Electronic synergy between NiCo2O4 and adjacent carbon defects to enhance styrene cracking activity for styrene-saturated activated carbon regeneration

Abstract

The design and construction of catalysts featuring abundant electron transfer and defect structures for the regeneration of styrene-saturated activated carbon (AC) pose a significant challenge. This paper presents successful preparation of a regenerated adsorbent with synergistic catalytic active sites of NiCo₂O₄ spinel and carbon defects using a simple impregnation method. Enhanced catalytic activity is attributed to the electronic synergy between NiCo₂O₄ and carbon defects in 2.5Ni-2.5Co-900, involving substantial electron transfers, abundant acidic sites, and strong reducibility. Reactive oxygen species and the robust oxygen transfer facilitate the deep oxidation of intermediates and carbon elimination. The optimized regenerated adsorbent achieved a styrene adsorption capacity of 446.4 mg/g with a recovery of 92.4 % of the fresh sample, which is 1.5 times higher than 5Ni-900 and about 1.8 times higher than the unmodified regenerated activated carbon sample, RAC-900. The mechanism of styrene cracking and the evolution of intermediates were investigated using thermogravimetric-infrared coupling, and the effects of catalytic temperature and active sites on styrene cracking, deep oxidation of intermediates, and elimination of carbon deposits were explored. Kinetic analysis demonstrated that the adsorption of styrene on regenerated samples entailed a multifaceted process integrating both physical and chemical adsorption. The significance of chemical adsorption became increasingly evident as the concentration of defective sites grew. Intra-pore diffusion is the main determining step of the adsorption process. After five cycles, the adsorption capacity of 2.5Ni-2.5Co-900 decreased to below 25 %, suggesting promising strategies to overcome challenges of regenerating styrene-saturated AC.