John P. Gabaldon, Chemical and Nuclear Engineering, Center for Micro-Engineered Materials, 1 University of New Mexico, Albuquerque, NM 87131, Mangesh T. Bore, Chemical & Nuclear Engineering, University of New Mexico, MSC 01 1120, Albuquerque, NM 87131-0001, and Abhaya Datye, Center for Micro-Engineereed Materials, University of New Mexico, 1 University of New Mexico, Albuquerque, NM 87131.
Mesoporous silica provides an ideal substrate to anchor nanoparticles in heterogeneous catalysts. With careful preparation, Au nanoparticles less than 1 nm in diameter can be uniformly distributed within the pore structure of the high surface silica. However, Au sinters readily on silica substrates at 200 ºC – 400 ºC, temperatures where Au is reactive for CO oxidation. One approach to improve the hydrothermal stability of Au is to add transition metal atoms to improve the anchoring to the silica surface. We have used two approaches, modification of the silica framework and silica surface modification. In both cases, we find that the transition metal atoms improve the stability of the Au nanoparticles. The Au particles within the pores do not show the same degree of growth as particles on the surface, which grow to sizes greater than 10 nm at these temperatures. The Au particles within the pores stay small, leading to higher reactivity for these mesoporous silica based catalysts. We find that surface functionalization is more effective than incorporating heteroatoms within the silica framework. Our current work is focused on understanding the mechanisms by which the transition metal oxide improves the stability of Au nanoparticles. We will contrast the behavior of mesoporous silica with model catalysts where the Au is deposited on the surface of a silica film. The role of factors such as adatom diffusion, adatom emission, and particle migration can be studied using this approach.