Synergy of thin catalytic layers and DBD plasma for coupling of methane

The conversion of methane into fuels and value-added chemicals shows a promising alternative for steam cracking of petroleum derivatives. Non-oxidative coupling of methane in a non-equilibrium plasma, generated in a dielectric barrier discharge (DBD) reactor, allows activation of methane in absence of oxygen at mild temperature, avoiding thereby formation of CO and CO2 in benefit of hydrocarbons (mainly alkanes and olefins). In this work, the DBD plasma for coupling of methane has been studied at ambient conditions assisted by a Pd/Al2O3 catalyst.

Combining catalysis and plasma is rather complex as introduction of a packed bed in a DBD plasma strongly influences the electrical field in the plasma, changing the properties of the plasma and the chemistry taking place in the plasma. Possible effects may include formation of microdischarges, changes in the discharge type, changes in the catalyst surface area, hot spot formation, changes in the catalytic surface properties and chemisorption of reactants. Besides, the morphology of the particles as well as the inter-particle space have a remarkable influence in the plasma features, as plasma is especially generated at the contact point between particles. Therefore, in this work, the catalyst was introduced as a thin layer on the internal wall in a DBD plasma reactor, minimizing the influence of the catalyst on the plasma. In addition, by this configuration with thin layers, the reactant volume occupied by the gas phase to create the plasma is very similar for the empty DBD reactor and the catalyst-assisted DBD reactor, making comparative tests possible.

Different amounts of catalysts have been tested up to loading of 15 mg. The thickness of catalyst layers are 20 µm or less to ensure the absence of any internal diffusional limitation. The catalytic wall reactors have been tested for methane coupling and compared to the non-catalytic DBD reactor. Important differences in product distribution are achieved in presence of the catalyst, i.e. more saturated hydrocarbons are formed and this effect increases with the amount of catalyst. In absence of catalyst, the selectivity to acetylene is ca. 20 %, whereas a loading below 6 mg of catalyst results in ethane selectivity of 20 % without any formation of acetylene. Also, the amount of carbonaceous deposits is strongly decreased by the presence of the catalyst. Indeed, the carbonaceous deposits are reduced up to an 80 % when 15 mg of catalyst are loaded on the reactor wall, compared to the empty DBD reactor in the same operation conditions. On the other hand, the methane conversion is determined by the plasma power and is not influenced by the amount of catalyst. For instance, the voltage input has been constant during all the experiments with different catalyst loadings providing the same plasma power (ca. 6.5 W). Hence, considering the same operation conditions, the amount of catalyst is only modifying the product distribution but not the plasma power or the methane conversion.

Remarkably, only very minor correction of the power source was needed in catalytic wall reactors to keep the plasma power the same as the non-catalytic reactor. This correction is significantly smaller compared to the correction that would be needed in a fixed bed reactor.

In short, the DBD plasma successfully activates the methane molecules at ambient conditions and the presence of the catalyst allows shifting the product distribution towards saturated hydrocarbons and less carbonaceous deposits.