283372 EXAFS Analysis of Supported Ag for Liquid-Phase Logistics Fuel Desulfurization
Liquid logistics fuels are an integral part of the mobility and electric-power paradigm, and will be for the foreseeable future. Applications such as military fuel cell systems must be able to operate on fuel sources where quality varies with location. Desulfurization is a critical step to ensure fuel quality for power units running on high sulfur logistic fuels. Liquid-phase desulfurization allows systems to become more compact, enabling simplified integration and higher levels of safety.
This paper explores dispersed silver oxides on TiO2 supports, and intends to identify the structures and characteristics of the silver phase. Significant analysis has been performed to understand how the silver structure changes with loading, with the ultimate goal to fully understand how the selective interaction of heterocyclic sulfur occurs, and how it can be harnessed and optimized to enhance capacity and ultimately desulfurization process size for integrated systems. To this end, characterization was performed by XPS, XRD and with a substantial amount of work studying the EXAFS spectra of sorbent materials with varying metallic loading. Because the technique allows for averaging across bulk properties (transmission spectra of Ag K-edge EXAFS), analysis of various sorbent material structures can be discerned on a progression of different oxidations states and crystallite morphologies as a function of loading, to better understand this complex heterogeneous system. The problems, benefits and different perspectives of the various characterization techniques we have employed will be discussed.
In summary, XPS analysis confirms that AgNO3, which is used as the precursor for preparing the sorbent material, is not present. Ag is mostly present in an oxide form in the near surface region. EXAFS was performed on the Ag K-edge (25514eV) on beamline X11A at the National Synchrotron Light Source, Brookhaven National Laboratory. Temperature dependent analysis of Ag references measured down to 19K supported the determination of the amplitude reduction factor, S02 to be 0.94, which enables accurate determination of coordination numbers. Analysis of reference materials verified Ag-Ag and Ag-O bond lengths, and found the Ag-Ag bond in a pure silver reference to be 2.876 Ċ, with a verified known coordination number of 12. The Ag-O bond lengths of Ag2O and Rhombohedral AgNO3 were also analyzed to be 2.047Ċ and 2.412 Ċ, respectively, with coordination numbers of 2 and 6, respectively. Samples of Ag/TiO2 sorbent materials were analyzed with differing silver loading, and a unique Ag-O distance (2.31(1) Ċ ) and coordination number of 4.1(1) was found consistently on the supported samples. Beyond 4wt% loading, a metallic contribution was found, with identical Ag-Ag bond length as was determined on the reference sample, but with coordination number growing with Ag loading. Linear combination of XANES data for supported Ag sorbents show that for all loadings, the XANES data can be analyzed with a combination of metallic Ag and 2wt% Ag/TiO2, which has only an Ag-O contribution. For low loadings, 4 wt% and below, Ag is similar, most likely in an oxide form. For high loadings, above 4 wt%, Ag chemistry consists of oxide form and metallic form. The metallic content increases with Ag loading.
Figure 1. XPS spectra of the Ag 3d region for the Ag supported TiO2 catalysts as a function of Ag loading.
Figure 2 Fourier Transforms of Ag K-edge EXAFS spectra for the Ag supported TiO2 catalysts as a function of Ag loading.
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