Synthesis of Bimetallic Ag-Ni-MOF-74 Catalyst with Excellent CO-SCR Performance in Low Temperature Range

Abstract

Selective catalytic reduction of NOx with CO technology (CO-SCR) is supposed to be a cost-effective and environmentally friendly technique for NOx abatement in the flue gas under CO-rich conditions. As a promising class of porous hybrid inorganic-organic materials, bimetallic metal-organic frameworks exhibit great physicochemical properties in catalysis area, whereas their application in low-temperature CO-SCR system are seldom reported. In this study, a series of bimetal organic-frameworks catalysts with different Ag contents were successfully prepared by a post-synthesis method and were assessed for NO reduction by CO. The typical experimental procedure for the synthesis of bimetallic Ag-Ni-MOF-74 catalysts is as follows: First, a light yellow Ni-MOF-74 sample was prepared by a hydrothermal method. Then 250 mg Ni-MOF-74, 1 mmol of NaBH4 and AgNO3 with different molar ratio (0.25, 0.5, 1 mmol) were added into 40 mL N,N-dimethylformamide (DMF) solution, and were stirred for 6 h. The mixtures were further moved into a Teflon-lined autoclave at 150 ℃ for 12 h. After washing with DMF and methanol, the obtained Agx-Ni-MOF-74 catalysts were dried at 60 ℃ under vacuum for 12 h. Totally, bimetallic Ag-Ni-MOF-74 catalysts exhibited a better low-temperature CO-SCR efficiency than monometallic Ni-MOF-74 catalysts. Especially, Ag1-Ni-MOF-74 achieved a nearly 100% NO conversion in the temperature range from 200 ℃ to 300 ℃. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field-emission scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and hydrogen temperature programmed reduction (H2-TPR) techniques were used to investigate the structure and properties of the samples. It was found that Ag addition not only enriched the form of more active sites, but also increased the specific surface area of the catalysts, which promote the activation and transfer of reactants. The synergistic effect between Ag and Ni species also contributed to enhancement of surface oxygen vacancies and accelerated the electron transfer in NO+CO reaction. By combining XPS and in situ FT-IR results, the mechanism of CO-SCR reaction over Ag-Ni-MOF-74 was proposed to follow Langmuir- Hinshelwood (L-H) mechanism, which exhibits a more readily low-temperature reaction rate and a lower reaction barrier than Eley-Rideal (E-R) mechanism.