Tracking the Mode of Carbon Deposition During Dry Reforming of Methane over Ni/<i>γ</i>-Al₂O₃

Dry reforming of methane was studied over Ni/Al2O3 catalyst at 600 degrees C with varying CO2/CH4 ratios (0.2, 0.5, 1, 2, 5). (CO2)-C-13 as well as (CO2)-C-12 were used along with (CH4)-C-12 and resulting carbon was characterized with C-13 solid state NMR spectroscopy in order to elucidate the role of CO2 in carbon build up. C-13 NMR results revealed that carbon in the deposited coke not only came from CH4, but also from CO2. CO2 saturation coverage is determined by adsorption calorimetry as 0.02 mole/site at 323 K, populated on Al2O3. The critical CO2/CH4 ratio for the onset of carbon growth at atmospheric pressure was determined as 1.8 in good agreement with the literature. The effect of CO2/CH4 ratio on the type of carbon formation was investigated by HCTEM. Long-term reaction tests resulted in octopus carbon structure with several fibers growing from one nickel crystal. Above the critical CO2/CH4 ratio, carbon growth was inhibited, only a small amount of amorphous carbon could form. At CO2/CH4 ratios below the critical value, whisker formation was clearly observed. When the steady state operation of reforming reaction was changed to a transient operation by injecting pure CO2 flow to the reactor, coke deposition could be inhibited at the expense of hydrogen stoichiometry.