Mechanistic aspects of plasma-enhanced catalytic methane decomposition by time-resolved operando diffuse reflectance infrared Fourier transform spectroscopy

Abstract

To study mechanistic aspects of plasma-enhanced catalysis, methane is decomposed by a supported Ni catalyst assisted by an Ar/O2 atmospheric pressure plasma jet (APPJ). The time-resolved surface response of the Ni catalyst is investigated by operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using various experimental settings. Catalyst temperatures of room temperature and 500 °C and nozzle-catalyst surface distances of 3, 5 and 8 mm were examined, and the amount of O2 of the Ar/O2 gas mixture flowing through the APPJ was either 0 or 0.5%. A synergistic effect of surface bonded C–O was observed during the exposure of the Ni catalyst to the APPJ for low oxygen operating conditions (pure Ar jet). Surface bonded C–O formed only when there was plasma present and the C–O signal was enhanced for higher catalyst temperature. When the supported Ni catalyst was subjected to the plasma-generated particle fluxes using highly oxidizing conditions, the presence of surface bonded C–O was suppressed. The plasma-catalytic CO and CO2 production in the gas phase measured downstream mirrored the surface behavior of C–O bonds when the plasma source operating condition was changed from a low oxygen portion to a high oxygen portion at high catalyst temperature (500 °C). CHn(n = 1, 2, 3) species on the catalyst surface were also studied by DRIFTS, and CHn destruction was found to correlate with C–O formation. In particular, the time-resolved CHn response showed a possible conversion process of CHn to C–O when the Ni catalyst was exposed to the plasma source. This finding may indicate a plasma-mediated regeneration of the catalyst by plasma-catalyst surface interactions.

Plasma-Catalytic Partial Oxidation of Methane on Pt(111): A Microkinetic Study on the Role of Different Plasma Species

“…The authors found that treatment with Ar plasma resulted in the formation of surface-bound CO, which was oxidized to CO 2 upon admixing of O 2 to the plasma. These results indicate that the CO selectivity can be altered by adjusting the flow of O 2 . Gibson et al investigated CH 4 oxidation using a Pd/Al 2 O 3 catalyst in a plasma.…”

“…However, they also suggested the presence of carboxylate groups or COO – , which are likely precursors to CO 2 . Zhang et al investigated the decomposition and oxidation of CH 4 exposed to a Ni catalyst on an Al 2 O 3 /SiO 2 support and an Ar or Ar/O 2 APPJ using time-resolved DRIFTS. The authors found that treatment with Ar plasma resulted in the formation of surface-bound CO, which was oxidized to CO 2 upon admixing of O 2 to the plasma.…”

Direct Observation of Plasma-Stimulated Activation of Surface Species Using Multimodal In Situ/Operando Spectroscopy Combining Polarization-Modulation Infrared Reflection-Absorption Spectroscopy, Optical Emission Spectroscopy, and Mass Spectrometry

“…Infrared (IR) spectroscopy has been one of the prominent analytical techniques to study surface science in thermal catalysis research but has only recently began to make inroads in plasma catalysis research. To date, there have been a few studies using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) − and transmission IR spectroscopy to study plasma–catalyst coupling. These studies highlight the possibility of using IR spectroscopy to understand the reaction pathways in plasma catalysis.…”

“…Thus, it is desirable to utilize surface-specific IR techniques in conjunction with plasma-phase characterization to better understand the fundamental interactions of the plasma with a surface. Finally, the majority of the in situ/operando IR spectroscopy techniques reported to date have focused on probing surface and gas-phase products − without simultaneously characterizing the plasma-phase. To the best of our knowledge, there has been no report on a spectroscopic tool that can study surface-adsorbed species, gas-phase products, and plasma characteristics simultaneously.…”

Quantifying the impact of vibrational nonequilibrium in plasma catalysis: Insights from a molecular dynamics model of dissociative chemisorption

“…Most notably, the microscopic plasma-induced phenomena at the catalyst surface remain elusive, as well as their potential impact on the activity of the catalyst [9]. Only very recently, experimental methods have been devised to access in situ insights on the surface processes and intermediates in plasma-catalytic setups [10,11].…”