X-ray Powder Diffraction
XRD patterns for the AgZ, Ag0Z, and aged
Ag0Z in different gas streams including dry air and
humid air are placed in Figure 5. The as-received AgZ exhibits a typical
pattern at 10° < 2θ < 36° for mordenite crystals.
The patterns of Ag0Z have two additional peaks at 2θ =
38.2° and 2θ = 44.3° corresponding to Ag crystals. This is in good
agreement with the formation of Ag particles in Ag0Z
during the reduction in H2. It also confirms that the
particles observed in the backscattered electron images in Figure 4 are
metallic Ag particles. Decreasing intensity of Ag crystal peaks is
observed with all aged Ag0Z in gas streams used in
this study, indicating that the amount of Ag0decreased during the aging process. This suggests that the
Ag0 was oxidized to other chemical forms when exposed
to the gas streams. It is noted that the decrease in the quantity of
Ag0 in the aged Ag0Z is consistent
with the decrease in iodine adsorption capacity shown in Figure 3. This
indicates that I2 is adsorbed only by reaction with the
Ag0 particles and the oxidized Ag+is not reactive or available to I2. Furthermore, all
aged Ag0Z have the similar patterns for mordenite
crystal as in Ag0Z and AgZ, indicating that the
crystal structure of mordenite is not affected by the aging process.
The XRD patterns also indicate that the aging gases have different
oxidation effects on the Ag0 particles in
Ag0Z. A portion of the silver in the
Ag0Z aged in dry air and humid air remained as
Ag0 which are consistent with the observations in the
backscattered electron images shown in Figure 4. Besides, no additional
characteristic peaks for crystals of Ag compounds (e.g.
Ag2O) are observed on the patterns of aged
Ag0Z, which is also consistent with the observations
in Figure 4. However, it is possible that Ag2O exists as
molecular species or small clusters, and thus are not detectable by the
XRD.