Preparation of Ni/Co composite materials based on Cucurbit[6]uril and their photothermal synergistic catalysis of nitrogen reduction to ammonia under mild conditions

Cucurbit[<i>n</i>]urils, owing to their unique structural features, exhibit versatility in performing tasks such as adsorption, detection, and catalysis. However, the scope of their catalytic applications remains limited, primarily because most cucurbituril-based catalytic reactions take place in either aqueous or organic phases. In this study, we have successfully synthesized porous honeycomb composites of cucurbit[6]uril with cobalt and nickel (denoted as Co@Q[6] and Ni@Q[6], respectively) and applied them in a photothermal synergistic heterogeneous gas-solid reaction for the reduction of nitrogen to ammonia under mild conditions. This represents a groundbreaking achievement, as it is the first instance where a cucurbit[<i>n</i>]uril-based material has demonstrated catalytic functionality in its solid-state form, thereby introducing a novel concept for the design and application of cucurbit[<i>n</i>]uril-based photocatalysts. To characterize the structure of these composites, we employed a range of techniques including X-ray absorption fine structure (XAFS) analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and hydrogen temperature-programmed reduction (H₂-TPR). Our findings revealed that Ni@Q[6] exhibits higher photothermal catalytic ammonia synthesis activity compared to Co@Q[6]. This enhanced activity is attributed to the strong metal-support interaction (MSI) between Ni and Q[6], which facilitates electron transfer and nitrogen activation. Furthermore, the thermal source promotes the transition of electrons from the valence band to the conduction band, thereby enhancing the cleavage of the NN bond. Notably, the band gaps of Co@Q[6] and Ni@Q[6] are significantly reduced. In particular, Ni@Q[6] demonstrates the highest efficiency in electron-hole pair separation, as evidenced by photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) measurements. Overall, Co/Ni@Q[6] provides an effective pathway for nitrogen reduction under mild conditions and advances the application of cucurbituril-based materials in photothermal catalysis. This work also contributes to the development of environmentally sustainable ammonia synthesis technology.