Minimizing carbon deposition in plasma-induced methane coupling with structured hydrogenation catalysts

The effect of temperature and hydrogen addition on undesired carbonaceous deposit formation during methane coupling was studied in DBD-plasma catalytic-wall reactors with Pd/Al2O3, using electrical power to drive the reaction. Experiments with thin catalyst layers allowed comparison of the performance of empty reactors and catalytic wall reactors without significantly influencing the plasma properties. The product distribution varies strongly in the temperature window between 25 and 200 °C. Minimal formation of deposits is found at an optimal temperature around 75 °C in the catalytic-wall reactors. The selectivity to deposits was c.a. 10% with only 9 mg of catalyst loading instead of 45% in the blank reactor, while decreasing methane conversion only mildly. Co-feeding H2 to an empty reactor causes a similar decrease in selectivity to deposits, but in this case methane conversion also decreased significantly. Suppression of deposits formation in the catalytic-wall reactor at 75 °C is due to catalytic hydrogenation of mainly acetylene to ethylene. In the empty reactor, H2 co-feed decreases conversion but does not change the product distribution. The catalytic-wall reactors can be regenerated with H2-plasma at room temperature, which produces more added-value hydrocarbons.