Homogenous photocatalytic decomposition of acetic acid using UV-Fe 2+/Fe3+ system in the absence of oxygen

Article abstract of DOI:10.1016/j.catcom.2010.06.003

Photocatalysis is a promising way to eliminate organic pollution. Here a novel homogenous non-oxygen photocatalytic system was studied. Acetic acid was used as a model substrate to be decomposed into hydrogen, methane, ethane and carbon dioxide continuously by photo-excited Fe²⁺ and Fe³⁺ ions. Fluorescence spectra and ultraviolet visible spectra showed that Fe ²⁺ and Fe³⁺ were excited by ultraviolet light. The rate of acetic acid decomposition increased along with the increase of temperature, Fe²⁺ and acetic acid concentration. The photocatalytic mechanism and kinetics were proposed and the effects of pH value, reaction temperature, concentration of FeSO₄ and CH₃COOH were investigated.

Full text of DOI:10.1016/j.catcom.2010.06.003

Catalysis Communications 11 (2010) 1099–1103
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
2
+
3+
Homogenous photocatalytic decomposition of acetic acid using UV-Fe /Fe
system in the absence of oxygen
Honghui Yang, Yubin Chen, Liejin Guo ⁎
State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi'an Jiaotong University, Xi'an 710049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Photocatalysis is a promising way to eliminate organic pollution. Here a novel homogenous non-oxygen
photocatalytic system was studied. Acetic acid was used as a model substrate to be decomposed into
Received 22 March 2010
Received in revised form 30 May 2010
Accepted 1 June 2010
2+
3+
hydrogen, methane, ethane and carbon dioxide continuously by photo-excited Fe
Fluorescence spectra and ultraviolet visible spectra showed that Fe and Fe were excited by ultraviolet
light. The rate of acetic acid decomposition increased along with the increase of temperature, Fe and acetic
acid concentration. The photocatalytic mechanism and kinetics were proposed and the effects of pH value,
and Fe
ions.
2+
3+
Available online 9 June 2010
2
+
Keywords:
Photocatalytic
reaction temperature, concentration of FeSO
4
and CH
3
COOH were investigated.
© 2010 Elsevier B.V. All rights reserved.
Acetic acid decomposition
2
+
3+
Fe /Fe
Activation energy
Decarboxylic reaction
2
+
1
. Introduction
some organic substances, and then itself is reduced into Fe [10]. If
3+
2+
2+
3+
2+
Fe
could be converted into Fe
efficiently, a Fe —Fe —Fe
Photocatalytic decomposition of organic pollution is recognized to
cycle can be established, and many kinds of organic acid could be
decomposed to produce some useful products in this cycle. But so far
as we know, no study was reported on this circular system in the
be one of the most promising ways to clean our environment. Studies
on eliminating organic pollution by heterogeneous or homogeneous
photocatalysts under ultraviolet (UV) light were widely published
absence of oxygen. Here we established the Fe2 —Fe —Fe circular
system with acetic acid as a model substrate. Its reaction mechanism
and kinetics were proposed. Furthermore, parameters in the reaction,
such as temperature, pH value, Fe2 concentration, and acetic acid
concentration were investigated.
+
3+
2+
[
1–6]. Most of these studies were carried out in the presence of
oxygen or H . Under these conditions, hydroxyl radicals were
generated from H splitting or the reaction between oxygen and
2 2
O
+
2 2
O
light-excited photocatalyst. Hydroxyl radical exhibits high standard
oxidation potential (ca. +2.8 V/SHE) and it decomposes organic
acid into carbon dioxide finally. But in photocatalytic reaction in
the absence of oxygen, protons can be reduced into hydrogen by
electrons transfered from the light-excited photocatalyst. Taking
acetic acid for example, its carbonyl is decomposed into carbon
dioxide, and the methyl radical reacts with hydrogen atom or another
methyl radical to produce methane or ethane. Hydrogen, methane
and ethane are excellent fuels. Thus, organic pollution can be
eliminated and gaseous fuel can be produced simultaneously by
photocatalysis in the absence of oxygen. But only a few studies
focused on non-oxygen condition [7,8].
2. Experimental
All of the reagents used were analytical grade and purchased from
4 2
market without further treatment. FeSO ·7H
O was used as Fe2
+
source. Photocatalytic reactions were carried out in a 450 mL home-
built inner irradiated photocatalytic reactor made with quartz as
shown in Fig. 1. The light source was a 500 W high-pressure mercury
lamp (mainly emits 365 nm light). Nitrogen was bubbled into the
reactor to eliminate oxygen before reaction. Fluorescence and
Ultraviolet visible spectra were obtained on a F-4500 (Hitachi) and
U-4100 (Hitachi) spectrophotometer, respectively. The volume of the
gas products was measured by volumeter and the components of the
gas products were analyzed by a gas chromatograph (GC, Beifen Co.
Ltd. Model, SP-2100) equipped with a thermal detector and a 4 m
Hayesep stainless column. Every experiment was carried out at least
three times independently to make sure the data were reliable. The
gas volumes data were converted into that under standard state (0 °C,
100 kPa) according to the ideal gas equation.
2+
Fe can be excited from ground state to excited state by UV light
to release an electron to an oxidant present in the solution, forming
3
+
2+
Fe . The electron released from Fe
can reduce proton into
hydrogen atom [9]. Fe3 produced from Fe
+
2+
can be excited by
light with a wavelength around 360 nm and extracts an electron from
⁎
Corresponding author. Tel.: +86 29 82663895; fax: +86 29 82669033.
E-mail address: lj-guo@mail.xjtu.edu.cn (L. Guo).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.06.003

1100
H. Yang et al. / Catalysis Communications 11 (2010) 1099–1103
Fig. 3. Ultraviolet visible absorption spectra of FeSO
4
–HAc (A) and HAc solution
(
B); and Fluorescence spectra of FeSO in H SO aqueous solution (D) and in HAc
4
2
4
aqueous solution (E) with the excitation light(C), Curve A and B share the left Y-axis,
while C, D and E share the same right Y-axis.
Fig. 1. Inner irradiation photocatalytic reactor and volumeter, 1, 2 cooling water inner
entrance and exit, 3 lamp, 4 reaction solution, 5 exit of gas for analysis.
Eqs. (1) to (7). Fe2+ and Fe3+ are used to represent all the species of iron
in the solution.
3
. Results and discussion
2
+
þ
hν
→
3 +
Fe
+ H
Fe
+ H•
ð1Þ
ð2Þ
3
.1. Reaction mechanism
−
þ
CH COOH
→
3
CH COO + H
Fe2+ in CH
such as [Fe (H
Ligand Field Theory [11], the scheme of molecule orbitals for these
3
3
COOH (HAc) aqueous solution can form many species,
2
+
2−x
2
O)
6
]
, [Fe (Ac)
x
(H
2
O) 6−x
]
, etc. According to the
3
+
−
hν
→
2 +
Fe
+ CH3COO
Fe
+ CH3COO•
ð3Þ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
2
− x
species is shown in Fig. 2. In the octahedral [Fe(Ac)
σ orbitals of the ligands mix with the 3d, 4 s and 4p orbitals then form
new molecule orbitals. The transition from 1t2g to 2e can be confirmed
by the near IR spectra shown in Fig. 3A. The absorption around 960 nmis
x
(H
2
O)6−x
]
ion,
CH3COO•
→CH3• + CO2
g
CH3• + H•→CH4
2H•→H2
Δ
−1
consistent with
60 nm), which is the transfer energy from 1t2g to 2e
be excited by UV light, and release hydrated electrons, then Fe ions
0
(the accepted value is 10400 cm , which is about
9
g
. These species can
2+
3
+
are oxidized into Fe ions [9,12,13]. The hydrated electron is reactive
and it can reduce proton into hydrogen. When the solution is irradiated
by UV light, the reaction processes are proposed and shown from
2CH3•→CH3CH3
When Fe2+ was irradiated by UV light, the electron transferred
from 1t2g to 2a1g (or 2t1u). The excitation energy of this transfer was
corresponding to the light absorption around 280 nm shown in the
UV spectra (Fig. 3A). Electron in 2a1g was released to reduce proton
2+
3+
3+
into hydrogen atom (H•). And then Fe was oxidized into Fe . Fe
was excited by another photon and one electron from CH
COO− was
COO was decomposed into CH
• reacted with each other to produce CH
. Although HAc itself absorbs the light with wavelength
shorter than 250 nm and get decomposed into CH and CO in the
absence of oxygen, Fe2 absorbs light with wavelength shorter than
80 nm and its absorbance is around 2 at 250 nm, and acetic acid
3
taken to regenerate Fe2 , while CH
and CO . CH •, H• and CH
and CO
+
−
3
3
•
2
3
3
4
,
2
C H
6
2
4
2
+
3
shows its absorbance only around 0.03 at 250 nm. This means most of
the photons will be absorbed by Fe2 other than HAc itself. The
+
4 4 2 4
fluorescence spectra of FeSO –HAc solution and FeSO –H SO
solution are displayed in Fig. 3 (E and D, respectively). The pH values
of the two solutions were 2.59 without adjustment. After
being excited by 280 nm light, the fluorescence emission intensity
of FeSO
4
–H
2
SO
4
solution was higher than that of FeSO
–H SO
generated by the
by proper electron donors
4
–HAc solution
and close to the intensity of exciting light. In FeSO
proper electron donor is present, and then Fe
4
2
4
solution, no
3
+
2+
UV light could not be reduced into Fe
except the emitted electron from the excited Fe2 , thus the electron
+
transferred from 2a1g back to 1t2g and accompanied with high intense
fluorescence emission. But in FeSO
4
–HAc solution, HAc reduced
2
+
O)6 − x]2 − x complex orbits.
3+
2+
Fig. 2. The scheme of Fe (H
2
O)
6
or [Fe (Ac)
x
(H
2
excited Fe
into Fe , and the electron in 2a1g orbital reduced

H. Yang et al. / Catalysis Communications 11 (2010) 1099–1103
1101
1
proton into hydrogen, thus the intensity of emitted fluorescence was
decreased.
where E
exponential factor; R is the ideal gas constant (8.314 J·mol ·K
a
is the activation energy (kJ·mol− ); A is the Arrhenius pre-
−
1
−1
)
and T is the absolute temperature (K). According to Eqs. (8) and (9),
3
.2. Effect of temperature and thermodynamic analysis
the following equation can be carried out as:
The effectof temperatureon therates of gasproduction isdepictedin
Ea 1
R T
ln r = ln A + n ln S−
ð10Þ
x
Fig. 4a. The production rates of CH
CO2 and rH•, and N (CH •)=N (CH
CH ), N represents the amount of the species in the bracket) increased
rapidly and almost linearly, and the production rates of H and C
C2H6,r 2) increased much more slowly when the temperature was
3
•, CO
2
and H• (abbreviated as rCH3•
,
r
(
3
4
)+2 N(C
2
H
6
), N (H•)=2 N(H )+N
2
4
The activation energy can be worked out using regression. The high
regression coefficients shown in Fig. 5a suggest that the Arrhennius
equation describes the relationships between temperature and
2
2 6
H
(
r
H
raised from 283 K to 343 K. The rapid increases of rCH3•, rCO2 and rH•
3
the production rates of H• or CH • properly. According to these
suggest that the decomposition rate of HAc was affected by temperature
remarkably, and the slow increases of rC2H6 and r 2indicate that the
H
3
data, the activation energy for H• and CH • production from light-
excited Fe2 and Fe was estimated to be 13.9 kJ/mol and 14.5 kJ/mol,
+
3+
reaction rates of Eqs. (6) and (7) were affected by temperature slightly.
The relationship between rCh3•, rH• and the concentration of HAc can be
expressed as follows [14]:
respectively.
3.3. Effect of pH value
n
The pH value is always an important factor in chemical or
photochemical reactions. Its influence on photocatalytic decomposi-
tion of acetic acid is shown in Fig. 4b. The pH values were adjusted by
rx = kS
where r
ð8Þ
x
represents rCH3• or rH•, and S is HAc concentration, and k is a
H
2
SO
4 4
and NaOH solution, and FeSO concentration was fixed to be
content. According to Arrhenius equation, the relationship between
constant (k) and temperature is expressed as follow:
−
1
0
.1 mol·L . When the pH value was below 2.0, the decomposition
rate of acetic acid decreased along with the decrease of pH, and when
the pH value was above 2.0, the decomposition rate of acetic acid
decreased along with the increase of pH. There were several
−
Ea =RT
k = Ae
ð9Þ
Fig. 4. The effect of parameters on acetic acid decomposition, (a) temperature, operated at HAc 0.444 mol L−1, pH unadjusted, FeSO
0.1 mol L−1; (b) pH, operated at T 323 K, HAc
4
.444 mol L− , FeSO
1
0.1 mol L ; (c) FeSO
−1
concentration, operated at T 323 K, HAc 0.444 mol L , pH unadjusted; (d) HAc concentration, operated at T 323 K, HAc 0.444 mol L
−1
−1
,
0
4
4
0.1 mol L− , and the open symbols indicate the result of blank experiments; Each reaction was carried out during 4 hours.
1
pH unadjusted, FeSO
4

1102
H. Yang et al. / Catalysis Communications 11 (2010) 1099–1103
3
.5 as shown in Fig. 4b. In summary, these factors decreased the reaction
rate of HAc decomposition when the pH value was higher than 2.0.
2
+
3
.4. Effect of Fe and acetic acid concentration
InFe2+/Fe3+/HAc system, thereaction wasactivated by UV light, and
2+
3+
the light harvest efficiency of Fe /Fe affected the HAc decomposition
rate. In the same reactor and under the same light intensity, the solution
with higher Fe2 concentration harvested more photons, and then
achieved higher reaction rate. Fig. 4c shows the relationship between
+
2
+
2+
Fe concentration and the amount of gas produced. When the Fe
−
1
−1
concentration increased from 0.025 mol·L
to 0.1 mol·L , the
increase ratio of gas production rates were 3.3 times higher than that
as Fe2 concentration increased from 0.1 mol·L to 0.2 mol·L . This
result shows that decomposition rate of HAc increased dramatically
along with the increase of Fe2 concentration when it was at low
concentration. When at higher concentration, due to the fixed intensity
+
−1
−1
+
of irradiation light, the number of photons trapped by Fe2 and Fe
+
3+
increased slightly, thus, decomposition rate of HAc increased slightly
2+
when Fe concentration increased further.
In the solutions with fixed Fe2 concentration and light intensity,
the acetic acid concentration affected the reaction rate of Eq. (1) for H•
production and Eq. (3) for HAc decomposition. The relationship
between gas production and acetic acid concentration is shown in
+
Fig. 4d. The rates of CH
4 2
and CO production increased near linearly
with the increase of HAc when the HAc concentration was lower than
−
1
−1
1
.333 mol·L . When the HAc concentration was 1.333 mol·L , rCH3•
−
1
was 14.27 mmol·h and the quantum yield was estimated to be 36%
by comparing the gas production rate and light intensity with
previous report [15]. According to Eq. (10), n can be resolved when
x x
r and S are given. The data of r can be obtained on Fig. 5a. S stands for
+
c (H ) in Eq. (1), and it represents c (HAc) in Eq. (2). Fig. 5b shows the
relationship between lnrH• and ln[c(HAc)]. The slope of the regressive
line was 0.5078 and the regression coefficient was 0.9968, thus the
value of n was 0.5078 in Eq. (10) in terms of rH•. The relationship
between lnrCH3• and ln[c(HAc)] is depicted in Fig. 5b. When c(HAc)
−
1
was lower than 1.333 mol·L , the relationship between ln rCH3• and
ln x was highly linearly correlated with a regression coefficient
of 0.9998. The slope of the line was 0.6066 and the value of n in
Eq. (10) in terms of rCH3• was 0.6066. According to the relationship
between rCH3• or rH• and ln[c(HAc)] shown by the equations in Fig. 4b,
1
T
Fig. 5. The relationship of lnrH• and lnrCH3
•
with (a) and (b) lnc(HAc).
backwards reactions in the photocatalytic process, which are listed in
Eqs. (11) and (12):
−
1
when the FeSO
4
concentration was 0.1 mol·L , the rate constant
and rH•(krH•) were resolved to be 3.65×10
.62×10 , respectively. Thus, the rate equations in this system
4
under 0.1 mol·L FeSO could be expressed as follows:
−
6
3
+
2 +
þ
of rCH3• (k
and
r
CH3•
Fe
+ H•
→
Fe
+ H
ð11Þ
−
6
3
−
1
3
+
2 +
þ
2
Fe
+ H2
→
2Fe
+ 2H
ð12Þ
−
6
0:6066
−1
−1
−1
rCH3• = 3:62 × 10
c
ðHAcÞðmol·s
Þ
Þ
ðcðHAcÞ ≤ 1:333mol·L
Þ
+
On the basis of Nernst equation, E (H /H
2
) is higher when the pH is
lower and this leads to higher hydrogen production rate as shown in
Eq. (1). Furthermore, the rate of the reaction expressed by Eq. (2) was
ð13Þ
−6
0:5078
−1
rH• = 3:65 × 10
c
ðHAcÞ ðmol·s
ðcðHAcÞ ≤ 1:333mol·L
Þ
repressed slightly by pH value. Thus, the concentration of H• and H
increased when the pH value reduced as shown in Fig. 4b. That means
2
ð14Þ
in a more acidic solution, Fe3 can be reduced faster by H• or H
the increase of H• and H increased the rates of these backwards
reactions, and then reduced the reaction rate of Eq. (3) by reducing
+
2
. Thus,
2
In order to evaluate the influence of HAc decomposition by the UV
light directly, a set of blank experiments was tested as shown in
Fig. 4d. The amount of HAc decomposed by UV light without Fe was
3
+
2+
the concentration of Fe . Hence, the HAc decomposition rate was
depressed. When pH value was raised above 2.0, the reaction rate of
Eq. (1) was depressed for the decreased concentration of proton.
Furthermore, when the pH value was between 2 and 3, the dominant
2+
about 5% of that decomposed by UV light in the presence of Fe
.
Considering that Fe2 shows much higher and broader region
absorbance of light than HAc does, we believe that the directly
decomposition of HAc by UV light has little effect on the results described
above.
+
3
+
2+
species of Fe in the solution was [Fe (OH)(H
2
O)
5
]
. Under this
condition, more OH▪ was produced once the Fe was activated by UV
light [13]. OH▪ reacted with H▪ to produce H O and therefore hindered
the positive reaction of HAc decomposition. Furthermore, most of Fe
precipitated as Fe (OH) when the pH value was higher than 3.0. The
Fe (OH) species blocked parts of light and its formation reduced Fe
concentration. This phenomenon was very clear when the pH value was
3
+
2
3+
4. Conclusions
3
3+
3
The mechanism of photocatalytic decomposition acetic acid by
Fe2+/Fe
3+
in the absence of oxygen was proposed. Fe
2+
could be

H. Yang et al. / Catalysis Communications 11 (2010) 1099–1103
1103
excited by ultraviolet light to decompose HAc and produce methane,
hydrogen. The increase of temperature increased rCH3 and rH•. And
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