| Metal particles deposited on oxide supports are used extensively as heterogeneous catalysts. By using a suitable combination of active metal phases and supports, the catalysts are designed for high activity, selectivity and mechanical strength. However, catalysts undergo deactivation, with poisoning, fouling, sintering and volatilization being some of the common reasons for loss of catalyst activity. For supported metal catalysts, sintering of metal particles is a major cause of catalyst deactivation. The rate and extent of sintering of supported metals depends upon temperature, atmosphere, support, promoter and metal. It is known that gold nanoparticles show high reactivity for CO oxidation at low temperature, but only when the Au particles are very small (<5 nm). Gold nanoparticles supported on silica show rapid sintering at 200°C--400°C. Porosity of support could play an important role in controlling the sintering of metal particles. But the role of pore size, pore curvature and structure is difficult to study with conventional supported metal catalysts. Surfactant templated mesoporous silica is a promising support material since it provides well defined pores of uniform size and structure. Hence, these silica supports provide ideal model systems for control of nanoparticle sintering. Limitations of mesoporous silica are its low hydrothermal stability at elevated temperatures and its inert nature. The pores of mesoporous silica reportedly collapse at temperatures above 500°C and gold nanoparticles supported on reducible oxides such as TiO2, CO3O4 and Fe2O 3 are more active compared to pure silica for CO oxidation.; In this work highly dispersed gold nanoparticles (<2 nm) were prepared within the pores of silica with pore sizes ranging from 2.2 nm to 6.5 nm and differing pore architecture (2D-hexagonal, 3D-hexagonal, cubic and pores coiled-up in spherical geometry). In the 2D-hexagonal pore structure, the pores are one dimensional and terminate on the particle surface. In such a pore structure, metal particles within a given pore can only interact with other particles in the same pore. In contrast, the 3D-hexagonal and cubic pore structures have interconnected pores, so the metal particles can interact with particles in neighboring pores. Curved pores in spherical, aerosol-derived silica provide a unique geometry with the particles protected inside thin silica walls which allow transport of gas phase molecules but retain the nanoparticles. After sintering studies it was found that the average Au particle size exceeded the diameter of the silica pores. (Abstract shortened by UMI.)... |
