Metalloporphyrin immobilized CeO2: in situ generation of active sites and synergistic promotion of photocatalytic water oxidation

Abstract

Inhibiting photo-generated charge recombination on molecular catalysts (MCs) and accelerating reaction kinetics on semiconductor catalysts are desired for promoting photocatalytic water oxidation. Coupling MCs with semiconductors to mimic natural photosynthetic systems may provide a smart strategy to simultaneously achieve the above targets. However, the lack of understanding for the interface interaction between the MCs and the semiconductors remains a key challenge to unlock the photocatalytic performance. Herein, we coupled two nonprecious-metal catalysts, cobalt porphyrin (CoTCPP) and cerium oxide (CeO₂ nanotubes), that are well matched in energy levels, to form CoTCPP@CeO₂ composites by facile solution synthesis. The interface interaction was systematically evaluated in terms of charge transfer and surface catalytic reaction. It was found that CoTCPP acts initially as a charge transporter to transfer photoexcited electrons to CeO₂ by d–f electron coupling, which effectively suppresses the charge recombination. Furthermore, the interfacial charge transfer results in reduction on CeO₂ accompanied with the creation of oxygen vacancies (OVs) and oxidation on CoTCPP that transforms into CoOOH. The in situ generated OVs and CoOOH species both act as the active sites to synergistically accomplish the photocatalytic water oxidation, thereby resulting in significantly boosted oxygen evolution reaction (OER) performance and long-term stability in neutral or even acidic environments. The CoTCPP@CeO₂NTs with an optimized composition ratio exhibit superior catalytic water oxidation characteristics in a neutral environment triggered by light, including a high oxygen production of 30 μmol g⁻¹ h⁻¹ and ultralong durability, which highly prevail over the performance of analogous systems such as H₂TCPP@CeO₂ NTs, Co₃O₄@CeO₂ NTs and TCPP@Co₃O₄@CeO₂ NTs under the same conditions. These findings disclosed the interfacial interactions between the heterogeneous components in terms of charge transfer and surface catalytically active sites, demonstrated the necessity of d–f electron coupling for effective charge transfer to inhibit charge recombination, and provided a smart strategy that highlights the in situ generation of catalytically active species, for instance, the OVs and oxyhydroxide, for promoting the photocatalytic OER and improving the catalytic durability.