Rational Design of Covalent Cobaloxime-Covalent Organic Framework Hybrids for Enhanced Photocatalytic Hydrogen Evolution.

Kerstin Gottschling,Petra Rovó,Tanmay Banerjee,Hugo Vignolo-González,Bettina V Lotsch,Gökcen Savasci,Christian Ochsenfeld,Philipp Mauker,Sandra Schmidt

doi:10.1021/jacs.0c02155

Abstract

Covalent organic frameworks (COFs) display a unique combination of chemical tunability, structural diversity, high porosity, nanoscale regularity, and thermal stability. Recent efforts are directed at using such frameworks as tunable scaffolds for chemical reactions. In particular, COFs have emerged as viable platforms for mimicking natural photosynthesis. However, there is an indisputable need for efficient, stable, and economical alternatives for the traditional platinum-based cocatalysts for light-driven hydrogen evolution. Here, we present azide-functionalized chloro(pyridine)cobaloxime hydrogen-evolution cocatalysts immobilized on a hydrazone-based COF-42 backbone that show improved and prolonged photocatalytic activity with respect to equivalent physisorbed systems. Advanced solid-state NMR and quantum-chemical methods allow us to elucidate details of the improved photoreactivity and the structural composition of the involved active site. We found that a genuine interaction between the COF backbone and the cobaloxime facilitates recoordination of the cocatalyst during the photoreaction, thereby improving the reactivity and hindering degradation of the catalyst. The excellent stability and prolonged reactivity make the herein reported cobaloxime-tethered COF materials promising hydrogen evolution catalysts for future solar fuel technologies.

Highlights

Identifying competitive alternatives to fossil-fuel-based energy constitutes one of the main research goals of this decade
Covalent organic frameworks (COFs)-4227 has been shown to be active in photocatalytic hydrogen evolution reactions with conventional hydrogen evolution cocatalysts such as platinum nanoparticles or molecular chloro(pyridine)cobaloxime.[14]
The COFs were characterized by FT-IR spectroscopy, sorption analysis, powder X-ray diffraction (PXRD), magic-angle-spinning solid-state NMR, and quantum-chemical calculations

Summary

■ INTRODUCTION

Identifying competitive alternatives to fossil-fuel-based energy constitutes one of the main research goals of this decade. Recent studies showed that the precious metal platinum can be replaced by earth-abundant molecular cocatalysts, namely, chloro(pyridine)cobaloxime and related complexes.[14−16] These cocatalysts are well-known and well-defined, while offering high tunability, which facilitate their incorporation into photoactive organic and inorganic systems.[17−19] Cobaloximes feature low overpotential for the hydrogen evolution reaction and have been used in heterogeneous systems with metal− organic frameworks[20,21] and carbon nitrides,[22,23] as well as physisorbed to COFs.[14] A major drawback of molecular proton reduction catalysts physisorbed to photosensitizers is their photodeactivation over time[24−26] and rate limitations due to diffusion-controlled mechanisms. These results combine the advantages of fully heterogeneous systems with the tunability of molecular cocatalysts and lead the way toward true single-site COFbased photocatalytic systems with a high level of interfacial control

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES