Synergy of Oxygen Vacancies and Confinement Effect in CO₂ Reforming of Toluene over Hydrotalcite-Derived Hollow-Sphere NiCo@Al₂O₃ Catalysts

CO2 reforming of tar is a promising pathway for the simultaneous conversion of undesirable tars and CO2 generated from biomass gasification, which is critical for syngas upgrading and utilization. However, catalyst deactivation caused by coking is a severe issue for supported catalysts in this application. In this study, hydrotalcite-derived NiCo alloys supported by Al2O3 nanosheet self-assembled hollow spheres were constructed by a template-sacrificial coprecipitation method. The hollow-sphere CS@NiCo(CP) catalyst exhibited superior activity and stability compared to NiCo(CP), NiCo(HT), and CS/NiCo(HT) catalysts synthesized by conventional coprecipitation, conventional hydrothermal, and template-sacrificial hydrothermal methods. The confinement effect of the hollow structure and porous shells enriched the local concentrations of CO2 relative to toluene adjacent to the catalytic sites, owing to the high diffusion resistance of toluene through the shell. Furthermore, the abundant oxygen defects and stronger basic sites in the CS@NiCo(CP) catalyst further facilitated the adsorption and activation of CO2 and provided higher quantities of active oxygen species for the gasification and elimination of surface carbon intermediates produced by toluene cleavage. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments suggest that abundant oxygen defects in the catalyst accelerated the critical steps of the ring-opening and oxidation of toluene.