Ce 1 − x Fe x VO 4 with Improved Activity for Catalytic Reduction of NO with NH 3

: A series of Ce 1 − x Fe x VO 4 (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 NH 3 . Among them, Ce 0.5 Fe 0.5 VO 4 showed the highest catalytic activity. The catalysts were characterized by X-ray diffraction (XRD), N 2 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 H 2 (H 2 -TPR), and temperature-programmed desorption of NH 3 (NH 3 -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 Fe 3+ into CeVO 4 , and the acidity of Ce 0.5 Fe 0.5 VO 4 differs from that of CeVO 4 and FeVO 4 . The presence of more acid sites and a sharp increase in active oxygen species in Ce 0.5 Fe 0.5 VO 4 effectively improved the selective catalytic reduction (SCR) activity.


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 NH 3 has been extensively studied as one of the most effective methods to remove NO x [1][2][3].V 2 O 5 -WO 3 -TiO 2 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 H 2 O/SO 2 , phosphorus, ash, and alkali metals [4,5].CeO 2 -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 CeO 2 -WO 3 -TiO 2 [7], CeO 2 -ZrO 2 -WO 3 [8], Mn/Ce-ZrO 2 [9], MnO x -CeO 2 [10], Sn-MnO x -CeO 2 [11], MnO x -CeO 2 [12], and V 2 O 5 /CeO 2 [13].However, CeO 2 -based catalysts may suffer from sulfation [14].
Generally, there are two approaches to design better NH 3 -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 CeVO 4 [18,22,24,28] separately to investigate their influences on NO conversion.For instance, Huang et al. developed Sn 0.20 Ce 0.80 VO 4 for NH 3 -SCR by citric-acid-assisted solvothermal synthesis and post-hydrothermal treatment [18].Zhao et al. developed Ce 0.85 Zr 0.15 VO 4 for NH 3 -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 δ Ce 1−δ VO 4 (δ ≤ 0.4) catalysts with good SO 2 tolerance and NO conversion activity by a hydrothermal method [14].The highest NO conversion appeared at δ = 0.3 for Fe δ Ce 1−δ VO 4 (δ ≤ 0.4).When the Fe content was higher, minor segregation of CeO 2 occurred.
In this paper, Ce 1−x Fe x VO 4 (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 NH 3 -SCR.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 0.5 Fe 0.5 VO 4 ) 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).

Catalytic Performance
To set the result in perspective, the catalytic activity of Ce 0.5 Fe 0.5 VO 4 was compared with those of other catalysts.Ce 0.5 Fe 0.5 VO 4 is more active than Ce 0.5 Fe 0.5 O x reported in our previous work [27] because Ce 0.5 Fe 0.5 O 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 0.5 Fe 0.5 VO 4 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 0.3 Ce 0.7 VO 4 developed by Zhang and coworkers 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/TiO 2 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].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 Fe 3+ (49 pm) is much smaller than that of Ce 3+ (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 Fe 3+ ions substitute some Ce 4+ 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 Fe 3+ ions are doped into the crystal cell, the cell volume will shrink.

Regular Characterization
Figure 3   HRTEM image of CeVO 4 is illustrated in Figure 4a.The (200) interplanar spacing is 0.369 nm, equal to that of un-doped CeVO 4 (0.369 nm) [34].The (101) interplanar spacing is 0.484 nm, identical to that of un-doped CeVO 4 (0.487 nm) [19,34].The angle between the (101) and ( 200) planes (90 • ) further confirms that the prepared sample is CeVO 4 .Figure 4b illustrates the HRTEM image of Ce 0.50 Fe 0.50 VO 4 .The d spacing of the (101) plane is 0.482 nm, slightly smaller than that of CeVO 4 (0.484 nm).The d spacing of the (200) plane is 0.367 nm, also slightly smaller than that of CeVO 4 (0.369 nm).The reason is that the ionic radius of Fe 3+ (49 pm) is smaller than that of Ce 3+ (103.8 pm) [32,33].It is noticeable that the angle between the (101) and ( 200) planes (101 • ) is obviously larger than that of CeVO 4 (90 • ).These results mean that some Fe 3+ substitute the Ce 4+ and slightly changes the parameters of crystal cells.The EDX-mapping data Ce 0.50 Fe 0.50 VO 4 (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 0.50 Fe 0.50 VO 4 is 0.507:0.493:1,close to the theoretical ratio.

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", u"' can be ascribed to surface Ce 4+ , whereas v and u can be attributed to surface Ce 3+ .The surface Ce 3+ /(Ce 4+ + Ce 3+ ) ratio of CeVO 4 is 39.1%, corresponding to an average oxidation state (AOS) of 3.61 (for Ce).For comparison, the surface Ce 3+ /(Ce 4+ + Ce 3+ ) ratio of Ce 0.50 Fe 0.50 VO 4 is 23.2%, corresponding to an AOS of 3.77.Figure 7 illustrates the Fe 2p XPS spectra.The surface Fe 2+ /(Fe 2+ + Fe 3+ ) ratio of FeVO 4 is 29.9%, corresponding to an AOS of 2.70 (for Fe).On the other hand, the Fe 2+ /(Fe 2+ + Fe 3+ ) ratio of Ce 0.50 Fe 0.50 VO 4 is 28.1%, corresponding to an AOS of 2.72. Figure 8 shows the V 2p XPS spectra.The surface V 4+ /(V 4+ + V 5+ ) ratio of Ce 0.50 Fe 0.50 VO 4 is 24.2%, while those of CeVO 4 and FeVO 4 are 18.7% and 18.9%, respectively, which means the AOSs of V in Ce 0.50 Fe 0.50 VO 4 , CeVO 4 , and FeVO 4 are 4.76, 4.81, 4.81, respectively.The above results manifest that when some Fe 3+ replace Ce 4+ , 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 0.5 Fe 0.5 VO 4 has the highest concentration of surface labile oxygen species among three representative catalysts.

H 2 -TPR and NH 3 -TPD Data
Figure 10 shows the H 2 -TPR profiles of samples.CeVO 4 exhibits a peak at 864 • C, corresponding to the reduction of CeVO 4 to CeVO 3 [24,36].In the H 2 -TPR profile of FeVO 4 , the obvious peak around 648 • C is assigned to the reduction of VO 4 3− [37], while the H 2 consumption at lower temperatures is ascribed to the reduction of Fe 3+ [16,37].The first peak of Ce 0.50 Fe 0.50 VO 4 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 Fe 3+ or Ce 4+ .The third peak at 740 • C is ascribed to the reduction of VO 4 3− [15,24].The data show that Ce 0.5 Fe 0.5 VO 4 owns more surface oxygen useful for NH 3 -SCR.Figure 11 shows the baseline-corrected NH 3 -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 0.50 Ce 0.50 VO 4 has more acid sites than CeVO 4 and FeVO 4 , which may be beneficial for NH 3 adsorption and NH 3 -SCR [24].

Synthesis
Ce 1−x Fe x VO 4 catalysts were synthesized by a modified sol-gel hydrothermal method.In a typical synthesis, 5 mmol NH 4 VO 3 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(NO 3 ) 3 •9H 2 O (98.5% purity, Sinopharm, Shanghai, China), Ce(NO 3 ) 3 •6H 2 O (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 NH 3 •H 2 O 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.

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), NH 3 (500 ppm), O 2 (3.0%), and balance N 2 .The total flow rate was 1000 mL/min.The gas concentrations were measured by an NO-NO 2 -NO x analyzer (42i-HL, High Level, Thermo Electron Corporation, Waltham, MA, USA).
NO x conversion was calculated using the given equation: where [NO x ] in and [NO x ] out are the concentrating of NO x entering the reactor and exiting the reactor, respectively.

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).
H 2 -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.% O 2 /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% H 2 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 .NH 3 -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.% O 2 /Ar (50 mL/min) for 0.5 h and cooled down to 30 • C followed by Ar purging for 0.5 h.Then, 10% NH 3 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 .

Conclusions
A series of Ce 1−x Fe x VO 4 catalysts were prepared by a modified hydrothermal method.It was found that that Ce 0.50 Fe 0.50 VO 4 exhibits the optimum NO x conversion efficiency compared with other catalysts.With the incorporation of Fe 3+ , the crystal structure of CeVO 4 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 H 2 -TPR data.Ce 0.50 Fe 0.50 VO 4 has more weak and medium acid sites compared with CeVO 4 and FeVO 4 .These factors contribute to the enhanced catalytic performance of Ce 0.50 Fe 0.50 VO 4 .The anti-SO 2 and anti-H 2 O performance of NH 3 -SCR catalysts should be studied in the future for applications under realistic environments.

Figure 1
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 0.5 Fe 0.5 O 4 gives the highest NO x conversion (75.3%) at 300 • C. The catalytic activities of FeVO 4 and CeVO 4 are lower than those of Ce 1−x Fe x VO 4 .The overall activity of these catalysts follows the trend of CeVO 4 < Ce 0.75 Fe 0.25 VO 4 < Ce 0.5 Fe 0.5 VO 4 > Ce 0.25 Fe 0.75 VO 4 > FeVO 4 , and the catalytic data are reproducible (FigureS1in 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].

Figure 2 .
Figure 2. XRD patterns of Ce 1−x Fe x VO 4 catalysts.The magnified (200) peak of CeVO 4 , Ce 0.75 Fe 0.25 VO 4 , Ce 0.5 Fe 0.5 VO 4 , and Ce 0.35 Fe 0.75 VO 4 are manifested in FigureS3from 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 Fe 3+ (49 pm) is much smaller than that of Ce 3+ (103.8 pm)[32,33].According to the Bragg's law, 2d (hkl) sinθ = nλ, the increase of 2θ value corresponds to the decrease shows the SEM images of samples.The sizes of CeVO 4 crystals are a fewhundred nanometers, and CeVO 4 is composed of irregular particles, rods, and plates.The sizes of FeVO 4 crystals are also a few hundred nanometers, and FeVO 4 is composed of agglomerated particles.Ce 0.75 Fe 0.25 VO 4 , Ce 0.50 Fe 0.50 VO 4 , and Ce 0.25 Fe 0.75 VO 4 generally exhibit morphologies more similar to that of CeVO 4 .

Figure 5 .
Figure 5. SEM image (a) and the corresponding EDX mapping images of O (b), Ce (c), Fe (d) and V (e) elements of Ce 0.50 Fe 0.50 VO 4 .The scale bar represents 20 µm.