Cold Nanoparticles Recent Advances in CO Oxidation

The oxidation of carbon monoxide at around room temperature is the most famous reaction known for gold catalysts. Haruta s group discovered in 1987 [1, 2] that gold is a unique catalyst for this reaction when gold metal particles are smaller than 5 nm and supported on oxides. Since then, extensive and intensive fundamental works have been published, and expanding new applications, from air purification (gas masks, gas sensors, indoor air quality control) to hydrogen purification for fuel cells (PROX, preferential selective oxidation of CO in the presence of Hj) have been developed. 

High activity in CO oxidation when gold particles are smaller than 5 nm, and supported on reducible oxides, such as iron oxide or titania, led Haruta et al. [3, 5-8] to propose the first mechanism of CO oxidation (Fig. 15.2) the reaction takes place at the interface between the gold metal particle and the oxide support, i.e., between CO adsorbed on the gold particles and O2 activated by the oxide support Later, based on the assumption that metal cations could be located at the metal-support interface. Bond and Thompson [9] proposed a mechanism rather similar. 

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On the other hand, the study of CO oxidation performed by Goodman et al. [10] on a Au/TiO2(110) model catalyst, led them to propose that the whole reaction takes place on the gold particles, providing that the latter are small and bi-dimensional they do not exhibit metallic properties any longer due to quantum size effect. 

According to these authors, the role of the support would only be to stabilise the small gold particles. Finally Kung et al. [11, 12] explained the activity of gold on alumina, although weaker than gold on titania, by another mechanism based on the presence of Au species at the gold-support interface and on a reaction between O2, which dissociatively adsorbs on Au , and CO, which reacts with Au -OH species (Fig. 15.4). 

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I 75 Cold Nanoparticles Recent Advances in CO Oxidation 15.5.2.2 Kung s Mechanism... 

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