5770
Ind. Eng. Chem. Res. 2003, 42, 5770-5774
Selective Catalytic Reduction (SCR) of NO with NH3 at Low Temperature Using Halogen Ions-Modified Al2O3, ZrO2, and TiO2 as Catalysts Caili Su,*,† Fuyu Notoya, and Eiji Sasaoka* Department of Environmental Chemistry and Materials, Faculty of Environmental Science and Technology, Okayama University, 3-1-1 Tsushima-Naka, Okayama 700-8530, Japan
The effects of halogen ions on the activities of Al2O3, TiO2, and ZrO2 for the low-temperature SCR of NO with NH3 have been studied. The results showed that Cl- and Br- ions increased the life of the catalysts from 1 to 2 h to several tens of hours (NO removal g90%), F- had no clear influence on the performances of the studied metal oxides, but I- was a fatal poison to the catalysts due to the formation of I2, which would cover the active sites needed for the reaction. The doping effect of Cl- and Br- ions on the catalysts was due to the increase of both the strength of acidity and the amount of acidic sites on the surface of the catalysts, which was confirmed by the temperature-programmed desorption of NH3 from the Al2O3-based samples. On the other hand, the accumulation of nitrate salts during the reaction decreased obviously over the Cland Br- ions-doped samples; as a result, the average selectivity to N2 during the reaction time increased correspondingly. Introduction NO is one of the main atmospheric pollutants that can cause acid rain, depletion of ozone, and photochemical smog. The main sources of NO are the flue gases of coal combustion and waste gases of automobiles. Thus, the removal of NO from the flue gases and the emission of automobiles has been one of the main topics in environmental science and technology. Among all the processes of removal of NO,1-5 the most ideal process should be direct decomposition of NO to N2 and O2. However, the dissociation of NO needs high energy, and no highly active catalyst has been found for this reaction so far. A well-known achievable process, which had been industrialized in the 1970s, is selective catalytic reduction (SCR) of NO with NH3.1 V2O5/TiO2 was applied as a catalyst; WO3 and MoO3 were used as additives. The disadvantage of the selective catalytic reduction of NO with NH3 is that V2O5/TiO2-based catalysts do not work when the temperature is lower than 250 °C, but the outlet temperature of large quantities of NOx generated from electric power plants, shaft furnaces, and waste incinerators is usually lower than 150 °C. Therefore, it is necessary to develop a lowtemperature SCR process. A successful low-temperature SCR process will permit a DeNOx unit to be set downstream of the air preheater and electrical dust precipitator in some practical processes so that attack by NH3 on the equipment of the preheater and electrical dust precipitator will be avoided and the leaked NH3 can be absorbed by the desulfurization process. Extensive attention has been paid to low-temperature SCR of NO with NH3 in recent years. The studied catalysts * To whom correspondence should be addressed. Tel./Fax: 086-251-8900. E-mail:
[email protected]. † Present address: Special Division of Green Life Technology, National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka, Ikeda city, Osaka 563-8577, Japan. Tel.: 072-751-9642. Fax: 072-753-9627. E-mail:
[email protected].
include active carbon fibers,6,7 metal oxides,8-10 and carbon fiber-supported metal oxides.11-14 The difficulty of the low-temperature process shown in the reported works10,14 is that different ammonia salts formed in the reaction will accumulate on the surface of the catalysts and decrease their activities. In this work, we try to modify the surface properties of Al2O3, TiO2, and ZrO2 and improve their activities to the reaction. Todo et al.15 and the precious work of this group16 reported the enhancement of halogen salts on the activities of Al2O3based catalyst and Cu(II)-NaY to the low-temperature SCR of NO with NH3, but they did not continue the work further, and the mechanisms were not studied either. In the present work, the effect of halogen ions on the activity of Al2O3, TiO2, and ZrO2 for the lowtemperature SCR of NO with NH3 has been studied and the results were compared with those obtained over Al2O3, TiO2, and ZrO2.10 It was found that Cl- and Brions increased the life of the catalysts significantly. The mechanism of the improvement of Cl- and Br- to the performance of the catalysts was studied by measuring the surface acidity of the metal oxides using temperature-programmed desorption (TPD) of NH3 and also by comparing the accumulation amount of different salts during the reaction over the surfaces. NO desorption profiles from the modified catalysts were also compared with the desorption of NO from the unmodified catalysts. 2. Experimental Procedure ZrO2 was prepared by a conventional precipitation method and TiO2 and Al2O3 were commercial samples (TiO2, Sakai Chemicals Co., Al2O3: Mizusawa Chem. Ind. Co.). A certain amount of these samples were pretreated using the same volume of solutions (0.2 N) of different halogen acids or salts for 3 h at room temperature, and then the samples were dried at 110 °C for 1 h. The supported amounts of different halogen acids were measured using an ion chromatograph and
10.1021/ie030235u CCC: $25.00 © 2003 American Chemical Society Published on Web 10/15/2003
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5771 Table 1. Amount of Different Halogen Acids Supported on the Metal Oxides supported amount/ mmol/g supports ZrO2 TiO2 Al2O3 Al2O3 Al2O3
modifying acids HBr HBr HBr HF HCl
before reaction
after reaction
reaction time/h
0.36 0.23 0.58 0.42 0.60
0.26 0.19 0.10 0.18 0.41
30 45 48 24 48
the data are shown in Table 1. For convenience of discussion, the samples were marked using the concentration of the acid or the halogen salt and the name of the metal oxides, for example, 0.2NHBr/Al2O3 means the catalyst was Al2O3, and it was pretreated using 0.2 N HBr solution. The SCR process was carried out using a flow-type packed-bed tubular reactor system under atmospheric pressure at 90-100 °C. The reactor consisted of a quartz tube of 1.5-cm i.d. in which 2 mL of catalyst was placed. A mixture of NO (250 ppm), SO2 (800 ppm), NH3 (800 ppm), O2 (5%), H2O (10%), CO2 (14%), and N2 balance was fed into the reactor at 200 cm3 STP/min. The inlet and outlet NO concentration was measured using a chemluminescent NO analyzer (Yanako Co., Model ES7) after NO2 was converted to NO using a KI + H2SO4 solution. The conversion of NO to N2 was analyzed using a GC (Yanako G3800) equipped with a thermal conductivity detector and a column of molecular sieve 5A. The calculation method is the same as that used in the literature.10 The acidic properties of the catalysts were studied by the temperature-programmed desorption (TPD) of NH3. The catalysts were pretreated in a flow of He (20 cm3 STP/min) at 100 °C for 3 h, and then a flow of NH3 (10 cm3 STP/min) was induced to the reactor at the same temperature for 1 h. A flow of He was fed again at 100 °C for 1 h to desorb the physically adsorbed NH3, and then the sample was cooled to room temperature. The TPD profiles of NH3 were measured using a quadrupole mass spectrometer when the sample was heated at a rate of 10 °C/min from room temperature to 800 °C. NO desorption profiles were obtained after the catalysts had been purged in the feed gas that was the same as that used in the reaction at 100 °C for 2 h. After the sample was cooled to room temperature, the TPD profiles of NO were measured using the same method as for NH3-TPD. 3. Results and Discussion Modifying Effect of Halogen Ions on the Activity and Selectivity of Al2O3, TiO2, and ZrO2. A comparison of the activities of Al2O3, TiO2, and ZrO2 with the modified samples using HBr solution is shown in Figure 1. It is clear that the activities and the life of all the three catalysts were improved by the pretreatment. It should be mentioned here that although the same amount of the acids were used for pretreating all the samples, the supported amount of HBr on different supports were not the same (shown in Table 1) due to the different densities and surface properties of the supports. It has been confirmed that the activity of Al2O3 was improved when the adsorbed amount of the acid was lower then 5.8 mmol/g. A higher amount of the acid would poison the active sites of the catalysts. On the other hand, a superacid catalyst, SO42-/ZrO2,
Figure 1. Activity of ZrO2, TiO2, Al2O3, and the corresponding samples pretreated with 0.2 N HBr solution for the low-temperature SCR of NO by NH3. Reaction conditions: inlet gases: SO2 800 ppm-NO-250 ppm-NH3-800 ppm-CO2 14%-H2O 10%O2 %-N2 balance. SV: 6000 h-1. 373 K for ZrO2- and Al2O3-based samples; 383 K for TiO2-based samples.
prepared according to the method described in the reference,17 did not show high activity in this reaction at 100 °C. (Fractional removal of NO was 40-60% and lasted only about 10 h.) This means that an acidic catalyst is necessary to this reaction, but the strength of the acid should be in a suitable range. The average selectivity to N2 during the reaction time was calculated using the following method:
average selectivity of N2/% ) [NO]decreased - [NO3 - ]formed - [NO2]formed [NO]decreased
× 100%
It was found that no NO2 was formed over Cl- or Brions-doped samples. The amount of NO3- salts accumulated on the catalysts decreased obviously (Table 2, column 6). As a result, the selectivity to N2 increased clearly as shown in Figure 2. This implies that the Cland Br- ions could depress the formation of NO3- salts on the surface of the catalysts. The total accumulating rate of the salts (Table 2, column 5) just decreased slightly due to the increase of NH4+ salts and SO42salts. These results indicate that NO3- salts may decrease the activity of the catalysts faster than SO42salts, although the formation of SO42- salts has been considered to be the main cause that deteriorates the catalysts.9,10,13 A comparison study about the adsorption of NH3 before and after modifying the catalyst with halogen ions will be discussed in the following part. As has been mentioned above, not all kinds of halogen ions could enhance the activity of the metal oxides to the reaction. The results in Figure 3 show that HF had no obvious positive effect and the HCl and HBr improved the activity greatly. Even a very small amount of HI poisoned the catalyst completely, probably due to the formation of I2 that would cover the active sites on the surface of the catalysts. On the other hand, the halogen salts (e.g., NiCl2 and NH4Cl) had the same effect on the activity of the studied metal oxides as shown in Figure 4. This implies that the anions (Cl- or Br-) played the role to improve the activity of the catalysts. NH3-TPD. It was supposed that the surface acidity of the metal oxides would be changed when they were
5772 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 Table 2. Weight Increase of Different Catalysts during the Reaction (Amount of NO3-, NH4+, SO42- over the Catalysts before and after Reaction Were Analyzed Using an Ion Chromatograph) sample weight/g catalysts
reaction time/h
before reaction
after reaction
weight increase with time g/h
ZrO2 TiO2 Al2O3 0.2NHBr/ZrO2 0.2NHBr/TiO2 0.2NHCl/Al2O3
24 19 22 26 45 22
2.26 1.66 1.29 2.44 1.69 1.28
2.74 2.07 1.78 2.88 2.44 2.05
0.022 0.020 0.022 0.017 0.017 0.016
Figure 2. Comparison of the average selectivities to N2 during the reaction time (ZrO2 26 h, TiO2 45 h, Al2O3 48 h) over different catalysts. Reaction conditions: inlet gases: SO2 800 ppm-NO-250 ppm-NH3-800 ppm-CO2 14%-H2O 10%O2 %-N2 balance. SV: 6000 h-1. 373 K for ZrO2- and Al2O3-based samples; 383 K for TiO2based samples.
Figure 3. The activity of Al2O3 pretreated using different kinds of halogen acids. Reaction conditions: inlet gases: SO2 800 ppmNO-250 ppm-NH3-800 ppm-CO2 14%-H2O 10%O2 %-N2 balance. SV: 6000 h-1. Temperature: 100 °C.
pretreated with halogen acids or salts; NH3-TPD from different samples are studied and the results are shown in Figure 5. It is obvious that Al2O3 pretreated with HCl and HBr adsorbed a larger amount of NH3 than that over untreated Al2O3. The main peaks of the desorption of NH3 shifted to a higher temperature, indicating the increased strength of the acidic sites by the modification of HCl and HBr. These sites would contribute to the adsorption and activation of NH3, which should be necessary steps for the reduction of NO by NH3, as what
NO3
×103mmol/g‚h NH4+
SO42-
18 13 32 6.5 5.4 16
120 110 200 140 300 220
47 40 71 54 64 85
-
Figure 4. The activity of Al2O3 pretreated using different solutions containing Cl- ions. Reaction conditions: inlet gases: SO2 800 ppm-NO-250 ppm-NH3-800 ppm-CO2 14%-H2O 10%O2 %-N2 balance. SV: 6000 h-1. Temperature: 100 °C.
has been proposed for the high-temperature SCR process.4 Although some NH3 desorbed from HF/Al2O3 at higher temperature than those of HCl/Al2O3 and HBr/ Al2O3, the entire activity of the catalyst was lower than that of HCl/Al2O3 and HBr/Al2O3. Combining the activity observed over the superacid H2SO4/ZrO2 (fractional removal of NO was 40-60% and lasted about 10 h), we can conclude that a too strong acidic site does not contribute to the activation of NH3 molecules in the reaction at low temperature. It probably needs higher temperature, as what has been studied by Yang et al. over SO42-/TiO2.18 For the sample of HI/Al2O3, since most of the I- was converted to I2 (the color of catalysts changed from white to brown), its adsorption for NH3 is difficult to compare with the other three samples and is not shown here. A mechanism proposed in Figure 6 describes how the Cl- and Br- ions increase the acidity of the catalysts. It is probably that the strength of both Lewis and Bronsted acid sites is increased due to the electronegativity of halogen elements. NO Desorption from the Used Catalysts. To compare the reaction mechanism at low temperature with that of high temperature, the desorption profiles of NO from the used catalysts (TiO2 and 0.2NHBr/TiO2) were studied and are shown in Figure 7. Both of the samples were pretreated in a feed gas which was the same as that used in the reaction for 2 h, and then the samples were heated from room temperature to 800 °C at a rate of 10 °C/min. The results showed that there were two desorption peaks of NO from TiO2, which are at 250 and 610 °C, respectively. The lower temperature peak is due to the desorption of NO adsorbed on the surface of the catalysts, but the higher temperature peak was considered to be caused by the decomposition of NO3- salts because it appeared together with O2 and
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5773
Figure 5. NH3-TPD profiles from Al2O3 pretreated using different halogen acids.
Figure 6. Proposed mechanism for the modifying effect of halogen ions on the surface acidity of the metal oxides.
H2O. It is interesting that no desorption peak of NO was observed from 0.2NHBr/TiO2. This means the adsorption of NO becomes difficult because of the changing of surface properties. Combining the results in Table 2, which shows that the accumulation of NO3- salts decreased over the catalysts pretreated using halogen acids, we can conclude that the decrease of NO3- salts formation over the doped samples was probably due to the difficulty of NO adsorption over the surface. The adsorbed NH3 over 0.2NHBr/TiO2 would act as basic sites for the adsorption of SO2, which should be easier
than the adsorption of NO. The adsorbed SO2 would be oxidized to SO3 and then form sulfate salts. Therefore, the accumulation rate of sulfates salts over the doped catalysts was higher than that over the pure metal oxide catalysts. If the low-temperature (
