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

Abstract

Cucurbit[n]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[n]uril-based material has demonstrated catalytic functionality in its solid-state form, thereby introducing a novel concept for the design and application of cucurbit[n]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 NN 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.