Photo-catalytic hydrogen production over Fe2O3 based catalysts

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 6 8 4 e7 6 8 9

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Photo-catalytic hydrogen production over Fe2O3 based catalysts A. Boudjemaa a,b,*, M. Trari c a

Technical and Scientific Research Centre of Physico-chemistry Analysis (CRAPC), BP 248, RP 16004, Algiers, Algeria Laboratory of Chemistry of Natural Gas, Faculty of Chemistry (USTHB) BP 32, 16111 Algiers, Algeria c Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry (USTHB) BP 32, 16111 Algiers, Algeria b

article info

abstract

Article history:

The hydrogen photo-evolution was successfully achieved in aqueous (Fe1
xCrx)2O3

Received 17 February 2010

suspensions (0  x  1). The solid solution has been prepared by incipient wetness

Received in revised form

impregnation and characterized by X-ray diffraction, BET, transport properties and

15 May 2010

photo-electrochemistry. The oxides crystallize in the corundum structure, they exhibit n-

Accepted 24 May 2010

type conductivity with activation energy of w0.1 eV and the conduction occurs via

Available online 18 June 2010

adiabatic polaron hops. The characterization of the band edges has been studied by the Mott Schottky plots. The onset potential of the photo-current is w0.2 V cathodic with

Keywords:

respect to the flat band potential, implying a small existence of surface states within the

Photo-catalytic

gap region. The absorption of visible light promotes electrons into (Fe1
xCrx)2O3-CB with

Hydrogen

a potential (w
0.5 VSCE) sufficient to reduce water into hydrogen. As expected, the

Corundum

quantum yield increases with decreasing the electro affinity through the substitution of

(Fe1
xCrx)2O3

iron by the more electropositive chromium which increases the band bending at the

Sulfite

interface and favours the charge separation. The generated photo-voltage was sufficient oxidation in the energetically to promote simultaneously H2O reduction and SO2
3
1 downhill reaction (H2O þ SO2
/ H2 þ SO2
). The best activity 3 4 , DG ¼
17.68 kJ mol
1 min
1 occurs over Fe1.2Cr0.8O3 in SO2
3 (0.1 M) solution with H2 liberation rate of 21.7 mmol g

and a quantum yield 0.06% under polychromatic light. Over time, a pronounced deceleration occurs, due to the competitive reduction of the end product S2O2
6 . ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The photo-electrochemical (PEC) conversion of light-to-electrical and/or chemical energy has been widely studied in the past and continues to attract much attention owing to the depletion of fossil energies and ecological problems [1e4]. Among the renewable energetic sources, the sun is clean, inexhaustible and remains the most promising. So, it is desirable to develop materials for specific objectives like the

energetic [5]. The photo-electrochemistry provides information about the energy band diagram of the interface semiconductor (SC)/electrolyte and enables the prevision of electrochemical reactions. It is based on the excitation of SC by energetic photons (hn > Eg) which generate electron/hole (e
/hþ) pairs that could be used for the water splitting. A great concern has been focused upon hydrogen as a clean energetic vector. It is recognized to be the fuel of the future which should reduce greenhouse emission, responsible of the

* Corresponding author. Technical and Scientific Research Centre of Physico-chemistry Analysis (CRAPC), BP 248, RP 16004, Algiers, Algeria. Tel.: þ213 21 24 79 50; fax: þ213 21 24 80 08. E-mail address: [email protected] (A. Boudjemaa). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.05.096

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 6 8 4 e7 6 8 9

climate change. Its photo-production is continuously growing, particularly in areas with favourable climatic conditions where the solar constant averages 1300 W m
2 (Algeria) and the sunshine duration exceeds 3000 h year
1. However, in spite of their good chemical stability, most oxides investigated for PEC conversion have large gap Eg and absorb only a small part of the sun spectrum resulting in low quantum yields. This result from the deep valence band, of O2
: 2p character owing to its large electron affinity and this restricts seriously SCs that can be employed in photo-catalysis. The semi-conductor is the key element of PEC device and some approaches to overcome the above handicap have been proposed [6]. The search of photo-active materials with new band structure led us to study the corundum family M2O3, in which M is commonly 3d metal [7]. For efficient PEC device, n-type SC requires not only a small gap and a long term chemical stability but also a flat band potential (Vfb) as cathodic as possible. The last requirement implies a small electro affinity. The hematite Fe2O3 and chromia Cr2O3 participate in a complete solid solution across the pseudo-binary phase system and the aim is to give insights on how the substitution on the M-sublattice influences the photo-activity. The passage from iron to chromium in M2O3 accounting by w1 eV in the atomic scale changes both the optical and transport properties. Literature survey reveals that they are employed in various applications such as electrochemical cells [8,9], photocatalysis [10] and opto-electronic devices [11]. These compounds, particularly with inherent low cost may be therefore attractive for solar applications. The direct optical gap and the chemical stability over a broad pH range make them attractive for PEC conversion. However, whereas Fe2O3 has been assessed photo-electrochemically [12], to our knowledge the PEC studies of Cr2O3 have not been reported and the limited work appears to have been focused on the physical properties [13,14]. The beneficial role of the porous structure on the catalytic performance has already been demonstrated [15e17] but little attention has been paid to the influence of the precursor on the surface properties of Fe2O3eCr2O3 photo-catalysts. In this paper, we report on the synthesis, the transport properties and the PEC characterization of the solid solution (Fe1
xCrx)2O3 and its application for the hydrogen production under visible illumination.

2.

Experimental method

The solid solutions (Fe1
xCrx)2O3: 0 Fe2O3/100 Cr2O3 (Cr), 30 Fe2O3/70 Cr2O3 (Fe3Cr7), 50 Fe2O3/50 Cr2O3 (Fe5Cr5), 70 Fe2O3/30 Cr2O3 (Fe7Cr3) and 100 Fe2O3/0 Cr2O3 (Fe) were prepared by incipient wetness impregnation. Fe2O3 was impregnated with a solution of Cr(NO3)3, 9H2O (purity, 99.5%) in appropriate proportions. After drying at 80  C overnight, the solids were calcined in air at 400  C for 2.5 h. To confirm the completion of the reaction, the end products were subjected to X-ray diffraction (XRD) using a monochromatic CuKa radiation, the data were collected for 10 s at each 0.02 step over a 2q range (10e80 ). The color changes from red (Fe2O3) to green (Cr2O3) passing by brownish and a set of complementary researches has been employed to characterize the powders. The Fe2þ content (oxygen deficiency) has been accurately determined

7685

by chemical analysis, the sample (50 mg) was dissolved in HCl (8 N) under inert atmosphere. The dissolution took several days and Fe2þ was titrated with K2Cr2O7. The specific surface areas were determined by the BET method using nitrogen gas as adsorbate at liquid nitrogen temperature on ASAP 2010 micromeritics apparatus. The diffuse reflectance spectra were recorded on the powder with a Cary 500 double beam spectrophotometer attached to a reflectance accessory. For PEC properties, the powder was cold pressed into 13 mm diameter pellets of w1 mm thickness and sintered at 400  C. The density of oxides calculated from the measured weights and dimensions, was found to be w80% of the X-ray density. Contact with copper wires was made onto the backside of the pellets with silver cement. The working electrodes were encapsulated in glass holders and isolated with epoxy resin. A three electrode cell was employed for the intensity potential J(V) characteristics including an emergency Pt counter electrode (Tacussel) and a saturated calomel electrode (SCE). The working electrode was irradiated by a tungsten lamp (200 W) through a flat optical window and the potentials were monitored by a Voltalab PGZ301 potentiostat (Radiometer). The electrolyte KOH (0.1 M) was continuously flushed with nitrogen and the experiments were performed at room temperature. The central part of the photo-catalytic system consists of 500 mL-Pyrex double jacketed reactor whose temperature was regulated at 50  2  C by a temperature controller (Julabo). The powder was dispersed under constant magnetic agitation. Visible light, produced by three tungsten lamps, was used as excitation source. Blank runs were carried out without the catalyst and in the dark. The outgoing gas has been positively identified as hydrogen by gas chromatography (Shimadzu IGC 121 ML) and the amount was determined volumetrically thanks to a water manometer with an accuracy of 0.05 mL. For the quantum efficiency (h), the flux intensity was measured by a light meter (Testo 545) with a relative error of 8%. The solutions were prepared from reagents of analytical quality and twice distilled water.

3.

Results and discussion

3.1.

Physical and photo-electrochemical properties

Fe2O3 and Cr2O3 are isostructural and form a complete solid solution. The XRD analysis of the samples revealed single phases; all the peaks index in the corundum structure with a hexagonal symmetry in agreement with the JPCDS N 34-0421. The patterns (Fig. 1) do not reveal any extra peak that could have resulted from a superstructure due to the existence of an ordering between the Cr3þ and Fe3þ ions which are randomly distributed in the crystal lattice. A linear change in the cell parameter was assumed for the solid solution according to the Vegard law owing to the closeness of ionic ratii of Cr3þ (0.061 nm) and Fe3þ (0.064 nm, HS) [18], the lattice constant changes slightly but monotonically and this is evidently a mono phase system. In the hexagonal description, the extension is more pronounced along the c-axis with an increase of 5% between the end members. The bulk density (4.20 g cm
3) corresponds toe80% of the theoretical value (5.24 g cm
2).

7686

4

log σ Ω-1 cm-1 )

(1010) (217)

(024) (116) (122) (214) (027)

(113)

(104) (110)

(012)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 6 8 4 e7 6 8 9

d

2 0

)

c b

-4

a 10

20

30

40 50 2θ degree )

60

70

-2

1.0

1.5

80

)

(1)

The transport properties are some of the common electrontransport properties of solids that characterize the nature of the charge carriers. They are enhanced by deviation from the stoichiometry, inducing variable valences. The n-type conductivity of (Fe(1
x)Crx)2O3 is due to vacancies and/or interstitial Fei, the oxides are soluble in acid solution and the chemical analysis gave an average oxygen deficiency of w0.5%. The increase of the conductivity (s) with temperature indicates semi conducting-like behavior (Fig. 3) which obeys to an Arrhenius type law:

Eg (eV)

d(Abs)/d λ

1.9

3.0

3.5

1000/T (K )

Over three weeks of continuous immersion, the compounds (Fe1
xCrx)2O3 showed no corrosion in alkaline solutions and the amount of dissolved ions would be too small for a reliable determination by UV visible. Although not a general rule, the photo-catalytic properties are governed by the active surface and the performance increases with decreasing the crystallite size [19]. The materials elaborated by impregnation technique have a submicron dimension crystallites. The Eg values of (Fe1
xCrx)2O3 were determined within error limits from the derivative curves of the reflectance diffuse spectra (Fig. 2, Insert) and the dependence Eg(x) is written as a linear function:

2.0

2.5 -1

Fig. 1 e XRD patterns of samples in the 2q range (10e80 ): (a) aeFe2O3, (b) Fe3Cr7, (c) Fe5Cr5 and (d) Fe7Cr3.

Eg ¼ 1:93
0:27x

2.0

Fig. 3 e Logarithm of the electrical conductivity vs. reciprocal temperature of: (,) Fe2O3, (6) Fe3Cr7, (;) Fe7Cr3, (A) Fe5Cr5 and (B) Cr2O3.

  DE s ¼ s0 exp
kT

(2)

The activation energy DE (Table 1) increases with increasing Cr-content, implying a charge transfer between cations with trapping of electrons in surface polaron states. The chemical inertness is a crucial parameter for a reliable catalytic performance. For most clarity, only Fe3Cr7 is reported (Fig. 4). The J(V) curve exhibits a dark current (Jd) smaller than 1 mA cm
2 due to the thermal excitation across DE, in conformity with a good electrochemical stability [20]. The potential at which the JdeV curve intercepts the potential-axis corresponds to the hydrogen evolution reaction (HER). The increase of the photo-current (Jph) along the anodic-going polarization is characteristic of n-type conductivity. The accurate flat band potential Vfb is obtained from the Mott Schottky plot:   1 2 kT V
V ¼
fb C2 330 eND e

(3)

where the symbols have their usual significations. The characteristic (C
2eV), depicted in Fig. 5, fits linearly with the experimental data, the flat region below w
0.6 V is due to the charges accumulation. The positive slope confirms the n-type behaviour of the electrode. The potential Vfb and the electrons density ND, provided from the potential intercept of the V-axis at C
2 ¼ 0 and the slopes of the straight lines respectively, are given in Table 2. The potential Vfb is w0.2 V more cathodic than the photo-current onset potential Von and this indicates a small existence of surface states within the gap region. The

1.8 600

675 750 λ (nm)

825

Table 1 e The physical properties of Fe2O3/Cr2O3 calcined at 400  C.

1.7

Catalysts

1.6 0.0

0.2

0.4 0.6 0.8 1.0 Composition (x)

1.2

Fig. 2 e Energy gap of (Fe1LxCrx)2O3 photo-catalysts. For most clarity, only Fe3Cr7 is reported in Inset.

Fe2O3 Fe3Cr7 Fe5Cr5 Fe7Cr3

Lattice constant (nm) a

c

0.503 0.430 0.434 0.485

1.369 1.372 1.371 1.304

S (BET) m2/g

DE (eV)

25.0 37.8 47.6 49.5

0.11 0.24 0.15 0.15

7687

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EH

2

J (μΑ/cm )

2 0

2O/H2

-1.2

Table 2 e The electrical and photo-electrochemical properties of Fe2O3/Cr2O3 calcined at 400  C in KOH (0.1 M) electrolyte.

= - 0.75 V

-0.8

-0.4

Catalysts

0.0

Fe2O3 Fe3Cr7 Fe5Cr5 Fe7Cr3

V(V) V = -0.61 V

-2

fb

-4

in the dark under illumination

Fig. 4 e The J(V) plots of the Fe3Cr7 electrode in KOH solution (0.1 M) under N2- atmosphere. Scanning rate 10 mV/s.

potential Vfb outlines the energetic position (P) of the conduction band with respect to vacuum: P ¼ 4:75 þ eVfb
DE

(4)

The problem for the hydrogen evolution was to find an oxide that has the band edges properly matched to the H2O/H2 level and a dn configuration may be used for this purpose. The P values (Table 2) are typical of materials in which CB is made up mainly of 3d orbital and it is of interest to discuss briefly the origin of this feature in terms of the band structure. The corundum M2O3 contains layers of hexagonal close packed ions with ded photo transitions. M3þ is octahedrally coordinated and the electronic band are defined by: 3d levels, the lower t2g levels providing VB is at w 6 eV below vacuum whereas CB is made up of half filled eg orbital of narrow width and whose potential is located below the H2O/H2 level (Table 2). Hence, the use of M2O3 as photo-cathodes for the hydrogen evolution does not require any bias assistance. When n-type SC is brought in contact with a solution, equilibrium is established by transfer of minority carriers (holes) from the bulk of SC to the interface and a bending of electronic bands near the surface occurs. This results in a junction electric field which promotes the separation of (e
/hþ) pairs. Fe2O3 has attractive photo-catalytic properties and many researches are being carried out to improve its performance. As already mentioned in introduction, n-type electrode material should have a potential Vfb as negative as possible

20

P (eV/V)

EH2 O=H2 (V)


0.72 e
0.52
1.15


0.51
0.61
0.33 e

4.13/
0.62 3.90/
0.85 4.27/
0.48 3.45/
1.30


0.51
0.75
0.40
0.88

3.2.

Photo-catalysis

The diffuse nature of the solar energy permits low capital investment and hydrogen can be obtained renewably in large amounts. The low electron mobility sets a limit for the doping level and the optimal crystallite size is controlled by the synthesis temperature. The photoelectrons generated in the bulk within the diffusion length migrate toward the interface to reduce water. Measurement done in our laboratory showed relatively large quantum yields in photo-induced water reduction on 25 nm sized Fe2O3 colloids in aqueous solution [22]. Fe2O3eVB is less anodic than the oxygen evolution, eliminating the possibility of unassisted water splitting. One way to prevent the holes accumulation consists to add being particularly favourable. It is a reducing agent, SO2
3 easier to oxidize than water, taking into account the large O2
over voltage over oxides and provides an absolute protection against photo-driven corrosion. The water reduction from aqueous suspension by parallel oxidation of SO2
3 has been yet investigated by some of us [22]. Oxygen is “by product” and is more interesting to oxidize because of its negative SO2
3 impact on the environment. Two mechanisms are equally possible: 2
2SO2
3 þ 2h / S2 O6

10

þ

5

-0.6

-0.4 -0.2 Potential (V)



E w
0:2 V

2

S2 O2
6 þ 2h þ 4OH /2SO4 þ 2H2 O

Vfb = - 0.66 V

0 -0.8

Vfb (V)

and this supposes a small SC electro affinity. The decrease of Vfb with the substitution rate x in (Fe(1
x)Crx)2O3 is due to the smaller electro negativity of chromium [21]. The photovoltage is imposed by the band bending at the interface and a maximal value is desirable in the darkness situation. As it is quite possible to substitute chromium into the Fe2O3 host lattice, this possibility could be envisaged to decrease the electron affinity of SC and to have an efficient separation of the pairs (e
/hþ) through a large band bending. In addition, the replacement of Fe3þ by Cr3þ raises the energy of VB, made up of 3d orbital and reduces the gap if we assume that CB, lies approximately at the same energy regardless of the M content. The broadening of the valence band would be responsible of the decrease of Eg.

þ

15

4

10 x C

-2

(nF-2 cm4 )

25

Von (V)

0.0

0.2

Fig. 5 e The Mott Schottky characteristic of Fe3Cr7 electrode in KOH solution (0.1 M).

(5) 

E w þ 1:7 V

(6)

SO2
3 is oxidized through the parallel reactions (5) and (6) 2
and not successively to S2O2
6 and then to SO4 because the 2
2
potential of the couple S2O6 /SO4 (þ1.7 V) is located far above cannot be converted to SO2
via holes VB VB and S2O2
6 4 process. The hydrogen production is temperature dependent, thus the experiment were performed at 50  C. Above, the

7688

B

25

-1

0.6

20

0.5

15

0.4 ι (%)

-1

A

H2 production rate ( μ mol g min )

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10

0.3 0.2

5

0.1 0.0

0 0

20

40 60 80 time (min)

100

120

0

20

40 60 time (min)

80

100

120

Fig. 6 e H2 production rate (A) and the corresponding yield (s) (B) on: (,) Fe2O3, (;) Fe7Cr3, (A) Fe5Cr5, (6) Fe3Cr7 in SO2L 3 (0. 1 M) electrolytes at 302 K and 1 atm.

vaporization losses become an increasing problem. The further criterion to eventuate in spontaneous photo-reductions is that the rest potential (Uf) of n-(Fe1
xCrx)2O3 must be more anodic than the potential Vfb (Table 2). The shiftless of the electronic bands of M2O3 with pH permits an adequate location of H2O/H2 level whose potential varies usually as
0.059 V pH
1. This property has been exploited and at pH 7, The compounds (Fe1
xCrx)2O3 have been tested successfully for hydrogen generation. They are thermodynamically stable over the pH range (6e14) [10] and provide an optimal band bending of w0.4 V, close to that previously claimed by Gerisher [23]. The best performance for H2 formation is achieved on Fe1.2Cr0.8O3 in SO2
3 electrolyte (0.1 M) with an evolution rate of 21.7 mmol g
1 min
1. The quantum yield (h) is defined as:   2 molecules of H2 finput number of photonsg The factor 2 arises because the hydrogen liberation is two electrons event. A quantum yield 0.06% under polychromatic light has been obtained (Fig. 6). The hydrogen evolution drops after less than 1 of continuous irradiation and reaches a plateau region; further illumination little increases the volume. This tendency to saturation is due to the reduction of the end product S2O2
6 which takes place in competition with the hydrogen evolution. In addition, HER proceeds on a limited number of active sites and can be rapidly saturated in a closed system. This was evident from the fact that after bubbling solution by nitrogen or using a new solution, the initial performance was entirely restored. The quantum yield remains relatively low, compared through recent study of nanorods structured Fe2O3 photo-catalysts under UV-light [24,25]. We have prepared mesoporous Fe2O3 for the photocatalytic hydrogen production with efficiency exceeding 1%; these issues are presently under way and will be considered in ongoing issues.

4.

Conclusion

The difference in the transport and optical properties of the hematite Fe2O3 and chromia Cr2O3 and the close structural

similarity prompted us to study the solid solution (Fe1
xCrx)2O3 to assess its photo-catalytic properties for the hydrogen production. The lattice constant changes slightly due to replacement of Fe3þ by Cr3þ with similar ionic radii. The electronic data suggest that the oxides have small polaron conductivity. The chemical analysis revealed an oxygen deficiency and (Fe1
xCrx)2O3 are low mobility, hopping n-type semi-conductors as confirmed by the Mott Schottky characteristics. The oxides are low cost, possess optical gap appropriately matched to the solar spectrum and have been tested successfully for hydrogen evolution. As expected, the quantum yield increases with the substitution rate x because of the large band bending originating from the smaller electro negativity of chromium. It was found that substitution of iron by chromium shifted the flat band potential towards negative is easier to oxidize than values. The sacrificial donor SO2
3 water and has been shown to be of interest because of its environmental impact. The volume of hydrogen rises linearly over illumination time as long as the concentration of S2O2
6 is weak and progressively drops to zero.

Acknowledgments The authors gratefully acknowledge the financial support of CRAPC. We also thank Dr S. Omeiri for his technical assistance.

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