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.