Figure 3-1 shows the flowchart of the experimental design in this study. Catalysts are synthesized through the evaporation induced self-assembly (EISA) process. Their microstructures and physiochemical properties are characterized by means of TEM, TGA, UV-vis, XRD, XPS, XAS and N2 adsorption isothermal. The adsorption and photoreduction experiments are employed to evaluate their photocatalytic activity, and EPR is carried out to clarify the mechanism of the photocatalytic reduction of CO2 with H2O.
3-1. Materials
Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic P123, EO20PO70EO20, M = 5800, Sigma-Aldrich) was used as the structure directing agent. Titanium isopropoxide (TTIP, Ti(OC3H7)4, Acros, 98.0 %), Zirconium (IV) tetra-propoxide (ZTP, Zr(OCH2CH2CH3)3, Acros, 70.0 %) and hydrogen tetrachloroaurate (III) trihydrate (HAuCl4·3H2O, Aldrich, 99.9 %) were used as the precursor of titanium, zirconium and gold, respectively. Absolute ethanol (CH3CH2OH, Sigma-Aldrich) was used as the solvent to dissolve the precursors. Acetylacetone (Acac, CH3COCH2COCH3, Fluka, 99.5 %) was used as the chelating agent to control the hydrolysis and condensation of the TiO2 and ZrO2. Hydrochloric acid (HCl, J. T. Baker, 36.5 %) and sodium hydroxide (NaOH, Riedel-de Haën, 99.0 %) were used to adjust the pH value of solution. They were shown in Table 3-1.
Table 3-1 The structural formula of materials used in this study.
Chemical Structural formula
Pluronic P123
x=20, y=70, z=20
Titanium isopropoxide O
Ti O O O
Zirconium (IV) tetra-propoxide O
Zr O O O
Hydrogen tetrachloroaurate (III)
trihydrate Cl Au Cl
Cl Cl
H+ OH2 OH2
OH2
Acetylacetone
Figure 3-1 Flowchart of experimental design in this study.
Preparation of mesoporous Au-loaded and Zr-doped TiO
2Characterization
Chemical property Microstructure Electronic structure
UV-vis
EPR
XPS TGA
BET
TEM ICP-MS
XRD
XAS
Adsorption and photoreduction test
Result and discussion
3-2. Preparation of mesoporous Au-loaded and Zr-doped TiO
2samples
Mesoporous Zr-doped TiO2 samples were prepared via an evaporation induced self-assembly (EISA) process. The preparation procedure is shown in Figure 3-2.
Pluronic P123 (4.56 g, 0.786 mmol) was firstly dissolved in absolute ethanol (46.8 mL).
Then, hydrochloric acid (0.14 m) and deionized water (4.16) were slowly added into the above mixture with vigorous magnetic stirring for 30 min. At the same time, titanium isopropoxide (12 mL), zirconium (IV) tetra-propoxide (x-y mL) together with acetylacetone (4.06 mL) were mixed in a brown glass vial form a reddish yellow complex solution. The complex was added to the surfactant solution to undergo hydrolysis under vigorous stirring for 1 hr. Subsequently, the gel was aged at ambient condition without any perturbation.
After approximately 2 days, yellow translucent glasslike xerogel was obtained upon solvent evaporation. The xerogel was heated about at 100 °C for 24 hr (ramp of 0.5 °C min-1) to improve the condensation of the inorganic network, and the surfactant was removed through calcination at 400 °C (ramp of 0.5 °C min-1) for 4 hr in air. The mesoporous TiO2 is called TiO2, and the Zr-doped TiO2 samples with various doping amount are named as ZrxTiO2, where x = 0.01, 0.02, 0.03, 0.04, 0.05 and 0.10. The introduction of gold nanoparticles on the mesoporous TiO2 support was achieved via a deposition-precipitation (DP) method, which the procedure is as shown in Figure 3-3. A HAuCl4 aqueous solution (4.0 × 10-3 M) was added to deionized water (20 mL) with vigorous stirring. The pH value was adjusted to 8.0 by dropwise addition of NaOH (1 M), and then 0.5 g of mesoporous TiO2 was dispersed in the mixture. The resulting solution was heated at 80 °C with continuously stirring for 1 h and the precipitates were separated by centrifugation. As-synthesized samples were dried and finally calcined at 300 °C for 2 hr in air. Based upon synthesis stoichiometry, the Au-loaded TiO2 catalysts are called x% Au-TiO2 where the x resents0.1, 0.5, 1, 2, 4 and 8 wt
% Au-loading.
Figure 3-2 Synthetic process of mesoporous TiO2 and ZrxTiO2 samples via an EISA method.
Table 3-2 Preparation mesoporous of TiO2 and ZrxTiO2 samples and corresponding names.
Sample name TTIP ZIP P123 Acac HCl H2O EtOH
TiO2 1 0 0.02 1 0.04 6 20
Zr0.01TiO2 1 0.01 0.02 1.01 0.04 6 20
Zr0.02TiO2 1 0.02 0.02 1.02 0.04 6 20
Zr0.03TiO2 1 0.03 0.02 1.03 0.04 6 20
Zr0.04TiO2 1 0.04 0.02 1.04 0.04 6 20
Zr0.05TiO2 1 0.05 0.02 1.05 0.04 6 20
Zr0.1TiO2 1 0.1 0.02 1.1 0.04 6 20
Figure 3-3 Synthetic process of Au-doped mesoporous TiO2 catalyst via a DP method
Table 3-3 Preparation conditions of Au-loaded TiO2 samples and corresponding names.
Sample name TTIP Au(a) P123 Acac HCl H2O EtOH
0.1% Au-TiO2 1 0.1 0.02 1 0.04 6 20
0.5% Au-TiO2 1 0.5 0.02 1 0.04 6 20
1.0% Au-TiO2 1 1.0 0.02 1 0.04 6 20
2.0% Au-TiO2 1 2.0 0.02 1 0.04 6 20
4.0% Au-TiO2 1 4.0 0.02 1 0.04 6 20
8.0 % Au-TiO2 1 8.0 0.02 1 0.04 6 20
(a) represent weight percent
3-3. Characterization
3-3-1. High Resolution Transmission Electron Microscopy (HTEM)
The inner structure of the catalyst was analyzed using a high resolution transmission electron microscopy (HRTEM, Philips TECNAI 20) working at a 200 keV accelerating voltage. The powders were dispersed into acetone with ultrasonication for 30 min, and then the suspension was dropped on a copper grid placed into the specimen stage.
3-3-2. Nitrogen adsorption and desorption isothermal
N2 adsorption and desorption isotherms at 77 K were obtained using a Micromeritics, TriStar 3000 instrument. The Brunauer-Emmet-Teller (BET) equation and Barret-Joyner-Halenda (BJH) model were used to calculate the specific surface area and pore size distributions, respectively. Prior to the N2 adsorption experiment, over 0.2 g of catalyst was degassed at 120 °C for 12 hr to remove physisorbed water.
3-3-3. X-ray Powder Diffractometry (XRPD)
X-ray diffraction (XRD) patterns of the catalysts were recorded with a X-ray powder diffractometer (XRPD, MAC Sience, MXP18) using Cu K radiation ( = 0.1546 nm) in the range of 20° to 70° (2θ) with a sampling width of 0.02° and scanning speed of X° min-1. The operating conditions of instrument were at an accelerating voltage of 30 kV and an emission current of 20 mA. The crystalline size (D) of all samples was calculated via a Scherrier’s equation:
cosθ λ
D K (3-1)
D: Crystalline size
K: Scherrier constant (0.89)
: Wavelength of X-ray source (Cu K radiation, = 0.1546 nm)
: Full width at half maximum (FWHM) θ: Scatting angle
3-3-4. 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 at 77 K in darkness and UV irradiation. A 250 W Hg lamp (Moritex ,MUV-250U-L) exhibiting a major output wavelength at 365 nm was positioned at a fixed distance from sample cavity. The conditions of the instrument were set at a center field of 3500 G and sweep width of 2000 G. The microwave frequency was 9.49 GHz and the power was 1.0 mW.
3-3-5. Thermo Gravimetric Analysis and Differential Scanning Calorimetry (TGA/DSC)
The measurements of organic content and thermal transition of the samples were carried out thermo gravimetric analysis (TGA, TA 5100) and differential scanning calorimetry (DSC, Netzsch 404). The samples were heated from temperature to 900 °C with a heating ramp of
5 °C min-1 under an air flow at 60 mL min-1.
3-3-6. X-ray Photoelectron Spectroscopy (XPS)
The surface chemical compositions and speciation on the catalysts were examined by X-ray Photoelectron Spectroscopy (XPS, ESCA PHI 1600) using an Al K radiation (1486.6 eV). The photoelectron was collected into the analyzer with pass energy of 23.5 eV. The collection step size in wide range scan (survey) and high resolution analysis (multiplex) were 1.0 and 0.1 eV, respectively. All analytic process was controlled under ultrahigh vacuum at the pressure below 1.0 × 10-8 Torr. The binding energies were referenced to the Ti 2p peak at 458.8 eV of the catalyst framework. For the qualification and quantification of each element, curve fitting of XPS spectra was performed on program with appropriate parameters including the binding energy, doublet separation and full-width at half maximum. The atomic ratio was calculated from the integrated peak area which was normalized by sensitive factors. The equation for atomic ration calculation is shown below:
2
Ix: intensity of species on XPS spectra ASFx: atomic sensitive factor of element Ax: peak area of XPS spectra
3-3-7. UV-vis Spectrometer
A diffuse reflectance UV-vis spectrophotometer (Hitachi, U-3010) was used to characterize the electronic structure of the catalysts. The UV-vis spectra were acquired from 900 to 200 nm with a scanning rate of 150 nm min-1. All the analysis used aluminum oxide (Al2O3) as the reference which was considered a total reflection material. The band gap of catalysts was determined from the absorption edges converted from the reflectance spectra using a Kubelka-Munk equation:
S k R
R R
F
2 )2 1 ) (
( (3-3)
k: absorption coefficient S: scatting coefficient R: %R reflectance
3-3-8. X-ray Absorption Spectroscopy (XAS)
The X-ray absorption spectroscopy (XAS) uses synchrotron radiation to investigate the structural and electronic properties of the X-ray absorbing atom and about its local environment. The Ti K-edge and Zr-edge X-ray absorption spectroscopy (XAS) spectra were recorded in transmission mode for synthesized powders mounted on a Scotch tape at beamline 01C1 and 16A1 with a double-crystal Si (111) monochromator for energy scanning, respectively..
3-3-9. CO
2Adsorption Test
CO2 adsorption-desorption isotherm was performed at 273 K using an adsorption apparatus (Micromeritics, TriStar 3000). Over 0.2 g catalyst was loaded into an adsorption tube and degassed with pure N2 gas at 120 °C for 12 hr. The saturated adsorption amount and the adsorption constant were calculated using the Langmuir equation:
0
P: gas pressure at 273K
P0: saturated vapor pressure at 273K θ: adsorbed amount on surface
Xm: saturated adsorbed amount on surface K: Langmuir equilibrium constant
3-3-10. CO
2Photoreduction Test
Before the photocatalytic reaction, the catalyst was heated at 120 °C for 1 hr and irradiated with UV light (305 nm, 30 W) in O2 for 12 hr to purify the surface. A batch reactor and a gaseous system were designed for the photocatalytic reduction of CO2 (as shown in Figure 3-4 and 3-5). First, a 0.05 g sample was dispersed on the glass fiber filter paper (ADVANTEC GC-50), and then the filter paper was placed on the stainless holder in the middle of reactor. The CO2 (99.99%) stream with different vapor content was obtained via mixing one CO2 stream with another one, which passed through a water bubbler
to be fully humidified (Humidity > 95 %), at different volume ratios. The reactant gas was then introduced to the cylindrical Pyrex glass reactor with a quartz window at the top. The cylindrical reactor has 55 mm in height and 85 mm in outside diameter, and the total volume is 220 ml. A stainless steel holder was placed into the reactor to support the glass fiber membrane loaded with the catalyst. To fulfill the reactor with the humidified CO2, the gas stream purged the reactor at a rate of 50 mL/min for 1 hr. The photocatalytic reduction of CO2 was carried out in a stainless box surrounded with 16 UV lamp (305 nm, 8 W for each lamp) for 8 hr. The gas (0.2 mL) was sampled using a side-port gaseous tight needle at various intervals, and the concentrations of CO2 and intermediates were measured by using a gas-chromatography (GC) equipped with a capillary column (30 m × 0.53 mm, Supel-QTM Plot), a thermal couple detector (TCD) and a flame ionization detector (FID). The yield of product at any given time in the reaction was determined by means of a calibration curve. A CH4 standard gas was prepared for the calibration curve. A 10 μL of CH4 gas was injected into the reactor fully filled with N2. Then, 20, 50 100, 200, 300, 400 and 500 μL of the mixture were analyzed by GC and calculated the injected CH4 amounts. The quantification of CH4 was performed by a FID based upon the apparent intensity of 2.41 min for CH4
compound, allowing the simultaneous analysis of N2 and CO2 by a TCD (see Appendix A).
Quantum efficiency (E) is an essential parameter to determine the photoreduction activity that can be evaluated with Equation 3-5. First, the total moles of photons were calculated from the period of photon flux by the Equation 3-6. The absorbed photon flux was detected by photometer of 120 mW/cm2 as shown in Figure 3-6. and the surface area received UV irradiation was 8.04 cm2. And the each photon energy at 305 nm wavelength is 6.5 × 10-19 J. The eight moles of electrons are required to produce 1 mol of methane from CO2. All methane yields of samples were compared by the initial and total quantum efficiency at 1 hr and 8 hr of UV illumination, respectively.
catalyst 100
Figure 3-4 Schematic illustration of the experimental setup for photoreduction of CO2.
Figure 3-5 Photographs of the (a) photocatalytic reactor and (b) illuminated system.
(b)
(a)
Figure 3-6 Photographs of the (a) photometer and (b) illuminated photometer system.