Pollutant photo-conversion strategy to produce hydrogen green fuel and valuable sulfur element using H2S feed and nanostructured alloy photocatalysts: Ni-dopant effect, energy diagram and photo-electrochemical characterization

https://doi.org/10.1016/j.cherd.2020.07.024Get rights and content

Highlights

  • Pollutant conversion to hydrogen clean fuel: Solar-based strategy for environmental remedy.

  • New Ni-doped effective/affordable/eco-friendly alloy solar-energy materials.

  • Solid-solution photocatalysts: empirical (photoelectrochemical) determination of energy diagram.

  • Mechanistic understanding of the Ni-dopant effect on the photocatalyst performance.

Abstract

Design and synthesis of effective, eco-friendly, low-price semiconductor-based photocatalyst/solar-energy materials and their application for photocatalytic transformation of plentiful/perilous hazardous chemicals (e.g. H2S) into hydrogen green fuel and other valuable elements is a promising/sustainable strategy, which is highly in demand from environmental, energy and fuel as well as chemical and technological standpoints. To this end, herein, a set of new quaternary, Ni-doped n-type solid-solution (alloy) semiconductor compounds, viz. NixIIFe0.2IIIZn0.7xIIS (x = 0, 0.02, 0.05 and 0.1) were synthesized through a facile one-pot hydrothermal route and employed for the photocatalytic conversion of alkaline H2S medium (pH = 11) to hydrogen fuel and elemental sulfur. The investigations revealed that by adding Ni into the ternary photocatalyst-base (Fe0.2Zn0.7S), with decreasing the size of constituting nanoparticles and enlarging the photocatalyst surface area, the absorption intensity was strengthened. Among the materials under consideration, the lowest charge recombination (photoluminescence emission), highest photocurrent, and greatest displacement in open circuit and flat-band potentials were witnessed for x = 0.05, the photocatalyst with maximum performance to produce H2 and S. Finally, by using photoelectrochemical data and depicting the energy-diagram of system, a detailed discussion was provided and the phenomenon was justified from physicochemical viewpoint.

Introduction

H2S is a very dangerous, lethal, corrosive and flammable gas which is produced in large scale, during various industrial and natural processes on the bio-globe (Apte et al., 2013, Preethi and Kanmani, 2018). Besides sour gas and oil resources, this harmful chemical is generated biologically through anaerobic respiration/metabolism of sulfate/sulfur reducing bacteria (SRB) inside sewage and groundwater zones (Delgado et al., 1999, Agrahari et al., 2013). A promising sustainable route for elimination of this noxious environmental pollutant and optimal utilization of H2S is its photocatalytic transformation into its constituents. In this green photo-degradation/transformation strategy, using sunlight (photon energy) and effective, low-price, eco-friendly semiconducting materials, H2S could be straightforwardly converted into hydrogen fuel and valuable sulfur element – which is important for agricultural use (sulfur fertilizer, fungicides, acaricides, etc.), chemical industries (e.g. H2SO4 production) and synthesis of S-containing biomolecules (Gurunathan et al., 2008, Lashgari and Ghanimati, 2019a, Chaudhari et al., 2011, Preethi and Kanmani, 2016, Paulsen, 2004). Regarding H2S photoconversion, sulfide semiconducting materials seem a good photocatalytic choice, due to their chemical stability in H2S media, strong photons absorption in a broad spectral region, and the ability of bond formation with proton species (Lashgari and Ghanimati, 2019a, Dan et al., 2017).

Concerning the photo-degradation of H2S, it is worth declaring that owing to the safety issue, this gas is firstly dissolved in an alkaline solution. Thereafter, the degradation process of the medium is carried out inside a photoreaction chamber containing the suspended photocatalyst (semiconductor) powder. In this regard, the present authors have introduced Fe0.2Zn0.7S, as an effective, narrow-bandgap, low-price, eco-friendly alloy photocatalyst for the production of hydrogen fuel from H2S media (Lashgari and Ghanimati, 2018). In that work, it was shown that the H2S photo-degradation is a pH-dependent process and the maximum activity is attained at pH = 11, where HS is the dominant species in the reaction medium.

Besides the degradation of H2S and production of H2, the present work attempts to introduce the matter of sulfur extraction to solid-solution photocatalysts. Furthermore, we should declare that in the nature, nickel in company with iron is found as Ni-Fe hydrogenase enzyme, being employed for the generation/storage of H atoms through NADPH route (Pavlov et al., 1998, Kim and Kim, 2011, Lashgari and Diarmand-Khalilabad, 2016). A glance at the literature also reveals that Ni is important for photocatalytic applications (Jin and Zhang, 2020), and its presence could enhance the photocatalyst activity by increasing the catalyst surface area as well as intensifying the absorption of photons (Chang et al., 2015, Gultom et al., 2017). Due to the possibility of existing at different oxidation states, moreover, the presence of this element in the photocatalyst materials could improve the charge (e/h) separation phenomenon (Amario et al., 2018, Chen et al., 2013, Wang et al., 2013). Most importantly, it is worth noting that Ni has a good interaction with H2S (Rostrup-Nielsen, 1968); by doping this element into the ternary photocatalyst (Fe0.2Zn0.7S), a boosted activity is anticipated for the resulting quaternary material [like heterogeneous catalysis, in semiconducting photocatalyst/solar-energy materials, in order to do a photo-conversion (redox) process, the reactant species should be first adsorbed on the photocatalyst surface; therefore, for good interactions, more reactants can be adsorbed on the catalyst surface and by the formation of a chemical bond between reactant and surface, the electron transfer and hence the redox process at the semiconductor/solution interface, could occur more effectively]. Owing to Ni importance in photocatalytic systems, this element has been noticed by the researchers and employed for the synthesis of NixZn1−xS/O photocatalysts for application in photoredox processes (Rajakarthikeyan and Muthukumaran, 2017, Zhao et al., 2011, Priyadharsini et al., 2016); in these works, the values of x = 0.02, 0.05 and 0.1 have been reported as optimum for the Ni quantity.

Based on points mentioned above, in this work, using the earth-abundant/eco-friendly iron, zinc and sulfur elements, a new set of Ni-doped alloy photocatalysts is synthesized and applied for the photocatalytic transformation of H2S pollutant into hydrogen fuel and elemental sulfur. Herein, through a systematic approach, the authors examine the power of Ni-dopant on enhancing the photocatalyst performance and the findings are explained in detail from physicochemical/photoelectrochemical perspectives. Quantitative plotting of the energy diagram of the solid-solution (alloy) photocatalysts using photoelectrochemical data is another interesting feature of this article. Here, we also measure the quantities photovoltage and photocurrent (being generated by striking photons onto the photocatalyst surface (Lashgari and Soodi, 2020)), as two determining thermodynamic and kinetic factors, playing the role of driving/electromotive force (of the redox process) and reaction (e-transfer) rate, respectively.

Section snippets

Synthesis and characterization

To synthesize the solid-solution semiconducting photocatalyst materials, viz. NixIIFe0.2IIIZn0.7xIIS (x = 0, 0.02, 0.05 and 0.1), a facile hydrothermal method was adopted (Lashgari and Ghanimati, 2018, Lashgari and Ghanimati, 2014). Briefly, based on molar ratios of the precursors, in the synthesis of, for instance Ni0.05Fe0.2Zn0.6S, we first prepared a 50 ml aqueous solution containing 20 mmol thioacetamide (CH3CSNH2; 98%, Scharlau), 1 mmol Ni2+ (Ni(CH3COO)2.6H2O; 98%, Fluka), 4 mmol Fe3+ (Fe(NO3)3

Photocatalyst materials

XRD patterns of the photocatalyst/solar-energy materials under consideration are shown in Fig. 1. Similar to our previous report, the observation of 3 characteristic peaks at 28.8, 47.7, and 56.6° confirms the synthesis of photocatalyst base (solid-solvent; x = 0) (for more details, please see Ref. Lashgari and Ghanimati (2018)). Furthermore, the XRD patterns indicate that by adding Ni-dopant into the ternary photocatalyst base (Fe0.2Zn0.7S), no additional peak or appreciable displacement in the

Conclusion

Through a facile hydrothermal route, one could effortlessly synthesize a set of new effective micro/nano-particulated Ni-doped quaternary alloy photocatalyst materials and effectually employ them for H2S degradation in alkaline media (pH = 11) to produce hydrogen fuel and sulfur element. While the dopant influence on bandgap alteration is negligible, XRD, BET and SEM analyses revealed that by doping Ni atoms into the photocatalyst base (Fe0.2Zn0.7S), the size of constituting nanoparticles

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

The authors would like to acknowledge anonymous Reviewers as well as the Editor of this manuscript for their useful comments to enhance the quality of work.

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