NiCo₂O₄ spinel for efficient toluene oxidation: The effect of crystal plane and solvent

Spinel oxides, e.g., NiCo₂O₄, is a promising catalyst for the catalytic oxidation of toluene. Understanding and designing versatile NiCo₂O₄ spinel is important for low-temperature toluene oxidation. Here, we investigated the surface-characteristic-dependent degradation activity of NiCo₂O₄ crystals through experiment and characterization. NiCo₂O₄ nanosheet using ethanol as solvent (named E--NiCo₂O₄) exposing {110} crystal planes exhibited the lowest temperature toluene oxidation. The T₉₉ of toluene conversion was 256 °C, which is much lower than that of NiCo₂O₄ nanosheet using ethylene glycol as solvent (named EG--NiCo₂O₄), NiCo₂O₄ octahedron (named O--NiCo₂O₄) and NiCo₂O₄ truncated octahedron (named TO--NiCo₂O₄). Characterization using various techniques such as XRD, TEM, BET, XPS, H₂-TPR and CO₂-TPD showed that Co³⁺ and surface adsorbed oxygen (O_sur) enriched surface, excellent redox properties and effective diffusion of the reaction product reasonably explain the enhancement in catalytic activity over the E--NiCo₂O₄. The research reveals that the effect of specific crystal planes and solvent was the key factor to govern the activity of low-temperature toluene oxidation.