Gold particles, catalyst supports

Titanium dioxide supported gold catalysts exhibit excellent activity for CO oxidation even at temperatures as low as 90 K [1]. The key is the high dispersion of the nanostructured gold particles over the semiconducting Ti02 support. The potential applications of ambient temperature CO oxidation catalysts include air purifier, gas sensor and fuel cell [2]. This work investigates the effects of ozone pretreatment on the performance of Au/Ti02 for CO oxidation. 

The Arrhenius plots of the CO conversion rate in Fig. 2 indicate that the activation energy for the Au/Nano-Ti02 catalysts is nearly zero. Haruta et al. [6] also reported similar observations. They suggest that this occurs when the CO adsorbed on gold particles reacts with adsorbed O2 on the support at the interfacial junction between the two surfaces. 

Figure 1. TEM image of a titania supported gold catalyst (1.7wt.% Au) prepared by deposition-precipitation (gold particle size = 5.3+ 0.3 nm, dispersion = 36%). (Reprinted from Reference [84], 2000, with permission from American Chemical Society).

A hypothesis that edge and corner sites work as active sites can explain why turn over frequency (TOF), which is defined as the reaction rate per one active site, in the case of metal catalysts, per surface exposed metal atom, increases with a decrease in the diameter of gold particles. However, it fails to explain the significant contribution of support materials and the contact structure of gold NPs. It seems to be reasonable that those edges and corners act as the sites for adsorption of one of the reactants, for example, CO in its oxidation. 

Figure 24. Size of metal particles and support pores in silver and gold catalysts for gas-phase propylene epoxidation.

Much more difficult is the interpretation of the behaviour of supported catalysts in the ethane-1,2-diol oxidation. In fact, despite the smaller gold particles, Au/XC72R displayed the worst performance either considering the 2 g preparation or the larger preparations (Table 14). 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

Xin X, Luo G, Zhao R (2005) Advances in preparation and application of supported gold nano-particles catalyst with high catalytic activity. Shiyou Eluagong 34 898-902... 

Carefully prepared Au catalysts have a relatively narrow particle size distribution, giving mean diameters in the range 2-10 nm with a standard deviation of about 30%. A major reason why Au particles remain as NPs even after calcination 573 K is the epitaxial contact of Au NPs with the metal oxide supports. Gold particles always expose its most densely packed plane, the (111) plane, in contact with a-Fe2O3(110), Co304(lll), anatase Ti02(112), and rutile TiO2(110). 

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