Sm-MnO_x catalysts for low-temperature selective catalytic reduction of NO_x with NH₃: Effect of precipitation agent

Highlights

• Sm_0.1Mn catalysts were synthesized with different precipitants and tested in NH₃-SCR.

• Precipitants greatly affected the surface acidity and redox capacity of the catalysts.

• H₂O and SO₂ resistance of the catalysts were significantly influenced.

• Sm_0.1Mn catalysts followed both Eley-Rideal and Langmuir–Hinshelwood mechanisms.

Abstract

A series of Sm–Mn mixed oxide catalysts were prepared via precipitation using various precipitants, namely Na₂CO₃ (NH₄)₂CO₃, and NH₃·H₂O, and evaluated for the selective catalytic reduction (SCR) of NO_x with NH₃ at low temperatures. Various characterisation techniques were used to determine the physicochemical properties of the catalysts, and it is found that their catalytic performance is greatly influenced by the nature of the precipitation agent used. It is found that Sm_0.1Mn–Na₂CO₃ and Sm_0.1Mn-(NH₄)₂CO₃ exhibit superior catalytic performance in the SCR reaction to that of Sm_0.1Mn–NH₃·H₂O due to an abundance of surface acid sites, high surface concentration of Mn⁴⁺, and high NO oxidation capacity. From in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) analysis, we conclude that the Sm–Mn catalysts follow both Eley-Rideal and Langmuir–Hinshelwood mechanisms, and that the Eley-Rideal mechanism is dominant at elevated temperatures.

Introduction

Nitrogen oxides (NO, NO₂, and N₂O) emitted from various industrial processes and diesel engines are major air pollutants that contribute to environmental issues, including acid rain, global warming, ozone depletion, and photochemical smog.1, 2, 3, 4 The selective catalytic reduction (SCR) of NO_x with NH₃ is the most reliable approach for controlling NO_x emissions and has thus been commercialised and widely implemented.^5,6

Vanadium-based SCR catalysts have been in commercial use since 1970s. However, their use presents several disadvantages, such as the biological toxicity of V₂O₅, generation of large amounts of N₂O, and a narrow operating temperature window (300–400 °C).⁷ Therefore, the development of a novel environmentally benign non-vanadium-based catalyst is crucial. Recently, it has been widely reported that Mn-based catalysts exhibit high SCR activity owing to their excellent redox capabilities and variable valence states. Kapteijn et al. compared the activity of various MnO_x catalysts and established that the activity per unit surface area was the highest for MnO₂.⁸ Therefore, we can conclude that SCR activity is correlated with the phase composition of the manganese species. For Mn-based mixed metal oxide catalysts, the preparation method is a key factor in determining the resultant phase of the Mn species. Chao et al. evaluated Mn–Ce/TiO₂ catalysts prepared by sol–gel, citric acid complexation, and co-precipitation methods.⁹ The results indicated that the Mn–Ce/TiO₂ catalyst prepared by the co-precipitation method exhibited the highest low-temperature SCR activity because this method led to the highest Mn⁴⁺ concentration. Liu et al. reported a series of Mn–Ti catalysts synthesised by the sol–gel method, and found that the synthesis conditions affected the physicochemical properties of the Mn–Ce/TiO₂ catalysts, which in turn influenced their SCR activity.¹⁰ However, the effect of precipitation agents on Mn-based catalysts has not been reported thus far. In our previous study, we found that Sm–Mn catalysts exhibited outstanding NH₃-SCR catalytic performance in the range of 50–300 °C.¹¹ Therefore, we selected the Sm–Mn mixed oxide as the standard sample to investigate the influence of precipitation agents on the catalyst crystalline size, morphology, reducibility, and surface oxygen species.

Herein, three different precipitants were selected for the preparation of Sm-doped MnO_x catalysts, resulting in a series of Sm-MnO_x mixed oxide catalysts after calcination. The physicochemical properties of the Sm-MnO_x catalysts were systematically evaluated by various characterization techniques, such as powder X-ray diffraction, scanning electron microscopy (SEM), N₂ adsorption–desorption analysis, X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD) of NH₃/NO_x, Raman spectroscopy, temperature-programmed H₂ reduction (H₂-TPR), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). It was established that the precipitants influenced the physicochemical properties of the catalysts and in turn their catalytic performance. The Sm_0.1Mn–Na₂CO₃ catalyst displayed the best NH₃-SCR performance at 50–350 °C.