Deactivation effects of potassium on a CeMoTiOx catalyst for the

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Deactivation effects of potassium on a CeMoTiOx catalyst for the selective catalytic reduction of NOx with NH3 Xiaoling Chen, Yang Geng, Wenpo Shan, and Fudong Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04444 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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Industrial & Engineering Chemistry Research

Deactivation effects of potassium on a CeMoTiOx catalyst for the selective catalytic reduction of NOx with NH3 Xiaoling Chen a, £, Yang Geng a, £, Wenpo Shan a,b*, Fudong Liu c, § a

Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse,

School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China b

Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China

c

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley 94720, California, United States

§

Current address: BASF Corporation, 25 Middlesex Essex Turnpike, Iselin, New Jersey 08830, United States

£

Equal Contribution

*Corresponding author. Fax: +86 25 84315173; Tel: +86 18012920637; E-mail: [email protected]

1

ACS Paragon Plus Environment

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Abstract In this study, we investigated the deactivation effects of potassium on CeMoTiOx for the selective catalytic reaction of NOx with NH3. CeMoTiOx catalyst exhibited high SCR activity and N2 selectivity during SCR reaction. Potassium is found to inhibit the SCR activity of CeMoTiOx, but the poisoned CeMoTiOx can be regenerated. Characterizations, including H2-TPR, TPD and in situ DRIFTS, revealed that potassium suppresses the reduction and adsorption capacity of CeMoTiOx. According to the separated oxidation results, potassium suppresses the oxidation of NO to NO2 and promotes the oxidation of NH3 to NOx and N2O, this may be the main reason for the decrease of low-temperature SCR activity and N2 selectivity respectively over CeMoTiOx. In addition, kinetic study demonstrated that potassium has inhibitory effect on SCR (4NH3 +4NO+O2 →4N2 +6H2 O) reaction rates, while enhances

both

NSCR

( 4NH3 +4NO+3O2 →4N2 O+6H2 O )

(4NH3 +5O2 →4NO+6H2 O) reaction rates in the high temperature region.

2


and

C-O

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1 Introduction Due to the hazardous effects of NOx, such as inducing haze and photochemical smog, the control of NOx has been paid more and more attentions. Selective catalytic reaction (SCR) has been commercially applied all over the world for flue gas de-nitration.1-3

Vanadium-based

catalysts,

such

as

V2O5-WO3/TiO2

and

V2O5-MoO3/TiO2, have been widely used for NH3-SCR for several decades.4,5 However, there are still some drawbacks, such as the toxic effect of V species and narrow temperature window.1,6,7 Therefore, many novel vanadium-free SCR catalysts have been developed to avoid these defects above. . Recently, ceria and ceria-containing materials have attracted more attention as NH3-SCR catalysts due to their remarkable oxygen storage capability and favorable redox property.6,8,9 Chang et al.10-12 developed a CeMoOx catalyst with favorable SCR activity. Meanwhile, the CeMoOx catalyst showed good catalytic performance with respect to the resistance to phosphorus and N2 selectivity. Geng et al.13 introduced Ti into Ce-Mo oxides and prepared a novel CeMoTiOx catalyst, which exhibited a good prospect because of the high activity in a wide temperature window and the less N2O formation. In recent years, renewable energy sources are paid more and more attentions by the researchers.14 Biomass, which is considered to be a novel environment-friendly fuel for power plant, shows great advantage due to its extensive distribution and low carbon emission.15,16 Nowadays, at least five biofuel-fired combined heat and power plants in Sweden have applied SCR at high-dust positions for NOx reduction.17 3



However, due to a major difference between typical fossil fuel and biomass that much higher content of alkali metals exists in the biomass, there are some drawbacks of biomass burning, one of which is the deactivation effect on SCR catalyst.16-20 The effects of alkali metals on commercial vanadium-based catalysts have been widely investigated.1,21-24 Chen et al.22 investigated the poisoning effect of alkali/alkaline earth metals on nano V2O5-WO3/TiO2, they found that the effect increased proportionately with the alkalescence. K could decrease the amount and stability of the Brønsted acid sites to a greater extent than Mg and Ca. Peng et al.25 prepared a series of V2O5/CeO2 catalysts with different K loadings, and found that the alkali poisoning effect is associated with the surface acidity, reducibility and NOx adsorption behaviors. They also designed different strategies to deal with the traditional SCR catalyst poisoned by alkali metals.1 Regeneration methods, such as washing and electrophoresis treatment, could remove most alkali metal from the catalyst and regenerate the SCR activities. In addition, ceria was found to be able to promote the alkali poisoning resistance of V2O5-WO3/TiO2 catalyst. To promote the applicability of ceria-based catalysts, the deactivation effects of potassium on our previously developed CeMoTiOx catalyst were investigated in this study by comparing the SCR reactivity, surface acidity and redox property between fresh and poisoned catalysts. Meanwhile, a kinetic study was also performed to investigate the potassium deactivation on the CeMoTiOx catalyst.

4


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2 Experimental 2.1 Catalyst Preparation The CeMoTiOx catalyst, with a Ce/Mo/Ti molar ratio of 1:1:5, was prepared by homogeneous precipitation method. The desired cerium nitrate, ammonium molybdate and titanium sulfate were co-dissolved into deionized water. After adding excess urea as precipitant, the solution kept stirring at 90 oC in a water bath for 10 h. The obtained precipitate was dried at 105 oC for 12 h, and then calcined at 500 oC for 3 h. Before tests, the powder catalysts were pressed, crushed and sieved into 40-60 mesh (for tests) and 200 mesh (for characterizations). 2.2 Catalyst Poisoning and Regeneration 0.3%K/CeMoTiOx was synthesized by impregnating the CeMoTiOx onto KNO3 solution. After being dried at 105 oC for 12 h and calcined at 500 oC for 3 h, the obtained catalyst was pressed, crushed and sieved into 40-60 mesh (for tests) and 200 mesh (for characterizations). The poisoned powder catalyst was regenerated in deionized water for 12 h at room temperature.26 After centrifugal washing for two times, the regenerated catalyst was dried at 105 oC for 12 h and then calcined at 500 oC for 3 h. Before tests, the powder catalysts were pressed, crushed and sieved into 40-60 mesh (for tests) and 200 mesh (for characterizations). 2.3 Catalytic Test The reaction was conducted in a fixed bed quartz tube and the temperature was controlled by a vertical electrical furnace. The SCR reaction conditions were 5



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controlled as follows: 500 ppm NH3, 500 ppm NO, 5 vol% O2, N2 balance and 200 mL min-1 total flow rate. The volume of catalyst (40-60 mesh) used for test was 80 mm3 (about 0.09 g), and GHSV was 150,000 h-1. The effluent gas component concentrations, including NH3, NO, NO2 and N2O were measured by a Fourier transform infrared spectrometer (FTIR, Thermo Scientific, Antaris, IGS analyzer). The data was collected after 40 min when the SCR reaction achieves a steady state. The NOx conversion, NH3 conversion and N2 selectivity are calculated as follows: NOx conversion=

[NOx ] -[NOx ] in

out

[NOx ]

×100%

(1)

in

NH3 conversion= N2 selectivity = 1-

[NH3 ]in -[NH3 ]out ×100% [NH3 ]in [N2 O]out

(2)

2 �[NOx ] +[NH3 ]in � -2 �[NOx ] +[NH3 ]out � in

×100%

(3)

out

2.4 Steady-State Kinetic Study

Steady-state kinetic study was conducted to gain the rate constants of SCR, NSCR and C-O reaction over CeMoTiOx and 0.3%K/CeMoTiOx. The reaction can be affected on account of the inner and external diffusion limitation, so the GHSV of reaction was raised to a polar altitude to keep NO conversion between 10% and 15%. The inlet NH3 concentration was maintained at 500 ppm, while NO concentration was varied from 300 to 700 ppm. The total gas flow rate was kept at 200 mL min-1, the catalyst mass varied from 1 to 60 mg, and GHSV varied from 200,000 to 12,000,000 cm-1g-1h-1. The rates of SCR reaction, NSCR reaction and C-O reaction can be described as follows:

δSCR =

([NOx ] +[NH3 ] -[NOx ] -[NH3 ] -2[N2 O] )GHSV in

in

out

out

out

2

6


(4)

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δNSCR = [N2 O]out GHSV δC-O =

δNH -δNO 3

(5) (6)

x

2

2.5 Characterization The X-ray diffraction (XRD) of the catalysts were carried out on a Bruker AXS D8 diffractometer with Cu Kα radiation. The data were collected with a step size of 0.08° 2θ/s. The temperature programmed reduction (TPR) tests of H2 were performed on Micromeritics AutoChem_Ⅱ_2920 instruments. After the pretreatment in a quartz reactor, the samples were heated at a rate of 10 oC min-1 in the 10 vol% H2/Ar gas flow. The temperature-programmed desorption (TPD) tests of NH3 and NOx were performed on the fixed bed quartz reactor using the same system as catalytic tests. 100 mg and 300 mg of catalysts were used for the NH3-TPD and NOx-TPD tests, respectively. The typical NH3-TPD experiment includes three procedures: (1) pretreatment of catalyst in N2 gas at 300 oC for 1 h; (2) saturation adsorption in 500 ppm NH3 at 50 oC for 1 h; (3) isothermal desorption in N2 gas from 50 oC to 600 oC at a heating rate of 10 oC min-1. The typical NOx-TPD experiment includes the similar procedures to NH3-TPD, while the adsorption was performed in 500 ppm NO + 5% O2. In situ DRIFTS tests were performed on a FTIR spectrometer (Thermo Scientific Nicolet IS50) equipped with a MCT detector cooled with liquid nitrogen. The samples were pretreated in N2 + 20% O2 at 300 oC for 0.5 h, and then cooled down to reaction 7



temperature (175 oC). The catalyst was presorbed by 500 ppm NH3 (500 ppm NO + 5% O2) gas, and then introduced by 500 ppm NO + 5%O2 (500 ppm NH3) gas. The background spectra were collected in N2 gas and subtracted from sample spectrum. All spectra were recorded by accumulating 100 scans with a resolution of 4 cm-1. 3 Results and discussion 3.1 SCR performance The CeMoTiOx catalyst exhibited high SCR activity in a wide temperature range for SCR reaction. The NOx conversion increased rapidly with the temperature and exceeded 80% from 200 oC to 425 oC (Figure 1A). After being poisoned by potassium, the SCR activity of 0.3%K/CeMoTiOx decreased apparently in the whole temperature range. The NOx conversion increased slowly with the reaction temperature and reached over 80% only in a narrow medium temperature range from 250 oC to 350 oC, then dropped rapidly at high temperature, with even below 40% at 450 oC. It clearly indicated that potassium has an inhibitory effect on the SCR activity of CeMoTiOx. The CeMoTiOx catalyst showed high N2 selectivity in the whole temperature range (Figure 1B). Almost no N2O was detected in the low and medium temperature region from 150 oC to 350 oC. Although the N2 selectivity decreased gradually in the high temperature region, it could archive about 80% at 450 oC. After being poisoned by potassium, the N2 selectivity of 0.3%K/CeMoTiOx dropped down significantly above 300 oC, and even lower than 60% at 450 oC. It indicated that potassium facilitates the formation of N2O during the SCR reaction, which resulted in the reduction of N2 selectivity over CeMoTiOx. 8


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After regenerating with water washing method, the SCR activity of poisoned catalyst showed substantial signs of recovery. The NOx conversion increased appreciably from 200 oC to 300 oC, and even approached 100% at 300 oC. The NOx conversion profiles of the fresh and regenerated catalysts almost overlapped in the high temperature region from 300 oC to 450 oC. In addition, the regenerated catalyst exhibited good recovered N2 selectivity in the whole reaction temperature, even slightly higher than CeMoTiOx. The result indicated that CeMoTiOx has great regeneration capacity after being poisoned by potassium. 3.2 XRD As shown in Figure 2, XRD patterns of 0.3%K/CeMoTiOx was similar to that of CeMoTiOx. Only anatase TiO2 (JCPDS File No. 21-1272) was detected in CeMoTiOx and 0.3%K/CeMoTiOx catalysts. No obvious diffraction peaks corresponding to Mo and Ce species were observed in CeMoTiOx and 0.3%K/CeMoTiOx. These results suggest that potassium has no obvious effects on the crystallization of CeMoTiOx and the Ce and Mo species on CeMoTiOx and 0.3%K/CeMoTiOx were probably existed as amorphous phase or crystallite phase with very small particle size.13 3.3 H2-TPR As shown in Figure 3, H2-TPR profiles of CeMoTiOx and 0.3%K/CeMoTiOx both exhibited three reduction peaks at 500/527

o

C, 535/568

o

C and 837/808

o

C

respectively. The peaks at 500/527 oC and 535/568 oC were ascribed to the reduction of Ce4+ on the surface, and the peak at 837/808 oC was attributed to the reduction of bulk CeO2.3,27-31 The profile showed the high reduction capacity of CeMoTiOx. After 9



being doped with potassium, the peak position of 0.3%K/CeMoTiOx shifted to higher temperature from 500 oC to 527 oC and from 535 oC to 568 oC, respectively. The peak intensity of 0.3%K/CeMoTiOx at 527 oC dropped significantly. The result suggested that potassium suppresses the reduction of Ce4+, which results in an obvious inhibition on the redox property of CeMoTiOx. 3.3 NH3 adsorption NH3-TPD were performed to investigate the adsorption capacities of the reactants for CeMoTiOx and 0.3%K/CeMoTiOx. Two NH3 desorption peaks at 150 oC and 200 o

C were shown respectively over CeMoTiOx in Figure 4A. While the peak at 200 oC

over 0.3%K/CeMoTiOx vanished. According to the result in Table 1, the NH3 adsorption capacity of CeMoTiOx (319 μmol g-1) was higher than that of 0.3%K/CeMoTiOx (262 μmol g-1), which indicated that potassium has an inhibitory effect on the NH3 adsorption capacity over CeMoTiOx. Figure 5A presents the DRIFT spectra of NH3 saturation adsorption over CeMoTiOx and 0.3%K/CeMoTiOx at 175 oC. The coordinated NH3 bound to Lewis acid sites (at 1601, 1251 and 1215 cm-1) and ionic NH4+ bound to the Brønsted acid sites (at 1722, 1676 and 1426 cm-1), as well as the bands at 3357 and 3266 cm-1 due to N-H stretching vibration modes were observed respectively over CeMoTiOx.32-36 After being doped with potassium, the spectra of NH3 saturation adsorption over 0.3%K/CeMoTiOx had no significant change in peak positions. 3.4 NOx adsorption As shown in Figure 4B, the NOx-TPD profiles over CeMoTiOx exhibited the single 10


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NOx desorption peak at 250

o

C, which was prevailingly associated with the

decomposition of chemisorbed NOx species. After being poisoned by potassium, the NOx desorption peak over 0.3%K/CeMoTiOx shifted to higher temperature at 270 oC, with the peak intensity decreased. It indicated that the NOx species were adsorbed stringer on 0.3%K/CeMoTiOx than those on CeMoTiOx. According to the result in Table 1, the NOx adsorption capacity of CeMoTiOx (15 μmol g-1) was slightly higher than that of 0.3%K/CeMoTiOx (14 μmol g-1). Figure 5B presents the DRIFT spectra of NOx saturation adsorption over CeMoTiOx and 0.3%K/CeMoTiOx at 175 oC. Two adsorption peaks at 1606 cm-1 (bidentate nitrate) and 1585 cm-1 (monodentate nitrate) were observed respectively over CeMoTiOx.25,37-39 After being doped with potassium, the spectra of NOx saturation adsorption over 0.3%K/CeMoTiOx changed visibly. The peak at 1585 cm-1 vanished and the intensity of peak at 1606 cm-1 was suppressed significantly. This result indicated that potassium restrains the adsorption of NOx, which is consistent with NOx-TPD results in Figure 4B. 3.5 Separated oxidation The separated NH3 oxidation results for CeMoTiOx and 0.3%K/CeMoTiOx were shown in Figure 6. The NH3 conversions over CeMoTiOx were higher than those over 0.3%K/CeMoTiOx. Both NOx selectivity and N2O selectivity during NH3 oxidation over the two catalysts increased with the increase of temperature. The NOx selectivity over CeMoTiOx was just 9% at 450 oC, which was much less than that over 0.3%K/CeMoTiOx (24% at 450 oC). In addition, the selectivity of N2O over 11



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CeMoTiOx was slightly lower than that over 0.3%K/CeMoTiOx. Therefore, N2 selectivity over CeMoTiOx was much higher than that over 0.3%K/CeMoTiOx. The results indicated that potassium has an inhibitory effect on the NH3 oxidization function of CeMoTiOx, while has a promotional effect on the transformation of NH3 to NOx and N2O. Thus, it could lead to a decrease of N2 selectivity. The separated NO oxidation results were shown in Figure 7. With the increase of temperature, NO was gradually oxidized to NO2 over both of the fresh and poisoned CeMoTiOx.

After

being

doped

with

potassium,

the

NO

conversion

of

0.3%K/CeMoTiOx decreased significantly, which suggests that the NO oxidization capacity of the catalyst could be inhibited by potassium. One important factor influencing the low temperature NH3-SCR performance is “fast SCR” process (2NH3 +NO+NO2 →2N2 +3H2 O),5,40,41 which could be promoted by the oxidization of

NO to NO2. Therefore, less NO2 production over 0.3%K/CeMoTiOx indicated that potassium suppresses the oxidation of NO to NO2, resulting in the inhibition of low-temperature SCR activity over CeMoTiOx, as shown in Figure 1A. 3.6 in situ DRIFTS The in situ DRIFTS results were performed to investigate the reaction mechanism

as shown in Figure 8 and Figure 9. After the introduction of NO and O2, the adsorbed ammonia species over CeMoTiOx including the coordinated NH3 (at 1601, 1251 and 1215 cm-1) and ionic NH4+ (at 1426 cm-1) went down gradually and then faded away in 10 min. Meanwhile, the adsorbed NOx species at 1606 cm-1 (assigned to bidentate nitrate) appeared (Figure 8A).37,40 These results indicated an Eley-Rideal mechanism 12


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for

SCR

reaction

over

CeMoTiOx.2,42-44

The

NH3

adsorption

peaks

of

0.3%K/CeMoTiOx shown in Figure 8B were similar to those of CeMoTiOx, which has been discussed above in section 3.3. After the introduction of NO+O2, the peak positions and disappearance tendency of the adsorbed ammonia species for 0.3%K/CeMoTiOx were almost the same as those for CeMoTiOx. It indicated that Eley-Rideal mechanism still existed after being doped with potassium. However, the disappearance speed of ammonia species on 0.3%K/CeMoTiOx was much slower, and no adsorbed nitrogen species obviously showed up in the 20 min. After introducing NH3 for 2 min as shown in Figure 9A, the NH3 adsorption peaks of CeMoTiOx at 1426 cm-1 (assigned to ionic NH4+) and 1215 cm-1 (assigned to coordinated NH3) showed up apparently, and the pre-existing NOx adsorption peaks decreased gradually and faded away until 5 min later. The result indicated a Langmuir-Hinshelwood mechanism for the SCR reaction over CeMoTiOx.45,46 After being doped with potassium, a peak at 1606 cm-1 (assigned to bidentate nitrate) for 0.3%K/CeMoTiOx was observed and the peak intensity was relatively weak (Figure 9B). The NOx adsorption peak of 0.3%K/CeMoTiOx disappeared immediately after the introduction of NH3, and the NH3 adsorption peaks at 1426, 1601 and 1215 cm-1 showed up respectively right away. This result indicated that Langmuir-Hinshelwood mechanism still existed after being doped with potassium. 3.7 Steady-State Kinetic Study 3.7.1 SCR reaction rates As shown in Figure 10A, the SCR reaction rates over CeMoTiOx increased 13



proportionately with the increase of NO concentration. After the calculation by linear fit, we can see that the correlation coefficients attained over 0.991. These results indicated that the SCR reaction order over CeMoTiOx with respect to gas concentration of NO is 1, which is consistent with our previous results for a Ce-W-Ti catalyst.4 After being doped with potassium, the trend of SCR reaction rates is in accordance with that of the fresh CeMoTiOx, and the correlation coefficients can reach 0.992 as well. However, the SCR reaction rates over 0.3%K/CeMoTiOx shown in Figure 10B were significantly less than those over CeMoTiOx, which indicates that potassium can restrain the SCR reaction rates. This result is in consistent with the SCR activity results in Figure 1. 3.7.2 NSCR reaction rates The NSCR reaction rates represented by the N2O formation rates over CeMoTiOx were shown in Figure 11A. With the increase of NO concentration, the NSCR reaction rates over CeMoTiOx remained more or less flat despite a negligibly rise at 450 oC, which indicates that the reaction order of NSCR with respect to NO concentration is nearly 0. After being doped with potassium, the NSCR reaction rates increased at 350 oC and 400 oC significantly and showed a different trend of increasing gradually with NO concentration at 450 oC (Figure 11B). This result indicated that potassium could slightly impact the N2O formation over CeMoTiOx, which was one of the reasons for the decline of N2 selectivity at high temperature, as shown in Figure 1. 14


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3.7.3 C-O reaction rates As shown in Figure 12A, the C-O reaction rates, representing the oxidation rates of NH3 to NO, were pretty low below 400 oC over CeMoTiOx. When the reaction temperature rose to 450 oC, the C-O reaction rates enhanced significantly. The C-O reaction rates decreased with the increase of NO concentration, which indicates that the reaction order of C-O with respect to NO concentration is below 0. After being doped with potassium, there was a clearly enhancement of the C-O reaction rates above 400 oC, as shown in Figure 12B, which indicates that more NH3 could be oxidized to NO in the high temperature region. Due to the competition with SCR reaction, the enhancement of C-O reaction should be an important reason for the decrease of NOx conversions at high temperature, as shown in Figure 1. 4 Conclusions The inhibition effects of potassium on the selective catalytic reaction of NOx with NH3 over a CeMoTiOx catalyst were investigated by characterization and kinetic methods. The CeMoTiOx catalyst exhibited high SCR activity and high N2 selectivity during SCR reaction. After being poisoned by potassium, the SCR activity and N2 selectivity both decreased in the whole temperature range. However, the SCR activity of the poisoned CeMoTiOx could be substantially regenerated by water washing method. The H2-TPR test showed that potassium induces an inhibition on the reduction capacity of CeMoTiOx, which is associated with the lower SCR activity of 0.3%K/CeMoTiOx. The separated NO oxidation results indicated that potassium could 15



suppress the oxidation of NO to NO2, which would result in the inhibition of low-temperature SCR activity over CeMoTiOx. The separated NH3 oxidation results showed that potassium promotes the oxidation of NH3 to NOx and N2O, which could lead to a decrease of N2 selectivity over CeMoTiOx. Meanwhile, TPD and in situ DRIFTS results indicated that 0.3%K/CeMoTiOx has lower NOx and NH3 adsorption capabilities, which should be an important reason for the inhibitory effects on NH3-SCR activity. In addition, the kinetic method results demonstrated that potassium has a great inhibitory effect on SCR reaction rates in the whole temperature range, while it could enhance both NSCR and C-O reaction rates in the high temperature region.

Acknowledgements We gratefully acknowledge the financial supports from the National Key R&D Program of China (2017YFC0212502) and the National Natural Science Foundation of China (51308296).

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Table 1. Adsorption Capacities of CeMoTiOx and 0.3%K/CeMoTiOx at 50 oC (μmol g-1) NH3

NOx

CeMoTiOx

319

15

0.3%K/CeMoTiOx

262

14

17



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Figure captions: Figure 1. (A) NOx conversion (B) N2 selectivity during NH3-SCR reactions over CeMoTiOx,

0.3%K/CeMoTiOx

and

regenerated

0.3%K/CeMoTiOx.

Reaction

conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol%, N2 balance, GHSV = 150,000 h-1. Figure 2. XRD patterns of CeMoTiOx and 0.3%K/CeMoTiOx. Figure 3. H2-TPR profiles of CeMoTiOx and 0.3%K/CeMoTiOx. Figure 4. (A) NH3-TPD and (B) NOx-TPD over CeMoTiOx and 0.3%K/CeMoTiOx. Figure 5. DRIFT spectra for CeMoTiOx and 0.3%K/CeMoTiOx at 175 oC of (A) NH3 saturation adsorption (B) NOx saturation adsorption. Figure 6. NH3 oxidation over CeMoTiOx and 0.3%K/CeMoTiOx. Reaction conditions: [NH3] = 500 ppm, [O2] = 5 vol%, N2 balance, GHSV = 150,000 h-1. Figure 7. NO oxidation over CeMoTiOx and 0.3%K/CeMoTiOx. Reaction conditions: [NO] = 500 ppm, [O2] = 5 vol%, N2 balance, GHSV = 150,000 h-1. Figure 8. DRIFT spectra taken at 175oC upon passing NO+O2 over NH3 presorbed (A) CeMoTiOx (B) 0.3%K/CeMoTiOx. Figure 9. DRIFT spectra taken at 175oC upon passing NH3 over NO+O2 presorbed (A) CeMoTiOx and (B) 0.3%K/CeMoTiOx. Figure 10. SCR reaction rates as a function of gaseous NO concentration over (A) CeMoTiOx and (B) 0.3%K/CeMoTiOx. Reaction conditions: [NO] = 300-700 ppm, [NH3] = 500 ppm, [O2] = 5 vol%, N2 balance, GHSV = 240,000-9,000,000 cm-1g-1h-1.

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Figure 11. NSCR reaction rates as a function of gaseous NO concentration over (A) CeMoTiOx and (B) 0.3%K/CeMoTiOx. Reaction conditions: [NO] = 300-700 ppm, [NH3] = 500 ppm, [O2] = 5 vol%, N2 balance, GHSV = 240,000-9,000,000 cm-1g-1h-1. Figure 12. C-O reaction rates as a function of gaseous NO concentration over (A) CeMoTiOx and (B) 0.3%K/CeMoTiOx. Reaction conditions: [NO] = 300-700 ppm, [NH3] = 500 ppm, [O2] = 5 vol%, N2 balance, GHSV = 240,000-9,000,000 cm-1g-1h-1.

19



Figure 1

20


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Figure 2

21



Figure 3

22


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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 9

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Figure 11

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Figure 12

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References (1) Peng, Y.; Li, J.; Shi, W.; Xu, J.; Hao, J. Design strategies for development of SCR catalyst: Improvement of alkali poisoning resistance and novel regeneration method. Environ. Sci. Technol. 2012, 46, 12623. (2) Xiong, S.; Xiao, X.; Liao, Y.; Dang, H.; Shan, W.; Yang, S. Global kinetic study of NO reduction by NH3 over V2O5-WO3/TiO2: relationship between the SCR performance and the key factors. Ind. Eng. Chem. Res. 2015, 54, 11011. (3) Li, X.; Li, X.; Li, J.; Hao, J. Identification of the arsenic resistance on MoO3 doped CeO2/TiO2 catalyst for selective catalytic reduction of NOx with ammonia. J. Hazard. Mater. 2016, 318, 615. (4) Geng, Y.; Huang, H.; Chen, X.; Ding, H.; Yang, S.; Liu, F.; Shan, W. The effect of Ce on a high-efficiency CeO2/WO3-TiO2 catalyst for the selective catalytic reduction of NOx with NH3. RSC Adv. 2016, 6, 64803. (5) Geng, Y.; Shan, W.; Xiong, S.; Liao, Y.; Yang, S.; Liu, F. Effect of CeO2 for a high-efficiency CeO2/WO3-TiO2 catalyst on N2O formation in NH3-SCR: a kinetic study. Catal. Sci. Technol. 2016, 6, 3149. (6) Liu, Z.; Zhang, S.; Li, J.; Ma, L. Promoting effect of MoO3 on the NOx reduction by NH3 over CeO2/TiO2 catalyst studied with in situ DRIFTS. Appl. Catal. B: Environ. 2014, 144, 90. (7) Liu, F.; He, H.; Ding, Y.; Zhang, C. Effect of manganese substitution on the structure and activity of iron titanate catalyst for the selective catalytic reduction of NO with NH3. Appl. Catal. B: Environ. 2009, 93, 194. 32


Page 32 of 39

Page 33 of 39 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


(8) Peng, Y.; Li, K.; Li, J. Identification of the active sites on CeO2-WO3 catalysts for SCR of NOx with NH3: An in situ IR and Raman spectroscopy study. Appl. Catal. B: Environ. 2013, 140-141, 483. (9) Ganduglia-Pirovano, M. V.; Hofmann, A.; Sauer, J. Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges. Surf. Sci. Rep. 2007, 62, 219. (10) Chang, H.; Wu, Q.; Zhang, T.; Li, M.; Sun, X.; Li, J.; Duan, L.; Hao, J. Design strategies for CeO2-MoO3 catalysts for DeNOx and Hg0 oxidation in the presence of HCl: The significance of the surface acid-base properties. Environ. Sci. Technol. 2015, 49, 12388. (11) Chang, H.; Jong, M. T.; Wang, C.; Qu, R.; Du, Y.; Li, J.; Hao, J. Design strategies for P-containing fuels adaptable CeO2-MoO3 catalysts for DeNOx: significance of phosphorus resistance and N2 selectivity. Environ. Sci. Technol. 2013, 47, 11692. (12) Chang, H.; Li, J.; Su, W.; Shao, Y.; Hao, J. A novel mechanism for poisoning of metal oxide SCR catalysts: base-acid explanation correlated with redox properties. Chem. Commun. 2014, 50, 10031. (13) Geng, Y.; Chen, X.; Yang, S.; Liu, F.; Shan, W. Promotional effects of Ti on a CeO2-MoO3 catalyst for the selective catalytic reduction of NOx with NH3. ACS Appl. Mater. Inter. 2017, 9, 16951. (14) Zheng, Y.; Jensen, A. D.; Johnsson, J. E. Laboratory investigation of selective catalytic reduction catalysts: deactivation by potassium compounds and catalyst 33



regeneration. Ind. Eng. Chem. Res. 2004, 43, 941. (15) Zheng, Y.; Jensen, A. D.; Johnsson, J. E. Deactivation of V2O5-WO3-TiO2 SCR catalyst at a biomass-fired combined heat and power plant. Appl. Catal. B: Environ. 2005, 60, 253. (16) Xiong, S.; Weng, J.; Liao, Y.; Li, B.; Zou, S.; Geng, Y.; Xiao, X.; Huang, N.; Yang, S. Alkali metal deactivation on the low temperature selective catalytic reduction of NOx with NH3 over MnOx-CeO2: A mechanism study. J. Phys. Chem. C 2016, 120, 15299. (17) Kling, A.; Andersson, C.; Myringer, A.; Eskilsson, D.; Jaras, S. Alkali deactivation of high-dust SCR catalysts used for NOx reduction exposed to flue gas from 100MW-scale biofuel and peat fired boilers: Influence of flue gas composition. Appl. Catal. B: Environ. 2007, 69, 240. (18) Huang, Z.; Li, H.; Gao, J.; Gu, X.; Zheng, L.; Hu, P.; Xin, Y.; Chen, J.; Chen, Y.; Zhang, Z.; Chen, J.; Tang, X. Alkali- and Sulfur-resistant tungsten-based catalysts for NOx emissions control. Environ. Sci. Technol. 2015, 49, 14460. (19) Lei, T.; Li, Q.; Chen, S.; Liu, Z.; Liu, Q. KCl-induced deactivation of V2O5-WO3/TiO2 catalyst during selective catalytic reduction of NO by NH3: Comparison of poisoning methods. Chem. Eng. J. 2016, 296, 1. (20) Wang, P.; Wang, H.; Chen, X.; Liu, Y.; Weng, X.; Wu, Z. Novel SCR catalyst with superior alkaline resistance performance: enhanced self-protection originated from modifying protonated titanate nanotubes. J Mater. Chem. A 2015, 3, 680. (21) Nicosia, D.; Czekaj, I.; Kröcher, O. Chemical deactivation of V2O5/WO3-TiO2 34


Page 34 of 39

Page 35 of 39 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


SCR catalysts by additives and impurities from fuels, lubrication oils and urea solution. Appl. Catal. B: Environ. 2008, 77, 228. (22) Chen, L.; Li, J.; Ge, M. The poisoning effect of alkali metals doping over nano V2O5-WO3/TiO2 catalysts on selective catalytic reduction of NOx by NH3. Chem. Eng. J. 2011, 170, 531. (23) Du, X.; Gao, X.; Qiu, K.; Luo, Z.; Cen, K. The Reaction of poisonous alkali oxides with vanadia SCR catalyst and the afterward influence: A DFT and experimental study. J. Phys. Chem. C 2015, 119, 1905. (24) Peng, Y.; Li, J.; Si, W.; Luo, J.; Wang, Y.; Fu, J.; Li, X.; Crittenden, J.; Hao, J. Deactivation and regeneration of a commercial SCR catalyst: Comparison with alkali metals and arsenic. Appl. Catal. B: Environ. 2015, 168-169, 195. (25) Peng, Y.; Li, J.; Huang, X.; Li, X.; Su, W.; Sun, X.; Wang, D.; Hao, J. Deactivation mechanism of potassium on the V2O5/CeO2 catalysts for SCR reaction: acidity, reducibility and adsorbed-NOx. Environ. Sci. Technol. 2014, 48, 4515. (26) Li, X.; Li, J.; Peng, Y.; Si, W.; He, X.; Hao, J. Regeneration of commercial SCR catalysts: probing the existing forms of arsenic oxide. Environ. Sci. Technol. 2015, 49, 9971. (27) Jiang, Y.; Xing, Z.; Wang, X.; Huang, S.; Liu, Q.; Yang, J. MoO3 modified CeO2/TiO2 catalyst prepared by a single step sol-gel method for selective catalytic reduction of NO with NH3. J. Ind. and Eng. Chem. 2015, 29, 43. (28) Zhan, S.; Shi, Q.; Zhang, Y.; Li, Y.; Tian, Y. Preparation of novel CeMo(x) hollow microspheres for low-temperature SCR removal of NOx with NH3. RSC Adv. 35



2016, 6, 59185. (29) Li, X.; Li, Y. Selective Catalytic reduction of NO with NH3 over Ce-Mo-Ox Catalyst. Catal. Lett. 2013, 144, 165. (30) Li, X.; Li, J.; Peng, Y.; Li, X.; Li, K.; Hao, J. Comparison of the structures and mechanism of arsenic deactivation of CeO2-MoO3 and CeO2-WO3 SCR catalysts. J. Phys. Chem. C 2016, 120, 18005. (31) Li, P.; Xin, Y.; Li, Q.; Wang, Z.; Zhang, Z.; Zheng, L. Ce-Ti amorphous oxides for selective catalytic reduction of NO with NH3: Confirmation of Ce-O-Ti active sites. Environ. Sci. Technol. 2012, 46, 9600. (32) Chen, L.; Li, J.; Ge, M. DRIFT study on cerium-tungsten/titiania catalyst for selective catalytic reduction of NOx with NH3. Environ. Sci. Technol. 2010, 44, 95990. (33) Wu, Z.; Jiang, B.; Liu, Y.; Wang, H.; Jin, R. DRIFT study of manganese/titania-based satalysts for low-temperature selective catalytic reduction of NO with NH3. Environ. Sci. Technol. 2007, 41, 5812. (34) Kijlstra, W. S.; Brands, D. S.; Poels, E. K.; Bliek, A. Mechanism of the selective catalytic reduction of NO by NH3 over MnOx/Al2O3. J. Catal. 1997, 171, 208. (35) Qi, G.; Yang, R. T.; Chang, R. MnOx-CeO2 mixed oxides prepared by co-precipitation for selective catalytic reduction of NO with NH3 at low temperatures. Appl. Catal. B: Environ. 2004, 51, 93. (36) Liu, C.; Chen, L.; Li, J.; Ma, L.; Arandiyan, H.; Du, Y.; Xu, J.; Hao, J. Enhancement of activity and sulfur resistance of CeO2 supported on TiO2-SiO2 for the selective catalytic reduction of NO by NH3. Environ. Sci. Technol. 2012, 46, 6182. 36


Page 36 of 39

Page 37 of 39 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


(37) Underwood, G. M.; Miller, T. M.; Grassian, V. H. Transmission FT-IR and Knudsen cell study of the heterogeneous reactivity of gaseous nitrogen dioxide on mineral oxide particles. J. Phys. Chem. A 1999, 103, 6184. (38) Peng, Y.; Si, W.; Li, X.; Chen, J.; Li, J.; Crittenden, J.; Hao, J. Investigation of the poisoning mechanism of lead on the CeO2-WO3 catalyst for the NH3-SCR reaction via in situ IR and Raman spectroscopy measurement. Environ. Sci. Technol. 2016, 50, 9576. (39) Chen, L.; Li, J.; Ge, M. DRIFT Study on cerium-tungsten/titiania catalyst for selective catalytic reduction of NOx with NH3. Environ. Sci. Technol. 2010, 44, 9590. (40) Shan, W.; Liu, F.; He, H.; Shi, X.; Zhang, C. A superior Ce-W-Ti mixed oxide catalyst for the selective catalytic reduction of NOx with NH3. Appl. Catal. B: Environ. 2012, 115-116, 100. (41) Shan, W.; Geng, Y.; Chen, X.; Huang, N.; Liu, F.; Yang, S. A highly efficient CeWOx catalyst for the selective catalytic reduction of NOx with NH3. Catal. Sci. Technol. 2016, 6, 1195. (42) Yang, S.; Guo, Y.; Chang, H.; Ma, L.; Peng, Y.; Qu, Z.; Yan, N.; Wang, C.; Li, J. Novel effect of SO2 on the SCR reaction over CeO2: Mechanism and significance. Appl. Catal. B: Environ. 2013, 136-137, 19. (43) Huang, H.; Shan, W.; Yang, S.; Zhang, J. Novel approach for a cerium-based highly-efficient catalyst with excellent NH3-SCR performance. Catal. Sci. Technol. 2014, 4, 3611. (44) Li, B.; Xiong, S.; Liao, Y.; Xiao, X.; Huang, N.; Geng, Y.; Zou, S.; Yang, S. 37



Why the Low-Temperature Selective Catalytic Reduction Performance of Cr/TiO2 Is Much Less than That of Mn/TiO2: A Mechanism Study. J. Phys. Chem. C 2016, 120, 23511. (45) Xiao, X.; Xiong, S.; Shi, Y.; Shan, W.; Yang, S. Effect of H2O and SO2 on the Selective Catalytic Reduction of NO with NH3 Over Ce/TiO2 Catalyst: Mechanism and Kinetic Study. J. Phys. Chem. C 2016, 120, 1066. (46) Chang, H.; Ma, L.; Yang, S.; Li, J.; Chen, L.; Wang, W.; Hao, J. Comparison of preparation methods for ceria catalyst and the effect of surface and bulk sulfates on its activity toward NH3-SCR. J. Hazard. Mater. 2013, 262, 782.

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Deactivation effects of potassium on a CeMoTiOx catalyst for the

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