Catalysis by Design - Theoretical and Experimental Studies of Model Catalysts

The development of new catalytic materials is still dominated by trial and error methods, even though the experimental and theoretical bases for their characterization have improved dramatically in recent years. Although it has been successful, the empirical development of catalytic materials is time consuming and expensive with no guarantee of success. We have been exploring computationally complex but experimentally simple systems to establish a “catalysis by design” protocol that combines the power of theory and experiment. We hope to translate the fundamental insights directly into a complete catalyst system that is technologically relevant. The essential component of this approach is that the catalysts are iteratively examined by both theoretical and experimental methods. This approach involves state-of-the-art first principle density functional theory calculations, experimental design of catalyst sites, and sub-Ångström resolution imaging with an aberration-corrected electron microscope to characterize the microstructure.

We are employing this approach to understand the catalytic sites in oxidation and lean NO_x catalysts. The model material for the oxidation catalyst system is Pt/Al₂O₃ and that for lean NO_x catalysis is Ag/Al₂O₃. We present our initial results on theoretical and experimental studies of the oxidation and reactivity of catalyst clusters towards O, CO, and NO_x. Our theoretical studies indicate that the reaction energetics are strongly dependent on the size of the clusters as well as the extent of oxidation of the clusters. We speculate that the energetics of CO and NO oxidation may be more favorable on the oxidized clusters than on the pure Pt clusters because of the weakened adsorption of the reactants. Experimentally, we have synthesized supported catalysts that contain small metal particles that mimic the theoretical models. We have also synthesized various supported catalysts with larger metal particles. We have studied CO oxidations on these catalysts and the results (including microstructural changes in Pt particles) will also be presented. In addition, we will summarize the results of our study of microstructural changes in supported catalysts, especially Ag/Al₂O₃, exposed to simulated lean NO_x exhaust via the ORNL ex-situ reactor in order to determine the impact of operating conditions on the catalyst.