Fe–Mn Mixed Oxide Catalysts Synthesized by One-Step Urea-Precipitation Method for the Selective Catalytic Reduction of NO x with NH3 at Low Temperatures

Abstract

A range of Fe–Mn mixed oxide catalysts, FeαMn1−αO x (α = 1, 0.25, 0.33, 0.50, 0 mol%) were prepared via one-step urea-precipitation method and applied to the selective catalytic reduction (SCR) of nitric oxide (NO x ) with NH3. The Fe0.33Mn0.66O x catalyst showed the highest activity in the NH3-SCR process within a broad operation temperature range (75–225 °C) with 90% NO x conversion. Series of characterization have been taken to investigate the physical and chemical properties of the catalysts. The BET results evidenced that the doping of iron species increased the surface area of the catalyst effectively, and this structure could provide more acid sites and active sites on the surface of catalysts for SCR reaction. X-ray powder diffraction results and Raman spectroscopy indicate that the active Mn and Fe species were in poorly crystalline or amorphous states which could increase lattice defects and oxygen vacancies on the surface of Fe0.33Mn0.66O x catalyst. The X-ray photoelectron spectra results suggested that more surface-adsorbed oxygen (OA), Fe3+, Mn4+ species existed on the surface of the Fe0.33Mn0.66O x catalyst compared with that of MnO x and FeO x catalysts, which is favorable to the NH3-SCR performance. Analysis by in situ Fourier transform infrared spectroscopy (FTIR) suggested that Fe-doping can enhance the absorption and the activation ability of NO which could promote the catalytic performance in the SCR process. Catalyst characterization Powder X-ray diffraction (XRD) was carried out on a PAN alytical powder X-ray diffractometer (Model EMPYREAN) with a monochromatic Cu Kα1 radiation (λ = 0.154056 nm) within a 2θ range of 10°–90° in the step of 0.02° at room temperature. The Brunauer–Emmett–Teller (BET) surface areas of the samples were computed by physical adsorption of N2 at − 196 °C using a NOVA 1200 (Quanta Chrome). The samples were pretreated at 300 °C for 5 h in a vacuum state previous to BET measurement. The high-resolution transmission electron microscopy (HR-TEM) determinations were conducted on a JEOL-2100 microscope. The Raman spectrums of samples were computed by a confocal Raman microscope (Labram DILOR) equipped with an Olympus BX-41 microscope (objective ×100) and TEcooled CCD detector (Andor), in a back-scattering configuration using an He–He laser (532 nm excitation line) with power of 510 mW at a sample and spectral resolution of 0.8 cm−1. X-ray photoelectron spectra (XPS) over the samples were by captured Al-Kα radiation (1486.7 eV). Binding energies of Mn 2p and O 1s were calibrated using C 1s (BE = 285.0 eV) as a standard. The in situ FTIR experiments of NO adsorption over MnO x and Fe0.33Mn0.66O x catalysts were performed on a VERTEX70-FTIR. Prior to NO adsorption, the samples (approximately 20 mg each) was pretreated at 250 °C in the He atmosphere for 1 h. The spectrum of NO adsorption at different temperatures were selected over each catalyst. The reaction condition was controlled as follows: 500 ppm NO and He as balance, and the flow rate was 100 mL/min.