Photoreduction of carbon dioxide to formic acid in aqueous suspension: A comparison between phthalocyanine/TiO2and porphyrin/TiO2 catalysed processes

Article abstract of DOI:10.3390/molecules20010396

Composite materials prepared by loading polycrystalline TiO ₂ powders with lipophilic highly branched Cu(II)- and metal-free phthalocyanines or porphyrins, which have been used in the past as photocatalysts for photodegradative processes, have been successfully tested for the efficient photoreduction of carbon dioxide in aqueous suspension affording significant amounts of formic acid. The results indicated that the presence of the sensitizers is beneficial for the photoactivity, confirming the important role of Cu(II) co-ordinated in the middle of the macrocycles. A comparison between Cu(II) phthalocyanines and Cu(II) porphyrins indicated that the Cu(II)- phthalocyanine sensitizer was more efficient in the photoreduction of CO₂ to formic acid, probably due to its favorable reduction potential.

Full text of DOI:10.3390/molecules20010396

Molecules 2015, 20, 396-415; doi:10.3390/molecules20010396
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Photoreduction of Carbon Dioxide to Formic Acid in Aqueous
Suspension: A Comparison between Phthalocyanine/TiO2 and
Porphyrin/TiO2 Catalysed Processes
Giuseppe Mele 1,*, Cosimo Annese 2,†, Lucia D’Accolti 3,†, Alberto De Riccardis 1,†
,
Caterina Fusco 2,†, Leonardo Palmisano 4,†, Anna Scarlino 1,† and Giuseppe Vasapollo 1,†
1
Department of Engineering for Innovation, University of Salento, via Arnesano, Lecce 73100, Italy;
E-Mails: alberto.dericcardis@unisalento.it (A.D.R.); anna.scarlino@unisalento.it (A.S.);
giuseppe.vasapollo@unisalento.it (G.V.)
2
CNR-ICCOM UOS of Bari, Chemistry Department, University of Bari, Via Orabona, 4,
Bari 70126, Italy; E-Mails: cosimo.annese@uniba.it (C.A.); fusco@ba.iccom.cnr.it (C.F.)
3
Chemistry Department, University of Bari “A. Moro”, via Orabona, 4, Bari 70126, Italy;
E-Mail: lucia.daccolti@uniba.it
4
Dipartimento di Energia, Ingegneria dell’Informazione e Modelli Matematici, DEIM,
Università di Palermo, Viale delle Scienze, Ed. 6, Palermo 90128, Italy;
E-Mail: leonardo.palmisano@unipa.it
†
These authors contributed equally to this work.
* Author to whom correspondence should be addressed; E-Mail: Giuseppe.mele@unisalento.it;
Tel.: +39-0832-297-281; Fax: +39-0832-297-733.
Academic Editor: Derek J. McPhee
Received: 15 November 2014 / Accepted: 22 December 2014 / Published: 30 December 2014
Abstract: Composite materials prepared by loading polycrystalline TiO2 powders with
lipophilic highly branched Cu(II)- and metal-free phthalocyanines or porphyrins, which
have been used in the past as photocatalysts for photodegradative processes, have been
successfully tested for the efficient photoreduction of carbon dioxide in aqueous
suspension affording significant amounts of formic acid. The results indicated that the
presence of the sensitizers is beneficial for the photoactivity, confirming the important role
of Cu(II) co-ordinated in the middle of the macrocycles. A comparison between Cu(II)
phthalocyanines and Cu(II) porphyrins indicated that the Cu(II)- phthalocyanine sensitizer

Molecules 2015, 20
was more efficient in the photoreduction of CO2 to formic acid, probably due to its
397
favorable reduction potential.
Keywords: CO2; heterogeneous photocatalysis; phthalocyanines; porphyrins; photo-reduction;
TiO2
1. Introduction
Carbon dioxide (CO2) is a well-known greenhouse gas which can be emitted to the environment
by natural sources as well as by human activities. It could be considered one of the simplest alternative
feedstocks, and has thus attracted the interest of industrial and academic researchers due to its low cost
and worldwide availability.
The production of chemicals from CO2 would not only preserve the oil reserves but also would
reduce the CO2 emissions [1,2]. As it is well known, CO2 is extremely thermodynamically stable, and
for this reason its reduction is a difficult task.
Some progress has been made to enhance the yield of the reduction by means of various strategies,
e.g., by reducing CO2 with H2 in supercritical fluids [3]. The catalytic activation of CO2, however, has
been the preferred approach in recent years. In particular the synthesis of formic acid (HCOOH) from
CO2 has reached such a state of knowledge that the continued progress of this research may well lead
to good practices the treatment of CO2 produced on an industrial scale. A fruitful interaction between
investigative work into the reaction mechanism(s) and the development of new catalytic systems
would therefore seem desirable [4].
Inoue and co-workers reported pioneering studies concerning the photocatalytic reduction of CO2 in
aqueous solution to produce HCOOH, along with other organic molecules, using various semiconductors
including titanium dioxide (TiO2) [5].
Photocatalytic reduction of CO2 using photosensitive semiconductor powders has been widely studied
in aqueous solutions [6–22] or supercritical fluid [23]. In particular, the photoreduction of CO2 with
water into organic compounds using various TiO2 photocatalysts has attracted the major interest.
TiO2 is one of the most widely used photocatalysts in the field of environmental applications due to
its low toxicity, ability to resist to photocorrosion, and low cost. However, the large band gap of titania
and massive recombination of photogenerated charge carriers limit its overall photocatalytic efficiency.
These problems could be overcome by modifying the electronic band structure of TiO2, which is
possible using various strategies like coupling with a narrow band gap semiconductor, metal
ion/nonmetal ion doping, co-doping with two or more foreign ions, surface sensitization by organic
dyes or metal complexes, and noble metal deposition [2,24].
The presence of suitable modifiers is important in order to perform the photoreduction processes
occurring under visible or solar light radiation [25–27]. For example CO2 was photocatalytically reduced
to produce methanol by using TiO2 and Cu-TiO2 in aqueous suspension under UV irradiation [28].
In other studies an efficient and clean photoreduction to CO was obtained by using enzyme-modified
TiO2 nanoparticles under visible light [29].

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An alternative strategy might be the use of sensitizers, in order to use visible radiation for the
activation of TiO2. Phthalocyanine metalloderivatives (MPcs) or their supramolecular arrangements
have recently attracted an increasing interest because they have been used not only as the main species
for the preparation of dyes and pigments, but also as the building blocks for the construction of new
molecular materials used in various technological applications [30–32], including some catalytic
ones [33].
In the last years, dye-sensitized TiO2 processes have been employed for solar energy conversion,
photocatalysis and (electro)-photography. These technologies can be actually considered affordable
because of their relatively low potential costs, low environmental impact, and efficient power
conversion [34,35]. The photocatalytic reduction of CO2 under anaerobic conditions, promoted by cobalt
and iron phthalocyanines, for instance, produced CO and formate in DMF and acetonitrile solutions [36].
The combination of TiO2 with porphyrins (Pps), metal porphyrins (MPps), phthalocyanines (Pcs),
metal phthalocyanines (MPcs) or other structurally similar species, allows one to obtain extremely
versatile composite sensitized materials with not only oxidizing properties suitable for photodegrading
organic pollutants in water [37–39], but also reductive properties to photoreduce CO2 [40,41].
Recently, the photoreduction of CO2 to formic acid in aqueous solution using cobalt (CoPc) and
zinc phthalocyanine (ZnPc)-loaded TiO2 synthesized under visible light irradiation has been reported;
the quantification of formic acid was performed by UV-Vis spectroscopy [42,43].
The main aim of this work was to compare hybrid photocatalysts, obtained by impregnation of
sensitizers (Pcs and Pps) onto the TiO2 surface, previously used successfully for photodegradation
reactions [37,38], and to demonstrate how some of them are suitable for an efficient photocatalytic
reduction of CO2 to formic acid. In addition a mechanism is proposed for the role of sensitizer, based
on the reduction potential values of the macrocycles.
This new class of multipurpose materials (Figure 1) can be actually considered, due to their relatively
low potential cost and the efficient power conversion. Their use has a low environmental impact and
employs new reaction conditions: in fact the use of organic hole scavengers, previously reported in
formic acid photosynthesis from CO2 [44] and also the use of strong alkaline pH values [42,43]
is avoided.
CO2
HCOOH
hv: 280-700nm
Sensitizer
impregnated
TiO2
OH
.-
H2O2, O2
O2
.
HO2
Oxidation
products
NO2
Figure 1. Double activity of CuPc-TiO2.

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2. Results and Discussion
2.1. Structures of the Sensitizers (MPcs, and MPps)
The molecular structures of the metal free and metallo-porhyrins and phthalocyanines used as
sensitizers in this work are reported in Figure 2. These compounds were opportunely synthesized,
characterized and loaded onto TiO2 as described in the Experimental Section.
O
N
N
N
N
M
N
N
N
M
O
N
O
N
N
N
N
O
M=Cu, Zn, [2H]
M=Cu, [2H]
CuPp, H2Pp
CuPc, ZnPc, H2Pc
Figure 2. Molecular structure of MPcs and MPps.
2.2. Photoreactivity Experiments
The photoreduction of CO2 to formic acid was chosen as a model reaction in order to evaluate
the photocatalytic capabilities of the Pcs/TiO2 and Pps/TiO2 composites in comparison with the
pristine TiO2. Three blank tests were carried out, according to the conditions of Process A (see
Experimental Section, 3.6.1), in order to ensure that formic acid production derived only from the
photoreduction of CO2. The first was carried out by irradiating the reaction system in the absence of
catalyst while the second one was run under dark in the presence of catalyst. The third test was
performed finally in the absence of CO2 to ensure that the formic acid was not obtained from some
other ubiquitous organic species. No hydrocarbon was detected in the above blank tests. Further
investigations were carried out in order to establish if the Pps or Pcs supported onto TiO2 were
photostable, i.e., if some decomposition of the supported sensitizers occurred under the same conditions
used during the photocatalytic experiments.
No significant release of organic degradation sensitizers was observed even after long irradiation
times (5–7 h) of the supported sensitizers used and also the sensitizer itself can be recovered
quantitatively from the TiO2 surface by extraction with chloroform or dichloromethane. The absence
of structural modifications for the phthalocyanine and porphyrins recovered was confirmed by the
analytical and spectral data.

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The use of ion chromatography to detect formic acid allowed its good separation from the interfering
species. Quantification of formic acid by UV-Vis spectroscopy is very difficult, in fact, due to the
interference of bicarbonate ions also at very acidic pHs. Optimization of the reaction conditions was
performed when CuPc was used as sensitizer. Notably HCOOH was formed when both NaHCO3 and
13
NaHCO3 C labelled were used.
A preliminary 13C GC/MS study on formic acid formation was performed using 13C enriched
NaHCO3 as described previously (Process B, see Experimental Section, 3.6.2). First, 99% enriched
CO2 was generated in situ from labelled bicarbonate by addition of phosphoric acid, then the reaction
mixture was irradiated for 8 h.
The use of this particular source of CO2 allowed us only to qualitatively detect the formation of the
product. The comparison with an authentic sample of labeled formic acid indicated that the produced
13
formic acid derived from CO2 (Figures 3 and 4). The reaction was monitored using the GC/MS in
SIM mode acquisition, in this case the main advantage was the increase of the sensitivity of the
instrument with respect to the acquisition in “full scan” acquisition.
H13COOH
46 (r.i 61.8 ± 3)
47 (r.i 100 ± 3)
m/z
Figure 3. Qualitative determination of formic acid performed by GC/MS in SIM Mode
analyzing an authentic sample of formic acid-13C 95 wt. % in H2O, 99 atom % 13C.
47 (r.i 100 ± 3)
46 (r.i 58.7 ± 3)
H13COOH
m/z
2.3 Effect of catalyst amount
Figure 4. Qualitative determination of formic acid performed by GC/MS in SIM Mode
13
analyzing formic acid-13C obtained by photoreduction of NaHCO3 C labelled.

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2.3. Effect of the Amount of Catalyst
In order to determine the effect of the catalyst amount, a set of experiments was carried out by
varying the amount of the 6.65-TiO2-CuPc photocatalysts in the range 0.025–0.300 g. The influence of
the photocatalyst quantity on formic acid formation is reported in Table 1. It can be observed that
0.025 g represents the amount of catalyst producing the most significant increase of formic acid
production if it is compared with bare TiO2 or higher loadings (0.075–0.300 g). Nevertheless a higher
quantity, i.e., 0.050 g, was used in the further experiments to avoid difficulties to reproduce the
measurements. In the presence of 0.050 g of CuPc-TiO2 sample the yield of formic acid reached the
value of 208.5 µmol/gcat after 8 h irradiation, and this figure is close to that obtained by using 0.025 g of
catalysts. The value obtained with 0.050 g is higher, however, than those obtained by Ishitani et al. [16],
Hirano et al. [17] and Kaneco [23].
Table 1. Effect of photocatalyst amount on formic acid production efficiency at pH = 3.
Catalyst a
TiO2-CuPc
TiO2-CuPc
TiO2-CuPc
TiO2-CuPc
TiO2-CuPc
TiO2
g of Catalyst
0.300
mg/L *(gcat)−1 b
μmol/gcat
37.6
39.2
90.6
208.5
239.5
131.0
184.7
-
34.6
36.0
83.4
191.8
220.4
120.5
169.9
-
0.150
0.075
0.050
0.025
0.050
TiO2
0.025
c
TiO2
0.050
TiO2-CuPc c
0.050
-
-
a Quantity of sensitizer = 6.65 μmol/g TiO2; b Irradiation time 8 h; c Reaction carried out in the absence of CO2.
2.4. Effect of the Initial pH
The influence of pH of the aqueous solution on the process efficiency for the formation of formic
acid was studied by using three different initial pH values (3, 7 and 13) without changing or checking
them throughout the experiments. In Table 2, the results of CO2 photoreduction at different initial pH’s
have been reported. The conversion of CO2 into formic acid increased by decreasing the pH value. The
highest conversion (CO2 to HCOOH) observed in this work was 0.65% and this value is reasonable
when compared with others reported in the literature (0.2%–0.4%) [42].
Table 2. Effect of initial pH on the production efficiency of formic acid.
Catalyst a,b
TiO2-CuPc
TiO2-CuPc
TiO2-CuPc
TiO2
pH
13
7
mg/L *(gcat)−1 c
60.0
μmol/gcat
65.2
58.3
63.4
3
191.8
208.5
3
120.5
131.0
a Quantity of sensitizer = 6.65 μmol/g TiO2; b Quantity of catalyst = 0.050 g; c Irradiation time 8 h.

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It was calculated as ratio of the concentration of formic acid and the concentration of carbon
dioxide in water by assuming the solution saturated by CO2 and by considering the relative constant of
solubility equal to 1.45 g/L at 298 K. The hydration equilibrium at pH = 3 was negligible, being the
constant of hydration equal to 1.70 × 10−3 at 298 K. This finding can be explained by considering both
the role of hole scavengers and the reaction mechanism.
It has been widely reported [45–49] that the presence of hole scavengers enhances the photoactivity
of TiO2. The role of scavengers is to prevent the hole-electron recombination and the interaction of h+
with precursor(s) of the desired product.
−
Under almost neutral or strongly alkaline conditions CO2 can be more easily converted into HCO3
and CO32− (see Equations (1) and (2)):
-
HCO3 + H+
CO2 + H2O
H2CO3
(1)
(2)
-
-
HCO
CO32 + H +
3
and both species are able to act as hole scavengers according to the following reactions:
−
•
HCO3 + h+ → HCO3
(3)
(4)
−
−
•
2
CO3 + h+ → CO3
−
2−
By taking into account that the amounts of HCO3 and CO3 anions, deriving from CO2, can be
considered negligible in acidic solution, their role as radical scavengers can be considered negligible at
the beginning of the process. However, their involvement throughout the process cannot be excluded,
at least when an increase of pH occurred.
The increased amount of formic acid observed in the case of the acidic solutions may be also
ascribed to protonation-deprotonation equilibria involving carbon dioxide or other related species
which have been reported to induce desorption of the product from the surface of TiO2, also
influencing the photoactivity. It is likely that the positive charge on the carbon atom as well as its
formation close to the surface of the catalyst facilitates the reduction.
2.5. Effect of Copper in the CuPc-TiO2 Composite
CuPc-TiO2 sample showed the highest efficiency when compared to pure TiO2, H2Pc and ZnPc
samples (Table 3).
Table 3. Effect of metal on formic acid production efficiency at pH = 3.
Catalyst a,b
TiO2-CuPc
TiO2-H2Pc
TiO2-ZnPc
TiO2
mg/L *(gcat)−1 c
191.8
μmol/gcat
208.5
75.0
69.0
81.5
88.5
120.5
131.0
a Quantity of sensitizer = 6.65 μmol/g TiO2; b Quantity of catalyst = 0.050 g; c Irradiation time 8 h.
The better activity of CuPc could be ascribed to the high capacity of Cu2+ to be reduced to Cu+ and
+
to transfer one electron to the [H-O-C=O]. species.

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A few years ago some of us demonstrated that the presence of copper coordinated in the middle of
the macrocycle could play an important role in photo-oxidation reactions, favouring the formation of
oxidant radical OH species [38,39], e.g., Equation (5):
−
TiO2[Cu(I)Sens] + 3O2 → TiO2[Cu(II)Sens] + •O2
(5)
Analogously, CO2 can be reduced by Cu(I) species present on the surface of TiO2 (see Equation (6))
producing the radical species hypothesized to be the precursor of HCOOH:
•−
TiO2[Cu(I)Sens] + CO2 → TiO2[Cu(II)Sens] + CO2
(6)
However, the electron transfer involving the protonated CO2 species generated in the acidic media
cannot be excluded a priori.
2.6. Effect of the Irradiation Source
The effect of irradiation source on the formic acid formation was investigated to select optimal
irradiation conditions. The results are reported in Table 4.
Table 4. Effect of irradiation sources on formic acid production efficiency at pH = 3.
Catalyst a,b
TiO2-CuPc
TiO2
Irradiation Source
Sanolux (UV/Vis) Halogen (Vis)
Sanolux (UV/Vis) Halogen (Vis)
Sanolux (UV/Vis)
mg/L *(gcat)−1 c
191.8
μmol/gcat
208.5
131.0
32.6
120.5
TiO2-CuPc
30.0
TiO2-CuPc
Halogen (Vis)
48.0
52.2
a Quantity of sensitizer = 6.65 μmol/g TiO2; b Quantity of catalyst = 0.050 g; c Irradiation time 8 h.
It can be seen that after 8 h the yield of formic acid increased when two irradiation sources were
used simultaneously. On the contrary, a negligible photoactivity was observed when a single lamp was
used as the irradiation source. Therefore, analogously to what observed for the photodegradation of
4-nitrophenol (4-NP) [37–39] it is essential to photoexcite both the components of the system, i.e.,
TiO2 and the sensitizer, in order to obtain substantial improvement of the photoreduction of CO2. The
beneficial effect on the photoreactivity is due probably to a cooperative mechanism as previously
reported in the literature [50].
2.7. Effect of the Sensitizers
In order to select the minimal loading of sensitizer able to produce a significant amount of formic
acid, three different quantities of CuPc have been supported on TiO2. Typically, porphyrin or
phtalocyanine molecules offer enormous potential as the light harvesting components of dye-sensitised
TiO2 hybrid systems. These are amongst the most used dyes tested for sensitisation of inorganic
semiconducting oxides improving the efficiency and thus the determining fundamental electron
transfer steps, from charge photogeneration to recombination.
As shown in Table 5, 6.65 µm ol was the amount of CuPc which gave a production of formic acid
significantly higher than the bare TiO2. Higher quantity of sensitizer did not give rise to a significant
increase of formic acid production. Notably a high quantity of sensitizer could cover the TiO2 surface

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hindering its photoexcitation. Other types of sensitizers (ZnPp-TiO2, ZnPc-TiO2 and H2Pp-TiO2)
showed negligible photoactivity in comparison to CuPc, and for this reason the results are not reported
for the sake of brevity.
Table 5. Effect of sensitizer’s loading on formic acid production efficiency at pH = 3.
Catalyst a
TiO2
µmol(CuPc)/gTiO2
mg/L *(gcat)−1 b
120.5
μmol/gcat
131.0
69.9
TiO2-CuPc
TiO2-CuPc
TiO2-CuPc
4.5
6.65
8.5
64.3
191.8
208.5
229.4
211.0
a Quantity of catalyst = 0.050 g; b Irradiation time 8 h.
In Table 6 only the H2Pp and CuPp samples are reported. They were shown to be efficient for the
photodegradation of 4-NP [38].
Table 6. Comparison between the photoactivity of phthalocyanine/TiO2 and porphyrin/TiO2
samples at pH = 3.
Catalyst a,b
TiO2-CuPc
TiO2-H2Pc
TiO2-CuPp
TiO2-H2Pp
TiO2
mg/L *(gcat)−1 c
191.8
μmol/gcat
208.5
75.0
120.5
30.0
26.5
48.0
32.6
120.5
131.0
a Quantity of sensitizer = 6.65 μmol/g TiO2; b Quantity of catalyst = 0.050 g; c Irradiation time 8 h.
Nevertheless, differently from what observed in the photooxidation of 4-NP, TiO2-CuPp and
TiO2-H2Pp showed a scarce photoactivity for CO2 photoreduction to formic acid. This behaviour might
be ascribed to the energy of their HOMO and LUMO.
As reported in literature [51] the First Oxidation Potential (FOP) and the First Reduction Potential
(FRP) (equivalent respectively to HOMO and LUMO) of some CuPp samples are about 0.75 and
−0.96 V. The above figures could be subjected to slight fluctuations, depending on the solvent in
which the measurements are carried out. The conduction band (CB) and the valence band (VB) of TiO2
are −0.5 and 2.7 V, respectively. FOP and FRP of CuPp together with the conduction band (CB) and
the Valence band (VB) of TiO2 are represented in Figure 5.
It can be hypothesized that the negligible activity of CuPp might be caused by the presence of the
redox potential of the couple H2O/H2 between the FRP of CuPp and the CB of TiO2. The electron
transfer between the sensitizer and the semiconductor, necessary for the occurrence of the photoreduction,
could be prevented, whereas reaction (7) can occur:
2 H2O + 2 e−→ H2(g) + 2 OH− (E0 = −0.82 V vs. NHE)
(7)

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Figure 5. Representation of energy level of TiO2, CuPp, CuPc and H2O/H2 vs. NHE.
The reason why the same composite (CuPp) works efficiently in photodegradation of 4-NP depends
on the role of the positive holes. Formation of Reactive Oxygen Species (ROS), like the radical species,
in fact, can occur as shown in the following equations:
OH− + h+VB → •OH
(8)
(9)
H2O + h+VB → •OH + H+
•
H2O2 + h+
→ HO2 + H+
(10)
(11)
VB
HO2 + h+VB → O2 + H+
•
To our knowledge the values of FRP and FOP of Cu(II) tetrakis[4-(2,4-bis-(1,1-
dimethylpropyl)phenoxy)]phthalocyanine are not known; however, these potentials could be considered
similar to that measured for the structurally analogous Cu(II) tetra-t-butylphthalocyanine compound [52].
With this assumption, being the FRP of CuPc lower than the potential of water reduction, it could be
reasonable to consider the reaction (12) less favored than the CO2 photoreduction (see Equation (6)):
2 H2O + 2 Cu(I)Pc-TiO2
H2(g) + 2 OH− + 2 CuPc (II)-TiO2
(12)
The possibility to modulate the energy level of porphyrins might prove this hypothesis.
2.8. Other Mechanistic Aspects
TiO2 has been selected as the photocatalyst to be sensitized because many studies have been
reported its effectiveness for the photocatalytic reduction of CO2 [3,23,26–29,53]. Previous papers on
photooxidation reactions report the classical mechanism involving the photoexcitation of the dye

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molecule used as sensitizer under visible light and the subsequent injection of electrons into the
conduction band (CB) of TiO2 [54,55].
Depending on the experimental conditions used, different reaction pathways can be hypothesized.
For example, in the presence of O2 into the suspensions the injected electrons (e−CB) can interact forming
•−
•
•
the superoxide anion radicals (O2 ) which is the precursor of OH and O2H radical species [56].
The latter are reported to have a key role in the photooxidation of many hazardous chemical compounds
(like 4-NP) (Equations (13–14)) [38,57–59]:
•
HO2 + 4-NP → oxidation products
HO• + 4-NP → oxidation products
(13)
(14)
In our case, although no oxygen was purging the reacting system, 62 μmol/gr (1.98 mg/L) of
molecular oxygen per gram of catalyst were detected. The presence of ubiquitous O2 was determined
by irradiating 8 h at pH = 3 with SANOLUX and RADIUM Xe-Halogen lamps an aqueous
suspension (100 mL) of TiO2-CuPc sample (0.100 g) containing the optimum quantity of sensitizer
(6.65 μmol/g TiO2). Photoreduction of CO2 by water is only theoretically readily available and
inexpensive, due to the high stability of this molecule. Two important species involved in CO2
•−
photo-reduction are carbon dioxide anion radical (CO2 ) and hydrogen radical (H•).
However, for both photoreduction and photodegradation, the key step is the formation of separated
charge (hole and electron) on the surface of TiO2 (Equation (15)):
TiO2 + hν → TiO2 (h+ + e−)
(15)
The photoinduced process of charge separation promoted by UV-Vis radiation and reported in
Figure 6 could be enhanced by the presence of a sensitizer.
Figure 6. Representation of charge separation enhancement by the use of sensitizer.
By taking into account the results of the present study and the literature data [54,60] the formation
of formic acid by irradiation of TiO2 powders can be tentatively explained by the following reaction
steps (Figure 7).

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Figure 7. Hypothesized mechanism for the formation of HCOOH on the surface of TiO2.
Both in the presence and in the absence of sensitizer, once the charges are separated (A), electrons
responsible for the photoreduction could migrate on the surface (B) or recombine with holes. Protonated
•−
CO2 can be adsorbed onto the surface of TiO2 where it can be reduced to the radical anion CO2 (C), to
which a close hydroxyl group donates a hydrogen (D), then the formed species can be reduced once
again and finally the acid pH favors the desorption of formic acid. At the same time the holes can
•−
migrate on the surface where they could react with hole scavengers or interfere with CO2 .
•−
The formation of CO2 , previously reported as a bent radical anion on a Rh/TiO2 composite [60,61],
is probably the first step as the aqueous solvent could partially stabilize the species. The radicals were
detected near a g value of 2.00 by ESR measurements [23]. Indeed when irradiation of the TiO2 powders
was performed in highly pure nitrogen medium in the absence of CO2, this signal was not observed.
The mechanism proposed is alternative to the bidentate linking of CO2 to TiO2 since no dimerization of
reaction intermediates took place during the protonation and formation of oxalic acid was not observed.
3. Experimental Section
3.1. Materials and Methods
Commercial phosphoric acid 85% (H3PO4) and sodium hydroxide (NaOH) (purchased by Carlo
Erba, Milan, Italy) were used for the preparation of the solutions. Titanium dioxide (pure anatase
phase, specific surface area 8 m2/g) was kindly provided by Tioxide Huntsman (Scarlino, Italy). CO2
purity was 99.999%. Formic acid, 2,4-bis-(1,1-dimethylpropyl)phenol, 4-nitrophthalonitrile,
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) 4-tert-butylbenzaldehyde, pyrrole, 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone and BF3·OEt2, were purchased from Sigma-Aldrich (Steinheim, Germany) and were
used without purification.
Formic acid analysis was performed by an Ion Chromatograph (IC) Dionex™ DX120 system
(Thermo Fisher Scientific Inc. Wyman Street Waltham, MA, USA) equipped with an Ion Pac ICE-AS6

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Analytical Column; heptafluorobutyric acid 0.8 mM (Sigma-Aldrich) was used as eluent (flow
1.00 mL/min) and tetrabutylammonium hydroxide 5.0 mM (Sigma-Aldrich) as regenerant (flow
4.5 mL/min). Formic acid 99% (Sigma Aldrich) was employed as standard for building IC calibration
curves. The analyses were carried out using a sample loop of 25 µL, with a column pressure less than
640 PSI and a background of 35–40 µS. UV spectra of sensitizers were obtained by using a UV
2401PC spectrometer (Shimadzu Italia S.r.l., Milano, Italy) after dissolving the samples in chloroform
(CHCl3). Molecular oxygen analysis was performed using a polarographic dissolved oxygen probes
Orion 0.86020A (Thermo Fisher Scientific Inc., Wyman Street Waltham, MA, USA). Analysis of
labelled formic acid was performed by GCMS-QP2010 Ultra Shimadzu in splitless and SIM mode,
using a MS 5 (30 m × 0.25 μm ID) capillary column (40 °C × 10 min, tr 1.82 min). An authentic
13
13
sample of formic acid C 95 wt. % in H2O, 99 atom % C (Sigma Aldrich) was used as standard.
Commercial sodium bicarbonate 13C 98 atom % 13C (Sigma Aldrich) was used as source of 13CO2.
4-[2,4-bis-(1,1-Dimethyl-propyl)phenoxy]phthalonitrile (DPP) was prepared following a procedure
reported in the literature [37]. Cu(II)[(5,10,15,20-tetra(4-tertbutylphenyl)] porphyrin (CuPp) and
[5,10,15,20-tetra(4-tert-butylphenyl)] porphyrin (H2Pp) were synthesized using a previously reported
procedure [38].
3.2. Synthesis of the Cu(II) Tetrakis[4-(2,4-bis-(1,1-dimethylpropyl)phenoxy)]phthalocyanine (CuPc)
The CuPc samples were synthesized according to a previously reported procedure [37] and shown
in the Scheme 1. Selected spectroscopic data consistent with the structure of CuPc are the following
ones: UV-VIS (CHCl3) λmax: 686, 617, 391, 339, 287 nm.
Scheme 1. Synthesis of MPcs (M = Cu, Zn) and H2Pc.
R
N
N
N
CN
CN
R
N
R
N
+
MX
M
N
2
O
DBU, ethanol
N
N
M = Cu (X=Cl)
M = Zn (X = OAc)
DPP
R
MPc ( M= Cu, Zn)
R
N
R
N
N
N
N
N
N
N
N
H
Py,HCl 4:1
R
R
N
R
R
N
Zn
N
H
N
48h
N
N
N
R
R
= R
H Pc
2
ZnPc
O

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3.3. Synthesis of the Zn(II) Tetrakis[4-(2,4-bis-(1,1-dimethylpropyl)phenoxy)]phthalocyanine (ZnPc)
A solution of DPP (300 mg, 0.83 mmol), 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU) (124 mg,
0.81 mmol), and Zn(OAc)2 (33 mg, 0.25 mmol) in absolute ethanol (2.5 mL, previously distilled from Mg)
was refluxed for 24 h under N2 atmosphere. The organic layer enabled the separation of a blue-green
solid, which was purified by chromatography (silica, ethyl acetate/hexane 1:9 v/v) and gave a mixture
of isomers of the Zn(II)-tetrakis[4-(2,4-bis-(1,1-dimethyl-propyl)-phenoxy)]phthalocyanine (ZnPc) in
50% yield. The structure was consistent with the following characterization: UV–Vis (CHCl3) λmax
:
679, 615, 347, 284.5, 248.
3.4. Synthesis of the Tetrakis[4-(2,4-bis-(1,1-dimethylpropyl)phenoxy)]phthalocyanine (H2Pc)
The tetrakis[4-(2,4-bis-(1,1-dimethylpropyl)phenoxy)]phthalocyanine (H2Pc) was prepared by
following a procedure reported in literature [62]. ZnPc (126 mg, 0.08 mmol) was dissolved in 10 mL
of pyridine and HCl (in molar ratio 4:1). The solution was continuously stirred for 36 h at 120 °C and
monitored by TLC (thin-layer chromatography). The reaction mixture was extracted by CH2Cl2 and
the crude product of the reaction, obtained after evaporation of the solvent, was purified by
chromatography (silica, ethyl acetate/hexane 0.5:9.5 v/v). A dark green solid (H2Pc) was recovered in
40% yield.
3.5. Preparation of TiO2-CuPc, TiO2-H2Pc, TiO2-CuPp, TiO2-H2Pp and TiO2-ZnPc Samples
The loaded samples used as photocatalysts for the photoreactivity experiments were prepared by
impregnating TiO2 (Tioxide, anatase phase) with various amounts of CuPp (6.65 μmol/g TiO2); H2Pp
(6.65 and μmol/g TiO2); CuPc (4.5, 6.65 and 8.5 μmol/g TiO2); H2Pc (6.65 μmol/g TiO2) and ZnPc
(6.65 μmol/g TiO2). The sensitizers were dissolved in 5 mL of CH2Cl2 and 500 mg of finely ground
TiO2 (previously sonicated for 30 min) were added to this solution. The mixture was stirred for 6 h and
the solvent was removed under vacuum. The code used for the samples is the following: CuPp and
CuPc indicate the copper porphyrin and the copper phthalocyanine, H2Pp and H2Pc the metal-free
porphyrin and the metal free phthalocyanine and ZnPc the zinc phthalocyanine. For instance
6.65-TiO2–H2Pc represents the sample prepared by using 6.65 μmol of metal free phthalocyanine used
to impregnate 1 g of TiO2 (anatase).
3.6. Photoreactivity Experiments
The set-up used for the photocatalytic experiments, reported in Figure 8, consisted of a three necked
Pyrex batch photoreactor of cylindrical shape containing 50 mL of aqueous suspension at various pH
values. The photoreactor was provided with a jacket for cooling water circulation and ports in its upper
section for the inlet and outlet of gases, for sampling and for pH and temperature measurements. A
HRC UV-VIS lamp 300 W (Vitalux, OSRAM, Milano, Italy) and Xe-Halogen lamp 400 W (Radium,
Wipperfurth, Germany) were placed in proximity (5–6 cm) of the reactor.

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Figure 8. Experimental set-up.
Two different processes (indicated as A or B) were carried out in order to reduce CO2 produced
from different sources.
3.6.1. Process A
CO2 was bubbled into the suspensions for approximately 0.5 h before switching on the lamps to
saturate the reaction mixture. The amount of catalyst used for the optimization of the reaction was
between 50 and 300 mg, whereas that to study the effect of the pH, the macrocycle, the metal and the
quantity of sensitizers on the formation of formic acid was 50 mg. The initial pH, except in the basic
(0.1 M NaOH) and neutral solution, was adjusted to 3 by addition of H3PO4. The reactor was tightly
sealed during the reaction, stirred continuously by a magnetic bar to prevent catalyst sedimentation and
the temperature inside the reactor was held at approximately 25 °C , due to a continuous circulation of
water in the jacket around the photoreactor. The photoreactivity runs lasted 8.0 h. Samples of 2.5 mL
volume were extracted from the suspension at the end of reaction and filtered through Teflon syringe
filters (1.0 mm, Alltech Associates). The quantitative determination of formic acid was performed by
measuring its conductivity with the Dionex DX120 Ion Chromatograph.
3.6.2. Process B
NaHCO3 solid (150 mg, 1.76 mmol) was mixed with 0.050 g of TiO2-CuPc catalyst (6.65 μmol/g
TiO2) and afterwards 15 mL of aqueous phosphoric acid solution at pH = 3 were added under vigorous
stirring in 15 min. Gaseous CO2 developed under these conditions. The resulting mixture was allowed
to react 8 h at 20–25 °C. The irradiation conditions were the same of those used for the process A.
The same procedure was brought using NaHCO3 13C labelled (150 mg, 1.76 mmol). Samples of 2.5 mL

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volume were extracted from the suspension at the end of reaction and filtered through Teflon syringe
filters (1.0 mm, Alltech Associates, Milano, Italy). The qualitative determination of formic acid was
performed by GC/MS working in the selected ion monitoring (SIM) mode.
4. Conclusions
In this paper, details of a low-cost, stable, effective catalyst capable of CO2 photoreduction with
production of formic acid has been described. TiO2 (anatase) samples impregnated with functionalized
metal phthalocyanines can be considered as a versatile class of photocatalysts that can be successfully
used not only for the photodegradation of organic micropollutants in aqueous suspension, but also for
the efficient photoreduction of CO2 under UV-Vis irradiation. To our knowledge, it is the first time
that Cu(II)–phthalocyanine samples are successfully used as sensitizers in photoreduction.
TiO2 (anatase) samples impregnated with functionalized copper phthalocyanines results to be 62%
more effective compared to bare TiO2 in the production of formic acid, an important product for its
multiple uses. The design of the composites has been inspired by previous studies focused on the
utilization of sensitized powders effective for photodegradation of 4-NP. The same materials were
tested for CO2 photo-reduction carried out in the absence of O2.
Negligible activity was observed with TiO2 sensitized porphyrins, although it can be excluded that
some types of these species could be effective. Further investigation is necessary to clarify the
mechanism based on reduction potential, which, if confirmed, could explain why the porphyrins are
less effective in the photoreduction than the phthalocyanines.
More details about the FOP and FOM values of the sensitizers could be important to design
TiO2 composites having photocatalytic properties oriented towards the production of ROS useful for
microorganic pollutants or CO2 photoreduction in water phases.
The absence of leaching, the mild conditions, the utilization of visible light, and the environmental
sustainability could stimulate the use of other hybrid photocatalysts, effective for photo-oxidation
reactions, to reduce CO2 to formic acid. It would be finally useful to broaden the present study to other
sensitizers supported on TiO2 both for the sake of comparison and to better understand some
mechanistic aspects.
Acknowledgments
L.P. and G.M. wish to acknowledge MIUR for economical support (PON01_02257); L.D., C.F.
and C.A. wish to thanks CNR ICCOM for economical support. D.B. is acknowledged for his
technical assistance.
Author Contributions
A.D.R., A.S. performed the synthesis, purification and characterization the porphyrins and
phthalocyanines used in this work. G.M., G.V. designed the composites TiO2 sensitizers. G.M., L.P.
designed the processes conditions. L.D. and C.F. designed the labeling experiments and analyzed
chemical assays. C.A. managed the analysis of labeled and unlabeled formic acid.

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Conflicts of Interest
The authors declare no conflict of interest.
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