Molecular structure and pH effects on the production of hydrogen from C1C4 aliphatic alcohols

ht. I. Hydrogen Energy, Printed in Great Britain.

Vol. 13, No. 9, pp. 569-572,

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1988 International

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MOLECULAR STRUCTURE AND pH EFFECTS ON THE PRODUCTION OF HYDROGEN FROM Cl-C4 ALIPHATIC ALCOHOLS 0. ENEA,A. A~1andD.

DUPREZ

Universite de Poitiers, Laboratoire de Chimie 4, U.A. CNRS 350,40 Avenue du Recteur Pineau, 86022Poitiers Cedex, France (Receivedforpublication

18Aprill988)

Abstract-The anodic photocurrents and the amounts of H2 produced under irradiation of aqueous suspensions of TiOl and Pt/SiOz + TiOz in 0.5 M alcohol were measured at various initial pH values. The photo-oxidation of alcohols on TiOz depends on the initial quantity of OH- ions. The transfer of photoproduced electrons to platinum deposits and the subsequent consumption of this charge for H, formation, strongly depends on the proton concentration in the bulk. The production of hydrogen can be optimized using a combination of photoelectrochemical and photocatalytic studies, determining the balance between electron production on illuminated semiconductors and consumption of these charges on the surface of the catalyst.

INTRODUCTION Dilute aqueous solutions of biomass constituents like alcohols, polyols, sugars, etc., appear to be economically attractive for the production of hydrogen via photocatalytic reactions. The efficiency of suspensions of various semiconductors (TiO,, CdS, Fea03 . . .) loaded by platinum were examined by Sakata, Kaway and Hashimoto [l-3] in the photodecomposition and photofermenting processes of some alcohols and sugars in order to find cheap and efficient photocatalysts for economically useful systems. The most promising photocatalysts were those based on Ti02 powders: a stable, reasonably efficient and inexpensive material. The production of hydrogen from aliphatic alcohols was recently examined by Borgarello and Pelizzeti [4] as a function of the characteristics (preparation procedure, anatase content, surface area) of TiOz particles and the nature (primary, secondary or tertiary) of the first alkylic alcohols. Hydrogen amounts produced by irradiated suspensions of Ti02 in concentrated (> 50% vol.) solutions of aliphatic (C, to Cd) monoalcohols were found by Pichat and co-workers [S-7] to be dependent on the content and spreading of the catalyst (Pt, Ni, Ru02 . . .) deposits as well as on the working temperature. None of these previous investigations on alcohols have established a link between the production of hydrogen by aqueous suspensions of Ti02 photocatalysts and the value of the initial pH in the bulk. Our preliminary experiments [g-10] with various polyols and sugars have shown a strong dependence between the photoactivity of Ti02 particles and the concentration of OH- ions in the suspension. In the present work we use a combination of photoelectrochemical and gas chromatography measurements, which we recently found [lo] very useful, in order 569

to obtain more specific information on the energetics and the photocatalytic properties ofHO particles.

EXPERIMENTAL Reagent grade chemicals of p.a. purity (alcohols, H$04, NaOH, KzS04) purchased from Merck and Degussa (Ti02 P 25, Si02 aerosil 300) were used as received to prepare all solutions and suspensions. The Pt/SiO, catalysts were prepared by impregnating a stirred suspension of Si02 in water with the required amount of a 10% wt H,PtCl, solution. After evacuation at 350 K and drying in an oven at 400 K, the powder was flushed with N2 while the temperature was progressively increased to 573 K and then the platinum oxide was reduced in Hz at 750 K for 15 h. A quantity of about 0.6% wt Pt was used for the whole batch in order to maintain a constant and high activity, but no attempt was made to maximize its catalytic activity. Initial pH values of vigorously stirred TiOZ (or Ti02 + Pt/Si02) suspension in alcoholic (0.5 M) solutions were adjusted with 0.25 M KOH or 0.25 M H2S04 and measured with a Tacussel pH Meter Minisis 5000. Photoelectrochemical experiments were performed at 350 < I < 420 nm in a two-compartment Pyrex cell separated by a large fine porosity glass frit. The larger compartment, having a flat optical window, was filled with 80 cm3 of a suspension containing0.2 gTi02, 0.0325 M K2S04, 10e3 M methylviologen and 0.5 M alcohols. A solution (50 cm3) having the same composition (except Ti03 and pH value was introduced into the smaller compartment where a high surface platinized platinum foil was used as counter-electrode. The collector electrode, a platinum flag (25 x 25 X 0.25 mm) introduced into the slurry compartment and held at 0.2 VISCE was used to measure the anodic

0. ENEA, A. ALI AND D. DUPREZ

510

photocurrent produced under irradiation with a 450 W lamp. In all experiments, the samples stirred at constant speed were degassed for more than 1 h with prepurified nitrogen’. A 10 cm water bath was used to remove IR radiation and a 350 nm cut-off filter to prevent direct photolysis of methylviologen. In photocatalytic experiments, the amount of hydrogen produced during the irradiation of stirred suspensions in a special (flat window) Pyrex flask of 80 cm3 volume, filled with 5 cm3 of 0.1 g TiOz + 0.1 g Pt/SiOa mixed suspensions containing 0.5 M alcohol, was detected by gas chromatography. In a typical experiment, the initial pH was adjusted to the desired value, the suspension thoroughly degassed (for at least30min) withN,, thevialclosedwithaseptumandthe sample irradiated at 350 < A < 420 nm with a 900 W Xenon lamp (Miiller). Gas aliquots (30 to 100~ 1) were analysed by means of a 5 m Porapak column at room temperature and a Gira gas chromatograph. No hydrocarbons (CH4, C2Hs, . . .) or CO2 were observed in the gas phase during the first hour of irradiation. The main photo-oxidation product of alcohols was a mixture of aldehydes and ketones, qualitatively detected by analytical standard methods.

RESULTS Photoelectrochemical

experiments

Illuminated suspensions of TiOz (2.5 g dme3) in aqueous solutions of 0.5 M alcohols have produced small anodic photocurrents (- 1 PA) at an inert (Pt) electrode immersed in the stirred suspension. The direct collection of photogenerated electrons by the inert (Pt) electrode, from the surface of TiOz (where the photoproduced holes are partially consumed by alcohol molecules acting as electron donors) is rather inefficient. The probability

I

I

0

50 -

2 t Z LOO-

150 -

I

CH,OH \

PH Fig. 2. AilAt vs pH plots for TiOz suspensions in 0.5 M alcohol.

of an electron transfer to a relatively small area (12.5 cm2) electrode is low and therefore the rate of e- h+ recombination increases with the electron build-up on the particles. The photocurrent has been significantly increased by the addition of small amounts of N, N’-dimethyl 4,4’dipyridyl dication (methylviologen or MV2+), an efficient electron trapping agent capable of reacting with protogenerated electrons to form the reduced radical MV+‘which is oxidizable at the collector electrode. The anodic photocurrents, always measured for the light intensity, area and imposed potential of collecting electrode, speed of stirring, etc., increase until a maximum steady-state value i,,,, is reached (see full curve in Fig. 1). Simultaneously, the initial pH decreases during the continuous illumination (see dotted curve in Fig. 1). On the contrary, the photocurrents are significantly lower, and the pH values stay almost unchanged, when the suspension contains a sufficient quantity of Si02 particles loaded by platinum deposits. The rate of photocurrents increase (AilAt), measured as the initial slope on the photocurrent-time curves as well as i,,, values reached after 30 to 40 min of irradiation, depend on the initial concentration of OHions in the bulk (see Fig. 2). The AilAt (and i,,,) values observed for Ti02-MV2+ -alcohol systems intercept with the pH axes at pH,,, 3.3 and exhibit a maximum at pH values ranging from 7 to 10. The AilAt values decrease in the sequence: CH,OH > CZHSOH > n-C3H,0H

Fig. 1, Photocurrent vs time (full line) and pH vs time (dotted line).

> n-C4H90H

when the length of the alkyl chain is increased from C, to Cd. This behaviour, similar to that of i,,, values, shows a decrease of the hole scavenger efficiency displayed by longer chain alcohols.

PRODUCTION

OF HYDROGEN

FROM ALIPHATIC

XHO-

ALCOHOLS

+ h+ + >C=O

571

+ H. (or H+ + e-) (4)

leads to the formation of aldehydes or ketones and produces a proportional quantity of protons. Simultaneously, hydroxide ions can also participate to the hole consumption OHad, + h+ -+ OH&

Illumination

(5) depending on the initial concentration of OH- in the bulk and of their hole scavenging efficiency in respect to that of alkoxide ions. The photoproduced electrons must be efficiently trapped on the surface of TiOz particles, otherwise the build-up of the negative charge will favor the recombination processes:

time ( min )

Ti02 (e- + h+) += TiOz Fig. 3. H2 amounts vs illumimation time for Ti02 + Pt/Si02 suspensions in 0.5 M methanol at various initial pH values.

The methylviologen trapping agent

(6) dication is an efficient electron

TiOz (e-) + MV*+ -+ TiOz + MV+’

Photocatalytic experiments The amounts of hydrogen produced by the mixed suspensions of Ti02 and Pt/SiO, in 0.5 M alcohol during the continuous illumination were found to depend on the initial concentration of OH- ions (see Fig. 3). The highest hydrogen production is observed at initial pH values close to 4 and in all cases the final pH values were significantly more acidic. During the first 50 min of irradiation the rate of hydrogen production with time is relatively small at pH 9 but it increases progressively until the maximum value observed at initial pH 4.3 is reached. In both cases a final pH value close to 4 was reached and stayed almost constant during long irradiation times. DISCUSSION When Ti02 particles are introduced in aqueous alcoholic solutions (before the illumination is started), alcohol and water molecules are dissociatively adsorbed on anatase basic sites in a simultaneous slow process, accompanied by a corresponding increase of proton concentration in the bulk (the initial pH 6 changes to pH 4): >CHOH+ HOH -+

XHO-

+ H+

(1)

OH- + H+

(2) As soon as the illumination of the stirred suspension is started, the creation of electron-positive hole pairs occurs: Ti02 + hv + TiO? (e- + h+)

(3) if hv > Eg, where Eg is the TiO:! band gap. The reaction of adsorbed alkoxide ions with the minority carriers:

(7) because of the adsorption of MV*+ on the Ti02 surface and of the formation of a relatively long-lived cation radical, MV” In photoelectrochemical experiments, the negative charge of these radicals is collected as an anodic photocurrent at an inert electrode, where they are reoxidized to methylviologen: MV+’ + Pt -j MV*+ Pt(e) (8) The increase of AilAt and i,,, values with the initial pH values (when pH < 9) shows that hole consumption by alkoxide ions is favored by an increase of OH- in the bulk. One of the reasons is the neutralization of H+ produced in reactions (1) and (2), another could be the neutralization of H’ radicals produced from reaction (4) by the hydroxyl radicals issued from reaction (5). If the initial (OH-) concentration in the bulk is too large (> 10m3 M) the hole scavenging efficiency of alkoxide ions decreases as well as AilAt and i,,, values, because of the competition with OH- ions in the adsorption processes at the Ti02 surface. The pH effects on the anodic photocurrent were also explained by Bard and co-workers [ 111 as a shift of the Fermi level energy EF EF = EF (pH = 0) -0.059 pH (at 25°C)

(9)

and of the flat band potential V, = -0.059 pH (IO) determined by the excess of OH- of H+ ions at the semiconductor. The hole scavenging action of alcohols decreases when the length of the alkyl chain is increased: presumably hydrophobic alkyl substituents make more difficult, and thus slower, the adsorption processes involving the polar hydroxyl groups and the desorption of the resulting aldehydes or ketones. In our photocatalytic experiments (performed in the

0. ENEA,

572

A. AL1 AND

absence of MV’+) the negative charge photoproduced under irradiation on the surface of Ti02 particles is collected by platinized silica particles TiOz (e-)

+ Pt/SiO,

--+ Pt(e-)/SiOz

+ Ti02 (11)

through an intra-particle electron transfer. The efficiency of Pt/SiO, suspensions, acting as high area, volumic electrodes, in trapping electrons depends on the consumption of the negative charge on the surface of platinum deposits which are cathodically charged until reduction processes can occur. The production of hydrogen from water Pt(ee)

+ 2 H+ -+ Pt + H2

(12)

or Pt(e-)

+ 2 Hz0 + Pt + HI + 2 OH-

(13)

is obviously dependent on the quantity of protons in the bulk. This explains why the hydrogen evolution rate is higher at pH 7 than at pH 9, contrary to what could be expected, if photoactivity only is considered. The consumption of the acidity issued from the photo-oxidation processes (reaction 4) in the catalytic reduction of water to hydrogen (reactions 12 and 13) explains why the pH shift during the illumination is much slower than without platinum loaded particles. The electron production at the illuminated surfaces of semiconductors can therefore be balanced by using an appropriate quantity of Pt/SiO, particles and pH value, with the subsequent consumption of this charge for useful reduction on the catalyst’s surface [ 121. CONCLUSIONS A combination of photoelectrochemical and gas chromatography measurements was necessary to elucidate the production of hydrogen by illuminated TiO, suspensions in aqueous alcoholic solutions. The main observations are the following: (I) the efficiency by which alkyl monoalcohols react with photoproduced holes reaches a maximum at neutral pH values and decreases in the order CH30H > C?H,OH > n-C3H,0H > n-CJH,OH; (2) the protons produced during the photo-oxidation of alcohols can be removed by using an appropriate

D. DUPREZ

catalyst for H2 production to keep more constant pH value and thus a high photoactivity; (3) significant electron transfer between the negatively charged Ti02 particles after irradiation and platinum deposits on SiO? particles occurs if the proton concentration is sufficiently high to favor the catalytic consumption of these electrons in water reduction. From the present study it results that dilute alcoholic solutions, as those existing in water or biomass effluents. can be virtually used for the production of hydrogen, a clean and useful fuel, by systems of particles. The major problems in using these systems for practical purposes are their rather low efficiency, the production of both oxidation and reduction products without separation, and the difficulty of holding slurry photocatalysts in place in large-scale flow systems. Improvements in their performance require new concepts in the design of these systems. Some of them, suggested in the present study, will be developed in our laboratory.

REFERENCES 1 T. Sakata and T. Kaway. Nouveuu J. Chim. 5. 275, (1981). 2 T. Sakata, T. Kaway and K. Hashimoto. Proc. Inr. Symp. Univ. Tokyo (Japan) 79 (1983). 3 T. Kaway. Proc. Ini. Symp. Univ. Tokyo (Japan) 52 (1983). 4 E. Borgarcllo and E. Pelizzetti. Chim. Ind. (Milan), 65, 474 (1983). 5 P. Pichat. J. M. Hermann, J. Disdier. H. Courbon and M. N. Mozzanega, Nouveau J. Chim. 5. 627 (1981). 6 L. T. Prahov, J. Disdier, J. M. Hermann and P. Pichat. Itit. J. Hydrogen Energy 9. 397 (1984). 7 P. Pichat, M. N. Mozzanega. J. Disdicr and J. M. Hermann, No14vem J. Chim. 6. SSY (lY82). 8. 0. Enea, Proc. 166th Meeting of Elactrochem. Sot. New Orleans (U.S.A.) (1984). 0. 0. Enea, Nouveuu J. Chim. 9. 281 (1985). 10. 0. Enea and A. J. Bard, Nouveau J. Chim. 9. 691 (1985). 11. M. D. Ward, J. R. White and A. J. Bard. J. Am. Chem. Tot.. 105. 27 (1983). 12. 0. Enea, A. Ah and D. Duprez, New J. Chem. 12, 27 (1988).