Date

2022-1-20

Author

Yarar, Melis

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In this work, the role of Pd loading on the Pd morphology and subsequent reactivity of Pd/TiO2 catalyst for CO oxidation and titania surface reduction were investigated. 2D patches of Pd metal were identified on titania surface at low loadings (≤2%). 2D patches give rise to mild-temperature and low pressure reduction of titania surface. In the presence of 2D Pd room temperature reduction was studied with quantitative Temperature Programmed Reduction (TPR) measurement which revealed that 70% of titania surface on 1%Pd/TiO2 was reduced at 300K, owing to Pd acting as a reduction promoter. Pd metal phase change from 2D to 3D with increasing Pd amount was identified by quantitative TPR analysis and HR-TEM measurements. Rate of room temperature reduction of titania surface by hydrogen spillover process disclosed by TPR analyses was found to be directly correlated with the surface area of Pd structures for ≤2% Pd loaded samples. Consequently for 2D particles, atomic hydrogen exhange between metal and support was taking place all through the surface area of metal. Paramagnetic centers formed during hydrogen exposure was monitored with in-situ room-temperature Electron Spin Resonance (ESR) Spectroscopy technique. The oxygen vacancies and Ti+3 species was only detected on low Pd loadings and under vacuum condition. The dependency of ESR signal on pressure was studied with saturation-recovery continous wave ESR experiments which revealed regulation of spin-lattice relaxation time with pressure decrease. Hydrogen spillover process was monitored with operando Nuclear Magnetic Resonance (NMR) spectroscopy and it was modelled using hydroxyl signal growth rate. CO oxidation measurements were conducted to reveal changes in catalytic activity by nanoscale nature of the Pd/TiO2 catalyst. In particular, increase in Pd loading in 2D region increased CO conversion substantially while increasing Pd loading when Pd is 3D no significant change occurred. On the other hand, at high temperature hydrogen reduction was found to led to phase change and Magnéli phase production. The phase boundary between Ti11O21 and anatase or rutile phases was found to be able to accommodate hydrogen and release when the temperature was changed.

Subject Keywords

titanium dioxide, 2D materials, electron spin resonance spectroscopy, magneli phase

URI

https://hdl.handle.net/11511/101188

Collections

Graduate School of Natural and Applied Sciences, Thesis