Construction of core–shell structured CrOx/NiCoxO4 catalyst for low temperature catalytic oxidation of toluene: Understanding the reactivity promotion mechanism of the core–shell interactions

Bimetallic-based spinels are potential materials for toluene oxidation ascribing to their remarkable stability and oxidation activity. However, the insufficient toluene adsorption capacity and limited reactive oxygen species severely restrict their low-temperature catalytic reactivity. This study reports a facile two-step solvothermal strategy for the synthesis of core–shell structured CrOx/NiCoxO4 catalyst, which exhibits excellent toluene oxidation performance with T50 and T90 values of 187 and 218 °C, respectively, and the toluene conversion remains above 90 % at 218 °C for 20 h. The in-situ DRIFTs indicate that the oxidation of toluene to CO2 and H2O follows Mars-van Krevelen (MvK) mechanism, and the rate limiting step is the oxidation of quinone to maleic anhydride. The strong interaction between core NiCoxO4 and shell CrOx regulates the morphology of CrOx from particles to porous flocculent structure, which increases the porosity of catalyst and promotes the adsorption of toluene for further oxidation. Moreover, the interaction promotes the charge transfer from NiCoxO4 to CrOx, which increases the adsorbed oxygen species contents and the redox capability of the catalyst. A further density functional theory (DFT) calculation confirms that the core NiCoxO4 is the main active component, where the oxygen vacancy formation energy of NiCoxO4 and CrOx decrease from 2.43 and 3.85 eV to 0.31 and 1.52 eV, while the toluene adsorption energy decrease from −0.04 and −0.94 eV to −5.29, and −4.48 eV, respectively, which evidently demonstrates that the formation of core–shell structured CrOx/NiCoxO4 would significantly promote the adsorption and oxidation of toluene, thereby exhibiting excellent low temperature toluene oxidation reactivity.