DOI: 10.1039/C5CC00978B
ChemComm
COMMUNICATION
Journal Name
experimental set-up can be found as Electronic Supplementary
As seen from equations 1-2, photolysis of NO2- can promote the
Information). Under those concentrations, the expected absorption in formation of •OH radicals but also can induce its consumption
the range of 250-400 nm is still low. However, at shorter
wavelengths the extinction coefficient greatly increases to around
ε=5500 M-1cm-1 at 205 nm.13, 18 In this sense, the UV-filtering effect
of the nitrite solution at λ ≥ 250 nm is not significant. With the use
of a Pyrex-well, for instance, almost all the radiation below 300 nm
would be filtered (Figure S1).
through reaction in equation 3. Additionally, the direct oxidation of
NO2- by the photogenerated holes (eq. 4) cannot be dismissed,
•
-
according to the reported potential of E(NO2 /NO2 ) = 1.0 V at pH
7.20 As CH4 is not expected to suffer direct oxidation from the holes,
the last reaction would also imply decrease in the formation of •OH
through the direct oxidation of H2O, which has a potential of
E(•OH/H2O) = 1.8 V at pH 7.
The XRD measurements indicate that only the monoclinic phase of
BiVO4 was obtained (Figure S3), which has been identified in
literature as the most photoactive one, in comparison to the
tetragonal scheelite.14 As reported and also confirmed from the UV-
vis diffuse reflectance results, the estimated band gap value for
monoclinic BiVO4 is around 2.4 eV (Figure S3). Thus, NO2- would
not have any important influence on the photonic transference to this
material, as it can be photoexcited at longer wavelengths.
NO2 h NO O
eq. 1
O H2O OH OH
eq. 2
NO2 OH NO2 OH
eq. 3
NO2 hVB NO2
Medium pressure Hg lamps have several lines of irradiation along
the visible and UV ranges, including more energetic irradiation able
to initiate the water photolysis (λ≥ 185 nm) and lead to the formation
of •OH radicals. This photolytic route might also oxidize CH4 into
CH3OH, C2H6 and CO2, as shown in Figure 1.12, 19 However, higher
methanol and CO2 rates are obtained in presence of BiVO4,
eq. 4
The lower concentration of hydroxyl radicals leads to lower
formation of methyl radicals and to a moderate oxidizing media,
which explain the slight decrease in methanol and CO2 rates. This
effect is particularly noticeable in the analysis of C2H6. As this
product is formed through an oxidative coupling mechanism that
requires two •CH3 radicals, its productivity is more affected by a
global decrease in the formation of these species. At the end, C2H6 is
completely inhibited during the rest of the test period.
indicating that a photocatalytic mechanism is actually involved.
From the arisen points above, when nitrite anions are present in the
system, a significant change in the products distribution is observed.
As seen in Figure 1, CO2 formation greatly drops. This effect is more
evident at longer stages of the reaction. According to our previous
results, the highest CO2 concentrations are reached during the first
60 min, after which, depending on the photocatalyst and on the
conditions of the system, the CO2 formation rates start to decrease. It
is interesting to observe that nitrite increases the initial formation of
CH3OH at expenses of significant decreases in those of CO2, which
is not surprising as probably most of the CO2 comes from oxidizing
stages of methanol.
Two additional tests with and without nitrite were carried out in
presence of an alternative photocatalyst. A similar trend was
observed when Bi2WO6 was used (see Figure S3): the complete
disappearance of C2H6 accompanied with an important decrease in
the CO2 formation in presence of NO2- species. These results
confirm the NO2- role on this process, although the specific
improvement depends on the features of the used material.
Despite the overall decrease in terms of formation rates, there is a
significant improvement in the selectivity in comparison to the tests
without nitrite ions. This effect can be better observed in Figure 2.
The corresponding selectivity to the different products for three tests
with BiVO4 is presented: one without nitrite ions and two with
increasing concentration of nitrite (0.5 and 1 mM). With the lower
-
concentration of NO2 , methanol selectivity improves at expenses of
the C2H6 disappearance, which is formed through a competitive
-
reaction, and of a decrease in the CO2 levels. At higher NO2
concentration, the effect is more evident and the selectivity to
CH3OH is even higher than 90% between 60 and 120 min of
reaction, which was never reached on previous works.
Some aliquots of the liquid phase were extracted at the end of the
tests, in order to analyze the remaining NO2- concentrations by
means of UV-vis spectrophotometry. Besides the previous ones, two
additional tests of only 60 min of illumination were carried out for
this purpose. The corresponding UV-vis spectra are presented in
Figure 3. There, it is possible to observe a certain decrease in the
-
absorption peak at 355 nm, which is a good indicator of NO2
consumption. It is interesting to find a faster decrease in the nitrite
concentration in the blank tests without catalyst (Figure 3A) in
comparison to the ones with BiVO4 (Figure 3B). Moreover, some
differences in the spectra are also seen: while the baseline keeps
increasing with time in the blank, in the test with BiVO4 the
apparition of a band of absorption at ca. 268 nm suggests a slightly
Figure 1. CH3OH, CO2 and C2H6 rates obtained during the blank and
photocatalytic tests with BiVO4, in absence and in presence of NO2
(1 mM).
-
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012
Murcia-López
