Single-Atom Catalysts Electrostatically Stabilized by Ionic Liquids

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 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, dehydrogenation, hydrocracking, isomerization, and so on.1Liu Y. Li Z. Yu Q. Chen Y. Chai Z. Zhao G. Liu S. Cheong W.-C. Pan Y. Zhang Q. et al.A general strategy for fabricating isolated single metal atomic site catalysts in Y zeolite.J. Am. Chem. Soc. 2019; 141: 9305-9311Crossref PubMed Scopus (139) Google Scholar As the particles are downsized to single atoms, the obtained single-atom catalysts (SACs) have the maximum atom efficiency and are strongly preferred for catalysis-related reactions.2Gu J. Hsu C.-S. Bai L. Chen H.M. Hu X. Atomically dispersed Fe3+ sites catalyze efficient CO2 electroreduction to CO.Science. 2019; 364: 1091-1094Crossref PubMed Scopus (775) Google Scholar 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 follows: constructing surface defects on supports as the anchoring sites for SAs to promote their connection,3Chen Y. Ji S. Chen C. Peng Q. Wang D. Li Y. Single-atom catalysts: synthetic strategies and electrochemical applications.Joule. 2018; 2: 1242-1264Abstract Full Text Full Text PDF Scopus (1168) Google Scholar spatially confining metal SAs within microporous supports,4Liu L. Díaz U. Arenal R. Agostini G. Concepción P. Corma A. Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D.Nat. Mater. 2017; 16: 132-138Crossref PubMed Scopus (393) Google Scholar,5He T. Chen S. Ni B. Gong Y. Wu Z. Song L. Gu L. Hu W. Wang X. Zirconium-porphyrin-based metal-organic framework hollow nanotubes for immobilization of noble-metal single atoms.Angew. Chem. Int. Ed. Engl. 2018; 57: 3493-3498Crossref PubMed Scopus (258) Google Scholar and introducing atoms with lone pairs of electrons onto the support.6Li H. Wang L. Dai Y. Pu Z. Lao Z. Chen Y. Wang M. Zheng X. Zhu J. Zhang W. et al.Synergetic interaction between neighbouring platinum monomers in CO2 hydrogenation.Nat. Nanotechnol. 2018; 13: 411-417Crossref PubMed Scopus (411) Google Scholar 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 NPs7Dupont J. Scholten J.D. On the structural and surface properties of transition-metal nanoparticles in ionic liquids.Chem. Soc. Rev. 2010; 39: 1780-1804Crossref PubMed Scopus (685) Google Scholar 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.8Babucci M. Fang C.-Y. Hoffman A.S. Bare S.R. Gates B.C. Uzun A. Tuning the selectivity of single-site supported metal catalysts with ionic liquids.ACS Catal. 2017; 7: 6969-6972Crossref Scopus (46) Google Scholar 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 IL-induced electrostatic stabilization to enhance the stability of Pt SAs dispersed on hydroxyapatite (Pt1@HAP) without compromising hydrogenation activity.9Ding S. Guo Y. Hülsey M.J. Zhang B. Asakura H. Liu L. Han Y. Gao M. Hasegawa J.-Y. Qiao B. et al.Electrostatic stabilization of single-atom catalysts by ionic liquids.Chem. 2019; 5: 3207-3219Abstract Full Text Full Text PDF Scopus (88) Google Scholar 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 [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).8Babucci M. Fang C.-Y. Hoffman A.S. Bare S.R. Gates B.C. Uzun A. Tuning the selectivity of single-site supported metal catalysts with ionic liquids.ACS Catal. 2017; 7: 6969-6972Crossref Scopus (46) Google Scholar 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 BmimTf2N-0.2Pt1@HAP offers higher stabilization against the aggregation of single-atom 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 single-atom Pt1 on other supports including CeO2, rutile TiO2, and monoclinic ZrO2 as well as other metal SACs such 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 on-stream 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.10Schöttle C. Guan E. Okrut A. Grosso-Giordano N.A. Palermo A. Solovyov A. Gates B.C. Katz A. Bulky calixarene ligands stabilize supported iridium pair-site catalysts.J. Am. Chem. Soc. 2019; 141: 4010-4015Crossref PubMed Scopus (26) Google Scholar The work was supported by the National Natural Science Foundation of China (21971145 and 21575137). Electrostatic Stabilization of Single-Atom Catalysts by Ionic LiquidsDing et al.ChemNovember 4, 2019In BriefElectrostatic interaction has been demonstrated as a simple and general strategy to protect atomically dispersed metal catalysts. Ionic liquid-stabilized single-atom catalysts (ILSSACs) exhibited considerably enhanced durability and hydrogenation activity for a series of catalysts. Full-Text PDF Open Archive