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Electrochimica Acta
jou rn al h om epa ge: www.elsev ier .com/ locate /e lec tac ta
lectrosynthesis of methanol from methane: The role of V2O5 in the reactionelectivity for methanol of a TiO2/RuO2/V2O5 gas diffusion electrode
obson S. Rochaa, Rafael M. Reisb, Marcos R.V. Lanzab, Rodnei Bertazzoli a,∗
Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica Rua Mendeleyev, 200, 13083-960 Campinas, SP, BrazilUniversidade de São Paulo, Instituto de Química de São Carlos Avenida Trabalhador São-carlense, 400, 13560-970 São Carlos, SP, Brazil
r t i c l e i n f o
rticle history:eceived 1 August 2012eceived in revised form6 September 2012ccepted 29 September 2012vailable online 13 October 2012
a b s t r a c t
Methane fed TiO2/RuO2/PTFE gas diffusion electrodes (GDEs) have been used for the electrosynthesisof methanol in 0.1 mol L−1 of Na2SO4 supporting electrolyte. By-products such as formaldehyde andformic acid are usually also formed during electrolyses, with the latter formed at a similar rate as thatof methanol. In this study, V2O5 was added to the composition of the GDE to improve its selectivityfor methanol. TiO2/RuO2/V2O5 powder hot-pressed with PTFE resulted in a GDE suitable for oxidativeelectrosynthesis with simultaneous oxygen evolution. By feeding the TiO2/RuO2/V2O5/PTFE GDE with
methane, it was possible to enhance its selectivity for methanol at low values of current density. Fur-thermore, the formation of formic acid and formaldehyde was suppressed, allowing for higher currentefficiencies. The addition of 5.6% of V2O5 increased the electrode’s current efficiency to 57% at 2.0 V, whichis 2-fold higher than the efficiency achieved in the absence of vanadium oxide.
Earlier papers have demonstrated the viability of oxidativelectrosynthesis using gas diffusion electrodes (GDEs) [1,2]. Gaseactants have been converted into more valuable reaction prod-cts using TiO2/RuO2 GDE. As an example, methane, a greenhouseas, has been oxidized to methanol, although formaldehyde andormic acid were formed simultaneously [1].
The GDEs have a porous structure with high efficiency in putting gas reactant in contact with a liquid reaction medium. It presents aonductive porous structure through which a pressure-driven gasercolates from one surface to another in contact with the elec-rolyte. The reaction takes place in the three phase section whereiquid, gas and catalyst are in close contact.
The gas flow rate can be chosen according to reactant to prod-ct reaction needs, which eliminates the mass transfer effects inhe reaction rate. Because the GDE is used as the anode in oxida-ive electrosynthesis, the choice of TiO2/RuO2 as the electrodease material, considering its physical stability under high posi-ive potentials. Furthermore, oxygen atoms resulting from water
lectrolysis remain chemically adsorbed as superoxides on the elec-rode surface, and are available for the oxidation of reactants toroducts [3].
On such oxide electrodes, the oxidation reaction takes place inmild conditions in which oxygen atoms are released and addedstep wise to the reactant molecule. This allows the operating con-ditions of the reaction to be adjusted to prevent the products fromtransforming into carbon dioxide.
The results reported in a previous paper demonstrated that aGDE composed of TiO2/RuO2 (70/30 w/w) hot-pressed with PTFEis stable and that the Ru redox couples maintain their reversibil-ity during electrolysis [1,4]. The oxide/PTFE GDE also showed afairly satisfactory performance in the oxidation of methane intomethanol, with a current efficiency of 30% [5]. Formic acid andformaldehyde were also formed, indicating the need to improve theelectrode’s selectivity for the synthesis of the targeted product [5].
In this work, V2O5 was included in the composition of the GDE toimprove its selectivity for methanol. In the literature, the vanadiumoxide is already used as a catalyst in the high temperature and pres-sure heterogeneous catalysis of alcohols by oxidation of methaneand ethane in the presence of sulfate [6]. In this process, methaneis transformed into methyl bisulfate, which is then hydrolyzed tomethanol mediated by vanadium redox couples [7].
2. Experimental
2.1. Preparation of gas diffusion electrodes
The oxide mass was prepared using a polymeric precursor solu-tion with a 1:1:3 ratio (titanium isopropoxide:citric acid:ethylene
lycol), to which a 0.2 mol L−1 solution of RuCl3 was added to reach final TiO2/RuO2 ratio of 70:30 w/w. Vanadium oxide was incor-orated as VCl3 0.2 mol L−1, thus, the final TiO2/RuO2/V2O5 ratiosere 68.9/29.6/1.5 w/w, 67.9/29.2/2.9 w/w and 66.1/28.3/5.6 w/w,
orresponding to percentages of 5%, 10% and 20% of V2O5. The per-entages of V2O5 were determined from the weight of RuO2, withhe ratio between RuO2 and TiO2 was kept fixed in 70/30 w/w. Theolutions were heated using a temperature ramp of 1 ◦C min−1 upo 450 ◦C and held at this temperature for 1 h under an O2 flux.
The calcined oxide mass was then milled thoroughly using mor-ar and pistil and the hydrophobic binder, a 60% PTFE dispersion3M Dyneon TF 3035 PTFE) was added for preparing the catalytic
ass. The ratio of oxide to Dyneon was 8/3.3, which is equivalento 20% of PTFE w/w. The mixture was then homogenized in a solu-ion of 4:1 bi-distilled water:isopropanol, after which it was driedt 120 ◦C for 12 h. A 10 mm diameter pressing tool was filled with.5 g of the precursor mass. A 3-mm-thick GDE was obtained afterintering for 2 h at 340 ◦C under a load of 200 kgf cm−2 [8].
Cyclic voltammetry experiments were performed to identifyhe metal redox couples present in the oxides. The electrode forhese experiments was a 0.12 cm2 vitreous carbon disc coated with
micro-porous layer of the metal oxide without the hydropho-ic binder. The micro-porous layer on the vitreous carbon discas applied by the drop wise deposition of 8 �L of a suspen-
ion consisting of 7 mg of metal oxide + 1 mL of H2O + 120 �L ofafion solution (30%). The suspension was dried under N2 flux.
conventional single-compartment three-electrode cell was usedor the cyclic voltammetry experiments. Potential was scannedt 20 mV s−1 from 0.0 V to 1.1 V vs SCE in a N2 purged solutionf 0.5 mol L−1 of H2SO4. The 100th cycle was recorded to ensureeproducibility.
.2. Methane electro-oxidation experiments
For the experiments of electrogeneration of methanol fromethane, a supporting electrolyte consisting of 0.1 mol L−1 Na2SO4as used, maintaining a gas pressure of 1.0 kgf cm−2 at the rear
f the GDE. The working electrode potential was controlled in theange of 1.3 V to 2.3 V vs SCE (Saturated Calomel Electrode). Aingle-compartment three-electrode cell was used, with the work-ng electrode placed at the bottom, as has been done previouslyn experiments involving oxygen reduction to hydrogen peroxide
ith a carbon/PTFE gas diffusion electrode [8].A conventional single-compartment cell with three electrodes
Pt as counter-electrode; SCE as reference; GDE as work-electrode)as used for the methane oxidation experiments. The exposed area
f the GDE was 0.5 cm2. The potential of the working electrode wasontrolled using a PGSTAT30 Metrohm Autolab potentiostat.
In experiments of 1-h, were collected samples every 5 min dur-ng the first half hour and every 15 min during the second half anour, samples were injected into a Varian 3800 GC equipped with aaturn 2200 ion trap mass spectrometer, by direct ex-column injec-ion into the chromatoprobe. The injector program consisted ofeating from 35 ◦C to 120 ◦C at 100 ◦C min−1 and holding for 5 minnd a split of 1/50. The same program was used for the column. Inhe mass spectrometer, 40 �A of energy emission was used with aeaction time of 120 �s for a mass range of 20–100 m/z.
. Results and discussion
Fig. 1A shows the X-ray diffractogram of TiO2/RuO2/V2O5 pow-
er containing the highest percentage of V2O5 (5.6%). Along withhe expected peaks of both phases of TiO2, anatase (A) and rutileR), well defined peaks of RuO2 and V2O5 are also visible. Fig. 1Bhows the distribution of RuO2 and Fig. 1C shows the distribution
Fig. 1. (A) X-ray diffractogram of TiO2/RuO2/V2O5 powder containing 5.6% of V2O5.Distribution of RuO2 (light color in B) and V2O5 (light color in C) on the GDE surfaceafter hot pressing with PTFE. Dark color represents other species in both figures.
of V2O5, in the GDE surface after hot pressing with PTFE. The SEMimage shows the species distribution on the electrode surface,thus, in Fig. 1B the RuO2 is represented by the light color in Fig. 1Cthe V2O5 is represented by the light color and in the figures B and Cthe other species are represented by the dark color. Despite someinhomogeneity, RuO2 and V2O5 are distributed over the entireelectrode surface.
Fig. 2 illustrates the voltammetric responses obtained bycyclic voltammetry on TiO2/RuO2 and TiO2/RuO2/V2O5 surfaces.As described above, TiO2/RuO2 70/30 w/w and TiO2/RuO2/V2O566.1/28.3/5.6 w/w coatings were deposited drop wise onto a vitre-ous carbon (VC) disc using a suspension of the metal oxides withNafion; the micro-porous layer were then dried under a N2 flow.As Fig. 2 shows, the I/E couples obtained on the bare VC surface arecharacteristic of the voltammetric response of an inert conductivematerial within the range of potentials used in the experiment, inthe absence of electroactive species in the supporting electrolyte.After the VC disc was coated with the oxides, its capacitive currentswere greatly increased due to the oxides’ semiconductive nature.
The voltammogram recorded on TiO2/RuO2 shows two distinctredox couples, as indicated in Fig. 2. The surface electrochemistry ofTiO2/RuO2 is governed by Ru redox couples. Two anodic peaks, oneat about 0.45 V vs SCE and the other at 0.90 V vs SCE, represent the
ig. 2. 100th cycle of a series of cyclic voltammograms obtained on vitreous carbonVC), TiO2/RuO2 and TiO2/RuO2/V2O5. Solution of 0.2 mol L−1 of Na2SO4. Scan ratef 20 mV s−1.
ransitions between the more stable oxidation states Ru3+ → Ru4+
nd Ru4+ → Ru6+, respectively, and the cathodic peaks at 0.90 Vnd 0.26 V represents the reduction of Ru6+ and Ru4+, respectively.hese oxidation states of transition metals have been identified pre-iously [9–12], although a good definition of the latter, which isighly dependent on the upper potential limit and on the potentialcan rate [9,10].
When V2O5 was added to TiO2/RuO2, the currents resulting fromotential scanning were slightly higher, as shown in Fig. 2. Thexpected anodic peak currents of vanadium correspond to the tran-itions of V3+ → V4+ and V4+ → V5+ [13,14]. However, only the peakf the last transition appeared at 0.60 V vs SCE. The expected peakorresponding to the V3+ → V4+ transition was found to be inac-ive in the coating compositions, possibly due to the conditionsmployed in these experiments.
Subsequently, GDEs were prepared with increasing amountsf V2O5 in TiO2/RuO2, as described above. These GDEs weresed for the oxidation of methane to methanol in a 0.2 mol L−1
olution of Na2SO4. The average permeability of the as-preparedDEs, recorded under a Darcian flow, was 5.2 × 10−4 Darcy, as
eported previously [1]. This parameter means that under a pres-ure drop of 1.0 kgf cm−2, such as the one used in this work, theH4 flow rate through the electrode is 6.8 × 10−3 mg s−1. Consid-ring the area of the electrode’s flat geometry, the flow rateas 1.1 × 10−2 mg s−1 cm−2. In each 1-h experiment, 24.5 mg ofethane was consumed.Fig. 3 shows the methanol concentration profiles obtained for
ontrolled potential experiments performed at 2.0 V vs SCE. Theower curve corresponds to the GDE without V2O5. As can beeen, the methanol concentrations increased with the percentagef V2O5. No apparent limit was detected for the concentration ofanadium oxide in the electrode. In other words, the more vana-ium oxide the electrode contains the more methanol is formed.owever, the GDE reached its maximum selectivity for methanol at.6% of V2O5. As already known, the main by-product of the oxida-ive electrosynthesis of methanol is formic acid, as well as minormounts of formaldehyde.
The apparent rate constants of the synthesis of methanol andormic acid were very similar, resulting in an equal concentrationt the end of the experiments [1]. During the controlled potentialxperiments, the concentrations of these compounds were deter-
ined by gas chromatography after 60 min of electrolysis at 2.0 V.
able 1 lists these results and indicates that the selectivity of theDE for methanol was significantly improved by the addition ofanadium oxide to TiO2/RuO2. Formic acid was not detected at
Fig. 3. Methanol concentration as a function of electrolysis time for the V2O5 per-centage as shown. Applied potential of 2.0 V. Solution of 0.2 mol L−1 of Na2SO4.
1.5% of V2O5, although very low concentrations of formaldehydepersisted at 5.6% of V2O5.
The route for the oxidative electrosynthesis of methanol andits by-products on conductive oxide electrodes (MOx) follows atraditional mechanism in which the discharge of water generateshydroxyl radicals, which, in turn, chemically adsorb onto the oxidesurface in the form of oxygen atoms. These oxygen atoms thenoxidize an organic compound, according to Eqs. (1)–(3) [3].
MOx + H2O → MOx(•OH) + H+ + e− (1)
MOx(•OH) → MOx O + H+ + e− (2)
CH4 + MOx O → CH3OH + MOx (3)
However, CH4 is also oxidized simultaneously by the redox cycleof ruthenium oxide. In the operational conditions of electrolysis,two ruthenium redox couples are active, as illustrated in Fig. 2:Ru3+/Ru4+ and Ru4+/Ru6+. These redox couples catalyze the forma-tion of methanol and formic acid. The first reaction steps consistof the dehydrogenation of CH4 to adsorbed CH3 and CH2, followedby further oxidation to methanol or to formic acid, respectively. Aspart of the process for the formation of methanol, Ru6+ is reducedto Ru4+, as shown in Eqs. (4) and (5).
CH4 + RuO22+ → CH3 RuO2
+ + H+ (4)
CH3 RuO2+ + H+ → CH3OH + RuO2+ (Ru6+ → Ru4+) (5)
Simultaneously, for the formation of formic acid, the dehydro-genation of CH4 to CH2 is followed by the reduction of Ru6+ to Ru4+
and then to Ru3+, as follows:
CH4 + 2RuO22+ → RuO2
+ CH2 RuO2+ +2H+ (6)
RuO2+ CH2 RuO2
+ → RuO+ O CH2 O RuO+
(Ru6+ → Ru4+) (7)
RuO+ O CH2 O RuO+ → HCOOH + 2RuO+
(Ru4+ → Ru3+) (8)
Although ruthenium oxide confers metallic conductivity on the
electrode, it is also responsible for the low selectivity for methanol.Table 1 indicates that significant amounts of formic acid andformaldehyde were formed in the absence of V2O5. The additionof 1.5–5.6% of vanadium oxide suppressed the formation of formic
Table 1By-products of the electrosynthesis of methanol from methane in 0.1 mol L−1 of Na2SO4 solution. Electrolysis time of 60 min at 2.0 V.
V2O5 in the TiO2/RuO2/V2O5 GDE (%) Methanol (mg L−1) Formaldehyde (mg L−1) Formic acid (mg L−1)
0% 103 24 1121.5% 179 36 nd
14 nd7 nd
n
acfirtf(
C
C
5otp
gfiiAcsp
iwelpmas
t
FaT
1.2 1.4 1.6 1.8 2.0 2.2 2.415
20
25
30
35
40
45
50
55
605,6% V
2O
5
0% V2O
5
Cur
rent
eff
icie
ncy
(%)
E vs SCE / V
2.9% 210
5.6% 297
d: not detected.
cid and formaldehyde. As Fig. 2 shows, only one vanadium redoxouple is available for the reaction, which may not produce a suf-cient amount of electrons for the formation of the double bondequired for the formation of formic acid and formaldehyde. Afterhe dehydrogenation of CH4, adsorbed CH3 is oxidized to methanol,ollowed by the reduction of V5+ to V4+, according to Eqs. (9) and10).
H4 + V2O42+ → CH3 V2O4
+ + H+ (9)
H3 V2O4+ + H+ → CH3OH + V2O3
2+ (V5+ → V4+) (10)
In view of the performance of the 66.1TiO2/28.3RuO2/.6V2O5 w/w electrode and its ability to suppress the formationf formic acid and greatly reduce the formation of formaldehyde,his electrode composition was selected for the subsequent com-lementary experiments.
In a series of electrolysis experiments at applied potentials ran-ing from 1.3 to 2.3 V vs SCE, methanol concentration profiles weretted linearly as a function of time. During the first 30 min of exper-
ment, the methanol concentration followed zero-order kinetics.t each value of applied potential, the methanol concentrationhanged over time by a zero-order rate constant, i.e., dC/dt = k. Fig. 4hows the apparent rate constant values as a function of appliedotential.
The zero-order rate constant for the synthesis of methanolncreased with potential in a constant slope up to about 2.0 V, at
hich point the slope leveled off somewhat. Needless to say, thentire oxidation process took place with simultaneous oxygen evo-ution. As the applied potential shifted towards a more positiveotential the adsorbed oxygen atoms (see Eq. (2)) evolved intoolecular oxygen rather than transferring to CH4 molecules. As
consequence, the current efficiency stabilized or even decayedtarting from 2.0 V.
Fig. 5 compares the current efficiencies of two GDEs, one ofhem containing V2O5. The data plotted in Fig. 4 refer to 30 min
1.2 1.4 1.6 1.8 2.0 2.2 2.46
8
10
12
14
16
18
(k 1
0-2)
/ mg
L-1s-1
E vs SCE / V
ig. 4. Zero-order apparent rate constants for the electrosynthesis of methanol as function of applied potential using 66.1TiO2/28.3RuO2/5.6 V2O5 w/w electrode.ime of electrolysis: 30 min.
Fig. 5. Current efficiency obtained during the electrosynthesis of methanol as afunction of applied potential for the TiO2/RuO2 70/30 w/w and TiO2/RuO2/V2O5
66.1/28.3/5.6 w/w GDEs.
of controlled potential electrolysis in which the final mass ofmethanol, which was formed by the exchange of two electrons (Eqs.(1) + (2) + (3)), was used in the Faraday equation. The electrical cur-rent values thus obtained were compared with the steady statecurrent resulting from the applied constant potential. An analysisof Fig. 5 reveals that, due to the non-formation of formic acid andformaldehyde, the current efficiency was higher in the presence ofV2O5, i.e., almost 2-fold higher than that achieved in the electrolysisperformed at 2.0 V.
4. Conclusions
TiO2/RuO2/V2O5 powder hot-pressed with PTFE resulted ina GDE suitable for oxidative electrosynthesis with simultaneousoxygen evolution. Feeding the TiO2/RuO2/V2O5/PTFE GDE withmethane increased its selectivity for methanol at low current den-sities. The main by-products, formic acid and formaldehyde, weresuppressed during the electrosynthesis of methanol, allowing forhigher current efficiencies. The addition of 5.6% of V2O5 to the elec-trode current raised its efficiency to 57% at 2.0 V, which is 2-foldhigher than the result obtained in the absence of vanadium oxide.
Acknowledgements
The authors gratefully acknowledge to Brazilian Funding Insti-tutions for financial support: Fundac ão de Amparo à Pesquisa doEstado de São Paulo (FAPESP), Coordenac ão de Aperfeic oamento dePessoal de Nível Superior (CAPES) and ConselhoNacional de Desen-volvimentoCientifico e TecnológicoCNPq).
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