TiO2 nanotube catalysts for selective NO reduction by NH3

Fuel xxx (2016) xxx–xxx

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Acidity, surface species, and catalytic activity study on V2O5-WO3/TiO2 nanotube catalysts for selective NO reduction by NH3 M. Aguilar-Romero a, R. Camposeco b, S. Castillo c,⇑, J. Marín c, V. Rodríguez-González b, Luz A. García-Serrano a, Isidro Mejía-Centeno d a

Centro Interdisciplinario de Investigaciones y Estudios sobre Medio Ambiente y Desarrollo, Instituto Politécnico Nacional, 07340 México, D.F., Mexico División de Materiales Avanzados, Instituto Potosino de Investigación Científica y Tecnológica, 78216 San Luis Potosí, S.L.P., Mexico Dirección de Tecnología del Producto, Instituto Mexicano del Petróleo, 07730 México, D.F., Mexico d Dirección de Investigación en Transformación de Hidrocarburos, Instituto Mexicano del Petróleo, 07730 México, D.F., Mexico b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


We analyze the acidity, redox

properties and catalytic activity of V-W/NT catalysts. 4+ 5+
Reduced V /V species and Lewis and Brønsted acid sites promote the NO conversion.
V2O5 increases Brønsted and Lewis acid sites. Lewis acid sites are promoted by WO3.
V-W/NT presents higher catalytic activity than W/NT and V/NT catalysts.
NO conversion is not affected by SO2, but water inhibits the catalytic activity.

a r t i c l e

i n f o

Article history: Received 13 May 2016 Received in revised form 16 November 2016 Accepted 22 November 2016 Available online xxxx Keywords: Titanate nanotubes Surface acidity V2O5-WO3 catalysts NOx SCR-process

NH3

N2

O2

NO

H2O

a b s t r a c t In this work, we report the catalytic activity of V2O5/TiO2, V2O5-WO3/TiO2 and WO3/TiO2-nanotube model catalysts in removing NO with NH3 via the SCR process. The catalytic activity includes the effect of SO2 and H2O. We also analyze the effect of WO3 and V2O5 loading upon the surface acidity of the nanotubes, and the effect of WO3 on the V4+/V5+ ratio, and its correlation with the catalytic activity. TiO2-nanotubes (NT), employed as support, were prepared by hydrothermal treatment of TiO2 with NaOH. The catalysts were characterized by X-ray diffraction, HR-TEM microscopy, N2 physisorption, FTIR, H2-TPR, Raman and XPS. In general, we found that ternary catalysts (V2O5-WO3/NT) showed a higher NO conversion versus V2O5/NT and WO3/NT model catalysts. In fact, we found a high NO conversion (93%) over 3V-10W/NT catalyst at low temperature (380 °C). In the presence of SO2 (50 ppm) and H2O (5 vol.%), NO conversion slightly decreases (from 93 to 80% at 380 °C). The surface acidity (Brønsted and Lewis) of the nanotubes is the main parameter improved by adding V2O5. WO3 preferably modifies the Lewis acid sites of the nanotubes. Additionally, the structure and morphology of the nanotubes as well as the V4+/V5+ ratio, which depend on the metal loading, play an important role in the removal of NO at intermediate temperatures even in the presence of SO2 and H2O. Ó 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. E-mail address: [email protected] (S. Castillo). http://dx.doi.org/10.1016/j.fuel.2016.11.090 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Aguilar-Romero M et al. Acidity, surface species, and catalytic activity study on V2O5-WO3/TiO2 nanotube catalysts for selective NO reduction by NH3. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.11.090

M. Aguilar-Romero et al. / Fuel xxx (2016) xxx–xxx

The Selective Catalytic Reduction (SCR) with NH3 is probably the best available technology to remove NOx from the stack gases of power plants and other stationary sources [1]. As for the catalysts used in SCR, V2O5/TiO2 and V2O5-WO3/TiO2-based materials are the most active and probably the most commonly used because of their high activity, good thermal stability, and resistance to poisoning by SO2 [2–7]. V2O5 is the active material for the catalytic activity in NO reduction. However, catalysts with V2O5 are usually limited at 2 wt.% in order to avoid the undesired SO2 oxidation to SO3. The addition of WO3 promotes the catalytic activity, improves the acid properties of TiO2, extend the operating temperature window for SCR, and stabilizes the anatase phase [8–10]. The interaction between TiO2 and supported vanadium oxide results in surface oxide phases with structures that are remarkably different from those of the corresponding bulk oxides [11,12]. In the case of vanadium oxides, for instance, a low content of species with a strongly distorted tetrahedral geometry was observed; likewise, for higher contents of vanadium oxide, oligomeric or polymeric metavanadate species were formed [11,12]. Furthermore, a direct correlation between high surface V3+ + V4+/Vn+ and V4+/V5+ ratio and the SCR activity has been reported [13,14]. However, the behavior of the oxygen species attached to the vanadium and the dispersion of the active species are influenced by the support [13]. Well-dispersed and isolated vanadium oxide species show low activity for the SCR reaction but present a high selectivity towards N2 [15]. The addition of tungsten modifies the surface acidity, which improves the ammonia supply for the SCR reaction and promotes the NO conversion [7]. However, the mechanistic aspects, such as the intrinsic nature of active sites and the reactive adsorbed NH3 species (bounded to Brønsted or Lewis acid sites) involved in the SCR process, are not clear yet [16,17]. In this sense of investigation, different transition metal oxides and supports have been used to modify the properties of the V2O5-based catalysts in order to improve the catalytic activity. The main goal is to obtain a high dispersion of vanadium and the formation of vanadate species on the support, as well as to modify the surface acidity and the strength of the acid sites [18]. Several catalysts such as CeO2/TiO2 [19], Mn/TiO2 [20], and NT structured catalysts such as Ce–carbon nanotubes [21]; Ce–titanium nanotubes [22], Cu/NT [23], and microspheres based on V2O5-WO3/Fe2O3/TiO2 [24] have been reported. The results consistently show that the combination of metal oxide catalysts modify the redox and acid properties of the catalytic materials to improve the NO reduction. It has been reported [7] that the redox properties of the catalysts are involved with the catalytic activity at low temperature. The surface acidity seems to be involved with the absorption and activation of NH3 at high temperature. However, the observed parallelism in the catalytic behavior of the catalysts reported in the literature (where the classic peak profile of the NO conversion reaches a maximum at a given temperature), and the competition of SCR and the ammonia oxidation reactions, suggest that the basic mechanism features of the reactions reported by different catalysts are probably the same [25]. In this case, the basic mechanism of the SCR system could be determined by the chemistry of the reactants [25]. The chemistry involved in the SCR process is complex and includes the typical stoichiometric reaction (1). The SCR process occurs when the selectivity to N2 is close to 100% and the NH3/NO ratio is close to 1. Oxygen does participate in the reaction (1), and NO is the main reactant. Adsorbed ammonia (molecularly adsorbed as unsaturated cations on Lewis acid sites or as ammonium ions over Brønsted acid sites) represents the active species.

4NH3 þ 4NO þ O2 ! 4N2 þ 6H2 O

ð1Þ

The two atoms of nitrogen arise, one from the ammonia and the other from the NO [25]. Water produced by the SCR process interacts with the surface of the catalysts and remains adsorbed on the reaction sites; avoiding the adsorption of ammonia. In fact, the presence of water, and SO2 in the gas stream not only modifies the chemistry involved in the SCR system but also the activity of the catalysts is severely affected [25]. In this work, we report the NO conversion on V2O5, WO3, and WO3/V2O5-supported on TiO2 nanotube model catalysts. The catalytic tests also include the presence of SO2 and H2O in the feed stream. We analyze the effect of 3 wt.% of V2O5, 3 wt.% of WO3, and 3 wt.% of the WO3/V2O5 ratio upon the surface acidity of the nanotubes. We also explore the redox properties and the vanadium species of the catalysts. Our main objective is to correlate the acid and redox properties of our catalysts with the catalytic activity of the SCR process. In general, we found that Brønsted and Lewis acid sites promotes the NO conversion at low temperature, up to 300 °C, but Lewis acid sites remains above 300 °C. Besides, the maximum NO conversion was reached when the ratio of the reduced V4+/V5+ species is close to 0.75.

[100]

Intensity (a.u)

1. Introduction

[110]

H2Ti3O7

[020]

[311]

A

A

3V/NT-used

3V/NT 2V/NT 1V/NT

A

A A

10

A

20

30

NT

A

[200] [105] [211] A [204]

[004]

[101]

TSG

A

40

50

60

70

80

2-Theta (degree)

[110]

[100]

[020]

H2Ti3O7

Intensity (a.u)

2

[311]

3V-10W/NT-used 3V-10W/NT 2V-5W/NT W

1V-3W/NT 3W/NT

B

A

A

[004]

[101]

10

20

30

A [105]A [211] A [204]

[200]

40

50

60

70

NT TSG

80

2-Theta (degree) Fig. 1. XRD patterns for (A) V/NT and (B) V-W/NT catalysts. H2Ti3O7 means the trititanic acid phase and A means anatase phase. TSG means titania obtained by solgel. NT means titania nanotubes.

Please cite this article in press as: Aguilar-Romero M et al. Acidity, surface species, and catalytic activity study on V2O5-WO3/TiO2 nanotube catalysts for selective NO reduction by NH3. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.11.090

3

M. Aguilar-Romero et al. / Fuel xxx (2016) xxx–xxx

and annealed at 400 °C for 4 h. The catalysts were identified as 1V/NT, 2V/NT, 3V/NT, 3W/NT, 1V-3W/NT, 2V-5W/NT and 3V-10W/NT.

2. Experimental 2.1. Preparation of titanic acid nanotubes Titanic acid nanotubes were prepared by the hydrothermal treatment of anatase TiO2 as a precursor that was synthesized by the sol–gel method. Anatase TiO2 was thermally treated with a 10 N NaOH aqueous solution in a closed cylindrical Teflon-lined autoclave Parr reactor at 140 °C for 24 h. The solution was filtered and the slurry was washed with a HCl solution until reaching a pH of 1. The slurry was then washed again with deionized water until getting neutral pH. Finally, the material was dried at 80 °C for 12 h, and then annealed in air at 400 °C for 4 h. The support was identified as NT.

2.3. Characterization X-ray diffraction (XRD) patterns of the samples packed in a glass holder were recorded at room temperature with Cu K-alpha radiation in a Bruker Advance D-8 diffractometer having a theta– theta configuration, and a graphite secondary-beam monochromator. The data were collected for scattering angles (2h) ranging from 4 to 80° with a step of 0.02° for 2 s per point. Transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) analyses were performed in a JEOL 2200FS microscope operating at 200 kV and equipped with a Schottky-type field emission gun, and an ultrahigh resolution pole piece (Cs = 0.5 mm, point-to-point resolution = 0.190 nm). The samples were ground, suspended in isopropanol at room temperature, and dispersed by ultrasonic stirring. Then, an aliquot of the solution was dropped on a 3 mm diameter-lacey-carbon-copper grid. Textural properties were obtained with an ASAP-2000 analyzer from Micromeritics. The specific surface area was calculated using the BET equation from the N2 physisorption at 196 °C. The pore size distribution was obtained by the BJH method from the desorption branch.

2.2. Deposition of vanadium and tungsten on NT The catalysts were prepared by wet impregnation in proper quantities of ammonium metavanadate (NH4VO3, Aldrich 99.9%), and tungstic acid (H2WO4, Aldrich 99%). The concentration of the NH4VO3 solution in water was adjusted to prepare catalysts with 1, 2, and 3 wt.% of V2O5; then, a proper quantity of an aqueous WO3 solution to obtain catalysts with 3, 5, and 10 wt.% of WO3 was added to the titanic acid nanotubes. Finally, the powder was filtered and washed with deionized water. The catalysts were dried

(A)

(B)

(C)

10 nm

2 nm 9 nm

10 nm

2 nm 4 nm

9 nm

2 nm

4 nm

5 nm

10 nm

(D)

3 nm

10 nm

10 nm

(E)

(F) 9 nm

8 nm

W 1 nm

8 nm

W W

8 nm

W

10 nm

20 nm

5 nm

Fig. 2. HRTEM images for (A) NT, (B) 3V/NT, (C) 3W/NT, (D) 1V-3W/NT, (E) 2V-5W/NT, and (F) 3V-10W/NT catalysts.

Please cite this article in press as: Aguilar-Romero M et al. Acidity, surface species, and catalytic activity study on V2O5-WO3/TiO2 nanotube catalysts for selective NO reduction by NH3. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.11.090

M. Aguilar-Romero et al. / Fuel xxx (2016) xxx–xxx

Fourier transform infrared spectroscopy (FTIR) spectra were recorded in a Nicolet 8700 spectrophotometer with a resolution of 4 cm1 accumulating 50 scans. In the cell, all the samples were treated in vacuum at 400 °C for 1 h; pyridine was then admitted for some seconds. Then, evacuations were performed from room temperature to 400 °C. The acidity per unit of surface area for Lewis and Brönsted sites was obtained following the corresponding procedure [26]. Hydrogen temperature-programmed reduction (H2-TPR) was carried out on a chemisorption analyzer (Quantachrome instruments, ChemBET TPR/TPD). Prior to the analysis, the samples (50 mg) were treated under nitrogen at 120 °C for 30 min. Then, the samples were heated from 30 to 900 °C at a 10 °C/min rate in a mixture (20 mL/min) of 10% H2/Ar. The Raman spectroscopy was performed in a Horiba-Jobin Yvon (Labram HR 800) equipped with a confocal Olympus Bx41 microscope, and a CCD detector with a resolution of 1024  256 pixels. An argon laser (43.4 mW) operating at 532 nm was used as the excitation source. Powders were placed in Linkam cell stages directly adapted to the instrument microscope. The thickness of the window glass cell was 1 mm. Each spectrum was acquired using a 50x objective. The software LabSpec (for Windows) was used to analyze the spectra. XPS was performed with a Thermo VG Scientific Escalab 250 spectrometer equipped with a hemispherical electron analyzer, and an Al Ka radiation source (1486.6 eV) powered at 20 kV and 30 mA, respectively. The binding energy was determined by using carbon C (1 s) as reference line (284.6 eV). The spectrometer was operated at pass energy of 23.5 eV, and the base pressure in the analysis chamber was maintained in the order of 3  108 mbar. Peak fitting was done by using XPSPEAK 41 with Shirley background.

With regard to WO3, small traces are observed as the load of this oxide increases at 2h = 25.6°. The result is reported in Fig. 1 (B). Characteristic peaks attributed to tungsten oxide located at 2h = 25.6° grew very small when the loading was increased to 10 wt.% in the 3V-10W/NT catalyst. These results suggest that WO3 crystallites were formed and slowly grew with the increasing WO3 loading and V2O5 concentration. The addition of V2O5 and WO3 did not modify the H2Ti3O7 phase, which preserved its structure up to 400 °C. It is important to note, in Fig. 1(B), that there is a shift of the band located at 9.7° of the 2h value for the 2V-5W/NT and 3V10W/NT catalysts. The band is shifted from 9.7° to 9.6°. The peak located around 2h = 9.7° is attributed to the interlayer distance [27]. It has been suggested [27] that the shift towards larger angles is due to a decrease in the interlayer spacing in the titanate

L

B

3V/NT 2V/NT

A 1400

NT 1450

1500

1550

1600

1700

Wavenumber (cm )

L L

L+ B

B

B

3V-10W/NT 2V-5W/NT

1V-3W/NT

3.1. XRD structure The XRD results of the two catalytic systems (V/NT and V-W/NT) with different content of vanadia and tungsten are shown in Fig. 1(A) and (B). The XRD patterns of both catalytic systems are dominated by the reflections of titanic acid (H2Ti3O7), which are located at 2h values of 9.7°, 24°, 28°, and 48.4° according to H2Ti3O7 (PDF = 036–0654). Characteristic peaks attributed to vanadium oxides were not observed in the 1V/NT, 2V/NT, and 3V/NT catalysts, even at higher vanadium loads. The same behavior was displayed by the 1V-3W/NT, 2V-5W/NT, and 3V-10W/NT catalyst reflections, where no V2O5 was detected.

1650 -1

Absorbance (a.u)

3. Results and discussion

B

1V/NT

2.4. Catalytic tests Catalyst samples of 100 mg were tested in a tubular quartz reactor. Before reaction, the catalysts were heated at 120 °C for 1 h under 100 cm3/min flow rate of N2. The feed stream composition was 500 ppm of NO, 500 ppm of NH3 and 2 v/v% of O2. When added, 50 ppm of SO2 and 5 v/v% of H2O were used in the feed stream. The total gas flow was 400 mL/min, and nitrogen was the balance gas. H2O was delivered by a syringe pump, and it was vaporized in heated lines at 120 °C before reaching the reactor. The analysis of reactants and products was made on-line by means of a NOx detector (Rosemount 951 A), and a FTIR spectrophotometer (Bruker Tensor 27) equipped with a 0.75 m pathlength gas cell heated at 120 °C to prevent condensation. Spectra were acquired at 4 cm1 resolution by averaging 44 scans. During the catalytic runs, the reactor temperature was raised from 25 to 500 °C at 4 °C/min. The NO conversion was obtained as follows: NO = [(NO)in  (NO)out] ⁄ 100/(NO)in.

L

L+ B

Absorbance (a.u)

4

3W/NT

B NT 1400

1450

1500

1550

1600

1650

1700

-1

Wavenumber (cm ) Fig. 3. Surface acidity for (A) NT, 1V/NT, 2V/NT and 3V/NT, (B) NT, 3W/NT, 1V-3W/NT, 2V-5W/NT and 3V-10W/NT catalysts, L and B stand for Lewis and Brönsted acid sites, respectively. The surface acidity of the catalysts is reported at 200 °C.

Please cite this article in press as: Aguilar-Romero M et al. Acidity, surface species, and catalytic activity study on V2O5-WO3/TiO2 nanotube catalysts for selective NO reduction by NH3. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.11.090

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M. Aguilar-Romero et al. / Fuel xxx (2016) xxx–xxx

nanotube walls accompanied by removal of water when nanotubes are calcined at high temperature. In our case, we suggest that the shift of the 2h value corresponding to the [1 0 0] reflection to lower angle is due to an increase in the interlayer spacing in the nanotube walls by the addition of vanadium and tungsten to the nanotubes. The corresponding d spacing for 2h = 9.7° is 9.11 nm, and for 2h = 9.6° is 9.20 nm. The corresponding d space for the peak located at 2h = 10.23°, reported in the literature [28], is 9.86 nm, and for 2h = 9.54° is 9.26 nm [29]. 3.2. HRTEM morphology The presence of nanotubes was determined by the HRTEM technique, and it is shown in Fig. 2(A), where well-defined nanotubes

2.50

with an inner diameter of 4 nm and external diameter close to 9 nm can be seen. The thickness of the nanotubular walls was around 2 nm. Fig. 2(B) shows the 3V/NT catalyst with an external diameter of 9 nm. The addition of 3 wt.% vanadium did not modify the inner and external diameters and thickness of the H2Ti3O7 structure. In Fig. 2(C)–(F), which correspond to the 3W-NT, 1V3W/NT, 2V-5W/NT, and 3V-10W/NT catalysts, it is possible to observe WO3 highly dispersed around the nanotube wall, where the crystal size of these WO3 nanoparticles was 1 nm on average. The particles were analyzed by EDX (not showed) to confirm the presence of WO3. The active components V2O5 and WO3 were well dispersed on the TiO2 nanotubes, and even when the load of the active components was increased, the tubular structure of H2Ti3O7 was

0.40

(a)

0.35

3V/NT 2.00

Lewis acid sites (μmol/m2)

Brønsted acid sites (μmol/m2)

(b)

2V/NT 1.50

1.00 1V/NT 0.50

3V/NT

0.30 0.25

NT

0.20 2V/NT 0.15 1V/NT 0.10 0.05

NT 0.00

150

200

250

300

350

400

0.00

450

150

200

250

2.00 3W/NT

3V-10W/NT

1.40 1.20

2V-5W/NT

1.00 0.80 1V-3W/NT

0.60 0.40 0.20 0.00

200

450

(d)

0.70

1V-3W/NT

0.60 2V-5W/NT

0.50 0.40 0.30 3W/NT 0.20

NT

0.10

NT 150

400

3V-10W/NT

0.80

Lewis acid sites (μmol/m2)

Brønsted acid sites (μmol/m2)

1.60

350

0.90

(c)

1.80

300

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Temperature (°C)

250

300

350

Temperature (°C)

400

450

0.00

150

200

250

300

350

400

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Temperature (°C)

Fig. 4. Brønsted and Lewis acid sites concentration for (a and b) 1V/NT, 2V/NT, 3V/NT and for (c and d) 3W/NT, 1V-3W/NT, 2V-5W/NT and 3V-10W/NT catalysts at 200, 300 and 400 °C.

Please cite this article in press as: Aguilar-Romero M et al. Acidity, surface species, and catalytic activity study on V2O5-WO3/TiO2 nanotube catalysts for selective NO reduction by NH3. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.11.090

M. Aguilar-Romero et al. / Fuel xxx (2016) xxx–xxx

3.3. Surface acidity The IR spectra of pyridine adsorbed at 200 °C on the V/NT and V-W/NT catalysts are shown in Fig. 3. The bands located at 1635 and 1545 cm1 are assigned to the pyridinium ion adsorbed on Brønsted acid sites. The bands located at 1621, 1576 and 1445 cm1 are assigned to Lewis acid sites [30,31]. Similar spectra were observed for the 1V/NT, 2V/NT, and 3V/NT catalysts. The main difference was the number of Brønsted and Lewis acid sites, where the 3V/NT catalyst showed the highest number of Brønsted acid sites with respect to the 1V/NT and 2V/NT catalysts due to the increased amount of vanadium. The number of acid sites for both Lewis and Brønsted acid sites is presented in Fig. 4. The pyridine adsorption rates were significantly decreased on both Brønsted and Lewis acid sites when the temperature increased from 200 to 400 °C. We can conclude that the vanadium oxide species on the TiO2 nanotubes mainly possess Brønsted acid sites. The addition of WO3 to the nanotubes increased the Brønsted acid sites in the following order: 3 > 5 > 10 wt.%. According to this result, the addition of V2O5 (1, 2, and 3 wt.%), and WO3 (3, 5, and 10 wt.%) to the nanotubes modify the surface acidity due to the Brønsted acid sites increases at 200 °C. However, the Lewis acid sites still remain between 300 and 400 °C. Lewis acid sites are essential for the NH3 SCR-NO process. We suggest that the number of Brønsted acid sites on the surface of the TiO2 nanotubes is due mainly to the rich dispersion of tungsten oxide and the presence of crystalline WO3 and V2O5 on the TiO2 nanotubes, as shown by TEM micrographs in Fig. 2. Several authors [30–33] have reported results about the surface acidity for V2O5-WO3/TiO2 catalysts. Yamazoe et al. [30], for instance, have reported the NH3-TPD for several WO3/TiO2 catalysts. They reported [30] that the TPD profiles for TiO2, 0.5 wt.% WO3/TiO2, 1 wt.% WO3/TiO2, and 2 wt.% WO3/TiO2 catalysts are similar. However, further addition of WO3, above 5 wt.%, made the NH3 desorption temperature high. They reported that the acid sites corresponding to the highest temperature peak (around 450 °C) are referred to the strong acid sites. Dongare et al. [32], in a study of NH3-TPD on V2O5/TiO2 catalysts, reported desorption of ammonia in two different temperature ranges, which indicates the presence of two types of acid sites with different strength. At low vanadia content (1 wt.%), the highest dispersion of vanadium lead to the highest acid strength. With the increase in vanadia content, the formation of bulk vanadia on anatase titania phase is observed, which shows lower acidity. Yang et al. [33] reported the NH3-TPD for TiO2, 10 wt.% TiO2-Al2O3, 3 wt.% TiO2-Al2O3, and Al2O3 catalysts. They found that alumina has two uptake patterns. The first one is located between 100 °C and 350 °C (type I), and the second one is located between 350 °C and 450 °C (type II). TiO2 shows a broad peak between 100 °C and 450 °C. The mixture TiO2-Al2O3 presents similar NH3 desorption pattern than that of Al2O3. The acid sites corresponding to the low/high temperature peak are referred to the weak/strong acid sites. They concluded [33] that the increase in acidity is ascribed to the distribution of an excess of negative/positive charge caused by the formation of bridge hetero metal-oxygen bonds. In our case, we found, in Fig. 4, that by adding 1, 2, and 3 wt.% of V2O5 to TiO2-nanotubes the Lewis acid sites increases almost by 2.6-fold of the nanotubes. The Brønsted acid sites of the nanotubes are increased by 7-fold. In this line, our results reported in Fig. 4, correlate with the highest temperature peak, referred to strong acid sites reported in the open literature [30–33]. Furthermore,

the addition of 3 wt.% of WO3 also modifies the surface acidity of the nanotubes. In fact, the Lewis acidity increased by 3.7-fold, and the Brønsted acid sites increased by 2.7-fold versus that of the nanotubes. These results are in agreement with that reported by Yamazoe et al. [30] in the sense that the addition of WO3 made the NH3-desorption temperature high. 3.4. Temperature programmed reduction The H2-TPR profiles to examine the reducibility of the V/NT and V-W/NT catalysts are shown in Fig. 5(A) and (B). The 1V/NT, 2V/NT, and 3V/NT catalysts display reduction peaks at 468 °C, 474 °C, and 518 °C, respectively, thus indicating different vanadium contents in the TiO2 nanotubes, which correspond to the reduction of V5+ to V3+; the presence of a reduction peak at around 400–500 °C is due to the surface vanadium oxide [34]. The reduction peak found

V+5

A

V+3

507 379 H2 consumption (a.u)

preserved. Finally, by the HRTEM technique, it was possible to confirm that the V/NT and V-W/NT catalysts preserved their tubular structure up to 400 °C.

637

3V/NT

499 643 414

2V/NT

462

100

560

378

1V/NT 200

300

400

500

600

700

Temperature (°C)

800

555

B

W +4--- W 0 W +6--- W +4

826

V+5--- V+3 10W-3V/NT

H 2 consumption (a.u)

6

521

5W-2V/NT 326

377 V+5--- V+3

3W-1V/NT

W +4--- W 0 768

V+5--- V+3

W +6--- W +4 656 W +4--- W 0 809

787

3W/NT

200

300

563

347

400

500

600

700

800

900

Temperature (°C) Fig. 5. TPR profiles for (A) V/NT and (B) V-W/NT loaded with different V2O5 and WO3 weight contents.

Please cite this article in press as: Aguilar-Romero M et al. Acidity, surface species, and catalytic activity study on V2O5-WO3/TiO2 nanotube catalysts for selective NO reduction by NH3. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.11.090

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above 700 °C indicates that vanadium species are dispersed on the TiO2 nanotubes. With regard to the 3W/NT catalyst, we observed three peaks located at 347 °C, 563 °C, and 800 °C. In this sense of investigation, Zhang et al. [35] in a study about the mechanism of SCR-NO with NH3 over W/TiO2 catalysts at high temperature, reported that the first peak (located at 347 °C) could be attributed to the reduction of W-OH groups. The second peak (located around 563 °C) might be attributed to the reduction of surface oxygen of WOx species. They also reported [35] that the peak located around 800 °C is probably due the reduction of amorphous tetrahedral WOx species, which are progressively reduced from W6+ to W4+. In the TPR profiles of the V-W/NT catalysts, reported in Fig. 5(B), the first peak was caused by the superimposed reduction of V5+ to V3+, and W6+ to W4+. The second peak located around 800 °C is assigned to the hydrogen consumption of W4+ to W0. The reduction peak located between 400 °C and 650 °C is shifted to higher temperatures with the increase in the loading content of vanadium and tungsten. In summary, the increase in the vanadia content, and the introduction of WOx into the TiO2 nanotubes enhances the reducibility of the catalysts from 200 °C to 400 °C.

3.5. Raman spectroscopy The surface structure of vanadium for the 1V/NT, 2V/NT, and 3V/NT catalysts was examined by Raman spectroscopy. The spectra are presented in Fig. 6(A). It is generally acknowledged that the band located at 1030 cm1 is associated with a V = O bridge of monomeric VOX species, and the band located around 920– 940 cm1 with a V–O–V bridge of polymeric VOX species [36,37]. In our case, the monomeric and polymeric VOx species were not observed for the 1V/NT, 2V/NT, and 3V/NT catalysts by Raman spectroscopy, probably due to the low content and the highly dispersed species of V2O5 on the TiO2 nanotubes. The Raman spectra for the 1V-3W/NT, 2V-5W/NT, and 3V-10W/NT catalysts were carried out to observe the role played by the addition of WO3 into the V/NT catalysts. The results are showed in Fig. 6(B). The catalysts featured in this work showed intense bands at 126, 153, 204, 277, 392, 448, 633, 701, 820, and 923 cm1. TiO2 nanotubes as support showed the characteristic bands at 272, 450, and 666 cm1, and other less intense bands at 232, 830, and 926 cm1 [36].

Intensity (a.u)

(c) 3V/NT (b) 2V/NT (a) 1V/NT

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Raman shift (cm )

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Intensity (a.u)

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Raman shift (cm ) Fig. 6. Raman spectra for (A) V/NT and (B) V-W/NT loaded with different V2O5 and WO3 weight contents.

Please cite this article in press as: Aguilar-Romero M et al. Acidity, surface species, and catalytic activity study on V2O5-WO3/TiO2 nanotube catalysts for selective NO reduction by NH3. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.11.090

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We found that the 2V-5W/NT and 3V-10W/NT catalysts display a weak defined band at 1030 cm1. The peak located at 1030 cm1 is due to the V@O stretching mode of dispersed vanadium species, which occur at low loadings predominantly as isolated

+4

V

A

V2p1/2

+5

V

+3

V

1V/NT

Table 1 Relative concentration of V3+, V4+ and V5+ species on the surface of the catalysts synthesized in this work. Catalyst 2V/NT 3V/NT 1V-3W/NT 2V-5W/NT 3V-10W/NT

W/V ratio

3.0 2.5 3.3

V3+

V4+

V5+

V4+/Vn+

18 26

72 57 37 48 43

10 17 63 52 57

2.57 1.32 0.58 0.92 0.75

Counts/s

V2p3/2

2V/NT

3V/NT

V2p 512

514

516

518

520

522

524

526

monovanadates in a distorted tetrahedral configuration with a short V@O terminal bond, and three bridging vanadium oxygen support (VAOAM) bonds, (MAO)3 V5+@O [36]. No WO3 signals were detected due to tungsten oxide species, which are significantly smaller (around 1 nm), and dispersed in the TiO2 nanotube support, as shown in the 2V-5W/NT and 3V10W/NT micrographs reported in Fig. 2. It was observed [38] that when the V2O5 loading is low, the isolated vanadium oxide species predominate on the surface of the TiO2 nanotube support due to the high surface area. In this line, we suggest that isolated vanadium oxide species predominate on the surface of the binary catalysts V/NT. The ternary catalysts, composed by V-W/NT, present isolated monovanadates on the surface of the nanotubes.

Binding Energy (eV) 3.6. XPS characterization

B

+5

+4

V2p 1/2

V

V

Counts/s

1V-3W/NT

2V-5W/NT

V2p 512

3V-10W/NT 514

516

518

520

522

524

526

Binding Energy (eV)

C

4f

7/2

4f

5/2

W

+5

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3V-10W/NT

Ti 3p

2V-5W/NT

Ti 3p

1V-3W/NT

3W/NT 32

34

36

38

40

42

Binding Energy (eV) Fig. 7. XPS high-resolution spectra over V 2p for (A) V/NT, (B) V-W/NT catalysts and (C) W 4f peaks for V-W/NT catalysts.

Fig. 7(A) and (B) display V2p X-ray photoelectron spectra for the 1V/NT, 2V/NT, 3V/NT, 1V-3W/NT, 2V-5W/NT, and 3V-10W/NT catalysts. In Fig. 7(A), the spectrum exhibits two peaks that are characteristic of the vanadium structure (2p3/2, 2p1/2). The spectra of the 1V/NT, 2V/NT, and 3V/NT catalysts show two peaks at 515.6 eV, and 516.5 eV. These values represent V2O4 species with V4+, and V2O5 species with V5+ oxidation states, respectively. The catalysts 1V-3W/NT, 2V-5W/NT, and 3V-10W/NT show V4+ and V5+ oxidation states. Fig. 7(C) shows spectra for the 3W/NT, 1V-3W/NT, 2V-5W/NT, and 3V-10W/NT catalysts. The doublet between 35.7 and 38 eV is attributed to W 4f5/2 and W 4f7/2, respectively. The peaks are located at the same binding energy of tungsten atoms with 6+ formal oxidation number, as in WO3 under oxidizing conditions. As the tungsten load is increased, a little shoulder corresponding to W 4f7/2 appears. It has been reported [39] that the vanadium surface species on V2O5-WO3/TiO2 catalysts are V4+ and V5+. The presence of WO3 stabilizes the surface V4+, and V2O5-WO3/TiO2 catalysts show a reducibility that is higher than the one shown by V2O5/TiO2 catalysts. The addition of WO3 between 3 and 6 wt.% in the V2O5/TiO2 catalysts increases the reduced V4+ species on the V2O5-WO3/TiO2 catalysts [40]. However, it has been reported that above 6 wt.% of WO3, the reduced V4+ species remain constant on the V2O5-WO3/TiO2 catalysts [40]. Furthermore, the addition of WO3 (6 wt.%) to V2O5/TiO2 provides strong Lewis and Brønsted acidity [41,42], and the ternary catalysts display higher catalytic activity for the SCR reaction with respect to the corresponding binary catalysts. In our case, the addition of vanadia seems to promote the V3+, V4+, and V5+ species on the surface of the nanotubes. However, the addition up to 5 wt.% of WO3 inhibits the formation of V3+ species and promotes the V4+ and V5+ species on the surface of the nanotubes. Above 5 wt.% of WO3, the relative concentration of V4+ and V5+ species diminishes. Table 1 presents the relative concentration of V3+, V4+, and V5+ species obtained by XPS for the catalysts used in this work. We found that the maximum of the reduced species (V4+/Vn+) ratio is reached at a value of 2.5 of the W/V ratio. Above 2.5 of the W/V ratio, the reduced species diminishes.

Please cite this article in press as: Aguilar-Romero M et al. Acidity, surface species, and catalytic activity study on V2O5-WO3/TiO2 nanotube catalysts for selective NO reduction by NH3. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.11.090

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M. Aguilar-Romero et al. / Fuel xxx (2016) xxx–xxx

100

100

1V/NT 2V/NT 3V/NT

NO Conversion (%)

60

40

60

40

20

20

B

A

0

0 100

200

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Temperature (°C) 100

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2V-5W/NT

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Temperature (°C)

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Temperature (°C)

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NO Conversion (%)

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20 Fresh with 50 ppm of SO2 with 5 vol.% of H2O and SO2

E

0

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500

Temperature (°C) Fig. 8. NO conversion as a function of the temperature over (A) 1V/NT, 2V/NT and 3V/NT. (B) NO conversion over 3V/NT catalyst with H2O and SO2 in the feed stream. (C) NO conversion over the 3W/NT, 1V-3W/NT, 2V-5W/NT and 3V-10W/NT catalysts as a function of the temperature. (D) NO conversion over 3V-10W/NT catalyst with H2O and SO2. (E) NO conversion over 2V-5W/NT catalyst with H2O and SO2. The composition of the mixture of gases was 500 ppm of NO, 500 ppm of NH3 and 2% per volume of O2 and (when added) 5 vol.% of H2O, 50 ppm of SO2. 100 mg of catalyst were used. The total flow was 400 cm3/min.

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3.7. Catalytic activity NO conversion as a function of the temperature is presented in Fig. 8. We found, in Fig. 8(A), that NO reduction with NH3 starts around 100 °C on the 1V/NT, 2V/NT, and 3V/NT catalyts. NO conversion increases by further increasing the temperature up to reach a NO conversions of 63% (for 1V/NT), 69% (for 2V/NT), and 74% for (3V/NT) at 350 °C. Above 350 °C, NO conversion diminishes as the temperature increases. We found a catalytic promotional effect by increasing the V2O5 loading from 1 to 3 wt.% on the NT. The effect of SO2 and water is presented in Fig. 8(B). We found that, in the presence of water (5 vol.%), and 50 ppm of SO2, NO conversion was slightly affected. However, this catalyst, 3V/NT, showed good tolerance to SO2 in the 100–350 °C interval of temperature, preserving the same conversion profile. It was also observed that NO conversion is affected principally by the presence of H2O when compared with that reported in the presence of SO2 for the 3V/NT catalyst. In fact, the addition to the feed stream of 5 vol.% of H2O, NO conversion on the 3V/NT catalyst decreased, from 74% to 65%. The competitive adsorption of H2O on the vanadia sites, which diminish the number of available sites for the adsorption and reaction of NH3 and NO, explains the lower NO conversion [42]. In contrast, the presence of 50 ppm of SO2 did not affect the NO conversion for the 3V/NT catalysts. It has been reported [43] that an increasing of the polymeric species on the surface of the catalyst supports the behavior of the 3V/NT catalyst in the presence of SO2. Fig. 8(C) presents the catalytic behavior for the 3W/NT, 1V-3W/NT, 2V-5W/NT, and 3V-10W/NT catalysts. The 3W/NT catalyst shows 57% of NO conversion at 440 °C, while for the 1V-3W/NT catalyst, the NO conversion was 70% at 440 °C. We found that the addition of 5 and 10 wt.% of WO3 to the NT support, promotes the NO conversion form 57% (for 3W/NT catalyst) to 85% (for 2V-5W/NT catalyst), and 94% (for 3V-10W/NT catalyst). Besides, the operation-temperature window is expanded from 80 to 400 °C when WO3 is added to the catalysts. The effect of SO2 (50 ppm) and water (5 vol.%) on the catalytic activity for the 3V-10W/NT catalysts is reported in Fig. 8(D). In the presence of water, the 3V-10W/NT catalyst reached a NO conversion close to 80% at 400 °C. Above this temperature, 400 °C, NO conversion diminishes. In the presence of SO2, we found that the 3V-10W/NT catalysts showed a good conversion of 95% at 400 °C, with no formation of N2O. The 2V-5W/NT catalyst reached a NO conversion of 70% at 400 °C in the presence of H2O and SO2. Therefore, we suggest that the V/NT catalysts with higher content of tungsten uniformly dispersed on the TiO2 nanotube support present a remarkable SCR performance at intermediate temperatures. Likewise, a lower content of tungsten resulted in a good catalytic performance shifting the operation window to high temperatures when compared with the V/NT catalysts. In general, we found, in Fig. 8(A) that the addition of V2O5 to the nanotubes promotes the catalytic activity to remove NO with NH3 under lean conditions. The NO conversion, reported in Fig. 8(A), as a function of the V2O5 loading seems to correlate with the surface acidity of the catalysts reported in Fig. 4(A) and (B). Brønsted and Lewis acid sites are involved in the NO reduction between 100 °C and 300 °C, but Lewis acid sites remain up to 400 °C. We suggest that Brønsted acid sites promote the NO conversion at low temperature, up to 300 °C. Lewis acid sites are involved in the NO reduction above 300 °C. Besides, as the V4+/Vn+ ratio diminishes on the catalysts containing only vanadia, NO conversion increases. It is then possible to conclude that values around 1 of the reduced V4+/Vn+ species, and the Brønsted and Lewis acid sites promote the NO conversion. We can also conclude that the ternary catalysts containing V-W supported on nanotubes present higher catalytic activity for the

SCR reaction than the corresponding V/NT and W/NT catalysts. We found that there is a possible correlation between the reduced V4+ species and the catalytic activity for the ternary catalysts. As the reduced V4+ species increase, the NO conversion also increases. In fact, the maximum NO conversion is reached when the reduced species is close to 0.75. Besides, the Lewis acid sites concentration on the surface of the nanotubes increases as the WO3 loading increases, but Brønsted acid sites remains constant. We suggest, then, that the Lewis acid sites promotes the NO conversion, by addition of WO3 to the V/NT catalysts, and are also involved in the shift of the NO conversion to lower temperatures. We can conclude that the reduced V4+/Vn+ species and the Lewis acid sites promote the NO conversion on the 3V-10W/NT catalyst. 4. Conclusions The catalytic activity for SCR-NO by NH3 over V2O5/NT (1, 2, and 3 wt.%), V2O5-WO3/NT (with a W/V ratio of 2.5, 3.0, and 3.3 wt.%), and W/NT (3 wt.% of WO3) catalysts and its correlation with the surface acidity and redox properties of the catalysts were analyzed. We found that the ternary catalysts (V-W/NT) present higher catalytic activity for the SCR reaction than the corresponding V/NT and W/NT catalysts. In fact, the catalytic activity correlates with the surface acidity and the redox properties of the ternary catalysts. In this line, the maximum NO conversion is reached on the 3V-10W/NT catalyst, when the relative concentration of reduced V4+/V5+ species on the surface of the nanotubes is close to 0.75. The addition of 10 wt.% of WO3 to the catalyst 3V/NT increased by 3.7-fold the Lewis acid sites concentration. The Brønsted acid sites remain constant. We suggest that Lewis acid sites increase and shift the NO conversion to lower temperature. For the V/NT catalysts, we found that Brønsted acid sites seem to promote the NO conversion at low temperature, up to 300 °C. Lewis acid sites are involved in the enhanced NO conversion above 300 °C. The highest NO conversion is reached when the value of the V4+/Vn+ species on the surface of the nanotubes is around of 1. The catalytic activity of the 3V/NT, 2V-5W/NT, and 3V-10W/NT catalysts for the SCR reaction is not affected in the presence SO2. However, in the presence of water, NO conversion is hindered. Acknowledgments The authors want to thank the financial support provided by the Molecular Engineering Program from the Mexican Institute of Petroleum (Project D.00477). R. Camposeco acknowledges the support provided by CONACYT, UAM-A, and LINAN-IPICYT. References [1] Forzatti P, Nova I, Tronconi E, Kustov A, Thogersen JR. Effect of operating variables on the enhanced SCR reaction over a commercial V2O5–WO3/TiO2 catalyst for stationary applications. Catal Today 2012;184:153–9. [2] Madia G, Elsener M, Koebel M, Raimondi F, Wokaun A. Thermal stability of vanadia-tungsta-titania catalysts in the SCR process. Appl Catal B: Environ 2002;39:181–90. [3] Djerad S, Crocoll M, Kureti S, Tifouti L, Weisweiler W. Effect of oxygen concentration on the NOx reduction with ammonia over V2O5–WO3/TiO2. Catal Today 2006;113:208–14. [4] Ramis G, Busca G, Cristiani C, Lietti L, Forzatti P, Bregani F. Characterization of tungsta-titania catalysts. Langmuir 1992;119:1744–9. [5] Kröcher O, Elsener M. Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils, and urea solution: I. Catalytic studies. Appl Catal B: Environ 2008;77:215–27. [6] Nicosia D, Czekaj I, Kröcher O. Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils and urea solution: Part II. Characterization study of the effect of alkali and alkaline earth metals. Appl Catal B: Environ 2008;77:228–36. [7] Alemany LJ, Lietti L, Ferlazzo N, Forzatti P, Busca G, Giamello E, et al. Reactivity and physicochemical characterization of V2O5-WO3/TiO2 De-NOx catalysts. J Catal 1995;155:117–30.

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Please cite this article in press as: Aguilar-Romero M et al. Acidity, surface species, and catalytic activity study on V2O5-WO3/TiO2 nanotube catalysts for selective NO reduction by NH3. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.11.090