3-1 Materials
The precursors of titania and vanadium were titanium isopropoxide (TTIP, Acros, 98 %+) and vanadium (V) oxytriisopropoxide (VTIP, Aldrich, 99 %+), respectively. 2-propanol (C3H7OH, J.Backer, 100 %) was used as solvent to dissolve the precursors. Rohdamine B (RhB, C28H31N2O3Cl, Sigma Aldrich, Dye content 95 %) was used to test the activities of the photocatalysts. Hydrogen acid (HCl, Crown, 35~37 %) was used to adjust the pH values of the hydrolyzed sol solution.
3-2 Preparation of vanadium doped TiO2 using a sol-gel method
In the beginning, VTIP and TTIP were dissolved in 70 mL isopropanol to obtain the V/Ti atomic ratios of 1×10-4 and 1×10-2. Afterward, 2 mL hydrochloric acid (24.5%, HCl) was injected into mixed solution at 4°C with stirring at 250 rpm for 9 hours to complete the hydrolysis of the VTIP and TTIP. Then, the solutions underwent gelation at ambient temperature for 4 hours. The doped TiO2 powders were obtained after solvent was evaporated at 100°C for 2 days followed by 150°C for 3 hours. The powders were then calcined under air at various temperatures (200~ 700°C).
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3-3 Characterization
3-3-1 Specific surface area
TriStar 3000 gas adsorption analyzer was used to measure the BET (Brunauer, Emmett, and Teller) surface area of catalysts by N2 physisorption. The surface area of the samples was estimated according to the N2 adsorption data by the BET model. Before the N2
adsorption, the sample was dried at 90 °C for 3 h and degassed at 120 °C for 6 h.
3-3-2 UV/Vis diffuse reflectance spectroscopy (UV/Vis-DRS)
The wavelength response range of the samples was recorded using an UV-vis spectrometer (HITACHI U-3010). The Al2O3 was used to be the reference. The spectra were recorded from 700 to 200 nm at a scanning rate of 300 nm/min. The bandgap of samples can be calculated by transformed the spectra into absorption according to Kubelka-Munk equation.[52]
S k R R R
F = − =
2 ) 1 ) ( (
2 (3-1)
k is an absorption coefficient, S is a scattering coefficient, and R is reflectance.
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3-3-3 Time-of-Flight Secondary Ion Mass Spectrometer (ToF-SIMS)
ToF-SIMS (ION-TOF, Munich, Germany) was used to analyze the surface composition of the sample. The primary ion source was a pulsed Ga+ source operated at 25 keV. The sputter time was 120 s. The pressure of the main chamber was around 10-9 mbar. The surface atomic ratio was calculated by sub-equation
I RSF I
m i i =
ρ (3-2)
Where RSF (relative sensitivity factor) is the conversion factor from secondary ion intensity to atom density, the unit of RSF is atoms/cm3,ρi is the impurity atom density in atoms/cm3, Im
is the matrix isotope (Ti, m/z= 50.8) secondary ion intensity in counts/s and Ii is the impurity isotope (V, m/z= 47.9) secondary ion intensity in counts/s.
3-3-4 Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
Bulk chemical compositions for V/Ti weight ratio was analyzed by Inductively coupled plasma mass spectrometry (ICP-MS, Perkin Elmer, SCIEX ELAN 5000). It is based on coupling together an inductively coupled plasma as a method of producing ions (ionization) with a mass spectrometer. All the samples were digested with acid solution coupled with microwave.
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3-3-5 X-ray diffractometry
The X-ray powder diffractometer (XRPD, MAC Sience, MXP18) was used to examined the crystal structure and grain size by using the CuKα radiation (λ= 0.15405 nm) and the operating conditions are at an accelerating voltage of 30 kV and an emission current of 20 mA. The range of the scanning 2θ is from 15° to 80° at sample width of 0.02° and scanning speed is 4°/min. The crystalline size (D) of all samples was estimated from Scherrer’s equation:[53]
Whereλis the x-ray wavelength (Cu Kα= 0.15406 nm), β is the width of the peak (full width at half maximum, FWHM), K is the Scherrer constant and θis the Bragg angle. The weight ratio of rutile phase (WR) can be estimated by the sub-equation
R
3-3-6 X-ray photoelectron spectroscopy (XPS)
The X-ray photoelectron spectroscopy (XPS, ESCA PHI 1600 spectrometer) was used to identify the surface chemical compositions and chemical state by using the AlKα radiation (1486.6 eV). All the analytical process in the chamber was controlled under ultrahigh vacuum at the pressure below 1.4 ×10-9 Torr. The collection step sizes in wide range scan
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and high-resolution analysis are 1.0 eV and 0.1 eV, respectively. The C 1s peak at 284.8 eV was used to be the reference. The integrated peak areas of spectra were estimated using sensitivity factors to determine the surface atomic ratios. The equation for atomic ratio is calculated by the sub-equation
2
where n means the atomic numbers, I means the intensity of species on XPS spectra, A is the peak area, ASF stands for the atomic sensitive factor of element and Arabic number represents elemental types.
3-3-7 Electron paramagnetic resonance (EPR)
The electron paramagnetic resonance spectrometer (EPR, Bruker EMX-10/12) was used to examine the photo-induced charge carriers at X-band frequency. The measurements were carried out at 77 K in darkness or under irradiation. The conditions of the instrument were set at a center field of 3500 G and a sweep width of 2000 G. The microwave frequency was 9.49 GHz and the power was 1.0 mW.
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3-3-8 Gas chromatograph (GC)
The gaseous sample after photolysis was analysed using gas chromatography (GC, Thermo TRACE GC Ultra) using a FS Cap Supel-Q PLOT column of 30 m length with a diameter of 0.53 mm. Helium was used as a carrier gas. Thermal conductivity detector (TCD) and flame ionization detector (FID) were used for analyzing the gaseous samples, such as CH4, CH3OH.etc.
The calibration curve is used to determine the concentration of the CH4 production.
First, the pure N2 gas purged the reactor for 1 hour to ensure the air was eliminated. After closed the valves, 10 μL CH4 was injected into the reactor. Then, 20 μL, 50 μL, 100 μL, 200 μL, 300 μL, 400 μL and 500 μL of gas in reactor were detected by GC to establish the calibration curve. Appendix D-1 shows the calibration curve of CH4.
3-3-9 X-ray absorption (XAS)
The X-ray absorption (XAS) spectra was used to identify the valence of the vanadium ions within the TiO2 lattice recorded at BL 16A at Taiwan Synchrotron Radiation Research Center (NSRRC). The measurements were carried out at Ambient temperature. The V K-edge spectrum was received using a fluorescence mode. The linear absorption coefficient (μ) was estimated in term of the ratio between incident (I0), fluorescence intensities (If). The following equation can be expressed by sub-equation:
μfluorescence(E)∞ If /I0 (3-6)
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3-4 Photocatalytic reduction of CO2
First, the 0.05 g samples were loaded on the Glass fiber filter paper(ADVANTEC GC-50), then filter paper was placed in the middle of reactor. The reactor has the size of 50 mm (length) × 74 mm (diameter) and the total volume of 220 ml. The reactor was covered by a quartz glass that can let UV light passing through as shown in Figure 3-1. Carbon dioxide (99.9%) is splitted into two pipelines, which one is for dry CO2 and the other one flowed through the DI water for humidification. The different split ratios of these two streams can be adjusted the humidity in the system, which was set about 90% in this study (as shown in Figure 3-2). In our case, the catalysts were pretreated by heated (heating) at 120
°C for 1h in air and were irradiated with the UV light (λ<305nm) for 12 hours. The magnet stirred to homogenize the gaseous system through the photocatalysis. Prior to the photoreduced experiments, the humidified CO2 gas purged the reactor for 1 hour to ensure the air was eliminated. After that, the valves located at outlet and inlet of the reactor were closed. Then, UV irradiation of the catalysts was carried out under irradiation of 16 UV lamps (8 W) at 305 nm for 8 hours. Figure 3-3 displays the photograph of the photocatalytic system.
Quantum efficiency is generally used to universally evaluate the photocatalytic performance of a certain photocatalyst and system design. The quantum efficiency therefore depends on both the collection of charges and the absorption of light. Since eight moles of electrons are required to produce 1 mol of methane from CO2, the quantum efficiency of a photocatalyst for photocatalytic conversion of CO2 to CH4 is expressed as Equation 3-7.
quantum efficiency (%) = × 100 (3-7) 8 × moles of methane yield moles of incident UV photon
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The equation for moles of UV photon absorbed by catalyst is calculated by the sub-equation.
In our study, the absorbed photon flux detected by photometer is 1.2 × 10-4 W/cm2. The surface area received UV-light is 8.042 cm2. T means the radiation time. The each photon energy at 305 nm wavelength is 6.517 × 10-19 J.
moles of incident UV photon = (3-8) Absorbed photon flux (W/cm2) × SA (cm2) × T (s) Each photon energy (J) × 6.02 × 1023
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Figure 3-1 A photograph of the reactor for photocatalytic reduction.
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Figure 3-2 An apparatus design for photocatalytic reduction of CO2.
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Figure 3-3 A photograph of the lamping system.
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