The Existence of Nitrate Radicals in Irradiated TiO2 Aqueous Suspensions in the Presence of Nitrate Ions

Article abstract of DOI:10.1002/anie.201804879

Evidence of the existence of nitrate radical in irradiated aqueous TiO₂ suspensions in the presence of nitrate ions are reported for the first time. The joint use of UV/Vis and EPR spectroscopy showed that nitrate radicals are formed by hole induced oxidation of nitrate ions. Photocatalytic degradation of a model alkene compound allowed to highlight the presence of an intermediate organic nitrate deriving from nitrate radical attack to the double bond of the substrate. These results not only allow deeper understanding of photocatalytic processes, but open the route to new green photocatalytic syntheses initiated by nitrate radicals and to new insights in the field of atmospheric chemistry.

Full text of DOI:10.1002/anie.201804879

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Accepted Article
Title: On the existence of nitrate radicals in irradiated TiO2 aqueous
suspensions in the presence of nitrate ions
Authors: Francesco Parrino, Stefano Livraghi, Elio Giamello, and
Leonardo Palmisano
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201804879
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Angewandte Chemie International Edition
10.1002/anie.201804879
COMMUNICATION
On the existence of nitrate radicals in irradiated TiO aqueous
2
suspensions in the presence of nitrate ions
[b]
*
[a]
[b]
[a]
Francesco Parrino , Stefano Livraghi , Elio Giamello , Leonardo Palmisano
Abstract: Some evidences of the existence of nitrate radical in
irradiated aqueous TiO suspensions in the presence of nitrate ions
prove, the hole scavenging effect of nitrate ions according to
equation (7)
2
are reported for the first time. The joint use of UV-vis and EPR
spectroscopy evidenced that nitrate radicals are formed by hole
induced oxidation of nitrate ions. Photocatalytic degradation of a
model alkene compound allowed to highlight the presence of an
intermediate organic nitrate deriving from nitrate radical attack to the
double bond of the substrate. These results not only allow deeper
understanding of photocatalytic processes, but open the route to
new green photocatalytic syntheses initiated by nitrate radicals and
to new insights in the field of atmospheric chemistry.
+
•
-
H
2
O
+ NO
3
→ H
2
O + NO
3
(7)
+
•
where H O may be formally considered as a hydrated hole, i.e.
a precursor of the OH radicals. The occurrence of this reaction
has been recently supported by picoseconds pulse radiolysis
experiments.
Although nitrate radical is one of the first species
spectroscopically observed ever,
2
[
14]
[
15]
its potentialities as an
oxidant in selective synthetic processes are under investigation.
To this aim few methods to allow nitrate radical accessibility on a
preparative scale have been proposed.
It is known that hydroxyl radical (OH) dominates the daytime
chemistry of the atmosphere. However, nitrate radical (NO ) is
3
[16,11]
[
1]
Heterogeneous photocatalysis has attracted attention as a
the most important nocturnal free oxidant radical responsible for
many oxidative reactions at night-time. In the earth's atmosphere,
sustainable tool for the green synthesis of valuable
[17]
compounds.
Reduction of nitrate ions is known to occur by
[
2]
NO
3
is mainly formed by reaction of NO
2
with ozone according
means of heterogeneous photocatalysis in anaerobic conditions,
to equation (1) or by dissociation of N
2
O
5
[Eq. (2)].
[18]
in the presence of hole scavengers
doped photocatalysts.
or with noble metals
[
19]
On the other hand, oxygen efficiently
NO
2
+ O
3
→ NO
3
+ O
2
(1)
(2)
competes with nitrate ions for the photogenerated electrons so
that nitrate ions concentration remains virtually unaltered in
N
2
O
5
→ NO
3
+ NO
2
irradiated TiO
2
aqueous suspensions [see the following Eqs. (8),
In general, NO
3
is less reactive than OH with volatile organic
[20]
(
13)-(15)]. The presence of nitrate ions only slightly influences
[
4]
compounds. Its oxidation potential (+2.00 V vs. SCE in
[21]
the photocatalytic degradation of many organic pollutants and
[
5]
[6]
[7]
[8]
MeCN) allows oxidation of alkenes, alcohols, esters,
[22]
the bactericidal activity of TiO
of our knowledge, nitrate ions have been considered as an “inert”
species in aqueous oxygenated TiO photocatalytic suspensions.
To get direct evidences of the presence of nitrate radicals in
irradiated TiO suspensions, UV-vis spectra of a rear side
irradiated mixture containing H O, P25 TiO (Evonik) and O with
2
. For these reasons, to the best
[
9]
[10]
ethers, and amines.
These reactions generally proceed
through direct electron transfer, by addition to π systems and by
2
[
11]
hydrogen atom abstraction.
It is reported that OH radicals react only with undissociated
2
[12]
nitric acid according to equation (3).
HNO + OH → NO + H
NO radical formation has been reported
2
2
2
or without nitrate ions have been recorded (see Figure S1 for
experimental details). Results are shown in Figure 1.
3
3
2
O
(3)
[
13]
3
to occur by pulse
radiolysis according to equations (4),(5) or by means of equation
6):
(
-
-
NO
3
→ NO
2
+ O
(4)
(5)
(6)
-
+
O + NO
3
3
+ H → NO + OH
3
+ e
-
-
NO
3
→ NO
[12a]
Katsumura et al.
mentioned, without giving a conclusive
[
a]
b]
Dr. F. Parrino*, Prof. L. Palmisano
Dipartimento di Energia, Ingegneria dell’Informazione e Modelli
Matematici (DEIM), University of Palermo, viale delle Scienze Ed. 6,
90128 Palermo (Italy)
*
E-mail: francesco.parrino@unipa.it
[
Dr. Stefano Livraghi, Prof. Elio Giamello
Figure 1. UV-vis spectra obtained on rear side illuminated mixture of water,
Dipartimento di Chimica and NIS, University of Torino, Via P. Giuria
2
TiO P25 in the presence of oxygen without (A) and with (B,C) nitrate ions.
7, 10125 Torino (Italy).
Spectrum C is obtained under the same experimental conditions of spectrum
B, but at a lower temperature (sample frozen by pouring liquid nitrogen).
Supporting information for this article is given via a link at the end of the
document.
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Angewandte Chemie International Edition
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COMMUNICATION
Spectrum A has been obtained when the sample consisting
Irradiation caused a steep increase of the signal for all of the
curves, indicating the photocatalytic production of OH radicals
according to equations (8)-(12).
of a mixture of TiO
blown with pure oxygen during 15 minutes, was irradiated from
the rear side and simultaneously analyzed against a P25 TiO
2
P25 and few drops of water, previously
2
–
–
O
2
+ e → O
2
(8)
reference. In the region between 420 and 700 nm no absorption
occurred. The LED irradiating from the rear side did not emit in
this range so that, although photons could reach the front side of
the sample, they did not influence the analysis. The wavelength
range in Figure 1 starts from 420 nm because signals below this
value derive from the LED emission. Blowing oxygen on the
sample prior to analysis was necessary in order to remove the
Ti centres. The latter ones would be otherwise formed under
irradiation, thus producing their characteristic bands in the
visible range. The presence of nitrate ions in the sample
–
+
O + H → HO
2
2
(9)
HO
2
+ HO
2
→ H
2
O
2
+ O
2
(10)
(11)
(12)
–
2
–
H
2
2
O
2
+ O
→ OH + OH + O
2
+
+
H
O + h → H + OH
3
+
The main information derived from the experiments of spin
trapping in Figure 2 are the following:
a)
Increasing the pH of the suspension results in a
(
spectra B and C) increased the absorbance in the range
generally higher amounts of detected hydroxyl radicals. This
may be, in principle, attributed to two distinct factors: (i) the OH
radicals trapping efficiency of DMPO is lower at acidic pH
between 430 and 700 nm. Notably, spectrum B is obtained at
ambient temperature while spectrum C is obtained on a sample
previously frozen by pouring liquid nitrogen on its surface.
[
23]
values,
and (ii) the OH radicals photogeneration increases
[24]
None of the species which, according to literature, may be
when pH rises from 3 to 7.
2
produced when TiO is irradiated in the presence of water and
b) In the initial steps of irradiation (from 0 to ca. 30-50 s)
nitrate, absorbs in this region (with the exception of blue
3
+
the hydroxyl radical generation rate is slightly higher in the
presence of nitrate ions than sulphate ions. This occurs for all of
the considered pH values.
coloured Ti species whose presence, as above mentioned, can
be neglected under the experimental conditions used). Spectra
B and C show more than 20 bands from 420 to 700 nm which
are in good agreement with those reported in the literature for
nitrate radical in gas or condensed phase (see Table S1 in the
Supporting Information).
c)
Upon further irradiation the two curves diverge. While
in the presence of sulphate ions, the OH radical production
increases attaining a plateau, a net decrease of the OH radical
detection is observed in the presence of nitrate ions. This effect
is more evident at pHs 4 and 6.5.
Figure 2 shows the trend of the EPR signal intensity versus
time corresponding to the DMPO-OH adduct [DMPO: 5,5-
dimethyl-1-pyrroline-N-oxide, Eq. (S1), Supporting Information]
Therefore, it can be concluded that the presence of nitrate
ions limits the production of OH radicals or, alternatively, nitrate
ions act as scavengers of the radical species.
2
for TiO suspensions in the presence of 4mM nitrate (black
curves) and 4mM sulphate (red curves) at pH 2.5, 4.0 and 6.5.
EPR intensity values have been recorded under irradiation
during ca. 200 seconds and in the dark for the remaining time. It
is worth to remind that a higher EPR intensity indicates higher
amount of hydroxyl radicals detected.
Nitrate radical formation by direct hydroxyl radical attack to
undissociated nitric acid [Eq. (3)] may be considered negligible
in the present experimental conditions (diluted HNO
3
, 4mM).
On the other hand, nitrate might be firstly reduced by the
-
photogenerated electrons (e CB) to NO
2
[Eq. (13)] which in turn
acts as the OH radical sink [Eq. (14)] affording peroxynitrite
+
-
(
(
HO
15)].
2
NO). The latter fastly decomposes back to H and NO
3
[Eq.
-
+
-
NO
3
2
2
+ 2H + e CB → NO
2
+ H
2
O
(13)
(14)
(15)
NO
HO
+ HO → HO
2
NO
+
-
NO → H + NO
3
If the latter mechanism holds, the rate of OH radicals decay
should depend on both pH and OH radical concentration. The
rate of disappearance of OH radicals in the presence of nitrate
ions is not accessible from Figure 2. In fact, although a decrease
of OH radicals concentration may be easily observed, their
generation simultaneously occurs under constant irradiation.
The slope of the curve in Figure 2 may be strongly affected by
the fact that the trapping efficiency of DMPO increases with
[
23]
Figure 2. EPR signals intensity versus time for TiO
presence of 4 mM nitrate (squares) and 4 mM sulphate (circles) ions at pH
.5, 4.0, and 6.5.
2
suspensions in the
increasing pH.
Moreover, the OH radicals generation and
related detection via spin trapping is more efficient at higher pH
values. These factors somehow mask the pH dependence of
[
24]
2
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Angewandte Chemie International Edition
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COMMUNICATION
-
+•
the OH radical decay in the presence of nitrate.
NO
3
+ Ti-OH → NO
3
+ Ti-OH
(17)
At constant pH the OH radical concentration at a given time
thereby competing with the formation of OH by reaction with
water [Eq. (12)]. The competition between water and nitrate ions
for the hole induced oxidation, previously reported in literature in
is determined by two opposite processes, i.e. OH generation
-
and OH consumption. By considering that the SO
4
radical is
[14]
produced only by reduction of peroxodisulfate ion with various
other systems, might be effective also in the present system
(at pH values lower than the zero point charge of TiO , i.e. ca.
[25]
one-electron reductants or via direct photolysis, the presence
of sulphate ions may be assumed as ineffective on OH
consumption. In fact, OH radical trapping experiments carried
out at different sulphate ions concentration (see Figure S2,
Supporting information) did not show significant differences in
terms of detected OH radicals. The effect of nitrate ions on the
consumption of OH radicals can be therefore simply expressed
as the difference between the curves in the presence of sulphate
and those in the presence of nitrate ions reported in each panel
of Figure 2. The results of this procedure are shown in Figure 3.
2
6.5). In fact, even if water is present in very large excess, the
density of nitrate anions on the positively charged surface can
be very high. Notably, both equations (16) and (17) imply a
dependence of the slope of the curves in Figure 3 from nitrate
ions concentration. Accordingly, increasing nitrate ions
concentration resulted in increased slope, keeping constant
other experimental conditions (Figure S3, Supporting
information). In fact, competition with water for the
photogenerated holes is higher
at
higher nitrate ions
concentration, thus producing faster decline of the amount of
detected OH radicals.
0
0
.01
.00
□
∆
□
Nitrate ions hole trapping effect has been recently
demonstrated by picoseconds pulse radiolysis measurements in
○
□
∆
○
−
−
[14]
pH = 2.5
○
∆
□○
the case of Cl and Br concentrated solutions. The capability
○
○
□
∆
○
□
∆
□
○
○
+•
∆
of H
equation (7) is reported to occur in ultrafast regime
compete with OH radical generation by water oxidation.
2
O
radical cation to oxidize nitrate ions according to
∆
[14]
-0.01
and to
□
pH = 6.5
□
[26]
∆
Recently, Shkrob et al.
3
reported NO radicals formation by
-0.02
pH = 4 ∆
hole scavenging of nitrate ions in imidazolium ionic liquids.
0
50
100
150
200
Equations (7), (16), and (17) allow to justify the results
shown in Figure 3, i.e. that the OH radical decay does not
depend on the OH radical concentration and pH. In fact, OH
radicals are not directly consumed by direct reaction with nitrate,
but through reaction of the latter with photogenerated holes.
Time / s
Figure 3. Difference between the EPR signals recorded in the presence of
nitrate and sulphate ions versus irradiation time at pH = 2.5 (circles), 4
triangles), and 6.5 (squares).
(
Therefore, the presence of nitrate radicals in irradiated TiO
2
aqueous suspensions in the presence of nitrate ions can be
proposed also on the basis of EPR indirect evidences. The EPR
The differences of the EPR signals in the presence of nitrate
and sulphate ions, reported in Figure
dependence with time. Remarkably, two of the lines (pH 4 and
.5) have the same slope which is also that of the first two points
recorded at pH 2.5. In this case, in fact, the slope changes after
0 seconds of irradiation, due to the lower trapping efficiency of
3
show
a
linear
3
signal of NO , in fact, could not be obtained in the present
working conditions as the direct observation of this radical
species has been performed at very low temperature in the case
6
[3]
of an irradiated crystalline solid.
8
Figure 4 reports the results of the photocatalytic oxidation of
DMPO at this acidic pH. The slope of the curves represents the
rate of the overall process consuming OH radicals in the
presence of nitrate ions. As the slope does not significantly
change with pH, the pH dependence of the OH radical decay in
the presence of nitrate ions may be considered negligible.
Therefore, the mechanism reported in equation (13)-(15) seems
to be less probable in these conditions as it would imply a pH
dependent OH radical consumption.
2 3 2 4
phenol in the presence of TiO P25 and HNO or H SO .
1.2
1
.0
0.8
HNO
3
0
0
0
.6
.4
.2
H
2
SO
4
Moreover, as the points in Figure 3 may be satisfactorily
fitted by a straight line in all cases, the kinetics of OH radicals
decay during irradiation is of zero order with respect to OH
radicals concentration. This suggests that the observed OH
radical decrease is not due to their direct reaction with nitrate
ions but rather to the reaction of the latters with a precursor of
0
.0
0
10
20
30
40
50
60
70
Time / min
+
OH radical i.e. with a photogenerated hole, in its free form [h ,
+
•
Figure 4. Normalized phenol concentration versus irradiation time in the
-1
2
Eq. (16)], hydrated [H O , Eq. (7)], or trapped on an active site
presence of TiO
squares) and H
2
P25 (0.5 g L ) at pH ca. 1.5 in the presence of HNO
3
(empty
+
•
at the TiO
2
surface [Ti-OH , Eq. (17)],
2
SO (full squares). Initial phenol concentration, C = 0.5 mM.
4
0
-
+
NO
3
+ h → NO
3
(16)
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COMMUNICATION
It is evident that the presence of sulphuric or nitric acid did
not significantly influence the photocatalytic degradation rate of
phenol. Although this finding is in accord with the relevant
whose mass spectrum was compatible with a nitric acid alkyl
ester was detected (see Figure S4, Supporting information). This
compound may be possibly obtained by preferential NO radical
3
[
21]
literature, it appears in conflict with the above reported EPR
results (Figure 2). In fact, the hydroxyl radical concentration
increases in the presence of sulphate ions eventually reaching a
plateau. On the contrary, it initially increases in the presence of
nitrate ions and after few seconds monotonically decreases. By
taking into account that the photocatalytic degradation of phenol
addition to the endocyclic double bond of limonene or of
possible unsaturated intermediates photocatalytically generated
[
31]
(Figure 5B).
In any case, this result is an additional
demonstration of the existence of nitrate radicals in the presence
of nitrate ions on irradiated semiconductors.
[27]
The presence of nitrate radicals may provide an additional
occurs mainly through hydroxyl radical attack,
the
[20]
mechanistic hypothesis to that reported by Parrino et al.
for
photocatalytic degradation rate should dramatically decrease in
the presence of nitric acid with respect to sulphuric acid.
Therefore, some other reaction paths involving nitrate ions must
be hypothesized to justify the results shown in Figure 4. The
presence of nitrate radicals may suggest an explanation. In fact,
the oxidizing power of nitrate radicals is close to that of hydroxyl
radicals. Furthermore, both nitrate and hydroxyl radical attacks
the photocatalytic bromine formation in the presence of nitrate at
acidic pHs. By taking into account the above mentioned
considerations, another possible “first step” might be the nitrate
ions oxidation to nitrate radical. Hole transfer to the nitrate ions
may be favoured by considering the positively charged TiO
2
surface at the given acidic conditions and the consequently
higher nitrate (and bromide ions) density close to the surface
with respect to the bulk. Furthermore, bromide ions are reported
[
28]
proceed through hydrogen abstraction
so that the oxidation
products in the two cases are possibly the same. Therefore, the
reaction mechanism may be very different from that occurring in
the presence of sulphuric acid, although the presence of nitrate
does not macroscopically affect the phenol degradation.
Notably, traces of paranitrophenol have been detected in the
[
32]
as efficient nitrate radical traps because of their fast reaction
affording elemental bromine [Eqs. (18)-(19)].
-
-
NO + Br → NO + Br
3
3
(18)
(19)
2
Br → Br
2
3
presence of HNO only within the first 10 minutes of reaction but
not at longer irradiation times. This is in agreement with Djengel
2
The existence of nitrate radicals in irradiated TiO suspensions
[29]
et al.
taken after 1 hour irradiation. Nitration may occur through NO
radicals generated according to equation (13), as described by
which did not observe nitration of phenol in samples
in the presence of nitrate ions can be firmly proposed on the
basis of UV-vis and EPR spectroscopy results. Furthermore,
photocatalytic tests in the presence of a model olefin allowed to
highlight the specific nitrate radical attack to the double bond of
the substrate affording the corresponding organic nitrate.
Formation of nitrate radical may be hypothesized to occur
through direct or mediated hole induced oxidation of nitrate ions.
This finding not only provides a deeper understanding of
photocatalytic processes, but it interdisciplinarily highlights the
possibility of nitrate radical production in mild conditions with
dramatic consequences in the field of atmospheric Chemistry,
green organic synthesis and environmental remediation.
2
[
30]
Bedini et al. , but also through NO
abstraction
3
initiated hydrogen
2
and subsequent NO addition. The probability of
[
28]
this occurrence could be very low in photocatalytic aqueous
systems, thus explaining the trace amounts of nitrophenol
detected.
Different is the case of the photocatalytic degradation of an
alkene like limonene (Figure 5A). In fact, NO radicals, unlike
3
OH radicals, are reported to add to double bond giving rise to
organic nitrates.
O
O
N
O
+
NO
B
3
Keywords: Photocatalysis • Radicals • Nitrate • TiO
2
A
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C=C double bond.
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[
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[
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3
from reaction of NO and limonene were not available, a species
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Angewandte Chemie International Edition
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COMMUNICATION
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Angewandte Chemie International Edition
10.1002/anie.201804879
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Evidences of the existence of
nitrate radicals in irradiated
Francesco Parrino*, Stefano
Livraghi, Elio Giamello, Leonardo
Palmisano
aqueous TiO
2
suspensions in
the presence of nitrate ions are
reported for the first time. The
joint use of UV-vis and EPR
spectroscopy evidenced that
nitrate radicals are formed by
direct or indirect oxidation of
nitrate ions by photogenerated
holes.
Page No. – Page No.
On the existence of nitrate
radicals in irradiated TiO
aqueous suspensions in the
presence of nitrate ions
2
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