Leveraging oxygen mobility with zirconia in low-temperature plasma for enhanced methane reforming to syngas†

Abstract

Despite extensive efforts to optimize the single-step production of syngas, hydrocarbons, and oxygenates via plasma catalysis, several challenges remain unresolved. In particular, understanding the various reaction pathways is hindered by the complexity of the reactions and the diverse range of chemical products formed. In this study, our main objective is to evaluate and compare the influence of zirconia on reaction pathways, methane (CH₄) and carbon dioxide (CO₂) conversions (%), and syngas selectivity (%) relative to the plasma-only route. Experiments were conducted at a low radio-frequency plasma power of 50 Watts without external heating. The results demonstrated significantly enhanced conversions of carbon dioxide and methane when the reaction chamber was packed with zirconia (ZrO₂). Methane conversion was observed to be the highest at a rich CO₂ feed [CO₂ : CH₄ (2 : 1)], while plasma only revealed conversion of 20.1%. After packing with zirconia, the conversion increased to 71.2% (3.5 times increment). On the other hand, carbon dioxide conversions were also observed to be the highest at a feed composition of CO₂ : CH₄ (2 : 1), with plasma only (13.6%) vs. with zirconia packing (60.9%) revealing a 4.4 times increase. Interestingly, at the rich CO₂ feed composition, the syngas product (CO + H₂) selectivity increased after packing ZrO₂ by 1.1 times for CO and 1.2 times for H₂. Optical emission spectroscopy (OES) analysis revealed important insights into the gas phase, with signatures of atomic oxygen (O) being the dominant plasma species in the gas phase under plasma-only conditions, while their intensities plummeted when zirconia was introduced, indicating active oxygen diffusion onto the surface of zirconia. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) confirmed important surface alterations after plasma exposure and most importantly provided experimental proof on zirconia's oxygen mobility. These findings provided an integral perspective into the design of catalytic materials that enhance oxygen mobility, enabling low-temperature and energy-efficient dry methane reforming for a sustainable future.