Improvement of catalytic activity over Mn-modified CeZrO_x catalysts for the selective catalytic reduction of NO with NH₃

Abstract

A series of MnCeZrO_x mixed oxide catalysts were facilely synthesized using the impregnation-NH₃·H₂O coprecipitation method and tested for selective catalytic reduction (SCR) of NO with NH₃. Doping manganese significantly improved the catalytic activity and the best performing SCR catalyst, Mn_0.25Ce_0.5Zr_0.25O_x, was shown to achieve NO conversion > 80% in the temperature range (60–350 °C), with the denitration effect up to 50% at room temperature (conditions: [NO] = [NH₃] = 500 ppm, [O₂] = 5 vol%, He as balance, flow rate  =  100 mL/min, GHSV  =  40, 000 h⁻¹). Characterization of the catalyst using BET, XRD, XPS, H₂-TPR, and in-situ FTIR proved that the improved SCR activity may be attributed to the large surface area, great reduction ability and increased amount of surface adsorbed oxygen afforded by the introduction of manganese. The SCR reaction mechanisms were also investigated by analyzing in-situ FTIR spectra and the SCR reaction pathway over the Mn_0.25Ce_0.5Zr_0.25O_x catalyst was shown to mostly follow the E-R mechanism.

Introduction

NO_x emissions from the combustion of fossil fuels have long been recognized as one of the most threatening atmospheric pollutants globally [1], [2], [3]. A variety of technologies have been developed to reduce NO_x emissions from both stationary and mobile combustion facilities and devices, among which selective catalytic reduction (SCR) with NH₃ as a reducing agent has attracted great attention for its high NO_x conversion, low cost, and being environmentally friendly [4], [5], [6]. The main reactions in a typical SCR process include:

4NO + 4NH₃ + O₂ → 4 N₂ + 6H₂O

4NO + 2NH₃ + 2O₂ → 3 N₂ + 6H₂O

2NO₂ + 4NH₃ + O₂ → 3 N₂ + 6H₂O

NO + NO₂ + 2NH₃ + 2O₂ → 2 N₂ + 3H₂O

The commonly adopted commercial denitration catalyst is the vanadium based catalyst (V₂O₅-MoO₃ (WO₃)/TiO₂), which has many shortcomings, such as high reaction temperature (300–400 °C), narrow operating temperature window and biological toxicity [7], [8], [9]. In recent years, V-free composite metal oxide catalysts have attracted increased interests by both academia and industry researchers for their high rates of NO_x conversion, and broadened operating temperature ranges [10], [11]. In particular, Ce-Zr mixed metal oxide catalysts have been extensively studied for their excellent SCR performance and low cost. Li et al. [12] synthesized a WO₃-CeO₂-ZrO₂ catalyst for selective catalytic reduction (SCR) of NO with NH₃, reaching complete conversion at 250–500 °C and the superb catalytic performance was attributed to the superior redox properties of CeO₂-ZrO₂, and the presence of highly dispersed or amorphous WO_x species. Masaaki et al. [13] loaded rhodium into a ceria-zirconia solid solution by incipient wetness impregnation for SCR by unburned hydrocarbons in the presence of O₂. However, there are still some deficiencies in the Ce-Zr catalysts, such as poor SCR performance at low temperatures and still narrow operating temperature window. Manganese oxides (MnO_x) involve several kinds of labile oxygen which is favorable to the redox cycle. Therefore, Mn-based catalysts are well-known for its distinguished SCR effect at low temperatures and may be an effective and economical choice as SCR catalysts [14].

Based on the hypothesis of the combined advantages of Mn-based and Ce-Zr mixed metal oxides catalysts, we synthesized a series of MnCeZrO_x catalysts and tested for NH₃-SCR in the present work. Specific attention was paid to the effect of Mn/Ce molar ratio on the SCR performance. The physical and chemical properties of the Mn-doping catalysts were systematically characterized using Brunauer-Emmertt-Teller (BET), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), H₂-Temperature Program (H₂-TPR) and in situ Fourier transform infrared spectroscopy (FTIR). The reaction pathway over the Mn_0.25Ce_0.5Zr_0.25O_x catalyst was also proposed based on the intermediate species observed in the SCR process.