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.