Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Ultrahydrophobicity wikipedia , lookup
Low-energy electron diffraction wikipedia , lookup
Surface tension wikipedia , lookup
Sessile drop technique wikipedia , lookup
Nanofluidic circuitry wikipedia , lookup
Flux (metallurgy) wikipedia , lookup
Energy applications of nanotechnology wikipedia , lookup
Applied Catalysis A: General 327 (2007) 261–269 www.elsevier.com/locate/apcata Manganese oxide catalysts for NOx reduction with NH3 at low temperatures Min Kang a, Eun Duck Park b, Ji Man Kim c, Jae Eui Yie d,* b a R&D Center, EnD Solutions Co. Ltd., 223-235, Seoknam-Dong, Seo-Gu, Incheon 405-220, Republic of Korea Department of Chemical Engineering, Division of Chemical Engineering and Materials Engineering, Ajou University, Wonchun-Dong, Yeongtong-Gu, Suwon 443-749, Republic of Korea c Functional Materials Laboratory, Department of Chemistry and Sungkyunkwan Advanced Institute of Nano Technology, Sungkyunkwan University, Cheoncheon-Dong, Jangan-Gu, Suwon 440-746, Republic of Korea d Catalyst and Surface Laboratory, Department of Applied Chemistry, Ajou University, Wonchun-Dong, Yeongtong-Gu, Suwon 443-749, Republic of Korea Received 20 December 2006; received in revised form 15 May 2007; accepted 17 May 2007 Available online 21 May 2007 Abstract Manganese oxide catalysts prepared by a precipitation method with various precipitants were investigated for the low temperature selective catalytic reduction (SCR) of NOx with NH3 in the presence of excess O2. Various characterization methods such as N2 adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and X-ray absorption near edge structure (XANES) were conducted to probe the physical and chemical properties of MnOx catalysts. The active MnOx catalysts, precipitated with sodium carbonate and calcined in air at moderate temperatures such as 523 K and 623 K, have the high surface area, the abundant Mn4+ species, and the high concentration of surface oxygen on the surface. The amorphous Mn3O4 and Mn2O3 were mainly present in this active catalyst. The carbonate species appeared to help adsorb NH3 on the catalyst surface, which resulted in the high catalytic activity at low temperatures. # 2007 Elsevier B.V. All rights reserved. Keywords: NO reduction; Manganese oxides; NH3-SCR; Precipitation method; Calcinations 1. Introduction The emission control of nitrogen oxides (NO, NO2 and N2O) from various combustion processes has been a major environmental concern related to the air quality because nitrogen oxides have been reported as one of the most serious pollutants causing acid rain along with sulfur dioxide and play a major role in the photochemical chain reaction leading to the formation of photochemical smog. The primary nitrogen oxides (NOx) produced from combustion processes with fossil fuel are nitric oxide (NO) and nitrogen dioxide (NO2), which are 90– 95% of the total NOx emission from automotive exhausts and stationary sources such as thermoelectric power plant and incinerator [1]. * Corresponding author. Tel.: +82 31 219 2513; fax: +82 31 219 2394. E-mail address: [email protected] (J.E. Yie). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.05.024 The removal of nitrogen oxides from stationary or mobile sources has become an important issue and a variety of reduction methods to minimize the emission of NOx such as a combustion control and a post-combustion control technology have been developed. Among the proposed post-combustion methods for NOx removal, the technologies using the catalysts are known as one of most efficient methods in terms of relatively low cost and high efficiency. Especially, the selective catalytic reduction (SCR) of nitrogen oxide has been generally recognized as the most effective and widely commercialized removal technology of NOx emitted from the stationary sources [1–3]. In order to convert NO contained in the flue gas into N2, the reducing agent must be employed. NH3, CO, H2 and a variety of hydrocarbons such as methane, propylene, and ethane have been employed as reductants for NO removal reaction [4]. Although a number of reducing agents can be utilized in SCR, ammonia is the most effective and widely commercialized, which is called NH3-SCR, for stationary sources such as power 262 M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269 plants and nitric acid plants [5]. The selective catalytic reduction (SCR) is a process in which a reducing agent, e.g. NH3, reacts selectively with NOx to produce N2 without consuming an excess O2. In the past few decades, the backbone of SCR technology is the development of SCR catalysts such as noble metals [6], supported metal oxides [7], zeolites [8] and others [1,9,10]. Among them, vanadia supported on titania is known to be the most effective and widely used commercial SCR catalyst due to its high activity and durability to sulfur compounds. Because this catalyst exhibits high conversions only in the temperature range of 573–673 K, the SCR should be applied before units for particle removal and desulphurization where the gas temperature decreases [11]. However, when the flue gas has high concentrations of particles and other contaminants which are deleterious for the catalyst, proper units should be located at the upstream of the catalyst bed to resolve above problems, which causes the decrease of the exit gas temperature. Therefore, there is a great interest in the development of SCR catalysts active at low temperatures (<573 K). A number of catalysts consisted of various transition metal (V, Cr, Mn, Fe, Co, Ni and Cu) oxides on different commercial supports such as silica and alumina have been studied. Among these catalysts, manganese oxides such as MnOx/Al2O3 [12], MnOx/NaY [13], MnOx/USY [14] and MnOx/TiO2 [15,16] have attracted much interests due to their high catalytic activities. These catalysts were prepared by the solution impregnation method on supports using manganese nitrate or acetate. In case of unsupported metal oxides, only limited works have been reported because unsupported MnOx catalysts suffer from very low surface areas [17]. Some additives such as citric acid were added in the preparation step to increase the surface area as well as the catalytic activity [18]. Recently, we found that MnOx catalysts prepared by a simple precipitation method with sodium carbonate showed the high catalytic activity for low temperature NH3-SCR [19]. This catalyst also appeared to be stable in the presence of excess water vapor. In the present study, we investigated the effect of preparation methods including kinds of precipitants and calcinations temperatures of the MnOx catalysts on their structural features and catalytic performance for selective catalytic reduction of NOx with ammonia. 2. Experimental 2.1. The preparation of catalysts The various kinds of manganese oxides were prepared by a precipitation method with different precipitants such as ammonium carbonate (AC), potassium carbonate (PC), sodium carbonate (SC), ammonium hydroxide (AH), potassium hydroxide (PH) and sodium hydroxide (SH). Each catalyst is denoted as MnOx-AC, MnOx-PC, MnOx-SC, MnOx-AH, MnOx-PH and MnOx-SH, respectively. 0.5 M ammonium carbonate ((NH4)2CO3, DAEJUNG, 99.5%) aqueous solution, 0.5 M potassium carbonate (K2CO3, DAEJUNG, 99.5%) aqueous solution, 0.5 M sodium carbonate (Na2CO3, SHINYO, 99.5%) aqueous solution, ammonium hydroxide (NH4OH, DAEJUNG, 25.0–28.0%) solution, potassium hydroxide (KOH, DAEJUNG, 99%) or sodium hydroxide (NaOH, SAMCHUN, 99%) was continuously added to 500 ml of 0.5 M manganese nitrate (Mn(NO3)2xH2O, Aldrich, 98.0+%) aqueous solution until the pH of the solution reached 8. The resulting precipitate was aged at 298 K for 1 h, filtered, and washed several times with distilled water. The cake was dried in air at 393 K for 12 h and calcined in static air at different temperatures such as 523 K, 623 K, 723 K and 823 K. 2.2. Catalyst characterization X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAC-III using Cu Ka radiation (l = 0.15406 nm), operated at 50 kV and 30 mA (1.5 kW). BET surface areas were calculated from N2 adsorption data that were obtained using Autosorb-1 apparatus (Quantachrome) at liquid N2 temperature. Before the measurement, the sample was degassed for 12 h at 150 8C. The amount of adsorbed NH3 and NO was measured at 300 K by a pulse adsorption method using helium as a carrier gas. X-ray photoelectron spectroscopy (XPS) data were obtained with an Mg Ka (1253.6 eV) X-ray source using ESCA2000 (VG Microtech) instrument. The binding energy of C 1s (284.7 eV) was used as an internal standard. The thermal gravimetric analysis (TGA) was carried out using a PerkinElmer Series 7 thermal analysis system under a flow of dry air. The temperature was raised from room temperature to 1173 K using a linear programmer at a heating rate of 40 K min1. The X-ray absorption near edge structure (XANES) spectra were taken in a transmission mode for the K-edge of Mn at beamline 3C1 of the pohang light source (PAL) operating at 2.5 GeV with ca. 100–150 mA of stored current. The detector gas was N2 for the incident beam and the transmitted beam. In addition to catalyst samples, XAFS data were also obtained for Mn foil, MnCO3, MnO, Mn2O3, Mn3O4, and MnO2 as references. They were analyzed by using ATHENA [20]. 2.3. Activity measurements Catalytic activities were measured over a fixed bed of catalysts in a tubular flow reactor of 8 mm i.d. Reactant gases were fed to the reactor by means of electronic mass flow controller (MKS). The reactant gas typically consisted of 500 ppm NO, 500 ppm NH3, 5 vol.% O2, and N2 or He. The NOx concentration in the inlet and outlet gas was analyzed by means of a NO/NO2 combustion gas analyzer (Euroton). The N2 and N2O in the effluent were separated at 353 K with HeySep D column and their concentrations were analyzed with a thermal conductive detector (TCD) in a gas chromatography (Hewlett-Packard, HP 5890). From the concentration of the gases at steady state both the conversion and the selectivity are calculated according to the following formula [12,17]: NO conversionð%Þ ¼ ½NOin ½NOout 100 ½NO in (1) M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269 N2 selectivityð%Þ ¼ ½N2 out 100 ½N2 out þ ½N2 Oout (2) The subscripts in and out indicated the inlet concentration and outlet concentration at steady state, respectively. 3. Results and discussion 3.1. The effect of kinds of precipitant Fig. 1 compared the catalytic activities of the MnOx catalysts precipitated with different kinds of precipitant in terms of NOx conversion and N2 selectivity as a function of reaction temperature. The NOx conversion was affected noticeably by kinds of its precipitant especially at low temperatures. The catalysts precipitated with precipitants containing carbonate as a anion showed the higher NOx conversion than those prepared with alkali or ammonium hydroxides. The former also exhibited the better N2 selectivity Fig. 1. NOx conversions and N2 selectivities at different reaction temperatures over various MnOx catalysts prepared by different precipitants. All catalysts were calcined in air at 623 K. Reactants: 500 ppm NO, 500 ppm NH3 and 5 vol.% O2 in N2. The gas hourly space velocity (GHSV) was 50,000 h1. 263 Table 1 N2 adsorption data of the MnOx catalysts precipitated with a different precipitant such as (NH4)2CO3 (AC), K2CO3 (PC), Na2CO3 (SC), NH4OH (AH), KOH (PH), and NaOH (SH) Catalystsa SBET (m2/g) VP (cm3/g) MnOx-AC (623) MnOx-PC (623) MnOx-SC (623) MnOx-AH (623) MnOx-PH (623) MnOx-SH (623) 132.7 154.3 173.3 25.9 29.1 31.0 0.32 0.38 0.35 0.26 0.29 0.27 a Catalysts were calcined in air at 623 K before the measurement. than the latter. For all catalysts, the N2 selectivity decreased with increasing reaction temperature. The NOx conversion and the N2 selectivity were also affected by kinds of cations such as sodium ion, potassium ion, and ammonium ion and decreased in this order. Therefore, the highest NOx conversion and N2 selectivity was observed over MnOx catalysts precipitated with sodium carbonate. Table 1 gives the BET surface area and pore volume of the various MnOx catalysts prepared with different precipitants. The surface areas and pore volumes of MnOx catalysts precipitated with alkali or ammonium hydroxides are much smaller than those of the MnOx catalysts prepared with precipitants containing carbonate. The kinds of cations in the precipitant also appeared to affect the surface area. The BET surface area decreased in the order when Na+, K+, and NH4+ was utilized as a cation in the precipitant, respectively. Therefore, the manganese oxide catalyst precipitated with sodium carbonate had the highest surface area. It is worth noting that the SCR activity closely correlates with the surface area. Based on the previous work [19], the additional factors besides the surface area must affect the SCR activity because MnOx-SC showed the higher specific activity per surface area than MnOx-AH. Fig. 2 shows the XRD patterns of the various MnOx catalysts just dried at 373 K and calcined in air at 623 K. All the asprepared MnOx-AH, MnOx-PH and MnOx-SH sample give sharp XRD peaks representing Mn3O4 phase. This phase was transformed into Mn5O8 phases after heat treatment at 623 K. For MnOx-AC, MnOx-PC and MnOx-SC catalysts, MnCO3 phase was observed when they were dried at 373 K. However, when they were calcined in air at 623 K, no noticeable crystalline phase was observed. This can be interpreted that asprepared MnCO3 was decomposed into the amorphous manganese oxide phase during calcinations. This partially decomposed and amorphous structure of the MnOx catalysts may also be a reason for the excellent catalytic activity at low temperatures, in addition to the high surface area. To find out the surface chemical states of MnOx catalysts prepared with different precipitants, XPS spectra of Mn 2p, O 1s, and C 1s were obtained as shown in Fig. 3. For all samples, two main peaks due to Mn 2p3/2 and Mn 2p1/2 were observed from 639 eV to 660 eV. It was reported that 2p3/2 binding energy of Mn(0) and Mn(IV)O2 was 639.0 eV and 642.1 eV, respectively [21,22]. Therefore, it is not easy to discern the 264 M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269 Fig. 2. XRD patterns of MnOx catalysts just dried (A) and calcined in air at 623 K (B) after precipitated with different precipitants: (a) MnOx-AC, (b) MnOx-PC, (c) MnOx-SC, (d) MnOx-AH, (e) MnOx-PH and (f) MnOx-SH (* MnCO3, ^ Mn3O4 and & Mn5O8). oxidation states of manganese species without peak deconvolution because there is only 3.1 eV energy difference between Mn(0) and Mn(IV). The peak position of Mn 2p3/2 for MnOx catalysts can exclude the possibility that there exist manganese species whose oxidation state is 0 or +2. Therefore, the deconvolution of Mn 2p3/2 was conducted based on the assumption that there are only Mn(III) and Mn(IV) species. The presence of satellite peak was also considered in the peak deconvolution. This satellite structure, noticeable on the higher binding energy side of the Mn 2p core-line in the XPS spectrum of the manganese compounds, originates from the charge transfer between outer electron shell of ligand and an unfilled 3d shell of Mn during creation of core-hole in the photoelectron process [23]. As shown in Fig. 3, the asymmetric peak was Fig. 3. XPS spectra for (A) Mn 2p, (B) O 1s, and (C) C 1s of the MnOx catalysts: (a) MnOx-AC (623), (b) MnOx-PC (623), (c) MnOx-SC (623), (d) MnOx-AH (623), (e) MnOx-PH (623) and (f) MnOx-SH (623). M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269 265 Table 2 Mn 2p and O 1s binding energies on MnOx catalysts precipitated with a different precipitant such as (NH4)2CO3 (AC), K2CO3 (PC), Na2CO3 (SC), NH4OH (AH), KOH (PH), and NaOH (SH) Catalystsa Mn MnOx-AC (623) MnOx-PC (623) MnOx-SC (623) MnOx-AH (623) MnOx-PH (623) MnOx-SH (623) a Mn4+/Mn3+ Mn 2p (eV) 4+ 643.7 643.7 643.7 643.8 643.8 643.7 3+ Mn 641.9 641.9 641.9 641.9 641.9 642.0 0.97 1.00 1.10 0.71 0.63 0.69 O 1s (eV) Oa/(Oa + Ob) Oa Ob 531.3 531.3 531.4 531.7 531.5 531.5 529.6 529.6 529.7 530.1 530.0 529.9 42.6 41.3 52.2 30.4 31.0 28.1 Catalysts were calcined in air at 623 K before the measurement. observed in XPS spectra of O 1s for all samples. The peak at 529.6–530.0 eV corresponds to lattice oxygen (O2) (hereafter denote as Ob) whereas the one at 531.3–531.7 eV corresponds to several O 1s states assigned to the surface-adsorbed oxygen such as O22 or O, in the form of hydroxyl, OH, and carbonate, CO32 (hereafter denoted as Oa) [24,25]. XPS spectra of C 1s were also obtained as shown in Fig. 3. For all samples, two adjacent peaks were observed from 282 eV to 292 eV. The peak at 285 eV, 286.5 eV, and 289 eV corresponds to contaminated carbon, CO bond, and surface carbonate species, respectively. The peak intensity at a higher binding energy in C 1s XPS spectra for MnOx prepared with precipitants containing carbonate anions was stronger than those for others. Table 2 lists the surface compositions from the XPS spectra of the MnOx catalysts prepared with different precipitants. The value of the Mn4+/Mn3+ ratio in the surface layer of MnOx catalyst was quite high (0.97–1.10) in catalysts precipitated with carbonate-containing precipitants. Especially, this value appeared to be the largest for MnOx-SC which showed the highest catalytic activity for the selective NOx reduction with NH3 at low temperatures. The relative concentration ratio of Oa/(Oa+Ob) was also higher in MnOx-SC, MnOx-PC, and MnOx-AC than that in MnOx-AH, MnOx-PH, and MnOx-SH. The manganese oxide prepared with sodium carbonate which was the best in NH3-SCR at low temperatures showed the highest relative concentration ratio of Oa/(Oa+Ob) among all catalysts. Therefore, the effect of calcinations temperature on the catalytic activity and structure and electronic state of catalysts were examined further for MnOx catalysts precipitated with sodium carbonate. temperatures. Therefore, the MnOx-SC catalysts calcined at moderate temperature such as 523 or 623 K have the highest catalytic activity for NOx reduction with NH3. The N2 selectivity gradually decreased with increasing calcination temperatures. Table 3 shows the BET surface areas, pore volumes, and average pore diameters of the MnOx-SC catalysts calcined at different temperatures. The BET surface areas and 3.2. The effect of calcination temperatures Fig. 4 compares the NOx conversions and N2 selectivity over MnOx-SC catalysts calcined at different temperatures. The MnOx-SC catalyst dried at 373 K showed less than 20% NOx conversion when the reaction temperature was below 400 K. This catalyst exhibited more than 80% NOx conversion only in the temperature range from 448 K to 498 K. However, 100% NOx conversion was obtained from 348 K to 448 K when MnOx-SC catalyst was calcined at 523 or 623 K. Further increase in calcinations temperature above 623 K caused decreasing NOx conversion especially at low reaction Fig. 4. NOx conversions and N2 selectivities at different reaction temperatures over various MnOx-SC catalysts calcined at different temperatures. Reactants: 500 ppm NO, 500 ppm NH3 and 5 vol% O2 in N2. The gas hourly space velocity (GHSV) was 50,000 h1. 266 M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269 Table 3 Surface area, pore volume and average pore diameter of the MnOx catalysts precipitated with sodium carbonate (SC) and calcined in air at different temperatures Catalysts MnOx-SC MnOx-SC MnOx-SC MnOx-SC (523) (623) (723) (823) SBET (m2/g) VP (cm3/g) DP (nm) 178.5 173.3 80.4 22.2 0.35 0.35 0.31 0.07 8.0 10.9 15.5 21.3 the pore volumes of the MnOx-SC catalysts monotonically decreased with increasing the calcination temperature from 523 K to 823 K. The corresponding average pore diameter gradually increased from 8.0 nm to 21.3 nm as expected. This can be due to the sintering to some extent at high calcinations temperature. Fig. 5 illustrates the XRD patterns of MnOx catalysts precipitated with sodium carbonate and calcined at different temperatures. For the MnOx-SC dried at 373 K, the strong peaks representing crystalline MnCO3 phase were observed. However, these strong peaks disappeared completely and very weak peaks which cannot be assigned to any manganese oxides were found when MnOx-SC catalysts were calcined at temperatures from 523 K to 723 K. This can be interpreted that MnCO3 was transformed into an amorphous phase during calcinations at this temperature. This partially decomposed and amorphous structure of the MnOx-SC catalyst may also be a reason for the high NOx conversion and N2 selectivity at low temperatures. This was also consistent with the previous observation that nitrous oxide formation occurred preferentially on crystalline phases [12]. When the MnOx-SC catalyst was calcined at 823 K, the formation of Mn2O3 crystalline phase was observed in XRD pattern. XPS measurements were performed to examine the effect of calcinations temperatures on the surface electronic state of MnOx-SC catalysts. Fig. 6 shows the XPS spectra of Mn 2p, O 1s, and C 1s of MnOx-SC catalysts with different calcination temperatures. For all samples, two main peaks due to Mn 2p1/2 and Mn 2p3/2 were observed from 639 eV to 660 eV, which can exclude the possibility of the presence of Mn(0) and Mn(II) species. Therefore, the deconvolution of Mn 2p3/2 was conducted based on the assumption that there are only Mn(III) and Mn(IV) species. The presence of satellite peak was also considered in the peak deconvolution. As shown in Fig. 6, the asymmetric peak was observed in XPS spectra of O 1s for all samples. As stated previously, the peak at 530.0 eV and 531.5 eV can be assigned to the lattice oxygen (Ob) and the Fig. 5. XRD patterns of MnOx-SC catalysts calcined at different temperatures. surface-adsorbed oxygen (Oa). XPS spectra of C 1s were also obtained as shown in Fig. 6. For all samples, two adjacent peaks were observed from 282 eV to 292 eV. The peak at 285 eV, 286.5 eV, and 289 eV corresponds to contaminated carbon, CO bond, and surface carbonate species, respectively. For MnOxSC catalysts, the peak intensity at a higher binding energy in C 1s XPS spectra decreased with increasing calcinations temperature. Table 4 lists the surface compositions from the XPS spectra of the MnOx catalysts calcined at different temperatures. The value of the Mn4+/Mn3+ ratio in the surface layer of MnOx catalyst was quite high in catalysts calcined at 523 or 623 K which showed the high catalytic activity for the selective NOx reduction with NH3 at low temperatures. The relative concentration ratio of Oa/(Oa+Ob) decreased with increasing calcinations temperature for manganese oxides catalysts precipitated with sodium carbonate. The most active manganese oxide prepared with sodium carbonate and calcined at 523 K showed the highest concentration of surface oxygen. Therefore, the active catalyst has the high Mn4+/Mn3+ ratio and surface oxygen concentration in the surface layer of MnOx catalyst. Table 4 Mn 2p and O 1s binding energies on MnOx-SC catalysts precipitated with sodium carbonate (SC) and calcined in air at different temperatures Catalysts Mn MnOx-SC MnOx-SC MnOx-SC MnOx-SC Mn4+/Mn3+ Mn 2p (eV) (523) (623) (723) (823) 4+ 643.6 643.7 643.5 643.5 3+ Mn 641.9 641.9 641.9 641.8 1.03 1.10 0.92 0.78 O 1s (eV) Oa/(Oa + Ob) Oa Ob 531.6 531.4 531.5 531.5 529.8 529.7 529.7 529.9 68.7 52.2 42.5 40.2 M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269 267 Fig. 6. XPS spectra for (A) Mn 2p, (B) O 1s, and (C) C 1s of the MnOx-SC catalysts calcined in air at different temperatures. The value of the Mn4+/Mn3+ ratio was previously considered as a significant parameter characterizing the intrinsic properties of other metal oxide catalysts. Machocki et al. [22] have investigated the catalytic combustion of methane over silver modified manganese–lanthanum oxides and found that the rate of methane oxidation is a linear function of the Mn4+/Mn3+ surface ratio. Tang et al. [26] also reached the same conclusion that MnOx–CeO2 catalysts showed higher catalytic activity for HCHO oxidation with increasing the Mn4+/Mn3+ surface ratio. Similar result was observed in the present study. In the general NH3-SCR process, NO is easily converted to N2 and H2O by NH3: 4NO þ 4NH3 þ O2 ! 4N2 þ 6H2 O (3) When both NO and NO2 are present, the following reaction proceeds much more rapidly than the above reaction (3): 4NH3 þ 2NO þ 2NO2 ! 4N2 þ 6H2 O (4) Wallin et al. [27] reported that the presence of NO2 in the gas mixture enhanced the SCR performance of a catalyst. Therefore, it can be inferred that the difference in the maximum NOx conversion between the catalysts is probably due to the difference in the amount of NO2 produced from the oxidation of NO by high Mn4+/Mn3+ surface ratio. The value of Oa/(Oa + Ob) in the surface layer of MnOx catalyst prepared with precipitants containing carbonate anions was higher than that of others. The high relative concentration ratio of Oa/(Oa+Ob) was previously found to be preferable for SCR reactions over the manganese-containing catalysts. Kapteijn et al. [17] indicated that the product distribution is determined by the concentration of reactive oxygen and nitric oxide, since both affect the relative distribution of the surface species. A high surface oxygen concentration facilitates the (– NH2) formation. Kijlstra et al. [10] studied the mechanism of SCR of NO with ammonia at low temperatures on MnOx/Al2O3 catalyst. They proposed that the reaction starts with the adsorption of NH3 on a Lewis acid and subsequently transforms into NH2; the NH2 would then react with gas-phase NO (an ER mechanism) and nitrite intermediates on the surface (a LH mechanism). This is supported by our results, which indicated that the N2 selectivity increased with the relative ratio between concentration of surface oxygen and that of lattice oxygen. The thermal gravimetric analysis was conducted to observe the change in the weight of manganese oxides precipitated with sodium carbonate with increasing temperature as shown in Fig. 7. The gradual weight loss occurred from room temperature to 850 K. This decreased weight (35 wt.%) is almost same to the calculated value assuming that MnCO3 was transformed into Mn2O3. Therefore, MnOx-SC catalyst calcined below 850 K must contain lots of carbon oxide species (COx). There is no significant weigh loss in the MnOxAH catalyst. It is reasonable that the presence of COx species in the MnOx-SC catalyst also affects the de-NOx activity at low temperatures because the residual COx species may act as acidic sites on the catalyst surface. These acidic sites can help Fig. 7. Thermal gravimetric analysis of manganese oxides precipitated with sodium carbonate and dried at 373 K. 268 M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269 Fig. 8. Mn K-edge XANES spectra of Mn reference samples and MnOx-SC catalysts calcined at different temperatures. The fitted spectra were plotted in dotted line. the basic reductant, NH3, adsorb on the surface at low temperatures, and therefore the reductant may be enriched compared with the surface of MnOx-AH catalyst. This was also supported by the fact that 0.207 mmol NH3/gcat and 0.011 mmol NH3/gcat could be adsorbed on MnOx-SC and MnOx-AH, respectively. This difference is much more than expected even if MnOx-SC has seven times larger surface area than that of MnOx-AH. The amount of chemisorbed NO on MnOx-SC and MnOx-AH at room temperature was 0.50 mmol NO/gcat and 0.12 mmol NO/gcat, respectively. This can be mainly due to difference in surface area between two catalysts. To determine the structural and electronic information of MnOx catalysts which were determined to be amorphous from XRD data, Mn K-edge XANES spectra were obtained. Fig. 8 presents Mn K-edge XANES spectra of MnOx-SC catalysts calcined at different temperatures with Mn reference compounds such as Mn foil, MnCO3, MnO, Mn3O4, Mn2O3, and MnO2. Easily discernible features of XANES spectra were observed for Mn reference samples and the edge energy shifted toward a higher energy with an increase in the oxidation state of Mn from metallic Mn foil to Mn(IV)O2. Mn K-edge XANES spectra of as-prepared MnOx-SC catalyst was similar to that of MnCO3, which is consistent with the fact that crystalline MnCO3 phase was observed from XRD. MnOx-SC catalysts calcined at 523 K, 623 K and 723 K appeared to be different compared with any XANES spectra of Mn reference samples measured. However, Mn K-edge XANES spectra of MnOx-SC calcined at 823 K was similar to that of Mn2O3, which is consistent with the fact that crystalline Mn2O3 phase was observed from XRD. To determine the quantitative amount of each manganese oxides, a linear XANES fitting was conducted for MnOx-SC catalysts calcined at different temperatures as shown in Table 5. Different weight fractions of manganese oxides appeared to be present in these catalysts. The manganese carbonate was determined to be mainly present with a small amount of Mn2O3 in MnOx-SC just dried at 373 K. When MnOx-SC was calcined at 523 K, the weight fraction of MnCO3 decreased noticeably and those of Mn3O4 and Mn2O3 increased. When this catalyst was calcined at 623 or 723 K, most of MnCO3 must be transformed into Mn2O3. Because Mn2O3 was determined to be mainly present with a small amount of Mn3O4 in MnOx-SC calcined at 823 K, some of Mn3O4 appeared to be further transformed into Mn2O3. Although no noticeable difference in the composition of manganese oxides for MnOx-SC calcined at 623 K compared with that of MnOx-SC calcined at 723 K from XANES fitting results, the former was superior to the latter in the catalytic activity at low temperatures. This indicates that the kind of manganese oxide as well as its crystallinity should be related to the catalytic activity in NH3-SCR at low temperatures. Table 5 The linear combination fitting result for MnOx catalysts precipitated with sodium carbonate (SC) and calcined in air at different temperatures Catalysts E0/(eV) Standard sample Weight fraction R-factora MnOx-SC (373) 6548.733 MnCO3 Mn2O3 0.966 0.034 0.15801 MnOx-SC (523) 6548.719 MnCO3 Mn3O4 Mn2O3 MnO2 0.424 0.279 0.286 0.011 0.02717 MnOx-SC (623) 6548.719 MnCO3 Mn3O4 Mn2O3 MnO2 0.135 0.280 0.510 0.075 0.52299 MnOx-SC (723) 6548.719 MnCO3 Mn3O4 Mn2O3 MnO2 0.128 0.314 0.487 0.071 0.12677 MnOx-SC (823) 6553.719 MnCO3 Mn3O4 Mn2O3 0.012 0.106 0.882 0.15782 a R-factor is defined as follows. R = sum((data-fit)^2)/sum(data^2)). M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269 [4] [5] [6] [7] The formation of ammonium nitrate on the catalyst surface during the course of the reaction has been claimed to be a main problem for low temperature NH3-SCR performance [28]. The temperature programmed desorption (TPD) was conducted from 373 K to 1073 K over the catalyst after a reaction and its effluent gas was analyzed with a mass spectrometer (PFEIFFER Vacuum Quadstar). During TPD experiment, no NH3 species was detected. This can exclude the formation of ammonium salts during a reaction. [10] 4. Conclusions [11] The manganese oxides catalyst prepared by a precipitation method using sodium carbonate and calcined at moderate temperatures such as 523 K and 623 K showed the high NOx conversion and N2 selectivity for low temperature selective catalytic reduction of NOx with NH3. This active MnOx catalyst has the high surface area, the abundant Mn4+ species, and the rich concentration of surface oxygen on the surface. The amorphous Mn3O4 and Mn2O3 were determined to be mainly present. The residual carbonate species on this catalyst also help NH3 adsorb on the surface, which resulted in the high catalytic activity at low temperatures. Acknowledgements The financial support by the Research Initiation Program at Ajou University (20041340) was appreciated by one of authors, Eun Duck Park. Experiments at PLS were supported in part by MOST and POSTECH. References [8] [9] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [1] H. Bosch, F.J.I.G. Janssen, Catal. Today 2 (1988) 369–379. [2] J.N. Armor, Appl. Catal. B 1 (1992) 221–256. [3] H.S. Rosenberg, L.M. Curran, A.U. Slack, J. Ando, J.H. Oxley, EPRIRP925, October.1978. [27] [28] 269 H. Hamada, Catal. Today 22 (1994) 21–40. J.M. Garcia-Cortes, Appl. Catal. B: Environ. 30 (2001) 399–408. G.A. Papapolymerou, L.D. Schmidt, Langmuir 1 (1985) 488–495. Y. Murakami, M. Inomata, K. Mori, K. Suzuki, A. Miyamoto, T. Hattori, in: G. Poncelet, P. Grange, P.A. Jacobs (Eds.), Preparation of Catalysis III, Elsevier, Amsterdam, 1983, pp. 531–534. J.R. Kiovsky, P.B. Koradia, C.T. Lim, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 218–225. G. Busca, L. Lietti, G. Ramis, F. Berti, Appl. Catal. B: Environ. 18 (1998) 1–36. W.S. Kijlstra, J.C.M.L. Daamen, J.M. van de Graff, B. van der Linden, E.K. Poels, A. Bliek, Appl. Catal. B: Environ. 7 (1996) 337–357. J. Muniz, G. Marban, A.B. Fuertes, Appl. Catal. B: Environ. 27 (2000) 27– 36. L. Singoredjo, R. Korver, F. Kapteijn, J. Moulijn, Appl. Catal. B: Environ. 1 (1992) 297–316. U. Bentrup, A. Bruckner, M. Richter, R. Fricke, Appl. Catal. B: Environ. 32 (2001) 229–241. G. Qi, R.T. Yang, R. Chang, Catal. Lett. 87 (2003) 67–71. P.G. Smirniotis, D.A. P na, B.S. Uphade, Angew. Chem. Int. Ed. 40 (2001) 2479–2482. G. Qi, R.T. Yang, Appl. Catal. B: Environ. 44 (2003) 217–225. F. Kapteijn, L. Singoredjo, A. Andreini, J.A. Moulijn, Appl. Catal. B: Environ. 3 (1994) 173–189. G. Qi, R.T. Yang, Chem. Commun. (2003) 848–849. M. Kang, T.H. Yeon, E.D. Park, J.E. Yie, J.M. Kim, Catal. Lett. 106 (2006) 77–80. B. Ravel, M. Newville, J. Synchrotron Radiat. 12 (2005) 537–541. S. Ponce, M.A. Pena, J.L.G. Fierro, Appl. Catal. B 24 (2000) 193– 205. A. Machocki, T. Ioannides, B. Stasinska, W. Gac, G. Avgouropoulos, D. Delimaris, W. Grzegorczyk, S. Pasieczna, J. Catal. 227 (2004) 282– 296. R.J. Iwanowski, M.H. Heinonen, W. Paszkowicz, R. Minikaev, T. Story, B. Witkowska, Appl. Surf. Sci. 252 (2006) 3632–3641. J.P. Holgado, G. Munuera, J.P. Espinós, A.R. González-Elipe, Appl. Surf. Sci. 158 (2000) 164–171. R. Larachi, J. Pierre, A. Adnot, A. Bernis, Appl. Surf. Sci. 195 (2002) 236– 250. X. Tang, Y. Li, X. Huang, Y. Xu, H. Zhu, J. Wang, W. Shen, Appl. Catal. B: Environ. 62 (2006) 265–273. M. Wallin, C. Karlsson, M. Skoglundh, A. Palmqvist, J. Catal. 218 (2003) 354–364. M. Koebel, M. Elsener, G. Madia, Ind. Eng. Chem. Res. 40 (2001) 52–59.