Cold atmospheric pressure plasma (CAP) can enhance thermal catalysis processes by a process called plasma-catalyst synergy (PCS). We will review work focused on plasma-catalytic decomposition of CH4 over a Ni-SiO2/Al2O3 catalyst aimed at clarification of the mechanistic basis of PCS. A well-studied atmospheric pressure plasma jet (APPJ) fed with Ar/O2 or Ar/CH4/O2 was interacted with the catalyst located downstream of the APPJ. At the catalyst position, Ar/CH4 was injected for Ar/O2 APPJ conditions, and catalyst temperature was varied from 25 oC to 500 oC. Fourier-transform infrared spectroscopy was used to quantitatively measure the downstream gas components. We also applied diffuse reflectance infrared Fourier transform spectroscopy for operando analysis of surface species such as CO and CHn. For unheated catalyst (25 oC) CO production was much greater than CO2 formation. In this case, the trend of surface CO was found to follow surface CHn, and comparisons with O atom flux measurements by our collaborators at University of Minnesota indicated that surface CHn is the intermediate species that is transformed to surface CO by plasma-produced O atoms. At 500 oC, CO2 gas phase production was dominant for Ar/O2. Little surface CHn was observed under these conditions, whereas surface CO absorption increased despite lower gas phase production of CO. This indicates that decomposition of CH4 to surface-adsorbed CHn is likely a rate-determining step in the temperature-dependent plasma-catalytic oxidation of CH4. Consistent with this, CH4/O2/Ar plasma exhibited a much stronger plasma-catalytic effect than Ar/O2 plasma with CH4 downstream injection. CH4 oxidation to CO and CO2 was ~7 times higher using Ar/CH4/O2 plasma than Ar/O2. In this case, the catalyst was found to be activated by plasma with large power, possibly due to reduction of catalyst. Systematic shifts of CO vibrational frequency were also seen that may offer insights into understanding PCS.

Infrared studies of the enhancement of catalytic methane decomposition by low temperature atmospheric pressure plasma

ACS Spring 2022

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