Low temperature selective catalytic reduction of NO with NH3 over Mn–Fe spinel: Performance, mechanism and kinetic study
Graphical abstract
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Highlights
► Mn–Fe spinel shows an excellent low temperature SCR activity. ► The SCR activity is promoted with the increase of Mn content. ► The SCR reaction over (Fe2.5Mn0.5)1−δO4 mainly follows the E–R mechanism. ► Catalyst deactivated by SO2 can be regenerated through water washing.
Introduction
Nitrogen oxides (NO and NO2), which result from automobile exhaust gas and industrial combustion of fossil fuels, have been a major pollutant for air pollution [1], [2], [3], [4]. They contribute to photochemical smog, acid rain, ozone depletion and greenhouse effect [1], [5], [6], [7]. Selective catalytic reduction (SCR) of NO with NH3 has been an efficient technique for the control of NOx emission from coal fired power plants and automobiles. V2O5–WO3(MoO3)/TiO2 has been widely used as a SCR catalyst to control the emission of NOx from stationary coal fired power plants and automobiles for several decades [3], [8], [9], [10]. The SCR unit is located upstream of the desulfurizer and electrostatic precipitator in order to avoid reheating of the flue gas. The problems of this system are as follows: the relatively narrow temperature window (350–400 °C), the low N2 selectivity in the high temperature range, the toxicity of vanadium pentoxide to the environment, the high conversion of SO2 to SO3, and the deposition of dust on the catalyst [3], [4], [8], [9], [10], [11], [12], [13].
For the above reasons, there has been strong interest in developing highly active catalysts for low temperature SCR, which would be placed downstream electrostatic precipitator and desulfurizer [14], [15], [16], [17]. Previous researches have demonstrated that some Mn based catalysts showed an excellent activity for the low temperature SCR reaction [10], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. So far, there is no agreement on the way the low temperature SCR reaction continues: by (1) reaction of gaseous NO with (activated) NH3 to an activated transition state and subsequent decomposition to N2 and H2O (i.e. the Eley–Rideal mechanism), or (2) adsorption of NO on the adjacent sites of adsorbed NH3, followed by reaction to an activated transition state and decomposition to the reaction products (the Langmuir–Hinshelwood mechanism) [21], [25].
Over the past few years, Mn–Fe spinel (Fe3−xMnx)1−δO4 nanoparticles have attracted considerable attention due to their reportedly improved performance as catalysts in Fischer–Tropsh synthesis [30]. Our previous research has demonstrated that (Fe3−xMnx)1−δO4 was an excellent magnetic catalyst/sorbent for the oxidization and capture of elemental mercury from the flue gas [31].
Herein, nanosized (Fe3−xMnx)1−δO4 was developed as a super catalyst for the low temperature SCR of NOx with NH3. (Fe3−xMnx)1−δO4 was synthesized using a co-precipitation method and then characterized using X-ray diffraction (XRD), NH3 temperature programmed desorption (NH3-TPD), N2 adsorption/desorption isotherm and X-ray photoelectron spectroscopy (XPS). Subsequently, a fixed-bed reactor system was used to investigate the low temperature SCR performance of (Fe3−xMnx)1−δO4. Furthermore, the mechanism of the low temperature SCR reaction over (Fe3−xMnx)1−δO4 was investigated using in situ DRIFTS study and kinetic analysis.
Section snippets
Catalyst preparation
Nanosized Fe3−xMnxO4, the precursor of (Fe3−xMnx)1−δO4 was prepared using a co-precipitation method [31], [32], [33]. Suitable amounts of ferrous sulfate, ferric chloride, and manganese sulfate were dissolved in distilled water (total cation concentration = 0.30 mol L−1). Then, the mixture was added to an ammonia solution, leading to an instantaneous precipitation of manganese ferrite. During the reaction, the system was continuously stirred at 800 rpm. The particles were then separated by
XRD
The characteristic reflections of synthesized catalysts (shown in Fig. 1) correspond very well to the standard card of maghemite (JCPDS: 39-1346). Additional reflections that would indicate the presence of other crystalline manganese oxides, such as Mn3O4, Mn2O3 or MnO2, were not present in the diffraction scan. Because the radiuses of Mn2+ (0.80 Å) and Mn3+ (0.66 Å) are bigger than those of Fe2+ (0.74 Å) and Fe3+ (0.64 Å) respectively, the lattice parameter of synthesized Fe2.5Mn0.5O4 (0.8446 nm)
Conclusion
Mn–Fe spinel, a low temperature SCR catalyst, was synthesized using a co-precipitation method. The adsorption of NO + O2 or/and the adsorption of NH3 on (Fe3−xMnx)1−δO4 were obviously promoted due to the incorporation of Mn into γ-Fe2O3. As a result, the SCR activity of (Fe3−xMnx)1−δO4 increased with the increase of Mn content. (Fe2.5Mn0.5)1−δO4 showed excellent activity and selectivity at 80–160 °C. The SCR reaction over (Fe2.5Mn0.5)1−δO4 mainly followed the Eley–Rideal mechanism. N2O formed on
Acknowledgments
This study was financially supported by the National Natural Science Fund of China (grant no. 51078203), the National High-Tech Research and Development (863) Program of China (grant nos. 2010AA065002 and 2009AA06Z301) and the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of China.
References (45)
- et al.
Appl. Catal. B: Environ.
(2009) - et al.
Appl. Catal. B: Environ.
(2009) - et al.
Appl. Catal. B: Environ.
(2009) - et al.
Appl. Catal. B: Environ.
(2006) - et al.
Appl. Catal. B: Environ.
(2009) - et al.
Appl. Catal. B: Environ.
(2008) - et al.
Appl. Catal. B: Environ.
(2010) - et al.
Appl. Catal. B: Environ.
(2009) - et al.
Appl. Catal. B: Environ.
(2006) - et al.
J. Catal.
(1997)
Appl. Catal. B: Environ.
J. Catal.
Appl. Catal. B: Environ.
Appl. Catal. B: Environ.
Appl. Catal. B: Environ.
Appl. Catal. B: Environ.
Mater. Res. Bull.
Appl. Catal. B: Environ.
J. Hazard. Mater.
Appl. Catal. B: Environ.
Appl. Catal. B: Environ.
J. Catal.
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