Effect of surface species and structure on the performance of CeO₂ and SO₄²⁻ doped MCM-41 catalyst toward NH₃-SCR

Abstract

The present work elucidated the effects of structures and surface species on the activity of CeO₂ and SO₄²⁻ doped MCM-41 catalysts toward NO reduction by NH₃. The results indicated that the sulfated species were generated over Cat-A (Ce(NO₃)₃·6H₂O and H₂SO₄ simultaneously doped on MCM-41) and Cat-B (Ce(NO₃)₃·6H₂O first immersed to MCM-41 and followed by impregnation of H₂SO₄) except for Cat-C (H₂SO₄ first doped onto MCM-41 and followed by Ce(NO₃)₃·6H₂O). The sulfate ion contributed to the formation of Ce³⁺ concentration, oxygen vacancies, and surface acidity, resulting in improvement of SCR activity. However, the positive influence on SCR activity was overwhelmed by the decomposition of sulfated species. The apparent activation energy of Cat-A (60 kJ mol⁻¹) was much lower compared with that of Cat-B and Cat-C (71 and 78 kJ mol⁻¹, respectively). Pore structures and surface species were responsible for the SCR activity over the CeO₂ and SO₄²⁻ doped MCM-41 catalysts. The best catalytic activity was obtained over Cat-A, followed by Cat-B, and Cat-C exhibited the least catalytic activity.

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1. Introduction

Nitrogen oxide species (NO_x) emission from flue gases of stationary sources are important pollutants in the air, which not only cause various environmental problems such as greenhouse effects, acid rain, and photochemical smog, but also affect human health.¹ With stringent NO_x emission legislation, many strategies have been implemented for controlling NO_x emission. Among NO_x removal technologies, selective catalytic reduction (SCR) of NO_x by NH₃ is one of the most widely used technologies for controlling NO_x from stationary resources, and the commercial catalyst is V₂O₅–WO₃ (MoO₃)/TiO₂.^2,3 However, the potential volatility and intense toxicity of vanadium oxide are harmful to human health and the environment.^4,5 Consequently, great efforts have been made to develop environmentally-friendly SCR catalysts.

In recent years, mesoporous materials have been extensively used for catalyst support due to their large specific surface area, favored pore diameter, and ordered framework.⁶ MCM-41, a typical representative in mesoporous materials, possesses potential catalytic applications. It is well known that pure MCM-41 mesoporous molecular sieves are restrained in industry application due to some disadvantages, such as inefficient catalytic activity and weak surface acidity. However, pure MCM-41 can be modified by some transition metals, remarkably improving the catalytic activity and surface acidity.^7–11 Liu et al.¹¹ reported that the Cu/MCM-41 catalysts exhibited excellent catalytic activity in the NO-SCR reaction. Wan et al.¹² also revealed that high catalytic activity of Cu–Al-MCM-41 modified by Cu and Al species was obtained in the C₃H₆-SCR reaction. Besides, Qiu et al.¹³ investigated the effect of Cu contents on NH₃-SCR activity over MCM-41 catalysts. Ziolek et al.¹⁴ reported that surface acidity could be improved by the introduction of Nb into MCM-41 in the NH₃-SCR reaction. Meanwhile, few studies have been done to elucidate the effect of structures and surface species on SCR activity over MCM-41 modified by some transition metals in the NH₃-SCR reaction.

Recently, CeO₂-based catalysts are extensively investigated because of their excellent oxygen storage capacity and redox ability in the NH₃-SCR reaction. Furthermore, supported SCR catalysts are dramatically influenced by the kinds of surface species and the structures of supporting materials. Excellent dispersion of active CeO₂ particles, which contributes to the redox ability and surface acidity, can be facilitated by the favored structure of MCM-41, resulting in improvement of catalytic activity in the NH₃-SCR reaction. Therefore, the structures and surface species of MCM-41 modified by CeO₂ should be intensively studied.

Sulfate modification is an effective method to increase catalytic activity for the improvement of surface acidity over CeO₂-based catalysts.^15–18 But the effect of sulfated CeO₂ on the structures and SCR performances over MCM-41 has been scarcely investigated. It is widely accepted that surface species and structures of the support catalysts are remarkably affected by the metal-loading procedure, resulting in different catalytic activities.^19,20 Thereby, the effect of CeO₂ and SO₄²⁻ doped MCM-41 on SCR activity can be investigated in the NH₃-SCR reaction.

In this work, the CeO₂ and SO₄²⁻ doped MCM-41 supports were prepared by an impregnation method. The specific surface area, pore structures, surface species, and catalytic activity were investigated. Furthermore, characteristics of the samples were explored by XRD, NH₃-TPD, TGA, N₂ adsorption-desorption, H₂-TPR, Raman, and XPS.

2. Experimental

2.1 Catalyst preparation

MCM-41 supports were purchased from Nanjing Xian-feng nanomaterials technology Co., Ltd. Ce(NO₃)₃·6H₂O and H₂SO₄ were purchased from Sinopharm Chemical Reagent Co., Ltd, and both were analytical grade. The CeO₂ and SO₄²⁻ doped MCM-41 were prepared by a wet impregnation method. First, the Ce(NO₃)₃·6H₂O (1.5 g) and 0.1 g mL⁻¹ H₂SO₄ (3 mL) were simultaneously dissolved in deionized water (3 mL) and then 2.1 g of the MCM-41 supports were immersed. The mixture was kept at 60 °C for 5 h and dried for 12 h at 105 °C. The powders were calcined at 550 °C for 5 h in air and the catalysts were named as Cat-A. Second, 0.1 g mL⁻¹ H₂SO₄ (3 mL) was dissolved in deionized water (3 mL) and then 2.1 g of the MCM-41 supports were added. This powder was maintained at 60 °C for 5 h, dried for 12 h at 105 °C, and calcined at 550 °C for 5 h in air. The obtained powders were placed into the 1.5 g Ce(NO₃)₃·6H₂O again. The above powders were immersed at 60 °C for 5 h, dried for 12 h at 105 °C, and calcined at 550 °C for 5 h. The catalysts were assigned as Cat-B. Third, the process was the same as Cat-B, but the impregnation sequence of Ce(NO₃)₃·6H₂O and H₂SO₄ was opposite. The 1.5 g Ce(NO₃)₃·6H₂O was first immersed, and followed by a 0.1 g mL⁻¹ H₂SO₄ (3 mL) solution. This catalyst was marked as Cat-C.

2.2 Catalyst characterization

Powder X-ray diffraction (XRD) measurements were made with a Rigaku D/Max 2500 system between 10° and 70° at a step of 6° min⁻¹ by using Cu Kα (40 kV, 100 mA) radiation to determine the crystal structure.

Raman spectra (RS) were performed on a Renishaw-2000 Raman spectrometer at a resolution of 2 cm⁻¹ by using the 514.5 nm line of an Ar ion laser as the excitation source.

TGA experiments were carried out on a Simultaneous Thermal Analyzer (DTG-60H) with 0.009 mg samples under a nitrogen flow of 10 mL min⁻¹. The temperature was increased from 50 to 900 °C at a rate of 10 °C min⁻¹.

The specific surface areas of the catalysts were obtained at −196 °C using a TristarII3020 automated gas sorption system. Prior to N₂ measurement, the catalysts were degassed at 300 °C for 4 h. The specific surface areas were measured with a Brunauer–Emmett–Teller (BET) model instrument. Average pore diameters and total pore volumes were obtained from desorption branches of the isotherms by the Barrett–Joyner–Halenda (BJH) method.

X-ray photoelectron spectroscopy (XPS) was performed in an ULVAC PHI 5000 Versa Probe-II equipment operating at 10⁻⁹ Pa with an Al Kα radiation (1486.6 eV) to analyze the chemical states and the surface atomic concentration of the catalysts. The observed spectra were corrected with the C 1s binding energy value of 284.8 eV.

The H₂ temperature-programmed reduction (H₂-TPR) experiment was conducted on a GC-9750 equipped with a thermal conductivity detector. Before the H₂-TPR experiment, the catalysts were pretreated in pure N₂ at 400 °C for 60 min and cooled to 100 °C. The H₂-TPR runs were performed in a flow of 5 vol% H₂/Ar (30 mL min⁻¹) from 100 to 900 °C with a heating rate of 10 °C min⁻¹.

The temperature programmed desorption of ammonia (NH₃-TPD) of samples was carried out in order to investigate the surface acidity using gas chromatography with a thermal conductivity detector (TCD) setup (Fuli, GC-9750, China). Before the test, the samples were pretreated in N₂ at 400 °C for 45 min, followed by introduction of 5% NH₃/N₂ at a flow rate of 30 mL min⁻¹ for about 60 min. Desorption was performed by heating from 100 to 450 °C with a heating rate of 10 °C min⁻¹.

2.3 Activity test

The SCR activity measurements were obtained using a fixed-bed quartz reactor (8 mm i.d.) with 0.4 mL catalysts. The experiments were performed at 150–450 °C and the gas flow was measured by mass flow meters. The reactant gas composition was as follows: 600 ppm NO, 600 ppm NH₃, 5 vol% H₂O (when used), 50 ppm SO₂ (when used), 5 vol% O₂ and N₂ balance. The total flow rate was 400 mL min⁻¹, corresponding to a gas hourly space velocity of 60 000 h⁻¹. The concentrations of NO_x and NH₃ were continuously measured by an ECOM J2KN and GXH-1050E flue gas analyzer, respectively. The N₂O species were analyzed with a gas chromatograph (Fuli, 9790). To minimize the effect of gas adsorption on the catalysts, the reaction system was kept for 30 min at each reaction temperature to reach a steady state before analysis.

3. Results and discussions

3.1 XRD analysis

The XRD patterns of Cat-A, Cat-B, and Cat-C catalysts are shown in Fig. 1a. The XRD pattern of Cat-A exhibited the presence of cubic CeO₂ phase (PDF: 43-1003), together with orthorhombic Ce₂(SO₄)₃ (PDF: 52-1494) peaks. The Cat-B and Cat-C catalysts merely showed XRD peaks corresponding to cubic CeO₂ phase (PDF: 43-1003). No sulfated species were observed with Cat-B and Cat-C catalysts, which indicated sulfated species were in the forms of a highly dispersed state or clustered state, which were too small to be detected by XRD over these two samples. Besides, strengths of the peaks of Cat-B and Cat-C were higher than that of Cat-A, and the value of FWHM (full width at half maximum) over Cat-A in Table 1 was larger than those of Cat-B and Cat-C, suggesting that Cat-B and Cat-C possessed higher crystallinity than Cat-A. According to Zhang et al.'s reports,²¹ the lattice parameter could serve as a function of the particle sizes of catalysts, and the lattice parameter would increase with a decrease in particle sizes of catalysts. Therefore, the lattice parameter a was calculated in the 2θ range of 27–30° in Fig. 1b for the catalysts, and the results are presented in Table 1. It was obvious that the lattice parameter a of Cat-A was larger than that of Cat-B and Cat-C. The phenomenon reflected that the Cat-A catalyst possessed smaller particle sizes in comparison with Cat-B and Cat-C.

Fig. 1 XRD patterns of different catalysts.

Table 1 Lattice parameter and FWHM of Cat-A, Cat-B and Cat-C catalysts

Sample	FWHM	2θ/°	Lattice parameter a/Å
Cat-A	1.35	28.55	5.408
Cat-B	1.20	28.67	5.386
Cat-C	1.28	28.64	5.393

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3.2 N₂ adsorption–desorption isotherms

Fig. 2 shows the nitrogen adsorption–desorption isotherms and the BJH pore size distribution curves of the catalysts. It was clear from Fig. 2 that all the catalysts showed type IV isotherms (according to IUPAC classification), which was typically characteristic of mesopores (2–50 nm).²² For the Cat-B and Cat-C samples, there were two regions of hysteresis: one at P/P₀ > 0.35 and the other at 0.2, which suggested that a greater abundance of micropores and mesoporous existed over these two samples. Furthermore, the appearance of a H₂ hysteresis loop was observed over pure MCM-41 with ink-bottle-type pores where large cavities were connected by narrow channels.^23,24 However, the hysteresis loop was absent over Cat-A, Cat-B, and Cat-C catalysts. In other words, the adsorption isotherm nearly coincided with the desorption isotherm over the three samples. This phenomenon indicated that the pores of Cat-A, Cat-B, and Cat-C were open pores.^25,26 After the simultaneous introduction of Ce(NO₃)₃·6H₂O and H₂SO₄, the profile of the curve over Cat-A changed. The volume of adsorbed N₂ also decreased, indicating a remarkable reduction of total pore volume and specific surface area, as shown in Table 2. In brief, the pore structures of Cat-A, Cat-B, and Cat-C changed due to the introduction of Ce(NO₃)₃·6H₂O and H₂SO₄ inside the framework of MCM-41. Fig. 2 also showed the BJH pore size distributions of the samples. The Cat-A catalyst mainly showed and exhibited distribution peaks near 2.8 and 4.2 nm, and desorption pore size distributions of Cat-B and Cat-C were 3.0 nm. Besides, a dramatic decrease in the N₂ desorption differential pore volumes per weight (cm³ g⁻¹ nm⁻¹) of Cat-A was observed when compared with Cat-B and Cat-C. This meant that the abundant mesopores disappeared over the Cat-A catalyst, resulting in a decrease in BET surface area.

Fig. 2 N₂ adsorption–desorption results of the catalysts.

Table 2 BET measurements for the catalysts

Samples	BET (m^2 g^−1)	Total pore volume (cm^3 g^−1)	Average pore diameter (nm)
MCM-41	952	1.0452	3.47
Cat-A	336	0.1302	6.75
Cat-B	555	0.4598	3.34
Cat-C	519	0.3665	3.84

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Table 2 shows structure properties such as the BET surface area, the average pore diameter, and the BJH desorption pore volume of the samples from N₂ physisorption results. The BET surface areas of the MCM-41, Cat-A, Cat-B, and Cat-C were 952, 336, 555, and 519 m² g⁻¹, respectively. After addition of CeO₂ and SO₄²⁻, the BET surface area of pure MCM-41 decreased remarkably. The total pore volume decreased according to the sequence: Cat-A (0.1302 cm³ g⁻¹) < Cat-C (0.3665 cm³ g⁻¹) < Cat-B (0.4598 cm³ g⁻¹) < MCM-41 (1.0452 cm³ g⁻¹). The average pore diameter was as follows: 6.67 nm for Cat-A > 3.84 nm for Cat-C > 3.34 nm for Cat-B. It was obvious that the BET surface areas and total pore volume of Cat-A, Cat-B, and Cat-C both remarkably decreased compared with those of pure MCM-41, which indicated that the mesopores of the MCM-41 support were blocked by the aggregated Ce₂(SO₄)₃ and the separated CeO₂ and SO₄²⁻ species, leading to the disappearance of small pores and then a dramatically decrease in BET surface area. Based on the above discussion, the catalysts with the same composition but different impregnation sequences showed significantly different pore structures. It was reasonably speculated that the structural properties of Cat-A, Cat-B, and Cat-C catalysts were affected by the surface species over the samples.

3.3 Raman

The Raman spectra are shown in Fig. 3. It was obvious that all of samples were dominated by the main band at 460 cm⁻¹, which was attributed to the F 2g mode of fluorite-type CeO_2.²⁷ The bands at 1005 cm⁻¹ correspond to the reflection of sulfate groups over Cat-A and Cat-B catalysts.¹⁶ However, the sulfate species were not observed over Cat-C. In comparison with the Cat-B and Cat-C catalysts, the Cat-A catalyst not only showed a decrease in the intensity of the band at 460 cm⁻¹, but also an increase in the FWHM value. Ilieva et al.²⁸ reported that an increase in the FWHM value was ascribed to defect formation or the small crystal size in the CeO₂ structure. In addition, these observables in width of the F 2g mode implied the effect of oxygen vacancies and inhomogeneous strain (related to the existence of low-valence cerium states). Wang et al.^29,30 also proved that the shift and width in the main F 2g mode band was closely related to the presence of reduced states of cerium. These indicated that the surface species and structures depended on the impregnation sequence of CeO₂ and SO₄²⁻ species over MCM-41.

Fig. 3 Raman profiles of the catalysts.

3.4 TGA analysis

In order to investigate the effect of structures and surface species on the SCR activity over the Cat-A, Cat-B, and Cat-C catalysts, thermo-gravimetric analysis (TGA) was applied and the results are shown in Fig. 4. It was obvious that the whole temperature range could be divided into two stages:

Fig. 4 Thermo-gravimetric analysis of the catalysts.

In the first stage at 30–250 °C, a remarkable weight loss was observed over the three samples, corresponding to desorption of physical adsorbed water and dehydroxylation,¹⁶ and the weight loss was shown as follows: MCM-41 (12.5%) < Cat-B (13%) < Cat-C (17%) < Cat-A (21.6%).

The second stage was at about 600–900 °C and for the Cat-A and Cat-B catalysts a weight loss was observed, respectively. This weight loss was attributed to the decomposition of Ce₂(SO₄)₃.³¹ However, no weight loss was found over the Cat-C catalyst, implying that there was no formation of Ce₂(SO₄)₃.

3.5 XPS analysis

Atomic concentrations of the catalysts are presented in Table 3. According to the XPS results, it was obvious that surface atomic concentration of Ce over Cat-C was the highest, followed by Cat-A and Cat-B which showed the lowest surface atomic concentrations. Interestingly, the surface S atomic concentration (provided in ESI†) was not detected over the three catalysts, which indicated that the sulfated species were likely to insert into the MCM-41 pores.

Table 3 Surface atomic concentration of the catalysts from XPS results (atomic%)

Samples	Surface atomic concentration (atomic%)
	Ce	S	Si	O	Ce^3+/(Ce^4+ + Ce^3+)	Oα/(Oα + Oβ)
Cat-A	2.8	0	20.85	76.35	24	67.7
Cat-B	1.13	0	25.07	73.80	17.8	73.7
Cat-C	4.87	0	19.67	75.46	16.3	59.4

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The Ce 3d XPS spectra of the Cat-A, Cat-B, and Cat-C samples are shown in Fig. 5a. The u, u′′, u′′′, v, v′′, and v′′′ peaks were attributed to the Ce⁴⁺ state while the u′ and v′ peaks corresponded to the Ce³⁺ state.^32,33 It was well known that the CeO₂-based catalysts showed outstanding reducibility via the redox shift between Ce⁴⁺ and Ce³⁺ which improved the oxygen storage/release capacity. Thereby, the surface Ce³⁺ and Ce⁴⁺ concentrations were calculated and the results are shown in Table 3. The Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio ranked in the sequence of Cat-A (24%) > Cat-B (17.8%) > Cat-C (16.3%). Besides, the Ce³⁺ enrichment over Cat-A was ascribed to the appearance of lattice defects or oxygen vacancies. Combined with XRD results, the Ce₂(SO₄)₃ species were observed but the S species were not detected over the Cat-A catalyst as seen from Table 3. These further confirmed that the sulfonation could not exist at the outer surface of MCM-41 modified by Ce(NO₃)₃·6H₂O and SO₄²⁻ species. The sulfonation not only served as a stabilizer of oxygen vacancies during the impregnation and calcination process, but also contributed to reducing the oxidation state of the surface cations.^34,35 Thus, the surface Ce³⁺ concentration of Cat-A was improved when compared with those of Cat-B and Cat-C.

Fig. 5 (a) Ce 3d XPS spectra of the catalysts. (b) O 1s XPS spectra of the catalysts.

Fig. 5b shows the O 1s XPS spectra of the Cat-A, Cat-B, and Cat-C catalysts. The O 1s peaks at 531.5–532.3 eV were assigned to the lattice oxygen (marked as O_β)^36,37 and the other peak at 532–533 eV could be attributed to the chemisorbed oxygen (marked as O_α).³⁸ In order to reflect the variation of oxygen species, the percentage of the chemisorbed oxygen and lattice oxygen were measured by Gaussian curve fitting and the data are shown in Table 3. The O_α/(O_α + O_β) ratio of Cat-A, Cat-B, and Cat-C was 67.7%, 73.7%, and 59.4%, respectively.

3.6 H₂-TPR analysis

The reducibility of Cat-A, Cat-C, and Cat-B was characterized by H₂-TPR experiments and the results are exhibited in Fig. 6. The reduction peaks of the surface CeO₂ and the bulk CeO₂ over pure CeO₂ are located at 509 and 800 °C, respectively.³⁹ For the Cat-C catalyst, TPR curve consisted of two peaks: a low-temperature peak and a high-temperature peak, which was ascribed to the reduction of surface CeO₂ and bulk CeO₂, respectively. But the intensity reduction peaks of Cat-C were much weaker than those of Cat-A and Cat-B. The Cat-A catalyst with a shoulder peak at 660 °C exhibited a sharp reduction peak at 694 °C, corresponding to the reduction of surface CeO₂ and sulfated species. It was obvious that the Cat-B catalyst showed merely one reduction peak at 665 °C, which was attributed to the reduction of sulfated species. Compared with Cat-B and Cat-C, the reduction temperature of surface CeO₂ and sulfated species over Cat-A both shifted to higher temperatures, the reason of which might be that the strongest interaction between Ce, SO₄²⁻ and support existed. Besides, no reduction of sulfated species was observed over Cat-C. Furthermore, the intensity reduction peak of sulfated species over Cat-A was stronger than that of Cat-B. This phenomena indicated that the redox properties of Cat-A, Cat-B, and Cat-C were dramatically affected by the surface species and structures.

Fig. 6 H₂-TPR results of the catalysts.

3.7 NH₃-TPD analysis

NH₃-TPD analysis was performed in order to investigate the effect of surface acidity over Cat-A, Cat-B, and Cat-C. As shown in Fig. 7, Cat-A showed two NH₃-desorption peaks: the first one centered at 175 °C, attributed to weak acidity; and the other located at 320 °C, corresponding to medium acidity.⁴⁰ Weaker peaks were observed at 100–250 °C on the Cat-B and Cat-C when compared with that of Cat-A, implying that Cat-A possessed a larger amount of weakly bound NH₃ sites. However, the intensity peaks of NH₃ desorption over Cat-A, Cat-B, and Cat-C were remarkably higher than that of pure MCM-41, which meant that the surface acidity over the catalysts was greatly improved by the introduction of CeO₂ and SO₄²⁻ species into MCM-41. In addition, the NH₃-TPD results also revealed that the amount and distribution of surface acidity were affected significantly by the different structures and surface species over Cat-A, Cat-B, and Cat-C.

Fig. 7 NH₃-TPD results of the catalysts.

3.8 The SCR activity of the catalysts

Catalytic activities of the Cat-A, Cat-B, and Cat-C catalysts are shown in Fig. 8. Pure MCM-41 exhibited inferior catalytic activity and its maximum NO_x conversion was about 35% at 400 °C. With the addition of Ce(NO₃)₃·6H₂O and SO₄²⁻ species into MCM-41, the SCR performance was dramatically enhanced. The Cat-C catalyst exhibited slight NH₃-SCR activity and 18.7% NO_x conversion was attained at 250 °C. With an increase of temperature, the Cat-C catalyst showed above 80% NO_x conversion at 337–450 °C. For the Cat-B catalyst, the SCR activity was dramatically promoted, its light-off temperature T50 (NO_x conversion was 50%) was about 268 °C and over 80% NO_x conversion was in the temperature range of 294–450 °C. The T50 of Cat-A decreased to 230 °C and more than 80% NO_x conversion was observed from 252–450 °C. It was obvious that the catalytic activity of Cat-A was evidently higher than Cat-B and Cat-C with the same components but different surface species and structures, which resulted in different SCR performance. Consequently, the catalytic activities of the catalysts were affected by the surface species and structures of Cat-A, Cat-B, and Cat-C, and Cat-A showed the best catalytic activity.

Fig. 8 NO_x conversion for the catalysts.

The results of N₂O production and N₂ selectivity of Cat-A, Cat-B, and Cat-C catalysts are exhibited in Fig. 9. The low N₂O production (below 12 ppm) was generated over the samples. Above 97% N₂ selectivity of Cat-A, Cat-B, and Cat-C was attained at 150–450 °C. The Cat-A, Cat-B, and Cat-C catalysts showed excellent N₂ selectivity in the temperature range of 150–450 °C. Hence, the CeO₂ and SO₄²⁻ species doped into MCM-41 is a promising environmentally-friendly SCR catalyst.

Fig. 9 N₂O production and N₂ selectivity of the catalysts.

3.9 NO oxidation

In order to investigate NO oxidation ability of the Cat-A, Cat-B, and Cat-C catalysts, the properties of NO oxidation to NO₂ are exhibited in Fig. 10. It was obvious that NO₂ productions of Cat-A and Cat-C were remarkably higher than that of Cat-B at 150–450 °C. These indicated that Cat-A and Cat-C showed excellent oxidation ability. Roy et al.⁴¹ reported that the evident difference of NH₃-SCR catalysts in NO oxidation ability and SCR performance could be assigned to the role of NH₃ during the SCR process. Therefore, surface acidity played an important role in NO oxidation activity. Combined with SCR activity and NH₃-TPD results, Cat-C with the weakest surface acidity showed the least SCR performance. This indicated that the NO oxidation ability over Cat-A, Cat-B, and Cat-C was not the only factor for determining the catalytic activity.

Fig. 10 NO oxidation activity over the Cat-A, Cat-B and Cat-C catalysts.

3.10 NH₃ oxidation

The NH₃ oxidation activity and by-products (N₂O, NO, and NO₂) of the Cat-A, Cat-B, and Cat-C catalysts are shown in Fig. 11. The NH₃ conversion of Cat-A was higher than those of Cat-B and Cat-C, especially for low temperature (below 300 °C) and the NH₃ conversion decreased as follows: Cat-A > Cat-B ≈ Cat-C. The concentration of NO and N₂O over Cat-C was dramatically higher than that of Cat-A and Cat-B. The NO₂ production was not detected at 150–450 °C and N₂O production was detected at higher temperatures (above 350 °C) over Cat-A, Cat-B, and Cat-C. It was well known that the excellent redox property could result in unwanted NH₃ unselective oxidation, causing the negative NO_x conversion at high temperatures. From the H₂-TPR results, Cat-C showed the lowest reduction temperature, which contributed to NH₃ unselective oxidation. Thereby, Cat-C possessed inferior catalytic activity at high temperatures. It was reported that surface acidity played an important role in activating NH₃, which was the necessary step in the NH₃-SCR reaction, resulting in enhancement of the catalytic activity and N₂ selectivity.⁴² The formation of sulfated species over Cat-A and Cat-B could contribute to surface acidity, remaining a proper redox ability, and then weaken the NH₃ unselective catalytic oxidation, leading to an enhanced N₂ selectivity and SCR activity. Consequently, the surface acidity could contribute to the NH₃ conversion and catalytic activity. Combined with the NH₃-TPD and SCR performance results, Cat-A possessed excellent NH₃ conversion and SCR activity in comparison with Cat-B and Cat-C, which might be ascribed to the abundance of weak and medium surface acidity.

Fig. 11 NH₃ oxidation activity (a) and by-products over the catalysts: NO concentration (b); and N₂O concentration (c).

In order to elucidate the effect of surface species and structures on SCR performance over the catalysts, pure MCM-41, Cat-A, Cat-B, and Cat-C were calcined at 650 °C for 4 h and named as A-MCM-41, A-Cat-A, A-Cat-B, and A-Cat-C, respectively. The BET surface areas, pore volumes, and average pore diameters of the A-Cat-A, A-Cat-B, and A-Cat-C catalysts results are shown in Table 4. The BET surface areas ranked in the sequence of A-Cat-C (478) > A-Cat-A (412) > A-Cat-B (100). From Fig. 12, A-Cat-A exhibited the best catalytic activity, followed by A-Cat-B, and A-Cat-C showed an inferior performance. When the calcination temperature was 650 °C, a portion of sulfated species would be decomposed from the TGA results; the structure would be destroyed under high temperature treatment, and this led to the decrease of catalytic activity in either event (the decomposition of sulfated species or the collapse of structure). Furthermore, pure A-MCM-41 possessed a lager BET surface area (921 m² g⁻¹) compared with that of the three aged samples, but it showed quite poor SCR activity. No obvious linear dependence between the catalytic activity and the BET surface areas was established. In addition, the catalytic activities of A-Cat-A, A-Cat-B, and A-Cat-C were decreased when compared with those of the fresh samples. Thereby, it was reasonable that decomposition of sulfated species could result in deactivation over the samples. Consequently, the formation of sulfated species over MCM-41 modified by Ce(NO₃)₃·6H₂O and SO₄²⁻ contributed to the improvement of SCR activity.

Table 4 BET measurements for the A-Cat-A, A-Cat-B and A-Cat-C catalysts

Samples	BET (m^2 g^−1)	Total pore volume (cm^3 g^−1)	Average pore diameter (nm)
A-MCM-41	921	0.9785	3.38
A-Cat-A	412	0.1468	5.83
A-Cat-B	100	0.1164	7.55
A-Cat-C	478	0.3703	3.57

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Fig. 12 NO_x conversion for the aged catalysts.

To investigate the effect of surface species and structures on catalytic activity over the Cat-A, Cat-B, and Cat-C catalysts, the SCR performance was studied by a macro-kinetic approach and the results are shown in Fig. 13. Generally speaking, with respect to NO, the NH₃-SCR reaction was a first-order reaction.^43,44 Therefore, the effective first-order rate constant was related to NO conversion by:

where k: reaction constant (cm³ g⁻¹ s⁻¹); V: the total gas flow rate (mL s⁻¹); W: the catalyst weight (g); x: the NO conversion (%); A: the pre-exponential factor; R: the gas constant (8.3145 J mol⁻¹ K⁻¹); T: the temperature (K); and E_a: the apparent activation energy (kJ mol⁻¹). The measured activation energies of NO conversion over Cat-A, Cat-B, and Cat-C were 60, 78, and 71 kJ mol⁻¹, respectively (Fig. 13). These differences originated from different samples with the same components but different surface species and structure. The different surface species and structures over Cat-A, Cat-B, and Cat-C should be further discussed because the performance of the samples was significantly affected by the different structures, redox properties, and surface species. Combined with the XRD and BET results, the particle sizes of CeO₂ over Cat-A was smaller than the other two samples. It was reported⁴⁵ that excellent dispersion of metal species on the support contributed to the catalytic activity of supported metal catalysts, which might be another reason that the Cat-A catalyst showed superior SCR performance. However, the Cat-A catalyst possessed the least BET surface area for the fresh samples. The reason was that the small pores were blocked or filled by the formation of sulfate species, resulting in the disappearance of small pores and then a decrease in the specific surface area. Meanwhile, the Cat-A catalyst exhibited the best catalytic activity due to the formation of sulfated species. From TGA results, the sulfated species existed on the Cat-A and Cat-B catalysts, and the weight loss of Cat-A was more evident than that of Cat-B and Cat-C at 30–150 °C, indicating occurrence of the largest number of hydroxyls over Cat-A, which contributed to the surface acidity, and then resulted in the excellent catalytic activity. Taking the SCR performance results into account, the catalytic activity ranked as follows: Cat-A > Cat-B > Cat-C. This phenomenon further confirmed that the SCR activity was dramatically improved by the formation of sulfated species. Considering the Raman, XPS, and H₂-TPR results, the occurrence of the interaction between Ce, SO₄²⁻ and MCM-41 led to a difficulty in reducing the surface CeO₂ and sulfated species over the Cat-A catalyst. However, the sulfonation could contribute to the formation of Ce³⁺ concentration and surface acidity. The higher Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio led to more oxygen vacancies, unsaturated chemical bonds, and a charge imbalance, which was beneficial to the activation and transportation of active oxygen species, resulting in the improvement of SCR activity. The surface acidity could contribute to adsorption of ammonia and then improve the catalytic activity. Hence, the sequence of catalytic activities over Cat-A, Cat-B, and Cat-C was the same as that of Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio and amount of surface acidity. These also confirmed that the formation of sulfated species on the surface of MCM-41 could contribute to the SCR activity. Meanwhile, the O_α/(O_α + O_β) of Cat-A (67.7%) was lower than that of Cat-B (73.7%), which was due to the formation of sulfated species and then covered on the surface of Cat-A, leading to a decrease in surface oxygen species. Therefore, the catalytic activity of Cat-A, Cat-B, and Cat-C was dependent on the different surface species and structures.

Fig. 13 Arrhenius plots for NO_x conversion over different catalysts.

3.11 The effect of H₂O and SO₂ on SCR activity

The emission of SO₂ and H₂O is unavoidable in exhaust gases from power plants. Therefore, the effect of SO₂ and H₂O on SCR performance should not be ignored. Fig. 14 presents the effect of SO₂ and H₂O on catalytic activity at 350 °C over the Cat-A, Cat-B, and Cat-C catalysts. As shown in Fig. 14, nearly 100% NO_x conversion of Cat-A was obtained in the presence of 5 vol% H₂O, and no decrease in SCR activity was observed for 9.5 h. When 5 vol% H₂O and 50 ppm SO₂ were added, the NO_x conversion of Cat-A was decreased. However, almost 80% NO_x conversion still was attained during the measured period although the NO_x conversion could not recover to its original value after 5 vol% H₂O and 50 ppm SO₂ were cut off. However, the Cat-A catalyst still exhibited a superior SCR performance and 97% NO_x conversion was obtained. Thereby, Cat-A possessed the best SO₂+H₂O durability.

Fig. 14 The effect of H₂O and SO₂ on SCR activity over Cat-A.

4. Conclusions

The present work investigated the effect of surface species and structures on SCR activity of a CeO₂ and SO₄²⁻ doped MCM-41 support. The results revealed that Cat-A showed the best SCR performance and above 80% NO_x conversion was obtained in the temperature range of 294–450 °C. The XRD results suggested that Cat-A possessed the smallest CeO₂ particle sizes. The BET results suggested that the specific surface area was not the only factor affecting catalytic activity. The Raman and H₂-TPR results implied that an interaction between Ce, SO₄²⁻ and MCM-41 existed and the sulfated species formed over Cat-A and Cat-B, which could affect the redox ability. The TGA results indicated that the sulfated species were generated over Cat-A and Cat-B. The XPS and NH₃-TPD results proved that Cat-A possessed higher surface Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio and a larger amount of surface acid sites. The apparent activation energy of Cat-A, Cat-B, and Cat-C was 60, 71, and 78 kJ mol⁻¹, respectively. These above factors depend on the structure and surface species over CeO₂ and SO₄²⁻ doped MCM-41, which could in turn affect the catalytic activity.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 21307047) and Academic Newcomer Award of Yunnan Province.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09505d

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