Abstract
Iron-based oxide catalysts for the NH3–SCR (selective catalytic reduction of NOx by NH3) reaction have gained attention due to their high catalytic activity and structural adjustability. In this work, iron–niobium, iron–titanate and iron–molybdenum composite oxides were synthesized by a co-precipitation method with or without the assistance of hexadecyl trimethyl ammonium bromide (CTAB). The catalysts synthesized with the assistance of CTAB (FeM0.3Ox-C, M = Nb, Ti, Mo) showed superior SCR performance in an operating temperature range from 150 °C to 400 °C compared to those without CTAB addition (FeM0.3Ox, M = Nb, Ti, Mo). To reveal such enhancement, the catalysts were characterized by N2-physisorption, XRD (Powder X-ray diffraction), NH3-TPD (temperature-programmed desorption of ammonia), DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy), XPS (X-ray Photoelectron Spectroscopy), and H2-TPR (H2-Total Physical Response). It was found that the crystalline phase of Fe2O3 formed was influenced by the presence of CTAB in the preparation process, which favored the formation of crystalline γ-Fe2O3. Owing to the changed structure, the redox-acid properties of FeM0.3Ox-C catalysts were modified, with higher exposure of acid sites and improved ability of NO oxidation to NO2 at low-temperature, both of which also contributed to the improvement of NOx conversion. In addition, the weakened redox ability of Fe prevented the over-oxidation of NH3, thus accounting for the greatly improved high-temperature activity as well as N2 selectivity.
Highlights
Selective catalytic reduction of NOx by NH3 (NH3–SCR) is an effective means of NOx abatement
High dispersion of sites having the same function and close coupling of redox-acid sites are important for the design of catalysts with superior NH3–SCR performance [2]
FeTi0.3Ox and FeMo0.3Ox catalysts with pure α-Fe2O3 phase exhibited almost the same reaction rate, which was lower than that of the FeM0.3Ox-C catalysts. This illustrated that the formation of the γ-Fe2O3 phase induced by CTAB addition during the catalyst preparation process improved the catalytic activity of FeM0.3Ox
Summary
Selective catalytic reduction of NOx by NH3 (NH3–SCR) is an effective means of NOx abatement. High dispersion of sites having the same function and close coupling of redox-acid sites are important for the design of catalysts with superior NH3–SCR performance [2]. Taking this principle into account, large numbers of iron-based composite oxide catalysts have been prepared and investigated for NH3–SCR, including Fe–Ti [6,7,8], Fe–Mo [9,10,11], Fe–W [12,13,14], Fe–Nb [15], and so on, in which the Fe component provides redox sites while a second component supplies acid sites. Qu et al [11] identified a di-nuclear entity (an isolated Mo ion and one adjacent surface Fe ion) as the active site in a Mo1/Fe2O3 single-atom catalyst, giving an explanation for the improved SCR reaction at acid-redox interfaces
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