Environ. Sci. Technol. 2000, 34, 5211-5214
copy (XPS). The emergence of activity for NO conversion under photoirradiation is discussed.
Characterization of Highly Active AgCl/Al2O3 Catalyst for Photocatalytic Conversion of NO
Experimental Section
Y O H I C H I Y A M A S H I T A , * ,† NAOKO AOYAMA,† NOBUTSUNE TAKEZAWA,‡ AND KIYOHIDE YOSHIDA† Research and Development Division, Riken Corporation, 4-14-1 Suehiro, Kumagaya, Saitama 360-8522, Japan, and Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, West 8 North 13 Sapporo, Hokkaido 060-8628, Japan
Conversion of NO under UV irradiation was evaluated at 310 K in the presence of O2 in a flow-type reactor over AgCl/ Al2O3 catalysts calcined at various temperatures. The AgCl/ Al2O3 catalysts were highly active for the conversion of NO. N2O was produced with a small amount of N2 and NO2 on the AgCl/Al2O3 catalysts under UV irradiation. The AgCl/ Al2O3 catalyst calcined at 673 K where crystallized AgCl coexisted with isolated Ag+ ions showed the best performance among catalysts studied. Initial conversion was achieved at 83%. However, the conversion levels of NO on the AgCl/ Al2O3 catalysts calcined below 673 K were rapidly lowered with increased photoirradiation time. High stability for NO conversion was attained on the AgCl/Al2O3 catalysts calcined at 773 and 873 K, where the crystallized AgCl were well dispersed on Al2O3.
Introduction Nitrogen oxides (NOx) exhausted from internal combustion engines and furnaces are now over tolerable levels for both health and environmental protection. Diesel engines and lean-burning gasoline engines are the major sources of NOx emission (1). Recently, the application of photocatalysis has been studied for removal of dilute NOx (2-5). Ibusuki et al. reported that dilute NOx in ambient air is effectively removed by photocatalytic oxidation on TiO2 (2). Anpo et al. found that NO is converted to N2 and O2 on Cu/ZSM-5 under the UV irradiation in the absence of oxygen (3). They also reported that NO is photocatalytically converted into N2, N2O, and NO2 on Ag/ZSM-5 in the presence of oxygen (4). More recently, we have found that AgCl/Al2O3 is highly active for photocatalytic conversion of NO (6). On the other hand, AgCl/ SiO2 and AgCl/TiO2 (7) were less active for stable NO conversion with photoirradiation time. In the present paper, NO conversion on AgCl/Al2O3 catalysts is evaluated in the presence of O2 under UV irradiation, and the effects of calcination temperature on the NO conversion is examined. The catalysts are characterized by X-ray diffraction (XRD) method, ultraviolet-visible (UV-vis) spectroscopy, and X-ray photoelectron spectros* Corresponding author fax:
[email protected]. † Riken Corporation. ‡ Hokkaido University. 10.1021/es001164i CCC: $19.00 Published on Web 11/10/2000
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2000 American Chemical Society
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The AgCl/Al2O3 catalysts were prepared by impregnating γ-Al2O3 powders (230 m2/g, Condea Chemie) with an aqueous solution of silver nitrate (Kokusan Kagaku, silver nitrate solution: 4.68 × 10-2 mol/L) and ammonium chloride (Kokusan Kagaku, ammonium chloride solution: 4.68 × 10-2 mol/L). The resulting samples were dried at 393 K for 3 h and then heated to various calcination temperatures with a temperature raising rate of 5 K min-1 in an electric furnace (S80G, Denken) in static air. Finally, they were kept at the calcination temperatures for 3 h. The Ag content was 2.0 wt %. For comparison, Ag/Al2O3 (2.0 wt %), Ag/Ferrierite (2.9 wt % Ag), and Al2O3 catalysts were also used for the experiments. The Ag/Al2O3 catalyst was prepared by impregnating γ-Al2O3 powders (230 m2/g, Condea Chemie) with an aqueous solution of silver nitrate. The Ag/ferrierite catalyst was prepared by ion exchange of ferrierite powders (210 m2/g, TOSOH) in an aqueous solution of silver nitrate, ammonium nitrate, and ammonium hydroxide. The Ag/Al2O3 and Ag/ ferrierite catalysts were also dried at 393 K for 3 h and then calcined at various temperatures for 3 h in air. The Al2O3 catalyst was prepared by calcining γ-Al2O3 powders (230 m2/ g, Condea Chemie) at 873 K for 3 h in air. Conversion of NO under UV irradiation was evaluated at 310 K using a flow-type photochemical reaction system as previously used (6). Each catalyst (3.0 g) was packed in a reactor made of quartz and irradiated using a 500-W Xe lamp (UXL-500D, USHIO) passed through both a band-pass filter (Toshiba UV-D33S, 250-400 nm) and a water filter. The irradiation area was ca. 6 cm2, and light intensity was ca. 36 mW/cm2 (at 365 nm). A reactant gas was composed of 10 ppm NO and 10% O2 (N2 balance). Total inflow was always maintained at 620 cm3/min (reactor pressure: ∼1 atm), and reactor temperature was 310 K during the reaction. The concentration of NO + NO2 in the outlet gas stream was measured with a chemiluminescence-based NO/NO + NO2 analyzer (Yanagimoto Co. Ltd). NO conversion is defined as follows:
NO conversion )
[NO]inlet - [NO + NO2]outlet [NO]inlet
Products were analyzed by mass spectroscopy (UPM-ST200P, ULVAC). The AgCl/Al2O3 catalysts calcined above 393 K were characterized by the XRD method using CuKR radiation (MXP3, MAC Science), XPS, and UV-vis spectroscopy (UV2400PC, Shimadzu). The dispersion of crystallized AgCl particles on Al2O3 was analyzed using the XRD method. The apparent contents of Ag and Cl on the AgCl/Al2O3 catalysts were determined by XPS spectra for Ag(3d) and Cl(2p) electrons. The chemical states of Ag species in the AgCl/ Al2O3 catalysts were analyzed by UV-vis spectra.
Results and Discussion Figure 1 shows typical NO conversion against the reaction time under Xe lamp irradiation on the AgCl/Al2O3 catalysts calcined at various temperatures. A total of 74% of NO is initially removed on the AgCl/Al2O3 catalyst calcined at 393 K. The NO conversion is enhanced with increased calcination temperature up to 673 K, and 83% of NO is initially removed. However, the conversion levels of NO on the catalysts calcined VOL. 34, NO. 24, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. UV-vis diffuse reflectance spectra of the AgCl/Al2O3 catalysts calcined at 393 (a), 673 (b), 773 (c), and 873 K (d) in air. FIGURE 1. NO conversion on the AgCl/Al2O3 catalysts under Xe lamp irradiation against calcination temperatures and irradiation time. The AgCl/Al2O3 catalysts calcined at 393 (9), 473 (2), 573 (4), 673 (1), 773 (3), and 873 K (b). Test conditions: 10 ppm NO, 10% O2, balance N2, flow rate ) 620 cm3/min, catalyst weight ) 3.0 g. below 673 K are rapidly lowered with increased photoirradiation time. The initial NO conversion on the AgCl/Al2O3 catalyst calcined at 773 K is lower than that on the catalyst calcined at 673 K. Calcination at 873 K, however, results in an increase of the NO conversion as compared with that of the catalyst calcined at 773 K. This leads to the initial NO conversion of 75%. The NO conversion on the AgCl/Al2O3 catalysts calcined at 773 and 873 K is also decreased with the increased photoirradiation time. However, lowering of the NO conversion with photoirradiation time is much slower than those on the catalysts calcined below 673 K. Some experiments were carried out on the Ag/Al2O3 catalysts calcined at 373-873 K and on the Ag/ferrierite. The activities of these catalysts were much lower than those of the Ag/Al2O3 catalysts. Among the Ag/Al2O3 catalysts used, the Ag/Al2O3 catalyst calcined at 673 K exhibited the best performance for the NO conversion. Initial conversion of NO on the Ag/Al2O3 catalyst calcined at 673 K was 14%. On the other hand, the Ag/ferrierite catalyst calcined at 673 K showed an NO conversion level of 12% at the initial period of the reaction. Figure 2 shows the UV-vis diffuse reflectance spectra of the AgCl/Al2O3 catalysts calcined at 393-873 K. An intense absorption band at ∼210 nm and a broad absorption band at ∼250 nm are observed for the AgCl/Al2O3 catalysts after calcination at 393-673 K. The intensity of the absorption band at ∼250 nm increases with increased calcination temperature; it increases markedly after calcination above 673 K. The absorption band at ∼210 nm, however, decreases greatly on calcination above 773 K. The absorption bands at ∼210 and ∼250 nm are attributed to 4d10 f 4d95s1 electron transition of isolated Ag+ ions (4, 8-10) and band gap of AgCl particles (11, 12) on Al2O3, respectively. Figure 3 illustrates the UV-vis diffuse reflectance spectra of the Ag/Al2O3 catalysts calcined at 393-873 K and Ag/ ferrierite calcined at 673 K. The Ag/Al2O3 catalysts exhibit an intense absorption band at ∼210 nm caused by isolated Ag+ ions (8-10) on calcination at 393-673 K. The absorption 5212
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FIGURE 3. UV-vis diffuse reflectance spectra of the Ag/Al2O3 catalysts calcined at 393 (a), 673 (b), 773 (c), and 873 K (d) in air, and the Ag/Ferrierite catalyst calcined at 673 K (e) in air. band at ∼210 nm, however, diminishes after calcination above 773 K. A broad absorption band at ∼230 nm appears at 873 K. The absorption band at ∼230 nm can be ascribed to the disordered silver oxides on Al2O3 since calcination of Ag/Al2O3 catalyst at 873 K in air leads to the formation of disordered silver oxides (6, 13). The Ag/ferrierite catalyst also shows an intense absorption band at ∼210 nm caused by isolated Ag+ ions on calcination at 673 K. These results show that isolated Ag+ ions are predominantly present for the Ag/Al2O3 catalysts after calcination below 673 K and for the Ag/ferrierite catalyst calcined at 673 K. On the other hand, isolated Ag+ ions and AgCl particles are formed for the AgCl/Al2O3 catalysts on calcination below 673 K while AgCl particles are mainly produced at the expense of isolated Ag+ ions by calcination above 773 K. Since the AgCl/Al2O3 catalysts were much more active for photocatalytic conversion of NO than the Ag/Al2O3 and the Ag/ferrierite catalysts, it is suggested that the presence of the band at 250 nm enhances the reaction. It is to be noted that NO conversion is very high on the AgCl/Al2O3 catalyst calcined at 673 K with absorption bands at ∼210 and 250 nm as shown
FIGURE 4. X-ray diffraction patterns of the AgCl/Al2O3 catalysts calcined at 393 (a), 473 (b), 573 (c), 673 (d), 773 (e), and 873 K (f) in air, and γ-Al2O3 calcined at 873 K (g).
TABLE 1. Characteristics of Silver or Silver Compounds in the AgCl/Al2O3 Catalysts Analyzed with XPS AgCl/Al2O3 catalyst
FIGURE 5. X-ray diffraction peak of crystallized AgCl from the AgCl/ Al2O3 catalysts observed at different calcination temperatures: 393 (a), 673 (b), 773 (c), and 873 K (d). XRD spectra are shown in 2θ range of 31-34°. in Figure 2. The high NO conversion at early stage suggests that AgCl particles in the presence of isolated Ag+ ions promote the NO conversion over the AgCl/Al2O3 catalysts more effectively than AgCl particles alone. Electron transfer from excited Ag+ to AgCl probably occurs as observed for the CdS/TiO2 couple (14) and thereby enhances photocatalytic conversion of NO on the AgCl/Al2O3 catalysts with the absorption band at ∼210 nm in addition to the absorption band at ∼250 nm. Figure 4 shows the XRD spectra of the AgCl/Al2O3 catalysts calcined at 393-873 K in air and of the Al2O3 catalyst calcined at 873 K. Distinct XRD peaks attributable to crystallized AgCl and Al2O3 are observed for the AgCl/Al2O3 catalysts. Figure 5 illustrates an XRD peak at 2θ ) 32.2° for crystallized AgCl formed on the Al2O3 after calcination at various temperatures. The peak attributed to AgCl decreases in intensity with increasing calcination temperature. The peak markedly decreases in intensity and becomes broader on calcination at 773 K. The decreasing and broadening of the peak are further progressed on calcination at 873 K. These results show
XPS
673 K
873 K
binding energy of Ag(3d5/2), eV Cl(2p), eV apparent surface Ag content (atom %) Cl content (atom %)
367.47 197.90 1.95 2.96
367.47 197.90 2.19 3.20
that the average size of AgCl particles markedly decreased on calcination at 773 K and the decreasing was further enhanced on calcination at 873 K. Surface contents of Ag and Cl on the AgCl/Al2O3 catalysts calcined at 873 and 673 K were analyzed by XPS spectra for Ag(3d) and Cl(2p) electrons. Table 1 shows the surface contents of Ag and Cl for the AgCl/Al2O3 catalysts calcined at 873 and 673 K. The surface contents of Ag and Cl on the AgCl/Al2O3 catalyst increase on calcination at 873 K. The results of XRD and XPS spectra suggest that the crystallized AgCl particles are well dispersed on calcination at higher temperatures. Thus, the increased intensity of the absorption band at ∼250 nm on calcination above 673 K (Figure 2) is ascribed to the increased dispersion of AgCl particles on Al2O3. These results suggest that photocatalytic conversion of NO is high on AgCl particles on Al2O3 and that it is further enhanced on well-dispersed AgCl particles. By mass spectrometric measurements, it was shown that UV irradiation on the AgCl/Al2O3 and Ag/Al2O3 catalysts previously calcined at 873 K in air led to the formation of N2O and a small amount of N2 and NO2 in the presence of NO (25 Torr). On the other hand, NO was mainly decomposed to N2 on the Ag/ferrierite catalyst calcined at 673 K. Anpo et al. have observed that NO is mainly converted to N2 on the Ag+/ZSM-5 catalyst calcined at 673 K (4). They have reported that electron transfer from excited Ag+ ion on ZSM-5 to the π antibonding molecular orbital of NO plays a significant role in the photocatalytic conversion of NO to N2 on Ag+/ ZSM-5. The Ag/ferrierite catalyst exhibited an intense absorption band at ∼210 nm caused by isolated Ag+ ions (8-10) on calcination at 673 K. Rate of NO conversion on the Ag/ferrierite calcined at 673 K, however, was very low as compared with those on the AgCl/Al2O3 catalysts. VOL. 34, NO. 24, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Rate of NO conversion on the AgCl/Al2O3 catalysts was lowered with the increased reaction time under UV irradiation. In particular, the lowering of the NO conversion was enhanced for the AgCl/Al2O3 catalysts calcined below 673 K where isolated Ag+ ions predominated. It was previously found that the formation of Ag0 atom, Agn0, and Agmn+ clusters suppresses photocatalytic conversion of NO on an AgCl/ Al2O3 catalyst UV irradiated (6). At irradiation time of 30 min, the formation of Ag0 atom, Agn0, and Agmn+ clusters was observed by UV-vis spectral measurements for the AgCl/ Al2O3 catalysts calcined below 673 K. Thus, low stability for NO conversion on the AgCl/Al2O3 catalysts calcined below 673 K is ascribed to the formation of Ag0 atom, Agn0, and Agmn+ clusters. On the other hand, the formation of isolated Ag+ was suppressed over the AgCl/Al2O3 catalysts calcined above 773 K, and the formation of Ag0 atom, Agn0, and Agmn+ clusters under UV irradiation was lowered. Under photoirradiation, electrons and holes are probably produced by photoactivation of the crystallized AgCl on Al2O3. The Ag0 atom, Agn0, and Agmn+ clusters (15) are formed by reaction between the created electron and Ag+ ions (16). Thus, it is suggested that the transformation of AgCl into isolated Ag+ is suppressed for the AgCl/Al2O3 catalysts calcined above 773 K. High stability of the AgCl/Al2O3 catalysts calcined above 773 K is ascribed to the formation of the crystallized AgCl particles well dispersed on Al2O3.
Literature Cited (1) Fritz, A.; Pitchon, V. Appl. Catal. B 1997, 13, 1. (2) Ibusuki, T.; Takeuchi, K. J. Mol. Catal. 1994, 88, 93.
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(3) Anpo, M.; Matsuoka, M.; Shioya, Y.; Yamashita, H.; Giamello, E.; Morterra, C.; Che, M.; Patterson, H. H.; Webber, S.; Outllete, S.; Fox, M. A. J. Phys. Chem. 1994, 98, 5744. (4) Anpo, M.; Matsuoka, M.; Yamashita, H. Catal. Today 1997, 35, 177. (5) Zhang, S.; Kobayashi, T.; Nosaka, Y.; Fujii, N. J. Mol. Catal. A 1996, 106, 119. (6) Yamashita, Y.; Aoyama, N.; Takezawa, N.; Yoshida, K. J. Mol. Catal. A 1999, 150, 233. (7) Yamashita, Y.; Takezawa, N.; Yoshida, K. Unpublished data. (8) Moore, C. E.; Atomic Energy Levels; National Bureau of Standards: Washington, DC, 1971; Vol. 3, p 48. (9) Truklin, A. N.; Etsin, S. S.; Shendrik, A. V. Izv. Akad. Nauk. SSSR, Ser. Fiz. 1976, 490, 2329. (10) Texter, J.; Kellerman, R.; Gonsiorwski, T. J. Phys. Chem. 1986, 90, 2118. (11) Okamoto, Y. Nachr. Akad. Wiss. Goettingen Math.-Phys. Kl. 1963, 14, 69. (12) Wong, J. S.; Shluster, M.; Cohen, M. L. Phys. Status. Solidi B 1976, 77, 295. (13) Aoyama, N.; Yoshida, K.; Abe, A.; Miyadera, T. Catal. Lett. 1997, 43, 249. (14) Serpone, N.; Borgarello, E.; Gratzel, M. J. Chem. Soc., Chem. Commun. 1984, 342. (15) Ozin, G. A.; Hber, H. Inorg. Chem. 1978, 17, 155. (16) Mess, C. E.; James, T. H. The Theory of Photographic Process, 3rd ed.; Macmillan Co.: New York, 1996; p 23.
Received for review April 7, 2000. Revised manuscript received September 1, 2000. Accepted September 13, 2000. ES001164I
