Why the Low-Temperature Selective Catalytic Reduction Performance

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Why the Low Temperature SCR Performance of Cr/TiO2 Much Less than That of Mn/TiO2: A Mechanism Study Bo Li, Shangchao Xiong, Yong Liao, Xin Xiao, Nan Huang, Yang Geng, Sijie Zou, and Shijian Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06697 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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The Journal of Physical Chemistry

Why the Low Temperature SCR Performance of Cr/TiO2 Much Less than That of Mn/TiO2: A Mechanism Study Bo Li, Shangchao Xiong, Yong Liao, Xin Xiao, Nan Huang, Yang Geng, Sijie Zou, 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: Although Mn/TiO2 and Cr/TiO2 had many similar physicochemical properties, the low temperature SCR performance of Mn/TiO2 was much better than that of Cr/TiO2. In this work, the physicochemical properties of Mn/TiO2 and Cr/TiO2 were characterized. Meanwhile, the mechanism of NO reduction over Mn/TiO2 and Cr/TiO2 was compared using the transient reaction. Furthermore, the kinetic parameters of NO reduction over Mn/TiO2 and Cr/TiO2 were obtained from the steady-state kinetic study. The Eley-Rideal mechanism contributed to NO reduction over both Mn/TiO2 and Cr/TiO2. As the SCR reaction through the Eley-Rideal mechanism needed to overcome the binding of activated NH3 species with the interface and the binding of activated NH3 with Cr/TiO2 was much stronger than that with Mn/TiO2, the rate constant of the SCR reaction over Cr/TiO2 through the Eley-Rideal mechanism was much lower than that over Mn/TiO2. Meanwhile, the rate of the catalytic oxidation of NH3 to NO (i.e. C-O reaction) over Cr/TiO2 was much higher than that of Mn/TiO2 due to the stronger oxidation and more Cr6+ on the surface. As a result, the low temperature SCR activity of Cr/TiO2 was much less than that of Mn/TiO2.

2


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1. Introduction So far, selective catalytic reduction (SCR) of NO by NH3 with V2O5-WO3(MoO3)/TiO2 as the catalyst is the most cost-effective technology to control NOx emission from coal-fired power plants.1, 2 As the operating temperature window of V2O5-WO3(MoO3)/TiO2 is 300-400 oC, the SCR unit is located upstream of the electrostatic precipitator (ESP). Because the space and access in many existing power plants are limited, it is very difficult to retrofit the SCR unit upstream of ESP.3 Therefore, there is a great demand to develop the low temperature SCR catalysts, which can be placed downstream of the electrostatic precipitator and desulfurizer.

4

Among the first row

transition metal, only Mn based catalysts, for example Mn/TiO2, 5 Mn-Fe/TiO2,4 MnOx-CeO2,6, 7 MnOx-CeO2/TiO2,8 Mn-Ce-Ti catalyst,9 Sm-MnOx,10 Mn promoted V2O5/TiO2,11 Mn-Fe spinel,

12

MnxCo3-xO4,13, 14 and Mn/Fe-Ti spinel,15 show an excellent SCR activity at low temperatures. The excellent low temperature SCR performance of Mn based catalysts is often attributed to its excellent oxidation ability and many acid sites on the surface.16 Cr based catalysts (for example Cr/TiO2) have more excellent oxidation ability and many acid sites on the surface. However, they generally show a poor low temperature SCR performance.17 Therefore, another physicochemical property of the catalyst may play an important role on the low temperature SCR reaction. Furthermore, the non-selective catalytic reduction reaction (i.e. the NSCR reaction) and the catalytic oxidation of NH3 to NO (i.e. the C-O reaction) may simultaneously happen during the low temperature SCR reaction,18 resulting in N2O formation and the decrease of NOx conversion at higher temperatures. Therefore, the SCR performance was dependent of the competition among the C-O reaction, the SCR reaction and the NSCR reaction.19 In this work, the kinetic parameters of the SCR reaction, the NSCR reaction and the C-O reaction during NO reduction over Mn/TiO2 and Cr/TiO2 were obtained from the steady-state kinetic study. After comparing the kinetic parameters of NO reduction over Mn/TiO2 and the Cr/TiO2, the reason why the low temperature SCR performance of Mn/TiO2 was much better than that of Cr/TiO2 was discovered.

2. Experimental 2.1 Catalyst preparation Mn/TiO2 and Cr/TiO2 (Mn and Cr loading were both 5 wt%) were prepared by the impregnation 3



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method using P25 (i.e. Degussa TiO2) as support and manganese nitrate or chromic nitrate as precursors.17, 18 The samples were dried at 110 oC for 12 h, and then calcined at 500 oC under air for 3 h. 2.2 Catalytic test NO reduction was performed on a fixed-bed quartz tube reactor with the internal diameter of 6 mm. 100 mL min-1 of the simulated flue gas containing 500 ppm of NO, 500 ppm of NH3 and 2% of O2 were used. The mass of catalyst with 40-60 mesh was 200 mg, and the corresponding gas hourly space velocity (GHSV) was 30000 cm3 g-1 h-1. The concentration of NH3, NO, NO2 and N2O in the inlet or outlet were monitored online by a Fourier transform infrared spectrometer (FTIR, Thermo, IGS Analyzer). NH3 conversion, NOx (including NO and NO2) conversion, N2 formation, and N2O selectivity were calculated as follows:

NH3 conversion=[NH3 ]in − [NH3 ]out

(1)

NO x conversion=[NO x ]in − [NOx ]out

(2)

N 2 formation=

[NH3 ]in + [NO x ]in − [NH3 ]out − [NO x ]out − 2[N 2O]out 2

(3)

2[N 2 O]out × 100% [NH 3 ]in + [NO x ]in − [NH 3 ]out − [NO x ]out

(4)

N 2 O selectivity=

2.3 Characterization X-ray diffraction pattern (XRD), BET surface area, H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectra (XPS) were performed on an X-ray diffractionmeter (Bruker-AXS D8 Advance), a nitrogen adsorption apparatus (Quantachrome, Autosorb-1), a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx) and an X-ray photoelectron spectroscopy (Thermo, ESCALAB 250), respectively. Temperature programmed desorption of ammonia (NH3-TPD) and temperature programmed desorption of NO (NO-TPD) were carried out on the packed-bed quartz tube reactor at a heating rate of 10 oC min-1. 2.4 Transient reaction study The transient reactions of the introduction of NH3 to the catalyst pretreated by NO+O2 and the introduction of NO+O2 to the catalyst pretreated by NH3 were both conducted at 200 oC. During 4


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the transient reaction, the concentrations of NH3, NO, NO2 and N2O in the outlet and in situ DRIFT spectra were both collected. In situ DRIFT spectra were recorded on another FTIR spectrometer (Nicolet IS 50) equipped with a MCT detector, collecting 32 scans with a resolution of 4 cm-1. 2.5 Steady-state kinetic study To obtain the kinetic parameters of NO reduction over Mn/TiO2 and Cr/TiO2, the steady-state kinetic study was performed. NO concentration in the inlet varied from 300-700 ppm, while NH3 concentration was kept at 500 ppm.15, 20, 21 To exclude the influence of the inner diffusion and external diffusion, less than 15% of NO conversion was obtained with a very high GHSV of 3.0×105-2.4×106 cm3 g-1 h-1.22, 23

3. Results 3.1 SCR performance Mn/TiO2 showed an excellent low temperature SCR activity and NOx conversion was higher than 80% at 150-250 oC (shown in Figure 1a). NH3 conversion over Mn/TiO2 above 175 oC was higher than NOx conversion. It suggests that the C-O reaction happened over Mn/TiO2 above 175 o

C,

24

resulting in a decrease of NOx conversion with the increase of reaction temperature from

200 to 250 oC. Meanwhile, some N2O formed during NO reduction over Mn/TiO2 and N2O selectivity of NO reduction over Mn/TiO2 gradually increased with the increase of reaction temperature. However, Cr/TiO2 showed a poor low temperature SCR activity and NOx conversion was lower than 70% at 100-250 oC (shown in Figure 1b). NH3 conversion over Cr/TiO2 was higher than NOx conversion above 150 oC and the difference between NH3 conversion and NOx conversion over Cr/TiO2 was much more remarkable than that over Mn/TiO2. It suggests that the C-O reaction during NO reduction over Cr/TiO2 was much more remarkable than that over Mn/TiO2. Meanwhile, some N2O also formed during NO reduction over Cr/TiO2 and N2O selectivity of NO reduction over Cr/TiO2 was much higher than that over Mn/TiO2. 3.2 Characterization 3.2.1 XRD and BET 5



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The characteristic peaks appeared in the XRD patterns of Mn/TiO2 and Cr/TiO2 mainly corresponded to anatase (JCPDS: 21-1272) and rutile (JCPDS: 21-1276).25 Meanwhile, any other characteristic peaks corresponding to manganese oxides and chromium oxides cannot be clearly observed (shown in Figure 2). It suggests that the loaded manganese oxides and chromium oxides were well dispersed on P25. BET surface areas of Mn/TiO2 and Cr/TiO2 were 42.3 and 33.0 m2 g-1, respectively. 3.2.2 XPS Figure 3 shows the XPS spectra of Mn/TiO2 and Cr/TiO2 over the spectral regions of Mn 2p, Cr 2p, Ti 2p and O 1s. The binding energies of Mn 2p 3/2 on Mn/TiO2 mainly appeared at 642.4 and 641.4 eV, and those of Mn 2p 1/2 mainly appeared at 653.8 and 652.9 eV. The binding energies at 653.8 and 642.4 eV were assigned to Mn4+, and those at 652.9 and 641.4 eV were attributed to Mn3+.26 The binding energies of Cr 2p 3/2 on Cr/TiO2 mainly appeared at 579.3 and 576.8 eV, and those of Cr 2p 1/2 mainly appeared at 587.8 and 586.3 eV. The binding energies at 587.8 and 579.3 eV were assigned to Cr6+, and those at 586.3 and 576.8 eV were attributed to Cr3+.27 The binding energies of Ti 2p on Mn/TiO2 and Cr/TiO2 both appeared at 458.4 and 461.1 eV, which were assigned to Ti4+ in TiO2.28 The binding energies of O 1s on Mn/TiO2 and Cr/TiO2 mainly centered at 529.8 and 531.0 eV, which were attributed to O2- in the transition metal oxides and O2in -OH, respectively.28 The percentages of Mn, Cr, Ti and O species on Mn/TiO2 and Cr/TiO2, which resulted from XPS analysis, were listed in Table 1. Table 1 shows that the concentration of Cr6+ on Cr/TiO2 was higher than that of Mn4+ on Mn/TiO2. 3.2.3 H2-TPR The peak fitting of Mn/TiO2 shows three obvious reduction peaks at 310, 344 and 410 oC (shown in Figure 4), which were attributed to the reduction of MnO2 to Mn2O3, the reduction of Mn2O3 to Mn3O4 and the reduction of Mn3O4 to MnO, respectively.29 The peak area of the first reduction peak corresponding to the reduction of MnO2 to Mn2O3 was approximately twice that of the second reduction peak corresponding to the reduction of Mn2O3 to Mn3O4. It suggests that the ratio of Mn4+ to Mn3+ of Mn/TiO2 was approximately 2, which was close to that resulted from 6


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XPS analysis (2.4 hinted in Table 1). Cr/TiO2 shows a strong reduction peak at 292 oC, which was assigned to the reduction of Cr6+.27, 30 In comparison with Mn/TiO2, the first reduction peak of Cr/TiO2 shifted approximately 18 oC to low temperature (shown in Figure 4). It suggests that the oxidation ability of Cr/TiO2 (i.e. the ability to obtain electron) was much better than that of Mn/TiO2. 3.2.4 NH3-TPD and NO-TPD Figure 5 shows NH3-TPD and NO-TPD profiles of Mn/TiO2 and Cr/TiO2. The integrations of NH3-TPD and NO-TPD profiles show that the capacities of Cr/TiO2 for the adsorption of NH3 and NO per gram were both less than those of Mn/TiO2. However, Table 2 shows that the capacities of Cr/TiO2 for the adsorption of NH3 and NO per BET surface area were close to those of Mn/TiO2. It suggests that the less capacities of Cr/TiO2 for NH3 and NO adsorption per gram were mainly related to its less BET surface area. 3.3 Transient reaction study 3.3.1 Mn/TiO2 Mn/TiO2 was mainly covered by coordinated NH3 bound to the Lewis acid sites (at 1603, 1175 and 1208 cm-1) after the adsorption of 500 ppm of NH3 at 200 oC for 30 min.5, 31 After the further introduction of 500 ppm of NO and 2% of O2, coordinated NH3 gradually diminished before the appearance of adsorbed NOx (shown in Figure 6a). It suggests that the reaction of adsorbed ammonia species with gaseous NO (i.e. the Eley-Rideal mechanism) can contribute to NO reduction over Mn/TiO2. At last, Mn/TiO2 was mainly covered by monodentate nitrite (at 1601 cm-1) and bidentate nitrate (at 1574 cm-1).

32

Figure 7a shows that N2O concentration in

the outlet rapidly increased to approximately 16 ppm after the introduction of NO+O2 to NH3 pretreated Mn/TiO2 and it then rapidly decreased. It suggests that the Eley-Rideal mechanism can contribute to N2O formation during NO reduction over Mn/TiO2. After the adsorption of 500 ppm of NO and 2% of O2 at 200 oC, Mn/TiO2 was mainly covered by monodentate nitrite (at 1601 cm-1) and bidentate nitrate (at 1574 cm-1). After NH3 was further introduced, the band at 1601 cm-1 corresponding to monodentate nitrite rapidly diminished, while the band at 1574 cm-1 corresponding to bidentate nitrate slowly decreased 7



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(shown in Figure 7b). It suggests that the reaction of adsorbed monodentate nitrite and bidentate nitrate with adsorbed NH3 (i.e. the Langmuir-Hinshelwood mechanism) can contribute to NO reduction over Mn/TiO2. At last, coordinated NH3 (at 1603, 1175 and 1208 cm-1) can be observed on Mn/TiO2. Figure 7b shows that N2O concentration in the outlet rapidly increased to approximately 20 ppm after the introduction of 500 ppm of NH3 to NO+O2 pretreated Mn/TiO2 and it then gradually decreased to 10 ppm in 40 min. The amount of N2O formed during the introduction of NH3 to fresh Mn/TiO2 was much less than that during the introduction of NH3 to NO+O2 pretreated Mn/TiO2 (shown in Figures 7b and 7c). It suggests that the Langmuir-Hinshelwood mechanism can contribute to N2O formation over Mn/TiO2. The formed N2O over Mn/TiO2 through the Langmuir-Hinshelwood mechanism may mainly result from the reaction of adsorbed bidentate nitrate with adsorbed NH3.5 3.3.2 Cr/TiO2 Cr/TiO2 was mainly covered by coordinated NH3 (at 1603 and 1208 cm-1) after the adsorption of 500 ppm of NH3 at 200 oC for 30 min.31 After the further introduction of 500 ppm of NO and 2% of O2, coordinated NH3 gradually decreased (shown in Figure 8a). It shows that the reaction of adsorbed ammonia species with gaseous NO (i.e. the Eley-Rideal mechanism) can contribute to NO reduction over Cr/TiO2. In comparison Figure 6a with 8a, the decrease of adsorbed NH3 over Cr/TiO2 due to the introduction of NO+O2 was much slower that that over Mn/TiO2. It implies that the rate of NO reduction over Cr/TiO2 through the Eley-Rideal mechanism was much less than that over Mn/TiO2. At last, monodentate nitrite (at 1610 cm-1) appeared on Cr/TiO2.

32

Figure 9a

shows that N2O concentration in the outlet rapidly increased to approximately 119 ppm during the introduction of NO+O2 to NH3 pretreated Cr/TiO2. It suggests that the Eley-Rideal mechanism can contribute to N2O formation over Cr/TiO2. The amount of N2O formed during the introduction of NO+O2 to NH3 pretreated Cr/TiO2 was much higher than that of Mn/TiO2 (shown in Figures 9a and 7a). It implies that the rate of N2O formation over Cr/TiO2 through the Eley-Rideal mechanism was much higher than that over Mn/TiO2. After the adsorption of 500 ppm of NO and 2% of O2 at 200 oC, Cr/TiO2 was mainly covered by monodentate nitrite (at 1610 and 1546 cm-1). After NH3 was further introduced, the band at 1610 8


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and 1546 cm-1 corresponding to monodentate nitrite rapidly diminished (shown in Figure 8b). It shows that the reaction of adsorbed monodentate nitrite with adsorbed NH3 (i.e. the Langmuir-Hinshelwood mechanism) can contribute to NO reduction over Cr/TiO2. At last, Cr/TiO2 was mainly covered by coordinated NH3 (at 1603 and 1208 cm-1). Figures 9b and 9c show that the amount of N2O formed during the introduction of NH3 to NO+O2 pretreated Cr/TiO2 was close to that during the introduction of NH3 to fresh Cr/TiO2. It suggests that N2O formed during the introduction of NH3 to NO+O2 pretreated Cr/TiO2 mainly resulted from the oxidation of NH3. Therefore, NO reduction over Cr/TiO2 through the Langmuir-Hinshelwood mechanism may not contribute to N2O formation, which was similar to NO reduction over MnOx-CeO2.21 3.4 Steady-state kinetic study Figures 10 and 11 show the relationship of the rates of NH3 conversion (δNH3), NOx conversion (δNOx), N2 formation (δN2) and N2O formation (δN2O) with gaseous NO concentration during NO reduction over Mn/TiO2 and Cr/TiO2. δNH3, δNOx and δN2 of Mn/TiO2 and Cr/TiO2 all obviously increased with the increase of gaseous NO concentration (shown in Figures 10a-10c and 11a-11c).

δN2O of Mn/TiO2 and Cr/TiO2 at 100-200 oC did not vary remarkably with the increase of gaseous NO concentration from 300 to 700 ppm (shown in Figures 10d ad 11d). However, δN2O of Cr/TiO2 above 200 oC obviously increased with the increase of gaseous NO concentration (shown in Figure 11d). Furthermore, Figures 10 and 11 both show that the δNH3 of Mn/TiO2 and Cr/TiO2 were generally higher than δNOx especially at higher temperatures. It suggests that the C-O reaction happened during NO reduction over Mn/TiO2 and Cr/TiO2. The C-O reaction contributed to NH3 conversion, while it contributed to NO formation. Therefore, the C-O reaction rate (δco) can be described as:33

δ C-O =

δ NH − δ NO 3

x

(5)

2

Figures 10e and 11e show that δc-o of Mn/TiO2 and Cr/TiO2 generally decrease with the increase of gaseous NO concentration. Meanwhile, δc-o of Cr/TiO2 was generally much higher than that of Mn/TiO2. 9



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4. Discussion 4.1 Reaction mechanism The transition reaction study demonstrated that both the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism can contribute to NO reduction over Mn/TiO2 and Cr/TiO2, and N2O formation over Mn/TiO2. However, only the Eley-Rideal mechanism can contribute to N2O formation over Cr/TiO2. The SCR reaction and the NSCR reaction through the Eley-Rideal mechanism, and the C-O reaction can be approximately described as:5, 16, 20, 22

NH3(g) → NH3(ad)

(6)

NH3(ad) +M n + =O → M ( n−1)+ -OH+NH 2

(7)

NH2 + NO(g) → N 2 +H 2 O

(8)

NH2 + Mn + =O → NH+M ( n−1)+ -OH

(9)

NH + M n+ =O+NO(g) → M( n−1)+ -OH+N 2O

(10)

1 NH+M n + =O+ O2 → NO+M (n -1)+ -OH 2

(11)

1 1 M (n -1)+ =O+ O 2 → M n + -OH+ H 2O 4 2

(12)

Firstly, gaseous NH3 adsorbed on the Lewis acid sites on Mn/TiO2 and Cr/TiO2 to form coordinated NH3 (i.e. Reaction 6), which was then activated by Mn+ (i.e. Cr6+ and Mn4+) on Mn/TiO2 and Cr/TiO2 to NH2 (i.e. Reaction 7). Then, gaseous NO was reduced by NH2 on the surface to N2 (i.e. Reaction 8). Meanwhile, NH2 on the surface can be further oxidized to NH (i.e. Reaction 9), which then reacted with gaseous NO to N2O (i.e. Reaction 10). Reaction 11 was the deep oxidation of NH to NO, which was the key step of the C-O reaction. Reaction 12 was the recovery of Mn+ on the surface. As the concentrations of the intermediates (i.e. NH and NH2) on Cr/TiO2 and Mn/TiO2 were very low, they cannot be clearly observed in the in situ DRIFT spectra.12, 16 Meanwhile, NO reduction over Mn/TiO2 and Cr/TiO2 through the Langmuir-Hinshelwood 10


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mechanism can be approximately described as follows: 16, 34

NO(g) → NO(ad)

(13)

NO(ad) + M n + =O → M (n -1)+ -O − NO

(14)

1 NO(ad) + M n + =O+ O2 → M (n -1)+ -O − NO2 2

(15)

M (n -1)+ -O-NO + NH3(ad) → M (n -1)+ -O-NO-NH 3 → M (n -1)+ -OH+H 2O+N 2

(16)

M ( n −1)+ -O-NO2 + NH3(ad) → M( n−1)+ -O-NO2 -NH3 → M ( n −1)+ -OH+H 2O+N2O

(17)

Gaseous NO firstly adsorbed on Mn/TiO2 and Cr/TiO2 to form physically adsorbed NO (i.e. Reaction 13). Then, physical adsorbed NO was oxidized by Mn+ on the surface to form NO2- and NO3- (i.e. Reactions 14 and 15), which then reacted with adsorbed NH3 to NH4NO2 and NH4NO3 (i.e. Reactions 16 and 17). At last, the formed NH4NO2 and NH4NO3 were decomposed to N2 and N2O, respectively. 4.2 Reaction kinetic study According to Reactions 8 and 10, the kinetic equations of N2 formation and N2O formation through the Eley-Rideal mechanism can be described as follows:20

d[N 2 ] dt

E-R

d[N 2 O] dt

=−

E-R

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

=−

(18)

d[NH] = k2 [NH][NO(g) ][M n + =O] dt

(19)

Where, k1, k2, [NH2], [NH], [Mn+=O] and [NO(g)] were the kinetic constants of Reactions 8 and 10, the concentrations of NH2, NH and Mn+ on the surface, and gaseous NO concentration, respectively. Meanwhile, the kinetic equations of NH and NH2 formation (i.e. Reactions 7 and 9) can be described as: 35

d[NH 2 ] = k3[NH3(ad) ][M n + =O] dt

(20)

d[NH 2 ] d[NH] =− = k 4 [NH 2 ][M n + =O] dt dt

(21) 11



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Where, k3 and k4 were the kinetic constants of Reactions 7 and 9, respectively. Furthermore, the kinetic equation of the deep oxidation of NH (i.e. Reaction 11) can be approximately described as:22, 36

d[NO] d[NH] =− = k5 [NH][M n + =O] dt dt

(22)

Where, k5 was the kinetic constant of Reactions 11. As the reaction reached the steady-state, NH concentration on the surface would not vary. Therefore,

d[NH] = k4 [NH 2 ][M n + =O] − k2 [NH][NO(g) ][M n + =O] − k5 [NH][M n + =O]=0 dt

(23)

Thus, [NH] can be described as:

[NH]=

k4 [NH 2 ] k2 [NO(g) ] + k5

(24)

Then, Equations 19 and 22 can be transformed as follows:

d[N2O] k4 [NH2 ] k [NH2 ][Mn + =O] = k2 [NO(g) ][Mn + =O] = 4 k5 k2[NO(g) ] + k5 dt E-R 1+ k2 [NO(g) ]

δ C-O =

d[NO] k4 [NH 2 ] k [NH 2 ][M n + =O] = k5 [M n + =O] = 4 k2 [NO (g) ] dt k2 [NO (g) ] + k5 +1 k5

(25)

(26)

The kinetic equations of N2 and N2O formation through the Langmuir-Hinshelwood mechanism (i.e. the decomposition of NH4NO2 and NH4NO3) can be described as follows:20

d[N 2 ] dt

L-H

d[N 2 O] dt

= k6 [M (n -1)+ -O-NO-NH 3 ]

L-H

(27)

= k7 [M (n -1)+ -O-NO 2 -NH 3 ]

(28)

Where, k6, k7, [M(n-1)+-O-NO-NH3] and [M(n-1)+-O-NO2-NH3] were the decomposition rate constants of NH4NO2 and NH4NO3, and the concentrations of NH4NO2 and NH4NO3 on Mn/TiO2 and Cr/TiO2, respectively. Then, δSCR and δNSCR can be described as follows: 12


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δ SCR =

d[N 2 ] dt

δ NSCR =

L-H

d[N 2 O] dt

+

d[N 2 ] dt

L-H +

E-R

= k6 [M (n -1)+ -O-NO-NH 3 ]+k1[NH 2 ][NO (g) ]

d[N 2 O] dt

E-R

= k7 [M (n -1)+ -O-NO 2 -NH 3 ]+

k4 [NH 2 ][M n + =O] k5 1+ k2 [NO (g) ]

(29)

(30)

The transient reaction demonstrates that the Langmuir-Hinshelwood mechanism did not contribute to N2O formation over Cr/TiO2. Therefore, k7[M(n-1)+-O-NO2-NH3] of Cr/TiO2 was approximately zero. NH2 concentrations on Mn/TiO2 and Cr/TiO2 at the steady-state were approximately independent of gaseous NO and NH3 concentrations (the deduction was shown in the Supporting Information),

20, 21

which were mainly related to k3, the concentration of NH3

adsorbed, and Mn+ concentrations on Mn/TiO2 and Cr/TiO2 (hinted by Equation 20). Meanwhile, the concentrations of NH4NO2 and NH4NO3 on the surface can be approximately regarded as constants at the steady-state (the deduction was shown in the Supporting Information),12, 20 which were not related to gaseous NO and NH3 concentration. Hinted by Equation 29, there should be an excellent linear relationship between δSCR and gaseous NO concentration, which was demonstrated in Figure 10c and Figure 11c. Therefore, k1[NH2] can be obtained after the linear regression of Figures 10c and 11c. If k5/(k2[NO(g)]) was close to zero, Equation 30 can be transformed as:

δ NSCR =k NSCR = k7 [M (n -1)+ -O-NO 2 -NH 3 ]+k4 [NH 2 ][M n + =O]

(31)

Equation 31 suggests that δNSCR can be approximately regarded as a constant as k5/(k2[NO(g)]) was close to zero, which was not related to the concentrations of gaseous NO and NH3.20, 21 Figure 10d shows that N2O formation over Mn/TiO2 was approximately independent of gaseous NO concentration. It suggests that k5/(k2[NO(g)]) of Mn/TiO2 was close to zero at 100-250 oC. Furthermore, Figure 11d shows that N2O formation over Cr/TiO2 was approximately independent of gaseous NO concentration at 100-200 oC. It suggests that k5/(k2[NO(g)]) of Cr/TiO2 was close to zero at 100-200 oC. However, N2O formation over Cr/TiO2 obviously increased with the increase of gaseous NO concentration above 200 oC. Therefore, k5/(k2[NO(g)]) of Cr/TiO2 was much higher than zero above 200 oC. It suggests that k5/k2 of Cr/TiO2 above 200 oC was much higher than that 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

of Mn/TiO2. Therefore, kNSCR of Mn/TiO2 and k4[NH2][Mn+=O] of Cr/TiO2 below 200 oC can be obtained from the average value of δNSCR in Figures 10d and 11d. Furthermore, k4[NH2][Mn+=O] and k5/k2 of Cr/TiO2 above 200 oC can be approximately obtained after the non-linear regression of Figure 11d according to Equation 25. Table 3 shows that k1[NH2] of Cr/TiO2 was much less than that of Mn/TiO2. It suggests that the rate of the SCR reaction over Cr/TiO2 through the Eley-Rideal mechanism was much less than that of Mn/TiO2 (hinted by Equation 18). Table 3 also shows that k4[NH2][Mn+=O] of Cr/TiO2 was much higher than that of Mn/TiO2 above 200 oC. 4.3 Mechanism of the SCR performance of Mn/TiO2 better than that of Cr/TiO2 Equation 20 suggests that NH2 concentration on the surface was proportional to the product of k3, the concentration of NH3 adsorbed, and Mn+ concentration on the surface. H2-TPR analysis suggests that the oxidation ability of Cr/TiO2 was much higher than that of Mn/TiO2. Therefore, k3 of Cr/TiO2 was much higher than that of Mn/TiO2. Furthermore, the concentration of Cr6+ on Cr/TiO2 was higher than that of Mn4+ on Mn/TiO2 (shown in Table 1). NH3-TPD analysis shows that the concentration of NH3 adsorbed on Cr/TiO2 was approximately 0.75 of that on Mn/TiO2 (shown in Figure 5a). Therefore, [NH2] on Cr/TiO2 at the steady-state was at least 0.75 of that on Mn/TiO2. However, k1[NH2] of Cr/TiO2 was much less than that of Mn/TiO2 (shown in Table 3). It suggests that k1 of Cr/TiO2 was much less than that of Mn/TiO2. Reaction 8 needed to overcome the binding of the interface with NH2. If the binding was stronger, k1 would be less. The binding enthalpies of NH3 with Cr and Mn are 183 and 147 kJ mol-1, respectively 37, 38 It suggests that the binding of Cr with adsorbed NH3 species was much stronger than that of Mn.39, 40 Therefore, k1 of Cr/TiO2 was much less than that of Mn/TiO2, which caused to the lower reaction rate constant of the SCR reaction over Cr/TiO2 through the Eley-Rideal mechanism. Although [NH3(ad)] obviously decreased with the increase of reaction temperature, the oxidation ability of Cr/TiO2 and Mn/TiO2 obviously increased. Meanwhile, the binding of the interface with activated NH3 species became weaker with the increase of reaction temperature, resulting in an increase of k1. As a result, k1[NH2] obviously increased with the increase of reaction temperature (shown in Table 3). As the binding of the interface with activated NH3 species became much weaker with the increase of reaction 14


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temperature, the difference between the binding of Cr/TiO2 with activated NH3 species and that of Mn/TiO2 became smaller. As a result, the ratio of k1[NH2] of Mn/TiO2 to that of Cr/TiO2 gradually decreased from 6 at 100 oC to 1.2 at 250 oC. As the oxidation ability of Cr/TiO2 was much better than that of Mn/TiO2 (hinted by the TPR analysis), k4 of Cr/TiO2 was much higher than that of Mn/TiO2. Meanwhile, [NH2] on Cr/TiO2 was at least 0.75 of that of Mn/TiO2. Furthermore, the concentration of Cr6+ on Cr/TiO2 was higher than that of Mn4+ on Mn/TiO2. Therefore, k4[NH2][Mn+=O] of Cr/TiO2 should be much higher than that of Mn/TiO2. Table 3 demonstrates that k4[NH2][Mn+=O] of Cr/TiO2 was much higher than kNSCR of Mn/TiO2 above 200 oC. Although k4[NH2][Mn+=O] of Cr/TiO2 was higher than that of Mn/TiO2, the Langmuir-Hinshelwood mechanism contributed to N2O formation over Mn/TiO2 while it did not contribute to N2O formation over Cr/TiO2 (hinted by the transition reaction study). Therefore, kNSCR of Mn/TiO2 below 200 oC was slightly higher than k4[NH2][Mn+=O] of Cr/TiO2 (shown in Table 3). Reaction 10 still needed to overcome the binding of the interface with adsorbed NH3 species. It suggests that k2 of Cr/TiO2 was much less than that of Mn/TiO2 due to the stronger affiliation of Cr/TiO2 with activated NH3 species. As the oxidation ability of Cr/TiO2 was much better than that of Mn/TiO2, k5 of Cr/TiO2 was much higher than that of Mn/TiO2. As a result, k5/k2 of Cr/TiO2 was much higher than that of Mn/TiO2. Both k4[NH2][Mn+=O] and k5/k2 of Cr/TiO2 was much higher than those of Mn/TiO2. Hinted by Equation 26, δC-O of Cr/TiO2 was much higher than that of Mn/TiO2 (shown in Figures 10e and 11e).

5. Conclusion As the binding of Cr/TiO2 with activated NH3 species was much stronger than that of Mn/TiO2, the rate of the SCR reaction through the Eley-Rideal mechanism over Cr/TiO2 was much less than that over Mn/TiO2 although the oxidation ability of Cr/TiO2 was much higher than that of Mn/TiO2, the concentration of Cr6+ on Cr/TiO2 was higher than that of Mn4+ on Mn/TiO2 and the amount of acid sites on Cr/TiO2 was close to that on Mn/TiO2. Meanwhile, the rate of the C-O reaction of Cr/TiO2 was much higher than that of Mn/TiO2 mainly due to the better oxidation ability and more Cr6+ on the surface. As a result, the SCR performance of Cr/TiO2 was much less than that of Mn/TiO2. 15



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Acknowledgements: This study was financially supported by the National Natural Science Fund of China (Grant No. 41372044), and the Natural Science Fund of Jiangsu Province (Grant No. BK20150036).

Supporting Information Available The deduction of the concentrations of NH2, NH4NO2 and NH4NO3 on the surface. This information is available free of charge via the Internet at http://pubs.acs.org/.

16


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Table 1 Percentages of Mn, Cr, Ti and O species on Mn/TiO2 and Cr/TiO2

/%

Ti4+

O2-

Mn4+/ Cr6+

Mn3+/Cr3+

Cr/TiO2

23.5

67.5

4.6

4.4

Mn/TiO2

28.8

66.5

3.3

1.4

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

Table 2 Capacities of Mn/TiO2 and Cr/TiO2 for NH3 and NO adsorption at 50 oC NH3

NO

Mn/TiO2

4.7

1.1

Cr/TiO2

4.5

1.2

22


Page 22 of 36

/µmol m-2

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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


Table 3 The kinetic parameters of NO reduction over Mn/TiO2 and Cr/TiO2

Temperature /oC

Mn/TiO2

Cr/TiO2

*

SCR reaction

/µmol g-1 min-1

NSCR reaction

k1[NH2]/106

R2

kNSCR/k4[NH2][Mn+=O]

k5/k2 /10-6

100

0.006

0.999

0.5

*

125

0.013

0.999

1.8

*

150

0.021

0.998

3.7

*

175

0.028

0.999

6.8

*

200

0.040

0.998

11.8

*

225

0.054

0.999

18.0

*

250

0.071

0.999

31.7

*

100

0.001

0.995

0.1

*

125

0.002

0.999

0.4

*

150

0.005

0.995

1.8

*

175

0.009

0.995

4.6

*

200

0.023

0.992

7.8

*

225

0.029

0.993

45.0

112

250

0.058

0.991

96.1

126

k5 ≈0 k2 [NO(g) ]

23



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 24 of 36

Figure captions Figure 1 SCR performance of: (a) Mn/TiO2; (b), Cr/TiO2. 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 Mn/TiO2 and Cr/TiO2 Figure 3 XPS spectral of Mn/TiO2 and Cr/TiO2 over the spectral regions of Mn 2p, Cr 2p, Ti 2p and O 1s Figure 4 H2-TPR profiles of Mn/TiO2 and Cr/TiO2 Figure 5 (a), NH3-TPD profiles of Mn/TiO2 and Cr/TiO2; (b), NOx-TPD profiles of Mn/TiO2 and Cr/TiO2. Figure 6 (a), DRIFT spectra taken at 200 oC upon passing NO+O2 over NH3 presorbed Mn/TiO2; (b), DRIFT spectra taken at 200 oC upon passing NH3 over NO+O2 presorbed Mn/TiO2. Figure 7 (a), Transient reaction taken at 200 oC upon passing NO+O2 over NH3 presorbed Mn/TiO2; (b), transient reaction taken at 200 oC upon passing NH3 over NO+O2 presorbed Mn/TiO2; (b), transient reaction taken at 200 oC upon passing NH3 over fresh Mn/TiO2. Figure 8 (a), DRIFT spectra taken at 200 oC upon passing NO+O2 over NH3 presorbed Cr/TiO2; (b), DRIFT spectra taken at 200 oC upon passing NH3 over NO+O2 presorbed Cr/TiO2. Figure 9 (a), transient reaction taken at 200 oC upon passing NO+O2 over NH3 presorbed Cr/TiO2; (b), transient reaction taken at 200 oC upon passing NH3 over NO+O2 presorbed Cr/TiO2; (c), transient reaction taken at 200 oC upon passing NH3 over fresh Cr/TiO2. Figure 10 Dependences of δNH3 (a), δNOx (b), δN2 (c), δN2O (d) and δC-O (e) during NO reduction over Mn/TiO2 on gaseous NO concentration. Reaction conditions: [NH3]=500ppm, [NO]=300-700 ppm,

[O2]=2%,

catalyst

mass=10-200

mg,

total

flow

rate=200

mL

min-1

and

GHSV=60000-1200000 cm3 g-1 h-1. Figure 11 Dependences of δNH3 (a), δNOx (b), δN2 (c), δN2O (d) and δC-O (e) during NO reduction over Cr/TiO2 on gaseous NO concentration. Reaction conditions: [NH3]=500ppm, [NO]=300-700 ppm,

[O2]=2%,

catalyst

mass=5-400

mg,

total

flow

rate=200

GHSV=30000-2400000 cm3 g-1 h-1. 24


mL

min-1

and

Page 25 of 36

100

NOx/NH3 conversion/%

100

80

80 NOx conversion 60

60

NH3 conversion N2O selectivity

40

40 20

20

N2O selectivity/%

0

0 100

125

150

175

200 o

225

250

Temperature/ C a 100

NOx/NH3 conversion/%

100

NOx conversion 80

NH3 conversion

80

N2O selectivity 60

60

40

40

20

20

N2O 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


0

0 100

125

150

175

200 o

225

250

Temperature/ C b

Figure 1 SCR performance of: (a) Mn/TiO2; (b), Cr/TiO2. 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.

25



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Page 26 of 36

Mn/TiO2

Cr/TiO2 10

20

30

40

50

60

70

80

2θ/degree

Figure 2 XRD patterns of Mn/TiO2 and Cr/TiO2

26


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Mn 2p

Mn/TiO2

Cr 2p

Cr/TiO2

576.8

579.3

642.4

587.8

653.8

665

660

641.4

652.9 644.8

656.2

655

650

586.3

645

640

595

635

590

585

580

575

570

Binding Energy/eV

Binding Energy/eV Mn/TiO2

Cr/TiO2

Ti 2p

Ti 2p

458.4 458.4 464.1

468

466

464.2

464

462

460

458

456

454

468

466

Binding Energy/eV

464

462

460

458

456

454

Binding Energy/eV

Mn/TiO2

O 1s

Cr/TiO2

O 1s

529.8

529.8 531.0

536

534

532

531.0

530

528

526

536

Binding Energy/eV

534

532

530

528

526

Binding Energy/eV

Figure 3 XPS spectral of Mn/TiO2 and Cr/TiO2 over the spectral regions of Mn 2p, Cr 2p, Ti 2p and O 1s

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 4 H2-TPR profiles of Mn/TiO2 and Cr/TiO2

28


Page 28 of 36

Page 29 of 36

NH3 concentration/ppm

120 Mn/TiO2 90

Cr/TiO2

60 30 0 100

200

300

400

500

o

600

Temperature/ C a 80

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


Mn/TiO2 60

Cr/TiO2

40 20 0 100

200

300

400 o

500

600

Temperature/ C b Figure 5 (a), NH3-TPD profiles of Mn/TiO2 and Cr/TiO2; (b), NOx-TPD profiles of Mn/TiO2 and Cr/TiO2.

29


0.05 NO+O2 20 min NO+O2 10 min NO+O2 5 min

1800

1700

1600

1208 1175

1603

NO+O2 3 min

1500

1400

1300

1200

NH3 1100

1000

-1

Wavenumber/cm

1208 1175

1603

a

0.05

NH3 20 min NH3 10 min NH3 5 min NH3 3 min 1601 1574

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

1601 1574


1800

1700

1600

NO+O2 1500

1400

1300

1200

1100

1000

-1

Wavenumber/cm

b Figure 6 (a), DRIFT spectra taken at 200 oC upon passing NO+O2 over NH3 presorbed Mn/TiO2; (b), DRIFT spectra taken at 200 oC upon passing NH3 over NO+O2 presorbed Mn/TiO2.

30


Page 30 of 36

Page 31 of 36

50


500

N2O concentration/ppm

40

400 NO NO2

300

30

N2O 200

20

100

10 0

0 -5

0

5

10

15

20

25

30

35

40

t/min


a 500

50

400

40 NH3

300


30

N2O 200

20

100

10

0

0 -5

0

5

10

15

20

25

30

35

40

t/min


b 500

50

400

40 NH3

300


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


30

N2O 200

20

100

10

0

0 -5

0

5

10

15

20

25

30

35

40

t/min c Figure 7 (a), Transient reaction taken at 200 oC upon passing NO+O2 over NH3 presorbed Mn/TiO2; (b), transient reaction taken at 200 oC upon passing NH3 over NO+O2 presorbed Mn/TiO2; (b), transient reaction taken at 200 oC upon passing NH3 over fresh Mn/TiO2. 31


0.05

NO+O2 20 min

NO+O2 10 min NO+O2 5 min

1603

1208

NO+O2 3 min

1800

1700

1600

1500

1400

1300

1200

NH3

1100

1000

-1

Wavenumber/cm

1603

1208

a

0.05 NH3 20 min NH3 10 min NH3 5 min

1610

NH3 3 min

1800

1700

1600

NO+O2

1546

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

1610


1500

1400

1300

1200

1100

1000

-1

Wavenumber/cm

b Figure 8 (a), DRIFT spectra taken at 200 oC upon passing NO+O2 over NH3 presorbed Cr/TiO2; (b), DRIFT spectra taken at 200 oC upon passing NH3 over NO+O2 presorbed Cr/TiO2.

32


Page 32 of 36

Page 33 of 36

250


500


200

400 NO NO2

300

150

N2O 200

100

100

50

0

0 -5

0

5

10

15

20

25

30

35

40

t/min


a 500

50

400

40

300

30 NH3

200


20

N2O

10

100

0

0 -5

0

5

10

15

20

25

30

35

40

t/min


b 500

50

400

40

300

30 NH3

200


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


20

N2O

100

10

0

0 -5

0

5

10

15

20

25

30

35

40

t/min c Figure 9 (a), transient reaction taken at 200 oC upon passing NO+O2 over NH3 presorbed Cr/TiO2; (b), transient reaction taken at 200 oC upon passing NH3 over NO+O2 presorbed Cr/TiO2; (c), transient reaction taken at 200 oC upon passing NH3 over fresh Cr/TiO2. 33



o

o

125 C o 225 C

o

150 C o 250 C

100

o

175 C

-1

40 20

o

100 C o 200 C

80

-1

100 C o 200 C

δNOx/µmol g min

-1

60

δNH3/µmol g min

80

-1

100

300

175 C

o

600

700

60 40 20

400

500

600

300

700

400

a o

o

150 C o 250 C

50

o

175 C

40 30

δN2O/µmol g min

125 C o 225 C

-1

o

100 C o 200 C

b

-1

80

500

NO concentration/ppm


-1

o

150 C o 250 C

0

0

60

-1

δN2/µmol g min

o

125 C o 225 C

40 20

o

100 C o 200 C

o

150 C o 250 C

500

600

125 C o 225 C

o

o

175 C

20 10 0

0 300

400

500

600

300

700

400

c 40

o

-1

100 C o 200 C

700



d o

125 C o 225 C

o

150 C o 250 C

o

175 C

δC-O/µmol g min

30

-1

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 34 of 36

20 10 0 300

400

500

600

700


e Figure 10 Dependences of δNH3 (a), δNOx (b), δN2 (c), δN2O (d) and δC-O (e) during NO reduction over Mn/TiO2 on gaseous NO concentration. Reaction conditions: [NH3]=500ppm, [NO]=300-700 ppm,

[O2]=2%,

catalyst

mass=10-200

mg,

total

flow

GHSV=60000-1200000 cm3 g-1 h-1.

34


rate=200

mL

min-1

and

o

125 C o 225 C

o

150 C o 250 C

150

o

175 C -1

o

100 C o 200 C

120 90

δNOx/µmol g min

-1

150

δNH3/µmol g min

200

-1

250

100 50

o

100 C o 200 C

400

500

600

300

700

500

600

o

175 C

400

o

100 C o 200 C

o

125 C o 225 C

o

150 C o 250 C

o

100

o

175 C

δN2O/µmol g min

80

b

-1

100

700


a

-1

o

30


-1

60

-1

150 C o 250 C

0 300

δN2/µmol g min

o

125 C o 225 C

60

0

40 20

100 C o 200 C

o

150 C o 250 C

o

500

600

125 C o 225 C

o

175 C

80 60 40 20 0

0 300

400

500

600

300

700

400

c 80

o

-1

100 C o 200 C

700



d o

125 C o 225 C

o

150 C o 250 C

o

175 C

60

-1

δC-O/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


-1

Page 35 of 36

40 20 0 300

400

500

600

700


e Figure 11 Dependences of δNH3 (a), δNOx (b), δN2 (c), δN2O (d) and δC-O (e) during NO reduction over Cr/TiO2 on gaseous NO concentration. Reaction conditions: [NH3]=500ppm, [NO]=300-700 ppm,

[O2]=2%,

catalyst

mass=5-400

mg,

total

flow

rate=200

GHSV=30000-2400000 cm3 g-1 h-1.

35


mL

min-1

and


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

Table of content

36


Page 36 of 36

Why the Low-Temperature Selective Catalytic Reduction Performance

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