Alkali Metal Deactivation on the Low Temperature Selective Catalytic

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Alkali Metal Deactivation on the Low Temperature Selective Catalytic Reduction of NO With NH Over MnO-CeO: A Mechanism Study x

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Shangchao Xiong, Jingxia Wen, Yong Liao, Bo Li, Sijie Zou, Yang Gen, Xin Xiao, Nan Huang, and Shijian Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05175 • Publication Date (Web): 01 Jul 2016 Downloaded from http://pubs.acs.org on July 1, 2016

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The Journal of Physical Chemistry

Alkali Metal Deactivation on the Low Temperature Selective Catalytic Reduction of NOx with NH3 over MnOx-CeO2: A Mechanism Study Shangchao Xiong, Jingxia Weng, Yong Liao, Bo Li, Sijie Zou, Yang Gen, Xin Xiao, Nan Huang, 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

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

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Abstract: The application of MnOx-CeO2 as the low temperature selective catalytic reduction (SCR) catalyst to control NOx emission from coal-fired power plants is extremely restricted due to the unrecoverable deactivation by SO2. There is little SO2 in the flue gas from biomass-fired power plants and the concentration of alkali metals in the flue gas after the electrostatic precipitator is very low, so the application of MnOx-CeO2 may be possible to control NOx emission from biomass-fired power plants. However, a very small amount of alkali metals showed a seriously negative effect on NO reduction over MnOx-CeO2 that both NOx conversion and N2 selectivity obviously decreased. In this work, the mechanism of NO reduction over MnOx-CeO2 and K-MnOx-CeO2 was investigated by the transient reaction study and the kinetic parameters of NO reduction were obtained from the steady-state kinetic study. After comparing the kinetic parameters, the mechanism of potassium deactivation on NO reduction over MnOx-CeO2 was discovered. The decrease of the SCR activity of MnOx-CeO2 after potassium deactivation was mainly attributed to the decrease of acid site and Mn4+ concentration on the surface and the increase of N2O selectivity was mainly related to the occurrence of N2O formation over K-MnOx-CeO2 through the Langmuir-Hinshelwood mechanism.

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1. Introduction Nitrogen oxides (NOx), which derive from the combustion of fuels at high temperatures under air, are one of the major atmospheric pollutants due to its contribution to photochemical smog, acid rain and ozone depletion.1, 2 Now, selective catalytic reduction (SCR) of NO by NH3 with V2O5-WO3/TiO2 as the catalyst has been successfully applied in coal-fired plants to control NOx emissions for several decades.3 The SCR unit is located upstream of electrostatic precipitator as the temperature window of V2O5-WO3/TiO2 is about 300-400 oC. However, it is difficult to retrofit the SCR devices in many existing power plants owing to the limitation of the space and access.4 Hence, there is a strong demand to develop low temperature SCR catalysts, which can be located downstream of the desulfurizer and electrostatic precipitator (ESP).5 Mn based catalysts, for example Mn/TiO2,

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Mn-Fe/TiO2,5 MnOx-CeO2,7, 8 MnOx-CeO2/TiO2,9 Mn-Ce-Ti catalyst,10 Mn

promoted V2O5/TiO2,11 Mn-Fe spinel, 12 and Mn/Fe-Ti spinel,13 show an excellent SCR activity at low temperatures. However, the application of the low temperature SCR unit in coal-fired power plants is currently extremely restricted for the unrecoverable deactivation by SO2.14, 15 Now, biomass is considered to be a more environmental-friendly alternative fuel than fossil fuel for power generation, which can significantly reduce the net CO2 emission.16 However, there is a small amount of alkali metals in the flue gas from biomass combustion,17-19 which shows a serious deactivation on the commercial SCR catalyst. The deactivation mechanism of V2O5-WO3/TiO2 by alkali metals is widely accepted that both the surface acidity and the reducibility decrease. 20-22 It is fortunate that little SO2 can be observed in the flue gas from biomass combustion since straw or wood chips have a very low content of sulfur.

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Meanwhile, the deactivation of the low

temperature SCR catalysts by alkali metals will be obviously alleviated as the concentration of alkali metals in the flue gas obviously decreases after the ESP. It suggests that the application of the low temperature SCR unit in the biomass-fired power plants is much more possible than that in coal-fired power plants. Although the concentration of alkali metals in the flue gas of biomass-fired power plants downstream of the ESP was very low, the effect of alkali metals on the low temperature SCR catalysts should be investigated. MnOx-CeO2 has been regarded as the most promising low temperature SCR catalyst due to its excellent performance for NOx removal. 3


7, 8, 24

Besides the


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decrease of SCR activity, N2O selectivity of NO reduction over MnOx-CeO2 obviously increased after the deactivation by alkali metals. N2O is now considered as a pollutant due to its contribution to global warming and stratospheric ozone depletion.25 However, this phenomena cannot be simply interpreted by the mechanism of the decrease of surface acidity and reducibility. Therefore, the deactivation mechanism of NO reduction over MnOx-CeO2 by alkali metals was studied in this work. The kinetic parameters of the SCR reaction and the non-selective catalytic reduction (NSCR) reaction during NO reduction over MnOx-CeO2 and alkali metals poisoned MnOx-CeO2 were obtained from the steady-state kinetic study. After comparing the kinetic parameters, the deactivation mechanism of alkali metals on the low temperature SCR reaction over MnOx-CeO2 was discovered.

2. Experimental 2.1 Catalyst preparation MnOx-CeO2 catalyst was synthesized by the citric acid method.7,

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Suitable amounts of

manganese nitrate, cerium nitrate and citric acid were dissolved in deionized water with the mole ratio of 3:7:10. Then, the obtained solution was dried at 100 oC in a water bath, resulting in a porous and foam-like solid. At last, the foam-like precursor was calcined at 650 oC for 6 h under air after drying at 120 oC for 12 h. K-MnOx-CeO2 (i.e. 0.3 wt% K poisoned MnOx-CeO2 catalyst) was synthesized by the traditional impregnation method.26 2.2 Catalytic test The reaction was carried out on a fix-bed quartz tube reactor with the internal diameter of 6mm, and the reaction temperature was controlled by a vertical electrical furnace.27, 28 The simulated flue gas contained 500 ppm of NH3 (when used), 500 ppm of NO (when used), 2% of O2 and N2 of balance. The total flow rate was 200 mL min-1 and 100 mg of catalyst with 40-60 mesh was used, resulting in a gas hourly space velocity (GHSV) of 120000 cm-3 g-1 h-1. The concentrations of gas compositions (including NO, NO2, NH3 and N2O) in the inlet and outlet were determined online by a Fourier transform infrared spectrometer (FTIR, Thermo SCIENTIFIC, ANTARIS, IGS Analyzer). The amount of N2 formed was calculated as:

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N 2 formation =

[NH 3 ]in -[NH 3 ]out +[NO x ]in -[NO x ]out -2[N 2O]out 2

(1)

2.3 Catalyst characterization BET surface area, X-ray diffraction pattern (XRD), H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectra (XPS) were measured on a nitrogen adsorption apparatus (Quantachrome, Autosorb-1), an X-ray diffractionmeter (Bruker-AXS D8 Advance), a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx) and an X-ray photoelectron spectroscopy (Thermo, ESCALAB 250), respectively. Furthermore, temperature programmed desorptions (TPD) of NO and NH3 were carried out on the fixed-bed quartz reactor. 2.4 Transient reaction study and steady-state kinetic study The transient reaction of the introduction of NO+O2 to NH3 presorbed catalyst and that of the introduction of NH3 to NO+O2 presorbed catalyst were performed at 180 oC to investigate the mechanism of NO reduction over MnOx-CeO2 and K-MnOx-CeO2. Before the transient reaction, the catalyst was pretreated at 300 oC under 200 mL min-1 of N2 to clean the surface. During the transient reaction, the concentrations of NO, NO2, NH3 and N2O in the outlet and in situ DRIFT spectra were simultaneously 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. To obtain the reaction rate constants of N2 and N2O formation (i.e. the SCR and the NSCR reaction) over MnOx-CeO2 and K-MnOx-CeO2, the steady-state kinetic study was performed. To overcome the diffusion limitation (including inner diffusion and external diffusion), an extremely high GHSV of 120000 to 4800000 cm-3 g-1 h-1 (the mass of catalyst was 5-100 mg and the total flow rate was 200-400 mL min-1) was used to keep NO conversion less than 15%.13, 29, 30 Gaseous NH3 concentration in the inlet was kept at 500 ppm during the steady-state kinetic study, while gaseous NO concentration varied from 300 to 700 ppm.25, 31

3. Results 3.1 SCR performance MnOx-CeO2 exhibited an excellent low temperature SCR activity and NOx conversion was higher than 80% at 120-200 oC (shown in Figure 1a). Furthermore, a large amount of N2O 5



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generated during NO reduction over MnOx-CeO2, resulting in a high N2O selectivity especially at higher temperatures (shown in Figure 1b). The SCR performance of synthetic MnOx-CeO2 was close to those reported in previous studies.8, 30 K-MnOx-CeO2 showed a poor low temperature SCR activity, and NOx conversion was less than 60% at 80-200 oC (shown in Figure 1a). It suggests that alkali metals showed a serious deactivation on NO reduction over MnOx-CeO2. This result was consistent with the deactivation of commercial V2O5-WO3/TiO2 by alkali metals.18, 22, 32 However, N2O selectivity of NO reduction over K-MnOx-CeO2 was much higher than that over MnOx-CeO2 (shown in Figure 1b). The similar result was seldom reported in previous studies. 3.2 Characterization 3.2.1 XRD and BET XRD patterns of MnOx-CeO2 corresponded well to the cubic fluorite structure (JCPDS, PDF 43-1002) and any other peaks corresponding to manganese oxides cannot be observed (shown in Figure 2). Meanwhile, the lattice parameter of synthetic MnOx-CeO2 (0.5414 nm) was slightly higher than that of pure CeO2 (0.5411 nm). It suggests that Mn cations may be introduced the cubic fluorite structure.24 After the doping of potassium, few changes can be observed in the XRD patterns of MnOx-CeO2 (shown in Figure 2). Meanwhile, the lattice parameter of K-MnOx-CeO2 was close to that of MnOx-CeO2 (shown in Table 1). It suggests the crystal structure of MnOx-CeO2 did not vary after the doping of potassium. Table 1 shows that the crystal sizes of MnOx-CeO2 and K-MnOx-CeO2 were both approximately 10 nm, which were calculated according to the Scherrer Equation.33 Therefore, the BET surface of MnOx-CeO2 did not vary obviously after the doping of potassium (shown in Table 1). 3.2.2 H2-TPR H2-TPR profiles of MnOx-CeO2 and K-MnOx-CeO2 both show two overlapped reduction peaks at approximately 277 and 391 oC (shown in Figure 3). The peak at 277 °C was attributed to the reduction of Mn4+ to Mn3+, and the peak at 391°C was assigned to the reduction of Mn3+ to Mn2+. 34

After the doping of potassium, the area of the first reduction peak of MnOx-CeO2 at 277 °C

slightly decreased (shown in Figure 3). It suggests that Mn4+ concentration on K-MnOx-CeO2 was less than that on MnOx-CeO2. 3.2.3 XPS 6


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XPS spectra of MnOx-CeO2 and K-MnOx-CeO2 over the spectral regions of Mn 2p, Ce 3d and K 2p are shown in Figure 4. The Mn 2p 3/2 binding energies of MnOx-CeO2 mainly appeared at 640.4, 641.4 and 642.4 eV (shown in Figure 4a), which were assigned to Mn2+, Mn3+ and Mn4+ on the surface, respectively.

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The Ce 3d species on MnOx-CeO2 mainly appeared at 882.2, 884.3,

888.5, 898.0, 900.7, 903.0, 907.2 and 916.4 eV (shown in Figure 4b). The peaks at 882.2, 888.5, 898.0, 900.7, 907.2 and 916.4 eV were attributed to Ce4+ on MnOx-CeO2, and the peaks at 884.3 and 903.0 eV were attributed to Ce3+ on MnOx-CeO2. 36, 37 After the doping of potassium, no significant changes can be observed in the spectral regions of Mn 2p and Ce 3d (shown in Figures 4c and 4d). Meanwhile, the binding energies of K species mainly appeared at 292.5 and 295.4 eV (shown in Figure 4e), which were assigned to K+ on the surface.38 The percentages of Mn, Ce, O and K species on MnOx-CeO2 and K-MnOx-CeO2 were calculated from the XPS spectra. As shown in Table 2, Mn4+ concentration on MnOx-CeO2 slightly decreased after the doping of potassium. This result was consistent with the result of H2-TPR analysis. 3.2.4 NH3-TPD and NO-TPD The capacities of MnOx-CeO2 and K-MnOx-CeO2 for NH3 and NOx adsorption can be obtained from the integration of NH3-TPD and NO-TPD profiles (shown in Figures S1 and S2). As shown in Table 3, the capacity of K-MnOx-CeO2 for NH3 adsorption was much less than that of MnOx-CeO2. It suggests that the amount of acid sites on MnOx-CeO2 significantly decreased after the doping of potassium. This result was consistent with the deactivation of commercial V2O5-WO3/TiO2 by alkali metals. 18, 22, 33 Table 3 also shows that the capacity of MnOx-CeO2 for NO adsorption obviously decreased after the doping of potassium. 3.2.5 NO and NH3 oxidation H2-TPR and XPS analyses both indicate that Mn4+ concentration on MnOx-CeO2 decreased after the doping of potassium. NH3-TPD and NO-TPD profiles indicate that the physical adsorption of NH3 and NO on MnOx-CeO2 were both restrained after the doping of potassium. As a result, the oxidation of NH3 and NO over MnOx-CeO2 were both restrained after the doping of potassium (shown in Figure 5). 7



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3.3 Transient reaction study 3.3.1 MnOx-CeO2 After the adsorption of NH3 at 180 oC, three bands at 1551, 1270 and 1182 cm-1 appeared on MnOx-CeO2 (shown in Figure 6a). The band at 1182 cm-1 was assigned to coordinated NH3 bound to the Lewis acid sites, and the bands at 1551 and 1270 cm-1 could be attributed to the oxidation products of adsorbed NH3 species.24, 30 After the further introduction of NO+O2, the band at 1182 cm-1 corresponding to coordinated NH3 rapidly diminished before the appearance of adsorbed NOx (shown in Figure 6a). Meanwhile, the band at 1622 cm-1 corresponding to adsorbed H2O, which is one of the products of the SCR reaction,

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once appeared during the transient reaction. These

results suggest that the reaction between adsorbed NH3 species and gaseous NO (i.e. the Eley-Rideal mechanism) can contribute to NO reduction over MnOx-CeO2. However, the bands at 1551 and 1270 cm-1 corresponding to the oxidation products of adsorbed NH3 species still existed after passing NO+O2. At last, five new bands at 1597, 1579, 1554, 1245 and 1200 cm-1 appeared on MnOx-CeO2 (shown in Figure 6a). The bands at 1597 and 1200 cm-1 were attributed to monodentate nitrite, and the bands at 1579, 1554 and 1245 cm-1 were attributed to bidentate nitrate.30 N2O concentration in the outlet rapidly increased to approximately 90 ppm and it then decreased to approximately zero in 3 min during the introduction of NO+O2 to NH3 presorbed MnOx-CeO2 (shown in Figure 7a). It suggests that the Eley-Rideal mechanism can contribute to N2O formation over MnOx-CeO2. MnOx-CeO2 was mainly covered by monodentate nitrite (at 1597 and 1200 cm-1) and bidentate nitrate (at 1579, 1554 and 1245 cm-1) after the adsorption of NO+O2 (shown in Figure 6b).30 After the further introduction of NH3, the bands corresponding to monodentate nitrite (at 1597 and 1200 cm-1) rapidly diminished. It suggests that the reaction between adsorbed monodentate nitrite and adsorbed NH3 (i.e. the Langmuir-Hinshelwood mechanism) can contribute to NO reduction over MnOx-CeO2. However, the bands at 1579, 1554 and 1245 cm-1 corresponding to bidentate nitrate shifted to 1554, 1500 and 1270 cm-1 due to the bonding with adsorbed NH3 and they did not decrease with the further introduction of NH3 (shown in Figure 6b). It suggests that the reaction between bidentate nitrate and adsorbed NH3 can not contribute to NO reduction over MnOx-CeO2. Figures 7b and 7c show that the amount of N2O formed during the introduction of NH3 to NO+O2 8


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pretreated MnOx-CeO2 was close to that formed during the introduction of NH3 to fresh MnOx-CeO2. It suggests that N2O formed during the introduction of NH3 to NO+O2 pretreated MnOx-CeO2 mainly resulted from the oxidation of NH3 by MnOx-CeO2 and the Langmuir-Hinshelwood mechanism can not contribute to N2O formation over MnOx-CeO2. 3.3.2 K-MnOx-CeO2 The peak at 1195 cm-1 can be clearly observed on K-MnOx-CeO2 after the adsorption of NH3 at 180 oC, which was assigned to coordinated NH3 bound to the Lewis acid sites. After the further introduction of NO+O2, coordinated NH3 on K-MnOx-CeO2 rapidly diminished before the appearance of adsorbed NOx (shown in Figure 8a). Meanwhile, the band at 1622 cm-1 corresponding to adsorbed H2O once appeared. They suggest that the reaction between adsorbed NH3 species and gaseous NO (i.e. the Eley-Rideal mechanism) can contribute to NO reduction over K-MnOx-CeO2. At last, four new bands at 1597, 1570, 1500 and 1257 cm-1 appeared on K-MnOx-CeO2. The band at 1597 cm-1 was assigned to monodentate nitrite, and the bands at 1570, 1500 and 1257 cm-1 were attributed to monodentate nitrate.39, 40 It suggests that the formation of bidentate nitrate on MnOx-CeO2 was substituted by monodentate nitrate after the doping of potassium. N2O concentration rapidly increased to approximately 31 ppm and it then decreased to approximately zero in 6 min after the introduction of NO+O2 to NH3 presorbed K-MnOx-CeO2 (shown in Figure 9a). It suggests that the Eley-Rideal mechanism can contribute to N2O formation over K-MnOx-CeO2. Figures 7a and 9a show that the amount of N2O formed during the introduction of NO+O2 to NH3 presorbed MnOx-CeO2 was much higher than that formed during the introduction of NO+O2 to NH3 presorbed K-MnOx-CeO2. It suggests that N2O formation over MnOx-CeO2 through the Eley-Rideal mechanism was restrained remarkably after the doping of potassium. K-MnOx-CeO2 was mainly covered by monodentate nitrite (at 1597cm-1) and monodentate nitrate (at 1570, 1500 and 1257 cm-1) after the adsorption of NO+O2 at 180 oC (shown in Figure 8b). After the further introduction of NH3, the bands at 1597 cm-1corresponding to monodentate nitrite rapidly diminished. Meanwhile, the bands at 1570, 1500 and 1257 cm-1corresponding to monodentate nitrate firstly shifted to 1540, 1480 and 1282 cm-1, and then they gradually decreased (shown in Figure 8b). They suggest that both the reaction between adsorbed monodentate nitrite 9



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and adsorbed NH3 and the reaction between adsorbed monodentate nitrate and adsorbed NH3 (i.e. the Langmuir-Hinshelwood mechanism) can contribute to NO reduction over K-MnOx-CeO2. There is generally agreement that the products of monodentate nitrite and monodentate nitrate with adsorbed NH3 were N2 and N2O, respectively.12 Figures 9b and 9c show that the amount of N2O formed during the introduction of NH3 to NO+O2 presorbed K-MnOx-CeO2 was higher than that formed during the introduction of NH3 to fresh K-MnOx-CeO2. It demonstrates that some N2O can form over K-MnOx-CeO2 through the Langmuir-Hinshelwood mechanism (i.e. the reaction between adsorbed monodentate nitrate and adsorbed NH3 species). 3.4 Steady-state kinetic study Figure 10 shows that there was a linear relationship between the rates of N2 formation over both MnOx-CeO2 and K-MnOx-CeO2 and gaseous NO concentration. However, the rates of N2O formation over MnOx-CeO2 and K-MnOx-CeO2 did not vary remarkably with the increase of gaseous NO concentration from 300 to 700 ppm. These results were consistent with the results of the steady-state kinetic studies on Mn-Fe spinel and MnOx-CeO2.25, 30 Figure 10 also shows that the rates of N2 formation and N2O formation over K-MnOx-CeO2 were much less than those over MnOx-CeO2.

4. Discussion 4.1 Reaction mechanism and reaction kinetic study Transient reaction study demonstrates that both the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism can contribute to NO reduction over MnOx-CeO2 and K-MnOx-CeO2 and N2O formation over K-MnOx-CeO2. However, only the Eley-Rideal mechanism can contribute to N2O formation over MnOx-CeO2. NO reduction over MnOx-CeO2 and K-MnOx-CeO2 through the Eley-Rideal mechanism can be approximately described as follows: 29, 30, 41

NH3(g) → NH 3(ad)

(2)

Mn 4+ =O + NH3(ad) → NH 2 + Mn 3+ -OH

(3)

NH 2 +NO (g) → N 2 +H 2 O

(4)

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Mn 4+ =O + NH 2 → NH + Mn 3+ -OH

(5)

NH+NO(g) +Mn 4+ =O → N 2O+Mn 3+ -OH

(6)

Ce 4+ =O+Mn 3+ -OH → Ce3+ -OH+Mn 4+ =O

(7)

Gaseous NH3 firstly adsorbed on MnOx-CeO2 and K-MnOx-CeO2 to form coordinated NH3 (i.e. Reaction 2). Then, adsorbed NH3 was activated by Mn4+ on the surface to form NH2 (i.e. Reaction 3), which then reacted with gaseous NO to N2 (i.e. Reaction 4). Meanwhile, some NH2 can be further oxidize to NH (i.e. Reaction 5), which then reacted with gaseous NO to N2O (i.e. Reaction 6). Reaction 7 was the recovery of Mn4+ by Ce4+ on the surface. NO reduction through the Langmuir-Hinshelwood mechanism over MnOx-CeO2 and K-MnOx-CeO2 can be approximately described as follows: 25, 30, 41

NO(g) → NO(ad)

(8)

NO(ad) + Mn 4+ =O → Mn 3+ -O-NO

(9)

1 NO(ad) + Mn 4+ =O + O 2 → Mn 3+ -O-NO2 2

(10)

NH3(ad) +Mn 3+ -O-NO → Mn 3+ -O-NO-NH3 → Mn 3+ -OH+N 2 +H 2O

(11)

NH3(ad) +Mn 3+ -O-NO2 → Mn 3+ -O-NO 2 -NH3 → Mn 3+ -OH+N 2O+H 2O

(12)

Reaction 8 was the physical adsorption of gaseous NO on the surface, which was then oxidized by Mn4+ on the surface to monodentate nitrite and monodentate nitrate (i.e. Reactions 9 and 10). Monodentate nitrite and monodentate nitrate can react with adsorbed NH3 to form NH4NO2 and NH4NO3 (i.e. reactions 11 and 12), which were then decomposed to N2 and N2O respectively. The rates of N2 formation and N2O formation through the Eley-Rideal mechanism (i.e. Reactions 4 and 6) can be described as:30

d[N 2 ] dt

E-R

d[N 2 O] dt

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

E-R

= −

(13)

d[NH] = k2 [NH][NO(g) ][Mn 4+ =O] dt

(14)

Where, k1, k2, [NO(g)], [NH2], [NH] and [Mn4+=O] were the reaction rate constants of Reactions 4 11



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and 6, gaseous NO concentration, and the concentrations of NH2, NH and Mn4+ on the surface, respectively. The rates of NH2 and NH formation on the surface (i.e. Reactions 3 and 5) can be described as follows:29

d[NH 2 ] = k3[NH3(ad) ][Mn 4+ =O] dt

(15)

d[NH] = k4 [NH 2 ][Mn 4+ =O] dt

(16)

Where, k3 and k4 were the reaction kinetic constants of reactions 3 and 5, respectively. NH concentration on the surface would not vary as the reaction reached the steady-state. 13, 25, 30 Therefore,

d[NH] = k4 [NH 2 ][Mn 4+ =O] − k2 [NH][NO(g) ][Mn 4+ =O]= 　0 dt

(17)

Thus,

[NH]=

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

(18)

Then, Equation 14 can be transformed as:

d[N 2 O] dt

E-R

= k2

k4 [NH 2 ] [NO( g ) ][Mn 4+ =O] = k4 [NH 2 ][Mn 4+ =O] k2 [NO( g ) ]

(19)

Meanwhile, the rates of N2 and N2O formation through the Langmuir-Hinshelwood mechanism (i.e. Reactions 11 and 12) can be described as follows:25, 31

d[N 2 ] dt

L-H

d[N 2 O] dt

= k5 [Mn 3+ -O-NO-NH3 ]

L-H

(20)

= k6 [Mn 3+ -O-NO2 -NH3 ]

(21)

Where, k5, k6, [Mn3+-O-NO-NH3] and [Mn3+-O-NO2-NH3] were the rate constants of NH4NO2 and NH4NO3 decomposition and the concentrations of NH4NO2 and NH4NO3 on the surface, respectively. Therefore, the rates of N2 and N2O formation can be described as follows:

12


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d[N 2 ] d[N 2 ] = dt dt

E-R

d[N 2 O] d[N 2 O] = dt dt

+

d[N 2 ] dt

E-R

+

L-H

= k1[NO(g) ][NH 2 ] + k5 [Mn 3+ -O-NO-NH3 ]

d[N 2 O] dt

L-H

(22)

= k4 [NH 2 ][Mn 4+ =O] + k6 [Mn 3+ -O-NO2 -NH3 ] (23)

Our previous studies demonstrated that NH2 concentration on the surface can be approximately regarded as a constant at the steady-state, which was independent of the concentrations of gaseous NH3 and NO.25, 29-31 Equation 15 suggests that NH2 concentration on the surface was mainly related to the product of k3, the concentrations of NH3 adsorbed and Mn4+ on the surface. Furthermore, our previous studies also demonstrated that the concentrations of NH4NO2 and NH4NO3 on the surface at the steady-state can be approximately regarded as constants,12, 25, 30 which were approximately not related to the concentrations of gaseous NH3 and NO. It suggests that the reaction orders of N2 and N2O formation through the Langmuir-Hinshelwood mechanism with respect to gaseous NO concentration were approximately zero (hinted by Equations 20 and 21). Furthermore, Mn4+ concentration on the surface at the steady-state can also be approximately regarded as a constant as it can be recovered rapidly through Reaction 7. Therefore, Equation 22 and 23 can be transformed as follows:

d[N 2 ] = kSCR-ER [NO(g) ] + kSCR-LH dt

(24)

d[N 2O] = kNSCR-ER + kNSCR-LH = k NSCR dt

(25)

Where,

kSCR-ER = k1[NH 2 ]

(26)

kSCR-LH = k5 [Mn 3+ -O-NO-NH 3 ]

(27)

k NSCR-ER = k4 [NH 2 ][Mn 4+ =O]

(28)

k NSCR-LH = k6 [Mn 3+ -NO3− -NH 3 ]

(29)

Equation 24 suggests that there would be an excellent linear relationship between the rate of N2 formation and gaseous NO concentration. This deduction was demonstrated in Figures 10a and 10c. Therefore, kSCR-ER and kSCR-LH can be obtained after the linear regression of Figures 10a and 10c. Meanwhile, Equation 25 suggests that the reaction order of N2O formation with respect to 13



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Page 14 of 36

gaseous NO concentration would be zero. Figures 10b and 10d demonstrated that N2O formation rates over both MnOx-CeO2 and K-MnOx-CeO2 did not vary remarkably with the increase of gaseous NO concentration. Therefore, kNSCR can be approximately obtained from the average value of N2O formation rates. Table 4 shows that kSCR-LH of MnOx-CeO2 and K-MnOx-CeO2 were both very low. It suggests that N2 formation over MnOx-CeO2 and K-MnOx-CeO2 mainly resulted from the Eley-Rideal mechanism (hinted by Equation 24). Table 4 also shows that both kSCR-ER and kNSCR of MnOx-CeO2 obviously decreased after the doping of potassium. However, the decrease of kSCR-ER of MnOx-CeO2 due to the doping of potassium was much more remarkable than that of kNSCR. 4.2 Mechanism of potassium effect on the SCR reaction over MnOx-CeO2 Figure 3 shows that the first reduction peaks of MnOx-CeO2 and K-MnOx-CeO2 both appeared at approximately 277 oC. It suggests that the oxidation ability of MnOx-CeO2 did not decrease obviously after the doping of potassium. k3 and k4 both depended on the oxidation ability of the catalyst, so k3 and k4 of K-MnOx-CeO2 were close to those of MnOx-CeO2. However, H2-TPR and XPS analyses both suggest that Mn4+ concentration on K-MnOx-CeO2 was less than that on MnOx-CeO2. Furthermore, NH3-TPD profiles suggest that the concentration of NH3 adsorbed on K-MnOx-CeO2 was much less than that on MnOx-CeO2. Hinted by Equation 15, [NH2] on K-MnOx-CeO2 was much less than that over MnOx-CeO2. Meanwhile, k1 was the reaction kinetic constant of Reaction 4, so k1 of K-MnOx-CeO2 may be close to that of MnOx-CeO2. As a result, kSCR-ER (k1[NH2]) of K-MnOx-CeO2 was much less than that of MnOx-CeO2 (shown in Table 4). N2O selectivity of NO reduction through the Eley-Rideal mechanism can be described as:

d[N 2 O] E-R dt N 2 O selectivity = d[N 2 ] d[N 2 O] E-R + dt dt

(30)

E-R

k4 [NH 2 ][Mn 4+ =O] k4 [Mn 4+ =O] = = k1[NO(g) ][NH 2 ] + k4 [NH 2 ][Mn 4+ =O] k1[NO (g) ] + k4 [Mn 4+ =O] Mn4+ concentration on K-MnOx-CeO2 was less than that on MnOx-CeO2. Hinted by Equation 30, N2O selectivity of NO reduction over K-MnOx-CeO2 through the Eley-Rideal mechanism should be less than that of MnOx-CeO2. It suggests that the decrease of kNSCR-ER of MnOx-CeO2 14


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due to the doping of potassium would be more remarkable than that of kSCR-ER. However, the transient reaction study demonstrated that the Langmuir-Hinshelwood mechanism can not contribute to N2O formation over MnOx-CeO2, while it can contribute to N2O formation over K-MnOx-CeO2. It suggests that kNSCR-LH of K-MnOx-CeO2 was much higher than that of MnOx-CeO2, which was equal to zero. As kNSCR of K-MnOx-CeO2 contained the contributions of both the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism (hinted by Equation 25), the decrease of kNSCR of MnOx-CeO2 due to the doping of potassium was much slower than that of kSCR-ER. As a result, N2O selectivity of NO reduction over K-MnOx-CeO2 was much higher than that over MnOx-CeO2 (shown in Figure 1).

5. Conclusion Potassium showed a negative effect on the low temperature SCR reaction over MnOx-CeO2 that the low temperature SCR activity obviously decreased and N2O selectivity obviously increased. The decrease of the SCR activity of MnOx-CeO2 due to the doping of potassium was mainly attributed to the decrease of acid site and Mn4+ concentration on the surface. However, the increase of N2O selectivity of NO reduction over MnOx-CeO2 due to the doping of potassium was mainly assigned to the occurrence of N2O formation over K-MnOx-CeO2 through the Langmuir-Hinshelwood mechanism.

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 NH3-TPD and NO-TPD profiles of MnOx-CeO2 and K-MnOx-CeO2. This information is available free of charge via the Internet at http://pubs.acs.org/.

16


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20


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Table 1 Crystal sizes, lattice parameters, and BET surface areas of MnOx-CeO2 and K-MnOx-CeO2 crystal size

lattice parameter

BET surface area

/nm

/nm

/m2 g-1

MnOx-CeO2

10

0.5414

50.5

K-MnOx-CeO2

10

0.5414

46.0

21



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Page 22 of 36

Table 2 Percentages of Mn, Ce, O and K species on MnOx-CeO2 and K-MnOx-CeO2 Mn 4+

/%

Ce 3+

2+

Mn

Mn

Mn

MnOx-CeO2

5.8

5.0

K-MnOx-CeO2

5.2

4.7

4+

O2-

K+

3+

Ce

Ce

1.5

20.9

1.8

65.0

-

2.0

21.5

1.4

64.7

0.5

22


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Table 3 Capacities of MnOx-CeO2 and K-MnOx-CeO2 for NH3 and NO adsorption at 50 oC /µmol g-1 NH3

NO

MnOx-CeO2

94

153

K-MnOx-CeO2

74

114

23



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Page 24 of 36

Table 4 The rate constants of the SCR reaction through the Eley-Rideal mechanism (kSCR-ER) and the Langmuir-Hinshelwood mechanism (kSCR-LH), and the rate constants of the NSCR reaction /µmol g-1 min-1

(kNSCR) during NO reduction MnOx-CeO2 and K-MnOx-CeO2 the SCR reaction

the NSCR reaction

Temperature/oC kSCR-LH

MnOx-CeO2

K-MnOx-CeO2

kSCR-ER/106

R2

kNSCR

120

6.3

0.101

0.996

9.3

140

0.8

0.150

0.991

15

160

0

0.164

0.997

26

180

0

0.254

0.999

64

200

0

0.202

0.996

77

120

1.1

0.013

0.996

3.7

140

0

0.022

0.998

7.9

160

0

0.023

0.998

14

180

0

0.026

0.999

30

200

1.6

0.024

0.994

45

24


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Figure Captions Figure 1 SCR performance of MnOx-CeO2 and K-MnOx-CeO2: (a), NOx conversion; (b), N2O selectivity. 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 2 XRD patterns of MnOx-CeO2 and K-MnOx-CeO2 Figure 3 H2-TPR profiles of MnOx-CeO2 and K-MnOx-CeO2 Figure 4 XPS spectra of MnOx-CeO2 and K-MnOx-CeO2 over the spectral regions of Mn 2p, Ce 3d and K 2p Figure 5 (a), NH3 oxidation over MnOx-CeO2 and K-MnOx-CeO2; (b), NO oxidation over MnOx-CeO2 and K-MnOx-CeO2. 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 6 DRIFT spectra taken at 180 oC upon: (a), passing NO+O2 over NH3 presorbed MnOx-CeO2; (b), passing NH3 over NO+O2 presorbed MnOx-CeO2. Figure 7 Transient reaction taken at 180 oC upon: (a), passing NO+O2 over NH3 presorbed MnOx-CeO2; (b), passing NH3 over NO+O2 presorbed MnOx-CeO2; (c), passing NH3 over fresh MnOx-CeO2. Figure 8 DRIFT spectra taken at 180 oC upon: (a), passing NO+O2 over NH3 presorbed K-MnOx-CeO2; (b), passing NH3 over NO+O2 presorbed K-MnOx-CeO2. Figure 9 Transient reaction taken at 180 oC upon: (a), passing NO+O2 over NH3 presorbed K-MnOx-CeO2; (b), passing NH3 over NO+O2 presorbed K-MnOx-CeO2; (c), passing NH3 over fresh K-MnOx-CeO2. Figure 10 Dependences of N2 formation rate (a) and N2O formation rate (b) during NO reduction over MnOx-CeO2 on gaseous NO concentration. Reaction conditions: [NH3]=500 ppm, [NO]=300-700 ppm, [O2]=2%, catalyst mass=5-20 mg, total flow rate=400 mL min-1 and GHSV=1200000-4800000 cm3 g-1 h-1; Dependences of N2 formation rate (c) and N2O formation rate (d) during NO reduction over K-MnOx-CeO2 on gaseous NO concentration. Reaction conditions: [NH3]=500 ppm, [NO]=300-700 ppm, [O2]=2%, catalyst mass = 10-50 mg, total flow rate = 200 mL min-1 and GHSV=240000-1200000 cm3 g-1 h-1.

25



NOx conversion/%

100 80 60 40 MnOx-CeO2

20

K-MnOx-CeO2 0 80

100

120

140

160 o

180

200

180

200

Temperature/ C a 80

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

MnOx-CeO2 K-MnOx-CeO2

60 40 20 0 80

100

120

140

160 o

Temperature/ C b

Figure 1 SCR performance of MnOx-CeO2 and K-MnOx-CeO2: (a), NOx conversion; (b), N2O selectivity. 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.

26


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(111) (220)

(200)

(311) (222) (400) (331)(420) MnOx-CeO2

K-MnOx-CeO2

20

30

40

50

60

70

80

2θ/degree Figure 2 XRD patterns of MnOx-CeO2 and K-MnOx-CeO2

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

o

391 C o

277 C MnOx-CeO2

K-MnOx-CeO2 100

200

300

400

500

o

600

700

800

Temperature/ C

Figure 3 H2-TPR profiles of MnOx-CeO2 and K-MnOx-CeO2

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


MnOx-CeO2

MnOx-CeO2

Mn 2p

Ce 3d

916.4

898.0 900.7

653.3

641.4 642.4

882.2

907.2 903.0

888.5 884.3

640.4 645.0

662

657

652

647

642

925

637

915

905

895

Binding Energy/eV

Binding Energy/eV

a

b

K-MnOx-CeO2

K-MnOx-CeO2

Mn 2p

897.9 900.6

641.4 642.4

882.2

907.3 902.9

640.4

888.4 884.3

645.0

657

875

Ce 3d

916.3 653.3

662

885

652

647

642

637

925

915

905

895

Binding Energy/eV

Binding Energy/eV

c

d

885

875

K 2p

K-MnOx-CeO2

292.5 295.4

297

295

293

291

Binding Energy/eV

e

Figure 4 XPS spectra of MnOx-CeO2 and K-MnOx-CeO2 over the spectral regions of Mn 2p, Ce 3d and K 2p

29



NH3 conversion/%

100 MnOx-CeO2 K-MnOx-CeO2

80 60 40 20 0 80

100

120

140

160 o

180

200

Temperature/ C a 30

NO 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

Page 30 of 36

MnOx-CeO2

25

K-MnOx-CeO2

20 15 10 5 0 80

100

120

140

160 o

180

200

Temperature/ C b Figure 5 (a), NH3 oxidation over MnOx-CeO2 and K-MnOx-CeO2; (b), NO oxidation over MnOx-CeO2 and K-MnOx-CeO2. 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.

30


1245 1200

0.1

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

1800

1700

1600

1500

1400

NH3

1182

1551

1270

1622

NO+O2 3 min

1300

1200

1100

1000

-1

Wavenumber/cm

1270 1182

0.1

1500

1554

a

NH3 15 min

1579

NH3 10 min NH3 5 min NH3 3 min

1800

1700

1600

1500

1400

1300

NO+O2 1200

1245

1597 1554

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


1597 1579 1554

Page 31 of 36

1200

1100

1000

-1

Wavenumber/cm

b Figure 6 DRIFT spectra taken at 180 oC upon: (a), passing NO+O2 over NH3 presorbed MnOx-CeO2; (b), passing NH3 over NO+O2 presorbed MnOx-CeO2.

31


Page 32 of 36

500

100

400

80

300

NO NO2

60

200

N2O

40

100

20

0

0 -3

0

3

6

9

12

15

N2O concentration/ppm

18

t/min

a

NH3 concentration/ppm

500

100 80

400 NH3

300

60

N2O

200

40

100

20 0

0 -3

0

3

6

9

12

15


18

t/min b


500

100

400

80

300

60

200

40 NH3

100

20

N2O 0

0 -3

0

3

6

9

12

15


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

NOx concentration/ppm


18

t/min c Figure 7 Transient reaction taken at 180 oC upon: (a), passing NO+O2 over NH3 presorbed MnOx-CeO2; (b), passing NH3 over NO+O2 presorbed MnOx-CeO2; (c), passing NH3 over fresh MnOx-CeO2.

32


1257

1570

1500

0.1

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

1622

NO+O2 3 min

1195

NH3

1800

1700

1600

1500

1400

1300

1200

1100

1000

-1

Wavenumber/cm

1195

1282

1540

NH3 15 min

1480

0.1

1612

a

NH3 10 min NH3 5 min NH3 3 min

1800

1700

1600

1500

NO+O2

1257

1500

1597 1570

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


1597

Page 33 of 36

1400

1300

1200

1100

1000

-1

Wavenumber/cm

b Figure 8 DRIFT spectra taken at 180 oC upon: (a), passing NO+O2 over NH3 presorbed K-MnOx-CeO2; (b), passing NH3 over NO+O2 presorbed K-MnOx-CeO2.

33



100

NOx concentration/ppm

500

80

400 NO NO2

300

N2O

200

60 40 20

100

0

0 -3

0

3

6

9

12

15


18

t/min

a 100


500


400

80

300

60 40

200 NH3 N2O

100

20 0

0 -3

0

3

6

9

12

15

18

t/min b


500

100

400

80

300

60

200

40 NH3

100

20

N2O 0

0 -3

0

3

6

9

12

15


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

Page 34 of 36

18

t/min c Figure 9 Transient reaction taken at 180 oC upon: (a), passing NO+O2 over NH3 presorbed K-MnOx-CeO2; (b), passing NH3 over NO+O2 presorbed K-MnOx-CeO2; (c), passing NH3 over fresh K-MnOx-CeO2.

34


160

o

140 C o 200 C

100

o

160 C

o

120 C o 180 C

-1

-1

o

120 C o 180 C

N2O formation rate/µmol g min

-1

N2 formation rate/µmol g min

200

80

120

o

140 C o 200 C

o

160 C

60 40

80

20

40 0 300

400

500

600

700

NO concentration/ppm

0 300

400

500

600

700

NO concentration/ppm b

o

-1

120 C o 180 C

o

140 C o 200 C

o

160 C

15 10 5 0 300

400

500

600


60

o

120 C o 180 C

-1

20

N2O formation rate/µmol g min

-1

-1

a

N2 formation rate/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

700

o

140 C o 200 C

o

160 C

40

20

0 300

400

500

600

700


c

d

Figure 10 Dependences of N2 formation rate (a) and N2O formation rate (b) during NO reduction over MnOx-CeO2 on gaseous NO concentration. Reaction conditions: [NH3]=500 ppm, [NO]=300-700 ppm, [O2]=2%, catalyst mass=5-20 mg, total flow rate=400 mL min-1 and GHSV=1200000-4800000 cm3 g-1 h-1; Dependences of N2 formation rate (c) and N2O formation rate (d) during NO reduction over K-MnOx-CeO2 on gaseous NO concentration. Reaction conditions: [NH3]=500 ppm, [NO]=300-700 ppm, [O2]=2%, catalyst mass = 10-50 mg, total flow rate = 200 mL min-1 and GHSV=240000-1200000 cm3 g-1 h-1.

35



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

TOC graphic

36


Page 36 of 36

Alkali Metal Deactivation on the Low Temperature Selective Catalytic

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