Preparation And Performance Of SnO₂-based Ultra-small Catalysts For Electrocatalytic Nitrogen Reduction To Ammonia

As one of the most widely used chemicals around the world,ammonia（NH₃）not only plays an irreplaceable role in many fields such as human life support,pharmaceuticals,and chemical industry,but also is expected to become a significant green energy carrier in the future.Nowadays,fossil fuels are increasingly scarce,and global warming is intensifying,global ammonia production still widely follows the energy-intensive Haber-Bosch process,resulting in huge energy consumption and greenhouse gas emissions,running counter to green development.In this regard,electrochemical nitrogen reduction to ammonia（ENRR）offers a promising solution:synthesizing ammonia under ambient conditions using electricity generated from clean energy sources.ENRR can avoid many drawbacks of traditional ammonia synthesis methods.However,the high reaction energy barrier and the fierce competition of hydrogen evolution reaction（HER）make it difficult to realize industrialization.ENRR needs to adsorb N₂ and break the highly stable N≡N bond to synthesize ammonia.Under ambient conditions,only relying on electric potential to break through the extremely high energy barrier results in extremely slow reaction kinetics.Therefore,high-performance catalysts are required to facilitate the reaction to proceed efficiently.Studies have shown that the reduction of catalyst size can not only increase its specific surface area and atom utilization to provide more abundant catalytic active sites,but also increase the number of unsaturated atoms to improve catalytic activity.In addition,size differences can also affect the formation of reaction intermediates,thereby affecting product selectivity.Therefore,ultra-small-scale catalysts,especially single-atom catalysts,exhibit excellent catalytic performance and thus become a current research focus.In order to break through the limitations of carbon-based ultra-small-scale catalysts in terms of loading and stability,in this paper,tin oxide（Sn O₂）was used as a substrate to support Cu and Ru to construct ultra-small-scale catalysts to obtain excellent ENRR catalytic activity.The main research contents are as follows:A high-temperature reduction method was used to load Cu on Sn O₂ nanosheets in the form of sub-nanoparticles.At the same time,a high concentration of oxygen vacancy defects（V_O）was created on the surface of Sn O₂,and the oxygen vacancy-rich catalyst 0.24%Cu/Sn O₂-300 was constructed.On the one hand,the introduction of Cu particles and V_O can increase the carrier concentration of Sn O₂ to accelerate the electron transfer rate in the electrochemical process to promote the catalytic reaction.On the other hand,the charge transfer between Cu and defect-rich tin oxide can adjust the electronic structure of Cu to make Cu in an electron-deficient state.The electron-deficient Cu can form more d-band holes,thereby enhancing the adsorption and activation of N₂ molecules by Cu to enhance the intrinsic catalytic activity of Cu.The catalyst achieved an ammonia yield of 14.6μg h^-1mg^-1_cat.and a Faradaic efficiency of8.3%,which was in the middle of the reported results.Moreover,the catalyst exhibits good electrochemical stability.0.22% Ru/SnO₂-300 was constructed by doping Ru mainly in the form of single atoms into the surface lattice of Sn O₂ by a high-temperature reduction method.On the one hand,the introduction of Ru and V_O enhanced the charge transfer rate of Sn O₂ and accelerated the catalytic reaction kinetics.On the other hand,the charge transfer between Ru and defect-rich Sn O₂ can tune the electronic structure of Ru and enhance its intrinsic catalytic activity.On this basis,theoretical calculations were carried out to reveal the catalytic mechanism of ENRR:（1）The synergistic effect of Ru single-atom active sites and oxygen vacancies can greatly reduce the rate-determining step energy barrier of the ENRR reaction;（2）The introduction of Ru can significantly reduce N₂The adsorption energy on the catalyst surface,thereby accelerating the ENRR process.The as-prepared catalyst achieved an ammonia yield of 32.4μg h^-1mg^-1_cat.and a Faradaic efficiency of 16.2%,exceeding most reported results.In addition,the binding energy between Ru single atoms and defect-rich tin oxide（-12.37 e V）is significantly stronger than that between nitrogen-doped graphene（-7.17 e V）,showing stronger stability.The catalysts also exhibit excellent electrochemical stability in electrochemical tests,and progress has been made by using tin oxide as a substrate to break through the limitations of carbon-based single-atom catalysts in electrochemical stability.