In Situ study of metal-oxide interface for engineering the metal-support interaction

Abstract

Heterogeneous catalysts have gained significant attention due to their substantial contribution to global domestic products, commodity chemicals, and global energy issues. Heterogeneous catalysts have advantages in remarkable stability, ease of separation, and efficient utilization of noble-metal atoms. Noble-metal nanoparticles (NPs) supported on metal oxides are the most widely used form of heterogeneous catalysts. Notably, when metal NPs are employed with reducible metal oxide supports (e.g., TiO2, CeO2, Co3O4, etc.), they exhibit a synergistic effect on catalytic activity, known as metal support interaction (MSI). To design high-performance catalysts, it is highly desirable to develop strategies for engineering the MSI, which can be applied through various methods. Investigating the interface of engineered MSI is crucial to developing insights into catalytic phenomena, which can guide the designing of efficient catalysts. The challenge in understanding the MSI comes from the pressure gap between analyzing pressure and reaction conditions, which show significant differences in Gibbs free energy. This substantial enough pressure gap can induce unexpected surface changes and can affect the catalytic mechanism. In this sense, utilizing in-situ techniques is crucial for in-depth characterization of the interactions at metal and metal-oxide support interface with a molecular level under a realistic environment. Therefore, I engineered MSI by modifying the interface of oxide supported metal NPs and investigated the change using in-situ techniques to reveal the key factors for the catalytic property. In this dissertation, Chapter 1 introduces the background of supported metal NPs for heterogeneous catalysis and MSI. The explanation of in-situ characterization techniques and lab-built ambient pressure X-ray photoelectron spectroscopy (AP-XPS) instrument is also covered. Chapter 2 introduces facet-dependent MSI at Pt NPs supported on cubic and octahedral Cu2O in CO oxidation reaction using transmission electron microscopy, scanning electron microscopy, AP-XPS, and diffusive reflectance infrared Fourier-transform (DRIFT) spectroscopy. The catalytic measurement showed facet-dependent CO oxidation activity for Pt NPs supported on cubic Cu2O and Pt NPs supported on octahedral Cu2O catalysts. Furthermore, in-situ AP-XPS and DRIFT measurement results revealed that the surface changes during the reaction depend on the facet of the support, leading to different catalytic performances. Chapter 3 introduces optimizing the MSI by varying the ratio of Cu and Ga species for CO2 hydrogenation. The CuGa on mesoporous SBA-15 catalysts exhibited enhanced methanol production than monometallic catalysts due to the synergistic interaction with Cu and GaOx. The role of each species was investigated by varying the molar ratio of Cu and Ga. XPS and DRIFT measurements revealed that the Cu+-GaOx interface is an active site where highly dispersed GaOx stabilizes the Cu+ state. Chapter 4 introduces morphology-dependent MSI using rod and plate ZnO with Pt NPs for CO2 hydrogenation reaction. The Pt NP on plate ZnO showed superior reactivity than rod ZnO, resulting from facet-dependent MSI. AP-XPS, DRIFT, and CO2 temperature programmed desorption measurement revealed that the CO2 adsorption capacity differed by the facet of ZnO, which was the key factor for the activity difference. Our strategies of engineering the MSI and unveiling key factors that are significantly involved in the reaction provide insight into designing high-performance catalysts via engineering the interface interaction.