J. Kaur, B. Pal / Catalysis Communications 53 (2014) 25–28
27
increased rutile content, and reached to 100% m-NA yield by pure (99%)
R-TiO2 after 4 h UV irradiation.
illumination. This fact suggests that the substituent and the position
of –NO2 group on the NB ring have an important role in the reduction pro-
cess because the electron withdrawing groups that lower the electron
density on a –NO2 group present on meta position favor the rapid conver-
sion of the –NO2 into the –NH2 group and hence reduce the nucleophilic-
ity of the resulting m-NA as observed in m-DNB reduction by P25-TiO2. As
the –NO2 group in the para position imparted less electronic induction
than the meta –NO2 group, the selectivity of p-DNB (24 μmol) reduction
to p-NA (17 μmol, 69%) is notably decreased without any production of
p-PDA by R-TiO2. This impact of –NO2 substituent is further supported
by the fact that almost no reduction (1–2 μmol) of NB to aniline
(1 μmol) occurs by R-TiO2 even after 8 h irradiation. However, P25-TiO2
being its mixed anatase–rutile phase has higher catalytic activity; hence,
25 μmol p-DNB is reduced to 20 μmol m-PDA (82%) and 5 μmol p-NA
(17%) relative to 14 μmol reduction of NB to 9 μmol aniline (66%) forma-
tion only after 4 h UV irradiation. It also observed that reduction of
25 μmol m-chloronitrobenzene by P25-TiO2 gives 100% m-chloroaniline
(25 μmol), whereas R-TiO2 gives only 4 μmol of m-chloroaniline after 4
and 8 h light irradiation, respectively, probably because of the poor
electron withdrawing nature of –Cl as compared to the –NO2 group.
The measured surface area 56 m2 g−1 of P25 is notably reduced with
increased rutile content on increasing sintering temperature i.e., 38, 30
and 18 m2 g−1 at 400, 600 and 800 °C, respectively. Therefore, although
the m-DNB reduction rate is decreased from 6.25 to 3.12 μmol/h, the
selectivity of m-NA yield is considerably improved because of the drastic
changes in the surface electronic properties of R-TiO2 with increased
crystallinity [25] where fewer defect sites appeared to promote m-NA
formation. The low photoreactivity of R-TiO2 may probably be due to
less surface OH concentration leading to poorer O2 adsorption essentially
required for proficient capturing of photoexcited electron [26–28] and
hence, exhibits fast recombination of e−/h+ pairs relative to P25-TiO2
catalyst. Many studies [20–29] have revealed that strong oxidation
of TiO2 at elevated temperatures leads to the formation of a metal-
deficient oxide and predominant defects are oxygen vacancies that
are important reactive agents for enhanced photocatalytic activity. The
active sites for –NO2 reduction on R-TiO2 are the Ti3+ atoms [22,29]
located at the oxygen vacancies on the R-TiO2 surface which behave as
The amount of m-DNB reduced is also subsequently decreased
because of lower photoactivity of R-TiO2. Fig. 2c demonstrated that
m-NA yield is highly improved with the increased amount of R-TiO2,
and exhibits maximum m-NA yield by 50 mg catalyst, and beyond this
amount, the second –NO2 group of m-NA starts reducing to give m-PDA
as a final product. This can be explained on the basis of increased per
molecule interactions of m-DNB with increasing amount of R-TiO2 and
availability of a higher number of active Ti3+ sites that imparted in
rapid reduction of both –NO2 groups.
The amount of m-DNB is gradually reduced with an increased amount
(17.9 μmol) of m-NA along with a little amount (3 μmol) of m-PDA
produced by P25-TiO2 during 2.5 h UV irradiation and, thereby, m-NA
gets converted into 100% m-PDA (25 μmol) after 4 h light exposure
(Fig. 3a). In contrary, complete reduction of m-DNB to m-NA by R-TiO2
is clearly observed after 8 h reduction and thereafter irradiation (N8 h)
led to less amount of m-PDA formation as shown in Fig. 3b. The efficiency
of –NO2 reduction to –NH2 group is further verified by simultaneous
analysis of acetone formed [11] during oxidation of iso-propanol under
photoirradiation. It found that the amount of acetone formed is higher
when both the –NO2 groups are reduced to m-PDA than one –NO2 reduc-
tion to m-NA formation as evident in the differences in peak area/height
of acetone (tR = 1.2 min) in the GC chromatogram (Fig. A2 in supporting
information).
The GC–MS analysis revealed that a single sharp peak at tR = 5.5 min
(Fig. 3c) for m-PDA and at tR = 8.1 min for m-NA (Fig. 3d) production by
P25 and R-TiO2 catalysts, respectively, evidencing cent percent yield
and selectivity of the obtained products whose mass (Fig. 3e and f)
fragmentation is also matched with the respective authentic samples,
confirmed the purity of m-NA and m-PDA. Thus, it was found that
m-DNB was efficiently and selectively reduced by the increased percent-
age of rutile content and reached to the highest rate by pure R-TiO2 as
compared to no appreciable reduction of NB under low intensity of UV
light. These findings are little different from the selective reduction [21]
of –NO2 to –NH2 group by R-TiO2 particles (obtained from P25-TiO2
with HF dissolution) using high power Xe lamp (2 kW, 27.3 W/m2)
Fig. 3. Time course of m-DNB reduction by (a) P25-TiO2 and (b) R-TiO2 under UV irradiation, (c–d) and (e–f) chromatograph and mass spectra of m-PDA and m-NA formation by 4 h and 8 h
photoreduction of m-DNB by P25-TiO2 and R-TiO2 catalysts, respectively.