X-ray Photoelectron Spectroscopy
XPS spectra for the AgZ, Ag0Z, and aged Ag0Z in dry air and humid air for 1 month and 2 months are shown in Figure 6. The results show that all samples have identical chemical compositions of Al, Si, and O, which when interpreted self-consistently with the aforementioned XRD results, confirm that the aging process does not impact on the chemical structure of mordenite crystals.
Ag 3d spectra for Ag standards and the Ag0Z and the aged Ag0Z samples are shown in Figure 6. The binding energy (BE) of Ag 3d5/2 measured for AgZ was 368.4 eV, and it shifted to 367.6 eV when the Ag+ in AgZ was reduced and formed Ag0 particles in Ag0Z. It is noted that the Ag 3d5/2 BE for the Ag0 particles in Ag0Z is different to that for Ag foil standard (368.2 eV), which should be attributed to the chemical/physical environment of the Ag0 particles as they locate in the macropores of the Ag0Z.4 These Ag 3d5/2 BEs are in agreement with those reported by Aspromonte and coworkers.15 An increasing Ag 3d BE is noted when Ag0Z was aged in aging gas streams used for this study. As highlighted in Figure 6, it shifts towards the Ag 3d BE for Ag2O (368.3 eV) and AgZ (368.4 eV). Since no significant Ag2O are detected by the XRD and XPS, this result suggests that the Ag0Z was mainly oxidized back to AgZ when being aged in gas streams. Combining the results from SEM and XRD, it is most likely that the Ag particles are dispersed during the aging process. That is, the oxidized Ag+ migrated back to the mordenite pores and channels, resulting in the same oxidation state as in AgZ. Since dry air and humid air are not as strong oxidants as NOx, a portion of silver remained as Ag particles, and accordingly, the Ag 3d BEs for the aged samples in dry air and humid air are in between AgZ and Ag0Z.