Effect of Catalyst Crystallinity on V-Based Selective Catalytic Reduction with Ammonia.

In this study, we synthesized V2O5-WO3/TiO2 catalysts with different crystallinities via one-sided and isotropic heating methods. We then investigated the effects of the catalysts' crystallinity on their acidity, surface species, and catalytic performance through various analysis techniques and a fixed-bed reactor experiment. The isotropic heating method produced crystalline V2O5 and WO3, increasing the availability of both Brønsted and Lewis acid sites, while the one-sided method produced amorphous V2O5 and WO3. The crystalline structure of the two species significantly enhanced NO2 formation, causing more rapid selective catalytic reduction (SCR) reactions and greater catalyst reducibility for NOX decomposition. This improved NOX removal efficiency and N2 selectivity for a wider temperature range of 200 °C-450 °C. Additionally, the synthesized, crystalline catalysts exhibited good resistance to SO2, which is common in industrial flue gases. Through the results reported herein, this study may contribute to future studies on SCR catalysts and other catalyst systems.

1. Introduction

Air pollution has recently become a critical, global issue [1]. In response, environmental regulations have been tightened to reduce the emissions of chemical impurities (such as NOX, SOx, CO, volatile organic compounds (VOCs), and particulate matter (PM)) from power plants, boilers, and mobile sources [2,3]. Among the numerous air pollutants, nitrogen oxides (NOX: NO, NO2, and N2O) are extremely dangerous as they can easily disperse over long distances and form secondary PM2.5 by reacting with water vapor, which causes acid rain and smog, contributes to global warming [4,5], and can deeply penetrate human lungs, causing adverse health effects such as increased cardiovascular and respiratory morbidity [6].

Owing to the harmful effects of NOx, several technologies, such as selective catalytic reduction (SCR), selective noncatalytic reduction (SNCR), and non-selective catalytic reduction (NSCR), have been used to reduce NOx emissions. SCR with ammonia, which converts NOX in fuel gas into N2 and H2O, is the most efficient NOX removal technology, as the process emits no secondary pollutants and can reduce NOx emissions by 80–100% at a relatively low temperature (approximately 350 °C) [7,8].

Commercially, V2O5-WO3/TiO2 has been used as an SCR catalyst due to the strong catalytic activity of V2O5, and lower oxidation activity for the conversion of SO2 to SO3 in fuel gas. However, it has a narrow, high activation temperature range (300–400 °C), and its performance is reduced at low temperatures (below 300 °C), which induce the oxidation of SO2 to SO3 [9,10]. The flue gas temperature of industrial processes is typically as low as 300 °C, and the temperature of diesel engines has a wide range (100–400 °C) [5,11]. Therefore, the use of V2O5-WO3/TiO2 is restricted and requires adjustment, such as in the form of upstream installation and desulfurization.

Extensive studies have been conducted to develop new catalysts that can be effective under a low and wide temperature range of 200–450 °C. For example, Liu et al. designed a W-promoted MnO catalyst (MnWO) composed of a unique core–shell structure with Mn3O4 surrounded by Mn5O8, and achieved a high NOx reduction efficiency from 60 °C to 250 °C [12]. Huang et al. fabricated multi-walled carbon nanotube (CNT)-supported vanadium catalysts, in which vanadium particles were highly dispersed on the walls of the carbon nanotubes, which exhibited excellent activity in the SCR of NO at 100–250 °C [13]. However, the utilization of these catalysts in industrial fields is limited, as the catalysts are only activated at low temperatures, and they are deactivated when they come in to contact with the sulfur and water in exhaust gas at low temperatures below 300 °C [14,15].

The activity of catalytic materials is closely related to their crystalline structure [16,17]. Wang et al. reported that the formation of crystalline tungsten oxide on the surface of titania results in higher water resistance and NOX removal efficiency at temperatures below 250 °C than those achieved by amorphous tungsten oxide [18]. Inomata et al. reported that V2O5 SCR catalysts with low crystallinity achieved better catalytic performance than that of V2O5 with high crystallinity [19]. They also reported that the crystalline V2O5 has a higher catalytic performance than amorphous V2O5 under the same sintering conditions [20]. Recently, many studies have been conducted on V-base SCR catalysts to enhance catalytic activity under low temperature [21,22]. However, the effect of catalyst crystallinity on the performance of V-based SCR catalysts remains unknown.

In this study, we explored the effect of crystallinity of V2O5-WO3/TiO2 on the NOX removal efficiency and improved the catalytic performance under temperatures ranging from 200 °C to 450 °C by controlling the crystallinity, with excellent thermal stability. The crystallinity of the V2O5 and WO3 catalysts was adjusted by altering the heating methods, and this was evaluated via transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman, and selected area electron diffraction (SAED) analyses.

2. Materials and Methods

2.1. Synthesis of V2O5-WO3/TiO2 Catalysts

Catalysts containing 2 wt.% and 10 wt.% of V2O5 and WO3/TiO2 were prepared following the impregnation method, respectively. NH4VO3 (0.256 g, 99.99%, Sigma-Aldrich Inc., St. Louis, MO, USA) and (NH4)6H2W12O40 xH2O (1.062 g, 99.99%, Sigma-Aldrich Inc., St. Louis, MO, USA) were dissolved in 100 mL of deionized water with oxalic acid (0.386 g, 99.999%, Sigma-Aldrich Inc., St. Louis, MO, USA), which acted as a solubility agent. TiO2 powder (8.800 g, NT-01, NANO Co., Ltd., Sang-ju, Republic of Korea) was mixed with the prepared solution, and the mixture was stirred for 2 h. The amorphous V2O5-WO3/TiO2 catalyst was prepared by drying the mixture on one side by heating the bottom of the beaker with a hot plate, while the crystalline V2O5-WO3/TiO2 catalyst was prepared following the isotropic heating method, in which the beaker was submerged in an oil bath. The prepared samples were dried at 110 °C for 12 h, and the obtained powders were then calcined at 500 °C in a furnace for 5 h under atmospheric pressure.

2.2. Catalyst Characterization

The surface morphology and elemental composition of the samples were investigated by field emission scanning electron microscopy (FE-SEM, model: SU8020/Hitachi, Tokyo, Japan), transmission electron microscope (TEM, model: JEM-2100F/JEOL Ltd., Tokyo, Japan), and electron energy loss spectroscopy (EELS) at an accelerating voltage of 10.0 kV. Additionally, we analyzed the extent of crystallinity using X-Ray Diffraction (XRD, model: Ultima IV/Rigaku, Tokyo, Japan) with Cu Kα (λ = 0.15406 nm) radiation in the 2θ range from 20° to 80° at a scan rate of 1°/min. The Raman spectra (Raman, model: alpha300s/WITec, Ulm, Germany) were measured using a 532 nm laser to generate an excited state to observe the structure of the catalysts. The textural properties were analyzed following the Brunauer–Emmett–Teller (BET, model: ASAP2020/Micromeritics Instrument Corp., Norcross, USA) method. X-ray photoelectron spectroscopy (XPS, model: K Alpha+/Thermo Scientific, Waltham, USA) was conducted with Al Kα radiation to confirm the oxidation states of the samples, and the binding energy of C1s was normalized as 284.8 eV. The reduction properties of the catalyst materials were measured by NH3-temperature-programmed desorption (NH3-TPD, model: AutoChem II 2920/Micromeritics Instrument Corp, Norcross, USA). The samples were pretreated at 150 °C in a current of N2 for 4 h to remove physiosorbed NH3 species and organic matters, and NH3 was then adsorbed with 10% NH3/He gas at 150 °C for 1 h. The TPD experiment was conducted under a temperature range of 100–900 °C. A H2-temperature-programmed reduction (H2-TPR, model: AutoChem II 2920/Micromeritics Instrument Corp, Norcross, USA) experiment was conducted, during which the samples were immersed in a current of 10% H2/Ar in the 150–900 °C temperature range.

2.3. Catalytic Measurement

The NH3-SCR activities were evaluated in a fixed-bed reactor under high atmospheric pressure. The operating temperature was varied from 200 °C to 500 °C, and the reactive gas was composed of 300 ppm each of NO, NH3 (NH3/NOX = 1.0), SO2, and 5 vol.% of O2 with a balance of N2 at a total flow rate of 500 sccm. During evaluation, 0.35 mg of the powder catalyst (sieved to 40–60 mesh) was tested, which yielded a gas hourly space velocity (GHSV) of 60,000 h−1. The reactive gas concentration was continuously monitored via Fourier transform-infrared spectroscopy (model: CX-4000/Gasmet, Vantaa, Finland) and O2 analyzer (Oxitec 5000, Marienheide, Germany). The NOX removal efficiency and N2 selectivity were calculated according to Equations (1) and (2), respectively.

3. Results and Discussion

Figure 1 shows the SEM (a, b) and TEM (c, d) images of the V2O5-WO3/TiO2 catalysts prepared following the one-sided heating (a–c) and isotropic heating (b–d) methods. Both prepared V2O5-WO3/TiO2 catalysts exhibited similar particle sizes, shapes with diameters ranging from 15 nm to 50 nm, specific surface areas, pore volumes, and pore sizes (Table 1). The catalyst particles were composed of V2O5 and WO3 nanoparticles on TiO2 supports. It should be noted that the prepared nanoparticles were similar in size to the TiO2 powders [23]. The insets of Figure 1c,d show the EELS elemental mapping of the prepared catalysts, in which the red, blue, and green areas indicate V, W, and Ti, respectively. The V2O5 and WO3 were uniformly distributed on the TiO2 supports with no agglomeration, confirming that the drying process did not affect the morphology of the prepared catalysts. Table 2 shows the V2O5, WO3, and TiO2 weight fractions of the catalysts, respectively.

Figure 1

SEM images of the V2O5-WO3/TiO2 prepared by the (a) one-sided heating and (b) isotropic heating methods. TEM images of the V2O5-WO3/TiO2 prepared by the (c) one-sided heating and (d) isotropic heating methods (insets show the EELS elemental mapping of V, W, and Ti).

Table 1

Brunauer–Emmet–Teller (BET) results of the V2O5-WO3/TiO2 prepared by the one-sided heating and isotropic heating methods.

Sample	BET Surface Area; SBET (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)
One-sided	69.6	0.252	14.48
Isotropic	70.2	0.257	14.67

Table 2

X-ray fluorescence analysis of the V2O5-WO3/TiO2 prepared by the one-sided heating and isotropic heating methods.

Sample	TiO2	WO3	V2O5	SO3	SiO2
One-sided	86.92	10.19	2.02	0.70	0.17
Isotropic	87.05	10.04	2.03	0.66	0.22

XRD measurements were taken to analyze the impact of the heating method on the crystalline structures of the prepared catalysts (Figure 2a). The prepared samples exhibit clear anatase TiO2 signals, V2O5 and WO3 signals were not observed because they are spread uniformly with low concentration [24]. Raman analysis was also conducted to determine how the heating conditions affected the structure of the V2O5-WO3/TiO2, as shown in Figure 2b,c. The spectra of the V2O5-WO3/TiO2 catalysts contained peaks at 144.7, 197.3, 401.5, 518.5, and 639.1 cm−1, in the spectra, which are typical of anatase TiO2 (Figure 2b) [25]. Figure 2c shows the structure of the vanadium and tungsten oxides in the 700–1100 cm−1 range. As active sites of V2O5-WO3/TiO2 catalysts in SCR reactions, the state of the vanadium oxide species on the surface of the V2O5-WO3/TiO2 plays a key role in its catalytic behavior [26]. The band at 988.7 cm−1 could be attributed to the V–O vibration of crystalline V2O5, and the bands at 800.5 cm−1 were associated with the W–O–W stretching of octahedrally coordinated W units [27,28,29]. The Raman spectra showed that the V2O5-WO3/TiO2 catalyst prepared by isotropic heating had high crystallinity, while that prepared by one-sided heating was mostly amorphous.

Figure 2

(a) XRD patterns and (b,c) Raman spectra of V2O5-WO3/TiO2 with different structures (black and red lines represent the V2O5-WO3/TiO2 prepared by the one-sided heating and isotropic heating methods, respectively).

To further investigate the crystallinity of the prepared catalysts, we also compared the SAED patterns of the catalysts prepared using the one-sided heating (Figure 3a–c) and isotropic (Figure 3d–f) heating methods. In the SAED patterns, single spots only become visible when the beam is diffracted by a single crystal; however, amorphous materials yield ring patterns [30,31]. The diffraction patterns of V2O5 and WO3 prepared by the one-sided heating method were ring-shaped (Figure 3a–b), indicating amorphous structures [32]. However, those prepared following the isotropic heating method exhibited clear crystalline diffraction (Figure 3d,e). The TiO2 nanoparticles maintained their anatase structure, even after the application of heat treatment (Figure 3c,f). This indicates that the crystallinity of the catalyst was greatly affected by the heating conditions. That is, one-sided heating produced an amorphous V2O5-WO3/TiO2 catalyst, while isotropic heating produced a crystalline V2O5-WO3/TiO2 catalyst.

Figure 3

TEM images and SAED patterns (inset) of the V2O5-WO3/TiO2 prepared by (a–c) one-sided heating and (d–f) isotropic heating methods.

To identify the effect of the crystallinity of V2O5-WO3/TiO2 on its SCR performance, its NOX removal efficiencies were measured in a fixed bed (Figure 4a–c). We found that the NOX removal efficiency of the amorphous V2O5-WO3/TiO2 catalyst was negatively impacted at temperatures below 300 °C; however, it exceeded 94% at 300–400 °C. The crystalline V2O5-WO3/TiO2 catalyst achieved a NOX removal efficiency of 82% at 200 °C; thus, it was 27% more efficient than the amorphous V2O5-WO3/TiO2 catalyst. Moreover, the efficiency increased to 100% in the temperature range of 240–400 °C (Figure 4a). NH3 conversion of amorphous and crystalline V2O5-WO3/TiO2 also showed a similar to the NOX conversion value (Figure S1). Figure 4b shows the N2 selectivity of the V2O5-WO3/TiO2 catalysts at different temperatures. A trace amount of N2O in the amorphous and crystalline V2O5-WO3/TiO2 was generated at 350 °C, and the N2 selectivity of the amorphous and crystalline V2O5-WO3/TiO2 catalysts reached 73% and 81% from 500 °C, respectively. Figure 4c shows that SO2 affected the NOX removal efficiency of the V2O5-WO3/TiO2 catalysts at 250 °C, which usually shows high deactivation caused by SO2. When SO2 gas was not added to the reactor, the NOX removal efficiencies of the amorphous and crystalline V2O5-WO3/TiO2 were maintained at 80% and 99%, and then rapidly decreased to 69% and 89% with the introduction of SO2, respectively. However, it returned to 80% and 99% when the SO2 was removed. When SO2 was introduced to the reactor, SO2 gas directly reacts with V2O5-WO3/TiO2 catalysts, and it produces the ammonium sulfates [15]. Ammonium sulfates slowly block the active sites of V2O5-WO3/TiO2 catalysts, and it leads to the decrease of NOx removal efficiency. When SO2 is removed, the produced ammonium sulfates were gradually removed, and the V2O5-WO3/TiO2 catalysts can be regenerated and return to the initial condition. The crystalline V2O5-WO3/TiO2 showed slightly high resistance against SO2 compared with amorphous V2O5-WO3/TiO2. Additionally, Figure S2 indicates that the crystalline V2O5-WO3/TiO2 catalyst showed higher NOX removal efficiency than the amorphous V2O5-WO3/TiO2 catalyst in the temperature range of 150–500 °C under gas conditions containing SO2.

Figure 4

(a) NOx removal efficiency; (b) N2 selectivity of the V2O5-WO3/TiO2 catalysts and (c) SO2 tolerance of the V2O5-WO3/TiO2 catalysts with different crystal structures at 250 °C (black and red lines represent the amorphous and crystalline V2O5-WO3/TiO2, respectively). Reaction conditions: [NO] & [NH3] = 300 ppm, [O2] = 5 vol.%, [GHSV] = 60,000 h−1.

We conducted XPS, NH3-TPD, and H2-TPR analyses to further elucidate the effect of crystallinity on the NOX removal performance of the V2O5-WO3/TiO2 catalysts, as shown in Figure 5. Figure 5a shows the survey peaks of the XPS results. The O 1s peaks can be fitted to two different peaks, i.e., chemisorbed oxygen (Oα) at 530.9 eV and lattice oxygen (Oβ) at 530.1 eV [24]. Surface chemisorbed oxygen plays a critical role in the oxidation of NH4+ in SCR reactions as it is more mobile than lattice oxygen and promotes the oxidation of NO to NO2 [33,34]. Therefore, the presence of NO2 induces a “fast SCR” and the Oα/(Oα+Oβ) concentration ratio is the important value for the SCR reaction [33]. Figure 5b clearly indicates that the Oα ratio of the crystalline V2O5-WO3/TiO2 exceeded that of the amorphous V2O5-WO3/TiO2. V 2p was mainly composed of V5+ and V4+, and the two fitted peaks at 517.1 and 516.1 eV could be attributed to V5+ 2p3/2 and V4+ 2p3/2, respectively [35]. According to previous studies, V4+ can promote the adsorption of oxygen and form reactive oxygen species on the surface of a catalyst, leading to fast redox cycles and improving the redox properties [36]. Figure 5c shows that crystalline V2O5-WO3/TiO2 contains a higher proportion of V4+ than amorphous V2O5-WO3/TiO2. The V4+/(V4++V5+) ratios of the crystalline and amorphous V2O5-WO3/TiO2 were 0.38 and 0.23, respectively (Table 3). Additionally, the W 4f on the surface of the catalyst was mainly composed of W 4f7 and W 4f5, while the Ti 3p was centered at 35.76, 38.12, and 37.50 eV, with a hexavalent state in the form of WO3 [37,38]. The W 4f XPS results of V2O5-WO3/TiO2 did not differ significantly, as shown in Figure 5d.

Figure 5

XPS spectra for (a) survey, (b) O1s, (c) V2p, and (d) W4f of amorphous V2O5-WO3/TiO2 and crystalline V2O5-WO3/TiO2 (e) NH3-TPD profiles of amorphous V2O5-WO3/TiO2 and crystalline V2O5-WO3/TiO2. B and L indicate Brønsted and Lewis acid sites, respectively. (f) H2-TPR profiles of amorphous V2O5-WO3/TiO2 and crystalline V2O5-WO3/TiO2.

Table 3

The ratio of Oα, V4+ of amorphous and crystalline V2O5-WO3/TiO2 measured by XPS, NH3-temperature-programmed desorption and H2-temperature-programmed reduction integral intensity of amorphous and crystalline V2O5-WO3/TiO2.

Sample	Oα/(Oα + Oβ)	V4+/(V4+ + V5+)	NH3 Desorption (cm3/g)	H2 Consumption (cm3/g)
Amorphous	0.30	0.23	13.74	40.17
Crystalline	0.33	0.38	16.97	46.06

Figure 5e shows the NH3-TPD results for the amorphous and crystalline V2O5-WO3/TiO2 catalysts, such as the effects of their structures on the contents and strengths of the surface acidic sites of the catalysts [39,40]. The NH3-TPD profile of the amorphous and crystalline V2O5-WO3/TiO2 significantly varied in the temperature range of 100–900 °C, in which NH3 desorption of 13.74 cm3/g and 16.97 cm3/g was measured, respectively (Table 3). The thermal conductivity detector (TCD) signals at the lower and higher temperatures were considered to be Brønsted and Lewis acid sites [35,41], and the concentration of desorbed NH3 of the crystalline V2O5-WO3/TiO2 catalyst was higher, indicating a higher capability for adsorption. According to these results, the crystalline active materials contained more Brønsted and Lewis acid sites. We also produced H2-TPR profiles to investigate the redox properties of the amorphous and crystalline V2O5-WO3/TiO2. (Figure 5f) The amorphous V2O5-WO3/TiO2 exhibited have three apparent reduction peaks centered at 387.9 °C, 466.5 °C, and 796.4 °C, which could be assigned to the co-reduction of V5+ to V3+, which corresponds to the surface vanadium species, reduction of W6+ to W4+, and reduction of W4+ to W0 in tungsten oxide [30,42]. However, the main reduction peaks of crystalline V2O5-WO3/TiO2 shifted to lower temperatures at 394.9 °C, 461.2 °C, and 780.7 °C, respectively, which could be because the higher crystallinity of the active materials reduced the large amount of NOX, which promoted the release of lattice oxygen to further reduce the vanadium and tungsten species [42]. Consequently, we can confirm that the crystalline V2O5-WO3/TiO2 catalysts exhibited enhanced performance when NH3 gas adsorption and the reduction of NO and NO2 gas increased.

4. Conclusions

In this study, amorphous and crystalline V2O5-WO3/TiO2 catalysts were synthesized following two different heating methods to investigate the effects of crystallinity on the acidity, surface species, and performance of the catalysts. The isotropic heating method formed crystalline V2O5 and WO3 structures that contained more Brønsted and Lewis acid sites. The crystalline V2O5-WO3/TiO2 catalyst also had higher chemisorbed oxygen and V4+ species ratios than the amorphous catalyst. The crystalline structure of the V and W species significantly enhanced the SCR reactions on the surface of the catalysts, resulting in high NOX removal efficiency and N2 selectivity over a wide temperature range of 200–450 °C. These results may contribute to future studies on SCR catalysts and other catalyst systems.