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TiO2 catalysts
Accepted Manuscript Title: Defect structure and evolution mechanism of O2 − radical in F-doped V2 O5 /TiO2 catalysts Author: Wei Zhao Qin Zhong Yanxiao Pan Rui Zhang PII: DOI: Reference:
S0927-7757(13)00658-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2013.08.047 COLSUA 18618
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
24-1-2013 18-8-2013 21-8-2013
Please cite this article as: W. Zhao, Q. Zhong, Y. Pan, R. Zhang, Defect structure and evolution mechanism of O2 minus radical in F-doped V2 O5 /TiO2 catalysts, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2013), http://dx.doi.org/10.1016/j.colsurfa.2013.08.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Defect structure and evolution mechanism of O2- radical in F-doped
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V2O5/TiO2 catalysts
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Wei Zhao, Qin Zhong*, Yanxiao Pan, and Rui Zhang
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School of Chemical Engineering, Nanjing University of Science and Technology,
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Nanjing 210094, P.R. China
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Fax: +86 025 8431 5517
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E-mail: [email protected]
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Abstract Fluorine (F)-doped titania (TiO2) nanoparticles were synthesized by the sol-gel
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method, which could create more oxygen vacancies on the TiO2 support. The
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experiments of X-ray diffraction (XRD) and high transmission electron microscopy
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(TEM) indicated that F-doping could inhibit anatase-to-rutile transition and active
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component V2O5 had a good dispersion on the support. This paper illustrated the
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preparation process of the F-doped V2O5/TiO2 catalyst and the generation of stabled
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superoxide radical anion (O2-) on the surface of the F-doped V2O5/TiO2 catalyst. The
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characterization and stability of O2- over V2O5/TiO2 was investigated by using
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electron paramagnetic resonance (EPR) spectroscopy. F-doping could produce more
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oxygen vacancies on the surface of the catalyst and increase the Ti3+ concentration.
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UV-vis spectrophotometer was employed to characterize the effect of F-doping on the
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band gap of the samples. Photoluminescence (PL) spectra strongly confirmed the
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enhancement of oxygen vacancies by F-doping. The presence of Ti3+ and V4+ in the
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samples was confirmed by X-ray photoelectron spectroscopy (XPS) spectra. The
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following mechanism was proposed according to the results from EPR spectra and
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reaction stoichiometry studies of radical species. Metal oxide catalysts might be
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viewed as electron donors while the adsorbed oxygen molecules behaved as surface
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electron acceptors. Over the isolated tetrahedral Ti species, a [Ti3+-OV-] radical pair
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was formed by F-doping. One possible explanation for the enhancement of selective
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catalytic reduction (SCR) catalytic activity was that the presence of Ti3+ and V4+
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formed by charge compensation, and the subsequently OV- moiety reacted with O2 to
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form O2-, which could react with NO to yield NO2.
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Keywords: SCR, F-doping, superoxide radical, EPR, evolution mechanism
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Introduction Environmental pollution has become a serious question regarding the protection of
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the environment in the past few decades. Low temperature selective catalytic
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reduction (SCR) of nitrogen oxides (NOX) with ammonia (NH3) is a promising
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technique to remove NOX in flue gases from stationary sources. Catalytic reactions on
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metal oxide surfaces are of significant importance for many technologies including
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water and gas purification. As a result, developing a fundamental understanding of the
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physical and chemical properties of oxide surfaces has been provided by a variety of
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experimental and theoretical techniques [1]. Because of its nontoxicity and long-term
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stability to corrosion, TiO2 has been of considerable interest using as the supports for
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NO-SCR by NH3, and intense research has been devoted to the preparation and
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modification of this semiconductor [2]. The catalytic reactivity of semiconductors is
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well-known to depend not only on their bulk energy band structure, but also to a great
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extent, on the point deficiency [3]. Many efforts have been made to improve its
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catalytic activity, for instance, by doping modification [4]. The oxygen vacancy is the
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common point defect and is affected by the preparation process. The incorporation of
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one or more doped elements in the support may improve the mechanical properties
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and catalytic performance of the catalysts. Recently, nonmental doping of TiO2 has
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received a lot of attention. Khan and Asahi reported that N-doped TiO2 could lower
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the band gap of TiO2 and shift its optical response to the visible region [5-6]. Zhao et
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al. [7] discovered that fluorine-doping on TiO2 greatly enhanced the acidity of Lewis
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acid sites, resulting in faster response but slower recovery than pure TiO2. Huang et al.
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[8] synthesized B-doped, Ni-doped and B-Ni-codoped TiO2 photocatalysts, and found
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that B-Ni-codoped TiO2 had a superior photocatalytic activity for removing NO. The
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effect of doping on the activity depended on many factors, e.g. the method of doping,
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and the type and the concentration of dopant [9]. Oxides are much more complex,
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since most surface properties depend not only on the structure, but also on the local
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stoichiometry of the surface. The ambition is to be able to functionalize oxide surfaces
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act as catalysts, photocatalysts, or act as supports, by controlling the composition and
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structure through defects or modifier atoms and molecules.
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In our previous work [10], we reported that the catalytic activity of V2O5/TiO2
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could be improved by introducing fluorine atoms in TiO2 using a sol-gel method. The
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addition of fluorine could create more O2-, and simultaneously enhanced the catalytic
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activity for NH3-SCR of NOX. The catalyst grain size was observed using the
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transmission electron microscopy (TEM), and high resolution TEM and X-ray
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diffraction (XRD) were used to investigate the effect of F-doping on the phase
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transformation of the samples. To the surface-stabilized radicals, the nature of the
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intrinsic oxygen anion vacancy defect sites was of significant interest as these sites
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were important in controlling the surface chemistry. Furthermore, the defect structure
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of phase had been studied, but there were no available data regarding the defect
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formation in the preparation process of the catalyst. In order to reveal the fundamental
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catalytic properties of F-doped V2O5/TiO2, it is necessary to investigate the defect
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states in it. This study aims to investigate the generating course of surface O vacancies
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(VO), in particular to the evolution mechanism of superoxide radicals (O2-).
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Experimental section
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Catalyst preparation
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F-doped TiO2 support was prepared by a sol-gel method. The tetrabutyl tianate
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(C16H36O4Ti, AR, purchased) and acet ylacetone (C5H8O2, AR, purchased) were mixed
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with magnetic stirring according to the molar ratio of 1:2. Ammonium fluotitanate
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((NH4)2TiF6, AR, purchased) used as the precursor of fluorine source was mixed with
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ethanol into the above solution. After vigorous stirring for 2 h at room temperature,
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the obtained gel was heated at 333 K in a water bath for 4 h and dried at 393 K for 6 h.
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Then the dried material was calcined at 773 K for 3 h in a furnace. The requisite
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amount of NH4VO3 (AR, purchased) was dissolved in distilled water at about 323-333
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K, and the pH of the suspension was adjusted to 1 using nitric acid solution and
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impregnated by contacting the support. The mixture was heated at 333 K in a water
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bath and refluxed for 4 h. Then the mixture was dried at 393 K for 6 h and calcined at
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623 K for 4 h. The catalysts contained 1 wt.% vanadium. The support and the catalyst
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were denoted as TiFX and VTiFX, respectively, where X represents the calculated
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initial molar ratio of F to Ti (RF/Ti×100). In this paper, VTiF1.35 was chosen for the
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study because it had optimal catalytic activity compared with other F-doped samples.
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Catalyst activity tests
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The activity measurements were performed in a fixed-bed flow reactor at 393-573
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K with a gas hourly space velocity (GHSV) of 27549 h-1. The total gas flow rate was 5
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100 mL/min. The simulated gas for these tests contained 0.05% NO, 0.05% NH3, 5%
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O2 in N2. The mixed reactants were pre-heated in a gas mixer before entering the
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reactor. The Ecom-JZKN flue gas analyzer (Germany) was used to monitor the SCR
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activity of the catalysts for NO removal.
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Catalyst characterization
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X-ray diffraction (XRD) patterns of all samples were recorded on a XD-3
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diffractometer with Cu Kα radiation (λ = 0.15418 nm) (Beijing Purkinje General
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Instrument Co., Ltd, China).
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Transmission electron microscopy (TEM) was carried out using a JEM-2100, the sample was deposited on a gold mesh by means of dip coating.
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Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker
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EMX-10/12 X-band (~9.7 GHz) spectrometer at ambient temperature. The sample (ca.
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0.3 g) was placed inside a quartz probe cell where they could be thermally treated and
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subjected to gas adsorption, and then transferred to the EPR spectrometer. The
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adsorption progress was monitored by EPR.
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X-ray photoelectron spectroscopy (XPS) measurements were carried out using a
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PHI Quantera Ⅱ (ULVAC-PHI, Japan) XPS System with monochromatic Al Kα
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excitation. All the bonding energies were calibrated to the C1s peak at 284.8 eV of the
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surface adventitious carbon.
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UV-vis diffuse reflectance (UV-Vis) spectra were measured using a Varian Cary
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500 spectrophotometer at room temperature in the range of 200-800 nm. A BaSO4
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pellet was used as a reference.
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Photoluminescence spectra (PL) of all samples were recorded using a
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visible-ultraviolet spectrophotometer (Labram-HR800) with a 325-nm He-Cd laser as
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the excitation light source at room temperature.
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Results and discussion
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XRD results and analysis
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Fig.1 displayed the XRD patterns of TiF0 (a), TiF1.35 (b), VTiF0 (c) and
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VTiF1.35 (d). As shown in Fig.1, the observed peaks at about 25.2°, 38.0°, 48.1°,
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54.5°, 62.8°, 70.3° and 75.3° were attributive indicator of anatase TiO2 phase, which
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were consistent with the values in the standard card (JCPDS, No. 21-1272). And the
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characteristic peaks at 2θ = 27.4°, 36.0° and 41.2° (JCPDS, No. 21-1276) were
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ascribed to the rutile phase TiO2. The crystalline phase detected in the F-doped
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samples (Fig.1b and Fig.1d) was pure-phase anatase. But the samples undoped by
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fluorine (Fig.1a and Fig.1c) showed trace amount of rutile phase, showing
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anatase-rutile mixed-phase. This suggested that F-doping could suppress the anatase
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to rutile phase transformation during heat treatment. Furthermore, the diffraction
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peaks of TiF1.35 (b) and VTiF1.35 (d) became broader and their relative intensity
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decreased after F-doping, which could be caused by a reduction in crystallite size and
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an increase in lattice strain [11]. No characteristic peaks for V2O5 species were
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observed in Fig.1, implying that V2O5 was either highly dispersed in the TiO2 matrix
152
or formed as tiny vanadia crystals having sizes and/or concentrations beyond the
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detection capacity of the diffractometer [12].
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(Please insert Fig.1 here) TEM results and analysis The grain size evolution of VTiF0 and VTiF1.35 was analyzed by TEM (Fig.2).
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From Fig.2a, the TEM image showed that the TiO2 grain was made of nanoparticles in
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the size range of 20-40 nm. A decrease in nanoparticle size (13-26 nm) and the
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agglomeration of the particles occurred when the sample was doped by fluorine
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(Fig.2b). This was conducive to the dispersion of the active component on its surface.
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In addition, the microstructure of the samples was also investigated by HRTEM. As
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shown in Fig.2c, the VTiF0 sample revealed two phases with separations of 0.31 and
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0.35 nm, corresponding to the spacing of lattice fringes distances of the (110) and
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(101) planes, respectively. This indicated that the undoped TiO2 contained the mixed
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phases of anatase and rutile. Fig.2d showed the spacing of the lattice fringe of the
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VTiF1.35 was 0.35 nm, corresponding to the (101) plane of the anatase particle. The
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formation of rutile phase was suppressed by F-doping due to the formation of Ti-F
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ligand [13]. Some studies had pointed out that TiO2 of anatase phase was the better
169
carrier of SCR catalyst [14]. No individual clusters/particles of V2O5 were seen on the
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surface of VTiF0 (a) and VTiF1.35 (b) using TEM, which indicated that the V2O5
171
species might have a good dispersion on the surface of TiO2 support, combined with
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the fact that small amount of vanadium species present on the support surface [15-18],
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which was in agreement with the XRD determination results.
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(Please insert Fig.2 here)
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EPR results and analysis EPR spectroscopy is a sensitive technique for investigation of paramagnetic
177
species having one or more unpaired electrons in the bulk or on the surface of the
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catalytic materials. The EPR spectra of the support’s intermediates were shown in
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Fig.3. A broad signal was observed at g=2.0047, which was assigned to the
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single-electron-trapped oxygen vacancies in TiF1.35. As shown in the spectrum of the
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support dried at 393 K for 6 h (Fig.3a), the signals (gxx=1.9903, gyy=1.9989, and
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gzz=2.0047) were ascribed to the triple-resonance peaks [19]. The anisotropy of these
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signals was indicative of the low symmetry of the surface sites during the drying
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process. However, only a signal peak was observed during the calcination process,
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indicating that the g tensor was isotropic (Fig.3b). These results demonstrated that
186
F-doped TiO2 (octahedral) existed mainly in the form of anatase phase during
187
calcinations. So the environment of superoxide radicals (O2-) was symmetrical space
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structure. Comparing the line (a) with line (b), it was found that the O2- signal peak of
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the calcining stage became wide and sharp, and the peak area of this signal increased
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significantly, indicating that the number of superoxide radicals was rapidly increased
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after the calcinations. Therefore, it could be speculated that oxygen vacancies and
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superoxide radicals mainly generated in the calcinations stage.
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(Please insert Fig.3 here)
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The EPR spectrum of the intermediates during the V-loading process was shown in
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Fig.4. The signal at g=2.0047 was assigned to superoxide radical. Because O2- was
196
present in the symmetrical anatase phase TiO2 lattice structure and g tensor was 9
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isotropic, and therefore only a split peak appeared. From Fig.4a, the signal peak of the
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catalyst’s intermediate after being dried up for 6 h became weak during the V-loading
199
process. This phenomenon indicated that there was only a small amount of O2- created
200
on the surface of the catalyst’s intermediate. Interestingly, this signal peak became
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high and sharp, and its integrated area increased after high temperature calcinations
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(Fig.4, b and c). Meanwhile, this peak intensity after being calcined for 4 h was
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significantly higher than that after being calcined for 2 h. This indicated that
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superoxide radicals were generated in the calcining stage and the number of O2-
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increased with an increasing calcinations time. Undoped sample was also studied and
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similar results were observed. The difference between undoped and F-doped sample
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was the signal intensity of the O2-.
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XPS results and analysis
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(Please insert Fig.4 here)
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Further analysis on the chemical composition and the valence states of various
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species of the samples was performed using XPS. Fig.5 showed the Ti2p XPS spectra
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of VTiF0 (a) and VTiF1.35 (b). After deconvolution with Gaussian distributions, four
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peaks were identified in the Ti2p spectra of two samples. From Fig.5a, four peaks of
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VTiF0 appearing at around 458.5, 457.5, 464.3, 462.9 eV were assigned to Ti4+2p3/2,
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Ti3+2p3/2, Ti4+2p1/2, and Ti3+2p1/2, respectively. However, these peaks for VTiF1.35
216
were observed at 458.3, 457.4, 464.0, 462.3 eV, respectively (as shown in Fig.5b).
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Compared Fig.5a with Fig.5b, it was found that the binding energy of the Ti2p peaks
218
shifted to the low binding energy, indicating that the doped F atoms converted Ti4+ to
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Ti3+ by charge compensation. From Ti2p spectra, the concentration of Ti4+2p3/2 and
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Ti3+2p3/2 in the samples was compared, the results being shown in Fig.6. Compared
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with VTiF0, more Ti3+ ions were produced on the VTiF1.35 catalyst, while fewer Ti4+.
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The charge transfer would promote the generation of the oxygen vacancy in the
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catalyst surface. These charge carriers migrated to the catalyst surface, where they
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could participate in redox reactions with adsorbed oxygen molecules, leading to the
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creation of the charged oxygen species, such as O2- [20]. The existence of a certain
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amount of Ti3+ surface states in TiO2 could reduce the electron and hole
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recombination rate and enhance catalytic activity [21]. The F-doped sample had the
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higher amount of surface oxygen vacancies than the undoped sample. O2 could be
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adsorbed to oxygen vacancies and formed O2-. Similar results were also observed in
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the O1s spectra. O1s-XPS studies of VTiF1.35 catalyst (see Fig.7 and Table 1)
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indicated that there existed mainly two types of surface oxygen, i.e. lattice oxygen (α)
232
whose binding energy was 529.7 eV, and adsorbed oxygen (β) whose binding energy
233
was 530.3 eV. The binding energies of O1s for the VTiF0 catalyst were 529.9 eV (α)
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and 530.4 eV (β) (see Fig.8 and Table 1). The ratio of adsorbed oxygen (β) of F-doped
235
catalyst was higher than that of undoped catalyst, suggesting that the F-doped catalyst
236
possessed higher oxygen vacancy ratio [22]. Due to calcinations treatment, the
237
electron of non-stoichiometry oxygen would be more likely to be bonded to titanium,
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and result in the formation of lower valence of Ti ions, most likely Ti3+. The migration
239
of oxygen in the catalyst would produce oxygen vacancy when being calcined. The
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oxygen vacancy could not only trap the dop-generated electrons but also increase the
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ratio of adsorbed oxygen (β) on the surface of the catalyst, which was in good
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agreement with the previous report by Zhang and coworkers [23]. (Please insert Fig.5, Fig.6, Fig.7, Fig.8 here)
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(Please insert Table 1 here)
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V2p3/2 XPS spectra of VTiF0 (a) and VTiF1.35 (b) were shown in Fig.9. The
246
peaks centered at 517 and 516.3 eV indicated that vanadium existing on the surface of
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the samples were present in the form of V5+ and V4+ oxidation state [18, 24]. It was
248
reported that the V species were quite reactive, which could give off oxygen during
249
the treatment process resulting in the formation of V4+ [25]. According to the area of
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XPS peak fitting, the ratios of V4+ to V5+ oxidation state in VTiF0 (a) and VTiF1.35 (b)
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was determined to be about 0.48 and 0.51, respectively, indicating that F-doping
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created more V4+ oxidation state. It seemed well established that there were quite
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strong interactions between the vanadia phase and the anatase surface. The studies
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discovered that the reoxidation of vanadium sites was the rate-limiting step in the
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SCR reaction at the temperature below 573 K and V4+ species could speed up this
256
rate-limiting step [26]. Thus, F-doped catalyst showed the better activity than the
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undoped V2O5/TiO2.
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(Please insert Fig.9 here)
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When V2O5/TiO2 was doped by F ion, the possible reactions might be described as
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follows:
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TiO
2
+q
F
→
4
3
2
1 q
q
2q
q
Ti Ti O F
+
q 2
O
2-
(1)
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(s) →
1 (ads) + 2 O2
O
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V
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O
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Ti
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Ti Ti O F
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O
0
+
O
e
→
V
O
V
+
-
(2)
e
(electron trapping in traps)
VO adsorbed oxygen ( , (ads) O2
2
3
(s) +
O
4
2
(ads) → Ti (s) +
4
3
2
1 q
q
2q
q
O
O
2 2
,
(3)
O
VO 2- (s) ) O
(4)
(5)
2
5
4
+ (1 p) 2 V 2 O5 → V 12 pV p
3
4
q p
1 q p
Ti Ti
2-
O
2 9 1 p q 2
F
q
+p
*
O
is the lattice oxygen (O2-),
V
0
O
(Kroger notation)
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(6)
In the above equations,
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O
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2-
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is an ionized oxygen vacancy level poised to rapidly trap an electron which
270
subsequently interacts with a valence band hole either radiatively or nonradiatively.
271
V
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metal oxide. The electron may be available via charge compensation effect caused by
273
foreign elements doping into the lattice (expressed as equation 1) and the reduction of
274
surface lattice oxygen O2- (expressed as equation 2). The impurities incorporated in
275
the TiO2 crystal could reduce energy consumption caused by the oxygen vacancy
276
formation, and induced the formation of oxygen vacancies [27]. The transition of the
277
oxygen-vacancy-trapped electron to the conduction band was a necessary prerequisite
278
for the transition of a valence-band electron to an oxygen vacancy. When the charge
279
centers generated from the lattice defects or impurities presented in TiO2, the electron
280
could be trapped in the oxygen vacancies of TiO2 support. With increasing the
281
concentration of oxygen defects, the surface ions might donate their electrons to the
282
neighboring surface vacancies and then transferred them to O2 adsorbed on the
283
surface of TiO2. The chemisorption of oxygen on oxygen-ion conducting solids might
is the oxygen vacancy where can trap the electron,
-
e
is the electron from
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result in the formation of electron-rich oxygen species (e.g., O2-, O22-, O- ions) on the
285
surface (expressed as equation 4). The trapped charges could easily release from Ti3+
286
ions and then migrate to the surface, and thus to initiate the catalytic oxidation
287
reaction. Ti3+ ions could be oxided to Ti4+ ions by transferring electrons to the surface
288
adsorption of O2 on the TiO2 or a neighboring surface Ti4+ ion (expressed as equation
289
5). Meanwhile, the adsorbed O2 was reduced to O2-, which could be further able to
290
oxidize NO. These chemisorbed active oxygen species could take part in the reaction,
291
and thus resulted in the enhancement of the catalytic activity.
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Then, during V2O5-loading process by impregnation method, the Ti3+ was still
293
observed on the catalyst (shown in Fig.5). Due to TiO2 is basic and ionic exchanger, it
294
could be supposed that the vanadate anions were not simply impregnated but better
295
exchanged at the surface of TiO2 during the loading process of V2O5. The strong
296
interaction between vanadia and titania resulted in the formation of a mixed metal
297
oxide compound rather than a stable surface vanadia overlayer on the TiO2 support
298
[28-30]. Since the ionic radius of V and Ti ions are very similar, it is possible that the
299
part of V ions was incorporated in the TiO2 lattice. V5+ could incorporate into the
300
crystal lattice of TiO2 to replace Ti4+ [31-33]. The charge imbalance caused by Ti4+
301
substituted V5+ could reduce Ti4+ to Ti3+ (expressed as equation 6) [34].
302
UV-vis diffuse reflectance spectra analysis
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Fig.10 showed the UV-vis diffuse reflectance spectra of TiF0 (a), TiF1.35 (b),
304
VTiF0 (c) and VTiF1.35 (d). As observed in Fig.10, the absorption edge of TiF0 (a)
305
and TiF1.35 (b) appeared at 432 nm, which might be ascribed to the O2p-Ti3d
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transition. The vanadium-containing catalysts showed the absorption edge at 590 and
307
575 nm, corresponding to VTiF0 (c) and VTiF1.35 (d), respectively. The curves of
308
F-doped samples exhibited a slight blue shift when compared to the undoped samples.
309
This might be caused by the harmonics and combination absorption edges of the
310
ligand-to-metal charge transfer (O2p-V3d-O2p) of V2O5 species and O2p-Ti3d charge
311
transfer of TiO2. From Fig.10 (c and d), it was found that a significant enhancement of
312
light absorption in the visible light region was observed when compared to Fig.10 (a
313
and b). This might be attributed to the electron transition from the valence band (O2p)
314
to the t2g level of V3d atomic orbital. And the increased visible light absorption (c and
315
d) might be related to the d-d transition of vanadium [35]. The electrons in the valence
316
band could easily move into the conduction band, and therefore generated the
317
electron-hole pairs. This was benefit for improving the catalytic ability of the catalysts.
318
The direct band gap energy of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d)
319
could be estimated from the plot of (αhυ)2 versus photon energy (hυ). The intercept of
320
the tangent to the plot would give a good approximation of the band gap energy for
321
the samples. The absorption coefficient α could be calculated from the measured
322
absorbance [36]. As shown in the inset of Fig.10, the band gap energy estimated from
323
the intercept of the tangent to the plot was 3.16 eV for TiF1.35, which was close to the
324
reported value of anatase TiO2 (3.2 eV) [37]. And the band gap energy of TiF0 showed
325
a blue shift to 3.01 eV, due to the fact that it also contained a rutile phase in addition
326
to anatase phase [38-40]. This indicated that the F-doping could effectively inhibit the
327
anatase-rutile phase transformation of TiO2, which was favorable to enhance the
Ac ce
pt
ed
M
an
us
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306
15
Page 15 of 41
328
catalytic activity of the SCR catalyst [41]. The results agreed with the results of XRD
329
and HRTEM.
330
PL spectra analysis
ip t
331
(Please insert Fig.10 here)
A great deal of attention has been given to the PL spectra with the aim to
333
investigate semiconductors as a useful probe for the efficiency of charge carrier
334
trapping, immigration, and transfer, and to understand the generation of electrons
335
and holes in the surface processes [42]. Superoxide radicals could be formed by
336
oxygen adsorbed to colour center of the catalyst surface [43]. And the catalyst
337
surface colour centers or F centers were detected by PL spectrum. To further reveal
338
the effect of F-doping on the separation of electron-hole pairs, the PL spectra of TiF0
339
(Fig.11a) and TiF1.35 (Fig.11b) were examined in the range of 350-800 nm. All the
340
samples showed a broad peak ranging from 400-800 nm. The main peak appeared at
341
about 523 nm which was assigned to the oxygen vacancy with one trapped electron,
342
F+ center [44]. This emission signal originated from the charge-transfer transition
343
from Ti3+ to oxygen anion in a TiO68- complex. The peak at 630 nm might be a
344
consequence of the polarizability of the lattice ions surrounding the vacancy [45].
345
Additionally, the PL emission intensity of TiF1.35 was weaker than that of TiF0. It
346
was known that the PL emission is resulted from the recombination of electrons and
347
holes, the lower PL intensity might indicate that F-doping slowed the recombination
348
rate of electron-holes in the TiO2 particles [46].
349
Ac ce
pt
ed
M
an
us
cr
332
After vanadium oxide was loaded on the supports, the colour center still existed, 16
Page 16 of 41
but decreased in intensity. As shown in Fig.11 (c and d), the emission signal intensity
351
at around 523 nm of VTiF1.35 (d) was weaker than VTiF0 (c). Due to oxygen
352
adsorbed on the colour centers could reduce the luminescence intensity of the sample
353
[43], this phenomenon illustrated that the differences of oxygen adsorption capacity in
354
the two samples, namely, the F-doped sample could absorb more oxygen than the
355
undoped sample. This inference has been verified in XPS characterization.
356
us
Catalytic activity in SCR-NO
an
357
(Please insert Fig.11 here)
cr
ip t
350
To examine the effect of F-doped TiO2 support on the activities of the catalyst for
359
NO removal by NH3, catalyst performance was evaluated in terms of NO conversion.
360
Fig.12 showed the results of the SCR reaction of NO by NH3 in the presence of
361
oxygen over VTiF0 and VTiF1.35 catalysts. It was clearly found that the VTiF1.35
362
catalyst exhibited higher NO removal activity than VTiF0 in the temperature range
363
between 393 and 573 K. And the NO conversion of VTiF1.35 reached nearly 100% at
364
543 K, however, 85% NO conversion observed for VTiF0. It was worth to note that
365
the NO conversion at low temperature (< 513 K) was highly enhanced by F-doping,
366
compared with undoped sample. These results indicate that there is a direct correlation
367
between the SCR activity and the physicochemical properties improved by F-doping.
Ac ce
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358
368
369 370
(Please insert Fig.12 here)
Conclusions In summary, the enhanced catalytic activity was observed for F-doped V2O5/TiO2 17
Page 17 of 41
catalyst due to the different structural and electronic stability, and the surface
372
superoxide radical (formed on the oxygen vacancies) could be produced in the doped
373
TiO2 system. The results indicated that the phase transition of anatase to rutile of TiO2
374
could be suppressed by F-doping, due to the formation of Ti-F ligand. The process of
375
F-doping was accompanied by the formation of oxygen vacancies. The most
376
important result was that the oxygen vacancies could be produced in the process of
377
drying and calcinations of the catalyst, and calcination conditions highly affected the
378
number of the oxygen vacancies. The electronic properties of TiO2 could be altered by
379
F-doping or defect structure and the electron bounded by oxygen vacancies could
380
reduce Ti4+ to Ti3+. The electrons in the energy level could be captured by adsorbed
381
oxygen to generate reactive oxygen species (O2-), thereby improving the oxidation
382
activity of the catalyst. In view of chemisorption of oxygen shown in XPS spectra, the
383
most profound feature was the presence of numerous oxygen defects on the surface of
384
the catalyst. These results indicated that if an impurity ion in a crystal structure had a
385
net charge difference from that of the replaced host ion, an ionic defect must be
386
introduced for charge compensation. And these results were of great significance for
387
understanding the surface physical chemistry and catalytic properties of the oxide.
cr
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388
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389 390 391 392
18
Page 18 of 41
393
Acknowledgments This work was financially supported by the National Natural Science Foundation
395
of China (51078185) and (U1162119), the research fund of Key Laboratory for
396
Advance Technology in Environmental Protection of Jiangsu Province (AE201001),
397
Research Fund for the Doctoral Program of Higher Education of China
398
(20113219110009), Industry-Academia Cooperation Innovation Fund Projects of
399
Jiangsu Province (BY2012025) and Scientific Research Project of Environmental
400
Protection Department of Jiangsu Province (201112). Special thanks are given to Prof.
401
Y.X. Sui (Nanjing University) for EPR measurement.
an
us
cr
ip t
394
M
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ip t
524
531
an
532
M
533 534
538 539 540 541
pt
537
Ac ce
536
ed
535
542 543 544 545
25
Page 25 of 41
546
Figure captions:
547
Fig.1. XRD patterns of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d).
548
Fig.2. TEM images of VTiF0 (a) and VTiF1.35 (b), and HRTEM micrographs of
551
ip t
550
VTiF0 (c) and VTiF1.35 (d). Fig.3. EPR spectra of the intermediates of TiF1.35 support in the air. (a) dried at 393 K for 6 h, (b) calcined at 773 K for 3 h.
cr
549
Fig.4. EPR spectra of the intermediates of V-loading to the F-doped TiO2 support
553
(VTiF1.35) in the air. (a) dried at 393 K for 6 h, (b) calcined at 773 K for 2 h, (c)
554
calcined at 773 K for 4 h.
an
us
552
Fig.5. Ti 2p-XPS spectra of VTiF0 (a) and VTi1.35 (b).
556
Fig.6. Ti4+ and Ti3+ concentrations obtained in VTiF0 and VTiF1.35.
557
Fig.7. O 1s-XPS spectra of VTiF1.35 catalyst.
558
Fig.8. O 1s-XPS spectra of VTiF0 catalyst.
559
Fig.9. V2p3/2-XPS spectra of VTiF0 (a) and VTiF1.35 (b).
560
Fig.10. UV-Vis adsorption spectra of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35
ed
pt
Ac ce
561
M
555
(d). The inset is the band gap of the samples.
562
Fig.11. PL spectra of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d).
563
Fig.12. NO conversions of VTiF0 and VTiF1.35. The total flow rate was 100 mL/min.
564
0.05% NO, 0.05% NH3, 5% O2 with N2 as the balance gas, GHSV=27549 h-1.
565 566 567 26
Page 26 of 41
Table 1 Molar ratios of lattice and adsorption oxygen of the catalysts (%) Oxygen species
VTiF0
VTiF1.35
Lattice oxygen (α)
63.21
41.41
Adsorption oxygen (β)
36.79
58.59
ip t
568
569
cr
570
us
571 572
an
573
M
574 575
579 580 581 582
pt
578
Ac ce
577
ed
576
583 584 585
27
Page 27 of 41
Graphical abstract
ip t
728
729
F-doping may convert some Ti4+ to Ti3+ by charge compensation. The Ti3+
731
surface states can trap the electrons and then transfer them to O2 adsorbed
732
on the surface of TiO2. Activity oxygen species can oxidize NO to NO2,
733
which is beneficial to the NH3-SCR reaction.
an
us
cr
730
Ac ce
pt
ed
M
734
40
Page 28 of 41
734
Highlights
735
The effects of F-doping on the creation of the oxygen vacancy are investigated.
736
Oxygen vacancies and superoxide radicals mainly generate in the calcinations
739
ip t
738
stage. O2- favours NO oxidation to NO2, which is beneficial to improve the NOX reduction.
cr
737
Ac ce
pt
ed
M
an
us
740
41
Page 29 of 41
ip t us
cr 587
an
586
Fig.1. XRD patterns of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d).
M
588
ed
589 590
593 594 595 596
Ac
592
ce pt
591
597 598 599 600 28
Page 30 of 41
ip t cr us an
601
VTiF0 (c) and VTiF1.35 (d).
604
608 609 610
Ac
607
ce pt
605 606
M
603
Fig.2. TEM images of VTiF0 (a) and VTiF1.35 (b), and HRTEM micrographs of
ed
602
611 612
29
Page 31 of 41
ip t us
cr 615
Fig.3. EPR spectra of the intermediates of the TiF1.35 support in the air. (a) dried at 393 K for 6 h, (b) calcined at 773 K for 3 h.
M
614
an
613
ed
616 617
620 621 622 623
Ac
619
ce pt
618
30
Page 32 of 41
ip t us
cr 625
an
624
Fig.4. EPR spectra of the intermediates of V-loading to the F-doped TiO2 support (VTiF1.35) in the air. (a) dried at 393 K for 6 h, (b) calcined at 773 K for 2 h, (c)
627
calcined at 773 K for 4 h.
ed
628
632 633 634
Ac
631
ce pt
629 630
M
626
635
31
Page 33 of 41
ip t us
cr 637 638 639
Ac
ce pt
ed
M
an
636
Fig.5. Ti 2p-XPS spectra of VTiF0 (a) and VTiF1.35 (b).
32
Page 34 of 41
ip t us
cr 641
an
640
Fig.6. Ti4+ and Ti3+ concentrations obtained in VTiF0 and VTiF1.35.
M
642
ed
643 644
647 648
Ac
646
ce pt
645
33
Page 35 of 41
ip t cr us an
649
Fig.7. O 1s-XPS spectra of VTiF1.35 catalyst.
650
M
651
ed
652 653
656 657 658 659
Ac
655
ce pt
654
34
Page 36 of 41
ip t cr us an
660
Fig.8. O 1s-XPS spectra of VTiF0 catalyst.
661
M
662
ed
663 664
667 668 669
Ac
666
ce pt
665
35
Page 37 of 41
ip t us
cr 671
an
670
Fig.9. V2p3/2-XPS spectra of VTiF0 (a) and VTiF1.35 (b).
M
672
ed
673 674
677 678 679 680
Ac
676
ce pt
675
681 682 683 684 36
Page 38 of 41
ip t us
cr 687
Fig.10. UV-Vis adsorption spectra of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d). The inset is the band gap of the samples.
M
686
an
685
ce pt Ac
689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709
ed
688
37
Page 39 of 41
ip t us
cr 711
an
710
Fig.11. PL spectra of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d).
M
712
ed
713 714
717 718 719 720
Ac
716
ce pt
715
721 722 723 724 38
Page 40 of 41
ip t cr us ed
M
0.05% NO, 0.05% NH3, 5% O2 with N2 as the balance gas, GHSV=27549 h-1.
ce pt
727
Fig.12. NO conversions of VTiF0 and VTiF1.35. The total flow rate was 100 mL/min.
Ac
726
an
725
39
Page 41 of 41
S0927-7757(13)00658-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2013.08.047 COLSUA 18618
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
24-1-2013 18-8-2013 21-8-2013
Please cite this article as: W. Zhao, Q. Zhong, Y. Pan, R. Zhang, Defect structure and evolution mechanism of O2 minus radical in F-doped V2 O5 /TiO2 catalysts, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2013), http://dx.doi.org/10.1016/j.colsurfa.2013.08.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Defect structure and evolution mechanism of O2- radical in F-doped
2
V2O5/TiO2 catalysts
3
Wei Zhao, Qin Zhong*, Yanxiao Pan, and Rui Zhang
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4 5
School of Chemical Engineering, Nanjing University of Science and Technology,
7
Nanjing 210094, P.R. China
8
Fax: +86 025 8431 5517
9
E-mail: [email protected]
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[email protected]
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Abstract Fluorine (F)-doped titania (TiO2) nanoparticles were synthesized by the sol-gel
25
method, which could create more oxygen vacancies on the TiO2 support. The
26
experiments of X-ray diffraction (XRD) and high transmission electron microscopy
27
(TEM) indicated that F-doping could inhibit anatase-to-rutile transition and active
28
component V2O5 had a good dispersion on the support. This paper illustrated the
29
preparation process of the F-doped V2O5/TiO2 catalyst and the generation of stabled
30
superoxide radical anion (O2-) on the surface of the F-doped V2O5/TiO2 catalyst. The
31
characterization and stability of O2- over V2O5/TiO2 was investigated by using
32
electron paramagnetic resonance (EPR) spectroscopy. F-doping could produce more
33
oxygen vacancies on the surface of the catalyst and increase the Ti3+ concentration.
34
UV-vis spectrophotometer was employed to characterize the effect of F-doping on the
35
band gap of the samples. Photoluminescence (PL) spectra strongly confirmed the
36
enhancement of oxygen vacancies by F-doping. The presence of Ti3+ and V4+ in the
37
samples was confirmed by X-ray photoelectron spectroscopy (XPS) spectra. The
38
following mechanism was proposed according to the results from EPR spectra and
39
reaction stoichiometry studies of radical species. Metal oxide catalysts might be
40
viewed as electron donors while the adsorbed oxygen molecules behaved as surface
41
electron acceptors. Over the isolated tetrahedral Ti species, a [Ti3+-OV-] radical pair
42
was formed by F-doping. One possible explanation for the enhancement of selective
43
catalytic reduction (SCR) catalytic activity was that the presence of Ti3+ and V4+
44
formed by charge compensation, and the subsequently OV- moiety reacted with O2 to
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45
form O2-, which could react with NO to yield NO2.
46
Keywords: SCR, F-doping, superoxide radical, EPR, evolution mechanism
47
Introduction Environmental pollution has become a serious question regarding the protection of
49
the environment in the past few decades. Low temperature selective catalytic
50
reduction (SCR) of nitrogen oxides (NOX) with ammonia (NH3) is a promising
51
technique to remove NOX in flue gases from stationary sources. Catalytic reactions on
52
metal oxide surfaces are of significant importance for many technologies including
53
water and gas purification. As a result, developing a fundamental understanding of the
54
physical and chemical properties of oxide surfaces has been provided by a variety of
55
experimental and theoretical techniques [1]. Because of its nontoxicity and long-term
56
stability to corrosion, TiO2 has been of considerable interest using as the supports for
57
NO-SCR by NH3, and intense research has been devoted to the preparation and
58
modification of this semiconductor [2]. The catalytic reactivity of semiconductors is
59
well-known to depend not only on their bulk energy band structure, but also to a great
60
extent, on the point deficiency [3]. Many efforts have been made to improve its
61
catalytic activity, for instance, by doping modification [4]. The oxygen vacancy is the
62
common point defect and is affected by the preparation process. The incorporation of
63
one or more doped elements in the support may improve the mechanical properties
64
and catalytic performance of the catalysts. Recently, nonmental doping of TiO2 has
65
received a lot of attention. Khan and Asahi reported that N-doped TiO2 could lower
66
the band gap of TiO2 and shift its optical response to the visible region [5-6]. Zhao et
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al. [7] discovered that fluorine-doping on TiO2 greatly enhanced the acidity of Lewis
68
acid sites, resulting in faster response but slower recovery than pure TiO2. Huang et al.
69
[8] synthesized B-doped, Ni-doped and B-Ni-codoped TiO2 photocatalysts, and found
70
that B-Ni-codoped TiO2 had a superior photocatalytic activity for removing NO. The
71
effect of doping on the activity depended on many factors, e.g. the method of doping,
72
and the type and the concentration of dopant [9]. Oxides are much more complex,
73
since most surface properties depend not only on the structure, but also on the local
74
stoichiometry of the surface. The ambition is to be able to functionalize oxide surfaces
75
act as catalysts, photocatalysts, or act as supports, by controlling the composition and
76
structure through defects or modifier atoms and molecules.
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In our previous work [10], we reported that the catalytic activity of V2O5/TiO2
78
could be improved by introducing fluorine atoms in TiO2 using a sol-gel method. The
79
addition of fluorine could create more O2-, and simultaneously enhanced the catalytic
80
activity for NH3-SCR of NOX. The catalyst grain size was observed using the
81
transmission electron microscopy (TEM), and high resolution TEM and X-ray
82
diffraction (XRD) were used to investigate the effect of F-doping on the phase
83
transformation of the samples. To the surface-stabilized radicals, the nature of the
84
intrinsic oxygen anion vacancy defect sites was of significant interest as these sites
85
were important in controlling the surface chemistry. Furthermore, the defect structure
86
of phase had been studied, but there were no available data regarding the defect
87
formation in the preparation process of the catalyst. In order to reveal the fundamental
88
catalytic properties of F-doped V2O5/TiO2, it is necessary to investigate the defect
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states in it. This study aims to investigate the generating course of surface O vacancies
90
(VO), in particular to the evolution mechanism of superoxide radicals (O2-).
91
Experimental section
92
Catalyst preparation
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F-doped TiO2 support was prepared by a sol-gel method. The tetrabutyl tianate
94
(C16H36O4Ti, AR, purchased) and acet ylacetone (C5H8O2, AR, purchased) were mixed
95
with magnetic stirring according to the molar ratio of 1:2. Ammonium fluotitanate
96
((NH4)2TiF6, AR, purchased) used as the precursor of fluorine source was mixed with
97
ethanol into the above solution. After vigorous stirring for 2 h at room temperature,
98
the obtained gel was heated at 333 K in a water bath for 4 h and dried at 393 K for 6 h.
99
Then the dried material was calcined at 773 K for 3 h in a furnace. The requisite
100
amount of NH4VO3 (AR, purchased) was dissolved in distilled water at about 323-333
101
K, and the pH of the suspension was adjusted to 1 using nitric acid solution and
102
impregnated by contacting the support. The mixture was heated at 333 K in a water
103
bath and refluxed for 4 h. Then the mixture was dried at 393 K for 6 h and calcined at
104
623 K for 4 h. The catalysts contained 1 wt.% vanadium. The support and the catalyst
105
were denoted as TiFX and VTiFX, respectively, where X represents the calculated
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initial molar ratio of F to Ti (RF/Ti×100). In this paper, VTiF1.35 was chosen for the
107
study because it had optimal catalytic activity compared with other F-doped samples.
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Catalyst activity tests
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The activity measurements were performed in a fixed-bed flow reactor at 393-573
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K with a gas hourly space velocity (GHSV) of 27549 h-1. The total gas flow rate was 5
Page 5 of 41
100 mL/min. The simulated gas for these tests contained 0.05% NO, 0.05% NH3, 5%
112
O2 in N2. The mixed reactants were pre-heated in a gas mixer before entering the
113
reactor. The Ecom-JZKN flue gas analyzer (Germany) was used to monitor the SCR
114
activity of the catalysts for NO removal.
115
Catalyst characterization
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X-ray diffraction (XRD) patterns of all samples were recorded on a XD-3
117
diffractometer with Cu Kα radiation (λ = 0.15418 nm) (Beijing Purkinje General
118
Instrument Co., Ltd, China).
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Transmission electron microscopy (TEM) was carried out using a JEM-2100, the sample was deposited on a gold mesh by means of dip coating.
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Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker
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EMX-10/12 X-band (~9.7 GHz) spectrometer at ambient temperature. The sample (ca.
123
0.3 g) was placed inside a quartz probe cell where they could be thermally treated and
124
subjected to gas adsorption, and then transferred to the EPR spectrometer. The
125
adsorption progress was monitored by EPR.
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X-ray photoelectron spectroscopy (XPS) measurements were carried out using a
127
PHI Quantera Ⅱ (ULVAC-PHI, Japan) XPS System with monochromatic Al Kα
128
excitation. All the bonding energies were calibrated to the C1s peak at 284.8 eV of the
129
surface adventitious carbon.
130
UV-vis diffuse reflectance (UV-Vis) spectra were measured using a Varian Cary
131
500 spectrophotometer at room temperature in the range of 200-800 nm. A BaSO4
132
pellet was used as a reference.
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Photoluminescence spectra (PL) of all samples were recorded using a
134
visible-ultraviolet spectrophotometer (Labram-HR800) with a 325-nm He-Cd laser as
135
the excitation light source at room temperature.
136
Results and discussion
137
XRD results and analysis
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Fig.1 displayed the XRD patterns of TiF0 (a), TiF1.35 (b), VTiF0 (c) and
139
VTiF1.35 (d). As shown in Fig.1, the observed peaks at about 25.2°, 38.0°, 48.1°,
140
54.5°, 62.8°, 70.3° and 75.3° were attributive indicator of anatase TiO2 phase, which
141
were consistent with the values in the standard card (JCPDS, No. 21-1272). And the
142
characteristic peaks at 2θ = 27.4°, 36.0° and 41.2° (JCPDS, No. 21-1276) were
143
ascribed to the rutile phase TiO2. The crystalline phase detected in the F-doped
144
samples (Fig.1b and Fig.1d) was pure-phase anatase. But the samples undoped by
145
fluorine (Fig.1a and Fig.1c) showed trace amount of rutile phase, showing
146
anatase-rutile mixed-phase. This suggested that F-doping could suppress the anatase
147
to rutile phase transformation during heat treatment. Furthermore, the diffraction
148
peaks of TiF1.35 (b) and VTiF1.35 (d) became broader and their relative intensity
149
decreased after F-doping, which could be caused by a reduction in crystallite size and
150
an increase in lattice strain [11]. No characteristic peaks for V2O5 species were
151
observed in Fig.1, implying that V2O5 was either highly dispersed in the TiO2 matrix
152
or formed as tiny vanadia crystals having sizes and/or concentrations beyond the
153
detection capacity of the diffractometer [12].
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154
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(Please insert Fig.1 here) TEM results and analysis The grain size evolution of VTiF0 and VTiF1.35 was analyzed by TEM (Fig.2).
157
From Fig.2a, the TEM image showed that the TiO2 grain was made of nanoparticles in
158
the size range of 20-40 nm. A decrease in nanoparticle size (13-26 nm) and the
159
agglomeration of the particles occurred when the sample was doped by fluorine
160
(Fig.2b). This was conducive to the dispersion of the active component on its surface.
161
In addition, the microstructure of the samples was also investigated by HRTEM. As
162
shown in Fig.2c, the VTiF0 sample revealed two phases with separations of 0.31 and
163
0.35 nm, corresponding to the spacing of lattice fringes distances of the (110) and
164
(101) planes, respectively. This indicated that the undoped TiO2 contained the mixed
165
phases of anatase and rutile. Fig.2d showed the spacing of the lattice fringe of the
166
VTiF1.35 was 0.35 nm, corresponding to the (101) plane of the anatase particle. The
167
formation of rutile phase was suppressed by F-doping due to the formation of Ti-F
168
ligand [13]. Some studies had pointed out that TiO2 of anatase phase was the better
169
carrier of SCR catalyst [14]. No individual clusters/particles of V2O5 were seen on the
170
surface of VTiF0 (a) and VTiF1.35 (b) using TEM, which indicated that the V2O5
171
species might have a good dispersion on the surface of TiO2 support, combined with
172
the fact that small amount of vanadium species present on the support surface [15-18],
173
which was in agreement with the XRD determination results.
174
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(Please insert Fig.2 here)
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175
EPR results and analysis EPR spectroscopy is a sensitive technique for investigation of paramagnetic
177
species having one or more unpaired electrons in the bulk or on the surface of the
178
catalytic materials. The EPR spectra of the support’s intermediates were shown in
179
Fig.3. A broad signal was observed at g=2.0047, which was assigned to the
180
single-electron-trapped oxygen vacancies in TiF1.35. As shown in the spectrum of the
181
support dried at 393 K for 6 h (Fig.3a), the signals (gxx=1.9903, gyy=1.9989, and
182
gzz=2.0047) were ascribed to the triple-resonance peaks [19]. The anisotropy of these
183
signals was indicative of the low symmetry of the surface sites during the drying
184
process. However, only a signal peak was observed during the calcination process,
185
indicating that the g tensor was isotropic (Fig.3b). These results demonstrated that
186
F-doped TiO2 (octahedral) existed mainly in the form of anatase phase during
187
calcinations. So the environment of superoxide radicals (O2-) was symmetrical space
188
structure. Comparing the line (a) with line (b), it was found that the O2- signal peak of
189
the calcining stage became wide and sharp, and the peak area of this signal increased
190
significantly, indicating that the number of superoxide radicals was rapidly increased
191
after the calcinations. Therefore, it could be speculated that oxygen vacancies and
192
superoxide radicals mainly generated in the calcinations stage.
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193
(Please insert Fig.3 here)
194
The EPR spectrum of the intermediates during the V-loading process was shown in
195
Fig.4. The signal at g=2.0047 was assigned to superoxide radical. Because O2- was
196
present in the symmetrical anatase phase TiO2 lattice structure and g tensor was 9
Page 9 of 41
isotropic, and therefore only a split peak appeared. From Fig.4a, the signal peak of the
198
catalyst’s intermediate after being dried up for 6 h became weak during the V-loading
199
process. This phenomenon indicated that there was only a small amount of O2- created
200
on the surface of the catalyst’s intermediate. Interestingly, this signal peak became
201
high and sharp, and its integrated area increased after high temperature calcinations
202
(Fig.4, b and c). Meanwhile, this peak intensity after being calcined for 4 h was
203
significantly higher than that after being calcined for 2 h. This indicated that
204
superoxide radicals were generated in the calcining stage and the number of O2-
205
increased with an increasing calcinations time. Undoped sample was also studied and
206
similar results were observed. The difference between undoped and F-doped sample
207
was the signal intensity of the O2-.
208
XPS results and analysis
ed
(Please insert Fig.4 here)
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209
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Further analysis on the chemical composition and the valence states of various
211
species of the samples was performed using XPS. Fig.5 showed the Ti2p XPS spectra
212
of VTiF0 (a) and VTiF1.35 (b). After deconvolution with Gaussian distributions, four
213
peaks were identified in the Ti2p spectra of two samples. From Fig.5a, four peaks of
214
VTiF0 appearing at around 458.5, 457.5, 464.3, 462.9 eV were assigned to Ti4+2p3/2,
215
Ti3+2p3/2, Ti4+2p1/2, and Ti3+2p1/2, respectively. However, these peaks for VTiF1.35
216
were observed at 458.3, 457.4, 464.0, 462.3 eV, respectively (as shown in Fig.5b).
217
Compared Fig.5a with Fig.5b, it was found that the binding energy of the Ti2p peaks
218
shifted to the low binding energy, indicating that the doped F atoms converted Ti4+ to
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Ti3+ by charge compensation. From Ti2p spectra, the concentration of Ti4+2p3/2 and
220
Ti3+2p3/2 in the samples was compared, the results being shown in Fig.6. Compared
221
with VTiF0, more Ti3+ ions were produced on the VTiF1.35 catalyst, while fewer Ti4+.
222
The charge transfer would promote the generation of the oxygen vacancy in the
223
catalyst surface. These charge carriers migrated to the catalyst surface, where they
224
could participate in redox reactions with adsorbed oxygen molecules, leading to the
225
creation of the charged oxygen species, such as O2- [20]. The existence of a certain
226
amount of Ti3+ surface states in TiO2 could reduce the electron and hole
227
recombination rate and enhance catalytic activity [21]. The F-doped sample had the
228
higher amount of surface oxygen vacancies than the undoped sample. O2 could be
229
adsorbed to oxygen vacancies and formed O2-. Similar results were also observed in
230
the O1s spectra. O1s-XPS studies of VTiF1.35 catalyst (see Fig.7 and Table 1)
231
indicated that there existed mainly two types of surface oxygen, i.e. lattice oxygen (α)
232
whose binding energy was 529.7 eV, and adsorbed oxygen (β) whose binding energy
233
was 530.3 eV. The binding energies of O1s for the VTiF0 catalyst were 529.9 eV (α)
234
and 530.4 eV (β) (see Fig.8 and Table 1). The ratio of adsorbed oxygen (β) of F-doped
235
catalyst was higher than that of undoped catalyst, suggesting that the F-doped catalyst
236
possessed higher oxygen vacancy ratio [22]. Due to calcinations treatment, the
237
electron of non-stoichiometry oxygen would be more likely to be bonded to titanium,
238
and result in the formation of lower valence of Ti ions, most likely Ti3+. The migration
239
of oxygen in the catalyst would produce oxygen vacancy when being calcined. The
240
oxygen vacancy could not only trap the dop-generated electrons but also increase the
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241
ratio of adsorbed oxygen (β) on the surface of the catalyst, which was in good
242
agreement with the previous report by Zhang and coworkers [23]. (Please insert Fig.5, Fig.6, Fig.7, Fig.8 here)
244
(Please insert Table 1 here)
245
V2p3/2 XPS spectra of VTiF0 (a) and VTiF1.35 (b) were shown in Fig.9. The
246
peaks centered at 517 and 516.3 eV indicated that vanadium existing on the surface of
247
the samples were present in the form of V5+ and V4+ oxidation state [18, 24]. It was
248
reported that the V species were quite reactive, which could give off oxygen during
249
the treatment process resulting in the formation of V4+ [25]. According to the area of
250
XPS peak fitting, the ratios of V4+ to V5+ oxidation state in VTiF0 (a) and VTiF1.35 (b)
251
was determined to be about 0.48 and 0.51, respectively, indicating that F-doping
252
created more V4+ oxidation state. It seemed well established that there were quite
253
strong interactions between the vanadia phase and the anatase surface. The studies
254
discovered that the reoxidation of vanadium sites was the rate-limiting step in the
255
SCR reaction at the temperature below 573 K and V4+ species could speed up this
256
rate-limiting step [26]. Thus, F-doped catalyst showed the better activity than the
257
undoped V2O5/TiO2.
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258
(Please insert Fig.9 here)
259
When V2O5/TiO2 was doped by F ion, the possible reactions might be described as
260
follows:
261
TiO
2
+q
F
→
4
3
2
1 q
q
2q
q
Ti Ti O F
+
q 2
O
2-
(1)
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Page 12 of 41
(s) →
1 (ads) + 2 O2
O
263
V
264
O
265
Ti
266
Ti Ti O F
267
O
0
+
O
e
→
V
O
V
+
-
(2)
e
(electron trapping in traps)
VO adsorbed oxygen ( , (ads) O2
2
3
(s) +
O
4
2
(ads) → Ti (s) +
4
3
2
1 q
q
2q
q
O
O
2 2
,
(3)
O
VO 2- (s) ) O
(4)
(5)
2
5
4
+ (1 p) 2 V 2 O5 → V 12 pV p
3
4
q p
1 q p
Ti Ti
2-
O
2 9 1 p q 2
F
q
+p
*
O
is the lattice oxygen (O2-),
V
0
O
(Kroger notation)
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(6)
In the above equations,
268
O
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2-
262
is an ionized oxygen vacancy level poised to rapidly trap an electron which
270
subsequently interacts with a valence band hole either radiatively or nonradiatively.
271
V
272
metal oxide. The electron may be available via charge compensation effect caused by
273
foreign elements doping into the lattice (expressed as equation 1) and the reduction of
274
surface lattice oxygen O2- (expressed as equation 2). The impurities incorporated in
275
the TiO2 crystal could reduce energy consumption caused by the oxygen vacancy
276
formation, and induced the formation of oxygen vacancies [27]. The transition of the
277
oxygen-vacancy-trapped electron to the conduction band was a necessary prerequisite
278
for the transition of a valence-band electron to an oxygen vacancy. When the charge
279
centers generated from the lattice defects or impurities presented in TiO2, the electron
280
could be trapped in the oxygen vacancies of TiO2 support. With increasing the
281
concentration of oxygen defects, the surface ions might donate their electrons to the
282
neighboring surface vacancies and then transferred them to O2 adsorbed on the
283
surface of TiO2. The chemisorption of oxygen on oxygen-ion conducting solids might
is the oxygen vacancy where can trap the electron,
-
e
is the electron from
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result in the formation of electron-rich oxygen species (e.g., O2-, O22-, O- ions) on the
285
surface (expressed as equation 4). The trapped charges could easily release from Ti3+
286
ions and then migrate to the surface, and thus to initiate the catalytic oxidation
287
reaction. Ti3+ ions could be oxided to Ti4+ ions by transferring electrons to the surface
288
adsorption of O2 on the TiO2 or a neighboring surface Ti4+ ion (expressed as equation
289
5). Meanwhile, the adsorbed O2 was reduced to O2-, which could be further able to
290
oxidize NO. These chemisorbed active oxygen species could take part in the reaction,
291
and thus resulted in the enhancement of the catalytic activity.
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Then, during V2O5-loading process by impregnation method, the Ti3+ was still
293
observed on the catalyst (shown in Fig.5). Due to TiO2 is basic and ionic exchanger, it
294
could be supposed that the vanadate anions were not simply impregnated but better
295
exchanged at the surface of TiO2 during the loading process of V2O5. The strong
296
interaction between vanadia and titania resulted in the formation of a mixed metal
297
oxide compound rather than a stable surface vanadia overlayer on the TiO2 support
298
[28-30]. Since the ionic radius of V and Ti ions are very similar, it is possible that the
299
part of V ions was incorporated in the TiO2 lattice. V5+ could incorporate into the
300
crystal lattice of TiO2 to replace Ti4+ [31-33]. The charge imbalance caused by Ti4+
301
substituted V5+ could reduce Ti4+ to Ti3+ (expressed as equation 6) [34].
302
UV-vis diffuse reflectance spectra analysis
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Fig.10 showed the UV-vis diffuse reflectance spectra of TiF0 (a), TiF1.35 (b),
304
VTiF0 (c) and VTiF1.35 (d). As observed in Fig.10, the absorption edge of TiF0 (a)
305
and TiF1.35 (b) appeared at 432 nm, which might be ascribed to the O2p-Ti3d
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Page 14 of 41
transition. The vanadium-containing catalysts showed the absorption edge at 590 and
307
575 nm, corresponding to VTiF0 (c) and VTiF1.35 (d), respectively. The curves of
308
F-doped samples exhibited a slight blue shift when compared to the undoped samples.
309
This might be caused by the harmonics and combination absorption edges of the
310
ligand-to-metal charge transfer (O2p-V3d-O2p) of V2O5 species and O2p-Ti3d charge
311
transfer of TiO2. From Fig.10 (c and d), it was found that a significant enhancement of
312
light absorption in the visible light region was observed when compared to Fig.10 (a
313
and b). This might be attributed to the electron transition from the valence band (O2p)
314
to the t2g level of V3d atomic orbital. And the increased visible light absorption (c and
315
d) might be related to the d-d transition of vanadium [35]. The electrons in the valence
316
band could easily move into the conduction band, and therefore generated the
317
electron-hole pairs. This was benefit for improving the catalytic ability of the catalysts.
318
The direct band gap energy of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d)
319
could be estimated from the plot of (αhυ)2 versus photon energy (hυ). The intercept of
320
the tangent to the plot would give a good approximation of the band gap energy for
321
the samples. The absorption coefficient α could be calculated from the measured
322
absorbance [36]. As shown in the inset of Fig.10, the band gap energy estimated from
323
the intercept of the tangent to the plot was 3.16 eV for TiF1.35, which was close to the
324
reported value of anatase TiO2 (3.2 eV) [37]. And the band gap energy of TiF0 showed
325
a blue shift to 3.01 eV, due to the fact that it also contained a rutile phase in addition
326
to anatase phase [38-40]. This indicated that the F-doping could effectively inhibit the
327
anatase-rutile phase transformation of TiO2, which was favorable to enhance the
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328
catalytic activity of the SCR catalyst [41]. The results agreed with the results of XRD
329
and HRTEM.
330
PL spectra analysis
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(Please insert Fig.10 here)
A great deal of attention has been given to the PL spectra with the aim to
333
investigate semiconductors as a useful probe for the efficiency of charge carrier
334
trapping, immigration, and transfer, and to understand the generation of electrons
335
and holes in the surface processes [42]. Superoxide radicals could be formed by
336
oxygen adsorbed to colour center of the catalyst surface [43]. And the catalyst
337
surface colour centers or F centers were detected by PL spectrum. To further reveal
338
the effect of F-doping on the separation of electron-hole pairs, the PL spectra of TiF0
339
(Fig.11a) and TiF1.35 (Fig.11b) were examined in the range of 350-800 nm. All the
340
samples showed a broad peak ranging from 400-800 nm. The main peak appeared at
341
about 523 nm which was assigned to the oxygen vacancy with one trapped electron,
342
F+ center [44]. This emission signal originated from the charge-transfer transition
343
from Ti3+ to oxygen anion in a TiO68- complex. The peak at 630 nm might be a
344
consequence of the polarizability of the lattice ions surrounding the vacancy [45].
345
Additionally, the PL emission intensity of TiF1.35 was weaker than that of TiF0. It
346
was known that the PL emission is resulted from the recombination of electrons and
347
holes, the lower PL intensity might indicate that F-doping slowed the recombination
348
rate of electron-holes in the TiO2 particles [46].
349
Ac ce
pt
ed
M
an
us
cr
332
After vanadium oxide was loaded on the supports, the colour center still existed, 16
Page 16 of 41
but decreased in intensity. As shown in Fig.11 (c and d), the emission signal intensity
351
at around 523 nm of VTiF1.35 (d) was weaker than VTiF0 (c). Due to oxygen
352
adsorbed on the colour centers could reduce the luminescence intensity of the sample
353
[43], this phenomenon illustrated that the differences of oxygen adsorption capacity in
354
the two samples, namely, the F-doped sample could absorb more oxygen than the
355
undoped sample. This inference has been verified in XPS characterization.
356
us
Catalytic activity in SCR-NO
an
357
(Please insert Fig.11 here)
cr
ip t
350
To examine the effect of F-doped TiO2 support on the activities of the catalyst for
359
NO removal by NH3, catalyst performance was evaluated in terms of NO conversion.
360
Fig.12 showed the results of the SCR reaction of NO by NH3 in the presence of
361
oxygen over VTiF0 and VTiF1.35 catalysts. It was clearly found that the VTiF1.35
362
catalyst exhibited higher NO removal activity than VTiF0 in the temperature range
363
between 393 and 573 K. And the NO conversion of VTiF1.35 reached nearly 100% at
364
543 K, however, 85% NO conversion observed for VTiF0. It was worth to note that
365
the NO conversion at low temperature (< 513 K) was highly enhanced by F-doping,
366
compared with undoped sample. These results indicate that there is a direct correlation
367
between the SCR activity and the physicochemical properties improved by F-doping.
Ac ce
pt
ed
M
358
368
369 370
(Please insert Fig.12 here)
Conclusions In summary, the enhanced catalytic activity was observed for F-doped V2O5/TiO2 17
Page 17 of 41
catalyst due to the different structural and electronic stability, and the surface
372
superoxide radical (formed on the oxygen vacancies) could be produced in the doped
373
TiO2 system. The results indicated that the phase transition of anatase to rutile of TiO2
374
could be suppressed by F-doping, due to the formation of Ti-F ligand. The process of
375
F-doping was accompanied by the formation of oxygen vacancies. The most
376
important result was that the oxygen vacancies could be produced in the process of
377
drying and calcinations of the catalyst, and calcination conditions highly affected the
378
number of the oxygen vacancies. The electronic properties of TiO2 could be altered by
379
F-doping or defect structure and the electron bounded by oxygen vacancies could
380
reduce Ti4+ to Ti3+. The electrons in the energy level could be captured by adsorbed
381
oxygen to generate reactive oxygen species (O2-), thereby improving the oxidation
382
activity of the catalyst. In view of chemisorption of oxygen shown in XPS spectra, the
383
most profound feature was the presence of numerous oxygen defects on the surface of
384
the catalyst. These results indicated that if an impurity ion in a crystal structure had a
385
net charge difference from that of the replaced host ion, an ionic defect must be
386
introduced for charge compensation. And these results were of great significance for
387
understanding the surface physical chemistry and catalytic properties of the oxide.
cr
us
an
M
ed
pt
Ac ce
388
ip t
371
389 390 391 392
18
Page 18 of 41
393
Acknowledgments This work was financially supported by the National Natural Science Foundation
395
of China (51078185) and (U1162119), the research fund of Key Laboratory for
396
Advance Technology in Environmental Protection of Jiangsu Province (AE201001),
397
Research Fund for the Doctoral Program of Higher Education of China
398
(20113219110009), Industry-Academia Cooperation Innovation Fund Projects of
399
Jiangsu Province (BY2012025) and Scientific Research Project of Environmental
400
Protection Department of Jiangsu Province (201112). Special thanks are given to Prof.
401
Y.X. Sui (Nanjing University) for EPR measurement.
an
us
cr
ip t
394
M
402
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530
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us
cr
ip t
524
531
an
532
M
533 534
538 539 540 541
pt
537
Ac ce
536
ed
535
542 543 544 545
25
Page 25 of 41
546
Figure captions:
547
Fig.1. XRD patterns of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d).
548
Fig.2. TEM images of VTiF0 (a) and VTiF1.35 (b), and HRTEM micrographs of
551
ip t
550
VTiF0 (c) and VTiF1.35 (d). Fig.3. EPR spectra of the intermediates of TiF1.35 support in the air. (a) dried at 393 K for 6 h, (b) calcined at 773 K for 3 h.
cr
549
Fig.4. EPR spectra of the intermediates of V-loading to the F-doped TiO2 support
553
(VTiF1.35) in the air. (a) dried at 393 K for 6 h, (b) calcined at 773 K for 2 h, (c)
554
calcined at 773 K for 4 h.
an
us
552
Fig.5. Ti 2p-XPS spectra of VTiF0 (a) and VTi1.35 (b).
556
Fig.6. Ti4+ and Ti3+ concentrations obtained in VTiF0 and VTiF1.35.
557
Fig.7. O 1s-XPS spectra of VTiF1.35 catalyst.
558
Fig.8. O 1s-XPS spectra of VTiF0 catalyst.
559
Fig.9. V2p3/2-XPS spectra of VTiF0 (a) and VTiF1.35 (b).
560
Fig.10. UV-Vis adsorption spectra of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35
ed
pt
Ac ce
561
M
555
(d). The inset is the band gap of the samples.
562
Fig.11. PL spectra of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d).
563
Fig.12. NO conversions of VTiF0 and VTiF1.35. The total flow rate was 100 mL/min.
564
0.05% NO, 0.05% NH3, 5% O2 with N2 as the balance gas, GHSV=27549 h-1.
565 566 567 26
Page 26 of 41
Table 1 Molar ratios of lattice and adsorption oxygen of the catalysts (%) Oxygen species
VTiF0
VTiF1.35
Lattice oxygen (α)
63.21
41.41
Adsorption oxygen (β)
36.79
58.59
ip t
568
569
cr
570
us
571 572
an
573
M
574 575
579 580 581 582
pt
578
Ac ce
577
ed
576
583 584 585
27
Page 27 of 41
Graphical abstract
ip t
728
729
F-doping may convert some Ti4+ to Ti3+ by charge compensation. The Ti3+
731
surface states can trap the electrons and then transfer them to O2 adsorbed
732
on the surface of TiO2. Activity oxygen species can oxidize NO to NO2,
733
which is beneficial to the NH3-SCR reaction.
an
us
cr
730
Ac ce
pt
ed
M
734
40
Page 28 of 41
734
Highlights
735
The effects of F-doping on the creation of the oxygen vacancy are investigated.
736
Oxygen vacancies and superoxide radicals mainly generate in the calcinations
739
ip t
738
stage. O2- favours NO oxidation to NO2, which is beneficial to improve the NOX reduction.
cr
737
Ac ce
pt
ed
M
an
us
740
41
Page 29 of 41
ip t us
cr 587
an
586
Fig.1. XRD patterns of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d).
M
588
ed
589 590
593 594 595 596
Ac
592
ce pt
591
597 598 599 600 28
Page 30 of 41
ip t cr us an
601
VTiF0 (c) and VTiF1.35 (d).
604
608 609 610
Ac
607
ce pt
605 606
M
603
Fig.2. TEM images of VTiF0 (a) and VTiF1.35 (b), and HRTEM micrographs of
ed
602
611 612
29
Page 31 of 41
ip t us
cr 615
Fig.3. EPR spectra of the intermediates of the TiF1.35 support in the air. (a) dried at 393 K for 6 h, (b) calcined at 773 K for 3 h.
M
614
an
613
ed
616 617
620 621 622 623
Ac
619
ce pt
618
30
Page 32 of 41
ip t us
cr 625
an
624
Fig.4. EPR spectra of the intermediates of V-loading to the F-doped TiO2 support (VTiF1.35) in the air. (a) dried at 393 K for 6 h, (b) calcined at 773 K for 2 h, (c)
627
calcined at 773 K for 4 h.
ed
628
632 633 634
Ac
631
ce pt
629 630
M
626
635
31
Page 33 of 41
ip t us
cr 637 638 639
Ac
ce pt
ed
M
an
636
Fig.5. Ti 2p-XPS spectra of VTiF0 (a) and VTiF1.35 (b).
32
Page 34 of 41
ip t us
cr 641
an
640
Fig.6. Ti4+ and Ti3+ concentrations obtained in VTiF0 and VTiF1.35.
M
642
ed
643 644
647 648
Ac
646
ce pt
645
33
Page 35 of 41
ip t cr us an
649
Fig.7. O 1s-XPS spectra of VTiF1.35 catalyst.
650
M
651
ed
652 653
656 657 658 659
Ac
655
ce pt
654
34
Page 36 of 41
ip t cr us an
660
Fig.8. O 1s-XPS spectra of VTiF0 catalyst.
661
M
662
ed
663 664
667 668 669
Ac
666
ce pt
665
35
Page 37 of 41
ip t us
cr 671
an
670
Fig.9. V2p3/2-XPS spectra of VTiF0 (a) and VTiF1.35 (b).
M
672
ed
673 674
677 678 679 680
Ac
676
ce pt
675
681 682 683 684 36
Page 38 of 41
ip t us
cr 687
Fig.10. UV-Vis adsorption spectra of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d). The inset is the band gap of the samples.
M
686
an
685
ce pt Ac
689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709
ed
688
37
Page 39 of 41
ip t us
cr 711
an
710
Fig.11. PL spectra of TiF0 (a), TiF1.35 (b), VTiF0 (c) and VTiF1.35 (d).
M
712
ed
713 714
717 718 719 720
Ac
716
ce pt
715
721 722 723 724 38
Page 40 of 41
ip t cr us ed
M
0.05% NO, 0.05% NH3, 5% O2 with N2 as the balance gas, GHSV=27549 h-1.
ce pt
727
Fig.12. NO conversions of VTiF0 and VTiF1.35. The total flow rate was 100 mL/min.
Ac
726
an
725
39
Page 41 of 41