Low Temperature Oxidation of CO and Hydrocarbons over Supported Gold Catalysts

Gold 2003
Veronique PITCHON, Veronique PITCHON, Corinne PETIT, Renaud COUSIN, Svetlana IVANOVA,

Until recently, gold has been considered as one of the least catalytically useful metal because of its chemical inertness and the difficulty to obtain a high dispersion on common support materials. However, in numerous works, it has been widely proved that it is possible to prepare gold nanoparticles deposited on metal-oxide supports and, in such conditions, gold exhibits high catalytic activity towards low temperature CO oxidation. The main objectives of this project are to characterize gold catalysts in terms of activity, selectivity and structure for the oxidation of carbon monoxide and hydrocarbons for low temperature applications.

Based upon a literature overview, we have developed a gold catalyst preparation by deposition-precipitation on wide range of supports (Al2O3, TiO2, CeO2, ZrO2, MgO) in order to do a screening of those formulations in the oxidation of carbon monoxide. In a first part, we have shown that gold over ceria catalyst seems to be the most effective for the CO oxidation. We have confirmed that Au/CeO2 is a potentially good catalyst for oxidation reactions. CO is converted below room temperature. The reason of the good activity lies in the fact that small particles are obtained by the HDP mode of preparation of Au/CeO2. Although this particles are not observable by TEM, this assumption is indirectly confirmed by the correlation obtained on the other catalysts between size and activity, the sequence of activity for CO oxidation being CeO2 > Al2O3 > TiO2 > ZrO2 > MgO. Also H2 TPR allows a good correlation between size and activity, but reveals itself complex due to the partial reduction of the support. The fraction of ceria reduced is always the same and equal to c.a. 30%

The conversion of propane appears to be more complicated : on a fresh catalyst, there are two zones of activity i.e. at T<300° with a conversion of c.a. 50% and then between 300 and 500°C, where the conversion is complete. The activity is enhanced in the presence of CO but the two zones of temperature remain. With an ageing treatment (air + 5% water at 600°C) a moderate deactivation is observed for both CO and propane conversion. Another indirect evidence is given by the fact that the activity decreases when the metal content is increased, i.e. when the probability of having larger particles is higher. Nevertheless, the electronic factor could also be invoked to explain the better activity. TPR has shown that there is indeed an effect of the interaction of the gold particles on the reductibility of ceria.

In the last, the different catalysts have been tested for several hydrocarbons (CH4, C2H4 and C2H2) oxidation, the results are shown in the figures below; The results will be discussed in terms of carbon bond actication, and structure sensitivity.

Methane oxidation

Ethylene oxidation

Acetylene oxidation

Propane oxidation

Keywords: CO oxidation, Gold Catalysis, Hydrocarbons oxidation
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