Acta Physico-Chimica Sinica ›› 2020, Vol. 36 ›› Issue (9): 1911050.

Special Issue: Precise Nanosynthesis

• Feature Article •

### Solid Frustrated Lewis Pairs Constructed on CeO2 for Small-Molecule Activation

Sai Zhang, Mingkai Zhang, Yongquan Qu()

• Received:2019-11-12 Accepted:2020-01-13 Published:2020-02-05
• Contact: Yongquan Qu E-mail:yongquan@mail.xjtu.edu.cn
• Supported by:
the National Natural Science Foundation of China(21872109);the China Postdoctoral Science Foundation(2018T111034);the Cyrus Tang Foundation through the Tang Scholar program. S. Zhang is supported by the Fundamental Research Funds for the Central Universities, China(xjj2018033);the Natural Science Foundation of Shaanxi Province, China(2019JQ-039)

Abstract:

Solid materials containing frustrated Lewis pairs (FLPs) as active sites have attracted much attention due to their ability to activate and transform small molecules. However, it is still highly challenging to precisely construct FLP sites on the surfaces of nanomaterials, thereby limiting the applications of these materials. Nanostructured ceria (CeO2) is commonly employed as a catalyst or functional support, and exhibits both Lewis acid and basic properties as well as abundant and easily regulated surface defects, which originate from the reversible Ce3+/Ce4+ redox pair. When the Lewis acid and base sites of CeO2 are independent of each other, the combined Lewis acid-base sites play a similar role to that of homogeneous FLP sites. Thus, the rich surface properties of nanostructured CeO2 provide significant potential for the construction of solid FLPs.

Herein, we demonstrate that solid FLP sites can be successfully constructed on the surface of CeO2(110) via the regulation of surface defect clusters, which can be used to create new Lewis acid sites composed of two adjacent Ce3+ atoms on the surface. Novel interfacial FLP sites can then be formed by combining these Lewis acid sites with neighboring surface lattice oxygens, which act as Lewis base sites. Porous CeO2 nanorods (PN-CeO2) with boundary surface defects were prepared by a special two-step hydrothermal process, and exhibited remarkable catalytic FLP properties. Hydrogen molecules could be effectively activated on the surface of PN-CeO2 with a low activation energy of 0.17 eV via a heterolytic cleavage process. Hydrogenation of alkenes and alkynes to alkanes could then be realized by the activated hydrogen under mild reaction conditions.

PN-CeO2 nanorods with FLP active sites were also able to activate CO2 molecules effectively. Unlike in other solid FLP sites, CO2 molecule activation was realized via a Lewis base site binding with the C atom while two Lewis acid sites bound the two O atoms, owing to the unique configuration of the FLP sites in PN-CeO2. When combined with the epoxidation of olefins by "isolated" Ce3+ sites in PN-CeO2, the FLP-inspired activated CO2 could be used to transform olefins and CO2 to cyclic carbonates through a selective tandem transformation route. In addition, density functional theory studies indicate that the FLP sites on CeO2(110) can activate the C―H bond of CH4 with activation energies as low as 0.63 eV, which can be attributed to the enhanced acidity and basicity of the FLP sites.

With this improved understanding of solid FLP sites constructed on ceria, we have also been able to summarize the challenges and prospects in this field, including their construction, characterization, and mechanism analysis.

MSC2000:

• O643