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.