Figure 2. Characterization of Cu-MOR catalysts prepared by ion
exchange and wetness impregnation methods. (a, b) XRD patterns; (c, d)
H2-TPR profiles; (e, f) UV-Vis spectra.
In Figure 2d, the Cu-MOR samples prepared by wetness impregnation
exhibit a distinct reduction peak in the temperature range of 200-400
°C, indicating a one-step reduction of bulk CuO particles
(CuO+H2→Cu+H2O).20This observation aligns with the predominant presence of copper as CuO
particles on the MOR surface, as evidenced by the XRD patterns in Figure
2b. With increasing Cu loading, the intensity of this reduction peak
strengthens and shifts toward higher temperatures, suggesting the
formation of bigger CuO particles at higher loading. Additionally, a
small reduction peak in the high-temperature region, corresponding to
the reduction of Cu+ to Cu0, is
observed. This implies that the Cu-MOR samples prepared by wetness
impregnation also contain a small amount of zeolite-confined
Cu2+ species.
The UV-Vis spectra of the Cu-MOR samples prepared by ion exchange and
wetness impregnation are presented in Figures 2e and 2f, respectively.
The absorption band at 200-300 nm is attributed to the charge transition
from the MOR framework coordinated O2- to
zeolite-confined Cu2+,
including mononuclear
Cu2+, mono(μ-oxo) di-copper and bis(μ-oxo) di-copper
species.21 The absorption band within the
300-500 nm range corresponds to the charge transition from coordinated
O2- to Cu2+ in small CuO clusters,
including oligomeric [Cu-O-Cu] species.22The absorption band within the 600-800 nm range is induced by the d-d
transition of Cu2+ within an octahedral coordination
environment in the bulk CuO particles.23 As
depicted in Figure 2e, with increasing number of ion exchanges, the peak
intensities within the 200-300 nm range (corresponding to
zeolite-confined Cu2+) and the 600-800 nm range
(corresponding to bulk CuO) both gradually increase. Notably, the peak
intensities of zeolite-confined Cu2+ are significantly
higher than those of bulk CuO, indicating a gradual increase in the
content of zeolite-confined Cu2+. Furthermore, the
absorption band within the 300-500 nm range is clearly observed for the
samples of Cu-MOR IE-4 and Cu-MOR IE-5, suggesting that an excessive
number of ion exchanges leads to the presence of small CuO clusters,
consistent with the H2-TPR profiles in Figure 2c.
The UV-Vis spectra of the Cu-MOR samples prepared through wetness
impregnation with various loadings are depicted in Figure 2f. In
comparison to MOR, the Cu-MOR samples exhibit distinct absorption bands
at 200-300 nm and 600-800 nm. Notably, with increasing loading, the
intensities of the former peak (200-300 nm) gradually rise (2 and 5
wt.% Cu-MOR) and subsequently stabilize at higher loadings (5, 10, 15
and 20 wt.% Cu-MOR). This observation suggests that the availability of
sites on MOR for anchoring zeolite-confined Cu2+species is limited. Conversely, the intensities of the latter peak
(600-800 nm) steadily increase with rising loading, indicating the
formation of more CuO particles on MOR. This trend aligns with the XRD
patterns in Figure 2b and the H2-TPR profiles in Figure
2d.