Abstract
The influence of the citric acid content on the structural and redox properties of a magnetic iron–cerium–tungsten mixed oxide catalyst prepared through a microwave-assisted citric acid sol–gel method is investigated via TG–DTG–DSC, XRD, N2 adsorption–desorption, XPS, H2-TPR and NH3-TPD. Additionally, the NH3-SCR activity of the magnetic FeCeW-m (m = 0.25, 0.5 and 1.0) catalysts are also studied. The results indicate that an increase in citric acid content strengthens the sol–gel reaction between citric acid and metal ions and promotes the formation of the γ-Fe2O3 crystallite not α-Fe2O3. Meanwhile, it decreases the BET surface area and pore volume of the catalyst. Furthermore, the surface concentration of iron species on the catalyst is enhanced when the molar ratio of citric acid/(Fe + Ce + W) increases from 0.25 to 1.0, but its surface absorbed oxygen and total oxygen concentration decrease. The magnetic FeCeW-0.5 catalyst shows the best reducibility at temperatures below 790 °C. The increase in the citric acid content inhibits the formation of acid sites in the catalyst, thus the magnetic FeCeW-0.25 catalyst possesses the most Lewis acid sites and Brønsted acid sites among the catalysts. The enhancement in citric acid content is beneficial to improve the SCR reaction rates normalized by the surface area of the catalyst. This catalyst exhibits high anti-SO2 and H2O poisoning, and the molar ratio of citric acid/(Fe + Ce + W) affects the adsorption of NOx species on its surface.
Highlights
Nitrogen oxide (NOx) emitted from the combustion of fossil fuel in coal- red power plants or automobile engines is a typical environmental pollutant, which causes serious problems to the environment and human health, such as acid rain, photochemical smog, pulmonary edema and tissue hypoxia.[1,2,3,4,5,6] many technologies have been developed to reduce the emission of NOx from coal- red power plants.[7,8,9] Compared with other de-nitrogen technologies, the selective catalytic reduction of NOx by NH3 (NH3-SCR) has drawn increasing attention due to its high efficiency.[7]
It can be concluded that the citric acid content plays an important role in the sol–gel process between citric acid and Fe/Ce/W ions, and the complex reaction between them is fully completed with the molar ratio of citric acid/(Fe + Ce + W) increasing from 0.25 to 1.0
No crystallite of cerium or/and tungsten species were detected for both the FeCeW-0.5 and FeCeW-1.0 catalysts. These results indicate that iron oxide is the main crystallite of the magnetic iron–cerium–tungsten mixed oxide catalyst prepared through the microwave-assisted citric acid sol–gel method, and Ce or W species probably existed in the crystallite phase with a small particle size or amorphous phase in the catalyst
Summary
Nitrogen oxide (NOx) emitted from the combustion of fossil fuel in coal- red power plants or automobile engines is a typical environmental pollutant, which causes serious problems to the environment and human health, such as acid rain, photochemical smog, pulmonary edema and tissue hypoxia.[1,2,3,4,5,6] many technologies have been developed to reduce the emission of NOx from coal- red power plants.[7,8,9] Compared with other de-nitrogen technologies, the selective catalytic reduction of NOx by NH3 (NH3-SCR) has drawn increasing attention due to its high efficiency.[7] V2O5–WO3(MoO3)/TiO2 is widely used as an NH3-SCR catalyst due to its high NOx conversion and high anti-SO2 poisoning It has some limitations, such as a relatively narrow temperature window. Due to their relatively high NH3-SCR activity, low cost and non-toxicity, iron-based catalysts have been receiving signi cant attention by many researchers.[17,18,19,20,21,22,23,24,25] Cerium or/and tungsten are widely used additives to optimize the NH3-SCR activity of iron-based catalysts owing to the high oxygen storage capacity and high redox ability of the Ce species by shi ing between Ce4+ and Ce3+, and the high surface acidity and excellent thermal stability of W species.[26,27,28,29,30] In our previous research, a novel magnetic iron–cerium–tungsten mixed oxide catalyst was proposed through a microwave-assisted citric acid sol–gel method with both Ce and W as additives, and the synergistic promotional effect of Ce and W on the NH3-SCR activity of iron oxide was investigated.[15,16] many researches have indicated that the amount of citric acid plays an important role in the sol–gel reaction between citric acid and metal ions, thereby in uencing the structural and redox properties of the powder obtained by the citric acid sol–gel method.[31,32,33,34,35] these properties are usually thought to be the important factors in the NH3-SCR activity of iron-based mixed oxide catalysts.[16,17]
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