Au Nanoparticle Enhancement of Plasma-Driven Methane Conversion into Higher Order Hydrocarbons via Hot Electrons

We demonstrate a more than 50-fold enhancement in the upconversion of methane to higher order hydrocarbons by discharging a nanosecond-pulsed plasma across Au nanoparticles. Here, the enhancement occurs as a result of the local field enhancement provided by the nanoparticles. The transient nature of the pulsed plasma enables the structure of these delicate nanoparticles to be preserved during the plasma discharge by producing a low-temperature plasma. Plasma emission spectroscopy shows signatures of the C2 Swan bands with and without the presence of these nanoparticles. Mass spectrometry demonstrates that methane is converted into higher order hydrocarbons with different groups of peaks representing species with molecular masses of 35–45, 45–60, and 60–70 amu, corresponding to C3, C4, and C5 species, respectively, under plasma discharge conditions. Electrostatic simulations show that a 3–4× enhancement in the field is produced at the nanoparticle surfaces. The exponential relation between electric field strength and plasma formation gives rise to a 50× increase in highly reactive, plasma-initiated radical species that are responsible for driving the methane upconversion process. This upconversion is important for several applications including mitigation of greenhouse emissions and improving the combustion of natural gas.