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Ceria supported on sulfated zirconia as a superacid catalyst for selective catalytic reduction of NO with NH3
Journal of Colloid and Interface Science 394 (2013) 515–521
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Ceria supported on sulfated zirconia as a superacid catalyst for selective catalytic reduction of NO with NH3 Shan Gao a, Xiongbo Chen a, Haiqiang Wang a,⇑, Jiansong Mo b,⇑, Zhongbiao Wu a,b, Yue Liu a, Xiaole Weng a a b
Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, China Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 29 September 2012 Accepted 15 December 2012 Available online 9 January 2013 Keywords: NO reduction SCR Ammonia Superacid Ceria Sulfated zirconia
a b s t r a c t In this paper, ceria supported on sulfated zirconia (CeSZ) as a superacid catalyst was synthesized and the resulted performances for selective catalytic reduction (SCR) of NO with NH3 were investigated. Experimental results revealed that the sulfation of zirconia supports could greatly improve the SCR activity of the catalysts. Among the tested samples, the CeSZ catalyst with Ce/Zr mole ratio at 0.095 possessed the highest NO conversion (i.e., 98.6% at ca. 420 °C and 180,000 h1). The sulfation had led to a formation of pure tetragonal phase of ZrO2, a well dispersion of CeO2, abundant stable superacid sites, increasing surface area and enrichment of Ce3+ on the surface, all of which were responsible for its excellent performance in SCR of NO with NH3. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Nitrogen oxides (NOx) from combustion of fossil fuels are one of the major pollutants to the atmosphere, which not only pose a serious threat to environment, but also cause photochemical smog, acid rain, and respiratory problems [1,2]. The selective catalytic reduction (SCR) of NOx with ammonia (NH3-SCR) is the most widely used technique for NOx emission control from stationary sources [3,4], where significant efforts have been devoted to develop novel SCR catalysts with promising catalytic activity and selectivity. Hamada et al. [5] had first found that the sulfated metal oxide catalysts (TiO2, ZrO2, Fe2O3, and Al2O3) were active in the propane-SCR reaction. Thereafter, the positive effect of sulfation treatment on SCR catalysts has attracted tremendous attentions, which is found to be connected with the increase in the surface area and the formation of acid sites on catalyst surface [6–8]. Recently, sulfated zirconia (SZ), a stronger solid acid than concentrated sulfuric acid, has been studied extensively [9–11]. Many elements supported on SZ were shown to be active for SCR reaction, including V [12], Pd [8,12], Pt [13], Co [6], Cu [14], Mn [15], and In [7,16]. On the other hand, ceria (CeO2) has been widely used as an essential ingredient in three-way catalysts (TWCs) for auto-
⇑ Corresponding authors. Address: Department of Environmental Engineering, Zhejiang University, Yuhangtang Road, No. 866, Hangzhou 310058, China. Fax: +86 571 87953088. E-mail addresses: [email protected] (H. Wang), [email protected] (J. Mo). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.12.034
motive emission control [17,18] and has also been considered as a promising candidate for selective catalytic reduction of NOx. In our previous work [4,19,20], ceria-based catalysts were proven to be active in the SCR of NO with ammonia. Furthermore, the catalytic performance for SCR reaction over ceria could be greatly enhanced after the surface sulfation by gaseous-phase SO2 [19]. Therefore, in this work, we developed a novel ceria catalyst supported on sulfated zirconia (SZ) for SCR reaction, which exhibited excellent catalytic activity. A series of characterizations such as XRD, TG–DSC, TEM, Raman, BET, XPS, FTIR, and Hammett indicator method were carried out to get insight into this superior performance. 2. Experimental methods 2.1. Catalyst preparation Pure ZrO2 (Z) was prepared by calcination of Zr(OH)4 (Sinopharm Chemical Reagent Co., Ltd., China) at a series of temperatures for 4 h in air. SO2 4 —ZrO2 (SZ) was synthesized in the same procedure by calcination of SO2 4 —ZrðOHÞ4 , which was obtained by impregnation of Zr(OH)4 with a solution of 0.25 N H2SO4 (immersion for 2 h, 5 ml/g). All the samples were dried at 80 °C overnight. The resulted supports were designated as Z-x or SZ-x, where x indicated the calcination temperature of precursors (ranging between 500 °C and 700 °C). The supported ceria-based catalysts were prepared by impregnating the zirconia supports with an aqueous solution of cerium
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nitrate. Based on the TG–DSC analysis of Zr(OH)4 and 2 SO2 4 —ZrðOHÞ4 , the calcination of SO4 —ZrðOHÞ4 at 600 °C was selected as an optimal calcination temperature for subsequent supports. After impregnation for 2 h, the samples were dried at 80 °C overnight and calcined at 450 °C for 3 h. The resulted catalysts were hereafter referred to CeSZ(y) or CeZ(y), where y indicated the Ce/Zr molar ratio (ranging between 0.024 and 0.190). Cu, Fe catalysts supported on sulfated zirconia were prepared by the same procedures, acting as reference catalysts. Copper nitrate and ferric nitrate were used as precursors. Resulted catalysts were referred to CuSZ(y) or FeSZ(y), where y indicated the Cu/Zr or Fe/Zr molar ratio.
pressure at 200–550 °C. A type K thermocouple was placed in the center of the reactor tube. The typical composition of inlet gas was 600 ppm NO, 600 ppm NH3, 3.5% O2, and N2 as balance gas. The gas hourly space velocity (GHSV) was about 180 000 h1. The concentration of NO, NO2, and O2 were monitored by a flue gas analyzer (Testo 335, Testo Inc., USA). N2O was detected by a FT-IR gas analyzer (Madur Photon Portable IR Gas Analyzers, Madur Ltd., Austria).
2.2. Characterizations
3.1.1. XRD of SO2 4 —ZrO2 X-ray diffraction patterns of zirconium supports after calcination were depicted in Fig. 1. The unsulfated ZrO2 support was a mixture of monoclinic (PDF-ICDD 37-1484: 2h = 28.2°, 31.5°) and tetragonal (PDF-ICDD 50-1089: 2h = 30.3°, 50.4°, and 60.2°) phases (mainly monoclinic). In contrast, only tetragonal phase was present in the sulfated ZrO2 supports. Based on the calculation of Scherrer formula, it could get that the crystallite size of unsulfated sample (Z-600) was 15.7 nm, and that of sulfated sample (SZ600) was 9.3 nm. This revealed that the sulfation had resulted in the decrease in crystallite size of the supports. When calcined at 500 °C, sulfated ZrO2 showed poorly resolved peaks of zirconia, which might indicate that the composition of SZ500 was mainly amorphous phase. Characteristic peaks of tetragonal ZrO2 were well resolved after higher temperature treatments at 600 °C or 700 °C due to the observed peak splitting at ca. 35.0° and 60.0°. The sharpened peaks of SZ calcinated at 700 °C indicated that the crystallite size was enlarged after the calcination at higher temperature. The absence of monoclinic phase in the sulfated samples, due to the delay in the phase transformation, could be affected by lots of factors. Some investigators have ascribed the stabilization of tetragonal ZrO2 to a particle size effect, with smaller particle size having greater surface energy contribution to their stability [23,24]. Besides, it was also connected with the pressure and the presence of phase stabilizers either in the bulk (usually as rareearth cationic dopants with intrinsic defects associated with them) or at the surface (such as water-derived or sulfate groups) [8,24]. In our case, the stabilization of tetragonal phase could be achieved by the smaller size of sulfated samples as resulted from sulfation, the introduction of cationic dopant (SO2 4 during sulfation treatment),
The crystal phases of the samples were identified by using X-ray diffraction (XRD) with Cu Ka radiation (model D/max RA, Rigaku Co., Japan). The scanning range (2h) was collected from 10° and 80° with a step size of 0.02°. Crystallite size of ZrO2 was determined from the characteristic peak (2h = 30.27° for the (011) reflection of tetragonal phase and 28.17° for the (1 1 1) reflection of monoclinic phase) by using Scherrer formula [21] as below:
Crystallite size ¼ K
k B1=2 cos h
where K is the shape factor (often assigned a value of 0.89 if the shape is unknown [22]), k is X-ray wavelength (at 1.54056), B1/2 the full-width-half-maximum (FWHM) of the ZrO2 characteristic peak, and h is the diffraction angle. Thermogravimetry and differential scanning calorimetry (TG– DSC: STA-409PC, NET-ZSCH, Germany) were performed. A solid sample was scanned at a heat rate of 10 °C /min in flowing N2. The surface acid strength of samples was measured by Hammett indicators, including p-nitrotoluene (pKa = 11.35), m-nitrotoluene (pKa = 11.99), and p-chloronitrobenzene (pKa = 12.70). The morphology, structure, and crystallite size of the samples were examined by transmission electron microscopy (TEM: JEM2010, Japan). The Raman spectra of samples were collected at a Raman Spectrometer (Ramen: Jobin–Yvon LabRAM HR800), using a laser at 514.5 nm line as the excitation source. The laser power of the 514.5 nm line at the samples was below 1.0 mW. The specific surface areas were determined by the Brunauer– Emmett–Teller (BET) method on a nitrogen adsorption apparatus (ASAP 2020, USA). All the samples were degassed at 300 °C prior to measurements. The data were collected in the relative pressure (P/P0) ranging from 0.05 to 0.30. X-ray photoelectron spectroscopy (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties of the samples, with Al Ka radiation (hv = 1486.6 eV). The shift of the binding energy due to relative surface charging was corrected using the C 1s level at 284.8 eV as an internal standard. The nature of bonding of sulfate ions with zirconia surface after calcination at 600 °C was studied by FT-IR spectrophotometer (Bruker ALPHA). The sample was diluted with KBr (100:3) and transferred to a 13 mm stainless steel die, compressed at a pressure of 10 MPa by a tablet press (769YP-15A) to obtain the selfsupporting pellet. The IR Transmission Spectra of samples were recorded in the range of 4000–400 cm1.
3. Results and discussion 3.1. Structure analysis of Z and SZ supports
2.3. SCR activity evaluation Selective catalytic reduction of NO with NH3 was carried out in a fixed-bed quartz reactor (1 cm i.d.) using a 0.5 g catalyst of 40–60 mesh. The experiments were performed under atmospheric
Fig. 1. XRD patterns of unsulfated and sulfated zirconia.
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and the subsequently doping of rare-earth cationic dopant (ceria addition during the catalyst preparation). 3.1.2. TG–DSC analysis of SO2 4 —ZrO2 It was reported in the previous work [9,25] that high temperature calcination was necessary for the formation of superacid sites, but it also resulted in loss of sulfates and concomitant transformation of tetragonal zirconia into monoclinic zirconia [25]. Tetragonal zirconia phase was suggested to be necessary for the generation of higher acidity in the sulfated zirconia as well as higher activity [26]. Therefore, there is necessary to seek out the optimal calcination temperature, which favored the retention of tetragonal phase of zirconia. In order to investigate the thermal stability of zirconium supports and superacid sites, thermal analysis were applied for Zr(OH)4 and sulfated Zr(OH)4. The results presented in Fig. 2 showed the characteristics of TG–DSC curves that were similar to those reported previously [27,28]. The whole temperature range could be divided roughly into three stages: In the first stage between 30 °C and 250 °C, the evident weight loss of samples accompanying a strong endothermic process at ca. 115 °C was observed, which was resulted from the desorption of physical adsorbed water and dehydroxylation during the thermal decomposition of zirconium hydroxide to zirconium oxide. The second stage could be observed from 250 °C to 650 °C related to a slight weight loss of 3–5%. One sharp exothermic peak centered at 432.5 °C (generally observed between 420 °C and 470 °C [8]) was investigated, which was attributed to the heat effect of the phase transformation of zirconia from an amorphous to crystalline state [8]. However, the corresponding peak was not detected in SO2 4 —ZrðOHÞ4 , which may shift to higher temperature and superimpos with endothermic peak of sulfate decomposition. This result indicated that the incorporation of sulfate was in favor of the stabilization of tetragonal zirconia phase, which agreed well with the XRD results. It has also been reported that the exothermic peak would shift to higher temperature when zirconia was doped with additives regardless of anions or cations [8]. The third stage in the curve began at ca. 650 °C, which was only observed in sulfated Zr(OH)4, where a weight loss of ca. 10% was observed. This weight loss was due to the decomposition of sulfate species [8,28], which confirmed that the sulfate species (superacid centers) on sulfated zirconia had a high thermal stability. Moreover, the overall weight loss of SO2 4 —ZrðOHÞ4 was much less than the unsulfated material (Zr(OH)4), suggesting that the incorporation of sulfate species had displaced part of hydroxide groups [8].
In summary, significant physicochemical changes were caused by the sulfation and heat treatment of the original zirconium hydroxide. The incorporation of sulfate helped to stabilize the metastable tetragonal zirconia phase, and the superacid centers were highly stable below 650 °C. Thus, although the two calcinations (i.e., at 600 °C, 700 °C) had all resulted in the presence of pure tetragonal structure, the calcination of SO2 4 —ZrðOHÞ4 at 600 °C was selected as the optimal calcination temperature due to the retention of superacid centers.
3.1.3. Hammett acidity of SO2 4 —ZrO2 The acidic strengths of the supports after calcination at 600 °C were examined by using Hammett indicator method (shown in Table 1). As is well known, the acidic strength of a solid was determined by the ability of the surface to convert an adsorbed neutral base into its conjugate acid [29]. From the table, it could be seen that the acid strength of Z-600 was H0 > 11.35, while that of SZ-600 was estimated to be H0 < 12.70, which was higher than that of 100% H2SO4 (H0 = 11.93), indicating that sulfated zirconia after calcination at 600 °C were superacids, further confirming the TG–DSC results.
3.2. Characterizations and performances of CeZ and CeSZ catalysts 3.2.1. Catalytic performances of NO over the catalysts The SCR performances of Ce catalysts supported on unsulfated or sulfated ZrO2 were depicted in Fig. 3. All of the used catalyst supports were calcined at 600 °C. As shown in Fig. 3, ceria supported on sulfated ZrO2 possessed much higher SCR activity than the corresponding unsulfated ones. More than 90% NO conversion had been achieved for CeSZ catalyst (Ce/Zr molar ratio at 0.095) in the temperature range from 330 °C to 510 °C. It was also noted that NO conversion over CeSZ(0.095) was higher than that over CeSZ(0.048), particularly in lower temperature. The influence of ceria contents on the SCR performance of CeSZ catalysts was shown in Fig. 4. It could be seen that the activity of CeSZ was strongly influenced by ceria contents. With increase in the ceria content, the conversion of NOAN2 was significantly enhanced. When Ce/Zr mol ratio got to 0.095, NO conversion up to 98.6% could be achieved at ca. 420 °C. After further increasing the Ce/Zr mol ratio to 0.190, the NO conversion went on increasing slightly in lower temperature region (below ca. 330 °C), but decreased in higher temperature region (above ca. 512 °C). Therefore, the Ce/Zr mole ratio at 0.095 of CeSZ catalyst was selected subsequently as the optimal loading content, which gave the optimal result in terms of catalytic performance and catalyst cost. To clearly demonstrate the advantages of CeSZ catalyst, the catalytic performance of ceria catalyst supported on SZ was compared with that of other elements (Cu, Fe), including NO conversion (Fig. S1), outlet N2O concentration (Fig. S2), and outlet NO2 concentration (Fig. S3). Actually, CeSZ catalyst obtained a comparatively higher SCR activity, yielding a 90% NO conversion within a broader temperature window, whereas CuSZ and FeSZ catalysts could only achieve ca. 78% (at 400 °C), ca. 85% (within 415–460 °C), respectively.
Table 1 The acid strengths of supports.
Fig. 2. TG–DSC profiles of Zr(OH)4 and SO2 4 —ZrðOHÞ4 .
Hammett indicator
pKa
SZ-600
Z-600
p-Nitrotoluene m-Nitrotoluene p-Chloronitrobenzene
11.35 11.99 12.70
+ + +
Note: Change in color (+) and no change in color ().
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Fig. 3. Temperature dependency of NO conversion over catalysts with or without sulfation treatment. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 3.5%, balance N2, catalyst 0.5 g, and GHSV about 180,000 h1.
Fig. 5. XRD patterns of unsulfated and sulfated catalysts.
47.5°, and 56.3°) could be observed in CeSZ until the Ce/Zr molar ratio was increased to 0.190. This result suggested that ceria had a better dispersion on the sulfated supports than that on the unsulfated supports, and CeO2 crystallites would form with the enlarged Ce loading. The aggregation of CeO2 would eventually lead to a bad dispersion of CeO2 on the sulfated ZrO2.
Fig. 4. Temperature dependency of NO conversion over CeSZ catalysts with various Ce/Zr mol ratios. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 3.5%, balance N2, catalyst 0.5 g, and GHSV about 180,000 h1.
N2O and NO2, the potential by-products in SCR reaction, had also been monitored in the experiments (Figs. S2 and S3). For CeSZ catalyst, the generated N2O concentration was no more than 10 ppm and NO2 concentration was less than 15 ppm within the whole temperature window, which were much lower than that of CuSZ and FeSZ catalysts. Overall, it could be concluded that ceria catalyst supported on sulfated zirconia showed the highest catalytic activity and the best N2 selectivity among the tested catalyst.
3.2.2. XRD XRD patterns of the ceria catalysts were illustrated in Fig. 5. The ceria supported on unsulfated ZrO2 support showed a mixture of monoclinic (PDF-ICDD 37-1484: 2h = 28.2°, 31.5°) and tetragonal (PDF-ICDD 50-1089: 2h = 30.3°, 50.4°, and 60.2°) phases (mainly monoclinic). However, all the catalysts supported on sulfated zirconia only presented tetragonal zirconia phase. Otherwise, the pattern of CeZ(0.095) catalyst showed an evident peak of CeO2 (PDF-ICDD 34-0394: 2h = 47.47), while no distinct peaks corresponding to crystalline CeO2 (PDF-ICDD 34-0394: 2h = 28.5°,
3.2.3. TEM The morphology and structure of the CeSZ and CeZ catalysts were investigated by TEM images. TEM micrograph of CeSZ (Fig. 6a) reflected well separated crystallites of rather irregular shape (averagely 9.28 nm, in the range of 6.0–13.1 nm), while the image of CeZ (Fig. 6c) displayed well-developed nanocrystals with average size of 15.28 nm (12.0–16.4 nm). The crystallite size observed by TEM was in good agreement with the result estimated by Scherrer equation. Fig. 6b and d were referred to the HR-TEM micrographs of CeSZ and CeZ, respectively. The CeZ displayed relatively better defined contours, exhibiting higher crystalline order. As shown in Fig. 6d, the d-spacing of CeZ was measured at ca. 0.320 nm, which was ascribable to monoclinic ZrO2 phase, and namely to the (1 1 1) crystal planes. Besides, the d-spacing of CeSZ was measured most frequently at ca. 0.304 nm, which was in proximity to the (0 1 1) plane of tetragonal ZrO2 (Fig. 6b). It was noted that the morphological feature was consistent with both XRD results and those reported in the previous sections. In addition, no CeO2 particles could be observed over the images of CeZ or CeSZ catalysts. 3.2.4. Raman spectra ZrO2 can exist in three crystalline forms: tetragonal, monoclinic, and cubic [24,30]. The tetragonal form has Raman bands at 149 [31], 269 [31,32], 314 [31,32], 454 [32] and 644 [32] cm1, and the monoclinic form has bands at 181 [31,32], 219 [31], 305 [31], 334 [31,32], 346 [32], 381 [32], 474 (strong) [31,32], 557 [31], 617 [32] and 639 [31] cm1. For unsulfated zirconia samples (Z, CeZ), bands at 181 [31,32], 221 [31], 305 [31], 334 [31,32], 346 [32], 381 [32], 474 [31,32], 557 [31], 617 [32], and 639 [31] cm-1 were assigned to the Raman-active modes for the monoclinic phase of ZrO2, while weak bands appeared at 269 and 454 cm1 were assigned to the Raman-active modes for tetragonal ZrO2. In the case of SZ support, no Raman bands could be observed. The reason of this phenomenon could be possibly due to the fluorescence of the sample, which
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Fig. 6. TEM and HR-TEM micrographs of (a) and (b)CeSZ(0.095); and (c) and (d)CeZ(0.095).
prevented the observation of possible bands of the crystalline form of ZrO2 [32,33]. After the ceria doping on sulfated zirconia, typical tetragonal phase were observed with no indication of the presence of monoclinic phase. A band at 1017 cm1 was ascribed to the reflection of sulfate group [32]. For pure ceria, which has a fluorite structure, the F2g mode is centered at ca. 456 cm1 in the Raman spectrum [34–36]. As shown in Fig. 7, the sharpened 474 nm band and a broad shoulder
Fig. 7. Raman spectra of ceria supported on sulfated or unsulfated zirconia.
(at. ca. 456 cm1) were observed in CeZ after the addition of ceria. Evidently, the band of monoclinic phase had some overlapping with the band of the cubic CeO2 phase [32]. And for CeSZ, only the characteristic bands of tetragonal phase were observed, which is consistent with the XRD results. 3.2.5. BET Pure zirconia calcined at 600 °C had a surface area of 36.94 m2/g. Compared with the pure zirconia, the surface area of CeZ had little change with the doping of ceria (36.19 m2/g). The surface area of zirconia was found to increase to 48.63 m2/g after sulfation treatment. This was in accord with the XRD results, which showed smaller particle sizes of sulfated samples than unsulfated ones. It has been reported that the presence of sulfate seemed to delay the formation of oxo bonds [23], which caused the great loss of surface area under high calcination temperature. Similar effect has also been reported for SO2 4 —TiO2 , where BET surface area is proportional to the amount of loaded sulfate [37]. The increased surface area would lead the better dispersion of the active components [26]. Furthermore, the surface area of CeSZ decreased to 44.18 m2/g after the introduction of ceria. 3.2.6. XPS analysis XPS was used to obtain further information about the valence/ oxidation state of the elements and surface composition of samples. The surface concentrations of various elements were summarized in Table 2. It was shown that the surface Ce/Zr molar ratios (CeSZ at 0.14, CeZ at 0.22) were apparently higher than the corresponding designed value (0.095), suggesting a preference of ceria
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Table 2 XPS results for samples.
CeSZ CeZ
Oa (%)
Ob (%)
Oc (%)
Zr (%)
S (%)
Ce (%)
37.79 49.98
24.21 15.86
10.31 5.20
20.54 23.81
4.25 –
2.91 5.16
aggregating on the surface of supports. Moreover, the Ce/Zr ratio for CeSZ was much lower than that on the CeZ. This result suggested that a less aggregation of ceria existed on the surface of sulfated zirconia, due to the higher surface area of SZ as confirmed by BET results. Fig. 8I showed the Ce 3d spectra of the sulfated catalyst (CeSZ(0.095)) and the unsulfated one (CeZ(0.095)). The peaks labeled u, u2, u3, v, v2, and v3 in black represented the 3d104f0 state of Ce4+, while the bands labeled u1 and v1 in red represented the 3d104f1 initial electronic state corresponding to Ce3+ [38]. It was worth notice that the intensities of Ce4+ characteristic peaks decreased apparently after sulfation treatment of the supports. For the calculation of Ce4+/Ce3+, the method proposed by Damyanova was used [35]. This method assumed that the ratio(r) of the area of the satellite at 916.7 eV to the total area was proportional to the surface concentration of Ce4+ and that this ratio attained 0.16 for all cerium atoms in the Ce4+ state [35]. By this method, the ratio(r) = Ce4+/(Ce3+ + Ce4+) was calculated. Comparing the calculated value of CeSZ(0.095) and CeZ(0.095), the CeSZ(0.095) displayed much lower r value (0.44) than that of the latter sample (approximately 0.77). The reason of the Ce3+ enrichment after sulfation treatment may be explained by the appearance of oxygen vacancies. Based on charge compensating mechanism, the sulfate could act as a stabilizer of oxygen vacancies during the calcination of the sulfated zirconia hydroxide [39]. Then, the stabilized oxygen vacancies not only stabilized the tetragonal crystalline phase, but also partially lowered the oxidation state of the surface cations [39,40]. Thus, for CeSZ catalysts, the tetragonal zirconia was the predominant phase of crystal, and Ce3+ was enriched on the surface of sulfated zirconia. The presence of Ce3+ species and oxygen vacancies could create a charge imbalance and unsaturated chemical bonds, which led to the increase in chemisorbed oxygen on catalyst surface and promoted the repeatable Ce4+/Ce3+ redox cycles in the final [38]. On the contrary, the Ce4+/Ce3+ redox cycles were seriously restrained once Ce3+ disappeared [38]. The abundance of Ce3+ in CeSZ could explain as the main reason for the SCR activity improvement of CeSZ catalyst. Fig. 8II showed O 1s spectra of sulfated catalyst and unsulfated one. The O 1s peak of CeZ(0.095) showed more than one species: crystal lattice oxygen Oa (529.4 eV), hydroxyl species Ob (531.0 eV), and chemisorbed water Oc (532.3 eV) [30,38]. In the case of CeSZ(0.095), the spectrum of oxygen became much broader, especially the enhanced Ob and Oc. The sulfation treatment could not only introduce an extra component of O in sulfates (enhanced Ob), but also enhance the content of surface chemisorbed water (enhanced Oc) [41,42]. The peak of Zr 3d doublet (Fig. 8III) was observed in the CeZ catalyst with BE values (181.7, 184.1 eV), relative to Zr(IV) species in the oxide phase [42–44]. In the case of CeSZ catalyst, a shift of these peaks to higher BE values by 0.6 eV was observed, which was related to the formation of a Zr(IV) species bound to a more electron attractive species as proposed in the literature [43]. Furthermore, the spectrum of S 2p (not shown) could be fitted for SZ support and CeSZ catalyst by a single component at BE = 169.1 eV, in agreement with the S (VI) oxidation state [42,44]. No appreciable presence of S-containing species with valence different from S6+ was ever observed.
Fig. 8. XPS spectra of (I) Ce 3d, (II) O 1s, and (III) Zr 3d.
3.2.7. FTIR FTIR spectra of samples were shown in Fig. 9. The peaks at 3415 cm1 and 1633 cm1 were attributed to the stretching vibration and distorting vibration of OH group, respectively [9,45]. Compared with the FT-IR spectra of unsulfated samples (Z,
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Natural Science Foundation of China (NSFC-51278458), Technological research for public welfare of Zhejiang Province (2012C23024), and Science Foundation of Chinese University. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.12.034. References [1] [2] [3] [4] [5] [6] [7] [8]
Fig. 9. FT-IR spectra of samples.
CeZ), the characteristic vibration peaks of OH group for sulfated samples (SZ, CeSZ) were stronger. Some bands existed at 490– 745 cm1 were assigned to ZrAOAZr bond [9]. The reason that these bands weakened after sulfation treatment was the formation of a Zr(IV) species bound to other species, which was in accord with the conclusion of XPS results. All the sulfated samples calcined at 600 °C showed IR bands of SO2 group in the region of 1300–900 cm1, with peaks at 4 1225.4, 1132.1 and 1046.1 cm1 (which were absent in spectra of unsulfated samples). These bands were typical characteristic of inorganic chelating bidentate sulfate ion coordinated to metal cation, which were assigned to asymmetric and symmetric stretching frequencies of S'O and SAO bonds [9,21,46]. The partially ionic nature of the S'O bond is responsible for the Bronsted acid sites in sulfated zirconia samples [21].
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
4. Conclusion [29]
A novel superacid catalyst, CeO2 supported on sulfated zirconia (CeSZ), was synthesized and used for the first time in the SCR of NO with ammonia. The catalyst with Ce/Zr molar ratio at 0.095 showed a significantly improved SCR performance by sulfation pretreatment of Zr(OH)4. The results of XRD, TG–DSC, Hammett indicator method, TEM, Raman, BET, XPS, and FT-IR spectra indicated that the incorporation of sulfate species into the zirconia supports led to (i) pure tetragonal phase of zirconia; (ii) smaller particle size and higher specific surface area (resulting in a well dispersion of ceria); (iii) enrichment of Ce3+ on the surface (leading to an increase in active oxygen content and accelerating the redox cycle of Ce3+/Ce4+); (iv) and stable superacidic properties (favoring the NH3 chemisorptions and activation). Consequently, the improved dispersion of ceria, the accelerated redox cycle of Ce3+/Ce4+ and the stable superacidic cites gave a promising SCR catalytic performance for CeO2 supported on sulfated zirconia catalysts. Acknowledgments This research was financially supported by Changjiang Scholar Incentive Program (Ministry of Education, China, 2009), National
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Ceria supported on sulfated zirconia as a superacid catalyst for selective catalytic reduction of NO with NH3 Shan Gao a, Xiongbo Chen a, Haiqiang Wang a,⇑, Jiansong Mo b,⇑, Zhongbiao Wu a,b, Yue Liu a, Xiaole Weng a a b
Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, China Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 29 September 2012 Accepted 15 December 2012 Available online 9 January 2013 Keywords: NO reduction SCR Ammonia Superacid Ceria Sulfated zirconia
a b s t r a c t In this paper, ceria supported on sulfated zirconia (CeSZ) as a superacid catalyst was synthesized and the resulted performances for selective catalytic reduction (SCR) of NO with NH3 were investigated. Experimental results revealed that the sulfation of zirconia supports could greatly improve the SCR activity of the catalysts. Among the tested samples, the CeSZ catalyst with Ce/Zr mole ratio at 0.095 possessed the highest NO conversion (i.e., 98.6% at ca. 420 °C and 180,000 h1). The sulfation had led to a formation of pure tetragonal phase of ZrO2, a well dispersion of CeO2, abundant stable superacid sites, increasing surface area and enrichment of Ce3+ on the surface, all of which were responsible for its excellent performance in SCR of NO with NH3. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Nitrogen oxides (NOx) from combustion of fossil fuels are one of the major pollutants to the atmosphere, which not only pose a serious threat to environment, but also cause photochemical smog, acid rain, and respiratory problems [1,2]. The selective catalytic reduction (SCR) of NOx with ammonia (NH3-SCR) is the most widely used technique for NOx emission control from stationary sources [3,4], where significant efforts have been devoted to develop novel SCR catalysts with promising catalytic activity and selectivity. Hamada et al. [5] had first found that the sulfated metal oxide catalysts (TiO2, ZrO2, Fe2O3, and Al2O3) were active in the propane-SCR reaction. Thereafter, the positive effect of sulfation treatment on SCR catalysts has attracted tremendous attentions, which is found to be connected with the increase in the surface area and the formation of acid sites on catalyst surface [6–8]. Recently, sulfated zirconia (SZ), a stronger solid acid than concentrated sulfuric acid, has been studied extensively [9–11]. Many elements supported on SZ were shown to be active for SCR reaction, including V [12], Pd [8,12], Pt [13], Co [6], Cu [14], Mn [15], and In [7,16]. On the other hand, ceria (CeO2) has been widely used as an essential ingredient in three-way catalysts (TWCs) for auto-
⇑ Corresponding authors. Address: Department of Environmental Engineering, Zhejiang University, Yuhangtang Road, No. 866, Hangzhou 310058, China. Fax: +86 571 87953088. E-mail addresses: [email protected] (H. Wang), [email protected] (J. Mo). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.12.034
motive emission control [17,18] and has also been considered as a promising candidate for selective catalytic reduction of NOx. In our previous work [4,19,20], ceria-based catalysts were proven to be active in the SCR of NO with ammonia. Furthermore, the catalytic performance for SCR reaction over ceria could be greatly enhanced after the surface sulfation by gaseous-phase SO2 [19]. Therefore, in this work, we developed a novel ceria catalyst supported on sulfated zirconia (SZ) for SCR reaction, which exhibited excellent catalytic activity. A series of characterizations such as XRD, TG–DSC, TEM, Raman, BET, XPS, FTIR, and Hammett indicator method were carried out to get insight into this superior performance. 2. Experimental methods 2.1. Catalyst preparation Pure ZrO2 (Z) was prepared by calcination of Zr(OH)4 (Sinopharm Chemical Reagent Co., Ltd., China) at a series of temperatures for 4 h in air. SO2 4 —ZrO2 (SZ) was synthesized in the same procedure by calcination of SO2 4 —ZrðOHÞ4 , which was obtained by impregnation of Zr(OH)4 with a solution of 0.25 N H2SO4 (immersion for 2 h, 5 ml/g). All the samples were dried at 80 °C overnight. The resulted supports were designated as Z-x or SZ-x, where x indicated the calcination temperature of precursors (ranging between 500 °C and 700 °C). The supported ceria-based catalysts were prepared by impregnating the zirconia supports with an aqueous solution of cerium
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nitrate. Based on the TG–DSC analysis of Zr(OH)4 and 2 SO2 4 —ZrðOHÞ4 , the calcination of SO4 —ZrðOHÞ4 at 600 °C was selected as an optimal calcination temperature for subsequent supports. After impregnation for 2 h, the samples were dried at 80 °C overnight and calcined at 450 °C for 3 h. The resulted catalysts were hereafter referred to CeSZ(y) or CeZ(y), where y indicated the Ce/Zr molar ratio (ranging between 0.024 and 0.190). Cu, Fe catalysts supported on sulfated zirconia were prepared by the same procedures, acting as reference catalysts. Copper nitrate and ferric nitrate were used as precursors. Resulted catalysts were referred to CuSZ(y) or FeSZ(y), where y indicated the Cu/Zr or Fe/Zr molar ratio.
pressure at 200–550 °C. A type K thermocouple was placed in the center of the reactor tube. The typical composition of inlet gas was 600 ppm NO, 600 ppm NH3, 3.5% O2, and N2 as balance gas. The gas hourly space velocity (GHSV) was about 180 000 h1. The concentration of NO, NO2, and O2 were monitored by a flue gas analyzer (Testo 335, Testo Inc., USA). N2O was detected by a FT-IR gas analyzer (Madur Photon Portable IR Gas Analyzers, Madur Ltd., Austria).
2.2. Characterizations
3.1.1. XRD of SO2 4 —ZrO2 X-ray diffraction patterns of zirconium supports after calcination were depicted in Fig. 1. The unsulfated ZrO2 support was a mixture of monoclinic (PDF-ICDD 37-1484: 2h = 28.2°, 31.5°) and tetragonal (PDF-ICDD 50-1089: 2h = 30.3°, 50.4°, and 60.2°) phases (mainly monoclinic). In contrast, only tetragonal phase was present in the sulfated ZrO2 supports. Based on the calculation of Scherrer formula, it could get that the crystallite size of unsulfated sample (Z-600) was 15.7 nm, and that of sulfated sample (SZ600) was 9.3 nm. This revealed that the sulfation had resulted in the decrease in crystallite size of the supports. When calcined at 500 °C, sulfated ZrO2 showed poorly resolved peaks of zirconia, which might indicate that the composition of SZ500 was mainly amorphous phase. Characteristic peaks of tetragonal ZrO2 were well resolved after higher temperature treatments at 600 °C or 700 °C due to the observed peak splitting at ca. 35.0° and 60.0°. The sharpened peaks of SZ calcinated at 700 °C indicated that the crystallite size was enlarged after the calcination at higher temperature. The absence of monoclinic phase in the sulfated samples, due to the delay in the phase transformation, could be affected by lots of factors. Some investigators have ascribed the stabilization of tetragonal ZrO2 to a particle size effect, with smaller particle size having greater surface energy contribution to their stability [23,24]. Besides, it was also connected with the pressure and the presence of phase stabilizers either in the bulk (usually as rareearth cationic dopants with intrinsic defects associated with them) or at the surface (such as water-derived or sulfate groups) [8,24]. In our case, the stabilization of tetragonal phase could be achieved by the smaller size of sulfated samples as resulted from sulfation, the introduction of cationic dopant (SO2 4 during sulfation treatment),
The crystal phases of the samples were identified by using X-ray diffraction (XRD) with Cu Ka radiation (model D/max RA, Rigaku Co., Japan). The scanning range (2h) was collected from 10° and 80° with a step size of 0.02°. Crystallite size of ZrO2 was determined from the characteristic peak (2h = 30.27° for the (011) reflection of tetragonal phase and 28.17° for the (1 1 1) reflection of monoclinic phase) by using Scherrer formula [21] as below:
Crystallite size ¼ K
k B1=2 cos h
where K is the shape factor (often assigned a value of 0.89 if the shape is unknown [22]), k is X-ray wavelength (at 1.54056), B1/2 the full-width-half-maximum (FWHM) of the ZrO2 characteristic peak, and h is the diffraction angle. Thermogravimetry and differential scanning calorimetry (TG– DSC: STA-409PC, NET-ZSCH, Germany) were performed. A solid sample was scanned at a heat rate of 10 °C /min in flowing N2. The surface acid strength of samples was measured by Hammett indicators, including p-nitrotoluene (pKa = 11.35), m-nitrotoluene (pKa = 11.99), and p-chloronitrobenzene (pKa = 12.70). The morphology, structure, and crystallite size of the samples were examined by transmission electron microscopy (TEM: JEM2010, Japan). The Raman spectra of samples were collected at a Raman Spectrometer (Ramen: Jobin–Yvon LabRAM HR800), using a laser at 514.5 nm line as the excitation source. The laser power of the 514.5 nm line at the samples was below 1.0 mW. The specific surface areas were determined by the Brunauer– Emmett–Teller (BET) method on a nitrogen adsorption apparatus (ASAP 2020, USA). All the samples were degassed at 300 °C prior to measurements. The data were collected in the relative pressure (P/P0) ranging from 0.05 to 0.30. X-ray photoelectron spectroscopy (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties of the samples, with Al Ka radiation (hv = 1486.6 eV). The shift of the binding energy due to relative surface charging was corrected using the C 1s level at 284.8 eV as an internal standard. The nature of bonding of sulfate ions with zirconia surface after calcination at 600 °C was studied by FT-IR spectrophotometer (Bruker ALPHA). The sample was diluted with KBr (100:3) and transferred to a 13 mm stainless steel die, compressed at a pressure of 10 MPa by a tablet press (769YP-15A) to obtain the selfsupporting pellet. The IR Transmission Spectra of samples were recorded in the range of 4000–400 cm1.
3. Results and discussion 3.1. Structure analysis of Z and SZ supports
2.3. SCR activity evaluation Selective catalytic reduction of NO with NH3 was carried out in a fixed-bed quartz reactor (1 cm i.d.) using a 0.5 g catalyst of 40–60 mesh. The experiments were performed under atmospheric
Fig. 1. XRD patterns of unsulfated and sulfated zirconia.
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and the subsequently doping of rare-earth cationic dopant (ceria addition during the catalyst preparation). 3.1.2. TG–DSC analysis of SO2 4 —ZrO2 It was reported in the previous work [9,25] that high temperature calcination was necessary for the formation of superacid sites, but it also resulted in loss of sulfates and concomitant transformation of tetragonal zirconia into monoclinic zirconia [25]. Tetragonal zirconia phase was suggested to be necessary for the generation of higher acidity in the sulfated zirconia as well as higher activity [26]. Therefore, there is necessary to seek out the optimal calcination temperature, which favored the retention of tetragonal phase of zirconia. In order to investigate the thermal stability of zirconium supports and superacid sites, thermal analysis were applied for Zr(OH)4 and sulfated Zr(OH)4. The results presented in Fig. 2 showed the characteristics of TG–DSC curves that were similar to those reported previously [27,28]. The whole temperature range could be divided roughly into three stages: In the first stage between 30 °C and 250 °C, the evident weight loss of samples accompanying a strong endothermic process at ca. 115 °C was observed, which was resulted from the desorption of physical adsorbed water and dehydroxylation during the thermal decomposition of zirconium hydroxide to zirconium oxide. The second stage could be observed from 250 °C to 650 °C related to a slight weight loss of 3–5%. One sharp exothermic peak centered at 432.5 °C (generally observed between 420 °C and 470 °C [8]) was investigated, which was attributed to the heat effect of the phase transformation of zirconia from an amorphous to crystalline state [8]. However, the corresponding peak was not detected in SO2 4 —ZrðOHÞ4 , which may shift to higher temperature and superimpos with endothermic peak of sulfate decomposition. This result indicated that the incorporation of sulfate was in favor of the stabilization of tetragonal zirconia phase, which agreed well with the XRD results. It has also been reported that the exothermic peak would shift to higher temperature when zirconia was doped with additives regardless of anions or cations [8]. The third stage in the curve began at ca. 650 °C, which was only observed in sulfated Zr(OH)4, where a weight loss of ca. 10% was observed. This weight loss was due to the decomposition of sulfate species [8,28], which confirmed that the sulfate species (superacid centers) on sulfated zirconia had a high thermal stability. Moreover, the overall weight loss of SO2 4 —ZrðOHÞ4 was much less than the unsulfated material (Zr(OH)4), suggesting that the incorporation of sulfate species had displaced part of hydroxide groups [8].
In summary, significant physicochemical changes were caused by the sulfation and heat treatment of the original zirconium hydroxide. The incorporation of sulfate helped to stabilize the metastable tetragonal zirconia phase, and the superacid centers were highly stable below 650 °C. Thus, although the two calcinations (i.e., at 600 °C, 700 °C) had all resulted in the presence of pure tetragonal structure, the calcination of SO2 4 —ZrðOHÞ4 at 600 °C was selected as the optimal calcination temperature due to the retention of superacid centers.
3.1.3. Hammett acidity of SO2 4 —ZrO2 The acidic strengths of the supports after calcination at 600 °C were examined by using Hammett indicator method (shown in Table 1). As is well known, the acidic strength of a solid was determined by the ability of the surface to convert an adsorbed neutral base into its conjugate acid [29]. From the table, it could be seen that the acid strength of Z-600 was H0 > 11.35, while that of SZ-600 was estimated to be H0 < 12.70, which was higher than that of 100% H2SO4 (H0 = 11.93), indicating that sulfated zirconia after calcination at 600 °C were superacids, further confirming the TG–DSC results.
3.2. Characterizations and performances of CeZ and CeSZ catalysts 3.2.1. Catalytic performances of NO over the catalysts The SCR performances of Ce catalysts supported on unsulfated or sulfated ZrO2 were depicted in Fig. 3. All of the used catalyst supports were calcined at 600 °C. As shown in Fig. 3, ceria supported on sulfated ZrO2 possessed much higher SCR activity than the corresponding unsulfated ones. More than 90% NO conversion had been achieved for CeSZ catalyst (Ce/Zr molar ratio at 0.095) in the temperature range from 330 °C to 510 °C. It was also noted that NO conversion over CeSZ(0.095) was higher than that over CeSZ(0.048), particularly in lower temperature. The influence of ceria contents on the SCR performance of CeSZ catalysts was shown in Fig. 4. It could be seen that the activity of CeSZ was strongly influenced by ceria contents. With increase in the ceria content, the conversion of NOAN2 was significantly enhanced. When Ce/Zr mol ratio got to 0.095, NO conversion up to 98.6% could be achieved at ca. 420 °C. After further increasing the Ce/Zr mol ratio to 0.190, the NO conversion went on increasing slightly in lower temperature region (below ca. 330 °C), but decreased in higher temperature region (above ca. 512 °C). Therefore, the Ce/Zr mole ratio at 0.095 of CeSZ catalyst was selected subsequently as the optimal loading content, which gave the optimal result in terms of catalytic performance and catalyst cost. To clearly demonstrate the advantages of CeSZ catalyst, the catalytic performance of ceria catalyst supported on SZ was compared with that of other elements (Cu, Fe), including NO conversion (Fig. S1), outlet N2O concentration (Fig. S2), and outlet NO2 concentration (Fig. S3). Actually, CeSZ catalyst obtained a comparatively higher SCR activity, yielding a 90% NO conversion within a broader temperature window, whereas CuSZ and FeSZ catalysts could only achieve ca. 78% (at 400 °C), ca. 85% (within 415–460 °C), respectively.
Table 1 The acid strengths of supports.
Fig. 2. TG–DSC profiles of Zr(OH)4 and SO2 4 —ZrðOHÞ4 .
Hammett indicator
pKa
SZ-600
Z-600
p-Nitrotoluene m-Nitrotoluene p-Chloronitrobenzene
11.35 11.99 12.70
+ + +
Note: Change in color (+) and no change in color ().
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Fig. 3. Temperature dependency of NO conversion over catalysts with or without sulfation treatment. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 3.5%, balance N2, catalyst 0.5 g, and GHSV about 180,000 h1.
Fig. 5. XRD patterns of unsulfated and sulfated catalysts.
47.5°, and 56.3°) could be observed in CeSZ until the Ce/Zr molar ratio was increased to 0.190. This result suggested that ceria had a better dispersion on the sulfated supports than that on the unsulfated supports, and CeO2 crystallites would form with the enlarged Ce loading. The aggregation of CeO2 would eventually lead to a bad dispersion of CeO2 on the sulfated ZrO2.
Fig. 4. Temperature dependency of NO conversion over CeSZ catalysts with various Ce/Zr mol ratios. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 3.5%, balance N2, catalyst 0.5 g, and GHSV about 180,000 h1.
N2O and NO2, the potential by-products in SCR reaction, had also been monitored in the experiments (Figs. S2 and S3). For CeSZ catalyst, the generated N2O concentration was no more than 10 ppm and NO2 concentration was less than 15 ppm within the whole temperature window, which were much lower than that of CuSZ and FeSZ catalysts. Overall, it could be concluded that ceria catalyst supported on sulfated zirconia showed the highest catalytic activity and the best N2 selectivity among the tested catalyst.
3.2.2. XRD XRD patterns of the ceria catalysts were illustrated in Fig. 5. The ceria supported on unsulfated ZrO2 support showed a mixture of monoclinic (PDF-ICDD 37-1484: 2h = 28.2°, 31.5°) and tetragonal (PDF-ICDD 50-1089: 2h = 30.3°, 50.4°, and 60.2°) phases (mainly monoclinic). However, all the catalysts supported on sulfated zirconia only presented tetragonal zirconia phase. Otherwise, the pattern of CeZ(0.095) catalyst showed an evident peak of CeO2 (PDF-ICDD 34-0394: 2h = 47.47), while no distinct peaks corresponding to crystalline CeO2 (PDF-ICDD 34-0394: 2h = 28.5°,
3.2.3. TEM The morphology and structure of the CeSZ and CeZ catalysts were investigated by TEM images. TEM micrograph of CeSZ (Fig. 6a) reflected well separated crystallites of rather irregular shape (averagely 9.28 nm, in the range of 6.0–13.1 nm), while the image of CeZ (Fig. 6c) displayed well-developed nanocrystals with average size of 15.28 nm (12.0–16.4 nm). The crystallite size observed by TEM was in good agreement with the result estimated by Scherrer equation. Fig. 6b and d were referred to the HR-TEM micrographs of CeSZ and CeZ, respectively. The CeZ displayed relatively better defined contours, exhibiting higher crystalline order. As shown in Fig. 6d, the d-spacing of CeZ was measured at ca. 0.320 nm, which was ascribable to monoclinic ZrO2 phase, and namely to the (1 1 1) crystal planes. Besides, the d-spacing of CeSZ was measured most frequently at ca. 0.304 nm, which was in proximity to the (0 1 1) plane of tetragonal ZrO2 (Fig. 6b). It was noted that the morphological feature was consistent with both XRD results and those reported in the previous sections. In addition, no CeO2 particles could be observed over the images of CeZ or CeSZ catalysts. 3.2.4. Raman spectra ZrO2 can exist in three crystalline forms: tetragonal, monoclinic, and cubic [24,30]. The tetragonal form has Raman bands at 149 [31], 269 [31,32], 314 [31,32], 454 [32] and 644 [32] cm1, and the monoclinic form has bands at 181 [31,32], 219 [31], 305 [31], 334 [31,32], 346 [32], 381 [32], 474 (strong) [31,32], 557 [31], 617 [32] and 639 [31] cm1. For unsulfated zirconia samples (Z, CeZ), bands at 181 [31,32], 221 [31], 305 [31], 334 [31,32], 346 [32], 381 [32], 474 [31,32], 557 [31], 617 [32], and 639 [31] cm-1 were assigned to the Raman-active modes for the monoclinic phase of ZrO2, while weak bands appeared at 269 and 454 cm1 were assigned to the Raman-active modes for tetragonal ZrO2. In the case of SZ support, no Raman bands could be observed. The reason of this phenomenon could be possibly due to the fluorescence of the sample, which
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Fig. 6. TEM and HR-TEM micrographs of (a) and (b)CeSZ(0.095); and (c) and (d)CeZ(0.095).
prevented the observation of possible bands of the crystalline form of ZrO2 [32,33]. After the ceria doping on sulfated zirconia, typical tetragonal phase were observed with no indication of the presence of monoclinic phase. A band at 1017 cm1 was ascribed to the reflection of sulfate group [32]. For pure ceria, which has a fluorite structure, the F2g mode is centered at ca. 456 cm1 in the Raman spectrum [34–36]. As shown in Fig. 7, the sharpened 474 nm band and a broad shoulder
Fig. 7. Raman spectra of ceria supported on sulfated or unsulfated zirconia.
(at. ca. 456 cm1) were observed in CeZ after the addition of ceria. Evidently, the band of monoclinic phase had some overlapping with the band of the cubic CeO2 phase [32]. And for CeSZ, only the characteristic bands of tetragonal phase were observed, which is consistent with the XRD results. 3.2.5. BET Pure zirconia calcined at 600 °C had a surface area of 36.94 m2/g. Compared with the pure zirconia, the surface area of CeZ had little change with the doping of ceria (36.19 m2/g). The surface area of zirconia was found to increase to 48.63 m2/g after sulfation treatment. This was in accord with the XRD results, which showed smaller particle sizes of sulfated samples than unsulfated ones. It has been reported that the presence of sulfate seemed to delay the formation of oxo bonds [23], which caused the great loss of surface area under high calcination temperature. Similar effect has also been reported for SO2 4 —TiO2 , where BET surface area is proportional to the amount of loaded sulfate [37]. The increased surface area would lead the better dispersion of the active components [26]. Furthermore, the surface area of CeSZ decreased to 44.18 m2/g after the introduction of ceria. 3.2.6. XPS analysis XPS was used to obtain further information about the valence/ oxidation state of the elements and surface composition of samples. The surface concentrations of various elements were summarized in Table 2. It was shown that the surface Ce/Zr molar ratios (CeSZ at 0.14, CeZ at 0.22) were apparently higher than the corresponding designed value (0.095), suggesting a preference of ceria
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Table 2 XPS results for samples.
CeSZ CeZ
Oa (%)
Ob (%)
Oc (%)
Zr (%)
S (%)
Ce (%)
37.79 49.98
24.21 15.86
10.31 5.20
20.54 23.81
4.25 –
2.91 5.16
aggregating on the surface of supports. Moreover, the Ce/Zr ratio for CeSZ was much lower than that on the CeZ. This result suggested that a less aggregation of ceria existed on the surface of sulfated zirconia, due to the higher surface area of SZ as confirmed by BET results. Fig. 8I showed the Ce 3d spectra of the sulfated catalyst (CeSZ(0.095)) and the unsulfated one (CeZ(0.095)). The peaks labeled u, u2, u3, v, v2, and v3 in black represented the 3d104f0 state of Ce4+, while the bands labeled u1 and v1 in red represented the 3d104f1 initial electronic state corresponding to Ce3+ [38]. It was worth notice that the intensities of Ce4+ characteristic peaks decreased apparently after sulfation treatment of the supports. For the calculation of Ce4+/Ce3+, the method proposed by Damyanova was used [35]. This method assumed that the ratio(r) of the area of the satellite at 916.7 eV to the total area was proportional to the surface concentration of Ce4+ and that this ratio attained 0.16 for all cerium atoms in the Ce4+ state [35]. By this method, the ratio(r) = Ce4+/(Ce3+ + Ce4+) was calculated. Comparing the calculated value of CeSZ(0.095) and CeZ(0.095), the CeSZ(0.095) displayed much lower r value (0.44) than that of the latter sample (approximately 0.77). The reason of the Ce3+ enrichment after sulfation treatment may be explained by the appearance of oxygen vacancies. Based on charge compensating mechanism, the sulfate could act as a stabilizer of oxygen vacancies during the calcination of the sulfated zirconia hydroxide [39]. Then, the stabilized oxygen vacancies not only stabilized the tetragonal crystalline phase, but also partially lowered the oxidation state of the surface cations [39,40]. Thus, for CeSZ catalysts, the tetragonal zirconia was the predominant phase of crystal, and Ce3+ was enriched on the surface of sulfated zirconia. The presence of Ce3+ species and oxygen vacancies could create a charge imbalance and unsaturated chemical bonds, which led to the increase in chemisorbed oxygen on catalyst surface and promoted the repeatable Ce4+/Ce3+ redox cycles in the final [38]. On the contrary, the Ce4+/Ce3+ redox cycles were seriously restrained once Ce3+ disappeared [38]. The abundance of Ce3+ in CeSZ could explain as the main reason for the SCR activity improvement of CeSZ catalyst. Fig. 8II showed O 1s spectra of sulfated catalyst and unsulfated one. The O 1s peak of CeZ(0.095) showed more than one species: crystal lattice oxygen Oa (529.4 eV), hydroxyl species Ob (531.0 eV), and chemisorbed water Oc (532.3 eV) [30,38]. In the case of CeSZ(0.095), the spectrum of oxygen became much broader, especially the enhanced Ob and Oc. The sulfation treatment could not only introduce an extra component of O in sulfates (enhanced Ob), but also enhance the content of surface chemisorbed water (enhanced Oc) [41,42]. The peak of Zr 3d doublet (Fig. 8III) was observed in the CeZ catalyst with BE values (181.7, 184.1 eV), relative to Zr(IV) species in the oxide phase [42–44]. In the case of CeSZ catalyst, a shift of these peaks to higher BE values by 0.6 eV was observed, which was related to the formation of a Zr(IV) species bound to a more electron attractive species as proposed in the literature [43]. Furthermore, the spectrum of S 2p (not shown) could be fitted for SZ support and CeSZ catalyst by a single component at BE = 169.1 eV, in agreement with the S (VI) oxidation state [42,44]. No appreciable presence of S-containing species with valence different from S6+ was ever observed.
Fig. 8. XPS spectra of (I) Ce 3d, (II) O 1s, and (III) Zr 3d.
3.2.7. FTIR FTIR spectra of samples were shown in Fig. 9. The peaks at 3415 cm1 and 1633 cm1 were attributed to the stretching vibration and distorting vibration of OH group, respectively [9,45]. Compared with the FT-IR spectra of unsulfated samples (Z,
S. Gao et al. / Journal of Colloid and Interface Science 394 (2013) 515–521
521
Natural Science Foundation of China (NSFC-51278458), Technological research for public welfare of Zhejiang Province (2012C23024), and Science Foundation of Chinese University. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.12.034. References [1] [2] [3] [4] [5] [6] [7] [8]
Fig. 9. FT-IR spectra of samples.
CeZ), the characteristic vibration peaks of OH group for sulfated samples (SZ, CeSZ) were stronger. Some bands existed at 490– 745 cm1 were assigned to ZrAOAZr bond [9]. The reason that these bands weakened after sulfation treatment was the formation of a Zr(IV) species bound to other species, which was in accord with the conclusion of XPS results. All the sulfated samples calcined at 600 °C showed IR bands of SO2 group in the region of 1300–900 cm1, with peaks at 4 1225.4, 1132.1 and 1046.1 cm1 (which were absent in spectra of unsulfated samples). These bands were typical characteristic of inorganic chelating bidentate sulfate ion coordinated to metal cation, which were assigned to asymmetric and symmetric stretching frequencies of S'O and SAO bonds [9,21,46]. The partially ionic nature of the S'O bond is responsible for the Bronsted acid sites in sulfated zirconia samples [21].
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
4. Conclusion [29]
A novel superacid catalyst, CeO2 supported on sulfated zirconia (CeSZ), was synthesized and used for the first time in the SCR of NO with ammonia. The catalyst with Ce/Zr molar ratio at 0.095 showed a significantly improved SCR performance by sulfation pretreatment of Zr(OH)4. The results of XRD, TG–DSC, Hammett indicator method, TEM, Raman, BET, XPS, and FT-IR spectra indicated that the incorporation of sulfate species into the zirconia supports led to (i) pure tetragonal phase of zirconia; (ii) smaller particle size and higher specific surface area (resulting in a well dispersion of ceria); (iii) enrichment of Ce3+ on the surface (leading to an increase in active oxygen content and accelerating the redox cycle of Ce3+/Ce4+); (iv) and stable superacidic properties (favoring the NH3 chemisorptions and activation). Consequently, the improved dispersion of ceria, the accelerated redox cycle of Ce3+/Ce4+ and the stable superacidic cites gave a promising SCR catalytic performance for CeO2 supported on sulfated zirconia catalysts. Acknowledgments This research was financially supported by Changjiang Scholar Incentive Program (Ministry of Education, China, 2009), National
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