Boosting Nitrogen Activation Via Bimetallic Organic Frameworks For Photocatalytic Ammonia Synthesis

Photocatalytic ammonia synthesis from N-2 is a carbon-neutral strategy, although its efficiency is impeded by the activation of inert N N triple bonds. In N-2 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 N-2 activation. The rationally designed BMOFs are composed of two functional metal nodes, in which the hard acid metal node with a high ionization potential (I-n) accepts the electron from N-2 and the soft acid metal node with a low I-n donates the electron to N-2. Owing to the bimetal synergistic effect, the potential activation energy barrier of N-2 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 (Delta I-n) between the two metals is >= 6 eV and the proportion of high I-n metal reaches similar to 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 mu mol 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.