ARTICLE pubs.acs.org/IECR
N2O and CO2 Formation during Selective Catalytic Reduction of NO with NH3 over V2O5/AC Catalyst Pan Li, Qingya Liu,* and Zhenyu Liu State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China ABSTRACT: Much research has shown that V2O5/AC is promising for the selective catalytic reduction (SCR) of NO with NH3 at low temperatures (180-250 °C). However, information on N2O formation and AC oxidation during the SCR process are limited especially under actual conditions in the presence of SO2 and H2O. These are studied in this work through specially designed experiments. Results show that under the conditions used, V2O5 loading, reaction temperature, and SO2 concentration have limited effects on N2O formation. N2O is generated from reduction of NO by both AC and NH3. SCR selectivity to N2 is promoted by V2O5, and less than 1 wt.% V2O5 is sufficient to yield a N2 selectivity of higher than 95%. Oxidation of AC to CO2 is promoted by temperature and SO2 and originates mainly from reaction of carbon-and-oxygen-containing functional groups with sulfuric acid formed from SO2 adsorption in the presence of SO2.
1. INTRODUCTION Selective catalytic reduction (SCR) of nitrogen oxides (NOx) with NH3 is a well-established technology for NOx abatement from flue gases in coal-fired power plants and industrial boilers.1,2 Although the main reaction is reduction of NOx to N2, N2O also forms over many catalysts.3-6 Selectivity to N2O on some V2O5/ TiO2-based mid temperature catalysts (usable at around 400 °C) was found even higher than that to N2.5 Formation of N2O on some low temperature catalysts (usable at around 200 °C), such as a carbon-supported MnOx, was also found to be significant.6 N2O formation not only lowers efficiency and selectivity of the SCR reaction but also contributes to depletion of the ozone layer.7,8 Thus, controlling its formation is an important task in research and development of SCR catalysts. Low temperature SCR catalysts have received more attention in recent years because they can be installed downstream of dust removal units and thus incur a minimal retrofitting cost for existing boilers. V2O5/TiO2-based catalyst was reported to have good performance at temperatures as low as 200 °C in the absence of SO2 but is deactivated quickly in the presence of SO2 because of formation of ammonia sulfates that cover its surface.9 Activated coke-supported V2O5 (V2O5/AC) was found to be promising at temperatures around 180-250 °C because of its high activity and resistance to SO2 poisoning when the moisture content of the flue gas is low.10-13 This catalyst was studied extensively in past decade including preparation techniques, reaction behaviors, the roles of the catalyst components, effects of SO2 and H2O, kinetics and mechanism, etc.13-16 Limited work, however, was reported on N2O formation except for that reported by Lazaro et al.17,18 which showed variation of N2O selectivity from 0 to 79% over different carbon precursors and at different reaction temperatures. Carbon-based catalysts have been used mainly in reductive environments. For SCR of NOx, however, the environment is oxidative because O2 is always present in flue gases. Carbon oxidation, r 2011 American Chemical Society
therefore, should also be considered in the process at least for stability of the catalyst. Carbon oxidation rate as high as 2.88 wt. %/h was reported for a MnOx/AC catalyst during the SCR at 200 °C6 and 0.66 wt.%/h for a SO2-adsorbed V2O5/AC catalyst during Ar purge at 300 °C,19 which are both worth concern. However, carbon loss was found to be insignificant over a V2O5/AC catalyst during SCR at 300 °C based on thermal balance data.17 Apparently, carbon oxidation of V2O5/AC catalyst is complex, which relates not only to the properties of the catalyst but also to reaction conditions. This paper studies formation of N2O and oxidation of AC during SCR of NO over a V2O5/AC catalyst under various conditions, including effects of V2O5 loadings, reaction temperature, and SO2. The unique feature of this study is collection of N2O and CO2 data during the SCR, which allows deeper understanding of the subject.
2. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of the Catalyst. The AC support (Xinhua Chemical Company, Taiyuan, China) was grounded and sieved to 40-60 mesh. Its proximate and ultimate analyses are shown in Table 1. V2O5/AC catalyst was prepared by pore volume impregnation using an aqueous solution containing ammonium metavanadate and oxalic acid, followed by drying at 50 °C for 12 h and 110 °C for 5 h. The sample was then calcined in a flow of Ar at 500 °C for 5 h and preoxidized in air at 250 °C for 5 h as described elsewhere.11 The V2O5 loadings of the catalyst were determined by concentration of the ammonium metavanadate solution and confirmed by inductively Received: May 25, 2010 Accepted: December 10, 2010 Revised: October 13, 2010 Published: January 4, 2011 1906
dx.doi.org/10.1021/ie101151d | Ind. Eng. Chem. Res. 2011, 50, 1906–1910
Industrial & Engineering Chemistry Research
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coupled plasma atomic emission spectroscopy (ICP-AES, AtomScan 16, Thermo Jarrel-Ash, Franklin, MA). The catalysts are termed Vn/AC where n is the integer most close to the weight percent of V2O5, such as V1/AC for 0.91 wt.% V2O5 in V2O5/AC. Specific surface area and pore volume of the AC and the catalysts were determined by N2 adsorption at 77 K on a Quadrasorb SI-MP (Quantachrome, Boynton Beach, FL) unit. Prior to the measurements, the samples were degassed under vacuum at 150 °C for 10 h. The specific surface area was obtained by the Multi-Point BET method. The micropore surface area and micropore volume were obtained by the empirical V-t plot method. These data are shown in Table 2. A thermogravimetric analyzer from Setaram (TGA, Setsys Evolution 24) was used for temperature-programmed decomposition (TPD) of the AC support. The TPD was carried out on 20 mg of AC in an Ar flow of 100 mL/min with a rate of 10 °C/min, from room temperature to 120 °C, at 120 °C for 1 h to remove H2O, and then to 300 °C. 2.2. Activity Measurements. SCR activity was tested in a fixed-bed quartz reactor (Φ6 500 mm) with 0.2 g of catalyst. The reactor was heated in an O2/Ar stream to the reaction temperature (150-250 °C) at a rate of 10 °C/min. At steady state, the O2/Ar stream was replaced by a model flue gas containing 500 ppm NO, 500 ppm NH3, 3.1 vol.% O2, 750 ppm SO2 (when used), 2.7 vol.% H2O (when used), and balance Ar. The total flow rate was 100 mL/min corresponding to a gas hourly space velocity (GHSV) of 15000 h-1 assuming a catalystbed density of 0.5 g/mL. A mass spectrometer (MS, Omnistar 200, Balzers) was used online to monitor the effluent gas concentration. Blank experiments were performed with 0.2 g quartz sand (40-60 mesh) under the same conditions that showed little SCR activity. Since MS cannot differentiate N2O from CO2 because of their same mass-to-electron (m/e) ratio of 44, the effluent gas was also analyzed off-line by gas chromatography-mass spectrometer (GC-MS) equipped with a TDX-01 column (1.5 m 3 mm) with an Ar carrier of 25 mL/min at 55 °C. Effluent gas was collected in a gas bag at a reaction time of 2 h and analyzed three times to yield average concentrations of N2O and CO2. The SCR behaviors of the catalyst are expressed by NO conversion (XNO, %) and selectivity to N2 (SN2, %) obtained
by eqs 1 and 2, respectively: CNO, in - CNO, out 100 XNO ¼ CNO, in
ð1Þ
! S N2 ¼
C N2 O 1CNO, in XNO
100
ð2Þ
where CNO,in and CNO,out are NO concentrations in the feed and the effluent, respectively, and CN2O is N2O concentration in the effluent.
3. RESULTS AND DISCUSSION 3.1. Physical Properties of the AC and the Catalysts. Because oxygen in carbon materials often leads to carbon loss during heating and the AC contains 5.5% oxygen (Table 1), the mass loss of AC itself was measured through TPD. Figure 1 shows that the AC support has a relatively constant mass loss rate (DTG) of smaller than 0.025%/min in a temperature range of 120-300 °C. The mass loss below 200 °C may be attributed to loss of oxygen-and-hydrogen-containing functional groups that yields H2O while that at higher temperatures to decomposition of oxygen-and-carbon-containing functional groups that yields CO2 and CO. The total mass loss (TG) of the AC in the temperature range is about 0.2%, which is much lower than its oxygen content, suggesting that most of the oxygen in the AC is inactive below 300 °C in Ar. Table 2 shows that the AC has a BET surface area of 954 m2/g and a micropore surface area of 807 m2/g, indicating dominance of micropores. The V2O5/AC catalysts’ surface areas are all smaller than that of the AC and are inversely proportional to V2O5 loading. The constant difference between BET and micropore surface areas indicates that V2O5 is mainly deposited in the micropores of the AC.
Table 1. Proximate and Ultimate Analyses of the ACa proximate (wt.%)
a
ultimate (wt.%)
Mad
Aad
Vad
1.60
10.43
3.96
Cad
Had
Oad
Nad
St,ad
80.29
1.03
5.50
0.65
0.50
M: moisture; A: ash; V: volatile matter; ad: air dry; t: total.
Figure 1. TG and DTG data for the AC during TPD in Ar at 10 °C/min.
Table 2. Physical Characteristics of the AC and V2O5/AC Catalysts name
V2O5 loading
BET surface
micropore surface
difference between BET and
total pore volume
average pore
(wt.%)
area (m2/g)
area (m2/g)
micropore surface area (m2/g)
(cm3/g)
diameter (nm)
AC
-
954
807
147
0.546
2.29
V1/AC
0.91
886
742
144
0.520
2.35
V3/AC
2.77
832
686
146
0.490
2.36
V5/AC
4.67
792
650
142
0.439
2.22
V7/AC
6.71
683
566
117
0.431
2.52
1907
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Figure 2. Effect of V2O5 on steady-state NO conversion at 200 °C. Conditions: 500 ppm NO, 500 ppm NH3, 3.1 vol.% O2 and balance Ar, GHSV of 15000 h-1.
Figure 3. N2O and CO2 formation and N2 selectivity for experiments in Figure 2 at time-on-stream of 2 h.
3.2. Effect of V2O5 Loading on SCR Behavior. Figure 2
shows the effect of V2O5 loading on steady-state NO conversion of the AC support and V2O5/AC catalysts at 200 °C. The AC has a very low SCR activity with a XNO of only 3%. The V2O5/AC catalysts show higher activities as indicated by increases in XNO to 47% for V1/AC and to 87% for V7/AC, which agrees with that reported by Zhu et al.10 Figure 3 shows the effects of V2O5 on N2O and CO2 formation and N2 selectivity. Clearly, N2O formation slightly increases from AC to V1/AC and almost keeps constant with a further increase of V2O5 loading. However, due to the too low XNO of AC, its SN2 is only 80%. V2O5 promotes SN2 because of the increased XNO and 0.91 wt.% V2O5 is sufficient to yield a SN2 of more than 98%. Besides promoting NO conversion, which is shown in Figure 2, V2O5 also greatly promotes CO2 formation. Calculation shows that the carbon loss rates are 0.006, 0.02, 0.03, and 0.05 wt.%/h for AC, V1/AC, V3/AC, and V7/AC, respectively. These carbon losses may be worrisome especially at high V2O5 loadings. Furthermore, literature indicated that catalyst with V2O5 loading of 1 wt.% shows the best resistance to SO2 poisoning at low temperatures among catalysts with V2O5 loadings from 1 to 17 wt.%,11 and Figure 3 indicates that V1/AC also shows the best resistance to O2 oxidation than any other V2O5/AC catalysts with V2O5 loadings more than 1 wt.%; thus, from the viewpoint
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Figure 4. Effect of reaction temperature on SCR activity of V1/AC.
Figure 5. N2O formation and N2 selectivity for experiments in Figure 4 at time-on-stream of 2 h. Open symbols: without SO2 and H2O. Filled symbols: with 750 ppm SO2 and 2.7% H2O.
of practical application, V1/AC is selected for a detailed study on N2 selectivity and CO2 formation. 3.3. Effects of Reaction Temperature and SO2 on N2 Selectivity. Figure 4 shows XNO of V1/AC at various temperatures in the presence or absence of SO2 and H2O. Obviously, XNO in both environments increases with increasing temperature. SO2þH2O inhibits NO conversion at 150 and 180 °C but promotes NO conversion at 200 °C or higher temperatures. These behaviors can be attributed to deposition of ammonium sulfates that cover the catalyst surface at lower temperatures and to NO reduction by ammonium sulfates at higher temperatures.13,20 As shown in Figure 5, N2O concentration generally increases with an increase in temperature in the absence of SO2þH2O, but its values are all lower than 10 ppm. SO2þH2O promotes N2O formation under all the conditions except at 250 °C. This behavior may indicate promotion of the redox ability by SO2þH2O at temperatures of 230 °C and below because N2O formation relates to the redox ability of the catalyst. Despite the different N2O formation, SN2 vary little with the temperature and are all higher than 95%. To understand the origin of N2O, reduction of NO and oxidation of NH3 experiments were performed at 200 °C over V1/AC and the AC with 500 ppm NO or 500 ppm NH3, 3.1 vol. % O2 and balance Ar. Results in Figure 6 show that no N2O is formed in NH3þO2 over both V1/AC and the AC, which agrees 1908
dx.doi.org/10.1021/ie101151d |Ind. Eng. Chem. Res. 2011, 50, 1906–1910
Industrial & Engineering Chemistry Research
Figure 6. N2O formation over V1/AC and the AC support at 200 °C under various environments at a reaction time of 2 h. (a) NH3þNOþ O2; (b) NOþO2; (c) NH3þO2.
Figure 7. CO2 formation for experiments in Figure 4 at a reaction time of 2 h.
with that reported for NH3 oxidation over V2O5/TiO2,21 V2O5-WO3/TiO2,22and V2O5/AC23 catalysts. The presence of N2O in NOþO2, although only 3 ppm over V1/AC and 2 ppm over the AC, indicates its formation via NO reduction by the AC because the AC is the only reductive agent in the reaction system. This agrees with that reported in the literature.24,25 The further increase in N2O concentration in NH3þNOþO2 suggests NO reduction by NH3 as well as by the AC. It is interesting to note that N2O formed over V1/AC is always more than that formed over the AC, suggesting the increased reducing capability by V2 O 5 . 3.4. Effects of Reaction Temperature and SO2 on CO2 Formation. Figure 7 shows CO2 concentration measured in the experiments in Figure 4 at a reaction time of 2 h, where the catalysts’ SCR activity is considered to be stable. Obviously, SO2þH2O promotes CO2 formation especially at high temperatures. Furthermore, CO2 concentration increases with an increase in reaction temperature especially in the presence of SO2þH2O. This temperature dependence is understandable because the carbon oxidation rate increases with an increase in temperature, but the effect of SO2þH2O is rather surprising because SO2 is thought to compete with AC for oxygen (to be oxidized into SO3) on the catalyst surface and H2O may inhibit carbon oxidation at the temperatures. To elucidate the promoting effects of SO2 and H2O on AC oxidation, SCR experiments were performed at 200 °C with and
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Figure 8. CO2 formation during SCR of NO over V1/AC at 200 °C under various environments.
without the presence of SO2 and H2O. Figure 8 shows again that AC oxidation is very limited in the absence of SO2 and H2O although 3.1 vol.% O2 is present in the feed. H2O does not promote AC oxidation significantly, yielding a CO2 concentration of 8 ppm at steady state. SO2 is responsible for AC oxidation especially at the early stage of the SCR as evidenced by the fast increases of CO2 concentration in curves c and d. These phenomena may possibly be attributed to sulfuric acid formed by SO2þO2þH2O (H2O is not only a flue gas component but also a product of the SCR reaction), which reacts with carbon-and-oxygen-containing functional groups on the AC surface. This explains the CO2 release behavior in Figure 8 well, i.e., gradual accumulation of H2SO4 through SO2 adsorption initially that increases its reaction with carbonand-oxygen-containing functional groups on the surface, and gradual consumption of these functional groups that becomes significant later. This may also indicate that some of carbonand-oxygen-containing functional groups do not react with O2 under the conditions used. With depletion of the carbon-andoxygen-containing functional groups, CO2 concentrations should decrease to a minimal level. If the 8 ppm CO2 concentration observed at 330 min in the presence of H2O and absence of SO2 was considered as the steady-state value, the carbon loss would reach to 49% in six months. It should also be noted that the 8 ppm steady-state CO2 concentration is obtained at a GHSV of 15 000 h-1, which is about 10 times more than that in the practice; thus, it is expected that the steady-state carbon loss should result in a CO2 concentration of lower than 8 ppm in the practice.
4. CONCLUSIONS The N2O formation over V2O5/AC catalyst during SCR of NO by NH3 is generally low, less than 10 ppm in a temperature range of 150-250 °C. V2O5 contributes little to the N2O formation while SO2þH2O slightly accelerates N2O formation. N2O is formed from reduction of NO by both AC and NH3. SCR selectivity to N2 is promoted by V2O5 and 0.91 wt.% V2O5 is sufficient to yield a N2 selectivity of higher than 95%. Oxidation of the AC to CO2 occurs in the SCR process, and its rate increases with an increase in V2O5 loading. SO2 promotes AC oxidation to CO2 because of formation of sulfuric acid on the surface, which reacts with carbon-and-oxygen-containing functional groups on the AC. 1909
dx.doi.org/10.1021/ie101151d |Ind. Eng. Chem. Res. 2011, 50, 1906–1910
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’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ86-10-64421077. Fax: þ86-10-64421077. E-mail: qyliu@ mail.buct.edu.cn.
’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (20736001 and 20821004). ’ REFERENCES (1) Morikawa, S.; Yoshida, H.; Takahashi, K.; Kurita, S. Improvement of V2O5-TiO2 catalyst for NOx reduction with NH3 in flue gases. Chem. Lett. 1981, 10, 251. (2) Bosch, H.; Janssen, F. Formation and control of nitrogen oxides. Catal. Today 1988, 2, 369. (3) Suarez, S.; Martin, J. A.; Yates, M.; Avila, P.; Blanco, J. N2O formation in the selective catalytic reduction of NOx with NH3 at low temperature on CuO-supported monolithic catalysts. J. Catal. 2005, 229, 227. (4) Li, Y.; Zhong, Q. The characterization and activity of F-doped vanadia/titania for the selective catalytic reduction of NO with NH3 at low temperatures. J. Hazard. Mater. 2009, 172, 635. (5) Odenbrand, C. U. I.; Gabrielsson, P. L. T.; Brandin, J. G. M.; Andersson, L. A. H. Effect of water vapor on the selectivity in the reduction of nitric oxide with ammonia over vanadia supported on silicatitania. Appl. Catal. 1991, 78, 109. (6) Valdes-Solis, T.; Marban, G.; Fuertes, A. B. Low-temperature SCR of NOx with NH3 over carbon-ceramic cellular monolith-supported manganese oxides. Catal. Today 2001, 69, 259. (7) Kramlich, J. C.; Linak, W. P. Nitrous oxide behavior in the atmosphere, and in combustion and industrial systems. Prog. Energy Combust. Sci. 1994, 20, 149. (8) Ruiz-Martinez, E.; Sanchez-Hervas, J. M.; Otero-Ruiz, J. Catalytic reduction of nitrous oxide by hydrocarbons over a Fe-zeolite monolith under fluidised bed combustion conditions. Appl. Catal., B 2004, 50, 195. (9) Choo, S. T.; Yim, S. D.; Nam, I.-S.; Ham, S.-W.; Lee, J.-B. Effect of promoters including WO3 and BaO on the activity and durability of V2O5/sulfated TiO2 catalyst for NO reduction by NH3. Appl. Catal., B 2003, 44, 237. (10) Zhu, Z.; Liu, Z.; Liu, S.; Niu, H. A novel carbon-supported vanadium oxide catalyst for NO reduction with NH3 at low temperatures. Appl. Catal., B 1999, 23, L229. (11) Zhu, Z.; Liu, Z.; Niu, H.; Liu, S. Promoting effect of SO2 on activated carbon-supported vanadia catalyst for NO reduction by NH3 at low temperatures. J. Catal. 1999, 187, 245. (12) Huang, Z.; Zhu, Z.; Liu, Z. Combined effect of H2O and SO2 on V2O5/AC catalysts for NO reduction with ammonia at lower temperatures. Appl. Catal., B 2002, 39, 361. (13) Huang, Z.; Zhu, Z.; Liu, Z.; Liu, Q. Formation and reaction of ammonium sulfate salts on V2O5/AC catalyst during selective catalytic reduction of nitric oxide by ammonia at low temperatures. J. Catal. 2003, 214, 213. (14) Sun, D.; Liu, Q.; Liu, Z.; Gui, G.; Huang, Z. An in situ DRIFTS study on SCR of NO with NH3 over V2O5/AC surface. Catal. Lett. 2009, 132, 122. (15) Boyano, A.; Galvez, M. E.; Moliner, R.; Lazaro, M. J. Carbon based catalytic briquettes for the reduction of NO: Catalyst scale-up. Catal. Today 2008, 137, 209. (16) Lazaro, M. J.; Boyano, A.; Galvez, M. E.; Izquierdo, M. T.; Garcia-Bordeje, E.; Ruiz, C.; Juan, R.; Moliner, R. Novel carbon based catalysts for the reduction of NO: Influence of support precursors and active phase loading. Catal. Today 2008, 137, 215.
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