Sustained production of H2O2 on irradiated TiO 2 - Fluoride systems

Article abstract of DOI:10.1039/b418789j

UV irradiation of fluorinated TiO₂ suspensions in water, in the presence of oxygen and a hole scavenger, leads to the production of H ₂O₂ with steady state concentration levels up to 1.3 millimolar; the H₂O₂ formation rate follows the TiO ₂ surface speciation, being maximum when the surface is completely covered by ≡Ti-F groups; these results outline the importance of surface speciation on the photocatalytic process. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b418789j

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Sustained production of H O on irradiated TiO – fluoride systems
2
2
2
Valter Maurino,* Claudio Minero, Giuseppe Mariella and Ezio Pelizzetti
Received (in Cambridge, UK) 15th December 2004, Accepted 18th March 2005
First published as an Advance Article on the web 6th April 2005
DOI: 10.1039/b418789j
UV irradiation of fluorinated TiO
2
suspensions in water, in the
chromatography or spectrophotometry). A careful cleaning of
TiO powder to eliminate organic and ionic impurities was carried
out by irradiating in air a TiO suspension for two days, then
presence of oxygen and a hole scavenger, leads to the
production of H O with steady state concentration levels up
2
2
2
2
washing the powder until no chloride ions were detected. The
H O was quantified by the peroxidase-catalysed oxidation of
2 2 2
to 1.3 millimolar; the H O formation rate follows the TiO
surface speciation, being maximum when the surface is
completely covered by MTi–F groups; these results outline the
importance of surface speciation on the photocatalytic process.
2 2
14a
14b
either p-phenylenediamine
when phenol is employed as hole scavenger).
The first striking feature of photocatalytic processes over TiO
in the presence of a hole scavenger is the production of H with
or phenol/4-aminoantipyrine
(
2
/F
In recent years photocatalytic reactions over semiconductor oxides
O
2 2
1
,2
have been investigated in depth. The formation of reactive
2
steady state concentration levels in the millimolar range (Fig. 1).
These concentrations are at least 100 times higher than those
reported so far. The maximum photonic efficiency attained for
?
2 2 2
oxygen species, namely OH , O , H O , during catalytic
processes on metal oxide semiconductors is of paramount
importance for practical application of both photocatalysis
2
H O photoproduction is 1.25% (pH 3.2, [HF] + [F ] 5 10 mM).
2
2
(e.g. water and air detoxification, self cleaning and self disinfecting
2 2
Without fluoride, the H O in the photocatalytic system is not
surfaces) and metal oxide catalysed oxidation of organic
detectable (below 0.1 mM). A possible role in the enhancement of
15
3,4
5–9
5,9,10
compounds. Formation of H O
and superoxide
particles has been reported. Concentration levels of
H O noticed so far are in the micromolar range. Although now
from
oxygen adsorption over TiO /F could be relevant. The second
2
2
2
irradiated TiO
2
feature is the bell shaped pH dependence of the initial rate of H O
2
2
2
2
production (Fig. 2). The maximum rate is obtained around pH 3.1
there is a consensus on the reductive mechanism involving O
2
and the curve is very similar to the dependence on the pH of MTi–F
coverage. Similar results were obtained by using phenol as hole
7,8
11
reaction with a conduction band electron (eCB), the detailed
pathway of H production and the influence of parameters like
2 2
O
2 2
scavenger. However, the ratio between the initial rates of H O
pH, the presence of anions, the nature and concentration of hole
scavenger, are still missing. Recently the strong influence of surface
adsorbed ions on the photocatalytic oxidation mechanism of
production and phenol degradation is half of that observed with
formic acid (0.16 vs. 0.31 at pH 3.2). This result can be explained
by the reducing ability of the radical generated by the one electron
oxidation of HCOOH, which could give the current doubling
effect in photoelectrochemical systems, or, as in the present case,
11–13
organic substrates over TiO
2
was demonstrated.
Surface
complexation by the redox inactive fluoride ion leads to an
increase in the degradation rate of organic substrates that react
2
16
produce an additional molecule of O
No H is formed in the absence of: 1) fluoride ions; 2) a hole
scavenger; 3) oxygen, even in the presence of fluoride and Ag as
2 2
from the reduction of O .
?
mainly through an OH radical mediated pathway, with a bell
2 2
O
11
shaped dependence on pH, reflecting the distribution of MTi–F;
kinetic analysis of competition experiments with different OH
a
+
?
scavengers allowed the quantification of the relative role of direct
?
electron transfer and mediated oxidation through OH radical (free
or adsorbed) in the photocatalytic degradation of phenol, showing
2
that over TiO /F the transformation proceeds almost entirely
?
through mediated oxidation by free OH , whereas on naked TiO
2
?
about 10% is due to a direct hole oxidation and the 90% to OH
12
(
adsorbed). In this note we present the results of the surface
fluorination of TiO on the reductive processes started by eCB
specifically the production of H O through oxygen reduction.
2
,
2
2
The irradiation experiments were carried out on 5 ml of aqueous
suspension containing the hole scavenger (formic acid or phenol)
2
1
and 0.5 g L of catalyst (TiO
0 W fluorescent lamp (TL K 40W 05 Philips, max. emission at
60 nm). Fluorides were added as HF. The pH before irradiation
was adjusted by adding HNO or NaOH solutions. Total photonic
2
P25 powder, Degussa), using a
4
3
3
25
21
flux was 1.0 6 10 Ein min . The filtered suspensions were
analysed by the appropriate analytical technique (HPLC, ion
Fig. 1
2 2
H O production in the photocatalytic degradation of formic acid
21
over fluorinated TiO
2
2 2
. Conditions: TiO 0.5 g L , HCOOH 1.0 6
2
2
22
*valter.maurino@unito.it
10 M, [HF] + [F ] 5 1.0 6 10 M. Air saturated suspension.
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Chem. Commun., 2005, 2627–2629 | 2627

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2
reduction by eCB, in the presence of F a release
produced from O
2
2?
2
2
in solution of HO /H
O
is achieved:
+
?
MTi–OH + O + e + H A MTi–OO + H O
2
CB
2
2
MTi–F + O + e A MTi–F + O2? (aq) A H O
2
CB
2
2
deg
ads
H O
2 2
The ratio between R
suggesting that the photocatalytic transformation of H
almost entirely the reaction of the surface MTi–OOH complexes
and not free H . Anions without surface complexing abilities
e.g. nitrate) do not lead to H formation. These results are
consistent with the reported production of H over irradiated
ZnO, the surface of which is not complexed by H
In conclusion, the modification of the TiO surface through
and R
is 19/22 # 0.86,
H
2
O
2
2 2
O involves
2 2
O
(
2 2
O
2 2
O
7
2 2
O .
2
Fig. 2 Influence of pH on the initial rate of the photocatalytic H
2
O
2
anion complexation has a strong influence on the reductive
pathways started by photogenerated eCB. The major effect of the
presence of oxygen as electron scavenger is the sustained
production of hydrogen peroxide, with steady state concentration
levels of 1–1.3 mM, nearly 100 times the levels reported so far.
Experimental results are explained in terms of a competition of the
fluoride with superoxide/peroxide species for the surface sites of
TiO , thus inhibiting H O degradation.{
production. Data are obtained from experiments like those reported in
Fig. 1.
electron scavenger. On the other hand, the loss of H
photocatalytic conditions is inhibited by the presence of fluoride.
Fig. 3 reports the disappearance curves of H at pH 4. The ratio
of the initial rate of H disappearance in the absence and in the
presence of fluoride ions (1 6 10 M) is R
The ability of peroxides to complex Ti(IV) compounds and the
2 2
O under
2 2
O
2 2
O
2
2 2
22
deg
5 19.
2 2
H O
The authors are grateful to MIUR (PRIN03, Contract No.
20030335534) and Universit a` di Torino (Ricerca locale) for
financial support.
17
1
8
surface of titanium dioxide has long been known. However, no
equilibrium adsorption data are reported. Recently it was noticed
that there was a pH dependence of the formation of MTi–OOH
Valter Maurino,* Claudio Minero, Giuseppe Mariella and Ezio Pelizzetti
Dipartimento di Chimica Analitica – Universit a` di Torino and NIS,
Nanostructured Interfaces and Surfaces Centre of Excellence, Via P.
Giuria 5, 10125, Torino, Italy. E-mail: valter.maurino@unito.it;
Fax: ++39 011 670734; Tel: ++39 011 6707615
1
9
species. The insert of Fig. 3 reports the adsorption isotherms of
H O at pH 4 over TiO P25 in the absence and in the presence of
2
2
2
2
2
fluoride ions (1 6 10 M). The competition of fluoride with
for the TiO surface sites is evident. Interestingly, the ratio of
adsorbed in the absence and in the presence of fluoride,
H O
2 2
2
the H
2 2
O
Notes and references
2
3
ads
when [H
2
O
2
]
free 5 1 6 10 M, is R H O # 22 (Fig. 3, insert).
2
2
{
Presented in part at ‘‘XIV Congresso Nazionale di Catalisi – GIC2004’’,
Lerici, La Spezia, Italy, 6–10 June 2004 and ‘‘The 9th Conference on TiO
Photocatalysis’’, San Diego, CA, USA, 24–28 October 2004.
Thus, a possible role of the redox inert ligand is the inhibition of
the formation of surface superoxo/peroxo species. When these are
2
1
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Fig. 3 Photocatalytic degradation of H
presence and in the absence of fluoride. Conditions: TiO
pH 4.0. Air saturated suspension. Insert: H Adsorption isotherm on
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O (C 5 1 6 10 M) in the
21
2
0.5 g L ,
1
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O
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(
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g L , pH 4.0.
2
628 | Chem. Commun., 2005, 2627–2629
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