Solar-Driven Hydrogen Peroxide Production Using Polymer-Supported Carbon Dots as Heterogeneous Catalyst
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Safe, sustainable, and green production of hydrogen peroxide is an exciting proposition due to the role of hydrogen peroxide as a green oxidant and energy carrier for fuel cells. The current work reports the development of carbon dot-impregnated waterborne hyperbranched polyurethane as a heterogeneous photo-catalyst for solar-driven production of hydrogen peroxide. The results reveal that the carbon dots possess a suitable band-gap of 2.98 eV, which facilitates effective splitting of both water and ethanol under solar irradiation. Inclusion of the carbon dots within the eco-friendly polymeric material ensures their catalytic activity and also provides a facile route for easy catalyst separation, especially from a solubilizing medium. The overall process was performed in accordance with the principles of green chemistry using bio-based precursors and aqueous medium. This work highlights the potential of carbon dots as an effective photo-catalyst.
KeywordsCarbon dot Photo-catalyst Heterogeneous catalyst Hydrogen peroxide
Polyurethane-supported carbon dots were developed as heterogeneous catalyst, and the developed catalyst was biodegradable.
Solar-driven H2O2 production from water, ethanol, and oxygen was achieved.
Recently, greener, inexpensive, and efficient water splitting techniques have garnered significant interest in the thrust toward sustainable energy conversion processes [1, 2, 3]. In this context, photo-catalysis has been adopted as a “green” and “sustainable” technique . In the last few years, much effort has been devoted toward sustainable production of hydrogen peroxide (H2O2) by photo-splitting of water (H2O). In the current scenario of rapid depletion of fossil fuels, sustainable production of H2O2 is very significant. H2O2 can be used as a fuel in one-compartment fuel cells with an output potential of 1.09 V, which is comparable to that of the hydrogen fuel cell with an output value of 1.23 V . Further, H2O2 serves as a clean and green oxidant with H2O as the only by-product . Thus, green and sustainable production of H2O2 is highly desirable for implementation of “green chemistry” principles and can provide a promising prospect for resolving global fuel woes.
In the conventional process, H2O2 is produced by the anthraquinone method, which is highly disadvantageous from the green chemistry point of view. The drawbacks include the multi-step process, and high energy consumption, et cetera . Catalytic systems such as Pd and bi-metallic Au–Pd have been studied for the direct production of H2O2 from H2 and O2 [8, 9, 10, 11]. Such direct methods are effective for quantitative production of H2O2, but have an attendant high risk because of the explosive nature of the H2/O2 mixture. Most recently, semiconductor-based nanocatalysts have been used for sustainable production of H2O2 from H2O and alcohols. Such methods involve irradiation of an oxygen (O2)-saturated alcohol or aqueous system under UV light in the presence of metallic semiconductors. State-of-the-art literature cites the use of different metal-based semi-conductor catalysts for sustainable production of H2O2 from H2O and alcohol. These include Ru-complexes, Ir-complexes, Ti nanoparticles, and Ni, et cetera [12, 13, 14, 15, 16, 17, 18, 19, 20]. However, the major concerns associated with these catalysts are their high cost, toxicity, metal-based origin, and non-degradable nature. In an effort to introduce metal-free and degradable catalysts, Shiraishi and his group reported polymeric graphitic carbon nitride (g-C3N4) for the production of H2O2 from ethanol . However, the high production cost and low band-gap of g-C3N4 hamper its successful use, especially for splitting H2O under solar irradiation. Thus, the development of an eco-friendly and sustainable catalyst system with the capacity to produce H2O2 is desirable to meet the current challenges.
Carbon dots (CDs), with unique physical, chemical, and optical properties, have attracted immense interest in the scientific community based on their utility in diverse areas, such as bio-imaging, biosensors, light-emitting diodes, and other opto-electrical devices [22, 23, 24, 25]. The unique traits of CDs include their excitation wavelength-dependent stable multi-color emission and excellent aqueous solubility with remarkable cytocompatibility. Like other quantum dots, exhibition of the “quantum confinement” effect is the typical signature of CDs. CDs consist predominantly of amorphous carbon along with sp 2 hybridized graphitic carbon. The most fascinating feature of CDs is their capability to harvest photons in the UV/visible spectral region. Such light-harvesting ability renders CDs as a potential candidate for photo-catalytic application [1, 26]. However, in practice, there are certain difficulties in the use of bare CDs as an effective photo-catalyst. CDs are soluble in water or organic solvents like ethanol. This creates serious difficulties in their separation when such media are used. However, in media where CDs are insoluble, CDs suffer solid-state quenching of the photo-activity . Such obstacles restrict the practical applicability of CDs as photo-catalysts.
In this context, the use of a polymeric matrix as a support can facilitate the application of CDs. The polymer matrix can prevent solid-state quenching and also provide sufficient mechanical strength and catalytic stability and facilitate application with easy separation. Such polymer/CD nanocomposite systems have found use in opto-electrical applications. Photo-catalytic applications have also been reported recently . In this regard, we found that the waterborne hyperbranched polyurethane (WPU) system is attractive as a polymer support. The solubility of both the nanomaterial and polymer in the same medium (aqueous) expedites homogeneous distribution of the CDs over the polymer matrix, thereby preserving their photo-harvesting ability. Further, the use of WPU offers additional advantages. During the polymerization reaction, WPU becomes soluble due to the creation of ionic centers along the polymer chains. Thus, it behaves like an ionomer with electron transfer capability. Hence, compared to conventional polymeric systems, WPU is expected to act as a better catalyst support for photo-catalytic reactions.
Herein, a CD-impregnated WPU nanocomposite film is used as a heterogeneous photo-catalyst for solar-driven production of H2O2 by using H2O, C2H5OH, and O2 as raw materials. Further, bacterial degradation of the catalyst is carried out by using Pseudomonas aeruginosa to highlight its eco-friendly behavior. Thus, the present study reports a metal-free, low-cost, easily separable, non-toxic, and biodegradable carbon dot-impregnated polymeric nanocomposite system as a heterogeneous catalyst for sustainable and green production of H2O2.
Isophorone diisocyanate (IPDI, Aldrich, Germany), poly(ethylene glycol) with a number average molecular weight (M n ) of 600 g mol−1 (PEG 600, Merck, India), 2,2-bis(hydroxymethyl) propionic acid (BMPA, Aldrich, Germany), 1,4-butanediol (BD, Merck, India), tannic acid (TA, Sigma-Aldrich, Belgium), triethylamine (TEA, Merck, India), and tetrahydrofuran (THF, Merck, India) were used for WPU preparation. Glycerol-based hyperbranched epoxy (HE) and vegetable oil-based poly(amide amine) (PAA) were used to cross-link the fabricated WPU/CD system [29, 30]. Corms of Colocasia esculenta were used to prepare the CDs. Moreover, ethanol (C2H5OH, Merck, India), Millipore water (H2O, obtained from MilliQ® Ultrapure Water Solutions, Type 1, water purifier system), and oxygen (O2, obtained from Jainex Gas Company, India) were used as starting materials in the photo-catalytic application. Potassium permanganate (KMnO4, Merck, India) was used in the estimation of hydrogen peroxide by following a titrimetric method. In the biodegradation study, the bacterial strain Pseudomonas aeruginosa (strain MTCC 7814) was used. This strain was procured from the Microbial Type Culture Collection and Gene Bank, Chandigarh, India. The bacterial culture was collected through the Department of Molecular Biology and Biotechnology, Tezpur University, India.
3.2 Catalyst Preparation
The CDs were prepared by using corms of C. esculenta as reported earlier . However, a slight modification was made to the synthetic procedure. In order to exert greater control over the particle size and size distribution, a hydrothermal method was employed. Briefly, an aqueous extract (100 mL) of corms of C. esculenta was heated at 150 °C for 5 h in a Teflon-lined hydrothermal reactor (250 mL capacity). The CDs formed were dried under reduced pressure. The CD-impregnated WPU polymer was prepared via an ex situ technique according to a method described elsewhere . However, in the present study, we used a high CD loading compared to that previously reported. Briefly, IPDI, PEG 600, and BD were reacted in the pre-polymerization step at a temperature of 85 ± 2 °C. In the next step, TA and BD were allowed to react with the pre-polymer formed in the first step at 70 ± 2 °C for 4–4.5 h. This process was followed by neutralization using TEA at room temperature. Finally, water was added and THF was removed under reduced pressure. Different weight percentages of CD were incorporated into the WPU matrix by using mechanical and ultra-sound energy. These nanocomposites were coded as PNC1.0, PNC2.0, PNC3.0, PNC4.0, and PNC5.0 for CD loadings of 1, 2, 3, 4, and 5 wt%, respectively. Further, the nanocomposites were cross-linked by using HE and PAA. Details of the fabrication technique are provided in Supplementary Information.
3.3 Catalyst Characterization
X-ray diffraction (XRD) patterns of the CDs and CD-impregnated WPU were obtained by using a Bruker AXS, Germany, Model: D8 Focus instrument (2θ = 10°–70° at a scanning rate of 2°min−1). The graphitic structure of the CDs was studied using Raman spectroscopy (Renishaw, UK, Renishaw Basis Series with 514 nm Laser). The shape and size of the CDs, as well as the distribution of the CDs over the WPU matrix, were visually studied by using high-resolution transmission electron microscopic (HRTEM) analysis. TEM images were obtained by using a JEOL, JEMCXII, Japan, microscope at an operating voltage of 200 kV using a Cu grid (TED PELLA INC, Ultrathin C, Type A, 400 mesh). The microscopic data were analyzed as inverse fast Fourier transform (IFFT) images by using Gatan Digital Micrograph software. A UV–visible photo-spectrometer (Thermo-Scientific, Evolve 300, USA) was used to determine the band-gap. The photoluminescence (PL) behavior was studied by using a PerkinElmer, USA, Model: LS-55 fluorescence spectrometer. The surface morphology of the biodegraded catalyst was studied by scanning electron microscopy (SEM, JEOL, JSM 6390LV) after platinum coating the surface.
Here α is the absorption coefficient, C is a constant, h is Plank’s constant, ν is the frequency, and E bulk is the energy gap. In practice, the band-gap is calculated by plotting hν versus (αhν)2. Extrapolation of the curve to (αhν)2 gives the value of E bulk.
3.5 Determination of Quantum Yield
Here, Q CD is the quantum yield of the CDs, Q R is the quantum yield of the reference compound (quinine sulfate), I CD is the intensity of the emission of the CDs, I R is the intensity of the luminescence of the reference compound (quinine sulfate), A CD is the absorbance of the CDs at the given excitation wavelength (360 nm), A R is the absorbance of the reference compound (quinine sulfate) at the given excitation wavelength (360 nm), and η is the refractive index of the solvent used (water). The quantum yield of quinine sulfate at 360 nm is 54%.
3.6 Photo-catalytic Reaction
Five different compositions, i.e., H2O, C2H5OH (100%), and mixtures of H2O and C2H5OH (25, 50, and 75% (v/v)) were used as the starting materials for the solar-driven production of H2O2. The reactants were placed into a two-necked glass reactor equipped with an O2 inlet. Initially, the mixture was saturated with molecular O2 by passing the gas for 10 min and the reactor was then sealed. The system was then irradiated under normal solar light with an intensity of 80,000–100,000 lx at ca. 25 °C with a continuous supply of O2 for different time intervals (5–50 h). The production of H2O2 was estimated via the redox titrimetric method using KMnO4 .
3.7 Biodegradation Study
Analysis of the biodegradability of the catalyst system was carried out by using P. aeruginosa as the test organism . A mineral salt solution containing 2.0 g (NH4)2SO4, 2.0 g Na2HPO4, 4.75 g KH2PO4, 1.2 g MgSO4·7H2O, 0.5 mg CaCl2·2H2O, 100 mg MnSO4·5H2O, 70 mg ZnSO4·7H2O, 10 mg H3BO3·5H2O, 100 mg CuSO4·7H2O, 1 mg FeSO4·7H2O, and 10 mg MoO3 in 1000 mL distilled water was prepared and sterilized at 120 °C under 15 lb pressure. The P. aeruginosa bacterial strain was cultured in a shaker incubator for 48 h using this medium. A 100-µL aliquot of the cultured bacteria was placed in a conical flask with 10 mL of the salt medium. UV-sterilized (wavelength 254 nm) catalyst films with a known weight were immersed in the medium and incubated at 37 °C for seven weeks. Growth of the bacteria was followed by measuring the absorbance of the medium at 600 nm at an interval of seven days. Similarly, a change in the weight of the catalyst was recorded after every seven days. SEM images of the biodegraded films were also taken after completion of the experimental period.
4 Results and Discussion
The synthesis of the CDs provides a green and sustainable route in which a large number of naturally available carbon resources are utilized . In the present study, a carbohydrate-rich aqueous extract of corms of C. esculenta was used as the precursor. The CDs were obtained through a hydrothermal method. It can be assumed that in the hydrothermal process, extensive carbonization and aromatization took place through condensation and polymerization of different carbohydrate molecules at elevated temperature and pressure [33, 36].
The Raman spectrum of the CDs displayed two distinct bands, i.e., the D band near 1360 cm−1 and the G band near 1580 cm−1 (Fig. 2b). The D band is characteristic of structural defects, and the G band is characteristic of symmetric sp 2 graphitic carbon stretching. UV–visible spectroscopy shows a peak near 270 nm, which is an indication of successful formation of the CDs (Fig. S1). Thus, TEM, XRD, Raman, and UV–visible spectroscopic analyses confirmed formation of the CDs. The CDs were incorporated into the WPU matrix for the development of a heterogeneous catalyst system for solar-driven H2O2 production.
Visual evidence obtained from the TEM image of the catalyst system confirmed the uniformity, even at high nanomaterial loading (4 wt%) (Fig. 1d). Comparison of the HRTEM images (Fig. 1b, e) and corresponding IFFT images of the CDs and PNC4.0 (Fig. 1c, f) revealed an increase in the interlayer spacing from 0.39 nm for the pristine CDs to 0.43 nm after incorporation into the polymer matrix. This may be due to the additional defects created within the CD structure during the fabrication process. Intercalation of the polymeric chains may also increase the interlayer distance. This indicates strong interaction between the polymer and nanomaterial.
The XRD pattern of PNC4.0 showed a peak corresponding to amorphous carbon near 2θ = 22°, ascribed to the (002) plane of graphitic carbon (Fig. 2a). Similar to the profile of the CDs, the Raman spectrum of PNC4.0 displayed D and G bands, albeit with a higher I D/I G ratio. This provides evidence of the generation of defects in the CDs during the fabrication process. On the other hand, modification of this system with HE and PAA is expected to provide better catalyst stability and prevent CD leaching by the formation of a cross-linked network structure.
Before studying the photo-catalytic activity of the prepared catalyst, effort was made to evaluate different parameters, which ensured suitability of the polymer-supported CD system as a photo-catalyst for H2O splitting. By virtue of their quantum dimension, the CDs possess the capability to harvest photons over the solar spectrum (covering the entire UV–visible spectral region, extending to the near IR) based on their photo-excited states . The associated transient species are responsible for the bright fluorescence as well as the redox processes, which are desirable for energy conversion applications [37, 38]. However, a precise mechanistic pathway to account for the light-harvesting capability of the CDs is yet to be established. Nevertheless, recent research gives the impression of charge separation in the form of holes (radical cation) and electrons (radical anion) via photo-excitation of the core carbon nanoparticles of the CDs. The radiative recombination of these holes and electrons, which are trapped at various surface sites, results in the observed fluorescence emission . Hence, to study the light-harvesting activity of the CDs, the photoluminescence behavior and HOMO–LUMO band-gap were evaluated.
Generally, H2O splitting requires the bottom level of the conduction band to be more negative than the redox potential of H+/H2 (0 V vs. NHE), while the top level of the valence band should be more positive than the redox potential of H2O/O2 (1.23 V vs. NHE). Thus, the minimum value of E bulk for any photo-catalyst should be 1.23 eV for H2O splitting. However, in practice, for effective H2O splitting, the band-gap should be larger than 2.0 eV. This is because, in reality, energy losses may occur due to kinetic overpotentials.
In the present study, E bulk was evaluated as 2.98 eV for the CDs. Hence, these species satisfy the criteria to act as a catalyst for solar-driven production of H2O2. In this context, the CDs appear to be the superior catalyst compared to g-C3N4. This may be because, g-C3N4 has a band-gap of 1.4 eV, which is good enough to split alcohols but is insufficient to split H2O under solar irradiation. The photo-catalytic production of H2O2 involves photo-irradiation of O2-saturated H2O or alcohol in the presence of a photo-catalyst, i.e., the CDs in the present case.
In the current study, we used H2O, mixtures of H2O and C2H5OH (25%, 50%, and 75% v/v), and C2H5OH (100%) saturated with molecular O2. In this regard, it is important to mention that many of the traditionally used semiconductors absorb only light energy or mostly in the UV region; hence, for a solar-driven process, dye sensitization is required . However, the CDs can also absorb photons in the visible region; hence, the CDs can be readily used under sunlight. In the actual reaction setup, the reaction mixtures were irradiated under normal solar light with an intensity of 80,000–100,000 lx at ca. 25 °C for different durations.
Nevertheless, the increased production of H2O2 with time clearly indicates the efficiency of the polymer-supported CDs as a photo-catalyst in the H2O splitting reaction. The rate of formation of H2O2 also supports this claim (Fig. S3). Some interesting observations emerge from this study. When H2O was used by itself, less H2O2 was produced (~46 µM after 50 h of reaction) compared to that obtained with 100% C2H5OH (~82 µM). On the other hand, in the case of the C2H5OH/H2O mixtures, the H2O2 production increased with an increase in the amount of C2H5OH in the mixture (~49 µM for 25%C2H5OH/75%H2O, ~61 µM for 50%C2H5OH/50%H2O, and ~70 µM for 75%C2H5OH/25%H2O). The increased H2O2 production in the presence of C2H5OH may be because of the favorable thermodynamic driving force, which is promoted by the suitable oxidation and reduction potential of the catalyst when alcohol is used (compared to water) . A previous report also suggests similar behavior of alcohol and H2O mixtures toward solar-driven water splitting . Thus, these data suggest that the photo-reaction of H2O with C2H5OH can enhance the H2O2 yield.
Here, it is pertinent to mention that CD alone can act as an effective catalyst with higher production yield (Fig. S4). However, the ease of separation makes the polymer-supported CDs more advantageous for use as a heterogeneous catalyst than the pristine CDs. This observation can be explained based on two factors. First, the increase in the size of the CDs after incorporation into the polymer matrix (due to intercalation of the polymeric chains in-between the layers of CD, which was confirmed by the increased d-spacing) may hamper the light-harvesting ability of the CDs. Secondly, the distribution of the CDs within the polymer matrix is not as uniform as in the solution state. This also contributed to reduction of the photo-catalytic activity of the polymer-supported CDs compared to the pristine CDs.
However, despite these facts, good photo-catalytic activity was observed for the polymer-supported CDs. The reusability of the CD-containing films was evaluated over several cycles. Reusability is an important factor for any catalyst from the green chemistry point of view. Accordingly, a model reaction was performed by using PNC4.0 as the photo-catalyst with a definite amount of reactants for 50 h. After completion of the reaction, the catalyst was recovered, dried, and used again under the same reaction conditions.
From the above results, it can be inferred that carbon dots dispersed in a polyurethane matrix can be used as an effective heterogeneous photo-catalyst for solar-driven hydrogen peroxide production. The overall process can be justified as a green practice. It offers the benefits of green chemistry in terms of availability, facile and sustainable preparation, catalyst stability, non-toxicity, recyclability, and biodegradability. The key components, i.e., carbon dots, were synthesized using bio-based raw material in a cost-effective, facile process. Further, preparation of the polymeric support, i.e., the polyurethane matrix, was performed in an aqueous medium with a low VOC content. The reusability test confirmed the effectiveness of the catalyst, even after several cycles of use. Compared to other metal-based catalysts that are used for hydrogen peroxide production, the present system offers the benefits of non-toxicity and biodegradability. Further, compared to graphitic carbon nitride, the present system is cost-effective with the additional capability to produce hydrogen peroxide from water under sunlight. This study may pave the way for recognition of polymer-supported carbon dots as heterogeneous photo-catalysts.
The authors acknowledge DBT (Grant No. BT/235/NE/TBP/2011 Dated April 30, 2012), India, for financial support and SAIF, NEHU, Shillong for TEM analysis.
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