Tailored Platinum Group Metal/Spinel Oxide Catalysts for Dynamically Enhanced Methane Oxidation

Abstract

A combined experimental and molecular modeling study identifies a family of spinel oxides that in combination with PGM (platinum group metals) provide enhanced methane oxidation activity. With a reduction in greenhouse gas (GHG) emissions urgently needed, there is renewed interest in the use of natural gas vehicles (NGVs) and engines (NGEs) for transportation, commerce, and industrial applications. NGVs and NGEs emit less CO₂ than their petroleum-derived counterparts but may emit uncombusted methane, an even more potent GHG. For stoichiometric engines, methane oxidation catalysts containing PGM and spinel oxide in layered architectures offer increased methane oxidation activity and lower light-off temperatures (T₅₀). The reducible spinel oxide has direct and indirect roles that are effectively described by the bulk oxygen vacancy formation energy (E_vac). We apply density functional theory (DFT) to identify several earth-abundant, cobalt-rich spinel oxides with favorable E_vac, shown to correlate with dynamic oxygen storage capacity (DOSC) and CO and H₂ oxidation activity. We experimentally rank-order the DFT-identified spinel oxides in combination with Pt+Pd for their methane oxidation activity measurements, under both time-invariant and modulated feed conditions. We show good agreement between the activity and the DFT-computed reducibility of the spinel oxide. The findings suggest spinel reducibility is a key factor in achieving enhanced low-temperature methane conversion, enabled through a balance of methane activation on the PGM sites and subsequent oxidation of the intermediates and byproducts on spinel oxides. In agreement with its computationally predicted E_vac, NiCo₂O₄ was confirmed to have the highest DOSC and lowest T₅₀ among the tested spinel samples.