# The Effect of Sulphate Doping on NanosizedTiO2 and MoOx/TiO2 Catalysts in Cyclohexane Photooxidative Dehydrogenation

Authors: P. Ciambelli; D. Sannino; V. Palma; V. Vaiano
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The effect of sulphate doping of titania in promoting activity and selectivity ofMoOx/TiO2  catalysts for the cyclohexane photooxidative dehydrogenation has been investigated in a gas-solid fluidized bed reactor. Sulphate and/or molybdate-modified titania catalysts were prepared by incipient wet impregnation of nanosized (5–10 nm crystallite size) samples. At 60% of titania surface coverage
by MoOx, sulphate surface density was obtained up to 19 μmol/m2 without formation of MoO3. The catalysts were characterized by N2 adsorption-desorption at −196∘C, micro-Raman and UV-visible reflectance spectroscopy, thermogravimetric analysis coupled with mass spectroscopy (TG-MS), and mass titration. Unsulphated and sulphated titania are both active in cyclohexane total oxidation, but sulphate doping of titania has a detrimental effect on the reaction rate. On Mo-based catalysts, polymolybdate species enabled sulphated titania to convert cyclohexane to benzene (99% selectivity) and cyclohexene, reducing at zero the formation of CO2. Cyclohexane conversion to benzene is almost linearly dependent on sulphate surface density, resulting in enhanced yield to benzene. The enhanced photooxidative dehydrogenation activity and benzene yield by sulphate doping could be attributed to the increase of surface acidity and, as a consequence, of cyclohexane adsorption.


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## Body

## 1. Introduction

Recently, growing
interest has been addressed to novel processes for the synthesis of chemicals
by reversing the final goal of the photocatalytic oxidation process—enhancing the
selectivity to partially oxidized products so much that the process could be
used for their production. Therefore, since one of the main goals of the 21st
century chemistry is to replace environmentally hazardous processes, selective
photocatalytic partial oxidation can play a key role in this evolution,
offering an alternative green route for the production of organics.Titanium dioxide is
the semiconductor most used in photocatalysis because of
its high photocatalytic activity allowing to realize the abatement of pollutants
both in liquid and gas phases [1–4]. It has been reported that the introduction of
foreign species into TiO2  leads to a dramatic change in activity of
the photocatalytic process. In particular, SO42−/TiO2  shows higher photocatalytic activity than unsulphated titania for a large
variety of organic compounds [5].  Fu et
al. [6] showed that the
improved photoactivity of sulphated titanium dioxide was due to a
greater surface area as well as a larger fraction of anatase phase which is
more active than rutile for the photocatalytic application. In the presence of
metal oxides such as WO3, the surface acidity of TiO2 was
increased, resulting in enhanced photocatalytic activity [7]. The formation of acid sites increases the
adsorption strength of different organics, that it is believed to contribute to
enhance the photocatalytic activity.The most of studies on photocatalytic reactions for organic synthesis deals
with slurry systems. In
contrast, selective oxidation in gas phase with molecular oxygen is still a
real challenge to current research. Few examples of selective
photocatalytic oxidative conversion of organic substrates in gas phase on
modified titania-based catalysts have been reported such as alkane and
cycloalkane oxidation [8–10], and olephin epoxidation [11, 12]. However, a significant
enhancement of catalytic activity and energy efficiency is necessary for such
application of photocatalysis. Recently, we have shown that benzene is obtained
with high selectivity by  photooxidative dehydrogenation of cyclohexane on MoOx/TiO2 catalysts in a gas-solid fixed [10] or,
better, fluidized bed reactor [13, 14]. The
active phases are octahedral polymolybdate species on titania surface [10]. Moreover,
we found that the presence of sulphate species on the surface of different
titanias (2.4 wt%
maximum SO4 content) enhances the benzene
yield as more as higher is the sulphate content evidencing a synergic effect of
sulphate and molybdate species [15]. It was also shown that the presence of sulphate increases surface acidity as the sulphate
surface density increases. The addition of sulphate to MoOx/γ-Al2O3 catalysts was found to promote the
selective mono-oxidative dehydrogenation of cyclohexane to cyclohexene, demonstrating
the possibility of finely tuning the process selectivity [16]. Maximum cyclohexane conversion and
cyclohexene yield of 11% were obtained for SO4 content of 2.4
wt% at 120°C. Physicochemical characterization
of catalysts indicated the presence of both octahedral polymolybdate and
sulphate species on alumina surface as previously found for titania. Increasing
sulphate load, thermogravimetry evidenced the presence of up to three sulphate
species of different thermal stability. We concluded that the lower activity
observed at high sulphate content is likely due to polymolybdate decoration by
sulphates [16].In this paper, we focus the attention on the
effect of sulphate doping of nanosized TiO2 on the photocatalytic
oxidative dehydrogenation of cyclohexane to cyclohexene and benzene on MoOx/TiO2 in a gas-solid fluidized bed reactor.


## 2. Experimental

Anatase titania
with crystallites size ranging from 5 to 10 nm (PC 500 provided by Millenium Inorganic Chemicals, Stallingborough, United Kingdom) was used as catalyst support. The preparation procedure for catalyst samples
(MoPCs) containing various amounts of molybdenum oxide and sulphate consisted of two steps. The first step was the impregnation of
the support with an aqueous solution of ammonium sulphate. The suspension was dried
under stirring at80°C to complete removal of excess water. The sample turned
into sulphate doped titania by calcination at 300°C for 3 hours. The second
step was the impregnation of the sulphated sample with an aqueous solution of
ammonium heptamolybdate (NH4)6Mo7O24·4H2O,
drying at 120°C for 12 hours and calcination in air at 400°C for 3 hours.Photocatalysts characterization
was performed by different techniques. Thermogravimetric analysis (TG-MS) of
powder samples was carried out in air flow with a thermoanalyzer (Q600, TA) in
the range 20–1100°C at 10°C/min heating rate. Specific surface area was evaluated
by N2 adsorption-desorption isotherms at −196°C with a Costech
Sorptometer 1040. Powder samples were treated at 180°C for 2 hours in He
flow (99.9990%) before testing. Laser Raman spectra of powder samples were
obtained with a dispersive micro-Raman (Invia, Renishaw), equipped with 785 nm
diode laser, in the range 100–2500 cm-1 Raman shift. UV-Vis reflectance spectra were obtained with a
Perkin Elmer Spectrometer Lambda 35 using an RSA-PE-20 reflectance spectroscopy
accessory (Labsphere Inc., North Sutton, NH, USA). All spectra were performed
using an 8° sample positioning holder, giving total reflectance relative to a
calibrated standard SRS-010-99 (Labsphere Inc.). The reflectance data were
reported as F(R∞) values
from Kubelka-Munk theory versus the wavelength [17]. Equivalent
band
gap determinations were made by plotting [F(R∞)*hν]2 versus hν
(eV) and calculating the x intercept of a line passing through 0.5 < F(R∞) < 0.8. The zero point charge (ZPC) of
supports and catalysts was determined by mass titration according to Noh and Schwarz
[18].Photocatalytic
tests were carried out feeding a
fluidized bed photoreactor [19] with 830 (stp) cm3/min N2 stream containing 1000 ppm cyclohexane, 1500 ppm oxygen, and 1600 ppm water.
The composition of the inlet and outlet gases was measured by an online
quadrupole mass detector (TraceMS, ThermoFinnigan) and a continuous CO-CO2 NDIR analyser (Uras 10, Hartmann & Braun). The reactor was illuminated by
four UV light sources (eye mercury lamp, 125 W) in a dark box. The temperature was set at 120°C after a
screening in the range 40–160°C [19]. In order to achieve fluidization conditions, 14 g of catalyst mixed with 63 g of α-Al2O3 (Aldrich, Milano, Italy 10 m2/g, 50 μm Sauter
diameter) were loaded to the reactor. UV sources were activated after the
complete adsorption of cyclohexane on catalyst surface in dark conditions.


## 3. Results and Discussion

### 3.1. Catalyst Characterization

The nominal MoO3 and sulphate
content together with the specific surface area (S.S.A) of the photocatalysts
are reported in Table 1.Table 1
List of catalysts
and their characteristics.
CatalystMoO3  wt%SO3* wt%Surface
area
m2/gMoO3 density μmol/m2SO4 density μmol/m2PC500—0.55345—0.17PC500S10—11241—4.8PC500S18—19169—11.724MoPC50023.80.32028.10.1517MoPC500S1016.6101448.07.312MoPC500S1811.718998.118.9*Evaluated by TG-MS analysis.Both SO4 and MoO3 surface
densities were evaluated with respect to the surface area values of the
calcined catalysts. All MoPCs were designed to reach about 60% monolayer coverage of titania surface (about
8 μmol/m2).Thermogravimetric curves of PC500,
PC500S10, and PC500S18 are reported in Figure1.TG (a) and DTG (b) curves of PC500,
PC500S10, and PC500S18 catalysts.
(a)(b)Three main steps of weight loss are recognized: (i) the first one associated with hydration water desorption, (ii) the second one related to the removal ofOH−
surface groups of titania, (iii) the last one attributed to the decomposition of sulphate species giving rise to gaseous SO3 as identified from its characteristic fragments m/z = 48 and 64 (not reported). The latter step ranges from 690°C to 840°C for PC500, while downshifts to 420–855°C and 375–850°C for PC500S10 and PC500S18,
respectively. By analyzing the first derivative curves in the region of sulphate loss, only one peak centered at 741°C is
observed on PC500, while PC500S10 and PC500S18 show three peaks at 552, 599, 717°C and at 477, 555, 730°C,
respectively. It has been reported [20] that sulphated TiO2 shows different
regions of sulphate loss, the lowest temperature peak being attributed to a
more weakly bonded sulphate species, the highest temperature peak to a more
strongly bonded sulphate. The structure of metal oxide bonded sulphate is still
subject of investigation in the literature. Jin et al. [20] proposed that sulphate would be coordinated to the titania
surface as bidentate anion, Bensitel et al. [21] and Waqif et al. [22] described a structure in which sulphate, at lower
content, would be bonded to the metal through three oxygen atoms. Moreover, the
sulphate coordination to the surface depends on the dehydration state of the surface
and then on the temperature. Increasing temperature and sulphate content,
polynuclear sulphates (polysulphates, S2O7−,
etc.) can be formed.TG and DTG curves of 24MoPC500, 17MoPC500S10, and 12MoPC500S18 are
displayed in Figure2. All the samples show a decomposition
step starting from about 700°C; the onset of weight loss shifts slightly at
higher temperature increasing the sulphate load. The estimated amount of weight
losses of the three samples well agrees to the nominal MoO3 content reported in Table 1, allowing the attribution of this
step to Mo oxide sublimation, reported to occur in the range 800–1000°C on MoOx/Al2O3 catalysts [23].TG (a) and DTG (b) curves of 24Mo PC500, 17MoPC500S10, and 12MoPC500S18 catalysts.
(a)(b)The DTG curve of 24MoPC500
shows a single high temperature peak, centered at624°C due to sulphate decomposition. We attribute the decreasing of decomposition temperature with
respect to PC500 to the presence of MoOx species, similarly as we previously
found for WOx/TiO2 catalysts [24]. For 17MoPC500S10, in addition to a DTG peak at
about 584°C two overlapping peaks appear, with maximum at 466°C and 525°C,
likely due to different surface sulphates. Similar DTG peaks, centered at 455°C,
573°C, and 630°C, are shown by 12MoPC500S18.UV-Vis DRS spectra of all the
photocatalysts are reported in Figure3. When supported on sulphated titania, Mo species
absorption bands are difficult to distinguish, as previously reported [16]. However, the presence of MoO3 crystallites (main band
at 360 nm) on the catalysts was not detected. Sulphate doping increases the overall UV absorption of all the samples (Figure
3), correspondent to a slight increase
in equivalent band gap energies, reported for MoPCs in Table 2. At
similar sulphate level, the partial surface coverage with MoOx leads
to a small decrease in the equivalent band gap energies with respect to the relevant support.Table 2
Equivalent band gap energy, ZPC values,
and amount of adsorbed cyclohexane in dark conditions for all catalysts.
CatalystEquivalent band gap energy
eVZPC
pH unitAdsorbed cyclohexane
μmol/m2PC5003.35.80.048PC500S103.34.10.034PC500S183.42.20.02624MoPC5003.03.40.0917MoPC500S103.22.20.1412MoPC500S183.31.20.22Figure 3
UV-Vis DRS spectra of PC500, PC500S10, PC500S18, 24MoPC500, 17MoPC500S10, and 12MoPC500S18 catalysts.Increasing the sulphate content of
titania, the values of ZPC (Table2) decrease from 5.8 for PC500 to 2.2 for
PC500S18. Also the presence of molybdenum confers, in all cases, stronger
acidity, indicated by the ZPC values of MoPCs catalysts, as reported in [15].The Raman spectra of PC and MoPC
samples are reported in Figures4(a) and 4(b), respectively. PC500S10
and PC500S18 exhibit a band around 986 cm-1 assigned to symmetric
stretching mode ν1 of sulphate belonging to point group Td [25]. The
signal at about 1012 cm-1 for PC500S10 and at 1018 cm-1 for PC500S18 is due to the S=O bonds of isolated
sulphates [26].Raman spectra of PC500, PC500S10, PC500S18 (a) and of 24MoPC500,
17MoPC500S10, 12MoPC500S18 (b) catalysts.
(a)(b)Broad
bands around 1050 and 1180 cm-1 are due to removal of degeneration of
ν3 mode, owing the symmetry lowering with the coordination to the titania surface
hydroxyl groups as bidentate anion. The upshift of S=O Raman band could indicate the
strengthening of double bond due to the formation of polynuclear sulphates [25]. On MoPCs, the absence
of bulk molybdena bands at 819 and 995 cm-1 [27] for
all Mo-based catalysts confirms the absence of MoO3 crystallites. Moreover,
a complex band with several contributions in the range 940–1030 cm-1 is observed. For 24MoPC500 the maximum is at 984 cm-1, with
shoulders at 960 and 967 cm-1. For 17MoPC500S10 the complex band is
sharpened, with an additional strong signal at 992 cm-1, overimposed
to 970, 980, and 984 cm-1 shoulders. For 12MoPC500S18, 978 and 982 cm-1 signals with a maximum at 996 cm-1 are present, characteristic
of polymolybdenyl species [28]. The Mo=O asymmetric stretching modes of hydrated
octahedrally polymeric species lay in the range 940–988 cm-1 [29] with nuclearity ranging between 7 and 12. Wachs [28] reported that in hydrated conditions Mo8O264− presents a Raman Mo=O band at 963 cm-1 on titania. The presence of
polymeric octahedral species and sulphate can be thus recognised. 984 and 996 cm-1 signals could be ascribed to the surface sulphates, considering the absence of
820 cm-1 band of MoO3 crystallites. Bands at 993, 1009,
and 1111 cm-1 have been assigned to the S–O stretching vibrations of SO42− species on CeO2 [30]. The
structure of sulphate species responsible for the band at 993 cm-1 cannot be definitely determined and thus requires further investigations.The upshift of
Mo=O band up to 980 cm-1 at increasing sulphate surface density indicates
a progressive polymerization of MoOx species.


### 3.2. Photocatalytic Tests

Preliminary tests carried  out in absence of UV light evidenced that no
reaction occurred in dark conditions. Cyclohexane conversion and CO2 outlet concentration on
PC500, PC500S10, and PC500S18 as a function of irradiation time are reported in
Figure 5. CO2 was the only product detected in the gas phase
(100% selectivity), reaching steady state values after about 30 minutes.
Cyclohexane conversion to CO2 decreases with the sulphate content.Figure 5
Cyclohexane
conversion (X) and CO2 outlet concentration (CO2) as a
function of irradiation time on PC500, PC500S10,
and PC500S18.For
all titania catalysts, the total oxidation of cyclohexane to CO2 and
H2O occurs without formation of by-products. Moreover, from Figure 6 it is evident that both
cyclohexane consumption and CO2 formation rates decreased with the
sulphate surface density as previously observed in a gas-solid fixed bed photo
reactor [15]. This
behavior may
be explained taking into account the reaction mechanism of gas-solid
photocatalytic oxidation of cyclohexane to CO2 on titania catalyst [31]. In the photocatalytic process, in fact, the surface
hydroxyl groups trap the charge transfer to produce very reactive surface
hydroxyl radicals. However, the presence of sulphate, grafted to the surface by
reaction with hydroxyl groups, decreases their surface concentration, resulting
in reduced activity. In addition, it is worthwhile to note that for total
oxidation of hydrocarbons on titania the rehydroxylation of the surface is an
essential step. Xie et al. [32], studying the gas-solid
photocatalytic oxidation of heptane on sulphated and unsulphated titania in the
presence of water, found that the presence of SO42− was
detrimental to the rehydroxylability of the catalyst.Figure 6
Cyclohexane
consumption rate and CO2 formation rate on
PC500, PC500S10, and PC500S18 as a function of sulphate
surface density.Cyclohexane
conversion and benzene outlet concentration as a function of irradiation time on
MoPCS catalysts are reported in Figure7. The analysis of outlet products disclosed the presence
of benzene and cyclohexene, the absence of carbon dioxide and the stability of
catalyst activity during the photocatalytic test. The trend of cyclohexene concentration
(not reported) was similar to that of benzene but its concentration was very
low (<1 ppm). It is evident that both cyclohexane total conversion and
partial conversion to benzene are strongly dependent of sulphate and molybdate
content.Figure 7
Cyclohexane conversion (X) and benzene outlet concentration (B) on
24MoPC500, 17MoPC500S10, and 12MoPC500S18 as a function of irradiation time.The
catalytic activity of MoPCs samples is reported in Figure8(a) in terms of reaction rate per unit surface
against sulphate surface density. Since the catalysts have
the same molybdate surface density, it is evident that the catalytic activity
is only dependent of the sulphate density. The values of cyclohexane
consumption rate and benzene formation rate are almost coincident (about 100 %
selectivity as the reaction rate to cyclohexene is two orders of magnitude
lower) and linearly increase with sulphate density. We  previously showed
that the very high selectivity to benzene of MoOx/TiO2 photocatalyst is related to the coverage extent of titania surface by the
polymolybdate species [10].Cyclohexane consumption rate, benzene and cyclohexene
formation rate (a) ZPC values and dark adsorption of cyclohexane (b) as
function of sulphate surface density on 24MoPC500, 17MoPC500S10, and
12MoPC500S18.
(a)(b)We retain that the mechanism for
the photooxidative dehydrogenation of cyclohexane [19] involves photoexcited octahedral molybdate surface species, which are able to initiate the oxidative dehydrogenation of adsorbed cyclohexane through hydrogen
abstraction forming cyclohexene, further dehydrogenated to benzene which finally
desorbs from catalyst surface. Photoreduced molybdate
is then regenerated by reactive oxygen species, as O−
formed by the
reaction between lattice oxygen and positive hole, O2−
formed
from gaseous oxygen and electron of the conduction band, H2O2 generated from OH• coupling. H2O2 is also responsible for water
formation by decomposition. In summary, the formation of benzene may occur mainly according to a photocatalytic cycle that involves
the molybdate both in the oxidized and in the photoreduced forms (redox
mechanism) [19]. Cyclohexane may be totally oxidized
into CO2 and H2O on unselective sites of the titania
surface, according to the mechanism presented by Einaga et al. [31]. The presence of sulphate on titania is
reported to give higher photoefficiency, in dependence of both
energy required to excite the electrons from the valence to the conduction band
(Eg) and limiting electron-hole pair recombination owing sulphate electron withdrawing properties. The equivalent band
gap energy trend with sulphate
content (Table 2) is in contrast with the first point. Further, hypotheses on the
role of surface sulphate consider it to participate in the reoxidation step of
the surface octahedrally coordinated
polymolybdate or to promote hydrogen abstraction from adsorbed cyclohexane
molecules, since its acid properties [19].A
strong indication on the main action of sulphate is suggested by Figure8(b),
where the amount of cyclohexane adsorbed in the dark on MoPCs catalysts is
reported as a function of sulphate surface density. The linear raise of adsorbed
amounts parallels the increase of cyclohexane reaction rate (Figure 8(a)).The values of ZPC reported
in Figure8(b) also indicate that the adsorption of cyclohexane is correlated to
the corresponding increase of catalyst acidity. These results suggest that sulphate
doping facilitates hydrogen abstraction from adsorbed cyclohexane, increasing
its storage.


## 3.1. Catalyst Characterization

The nominal MoO3 and sulphate
content together with the specific surface area (S.S.A) of the photocatalysts
are reported in Table 1.Table 1
List of catalysts
and their characteristics.
CatalystMoO3  wt%SO3* wt%Surface
area
m2/gMoO3 density μmol/m2SO4 density μmol/m2PC500—0.55345—0.17PC500S10—11241—4.8PC500S18—19169—11.724MoPC50023.80.32028.10.1517MoPC500S1016.6101448.07.312MoPC500S1811.718998.118.9*Evaluated by TG-MS analysis.Both SO4 and MoO3 surface
densities were evaluated with respect to the surface area values of the
calcined catalysts. All MoPCs were designed to reach about 60% monolayer coverage of titania surface (about
8 μmol/m2).Thermogravimetric curves of PC500,
PC500S10, and PC500S18 are reported in Figure1.TG (a) and DTG (b) curves of PC500,
PC500S10, and PC500S18 catalysts.
(a)(b)Three main steps of weight loss are recognized: (i) the first one associated with hydration water desorption, (ii) the second one related to the removal ofOH−
surface groups of titania, (iii) the last one attributed to the decomposition of sulphate species giving rise to gaseous SO3 as identified from its characteristic fragments m/z = 48 and 64 (not reported). The latter step ranges from 690°C to 840°C for PC500, while downshifts to 420–855°C and 375–850°C for PC500S10 and PC500S18,
respectively. By analyzing the first derivative curves in the region of sulphate loss, only one peak centered at 741°C is
observed on PC500, while PC500S10 and PC500S18 show three peaks at 552, 599, 717°C and at 477, 555, 730°C,
respectively. It has been reported [20] that sulphated TiO2 shows different
regions of sulphate loss, the lowest temperature peak being attributed to a
more weakly bonded sulphate species, the highest temperature peak to a more
strongly bonded sulphate. The structure of metal oxide bonded sulphate is still
subject of investigation in the literature. Jin et al. [20] proposed that sulphate would be coordinated to the titania
surface as bidentate anion, Bensitel et al. [21] and Waqif et al. [22] described a structure in which sulphate, at lower
content, would be bonded to the metal through three oxygen atoms. Moreover, the
sulphate coordination to the surface depends on the dehydration state of the surface
and then on the temperature. Increasing temperature and sulphate content,
polynuclear sulphates (polysulphates, S2O7−,
etc.) can be formed.TG and DTG curves of 24MoPC500, 17MoPC500S10, and 12MoPC500S18 are
displayed in Figure2. All the samples show a decomposition
step starting from about 700°C; the onset of weight loss shifts slightly at
higher temperature increasing the sulphate load. The estimated amount of weight
losses of the three samples well agrees to the nominal MoO3 content reported in Table 1, allowing the attribution of this
step to Mo oxide sublimation, reported to occur in the range 800–1000°C on MoOx/Al2O3 catalysts [23].TG (a) and DTG (b) curves of 24Mo PC500, 17MoPC500S10, and 12MoPC500S18 catalysts.
(a)(b)The DTG curve of 24MoPC500
shows a single high temperature peak, centered at624°C due to sulphate decomposition. We attribute the decreasing of decomposition temperature with
respect to PC500 to the presence of MoOx species, similarly as we previously
found for WOx/TiO2 catalysts [24]. For 17MoPC500S10, in addition to a DTG peak at
about 584°C two overlapping peaks appear, with maximum at 466°C and 525°C,
likely due to different surface sulphates. Similar DTG peaks, centered at 455°C,
573°C, and 630°C, are shown by 12MoPC500S18.UV-Vis DRS spectra of all the
photocatalysts are reported in Figure3. When supported on sulphated titania, Mo species
absorption bands are difficult to distinguish, as previously reported [16]. However, the presence of MoO3 crystallites (main band
at 360 nm) on the catalysts was not detected. Sulphate doping increases the overall UV absorption of all the samples (Figure
3), correspondent to a slight increase
in equivalent band gap energies, reported for MoPCs in Table 2. At
similar sulphate level, the partial surface coverage with MoOx leads
to a small decrease in the equivalent band gap energies with respect to the relevant support.Table 2
Equivalent band gap energy, ZPC values,
and amount of adsorbed cyclohexane in dark conditions for all catalysts.
CatalystEquivalent band gap energy
eVZPC
pH unitAdsorbed cyclohexane
μmol/m2PC5003.35.80.048PC500S103.34.10.034PC500S183.42.20.02624MoPC5003.03.40.0917MoPC500S103.22.20.1412MoPC500S183.31.20.22Figure 3
UV-Vis DRS spectra of PC500, PC500S10, PC500S18, 24MoPC500, 17MoPC500S10, and 12MoPC500S18 catalysts.Increasing the sulphate content of
titania, the values of ZPC (Table2) decrease from 5.8 for PC500 to 2.2 for
PC500S18. Also the presence of molybdenum confers, in all cases, stronger
acidity, indicated by the ZPC values of MoPCs catalysts, as reported in [15].The Raman spectra of PC and MoPC
samples are reported in Figures4(a) and 4(b), respectively. PC500S10
and PC500S18 exhibit a band around 986 cm-1 assigned to symmetric
stretching mode ν1 of sulphate belonging to point group Td [25]. The
signal at about 1012 cm-1 for PC500S10 and at 1018 cm-1 for PC500S18 is due to the S=O bonds of isolated
sulphates [26].Raman spectra of PC500, PC500S10, PC500S18 (a) and of 24MoPC500,
17MoPC500S10, 12MoPC500S18 (b) catalysts.
(a)(b)Broad
bands around 1050 and 1180 cm-1 are due to removal of degeneration of
ν3 mode, owing the symmetry lowering with the coordination to the titania surface
hydroxyl groups as bidentate anion. The upshift of S=O Raman band could indicate the
strengthening of double bond due to the formation of polynuclear sulphates [25]. On MoPCs, the absence
of bulk molybdena bands at 819 and 995 cm-1 [27] for
all Mo-based catalysts confirms the absence of MoO3 crystallites. Moreover,
a complex band with several contributions in the range 940–1030 cm-1 is observed. For 24MoPC500 the maximum is at 984 cm-1, with
shoulders at 960 and 967 cm-1. For 17MoPC500S10 the complex band is
sharpened, with an additional strong signal at 992 cm-1, overimposed
to 970, 980, and 984 cm-1 shoulders. For 12MoPC500S18, 978 and 982 cm-1 signals with a maximum at 996 cm-1 are present, characteristic
of polymolybdenyl species [28]. The Mo=O asymmetric stretching modes of hydrated
octahedrally polymeric species lay in the range 940–988 cm-1 [29] with nuclearity ranging between 7 and 12. Wachs [28] reported that in hydrated conditions Mo8O264− presents a Raman Mo=O band at 963 cm-1 on titania. The presence of
polymeric octahedral species and sulphate can be thus recognised. 984 and 996 cm-1 signals could be ascribed to the surface sulphates, considering the absence of
820 cm-1 band of MoO3 crystallites. Bands at 993, 1009,
and 1111 cm-1 have been assigned to the S–O stretching vibrations of SO42− species on CeO2 [30]. The
structure of sulphate species responsible for the band at 993 cm-1 cannot be definitely determined and thus requires further investigations.The upshift of
Mo=O band up to 980 cm-1 at increasing sulphate surface density indicates
a progressive polymerization of MoOx species.


## 3.2. Photocatalytic Tests

Preliminary tests carried  out in absence of UV light evidenced that no
reaction occurred in dark conditions. Cyclohexane conversion and CO2 outlet concentration on
PC500, PC500S10, and PC500S18 as a function of irradiation time are reported in
Figure 5. CO2 was the only product detected in the gas phase
(100% selectivity), reaching steady state values after about 30 minutes.
Cyclohexane conversion to CO2 decreases with the sulphate content.Figure 5
Cyclohexane
conversion (X) and CO2 outlet concentration (CO2) as a
function of irradiation time on PC500, PC500S10,
and PC500S18.For
all titania catalysts, the total oxidation of cyclohexane to CO2 and
H2O occurs without formation of by-products. Moreover, from Figure 6 it is evident that both
cyclohexane consumption and CO2 formation rates decreased with the
sulphate surface density as previously observed in a gas-solid fixed bed photo
reactor [15]. This
behavior may
be explained taking into account the reaction mechanism of gas-solid
photocatalytic oxidation of cyclohexane to CO2 on titania catalyst [31]. In the photocatalytic process, in fact, the surface
hydroxyl groups trap the charge transfer to produce very reactive surface
hydroxyl radicals. However, the presence of sulphate, grafted to the surface by
reaction with hydroxyl groups, decreases their surface concentration, resulting
in reduced activity. In addition, it is worthwhile to note that for total
oxidation of hydrocarbons on titania the rehydroxylation of the surface is an
essential step. Xie et al. [32], studying the gas-solid
photocatalytic oxidation of heptane on sulphated and unsulphated titania in the
presence of water, found that the presence of SO42− was
detrimental to the rehydroxylability of the catalyst.Figure 6
Cyclohexane
consumption rate and CO2 formation rate on
PC500, PC500S10, and PC500S18 as a function of sulphate
surface density.Cyclohexane
conversion and benzene outlet concentration as a function of irradiation time on
MoPCS catalysts are reported in Figure7. The analysis of outlet products disclosed the presence
of benzene and cyclohexene, the absence of carbon dioxide and the stability of
catalyst activity during the photocatalytic test. The trend of cyclohexene concentration
(not reported) was similar to that of benzene but its concentration was very
low (<1 ppm). It is evident that both cyclohexane total conversion and
partial conversion to benzene are strongly dependent of sulphate and molybdate
content.Figure 7
Cyclohexane conversion (X) and benzene outlet concentration (B) on
24MoPC500, 17MoPC500S10, and 12MoPC500S18 as a function of irradiation time.The
catalytic activity of MoPCs samples is reported in Figure8(a) in terms of reaction rate per unit surface
against sulphate surface density. Since the catalysts have
the same molybdate surface density, it is evident that the catalytic activity
is only dependent of the sulphate density. The values of cyclohexane
consumption rate and benzene formation rate are almost coincident (about 100 %
selectivity as the reaction rate to cyclohexene is two orders of magnitude
lower) and linearly increase with sulphate density. We  previously showed
that the very high selectivity to benzene of MoOx/TiO2 photocatalyst is related to the coverage extent of titania surface by the
polymolybdate species [10].Cyclohexane consumption rate, benzene and cyclohexene
formation rate (a) ZPC values and dark adsorption of cyclohexane (b) as
function of sulphate surface density on 24MoPC500, 17MoPC500S10, and
12MoPC500S18.
(a)(b)We retain that the mechanism for
the photooxidative dehydrogenation of cyclohexane [19] involves photoexcited octahedral molybdate surface species, which are able to initiate the oxidative dehydrogenation of adsorbed cyclohexane through hydrogen
abstraction forming cyclohexene, further dehydrogenated to benzene which finally
desorbs from catalyst surface. Photoreduced molybdate
is then regenerated by reactive oxygen species, as O−
formed by the
reaction between lattice oxygen and positive hole, O2−
formed
from gaseous oxygen and electron of the conduction band, H2O2 generated from OH• coupling. H2O2 is also responsible for water
formation by decomposition. In summary, the formation of benzene may occur mainly according to a photocatalytic cycle that involves
the molybdate both in the oxidized and in the photoreduced forms (redox
mechanism) [19]. Cyclohexane may be totally oxidized
into CO2 and H2O on unselective sites of the titania
surface, according to the mechanism presented by Einaga et al. [31]. The presence of sulphate on titania is
reported to give higher photoefficiency, in dependence of both
energy required to excite the electrons from the valence to the conduction band
(Eg) and limiting electron-hole pair recombination owing sulphate electron withdrawing properties. The equivalent band
gap energy trend with sulphate
content (Table 2) is in contrast with the first point. Further, hypotheses on the
role of surface sulphate consider it to participate in the reoxidation step of
the surface octahedrally coordinated
polymolybdate or to promote hydrogen abstraction from adsorbed cyclohexane
molecules, since its acid properties [19].A
strong indication on the main action of sulphate is suggested by Figure8(b),
where the amount of cyclohexane adsorbed in the dark on MoPCs catalysts is
reported as a function of sulphate surface density. The linear raise of adsorbed
amounts parallels the increase of cyclohexane reaction rate (Figure 8(a)).The values of ZPC reported
in Figure8(b) also indicate that the adsorption of cyclohexane is correlated to
the corresponding increase of catalyst acidity. These results suggest that sulphate
doping facilitates hydrogen abstraction from adsorbed cyclohexane, increasing
its storage.


## 4. Conclusions

Nanosized titania was employed to explore the effect of sulphate doping
on TiO2 and MoOx/TiO2 catalysts for the photooxidative
dehydrogenation of cyclohexane in a
photocatalytic gas-solid fluidized bed reactor. The high specific surface area of titania,
even at 60% of MoOx surface coverage, allowed to vary sulphate
surface density up to 19 μmol/m2 without
formation of segregated MoO3 crystallites.On nanosized
titania, deep oxidation of cyclohexane only occurred. The cyclohexane consumption
rate and carbon dioxide formation rate decreased as the sulphate load increases
likely due to the detrimental decrease of surface free hydroxyls.Photocatalytic selective
oxidation of cyclohexane to benzene on MoOx/nanosized titania was
enhanced by the sulphation. The presence of sulphate species on the surface of
titania promoted benzene yield as more as higher was the sulphate content. Cyclohexene
was produced in low concentration and CO2 was not detected in
gas-phase. The enhanced photooxidative
dehydrogenation activity of MoOx/TiO2 catalysts is attributed
to the increase of surface acidity due to sulphate doping that induces the formation of acid sites able to
increase the hydrocarbon adsorption.



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*Source: 258631-2008-04-07.xml*