Cleanup of Biomass Gasification Effluent Using M-doped Ceria Catalysts

Open Access
Author:
Krcha, Matthew David
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 01, 2014
Committee Members:
  • Michael John Janik, Dissertation Advisor
  • Michael John Janik, Committee Chair
  • Robert Martin Rioux Jr., Committee Member
  • Scott Thomas Milner, Committee Member
  • Adrianus C Van Duin, Committee Member
Keywords:
  • Density Functional Theory
  • Catalysis
  • Ceria
  • Dopant
  • Hydrocarbon
  • Sulfur
Abstract:
Biomass conversion to liquid fuels may be accomplished through gasification to syngas followed by fuel synthesis processes, enabling a renewable energy source of liquid fuels. Prior to fuel synthesis catalysis, the syngas must be cleaned of sulfur and tar species. In a Department of Energy forecast for 2012, approximately 50% of the cost to produce ethanol through biomass gasification is involved in syngas cleanup. A tar reforming catalyst that can operate at high gasifier effluent temperatures and tolerate or, ideally act as a sulfur sorbent as well would help to reduce this cost associated with syngas cleanup. Ceria-based mixtures have shown promise in being able to reform hydrocarbons and remove sulfur at high temperatures. This dissertation employs computational chemistry methods to design a ceria-based catalyst that can reform hydrocarbons into CO and H2 as well as remove sulfur at high temperatures, thus making biomass gasification-based processes viable. Density functional theory (DFT+U) is used to generate structure-composition-function relationships for H2S adsorption and hydrocarbon conversion. Transition metals can be doped into ceria to alter its reducibility and hydrocarbon activity. Considering all of the transition metals, we find the methane adsorption energy correlates with the oxygen vacancy formation energy for M-doped CeO2. Using this correlation, the methane conversion rate follows a volcano relationship with surface reducibility. One of the dopants near the top of this volcano relationship is Mn, and to correctly represent the electronic structure of Mn-doped ceria, a correction to the energy of electrons in localized Mn d-states must be used. We utilized Mn-doped ceria as a system of focus to evaluate how this correction parameter, as well as the concentration and structure of Mn in the surface, affects the hydrocarbon activity. Employing the most stable structure of Mn-doped ceria and Zr-doped ceria, the reforming of propane over the surfaces are examined, along with developing an approach to model the complete oxidation of large hydrocarbons over an oxide. To clean up the effluent from biomass gasification, sulfur must also be removed and Mn-doped ceria also shows promise in being able to remove sulfur.