CHEMICAL-VAPOR-DEPOSITION AS A NOVEL TECHNIQUE FOR CATALYST PREPARATION - MODELING OF ACTIVE PHASE PROFILES

Chemical vapour deposition (CVD) has great potential for applying metals to catalyst supports and structured catalytic reactors via the gas phase. This technique increases the flexibility in process conditions and decreases the number of steps necessary to obtain a loaded catalyst carrier. In this early stage of research a model has been developed for the mathematical description of the deposition of catalytic material from the gas phase in porous structures. Thus a catalyst pellet and a monolith are infiltrated with NiCl2 which decomposes to Ni at elevated temperatures. On the basis of rate equations and kinetic data for the deposition of ceramic materials and semiconductors reported in the literature, possible Langmuir-Hinshelwood-type kinetic equations for metal deposition have been derived. The simulations indicate that the preparation of catalysts with homogeneous and inhomogeneous active site distributions is possible. Associative adsorption of the active species gives rise to catalysts with active site distributions which vary between a degenerated egg-shell and a well-formed egg-shell. Dissociative adsorption results in egg-shell, egg-white and (nearly) egg-yolk activity profiles. The same considerations are valid for the CVD of metals in monolithic reactors. Application of the concept of a generalized Thiele modulus shows a correlation between the Thiele modulus and the location maximum deposition when dissociative adsorption is assumed. This concept can be a convenient tool in catalyst design.