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 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}