3-1. Chemicals
Zirconium sulfate tetrahydrate (Zr(SO4)2·4H2O, Alfa, 98%), hexadecyltrimethyl ammonium bromide (C16TAB, Sigma Aldrich, ≥ 98% ) and trimethyl-1-octylammunium bromide (C8TAB, Alfa, 97%) were used as the precursor and the templates respectively, to prepare porous ZrO2. Sulfuric acid (H2SO4, Showa, 98%) was used for post sulpnation.
Ethanol (C2H5OH, Echo, 99.5%) and ammonium nitrate (NH4NO3, Riedel-de Haën, 98%) were used as an extraction solution to remove the template from the ZrO2 in terms of ion exchange.
3-2. Preparation of porous sulfated zirconium
Except for S-ZrO2 sample, which was obtained from calcination of Zr(SO4)2·4H2O at 750 °C for 5 hours. Besides, the other samples will be discussed in section 3-2-1 and 3-2-2.
3-2-1.
Precipitation process
The surfactant (C16TAB, 1.47g) was dissolved in water (50ml), and Zr(SO4)2·4H2O (2.68g) was dissolved in water (14.7 ml) was added. This led to a white precipitate. The molar ratio of the individual components of CTAB to Zr was 0.50. The mixture was stirred for 2h at room temperature and then heated at 100 °C for 1 day in a closed beaker with Teflon covering. After filtration, the precipitate was calcined at 500 °C for 5 hours. This product was called C16S-ZrO2. A serious molar ratio (0.25, 0.38, and 0.63) by changing the mass of surfactant was prepared also.
The sample C16S-ZrO2 which molar ratio is 0.5 (0.5g) was added to different concentration sulfuric acid solutions (0.3, 0.6, 0.9 and 1.2 M) for further coating. The mixture was stirred at room temperature for 2 hours. After pumping, the powders were calcined at 500 °C for 5 hours once again.
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3-2-2.
Hydrothermal process
The surfactant (C8TAB, 1.9g) was dissolved in water (35ml), and Zr(SO4)2·4H2O (2.68g) was dissolved in water (14.7 ml) was added. The molar ratio of the individual components of C8TAB to Zr was 0.50. The mixture was stirred for 2h at room temperature and then under hydrothermal processing at 150°C for 36 hours. This process led to a white precipitate. To remove the surfactant efficiently, the precipitant was filtered and extracted with 150 ml alcoholic solutions (99%) and this product was called C8S-ZrO2 (EtOH). Another one which was filtered and extracted with alcoholic solutions with ammonium nitrate (150 ml of ethanol containing 1.6 g of NH4NO3) was called C8 S-ZrO2 (IEE). Both of them were under stirred condition at 60°C for 15 mins for three times. The amount of NH4NO3 corresponded to NH4+/CTAB+ molar ratio is 2. The extracted sample after filtration was calcinated at 500°C for 5 hours.
3-3. Characterization
3-3-1.
Nitrogen adsorption and desorption isothermal
The specific surface area, SBET, and pore sizes were determined from Brunaner, Emmett and Teller model (BET) and Barrett, Joyner and Halenda formula (BJH), respectively. This is based on the N2 adsorption and desorption isothermal at 77K by Micromeritics, Tristar 3000.
Prior to the adsorption/desorption, the samples were degassed at 150°C under vacuum for 12 hours.
3-3-2.
Fourier Transform Infrared Spectrometer (FTIR)
The S-ZrO2 samples were identified using Fourier transform infrared spectrometer (FTIR, Thermo Scientific Nicolet iS10) scan from 400 to 4000 cm-1 with the resolution of 4 cm-1 for 100 cycles. The samples were mixed with KBr (Merck) and then pressed as a flake for the FTIR measurement.
38
3-3-3.
X-ray photoelectron Spectroscopy (XPS)
The surface chemical compositions and chemical states of the S-ZrO2 samples were characterized using X-ray photoelectron spectroscopy (XPS, ESCA PHI 1600) using an Al Kα X-ray source (1486.6 eV). The photoelectron was collected into the analyzer with pass energy of 23.5 eV. In the collection step, size in wide range scan and high-resolution analysis were 0.1 eV. All analytical process was controlled under ultrahigh vacuum conditions at pressure less than 1.4 × 10-9 Torr. The chemical shift in binding energy of XPS spectra was reference to the Zr (3d) line at 182.4 eV. In order to quantify and qualify each element, curve fitting of the XPS spectra was performed. After subtraction of the “Shirley-shaped”
background, the original spectra were fitted using a nonlinear least-square fitting program and combination of Gaussian-Lorentzian peak shapes were adapted for all peaks. The parameters used for the curve fitting of the Zr 3d, S 2p, O 1s, C 1s, including the binding energies, doublet separation, and full-width at half maximum. The integrated peak areas were normalized atomic sensitive factors to calculate atomic ratios.
The equation for atomic ratio calculation is shown below,
n1/n2 = (I1/ASF1)/( I2/ASF2) = (A1/ASF1)/(A2/ASF2) (3-1)
where as,
n: atomic number
I: intensity of XPS spectra ASF: atomic sensitivity factor A: peak area of XPS spectra
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3-3-4.
Thermo Gravimetric Analysis (TGA)
The organic volume and energy flow of the samples were measured using thermo gravimetric analysis (TGA, TA5100). The samples were heated from room temperature to 900°C at a heating rate of 10°C/min under an air flow with a flow rate of 50 ml/min.
3-3-5.
Water content measurement
Water contents were measured using thermal gravimetric analysis (Perkin Elmer SII, pyris diamond TG/DTA analyzer). The sulfated ZrO2 powders (0.08g) were pressed into pellets with 1.2 cm in diameter and 25 mm in thickness at 1960 kPa. A 0.05 ml of water droplets was added to the pellet to wet the samples. Samples were pat-dried with tissue paper to remove the surface water. Samples were heated from room temperature to 400 °C at 10 °C min-1 under N2 flow with 10 ml min-1. The weight of dry membranes was determined from the point at which degradation of sulfonic acid begins (400 °C). Water content was calculated according to the follow equation
Water content =
weight of wet pellet - weight of dry pellet
×100%
weight of wet pellet
3-3-6.
Proton conductivity measurement
For 2-probe measurements in the direction of thickness, specimens were prepared by specimen sandwiched into electrodes consisting of two pieces of gold sheets situated opposite each other. A specimen fixed to a measuring cell was placed inside a temperature and humidity chamber under constant temperature and humidity (Room temperature, 100%RH).
AC impedance (PGSTAT 30) measurements were taken using a computer-controlled Autolab model, and Cole-Cole (Z’-Z’’) plots were obtained. The frequency limits of the sinusoidal signals were typically set between 100Hz and 10000Hz, with an oscillation of 100 mA. A best-fitting curve was overlaid onto the measurements taken by the 2-probe method. The
40
sulfated ZrO2 powders were pressed into pellets with 1.2 cm in diameter and 25 mm in thickness at 1960 kPa. In contrast to Nafion117, the membrane was cut with 1.2cm in diameter and 20 mm in thickness. A 0.05 ml of water droplets was added to the pellet and Nafion117 to wet the samples. The pellet was sandwiched into electrodes consisting of two pieces of gold sheets situated opposite each other. A specimen fixed to a measuring cell was placed inside a temperature and humidity chamber under constant temperature and humidity (Room temperature, 100% RH). The frequency limits of the sinusoidal signals were typically set between 100Hz and 10000Hz, with an oscillation of 100 mA. Conductivity was calculated from the obtained membrane resistance, Rbulk, by using the following formula.
σ=L/(R·A) (3-2)
Where
σ is conductivity (Scm-1), L is membrane thickness (cm), A is an electrode area (cm2) and R is resistance (Ω).
3-3-7.
Temperature programmed desorption (TPD)
The TPD (Auto Chem II 2920) experiments were used to measure the acidity by using probe of ammonia. The samples (0.10-0.15 g) were pretreated in He for 1 h at 200°C and then cooled to 100°C. At this temperature, NH3 was pulsed over the samples and adsorbed for 1hr.
The physically adsorbed NH3 was desorbed in He at a rate of 25ml min-1 for 30 min.
Desorption was programmed form 100 to 600°C at a heating rate of 10°C min-1. A thermal conductivity detector (TCD) was used to monitor the desorption volume of NH3.
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3-3-8.
High Resolution Transmission Electron Microscope (TEM)
The particle size and shape of nanocrystals were examined by a high resolution transmission electronic microscopy (HR-TEM, JEOL JEM-2010) at an accelerating voltage of 200 KV. The specimen was prepared by dispersing of powders into acetone with ultrasonic vibration. The colloid was dropped on a holey carbon film supported on a Cu grid (Ted Pella, Inc., 200 meshes).
3-3-9.
Inductively Coupled Plasma- Mass Spectrometry (ICP-MS)
ICP-MS (Perkin Elmer, SCIEX ELAM 5000) was used to analyze bulk chemical compositions including S/Zr ratio of samples. All solid samples were digested with acid solution coupled with microwave.
4-1.
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0 200 400 600 800 1000
20 40 60 80 100
C16SS-ZrO2 C16S-ZrO2
Zr(SO4)2 4H2O C8S-ZrO2 (EtOH)
C8S-ZrO2 C8S-ZrO2 (IEE)
Weight (%)
Temperature (
oC)
Figure 4-1 The TGA curve of Zr(SO4)2 • 4H2O, and as-prepared C16S-ZrO2, C16S-ZrO2, C8S-ZrO2, C8S-ZrO2 (EtOH) and C8S-ZrO2 (IEE).
In this study, the surfactant molecules in the C16S-ZrO2 and C8S-ZrO2 samples were removed thorough calcination at 500 °C for 5 hr. In addition, the S-ZrO2 sample was obtained from calcination of Zr(SO4)2 · 4H2Oat 750 °C for 5 hr. To determine the quantity of the sulfated species remaining in the C16S-ZrO2, C8S-ZrO2, and S-ZrO2 samples, their TGA curves were recorded and shown in Figure 4-2. The S-ZrO2 sample exhibited little mass loss in the range of 25-900 °C, indicating that the calcination at 750 °C effectively eliminated sulfated species from the Zr(SO4)2 · 4H2O. This finding is in agreement with the TGA result of the Zr(SO4)2 · 4H2O sample that sulfated species was completely removed above 750 °C.
The C16S-ZrO2 and C8S-ZrO2 samples started to undergo 4 and 8 % weight loss at 590 and 690 °C, respectively. The weight losses represent the amounts of the sulfated species remained in these two samples because the calcination temperature (500 °C) was lower than the temperatures (570-620 °C) required for entirely removing the sulfated species from the
44
samples. In fact, S elements were also identified in the samples from XPS and ICP-MS.
These results indicate that sulfated zirconia powders were successfully obtained through the preparation processes in this study. The C16S-ZrO2 sample contained about 4 % mass from adsorbed water. In contrast, only few amounts of water were present in the C8S-ZrO2
sample. The difference in the water contain in these two porous samples is attributed to their different pore sizes.
0 200 400 600 800 1000
85 90 95 100
C8S-ZrO2 (EtOH) C16SS-ZrO2 C16S-ZrO2 S-ZrO2
C8S-ZrO2 (IEE)
Weight (%)
Temperature (
oC)
Figure 4-2 The TGA curves of C16S-ZrO2, C16SS-ZrO2, C8S-ZrO2(IEE), C8S-ZrO2(EtOH), and S-ZrO2 samples.
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4-2. Surface functional group
Figure 4-3 shows the FTIR spectra of S-ZrO2, C16S-ZrO2, C16SS-ZrO2, C8S-ZrO2 (IEE), and C8S-ZrO2(EtOH) samples. Intensive Zr-O absorptions in the region of 400-800 cm-1 indicate that calcination turned all the samples into ZrO2 forms[54]. In addition, almost hydrocarbons were burned out from the templated C16S-ZrO2, C16SS-ZrO2, C8S-ZrO2 (IEE), and C8S-ZrO2(EtOH) samples after the thermal treatment at 500 °C because the C-H absorptions were absence in their IR spectra. Except for the S-ZrO2 sample, all the templated ZrO2 powders showed the significant S-O and S=O stretching absorptions in the region of 850-1450 cm-1.[64] The C16S-ZrO2, C16SS-ZrO2, and C8S-ZrO2(EtOH) samples contained the S=O symmetric stretching absorption at 1270 cm-1. In addition, the S-O asymmetric stretching bands at 1220, 1140, 1070, 991 cm-1 were observed, revealing that SO4
species bidentately complexes to the ZrO2 matrix.[54] The C8S-ZrO2 (IEE) sample showed a different sulfated absorption feature. The S-O absorption at 1140 cm-1 became the most intensive in the SO4 absorption set. In addition, the S=O stretching mode blue shifted to 1345 cm-1.[64] These phenomena suggest a polysulfated species.
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4000 3500 3000 2500 2000 1500 1000 500
Zr-O 800-400 cm-1
2200 2000 1800 1600 1400 1200 1000 800
991 cm-1 C8S-ZrO2 (EtOH) samples. (a) the original spectra, and (b) the zoom-in spectra in the 700-2200 cm-1.
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4-3. Chemical compositions
The quantities and bonding of the sulfated species on the surface of the ZrO2 samples determine proton-transfer efficiency. To understand the surface properties of the sulfated ZrO2 powders, all the samples were characterized by using XPS and ICP-MS. Figure 4-4 shows the S (2p) XP spectra of the S-ZrO2, C16S-ZrO2, C16SS-ZrO2, C8S-ZrO2 (IEE), and C8S-ZrO2(EtOH) samples. Theoretically, the S (2p) photoelectron line appears in the binding energy of 168.5-171.3 eV.[65] However, the S (2p) peak in the ZrO2 matrix in this study was significantly interfered with other photoelectron lines. After deconvolution, there were four peaks contained in the broad signal ranging from 166 to 177 eV. Similar phenomenon was observed in Marcus’s[65] and Milburn’s[66] research. To ascertain the S (2p) peak from the interferences, three samples including pure ZrO2, sulfuric-acid soaked ZrO2 and TiO2 powders were additionally prepared. The pure ZrO2 powders showed the broad peak in the range of 168-177 eV (see Appendix A-1). After soaking with sulfuric acid, the ZrO2 sample exhibited an additional peak centered at 168.64 eV. Moreover, the soaked TiO2 sample showed the single peak at 169.0 eV. These results clearly evidence that the deconvoluted peaks centered at 168.64-169.0 eV belong to S (2p) photoelectron line, and the other peaks at higher binding energies arise from ZrO2 matrix effect. All the sulfated samples exhibited one S (2p) photoelectron peak except for the C8S-ZrO2 (IEE) sample. In addition to the S (2p) peak at 168.2 eV, the C8S-ZrO2 (IEE) sample had another S (2p) peak at 169.4 eV, which is attributed to poly-sulfated species.
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178 176 174 172 170 168 166
C8S-ZrO
2(IEE)
C8S-ZrO
2(EtOH)
C16SS-ZrO2
C16S-ZrO
2
S-ZrO2
Binding Energy (eV)
Intensity (A.U.)
Figure 4-4 The S (2p) XPS of the S-ZrO2, C16S-ZrO2, C16SS-ZrO2, C8S-ZrO2 (EtOH), and C8S-ZrO2 (IEE) samples.
The surface and total S/Zr atomic ratios of the sulfated ZrO2 samples are summarized in Table 4-1. Calcination of Zr(SO4)2•4H2O at 750 °C removed most of sulfated ions from the S-ZrO2 sample. However, the XPS results showed 0.1 of S/Zr ratio on its surface, indicating that the remaining sulfated species is primarily incorporated in the surface layer. The C16S-ZrO2 and C16SS-ZrO2 samples had similar bulk and surface S/Zr ratios of 1.9×10-2-2.1×10-2 and 0.15-0.18, respectively. Addition of sulfuric acid in the precipitation process only assists the formation of regular pore sizes, but has little effect on the surface composition. The C8S-ZrO2 (EtOH) powders had the bulk and the surface S/Zr ratios of
49
20.3×10-2 and 0.38, respectively. These values are larger than the S/Zr ratios of the mesoporous samples. Because the C8S-ZrO2 (EtOH) sample had microporous structure, the micropore confinement presumably inhibits S-O-Zr bond vibrations and breaking. The C
8-S-ZrO2 (IEE) sample exhibited the highest S/Zr ratios both in the bulk (94.8×10-2) and at the surface (1.02). Replacement of octylammonium ions with NH4+ though ion exchange treatment enhanced the stability of sulfated species against thermal treatment. This result is in agreement with its TGA curve which shows the higher temperature for removal of the sulfated species. We further compared the surface and the bulk S/Zr ratios and found that the surface ratios were 10, 7, and 2 times higher than the bulk ratios in the C16S-ZrO2, C16SS-ZrO2, C8S-ZrO2 (EtOH) samples, respectively. Like the S-ZrO2 sample, this finding indicates that the sulfated species was mainly incorporated on the surface. The C8S-ZrO2
(IEE) sample showed similar surface and bulk S/Zr ratios, revealing the bulk doping.
Table 4-1 Sulfur-to-zirconium atomic ratios in the different sulfated ZrO2 samples.
S/Zr atomic ratio
Samples ICP-MS XPS
S-ZrO2 -a 0.10
C16S-ZrO2 1.9×10-2 0.18
C16SS-ZrO2 2.1×10-2 0.15
C8S-ZrO2 (EtOH) 20.3×10-2 0.38 C8S-ZrO2 (IEE) 94.8×10-2 1.02
a- represents unavailable.
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Figure 4-5 shows the O (1s) XP spectra of the sulfated ZrO2 samples. The O (1s) peak in all the sulfated samples can be fitted into O-Zr (BE= 530.0 eV), O-S (BE= 531.6 eV) and O-H (532.6 eV) states.[67] To derive the sulfated species and surface chemical structure of the sulfated samples, the O-S/S, O-Zr/Zr, and O-H/Zr atomic ratios were calculated and listed in Table 4-2. The S-ZrO2 sample had the O-S/S ratio of 3.89, suggesting 4-coordinated S elements. The three templated samples, including C16S-ZrO2, C16SS-ZrO2, and C8S-ZrO2
(EtOH), showed the S-O/S ratios in the range of 3.54-3.66, revealing a pyrosulfate species (S2O72-). A low S-O/S ratio of 3.10 was found in the C8S-ZrO2 (IEE) sample. Polysulfated species was considered to be formed at the ZrO2 sample.[68] In addition, bulk doping resulted the C8S-ZrO2 (IEE) sample in a non-stochiometric Zr-O/Zr ratio of 1.44 and a high O-H/Zr ratio of 0.29. These features imply its higher surface Lewis and Bronsted acidity over the other samples.
Table 4-2 The surface O-S/S, O-Zr/Zr, and O-H/Zr atomic ratios
Samples O-S/S (O-Zr)/Zr O-H/Zr
S-ZrO2 3.89 1.77 0.18
C16S-ZrO2 3.54 2.39 0.20
C16SS-ZrO2 3.66 2.01 0.14
C8S-ZrO2 (EtOH) 3.60 1.95 0.20
C8S-ZrO2 (IEE) 3.10 1.44 0.29
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540 538 536 534 532 530 528 526 524 C8S-ZrO
2(IEE)
C8S-ZrO2(EtOH)
C16SS-ZrO2
C16S-ZrO
2
S-ZrO2
Binding Energy (eV)
Iintensity (A.U.)
Figure 4-5 The O (1s) XPS of the S-ZrO2, C16S-ZrO2, C16SS-ZrO2, C8S-ZrO2 (EtOH), and C8S-ZrO2 (IEE) samples.
According to the XPS and FTIR results, we propose the different chemical structures of sulfated species on the ZrO2 surface. The sulfated group (SO4) monodentately or bidentately bonds to the Zr4+ ions on the surface of the S-ZrO2 sample. Its oxygen deficient property (O-Zr/Zr= 1.77) infers to the Lewis acidity. Templating methods lead pyrosulfated groups bidentately binding to the ZrO2 surface. Poly-sulfated moieties are introduced into the ZrO2
surface lattice after the ion exchange treatment.
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Figure 4-6 The chemical structures of the sulfated groups on the different ZrO2 samples. (a) the S-ZrO2 powder, (b) the C16S-ZrO2, C16SS-ZrO2, and C8S-ZrO2 (EtOH) samples, (c) the C8S-ZrO2 (IEE) sample.
Through extraction of MCM-41 materials with an ethanolic ammonium nitrate solution, Lang and Tuel[69] efficiently remove the organic template from the templated samples. The exchange of the organic cations with the NH4+ ions preserved the porous structure and led to the porous structure similar to those obtained after calcination. In this study, we used the similar procedure to remove the C8TAB from the ZrO2 sample before calcination to prevent incomplete oxidation during the thermal treatment. However, the microporous structure was destroyed by the ion exchange treatment. This result is attributed to the dehydration of the sample induced by the high ionic strength of the salt solution. The dehydration shrinks the micropores, thus causing the disappearance of the pores in the following calcination. The sulfated species on the pore wall surface was incorporated into the ZrO2 matrix through the elemental rearrangement, and some of them were segregated from the lattice to the particle
53
surface due to the limitation of solubility. A high concentration of the surface sulfated species formed polysulfated moiety.
Figure 4-7 Formation of C8S-ZrO2 (IEE).
54
4-4. NH
3adsorption and desorption
To examine the surface acidity of the sulfated samples, their TPD measurements were carried out, and the results are shown in Figure 4-8. The S-ZrO2 sample showed a desorption peak at 235 °C, which is devoted to the adsorption at the Lewis acid sites with medium strength. Substantial amounts of medium Lewis acid and strong Bronsted acid sites were found in the C8S-ZrO2 (IEE) sample at 290 and 510 °C, respectively. Its low O-Zr/Zr and high O-H/Zr ratios support the TPD result. The C16S-ZrO2, C16SS-ZrO2, and C8S-ZrO2
(EtOH) had intensive desorption peaks at 176-188 °C, which is ascribed to the NH3 adsorbed on weak Lewis acid sites.[70]
100 200 300 400 500 600
0.00 0.01 0.02 0.03 0.04 0.05 0.06
C16S-ZrO2
C8S-ZrO2(IEE) C8S-ZrO2(EOH)
C16SS-ZrO2
S-ZrO2
TCD Intencity
Temperature (oC)
Figure 4-8 NH3 TPD patterns of the S-ZrO2, C16S-ZrO2, C16SS-ZrO2, C8S-ZrO2 (EtOH), and C8-S-ZrO2 (IEE) samples.
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4-5. Textures and proton conductivities of the sulfated zirconia
To explore the effect of porous structures on the proton conductive capability, the textures of the sulfated samples were characterized in terms of N2 physisorption at 77 K. The S-ZrO2 sample (Appendix B-1) showed a Type IV adsorption isotherm[71] and a H3 hysteresis loop.[72] The mesoporous feature resulted from its interparticle voids. Without templating effect, the sample had a small surface area of 64 m2/g and relatively large pore size of 16.4 nm. In addition to the C16S-ZrO2 sample that was prepared with the CTAB/Zr molar ratio of 0.5, the samples with the CTAB/Zr ratios of 0.25, 0.38, and 0.63 were also prepared. Figure 4-9 and Figure 4-10 show the N2 adsorption/desorption isotherms and pore size distributions of the C16S-ZrO2 samples prepared with different CTAB/Zr molar ratios.
Typical Type IV adsorption isotherms were observed in the C16S-ZrO2 samples when the CTAB/Zr ratio was higher than 0.50, indicating mesoporous structures. In addition, the hysteresis loop (H2 or H3) was observed in the relative pressure range from 0.4 to 0.8. The inflection was not sharp, indicated that the pores are not a uniform size and has broad distribution. The gentle inflection reveals wide pore-size distributions. All these C16TAB-templated samples exhibited continuous increase in the pore volume when the pore size was smaller than 4.4 nm. Moreover, higher amounts of the template resulted in larger numbers of the meso/micro pores. The samples with the CTAB/Zr ratios higher than 0.5 showed a typical pore size of 3.9 nm, which is resulted from self-assembled micelles.
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0.0 0.2 0.4 0.6 0.8 1.0
CTAB/Zr=0.63
CTAB/Zr=0.38
CTAB/Zr=0.25 CTAB/Zr=0.50
Volume N 2 adsorbed (A.U.)
P/P0
Figure 4-9 Nitrogen adsorption-desorption isotherms of the C16S-ZrO2 samples synthesized with different CTAB/Zr molar ratios.
1 2 3 4 5 6 7 8
0.000 0.001 0.002 0.003 0.004 0.005 0.006
dV/dR (cm3 g-1 nm-1 )
Pore diameter (nm)
CTAB/Zr = 0.25 CTAB/Zr = 0.38 CTAB/Zr = 0.50 CTAB/Zr = 0.63
Figure 4-10 Pore size distributions of the C16S-ZrO2 samples synthesized with different CTAB/Zr molar ratios.
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Table 4-3 summarizes the surface areas, pore volumes, average pore diameters, and proton conductivities of the sulfated ZrO2 samples templated with C16TAB. The sulfated ZrO2 sample had the smallest surface area of 78 m2/g at the C16TAB/Zr= 0.25. Its large pore volume (0.16 cm3/g) and mean pore size (8.2 nm) were mainly from inter-particle pores.
Increased numbers of templated pores raised the surface area to 114 m2/g and reduced the mean pore size to 2.4 nm when the C16TAB/Zr= 0.38. Further increase in the C16TAB/Zr ratio to 0.50-0.63 increased the surface area to 122-128 m2/g. The appearance of the typical pores at 3.9 nm also resulted in larger mean pore size of 2.8-3.0 nm. The sample prepared with the C16TAB/Zr ratio= 0.50 performed the highest proton conductivity of 20 mS/cm, whereas the C16TAB/Zr= 0.25 led the sample exhibiting the lowest proton conductivity of 12 mS/cm. This phenomenon suggests that large surface area and small pore diameter are beneficial to the proton transportation. The texture effect is supported by the low proton conductivity of the S-ZrO2 sample (9 mS/cm, shown in Table 4-4). The samples with the C16TAB/Zr= 0.38 and 0.63 showed the similar proton conductivities of 16-17 mS/cm because of the compromising effects of their pore sizes and surface areas.
Table 4-3 Summaries of the BET properties and the proton conductivities of the C16S-ZrO2
synthesized with different CTAB/Zr molar ratios.
synthesized with different CTAB/Zr molar ratios.