Experimental Study on the Deactivating Effect of KNO 3 , KCl , and K 2 SO 4 on Nanosized Ceria / Titania SCR Catalyst

1South China Institute of Environmental Sciences, The Ministry of Environmental Protection of PRC, No. 7 West Street, Yuancun, Guangzhou 510655, China 2The Key Laboratory of Water and Air Pollution Control of Guangdong Province, No. 7 West Street, Yuancun, Guangzhou 510655, China 3School of Chemical Engineering and Technology, Tianjin University, Tianjin Weijin Road, No. 92, Nankai District 300072, China

Recently, Ce/TiO 2 catalyst has gained recognition because of its excellent activity and selectivity.Xu et al. reported that Ce/TiO 2 catalyst is highly efficient at 275-400 ∘ C, and the undesired by-product, N 2 O, 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/TiO 2 catalysts prepared using the single step sol-gel method had the best SCR activity and SO 2 resistance.Liu et al. demonstrated the feasibility of a supercritical water synthesis route in the syntheses of Ce/TiO 2 catalysts by a strong metalsupport interaction [9].Chen et al. found that tungsten modification could further improve the activity of Ce/TiO 2 [10], and Liu et al. used MoO 3 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/TiO 2 -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 Cubased catalysts supported on TiO 2 or ZrO 2 were reported by Kustov et al. [14].With respect to CeO 2 /TiO 2 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 MnO x /TiO 2 , 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/TiO 2 catalysts in the SCR application.KCl and K 2 SO 4 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].KNO 3 was selected as the precursors of potassium oxide.Fresh and K-poisoned Ce/TiO 2 catalysts were subjected to a range of characterizations (e.g., XRD, XPS, NH 3 -TPD, and H 2 -TPR), and the deactivation mechanism was discussed.

Experimental
2.1.Preparation of Ce/TiO 2 Catalyst.Commercially produced P25 TiO 2 (Degussa, Germany) was used as the catalyst support.Cerous nitrate (AR, Ce(NO 3 ) 3 ⋅6H 2 O) was used as the precursor of ceria.Ceria was loaded on P25 TiO 2 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.

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, KNO 3 , KCl, and K 2 SO 4 were dissolved in distilled water and impregnated with Ce/TiO 2 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/TiO 2 -x-y, where  are the precursors (KNO 3 , KCl, and K 2 SO 4 ) and  is the K/Ce molar ratios (0.25, 0.5, 1, and 2).

SCR Activity
Evaluation.SCR activities on fresh and Kpoisoned Ce/TiO 2 catalysts were tested in a fixed-bed reactor.The typical reactant gas composition was as follows: 750 ppm NO, 750 ppm NH 3 , 1.5% H 2 O, 3.5% O 2 , and balanced N 2 .The catalyst dosage was 0.5 g.The gas hourly space velocity (GHSV) was approximately 100,000 h −1 .NO, NO 2 , and O 2 concentrations were monitored by a flue gas analyzer (KM9106, Quintox Kane International Limited).N 2 O 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 K radiation (model D/max RA, Rigaku Co., Japan).The data were collected for scattering angles (2) ranging between 5 ∘ and 80 ∘ with a step size of 0.02 ∘ .X-ray photoelectron spectroscopy with Al K X-ray (ℎ] = 1486.6eV) 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 (NH 3 -TPD) and temperature programmed reduction with hydrogen (H 2 -TPR) experiments were carried out using a TP-5080 instrument (Tianjin Xianquan Industry and Trade Development Co. Ltd., China).Prior to the NH 3 -TPD experiments, 100 mg samples were pretreated in pure N 2 at 350 ∘ C for 1 h and then saturated with anhydrous NH 3 (4% in N 2 ) at room temperature.Desorption was carried out by heating the samples in N 2 (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 N 2 at 350 ∘ C for 1 h and cooled to 70 ∘ C. The H 2 -TPR runs were carried out with a linear heating rate of 10 ∘ C/min from 70 ∘ C to 800 ∘ C in H 2 (4% in N 2 ).

Results and Discussion
3.1.SCR Performance.Figures 1-3 show NO conversions as a function of the reaction temperature over catalysts with different loading amounts of KNO 3 , KCl, and K 2 SO 4 , respectively.The fresh Ce/TiO 2 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 K 2 SO 4 is more harmful to the SCR activity of Ce/TiO 2 than that of KNO 3 .The NO conversion at 370 ∘ C in Ce/TiO 2 -KNO 3 -0.25 is consistent at 45%, but it is kept at 16% and 24% in Ce/TiO 2 -K 2 SO 4 -0.25 and Ce/TiO 2 -KCl-0.25,respectively.
To find the internal reasons for the deactivation by potassium salts, Ce/TiO 2 , Ce/TiO 2 -KNO 3 -0.25,Ce/TiO 2 -K 2 SO 4 -0.25, and Ce/TiO 2 -KCl-0.25 were subjected to a range      Experimental, the Ce/Ti molar ration was designed at 1 : 19; hence the weight percentage of ceria in the Ce/TiO 2 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/TiO 2 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 7.6-7.9nm in the three K-poisoned catalysts.Notably, the intensity of ceria peaks follows the following sequence: Ce/TiO 2 -KCl-0.25 Morphology, structure, and particle size of the fresh and K-poisoned Ce/TiO 2 catalysts further observed by TEM are shown in Figure 6.Aggregation can also be observed in Kpoisoned catalysts like SEM images.Some ceria particles are circled in red.A small number of ceria particles can be observed in the fresh Ce/TiO 2 catalyst.However, in Kpoisoned catalysts, the number of ceria particles increases significantly, and overgrowth of ceria particles occurs in Ce/TiO 2 -KCl-0.25 and Ce/TiO 2 -K 2 SO 4 -0.25.The obvious difference in the number of ceria particles between fresh and K-poisoned Ce/TiO 2 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.

Physical Characterizations.
Table 1 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/TiO 2 , those of Ce/TiO 2 -K 2 SO 4 -0.25 reduced slightly, indicating that part of the K 2 SO 4 may deposit on the catalyst surface and cover a few pores.For Ce/TiO 2 -KCl-0.25 and Ce/TiO 2 -KNO 3 -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/TiO 2 .The changes in physical characterizations are not the primary reason for deactivation.

Surface Species.
The surface atomic concentrations of Ce, Ti, O, K, Cl, and S acquired with XPS are shown in Table 2.As revealed by XRD, highly dispersed amorphous ceria transforms into ceria crystals after adding potassium.Generally, amorphous ceria interacts closely with the TiO 2 support; however ceria crystals are more independent and tend to agglomerate.Hence the surface atomic concentrations  The XPS spectra of Ce 3d are shown in Figure 7, where peaks labeled as u, u2, u3, v, v2, and v3 represent the 3d 10 4f 0 state of Ce 4+ species, and those labeled as u1 and v1 represent the 3d 10 4f 1 initial electronic state corresponding to Ce 3+ species [21,22].Generally, Ce 4+ is dominant in fine ceria crystals, and Ce 3+ accompanied with ceria defects is abundant in small ceria particles such as amorphous ceria.The intensity of Ce 4+ 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.
The fitted XPS spectra of O 1s are shown in Figure 8.The O 1s peaks mainly contain two different species: crystal lattice oxygen (O  ) and chemisorbed oxygen (O  ) [21,23,24].The O  peak is located at 529.92 eV in the spectrum of Ce/TiO 2  but shifts to a lower binding energy in the spectra of Kpoisoned catalysts.This shift can often be observed during the transformation of Ce 3+ into Ce 4+ [21,23,24].Chemisorbed oxygen has been proven to be active in oxidation reactions and will take part in the oxidation of Ce 3+ to Ce 4+ .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/TiO 2 -KCl-0.25 and Ce/TiO 2 -KNO 3 -0.25 are lower than that of Ce/TiO 2 , as expected.However, the chemisorbed oxygen concentration in Ce/TiO 2 -K 2 SO 4 -0.25 is slightly higher and may be contributed by the SO 4 2− groups.As reported by Gao et al., sulfurization treatment could provide new chemisorbed oxygen in the form of -OH or H 2 O groups [25].

Surface Acidity and Reducibility.
According to the widely accepted Eley-Rideal and Langmuir-Hinshelwood mechanism of SCR reaction [26,27], the adsorption of NH 3 on the catalyst surface s is considered a prerequisite.The NH 3 -TPD profiles of the four catalysts are shown in Figure 9, where the decreased NH 3 -desorption can be observed in the Kpoisoned samples.The total amount of desorbed ammonia is calculated at 209 mol/g, 141 mol/g, 119 mol/g, and 157 mol/g over Ce/TiO 2 , Ce/TiO 2 -KNO 3 -0.25,Ce/TiO 2 -KCl-0.25,and Ce/TiO 2 -K 2 SO 4 -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.
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/TiO 2 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/TiO 2 , 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/TiO 2 -KNO 3 -0.25,Ce/TiO 2 -KCl-0.25,and Ce/TiO 2 -K 2 SO 4 -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 H 2 -TPR profiles of Ce/TiO 2 -KCl-0.25 and Ce/TiO 2 -K 2 SO 4 -0.25, respectively.Similar peaks are not found in the H 2 -TPR profile of Ce/TiO 2 -KNO 3 -0.25,demonstrating that the crystallinity of ceria in Ce/TiO 2 -KCl-0.25 and Ce/TiO 2 -K 2 SO 4 -0.25 is higher.

Deactivation Mechanism.
Based on the analysis above, the changes that take place after adding potassium are summarized in Table 4.We can find two main reasons for the deactivation of Ce/TiO 2 catalyst by potassium: the structural and chemical state changes of ceria and the degradation of surface acidity.
In terms of ceria catalysts, good redox behavior in catalysis reactions involves high-speed Ce 4+ /Ce 3+ redox cycles.Previous researches on the redox behavior of ceria have shown that the oxidation of Ce 3+ to Ce 4+ is very fast, whereas the reduction of Ce 4+ to Ce 3+ is slow in most cases [30].Oxygen diffusion that depends on the type, size, and concentration of oxygen vacancies is proposed to be the ratecontrolling step of ceria reduction [30][31][32].Therefore, the nature of oxygen vacancy highly affects the redox behavior of ceria.It is known that once Ce 3+ 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 Ce 4+ /Ce 3+ redox cycle.For the Ce/TiO 2 catalyst, ceria mainly exists in a highly dispersed amorphous form with surface having abundant defects; hence   Reduced NH 3 adsorption

Serious passivating of ceria
Serious deactivation the reduction of ceria can start at very low temperature (261 ∘ C), and the Ce/TiO 2 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 K 2 SO 4 ; hence the deactivation by the addition of KCl and K 2 SO 4 is more significant than adding KNO 3 .
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/TiO 2 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 SO 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 CeO 2 /TiO 2 catalyst.In the three catalysts, the largest decrease of NH 3 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 K 2 SO 4 -doped catalysts.The aggregation that attributed to the deposition of K 2 SO 4 on catalyst surface will reduce the exposure of active sites for the SCR reaction.

Conclusions
The addition of KNO 3 , KCl, and K 2 SO 4 could deactivate the Ce/TiO 2 catalyst in a SCR reaction.After adding KNO 3 , KCl, or K 2 SO 4 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 K 2 SO 4 showed greater effect on the changes of ceria state than KNO 3 .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 K 2 SO 4 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/TiO 2 utilized in biofuel boilers will be more significant than that in coal-fired boilers.

Ce/TiO 2 2 NOFigure 1 :
Figure 1: Variation of NO conversion with reaction temperature of catalysts with different KNO 3 loading.

Figure 2 :
Figure 2: Variation of NO conversion with reaction temperature of catalysts with different K 2 SO 4 loading.

2 Figure 3 :
Figure 3: Variation of NO conversion with reaction temperature of catalysts with different KCl loading.

Table 2 :
Surface atomic concentrations of various elements acquired with XPS.
2 -KNO 3 -0.25,indicating that KNO 3 has decomposed into K 2 O.The atomic concentration of the K surface is higher in Ce/TiO 2 -K 2 SO 4 -0.25 than in Ce/TiO 2 -KCl-0.25 and Ce/TiO 2 -KNO 3 -0.25 which may be attributed to the deposition of K 2 SO 4 on the surface of Ce/TiO 2 -K 2 SO 4 -0.25.The atomic concentration of K in Ce/TiO 2 -KCl-0.25 is more than twice that of Cl, suggesting that Cl can enter into the catalyst bulk.

Table 3 :
Peak areas of the fitted O 1s and Ce 3d XPS spectra.

Table 4 :
Sum of the property changes after potassium addition.