- Email: [email protected]
Single-Atom Catalysts Electrostatically Stabilized by Ionic Liquids
forward by Chuang et al., single-molecule spectroscopy, linear and circular dichroism, and non-linear optical experiments could provide complementary information to allow strong constraints on the parameters of the molecular packing. Imaging techniques such as transmission electron cryomicroscopy could provide further crucial evidence on morphology. The work of Chuang et al. focuses on the thermal effect on the energetic disorder. It is important to remember that other effects of temperature could affect the spectra. For example, structural phase transitions could lead to significant spectral changes for small changes in temperature. Such effects must be kept in mind in the analysis of experimental spectra. Other effects that could affect the interpretation are the presence of charge-transfer states, couplings beyond the dipole approximation, and strong vibronic coupling. For example, for linear aggregates, the presence of charge-transfer states is known to introduce an effective
coupling that could lead to a difference in sign between short- and long-range couplings. In summary, the work of Chuang et al. highlights the sensitivity of optical experiments combined with theoretical modeling as probes of molecular packing in supramolecular structures on the path to designing functional optical materials. 1. Chuang, C., Bennett, D.I.G., Caram, J.R., Aspuru-Guzik, A., Bawindi, M.G., and Cao, J. (2019). Generalized Kasha’s model: tdependent spectroscopy reveals shortrange structures of two-dimensional excitonic systems. Chem 5, this issue, 3135– 3150. 2. Gu¨nther, L.M., Lo¨hner, A., Reiher, C., Kunsel, T., Jansen, T.L.C., Tank, M., Bryant, D.A., Knoester, J., and Ko¨hler, J. (2018). Structural variations in chlorosomes from wild-type and a bchQR mutant of Chlorobaculum tepidum revealed by single-molecule spectroscopy. J. Phys. Chem. B 122, 6712–6723. 3. Lo¨hner, A., Kunsel, T., Ro¨hr, M.I.S., Jansen, T.L.C., Sengupta, S., Wu¨rthner, F., Knoester, J., and Ko¨hler, J. (2019). Spectral and structural variations of biomimetic lightharvesting nanotubes. J. Phys. Chem. Lett. 10, 2715–2724. 4. Li, X., Buda, F., de Groot, H.J., and Sevink, G.J.A. (2019). Molecular insight in the
optical response of tubular chlorosomal assemblies. J. Phys. Chem. C 123, 16462– 16478. 5. Kirstein, S., and Daehne, S. (2006). J-aggregates of amphiphilic cyanine dyes: self-organization of artificial light harvesting complexes. Int. J. Photoenergy 2006, 20363. 6. Dijkstra, A.G., Duan, H.G., Knoester, J., Nelson, K.A., and Cao, J. (2016). How two-dimensional brick layer J-aggregates differ from linear ones: Excitonic properties and line broadening mechanisms. J. Chem. Phys. 144, 134310. 7. Yamaguchi, A., Kometani, N., and Yonezawa, Y. (2006). Spectroscopic properties of the mixed J-aggregate of unsymmetric merocyanine dyes in wide temperature range. Thin Solid Films 513, 125–135. 8. Kriete, B., Lu¨ttig, J., Kunsel, T., Maly´, P., Jansen, T.L.C., Knoester, J., Brixner, T., and Pshenichnikov, M.S. (2019). Interplay between structural hierarchy and exciton diffusion in artificial light harvesting. Nat. Commun. 10, 4615. 9. Kasha, M. (1963). Energy transfer mechanisms and the molecular exciton model for molecular aggregates. Radiat. Res. 20, 55–70. 10. Heijs, D.J., Malyshev, V.A., and Knoester, J. (2005). Thermal broadening of the J-band in disordered linear molecular aggregates: a theoretical study. J. Chem. Phys. 123, 144507.
Preview
Single-Atom Catalysts Electrostatically Stabilized by Ionic Liquids Baojuan Xi1 and Xuping Sun2,* The prohibition of single-atom catalysts (SACs) to aggregate is challenging for further investigations. In this issue of Chem, Ding et al. establish a novel and general stabilization strategy based on the electrostatic interaction between ionic liquids and SACs. The supported metal catalysts exhibit excellent activity and/or selectivity in various industrial reactions, such as hydroalkylation, hydrogenation, dehydro-
genation, hydrocracking, isomerization, and so on.1 As the particles are downsized to single atoms, the obtained single-atom catalysts (SACs)
3012 Chem 5, 3006–3019, December 12, 2019 ª 2019 Elsevier Inc.
have the maximum atom efficiency and are strongly preferred for catalysis-related reactions.2 However, the SACs suffer from the high tendency to self-aggregate into nanoparticles (NPs) because of high surface energy. To this end, gigantic efforts are devoted to strengthening the stability of SACs. The effective methods presented can be summarized as
1Key
Laboratory of the Colloid and Interface Chemistry, Ministry of Education and School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, Shandong, China
2Institute
of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2019.11.013
Figure 1. The Effect of ILs on the Stability and Activity of Pt1@HAP and Pd1/HAP SACs in Catalytic Reactions (A) HAADF-STEM image of 0.2 Pt 1 @HAP. (B) Activity of 0.2Pt 1 @HAP and ILs-0.2Pt 1 @HAP. (C) Arrhenius plots of reactions over 0.2Pt 1 @HAP and BmimTf 2 N-0.2Pt 1 @HAP. (D) The energy profiles of the dimerization process for two Pt atoms on HAP surface without (black) and with (orange) [Bmim][BF 4 ]. (E) The acetylene conversion and ethylene selectivity over 0.02Pd 1 @HAP and BmimBF 4 -0.02Pd 1 @HAP catalysts.
follows: constructing surface defects on supports as the anchoring sites for SAs to promote their connection,3 spatially confining metal SAs within microporous supports,4,5 and introducing atoms with lone pairs of electrons onto the support.6 However, these avenues always require special supports or particular synthetic conditions and thus, to some extent, can’t be applied broadly. Ionic liquids (ILs) have early been demonstrated to efficiently improve the stability of metal NPs7 such as Pd, Ir, and transition-metal NPs. Furthermore, the catalytic properties can be tuned by the application of a layer of
ILs outside the support to immobilize homogeneous catalysts.8 What will happen after the modification of ILs to the SACs? Until now, there has been no document about the application of ILs to enhance the stability and even the activity of SACs. In this issue of Chem, Ding et al. have reported the first example of ILinduced electrostatic stabilization to enhance the stability of Pt SAs dispersed on hydroxyapatite (Pt1@HAP) without compromising hydrogenation activity.9 The modification by ILs not only influences the stability of Pt1@HAP, but also affects its catalytic performance.
As shown in Figure 1A, the single-atom Pt1 catalysts adopted can well be fabricated using (1,5-Cyclooctadiene)dimethylplatinum(II) (PtCODMe2) as the precursor and HAP as the support by a facile impregnation process. The obtained 0.2Pt1@HAP catalyst with 0.2 wt% Pt loading is chosen as the example to systematically characterize the dispersion state of Pt before and after hydrogenation reaction and examine the effect of ILs on Pt1 SAs. In the connection, three ILs are used as the modifier to attain IL-0.2Pt1@HAP, bearing 1-n-butyl-3-methylimidazolium [Bmim+] cation combined with non-coordinating anions, either tetrafluoroborate
Chem 5, 3006–3019, December 12, 2019 3013
[BF4-], bis(trifluoromethanesulfonyl)imide [Tf2N ], or trifluoromethanesulfonate [CF3SO3 ]. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) confirms the atomic dispersion of Pt on HAP. According to the Fourier and wavelet transform spectra derived from X-ray absorption spectroscopy (XAS), the Pt-O contribution dominates instead of Pt-Pt contribution further indicating the dominant presence of single-atom Pt species on HAP. Both normalized XANES results and the detailed in situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) suggest that, after modification by ILs, the electronic properties of single-atom Pt are slightly adjusted and the Pt oxidation state decreases, favoring the enhancement of turnover frequency (TOF).8 To verify the promotional effect of ILs on the stability of Pt SACs, the authors tested the BmimTf2N-0.2Pt1@HAP and 0.2Pt1@HAP in propylene hydrogenation or H2 at 90 C for 1 h. Characterization results show that BmimTf2N0.2Pt1@HAP offers higher stabilization against the aggregation of singleatom Pt than 0.2Pt1@HAP. Furthermore, ILs exert beneficial action on the activity of Pt SACs. 0.2Pt1@HAP shows a TOF of only 8 h 1, whereas the TOF increases substantially to 35 h 1 for BmimBF4-0.2Pt1@HAP, 67 h 1 for BmimTf2N-0.2Pt1@HAP, and 81 h 1 for BmimCF3SO3-0.2Pt1@HAP under the same condition. The IL stabilization strategy developed by the authors has good workability and versatility. It also finds its application in stabilizing the singleatom Pt1 on other supports including CeO2, rutile TiO2, and monoclinic ZrO2 as well as other metal SACs such
3014 Chem 5, 3006–3019, December 12, 2019
as Pd1@HAP. The BmimBF4-Pd1@HAP exhibits higher activity and selectivity than Pd1@HAP in the semi-hydrogenation of acetylene. Even in a >90 h onstream test, the acetylene conversion only drops to 92%, accompanied by a high ethylene selectivity of > 75%, versus the conversion of 92% degraded rapidly within 17 h over Pd1@HAP. In conclusion, in this issue of Chem, the work of Ding et al. nicely opens up an adaptable route to stabilize SACs via the coating of ILs. It’s demonstrated that the ILs can provide sufficient protection via the electrostatic interaction to the isolated metal atoms such as Pt and Pd SAs on HAP or other supports, which is considerably and quantitatively verified by the density functional theory (DFT) calculation. With this enlightening work, more explorations about SACs will be encouraged, involving the rational enhancement of stability of SACs by ILs and the exploitation of new stabilization methodologies. In this study, the authors also deploy the DFT simulation to quantify the electrostatic stabilization effect of ILs on the Pd SACs. Considering the formation of a Pt dimer from two isolated Pt1 SAs on HAP, Pt2-BF4@HAP must weaken both Pt-BF4 interaction and Pt-O bonds with an activation energy of 0.72 eV, 6 times higher than 0.11 eV for the bare 0.2Pt1@HAP. The results shed some insights from the molecular level on the fundamentals of ILs protecting the isolated Pt atoms. Specifically, the anions mainly function in stabilization and electronic modulation by direct interacting with Pt1 species on the support, while the peripheral cations balance the charge and possibly offer additional steric inhibition.10
ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (21971145 and 21575137). 1. Liu, Y., Li, Z., Yu, Q., Chen, Y., Chai, Z., Zhao, G., Liu, S., Cheong, W.-C., Pan, Y., Zhang, Q., et al. (2019). A general strategy for fabricating isolated single metal atomic site catalysts in Y zeolite. J. Am. Chem. Soc. 141, 9305–9311. 2. Gu, J., Hsu, C.-S., Bai, L., Chen, H.M., and Hu, X. (2019). Atomically dispersed Fe3+ sites catalyze efficient CO2 electroreduction to CO. Science 364, 1091–1094. 3. Chen, Y., Ji, S., Chen, C., Peng, Q., Wang, D., and Li, Y. (2018). Single-atom catalysts: synthetic strategies and electrochemical applications. Joule 2, 1242–1264. 4. Liu, L., Dı´az, U., Arenal, R., Agostini, G., Concepcio´n, P., and Corma, A. (2017). Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D. Nat. Mater. 16, 132–138. 5. He, T., Chen, S., Ni, B., Gong, Y., Wu, Z., Song, L., Gu, L., Hu, W., and Wang, X. (2018). Zirconium-porphyrin-based metal-organic framework hollow nanotubes for immobilization of noble-metal single atoms. Angew. Chem. Int. Ed. Engl. 57, 3493–3498. 6. Li, H., Wang, L., Dai, Y., Pu, Z., Lao, Z., Chen, Y., Wang, M., Zheng, X., Zhu, J., Zhang, W., et al. (2018). Synergetic interaction between neighbouring platinum monomers in CO2 hydrogenation. Nat. Nanotechnol. 13, 411–417. 7. Dupont, J., and Scholten, J.D. (2010). On the structural and surface properties of transition-metal nanoparticles in ionic liquids. Chem. Soc. Rev. 39, 1780–1804. 8. Babucci, M., Fang, C.-Y., Hoffman, A.S., Bare, S.R., Gates, B.C., and Uzun, A. (2017). Tuning the selectivity of single-site supported metal catalysts with ionic liquids. ACS Catal. 7, 6969–6972. 9. Ding, S., Guo, Y., Hu¨lsey, M.J., Zhang, B., Asakura, H., Liu, L., Han, Y., Gao, M., Hasegawa, J.-Y., Qiao, B., et al. (2019). Electrostatic stabilization of single-atom catalysts by ionic liquids. Chem 5, this issue, 3207–3219. 10. Scho¨ttle, C., Guan, E., Okrut, A., GrossoGiordano, N.A., Palermo, A., Solovyov, A., Gates, B.C., and Katz, A. (2019). Bulky calixarene ligands stabilize supported iridium pair-site catalysts. J. Am. Chem. Soc. 141, 4010–4015.
coupling that could lead to a difference in sign between short- and long-range couplings. In summary, the work of Chuang et al. highlights the sensitivity of optical experiments combined with theoretical modeling as probes of molecular packing in supramolecular structures on the path to designing functional optical materials. 1. Chuang, C., Bennett, D.I.G., Caram, J.R., Aspuru-Guzik, A., Bawindi, M.G., and Cao, J. (2019). Generalized Kasha’s model: tdependent spectroscopy reveals shortrange structures of two-dimensional excitonic systems. Chem 5, this issue, 3135– 3150. 2. Gu¨nther, L.M., Lo¨hner, A., Reiher, C., Kunsel, T., Jansen, T.L.C., Tank, M., Bryant, D.A., Knoester, J., and Ko¨hler, J. (2018). Structural variations in chlorosomes from wild-type and a bchQR mutant of Chlorobaculum tepidum revealed by single-molecule spectroscopy. J. Phys. Chem. B 122, 6712–6723. 3. Lo¨hner, A., Kunsel, T., Ro¨hr, M.I.S., Jansen, T.L.C., Sengupta, S., Wu¨rthner, F., Knoester, J., and Ko¨hler, J. (2019). Spectral and structural variations of biomimetic lightharvesting nanotubes. J. Phys. Chem. Lett. 10, 2715–2724. 4. Li, X., Buda, F., de Groot, H.J., and Sevink, G.J.A. (2019). Molecular insight in the
optical response of tubular chlorosomal assemblies. J. Phys. Chem. C 123, 16462– 16478. 5. Kirstein, S., and Daehne, S. (2006). J-aggregates of amphiphilic cyanine dyes: self-organization of artificial light harvesting complexes. Int. J. Photoenergy 2006, 20363. 6. Dijkstra, A.G., Duan, H.G., Knoester, J., Nelson, K.A., and Cao, J. (2016). How two-dimensional brick layer J-aggregates differ from linear ones: Excitonic properties and line broadening mechanisms. J. Chem. Phys. 144, 134310. 7. Yamaguchi, A., Kometani, N., and Yonezawa, Y. (2006). Spectroscopic properties of the mixed J-aggregate of unsymmetric merocyanine dyes in wide temperature range. Thin Solid Films 513, 125–135. 8. Kriete, B., Lu¨ttig, J., Kunsel, T., Maly´, P., Jansen, T.L.C., Knoester, J., Brixner, T., and Pshenichnikov, M.S. (2019). Interplay between structural hierarchy and exciton diffusion in artificial light harvesting. Nat. Commun. 10, 4615. 9. Kasha, M. (1963). Energy transfer mechanisms and the molecular exciton model for molecular aggregates. Radiat. Res. 20, 55–70. 10. Heijs, D.J., Malyshev, V.A., and Knoester, J. (2005). Thermal broadening of the J-band in disordered linear molecular aggregates: a theoretical study. J. Chem. Phys. 123, 144507.
Preview
Single-Atom Catalysts Electrostatically Stabilized by Ionic Liquids Baojuan Xi1 and Xuping Sun2,* The prohibition of single-atom catalysts (SACs) to aggregate is challenging for further investigations. In this issue of Chem, Ding et al. establish a novel and general stabilization strategy based on the electrostatic interaction between ionic liquids and SACs. The supported metal catalysts exhibit excellent activity and/or selectivity in various industrial reactions, such as hydroalkylation, hydrogenation, dehydro-
genation, hydrocracking, isomerization, and so on.1 As the particles are downsized to single atoms, the obtained single-atom catalysts (SACs)
3012 Chem 5, 3006–3019, December 12, 2019 ª 2019 Elsevier Inc.
have the maximum atom efficiency and are strongly preferred for catalysis-related reactions.2 However, the SACs suffer from the high tendency to self-aggregate into nanoparticles (NPs) because of high surface energy. To this end, gigantic efforts are devoted to strengthening the stability of SACs. The effective methods presented can be summarized as
1Key
Laboratory of the Colloid and Interface Chemistry, Ministry of Education and School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, Shandong, China
2Institute
of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2019.11.013
Figure 1. The Effect of ILs on the Stability and Activity of Pt1@HAP and Pd1/HAP SACs in Catalytic Reactions (A) HAADF-STEM image of 0.2 Pt 1 @HAP. (B) Activity of 0.2Pt 1 @HAP and ILs-0.2Pt 1 @HAP. (C) Arrhenius plots of reactions over 0.2Pt 1 @HAP and BmimTf 2 N-0.2Pt 1 @HAP. (D) The energy profiles of the dimerization process for two Pt atoms on HAP surface without (black) and with (orange) [Bmim][BF 4 ]. (E) The acetylene conversion and ethylene selectivity over 0.02Pd 1 @HAP and BmimBF 4 -0.02Pd 1 @HAP catalysts.
follows: constructing surface defects on supports as the anchoring sites for SAs to promote their connection,3 spatially confining metal SAs within microporous supports,4,5 and introducing atoms with lone pairs of electrons onto the support.6 However, these avenues always require special supports or particular synthetic conditions and thus, to some extent, can’t be applied broadly. Ionic liquids (ILs) have early been demonstrated to efficiently improve the stability of metal NPs7 such as Pd, Ir, and transition-metal NPs. Furthermore, the catalytic properties can be tuned by the application of a layer of
ILs outside the support to immobilize homogeneous catalysts.8 What will happen after the modification of ILs to the SACs? Until now, there has been no document about the application of ILs to enhance the stability and even the activity of SACs. In this issue of Chem, Ding et al. have reported the first example of ILinduced electrostatic stabilization to enhance the stability of Pt SAs dispersed on hydroxyapatite (Pt1@HAP) without compromising hydrogenation activity.9 The modification by ILs not only influences the stability of Pt1@HAP, but also affects its catalytic performance.
As shown in Figure 1A, the single-atom Pt1 catalysts adopted can well be fabricated using (1,5-Cyclooctadiene)dimethylplatinum(II) (PtCODMe2) as the precursor and HAP as the support by a facile impregnation process. The obtained 0.2Pt1@HAP catalyst with 0.2 wt% Pt loading is chosen as the example to systematically characterize the dispersion state of Pt before and after hydrogenation reaction and examine the effect of ILs on Pt1 SAs. In the connection, three ILs are used as the modifier to attain IL-0.2Pt1@HAP, bearing 1-n-butyl-3-methylimidazolium [Bmim+] cation combined with non-coordinating anions, either tetrafluoroborate
Chem 5, 3006–3019, December 12, 2019 3013
[BF4-], bis(trifluoromethanesulfonyl)imide [Tf2N ], or trifluoromethanesulfonate [CF3SO3 ]. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) confirms the atomic dispersion of Pt on HAP. According to the Fourier and wavelet transform spectra derived from X-ray absorption spectroscopy (XAS), the Pt-O contribution dominates instead of Pt-Pt contribution further indicating the dominant presence of single-atom Pt species on HAP. Both normalized XANES results and the detailed in situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) suggest that, after modification by ILs, the electronic properties of single-atom Pt are slightly adjusted and the Pt oxidation state decreases, favoring the enhancement of turnover frequency (TOF).8 To verify the promotional effect of ILs on the stability of Pt SACs, the authors tested the BmimTf2N-0.2Pt1@HAP and 0.2Pt1@HAP in propylene hydrogenation or H2 at 90 C for 1 h. Characterization results show that BmimTf2N0.2Pt1@HAP offers higher stabilization against the aggregation of singleatom Pt than 0.2Pt1@HAP. Furthermore, ILs exert beneficial action on the activity of Pt SACs. 0.2Pt1@HAP shows a TOF of only 8 h 1, whereas the TOF increases substantially to 35 h 1 for BmimBF4-0.2Pt1@HAP, 67 h 1 for BmimTf2N-0.2Pt1@HAP, and 81 h 1 for BmimCF3SO3-0.2Pt1@HAP under the same condition. The IL stabilization strategy developed by the authors has good workability and versatility. It also finds its application in stabilizing the singleatom Pt1 on other supports including CeO2, rutile TiO2, and monoclinic ZrO2 as well as other metal SACs such
3014 Chem 5, 3006–3019, December 12, 2019
as Pd1@HAP. The BmimBF4-Pd1@HAP exhibits higher activity and selectivity than Pd1@HAP in the semi-hydrogenation of acetylene. Even in a >90 h onstream test, the acetylene conversion only drops to 92%, accompanied by a high ethylene selectivity of > 75%, versus the conversion of 92% degraded rapidly within 17 h over Pd1@HAP. In conclusion, in this issue of Chem, the work of Ding et al. nicely opens up an adaptable route to stabilize SACs via the coating of ILs. It’s demonstrated that the ILs can provide sufficient protection via the electrostatic interaction to the isolated metal atoms such as Pt and Pd SAs on HAP or other supports, which is considerably and quantitatively verified by the density functional theory (DFT) calculation. With this enlightening work, more explorations about SACs will be encouraged, involving the rational enhancement of stability of SACs by ILs and the exploitation of new stabilization methodologies. In this study, the authors also deploy the DFT simulation to quantify the electrostatic stabilization effect of ILs on the Pd SACs. Considering the formation of a Pt dimer from two isolated Pt1 SAs on HAP, Pt2-BF4@HAP must weaken both Pt-BF4 interaction and Pt-O bonds with an activation energy of 0.72 eV, 6 times higher than 0.11 eV for the bare 0.2Pt1@HAP. The results shed some insights from the molecular level on the fundamentals of ILs protecting the isolated Pt atoms. Specifically, the anions mainly function in stabilization and electronic modulation by direct interacting with Pt1 species on the support, while the peripheral cations balance the charge and possibly offer additional steric inhibition.10
ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (21971145 and 21575137). 1. Liu, Y., Li, Z., Yu, Q., Chen, Y., Chai, Z., Zhao, G., Liu, S., Cheong, W.-C., Pan, Y., Zhang, Q., et al. (2019). A general strategy for fabricating isolated single metal atomic site catalysts in Y zeolite. J. Am. Chem. Soc. 141, 9305–9311. 2. Gu, J., Hsu, C.-S., Bai, L., Chen, H.M., and Hu, X. (2019). Atomically dispersed Fe3+ sites catalyze efficient CO2 electroreduction to CO. Science 364, 1091–1094. 3. Chen, Y., Ji, S., Chen, C., Peng, Q., Wang, D., and Li, Y. (2018). Single-atom catalysts: synthetic strategies and electrochemical applications. Joule 2, 1242–1264. 4. Liu, L., Dı´az, U., Arenal, R., Agostini, G., Concepcio´n, P., and Corma, A. (2017). Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D. Nat. Mater. 16, 132–138. 5. He, T., Chen, S., Ni, B., Gong, Y., Wu, Z., Song, L., Gu, L., Hu, W., and Wang, X. (2018). Zirconium-porphyrin-based metal-organic framework hollow nanotubes for immobilization of noble-metal single atoms. Angew. Chem. Int. Ed. Engl. 57, 3493–3498. 6. Li, H., Wang, L., Dai, Y., Pu, Z., Lao, Z., Chen, Y., Wang, M., Zheng, X., Zhu, J., Zhang, W., et al. (2018). Synergetic interaction between neighbouring platinum monomers in CO2 hydrogenation. Nat. Nanotechnol. 13, 411–417. 7. Dupont, J., and Scholten, J.D. (2010). On the structural and surface properties of transition-metal nanoparticles in ionic liquids. Chem. Soc. Rev. 39, 1780–1804. 8. Babucci, M., Fang, C.-Y., Hoffman, A.S., Bare, S.R., Gates, B.C., and Uzun, A. (2017). Tuning the selectivity of single-site supported metal catalysts with ionic liquids. ACS Catal. 7, 6969–6972. 9. Ding, S., Guo, Y., Hu¨lsey, M.J., Zhang, B., Asakura, H., Liu, L., Han, Y., Gao, M., Hasegawa, J.-Y., Qiao, B., et al. (2019). Electrostatic stabilization of single-atom catalysts by ionic liquids. Chem 5, this issue, 3207–3219. 10. Scho¨ttle, C., Guan, E., Okrut, A., GrossoGiordano, N.A., Palermo, A., Solovyov, A., Gates, B.C., and Katz, A. (2019). Bulky calixarene ligands stabilize supported iridium pair-site catalysts. J. Am. Chem. Soc. 141, 4010–4015.