Development of Multi-scale Computational Methods for Modeling Phase Formation in Pd-Based Catalysts

Open Access
Author:
Senftle, Thomas Patrick
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 02, 2015
Committee Members:
  • Michael John Janik, Dissertation Advisor/Co-Advisor
  • Michael John Janik, Committee Chair/Co-Chair
  • Adrianus C Van Duin, Committee Chair/Co-Chair
  • Robert Martin Rioux Jr., Committee Member
  • Scott Thomas Milner, Committee Member
Keywords:
  • Catalysis
  • DFT
  • ReaxFF
  • Palladium
  • Ceria
Abstract:
In Pd/ceria catalysts, mixed Pd-Ce oxide formations at the cluster-support interface offer unique activity toward hydrocarbon activation. Both experimental and computational investigations in the literature suggest that unique Pd-O-Ce surface formations yield highly reactive sites on the catalyst surface. However, structural details and the corresponding reaction mechanisms governing the behavior of such sites are not well understood. As such, this dissertation employs Density Functional Theory (DFT) in tandem with classical ReaxFF modeling to determine the stability and activity of methane activation sites at the Pd/ceria interface. The ReaxFF interaction potential is used to investigate cluster-support interactions at length and time scales inaccessible to quantum methods. In particular, this dissertation develops a hybrid grand canonical Monte Carlo/molecular dynamics (GC-MC/MD) approach that is suited to assess oxide formation in regions where the Pd cluster contacts the underlying ceria support. When coupled with DFT, this multi-scale approach demonstrates that Pd atoms incorporated in the fluorite lattice structure of ceria alternate between Pd4+ and Pd2+ oxidation states during operation. In general, Pd4+ states are effective hydrocarbon activation sites, as the dissociative adsorption of the hydrocarbon reduces Pd4+ to its thermodynamically preferred Pd2+ state. Therefore, maximizing the number of transient Pd4+ states under operating conditions is essential for achieving optimal catalytic performance. The multi-scale simulation methodology developed and applied in this dissertation can evaluate the trade-off between stability and activity exhibited by Pd/CeO2, and can be readily extended to other catalytic systems.