New Materials for Environmental Applications:Thermoelectrics and Low-Temperature Oxidation Catalysts

Doctoral thesis, 2007

New materials have been researched for environmental applications as thermoelectrics and low-temperature oxidation catalysts. One of the main objectives of this thesis was to prepare and characterize new efficient thermoelectric materials (TM’s). Large single crystals of the phonon-glass electron-crystal (PGEC) material clathrate Ba8Ga16Ge30 was grown for the first time using the Czochralski method. For n-doped clathrate Ba8Ga16Ge30, thermoelectric evaluations showed a record high figure-of-merit, ZT above 800 K and evidence of a transition from extrinsic to intrinsic semiconductor behaviour with increasing temperature. Furthermore, a series of seven samples of YMn4+xAl8-x (-2.0 £ x £ 2.5) was prepared and evaluated for thermoelectric performance. Evaluation of the YMn4+xAl8-x samples showed that structurally ordered YMn4Al8 exhibits metallic conductivity, whereas off-stoichiometric compounds displayed semiconductor-like behavior at low temperatures. The second objective of this thesis was to evaluate new catalyst materials for low-temperature oxidation reactions. Hexagonal phase mesoporous Pt particles (H1-Pt) expose mainly concave surfaces with expected different catalytic properties compared to conventional Pt particles with convex surfaces. A series of Pt/Al2O3 catalysts was prepared using H1-Pt, Pt-black and wet-impregnated Pt. The catalysts were evaluated for CO oxidation. The H1-Pt/Al2O3 catalyst appeared less sensitive to CO poisoning but more sensitive to mass-transport limitations than the other two catalysts. Finally, a study of catalysts for selective CO oxidation in a hydrogen-rich feed gas stream was performed in a high throughput screening system. The study showed that Pt/Al2O3 and Rh/Al2O3 catalysts were most active and selective for CO oxidation in the presence of H2.