First principles and experimental study of NH3 adsorptions on MnOx surface

doi:10.1016/j.apsusc.2013.08.039

Applied Surface Science

Volume 285, Part B, 15 November 2013, Pages 215-219

https://doi.org/10.1016/j.apsusc.2013.08.039 Get rights and content

Highlights

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The E_ads values of Mn₂O₃–NH₃ and Mn₃O₄–NH₃ were more negative.
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The shorter MnN bonds were obtained on Mn₂O₃ (2 2 2) and Mn₃O₄ (2 1 1) surfaces.
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According to the DFT study, Mn₂O₃ (2 2 2) and Mn₃O₄ (2 1 1) surfaces were active for NH₃ adsorption.
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Mn₂O₃ and Mn₃O₄ showed better NH₃ adsorption capability and catalytic activity.

Abstract

The oxidative abstraction of hydrogen from adsorbed ammonia is the first step in selective catalytic reduction (SCR) and NH₃ adsorptions on the MnO_x surfaces played a significant role in the mechanism of SCR with NH₃. NH₃ adsorptions on the Mn₂O₃ (2 2 2), Mn₃O₄ (2 1 1) and MnO₂ (1 1 0) surfaces were investigated by the density functional theory (DFT) method. With more negative adsorption energy values, the Mn₂O₃ (2 2 2) and Mn₃O₄ (2 1 1) surfaces tended to be the favorable adsorption sites for NH₃ molecule. In addition, the shorter Mn single bond N bonds indicated that NH₃ adsorptions took place easily on Mn₂O₃ (2 2 2) and Mn₃O₄ (2 1 1) surfaces. According to the ammonia temperature programmed desorption (NH₃-TPD) performance, Mn₂O₃ and Mn₃O₄ showed significantly higher amount of desorbed NH₃, which was in good agreement with the DFT study. Meanwhile, the NH₃-SCR performances of Mn₂O₃ and Mn₃O₄ for NO conversion below 433 K show higher performance than that of MnO₂.

Graphical abstract

The shorter Mn single bond N bonds and the more negative E_ads values indicated that NH₃ adsorptions took place easily on Mn₂O₃ (2 2 2) and Mn₃O₄ (2 1 1) surfaces, which was in line with the NH₃-TPD and NH₃-SCR performances.

Introduction

The difficulty of nitrogen oxide removal has become a serious concern [1], [2] and selective catalytic reduction (SCR) with NH₃ is widely applied to remove NO_x [3], [4]. Moreover, MnO_x on different supports have excellent catalytic activities in the low-temperature SCR reaction for the removal of NO_x from stationary sources [5], [6], [7], [8]. Recently, the MnO_x and CeO_x supported on carbon nanotubes displayed better NH₃-SCR activity, higher SO₂-tolerance and improved water-resistance. On the other hand, the catalyst obtained from the pyridine-thermal route presents excellent SO₂ resistance, better NH₃-SCR activity as well as favorable stability [9], [10]. In addition, the MnO_x contains various types of labile oxygen to complete a catalytic cycle [11]. The activity and selectivity of pure manganese oxides in SCR at significantly low temperatures have been studied by many researchers [12], [13], [14]. However, doping metals can enhance NO conversion and N₂ selectivity of the manganese oxide-based catalysts in low-temperature SCR with NH₃. With a broad operation temperature window, NiMnFe mixed oxides show enhanced performance for the SCR of NO with NH₃ [15]. MnO_x/ZrO₂–CeO₂ nanorods show the excellent catalytic activity that should be attributed to the adsorbed surface oxygen and oxygen vacancies [16]. It was widely accepted that the first step in SCR was an oxidative abstraction of hydrogen from adsorbed ammonia [4], [13]. Therefore, NH₃ adsorptions played a significant role in the mechanism of SCR with NH₃ [13], [17], [18], [19], [20]. NH₃ was adsorbed on both Bronsted and Lewis acid sites and amide species were present at the surface, mainly on Mn sites [19], [21]. Most performance and mechanism studies concentrated on the SCR reaction process [5], [7], [18], while little attention was paid to NH₃ adsorptions on the MnO_x surfaces. In fact, the surface reactivity of the metal oxide was closely connected to the presence of adsorption capacity. The adsorption of gas-phase reagent at surfaces played an important part in many types of heterogeneous catalysis [22]. Consequently the adsorptions of NH₃ molecules on metallic oxides surfaces were studied by DFT method in previous researches [23], [24], [25], [26]. Based on the combination of experimental and theoretical investigations, Maitarad found the Mn@CeO₂(1 1 0) model in the DFT analysis showed a prominent effect on the NO and NH₃ adsorption and the catalysts exhibited a better catalytic performance [27].

In this study, MnO_x catalysts were characterized by X-ray diffraction (XRD) and ammonia temperature programmed desorption (NH₃-TPD). NH₃ adsorptions on the Mn₂O₃ (2 2 2), Mn₃O₄ (2 1 1) and MnO₂ (1 1 0) surfaces were investigated using a DFT method, systematically. By the means of the NH₃-TPD and SCR performance below 433 K, Mn₂O₃ and Mn₃O₄ showed significantly higher amount of adsorption of NH₃ and NO conversion, except MnO₂. At the same time, the adsorptions of NH₃ molecules on metallic oxides surfaces were studied by DFT method using the generalized gradient approximation (GGA).

Section snippets

Surface methods

The calculations in this work are performed using the program package CASTEP, which is a DFT code, working in a plane wave basis set. The electron-ion interaction is described using the ultrasoft Vanderbilt-type pseudopotentials, with a plane wave energy cutoff of 340 eV. We used the GGA of PW91 for the exchange-correlation energy. The subatrate is modeled by six layers of MnO_x separated by a vacuum layer of 10 Å. The two uppermost substrate layers and the NH₃ molecule are allowed to relax. This

XRD patterns

XRD patterns of pure MnO_x were shown in Fig. 2, indicating that the peaks attributed to oxidized species of manganese. The diffractions of Mn₂O₃ [JCPDS No. 89-4836, Cubic, space group: Ia-3 (206)], Mn₃O₄ [JCPDS No. 75-1560, Tetragonal, space group: I41/amd (141)] and MnO₂ [JCPDS No. 24-735, Tetragonal, space group: P42/mnm (136)] were observed. There are some published works on the Mn₂O₃ (2 2 2), Mn₃O₄ (2 1 1) and MnO₂ (1 1 0) surfaces [28], [29], [30], but few of them are on MnO_x/NH₃ system. Clear

Conclusion

Density functional theory calculations have been carried out to investigate the NH₃ adsorption on Mn₂O₃ (2 2 2), Mn₃O₄ (2 1 1) and MnO₂ (1 1 0) surfaces. The optimized geometries and adsorption energies showed that Mn₂O₃ (2 2 2) and Mn₃O₄ (2 1 1) surfaces were active for NH₃ adsorption. However, NH₃ adsorption carried out difficultly on the MnO₂ (1 1 0) surface. The calculations also suggested that the more negative E_ads values of Mn₂O₃–NH₃ and Mn₃O₄–NH₃ were −1.31 eV and −1.44 eV, which led to the easiness

Acknowledgements

This work was financially supported by the National “Twelfth Five-Year” Plan for Science & Technology Support of China (2011BAE29B02). And the tests of XRD were supported by Research and Test Center of Materials, Wuhan University of Technology. TPD tests were supported by State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology.

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First principles and experimental study of NH3 adsorptions on MnOx surface

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Surface methods

XRD patterns

Conclusion

Acknowledgements

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