Photooxidation mechanism of nitrogen-containing compounds at TiO2/H2O interfaces: An experimental and theoretical examination of hydrazine derivatives

Article abstract of DOI:10.1016/S0045-6535(99)00500-7

The photocatalytic oxidation of oxalyldihydrazide, N,N'- bis(hydrazocarbonyl)hydrazide, N,N'-bis(ethoxycarbonyl)hydrazide, malonyldihydrazide, N-malonyl-bis[(N'-ethoxycarbonyl)hydrazide] was examined in aqueous TiO₂ dispersions under UV illumination. The photomineralization of nitrogen and carbon atoms in the substrates into N₂ gas, NH₄/⁺ (and/or NO₃/^-) ions, and CO₂ gas was determined by HPLC and GC analysis. The formation of carboxylic acid intermediates also occurred in the photooxidation process. The photocatalytic mechanism is discussed on the basis of the experimental results, and with molecular orbital (MO) simulation of frontier electron density and point charge. Substrate carbonyl groups readily adsorb on the TiO₂ surface, and the bonds between carbonyl group carbon atoms and adjacent hydrazo group nitrogen atoms are cleaved predominantly in the initial photooxidation process. The hydrazo groups were photoconverted mainly into N₂ gas (in mineralization yields above 70%) and partially to NH₊⁴ ions (below 10%). The formation of NO⁺₃ ions was scarcely recognized. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0045-6535(99)00500-7

Chemosphere 41 (2000) 337±343
Photooxidation mechanism of nitrogen-containing compounds
at TiO /H O interfaces: an experimental and theoretical
examination of hydrazine derivatives
2
2
a
Kunio Waki , Jincai Zhao , Satoshi Horikoshi , Natsuko Watanabe ,
b
c
c
c,
*
Hisao Hidaka
a
Faculty of Engineering, Department of Applied Chemistry, Toyo University, 2100 Nakanodai, Kujirai, Kawagoe,
Saitama 350-0815, Japan
b
Institute of Photographic Chemistry, Chinese Academy of Sciences, Beishatan, Beijing 100101, People's Republic of China
Frontier Research Center for the Global Environment Protection, Meisei University, 2-1-1 Hodokubo, Hino, Tokyo 191-8506, Japan
c
Received 23 June 1999; accepted 6 October 1999
Abstract
0
0
The photocatalytic oxidation of oxalyldihydrazide, N; N -bis(hydrazocarbonyl)hydrazide, N; N -bis(ethoxycarbon-
0
yl)hydrazide, malonyldihydrazide, N-malonyl-bis[(N -ethoxycarbonyl)hydrazide] was examined in aqueous TiO
2
dis-
gas,
persions under UV illumination. The photomineralization of nitrogen and carbon atoms in the substrates into N
�
2
‡
2
NH (and/or NO ) ions, and CO gas was determined by HPLC and GC analysis. The formation of carboxylic acid
4
3
intermediates also occurred in the photooxidation process. The photocatalytic mechanism is discussed on the basis of
the experimental results, and with molecular orbital (MO) simulation of frontier electron density and point charge.
2
Substrate carbonyl groups readily adsorb on the TiO surface, and the bonds between carbonyl group carbon atoms
and adjacent hydrazo group nitrogen atoms are cleaved predominantly in the initial photooxidation process. The
‡
hydrazo groups were photoconverted mainly into N
�
2
gas (in mineralization yields above 70%) and partially to NH4
ions (below 10%). The formation of NO3 ions was scarcely recognized. Ó 2000 Elsevier Science Ltd. All rights
reserved.
Keywords: Photooxidation; Photodegradation; Hydrazine; Photocatalysis; Titanium dioxide; Gas-generating agent
‡
�
‡
�
1
. Introduction
NH and/or NO3 ions. The ratio of NH4 to NO3 for-
4
mation is dependent on the molecular structure of the
substrate. The photocatalytic decomposition of com-
pounds containing one or more hydrazo moieties has
rarely been reported.
The photodegradation of a series of nitrogen-con-
taining compounds has been investigated in our previ-
ous papers (Hidaka et al., 1994, 1996; Waki et al., 1995;
Nohara et al., 1996, 1997) and other publications (Low
et al., 1989, 1991; Pellizzetti et al., 1989, 1992, 1993). The
photomineralized products of nitrogen atoms in those
N-containing compounds examined were found to be
2
TiO particles absorb UV light with an energy greater
than the bandgap (ca. 3.2 eV) to generate electron/hole
‡
�
pairs. The holes (h ) are ultimately trapped by HO or
O at the TiO
H
2
2
particle surface to yield áOH radicals
and H . Concomitantly, dioxygen molecules react with
‡
�
electrons (e ) in the conduction band to produce su-
á�
*
peroxide radical anions, O , which combine with pro-
tons to generate hydroperoxy radicals (áOOH). The
Corresponding author.
E-mail address: hidaka@epfc.meisei-u.ac.jp (H. Hidaka).
2
0045-6535/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 ( 9 9 ) 0 0 5 0 0 - 7

3
38
K. Waki et al. / Chemosphere 41 (2000) 337±343
charge of the TiO
to an excess of protons from H
2
particle surface becomes positive due
O or from the solution
0.6% of the calculated values. The photocatalyst titani-
um dioxide was Degussa P-25 (particle size, 20±30 nm by
TEM; 83% anatase and 17% rutile by X-ray di€raction;
2
upon irradiation. Accordingly, the negatively charged
atoms of organic compounds may be adsorbed easily on
2
� 1
surface area, 53 m g by BET method).
the surface of TiO
and the áOH radicals photogenerated on the TiO
2
particles due to Coulombic forces,
sur-
2
2.2. Photodegradation procedures and analytical methods
face are expected to attack predominantly at the posi-
tions having the highest frontier electron density.
In this study, the photocatalytic mineralization of
compounds containing one or more hydrazo moieties,
For the photocatalyzed conversion of the gas-gener-
ating agents, an aqueous dispersion (50 ml) of the sub-
2
strates (1 mM) and 100 mg of TiO particles were placed
0
i.e., oxalyldihydrazide, N; N -bis(hydrazocarbonyl)-
in a 124 ml pyrex reactor; the mixture was then dispersed
by sonication for 5 min and then was illuminated with a
75 W Hg lamp under continuous agitation, which gave a
hydrazide, N; N-bis(ethoxycarbonyl)hydrazide, mal-
0
onyldihydrazide and N-malonyl-bis[(N -ethoxycarbonyl)
hydrazide] was examined under UV irradiation in
�
2
light intensity of 2.01 mW cm in the wavelength range
310±400 nm (the maximum emission: k ˆ 365 nm). The
headspace volume in the reaction vessel was purged with
oxygen for 15 min prior to irradiation.
‡
aqueous TiO
3
2
dispersions. The formation of NH4 (or
�
2 2
NO ) ions, the evolution of CO and N gases, and the
generation of carboxylic acid intermediates were moni-
tored during the photodegradation process. Simulation
of frontier electron density and point charge was also
carried out. Mechanistic steps are discussed from the
standpoint of the chemical structure of the substrates,
the point charges and the frontier electron densities. The
compounds investigated in this work are commonly used
as gas-generating agents. They have characteristic mo-
lecular structures, and therefore studies of their pho-
tooxidation should provide much information enabling
the details of the photocatalytic mechanism for N-con-
taining compounds to be understood.
2 2
The temporal evolution of N and CO gases was
monitored by gas chromatography with an Ookura
Riken chromatograph (model 802; TCD detector)
through a Molecular sieve 5A (for N
pack Q (for CO gas) column with helium as the carrier
2
gas) or a Pora-
2
‡
�
gas. The concentration of NH4 and NO3 ions formed
was assayed with a JASCO HPLC equipped chro-
matograph with a CD-5 conductivity detector using a
Y-521 cationic column or an I-524 anionic column.
Molecular orbital (MO) calculations were carried out
at the single determinant (Hartree±Fock) level with the
optimal conformation having minimum energy obtained
at the AM1 level (Nohara et al., 1997). All semi-
empirical calculations were performed with MOPAC
version 6 in the CAChe package implemented on a
Power Macintosh system. All MO calculations were
simulated for the initial processes of áOH radical attack
2
. Experimental
2
.1. Chemicals and reagents
Oxalyldihydrazide was supplied by Tokyo Kasei Ind.
and adsorption of molecules on the TiO
2
surface.
0
and was recrystallized from water. N; N -bis(hydrozo-
carbonyl)hydrazide was synthesized by addition of
bis(ethoxylcarbonyl) hydrazide to an excess of hydrazine
3. Results and discussion
(
10 times by weight) under slow agitation for 24 h (yield,
0
‡
�
5
was prepared by elimination of HCl from hydrazine
5%, m.p. 214°C). N; N -bis(ethoxycarbonyl)hydrazide
The formation of CO
2
, N
2
, NH4 and NO3 during
photodegradation of the substrates and the detected
intermediates of carboxylic acids in the photodegrada-
tion of hydrazine substrates is summarized in Table 1.
The photomineralization in the photodegradation of
oxalyldihydrazide is illustrated in Fig. 1. The apparent
(
1 mol) and ethyl chloroformate (2 mol) in the presence
of NaOH (2 mol). The precipitate was washed with iced
water to remove NaCl and then was dried (yield 80%).
The obtained powder was recrystallized twice from
ethanol (m.p. 214°C). Malonyldihydrazide was prepared
by the reaction of malonodiethyl ester with an excess of
hydrazine under stirring for 24 h. Recrystallization was
®rst-order rate constant of
N
2
gas formation
(k ˆ 1.1 ´ 10 min ) was higher than that of CO gas
(k ˆ 6.0 ´ 10 min ).
The two hydrazo moieties of oxalyldihydrazide were
converted to N gas with a yield of 81% after 4 h of il-
�
�
2
� 1
� 1
2
3
0
from water (m.p. 156°C). N-malonyl-bis[(N -ethoxycar-
bonyl)hydrazide] was prepared by elimination of HCl
from malonodihydrazide (1 mol) and ethyl chlorofor-
mate (2 mol) in the presence of NaOH (2 mol). The
sample was recrystallized with ethanol and tetra-
hydrofuran (yield 19%, m.p. 190°C). The purities of all
compounds prepared were checked by elemental analy-
sis of C, H and N atoms, which were found to be within
2
‡
lumination. NH4 ions were also generated in a small
�
amount (ca. 10%) and no formation of NO3 ions was
observed in the photodegradation process. The fronter
electron densities and point charges for oxalyldihyd-
razide were calculated to estimate the position adsorbed
of the substrates over TiO
2
catalyst as well as the

K. Waki et al. / Chemosphere 41 (2000) 337±343
339
Table 1
Formation of NH4 and NO3 ions, CO
‡
�
2
and N
2
gases, and intermediate carboxylic acids in the photodegradation of hydrazine
a
derivatives (1 mM) in aqueous TiO
Substrates
2
(100 mg) dispersions (50 ml)
Products (mM) after 4 h
Products (mM) after 8 h
Detected
‡
�
‡
�
carboxylic acids
N
2
CO
2
NH
NO
N
2
CO
2
NH
NO
4
3
4
3
Oxalyldihydrazide
1.64
1.20
0.53
2.42
0.84
1.74
0.37
0.25
0.01
0.30
0.03
0.01
0.02
0.001
0
1.68
2.52
0.68
1.68
1.87
1.32
1.36
3.89
2.80
3.37
0.44
0.58
0.01
0.30
0.05
0.02
0.20
0.002
0.01
0.02
HCOOH
(COOH)
2
HCOOH
0
N; N -bis(hydrazocarbonyl)
hydrazide
1.50
0.58
1.66
0.75
0
N; N -bis(ethoxycarbonyl)
hydrazide
Malonyldihydrazide
HCOOH
CH
HCOOH
CH (COOH)
HCOOH
3
COOH
2
2
0
N-malonyl-bis[(N -ethoxy-
carbonyl) hydrazide]
0
CH
CH
2
(COOH)
COOH
2
3
a
2 2
The amount of N and CO formed was calculated as moles from a liter of the dispersion.
Table 2
Frontier electron densities and point charge calculations of the
oxalyldihydrazide using the MOPAC system in the CAChe
package
Atom
Point charge
Frontier electron density
Cl
C2
0.256
0.257
0.298
0.295
0.282
0.282
0.223
0.218
0.150
0.017
0.018
0.014
0.020
0.014
0.020
O3
O4
)0.347
)0.347
)0.293
)0.291
)0.185
0.269
N5
‡
�
Fig. 1. Mineralization yield into CO
2
, N
2
, NH4 and NO3 in
the photocatalyzed decomposition of oxalyldihydrazide (1 mM)
N6
N7
in aqueous TiO
illumination.
2
(100 mg) dispersion (50 ml) under UV-
H9
H10
H11
H12
H13
H14
0.268
0.146
0.154
0.146
0.154
position attacked by áOH radicals. The atoms with the
largest negative point charges are O3 and O4, and the
two carbonyl moieties have the highest frontier electron
density (see Table 2). The O3 and O4 atoms are easily
adsorbed on the positive TiO
2
surface in the initial
of irradiation as shown in Table 1. The mineraliza-
tion yield into N gas was about 90% after illumination
of 8 h.
photoreaction. The subsequent reaction is attack of áOH
radicals on the two carbonyl moieties to generate formic
2
acid and oxalic acid, and ®nally to evolve CO
2
gas. It
The total mineralization yield of the N-atoms in
0
was presumed that the abstraction of hydrogen atoms
from the two nitrogen atoms by OH radicals occurs to
generate N gas after the cleavage of C±N bonds.
N; N -bis(hydrazocarbonyl)hydrazide into N gas and
2
‡
�
3
NH ions (no NO ions) was almost 100% and no other
4
2
N-containing organic intermediates were detected in the
degraded TiO dispersion. The MO calculation indicat-
0
Fig. 2 shows the photomineralization of N; N -
2
0
0
bis(hydrazocarbonyl)hydrazide. Since N; N -bis(hydra-
zocarbonyl)hydrazide has an additonal hydrazo moiety
ed that the adsorption of N; N -bis(hydrazocarbonyl)
hydrazide on the TiO surface occurs preferentially via
the two oxygen atoms (O5 and O6), and the two carbons
(C3 and C4) in the carbonyl group are easily attacked by
2
0
between the two carbonyl groups, N; N -bis(hydrazo-
carbonyl)hydrazide produced about 1.5 times more N
gas than oxalyldihydrazide by photooxidation after 8 h
2
OH radicals. It seems that the initial evolution of N gas
2

3
40
K. Waki et al. / Chemosphere 41 (2000) 337±343
is from the decomposition of the two terminal hydrazo
moieties, and subsequently the central hydrazo group is
both compounds was almost the same after irradiation
of 8 h. Only formic acid was detected as a carboxylic
acid intermediate in the photooxidation process.
2
to be converted into N because the frontier electron
density of the central N±N atoms is lower than that of
the terminal hydrazo moieties (see Table 3). The evolu-
0
tion of CO
was slower than that from oxalyldihydrazide within 6 h
2
from N; N -bis(hydrazocarbonyl)hydrazide
2
of irradiation, but the ®nal amount of CO evolved from
0
Fig. 3. Photomineralization of N; N -bis(ethoxycarbonyl)hy-
drazide. (The experimental conditions are the same as in Fig. 1.)
Table 4
Frontier electron densities and point charge calculations of the
hydrazo diethylester using the MOPAC system in the CAChe
package
0
Fig. 2. Photomineralization of N; N -bis(hydrazocarbonyl)hy-
drazide. (The experimental conditions are the same as in Fig. 1.)
Table 3
Frontier electron densities and point charge calculations of the
hydrazocarbodihydrazide using the MOPAC system in the
CAChe package
Atom
Point charge
Frontier electron density
N1
N2
)0.218
)0.219
0.735
0.736
0.104
)0.103
0.103
)0.103
)0.629
)0.389
)0.628
)0.389
0.229
0.229
0.075
0.073
0.042
0.042
0.035
0.075
0.073
0.045
0.042
0.035
0.290
0.290
0.356
0.351
0.003
0.001
0.003
0.001
0.305
0.043
0.304
0.043
0.004
0.003
0.001
0.001
0.000
0.000
0.001
0.001
0.001
0.000
0.000
0.001
C3
C4
C5
C6
Atom
Point charge
Frontier electron density
N1
N2
)0.230
)0.229
0.633
0.171
0.169
0.335
0.345
0.270
0.269
0.161
0.040
0.161
0.042
0.003
0.003
0.004
0.005
0.006
0.004
0.006
0.005
C7
C8
C3
C4
O9
0.633
0.654
O10
O11
O12
H13
H14
H15
H16
H17
H18
H19
H20
H21
H22
H23
H24
O5
O6
)0.653
)0.238
)0.340
)0.237
)0.340
0.221
N7
N8
N9
N10
H11
H12
H13
H14
H15
H16
H17
H18
0.219
0.219
0.198
0.191
0.218
0.198
0.191

K. Waki et al. / Chemosphere 41 (2000) 337±343
341
0
The mineralization yield for N; N -bis(ethoxycar-
low frontier electron density (0.059). The adsorption of
malonyldihydrazide on the TiO surface proceeds via the
two oxygen atoms. N was evolved rapidly since the
bonyl)hydrazide in a TiO
2
dispersion is depicted in
2
Fig. 3. The conversion yield to N
illumination for 8 h and the apparent ®rst-order rate
2
gas was 70% after
2
C±N bond is photocleaved more easily than the C±C
bond. The formation of carboxylic acid intermediates in
the photooxidation of malonydihydrazide was also de-
termined as shown in Fig. 4(b). Malonyldihydrazide was
photooxidized to generate formic acid and malonic acid,
constant was k ˆ 3.6 ´ 10� min . A high frontier elec-
3
� 1
tron density, and the most negative point charge in
0
N; N -bis(ethoxycarbonyl)hydrazide were at O9 and O11
as listed in Table 4. The formation of NH4 and NO3
ions was scarcely observed as shown in Fig. 3. The
‡
�
2
and ®nally to evolve CO . The concentration of malonic
photodecomposition of the ±CH
pected and evidenced to generate both acetic acid and
formic acid, and ®nally to evolve CO gas (see Table 1).
In the photodegradation of malonyldihydrazide, N
gas was rapidly evolved upon illumination for less than
h, but CO generation from the photooxidation of
2
CH
3
moiety was ex-
acid increased with increasing irradiation time to reach a
maximum at about 2 h of irradiation, and then decreased
2
2
Table 5
Frontier electron densities and point charge calculations of the
malono carbodihydrazide using the MOPAC system in the
CAChe package
2
2
carbon atoms in the substrate was observed to increase
gradually as shown in Fig. 4(a). The two oxygen atoms
have a large negative point charge, and the C2, C3, O4,
O5, N6 and N8 atoms possess a high frontier electron
density (see Table 5), resulting in easy attack by áOH
radicals. In contrast to oxalyldihydrazide, C1 had a very
Atom
Point charge
Frontier electron density
C1
C2
)0.212
0.269
0.269
0.059
0.291
0.289
0.224
0.224
0.256
0.141
0.248
0.135
0.008
0.008
0.020
0.013
0.027
0.020
0.014
0.024
C3
O4
)0.358
)0.358
)0.316
)0.185
)0.316
)0.184
0.147
O5
N6
N7
N8
N9
H10
H11
H12
H13
H14
H15
H16
H17
0.146
0.255
0.142
0.151
0.255
0.142
0.152
Fig. 4. (a) Photomineralization of malonyldihydrazide. (b)
Formation of intermediates of formic acid and amalonic acid in
the photooxidation of malonyldihydrazide. (The experimental
conditions are the same as in Fig. 1.)
0
Fig. 5. Photomineralization of N-malonyl-bis[(N -ethoxycar-
bonyl)hydrazide]. (The experimental conditions are the same as
in Fig. 1.)

3
42
K. Waki et al. / Chemosphere 41 (2000) 337±343
with further irradiation. Formic acid was also produced
in the photodegradation process. The amount of for-
mic acid formed was more than that of malonic acid,
but was also decomposed at longer irradiation times.
The photomineralization yields and the frontier electron
electron densities of the C2 and C3 atoms of the car-
bonyl groups are larger than those of all the other at-
oms. The initial photooxidation reaction proceeds easily
from the cleavage of either the C3±N8 or the C2±N6
0
bond in N-malonyl-bis[(N -ethoxycarbonyl)hydrazide].
0
density simulation for N-malonyl-bis[(N -ethoxycar-
2
The formation of N gas in the photodegradation of
bonyl)hydrazide] are depicted in Fig. 5 and Table 6.
The four carbonyl oxygen atoms have the most
negative point charges in the structure, and the frontier
this compound was lower than that of malonyldihyd-
razide within 4 h irradiation owing to the generation of
some complicated intermediates. As listed in Table 1, the
intermediates of formic acid, acetic acid and malonic
acid were also generated in the photooxidation process
Table 6
2
prior to CO evolution.
Frontier electron densities and point charge calculations of the
malono dicarbodihydrazide diethylester using the MOPAC
system in the CAChe package
4
. Conclusion
The nitrogen atoms in the hydrazine derivatives in-
vestigated were photocatalytically converted predomi-
nantly to N
than 70% and partially to NH4 ions with less than 10%
2
gas with a mineralization yield of more
‡
Atom
Point charge
Frontier electron density
�
mineralization yield. The formation of NO3 ions was
scarcely observed in the photooxidation of any sub-
strates used. The carbon atoms in the substrates were
C1
C2
)0.209
0.257
0.256
0.067
0.194
0.194
0.162
0.163
0.181
0.169
0.178
0.169
0.084
0.083
0.014
0.004
0.016
0.004
0.034
0.032
0.072
0.070
0.018
0.016
0.019
0.011
0.019
0.012
0.002
0.002
0.001
0.001
0.002
0.002
0.003
0.001
0.001
0.002
0.018
0.016
0.019
C3
O4
2
photomineralized into CO via intermediates of various
)0.275
)0.276
)0.251
)0.150
)0.248
)0.155
0.321
carboxylic acids. Owing to the higher point charges of
oxygen atoms in the carbonyl groups, adsorption of the
O5
N6
2
substrates onto the TiO surface takes place via the
N7
N8
carbonyl groups and the initial photooxidation proceeds
to cleavage of the bond between the carbonyl carbon
and the neighboring atoms by attack of áOH and/or
áOOH radicals. The methylene moiety between the two
carbonyl groups in malonyldihydrazide and N-malonyl-
N9
C10
C11
C12
C13
C14
C15
O16
O17
O18
O19
H20
H21
H22
H23
H24
H25
H26
H27
H28
H29
H30
H31
H32
H33
H34
H35
H20
H21
H22
0.317
)0.004
)0.221
)0.003
)0.221
)0.266
)0.265
)0.375
)0.375
0.137
0
bis[(N -ethoxycarbonyl)hydrazide] has a lower frontier
2
electron density and the generation of CO gas from the
methylene carbon was very slow, since the attack of áOH
radicals is dicult.
Acknowledgements
0.150
0.231
This research was supported by the Grants-in-aid for
Scienti®c Research from the Ministry of Education,
Science, Culture (no. 10640569) and from the Environ-
mental Protection Project Foundation from the Envi-
ronment Agency of Japan (to H. H). Financial support
from the National Natural Science Foundation of China
(no. 29677019 and no. 29725715) is also acknowledged
(by J. Z).
0.232
0.229
0.233
0.097
0.096
0.093
0.090
0.091
0.098
0.094
0.090
0.092
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0.091
0.137
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