Photocatalytic Reforming of Biomass Derived Crude Glycerol in Water

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Photocatalytic Reforming of Bio-mass 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, Kanakampalayam Krishnanan Cheralathan, Valluri Durga Kumari, Bernaurdshaw Neppolian, and Shankar Muthukonda Venkatakrishnan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04118 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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ACS Sustainable Chemistry & Engineering

Photocatalytic Reforming of Bio-mass 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 Reddy1, Kanakampalayam Krishnanan Cheralathan2, Valluri Durga Kumari3, Bernaurdshaw Neppolian4, Shankar Muthukonda Venkatakrishnan1* 1

Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science &

Nanotechnology, Yogi Vemana University, Vemanapuram, Kadapa-516003, Andhra Pradesh, INDIA. 2

3

4

Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Near Katpadi Road, Vellore-632014, Tamil Nadu, INDIA.

Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (CSIRIICT), Uppal Road, Tarnaka, Hyderabad 500007, Telangana, INDIA.

SRM Research Institute, SRM University, SRM Nagar, Potheri, Kattankulathur- 603203,

Tamil Nadu, INDIA.

* Corresponding author. Tel.:+91-9966845899; Fax: +91-8562225419 E-mail address: [email protected] (M.V.Shankar). 1

ACS Paragon Plus Environment

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ABSTRACT Crude glycerol (10% w/w) is produced as a substantial by-product during the industrial production of biodiesel via transesterification process. Catalytic hydrogen (H2) generation by utilizing the crude glycerol and solar light is considered as a promising avenue. The present work illustrates enhanced rates of H2 generation and co-catalyst behaviour 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 diameter varies 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 4,719 µmol h-1 g-1cat 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 electro-negativity of Ni(OH)2 quantum dots present on the surface of TNT may facilitate effective shuttling of photoexcitons and, thereby largely preventing electron-hole recombination in TiO2 during photocatalysis.

Key words: Crude glycerol, titanium dioxide, hydrogen, photocatalysis, water splitting, solar light.

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INTRODUCTION Hydrogen (H2) generation from biomass derived by-products (crude glycerol) is a sustainable approaches to partially full-fill the global energy demand and also an alternative route for environmental remediation. Glycerol market has been expecting to reach USD 2.52 billion by 2020 globally, and biodiesel industry has emerged as the most important source of glycerol, accounting for over 1400 kt production in 20131, and its production increases steadily as about 4.53 kg of glycerol is formed for every 45.3 kg of biodiesel prepared2,3. Therefore, continuous supply of crude glycerol in larger quantities is possible especially at lower price from biomass feed-stock. According to some reports, crude glycerol may contain impurities such as carbon content at an average of about 25%, which includes methanol, fatty acid methyl esters, mono-glycerides, di-glycerides, glycerol oligo-mers, polymers, and unreacted tri-acylglycerols. Apart from that, water and a small quantity of metals like Na, Ca, K, Mg, Na, 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 process

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 comply with the 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 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 generation5–7. 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 photo corrosion, and suitable band edge potentials for both proton reduction and water oxidation reactions

8–12

. However, conventional TiO2 nanoparticles (NP) are

unfavourable for the photocatalysis applications, due to fast recombination of photo-formed electron and hole pairs. So the hierarchical nanostructures of TiO2, especially 1-D nanotubes, nanorods, nanobelts have become important class of nanomaterials to improve the photocatalytic efficiency13,14 as they can hinder the 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, more number of active 3



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sites on the outer as well as the inner walls of the nanotubes and even higher mechanical strength 15–17. Further to improve the efficiency of TiO2, various other strategies have been followed such as, modification of metal oxides noble metals

22,23

, noble metals

18,19

24,25

with other semiconductors (mixed oxides)

using different phases of TiO2

6,26,27

20,21

, non-

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 co-catalyst with TiO2 has been proven to be effective way for improved photocatalytic H2 production 28–31. However this system has some restrictions owing to 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 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 co-catalyst along with TiO2 for efficient H2 production 39. This is due to its beneficial intrinsic properties like its work functions that promote the separation of photo-excited 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 SHE, 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 favoured 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, 1-D nanomaterials show more beneficial properties than 0-D particles. Hence in this study, the 1-D TiO2 nanotubes based Ni(OH)2 photocatalyst was synthesized and used to improve the catalytic activity by harvesting the beneficial properties of these nano structures (vide supra), as the reports on the TiO2 based Ni(OH)2 photocatalysts for H2 generation are very limited46. In the present work, the industrial by-product, “crude glycerol” was used as a sacrificial agent to improve the photocatalytic efficiency.

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EXPERIMENTAL

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, 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, then transferred into a Teflon-lined autoclave (capacity 250 mL) and heated @ 130 ⁰C/ 20 h. The white colour precipitate thus obtained was washed twice at each stage with distilled H2O, dil. HCl and ethanol. Then the solid compound was dried at 80 ⁰C for 12 h to get a bright white powder. It was further spread in a ceramic boat and calcined at 350 ⁰C for 5 h @ 5 ⁰C/min. The resulted catalyst was denoted as TNT.

Preparation of Ni(OH)2/TiO2 nanotube (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 Ni(OH)2/TNT 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 ⁰C/12 h followed by calcination @ 350 ⁰C/3 h. Same procedure was followed for preparation of varying wt % of Ni (0.5, 1.0, 2.0, 3.0, 4.0) loaded TNT and the samples were

5



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labelled 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 Xray diffractometer. Transmission electron microscopy (TEM) images were captured using a 200 KV FEI-Tecnai, G2 20 S-Twin 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 photon with 1486.6 eV energy. Solar Photocatalytic H2 Production experiments All the photocatalytic experiments were performed as per the reported procedure

36

. Here

industrial by-product, “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 hours 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 6


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nanotubes, at this same stage the rolling of nanosheets also simultaneously occur to form tubular morphology17. During ethanol washing, the impurities (Na+) are removed and pure H2Ti3O7 nanotubes are obtained and finally calcined at 350 ⁰C 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 1(a) 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 with white color (shown inset of Figure 1(a)). The Figure 1(b) shows 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 Figure 1(b). HR-TEM image of TNT is given in Figure 1(c) 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 1(d)) confirms the anatase planes of TiO2. The Figure 2(a) 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 colour of the nanocomposite powder looks pale yellow (see the inset of photograph in Figure 1(a)). Figure 2(b) 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 2(c)). The 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 7



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dots, in that case, the detection limit is lower 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 al48 reported the synthesis of Ni(OH)2/TiO2 nanoparticles for H2 production in which no NiO XRD pattern was identified for lower wt% 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 interface between reaction intermediates and catalyst surface. Hence, pure glycerol has shown high 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 formula, 66.

The results of the calculations are given in Table 1. In presence of crude glycerol, the NT-3 catalyst shows a solar energy conversion efficiency of 15.19%, whereas in presence of crude glycerol, the efficiency is only 1.57 %, much lesser 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 pure glycerol leads to higher H2 production and solar conversion efficiency, the higher cost makes it less favourable than the crude glycerol for photocatalytic reforming.

Plausible reaction mechanism of Ni(OH)2/TNT for improved H2 generation

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Based on the experimental evidences obtained, a plausible reaction mechanism is proposed for photocatalytic H2 generation process on the Ni(OH)2-TNT catalyst as given in Figure 8. Though the conduction band edge of anatase in TiO2 is higher (or more negative) than the reduction potential of H+/H2, the rate of H2 production is low for TNT in the absence of Ni(OH)2 quantum dots, due to rapid recombination of CB electrons and VB holes and presence of large H2 production over potential. Once Ni(OH)2 quantum dots were deposited on TNT, the photo-excited electrons from conduction band (CB) of TiO2 migrate to CB of Ni(OH)2 quantum dots, and reduce them to Ni0. It is plausible because the CB potential of (0.23 V vs SHE, pH = 0) of Ni2+/Ni is slightly lower than the CB level (about -0.26 V) of anatase TiO2. The glycerol consists of primary, secondary and tertiary alcohols and it acts as hole scavenger to get oxidised into H+ ions and oxidized intermediates, the H+ ions in-turn get reduced with photogenerated electrons at Ni(OH)2 surface to generate H2 gas. The in-situ formed Ni0 particles may transfer the gained electrons to H+ ions to reduce them to H2, so that the Ni(OH)2 co-catalyst is regenerated. The co-catalytic effect of Ni(OH)2 quantum dots could be confirmed further by conducting the photocatalytic experiment under Xe lamp illumination with visible light cut-off filter. In the absence of visible light, the NT-3 catalyst did not show any H2 generation activity which indicates that the catalyst is not active in the absence of UV light. Moreover, neither TNT nor Ni(OH)2/TNT are active under visible light. But under solar light, which is having both UV and visible light photons, the NT-3 catalyst exhibites improved activity which confirms that TNT is utilizing UV portion of solar light and generates charge carriers which in turn, transferred to Ni(OH)2 quantum dots for H2 generation. Some of the previous reports support the reaction mechanism proposed in this study 40,42,45. CONCLUSIONS Efficient photocatalytic reforming of “crude glycerol”, a by-product from biodiesel industry was achieved by using Ni(OH)2/TiO2 nanotubes photocatalysts. The characterization results indicated that Ni(OH)2 quantum dots decorated the surface of 1-D anatase TiO2 nanotubes, which improved visible light absorption property of the nanotubes. The Ni(OH)2 quantum dots played a major role as co-catalysts and remarkably enhanced the H2 production rate, 12 fold higher H2 production when compared to pristine TiO2 nanotubes. Thus the low cost crude glycerol, a biodiesel by-product was successfully used to generate H2 by improving photocatalytic efficiency of TiO2 nanotubes with low cost Ni(OH)2 quantum dots as a co13



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catalyst (substitute for noble metals) by utilizing sunlight, thus fulfilling the theme of “converting waste into clean fuels”.

ACKNOWLEDGMENTS N. Lakshmana Reddy greatly acknowledges Department of Science and Technology (DST), New Delhi, India for financial support through INSPIRE fellowship (IF 131053) to carryout Ph.D program. Dr. V. Durga Kumari is highly thankful to UGC, New Delhi (No. F.6-6/201517/EMERITUS-2015-17-GEN-5524/(SA-II)) for the grant of Emeritus fellowship. Authors thankful to RUSA (YV University), MHRD, Govt. of India for providing facilities.

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Figure captions Scheme 1. Schematic representation for preparation of Ni(OH)2/TNT nanocomposite by wet impregnation method. Scheme 2. Schematic representation of formation of TiO2 nanotubes prepared by hydrothermal method. Figure 1. (a-b) TEM images of TNT, (c) HR-TEM image of TNT, (d) SAED pattern of TNT. Figure 2. (a-c)TEM, HR-TEM and TEM-EDS results of NT-3. Figure 3. X-ray diffraction patterns of TNT and Ni(OH)2-TNT (NT-1 to NT-5) catalysts. Figure 4. DR UV-vis spectra of TNT and Ni(OH)2/TNT (NT-1 to NT-5) catalysts. Figure 5. X-ray photoelectron spectrum of Ni(OH)2/TNT (NT-3). (a) survey spectrum, (b-d) narrow scan of Ti 2p, Ni 2p and O 1s. Figure 6. Photoluminescence (PL) spectra of TNT and Ni(OH)2/TNT catalysts Figure 7. Photocatalytic H2 generation performance of Ni(OH)2/TNT catalysts with crude glycerol and pure glycerol, (a) effect of Ni-loading on TNT in the presence of crude glycerol, (b) stability of NT-3 in the presence of crude glycerol, (c) effect of Ni-loading on TNT in the presence of pure glycerol. Figure 8. Plausible reaction mechanism for H2 generation on Ni(OH)2/TNT from biomass derived glycerol. Table Captions Table 1. Solar Energy Conversion Efficiency of Photocatalysts

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Scheme 1

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Scheme 2

23



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Figure 1

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Figure 2

25



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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Table 1. S.No

Sample ID

Amount of H2 Energy of Solar energy produced mmol. Incident Solar conversion h-1. g-1cat. Light (mW/cm2) efficiency (%)

1.

NT-3*

4.71

300

1.57

2.

NT-3+

45.57

300

15.19

* = Crude glycerol, + = Pure glycerol

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For Table of Contents Use Only

Synopsis: Sustainable H2 fuel generation process from the crude glycerol a by-product from biodiesel industry through photocatalytic process under solar irradiation using Ni(OH)2/TNT catalyst.

33


Photocatalytic Reforming of Biomass Derived Crude Glycerol in Water

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