Photocatalytic removal of nitrogen oxides from air on TiO2 modified with bases and platinum

Article abstract of DOI:10.1134/S002315841104015X

The efficiency of TiO₂ (Degussa P-25) modified with an alkaline admixture (urea, BaO), sulfuric acid, or platinum in the photocatalytic oxidation of NO (50 ppm) with a flowing 7% O₂ + N₂ mixture under UV irradiation in a flow reactor at room temperature and atmospheric pressure is reported. Because of the progressive blocking of active sites of the photocatalyst by the reaction products (NO₂, NO ₃ ^- ), it is impossible to realize prolonged continuous removal of NO _x (NO + NO₂) from air without catalyst regeneration at elevated temperatures. The efficiency of the photocatalysts is characterized by specific photoadsorption capacity (SPC) calculated from the total amount of NO _x adsorbed during 2-h-long irradiation. Modification of TiO ₂ with 5% BaO or 5% urea raises the SPC of the catalyst by a factor of 2-3. Presumably, this promoting effect is due to the basic properties of these dopants, which readily sorb NO₂ and NO ₃ ^- . A considerable favorable effect on SPC is also attained by adding 0.5% Pt to (5% BaO)/TiO₂. The SPC of the (0.5% Pt)/TiO₂ catalyst depends on the state of the platinum. The samples calcined in air at 500°C, which contain Pt⁺ and Pt²⁺, have an approximately 2 times higher SPC than unpromoted TiO₂ and ensure a much larger NO₂/NO ratio at the reactor outlet. Conversely, the samples reduced in an H₂ atmosphere at 200°C, whose platinum is in the Pt0 state, show a lower SPC than the initial TiO₂ and cause no significant change in the NO₂/NO ratio.

Full text of DOI:10.1134/S002315841104015X

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2011, Vol. 52, No. 4, pp. 518–524. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © B.N. Shelimov, N.N. Tolkachev, G.N. Baeva, A.Yu. Stakheev, V.B. Kazanskii, 2011, published in Kinetika i Kataliz, 2011, Vol. 52, No. 4, pp. 529–535.
Photocatalytic Removal of Nitrogen Oxides from Air
on TiO2 Modified with Bases and Platinum
B. N. Shelimov*, N. N. Tolkachev, G. N. Baeva, A. Yu. Stakheev, and V. B. Kazanskii
Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
*eꢀmail: bns@ioc.ac.ru
Received July 9, 2010
Abstract—The efficiency of TiO2 (Degussa Pꢀ25) modified with an alkaline admixture (urea, BaO), sulfuric
acid, or platinum in the photocatalytic oxidation of NO (50 ppm) with a flowing 7% O2 + N2 mixture under
UV irradiation in a flow reactor at room temperature and atmospheric pressure is reported. Because of the
ꢀ
progressive blocking of active sites of the photocatalyst by the reaction products (NO ,
), it is impossible
NO3
2
to realize prolonged continuous removal of NOx (NO + NO2) from air without catalyst regeneration at eleꢀ
vated temperatures. The efficiency of the photocatalysts is characterized by specific photoadsorption capacity
(SPC) calculated from the total amount of NOx adsorbed during 2ꢀhꢀlong irradiation. Modification of TiO2
with 5% BaO or 5% urea raises the SPC of the catalyst by a factor of 2–3. Presumably, this promoting effect
ꢀ
is due to the basic properties of these dopants, which readily sorb NO2 and
. A considerable favorable
NO3
effect on SPC is also attained by adding 0.5% Pt to (5% BaO)/TiO2. The SPC of the (0.5% Pt)/TiO2 catalyst
depends on the state of the platinum. The samples calcined in air at 500
C, which contain Pt+ and Pt2+, have
an approximately 2 times higher SPC than unpromoted TiO2 and ensure a much larger NO2/NO ratio at the
reactor outlet. Conversely, the samples reduced in an H2 atmosphere at 200
C, whose platinum is in the Pt0
°
°
state, show a lower SPC than the initial TiO2 and cause no significant change in the NO2/NO ratio.
DOI: 10.1134/S002315841104015X
Photocatalytic methods of removing organic and
inorganic pollutants from air are being intensively
developed now (see relevant reviews [1–7]). The most
popular photocatalyst is titanium dioxide, which is
catalytically very active, biologically and chemically
inert, and fairly cheap. As TiO2 is irradiated with UV
light with a quantum energy exceeding its band gap
(>3.0 eV for rutile and >3.2 eV for anatase), various
organic atmospheric pollutants adsorbed on its surface
are oxidized (mineralized) by atmospheric oxygen to
СО2 and water even at room temperature. When the
initial organic pollutant molecule contains nitrogen,
halogen, or sulfur atoms, there can be the undesired
formation of oxides of these elements, which are toxic
compounds. Both commercial TiO2 brands and laboꢀ
ratoryꢀmade samples are used in photocatalysis. The
latter are prepared by the hydrolysis of various organꢀ
otitanium compounds followed by drying and calcinaꢀ
tion of the resulting TiO2. The most active systems are
those which have nanosized structure elements and,
accordingly, are characterized by a developed surface and
contain micropores. The syntheses of TiO2ꢀbased nanoꢀ
materials, and their properties, modification methods,
and applications are considered in recent reviews [8, 9].
The removal of nitrogen oxides, which are highly
toxic compounds, from air is a challenging problem of
environmental catalysis. It was demonstrated in a
number of studies [10–16] that the irradiation of TiO2
with UV light in air or in NOꢀcontaining O2–N2 mixꢀ
tures yields NO2. Part of the nitrogen dioxide remains
adsorbed on the surface, and the rest of it is released
into the gas phase. In addition, surface nitrate strucꢀ
−
tures containing the
fragment were identified by
NO3
IR spectroscopy [17, 18]. At the early stages of UV irraꢀ
diation, the concentration of NOx (NOх = NO + NO2)
at the outlet of the photocatalytic reactor is far below
the initial NO concentration in the feed stream. Howꢀ
ever, the catalyst gradually loses activity with time
because its active sites are blocked by the NO2 and
NO3−
resulting from NO photooxidation, and the NOх
concentration at the outlet of the photocatalytic cell
gradually approaches the initial NO concentration in
the feed. The service life of the catalyst for NOх
removal from air depends on many factors, such as the
initial NO concentration, the amount of TiO2 in the
reactor, the feed flow rate, and the humidity of the polꢀ
luted air. TiO2 fully restores its photocatalytic activity
on being heatꢀtreated in flowing air or inert gas at 400–
500°С or washed with water. In the latter case, the
518

PHOTOCATALYTIC REMOVAL OF NITROGEN OXIDES FROM AIR
519
−
−
adsorbed
and
ions are washed off from the
The BaO/TiO2 sample (5% Ba) was prepared by
incipientꢀwetness impregnation of TiO2 with a soluꢀ
tion of Ba(NO3)2 (analytical grade) followed by drying
NO3
NO2,
surface and the active sites become available for
adsorption and subsequent photoreaction [12].
at 60°С and calcination in flowing air at 550°С for 2 h.
Because of the gradual poisoning of the TiO2ꢀbased
photocatalysts by the reaction products, it is impossiꢀ
ble to carry out prolonged continuous photocatalytic
NOх removal from air without catalyst regeneration.
However, the efficiency of the photocatalyst, i.e., the
total amount of NOх removed from air can be markꢀ
edly increased by means of chemical modification.
Some authors used mixtures of TiO2 with adsorbents
having a high adsorption capacity, such as type A and
Y zeolites [19], montmorillonite [20], and activated
The Pt/BaO/TiO2 (0.5% Pt, 5% Ba) and Pt/TiO2
(0.5% Pt) samples were prepared by incipientꢀwetness
impregnation of BaO/TiO2 (5% Ba) and TiO2 with a
solution of [Pt(NH3)4]Cl2 (Aldrich) followed by calciꢀ
nation in air at 500°С.
CO(NH2)2/TiO2 (5% urea) was obtained by incipiꢀ
entꢀwetness impregnation of TiO2 with a urea solution
and was dried at room temperature.
The SO4/TiO2 sample (2 wt % S) was prepared by
carbon [21–23]. In these composite systems, part of double incipientꢀwetness impregnation of TiO2 with
the NO2 desorbs from the TiO2 surface and binds 1 N H2SO4 followed by drying and calcination at
strongly to adsorption sites of the zeolite or carbon, 625
thus vacating photocatalytic sites of TiO2. Ichiura et al.
[24] used thin composite plates prepared from a mixꢀ
ture of suspensions of TiO2 and basic oxides (CaO,
MgO) or CaCO3. The nitrogen dioxide and HNO3 that
°
С
for 4 h.
Determination of Photocatalytic Activity
The catalysts as a suspension of a fine powder in
distilled water were applied onto the inner walls of a
cylindrical, quartz, optically transparent, flow photoꢀ
reactor (30 mm in diameter, 250 mm in height).
resulted from the photoreaction reacted with the
oxides or carbonate. As a result, the amount of
removed NOx per unit weight of TiO2 was substantially
larger. We demonstrated earlier [25] that an efficient
way of enhancing the specific photoadsorption capacꢀ
ity (SPC) of TiO2 for nitrogen oxides is its dispersing
on a support with a large specific surface area (e.g.,
Al2O3). The SPC of 50% TiO2/Al2O3 per unit weight of
titanium dioxide turned out to be >5 times higher than
that of massive TiO2. This effect is seemingly due to the
fact that dispersion makes most of the TiO2 atoms
accessible to the adsorbing reactants and that part of
the reaction products is captured by the support.
Thereafter, the reactor was dried at 120°С overnight. The
weight of the photocatalyst in the reactor was 0.2 g.
Photocatalytic activity in NO oxidation with oxyꢀ
gen was measured using a continuousꢀflow setup (Fig. 1)
at atmospheric pressure near room temperature. The
reactor, with a photocatalyst on its walls, was conꢀ
nected to the setup, and the photocatalyst was actiꢀ
vated by heating it in flowing purified air at 500°С for
1 h. Next, the reactor was cooled to room temperature
and a mixture modeling polluted air (45–50 ppm NO +
7 vol % О2
+ N2) was passed through the reactor until
Thus, the problem of enhancing the specific activꢀ
ity, selectivity, and stability of titanium dioxide–based
systems used in the photocatalytic removal of NOx
from air is still a challenge. Here, we report the prepaꢀ
ration and testing of Degussa Pꢀ25 TiO2 samples modꢀ
ified with alkaline admixtures and platinum, which is
widely used as a promoter of various catalysts.
the inlet and outlet NO concentrations were equal.
The gas mixture flow rate was 440 cm3/min.
The NO, NO2, and NOх (i.e., NO + NO2) concenꢀ
trations were measured in 5ꢀs intervals on a luminesꢀ
cent NOх analyzer (Eco Physics, Model CD70S) or on
an IR analyzer (Temet Instruments, Model GasmetTM
Dxꢀ4000n). This IR analyzer was also used to deterꢀ
mine the N2O concentration in the gas.
EXPERIMENTAL
After the removal of the furnace, the catalyst in the
reactor was irradiated with UV light from two 26ꢀW
black light lamps. The radiant intensity of the lamp,
Sample Preparation
The photocatalysts were unmodified and modified measured with a Photex radiometer and a set of light
Degussa Pꢀ25 titanium dioxide samples (S
sp = 48 m2/g). filters, was distributed as follows:
Wavelength range, nm
420–390
3.5
390–370
19.5
370–350
47.0
350–320
25.6
320–290
5.0
Fraction of radiant intensity in the range, %
For a more uniform irradiation of the photocataꢀ reactor; in addition, the reactor and lamps were surꢀ
lyst, the UV lamps were placed on both sides of the rounded with an aluminum shield.
KINETICS AND CATALYSIS Vol. 52
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2011

520
SHELIMOV et al.
Concentration, ppm
4
60
50
40
30
20
10
1 2
3
8
NO /NO
NOx
NO
0.4% NO/N2
x
Air
N2
5
6
NO2
N2O
7
9
0
10
20
Time, min
30
40
Fig. 1. Setup for photocatalytic activity measurements:
) valve, ( ) needle valve, ( ) flow meter, ( ) water separaꢀ
tor, ( ) quartz reactor, ( ) aluminum shield, ( ) UV lamps,
NO analyzer, and ( ) personal computer.
Fig. 2. Outlet NO, NO NO , and N O concentrations as
(
1
2
3
4
2,
x
2
a function of the UV irradiation time for the TiO
5
6
9
7
2
(Degussa Pꢀ25) photocatalyst.
(8
)
x
HNO2(ads). At the second, transitional stage, NO2
appears in the gas phase and its concentration
The state of supported platinum was determined by
diffuse reflectance IR spectroscopy of adsorbed CO.
Spectra were recorded on a Nicolet Protégé 460 IR increases with time. Therefore, HNO2 is further oxiꢀ
spectrophotometer using a diffuse reflectance attachꢀ
ment. Before taking the spectra, the samples were
heatꢀtreated for 1 h at 500oС and a residual gas presꢀ
sure of 0.01 Pa. The IR spectra were recorded at room
temperature in the presence of CO (8 Torr) and after
dized to NO2, which can subsequently be oxidized to
NO3. This is accompanied by a marked increase in
H
the outlet NO concentration. At the third stage, the
reaction reaches a steady state in which the NOx conꢀ
centration is nearly equal to the inlet NO concentraꢀ
tion; however, two nitrogenꢀcontaining compoꢀ
nents—NO and NO2—are observed at the outlet. At
this stage, nitrogen oxides are practically not removed
from the gas. Moreover, since the singleꢀexposure
maximum allowable NO2 concentration in the air for
inhabited areas is 4.7 times lower than the NO conꢀ
centration, according to sanitary regulations, this
“purified” air is even more toxic than the initial air.
pumping the samples at 20°С (30 min) and 100°С (1 h).
RESULTS AND DISCUSSION
It was established by preliminary experiments that,
without UV irradiation, the NO concentration in the
NO–O2–N2 stream at the inlet and outlet of the phoꢀ
toreactor with TiO2 remains invariable for at least 2 h.
After UV light is switched on, the outlet NO concenꢀ
tration falls sharply, and, after some period of time,
whose length depends on the sample, NO2 appears at
the outlet, whose concentration then varies with time.
No other products of the photoreaction were
observed. Figure 2 presents the typical plots of the
NO, NO2, and NOх concentrations as a function of the
irradiation time for the TiO2 photocatalyst. The zero
point corresponds to the beginning of irradiation.
The washings obtained from the catalyst that is in
NO3−
the steady state are dominated by
ions [12],
whose concentration varies insignificantly with time at
this stage. Therefore, NO is oxidized to NO2, while
HNO3 remains on the surface, blocking active sites.
Note that, once the steady state is reached, the outlet
NO concentration is well below the inlet NO concenꢀ
tration. Hence, NO photooxidation does not cease at
this stage, but the process is at equilibrium so that the
NO2 formation rate is equal to the rate of TiO2 desorpꢀ
tion from the photocatalyst surface. After the terminaꢀ
tion of the photoreaction, the initial activity of TiO2
can be fully restored by removing the NO oxidation
products via the heat treatment of the catalyst in flowꢀ
ing nitrogen at 650–773 K.
The NO photooxidation reaction can be divided
into three stages. At the initial stage, immediately after
irradiation is started, the outlet NO concentration
decreases rapidly to its minimum (nearꢀzero) value
and stays at this level for some time (Fig. 2). As this
takes place, the NO2 concentration in the gas is very
low and almost invariable. According to ion chromaꢀ
tography data, the most abundant ion in the water that
−
Thus, the overall NO photooxidation process can
was used to wash irradiated TiO2 is
[12]. Thereꢀ
NO2
fore, at this stage, NO is mainly oxidized to be represented as follows:
KINETICS AND CATALYSIS Vol. 52
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PHOTOCATALYTIC REMOVAL OF NITROGEN OXIDES FROM AIR
521
NO2(gas)
•
•
+OH
+ OH
+ OH
NO(gas)
NO(ads)
HNO2(ads)
NO2(ads)
HNO3(ads)
+ O–2
N(II)
N(III)
N(IV)
N(V)
In this process, the oxidation state of nitrogen
increases progressively from +2 (NO) to +5 (HNO3).
Other NO oxidation mechanisms involving −• are
−•
O
O2
also possible. Initially,
may react with the proton
O2
The mechanism of NO photooxidation on TiO2
was discussed in a number of publications [12, 24–29].
All researchers, based on the analogy between this proꢀ
cess and gasꢀphase freeꢀradical processes, agree that
the key role in photooxidation is played by the •ОН
radicals that result from the capture of holes photogeꢀ
nerated in TiO2 by surface hydroxyl groups and
adsorbed water molecules:
liberated by adsorbed water via reaction (III):
O2−• + H+ ꢀ HO•2.
(IX)
•
Next, the
radical may oxidize NO to HNO3:
НО2
NO+НО•2 ꢀ HNO3.
(X)
Figure 3 plots the NOx concentration at the outlet
of the photoreactor versus the UV irradiation time for
modified TiO2ꢀbased catalysts. The time dependence
of the NOx concentration for the initial TiO2 is also
presented here for comparison. These dependences
are similar in character for all catalysts, but they differ
significantly in two parameters, namely, the minimum
NOx concentration attained soon after the beginning
of irradiation and the time to reach the steady state.
The efficiency of the photocatalysts in the removal of
nitrogen oxides from air was estimated as the total
amount of NOх adsorbed by the photocatalyst during
2ꢀhꢀlong irradiation, which was determined by inteꢀ
grating the kinetic curves shown in Fig. 3. The calcuꢀ
lated SPCs per gram of catalyst are listed in the table.
The modification of TiO2 by impregnation it with
an aqueous Ba(NO3)2 or urea solution followed by dryꢀ
ing and heat treatment yields BaO/TiO2 and
(NH2)2CO/TiO2 samples having, respectively, a 2 and
3.2 times higher SPC than unmodified TiO2 (table).
This promoting effect is likely due to the alkaline
admixtures efficiently sorbing the resulting NO2 and
HNO3. A significant increase in the SPC is also
attained by adding 1% Pt to (5% BaO)/TiO2. However,
+
−
hν
ꢀꢀꢀꢁꢀ
ꢂꢀꢀꢀ
(I)
(II)
TiO2
h + e ,
−
+
•
OH + h ꢀ OH,
+
•
+
(III)
H2O + h ꢀ OH + H ,
where h+ is a hole and e– is an electron site.
Reaction (III) was unlikely under our experimental
conditions because the photocatalysts were preꢀheatꢀ
treated in flowing air at 500°С.
The oxidation of NO with the •ОН radical takes
place via the following consecutive reactions:
•
(IV)
(V)
NO + OH ꢀ HNO2,
•
HNO2 + OH ꢀ NO2 + H2O,
•
(VI)
NO2 + OH ꢀ HNO3.
The electrons liberated in reaction (I) are likely capꢀ
tured by the electron acceptor molecules of oxygen:
O2 + e− ꢀ O2−.
(VII)
−
O2
The subsequent fate of
is not usually discussed
in the literature. It is quite obvious, however, that this
species is involved in the photocatalytic process; othꢀ this leads to a marked increase in the NO2/NO ratio
erwise, the catalytic cycle would be unrealizable.
We hypothesized earlier [25] that the radical anion
under steadyꢀstate conditions. This is an undesired
side effect in view of NO2 being more toxic than NO.
The “acidic” TiO2 prepared by impregnating the
initial titanium dioxide with H2SO4 shows low effiꢀ
ciency in NO oxidation (Fig. 3). The causes of this
O2−•
directly oxidizes NO into the nitrate ion:
NO + O2−• ꢀ NO3−.
(VIII)
As was mentioned above, the nitrate ion results greatly decreased efficiency of TiO2/H2SO4 were not
from contact between TiO2 and NO in the presence of studied in this work because this photocatalyst is of no
oxygen. It was identified by IR spectroscopy [17, 18] practical interest. Apparently, its low activity is due to the
and was also detected in the washings from TiO2 after lower yield of •ОН radicals in reaction (II) because of the
NO photooxidation.
blocking of hydroxyl groups by H2SO4 molecules
KINETICS AND CATALYSIS Vol. 52
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522
NOx concentration, ppm
SHELIMOV et al.
Kubelka–Munk units
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
60
50
40
30
20
10
2094
2086
3
1
2
2187
4
3
2123
2100
2069
2050
2206
2
1
2250
2200
2150
2100
2000
0
30
60
90
Wavenumber, cm–1
Time, min
Fig. 3. Outlet NO concentration as a function of the UV
Fig. 4. IR spectra of CO adsorbed (
1
) on TiO and (
2
,
3
) on
and
x
2
Pt/TiO after ( ) heat treatment in air at 500°С
2
SO24–/TiO2,
2
irradiation time for the (
BaO/TiO , and ( (NH ) CO/TiO photocatalysts.
1) (2) TiO ,
2
(
3
) subsequent reduction in an
Н
atmosphere at 200
°С.
2
(3)
4)
Р
= 8 Torr; room temperature.
2
3 2
2
СО
and/or because equilibrium (III) shifts to the left with Pt/TiO2 preheated in air at 500
°С
and brought into conꢀ
an increasing proton concentration.
tact with CO shows, along with the above absorption
bands (which are assignable in this case to Pt2+–CO
complexes as well), an absorption band at 2123 cm–1
The table also presents data illustrating the effect of
Pt on the SPC of TiO2. Clearly, the SPC depends on
the state of platinum in TiO2, which was monitored by
recording the IR spectra of adsorbed carbon monoxide
(Fig. 4). The spectrum of platinumꢀfree TiO2 after CO
adsorption shows absorption bands at 2206 and 2187 cm–1
(Fig. 4, curve 2
). This band is characteristic of Pt+–CO
complexes. The lower frequency bands are convenꢀ
tionally assigned to complexes between CO and Pt0
[30]. The changes in the intensity of absorption bands
of adsorbed CO caused by pumping of the samples at
different temperatures are illustrated in Fig. 5. At room
temperature, the absorption bands of Pt2+–CO (2206
and 2187 cm–1) disappear entirely, while the band due
to the Pt+–CO complexes (2123 cm–1) disappears only
(Fig. 4, curve 1), which disappear entirely upon the
pumping of the sample at room temperature. These
bands occur at lower frequencies than the band of free
CO)
CO molecule (ν( = 2143 cm–1) and are due to the
stretching vibrations of the C–O bond in weakly
bonded Ti4+–СО complexes [30]. The IR spectrum of partially. These complexes decompose completely
Specific NOx photoadsorption capacity of TiO2ꢀbased photocatalysts
SPC
NO2/NO in the
steady state
Inlet NO concentraꢀ
tion, ppm
Photocatalyst
µmole NOx
mol NOx
mol TiO2
×10−2
gCat
TiO (“Degussa P25”)
55
53
55
52
50
50
60
121
194
190
110
44
0.48
1.02
1.56
1.60
0.88
0.35
0.25
0.35
5.09
0.27
4.49
0.20
2
5% BaO/TiO
2
0.5% Pt/5%BaO/TiO
2
5% (NH ) CO/TiO
2 2
2
0.5% Pt/TiO (oxidized)
2
0.5% Pt/TiO (reduced)
2
KINETICS AND CATALYSIS Vol. 52
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2011

PHOTOCATALYTIC REMOVAL OF NITROGEN OXIDES FROM AIR
523
Kubelka–Munk units
Kubelka–Munk units
1.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2079
2090
0.9
0.8
0.7
2094
2187
2079
2187
0.6
0.5
0.4
0.3
0.2
0.1
0
2100
2121
1
2
1
2
3
3
2250
2200
2150
2100
2050
2000
2250
2200
2150
2100
2050
2000
Wavenumber, cm–1
Wavenumber, cm–1
Fig. 5. IR spectra of CO adsorbed on the oxidized Pt/TiO
Fig. 6. IR spectra of CO adsorbed on the reduced Pt/TiO
2
2
sample (
1
) after CO adsorption at room temperature and
) after subsequent pumping of the
sample containing adsorbed CO at ( ) room temperature
and ( 100
sample (
1
) after CO adsorption at room temperature and
) after subsequent pumping of the
sample containing adsorbed CO at ( ) room temperature
and ( 100
Р
= 8 Torr and (
2,
3
Р
= 8 Torr and (2, 3
СО
СО
2
2
3
)
°С.
3)
°С.
upon pumping at 100
thermally more stable and survive partially upon
pumping at 100 . Thus, the СО–Pt/TiO2 system proꢀ
°С
. The Pt0–CO complexes are effect of Pt0 is not as significant as the effect of the platꢀ
inum ions.
It was reported that the Pt/TiO2 photocatalyst preꢀ
°С
vides another example of the familiar correlation
between the strength of the CO–Pt bond and the νCO of
the adsorbed carbon monoxide molecule [30].
pared by impregnation of titanium dioxide with a
Pt(NH3)4(NO3)2 solution followed by calcination at
200–500
tion into NO2 and HNO3 under irradiation with visible
light (up to = 546 nm) and that the reaction rate
°С exhibits appreciable activity in NO oxidaꢀ
In the IR spectrum of the Pt/TiO2 sample preꢀ
λ
reduced with hydrogen at 200°С (Fig. 4, curve 3), the
decreases markedly upon the reduction of platinum to
platinum metal [31].
absorption band of the Pt+–CO complexes is missing,
indicating that the entire Pt+ is reduced, and the
absorption bands of the Pt0–CO complexes (2094 and
2086 cm–1) are much stronger than the same bands in the
Thus, the data presented here suggest that the phoꢀ
tocatalytic activity of titanium dioxide in the removal
of nitrogen monoxide from air can be substantially
enhanced by introducing a basic promoter or platiꢀ
num. The presence of more oxidized platinum species
affords a significant increase in SPC, while the
reduced platinum species diminish SPC.
spectrum represented by curve 2
. The band at 2187 cm–1
is likely due to Ti4+–СО. The Pt0–CO complexes posꢀ
sess the highest thermal stability and decompose upon
pumping at 100°С (Fig. 6).
Now we will discuss the effect of different forms of
Pt in TiO2 on its photocatalytic properties. The presꢀ
ence of Pt+ and Pt2+ in the oxidized samples affords a
nearly twofold enhancement of the SPC (table). In
addition, the NOx leaving the reactor in this case is
dominated by NO2. This is possibly due to the more
intensive direct NO oxidation by oxygen adsorbed on
the Pt ions. Conversely, after the reduction of platiꢀ
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