Materials Science & Engineering
Influence of ZrO2 Nanoligands on the Catalytic Performance of Supported Pt/ZrO2/SiO2 Catalysts
A series of double-supported Pt/ZrO2/SiO2 catalysts were prepared to determine the influence of ZrO2 nanoparticle (NP) domain size on the reactivity of the catalytic active Pt component. The catalysts were synthesized by first impregnating zirconium tert-butoxide, Zr[OC(CH3)3]4, in toluene, drying and then calcining at 500 C to form ZrO2. In a second step, aqueous platinum tetra-ammine nitrate, Pt(NH3)4(NO3)2, was impregnated, dried and calcined in air at 500 C to form the final double-supported Pt/ZrO2/SiO2 catalyst. The ZrO2 loading was varied between 1% and 50% and the Pt loading was maintained constant at 0.1%. In situ Raman and UV-vis spectroscopy and TEM microscopic characterization revealed that the supported ZrO2 phase varied its domain size from isolated surface species to polymeric surface species to NPs (1-3 nm). TEM microscopy revealed that the supported Pt phase was present as 10-70 nm NPs for 1-20% ZrO2/SiO2, where the surface ZrOx species was present (1-12%) and a combination of surface ZrOx species and ZrO2 NPs (15-20%). The Pt NPs, however, were completely absent for higher zirconia loading where ZrO2 NPs are present and reflect the presence of a highly dispersed Pt phase on the ZrO2 NPs. The reactivity of the supported Pt phase was chemically probed with CH3OH oxidation (both steady-state and CH3OH-temperature programmed surface reaction (TPSR) spectroscopy). The reactivity of the methanol oxidation reaction was found to increase with the dimension of the Pt NPs, which also corresponds to lower ZrO2 domain size. Thus, the reactivity of the Pt catalytic active sites could be tuned by the domain size of the ZrO2 nanoligands.
The Role of Nanosilica Dispersion and Particle Size in Hybrid Epoxy-Silica Nanocomposites
Hybrid epoxy-silica nanocomposites (HESN) have been shown to achieve higher fracture energy by adding a small amount of nanosilica with a reactive liquid rubber (CTBN). Previously, we have shown that smaller nanosilica particles (20 nm in diameter) are more effective than larger nanosilica particles (80 nm in diameter). This present study focuses on the effects of rubber particle size and nanosilica dispersion on the toughening mechanism in epoxy based nanocomposites. Core/shell rubber particles consisting of methacrylated butadiene-styrene (MBS) are used to reduce the size of the rubber phase. The fracture toughness and toughening mechanisms have been examined. Interestingly, preliminary results show no substantial toughening synergism in 20HESN/MBS system. Moreover, no considerable nanosilica clustering was detected on the fracture surface. The above observations suggest the importance of nanosilica dispersion status in toughening of HESN and further studies are in progress.
Grain Growth Kinetics in Co-doped Aluminas: ReIation with Grain Boundary Complexions
It is being gradually recognized that there are various types of dopant/impurity segregation induced grain boundary complexions in polycrystalline oxides; starting from an ordered segregation of dopants, going through a series of changes in decreasing crystalline order and finally forming an intergranular glassy film. It is also understood that the formation and stability of these boundary complexions are functions of thermodynamic variables like dopant concentration, grain boundary energy and temperature etc. In a series of grain growth kinetics experiments in our laboratory, ultrahigh purity polycrystalline alumina doped with controlled amounts of Ca, Si, Nd, Y, La etc. has shown discontinuities in the grain boundary mobility that could be linked to the various types of grain boundary complexions observed. In alumina co-doped with Y and Si, the boundary mobility increases by orders of magnitude as compared to alumina doped with Y alone. The present work aims to discuss the outcomes of detailed boundary mobility measurements in alumina co-doped with Cu-Ti, Zr-Si and Y-Si under similar conditions over a wide temperature range and relate the boundary mobility values to various grain boundary complexions.