Novel MnS/(InxCu1-x)2S3 composite for robust solar hydrogen sulphide splitting via the synergy of solid solution and heterojunction

https://doi.org/10.1016/j.apcatb.2018.11.016Get rights and content

Highlights

  • A “Solid solution-Heterojunction” strategy was applied to design the MnS/(InxCu1-x)2S3 composites for H2S splitting.

  • The highest H2 production i.e. 29,252 μmol h−1 g−1 was obtained over MnS/(InxCu1-x)2S3 composite.

  • EXAFS and DFT calculations confirm that the formation of (InxCu1-x)2S3 solid solution.

  • The (InxCu1-x)2S3 solid solution plays dual roles: (i) enhancing light absorption, and (ii) promoting the desorption of S.

  • The 2D layered heterostructure between γ-MnS and (InxCu1-x)2S3 is crucial to the electron–hole separation efficiency.

Abstract

Large photocatalytic hydrogen (H2) production from copious waste hydrogen sulphide (H2S) can meet the increasing demand for H2 in a sustainable manor which is beneficial from both environmental and energy standpoints. In this work, we reported a robust MnS/(InxCu1-x)2S3 composite photocatalyst. Both experimental results and density functional theory (DFT) calculations proved that Cu does not act as a cocatalyst but forms a solid solution ((InxCu1-x)2S3) in the composites, which plays dual roles in improving the photocatalytic performance of H2S splitting: (i) enhancing solar light absorption, and (ii) promoting the desorption of sulfur (S) adsorbed on the catalyst surface. Moreover, the formation of a heterojunction between γ-MnS and (InxCu1-x)2S3 can significantly improve charge separation and migration in the composites. As a result, the MnS/(InxCu1-x)2S3 exhibits greatly extended visible light absorption up to 599 nm and extraordinarily high photocatalytic H2 production under visible light from H2S with a maximum rate of 29,252 μmol h−1 g−1. The corresponding apparent quantum efficiencies (AQE) at 420 and 450 nm are as high as 65.2% and 62.6%, respectively. They are the highest so far for the visible light photocatalytic splitting of H2S in the absence of noble-metal co-catalysts.

Introduction

High-quality oil and natural gas reservoirs have been excessively used and depleted. Meanwhile, the energy consumption increased remarkably over the past years. To satisfy the fast growing energy demands, the oil and gas developments have to turn to high acid oil and gas reservoirs, in which hydrogen sulphide (H2S) is an important component [[1], [2], [3]]. Every year, millions of tons (>4 × 107 t) of H2S are produced around the world from oil-refinery plants and natural-gas extraction, and this trend will increase further in the future [4]. H2S, owing to the extremely toxic, malodorous and corrosive nature, is a huge obstacle for exploiting acid oil and gas reservoirs. Hence, the effective removal of H2S is highly desired. Most of the current industrial technology for removing H2S is the Claus process [5], which is not an eco-friendly strategy because of the high temperature (ca. 1200 °C) involved. Meanwhile, this process results in further environmental problems due to the generation of hazardous by-products, e.g. SOx. More importantly, instead of capturing hydrogen (H2) from H2S, the Claus process ends up converting it to sulfur (S) and H2O [6]. As a clean energy carrier, H2 has the highest energy density (120–142 MJ/kg) in comparison to any other fuels without carbon trace [[7], [8], [9], [10]]. Therefore, the production of H2 from H2S is beneficial for both the abatement of the toxic pollutant and producing clean energy.

Over the past years, various strategies have been proposed for converting H2S to H2 and S [1]. Among them, the photocatalytic method, which can directly utilize abundant solar energy, has attracted considerable attention [[11], [12], [13], [14], [15]]. Nevertheless, the quest for suitable photocatalysts for H2 production from H2S is still a tremendous challenge as the catalyst deactivation was commonly observed during H2S splitting process [16]. Recently, metal sulfides have been explored and verified to be more suitable photocatalysts compared to oxides for splitting H2S as metal sulfides are more stable in the S-containing medium and can effectively reduce the catalyst deactivation [11,12,[17], [18], [19], [20]]. Moreover, the serious photocorrosion of metal sulfides could be inhibited during photocatalytic splitting of H2S process, because the photo-generated hole can be consumed by the excess of H2S (H2S + 2 h+ → S + 2H+) [17]. Unfortunately, single-phase metal sulfides usually suffer from low photocatalytic efficiency due to the poor solar light absorption and fast recombination rate of photo-induced excitons [12,[21], [22], [23], [24]]. To overcome these drawbacks, guiding the rational design of suitable photocatalysts to establish an efficient pathway to improve the efficiency of H2 evolution from H2S is essential.

To date, fabrication of heterojunction with other semiconductors was frequently used to effectively inhibit the electron-hole recombination [[22], [23], [24]], Numerous metal sulfide heterojunctions, such as In2S3/MoS2/CdS [25], CuS@CuInS2:In2S3 [26], and CdS/PANI [27], etc. have been reported for highly efficiency photocatalytic H2 production due to their remarkable charge-separation and transfer efficiency. For instance, we previously reported an efficient MnS/In2S3 heterostructure photocatalyst with an apparent quantum yield (AQE) of 34.2% at 450 nm [17]. However, the light absorption ability of semiconductor heterojunctions was limited by the band gap position of the original single-phase photocatalyst. On the other hand, the formation of solid solution has been proposed as an elegant strategy to extend visible light absorption [[28], [29], [30], [31]]. Among them, solid solutions containing copper have became a research hotspot. Akihiko Kudo et al. has reported a series of solid solutions containing copper with remarkable solar light absorption ability, such as ZnS-CuInS2-AgInS2 [28,29], (CuIn)xZn2(1-x)S2 [30], (CuGa)1−xZn2xS2 [31]. The density functional theory (DFT) calculations revealed that the contribution of Cu 3d orbital to the valence band is crucial to realize the superior visible-light-response. Therefore, the synergy of solid solution and heterojunction could offer a highly active visible light photocatalyst for H2 production from H2S. In addition, the formed S during the photocatalytic reaction can be adsorbed on the catalyst surface to block active sites [12,17]. Hence, the desorption ability of S on the catalyst surface is also crucial for long-term photocatalytic H2 production from H2S. To the best of our knowledge, most of the previous work only focused on one or two above aspects to enhance the photocatalytic performance, but very few non-noble metal photocatalysts can meet these strict demands.

In this work, a series of novel MnS/(InxCu1-x)2S3 composites with hierarchical porous structures were successfully fabricated via a facile one-pot solvothermal method. They exhibit greatly extended light absorption up to 599 nm and outstanding visible light photocatalytic H2 production from H2S with a maximum H2 production rate of 29,252 μmol h−1 g−1. The corresponding AQE at 420 and 450 nm are as high as 65.2% and 62.6%, respectively. The local structure of Cu is identified through the combination of synchrotron-based experimental study and DFT calculations. Notably, CuS is not the active phase, instead, Cu is incorporated in the crystal structure of β-In2S3 to form (InxCu1-x)2S3 solid solution, which can enhance the visible light absorption and promote the desorption of S adsorbed on the catalyst surface. Moreover, the formation of heterojunction between γ-MnS and (InxCu1-x)2S3 can further improve charge separation and migration in the composites. These findings provide a new strategy for the design of robust photocatalysts for H2S splitting via the synergy of solid solution and heterojunction.

Section snippets

Synthesis of the composites

In a typical procedure, Mn(Ac)2·4H2O (1.4 mmol), InCl3 (0.6 mmol) and thioacetamide (TAA) (9.0 mmol) were dissolved in 23.8 mL pyridine (VPy) to form a homogeneous solution. Subsequently, 1.2 mL of 0.05 M Cu(Ac)2·H2O/pyridine (VCu/Py) solution was added dropwise in the above solution under constant stirring. The obtained solution was immediately transferred to a 50 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 30 h. After cooling to room temperature, the prepared

Results and discussion

The photocatalytic performance of all the as-obtained samples was evaluated by H2 production from H2S under visible-light irradiation (λ > 420 nm). The samples synthesized at 180 °C for 30 h with different compositions were denoted as MIC0.6, MIC1.2, MIC1.8, and MIC3.0, using an increasing volume of 0.05 M Cu(CH3COO)2·H2O precursor as described in the Experimental section. The blank experiments revealed that H2 was not detected in the absence of photocatalyst or light irradiation. As shown in

Conclusion

In summary, we designed and reported a highly active noble-metal free MnS/(InxCu1-x)2S3 composite based on the synergy of solid solution and heterojunctions. The maximum H2 production rate of 29,252 μmol h−1 g−1 is achieved over the optimized MnS/(InxCu1-x)2S3 composite under visible-light irradiation (λ > 420 nm), which is ca. 2.5 times higher than that of the pristine MnS/In2S3 (11,945 μmol h−1 g−1) and 1400 times higher than that over CuS (21 μmol h−1 g−1) alone. The corresponding AQE at 420

Acknowledgements

This research was financially supported by the Sichuan Provincial International Cooperation Project (2017HH0030), the Innovative Research Team of Sichuan Province (2016TD0011) and the National Natural Science Foundation of China (U1232119). We thank the Synchrotron radiation source at KIT, Karlsruhe, for providing beamtime and the beamline scientists Dr. Anna Zimina and Dr. Tim Prüssmann at CAT-ACT for the support during measurements.

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