ARTICLE pubs.acs.org/IECR
Kinetics and Mechanism Study of Low-Temperature Selective Catalytic Reduction of NO with Urea Supported on Pitch-Based Spherical Activated Carbon Zhi Wang,† Yanli Wang,† Donghui Long,† Isao Mochida,‡ Wenming Qiao,*,† Liang Zhan,† Xiaojun Liu,† Seong-Ho Yoon,‡ and Licheng Ling† † ‡
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 816-8580, Japan
bS Supporting Information ABSTRACT: The kinetics and mechanism of selective catalytic reduction (SCR) of NO with urea supported on pitch-based spherical activated carbons (PSACs) were studied at low temperatures. NO oxidation to NO2 catalyzed by the 0.50.8 nm micropores in PSACs was found to be the rate-limiting step in ureaSCR reaction, which was confirmed by both the apparent activation energy calculations and the kinetics results of ureaSCR reaction and NO oxidation on PSAC. These two reactions gave very similar negative apparent activation energies (16.5 kJ/mol for ureaSCR reaction and 15.2 kJ/mol for NO oxidation), indicating that the adsorption of reactants on PSAC is of key importance in these two reactions. Moreover, these two reactions were both approximately first-order with respect to NO and one-half order with respect to O2. It was found that NO3 from the disproportionation of the produced NO2 was quickly reduced by supported urea into N2. After the complete consumption of supported urea, NO2 started to release, and the carbon surface was gradually oxidized by adsorbed NOx species. NO3 was found to be stably adsorbed on the oxidized carbon surface. On the basis of the results obtained, a reaction mechanism of low-temperature urea-SCR reaction on PSAC was proposed and discussed.
1. INTRODUCTION Nitrogen oxides (NOx) emitted from the combustion of fossil fuels are considered to be responsible for many serious environmental problems, such as acid rain, photochemical smog, ozone depletion, and the greenhouse effect.1 Many technologies have been developed to remove NOx. Among them, selective catalytic reduction (SCR) with NH3 is the most widely used method for the removal of NOx from stationary sources. The industrial SCR operations are carried out on V2O5/TiO2-based catalysts in an optimum temperature range of 300400 °C.28 However, NH3 does not appear to be an ideal reducing agent when considering its corrosiveness and toxicity. Furthermore, it is very difficult to exactly control an appropriate NH3 input because of the fluctuating NOx concentration in exhaust gas, which is very likely to cause additional environmental problems due to NH3 slip.9,10 Therefore, it is of great significance to develop new SCR technologies with other proper reducing agents as a substitution for NH3. A variety of reducing agents, such as hydrocarbons,x11,12 CO,13,14 ethanol,15 H2,16,17 and carbon,18,19 have been reported for the SCR application. They show various reducing abilities toward NOx under different conditions. Among them, the use of hydrocarbons, CO, and ethanol, however, inevitably causes leakage. H2 and carbon are more easily oxidized by O2 than by NO in an oxidizing atmosphere, resulting in the fast consumption of reducing agents. Recently, Shirahama et al.20,21 proved that urea supported on activated carbon fibers (ACFs) could efficiently reduce NOx to N2 at 30 °C in the presence of O2. The r 2011 American Chemical Society
possible reaction steps were proposed as follows: NO þ 0:5O2 f NO2 NO2 þ NO þ ðNH2 Þ2 CO f 2N2 þ CO2 þ 2H2 O
ð1Þ ð2Þ
Here, urea was used as a reducing agent instead of NH3 to avoid the above-mentioned difficulties, which makes this lowtemperature ureaSCR on activated carbon materials a new promising NOx removal technology in commercial application. Much valuable information has been gained about the influences of operation conditions on the ureaSCR reaction. However, relatively little is still known referring to the mechanical details, including the following: What is the type of active sites? How does the reaction temperature affect the SCR activity? What role does O2 play in the ureaSCR reaction? In addition, it is not clear whether the reaction proceeds through a LangmuirHinshelwood (LH) mechanism that NO is first adsorbed and oxidized on the carbon surface and then reacts with supported urea to produce N2, or an EleyRideal (ER) mechanism that a gas phase NO is directly reduced by supported urea to form the products. These questions which have been extensively studied for other SCR processes are important and need to be clarified for the ureaSCR reaction. Received: December 15, 2010 Accepted: March 30, 2011 Revised: March 25, 2011 Published: April 08, 2011 6017
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Industrial & Engineering Chemistry Research In this paper, we studied the kinetics and mechanism of lowtemperature ureaSCR reaction on pitch-based spherical activated carbon (PSAC). The research described the effects of PSAC pore structure, reaction temperature, NO, and O2 feed concentrations on NOx removal efficiency. It was found that NO oxidation is crucial in ureaSCR reaction. Both the apparent activation energies and the reaction orders of urea SCR reaction and NO oxidation on PSAC were calculated respectively. The role of O2 was further illustrated by a transient response experiment. The variation of surface species on PSAC during the reaction process was characterized by Fourier transform infrared (FTIR) spectra. This work is expected to deepen the understanding of low-temperature ureaSCR reaction on activated carbon materials and provide important fundamental knowledge of this NOx removal technology for its commercial application.
2. EXPERIMENTAL SECTION 2.1. Preparation of PSACs. A commercial PSAC (PSAC-w) supplied by Shanghai Heda Carbon Company was used in this work. It has a BrunauerEmmettTeller (BET) surface area of 1182 m2/g, and a pore volume of 0.47 cm3/g calculated from the nitrogen adsorptiondesorption isotherm at 196 °C. The composition of PSAC-w is 0.37 wt % mineral matter (ash), 93.00 wt % C, 1.40 wt % N, 3.91 wt % O, 0.89 wt % H, and 0.43 wt % S. In addition, a kind of oxidized pitch spheres supplied by Shanghai Heda Carbon Company was used as the starting material for the preparation of PSACs with different pore structures. The as-received oxidized pitch spheres were carbonized in N2 at 1000 °C for 2 h and further activated with CO2 at 1000 °C for 15 h in a tube furnace. The total flow rate was maintained at 100 mL/min through the preparation process. Finally, the obtained samples were sieved to 7001000 μm. The PSACs with different pore structures are designated as PSACMh, where M represents the activation period with CO2, for example, PSAC-2h. 2.2. Characterization. Nitrogen adsorptiondesorption isotherms of PSACs were measured at 196 °C using a Micromeritics ASAP 2020 M analyzer to determine the texture properties. Before any such analysis, the samples were degassed at 200 °C under a vacuum of 103 Torr. The specific surface area (SBET) was determined according to the BET method. The total pore volume (Vt), micropore volume (Vmic, micropore: pore size below 2 nm), and mesopore volume (Vmes, mesopore: pore size between 2 and 50 nm) were calculated according to the density functional theory (DFT). Elemental analysis was carried out using a Vario EL elemental analyzer. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface compositions of PSACs on a PHI 5000 VersaProbe system at room temperature and at a pressure lower than 108 Torr with Al KR radiation (1486.6 eV). The contents of C, O, N, and S on the surface of PSACs were calculated from the corresponding peak areas of the XPS data divided by each sensitivity factor of the elements. Temperature-programmed desorption (TPD) tests of PSACs were carried out in a quartz flow tube reactor with an inner diameter of 4 mm. A 0.1 g sample was packed into the reactor with a thermocouple inserted next to the sample to monitor the temperature. Before each test, the sample was first purged with He (100 mL/min) at 50 °C for 1 h. Then, a TPD test was run immediately in He (100 mL/min) at 10 °C/min to 950 °C, and
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the temperature was held at 950 °C for 30 min. The concentrations of CO and CO2 in the effluent gas during the TPD process were measured simultaneously using an online quadrupole mass spectrometer (QIC-20 gas analysis system) with a minimum detectable concentration of 0.1 ppmv. 2.3. Supporting Urea on PSACs. Urea was supported on PSACs by pore volume impregnation using an aqueous solution of urea. After impregnation, the obtained samples were kept at room temperature for 24 h and, then, were vacuum-dried at 60 °C for 24 h. The urea loading was controlled at 8 wt %. 2.4. Activity Test. The reactivities of NO with urea supported on PSACs were measured in a vertical fixed-bed glass reactor with an internal diameter of 10 mm. The feed gas consisted of 1001000 ppmv NO, 121 vol % O2, and a balance of N2. In all tests, the total flow rate was maintained at 100 mL/min. The reaction temperature was controlled from 30 to 90 °C. The concentrations of NO, NO2, and NOx were continually measured by an ECO PHYSICS CLD62 chemiluminescence NO/ NOx analyzer with a minimum detectable concentration of 0.5 ppmv. NO conversion, percentage yield of NO2 produced, and NOx conversion during SCR tests were defined by the following equations: NO conversion ¼ ðCNO, in CNO, out Þ=CNO, in 100% ð3Þ
percentage yield of produced NO2 ¼ CNO2, out =CNO, in 100%
ð4Þ
NOx conversion ¼ ðCNO, in CNOx, out Þ=CNO, in 100%
ð5Þ
2.5. Transient Response Experiment. Transient response experiment upon switching off and on O2 was also carried out in the above-mentioned fixed-bed glass reactor as follows. A feed gas of 500 ppmv NO, 21 vol % O2, and a balance of N2 passed through the carbon bed containing 1.0 g PSAC-w with 8 wt % urea loading at 30 °C. The total flow rate was maintained at 100 mL/min. Correspondingly, a space velocity of 3000 1/h was obtained. When the ureaSCR reaction reached the stationary state of NOx conversion (about 12 h), O2 was shut off and replaced by a makeup N2 of the same flow rate to maintain the total flow rate constant. When the NOx conversion reached a new stationary state, O2 of the original flow rate was reintroduced to the feed gas to replace the makeup N2. 2.6. FTIR Studies of UreaSCR Process. The UreaSCR reaction was first carried out by flowing a feed gas of 500 ppmv NO, 21 vol % O2, and a balance of N2 over 1.0 g PSAC-w with 8 wt % urea loading at 30 °C for various periods, and then, N2 passed through the carbon bed until NOx in the effluent was below 5 ppmv. In each experiment, the total flow rate was 100 mL/min. The FTIR spectra of these samples were collected by a Nicolet 5S C FTIR system. Each sample was first finely ground and then diluted with KBr to give a KBr/PSAC-w weight ratio of 200. The FTIR spectra were recorded by accumulating 400 scans at a spectra resolution of 4 cm1. In addition, FTIR spectra of pure urea, glass fiber with 8 wt % urea loading, and PSAC-w were also collected. 6018
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Table 1. Pore Parameters of PSACs with Different Pore Structures SBET samples
Vt
Vmic
(m2/g) (cm3/g) (cm3/g)
V (0.50.8 nm pores)
Vmes
(cm3/g)
(cm3/g)
PSAC-1h PSAC-2h
541 846
0.22 0.35
0.22 0.35
0.10 0.15
0 0
PSAC-3h
1116
0.50
0.46
0.13
0.04
PSAC-4h
1638
0.73
0.56
0.12
0.17
PSAC-5h
1870
0.87
0.52
0.09
0.35
Figure 1. Reactivity of NO with 8 wt % urea supported on PSAC-w. Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, reaction temperature = 30 °C, space velocity = 3000 1/h, carbon particle size = 200300 μm.
Figure 3. Reactivities of NO with 8 wt % urea supported on PSACs with different pore structures. Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, reaction temperature = 30 °C, space velocity = 6000 1/h, carbon particle size = 7001000 μm.
Figure 2. Nitrogen adsorptiondesorption isotherms of PSACs with different pore structures.
3. RESULTS AND DISCUSSION 3.1. Reactivity of NO with Urea Supported on PSAC-w. Figure 1 shows the reactivity of NO with 8 wt % urea supported on PSAC-w at 30 °C and with a space velocity of 3000 1/h. The NO conversion decreased from an initial 98% to about 87% in the first 7 h and maintained this level for 23 h (defining this stable NOx removal period as the stationary state). After that, it decreased to the minimum value of 77% at 51 h and then increased to reach a stable value of 91% (defining this stable NO oxidation period as the final state). It is important to note that no NO2 was detected at the outlet of the reactor in the initial 40.9 h, suggesting that the adsorbed nitrogen oxides are quickly reduced by supported urea on the surface of PSAC-w. After the start of NO2 emission, the outlet percentage yield of produced NO2 quickly increased with time on stream and finally stabilized at 91% which was equal to the final-state NO conversion, indicating that at the end of the experiment all consumed NO is catalytically oxidized to NO2 by O2 in the presence of PSAC-w. The final-state NO conversion reflects the activity of PSAC for NO catalytic oxidation.
Figure 1 also shows NOx conversion vs reaction time over the urea-supported PSAC-w. It was observed that, at the early stage of the reaction, the NOx conversion was equal to the NO conversion, and then, it gradually decreased to 4% at 60 h and finally to zero at 70 h. 3.2. Effect of Pore Structure on UreaSCR Activity and NOx Removal Amount. In order to illuminate the effect of pore structure on the ureaSCR activity, PSACs from the same origin with various extents of burn off were prepared by altering the activation period, and CO2 was used as the activating agent. Figure 2 shows the nitrogen adsorptiondesorption isotherms of PSACs with different pore structures. The pore parameters of the PSACs prepared in this work are summarized in Table 1. With increasing activation period, both the BET surface area and the total pore volume monotonically increased, while the pore volume of 0.50.8 nm pores reached the maximum value of 0.15 cm3/g at an activation period of 2 h. Figure 3 shows the NOx conversions vs reaction time on 8 wt % urea-supported PSACs with different pore structures at 30 °C and with a space velocity of 6000 1/h. The result indicated that the pore structure influenced the ureaSCR activity significantly. From Figure 3, it was observed that urea-supported PSAC-2h had the highest activity. It is worth noting that owing to a short activation period PSAC-2h had almost no mesopores, suggesting that the ureaSCR reaction can efficiently take place in micropores. The stationary-state NOx conversion on these ureasupported PSACs decreased in the order of PSAC-2h > PSAC3h > PSAC-4h > PSAC-1h > PSAC-5h. 6019
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Figure 4. Influences of (a) BET surface area and (b) pore volume of 0.50.8 nm pores on stationary-state NOx conversion and final-state NO conversion. Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, urea loading = 8 wt %, reaction temperature = 30 °C, space velocity = 6000 1/h, carbon particle size = 7001000 μm.
NO oxidation was found to be an important step in NH3SCR technologies.2224 Herein, in order to illuminate the role of NO oxidation in the ureaSCR reaction, the activities of PSACs for NO catalytic oxidation were also studied in the present work. Influences of BET surface area and pore volume of 0.50.8 nm pores on final-state NO conversion are presented in Figure 4. It was found that the activity of PSAC for NO catalytic oxidation did not directly correlate with the BET surface area but had a great dependence on the pore volume of 0.50.8 nm pores. The activity of PSAC for NO catalytic oxidation increased with larger pore volume of 0.50.8 nm pores, which is consistent with the report of Zhang et al.25 Influences of PSAC pore structure on stationarystate NOx conversion are also presented in Figure 4. It is interesting to note that stationary-state NOx conversion and final-state NO conversion always showed similar variations on PSACs with various extents of burn off. PSAC which had a better NO catalytic oxidation activity achieved a higher ureaSCR activity, indicating that NO oxidation is also crucial in ureaSCR reaction as in the case of NH3SCR reaction. The effects of pore parameters on the NOx removal amount were investigated in this work. And the results are shown in Figure 5. Herein, the NOx removal amount was obtained by an
Figure 5. Influences of (a) BET surface area, (b) total pore volume, and (c) pore volume of 0.50.8 nm pores on the NOx removal amount. Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, urea loading = 8 wt %, reaction temperature = 30 °C, space velocity = 6000 1/h, carbon particle size = 7001000 μm.
integration method. From Figure 5a and b, it was found that the NOx removal amount showed no direct relationships with BET surface area and total pore volume. The increased carbon surface area and total pore volume did not seem to benefit the NOx removal amount when CO2 activation period was above 2 h. It was interesting to find that similar to the case in NO catalytic oxidation, the NOx removal amount increased with larger pore volume of 0.50.8 nm pores, as shown in Figure 5c. Considering that the same amounts of urea were consumed for SCR reaction on PSACs with different pore structures, the increased NOx 6020
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Table 2. Elemental Surface Composition According to XPS Analysis
samples
total C atomic content (%)
total O atomic content (%)
total N atomic content (%)
total S atomic content (%)
O/C
PSAC-1h PSAC-2h PSAC-5h
76.88 77.69 78.97
20.77 20.27 18.64
1.89 1.58 1.92
0.46 0.46 0.47
0.27 0.26 0.24
Figure 6. TPD profiles of CO and CO2 from PSAC-1h, PSAC-2h, and PSAC-5h.
removal amount indicated a larger amount of nitrogen oxides adsorbed on PSACs. Thus, the above results suggested that the 0.50.8 nm micropores could serve as a reservoir for adsorbed nitrogen oxides after the urea consumption. 3.3. Effect of Surface Composition on UreaSCR Activity. As surface composition may also affect the activity of ureaSCR reaction, the surface compositions of the above-mentioned PSACs were determined by XPS analysis and are summarized in Table 2. It was noted that the surface content of O decreased slightly in the order of PSAC-1h > PSAC-2h > PSAC-5h. Considering the behaviors of these PSACs in the ureaSCR reaction and NO catalytic oxidation as shown in Figures 3 and 4, both the ureaSCR reaction and NO oxidation do not seem to directly relate with the content of surface oxygen-containing functional groups, at least in the studied experimental conditions. Besides, since the CO2 activation process did not introduce N and S onto the PSAC surface, the surface contents of both N and S were almost constant at very low levels. Thus, the marked change of ureaSCR activity on PSACs with various extents of burn off obviously can not be ascribed to surface nitrogen and sulfur-containing functional groups. Since oxygen-containing functional groups on carbon surface have many types, the TPD tests were performed to examine the nature of these oxygen-containing functional groups on PSACs with various extents of burn off. And then, their effect on the activity of ureaSCR reaction could be further illuminated. It has been well-known that oxygen-containing functional groups on the carbon surface decompose as CO2 (from carboxyl groups, lactones, and anhydrides) and CO (from carbonyl groups, quinones, and phenols)2628 during the thermal desorption process. Figure 6 shows the TPD curves for CO and CO2 evolution from PSAC-1h, PSAC-2h, and PSAC-5h, respectively.
Table 3. Amounts of CO and CO2 Desorbed from PSACs with Different Pore Structures during the TPD Process to 950 °C samples
CO (mmol/g)
CO2 (mmol/g)
total (mmol/g)
PSAC-1h
1.93
0.46
2.39
PSAC-2h
1.51
0.31
1.82
PSAC-5h
1.08
0.24
1.32
Figure 7. Effect of reaction temperature on stationary-state NOx conversion and final-state NO conversion. Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, urea loading = 8 wt %, space velocity = 6000 1/h, carbon particle size = 6080 μm.
The total amounts of CO and CO2 desorbed were obtained by integration under the curve after calibration with known concentrations of each gas. And the results are presented in Table 3. It was found that the desorption amounts of both CO and CO2 decreased in the same order of PSAC-1h > PSAC-2h > PSAC-5h, suggesting that a long activation period simultaneously reduced the contents of various types of oxygen-containing functional groups. Such results were in line with the XPS analysis, which further conformed that both the ureaSCR reaction and NO oxidation have no direct relationships with the content of surface oxygen-containing functional groups in the studied experimental conditions. 3.4. Effect of Reaction Temperature on UreaSCR Activity. To further clarify the relation between the ureaSCR reaction and NO oxidation, the following experiment was designed to obtain the effect of reaction temperature on these two reactions and the corresponding apparent activation energies of these two reactions on PSAC-w were calculated, respectively. Before such experiments, preliminary tests were carried out to ensure the elimination of external and internal diffusion effects. The results showed that when the total flow rate was above 80 cm3/min, the external diffusion effect can be eliminated. And the particle size of PSAC below 80100 μm can guarantee the elimination of the internal diffusion effect in the present work (see details in the Supporting Information). Thus, the total flow rate of 100 cm3/min and the crushed PSAC-w with a particle size in the range of 6080 μm were used to ensure the obtained apparent activation energies and kinetics results free from the effects of mass transfer. The effect of reaction temperature on ureaSCR reaction and NO oxidation is shown in Figure 7. It 6021
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Figure 8. Effect of reaction temperature on ln r (r: reaction rate). Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, urea loading = 8 wt %, space velocity = 6000 1/h, carbon particle size = 6080 μm.
was observed that stationary-state NOx conversion and finalstate NO conversion were affected noticeably by reaction temperature, and they both decreased with increasing reaction temperature, from 82% and 86% at 30 °C to 27% and 32% at 90 °C, respectively. The effect of reaction temperature on the reaction rates of stationary-state ureaSCR reaction and final-state NO oxidation is presented in Figure 8. According to the Arrhenius equation, the apparent activation energies of these two reactions were calculated from Figure 8 to be 16.5 and 15.2 kJ/mol for the stationary-state ureaSCR reaction and final-state NO oxidation, respectively. The very similar apparent activation energies suggest that the oxidation of NO to NO2 is the rate-limiting step in ureaSCR reaction. 3.5. Kinetics Studies of the UreaSCR Reaction. In order to determine the order of reaction with respect to NO, the feed O2 concentration and urea loading were fixed at 21 vol % and 8 wt %, respectively, while the feed NO concentration was varied within a range of 100 to 1000 ppmv. Similarly, in order to determine the order with respect to O2, the feed NO concentration and urea loading were fixed at 500 ppmv and 8 wt %, respectively, while the feed O2 concentration was varied within a range of 121 vol %. Figure 9 illustrates the effect of feed NO concentration on the reaction rates of stationary-state ureaSCR reaction and finalstate NO oxidation on PSAC-w. It was found that higher feed NO concentration remarkably increased the reaction rates of these two reactions. The orders of stationary-state urea-SCR reaction and final-state NO oxidation with respect to NO were both calculated to be about 1. The effect of feed O2 concentration on the reaction rates of stationary-state ureaSCR reaction and final-state NO oxidation on PSAC-w is illustrated in Figure 10. The result indicated the importance of gaseous O2 in ureaSCR reaction. With increasing feed O2 concentration, the reaction rates of stationary-state ureaSCR reaction and final-state NO oxidation both increased significantly. The orders of these two reactions with respect to O2 were calculated to be 0.5114 for stationary-state ureaSCR reaction and 0.4925 for final-state NO oxidation, respectively. Herein, the very similar kinetics results of these two reactions further confirms the above conclusion that NO oxidation is the rate-limiting step in the ureaSCR reaction.
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Figure 9. Effect of feed NO concentration on reaction rate. Reaction conditions: 21 vol % O2, balance N2, urea loading = 8 wt %, reaction temperature = 30 °C, space velocity = 6000 1/h, carbon particle size = 6080 μm.
Figure 10. Effect of feed O2 concentration on reaction rate. Reaction conditions: 500 ppmv NO, balance N2, urea loading = 8 wt %, reaction temperature = 30 °C, space velocity = 6000 1/h, carbon particle size = 6080 μm.
3.6. Transient Response upon Switching Off and On O2. The role of gaseous O2 was further illustrated by a transient response experiment which was carried out by switching O2 off and on in the gas phase. The transient NO conversion is shown in Figure 11. The NO conversion first declined quickly after O2 was shut off, followed by a slow decline to zero. And then it sharply increased to 77% in 3 min after the reintroduction of O2 and finally to the original value. This result further strongly suggested that gaseous O2 plays a determinant role in the ureaSCR reaction. After O2 was shut off, the surface concentration of O2 on PSAC-w sharply decreased. The residual adsorbed O2 continued to participate in ureaSCR reaction until all O2 was exhausted. 3.7. FTIR Studies of Urea Activation and UreaSCR Process on PSAC. To investigate the effect of support texture on ureaSCR reaction, 8 wt % urea was supported on PSAC-w and glass fiber, respectively. And then, comparative activity tests were performed under a feed gas of 500 ppmv NO, 21 vol % O2, and balance N2 at 30 °C with a space velocity of 3000 1/h. The result 6022
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Figure 11. Transient response upon switching off and on O2 on PSACw with 8 wt % urea loading. Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, reaction temperature = 30 °C, space velocity = 3000 1/h, carbon particle size = 200300 μm.
showed that urea supported on glass fiber did not react with NO (not shown), while that supported on PSAC-w could effectively reduce NO to N2 (shown in Figure 1). Figure 12 illustrates FTIR spectra of pure urea and glass fiber with 8 wt % urea loading. In the spectrum of urea-supported glass fiber, several bands at 3448, 3348, 1680, 1624, and 1466 cm1 were observed. The bands at 3448 and 3348 cm1 can be ascribed to asymmetric and symmetric —NH2 stretching, respectively.29 The bands at 1680, 1624, and 1466 cm1 can be ascribed to CdO stretching, —NH2 bending, and asymmetric N—C—N stretching, respectively.29,30 Comparison of the spectrum of urea-supported glass fiber with that of pure urea showed that glass fiber as a support did not change the positions and intensities of the FTIR bands ascribed to urea, indicating that urea is not activated by glass fiber. This result may be a reason for no ureaSCR reaction observed on glass fiber with 8 wt % urea loading. Figure 13a and b show the FTIR spectra of PSAC-w and ureasupported PSAC-w. On PSAC-w with 8 wt % urea loading, bands at 1680, 1624, and 1466 cm1 shifted to lower wavenumbers at 1657, 1620, and 1441 cm1, respectively. The red-shift frequency of CdO stretching may be due to the conjugative effect between graphite layers on carbon surface and CdO bond in urea molecule, resulting in the elongation of CdO bond. Moreover, due to the possible electron-donating effect of graphite layers the electron density of —NH2 group increased, which enhanced the reducing ability of —NH2 group toward NOx. The variation of surface species on PSAC-w during the reaction process is shown in Figure 13be. After 42 h of ureaSCR reaction when NO2 just started to release, bands at 1657, 1620, and 1441 cm1 disappeared, indicating the consumption of urea. Meanwhile, a very small band at 1383 cm1 which is ascribed to asymmetric NO3 stretching appeared.31,32 Such a result agrees well with the above conclusion that supported urea can reduce adsorbed nitrogen oxides quickly to produce N2 on the PSAC surface and NO2 is not released until the complete consumption of urea. After 55 h when the outlet percentage yield of produced NO2 reached 54% and continued to increase, the band at 1383 cm1 significantly increased, suggesting more NO3 molecules were adsorbed on the surface
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Figure 12. FTIR spectra of urea and glass fiber with 8 wt % urea loading.
Figure 13. FTIR spectra of (a) PSAC-w, and PSAC-w with 8 wt % urea loading after exposure to a feed gas of 500 ppmv NO, 21 vol % O2, and balance N2 at 30 °C and with a space velocity of 3000 1/h for (b) 0, (c) 42, (d) 55, and (e) 70 h, carbon particle size = 200300 μm.
of PSAC-w. Meanwhile, a strong band at 15201590 cm1 which has been observed by many researchers and has not been interpreted unequivocally appeared. This band is probably a composite band due to CdO stretching vibrations in conjugated systems such as diketone, ketoester, and quinone,3236 which indicated the surface oxidation of PSAC-w. After 70 h, the intensities of bands at 1383 and 15201590 cm1 increased. At this moment, the outlet NOx concentration reached the inlet value, suggesting the adsorption saturation of nitrogen oxides. It seemed that NO3 and oxygen-containing groups were the main species present on the surface of the exhausted sample when comparing the FTIR spectrum of the exhausted sample with that of the origin PSAC-w. 3.8. Mechanism Consideration of the UreaSCR Reaction on PSAC. UreaSCR reaction on PSAC was studied in the present work. It was found that the NO removal process which involves a series of elementary reactions is very complex. In order to clarify this NO removal process, the detailed ureaSCR mechanism is discussed below. 3.8.1. Adsorption and Oxidation of NO on PSAC. Although it is generally agreed that the presence of gaseous O2 can enhance 6023
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Industrial & Engineering Chemistry Research the adsorption and selective catalytic reduction of NO on the carbon surface,25,3741 the detailed mechanism has not been fully understood and represented. Three main possible explanations have been reported for the role of O2 in the NO adsorption process. Ahmed et al.38 proposed that NO is oxidized by O2 to produce NO2 in the gas phase and then NO2 is adsorbed on the activated carbon (AC) surface. However, the NO oxidation by O2 in the gas phase is very slow, so this statement can not explain the observation that 91% NO was oxidized into NO2 in the presence of PSAC-w at 30 °C and with a space velocity of 3000 1/ h as shown in Figure 1. Similar results were observed by Mochida et al.42 on ACFs. The second explanation which was proposed by Richter et al.43 suggests that NO is adsorbed on the oxidized carbon surface formed by the reaction between gas phase O2 and AC. However, this statement also has a problem to account for the observation that preadsorption of O2 on the AC surface only slightly improved the NO adsorption.37 The importance of micropores of AC to gas adsorption and reaction has been known for many gas molecules, such as NO2, SO2, and CH4.4446 Recently, the third explanation proposed by Zhu et al.37 and Zhang et al.25 is that NO and O2 molecules are coadsorbed in the micropores of AC and then adsorbed NO is oxidized into adsorbed NO2 on the AC surface. According to the result of Zhu et al.,37 the presence of gaseous O2 significantly enhanced the adsorption of NO, and a considerable amount of NO2 was found to be desorbed from the AC surface during the subsequent TPD process. Zhang et al.25 further reported that NO and O2 were coadsorbed in the micropores with a pore size of about 0.7 nm where they could contact each other very closely, and these micropores acted as nanocatalytic reactors for NO oxidation. For the PSACs, their proposition is also true as it was verified by the NO catalytic oxidation results on PSACs with different pore structures in our present work. As shown in Figure 4, PSAC-2h with only micropores efficiently catalyzed the NO oxidation to produce NO2. Moreover, PSAC with larger pore volume of 0.50.8 nm pores had a higher activity for NO catalytic oxidation irrespective of the BET surface area and the surface composition, at least in the studied experimental conditions, suggesting that these 0.50.8 nm micropores are the active sites for NO oxidation by O2. Herein, it should be mentioned that, except for the crucial role of the 0.50.8 nm micropores in NO catalytic oxidation, these micropores also obviously affected the adsorption amount of nitrogen oxides on PSACs as shown in Figure 5. The adsorption amount of nitrogen oxides was increased with the larger pore volume of 0.50.8 nm pores while it had no direct relationships with the BET surface area and total pore volume, which further confirmed the importance of these micropores in the adsorption of nitrogen oxides. It is very important to note that ureaSCR reaction had a great dependence on NO oxidation as shown in Figure 4. To elucidate the relation between ureaSCR reaction and NO oxidation, we calculated the apparent activation energies and studied the kinetics of these two reactions on PSAC-w. The very similar apparent activation energies and kinetics results of these two reactions strongly suggest that the oxidation of NO is the rate-limiting step in ureaSCR reaction. Herein, the apparent activation energies of ureaSCR reaction and NO oxidation on PSAC-w are both negative (16.5 kJ/mol for ureaSCR reaction and 15.2 kJ/mol for NO oxidation), which means that the reaction temperature has a negative effect on the reaction process as indicated in Figure 7. Generally, in a given catalytic reaction, a high reaction temperature benefits the
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activation of reactants and the desorption of products, but inhibits the adsorption of reactants on the surface of catalysts. Therefore, the data found in this work strongly suggest that the coadsorption of NO with O2 on PSAC is a key step in both the ureaSCR reaction and NO oxidation. Interestingly, similar observations were also reported for NH3SCR,37,38,43,47 which gives some clues in the mechanism similarity among a variety of SCR technologies on AC in the low temperature region. According to the kinetics results, both ureaSCR reaction and NO oxidation are approximately first-order with respect to NO, and one-half order with respect to O2. Thus, the adsorbed O2 molecule may first split into two oxygen atoms, and then, the newly produced oxygen atom oxidizes the adsorbed NO into NO2 in the micropores of PSAC. Since the apparent activation energies of both ureaSCR reaction and NO oxidation are much lower than the dissociation energy of O2 (493.4 kJ/mol) at 273 K,48 the dissociation of adsorbed O2 in the micropores of PSAC may be catalyzed by a composite effect of the presence of NO, the carbon surface and the overlapping force fields of the opposite walls within the micropores.25,4952 The determinant role of gaseous O2 in the ureaSCR reaction was first indicated in the kinetics experiment by varying the feed O2 concentration, and was further verified by the O2 transient response experiment. As shown in Figure 11, the ureaSCR reaction did not take place without the presence of O2. In addition, the O2 transient response experiment also gives some important information on the ureaSCR reaction mechanism. If the ureaSCR reaction follows the ER mechanism, after O2 was turned off, gaseous NO would still be reduced by supported urea on PSAC to give a stable NO conversion in the stationary-state NOx removal period. However, the result was on the contrary. The NO conversion decreased to zero after O2 was turned off for a while, which strongly suggests that the ureaSCR reaction proceeds through a LH mechanism which involves the coadsorption of NO with O2 and the oxidation of NO to form NO2. 3.8.2. Disproportionation of NO2 and Reduction of Adsorbed Nitrogen Oxides. Because NO2 can be more easily adsorbed on the carbon surface than NO, many researchers consider NO2 to be the main adsorbed species on AC.37,5356 However, the FTIR spectrum of the exhausted sample (Figure 13e) showed that abundant adsorbed NO3 existed on the carbon surface. It is wellknown that NO2 can interact with catalytic surfaces to form NO and NO3 through the following reaction:5762 2NO2 f NO3 þ NO
ð6Þ
Systematic investigations of NO2 adsorption on Na and BaY zeolites have shown that the adsorbed NOx species from the disproportionation of NO2 gave two characteristic IR bands.6365 The band between 1370 and 1420 cm1 at the lower frequency range was assigned to the adsorbed NO3 species associated with the positively charged adsorption centers of the zeolites, and the higher frequency band in the 20002200 cm1 range was assigned to the adsorbed NOþ species on Lewis base O sites. While from our experiments, it is clear that adsorbed NO3 species was the dominant NOx species on the surface of the exhausted sample as shown in Figure 13e. A possible explanation for the absence of the band between 2000 and 2200 cm1 is that NO has a rather low adsorption coefficient on the carbon surface, thus most of the NO molecules are released as gas during the reaction process or under the N2 purge before the FTIR studies. Such explanation agrees well with the observations based on the NO2 adsorption on ACFs31 and 6024
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Industrial & Engineering Chemistry Research
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single-wall carbon nanotubes (SWNTs),60,61 respectively. And it is also in line with the report by Peng et al.66 using ab initio simulations. Their simulation results indicated that NO3 was the most likely long-lived adsorbed species based on the adsorption of NO2 on SWNTs. In the present work, urea showed a very high reducing ability toward adsorbed NOx species. Once NO3 was produced, it was reduced very rapidly by supported urea to produce N2, which was suggested by the activity tests in Figure 1 and was conformed by the FTIR results in Figure 13. It is worth noting that, in most ureaSCR technologies, a temperature above 350 °C is required for the sufficient decomposition of urea into NH3 and HNCO according to the following reaction:6769 ðNH2 Þ2 CO f NH3 þ HNCO
ð7Þ
HNCO is then hydrolyzed to form NH3 and CO2 on common SCR catalysts, like V2O5/WO3TiO2 and Fe/ZSM-5.70,71 HNCO þ H2 O f NH3 þ CO2
ð8Þ
The product NH3 rather than urea is considered to be the real reducing agent for NOx removal in these ureaSCR reactions. But in this work, urea can not be decomposed at such a low temperature of 30 °C, so there must be other reaction pathways for the low-temperature ureaSCR reaction. Chen et al.72 reported that the amide group was very reactive toward NO þ O2 over Fe/ZSM-5 at 200 °C. While Joubert et al.73 gave a different opinion that the amide species should undergo Hofmann rearrangement to produce isocyanate. The hydrolysis of isocyanate then led to the formation of CO2 and amine which was considered to be the real reducing agent toward NOx over Pt/Al2O3 in the temperature range of 150350 °C. Herein, it should be mentioned that the reaction temperatures used in these literatures are much higher than that in the present work. To our knowledge, so far, very little is known about the reaction between amide species and NOx on the AC surface in the low temperature range. From our experimental results, it seems that the amide group also shows a very high reactivity toward adsorbed NOx species at low temperatures. Detailed and systematic studies on the interaction between the amide group and the adsorbed NOx species are in progress with the aid of in situ FTIR. 3.8.3. Surface Oxidation of PSAC and NO3 Adsorption. After the complete consumption of supported urea, the outlet conversion of NO declined to a minimum value, and then slowly increased and finally stabilized. A possible explanation for this observation is that the produced NO3 can not exist on reductive carbon surface (C*) and it will gradually oxidize the carbon surface to produce oxygen-containing functions with NO release. The reaction is expressed as follows: NO3 ðadÞ þ 2C f 2CðOÞ þ NOðgÞ
ð9Þ
The oxidation of the carbon surface was confirmed by the FTIR results that bands ascribed to oxygen-containing functions appeared and increased remarkably after the complete consumption of supported urea as shown in Figure 13. Similar observations were previously reported based on the adsorption of NO þ O2 on various activated carbon materials in many literature reports.21,25,42,74,75 With the oxidation of carbon surface, more NO3 molecules were adsorbed on the surface of PSAC, which was indicated by the FTIR results as shown in Figure 13d and e. It seems that NO3
Figure 14. Reaction scheme of urea-SCR on PSAC.
can be stably adsorbed on the oxidized carbon surface. After the carbon surface was oxidized to some extent, as the remaining reductive carbon surface became less and less, the rate of reaction 9 was significantly decreased. As a result, the outlet NO conversion was increased. Meanwhile, more and more NO3 molecules stably occupied the carbon surface, which inhibited the disproportionation of NO2. Thereby the newly produced NO2 from NO oxidation was liberated to the gas phase with an increasing outlet percentage yield. After the carbon surface was extensively oxidized and the NO3 adsorption reached saturation, both the outlet NO conversion and NO2 percentage yield kept constant as shown in Figure 1. Herein, it should be mentioned that the activity of carbon surface oxidation by nitrogen oxides was decreased by the pre-existing oxygen-containing functional groups. As shown in Figure 3, PSAC-1h with the highest surface content of O exhibited an obviously delayed decrease of NOx conversion after complete urea consumption. Similar observations were reported in many literature reports25,31,44,76 concerning the adsorption of nitrogen oxides (NO þ O2 or NO2) on the AC surface. The whole ureaSCR process includes multiple steps, such as (1) the coadsorption of NO and O2, (2) the oxidation of adsorbed NO, (3) the disproportionation of NO2, (4) the reaction between supported urea and NO3, (5) the oxidation of carbon surface, (6) the adsorption of NO3 on the oxidized carbon surface, and (7) NO2 release to the gas phase, which are summarized in Figure 14.
4. CONCLUSION Selective catalytic reduction of NO with urea catalyzed by PSAC was studied at low temperatures with a special focus on the kinetics and mechanism aspects. The O2 transient response experiment clearly indicates that the ureaSCR reaction proceeds through the LH mechanism, because no reaction was observed between gaseous NO and supported urea in the absence of O2. During the reaction process, NO was first coadsorbed with O2 in the 0.50.8 nm micropores where it was then catalytically oxidized into NO2. It was found that NO catalytic oxidation is the rate-limiting step in the ureaSCR reaction, which was confirmed by the very similar apparent activation energies and kinetics results of these two reactions. The produced NO2 then disproportionated to NO3 and NO. It is important to point out that the amide groups in urea molecules are very reactive toward the adsorbed NO3 to produce N2 in the low temperature range on PSAC, thus only nonadsorbed NO was detected until the complete consumption of supported urea. After that, the carbon surface was gradually oxidized by adsorbed NOx species, and NO3 was found to be stably adsorbed on the oxidized carbon surface. In conclusion, 6025
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Industrial & Engineering Chemistry Research understanding the mechanism of such low-temperature urea SCR reaction will likely motivate further significant research in its commercial application, for example, for NOx removal from low-temperature exhaust gas emissions by stationary sources and foul air in tunnels.
’ ASSOCIATED CONTENT
bS
Supporting Information. Preliminary tests to eliminate the effects of external and internal diffusion. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ86-21-64253730. Fax: þ86-21-64252914. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National High-Tech Research and Development Program (No. 2007AA05Z311), the Natural Science Foundation of China (No. 20806024 and No. 20977028), and the Research Fund for the Doctoral Program of Higher Education (No. 20090074110009). ’ REFERENCES (1) Bosch, H.; Janssen, F. Catalytic Reduction of Nitrogen Oxides: A Review on the Fundamentals and Technology. Catal. Today 1988, 2, 369. (2) Janssen, F.; Kerkhof, F.; Bosch, H.; Ross, J. R. H. Mechanism of the Reaction of Nitric Oxide, Ammonia, and Oxygen over Vanadia Catalysts. I. The Role of Oxygen Studied by Way of Isotopic Transients under Dilute Conditions. J. Phys. Chem. 1987, 91, 5921. (3) Chen, J. P.; Yang, R. T. Role of WO3 in Mixed V2O5-WO3/TiO2 Catalysts for Selective Catalytic Reduction of Nitric Oxide with Ammonia. Appl. Catal. A: Gen. 1992, 80, 135. (4) Ozkan, U. S.; Cai, Y. P.; Kumthekar, M. W.; Zhang, L. P. Role of Ammonia Oxidation in Selective Catalytic Reduction of Nitric Oxide over Vanadia Catalysts. J. Catal. 1993, 142, 182. (5) Ozkan, U. S.; Kumthekar, M. W.; Cai, Y. P. Selective Catalytic Reduction of Nitric Oxide over Vanadia/Titania Catalysts: Temperature-Programmed Desorption and Isotopically Labeled OxygenExchange Studies. Ind. Eng. Chem. Res. 1994, 33, 2924. (6) Dumesic, J. A.; Topsøe, N. -Y.; Topsøe, H.; Chen, Y.; Slabiak, T. Kinetics of Selective Catalytic Reduction of Nitric Oxide by Ammonia over Vanadia/Titania. J. Catal. 1996, 163, 409. (7) 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: Environ. 2003, 44, 237. (8) Giakoumelou, I.; Fountzoula, C.; Kordulis, C.; Boghosian, S. Molecular Structure and Catalytic Activity of V2O5/TiO2 Catalysts for the SCR of NO by NH3: In Situ Raman Spectra in the Presence of O2, NH3, NO, H2, H2O, and SO2. J. Catal. 2006, 239, 1. (9) Armor, J. N. Catalytic Removal of Nitrogen Oxides: Where Are the Opportunities?. Catal. Today 1995, 26, 99. (10) Bhattacharyya, S.; Das, R. K. Catalytic Control of Automotive NOx: A Review. Int. J. Energy Res. 1999, 23, 351. (11) Haneda, M.; Shinoda, K.; Nagane, A.; Houshito, O.; Takagi, H.; Nakahara, Y.; Hiroe, K.; Fujitani, T.; Hamada, H. Catalytic Performance of Rhodium Supported on CeriaZirconia Mixed Oxides for Reduction of NO by Propene. J. Catal. 2008, 259, 223.
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