, 2004, 14(5), 197–199
UV light for 1 min. The spectrum of adsorbed formic acid was
obtained in a separate experiment. A band at 1570 cm–1
corresponds to asymmetric COO stretching vibrations, and a
band at 1360 cm–1 corresponds to symmetric COO stretching
vibrations for bridging formate.8 In the high-frequency region,
bands at 2954 and 2868 cm–1 most likely correspond to O–H
and C–H stretching vibrations. Thus, the photocatalytic oxidation
of DMMP proceeds via formic acid mainly in a bridging
bidentate form. Formic acid appears after 1 min oxidation and
undergoes almost total oxidation into H2O and CO2 in 15 min.
Phosphoric acid was found as the final surface product of the
photocatalytic oxidation of DMMP.
2824
MeO–Ti
7
2954
2868
4
3
6
5
1360
1570
7
2
6
5
8
4
3
2
1
In an experiment at 1% relative humidity and an excess of
DMMP, after the evaporation of 5 µl (5.7 mg, 46 mmol) of
liquid DMMP, vapour-phase DMMP underwent exponential
decay (k = 0.071 min–1) to the value of 12 µmol, which cor-
responds to the adsorption of 4.2 mg of DMMP. Gaseous
methanol grew up to 1.8 µmol, which corresponds to the
hydrolysis of 0.2 mg of DMMP. The total methanol quantity
increased with the reaction rate constant k = 0.0077 min–1. The
maximum adsorbed methanol amount was observed at t = 5 min
and was equal to 0.4 µmol, whereas after t = 15 min adsorbed
methanol was not observed. Therefore, DMMP from a gas
phase replaces methanol adsorbed on the TiO2 surface; how-
ever, not all the adsorbed DMMP undergoes hydrolysis.
After the photocatalytic oxidation was started, the total
DMMP quantity and methanol concentration that had formed in
hydrolysis decreased, the gas phase CO2 and CO concentration,
as well as the total formic acid quantity, increased (Figure 3).
DMMP displaces into the gas phase surface intermediate photo-
catalytic oxidation products such as formic acid and CO. Total
carbon amount in the gas phase made up 25% of the total
carbon amount present in the system.
8
1
3000
2900
2800
1600 1400 1200 1000
v/cm–1
Figure 2 IR spectra of the TiO2 surface during the photocatalytic oxida-
tion after the completion of hydrolysis of 0.1 µl of DMMP after (1) 0, (2) 1,
(3) 2, (4) 4, (5) 10, (6) 14 and (7) 30 min. For comparison, the spectrum
of adsorbed formic acid is also given (8).
Before the experiments, the TiO2 sample inside the reactor
was exposed to UV light with purging air at room temperature
for several hours until its FTIR spectrum became constant. The
hydrolysis and photocatalytic oxidation were carried out at
1 and 50% initial relative humidities at 300 K; the DMMP
quantity was 0.1 or 5 µl.
In the experiment with 1% relative humidity, after the
injection of 0.1 µl (0.11 mg, 0.92 µmol) of liquid DMMP into
the reactor, its full evaporation finished in about 1 min. Then,
the gas-phase DMMP concentration exponentially decreased
below the detection limit with the first-order reaction constant
k = 0.071 min–1. No gaseous hydrolysis products were observed;
thus, all the products were adsorbed on the TiO2 surface. The
To study the influence of humidity, the hydrolysis and photo-
catalytic oxidation of 0.1 µl of DMMP were carried out at a
50% relative humidity. After the evaporation, gaseous DMMP
underwent an exponential decay (k = 0.05 min–1) until its con-
sumption below the detection limit. In contrast to low humidity
hydrolysis, gaseous methanol was formed and its concentration
IR spectra of the TiO2 surface are shown in Figure 1.†,3,6,7
A
band at 2824 cm–1 corresponds to the stretching vibrations of
the MeO–Ti moiety of adsorbed methanol that is formed in
DMMP hydrolysis.3 The quantity of adsorbed methanol grows
according to an exponential law with the first-order reaction
rate constant k = 4.2×10–3 min–1. It takes about 700 min for the
hydrolysis to finish and by that time the adsorbed methanol
quantity is equal to the initial DMMP quantity within the
experimental accuracy ( 10%).
–1
exponentially increased (k = 0.003 6 min ) until it became
equal to the initial DMMP amount. A negligible amount of
methanol was detected on the TiO2 surface. Hence, water replaces
methanol adsorbed on the TiO2 surface. After UV light was
turned on and photocatalytic oxidation started, the gas-phase
CO2 concentration exponentially increased (k = 0.25 min–1).
The final quantity of CO2 was equal to the triple initial DMMP
amount within the experimental accuracy. Gas-phase methanol
exponentially decreased (k = 0.3 3 mi–n1) until its total con-
sumption.
To prove that the DMMP adsorption and hydrolysis take
place on the TiO2 surface only, an experiment was carried out
under the above conditions without TiO2 inside the reactor. This
experiment showed that, after evaporation, the DMMP concen-
tration in the gas phase remains constant.
This study demonstrated the rapid DMMP removal from a
gas phase, the subsequent hydrolysis to the bonded products
and total mineralization under UV radiation on the TiO2 sur-
face. Water was shown to displace the DMMP hydrolysis
To prove that hydrolysis involves mainly only one methoxy
group of the DMMP molecule, hydrolysis products were washed
off the TiO2 surface with water and dimethyl sulfoxide and
analysed by GC-MS. This analysis showed that, in addition to
main hydrolysis products, methanol and DMMP with one
methoxy group hydrolysed, trace DMMP with both methoxy
groups hydrolysed occurred on the TiO2 surface.
After the hydrolysis of 0.1 µl of DMMP was completed,
the TiO2 surface was exposed to UV radiation. The photo-
catalytic oxidation of adsorbed products started and the CO2
quantity in the gas phase increased from zero to 3 µmol with
the first-order reaction rate constant k = 0.28 min–1. The final
amount of CO2 equals to the triple amount of initial DMMP
within the experimental accuracy ( 10%).
10
20
5
1
8
16
4
6
4
2
0
12
3
8
4
The TiO2 surface spectra during the photocatalytic oxidation
are shown in Figure 2. The spectrum of the TiO2 surface
exposed to UV light for 1 min can be approximated by the sum
of the TiO2 surface spectrum before photocatalytic oxidation
and the spectrum of adsorbed formic acid that was exposed to
2
0
0
200
400
600
t/min
†
Assignment of the IR bands in the spectra of DMMP and adsorbed
methanol (cm–1): 3010 [νa(MeP)], 2962 [νa(MeO)], 2930 [νs(MeP)],
2858 [νs(MeO)], 2824 (MeO–Ti), 1467 [δs(MeO)], 1313 [δ(MeP)], 1275
[ν(P=O)], 1188 [ρ(MeP)], 1075 [νa(C–O)], 1050 [νs(C–O)].
Figure 3 Changes of gas-phase compound quantities during the photo-
catalytic oxidation of 5 µl of previously hydrolysed DMMP: (1) DMMP,
(2) methanol, (3) formic acid, (4) CO2 and (5) CO.
198 Mendeleev Commun. 2004
Trubitsyn, Dmitry A.
