Article pubs.acs.org/IECR
Selective Catalytic Reduction of NOx by NH3 over Mn-Promoted V2O5/ TiO2 Catalyst Zhiming Liu,*,† Yuan Li,† Tianle Zhu,*,‡ Hang Su,† and Junzhi Zhu† †
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China School of Chemistry and Environment, Beihang University, Beijing 100191, China
‡
S Supporting Information *
ABSTRACT: The effect of Mn on the catalytic performance of V2O5/TiO2 catalyst for the selective catalytic reduction of NOx by NH3 (NH3-SCR) has been investigated in this study. It was found that the added Mn significantly enhanced the activity of V2O5/TiO2 catalyst for NH3-SCR below 400 °C. The redox cycle (V4+ + Mn4+ ↔ V5+ + Mn3+) over Mn-promoted V2O5/TiO2 catalyst plays a key role for the high catalytic deNOx performance. The redox cycle promotes the adsorption and activation of NH3 and NO, forming more reactive intermediates (NH4+, coordinated NH3, NO2, and monodentate nitrate species), thus promoting the NH3-SCR to proceed.
1. INTRODUCTION Nitrogen oxides (NOx) emitted from mobile and stationary sources are major air pollutants, as they cause a variety of environmentally harmful effects such as photochemical smog, acid rain, and ozone depletion.1,2 Selective catalytic reduction of NOx with NH3 (NH3-SCR) is well-established and used to control NOx in flue gas from stationary sources, and the most widely used catalyst system for this process is V2O5/TiO2-based catalyst.1,3 Since vanadia is active not only in the reduction of NOx but also in the undesired oxidation of SO2 to SO3, its content is generally kept low.3 The activity of V2O5/TiO2 catalyst is closely related to the loading of vanadia.4 In the case of low vanadia loading, the low-temperature activity of V2O5/ TiO2 needs to be improved. WO3 is employed in larger amounts (about 10 wt %) to increase the activity of V2O5/TiO2 catalyst.5 Recently, the price of tungsten has risen about 30 times compared to 10 years ago. SCR catalyst producers began to develop substitutes for tungsten. Park et al.6 found that the addition of MoO3 to V2O5/Al2O3 catalyst led to an enhancement of the activity. The additions of WO3 and MoO3 to V2O5/TiO2 are similar, and both oxides act as “chemical” promoters besides playing a “structural” function as well.7 Previous research showed that Mn-based catalysts are active for the NH3-SCR of NOx in the low-temperature range.8−10 Yang et al.11,12 reported that MnOx−CeO2 mixed oxide was a superior catalyst for the low-temperature NH3-SCR reaction. The introduction of Mn to Fe-containing catalyst could obviously enhance the lowtemperature activity,13 probably due to the synergetic effect between iron and manganese species. Considering that the reducibility of the vanadia is closely related to the NH3-SCR performance14 and manganese oxides contain various valence states, which is important to complete the catalytic cycle in SCR,15 the present work attempts to improve the activity of V2O5/TiO2 catalyst by adding manganese oxide. It was found that the addition of Mn exhibited a noticeable promoting effect on the activity of V2O5/ TiO2 for the NH3-SCR. On the basis of the characterization © 2014 American Chemical Society
results, the cause of the promoting effect of Mn has been elucidated.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The catalysts were prepared by the impregnation method, and Degussa AEROSIL TiO2 P25 was used as the support. MnOx/TiO2 catalysts with Mn loading varying from 1 to 10 wt % (MnxTi) were prepared by impregnating TiO2 with a proper amount of the manganese nitrate solution; then it was for 4 h followed by drying at 120 °C. Subsequently the prepared MnOx/TiO2 catalyst was prepared by impregnating a proper amount of ammonium metavanadate; then it was stirred for 4 h, subsequently dried at 120 °C, and calcined at 500 °C for 4 h in air. The loading of V2O5 is fixed at 1 wt %. Hereinafter these catalysts are designated by V1MnxTi. For comparison, the 1 wt %V2O5/ TiO2 (V1Ti) and 1 wt %V2O5−2 wt %WO3/TiO2 (V1W2Ti) catalysts were also prepared by the same preparation method as described above using NH 4 VO 3 or NH 4 VO 3 and (NH4)10W12O41 as precursors (H2C2O4·2H2O was used to facilitate the solution of the precursors), respectively. 2.2. Catalytic Activity Measurement. The activity measurements were carried out in a fixed-bed quartz reactor using a 0.12 g catalyst of 40−60 mesh. The feed gas mixture contained 500 ppm NO, 500 ppm NH3, 5% O2, 0 or 5% H2O, 0 or 50 ppm SO2, and helium as the balance gas. Water vapor was generated by passing helium gas through a heated gas-wash bottle containing deionized water. The total flow rate of the feed gas was 300 cm3·min−1, corresponding to a gas hourly space velocity (GHSV) of 128 000 h−1. The reaction temperature was increased from 200 to 450 °C in a heating rate of 5 °C·min−1. The composition of the gas in the inlet and outlet streams was analyzed by a chemiluminescence NO/NO2 Received: Revised: Accepted: Published: 12964
May 8, 2014 July 25, 2014 July 28, 2014 July 28, 2014 dx.doi.org/10.1021/ie501887f | Ind. Eng. Chem. Res. 2014, 53, 12964−12970
Industrial & Engineering Chemistry Research
Article
analyzer (Thermo Scientific, Model 42i-HL) and gas chromatograph (Shimadzu GC 2014 equipped with Porapak Q and molecular sieve 5A columns). A molecular sieve 5A column was used for the analysis of N2, and a Porapak Q column was used for that of N2O. The activity data were collected when the catalytic reaction practically reached steady-state condition at each temperature. The oxidation of SO2 to SO3 over V1Ti and V1Mn2Ti catalysts was also evaluated. The reactant gas typically consisted of 500 ppm SO2, 5% O2, 5% H2O, and helium as the balance gas. The concentrations of SO2 and SO3 were detected by a Fourier transform infrared (FT-IR) gas analyzer (Gasmet Dx4000). The formation of SO3 was found negligible (
