A Multi-Technique Comparative Evaluation of Silver Dispersion on Polycrystalline Titania
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Energy independence, coupled with the increasing demands on energy supply, and rising environmental, health, and efficiency concerns have fueled the need for more appropriate refining processes in the petroleum industry. The North American pursuit of energy inde- pendence can be advanced through the refining of regionally available petroleum reserves – deposits of shale oil, tar sands, and bitumen. These unconventional deposits typically contain extra heavy oils that are extremely viscous and high in concentrations of sulfur. The processing of these sulfur rich feedstock presents many new challenges. One of these is the removal of heterocyclic sulfur compounds and their alkylated derivatives. Sulfur removal is necessary for the evolution of fuel cell technology and for cleaner fuel oil. Catalytic and adsorption processes that target undesirable refractory sulfur compounds are being investigated for use in the desulfurization of hydrocarbon streams. A silver-titania adsorbent developed at Auburn University is one such compound that can be used in the desulfurization processes. The readily available raw materials, facile synthesis, and multi- cycle regenerability at ambient conditions make this adsorbent appealing. Ag/TiO2adsor- bents are capable of removing various refractory species in the presence of 160 fold excess of competing aromatics in logistic fuels, such as JP5. Ag/TiO2adsorbents showed a consistent capacity of 8.5 mg S/g of competing aromatics for JP5 fuels (∼1200 ppmw sulfur) for 10 cycles. There is a continual effort to improve the sulfur sorption capacity of Ag/TiO2adsor- bents. The first step in improving the quality of an adsorbent is to have a fundamental understanding of its properties. Literature has shown that there is a strong correlation between the efficacy of supported catalyst and particle dimension and dispersion. It is im- portant to note that dispersion is important to gain the greatest access to the active atoms ii of the adsorbent. Thus, the size of metal particles is crucial to the activity and selectivity of such systems. The purpose of this study was to perform fundamental studies to acquire a molecular level understanding of Ag/TiO2adsorbents used in the liquid phase desulfur- ization of logistics fuels. The ultimate goal of this study is the enhancement of performance capabilities and development of new sorbent materials. In the literature, various techniques for the estimation of crystallite size are described. The intrinsic strengths and weaknesses of the four such techniques, Electron Microscopy (EM), X-ray Diffraction (XRD), Gaseous Chemisorption, and X-ray Photoelectron Spec- troscopy (XPS) have been analyzed in their application to the Ag/TiO2adsorbent system. Oxygen chemisorption and XPS have proven to be useful in defining the catalytic system. Both techniques have demonstrated respective nanometer scale and sub-nanometer scale oc- currence of Ag supported on titania. The results from these techniques complemented with information from Electron Paramagnetic Resonance (EPR) support the hypothesis that well dispersed silver oxide crystallite decorate defects on the titania support at low loading (<4 wt%). These crystallites are responsible for the preferential selectivity of Ag/TiO2for sulfur removal at low loadings. The XPS particle size estimation was approximately a factor of approximately 5 less than the particle size found estimated by oxygen chemisorption for the same range. Particle size estimation using XRD was not achieved. Silver atoms are mono–dispersed between 0 – 4 wt % loading, after this loading there is a critical point beyond which particle growth occurs.