Effect of manganese substitution on the structure and activity of iron titanate catalyst for the selective catalytic reduction of NO with NH3

doi:10.1016/j.apcatb.2009.09.029

Applied Catalysis B: Environmental

Volume 93, Issues 1–2, 25 November 2009, Pages 194-204

https://doi.org/10.1016/j.apcatb.2009.09.029 Get rights and content

Abstract

Selective catalytic reduction (SCR) of NO with NH₃ over manganese substituted iron titanate catalysts was fully studied. The low temperature SCR activity was greatly enhanced when partial Fe was substituted by Mn, although the N₂ selectivity showed some decrease to a certain extent. The Mn substitution amounts showed obvious influence on the catalyst structure, redox behavior and NH₃/NO_x adsorption ability of the catalysts. Among Fe_aMn_1−aTiO_x (a = 1, 0.75, 0.5, 0.2, 0) serial catalysts, Fe_0.5Mn_0.5TiO_x with the molar ratio of Fe:Mn = 1:1 showed the highest SCR activity, because the interaction of iron, manganese and titanium species in this catalyst led to the largest surface area and the highest porosity, the severest structural distortion and most appropriate structural disorder, the enhanced oxidative ability of manganese species, the highest mobility of lattice oxygen, the proper ratio of Brønsted acid sites and Lewis acid sites together with the enhanced NO_x adsorption capacity.

Introduction

Selective catalytic reduction (SCR) of NO with NH₃ is one of the most efficient and economic technologies for the removal of nitrogen oxides (NO_x) from stationary and mobile sources, and the most widely used catalyst system is V₂O₅–WO₃ (MoO₃)/TiO₂ [1]. Because of some inevitable disadvantages in practical application, such as the narrow operation temperature window [2], high conversion of SO₂ to SO₃ at high temperatures [3] and the toxicity of vanadium pentoxide to environment and human health [4], more and more researchers are focusing on the development of new SCR catalysts. In our previous study [5], [6], we have developed an environmentally friendly novel iron titanate catalyst in crystallite phase with specific Fe–O–Ti structure, which showed excellent SCR activity, N₂ selectivity and H₂O/SO₂ durability in the medium temperature range. However, the catalytic activity was not high enough for the application in denitrogenation of exhaust gas with low temperature, such as the flue gas after dust removal and desulfurization from coal-fired power plants and the exhaust gas from diesel engines in cold-start process. Therefore, it is very necessary to modify this iron titanate catalyst to improve the low temperature activity, which is crucial for the practical utilization.

Manganese oxides usually show good SCR activity in the low temperature range, such as pure MnO_x [7], [8], MnO_x loaded on TiO₂/Al₂O₃/SiO₂/AC (activated carbon) [9], [10], [11], [12] and Mn–Ce, Mn–Cu mixed oxides [13], [14], [15]. Previous studies showed that in Fe-containing SCR catalysts, the introduction of Mn could obviously enhance the low temperature activity [16], [17], probably due to the synergistic effect between iron and manganese species. It was reported that the introduction of lanthanide elements (such as La, Ce and Pr) and the third main group element In could also improve the activity, stability or SO₂ durability of the SCR catalysts using NH₃ or hydrocarbons as reducing agent [18], [19], [20], [21], [22]. Therefore, based on our iron titanate catalyst, we can also substitute partial Fe by other elements to adjust its physicochemical properties, expecting to enhance the low temperature SCR activity.

In this paper, five kinds of elements including La, Ce, Pr, In and Mn were introduced into the iron titanate catalyst, among which Mn showed the best promoting effect. Based on this result, we further investigated the influence of Mn substitution amounts on the catalyst structure and catalytic activity using various characterization methods. The structural properties were characterized using N₂ physisorption, powder X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) methods. Then, X-ray photoelectron spectra (XPS) and H₂-temperature programmed reduction (H₂-TPR) were conducted to evaluate the variation of redox properties during the substitution process. Finally, temperature programmed desorption of NH₃ and NO_x (NH₃-TPD and NO_x-TPD) together with in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) of NH₃ and NO_x adsorption was carried out to reveal the evolution of adsorption ability of reactants, which is important for the SCR reaction. The promoting mechanism of Mn on the low temperature SCR activity of iron titanate catalyst was proposed accordingly.

Section snippets

Catalyst synthesis and activity test

Fe_0.9M_0.1TiO_x (M = La, Ce, Pr, In, Mn) and Fe_aMn_1−aTiO_x with different Mn substitution amounts (a = 1, 0.75, 0.5, 0.2, 0) were prepared by co-precipitation method using Fe(NO₃)₃·9H₂O, Ti(SO₄)₂, relevant metal nitrates as precursors and NH₃·H₂O (25 wt%) as precipitator. The precipitate cake was filtrated and washed using distilled water, followed by desiccation at 100 °C for 12 h and calcination at 400 °C for 6 h in air condition. Pure oxides including Fe₂O₃, MnO_x and TiO₂ were prepared from Fe(NO₃)₃·9H₂

SCR activity of Fe_0.9M_0.1TiO_x catalysts (M = La, Ce, Pr, In, Mn)

Fig. 1 shows the NO_x conversion as a function of temperature in the NH₃-SCR reaction over Fe_0.9M_0.1TiO_x catalysts (M = La, Ce, Pr, In, Mn). From the results we can see that, the substitution of partial Fe with other elements could indeed influence the SCR activity of iron titanate catalyst. At temperatures below 250 °C, the Mn and Ce substitutions could obviously enhance the NO_x conversions, while the La, Pr and In substitutions decreased the NO_x conversions to a certain extent. Moreover, the Ce

Conclusions

The substitution of partial Fe by Mn could significantly promote the SCR activity of iron titanate catalyst, especially in the low temperature range. Fe_0.5Mn_0.5TiO_x with the molar ratio of Fe:Mn = 1:1 showed the best activity, over which NO_x was totally eliminated at 175 °C at GHSV = 50 000 h⁻¹. However, the N₂ selectivity showed an obvious decrease with the increasing of Mn substitution amounts, and there should be a compromise between the SCR activity and N₂ selectivity when we determine on the Mn

Acknowledgements

We sincerely appreciate the help from National Synchrotron Radiation Laboratory, University of Science and Technology of China for supplying the beam time to carry out XAFS experiments. This work was financially supported by Chinese Academy of Sciences (KZCX1-YW-06-04) and the National High Technology Research and Development Program of China (2006AA06A304, 2009AA06Z301).

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Effect of manganese substitution on the structure and activity of iron titanate catalyst for the selective catalytic reduction of NO with NH3

Abstract

Introduction

Section snippets

Catalyst synthesis and activity test

SCR activity of Fe0.9M0.1TiOx catalysts (M = La, Ce, Pr, In, Mn)

Conclusions

Acknowledgements

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Effect of manganese substitution on the structure and activity of iron titanate catalyst for the selective catalytic reduction of NO with NH₃

SCR activity of Fe_0.9M_0.1TiO_x catalysts (M = La, Ce, Pr, In, Mn)