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.