusters consisted of several Pt atoms, which provided multiple active sites and favored the co-adsorption and reaction of CO and O₂. The attenuated CO adsorption and enhanced O₂ dissociative adsorption compared to Pt nanoparticles help to reduce the CO poisoning phenomenon and make it easier for the active sites to adsorb O₂, thus enhancing the catalytic activity.

The nature of the oxide carrier is found to be a critical factor in the catalytic reaction of highly dispersed precious metal catalysts. In the context of a given particle size, the carrier effect is typically explained through the concept of a reaction occurring at the metal/oxide interface or through the process of reactant spillover onto or from the oxide support [23]. Qiao et al. [24] loaded Pt onto Fe(OH)x using co-precipitation and found that the mixed carrier of Fe(OH)x and Fe₃O₄ contained a large number of OH groups. The function of these groups is hypothesized to be significant in the processes of adsorption and activation of O₂, thereby enhancing the efficiency of CO oxidation. Bae et al. [25] loaded Pt onto SnO₂, SiO₂, and Sb-SnO₂ carriers by means of wet impregnation. The Pt/Sb-SnO₂ catalysts exhibited optimal activity and durability in CO oxidation. This phenomenon was attributed to the fact that Pt doping altered the nature of the oxygen on the surface of the Sb-SnO₂, thereby facilitating the transfer of oxygen and reducing the formation of surface carbonates. The integration of heterogeneous metal elements has been demonstrated to induce electron transfer and strain effects between metals. These effects have the potential to modify the surface electronic structure of the catalyst and consequently alter the CO binding strength [26]. Lee et al. [27] deposited MnOx on Pt/Al₂O₃ catalysts by atomic layer deposition technique, and the introduction of Mn inhibited the sintering of Pt particles at 1073 K and enhanced the catalytic activity through the formation of Pt-MnOx interface. MnOx participates in the surface reaction by releasing lattice oxygen (O_latt), which induces the CO oxidation pathway of the Pt catalytic site to shift from the L-H mechanism to the Mars–van Krevelen (MvK) mechanism, and its CO oxidation rate is increased by 3–4 times compared with that of unmodified Pt/Al₂O₃. Wang et al. [28], to solve the problem of insufficient activity of Pt²⁺ species due to overstabilization by CeO₂, proposed a strategy to regulate the local environment of Pt by introducing W additive. The high electronegativity of W promotes electron transfer from Pt to W, weakens the strong interaction between Pt and CeO₂, lowers the energy barrier for the reduction of Pt²⁺ to active Pt0, and W induces the formation of low-oxygen vacancies in the active sites on the surface of CeO₂, which promotes interfacial oxygen activation.

Surface oxygen vacancies have been identified as playing a pivotal role as active sites in CO oxidation. The concentration and fluidity of Ov are usually one of the key factors determining the CO oxidation activity of metal oxide catalysts. Designing catalysts with abundant and reversible oxygen vacancy generation capabilities is an important strategy for improving the oxidation performance of CO [29,30]. Liu et al. [31] used H₂ reduction treatment to introduce a high concentration of Ov on the TiO₂ surface to provide uniformly dispersed sites for Pt nanoparticles (NPs) and Pt mono-atoms, where Pt NPs are preferentially enriched with CO molecules to diminish the competing adsorption effect, and atomically dispersed Pt sites are highly efficiently dissociated from the activated molecular oxygen via SMSI. Combining in situ characterization and theoretical calculations, the study reveals a unique dual-mechanism coupling pathway: the Pt NP-dominated L-H mechanism completes the adsorption and dissociation of O₂, while the lattice distortion triggered by the Pt-Ti ISAA promotes the direct participation of neighboring surface O_latt in CO oxidation via the MvK mechanism.

2.2. Pd-Based Catalysts

Owing to their superior efficiency, selectivity, and exceptional low-temperature activity in CO oxidation, palladium-based catalysts have attracted considerable research interest in recent years [32]. Shi et al. [33] revealed the structural synergistic mechanism of palladium–nickel (PdNi) nano-alloys in CO oxidation reaction by modulating their chemical composition, phase state, and atomic structure. The optimal catalytic activity at Pd:Ni ≈ 50:50 originated from the expanded interatomic distance between surface Pd atoms and elevated coordination number, which collectively enhanced CO and O₂ adsorption and activation. In the study by Wu et al. [34], monodisperse CoPd NPs were synthesized through colloidal chemistry. It was observed that the Co component exhibited a tendency to migrate to the surface, resulting in the formation of a cobalt oxide active layer. This active layer is believed to contribute to an interfacial synergistic effect with the metallic Pd core. The mechanisms by which this effect occurs include: (1) Pd adsorption of CO, (2) CoOx provision of O_latt, and (3) Ov at the interface, which promote the O₂ adsorption and reaction cycle. The collective effect of these mechanisms is said to significantly enhance the catalytic capacity. As shown in Figure 1b, Huang et al. [35] successfully achieved continuous control over the size of Pd nanoparticles in the Pd/Co₃O₄ catalyst by regulating the calcination temperature from 200 °C to 600 °C, causing it to gradually increase from 2.5 nm to 10.6 nm. Research indicates that smaller particle sizes significantly enhance CO oxidation catalytic activity. The fundamental mechanism underlying this size dependence lies in the fact that smaller Pd particles are more conducive to O₂ dissociation adsorption and CO₂ formation, while effectively preventing poisoning caused by excessive CO adsorption.

By modulating the nanostructures of the palladium species, the surface chemisorption behaviors were significantly altered when the particle size was reduced from the conventional nanoscale to sub-nanometer clusters: the linear CO adsorption conformation gradually became the dominant form, while the bridge-type and multidentate adsorption modes were reduced accordingly. This adsorption configuration shift reduced CO binding strength to active sites, accelerating intermediate desorption and mitigating CO poisoning at low temperatures [36]. To explore the nanoparticle size effect, Ma et al. [37] compared Pd/CeO₂ catalysts ranging from single atoms to nanoclusters (1–2 nm) and NPs (>2 nm) for CO oxidation. The catalytic performance of Pd/CeO₂ catalysts exhibited a discernible size dependence, with single-atom Pd/CeO₂ catalysts demonstrating the highest intrinsic activity. Soni et al. [38] synthesized Pd NPs of different sizes (1–2 nm, 4–6 nm, and 7–10 nm) using a precipitation method and found that the CO oxidation activity of the catalysts exhibited a clear size dependence, with the smallest-sized Pd-S-N catalysts exhibiting the lowest ignition temperature and the highest activity. This may be because small-sized Pd NPs are more likely to desorb CO at low temperatures, thereby liberating active sites for enhanced O₂ adsorption and activation. Satsuma et al. [39] compared Pd supported on different carriers for low-temperature CO oxidation, revealing the following activity order: Pd/CeO₂ > Pd/TiO₂ > Pd/Al₂O₃ > Pd/ZrO₂ ≥ Pd/SiO₂. This may be due to the ability of Ce to reduce PdO to Pd at low temperatures and the remarkable oxygen storage capacity (OSC) of Ce, which makes them show higher activity in CO oxidation reactions. Especially at high CO coverage, Ce can provide enough oxygen to sustain the CO oxidation reaction, and in addition, CeO₂ stabilizes precious metals and prevents them from sintering at higher temperatures [40]. Liu et al. [41] to enhance the synergistic effect between Ti and Ce, successfully prepared a homogeneous TiO₂-CeO₂ solid solution using an in situ capture strategy and prepared a Pd/TiO₂-CeO₂ catalyst. The PTC-iC catalysts were found to contain elevated levels of reactive oxygen species (ROS) on their surface. These ROS play a crucial role in activating and dissociating gaseous O₂, thereby replenishing the O_latt and enhancing the CO oxidation activity. Muravev et al. [42] identified CeO₂ crystallite size as a key factor controlling Pd/CeO₂ performance in CO oxidation. Their results show that smaller-sized CeO₂ nanocrystals can improve the stability of noble metals. This phenomenon can be attributed to the fact that smaller CeO₂ nanocrystals exhibit stronger oxygen mobility, enabling them to provide active oxygen more effectively during the reaction and thereby improving the redox performance of the catalyst.

2.3. Ru-Based Catalysts

Ru-based has analogous properties to Pt, yet it is significantly more economical, rendering it competitive for CO oxidation [11]. As shown in Figure 1c, Sunmi Kim et al. [43] prepared Ru NPs in two sizes, 2.8 nm and 6 nm, by modulating the precursor concentration and seed growth method. The 6 nm particles were found to maintain high activity through a stable core–shell structure, and smaller Ru NPs were more likely to be fully oxidized due to their large specific surface area, but the thin layer of RuO₂ was unstable under the reaction conditions and susceptible to further oxidation to inactive oxides, which reduced catalytic reaction activity. Sang Hoon Joo et al. [44] reached a similar conclusion regarding the underlying cause of the observed discrepancy in catalytic activity. They determined that this variation is attributable to the kinetic stability of the Ru@RuO₂ core–shell structure. Specifically, they ascertained that large particles possess a greater propensity to preserve an active thin oxide layer on their surface. In contrast, small particles are less active due to the formation of an inactive thick oxide layer resulting from complete oxidation. Yang et al. [45] successfully prepared unique RuO₂ wire-like structures on TiO₂(110) surface by precursor deposition, pyrolysis, and oxidation. These structures exhibit excellent stability at low temperatures and are capable of reversible reduction and re-oxidation by temperature modulation. Li et al. [46] examined the CO oxidation performance of three types of CeO₂-supported ruthenium catalysts with distinct morphologies: nanorods (NR), nanocubes (NC), and nanooctahedrons (NO). Their findings indicated that the carrier morphology is instrumental in determining the catalytic activity by regulating the distribution and valence state of Ru species, as well as the SMSI. It is shown that CeO₂ NR significantly promotes the high dispersion of Ru in the cationic state (Ruⁿ⁺, 4 ≤ n ≤ 6) due to the exposure of defect-rich 111 crystalline surfaces and the formation of a strong SMSI via Ru-O-Ce bonds or Ru embedded in the CeO₂ lattice, as well as inducing a high concentration of Ov. This structure enables 5Ru/CeO₂ NR-r to exhibit excellent low-temperature activity after reduction treatment.

2.4. Au-Based Catalysts

Gold is usually considered to be chemically stable and chemically inert; however, when dispersed on a nanoscale, it exhibits significant catalytic activity in low-temperature CO oxidation reactions [47]. In the context of noble metal catalysts, it is widely acknowledged that oxygen adsorption and activation are pivotal rate-limiting steps. The incorporation of reducible oxides as carriers or co-catalysts has been demonstrated to enhance oxygen adsorption sites, thereby augmenting the activity of noble metal catalysts for CO oxidation [48]. TiO₂ is a frequently utilized carrier that facilitates the participation of oxygen atoms or molecular oxygen in the low-temperature oxidation of CO. Yates et al. [49] revealed a two-site catalytic mechanism for the CO oxidation reaction on Au/TiO₂ catalysts, where CO molecules are preferentially anchored to the five-coordinated titanium (Ti5C) active site on the TiO₂ surface and form CO-O₂ linear transition state complexes with activated oxygen species in the interfacial region between Au NPs and the carrier. The complex significantly reduces the O-O bond breaking energy barrier through the electronic synergistic effect of Ti5c-Au bimetallic sites to realize the efficient oxidation of CO under low-temperature conditions (120 K). As shown in Figure 1d, Du et al. [50] synthesized a series of Au/TiO₂ catalysts with identical Au loading but varying particle size distributions by modulating the roasting temperature and plant extract concentration. The experimental results revealed that the CO oxidation activity of the Au catalysts was strongly dependent on the particle size. The most active catalysts possessed an average Au particle size of 3.8 nm. This optimal activity is due to the maximized interfacial contact between the hemispherical Au NPs and the TiO₂ support. This resulted in the formation of the longest perimeter interfaces, thus indicating that the interface is the key determinant for CO oxidation. A comparable discovery was made by Tana et al. [51], in which a substantial enhancement in catalyst activity was witnessed when the catalyst crystal size was diminished from 7.5 nm to 3.9 nm, from 12% to 63% at 273 K. The activity of the catalyst was also found to exceed that of the other catalysts. This phenomenon may be attributed to the propensity of small Au particles to exhibit a greater proportion of low-coordination sites, thereby facilitating efficient adsorption of CO and expediting surface reactions of CO.

Lu et al. [52] proposed a strategy of modifying alumina carriers by nickel doping to address the low activity of traditional Au/Al₂O₃ catalysts in CO oxidation due to carrier inertness. Alumina nanosheet carriers with varying Ni/Al molar ratios (0.01–0.1) were synthesized via a precipitation–hydrothermal method. Nickel doping exhibited a dual effect: it reduced the average particle size of gold particles from 3.6 nanometers to 2.4 nanometers and enhanced oxygen adsorption and activation by forming Ov. In terms of carrier effect studies. Widmann et al. [53] systematically investigated Au catalysts on various metal oxide supports and found a positive correlation between the carrier’s reducibility and its OSC. Catalytic activity decreased in the order Au/TiO₂ > Au/ZrO₂ > Au/ZnO > Au/Al₂O₃, paralleling the trend in support reducibility. Specifically, it has been demonstrated that the reduction in the support material facilitates the formation of Ov, thereby enhancing the adsorption and activation of oxygen. These active sites have been shown to promote the adsorption and dissociation of O₂, in addition to modulating the electronic structure of Au through SMSI. As a typical representative of “active carriers” in catalytic materials, cerium-based carriers show significant advantages in catalytic reactions involving redox processes compared to inert carriers such as alumina, which are irreducible. Its excellent redox properties can effectively participate in the catalytic cycle, and through the dynamic formation and repair mechanism of surface Ov, it can significantly increase the rate of catalytic reactions, including redox steps, such as CO oxidation [54,55]. Wang et al. [56] constructed an enriched Au-CeOx interface by uniformly anchoring CeOx NPs on the Au surface, which significantly enhanced the concentration of the electron-deficient state (Au⁴⁺) of Au, and the formation of [Au(CO)₂]⁸⁺ intermediates at the interface and the ROS supplied by CeOx synergistically facilitated the highly efficient conversion of CO, and the activity was enhanced by 4-fold compared with that of unmodified Au/SiO₂. A reduction in the particle size of the carrier to the nanometer level results in a significant change in the properties of the support. Carrettin et al. [57] significantly enhanced the activity of Au/CeO₂ catalysts in CO oxidation reaction by depositing Au on nanoparticulated CeO₂, which was two orders of magnitude higher than the catalysts prepared by conventional co-precipitation method or by depositing Au on conventional CeO₂ support.

2.5. Ag-Based Catalysts

Unlike platinum group metals and Au, Ag, as a group IB metal, has its d energy band completely filled. Ag catalysts are more likely to absorb and activate oxygen molecules. The highest activity was observed for Ag clusters with a diameter of ~5 nm; smaller clusters inhibited the reaction due to over-adsorption at low-coordination sites, and larger clusters showed reduced activity due to decreased O₂* adsorption capacity. Compared with Pt and Pd, Ag’s strong oxygen adsorption capacity makes it easier to form an oxygen cover layer under high O₂ partial pressure conditions, effectively circumventing the CO poisoning effect. Compared to Ru, Ag has a relatively low critical pressure, which allows for a kinetic regional shift from CO-dominated to O*-dominated at a wider [O₂]/[CO]² pressure ratio, resulting in a higher oxidation activity [58]. Wang et al. [59] investigated the impact of Ag particle size on the activity and stability of the CO oxidation reaction. Through a meticulous combination of experimental and theoretical calculations, they ascertained that nano-sized Ag particles exhibit intrinsic activity that surpasses that of monoatomic Ag in this reaction. As the size of the silver particles increases, their activation capacity for oxygen is enhanced, resulting in an increase in CO oxidation activity; however, when the particle size is too large, the number of active sites on the surface decreases, leading to a decrease in apparent activity. E. Kolobova et al. [12] prepared Ag/ZSM-5 catalysts with different silver loadings (1–7 wt.%) by impregnation, where 5–7 wt.% silver loading catalysts showed optimal activity Ag⁺ and Ag^nδ+ charged clusters are the key active sites for CO oxidation, whereas silver NPs indirectly participate in the reaction by generating active Ag⁺ through surface oxidation. Zhang et al. [60] found that the pretreatment condition of low-temperature H₂ reduction (≤200 °C) not only effectively modulates the dispersion of Ag particles but also maintains the stable presence of catalytically active surface oxygen species (Ag^nδ+ clusters). In contrast, when the reduction temperature exceeded 300 °C, the surface oxygen species completely disappeared, while significant aggregation of silver particles (particle size >9 nm) occurred, leading to a dramatic decrease in catalytic activity. In-depth analysis revealed that the synergistic effect of silver particle size (4.5–5.5 nm) and surface oxygen species (Ag₂O and Ag^nδ+ clusters) is the key to achieving low-temperature, efficient catalysis in this system. Among them, Ag^nδ+ clusters acted as active sites to promote the adsorption–activation of CO molecules, whereas the metallic silver particles of specific sizes provided sufficient active specific surface area as well as maintained the stable presence of oxygen species on the surface, and the synergistic effect of the two significantly enhanced the CO oxidation reaction kinetics. As shown in Figure 1e, Tang et al. [61] systematically investigated the optimization of silver catalyst performance in the CO oxidation reaction by regulating the crystalline phase of the TiO₂ support (anatase, rutile, and their mixed phases). The study revealed that mixed-phase m-TiO₂ most effectively promotes Ag species dispersion due to its balanced surface hydroxyl and defect concentrations, yielding fine Ag particles with an average size of merely 3.5 nm. In contrast, Ag particles on single-phase supports exhibited larger dimensions. The m-TiO₂ support not only provides abundant ROS but also enhances the catalyst’s reductive capacity, thereby promoting CO activation and oxidation.

Figure 1. (a) Effect of different forms of Pt on the oxidative capacity for CO oxidation. Reprinted with permission from [22]. Copyright 2022 American Chemical Society. (b) The influence of Pd with different particle sizes on the oxidation capacity of CO [35]. Reprinted with permission from Copyright 2019 American Chemical Society. (c) The influence of different Ru particle sizes on the oxidation capacity of CO. (The blue dots represent TOF values) [44]. Reprinted with permission from Copyright 2010 American Chemical Society. (d) The influence of Au with different particle sizes on the oxidation capacity of CO [50]. Reprinted with permission from Copyright 2014 American Chemical Society. (e) The influence of different support crystal planes on Ag oxidation of CO [61]. Copyright 2025 American Chemical Society.

Figure 1. (a) Effect of different forms of Pt on the oxidative capacity for CO oxidation. Reprinted with permission from [22]. Copyright 2022 American Chemical Society. (b) The influence of Pd with different particle sizes on the oxidation capacity of CO [35]. Reprinted with permission from Copyright 2019 American Chemical Society. (c) The influence of different Ru particle sizes on the oxidation capacity of CO. (The blue dots represent TOF values) [44]. Reprinted with permission from Copyright 2010 American Chemical Society. (d) The influence of Au with different particle sizes on the oxidation capacity of CO [50]. Reprinted with permission from Copyright 2014 American Chemical Society. (e) The influence of different support crystal planes on Ag oxidation of CO [61]. Copyright 2025 American Chemical Society.

Noble metal catalysts demonstrate outstanding performance in CO oxidation reactions. Their unique d-electron orbital structure facilitates strong CO adsorption and stabilizes key carbonyl intermediates, thereby markedly lowering the activation energy barriers of the reaction. However, the catalytic efficiency is significantly limited by the competition between CO and O₂ for adsorption on the metal active sites. The strong binding affinity of CO leads to site poisoning, which suppresses O₂ adsorption and its subsequent dissociative activation. In order to address this critical issue, the present study has sought to optimize the reaction pathway through the precise modulation of nanostructure and the design of synergistic effects among carriers. For Pt-, Pd-, and Au-based catalysts, a decrease in particle size has been shown to expose a greater number of low-coordinated active sites, thereby enhancing CO/O₂ co-adsorption and weakening the CO poisoning effect. In contrast, Ru- and Ag-based catalysts have exhibited the opposite size-dependent characteristics. Furthermore, the incorporation of reducible oxides as carriers or co-catalysts can markedly enhance the provision of oxygen species at the reaction interface. This is achieved by furnishing surface oxygen adsorption sites, fostering the formation of Ov, and facilitating the dynamic OSC mechanism. Consequently, this mitigates the inhibition of O₂ activation by strong CO adsorption, thereby circumventing the bottleneck. The performance summary of the precious metal catalyst is presented in

Table 1. Common Precious Metal CO Catalysts.

Catalysts	Reaction Conditions		CO Conversion (%)	Temperature (°C)	Ref.
	Reaction Atmosphere	WHSV (mL g−1 h−1)
Pt/TiO2	79.8%N2 + 10%O2 + 0.2%CO	30,000	50	213	[62]
Pt(N)/Al2O3	1 vol% CO in air	10,000	90	220	[63]
Pt/CeO2-CA	0.4%CO/10%O2	240,000	90	136	[64]
PtNP/TiO2-R	1% CO + 16% O2	60,000	50	106	[31]
Pt/TiO2	1% CO + 16% O2	47,900	100	100	[65]
4.5%Pt/CeO2	1000 ppm CO + 8% O2	60,000	50	85	[66]
1%Pt-2%Bi/SiO2	1% CO + 20% O2	134,000	100	100	[67]
1%Pt/SiO2	1% CO + 20% O2	134,000	100	150	[67]
Pt/CeO2	0.6% CO + 0.6% O2	80,000	100	180	[68]
0.1%Pt/SiO2	1% CO + 1% O2	120,000	100	270	[25]
Pd@CeO2	4% CO + 20% O2	40,000	100	253	[69]
Pd/CeO2(111)	2% CO + 2% O2	60,000	100	150	[70]
Pd/CeO2(100)	2% CO + 2% O2	60,000	100	200	[70]
PdCu-CeO2	1% CO + 20% O2	60,000	100	140	[71]
PdCu-CeO2	1% CO + 20% O2	60,000	100	140	[71]
Pd/CeO2(100)	2% CO + 2% O2	60,000	100	200	[70]
Pd/α-Fe2O3	1% CO + 20% O2	15,000	100	120	[72]
Pd/γ-Fe2O3	1% CO + 20% O2	15,000	100	80	[72]
Pd/MnOx–CeO2	2% CO + 5% O2	40,000	100	50	[73]
Ru/Cu-SSZ-13	5000 ppm CO + 5% O2	3,000,000	90	190	[74]
Ru NWs/TiO2	1.0% CO + 1.0% O2 + 98% He	12,600	100	150	[75]
5Ru/CeO2(110)	1% CO + 20% O2	36,000	50	135	[46]
5Ru/CeO2(111)	1% CO + 20% O2	36,000	50	157	[46]
5Ru/CeO2 (100)	1% CO + 20% O2	36,000	50	135	[46]
Ru/γ-Al2O3	1% CO + 1% O2	20,000	50	155	[11]
Ru-Co3O4	1% CO + 20% O2	30,000	100	75	[76]
Au/SiO2	2% CO + 16% O2	16,000	100	340	[56]
Au/CuO	1% CO + 21% O2	15,000	100	55	[77]
Au/Cu2O	1% CO + 21% O2	15,000	100	200	[77]
Au/CeO2-NR	1% CO + 5% O2	20,000	99	25	[78]
CeOₓ@Au/SiO2	2% CO + 16% O2 + 82% He	16,000	100	150	[56]
111-SC	5000 ppm CO + 20% O2	36,000	100	25	[79]
Au-MLMO	5000 ppm CO + 20% O2	/	100	124	[80]
Ag/SiO2-700	1.0% CO + 1.0% O2 + 98% He	12,000	98	58	[81]
Ag/Co3O4	1.6% CO + 21.4% O2	30,000	100	120	[82]
Ag/SiO2-900	1.0% CO + 1.0% O2 + 98% He	12,000	98	57	[83]

.

3. Non-Precious Metal Catalysts and Their Supported Precious Metal Systems

The practical deployment of precious metal catalysts has long been challenged by their high cost. Consequently, significant endeavors have been undertaken to engineer cost-effective alternatives. Among the non-precious metal oxides, Mn₂O₃, Cu₂O, Fe₂O₃, CeO₂, and Co₃O₄ demonstrate superior catalytic efficacy in the oxidation of CO. In comparison with precious metals, non-precious metals characteristically exhibit lower electron affinities and reduced atomic radii. This property renders them more susceptible to chemical reactions with oxygen [19].

3.1. Fe-Based Catalysts

In the field of chemical engineering, there is growing interest in the potential of iron-based catalysts to replace precious metal catalysts in CO oxidation reactions. This interest is driven by several factors, including the catalysts’ environmental friendliness, cost advantages, and abundant natural reserves [84,85].

The findings demonstrated that the synergistic effect of the Fe²⁺/Fe³⁺ redox electron pair significantly enhanced the activation of surface oxygen species by modulating the catalyst’s electronic structure. This effect also provided an optimized electronic environment for the adsorption-dissociation of reaction intermediates, resulting in a substantial enhancement of catalytic performance [86]. K.S. Abdel Halim et al. [87] successfully prepared nanocrystalline Fe₂O₃ with crystal sizes ranging from 75 to 150 nm using a combined co-precipitation and ball-milling approach. Their study revealed a notable size effect: as the crystal size decreases, the CO conversion increases. Fe₂O₃ with a crystal size of 75 nm achieved 98% CO conversion at 500 °C, demonstrating good catalytic performance. Nanocrystalline Fe₂O₃ powders exhibit excellent performance in CO-catalyzed oxidation reactions due to their smaller crystal size and higher density of active sites. Furthermore, Liu et al. [88] demonstrated that Fe₂O₃ NR with a predominant (110) crystalline facet exhibits the optimal CO oxidation efficiency. The oxidation mechanism of CO on the surface of Fe₂O₃ under aerobic conditions involves the preferential adsorption of CO on the active site of Fe³⁺. This process results in the combination of the O_latt with unsaturated surface coordination, leading to the generation of CO₂ and the formation of Ov. Subsequently, gas-phase oxygen molecules fill these Ov through dissociative adsorption, thereby completing the catalyst regeneration cycle. In an anaerobic environment, Fe₂O₃ directly oxidizes CO through a stepwise self-reducing path (Fe₂O₃→Fe₃O₄→FeO→Fe), and the simultaneously generated metal Fe further catalyzes CO disproportionation (2CO→C + CO₂), which multiplies the efficiency of total CO removal [89]. li et al. [90] successfully prepared porous spongy Fe₂O₃ microspheres by spray pyrolysis combined with a sodium nitrate fusion template, whose surface-rich OH groups dominated the bicarbonate reaction pathway and accelerated the CO oxidation reaction rate. Song et al. [91] introduced surface defect charge state analysis on the basis of traditional AITM and constructed a surface phase diagram of α-Fe₂O₃(001) combining oxidation potential and Fermi level. Surfaces with iron-based ends exhibit the highest CO oxidation catalytic activity. Yu et al. [92] systematically revealed the mechanism of CO oxidation on the surface of Fe₃O₄ using a density functional theory method, where CO is preferentially adsorbed on the octahedral Fe³⁺, forming an inclined or vertical adsorption structure. The uncoordinated surface oxygen atom functions as a pivotal active site, instigating the oxidation reaction through a nucleophilic attack on the C atom of the adsorbed state CO. In the Fe₃O₄(001)-B termination surface, this process culminates in the formation of a linear transition state of CO₂. While CO multimolecular adsorption changes the surface electronic environment, leading to a weakening of the Fe-O bond, which reduces the oxidation energy barrier of CO and creates a significant thermodynamic driving force.

Gao et al. [93] systematically compared gold catalysts supported on nanorod-shaped versus nanosphere-shaped hematite, revealing the crucial regulatory role of support morphology in CO oxidation performance. They observed that nanorod-shaped α-Fe₂O₃ exhibits a richer surface defect structure. These defects serve as effective anchoring sites for gold particles, enabling the formation of smaller, more uniformly distributed hemispherical structures that exhibit SMSI. In contrast, the relatively smooth surface of nanospheric α-Fe₂O₃ supports larger gold particles with poorer dispersion. Further investigations revealed that nanorod-shaped carriers not only enhance the binding strength between gold and the support but also significantly boost the activation capacity of interfacial oxygen. This synergistic effect collectively promotes the high activity and stability of the CO oxidation reaction. Shi et al. [94] systematically investigated the surface structure of iron oxide NR to elucidate its influence on the CO oxidation catalytic performance of supported gold NPs. The Au/FeOOH catalyst exhibited the most outstanding CO oxidation activity, potentially due to the abundant hydroxyl groups on the FeOOH support surface promoting oxygen activation. Furthermore, in situ transmission electron microscopy during the reaction process visually revealed the structural evolution of the catalysts: gold particles remained well dispersed in both Au/FeOOH and Au/α-Fe₂O₃, whereas Au/FeOx exhibited Ostwald ripening, forming larger particles that led to decreased activity. Zhao et al. [95] systematically compared gold catalysts supported on γ-Fe₂O₃ and α-Fe₂O₃, revealing a key mechanism whereby the crystalline phase of the support determines CO oxidation performance by regulating redox properties. As illustrated in Figure 2, the Au/γ-Fe₂O₃ catalyst exhibits outstanding activity at low temperatures, centered on its unique MvK redox cycle: CO molecules first adsorb and activate at gold active sites, subsequently reacting with carrier O_latt to form CO_2, leading to local surface reduction to the Fe₃O₃ phase. Subsequently, gas-phase O₂ rapidly replenishes the consumed O_latt, dynamically regenerating the catalytic cycle. Notably, the inverse spinel structure of γ-Fe₂O₃ and its cation vacancies provide strong anchoring sites for gold particles. This not only enhances metal–support interactions but also optimizes interfacial oxygen migration, enabling complete CO conversion at −20 °C with a turnover frequency of 2.02 s⁻¹.

3.2. Cu-Based Catalysts

Copper-based catalysts show remarkable potential in CO catalytic oxidation due to their unique 3d electronic configuration, multivalent properties and low-cost advantages. It is shown that the Cu(II)/Cu(I) redox cycle plays a central role in regulating the adsorption–desorption equilibrium of ROS [96]. Under high-temperature conditions, CO reacts with CuO O_latt to produce CO₂, leading to surface reduction to Cu₂O, which is subsequently re-oxidized to CuO by ambient oxygen, forming a dynamic cyclic mechanism [97]. The adsorption of CO on the CuO surface at room temperature results in the formation of a carbonyl complex. This process is accompanied by reactions with surface and gas-phase oxygen, resulting in the formation of carbonate intermediates. Concurrently, this series of reactions initiates the valence transition of CuO to Cu₂O [98]. Notably, the Cu⁺ cations at the grain boundaries on the Cu₂O surface have been identified as highly active sites for low-temperature CO conversion [99,100]. Catalyst structural features also profoundly affect the performance. Zhang et al. [101] prepared five sizes of cubic Cu₂O (1029, 682, 446, 109, 34 nm) to elucidate the contribution of different surface sites (face sites, edge sites) of catalyst NPs to the catalytic activity. The results revealed that the BET surface area was inversely proportional to the particle size. The activity of large-size Cu₂O (>100 nm) is predominantly driven by the facet sites (CuO/Cu₂O(100)). The reaction proceeds via the MvK mechanism. First, CO adsorbs onto surface oxygen sites of copper oxide. This then forms a carbonate intermediate, which subsequently decomposes to release CO₂ and generate an oxygen vacancy. In contrast, the catalytic activity of small-sized Cu₂O (<100 nm) switches to the edge site due to a significant increase in the density of the edge site (CuO/Cu₂O(110)), which also proceeds via the MvK mechanism, but the reaction pathway in which CO reacts directly with the neighboring threefold-coordinated O ions site to produce CO₂, reduces the activation energy to 0.60 eV. In-depth analysis showed that Cu-based catalysts follow a dual path in CO oxidation—direct oxidation dominates on the metallic copper surface, while Cu₂O mediates the redox mechanism through Cu⁺/Cu²⁺ valence oscillations. Burcu Karagoz et al. [102] revealed the decisive role of surface atomic arrangement in catalytic activity. Whilst conventional Cu₂O(111) crystal planes exhibit inert behavior towards CO oxidation at sub−100 K cryogenic temperatures, they demonstrate exceptional low-temperature catalytic performance when forming a unique PY reconstruction structure composed of nanopyramidal units. This activity stems from the unsaturated oxygen atoms exposed on the PY-reconstructed surface, which enable highly efficient reactions with CO molecules. Theoretical calculations further elucidate the intrinsic mechanism of this reaction: the energy barrier for the oxidation process is exceptionally low (merely 0.10 eV), while the resulting CO₂ product adsorbs weakly in a configuration nearly parallel to the surface. This unique adsorption configuration facilitates rapid desorption of the product, thereby ensuring both the efficiency and sustainability of the reaction. Qiu et al. [103] systematically compared cubic and octahedral morphologies of Au-CuO/Cu₂O catalysts, discovering that catalysts prepared based on the cubic morphology exhibited stronger Au-CuO interface interactions. This led to a reduction in the electron density of Au species and an increase in the proportion of surface hydroxyl oxygen, significantly enhancing catalytic performance. As illustrated in Figure 3, Hong et al. [104] reported similar findings: although octahedral supports exhibited higher activity, loading Pt onto cubic Cu₂O catalysts yielded substantially greater activity gains than octahedral Cu₂O, demonstrating a pronounced enhancement effect dependent on crystal plane orientation. This may arise because in Pt/cubic Cu₂O, the (100) plane terminating at oxygen atoms dominates the formation of Pt–O–Cu interfacial bonding. This structure effectively suppresses oxygen adsorption, thereby forming only a thin layer of CuO and preserving the high activity of Pt.

3.3. Mn-Based Catalysts

Manganese oxide systems (MnO, Mn₂O₃, MnO₂, Mn₃O₄) show excellent performance in the catalytic oxidation of CO, attributed to their unique redox multivalent (+2/+3/+4) and dynamic oxygen storage properties. This multivalent synergistic effect confers a flexible electron transfer capability: the Mn³⁺/Mn⁴⁺ oxidation pair rapidly activates gas-phase oxygen molecules, while the Mn²⁺/Mn³⁺ reduction electron pair promotes the release and regeneration of O_latt [105]. Gao et al. [106] thoroughly examined the carbon monoxide oxidation mechanism of α-MnO₂ nanowires. This process involves the chemisorption of CO on the metal oxide surface and its subsequent reaction with O_latt, yielding MCO₂ intermediates and concurrently generating Ov. Following the formation and desorption of CO₂, an O₂ molecule replenishes the oxygen vacancy, reforming the MO₂ site. Subsequently, another CO molecule is adsorbed onto this site to form an MO₂CO intermediate, which ultimately desorbs as CO₂, thus completing the catalytic cycle. Liang et al. [107] prepared α-, β-, γ-, and δ-MnO₂ NR via a hydrothermal method. They then systematically explored the catalytic performance and mechanism for CO oxidation. The catalytic activities were in the order of α ≈ δ > γ > β. The difference in activities was mainly attributed to the modulation effect of the crystalline phase structure on CO adsorption, O_latt activity, and intermediate conversion. Although the layered structure of δ-MnO₂ enables optimal CO adsorption capacity, the α-phase exhibits a greater propensity to release O_latt and engage more actively in oxidation reactions. This tendency is attributed to the elongation of the Mn-O bond lengths, which results in a weakened binding energy of the O_latt. The reaction proceeds via the Mars–van Krevelen mechanism: CO is preferentially adsorbed onto surface hydroxyl sites and reacts with O_latt to form CO₂. Concurrently, Mn⁴⁺ is reduced to Mn³⁺ or Mn²⁺, and subsequently O₂ re-oxidizes Mn₂O₃/Mn₃O₄ to MnO₂ to complete the cycle.

Liu et al. [108] successfully synthesized two α-MnO₂ catalysts (MnO₂-D and MnO₂-P) with distinct oxygen vacancy concentrations via morphological control strategies, systematically investigating their low-temperature CO oxidation performance. The study revealed that MnO₂-D, rich in defect-rich Mn^δ+-Ov structures, exhibited outstanding catalytic activity. This may be attributed to the predominant exposure of the (310) crystal plane in MnO₂-D, where the oxygen vacancy formation energy (0.69 eV) is significantly lower than that in MnO₂-P (1.59 eV), thereby promoting the generation and activation of adsorbed oxygen species on the surface.

As shown in Figure 4a, Zhang et al. [109] loaded gold NPs onto β-MnO₂ and γ-MnO₂ via the precipitation deposition method. They observed that Au/β-MnO₂ significantly enhanced CO oxidation activity, attributed to the SMSI effect promoting the formation of Au⁺ ions, Mn²⁺ species, and adsorbed oxygen (O_ads), thereby enhancing oxygen vacancy regeneration and reaction sites. In contrast, Au/γ-MnO₂ exhibited reduced catalytic efficiency due to gold NPs being trapped within the support pores, leading to a decreased Au⁺ fraction and Au³⁺ formation. As depicted in Figure 4b, Zhang et al. [88] constructed a 1 wt% Pt/β-MnO₂ catalyst via the deposit-precipitation method. Pt/β-MnO₂ catalyst via a deposit-precipitation method. They observed that Pt loading suppressed CO oxidation in the low-temperature range (<210 °C) but significantly enhanced activity at elevated temperatures (>210 °C). At low temperatures, the catalyst surface predominantly featured Pt⁴⁺-O-Mn structures, which, while relatively stable, hindered the oxidation of CO by the MnO₂ support, resulting in low activity. As temperature increases, Pt⁴⁺ is reduced to Pt²⁺, shifting the structure towards Pt-Pt²⁺-Pt. This configuration facilitates oxygen activation and reactant transfer, substantially enhancing CO oxidation rates.

3.4. Ce-Based Catalysts

As a typical rare-earth metal oxide, cerium dioxide finds significant application in the domain of environmental catalysis by virtue of its unique Ce⁴⁺/Ce³⁺ redox pair property. The material is endowed with excellent OSC through a reversible oxygen vacancy formation mechanism, a property that stems from the dynamic migration behavior of O_latt during Ce³⁺/Ce⁴⁺ valence state transitions. Compared with noble metal catalysts, CeO₂ systems are more advantageous in terms of cost-effectiveness and environmental friendliness, which makes them of interest in oxygen-demanding reaction systems such as CO catalytic oxidation. The exposed crystal facets of CeO₂ significantly determine its catalytic performance. This phenomenon is primarily attributed to the differences in structural stability and oxygen defect formation ability of different crystal facets. CeO₂ with 110 and 100 facets exhibits superior catalytic activity [111]. Lykaki et al. [18] synthesized CeO₂ exposing specific crystal planes by the hydrothermal method. CeO₂ NR with dominant exposure of 100 and 110 crystal planes showed optimal activity in CO oxidation reaction. Because they possess a lower energy barrier for forming Ov and higher mobility of surface oxygen. According to Breysse’s hypothesis [112], the initial interaction between CO and CeO₂ entails the adsorption of CO on the CeO₂ surface, followed by a reaction with O_latt, resulting in the formation of CO₂. This process is concomitant with the formation of Ov. Subsequently, gas-phase O₂ rapidly fills these vacancies, thereby restoring the O_latt. According to the kinetic study, the oxidation reaction is limited by the rate of CO adsorption and activation. Similar findings were made by Zheng et al. [113]. The adsorption of CO onto the ceria CeO₂ surface leads to the generation of CO through the capture of O_latt. The subsequent formation of Ov initiates the reduction in neighboring Ce⁴⁺ to Ce³⁺. Pablo G. He et al. [114] systematically investigated the influence of CeO₂ support morphology (nano-octahedra, cubes, particles, spheres, and rods) on the exposure of (111), (100), and (110) crystal planes and their catalytic mechanisms, thereby regulating the low-temperature CO oxidation activity of Pd/CeO₂ catalysts through crystal plane engineering. Characterization revealed that the spherical 1Pd/CeO₂-S catalyst, exhibiting simultaneous exposure of (111) and (100) planes, significantly promoted the formation of Pd_xCe_1−xO_2−y solid solutions. This generated the highest concentration of surface Ov, conferring exceptional low-temperature activity in the CO oxidation reaction.

As shown in Figure 5, Ha et al. [115] synthesized morphologically controllable CeO₂ supports via hydrothermal methods and loaded them with 3 nm Au particles. They discovered that the Au/CeO₂ cubic structure exhibited significantly higher performance than the Au/CeO₂ cubic structure. The fundamental reason lies in the interfacial structure: the cubic structure primarily exposes. The (100) crystal plane of the cube exhibits greater flexibility in its interface structure with the Au NPs. Theoretical calculations reveal that the Au/CeO₂ (100) interface spontaneously forms an O-C-O intermediate (with an energy barrier of merely 0.4 eV), whereas the octahedral (111) interface requires overcoming a 0.86 eV energy barrier. This significantly reduces the overall reaction energy barrier, substantially enhancing the reaction rate.

3.5. Co-Based Catalysts

Cobalt-based oxides are one of the most competitive candidates for catalytic oxidative degradation of CO owing to their abundance of ROS, excellent redox properties, and multivalent metal character [116]. By controlling the crystal morphology and crystallographic orientation of Co₃O₄ nanostructures, Co-O active sites with specific coordination structures can be exposed in a targeted manner, thus optimizing their catalytic performance. The oxidation state of cobalt governs the catalytic activity, wherein Co³⁺ in octahedral coordination plays the role of the key active center. In contrast, tetrahedrally coordinated Co²⁺ contributes minimally to the reaction [117]. Co₃O₄ NR with predominantly exposed (110) planes demonstrates outstanding catalytic performance in low-temperature CO oxidation, attributed to their surface enrichment of active Co³⁺ species. Xie et al. [118] achieved the targeted exposure of 110 crystallites rich in octahedrally coordinated Co³⁺ active sites by modulating the synthesis of cobalt hydroxyl carbonate precursors with the calcination process, enabling them to exhibit excellent CO oxidation capability even at temperatures as low as −77 °C. Liu et al. [119] achieved a substantial increase in the Co³⁺ content and oxygen vacancy concentration on the catalyst surface through a hydrogen pre-reduction treatment. Concurrently, the Co₃O₄ particle size underwent a reduction from 32.7 nm to 14.3 nm. This change resulted in the exposure of a greater number of active sites, thereby enhancing the adsorption of Co³⁺ on CO and hindering the competition between O₂ and CO adsorption. Liliana Lukashuk [120] systematically revealed the underlying mechanism and pathways of CO oxidation on Co₃O₄ catalysts. At temperatures above 100 °C, CO is preferentially adsorbed onto Co³⁺ active sites, where it reacts with surface O_latt to form CO₂ and generate Ov. Subsequently, O₂ rapidly refills these vacancies, enabling a continuous redox cycle. The dynamic equilibrium of surface Co³⁺ as the key active site Ov is essential for maintaining catalytic activity. The MvK mechanism’s efficacy hinges upon the activity of surface oxygen sites and its dynamic capacity for regeneration. Li et al. [121] exposed the Co-O layer by means of NaBH₄ reduction and acid etching to effect a change in Co from six-coordinated (CoO6) to five-coordinated (CoO5). The geometrical unsaturation of the five-coordinated Co promotes the generation of Ov, which not only acts as an active site to adsorb O₂ molecules but also enhances the MvK reaction rate by lowering the dissociation energy of O-O bonds. The CO oxidation reaction over cobalt-based catalysts follows the MvK mechanism. CO preferentially adsorbs onto Co²⁺ sites and is oxidized by O_latt from adjacent Co³⁺ sites, yielding CO₂ in a vibrationally excited state. Subsequent to this, surface Ov are repaired by the rapid replenishment of oxygen, which realizes dynamic repair while completing the re-oxidation cycle from Co²⁺ to Co³⁺ [122].

As depicted in Figure 6a, Li et al. [82] successfully synthesized hollow porous Co₃O₄ polyhedra by pyrolyzing ZIF-67 crystals. They then constructed an Ag/Co₃O₄ interfacial catalyst by loading silver NPs, which demonstrated highly efficient catalysis for CO oxidation. Characterization revealed that Ag incorporation substantially increased the proportion of surface Ov and adsorbed oxygen species, enhancing oxygen species migration and thereby significantly boosting catalytic activity. As depicted in Figure 6b, Sami Barkaoui et al. [123] employed a hydrothermal approach to synthesize nano-cubic Co₃O₄-C primarily exposing the 001 crystal plane, alongside nano-floral Co₃O₄-F predominantly displaying the 111 crystal plane. Subsequently, 2 nm gold NPs were loaded onto different morphologies of Co₃O₄, with the Au/Co₃O₄-C catalyst exhibiting superior performance. This may be attributed to the (001) plane providing a higher Co³⁺/Co²⁺ ratio and adsorbed oxygen content, thereby promoting oxygen activation and reaction rates. Catalyst deactivation primarily stems from the accumulation of the reaction product carbon dioxide at active sites, rather than metal sintering phenomena. The synergistic interaction between the (001) plane and gold interface effectively enhances oxygen activation and carbon monoxide adsorption processes, whereas the (111) plane is more prone to adsorbing carbon dioxide molecules.

Non-precious metal catalysts (Fe, Cu, Mn, Ce, Co) have emerged as promising alternatives to precious metals in CO oxidation due to their cost-effectiveness, environmental sustainability, and distinctive redox characteristics. The performance of these catalysts is strongly dependent on their capacity to chemisorb CO, which creates an essential precursor state for oxidation by enriching reactant concentration on the catalyst surface. However, the chemisorption of CO molecules needs to overcome certain energy barriers, which directly affects the overall activation energy requirement of the catalytic reaction. It is important to acknowledge that while non-precious metal catalysts can effectively mitigate the risk of carbon monoxide poisoning, which is a concern with conventional precious metal catalysts, the electronic structure properties of their surfaces may result in a reduction in the activation efficiency of CO molecules. This paradoxical characterization stems from the adsorption–activation equilibrium relationship between surface active sites and CO molecules. Overly strong chemisorption, while increasing the surface coverage, may weaken the subsequent activation process of the CO molecules. Conversely, weaker adsorption may lead to insufficient concentration of the effective reactants. Therefore, optimizing the surface electronic structure of non-precious metal catalysts to balance the adsorption strength and activation capacity becomes a key research direction to enhance their catalytic performance. A summary of the properties of common non-precious metal catalysts is provided in

Table 2. Common non-precious metal CO catalysts.

Catalysts	Reaction Conditions		CO Conversion (%)	Temperature (°C)	Ref.
	Reaction Atmosphere	GHSV(h−1)
CuO/Al2O3–CeO2	0.3% CO/5% O2/0.02% SO2/N2	60,000	50	67	[124]
CeO2/o-Cu2O	1.8% CO/10% O2/88.2% N2	40,000	50	190	[125]
CuO-Co3O4-CeO2	0.4%CO/15%O2	24,000	100	125	[126]
MCO-300	1%CO/20%O2	24,000	100	80	[127]
γ-MnO2-0.5 h	1%CO/20%O2	18,000	100	156	[128]
Cu-OMS-2	0.7%CO/10%O2	13,200	100	148	[129]
13.2%CeO2–CuO/c-Cu2O	1%CO/air	36,000	100	140	[130]
CuO/Cu2O	1%CO/20%O2	30,000	100	150	[96]
SLC-Omv-OL	1% CO/20% O2/He.	6000	100	150	[121]
CeO2@TiO2	1000 ppm CO/5%O2	15,000	100	300	[131]
Cu0.1MnOx	1%CO/20%O2	36,000	95	80	[106]
Ce–Co–O 600	CO: O2 = 1:20 in He	12,000	100	153	[7]
CeO2	5000 ppm CO + 10% O2 in N2	25,000	100	400	[132]
Co0.9Ce0.1/RSAC	2500 ppm CO + 21% O2 in air	8000	100	110	[133]
Mn5Co1Ox	1% CO in air	30,000	50	183	[134]
CuO/γ-Fe2O3	1% CO + 20% O2 in 79% N2	/	98.5	180	[135]
Co3O4/TiO2(A)	1% CO/10% O2/89% N2	6000	100	−43	[136]
FeCe/Al–Cl	1.6% CO + 20.8% O2 in 77.6% N2	30,000	100	295	[137]

.

4. The Effect of H₂O and SO₂ on CO Oxidation

The dual role of water vapor in catalytic reaction systems has become an active area of research, as variations in its concentration profoundly influence the CO oxidation process. The presence of an optimal amount of water enhances catalytic activity mainly through three key pathways: (1) promoting catalyst surface remodeling to form new active sites [138,139]; (2) directly participating in the CO to CO₂ oxidation pathway via a surface hydroxyl-mediated co-catalytic mechanism [140]; (3) facilitating the generation of OOH⁻ to effectively activate O₂ molecules [141,142]. Figure 7a reveals that water molecules dynamically alter the local structure of Pd active sites through solvation, inducing partial dissociation of the Pd²⁺ cation originally firmly anchored within the SSZ-13 zeolite hexagonal ring. This forms a mobile Pd–H₂O complex. This structural transformation markedly enhances the mobility of the Pd site and weakens the strong interaction between Pd and the zeolite framework oxygen through ligand exchange and charge shielding, thereby optimizing the coordination environment of the active site [143]. Figure 7b illustrates a co-catalytic mechanism mediated by surface hydroxyl groups, wherein water molecules at the Au–Fe₂O₃ interface promote O₂ activation to generate highly reactive surface hydroxyl species. Subsequently, CO reacts with this hydroxyl to form a crucial carboxylate intermediate. This intermediate further decomposes into CO₂, regenerating the hydroxyl, thereby establishing an efficient water-assisted cyclic catalytic process [144]. Johnny Saavedra et al. [145] discovered that water acts as a potent promoter within low to moderate water pressure ranges (approximately 300–600 Pa). As illustrated in Figure 7c, it aids in the activation of O₂ at the metal–support interface of gold NPs to form the key intermediate (Au-OOH) by adsorbing onto the support surface and participating in the proton transfer mechanism. This also accelerates the decomposition of the rate-determining intermediate, thereby significantly enhancing the reaction rate.

Masakazu Daté et al. [146] found that the degree of OH dependence of different carriers was determined by their surface oxygen mobility and acidity, leading to differences in moisture sensitivity. TiO₂ can still participate in the reaction via carrier oxygen at low humidity due to its high surface oxygen mobility, and the activity is further optimized at high humidity; Al₂O₃ is dependent on OH-mediated O₂ activation, with activity significantly elevated at 200 ppm H₂O; SiO₂ has no surface oxygen mobility, and its activity is completely dependent on OH supply, which decreases abruptly at very low humidity (<1 ppm). Johnny Saavedra et al. [147] demonstrated that water plays a central role via a proton transfer-mediated interfacial reaction mechanism. Meanwhile, OH groups on the TiO₂ surface primarily serve as anchoring sites for water molecules, indirectly modulating catalytic activity. This facilitation promotes the activation of O₂ to form the key intermediate OOH through proton transfer, thereby enhancing the efficiency of CO oxidation. The inhibitory effect of water on the catalytic reaction manifests in two main aspects: first, surface lattice OH groups generated by the dissociative adsorption of H₂O on Ov prevent the adsorption of gaseous O₂ [148]; second, these species facilitate the formation of stable bicarbonate compounds near the active site, thereby blocking it [146,149]. Xu et al. [150] enhanced the water resistance of the catalyst by utilizing the strong interaction between Co and Sn. This effectively inhibited water molecule adsorption on the catalyst surface and reduced its hydrophilicity.

As shown in Figure 8a, Junemin Bae et al. [151] utilized La³⁺ to selectively replace Co²⁺ at the tetrahedral coordination site on the surface of Co₃O₄. This resulted in a significant increase in the formation energy of Ov by 0.3 eV. It also inhibited the dissociative adsorption of H₂O molecules at Ov, blocking the generation of surface hydroxyls and the formation pathway of bicarbonate species, thereby enhancing water resistance. Water adsorption can be considerably reduced by establishing a hydrophobic interface. This is commonly accomplished through the incorporation of hydrophobic substances, for instance, polydimethylsiloxane and carbon nanotubes, as shown in Figure 8b [152].

SO₂ poisoning primarily results from the competitive adsorption and chemical transformation processes that occur on the catalyst surface. In these processes, the active site combines with SO₂ to form a non-volatile metal sulfate. Consequently, the catalytic performance deteriorates and ultimately progresses to complete deactivation [153]. The use of strongly acidic carriers has been found to be an effective method of preventing SO₂ uptake. Sulfate species deposited on the surface of metal oxides (e.g., SiO₂, TiO₂, ZrO₂, etc.) exhibit lower thermodynamic stability compared to other types of carriers and thus are more readily desorbed or removed from the surface of such metal oxide carriers [154,155,156,157]. Young Seok Ryou et al. [155] found that the sulfur adsorption decreased drastically with the decrease in Ce content after the introduction of Zr, which inhibited the formation of cerium sulfate. In addition, Zr stabilized the CeO₂ lattice to maintain the pore structure, reducing sulfur enrichment in the collapsed pores. F.J. Gracia et al. [157] found that Al₂O₃ carriers exhibit short-term sulfur resistance advantages and that their surfaces can oxidize SO₂ to sulfate storage, delaying the direct toxicity of sulfur to Pt. Liu et al. [158] methodically examined the mechanism of four carriers, SnO₂, WO₃, Al₂O₃ and CeO₂, on the active structure and sulfur resistance of Pt catalysts. SnO₂ carriers promote sulfur migration to the carrier surface through the formation of PtS₂ and SnS species, reducing the active site coverage; WO₃ inhibits sulfur adsorption and stabilizes sulfur to the WS₃ structure by using its strong Brønsted acidic site. Al₂O₃ carriers trigger progressive deactivation, although they retard sulfur adsorption by virtue of their intermediate Lewis acidity. CeO₂ reacts with SO₂ to produce Ce(SO₄)₂ by the Ov-mediated sulfation pathway and triggers structural collapse and Pt particles. CeO₂ reacts with SO₂ to form Ce(SO₄)₂ due to the oxygen vacancy-mediated sulfation pathway, which triggers structural collapse of the carrier and agglomeration of the platinum particles, leading to a drastic decrease in activity. Constructing sacrificial sites to mitigate SO₂ adsorption or modulating the structure and morphology of catalysts are considered to be effective methods to improve their SO₂ resistance properties [159]. Gao et al. [131] fabricated a CeO₂@TiO₂ core–shell structure. The TiO₂ shell layer not only reduces the direct contact between SO₂ and CeO₂ core through physical isolation but also inhibits the generation of Ce³⁺ and the formation of cerium sulfate. Additionally, its higher specific surface area (81.39 m²/g) and abundant surface Ov promote the CO oxidation reaction.

5. CO Oxidation Mechanism

The catalytic oxidation mechanisms of CO have been extensively studied in recent years and are primarily categorized into three models: the Langmuir–Hinshelwood (L-H), Eley–Rideal (E-R), and MvK mechanisms. However, the current understanding of the CO oxidation mechanism, the role of vacancies and the source of oxygen for the reaction is somewhat lacking. Figure 9 summarizes common CO oxidation mechanisms.

5.1. The Langmuir–Hinshelwood Mechanism

The L-H mechanism is applicable to the process where CO and O₂ are co-adsorbed and subsequently activated on the catalyst surface. In a typical reaction pathway, CO and O₂ molecules first co-adsorb at adjacent active sites, forming OCOO* transition state intermediates with a peroxygen structure via electron transfer, followed by dissociation to produce CO₂ molecules in the adsorbed state and a residual O species. The surface oxygen atom further reacts with another adsorbed state CO molecule via C-O bond recombination to produce CO₂ [160]. As demonstrated in reaction Equations (1)–(5), the active site is denoted by M, while □ refers to the oxygen vacancy.

CO + M → CO*

(1)

O₂ + M → O₂*

(2)

CO* + O₂* → OOCO*

(3)

OOCO* → CO₂ + O*

(4)

CO* + O* → CO₂

(5)

As shown in Figure 10a, the categorization of L-H mechanisms, based on the differences in reactant adsorption sites, can be divided into competitive and noncompetitive types. The competitive L-H mechanism (unit point model) is particularly typical in loaded metal catalyst systems, especially when noble metals (e.g., Pt, Pd) are loaded on irreducible carriers (e.g., Al₂O₃, SiO₂) that are chemically inert and have low oxygen mobility; the competitive adsorption characteristics are more significant. The core feature of this mechanism is that CO and O₂ need to compete to occupy the same metal active site to complete the adsorption–activation process [161]. At low temperatures, CO preferentially occupies the metal sites by virtue of its stronger adsorption affinity, resulting in the blocking of O₂ adsorption, whereas as the temperature increases, CO adsorption decreases and desorption occurs, and the active sites are released for O₂ activation, at which time the adsorption and dissociation of O₂ become the quick-control steps of the reaction [162,163].

The noncompetitive L-H mechanism (two-site model) is observed in catalysts with multiple active sites. In these catalysts, CO is preferentially adsorbed to metal sites (e.g., Pt0 or PtOx), while O₂ completes its adsorption and dissociation at the oxygen vacancy site on the surface of the carrier or at the interfacial site. This results in the formation of a highly ROS (e.g., O⁻ or O²⁻). The rate-limiting step in this process is the adsorption and activation of the reactants [161,164]. This approach offers the advantage of bypassing competitive adsorption-induced inhibition at the active site [165].

5.2. Eley–Rideal Mechanism

According to the E-R mechanism, O₂ first chemisorbs on catalyst active centers, generating pre-adsorbed oxygen. Subsequently, gas-phase CO molecules collide with and react directly with O_ads at the interface. The formation of C-O bonds during the interaction of O₂ and CO is likely rate-limiting [166,167], as shown in reaction Equations (6)–(8)

O₂ + M* → O₂*

(6)

CO + O₂* → CO₂ + O*

(7)

CO + O* → CO₂

(8)

As shown in Figure 10b, under dry conditions, the Pd²⁺ cation position within the molecular sieve’s hexagonal ring remains fixed. Adsorbed CO reacts directly with O₂ molecules in the gas phase, following the E-R mechanism. In the presence of H₂O, water molecules coordinate with Pd²⁺ to form partially framework-displaced, mobile Pd-H₂O complexes. This reconfiguration of active sites enables both CO and O₂ to adsorb simultaneously on the same Pd active center, thereby following the L-H mechanism [143].

Gong et al. [168] discovered that when Ov are in proximity to CuO, Cu⁺ exhibits a preference for adsorbing CO to form CO-Cu⁺. Subsequently, gas-phase O₂ directly attacks the adsorbed state CO, resulting in the generation of CO₂. The O₂ molecule dissociatively participates in dual roles: one oxygen atom replenishes Ov, while its counterpart oxidizes CO. This process is followed by a second CO molecule combining with O-Cu⁺ to dissociate. The reaction unfolds in accordance with the E-R mechanism. Conversely, when the Ov moves away from CuO, O₂ exhibits a preference for adsorbing to Ov and dissociating. The process entails the presence of one oxygen atom, which occupies the vacancies and subsequently forms O_latt. Concurrently, another oxygen atom undergoes a conversion to surface-active oxygen (O*). The O* then reacts directly with gas-phase CO, resulting in the generation of CO₂. The reaction transforms into the Mvk mechanism. Mehdi D. Esrafili et al. [169] investigated the catalytic performance of Pd-doped graphene for CO oxidation reaction by DFT and found that Pd-graphene efficiently catalyzes CO oxidation through a synergistic LH-ER mechanism. Among them, the ER mechanism critically governs the second step, wherein gas-phase or weakly adsorbed CO directly reacts with oxygen adatoms generated via the initial L-H step to form a second CO₂ molecule. The low-energy barrier property of the ER mechanism stems from the strong activation of O₂ by Pd doping. As the study progressed, on the basis of the ER mechanism, the researchers identified the TER mechanism. The TER mechanism, in contrast, entails the synergistic activation of O₂ by two pre-adsorbed CO molecules, resulting in the formation of the intermediate OOC-x-COO. This process is accompanied by the concurrent generation of two CO₂. The OCO-x-OCO intermediate’s formation or breakdown could act as the rate-limiting step within the reaction sequence [170], as shown in reaction Equations (9)–(11).

CO + M → CO*

(9)

O₂ + 2CO* → OOC-M-COO

(10)

OOC-M-COO → 2CO₂ + M

(11)

5.3. Mars–Van Krevelen Mechanism

For multiphase catalytic systems with surfaces rich in O_latt atoms, the MvK mechanism exhibits a unique oxidation reaction pathway [171]. The mechanism is characterized by two key stages. Initially, CO molecules oxidize with the active O_latt on the catalyst surface, resulting in the production of CO₂, while surface Ov are formed at the oxygen de-embedding sites. Subsequently, O₂ replenishes Ov via chemisorption, followed by dissociative adsorption that regenerates O_latt while simultaneously oxidizing a second CO molecule [172]. As shown in reaction Equations (12)–(15) (where [] denotes an Ov). Based on the dependence on O_latt, the CO oxidation pathway on catalysts with high OSC is primarily governed by the MvK mechanism.

CO + X → CO*

(12)

CO∗ + O_latt→CO₂* + []

(13)

CO₂-→ CO₂ + X*

(14)

[] + O₂ → O_latt

(15)

Zou et al. [173] employed a defect-engineered Pt/Ti-D catalyst wherein Pt was robustly anchored in atomic form at defect sites, forming a unique Pt-O₅ coordination environment. The pivotal role of this structure lies in transferring the adsorption and activation functions of O₂ from the noble metal Pt sites to the Ti³⁺ defect sites on the support. This not only avoids competitive adsorption with CO but also substantially reduces the activation energy barrier for O₂ to 0.38 eV. Consequently, the mechanism follows the MVK pathway, as illustrated in Figure 10c. Gu et al. [174] decomposed the CO oxidation reaction into two half-reactions: oxygen vacancy formation (Ce⁴⁺ reduction) and conversion (Ce³⁺ oxidation). They then quantified the kinetic rate of oxygen vacancy formation in CeO₂-ZrO₂-based catalysts using time-resolved energy dispersive X-ray absorption spectroscopy. Their findings revealed that in tetragonal-phase CZ solid solution (Ce/Zr = 1:1), the rate-determining step of the MvK mechanism is Vo formation. The addition of noble or transition metals has been shown to enhance the kinetics of Vo formation, reduce the activation energy, and improve the catalytic performance. Xu et al. [175] studied CO oxidation mechanism on α-Mn₂O₃ by systematically varying the partial pressures of CO and O₂. Their results revealed a distinct temperature-dependent kinetic behavior: below 190 °C, the reaction rate showed dual dependence on the concentrations of both CO and O₂. At temperatures above 190 °C, the reaction rate became independent of the O₂ concentration, and the consumption rate of O_latt was controlled primarily by the diffusion of CO. This phenomenon can be attributed to the temperature-induced desorption of adsorbed oxygen, concurrent with an increase in the mobility of O_latt. Consequently, this results in a shift in the reaction path from surface-adsorbed oxygen to bulk O_latt. A similar phenomenon was also identified by Tian et al. [13]. At temperatures ranging from 50 to 90 °C, CO was adsorbed at the Mn³⁺-Ov site to form carbonate intermediates, following the Langmuir–Hinshelwood pathway. When the temperature was >190 °C, it transformed into the MvK pathway. The reaction order of O₂ approaches 0, and O_latt directly participates in oxidation. Fan et al. [164] found that the inhomogeneity in the size of Pt NPs triggers an insufficient concentration of carrier Ov, leading to an inefficient O₂ activation. Consequently, this inefficient O₂ activation suppresses the adsorption of CO on Pt, which in turn retards the MvK mechanism.

Figure 10. (a) Effect of different natures of carriers on CO oxidation mechanism. Reprinted with permission from [161]. Copyright 2019 American Chemical Society. (b) Water causes the CO oxidation mechanism to shift from E-R to L-H. Reprinted with permission from [143]. Copyright 2025 American Chemical Society. (c) The MVK mechanism of CO oxidation. Reprinted with permission from [173]. Copyright 2025 American Chemical Society.

Figure 10. (a) Effect of different natures of carriers on CO oxidation mechanism. Reprinted with permission from [161]. Copyright 2019 American Chemical Society. (b) Water causes the CO oxidation mechanism to shift from E-R to L-H. Reprinted with permission from [143]. Copyright 2025 American Chemical Society. (c) The MVK mechanism of CO oxidation. Reprinted with permission from [173]. Copyright 2025 American Chemical Society.

6. Conclusions and Outlook

In recent years, CO catalytic oxidation research has made remarkable progress in the design and mechanism exploration of precious and non-precious metal catalysts. Precious metals excel in low-temperature reactions by virtue of SMSI and efficient oxygen activation, but their high cost and anti-poisoning issues remain bottlenecks; non-precious metals show substitution potential through multivalent redox cycling and dynamic migration of O_latt. However, the dynamic resolution of the reaction mechanism (e.g., coupling of Langmuir–Hinshelwood (L-H) and MvK mechanisms and how water alters the catalyst reaction path) and the enhancement of anti-poisoning ability (e.g., hydrophobic modification, core–shell isolation) still need to be explored in depth. Therefore, future research should focus on the following directions: developing low-cost single-atom/cluster catalysts and catalysts for non-precious metal composite systems; combining in situ characterization techniques and machine learning simulations to reveal dynamic reaction pathways; designing hydrophobic-sulfur-resistant bifunctional shell layers to physically segregate H₂O/SO₂ and protect the catalyst active sites to enhance durability in complex environments; exploring photothermal synergistic catalysis to reduce energy consumption and enhance mass transfer efficiency; and constructing a highly efficient, stable and environmentally friendly CO catalytic oxidation system to support environmental governance and carbon neutrality goals.