| A novel approach for directly adding precise numbers of metal atoms in submonolayer amounts to the surface of micron-sized particles (catalysts or support particles) using a new apparatus of original construction that combines an Atomic Beam Deposition (ABD) system with a Temporal Analysis of Products (TAP-2) reactor system is presented. This technique provides a method to "atomically tailor" any catalyst surface to improve its performance. The method involves: (1) direct deposition of atomic species in precise known amounts, (2) well-defined, reproducible thermal processes to kinetically manipulate surface reactions, and (3) precise kinetic characterization of each catalyst state, which can be directly related to the intrinsic properties of the catalyst.; The new approach is illustrated by the example of CO oxidation over a series of Pd/PdO catalysts. Catalysts of different loadings were prepared by direct atomic deposition of Pd and PdO in monolayer and submonolayer amounts on 250-micron silica particles. Preliminary characterization of the Pd deposits was performed using XPS, SEM and TEM. Precise kinetic characterization, was performed using pulse-response methods, temperature programmed reaction (TPR) and steady-state experiments. CO2 production during TPR experiments exhibited oscillatory behavior, which was attributed to structure forming reactions. Based on TPR and adsorption data a detailed mechanism of the reaction and a qualitative model of the evolution of catalytic structure under the influence of a reactive media were proposed. CO oxidation over monolayer Pd/PdO catalysts provokes a self-assembly process, and Pd clusterization along with the generation of CO2. The results provide an interesting and surprising kinetic map of the evolution of Pd reactivity. |
