TiO2-based catalysts with high Fe2O3-to-V2O5 ratios

Journal Pre-proof Selective catalytic reduction of NO by NH3 over Fe2 O3 -promoted V2 O5 /TiO2 -based catalysts with high Fe2 O3 -to-V2 O5 ratios Thi Phuong Thao Nguyen, Ki Hyuck Yang, Moon Hyeon Kim, Yong Seok Hong

PII:

S0920-5861(20)30085-7

DOI:

https://doi.org/10.1016/j.cattod.2020.02.021

Reference:

CATTOD 12689

To appear in:

Catalysis Today

Received Date:

30 September 2019

Revised Date:

14 February 2020

Accepted Date:

19 February 2020

Please cite this article as: Thao Nguyen TP, Yang KH, Kim MH, Hong YS, Selective catalytic reduction of NO by NH3 over Fe2 O3 -promoted V2 O5 /TiO2 -based catalysts with high Fe2 O3 -to-V2 O5 ratios, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.02.021

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Selective catalytic reduction of NO by NH3 over Fe2O3-promoted V2O5/TiO2-based catalysts with high Fe2O3-to-V2O5 ratios

Thi Phuong Thao Nguyena, Ki Hyuck Yanga, Moon Hyeon Kima,*, Yong Seok Hongb

a

Department of Environmental Engineering, College of Engineering, Daegu University

201 Daegudae-ro, Jillyang, Gyeongsan 38453, Republic of Korea Department of Environmental Systems Engineering, College of Science and Technology,

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b

Korea University, 2511 Sejong-ro, Chochiwon, Sejong 30019, Republic of Korea

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Submitted to Catalysis Today, September 30, 2019, Revised December 17, 2019, 2nd Revised

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February 14, 2020

Tel: +82-(0)53 850-6693

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*Corresponding author.

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E-mail: [email protected]

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Graphical abstract

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Highlights A transformation of surface VOx species by the presence of Fe2O3

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A key role of Fe-O-V species for the reduction of N2O by strongly-adsorbed NH3

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Depression of N2O emissions using supported catalysts with high Fe2O3/V2O5 ratios

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Reduction of N2O by surface NH3 species strongly adsorbed on the Lewis sites

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

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Abstract

Fe2O3-promoted V2O5-WO3/TiO2 catalysts with high Fe2O3-to-V2O5 ratios (3.4 – 6) have

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been prepared for the selective reduction of NO with NH3 (NH3-SCR) and extensively

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characterized using various spectroscopic measurements. No crystalline iron oxides were indicated even for 8% Fe2O3-promoted V2O5-WO3/TiO2 but distorted α-Fe2O3 crystallites

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were observed in a XRD pattern for 7.04% Fe2O3-promoted WO3/TiO2. Both Raman and XPS measurements suggest the formation of Fe-O-V species which may make a better N2 selectivity in NH3-SCR reaction at high temperatures. All Fe2O3-promoted catalysts showed a

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great depression on N2O formation. At temperatures > 400oC, samples with 5.46 and 8%

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Fe2O3 (Fe2O3/V2O5 ratio = 3.4 and 5) gave significantly lower N2O production levels compared with the unpromoted and lower Fe2O3 loading catalysts. However, increasing in Fe2O3 loading resulted in a decrease in high temperature deNOx activity due to the oxidation of NH3 into NO. All the 3.4 and 5 ratio catalysts after a hydrothermal aging at 550oC for 10 h gave NO removal activity and N2O formation similar to those measured over non-aged ones. However, such an aging at 750oC could lead to a considerable increase in N2O formation

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even for the 5.46% Fe2O3-promoted catalyst. Fe2O3-promoted V2O5/TiO2-based catalysts could catalyze the reduction of gas-phase N2O by NH3 adsorbed strongly, suggesting that this reaction may be a major route to greatly suppress N2O formation, consistent with infrared studies showing a reaction between N2O and surface NH3 species strongly adsorbed on the catalysts. This surface reaction could readily occur from a temperature as low as 250oC.

Key words: Fe2O3-promoted V2O5/TiO2-based catalyst; high Fe2O3-to-V2O5 ratio; NH3-SCR;

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N2O formation; N2O reduction with NH3, strongly adsorbed NH3

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1. Introduction

Titania-supported V2O5-based systems are commercially employed in the selective

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catalytic reduction of NO by NH3 (NH3-SCR) for stationary and mobile applications [1].

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Regrettably, the catalysts can give a considerable amount of N2O due to the direct oxidation of NH3 into N2O and a reaction between NH3 and NO,

(1)

4NH3 + 4NO + 3O2 → 4N2O + 6H2O,

(2)

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2NH3 + 2O2 → N2O + 3H2O,

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which predominantly occurs at high temperatures as side reactions, beside the standard NH3SCR reaction [2,3],

4NH3 + 4NO + O2 → 4N2 + 6H2O.

(3)

N2O is an undesired side product since it is a high global warming potential

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greenhouse gas [2]. According to previous proposals that Fe-exchanged zeolites catalysts are highly active for NH3-SCR reaction, direct N2O decomposition, and N2O reduction with NH3 [4-8], many studies on the modification of V2O5/TiO2-based catalysts using Fe-zeolites, and Fe oxides have been conducted to improve their selectivity at high reaction temperatures, representatively a series of Fe-zeolite coated V2O5-WO3/TiO2 catalysts [9], a promotion of Fe2O3 in a TiO2 support [10], and a substitution of WO3 with Fe2O3 [11]. In our earlier work [12], we have reported a significant role of Fe2O3 in V2O5-WO3/TiO2 catalysts to efficiently

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depress the formation of N2O in NH3-SCR reaction. All of a 2.73% Fe2O3-promoted inhouse-made-V2O5-WO3/TiO2 and a 2.73% Fe2O3-promoted commercial-V2O5-WO3/TiO2 catalysts exhibited significantly lower N2O levels at temperatures > 350oC, compared with

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unpromoted ones. The extent of the N2O formation was reduced by 60% for the former Fe2O3-promoted sample. X-ray diffraction (XRD) and Raman spectroscopy measurements

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suggested the presence of Fe2O3-induced, tetrahedral polymeric vanadates and/or V-O-Fe

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species that could be responsible for the reduction in N2O production. On the other hand, NH3 temperature-programmed desorption (NH3 TPD) studies showed an increase of thermally-stable surface NH3 species after the Fe2O3 promotion. Based on these previous

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results with the Fe2O3-promoted V2O5/TiO2-based systems, we have proposed a major pathway to greatly depress the formation of N2O in NH3-SCR reaction at high temperatures

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that is a reaction between the N2O and strongly-adsorbed NH3 residues [12].

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This study is a continuation of our earlier report to see if Fe2O3-promoted V2O5WO3/TiO2 catalysts with high Fe2O3-to-V2O5 ratios have a further depressive effect on N2O production in NH3-SCR reaction and if they can successfully catalyze the reduction of N2O with NH3, and the N2O decomposition. Not only these N2O-related reactions were directly measured using the high Fe2O3-promoted catalysts, but surface reactions between gaseous N2O and NH3 species adsorbed strongly on their surface were also studied using an in situ

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infrared spectroscopic technique.

2. Experimental

2.1 Catalyst preparation

A commercial powder type TRONOX 10% WO3/TiO2, here after designated to WT,

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was calcined at 500oC for 4 h in air to use it as a support. Two WT-supported catalysts with 0.46 and 1.6% V2O5 were prepared by impregnating the WT with an aqueous NH4VO3 solution which had been obtained by dissolving corresponding NH4VO3 (99.99%, Sigma-

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Aldrich) amounts in an aqueous solution of oxalic acid (≥ 99%, Sigma-Aldrich). These catalysts were calcined as for the WT, prior to preparing Fe2O3-promoted catalysts. Each

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aqueous solution of Fe(NO3)·9H2O (≥ 99.95%, Sigma-Aldrich) containing Fe amounts equivalent to 2.73 – 8% Fe2O3 was loaded on the WT, 0.46% V2O5/WT and 1.6% V2O5/WT

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using a wet impregnation method. All the resulting samples were dried at 110oC overnight in an oven and then calcined at 500oC for 1 h in air prior to conducting catalytic activity tests

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and spectroscopic characterization. A WO3-free TiO2-supported 8% Fe2O3-1.6% V2O5 sample was obtained in a similar fashion as described above using a Millennium DT51 TiO2

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as a support, referred to as 8% Fe2O3-1.6% V2O5/T.

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In order to test the hydrothermal stability of selected catalysts, 1.6% V2O5/WT, 5.46% Fe2O3-1.6% V2O5/WT, 8% Fe2O3-1.6% V2O5/WT, and 8% Fe2O3-1.6% V2O5/T, all these were subjected to a hydrothermal aging (HA) at 550 and 750oC for 10 h under a flowing mixture of 10% H2O and 5% O2 in flowing N2 at a total flow rate of 200 cm3/min. These aged samples were indicated by “-nHA” next to each catalyst to distinguish them from the fresh ones, where n represents the aging temperature.

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2.2 Catalyst characterization

A Rikagu Model D/MAX 2500 PC diffractometer with a Cu Kα (λ = 1.5405 Å) radiation source was utilized to collect XRD patterns of catalyst samples from a 2θ value of 5 to 80o. Each sample was measured at a scanning rate of 0.1 o/min with an X-ray voltage and current of 40 kV and 20 mA, respectively.

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To determine representative textural properties of the catalysts, specific BET surface area (SBET), mesopore size (dm) and total pore volume (Vt), nitrogen sorption experiments were conducted using a Micromeritics Model 3 Flex Version 3.01 system. An appropriate

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amount of each calcined sample (ca. 50 mg) was charged into the system sample cell and allowed a 1-h evacuation at room temperature following a further evacuation at 90oC before

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finally evacuatation at 300oC overnight under a high dynamic vacuum below 10-7 Torr (1

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Torr = 133.3 Pa). After this, N2 was introduced into the sample cell at the temperature of liquid nitrogen (-196oC). Pore size distributions (PSD) for the catalyst samples were calculated using the Barret-Joyner-Halenda (BJH) mesopore model.

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Raman measurements for the catalyst samples were conducted using a Thermo Scientific DXR 2xi Raman spectrometer equipped with an EM-CCD detector. Each sample

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was excited by a 514-nm diode laser with a laser power of ca. 2mW. Exposure of each

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sample to the laser beam was 0.01 – 0.03 s. All Raman spectra were recorded in the region 100 – 2000 cm-1 with a scan number of 500 – 800 and a resolution of 1.5 cm-1. A Thermo Scientific K-Alpha+ high performance X-ray photoelectron (XPS) was

employed to obtain Fe 2p, V2p, O 1s and W 4f core level spectra of the samples. The spectrometer was equipped with a microfocussed monochromatic Al Kα X-ray source with a radiation energy of 1486.6 eV. Approximately 10 mg of each sample was pressed into a 5-

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mm diameter thin disc and introduced to a prevacuum before the measurement. The X-ray spot size and sample step size used for element-specific energy ranges were 400 μm and 0.05 eV, respectively. A total number of scan was 100 – 150 for Fe 2p and 30 for V 2p, O 1s and W 4f. All core level binding energies were corrected using a 284.8 eV peak for C 1s.

2.3 Catalytic reactions

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NH3-SCR reaction with Fe2O3-promoted catalysts was performed in an on-line system consisting of a 3/8” quartz reactor placed in a temperature adjustable furnace and a modified Zemini Model MARS 0.75 L/8.0 V White gas cell coupled with a Thermo Electron

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Nicolet 6700 Fourier-transformation infrared (FT-IR) spectrometer [12-14]. A total gas flow rate in all measurements was 1000 cm3/min which corresponds to a gas hourly space velocity

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(GHSV) of 76,200 h-1. A mixture of 21% O2/N2 was flowed through the reactor containing

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ca. 0.4 g of each catalyst to calcine it at 500oC for 1 h before allowing the reaction. The reaction gas stream was a mixture of 500 ppm NO (99.99%, Scott Gas Specialty), 500 ppm NH3 (99.999%, Scott Gas Specialty) and 5% O2 (99.999%, Praxair) in N2 (99.999%, Praxair).

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In order to test activity of the catalyst samples in N2O removal reactions, such as N2O reduction with NH3, and N2O direct decomposition, a similar experimental procedure as for

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the NH3-SCR reaction was carried out with flowing mixtures of 300 ppm N2O (99.999%,

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Scott Gas Specialty) and 5% O2 or of 300 ppm N2O, 300 ppm NH3 and 5% O2 in N2. The NO, NH3 and N2O were used without further purification while the N2 and O2 were passed through moisture traps.

2.3 In situ DRIFTS measurements

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In situ DRIFTS measurements were conducted using a Thermo Electron Nicolet 6700 FT-IR spectrometer coupled with a DRIFT dual-cup environmental chamber. All spectra were recorded from 650 to 4000 cm-1 using a MCT A detector. The environmental chamber was equipped with ZnSe windows. The whole system was fully purged with pure N2 obtained by boiling liquid N2 up at a flow rate of 20 L/min. The temperature of each sample cell was precisely controlled by a CAL 9500P programmable process controller. An appropriate amount of each catalyst sample (ca. 10 mg) was loaded into the

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sample cell and in situ pretreated at 500oC for 1 h in a flowing mixture of 5% O2/N2 with a total flow rate of 40 cm3/min. Then, the temperature of the cell and the concentration of O2 were reduced to 100oC and 0%, respectively. A spectrum of the bare sample for Fourier-

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transforming a spectrum taken after NH3 adsorption was recorded after 30 min. The sample was then exposed to 700 ppm NH3 in N2 at 100oC for 30 min. After this, the gas flow was

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switched to a pure N2 to purge the sample for 1 h and a spectrum was collected to use as a

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background spectrum. For NH3 temperature programmed desorption (NH3 TPD), the cell was heated from 100 to 700oC at a rate of 10 oC/min in a flow of pure N2. In the case of N2O temperature programmed surface reaction (N2O TPSR), 175 ppm N2O was added into the gas

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stream. During each process, spectra were recorded successively. The N2O (99.999%, Scott Gas Specialty) and NH3 (99.999%, Scott Gas Specialty) were used without a further

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purification. The N2 (99.999%, Praxair) and O2 (99.999%, Praxair) were further purified

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using moisture and oxygen traps. All spectra were collected with a total scanning number of 500 and a resolution of 4 cm-1. Gas-phase concentrations of N2O removed in the N2O TPSR with the Fe2O3-promoted catalysts were determined by integrating bands at 2327 and 2215 cm-1 by gas-phase N2O using the OMNIC software.

3. Results and discussion

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3.1 Physicochemical properties of WT-supported Fe2O3-V2O5 catalysts

XRD patterns of Fe2O3-promoted WT-supported V2O5 catalysts with high Fe2O3-toV2O5 ratios are shown in Fig. 1. All WT-supported samples exhibited a strong peak near 25.3o with other weaker ones in higher 2θ region (Fig. 1(a) – (e)). These features are consistent with an XRD pattern of a JCPDS # 84-1286 TiO2 (Fig. 1(f)). All the samples

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showed no peaks at 2θ = 23.2, 23.6, and 24.6o by crystalline WO3 [15], proposing that they contain amorphous WO3 species. A sample of 2.73% Fe2O3/WT (Fig. 1(a)) had no peaks corresponding to wustite (FeO), hematite (α-Fe2O3), maghemite (γ-Fe2O3), magnetite

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(Fe3O4), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and feroxyhyte (δ-FeOOH) in respective JCPDS #s 75-1150, 33-0664, 39-1346, 02-1035, 29-0713, 34-1266, 08-98, and 29-

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712. However, 7.04% Fe2O3/WT gave peaks at 33.2, and 35.6o with weaker peaks at 40.9,

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and 49.5, as seen in a magnified XRD pattern (Fig. 1(b)). All these reflections are matched to those indicated for pristine α-Fe2O3 (Fig. 1(g)) and their upward shift did not occur unlike an observation reported for a series of Fe2O3/Al2O3 with relatively high Fe2O3 contents (15 –

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95%) [16]. All of catalysts containing V2O5, such as 2.73% Fe2O3-0.46% V2O5/WT, 5.46% Fe2O3-1.6% V2O5/WT, and 8% Fe2O3-1.6% V2O5/WT, showed no XRD peaks related to

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crystalline V2O5 (JCPDS # 41-1426) as well as to the aforementioned iron oxides and

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oxyhydroxides, as provided in (c) – (e) in Fig. 1. The coexistence of Fe2O3 and V2O5 on a WT support has created not only the redispersion of the V2O5 but also the formation of Fe-OV-like species [12]. This suggests that the lateral interaction of iron oxides with neighboring vanadate species may suppress the formation of their crystallites and may be the reason why the Fe2O3-V2O5/WT catalysts gave no α-Fe2O3 peaks unlike the 7.04% Fe2O3-promoted sample, which will be discussed with Raman and XPS results below. Since iron

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oxyhydroxides can be easily transformed into α-Fe2O3 at a relatively low thermal energy [12,17] and all catalyst samples studied here were calcined at 500oC for 1 h in a flowing air, it is reasonable that iron oxide species existing in the samples even with high Fe2O3-to-V2O5 ratios are in the form of highly dispersed non-crystallites. Sorption isotherms and PSDs for Fe2O3-promoted V2O5/WT catalysts with high Fe2O3-to-V2O5 ratios are provided in Fig. 2. A hysteresis loop with a closing point at P/Po = 0.63 – 0.68 appeared irrespective to the catalyst, as shown in (S1) – (S5) in Fig. 2(a), which is

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a common feature of mesoporous materials [18]. All the catalyst samples represented the PSDs with maxima around 6 – 7.5 nm, depending on the catalyst (Fig. 2(b)). A PSD for V2O5 containing catalysts was shifted toward larger pore sizes ((S3) – (S5) in Fig. 2(b)), indicating

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that the blockage of relatively small pores occurred. On the other hand, textural data of the catalyst samples are listed in Table 1. WT-supported Fe2O3 and Fe2O3-V2O5 samples had

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SBET and Vt values lower than those (SBET = 91 m2/g, Vt = 0.30 cm3/g) of a sample of WT

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reported already [12]; however, the SBET and Vt values of the latter catalysts are similar to those for WT-supported 1.6% V2O5 and 2.73% Fe2O3-1.6% V2O5 that have been, respectively, denoted to VWT and 2.73% Fe2O3/VWT [12]. Meanwhile, values for dm of the

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catalysts employed in this work (Table 1) were slightly larger than that (dm = 13.9 nm) of the

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WT-supported 2.73% Fe2O3-1.6% V2O5 determined previously [12].

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3.2 Surface molecular structures of WT-supported Fe2O3-V2O5 catalysts

Raman spectra of WT-supported Fe2O3 and Fe2O3-V2O5 catalysts are presented in

Fig. 3. All these samples gave bands at 398, 518, and 637 cm-1 which are characteristic Raman signals for anatase TiO2 [19,20]. Intensities of these bands decreased significantly when a Fe2O3 loading was 5.46 – 8% in WT-only and 1.6% V2O5 catalysts, as shown in

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spectra (b), (d) and (e) in Fig. 3, and this is because the TiO2 surface was further covered by FeOx species. None of bands by crystalline V2O5 and WO3 [12] were indicated, which is consistent with the XRD measurements (Fig. 1). Characteristic bands of crystalline α-Fe2O3 at 615, 500, 410, and 300 cm-1 [21] were not observed even for high Fe2O3 samples, such as 7.04% Fe2O3/WT, and 8% Fe2O3-1.6% V2O5. This is due to that these signals would be easily overshadowed by strong Raman modes of TiO2 at the region. A sample of 2.73% Fe2O3/WT exhibited bands at 795, 880, and 981 cm-1 in the

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region 700 – 1100 cm-1, as seen in its magnified spectrum (dash line) in Fig. 3(a). The band at 795 cm-1 is assigned to the first overtone of the 398 cm-1 band [12,22], while the latter two ones are attributed to the asymmetric W-O-W and terminal W=O in surface polytungstate

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species with an octahedral environment, respectively [12,22,23]. When a Fe2O3 content increased to 7.04%, all these bands were hardly detected even after a magnification, because

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of a surface fully covered by the iron oxide species (Fig. 3(b)). The 2.73% Fe2O3/WT gave a

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peak near 1330 cm-1 (Fig. 2(a)) but no Raman bands regarding iron oxides and oxyhydroxides reported in the literature [12,17,24]. A similar peak centered at 1320 cm-1 appeared in the 7.04% Fe2O3/WT (Fig. 3(b)). The signal at 1320 – 1330 cm-1 is attributed to

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the second order scattering in α-Fe2O3 [17,24].

Bands at 795, 880, and 981 cm-1 were observed for samples of 2.73% Fe2O3-0.46%

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V2O5/WT and 5.46% Fe2O3-1.6% V2O5/WT, as shown in spectra (c) and (d) in Fig. 3. The

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latter two bands are associated with V2O5 but not with WO3, since surface V2O5 species give rise to bands at similar frequencies and the Raman cross section of vanadium oxide species is approximately 4 times of that of tungsten oxide species [12,22,23,25-27]. Therefore, the 880 and 981 cm-1 bands can be assigned to the bridging V-O-V and terminal V=O bonds in distorted octahedrally coordinated vanadium oxide species, respectively, indicating the presence of polymeric mono-oxo vanadates on the surface of the catalysts [12,25-27]. The

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2.73 and 5.46% Fe2O3-promoted V2O5 catalysts had no bands related to the iron oxides and oxyhydroxides mentioned previously, unlike a sample of 8% Fe2O3-1.6% V2O5/WT that gave a weak broad band at ca. 1327 cm-1 (Fig. 3(e)) due to the second order scattering in α-Fe2O3 [17,24]. On the other hand, the 8% Fe2O3-promoted sample gave only a broad band near 825 cm-1. A similar Raman spectrum was shown for the 5.46% Fe2O3-1.6% V2O5/WT sample in which the bands at 880, and 981 cm-1 became weak (Fig. 3(d)). A comparative study of Raman spectra between 5% V2O5/Al2O3 and 5% Fe2O3-5% V2O5/Al2O3 has reported that the

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iron oxide species can transform surface metavanadates (VO3) into orthovanadates (VO4) whose characteristic Raman signal appears around 820 cm-1 [21]. Sharp phonon signals at 1020 – 1030 cm-1 by the isolated VO3 species when collected under dehydrated conditions

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exist even in high V2O5 samples, representatively 8% V2O5/TiO2, but do not under hydrated ones such as ambient measurements [25,28,29]. The 8% Fe2O3-1.6% V2O5/WT sample would

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contain such metavanadates although these species were not visible because of the data

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collection at ambient conditions. Based on these reports, and the previous discussion, the 825 cm-1 peak can be assigned to the VO4 species, disclosing that the two-dimensional VO3 species in the 8% Fe2O3-1.6% V2O5/WT can be transformed to tetrahedrally-coordinated VO4

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species and that the iron oxide species are responsible for this surface molecular structure change.

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XPS spectra of Fe 2p core level in WT-supported Fe2O3-promoted V2O5 catalysts are

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shown in Fig. 4(a). The catalyst samples exhibited the main Fe 2p3/2 and Fe 2p1/2 peaks at 710.4 – 710.9 eV and 723.7 – 724.0 eV, respectively, consistent with binding energy (BE) values for Fe3+ reported in the literature [30,31]. The Fe 2p3/2 peak of each sample accompanied the corresponding satellites whose BEs are higher, by 5.1 – 6.1 eV, than its main peak one, depending on the sample. The Fe 2p3/2-to-satellite energy split values (ΔEMS) for the catalysts were considerably lower than that for bulk Fe2O3, which is approximately 8

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eV [30], implying that the chemical environments of Fe3+ oxide species in the catalyst samples are different from that in the bulk oxide. The ΔEMS decreases with a decrease in the electronegativity of ligands [31]. The lower ΔEMS observed for the catalysts may be due to the difference in the electronegativity between O2- ligands in the catalysts and in the bulk iron oxide [31,32]. An interesting point is that 5.46% Fe2O3-1.6% V2O5/WT and 8% Fe2O3-1.6% V2O5/WT gave a shoulder around 709.3 eV, which is similar to a Fe 2p3/2 BE value of Fe2+ oxide species [30,31]. This suggests that there might be a change in an electron distribution in

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Fe-O bonds in the catalysts surface. XPS spectra of V 2p, W 4f, and O 1s in WT-supported Fe2O3-V2O5 catalysts and the WT itself are shown in spectra (b) – (d) in Fig. 4. As seen from Fig. 4(b), a 2.73% Fe2O3-

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0.46% V2O5/WT catalyst gave a main V 2p3/2 peak at 516.2 eV while samples with 1.6% V2O5 exhibited a higher BE by 0.2 eV. The indicated V 2p BE values were somewhat lower

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than those (516.7 – 517.2 eV) reported for bulk V2O5 [33,34]. Since the V2O5 amounts in the

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catalyst samples were below monolayer coverage, the low V 2p3/2 BE is due to the high dispersion of VOx species [35]. A peak near 524.0 eV existed in V 2p spectra of the catalysts and could be attributed to V 2p1/2 [34]. The WT support showed W 4f7/2 and W 4f5/2 main

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peaks at 36.2 and 38.0 eV which are similar to those for W6+ in WO3 [36]. The 2.73% Fe2O3/WT had higher W 4f7/2 and W 4f5/2 BEs, by 0.2 eV, compared with the support (Fig.

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4(c)-(S2)). The BEs approached the values for the support when a Fe2O3 loading was 7.04%.

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The samples with 0.46 – 1.6% V2O5 exhibited W 4f main peaks whose BEs was almost the same as those of the WT-only sample. All catalysts studied here gave relatively strong O 1s peaks around 530.3 – 530.5 eV (Fig. 4(d)), which is typical for the lattice oxygen [37]. Based on the XPS and Raman results, the previous discussion, and a strong affinity of vanadium to oxygen, it is proposed that surface iron oxides may interact with their adjacent surface vanadate and tungstate species to form Fe-O-V- and Fe-O-W-like moieties on the catalyst

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surface. These species can make a better N2 selectivity in NH3-SCR reaction at high temperatures [38-40]. This is consistent with our earlier proposal for 2.73% Fe2O3-promoted V2O5-WO3/TiO2 catalysts [12].

3.3 N2O formation and NH3-SCR performances of WT-supported Fe2O3-V2O5 catalysts

It is of our particular interest to the extent of the formation of N2O in NH3-SCR

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reaction over WT-supported Fe2O3-promoted V2O5 catalysts with higher Fe2O3-to-V2O5 ratios (3.4 – 5.9), compared to a laboratory-made sample of 2.73% Fe2O3-1.6% V2O5/WT (Fe2O3/V2O5 ratio = 1.7) used in our previous study [12]. Their N2O production levels are

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provided in Fig. 5 (a) in which the earlier data of N2O formation have been included for a readily comparison as indicated by a dash line. All the catalysts showed significant N2O

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production at high temperatures > 400oC. Compared with a 1.6% V2O5/WT catalyst that has

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given very high N2O formation, for example, 60 and 115 ppm at 450 and 480oC, respectively [12], the high Fe2O3-promoted V2O5/WT catalysts employed here showed considerably lower N2O emissions even at the temperatures, regardless of the Fe2O3-to-V2O5 ratio, as revealed in

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Fig. 5(a). A 5.46% Fe2O3-1.6% V2O5/WT catalyst with a ratio of Fe2O3/V2O5 = 3.4 has the highest N2O production and yields 27 ppm N2O even at 480oC. This value is approximately a

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half of the concentration obtained for a 2.73% Fe2O3-1.6% V2O5/WT catalyst reported earlier

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[12], disclosing that a depressive effect on N2O production in NH3-SCR reaction increases with V2O5-WO3/TiO2 catalysts having high Fe2O3/V2O5 ratios. Recent studies on the formation of N2O over V2O5/TiO2 catalysts with different V2O5 loadings have proposed that surface VOx species act as active sites for N2O formation and that this may be affected by changes in their surface molecular structure [41,42]. Combining this and the previous discussion of the Raman results with the indicated trend in the extent of N2O formation, it

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suggests that increasing Fe2O3 loading probably gives rise to a rearrangement of surface VOx structures, thereby resulting in a lower N2O formation. Another interest to us is the influence of other surface oxides, such as V2O5 and WO3, on the production of N2O in NH3-SCR reaction. The 2.73% Fe2O3-promoted WT catalyst containing 0.46% V2O5 gave a N2O formation around 15 ppm at 480oC (Fig. 5(a)), which is much lower than that (ca. 45 ppm) reported for a 2.73% Fe2O3-1.6% V2O5/WT catalyst [12]. A similar effect can be shown for the 7.04% Fe2O3/WT sample whose N2O

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concentration even at 480oC was less than 15 ppm that is almost a half of a N2O concentration measured for the 8% Fe2O3-1.6% V2O5/WT catalyst with the Fe2O3 content reasonably comparable with the V2O5-free one, as shown in Fig. 5(a). Thus, N2O formation

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levels increase with V2O5 amount loaded onto the WT. This is in good agreement not only with the previous discussion, but also with the dependence reported for V2O5-WO3/TiO2

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systems with different V2O5-to-WO3 ratios [42]. Finally, we can address a role of WO3 in

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suppressing the formation of N2O in NH3-SCR reaction. The 8% Fe2O3-1.6% V2O5/T without WO3 showed N2O concentrations comparable with those over the 8% Fe2O3-1.6% V2O5/WT (Fig. 5(a), implying that WO3 may not affect crucially the formation of N2O.

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Figure 5(b) and (c) show conversions of NO and NH3 in NH3-SCR reaction over Fe2O3-promoted V2O5/WT catalysts with high Fe2O3-to-V2O5 ratios. At all temperatures

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below 350oC, values for NO conversion of the catalysts were comparable with corresponding

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conversions for NH3 according to Eq. (3). The extent of NO removal varies with a ratio of Fe2O3/V2O5 loaded onto WT, as shown in Fig. 5(b). A 5.46% Fe2O3-1.6% V2O5/WT catalyst with a Fe2O3-to-V2O5 ratio of 3.4 showed NO conversions at 300 – 480oC that are very similar to those reported for 2.73% Fe2O3-1.6% V2O5/WT and a commercial 1.44% V2O59.42% WO3/TiO2 catalyst [12] but the laboratory-made Fe2O3-promoted sample gave lower N2O production levels by 50% even at 480oC (Fig. 5(a)). Based on these points, an optimal

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Fe2O3/V2O5 ratio may be 3.4 for good SCR reaction and low N2O emissions. On the other hand, 8% Fe2O3-1.6% V2O5/WT possessed higher NO and NH3 conversions by max. 20% at temperatures < 350oC, compared with 8% Fe2O3-1.6% V2O5/T. This is mainly due to the absence of WO3 in the latter sample, since it has been well-known that the addition of WO3 to V2O5/TiO2 noticeably increases deNOx activity at low temperatures, with a much wider temperature window for high deNOx rate [43]. However, the both catalysts gave comparable deNOx performances at 300 – 400oC at which most of stationary industrial SCR systems are

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operated [15], suggesting that it may be probable to successfully replace the WO3, typically 7 – 10% [9,12,13,15,29], in titania-supported V2O5 catalysts to such iron oxide species. In addition, a loss in deNOx activity at temperatures > 400oC occurred for all catalysts studied

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here. This is because of the oxidation of NH3 to N2O, NO, and N2, which can predominantly take place at such temperatures [2,9,12,13,15,29]. The NO has increased after the promotion

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of Fe2O3 to V2O5-WO3/TiO2 catalysts, as reported in our previous study [12].

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The extent of N2O production and NO conversion in NH3-SCR reaction with WTsupported Fe2O3-promoted 1.6% V2O5 analogues at chosen temperatures as a function of the Fe2O3 loading is provided in Fig. 6. N2O formation at 450 and 480oC greatly decreased when

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the Fe2O3 loading increased up to 5.46%, as seen in Fig. 6(a), which is represented that the 5.46% Fe2O3 (Fe2O3/V2O5 ratio = 3.4) is optimal to efficiently depress such a N2O production

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at high temperatures. When V2O5/WT samples were promoted by Fe2O3 up to 8%, there was

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no noticeable decrease in the conversion of NO at 400 and 450oC, although this was significant at 480oC (Fig. 6(b)). Consequently, it is proposed that an optimum Fe2O3 loading in WT-supported 1.6% V2O5 may be 5.46% in which a maximum suppressant effect on N2O production can be achievable without significant deNOx activity loss. V2O5/TiO2-based catalysts for diesel automotive applications essentially require a good hydrothermal stability since the exhaust of diesel engines typically has a temperature >

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600oC during the regeneration of diesel particulate filter and contains ca. 10% H2O [44,45]. N2O formation, and NO and NH3 conversions in NH3-SCR reaction over 5.46% Fe2O3-1.6% V2O5/WT-550HA, 8% Fe2O3-1.6% V2O5/WT-550HA, 8% Fe2O3-1.6% V2O5/T-550HA, 5.46% Fe2O3-1.6% V2O5/WT-750HA, and 1.6% V2O5/WT-750HA are shown in Fig. 7. All the 550oC aged catalysts gave no significant difference in the N2O formation and deNOx activity, compared with their fresh ones in Fig. 5, which is in good agreement with a previous study reported for 2% V2O5-10% WO3/TiO2 whose hydrothermal aging at 550oC have

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showed no effects on NO conversion [45]. At 400 – 480oC, the 5.46% Fe2O3-1.6% V2O5/WT750HA exhibited a N2O production of 123 – 216 ppm although no noticeable decrease in NO conversion was observed (Fig. 7(a) and (b)). Surprisingly, this catalyst had N2O formation,

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deNOx activity and NH3 conversion profiles similar to those measured over the 1.6%

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V2O5/WT-750HA sample (Fig. 7(a) – (c)).

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3.4 N2O removal reactions with WT-supported Fe2O3-V2O5 catalysts

To examine whether or not the promotion of Fe2O3 in WT-supported V2O5 catalysts

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can facilitate the reduction of N2O by NH3 that has been proposed as a major pathway to decrease N2O production in NH3-SCR deNOx catalysis with 2.73% Fe2O3-1.6% V2O5/WT

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catalysts [12], the N2O-NH3-O2, and N2O-O2 reactions were allowed, as provided in Fig. 8.

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The N2O reduction with NH3 is shown in Fig. 8(a). This reaction took place over 8% Fe2O31.6% V2O5/WT at temperatures < 400oC but the extent was small. While, 5.46% Fe2O3-1.6% V2O5/WT gave N2O concentrations at 400 – 480oC that is greater than the upstream one (300 ppm) as indicated by a dash line. This is also observed over 8% Fe2O3-1.6% V2O5/T with the same Fe2O3/V2O5 ratio as that of the 8% Fe2O3-1.6% V2O5/WT catalyst but not with 10% WO3. These indicate that the NH3 fed with the 300 ppm N2O over the catalysts was oxidized

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to N2O. Such an oxidation not only over V2O5-WO3/TiO2 systems but also over metal oxides can easily occur [2,15,29,46,47]. Even for the 5.46% Fe2O3-1.6% V2O5/WT with the highest additional N2O concentration (ca. 15 ppm) at 480oC among the catalysts used (Fig. 8(a)), the measured N2O level was significantly lower than that (ca. 30 ppm) in NH3-SCR reaction, as seen in Fig. 5(a). Figure 8(b) discloses that all catalysts used had no significant difference in the extent of NH3 conversion in the reduction of N2O with NH3 at all temperatures. In addition to this, the N2O-O2 reaction (N2O decomposition) did not occur irrespective to the

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catalyst used here (Fig. 8(a)). A commercial V2O5-WO3/TiO2 catalyst coated by a Fe-zeolite could well catalyze the direct decomposition of N2O at temperatures > 350oC [9]. These strongly suggest that highly ionic Fe species are active for the reaction. Based on the results

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of the reduction of N2O with NH3 and its decomposition, and the previous discussion regarding the role of Fe2O3 in depressing N2O production in NH3-SCR reaction, the N2O

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reduction in the presence of NH3 may take place over the Fe2O3-promoted V2O5-WO3/TiO2

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catalysts, according to the overall stoichiometry [9,12,13,15]:

(4)

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4NH3 + 4N2O + O2 → 6N2 + 6H2O.

It is worthy to be noted that this reaction is probable when the NH3 was strongly adsorbed on

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such Fe2O3-promoted V2O5/WT catalysts as proposed in our earlier work [12], which will be

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discussed for the reaction between such strongly-adsorbed NH3 species and gas-phase N2O using a DRIFTS technique below. In situ DRIFTS measurements were conducted to monitor changes in a gas-phase

N2O (“N2O(g)”) concentration of 175 ppm continuously fed over WT-supported Fe2O3promoted V2O5 catalysts on which 700 ppm NH3 had been adsorbed at 100oC following a fully purge by flowing N2. The changes with respect to the ramping temperature were

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calculated by integrating a predominant IR absorption of the N2O(g) at 2237 and 2215 cm-1 [13,14] and the results are provided in Fig. 9. The reactivity between preadsorbed NH3 species (“NH3(ad)”) and N2O(g) can be determined by two points: one is a starting temperature giving a N2O concentration less than the upstream one that was marked by a horizontal bar in Fig. 9, and another is a N2O concentration below the 175 ppm N2O. All these two indicators were much lower over 5.46% Fe2O3- and 8% Fe2O3-1.6% V2O5/WT catalysts than over 7.04% Fe2O3/WT and 8% Fe2O3-1.6% V2O5/T. This indicates that the coexistence of Fe2O3

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and V2O5 on WT may further facilitate the N2O consumption. That N2O concentrations > 175 ppm at 130 – 220 and 130 – 350oC were, respectively, measured for the former two samples and the latter two ones imply an additional formation of N2O via a surface reaction between

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NH3(ad) and surface oxygen atoms. The two Fe2O3-promoted 1.6% V2O5/WT catalysts had no noteworthy difference in the surface reactivity to the NH3(ad)-N2O(g) reaction. Consequently, it

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is clear that the reduction of N2O by strongly-adsorbed NH3 species readily takes place over

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WT-supported Fe2O3-V2O5 catalysts, consistent with our earlier proposal [12], and that the iron oxide species can facilitate this reaction.

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3.5 Interaction of N2O(g) with surface NH3 species

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In order to further understand the interaction between N2O(g) and NH3(ad), DRIFTS

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spectra of the NH3(ad) species on the Fe2O3-promoted catalysts during NH3 TPD and N2O TPSR runs were recorded, as shown in Figs. 9 and 10. A 7.04% Fe2O3/WT sample showed bands at 1670,1600, 1438, and 1255 cm-1 (Fig. 10(a)-(S1)) with NH stretching bands in the frequency region above 2400 cm-1 (Fig. 10(b)-(S1)). The 1670 and 1438 cm-1 absorptions are due to protonated NH3 species (NH4+) on the Bronsted acid sites while the 1600 and 1255 cm1

bands are assigned to the respective asymmetric and symmetric stretching vibrations for

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NH3 molecularly adsorbed on the Lewis acid sites [48-50]. Similar features appeared for the other samples, although on samples of 5.46% Fe2O3-1.6% V2O5/WT and 8% Fe2O3-1.6% V2O5/WT, the symmetric NH3 peak was observed at 1235 cm-1, as shown in spectra (S2) and (S3) in Fig. 10(a). The 8% Fe2O3-1.6% V2O5/T exhibited a shoulder near 1220 cm-1 which could be related to coordinated ammonia species. An intense inverse band near 1363 cm-1 is attributed to the S=O bond in sulfate species that exist in the commercial WT and TiO2 studied because of the preparation using sulfate routes [15,51,52]. The presence of the inverse

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peak suggests the formation of ammonium (bi)sulfates thereby losing the vibration mode. The negative band above 3500 cm-1 may indicate the depletion of OH groups [51]. On the other hand, a comparison of DRIFTS spectra collected in N2O TPSR and NH3 TPD with WT-

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supported Fe2O3-V2O5 catalysts is provided in Fig. 11. The removal of adsorbed NH3 was readily observed from a temperature near 100oC in both cases. A positive band in the region

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1300 – 1400 cm-1 could be due to a recovery of sulfate groups upon NH3 desorption

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[15,51,52]. All samples showed a split of the δs-NH3 band upon heating in NH3 TPD measurements. The band split into two broad bands centering around 1250 and 1225 cm-1, and the latter decreased at temperatures ≤ 250oC implying the desorption of NH3 species

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which weakly bonded on Lewis sites. The temperature vs NH3 removal patterns in NH3 TPD and N2O TPSR for the 7.04% Fe2O3/WT analogue were almost identical (Fig. 11(a)). In

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contrast, when 175 ppm N2O(g) was flowed over the three V2O5-containing samples after NH3

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adsorption, differences in the bending vibration region of coordinated NH3 was observed. The intensity of band at 1250 cm-1 decreased from ca. 250oC and the extent increased with the temperature, as seen in IR spectra (b) – (d) in Fig. 11. At temperatures > 350oC, the symmetric NH3 band was significantly more negative compared with spectra for NH3 TPD runs. These observations strongly propose that some thermally-stable NH3(ad) on Lewis sites reacted with the N2O(g). The surface NH3(ad) species could not be desorbed even at 500oC as

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indicated during NH3 TPD runs. That the peaks inversely appeared is because a background for the spectra was collected after NH3 adsorption. In the case of 8% Fe2O3-1.6% V2O5/T (Fig. 11(d)), the intensity of a band at 1438 cm-1 also decreased when flowing N2O, indicating that this sample can catalyze the reaction between gaseous N2O and NH3 species adsorbed on the Bronsted sites. Consequently, results here suggest the presence of stronglyadsorbed NH3 species on some Lewis and possibly Bronsted sites of the Fe2O3-promoted

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catalysts and these species could readily react with N2O(g) at a temperature near 250oC.

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4. Conclusions

Fe2O3-promoted V2O5-WO3/TiO2 catalysts with Fe2O3-to-V2O5 ratios of 3.4 – 6

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studied here show higher potentials in depressing N2O formation in NH3-SCR reaction

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compared with an earlier Fe2O3-promoted V2O5/TiO2-based sample with a Fe2O3/V2O5 ratio = 1.7. A 5.46% Fe2O3/1.6% V2O5 ratio may be optimum to obtain a maximum depression of N2O formation without a significant decrease in deNOx activity. The catalyst exhibits a good

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hydrothermal stability even after a hydrothermal aging at 750oC for 10 h but unfortunately, they give very high N2O production at 350 – 480oC. Raman and XPS measurements propose

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the formation of Fe-O-V and possibly Fe-O-W species which is probably improve high

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temperature N2 selectivity in NH3-SCR reaction with Fe2O3-promoted V2O5-WO3/TiO2 catalysts with high Fe2O3-to-V2O5 ratios. Gas-phase N2O molecules can be easily reduced by surface NH3 moieties strongly adsorbed on the Fe2O3-promoted V2O5-WO3/TiO2 catalysts with high Fe2O3-to-V2O5 ratios, suggesting that a reduction of N2O by thermally-stable surface NH3 may be probable as a main pathway to decrease N2O production in NH3-SCR reaction with the catalysts.

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Author Contributions Section The original and revised manuscripts were written by T.P.T. Nguyen who also performed the instrumental characterizations. The key approach to this study was designed by M.H. Kim and Y.S. Hong who also determined a significance of all data. K.H. Yang performed the experiments regarding activity measurements. All the co-authors have made an approval of the final version of this revised manuscript.

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Declaration of Interest Statement All the authors declare no conflict of interest.

Acknowledgement

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A grant-in-aid for this study was provided not only by the Korea Research Foundation via a Grant 2016R1D1A3B03936098 but also by the Korea Institute of Energy Technology

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Evaluation and Planning (KETEP) via Grant 20193410100050.

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Figure captions

Fig. 1. XRD patterns for WT-supported Fe2O3-promoted V2O5 catalysts with high Fe2O3-toV2O5 ratios: (a) 2.73% Fe2O3/WT; (b) 7.04% Fe2O3/WT; (c) 2.73% Fe2O3-0.46% V2O5/WT; (d) 5.46% Fe2O3-1.6% V2O5/WT; (e) 8% Fe2O3-1.6% V2O5/WT; (f) anatase TiO2 (JCPDS # 84-1286); (g) α-Fe2O3 (JCPDS # 33-0664). Fig. 2. (a) N2 sorption isotherms, and (b) pore size distributions of WT-supported Fe2O3-

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promoted V2O5 catalysts with high Fe2O3-to-V2O5 ratios. In both (a) and (b): (S1) 2.73% Fe2O3/WT; (S2) 7.04% Fe2O3/WT; (S3) 2.73% Fe2O3-0.46% V2O5/WT; (S4) 5.46% Fe2O3-1.6% V2O5/WT; (S5) 8% Fe2O3-1.6% V2O5/WT. In (a): Closed and

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open symbols represent the respective adsorption and desorption processes.

Fig. 3. Raman spectra for WT-supported Fe2O3-promoted V2O5 catalysts with high Fe2O3-to-

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V2O5 ratios: (a) 2.73% Fe2O3/WT; (b) 7.04% Fe2O3/WT; (c) 2.73% Fe2O3-0.46%

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V2O5/WT; (d) 5.46% Fe2O3-1.6% V2O5/WT; (e) 8% Fe2O3-1.6% V2O5/WT. Fig. 4. XPS core level spectra of (a) Fe 2p, (b) V 2p, (c) W 4f, and (d) O 1s in WT-supported Fe2O3-promoted V2O5 catalysts with high Fe2O3-to-V2O5 ratios. (S1) WT; (S2) 2.73%

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Fe2O3/WT; (S3) 7.04% Fe2O3/WT; (S4) 2.73% Fe2O3-0.46% V2O5/WT; (S5) 5.46% Fe2O3-1.6% V2O5/WT; (S6) 8% Fe2O3-1.6% V2O5/WT.

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Fig. 5. Reduction of NO by NH3 with WT-supported Fe2O3-promoted V2O5 catalysts with

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high Fe2O3-to-V2O5 ratios: (a) N2O formation; (b) deNOx activity; (c) NH3 conversion. Reaction conditions: [NO] = 500 ppm, [NH3] = 500 ppm, [O2] = 5%, and GHSV = 76,200 h-1.

Fig. 6. Influence of Fe2O3 loadings on (a) N2O formation and (b) NO conversion in the reduction of NO by NH3 over WT-supported Fe2O3-promoted 1.6% V2O5 catalysts as a function of reaction temperature.

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Fig. 7. Reduction of NO by NH3 over WT-supported Fe2O3-promoted V2O5 catalysts with high Fe2O3-to-V2O5 ratios after a hydrothermal aging at 550 and 750oC for 10 h in 10 H2O in flowing N2: (a) N2O formation; (b) deNOx activity; (c) NH3 conversion. Reaction conditions: [NO] = 500 ppm, [NH3] = 500 ppm, [O2] = 5%, and GHSV = 76,200 h-1. Fig. 8. Activity of WT-supported Fe2O3-promoted V2O5 catalysts with high Fe2O3-to-V2O5 ratios in N2O removal reaction in the presence and absence of NH3: (a) N2O

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concentration; (b) NH3 conversion. Reaction conditions: [N2O] = 300 ppm, [NH3] = 0 or 300 ppm, [O2] = 5%, and GHSV = 76,200 h-1. Closed and open symbols represent the respective N2O decomposition and N2O reduction by NH3.

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Fig. 9. Changes in concentrations of N2O at temperatures selected during N2O-TPSR with WT-supported Fe2O3-promoted V2O5 catalysts with high Fe2O3-to-V2O5 ratios.

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Fig. 10. In situ DRIFTS spectra of NH3 adsorbed on WT-supported Fe2O3-promoted V2O5

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catalysts with high Fe2O3-to-V2O5 ratios: (a) 1900 – 1100; (b) 3800 – 2400 cm-1. (S1) 7.04% Fe2O3/WT; (S2) 5.46% Fe2O3-1.6% V2O5/WT; (S3) 8% Fe2O3-1.6% V2O5/WT; (S4) 8% Fe2O3-1.6% V2O5/T. The adsorption on each catalyst calcined at

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500oC was allowed at 100oC for 30 min using 700 ppm NH3 in flowing N2. Fig. 11. In situ temperature-resolved DRIFTS spectra collected during N2O-TPSR and NH3-

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TPD with WT-supported 1.6% V2O5 catalysts with different Fe2O3 amounts: (a)

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5.46% Fe2O3-1.6% V2O5/WT; (b) 8% Fe2O3-1.6% V2O5/WT. Solid and dash lines are for the respective TPSR and TPD runs with a heating rate of 10oC/min. All the samples were exposed to a flowing mixture of 700 ppm NH3 in N2 at 100oC for 30 min, prior to allowing a N2 purge following a flowing mixture of 175 ppm N2O in N2.

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Fig. 1

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Table

SBET (m2/g)

dmb (nm)

Vtc (cm3/g)

Ref.

Td

88

16.5

0.39

This work

WTe

91

12.7

0.30

[12]

2.73% Fe2O3/WT

73

14.8

0.27

This work

7.04% Fe2O3/WT

72

13.4

0.24

This work

1.6% V2O5/WT

67

14.2

0.26

[12]

2.73% Fe2O3-0.46% V2O5/WT

65

14.9

0.24

This work

2.73% Fe2O3-1.6% V2O5/WT

60

13.9

0.23

[12]

5.46% Fe2O3-1.6% V2O5/WT

65

15.1

0.25

This work

61

14.8

0.22

This work

-

-

-

This work

5.46% Fe2O3-1.6% V2O5/WT-550HAf

-

-

-

This work

8% Fe2O3-1.6% V2O5/WT-550HAf

-

-

-

This work

8% Fe2O3-1.6% V2O5/T-550HAf

-

-

-

This work

5.46% Fe2O3-1.6% V2O5/WT-750HAf

-

-

-

This work

1.6% V2O5/WT-750HAf

-

-

-

This work

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8% Fe2O3-1.6% V2O5/WT 8% Fe2O3-1.6% V2O5/T

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Catalysta

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Table 1 Surface area, pore size and total pore volume of WT-supported Fe2O3-V2O5 catalysts.

Note. SBET: specific BET surface area; dm: mesopore size; Vt: total pore volume. All %s are nominal values.

b

Using the Barrett-Joyner-Halenda (BJH) mesopore model.

c

Calculated using N2 sorption amounts at P/Po ≈ 0.994.

d

A commercial TiO2.

e

A commercial 10% WO3/TiO2.

f

Hydrothermally aged at 550 and 750oC for 10 h in 10% H2O in flowing N2.

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a

41