Characterization of plasma catalytic decomposition of methane: role of atomic O and reaction mechanism

Abstract

In this work, we investigated atmospheric pressure plasma jet (APPJ)-assisted methane oxidation over a Ni-SiO2/Al2O3 catalyst. We evaluated possible reaction mechanisms by analyzing the correlation of gas phase, surface and plasma-produced species. Plasma feed gas compositions, plasma powers, and catalyst temperatures were varied to expand the experimental parameters. Real-time Fourier-transform infrared spectroscopy (FTIR) was applied to quantify gas phase species from the reactions. The reactive incident fluxes generated by plasma were measured by molecular beam mass spectroscopy (MBMS) using an identical APPJ operating at the same conditions. A strong correlation of the quantified fluxes of plasma-produced atomic oxygen with that of CH4 consumption, and CO and CO2 formation implies that O atoms play an essential role in CH4 oxidation for the investigated conditions. With the integration of APPJ, the apparent activation energy was lowered and a synergistic effect of 30% was observed. We also performed in-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) to analyze the catalyst surface. The surface analysis showed that surface CO abundance mirrored the surface coverage of CHn at 25 oC. This suggests that CHn adsorbed on the catalyst surface as an intermediate species that was subsequently transformed into surface CO. We observed very little surface CHn absorbance at 500 oC, while a ten-fold increase of surface CO and stronger CO2 absorption were seen. This indicates that for a nickel catalyst at 500 oC, the dissociation of CH4 to CHn may be the rate-determining step in the plasma-assisted CH4 oxidation for our conditions. We also found the CO vibrational frequency changes from 2143 cm-1 for gas phase CO to 2196 cm-1 for CO on a 25 oC catalyst surface, whereas the frequency of CO on a 500 oC catalyst was 2188 cm-1. The change in CO vibrational frequency may be related to the oxidation of the catalyst.

Plasma catalysis: separating plasma and surface contributions for an Ar/N2/O2 atmospheric discharge interacting with a Pt catalyst

“…The jet is operated at 14.3 MHz with a duty cycle of 20%. This operating frequency was found in prior work to maximize power dissipation in the plasma for this APPJ with similar gas flow conditions [8].…”

“…One promising method is plasma catalysis [6]. Plasma catalysis, the coupling of a plasma with a heterogenous catalytic reaction, is being researched for several different applications including air treatment [7], methane decomposition [8], and nitrogen fixation, and shows promise for enhancing the catalytic yield [9]. Plasma catalysis also opens the door for reactions at atmospheric pressure, which is attractive for large-scale production.…”

“…In plasma catalysis, the plasma creates a variety of excited gas species, electronically excited species, atomic radicals, ions, electrons, photons, or intermediate gas compounds [10]. These species may interact with the catalyst and lower the activation energy of reactions or improve product yields [8]. With the wide variety of species potentially arriving at the catalyst surface, the interactions occurring can be complex.…”

Investigation of Ni catalyst activation during plasma-assisted methane oxidation

“…It is worth noting that the generated plasma plume did not directly touch the catalyst to avoid arcing and excessive heating induced by plasma. More information about the plasma jet and catalyst material used here can be found in [25,26,27].…”

“…In previous work, CH 4 was injected downstream of the APPJ [25,26,27] rather than directly being admitted to the plasma jet. Hence, entrainment of CH 4 into the ionized plasma plume, where interaction with high-energy electrons was possible, was required to break C-H bonds.…”