The escalating global CO₂ emissions necessitate innovative strategies for CO₂ utilization. CO₂ can be a valuable carbon source for synthesizing chemicals and fuels. Carbonyl (*CO) is a crucial intermediate in many CO₂-based synthesis processes. However, activating CO₂, particularly cleaving its stable C=O double bond, presents a significant challenge. Conventional CO₂ activation methods typically involve H₂-assisted activation, a complex pathway with redundant catalyst requirements. Direct CO₂ dissociation to *CO is a more desirable yet challenging alternative. Oxygen vacancies in reducible oxides can modulate the electronic structure, facilitating CO₂ activation. However, conventional reducible oxides often have insufficient electron-donating capacity to effectively cleave the C=O bond. Molybdenum nitride and carbide catalysts show high activity in C1 chemistry, but their activity is typically attributed to the bulk structure, overlooking the role of surface MoOₓ layers. This study aims to leverage the unique properties of a MoO₃/Mo₂N heterostructure, specifically its low oxygen vacancy formation energy, to create a highly disordered surface with strong electron-donating capacity for direct CO₂ dissociation.

The literature highlights the importance of CO2 utilization for mitigating climate change and producing value-added chemicals. Previous studies have explored H2-assisted CO2 activation using various metal catalysts, but these methods often involve complex reaction pathways. The use of oxygen vacancies in reducible metal oxides has been investigated as a means to improve CO2 activation. However, challenges remain in achieving efficient C=O bond cleavage. Molybdenum-based catalysts, including nitrides and carbides, have shown promise in C1 chemistry; however, the role of surface oxides, such as MoOx, has not been fully investigated. This research builds upon prior work demonstrating the role of oxygen vacancies in Pt/Mo2N catalysts but seeks to develop a catalyst system that achieves direct CO2 dissociation without the need for noble metals.

The MoO₃/Mo₂N heterostructure was synthesized via a two-step process. First, MoO₃ was prepared by a hydrothermal method using a triblock copolymer (P123) as a template. Second, MoO₃ was nitrided under NH₃ at 650 °C, followed by passivation with 1% O₂/Ar. Various characterization techniques were employed to investigate the catalyst's structure and properties. X-ray diffraction (XRD) confirmed the bulk γ-Mo₂N structure. Raman spectroscopy revealed the presence of MoO₃ on the surface, which dynamically transformed to MoOₓ (x < 3) under reducing conditions. In situ Raman spectroscopy and H₂-TPR (Temperature Programmed Reduction by Hydrogen) studies explored the redox properties of the surface MoOₓ. Four-step surface reaction experiments (Ar → CO₂ → Ar → H₂) were conducted to investigate CO₂ adsorption and activation. Temperature-programmed desorption (TPD) experiments were carried out to study CO and CO₂ desorption. The catalytic performance was evaluated in a fixed-bed reactor under various reaction conditions. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used for surface and bulk structure analysis. Density functional theory (DFT) calculations were performed to elucidate the CO₂ activation mechanism. The catalytic performance was compared with that of other catalysts (e.g., CeO₂, Pt/CeO₂, etc.).

The MoOₓ/Mo₂N catalyst demonstrated exceptional activity and stability for CO₂ hydrogenation to CO under harsh reaction conditions (600 °C, GHSV = 200,000 mL/gcat/h). A CO₂ conversion of ~48% with 100% CO selectivity was achieved. Long-term stability tests (900 h) showed no significant deactivation. In situ characterization revealed the formation of a dynamically changing subnano MoOₓ surface with abundant oxygen vacancies under reaction conditions. These oxygen vacancies acted as active sites for direct CO₂ dissociation via electron transfer mediated by exposed Mo sites. H₂ played a crucial role in removing leached oxygen atoms, regenerating the active vacancies and preventing over-oxidation of the bulk Mo₂N. DFT calculations confirmed that the high density of oxygen vacancies significantly enhanced CO₂ adsorption and promoted C=O bond cleavage, with CO₂ dissociation being the rate-limiting step. Compared to other catalysts, including CeO₂, the MoOₓ/Mo₂N catalyst exhibited superior activity and stability. The apparent activation energy of the MoOₓ/Mo₂N catalyst was significantly lower (45.0 kJ/mol) than other catalysts, indicating a facile reaction pathway. The high activity of the MoOₓ/Mo₂N catalyst was attributed to the synergistic effect of the subnano MoOₓ layer with high oxygen vacancy concentration and the stable Mo₂N bulk structure. The catalyst maintained stability even after multiple cycles and extended exposure to air, highlighting its excellent recyclability.

This study demonstrates a novel approach to CO₂ activation based on the unique redox properties of a subnano MoOₓ surface on Mo₂N. The direct cleavage of the C=O bond, achieved through the high electron-donating capacity of oxygen vacancies, represents a significant advancement in CO₂ hydrogenation catalysis. The findings address the limitations of traditional H₂-assisted activation methods by providing a simplified and highly efficient pathway. The exceptional activity and stability of the MoOₓ/Mo₂N catalyst, even under harsh reaction conditions, highlight its potential for industrial applications. The observed superior performance compared to other catalysts underscores the significance of defect engineering in catalyst design. This work provides insights into the design of high-performance catalysts for CO₂ conversion, offering a path towards sustainable chemical synthesis.

This work demonstrates the exceptional catalytic performance of a MoOₓ/Mo₂N catalyst for CO₂ hydrogenation to CO, achieving high activity and stability without the need for supported active metals. The direct cleavage of the C=O bond, facilitated by abundant oxygen vacancies on the subnano MoOₓ surface, represents a significant advancement in CO₂ conversion catalysis. Future research could explore the optimization of the catalyst synthesis and the application of this approach to other CO₂-related reactions. Investigating the influence of various dopants on catalyst performance and extending the study to other metal nitride/oxide heterostructures are also promising avenues for future research.

The study primarily focuses on the reverse water-gas shift (RWGS) reaction. The applicability of the MoOₓ/Mo₂N catalyst to other CO₂ conversion reactions needs further investigation. The DFT calculations are performed at 0K, which might not fully capture the dynamic behavior of the catalyst at reaction temperatures. While the long-term stability is demonstrated, potential long-term deactivation mechanisms under industrial conditions require further study.