# Experimental Study on the Deactivating Effect of KNO3, KCl, and K2SO4 on Nanosized Ceria/Titania SCR Catalyst

Authors: Xiongbo Chen; Ping Fang; Wenhao Zeng; Kunyuan Liu; Zhixiong Tang; Chaoping Cen
Journal: Journal of Nanomaterials
 (2015)

---

## Abstract

Nanosized Ce/TiO2 is effective in selective catalytic reduction of NO with NH3. The NO conversion of Ce/TiO2 is 93% at 370°C. However, addition of potassium using KNO3, KCl, or K2SO4 as precursors effectively deactivates Ce/TiO2. NO conversion at 370°C is reduced to 45%, 24%, and 16% after addition of KNO3, KCl, and K2SO4, respectively, with a controlled K/Ce molar ration at 0.25. The deactivation may be attributed to the changes in the structural and chemical state of ceria and the degradation of surface acidity. The transformation of amorphous ceria into ceria crystals after potassium addition, together with the decrease of surface defects, is also determined. Oxygen diffusion in the process of ceria reduction is slow, and the redox cycle is slowed down. Moreover, the surface acid sites are markedly destroyed, leading to the reduced capacity of ammonia adsorption. These results may provide useful information for the application and life management of CeO2/TiO2 in potassium-rich environments such as biofuel-fired boilers.


---

## Body

## 1. Introduction

Selective catalytic reduction (SCR) of NO with ammonia is the most efficient and reliable technology to remove NOx from stationary sources. V2O5(WO3)/TiO2 or V2O5(MoO3)/TiO2 catalysts are widely used in the SCR process. These vanadium-based catalysts are highly efficient. However, V2O5 is an ecotoxic material that is harmful to the environment [1]. To replace vanadium-based catalysts, environment-friendly, nonvanadium catalysts such as Ce/TiO2, Mn2Nb1Ox, MnOx-CeOy, Fe-ZSM-5, Cu-ZSM-5, and FeTiOx have been developed in the past years [2–8].Recently, Ce/TiO2 catalyst has gained recognition because of its excellent activity and selectivity. Xu et al. reported that Ce/TiO2 catalyst is highly efficient at 275–400°C, and the undesired by-product, N2O, could be hardly detected [3]. Gao et al. compared three preparation methods, namely, single step sol-gel method, impregnation method, and coprecipitation method, and found that the Ce/TiO2 catalysts prepared using the single step sol-gel method had the best SCR activity and SO2 resistance. Liu et al. demonstrated the feasibility of a supercritical water synthesis route in the syntheses of Ce/TiO2 catalysts by a strong metal-support interaction [9]. Chen et al. found that tungsten modification could further improve the activity of Ce/TiO2 [10], and Liu et al. used MoO3 modification to enhance this activity. Chen et al. investigated a series of ceria catalysts supported on titanates with various morphologies and structures, including nanoparticle, nanotube, fragment, nanowire, and nanorod; the investigation revealed a good SCR performance of the former three catalysts. Moreover, Ce/TiO2-based catalysts have been commercially produced in rare earth-rich regions such as Shandong, China, and utilized in deNOx facilities in power plants.While nonvanadium SCR catalysts were developed, the deactivation of SCR catalysts by alkali metals and alkaline earth metals has gained popularity. This problem has been proven to be more serious in biofuel boilers because alkali metal content is higher in biofuels than in coal. For vanadium-based catalysts, many studies in literature noted the decrease of surface acidity by potassium, sodium, and calcium compounds, and a few works found the interaction between poison and vanadium sites [11, 12]. Moreover, Strege et al. proposed that the blocking of surface pores is an important reason for this observation. As for Fe-ZSM-5, Kern et al. attributed alkali deactivation to the decreased capability of ammonia adsorption [13]. Similar results of Fe- and Cu-based catalysts supported on TiO2 or ZrO2 were reported by Kustov et al. [14]. With respect to CeO2/TiO2 catalysts, Wang et al. investigated the significant deactivation by sodium and calcium salts and proposed a deactivation mechanism based on the change of the ceria state [15]. Some other catalysts, such as Cu-SAPO-34 and MnOx/TiO2, also encountered a similar deactivation [16, 17]. Considering the extensive knowledge of alkali deactivation of various catalysts, the application of SCR technology to the purification of biofuel flue gas is questionable.In this paper, we investigated the poisoning effect of various potassium compounds on nanosized Ce/TiO2 catalysts in the SCR application. KCl and K2SO4 were selected as the precursors of potassium because the potassium content is very high in the flue gas of boilers firing biofuels, and Cl and S elements always coexist [18, 19]. KNO3 was selected as the precursors of potassium oxide. Fresh and K-poisoned Ce/TiO2 catalysts were subjected to a range of characterizations (e.g., XRD, XPS, NH3-TPD, and H2-TPR), and the deactivation mechanism was discussed.


## 2. Experimental

### 2.1. Preparation of Ce/TiO2 Catalyst

Commercially produced P25 TiO2 (Degussa, Germany) was used as the catalyst support. Cerous nitrate (AR, Ce(NO3)3·6H2O) was used as the precursor of ceria. Ceria was loaded on P25 TiO2 using the wet-impregnation method with a controlled Ce/Ti molar ratio of 1 : 19. In summary, P25 was impregnated in a cerous nitrate solution, and the mixture was stirred for 4 h, dried at 80°C for 12 h, and calcined at 450°C for 3 h.


### 2.2. Addition of Potassium Compounds

As described in previous reports, adding potassium in various concentrations was always conducted by the wet-impregnation method to simulate the poisoning mechanism of potassium in real flue gas at laboratory [16, 20]. In this paper, KNO3, KCl, and K2SO4 were dissolved in distilled water and impregnated with Ce/TiO2 catalysts. The mixture was stirred vigorously for 4 h, dried at 80°C for 12 h, and calcined at 450°C for 3 h. The K/Ce molar ratio was controlled at 0.25, 0.5, 1, and 2. The prepared catalysts were denoted as Ce/TiO2-x-y, where x are the precursors (KNO3, KCl, and K2SO4) and y is the K/Ce molar ratios (0.25, 0.5, 1, and 2).


### 2.3. SCR Activity Evaluation

SCR activities on fresh and K-poisoned Ce/TiO2 catalysts were tested in a fixed-bed reactor. The typical reactant gas composition was as follows: 750 ppm NO, 750 ppm NH3, 1.5% H2O, 3.5% O2, and balanced N2. The catalyst dosage was 0.5 g. The gas hourly space velocity (GHSV) was approximately 100,000 h−1. NO, NO2, and O2 concentrations were monitored by a flue gas analyzer (KM9106, Quintox Kane International Limited). N2O was detected by a FT-IR gas analyzer (Madur Photon Portable IR Gas Analysers, Madur Ltd., Austria).


### 2.4. Characterization Methods

XRD analysis was performed using X-ray diffraction with Cu Kα radiation (model D/max RA, Rigaku Co., Japan). The data were collected for scattering angles (2θ) ranging between 5° and 80° with a step size of 0.02°. X-ray photoelectron spectroscopy with Al Kα X-ray (h
ν
=
1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties and probe the electronic state of the elements. The microstructures were observed using a scanning electron micrograph (SEM) in a Phillips XL-30-ESEM system with a voltage of 15 kV. Nitrogen adsorption-desorption isotherms were obtained using a nitrogen adsorption apparatus (ASAP 2020, USA). All the samples were degassed at 200°C prior to measuring. The Brunauer-Emmett-Teller (BET) specific surface area (SBET) was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range from 0.05 to 0.30. Temperature programmed desorption with ammonia (NH3-TPD) and temperature programmed reduction with hydrogen (H2-TPR) experiments were carried out using a TP-5080 instrument (Tianjin Xianquan Industry and Trade Development Co. Ltd., China). Prior to the NH3-TPD experiments, 100 mg samples were pretreated in pure N2 at 350°C for 1 h and then saturated with anhydrous NH3 (4% in N2) at room temperature. Desorption was carried out by heating the samples in N2 (30 mL/min) from 70°C to 800°C with a heating rate of 10°C/min. Before raising the temperature, a preheat treatment at 70°C for 1 h was conducted. For the TPR experiments, 50 mg samples were pretreated in pure N2 at 350°C for 1 h and cooled to 70°C. The H2-TPR runs were carried out with a linear heating rate of 10°C/min from 70°C to 800°C in H2 (4% in N2).


## 2.1. Preparation of Ce/TiO2 Catalyst

Commercially produced P25 TiO2 (Degussa, Germany) was used as the catalyst support. Cerous nitrate (AR, Ce(NO3)3·6H2O) was used as the precursor of ceria. Ceria was loaded on P25 TiO2 using the wet-impregnation method with a controlled Ce/Ti molar ratio of 1 : 19. In summary, P25 was impregnated in a cerous nitrate solution, and the mixture was stirred for 4 h, dried at 80°C for 12 h, and calcined at 450°C for 3 h.


## 2.2. Addition of Potassium Compounds

As described in previous reports, adding potassium in various concentrations was always conducted by the wet-impregnation method to simulate the poisoning mechanism of potassium in real flue gas at laboratory [16, 20]. In this paper, KNO3, KCl, and K2SO4 were dissolved in distilled water and impregnated with Ce/TiO2 catalysts. The mixture was stirred vigorously for 4 h, dried at 80°C for 12 h, and calcined at 450°C for 3 h. The K/Ce molar ratio was controlled at 0.25, 0.5, 1, and 2. The prepared catalysts were denoted as Ce/TiO2-x-y, where x are the precursors (KNO3, KCl, and K2SO4) and y is the K/Ce molar ratios (0.25, 0.5, 1, and 2).


## 2.3. SCR Activity Evaluation

SCR activities on fresh and K-poisoned Ce/TiO2 catalysts were tested in a fixed-bed reactor. The typical reactant gas composition was as follows: 750 ppm NO, 750 ppm NH3, 1.5% H2O, 3.5% O2, and balanced N2. The catalyst dosage was 0.5 g. The gas hourly space velocity (GHSV) was approximately 100,000 h−1. NO, NO2, and O2 concentrations were monitored by a flue gas analyzer (KM9106, Quintox Kane International Limited). N2O was detected by a FT-IR gas analyzer (Madur Photon Portable IR Gas Analysers, Madur Ltd., Austria).


## 2.4. Characterization Methods

XRD analysis was performed using X-ray diffraction with Cu Kα radiation (model D/max RA, Rigaku Co., Japan). The data were collected for scattering angles (2θ) ranging between 5° and 80° with a step size of 0.02°. X-ray photoelectron spectroscopy with Al Kα X-ray (h
ν
=
1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties and probe the electronic state of the elements. The microstructures were observed using a scanning electron micrograph (SEM) in a Phillips XL-30-ESEM system with a voltage of 15 kV. Nitrogen adsorption-desorption isotherms were obtained using a nitrogen adsorption apparatus (ASAP 2020, USA). All the samples were degassed at 200°C prior to measuring. The Brunauer-Emmett-Teller (BET) specific surface area (SBET) was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range from 0.05 to 0.30. Temperature programmed desorption with ammonia (NH3-TPD) and temperature programmed reduction with hydrogen (H2-TPR) experiments were carried out using a TP-5080 instrument (Tianjin Xianquan Industry and Trade Development Co. Ltd., China). Prior to the NH3-TPD experiments, 100 mg samples were pretreated in pure N2 at 350°C for 1 h and then saturated with anhydrous NH3 (4% in N2) at room temperature. Desorption was carried out by heating the samples in N2 (30 mL/min) from 70°C to 800°C with a heating rate of 10°C/min. Before raising the temperature, a preheat treatment at 70°C for 1 h was conducted. For the TPR experiments, 50 mg samples were pretreated in pure N2 at 350°C for 1 h and cooled to 70°C. The H2-TPR runs were carried out with a linear heating rate of 10°C/min from 70°C to 800°C in H2 (4% in N2).


## 3. Results and Discussion

### 3.1. SCR Performance

Figures1–3 show NO conversions as a function of the reaction temperature over catalysts with different loading amounts of KNO3, KCl, and K2SO4, respectively. The fresh Ce/TiO2 catalyst shows good catalytic activity in the temperature range of 285°C to 460°C. The NO conversion reaches up to 93% at 370°C. However, the NO conversion rapidly decreases after adding potassium. When the K/Ce molar ration exceeds 0.25, NO conversion becomes negligible in the entire temperature range regardless of the precursors. Relatively, the addition of KCl or K2SO4 is more harmful to the SCR activity of Ce/TiO2 than that of KNO3. The NO conversion at 370°C in Ce/TiO2-KNO3-0.25 is consistent at 45%, but it is kept at 16% and 24% in Ce/TiO2-K2SO4-0.25 and Ce/TiO2-KCl-0.25, respectively.Figure 1
Variation of NO conversion with reaction temperature of catalysts with different KNO3 loading.Figure 2
Variation of NO conversion with reaction temperature of catalysts with different K2SO4 loading.Figure 3
Variation of NO conversion with reaction temperature of catalysts with different KCl loading.To find the internal reasons for the deactivation by potassium salts, Ce/TiO2, Ce/TiO2-KNO3-0.25, Ce/TiO2-K2SO4-0.25, and Ce/TiO2-KCl-0.25 were subjected to a range of characterizations including XRD, BET-BJH, XPS, NH3-TPD, and H2-TPR.


### 3.2. Crystal Structure and Morphology

The powder XRD patterns of fresh and K-poisoned Ce/TiO2 catalysts are shown in Figure 4. Characterization peaks in the anatase phase (PDF-number 21-1272, 2θ = 25.28°, 37.80°, 48.05°, 53.89°, 55.06°, and 62.69°) and rutile phase (PDF-number 21-1276, 2θ = 27.45°, 36.09°, and 54.32°) appear in all the four catalysts. The characterization peaks of ceria (PDF-number 43-1002, 2θ = 28.55°, 33.08°, 47.48°, and 56.33°) are not noticeable in Ce/TiO2 but become clear after adding potassium. As described in Experimental, the Ce/Ti molar ration was designed at 1 : 19; hence the weight percentage of ceria in the Ce/TiO2 catalyst is approximately 10% which is above the detection limit of XRD. As such, the changes of ceria peaks suggest that highly dispersing amorphous ceria as very small nanoparticles is the dominant structure of ceria in the fresh Ce/TiO2 catalyst but transforms into ceria crystals in the K-poisoned catalysts. The particle size of ceria, which is calculated by plane [
111
] using the Scherrer Equation, grows to 7.6–7.9 nm in the three K-poisoned catalysts. Notably, the intensity of ceria peaks follows the following sequence: Ce/TiO2-KCl-0.25 > Ce/TiO2-K2SO4-0.25 > Ce/TiO2-KNO3-0.25, indicating that the crystallinity of ceria in the three catalysts may also follow the same order.Figure 4
Powder XRD patterns of fresh and K-poisoned Ce/TiO2 catalysts.Microstructures of the fresh and K-poisoned Ce/TiO2 catalysts observed by SEM are shown in Figure 5. The surface of fresh Ce/TiO2 catalyst is smooth when viewed through SEM (Figure 5(a)). The aggregation of particles cannot be observed. However, different degrees of aggregation can be observed in K-poisoned catalysts. Interstices between particles are enlarged. Variant large particles appear in the SEM image of Ce/TiO2-KCl-0.25. Considering the highest intensities of ceria and titania XRD peaks in Ce/TiO2-KCl-0.25, the destructive effect of KCl may have possibly occurred on ceria and titania particles. High-grade aggregation was observed in Ce/TiO2-K2SO4-0.25, and the surface becomes bumpy like heaped-up hills. Microstructures indicate that the dispersion of constituent particles worsened in K-poisoned catalysts, especially in KCl- and K2SO4-poisoned catalysts.Figure 5
SEM images of Ce/TiO2 (a), Ce/TiO2-KNO3-0.25 (b), Ce/TiO2-KCl-0.25 (c), and Ce/TiO2-K2SO4-0.25 (d).
(a)
(b)
(c)
(d)Morphology, structure, and particle size of the fresh and K-poisoned Ce/TiO2 catalysts further observed by TEM are shown in Figure 6. Aggregation can also be observed in K-poisoned catalysts like SEM images. Some ceria particles are circled in red. A small number of ceria particles can be observed in the fresh Ce/TiO2 catalyst. However, in K-poisoned catalysts, the number of ceria particles increases significantly, and overgrowth of ceria particles occurs in Ce/TiO2-KCl-0.25 and Ce/TiO2-K2SO4-0.25. The obvious difference in the number of ceria particles between fresh and K-poisoned Ce/TiO2 catalysts further confirms the increasing crystallization of ceria after K-loading. The TEM results are in good agreement with the XRD and SEM discussions. Therefore, the transformation of highly dispersing amorphous ceria to worse dispersing ceria crystals is certain.Figure 6
TEM images of Ce/TiO2 (a), Ce/TiO2-KNO3-0.25 (b), Ce/TiO2-KCl-0.25 (c), and Ce/TiO2-K2SO4-0.25 (d).
(a)
(b)
(c)
(d)


### 3.3. Physical Characterizations

Table1 shows the physical characterizations including BET surface area, pore volume, and average pore diameter. Compared with the BET surface area, pore volume, and average pore diameter of Ce/TiO2, those of Ce/TiO2-K2SO4-0.25 reduced slightly, indicating that part of the K2SO4 may deposit on the catalyst surface and cover a few pores. For Ce/TiO2-KCl-0.25 and Ce/TiO2-KNO3-0.25, the BET surface areas are reduced further, pore volumes are not reduced, and average pore diameters slightly increased, suggesting that a few pores agglomerate and enlarge along with the growth of ceria crystals. All the changes mentioned in the physical characterizations accorded well with the microstructures showed by SEM. Notably, the changes in physical characterizations were slight. Hence adding potassium has very limited effect on the physical characterizations of Ce/TiO2. The changes in physical characterizations are not the primary reason for deactivation.Table 1
BET surface area, pore volume, and average pore diameter.
Sample
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore diameter (nm)
Ce/TiO2
45.9
0.40
28.55
Ce/TiO2-KNO3-0.25
42.8
0.40
30.57
Ce/TiO2-K2SO4-0.25
45.0
0.39
28.03
Ce/TiO2-KCl-0.25
41.7
0.41
31.59


### 3.4. Surface Species

The surface atomic concentrations of Ce, Ti, O, K, Cl, and S acquired with XPS are shown in Table2. As revealed by XRD, highly dispersed amorphous ceria transforms into ceria crystals after adding potassium. Generally, amorphous ceria interacts closely with the TiO2 support; however ceria crystals are more independent and tend to agglomerate. Hence the surface atomic concentrations of Ce increase after adding potassium. The atomic concentrations of K, Cl, N, and S gave some evidence of the final form of potassium poisons. N atoms are hardly detected in Ce/TiO2-KNO3-0.25, indicating that KNO3 has decomposed into K2O. The atomic concentration of the K surface is higher in Ce/TiO2-K2SO4-0.25 than in Ce/TiO2-KCl-0.25 and Ce/TiO2-KNO3-0.25 which may be attributed to the deposition of K2SO4 on the surface of Ce/TiO2-K2SO4-0.25. The atomic concentration of K in Ce/TiO2-KCl-0.25 is more than twice that of Cl, suggesting that Cl can enter into the catalyst bulk.Table 2
Surface atomic concentrations of various elements acquired with XPS.
Sample
Ce
Ti
O
K
Cl
S
Ce/TiO2
1.86
31.60
66.54
—
—
—
Ce/TiO2-KNO3-0.25
2.01
32.22
65.00
0.76
—
—
Ce/TiO2-KCl-0.25
2.13
31.49
64.92
0.99
0.47
—
Ce/TiO2-K2SO4-0.25
2.29
30.41
64.43
1.76
—
1.11The XPS spectra of Ce 3d are shown in Figure7, where peaks labeled as u, u2, u3, v, v2, and v3 represent the 3d104f0 state of Ce4+ species, and those labeled as u1 and v1 represent the 3d104f1 initial electronic state corresponding to Ce3+ species [21, 22]. Generally, Ce4+ is dominant in fine ceria crystals, and Ce3+ accompanied with ceria defects is abundant in small ceria particles such as amorphous ceria. The intensity of Ce4+ is always measured in terms of peak area proportion of u3 in the whole spectrum. As shown in Table 3, the peak area proportion of u3 increases after adding potassium. This finding reveals that the concentration of ceria defects is reduced and the XRD results have confirmed the transformation of amorphous ceria into ceria crystals.Table 3
Peak areas of the fitted O 1s and Ce 3d XPS spectra.
Sample
Peak area of O 1s
Peak area of Ce 3d
Oα
Oβ
Proportion of Oβ
u3
Whole spectrum
Proportion of u3
Ce/TiO2
130585
19085
12.75%
5283
77795
6.79
Ce/TiO2-KNO3-0.25
135300
17500
11.45%
6186
73603
8.40
Ce/TiO2-KCl-0.25
128985
14285
9.97%
6783
80462
8.43
Ce/TiO2-K2SO4-0.25
146085
21501
12.83%
9547
89451
10.67Figure 7
XPS spectra of Ce 3d.The fitted XPS spectra of O 1s are shown in Figure8. The O 1s peaks mainly contain two different species: crystal lattice oxygen (O
α) and chemisorbed oxygen (O
β) [21, 23, 24]. The O
α peak is located at 529.92 eV in the spectrum of Ce/TiO2 but shifts to a lower binding energy in the spectra of K-poisoned catalysts. This shift can often be observed during the transformation of Ce3+ into Ce4+ [21, 23, 24]. Chemisorbed oxygen has been proven to be active in oxidation reactions and will take part in the oxidation of Ce3+ to Ce4+. The concentration of chemisorbed oxygen is assumed to be positively related with the ceria defects. As a result, the concentration of chemisorbed oxygen should decrease after adding potassium addition (see Table 3). The concentrations of chemisorbed oxygen in Ce/TiO2-KCl-0.25 and Ce/TiO2-KNO3-0.25 are lower than that of Ce/TiO2, as expected. However, the chemisorbed oxygen concentration in Ce/TiO2-K2SO4-0.25 is slightly higher and may be contributed by the S
O
4
2
- groups. As reported by Gao et al., sulfurization treatment could provide new chemisorbed oxygen in the form of –OH or H2O groups [25].Figure 8
XPS spectra of O 1s: (a) Ce/TiO2, (b) Ce/TiO2-KNO3-0.25, (c) Ce/TiO2-KCl-0.25, and (d) Ce/TiO2-K2SO4-0.25.
(a)
(b)
(c)
(d)


### 3.5. Surface Acidity and Reducibility

According to the widely accepted Eley-Rideal and Langmuir-Hinshelwood mechanism of SCR reaction [26, 27], the adsorption of NH3 on the catalyst surface s is considered a prerequisite. The NH3-TPD profiles of the four catalysts are shown in Figure 9, where the decreased NH3-desorption can be observed in the K-poisoned samples. The total amount of desorbed ammonia is calculated at 209 μmol/g, 141 μmol/g, 119 μmol/g, and 157 μmol/g over Ce/TiO2, Ce/TiO2-KNO3-0.25, Ce/TiO2-KCl-0.25, and Ce/TiO2-K2SO4-0.25, respectively, showing a noticeable decrease. The desorbed ammonia corresponds to the ammonia adsorbed on the Lewis and Brønsted acid sites. Hence these acid sites were partly destroyed by potassium after addition.Figure 9
NH3-TPD profiles of Ce/TiO2, Ce/TiO2-KNO3-0.25, Ce/TiO2-KCl-0.25, and Ce/TiO2-K2SO4-0.25.It is widely accepted that the reduction of ceria can be divided into two processes: the initial reduction of surface ceria species at low temperature and the further reduction of bulk ceria at high temperature. Considering the XRD and XPS results, we can deduce that amorphous ceria with abundant defects in Ce/TiO2 tends to be reduced at low temperatures, whereas ceria crystals in K-poisoned catalysts reduce at high temperatures [28, 29]. As depicted in Figure 10, the reduction of ceria starts at 261°C in Ce/TiO2, and the reduction maximum appears at 487°C. However, the starting temperature shifts to the right at 444°C, 457°C, and 377°C in the profiles of Ce/TiO2-KNO3-0.25, Ce/TiO2-KCl-0.25, and Ce/TiO2-K2SO4-0.25, respectively. This finding demonstrates that the addition of potassium leads to the passivating of ceria, similar to the transformation of amorphous ceria into ceria crystals. Moreover, there are sharp reduction peaks centering at 718°C and 652°C in the H2-TPR profiles of Ce/TiO2-KCl-0.25 and Ce/TiO2-K2SO4-0.25, respectively. Similar peaks are not found in the H2-TPR profile of Ce/TiO2-KNO3-0.25, demonstrating that the crystallinity of ceria in Ce/TiO2-KCl-0.25 and Ce/TiO2-K2SO4-0.25 is higher.Figure 10
H2-TPR profiles of Ce/TiO2, Ce/TiO2-KNO3-0.25, Ce/TiO2-KCl-0.25, and Ce/TiO2-K2SO4-0.25.


### 3.6. Deactivation Mechanism

Based on the analysis above, the changes that take place after adding potassium are summarized in Table4. We can find two main reasons for the deactivation of Ce/TiO2 catalyst by potassium: the structural and chemical state changes of ceria and the degradation of surface acidity.Table 4
Sum of the property changes after potassium addition.
Sample
Crystal structure of ceria
Microstructure
Chemical state of cerium
Ammonia adsorption
Reducibility of ceria
SCR performance
Ce/TiO2-KNO3-0.25
Amorphous to crystal
Enlarged interstices
Decrease of Ce3+
Reduced NH3 adsorption
Passivating of ceria
Deactivation
Ce/TiO2-KCl-0.25
Highest crystallinity
Variant particles
Decreased of Ce3+
Largest decrease of NH3 adsorption
Serious passivating of ceria
Serious deactivation
Ce/TiO2-K2SO4-0.25
High crystallinity
High aggregation
Largest decrease of Ce3+
Reduced NH3 adsorption
Serious passivating of ceria
Serious deactivationIn terms of ceria catalysts, good redox behavior in catalysis reactions involves high-speed Ce4+/Ce3+ redox cycles. Previous researches on the redox behavior of ceria have shown that the oxidation of Ce3+ to Ce4+ is very fast, whereas the reduction of Ce4+ to Ce3+ is slow in most cases [30]. Oxygen diffusion that depends on the type, size, and concentration of oxygen vacancies is proposed to be the rate-controlling step of ceria reduction [30–32]. Therefore, the nature of oxygen vacancy highly affects the redox behavior of ceria. It is known that once Ce3+ appears, oxygen vacancies will be generated to maintain electrostatic balance [32, 33]. Consequently, ceria defects on the catalyst surface are the most active species with fast Ce4+/Ce3+ redox cycle. For the Ce/TiO2 catalyst, ceria mainly exists in a highly dispersed amorphous form with surface having abundant defects; hence the reduction of ceria can start at very low temperature (261°C), and the Ce/TiO2 shows good SCR performance. For K-poisoned catalysts, amorphous ceria transforms into ceria crystals, and the size of ceria particles enlarges. As a result, the amount of surface defects, as well as oxygen vacancies, is reduced which hinders the reduction of ceria in the redox cycle. The reduction of ceria only occurred at higher temperatures. The K-doped catalysts showed the worst SCR performance. Notably, a more complete transformation of the structural and chemical state of ceria can be observed after adding KCl and K2SO4; hence the deactivation by the addition of KCl and K2SO4 is more significant than adding KNO3.The degradation of the surface acidity is considered a common reason for all the SCR catalysts that underwent alkali deactivation. It is widely accepted that the acid sites on the catalyst surface can be easily destroyed by alkaline species. Hence the decreasing capacity of ammonia adsorption by potassium has always been observed in V-, Fe-, Cu-, and zeolite-based catalysts [11–14, 16, 17]. For the Ce/TiO2 catalyst, we have found apparent degradation of the surface acidity regardless of the difference of the potassium precursor.It is interesting to note that the Cl− and S
O
4
2
- anions have additional poisoning effects. Lisi et al. have reported that acidic HCl can promote the formation of new acid sites on the vanadium-based catalysts [34], indicating that Cl− may be beneficial to surface acidity. However, this positive effect of Cl− is not found on KCl-doped CeO2/TiO2 catalyst. In the three catalysts, the largest decrease of NH3 adsorption is observed in the KCl-doped catalyst. The extra decrease may be associated with Cl−. From XPS results, we have deduced that Cl− can enter freely into the catalyst bulk. Cl− may combine with Ce to form cerium chloride and destroy the Ce-centered Lewis acid sites. High-grade aggregation of the whole catalyst sample is observed in K2SO4-doped catalysts. The aggregation that attributed to the deposition of K2SO4 on catalyst surface will reduce the exposure of active sites for the SCR reaction.


## 3.1. SCR Performance

Figures1–3 show NO conversions as a function of the reaction temperature over catalysts with different loading amounts of KNO3, KCl, and K2SO4, respectively. The fresh Ce/TiO2 catalyst shows good catalytic activity in the temperature range of 285°C to 460°C. The NO conversion reaches up to 93% at 370°C. However, the NO conversion rapidly decreases after adding potassium. When the K/Ce molar ration exceeds 0.25, NO conversion becomes negligible in the entire temperature range regardless of the precursors. Relatively, the addition of KCl or K2SO4 is more harmful to the SCR activity of Ce/TiO2 than that of KNO3. The NO conversion at 370°C in Ce/TiO2-KNO3-0.25 is consistent at 45%, but it is kept at 16% and 24% in Ce/TiO2-K2SO4-0.25 and Ce/TiO2-KCl-0.25, respectively.Figure 1
Variation of NO conversion with reaction temperature of catalysts with different KNO3 loading.Figure 2
Variation of NO conversion with reaction temperature of catalysts with different K2SO4 loading.Figure 3
Variation of NO conversion with reaction temperature of catalysts with different KCl loading.To find the internal reasons for the deactivation by potassium salts, Ce/TiO2, Ce/TiO2-KNO3-0.25, Ce/TiO2-K2SO4-0.25, and Ce/TiO2-KCl-0.25 were subjected to a range of characterizations including XRD, BET-BJH, XPS, NH3-TPD, and H2-TPR.


## 3.2. Crystal Structure and Morphology

The powder XRD patterns of fresh and K-poisoned Ce/TiO2 catalysts are shown in Figure 4. Characterization peaks in the anatase phase (PDF-number 21-1272, 2θ = 25.28°, 37.80°, 48.05°, 53.89°, 55.06°, and 62.69°) and rutile phase (PDF-number 21-1276, 2θ = 27.45°, 36.09°, and 54.32°) appear in all the four catalysts. The characterization peaks of ceria (PDF-number 43-1002, 2θ = 28.55°, 33.08°, 47.48°, and 56.33°) are not noticeable in Ce/TiO2 but become clear after adding potassium. As described in Experimental, the Ce/Ti molar ration was designed at 1 : 19; hence the weight percentage of ceria in the Ce/TiO2 catalyst is approximately 10% which is above the detection limit of XRD. As such, the changes of ceria peaks suggest that highly dispersing amorphous ceria as very small nanoparticles is the dominant structure of ceria in the fresh Ce/TiO2 catalyst but transforms into ceria crystals in the K-poisoned catalysts. The particle size of ceria, which is calculated by plane [
111
] using the Scherrer Equation, grows to 7.6–7.9 nm in the three K-poisoned catalysts. Notably, the intensity of ceria peaks follows the following sequence: Ce/TiO2-KCl-0.25 > Ce/TiO2-K2SO4-0.25 > Ce/TiO2-KNO3-0.25, indicating that the crystallinity of ceria in the three catalysts may also follow the same order.Figure 4
Powder XRD patterns of fresh and K-poisoned Ce/TiO2 catalysts.Microstructures of the fresh and K-poisoned Ce/TiO2 catalysts observed by SEM are shown in Figure 5. The surface of fresh Ce/TiO2 catalyst is smooth when viewed through SEM (Figure 5(a)). The aggregation of particles cannot be observed. However, different degrees of aggregation can be observed in K-poisoned catalysts. Interstices between particles are enlarged. Variant large particles appear in the SEM image of Ce/TiO2-KCl-0.25. Considering the highest intensities of ceria and titania XRD peaks in Ce/TiO2-KCl-0.25, the destructive effect of KCl may have possibly occurred on ceria and titania particles. High-grade aggregation was observed in Ce/TiO2-K2SO4-0.25, and the surface becomes bumpy like heaped-up hills. Microstructures indicate that the dispersion of constituent particles worsened in K-poisoned catalysts, especially in KCl- and K2SO4-poisoned catalysts.Figure 5
SEM images of Ce/TiO2 (a), Ce/TiO2-KNO3-0.25 (b), Ce/TiO2-KCl-0.25 (c), and Ce/TiO2-K2SO4-0.25 (d).
(a)
(b)
(c)
(d)Morphology, structure, and particle size of the fresh and K-poisoned Ce/TiO2 catalysts further observed by TEM are shown in Figure 6. Aggregation can also be observed in K-poisoned catalysts like SEM images. Some ceria particles are circled in red. A small number of ceria particles can be observed in the fresh Ce/TiO2 catalyst. However, in K-poisoned catalysts, the number of ceria particles increases significantly, and overgrowth of ceria particles occurs in Ce/TiO2-KCl-0.25 and Ce/TiO2-K2SO4-0.25. The obvious difference in the number of ceria particles between fresh and K-poisoned Ce/TiO2 catalysts further confirms the increasing crystallization of ceria after K-loading. The TEM results are in good agreement with the XRD and SEM discussions. Therefore, the transformation of highly dispersing amorphous ceria to worse dispersing ceria crystals is certain.Figure 6
TEM images of Ce/TiO2 (a), Ce/TiO2-KNO3-0.25 (b), Ce/TiO2-KCl-0.25 (c), and Ce/TiO2-K2SO4-0.25 (d).
(a)
(b)
(c)
(d)


## 3.3. Physical Characterizations

Table1 shows the physical characterizations including BET surface area, pore volume, and average pore diameter. Compared with the BET surface area, pore volume, and average pore diameter of Ce/TiO2, those of Ce/TiO2-K2SO4-0.25 reduced slightly, indicating that part of the K2SO4 may deposit on the catalyst surface and cover a few pores. For Ce/TiO2-KCl-0.25 and Ce/TiO2-KNO3-0.25, the BET surface areas are reduced further, pore volumes are not reduced, and average pore diameters slightly increased, suggesting that a few pores agglomerate and enlarge along with the growth of ceria crystals. All the changes mentioned in the physical characterizations accorded well with the microstructures showed by SEM. Notably, the changes in physical characterizations were slight. Hence adding potassium has very limited effect on the physical characterizations of Ce/TiO2. The changes in physical characterizations are not the primary reason for deactivation.Table 1
BET surface area, pore volume, and average pore diameter.
Sample
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore diameter (nm)
Ce/TiO2
45.9
0.40
28.55
Ce/TiO2-KNO3-0.25
42.8
0.40
30.57
Ce/TiO2-K2SO4-0.25
45.0
0.39
28.03
Ce/TiO2-KCl-0.25
41.7
0.41
31.59


## 3.4. Surface Species

The surface atomic concentrations of Ce, Ti, O, K, Cl, and S acquired with XPS are shown in Table2. As revealed by XRD, highly dispersed amorphous ceria transforms into ceria crystals after adding potassium. Generally, amorphous ceria interacts closely with the TiO2 support; however ceria crystals are more independent and tend to agglomerate. Hence the surface atomic concentrations of Ce increase after adding potassium. The atomic concentrations of K, Cl, N, and S gave some evidence of the final form of potassium poisons. N atoms are hardly detected in Ce/TiO2-KNO3-0.25, indicating that KNO3 has decomposed into K2O. The atomic concentration of the K surface is higher in Ce/TiO2-K2SO4-0.25 than in Ce/TiO2-KCl-0.25 and Ce/TiO2-KNO3-0.25 which may be attributed to the deposition of K2SO4 on the surface of Ce/TiO2-K2SO4-0.25. The atomic concentration of K in Ce/TiO2-KCl-0.25 is more than twice that of Cl, suggesting that Cl can enter into the catalyst bulk.Table 2
Surface atomic concentrations of various elements acquired with XPS.
Sample
Ce
Ti
O
K
Cl
S
Ce/TiO2
1.86
31.60
66.54
—
—
—
Ce/TiO2-KNO3-0.25
2.01
32.22
65.00
0.76
—
—
Ce/TiO2-KCl-0.25
2.13
31.49
64.92
0.99
0.47
—
Ce/TiO2-K2SO4-0.25
2.29
30.41
64.43
1.76
—
1.11The XPS spectra of Ce 3d are shown in Figure7, where peaks labeled as u, u2, u3, v, v2, and v3 represent the 3d104f0 state of Ce4+ species, and those labeled as u1 and v1 represent the 3d104f1 initial electronic state corresponding to Ce3+ species [21, 22]. Generally, Ce4+ is dominant in fine ceria crystals, and Ce3+ accompanied with ceria defects is abundant in small ceria particles such as amorphous ceria. The intensity of Ce4+ is always measured in terms of peak area proportion of u3 in the whole spectrum. As shown in Table 3, the peak area proportion of u3 increases after adding potassium. This finding reveals that the concentration of ceria defects is reduced and the XRD results have confirmed the transformation of amorphous ceria into ceria crystals.Table 3
Peak areas of the fitted O 1s and Ce 3d XPS spectra.
Sample
Peak area of O 1s
Peak area of Ce 3d
Oα
Oβ
Proportion of Oβ
u3
Whole spectrum
Proportion of u3
Ce/TiO2
130585
19085
12.75%
5283
77795
6.79
Ce/TiO2-KNO3-0.25
135300
17500
11.45%
6186
73603
8.40
Ce/TiO2-KCl-0.25
128985
14285
9.97%
6783
80462
8.43
Ce/TiO2-K2SO4-0.25
146085
21501
12.83%
9547
89451
10.67Figure 7
XPS spectra of Ce 3d.The fitted XPS spectra of O 1s are shown in Figure8. The O 1s peaks mainly contain two different species: crystal lattice oxygen (O
α) and chemisorbed oxygen (O
β) [21, 23, 24]. The O
α peak is located at 529.92 eV in the spectrum of Ce/TiO2 but shifts to a lower binding energy in the spectra of K-poisoned catalysts. This shift can often be observed during the transformation of Ce3+ into Ce4+ [21, 23, 24]. Chemisorbed oxygen has been proven to be active in oxidation reactions and will take part in the oxidation of Ce3+ to Ce4+. The concentration of chemisorbed oxygen is assumed to be positively related with the ceria defects. As a result, the concentration of chemisorbed oxygen should decrease after adding potassium addition (see Table 3). The concentrations of chemisorbed oxygen in Ce/TiO2-KCl-0.25 and Ce/TiO2-KNO3-0.25 are lower than that of Ce/TiO2, as expected. However, the chemisorbed oxygen concentration in Ce/TiO2-K2SO4-0.25 is slightly higher and may be contributed by the S
O
4
2
- groups. As reported by Gao et al., sulfurization treatment could provide new chemisorbed oxygen in the form of –OH or H2O groups [25].Figure 8
XPS spectra of O 1s: (a) Ce/TiO2, (b) Ce/TiO2-KNO3-0.25, (c) Ce/TiO2-KCl-0.25, and (d) Ce/TiO2-K2SO4-0.25.
(a)
(b)
(c)
(d)


## 3.5. Surface Acidity and Reducibility

According to the widely accepted Eley-Rideal and Langmuir-Hinshelwood mechanism of SCR reaction [26, 27], the adsorption of NH3 on the catalyst surface s is considered a prerequisite. The NH3-TPD profiles of the four catalysts are shown in Figure 9, where the decreased NH3-desorption can be observed in the K-poisoned samples. The total amount of desorbed ammonia is calculated at 209 μmol/g, 141 μmol/g, 119 μmol/g, and 157 μmol/g over Ce/TiO2, Ce/TiO2-KNO3-0.25, Ce/TiO2-KCl-0.25, and Ce/TiO2-K2SO4-0.25, respectively, showing a noticeable decrease. The desorbed ammonia corresponds to the ammonia adsorbed on the Lewis and Brønsted acid sites. Hence these acid sites were partly destroyed by potassium after addition.Figure 9
NH3-TPD profiles of Ce/TiO2, Ce/TiO2-KNO3-0.25, Ce/TiO2-KCl-0.25, and Ce/TiO2-K2SO4-0.25.It is widely accepted that the reduction of ceria can be divided into two processes: the initial reduction of surface ceria species at low temperature and the further reduction of bulk ceria at high temperature. Considering the XRD and XPS results, we can deduce that amorphous ceria with abundant defects in Ce/TiO2 tends to be reduced at low temperatures, whereas ceria crystals in K-poisoned catalysts reduce at high temperatures [28, 29]. As depicted in Figure 10, the reduction of ceria starts at 261°C in Ce/TiO2, and the reduction maximum appears at 487°C. However, the starting temperature shifts to the right at 444°C, 457°C, and 377°C in the profiles of Ce/TiO2-KNO3-0.25, Ce/TiO2-KCl-0.25, and Ce/TiO2-K2SO4-0.25, respectively. This finding demonstrates that the addition of potassium leads to the passivating of ceria, similar to the transformation of amorphous ceria into ceria crystals. Moreover, there are sharp reduction peaks centering at 718°C and 652°C in the H2-TPR profiles of Ce/TiO2-KCl-0.25 and Ce/TiO2-K2SO4-0.25, respectively. Similar peaks are not found in the H2-TPR profile of Ce/TiO2-KNO3-0.25, demonstrating that the crystallinity of ceria in Ce/TiO2-KCl-0.25 and Ce/TiO2-K2SO4-0.25 is higher.Figure 10
H2-TPR profiles of Ce/TiO2, Ce/TiO2-KNO3-0.25, Ce/TiO2-KCl-0.25, and Ce/TiO2-K2SO4-0.25.


## 3.6. Deactivation Mechanism

Based on the analysis above, the changes that take place after adding potassium are summarized in Table4. We can find two main reasons for the deactivation of Ce/TiO2 catalyst by potassium: the structural and chemical state changes of ceria and the degradation of surface acidity.Table 4
Sum of the property changes after potassium addition.
Sample
Crystal structure of ceria
Microstructure
Chemical state of cerium
Ammonia adsorption
Reducibility of ceria
SCR performance
Ce/TiO2-KNO3-0.25
Amorphous to crystal
Enlarged interstices
Decrease of Ce3+
Reduced NH3 adsorption
Passivating of ceria
Deactivation
Ce/TiO2-KCl-0.25
Highest crystallinity
Variant particles
Decreased of Ce3+
Largest decrease of NH3 adsorption
Serious passivating of ceria
Serious deactivation
Ce/TiO2-K2SO4-0.25
High crystallinity
High aggregation
Largest decrease of Ce3+
Reduced NH3 adsorption
Serious passivating of ceria
Serious deactivationIn terms of ceria catalysts, good redox behavior in catalysis reactions involves high-speed Ce4+/Ce3+ redox cycles. Previous researches on the redox behavior of ceria have shown that the oxidation of Ce3+ to Ce4+ is very fast, whereas the reduction of Ce4+ to Ce3+ is slow in most cases [30]. Oxygen diffusion that depends on the type, size, and concentration of oxygen vacancies is proposed to be the rate-controlling step of ceria reduction [30–32]. Therefore, the nature of oxygen vacancy highly affects the redox behavior of ceria. It is known that once Ce3+ appears, oxygen vacancies will be generated to maintain electrostatic balance [32, 33]. Consequently, ceria defects on the catalyst surface are the most active species with fast Ce4+/Ce3+ redox cycle. For the Ce/TiO2 catalyst, ceria mainly exists in a highly dispersed amorphous form with surface having abundant defects; hence the reduction of ceria can start at very low temperature (261°C), and the Ce/TiO2 shows good SCR performance. For K-poisoned catalysts, amorphous ceria transforms into ceria crystals, and the size of ceria particles enlarges. As a result, the amount of surface defects, as well as oxygen vacancies, is reduced which hinders the reduction of ceria in the redox cycle. The reduction of ceria only occurred at higher temperatures. The K-doped catalysts showed the worst SCR performance. Notably, a more complete transformation of the structural and chemical state of ceria can be observed after adding KCl and K2SO4; hence the deactivation by the addition of KCl and K2SO4 is more significant than adding KNO3.The degradation of the surface acidity is considered a common reason for all the SCR catalysts that underwent alkali deactivation. It is widely accepted that the acid sites on the catalyst surface can be easily destroyed by alkaline species. Hence the decreasing capacity of ammonia adsorption by potassium has always been observed in V-, Fe-, Cu-, and zeolite-based catalysts [11–14, 16, 17]. For the Ce/TiO2 catalyst, we have found apparent degradation of the surface acidity regardless of the difference of the potassium precursor.It is interesting to note that the Cl− and S
O
4
2
- anions have additional poisoning effects. Lisi et al. have reported that acidic HCl can promote the formation of new acid sites on the vanadium-based catalysts [34], indicating that Cl− may be beneficial to surface acidity. However, this positive effect of Cl− is not found on KCl-doped CeO2/TiO2 catalyst. In the three catalysts, the largest decrease of NH3 adsorption is observed in the KCl-doped catalyst. The extra decrease may be associated with Cl−. From XPS results, we have deduced that Cl− can enter freely into the catalyst bulk. Cl− may combine with Ce to form cerium chloride and destroy the Ce-centered Lewis acid sites. High-grade aggregation of the whole catalyst sample is observed in K2SO4-doped catalysts. The aggregation that attributed to the deposition of K2SO4 on catalyst surface will reduce the exposure of active sites for the SCR reaction.


## 4. Conclusions

The addition of KNO3, KCl, and K2SO4 could deactivate the Ce/TiO2 catalyst in a SCR reaction. After adding KNO3, KCl, or K2SO4 with a K/Ce molar ration of 0.25, the NO conversion at 370°C dropped sharply from 93% to 45%, 24%, and 16%. Further increase in the amount of potassium led to complete deactivation. Changes in the structural and chemical state of ceria and the degradation of surface acidity were the primary reasons for the deactivation. Ceria particles grew and amorphous ceria transformed into ceria crystals after adding potassium. As a result, the amount of ceria defects as well as oxygen vacancies was reduced which ultimately lowered the rate of ceria reduction and redox cycle. KCl and K2SO4 showed greater effect on the changes of ceria state than KNO3. Potassium could destroy the acid sites, leading to the decline of ammonia adsorption capability. The introduction of Cl− from KCl could be combined with Ce to form cerium chloride; hence Ce-centered Lewis acid sites were destroyed by Cl−. The deposition of K2SO4 on catalyst surface will reduce the exposure of active sites for the SCR reaction. Cl and S have always coexisted with K biofuels and their concentrations are always high; thus the deactivation of Ce/TiO2 utilized in biofuel boilers will be more significant than that in coal-fired boilers.



---

*Source: 513814-2015-10-26.xml*