Photocatalytic Reforming of Biomass Derived Crude Glycerol in Water

Feb 3, 2018 - The solar light energy conversion efficiency of the optimized catalyst and cost benefit analysis by using crude glycerol are also evalua...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 3754−3764

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Photocatalytic Reforming of Biomass Derived Crude Glycerol in Water: A Sustainable Approach for Improved Hydrogen Generation Using Ni(OH)2 Decorated TiO2 Nanotubes under Solar Light Irradiation Nagappagari Lakshmana Reddy,† Kanakkampalayam Krishnan Cheralathan,‡ Valluri Durga Kumari,§ Bernaurdshaw Neppolian,∥ and Shankar Muthukonda Venkatakrishnan*,† †

Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science & Nanotechnology, Yogi Vemana University, Vemanapuram, Kadapa−516003, Andhra Pradesh, India ‡ Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology (VIT), Thiruvalam Road, Vellore−632014, Tamil Nadu, India § Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (CSIR-IICT)62391, Uppal Road, Tarnaka, Hyderabad−500007, Telangana, India ∥ SRM Research Institute, SRM University, SRM Nagar, Potheri, Kattankulathur−603203, Tamil Nadu, India ABSTRACT: Crude glycerol (10% w/w) is produced as a substantial byproduct during the industrial production of biodiesel via transesterification processes. Catalytic hydrogen (H2) generation by utilizing crude glycerol and solar light is considered as a promising avenue. The present work illustrates enhanced rates of H2 generation and cocatalyst behavior of Ni(OH)2 decorated on TiO2 nanotubes dispersed in aqueous crude glycerol solution (industrial byproduct) under solar light irradiation. The catalyst characterization reveals that the TiO2 nanotubes (TNT) are of anatase phase with length ranges from 100 to 300 nm and diameters from 4.9 to 9.8 nm. The Ni(OH)2 quantum dots deposited on TNT have an average particle size of 8.4 nm. The presence of Ni(OH)2 on TNT and oxidation states of Ti4+ and Ni2+ cations are confirmed by XPS analysis. The optimal loading of Ni (2.0 wt %) leads to a high rate of photocatalytic H2 generation of 4719 μmol h−1 gcat−1 and it is ∼12-fold higher than pristine TNT. The solar light energy conversion efficiency of the optimized catalyst and cost benefit analysis by using crude glycerol are also evaluated. The high electronegativity of Ni(OH)2 quantum dots present on the surface of TNT may facilitate effective shuttling of photoexcitons, thereby largely preventing electron−hole recombination in TiO2 during photocatalysis. KEYWORDS: Crude glycerol, Titanium dioxide, Hydrogen, Photocatalysis, Water splitting, Solar light

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INTRODUCTION Hydrogen (H2) generation from biomass derived byproducts (crude glycerol) is a sustainable approache to partially fullfill the global energy demand and also an alternative route for environmental remediation. The glycerol market has been expected to reach USD 2.52 billion by 2020 globally, and the biodiesel industry has emerged as the most important source of glycerol, accounting for over 1400 kt production in 2013,1 and its production has increased steadily as about 4.53 kg of glycerol is formed for every 45.3 kg of biodiesel prepared.2,3 Therefore, a continuous supply of crude glycerol in larger quantities is possible especially at a lower price from biomass feedstock. According to some reports, crude glycerol may contain impurities such as carbon content at an average of © 2018 American Chemical Society

about 25%, which includes methanol, fatty acid, methyl esters, monoglycerides, diglycerides, glycerol oligo-mers, polymers, and unreacted triacylglycerols. Apart from that, water and a small quantity of metals like Na, Ca, K, Mg, P and S could also be present along with salts left over from the transesterification reaction which are soluble in the glycerol layer.3,4 As the refining leads to complex processes with a high operating cost, integration of crude glycerol for value-added-products (VAP) especially photocatalytic reforming to generate H2 fuel can be considered as a sustainable approach, which complies with the Received: November 8, 2017 Revised: December 27, 2017 Published: February 3, 2018 3754

DOI: 10.1021/acssuschemeng.7b04118 ACS Sustainable Chem. Eng. 2018, 6, 3754−3764

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Preparation of Ni(OH)2/TNT Nanocomposite by Wet Impregnation Method

efficiency of TiO2.32,33 In this connection, many metal oxides like Cu2O, WO3, Bi2O3, Fe2O3, CoO, NiO, and Ni(OH)2 have been used along with TiO2 to improve the H2 production.34−38 In particular, NiO/Ni(OH)2 has recently emerged as an efficient cocatalyst along with TiO 2 for efficient H 2 production.39 This is due to its beneficial intrinsic properties like its work function that promotes the separation of photoexcited charge carriers on the photocatalyst surface, which provide sufficient active species for reduction of H+ into H2. Moreover the conduction band (CB) potential of NiO (−0.23 V vs NHE, pH = 0) is lower than the CB level (about −0.26 V) of anatase TiO2 and also more negative than water reduction potential.40 Hence, shuttling of photoexcitons from TiO2 to NiO and then to H+ ions is favored and it further leads for an enhanced rate of H2 production.41 NiO/Ni(OH)2 supported over TiO2 nanoparticles has been widely explored.42−45 At the same time, one nanomaterials show more beneficial properties than zero dimensional particles. Hence in this study, TiO2 nanotube based Ni(OH)2 photocatalyst was synthesized and used to improve the catalytic activity by harvesting the beneficial properties of these nanostructures (vide supra), as the reports on the TiO2 based Ni(OH)2 photocatalysts for H2 generation are very limited.46 In the present work, the industrial byproduct, “crude glycerol” was used as a sacrificial agent to improve the photocatalytic efficiency.

concept of converting waste into clean energy. Though steam reforming of glycerol using heterogeneous catalysts to H2 is a known process, it is being carried out at high temperature (>500 °C), but photocatalysis is an energy efficient process which works under ambient conditions, often facilitated under solar light that encourages more research work for the utilization of crude glycerol. In this direction, many semiconductor photocatalysts have been used in water splitting reaction for H2 generation.5−7 Among them, titanium dioxide (TiO2) is one of the earliest and most extensively studied catalysts for H2 generation due to its abundance, strong chemical stability against photocorrosion, and suitable band edge potentials for both proton reduction and water oxidation reactions.8−12 However, conventional TiO2 nanoparticles (NP) are unfavorable for the photocatalysis applications, due to fast recombination of photoformed electron and hole pairs. So one dimensional nanostructures of TiO2, especially nanotubes, nanorods and nanobelts have become important class of nanomaterials to improve the photocatalytic efficiency13,14 as they can suppress the exciton recombination process. More importantly, TiO2 nanotubes have attracted much attention due to their unique properties like higher surface area, quantum confinement, unidirectional flow of electrons, greater number of active sites on the outer as well as the inner walls of the nanotubes, and even higher mechanical strength.15−17 To further improve the efficiency of TiO2, various other strategies have been followed such as, modification of metal oxides18,19 with other semiconductors,20,21 non-noble metals,22,23 noble metals24,25 and using different phases of TiO26,26,27 in order to tune their electronic band gaps to tackle the rapid recombination of photogenerated electron−hole pairs and improve the broad range of light absorption. Among these strategies, deposition of noble metals (such as Pt, Pd, Au, Ag, and Rh) as cocatalyst with TiO2 has been proven to be effective way for improved photocatalytic H2 production.28−31 However, this system has some restrictions owing to the high cost of the noble metals which limit its practical applications. Hence there is an urgent need to replace noble metals with some other earth abundant metal or metal oxides to improve the photocatalytic

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EXPERIMENTAL SECTION

Chemicals and Reagents. Analytical reagent (AR grade) chemicals were used throughout this work. Commercial titanium(IV) oxide (anatase TiO2), sodium hydroxide pellets (NaOH), Nickel(III) nitrate hexahydrate crystals (Ni(NO3)2·6H2O), hydrochloric acid (HCl), and glycerol were received from Merck India. Ethanol (C2H5OH) was obtained from C.S.F Chemicals, China. Crude glycerol (research sample) was provided by M/s. Kaleesuwari Refinery Pvt. Ltd., Visakapatnam, Andhra Pradesh, India. Distilled water was used for synthesis of materials and photocatalytic experiments. Synthesis of TiO2 Nanotubes. The H2Ti3O7 nanotubes were synthesized by a hydrothermal method based on our previous report.47 In a typical synthesis, Merck TiO2 particles (2.5 g) was dispersed into 10 M NaOH solution (200 mL) and stirred for 1 h and, then, 3755

DOI: 10.1021/acssuschemeng.7b04118 ACS Sustainable Chem. Eng. 2018, 6, 3754−3764

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ACS Sustainable Chemistry & Engineering Scheme 2. Formation of TiO2 Nanotubes Prepared by Hydrothermal Method

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transferred into a Teflon-lined autoclave (capacity 250 mL) and heated @ 130 °C/20 h. The white color precipitate thus obtained was washed twice at each stage with distilled H2O, dil. HCl, and ethanol. Then, the solid compound was dried at 80 °C/12 h to get a bright white powder. It was further spread in a ceramic boat and calcined at 350 °C for 5 h @ 5 °C/min. The resulted catalyst was denoted as TNT. Preparation of Ni(OH)2/TNT Composite Photocatalyst. Ni(OH)2/TNT nanocomposites were prepared by a wet impregnation method as reported elsewhere.36 The schematic representation of formation of nanocomposite is shown in Scheme 1. Nickel was loaded onto TNT by using an appropriate amount of nickel(III) nitrate hexahydrate aqueous solution. For instance, 0.5 g of TNT was dispersed into an aqueous solution containing requisite amount of [Ni(NO3)2·6H2O], and the solvent was evaporated by heating under constant magnetic stirring. The obtained fine powder was dried @80 °C/12 h followed by calcination @ 350 °C/3 h. Same procedure was followed for preparation of varying weight percent of Ni (0.5, 1.0, 2.0, 3.0, 4.0) loaded TNT and the samples were labeled as NT-1, NT-2, NT-3, NT-4, and NT-5, respectively. Ni(OH)2 alone was prepared in similar method without TNT and named as N-1. Characterization of the Photocatalysts. Powder X-ray diffraction patters of the catalysts were recorded using Bruker D8 advance X-ray diffractometer. Transmission electron microscopy (TEM) images were captured using a 200 kV FEI-Tecnai, G2 20 STwin high resolution TEM instrument. The samples were initially dispersed in isopropanol and then a drop of the dispersion was placed on a carbon coated copper grid and allowed to dry. The dimensions of nanotubes and particles were measured using Image j software. UV−vis DRS spectra were obtained using a Jasco V-670 UV−vis spectrophotometer equipped with an integrating sphere. Photoluminescence spectra were collected using Hitachi F-7000 fluorescence spectrophotometer using an excitation wavelength of 280 nm. X-ray photoelectron spectrum was collected using an Axia Ultra (Kratos Analyticals UK) XPS instrument using Al Kα photons of 1486.6 eV energy. Solar Photocatalytic H2 Production Experiments. All the photocatalytic experiments were performed as per the reported procedure.36 Here industrial byproduct, “crude glycerol” as sacrificial (organic) agent. Typically, catalyst (5 mg) was suspended into a quartz reactor containing crude glycerol−water (5% v/v) mixture. After dark experiments the reactor was irradiated with solar light for 4 h in a sunny day (average intensity = 300 ± 10 mW/cm2 using Newport power meter, model 843-R) and H2 gas generated was quantified against standard samples at every hour using a gas chromatograph (GC), Shimadzu GC-2014 equipped with a packed column (molecular sieve/5A) and thermal conductivity detector (TCD).

RESULTS AND DISCUSSION Proposed Mechanism of TNT Formation. The mechanism of TNT formation is shown in Scheme 2. After hydrothermal treatment (HT), the resulting compound is washed with water, at this stage it is expected to form Na2Ti3O7 nanosheets. After acid (HCl) washing, Na+ ions are replaced with H+ ions to form H2Ti3O7 nanotubes, at this same stage the rolling of nanosheets also simultaneously occur to form tubular morphology.17 During ethanol washing, the impurities (Na+) are removed and pure H2Ti3O7 nanotubes are obtained and finally calcined at 350 °C for 5 h to get pure TNT. Size and Morphology. The morphology and size of the prepared TNT and Ni(OH)2/TNT (NT-3) catalysts were investigated through TEM analysis. Figure 1a shows a representative, low magnification TEM image of TNT, which displays randomly arranged one-dimensional nanotubes having hollow space inside. The length of the TiO2 nanotubes varies from 100 to 300 nm (shown inset in Figure 1a). Figure 1b

Figure 1. (a and b) TEM images of TNT, (c) HR-TEM image of TNT, and (d) SAED pattern of TNT. 3756

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Figure 2. (a−c)TEM, HR-TEM, and TEM-EDS results of NT-3.

table attached with the spectrum indicates the composition of the elements present in NT-3 and confirms the deposition of Ni(OH)2 on TNT. Crystal Structure and Phase Analysis. The crystal structure and phase of the prepared TNT and Ni(OH)2/ TNT samples were studied through XRD analysis. Figure 3 displays the X-ray diffraction patterns of TNT and Ni(OH)2/ TNT (NT-1 to NT-5) catalysts. The XRD patterns show pure anatase TiO2 phase (JCPDS card no. 21-1272) of TNT and NT samples which are well supported by the TEM results. In the XRD patterns, no diffraction peaks of Ni(OH)2 are identified. It may be due to lower quantity and fine dispersion of Ni(OH)2 on TNT. Also, Ni(OH)2 may exist as quantum dots, in that case, the detection of the same is difficult due to very small sizes of the crystallites. The presence of Ni(OH)2 quantum dots was clearly observed in TEM images as discussed in previous section. Similar results were reported in the literature, for example Fujita et al.48 reported the synthesis of Ni(OH)2/TiO2 nanoparticles for H2 production in which no

shows a magnified TEM image of TiO2 nanotubes which clearly indicates hollow space inside the nanotubes. The outer diameter of the tube is about 4.9−9.8 nm, whereas the inner diameter is between 2.6 and 4.0 nm. The wall thickness of the tube varies from 1.9 to 2.4 nm (see inset in Figure 1b). HRTEM image of TNT is given in Figure 1c which indicates that the d-spacing of the lattice fringes is 0.358 nm. The d-spacing matches well with (101) plane of anatase TiO2. The SAED pattern of TNT (Figure 1d) confirms the anatase planes of TiO2. Figure 2a depicts higher magnification image of Ni(OH)2/TNT (NT-3) catalyst. Here small dark spots are present on TNT surfaces, which could be Ni(OH)2 quantum dots. The average size of these NPs is found to be 8.4 nm. The color of the nanocomposite powder looks pale yellow (see the inset photograph in Figure 1a). Figure 2b clearly shows the presence of Ni(OH)2 quantum dots in the form of black spots on TNT and the d-spacing value of 0.354 nm matches with anatase TiO2. The energy-dispersive X-ray spectrum confirms the presence of Ti, O, and Ni in NT-3 catalyst (Figure 2c). The 3757

DOI: 10.1021/acssuschemeng.7b04118 ACS Sustainable Chem. Eng. 2018, 6, 3754−3764

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light absorption from 400 to 500, extended up to 800 nm range, which can be assigned to Ni(II) d−d transition.50,51 An absorption shoulder can be seen around 410 to 450 nm for Ni(OH)2/TiO2, this is due to the direct interfacial charge transfer from the VB of TiO2 to CB of Ni(II).42 The absorption in the higher wavelength region is gradually increased with increasing amount (wt %) of the deposited Ni(OH)2 (Figure 4). The corresponding band gap values of the composite photocatalyst were calculated using the Kubelka−Munk function. The intercepts of the tangents to the plots of photon energy (hυ) versus (αhυ)2 were shown inset in Figure 4. This observation reveals Ni(OH)2 deposition on TiO2 lattices which is in accordance with the results reported by Yu et al.42 At the same time, only slight changes were observed for the absorption thresholds from 500 to 800 nm range for NiO/ TiO2.40 On the other hand, in the case of the Ni/TiO2 system no obvious change in the absorption edge around 400 nm can be seen unlike in the Ni(OH)2/TiO2 system.45 Hence it can be concluded from the above observations that Ni, NiO, and Ni(OH)2 show some unique influence on the corresponding semiconductors. The oxygen defects sites created by deposition of Ni(OH)2 on TiO2 may also be one of the reasons for enhanced light absorption toward higher wavelengths.43 Similar type of high wavelength light absorption can be seen in other noble metal based photocatalytic systems wherein Pt and Pd were used as cocatalysts. For example Meng et al.52 observed visible light absorption of Pt deposited photocatalysts for H2 generation. A similar type of light absorption of Pt/TiO2 was reported by Yu et al.53 Based on the characterization results it is clear that Ni(OH)2 deposited on TNT does not involve in band gap narrowing but it is present on the surface and enhances light absorption in the higher wavelength region (vide inf ra). This interesting result makes the Ni(OH)2/TNT material a promising candidate for solar energy applications.42,46 X-ray Photoelectron Spectroscopy (XPS) Studies. XPS was used to find the chemical oxidation states of the elements present on the surface of Ni(OH)2/TiO2 photocatalysts as depicted in Figure 5. The XPS survey spectrum shown in Figure 5a clearly indicates the existence of Ti, Ni, O, and C in Ni(OH)2/TiO2 catalyst. Figure 5b shows the narrow scan spectrum of Ti with peaks at binding energies 458.9 and 464.7 eV corresponding to Ti 2p3/2 and Ti 2p1/2 transitions, respectively.54 The narrow scan Ni 2p3/2 XPS spectrum exhibits a peak at 855.6 eV and a satellite at 861.0 eV which can be correlated to Ni(OH)2;42,55,56 (see Figure 5c). Particularly, the Ni 2p3/2 peak at 855.6 eV indicates that deposited nickel compound on the surface of TiO2 nanotubes consists mainly of Ni(OH)2.57 Ran et al.58 reported Ni(OH)2/CdS nanorods for H2 generation, and confirmed the existence of Ni(OH)2 from XPS data with Ni 2p3/2 peak appearing at a binding energy (BE) value of 855.9 eV. Hao et al.59 reported ZnS/ZnO/ Ni(OH)2 composite, where the Ni 2p3/2 peak corresponding to Ni(OH)2 can be found at 856.8 eV. It can be observed that the optimized catalyst does not contain any NiO or metallic Ni0 because, in the XPS data, the BE values corresponding to NiO (853.7 eV) and Ni0 (852.6 eV) are absent. There is enough evidence in the literature to support this conclusion. For example, Simon et al.60 reported Ni/CdS catalyst where the metallic Ni0 peak in XPS appears at 852.7 eV. Cheng et al.61 found an Ni 2p3/2 peak at 854.3 eV that corresponds to NiO. Liu et al.62 observed a NiO peak at 854.8 eV and a Ni(OH)2 peak at 855.8 eV in their NiO/g-C3N4 catalysts. Hence, based

Figure 3. X-ray diffraction patterns of TNT and Ni(OH)2-TNT (NT1 to NT-5) catalysts.

NiO XRD pattern was identified for lower weight percent of Ni(OH)2 (98%) can generate 7 H2 per molecule through photocatalytic reforming process. At the same time, impurities in crude glycerol (purity >70%) may hinder the efficiency of catalytic process, therefore, produces less number of H2 molecules ascribed to complex interactions between reaction intermediates and catalyst surface. Hence, pure glycerol has shown higher activity in the present study. Efficiency Calculation and Cost Benefit Analysis. The solar energy conversion efficiency of NT-3 with pure and crude glycerol was calculated using the following formula66 solar energy conversion efficiency (%) =

output energy (H 2) × 100 energy of incident solar light

(1)

The results of the calculations are given in Table 1. In presence of crude glycerol, the NT-3 catalyst shows a solar energy Table 1. Solar Energy Conversion Efficiency of Photocatalysts s. no.

sample ID

amount of H2 produced (mmol h−1 gcat−1)

energy of incident solar light (mW/cm2)

solar energy conversion efficiency (%)

1 2

NT-3a NT-3b

4.71 45.57

300 300

1.57 15.19

a

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Crude glycerol. bPure glycerol.

CONCLUSIONS Efficient photocatalytic reforming of crude glycerol, a byproduct from the biodiesel industry, was achieved by using Ni(OH)2/TiO2 nanotube photocatalysts. The characterization results indicated that Ni(OH)2 quantum dots decorated the surface of anatase TiO2 nanotubes, which improved visible light absorption property of the nanotubes. The Ni(OH)2 quantum

conversion efficiency of 15.19%, whereas in the presence of crude glycerol, the efficiency is only 1.57%, much less than pure glycerol. While comparing the cost of crude glycerol, it is very low (Rs. 25/L ≈ $0.375/L) than that of pure glycerol (purchased from Merck India), (Rs.1,170/L ≈ $18/L) that is about 47 times higher than the crude glycerol. Though usage of

Figure 8. Plausible reaction mechanism for H2 generation on Ni(OH)2/TNT from crude glycerol. 3761

DOI: 10.1021/acssuschemeng.7b04118 ACS Sustainable Chem. Eng. 2018, 6, 3754−3764

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dots played a major role as cocatalysts and remarkably enhanced the H2 production rate for about 12-fold higher H2 production compared to pristine TiO2 nanotubes. Thus, the low cost crude glycerol, a biodiesel byproduct was successfully used to generate H2 by improving photocatalytic efficiency of TiO2 nanotubes with low cost Ni(OH)2 quantum dots as a cocatalyst (substitute for noble metals) by utilizing sunlight, thus fulfilling the theme of “converting waste into clean fuels”.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-9966845899. Fax: +91-8562225419. E-mail address: [email protected] (S.M.V.). ORCID

Shankar Muthukonda Venkatakrishnan: 0000-0002-5284-1480 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.L.R. greatly acknowledges Department of Science and Technology (DST), New Delhi, India, for financial support through INSPIRE fellowship (IF 131053) to carryout the Ph.D. program. V.D.K. is highly thankful to UGC, New Delhi (No. F.6-6/2015-17/EMERITUS-2015-17-GEN-5524/(SA-II)) for the grant of the Emeritus fellowship. The authors thankful to RUSA (YV University), MHRD, Govt. of India, for providing facilities.

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b04118 ACS Sustainable Chem. Eng. 2018, 6, 3754−3764

Research Article

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DOI: 10.1021/acssuschemeng.7b04118 ACS Sustainable Chem. Eng. 2018, 6, 3754−3764

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DOI: 10.1021/acssuschemeng.7b04118 ACS Sustainable Chem. Eng. 2018, 6, 3754−3764