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Experimental design as a tool to study the reactionparameters in hydrogen production fromphotoinduced reforming of glycerol over CdSphotocatalyst

Samantha A.L. Bastos a, Paula A.L. Lopes a, F�abio N. Santos b,Luciana Almeida Silva a,c,*

a Instituto de Quımica, Universidade Federal da Bahia, Campus de Ondina, Salvador 40170-290, BA, Brazilb ThoMSon Mass Spectrometry Laboratory, Instituto de Quımica, Universidade de Campinas, 13084-971 Campinas,

SP, Brazilc Instituto Nacional de Ciencia e Tecnologia, INCT, de Energia e Ambiente, Universidade Federal da Bahia,

40170-290 Salvador, BA, Brazil

a r t i c l e i n f o

Article history:

Received 28 March 2014

Accepted 14 July 2014

Available online 7 August 2014

Keywords:

Experimental design

CdS

Hydrogen

Photoinduced reforming

Seawater

* Corresponding author. Instituto de Quımicþ55 71 3283 6881.

E-mail address: las@ufba.br (L.A. Silva).http://dx.doi.org/10.1016/j.ijhydene.2014.07.00360-3199/Copyright © 2014, Hydrogen Energ

a b s t r a c t

The aim of this study was to set the reaction conditions of the photoinduced reforming of

glycerol aqueous solution over Pt/hex-CdS under visible light irradiation for enhancement

of hydrogen production by using a fractional factorial experimental design followed by a

BoxeBehnken design. The parameters assessed were irradiation time, mass of photo-

catalyst, concentration of glycerol, pH and electrolyte concentration (NaCl). The pre-

liminary two-level fractional factorial design (25�1) showed that all of the investigated

factors have significant effect in hydrogen production, being pH the most important

parameter. The three factors BoxeBehnken design showed maximum response for

hydrogen production in pH 4.0, 55% glycerol and 1.5 mol L�1 NaCl. The amount of hydrogen

obtained under these conditions was 270% higher than our previous result, using the same

photocatalyst and electron donor. In the ideal pH, >CdSH2þand >CdOH species are pre-

dominant before irradiation, indicating that such species play an important role in the

primary steps of the photoelectrochemical mechanism, which served as the basis for

proposing a mechanism for hydrogen generation as well as glycerol photooxidation. Based

on the surface response [NaCl] � [glycerol], a solution with salinity equivalent to approx-

imately the natural seawater was tested and the result for hydrogen production was

comparable to the best condition; besides, under this condition, the solubility of CdS in

aqueous solution is reduced.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

a, Universidade Federal da Bahia, Campus de Ondina, Salvador 40170-290, BA, Brazil. Tel.:

73y Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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 en e r g y 3 9 ( 2 0 1 4 ) 1 4 5 8 8e1 4 5 9 5 14589

Introduction

Renewable hydrogen can be produced in an efficient process

via the photocatalytic reforming of biomass components and

derivatives in ambient conditions. The photoinduced

reforming process combines the water splitting and oxidation

reactions of organic compounds under non-aerated condi-

tions in a photocatalytic cell [1e8]. Most of the systems

developed until now use Pt/TiO2 as the photocatalyst under

UV light irradiation, which is inefficient to drive the photoin-

duced reforming under sunlight irradiation, because the

incoming solar energy spectrumaccounts only about 3% of UV

light against approximately 43% of visible light.

Several studies have assessed the different reaction pa-

rameters that can influence the efficiency of the photoinduced

reforming of biomass derivatives in an attempt to find a highly

efficient system for hydrogen production [6e13]. The reaction

parameters most frequently evaluated are pH, mass of the

photocatalyst, type and concentration of co-catalyst, the

presence and concentration of electrolytes and type and

concentration of the biomass derivative. However, in all sys-

tems evaluated optimization of the reaction conditions are

usually carried out by a traditional univariate approach, in

which each factor is studied separately. This procedure does

not consider the interactions among the factors that can limit

the performance of the process. In this case, a multivariate

experimental design strategy is recommended because it al-

lows for simultaneous variation of all evaluated factors,

making it possible to distinguish interactions among them

that would not be detectable by the classical univariate

experimental design [14e16]. Besides, the multivariate pro-

cedure reduces considerably, since only two levels of each

factor are investigated, the number of required experiments

without loss of information about the system and it is

considered a good strategy to screen themore relevant factors

for a high efficiency of the reaction.

To perform the multivariate optimization, either a full

factorial design or a fractional factorial design in two levels

can be employed. In a full factorial design the number of ex-

periments is 2K þ C, where K is the number of variables and C

is the number of replicates at the central point. In this pro-

cedure, the effects of all the factors and their interactions can

be obtained, since all experiments are performed. However, in

cases in which the number of variables that affect the

chemical system is greater than four, it is recommended a

fractional factorial design in order to reduce the number of

experiments without loss of information about the effects of

all factors. In a fractional factorial design the number of ex-

periments required is 2K�P þ C, where P indicates how frac-

tionated the experimental design is. Some interactions among

the factors, for example, interactions among three or four

factors are not estimated in this case, since a half (2K�1) or a

quarter (2K�2) of the experiments in the full factorial design

are performed. After the screening of relevant variables one

can apply an experimental design with different level values,

according to the tendencies of each factor that resulted in an

enhancement in hydrogen production. Another alternative is

to apply a response surface methodology, such as Central

Compound Design (CCD), BoxeBehnken design or Doehlert, to

find the critical conditions, maximum andminimumpoints of

the response (in this case hydrogen production) in the

experimental domain studied [15].

Recently, we evaluated binary (Pt/hex-CdS) and ternary

(Pt/CdS/TiO2 and Pt/TiO2/hex-CdS) hybrid photocatalysts in

photoinduced reforming of glycerol under visible light irra-

diation (l > 418 nm) [17]. All of the hybrid materials demon-

strated photocatalytic activity with respect to hydrogen gas

production. The systems with CdS/aqueous solution interfa-

cial contact showed higher activity, with relative order of

reactivity to the hybrid photocatalysts: Pt/hex-CdS > Pt/CdS/

TiO2 > Pt/TiO2/CdS. Those results suggest that the hydrogen

production mechanism can be strongly influenced by hy-

drolytic surface reactions on CdS. The functional groups

developed on CdS surface, such as hydroxyl (on the cadmium

site) and thiol (on the sulfur site), are very susceptible to

variations on the solution conditions. Thus, the aim of this

study was to set the reactional conditions for hydrogen pro-

duction enhancement from photoinduced reforming of

glycerol aqueous solution over Pt/hex-CdS under visible light

irradiation using both factorial fractional and BoxeBehnken

designs. The parameters evaluated were irradiation time,

mass of photocatalyst, concentration of glycerol, pH and

electrolyte concentration (NaCl). Since NaCl is a major

component of natural seawater, the concentration of this

electrolyte was inserted as a factor on the experimental

design in order to evaluate the viability of the use of natural

seawater for producing hydrogen. Each factor and their in-

teractions were evaluated employing a fractional factorial

design and BoxeBehnken design to establish the best reac-

tion conditions with respect of hydrogen production. In this

work, we also evaluated CdS stability under reaction

conditions.

Experimental

Photocatalyst preparation

Hexagonal CdS (named hex-CdS) was prepared according to a

well established procedure in literature by heat-treating

commercial-grade CdS (Aldrich, yellow powders) at 700 �Cunder a nitrogen flow for one hour [17].

Photocatalytic reactions

A high-pressure 500 W HgeXe arc lamp was used as the light

source for the photocatalytic reactions. The collimated light

beam was passed through an IR filter, a focusing lens and a

418 nm cutoff filter before reaching the photocatalytic cell,

which was air cooled to maintain a constant temperature.

Before each experiment, the photocatalytic cell was purged

with argon for 30 min to eliminate O2. The photocatalytic cell

(total volume ¼ 180 mL) was equipped with a flat window and

argon gas inlet/outlet tubes, which serve to collect and

transfer gaseous products to the analytical system.

Hydrogen gas evolution was measured by gas chromatog-

raphy SHIMADZU (GC2014) with thermal conductivity detec-

tion (TCD) and CO and CO2 were measured simultaneously in

ionization flame detection (FID) with methanator. Because He

Table 2 e Factor levels of the first fractional factorialdesign 2(5¡1).

Parameter Level (�1) Level (0) Level (þ1)

Irradiation time (h) 3.0 5.0 7.0

Mass of CdS (G) 60 90 120

Glycerol concentration (v/v%) 30 50 70

pH 4.0 7.0 10.0

NaCl concentration (mol L�1) 0.1 0.55 1.0

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 5 8 8e1 4 5 9 514590

and H2 have similar conductivity values, argon was used as a

carrier gas.

An inductively coupled plasma optical emission spec-

trometer (ICP OES) (Optima 8300, Perkin Elmer, Inc., Massa-

chussets, USA) equipped with a cyclonic nebulizer chamber

(Perkin Elmer, Inc., Massachussets, USA) and concentric

nebulizer (Perkin Elmer, Inc., Massachussets, USA) was

employed for Cd determination in the liquid effluents. The

operating conditions are summarized in Table 1. The emission

line for the analysis by ICP OES was the main emission line of

Cd (228,802 nm). To obtain the analytical curve, a high-purity

grade 1 g L�1 stock solution of Cd (Accu Standard) was suc-

cessively diluted with a 55% glycerol aqueous solution at pH 4

and 1 mol L�1 NaCl, previously diluted 50 times, to give rise

standard solutions in the concentration ranging of

0.1e20 mg L�1. Samples were prepared by dilution of 200 mL of

each effluent solution in 10 mL of water.

In the photolysis experiment, the photocatalyst was

dispersed in an aqueous solution (total volume ¼ 60 mL)

containing an appropriate concentration of glycerol and NaCl,

with pH adjusted by the addition of NaOH or HCl solution.

Metallic platinum was deposited in situ on the photocatalyst

surface by the photodecomposition of PtCl62� with addition of

40 mL of 8% H2PtCl6.6H2O before pH adjustment. Aliquots of

1 mL of the gas phase were injected in the GC system in in-

tervals of one hour. In order to ensure the accuracy of the

determination and quantification of the hydrogen produced

during the photoinduced reforming reaction, a standard

mixture of 5%H2, 2%CO2 and 500 ppmCOdiluted in argonwas

injected before each experiment.

Experimental design

A fractional factorial design 2(5�1) (Table 2 in Section 3) was

done within the Statistica software in order to evaluate the

factors irradiation time, mass of photocatalyst, concentration

of glycerol, pH and electrolyte concentration (NaCl) in two

levels. A BoxeBenhken design was also done aiming the more

significant factors, such as concentration of glycerol, pH and

electrolyte concentration (NaCl), in three levels (Table 4 in

Section 3).

Point of zero charge

The zero point of charge of hydrous CdS under different

concentrations of NaCl was determined by the solid addition

method [18,19]. For each concentration of NaCl (0.1; 0.5; 1.0

and 1.5 mol L�1) a series of twelve flasks containing 10 mL of

NaCl solution had the pH adjusted by adding hydrochloric acid

or sodium hydroxide solutions to give rise pH varying of 1e12.

After adjusted the initial pH of the solutions, 0.040 g of hex-

Table 1 e Instrumental conditions of the ICP OES.

Parameter Value

RF incident power 1.5 kW

Plasma argon flow rate 12 L min�1

Auxiliary argon flow rate 0.8 L min�1

Nebulizer argon flow rate 0.7 L min�1

Sample injecting flow rate 1.0 L min�1

CdS was added to each flask. The suspensions were then

manually shaken, allowed to equilibrate for 24 h, and the pH

values of the supernatant were measured. The difference

between the initial and final pH value (DpH) was plotted

against initial pHs. The point of intersection of the resulting

null pH corresponds to the zero point of charge, pHZPC.

Results and discussion

The first fractional factorial design 2(5�1) in two levels con-

structed for the five factors selected for study (Table 2) resul-

ted in a matrix with sixteen experiments and three central

points (total 19 experiments) with the amount of hydrogen

produced in the photoinduced reforming reaction as the

response (Table 3). The levels were established based on our

previouswork [17] that evaluated the photocatalytic activity of

Pt/hex-CdS under the following conditions: 60 mg of photo-

catalyst dispersed in an aqueous solution (total

volume ¼ 60 mL) containing 30% glycerol and 40 mL of 8%

H2PtCl6.6H2O, with pH adjusted to 11 by the addition of a

NaOH solution without NaCl addition.

The Pareto's chart generated from Statistica software (Fig. 1)

shows that all of the investigated factors have significant ef-

fect in hydrogen production, considering 95% confidence

(p < 0.05). The factors pH and irradiation time have negative

effect, exerting more influence in level (�1). The factors glyc-

erol concentration, mass of CdS and NaCl concentration have

positive effect, influencing more in level (þ1). The larger

amounts of hydrogenwere obtained from experiments 6 and 7

(Table 3), of which the three factors with most strong effects

are located in most influent levels. The mass of CdS and

irradiation time factors showed minor effects as compared to

the others, but they also contributed for an increase of

hydrogen production in experiment 7 (Table 3).

The interaction effects depends on their levels, as seen in

Table 3 comparing experiments 5 and 8, in which the amounts

of hydrogen produced are very similar, with mass of CdS in

level (�1) in experiment 5 and irradiation time in level (þ1) in

experiment 8. This is an evidence that mass of CdS and irra-

diation time factors have similar effects in hydrogen produc-

tion, however these effects are opposite. Additionally, in

experiments 5 and 8, with NaCl concentration in level (�1), in

experiments 1 and 4, with glycerol concentration in level (�1),

and experiments 13 and 16, with pH in level (þ1), the amounts

of hydrogen decreased in this order and were proportional to

the increase in effects of the factors. It is worth to note that an

interaction two to two was observed among all factors eval-

uated as showed in the first order interactions in Pareto's chart(Fig. 2).

Table 3 e Experiments matrix and results obtained of the 2(5¡1) fractional factorial design.

Experiments Irradiation time (h) CdS (mg) Glycerol (v/v%) pH NaCl (mol L�1) H2 (mmol)

1 �1 �1 �1 �1 þ1 11.85

2 þ1 �1 �1 �1 �1 2.91

3 �1 þ1 �1 �1 �1 5.64

4 þ1 þ1 �1 �1 þ1 9.21

5 �1 �1 þ1 �1 �1 19.22

6 þ1 �1 þ1 �1 þ1 36.83

7 �1 þ1 þ1 �1 þ1 48.14

8 þ1 þ1 þ1 �1 �1 18.22

9 �1 �1 �1 þ1 �1 2.36

10 þ1 �1 �1 þ1 þ1 7.87

11 �1 þ1 �1 þ1 þ1 8.51

12 þ1 þ1 �1 þ1 �1 5.52

13 �1 �1 þ1 þ1 þ1 3.59

14 þ1 �1 þ1 þ1 �1 0.00

15 �1 þ1 þ1 þ1 �1 8.61

16 þ1 þ1 þ1 þ1 þ1 0.01

17 0 0 0 0 0 0.02

18 0 0 0 0 0 0.01

19 0 0 0 0 0 0.01

2233.27

-3179.12

7389.01

9391.70

-13439.70

p=0,05

Standardized Effect Estimate (Absolute Value)

(2) CdS (mg)

(1) Irradiation time (h)

(5) [NaCl] Mol L-1

(3) [Glycerol] %

(4) pH

Fig. 1 e Pareto's chart of the factors effects.

-182.06

-903.54

931.82

-1046.13

1329.82

2233.27

2500.73

-2518.95

-3179.12

-5645.24

-6576.91

7389.01

9391.70

-12194.70

-13439.70

p=0,05

Standardized Effect Estimate (Absolute Value)

2by42by51by41by52by3

(2) CdS (mg)3by51by3

(1) Irradiation time (h)1by24by5

(5) [NaCl] Mol L-1

(3) [Glycerol] %3by4

(4) pH

Fig. 2 e Pareto's chart of the first order interactions.

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 en e r g y 3 9 ( 2 0 1 4 ) 1 4 5 8 8e1 4 5 9 5 14591

The levels of the factors in experiment 7 represent the

optimal condition in the experimental domain studied.

However, the curvature parameter showed relevant and

negative effect, indicating that there is an inflection point

(maximum) in the experimental range investigated. There-

fore, it is possible an enhancement of the hydrogen amount

produced from experiment 7, fixing the less significant fac-

tors (irradiation time and mass of CdS) and modifying the

levels of the factors following the tendency observed in Par-

eto's chart.

The response surface method is a strategy frequently

used to obtain the maximum response condition (curve in-

flection). In the present system, a BoxeBehnken design was

applied with three of the more significant factors and three

replications in the central point. The factors and levels of

the BoxeBehnken design are described in Table 4 and the

matrix of the experiments with respective results is shown

in Table 5.

From the analysis of the surface response of the experi-

mental design (Fig. 3) the bestmodel that wasmore adjustable

to the experimental data was one with linear and quadratic

factors (terms) and an interaction two by two. The response

surface enables us to conclude that the estimated critical

point is as follows: pH 3.6, 84% glycerol and 0.6 mol L�1 NaCl.

However, such conditions correspond to theminimal point, as

seen in the response surface, where regions with high con-

centrations of glycerol represent low hydrogen production

(Fig. 3(a) and (b)). The results in Table 5 reveal that experiment

11 produced the highest amount of hydrogen; thus, pH 4, 55%

glycerol and 1.5 mol L�1 NaCl was established as the best

Table 4 e Factors and levels investigated in the 33

BoxeBehnken experimental design.

Factors Level (�1) Level (0) Level (þ1)

Glycerol concentration (v/v%) 55 70 85

pH 2.0 4.0 6.0

NaCl concentration (mol L�1) 0.5 1.0 1.5

Table 5 e Experiments of matrix for 33 BoxeBehnkenexperimental design.

Experiments pH Glycerol(v/v%)

NaCl(mol L�1)

H2 (mmol)

1 �1 �1 0 113.17

2 þ1 �1 0 0.02

3 �1 þ1 0 51.30

4 þ1 þ1 0 0.02

5 �1 0 �1 38.92

6 þ1 0 �1 0.06

7 �1 0 þ1 60.50

8 þ1 0 þ1 0.02

9 0 �1 �1 75.79

10 0 þ1 �1 34.87

11 0 �1 þ1 141.82

12 0 þ1 þ1 9.49

13 0 0 0 64.78

14 0 0 0 67.67

15 0 0 0 68.83

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 5 8 8e1 4 5 9 514592

condition to produce the maximum amount of hydrogen,

since it was not possible to estimate themaximumpoint from

response surfaces. The amount of hydrogen produced using

this reaction condition was as high as 141.82 mmol in three

hours, which is equivalent to 394 mmol gcat�1 h�1 against

107 mmol gcat�1 h�1 obtained in our previous work [17], using the

same photocatalyst and electron donor. Thus, the amount of

hydrogen obtained in this work after optimization of the

reactional parameters using multivariate design was almost

270% higher than our previous result.

In order to evaluate the suitability of seawater to compose

the reactional medium in the photoinduced reforming of

glycerol, experimental tests with concentration of NaCl in

0.6 mol L�1, which corresponds to approximately the salinity

of natural seawater, were done. In these tests, we used 45%

glycerol instead of 55% glycerol (optimal condition) based on

the surface response [NaCl] � [glycerol], Fig. 3(a), which in-

dicates the former as a better response when pH is fixed at 4.

Under this condition, the amount of hydrogen obtained was

117 mmol, which corresponds to 324 mmol gcat�1 h�1. This result

is comparable to the result of the optimal point, confirming

that it is possible to use natural seawater to produce hydrogen

without significantly decreasing the efficiency of the process.

The photocatalytic treatment of aqueous solutions of

biomass components and derivatives results in the oxidation

of the organic substrate by holes toward CO2, which is

Fig. 3 e Response surfaces type BoxeBehnken design: (a)

accompanied by the production of gas-phase hydrogen from

water [1]. Considering both reactions, the overall reaction for

photoinduced reforming of glycerol can be described as

follows:

C3H8O3 þ 3H2O / 7H2 þ 3CO2 (1)

The theoretical stoichiometry ratio H2/CO2 for equation (1)

is 2.33. However, no experiment has presented ratio near the

theoretical value (Fig. 4). The conditions closer to the theo-

retical ratio were those in which glycerol concentrationwas at

level (þ1), that is, as high as 85% concentration. For the best

condition to produce the maximum amount of hydrogen,

experiment 11, the ratio H2/CO2 was 97, which means glycerol

is not being totally mineralized in the evaluated time of re-

action. CO concentrations were also determined for all ex-

periments and varied in the range of 0e37 ppb. Under the best

condition to produce hydrogen CO is present in ultra-low

concentration of 1.7 ppb, which makes it possible to use the

obtained hydrogen in proton exchange membrane fuel cells

(PEMFCs), in which CO evenwith a very low concentration can

easily poison the catalyst.

In both fractional factorial and BoxeBehnken designs, pH

was the most important factor for hydrogen production. It is

interesting to note that at pH 7 in the experiments matrix of

the fractional factorial design 2(5�1) (Table 3) and at pH 6 in the

experiments matrix for BoxeBehnken design (Table 5) the

amount of hydrogen falls to near zero. This fact must be

associated with the variation of zeta potential on CdS surface

under different pH conditions, which implies in formation of

different functional groups on the surface. Park and Huang

[20] studied the zeta potential of hydrous CdS(s) as a function

of pH in the presence of NaClO4 electrolytes. The zeta poten-

tials at different electrolytes concentrations converges to zero

near neutral pH and is equal zero at pH 7.5, which means that

the ions are inert electrolytes at this condition. The surface

neutral charge in pH around 7 can decrease the interactions

between the substrate and functional groups on the CdS sur-

face, hindering charge-transfer processes. In the present

work, we determined the point of zero charge of hydrous CdS

under different concentrations of NaCl by the solid addition

method (Fig. 5). The point of intersection of the resulting null

pH corresponds to the zero point of charge, pHZPC, at this point

the surface charge on hydrous CdS is neutral. The already

known functional groups developed on CdS surface, hydroxyl

and thiol (protonated and deprotonated forms) [21], can be

[NaCl] £ [Glycerol]; (b) [Glycerol] £ pH; (c) [NaCl] £ pH.

Fig. 4 e Ratios of H2/CO2 and amount of CO produced in each experiment of BoxeBehnken experimental design.

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 en e r g y 3 9 ( 2 0 1 4 ) 1 4 5 8 8e1 4 5 9 5 14593

identified in some regions of the curves in Fig. 5. For suspen-

sion without the addition of NaCl the pHZPC is 6.4, which

means that the functional groups on surface are >CdSH and

>CdOH. Fig. 5 shows that, in general, the pHZPC progressively

increases as increases the NaCl concentration, varying from

6.2 to 8.4. At lower pH, the protonated functional groups are

predominant, >CdSH2þ (pH 4.1e4.3), more acid group, and

>CdOH2þ (pH 5.8e7.7); while at higher pH are present the

deprotonated forms, >CdS� (pH 7.3e7.7) and >CdO� (pH

8.9e9.9). The equilibrium among the different forms of func-

tional groups on hydrous CdS can be expressed as follows:

>CdSHþ2%>CdSH þ Hþ (2)

>CdSH%>CdS�þHþ (3)

Fig. 5 e Functional groups and zero point of charge of

hydrous CdS in presence of different concentrations of

NaCl: (a) >CdSH2þ and >CdOH; (b) >CdOH2

þ and >CdSH; (c)

>CdS¡ and >CdOH; (d) >CdO¡ and >CdS¡; (e) pHZPC.

>CdOHþ2%>CdOHþHþ (4)

>CdOH%>CdO�þHþ (5)

As can be seen in Fig. 4, at strong acid conditions, there is

no considerable difference between initial and final pH

(DpH z 0), indicating that there is no considerable surface

charge on hydrous CdS. However, at pH around 4, a large DpH

was observed and increases with increasing NaCl concentra-

tion and it becomes maximum at 1.5 mol L�1. The progressive

increasing in positive charge surface on hydrous CdS suggests

that the presence of electrolytes favors the surface proton-

ation and chloride ions are not efficiently adsorbed under high

NaCl concentration. However, the effect at high pH is oppo-

site, the negative charge decreases as increases NaCl con-

centration, suggesting that sodium ions are efficiently

adsorbed under these conditions.

The optimal experimental conditions found using multi-

variate design established pH 4 as the ideal pH for the

maximum hydrogen production. According to surface chem-

ical study of hydrous CdS the predominant species at pH 4,

before irradiation, are > CdSH2þand >CdOH, indicating that

such species play an important role in the primary steps of the

photoelectrochemical mechanism. Under irradiation,

>CdSH2þand >CdOH sites can combine with photogenerated

electron-hole pair according to reactions (6) and (8), yielding

trapped electron and hole, >CdSH and >CdOHþ, which are

involved in generation of reactive radicals, reactions (7) and

(10). Additionally, in the present work, the hydrogen produc-

tion is lower at high pH and is higher at low pH, indicating that

oxidation mechanism of the electron donor does not take

place via dissociation in aqueous solution, probably, because

of the low acidity of glycerol (pKa ¼ 14.2). A possible mecha-

nism for glycerol photooxidation is an initiation at surface-

trapped holes, >CdSHþ and >CdOHþ, via primary carbon to

yield glyceraldehydes [17], equations (7)e(9). A proposed

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 4 5 8 8e1 4 5 9 514594

mechanism compatible with this discussion is presented

below:

>CdSHþ2 ���!

e�>CdSHþ $H (6)

>CdSH���!hþ>CdSHþ

���������������!H2COHCHOHCH2OH>CdSHþ

2

þH2COHCHOH$CHOH(7)

>CdOH���!hþ>CdOHþ

���������������!H2COHCHOHCH2OH>CdOHþ

2

þH2COHCHOH$CHOH(8)

2 H2COHCHOH$CHOH / H2COHCHOHCHO þ H2 (9)

>CdOHþ2 ���!

eþ>CdOHþ $H (10)

2$H/H2 (11)

The generated hydrogen radicals, equations (6) and (10),

can combine at platinum nanoparticles photodeposited on

CdS surface to produce molecular hydrogen, equation (11).

The active sites, >CdSH2þ and >CdOH, are then regenerated

in reactions (7) and (10). However, it is well known that CdS

is prone to photocorrosion in aqueous environments by the

photogenerated holes [22,23]. In order to evaluate the cad-

mium leaching three selected samples of liquid phase were

submitted to cadmium analysis by ICP-OES. Such samples

were the liquid effluents from experiments 1 and 11 of the

BoxeBehnken experimental design with higher hydrogen

production, samples A and B, respectively, and the experi-

ment with seawater simulation, sample C. The results

shown in Table 6 reveal high cadmium leaching, mainly in

lower pH condition. Meanwhile, one can also see a clear

tendency in an increase of CdS stability by adjustment of

pH, salinity and glycerol concentration because it reduces

the solubility of CdS in aqueous solution. This tendency

confirms the results obtained from the first fractional

factorial design that revealed the electrolyte and glycerol

ratio in the mixture influences the amount of CdS required

for higher production of hydrogen. The association of the

results obtained in this study with alternatives that have

been proposed to improve the resistance to CdS photo-

corrosion, such as the replacement of some cadmium atoms

by zinc atoms in the crystal structure [11,22,23], may result

in a highly efficient system regarding production of renew-

able hydrogen and with the desired stability at the reaction

conditions.

Table 6 e Results of cadmium leachinganalysis from liquid effluents of selectedsamples. A: Experiment 1 (BoxeBehnkenexperimental design); B: Experiment 11(BoxeBehnken experimental design); andC: Experiment with Seawater simulation.

Sample Leached Cd (%)

A 10.3 ± 0.2

B 5.24 ± 0.01

C 3.8 ± 6.1 � 10�3

Conclusions

Our results demonstrated that the experimental design is a

powerful tool to evaluate the influence of each reaction

parameter and their interactions in the photoinduced

reforming of glycerol over hex-CdS photocatalyst. With this

study it was possible to set the ideal conditions for enhance-

ment of hydrogen production without any structure,

morphology or composition modification in the photocatalyst

as well as confirm the positive effect of electrolytes addition

for producing hydrogen and the viability for using natural

seawater in photoinduced reforming of a biomass derivative.

Additionally, our results open up perspectives to increase CdS

stability in hydrous environments by adjustment of pH,

salinity and glycerol concentration to reduce the solubility of

CdS in aqueous solution.

Acknowledgments

The authors acknowledge the Brazilian research funding

agencies Conselho Nacional de Desenvolvimento Cientıfico e

Tecnol�ogico (CNPq) and Fundac~ao de Amparo �a Pesquisa do

Estado da Bahia (FAPESB), grant number APP0046/2011, for

financial support.

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