Strained few-layer MoS₂ with atomic copper and selectively exposed in-plane sulfur vacancies for CO₂ hydrogenation to methanol

The escalating global warming crisis, driven by excessive anthropogenic CO₂ emissions, necessitates sustainable solutions. Catalytic CO₂ hydrogenation, producing clean fuels and valuable chemicals, presents a promising approach. Methanol synthesis from CO₂ is particularly attractive due to methanol's high energy density and its role as a feedstock in the methanol economy. While various catalysts have been explored, including metal oxides, solid solutions, and metal alloys, few-layer MoS₂ nanosheets have recently shown promise, particularly at lower reaction temperatures. However, conventional MoS₂ often exhibits poor performance due to multilayer/thick-layer structures with inert basal planes and randomly exposed sulfur vacancies (SVs). Selective exposure and activation of the basal plane, while minimizing edge SV exposure (which promotes methane formation), is crucial for improving methanol selectivity. This study hypothesizes that a few-layer fullerene-like MoS₂ structure, with in-plane SVs and minimal edge SVs, will selectively catalyze CO₂ to methanol. Strain engineering is proposed as a method to activate the basal plane of MoS₂, controlling the electronic structure and facilitating SV formation. While strain can be introduced using various methods, many are complex, expensive, and unsuitable for high-pressure gas-solid systems. This research aims to develop a novel synthesis method to create strained few-layer MoS₂ with selectively exposed in-plane SVs, enhanced by atomic copper, for efficient and selective methanol synthesis from CO₂ hydrogenation.

Extensive research has been devoted to developing efficient catalysts for methanol synthesis from CO₂ hydrogenation. These catalysts include Cu-metal oxides (Cu/ZnO/Al₂O₃, Cu/ZrO₂), In₂O₃-based oxides, solid solutions (ZnO/ZrO₂, GaZrOₓ, In₂O₃/ZrO₂), metal alloys (NiGa, MnCo, PdZn, PdIn), and Mo-containing solids (MoP, β-Mo₂C, MoS₂). Few-layer MoS₂ nanosheets, unveiled for CO₂ hydrogenation in 2021, have attracted significant attention due to their relatively low reaction temperatures and satisfactory catalytic performance and stability for methanol synthesis. The different polytypes (1T, 2H, and 3R), layered structures (multilayer, few-layer, single-layer), active sites (in-plane and edge sites), and vacancies (Mo and S vacancies) of MoS₂ greatly impact its catalytic activity. Only few-layered 2H-phase MoS₂ with sufficiently exposed in-plane SVs effectively catalyzes methanol synthesis, while abundant edge SVs promote methane production. Conventional methods often result in multilayer/thick-layer MoS₂ with poor performance, necessitating strategies to improve MoS₂ dispersity, reduce layer stacking, and selectively expose and activate the basal plane while minimizing edge sites. Strain engineering in two-dimensional transition metal dichalcogenides has shown promise in basal plane activation, but many existing methods are complicated and difficult to scale up. This research builds on prior work by the team on silica-encapsulated nanostructures to address the challenges in controlling the synthesis of strained, few-layer MoS₂ with a fullerene-like structure for selective CO₂ hydrogenation.

The synthesis involved a multi-step process. First, uniform MoO₂ nanocores were synthesized hydrothermally using polyvinylpyrrolidone (PVP) as a capping agent in a water-ethanol cosolvent. The MoO₂ nanocores were then used as templates to synthesize MoO₂@SiO₂ by depositing a uniform silica shell containing mesoporous channels using cetyltrimethylammonium chloride (CTAC) surfactant. The MoO₂ core was then sulfurized to MoS₂ using thioacetamide (TAA) under hydrothermal conditions, resulting in MoS₂@SiO₂ with a fullerene-like hollow structure. Finally, atomic copper was introduced via impregnation, forming Cu/MoS₂@SiO₂. Control samples, including MoS₂ nanoparticles (MoS₂-NPs) without silica encapsulation and multilayered MoS₂ synthesized by traditional hydrothermal methods (MoS₂-HT), were also prepared for comparison. The morphology and structure were analyzed using FESEM and TEM, revealing the uniform spherical MoO₂ and MoO₂@SiO₂ core-shell structures. The sulfurization resulted in a fullerene-like MoS₂ hollow structure with 2-4 layers. HAADF-STEM-EDS confirmed the elemental composition. Optimization studies investigated the influence of parameters such as reaction temperature, time, solvents, and sulfur sources on MoS₂@SiO₂ morphology. Cu incorporation was confirmed using HAADF-STEM-EDS, showing Cu localization within the MoS₂ phase. XPS analysis confirmed the chemical state and electronic structure, and EPR measurements quantified the SV concentration. XRD patterns confirmed the crystal structure of the different samples. N₂ physisorption revealed the mesoporous nature of Cu/MoS₂@SiO₂. Synchrotron radiation-based XAS (XANES and EXAFS) examined the electronic structure and coordination environment of the Cu species, confirming atomic Cu dispersion and the Cu-S bond formation. Catalytic performance was evaluated using a high-pressure fixed-bed flow reactor for CO₂ hydrogenation to methanol in the temperature range of 180–260 °C, examining CO₂ conversion, methanol selectivity, and specific methanol yield. Long-term stability tests were conducted for 150 h. In-situ DRIFTS and operando XAS studies investigated the reaction mechanism and the role of Cu. DFT calculations were performed using VASP, examining SV formation energies, H₂ dissociation, and CO₂ reduction on strained MoS₂ monolayers and nanotubes to understand the influence of strain.

The synthesized Cu/MoS₂@SiO₂ catalyst displayed superior catalytic performance in CO₂ hydrogenation to methanol. The fullerene-like structure of the few-layer MoS₂, created through a physically constrained topological conversion within a silica shell, led to the selective exposure of in-plane sulfur vacancies (SVs), crucial for methanol synthesis, while minimizing edge SVs which promote methane formation. The introduction of atomically dispersed copper further enhanced the catalytic activity by facilitating H₂ activation and increasing SV formation. The catalyst exhibited a remarkable specific methanol yield of 6.11 molMeOH molMo⁻¹ h⁻¹ with 72.5% selectivity at 260 °C, significantly exceeding previously reported MoS₂-based catalysts and commercial Cu/ZnO/Al₂O₃. This high performance was attributed to the unique combination of several factors: (1) the few-layer, fullerene-like MoS₂ structure with high density of SVs; (2) the selective exposure of in-plane SVs and reduction in edge SVs; (3) the enhancement of H₂ activation and spillover facilitated by atomic copper; and (4) the strain introduced by the spherical curvature of MoS₂ that promoted the formation of SVs and the subsequent reaction steps. The catalyst demonstrated excellent long-term stability for 150 h at 260°C, showing no significant decline in activity. In-situ DRIFTS revealed the formation of key intermediates (CO, CH₃O) during the reaction, providing insights into the reaction mechanism. Operando XAS studies confirmed the presence of atomically dispersed copper coordinated to sulfur atoms throughout the reaction, further supporting the role of Cu in promoting methanol formation. DFT calculations corroborated the experimental findings, indicating that compressive strain facilitates SV formation and CO₂ hydrogenation, while tensile strain helps regenerate active sites, highlighting the critical role of strain in enhancing catalytic activity. The optimized Cu loading was found to be 1.5 wt%.

The findings directly address the research question by demonstrating the successful synthesis and application of a highly efficient and selective catalyst for CO₂ hydrogenation to methanol. The superior performance of Cu/MoS₂@SiO₂ compared to other MoS₂-based catalysts and commercial Cu/ZnO/Al₂O₃ highlights the significance of the unique structural features and the synergistic effect of strain and atomic copper. The observed high selectivity toward methanol over methane demonstrates the effectiveness of the strategy to selectively expose in-plane SVs while minimizing edge SVs. The excellent stability observed over an extended reaction time underscores the potential for practical applications in CO₂ conversion technologies. The in-situ and operando spectroscopic studies, along with the DFT calculations, provide a comprehensive understanding of the reaction mechanism and the role of the structural features and atomic copper. These findings provide valuable insights for the design and development of next-generation catalysts for CO₂ hydrogenation and contribute to the broader efforts towards sustainable energy solutions.

This study successfully synthesized a highly efficient and stable catalyst, Cu/MoS₂@SiO₂, for selective CO₂ hydrogenation to methanol. The unique fullerene-like structure, achieved through physically constrained sulfidation of MoO₂, introduced strain and selectively exposed in-plane sulfur vacancies, crucial for methanol synthesis. The incorporation of atomic copper further enhanced catalytic activity. The combined experimental and theoretical results provided a comprehensive understanding of the reaction mechanism and the role of strain and copper. Future research could explore other transition metal dopants and different support materials to further optimize the catalyst performance and investigate the effects of different strain levels on catalytic activity. Furthermore, scaling up the synthesis method for industrial applications is a crucial next step.

While the Cu/MoS₂@SiO₂ catalyst demonstrated excellent performance and stability, some limitations exist. The synthesis method involves multiple steps, which could potentially increase the production cost and complexity. The study primarily focused on laboratory-scale experiments; further investigation is required to scale up the synthesis for industrial applications. The DFT calculations involved simplified models; more complex models could offer further insights into the reaction mechanism. The long-term stability tests were conducted under specific reaction conditions; the catalyst's performance under varying conditions (temperature, pressure, gas composition) warrants further investigation.