Solar-Driven Reduction of Aqueous Protons Coupled to Selective Alcohol Oxidation with a Carbon Nitride–Molecular Ni Catalyst System

Abstract

Solar water-splitting represents an important strategy toward production of the storable and renewable fuel hydrogen. The water oxidation half-reaction typically proceeds with poor efficiency and produces the unprofitable and often damaging product, O2. Herein, we demonstrate an alternative approach and couple solar H2 generation with value-added organic substrate oxidation. Solar irradiation of a cyanamide surface-functionalized melon-type carbon nitride (NCNCNx) and a molecular nickel(II) bis(diphosphine) H2-evolution catalyst (NiP) enabled the production of H2 with concomitant selective oxidation of benzylic alcohols to aldehydes in high yield under purely aqueous conditions, at room temperature and ambient pressure. This one-pot system maintained its activity over 24 h, generating products in 1:1 stoichiometry, separated in the gas and solution phases. The NCNCNx–NiP system showed an activity of 763 μmol (g CNx)−1 h–1 toward H2 and aldehyde production, a Ni-based turnover frequency of 76 h–1, and an external quantum efficiency of 15% (λ = 360 ± 10 nm). This precious metal-free and nontoxic photocatalytic system displays better performance than an analogous system containing platinum instead of NiP. Transient absorption spectroscopy revealed that the photoactivity of NCNCNx is due to efficient substrate oxidation of the material, which outweighs possible charge recombination compared to the nonfunctionalized melon-type carbon nitride. Photoexcited NCNCNx in the presence of an organic substrate can accumulate ultralong-lived “trapped electrons”, which allow for fuel generation in the dark. The artificial photosynthetic system thereby catalyzes a closed redox cycle showing 100% atom economy and generates two value-added products, a solar chemical, and solar fuel.

Visible‐Light‐Driven Chemoselective Hydrogenation of Nitroarenes to Anilines in Water through Graphitic Carbon Nitride Metal‐Free Photocatalysis

“…More importantly, it will offer insights into the light‐induced mechanism . A positive dependence of the photocatalytic activity on the incident light intensity has been observed for both semiconductors and metal‐nanoparticle photocatalytic processes, and is considered as an experimental signature of light‐induced processes mediated by energetic charge carriers (hot electrons and holes) . This relationship was also observed for the g‐C 3 N 4 photocatalytic hydrogenation of nitrobenzene to aniline, with an almost linear dependence ( R 2 =0.991; Figure ).…”

Comprehensive review and future perspectives on the photocatalytic hydrogen production

“…Different examples also have been reported, but avoiding the use of a sacrificial electron donor remains a key issue. In this context, few demonstrations that combine H 2 production with oxidation of organic compounds, including lignocelluloses, have been reported recently . The gap between heterogeneous and homogeneous catalysis obviously blurs when molecular catalysts are immobilized on the surface of semiconductors.…”

Enhancing Light Absorption and Charge Transfer Efficiency in Carbon Dots through Graphitization and Core Nitrogen Doping

“…However,like g-N-CD,asaresult of increased light absorption, lower amounts of g-CD are required than a-CD and the specific activity is significantly higher (1549 vs. 226 mmol H2 (g CD ) À1 h À1 ;F igure 2b). [3,27] Thea ugmented performance agrees with the greatly increased visible light absorption of g-N-CD. [3] The g-N-CD/NiP photocatalytic system was further studied under conditions of visible-only light irradiation by equipping the solar light simulator with various longpass filters (l > 400, 455, 495 nm) ( Figure S20).…”