Miao Liu,Zhenghao Lu,Linghan Yang,Renmin Gao,Xinying Zhang,Yongjing Wang,Yonghao Wang
Electrocatalytic reduction of nitrate to ammonia is an effective method for the recycling of NO 3 - -N in the field of nitrate polluted water treatment . In this work, Co-N-C was prepared in this work by pyrolysis of Co-based metal organic framework at 700 °C and it can achieve 100 % selectivity to ammonia under the optimum experiment conditions. Impressively, no harmful nitrite generated in the reduction process because the reduction rate constant of Co-N-C for NO 2 - -N is higher than that of NO 3 - -N. Experiment results show that the chemical adsorption rather than electrostatic adsorption dominated among Co-N-C and free NO 3 - , which is significant for the subsequent electronic reduction process. Mechanism analysis indicated the active hydrogen (H*) reduction mechanism is predominate for Co-N-C but the direct electron mechanism is predominate for Co-C during the reduction from NO 3 - to NO 2 - . While for Co-N-C and Co-C, the subsequent reduction of NO 2 - to NH 4 + was conducted by the direct electron mechanism. The present work not only explores high performance for the selectively electrocatalytic reduction from nitrate to ammonia but also promote new inspiration for the preparation of other Co-based functional materials for reducing the production of nitrite.
In this paper, ultrafine platinum nanoparticles (Pt/Co-N-C) were prepared by irradiating the mixture of Co-N-C and K2[PtCl4] with near-ultraviolet (UV) light at a wavelength of 395 nm and visible light at 450 and 550 nm without adding any reducing and stabilizing agents using MOFs-modified carbon materials (Co-N-C) as the carrier. The composition and morphology of the Pt/Co-N-C catalysts were investigated using characterization tools such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HR-TEM), etc. The results showed that the Pt nanoparticles were well dispersed on the surface of the Co-N-C, with average particle sizes of about 2.39, 2.86, and 3.53 nm, respectively. In the catalytic 4-NP reduction experiments, the catalyst (Pt/Co-N-C-1) prepared under near-ultraviolet light (395 nm) irradiation showed the highest catalytic activity with a rate constant k=0.64 min-1. After five cycles, the conversion rate was still maintained at about 90%, which was highly reusable. It was demonstrated that 395 nm near-ultraviolet light was more favorable for the modulation of the distribution of the species during the hydrolysis and reduction of platinum in this reaction to form smaller-sized platinum nanocrystals, which was conducive to the catalyst to exhibit higher activity.
A series of Co@N-C materials are synthesized from pyrolysis of ZIF-67 at different temperatures and employed as catalyst for electrochemical reduction of CO 2 (CO 2 RR) to syngas. Through a systematical investigation of the relationship between structure of Co@N-C catalysts and their catalytic activity, it could be found that the Co species exposed on the material surface mainly promote the hydrogen evolution reaction (HER) process to produce H 2 , while the Co particles packaged by N-C skeleton provide electrons and transfer them to CO 2 molecule through the Co -N- C skeleton, promoting the CO 2 reduction reaction. Following this thinking, the pristine Co@N-C materials are treated by aqua regia to remove the surface exposed Co species. After acid treatment, the affinity of H species sharply decreases, and adsorption of CO 2 and transfer of electron to CO 2 are successfully improved. The acid-treated Co@N-C catalysts exhibit improved catalytic activity, faradic efficiency of CO, and stability in CO 2 RR process. Among all these catalysts, the obtained Co@N-C-700-A shows highest CO 2 reduction activity and stability, achieving a specific current of 25 A∙g (cat) −1 and a productivity of 0.266 mol (CO) h −1 ∙ g(cat) −1 at a mild potential of -0.57 V vs. RHE. The ratio of CO/H 2 in products could be adjusted in the range of 1/4 to 1.5/1.
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