3.4 In-situ detection of surface species
Figure 6 illustrates the surface species of Cu-MOR IE-3 and the 20 wt.% Cu/MOR catalysts, monitored during plasma-catalytic DOMtM usingin-situ Fourier Transform Infrared (FTIR) spectroscopy. (In-situ DRIFTS reaction cell shown in Figure S5) Following plasma activation, the peaks corresponding to surface CH3O* species are readily observed at 1054, 2910, and 2940 cm-1, serving as key intermediates in the process of CH3OH formation.31 It is noteworthy that CH3O* is formed through combination of CH3* (arising from CH4 dissociation) and O (via O2 dissociation), consistent with the OES results in Figure 5d. In addition, the peaks of DOMtM by-products are observed between 2400-1700 cm-1, including CO2(2347 cm-1), CO (2170 and 2115 cm-1), aldehyde, and carboxylic acid products (1780 and 1750 cm-1).32,33 In Figure 6a, Cu/MOR IE-3 exhibits significantly higher FTIR intensity for adsorbed CH3O* and for CH3OH (1015 and 1030 cm-1) compared to the 20 wt.% Cu/MOR catalyst (Figure 6b).34 This emphasizes the crucial involvement of exchanged Cu2+ in plasma-catalytic DOMtM. Conversely, the 20 wt.% Cu/MOR catalyst obtained through wetness impregnation shows higher FTIR intensity of by-products (CO2, CO, -C=O, and -COOH). Notably, in the case of 20 wt.% Cu/MOR, a new absorption band of HCOO* at 1585 cm-1 indicates the over-oxidation of CH4 to CO2.28Based on the above characterizations and in-situ FTIR results, we can again conclude that zeolite-confined Cu2+ species with octahedral coordination, including mono(μ-oxo) di-copper and bis(μ-oxo) di-copper species, are the active sites for plasma-catalytic DOMtM over Cu/MOR catalysts.