Article pubs.acs.org/IECR
Dual Effect of Sulfation on the Selective Catalytic Reduction of NO with NH3 over MnOx/TiO2: Key Factor of NH3 Distribution Shijian Yang,*,† Feihong Qi,† Yong Liao,† Shangchao Xiong,† Yi Lan,† Yuwu Fu,† Wenpo Shan,† and Junhua Li*,‡ †
School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing, 210094 P. R. China State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), School of Environment, Tsinghua University, Beijing, 100084 P. R. China
‡
ABSTRACT: The sulfation showed a dual effect on the selective catalytic reduction (SCR) reaction over MnOx/TiO2. The activation of adsorbed NH3 and the adsorption of NOx on MnOx/TiO2 were both restrained after the sulfation. Therefore, the SCR reaction through the Eley−Rideal mechanism and that through the Langmuir−Hinshelwood mechanism were both restrained, resulting in an obvious decrease of NO conversion below 300 °C. During NO reduction over MnOx/TiO2 at higher temperatures, there was a competition among the SCR reaction, the nonselective catalytic reduction (NSCR) reaction and the catalytic oxidation of NH3 to NO. The NSCR reaction and the catalytic oxidation of NH3 to NO over MnOx/TiO2 at higher temperatures were both restrained after the sulfation, so more NH3 adsorbed on sulfated MnOx/TiO2 was used to reduce NO to N2. Therefore, the SCR activity and N2 selectivity of sulfated MnOx/TiO2 at higher temperatures were much higher than those of MnOx/TiO2.
1. INTRODUCTION The emission of nitrogen oxides (NO and NO2) from mobile and stationary sources is a serious concern because of the contribution of these oxides to the formation of smog, acid rain, and ozone.1 So far, the most efficient technique to control NOx emissions from coal-fired power plants and automobiles is the selective catalytic reduction (SCR) of NO with NH3 using V2O5−WO3/TiO2 as the catalyst.2 V2O5−WO3/TiO2 is generally located upstream of the electrostatic precipitator because its temperature window is 300−400 °C. However, retrofitting the SCR devices in many existing power plants is extremely difficult due to the limitation of the space before the electrostatic precipitator.3 Therefore, the low temperature SCR catalyst, which can be placed downstream of the electrostatic precipitator and desulfurizer, has been in strong demand.4 Mn- based catalysts, such as MnO2/TiO2,5−9 MnOx− CeO2,1,10 MnOx−CeO2/TiO211 and Fe2O3−MnO2/TiO2,12 show an excellent low temperature SCR activity among the first row transition metal-based catalysts.13−17 However, Mnbased catalysts are currently extremely restricted in their applications for at least two reasons: the formation of N2O and the deactivation of SO2.18,19 N2O is now considered a pollutant because of its contribution to the greenhouse effect and the depletion of the ozone layer.20−23 The deactivation of SO2 on the low temperature SCR reaction was generally attributed to the deposition of NH4HSO4 and/or (NH4)2SO4.24 However, the presence of SO2 would cause to the sulfation of the catalyst,25−27 which may contribute to the deactivation. Therefore, the effect of sulfation on the SCR reaction over MnOx/TiO2 (including the SCR activity and N2 selectivity) should be studied in detail. The results show that there was a competition over MnOx/TiO2 among the SCR reaction, the nonselective catalytic reduction (NSCR) reaction, and the catalytic oxidation of NH3 to NO, and the SCR activity and N2 © 2014 American Chemical Society
selectivity mainly depended on the activation of adsorbed NH3 and NH3 distribution.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. MnOx/TiO2 (Mn loading was 5 wt %) was prepared by the impregnation method using Degussa TiO2 P25 as support and manganese nitrate as precursor.28 The sample was dried at 110 °C for 12 h, and it was then calcined at 500 °C under air atmosphere for 3 h. Sulfated MnOx/TiO2 was obtained by pretreating MnOx/TiO2 in a flow of 400 ppm of SO2 and 2% O2 (200 mL min−1) at 300 °C for 8 h.25 2.2. Catalytic Test. NO reduction and the catalytic oxidation of NH3 were both performed on a fixed-bed quartz tube reactor with the internal diameter of 6 mm.28 The catalyst with 40−60 mesh was placed on the quartz wool held in the reactor, and the reactor was heated by a vertical electrical furnace. The total flow rate was 200 mL min−1 (room temperature), the mass of catalyst was 200 mg, and the corresponding gas hourly space velocity (GHSV) was 6 × 104 cm3 g−1 h−1 (i.e., 75000 h−1). The feed contained 500 ppm of NO (when used), 500 ppm of NH3, 2% of O2, and balance of N2. The concentrations of NH3, NO, NO2, and N2O in the outlet were continually monitored by a Fourier transform infrared spectrometer (FTIR, MKS Instruments). The ratios of NH3 conversion, NOx conversion, and the amount of N2 formed were calculated as follows:25 Received: Revised: Accepted: Published: 5810
January 10, 2014 March 11, 2014 March 14, 2014 March 14, 2014 dx.doi.org/10.1021/ie5001357 | Ind. Eng. Chem. Res. 2014, 53, 5810−5819
Industrial & Engineering Chemistry Research
Article
NH3 conversion =
[NH3]in − [NH3]out [NH3]in
(1)
NOx conversion =
[NOx ]in − [NOx ]out [NOx ]in
(2)
N2 formation [NH3]in − [NH3]out + [NOx ]in − [NOx ]out − 2[N2O]out = 2 (3)
2.3. Characterization. XRD patterns were recorded on an X-ray diffractionmeter (Rigaku, D/max-2200/PC) between 20° and 80° at a step of 7° min−1 operating at 30 kV and 30 mA using Cu Kα radiation. BET surface area was determined using a nitrogen adsorption apparatus (Quantachrome, Autosorb-1). H2-temperature programmed reduction (H2-TPR) was recorded on a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx). Before the experiment, approximately 0.050 g of catalyst was pretreated under He atmosphere at 300 °C for 60 min. After the catalyst was cooled to room temperature, H2TPR was performed at a rate of 10 °C min−1 to 800 °C under a 10%H2/He gas flow (50 cm3 min−1). Temperature programmed desorption of ammonia (NH3-TPD) and temperature programmed desorption of NO (NO-TPD) were carried out on the packed-bed microreactor. Before the experiment, approximately 0.100 g of catalyst was pretreated under N2 atmosphere (200 mL min−1) at 300 °C for 60 min. After the catalyst was cooled to 50 °C, the N2 flow was switched to a flow of 500 ppm of NH3/N2 or 500 ppm of NO/N2 +2% of O2 (200 mL min−1) for 60 min. The sample was then purged by N2 (200 mL min−1) for another 60 min. At last, NH3-TPD or NOTPD was performed at a heating rate of 10 °C min−1 to 600 °C under a N2 gas flow with 200 mL min−1. Mn 2p, S 2p, Ti 2p, and O 1s binding energies were recorded on an X-ray photoelectron spectroscopy (Thermo, ESCALAB 250) with Al Kα (hv = 1486.6 eV) as the excitation source and C 1s line at 284.8 eV as the reference for the binding energy calibration. In situ DRIFT spectra were performed on another FTIR spectrometer (Nicolet NEXUS 870) equipped with a liquidnitrogen-cooled MCT detector, collecting 100 scans with a resolution of 4 cm−1. Prior to each experiment, the catalyst (
