Photocatalytic synthesis of urea from in situ generated ammonia and carbon dioxide

Article abstract of DOI:10.1111/j.1751-1097.2011.01037.x

TiO₂ and Fe-titanate (different wt%) supported on zeolite were prepared by sol-gel and solid-state dispersion methods. The photocatalysts prepared were characterized by X-ray diffraction, scanning electron microscopy and ultraviolet (UV)-visible diffuse reflectance spectroscopy techniques. Photocatalytic reduction of nitrate in water and isopropanol/oxalic acid as hole scavengers are investigated in a batch reactor under UV illumination. The yield of urea increased notably when the catalysts were supported on zeolite. The Fe-titanate supported catalyst promotes the charge separation that contributes to an increase in selective formation of urea. The product formation is because of the high adsorption of in situ generated CO₂ and NH₃ over shape-selective property of the zeolite in the composite photocatalyst. The maximum yield of urea is found to be 18 ppm while 1% isopropanol containing solution over 10 wt% Fe-titanate/HZSM-5 photocatalyst was used.

Full text of DOI:10.1111/j.1751-1097.2011.01037.x

Photochemistry and Photobiology, 2012, 88: 233–241
Photocatalytic Synthesis of Urea from in situ Generated Ammonia and
Carbon Dioxide
1
Basavaraju Srinivas , Valluri Durga Kumari , Gullapelli Sadanandam , Chilumula Hymavathi ,
*1
1
1
1
Machiraju Subrahmanyam and Bhudev Ranjan De
2
1
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad, India
Department of Chemistry and Chemical Technology, Vidyasagar University, Midnapur, West Bengal, India
2
Received 13 July 2011, accepted 6 November 2011, DOI: 10.1111/j.1751-1097.2011.01037.x
temperature (180ꢀ–185ꢀC) to form ammonium carbamate,
which is dehydrated subsequently into urea and water.
ABSTRACT
TiO and Fe-titanate (different wt%) supported on zeolite were
2
prepared by sol-gel and solid-state dispersion methods. The
photocatalysts prepared were characterized by X-ray diffraction,
scanning electron microscopy and ultraviolet (UV)–visible diffuse
reflectance spectroscopy techniques. Photocatalytic reduction of
nitrate in water and isopropanol ⁄ oxalic acid as hole scavengers
are investigated in a batch reactor under UV illumination. The
yield of urea increased notably when the catalysts were supported
on zeolite. The Fe-titanate supported catalyst promotes the
charge separation that contributes to an increase in selective
formation of urea. The product formation is because of the high
Pressure
NH3 þ CO2 ꢀ! H2NCOONH4 ꢀ! COðNH2Þ :
ꢀH2O
2
2
Temp:
Therefore, it is of high basic science interest to develop a
simplified method for the production of nitrogen containing
fertilizers, especially at a small to middle scale of operation. A
single-step process that can convert CO , N and H sources to
2
2
2
nitrogen fertilizers at decreased pressure and ⁄ or temperature
has the potential for development. The report of Shibata et al.
(16) is worth mentioning here where urea was synthesized by
ꢀ
electrochemical reduction of CO in the presence of NO using
adsorption of in situ generated CO
2
and NH
3
over shape-
2
2
a Cu-loaded gas diffusion electrode. Furthermore, the photo-
catalytic synthesis process reported in the literature allows urea
formation under room temperature and pressure (17–19). The
selective property of the zeolite in the composite photocatalyst.
The maximum yield of urea is found to be 18 ppm while 1%
isopropanol containing solution over 10 wt% Fe-titanate ⁄
HZSM-5 photocatalyst was used.
ꢀ
photo-induced conversion of CO
2
and NO3 to urea was
attempted under many permutations and combinations of
photocatalysts and solvents. Here, we have investigated the
photocatalytic reduction of CO2 from CO -purged gas, or
INTRODUCTION
2
2
in situ-generated CO through mineralization of isopropyl
The major industrial source of nitrate and nitrite ions are in
steel mill, fertilizer and explosives plants, mines, paper mills
and food production. They are the common pollutants in
water and remediation by photocatalytic mineralization using
semiconductors is one of the possible methods (1–15). How-
ever, instead of mineralization, transforming into useful
chemicals is a challenging task. In the present work, transfor-
alcohol ⁄ oxalic acid in the presence of TiO -based semicon-
2
ductor-supported zeolite systems in aqueous medium at room
temperature under normal pressure conditions.
MATERIALS AND METHODS
2
)1
2
Materials. TiO P-25 (anatase-80%, rutile-20%, surface area 50 m g
)1
mations of NO , CO and H O into urea using photocatalysis
2
3
2
and particle size 27 nm) from Degussa Corporation, Germany;
Fe(NO3)2 is procured from Loba Chemie Pvt. Ltd., India. Titanium
isopropoxide was from Fluka. Acetonitrile used was HPLC grade from
Rankem, India. Zeolites are procured from SUD-CHIEMIE, India.
technique is explored.
Industrial production of ammonia is mainly based on the
Haber process, which involves the heterogeneous reaction of
nitrogen and hydrogen on an iron-based catalyst at high
pressure (150–300 atm) and temperature (400ꢀ–500ꢀC). The
equilibrium conversion of hydrogen and nitrogen gas into
ammonia in the process is only in the order of 10–15%. Such
low conversion efficiencies in large-scale chemical plants
warrant costly operating conditions required to commercially
produce hundreds to thousands of tons per day of ammonia.
Urea is commercially produced via the reaction of carbon
dioxide and ammonia at high pressure (100–200 atm) and
2
Techniques. 1 About 2, 5, 10 and 15 wt% of TiO were loaded on
HZSM-5 by solid-state dispersion method. Required amount of TiO
2
(P-25) was added to HZSM-5 (Si ⁄ Al = 30) powder and thoroughly
mixed in a mortar and pestle. Minimum amount of ethanol was added
to it and mixed thoroughly. The sample was evaporated to dryness at
)
1
1
10ꢀC and calcined at 500ꢀC for 5 h at the rate of 2ꢀC min
.
Fe TiO and Fe Ti were prepared by sol-gel method. Ferric
2
2
5
2
2 7
O
nitrate and titanium-isopropoxide (Fe ⁄ Ti = 2:1 and 1:1) were dis-
solved in required quantity of isopropanol with constant stirring and
dilute HNO was added to form a sol and fine gel was obtained in two-
3
and-a-half days. The gel thus obtained was dried at 110ꢀC and calcined
)1
at 600–700ꢀC for 5 h at the rate of 2ꢀC min . The surface ratio of
Fe ⁄ Ti was estimated by scanning electron microscopy (SEM)-energy
dispersive X-ray spectroscopy (EDAX) technique and the overall
composition (bulk) was estimated by atomic absorption spectroscopy
(AAS) technique.
*
Corresponding author email: durgakumari@iict.res.in (Valluri Durga Kumari)
2011 Wiley Periodicals, Inc.
ꢁ
Photochemistry and Photobiology ꢁ 2011 The American Society of Photobiology 0031-8655/12
2
33

2
34 Basavaraju Srinivas et al.
3
dispersion method. About 5, 10 and 15 wt% of Fe
Fe
2
TiO
5
and Fe
2
Ti
2
O
7
⁄ HZSM-5 were prepared by solid-state
TiO and Fe Ti
very low conversions of nitrate were observed with no scaven-
ger. Moreover, the absence of carbon source was lacking in the
electrolyte, consequently no urea formation was observed.
2
5
2
2 7
O
systems were added to HZSM-5 (Si ⁄ Al = 30) powder and thoroughly
mixed in a mortar and pestle. Minimum amount of ethanol was added
and mixed thoroughly. The sample was evaporated at 110ꢀC and
)
1
calcined at 500ꢀC for 5 h at the rate of 2ꢀC min
Characterization. The X-ray diffraction (XRD) patterns of the
TiO2, Fe TiO and Fe Ti and their respective samples loaded on
.
ꢀ
NO and oxalic acid in urea synthesis
3
2
5
2
2 7
O
In the case of oxalic acid as hole scavenger, the oxalate anion is
HZSM-5 catalysts were recorded with Siemens D-5000 XRD using Ni-
+
oxidized by holes (h ) to produce two moles of carbon dioxide
(CO ) as expressed in Eq. (1).
0
filtered Cu Ka radiation (k = 1.5409 A ) and 2h range between 2ꢀ and
o
0 . The ultraviolet–visible (UV–Vis) diffuse reflectance spectra (DRS)
6
2
e
were recorded on a GBC UV-Visible Cintra 10 spectrometer in the
wavelength range of 200–800 nm. The SEM analyses of the catalysts
were performed on Hitachi S-520 SEM unit. Elemental analysis was
carried out using Link, ISIS-300, Oxford, EDAX detector.
C2O þ hþ ꢀ! CO2 þ CO ꢀ:
2ꢀ
ꢁ
ð1Þ
4
2
The generated CO
2
dissolves in water and produces
Analysis. Photocatalytic reactions were carried out in a closed and
deaerated condition in a quartz reactor (250 mL capacity) under UV
irradiation. The reaction was performed by taking known amount of
carbonic acid which will dissociate and release protons in the
solution. The protons thus generated undergo reduction at
the conduction band of TiO2. In the present investigation, the
1
KNO
3
as nitrogen source in 225 mL water containing 3 g L) catalyst
suspensions. The progress of the reaction was monitored by high-
pressure liquid chromatography with Zorbax Rx-SIL column, UV–VIS
detector, mobile phase (acetonitrile:water in the ratio 9:1), flow
rate = 1 mL min) . HPLC (high pressure liquid chromatography)
formation of ammonia is attributed to the following steps.
ꢀ
The NO3 ion adsorbed over the surface of TiO is reduced by
2
)
1
eight photogenerated electrons (e ) to give NH
ꢀ
3
. In the overall
reaction, one NO3 ion reacts with four oxalate anions to yield
NH and eight CO molecules as shown in Eq. (2).
)
1
)1
analysis was carried out for the analysis of NO
3
, NO
2
ions present
during the course of reaction using ion conductivity detector IC-A1 and
with (Shim-pack) HPLC column, 2.5 Mm pthalic acid (pH = 4.7) as
3
2
)
1
mobile phase, flow rate = 1mL min . Urea was also detected by
enzymatic method (blood urea nitrogen; BUN) using diagnostic kits.
Ammonia liberated was cross-checked with indophenol blue; a popular
NO þ 4C2O þ 10Hþ ꢀ! NH þ 8CO2 þ 3H2O:
ꢀ
2ꢀ
þ
ð2Þ
3
4
4
ꢀ
method for the analysis of ammonia and the determination of NO2 was
performed by diazotization method.
When TiO
ꢀ
2
bare was used as a photocatalyst, conversion of
ꢀ
NO to traces of NO2 and 0.8 ppm of NH was observed in
3
3
All the reactions were carried out at the original pH of the solution.
Samples were collected at periodic intervals, filtered through millipore
membrane filters and centrifuged to remove the catalyst particles.
Control experiments were performed in the absence of respective
catalysts and it revealed that no conversion to nitrate was observed.
pure water. When oxalic acid was used as a hole scavenger,
about 2.0 ppm of ammonia generation was noted. It seemed
ꢀ
that the reduction of NO3 was an indirect reaction path and
probably initiated by the oxidation of oxalate ions to the
ꢁꢀ
species CO
In the presence of oxalic acid, NO3 reduction follows as
expressed in Eq. (2).
2
and CO2 at the expense of photogenerated holes.
ꢀ
RESULTS AND DISCUSSION
ꢁꢀ
In the case of bulk semiconductor particles of high stability,
such as TiO , several kinds of reduction products of carbon
It is well established that CO2 species are very active and
0
ꢁꢀ
2
2
have stronger reduction ability as E of (CO ⁄ CO ) =
2
ꢀ
ꢀ
0
dioxide including formate and formaldehyde are obtained in
aqueous solutions in the absence of hole scavenger. However,
the amounts of the products are very small. To improve the
yields, various scavengers were used along with photocatalysts
to minimize the electron-hole recombination process (6). The
present investigation was carried out with isopropanol ⁄ oxalic
)1.8 V as compared with NO and NO2 ions whose E
3
0
ꢀ
þ
ꢀ
þ
4
(NO ⁄ NH ) = 1.203 V and E of (NO ⁄ NH ) = 0.897 V,
3
4
2
respectively, for the formation of ammonia (4). The reaction
was aimed and expected to utilize the in situ-generated
ammonia for the formation of urea. When oxalic acid was
used as hole scavenger, there was the possibility of formation
of ammonium oxalate in a parallel reaction which was
observed and confirmed by the mass spectral analysis. This
acid as a hole scavenger and also to act as a CO source
2
ꢀ
whereas the NO3 ion was used as the nitrogen source.
To begin with, all the experiments are carried out on bare
TiO and the results obtained are depicted in Table 1. From the
ꢀ
might be expected during the course of NO3 reduction to
2
ammonia. The ammonia formed reacts with oxalic acid and it
may retard the rate of urea formation. To avoid the formation
of unwanted ammonium oxalate under the set of experimental
conditions, further experiments were conducted with propan-
2-ol as hole scavenger and also as a source for CO2.
data in Table 1, it is clearly seen that when no scavenger was
used, the reduction of nitrate was minimal as the oxidation of
water and hence it was difficult to generate hydrogen radicals
that will be subsequently utilized in nitrate reduction. Therefore,
Table 1. Influence of hole scavenger on nitrate reduction for urea formation over TiO
2
catalyst.
Product distribution (ppm)
Ammonia
Percentage
conversion of nitrate
S. no.
Scavenger
Nitrite
Urea
1
2
3
Oxalic acid
Isopropanol
–
80
60
5
–
–
2.0
2.5
0.8
10.75
12.2
–
Trace
Reaction conditions: catalyst = 3 g L) , nitrogen source = KNO (1.6 x 10)3 M), pH = original, batch capacity = 100 mL, illumination
1
3
source = high-pressure Hg vapor lamp (Phillips 250 W), time = 6 h, pressure = 1 atmosphere, temperature = 40ꢀC, CO
2
source = 1% oxalic
acid (w ⁄ v), isopropanol (v ⁄ v) in water (s. nos. 1 and 2).

Photochemistry and Photobiology, 2012, 88 235
ꢀ
150
40
30
120
10
00
NO and propan-2-ol in urea synthesis
3
1
1.4
1.2
It is known that the recombination of photoexcited electron-
hole pairs is the main energy wastage step of photocatalytic
reaction. To inhibit the recombination, the reaction is con-
ducted with different scavengers such as methanol, ethanol and
1
1
.0
0.8
.6
.4
.2
0.0
1
3 3 2
isopropanol. It is reported that CH OH and (CH ) CHOH are
0
1
known to act as the best hole-capturing agents to accelerate
various photocatalytic reactions. The holes in the valence band
0
90
+
•
0
(
3 2
h vb) are captured by isopropanol to yield (CH ) COH
80
radicals and these further react with electron and holes
generated releasing hydrogen and a ketone. The ketone thus
formed undergoes further oxidation to carbon dioxide and
water as shown in Eq. (3).
70
1
2
3
4
5
6
Time (h)
60
50
40
30
20
hv
TiO2 ꢀ! TiO2ðe þ h Þ
ꢀ
þ
þ
þ
ꢁ
2
ðCH3Þ CHOH þ h ꢀ! H þ ðCH3Þ C-OH
2
ð3Þ
þ
ꢀ
; e
h
ꢁ
ðCH3Þ2 C-OH ꢀ! H2 þ COðCH3Þ
2
0
1
2
3
4
5
6
7
8
9
10
þ
COðCH3Þ þ h ꢀ! CO2 þ H2O:
Time (h)
2
Figure 1. Photo catalytic nitrate reduction and (inset) ammonia
formation with time over bare TiO catalyst. Reaction conditions:
2
a-hydroxyl radicals are generated during the process of
isopropanol oxidation with holes of TiO2. These radicals have
strong reducing power and can inject another electron into the
)
1
)3
catalyst = 3 g L
,
nitrogen
source = KNO
3
(1.6 · 10 M),
pH = original, batch capacity = 100 mL, illumination source =
high-pressure Hg vapor lamp (Phillips 250 W), time = 6 h, pres-
conduction band of TiO2 (as depicted in Scheme 1) This
.
sure = 1 atmosphere, temperature = 40ꢀC. CO
2
sources = 1%
oxalic acid (w ⁄ v) and isopropanol (v ⁄ v) in water (s. nos. 1 and 2).
additional electron subsequently utilized to improve the rate of
proton reduction as expressed in equation 4 (6).
þ
H þ 2eꢀ ꢀ! H2:
thermal stability and resistance to acids and bases is fairly
good. Zeolite can be used as support material for the
semiconductors. TiO2 supported on zeolite for various
2
ð4Þ
It is assumed that in situ-generated carbon dioxide and
ammonia react to form ammonium carbamate, which on
dehydration forms urea as shown in Eq. (5).
cyclization reactions, mineralization studies and also CO
reduction reaction was reported earlier from our group (20–
4). In the present investigation, to improve the adsorption of
2
2
nitrate and to prevent the oxidation of ammonia, which was
formed during nitrate reduction, the immobilization of TiO2
on zeolite was envisaged and a speculative mechanism is
proposed (Scheme 2).
Different TiO2 (wt%) supported on HZSM-5 zeolite are
prepared by the solid-state dispersion method (25–27). The
XRD patterns of the samples are shown in Fig. 2. The
ꢀ
H2O
CO2 þ NH3 ꢀ! H2NCOONH4 ꢀ! COðNH2Þ :
ð5Þ
2
The course of the reduction reaction of nitrate and
formation of ammonia over bare TiO is illustrated in
Fig. 1a,b. From the reported experiments, the optimized
required concentration of the hole scavenger is arrived at
and further experiments are conducted with the same.
2
characteristic 101, 200 planes of TiO anatase and 110 of rutile
2
are identified from the XRD results. The reduction of nitrate is
carried out in the presence of isopropanol as the hole
scavenger. The results obtained are shown in Table 2.
SEM photographs of different TiO (wt%) supported on
HZSM-5 catalysts (prepared by solid-state dispersion) are
provided in Fig. 3. It is observed that HZSM-5 morphology is
2
Influence of TiO -zeolite on urea formation
Zeolites are widely used as desiccant and sorbent. Neverthe-
less, they are often used as catalysts possibly because of their
2
not changed, wherein preformed TiO
zeolite support.
It is seen clearly from the results that formation of nitrite
and ammonia are very evident. Although the conversion of
2
was dispersed on the
(
CH ) C-OH
+
-
3
2
H + NO3
e-
e-
-
-
NO
2
NH
3
e-
e-, H+
2
nitrate is less on TiO -supported zeolites compared with bare
hv
TiO , it is a noteworthy observation of stable ammonia being
2
formed in the reaction and not undergoing oxidation. Further
oxidation of ammonia is prevented; probably ammonia
formed is adsorbed over the zeolite. The in situ-generated
h+
CH ) CHOH
.
2
+
2 2
ammonia and CO adsorbed are on the TiO -supported zeolite
H + (CH ) C-OH
3
and further condensation reaction on the surface led to the
formation of urea. The difference in the photocatalytic activity
of bare titania and different TiO2 (wt%) loaded on zeolite
(
3
2
Scheme 1. Ammonia formation.

2
36 Basavaraju Srinivas et al.
-
+
2
Table 2. Effect of TiO -zeolite for urea formation.
TiO2
TiO ( e + h )
2
h+ + (CH
+
Product distribution (ppm)
)
CHOH
2 H + (CH )CO
3
Percentage
conversion
of nitrate
3
2
TiO
(Wt %)
2
+
-
H2
Nitrite
Ammonia
Urea
2
H + 2 e
S. no.
+
(
CH ) CO + h
CO + H O
2 2
3
2
1
2
3
5
10
15
25
22
10
4.7
6.3
3.9
3.61
1.88
0.8
4
6
3
+
-
-
-
2
H + 2 e + NO
NO + H O
2 2
3
+
H + 2 e-
Reaction conditions: catalyst = 3 g L)1, N
2
2
source = KNO
3
(1.6 ·
)
3
1
0
2
M), CO source = 1% (vol.) isopropanol, pH = original, batch
capacity = 100 mL, illumination source = high-pressure Hg vapor
lamp (Phillips 250 W), time = 6 h, pressure = 1 atmosphere, temp-
erature = 40ꢀC.
[
HO-N-OH]
-
H O
2
[N = O]
2
However, the higher loadings of TiO over zeolite samples
H + 2 e-
+
2
have shown decreasing trends in photocatalytic activity, as
they show more red shifts in the absorption edge as seen from
Fig. 4. It might be owing to the fact that the excess loading
improves the electron-hole recombination centers resulting in
lowering of the activity. Therefore, the photocatalytic activity
[
H-N-OH]
+
-
2
H + 2 e , - H O
2
is achieved by the dispersion of TiO on zeolite to a certain
2
[
NH2]
extent. The high activity of the TiO
zeolite sample can be ascribed to the high dispersion of TiO
on the surface, strong adsorption of substrates and
also because of lower recombination of electron-hole pairs
generated.
2
(10 wt%) supported over
2
+
H + e-
NH3
The DRS of TiO ⁄ HZSM-5 photocatalysts are shown in
2
Scheme 2. Schematic representation and stabilization of intermediate
species during nitrate reduction and prevention of ammonia oxidation
over TiO -zeolite.
Fig. 4. These spectra are examined to understand the size
2
quantization effect and the TiO
The position of the absorption edge for anatase form of TiO
around 380 nm corresponds to band gap energy of 3.2 eV. A
blue shift of about 40–50 nm at low loadings of TiO was
loading.
2
interaction with the zeolite.
2
2
observed and the shift decreased with increasing TiO
2
Fe-titanates
In view of the aforesaid observations, it is inferred that bare
TiO shows better activity than when supported over zeolite.
To improve the activity of TiO further, iron titanates were
2
2
made using sol-gel technique. The prepared catalysts are tested
under similar conditions as with TiO2. The reduction of nitrate
was performed using isopropanol, which played a dual role: (1)
as a hole scavenger and (2) in releasing CO by undergoing
2
oxidation. Thus, in situ formed ammonia and carbon dioxide
react under illuminated conditions in the presence of photo-
catalyst to form an unstable C–N linkage and subsequent
reaction with protons forms an amide. At this stage, there are
two further courses of the reaction for the formation of urea:
(
1) oxidation to an amino acid and reaction with ammonia and
Figure 2. X-ray diffraction spectra of: (i) HZSM-5, TiO
iii) 5 wt%, (iv) 10 wt%, (v) 15 wt% supported over HZSM-5 and (vi)
TiO
2
(ii) 2 wt%,
subsequent dehydration; (2) amide undergoes hydroxylation
by hydroxyl radical and amination leads to the formation of
carbamate and subsequent dehydration to urea as shown in
Scheme 3. All the catalyst systems are prepared by sol-gel
method with varying iron to titanium ratios. These systems are
supported on zeolites and their activities are evaluated under
UV irradiation.
(
2
.
catalysts may be attributed to the following reasons. It is
known that the loading of TiO on zeolite brings blue shift in
2
the absorption thereby showing better photocatalytic activity
than bare titania catalyst. This may be owing to the shift
toward more energetic UV region. It is evident from the DRS
Fe TiO prepared by sol-gel method was reported as an
2
5
anode for photoelectrolysis of water and the quantum effi-
ciency was 15% at 350 nm (28). The characteristics of Fe TiO
are identified from the XRD results (JCPDS 76-1158). This
2
spectra that TiO loading resulted in a decrease in absorption
and a blue shift in the absorption edge of the titania (Fig. 4).
2
5

Photochemistry and Photobiology, 2012, 88 237
Figure 3. Scanning electron micrographic photographs of TiO
2
: (a) 5 wt%, (b) 10 wt%, (c) 15 wt% supported over HZSM-5 and (d) HZSM-5.
+
-
2
H , 2e
-
H2O
[HN = C = O]
H-C = O
NH +CO
3
2
.
+
+
-
H , OH
NH3
2H , 2e , (O)
+
NH COONH
H2N-COOH
2
4
-
H2O
+ NH3 ,
- H O
2
CO(NH2)2
Scheme 3. Speculative mechanism for urea formation.
Figure 4. Ultraviolet-diffuse reflectance spectroscopy of: (a) HZSM-5,
TiO , (b) 2 wt%, (c) 5 wt%, (d) 10 wt%, (e) 15 wt% supported over
HZSM-5 and (f) TiO
Table 3. Effect of Fe ⁄ Ti ratio on nitrate reduction for urea formation.
2
2
.
Product distribution
(ppm)
Percentage
conversion
of nitrate Ammonia Nitrite Urea
S.
no. Catalyst
material is known to realize higher energy conversion specially
when used as a film. Therefore, in the present investigation, the
photocatalytic properties of Fe-titanate are optimized either by
employing it directly in the powder form or by dispersing it on
a support such as HZSM-5. The reported light adsorption edge
for this material in a film form is around 430 nm. Photo-
catalytic reduction of nitrate was carried out under UV
irradiation in the presence of isopropanol and the results
obtained are depicted in Table 3.
Scavenger
1
2
Fe
Fe
2
TiO
Ti
5
Isopropanol
Isopropanol
35
20
0.8
2.0
0.1
0.5
12.19
5.3
2
2
O
7
)
1
Reaction conditions: catalyst = 3 g L , N
1
2
source = KNO
3
(1.6 ·
)
3
0
2
M), CO source = 1% (vol.) isopropanol, pH = original, batch
capacity = 100 mL, illumination source = high-pressure Hg vapor
lamp (Phillips 250 W), time = 6 h, pressure = 1 atmosphere, temp-
erature = 40ꢀC.

2
38 Basavaraju Srinivas et al.
(a)
(
c)
(
d)
(b)
Figure 5. (a) X-ray diffraction spectra of: (a) Fe
2
TiO
5
, (b) HZSM-5, (c) 5 wt% Fe
2
TiO
5
2 5
⁄ HZSM-5 and (d) 10 wt% Fe TiO ⁄ HZSM-5.
Figure 6. Scanning electron micrographic photographs of: (a) Fe
2
5 2 5
TiO and (b) 10 wt% Fe TiO ⁄ HZSM-5.
It is clear from the data that on bare Fe-titanates
XRD patterns of bare Fe TiO HZSM-5 and 5 and 10 wt%
2 5,
maximum conversion of nitrate observed was 35%.
Furthermore, when the formation of urea content is taken
into account, Fe ⁄ Ti = 1:1 shows better activity than
Fe ⁄ Ti = 2:1. Fe-titanates show comparatively better activity
than bare TiO2. However, the yield of urea was not
appreciable. This may be attributed to the generated
ammonia in the reaction undergoing further oxidation. In
view of this, it is suggested to suppress the oxidation of
ammonia with the modification of the catalyst. Thus, the
reactions are performed with Fe-titanates supported over
zeolites.
2 5
Fe TiO supported over HZSM-5, are shown in Fig. 5a–d.
Supported Fe-titanates
Keeping in view the aforesaid observations from the activity of
TiO
TiO
2
2
for the formation of urea, the optimum loading of the
on zeolite was 10 wt%. Furthermore, taking into account
the observations made, Fe TiO or Fe Ti O (10 wt%) sup-
2 5 2 2 7
ported on HZSM-5 were investigated for the formation of urea
during reduction of nitrate with isopropanol as hole scavenger.
The SEM photographs of Fe
2
TiO
5
and 10 wt% Fe2-

Photochemistry and Photobiology, 2012, 88 239
CO(NH2)2
5
00
-
H O
450
400
350
300
2
NO3 , H , e-
-
+
NH COONH4
2
NH3 + CO2
-
NH3
e
+
-
H , e
hv
TiO2
2
50
h+
0
2
4
6
8
10
12
(
CH ) CHOH
ZEOLITE
3
2
Tim e (h)
(
a)
+
(
CH ) CO, H , CO
3
2
2
3
0
Scheme 4. Schematic representation of urea formation over TiO
supported on zeolite photo catalyst under ultraviolet illumination.
2
25
2
1
1
0
5
0
5
0
Table 4. Effect of Fe:Ti ratio on the formation of urea.
Product distribution (ppm)
Nitrite Ammonia Urea
Percentage
conversion
of nitrate
S. no.
Catalyst
1
2
3
10 wt% Fe
2
2
2
TiO
5
5
⁄
⁄
90
88
30
10
9
2.6
1.5
0.2
18.77
9.26
7.0
HZSM-5
10 wt% Fe
HZSM-5
10 wt% Fe
HZSM-5
TiO
Ti
0
2
4
6
8
10
12
Tim e (h)
2
O
7
⁄
1.2
(
b)
)
1
Reaction conditions: catalyst = 3 g L , N
2
source = KNO
3
(1.6 x
)
3
10
2
M), CO source = 1% (vol.) isopropanol, pH = original, batch
0
0
0
.8
.7
.6
capacity = 100 mL, illumination source = high-pressure Hg vapor
lamp (Phillips 250 W), time = 6 h, pressure = 1 atmosphere, tem-
perature = 40ꢀC; hole scavenger: serial nos. 1, 3 = isopropanol; serial
no. 2 = oxalic acid.
TiO ⁄ HZSM-5 are shown in Fig. 6 and the zeolite was found
5
to play a dual role in the adsorption of both the reactants and
products (Scheme 4) by virtue of uniform regular pore
geometry of zeolites. The ammonia and urea (as evidenced
from authentic sample and mass spectral analysis) formed in
the reaction do not undergo further polymerization. The
polymerization and condensation products of urea were
observed under the experimental study whereas when the
0.5
0.4
0.3
0.2
reaction was performed on bare TiO , the urea yields are
2
0
1
2
3
4
5
6
7
8
9
10
found to be decreased (evidenced from high pressure liquid
chromatography analysis). The condensation of products was
reduced on Fe-titanates supported on zeolites, which may be
because of the virtue of shape-selectivity property of the
zeolite. The results obtained are depicted in Table 4. The
overall reaction is as follows:
Time (h)
(
c)
Figure 7. Scale-up experiment. (a) Nitrate reduction over time vari-
able; (b) formation of nitrite over time variable; (c) NH formation
with time on illumination. Reaction conditions: catalyst = 3 g L
N source = KNO (8.3 x 10 M), CO source = 5% (vol.) isopro-
2 3 2
panol, pH = original, batch capacity = 500 mL, illumination
source = high-pressure Hg vapor lamp (Phillips 250 W), time =
3
)
1
,
)
3
ꢀ
þ
ꢀ
2
NO þ CO2 þ 18H þ 16e ꢀ! ð NH2Þ C ¼ O þ 7H2O:
3
2
The optimized parameter of initial feasibility of the reaction
at lab scale was scaled-up to five-fold. During the course of the
reaction, nitrate reduction is depicted in Fig. 7a and the
formation of nitrite over period of time is illustrated in Fig. 7b.
The formation of ammonia is seen in Fig. 7c with time on
1
2 h, pressure = 1 atmosphere, temperature = 40ꢀC.
stream. The maximum conversion of nitrate achieved was 90%
and the urea yields are ꢂ18% in scale-up experiments as
shown in Table 5.

2
40 Basavaraju Srinivas et al.
Table 5. The trend in formation of urea with time in a five-fold scale-
up experiment.
7. Liu, F. and H. He (2010) Structure–activity relationship of iron
titanate catalysts in the selective catalytic reduction of NO with
NH . J. Phys. Chem. C, 114, 16929–16936.
x
3
S. no.
Time (h)
Urea (ppm)
8. Constantinou, C. L., C. N. Costa and A. M. Efstathiou (2010)
Catalytic removal of nitrates from waters. Catal. Today 151, 190–
194.
9. Kominami, H., A. Furusho, S. Murakami, H. Inoue, Y. Kera and
B. Ohtani (2001) Effective photocatalytic reduction of nitrate to
ammonia in an aqueous suspension of metal-loaded titanium (IV)
oxide particles in the presence of oxalic acid. Catal. Lett. 76,
31–33.
10. Gao, W., R. Jin, J. Chen, X. Guan, H. Zeng, F. Zhang and N.
Guan (2004) Titania-supported bimetallic catalysts for photocat-
alytic reduction of nitrate. Catal. Today 90, 331–336.
11. Gao, W., J. Chen, X. Guan, R. Jin, F. Zhang and N. Guan
(2004) Catalytic reduction of nitrite ions in drinking water
1
2
3
4
5
6
8
0
1
2
3
4
5
8
10
–
5.2
9.41
3.85
18.92
1.92
3.0
10
3.5
)
1
Reaction conditions: catalyst = 3 g L , N
2
source = KNO
3
(8.3 ·
M), CO source = 5% (vol.) isopropanol, pH = original, batch
)
3
10
2
over Pd-Cu ⁄ TiO
39.
2. Sa, J., T. Berger, K. Fottinger, A. Riss, J. A. Anderson and H.
Vinek (2005) Can TiO promote the reduction of nitrates in water?
J. Catal. 234, 282–291.
3. Ranjit, K. T. and B. Viswanathan (1997) Photocatalytic reduction
2
bimetallic catalyst. Catal. Today 93–95, 333–
capacity = 500 mL, illumination source = high-pressure Hg vapor
lamp (Phillips 250 W), time = 12 h, pressure = 1 atmosphere, temp-
erature = 40ꢀC.
3
1
1
2
CONCLUSIONS
2
of nitrite and nitrate ions over doped TiO catalysts. J. Photochem.
Photobiol A: Chem. 107, 215–220.
14. Ranjit, K. T. and B. Viswanathan (1997) Photocatalytic reduction
Urea was synthesized using TiO
2
(P25) and Fe-titanate sup-
ported over zeolite photocatalysts under atmospheric pressure
and ambient temperature in a batch reactor. The reactions are
of nitrite and nitrate ions to ammonia on M ⁄ TiO
2
catalysts.
J. Photochem. Photobiol A: Chem. 108, 73–78.
ꢀ
15. Ranjit, K. T., B. Viswanathan and T. K. Varadarajan (1996)
Photo catalytic reduction of nitrite and nitrate ions over TiO2
semiconductors. J. Mater. Sci. Lett. 15, 874–877.
performed in aqueous medium containing NO3 ion and 2-
2
propanol (acts as hole scavenger and source for CO ) under UV
light illumination. It is investigated that an optimum of TiO or
2
16. Shibata, M., K. Yoshida and N. Furuya (1995) Electrochemical
synthesis of urea on reduction of carbon dioxide with nitrate and
nitrite ions using Cu-loaded gas-diffusion electrode. J. Electroanal.
Chem. 387, 143–145.
Fe-titanate (10 wt%) supported over zeolite is required to
achieve 90% reduction of nitrate to ammonia. The photoactive
component and zeolite combinate inhibits polymerization of the
product and facilitates the condensation to form urea. A
maximum of 18 ppm of urea was obtained at 90% conversion
of nitrate. The reaction conducted under solar illumination was
found to have lower rates of urea formation. The optimized
parameter of initial feasibility of the reaction at lab scale was
scaled-up to five-fold with a greater success by achieving the urea
production in milder reaction conditions compared with the
existing conditions for urea formation.
1
1
1
7. Shchukin, D. G. and H. Mohwald (2005) Urea photosynthesis
inside polyelectrolyte capsules: Effect of confined media. Langmuir
21, 5582–5587.
8. Kuwabata, S., H. Yamauchi and H. Yoneyama (1998) Urea
photosynthesis from inorganic carbon and nitrogen compounds
2
using TiO as photo catalyst. Langmuir 14, 1899–1904.
9. Ustinovich, E. A., D. G. Shchukin and D. V. Sviridov (2005)
Heterogeneous photocatalysis in titania-stabilized per fluorocar-
bon-in-water emulsions: Urea photosynthesis and chloroform
photo degradation. J. Photochem. Photobiol. A: Chem. 175, 249–
2
52.
2
0. Subba Rao, K. V., B. Srinivas and M. Subrahmanyam (2004)
Photocatalytic reactions over semiconductor based zeolites for
organic synthesis: A review. In Photo ⁄ Electrochemistry & Photo
Biology in the Environment, Energy and Fuel (Edited by S. Kaneco,
B. Viawanathan and K. Funasaka), pp. 31–89. Research Signpost,
Trivandrum, Kerala, India.
Acknowledgements—The authors thank MCF New Delhi for the
project grant and MS thanks CSIR Emeritus Scientist Scheme.
REFERENCES
2
2
1. Subrahmanyam, M., S. Kaneco and N. A. Vante (1999) A screening
for the photo reduction of carbon dioxide supported on metal oxide
catalysts for C –C selectivity. Appl. Catal. B 23, 169–174.
1 3
2. Srinivas, B., B. Shubhamangala, K. Lalitha, V. Durga Kumari,
M. Subrahmanyam and B. R. De (2011) Photo catalytic reduction
1
. Hamanoi, O. and A. Kudo (2002) Reduction of nitrate and nitrite
ions over Ni-ZnS photocatalyst under visible light irradiation in
the presence of a sacrificial reagent. Chem. Lett. 8, 838–839.
. Tawkaew, S., S. Yin and T. Sato (2001) Photo reduction of nitrate
2
2 2
of CO over Cu–TiO ⁄ molecular sieve-5A composite. Photochem.
ion and photo evolution of hydrogen on unsupported TiO
TiO pillared H Nb 17 nano composites. Int. J. Inorg. Mater. 3,
55–859.
2
and
Photobiol. 87, 995–1001.
3. Sandhya, P., C. H. Hymavathi, V. Durga Kumari and M.
Subrahmanyam (2006) Photo Catalytic Urea Synthesis from
2
4
6
O
2
8
3
. Mori, T., J. Suzuki, K. Fujimotoa, M. Watanabe and Y. Hase-
gawa (1999) Reductive decomposition of nitrate ion to nitrogen in
water on a unique hollandite photo catalyst. Appl. Catal B: Env.
2
Nitrate, CO and Water, 8th Asian Academic Network for
Environmental Safety and Waste Management (AANESWM).
Editors: V. Murugesan, R. Jayabalou, S. Nanjundan and M.
Palanichamy, organized by the Department of Chemistry and
Centre with Potential for Excellence in Environmental Science,
Anna University, Chennai, India, December.
2
3, 283–289.
. Li, Y. and F. Wasgestian (1998) Photo catalytic reduction of
nitrate ions on TiO by oxalic acid. J. Photochem. Photobiol. A:
Chem. 112, 255–259.
. Jin, R., W. Gao, J. Chen, H. Zeng, F. Zhang, Z. Liu and N. Guan
2004) Photocatalytic reduction of nitrate ion in drinking water by
using metal-loaded MgTiO -TiO composite semiconductor cata-
4
5
2
2
2
4. Sangeetha Kumari, N., P. Saritha, V. Durga Kumari and M.
Subrahmanyam (2006) Photo Reduction of Nitrate Assisted by
Photocatalytic Degradation of Organic Waste. National Workshop
on Catalysis for Energy at Banaras Hindu University, Varanasi,
February.
(
3
2
lyst. J. Photochem. Photobiol. A: Chem. 162, 585–590.
. Yamagata, S., S. Nakabayashi, K. M. Sancier and A. Fujishima
6
5. Zhang,S.,N.FujiiandY.Nosaka(1998)ThedispersioneffectofTiO
loaded over ZSM-5 zeolite. J. Mol. Catal. A: Chem. 129, 219–224.
2
(1998) Photo catalytic oxidation of alcohols on TiO
Soc. Jpn. 61, 3429–3434.
2
. Bull. Chem.

Photochemistry and Photobiology, 2012, 88 241
2
6. Panpa, W., P. Sujaridworakun and S. Jinawath (2008) Photocat-
TiO +HZSM-5 zeolite combinate in the photo degradation of
aqueous phenol and p-chlorophenol. Appl. Catal. A: Gen. 234,
155–165.
2
2
)
alytic activity of TiO
2
⁄ ZSM-5 composites in the presence of SO
4
ion. Appl. Catal. B: Env. 80, 271–276.
2
7. Durgakumari, V., M. Subrahmanyam, K. V. Subba Rao, A.
Ratnamala, M. Noorjahan and K. Tanaka (2002) An easy
28. Rusina, O., W. Macyk and H. Kisch (2005) Photo electrochemical
properties of a dinitrogen-fixing iron titanate thin film. J. Phys.
Chem. B. 109, 10858–10862.
and efficient use of TiO
2
supported HZSM-5 and