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Catalysis and Kinetics
Low-temperature pyrolysis-catalysis coupled system for tetrachlorobenzene efficient removal: Condition optimization and decomposition mechanism Pingping Liu, Xiaosheng Yuan, Huarui Ren, Yanke Yu, Ning Xu, Jinglian Zhao, and Chi He Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00095 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018
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Low-temperature pyrolysis-catalysis coupled system 1
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2 5 6 7 8
3 1. Rotor flowmeter; 2. Gas inlet; 3. Ice bath; 4. Fly ash and TeCB 5. Quartz sand; 6. Catalyst bed; 7. Quartz wool; 8. Glass beads
Graphical abstract
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1
Low-temperature pyrolysis-catalysis coupled system for tetrachlorobenzene
2
efficient removal: Condition optimization and decomposition mechanism
3
Pingping Liu, Xiaosheng Yuan, Huarui Ren, Yanke Yu, Ning Xu, Jinglian Zhao, Chi He*
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Department of Environmental Science and Engineering, State Key Laboratory of Multiphase
5
Flow in Power Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University,
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Xi'an 710049, Shaanxi, P.R. China
7 8
*
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Tel./Fax: 86 29 82663857; E-mail:
[email protected] (C. He)
To whom correspondence should be addressed:
1
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ABSTRACT
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The decomposition of tetrachlorobenzene (TeCB) emitted from simulated fly ash was studied
12
in a low-temperature pyrolysis-catalysis coupled system. The influences of catalyst support,
13
active component type, active phase/assistant ratio, vanadium loading, and catalyst calcination
14
temperature on TeCB conversion were comprehensively investigated. The optimal catalyst
15
composition and preparation condition were determined through single factor experiments.
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Moreover, the effects of reaction temperature, space velocity, and pollutant initial concentration
17
on TeCB decomposition efficiency were explored by orthogonal experiments. The possible
18
mechanism for PeCB decomposition over prepared V2O5-WO3/TiO2 catalysts was proposed
19
based on reaction byproduct composition and distribution. It is found that the TeCB
20
decomposition efficiency is positively correlated with reaction temperature, while negatively
21
correlated with gas hourly space velocity (GHSV) and TeCB initial concentration. GHSV has the
22
most significant effect on TeCB decomposition, followed by reaction temperature and TeCB
23
concentration. Under optimum condition of space velocity of 600 h-1, reaction temperature of
24
350 °C, and TeCB initial concentration of 0.5 vol.%, the TeCB conversion can reach up to 94.1%.
25
The key substeps of TeCB decomposition are aromatic pollutant adsoprtion and nucleophilic
26
substitution and intermediates electrophilic substitution. The TeCB molecules were first adsorbed
27
on catalyst surface, and then decomposed into low chlorinated aromatics and aromatic and
28
aliphatic hydrocarbons as phenolates, benzoquinone, aldehydes, and carboxylic acids.
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Keywords: Tetrachlorobenzene; Low-temperature pyrolysis; Catalytic destruction; Condition
30
optimization; Decomposition mechanism
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INTRODUCTION
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The rapid economic development and improved living standards have led to great increase of
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municipal solid waste (MSW).1 As one of waste-to-energy technologies, incineration is
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considered as a strategic option for MSW reduction and disposal because it is possible to obtain
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70% mass and 90% volume reduction in waste treatment.2,3 However, the emission of fly ash
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from incineration is one of major and serious threats towards the environment and society.4
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Municipal solid waste incineration (MSWI) produces fly ash, which accounts for approximately
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3-5 wt.% of the original waste amount. The MSWI fly ash contains a large amount of toxic heavy
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metals, such as mercury, plumbum, cadmium, and chromium, and highly toxic dioxins such as
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polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). These
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pollutants are extremely harmful to soil and fresh water ecosystems, especially when they are
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bioaccumulated through earthy and aquatic food webs.5 In China, such PCDD/Fs emissions have
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been restricted to 0.1 abd 0.5 ng I-TEQ Nm-3 for municipal incinerators.
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Numerous methods for decomposing PCDD/Fs have been developed, such as activated carbon
45
adsorption, UV photolysis and photocatalytic oxidation, plasma discharge technology, and
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selective catalytic reduction (SCR) technology.6-13 Only the activated carbon adsorption and SCR
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technology has entered industrial applications while the other two technologies are still in
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laboratory research stage or semi-industrial experiment stage. Activated carbon adsorption with
49
bag dedusting technology only entraps dioxins in the gas phase and has no substantial reduction
50
effect on dioxin; SCR technology will affect the stability of operation due to catalyst poisoning;
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UV photolysis is less effective due to energy efficiency issues. Plasma technology is difficult to
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bring to the market because of its high equipment costs and operating costs. In addition, low 3
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temperature pyrolysis technology was widely used to degrade dioxins in waste incineration fly
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ash due to simple operation, low energy consumption, and high removal rate, however, there is
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still possible to cause secondary pollution.14 Therefore, it is necessary to add a process to reduce
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secondary pollution after the pyrolysis process. Catalytic oxidation shows several advantages in
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complete and direct decomposition of chlorinated volatile organic compounds.15 It can be
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anticipated that the low-temperature pyrolysis-catalysis combined technology can significantly
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reduce the activation energy of dioxins and decomposition of dioxin at lower temperature, which
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is a promising method with great potential to integrate energy saving and reduce the formation of
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secondary pollutant.
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During the past decades, a number of catalyst systems for the total oxidation of chlorinated
63
compounds were reported. Most of them were focused on three types of catalysts based on noble
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metals (e.g., Pd, Pt, Rh, Ni, Au, and Ir),16,17 transition metals (e.g., V, W, Mn, Fe, and Cu)18-20 and
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zeolites.21-23 Noble metal-based catalysts are catalytically more active at low temperature in
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comparison to transition metal oxides and zeolites, however, they are expensive and susceptible to
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fast deactivation by chlorine poisoning.21 The activity of zeolites was related to their acid
68
properties. However, the formation of polychlorinated compounds and deposition of coke on
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these acidic catalysts are to be resolved. In general, transition metal oxides are less active than the
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noble metals, but they can resist chlorine deactivation to a larger extent and the cost of transition
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metal oxides is much lower.24 Amongst, the V2O5-WO3/TiO2 catalysts are successfully applied to
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destroy chlorinated volatile organic compounds effectively.25 For instance, Xu et al. revealed that
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the V2O5-WO3/TiO2 catalysts were effective in decomposition of pentachlorobenzene.26 Yang and
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co-workers proposed that V2O5-WO3/TiO2 catalyst had superior activity for PCDD/F 4
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decomposition.27
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Due to high toxicity of PCDD/PCDFs, some model compounds such as tetrachlorobenzene,
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pentachlorobenzene, 1,2-dichlorobenzene, and 2-chlorophenol were employed to predict the
78
dioxin destruction behaviors in laboratory study.26,
79
feasibility of low-temperature pyrolysis-catalysis coupled technology for TeCB decomposition,
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and to figure out the effects of catalyst type and composition and operating conditions such as
81
reaction temperature, space velocity, and pollutant initial concentration on TeCB conversion.
82
Moreover, the optimum reaction conditions and mechanism for TeCB decomposition were also
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put forward.
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2. MATERIALS AND METHODS
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2.1. Chemicals
28-30
This paper aims to investigate the
86
Tetrachlorobenzene (TeCB) was purchased from Shanghai Shanpu Chemical Co., Ltd.
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Ammonium tungstate and ammonium metavanadate were purchased from Tianjin Fu Chen
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Chemical Reagent Plant. Activated alumina and coconut shell activated carbon were purchased
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from Xiangyuan water purification materials plant, Henan Province. Titanium dioxide was
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purchased from Xingtai chemical plant, Shanghai Jinshan. The reagents are of analytical grade
91
and other reagents are commercially available.
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2.2. Catalyst preparation
93
All catalysts used in the experiments were prepared by an impregnation method. Typically, a
94
certain amount of oxalic acid was dissolved in deionized water at 60 °C. Ammonium tungstate
95
and ammonium metavanadate were then added in batches and kept stirring for 1 h. After that, the
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TiO2 support (dried at 105 °C for 2 h in advance) was added and stirred at 60 °C for 4 h. The 5
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sample was then transferred to an oven and dried at 105 °C for 12 h, followed by calcined in
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muffle furnace at 450 °C for 5 h to obtained the V2O5-WO3/TiO2 (VWTi) catalyst (contents of
99
V2O5 and WO3 are 5.0 and 10.0 wt. %, respectively). In addition, the VWTi catalysts with
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different calcination temperatures, vanadium loadings, and V/W ratios (w/w) were also
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synthesized. For comparison, CuO-WO3/TiO2 (CuWTi) and MnOx-WO3/TiO2 (MnWTi) catalysts
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(contents of CuO/MnOx and WO3 are 5.0 and 10.0 wt. %, respectively) were further prepared by
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the same method using copper nitrate and manganese acetate as metal precursors, respectively.
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2.3. Catalyst characterizations
105
N2 sorption isotherms were measured at 77 K on a SSA-4000 apparatus (Builder, China). Prior
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to the measurements, the samples were evacuated for 4 h under vacuum at 473 K. The total pore
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volume was estimated from the amount of nitrogen adsorbed at a relative pressure (P/P0) of ca.
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0.99. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method,
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and the pore size distribution was derived from the desorption branch of the N2 isotherm using the
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Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) measurements were performed
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using a powder diffractrometer (XRD-7000, Shimadzu Ltd., Japan) with Cu-Kα radiation. The
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tube voltage was 40 kV, and the current was 40 mA. XRD diffraction patterns were obtained in 2θ
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range of 10-80° (scanning rate of 4°/min). Fourier transform infrared (FT-IR) spectra were
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recorded at room temperature on a Bruker Tensor 37 FT-IR spectrometer with 64 scans at an
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effective resolution of 4 cm-1. The morphology of the adsorbents was observed by field emission
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electron scanning microscopy (FE-SEM, JSM-6700F, JOEL, Japan).
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2.4. Decomposition activity
118
Schematic diagram of the low-temperature pyrolysis-catalysis combined system is displayed in 6
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Scheme 1. The low-temperature pyrolysis-catalysis combined reactor is a three-section heating
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tube furnace actually, and the tubes were charged with simulated TeCB-containing fly ash, quartz
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sand, catalyst and underfill from top to bottom. The upper section is heated to control the
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pyrolysis temperature, the middle section is heated to control the catalytic reaction temperature,
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and the lower section is heated to ensure that the gas is kept in the vaporized state and absorbed
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by the absorption liquid. During the experiment, aeration was performed for 10 min before
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warming, and the residual gas in the reactor was discharged. First of all, the catalyst (40-60 mesh)
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in the middle part of the reactor was heated to bring the catalyst into the activation temperature.
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The specific temperature was set according to the experimental requirements. Then heating the
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simulated fly ash in the upper part the reactor, and the pyrolysis temperature was set according to
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the experiment. In this experiment, K type thermocouple is connected with the display instrument
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and electronic regulator. We can set the temperature through the electronic regulator, and measure
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the temperature by the K-type galvanometer. After the required temperature was reached, the air
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pump was turned on and the flow rate was adjusted according to the experimental requirements
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and the flow rate was controlled by rotor flow meter and stable-flow valve. The reaction
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conditions of single factor experiments are total flow rate of 200 mL min-1, pyrolysis temperature
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of 300 °C, TeCB concentration of 1.0 vol.%, reaction time of 60 min, and catalyst dosage of 10
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mL. The chlorine-containing fly ash was extracted with Soxhlet for 24 hours after the reaction.
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The concentration of TeCB in the absorption liquid was determined by gas chromatograph
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(Beifen 3420A, China) with external standard method. A certain amount of TeCB was dissolved
139
in a toluene solution to prepare a standard solution of TeCB from 10 to 400 mg·L-1 to draw the
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standard curve by using the gas chromatograph (Table S1 and Fig. S1). Agilent 6890-5973 7
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GC-MS system equipped with a six-port valve (0.25 mL), capillary column (DB-dioxin, 60 m ×
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0.25 mm (i.d.) × 0.15 µm (film)), and
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detection of the tail gas composition with an injection volume of 10 µL and the fragment size
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ranges from 33 to 450 amu. Surface groups of the catalyst were characterized on a Bruker
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tensor37 FT-IR with wavenumber ranges from 4000 to 400 cm-1.
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63
Ni electroncapture detector (µ-ECD), was employed for
The TeCB conversion (η) is calculated as: m0 − m1 ×100% m1
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η=
148
where m0 and m1 are the initial and residual masses of TeCB (mg), respectively.
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3. RESULTS AND DISCUSSION
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3.1. Optimization of catalyst
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3.1.1. Effect of catalyst support
(1)
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In this work, three typical supports, that is, activated carbon, TiO2, and γ-Al2O3, were selected
153
to study the effect of catalyst supports on TeCB conversion, as shown in Fig. 1. Only 54% of
154
TeCB can be converted with single pyrolysis process. The decomposition efficiency of TeCB
155
respectively increases by 8.1% and 7.6% when activated carbon or γ-Al2O3 is added, while the
156
decomposition efficiency of TeCB increases by 14.9% with the assistance of TiO2. Which
157
indicates that TiO2 is an efficient support in catalytic decomposition of TeCB. Bertinchamps et
158
al.31 believed that titania induces a spreading of the entire set of active phase as a well-dispersed
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monolayer over its surface. Xiang et al.32 reported that TiO2 has a certain catalytic effect on the
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decomposition of TeCB, while SiO2 prevents the homogeneous spreading of the active phase in
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the form of a well-dispersed monolayer but promotes the formation of poorly dispersed
8
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crystallites of active phase. This observation was also proved by Bond et al.33 who compared VOx
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monolayer supported on TiO2, Al2O3, and SiO2. They pointed out the preferential spreading of
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VOx on TiO2 rather on Al2O3. Moreover, they demonstrated the formation of V2O5 crystallites on
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SiO2, even below the theoretical monolayer coverage.
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3.1.2. Effect of active component
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V2O5-WO3 (VW), MnOx-WO3 (MnW), CuO-WO3 (CuWi) were used as the active phase and
168
introduced to TiO2 support by the impregnation method. The effect of different active components
169
on TeCB conversion is shown in Fig. 2. The activity difference of these catalysts is not obvious in
170
low temperature region (200-250 °C), while the effect of active component becomes significant
171
when further increase the reaction temperature. For instance, 98.6 % and 89% of TeCB can be
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respectively converted over VWTi and CuWTi at 400 °C, much higher than that over MnWTi
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catalyst (79.2%).
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3.1.3. Effect of catalyst assistant
175
The effect of V2O5/WO3 ratio (1 : 1, 1 : 2, and 1 : 3, w/w) on decomposition efficiency of TeCB
176
was investigated, as shown in Fig. 3. It is shown that all V2O5/WO3-contained catalysts have
177
much higher catalytic activity than that of TiO2. TeCB decomposition efficiency increases firstly
178
and then decreases to some extend with the increasing of V2O5/WO3 ratio. Catalysts with
179
V2O5/WO3 ratio of 1 : 1 and 1 : 3 possess similar catalytic activity, while the activity of catalyst
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with V2O5/WO3 ratio of 1 : 2 is obviously higher than the other two groups. Albonetti et al.34
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proposed that the TiO2/WO3 catalyst has a higher number of Brønsted acid sites compared to that
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of TiO2 sample, which play an important role in the oxidation of o-dichlorobenzene over
183
TiO2-based catalysts. Wang et al.35 found that WO3 has a positive effect on the dispersion of 9
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vanadium sites on the surface of V2O5/TiO2, and strengthen the interaction between vanadium
185
oxide and TiO2 support.
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3.1.4. Effect of vanadium loading
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As shown in Fig. 4, the activity of the catalyst increases firstly with the increasing of vanadium
188
loading, and the catalyst with vanadium loading of 5.0 wt.% possesses the highest activity with
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more than 98% of TeCB converted at 400 °C. Compared with pure TiO2, all catalysts present
190
higher TeCB oxidation capacity in the presence of vanadium oxide. However, the catalytic
191
activity decreases to some extend when the loading of vanadium is 7.0 wt.% (about 94.9% TeCB
192
converted at 400 °C). Yu et al.36 found that excessive vanadium loading could block the pore
193
space on TiO2 surface and suppressed the adsorption capacity of catalyst.
194
3.1.5.Effect of calcination temperature
195
The effect of calcination temperature (400, 450, and 500 °C) of catalyst on decomposition
196
efficiency of TeCB is shown in Fig. 5. It can be found that the catalyst activity is enhanced
197
continuously with the increasing of calcination temperature, and the best sintering temperature is
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500 °C. The activity of catalyst calcined at 400 °C is almost the same as that of TiO2 support as
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the V2O5 active phase is derived from the decomposition of NH4VO3 precursor in air at high
200
temperature. Zhang et al.37 revealed that the final decomposition product of NH4VO3 under Ar
201
conditions is V2O5 in temperature range of 350-450 °C, while it is very difficult to decompose
202
NH4VO3 under air conditions at 400 °C. Chen et al.38 found that when the calcination temperature
203
is lower than 500 °C, the active vanadium component cannot completely converted into V2O5,
204
and some other vanadium oxides such as VO, VO2, and V2O3 are formed, which would decrease
205
the whole catalytic activity. 10
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3.2. Catalyst characterizations
207
The catalyst with optimal formula was analyzed by XRD and SEM, as shown in Figs. 6 and 7.
208
It can be observed that all studied catalysts exhibit characteristic diffraction peaks of anatase TiO2,
209
and rutile TiO2 cannot observed in TiO2 support. The characteristic peaks of V2O5 and WO3 can
210
be observed, indicating the presence of V2O5 and WO3 crystals on the surface of catalyst. Rutile
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TiO2 is observed over the fresh and used V2O5-WO3/TiO2 catalysts, indicating that the anatase
212
TiO2 is partly transformed to rutile phase after loading vanadium oxide even the presence of WO3
213
plays a role of inhibiting the transfer of anatase TiO2 to rutile phase, which has a negative effect
214
on catalytic activity.39 The specific surface area and porosity of prepared catalysts are shown in
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Table S2. The specific surface area of pure TiO2 is larger than that of V2O5-WO3/TiO2 catalyst,
216
while both the average pore size and pore volume of V2O5-WO3/TiO2 increase to some degree
217
compared with TiO2 support. The FE-SEM images of various catalysts are shown in Fig. 7. It can
218
be seen that all particles are evenly distributed over TiO2 support. Small aggregates can be
219
observed after loading of vanadium and tungsten, which results in the formation of some
220
interparticle large holes. No significant change can be found in morphology of the fresh and used
221
catalysts.
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3.3. Optimization of reaction condition
223
According to the actual combustion condition of municipal solid waste incineration reactors, a
224
series of tests were carried out at different temperatures (250-400 °C), TeCB concentrations (0.5
225
vol.%-1.5 vol.%), and space velocities (300-1500 h-1), as shown in Fig. 8. Experimental settings
226
are shown in Table 1. The effect of reaction temperature on TeCB decomposition efficiency is
227
shown in Fig. 8A. TeCB conversion increases from 78.2% to 98.9% when reaction temperature 11
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increases from 200 to 400 °C. Gao et al.40 believed that the dechlorination of hexachlorobenzene
229
on solid supports is based on surface solid-gas reaction. According to the classic crystal chemistry
230
theory, metallic ions and O2- on the surfaces of crystal are unsaturated, which can form free
231
electrons in vacancies for chlorine abstraction. The free electrons increase with increasing of
232
temperature, resulted in much higher dechlorination efficiency. In catalytic decomposition of
233
PCDD/Fs, the polyaromatic compounds are strongly adsorbed on the Lewis acid sites of
234
V2O5-WO3/TiO2 catalysts by interacting with the p-orbitals of aromatic systems.41,42 VOx is
235
considered as the active phase of vanadium-tungsten supported catalysts. The active lattice
236
oxygen over V5+Ox firstly oxidize pollutants on catalyst surface with V5+Ox reduced to V4+Ox, and
237
then the reduced vanadium is reoxidized in the present of oxygen. Higher temperature enhances
238
catalyst activity, and the TeCB adsorption and decomposition cycle proceeds more quickly.
239
The effect of gas hourly space velocity (GHSV) on TeCB decomposition is shown in Fig. 8B.
240
In general, the increase of GHSV reduces TeCB decomposition efficiency due to the decrease of
241
contact time between pollutant molecules and catalyst. As show in Fig. 8B, the TeCB conversion
242
decreases from 95.0% to 80.7% with increasing of space velocity from 300 to 1800 h-1, similar
243
with the results reported by Wu et al.43 and Xu et al.26. Considering the pyrolysis process, the
244
combined process can achieve better synergistic effect with a relative low space velocity.
245
The influence of TeCB concentration on TeCB conversion is illustrated in Fig. 8C. TeCB
246
concentration has a significant negative influence on the decomposition of TeCB, which reduces
247
from 94% to 80% when the TeCB concentration increases from 0.5 vol.% to 1.5 vol.% due to the
248
limited pyrolysis energy and catalyst active site, in agreement with the results proposed by Ren et
249
al.44 As is known, the TeCB content in actual waste fly ash is much lower than 0.5 vol.%, which 12
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250
means that the reported technology could achieve higher decomposition rate toward
251
polychlorinated benzene substances in fly ash under actual treatment process.
252
According to the results of single factor experiment, the effects of reaction temperature, gas
253
hourly space velocity, and TeCB concentration were analyzed by orthogonal test, as shown in
254
Tables 2 and 3. It can be revealed that the TeCB decomposition efficiency decreases continuously
255
with the increasing of gas hourly space velocity from 300 to 1800 h-1 and TeCB inlet
256
concentration from 0.5 to 1.5 vol.%, while the TeCB decomposition efficiency increase with the
257
increasing of reaction temperature from 250 to 400 °C. The larger the range is, the more obvious
258
the effect is. The optimum conditions for TeCB decomposition are the reaction temperature of
259
350 °C, the space velocity of 600 h-1, and the concentration TeCB of 0.5 vol.%, under which
260
around 94.1% of TeCB can be removed.
261
3.4. Product distribution and proposed TeCB decomposition mechanism
262
The main products in tail gas absorption solution are illustrated in Fig. 9. Many dechlorination
263
products such as 1,3-dichlorobenzene (12.1 min), trichlorobenzene homologue (13.5/13.9 min),
264
and 1,2,3,5-tetrachlorobenzene (17.9 min) can be detected in the tail gas. Besides, other reaction
265
intermediates as cyclosiloxane hydrocarbon compounds (15.6/16.5 min), phenol (18.8 min),
266
benzoquinone (20.3 min), and phthalic acid (22.1 min) are also found (Table 4). Short-chain
267
alkanes and olefins can combine and react with SiO2 in fly ash due to the occurrence of
268
dechlorination/hydrogenation and partial cracking of TeCB in pyrolysis and catalysis process,
269
resulting in the formation of cyclosiloxane hydrocarbons. The presence of phenol, benzoquinone,
270
and phthalic acid indicates that chlorine in TeCB is completely removed as well as the occurrence
271
of catalytic oxidation reactions. FT-IR spectra of the fresh and used V2O5-WO3/TiO2 catalysts 13
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were collected and displayed in Fig. 10. The band at 500-800 cm-1 can be assigned to the
273
vibration of Ti-O bond of TiO2 and the small band near 3500 cm-1 corresponds to the -OH
274
group.45 The band at 2350 cm-1 represents the C=O bond,46 assigning to the adsorbed CO2 and CO
275
over catalyst surface. The bands at 1750 and 1700 cm-1 also represent the C=O, which can be
276
attributed to the presence of carbonyl intermediates as quinone and aldehydes.47 The bands
277
between 1475 and 1395 cm-1 represent the C-O and the C-H.48
278
Based on the results of GC-MS and FT-IR, a possible TeCB decomposition route over
279
synthesized catalyst was proposed, as shown in Fig. 11. The first step of TeCB decomposition is
280
believed to be a nucleophilic substitution with the formation of surface phenolates (A), in
281
agreement with the results reported by Lichtenberger and Amiridis49 and Xu et al.26. Since the
282
C-Cl bond in aromatic halide is weaker than the C-H bond, it is more prone to be attacked by
283
nucleophiles.50 Firstly, the aromatic compounds are adsorbed on catalyst via nucleophilic attacks
284
on chlorine positions of aromatic rings. The remaining aromatic rings are subsequently oxidized
285
or opened, resulting in the occurrence of some non-aromatic intermediates. TeCB offers four
286
positions for such a nucleophilic attack. Consequently, the oxidation of TeCB is their dissociative
287
adsorption on a vanadium oxide site via one or more Cl abstracted simultaneously or in sequence.
288
The presence of oxygen molecules in nucleophilic reagent plays an active role in the oxidation of
289
chlorinated compounds, and chlorine atoms in adsorbed TeCB can be gradually substituted by
290
surface oxygen. As such, the first step in the oxidation of TeCB on catalyst is the cleavage of C-Cl
291
bond, in consistent with the conclusions reported by Brink et al..50
292
The second step is the electrophilic substitution of the adsorbed partially dechlorinated species
293
(C-E). The proposed electrophilic attack on this species is expected to be faster than the 14
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Page 16 of 41
294
corresponding attack on the adsorbed tetrachlorobenzene derivatives, since halogen substituents
295
are deactivating toward electrophilic substitution reactions. Chlorine is known to be an ortho-para
296
director during electrophilic substitutions. One of chlorine atoms is abstracted and replaced by a
297
surface oxygen species during nucleophilic substitution, and the ortho-para-directing effect of the
298
Cl substituent is then enhanced by the surface oxygen. The electrophilic substitution could also
299
result in the bond breaking of the aromatic ring to give a nonaromatic intermediate (B), which
300
reacts rapidly to form surface maleates (F), acetates (G), and aldehydes (H). Some of the partial
301
oxidation products formed on the surface can undergo further reaction to form the final reaction
302
products (i.e., CO, CO2, HCl, and H2O).
303
The reaction mechanism of TeCB decomposition can be determined as a coexistence of
304
multiple reactions. Amongst, the catalysis of heavy metal on surface of fly ash, the generation of
305
oxygen on surface of the catalyst, and the cleavage of C-Cl bond of TeCB are three key processes
306
in the whole decomposition reaction. The catalyst provides a place for decomposition reaction,
307
and also enhances the generation of oxygen and reactive oxygen species, promoting the
308
decomposition efficiency of pollutants.
309
4. CONCLUSIONS
310
In summary, the feasibility of decomposition of tetrachlorobenzene by using a low-temperature
311
pyrolysis-catalysis combined technology was demonstrated. The V2O5-WO3/TiO2 has higher
312
catalytic activity than that of CuO-WO3/TiO2 and MnOx-WO3/TiO2 catalysts, and TeCB in the
313
simulated fly ash can be effectively decomposed by the V2O5-WO3/TiO2 catalyst. Amongst,
314
V2O5-WO3/TiO2 catalyst with V2O5 loading of 5.0 wt.% and V2O5/WO3 weight ratio of 1 : 2
315
possesses the best TeCB decomposition efficiency. TeCB conversion increases from 78.2% to 15
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98.9% when the reaction temperature increases from 200 to 400 °C, while the TeCB
317
decomposition efficiency shows an obvious reduction when increase the gas hourly space velocity
318
or TeCB inlet concentraion. Orthogonal test indicates that more than 94% of TeCB can be
319
removed under condition of reaction temperature of 350 °C, TeCB concentraion of 0.5 vol.%, and
320
gas hourly space velocity of 600 h-1. The main substeps of TeCB decomposition are aromatic
321
pollutant nucleophilic substitution and intermediates electrophilic substitution. It is shown that
322
TeCB can be finally converted to CO, CO2, H2O, and HCl, while several kinds of reaction
323
byproducts such as low chlorinated aromatics and aromatic and aliphatic hydrocarbons.
324
ASSOCIATED CONTENT
325
Supporting Information: GC peak areas of TeCB-toluene standard solutions, standard curve of
326
TeCB, and textural property of catalysts. This material is available free of charge via the Internet
327
at http://pubs.acs.org.
328
ACKNOWLEDGMENTS
329
This work was financially supported by the National Natural Science Foundation of China
330
(21477095,
21677114),
the
National
Key
Research
and
Development
Program
331
(2016YFC0204201), and the Fundamental Research Funds for the Central Universities
332
(xjj2017170). The valuable comments from the editor and anonymous reviewers are much
333
appreciated.
334
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Energy & Fuels
Tables Table 1 Parameters for single-factor experiments. Experiment number
Reaction temperature/°C
GHSV/h-1
TeCB concentration/vol.%
1
250
600
1
2
275
600
1
3
300
600
1
4
325
600
1
5
350
600
1
6
375
600
1
7
400
600
1
8
300
300
1
9
300
600
1
10
300
900
1
11
300
1200
1
12
300
1500
1
13
300
1800
1
14
300
600
0.5
15
300
600
0.75
16
300
600
1.0
17
300
600
1.25
18
300
600
1.5 24
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Page 26 of 41
Table 2 Parameters for orthogonal experiments. Factor
GHSV/h-1
Temperature/°C
TeCB concentration/%
Level 1
300
300
0.5
Level 2
600
320
0.75
Level 3
1200
350
1
25
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Energy & Fuels
Table 3 Results of multi-factor orthogonal experiments. Experiment number
GHSV/h-1
Reaction temperature/°C
TeCB concentration/vol.%
Decomposition efficiency/%
1
300
300
1.0
90.2
2
300
325
0.5
92.4
3
300
350
0.75
93.5
4
600
300
0.75
89.7
5
600
325
1.0
90.6
6
600
350
0.5
94.1
7
1200
300
0.5
87.3
8
1200
325
0.75
89.2
9
1200
350
1.0
88.6
The average of indicator 1
92.1
89.1
89.8
The average of indicator 2
91.5
90.8
90.8
The average of indicator 3
88.4
92.1
91.3
Range
3.1
3.0
1.5 26
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Page 28 of 41
Table 4 Composition of tail gas absorption solution. Retention time/min
Substance
Molecular formula
12.1
1,3-dichlorobenzene
C6H4Cl2
13.5
1,2,4-trichlorobenzene
C6H3Cl3
13.9
1,3,5-trichlorobenzene
C6H3Cl3
15.6/16.5
Tetradecamethylcycloheptasiloxane
C14H42O7Si7
17.9
1,2,3,5-tetrachlorobenzene
C6H2Cl4
18.8
Phenol
C6H5OH
20.3
Benzoquinone
C6H4O2
22.1
Phthalic acid
C8H6O4
27
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Energy & Fuels
Figure captions Fig. 1 Effect of catalyst support on TeCB decomposition efficiency. Fig. 2 Effect of active phase composition on TeCB decomposition efficiency. Fig. 3 Effect of V2O5/WO3(w/w) on TeCB decomposition efficiency. Fig. 4 Effect of vanadium loading onTeCB decomposition efficiency. Fig. 5 Effect of catalyst calcination temperature on TeCB decomposition efficiency. Fig.6 XRD patterns of various samples. Fig. 7 FE-SEM images of (a) TiO2, (b) Fresh VWTi, and (c) Used VWTi. Fig. 8 TeCB decomposition under different reaction conditions. Fig. 9 Reaction product distribution of TeCB decomposition over VWTi catalyst at 350 °C. Fig. 10 FT-IR spectra of the fresh and used VWTi catalysts. Fig. 11 Proposed TeCB decomposition mechanism over VWTi catalyst. Scheme 1 Schematic diagram of the low-temperature pyrolysis-catalysis combined system.
28
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Page 30 of 41
Fig. 1
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Page 31 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Fig. 2
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ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Fig. 3
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ACS Paragon Plus Environment
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Energy & Fuels
Fig. 4
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ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Fig.5
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ACS Paragon Plus Environment
Page 35 of 41
Anatase TiO2 Rutile TiO2 V2O5 WO3
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Used-VWTi
Fresh-VWTi
TiO2 support
10
20
30
40
50
60
70
80
2θ (°) Fig. 6
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ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
a
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b
100 nm
100 nm
c
100 nm
Fig. 7
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ACS Paragon Plus Environment
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Energy & Fuels
Fig. 8 36
ACS Paragon Plus Environment
Energy & Fuels
Cl O Si O
30
Si O
Si
Si
Si O
Cl
O
O
Cl
Cl
Si O Si
25
Intensity/a.u. (105)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Cl
OH
Cl
20
O
O
Cl Cl
OH
15
O HO Cl
10
Cl O
Cl
5 Cl
0 9
12
15
18
21
Retention time/min
Fig. 9
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ACS Paragon Plus Environment
Page 39 of 41
Fresh VWTi
Transmittance/%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
4000
3500 (-OH)
Used VWTi 2350 (C=O)
1750 (C=O)
1395 (C-H)
690 (Ti-O)
3500
3000
2500
2000
1500
1000
500
Wavenumber/cm-1 Fig. 10
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ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Fig. 11
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ACS Paragon Plus Environment
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Energy & Fuels
Scheme 1
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ACS Paragon Plus Environment