Theoretical Study of CO Oxidation on Pt Single-Atom Catalyst Decorated C₃N Monolayers with Nitrogen Vacancies

Abstract

Carbon monoxide (CO) is a major toxic gas emitted from vehicle exhaust, industrial processes, and incomplete fuel combustion, posing serious environmental and health risks. Catalytic oxidation of CO into less harmful CO₂ is an effective strategy to reduce these emissions. In this study, we investigated the catalytic performance of platinum (Pt) single atoms doped on C₃N monolayers with various vacancy defects, including single carbon (C_V) and nitrogen (N_V) vacancies, using density functional theory (DFT) calculations. Our results demonstrate that Pt@N_V-C₃N exhibited the most favorable catalytic properties, with the highest O₂ adsorption energy (−3.07 eV). This performance significantly outperforms Pt atoms doped at other vacancies. It can be attributed to the strong binding between Pt and nitrogen vacancies, which contributes to its excellent resistance to Pt aggregation. CO oxidation on Pt@N_V-C₃N proceeds via the Eley–Rideal (ER2) mechanism with a low activation barrier of 0.41 eV for the rate-determining step, indicating high catalytic efficiency at low temperatures. These findings suggest that Pt@N_V-C₃N is a promising candidate for CO oxidation, contributing to developing cost-effective and environmentally sustainable catalysts. The strong binding of Pt atoms to the nitrogen vacancies prevents aggregation, ensuring the stability and durability of the catalyst. The kinetic modeling further revealed that the ER2 mechanism offers the highest reaction rate constants over a wide temperature range (273–700 K). The low activation energy barrier also facilitates CO oxidation at lower temperatures, addressing critical challenges in automotive and industrial pollution control. This study provides valuable theoretical insights for designing advanced single-atom catalysts for environmental remediation applications.

1. Introduction

Carbon monoxide (CO) is one of the most common toxic gases in the atmosphere. It originates from car exhausts, industrial processes, and incomplete fuel or petroleum product burning [1,2,3,4,5,6]. CO is a colorless, odorless, tasteless, flammable component in the air, which, however, is toxic and harmful to the human body [1,7,8,9,10]. When inhaled, CO can irreversibly attach to hemoglobin and hence reduce its capacity to transfer oxygen to body tissues. It may even cause personal death, as carbon monoxide gas replaces oxygen by binding to the iron atom in blood hemoglobin (Hb). Hence, Hb becomes unqualified to carry oxygen to the brain [6,10]. Toxicity by CO is accompanied by some symptoms, including nausea, dizziness, loss of consciousness, and headache, and may lead to death. Therefore, developing effective methods to reduce or remove CO emissions from vehicle and industrial sources is critical and requires urgent attention. One of the most widely used methods for removing CO as a highly toxic gas is its conversion into carbon dioxide (CO₂), which is significantly less harmful. This conversion commonly occurs via a catalytic oxidation reaction using suitable catalysts.

In recent decades, significant attention has been devoted to low-temperature CO oxidation due to its crucial role in mitigating the environmental pollution associated with harmful emissions. Noble metal catalysts such as Pd, Pt, and Au have been extensively employed in catalytic processes due to their excellent catalytic activities, stability, and reliability in practical applications such as vehicle catalytic converters and industrial reactions [11,12,13,14,15,16,17]. Nevertheless, these traditional noble metal catalysts typically require elevated temperatures to achieve optimal catalytic efficiency, posing substantial drawbacks including high operational costs, limited tunability, scarcity, and availability constraints. Consequently, there is an urgent need for alternative catalysts capable of efficiently catalyzing CO oxidation at lower temperatures and lower material costs. Recently, two-dimensional (2D) materials have emerged as promising candidates for catalytic applications owing to their remarkable physicochemical properties, including exceptional surface reactivity, tunable electronic structures, cost-effectiveness, and environmental compatibility. Among these, carbon nitride (C₃N)-based catalysts have attracted considerable research interest for catalytic reactions [1,18,19,20,21,22,23]. However, pristine C₃N monolayers exhibit intrinsic chemical inertness toward the activation and adsorption of small molecules such as O₂, severely limiting their practical catalytic applications. Recent studies have shown that introducing single atomic vacancies, including carbon vacancies (C_V) and nitrogen vacancies (N_V), into C₃N monolayers can significantly enhance their chemical reactivity. These vacancy sites create unsaturated coordination environments that facilitate stronger adsorption and activation of reactant molecules. Moreover, such vacancies serve as effective anchoring sites for single metal atoms, improving their stability and catalytic performance. Due to their unique structural attributes, single-atom catalysts (SACs), featuring isolated metal atoms anchored onto support materials, have shown remarkable promise. SACs maximize metal utilization efficiency, significantly reducing costs, and exhibit enhanced catalytic properties from well-defined active sites, distinct electronic structures, and minimized aggregation effects. Additionally, SACs offer improved selectivity and stability compared to traditional nanoparticle-based catalysts, making them particularly advantageous in catalytic applications.

Previous studies have categorized the CO oxidation reaction into four primary mechanisms based on extensive theoretical and experimental investigations [24,25,26,27,28,29,30,31]: (i) the Eley–Rideal (ER) mechanism, where gaseous CO directly reacts with adsorbed O₂ (O₂^*_(ads) + CO_(g)→CO_2(g) + O*_(ads)), (ii) the Langmuir–Hinshelwood (LH) mechanism, involving the simultaneous adsorption of both CO and O₂ molecules onto the catalyst surface before reaction, (O₂*_(ads) + CO*_(ads)→CO_2(g) + O*_(ads)), (iii) the new Eleye Rideal (NER) mechanism, characterized by two gaseous CO molecules reacting directly with adsorbed O₂ (2CO_(g) + O₂^*_(ads)→2CO_2(g)), and (iv) the termolecular Eley–Rideal (TER) mechanism, in which gaseous O₂ reacts directly with two adsorbed CO molecules (2CO^*_(ads) + O_2(g)→2CO_2(g)). However, this study specifically focuses on the ER and LH mechanisms, as single-atom metal doping typically exhibits pronounced reaction specificity toward these pathways. Furthermore, prior investigations indicate that CO oxidation on C₃N-based surfaces primarily proceeds via these mechanisms, achieving relatively low activation energy barriers. It has also been established that non-metal doping on C₃N surfaces predominantly facilitates CO oxidation through the LH mechanism with notably low activation energies [1,21]. Conversely, doping metal atoms onto C₃N tends to promote the ER mechanism preferentially [18,20], usually characterized by lower activation barriers than the LH pathway, although somewhat higher than those observed in non-metal doping scenarios. Consequently, a promising research avenue involves incorporating isolated single metal atoms to further reduce the energy barriers for CO oxidation reactions.

Motivated by these insights, the present work theoretically investigates the strategic introduction of nitrogen vacancies into the C₃N monolayer, followed by the doping of platinum (Pt) single atoms, to create an innovative catalytic platform. This combined approach aims to exploit synergistic interactions between Pt single atoms and nitrogen-deficient sites on the C₃N surface, significantly enhancing the adsorption and activation of reactant molecules. Ultimately, our objective is to achieve an effective low-temperature catalytic oxidation of CO, demonstrating the considerable potential of these modified C₃N-based single-atom catalyst (SACs) systems for efficient, cost-effective, and sustainable catalytic technologies.

2. Computational Method

All calculations were performed using the plane-wave-based periodic DFT method implemented in the Vienna ab initio simulation package (VASP) [32,33], where the ionic cores are described by the projector augmented wave (PAW) method [34,35,36]. The exchange and correlation energies are computed using the Perdew–Burke–Ernzerhof (PBE) functional [37] and the dispersion correction to account for the van der Waals interaction (PBE-D3) method [38,39]. To have accurate energies, a plane-wave cutoff energy of 400 eV, the Gaussian electron smearing method with σ = 0.10 eV was used. The Brillouin zone integrations were performed using the Monkhorst–Pack grids of 15 × 15 × 15 and 3 × 3 × 1 for the bulk and surface calculations, respectively. All transition state structures were located using the climbing image nudged elastic band (CI-NEB) method [40]. Geometry optimization was performed until the forces acting on each atom were smaller than 0.05 eV/Å with the electronic self-consistent field iteration threshold of 10⁻⁶ eV. A vacuum spacing of 21 Å was applied to prevent spurious interactions between periodic images.

The binding energy (E_b) of a Pt atom doped on the C₃N surface was calculated using the following equation.

$$E_b=E_{Pt@Cv-C3N }-E_{C3N}-E_{Pt}$$

(1)

$$E_b=E_{Pt@Nv-C3N}-E_{C3N}-E_{Pt}$$

(2)

where $E_{Pt@Cv-C3N or Pt@Nv-C3N}$ is the total energy of the C₃N surface with a Pt atom doped at either a carbon vacancy (C_V) or a nitrogen vacancy (N_V) site; $E_{C3N}$ is the total energy of the bare C₃N surface; $E_{Pt}$ is the total energy of the Pt atom. The stability of the Pt-doped C₃N surface was considered by the $E_b$ value. The less-positive E_b value refers to high stability or strong interaction of the promoted Pt on the C₃N surface (Pt@C_V-C₃N or Pt@N_V-C₃N).

The adsorption energies (E_ads) of CO, O₂, and CO+O₂ adsorption on the Pt@C_V-C₃N or Pt@N_V-C₃N surface were calculated using the following equation.

$$E_{ads}=E_{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{e}/\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}}-E_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}}-E_{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{e}}$$

(3)

where $E_{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{e}/\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}}$ is the total energy of the CO, O₂, and CO+O₂ molecule adsorption on a Pt@C_V-C₃N or Pt@N_V-C₃N surface; $E_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}}$ is the total energy of the Pt@C_V-C₃N or Pt@N_V-C₃N surface; and $E_{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{e}}$ is the total energy of the CO, O₂, and CO+O₂ molecule. The negative E_ads value represents the strong interaction between the CO, O₂, and CO+O₂ molecules and the Pt@C_V-C₃N or Pt@N_V-C₃N surface.

The reaction rate constants ($k$) of the reaction mechanisms on the Pt@C₃N surface were estimated using the standard Arrhenius equation [41,42,43,44,45,46].

$$k={A\mathrm{e}}^{(-{\displaystyle \frac{Ea}{RT}})}$$

(4)

where $k$ is the reaction rate constant, $\mathrm{A}$ is the pre-exponential factor, $Ea$ is the activation energy required for the reaction in J/mol, $\mathrm{R}$ is the gas constant (8.314 J/K⁻¹mol⁻¹), and $T$ is the absolute temperature in K.

3. Results

3.1. Structural Model of Pristine C₃N

Figure 1a presents the optimized structure of a pristine C₃N monolayer, which exhibits a hexagonal lattice and belongs to the P6/mmm space group [47,48,49]. In our study, the pristine C₃N was modeled using the 5 × 5 pristine C₃N supercell to capture the structural and electronic characteristics of the material accurately. Within this monolayer, two distinct types of hexagonal rings are observed. As shown in Figure 1b, (1) the C-N rings are composed of four carbon and two nitrogen atoms, and (2) the C-C rings comprise six carbon atoms. Both the C-N and C-C bonds exhibit a bond length of approximately 1.41 Å, which aligns well with previously reported values in the literature [19,50,51,52].

Despite the structural stability and periodicity of pristine C₃N, its chemical inertness toward small gas molecules, particularly O₂, limits its potential for catalytic applications such as CO oxidation. To address this shortcoming, we further investigate the effect of introducing structural defects, specifically nitrogen vacancies (N_V), into the C₃N monolayer. Defect engineering has proven to be an effective strategy for tuning the electronic properties and enhancing the surface reactivity of 2D materials. In the case of C₃N, N_V can create localized electronic states that serve as active sites, improving the adsorption and activation of gas molecules. By focusing on the defective structure of C₃N, we aim to explore how N_V can synergistically interact with doped Pt single atoms to enhance catalytic activity further. This defect-engineered approach not only increases the number of active sites but also modifies the local electronic environment to favor the stronger binding of reactants and lower energy barriers for the CO oxidation reaction. Therefore, investigating nitrogen-deficient C₃N is crucial to designing high-performance, single-atom catalyst systems for low-temperature and energy-efficient CO oxidation.

3.2. The Defective Structure of C₃N

Decorating C₃N monolayers with single-atom catalysts (SACs) has emerged as a promising strategy to overcome the intrinsic chemical inertness of pristine C₃N toward gas molecules. In particular, introducing atomic-scale defects, such as vacancies, can significantly enhance the interaction between the surface and adsorbates by providing reactive sites and modulating the local electronic environment. In this study, we focus on exploring the doping of a Pt atom into the defective C₃N monolayer to identify the most thermodynamically stable configurations and to assess their potential for improved gas adsorption and catalytic performance in CO oxidation. We specifically consider single vacancies, which include two types: (i) a carbon vacancy (C_V), created by removing a single carbon atom from the C₃N monolayer (Figure 2a), and (ii) a nitrogen vacancy (N_V), formed by removing a single nitrogen atom (Figure 2b). These vacancy sites serve as anchoring points for Pt atoms, forming Pt-doped C₃N systems (Pt@C₃N). The introduction of such defects not only alters the electronic structure of the C₃N surface but also improves the binding affinity of the Pt atom, thus enhancing its catalytic potential. To evaluate the stability and interaction strength between the Pt atom and the defective C₃N monolayer, we calculated the binding energy of the Pt atom at each vacancy site, as defined in Equation (2). A lower binding energy indicates a stronger interaction and higher structural stability. The binding energies for Pt atoms anchored at the C_V and N_V sites are presented in the upper panel of Figure 3. Based on these results, the most stable Pt@C₃N configuration is identified and further analyzed in the context of CO oxidation mechanisms in the subsequent sections. Furthermore, to evaluate the feasibility of defect formation in C₃N, we calculated the vacancy formation energies $E_{form}$ for both carbon (C_V) and nitrogen (N_V) vacancies using the following equation:

$$E_{form}=E_{{\mathrm{C}}_3\mathrm{N}-\mathrm{vacancy}}-E_{{\mathrm{C}}_3\mathrm{N}}+{\mu }_{\mathrm{N} \mathrm{o}\mathrm{r} \mathrm{C}}.$$

(5)

where $E_{{\mathrm{C}}_3\mathrm{N}-\mathrm{v}\mathrm{a}\mathrm{c}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{y}}$ is the total energy of the C₃N monolayer with one vacancy (either C_V or N_V); $E_{{\mathrm{C}}_3\mathrm{N}}$ is the total energy of the pristine C₃N monolayer; and ${\mu }_{\mathrm{N} \mathrm{o}\mathrm{r} \mathrm{C}}$ is the chemical potential of the removed atom (C or N). The results show that the formation energy of C_V is 5.03 eV, whereas that of N_V is 6.21 eV. These positive values indicate that both vacancies require energy input to form, consistent with the chemically stable nature of the C₃N surface. However, both vacancies have sufficiently low formation energies to be accessible under realistic experimental conditions, such as thermal annealing. Although nitrogen vacancies are less likely to form spontaneously due to their higher formation energy, subsequent Pt doping stabilizes both vacancy types effectively. As demonstrated in the following section, Pt atoms bind strongly to both carbon and nitrogen vacancies with comparable binding energies, making both defect types viable anchoring sites for single-atom Pt catalysts on the C₃N monolayer.

We then investigated the Pt-doped C₃N (Pt@C₃N) monolayer for its potential in catalyzing the CO oxidation reaction. The optimized structures and electronic properties of Pt atoms anchored at single-vacancy sites, specifically carbon (C_V) and nitrogen (N_V) vacancies, are shown in Figure 3. The binding energies (E_b) for Pt@C_V-C₃N and Pt@N_V-C₃N configurations are significantly negative, −5.77 and −5.79 eV, respectively, indicating strong interactions between the Pt atom and the defective C₃N surface. This strong binding suggests that Pt atoms are thermodynamically stable at these vacancies and are unlikely to migrate or aggregate into clusters, a common issue in nanoparticle-based catalysts. To further assess the electronic characteristics of these systems, we performed a Bader charge analysis to quantify the charge transfer between the Pt atom and the C₃N monolayer. The Bader charge analysis (Q_Pt) indicates that the Pt atom exhibits electron donation to the surrounding lattice, with Q_Pt values of 0.31 e⁻ for Pt@C_V-C₃N and 0.21 e⁻ for Pt@N_V-C₃N, respectively. This charge transfer confirms that the Pt atom becomes partially oxidized upon incorporation into the defect site, which can play a crucial role in enhancing its catalytic activity by modifying its electronic structure and increasing its interaction with reactant molecules. Given their strong binding and favorable charge characteristics, the Pt@C_V-C₃N and Pt@N_V-C₃N configurations are selected for further analysis in the subsequent sections to evaluate their adsorption behavior and catalytic performance in CO oxidation. To evaluate the influence of Pt decoration and carbon or nitrogen vacancies on the electronic structure of C₃N monolayers, the partial density of states (PDOS) was calculated and compared among the pristine C₃N, and Pt-decorated C₃N with carbon or nitrogen vacancies [53,54], as shown in Figure S6. As observed in the pristine C₃N monolayer (Figure S6a), a clear band gap exists near the Fermi level, indicating its semiconducting nature. Upon decoration with a Pt single atom (Figure S6b,c), new states emerge near the Fermi level, which are mainly contributed by the Pt d-orbitals. These states significantly enhance the electronic conductivity of the system. Furthermore, the hybridization between the Pt d-orbital and the surrounding N p-orbital or C p-orbital orbitals suggests a strong interaction between the Pt atom and the defective C₃N surface, which facilitates the charge transfer processes that are essential for CO oxidation. These electronic modifications are expected to lower the reaction barriers and enhance the overall catalytic performance.

3.3. The O₂ and CO Adsorption on Pt-Doped C₃N Monolayers with Single Vacancies

In this section, we use DFT calculations to investigate the adsorption behavior of CO and O₂ molecules on Pt-doped C₃N monolayers containing single vacancies, namely, carbon vacancies (C_V) and nitrogen vacancies (N_V). The adsorption energy (E_ads) was used as the key parameter to assess the interaction strength between gas molecules and the modified C₃N surface [55,56] and to help identify possible CO oxidation mechanisms. Only the most stable adsorption configurations, characterized by the most significant E_ads values, are discussed herein, though multiple initial configurations were evaluated. For CO adsorption on the Pt@C₃N monolayers with C_V and N_V vacancies, the carbon atom of the CO molecule binds directly to the Pt atom located at the vacancy site (Figure 4a). The E_ads were calculated to be −2.52 and −2.24 eV for Pt@C_V-C₃N and Pt@N_V-C₃N. These interactions produce strong chemisorption, indicating a robust binding between CO and the Pt site. This strong interaction suggests a favorable environment for the initial step in the CO oxidation process.

For O₂ adsorption on the same Pt-doped single-vacancy systems, the O₂ molecule forms a bidentate configuration with the Pt atom, where both oxygen atoms participate in bonding. These interactions also exhibit strong chemisorption, with E_ads of −2.64 and −3.07 eV for Pt@C_V-C₃N and Pt@N_V-C₃N, respectively (Figure 4b). The stronger adsorption of O₂ compared to CO suggests that the Pt@C_V-C₃N and Pt@N_V-C₃N systems satisfy the energetic requirement for the Eley–Rideal (ER) mechanism, where pre-adsorbed O₂ reacts with incoming CO molecules. To compare O₂ and CO adsorption on various vacancy sites of the C₃N monolayer, the adsorption energy was used to identify the most favorable configurations and to determine the possible oxidation mechanism of CO on the C₃N monolayer. The criterion for CO oxidation via the ER mechanism is that the adsorption energy of the O₂ molecule must be stronger than that of the CO molecule. Our study found that O₂ molecules can significantly better adsorb onto Pt-doped single vacancies on the C₃N monolayer (Pt@C_V-C₃N and Pt@N_V-C₃N) than CO molecules on other surfaces. Therefore, this research focuses exclusively on Pt@C_V-C₃N and Pt@N_V-C₃N. Furthermore, co-adsorption of CO and O₂ was observed to be thermodynamically favorable on both Pt@C_V-C₃N and Pt@N_V-C₃N surfaces, indicating the potential for the Langmuir–Hinshelwood (LH) mechanism, where both reactants are adsorbed before reaction (Figure S1). Among all configurations studied, Pt@C_V-C₃N and Pt@N_V-C₃N show the most promising properties for CO oxidation. This is attributed to their significantly stronger O₂ adsorption compared to CO, co-adsorption feasibility, and the favorable energetics supporting both ER and LH pathways. These features underscore the catalytic potential of Pt-doped single-vacancy C₃N monolayers as efficient, low-temperature CO oxidation catalysts. Therefore, Pt@C_V-C₃N and Pt@N_V-C₃N systems have been selected for further in-depth mechanistic investigation, given their superior reactivity and thermodynamic favorability for CO oxidation.

Moreover, prior to investigating the charge density differences, the co-adsorption of CO and O₂ on the Pt@C_V-C₃N and Pt@N_V-C₃N surfaces was also studied to support the possibility of a Langmuir–Hinshelwood (LH) mechanism. The results indicate that CO+O₂ can be chemisorbed on both surfaces, with adsorption energies of −3.44 eV for Pt@C_V-C₃N and −4.14 eV for Pt@N_V-C₃N, respectively. These findings imply that the simultaneous adsorption of both reactants on the Pt@C_V-C₃N and Pt@N_V-C₃N surfaces is possible, offering initial support for the occurrence of the Langmuir–Hinshelwood (LH) mechanism. Moreover, the charge density differences of the adsorption of CO, O₂, and the co-adsorbed CO+O₂ species on Pt@C_V-C₃N and Pt@N_V-C₃N surfaces are illustrated in Figure 5 and Figure 6, where the yellow and blue regions represent charge accumulation and depletion, respectively. These visualizations reveal pronounced charge redistribution in the CO+O₂ co-adsorbed systems, indicating strong electronic interactions between the adsorbates and the Pt-doped C₃N surfaces. This observation is further supported by the calculated net charge transfer values ∆Q_CO+O2, which show substantial electron donation from the surfaces to the co-adsorbed species: −0.97 e⁻ for Pt@C_V-C₃N and −0.83 e⁻ for Pt@N_V-C₃N. In comparison, the individual adsorption of O₂ leads to moderately lower charge transfer values of −0.18 e⁻ and −0.72 e⁻, respectively, for the same surfaces. CO adsorption shows the weakest charge transfer, with values of −0.17 e⁻ for Pt@C_V-C₃N and −0.25 e⁻ for Pt@N_V-C₃N, indicating relatively weaker interaction. These results suggest a preference for O₂ adsorption on the Pt-doped C₃N surfaces, which may then undergo a reaction with incoming CO molecules, a behavior characteristic of the Eley–Rideal (ER) mechanism. The stronger interaction and greater electron transfer for O₂ compared to CO further support the conclusion that CO oxidation is more likely to proceed effectively via the ER pathway on both Pt@C_V-C₃N and Pt@N_V-C₃N surfaces.

3.4. Reaction Mechanism of CO Oxidation on Pt@N_V-C₃N

Based on previous theoretical and experimental studies on CO oxidation on C₃N-based catalysts, it primarily proceeds via two dominant mechanisms: the Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) pathways. In this work, we evaluated the feasibility of these mechanisms on the Pt@N_V-C₃N surface by analyzing the adsorption energies of CO, O₂, and co-adsorbed CO+O₂ species, identifying the most energetically favorable pathways. For the ER mechanism, the reaction begins with the pre-adsorption of an O₂ molecule on the Pt active site. The absorbed O₂ then reacts directly with a gas-phase CO molecule, resulting in either a transient carbonate-like intermediate (CO₃) or the immediate formation of CO₂ and a surface-bound oxygen atom (O*). In contrast, the LH mechanism involves the co-adsorption of both CO and O₂ molecules at the Pt site on the N_V defective C₃N surface. This leads to the formation of a peroxo-type intermediate (OO-CO), which subsequently decomposes to produce a CO₂ molecule and an activated oxygen atom (O*) remaining on the surface. To better understand the full catalytic process, we examined the detailed stepwise pathways for both mechanisms, including the structural configurations of each intermediate and transition state. The energetic profiles and geometric changes along these reaction pathways are discussed in the following sections to elucidate the role of Pt doping and nitrogen vacancies in facilitating low-energy CO oxidation.

3.4.1. Eley–Rideal Mechanism (ER) on Pt@N_V-C₃N

The Eley–Rideal (ER) mechanism involves a reaction between a gas-phase CO molecule and a pre-adsorbed O₂ molecule on the surface of the catalyst. On the Pt@N_V-C₃N system, various physisorption configurations of the CO molecule, floating above or approaching different orientations of the adsorbed O₂, can significantly influence the reaction pathway and energetics. This study examines two distinct ER reaction pathways: (i) ER1 Mechanism: The CO molecule approaches and reacts with the center of the adsorbed O₂ molecule, forming a transient carbonate-like intermediate (CO₃). This intermediate subsequently decomposes to yield carbon dioxide (CO₂) and a surface-bound activated oxygen atom (O*). The overall reaction can be summarized as: ER1: O₂ + CO → CO₃ → CO₂ + O*.ER2: The CO molecule attacks the side of the adsorbed O₂ molecule, bypassing the CO₃ intermediate and leading directly to the formation of CO₂ and a remaining O* on the surface: ER2: O₂ + CO → CO₂ + O*. The specific configurations and structural evolution involved in both ER1 and ER2 mechanisms will be illustrated in Figure 7. Corresponding energy profiles detailing the reaction steps and associated energy barriers are presented in Figure 8 and Figure 9, providing insight into the relative favorability and activation energies of each pathway.

ER1 mechanism: Figure 8 presents the energy profile for the CO oxidation on Pt@N_V-C₃N via the ER1 mechanism. In this pathway, O₂ and CO molecules do not react in a single elementary step. Instead, the reaction proceeds through a carbonate-like intermediate (CO₃). The process begins with the physisorption of a CO molecule above pre-adsorbed O₂ species on the Pt@Nv-C₃N surface (IS-1A), with an initial CO-O₂ distance of 3.14 Å, indicating weak initial interaction. The O₂ molecule interacts with the surface metal atom through both of its oxygen atoms, exhibiting O–metal bond lengths of 2.06 Å and 1.95 Å, respectively, while the O-O bond remains intact. The reaction progresses as the CO approaches and inserts itself between the O atoms of O₂, leading to O-O bond cleavage and the formation of the CO₃ intermediate (IM-1A). This step requires an activation energy of 0.94 eV (TS-1A). A vibrational frequency of 428.58i cm⁻¹ is associated with this process, and the direction of this vibrational mode is consistent with the transition state structure and aligns well with the motion along the reaction coordinate. Subsequently, the CO₃ structure decomposes into a CO₂ molecule and an activated oxygen atom (O*) adsorbed on the Pt@N_V-C₃N surface (FS-1A). However, this decomposition step is kinetically hindered, as it involves breaking a strong intermolecular bond with a stable CO₃ structure. This is reflected in the higher activation energy barrier of 1.32 eV (TS-2A), identifying this step as the rate-limiting step of the ER1 pathway. The calculated imaginary frequency of 280.74i cm⁻¹ corresponds to the transition state and reaction coordinate, indicating that the CO₃ bond cleavage is kinetically unfavorable due to the structural stability of the CO₃ intermediate. This result is further supported by density of states (DOS) analysis (Figure S2), which reveals significant electronic interaction during CO₃ formation and its subsequent breakdown. The strong binding within the intermediate is consistent with the observed energy profile, confirming the critical role of CO₃ stability in governing the kinetics of this pathway.

ER2 mechanism: In the CO oxidation on Pt@N_V-C₃N via the ER2 mechanism (Figure 9), the reaction starts with the physisorption of CO molecules onto pre-adsorbed O₂ on the Pt@N_V-C₃N monolayers and is defined as the initial state (IS-1B) of the ER2 mechanism, where the distance between the O atoms in O₂ and the CO molecule in IS-1B is 2.81 Å. Then, the CO molecules react with the O atom of the O₂ molecule to form CO₂, which desorbs from the surface, leaving an activated O atom on the surface. This contrasts with the ER1 mechanism, where the CO molecule interacts with the terminal site of the O₂ molecule. In the ER2 mechanism, the CO molecule will react on the side of the O₂ molecule, allowing for a concerted process in which CO₂ formation and O-O bond cleavage co-occurs. As a result, the reaction along this pathway is more favorable, consistent with the low energy barrier values of 0.41 eV (TS-1B). This represents a critical point of reaction where bond cleavage and formation occur concurrently. The observed variations in bond lengths are consistent with the structure of the transition state (TS-1B). Specifically, the Pt-O bond length ranges from 1.99 to 2.02 Å, while the O-O bond length is 1.68 Å, suggesting bond elongation and weakening. In addition, the direction and magnitude of the vibrational mode align with the TS structure, as evidenced by a calculated imaginary frequency of 531.28i cm⁻¹. This is consistent with the DOS analysis, which presents a weak interaction between the CO molecule and the side-on adsorbed O₂ on the surface, followed by the decomposition into CO₂ and atomic O, as illustrated in Figure S3.

3.4.2. Langmuir–Hinshelwood Mechanism (LH) on Pt@N_V-C₃N

The Langmuir–Hinshelwood (LH) mechanism for CO oxidation on the Pt@N_V-C₃N surface begins with the co-adsorption of O₂ and CO molecules at the Pt active site. After co-adsorption, the O₂ molecule interacts with a neighboring CO molecule to form a peroxo-type intermediate species (OC-OO). This intermediate represents a crucial transition state in which the O-O bond is weakened, facilitating the subsequent cleavage. Following the formation of the OC-OO intermediate, the reaction proceeds via decomposition of the peroxo-complex, resulting in the formation of one CO₂ molecule and a remaining adsorbed oxygen atom (O*), which remains available for a further reaction with an additional CO molecule in the next catalytic cycle. We will illustrate the specific characteristics of the LH reaction mechanisms in Figure 10, and we will present the energy profiles for CO oxidation via the LH mechanisms in Figure 11.

In the CO oxidation process on Pt@N_V-C₃N via the LH mechanism (Figure 11), the reaction begins with CO and O₂ co-adsorbed on the Pt site of the Pt@N_V-C₃N monolayer, referred to as the initial state (IS-1C) of the LH mechanism. The distance between the O atom in the O₂ molecule and the Pt atom on the surface in IS-1 is calculated to be 2.09 Å, while the distance between the CO molecule and the Pt atom on the surface in IS-1C is 1.95 Å. Then, one O atom from the O₂ molecule approaches the CO molecule, forming a peroxo-type OC-OO intermediate (IM-1C). The OC-OO intermediate forms over the Pt atom on the surface, passing through the TS-1C transition state with a high activation energy of 1.46 eV. The direction and magnitude of the vibrational mode are consistent with the structure of this transition state, as indicated by the calculated imaginary frequency of 293.15i cm⁻¹. Afterward, the O-O bond in the O₂ molecule decomposes into a CO₂ product, and an O atom is adsorbed on the surface (FS-1C). The cleavage of the O-O bond leading to CO₂ formation is kinetically hindered, necessitating a high activation energy barrier at this stage. The activation energy required for this reaction step is 1.61 eV (TS-2C), making it unlikely to occur at room temperature. The direction and vibrational mode are consistent with the structure of this transition state, as confirmed by a frequency of 311.21i cm⁻¹ According to the density of states (DOS) analysis, the strong interaction involved in the formation of the peroxo-type OC-OO intermediate inhibits its decomposition into CO₂ and O*, indicating that this step is energetically unfavorable, as shown in Figure S4.

3.4.3. Eley–Rideal Mechanism (ER2) on Pt@C_V-C₃N

Previous studies have shown that the rate-determining step (RDS) for single-atom metal-doped catalysts typically involves an activation energy barrier below 0.80 eV [57,58], indicating that the formation of CO₂ can readily occur at room temperature. This observation provides a valuable benchmark for evaluating the catalytic activity of Pt-doped C₃N systems investigated in this study. Therefore, we selected only the ER2 mechanism for further study of CO oxidation on Pt@C_V-C₃N.

ER2 mechanism: In the CO oxidation process on Pt@C_V-C₃N via the ER2 mechanism, the corresponding energy profile and reaction pathway are illustrated in Figure 12. In this pathway, a CO molecule in the gas phase reacts directly with a pre-adsorbed O₂ species on the Pt@C_V-C₃N surface. The reaction proceeds through the formation of a transient CO-O₂ complex, followed by O-O bond cleavage and the subsequent formation of a CO₂ molecule and an adsorbed atomic oxygen species (O*). The energy barrier for this step is calculated to be 0.91 eV (TS-1D), which is significantly higher than that of the ER2 mechanism on Pt@N_V-C₃N (0.41 eV), suggesting that the C_V site is less kinetically favorable for CO oxidation. Furthermore, the direction and vibrational mode associated with the imaginary frequency of 697.23i cm⁻¹ are consistent with the transition state structure, confirming its validity. This observation aligns with the DOS analysis, which reveals a weak interaction between the CO molecule and side-on adsorbed O₂ on the surface, leading to the formation of CO₂ and atomic O*, as illustrated in Figure S5. In comparison, the ER2 mechanism on the Pt@N_V-C₃N surface exhibits an even weaker interaction, suggesting that the presence of vacancy sites significantly influences the reaction process.

4. Discussion

4.1. Comparison of the Reaction Mechanism of CO Oxidation on Pt@N_V-C₃N

To elucidate the reaction mechanism of CO oxidation on Pt-based single-atom catalysts, we systematically investigated four distinct reaction pathways using DFT calculations: ER1-Pt@N_V-C₃N, ER2-Pt@N_V-C₃N, LH-Pt@N_V-C₃N, and ER2-Pt@C_V-C₃N. These pathways were analyzed based on their energy profiles, focusing on the activation energy barriers (E_a) and the thermodynamic stability of the products, as illustrated in Figure 13. Among the mechanisms explored, the ER2 pathway on Pt@C₃N, where a CO molecule reacts with the side of a pre-adsorbed O₂ molecule, was found to be the most favorable. This reaction forms a CO₂ molecule, desorbing from the surface, leaving behind an adsorbed O atom (O*). This step exhibited the lowest activation barrier, making it the kinetically preferred route for CO oxidation on the Pt@C₃N system. In comparison, the ER1 mechanism, which involves the formation of a carbonate-like intermediate (CO₃), was found to be less favorable. Although the CO₃ intermediate is thermodynamically stable, its decomposition requires relatively high activation energy ($E_a$ = 1.32 eV) due to the strength of its internal bonds. This is consistent with charge density difference analyses, which show a high degree of localized electron density within the CO₃ species, indicating strong electronic stabilization and significant charge redistribution relative to the transition state (TS) in the ER2 mechanism. Furthermore, our analysis highlights that the type of vacancy (C_V vs. N_V) on the C₃N surface plays a critical role in determining the reaction pathway. The ER2 mechanism on Pt@N-C₃N emerged as the most favorable route, both kinetically and thermodynamically, among all studied cases. The detailed variations in bond lengths and bond angles for all key structures involved in the CO oxidation process, including reactants, intermediates, transition states, and products, are summarized in Table S1 in the Supporting Information. These findings suggest that engineering nitrogen vacancies on C₃N supports can significantly enhance the catalytic activity of Pt single-atom systems for CO oxidation. Thus, tailoring vacancy types offers a promising strategy for the rational design of efficient, low-temperature oxidation catalysts based on 2D materials.

To evaluate the catalytic performance of the designed Pt@N_V-C₃N system for CO oxidation, a comparative analysis was conducted against previously reported transition metal-doped C₃N systems, as summarized in

Table 1. The highest energy barrier of CO oxidation in the best mechanism by various catalysts.

Surface	Position	Type of CO Oxidation Reaction	Calculation
Si-C3N [21]	C vacancy	LH ($E_a$ = 0.38 eV)	Dmol3
B-C3N [1]	C vacancy	LH ($E_a$ = 0.32 eV)	Dmol3
Gr-BCN2 [2]	C vacancy	LH ($E_a$ = 0.46 eV)	Dmol3
Gr-BN3 [2]	C vacancy	LH ($E_a$ = 0.01 eV)	Dmol3
CuN3-Gr [3]	C vacancy	ER ($E_a$ = 3.20 eV)	Dmol3
Mn-Gr [59]	C vacancy	ER ($E_a$ = 0.83 eV)	Dmol3
Pd-C3N [20]	N vacancy	ER ($E_a$ = 0.64 eV)	Dmol3
Mn-C3N [60]	N vacancy	ER ($E_a$ = 0.57 eV)	Dmol3
Fe-C3N [18]	N vacancy	ER ($E_a$ = 0.47 eV)	Dmol3
Pt-C3N*	C vacancy	ER ($E_a$ = 0.91 eV)	VASP
Pt-C3N*	N vacancy	ER ($E_a$ = 0.41 eV)	VASP

* This work.

. According to previous research, Fe-C₃N with nitrogen vacancies exhibited the lowest activation energy ($E_a$) of 0.47 eV among the reported transition metal-doped systems. These studies also revealed that, in metal-doped C₃N surfaces, the CO oxidation reaction generally proceeds via the Eley–Rideal (ER) mechanism. In contrast, our analysis shows that Pt-C₃N at nitrogen vacancy sites demonstrates a superior catalytic performance, with an $E_a$ of only 0.41 eV, which is lower than that of Fe-C₃N and other reported systems. Furthermore, the corresponding adsorption energies for CO and O₂ are −2.24 eV and −3.07 eV, respectively, indicating significantly enhanced adsorption strength, especially for O₂ adsorption. This strong adsorption of O₂ indicates efficient activation and charge transfer, which are essential steps in the CO oxidation process. Furthermore, to confirm that the CO oxidation reaction proceeds more favorably on nitrogen vacancies than on carbon vacancies on the metal-doped C₃N surface, a comparative study was conducted using Pt doped at the carbon vacancy site of the C₃N surface (Pt@C_V-C₃N). The results showed that Pt@C_V-C₃N exhibits an increased activation energy ($E_a$) of 0.91 eV and significantly stronger adsorption energies (−2.52 eV for CO and −2.64 eV for O₂). This may be attributed to overly stable intermediate species forming, which can block the reaction progress and lead to a high energy barrier. In contrast, the Pt@N_V-C₃N surface offers a more promising balance between activation energy and adsorption strength, making it a highly efficient candidate for low-temperature CO oxidation applications. These results highlight the superior catalytic performance of Pt@N_V-C₃N compared to previously reported TM-doped C₃N systems, especially in terms of the thermodynamics and kinetics that are important for the catalytic activity.

4.2. Kinetic Studies and Microkinetic Modeling

To comprehensively evaluate the temperature dependence of CO oxidation on Pt@C₃N systems, we performed kinetic studies based on the Arrhenius equation for the four most relevant pathways: ER1, ER2, LH mechanisms on Pt@N_V-C₃N, and the ER2 mechanism on Pt@C_V-C₃N. The results are presented in Figure 14a, which shows the natural logarithm of the rate constant ($ln k$) versus the inverse temperature (1000/$T$). Figure 14b depicts the corresponding reaction rate trends across the temperature range of 273–700 K. As shown in Figure 14a, the slope of each line reflects the activation energy ($E_a$) of the respective mechanism. The ER2 mechanism on the Pt@N_V-C₃N surface exhibits the shallowest slope, consistent with its lowest activation barrier (0.41 eV), indicating that this pathway is kinetically the most accessible. In contrast, the LH and ER1 mechanisms display steeper slopes due to their higher activation energies (1.46 eV and 1.32 eV, respectively), which correspond to slower reaction rates, particularly at lower temperatures. Figure 14b further reinforces these findings through microkinetic modeling. The ER2 pathway on Pt@N_V-C₃N demonstrates a higher reaction rate across a broader temperature window, with the peak activity occurring between 500 and 600 K. This peak suggests an optimal operation temperature range for catalytic performance under realistic conditions [61,62]. The LH and ER1 pathways on the same surface show much narrower and lower rate profiles, confirming that their high activation barriers make them less effective at moderate temperatures. Additionally, the ER2 mechanism on Pt@C_V-C₃N shows an activity peak at a higher temperature (~670 K). This further supports the conclusion that N_V creates a more reactive environment for low-temperature CO oxidation than C_V. These findings indicate that ER2 on Pt@N_V-C₃N is the most kinetically favorable pathway across the full temperature range. Pt@C_V-C₃N requires a significantly higher temperature to achieve comparable catalytic activity. ER1 and LH mechanisms are kinetically limited due to their higher energy barriers and reduced charge transfer during key transition steps. Thus, combining Arrhenius analysis and microkinetic modeling clearly demonstrates that the ER2 mechanism on Pt@N_V-C₃N offers superior catalytic performance for CO oxidation under both ambient and elevated temperatures. This insight emphasizes the importance of vacancy engineering, specifically nitrogen vacancies, in tuning the activity of single-atom catalysts and guiding the design of next-generation, low-temperature catalytic materials.

5. Conclusions

In this study, the catalytic performance of single platinum (Pt)-doped C₃N monolayers for CO oxidation was thoroughly investigated using density functional theory (DFT) calculations. Among all Pt-doped vacancy configurations in C₃N monolayers, Pt doped at a nitrogen vacancy (Pt@N_V-C₃N) exhibited the most favorable adsorption properties for both CO and O₂ molecules, making it the most promising catalyst for CO oxidation. Furthermore, mechanistic studies of CO oxidation via the Eley–Rideal (ER1 and ER2) and Langmuir–Hinshelwood (LH) pathways on the Pt@N_V-C₃N surface revealed that the ER2 pathway is more efficient. In particular, the ER2 mechanism exhibits low energy barriers (rate-limiting step of 0.41 eV) and high reaction rate constants at temperatures between 273 K and 700 K, indicating that CO oxidation via the ER2 route is the most suitable reaction mechanism for CO oxidation on the Pt@N_V-C₃N surface. Furthermore, microkinetic modeling confirmed that the ER2 mechanism dominates the CO oxidation process at optimal temperatures of 500–600 K. By comparison, we found that Pt@Nv-C₃N is predicted to be a potential material for CO oxidation among these materials. Therefore, we hope that Pt@N_V-C₃N might be a highly effective material for the CO oxidation reaction.