Revealing the active sites of the structured Ni-based catalysts for one-step CO2/CH4 conversion into oxygenates by plasma-catalysis

Abstract

The direct conversion of CO2/CH4 into liquid chemicals is an attractive way for using carbon resources, but it is still unclear which types of catalysts are suitable for plasma-involved catalysis. Herein, a serious of nickel foam (NF) supported Ni-based catalysts derived from Layered double hydroxides (LDH) were newly designed by a green hydrothermal strategy. The crystal structures, morphologies, and the redox behaviors of these catalysts (NiAl-LDH/NF, NiO/NF, Ni/NF and NiGa/NF) were systemically characterized. Plasma-enabled direct conversion of CO2/CH4 was performed in a co-axial dielectric barrier discharge reactor using a nanosecond repetitive-pulse power, and the results showed that the selectivities of the liquid oxygenates were strongly depended on their microstructures, surface compositions (valence states) and reducibilities. The zero-valent Ni/NF and NiGa/NF showed high total liquid selectivity (> 30 %) and dominant CH3COOH (> 15 %), while the Ni2+ in NiO/NF gave the comparable activities for CH3OH (8.9 %) and CH3COOH (9.6 %). Unexpectedly, NiAl-LDH/NF exhibited the highest CH3OH selectivity (12.3 %) and negligible carbon deposition. According to systemic characterizations, an “−OH reservoir” mechanism was proposed that numerous −OH groups over the ultrathin nanosheets (∼60 nm) of NiAl-LDH may react with the interfacial CHx* radicals and, on the other hand, possess inferior electron affinity for electronegative C atom, thus endowing NiAl-LDH the prominent CH3OH selectivity and high resistance to carbon deposition.