Boosting Nitrogen Activation via Bimetallic Organic Frameworks for Photocatalytic Ammonia Synthesis

Abstract

Photocatalytic ammonia synthesis from N2 is a carbon-neutral strategy, although its efficiency is impeded by the activation of inert N≡N triple bonds. In N2 activation, the electron acceptance process is often strongly coupled with the electron donation process, leading to a high potential activation energy barrier and low photocatalytic activity. Herein, we proposed a strategy to decouple these two processes by bimetallic organic frameworks (BMOFs) for boosting N2 activation. The rationally designed BMOFs are composed of two functional metal nodes, in which the hard acid metal node with a high ionization potential (In) accepts the electron from N2 and the soft acid metal node with a low In donates the electron to N2. Owing to the bimetal synergistic effect, the potential activation energy barrier of N2 is reduced, as confirmed by the in situ Fourier transform infrared (FTIR) spectra and density functional theory (DFT) calculations. Via testing six kinds of bimetal combinations, it is found that, as the ionization potential difference (ΔIn) between the two metals is ≥6 eV and the proportion of high In metal reaches ∼20%, the bimetal synergistic effect becomes dominant. In all the as-prepared BMOFs, the optimal BMOF(Sr)–0.2Fe photocatalyst exhibits an NH3 evolution rate up to 780 μmol g–1 h–1. This work may unveil a corner of the hidden mechanism for the chemical bond activation in a broad range of catalytic processes.