|The removal of sulfur from distillate fuels is a difficult operation, which carries a cost associated with the capital equipment and production of hydrogen or other reagents necessary to drive the process. While specifications and environmental regulations have driven the allowable limits of sulfur content to lower levels, the content of sulfur present in military distillate and jet fuels in particular is too high to support utilization in high efficiency power generation systems such as fuel cells. Thus additional processes are necessary to provide suitable clean and usable fuel to ensure long life and continued high efficiency operation. Adsorption has been studied for a variety of materials and conditions, as an alternate to more severe or complex processes. The work described herein focuses upon unique approaches to study adsorption from the standpoint of all key atomic species to facilitate a better understanding of the material as well as the mechanisms for which they operate. Given the complexity of liquid fuels, this work attempts to utilize techniques which can provide insight based upon local atomic structure, and materials changes associated with the desulfurization process under selective adsorption.
The system of silver dispersed on anatase TiO2 was studied because of its relevance as a practical, scalable adsorbent material that exhibited high capacity. Studies utilizing X-Ray Photoelectron Spectroscopy (XPS), and X-Ray Absorption Spectroscopy (XAS) were performed to better understand surface and overall structural characteristics, and were applied such that silver, sulfur and titanium could be studied, both in an unused form to understand basic structure, as well as in situ under calcination/activation and an adsorbed state to understand the basis of materials structure changes for both the adsorbent surfaces and the adsorbed species. XPS was utilized to first probe the surface structure and better understand the differences in surface and lattice oxygen and other ratios to better understand stoichiometry and changes caused due to the loading of silver on the TiO2 surface. XAS was utilized to study the structure of the dispersed silver and then also explore changes when it underwent adsorption. Application of XAS found a unique 2.32Å Ag-O bond structure which is consistent with a fully hydrated Ag+ ion in fourfold coordination with oxygen, and with a highly ionic character as compared to solid references. The loading of silver did not change the surface structure of TiO2, further validating that the silver is existing as a highly dispersed phase on a wet surface. High loading of silver causes a metallic phase to form, which is not interacting with the TiO2 surface.
Upon adsorption of sulfur, XAS analysis indicates minor observable changes to the titanium and silver species, showing that they both appear to take part in desulfurization. Studies of the structure of sulfur via an in situ method has provided distillate fuel analysis by XAS, and clearly shows changes in the fuel from a benzothiophene type structure to both oxidized sulfate and reduced sulfide under adsorption. The behavior of conversion of the sulfur changed with different silver loading levels, helping to identify the effect of loading and dispersion of silver, as well as the support’s own interaction. These results help to bring forth a notional adsorption mechanism.
Formation is also studied to determine the changes to the silver upon calcination, and means by which the sorbent might be damaged. These in situ studies indicate a complete decomposition from nitrate to metal, with subsequent dispersion to the new oxide form. When performed within the standard temperature range specified, the oxide appears stable, however higher temperature conditions induces metal agglomeration, even at low loading.