Defect engineering for high-selection-performance of N2 activation over CeO2(111) surface

Dual-Shelled CeO2 Hollow Spheres Decorated with MXene Quantum Dots for Efficient Electrocatalytic Nitrogen Oxidation

Electrocatalytic nitrogen oxidation (NOR) provides a promising alternative strategy for synthesizing nitric acid from widespread N2, which overcomes the disadvantages of Haber-Bosch-Ostwald process. However, the NOR process suffers from the limitation of high N≡N bonding energy, sluggish kinetics, and low efficiency. It is prerequisite to develop more efficient NOR electrocatalysts. Herein, dual-shelled CeO2 hollow spheres (D-CeO2) are synthesized and modified with Ti3C2 MXene quantum dots (MQDs) for NOR, which exhibited a NO3 - yield rate of 71.25 µg h-1 mgcat -1 and Faradic Efficiency (FE) of 31.80% at 1.7 V versus RHE. The unique quantum size effect and abundant edge active sites lead to more effective capture of nitrogen. Moreover, the dual-shelled hollow structure will gather intermediate products in the interlayer of the core-shell to facilitate N2 fixation. The in situ Fourier transform infrared (FTIR) spectroscopy confirmed the formation of *NO and NO3 - species during the NOR, and the kinetics and possible pathways of NOR are calculated by density functional theory (DFT). In addition, a Zn-N2 reaction device is assembled with D-CeO2/MQDs as anode and Zn plate as cathode, obtaining an extremely high NO3 - yield rate of 104.57 µg h-1 mgcat -1 at 1 mA cm-2.

Cerium Oxide with Specific Surface Oxygen Vacancy and Facet for Nitrate Synthesis through Electrocatalytic Nitrogen Oxidation

Recently, the green and sustainable synthesis of nitric acid (HNO3) through the electrochemical nitrogen oxidation reaction (NOR) has attracted significant attention. Developing high-efficiency electrocatalysts to overcome the challenges caused by the chemical inertness of N2 and the slow, 10-electron transfer kinetics is highly desirable. In this work, we investigated the NOR performance of cerium oxide (CeO2), which is known for its excellent oxygen storage and release capabilities. The concentration of oxygen vacancies and the morphology with different specific surface areas and exposed crystal facets have a significant influence on NOR activity. The hollow nanosphere with primarily exposed {111} facets exhibits the highest oxygen vacancy content (35.07%) and the largest surface area (101.26 m2/g), leading to an optimal NO3- yield of 383.06 μmol·h-1·gcat-1 with a Faradic efficiency (FE) of 5.17% at 1.7 V vs RHE. In situ Fourier transform infrared (FTIR) spectroscopy confirmed the formation of *NO2, *NO, and NO3- species on the CeO2 hollow nanosphere electrode during the NOR. Theoretical calculations were also performed to confirm the effects of oxygen vacancies and the {111} crystal plane on CeO2 hollow nanospheres, leading to excellent NOR catalytic activity of CeO2 materials.

The novel π-d conjugated TM2B3N3S6 (TM = Mo, Ti and W) monolayers as highly active single-atom catalysts for electrocatalytic synthesis of ammonia

Recently, single-atom catalysts (SACs) are receiving significant attention in electrocatalysis fields due to their excellent specific activities and extremely high atomic utilization ratio. Effective loading of metal atoms and high stability of SACs increase the number of exposed active sites, thus significantly improving their catalytic efficiency. Herein, we proposed a series (29 in total) of two-dimensional (2D) conjugated structures of TM2B3N3S6 (TM means those 3d to 5d transition metals) and studied the performance as single-atom catalysts for nitrogen reduction reaction (NRR) using density functional theory (DFT). Results show that TM2B3N3S6 (TM = Mo, Ti and W) monolayers have superior performance for ammonia synthesis with low limiting potentials of -0.38, -0.53 and -0.68 V, respectively. Among them, the Mo2B3N3S6 monolayer shows the best catalytic performance of NRR. Meanwhile, the π conjugated B3N3S6 rings undergo coordinated electron transfer with the d orbitals of TM to exhibit good chargeability, and these TM2B3N3S6 monolayers activate isolated N2 according to the "acceptance-donation" mechanism. We have also verified the good stability (i.e., Ef < 0, and Udiss > 0) and high selectivity (Ud = -0.03, 0.01 and 0.10 V, respectively) of the above four types of monolayers for NRR over hydrogen evolution reaction (HER). The NRR activities have been clarified by multiple-level descriptors (ΔG*N2H, ICOHP, and Ɛd) in the terms of basic characteristics, electronic property, and energy. Moreover, the aqueous solution can promote the NRR process, leading to the reduction of ΔGPDS from 0.38 eV to 0.27 eV for the Mo2B3N3S6 monolayer. However, the TM2B3N3S6 (TM = Mo, Ti and W) also showed excellent stability in aqueous phase. This study proves that the π-d conjugated monolayers of TM2B3N3S6 (TM = Mo, Ti and W) as electrocatalysts show great potentials for the nitrogen reduction.