Active CuxCo1−xAl2O4 spinel catalysts with potent SO2 tolerance for toluene combustion: measuring the Cu substitution capacity in the spinel lattice with XRD extrapolation

Abstract

To obtain more applicable catalysts with sulphur and water vapor tolerance for toluene combustion, Cu_xCo_1−xAl₂O₄ compounds have been purposely designed by substituting the Co site of the CoAl₂O₄ spinel with different amounts of Cu. For the first time, the XRD extrapolation method has been adopted to measure the substitution capacity of Cu²⁺ in the spinel matrix, which is x = 0.53. Below this capacity, Cu²⁺ is fully incorporated in the lattice, thus forming substituted Cu_xCo_1−xAl₂O₄ with a pure spinel phase. Above this capacity, the excess Cu²⁺ overflows to the surface to generate CuO crystallites. Meanwhile, the structure transforms gradually from the CoAl₂O₄ phase to the CuAl₂O₄ phase. The Cu_0.5Co_0.5Al₂O₄ sample near the substitution capacity exhibits the highest activity, with an evident threshold effect being observed. Combining experiments and DFT calculations, it has been discovered that this catalyst possesses the richest amount of surface defects, active oxygen and acidic sites, which promotes the activity synergistically. In situ DRIFTS results have disclosed that toluene combustion could follow a Mars–van Krevelen mechanism, with a possible pathway of toluene → benzyl alcohol → benzaldehyde → benzoic acid → CO₂. Compared to the unmodified CoAl₂O₄, Cu_0.5Co_0.5Al₂O₄ exhibits remarkably improved and potent SO₂ and H₂O tolerance. This work offers new insights into designing high-performance non-noble metal catalysts for real VOC combustion.