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.