Plasma-Driven Efficient Conversion of CO2 and H2O into Pure Syngas with Controllable Wide H2/CO Ratios over Metal-Organic Frameworks Featuring In Situ Evolved Ligand Defects.

While the mild production of syngas (a mixture of H2 and CO) from CO2 and H2O is a promising alternative to the coal-based chemical engineering technologies, the inert nature of CO2 molecules, unfavorable splitting pathways of H2O and unsatisfactory catalysts lead to the challenge in the difficult integration of high CO2 conversion efficiency with produced syngas with controllable H2/CO ratios in a wide range. Herein, we report an efficient plasma-driven catalytic system for mild production of pure syngas over porous metal-organic framework (MOF) catalysts with rich confined H2O molecules, where their syngas production capacity is regulated by the in situ evolved ligand defects and the plasma-activated intermediate species of CO2 molecules. Specially, the Cu-based catalyst system achieves 61.9 % of CO2 conversion and the production of pure syngas with wide H2/CO ratios of 0.05 : 1-4.3 : 1. As revealed by the experimental and theoretical calculation results, the in situ dynamic structure evolution of Cu-containing MOF catalysts favors the generation of coordinatively unsaturated metal active sites with optimized geometric and electronic characteristics, the adsorption of reactants, and the reduced energy barriers of syngas-production potential-determining steps of the hydrogenation of CO2 to *COOH and the protonation of H2O to *H.

Regulated Dual Defects of Bridging Organic and Terminal Inorganic Ligands in Iron-based Metal-Organic Framework Nodes for Efficient Photocatalytic Ammonia Synthesis.

Engineering advantageous defects to construct well-defined active sites in catalysts is promising but challenging to achieve efficient photocatalytic NH3 synthesis from N2 and H2O due to the chemical inertness of N2 molecule. Here, we report defective Fe-based metal-organic framework (MOF) photocatalysts via a non-thermal plasma-assisted synthesis strategy, where their NH3 production capability is synergistically regulated by two types of defects, namely, bridging organic ligands and terminal inorganic ligands (OH- and H2O). Specially, the optimized MIL-100(Fe) catalysts, where there are only terminal inorganic ligand defects and coexistence of dual defects, exhibit the respective 1.7- and 7.7-fold activity enhancement comparable to the pristine catalyst under visible light irradiation. As revealed by experimental and theoretical calculation results, the dual defects in the catalyst induce the formation of abundant and highly accessible coordinatively unsaturated Fe active sites and synergistically optimize their geometric and electronic structures, which favors the injection of more d-orbital electrons in Fe sites into the N2 π* antibonding orbital to achieve N2 activation and the formation of a key intermediate *NNH in the reaction. This work provides a guidance on the rational design and accurate construction of porous catalysts with precise defective structures for high-performance activation of catalytic molecules.

Highly Efficient Nitrogen Reduction to Ammonia through the Cooperation of Plasma and Porous Metal-Organic Framework Reactors with Confined Water.

While the ambient N2 reduction to ammonia (NH3) using H2O as hydrogen source (2N2+6H2O=4NH3+3O2) is known as a promising alternative to the Haber-Bosch process, the high bond energy of N≡N bond leads to the extremely low NH3 yield. Herein, we report a highly efficient catalytic system for ammonia synthesis using the low-temperature dielectric barrier discharge plasma to activate inert N2 molecules into the excited nitrogen species, which can efficiently react with the confined and concentrated H2O molecules in porous metal-organic framework (MOF) reactors with V3+, Cr3+, Mn3+, Fe3+, Co2+, Ni2+ and Cu2+ ions. Specially, the Fe-based catalyst MIL-100(Fe) causes a superhigh NH3 yield of 22.4 mmol g-1 h-1. The investigation of catalytic performance and systematic characterizations of MIL-100(Fe) during the plasma-driven catalytic reaction unveils that the in situ generated defective Fe-O clusters are the highly active sites and NH3 molecules indeed form inside the MIL-100(Fe) reactor. The theoretical calculation reveals that the porous MOF catalysts have different adsorption capacity for nitrogen species on different catalytic metal sites, where the optimal MIL-100(Fe) has the lowest energy barrier for the rate-limiting *NNH formation step, significantly enhancing efficiency of nitrogen fixation.