Deactivation of Ce-Ti Oxide Catalyst by K3PO4 for the Selective Catalytic Reduction of NO with NH3

Received: July 2, 2018
Revised: November 20, 2018
Accepted: December 10, 2018

Download Citation: RIS|BibTeX|https://doi.org/10.4209/aaqr.2018.06.0215  

Cite this article:

Jiang, Y., Lai, C., Liu, S., Liang, G., Bao, C., Shi, W. and Ma, S. (2019). Deactivation of Ce-Ti Oxide Catalyst by K3PO4 for the Selective Catalytic Reduction of NO with NH3. Aerosol Air Qual. Res. 19: 422-430. https://doi.org/10.4209/aaqr.2018.06.0215

HIGHLIGHTS

Keywords: Selective catalytic reduction; NO; NH3; K3PO4; Ce-Ti oxide; Activity.

INTRODUCTION

 Anthropogenic emission of nitrogen oxides (NO_x, x = 1, 2) has attracted extensive concern in recent years because of its serious threat to environment and human health (Li et al., 2017; Yu et al., 2017). Selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) has been considered as one of most effective techniques for NO_x removal in stationary sources. Despite the fact that V₂O₅/TiO₂ mixed with WO₃ or MoO₃ has been industrially applied during the past decades, several drawbacks associated with this type of catalyst have to be noticed, such as toxicity of vanadium species, high oxidization activity of SO₂ to SO₃ (Dunn et al., 1998; Du et al., 2018), formation of greenhouse gas N₂O at high temperatures (Yates et al., 1996) and catalyst deactivation by the exposure to ash compounds present in flue gas (Chen et al., 2011; Jiang et al., 2014; Chen et al., 2018; Wang et al., 2018). Among promising substitutes for commercial V₂O₅/TiO₂-based catalysts, in the last years, considerable attention has been paid to the activities of ceria-based catalysts due to high oxygen storage capacity and outstanding redox property of ceria (Guo et al., 2013; Jiang et al., 2017a; Jiang et al., 2018). From the point of view of industrial applications, it is of great interest to study the influence of flue gas components (e.g., alkali metal, alkaline earth metal, heavy metal, etc.) on the catalytic behaviors of ceria-based catalysts.

As far as the effect of potassium on the SCR of NO with NH₃ over ceria-based SCR catalysts is concerned, previous studies mainly focused on K₂O. Wang et al. (2013) reported the strong inhibition by K₂O for the activity of Ce/TiO₂ catalyst. Du et al. (2012) attributed the deactivation by K₂O of Ce-Ti oxide to the decrease in its reducibility and surface acidity. Wang et al. (2017) revealed that the deposition of K₂O could suppress the adsorption of NH₃ on the surface of Ce/TiO₂ catalyst, thereby resulting in the loss of its activity. Gao et al. (2014) compared the effect of K₂O on the SCR activities of ceria supported on zirconia with and without sulfation treatment. Peng et al. (2012) proposed that the decrease in the quantity of Brønsted acid sites was responsible for the deactivation by K₂O of CeO₂-WO₃ catalyst. In fact, potassium can be also bound as chloride, sulfate or phosphate, which is largely determined by fuel composition and reaction conditions (Beck et al., 2004; Zhuo et al., 2012; Jang et al., 2016).

Recently, co-combustion of coal with secondary fuels such as municipal sewage sludge, straw, wood or meat and bone meal (MBM) received increasing attention. Considering that these secondary fuels are rich in phosphorus and potassium, their interaction could result in the formation of more potassium phosphate in flue gas during the co-combustion of coal with secondary fuels, compared with coal combustion (Zhuo et al., 2012). Potassium phosphate will be deposited on the surface of SCR catalysts and unavoidably have a further effect on their activities. Because of practical interests, quite a few researchers have investigated the influence of K₃PO₄ on V₂O₅/TiO₂-based catalysts (Beck et al., 2004; Castellino et al., 2009). However, the effect of K₃PO₄ on ceria-based catalysts has been ignored and hardly ever studied until our present work. In addition, different K species would exert a different effect on catalytic behaviors (Zhang et al., 2015; Du et al., 2017). Consequently, it is indispensable to further investigate the effect of K₃PO₄ on the SCR of NO with NH₃ over Ce-Ti oxide catalyst. These catalysts were characterized by means of BET, XRD, XPS, NH₃-TPD, H₂-TPR and catalytic activity measurements in this work.

EXPERIMENTAL

 
Catalyst Preparation

Ce-Ti oxide was prepared by a single-step sol-gel method and denoted as CT. The CeO₂ loading was set to 20% (the mass ratio of CeO₂/TiO₂). Butyl titanate (0.1 mol), anhydrous ethanol (3.5 mol), deionized water (1.9 mol), nitric acid (0.2 mol) and cerium nitrate (0.01 mol) were mixed and stirred violently at room temperature. After 3 h, the mixture was dried at 80°C for 24 h to form xerogel. At last, the xerogel was calcined at 500°C for 6 h.

The K₃PO₄- and H₃PO₄-doped catalysts were prepared by impregnation via incipient wetness with the aqueous solutions of required concentrations of K₃PO₄ and H₃PO₄ on the Ce-Ti oxide, respectively. The samples were impregnated at room temperature for 4 h. Then the samples were dried at 110°C for 12 h. As for the P₂O₅-doped catalyst, H₃PO₄ was used as the precursor of P₂O₅ and impregnated on the Ce-Ti oxide at room temperature for 4 h. After drying at 110°C for 12 h, the sample was calcined at 500°C for 5 h. A series of K₃PO₄-doped samples were denoted by “KPCT(x)” and “x” was the molar ratio of K to Ce.

Characterization of Catalysts

The physical properties of the samples were measured by N₂ adsorption and desorption at 77 K with ASAP 2020-M (Micromeritics Instrument Corp.). According to the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, BET surface area, total pore volume and average pore diameter could be determined.

X-ray diffraction (XRD) patterns were obtained on an X’Pert PRO MPD system (Panalytical Corp.) with Cu Kα radiation operating at 40 kV and 40 mA. The XRD data were collected in the scattering angles (2θ) ranging between 10 and 90° with a scanning rate of 5° min^–1.

The X-ray photoelectron spectroscopy (XPS) spectra were collected on a Thermo ESCALAB 250 spectrometer using monochromated Al Kα X-ray radiation (hv = 1486.6 eV) at 150 W. The binding energies of K 2p, Ti 2p, Ce 3d and O 1s were calibrated by measuring the reference peak of C 1s (BE = 284.6 eV) from adventitious carbon in order to eliminate sample charging effect.

The analysis on temperature-programmed desorption (NH₃-TPD) and temperature-programmed reduction (H₂-TPR) were carried out on a FINESORB-3010 chemisorption analyzer (FINETEC Instruments Corp.) with a thermal conductivity detector (TCD).

Catalytic Activity Tests

The SCR activities of the samples were tested in a fixed-bed quartz tubular flow reactor (i.d. = 8 mm) containing 0.34 g catalyst with 60–100 mesh in the temperature range of 150–500°C. The feed gas consisted of 1000 ppm NO, 1000 ppm NH₃, 3% O₂ and balance N₂. The total flow rate was 500 mL min^–1, corresponding to a gas hourly space velocity (GHSV) of 90,000 h^–1. The concentrations of NO and O₂ were monitored by a flue gas analyzer (350 Pro, Testo). The concentrations of NH₃, N₂O and NO₂ were analyzed by a FT-IR gas analyzer (DX-4000, Gasmet). The activity data were collected and recorded after 30 min when the SCR reaction reached a steady state at each temperature.

RESULTS AND DISCUSSION

 
SCR Activity Tests

Fig. 1 compares NO conversion as a function of temperatures over Ce-Ti oxides doped with different P species. As for H₃PO₄- and P₂O₅-doped samples, the molar ratio of P/Ce was 0.5:1. Over CT-K₃PO₄, the molar ratio of K/P/Ce was 0.5:0.2:1. It could be seen that the deactivation of CT followed the sequence: CT-P₂O₅ < CT-H₃PO₄ < CT-K₃PO₄. It was clear that different P species would exert a different effect on catalytic behaviors of CT. K₃PO₄ led to most serious deactivation of CT compared with the other two P species. After that, the effect of K₃PO₄ loadings on the catalytic behaviors of CT was investigated and the results are shown in Fig. 2. The catalytic activity of CT deceased with increasing K₃PO₄loadings in the temperature range of 150–500°C. When the molar ratio of K/Ce reached 0.25, the maximum NO conversion fell to only less than 30% and CT had lost most of its activity. These results indicated that K₃PO₄ acted as a strong inhibitor on the catalytic activity of CT. 

Fig. 1. Variation of NO conversion with temperatures over Ce-Ti oxides doped with different P species. Reaction condition: [NO] = [NH₃] = 1,000 ppm, [O₂] = 3%, N₂ balance, total flow rate = 500 mL min^–1, and GHSV = 90,000 h^–1.

Fig. 2. Variation of NO conversion with temperatures over Ce-Ti oxides with different K₃PO₃ loadings. Reaction condition: [NO] = [NH₃] = 1,000 ppm, [O₂] = 3%, N₂ balance, total flow rate = 500 mL min^–1, and GHSV = 90,000 h^–1.

 The oxidation activities of NO to NO₂ by O₂ were measured over the fresh CT and KPCT(0.5) and the results are presented Fig. 3. As for the fresh CT, the NO oxidation increased with increasing temperatures and reached the maximum of 13.9% at 450°C. Further increasing temperatures led to the drop in the NO oxidation. After adding K₃PO₄, the NO oxidation decreased obviously. It is established that the existence of NO₂ can promote the SCR reaction via the “fast SCR reaction” of 2NH₃ + NO + NO₂ → 2N₂ + 3H₂O (Fedeyko et al., 2010; Ma et al., 2013). It implied that K₃PO₄ could inhibit the oxidation of NO to NO₂, thereby decreasing the SCR activity of CT.

Fig. 3. Oxidation of NO to NO₂ by O₂ over CT and KPCT(0.5). Reaction condition: [NO] = 1,000 ppm, [O₂] = 3%, N₂ balance, total flow rate = 500 mL min^–1, and GHSV = 90,000 h^–1.

NH₃ oxidation, as an important side reaction, can cause the decrease in SCR activity at high temperatures (Ma et al., 2013). NH₃ oxidation tests were performed in the feed gas without NO. The concentrations of N₂O and NO₂ were lower than 7 ppm over the two samples in the temperature range of 150–500°C. Therefore, the NH₃ oxidation results only contained the NO concentration curves (see Fig. 4). It could be seen from Fig. 4 that the NH₃ oxidation started at about 350°C and increased sharply with temperatures over the fresh CT. After doping K₃PO₄, NO concentration increased at 350–500°C obviously, while no obvious change was observed below 350°C. This indicated that the presence of K₃PO₄ could result in the formation of more NO at high temperatures, which led to the decrease in the SCR activity of CT.

Fig. 4. NO formation over CT and KPCT(0.5). Reaction condition: [NH₃] = 1,000 ppm, [O₂] = 3%, N₂ balance, total flow rate = 500 mL min^–1, and GHSV = 90,000 h^–1.

 
Characterization of Catalysts

XRD Results

The XRD patterns of Ce-Ti oxides with various K₃PO₄ loadings are illustrated in Fig. 5. For the fresh CT, only anatase TiO₂ could be detected while the characteristic diffraction peaks of Ce species were not found. This indicated that Ce species were well dispersed and existed as an amorphous or highly dispersed phase on the surface of anatase TiO₂. After doping K₃PO₄, the characteristic peaks belonging to Ce, K or P species were not observed. Du et al. (2012) found similar phenomena when they studied the effect of K and Na on Ce-Ti oxide prepared by a co-precipitation method. It meant that the dispersion of CeO₂was barely influenced by K₃PO₄. However, the intensity of the characteristic peaks of anatase TiO₂ was found to decrease over KPCT(0.5). Our previous study found a similar phenomenon over Ca-doped Ce-Ti oxide catalysts (Jiang et al., 2017b). The mean crystallite size of anatase was evaluated using the Scherrer equation. When the molar ratio of K/Ce was lower than 0.5, the grain size of anatase was about 4.8 nm. As for KPCT(0.5), the grain size of anatase decreased to 3.4 nm. The mean crystallite sizes of anatase in different samples agreed well with the XRD results. It indicated that there existed a strong interaction between K₃PO₄ and TiO₂. 

Fig. 5. XRD patterns of Ce-Ti oxides with different K₃PO₃ loadings.

 
N₂ Adsorption-Desorption Results

Fig. 6 shows the N₂ adsorption-desorption isotherms and corresponding pore size distribution curves of the fresh and K₃PO₄-doped CT catalysts. Both samples had a typical “IV” isotherm, suggestive of mesoporous structures (Leofantia et al., 1998). As shown in Fig. 6(b), the pore distribution of the fresh CT was centered at around 6.2 nm, which could be attributed to the pore structure of the TiO₂ support. After doping K₃PO₄, the pore size was slightly shifted to a higher value while the pore volume decreased a little. It confirmed that the impregnation of K₃PO₄only caused slight pore blocking.

Fig. 6. (a) N₂ adsorption-desorption isotherms and (b) pore size distribution curves of CT and KPCT(0.5).
 

 
XPS Results

Table 1 lists the concentration of various atoms on the fresh and K₃PO₄-doped CT catalysts. The presence of K₃PO₄ resulted in the slight decrease in the concentration of active Ce atom from 30.53 at.% to 30.18 at.%. The concentrations of Ti and O were also found to fall, compared with those on the surface of the fresh CT. It seemed probably to be originated from the fact that the catalyst surface might be covered with K₃PO₄during its impregnation process. K₃PO₄ hindered the XPS detection of Ce, Ti and O atoms, which lay under K₃PO₄. In our previous study on the effect of PbCl₂ on V₂O₅/TiO₂, similar phenomena were also observed (Jiang et al., 2014).

Fig. 7 shows XPS spectra of K 2p, Ti 2p, Ce 3d and O 1s. As shown in Fig. 7(a), the binding energy of the 2p_3/2 peak was 293.2 eV in the K 2p XPS spectra of KPCT(0.5). Referring to the handbook of XPS (Wagner et al., 1979), it could be inferred that K existed in the form of K₃PO₄ on the surface of KPCT(0.5).

Fig. 7. XPS spectra of (a) K 2p, (b) Ti 2p, (c) Ce 3d, and (d) O 1s of CT and KPCT(0.5).

For the fresh CT, two main peaks ascribed to Ti 2p_1/2 and Ti 2p_3/2 were observed at about 464.5 eV and 458.4 eV, respectively (see Fig. 7(b)). It indicated that Ti was present in the form of Ti⁴⁺ (Wagner et al., 1979). After doping K₃PO₄, the binding energies of Ti 2p were hardly changed. It implied that the presence of K₃PO₄ had almost no impact on the chemical state of Ti. Ti still existed as Ti⁴⁺ in KPCT(0.5). However, the intensity of the Ti 2p XPS peaks was found to decrease to a certain extent. This might be ascribed to an interaction between K₃PO₄ and Ti, which was also demonstrated in XRD results.

According to the convention established by Burroughs et al. (1976), the XPS spectra of Ce 3d could be deconvoluted into eight overlapped peaks. The peaks labeled as uʹ and vʹ represented the 3d¹⁰4f¹ initial electronic state corresponding to surface Ce³⁺, while the other ones were assigned to the 3d¹⁰4f⁰ initial electronic state of surface Ce⁴⁺ species (Burroughs et al., 1976). As exhibited in Fig. 7(c), Ce existed in both forms of Ce³⁺ and Ce⁴⁺ on the surface of the fresh CT. It is widely accepted that the Ce³⁺/Ce⁴⁺ pairs contribute to the storage and release of active oxygen species and the oxidation of NO to NO₂ (Geng et al., 2017). Furthermore, a higher Ce³⁺ ratio is indicative of more oxygen vacancies, which help to adsorb reactants (Liu et al., 2013a; Liu et al., 2013b). All of these factors are favorable to the SCR reaction. According to the area ratio of peaks ascribed to Ce³⁺ and Ce⁴⁺, the ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) could be calculated. After doping K₃PO₄, the ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) decreased from 0.48 to 0.29. It was clear that the amount of Ce³⁺ decreased after doping K₃PO₄. The oxidation state of Ce species on the catalyst surface was changed remarkably. The decrease in the amount of Ce³⁺ resulted from the fact that the interaction between K₃PO₄ and Ce species suppressed the reduction of Ce⁴⁺ to Ce³⁺ in SCR reaction. Consequently, the reducibility of the CT was lessened. In addition, K₃PO₄ acted as an inhibitor on the electron transfer and resulted in the decrease in the amount of oxygen vacancies, which was disadvantageous to the adsorption and activation of reactive species (Geng et al., 2017). In a word, the decrease in the amount of Ce³⁺ and the degradation of reducibility should be responsible for a strong inhibition of K₃PO₄ in the activity of the Ce-Ti oxide.

The O 1s XPS curves of different samples were made up of three overlap peaks, as illustrated in Fig. 7(d). The peaks at 527.7–530.6 eV corresponded to lattice oxygen O^2– in the metal oxides (denoted as O_α), the peaks at 530.6–531.1 eV belonged to surface labile oxygen such as O^–or OH from defect-oxide or hydroxyl-like groups (denoted as O_β), and the peaks at 531.1–533.5 eV were assigned as chemisorbed water (denoted as O_γ) (Dupin et al., 2000; Eom et al., 2008; Lee and Bai, 2018). Owing to its higher mobility, O_β is considerably more active than O_α and O_γ(Jiang et al., 2018). High O_β ratio is beneficial for the oxidation of more NO to NO₂ in the SCR process (Wu et al., 2008; Chen et al., 2009). It was reported that the reaction rate of NH₃ with the mixture of NO and NO₂ was faster than that with NO alone, especially at low temperature (Long and Yang, 1999). Therefore, high O_β ratio is indicative of superior SCR activity. After doping K₃PO₄, the ratio of O_β/(O_α + O_β + O_γ) was found to decrease from 0.35 to 0.23, which indicated less surface chemisorbed oxygen on the surface of K₃PO₄-doped CT. This might be due to the fact that potassium ions easily occupied oxygen vacancies since they had similar ionic radius with oxygen ions (Zhang et al., 2014). These results confirmed that the decrease of Ce³⁺ was accompanied by the decrease in oxygen vacancies and active oxygen species, which played a negative role in the SCR activity of CT.

NH₃-TPD Results

NH₃-TPD analysis was performed to study the effect of K₃PO₄ on the surface acidity of CT and the results are shown in Fig. 8. The fresh CT exhibited a broad NH₃ desorption peak in the temperature range of 100–500°C and a small peak at 500–700°C. After deconvolution and peak-fitting, the broad peak could be separated into two peaks centered at about 205°C and 310°C. It is known that the ammonia adsorbed on Brønsted acid sites has less thermal stability than that adsorbed on Lewis acid sites during TPD process (Phil et al., 2008). As a result, the low- and medium-temperature peaks could be assigned to the weak and moderate adsorption of ammonia on Brønsted acid sites (Cai et al., 2014). The small high-temperature peak could be attributed to the NH₃ species strongly adsorbed on Lewis acid sites (Zhao et al., 2016; Duan et al., 2017). It was clear that most of adsorbed NH₃ was coordinated to Brønsted acid sites, which played a predominant role in SCR reaction. After doping K₃PO₄, no significant change was observed on the high-temperature NH₃ desorption peak. This indicated that the Lewis acid sites on the surface of CT were hardly influenced by K₃PO₄. However, the other two peaks shifted to lower temperatures, while their peak areas decreased considerably, especially the medium-temperature peak. Tang et al. (2010) also found similar changes on the NH₃-TPD profiles of Na⁺-V₂O₅/TiO₂ catalysts. It is accepted that the peak location is closely related to the adsorption strength and the peak area corresponds to the amount of the adsorbed NH₃ (Cai et al., 2014). After doping K₃PO₄, though the NH₃-adsorbed species could be more easily desorbed at low temperatures, the tremendous decrease in the Brønsted acidity resulted in the great weakness of the ability of CT to adsorb NH₃. Ce³⁺ was reported to be related to the formation of Brønsted acid sites (Shu et al., 2012). It would be worthwhile to notice that the decrease in the amount of Ce³⁺ on the surface of K₃PO₄-doped CT had been demonstrated by the above XPS results. Therefore, the significant decrease in the amount of Brønsted acid sites is one of the main reasons for the loss of the activity of CT after doping with K₃PO₄.

 Fig. 8. NH₃-TPD profiles of CT and KPCT(0.5).

H₂-TPR Results

Fig. 9 displays the H₂-TPR profiles of the fresh and K₃PO₄-doped CT catalysts. As for the fresh CT, the onset temperature of reduction was at about 320 °C and there were two overlapped reduction peaks centered at around 515°C and 680°C. The peak at low temperature was probably related to the reduction of the surface oxygen of ceria (Ce⁴⁺-O-Ce⁴⁺) and the one at high temperature could be attributed to the reduction of Ce⁴⁺to Ce³⁺ (Liu et al., 2013a; Liu et al., 2018). In contrast, the onset temperature of reduction shifted to a higher value (about 370°C) and only one reduction peak centered at about 600°C was observed over KPCT(0.5). Higher onset and reduction peak temperatures are generally regarded as indicators of the decrease in the reducibility (Liu et al., 2008; Gao et al., 2010; Zhao et al., 2016). It was clear that the presence of K₃PO₄ led to the remarkable drop in the reducibility of CT. It should be noted that this result was exactly in line with that of the above XPS analysis. In correlation with the results of BET, XRD and XPS, the loading of K₃PO₄ made the reduction of CT harder probably because of decreased surface area, less surface oxygen, change in chemical state of Ce species and the interaction of K₃PO₄ with Ce species.

Fig. 9. H₂-TPR profiles of CT and KPCT(0.5).

CONCLUSIONS

K₃PO₄ deactivated the Ce-Ti oxide catalyst seriously for the selective catalytic reduction of NO with NH₃. The fresh and K₃PO₄-doped Ce-Ti oxides were characterized using XRD, BET, XPS, H₂-TPR and NH₃-TPD. The results indicated that K₃PO₄ had no significant effect on the dispersion and crystalline form of Ce species on anatase TiO₂. After doping the catalyst with K₃PO₄, the BET surface area and total pore volume decreased, and the pore were found to be slightly blocked. The XPS results revealed that the interaction between K₃PO₄ and Ce species hindered the transformation of Ce⁴⁺ into Ce³⁺. As a result, the amount of Ce³⁺ decreased, accompanied by a decline in oxygen vacancies and active oxygen species, which played a negative role in the SCR reaction over the Ce-Ti oxide. Both the XPS and the H₂-TPR results demonstrated a decrease in the reducibility of the Ce-Ti oxide after K₃PO₄ doping. In addition, the reduction in surface acidity, mainly due to the decreased number of Brønsted acid sites, may lead to the marked drop in the amount of NH₃ adsorbed on the catalyst surface. Based on these results, the decrease in the amount of Ce³⁺, oxygen vacancies and surface chemisorbed oxygen and the reduction in surface Brønsted acidity and reducibility were primarily responsible for the deactivation of the Ce-Ti oxide by K₃PO₄.

 
ACKNOWLEDGEMENTS

 This work was supported by the National Natural Science Foundation of China (No. 51506226), Natural Science Foundation of Shandong Province (No. ZR2015EM010), the Fundamental Research Funds for the Central Universities (No. 15CX05005A) and the scholarship from China Scholarship Council, China (CSC) (No. 201706455013).

Aerosol Air Qual. Res. 19 :422 -430 . https://doi.org/10.4209/aaqr.2018.06.0215