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Electro-oxidative desulfurization of a thiophene-containing model fuel using a square wave potentiometery technique Zeinab Alipoor, Amir Behrouzifar, Soosan Rowshanzamir, and mansour Bazmi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b00354 • Publication Date (Web): 20 Apr 2015 Downloaded from http://pubs.acs.org on May 6, 2015
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Electro-oxidative desulfurization of a thiophene-containing model fuel using a
2
square wave potentiometery technique
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Zeinab Alipoor,a,b Amir Behrouzifar,a,b Soosan Rowshanzamir,*,a,b Mansour Bazmi,c
4
a
5 6
Fuel Cell Laboratory, Green Research Center, Iran University of Science and Technology (IUST), Narmak, Tehran, 16846-13114, Iran
b
c
School of Chemical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, 16846-13114, Iran
Faculty of Research and Development in Downstream Petroleum Industry, Research Institute of Petroleum Industry (RIPI), Tehran, 14857-33111, Iran.
7
* Tel./fax: +98 21 77491242, E-mail address:
[email protected] (Corresponding author)
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ABSTRACT
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A dynamic electrochemical technique was used for desulfurization of thiophene containing solutions.
10
Firstly, adsorption of thiophene from an aqueous solution onto a platinum electrode surface and electro-
11
oxidative behavior of thiophene were studied using cyclic voltammetry technique and then, a square
12
wave potentiometery method was utilized to electrochemically desulfurize the aqueous thiophene
13
solution and a thiophene-containing model fuel. Results indicated that, for thiophene molecules the best
14
adsorption potential is 0.2 V and the maximum electro-oxidation rate occurs at 1.1 V. Also, the optimal
15
square wave frequency was found as 50 Hz. Ion chromatographic measurement of sulfate ion
16
concentration revealed that reaction conversion was 100% in the electro-oxidative desulfurization of
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aqueous thiophene solution. The high desulfurization efficiency can be attributed by excellent electro-
18
catalytic activity of platinum electrode and performing experiments at the best operating conditions.
19
Based on gas chromatographically quantitated thiophene concentration in the hydrocarbon phase,
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desulfurization reactions reached a conversion of about 88% during electro-oxidative desulfurization of
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a model fuel/aqueous electrolyte emulsion. The lower desulfurization efficiency can be attributed by
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addition of a mass transfer (from hydrocarbon to aqueous phase) resistance and lessening overall
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thiophene concentration. Ion chromatographic analysis of the aqueous phase revealed that only 38% of
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thiophene molecules were completely oxidized to sulfate ions. Fourier transform infrared spectroscopy
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studies of the hydrocarbon and the aqueous phases showed that remaining part of converted thiophene
7
molecules were partially oxidized to a sulfone and an organic sulfate compounds. Due to polarity of
8
these components, which results in their aqueous phase solubility, no more steps were required to
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separate the products of desulfurization from the hydrocarbon phase. Based on the obtained results,
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electrochemical desulfurization can proposed as an effective alternative for the commercial
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hydrodesulfurization processes.
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Keywords: Electro-oxidation; Desulfurization; Thiophene; Square wave potentiometery.
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1. Introduction
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Combustion of fossil fuels with high sulfur content release detrimental sulfur oxides into the atmosphere
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and causes numerous environmental problems. Accordingly, increasingly severe environmental
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regulations have been legislated in many countries to lessen sulfur levels in the fossil fuels.
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Hydrodesulfurization (HDS)
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hydrocarbon mixtures. Using HDS it is possible to reduce sulfur content of typical fossil fuels to lower
19
than 10 ppm and simultaneously remove nitrogen, oxygen and metals. Unfortunately, investment
20
required and operating costs of HDS units are too high and also HDS units possess some operational
21
difficulties such as possibilities of losing octane number (due to saturation of olefin and aromatic
22
compounds) 3, reacting produced hydrogen sulfide with olefins and formation of mercaptans 4, cracking
1, 2
is the exclusive commercial process for eliminating sulfur from the
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of valuable hydrocarbons at high temperatures, and deactivation of catalyst with coke and metal
2
deposition 5. Consequently, an extreme efforts have been put by research and development communities
3
to find an economical and convenient substitute for hydrodesulfurization. Several review articles were
4
addressed the developments of non-HDS routes for desulfurization of liquid fuels
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desulfurization (ODS)
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pervaporative desulfurization
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novel routes for liquid fuels desulfurization. All of these methods are in research and development steps
8
and retain their own drawbacks, e.g. low desulfurization efficiency, deteriorating the fuel quality, high
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cost, difficulty in industrial application, etc. Hence, it is necessary to make much more endeavor to
8, 9
, biodesulfurization (BDS) 14, 15
10, 11
6, 7
. Oxidative
, adsorptive desulfurization (ADS)
and electrochemical desulfurization (ECDS)
16, 17
12, 13
,
are some of the
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increase effectiveness of these methods for practical uses.
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Electrochemical desulfurization (ECDS) technology has been proposed to remove sulfur by the
12
electrochemical oxidation or reduction of sulfur compounds in the fossil fuels. This technology is able to
13
remove sulfur at quite low temperature and pressure. Using this method, it is possible to control products
14
distribution by adjusting the applied potential or to control overall conversion rate via adjusting the
15
applied current. Besides, various choice of process schemes (i.e. static or dynamic), reactors (i.e. divided
16
or undivided), electrolytes (i.e. acids, bases, salts, or ionic liquids), and electro-catalysts (i.e. both
17
precious and non-precious metals), make the ECDS very flexible where it can be used to both solid and
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liquid fuels
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applied potential varies over time. By changing the electrode potential, it is likely to subject the
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electrode in both adsorption and oxidation conditions
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desulfurization techniques are possibility of undesired side reactions (e.g. hydrogen and/or oxygen
22
evolution) occurrence, which consume energy and diminishes the overall efficiency, and electro
23
catalytic activity decline by contaminants deposition on the electrodes surface
16
. In dynamic electrochemical methods, such as linear or square wave voltammetry, the
18
. The main disadvantages of electrochemical
16
. Furthermore,
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polymerization of hydrocarbons and formation of sulfur oligomers, which may be trapped in the
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polymerized hydrocarbon clusters, could results in lowering fuel quality 16.
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Wang et al. 19, 20 reported deep desulfurization of gasoline using the electrochemical catalytic oxidation
4
on particle group anodes of β-PbO2/C and CeO2/C with an electrochemical fluidized-bed reactor. They
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concluded that the particle group anode can remarkably accelerate the reaction rate and promote the
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electrochemical catalysis performance. They reported sulfur concentration reduction in gasoline from
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310 to 40 and 310 to 50 parts per million by weight for β-PbO2/C and CeO2/C anodes, respectively.
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Korotaeva et al.
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compounds on a mercury cathode at various compositions of the supporting electrolyte and finally found
21
studied the electro-reduction of complicated nitrogen and sulfur containing organic
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the conditions for selective electrochemical desulfurization.
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Shen et al.
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slurry (CWS) under the ambient conditions. They reported desulfurization efficiency improvement with
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the increasing NaBO2 concentration and up to 58.5% total sulfur reduction for CWS sample. Also, Shen
14
et al.
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borohydride chemical and electrochemical reduction. Later, Shu et al.
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combination with pulse voltage usage for coal desulfurization process using a boron-doped diamond
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(BDD) thin film electrode and reported a higher desulfurization efficiency (64%) compared with the
18
common electrochemical desulfurization techniques.
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Zhang et al. 25 reported the enhanced desulfurizing flotation of high sulfur coal by sonoelectrochemical
20
method. They found the optimal conditions (i.e. additive concentration, sonoelectrolytic time, current
21
density, and ultrasound intensity) for the coal samples with different size fractions and reported a sulfur
22
reduction of up to 75.4%. Gong et al. 26 studied the effects of ionic liquid addition on desulfurization of
17
22, 23
used sodium metaborate (NaBO2) in the electro-reductive desulfurization of coal water
reviewed recent developments in the desulfurization of coal water slurry using sodium 24
applied the same concept in
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a coal water slurry by constant-current electrolysis and reported more removal of organic sulfur in coal
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via increasing ionic liquids content. Recently, Hammad and coworkers 27 presented a simplified process
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flow diagram (PFD) for in-situ electrochemical hydrodesulfurization of crude oil and its fractions, which
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can be a very useful concept for industrial developments of ECDS technology.
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The objective of this paper was the desulfurization of a thiophene containing model fuel using a
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dynamic electrochemical method. For electrochemical desulfurization of thiophene containing solutions,
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two sequential processes should be occurred: adsorption of thiophene molecules from the bulk solution
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onto the electrode surface and then oxidation of the adsorbed thiophene molecules. These phenomena
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were studied separately in this work. Firstly, sulfur removal from an aqueous thiophene solution was
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investigated. For this purpose, adsorptive and oxidative behavior of thiophene at surface of a platinum
11
electrode were studied experimentally using cyclic voltammetry technique. After that a square wave
12
potential program was applied for electrochemical desulfurization of the aqueous thiophene solution.
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Finally, the thiophene containing model fuel (which was emulsified in an aqueous electrolyte solution
14
before) was desulfurized by the square wave potentiometery technique. At each stage the reactions
15
products were analyzed with ion chromatography, gas chromatography and Fourier transform infrared
16
(FT-IR) spectroscopy to measure the desulfurization degree and determine the products distribution.
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2. Materials and methods
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2.1. Materials and setup
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All the reagents used (thiophene, sulfuric acid, perchloric acid, n-decane, Tween 20, Span 20) were
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purchased from Merck Company. A silicon carbide 5000 mesh sandpaper (991A Softflex, Matador
21
GmbH & Co. KG) and 0.05 µm alumina powder were used for electrode polishing.
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A potentiostat (SP-150 with EC-Lab software, Bio-Logic SAS) was utilized for electronic control and
2
data acquisition. A conventional three electrodes cell was used to carry out the experiments. The
3
reference electrode was Ag/AgCl saturated with KCl and all the potentials reported in this work are
4
respect to this electrode. The counter electrode was a 12 cm2 platinum foil in the all experiments. The
5
working electrode for the voltammetry measurements was a platinum disk with 2 mm diameter, where
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just one side of the disk was exposed to the solution. Another 1.28 cm2 platinum foil electrode was used
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for the bulk desulfurization experiments. The electrodes material was 99.99% platinum. All the working,
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counter and reference electrodes were supplied by Azar Electrode Company.
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Ion chromatography (819 IC detector/820 IC separation center, Metrohm AG with Metrosep A Supp 5 –
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250/4.0 anion separation column), gas chromatography (CP-3800 GC, Varian Inc. equipped with sulfur
11
chemiluminescence detector (SCD) for determining sulfur containing compounds concentration and
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7890A GC, Agilent Technologies equipped with thermal conductivity detector (TCD) for measuring
13
hydrocarbon compounds concentration), and FT-IR spectroscopy (VERTEX 70, Bruker Corporation)
14
were used to investigate composition of the reactant and product solutions.
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2.2. Experimental procedures
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Aqueous solution of 0.5 M sulfuric acid (H2SO4) and 1.0 M perchloric acid (HClO4) were used as the
17
supporting electrolyte. When quantitative measurement of sulfate ion was required, perchloric acid was
18
used as the electrolyte. All the solutions were made from deionized water. Before each experiment, the
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solutions were deoxygenated by bubbling with ultra-pure nitrogen stream for a while. All the
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experiments were conducted at ambient temperature and pressure.
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2.2.1. Thiophene electro-adsorption from aqueous solution
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Chronoamperometry technique with fixed potential was utilized to study thiophene adsorption (from a
2
solution of 1.0 mM thiophene in 0.5 M H2SO4) onto surface of the platinum disk working electrode.
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Effects of electrode potential (-0.2 to 0.4 V) on the thiophene adsorption were studied experimentally.
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After applying a constant potential for about five minutes, the solutions were drained and the cell was
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refilled with the fresh (thiophene-free) 0.5 M H2SO4 solution. Finally voltammogram of the thiophene-
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adsorbed platinum electrodes were logged in the fresh sulfuric acid solution using cyclic voltammetry
7
technique with a scan rate of 50 mV/s.
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In the cyclic voltammetry studies, the reactants concentration preferred to be invariant. To prevent major
9
changes in the reactants composition, ratio of the working electrode surface to the solution volume must
10
be kept small enough; in this manner the reactions rate became extremely small and hence the whole
11
process can be considered as quasi steady state. Therefore, in these experiments the small platinum disk
12
was used as the working electrode and the solution volume was about 12 cm3.
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2.2.2. Thiophene electro-oxidation in aqueous solution
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To study dynamic electrochemical adsorption/oxidation behavior of thiophene molecules in the aqueous
15
solution, a square wave potential program was designed using the stepwise potential fast
16
chronoamperometry (SPFC) technique which was supplied from the potentiostat. The lower and upper
17
limits of the square wave potential program were the optimum thiophene adsorption and oxidation
18
potentials, respectively. Effects of the square wave potential program frequency (1 to 100 Hz) on the
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desulfurization efficacy was investigated. These experiments were performed with the 1.28 cm2
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platinum foil working electrode in the solution of 1.0 mM thiophene in 0.5 M H2SO4. The square wave
21
potential program was applied to a bulk cell with 100 cm3 of the solution. Ratio of the working electrode
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surface to the solution volume in these experiments was about five times greater than that of cyclic
2
voltammetry studies.
3
A 12 cm3 portion of the solution was transferred to another cell after every 10 minutes. The separated
4
samples were studied using cyclic voltammetry. The platinum disk was utilized as the working electrode
5
and the scan rate was 50 mV/s. This trend was continued until the last voltammogram became similar to
6
the fresh platinum electrode voltammogram, which was recorded previously. When these
7
voltammograms became approximately analogous, the reactions are completed, because the whole
8
adsorbed thiophene molecules are reacted, the electrode surface is fresh, and there are no more
9
thiophene molecules present in the solution to be further adsorbed and oxidized.
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Finally, to measure the concentration of desulfurization products (i.e. sulfate ion), the previous
11
experiment was repeated with the 1.0 M HClO4 electrolyte solution. Concentration of sulfate ion in the
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reactant and product solutions were measured via ion chromatography technique.
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2.2.3. Model fuel preparation
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To study electrochemical desulfurization from a hydrocarbon mixture, a model fuel with 466 parts per
15
million by weight (ppmw) thiophene was prepared by dissolving an appropriate amounts of thiophene in
16
pure n-decane. To perform an electrochemical reaction, the solution must be electrically conductive. To
17
bring conductivity to the solution, the model fuel was emulsified in the electrolyte solution (i.e. 1.0 M
18
HClO4). Span 20 and Tween 20 were used as the emulsifier and an ultrasonic bath (WUC-D10H,
19
WiseClean) was utilized for mixing. Prior to desulfurization of the model fuel, optimum conditions of
20
the emulsification process were determined experimentally.
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Effects of the aqueous to hydrocarbon phase volume ratio, the emulsifier weight percent, the emulsifier
2
hydrophilic-lipophilic balance (HLB), mixing duration and intensity (i.e. power of ultrasonic bath) on
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the emulsions stability were investigated. As shown in the Table 1, a L16 Taguchi orthogonal array was
4
used to study these variables. In each experiment, initially Span 20 and Tween 20 were dissolved in the
5
hydrocarbon and aqueous phases, respectively and then the aqueous phase was added slowly to the
6
hydrocarbon phase under stirring at 1500 rpm via a magnetic stirrer. After that, the mixture was mixed
7
again for about ten minutes at 1500 rpm and finally was placed into the ultrasonic bath. The prepared
8
emulsions were transferred into a decanter and the emulsions stability were determined from volume
9
percent of the separated aqueous phase after 24 hours.
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2.2.4. Electrochemical desulfurization of model fuel
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After determining the optimal emulsification conditions, 100 cm3 emulsion of 1.0 M HClO4 electrolyte
12
solution and the model fuel was prepared. Electrochemical desulfurization of this emulsion was
13
performed in the three electrodes cell with the 1.28 cm2 platinum foil working electrode. Limits and
14
frequency of the square wave potential program were those found in the previous steps. To prevent
15
phase separation, the emulsion was mixed using the magnetic stirrer during the reactions. After 300
16
minutes, the cell contents were transferred into a decanter. After complete separation of the two phases,
17
composition of the hydrocarbon phase was determined via gas chromatography and concentration of
18
sulfate ion in the aqueous phase was measured by ion chromatography. Besides, FT-IR spectroscopy
19
was used to determine the possible products formed in the both hydrocarbon and aqueous phases.
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2.2.5. Electrode cleaning
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As the electrochemical methods are very sensitive to even small quantities of contamination, it is vital to
22
clean the electrode surface at the beginning of each experiment. For this reason, voltammograms of the
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as received (fresh) polycrystalline platinum disk electrode surface were obtained in 0.5 M H2SO4
2
electrolyte using cyclic voltammetry technique with a scan rate of 50 mV/s. Before starting each
3
experiment, the electrodes were polished by the silicon carbide sandpaper and then by the alumina
4
powder and finally rinsed with the deionized water. Moreover, an electrochemical route was utilized to
5
further clean the working electrode surface. To perform the electrochemical cleaning, the working
6
electrode was immersed in 0.5 M H2SO4 electrolyte solution and a cyclic voltammetry test (-0.2 to 1.2 V
7
with a scan rate 50 mV/s) was conducted. The voltammetry test was continued until the obtained
8
voltammogram became stable and similar to that of fresh electrode which was recorded before.
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3. Result and discussion
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3.1. Voltammogram of fresh platinum electrode
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Voltammogram of the platinum electrode surface shows the electron transfer surface reactions. In the
12
Figure 1 presents voltammogram of the fresh platinum surface in 0.5 M H2SO4 electrolyte with a scan
13
rate 50 mV/s. This voltammogram was used as the reference for comparing to other voltammograms
14
obtained at the adsorption/oxidation states. As in the potentials positive than 1.2 V oxygen evolution
15
reaction occurs and in the potentials negative than -0.2 V hydrogen gas releases, the potential range of -
16
0.2 to 1.2 V was used for cyclic voltammetry studies.
17
As can be seen in Figure 1, there are six peaks in the platinum electrode surface voltammogram. Each of
18
these peaks corresponds to an electron transfer reaction on the electrode surface. Peak 1 represents the
19
platinum surface oxidation which can be expressed by the following reactions 28:
20
Pt + H‒O‒H → Pt‒O‒H + H+ + e⁻
(1)
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Pt ‒O‒H + H‒O‒H → Pt‒(O‒H)2 + H+ + e⁻
(2)
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Pt‒(O‒H)2 → Pt‒O + H‒O‒H
(3)
2
Peak 2 represents the platinum oxide reduction 28:
3
Pt‒O + 2H+ +2e⁻ → Pt + H‒O‒H
4
Peaks 3 and 4 are related to hydrogen electro-adsorption on Pt (100) and Pt (111) crystal surfaces,
5
respectively 28:
6
Pt + H+ + e⁻ → Pt‒Hads
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Peaks 5 and 6 are correspond to hydrogen electro-desorption from Pt (111) and Pt (100) crystal surfaces,
8
respectively 28:
9
Pt‒Hads → Pt + H+ + e⁻
(4)
(5)
(6)
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3.2. Effects of potential on thiophene adsorption
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When a fixed potential applies to the solution of thiophene in the electrolyte, surface adsorption of
12
thiophene molecules partially inhibits hydrogen adsorption, therefore any changes in the peaks 3, 4, 5,
13
and 6, shown in Figure 1, must be related to the thiophene adsorption. To experimentally study this
14
phenomenon, a series of fixed potentials (-0.2, -0.1, 0, 0.1, 0.2, 0.3, 0.4 V) were applied to the aqueous
15
thiophene solution. Then, thiophene solution was rinsed out from the cell and voltammogram of the
16
electrode surfaces was recorded in 0.5 M H2SO4 fresh electrolyte.
17
Figure 2 shows first voltammogram cycle of the platinum electrode surface after adsorption of thiophene
18
at 0.2 V. Also, for ease of comparison, voltammogram of the fresh platinum surface is shown again in
19
this figure. As can be seen, the peaks which are belong to hydrogen adsorption and desorption are
20
disappeared in voltammogram of the thiophene adsorbed platinum electrode. The reason is partial
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coverage of the electrode surface with thiophene molecules which to some extent inhibits the hydrogen
2
adsorption and desorption reactions. Also, it can be seen that oxidation peak of the platinum surface
3
covered by a larger peak (with maximum around 1.0 V) which is for thiophene electro-oxidation.
4
However, there is no significant difference between the oxygen desorption counter peaks for the two
5
voltammograms. This phenomenon indicates that the thiophene oxidation does not interfere with the
6
platinum electrode surface oxidation.
7
Figure 3 shows the subsequent cycles of platinum electrode surface voltammogram after thiophene
8
adsorption at 0.2 V. With progress of process peaks which are related to the thiophene oxidation are
9
vanished, while the hydrogen adsorption and desorption peaks are gradually appeared. Oxidation of
10
adsorbed thiophene which leads to release of the electrode surface is the cause.
11
Figure 4 shows the first cycles of platinum disk surface voltammogram after thiophene adsorption at
12
different potentials. Using the above mentioned facts, it can be inferred that the best adsorption potential
13
is the one for which the hydrogen adsorption and desorption peaks be the smallest while the thiophene
14
oxidation peak be the largest. To quantitatively determine the optimal adsorption potential two criteria
15
were used here: extent of the electrode surface that is covered by thiophene molecules and the charge
16
consumed during thiophene oxidation.
17
Amount of electrical charge consumed per electron transfer reaction can be determined by dividing the
18
area under the peak to the applied potential scan rate. Hence, it is possible to calculate the relative
19
fraction of electrode surface which is covered by thiophene molecules using the ratio of charge
20
consumed during hydrogen adsorption/desorption in the absence and presence of thiophene molecules.
21
Also, as stated before, similarity of oxygen desorption peaks for the whole cycles of voltammogram,
22
indicates that the thiophene electro-oxidation does not interfere with the surface oxidation. Therefore,
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difference between charged consumed in the first oxidation peak of the platinum electrode surface
2
voltammogram after thiophene adsorption and that of the fresh platinum electrode is a measure of
3
charge consumed during the electro-oxidation of thiophene at electrode surface.
4
Figure 5 shows coverage of the platinum electrode surface with thiophene molecules and also charge
5
consumed during the thiophene electro-oxidation versus adsorption potentials. Both curves are relative
6
to an arbitrary (i.e. the maximum value calculated) scale. As can be seen, maximum surface coverage
7
and oxidation charge were established at adsorption potential of 0.2 V, which is in the double layer
8
region. Also, the trends of two curves are in excellent agreement. The observed behavior was expected
9
since there is a competition between thiophene adsorption and hydrogen evolution at more negative
10
(than 0.2 V) potentials and between oxygen evolution and thiophene oxidation at more positive (than 0.2
11
V) potentials. It can be concluded that 0.2 V is the optimum thiophene adsorption potential in the
12
solution of 0.5 M H2SO4.
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3.3. Effects of frequency on thiophene desulfurization
14
To remove sulfur from the aqueous thiophene solution, the square wave potentiometery technique was
15
utilized. A stepwise potential fast chronoamperometry technique was used to apply the square wave
16
potential program. The lower and upper limits of the square wave potential program were selected based
17
on the potential of thiophene adsorption (i.e. 0.2 V) and oxidation (i.e. 1.1 V), respectively. In addition
18
to limits of the square wave potential program, times of repeating a single potential wave per time unit,
19
which known as the frequency, strongly affects the desulfurization efficiency. The frequencies tested
20
here were 1, 10, 50 and 100 Hz.
21
As stated before, after every ten minutes a cyclic voltammetry tests conducted on a separated portion of
22
the solution. Figures 6a and 6b shows the corresponding voltammograms after 10 and 40 minutes of
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1
carrying the desulfurization reactions. As can be seen, shape of voltammograms became similar to that
2
of fresh platinum electrode with the lapse of time. Although, trend of the voltammograms were similar
3
for different frequencies, however the conversion rates were different. To quantitatively determine the
4
optimum frequency of square wave potential program, the charge under the oxidation peaks were
5
calculated and compared with the voltammogram of fresh platinum electrode.
6
After 50 minutes of applying the square wave potential program to the solutions, ratio of charge under
7
the oxidation peak to that of fresh platinum electrode for the frequencies of 1, 10, 50, and 100 Hz was
8
1.45, 1.64, 1.01, and 2.10, respectively. As can be conclude, at the frequency of 50 Hz the charge under
9
the oxidation peak was approximately equal to that of fresh platinum electrode. At the other frequencies,
10
more time was needed to reactions undergo completion. Consequently, 50 Hz was the best frequency for
11
stepwise thiophene electro-oxidation. Total reaction extent is the sum of its progress in the each step.
12
The decrease in desulfurization efficiency at higher and lower frequencies than 50 Hz can be explained
13
by taking two factors into account: the thiophene desulfurization efficiency in a single potential step and
14
the number of available potential steps per unit time.
15
If frequency of the square wave potential program be high, i.e. a short time epoch in each step, amounts
16
of thiophene that can be adsorbed and oxidized in a single potential step will be finite. Though the total
17
number of potential steps is high, the overall thiophene desulfurization extent will be limited. On the
18
other hand, when frequency of the square wave potential program is low, i.e. a long period of time in
19
each step, although the available time is enough for complete oxidation of the thiophene adsorbed on
20
electrode surface in each step, but the low number of potential steps per unit time, limits the overall
21
desulfurization reaction progress.
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It is expected that the following reaction will apply, if the thiophene desulfurization undergoes complete
2
oxidation:
3
C4H4S + 12H2O → SO42⁻ + 4CO2 + 28H+ + 26e⁻
4
If this reaction govern the desulfurization process, the sulfur will be converted to the sulfate ion. Hence,
5
if the sulfate ion concentration was measured, overall reaction conversion can be determined. Since, the
6
sulfuric acid contain large amounts of sulfate ion respect to that of produced in the reaction, it cannot be
7
used as the electrolyte. Hence, to experimentally determine the reaction progress (through measuring
8
sulfate ion concentration), perchloric acid was used as the electrolyte. Revealing the optimal condition
9
of thiophene electro-oxidation, the square wave potential program (i.e. 0.2 and 1.1 V as the potential
10
limits and the frequency of 50 Hz) was applied to the solution of 1.0 mM thiophene in 1.0 M HClO4 for
11
50 minutes. Sulfate ion concentration in both feed and product solutions were determined via ion
12
chromatography. The result showed a desulfurization conversion of about 100%.
13
3.4. Emulsion stability
14
Due to low conductivity of the hydrocarbons, electrochemical desulfurization of the model fuel was not
15
possible without using the electrolyte. However, the model fuel and the electrolyte (i.e. 1.0 M HClO4)
16
are not miscible and form two distinct phases. To bring these into one phase, a mixture of two
17
emulsifiers were used. To prepare an emulsion, combination of two emulsifiers with high and low HLB,
18
is often more effective than use of a single emulsifier. The required emulsion must be semi-stable; i.e.
19
should be stable during the desulfurization reactions and then should be easily broke so that the
20
hydrocarbon and aqueous phases can be separated from each other.
(7)
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Results of emulsion stability tests were summarized in the last column of the Table 1. As can be seen, in
2
most cases no separation was occurred after 24 hours. Analyzing the obtained results showed that the
3
volume ratio of aqueous to hydrocarbon phase has most effect in the emulsion stability and thus the
4
conditions of first run was the optimal condition for emulsification.
5
3.5. Desulfurization of model fuel
6
Using the optimum adsorption potential (i.e. 0.2 V), frequency of the square wave potential program
7
(i.e. 50 Hz), and model fuel/electrolyte emulsion preparation conditions, the model fuel was desulfurized
8
using the dynamic electro-oxidative method and finally the products were analyzed to determine the
9
overall reaction progress and the products distribution.
10
Gas chromatography results showed that about 88% of thiophene was reacted, but the ion
11
chromatography results revealed that the amount of produced sulfate ion was less than that expected if
12
the thiophene undergoes complete electrochemical oxidation. The gas chromatography and ion
13
chromatography spectra were supplied as the attachments. Calculation revealed that about 38% of sulfur
14
in thiophene was converted to the sulfate ion. It seems that the reaction route was different from
15
Equation 7 and thiophene was partially oxidized. To prove this, the hydrocarbon and aqueous phases
16
were analyzed with FT-IR spectroscopy. Figures 7a and 7b show the FT-IR spectrum of the hydrocarbon
17
and aqueous phases before and after electrochemical desulfurization reaction, respectively. Comparison
18
of the spectrums obtained before and after the reactions for each phase, revealed that there was no
19
obvious change in available compounds in the hydrocarbon phase. However, in the aqueous phase some
20
new peaks were detected.
21
The peaks at wavenumber of 1288.6 and 1418.5 cm-1 are related to sulfur-oxygen double bond in a
22
sulfone and in an organic sulfate compounds, respectively 29, 30. Therefore, it can be concluded that some
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parts of the thiophene molecules were completely oxidized to the sulfate ion and some other parts were
2
partially oxidized to a sulfone (i.e. thiophene oxide) and an organic sulfate (i.e. probably divinyl sulfate)
3
compounds. The following reactions describe conversion of thiophene to theses organic oxy-sulfur
4
compounds:
+
2H2O
S
O
5
+ 6
+
S
4H2O
S
4H+ +
4e-
O
O O
(8)
S
O O
+
6H+ +
6e(9)
7
The reactions described by Equations 7, 8, and 9 were occurred simultaneously in the solution. As
8
mentioned before, conversion of reaction 7 was 38% and because the overall conversion of thiophene
9
was 88%, combined conversion of reactions 8 and 9 was 50%. As quantitative measurement of produced
10
sulfone and organic sulfate compounds concentration was complicated and was not performed in this
11
work, it was impossible to determine conversion of reactions 8 and 9 separately.
12
As the produced sulfone and organic sulfate compounds are polar and concentration of them was very
13
low in the bulk, these species were dissolved in the aqueous phase. Consequently, no further separation
14
steps (e.g. liquid extraction) were needed to remove these compounds from the model fuel.
15
4. Conclusion
16
In this work, thiophene adsorption from its aqueous solution on the platinum electrode surface was
17
investigated in different potentials (i.e. -0.2, to 0.4 V vs. Ag/AgCl reference electrode). Results of the
18
cyclic voltammetry studies showed that maximum thiophene adsorption occurs in the double-layer
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region at 0.2 V. The square wave potential program was applied to the working electrode to provide a
2
means for the thiophene adsorption and oxidation. The lower and upper potential limits of the square
3
wave potential program were 0.2 V (adsorption potential) and 1.1 V (oxidation potential), respectively.
4
Frequency of the square wave potential program was studied at several points (i.e. 1, 10, 50, and 100
5
Hz). The samples were analyzed with cyclic voltammetry technique every ten minutes to determine the
6
optimum frequency. The results indicated that an intermediate frequency of 50 Hz was optimal for the
7
desulfurization of thiophene from its aqueous solution. To measure experimentally thiophene
8
conversion, the square wave potential program was applied to the solution of 1.0 mM thiophene in 1.0
9
M HClO4. Based on ion chromatographic determination of sulfate ion concentration in the feed and
10
product solutions, 100% conversion was achieved.
11
To study the electrochemical desulfurization from a hydrocarbon mixture, the model fuel (thiophene in
12
n-decane) was prepared. To perform the electrochemical reactions, the model fuel was emulsified with
13
the aqueous electrolyte solution. Before, executing the desulfurization experiments, effects of the
14
emulsification parameters (i.e. aqueous/ hydrocarbon phase volume ratio, emulsifier (Span 20 and
15
Tween 20) weight percent, emulsifier HLB and mixing extent and time) on the emulsion stability were
16
investigated and the optimum conditions were found. Finally, the emulsion was putted in the
17
adsorption/oxidation environment using the square wave potential program and the phases were
18
separated and analyzed. Gas chromatography results showed about 88% conversion of the model fuel.
19
FT-IR spectroscopy results revealed that thiophene was converted to inorganic sulfate ion, a sulfone and
20
an organic sulfate compound. All of these compounds are water soluble and can be easily separated
21
from the model fuel. The obtained results confirmed that the electrochemical desulfurization has an
22
excellent potential to be a competitor for the conventional hydrodesulfurization processes.
23
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REFERENCES 1. Mochida, I.; Choi, K. H., An overview of hydrodesulfurization and hydrodenitrogenation. Journal of the Japan Petroleum Institute 2004, 47, 145-163. 2. Shang, H.; Du, W.; Liu, Z.; Zhang, H., Development of microwave induced hydrodesulfurization of petroleum streams: A review. Journal of Industrial and Engineering Chemistry 2013, 19, 1061–1068. 3. Song, C., An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catalysis Today 2003, 86, 211–263. 4. Gilbert, W. R., Formation of thiophenic species in FCC gasoline from H2S generating sulfur sources in FCC conditions. Fuel 2014, 121, 65–71. 5. Furimsky, E.; Massoth, F. E., Deactivation of hydroprocessing catalysts. Catalysis Today 1999, 52, 381–495. 6. Srivastava, V. C., An evaluation of desulfurization technologies for sulfur removal from liquid fuels. RSC Advances 2012, 2, 759–783. 7. Pawelec, B.; Navarro, R. M.; Campos-Martin, J. M.; Fierro, J. L. G., Towards near zero-sulfur liquid fuels: a perspective review. Catalysis Science & Technology 2011, 1, 23–42. 8. Campos-Martin, J. M.; Capel-Sanchez, M. C.; Perez-Presas, P.; Fierro, J. L. G., Oxidative processes of desulfurization of liquid fuels. Journal of Chemical Technology and Biotechnology 2010, 85, 879–890. 9. Shang, H.; Zhang, H.; Du, W.; Liu, Z., Development of microwave assisted oxidative desulfurization of petroleum oils: A review. Journal of Industrial and Engineering Chemistry 2013, 19, 1426–1432. 10. Soleimani, M.; Bassi, A.; Margaritis, A., Biodesulfurization of refractory organic sulfur compounds in fossil fuels. Biotechnology Advances 2007, 25, 570–596. 11. Kilbane II, J. J., Microbial biocatalyst developments to upgrade fossil fuels. Current Opinion in Biotechnology 2006, 17, 305–314. 12. Shen, Y.; Li, P.; Xu, X.; Liu, H., Selective adsorption for removing sulfur: a potential ultra-deep desulfurization approach of jet fuels. RSC Advances 2012, 2, 1700–1711. 13. Hernandez-Maldonado, A. J.; Yang, R. T., Desulfurization of transportation fuels by adsorption. Catalysis Reviews: Science and Engineering 2004, 46, 111–150. 14. Mortaheb, H. R.; Ghaemmaghami, F.; Mokhtarani, B., A review on removal of sulfur components from gasoline by pervaporation. Chemical Engineering Research and Design 2012, 90, 409–432. 15. Mitra, D., Desulfurization of gasoline by pervaporation. Separation & Purification Reviews 2012, 41, 97–125. 16. Lam, V.; Li, G.; Song, C.; Chen, J.; Fairbridge, C.; Hui, R.; Zhang, J., A review of electrochemical desulfurization technologies for fossil fuels. Fuel Processing Technology 2012, 98, 30– 38. 17. Shen, Y.; Liu, X.; Sun, T.; Jia, J., Recent advances of sodium borohydride reduction in coal water slurry desulfurization: integration of chemical and electrochemical reduction. RSC Advances 2012, 2, 8867–8882. 18. Hourani, M., Desulfurization of thiophene by electrochemical perturbation. Journal of Electroanalytical Chemistry 1994, 368, 139-142. 19. Wang, W.; Wang, S.; Wang, Y.; Liu, H.; Wang, Z., A new approach to deep desulfurization of gasoline by electrochemically catalytic oxidation and extraction. Fuel Processing Technology 2007, 88, 1002–1008.
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20. Wang, W.; Wang, S.; Liu, H.; Wang, Z., Desulfurization of gasoline by a new method of electrochemical catalytic oxidation. Fuel 2007, 86, 2747–2753. 21. Korotaeva, L. M.; Rubinskaya, T. Y.; Rybakova, I. A.; Gultyai, V. P., Electrochemical desulfurization of (1-adamantyl)(alkylsulfonyl)pyridines. Russian Chemical Bulletin, International Edition 2008, 56, 90–94. 22. Shen, Y.; Yang, X.; Sun, T.; Jia, J., Innovative desulfurization process of coal water slurry under atmospheric condition via sodium metaborate electroreduction in the isolated slot. Energy & Fuels 2011, 25, 5007–5014. 23. Shen, Y.; Sun, T.; Jia, J., A novel desulphurization process of coal water slurry via sodium metaborate electroreduction in the alkaline system. Fuel 2012, 96, 250–256. 24. Shu, C.; Sun, T.; Jia, J.; Lou, Z.; Shen, Y., A coal desulfurization process via sodium metaborate electroreduction with pulse voltage using a boron-doped diamond thin film electrode. RSC Advances 2013, 3, 1476–1485. 25. Zhang, H. X.; Hou, X. Y.; Xu, S. X.; Li, Z. L.; Yu, H. F.; Shen, X. H., Enhanced desulfurizing flotation of coal using sonoelectrochemical method. Ultrasonics Sonochemistry 2013, 20, 1176–1181. 26. Gong, X.; Wang, M.; Wang, Z.; Guo, Z., Desulfuration of electrolyzed coal water slurry in HCl system with ionic liquid addition. Fuel Processing Technology 2012, 99, 6–12. 27. Hammad, A. D.; Yusuf, Z.; Al-Rasheedi, N., Sulphur removal from crude oil and its fraction. In Encyclopedia of Applied Electrochemistry, Savinell, R. F.; Ota, K.; Kreysa, G., Eds. Springer-Verlag: 2012. 28. Zhang, J., PEM Fuel Cell Electrocatalysts and Catalyst Layers: Fundamentals and Applications. Springer-Verlag: 2008. 29. Silverstein, R. M.; Webster, F. X.; Kiemle, D. J., Spectrometric Identification of Organic Compounds. 7th ed.; John Wiley & Sons Inc.: 2005. 30. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Vyvyan, J. R., Introduction to Spectroscopy. 5th ed.; Cengage Learning: 2015.
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Table 1: Experimental conditions and results of the emulsion stability study. Volume Run
Wt %
Power
Time
Separation
(%)
(minute)
(%)
HLB
ratio 1
50
0.20
13
50
15
73.3
2
50
0.75
11
40
10
66.7
3
50
1.00
10
70
20
43.3
4
30
0.20
12
70
10
NA
5
20
0.20
11
60
20
NA
6
30
0.75
10
60
15
NA
7
20
0.75
13
70
5
NA
8
50
0.50
12
60
5
70.0
9
20
1.00
12
40
15
NA
10
10
1.00
13
60
10
NA
11
10
0.75
12
50
20
NA
12
20
0.50
10
50
10
NA
13
30
0.50
13
40
20
NA
14
10
0.20
10
40
5
NA
15
10
0.50
11
70
15
NA
16
30
1.00
11
50
5
NA
NA: no separation occurred after 24 hours. 2
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Captions of Figures:
2
Figure 1. Voltammogram of the fresh platinum disk electrode surface.
3
Figure 2. First cycle of the platinum disk electrode surface voltammogram after thiophene adsorption at
4
0.2 V.
5
Figure 3. Voltammogram of the platinum disk electrode surface after thiophene adsorption at 0.2 V.
6
Figure 4. First cycle of the platinum disk electrode surface voltammogram after thiophene adsorption at
7
different potentials.
8
Figure 5. Platinum electrode surface coverage with the thiophene molecules and also charge consumed
9
during thiophene electro-oxidation (both curves are based on an arbitrary scale).
10
Figure 6. Voltammogram of the platinum electrode surface after applying the square wave potential
11
program for (a) 10 minutes and (b) 40 minutes.
12
Figure 7. FT-IR spectrum of (a) the hydrocarbon phase and (b) the aqueous phase before and after
13
electrochemical desulfurization of the model fuel.
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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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Figure 5
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Figure 6a
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Figure 6b
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Figure 7a
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Figure 7b
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