ATOMIC LAYER EPITAXY (ALE) ON POROUS SUBSTRATES

Atomic layer epitaxy (ALE), commonly used for growing single crystals and thin films, has been applied to process catalysts on porous high s urface area substrates. Unlike many compound growth methods ALE growth is not controlled by the dose of the reactant but rather by the surfa ce itself. The self-controlling feature of ALE allows the growth of co mpounds on porous, heterogeneous surfaces as well. The desired surface structures are formed in the chemisorption reactions, and no heat tre atments afterwards are needed. The basic ALE reactions with porous hig h surface area substrates, including the chemisorption and surface sat uration, will be presented. The surface densities of metal compounds o n alumina and silica can be controlled by various means. The number of bonding sites can be regulated by heat treatment or by using reactant s able to block selected bonding sites before binding the active metal species. The reaction temperature can sometimes be used for controlli ng the densities of the metal compounds. The size of the reactant mole cule or its chemical character may control the saturation level obtain ed. The growth of compound layers can be used to increase the metal co ncentration in the catalyst. In addition to the binding of one metal c ompound on the surface, two or several metal compounds can be bound on the substrates in a controlled way.