Comparative study of coal based catalysts for NO adsorption and NO reduction by CO

Abstract

A series of Fe-Co catalysts supported on coal based activated semicoke were tested for the catalytic performance of NO adsorption and NO reduction by CO. The structural property, surface chemistry and reaction mechanism were then investigated by nitrogen adsorption, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman scattering, and in situ diffuse reflectance infrared Fourier transform spectra (DRIFT). The catalysts were prepared by changing type of coal, carbonization temperature, and activation method. For type of coal, lignite and bitumite were used as the raw materials and the experimental results showed that catalyst based on lignite exhibited superior performance of both NO adsorption and reduction than the catalyst based on bitumite. It was explained that less volatile and moisture content and higher level of graphitization of bitumite inhibit its pore opening process, and further resulted in smaller surface area, less metal loading and less surface Oβ, which are determining for the catalytic behavior. Carbonization temperatures of 500, 700 and 900 °C were tried. Higher carbonization temperature could enhance pores-opening and increase surface area where adsorbed NOx could be stored, resulting in higher adsorption capacity. But higher carbonization temperature could lead to the depletion of surface oxygen and decrease the amount of loaded metals, lowering the catalytic activity. Carbonization temperature of 700 °C was found to the optimum one considering both NO adsorption and reduction. Three activation methods were studied, i.e., HNO3, KOH and vapor activation. Catalyst activated by KOH exhibited the best performance of both NO adsorption and reduction. It is explained that KOH activation could help to retain surface oxygen, and thus increase metal loading and vacancy Oβ, which are essential for the catalytic performance.