Effect of H2O and SO2 on the Selective Catalytic Reduction of NO with

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Effect of HO and SO on the Selective Catalytic Reduction of NO with NH Over Ce/TiO Catalyst: Mechanism and Kinetic Study 3

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Xin Xiao, Shangchao Xiong, Yijie Shi, Wenpo Shan, and Shijian Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10577 • Publication Date (Web): 21 Dec 2015 Downloaded from http://pubs.acs.org on December 21, 2015

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The Journal of Physical Chemistry

Effect of H2O and SO2 on the Selective Catalytic Reduction of NO with NH3 Over Ce/TiO2 Catalyst: Mechanism and Kinetic Study

Xin Xiao, Shangchao Xiong, Yijie Shi, Wenpo Shan, Shijian Yang ∗ Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing, 210094 P. R. China

∗

Corresponding author phone: 86-18-066068302; E-mail: [email protected] (S. J. Yang). 1

ACS Paragon Plus Environment


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Abstract: Ce based catalysts now were regarded as a promising alternative to substitute V2O5/WO3-TiO2 catalyst for the abatement of NOx in the flue gas. However, H2O and SO2 showed a notable effect on NO reduction over Ce based catalysts and the mechanism was not very clear. In this work, the mechanism of H2O and SO2 effect on NO reduction over Ce/TiO2 was studied using the steady state kinetic study and the reaction kinetic parameters were compared. Although H2O and SO2 both showed a promotion on NO reduction over Ce/TiO2 at higher temperatures, their mechanisms were quite different. The effect of SO2 on NO reduction over Ce/TiO2 was mainly attributed to the sulfation. The selective catalytic reduction (SCR) reaction over Ce/TiO2 at higher temperatures was promoted after the sulfation due to the promotion of NH3 adsorption. Meanwhile, the catalytic oxidation of NH3 to NO (i.e. the C-O reaction) over Ce/TiO2 at higher temperatures was suppressed after the sulfation. Therefore, the promotion of NO reduction over Ce/TiO2 at higher temperatures due to the introduction of SO2 was related to not only the inhibition of the C-O reaction but also the promotion of the SCR reaction. However, the SCR reaction over Ce/TiO2 at higher temperatures was restrained in the presence of H2O due to the inhibition of NH3 adsorption and the decrease of oxidation ability. Therefore, the promotion of the SCR reaction over Ce/TiO2 at higher temperatures due to the presence of H2O was mainly attributed to the inhibition of the C-O reaction.

Keywords: the SCR reaction; the C-O reaction; sulfation; NH3 adsorption; the steady state kinetic study.

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1. Introduction Selective catalytic reduction (SCR) of NO by NH3 with V2O5/WO3-TiO2 as the catalyst is now the commercial technology to control the emission of nitrogen oxides from coal fired plants and diesel engines. 1 However, the SCR performance of V2O5/WO3-TiO2 is still not highly satisfactory due to N2O formation at high temperatures, the relatively narrow temperature window,

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and the

toxicity of V2O5 to the environment. 3 Therefore, many researchers strove to develop a better N2 selectivity and more environmental-friendly SCR catalyst. Recently, Ce-based catalysts, for example CeO2/TiO2, 4 CeO2-WO3/TiO2, 3, 5 CeO2-MO3/TiO2, 6, 7

CeO2-WO3

8, 9

and Ce-W-Ti mixed oxides,

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which showed excellent SCR activity and N2

selectivity, were developed to substitute V2O5/WO3-TiO2 catalyst for the abatement of NOx. However, H2O and SO2, which are inevitable in the flue gas, often present a remarkable effect on NO reduction over Ce based catalysts. NOx conversion over Ce/TiO2 at lower temperatures obviously decreased after the introduction of SO2 and H2O, while it obviously increased at higher temperatures.11,

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SO2 effect on the SCR reaction over Ce/TiO2 was once attributed to the

formation of sulfate on Ce/TiO2, resulting in the disruption of the redox properties between Ce(IV) and Ce(III) and the inhibition of the adsorption of nitrate species.

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However, the promotion of

SO2 on NO reduction over Ce/TiO2 at higher temperatures cannot be well interpreted by this hypothesis. Meanwhile, the effect of H2O on NO reduction over Ce/TiO2 was seldom studied in detail. Therefore, the mechanism of H2O and SO2 effect on NO reduction over Ce/TiO2 was not very clear. This lack presented a severe limitation in improving the durability of H2O and SO2. The catalytic oxidation of NH3 to NO (i.e. the C-O reaction), which contributes to NO formation, may simultaneously happen during the SCR reaction over Ce/TiO2 especially at higher temperatures. Therefore, NO reduction over Ce/TiO2 was related to not only the SCR reaction but also the C-O reaction. In this work, the reaction kinetic parameters of NO reduction over Ce/TiO2 (including the SCR reaction and the C-O reaction) in the presence/absence of H2O and SO2 were obtained from the steady state kinetic study. Then, the mechanism of SO2 and H2O effect on NO reduction over Ce/TiO2 was studied after comparing the kinetic parameters. Although both H2O and SO2 showed a remarkable promotion on NOx conversion over Ce/TiO2 at higher temperatures, their mechanisms were quite different. The promotion of NOx conversion at higher temperatures 3



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due to the presence of H2O was mainly attributed to the inhibition of the C-O reaction, while the SCR reaction over Ce/TiO2 was restrained. However, the SCR reaction over Ce/TiO2 at higher temperatures was promoted in the presence of SO2 due to the promotion of NH3 adsorption. Therefore, the promotion of NOx conversion at higher temperatures due to the introduction of SO2 was attributed to both the suppression of the C-O reaction and the promotion of the SCR reaction.

2. Experimental 2.1 Catalyst preparation Ce/TiO2 (CeO2 loading was 10 wt%) was prepared by the impregnation method using cerium nitrate hexahydrate (i.e. Ce(NO3)3·6H2O) as precursor and Degussa TiO2 (i.e. P25) as support.

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After dried at 110 oC for 12 h, the sample was calcined at 500 oC for 3 h under air atmosphere. Then, Ce/TiO2 was treated at 300 oC under a flow of 200 mL min-1 of 500 ppm of SO2 and 2% O2 for 8 h to obtain sulfated Ce/TiO2.

2.2 Catalytic reaction The catalytic reaction was performed on a fixed-bed quartz tube reactor, which was described in detail in our previous works.14-16 The concentrations of reactants (including NO, NO2, NH3 and N2O) in the inlet or outlet were determined online by an IGS infrared Analyzer (Thermo SCIENTIFIC, ANTARIS). In the catalytic reaction, 100 mL min-1 of the simulated flue gas containing 500 ppm of NO, 500 ppm of NH3, 2% of O2, 100 ppm of SO2 (when used), 5% of H2O (when used) and balance of N2 was used. 200 mg of the catalyst with 40-60 mesh was used for the test, and the corresponding gas hourly space velocity (GHSV) was 3.0×104 cm3 g-1 h-1. To obtain the reaction kinetic parameters, the steady state kinetic study was performed. NOx conversion was kept less than 15% to overcome the diffusion limitation (including the inner diffusion and external diffusion).17 Therefore, a very high GHSV of 60000-6000000 cm3 g-1 h-1 was used (i.e. the catalyst mass was 2-200 mg, and the total flow rate was 200 mL min-1). Gaseous NH3 concentration in the inlet was kept at 500 ppm during the steady state kinetic study, while gaseous NO concentration varied from 0 to 700 ppm.16, 18

2.3 Characterization

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X-ray diffraction patterns (XRD), BET surface areas and H2-temperature programmed reduction (H2-TPR) were performed on an X-ray diffractionmeter (Bruker-AXS D8 Advance), a nitrogen adsorption apparatus (Quantachrome, Autosorb-1) and a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx), respectively. NH3-TPD, NO-TPD, NH3 oxidation and NO oxidation were all carried out on the packed-bed quartz tube reactor. X-ray photoelectron spectra (XPS) over the spectral regions of Ce 3d, O 1s, Ti 2p and S 2p were recorded on a X-ray photoelectron spectroscopy (Thermo, ESCALAB 250) with Al Kα (hv=1486.6 eV) as the excitation source and C 1s line at 284.6 eV as the reference. In situ DRIFT spectra during the reaction of adsorbed NH3 with gaseous NO and that of adsorbed NOx with adsorbed NH3 were recorded on an infrared spectrometer (Nicolet NEXUS 870) collecting 32 scans with a resolution of 4 cm-1.

3. Results 3.1 SCR performance of Ce/TiO2 NOx conversion over Ce/TiO2 obviously increased with the increase of reaction temperature from 150 to 300 oC, while it gradually decreased with the further increase of reaction temperature to 500 oC (shown in Figure 1a). Meanwhile, little N2O formed during NO reduction over Ce/TiO2, resulting in an excellent N2 selectivity (the data was not shown). Figure 1 also shows that NH3 conversion over Ce/TiO2 above 350 oC was much higher than NOx conversion. It suggests that the C-O reaction happened during NO reduction over Ce/TiO2 above 350 oC. 19 NOx conversion over Ce/TiO2 below 300 oC decreased notably after the introduction of 5% of H2O, while it obviously increased above 400 oC (shown in Figure 1a). After the introduction of SO2, NOx conversion over Ce/TiO2 below 250 oC obviously decreased (shown in Figure 1a). However, NOx conversion over Ce/TiO2 above 300 oC was close to 100% in the presence of SO2, which was much higher than that in the absence of SO2 (shown in Figure 1a). The inhibition of SO2 on the SCR reaction at low temperatures was often attributed to the deposit of NH4HSO4 or NH4HSO3, which covered the active sites for the SCR reaction.2, 20 However, SO2 can react with Ce/TiO2, resulting in a sulfation of the catalyst.13, 21 Figure 1a shows that NOx conversion over sulfated Ce/TiO2 below 450 oC was close to that of Ce/TiO2 in the presence of SO2. Meanwhile, the further introduction of SO2 showed a neglectable effect on NOx conversion over sulfated 5



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Ce/TiO2 below 450 oC. It suggests that the effect of SO2 on NO reduction over Ce/TiO2 may be mainly related to the sulfation of the catalyst.21 After the simultaneous introduction of H2O and SO2, NOx conversion over Ce/TiO2 below 350 oC decreased remarkably, which was much less than that in the presence of SO2 or that in the presence of H2O (shown in Figure 1a). It suggests that H2O and SO2 showed a synergetic inhibition on NO reduction over Ce/TiO2 at lower temperatures. However, NOx conversion over Ce/TiO2 in the presence of SO2 and H2O above 350 o

C was approximately 100% (shown in Figure 1a), which was similar to that in the presence of

SO2.

3.2 Characterization 3.2.1 XRD and BET As P25 contains both rutile and anatase, 22 the characteristic peaks appeared in the XRD pattern of Ce/TiO2 mainly corresponded to anatase (JCPDS: 21-1272) and rutile (JCPDS: 21-1276) (shown in Figure 2). Meanwhile, only two slightly broad peaks (at approximately 28.5o and 33.0o) corresponding to the (111) and (200) lattice planes of the cubic fluorite structure of CeO2 (JCPDS: 34-0394) appeared in the XRD pattern of Ce/TiO2 although the content of CeO2 in Ce/TiO2 was 10%. It suggests that the loaded CeO2 was well dispersed on P25. After the sulfation, no changes happened in the XRD pattern of Ce/TiO2 (shown in Figure 2). It suggests that the crystal structure of Ce/TiO2 was not destroyed after the sulfation. The BET surface areas of Ce/TiO2 and sulfated Ce/TiO2 were 46.3 and 41.0 m2 g-1, respectively. 3.2.2 XPS To investigate the effect of sulfation on the surface of Ce/TiO2, XPS spectra of Ce/TiO2 and sulfated Ce/TiO2 over the spectral regions of Ce 3d, O 1s, Ti 2p and S 2p were evaluated (shown in Figure 3). The Ce 3d binding energies of Ce/TiO2 mainly centered at 882.1, 885.1, 888.1, 898.3, 900.7, 903.6, 907.2 and 916.5 eV (shown in Figure 3a). The binding energies at 882.1, 888.1, 898.3, 900.7, 907.2 and 916.5 eV was assigned to Ce4+on Ce/TiO2, and those at 885.1 and 903.6 eV were attributed to Ce3+ on Ce/TiO2.23 The O 1s species on Ce/TiO2 mainly appeared at 529.9 and 531.5 eV (shown in Figure 3b), which were assigned to O in metal oxides (i.e. CeO2 and TiO2) and O in -OH, respectively.

24

The Ti 2p species on Ce/TiO2 mainly appeared at 458.7 and 464.4

eV (shown in Figure 3c), which was assigned to Ti4+ in TiO2. The S species on Ce/TiO2 was not 6


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observed (shown in Figure 3d). After the sulfation, two new binding energies at 886.0 and 904.4 eV appeared on Ce/TiO2 in the spectral region of Ce 3d (shown in Figure 3e), which could be attributed to Ce2(SO4)3.23 Meanwhile, a new binding energy at 532.2 eV appeared on Ce/TiO2 in the spectral region of O 1s (shown in Figure 3f), which was assigned to O in SO42-.19, 25 Furthermore, the Ti 2p species on Ce/TiO2 shifted to 459.1 and 464.8 eV due to the sulfation of TiO2 (shown in Figure 3g). The S species appeared on sulfated Ce/TiO2 at 168.8 and 170.0 eV (shown in Figure 3h), which were attributed to S in HSO4- and SO42-, respectively.

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They all suggest that sulfate appeared on

Ce/TiO2 after the sulfation. The percentages of Ce species and S species on Ce/TiO2 and sulfated Ce/TiO2 were collected from the XPS analysis. Table 1 shows that Ce concentration on Ce/TiO2 obviously decreased after the suflation due to the coverage by SO42-. Meanwhile, Ce4+ concentration on Ce/TiO2 obviously decreased after the sulfation. 3.2.3 H2-TPR Ce/TiO2 showed two slight reduction peaks at 629 and 786 oC (shown in Figure 4), which were assigned to the reduction of oxygen species on the surface of CeO2 and that of oxygen in the bulk of CeO2, respectively. 21, 27 Sulfated Ce/TiO2 showed a strong reduction peak at 682 oC, which was assigned to the reduction of SO42-.21 Meanwhile, a slight reduction peak at 821 oC appeared in the H2-TPR profile of sulfated Ce/TiO2, which may be related to the reduction of oxygen in the bulk of CeO2. The shift of the first reduction peak to high temperature suggests that the oxidation ability of Ce/TiO2 obviously decreased after the sulfation.28 3.2.4 NH3 and NO adsorption Figure 5 shows NH3-TPD and NO-TPD profiles of Ce/TiO2 and sulfated Ce/TiO2. The capacities of Ce/TiO2 and sulfated Ce/TiO2 for NH3 and NO adsorption at 50 oC were calculated according to the NH3-TPD and NO-TPD profiles,. Table 2 shows that the capacity of Ce/TiO2 for NH3 adsorption obviously increased after the sulfation, while that for NO adsorption decreased notably. It suggests that the adsorption of NH3 on Ce/TiO2 was promoted after the sulfation especially at higher temperatures (shown in Figure 5a), while the adsorption of NO on Ce/TiO2 was almost completely suppressed (shown in Figure 5b). 7



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Figure 6 shows in situ DRIFT spectra of the adsorption of NH3 and NO on Ce/TiO2 and sulfated Ce/TiO2 at 300 oC. After the adsorption of NH3, two characteristics vibrations (at 1600 and 1176 cm-1) appeared on Ce/TiO2 (shown in Figure 6a), which were assigned to coordinated NH3 bound to the Lewis acid sites.29 However, the characteristic vibrations appeared on sulfated Ce/TiO2 at 1608, 1414 and 1257 cm-1 after the adsorption of NH3. The band at 1608 cm-1 was assigned to coordinated NH3 bound to the Lewis acid sites, while the bands at 1414 and 1257 cm-1 were attributed to ionic NH4+ bound to the Brønsted acid sites.21 Meanwhile, a negative peak appeared on sulfated Ce/TiO2 at 1356 cm-1. The peak was assigned to SO42- on sulfated Ce/TiO2, which was covered by adsorbed NH3.28 They suggest that NH3 species adsorbed on Ce/TiO2 mainly adsorbed on the unsaturated coordination of Ce-O-Ti, while NH3 species adsorbed on sulfated Ce/TiO2 mainly adsorbed on SO42-, which is the typical Brønsted acid sites.25, 30 After the adsorption of NO+O2 on Ce/TiO2, four characteristic vibrations appeared at 1605, 1574, 1539 and 1240 cm-1 (shown in Figure 6b). The band at 1605 cm-1 was assigned to monodentate nitrite, and the bands at 1574, 1539 and 1240 cm-1 were assigned to bidentate nitrate. 13, 31

However, the characteristic vibrations of sulfated Ce/TiO2 after NO+O2 adsorption mainly

appeared at 1625 and 1375 cm-1, which were attributed to nitro.19, 21, 25, 31 The adsorption of nitrite on Ce/TiO2 and sulfated Ce/TiO2 can be approximately described as follows:

NO (g) → NO (ad)

(1)

NO(ad) + Ce4+ =O → Ce3+ -O-NO

(2)

≡ O + NO(ad) +Ce4+ → O2 N-Ce3+

(3)

Reaction 1 was the physical adsorption of gaseous NO on Ce/TiO2 and sulfated Ce/TiO2. Then, physically adsorbed NO was oxidized to nitrite by Ce4+ on Ce/TiO2 and sulfated Ce/TiO2 (i.e. Reactions 2 and 3). Nitrite adsorbed on Ce/TiO2 as monodentate nitrite via O atom (shown in Reaction 2).32 However, the bare O atom on Ce/TiO2 disappeared after the suflation, so nitrite adsorbed on sulfated Ce/TiO2 as nitro via N atom (shown in Reaction 3).21, 32 3.2.5 NH3 and NO oxidation The oxidation of NH3 and NO over Ce/TiO2 and sulfated Ce/TiO2 were shown in Figure 7. H2-TPR analysis suggests that the oxidation ability of Ce/TiO2 obviously decreased after the 8


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sulfation. Meanwhile, NO-TPD profiles suggest that the physical adsorption of NO on Ce/TiO2 was almost completely suppressed after the sulfation. As a result, the oxidation of NO over Ce/TiO2 was restrained after the sulfation (shown in Figure 7a). Although the adsorption of NH3 over Ce/TiO2 was promoted notably after the sulfation, the oxidation ability obviously decreased. As a result, NH3 oxidation over Ce/TiO2 was still restrained after the sulfation (shown in Figure 7b).

3.3 In situ DRIFT spectra study 3.3.1 NO reduction over Ce/TiO2 After the adsorption of NH3 at 300 oC, Ce/TiO2 was mainly covered by coordinated NH3 (at 1600 and 1176 cm-1). Coordinated NH3 on Ce/TiO2 rapidly diminished after the introduction of NO+O2. Meanwhile, adsorbed H2O, which is the product of the SCR reaction, once appeared at 1625 cm-1 (shown in Figure 8a). It suggests that the reaction of adsorbed NH3 with gaseous NO (i.e. the Eley-Rideal mechanism) contributed to NO reduction over Ce/TiO2. After the adsorption of NO+O2 at 300 oC, Ce/TiO2 was mainly covered by monodentate nitrite and bidenate nitrate. After the introduction of NH3, the band corresponding to monodentate nitrite at 1605 cm-1 rapidly diminished (shown in Figure 8b). However, the bands at 1574, 1539 and 1240 cm-1 corresponding to bidentate nitrate shifted to 1558, 1524 and 1247 cm-1 due to the combination with adsorbed NH3 21 and their intensities almost did not decrease. They suggests that the reaction of adsorbed monodentate nitrite with adsorbed NH3 species (i.e. the Langmuir-Hinshelwood mechanism) contributed to NO reduction over Ce/TiO2, while the reaction of adsorbed bidentate nitrate with adsorbed NH3 may not take part in NO reduction over Ce/TiO2 due to the strong binding energy of bidentate nitrate with the interface.14 3.3.2 NO reduction over sulfated Ce/TiO2 After the adsorption of NH3 at 300 oC, sulfated Ce/TiO2 was mainly covered by coordinated NH3 (at 1608 cm-1) and ionic NH4+ (1414 and 1257 cm-1). These bands corresponding to adsorbed NH3 species rapidly diminished after the introduction of NO+O2 (shown in Figure 9a). It suggests that the reaction of adsorbed NH3 with gaseous NO (i.e. the Eley-Rideal mechanism) contributed to NO reduction over sulfated Ce/TiO2.

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After the adsorption of NO+O2 at 300 oC, sulfated Ce/TiO2 was mainly covered by nitro (at 1625 and 1375 cm-1). Adsorbed nitro on sulfated Ce/TiO2 at 1375 cm-1 first shifted to 1389 cm-1 due to the combination with NH3, which was attributed to NH4NO2. Then, adsorbed NH4NO2 rapidly diminished (shown in Figure 9b). It suggests that the reaction of adsorbed nitro with adsorbed NH3 (i.e. the Langmuir-Hinshelwood mechanism) contributed to NO reduction over sulfated Ce/TiO2.

4. Discussion 4.1 Reaction mechanism and kinetics In situ DRIFT study demonstrates that both the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism contributed to NO reduction over Ce/TiO2 and sulfated Ce/TiO2. NO reduction over Ce/TiO2 and sulfated Ce/TiO2 through the Eley-Rideal mechanism can be approximately described as follows:5, 33-35

NH 3(g) → NH 3(ad)

(4)

NH3(ad) + Ce4+ =O → NH2 +Ce3+ -OH

(5)

NO (g) +NH 2 → N 2 +H 2 O

(6)

The SCR reaction over Ce/TiO2 and sulfated Ce/TiO2 started with the adsorption of gaseous NH3 on the surface to form coordinated NH3 and ionic NH4+ (i.e. Reaction 4). Then, adsorbed NH3 species was activated by Ce4+ on the surface to NH2 (i.e. Reaction 5). At last, NH2 on the surface reacted with gaseous NO to form N2 (i.e. Reaction 6). Meanwhile, NH2 on the surface can be deeply oxidized to gaseous NO at high temperatures (i.e. Reaction 7), which was the so-called C-O reaction. 19, 36 The formed NO from the C-O reaction can then react with NH2 to form N2 (i.e. Reaction 6), which was the so-called the selective catalytic oxidation of NH3. 4+

Ce NH 2 +O 2  → NO(g) + H 2 O

(7)

NO reduction over Ce/TiO2 and sulfated Ce/TiO2 through the Langmuir-Hinshelwood mechanism can be approximately described as follows:4, 21, 29, 37

Ce3+ -NO2- + NH3(ad) → Ce3+ -NO2- -NH3 → Ce3+ -OH+N2 +H2O

(8)

Adsorbed nitrite (including monodentate nitrite over Ce/TiO2 and nitro over sulfated Ce/TiO2) 10


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firstly reacted with adsorbed NH3 species to form NH4NO2, which was then decomposed to H2O and N2. According to Reaction 6, the kinetics of NO reduction over Ce/TiO2 and sulfated Ce/TiO2 through the Eley-Rideal mechanism can be described as:

-

d[NO(g) ] dt

= k1[NO(g) ][NH 2 ]

(9)

Where, k1, [NO(g)] and [NH2] were the reaction kinetic constant of Reaction 6, gaseous NO concentration and NH2 concentration on the surface, respectively. Our previous studies demonstrated that NH2 concentration on the surface at the steady state was approximately independent of gaseous NO and NH3 concentrations and NH2 concentration on the surface was mainly related to the oxidation ability of Ce4+ on the surface, Ce4+ concentration on the surface and the concentration of NH3 adsorbed.14, 17, 18 Meanwhile, the kinetics of NO reduction over Ce/TiO2 and sulfated Ce/TiO2 through the Langmuir-Hinshelwood mechanism can be described as:

-

d[N2 ] = k2 [Ce3+ -NO2 NH3 ] dt

(10)

Where, k2 and [Ce3+-NO2NH3] were the decomposition rate constant of NH4NO2 and NH4NO2 concentration on the surface, respectively. Our previous studies demonstrated that NH4NO2 concentration on the surface at the steady state was approximately independent of gaseous NO and NH3 concentrations, which was mainly related to the concentration of nitrite adsorbed and the concentration of NH3 adsorbed.14, 20, 38 Therefore, the SCR reaction rates (i.e. δSCR) of Ce/TiO2 and sulfated Ce/TiO2 can be approximately described as:

δSCR =k1[NO(g) ][NH2 ] + k2 [Ce3+ -NO2 NH3 ]=kE-R [NO(g) ]+kL-H

(11)

kL-H =k2 [Ce3+ -NO 2 NH 3 ]

(12)

kE-R =k1[NH 2 ]

(13)

Where, kL-H and kE-R were the reaction rate constant of the SCR reaction through the Langmuir-Hinshelwood mechanism and that through the Eley-Rideal mechanism, respectively. 11



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Equation 11 suggests that there would be an excellent linear relationship between the SCR reaction rate and gaseous NO concentration, and the intercept and slope can be used to describe kL-H and kE-R respectively.16-18, 39 The kinetics of the C-O reaction can be approximately described as:

d[NO(g) ] dt

=k3 [NH 2 ][Ce 4+ ]α

(14)

Where, k3 and α were the reaction rate constant of Reaction 7 and the reaction order of Reaction 7 with respect to Ce4+ concentration on the surface, respectively. To investigate the mechanism of H2O and SO2 effect on NO reduction over Ce/TiO2, the reaction kinetic parameters of NO reduction over Ce/TiO2 (including the SCR reaction and the C-O reaction) were calculated from the steady state kinetic study (shown in Figures 10-12). The SCR reaction contributed to both NO conversion and NH3 conversion. The C-O reaction contributed to NH3 conversion, while it contributed to NO formation. Therefore, the SCR reaction rate and the C-O reaction rate (i.e. δC-O) can be calculated as follows: 22, 27

δ SCR =

δ C-O =

δ NH + δ NO 3

x

(15)

2

δ NH − δ NO 3

x

(16)

2

Where, δNH3 and δNOx were the rates of NH3 conversion and NOx conversion, respectively. According to the rates of NH3 conversion and NOx conversion during the steady state kinetic study (shown in Figures 10a, 10b, 11a and 11b), the rates of the SCR reaction and the C-O reaction were calculated according to Equations 15 and 16. Figures 10c and 11c show that the SCR reaction rates of Ce/TiO2 and sulfated Ce/TiO2 were both linearly dependent with gaseous NO concentration (the correlation coefficients were all higher than 0.98), which were consistent with the hint of Equation 11. Then, the parameters of kL-H and kE-R of the SCR reaction over Ce/TiO2 and sulfated Ce/TiO2 were calculated after the linear regression of Figures 10c and 11c. Figures 10d and 11d show that the C-O reaction rates of Ce/TiO2 and sulfated Ce/TiO2 both decreased with the increase of gaseous NO concentration. It suggests that the reaction order of the C-O reaction with respect to gaseous NO concentration was less than zero. Our previous study 12


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demonstrated that NH2 concentration on the surface was approximately independent of gaseous NO concentration at the steady state.14, 15 Meanwhile, Equation 9 suggests that the amount of NH2 consumed for the SCR reaction was directly proportional to gaseous NO concentration. Therefore, the amount of NH2 on the surface, which was deeply oxidized to gaseous NO, would obviously decrease after the increase of gaseous NO concentration. As a result, the C-O reaction rate decreased notably with the increase of gaseous NO concentration (shown in Figures 10d and 11d). Gaseous NH3 concentration in the outlet during the steady state kinetic study was difficult to be accurately determined in the presence of H2O. Figure 1 shows that the difference of NH3 conversion and NOx conversion over Ce/TiO2 in the presence of H2O was less than 10% below 450 oC. It suggests that the contribution of the C-O reaction to NH3 conversion over Ce/TiO2 in the presence of H2O was less than 5% below 450 oC. Therefore, the SCR reaction rate during the steady state kinetic study in the presence of H2O was approximately calculated according to NOx conversion (shown in Figure 12) and the contribution of the C-O reaction was approximately neglected. Then, kL-H and kE-R of the SCR reaction over Ce/TiO2 in the presence of H2O were calculated after the linear regression of Figure 12, which were shown in Table 3.

4.2 Mechanism of H2O and SO2 effect Equation 13 suggests that kE-R was in direct proportion to the product of k1 and NH2 concentration on the surface. k1 increased with the increase of reaction temperature. NH2 concentrations on the surface was mainly related to the oxidation ability of Ce4+ on the surface, Ce4+ concentration on the surface and the concentration of NH3 adsorbed.

14, 21

The oxidation

ability of Ce4+ on the surface increased with the increase of reaction temperature, while the concentration of NH3 adsorbed decreased. As a result, there may be a peak value of kE-R with respect to reaction temperature (shown in Table 3). Table 1 shows that Ce4+ concentration on Ce/TiO2 obviously decreased after the sulfation. Meanwhile, H2-TPR analysis suggests that the oxidation ability of Ce/TiO2 obviously decreased after the sulfation. As a result, NH2 concentration on Ce/TiO2 below 350 oC obviously decreased after the sulfation resulting in a decrease of kE-R (shown in Table 3) although the adsorption of NH3 on Ce/TiO2 was promoted after the sulfation. However, the promotion of NH3 adsorption on Ce/TiO2 due to the sulfation at higher temperatures was much more remarkable than that at low temperatures (shown in Figure 13



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5a). As a result, NH2 concentration over Ce/TiO2 above 350 oC obviously increased after the sulfation resulting in an increase of kE-R (shown in Table 3) although Ce4+ concentration and the oxidation ability on/of sulfated Ce/TiO2 were both less than those on/of Ce/TiO2. Equation 14 suggests that the rate of the C-O reaction was mainly related to NH2 concentration, the oxidation ability of Ce/TiO2 (i.e. k3), and Ce4+ concentration on the surface. Both the oxidation ability and Ce4+ concentration of/on Ce/TiO2 obviously decreased after the sulfation. Meanwhile, NH2 concentration on Ce/TiO2 below 350 oC obviously decreased after the sulfation. Therefore, δC-O of Ce/TiO2 below 350 oC obviously decreased after the sulfation (shown in Figures 10d and 11d). Although NH2 concentration on Ce/TiO2 at 350-450 oC increased after the sulfation, both the oxidation ability and Ce4+ concentration of/on Ce/TiO2 obviously decreased. As a result, δC-O of Ce/TiO2 at 350-450 oC still decreased after the sulfation (shown in Figures 10d and 11d). However, the promotion of NH3 adsorption on Ce/TiO2 due to the sulfation became more remarkable at 500 o

C, resulting in an obvious increase of NH2 concentration on the surface. Therefore, δC-O of

Ce/TiO2 at 500 oC was less than that over sulfated Ce/TiO2 (shown in Figures 10d and 11d) although Ce4+ concentration and the oxidation ability on/of Ce/TiO2 were both higher than those on/of sulfated Ce/TiO2. Equation 12 suggests that kL-H was related to both the stability of NH4NO2 adsorbed (i.e. k2) and the concentration of NH4NO2 on the surface. The concentration of NH4NO2 on the surface was mainly related to the concentrations of NH3 adsorbed and NO2- adsorbed.15, 20 NH4NO2 adsorbed on Ce/TiO2 via O atom, while it adsorbed on sulfated Ce/TiO2 via N atom. It suggests that NH4NO2 adsorbed on Ce/TiO2 was much more stable than that on sulfated Ce/TiO2. Therefore, k2 of Ce/TiO2 was less than that of sulfated Ce/TiO2. Although NH3 adsorption on Ce/TiO2 was promoted after the sulfation, NO2- adsorption on Ce/TiO2 was restrained due to the decrease of the oxidation ability and the inhibition of NO adsorption. As a result, the variation of kL-H of Ce/TiO2 due to the sulfation was slight and irregular (shown in Table 3). There is generally agreement that both NH3 adsorption and NO adsorption would be restrained after the introduction of H2O.16,

39-41

Meanwhile, our previous study demonstrated that the

oxidation ability of the SCR catalyst would decrease after the introduction of H2O.16, 39 As the adsorption of NH3 on Ce/TiO2 was restrained and the oxidation abilities of Ce/TiO2 decreased, 14


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NH2 concentration on Ce/TiO2 obviously decreased after the introduction of H2O, resulting in a decrease of kE-R (shown in Table 3). As the adsorption of NO on Ce/TiO2 was restrained and its oxidation ability decreased, the concentration of NO2- adsorbed on Ce/TiO2 decreased notably after the introduction of H2O. Meanwhile, the adsorption of NH3 on Ce/TiO2 was restrained after the introduction of H2O, resulting in a decrease of NH3 concentration on the surface. As a result, the concentration of NH4NO2 on Ce/TiO2 decreased notably after the introduction of H2O. Hinted by Equation 12, kL-H of Ce/TiO2 obviously decreased after the introduction of H2O (shown in Table 3). NH2 concentration on Ce/TiO2 obviously decreased after the introduction of H2O. Meanwhile, the oxidation ability of Ce/TiO2 (i.e. k3) decreased after the introduction of H2O. Hinted by Equation 14, the C-O reaction over Ce/TiO2 was obviously restrained after the introduction of H2O, which was consistent with the result in Figure 1. kL-H of Ce/TiO2 generally did not vary remarkably after the sulfation (shown in Table 3). However, kE-R of sulfated Ce/TiO2 below 350 oC was much less than that of Ce/TiO2. Therefore, δSCR of Ce/TiO2 below 350 oC obviously decreased after the sulfation (shown in Figures 10c and 11c). Furthermore, the contributions of the C-O reaction during NO reduction over both Ce/TiO2 and sulfated Ce/TiO2 below 350 oC were very low (shown in Figures 10b and 11b), which can be approximately neglected. As a result, δNOx of Ce/TiO2 below 350 oC obviously decreased after the sulfation (shown in Figures 10b and 11b), resulting in a decrease of NOx conversion over Ce/TiO2 after the sulfation (shown in Figure 1). However, kE-R of Ce/TiO2 increased remarkably above 350 o

C after the sulfation, resulting in an obvious increase of δSCR (shown in Figures 10c and 11c).

Meanwhile, Figures 10d and 11d show that the C-O reaction rate of Ce/TiO2 below 450 oC obviously decreased after the sulfation. Therefore, δNOx of Ce/TiO2 above 350 oC obviously increased after the sulfation (shown in Figures 10b and 11b), resulting in a promotion of NOx conversion over Ce/TiO2 after the surfation (shown in Figure 1). They suggest that the promotion of NOx conversion over Ce/TiO2 after sulfation above 350 oC (shown in Figure 1a) was not only related to the inhibition of the C-O reaction but also related to the promotion of the SCR reaction. Table 3 also shows that the reaction rate constants (i.e. kL-H and kE-R) of the SCR reaction over Ce/TiO2 both obviously decreased after the introduction of H2O. Meanwhile, the C-O reaction rate 15



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of Ce/TiO2 below 350 oC was very low (shown in Figure 10d). Therefore, δNOx of Ce/TiO2 below 350 oC obviously decreased after the introduction of H2O (shown in Figures 10b and 12), resulting in an obvious decrease of NOx conversion (shown in Figure 1). However, Figures 10d and 12 shows that δNOx of Ce/TiO2 above 400 oC still decreased after the introduction of H2O, which was not consistent with the promotion of NOx conversion over Ce/TiO2 above 400 oC due to the introduction of H2O (shown in Figure 1a). NO concentration at each catalyst column would gradually decrease due to NO reduction over Ce/TiO2. Figure 10 indicates that δSCR of Ce/TiO2 in the absence of H2O obviously decreased with the decrease of gaseous NO concentration, while δC-O obviously increased. Therefore, δNOx of Ce/TiO2 in the absence of H2O decreased remarkably with the decrease of gaseous NO concentration. Figure 10b also shows that δNOx of Ce/TiO2 in the absence of H2O was even less than zero above 400 oC if gaseous NO concentration was very low. As the C-O reaction over Ce/TiO2 was almost suppressed in the presence of H2O, the decrease of δNOx of Ce/TiO2 in the presence of H2O due to the decrease of gaseous NO concentration was much slower than that in the absence of H2O. Therefore, δNOx of Ce/TiO2 in the presence of H2O with low NO concentration would be higher than that in the absence of H2O. As a result, δNOx of the whole Ce/TiO2 catalyst column above 400 oC in the presence of H2O was higher than that in the absence of H2O, resulting in a promotion of NOx conversion over Ce/TiO2 after the introduction of H2O. Therefore, the increase of NOx conversion over Ce/TiO2 above 400 oC due to the introduction of H2O was mainly related to the inhibition of the C-O reaction.

5. Conclusions Both SO2 and H2O showed a promotion on NO reduction over Ce/TiO2 at higher temperatures. However, their mechanisms were quite different. The promotion of NOx conversion at higher temperatures due to the presence of H2O was mainly related to the inhibition of the C-O reaction while the SCR reaction over Ce/TiO2 was still restrained. However, the SCR reaction over Ce/TiO2 at higher temperatures was promoted after the introduction of SO2 due to the promotion of NH3 adsorption. Therefore, the promotion of NOx conversion at higher temperatures due to the introduction of SO2 was related to not only the inhibition of the C-O reaction but also the promotion of the SCR reaction.

16


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Acknowledgements: This study was financially supported by the National Natural Science Fund of China (Grant No. 21207067, 41372044 and 51308296), the Natural Science Fund of Jiangsu Province (Grant No. BK20150036) and the Zijin Intelligent Program, Nanjing University of Science and Technology (Grant No. 2013-0106).

17



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H. Novel Effect of SO2 on the SCR Reaction Over CeO2: Mechanism and Significance. Appl. Catal. B-environ 2013, 136, 19-28. (22) Wang, C. Z.; Yang, S. J.; Chang, H. Z.; Peng, Y.; Li, J. H. Structural Effects of Iron Spinel Oxides Doped with Mn, Co, Ni and Zn on Selective Catalytic Reduction of NO with NH3. J. Mol. Catal. A-chem. 2013, 376, 13-21. (23) Smirnov, M. Y.; Kalinkin, A. V.; Pashis, A. V.; Sorokin, A. M.; Noskov, A. S.; Kharas, K. C.; Bukhtiyarov, V. I. Interaction of Al2O3 and CeO2 Surfaces with SO2 and SO2+O2 Studied by X-ray Photoelectron Spectroscopy. J. Phys. Chem. B 2005, 109, 11712-11719. (24) Yang, S.; Guo, Y.; Yan, N.; Qu, Z.; Xie, J.; Yang, C.; Jia, J. Capture of Gaseous Elemental Mercury from Flue Gas Using a Magnetic and Sulfur Poisoning Resistant Sorbent Mn/γ-Fe2O3 at Lower Temperatures. J. Hazard. Mater. 2011, 186, 508-515. (25) Huang, H.; Lan, Y.; Shan, W.; Qi, F.; Xiong, S.; Liao, Y.; Fu, Y.; Yang, S. Effect of Sulfation on the Selective Catalytic Reduction of NO with NH3 Over γ-Fe2O3. Catal. Lett. 2014, 144, 578-584. (26) Fu, H. B.; Wang, X.; Wu, H. B.; Yin, Y.; Chen, J. M. Heterogeneous Uptake and Oxidation of SO2 on Iron Oxides. J. Phys. Chem. C 2007, 111, 6077-6085. (27) Sun, M.; Zou, G.; Xu, S.; Wang, X. Nonaqueous Synthesis, Characterization and Catalytic Activity of Ceria Nanorods. Mater. Chem. Phys. 2012, 134, 912-920. (28) Yang, S. J.; Li, J. H.; Wang, C. Z.; Chen, J. H.; Ma, L.; Chang, H. Z.; Chen, L.; Peng, Y.; Yan, N. Q. Fe-Ti Spinel for the Selective Catalytic Reduction of NO with NH3: Mechanism and Structure-activity Relationship. Appl. Catal. B-environ 2012, 117, 73-80. (29) Shu, Y.; Sun, H.; Quan, X.; Chen, S. O. Enhancement of Catalytic Activity Over the Iron Modified Ce/TiO2 Catalyst for Selective Catalytic Reduction of NOx with Ammonia. J. Phys. Chem. C 2012, 116, 25319-25327. (30) Ma, L.; Li, J. H.; Ke, R.; Fu, L. X. Catalytic Performance, Characterization, and Mechanism Study of Fe2(SO4)3/TiO2 Catalyst for Selective Catalytic Reduction of NOx by Ammonia. J. Phys. Chem. C 2011, 115, 7603-7612. (31) Hadjiivanov, K. I. Identification of Neutral and Charged NxOy Surface Species by IR Spectroscopy. Catal. Rev. 2000, 42, 71-144. 20


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Table 1. Percentages of Ce species and S species on Ce/TiO2 and sulfated Ce/TiO2 Ce 3+

4+

Ce-SO4

2-

S(SO42-)

Ce

Ce

Ce/TiO2

0.7

2.2

-

-

sulfated Ce/TiO2

0.2

1.6

0.5

3.3

22


/%

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Table 2. Capacities of Ce/TiO2 and sulfated Ce/TiO2 for NH3 and NO adsorption at 50 oC /µmol g-1 NH3

NO

Ce/TiO2

193

63

Sulfated Ce/TiO2

274

13

23



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Table 3. Kinetic parameters of the SCR reaction over Ce/TiO2 and sulfated Ce/TiO2 /µmol g-1 min-1

δSCR =kL-H +kE-R [NO(g) ]

Temperature/oC

Ce/TiO2

sulfated Ce/TiO2

Ce/TiO2 with H2O

kL-H

kE-R/106

R2

200

0.6

0.009

0.991

250

4.4

0.042

0.997

300

46

0.130

0.986

350

52

0.236

0.986

400

114

0.398

0.983

450

131

0.389

0.986

500

130

0.321

0.994

200

0.7

0.004

0.998

250

5.7

0.019

0.997

300

29

0.074

0.994

350

51

0.183

0.986

400

101

0.459

0.995

450

156

0.507

0.990

500

129

0.368

0.996

300

6.6

0.077

0.994

350

20.8

0.110

0.993

400

28.7

0.201

0.997

450

53.0

0.229

0.997

500

23.7

0.249

0.996

24


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Figure captions Figure 1. SCR performance of Ce/TiO2: (a), NOx conversion; (b), NH3 conversion. Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=2%, catalyst mass=200 mg, total flow rate=100 mL min-1 and GHSV=30000 cm3 g-1 h-1. Figure 2. XRD patterns of Ce/TiO2 and sulfated Ce/TiO2 Figure 3. XPS spectra of Ce/TiO2 and sulfated Ce/TiO2 over the spectral regions of Ce 3d, S 2p, Ti 2p and O 1s Figure 4. TPR profiles of Ce/TiO2 and sulfated Ce/TiO2 Figure 5. (a) NH3-TPD profiles of Ce/TiO2 and sulfated Ce/TiO2; (b), NOx-TPD profiles of Ce/TiO2 and sulfated Ce/TiO2. Figure 6. (a), DRIFT spectra of the adsorption of NH3 on Ce/TiO2 and sulfated Ce/TiO2 at 300 oC; (b), DRIFT spectra of the adsorption of NO+O2 on Ce/TiO2 and sulfated Ce/TiO2 at 300 oC. Figure 7. (a), NO oxidation over Ce/TiO2 and sulfated Ce/TiO2; (b), NH3 oxidation over Ce/TiO2 and sulfated Ce/TiO2. Reaction conditions: [NH3]/[NO]=500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate=200 mL min-1 and GHSV=120000 cm3 g-1 h-1. Figure 8. (a), DRIFT spectra taken at 300 oC upon passing NO+O2 over NH3 presorbed Ce/TiO2; (b), DRIFT spectra taken at 300 oC upon passing NH3 over NO+O2 presorbed Ce/TiO2. Figure 9. (a), DRIFT spectra taken at 300 oC upon passing NO+O2 over NH3 presorbed sulfated Ce/TiO2; (b), DRIFT spectra taken at 300 oC upon passing NH3 over NO+O2 presorbed sulfated Ce/TiO2. Figure 10. Dependences of δNH3 (a), δNOx (b), δSCR (c), and δC-O (d) during NO reduction over Ce/TiO2 on gaseous NO concentration. Reaction conditions: [NH3]=500 ppm, [NO]=0-700 ppm, [O2]=2%, catalyst mass=2-200 mg, total flow rate=200 mL min-1 and GHSV=60000-6000000 cm3 g-1 h-1. Figure 11. Dependences of δNH3 (a), δNOx (b), δSCR (c), and δC-O (d) during NO reduction over sulfated Ce/TiO2 on gaseous NO concentration. Reaction conditions: [NH3]=500 ppm, [NO]=0-700 ppm, [O2]=2%, catalyst mass=2-200 mg, total flow rate=200 mL min-1 and GHSV=60000-6000000 cm3 g-1 h-1. 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

Figure 12. Dependences of δNOx during NO reduction over Ce/TiO2 in the presence of 5% H2O on gaseous NO concentration. Reaction conditions: [NH3]=500 ppm, [NO]=300-700 ppm, [O2]=2%, catalyst mass=5-10 mg, total flow rate=200 mL min-1 and GHSV=120000-2400000 cm3 g-1 h-1.

26


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NOx conversion/%

100 80 Ce/TiO2

60

Ce/TiO2 with H2O Ce/TiO2 with SO2

40

Ce/TiO2 with H2O and SO2

20

sulfated Ce/TiO2 sulfated Ce/TiO2 with SO2

0

150 200 250 300 350 400 450 500 o

Temperature/ C a 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

Ce/TiO2 Ce/TiO2 with H2O

40

Ce/TiO2 with SO2

20

Ce/TiO2 with H2O and SO2

0

sulfated Ce/TiO2 with SO2

sulfated Ce/TiO2

150 200 250 300 350 400 450 500 o

Temperature/ C b

Figure 1. SCR performance of Ce/TiO2: (a), NOx conversion; (b), NH3 conversion. Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=2%, catalyst mass=200 mg, total flow rate=100 mL min-1 and GHSV=30000 cm3 g-1 h-1.

27



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 2. XRD patterns of Ce/TiO2 and sulfated Ce/TiO2

28


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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


Ce/TiO2

Ce/TiO2

Ce 3d

O 1s

Ce/TiO2

529.9

900.7 898.3

916.5

903.6 907.2

Ti 2p 458.7

882.1 885.1

888.1 464.4

531.5

925

915

905

895

885

875

534

532

530

528

468

466

464

462

460

458

Binding Energy/eV

Binding Energy/eV

Binding Energy/eV

a

b

c

Ce/TiO2

S 2p

sulfated Ce/TiO2

sulfated Ce/TiO2

Ce 3d

456

454

O 1s 530.1

916.7

900.9 885.1 882.3 903.5 898.5 886.0 904.4 907.1 888.0

531.6 532.2

172

170

168

925

166

915

905

895

885

875

534

532

530

Binding Energy/eV

Binding Energy/eV

Binding Energy/eV

d

e

f

sulfated Ce/TiO2

Ti 2p

sulfated Ce/TiO2

528

S 2p 168.8

459.1

170.0

464.8

468

466

464

462

460

458

456

454

172

170

168

Binding Energy/eV

Binding Energy/eV

g

h

166

Figure 3. XPS spectra of Ce/TiO2 and sulfated Ce/TiO2 over the spectral regions of Ce 3d, S 2p, Ti 2p and O 1s

29



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 4. TPR profiles of Ce/TiO2 and sulfated Ce/TiO2

30


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NH3 concentration/ppm

120 Ce/TiO2

100

sulfated Ce/TiO2

80 60 40 20 0 100

200

300

400

500

o

600


NOx concentration/ppm

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


Ce/TiO2 80

sulfated Ce/TiO2

60 40 20 0 100

200

300

400 o

500

600

Temperature/ C b

Figure 5. (a) NH3-TPD profiles of Ce/TiO2 and sulfated Ce/TiO2; (b), NOx-TPD profiles of Ce/TiO2 and sulfated Ce/TiO2.

31



1176

0.1

1257

1600

Ce/TiO2

1700

1600

sulfated Ce/TiO2

1414

1608 1800

1500

1400

1300

1200

1100

1000

-1

wavenumber/cm

a

1605 1574 1539

1240

0.1

Ce/TiO2

1800

1700

1600

sulfated Ce/TiO2

1375

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

1500

1400

1300

1200

1100

1000

-1

wavenumber/cm

b

Figure 6. (a), DRIFT spectra of the adsorption of NH3 on Ce/TiO2 and sulfated Ce/TiO2 at 300 oC; (b), DRIFT spectra of the adsorption of NO+O2 on Ce/TiO2 and sulfated Ce/TiO2 at 300 oC.

32


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NO conversation/%

15 Ce/TiO2

12

sulfated Ce/TiO2

9 6 3 0 150

200

250

300

350

400

o

450

500

450

500


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


Ce/TiO2 sulfated Ce/TiO2

80 60 40 20 0 150

200

250

300

350

400

o

Temperature/ C b

Figure 7. (a), NO oxidation over Ce/TiO2 and sulfated Ce/TiO2; (b), NH3 oxidation over Ce/TiO2 and sulfated Ce/TiO2. Reaction conditions: [NH3]/[NO]=500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate=200 mL min-1 and GHSV=120000 cm3 g-1 h-1.

33


Page 34 of 39

1240

0.1

NO+O2 10 min

1185

1625

NO+O2 15 min

NO+O2 5 min NO+O2 3 min

1700

1600

NH3

1176

1600 1800

1500

1400

1300

1200

1100

1000

-1

wavenumber/cm

1176

1247

1524

0.1

1558

1600

a

NH3 15 min NH3 10 min NH3 5 min

1186

NH3 3 min NO+O2

1800

1700

1600

1500

1240

1605 1574 1539

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

1605 1574 1539


1400

1300

1200

1100

1000

-1

wavenumber/cm

b Figure 8. (a), DRIFT spectra taken at 300 oC upon passing NO+O2 over NH3 presorbed Ce/TiO2; (b), DRIFT spectra taken at 300 oC upon passing NH3 over NO+O2 presorbed Ce/TiO2.

34


0.1

1375

NO+O2 15 min NO+O2 10 min NO+O2 5 min

1800

1700

1600

1500

NH3

1257

1608

1414

NO+O2 3 min

1400

1300

1200

1100

1000

-1

0.1

1257

1608

1414

wavenumber/cm

NH3 15 min NH3 10 min

1389

NH3 5 min

1800

1700

1600

NH3 3 min NO+O2

1375

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


1625

Page 35 of 39

1500

1400

1300

1200

-1

1100

1000

wavenumber/cm

a

b Figure 9. (a), DRIFT spectra taken at 300 oC upon passing NO+O2 over NH3 presorbed sulfated Ce/TiO2; (b), DRIFT spectra taken at 300 oC upon passing NH3 over NO+O2 presorbed sulfated Ce/TiO2.

35



400

o

o

250 C o 450 C

500

o

300 C o 500 C

350 C

o

o

200 C o 400 C

400

-1

o

200 C o 400 C

δNOx/µmol g min

o

250 C o 450 C

o

300 C o 500 C

350 C

300

300

-1

-1

δNH3/µmol g min

-1

500

200

200

100

100 0 300

400

500

600

0

-100

700

0

100

200

300

400

500

600

NO concentration/ppm


a

b

o

200 C o 400 C

o

o

250 C o 450 C

200

o

300 C o 500 C

350 C

o

o

200 C o 400 C

250 C o 450 C

100

300

o

700

o

300 C o 500 C

350 C

150

-1

300

200

δC-O/µmol g min

-1

400

-1

500

100

-1

0

δSCR/µmol g min

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

Page 36 of 39

200 100

100 50 0

0 0

100

200

300

400

500

600

0

700

200

400

500



c

d

600

700

Figure 10. Dependences of δNH3 (a), δNOx (b), δSCR (c), and δC-O (d) during NO reduction over Ce/TiO2 on gaseous NO concentration. Reaction conditions: [NH3]=500 ppm, [NO]=0-700 ppm, [O2]=2%, catalyst mass=2-200 mg, total flow rate=200 mL min-1 and GHSV=60000-6000000 cm3 g-1 h-1.

36


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500

o

o

250 C o 450 C

600

o

300 C o 500 C

350 C

-1

400 300 200 100 0

600

100

200

400

500

600

o

200 C o 400 C

250 C o 450 C

100

300

o

300 200 100 0

-100

700

0

200

400

500

a

b

o

o

o

200

o

300 C o 500 C

350 C

δC-O/µmol g min

-1

250 C o 450 C

350 C

400


o

o

300 C o 500 C


200 C o 400 C

500

300

-1

0

o

500

-1

o

200 C o 400 C

δNOx/µmol g min

-1

δNH3 /µmol g min

-1

600

o

200 C o 400 C

250 C o 450 C

100

300

600

o

700

o

300 C o 500 C

350 C

150

-1

400

-1

δSCR/µmol g min

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


300 200 100 0

100 50 0

0

100

200

300

400

500

600

700

0

200

400

500



c

d

600

700

Figure 11. Dependences of δNH3 (a), δNOx (b), δSCR (c), and δC-O (d) during NO reduction over sulfated Ce/TiO2 on gaseous NO concentration. Reaction conditions: [NH3]=500 ppm, [NO]= 0-700 ppm, [O2]=2%, catalyst mass=2-200 mg, total flow rate=200 mL min-1 and GHSV=60000-6000000 cm3 g-1 h-1.

37



-1

200

-1

250

150

δNOx/µmol g min

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

o

300 C o 450 C

o

350 C o 500 C

Page 38 of 39

o

400 C

100 50 0 300

400

500

600

700


Figure 12. Dependences of δNOx during NO reduction over Ce/TiO2 in the presence of 5% H2O on gaseous NO concentration. Reaction conditions: [NH3]=500 ppm, [NO]=300-700 ppm, [O2]=2%, catalyst mass=5-10 mg, total flow rate=200 mL min-1 and GHSV=120000-2400000 cm3 g-1 h-1.

38


Page 39 of 39

TOC

sulfated Ce/TiO2

0.4

Ce/TiO2 with H2O

-1

0.5

-1

-1 -1

200 Ce/TiO2

160

kL-H/µmol g min

0.6

kE-R/mol g min

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


0.3 0.2 0.1

Ce/TiO2 sulfated Ce/TiO2 Ce/TiO2 with H2O

120 80 40 0

0.0 200 250 300 350 400 450 500

200 250 300 350 400 450 500

Temperature/ C

Temperature/ C

o

o

39


Effect of H2O and SO2 on the Selective Catalytic Reduction of NO with

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