Improvement of the Activity of Î³-Fe2O3 for the Selective Catalytic

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Improvement of the activity of #-Fe2O3 for the selective catalytic reduction of NO with NH3 at high temperatures: NO reduction versus NH3 oxidization Shijian Yang, Huazhen Chang, Lei Ma, Zan Qu, Naiqiang Yan, Chizhong Wang, and Junhua Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie303272u • Publication Date (Web): 05 Mar 2013 Downloaded from http://pubs.acs.org on March 28, 2013

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Industrial & Engineering Chemistry Research

Improvement of the activity of γ-Fe2O3 for the selective catalytic reduction of NO with NH3 at high temperatures: NO reduction versus NH3 oxidization Shijian Yang ┼, ╪, §, Huazhen Chang ╪, Lei Ma ╪, Zan Qu §, Naiqiang Yan §, Chizhong Wang ╪, and Junhua Li ╪, ∗ ┼

School of Environmental and biological Engineering, Nanjing University of Science and

Technology, Nanjing, 210094 P. R. China ╪

State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC),

School of Environment, Tsinghua University, Beijing, 100084 P. R. China §

School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai,

200240 P. R. China

∗

Corresponding author phone and fax: 86-10-62771093; E-mail: [email protected] (J. H.

Li), [email protected] (S. J. Yang). 1

ACS Paragon Plus Environment


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Abstract: Lignite is widely used as the fuel for coal-fired power plants, and its flue gas temperature is about 50-100 oC higher than others. V2O5-WO3/TiO2 is extremely restricted in the selective catalytic reduction (SCR) of NO from the coal-fired power plants burning lignite due to the drop of NOx conversion, low N2 selectivity and volatility of vanadium pentoxide at high temperatures. Therefore, a more environmental-friendly SCR catalyst with excellent SCR activity and better N2 selectivity at 350-450 oC should be developed for this application. In this work, sulfated Fe-Ti spinel catalyst was developed for the SCR of NO from the coal-fired power plants burning lignite. The drop of NOx conversion at high temperatures was mainly related to the simultaneous occurrence of the catalytic oxidization of NH3 to NO during the SCR reaction. Ti was incorporated into γ-Fe2O3 to decease the oxidization ability of Fe3+ on the surface, and the sites for -NH2 adsorption and the active components for -NH2 oxidization were separated after the sulfation to decrease the probability of the collision between -NH2 adsorbed and Fe3+ on the surface. They both inhibited the catalytic oxidization of NH3 to NO over γ-Fe2O3. However, the SCR reaction over γ-Fe2O3 was simultaneously restrained after the incorporation Ti and the sulfation. Therefore, NOx conversion over γ-Fe2O3 at high temperatures depended on the ratio of NH3 conversion through the catalytic oxidization of NH3 to NO to that through the SCR reaction. This ratio decreased after the incorporation of Ti, and it further decreased after the sulfation, resulting in an obvious promotion of NOx conversion at high temperatures. Therefore, Sulfated Fe-Ti spinel showed excellent SCR activity, N2 selectivity, and H2O+SO2 durability at 300-450 oC, which was suitable for the application in the coal-fired power plants burning lignite. Keywords: Selective catalytic reduction; Catalytic oxidization of NH3 to NO; Sulfated Fe-Ti spinel; Ti incorporation; Sulfation.

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Table of content

2.5

NH3 oxidization to NO/SCR

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2.0 1.5

γ-Fe2O3

Fe-Ti spinel sulfated Fe-Ti spinel

1.0 0.5 0.0

150 200 250 300 350 400 450 500 o

Temperature/ C

3



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1. Introduction Because Nitrogen oxides (NO and NO2) contribute to photochemical smog, acid rain, ozone depletion and greenhouse effect, they have been major pollutants for air pollution.1 So far, the most promising technology to control the emission of nitrogen oxides from automobile exhaust gas and industrial combustion of fossil fuels is selective catalytic reduction (SCR) with NH3.2 V2O5-WO3/TiO2 has been widely employed as a SCR catalyst to control the emission of NOx from coal-fired power plants for several decades.3 The temperature window of V2O5-WO3/TiO2 is about 300-400 oC. Recently, lignite is widely used as the fuel for coal-fired power plants.4 The temperature of flue gas from the coal-fired power plants burning lignite is about 50-100 oC higher than those burning anthracite and bituminous coal. If the temperature of flue gas is higher than 400 o

C, a large amount of N2O will form over V2O5-WO3/TiO2 and NOx conversion will decrease.5

Furthermore, vanadium pentoxide will volatize at high temperatures, which is a serious concern due to its toxicity to the environment.6 Therefore, a more environmental-friendly SCR catalyst with excellent SCR activity and better N2 selectivity at 350-450 oC should be developed for the coal-fired power plants burning lignite. As is well known, iron oxides for example γ-Fe2O3 and α-Fe2O3 are environmental-friendly and low cost.7 α-Fe2O3 shows a poor SCR activity, while γ-Fe2O3 has excellent SCR activity and N2 selectivity at 200-350 oC.8 However, NOx conversion over γ-Fe2O3 will rapidly decrease with the further increase of reaction temperature from 350 to 500 oC.9 The drop of NOx conversion at high temperatures was mainly related to the simultaneous occurrence of the catalytic oxidization of NH3 to NO during the SCR reaction.10 Therefore, the inhibition of the catalytic oxidization of NH3 to NO over γ-Fe2O3 was very important for the SCR reaction at high temperatures. In this work, Ti was incorporated into γ-Fe2O3 to decease the oxidization ability of Fe3+ on the surface, and the sites for -NH2 adsorption and the active components for -NH2 oxidization were separated after the sulfation to decrease the probability of the collision between -NH2 adsorbed and Fe3+ on the surface. They both inhibited the catalytic oxidization of NH3 to NO over γ-Fe2O3. Therefore, sulfated Fe-Ti spinel showed excellent SCR activity, N2 selectivity, and H2O+SO2 durability at 300-450 oC, which was suitable for the SCR of NO from the coal-fired power plants burning lignite. 4


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2. Experimental 2.1 Preparation γ-Fe2O3 and Fe-Ti spinel were obtained after the calcination of Fe3O4 and Fe2TiO4 under air for 3 h at 400 and 500 oC, respectively. Fe3O4 and Fe2TiO4 were prepared using a co-precipitation method, which was described in our previous work.11 Fe-Ti spinel (1.0 g) was sulfated under 400 ppm of SO2 and 2% of O2 (200 mL min-1) at 300 oC for 8 h.12 V2O5-WO3/TiO2 with 2 wt.% V2O5 and 10 wt.% WO3 were prepared by the conventional impregnation method.8

2.2 Catalytic test The SCR reaction, the catalytic oxidization of NH3, temperature programmed desorption of ammonia (NH3-TPD) and temperature programmed desorption of NO (NO-TPD) were carried out on a fixed-bed quartz tube reactor. The catalyst with 40-60 mesh was placed on the quartz wool held in the reactor, which was heated by a vertical electrical furnace. The feed contained 500 ppm of NO (when used), 500 ppm of NH3, 2% of O2, 10% of H2O (when used), 400 ppm of SO2 (when used), and balance of N2. The concentrations of NO, NO2, NH3, N2O and SO2 were continually monitored by an FTIR spectrometer (MKS Instruments).

2.3 Characterization XRD pattern was recorded on an X-ray diffractionmeter (Rigaku, D/max-2500HB+/PC) between 10° and 80° at a step of 7° min-1 operating at 30 kV and 30 mA using Cu Kα radiation. A nitrogen adsorption apparatus (Quantachrome, Autosorb-1) was used to determine the BET surface area. H2-TPR (temperature programmed reduction) was recorded on a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx). Fe 2p, Ti 2p, S 2p and O 1s binding energies on synthetic samples were determined by an X-ray photoelectron spectroscopy (Thermo, ESCALAB 250) using Al Kα (hv=1486.6 eV) as the excitation source. A Fourier transform infrared spectrometer (FTIR, Nicolet NEXUS 870) equipped with a liquid-nitrogen-cooled MCT detector was used to collect the in situ DRIFT spectra.

3. Result 3.1 SCR performance 3.1.1 SCR activity 5



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As shown in Figure 1a, 2% V2O5-WO3/TiO2 showed an excellent SCR activity at 250-450 oC (NOx conversion was higher than 80%). However, a large amount of N2O formed over 2% V2O5-WO3/TiO2 above 350 oC (shown in Figure 1b). NOx conversion over γ-Fe2O3 was close to 90% at 200-300 oC. With the increase of reaction temperature from 300 to 500 oC, NOx conversion over γ-Fe2O3 gradually decreased. Especially, NOx concentration in the outlet of γ-Fe2O3 was much higher than that in the inlet at 500 oC (shown in Figure 1a). Meanwhile, the conversion of NOx was much less than that of NH3 above 300 oC, which almost reached 100% (shown in Figure 1c), They suggest that some NH3 was oxidized to NO by γ-Fe2O3 above 300 oC. After the incorporation of Ti into γ-Fe2O3, NOx conversion at 150-250 oC obviously decreased. However, Fe-Ti spinel showed excellent SCR activity (NOx conversion was higher than 80%) and N2 selectivity (close to 100%) at 300-400 oC. NOx conversion over Fe-Ti spinel above 350 oC was much higher than that over γ-Fe2O3 (shown in Figure 1a). NOx conversion over Fe-Ti spinel gradually decreased with the increase of reaction temperature from 350 to 500 oC. After the sulfation, NOx conversion over Fe-Ti spinel further decreased at 150-250 oC (shown in Figure 1a). However, sulfation Fe-Ti spinel showed an excellent SCR activity at 300-450 oC, which was close to 2% V2O5-WO3/TiO2. Furthermore, little N2O formed over sulfated Fe-Ti spinel, which was much better than that over 2% V2O5-WO3/TiO2 (shown in Figure 1b). 3.1.2 Effect of H2O and SO2 H2O and SO2, which are permanently and abundantly present in the flue gas, often lead to the deactivation of SCR catalyst.13 Therefore, the effect of 10% of H2O and/or 400 ppm of SO2 on the SCR activity was investigated with a 24 h test. The sulfation of Fe-Ti spinel simultaneously happened during the SCR reaction over Fe-Ti spinel in the presence of SO2. Figure 1a shows that the SCR activity of sulfated Fe-Ti spinel was worse than that of Fe-Ti spinel at 150-250 oC. Furthermore, the presence of SO2 caused the deposition of NH4HSO4 and (NH4)2SO4 on the catalyst blow 250 oC.14 As a result, the presence of SO2 showed an obvious interference with the SCR reaction over Fe-Ti spinel at 150-250 oC (shown in Figure 2). The SCR activity of sulfated Fe-Ti spinel was much better than that of Fe-Ti spinel above 400 oC, so the presence of SO2 showed an obvious promotion on the SCR reaction over Fe-Ti spinel above 400 oC (shown in Figure 2). Because H2O could compete with gaseous NH3 for the active sites,15 the presence of 6


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H2O showed a severe interference with the SCR reaction over Fe-Ti spinel with SO2 at 150-250 oC. However, the effect of H2O gradually decreased with the increase of reaction temperature. Fe-Ti spinel showed excellent SCR activity, N2 selectivity and H2O+SO2 durability at 300-450 oC (shown in Figure 2), which was suitable for the SCR of NO from the coal-fired power plants burning lignite.

3.2 Characterization 3.2.1 XRD and BET After the incorporation of Ti, few changes can be observed in the diffraction scan and the characteristic peaks corresponding to rutile and anatase did not appear (shown in Figure 3). If there were some amorphous TiO2 in Fe2TiO4, they should transform to rutile (or anatase) after the calcination at 500 oC for 3 h.16 Therefore, there was no amorphous TiO2 in Fe2TiO4 and Ti was introduced into the spinel structure, which was consistent with the result in previous research using electron energy loss spectroscopy (EELS), X-ray adsorption near edge structure (XANES) and extended X-ray adsorption fine structure (EXAFS).17 After the sulfation, few changes can be observed in the diffraction scan of Fe-Ti spinel (shown in Figure 3). Therefore, the structure of Fe-Ti spinel was not destroyed after the sulfation. The BET surface areas of γ-Fe2O3, Fe-Ti spinel and sulfated Fe-Ti spinel were 74.7, 85.5 and 78.7 m2 g-1, respectively. 3.2.2 XPS XPS spectra of γ-Fe2O3, Fe-Ti spinel and sulfated Fe-Ti spinel over the spectral regions of Fe 2p, O 1s, Ti 2p and S 2p were shown in Figure 4. The Fe 2p 3/2 peaks on γ-Fe2O3 mainly centered at 710.2, 711.1 and 712.5 eV (shown in Figure 4a), which were assigned to Fe3+ in the spinel structure and Fe3+ bonded with hydroxyl groups, respectively.18 It suggests that Fe species on γ-Fe2O3 were mainly Fe3+ type species.11 The O 1s species on γ-Fe2O3 were assigned to O2- in transition metal oxides (at 530.2 eV) and that in -OH (at 531.6 eV).19 Meanwhile, the S species on γ-Fe2O3 was not observed (shown in Figure 4c). After the incorporation of Ti, few changes were observed in the spectral regions of Fe 2p and O1s (show in Figures 4d and 4e). The Ti peaks were assigned to Ti 2p 1/2 (464.1 eV) and Ti 2p 3/2 (458.4 eV) of Ti4+ (shown in Figure 5f). 11 7



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The S species on sulfated Fe-Ti spinel (shown in Figure 4h) were assigned to SO42- (168.9 eV) and HSO4- (170.0 eV).20 The presence of SO42- on sulfated Fe-Ti spinel can also be demonstrated by the XPS spectra over Fe 2p, O 1s and Ti 2p regions. After the sulfation, two new peaks (713.5 and 533.0 eV) appeared in the spectral regions of Fe 2p and O 1s (shown in Figures 4i and 4j), which could be attributed to Fe3+ bound by SO42- and O2- in SO42-, respectively. 9 Meanwhile, the binding energies of Ti 2p 1/2 (464.1 eV) and Ti 2p 3/2 (458.4 eV) of Ti4+ shifted to 464.6 and 458.9 eV, respectively (shown in Figure 4k). It suggests that Ti4+ on Fe-Ti spinel could be bound by SO42- after the sulfation. The percents of Fe, Ti, O and S on γ-Fe2O3, Fe-Ti spinel and sulfated Fe-Ti spinel collected from XPS spectra are shown in Table 1. As shown Table 1, the percent of Fe3+ on the surface obviously decreased after the incorporation of Ti and the sulfation. 3.2.3 H2-TPR The reduction peaks of γ-Fe2O3 mainly centered at 317 and 680 oC, which were assigned to the reduction of γ-Fe2O3 to Fe3O4 21 and the reduction of Fe3O4 to Fe 11, respectively. TPR profile of Fe-Ti spinel also showed two obvious reduction peaks. The first peak centered at 410 oC corresponded to the reduction of (Fe2Ti)0.8O4 to Fe2TiO4, and the broad peak at higher temperature was attributed to the reduction of Fe2TiO4 to Fe and TiO2.21 In comparison with γ-Fe2O3, a strong shift of the first peak to high temperature happened in the TPR profile of Fe-Ti spinel (shown in Figure 5). It suggests that the oxidization ability of Fe3+ on Fe-Ti spinel was much less than that on γ-Fe2O3. After the sulfation of Fe-Ti spinel, the first reduction peak further shifted to high temperature (shown in Figure 5). It suggests that the oxidization ability of Fe3+ on Fe-Ti spinel further decreased after the sulfation. TPR analysis shows that the area of the first peak for Fe-Ti spinel reduction was much less than that for sulfated Fe-Ti spinel. It suggests that the reduction of SO42could contribute to the first reduction peak of sulfated Fe-Ti spinel. 3.2.4 Adsorption of NH3 and NO The capacities of γ-Fe2O3, Fe-Ti spinel and sulfated Fe-Ti spinel for NH3 and NO adsorption at 50 oC can be calculated from NH3-TPD and NO-TPD, which are shown in Table 2. The capacity of Fe-Ti spinel for NH3 adsorption was about twice that of γ-Fe2O3, and it further increased after 8


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the sulfation. However, the capacity of γ-Fe2O3 for NO adsorption obviously decreased after the incorporation of Ti, and it further decreased after the sulfation (shown in Table 2). They indicate that the adsorption of NH3 on γ-Fe2O3 was promoted after the incorporation of Ti and the sulfation, while the adsorption of NO on γ-Fe2O3 was restrained. The characteristic vibrations of NH3 adsorption on γ-Fe2O3 and Fe-Ti spinel at 300 oC were mainly assigned to coordinated ammonia bound to the Lewis acid sites (at 1203 and 1606 cm-1).22 The intensity of the adsorption of NH3 on Fe-Ti spinel was higher than that on γ-Fe2O3 (shown in Figure 6a). It indicates that the adsorption of NH3 on γ-Fe2O3 was promoted after the incorporation of Ti, which was consistent with the results of NH3-TPD (shown in Table 2). The characteristic vibrations of NH3 adsorption on sulfated Fe-Ti spinel appeared at 1430 and 1278 cm-1 (shown in Figure 6a), which could be assigned to NH4+ bound to the Brønsted acid sites.23 Furthermore, a slight vibration at 1606 cm-1 corresponding to coordinated ammonia bound to the Lewis acid sites still appeared. XPS spectra show that sulfated Fe-Ti spinel was covered by SO42-. As is well known, SO42- is a typical Brønsted acid.24 Therefore, NH3 could mainly adsorb on SO42on sulfated Fe-Ti spinel. The characteristic vibrations of NO+O2 adsorption on γ-Fe2O3 at 300 oC were assigned to monodentate nitrite (at 1601cm-1) and monodentate nitrate (at 1579 cm-1).25 The characteristic vibrations of NO+O2 adsorption on Fe-Ti spinel could be assigned to bridging nitrate (at 1579 and 1622 cm-1).26 The characteristic vibration of NO+O2 adsorption on sulfated Fe-Ti spinel was assigned to nitro (at 1385 cm-1) .27 Nitro adsorbed on sulfated Fe-Ti spinel was coordinated via its N atom,27 which was quite different from those on γ-Fe2O3 and Fe-Ti spinel. Monodentate nitrite and monodentate nitrate adsorbed on γ-Fe2O3 and bridging nitrate adsorbed on Fe-Ti spinel were coordinated by one or two of its oxygen atoms. The chemical adsorption of NO+O2 on γ-Fe2O3, Fe-Ti spinel and sulfated Fe-Ti spinel can be approximately described as follows: 8 (1)

≡ Fe 3 + + ≡ O 2 − + NO (ad) → ≡ Fe 2 + + NO x (ad)

(2)

Reaction 1 was the physical adsorption of gaseous NO on the surface. Then, the physically

9



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adsorbed NO was oxidized by Fe3+ on the surface to form adsorbed NOx species (Reaction 2). The kinetic equation of the oxidization of adsorbed NO by Fe3+ on the surface (Reaction 2) can be approximately described as:8

-

d[N O (ad) ] dt

=-

d[ ≡ Fe 3+ ] d[N O x (ad) ] = = k1 [N O (ad) ][ ≡ Fe 3+ ] dt dt

(3)

Where, k1, [NO(ad)], [NOx(ad)] and [≡Fe3+] were the kinetic constant of Reaction 2 and the concentrations of adsorbed NO, NOx and Fe3+ on the surface, respectively. H2-TPR analysis demonstrates that the oxidization abilities of Fe3+ on Fe-Ti spinel and sulfated Fe-Ti spinel were much less than that on γ-Fe2O3. It suggests that k1 of Fe-Ti spinel and sulfated Fe-Ti spinel were much less than that of γ-Fe2O3. The physical adsorption of NO on γ-Fe2O3 was about 3.8-5.7 times those of Fe-Ti spinel and sulfated Fe-Ti spinel (shown in Table 2). Moreover, the concentrations of Fe3+ on Fe-Ti spinel and sulfated Fe-Ti spinel were less than that on γ-Fe2O3 (shown in Table 1). Hinted by Equation 3, the amounts of adsorbed NOx on Fe-Ti spinel and sulfated Fe-Ti spinel were much less than that on γ-Fe2O3, which was demonstrated by Figure 6b. 3.2.5 Oxidization of NH3 Figure 7a shows that little NH3 can be oxidization over γ-Fe2O3 below 200 oC. With the increase of reaction temperature, NH3 oxidization over γ-Fe2O3 was obviously promoted. NH3 over γ-Fe2O3 was mainly oxidized to N2 below 350 oC. However, the amount of NO from NH3 oxidization over γ-Fe2O3 obviously increased with the increase of reaction temperature from 350 to 500 oC, resulting in an obvious decrease of N2 selectivity. After the incorporation of Ti, NH3 oxidization over γ-Fe2O3 was obviously restrained (shown in Figure 7b). Meanwhile, the amount of NO from NH3 oxidization over Fe-Ti spinel was much less than that over γ-Fe2O3 at high temperatures. NH3 oxidization over Fe-Ti spinel was further restrained after the sulfation, while its N2 selectivity was obviously promoted (shown in Figure 7c). Sulfated Fe-Ti spinel shows an excellent N2 selectivity (>95%) for NH3 oxidization below 450 oC, and only a small amount of N2O and NO formed.

3.3 In situ DRIFTS study Sulfated Fe-Ti spinel was first treated with NH3 /N2, NO+O2/N2 was then introduced into the IR cell (shown in Figure 8a). After the adsorption of NH3 at 300 oC, sulfated Fe-Ti spinel was mainly 10


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covered by NH4+ bound to SO42- (1430 and 1278 cm-1). After NO+O2/N2 passed over NH3/N2 pretreated sulfated Fe-Ti spinel, NH4+ diminished, and sulfated Fe-Ti spinel was mainly covered by nitro (at 1385 cm-1). Meanwhile, a new band at 1625 cm-1 appeared, which could be assigned to adsorbed H2O resulting from the SCR reaction.22 They both demonstrate that the reaction between adsorbed NH3 and NO could contribute to the SCR reaction over sulfated Fe-Ti spinel. Then, the reactants were introduced to sulfated Fe-Ti spinel in the reverse order (shown in Figure 8b). After the adsorption of NO+O2 on sulfated Fe-Ti spinel at 300 oC, sulfated Fe-Ti spinel was mainly covered by nitro (at 1385 cm-1). After NH3/N2 passed over NO+O2/N2 pretreated sulfated Fe-Ti spinel, the band at 1385 cm-1 corresponding to nitro swiftly diminished, and sulfated Fe-Ti spinel was mainly covered by NH4+ bound to SO42- (at 1430 and 1278 cm-1). It indicates that the reaction between adsorbed NOx (nitro) and NH3 could contribute to the SCR reaction over sulfated Fe-Ti spinel. At last, NH3 and NO+O2 were simultaneously introduced at 300 oC and the IR spectra were recorded. As shown in Figure 8c, NH4+ bound to SO42- (at 1430 and 1278 cm-1) and adsorbed H2O (at 1625 cm-1) both appeared. However, nitro (at 1385 cm-1) was not observed. They suggest that nitro could not form during the SCR reaction over sulfated Fe-Ti spinel. Both NH3 and NO adsorbed on sulfated Fe-Ti spinel can be oxidized by Fe3+ on sulfated Fe-Ti spinel to -NH2 and nitro respectively, so NH3 adsorbed would compete with NO adsorbed for the oxidization agents (i.e. Fe3+ on sulfated Fe-Ti spinel). Table 2 shows that the capacity of sulfated Fe-Ti spinel for NH3 adsorption was about 17 times that of NO. It suggests that Fe3+ on sulfated Fe-Ti spinel preferred to activate adsorbed NH3 rather than to oxidize adsorbed NO. As a result, nitro could not form on sulfated Fe-Ti spinel during the SCR reaction and the SCR reaction over sulfated Fe-Ti spinel mainly followed the Eley-Rideal mechanism (i.e. the reaction between activated NH3 and gaseous NO), which was consistent with the effect of sulfation on iron titanate.23 The Langmuir-Hinshelwood pathway of the SCR reaction over iron titanate catalyst was cut off by the sulfation process, and only the Eley–Rideal reaction pathway between adsorbed NH3 species and gaseous NO dominated in the SCR reaction.23

4. Discussion 4.1 Reaction mechanism 11



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Our previous study using in situ DRIFTS study and kinetic analysis demonstrated that both the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism happened during the SCR reaction over γ-Fe2O3 below 300 oC.25 However, the SCR reaction over γ-Fe2O3 above 300 oC mainly followed the Eley-Rideal mechanism.8 The SCR reaction over γ-Fe2O3 below 300 oC through the Langmuir-Hinshelwood mechanism was cut off after the incorporation of Ti, and the SCR reaction over Fe-Ti spinel mainly followed the Eley-Rideal mechanism.8 The SCR reaction through the Eley-Rideal mechanism can be approximately described as follows:8 (4)

NH 3 (ad) + ≡ Fe 3+ → -NH 2 + ≡ Fe 2+ + H +

(5)

-NH 2 + NO (g) → N 2 + H 2 O

(6)

≡ Fe 2+ +

1 1 O 2 → ≡ Fe 3+ + ≡ O 2 − 4 2

(7)

There is general agreement that the SCR reaction starts with the adsorption of gaseous NH3 on the acid sites (i.e. Reaction 4), which is very strong compared to the adsorption of NO+O2 and the reaction products.28 Then, the adsorbed NH3 was activated by Fe3+ on the surface to form -NH2 (i.e. Reaction 5). -NH2 on the surface can reduce gaseous NO to form N2 and H2O (Reaction 6). At last, the reduced Fe3+ can be regenerated through Reaction 7. During the reduction of gaseous NO by -NH2 (Reaction 6), -NH2 can be simultaneously catalytically oxidized to NO by Fe3+ on the surface. Therefore, NH3 can be catalytically oxidized in the absence of NO at high temperatures (shown in Figure 7). The oxidization of -NH2 can be approximately described as:24 3+

≡ Fe -NH 2 + O 2  → NO+H 2 O

(8)

NO from NH3 oxidization can also be reduced by -NH2 to N2 (Reaction 6), which is the so-called selective catalytic oxidization (SCO) of NH3.29 The kinetic equations of the Reactions 5, 6 and 8 can be approximately described as:8

−

d[N H 3(ad) ] dt

=

−

d[ ≡ Fe 3+ ] d[-N H 2 ] = = k 2 [N H 3(ad) ][ ≡ Fe 3+ ] dt dt 12


(9)

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−

d[NO(g) ] d[-NH 2 ] =− = k3 [ − NH 2 ][NO(g) ] dt dt

(10)

−

d[-NH 2 ] d[ ≡ Fe3+ ] =− = k4 [-NH 2 ][ ≡ Fe3+ ] dt dt

(11)

Where, k2, k3, k4, [NH3(ad)], [-NH2] and [NO(g)] were the kinetic constants of Reactions 5, 6 and 8, the concentrations of adsorbed NH3, and -NH2 on the surface and the concentration of gaseous NO, respectively.

4.2 Contributions of the catalytic oxidization of NH3 to NO and the SCR reaction -NH2 on the surface can be oxidized through both Reactions 6 and 8, so the consumption of -NH2 (i.e. NH3 conversion) can be described as:

−

d[-NH 3(g ) ] dt

=−

d[-NH 2 ] = ηSCR + ηCO = k3 [-NH 2 ][NO (g) ] + k4 [-NH 2 ][ ≡ Fe3+ ] dt

(12)

Where, ηSCR and ηCO were the contributions of the SCR reaction (mainly the Eley-Rideal mechanism) and the catalytic oxidization of NH3 to NO to NH3 conversion. Meanwhile, the reduction of gaseous NO can be described as:

−

d[NO (g) ] dt

= ηSCR -ηCO = k3 [-NH 2 ][NO (g) ] − k4 [-NH 2 ][ ≡ Fe3+ ]

(13)

According to Equations 12 and 13, ηSCR and ηCO can be calculated according to the difference between the ratio of NOx conversion and that of NH3, which was shown in Figure 9. H2-TPR analysis (Figure 5) demonstrates that the oxidization ability of Fe3+ on γ-Fe2O3 obviously decreased after the incorporation of Ti. It suggests that k2 of Fe-Ti spinel was much less than that of γ-Fe2O3. Moreover, the concentration of Fe3+ on Fe-Ti spinel was much less than that on γ-Fe2O3 (shown in Table 1). Although the concentration of NH3 adsorbed on Fe-Ti spinel was higher than that on γ-Fe2O3 (shown in Table 2), the activation of NH3 over γ-Fe2O3 (Reaction 5) was obviously restrained after the incorporation of Ti due to the remarkable decrease of both k2 and Fe3+ concentration on the surface (hinted by Equation 9). It suggests that the concentration of -NH2 on γ-Fe2O3 decreased after the incorporation of Ti. Therefore, the SCR reaction over γ-Fe2O3 through the Eley-Rideal mechanism was obviously restrained after the incorporation of Ti (hinted by Equation 10). Furthermore, The SCR reaction over γ-Fe2O3 below 300 oC through the

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir–Hinshelwood mechanism was cut off after the incorporation of Ti due to the inhibition of the adsorption of NO+O2.8 As a result, the SCR reaction over γ-Fe2O3 was obviously restrained after the incorporation of Ti, resulting in an obvious decrease of NOx conversion below 300 oC (shown in Figure 1a). The acid sites on Fe-Ti spinel (Lewis acid) mainly resulted from the unsaturated coordination of Fe3+, Ti4+ and O2-, so NH3 mainly adsorbed on Fe-O-Ti band on Fe-Ti spinel. However, the acid sites on sulfated Fe-Ti spinel mainly resulted from SO42- on the surface, so NH3 mainly adsorbed on SO42- on sulfated Fe-Ti spinel. It suggests that the sites for NH3 adsorption and the active components for NH3 activation (Fe3+) on Fe-Ti spinel were separated after the sulfation. Therefore, the probability of the collision between NH3 adsorbed and Fe3+ on Fe-Ti spinel obviously decreased after the sulfation. Meanwhile, the oxidization ability of Fe3+ on Fe-Ti spinel slightly decreased after the sulfation (shown in Figure 5). Therefore, k2 of Fe-Ti spinel was higher than that of sulfated Fe-Ti spinel. Moreover, the concentration of Fe3+ on Fe-Ti spinel slightly decreased after the sulfation (shown in Table 1). As a result, the SCR reaction over Fe-Ti spinel through the Eley-Rideal mechanism was further restrained after the sulfation (hinted by Equation 9), resulting in an obvious decrease of NOx conversion at 150-300 oC (shown in Figure 1a). Because the oxidization ability of Fe3+ on Fe-Ti spinel was much less than that on γ-Fe2O3, k4 of Fe-Ti spinel was much less than that of γ-Fe2O3. Meanwhile, -NH2 on Fe-Ti spinel was less than that on γ-Fe2O3. Moreover, the concentration of Fe3+ on γ-Fe2O3 obviously decreased after the incorporation of Ti (shown in Table 1). According to Equation 11, the catalytic oxidization of -NH2 to NO over γ-Fe2O3 was obviously restrained after the incorporation of Ti (shown in Figures 9a and 9b). -NH2 resulted from the activation of adsorbed NH3, so the sites for -NH2 adsorption were the same as those for NH3. Therefore, the sites for -NH2 adsorption and the active components for -NH2 oxidization (Fe3+) on Fe-Ti spinel were separated after the sulfation. It suggests that the probability of the collision between -NH2 adsorbed and Fe3+ on Fe-Ti spinel obviously decreased after the sulfation, which was similar to that of NH3 activation. Meanwhile, the oxidization ability of Fe3+ on sulfated Fe-Ti spinel was slightly less than that on Fe-Ti spinel. Therefore, k4 of Fe-Ti spinel decreased after the sulfation. Meanwhile, the activation of adsorbed NH3 over Fe-Ti spinel 14


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was restrained after the sulfation, resulting in the decrease of -NH2 on Fe-Ti spinel. Moreover, the concentration of Fe3+ on Fe-Ti spinel slightly decreased after the sulfation (shown in Table 1). According to Equation 11, the catalytic oxidization of -NH2 to NO over Fe-Ti spinel was further restrained after the sulfation (shown in Figures 9b and 9c).

4.3 NH3 oxidization versus NO reduction As shown in Figure 7, NH3 oxidization did not happen at low temperatures (below 200 oC over γ-Fe2O3, below 250 oC over Fe-Ti spinel and below 300 oC over sulfated Fe-Ti spinel). Therefore, Reaction 6 predominated over NH3 conversion at low temperatures (shown in Figure 9). However, the catalytic oxidization of NH3 to NO was obviously promoted with the increase of reaction temperature. Therefore, Reaction 8 would compete with Reaction 6 for the consumption of -NH2. Figure 1b shows that most of NH3 was oxidized above 300 oC. It suggests that the inhibition of NH3 activation due to the incorporation of Ti and the sulfation can be neglected above 300 oC. Therefore, NOx conversion over γ-Fe2O3, Fe-Ti spinel and sulfated Fe-Ti spinel at high temperatures depended on the ratio of ηCO to ηSCR (k5), which can be described as:

k5 =

ηCO k4 [-NH 2 ][≡ Fe3+ ] k4 [≡ Fe3+ ] = = ηSCR k3[-NH 2 ][NO (g) ] k3[NO (g) ]

(14)

k5 of γ-Fe2O3, Fe-Ti spinel and sulfated Fe-Ti spinel can be calculated from Figure 9. As shown in Figure 10, k5 of γ-Fe2O3, Fe-Ti spinel and sulfated Fe-Ti spinel all increased with the increase of reaction temperature. Meanwhile, k5 obviously decreased in the following sequence: γ-Fe2O3> Fe-Ti spinel>sulfated Fe-Ti spinel. Equation 14 shows that k5 was directly proportional to the product of k4 and the concentration of Fe3+ on the surface. The concentration of Fe3+ on γ-Fe2O3 decreased after the incorporation of Ti, and it further decreased after the sulfation (shown in Table 1). Furthermore, k4 obviously decreased in the following sequence: γ-Fe2O3>Fe-Ti spinel>sulfated Fe-Ti spinel. Hinted by Equation 14, k5 obviously decreased in the same sequence. Therefore, NOx conversion over γ-Fe2O3 at high temperatures obviously increased after the incorporation of Ti, and it further increased after the sulfation.

5. Conclusion The SCR reaction over γ-Fe2O3 mainly followed the Eley-Rideal mechanism at high 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

temperatures. However, the catalytic oxidization of NH3 to NO simultaneously happened, resulting in a drop of NOx conversion. NOx conversion over γ-Fe2O3 at high temperatures depended on the ratio of NH3 conversion through the catalytic oxidization of NH3 to NO to that through the SCR reaction, which was directly proportional to the product of the concentration of Fe3+ on the surface and the ability of Fe3+ on the surface for the catalytic oxidization of -NH2 to NO. After the incorporation of Ti, the ability of Fe3+ on γ-Fe2O3 for the catalytic oxidization of -NH2 to NO obviously decreased due to the decrease of oxidization ability. The sites for -NH2 adsorption and the active components for -NH2 oxidization on Fe-Ti spinel were separated after the sulfation, resulting in a further decrease of the ability of Fe3+ for the catalytic oxidization of -NH2 to NO. Meanwhile, the concentration of Fe3+ on γ-Fe2O3 obviously decreased after the incorporation of Ti and the sulfation. Therefore, the ratio of NH3 conversion through the catalytic oxidization of NH3 to NO to that through the SCR reaction obviously decreased after the incorporation of Ti and the sulfation, resulting in an obvious promotion of NOx conversion at high temperatures. However, the activation of NH3 over γ-Fe2O3 was simultaneously restrained, so the low temperature SCR activity of γ-Fe2O3 obviously decreased after the incorporation of Ti and the sulfation.

16


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Acknowledgments This study was financially supported by the National Natural Science Fund of China (Grant Nos. 21207067 and 51078203), special fund of State Key Joint Laboratory of Environment Simulation and Pollution Control, and the National High-Tech Research and Development (863) Program of China (Grant No. 2012AA062506).

17



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(20) Fu, H. B.; Wang, X.; Wu, H. B.; Yin, Y.; Chen, J. M. J. Phys. Chem. C 2007, 111, 6077. (21) Ayub, I.; Berry, F. J.; Crabb, E.; Helgason, O. J. Mater. Sci. 2004, 39, 6921. (22) Qi, G. S.; Yang, R. T. J. Phys. Chem. B 2004, 108, 15738. (23) Liu, F. D.; Asakura, K.; He, H.; Shan, W. P.; Shi, X. Y.; Zhang, C. B. Appl. Catal. B-environ 2010, 103, 369. (24) Xie, G. Y.; Liu, Z. Y.; Zhu, Z. P.; Liu, Q. Y.; Ge, J.; Huang, Z. G. J. Catal. 2004, 224, 42. (25) Yang, S.; Wang, C.; Li, J.; Yan, N.; Ma, L.; Chang, H. Appl. Catal. B-environ 2011, 110, 71. (26) Liu, F. D.; He, H.; Zhang, C. B.; Feng, Z. C.; Zheng, L. R.; Xie, Y. N.; Hu, T. D. Appl. Catal. B-environ 2010, 96, 408. (27) Hadjiivanov, K. I. Catal. Rev. 2000, 42, 71. (28) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Appl. Catal. B-environ 1998, 18, 1. (29) Chmielarz, L.; Kustrowski, P.; Rafalska-Lasocha, A.; Dziembaj, R. Appl. Catal. B-environ 2005, 58, 235.

19



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Page 20 of 32

Table 1 Percents of Fe, Ti, O and S on synthetic catalysts

/%

Fe3+

Ti4+

O2-

S (SO42-)

γ-Fe2O3

40.0

-

60.0

-

Fe-Ti spinel

18.7

17.8

63.5

-

sulfated Fe-Ti spinel

14.1

14.7

66.1

5.1

20


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Table 2 Capacities of synthetic catalysts for NO and NH3 adsorption at 50 oC

/mmol g-1

NO

NH3

γ-Fe2O3

1.7

1.2

Fe-Ti spinel

0.44

2.4


0.30

5.0

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure captions Figure 1 SCR performance of synthetic catalysts: (a), NOx conversion; (b), N2O concentration; (c), NH3 conversion. Reaction condition: [NH3]=[NO]= 500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate =200 mL min-1 and GHSV= 1.2×105 cm3 g-1 h-1. Figure 2 Effect of H2O and SO2 on the SCR reaction over Fe-Ti spinel. Reaction condition: [NH3]=[NO]= 500 ppm, [O2]=2%, [SO2]=400 ppm, [H2O]=10%, catalyst mass=200 mg, total flow rate =200 mL min-1 and GHSV= 6×104 cm3 g-1 h-1. Figure 3 XRD patterns of synthetic catalysts Figure 4 XPS spectra of synthetic catalysts over the spectral regions of Fe 2p, Ti 2p, O 1s and S 2p Figure 5 H2-TPR of synthetic catalysts Figure 6 (a), DRIFT spectra of the adsorption of NH3 on synthetic catalysts at 300 oC; (b), DRIFT spectra of the adsorption of NO+O2 on synthetic catalysts at 300 oC. Figure 7 NH3 oxidization over synthetic catalysts: (a), γ-Fe2O3; (b), Fe-Ti spinel; (c), sulfated Fe-Ti spinel. Reaction condition: [NH3] = 500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate =200 mL min-1 and GHSV= 1.2×105 cm3 g-1 h-1. Figure 8 (a), DRIFT spectra taken at 300 oC upon passing NO+O2 over NH3 presorbed sulfated Fe-Ti spinel; (b), DRIFT spectra taken at 300 oC upon passing NH3 over NO+O2 presorbed sulfated Fe-Ti spinel; (c), DRIFT spectra taken at 300 oC upon passing NH3+NO+O2 over sulfated Fe-Ti spinel. Figure 9 Contributions of the SCR reaction (SCR) and the catalytic oxidization of NH3 to NO (CO) to NH3 conversion during the SCR reaction over: (a), γ-Fe2O3; (b), Fe-Ti spinel; (c), sulfated Fe-Ti spinel. Reaction condition: [NH3]=[NO]= 500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate =200 mL min-1 and GHSV= 1.2×105 cm3 g-1 h-1. Figure 10 The ratio of NH3 conversion through the catalytic oxidization of NH3 to NO to that through the SCR reaction. Reaction condition: [NH3]=[NO]= 500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate =200 mL min-1 and GHSV= 1.2×105 cm3 g-1 h-1.

22


Page 22 of 32

Page 23 of 32

200

N2O concentration/ppm

100

NOx conversion/%

80

160

60 40 20 0 -20

120

γ-Fe2O3

Fe-Ti spinel sulfated Fe-Ti spinel 2% V2O5-WO3/TiO2

-40

γ-Fe2O3


80 40

150 200 250 300 350 400 450 500

0 150 200 250 300 350 400 450 500

o

Temperature/ C

o

Temperature/ C

a

b 100

NH3 conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60

γ-Fe2O3


40 20 0

150 200 250 300 350 400 450 500 o

Temperature/ C

c

Figure 1 SCR performance of synthetic catalysts: (a), NOx conversion; (b), N2O concentration; (c), NH3 conversion. Reaction condition: [NH3]=[NO]= 500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate =200 mL min-1, GHSV= 1.2×105 cm3 g-1 h-1.

23



Fe-Ti spinel

Fe-Ti spinel with SO2

Fe-Ti spinel with SO2+H2O

100

NOx conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80 60 40 20 0

150

200

250

300

350 o

Temperature/ C

400

450

Figure 2 Effect of H2O and SO2 on the SCR reaction over Fe-Ti spinel. Reaction condition: [NH3]=[NO]= 500 ppm, [O2]=2%, [SO2]=400 ppm, [H2O]=10%, catalyst mass=200 mg, total flow rate =200 mL min-1 and GHSV= 6×104 cm3 g-1 h-1.

24


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(311) (220)

(511)(440) (422)

(400)


Fe-Ti spinel

γ-Fe2O3 10

20

30

40

o

2θ/

50

60

70

80

Figure 3 XRD patterns of synthetic catalysts

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

γ-Fe2O3

Fe 2p

γ-Fe2O3

O 1s

γ-Fe2O3

S 2p

530.2

724.8 712.5

719.0

711.1

710.2

730

Page 26 of 32

725

720

715

710

531.6

705

536

534

532

530

528

526

174

172

Binding Energy/eV

Binding Energy/eV

a

170

b

Fe-Ti spinel

O 1s

Fe-Ti spinel

Ti 2p

724.5 712.2

719.0

458.4

529.9

711.1 710.2

464.1

531.8

730

725

720

715

710

705

Binding Energy/eV

536

534

532

530

528

Binding Energy/eV

526

468

466

464

462

Fe-Ti spinel

458

456

454

f S 2p


S 2p

460

Binding Energy/eV

e

d

166

c

Fe-Ti spinel

Fe 2p

168

Binding Energy/eV


Fe 2p

724.9 168.9

719.4

713.5

712.1

711.0

170.0

174

172

170

168

166

174

172

Binding Energy/eV

710.1

170

168

166

h

g O 1s


730

725

720

715

710

705

Binding Energy/eV

Binding Energy/eV

i


Ti 2p

530.3

458.9

531.9

464.6

533.0

536

534

532

530

Binding Energy/eV

j

528

526

468

466

464

462

460

458

Binding Energy/eV

456

454

k

Figure 4 XPS spectra of synthetic catalysts over the spectral regions of Fe 2p, Ti 2p, O 1s and S 2p

26


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317 γ-Fe2O3

410

Fe-Ti spinel 436 sulfated Fe-Ti spinel 100

200

300

400

500

600

700

o

Temperature/ C

800

900

Figure 5 H2-TPR of synthetic catalysts

27



1606

1203

0.2

γ-Fe2O3

2000

1800

1600


1278

1430

Fe-Ti spinel

1400

-1

1200

1000

Wavenumber/cm

1601 1579

a

0.2

γ-Fe2O3

Fe-Ti spinel

1622


1385

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2000

1800

1600

1400

-1

1200

1000

Wavenumber/cm

b

Figure 6 (a), DRIFT spectra of the adsorption of NH3 on synthetic catalysts at 300 oC; (b), DRIFT spectra of the adsorption of NO+O2 on synthetic catalysts at 300 oC.

28


Page 28 of 32

100

Conversion/%

NH3

100

80

80

60

NO NO2

60

40

N2O

40

20

N2

20

0

0

100 80

NH3 NO NO2

60 40 20

60

N2O

40

N2

20 0

0

150 200 250 300 350 400 450 500

Selectivity/%

80

100

Selectivity/%

150 200 250 300 350 400 450 500 o

o

Temperature/ C

Temperature/ C

a

b

NH3 conversion/%

100 80

100 80

NH3

60

NO NO2

60

40

N2O

40

20

N2

20

0

Selectivity/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NH3 conversion/%

Page 29 of 32

0

150 200 250 300 350 400 450 500 o

Temperature/ C

c

Figure 7 NH3 oxidization over synthetic catalysts: (a), γ-Fe2O3; (b), Fe-Ti spinel; (c), sulfated Fe-Ti spinel. Reaction condition: [NH3] = 500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate =200 mL min-1 and GHSV= 1.2×105 cm3 g-1 h-1.

29


Page 30 of 32

0.2

1625

0.2

NH3 10 min

NO+O2 10 min

NH3 5 min

NO+O2 5 min

2000

1800

1600

1400

1200

1278

1430

NH3 3 min NO+O2

1385

NH3

1278

1430

NO+O2 3 min 1606

2000

1000

1800

1600

1400

1200

1000

-1

-1

Wavenumber/cm

Wavenumber/cm

a

0.1

1430

1278

b

1625

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1385


NH3+NO+O2 10 min NH3+NO+O2 5 min NH3+NO+O2 3 min NH3+NO+O2 1 min 2000

1800

1600

1400

-1

1200

1000

Wavenumber/cm

c

Figure 8 (a), DRIFT spectra taken at 300 oC upon passing NO+O2 over NH3 presorbed sulfated Fe-Ti spinel; (b), DRIFT spectra taken at 300 oC upon passing NH3 over NO+O2 presorbed sulfated Fe-Ti spinel; (c), DRIFT spectra taken at 300 oC upon passing NH3+NO+O2 over sulfated Fe-Ti spinel.

30


Page 31 of 32

100

100

CO SCR

80

NH3 conversion/%

NH3 conversion/%

60 40 20 0

CO SCR

80 60 40 20 0

150 200 250 300 350 400 450 500

150 200 250 300 350 400 450 500 o

o

Temperature/ C

Temperature/ C

a

b 100

NH3 conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CO SCR

80 60 40 20 0

150 200 250 300 350 400 450 500 o

Temperature/ C

c

Figure 9 Contributions of the SCR reaction (SCR) and the catalytic oxidization of NH3 to NO (CO) to NH3 conversion during the SCR reaction over: (a), γ-Fe2O3; (b), Fe-Ti spinel; (c), sulfated Fe-Ti spinel. Reaction condition: [NH3]=[NO]= 500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate =200 mL min-1 and GHSV= 1.2×105 cm3 g-1 h-1.

31



2.5

NH3 oxidization to NO/SCR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2.0 1.5

γ-Fe2O3

Fe-Ti spinel sulfated Fe-Ti spinel

1.0 0.5 0.0

150 200 250 300 350 400 450 500 o

Temperature/ C

Figure 10 The ratio of NH3 conversion through the catalytic oxidization of NH3 to NO to that through the SCR reaction. Reaction condition: [NH3]=[NO]= 500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate =200 mL min-1 and GHSV= 1.2×105 cm3 g-1 h-1.

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Improvement of the Activity of Î³-Fe2O3 for the Selective Catalytic

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