Crystal facet engineering of spinel NiCo2O4 with enhanced activity and water resistance for tuneable catalytic methane oxidation†

Spinel NiCo₂O₄ are excellent catalysts for complete methane oxidation. Nevertheless, the spinel structure is thermally unstable and its activity is negatively affected by humidity. Herein, we report crystal facet engineering synthesis of spinel NiCo₂O₄ hexagonal nanosheets with different exposed facets. Density functional theory (DFT) simulations predict that a more viable reaction mechanism for methane oxidation occurs on 112-NiCo₂O₄ with {112} exposed facets compared with 111-NiCo₂O₄ with {111} exposed facets. Detailed material characterization and catalytic oxidation testing verified the DFT results showing that 112-NiCo₂O₄ has better thermal stability as well as higher catalytic activity towards methane oxidation than 111-NiCo₂O₄. Conversely, 111-NiCo₂O₄ has the enhanced water resistance of the two catalysts. DFT calculations suggest that OH groups tend to preferentially adsorb onto metal sites, which (1) reduces the number of active sites available and (2) makes CH₄ adsorption and activation a more arduous process. This study offers insights on the behavior of spinel oxide catalysts towards methane combustion in dry and humid conditions, further demonstrating that crystal facet engineering can be a practical strategy to tune the activity and water resistance of metal-oxide catalysts.