Figure 4. XPS of the Cu-MOR catalysts. (a) Cu
2p3/2 spectra; (b) Proportion of highly dispersed
Cu2+ and CuO species on Cu-MOR surface; (c)
Relationship between highly dispersed
Cu2+ content and reaction performance. The standard
charge was calibrated by C 1 s binding energy of 284.8 eV.
The above catalyst characterization results indicate that CuO particles
dominate the composition of Cu-MOR catalysts prepared through wetness
impregnation. The size of CuO particles gradually increases with higher
loading, even though a limited quantity of zeolite-confined
Cu2+ is also present. Our catalytic tests (Figure 1b)
reveal that the CH4 conversion gradually increases, but
the CH3OH selectivity decreases with rising Cu loading
from 2 to 20 wt.%. Additionally, the CO2 selectivity
dramatically increases with higher Cu loading. These outcomes reaffirm
that CuO particles facilitate the oxidation of CH4 to
CO2, while zeolite-confined Cu2+species promote CH3OH production. In summary, both small
CuO clusters (including [Cu-O-Cu] oligomers) and bulk CuO particles
are unfavorable for CH4 conversion to
CH3OH. Therefore, we can conclude that the active sites
on Cu-MOR catalysts for the selective oxidation of CH4to CH3OH, driven by
CH4/O2 plasma, are the zeolite-confined
Cu2+ species.
To elucidate the active sites of zeolite-confined Cu2+species, XPS analysis was employed to characterize the Cu-MOR catalysts
prepared through ion exchange. Figure 4a shows the Cu
2p3/2 spectra, in which we observe four peaks,
corresponding to a binding energy of 944.4, 936.2, 934.8 and 933.6 eV.
The peak at 944.4 eV is attributed to the satellite peak of
Cu2+ species, confirming the presence of divalent Cu
species (CuO and Cu2+) on the Cu-MOR catalysts.
Generally, the Cu 2p3/2 peaks at ca. 933.6 and 936.2 eV
correspond to zeolite-confined Cu2+ species with
tetrahedral and octahedral coordination,
respectively.24 The binding energy of CuO
nanoparticles is within the range of 933.5-934.5 eV. Therefore, the peak
at 933.6 eV could be assigned to both zeolite-confined
Cu2+ species with tetrahedral coordination and CuO
nanoparticles, while the peak at 936.2 eV should be attributed to
zeolite-confined Cu2+ species with octahedral
coordination, such as mono(μ-oxo) di-copper and bis(μ-oxo) di-copper
species. The peak at 934.8 eV is assigned to small CuO clusters,
including [Cu-O-Cu] oligomers.25
In Figure 4b, the relative contents of different copper species are
presented for the Cu-MOR samples prepared with a different number of ion
exchanges. The Cu-MOR IE-3 catalyst exhibits the highest abundance of
zeolite-confined Cu2+ species with octahedral
coordination. Conversely, Cu-MOR IE-4 and IE-5 show the presence of
small CuO clusters (including [Cu-O-Cu] oligomers), consistent with
the H2-TPR results (Figure 2c) and UV-Vis spectra
(Figure 2e). Notably, Figure 4c illustrates a linear increase in
CH3OH selectivity and CH4 conversion
with the zeolite-confined Cu2+ species having
octahedral coordination. Cu-MOR IE-3, with the most abundant
zeolite-confined Cu2+oct. species,
exhibits the highest CH3OH selectivity and
CH4 conversion. Conversely, Cu-MOR IE-4 and IE-5,
showing the presence of small CuO clusters with decreased
zeolite-confined Cu2+oct. species,
exhibit reduced CH4 conversion and CH3OH
selectivity. These findings further underscore that zeolite-confined
Cu2+ species with octahedral coordination, including
mono(μ-oxo) di-copper and bis(μ-oxo) di-copper species, serve as the
active sites for plasma-catalytic DOMtM.