Magnetically separable nanocomposites with photocatalytic activity under visible light for the selective transformation of biomass-derived platform molecules

Article abstract of DOI:10.1039/c1gc15692f

Novel magnetically separable TiO₂-guanidine-(Ni,Co)Fe ₂O₄ nanomaterials were prepared and characterised by a series of techniques including XRD, SEM, TEM, N₂ physisorption as well as XPS and subsequently tested for their photocatalytic activities in the selective transformation of malic acid in aqueous solution. The modification with guanidine, which remarkably decreases the band gap of the metal oxide, was found to have a significant effect in the photocatalytic activity of the materials under visible light, showing a remarkably superior activity to that of the commercial Degussa P25. These materials are also envisaged to have interesting photocatalytic activities under sunlight.

Full text of DOI:10.1039/c1gc15692f

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PAPER
Magnetically separable nanocomposites with photocatalytic activity under
visible light for the selective transformation of biomass-derived platform
molecules
a
b
c
d
c
Alina M. Balu, Babita Baruwati, Elena Serrano, Jaume Cot, Javier Garcia-Martinez,
b
a,b
Rajender S. Varma* and Rafael Luque*
Received 14th June 2011, Accepted 7th July 2011
DOI: 10.1039/c1gc15692f
Novel magnetically separable TiO
characterised by a series of techniques including XRD, SEM, TEM, N
2
-guanidine-(Ni,Co)Fe
2
O
4
nanomaterials were prepared and
physisorption as well as
2
XPS and subsequently tested for their photocatalytic activities in the selective transformation of
malic acid in aqueous solution. The modification with guanidine, which remarkably decreases the
band gap of the metal oxide, was found to have a significant effect in the photocatalytic activity of
the materials under visible light, showing a remarkably superior activity to that of the commercial
Degussa P25. These materials are also envisaged to have interesting photocatalytic activities under
sunlight.
1
. Introduction
Nanometric size TiO has been widely employed as photo-
2
catalyst in many of these previously mentioned processes due
to its inherent ability of generating charged carriers, thereby
Photocatalytic processes have attracted a great deal of attention
in recent years in our aim to switch to more benign, atom
efficient and sustainable processes that make use of environmen-
tally friendly reagents under mild conditions. Heterogeneous
photocatalysis can indeed offer a significant versatility to
assist scientists in finding the appropriate solutions for key
issues affecting our society (i.e. energy issues). This versatility
is reflected in the remarkable number of applications which
have been to oxidations and oxidative breaks (i.e. removal of
organic pollutants from wastewater), reductions, isomerizations,
7
inducing reductive and oxidative processes. It also has a high
photocatalytic activity, low toxicity, chemical stability and very
1–3
8
low cost. The anatase form of titania has been reported to
4
give the best combination of photoactivity and photostability.
Practically, TiO
00–388 nm.
Nevertheless, there are important drawbacks that severely
2
photoactivation takes place in the range of
3
limit the application of titanium dioxide photocatalysts as such
to degrade organic pollutants in the gas or liquid phase and/or
2–5
substitutions, condensations and polymerizations.
1,9,10
to perform useful transformations of organic compounds.
In general, an ideal photocatalyst should feature photo-
stability under the source of irradiation, a chemical and/or
biological inert nature, low cost and availability and most
importantly the capability to adsorb reactants under efficient
photonic activation (hu ≥ E ) as well as being as much selective
as possible in the target reaction.
One of its most important limitations is the lack photocatalytic
11,12
activity under visible light.
The anatase form of TiO
3.2 eV bandgap in most media), corresponding to an onset of
the optical absorption band at about 350 nm. This onset of the
TiO absorption is also inadequate to achieve efficient solar-light
photocatalytic activity (only ca. 5% of the solar light energy can
be absorbed by TiO ).
2
is a wide band gap semiconductor
(
g
3,6
2
2
In view of these premises, there has been a continued interest
a
Dpto. Qu ´ı mica Org a´ nica, Universidad de C o´ rdoba, Campus de
Rabanales, Edif. Marie Curie, Ctra. Nnal. IV, Km 396, E-14014,
C o´ rdoba, Spain. E-mail: q62alsor@uco.es
in improving the photocatalytic efficiency of TiO catalysts
2
13
for visible and/or sunlight irradiation by different strategies,
14
namely the use of an organic dye as photosensitizer or its
b
Sustainable Technology Division, National Risk Management Research
15
modification and/or doping with metallic and non-metallic
Laboratory, US Environmental Protection Agency, MS 443, 26 West
Martin Luther King Drive, Cincinnati, Ohio, 45268, USA.
E-mail: Varma.Rajender@epamail.epa.gov
1
1,16
elements.
TiO
Asahi et al. firstly showed an absorption increase of
11
2
in the visible region upon nitrogen doping, paving the way
c
Molecular Nanotechnology Lab, Department of Inorganic Chemistry,
to further studies of titania doping with non-metallic elements
including C and S. Doping with these elements rendered TiO
University of Alicante, Ctra. San Vicente s/n, E-03690, Alicante, Spain
d
2
Institut de Qu ´ı mica Avan c¸ ada de Catalunya CID-CSIC, C/Jordi Girona
17
1
8 -26, 08034, Barcelona
materials that were able to work under visible light.
2
750 | Green Chem., 2011, 13, 2750–2758
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Another important issue of current photocatalytic research is
related to the efficient separation of the catalyst upon reaction
Procedure for the preparation of the magnetically separable
photocatalysts TiO -G-(Ni, Co)Fe The catalysts (identical
procedure for Ni and Co) were prepared in two steps, namely the
preparation of the magnetic nanoparticle support (nanoferrites)
and subsequent deposition of the N-modified titanium dioxide.
2
2
4
O .
18
completion. Generally, conventional photocatalysts need an
additional filtration (settling) step and/or tedious workup to be
recovered from the reaction mixture that often leads to loss of
catalyst (due to its fine particles in the nanometre scale) and
difficulties in separation. Despite several reports that deal with
the immobilisation of the photocatalysts onto various supports
A typical procedure for the preparation of the TiO
material was as follows:
2
-G-NiFe
2
O
4
(
a) Synthesis of NiFe
O and 6.88 g Fe (NO
2
O
4
nanoparticles. 2.28 g Ni (NO
9H
3
)
2
19
(
e.g. beads, zeolites, carbons), this stabilisation rendered less
6
H
2
3
)
3
2
O was dissolved in 200 mL of
efficient and therefore active catalysts due to a reduction in their
effective surface areas and reduced mass-transfer rates.
deionized water. Ammonium hydroxide (25%) was added into
the solution to bring the pH up to 9 under magnetic stirring.
The mixture was then further stirred for 1 h at room temperature
and transferred to a Teflon lined stainless steel autoclave. The
The design of magnetically separable catalysts is envisaged
to be a promising alternative to improve the efficient sepa-
ration of photocatalysts from solutions by applying a simple
magnet, meeting both the requirements of high accessibility
◦
temperature of the autoclave was then raised to 220 C at a rate
◦
-1
1
0 C min and maintained at that temperature for 2 h. The
20
with improved reusability. The combination of these features
with TiO can also offer important alternative nanomaterials
reaction mixture was then cooled to room temperature, filtered
2
off, washed 3 times with water and dried overnight in an oven at
◦
which find applications in a wide range of reactions due to their
potential in designing surface functionalities along with catalyst
1
00 C to yield the NiFe
2
O
4
nanoparticles.
(
b) Synthesis of the TiO
2
-G-NiFe catalyst. 2.1 g NiFe
2
O
4
2
O
4
2
1–23
preparation.
reported revisions of the topic by Cozzoli et al.
Following our initial work in both the preparation of
For more information, please refer to recently
nanoparticles were dispersed in 1000 mL deionized water by
ultrasonication for 1 h in order to break up the aggregates that
might have formed during the processing steps. 40 mL (50% in
24,25
26,27
magnetically separable catalysts
and visible-light N-doped
herein, we report a simple methodology for
the preparation of magnetically separable N-modified TiO
Ni,Co)Fe materials. These materials combine the advantage
HCl) TiCl
mixture was stirred for 15 min and then ammonium hydroxide
25%) was added to hydrolyse the mixture bringing the pH
up to 9. The nucleation and growth of the TiO particles on
4
and 80 g guanidine nitrate were then added. The
28,29
photocatalysts,
2
-
(
(
2
O
4
2
of being easily recoverable by using a simple magnet as well
as offering the possibility to work under visible and sunlight
irradiation. The activity of the materials was tested in the
selective photocatalytic transformation of 2-hydroxy-butandioic
acid (malic acid) in aqueous solutions to C1 and C2 chemicals.
Selective oxidation catalysts are of utmost importance in het-
erogeneous photocatalysis as these reactions tend to generate a
series of byproducts along with the target compound, especially
in photocatalytic processes where the photo-transformation
often leads to complete mineralisation of the starting
the surface of the magnetic nanoparticles restricted them from
agglomeration during the stirring process, in a similar way
as previous reports for other supported nanoparticles. The
31
mixture was then stirred magnetically overnight, filtered off,
washed several times with water and dried overnight in the
◦
oven at 100 C. The powder eventually obtained was calcined at
◦
3
50 C for 2 h to give the final TiO
2
-G-NiFe
material was synthesized using an
identical methodology to that of the NiFe -G changing
the precursor to Co(NO 6H O.
2
O
4
catalyst.
The TiO
2
-G-CoFe
2
O
4
2
O
4-TiO
2
3
)
2
2
30
material.
2.2. Materials characterization
2
. Experimental
The phases of the as-synthesized catalysts were determined by
X-ray diffraction in an MMS X-ray diffractometer with a Cu-
All chemicals are purchased from Aldrich Chemicals and are
used without further purification.
◦
◦
Ka source in the 2q range 20 to 80 . The data were collected
-
1
with a step of 1 K min . A few drops of the as-synthesized
nanoparticles in isopropyl alcohol were added to a quartz plate
and dried at room temperature before recording the X-ray
patterns.
2
.1. Synthesis of materials
Synthesis of the TiO
2
guanidine nitrate (TiO
2
-G) [non
magnetic]. In a typical procedure, 20 g of TiCl
0% in HCl) was added to 1000 mL of water. 80 g of guanidine
4
(digested to
Nitrogen physisorption was measured with a Micromeritics
◦
5
instrument model ASAP 2000 at -196 C. The samples were
◦
-2
nitrate was then added to the solution and magnetically stirred
for 30 min. An ammonium hydroxide solution was then added to
increase the pH of the solution to 9. The reaction was continued
at room temperature for 24 h. The ensuing precipitate was then
centrifuged and washed with water three times to remove the
chlorides and any other water-soluble reactants. The product
outgassed for 2 h at 100 C under vacuum (P < 10 Pa) and
subsequently analyzed. The linear part of the BET equation
(relative pressure between 0.05 and 0.22) was used for the
determination of the specific surface area. The pore size
distribution was calculated from the adsorption branch of the N
2
physisorption isotherms and the Barret-Joyner-Halenda (BJH)
formula. The cumulative mesopore volume was obtained from
the PSD curve.
◦
was then dried at 100 C overnight, powdered, and calcined at
◦
3
50 C to obtain the final bright yellow colored products.
A commercial Degussa P25 catalyst (AeroxideꢀR TiO
,
TEM micrographs were recorded on a Phillips CM 20 TEM
microscope at an operating voltage of 200 kV. A drop of the
as-synthesized nanoparticles in ethanol was loaded on a carbon
2
Sigma–Aldrich) was also characterised and subsequently
utilised for comparative purposes.
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coated copper grid and then allowed to dry at room temperature
before recording the micrographs.
UV-DRS spectra were recorded on a Shimadzu UV-250IPC
instrument in the 200 to 800 nm range.
XPS measurements were performed in a ultra high vacuum
(
UHV) multipurpose surface analysis system (SpecsTM model,
-
10
Germany) operating at pressures <10 mbar using a conven-
tional X-ray source (XR-50, Specs, Mg-Ka, 1253.6 eV) in a
“
stop-and-go” mode to reduce potential damage due to sample
irradiation. The survey and detailed metal high-resolution
spectra (pass energy 25 and 10 eV, step size 1 and 0.1 eV,
respectively) were recorded at room temperature with a Phoibos
150-MCD energy analyser. Powdered samples were deposited
on a sample holder using double-sided adhesive tape and
-
6
subsequently evacuated under vacuum (<10 Torr) overnight.
Eventually, the sample holder containing the degassed sample
was transferred to the analysis chamber for XPS studies. Binding
energies were referenced to the C 1s line at 284.6 eV from
adventitious carbon.
Fig. 1 XRD pattern of the TiO
tion pattern of the anatase phase (a) as well as of TiO2-G-NiFe
2
-G catalyst showing the clear diffrac-
2 4
O (b)
and TiO -G-CoFe (c).
2
2 4
O
The magnetic properties of the nanomaterials were recorded
on a Lakeshore 7000 vibrating sample magnetometer (VSM) up
to a field of 10 000 G at room temperature.
The magnetically separable materials synthesized from TiO
G did not correspond to a single phase compound but confirmed
to be a mixture of Ni Fe or CoFe and TiO by X-ray
diffraction and TEM (TiO -G was growth and deposited on
2
-
2
O
4
2
O
4
2
2
the surface of the Ni or Co ferrite nanoparticles, the obtained
photocatalyst being N-modified titanium dioxide nanoparticles
supported on the ferrites). Fig. 1 also shows the corresponding
X-ray diffraction patterns for the as synthesized samples in
2
.3. Photocatalytic experiments
All catalytic tests were performed in a Pyrex cylindrical double-
walled immersion well reactor open to air. In a typical exper-
iment (see Fig. 7 for more details), an aqueous solution (50
mL) containing 0.5 mmol malic acid and 0.1 g catalyst was
maintained in the dark for 1 h (to reach complete adsorption
at equilibrium) and subsequently exposed to a 150 W Quartz
Halogen Lamp (Fiber-Lite, Dolar Jenner Industries), which
produces strong visible light (l > 400 nm) to start the pho-
tocatalytic reaction. An adequate UV-filter to remove most of
the emission before 400 nm was utilised in the reaction (to use
P25 as comparison in the photocatalytic reaction). Under these
conditions, a very low conversion of P25 (which does not absorb
in the visible range) was obtained. A water bath was used to
32
which the typical diffractions of the anatase phase (T) and the
33
respective nanoferrites (NiFe
other TiO phase contamination (rutile, brookite) was observed
in the synthesized materials. The Scherrer equation gives a
nanoparticle size of 5.99 nm (NiFe particle) and 11.9 nm
TiO ) for TiO -G-NiFe as well as 8.3 nm (CoFe particle)
and 12 nm (TiO -G-CoFe ), respectively, in good agreement
2
O
4
, CoFe
2
O
4
) are depicted. No
2
2
O
4
(
2
2
2
O
4
2
O
4
2
2
O
4
with those observed in the TEM micrographs (Fig. 2, top
images). These show that the particles are in the 5–10 nm range
with a very narrow size distribution and a slight aggregation for
both the TiO -G and the magnetically separable nanomaterials,
2
◦
respectively. The aggregation might have occurred in the absence
of any coating agent or organic moieties on the surface of the
maintain the solution at RT (25 C). Samples were periodically
withdrawn (every 0.25 h) and analysed by HPLC, GC and
GC/MS. Dark experiments (in the absence of visible light) were
carried out in a similar way to those under visible light, providing
no conversion of starting material after 2 h reaction.
Total mineralisation was followed by measuring the total
organic carbon (TOC) at different times in solution. Under
optimised conditions, a >98% mineralisation was obtained
after 12 h and TOC of actual experiments performed (0.5–2
h) was found to be over 90% of the initial content (less 10%
mineralisation).
particles. The larger size of the TiO
as compared to TiO -G correlates well with the deposition of the
TiO -G phase on the nanoferrites (thus increasing nanoparticle
2
-(Ni or Co)Fe
2
4
O materials
2
2
sizes in the final photocatalysts).
The morphology of the materials is quasi-spherical as found
in SEM micrographs (Fig. 3), which was almost identical
regardless of the preparation method for both non magnetic
and magnetically separable catalysts.
Textural properties of the materials are summarised in Table
1
. As compared to the commercial TiO
2
P25, the synthesized
2
materials possessed significantly higher surface areas (>200 m
-
1
g ) and a narrow pore size distribution (5–6 nm).
3
. Results and discussion
UV-DRS spectra of the synthesized sample clearly shows the
extension of the absorption band to the visible region (Fig. 4).
The corresponding X-ray diffraction pattern for the as-
synthesized TiO -G sample correlated well with that of a pure
2
The TiO -G sample has two broad absorption bands at 390 nm
2
anatase form (Fig. 1). Crystallite sizes as calculated using the
Scherrer equation were found to be 6.77 nm, in good agreement
to the particle size obtained from TEM measurements (Fig. 2,
bottom).
and 462 nm respectively, as well as a very small contributions at
ca. 550 nm, clearly pointing to a potential photocatalytic activity
under visible light. The intense yellow colour of the N-modified
titania materials can therefore be ascribed to the N-doping, in
2
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2 2 4 2 2 4 2
Fig. 2 TEM micrographs of TiO -G-CoFe O (top, left) and TiO -G-NiFe O (top, right) as compared to the parent TiO -G (bottom) catalyst.
28,34,35
good agreement with previous reports.
only exhibited the fundamental absorption edge of TiO
87 nm, showing negligible absorption in the visible range.
Comparatively, P25
Cr (as potential impurity present in the Ti precursor) in both
materials as well as appreciable traces of Cl in the magnetically
2
at ca.
34
3
separable composites, which were not observed in the TiO -
2
UV spectra of the magnetically separable catalysts shows a
continuous absorption from the visible to the UV range. This
is due to the coloured ferrite particles which absorb throughout
the visible range.
XPS analysis was subsequently employed to determine the
chemical states and elements present in the materials. Fig. 5
G, coming from the TiCl
synthesis procedure.
4
utilised as titania precursor in the
O 1s XP spectra of the different materials are shown in Fig. 6.
Four distinctive peaks can be observed in the TiO -G spectrum.
The peak at 529.9 eV is the characteristic peak of the Ti–O bond
in TiO , corresponding to around 50% of the O species in the
material, while the peak at ca. 531.2 eV has been reported to
2
2
depicts the survey spectra of the TiO
2
-G and TiO
2
-G-NiFe
2
O
4
36
materials, showing the presence of Ti, O, N, C and traces of
be due to N–Ti–O species, accounting in this case for over
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2
Table 1 Textural properties of various TiO modified materials
a
b
2
-1
c
3
-1
Material
d
p
(nm)
A
BET (m g )
p
V (cm g )
TiO
TiO
TiO
TiO
2
2
2
2
P25 (AeroxideꢀR P25) 21
50
0.30
0.35
0.30
0.31
-G
5.0
4.9
5.8
280
217
200
2 4
-G-NiFe O
-G-CoFe O
2 4
a
Average mesopore diameters were estimated from the adsorption
b
branch of the nitrogen isotherm using the BJH method. The BET
surface area was estimated by using multipoint BET method using
the adsorption data in the relative pressure (P/P
Cumulative mesopore volume obtained from the PSD curve.
0
) range of 0.05–0.22.
c
Fig. 4 UV-DRS spectrum of the TiO
P25 and TiO -G-NiFe
2 2
-G material as compared to TiO
2
2 4
O .
reduced and shifted Ti–O–N component (<3% total O species,
528.3 eV).
The N 1s spectra obtained for these materials generally shows
a broad peak from 396 to 403 eV, which is typical of nitrogen-
1
1,34
doped titanium dioxide (Fig. 7).
However, in contrast with
-G and TiO -G-NiFe
the similar O 1s spectra found for TiO
2
2
2
O ,
4
the observed N 1s spectra are remarkably different in shape
and contribution. After fitting of the curve data, four main
peaks were obtained for both magnetically and non magnetically
separable materials at ca. 400.9, 399.8, 398.9 and 397.6 eV.
Generally, two peaks have been reported in N 1s core levels for
1
1,34,37
N-doped TiO
to the anionic N- in Ti–O–N, that is, the interstitial N
the peak at 397.6 eV has been assigned to the substitutional
2
materials
The peak at 399.8 eV is attributed
11,34,38
while
1
1,34,37
N in O–Ti–N linkages.
The binding energy of the second
39
peak is notably higher than that of TiN appearing at 396.9 eV,
which may be due to the higher electron negativity of oxygen
than nitrogen, so the electron density on the nitrogen decrease
(therefore binding energy increases).
However, due to the peculiar structure of guanidine (which
potentially has three N groups available), two alternative bands
appeared which are most probably related to the formation
of similar Ti–O–N (400.9 eV) and O–Ti–N (398.9 eV) species
through binding to the two different types of N (imine and ter-
minal amine groups). These observations are in good agreement
with peaks observed in the O 1s spectra and suggest the chemical
state of N is primarily as terminal N (interacting with Ti and O
in the structure of the material) and not as bulk N doping. The
significantly higher binding energies found for the N 1s bands as
compared to previous reports (397.6 vs. 396 eV) is believed to be
due to the strong interaction of the N groups of guanidine with
Fig. 3 SEM micrographs of TiO
2 2 4
-G-NiFe O (top) as compared to
TiO -G (bottom).
2
3
5% of the total O species. An additional peak appears at ca.
5
27.7 eV (10% O species) and could be attributed to Ti–O–N
36
species in the materials, most probably established between
the hydroxyl groups of TiO and the functional N atoms of
guanidine (terminal NH groups). Last, the small contribution
2
2
at 532.1 eV (<5% of O species) can be assigned to a different
type of N–Ti–O environment to that observed at 531.2 eV, which
could be due to the linkage of a different N from guanidine (N
from the imine part of the molecule). Interestingly, the O 1s
the TiO in a similar way to previous reports, which support the
2
theories on the chemical state of the N species.
spectra of TiO
of TiO -G, which indicates the N-TiO
present in the surface of the materials. This finding is also in
good agreement with the observed deposition of the TiO -G
phase on the nanoferrites as confirmed by XRD. Particularly, the
TiO -G-NiFe material exhibited a higher contribution of the
characteristic Ti–O bond in titania (>60%) and a significantly
2
-G-(Ni or Co)Fe
2
O
4
were very similar to that
Furthermore, these findings were also supported by the results
of XPS for the Ti 2p region (not shown). The binding energy
2
2
phase is the main phase
of the Ti 2p3/2 core levels of TiO
2
-G were found to be 458.4
34
2
eV, while pure TiO has been reported to appear at 459.05 eV
2
The binding energy of Ti 2p3/2 after nitrogen doping decreases
and suggests different electronic interactions of Ti with anions,
which causes partial electron transfer from the N to the Ti and
2
2
O
4
2
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Fig. 6 O 1s XPS of TiO
2 2 2 4
-G (top) and TiO -G-NiFe O (bottom)
showing the different O species present in the materials.
for the as-synthesized NiFe
2
O
4
. Supported TiO -G materials
2
showed similar properties which were in good agreement with
their facile magnetic separation by a simple magnet at room
temperature.
Fig. 5 XPS survey of TiO
2 2 2 4
-G material (top) and TiO -G-NiFe O
(
bottom) materials.
Characterised materials were subsequently investigated in the
photocatalytic transformation of a biomass-derived platform
molecule (malic acid, MA) in aqueous solution under visible
an increase in the electron density of Ti because of the lower
electron negativity of nitrogen compared to oxygen. These facts
also verified the partial incorporation of nitrogen into the lattice
via oxygen substitution.
The magnetic properties of the synthesized nanomaterials
were also investigated. The magnetization versus applied field
light. Malic acid (COOH–CHOH–CH
2
–COOH) is a C4 di-
carboxylic acid that was selected by the USDOE in 2004 as
40
one of the top building blocks to focus at in the near future.
Herrmann et al. previously investigated the different reaction
pathways of this molecule leading to total mineralisation and
established a series of parallel degradation routes in which
several intermediates (up to 15) were identified prior to achieve
studies of the Ni and CoFe
2
4
O nanoparticles show that the
as-synthesized particles are superparamagnetic at room tem-
perature, which is the main criterion for magnetic separation.
30
The saturation magnetization and coercivity of the NiFe
2
O
4
total mineralisation of the organic diacid.
were found to be 42.24 emu g-1 and 12 Oe, respectively.
Our aim was to selectively obtain some of these intermediates
Fig. 8 shows the magnetization versus applied field plot
via photocatalytic activation avoiding as much as possible total
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2 4
Fig. 8 Magnetization vs. applied field magnetic studies of NiFe O .
selectivities. TOC show than less than 15% total mineralisation
is obtained under these conditions at times of reaction of 90
min. This high selectivity to formic acid has been reported for
the first time. Blank runs (dark experiments) were performed
and provided virtually no conversion of starting material under
the investigated reaction conditions.
The magnetically separable materials could also be easily
separated from the reaction mixture by using a simple magnet,
21,22
similarly to previous reports by our group,
and reused in the
photocatalytic process several times, preserving almost intact
(
<90%) its initial activity.
Further experiments are ongoing in our laboratories to
complete these results as well as to try to extend this protocol
to the production of other intermediates from the degradation
pathway from malic acid proposed by Herrmann et al. (e.g.
3
-oxopropanoic acid, acetaldehyde, 2-hydroxypropanoic acid)
as well as to related platform molecules (e.g. succinic acid).
Attempts to extend the protocol to the use of sunlight as
irradiation source have also shown a promising results in the
degradation of dyes (for instance over a 60% methylene blue
degradation after 1 h irradiation on a sunny day). These
and other complementary studies will be reported in due
course.
Fig. 7 N 1s XPS of TiO
2 2 2 4
-G (top) and TiO -G-NiFe O (bottom)
showing the different N species present in the materials.
4
. Conclusions
mineralisation of malic acid. For this, a series of operational
parameters known to influence the photocatalytic activity of a
material were optimised. These include light intensity, nature
and concentration of the substrate, time of irradiation and
concentration of the photocatalyst. Optimised results have been
included in Fig. 9.
A series of visible-light effective photocatalysts were synthesized
following a simple methodology. Both magnetic and non mag-
netic materials were found to have various Ti–N environments
which in turn rendered useful catalysts to work under visible
light and potentially sunlight. Materials were found to be
photocatalytically active in the selective transformation of malic
acid to high added value compounds, avoiding complete min-
eralisation. In particular, the magnetically separable catalysts
were comparably more active to P25 (used as reference) as
well as being easily recoverable from the final solution and
recycled several times preserving most of their initial activities.
We envisage these materials could also be highly useful for
related transformations of biomass-derived platform molecules
at mild conditions.
These pointed to the formation of formic (HCOOH) and
acetic acids (CH
3
–COOH) as main products prior to mineralisa-
tion to CO (Scheme 1). Traces of oxalic acid (HCOOH–COOH)
2
were also detected by GC/MS. Under the investigated reaction
conditions, a selectivity close to 80% to formic acid could be
achieved in less than 2 h of reaction (Fig. 8). The synthesized
materials also possessed remarkably improved activities to those
of P25 under visible light, with the magnetically separable
TiO
2
-G-NiFe
2
4
O material showing optimum activity at high
2
756 | Green Chem., 2011, 13, 2750–2758
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Fig. 9 Photocatalytic transformation studies of malic acid under visible light irradiation using various titania catalysts. Reaction conditions: 0.5
◦
mmol malic acid in 50 mL water, 0.1 g catalyst, 25 C, 150 W Quartz Halogen Lamp (l > 400 nm) irradiation for 90 min.
Scheme 1 Photocatalytic transformation of malic acid into different chemicals.
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as well as for funding a short stay fellowship Incentivos para
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acknowledges funds from the research group FQM-162, Depar-
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