Effect of epimerization of d-glucose on photocatalytic hydrogen generation over Pt/TiO2

Article abstract of DOI:10.1016/j.catcom.2011.11.017

Effects of α-d- and β-d-glucose as electron donors on photocatalytic hydrogen generation over Pt/TiO₂ have been investigated. α-d-Glucose exhibits better photocatalytic activity for hydrogen evolution than β-d-glucose. The effects of initial pH of the anomer solutions on photocatalytic hydrogen generation have also been studied. Weak basic condition is favorable for the photocatalytic hydrogen generation. There is a large activity difference between α-d- and β-d-glucose under neutral condition, while the difference is very small under basic and acidic conditions. The possible mechanism was discussed.

Full text of DOI:10.1016/j.catcom.2011.11.017

Catalysis Communications 18 (2012) 21–25
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Effect of epimerization of d-glucose on photocatalytic hydrogen generation
over Pt/TiO2
a
a,
a
b
b
Meihua Zhou , Yuexiang Li ⁎, Shaoqin Peng , Gongxuan Lu , Shuben Li
a
Department of Chemistry, Nanchang University, Nanchang 330031, PR China
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Effects of α-D- and β-D-glucose as electron donors on photocatalytic hydrogen generation over Pt/TiO2 have
been investigated. α-D-Glucose exhibits better photocatalytic activity for hydrogen evolution than β-D-
glucose. The effects of initial pH of the anomer solutions on photocatalytic hydrogen generation have also
been studied. Weak basic condition is favorable for the photocatalytic hydrogen generation. There is a
large activity difference between α-D- and β-D-glucose under neutral condition, while the difference is
very small under basic and acidic conditions. The possible mechanism was discussed.
Received 2 September 2011
Received in revised form 4 November 2011
Accepted 10 November 2011
Available online 20 November 2011
Keywords:
©
2011 Elsevier B.V. All rights reserved.
Photocatalysis
Hydrogen generation
α-D-Glucose
β-D-Glucose
α-β Equilibrium
1
. Introduction
Glucose as an electron donor for photocatalytic hydrogen produc-
tion has been studied [5,20–24], but these researches involve only the
equilibrated solution of the anomers. It is important to understand
Photocatalytic water splitting by solar light has been considered as
one of the most promising routes for renewable hydrogen produc-
tion. Many researches have been done in this field [1–8]. However,
in the absence of an electron donor, the efficiency of photocatalytic
hydrogen generation is very low because of the recombination of
photoinduced electrons and holes on semiconductor surface [9]. In
order to achieve a higher efficiency, many researches in this field
have involved electron donors as sacrificial agents, such as alcohols
2
the effect of structure of the anomers on photocatalytic H generation.
Temperature, glucose concentration [25] and metal ions [26] etc. can
change the epimerization equilibrium of D-glucose through the open
chain form. Therefore, if one can optimize the reaction conditions to
change the ratio of α-D- to β-D-glucose in an equilibrated solution, a
more effective reaction system for the hydrogen evolution would be
constructed. Since open chain D-glucose in an equilibrium solution
or a non-equilibrium solution is minor, its effect on the hydrogen evo-
lution should be negligible. Thus, effects of α-D- and β-D-glucose as
[
[
10], carbohydrates [11], hydrocarbons [12] and organic pollutants
13–19], which can react irreversibly with the holes. From a practical
point of view, the electron donor should be cheap and easy to obtain.
Glucose is a good electron donor because it is a cheap and renewable
biomass from natural cellulose and starch.
electron donors on photocatalytic hydrogen generation over Pt/TiO
have been investigated.
2
Natural glucose occurs in D-glucose form. At room temperature,
the free linear aldehyde form is the minor form in aqueous solution
2
. Experimental
All chemicals were of analytical grade and were used without
(
0.024%). The two remaining possibilities are the α and β glucopyra-
nose forms. In aqueous solution and at room temperature, the two
forms correspond to equilibrium of ca. 33% α-D-glucose and ca. 67%
β-D-glucose. The difference between two anomers originates from
further purification. β-D-Glucose was prepared by using α-D-
glucose as described in the literature [27]. The equilibrated D-
glucose solution was obtained by dissolving a given amount of
α-D-glucose in distilled water, and setting aside for 24 h at room
temperature.
the anomeric carbon C
1
, the OH functional group being axial for the
α form and equatorial for β form (see Fig. 1).
1
.0 wt.% Pt/TiO
16]: 1.00 g anatase TiO
sizes 20 nm, BET surface area 124 m g ), 26.7 mL 1.93×
2
was prepared by the photoreduction deposition
[
2
(Shanghai Kangyu Co Ltd, anatase, particle
2
−1
⁎
Corresponding author. Tel.: +86 791 83969983; fax: +86 791 83969983.
−
3
−1
E-mail address: liyx@ncu.edu.cn (Y. Li).
10 mol L
2 6
H PtCl , 2.0 mL anhydrous ethanol and 71.3 mL distilled
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.11.017

22
M. Zhou et al. / Catalysis Communications 18 (2012) 21–25
Fig. 1. Mutarotation of D-glucose.
water were added to a Pyrex cell with a flat side window for illumi-
nation. The other conditions are the same as the photocatalytic reac-
tions (see below). After 3 h irradiation, Pt was deposited on the
3. Results and discussion
2
3.1. Photocatalytic activity of D-glucose anomers for H evolution
2
surface of TiO .
Photocatalytic reactions were conducted in a 180 mL Pyrex cell.
The top of the cell was sealed with a silicone rubber septum. A
Because single α and β glucopyranose forms of D-glucose in aque-
ous solution are very unstable and can transform each other quickly,
in order to detect the activity of the two anomers, the photocatalytic
reaction for the hydrogen evolution has been conducted under non-
equilibrium of isomerism, namely α-D- or β-D-glucose being dis-
solved quickly and the reaction time being very short (see experi-
mental section). Fig. 2 shows that compared with pure water, low
initial concentration of glucose anomers and the equilibrated D-
2
50 W high pressure mercury lamp was used as the light source.
The reaction mixture inside the cell was maintained in suspension
by means of magnetic stirring. When photocatalytic activity of α-D-
or β-D-glucose was investigated, the experimental process was as
follows: 50 mg 1.0 wt.% Pt/TiO
added, N was bubbled through the mixture for 30 min to remove
oxygen, and a given amount of α-D- or β-D-glucose crystal was
added quickly. N was bubbled for another 8 min, the cell was illumi-
2
and 100 mL distilled water were
2
glucose can significantly increase the activity for photocatalytic H
evolution. Their activities decrease in the order: α-D-glucose>equili-
2
2
(glucose)b0.075 mol L−
1
,
nated immediately. The photocatalytic activity was determined by
measuring the amount of hydrogen production for 0.5 h irradiation
brated D-glucose>β-D-glucose. When C
the activity increases with increase of the glucose concentration;
0
−
1
on a gas chromatograph (TCD, 13× molecular sieve column, N
2
as
while when C
of the initial concentration. A control experiment showed that
when only with equilibrated glucose solution (without Pt/TiO ), no
0
(glucose)>0.075 mol L , it is almost independent
gas carrier). Sampling was conducted intermittently through the
septum during experiments. When the photocatalytic activity of
equilibrated D-glucose solution was evaluated, the experimental
process was as follows: 100 mL equilibrated D-glucose solution and
2
hydrogen evolution could be detected under the same irradiation
condition, indicating the hydrogen evolution in Fig. 2 is a photocata-
lytic process.
5
2 2
0 mg 1.0 wt.% Pt/TiO were added to the cell. After N was bubbled
through the mixture for 30 min, the irradiation was conducted.
Other procedures were similar to those for α-D- or β-D-glucose. The
As shown in Fig. 3, for both α-D- and β-D-glucose, the hydrogen
evolution increases with increasing initial pH, reaches a maximum
at pH 11.2, and then decreases quickly after pH 11.2. Stable hydrogen
evolution occurs in the pH range from 3.2 to 9.6. Thus, weak basic
condition is favorable to the photocatalytic hydrogen evolution,
while acidic or strong basic condition is unfavorable. The results are
similar to that obtained from the equilibrated D-glucose solution [22].
−
1
pH value of the solution was adjusted with 0.50 mol L
2 4
H SO and
−
1
0
.50 mol L
Different concentration solutions were obtained by dissolving
α-D-glucose crystals in 25 mL distilled water. 500 mg TiO was
quickly added to the above solutions, stirred at room temperature
for 30 min. TiO was removed by centrifugation. 10 mL supernatant
NaOH, and determined by a pH meter.
2
2
2
For the H evolution, obvious difference between α-D- and β-D-
was diluted to 100 mL. The chemical oxygen demand (COD) of the
diluted solution was measured on a COD-571 Meter using potassium
dichromate method, and the results were calculated in mg L− 1. Ad-
glucose can be observed under neutral condition, while the difference
is very small under acidic and basic conditions. The results can be at-
tributed to the fact that α-D- and β-D-glucose transform quickly into
2
sorption of equilibrated D-glucose on TiO was also measured simi-
larly by using the equilibrated solutions directly. Because there
was no other reductive substance in this system, COD could be
used to express the relative concentration of glucose. The adsorption
amounts of glucose onto the catalyst were calculated based on the
concentration difference (ΔCOD) before and after adsorption.
100
80
•
OH radicals react with terephthalic acid (TA) to produce highly
6
4
0
0
fluorescent 2-hydroxyterephthalic acid (TAOH) which emits fluores-
cence at around 426 nm on the excitation of its own 312 nm absorp-
tion band [28]. The measurement of •OH radicals was performed by
this TA fluorescence (FL) probe method as follows: 50 mg 1.0 wt.%
α-D-glucose
equilibrated-D-glucose
β-D-glucose
Pt/TiO
2
and 100 mL 5.0 mmol L−1 aqueous TA solution were added
20
0
to a 180 mL Pyrex cell. The other conditions are the same as the
above photocatalytic reactions. The photocatalyst was removed by
centrifugation. The photoluminescence intensity of the supernatant
was measured on a Hitachi F-4600 fluorescence spectrophotometer.
The reactivity of glucose anomers was performed by a similar method
for measurement of •OH radicals using 100 mL of 5.0 mmol L
ous TA solution containing 0.050 mol L
equilibrated D-glucose, respectively.
without glucose
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
-1
Concentration of glucose (mol L )
−
1
aque-
Fig. 2. Effect of the initial concentration of D-glucose anomers and equilibrated D-glu-
cose on photocatalytic hydrogen evolution.Reaction conditions: 50 mg 1.0 wt.% Pt/TiO
100 mL glucose solution, 0.5 h irradiation, room temperature.
−
1
α-D-, β-D-glucose and the
2
,

M. Zhou et al. / Catalysis Communications 18 (2012) 21–25
23
1
00
(glucose) b0.050 mol L−1, and reaches
0
the initial concentration for C
a flat for C
ΔCODα-
cates that more α-D-glucose can be absorbed on TiO
−
1
0
(glucose) >0.075 mol L . For all initial concentrations,
-glucose can be observed. This indi-
than β-D-
D
-glucose >ΔCODequilibrated-
D
8
6
4
2
0
0
0
0
0
2
glucose, which can explain why α-D-glucose has higher photocataly-
tic activity than β-D-glucose (Fig. 2). This suggests that adsorption dif-
ference between the two glucose anomers is a key factor to the H
evolution.
2
Fig. 5 shows the •OH-trapping photoluminescence spectra of Pt/
TiO in TA solution in the absence of and presence of D-glucose anom-
ers and the equilibrated D-glucose after UV light irradiation. A strong
photoluminescence emission peak of TAOH can be observed at about
2
4
26 nm for the TA solution (curve A). When D-glucose anomers and
their equilibrated D-glucose were added into the reaction system, in-
tensities of the photoluminescence emission peaks decrease marked-
ly (curve B-D), suggesting that both α-D- and β-D-glucose can
compete effectively with TA to trap •OH generated by illuminating
Fig. 3. Effect of initial pH on photocatalytic hydrogen evolution from D-glucose anomer
solution. Reaction conditions as in Fig. 2 except for 0.050 mol L−1 glucose solution.
2
TiO . The intensities of the peaks decrease in the order: none (curve
A)>β-D-glucose (curve B)>equilibrated D-glucose (curve C)~α-D-
glucose (curve D), indicating that α-D-glucose can compete more ef-
fectively with TA than β-D-glucose. The result is in good agreement
with the result from Fig. 2. The peak intensity of curve C is very
near to that of curve D, which can be attributed to the fact that the ad-
the equilibrated D-glucose under acidic and basic conditions, while
the transformation takes place slowly under neutral condition [29].
Fig. 4 shows time courses for photocatalytic H
TiO . For the equilibrated solution, the rate of H
2
evolution over Pt/
evolution (slope of
2
2
sorption time of the equilibrated D-glucose on TiO
2
before irradiation
the curve) for 15 min irradiation is the largest, decreases slowly for
following time intervals until 45 min, and then keeps almost con-
stant. This would be due to a larger pH change of the solution at initial
reaction stage and a smaller change at later reaction stage (initial pH:
(30 min) is longer than that of α-D-glucose (8 min).
3.3. Mechanism
4
.88, 45 min irradiation: 4.12, 180 min irradiation: 3.98). For α-D-
There are a large amount of surface hydroxyl groups for TiO
aqueous solution [30]. The superficial ionization equilibriums of
TiO can be expressed by
2
in
glucose solution, the rate of H evolution for 15 min irradiation is
close to that for the equilibrated D-glucose solution, which can be at-
tributed to the fact that the adsorption time for the equilibrated D-
2
2
2 2
glucose on TiO (30 min bubbling N before irradiation) is longer
than that for α-D-glucose (8 min). This suggests that adsorption equi-
librium affects the photocatalytic activity. With increasing irradiation
time, adsorption amount of α-D-glucose on TiO enhances, and thus
2
the rate increases compared to that for the equilibrated D-glucose.
After 60 min irradiation, the adsorption and transformation of α-D-
glucose would reach equilibrium. As a result, the rate is close to that
for the equilibrated D-glucose (two parallel lines).
þ
Ka1
↔
> TiOH þ Hþ
> TiO þ Hþ
>
>
TiOH2
ð1Þ
ð2Þ
Ka2
↔
−
TiOH
where >TiOH represents the “titanol” surface group. Ka1 is acidity
constant for the first dissociation of Eq. (1), and Ka2 is acidity con-
stant for the second dissociation of Eq. (2). The pH of zero point of
3
.2. Adsorption performance and reactivity of D-glucose anomers
As shown in inset of Fig. 4, for α-D-glucose and the equilibrated
D-glucose, the adsorption amount (ΔCOD) increases sharply with
charge of the used TiO
TiO mainly occurs in >TiOH
tral and basic conditions, respectively.
2
is 4.77 [19]. Thus, the surface species of
+
−
2
2
, >TiOH and >TiO under acidic, neu-
200
2
1
1
00
50
00
1
50
100
50
50
0
0
30
60
90
120
150
180
0
400
450
500
550
600
Irridation time (min)
Wavelenght (nm)
Fig. 4. Time courses for photocatalytic H
2
evolution over Pt/TiO
2
form α-D-glucose and
except for
.075 mol L− glucose solution and irradiation time. Inset shows adsorption amount
of α-D-glucose and equilibrated D-glucose (ΔCOD) on TiO
the equilibrated D-glucose solutions. Reaction conditions as in Fig.
2
2
Fig. 5. •OH-trapping photoluminescence spectra of Pt/TiO in various solutions. (A) TA,
1
0
(B) TA+β-D-glucose, (C) TA+equilibrated D-Glucose, (D) TA+α-D-glucose. Reaction
conditions as in Fig. 3 except for 5.0 mmol L− TA.
1
2
.

24
M. Zhou et al. / Catalysis Communications 18 (2012) 21–25
Glucose molecule (RHCOH) can be adsorbed on the catalyst
through hydrogen bond. The adsorption reaction can be expressed by
increases, leading to increasing the hydrogen generation activity.
−
However, with increasing pH, the surface TiO species increases.
As a result, electrostatic repulsion between TiO and RHCO− in-
creases, which is disadvantage to efficient hydrogen generation.
Thus, there is an optimal value at pH 11.2.
−
>
TiOH þ RHCOH→ > TiOH⋯OHCHR
ð3Þ
where ⋯ represents a hydrogen bond. Because of the cyclic oxygen of
Primary product of D-glucose oxidation is gluconate or gluconic
acid [5,20–23]. The reaction can be expressed by
glucose (Fig. 1) can affect strongly hydroxyl group at C due to the
shortest distance, the strength of hydrogen bond between >TiOH
and the hydroxyl group at C is much larger than those between
TiOH and other hydroxyl groups. Thus, we could assume that a glu-
cose molecule can be adsorbed mainly at TiO surface by the hydroxyl
group at C . Based on the configuration difference (Fig. 1), β-D-
glucose allows the most planar approach to TiO surface among the D-
1
RHCOH þ H2O→RHCOOH þ 2e þ 2Hþ
ð9Þ
1
>
With decreasing pH, redox potential of RHCOOH/RHCOH increases
2
and thus the activity of D-glucose decreases. In addition, with de-
1
+
creasing pH, the >TiOH
2
species enhances and >TiOH specie de-
2
glucose anomers. Similar explanation about β-D-glucose adsorption on
Pt electrode surface has been suggested [29]. Thus, less β-D-glucose
creases. The latter can trap effectively photogenerated hole to form
the surface hydroxyl radical (>TiOH• ), but the former cannot,
+
molecules could be adsorbed on TiO
The photocatalytic reaction is initiated by the photoexcitation of
TiO particles, which leads to the formation of electron-hole pairs. Va-
2
than α-D-glucose molecules.
which decreases also the activity. Thus, the activities for photocataly-
tic hydrogen evolution in both α-D- and β-D-glucose reaction system,
decrease similarly at low pH range (Fig. 3).
2
lence band holes can be filled by surface hydroxyl groups on TiO
2
to
The pH values of the reaction solutions in Fig. 2 were natural. Due
form hydroxyl radicals.
2
to the acidic products such as gluconic acid and CO , the pH decreased
with the reaction time [34]. This suggests that the pH effect would
also play a role for the hydrogen evolution.
hν
→
hvb þ •ecbꢀ
þ
TiO2
ð4Þ
ð5Þ
þ
þ > Ti ꢀ OH→ > TiOH•þ
h
vb
4. Conclusions
2
Pt deposited on TiO can trap the photogenerated electrons, and
electron acceptor H ions can obtain the electrons to produce hydro-
gen on Pt due to low overpotential.
α-D- and β-D-glucose are good electron donors for photocatalytic
hydrogen generation over Pt/TiO . α-D-Glucose exhibits better photo-
2
+
catalytic activity for hydrogen evolution than β-D-glucose, which has
been confirmed by COD and TA-FL. Weak basic condition is favorable
to the hydrogen generation. There is a large activity difference be-
tween α-D- and β-D-glucose under neutral condition, while the differ-
ence is small under basic and acidic conditions.
þ
−
H
þ •ecb→1=2H2↑
ð6Þ
Adsorbed glucose (electron donor) can react with the formed
surface hydroxyl radicals to improve the hydrogen evolution. Photo-
catalytic hydrogen evolution takes place with simultaneous photo-
Acknowledgments
degradation of D-glucose [21]. The oxidation of glucose at C
1
through •OH radicals is the easiest at a gold electrode [31]. Thus,
we could suggest that photocataltytic oxidation of glucose through
photoinduced hydroxyl radical takes place also at C . Glucose can
1
The financial supports of NSFC (20763006 and 21163012), 973
project (2009CB220003), the Natural Science Foundation of the
Jiangxi Province (2010JZH0096), and Research Fund of Education
Ministry of Jiangxi, China (GJJ09041) are gratefully acknowledged.
react with the hydroxyl radical over TiO
2
by H atom abstraction
[
30] as follows:
TiOH• þ RHCOH→ > TiOHþ þ RC•OH
þ
ð7Þ
>
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1