Full paperAmmonia photosynthesis under ambient conditions using an efficient nanostructured FeS2/CNT solar-energy-material with water feedstock and nitrogen gas
Graphical abstract
Introduction
Doubtless, nitrogen is a crucial element for living organisms, being consumed in the synthesis of proteins and nucleotides [1]. Although the main source of nitrogen in the Nature is atmospheric N2 gas (about four-fifths of the atmosphere), the diatomic molecule is inactive and cannot directly be used up by living cells through their respiratory/metabolic processes [2]. Therefore, activation of N2 molecules is a vital process which is the demand of all organisms and essential for the survival of life on the bio-globe. Concerning this issue – which is commonly referred to as nitrogen fixation – ammonia () is a strategic material that can be directly consumed by plants [3], thereby eventually being used up by other living creatures.
In Nature, through an associative hydrogenation (H+/e−) pathway of N2 molecules, ammonia is biologically produced under ambient conditions via metabolic activities of certain types of bacteria possessing nitrogenase enzyme [1], [2], [4]. Ammonia is also synthesized abiotically through the Haber-Bosch industrial process under high temperature and pressure conditions [5], [6], by breaking the triple bonds in nitrogen molecules and reacting with H2 feed upon appropriate iron-based catalysts.
Besides its importance in agricultural use, ammonia is also considered as a modern, green fuel (zero-carbon footprint) [7], [8], which can be indirectly oxidized in a fuel cell reactor and converted into nitrogen and water products:
The reverse reaction, i.e. direct synthesis of ammonia (as well as O2 by-product) from N2 and H2O molecules, is a novel alternative strategy for the conventional Haber-Bosch process, in which water molecules serve as hydrogen source (reducing agent [9]) and the energy of process could be supplied by a renewable and clean source. Regarding sustainable fixation of N2 molecules [10], [11], [12], we have recently reported [6] that by using a semiconductor-assisted water photosplitting setup, H atoms can be transiently generated upon the photocatalyst surface [13] and applied for atomistic hydrogenation of the reactant species at the interface region to synthesize ammonia as the major product. With reference to the mechanistic aspects, moreover, it is worth noting that the major energy barrier in the nitrogen photo-fixation phenomenon is the transfer of the first two electrons (as e−/H+ [14], [15]) to the stable N2 molecules at the photocatalyst/solution interface [16], [17].
In this paper, using two earth-abundant, eco-friendly iron and sulfur elements [18], [19], pyrite (FeS2; iron disulfide) photocatalyst/solar-energy material is synthesized in the absence and presence of carbon nanotube (CNT) and examined as effective, hole-scavenger-free photocatalyst of N2-photofixation process [to the best of our knowledge, the application of CNT to N2-photofixation photocatalysts is introduced for the first time in this work; concerning the presence of carbon – as fullerene, diamond, graphene, carbon nitride, etc., however, it is worth mentioning that some photocatalytic as well as electrocatalytic properties were also reported in the literature for these carbonous photocatalyst/electro-catalyst materials, Refs. [20], [21], [22], [23], [24], [25], [26], [27]]. In the selection of the photocatalyst elements, we noticed the ability of iron to interact with nitrogen molecules as well as sorb H-atoms [18], [19], [28], [29], [30], [31]. Sulfur can also combine with iron and produce pyrite – a stable narrow bandgap semiconductor/solar-energy material ( and 1.6 eV [32], [33], [34]) with good potency to strongly absorb photons throughout the UV–Vis–NIR spectral region [35]. Moreover, sulfur in the anionic form is able to be protonated upon the photocatalyst surface [36], [37]; protons in the reaction medium, are naturally produced via water auto-ionization or deliberately generated via water photo-oxidation processes [13], [38].
Concerning the use of CNT, it is worth noting that not only can CNT serve as H-atom reservoir for the hydrogenation process [38], [39], [40], its presence in the photocatalyst/solar-energy material can also retard the charge (e/h) recombination process [13]; therefore, a greater photocatalytic activity is anticipated for this nanocomposite solar-energy material. From the more basic standpoint, the extraordinary properties of CNT can be originated in the molecular orbital/geometrical structure of this cylindrical/pyramidal π-electron resonating phononic system, consisting of sp2 C–C σ-bonds and radially oriented as well as polarized pz orbitals which are perpendicular to the nanotube surface [41], [42].
Finally, we should declare that in the present article, the authors have attempted to provide a comprehensive discussion for their findings on this nanocomposite energy-material and scrutinize the N2-photofixation phenomenon from different perspectives.
Section snippets
Photocatalyst preparation
The synthesis of FeS2 was solvothermally carried out according to the literature [43] with a little modification in washing of the photocatalyst. Briefly, 0.634 g FeCl2 (Sigma-Aldrich; 99.9%) and 0.962 g sulfur powder (Merck; 99%) were added to a 40 ml absolute ethanol (Merck; 99.9%) and stirred to form a homogeneous mixture. Then 20 ml oleylamine (C18H35NH2, as a capping agent and sulfur reductant [44], purchased from Acros; C18-content: ~ 80–90%) was added to the mixture and sonicated
Synthesis of the photocatalyst/solar-energy materials
XRD patterns of the solar-energy materials synthesized in this work are presented in Fig. 1. The pattern of the semiconductor compound in the absence of CNT completely matches that of pyrite (JCPDS card no. 71-0053) [53] and it confirms the synthesis of FeS2 solar-energy material. In the case of FeS2/CNT nanocomposite, besides XRD peaks of FeS2, the peaks of CNT are also weakly visible. Moreover, the remaining peaks with slight intensity () signify the possibility of the formation of
Conclusion
Based on the present study, we can conclude that:
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Using a facile solvothermal route, one could synthesize pyrite semiconductor nanoparticles in the absence and presence of CNT and employ them as efficient narrow-bandgap solar-energy-materials. These nanostructured energy materials can be utilized in contemporaneous oxidization of water molecules and N2 conversion into ammonia.
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Pyrite and its CNT nanocomposite are among eco-friendly, low-price, abundant solar-energy-materials with appropriate band
Acknowledgements
The authors would like to acknowledge the research council of IASBS for the financial support of this project (G2017IASBS32603). We should also thank the anonymous Referees for their useful comments to enhance the quality of work.
Mohsen Lashgari, Associate Professor of Physical Electrochemistry at IASBS. The author of several fundamental works in the field of artificial photosynthesis, semiconductor-based photocatalyst solar-energy materials (thin-film and bulk), generation of H-based solar-fuels, electrodic respiration/metabolism, corrosion, quantum and interfacial electrochemistry. He has supervised several master and Ph.D. students and invited internationally concerning the nanostructured energy materials,
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Mohsen Lashgari, Associate Professor of Physical Electrochemistry at IASBS. The author of several fundamental works in the field of artificial photosynthesis, semiconductor-based photocatalyst solar-energy materials (thin-film and bulk), generation of H-based solar-fuels, electrodic respiration/metabolism, corrosion, quantum and interfacial electrochemistry. He has supervised several master and Ph.D. students and invited internationally concerning the nanostructured energy materials, pollutants, global warming and climate change issues.
Parisa Zeinalkhani is currently pursuing her Ph.D. degree in physical chemistry at the Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran, under the supervision of Dr. Mohsen Lashgari. Her interests focus on photochemical/photoelectrochemical nitrogen photo-fixation and solar fuel production via artificial photosynthesis.