Bond-Thompson mechanism

The PD method provides a simple and effective route to produce supported Au and Pt catalysts efficient in CO oxidatiom With P25 supported catalysts, the best results were obtained for An at pH 10, while for ZnO supported catalysts toe most active were those prepared with no pH adjustment. On P25, Au is more active than Pt, while Pt provides better resnlts for ZnO. This might be related to toe smaller particle size of Au when supported on P25 and a stronger SMSl effect, which are important factors in the Bond-Thompson mechanism preferred by Au, which requires a significant participation of toe support lattice oxygen. With Pt catalysts, toe dependence on toe support is not so high, reflecting toe preference for a Langmuir-Hinshelwood mechanism, which involves primarily the bare active metal centre. 

The ubiquitous electron was discoveied by J. J. Thompson in 1897 some 25 y after the original work on chemical periodicity by D. I. Mendeleev and Lothar Meyer however, a further 20 y were to pass before G. N. Lewis and then I. Langmuir connected the electron with valency and chemical bonding. Refinements continued via wave mechanics and molecular Orbital theory, and the symbiotic relation between experiment and theory still continues... 

In a recent review, Haruta has reassessed the mechanism of CO oxidation on the basis of the Bond and Thompson and Kung models, i.e. that the CO is activated by adsorption on Au° on the surface of the gold nanoparticles and that dioxygen is activated by the atoms at the periphery between the support and the gold nanocrystals (Fig. 4.7). The atoms at the periphery are proposed by Haruta to be cationic in nature, possibly Au(OH)s or Au(OH) formed by the presence of water vapor that is essential for the observed high activity catalysts. It is clear that the debate will continue for the immediate future. There are two reasons why finding an answer to the key question of the nature of the active site in gold catalysts for CO oxidation. The first is purely scientific... 

Figure 6.10 Mechanism proposed by Bond and Thompson.4 (Reprinted from G.C. Bond and D.T. Thompson, Gold Bull. 33 (2000) 41, with permission from World Gold Council.)...

The mechanisms of Bond and Thompson [2] and Kung and coworkers [5,17,38,39] are similar. They are derived from the picture of an active site consist of Au-OH and metallic Au, as shown in Figure 1. Figure 3 shows our proposed reaction mechanism [5,38]. CO is adsorbed on the Au cation and is inserted into... 

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. 

Fig. 15.3 Mechanism of co oxidation on supported fold catalysts, as proposed by Bond and Thompson [9].

Two mechanisms of CO oxidation reaction among those proposed in the Introduction (Section 15.1) involve reducible oxide supports Haruta s mechanism with fully metallic gold particles (Fig. 15.2) and Bond and Thompson s mechanism involving unreduced gold species at the metal-support interface (Fig. 15.3). 

Bond and Thompson s Mechanism Unreduced Cold at the Interfiice... 

In the mechanism proposed by Bond and Thompson [9] (Fig. 15.3), CO adsorbed on Au reacts with a hydroxy group located on a peripheral unreduced Au " species, to form the carboxylate. The latter in turn reacts with a superoxide O2" ion formed on the oxide support like Haruta, the authors emphasise that the interaction between gold and the support is particularly important, since it dictates the nature of the interface at which the reaction takes place. The hypothesis of the presence... 

---