Low temperature selective catalytic reduction of NO with NH₃ over Mn–Fe spinel: Performance, mechanism and kinetic study

Abstract

(Fe_3−xMn_x)_1−δO₄ was synthesized using a co-precipitation method and then developed as a catalyst for the low temperature selective catalytic reduction (SCR) of NO with NH₃. The SCR activity of (Fe_3−xMn_x)_1−δO₄ was clearly enhanced with the increase of Mn content. The results of in situ DRIFTS study demonstrated that both the Eley–Rideal mechanism (i.e. reaction of activated ammonia with gaseous NO) and the Langmuir–Hinshelwood mechanism (i.e. reaction of adsorbed ammonia species with adsorbed NO_x species) might happen during the SCR reaction over (Fe_3−xMn_x)_1−δO₄. According to the kinetic analysis, the respective contribution of the Langmuir–Hinshelwood mechanism and the Eley–Rideal mechanism on the SCR reaction was studied. Only the adsorption of NO + O₂ on (Fe_2.8Mn_0.2)_1−δO₄ was promoted, so the Langmuir–Hinshelwood mechanism predominated over NO conversion on (Fe_2.8Mn_0.2)_1−δO₄ especially at lower temperatures. Both the adsorption of NO + O₂ and the adsorption of NH₃ on (Fe_2.5Mn_0.5)_1−δO₄ were obviously promoted, so NO conversion on (Fe_2.5Mn_0.5)_1−δO₄ mainly followed the Eley–Rideal mechanism especially at higher temperatures. Both the nitrate route and the over-oxidization of adsorbed ammonia species contributed to the formation of N₂O on (Fe_2.8Mn_0.2)_1−δO₄ above 140 °C. However, the formation of N₂O on (Fe_2.5Mn_0.5)_1−δO₄ mainly resulted from the over-oxidization of adsorbed ammonia species. Although the activity of (Fe_2.5Mn_0.5)_1−δO₄ was suppressed in the presence of H₂O and SO₂, the deactivated catalyst can be regenerated after the 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 NO_x 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 NO_x 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 (Fe_3−xMn_x)_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 (Fe_3−xMn_x)_1−δO₄ was an excellent magnetic catalyst/sorbent for the oxidization and capture of elemental mercury from the flue gas [31].

Herein, nanosized (Fe_3−xMn_x)_1−δO₄ was developed as a super catalyst for the low temperature SCR of NO_x with NH₃. (Fe_3−xMn_x)_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 (Fe_3−xMn_x)_1−δO₄. Furthermore, the mechanism of the low temperature SCR reaction over (Fe_3−xMn_x)_1−δO₄ was investigated using in situ DRIFTS study and kinetic analysis.