On the role of WO3 surface hydroxyl groups for the photocatalytic partial oxidation of methane to methanol

Article abstract of DOI:10.1016/j.catcom.2014.09.025

Highlights Photocatalytic conversion of CH₄ over WO₃ and WO₃/F was studied.Surface fluorinated WO₃ led to low yield of methanol.Reaction pathway was proposed for the photocatalytic conversion of CH₄ o

Full text of DOI:10.1016/j.catcom.2014.09.025

On the role of WO3 surface hydroxyl groups for the photocatalytic partial
oxidation of methane to methanol
Katherine Villa, Sebasti a´ n Murcia-L o´ pez, Teresa Andreu, Joan Ram o´ n
Morante
PII:
DOI:
Reference:
S1566-7367(14)00383-5
doi: 10.1016/j.catcom.2014.09.025
CATCOM 4062
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
5 August 2014
1 September 2014
2 September 2014
Please cite this article as: Katherine Villa, Sebasti ´a n Murcia-L ´o pez, Teresa Andreu,
Joan Ram o´ n Morante, On the role of WO3 surface hydroxyl groups for the photocat-
alytic partial oxidation of methane to methanol, Catalysis Communications (2014), doi:
1
0.1016/j.catcom.2014.09.025
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On the role of WO surface hydroxyl groups for the photocatalytic partial
3
oxidation of methane to methanol
a*
a
a
Katherine Villa , Sebastián Murcia-López , Teresa Andreu and Joan Ramón Morante
a,b
a
Catalonia Institute for Energy research (IREC), Jardins de les Dones de Negre 1,
08930 Sant Adrià de Besòs, Spain.
b
Department of Electronics, University of Barcelona (UB), Marti i Franquès 1,
8028 Barcelona, Spain.
0
*
Corresponding autor: kvilla@irec.cat, tel. 34 933 562 615

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Abstract
The photocatalytic partial oxidation of methane to methanol has been investigated on
WO . The surface fluorination of the catalyst has given an insight on the reaction
3
mechanism which proceeds mainly through the interaction with surface hydroxyl
groups.
Keywords
Surface fluorinated WO ; Photocatalysis; CH conversion; Mechanism
3
4

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1
. Introduction
For several years great effort has been devoted to the study of the partial oxidation of
methane into useful oxygenates compounds such as methanol [1], [2] and [3]. Given the
low reactivity of methane, this reaction requires high temperatures and pressures to
proceed [4] and [5]. Nevertheless, previous research has demonstrated that is possible to
perform this methane conversion under mild conditions by photocatalysis [6]. Among
the candidate materials that have been examined as possible photocatalysts, tungsten
oxide (WO ) is one of the most promising, due to its relative chemical stability, non-
3
toxicity, and moderate oxidizing power, as previously reported by Taylor and Nocetti
[
7] and Gondal et al. [8].
The mechanism of this photocatalytic process involved the generation of hydroxyl
radicals that are responsible for the activation of methane through the H-abstraction.
However, the contribution of the bulk or surface of the catalyst in the mechanism is
neither fully understood nor reported earlier. The fact that CH is not adsorbed on the
4
catalyst tends to support the assumption that the oxidation of methane is essentially a
bulk reaction, but there is no direct evidence about this procedure. In the literature,
several studies have demonstrated that surface fluorination of photocatalysts gives
information about the influence of free OH radicals in the solution bulk and surface OH
groups in the mechanism reaction [9], [10], [11] and [12]. Based on this approach, the
purpose of the present work is to evaluate the performance of WO surface modified by
3
fluorine anions and their implications in the mechanistic process of the selective
oxidation of methane to methanol.

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2
2
. Experimental
.1. Catalyst synthesis
KIT-6 mesoporous silica material was synthesized following the procedure reported in
literature [13]. Ordered mesoporous WO was prepared as follows: a solution of 5 mmol
3
of phosphotungstic acid hydrate (Alfa-Aesar) in ethanol (Aldrich) was incorporated into
0
.75 g of as-prepared KIT-6 silica under stirring. The sample was dried and then
calcined at 350ºC for 4h to decompose the precursors, and then further at 550 ºC for 6h
to obtain WO inside the hosting silica. Later, the obtained material was suspended
3
under stirring in a 10 % wt HF solution to remove the KIT-6 silica template. Finally, the
mesoporous WO catalyst was separated by centrifugation, washed sequentially with
3
water and ethanol and dried at room temperature. The WO /F was prepared by treatment
3
of the surface of the as-prepared mesoporous WO with hydrofluoric acid. First, 33 µL
3
of 50% HF were added into 15 mL of the aqueous slurry containing 0.3 g of WO under
3
magnetic stirring for 4 h. Later, the suspension was centrifuged, rinsed with water for
several times, and finally, the powder was dried at 50ºC.
2
.2. Catalyst characterization
The crystallinity was determined by X-ray powder diffraction (XRD) using a
diffractometer with Cu Kαradiation source, a LYNXEYE super speed detector and a Ni
filter (Bruker D8 Advance). The light absorption properties were measured using UV–
vis diffuse reflectance spectrophotometer (Perkin Elmer Lamba 950 UV–Vis) with a
wavelength range of 250–800 nm.

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2
.3. Photocatalytic setup
The photocatalytic partial oxidation of methane tests were performed in a
photochemical reactor (Ace Glass) of 500 mL volume equipped with gas inlet and
outlet. A medium pressure quartz mercury-vapor lamp (immersion-type Ace Glass)
inside the reactor was used to provide UVC-visible light irradiation. The reaction
temperature was maintained at 55 ˚Cby recirculation of cold water in the outer jacket of
-1
-1
the lamp. A mixture of methane (4.5 mL min ) and helium (17.9 mL min ) was
sparged continuously through the photocatalytic reactor. In every experiment, 0.3 g of
the photocatalyst was added into 300 mL of water (milli-Q). Prior to irradiation, the
suspension was magnetically stirred in the dark for 30. After that, the lamp was turned
on and gas samples were periodically taken for analysis. Blank test with water under
UVC-visible irradiation was conducted to examine the products due to photolysis. The
products were analyzed using a Shimadzu GC-2010 chromatograph equipped with a
capillary column (HP-PLOT Q), a thermal conductivity detector (TCD) and a flame
ionization detector (FID).
3
. Results and discussion
Fig. 1 shows the powder XRD patterns of pure WO and WO /F. All the diffraction
3
3
peaks of both samples indicate the formation of monoclinic structure of WO (JCPDS
3
Card No. 43-1035). Clearly, the fluorination process does not affect the crystal structure
of WO . The crystal size was estimated to be 11.7 nm and 11.9 nm for WO and WO /F,
3
3
3
respectively, based on the Scherrer equation [14].

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The diffuse reflectance spectra of the present two samples are shown in Fig. 2.
Compared with pure WO , the WO /F exhibits a slightly higher visible light absorption.
3
3
Therefore, fluorination affected the optical absorption edge of WO . This influence in
3
the light absorption characteristics has also been reported for fluorinated surface TiO2
-
2-
and SrTiO . In the latter, it has been explained that the substitution of F to O must be
3
4
+
3+
compensated for by changing an equivalent number of Ti to Ti to maintain the
3
+
electroneutrality of the sample. As a result, the presence of Ti shifts the absorption
edge of SrTiO to visible light range [15] and [16].
3
The photocatalytic conversion of methane to methanol was studied from aqueous
suspension containing the as-synthesized WO and WO /F samples under UVC-visible
3
3
light irradiation. A blank experiment consisting on the irradiation of methane in the
presence of water (no catalyst) evidences the product formation (Fig 3). It is well-
known that the irradiation of water with a deep-UV lamp (185nm) leads to the
generation of hydroxyl radicals by photolysis [17] and [18].
ꢂꢃꢄꢅ ꢆꢇꢈꢉꢊꢋꢌ
ꢀꢁ
ꢍꢎꢎꢎꢎꢎꢎꢎꢎꢎꢏꢐꢑꢒ ꢓ ꢁꢀ
(1)
ꢀ
These free hydroxyl radicals can initially perform the activation of methane to produce
methyl radicals and then, a series of subsequent reactions involve the production of
methanol, ethane, CO and other minor products.
2
ꢀ
ꢀ
ꢔ
ꢓ ꢁ ꢖꢔ
ꢓ
ꢗ
ꢀꢁ
(2)
ꢕ

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As can be seen in Fig. 3 when the photocatalytic reaction is carried out in the presence
of WO , the yield of methanol increases 2.4-fold. The reaction is initiated by irradiation
3
of the WO slurries with light energy higher than ∼2.7 eV (that is, wavelengths of <459
3
-
+
nm) that generates an electron (e ) and hole (h ) pair in the catalyst as described in
scheme I. In principle, these photogenerated pairs can undergo reductive and oxidative
reactions on the catalyst surface and in the bulk.
+
Since the conduction band (CB) potential of WO is not negative enough to reduce H
3
6
+
5+
to H , the formed electrons react with WO to reduce the W to W . On the other
2
3
hand, this process also involves the production of HO⦁ by the reaction of
photogenerated holes with water or hydroxide ions adsorbed on the surface. According
to Hameed et al., methane is an inert molecule that is not adsorbed on the catalyst, then,
the photocatalytic oxidation reactions occur in the bulk of the catalyst [19]. However,
the mechanism of the generation of methanol in the presence of the catalyst with water
under UVC-visible irradiation is not clear.
Scheme I
ꢂꢃ
ꢜ
ꢟ
ꢘꢁ ꢍꢏ ꢙ ꢓꢝ
ꢗ
ꢚꢛ
ꢞꢛ
ꢂꢃ
ꢜ
ꢜ
ꢢꢟ
ꢖꢘꢉꢟ
ꢙ
ꢍꢏ ꢙ ꢓꢘ
ꢚꢛ
ꢠꢡ
ꢟ
ꢀꢁꢣꢤꢥ ꢖ ꢁꢀ
ꢟ
ꢝ
ꢝ
ꢔ
ꢓ
ꢓ
ꢣꢤꢥ
ꢞꢛ
ꢟ
ꢓ ꢁꢜ ꢖ ꢁꢀ
ꢞꢛ
ꢣꢤꢥ ꢣꢤꢥ
ꢓ ꢁꢀ ꢖꢔ
ꢀ
ꢓ ꢀꢁ
ꢕ
ꢣꢤꢥ
ꢗ
ꢐ
ꢀ
ꢄꢔ
ꢓ
ꢀꢁ
ꢣꢤꢥ
ꢖꢔ ꢗꢁ ꢓ
ꢗ
ꢀ
ꢒ
In fact, if the mechanism of photocatalytic conversion of methane occurs via
homogeneous radical reaction in bulk solution, the addition of a source of OH radicals
as H O would increase the production of CH OH. Taylor and Nocetti [7] reported a
2
2,
3
remarkable increase in the CH OH production after addition of this chemical to the
3

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suspension. Even though, in reference [20] and in the present work, the opposite effect
was observed (Fig. 3). Gondal et al. suggested that the decrease of the production of
methanol in the system WO /H O is due to its type of irradiation (visible laser). Since
3
2
2
the laser irradiation emits a high flux density monochromatic light, it is not necessary to
add external hydroxyl radical generator. However, although the lamp used in these
experiments exhibits the same energy distribution of the lamp described by Taylor and
Noceti, it was not observed such an increase of yield of CH OH after the addition of
3
H O .
2
2
In addition, a treatment of the surface of WO with hydrofluoric acid was carried out to
3
partially occupy the interaction sites of WO . As shown in scheme II, the surface OH
3
would be replaced by fluoride, then, on this WO /F, the holes photogenerated, directly
3
would react with water molecules to produce only free hydroxyl radicals [21] and [22].
Scheme II
ꢟ
ꢜ
ꢘꢦꢁ ꢓ
ꢓꢧ ꢖꢘꢦꢧꢓ ꢀꢁ
ꢘꢦꢧꢓꢝ ꢓ ꢁꢖꢘꢦꢧꢓ ꢁꢀ
ꢟ
ꢓ
ꢨꢡꢩꢩ
ꢟ
ꢀ
As can be seen from Fig. 3, the fluorination of WO led to a decrease in the yield of
3
-1
methanol. Strikingly, the methanol generated (2.4 µmol h ) was almost the same that
-1
the blank experiment without catalyst (2.1 µmol h ). This result suggests that about 6%
of the methanol photogenerated is due to direct hole oxidation (solution bulk) and 94%
due to adsorbed hydroxyl radicals. Hence, a surface catalyst with active sites is essential
for the improvement of the generation of methanol under UVC-visible light irradiation
with water. On the other hand, considering the yields of ethane corresponding to each
system (Fig. 3), it seems clear that increasing the content of free OH radicals in the
aqueous slurry leads to a high formation of methyl radicals that stimulate the increase of
ethane. Hence, in the system WO /H O is observed the highest selectivity toward C H .
3
2
2
2
6

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(See the Supplementary data, Table S1).
Based on the above, the reaction scheme shown in Fig. 4 has been proposed for the
photocatalytic conversion of CH on WO . It can be assumed that although methane is
4
3
not adsorbed on the catalyst (See the Supplementary data, Fig. S1), the mechanism in
this photocatalytic conversion would imply a synergistic effect between water and
catalyst that improves the generation of methanol. Thus, the hydroxyl radical formed in
the photolysis of water activate the methane by hydrogen abstraction to produce mainly
methyl radicals and then these reactive species would react with OH radicals adsorbed
on the catalyst to increase the yield of methanol. On WO /F the reaction proceeds
3
almost entirely through homogeneous hydroxyl radicals. As a result, the generation of
ethane and total oxidation is favored.
4
. Conclusions
Summing up the results, it can be concluded that the modification of the surface of WO3
by fluorination affects negatively the yield of methanol. This effect is attributed to a
change in the reaction mechanism that favors the free OH radical formation in the
aqueous medium. Also, the addition of a hydroxyl radical generator to the reaction
results in a decrease of the generation of methanol compared to pure WO . Then, our
3
findings describe for the first time that OH radical groups on the catalyst surface are the
main responsible to enhance the performance of WO in the selective oxidation of
3
methane to methanol while a larger amount of free OH radicals favor the formation of
ethane.

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Acknowledgements
This work was partially supported by the European Regional Development Funds
ERDF, FEDER Programa Competitivitat de Catalunya 2007-2013), Ministerio de
(
Economia y Competitividad (CSD2009-00050), and the Framework 7 program under
the project CEOPS (FP7-NMP-2012-309984).

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Figure captions
Fig. 1. XRD patterns of WO , WO /F and the corresponding diffraction pattern of
3
3
monoclinic phase (JCPDS 43-1035).
Fig. 2. Diffuse reflectance spectra of WO and WO /F samples and spectral energy
3
3
distribution of the quartz mercury-vapor lamp.
Fig. 3. Yields of products in the photocatalytic oxidation of CH on blank, WO /F,
4
3
WO /H O and WO at ~50 ºC under UVC-visible light irradiation. Catalyst dosage is
3
2
2
3
-1
1
g·L . Data corresponding to 2 h of irradiation in continuous methane flow of 4.5 mL
-1
min . Error bars represent standard deviation.
Fig. 4. Proposed reaction pathway for the selective oxidation of methane to methanol on
WO and WO /F.
3
3

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Graphical abstract

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Highlights



Photocatalytic conversion of CH over WO and WO /F was studied.
4
3
3
Surface fluorinated WO led to low yield of methanol.
3
Reaction pathway was proposed for the photocatalytic conversion of CH on
4
WO3.