4. U NRAVELING C ATALYTIC T RENDS OF A TOMICALLY D ISPERSED C ATALYSTS FOR
4.5. R EFERENCES
(1) Schlögl, R. Angew. Chem. Int. Ed. 2010, 54, 3465–3520.
(2) Liu, L.; Corma, A. Chem. Rev. 2018, 118, 4981–5079.
(3) Thomas, J. M. Phil. Trans. R. Soc. A 2016, 374, 20150226.
(4) Wang, A.; Li, J.; Zhang, T. Nat. Rev. Chem. 2018, 2, 65–81.
(5) Freyschlag, C. G.; Madix, R. J. Mater. Today 2011, 14, 134–142.
(6) Kim, J.; Kim, H.-E.; Lee, H. ChemSusChem 2018, 11, 104–113.
(7) Su, J.; Ge, R.; Dong, Y.; Hao, F.; Chen, L. J. Mater. Chem. A 2018, 6, 14025–14042.
(8) Yang, X.-F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T. Acc. Chem. Res. 2013, 46, 1740–
1748.
(9) Zhang, H.; Liu, G.; Ye, J. Adv. Energy Mater. 2018, 8, 1701343.
(10) Kwak, J. H.; Hu, J.; Mei, D.; Yi, C.-W.; Kim, D. H.; Peden, C. H. F.; Allard, L. F.; Szanyi, J.
Science 2009, 325, 1670–1673.
(11) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935–938.
(12) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T.. Nat.
Chem. 2011, 3, 634–641.
(13) Lin, L.; Zhou, W.; Gao, R.; Yao, S.; Zhang, X.; Xu, W.; Zheng, S.; Jiang, Z.; Yu, Q.; Li, Y.-W.;
Shi, C.; Wen, X.-D.; Ma, D. Nature 2017, 544, 80–85.
(14) Fang, X.; Shang, Q.; Wang, Y.; Jiao, L.; Yao, T.; Li, Y.; Zhang, Q.; Luo, Y.; Jiang, H.-L.. Adv.
Mater. 2018, 30, 1705112.
(15) Chen, Z.; Vorobyeva, E.; Mitchell, S.; Fako, E.; Ortuño, M. A.; López, N.; Collins, S. M.;
Midgley, P. A.; Richard, S.; Vilé G.; Pérez-Ramírez, J. Nat. Nanotechnol. 2018, 13, 702–707.
(16) Guo, X.; Fang, G.; Li, G.; Ma, H.; Fan, H.; Yu, L.; Ma, C.; Wu, X.; Deng, D.; Wei, M.; Tan, D.;
Si, R.; Zhang, S.; Li, J.; Sun, L.; Tang, Z.; Pan, X.; Bao, X. Science 2014, 344, 616–619.
(17) Zhang, Y.; Pang, S.; Wei, Z.; Jiao, H.; Dai, X.; Wang, H.; Shi, F. Nat. Commun. 2018, 9, 1465.
(18) Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani-Stephanopuoulos, M.; Sykes, E. C. H. Science 2012, 335, 1209–1212.
(19) Yang, S.; Kim, J.; Tak, Y. J.; Soon, A.; Lee, H. Angew. Chem. Int. Ed. 2016, 55, 2058–2062.
(20) Choi, C. H.; Kim, M.; Kwon, H. C.; Cho, S. J.; Yun, S.; Kim, H.-T.; Mayrhofer, K. J. J.; Kim, H.; Choi, M. Nat. Commun. 2016, 7, 10922.
(21) Yan, H.; Cheng, H.; Yi, H.; Lin, Y.; Yao, T.; Wang, C.; Li, J.; Wei, S.; Lu, J. J. Am. Chem. Soc.
2015, 137, 10484–10487.
(22) Yang, S.; Tak, Y. J.; Kim, J.; Soon, A.; Lee, H. ACS Catal. 2017, 7, 1301–1307.
(23) Kwon, Y.; Kim, T. Y.; Kwon, G.; Yi, J.; Lee, H. J. Am. Chem. Soc. 2017, 139, 17694–17699 (2017).
(24) Mitchell, S.; Thomas, J. M.; Pérez-Ramírez, J. Catal. Sci. Technol. 2017, 7, 4248–4249.
(25) Gates, B. C.; Flytzani-Stephanopoulos, M.; Dixon, D. A.; Katz, A. Catal. Sci. Technol. 2017, 7, 4259–4275.
(26) Liu, J. ACS Catal. 2017, 7, 34–59.
(27) Liu, J.; Jiao, M.; Lu, L.; Barkholtz, H. M.; Li, Y.; Wang, Y.; Jiang, L.; Wu, Z.; Liu, D.-J.;
Zhuang, L.; Ma, C.; Zeng, J.; Zhang, B.; Su, D.; Song, P.; Xing, W.; Xu, W.; Wang, Y.; Jiang, Z.; Sun, G. Nat. Commun. 2017, 8, 15938.
(28) Li, T.; Liu, J.; Song, Y.; Wang, F. ACS Catal. 2018, 8, 8450–8458.
(29) Li, S.; Liu, J.; Yin, Z.; Ren, P.; Lin, L.; Gong, Y.; Yang, C.; Zheng, X.; Cao, R.; Yao, S.; Deng, Y.; Liu, X.; Gu, L.; Zhou, W.; Zhu, J.; Wen, X.; Xu, B.; Ma, D. ACS Catal. 2020, 10, 907–913.
(30) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169–11186.
(31) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865–3868.
(32) Man, I. C.; Su, H.-Y.; Calle-Vallejo, F.; Hansen, H. A.; Martínez, J. I.; Inoglu, N. G.; Kitchin, J.;
Jaramillo, T. F.; Nørskov, J. K.; Rossmeisl, J. ChemCatChem 2011, 3, 1159–1165.
(33) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Science 2009, 323, 760–764.
(34) Hansen, H. A.; Viswanathan, V.; Nørskov, J. K. J. Phys. Chem. C 2014, 118, 6706–6718.
(35) Siahrostami, S.; Verdaguer-Casadevall, A.; Karamad, M.; Deiana, D.; Malacrida, P.; Wickman, B.; Escudero-Escribano, M.; Paoli, E. A.; Frydendal, R.; Hansen, T. W.; Chorkendorff, I.;
Stephens, I. E. L.; Rossmeisl, J. Nat. Mater. 2013, 12, 1137–1143.
(36) Xu, H.; Cheng, D.; Cao, D.; Zeng, X. C. Nat. Catal. 2018, 1, 339–348.
(37) Fei, H.; Dong, J.; Feng, Y.; Allen, C. S.; Wan, C.; Volosskiy, B.; Li, M.; Zhao, Z.; Wang, Y.;
Sun, H.; An, P.; Chen, W.; Guo, Z.; Lee, C.; Chen, D.; Shakir, I.; Liu, M.; Hu, T.; Li, Y.;
Kirkland, A. I.; Duan, X.; Huang, Y. Nat. Catal. 2018, 1, 63–72.
(38) Kulkarmi, A.; Siahrostami, S.; Patel, A.; Nørskov, J. K. Chem. Rev. 2018, 118, 2302–2312.
(39) Viswanathan, V.; Hansen, H. A.; Rossmeisl, J.; Nørskov, J. K. ACS Catal. 2012, 2, 1654–1660.
5
L IGAND E XCHANGE R EACTION OF A TOMICALLY
D ISPERSED C ATALYSTS TO C ONTROL T HEIR
E LECTROCATALYTIC P ROPERTIES
5.1.INTRODUCTION
Catalysts have been utilized over 80% of industrial chemical processes to improve the productivity of the desired processes.1–4 In general, catalysts have been categorized into two classes:
homogeneous catalysts based on metal salts or organometallic complexes, and heterogeneous catalysts in the form of metal or metal oxide nanoparticles (NPs) dispersed on a solid support.5 Homogeneous catalysts have well-defined, tunable, and fully accessible catalytic sites, thereby exhibiting highly active and selective catalytic properties, but suffer from the stability of the catalytic sites and difficulty of separation and recyclability. On the other hand, heterogeneous catalysts are easy to separate and have excellent durability, however they show low atom-efficiency and selectivity. Given the complementary nature of homo- and heterogeneous catalysts, there has been a long-standing effort to develop new catalysts that combine the virtues of the two-class of catalysts.6–10
Recently emerged atomically dispersed catalysts6 (or single atom catalyst11) are a class of catalysts where metal atoms are fully dispersed on a solid support. Inheriting the merits of homo- and heterogeneous catalysts, atomically dispersed catalysts have exhibited excellent catalytic performance and lifetimes for various thermo-,12–14 photo-,15,16 and electrocatalytic reactions,17–19 and therefore attracted great attention. However, the full dispersion of metal atoms on supports does not always guarantee excellent catalytic properties.8,20 Like molecular catalysts, proper coordination environments and oxidation states are necessary to exhibit outstanding catalytic performance.6,21 Thus, it is essential to understand the optimal coordination structure of atomically dispersed catalysts at a molecular level.
Unfortunately, in atomically dispersed catalysts, metal atoms are coordinated with complex support surface sites; it is difficult to figure out and control the coordination environment of the catalytic sites, hindering the investigation of their structure-activity correlation. Nevertheless, there have been attempts to modulate the coordination environments of atomically dispersed catalysts. By controlling organic precursors22–24 or temperatures25–29 in the thermal activation process, coordination environments can be tuned to have optimal configurations for target catalytic reactions. In the other way, the tailoring types
of supports can afford highly active catalyst-support interface structures.30–33 However, such methods are still far from the exquisite control of ligands realized in homogeneous catalysts.34
Herein, we report the ligand exchange reactions35 on atomically dispersed Rh catalysts, which allows the elaborate control of ligand types and the consequent modulation of catalytic activity and selectivity for the ORR. Thermal treatments of a RhCl3-impregnated carbonaceous support under NH3
and CO gas afforded NH3 and CO dominantly ligated atomically dispersed Rh catalysts, respectively.
The CO-ligated atomically dispersed Rh catalyst exhibited approximately 30-fold higher ORR activity than NH3-ligated Rh catalyst, whereas the latter showed three times higher H2O2 selectivity than the former. Notably, the NH3 and CO ligands could be reversibly exchanged around atomically dispersed Rh atoms, and the oxidation states could also be tuned by the change of ligands. Accordingly, ORR activity and selectivity were reversibly tuned.
5.2.EXPERIMENTAL METHODS 5.2.1 Synthesis of M/CNT_IL_G
Before using the carbon nanotubes (CNT, Carbon Nano-material Technology Co. LTD), CNT was purified through heat treatment and acid leaching to remove metallic impurities. 38.0 g of CNT was calcined at 500 °C for 1 h at a ramping rate of 7.9 °C min−1. The heated CNT was dispersed in 810 g of 6 M HNO3 (diluted from 60% HNO3, Samchun Chemicals) and stirred at 80 °C for 12 h. The suspension was vacuum-filtered and washed with copious amounts of deionized (DI) water until the filtrate became neutral. The resulting CNT was treated in the same manner as described above with 720 g of 6 M HCl (diluted from 36% HCl, Samchun Chemicals) and then dried at 60 °C overnight. To coat ionic liquid (IL) onto CNT, 0.1 g of acid-treated CNT was mixed with 1.44 g of 1-butyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide (Aldrich) on mortar for 15 minutes. The mixture was then pyrolyzed at 450 °C for 1 h at a ramping rate of 5.3 °C min−1 under N2 condition. The annealed mixture was ground on a mortar and washed with copious amounts of acetone through vacuum filtration to eliminate physisorbed-IL on CNT surfaces. The resulting powder was dried at 60 °C overnight and labeled as CNT_IL.
Atomically dispersed metal catalysts were prepared by an incipient-wetness impregnation of metal precursor on CNT_IL, followed by thermal activation. For atomically dispersed Rh catalysts, 3.9 mg of RhCl3 (Alfa Aesar) was dissolved in 1 mL of acetone. The solution was mixed with 100 mg of CNT_IL by hand scrubbing in a plastic bag, and the mixture was dried at room temperature (RT) overnight. The dried powder was heated at 200 °C for 2 h at a ramping rate of 0.6 °C min−1 under various gas conditions. Same as Rh, atomically dispersed Ir and Pt catalysts were fabricated using H2IrCl6 (99%, Alfa Aesar) and H2PtCl6 (99.95%, Umicore) as precursors for Ir and Pt, respectively.
Depending on the used metal (M) and gases (G), the labels of catalysts were determined. The nominal
metal loading was written in front of the metal. For example, 1.5Rh/CNT_IL_NH3 is a label for NH3- treated 1.5 wt% Rh catalysts. When sequential ligand exchange reactions were performed, all the used gases (G) were written in the order of use.
5.2.2 Characterization Methods
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were taken on a JEOL JEM-2100F electron microscope at an acceleration voltage of 200 kV. X- ray photoelectron spectroscopy (XPS) measurements were carried out with an ESCLAB 250Xi system (Thermo Scientific) equipped with a monochromatic Al Kα X-ray source (1486.6 eV). The obtained XPS spectra were analyzed using the XPSPeak41 software. The Rh 3d binding energy region were fitted to the spectra using the Gaussian (Gaussian 70, Lorentzian 30) function after Shirley-type background subtraction.
5.2.3 XAS Experiment
X-ray absorption spectroscopy (XAS) was measured at 8C nano-probe XAS beamline (BL8C) of Pohang Light Source (PLS-II) in the 3.0 GeV storage ring, with a ring current of 250 mA. X-ray beam was monochromated by a Si(111) double crystal. The X-ray beam was then delivered to a secondary source aperture where the beam size was adjusted to be 0.5 mm (v) × 1 mm (h). XAS spectra were collected in both transmission and fluorescence modes. The obtained spectra were processed using Demeter software. Extended X-ray absorption fine structure (EXAFS) spectra were fitted in a Fourier- transform range of 4–14 Å-1 with a Hanning window applied between 1 Å and 2.3 Å or between 1 Å and 3.0 Å. The amplitude reduction factor (So2) was set to be 0.85 during the fitting.
5.2.3 Electrochemical characterization
Electrochemical characterization was performed by constructing a three-electrode cell using an electrochemical workstation (CHI760E, CH Instruments) at atmospheric pressure on RT. Three- electrode cell was built with a rotating ring disk electrode (RRDE, AFE7R9GCPT, Pine Research Instrumentation), a graphite counter electrode, and an Ag/AgCl (ALS) reference electrode. The 0.1 M HClO4 electrolytes were prepared from the dilution of 70% HClO4 (Veritas double distilled, GFS Chemicals) in 18.2 MΩ cm Millipore water. Prior to measurements, the RRDE was polished sequentially with 1.0 and 0.3 µm alumina suspensions to afford a mirror finish. The catalyst ink was prepared by mixing of 7.4 mg of catalyst, 50 µl of deionized water, 60 µl of Nafion (D521, DuPont), and 490 µl of ethanol (Samchun Chemicals, 99.9%), followed by ultrasonication for 20 min. Then, 8 µl of the prepared ink was drop-cast onto the glassy carbon disk (0.247 cm−2) of the RRDE. The catalyst- coated RRDE was used as the working electrode. The ORR activities and selectivities of the catalysts were measured by linear sweep voltammetry (LSV) at a scan rate of 5 mV s−1 with an electrode rotation
speed of 1,600 rpm in O2-saturated electrolyte. The capacitive currents were corrected by subtracting the LSV data obtained in N2-saturated electrolyte from those measured under O2-saturated conditions.
Electrochemical impedance spectroscopy was conducted at 0.4 V with an AC potential amplitude of 0.01 V from 100,000 to 1 Hz at an electrode rotation speed of 1,600 rpm to correct a solution resistance for iR-compensation. The solution resistance was determined from the x-axis intercept in the high- frequency region of the Nyquist plot. All the currents were collected after iR-compensation. The H2O2
yields were calculated using the following equation: H2O2 yield (%) = 200/(1 + (N × id) /ir), where N, id, and ir are the collection efficiency (0.37, provided by the manufacturer), disk current, and ring current, respectively. The accelerated degradation test (ADT) was conducted with cyclic voltammetry in a potential range of 0.6–1.0 V in N2-saturated electrolyte at a scan rate of 50 mV s−1 for 5,000 cycles. The measurements were repeated at least three times and the averaged data were reported.
5.3 RESULTS AND DISCUSSION
5.3.1 Synthesis and Characterization of 1.5Rh/CNT_IL_G Catalysts
A prerequisite of ligand exchange reaction on atomically dispersed catalysts is that metal sites are anchored on supports with the complete isolation. In our previous work,36 we found that IL coating onto CNT provides abundant anchoring sites of metal atoms, which suits the isolation of metal precursors. Using CNT_IL as a support, we carried out incipient-wetness impregnation of RhCl3 to load 1.5 wt% Rh as single metal sites, and the resulting catalyst was named as 1.5Rh/CNT_IL_Imp. Through HAADF-STEM images of 1.5Rh/CNT_IL_Imp, we observed that bright dots are apart from each other (Figure 5.1), confirming isolated anchoring of Rh on CNT_IL. Next, we performed the ligand exchange reaction with heat treatment at 200 °C in NH3 or CO gas flows (Figure 5.2). As a comparison group, heat treatment in an Ar condition was also conducted.
Figure 5.1. (a) Low and (b) high magnification HAADF-STEM images of 1.5Rh/CNT_IL_Imp.
(a) (b)
40 nm 2 nm
Figure 5.2. A schematic illustration of ligand exchange reaction on atomically dispersed catalysts.
HAADF-STEM images show that even after heat treatment, atomically dispersed Rh sites are firmly preserved without agglomeration, regardless of gas condition (Figure 5.3). The ligand substitution reaction is identified by the XAS. When Rh is impregnated onto CNT_IL (1.5Rh/CNT_IL_Imp), two scattering peaks corresponding to Rh–N and Rh–Cl can be identified by the Rh K-edge k2-weighted EXAFS fitting results (Figure 5.4 and Table 5.1). The Rh–Cl bonds originate from RhCl3 precursor and Rh–N bonds are generated by the interaction between Rh precursor and support. The heat treatment in an inert gas condition cannot induce a ligand exchange reaction. Only partial pyrolysis, observed from the reduction of the number of Rh–N bonds, can be found through EXAFS fitting results (Table 5.1). There is also no distinctive change in Rh K-edge X-ray absorption near edge structure (XANES) spectrum of 1.5Rh/CNT_IL_Ar compared to that of 1.5Rh/CNT_IL_Imp.
However, when NH3 and CO were respectively treated, differentiated EXAFS and XANES spectra are shown. For the EXAFS spectrum of 1.5Rh/CNT_IL_NH3, one additional Rh–N coordination is generated with loss of one Rh–Cl bond compared to that of 1.5Rh/CNT_IL_Ar, indicating the ligand exchange from Cl to NH3. In a similar manner, in case of CO treatment, scattering peaks for Rh–C and in relatively longer atomic distance Rh–O evolved with reduction of one Rh–Cl bond compared to that of 1.5Rh/CNT_IL_Ar, which demonstrate the ligand exchange from Cl to CO. Owing to the changes in the coordination structure of Rh, distinguished XANES spectra of 1.5Rh/CNT_IL_NH3 and 1.5Rh/CNT_IL_CO can be observed.
1.5 Rh/CNT_IL_Imp 1.5 Rh/CNT_IL_CO 1.5 Rh/CNT_IL_NH3
NH3 Gas CO Gas
5.3.2 Reversible Ligand Exchange
The ligand exchange reaction can take place reversibly (Figure 5.5). When CO was treated on the 1.5Rh/CNT_IL_NH3 (1.5Rh/CNT_IL_NH3_CO), its XANES and EXAFS spectra are transformed into similar form to that of 1.5Rh/CNT_IL_CO. Subsequently, when NH3 was retreated on 1.5Rh/CNT_IL_NH3_CO (1.5Rh/CNT_IL_NH3_CO_NH3), the XANES and EXAFS spectra are recovered into original form of 1.5Rh/CNT_IL_NH3. It is analogous to 1.5Rh/CNT_IL_CO, demonstrated by the alteration of XANES and EXAFS spectra of subsequent NH3
(1.5Rh/CNT_IL_CO_NH3) and CO-treated samples (1.5Rh/CNT_IL_CO_NH3_CO) on 1.5Rh/CNT_IL_CO. Despite such repetitive heat treatments, neither agglomeration of metals into NPs nor loss of metals were found (Figure 5.6 and Table 5.1).
Figure 5.3. HAADF-STEM images of (a) 1.5Rh/CNT_IL_Ar, (b) 1.5Rh/CNT_IL_NH3, and (c) 1.5Rh/CNT_IL_CO.
Figure 5.4. Rh K-edge (a) XANES and (b) EXAFS spectra of 1.5Rh/CNT_IL_Imp, 1.5Rh/CNT_IL_Ar, 1.5Rh/CNT_IL_NH3, and 1.5Rh/CNT_IL_CO.
(b) (c)
(a)
2 nm 2 nm 2 nm
0 1 2 3 4 5 6
FT Magnitude (a.u.)
1.5Rh/CNT_IL_Imp 1.5Rh/CNT_IL_CO
1.5Rh/CNT_IL_NH3
1.5Rh/CNT_IL_Ar
Photon Energy (eV) 23180 23220 23260 23300 23340
Normalized Absorption
1.5Rh/CNT_IL_Imp 1.5Rh/CNT_IL_Ar 1.5Rh/CNT_IL_NH3 1.5Rh/CNT_IL_CO
(a) (b)
Rh K
Reduced Distance (Å )
Table 5.1. Summary of EXAFS fitting parameters of 1.5Rh/CNT_IL_G catalysts.
Sample Shell CNa ΔE0
(eV)b
R (Å)c
σ2 (10−3Å2)d
R factor (%)e 1.5Rh/CNT_IL
_Imp
Rh–N 3
−1.41 ± 1.27 2.08 ± 0.01 2 ± 1
Rh–Cl 3 2.34 ± 0.01 2 ± 1 0.5
1.5Rh/CNT_IL _Ar
Rh–N 2
−1.23 ± 1.25 2.03 ± 0.02 4 ± 1
Rh–Cl 3 2.34 ± 0.01 5 ± 1 0.4
1.5Rh/CNT_IL _NH3
Rh–N 3
−4.79 ± 1.39 2.04 ± 0.01 1 ± 1
Rh–Cl 2 2.30 ± 0.02 6 ± 1 0.5
1.5Rh/CNT_IL _NH3_CO
Rh–C 1
−3.43 ± 2.69
1.88 ± 0.04 5 ± 5
Rh–N 3 2.10 ± 0.03 6 ± 2 1.5
Rh–Cl 1 2.36 ± 0.02 2 ± 1
Rh–O 1 2.78 ± 0.08 6 ± 5
1.5Rh/CNT_IL _NH3_CO_NH3
Rh–N 4
−5.58 ± 5.15 2.05 ± 0.03 3 ± 2
Rh–Cl 1 2.33 ± 0.04 3 ± 3 3.3
1.5Rh/CNT_IL _CO
Rh–C 1
−1.90 ± 1.88
1.84 ± 0.03 2 ± 2
Rh–N 1 2.06 ± 0.03 2 ± 2 1.2
Rh–Cl 2 2.35 ± 0.03 6 ± 3
Rh–O 1 3.03 ± 0.03 3 ± 3
1.5Rh/CNT_IL_CO _NH3
Rh–N 4
−6.33 ± 1.86 2.05 ± 0.02 3 ± 1
Rh–Cl 1 2.31 ± 0.03 5 ± 2 0.8
1.5Rh/CNT_IL _CO_NH3_CO
Rh–C 1
−3.43 ± 3.39
1.86 ± 0.04 3 ± 2
Rh–N 2 2.06 ± 0.03 3 ± 2 1.2
Rh–Cl 1 2.34 ± 0.03 4 ± 2
Rh–O 1 3.04 ± 0.05 2 ± 1
aCoordination number. Fixed value bEnergy shift. cBond distance. dDebye–Waller factor. eR factor was obtained from the best fit for the respective catalyst.
These changes in coordination environments are accompanied by the modification of the oxidation state of Rh. Rh 3d XPS spectra of samples showed doublet peaks assigned to Rh 3d5/2 and Rh 3d3/2 of Rh3+ (Figure 5.7). The XPS peaks of 1.5Rh/CNT_IL_NH3 is shifted to higher binding energy by 0.4 eV compared to that of 1.5Rh/CNT_IL_CO. These results clearly showed the alteration of oxidation state of atomically dispersed Rh depending on the types of ligands. Interestingly, as we alternately treated NH3 and CO, the oxidation state of Rh is also fluctuated in accordance with the used gas, substantiating reversible ligand exchange reactions in our systems.
Figure 5.5. Rh K-edge (a) XANES and (b) EXAFS spectra of 1.5Rh/CNT_IL_NH3, 1.5Rh/CNT_IL_NH3_CO, and 1.5Rh/CNT_IL_NH3_CO_NH3. Rh K-edge (c) XANES and (d) EXAFS spectra of 1.5Rh/CNT_IL_NH3, 1.5Rh/CNT_IL_NH3_CO, and 1.5Rh/CNT_IL_NH3_CO_NH3.
0 1 2 3 4 5 6
FT Magnitude (a.u.)
Photon Energy (eV) 23180 23220 23260 23300 23340
Normalized Absorption
(c) (d)
Rh K
Reduced Distance (Å )
0 1 2 3 4 5 6
FT Magnitude (a.u.)
Photon Energy (eV) 23180 23220 23260 23300 23340
Normalized Absorption
(a) (b)
Rh K
Reduced Distance (Å ) NH3
NH3_CO NH3_CO_NH3
NH3
NH3_CO NH3_CO_NH3
CO CO_NH3 CO_NH3_CO
CO CO_NH3 CO_NH3_CO
Figure 5.6. (a,b) Low and (c,d) high magnification HAADF-STEM images of (a,c) 1.5Rh/CNT_IL_NH3_CO_NH3 and (b,d) 1.5Rh/CNT_IL_CO_NH3_CO.
Figure 5.7. Rh 3d XPS Spectra of (a) 1.5Rh/CNT_IL_NH3, 1.5Rh/CNT_IL_CO, (b) 1.5Rh/CNT_IL_NH3, 1.5Rh/CNT_IL_NH3_CO, 1.5Rh/CNT_IL_NH3_CO_NH3, (c) 1.5Rh/CNT_IL_CO, 1.5Rh/CNT_IL_CO_NH3, and 1.5Rh/CNT_IL_CO_NH3_CO.
(a)
(c)
(b)
(d)
40 nm
2 nm
40 nm
2 nm
Binding Energy (eV) 307 309 311 313 315 317 319
Intensity (a.u.)
Binding Energy (eV) 307 309 311 313 315 317 319
Intensity (a.u.)
Binding Energy (eV) 307 309 311 313 315 317 319
Intensity (a.u.)
(a) Rh 3d (b) (c)
Rh3+
1.5Rh/CNT_IL 1.5Rh/CNT_IL Rh 3d
Rh3+
Rh 3d 1.5Rh/CNT_IL
Rh3+
CO CO_NH3 CO_NH3 _CO
NH3 NH3_CO NH3_CO _NH3 CO
NH3
5.3.3 Electrochemical Properties of Ligand-Modulated Catalysts
Figure 5.8. (a) ORR polarization curves, (b) Tafel plot, (c) kinetic current density at 0.4 V, and (d) H2O2
yield at 0.4 V of 1.5Rh/CNT_IL_NH3 and 1.5Rh/CNT_IL_CO.
Exploiting the ligand substituted Rh catalysts, we investigated the ligand effects of atomically dispersed catalysts on their ORR catalytic properties. In a three-electrode cell, we evaluated their electrocatalytic properties using a rotating ring-disk electrode technique in an acidic medium (0.1 M HClO4). The ORR polarization curves and H2O2 yields display that huge difference in ORR activity and selectivity between 1.5Rh/CNT_IL_NH3 and 1.5Rh/CNT_IL_CO (Figure 5.8a). At 0.4 V, kinetic current density of 1.5Rh/CNT_IL_CO is 36.4 mA cm−2, which is approximately 30-fold higher than that of 1.5Rh/CNT_IL_NH3 (1.2 mA cm−2). For Tafel analyses, 1.5Rh/CNT_IL_CO showed a Tafel slope of 141 mV dec−1 in an potential range of 0.50–0.60 V, whereas 1.5Rh/CNT_IL_NH3 exhibited a higher Tafel slope of 172 mV dec−1 in an potential range of 0.30–0.43 V, indicating faster ORR kinetics on CO-ligated atomically dispersed catalysts than that on NH3-ligated catalyst. On the other hand, H2O2
yield of 1.5Rh/CNT_IL_CO is 21.7 %, about one-third of that of 1.5Rh/CNT_IL_NH3 (61.5 %), showing dramatic changes in ORR selectivity only by the simple ligand modulation of atomically dispersed catalysts. The ORR activity and selectivity of 1.5Rh/CNT_IL_NH3 and 1.5Rh/CNT_IL_CO are stably preserved (Figure 5.9a) without agglomeration of Rh (Figure 5.9b,c) after accelerating
jk (mA cm-2 )
0 10 20 30 40 50
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 j (mA cm-2 )
-6 -5 -4 -3 -2 -1 HO Yield (%)22 0
20 40 60 80 100
1.5Rh/CNT_IL_NH3 1.5Rh/CNT_IL_CO
H2O2 Yield (%)
0 20 40 60 80 100
jk (mA cm-2)
0.1 1 10 100
E - iR (V vs. RHE)
0.2 0.3 0.4 0.5 0.6
0.7 1.5Rh/CNT_IL_NH3
1.5Rh/CNT_IL_CO
141 mV dec-1
172 mV dec-1
Rh_NH3 Rh_CO
×~1/3
(a) (b)
Rh_NH3 Rh_CO
×~30
(c) (d)
E −iR(V vs. RHE)