Elsevier

Molecular Catalysis

Volume 515, October 2021, 111856
Molecular Catalysis

A strategy for integrating transition metal-complex cocatalyst onto g-C3N4 to enable efficient photocatalytic hydrogen evolution

https://doi.org/10.1016/j.mcat.2021.111856Get rights and content

Highlights

  • A novel strategy of metal functionalized g-C3N4 is developed for photocatalytic hydrogen evolution without noble metals.

  • A hydrogen evolution rate of 68.6 μmol h−1 (λ ≥ 420 nm) on Ni/DBM(0.12%)-CN exceeds most reported values.

  • The coordination-introduced metal ion improves the utilization of visible light through the metal to ligand charge transfer mechanism.

  • A unique hydrogen evolution reaction pathway is proposed based on both experiments and theoretical calculations.

Abstract

Achieving efficient and stable solar-to-hydrogen conversion is of great significance to the sustainable development of global energy. Herein, we report a novel strategy for metal functionalization via in-situ construction of metal coordination sites in graphitic carbon nitride (CN), in which pyrimidinone derivative ligands are incorporated onto CN framework through one-step vapor diffusion process, and then transition metal ions (such as Ni, Cu, Co) are covalently immobilized by strong coordination interaction. The coordination-introduced metal ions not only serve as catalytic reaction sites, but also improve the utilization of visible light through the metal ion to ligand charge transfer (MLCT) mechanism. Specially, the strong interaction between metal ion and ligand promotes charge separation by expanding the π-conjugated delocalization system of CN, which makes it possible to continuously generate hydrogen without noble metals. The optimal sample Ni/DBM(0.12%)-CN shows a hydrogen evolution rate of 68.6 μmol h−1 (λ ≥ 420 nm) and a remarkable quantum conversion efficiency of 5.57% under irradiation of the light with wavelength of 450 nm. The mechanism of high-activity hydrogen production was studied through both experiments and theoretical calculations. This study provides a new strategy for designing cost-effective CN photocatalyst with alternative metal coordination sites for efficient visible light utilization.

Introduction

Energy sustainability is essential to global economic development and human social progress [1,2]. As one of sustainable clean energy, hydrogen has attracted more and more attentions. Generation of hydrogen from solar energy is becoming a research focus [3]. Learning form photosynthesis of green plants, a variety of photocatalysts for solar-to-hydrogen conversion was developed. Among them, graphitic carbon nitride (CN), as a layered polymer semiconductor, was first reported for photocatalytic water splitting in 2009 [4]. In view of its many advantages such as metal-free composition, high thermal stability and low-cost [5], graphitic carbon nitride-based catalysts have been widely developed for water splitting, CO2 reduction and organic photosynthesis [6].

Nevertheless, pristine CN suffers from limited utilization of visible light, sluggish charge separation and retarded surface photoreaction kinetics, which prohibit its possible industrial applications [[7], [8], [9]]. Under the efforts of researchers, several strategies such as combining with other semiconductors [10,11], modifying microstructure [12,13], copolymerizing with π-conjugated aromatic molecules and introducing heteroatoms have been successfully applied to improve the photocatalytic performance of CN [[14], [15], [16]]. Very importantly, introducing π-electron-rich aromatic organic molecules to CN skeleton is able to effectively enlarge the π-conjugated system, thus improves the utilization of long-wavelength light [17]. For example, Li and his colleagues prepared aromatic rings-embedded CN through copolymerization of urea precursor and benzene compounds, which effectively adjusted the bandgap energy and improved the photocatalytic activity of CN [16]. Unfortunately, the above modification strategies still require extra cocatalysts such as noble metals (Pt, Ru or Pd) to reduce the hydrogen evolution overpotential and activate protons to generate hydrogen. The high price of precious metals hinders the practical application of CN for hydrogen evolution [18,19]. Therefore, developing an efficient but simple modification method is the primary task for its industrialization.

Among them, establishment of coordination sites for anchoring transition metals in catalysts to achieve functionalization is an effective way to enhance photocatalytic activity without extra noble metal cocatalysts, which is ubiquitous in natural photosynthesis [20,21]. In particular, this strategy can induce metal to ligand charge transfer (MLCT) process, which generates more electrons through the low-energy electron transfer pathway between metal center and ligand skeleton [22,23]. Meanwhile, more catalytic active sites are exposed to accelerate surface reaction. Based on this, Yang et al. developed a single-atom cobalt modified CN photocatalyst through one-step synthesis strategy, demonstrating that Co atoms are immobilized on CN by forming covalent Co−O and Co−N bonds [24]. To improve the sluggish kinetics in hydrogen evolution reaction (HER), Qi and co-workers introduced single-site copper to CN matrix by in-situ synthesis, achieving optimal hydrogen generation rate of 4.5 times of that of pure CN [25]. Kong et al. also proposed a photo-assisted method to prepare Ni/g-C3N4 composite photocatalysts [26]. Due to the improved electron mobility and faster charge dynamics, the prepared Ni(0)-based CN catalyst exhibits excellent performance for hydrogen evolution without any other cocatalyst. Furthermore, to design metal-functionalized photocatalysts for efficient hydrogen production, the selection of metal center is the foremost. Nickel is considered an ideal choice to replace noble metals in metal functionalization due to its inherent high activity and low-cost in HER [27]. Accordingly, Zhao and his colleagues successfully introduced well-dispersed Ni-EDTA complex onto the surface of CdS for photocatalytic hydrogen evolution. The activity of the hexacoordinated Ni(II) center as a cocatalyst even exceeds that of Pt [28]. We have successfully synthesized nickel-functionalized CN catalyst by combining well-designed nickel complex and benzene-embedded CN framework, demonstrating that the rich redox properties of nickel(II) center in the metal-functionalized catalyst endow it excellent hydrogen evolution performance. As being reported, the easily accessible oxidation states of nickel can be Ni(0), Ni(I), Ni(II), Ni(III), and Ni(IV) [29,30]. HER over nickel metal sites usually involve the conversion of Ni(II) to Ni(I), which corresponds to the transition from tetradentate planar coordination mode suitable for Ni(II) to tetrahedral coordination mode suitable for Ni(I) [31,32]. The overall HER cycle including one-electron reduction and protonation of nickel center from Ni(II) to H−Ni(III), which further combines with protons to form hydrogen [33,34]. Therefore, the strategy of designing photocatalysts with alternative metal coordination sites is reasonable and attractive, especially for the development of catalysts with nickel active sites.

In this work, we proposed a novel vapor diffusion method to synthesize metal-functionalized photocatalyst by in-situ constructing localized metal coordination sites in CN framework. Through keto-enol cyclization reaction between urea precursor and phenyl-containing molecules (including acetylacetone (AA), benzoylacetone (BA), and dibenzoylmethane (DBM)), the newly-formed pyrimidinone derivatives in CN skeleton act as bidentate ligands (providing both O- and N- coordination sites) to coordinate with transition metals. Finally, the metal-coordinated pyrimidinone derivatives that introduced to CN framework can replace Pt cocatalyst to generate hydrogen under visible light [[35], [36], [37]]. In addition, pyrimidinone derivative with different aromatic structures can change the strength of the covalent bond formed with metal ions, and the level of π-conjugated effect as well. As a proof of concept, nickel was chosen as the main model metal. The result shows that all catalysts exhibit excellent HER performance, among which the highest activity of Ni/DBM(0.12%)-CN reaches 68.6 μmol h−1, and no obvious decay during 15 h cyclic test. According to the results of experiments and DFT computations, the introduced pyrimidinone derivatives with abundant π-electrons can amplify the metal to ligand charge transfer (MLCT) effect and enhance the delocalization of the π-conjugated system. This is beneficial for the long-wavelength light absorption and photogenerated carriers separation in CN [22]. The excellent HER performance of the prepared catalysts proves that this metal-functionalized strategy is feasible, and the performance of the optimized catalyst exceeds most reported noble-metal-free CN-based photocatalysts.

Section snippets

Synthesis of CN and pHsingle bondCN catalysts

Pristine carbon nitride (CN) was prepared by annealing urea at 550 °C with a heating rate of 5 °C min−1 for 4 h. To introduce pyrimidinone derivatives with localized metal coordination sites to CN framework, a novel in-situ approach of vapor diffusion was developed (Figs. S1 and 1a). As shown in Fig. S2, a certain amount of phenyl-containing molecules (denoted as pH), namely acetylacetone (AA), benzoylacetone (BA) and dibenzoylmethane (DBM) were placed in a small quartz boat which was hung over

Formation process of Ni/pH-CN

As a family of diimine ligands, pyrimidinone derivatives are widely used in ligand exchange barrier studies and heavy metal ion recognition [40]. Experimentally, the framework of pyrimidinone derivatives can provide abundant N- and O- complexing ligands and form thermodynamically favorable complex through strong metal-ligand interactions [37]. Therefore, a keto-enol cyclization reaction between urea and phenyl-containing molecules is applied to in-situ build metal coordination sites in CN

Conclusion

In summary, this work developed a one-step approach to in-situ construct metal coordination sites in CN matrix, and then coordinate with transition metal for functionalization to achieve noble-metal-free photocatalysts. The nickel-coordinated pyrimidinone derivatives in CN framework promote the photocatalytic activity of Ni/pHsingle bondCN through the proposed MLCT mechanism. Meanwhile, three phenyl-containing molecules (acetylacetone, benzoylacetone, and dibenzoylmethane) are used to tune the conjugated

CRediT authorship contribution statement

Peng Zeng: Writing – original draft, Data curation, Conceptualization, Methodology, Visualization, Investigation, Formal analysis. Wei-De Zhang: Writing – review & editing, Supervision, Validation, Project administration, Resources, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (21773074).

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