Two Novel Case Studies of the Crucial Role of Heterogeneous Catalyst Supports: Core@Shell Nanostructure and Photocatalysis.

Abstract

Catalytic fuel reforming and photocatalysis are two promising technologies for reducing the use of carbon-based fuels. Each has well-known performance limitations, but novel catalyst support engineering strategies may offer effective solutions. Metal particle growth is the key problem for Ni-based reforming catalysts. Nickel@Silica (core@shell) nanostructured materials, having characteristic resistance to metal nanoparticle growth, were synthesized in a water-in-oil microemulsion template. Both nanospheres and nanotubes could be synthesized and were characterized by TEM, hydrogen chemisorption, and nitrogen physisorption. Ni@Silica nanotubes had tunable lengths of up to 2 microns, tunable shell thicknesses of 5.1 to 12.4 nanometers, and uniform cavity diameters. Ni@Silica nanostructures had stable hydrogen selectivity during propane autothermal reforming experiments, whereas impregnated Ni/Silica deactivated continuously. TGA and TEM showed that Ni@Silica materials resisted particle growth and carbon deposition, but also experienced shell sintering with unknown long-term consequences for performance. Interestingly, size selectivity characteristics were indicated by the relatively higher hydrogen, higher carbon dioxide, and lower carbon monoxide selectivities observed with Ni@Silica catalysts. These results demonstrate core@shell nanostructured catalysts’ major promise for reforming applications, especially if other shell materials less prone to sintering are utilized, such as Alumina and Zirconia.

The primary limitation in photocatalysis is low photo efficiency caused by charge carrier recombination. The deposition of Au nanoparticles may improve the photo efficiency of Titania at elevated temperatures by decreasing the rate of recombination in Titania and by causing an electrochemical promotion of Au catalytic activity. Titania prepared by flame-spray pyrolysis and Au/Titania prepared by photodeposition were evaluated for photocatalytic ethylene oxidation in a novel quartz plate reactor and in-situ DRIFTS. Photochemical promotion of Au was not observed due to a loss of surface hydroxyl species and Au particle growth at elevated temperatures. A new photocatalytic reaction pathway on Titania was discovered, enhanced by Au nanoparticles in the presence of water and achieving a maximum rate at unprecedented temperatures. Other reactions that may be catalyzed by its chemistry should be investigated. Moreover, photocatalytic promotion of catalysis may be achieved by stabilizing a high metal dispersion in close contact with the semiconductor, such as with a Au@Titania nanostructure.