V2AlC MAX-mediated 2D/0D ZnCo2O4 NS/TiO2 Z-scheme heterojunctions with enhanced photocatalytic CO2 bireforming via synergistic charge transfer in a monolith and flow reactor

The exfoliated 2D V₂AlC MAX (VM) and 2D ZnCo₂O₄ nanoslabs (ZNS) were designed to construct 2D/2D/0D V₂AlC-M/ZnCo₂O₄-NS/TiO₂ monolithic nanotextures for photocatalytic CO₂ reduction. The photocatalytic efficiency of VM-ZNS/TiO₂ nanocomposite was systematically studied in fixed-bed, monolith, and flow photoreactors, and the roles of catalyst, reactor, and process on yield and selectivity were disclosed. Incorporating 2D V₂AlC provides a conductive surface between the core–shell structure of 2D/0D ZNS/TiO₂, which prevents the recombination of photo-induced charges with higher charge transfer mobility. Using a fixed bed, V₂AlC/TiO₂ (VT) was beneficial to maximize the formation of CO, whereas ZVT (ZnCo₂O₄/V₂AlC/TiO₂) promoted the yield of CH₄ and H₂ during the bireforming of CO₂ under a batch process. The ZVT has much higher photocatalytic efficiency, which can be linked to the efficient generation and transfer of charges within the composite. The monolith reactor with batch operation exhibited exceptional performance, producing 49313, 32,429 and 14,060 µmol g⁻¹ of CO, CH₄ and H₂, a 2.82, 6.17 and 2.68-fold increase compared to the fixed-bed reactor. Comparatively, the fixed bed and flow reactor favoured CO and H₂ production, whereas the monolith was beneficial for CH₄ and H₂ formation. The monolith photoreactor showed the highest overall efficiency due to its superior light and mass transfer design, facilitated generation and utilization of photogenerated electrons. Thus, ZVT favours selectively producing CO, CH₄ and H₂ depending on the type of reactors, which confirms that yield and selectivity depend on the catalyst and reactor and the type of operation due to different mass transfer and reaction kinetics limitations. The stability of ZVT was also different for each reactor, and it was stable in multiple consecutive cycles, showcasing its potential for sustainable CO₂ reduction applications.