2. I MPACT OF T EXTURAL P ROPERTIES OF M ESOPOROUS P ORPHYRINIC C ARBON
3.2. E XPERIMENTAL M ETHODS
3.2.1. Synthesis of Fe–N/C Catalysts with Silica Protective Layer
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heat-treatment at 800 °C and HF etching steps were carried out in the same manner as described above. Fe-PANI/CNT was synthesized using FeCl3 and polyaniline as the Fe and N precursors, respectively.35,39,67 In the optimized synthesis, 0.4 g of acid-treated CNT and 1 ml of aniline were added to 125 ml of 0.5 M HCl, and the mixture was stirred for 1 h to disperse aniline. 1.5 g of FeCl3
and 1.25 g of APS were added to this solution at a temperature below 10 °C, and the mixture was stirred for 24 h. Next, the solution was evaporated with a rotary evaporator at 90 °C, and the resulting wet powder was further dried overnight in an oven at 60 °C. The dried powder was mixed with 2 mL of TEOS in a mortar, followed by mixing with 2 mL of formic acid. The composite was placed in an alumina crucible and dried in an oven at 60 °C for 3 h. The dried composite was subsequently heated to 900 °C at a ramping rate of 2 °C min−1 and maintained at that temperature for 3 h under N2 gas at a flow rate of 1 L min−1. Finally, the resulting composite was washed with at 10% HF/ethanol solution in the same manner as described above. A series of Fe–N/CNT catalysts without the silica coating were also prepared for comparison purpose, except only for the silica coating step, and were denoted Fe–N/CNT_w/o SiO2 (N = Por, Phen, and PANI) catalysts.
3.2.1.4. Fe–N/C Catalysts with Different Carbon Supports. The efficacy of silica coating- mediated synthesis was explored with different carbons supports (CNT, KB, and VC) while fixing Fe and N precursors with FeAc and 1,10-phenanthroline, respectively. The resulting catalysts were referred to as Fe-Phen/C (C = CNT, KB, or VC). The synthesis of Fe-Phen/KB and Fe-Phen/VC was carried out with the same method used for Fe-Phen/CNT, except KB and VC were used instead of CNT. The samples prepared without the silica coating were denoted Fe-Phen/C_w/o SiO2 (C = CNT, KB, or VC) catalysts.
3.2.1.5. Fe–N/C Catalysts with Controlled Fe Contents. A series of Fe-Phen/CNT catalysts containing differing amount of Fe were prepared by controlling the nominal amounts of FeAc/1,10- phenanthroline precursor. In the precursor mixture, the atomic ratio between FeAc and 1,10- phenanthroline was fixed at 1:3, and the amount of acid-treated CNT was 0.2 g in each sample. The synthesis was carried out with the same method used for Fe-Phen/CNT, except different amounts of FeAc and 1,10-phenanthroline were used. The resulting catalysts were denoted Fe-Phen/CNT_X wt%
(X: nominal iron content in the precursor mixture = 1, 3, 5, and 6), and the counterparts prepared without the silica coating step were designated Fe-Phen/CNT_X wt%_w/o SiO2. Note that Fe- Phen/CNT_5 wt% and Fe-Phen/CNT_5 wt%_w/o SiO2 were the same samples with Fe-Phen/CNT and Fe-Phen/CNT_w/o SiO2, respectively.
3.2.1.6. Fe–N/C Catalysts Pyrolyzed at Different Temperatures. A series of Fe-Phen/CNT catalysts were prepared by following the same procedure for Fe-Phen/CNT catalyst, except pyrolysis was performed at different temperatures (600 °C, 700 °C, 800 °C, 900 °C, and 1000 °C). The resulting catalysts were designated by Fe-Phen/CNT_Y °C (Y: pyrolysis temperature), and the counterparts prepared without the silica coating step were designated by Fe-Phen/CNT_ Y °C_w/o SiO2. Note that
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Fe-Phen/CNT_800 °C and Fe-Phen/CNT_800 °C_w/o SiO2 were the same samples with Fe- Phen/CNT and Fe-Phen/CNT_w/o SiO2, respectively.
3.2.1.7. Fe–N/C Catalysts Prepared with Additional S or P Precursor. S-doped and P-doped Fe- Phen/CNT catalysts (Fe-S-Phen/CNT and Fe-P-Phen/CNT) were prepared by using BTD and triphenylphosphine as the S and P sources, respectively. For Fe-S-Phen/CNT synthesis, 0.40 g of acid- treated CNT was mixed with 0.149 g of FeAc, 0.231 g of 1,10-phenanthroline, and 0.174 g of BTD.
In case of Fe-P-Phen/CNT, 0.40 g of acid-treated CNT was mixed with 0.249 g of FeAc, 0.387 g of 1,10-phenanthroline, and 0.564 g of triphenylphosphine. Another set of S-doped and P-doped Fe- Phen/CNT catalysts (Fe-S-Phen/CNT_2 and Fe-P-Phen/CNT_2) was prepared using diphenyl disulfide and tris(diethylamino)phosphine as the S and P sources, respectively. For Fe-S-Phen/CNT_2, 0.40 g of acid-treated CNT was mixed with 0.202 g of FeAc, 0.313 g of 1,10-phenanthroline, and 0.380 g of diphenyl disulfide. In the synthesis of Fe-P-Phen/CNT_2, 0.40 g of acid-treated CNT was mixed with 0.231 g of FeAc, 0.359 g of 1,10-phenanthroline, and 0.492 g of tris(diethylamino)phosphine. The atomic ratio of 1,10-phenanthroline to each heteroatom precursor was 1:1 and the nominal Fe content was fixed at 5 wt%. The subsequent steps after precursor mixing were the same as those used for the optimized Fe-Phen/CNT catalyst described above.
Table 3.1. List of catalysts presented in Chapter 3.
Catalyst Name a Remarks
Effect of Fe and N precursors (Fe−N/CNT)
Fe−Por/CNT FeTMPPCl, CNT
Fe−Phen/CNT FeAc, Phen,b CNT
Fe−PANI/CNT FeCl3, polyaniline, CNT Effect of carbon supports
(Fe−Phen/C) Fe−Phen/CNT FeAc, Phen, CNT
Fe−Phen/KB FeAc, Phen, KB
Fe−Phen/VC FeAc, Phen, VC
Effect of Fe contents (Fe−Phen/CNT_X wt %)
Fe−Phen/CNT_1 wt% Fe loading, 1 wt%
Fe−Phen/CNT_3 wt% 3 wt%
Fe−Phen/CNT_5 wt% 5 wt%
Fe−Phen/CNT_6 wt% 6 wt%
Effect of pyrolysis temperature
Fe−Phen/CNT_600 °C Pyrolysis temperature, 600 °C
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(Fe−Phen/CNT_Y °C) Fe−Phen/CNT_700 °C 700 °C
Fe−Phen/CNT_800 °C 800 °C
Fe−Phen/CNT_900 °C 900 °C
Fe−Phen/CNT_1000 °C 1000 °C
Effect of heteroatom precursor (Fe−H−Phen/CNT)
Fe−S−Phen/CNT FeAc, BTD,c Phen, CNT Fe−P−Phen/CNT FeAc, triphenylphosphine, Phen, CNT Fe−S−Phen/CNT_2 FeAc, diphenyl disulfide, Phen, CNT Fe−P−Phen/CNT_2 FeAc, tris(diethylamino)phosphine, Phen, CNT
a Catalysts prepared without the silica-coating step are denoted as “catalyst name”_w/o SiO2. b 1,10- Phenanthroline. c 2,1,3-benzothiadiazole.