Distribution of La and O Vacancies in Bulk and Surface Models of Ceria

Restricted (Penn State Only)
Author:
Akkullu, Monica Roy
Graduate Program:
Chemical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
December 15, 2016
Committee Members:
  • Michael Janik, Thesis Advisor
  • Kristen Fichthorn, Committee Member
  • Xueyi Zhang, Committee Member
Keywords:
  • Lanthanum doped Ceria
  • Ceria
  • DFT plus U
  • Dopant and O vacancy distribution
Abstract:
Dry reforming of methane is a significant alternative energy technology that can valorize two major greenhouse gases, methane and carbon dioxide, by converting them to synthesis gas. One of the major challenges involved in making dry reforming of methane a commercially viable process is to design a catalyst that would have high activity, selectivity, sulfur tolerance, and resistance to coking. Preliminary experimental and density functional theory (DFT) studies have shown that transition metal (TM) doped rare earth oxides (REOs) can provide better coking resistance and sulfur tolerance than conventional reforming catalysts by forming oxysulfide surfaces. Further atomistic scale understanding of these catalytic systems for dry reforming of methane calls for a thorough understanding of dopant and doping induced O vacancy distribution in the REO, to develop a reasonable model of the TM-doped REO. This thesis utilizes DFT to study the distribution of La and O vacancies in bulk and surface models CeO2. The La dopant and O vacancy preferences have been studied in the bulk by low and high level doping of La atoms in the CeO2 fluorite supercell. We also consider La and O vacancy distribution on a 5 layered CeO2 (111) surface slab. The energies and the probabilities for different possible configurations have been evaluated and used to identify the most stable arrangements of La and O vacancies in CeO2 bulk and surfaces. Future reactivity studies using the most stable surface model developed from this study are discussed.