Figure 3. HRTEM images of (a, b) Cu-MOR IE-3 (ion exchange) and
(c-g) Cu-MOR prepared through wetness impregnation with various
loadings.
Figure 3 presents the HRTEM images of the Cu-MOR IE-3 sample and Cu-MOR
samples with various loadings (2, 5, 10, 15 and 20 wt.%). For the
Cu-MOR IE-3 sample, the XRD, H2-TPR and UV-Vis results
demonstrate that most of the copper exists on MOR as zeolite-confined
Cu2+ species, which may not be readily discerned by
HRTEM. Nevertheless, some highly dispersed CuO particles are directly
observed. The crystalline spacing of 0.196 nm, 0.186 nm, 0.231 nm, and
0.233 nm corresponds to the (-112), (-202), (200), and (111) crystalline
planes of CuO, respectively (Figure 3a). Although the average size of
the CuO particles is approximately 3.2 nm (Figure S4), which is not
large enough to be detected by XRD, it explains the absence of
characteristic diffraction peaks of CuO in Figure 2a.
As depicted in Figures 3(c-g), copper is also highly dispersed on MOR in
the Cu-MOR samples prepared through wetness impregnation. However, the
size of CuO particles significantly increases with increasing copper
loading. The particle size distribution presented in Figure S4 indicates
that the average size of the CuO particles on Cu-MOR samples with 2, 5,
10, 15 and 20 wt.% loadings is estimated to be around 4.7, 5.2, 6.8,
8.6, and 13.3 nm, respectively. Hence, it is evident that larger CuO
particles are formed on Cu-MOR samples with higher loading, consistent
with the findings from XRD, H2-TPR, and UV-Vis analyses.
However, small CuO clusters, including [Cu-O-Cu] oligomers, are
detected in the Cu-MOR IE-4 and Cu-MOR IE-5 samples. Our catalytic tests
highlight that Cu-MOR IE-3 exhibits superior catalytic performance,
while Cu-MOR IE-4 and Cu-MOR IE-5 show diminished CH4conversion and CH3OH selectivity (Figure 1a). These
findings suggest that zeolite-confined Cu2+ species
promote CH4 conversion to CH3OH, whereas
small CuO clusters, including [Cu-O-Cu] oligomers, are less
favorable for DOMtM.