Studies of model systems for the low temperatureoxidation of CO bysupported gold catalysts

• Main Author: Braun, Simon
• Published: University of Manchester 2008
• Subjects: 541.39
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.504722

The preparation and reactivity of a range of model systems for the low temperature oxidation of CO by gold catalysts have been investigated with a view to assessing their feasibility for bridging the 'materials gap' between well-defined single crystalline models and practical supported catalysts. The studied systems included gold powder, electrochemically oxidized gold foil, gold nanoparticles prepared on Ti02 single crystal surfaces by spin-coating and gold nanoparticles prepared on Ti02 powder by deposition-precipitation. Particular attention was paid to the reduction of oxidised gold species, since these are the common precursors in the preparation of practical nanoparticular catalysts. Oxidised gold species may also be mechanistically relevant for the catalytic low-temperature CO oxidation. A low volume reactor was designed for studies of flat gold model catalysts. It was tested with in situ X-ray absorption spectroscopy (XAS). The design of the reactor cell allows the use of further techniques including UVNis spectroscopy, X-ray differaction (XRD), Raman scattering and Infrared (IR) Spectroscopy. The reactor was also designed to permit reaction rate measurements with a mass spectrometer. In comparison to other reactors reported in the literature, this design is very cost efficient, not only in its construction but also due to its compatibility with a wide range of spectroscopic techniques. In situ X-ray photoelectron spectroscopy (XPS) on gold particles supported on Ti02 powder indicated that different atmosphere could change the morphology of the gold particles. This change is more prominent for smaller particles. The formation of graphite like spices under a CO atmosphere was observed as well. X-ray photoelectron spectroscopy (XPS) and in situ Au L-edge XAS indicated that suitable gold nanoparticles could be prepared by spin coating of tetrachloroaurlc acid solutions onto rutile single crystals. Gold loadings as low as 0.1 and 0.03 monolayers could be achieved. The XAS data indicated the presence of low average Au-Au coordination numbers, suggesting the presence of very small nanoparticles. XA spectra taken under CO and O2atmosphere indicated that CO interacts with the gold particles. In situ X-ray absorption near edge structure (XANES) C K-edge studies were carried out on electrochemically oxidised gold foils. When exposed to carbon monoxide at pressures up to 10-2 mbar the anodic oxide layers reduce rapidly. This process is accompanied by the formation of a carbonaceous layer at the surface. XPS studies revealed that the foils were superficially not fully reduced. Au Lm-edge spectra showed that the reduction of the samples proceeds also in oxygen at elevated temperatures, with an ultrathin oxidic layer remaining near the surface, similar to the samples treated with CO at 10-2 mbar pressure. The_catalytic.actiyity_ofuIlsllPported bulk gold, in form of gold powder with particle sizes between 0.5 ~m and 2 ~m, was determined. Pure gold was found to be generally active at temperatures above 570 K. Heating to 700 K in a 1:1 COI02 mixture resulted in a conditioning ---effect,-leading to-higher catalytic activity of the gold powder. An activation energy of 53 kJ/mol for the CO oxidation reaction was determined from the temperature dependence of the reaction. Scanning electron microscopy and atomic force microscopy images revealed mesoscopic surface morphology changes caused by the catalytic, which seems to be associated with the conditioning of the catalyst.