Catal Lett (2009) 132:253–258 DOI 10.1007/s10562-009-0096-7
Phase structure of V2O5/TiO2 catalyst and catalytic behavior with removal of NO by ammonia Xin Zhang Æ Xiaogang Li Æ Junsheng Wu Æ Renchun Yang Æ Zhihua Zhang
Received: 13 June 2009 / Accepted: 9 July 2009 / Published online: 28 July 2009 Ó Springer Science+Business Media, LLC 2009
Abstract V2O5/TiO2 catalyst with 3% (w/w) V loading has been prepared by sol–gel method. The characterization results of the catalyst structure and catalytic activity show that VOX state is strongly dependent on the calcination temperature. Little effect is found for phase structure of TiO2 support on catalytic activity. High catalytic activity in wide temperature range (240–420 °C) is observed for the catalysts calcinated at different temperatures at a space velocity of 50,000 h-1. Space velocity and alkali metal oxides strongly influence the catalytic activity of the catalyst which was calcinated at 450 °C, furthermore, the one has high tolerance to SO2 in our test conditions. Keywords V2O5/TiO2 Phase structure Catalytic activity
1 Introduction Anatase titania has been widely used in photocatalyst [1–4], solar cell [5–8] and catalyst support [9, 10] owning to its cheapness, stability and nontoxicity. In nature, titania exists as three phases, that is, brookite, anatase and rutile, and rutile is more stable than brookite and anatase thermodynamically. X. Zhang X. Li (&) J. Wu R. Yang Z. Zhang School of Material Science & Engineering, University of Science & Technology Beijing, 100083 Beijing, People’s Republic of China e-mail:
[email protected]
In the previous studies of the catalysts of removal of NO by ammonia, anatase titania is widely used as catalyst support. Many approaches have been tested to improve the catalytic activity and thermal stability of the V2O5/TiO2 catalyst, for example the addition of catalytic and structural promoter such as WO3 [11–13], MO3 and rare earth elements [14], etc. Moreover, V2O5/TiO2 and V2O5–WO3/TiO2 catalysts have been prepared using sulfated TiO2 support by the method of impregnation to improve the catalytic activity due to increased acid site and to enhance thermal stability. Above all the studies of V2O5/TiO2 catalysts of removal NO by ammonia, the TiO2 support is the monophasic anatase. Besides, the addition of structural promoter is more favor of maintaining the monophasic anatase. However, in the photocatalysis, mixed-phase TiO2 revealed high activity due largely to the synergistic effect between anatase and rutile phase [15, 16]. But, it is unclear of the effect for the TiO2 support with mixed phases or monophasic rutile phase on the catalytic activity of V2O5/TiO2 catalyst with removal of NO by ammonia. In view of these, the V2O5/TiO2 catalyst with 3% (w/w) vanadia loading was prepared in nitric acid solution by sol– gel method, and then it was calcinated at various temperatures to obtain catalysts with different TiO2 phase compositions. The characterization and discussions were also made on the distribution of VOX state, the resultant effects on the structure as well as the catalytic activity of the catalysts.
2 Experimental
X. Zhang e-mail:
[email protected]
2.1 Catalyst preparation
Z. Zhang PetroChina Daqing Petrochemical Research Center, 163714 Daqing, People’s Republic of China
A typical procedure to prepare V2O5/TiO2 catalyst was as follows: 0.228 g cetrimonium bromide and 2 mol L-1
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quantitative vanadium oxalate were dissolved in 10 g 1 mol L-1 nitric acid with continuous stirring for 20 min. Then, 4.26 g tetrabutylorthotitanate was added into the solution under stirring for another 10 min. After that, the solution was stratified and the lower layer solution was collected by separatory funnel. Then 0.45 g formamide was added into the solution to adjust the pH under continuous stirring for 5 min. The resultant sol was loaded into tetrafluoroethane-sealed box in water bath at 60 °C and it changed to gelatin gradually. After aging at 60 °C for 72 h, the wet gelatin was then dried in an oven at 60 °C for another 72 h. Finally, the catalyst powders were collected after calcinated in a furnace at 450, 550 and 650 °C for 2 h with the heating rate 1 °C min-1. 2.2 Characterization of catalysts X-ray diffraction (XRD) was preformed on a D/MAX-RB X-ray diffractometer (Rigaku Co., Japan, Cu Ka radiation, ˚ ). Scans were taken over a 2h range of k = 1.5406 A 10–90 ° at a speed of 6 ° min-1. X-ray photoelectron spectroscopy (XPS) was preformed with an ESCALAB 250 system (Thermalfisher Co., USA) equipped with an Al anode (Al Ka = 1,486.6 eV) as excitation source. The pressure in the main chamber was about 10-9 mbar. The C 1s line was used as an internal standard to calibrate the binding energy. XPS measurements were carried out only with samples after calcination. Nitrogen adsorption/ desorption was used to determined specific surface area of catalyst on an autosorb 1C apparatus (Quantachrome Instruments Co., USA). Raman spectra were measured with a Nicolet Almega XR spectrometer (Thermo Electron Co., USA) with 632 nm line of Ar ion laser as excitation source under ambient condition. SupraTM 55 field emission scanning electron microscopy (FESEM, ZEISS, Germany) was employed to investigate the structure of the catalysts.
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each reaction temperature to reach a steady state before each test was taken.
3 Results and discussions 3.1 Activity test Figure 1 shows the results of the catalytic activity tests in removal of NO by ammonia performed over the catalysts which were calcinated at 450, 550 and 650 °C, respectively. It’s very obvious that there is the same trend in catalytic activity of the catalysts. The NO conversion was greater than 95% in a wide temperature range (270–420, 240–420 and 270–390 °C respectively calcinated at 450, 550 and 650 °C). The differences are the original temperature with high catalytic activity (greater than 90%) and the decrease temperature inflexion of NO conversion in the high temperature range. 3.2 Effect of GHSV, SO2 and alkali metal oxides on NO reduction by ammonia The influences of the GHSV, SO2 and alkali metal oxides on the NO conversion over the catalyst calcinated at 450 °C were studied in the paper. As shown in Fig. 2a, the catalytic activity decreased dramatically with increasing the space velocity from 10,000 to 100,000 h-1 especially under 300 °C. It is obvious that the catalytic activity decreased dramatically with the space velocity range (10,000–300,000 h-1) at fixed test temperature of 330 °C in the Fig. 2b. These results indicate that this catalyst is highly effective for NO reduction reaction within a wide
2.3 Activity test Catalyst activity was evaluated in a fixed bed flow reactor at atmospheric pressure. Typically, about 0.9 cm3 catalyst (60–120 mesh) was charged in a diameter of 12 mm stainless steel tube reactor. A feed gas stream containing NH3 (1,000 ppm), NO (1,000 ppm), O2 (5%) and N2 (balance) was adjusted by mass flow controllers and introduced to the reactor with a total flow rate of 750 cm3 min-1, yielding a gas hourly space velocities (GHSV) of 50,000 h-1. The test temperature was monitored and controlled by two K type thermocouples located just on the top and the bottom of catalyst bed. Analysis of NO concentration was carried out using Testo 350Xl (Testo Co., Germany). The reaction system was kept for 30 min at
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Fig. 1 NO conversion of the catalysts after calcinated at different temperatures. Reaction conditions: NO 1,000 ppm, NH3 1,000 ppm, O2 5%, balance N2, GHSV = 50,000 h-1 total flow rate 750 cm3 min-1
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Fig. 2 Different factors influencing the catalytic activity of the catalyst after calcinated at 450 °C a, b GHSV; c 500 ppm SO2 and d alkali metal oxide. Reaction condition: NO 1,000 ppm, NH3 1,000 ppm, O2 5%, balance N2
120 min at 330 °C and after SO2 addition, was completed, the test continued for another 60 min. When 500 ppm SO2 was added into the reaction system, the NO conversion decreased quickly from 99% to approximate 97%, and then remained stable. It turned back to approximate 99% when SO2 addition was stopped. The results reveal that this catalyst has a good sulfur tolerance under our test conditions. Figure 2d shows the influence of alkali metal oxides (K, Na) on removal of NO by ammonia in the presence of O2. It can be seen that the NO conversion is strongly affected by addition of the alkali metal oxides with 1% (w/w) potassium and sodium oxide loadings in the catalyst in our test conditions. 3.3 Characterization of the catalysts Fig. 3 XRD pattern of the catalysts after calcinated at different temperatures
range of GHSV. Figure 2c shows the influence of SO2 on reaction of NO by ammonia in the presence of O2. Before addition of SO2, the SCR reaction was stabilized for Table 1 Morphological properties
A anatase, R rutlie, Dc mean grain size at (110) plane
3.3.1 XRD Figure 3 shows the XRD patterns of the catalyst after calcinated at different temperatures. Based on the XRD pattern, the powders were composed of mixed phase (anatase and rutile) after calcinated at 450 and 550 °C,
Catalyst
T calcinated ( °C)
SBET (m2 g-1)
Phase
Dc (nm)
V4? (%)
V5? (%)
CAT-1
450
90.09
82%A ? 18%R
24.3
100
CAT-2
550
29.80
50%A ? 50%R
31.2
68
32
CAT-3
650
16.35
R
37.1
65
35
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respectively. However, it consisted of monophasic rutile after calcinated at 650 °C. The relative proportion of anatase and rutile was determined according to the equation reported in literature [17]: Að%Þ ¼
1 1 þ ð1:4Ið110ÞR =Ið101ÞA Þ
where I is the intensity of the corresponding XRD peak. The crystalline diameters were calculated using Scherrer method. Table 1 lists the structure nature of all the samples after calcinated at different temperatures. The proportion of rutile increases with increasing of the calcination temperature, and all the anatase phase transforms into rutile phase when the calcination temperature is up to 650 °C. As shown in Table 1, the specific surface area decreased dramatically with increasing the calcination temperature and the lowest one is 16.35 m2 g-1 which was calcinated at 650 °C. The specific surface area reduced due to particles sintering and pore collapse when the catalysts were calcinated at high temperature. 3.3.2 XPS
Fig. 4 XPS spectra of the catalysts after calcinated at different temperatures, a V 2p3/2; bTi 2p3/2 and c O 1s
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SCR reaction is a process of oxidation/reduction. Casagrande found that the catalytic activity is affected by atomic state of active ingredient on the surface of catalyst [18]. XPS is used to investigate the atomic state of the catalyst. It can be observed that the XPS signals of V 2p3/2 appear at the binding energy of approximately between 515 and 518 eV, which can be fitted with three kinds of vanadium state. Moreover, the peak of binding energy is shifted after calcinated at different temperatures. The peak at 515.8 eV [19] of V 2p3/2 is attributed to V3? surface species and the one at 516.3 eV [20] is assigned to V4? surface species. The peaks at 516.6 [20], 516.9 [21] and 517.1 eV [22] is all ascribed to V5? surface species. Figure 4a shows V 2p3/2 peaks obtained from XPS analysis. According to Fig. 4a, the vanadium state on the support surface is mainly the V4? and V5? species and the evolution of vanadium surface species is shown in Table 1. To study the effect of calcination temperature and vanadium element on the surface of catalyst, the XPS analysis of Ti 2p3/2 and O1s are studied. The Ti 2p3/2 and O1s peaks of the catalyst are shown in Fig. 4b and c, respectively. The Ti 2p3/2 and O1s peaks indicate Ti4? due to TiO2 crystallite. In addition, the obvious peak shift appeared with increase of the calcination temperatures is due to the increase of rutlie phase proportion and the electronic interaction of V–Ti atoms.
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Fig. 5 Raman spectra of the samples
3.3.3 Raman spectra The surface structure of vanadium on catalysts was examined by Raman spectroscopy, as shown in Fig. 5. It is generally acknowledged that the band around 1,030 cm-1 is associated with V=O bridge of monomeric VOX [23–25], and the band around 920–940 cm-1 with V–O–V bridge of polymeric VOX species [26–29]. According to the previous studies [30, 31], V=O band of monomeric VOX spices is advantage for the SCR reaction. The present result shows the broad peaks around 1,019 and 917 cm-1 that are attributed to V=O bridge monomeric and V–O–V band polymeric VOX species, respectively. The bands shift is due to strong electronic interaction of V–Ti atoms. However, the ratio between monomeric and polymeric species can not be determined by Raman shift because of Raman intensity change and bands shift.
3.3.4 FESEM Figure 6 shows the typical images of the catalysts and the inserted image of each FESEM image is the line profile of the samples. The catalysts were sintered at different temperature and the diameter of the particles is approximately 300 nm. The line profile was drawn in each FESEM image where was labeled using the rectangle. The surface of the sample calcinated at lower temperature more roughness than the one at higher temperature due to the particles is consisted of different scale crystal gains. The crystal gain is growth with increasing the calcination temperature. The rough surface can provide more active react site for the reaction of removal NO by ammonia.
Fig. 6 FESEM images and profiles of the samples calcinated at a 450 °C; b 550 °C and c 650 °C
4 Conclusions V2O5/TiO2 catalysts with 3% (w/w) V loading have been prepared in 1 mol L-1 nitric acid solution with adding formamide by sol–gel method. The different phase composition of the catalysts was obtained after calcinated at different temperatures. The V2O5/TiO2 catalyst is highly active for the catalytic reduction of NO by ammonia in our test conditions. Phase structure of the catalyst and SO2 has slight influence on catalytic activity. The catalytic activity is related to the distribution and the state of vanadium on
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the surface of the catalyst. Vanadium was well dispersed in the form of V4? and V5? V=O bridge monomeric VOX species which is favor for the catalytic activity in our test conditions. In addition, space velocity and alkali metal oxides have obvious influence on the catalytic activity. In future work, the reaction mechanisms on phase structure of the support and vanadium state are worthy of further investigation. Acknowledgments The authors are grateful for the financial support of Chinese National Programs for High Technology Research and Development (No.2006AA05Z306). Meanwhile, the correction of English language by Dr. Yufeng Cheng, Jianlong Zhou and Lin Lu is gratefully acknowledged.
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