Novel Ti based mixed metal oxide systems for adsorptive desulfurization: A combined ab initio and experimental study

Open Access
Author:
Sitamraju, Siddarth
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Education
Document Type:
Dissertation
Date of Defense:
August 29, 2014
Committee Members:
  • Chunshan Song, Dissertation Advisor
  • Chunshan Song, Committee Chair
  • Michael John Janik, Committee Member
  • Xiaoxing Wang, Committee Member
  • Jonathan P Mathews, Special Member
Keywords:
  • DFT
  • Titania
  • Anatase
  • Adsorptive desulfurization
  • Air promoted desulfurization
Abstract:
This work examines the mechanism for the adsorptive desulfurization of liquid hydrocarbon fuels over TiO2 based adsorbents. The adsorbent was doped with several metals for enhanced adsorption performance. The undoped and the doped adsorbents were modeled and analyzed using density functional theory (DFT) calculations to explore structure and functional correlations of these systems. Experimental evidence is also offered to support the theoretical results. The TiO2 based systems were chosen as adsorbent materials because of their adsorption capacity and selectivity towards sulfur. Ce doping is shown to have improve the adsorption capacity and selectivity using batch and flow experiments. Herein, the stable facets of the TiO2 crystal are used to model the adsorption of thiophene and its derivatives. Doping was carried out by substituting a surface Ti atom with another metal atom (Ce, Zr, Fe, Mo, etc.). Substitutional doping not only affected the physical properties of the crystals (surface area, pore size), but also affected the electronic properties, leading to variations in the adsorption capacity. The changes in the performance are studied in detail and a unique adsorbent for superior adsorption performance was predicted using DFT and validated through experiments. The presence of oxygen in the atmosphere during the adsorption process leads to a significant increase in adsorption capacity. Experimental characterization determined that the oxidation of thiophenes was the source of the increase. To this effect, three different oxidation states of surface Ti atoms were modeled. A stoichiometric surface having Ti atoms in a 4+ oxidation state, an O-poor surface with surface O-vacancies having Ti atoms in a 3+ state and an O-rich surfaces with an extra O-molecule on the surface were constructed. Results showed that the oxygen rich surfaces have higher affinity towards the thiophenes than the other types of surfaces, due to the formation and adsorption of a sulfone species on such surfaces. Though these sites contribute to improved adsorption, they cannot be formed spontaneously under reaction conditions. Further mechanisms to form such sites are explored and a novel adsorption cycle which allows continuous adsorption of thiophenes is proposed. The mechanism proposed uses surface O-vacancies as active sites for activating the oxygen molecules for oxidation of the thiophenic molecules. The vacancy sites are then regenerated by the formation of a sulfone. Each step in the mechanism is analyzed using DFT and dopants are screened based on the energetics for the critical steps in the cycle viz. vacancy formation, oxygen adsorption and sulfoxide formation. Experimental evidence is provided to validate the predicted adsorbents. This work paves the way for characterization and design of doped metal oxide adsorbents capable of adsorbing high amounts of sulfur selectively.