Aging mechanism
Based on experimental results and material characterizations, the aging
processes on Ag0Z occur through the Ag oxidation
followed by the migration of Ag+ into the pores or channels of mordenite
crystalline framework. Similar phenomena of Ag cluster dispersion in
microporous zeolites including MFI12 and Y
zeolite16 were also observed in previous studies.
Thus, the pathways of the aging processes can be expressed as in the
following steps; firstly, Ag0 is oxidized to
Ag+ by oxygen which has existed in air gas streams,
then the oxidized Ag+ migrated into the pores of
mordenite crystalline structure from the surface. After that,
Ag+ was bound to the mordenite crystalline structure
through substitution with hydrogen. Finally, the generated hydrogen
produced water by binding with oxygen or hydroxide.
In dry air as shown in Eqns. (3) ~ (6),
Ag0 is oxidized by O2 in dry air to
Ag+ on the surface of the mordenite, then
Ag+ is migrated into the pores and bound to the
crystalline structure in mordernite through subtitution reaction with
H+. The replaced H+ was bound to the
O2- in order to produce H2O. The
different influences of the aging gas streams on Ag0Z
should depend on the different oxidation reactions of
Ag0Z. That is, the aging effects rely on the strength
of the oxidants. The subsequent migration process, which is reactions
(4) - (6) can be similar for the aging process in both gas streams.
Considering the similar reactions (4) - (6) in both gas streams and the
faster aging process in humid air (Figure 3) which shows higher iodine
capacity loss after 1 week, it can be concluded that the aging processes
in all gas streams used in this study are controlled by the oxidation
reaction as shown in Eqns. (1) for dry air and (6) for humid air. The
overall reaction, Eqn. (7), is proposed for the oxidation process of Ag
cluster which is similar reaction as described by previous
study.12
\begin{equation}
\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }O_{2}+4Ag{4\text{Ag}_{s}}^{+}+{2O}^{2-}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (3)\nonumber \\
\end{equation}\begin{equation}
\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }{\text{Ag}_{s}}^{+}{\text{Ag}_{p}}^{+}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (4)\nonumber \\
\end{equation}\begin{equation}
\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }{\text{Ag}_{p}}^{+}+HZ\ AgZ+H^{+}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (5)\nonumber \\
\end{equation}\begin{equation}
\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }{2H}^{+}+O^{2-}H_{2}O\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (6)\nonumber \\
\end{equation}\begin{equation}
\ \ \ \ \ \ \ \ Overall\ reaction:\ \ O_{2}+4Ag+4HZ4AgZ+{2H}_{2}O\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (7)\nonumber \\
\end{equation}Where Z representes mordenite, the subscript \(s\) denotes the
Ag+ on the external surface and p denotes
the Ag+ in the pores and channels of mordenite
crystals.
In case of the aging process with humid air, an air stream containing
water can promote the oxidation process based on experimental data and
material analyses. The oxidation of Ag to Ag+ with
humid air takes place faster than with dry air by forming
OH- instead of O2-. Then
H2O can be formed through a similar pathway in dry air
as shown Eqns. (9) - (11). The overall reaction shown in Eqn. (12) is
the same as that in dry air, but is catalyzed by water. Therefore, it
can be expected that the effect of humid air on Ag0Z
will increases as the water vapor concentration increases.
\begin{equation}
\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }{{2H}_{2}O+O}_{2}+4Ag{4\text{Ag}_{s}}^{+}+{4OH}^{-}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (8)\nonumber \\
\end{equation}\begin{equation}
\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }{\text{Ag}_{s}}^{+}{\text{Ag}_{p}}^{+}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (9)\nonumber \\
\end{equation}\begin{equation}
\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }{\text{Ag}_{p}}^{+}+HZ\ AgZ+H^{+}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (10)\nonumber \\
\end{equation}\begin{equation}
\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }H^{+}+\text{HO}^{-}H_{2}O\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (11)\nonumber \\
\end{equation}\begin{equation}
\ \ \ \ \ \ \ \ \ Overall\ reaction:\ O_{2}+4Ag+4HZ4AgZ+{2H}_{2}O\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (12)\nonumber \\
\end{equation}