Natural Sunlight Harvesting Benzothiadiazole-based Molecular Photocatalyst for H2O2 Production: Recyclable Homogeneous Biphasic System

Abstract

Photocatalytic hydrogen peroxide (H2O2) synthesis offers a sustainable route for solar-to-chemical energy conversion to address the increasing global energy demand. However, developing an efficient and sustainable photocatalytic system for H2O2 synthesis remains a critical bottleneck. Herein, we report a unique toluene-water two-phase system for highly efficient H2O2 synthesis employing benzothiadiazole-based small organic photocatalyst Py-BT-Th, which remained nearly unseen. Py-BT-Th shows a high H2O2 production rate of 107.18 mmol g-1 h-1 with turnover frequency of ~ 32 h-1 under natural sunlight irradiation in the toluene-water biphasic system using triethylamine as a sacrificial agent. The biphasic system furnishes unparalleled opportunity for high H2O2 mass transport and hence rapid kinetics across the interface formed between the toluene-water two-phase system. Experimental studies reveal that the superior photocatalytic performance of the catalyst Py-BT-Th is attributed to its donor-acceptor type molecular architecture, which promotes efficient exciton separation, high molar absorptivity (Ɛ = 12.27 × 103 M-1 cm-1), long exciton lifetime (12.9 ns) and broad light-harvesting capacity. Scavenger and in-situ EPR spectroscopic studies confirm that Py-BT-Th produces H2O2 following a dual mechanistic route: (1) Indirect two-step one-electron ORR via electron transfer pathway, (2) Singlet oxygen generation via energy transfer pathway. In both routes, •O2− is the key intermediate that ultimately gets converted into hydrogen peroxide, in association with the oxidation of triethylamine by photogenerated holes. Overall, this work disentangles the dilemma associated with small organic photocatalysts for H2O2 synthesis through a recyclable toluene-water biphasic system.