Abstract
The two-dimensional morphology of molybdenum oxycarbide (2D-Mo2COx) nanosheets dispersed on silica is found vital for imparting high stability and catalytic activity in the dry reforming of methane. Here we report that owing to the maximized metal utilization, the specific activity of 2D-Mo2COx/SiO2 exceeds that of other Mo2C catalysts by ca. 3 orders of magnitude. 2D-Mo2COx is activated by CO2, yielding a surface oxygen coverage that is optimal for its catalytic performance and a Mo oxidation state of ca. +4. According to ab initio calculations, the DRM proceeds on Mo sites of the oxycarbide nanosheet with an oxygen coverage of 0.67 monolayer. Methane activation is the rate-limiting step, while the activation of CO2 and the C–O coupling to form CO are low energy steps. The deactivation of 2D-Mo2COx/SiO2 under DRM conditions can be avoided by tuning the contact time, thereby preventing unfavourable oxygen surface coverages.
Highlights
The two-dimensional morphology of molybdenum oxycarbide (2D-Mo2COx) nanosheets dispersed on silica is found vital for imparting high stability and catalytic activity in the dry reforming of methane
Deactivation of Mo2C in DRM conditions proceeds oxidatively according to Mo2C + 5 CO2 → 2 MoO2 + 6 CO11,17,18 and in order to mitigate it, dry reforming of methane is often conducted at elevated pressures (2–10 bar)[11,14,16,18,21]
A dried aliquot of this solution was analyzed by transmission electron microscopy (TEM) revealing single and a few-layer thin flakes of Mo2CTx, the latter morphology is identified by the characteristic scrolling of edges of the few-layer thin MXene nanosheets (Supplementary Fig. 3)[34]
Summary
The two-dimensional morphology of molybdenum oxycarbide (2D-Mo2COx) nanosheets dispersed on silica is found vital for imparting high stability and catalytic activity in the dry reforming of methane. Consistent with XPS analysis, the XANES spectrum of 2D-Mo2COx/SiO2 reveals a significant shift of the Mo K-edge towards higher energies compared to 2D-Mo2C/SiO2 (i.e., 20015.7 vs 20000.7 eV), suggesting an average Mo oxidation state for 2D-Mo2COx/SiO2 between Mo5+ and Mo6+ (Fig. 1e inset, Supplementary Table 1).
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