(600ad) 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 Å, with a verified known coordination number
of 12.  The Ag-O bond lengths of Ag₂O and Rhombohedral AgNO₃
were also analyzed to be 2.047Å and 2.412 Å, 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) Å ) 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.