The molecular level understanding of the strong adsorption
of Xe
to silver-modified zeolites remains elusive. Here, we probe the effect
of silver oxidation state on the thermodynamics of Xe sorption in
silver-functionalized zeolites by measuring the enthalpies of adsorption
after various treatments using inverse gas chromatography (IGC). The
enthalpy of adsorption was measured for silver-functionalized chabazites
(AgCHA) before and after hydrogen reduction and subsequent reoxidation.
The sorption enthalpy (ΔH) for AgCHA was 35.2
kJ/mol, which decreased to 25.8 kJ/mol with hydrogen reduction. After
reoxidation (O2-AgCHA), 95% of the binding strength was
restored. Hydrogen reduction of the base chabazite (CHA) did not influence
Xe adsorption. Henry’s law constant for Xe adsorption increased
in the order AgCHA > O2-AgCHA > H2-AgCHA
> CHA.
A decrease in enthalpy and Henry’s constant with silver reduction
and increase with reoxidation suggest that ionic silver is playing
a role in Xe binding. The effect of reduction and reoxidation on the
zeolite microstructure was analyzed using surface area analysis, powder
X-ray diffraction (p-XRD), scanning electron microscopy/energy-dispersive
X-ray spectroscopy (SEM/EDS), and X-ray photoelectron spectroscopy
(XPS). These results lay the groundwork for better material design
of strong noble gas adsorbents.
Separation
of the noble gases from air is typically done
through
a cryogenic distillation process that is both energy intensive and
expensive. Ag-functionalized zeolites and MOFs have a well-documented
affinity for Xe and, to a lesser extent, Kr that could serve as an
economical alternative to this process on a commercial scale. The
mechanism driving the Ag–Xe interaction, however, is still
a matter of debate, and the use of other metals in place of Ag is
not as thoroughly documented. In this study, Ag, Cu, and Pd functionalized
chabazite specimens were prepared, and their affinities for the noble
gases Xe, Kr, and Ar were investigated and compared to each other
and an unexchanged Na-chabazite. From these analyses, Ag-functionalized
chabazite displayed the highest affinity for Xe among these samples,
but there was not a similar affinity for Kr or Ar. From the results,
a mechanism is proposed such that the strong Ag–Xe interaction
contains both an underlying physical and electronic aspect related
to the formation of Ag nanoclusters within the chabazite pore geometry
that alters the chabazite surface state such that Xe is preferentially
adsorbed.
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