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Journal of Photochemistry and Photobiology A: Chemistry 281 (2014) 35–39

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

jo ur nal homep age: www.elsev ier .com/ locate / jphotochem

ydrogen photoproduction from bio-derived alcohols in an opticalber honeycomb reactor loaded with Au/TiO2

lena Taboadaa, Inma Angurellb, Jordi Llorcaa,∗

Institut de Tècniques Energètiques and Centre for Research in NanoEngineering, Universitat Politècnica de Catalunya, Av. Diagonal 647, Ed. ETSEIB, 08028arcelona, SpainDepartament de Química Inorgànica, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain

r t i c l e i n f o

rticle history:eceived 26 December 2013eceived in revised form 22 February 2014ccepted 1 March 2014

a b s t r a c t

Cordierite honeycombs loaded with 1 wt.% Au/TiO2 have been used in an optical fiber photoreactorilluminated with UV LEDs to produce hydrogen at 298 K from aqueous methanol, ethanol, glycerol,bio-ethanol, and bio-glycerol in liquid phase. The amount of alcohol has been varied between 1% and100%. Under the same photoreaction conditions, the amount of hydrogen produced follows the trend:

vailable online 12 March 2014

eywords:io-ethanolio-glycerol

methanol ∼ ethanol ∼ glycerol > bio-ethanol � bio-glycerol. Hydrogen photogeneration is significantlyenhanced (4–5-fold increase) in the optical fiber honeycomb photoreactor with respect to a conventionalslurry photoreactor.

© 2014 Elsevier B.V. All rights reserved.

itaniaold

. Introduction

In the viewpoint of hydrogen production, alcohols representn emerging source since they can be produced renewably fromiomass [1,2]. Among them, bio-ethanol constitutes an importantource that seems to be particularly suitable due to its easy androadly implemented production from a variety of plants. Also,io-glycerol is the main byproduct of biodiesel obtained via trans-sterification of vegetable oils. Biodiesel production, as a surrogatef fossil diesel, could be more economical viable using glycerols a value added product. Thus, it could be directly used for pro-ucing hydrogen. Alcohol catalytic reforming processes aimed tohe production of hydrogen have been widely reported in the lit-rature [3,4]. More recently, alcohols and renewable bio-alcoholsave been used in photochemical processes [5–10]. Althoughhe hydrogen yields reported in catalytic reforming are higherhan those obtained by photocatalytic routes, the later is receiv-ng particular attention because is based on the Sun, which is

perpetual source of energy, and it is an environmentally safeechnology operating at ambient conditions without undesirableyproducts.

Photocatalytic processes are based on electron–hole pairs thatre generated in semiconductors upon bandgap excitation by light.epending on the excitation lifetime relative to that of charge

∗ Corresponding author. Tel.: +34 93 401 17 08; fax: +34 93 401 71 49.E-mail address: jordi.llorca@upc.edu (J. Llorca).

ttp://dx.doi.org/10.1016/j.jphotochem.2014.03.004010-6030/© 2014 Elsevier B.V. All rights reserved.

recombination a net fraction of photocharges are present, which aretrapped at defect sites or migrate toward the photocatalyst surface.These surface electrons and holes can reduce and oxidize sur-face adsorbed molecules, respectively. TiO2-based photocatalystsare among the most attractive due to their availability, corrosionresistance, non-toxicity, low price and high photoactivity and sta-bility. Incorporating organics such as alcohols as sacrificial electrondonors into the photocatalytic process increase charge-separationefficiency and give higher H2 generation rates [6–10]. The rate ofhydrogen evolution is much higher, compared to that obtained inthe absence of organics in the irradiated photocatalyst, due to theirreversible oxidation of the organic molecule with photogeneratedholes and the concomitant suppression of electron–hole recombi-nation and H2–O2 back reaction, which are the most importantprocesses for decreasing the light-to-hydrogen conversion effi-ciency. The effect of the addition of noble metal nanoparticles(cocatalyst) to TiO2 is well known as it considerably increases thehydrogen production rate [5]. Deposition of Au nanoparticles ontoTiO2 has been found to enhance hydrogen production by facilitatingelectron transfer and therefore inhibiting electron–hole recombi-nation as well as by reducing the overpotentials for H2 generation[7–10].

It is important to recall that, whereas photocatalyst develop-ment constitutes an active area of research, if photons are not

effectively transmitted to the photocatalyst then its full potentialwill not be reached and system efficiency will be lowered [11].Slurry photoreactors, which are the most used in photocatalysisresearch in liquid phase, facilitate mass transfer, but they are

3 and Photobiology A: Chemistry 281 (2014) 35–39

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6 E. Taboada et al. / Journal of Photochemistry

imited by poor light penetration in the suspension. In many pho-oreactor designs (slurry reactors, annular reactors, spinning diskeactors, optical tube reactors and immersion reactors) the lightas to travel through the fluid containing reagents and also through

transparent wall. In addition, it is very difficult to achieve uniformrradiance on the entire catalyst surface, which is critical because a

inimum energy level is needed to activate the photocatalyst. Oncehe photocatalyst is activated, maximized contact between thehotocatalyst and reagents should be achieved, as well as removalf the reaction products formed. An ideally intensified photoreac-or should thus be able to integrate both maximized light efficiencynd mass transfer simultaneously. We have recently reported pho-ocatalytic generation of hydrogen from gaseous water–ethanol

ixtures in an optical fiber honeycomb reactor, where Au/TiO2hotocatalysts were immobilized on the monolith walls [12].ith this configuration, high light efficiency and availability

ombined with optimum mass transfer and scale-up potential waschieved.

In this work, we extend our work on photocatalytic produc-ion of hydrogen in optical fiber honeycomb reactors by studyingheir performance in liquid phase with a variety of bio-related alco-ols. Different mixtures of water and alcohol (methanol, ethanol,lycerol, bio-ethanol and bio-glycerol) have been tested over

wt.% Au/TiO2. We have used dendrimer-protected pre-formedu nanoparticles to ensure a constant Au nanoparticle size and torevent Au agglomeration [13,14].

. Experimental

.1. Photocatalyst preparation

Cordierite (Al4Mg2Si5O18) honeycombs (Rauschert, 100 cellser square inch, pore volume 210 ± 30 mm3 g−1, average poreiameter 3 ± 1.5 �m) measuring 16 mm in diameter and 20 mm

ength were used. They contained exactly 21 square channels with.1 mm channel width. The preparation of the Au/TiO2 photocat-lytic monoliths involved two steps. First, the honeycombs wereoated with a pure TiO2 layer by soaking them into pure titaniumsopropoxide, Ti(OCH(CH3)2)4, dried under continuous rotation andalcined at 723 K for 4 h (10 K min−1). Titania mass was monitoredy weight gain and was 30 ± 5 mg, which corresponds to a catalyst

oading of about 0.7 mg cm2.Gold nanoparticles were loaded onto the TiO2 layer by incipient

etness impregnation from a toluene solution containing pre-ormed Au nanoparticles measuring ca. 2.5 nm and capped withhiol-derived carboxilane dendrons HS(CH2)3Si((CH2)3Si(CH3)3)3,hich were prepared as reported previously [13]. The Au load-

ng was 1 wt.% with respect to TiO2. Honeycombs were finallyalcined at 673 K for 2 h (2 K min−1) to eliminate the carboxi-ane dendron shell and to assure a tight contact between theu nanoparticles and the TiO2 support. This temperature waselected following a detailed in situ XPS and HRTEM studyeported in [15]. After calcination the monoliths exhibitedhe characteristic purple color of Au/TiO2 containing small Auanoparticles.

.2. Photocatalyst characterization

Surface area measurements (BET) of the TiO2 powders were car-ied out with a Micromeritics ASAP 2000 apparatus after degassingt 443 K. The photocatalytic Au/TiO2 cordierite honeycombs were

haracterized by X-ray diffraction (XRD), high resolution trans-ission electron microscopy (HRTEM), UV-vis spectroscopy, and-ray photoelectron spectroscopy (XPS). XRD patterns were col-

ected between 10◦ and 90◦ of 2� with a step width of 0.02◦ and

Fig. 1. Scheme of the photoreactor used in this work.

a step time of 1 s using a Bruker D8 instrument equipped with CuK� incident radiation and a graphite monochromator. HRTEM wascarried out with a JEOL J2010F electron microscope equipped witha field emission gun operating at an accelerating voltage of 200 kV.Samples were dispersed in alcohol in an ultrasonic bath and a dropof supernatant suspension was poured onto a holey carbon coatedgrid. UV–vis reflectance spectra were collected with a ShimadzuUV3600 UV-Vis-NIR spectrometer equipped with an integratingsphere. XPS was performed on a SPECS system equipped with an Alanode XR50 source operating at 150 mW and a Phoibos 150 MCD-9detector. The pass energy of the hemispherical analyzer was set at25 eV and the energy step was set at 0.1 eV. Charge stabilization wasachieved by using a SPECS Flood Gun FG 15/40. The binding energy(BE) values were referred to the C 1s peak at 284.8 eV. Stability ofthe optical fibers was checked before and after the photocatalytictests by scanning electron microscopy (SEM) with a Zeiss NEON40crossbeam instrument operating at 5 kV.

2.3. Photoreaction experiments

The optical fiber catalytic wall reactor used in this work wasdescribed in detail in [12]. The light source consisted on four highefficiency UV LEDs emitting at a wavelength of 365 ± 5 nm (12 W,0.7 A) coupled to a bunch of optical fibers (Fig. 1). The opticalfibers measured 0.8 mm in diameter and were made of PMMA(poly(methyl methacrylate)). The fibers were originally coated toavoid light losses, so they were treated individually to obtain lat-eral irradiation exactly where the monoliths were placed. To attainuniform illumination, the end of the optical fibers was capped witha zinc-based paint to attain back-reflection of light. The photore-actor casing was made of glass and consisted of two pieces sealedby an O-ring that allowed an easy exchange of the photocatalyticmonoliths. The nominal distance between the surface of the fiberand the honeycomb walls was 0.9 ± 0.2 mm.

Photoreactions were carried out in liquid phase at 298 K inbatch mode (20 mL). In a typical experiment, the optical fibers

were first inserted into the channels of the photocatalytic monolithinside the photoreactor. Then, the photoreactor was loaded withthe water–alcohol mixture and the system was purged with Aruntil no air was detected and the experiment started. Agitation was

and Photobiology A: Chemistry 281 (2014) 35–39 37

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into the reaction, but no hydrogen was detected.Fig. 4 compiles the hydrogen photoproduction rates obtained at

steady state using different alcohols and alcohol–water mixtures.

E. Taboada et al. / Journal of Photochemistry

rovided by bubbling Ar through the liquid inside the photoreactor,hich also served as sweep gas. Several alcohol–water mixturesere tested (100%, 80%, 50%, 30%, 10%, and 1% alcohol on a molar

asis). The alcohols used were methanol (Panreac 99.9%), ethanolPanreac 99.9%), glycerol (Panreac 99.5%), bio-ethanol (Deulep6%), and bio-glycerol (from sunflower, 94%). Slurry reactor opera-ion was carried out strictly under the same conditions by replacinghe photocatalytic monolith with photocatalyst powder. The pho-oreactor effluent was monitored on-line every 1.5 min by gashromatography (Agilent 3000 A MicroGC) using MS 5 A, Plot U andtabilwax columns. Light irradiation was measured directly with aV-A radiation monitor from Solar Light Co. before and after eachhotocatalytic test. Blank experiments were carried out with theordierite monolith support and no photoactivity was measured.

. Results and discussion

.1. Photocatalyst characterization

The BET surface area of the TiO2 support was 41 ± 6 m2 g−1. XRDrofiles showed the presence of both anatase and rutile with a ratiof anatase:rutile ∼93:7, as determined according to the methodescribed in [16]. The microstructure of the Au/TiO2 photocatalystas studied by HRTEM and HAADF. The photocatalysts were consti-

uted by anatase and rutile crystallites of about 20–50 nm and Auanoparticles measuring 3.8 ± 0.5 nm in diameter (Fig. 2a and b).his is slightly larger than the size of Au nanoparticles capped witharboxilane dendrons in the precursor solution (Fig. 2c) as a resultf the calcination treatment, in accordance to [15]. The homogene-ty in size and good dispersion of the Au nanoparticles merits toe highlighted, which are a direct consequence of the preparationethod employed. UV–vis reflectance spectra showed a localized

urface plasmon resonance (LSPR) of Au nanoparticles supportedn TiO2 at about 567 nm (Fig. 2d), which is fairly similar to valueseported in the literature for Au nanoparticles measuring 3–5 nmupported on anatase [9]. The Au 4f core-level spectra showedhotoemitted electrons with binding energies at 83.7–83.9 eV (Auf7/2) and 87.2–87.4 eV (Au 4f5/2) eV, indicating that metallic Auas the only gold species on the near surface region of the photo-

atalysts.

.2. Photocatalytic tests

Fig. 3 shows the results of a typical experiment over a photocat-lytic 1 wt.% Au/TiO2 honeycomb. In this case, the alcohol–waterixture employed was EtOH:H2O = 1:1 (molar basis). After purging

he system, light was turned on and, immediately, the photopro-uction of H2 started and shortly after steady state was reached.y using a sweep flow gas (Ar) of 10 mL min−1 the concentrationf H2 was about 0.05% and the rate of photoproduced hydrogen1.8 �mol H2 min−1 g−1. When the sweep gas flow was doubled20 mL min−1) the concentration of hydrogen halved, but theate of photoproduction of hydrogen was maintained. There-ore, the sweep gas flow had an effect on the concentration ofydrogen at the photoreactor outlet but no effect on the rate ofhotoproduction of hydrogen (no diffusion limitations). From nownwards, only the photoproduction rates will be then considered,hich are independent of the sweep gas flow used. At the end

f the experiment the light was turned off and the production ofydrogen stopped. However, the other products of the photore-ction (mostly acetaldehyde) remained in the outlet stream of the

hotoreactor since they were dissolved in the liquid medium. Forhat reason, no mass balance calculations were possible since theroduct distribution between the liquid and gas phases varied. Alank experiment with a honeycomb loaded only with TiO2 did not

Fig. 2. (a) Bright field TEM and (b) HAADF images of the Au/TiO2 photocatalystcalcined at 673 K. (c) HRTEM images of the dendrimer-capped pre-formed Aunanoparticles. (d) UV–vis spectrum of the Au/TiO2 photocatalyst calcined at 673 K.

show appreciable hydrogen photoproduction. Also, the Au/TiO2photocatalyst was tested under visible light in order to assess ifthe Au Plasmon resonance at about 567 nm (Fig. 2d) contributed

Fig. 3. Hydrogen evolution over 1 wt.% Au/TiO2 (� and �) and TiO2 (©) honey-combs with time on stream from an ethanol:water = 1:1 molar liquid mixture usingdifferent sweep gas flows (� and © %H2, � �mol H2 min−1 g−1).

38 E. Taboada et al. / Journal of Photochemistry and Photobiology A: Chemistry 281 (2014) 35–39

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apparent quantum efficiency (AQE) for hydrogen photogenera-tion was calculated from the ratio of the double amount of H2(rH2 ) and the overall amount of photons irradiated by the optical

ig. 4. Hydrogen photoproduction rates over 1 wt.% Au/TiO2 honeycombs with dif-erent alcohol–water liquid mixtures.

or each alcohol–water mixture ratio tested, the photoproductionf hydrogen using pure methanol, ethanol and glycerol was alwaysigher than that obtained with bio-ethanol and bio-glycerol. Themount of hydrogen photoproduced was virtually indistinguish-ble when comparing methanol and ethanol, and slightly higherhan that obtained with glycerol. Other authors have reported17,18] or theoretically predicted [19] higher hydrogen yields withlycerol with respect to methanol and ethanol, but in any casehey all give good rates. The photoproduction of H2 from bio-thanol was roughly 25% less than that from the pure alcohol. Theowest hydrogen photoproduced rate was obtained when usingio-glycerol, which was about 60% less than that from pure glycerol.he lower H2 photoproduction rates obtained from the bio-alcoholsith respect to the pure alcohols can be explained by the pres-

nce of impurities such as aldehydes, organic acids and FAMEs.he strong adsorption of aldehydes on the surface of titania-basedhotocatalysts, for instance, has been shown to compete withhe adsorption of alcohol molecules, lowering the overall hydro-en photoproduction rate [12,18]. Concerning the effect of thelcohol–water mixture composition on the photoproduction ratef H2, although the rate behavior as a function of alcohol concen-ration is shallow, it is observed that the higher the water contentn the mixture the lower the H2 photoproduction rate. This haseen already reported in the literature [12] and related to a bene-cial effect of alcohol concentration on hole scavenging on TiO2.

nterestingly, whereas the trend between alcohol concentrationnd H2 photoproduction rate was maintained approximately lin-ar for the pure alcohols, a different behavior was observed for theio-alcohols. In this case, the scavenging effect was progressivelyuppressed at high water concentrations (1–10% alcohol) and thehotoproduction of H2 decreased markedly. Since CO2 productionas almost inexistent, photo-induced reforming of alcohols was

uled out.Fig. 5 shows the hydrogen photoproduced rates at steady state

btained from the different alcohol:water = 1:1 molar mixtures bysing the optical fiber honeycomb photoreactor and the slurryhotoreactor configuration with the same amount of 1 wt.%u/TiO2 photocatalyst. Clearly, in all cases the photocatalytic per-

ormance of the honeycomb photoreactor is much better thanhat of the conventional slurry reactor configuration (ca. 4–5-foldncrease). Taking into account that in both cases the light source

as exactly the same (optical fibers), either inside the monolithhannels (catalytic wall photoreactor) or without them (slurryeactor), the better photocatalytic performance of the optical fiberoneycomb photoreactor is ascribed to a better photon delivery.

Fig. 5. Hydrogen photoproduction rates and AQE values over 1 wt.% Au/TiO2 in acatalytic wall photoreactor (dark gray) or a slurry reactor (light gray) configuration.Alcohol:water = 1:1.

Both the immobilization of the photocatalyst onto the honeycombstructure and the insertion of optical fibers inside the channelslikely result in enhanced photon delivery to the photocatalyst,which yields a higher H2 photoproduction rate with respect tothat obtained in the slurry photoreactor configuration, where pho-ton delivery is less controlled. Assuming optimal light absorption,operation in a non-diffusion-limited regime, and that two pho-tons are required for liberation of one hydrogen molecule, the

Fig. 6. SEM images of an optical fiber before (a) and after (b) the photocatalytic tests.

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bers (N�, 2.4 �mol photons min−1) using the equation AQE =2rH2 /N�) × 100 [20,21]. The AQE values corresponding to the opti-al fiber honeycomb and the slurry photoreactors are included inig. 5.

Light irradiation was measured directly on the optical fibersefore and after each photocatalytic test and was maintainedonstant within 3% error. All the hydrogen photogeneration ratesere normalized to the initial value N�. However, a progressiveecrease of the overall amount of photons irradiated by the opticalbers was observed over all the different experiments (decreasef ca. 60%). Fig. 6 shows SEM images of a fresh optical fiber andn optical fiber at the end of the experiments. It is evident thathe roughness of the optical fiber increased markedly at thend of the experiments (Fig. 6b), which explains the decrease ofhotons irradiated. The deterioration of the optical PMMA fibersas mostly due to the adverse effect of aldehydes, as tested in a

eparate experiment. Therefore, chemical-resistant optical fibersi.e. quartz) would be more adequate for longer durability in thehotoproduction of hydrogen from water-alcohol/bio-alcoholixtures.

. Conclusions

Au/TiO2 photocatalyst (1 wt.% Au, 93% anatase, 7% rutile)repared from pre-formed Au nanoparticles was tested for the pho-oproduction of hydrogen from various alcohols and bio-alcohols.he photocatalyst was used in powdered form in a slurry reactor,r was deposited onto catalytic walls (cordierite honeycomb) in anptical fiber photoreactor. The hydrogen photoproduction rate wasigher when pure methanol, ethanol and glycerol were used, fol-

owed by that of bio-ethanol and bio-glycerol, thus suggesting thatmpurities that commonly accompany bio-alcohols are adsorbednto the photocatalyst surface and compete with alcohol holecavenging capacity. Alcohol–water mixtures were also tested andhe hydrogen photoproduction rate progressively decreased with

ater content. Aqueous bio-alcohol showed a stronger decline

f hydrogen photoproduction at increasing water content. Theptical fiber honeycomb photoreactor showed better performance4–5-fold enhancement of hydrogen photoproduction rates)

[[[[[

hotobiology A: Chemistry 281 (2014) 35–39 39

than the conventional slurry reactor under the same operationalconditions, allowing a higher degree of process intensification.

Acknowledgements

This work has been funded through grant MINECO ENE2012-36368. E.T. acknowledges UPC for a postdoc grant. J.L. is grateful toICREA Academia program.

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