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

To obtain more applicable catalysts with sulphur and water vapor tolerance for toluene combustion, CuxCo1−xAl2O4 compounds by substituting Co−site of CoAl2O4 spinel with different amount of Cu have been purposely designed. For the first time, the XRD extrapolation method has been adopted to measure the substitution capacity of Cu2+ in the spinel matrix, which is with an x = 0.53. Below the capacity, Cu2+ is fully incorporated in the lattice, thus forming substituted CuxCo1−xAl2O4 with spinel pure phase. Above the capacity, the excess Cu2+ overflows to the surface to generate CuO crystallites. Meanwhile, the structure transforms gradually from CoAl2O4 to CuAl2O4 phase. Cu0.5Co0.5Al2O4 sample near the substitution capacity exhibits the highest activity, with an evident threshold effect being observed. Combining experiments and DFT calculation, 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 follows a Mars−van−Krevelen mechanism, with a possible pathway of toluene → benzyl alcohol → benzaldehyde → benzoic acid → CO2. Compared to the unmodified CoAl2O4, Cu0.5Co0.5Al2O4 exhibits remarkably improved and potent SO2 and H2O tolerance. This work offers new insights into designing high−performance non−noble metal catalysts for real VOCs combustion.