The conversion of CO₂ into valuable chemicals and fuels is crucial for mitigating climate change and achieving sustainable energy. Photocatalysis offers a promising approach to this challenge, leveraging sunlight to drive CO₂ reduction reactions (CO₂RR). Single-atom catalysts (SACs) have emerged as a highly efficient class of photocatalysts due to their maximized atom utilization and unique electronic properties. However, designing SACs with high stability and selectivity for CO₂RR remains a significant challenge. This study focuses on a novel SAC, Ti@C₄N₃, which incorporates a single titanium atom supported on a two-dimensional C₄N₃ substrate. The combination of Ti's catalytic properties and the unique electronic structure of C₄N₃ offers potential for enhanced CO₂RR activity. The primary research question is to explore the structural stability, electronic properties, and photocatalytic performance of Ti@C₄N₃ for CO₂RR, aiming to understand its mechanism and unlock potential for efficient CO₂ conversion. Understanding the detailed mechanism of CO₂ activation and the subsequent reduction pathways is key to designing highly efficient and selective catalysts. This study addresses the importance of addressing both thermodynamic and kinetic limitations in CO₂RR through a multi-faceted computational approach, combining density functional theory (DFT) calculations and ab initio non-adiabatic molecular dynamics (NAMD) simulations.

Extensive research has been devoted to developing efficient photocatalysts for CO₂RR. Various materials, including metal oxides, metal-organic frameworks (MOFs), and SACs, have been explored. Studies have highlighted the importance of factors such as band gap, electronic structure, and active site configuration in determining catalytic activity and selectivity. The literature demonstrates that SACs, due to their unique electronic properties and maximized atom utilization, hold significant promise for efficient CO₂RR. However, many SACs suffer from limitations in stability and selectivity. The incorporation of single metal atoms into two-dimensional (2D) substrates like C₄N₃ has shown potential for enhanced catalytic performance. Previous studies have examined the use of C₄N₃ and its derivatives in photocatalysis, often modified with metal nanoparticles or dopants to tailor their electronic and optical properties. This study extends upon this work by investigating the precise role of a single Ti atom supported on C₄N₃ for CO₂RR, investigating not only C1 product formation but also focusing on the less-explored C-C coupling pathways and C2+ product generation. The importance of understanding not only the thermodynamic feasibility of these reactions but also the associated kinetic barriers is emphasized to enable the design of practical and efficient CO₂RR photocatalysts.

This study employed a combination of density functional theory (DFT) calculations and ab initio non-adiabatic molecular dynamics (NAMD) simulations to investigate the properties and photocatalytic performance of the Ti@C₄N₃ catalyst. DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) with the Perdew-Burke-Ernzerhof (PBE) functional and projector-augmented wave (PAW) pseudopotentials. The calculations covered a range of aspects, including: 1. **Structural Optimization:** To determine the most stable configuration of Ti@C₄N₃. 2. **Electronic Properties:** To investigate band structure, density of states, and optical absorption spectra. 3. **Thermodynamic Stability:** Assessing the chemical and mechanical stability through calculations of adsorption energies, activation barriers for aggregation, and elastic constants. 4. **Kinetic Analysis:** Using the climbing image nudged elastic band (CI-NEB) method to determine diffusion pathways and activation energies for Ti atom migration. Free energy profiles were also calculated to understand the stability of Ti on the surface in aqueous environments. 5. **Reaction Mechanism:** DFT+U calculations (with Ueff = 3.79 eV) were conducted to study the CO₂RR reaction pathways for both single and dual CO₂ molecules. Ab initio NAMD simulations, using the Hefei-NAMD program, were employed to study photocarrier dynamics and the activation mechanism of CO₂. Real-time time-dependent density functional theory (rt-TDDFT) molecular dynamics simulations, utilizing the TDAP program, were used to explore the dual CO₂ activation under light irradiation. The simulations analyzed the time evolution of geometrical factors (bond angles and lengths) and electronic populations to unveil the underlying mechanism of dual activation and C-C coupling. Hirshfeld charge analysis was used to track charge transfer during the reaction process. The methodology involved a multi-step approach, combining static DFT calculations to establish the fundamental properties of the catalyst with dynamic NAMD simulations to investigate the time-dependent processes occurring during photocatalysis. This approach provided a comprehensive understanding of both the static and dynamic aspects of the catalyst and reaction mechanism. The utilization of multiple computational techniques, and the consideration of both thermodynamic and kinetic factors, are key strengths of the methodology employed.

The key findings of this research demonstrate the exceptional properties of the Ti@C₄N₃ catalyst and its efficiency in catalyzing CO₂RR. **Structural and Thermodynamic Stability:** * Ti@C₄N₃ exhibits remarkable thermal, chemical, and mechanical stability. AIMD simulations showed minimal energy fluctuation at 500 K, indicating thermal robustness. Free energy calculations revealed a high activation energy barrier (2.47 eV) for Ti leaching from the active site in aqueous solutions. The calculated elastic constants met the Born-Huang stability criteria, and the Young's modulus (136.25 GPa) was considerably lower than many other 2D materials, suggesting flexibility. **Electronic and Optical Properties:** * The introduction of Ti atoms modifies the electronic structure of C₄N₃, transforming it from a conductor to a semiconductor with a band gap of 0.97 eV. This change in electronic structure gives rise to two distinct absorption peaks (327.77 nm and 529.61 nm) in the visible light region, significantly enhancing the catalyst's ability to absorb sunlight. * The photogenerated electrons have a long lifetime (38.21 ps), providing ample time for migration to the surface and participation in reduction reactions. **CO₂ Activation Mechanism:** * Ti@C₄N₃ exhibits dual CO₂ activation: thermal activation of one CO₂ molecule through back-donation from the high-valence Ti(IV) ion and photo-induced activation of another weakly adsorbed CO₂ molecule from photoelectrons provided via the CBM. * Ab initio NAMD simulations revealed that the light-induced activation of the second CO₂ molecule involves a three-stage process: initial electron excitation, charge transfer from both Ti and C₄N₃ substrates, and deep activation of CO₂ leading to structural bending and bond elongation. Temperature plays a key role in facilitating charge transfer through electron-phonon coupling, enhancing the activation process. **CO₂RR Mechanism and Selectivity:** * DFT+U calculations identified several possible reaction pathways for CO₂RR, including single and dual CO₂ reduction. For single CO₂ reduction, the formation of CH₄ was found to be kinetically favored. However, the dual activation of CO₂ allows for direct C-C coupling to form oxalate with a low energy barrier (0.19 eV), which is followed by subsequent reduction to produce C₂H₆. The overall rate-determining step for C₂H₆ production is the formation of the HOHOCCOHOH intermediate with an energy barrier of 1.09 eV. This is a significant finding, as it deviates from the common belief that CO intermediates are necessary for multi-carbon product formation.

This work provides significant insights into the design and application of highly efficient photocatalysts for CO₂RR. The dual activation mechanism observed in Ti@C₄N₃ offers a new paradigm for improving CO₂RR efficiency. By combining both thermal and photo-induced activation, the catalyst overcomes the limitations often associated with single activation pathways. The observation that C-C coupling can occur directly from activated CO₂ molecules without the intermediate formation of CO is also groundbreaking, offering new strategies for designing catalysts capable of producing multi-carbon products. The superior performance of Ti@C₄N₃ highlights the potential of single-atom catalysts in addressing the challenges of CO₂ reduction. The findings have implications for the broader field of photocatalysis, suggesting that strategically designed SACs with dual activation mechanisms can significantly enhance catalytic efficiency and selectivity. The detailed mechanistic understanding gained in this study lays the foundation for developing advanced photocatalytic systems for CO₂ conversion.

This study demonstrates the remarkable potential of Ti@C₄N₃ as a highly efficient and stable single-atom photocatalyst for CO₂RR. The dual activation mechanism, combining thermal and photo-induced activation, leads to direct C-C coupling and the selective production of C₂H₆. The detailed mechanistic understanding gained through a combined DFT and ab initio NAMD simulation approach provides valuable insights for the design of future CO₂RR photocatalysts. Future research could explore other SACs with similar dual activation mechanisms, investigate the impact of different support materials and the optimization of reaction conditions for further improving the efficiency and selectivity of the reaction.

While this study provides a comprehensive computational investigation of Ti@C₄N₃ for CO₂RR, some limitations exist. The study is purely computational, and experimental validation is necessary to confirm the findings. The accuracy of the computational methods relies on the chosen approximations and parameters. The simulations consider a simplified model, and real-world conditions could introduce complexities not fully captured in the simulations. Furthermore, the focus is primarily on the mechanistic aspects, and a broader evaluation of the catalyst's performance under various operating conditions would enhance the scope of the study.