Visible-light-induced oxidation of trans-ferulic acid by TiO2 photocatalysis

Article abstract of DOI:10.1016/j.jcat.2012.08.018

The oxidation of trans-ferulic acid (C₁₀H₁₀O ₄) in aqueous TiO₂ dispersion occurs via the formation of a charge-transfer complex on the TiO₂ surface that is able to absorb visible light (λ ≥ 400 nm). The main product is CO₂, whereas secondary oxidation products are organic species such as vanillin, caffeic acid, homovanillic acid, and vanillylmandelic acid. Oxidation through the formation of a charge-transfer complex occurs only in the presence of specific TiO₂ samples. Experiments in the absence of oxygen, in the presence of bromate ions and by using a phosphate-modified TiO₂, have been carried out for investigating the reaction mechanism. In order to study the interaction between trans-ferulic acid and TiO₂ surface and to characterize the charge-transfer complex, UV-Vis diffuse reflectance and FT-IR spectroscopies have been used. FT-IR characterization of TiO₂ samples in contact with the aqueous trans-ferulic acid solution indicates that the charge-transfer complex formation occurs via adsorption of bidentate ferulate species.

Full text of DOI:10.1016/j.jcat.2012.08.018

Journal of Catalysis 295 (2012) 254–260
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Visible-light-induced oxidation of trans-ferulic acid by TiO photocatalysis
2
a
a
b
a
c
d
F. Parrino , V. Augugliaro , G. Camera-Roda , V. Loddo , M.J. López-Muñoz , C. Márquez-Álvarez ,
a
a,
⇑
a
G. Palmisano , L. Palmisano , M.A. Puma
a
‘
‘Schiavello-Grillone’’ Photocatalysis Group, Dipartimento di Ingegneria Elettrica Elettronica e delle Telecomunicazioni, di tecnologie Chimiche, Automatica e modelli
Matematici (DIEETCAM), University of Palermo, Viale delle Scienze, Ed. 6, 90128 Palermo, Italy
b
Dipartimento di Ingegneria Chimica, University of Bologna, Via Terracini 28, 40131 Bologna, Italy
Department of Chemical and Environmental Technology, ESCET, Rey Juan Carlos University, C/Tulipán s/n, 28933 Móstoles, Madrid, Spain
Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, 28049 Cantoblanco, Madrid, Spain
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 July 2012
Revised 20 August 2012
Accepted 21 August 2012
Available online 25 September 2012
The oxidation of trans-ferulic acid (C10H10O ) in aqueous TiO2 dispersion occurs via the formation of a
4
charge-transfer complex on the TiO
product is CO , whereas secondary oxidation products are organic species such as vanillin, caffeic acid,
homovanillic acid, and vanillylmandelic acid. Oxidation through the formation of a charge-transfer com-
plex occurs only in the presence of specific TiO samples. Experiments in the absence of oxygen, in the
presence of bromate ions and by using a phosphate-modified TiO , have been carried out for investigating
the reaction mechanism. In order to study the interaction between trans-ferulic acid and TiO surface and
to characterize the charge-transfer complex, UV–Vis diffuse reflectance and FT-IR spectroscopies have
been used. FT-IR characterization of TiO samples in contact with the aqueous trans-ferulic acid solution
2
surface that is able to absorb visible light (k P 400 nm). The main
2
2
2
Keywords:
trans-Ferulic acid photo-oxidation
Visible-light photocatalysis
2
TiO
Charge-transfer complex
2
2
indicates that the charge-transfer complex formation occurs via adsorption of bidentate ferulate species.
Ó 2012 Elsevier Inc. All rights reserved.
1
. Introduction
Among the advanced oxidation processes, photocatalysis in the
irradiation [15]. The main drawback of these methods, however,
is that the doping sites could act as recombination centers for
the photoexcited electron–hole pairs. Consequently, sometimes
the activity of these TiO -based photocatalysts does not improve.
In the dye sensitization, organic molecules coated on the TiO
presence of an irradiated semiconductor has proven to be very effec-
tive in the field of environmental remediation [1–6]. Despite the
great research activity in the last two decades for obtaining a photo-
catalyst with optimal features, TiO
which any alternative photocatalyst must be compared. TiO
2
2
surface are used to harvest incident light. Light absorption by the
dye is followed by an electron injection from the excited state of
the dye molecule into the conduction band of the semiconductor.
The remaining positive hole is scavenged by a redox couple on
2
remains a benchmark against
is
2
indeed one of the most attractive photocatalytic materials due to
its low cost, good chemical and photochemical stability, and
2
the TiO surface [16]. In the majority of dye-sensitized photoreac-
nontoxicity. In general, pure TiO
2
with its large band gap (3.2 eV
tions, the dye molecule does not change its chemical nature in the
course of the whole process. Another mechanism, called surface-
complex charge-transfer mechanism, can be effective for the
utilization of visible light. Dimitrijevic et al. [17] showed that the
for anatase) can be activated only by near-UV light (k < 385 nm),
which represents a small fraction of solar light (about 3–4%). This
inactivity under visible light strongly limits the practical application
of TiO
2
photocatalyst. In order to overcome this drawback, pure TiO
2
modification of TiO
ing of visible light and promotes spatial separation of charges.
Fullerol-modified TiO exhibits visible-light photocatalytic activity
[18]; in this case, the sensitization works through a single-step
process, in which an electron is injected from a surface fullerol–
2
nanoparticles with dopamine enables harvest-
has been modified by impurity doping and dye sensitization.
The doping has been done with ions of transitional metals, such
as Co, Cr, Cu, Fe, Mn, Ni and V [7–10], and also of nonmetals such as
B, C, N and S [11–14]. Recently, new visible responsive photocata-
2
lysts were prepared by modification of TiO
metal complexes or nanoparticles. In particular, Au nanoparticles
supported on TiO have been successfully used for the oxidation
of alcohols and for the reduction of nitrobenzene under visible
2
surface with noble
2 2
TiO complex to TiO without involving the excited state of fullerol.
This system is able to drive both photooxidative and photoreduc-
2
tive conversions, including the redox conversion of 4-chlorophe-
ꢀ
nol, I , and Cr(VI) and also the H
2
production. The degradation of
salicylic acid under visible light [19] is ascribed to the formation
of surface complexes of H /TiO ; the proposed reaction mecha-
nism involves a photoinduced electron transfer of the surface
complexes Ti–OOH. A complex between the TiO surface and the
2
O
2
2
⇑
Corresponding author.
E-mail address: leonardo.palmisano@unipa.it (L. Palmisano).
2
0
021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.08.018

F. Parrino et al. / Journal of Catalysis 295 (2012) 254–260
255
nonionic surfactant having polyoxyethylene groups (Brij) exhibits
visible-light activity for the reduction of CCl and Cr(VI) [20]; the
Brij/TiO complex shows a broad absorption band in the 320–
00 nm range. In the radical chain reaction of photosulfoxidation
of alkanes, the starter radical is generated through absorption of
visible light by the TiO –SO complex [21]. Kim and Choi [22]
investigated on the formation of surface complex between TiO
chromatography) and SiO
Riedel-de Haën) were used for blank experiments.
The amount of catalyst used for the runs was 0.6 g L for HPC3,
HP1/50, and HP0.5, while it was 0.8 g L for Merck and 1 g L for
the other powders. A systematic study on the dependence of the
reactivity on the catalyst amount was performed only in the case
2
(silica gel for column chromatography,
4
ꢀ
1
2
ꢀ1
ꢀ1
5
2
2
2
2
of P25 TiO .
and EDTA, a common electron donor. The complex can absorb
visible light through ligand-to-metal charge-transfer (LMCT)
mechanism, and its activity is outstanding among all tested
All the preparation details of HPC3, HP1/50, and HP0.5 samples
are reported elsewhere [30–32]. In the following, only some essen-
tial information is reported. The HPC3 sample was prepared
TiO
2
–substrate complexes, giving rise to the photoreduction of
4
through a sol–gel route ex TiCl (purity > 97%, Fluka) whose hydro-
Cr(VI) and production of H
2
.
lysis in water (TiCl /water = 1:11 v:v) produced a white sol that
4
In some cases of dye-sensitized photoreactions, however, the
same dye molecules change their chemical nature in the course
of the whole process. Trichlorophenol in water forms a charge-
upon stirring for 10 h became a clear solution. Then, the suspen-
sion was dried and the sol was heated at 673 K for 3 h producing
2
ꢀ1
a powder with SSA of 35 m g . For HP1/50 sample, the precursor
solution was obtained by adding 20 mL of TiCl to 1000 mL water
transfer complex with TiO
2
that is activated by light wavelengths
4
as long as 520 nm [23]. The trichlorophenoxyl radicals, resulting
from charge-transfer, couple with each other to form a set of poly-
aromatic chlorinated products, whose distribution is also affected
by the light energy [24]. The degradation reaction of phenol, 2,4-
dichlorophenol, and 4-chlorophenol in aqueous suspension of pure
contained in a volumetric flask; the addition was carried out very
slowly without agitation in order to avoid the overheating of solu-
tion as the TiCl hydrolysis is highly exothermic. At the end of the
4
addition, the resulting solution was mixed for 2 min by a magnetic
stirrer and then the flask was sealed and maintained at room tem-
perature (ca. 298 K) for a total aging time ranging from 4 to 9 days.
The sol became a transparent solution after ca. 12 h aging, and
then, after waiting a few days, the precipitation process started.
The colloidal dispersion was then dialyzed by using a dialysis
tubing cellulose membrane (average flat width: 76 mm, Sigma
Aldrich) until only a negligible amount of chloride ions was
detected. The solid was then separated by centrifugation (20 min
at 5000 rpm) and dried at room temperature by obtaining a pow-
TiO
chlorides and CO
lic compounds and TiO
2
occurs under visible illumination (k > 420 nm) by generating
[25]. The surface complexation between pheno-
appears to be responsible for the reactiv-
2
2
ity. The selective oxidation of benzyl alcohol to benzaldehyde
under visible light in acetonitrile [26] proceeds through the forma-
tion of a characteristic complex of a benzyl alcoholic compound on
the TiO
in the degradation of RS-2-(4-chloro-o-tolyloxy)propionic acid
mecoprop) on TiO under visible light [27]. TiO treated with
2
surface. The surface complexation mechanism is operative
2
ꢀ1
(
2
2
der with SSA of 118 m g . For the HP0.5 sample, the precursor
solution was obtained by slowly adding 5 mL of TiCl drop wise
mecoprop absorbs also in the visible region; the corresponding
FTIR investigation showed that a charge-transfer complex between
4
into 50 mL of water in a beaker under magnetic stirring. After that,
the beaker was closed and mixing was prolonged for 12 h at room
temperature, eventually obtaining a clear solution. This solution
was boiled at 373 K for 0.5 h obtaining a white suspension that
was then dried at 323 K in order to obtain the final solid consisting
TiO
This paper reports the results of a study in which trans-ferulic
acid (FA, C10 , Fig. 1) is oxidized in aqueous TiO dispersion
2
and mecoprop is formed through carboxylate formation.
H
10
O
4
2
irradiated by visible light. The oxidation process occurs through
the formation of a charge-transfer complex able to absorb visible
light (k P 400 nm); it must be outlined that FA not only forms a
2
mainly of amorphous TiO and crystals of anatase (75%) and rutile
(25%). The powder was finally washed with distilled water and
centrifuged several times until the chloride ion concentration in
the washing water reached a negligible value.
2
complex on TiO surface, but also itself undergoes the oxidation
reactions. The photocatalytic oxidation of FA has been recently
investigated under near-UV irradiation as a method both for water
detoxification [28] and for chemical production of vanillin [29].
Modification of P25 TiO
2
with phosphate was carried out as fol-
PO (Sigma–Aldrich, >98%
lows: 1 g of P25 TiO and 2.76 g of NaH
2
2
4
In this study, different commercial and home-prepared TiO
samples have been tested and a tentative mechanism to explain
the occurrence of the photo-oxidation of FA up to CO has been
hypothesized. The interactions between FA and TiO surface have
been studied by means of UV–Vis diffuse reflectance and FT-IR
spectroscopies.
2
assay) were suspended in 50 mL water and mixed for 12 h. Then,
the powder was recovered by centrifugation and suspended again
in 50 mL of fresh NaH PO solution. This operation was repeated
2 4
two more times and then the powder was dried under vacuum
at room temperature and finally ground.
Photocatalytic runs were performed by using a cylindrical batch
photoreactor (CPR, internal diameter: 32 mm; height: 188 mm) of
Pyrex glass. The photoreactor was provided with ports in its upper
section for the inlet and outlet of gases and for sampling. A 100 W
2
2
2
. Experimental
1
2 V halogen display/optic lamp (Osram GmbH, Germany) was axi-
The photoreactivity experiments were performed by using two
ally positioned within the photoreactor. In order to cool the lamp
and to cut off light emitted at k < 400 nm (which was actually pres-
ent in very small amount in the emission spectrum of the lamp), a
commercial TiO
2
samples, that is, Degussa P25 (ca. 80% anatase and
2
ꢀ1
2
(
0% rutile, BET specific surface area, SSA, 50 m g ) and Merck
2
ꢀ1
100% anatase, SSA: 10 m g ), and three home-prepared anatase
TiO samples (hereafter denoted as HPC3, HP1/50, and HP0.5).
P25 TiO sample was modified by surface impregnation
1
2
M aqueous solution of NaNO (Sigma–Aldrich, >98% assay) was
2
recirculated by means of a peristaltic pump through the Pyrex
thimble surrounding the lamp. The radiation energy impinging
on the suspension had an average value of 0.4 W cm ; it was
measured between 450 and 950 nm by using a Delta Ohm 9721
A
2
with phosphate ions. Also Al
O
2 3
(Fluka type 507 C neutral, for
ꢀ
2
O
radiometer. The aqueous suspension of TiO
2
(temperature of ca.
CH O
ꢀ1
3
300 K; volume of 150 mL; catalyst amount: 0.6–3.0 g L ) was
magnetically stirred, and air at atmospheric pressure was bubbled
for 0.5 h before starting the runs and during the runs; the initial FA
concentration was always 0.5 mM and initial pH ranged between
3.2 and 3.9. For a few runs, pure nitrogen was bubbled into the
OH
HO
Fig. 1. Chemical structure of ferulic acid.

2
56
F. Parrino et al. / Journal of Catalysis 295 (2012) 254–260
suspension and bromate ion (e.g. KBrO
added to act as electron trap. During the photoreactivity runs,
samples for analyses were withdrawn at fixed times; they were
immediately filtered through 0.25 or 0.45
Millipore) to separate catalyst particles.
3
, Sigma–Aldrich, >98%) was
crystal was recorded and ratioed against a background spectrum
corresponding to deionized water deposited on the bare crystal.
For reference purposes, the spectrum of FA dissolved in chloroform
(Panreac, 99.0% trichloromethane stabilized with ethanol) was also
recorded using the bare ATR plate. Contribution of bands due to
chloroform was removed by relating this spectrum against the
spectrum of chloroform.
lm membranes (HA,
Measurements of FA adsorption onto the catalysts in the dark
ꢀ1
were performed by suspending 1.11 g L of the catalysts in solu-
tions with different substrate concentrations. After 5 h stirring,
the equilibrium was safely reached and the concentration of FA
was measured.
The quantitative determination and identification of FA and its
oxidation products were performed by means of a Beckman Coul-
ter HPLC (System Gold 126 Solvent Module and 168 Diode Array
Simultaneous thermal gravimetric (TG/DTG) and differential
thermal analysis (DTA) measurements were performed on cata-
lysts by using a Perkin Elmer STA 6000 system, in the 30–750 °C
ꢀ1
range under a nitrogen flux of ca. 20 mL min . The temperature
program consisted of three steps: temperature scan from 30 to
ꢀ1
120 °C at 10 °C min , 15 min in isothermal condition at 120 °C,
ꢀ1
Detector), equipped with a Phenomenex Synergi 4
0A column at 298 K. The eluent consisted of a mixture of metha-
nol and 1 mM trifluoroacetic acid aqueous solution (55:45 v:v); its
l
m Hydro-RP
and temperature scan from 120 to 750 at 10 °C min
. Results and discussion
Table 1 reports BET specific surface areas and the degree of
.
8
3
ꢀ1
flow rate was 0.6 mL min
. A UV detector measured the
absorbance at 260 nm. Species were identified by comparing their
retention times and UV–Vis spectra with those of standards. In
order to determine the concentration of bromide and bromate ions,
ion chromatography (IC) analyses were carried out by a Dionex DX
surface hydroxylation of catalysts, along with their photoactivity
performances expressed by the percentage values of FA conversion
and mineralization after 180 min of visible irradiation.
According to literature [34], the weight losses of hydroxylated
2
TiO samples in the range 120–600 °C can be related to weakly
bonded and strongly bonded OH groups. In Table 1, the global
weight losses of OH groups are reported. It can be seen that the
home-prepared samples are the most hydroxylated ones with
values of 10.75% w/w for HP0.5, 4.66% w/w for HP1/50, and
1
20 instrument equipped with an IonPacAS14A column
CO /1.0 mM NaHCO
(
4 ꢁ 250 mm). The eluent was an 8.0 mM Na
2
3
3
buffer solution. Mineralization extent was evaluated by total
organic carbon (TOC) determination carried out by a 5000 A
Shimadzu analyzer.
Standards of FA, vanillylmandelic acid, homovanillic acid, vanil-
lin, caffeic acid, 3-methoxycinnamic acid, KBrO , KBr, and NaH PO
3 2 4
were purchased from Sigma–Aldrich and used as received. All the
other used compounds were reagent grade.
In order to investigate the electronic properties of the charge-
transfer complex produced by the interaction between FA and
1
.81% w/w for HPC3. The commercial samples showed values of
.78% w/w for P25 TiO and 0.11% w/w for Merck TiO
The results of adsorption experiments have been modelled by
applying the Langmuir model linearized in the following way:
0
2
2
.
the TiO
2
surface, 50 mg of the catalysts was suspended in 50 mL
1
N
1
1
¼ N
þ N
0
ð1Þ
of 0.5 mM FA aqueous solution for 1 h; then, the suspension was
dried under vacuum at 323 K and the resulting sample, spread onto
0
KC
L
BaSO
4
plates, was characterized by diffuse reflectance spectra re-
where N are the adsorbed moles, N
and K the adsorption equilibrium constant. Fig. 2 shows that the
Langmuir isotherm fits satisfactorily the experimental adsorption
0
the monolayer adsorbed moles,
corded on a Shimadzu UV-2401PC UV–Vis spectrophotometer.
Reflectance data were converted by the instrument software to
1
F(R ) values proportional to absorbance according to the Kub-
data obtained with HP0.5, HPC3, HP1/50, and P25 TiO
SiO and Al also adsorbed the substrate while TiO
surprisingly, did not adsorb. The K values were found equal to
6207.98, 7709.38, 12903.47, and 12346.32 M for HP0.5, HP1/50,
HPC3, and P25 TiO , respectively. The maximum amount of FA that
could be adsorbed per gram of catalyst, N , was found to be equal to
0.55, 0.21, 0.10, and 0.08 mmol g for HP0.5, HP1/50, HPC3, and
P25 TiO , respectively. A qualitative effect of temperature was also
studied and, notably, the amount of adsorbed substrate increased
by increasing the suspension temperature from room temperature
to 323 K. The latter behavior can be ascribed to the occurrence of
chemisorption rather than physisorption, as the predominant
phenomenon.
2
samples.
Merck,
elka–Munk theory [33].
2
2
O
3
2
2
The adsorption in the dark of FA on TiO catalysts was studied
ꢀ1
by attenuated total reflection-Fourier transform infrared (ATR-
FTIR) spectroscopy. ATR-FTIR spectra were recorded using a Ther-
mo Nicolet Nexus 670 FTIR spectrometer equipped with a liquid
nitrogen-cooled MCT detector. A SensIR Technologies DurasamplIR
horizontal ATR accessory with a stainless steel trough plate was
used. The internal reflection element was a diamond-faced ZnSe
prism, the incidence angle was 45° and total number of reflections
was 9. The trough above the ATR crystal was ca. 4 mm in diameter
and had an approximate capacity of 50
particles were deposited by placing 20
in Milli-Q deionized water) on the surface of the ATR crystal and
evaporating to dryness at room temperature under vacuum. The
procedure was repeated four times to obtain thin films containing
2
0
ꢀ1
2
l
L. Thin films of TiO
2
1
ꢀ
l
L of suspension (2.5 g L
One can observe that the higher the amount of surface hydrox-
ylation of the catalyst, the higher FA adsorption. On the other hand,
ca. 200
l
g TiO
2
. Adsorption of FA was carried out at room temper-
L, 0.5 mM) on
Table 1
ature by depositing a stock aqueous solution (40
the coated crystal. Infrared spectra were recorded while keeping
l
BET specific surface areas (SSA), % w/w of global surface OH groups, conversion (X)
and mineralization after 180 min irradiation.
the sample in the dark for 2 h. Each spectrum was obtained as
the average of 1000 scans, with 4 cm resolution. Spectra were
ratioed against a background spectrum recorded after depositing
ꢀ1
Powder
P25 TiO
SSA (m2
g
)
OH tot. (% w/w)
X (%)
Mineralization (%)
ꢀ
1
2
50
10
0.78
0.11
70
0.0
18
0.0
Merck TiO
2
2
20 lL Milli-Q deionized water on the TiO -coated crystal. This pro-
cedure allowed to eliminate from the spectra absorption bands due
to water and titania and thus to reveal only bands corresponding to
organic species both adsorbed and in solution. In order to evaluate
the contribution of species present in the solution phase, the spec-
trum of the 0.5 mM FA aqueous solution deposited on the bare ATR
HP1/50
HPC3
HP0.5
118
35
235
–
4.66
1.81
10.75
–
80
84
100
0.0
0.0
72
66
89
0.0
0.0
SiO
Al
2
2
O
3
–
–

F. Parrino et al. / Journal of Catalysis 295 (2012) 254–260
257
6
5
4
3
2
1
0
0
0
0
0
0
0
0.6
HP0.5
HPC3
HP1/50
P25
0
0
0
0
0
.5
.4
.3
.2
.1
FA
CO
2
0
-
30
0
30
60
90 120 150 180 210 240
Time [min]
Fig. 3. Concentration of trans-ferulic acid (FA) and of normalized carbon dioxide
(i.e., divided by 10) versus irradiation time in the presence of air for a representative
0
10
20
30
/C [mM ]
40
50
60
run. Photocatalyst: HP1/50 (0.6 g Lꢀ1). Ignition of the lamp at time = 0.
-
1
1
L
no reaction occurred in the presence of Merck TiO
2 2
, SiO , and
2
Fig. 2. Adsorption isotherms of FA on HP0.5, HPC3, HP1/50, and P25 TiO . Plots
represent Langmuir theoretical model, whereas symbols stand for experimental
values.
Al . The vanillin concentration reached a maximum of ca.
2 3
O
0.01 mM in the first hours and remained virtually constant during
the course of reaction. The concentrations of homovanillic acid,
vanillylmandelic acid, and vanillic acid varied between 0.5 and
photocatalytic mineralization (reported in Table 1) notably follows
the trends of FA adsorption (Fig. 2), thus showing that the higher
the amount of the adsorbed substrate, the higher the mineraliza-
tion is. A similar trend may be also observed for conversion data.
The oxidation of FA in aqueous medium under visible-light
5
l
M for runs carried out using the different catalysts. Formic acid
reached a concentration between 7 and 10 M for runs carried out
with HP1/50, HP0.5, and HPC3, whereas its concentration reached
a value of 2 M by using P25 TiO . Acetic acid maximum concen-
tration was 20 M in runs carried out by using HP1/50 and
HPC3, whereas the concentration reached in the presence of
HP0.5 and P25 TiO was about 1 M. The concentrations of oxalic
acid ranged between 0.2 and 2 M for all the used catalysts.
By considering that bare TiO is unable to absorb visible light
and FA does not react in the absence of TiO (neither in homoge-
neous system nor in the presence of SiO and Al ), the experi-
l
l
2
irradiation occurred only in the presence of home-prepared TiO
samples and P25 TiO ; Merck TiO , SiO , and Al samples were
2
l
2
2
2
2 3
O
inactive. All the active powders, when suspended in FA solution,
turned their color from white to different yellow nuances depend-
ing on the catalyst. These suspensions were stable under dark con-
ditions. Irradiation by visible light of suspensions of yellow-colored
2
l
l
2
2
P25 TiO
2
, HP0.5, HP1/50, and HPC3 samples produced the complete
2
2 3
O
mineralization of FA after a maximum time of 10 h, eventually
decoloring the catalyst surface. The main oxidation product was
CO . Vanillin was the most abundant intermediate product de-
2
mental findings that (i) active samples change their color in the
presence of FA; (ii) the mineralization percentage increases by
increasing the amount of adsorbed FA; and (iii) photo-oxidation
of FA occurs under visible-light irradiation (k P 400 nm) may be
explained by hypothesizing the formation of a charge-transfer
tected by means of HPLC; other detected aromatic intermediates
were caffeic acid, obtained from FA by substitution of a methoxy
group with an –OH one, homovanillic acid and vanillylmandelic
acid, both intermediates of vanillin production, and vanillic acid,
which derives from vanillin overoxidation. Among open-ring spe-
cies, only oxalic, formic, and acetic acids were identified by means
of ion chromatography. It is worth noting that the products
deriving from FA oxidation with the same catalysts but under
near-UV irradiation [29], identified by HPLC chromatography too,
were almost the same as in the case of visible irradiation.
complex between TiO
a light absorbing species on the TiO
spectra were recorded. In Fig. 4A, the absorbance spectra of pure
FA and bare TiO HP1/50 are compared with that of the same
TiO after wet impregnation with FA solution (spectrum HP1/
2
and FA. In order to verify the presence of
2
surface, diffuse reflectance
2
2
50&FA). The spectrum reported in Fig. 4B is obtained by difference
between the spectrum HP1/50&FA and that obtained by the sum of
HP1/50 and FA spectra.
For a representative run, Fig. 3 shows the concentration of FA
and CO versus irradiation time. Notably, after starting visible irra-
2
diation of the suspensions, no photo-isomerization of FA was
noticed, whereas in the case of near-UV irradiation, trans-ferulic
acid partially transformed in cis-ferulic acid after few minutes
Comparison of the reflectance spectra in Fig. 4A clearly indi-
cates broad visible absorption between 400 and 600 nm with a
maximum between 400 and 420 nm (see Fig. 4B). The other
2
home-prepared catalysts and P25 TiO showed similar behavior.
It is worth noting that this absorption widely matches with the so-
lar emission spectrum, although the latter presents a maximum at
ca. 600 nm.
[
29]. Fig. 3 shows that FA almost completely reacted after 4 h
visible irradiation. The concentration of produced CO has been
calculated by the measurement of the total organic carbon amount.
It can be observed that the production of CO starts with the igni-
tion of the lamp. This behavior indicates that a part of the TiO
2
The diffuse reflectance spectra obtained indeed by using TiO
2
2
Merck (Fig. 4C and D), SiO
showed similar features. For Merck TiO
2
, and Al
2
O
3
(spectra not reported)
, it can be noticed that
2
2
surface is able to directly perform the mineralization of the organic
substrate through the consecutive oxidation of adsorbed interme-
diate species, which do not desorb from the surface [35]. The data
of Fig. 3 also show that FA desorption or adsorption phenomena do
not occur after switching on the lamp, thus indicating that the
catalyst is not able to be excited by the visible light and therefore
to give rise to photoadsorption or photodesorption.
no novel absorption, coming from the difference between the spec-
trum Merck&FA and the sum of Merck and FA, is present. Thus,
TiO Merck, as also SiO and Al O samples, did not give rise to
2 2 2 3
charge-transfer complex formation with a new band absorption
and notably they did not show any photocatalytic activity under
visible-light irradiation.
2
Merck TiO was active under near-UV irradiation, but in this
P25 TiO
2
and home-prepared catalysts (HP1/50, HPC3, and
condition, it works according to the well-known redox mechanism
involving the photoexcitation of the solid [29]. Despite a strong
HP0.5) gave rise to conversions in the 70–100% range, whereas

2
58
F. Parrino et al. / Journal of Catalysis 295 (2012) 254–260
0.15
0.10
0.05
0.00
HP1/50&FA
HP1/50
FA
0
0
0
0
0
0
.25 (A)
.20
(B)
.15
.10
.05
.00
2
00
300
400
500
600
400
500
600
Wavelength [nm]
Wavelength [nm]
Merck&FA
Merck
FA
0.4
(
C)
(D)
0.02
0
0
0
0
.3
.2
.1
0
.00
.0
2
-0.02
00
300
400
500
600
400
500
600
Wavelength [nm]
Wavelength [nm]
Fig. 4. (A) Reflectance spectra of pure FA (black), pure HP1/50 (red), and HP1/50 upon wet impregnation with FA solution (blue); (B) Spectrum obtained by [HP1/50&FA –
(
2 2
HP1/50 + FA)]; (C) Reflectance spectra of pure FA (black), pure Merck TiO (red) and Merck TiO upon wet impregnation with FA solution (blue); (D) Spectrum obtained by
[
Merck&FA – (Merck + FA)]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2 2 3
adsorption of FA onto SiO and Al O samples, no change in the
reflectance spectra of the powders as well as no color modification
were noticed, then evidencing the absence of complex formation.
0.02
Indeed, SiO
differently from TiO
to TiO
Interaction between FA and TiO
of an FTIR investigation. ATR-FTIR spectra of the photocatalysts in
contact with an aqueous solution of FA at room temperature and in
the dark were recorded at given time intervals for a 2-h period. A
fast initial development of absorption bands was observed for
2
and Al
2
O
3
do not have a low lying conduction band,
a
b
2
, compatible with electron transfer from FA
2
.
2
surface was studied by means
c
d
e
f
2
P25 TiO as well as for home-prepared samples. Bands’ growth de-
g
cayed quickly after several minutes, and a nearly constant intensity
was reached after 2 h, while the relative intensity of bands was
maintained. Fig. 5 shows ATR-FTIR spectra of these samples in
contact with the FA aqueous solution for 2 h (spectra a–d). A
contribution of FA in solution to these spectra can be disregarded
taking into account the very weak intensity shown by the
spectrum of the aqueous solution of FA (Fig. 5g). Therefore, it can
be concluded that all the bands observed in the spectra of home-
1800
1600
1400
1200
1000
Wavenumber (cm-1)
Fig. 5. ATR-FTIR spectra of titania photocatalysts HP0.5 (a), HPC3 (b), Degussa P25
c), HP1/50 (d) and Merck anatase (f) in contact with a 0.5 mM aqueous solution of
FA at room temperature in the dark for 2 h. The spectra of the 0.5 mM aqueous
solution of FA (g) and FA dissolved in chloroform (e) are also shown for comparison.
(
prepared samples and P25 TiO
On the other hand, the spectrum of TiO
2
correspond to adsorbed species.
Merck (Fig. 5f) shows only
2
ferulate. These results indicate that adsorption of FA on P25 TiO
2
the weak contributions of ferulic acid solution and is virtually free
from bands due to adsorbed species.
and the three home-prepared titania samples takes place via
carboxylate formation, in agreement with previous reports show-
ing that adsorption of carboxylic acids on titania proceeds very fast
via titanium carboxylate formation [36–41]. In general, this kind of
adsorption has been shown to occur through bridging bidentate
interactions. The splitting of the carboxylate bands of ferulate spe-
cies adsorbed on the series of titania samples reported in this study
(ca. 100 cm ) strongly supports a bidentate binding, although the
relatively small value might suggest a chelate rather than bridging-
type of linkage [42]. FTIR results indicate that the charge-transfer
2
The spectra of P25 TiO and the three home-prepared titania
samples retain essentially most of the features of FA in the
ꢀ1
1
800–900 cm wavenumber region (Fig. 5e). Two main differ-
ences can be noticed in comparison with the spectrum of the
unbounded acid. On one hand, the carbonyl stretching band of
ꢀ1
ꢀ1
the carboxylic group at ca. 1695 cm is completely removed in
the spectra of home-prepared samples and only shows very weak
2
intensity for P25 TiO . On the other hand, the spectra of the four
samples show two additional overlapping broad bands centered
at around 1500 and 1400 cm , which can be respectively ascribed
to the antisymmetric and symmetric OCO stretching of adsorbed
2
complex formed by adsorption of ferulic acid on P25 TiO , HP0.5,
HP1/50, and HPC3 corresponds to a bidentate titanium ferulate.
Adsorbed ferulate species were not detected on Merck titania, in
ꢀ1

F. Parrino et al. / Journal of Catalysis 295 (2012) 254–260
259
4
3
2
1
0
0.6
Bromate
Bromide
ferulic acid
0
0
0
0
0
0
.5
.4
.3
.2
.1
.0
0
50
100
150
200
0
1
2
1
3
Irradiation time [min]
-
P25 amount [g L ]
2 3
Fig. 8. FA oxidation in the absence of O and in the presence of KBrO .
2
Fig. 6. First-order rate constant versus P25 TiO amount.
chlorophenols [44], could occur through the following reaction
steps:
agreement with UV–Vis data, which showed the lack of charge-
transfer complex formation on that sample.
Photocatalytic reactions are generally carried out by using a
limited amount of catalyst, since an excess can produce a screening
effect of particles in the nearby of the irradiation source with re-
spect to the backward particles that remain inactive. For runs car-
TiO
2
þ FA ! ½TiO
2
ꢀ FAꢂ
ð3Þ
À
Á
h
m
ꢀ FAÅþ
ꢀ
½
TiO
2
ꢀ FAꢂ ! TiO
2
e
ð4Þ
ð5Þ
ð6Þ
CB
þ eꢀ !! OH
Å
O
2
CB
ried out by using increasing amounts of P25 TiO
degradation well fitted first-order kinetics:
2
, the rate of FA
Åþ
Å
þ
FA þ H
2
O ! OH þ H þ FAðsÞ
ðsÞ
dC
dt
L
ꢀ
¼ kC
L
ð2Þ
Åþ
h
m
h
m
hm
FA ! ! oxidation intermediates ! CO
2
ð7Þ
ðsÞ
where C
L
is the FA concentration in the liquid phase, t the irradia-
In order to support the reported mechanism, some experiments
have been performed by oxidizing FA in the presence of a phos-
phate-modified P25 TiO . The runs showed that the FA oxidation
2
tion time, and k is the first-order rate constant. By applying a
least-squares best-fitting procedure to the C
L
ꢀ t data, the values
of k have been determined; they are reported in Fig. 6 as a function
of the catalyst amount. It may be noted that the values of k linearly
increase with the catalyst amount up to significant levels. This
behavior is in agreement with the occurrence of a complete irradi-
ation of all the catalyst particles inside the reactor. In the presence
of the highest amount of suspended catalyst (3 g L ) indeed, the
irradiance measured outside the reactor was equal to 50% of the
emitted one. As reported by Camera-Roda et al. [43], the reaction
rate increases linearly with the amount of the photocatalyst regard-
less of the intensity of the irradiation provided that the value of the
optical thickness of suspension is smaller than 1, as it is the case of
the used experimental conditions.
rate decreases of ca. 50%. It is indeed well known that phosphate
ions adsorb strongly thanks to electrostatic interactions between
anions and catalyst surface and react with the surface hydroxyl
groups, without modifying the electronic features of the catalysts
[45]. As a final result, they are able either to block active sites or
to compete with organic species during the photocatalytic
processes, thus reducing the global rate of oxidation of target
compounds.
For an initial pH in the 3.2–3.9 range, the catalyst color got yel-
lowish in the presence of FA, whereas water remained uncolored.
By increasing the initial pH by using NaOH 1 M solution, a decrease
in conversion was observed down to 0% at pH = 12, due to the lack
ꢀ1
Irradiating by visible light (k P 400 nm), a cis- and trans-ferulic
acids mixture (40:60 M ratio), prepared by a 1 h-lasting homoge-
neous UV photoisomerization of trans-ferulic acid [29],
allowed to observe that the two isomers exhibit a degradation
kinetics of first order expressed by Eq. (2). The values of k were
2
of adsorption of the anionic species on TiO surface. FA is indeed
characterized by the equilibriums reported in Fig. 7, and the pH
strongly influences the species existing in water.
2
Nevertheless, Merck TiO did not give rise neither to conversion
nor to adsorption of FA at any initial pH values. A prominent role in
the absence of activity is probably played by characteristic features
of the solid surface (such as limited hydroxylation).
ꢀ
3
ꢀ1
ꢀ3
ꢀ1
4
.3 ꢁ 10 mM min
and 5.2 ꢁ 10 mM min
for cis- and
trans-ferulic acids, respectively, indicating that both isomers show
the same reactivity. Furthermore, values of conversion and selec-
tivity to vanillin were comparable, starting just from trans-ferulic
acid or from a cis–trans mixture. Indeed, after 1-h visible irradia-
tion in both cases, the conversion was about 60% and the selectiv-
In order to better investigate the reaction mechanism, some runs
were carried out in the virtual absence of O
2
, by continuously bub-
bling N in the suspension. As expected, the anaerobic conditions
2
determined the total inhibition of the FA degradation. In anaerobic
conditions but in the presence of bromate ions, high FA conversions
were observed. Bromate ions played indeed the role of oxidant
agent, reducing to bromide ions, by accepting electrons from the
ity to vanillin was about 2% for a standard reaction using P25 TiO
2
.
The optical electron transfer from FA to the conduction band of
TiO
2
, known in literature for molecules like cathecol, enediols, and
O
O
O
CH O
pKa1
CH O
pKa2
CH O
3
3
3
OH
O
O
_
HO
HO
O
Fig. 7. Ferulic acid acid–base equilibrium.

2
60
F. Parrino et al. / Journal of Catalysis 295 (2012) 254–260
excited charge-transfer complex formed between FA and TiO
2
.
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782.
4
Accordingly, Fig. 8 shows that once FA has been completely
converted, bromate ions concentration did not change, thus demon-
strating that the reactivity is due to the complex activity.
[
[
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[
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FA has been oxidized through the formation of a charge-transfer
complex with some specific TiO
The most abundant detected products were CO
action between FA and TiO has been deeply investigated in order
2
samples under visible irradiation.
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to hypothesize adsorption and reaction mechanisms. The former
one was modelled by means of Langmuir adsorption isotherms
and detailed by means of ATR FT-IR, allowing to understand that
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FA adsorption on TiO
and that the absence of adsorption in the presence of TiO
SiO , and Al is the reason of the lack of reactivity. Accordingly to
the reported evidence, reactivity was found to follow the same
trend of adsorbed FA amounts on TiO samples. Finally, the more
hydroxylated was TiO surface (found by means of TGA measure-
2
surface occurs via carboxylate formation,
2
Merck,
[
[
[
[
[
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2 3
O
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