Ce_1−xFe_xVO₄ with Improved Activity for Catalytic Reduction of NO with NH₃

Abstract

A series of Ce_1−xFe_xVO₄ (x = 0, 0.25, 0.50, 0.75, 1) catalysts prepared by modified hydrothermal synthesis were used for selective catalytic reduction (SCR) of NO_x with NH₃. Among them, Ce_0.5Fe_0.5VO₄ showed the highest catalytic activity. The catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction using H₂ (H₂-TPR), and temperature-programmed desorption of NH₃ (NH₃-TPD). The results indicated the formation of Ce-Fe-V-O solid solutions. The average oxidation states (AOS) of Ce, Fe, V, and O atoms changed obviously with the incorporation of Fe³⁺ into CeVO₄, and the acidity of Ce_0.5Fe_0.5VO₄ differs from that of CeVO₄ and FeVO₄. The presence of more acid sites and a sharp increase in active oxygen species in Ce_0.5Fe_0.5VO₄ effectively improved the selective catalytic reduction (SCR) activity.

1. Introduction

NO_x pollutants released from power plants and car emissions have attracted much concern for their serious harm to the atmospheric environments and human health. The selective catalytic reduction (SCR) of NO_x with NH₃ has been extensively studied as one of the most effective methods to remove NO_x [1,2,3]. V₂O₅-WO₃-TiO₂ is the most widely used SCR catalyst for its extraordinarily high activity in NO reduction. However, it generally operates in a high-temperature window (300–400 °C), and it can be deactivated by the flue gas containing H₂O/SO₂, phosphorus, ash, and alkali metals [4,5]. CeO₂-based materials have been studied as either catalysts or catalyst supports due to their excellent redox property and oxygen-storage capacity [6]. These catalysts include CeO₂-WO₃-TiO₂ [7], CeO₂-ZrO₂-WO₃ [8], Mn/Ce-ZrO₂ [9], MnO_x-CeO₂ [10], Sn-MnO_x-CeO₂ [11], MnO_x-CeO₂ [12], and V₂O₅/CeO₂ [13]. However, CeO₂-based catalysts may suffer from sulfation [14].

Metal vanadates (AVO₄, A = La, Fe, Ce, etc.) is another kind of catalysts attracting attention. The reported AVO₄-based catalysts include FeVO₄/TiO₂ [15], Ni-FeVO₄/TiO₂ [16], Fe_xEr_1−xVO₄ [17], Sn-CeVO₄ [18], Sb-CeVO₄ [19], Ce_0.5RM_0.5VO₄ (RM = Tb, Er, or Yb) [20], LaVO₄/TiO₂-WO₃-SiO₂ [21], CeV_1−xW_xO₄ [22], and Zr-CeVO₄/TiO₂ [23]. For instance, Zr-CeVO₄/TiO₂ has high N₂ selectivity and H₂O/SO₂ durability in NH₃-SCR [23]. However, further promotion of AVO₄ is needed [24,25].

Generally, there are two approaches to design better NH₃-SCR catalysts. One approach is to modify the primary catalyst with one metal oxide or multiple metal oxides. The other approach is to modify supports used to disperse transition metal oxides [26]. In particular, the formation of metal oxide solid solutions may lead to better catalytic properties [27], since it may increase the catalyst’s surface area, thermal stability, and the number of active oxygen sites.

Previous research reported that Sn, Zr, La, etc., were doped into CeVO₄ [18,22,24,28] separately to investigate their influences on NO conversion. For instance, Huang et al. developed Sn_0.20Ce_0.80VO₄ for NH₃-SCR by citric-acid-assisted solvothermal synthesis and post-hydrothermal treatment [18]. Zhao et al. developed Ce_0.85Zr_0.15VO₄ for NH₃-SCR [24]. However, the content of Sn or Zr in the solid solutions was not very high. Kang et al. synthesized a series of Fe_δCe_1−δVO₄ (δ ≤ 0.4) catalysts with good SO₂ tolerance and NO conversion activity by a hydrothermal method [14]. The highest NO conversion appeared at δ = 0.3 for Fe_δCe_1−δVO₄ (δ ≤ 0.4). When the Fe content was higher, minor segregation of CeO₂ occurred.

In this paper, Ce_1−xFe_xVO₄ (x ≤ 0.75) solid solutions were prepared by a modified sol-gel hydrothermal method. The objective of our research was to incorporate more Fe into the solid solution to increase the number of active oxygen sites, facilitate the interactions among Ce, Fe, and V in fully mixed solid solutions, and achieve high activity in NH₃-SCR.

2. Results and Discussion

2.1. Catalytic Performance

Figure 1 depicts the NO_x conversions on five catalysts, in the reaction temperature range of 150–400 °C. The NO_x conversion on these catalysts increases as the temperature increases from 150 to 300 °C, and then decreases. Among them, Ce_0.5Fe_0.5O₄ gives the highest NO_x conversion (75.3%) at 300 °C. The catalytic activities of FeVO₄ and CeVO₄ are lower than those of Ce_1−xFe_xVO₄. The overall activity of these catalysts follows the trend of CeVO₄ < Ce_0.75Fe_0.25VO₄ < Ce_0.5Fe_0.5VO₄ > Ce_0.25Fe_0.75VO₄ > FeVO₄, and the catalytic data are reproducible (Figure S1 in Supplementary Materials). It should be mentioned that the objective of this research was to identify the activity trends among these catalysts, i.e., to identify the best catalyst and understand why it is better than other catalysts. In addition, a small catalyst weight (200 mg) and a high flow rate (1000 mL/min) were adopted herein. Thus, the highest conversion was not 100%. Nevertheless, complete conversion can usually be achieved easily when more catalyst is used or the flow rate of the gas is decreased [27].

The stability of the optimal catalyst (Ce_0.5Fe_0.5VO₄) at 300 °C was tested as a function of time on stream. As shown in Figure S2 from Supplementary Materials, the NO_x conversion increases slightly in the initial 100 min, and it then becomes stable (74–75% conversion).

To set the result in perspective, the catalytic activity of Ce_0.5Fe_0.5VO₄ was compared with those of other catalysts. Ce_0.5Fe_0.5VO₄ is more active than Ce_0.5Fe_0.5O_x reported in our previous work [27] because Ce_0.5Fe_0.5O_x can only achieve about 70% NO_x conversion at 300 °C under relatively milder conditions (200 mg catalyst, [NO] = 500 ppm, flow rate = 500 mL/min) [27], whereas Ce_0.5Fe_0.5VO₄ can achieve 75.3% NO_x conversion at 300 °C under relatively harsher conditions (200 mg catalyst, [NO] = 500 ppm, flow rate = 1000 mL/min). For comparison, Fe_0.3Ce_0.7VO₄ developed by Zhang and co-workers can lead to >95% NO_x conversion at 250–300 °C under relatively milder conditions (300 mg catalyst, [NO] = 500 ppm, flow rate = 250 mL/min) [14]. In a recent work by Tang and co-workers, Ce-W/TiO₂ can leads to ~90% NO conversion at 300 °C under conditions identical to those in our current work (200 mg catalyst, [NO] = 500 ppm, flow rate = 1000 mL/min) [29].

2.2. Regular Characterization

Figure 2 gives the XRD patterns of samples. CeVO₄ shows peaks at 2θ = 18.4, 24.3, 32.7, 34.6, 39.3, 43.8, 48.2, and 55.9°, attributed to the (101), (200), (112), (220), (301), (103), (312), and (420) planes of tetragonal CeVO₄, respectively [30]. The other minor peaks are all consistent well with the standard pattern of CeVO₄ (JCPDS: 12-0757). The XRD patterns of FeVO₄ shows characteristic peaks at 25.2, 27.1, 27.8, 28.0, 42.1, 42.2, 49.8, 52.4, 54.8 and 59.2°, are ascribed to the (012),$(\overline{2}$01),$(1\overline{1}$2), $(\overline{2}$10), $(\overline{3}$10), $(0\overline{3}$3), (123), $(3\overline{1}$2), $(\overline{4}$12) and$(2\overline{2}$3) lattice planes of triclinic FeVO₄ (JCPDS: 71-1592) [31]. The XRD patterns of Ce_0.75Fe_0.25VO₄, Ce_0.5Fe_0.5VO₄, and Ce_0.25Fe_0.75VO₄ are similar to that of CeVO₄, indicating that the former samples keep the structure of CeVO₄.

The magnified (200) peak of CeVO₄, Ce_0.75Fe_0.25VO₄, Ce_0.5Fe_0.5VO₄, and Ce_0.35Fe_0.75VO₄ are manifested in Figure S3 from Supplementary Materials. It is clear to see that the 2θ value right shifts to the lager value with the increase in Fe content, because the ion radius of the doped Fe³⁺ (49 pm) is much smaller than that of Ce³⁺ (103.8 pm) [32,33]. According to the Bragg’s law, 2d_(hkl)sinθ = nλ, the increase of 2θ value corresponds to the decrease in the d spacing d_(hkl) value, that is, lattice constraint. This fact implies that the Fe³⁺ ions substitute some Ce⁴⁺ sites. With the increase in Fe contents, the crystal parameter “a” gradually decreases from 0.7399 to 0.7349 nm, while the crystal parameter “c” gradually decreases from 6.496 to 6.469 nm (Table S1 in Supplementary Materials). Once the Fe³⁺ ions are doped into the crystal cell, the cell volume will shrink.

Figure 3 shows the SEM images of samples. The sizes of CeVO₄ crystals are a few-hundred nanometers, and CeVO₄ is composed of irregular particles, rods, and plates. The sizes of FeVO₄ crystals are also a few hundred nanometers, and FeVO₄ is composed of agglomerated particles. Ce_0.75Fe_0.25VO₄, Ce_0.50Fe_0.50VO₄, and Ce_0.25Fe_0.75VO₄ generally exhibit morphologies more similar to that of CeVO₄.

HRTEM image of CeVO₄ is illustrated in Figure 4a. The (200) interplanar spacing is 0.369 nm, equal to that of un-doped CeVO₄ (0.369 nm) [34]. The (101) interplanar spacing is 0.484 nm, identical to that of un-doped CeVO₄ (0.487 nm) [19,34]. The angle between the (101) and (200) planes (90°) further confirms that the prepared sample is CeVO₄. Figure 4b illustrates the HRTEM image of Ce_0.50Fe_0.50VO₄. The d spacing of the (101) plane is 0.482 nm, slightly smaller than that of CeVO₄ (0.484 nm). The d spacing of the (200) plane is 0.367 nm, also slightly smaller than that of CeVO₄ (0.369 nm). The reason is that the ionic radius of Fe³⁺ (49 pm) is smaller than that of Ce³⁺ (103.8 pm) [32,33]. It is noticeable that the angle between the (101) and (200) planes (101°) is obviously larger than that of CeVO₄ (90°). These results mean that some Fe³⁺ substitute the Ce⁴⁺ and slightly changes the parameters of crystal cells.

The EDX-mapping data Ce_0.50Fe_0.50VO₄ (Figure 5) show the even distribution of Ce, Fe, V, and O elements. XRF data show that the Ce: Fe: V molar ratio of Ce_0.50Fe_0.50VO₄ is 0.507:0.493:1, close to the theoretical ratio.

2.3. XPS Data

Figure 6 gives the Ce 3d XPS spectra. The Ce 3d_3/2 and Ce 3d_5/2 signals are marked as “u” and “v”, respectively. The v, v″, v‴, u, u″, and u‴ can be ascribed to surface Ce⁴⁺, whereas v′ and u′ can be attributed to surface Ce³⁺. The surface Ce³⁺/(Ce⁴⁺ + Ce³⁺) ratio of CeVO₄ is 39.1%, corresponding to an average oxidation state (AOS) of 3.61 (for Ce). For comparison, the surface Ce³⁺/(Ce⁴⁺ + Ce³⁺) ratio of Ce_0.50Fe_0.50VO₄ is 23.2%, corresponding to an AOS of 3.77.

Figure 7 illustrates the Fe 2p XPS spectra. The surface Fe²⁺/(Fe²⁺ + Fe³⁺) ratio of FeVO₄ is 29.9%, corresponding to an AOS of 2.70 (for Fe). On the other hand, the Fe²⁺/(Fe²⁺ + Fe³⁺) ratio of Ce_0.50Fe_0.50VO₄ is 28.1%, corresponding to an AOS of 2.72.

Figure 8 shows the V 2p XPS spectra. The surface V⁴⁺/(V⁴⁺ + V⁵⁺) ratio of Ce_0.50Fe_0.50VO₄ is 24.2%, while those of CeVO₄ and FeVO₄ are 18.7% and 18.9%, respectively, which means the AOSs of V in Ce_0.50Fe_0.50VO₄, CeVO₄, and FeVO₄ are 4.76, 4.81, 4.81, respectively. The above results manifest that when some Fe³⁺ replace Ce⁴⁺, the oxidation states of Fe, Ce, and V are all changed.

Figure 9 shows the O 1s XPS spectra. The O 1s spectra can be divided into two peaks at 529.9, and 531.1 eV corresponding to lattice oxygen (denoted as O_α) as well as the surface oxygen and oxygen vacancies (denoted as O_β), respectively [19,35]. Owing to greater mobility, O_β (surface labile oxygen) is more active compared to O_α (bulk oxygen). As shown in Figure 9, Ce_0.5Fe_0.5VO₄ has the highest concentration of surface labile oxygen species among three representative catalysts.

2.4. H₂-TPR and NH₃-TPD Data

Figure 10 shows the H₂-TPR profiles of samples. CeVO₄ exhibits a peak at 864 °C, corresponding to the reduction of CeVO₄ to CeVO₃ [24,36]. In the H₂-TPR profile of FeVO₄, the obvious peak around 648 °C is assigned to the reduction of VO₄³⁻ [37], while the H₂ consumption at lower temperatures is ascribed to the reduction of Fe³⁺ [16,37]. The first peak of Ce_0.50Fe_0.50VO₄ at 550 °C is ascribed to the reduction of active surface oxygen. The second peak at 632 °C may be ascribed to the reduction of Fe³⁺ or Ce⁴⁺. The third peak at 740 °C is ascribed to the reduction of VO₄³⁻ [15,24]. The data show that Ce_0.5Fe_0.5VO₄ owns more surface oxygen useful for NH₃-SCR.

Figure 11 shows the baseline-corrected NH₃-TPD data obtained by subtracting the “blank” TPD profile. The four peaks in low, medium, high, and superhigh temperature regions are ascribed to weak, medium, medium strong, and strong acid sites, as divided by peakfit V4.12 software. The distribution of four kinds of acid sites can be estimated by the relative peak areas. It is observed that the Fe_0.50Ce_0.50VO₄ has more acid sites than CeVO₄ and FeVO₄, which may be beneficial for NH₃ adsorption and NH₃-SCR [24].

3. Materials and Methods

3.1. Synthesis

Ce_1−xFe_xVO₄ catalysts were synthesized by a modified sol-gel hydrothermal method. In a typical synthesis, 5 mmol NH₄VO₃ was added in 80 mL hot water and heated at 60 °C in a water bath with constant stirring to prepare solution A. Stoichiometric Fe(NO₃)₃·9H₂O (98.5% purity, Sinopharm, Shanghai, China), Ce(NO₃)₃·6H₂O (99.95%, Aladdin, Beijing, China), and 5 mmol citric acid were separately dissolved in 30 mL ethanol and then these three solutions (about 30 mL each) were mixed and magnetically stirred to form solution B. After 30 min of constant stirring plus heating in a water bath at 60 °C, the solution’s color turned from yellowish to transparent and clean. Solution B was added into solution A drop by drop, while the whole solution was stirred vigorously. Subsequently, a moderate amount of propylene oxide was added dropwise into the mixed solution heated in a water bath at 60 °C until a duck-blood-like sol solution was formed. Finally, 2 M NH₃·H₂O or 2 M HCl solution was added slowly into the above mixture with constant stirring. The final pH value of the sol solution, monitored by a pH meter, was 7. The sol solution was transferred to two sealed autoclaves (with 100 mL volume each) and hydrothermally treated at 180 °C for 24 h. Then, the solids in the autoclaves were isolated via filtration and washed with deionized water until the pH value of the solvent became 7. The solids were collected and dried at 110 °C for 6 h. The dried solids were carefully grinded in a mortar and transferred into a porcelain bowl and heated to 750 °C (heating rate: 5 °C/min) in a muffle oven (with static air), and then heated at 750 °C for 12 h.

3.2. Activity Measurement

Catalytic activity measurement was performed in a quartz tube reactor (i.d. = 8 mm). The 200 mg catalyst (40–60 mesh) was added in the reactor for each test. The feed gas contained NO (500 ppm), NH₃ (500 ppm), O₂ (3.0%), and balance N₂. The total flow rate was 1000 mL/min. The gas concentrations were measured by an NO–NO₂–NO_x analyzer (42i-HL, High Level, Thermo Electron Corporation, Waltham, MA, USA).

NO_x conversion was calculated using the given equation:

$$X_{{\mathrm{NO}}_x}(\%)=\left(\frac{{\left[{\mathrm{NO}}_x\right]}_{\mathrm{in}}-{\left[{\mathrm{NO}}_x\right]}_{\mathrm{out}}}{{\left[{\mathrm{NO}}_x\right]}_{\mathrm{in}}}\right)\times 100\%$$

(1)

where [NO_x]_in and [NO_x]_out are the concentrating of NO_x entering the reactor and exiting the reactor, respectively.

3.3. Characterization

XRD patterns were recorded on an Advanced D8 (Bruker) powder diffractometer using Cu K_α radiation. TEM images were obtained with a JEOL JEM-2100F field emission TEM instrument equipped with an EDAX Genesis XM4-Sys60 system (EDAX Inc., Mahwah, NJ, USA). The elemental compositions of powders were determined by XRF (Thermo-3600, Thermo Fisher Scientific Inc., Waltham, MA, USA). The specific BET surface data were measured using a Micromeritics analyzer (Tristar II 3020M, Micromeritics, Norcross, GA, USA). XPS data were obtained on a thermoESCLAB 250XI instrument using monochromatic Al Kα radiation (Thermo, Waltham, MA, USA).

H₂-TPR experiments were carried out using a Micromeritics AutoChem 2920 (Micromeritics, Norcross, GA, USA) chemisorption instrument, following the procedure reported [38,39]. Firstly, the sample (40–60 mesh, 100 mg) was placed in a quartz tube reactor. Secondly, the sample was pretreated at 300 °C in 20 vol.% O₂/Ar (50 mL/min) for 0.5 h and cooled down to 30 °C followed by purging by Ar for 0.5 h. Then, a gas flow (10% H₂ in Ar, 50 mL/min) passed through the sample. Finally, the temperature was increased from 50 to 950 °C at a rate of 10 °C min⁻¹.

NH₃-TPD experiments were carried out on a Micromeritics AutoChem 2920 chemisorption instrument. A sample (40–60 mesh, 50 mg), placed in a quartz reactor, was pretreated at 300 °C in 20 vol.% O₂/Ar (50 mL/min) for 0.5 h and cooled down to 30 °C followed by Ar purging for 0.5 h. Then, 10% NH₃ in Ar (50 mL/min) flowed through the sample, and the temperature was ramped from 50 to 650 °C at a rate of 10 °C min⁻¹.

4. Conclusions

A series of Ce_1−xFe_xVO₄ catalysts were prepared by a modified hydrothermal method. It was found that that Ce_0.50Fe_0.50VO₄ exhibits the optimum NO_x conversion efficiency compared with other catalysts. With the incorporation of Fe³⁺, the crystal structure of CeVO₄ crystal gradually distorts and the average oxidation states of Ce, Fe, and V change accordingly. The surface oxygen and oxygen vacancy contents increase, as proved by XPS and H₂-TPR data. Ce_0.50Fe_0.50VO₄ has more weak and medium acid sites compared with CeVO₄ and FeVO₄. These factors contribute to the enhanced catalytic performance of Ce_0.50Fe_0.50VO₄. The anti-SO₂ and anti-H₂O performance of NH₃-SCR catalysts should be studied in the future for applications under realistic environments.