Fabrication and Characterization of Au/TiO2 Catalysts for Low Temperature CO Oxidation

Abstract

CO oxidation is the reaction between carbon monoxide and oxygen forming carbon dioxide, as described by 2CO + O2 → 2CO2. Gold (Au) being chemically highly inert is not expected to be a good catalyst but exhibits an unexpectedly high activity towards CO oxidation even at room temperature. Au nanoparticles (NPs) with a diameter of 3 nm supported on TiO2 exhibit the highest activity towards CO oxidation. In this work, a method of fabricating such a catalyst in a clean room facility, relying on solid state dewetting, was developed. AuNPs were be formed by annealing 5 Å thin films of Au deposited on ~10 nm films of TiO2 produced by reactive sputtering onto fused silica. The performance towards CO oxidation was evaluated by employing mass spectrometry in combination with a gas reactor to measure the production of CO2. Comparing the CO2 signal pre- and post annealing, an increase of one order of magnitude was observed which could be attributed to the formation of highly active AuNPs. The influence of varying parameters, such as annealing time and temperature, were investigated. It was found that the activity of the catalyst greatly depends on the temperature at which the reaction is carried out and the annealing time. Short annealing times (900 s) was preferred. The catalyst suffered from CO poisoning at reaction temperatures of 40, 100 and 130 ◦C, but the effects of this was reduced at 100 ◦C and almost disappeared completely at 130 ◦C. There was no clear trend between annealing temperature and activity, although temperatures below 450 ◦C seem to be favorable. The localized surface plasmon resonance (LSPR) of the AuNPs was studied in-operando, finding a correlation between spectral shift and activity, most likely as a result of the formation of surface oxygen vacancies. To gain a better understanding of the morphology of the samples, scanning electron microscopy (SEM) was used, finding that particles were randomly dispersed. The portion of particles exhibiting high activity towards CO Oxidation, meaning being close to 3 nm in diameter, was only 22%. It is unclear how the distribution changes under different annealing conditions. The surface chemistry was studied using X-ray photoelectron spectroscopy (XPS), finding that annealing in an oxidizing environment results in a chemical shift of Ti. This could be linked to a decreased catalytic activity caused by the loss of surface oxygen vacancies in the TiO2 support.