(352b) Controlled Metal@Metal Oxide Core-Shell Structures for Selective Heterogeneous Catalysis

Metal@metal oxide core-shell catalysts have attracted significant attention as compared to conventional supported catalysts. These structures facilitate construction of promising heterogeneous catalysts due to: (i) the high concentration of catalytically unique metal/metal oxide interfacial sites; (ii) improved chemical and thermal stability of small metal nanoparticles (NPs) by encapsulation with the metal oxides¹; and (iii) spatially regulated delivery of reactants to the interfacial sites². However, the controlled synthesis of these materials is challenging due to the large lattice mismatch between metal and metal oxide crystal structures, and the poor control over the synthesis of reducible metal oxide shells (such as TiO₂ and CeO₂) at room temperature induced by the highly reactive nature of the precursors, which leads to excessively fast, uncontrolled hydrolysis. We discuss a two-step synthesis method, known as “seeded growth”³, as a feasible approach to overcome these challenges. A layer of ligand or surfactant molecules is introduced on the surface of the metal cores prior to encapsulation with the metal oxide shell. The molecules are able to reduce the large interfacial energy, as well as, minimize the aggregation of the NPs in solution. Pd@TiO₂ is used as a probe system to develop a controlled methodology for synthesizing metal@metal-oxide catalytic structures. The parameters that affect Pd NPs synthesis (such as reaction temperature, the amount of reducing agent and surface ligand) are investigated to achieve controlled Pd NP size and distribution. We find that the ligand coverage on the surface of Pd NPs can be affected by different reaction conditions leading to changes in the core-shell structures. Controlled synthesis of TiO₂ porous shell structures is achieved by varying parameters that affect the sol-gel reaction of titanium alkoxide precursor molecules (TAPs). We show that catalytic structures containing Pd NPs encapsulated within a porous TiO₂ film with small pores lead to high hydrodeoxygenation (HDO) reaction selectivity, while maintaining high catalytic activity. Catalyst selectivity is found to depend on both the presence of Pd-TiO₂ interfacial sites and the pore size of the TiO₂ shell². The insights obtained from this systematic study provide a platform for controlled synthesis of metal@metal oxide core-shell structures with varying metal and metal oxide compositions.

REFERENCES

[1] Gould, T. D., Izar, A., Weimer, A. W., Falconer, J. L. and Medlin, J. W. ACS Catal. 4 (2014), 2714.

[2] J. Zhang, B. Wang, E. Nikolla, and J. W. Medlin, Angew. Chem. Int. Ed. 56 (2017), 6594.

[3] Li, G. & Tang, Z. Nanoscale 6 (2014), 3995.