Scanning Electron Microscopy
SEM backscattered electron images for unaged AgZ,
Ag0Z, and aged Ag0Z in dry air and
humid air (d.p. -15 oC) for 1 month are shown in
Figure 4. Small particles are observed on the mordenite crystal surface
as shown in Figure 4b. White dots in the image indicates that they
comprise elements of higher atomic mass than the Al, Si, and O that
comprise the zeolite framework. According to the chemical composition of
Ag0Z,11 these particles are
necessarily composed of Ag atoms. This observation is in agreement with
previous works that indicate Ag ions (Ag+) in AgZ are
reduced to Ag0 upon reduction in H2with concomitant migration to the mordenite surface and formation of
aggregate clusters.14 This is also consistent with
their absence in the image of AgZ (Figure 5a). The observed diameter of
the Ag0 particles ranged from 0.1 to 1.2 μm. While we
expect particles smaller than 0.1 μm also exist, they cannot be observed
under the current magnification. Although the I2 loading
capacity of Ag0Z aged in dry air and humid air are
decreased (Figure 3), a significant amount of Ag0particles are observed on the surface of aged Ag0Z in
dry air (Figure 4c) and humid air (Figure 4d). It is because a reduction
in amount or size of the Ag0 particles cannot be
quantitively observed by the SEM images.