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Photo-Assisted Desulfurization Induced by Visible Light Irradiation for the Production of Ultra-Low Sulfur Diesel Fuel Using Nanoparticles of CdO Asmaa S. Morshedy, Ahmed M.A. El Naggar, Sahar M. Tawfik, Omar I. Sif El-Din, Sana I. Hassan, and Ahmed I. Hashem J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09057 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on November 2, 2016
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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Photo-Assisted Desulfurization Induced by Visible Light Irradiation for the Production of
1
Ultra-Low Sulfur Diesel Fuel using Nanoparticles of CdO.
2
Asmaa S. Morshedy*a, Ahmed M.A. El Naggara, Sahar M. Tawfika, Omar I. Sif El-Dina, Sana I. Hassana, Ahmed I. Hashemb
3 4 5 6 7 8 9
a b
Petroleum Refining Division, Egyptian Research Institute (EPRI), Cairo, 11727 Egypt Chemistry Department, Faculty of Science, Ain Shams University, Egypt.
Abstract The heterogeneous photocatalytic desulfurization processes have been paid wide
10
attention due to its effectiveness in removing the condensed organo-sulfur compounds. Such
11
methods may gain greater consideration via utilizing the visible light in general and sun spectrum
12
in particular. This research work aims to produce low sulfur diesel fuel through a catalyzed
13
photochemical route using nanoparticles (NPS) of CdO under the visible light irradiation. Two
14
various structures of CdO were prepared in this study by both the chemical precipitation and
15
auto-ignition techniques. The structural and morphological characteristics of the obtained
16
cadmium oxides were determined via different tools of analyzes. The production of a low sulfur
17
diesel fuel was then investigated under various operating parameters, such as type source of
18
light, catalyst- to- feed dosage and reaction time. The effect of adding oxidizing agents at
19
different concentrations on the desulfurization process was also studied. After the maximum
20
sulfur removal had been detected at the optimum conditions, the ultimate removal of sulfur was
21
attained through a subsequent solvent extraction step. A diesel fuel with a Sulfur content of 45
22
ppm was acquired at the end of this research study. A total sulfur removal of 99.6 wt% was
23
obtained since the original diesel fuel feedstock has an overall concentration of the sulfur
24
compounds = 11500 ppm.
25 26
____________________________________________
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*Corresponding author: Email address:
[email protected] (Asmaa Morshedy)
28
Fax: 0020222747433 Telephone: 00201095036044.
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31 1. Introduction
32
The presence of organo-sulfur compounds such thiols, sulfides, disulfides and thiophenes in the
33
petroleum products are superior sources of the environmental contaminations. The sulfur
34
compounds can lead to the release of the sulfur dioxide (SO2) which can be considered as an
35
important air pollutant and is one of the responsible for the acid rain.1 Therefore, the removal of
36
these compounds is paramount in the petroleum processing industry. In the last decades, several
37
processes have reported the elimination of such compounds to produce low sulfur diesel fuels.
38
Among those processes, the hydro-desulfurization (HDS) which is the most common industrially
39
but it requires high temperatures and pressures with an enormous consumption of the hydrogen
40
gas. This, in fact, makes the operating costs of such processes highly elevated; hence, it may be a
41
non-cost-effective process. On the other hand, HDS is less efficient in removing refractory sulfur
42
compounds
43
such
as
dibenzothiophene
and
other
alkyl-substituted
derivatives
of
dibenzothiophene.2
44
The future technologies are aiming to produce ultra-low-sulfur diesel fuel (ULSD) and ultimately
45
to attain fuels with zero sulfur content.3 In 2013, the Environmental Protection Agency (EPA)
46
has issued a series of tight regulations to reduce the sulfur content in diesel fuel to less than15
47
ppm. Photocatalytic desulfurization is one of the promising techniques which can be an
48
alternative technology for the deep desulfurization of fuel oils. In comparison to the conventional
49
HDS, the photocatalytic desulfurization has got attractive features particularly; high removal
50
efficiency and the low operational costs owing to the mild temperature and pressure conditions.4
51
The photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole
52
pairs which subsequently generate free radicals due to the effect of light irradiation. These
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radicals can undergo secondary reactions such as the photo-oxidation of sulfur compounds into
54
sulfoxides and sulfones5 as proposed in the current study.
55
Many semiconducting photocatalytic materials have been recently developed for versatile
56
applications under the effect of light irradiation. The most common photocatalysts and
57
semiconductors are the transition metal oxides which have unique characteristics. The
58
semiconductors possess avoid energy region where no energy levels are available to promote
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recombination of an electron and hole that are produced by photo-activation in the solid.6 Among
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these transition metal oxides, CdO can be a promising candidate for optoelectronics applications
61
and other applications such as solar cells7 phototransistors8, photodiodes9, transparent
62
electrodes10 and gas sensors. According to literature, one of the cadmium structures namely CdS,
63
has been known as one of the most active photo-catalysts and has been extensively used while
64
the CdO has not been widely used in photocatalysis.11 CdO has got an outstanding characteristic
65
based on having a band gap of 2.3 eV with a simultaneous indirect band gap of 1.36 eV; known
66
as n-type semiconductor.12
67
CdO is not only having the unique optical characteristics but also has selective catalytic
68
properties. This in turn can motivate the usage of such oxide in the photo-degradation of some
69
organic compounds such as dyes and the environmental pollutants.13 Therefore, this can be
70
highly beneficial to the designated desulfurization process during this research work since CdO
71
can crack the high molecular weight condensed sulfur compounds. So the removal of such sulfur
72
structures can be facilitated, and low sulfur products can be obtained.
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It has been reported that the physical and chemical properties of CdO are about its particle shape
74
and size. Therefore, these properties are strongly dependent on the CdO preparation methods and
75
conditions.14 Several investigations have studied the preparation of different structures of CdO
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by various techniques. Nanometer-sized CdO organo-sol was frugally and interestingly produced
77
from an aqueous solution of Cd(NO3)2 as reported15 while Nano-needles of CdO was obtained
78
by the chemical vapor deposition.16 Synthesis of CdO nano-wires by decomposing CdCO3 in a
79
KNO3 was also reported in.17 The micro-emulsion and solvothermal techniques can be used for
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the preparation of CdO nanoparticles.18
81
In general, photo-catalysts can provide adequate desulfurization activity19 however they can be
82
much more efficient if coupled with oxidizing agents such as H2O2.20 The oxidizing agent
83
generates free radicals which subsequently convert the sulfur compounds into high polar
84
oxidized compounds.21 A solvent extraction step is next required to produce low sulfur products
85
via the separation of the oxidized components.22
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The presented research paper reports the preparation of CdO nanoparticles by both chemical
87
precipitation23 and auto-ignition methods. The obtained oxides out of these two procedures were
88
employed to produce low sulfur diesel fuel via a photocatalytic desulfurization process of diesel
89
fuel fraction. The removal of the sulfur compounds was carried out under visible light
90
irradiation, specifically by using a linear halogen lamp. The utilization of such source of light
91
radiation in the photo-desulfurization processes has not been yet reported. The photo-catalyst is
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coupled with an oxidizing agent (H2O2) to be much more efficient for desulfurization process.24
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The eventual sulfur content in the diesel fuel was acquired via the solvent extraction technique.
94
2. Experimental
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3. 2.1. Feedstock
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A straight run diesel fuel fraction was conducted from Suez oil processing company (SOCo),
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Suez-Egypt. The physicochemical characteristics of the feedstock are listed in Table 1.
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Table 1. General characteristics of the diesel fuel fraction.
Characteristics
Measured value
Test method
Refractive index,20 oC
1.4866
ASTM-1218
Density,20 oC, gm/cm3
0.8575
Mettler Toledo DE40
108
ASTM D-92
1.15 (11500 ppm)
ASTM D-4294
Pour point, oC
-8
ASTM D-97
Aniline point, oC
74
ASTM D-611-82
Diesel index
54.53
ASTM D 611
API gravity
36.01
ASTM D 1298
o
Flash point, C Sulfur content, wt.%
100 101
Component analysis Total saturates, wt.%
70
Total aromatics, wt.%
30 ASTM D-86
ASTM distillation, vol.% Intial boiling point, oC
200
10% distillate at
220
20% distillate at
240
30% distillate at
260
40% distillate at
288
50% distillate at
304
60% distillate at
320
70% distillate at
330
80% distillate at
340
Final boiling point, oC
340
Loss
2
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2.2. Catalyst preparation
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2.2.1 Materials
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Cd (NO3)2 and citric acid were purchased from LOBA Chemie (India) and El-Nasr chemicals
108
Ltd-Egypt respectively. Highly pure NaOH was also utilized for the catalyst preparation and was
109
obtained from Sigma-Aldrich, UK. All the reagents were used as received without any treatment.
110
2.2.2 Synthesis procedures.
111
The Cadmium Oxide (CdO) nanoparticles were prepared by two methods namely; (a)The
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chemical precipitation25 and (b) The auto-ignition methods.26
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In the chemical precipitation, 0.1N solution of Cd (NO3)2.6 H2O was prepared using de-ionized
114
water. The solution was then heated up to 60 oC to enhance the dissolution of the cadmium salt.
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An aqueous solution of NaOH (0.5N) was afterward added drop-wise to the cadmium solution
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under vigorous stirring until the reaction had completed. During the addition of NaOH, a cloud
117
of suspended molecules was initially observed. The solution was then turned to bright white due
118
to the formation of the Cd-hydroxide particles. The stirring was afterward stopped, and the
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hydroxide particles were allowed to precipitate at the bottom of the preparation vessel. The
120
precipitate was then filtered on a Buchner funnel and repeatedly washed with de-ionized water.
121
The obtained cadmium hydroxide was dried in an oven for overnight at 120oC. Finally, the
122
hydroxide particles were converted to Cd-oxide (catalyst A) via a calculation step for 4h at 500
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o
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(b) In auto-ignition, cadmium nitrate [Cd (NO3)3.6H2O] and citric acid (C6H8O7.H2O) were first
125
dissolved in a minimum volume of de-ionized water on a different basis. Both solutions were
126
then mixed with certain molar ratios (according to equation 1). The mixing was executed at 80o C
127
under energetic stirring 600 rpm.
128
C.
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9 Cd (NO3)2.4H2O(c) + 5 C6H8O7(c)
9 CdO (C) + 9 N2(g) + 30 CO2(g) + 24 H2O(g)
(1)
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At the first place, a transparent solution was detected while a highly viscous snow-white liquid
130
was then obtained after a short time (ca 20 min). The temperature was next increased to 200o C
131
where the viscous liquid had started to swell and was simultaneously auto-ignited. A Large
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volume of gasses was immediately generated due to the effect of auto-ignition. The evolution of
133
gasses had left behind a voluminous amount of solid powder. The obtained powder was finally
134
calcined at 500o C for 4 h to produce pure cadmium oxide (catalyst B).
135
2.3. Catalysts characterizations
136
The essential structural and morphological characteristics of the synthesized cadmium oxides
137
were examined through various analysis tools. The X-ray diffraction patterns of both oxides were
138
recorded by Brucker AXS-D8 Advance XRD instrument (Germany) with nickel-filtered copper
139
radiation (λ =1.5405Å) at scanning speed of 0.4 degrees/ min. The N2 adsorption-desorption
140
isotherms were performed with Quanta chrome Nova 3200 instrument (USA). The surface area
141
and total pore volume were calculated throughout the BET plot and BJH equation respectively.
142
The surface, as well as the inner morphology of the prepared oxides, was obtained by scanning
143
electron microscope (SEM) model JEOL 5300 (Japan) and Transmission electron microscope
144
(TEM), model JEOL 1230, Japan. The UV- reflectance analysis of the prepared photo-catalysts
145
was acquired via UV-spectrophotometer model V-570 manufactured by JASCO (Japan). Photo-
146
luminance (PL) analysis (as one of the essential characteristics for photo-catalysts) was measured
147
at room temperature using Spectrofluorometer, model JASCO FP-6500-Japan. Fourier transform
148
infrared spectroscopy was used to obtain FTIR spectra in the 4000-400 cm-1 range were recorded
149
at room temperature by Perkin Elmer (model spectrum one FT-IR spectrometer, USA). Samples
150
were prepared using the standard KBr pellets. The Sulfur content (wt.%) of the feedstock as well
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as the obtained products after the desulfurization stages was measured via an X-ray fluorescence
152
spectrometer.
153
2.4. Photocatalytic desulfurization activity
154
After the full characteristics of the prepared semiconductor photo-catalysts were determined, it
155
was transferred to the sulfur removal stage. All experiments were carried out in a batch double
156
jacketed photo-reactor with a total capacity of 1 L. The temperature during the photo-reaction
157
was controlled using a water-cooling system. The desulfurization processes have started by
158
charging both the semiconductor and the feedstock into the reaction vessel. The whole system
159
was then exposed to the irradiation source to get the process started. Two sources of radiation
160
namely; linear halogen and Xenon lamp, with a power of 500W, were utilized. Figure 1 shows
161
the setting up that was used in the sulfur removal process. This design was originally built up
162
by the authors of this research work.
163
164
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Figure 1. Photocatalytic desulfurization set up fitted with irradiation source. (1) Woody box, (2) Linear halogen lamp, (3) Glass double jacket, (4) Water inlet, (5) Magnetic stirrer, (6) Water outlet, (7) Diesel Feed, (8) Stirrer bar.
165 166 167
Different parameters could affect the degree of sulfur removal from diesel fuel photo-
168
catalytically. In this study, several factors were examined, specifically the effect of the light
169
source, catalyst dose and the reaction time. The effect of adding an oxidizing agent at different
170
ratios to execute a simultaneous chemical reaction was also investigated. At the end of this
171
stage, the determined optimum conditions were applied to the desulfurization of diesel fuel
172
while under the effect of sunlight. The obtained product after this step had received a
173
subsequent extraction at the optimum S/F ratio, as determined.
174
2.5. Extraction Procedure
175
Promptly after the optimization of the best desulfurization result through the parameters above, a
176
subsequent solvent extraction process was carried out by removing the more polar sulfur
177
compounds. The extraction step was urgent in the attention of reaching to as low as possible in
178
sulfur content in the obtained diesel fuel. The extraction process was done using acetonitrile as a
179
solvent.22
180
Both the feed stock and oxidized products which obtained after the oxidation process were
181
subjected to solvent extraction in a jacketed mixer-settler batch apparatus. The extraction
182
temperature was adjusted to 50 oC with an accuracy of ± 1oC by using an ultra-thermostat. The
183
feed and the solvent were kept in good contact with continuous agitation for 45 minutes, and
184
then, the phases were left to settle for 45 minutes, and then, separated. The solvent was removed
185
from the raffinate phase by washing several times with hot distilled water. The raffinate was then
186
dried over anhydrous calcium chloride. The solvent was eliminated from the extract phase by
187
distillation under reduced pressure.
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All the experimental recordings which are related to the sulfur removal processes as well as the
189
characterization of the catalysts using the different tools were done in triplicate. This was
190
necessary in order to ensure the presented data through this research work. On the other hand, the
191
% accuracy of the results which were achieved by the BET surface area analyzer is 99.7% and
192
the percentage of measurement error does not exceed 0.3%. For the XRD instrument, the
193
potential error is not exceeding 0.01 (d-spacing). The TEM microscope had provided high
194
resolution images with an achieved quality of 99%. The measurement error of the X-ray
195
florescence spectrometer ±3 ppm.
196 197
3. Results and Discussions The structural, morphological and surface characteristics of catalysts A and B are discussed
198
throughout this section. The photo properties, as well as the photocatalytic exploit of both
199
catalysts, are also illustrated.
200
3.1 X-ray analysis
201
Figure 2 shows the XRD patterns of the as-synthesized catalysts A and B. Both catalysts had
202
exhibited similar spectrums. For catalyst A, consisting of five main reflections centered around
203
2θ of 33.04°, 38.33°, 55.33°, 65.98°, and 69.31° corresponding to (111), (200), (220), (311) and
204
(222) planes, respectively (JCPDS card number 04-016-6410) and lattice constant ao=4.6920Å.
205
Also, for Catalyst B, consisting of five main reflections centered around 2θ of 33.11°, 38.41°,
206
55.45°, 66.13°, and 69.48° matching with the (111), (200), (220), (311), and (222) planes of
207
faced centered cubic (FCC) CdO (Ref. Code: 04-016-6360) with a lattice constant ao = 4.6825
208
Å. All the detected peaks are indicative of the formation of CdO however at different phases.
209
Specifically, the obtained peaks at 2θ of 33°, 38°, 55° is characteristics to the CdO hexagonal
210
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closed pack (HCP) phase. While the face-centered cubic (FCC) phase was detected via the given
211
signals at 65° and 69°.27
212
Although both catalysts showed a structure of mixed phases, the FCC phase is more dominant
213
owing to the number of its significant peaks, according to the given spectrum. The exhibited
214
sharp and intense peaks in the spectrum can explicitly reveal the high crystalline structures of
215
both the prepared catalysts. Also, the absence of any additional peaks confirms the high purity of
216
the prepared CdO nanoparticles.
217 218
219
220 Figure 2. XRD patterns of CdO nanoparticles Catalyst A and Catalyst B.
221 222 223 224
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3.2 Surface identifications
225
The surface characteristics of catalysts A and B are illustrated in Table 2*. The average particles
226
size of both catalysts, as calculated from the XRD and surface analyzes.
227
Table 2*. Surface characteristics and particles size values of the CdO catalysts XRD Results
228
Surface characteristics
CdO Nanoparticles
ao (Å)
DXRD
SBET
DBET
VP
rp
(nm)
(m2/g)
(nm)
(cm3/g)
(nm)
Catalyst A
4.6920
70.15
17.77
47.34
0.026
1.1
Catalyst B
4.6825
38.27
22.59
37.24
0.029
1.24
229 (*ao= unit cell size; DXRD= Crystal size; SBET= Surface area; Vp= Total pore volume; rP= Pore radius).
230
Table 2* shows low surface area values for both catalysts A and B. Nevertheless, the surface
231
area of catalyst B is approximately 30% higher than that of Catalyst A. In general; the catalysts
232
with the higher surface area could provide better catalytic activity although photocatalysts it
233
might be a bit different. The photocatalytic activity is much more relevant to the catalyst ability
234
to absorb radiation and its photo-optical properties. Both catalysts had exhibited a nearly similar
235
low total pore volume, however, a little bit larger average pore radius was detected in catalyst B.
236
In particular, the average pore radius of catalyst A was 12% smaller than that of catalyst B.
237
Unlike the metal oxides which are produced by similar methods, the detected pores system of the
238
two catalysts is uniquely micro-porous structure. The low total pore volume of the synthesized
239
catalysts can confirm that both of them had been formed in dense-like structures during the
240
preparation procedures.
241
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According to Table 2*, catalysts A and B had shown similar unit cell as calculated from the
242
following equation: ao= d111√3, reported in.28 On the other hand, both catalysts showed unlike
243
average crystal sizes. Mainly, catalyst A exhibited a bigger particle size of 10 nm than that of
244
catalyst B, as calculated from the relationship: DBET= 6/ρ. SBET. In line with, catalyst B has
245
presented an average particle size equal to nearly half of the detected grain size of catalyst A, as
246
given by Scherer’s equation.28 This, in turn, could explain the reason behind the slight increase in
247
the surface area value of catalyst B over catalyst A. This also can confirm the good match
248
between the XRD data and the surface characteristics of the synthesized catalysts owing to the
249
linked SBET values to the familiar crystal size by XRD. Generally, the different DXRD between
250
Catalyst A and Catalyst B, as acquired from Table 2*, are referred to the different method of
251
preparing each catalyst. In particular, the use of citric acid as a capping agent during the
252
synthesis of catalyst B could strictly control the crystallization of its grains, hence a catalyst with
253
a smaller particle size was obtained.
254
3.3 Morphological structure
255
The surface and internal morphology of the prepared CdO structures are investigated
256
respectively through the displayed SEM and TEM; Figures 3 & 4. Both catalysts A and B had
257
exhibited low porous nature (Figure 3) which is in a high harmony with the acquired total pore
258
volume via the surface analysis. Nevertheless, the two catalysts showed different surface
259
morphology. Specifically, a non-smooth and non-uniform surface was detected for catalyst A.
260
Moreover, grains of the cadmium oxide with different shapes as well as some aggregated
261
particles were observed through the given SEM images. On the other hand, catalyst B had
262
displayed smooth morphology and uniformly well-dispersed particles of CdO along the whole
263
surface. The given SEM images also exhibit similarly shaped particles of CdO with almost the
264
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same crystal size. The different surface area (SBET values-Table 2*) of the two catalysts can be
265
referred to the non-similar morphology of each of them,
266 267
Catalyst A
268 269 270 271
Catalyst B
272 273 274
Figure 3. Surface morphology via SEM image of the as-prepared catalysts A and B. Catalyst A
Catalyst B
Figure 4. TEM micrographs of the as-prepared catalysts A and B.
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The displayed TEM micrographs are strongly matching with the data given by the surface area
296
analysis in terms of detecting low porous structure in the crystals of both catalysts. The
297
morphologies of both catalysts are also following the prior given XRD patterns due to the
298
various CdO phases that are clearly observed from Figure 4. In particular, nanoparticles of the
299
CdO with different shapes, namely hexagonal beehive-like and FCC structures were seen. The
300
particles of the FCC phase can also be detected as embedded in the middle of the other structure.
301
Figure 4 also shows uniform well-dispersed nanoparticles of the cadmium oxides along the
302
whole structure, for both catalysts. The HCP phase had also been clearly detected, as indicated in
303
Figure 4. However, catalyst B exhibited much more domination of the crystal structure in the
304
FCC phase than catalyst A. This can be referred to the usage of the citric acid as a capping agent
305
during the synthesis of catalyst B. The citric acid could help in controlling both the particle size
306
and the crystallization of the obtained CdO during the preparation. The noted different
307
morphological and surface characteristics in catalysts A and B can be attributed to the utilized
308
preparation methodology for each of them.29 About Figure 4, nano-crystallites with average
309
sizes of ca 70 and ≥ 38 nm25 were observed for catalysts A and B respectively. The given
310
measurements by the TEM are genuinely matched with the calculated gain sizes from the XRD
311
data, by Scherer’s equation.
312
3.4 UV–visible absorption
313
The influence of the preparation method on the photo-optical of the produced cadmium oxides is
314
studied through their capability of absorbing the UV radiation. The spectra of the UV–visible
315
absorption by the Cd oxides nanoparticles; both catalysts A and B, are shown in Figure 5.
316 317 318
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319 320 321 322 323
Figure 5. Electronic absorption of UV-visible spectra catalyst A and catalyst B.
324 325 326 327 328 329 330 331 332 333 334
Catalyst A has shown an absorption band as a red shift while the band of catalyst B was noticed
335
at blue shift area. About the absorption spectra, the effective wavelengths of catalysts A and B
336
are 465 and 370 nm respectively. In line with, respective energy band gaps of 2 and 3.15 eV for
337
catalyst A and B were detected. The direct band gap of CdO is estimated from the plot of (αh)ט2
338
versus hט, where h טis the photon energy and α is the ratio of the absorption coefficient to the
339
scattering coefficient. These band gaps can explicitly ensure the red and blue shifts which have
340
occurred for the as-prepared CdO NPs Catalyst A and Catalyst B respectively owing to the
341
inverse proportion of energy gap and wavelength30 as shown in Figure 5.
342
3.5 Catalysts photoluminescence (PL)
343
The photoluminescence spectra study was carried out to find the ability of each of catalyst for the
344
photocatalytic reaction. The PL of as-prepared CdO catalysts are shown in Figure 6.
345 346
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347 348 349 350
Figure 6. Room temperature PL spectra of CdO nanoparticles catalyst A and catalyst B .
351 352 353 354 355 356 357 358 359
Three similar emission peaks are observed for both catalysts A and B however at different
360
wavelength value. The peaks centered on wavelength values of 420, 460 and 570 nm were
361
detected for catalyst A while they were 435, 465, and 550 nm for catalyst B. for both catalysts,
362
the first peak corresponds to the band-edge emission while the second and third ones are
363
referring to the artifact and the deep-level (trap-state) emissions.13 The last mentioned type of
364
emission is a green emission which takes place because of the recombination of photo-generated
365
holes and the ionized electron in the valence band. The different wavelengths, as detected from
366
Figure 6, for both catalysts are strongly dependent on the energy band gap of each of them. The
367
PL intensity of catalyst A is lower than that of catalyst B which apparently means that
368
recombination of (h+& e-) system in case of catalyst A is slow compared to B. This, in turn, will
369
undoubtedly effect on the photocatalytic activity of the synthesized catalysts, specifically, higher
370
activity can be expected for catalyst A.
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374
3.6 Catalysts photocatalytic activity
After the full characterization of catalysts A and B, both catalysts had exhibited nearly similar 375 structural, surface and morphological properties according to XRD, surface analysis, and TEM. 376 Nevertheless, each catalyst showed remarkably different photo-optical characteristics, about their 377 UV absorbance and PL. Therefore, CdO-NPS which were prepared by the precipitation method 378 (Catalyst A) was selected to study the effect of the different parameters on the photocatalytic 379 desulphurization of diesel fuel under visible irradiation.
380
3.6.1 Influence of the radiation source
381
At the first place, the effect of the radiation source on the desulfurization process was
382
investigated at a constant dosage of catalyst and reaction time. The sulfur removal activity was
383
tested under the effect of two sources of light namely; Xenon and linear halogen (LHL) lamps on
384
an individual basis. The radiation power was kept constant at 500 W for both lamps. The process
385
was then carried out at room temperature (around 30 oC) while using a catalyst dosage of 3 g/L
386
of diesel fuel and a reaction time of 3hours. The effect of the radiation source of the sulfur
387
removal exploit is presented in Figure 7a, which shows that the CdO has a limited sulfur
388
removal activity under the effect of both types of irradiation.
389
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S removal, Wt. %
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The Journal of Physical Chemistry
6 5 4 3 2 1 0 Dark
Xenone Lamp
Linear halogen lamp
Effect of light source
390 Figure 7a. Effect of the light source on the sulfur removal %
391
The sulfur content of the two products obtained after exposing the feed to irradiation with Xenon
392
and LHL lamps are reduced from11500 ppm to 10925 ppm and 10672 ppm respectively. So that
393
LHL shows a slight increase in sulfur removal (2.2%) than Xenon lamp. This increase in the
394
sulfur removal percentage in case of LHL can be referred to different wavelengths of each source
395
of radiation. In practical, Xenon lamp has got a wavelength of 420 nm while that of the LHL is
396
approximately 500-550nm. Thus, the CdO nanoparticles were able to perform efficiently under
397
the LHL owing to the catalyst UV absorbance measurements, as previously indicated. To
398
apparently find out about the removal of sulfur by the prepared CdO through the photocatalytic
399
desulfurization, the adsorption capacity of the catalyst was tested in the absence of light
400
radiation. The result had indicated that less than 5 wt. % of the sulfur was adsorbed by the
401
synthesized CdO over 24h.
402 403 404
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3.6.2 Influence of catalyst dosage
405
The effect of the catalyst-to-feedstock dosage on the removal of sulfur compounds was tested
406
(figure 7b). Various dosages 3, 5, 7, 9 and 11 g/L were used at a constant operational time of 3h.
407
The sulfur removal was induced by the LHL since it had exhibited better efficiency with the CdO
408
nanoparticles, as given in 3.6.1.
409
15
S removal, wt.%
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12 9 6
y = -0.1399x2 + 2.7668x - 0.0023 R² = 0.9966
3 0 0
2
4
6
8
10
12
CdO photocatalyst/feed dosage (gm/L)
Figure 7b. Effect of CdO-to-feed dosage on the sulfur removal.
410 411 412
Figure 7b, the percentages of sulfur compounds in the obtained diesel fuel had decreased by
413
increasing the dosage of the catalyst. In practical, the catalyst showed a continuous increase in
414
removing the sulfur compounds up to a catalyst-to-feed dosage of 9 g/L. This was then followed
415
by the detection of a steady-state of the sulfur removal. The detected increase in the sulfur
416
removal % by increasing the catalyst dosage up to 9 g/L can be attributed to the rise in the
417
number of the provided active sites by the catalyst. Nevertheless, a very slight increase in the
418
sulfur removal% was observed as the catalyst dosage elevated from 7 to 9 g/L. It might be as a
419
result of increasing the scattered radiation among the particles of catalyst within in the reaction
420
system. This, in turn, could restrict the removal of sulfur compounds due to the limited
421
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photocatalytic activity of the CdO particles. In line with, the noted stable sulfur removal by
422
increasing the catalyst-to-feed dosage to 11 g/L is referred to the certain photon flux which is
423
provided with the reaction system. Practically, the number of the absorbed photons increases by
424
increasing the amount of the photocatalyst up to a certain point. However, the presence of
425
additional amount at that point does not provide further absorption of photons. Therefore, the
426
catalyst to feed dosage of 7g/L was determined as the optimum dosage.
427
3.6.3. Effect of reaction time
428
At the completion of the prior stage, the catalyst-to-feed dosage of 7 g/L was used to investigate
429
the influence of the operational time on the removal of sulfur compounds from feedstock under
430
the LHL. The relationship between the percentage of the removed sulfur compounds and the
431
operational time is presented in Figure 7c.
432
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S removal, wt.%
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The Journal of Physical Chemistry
15 12 y = -0.1875x2 + 3.299x + 4.7415 R² = 1
9 6 0
2
4
6
8
Time of reaction (Hour)
Figure 7c. Effect of the operational time on sulfur removal%.
433 434
The percent of the sulfur compounds in the produced diesel fuel had decreased by the increase of
435
the reaction time. This decrease can be referred to the inflation of the photocatalytic activity of
436
the CdO nanoparticles by the time growth. This is due to the exposure of the catalyst particles
437
continuously to the radiation source. This might explain the noticeable rise in the sulfur
438
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removal% by increasing the reaction time from 1 to 5h. On the other hand, a slight increase in
439
the removed sulfur% was detected as the operating time increased to 7h. This can be attributed
440
the coverage of the most of the catalyst surface by a layer of the adsorbed sulfur compounds after
441
5h of the reaction. It may reduce the photocatalytic activity of CdO, however, this decrease
442
might be compromised by an increase in the sulfur removal%. It obviously is due to the
443
interaction between the adsorbed sulfur and the sulfur compounds of the feedstock, based on the
444
known rule of like attract like.
445
3.6.4 Sulfur removal via catalytic photochemical reaction
446
As soon as the optimum conditions were determined out of the photocatalytic desulfurization
447
process, these conditions were utilized in the next stage of the investigation. Since the
448
combination of photocatalytic and photochemical oxidation increases the desulfurization
449
process.21 So that, This part will include the addition of hydrogen peroxide as oxidizing agents to
450
the photocatalytic desulfurization of the diesel fuel. H2O2 was frequently selected as an
451
additional oxidant due to its commercial availability, infinite solubility in water, high-cost
452
.
effectiveness for ( OH) free radical production and simple operation for attacking most organic
453
substances.22 Here the oxidation of the sulfur compounds using H2O2 and H2O2 containing acetic
454
acid to improve the sulfur removal%.
455 456
The strength of the H2O2 The effect of the various strength of H2O2 on the oxidation of sulfur compounds was tested at a
457
constant H2O2-to-feed ratio of 1:1. Figure 8a.
458
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30 27.5
S removal, wt.%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
25 22.5 20 17.5 15
y = 0.0048x2 - 0.107x + 18.697 R² = 0.9949
12.5 10 0
10
20
30
40
50
60
Strength of Hydrogen peroxide (%)
459 Figure 8a. Effect of strength of H2O2 on sulfur removal % at H2O2 /feed (1/1).
460
It shows that the photocatalytic-oxidative desulfurization decreases the sulfur compounds
461
percentages in the acquired diesel fuel to high levels, compared to the non-inclusion of an
462
oxidizing agent. Also, it shows that the increase in the H2O2 strength could positively influence
463
the desulfurization of the diesel fuel fraction. In practical, the increase of the oxidizing agent
464
power by about 2.5 fold increases the sulfur removal up to 32 wt.%. This increase of sulfur
465
removal might be attributed to the increased number of the radicals that could be provided by the
466
hydrogen peroxide when its strength was increased. These radicals could subsequently attack the
467
organic substances and participate in oxidizing the sulfur compounds in the diesel fuel. The
468
oxidized sulfur compounds would be afterward readily eliminated either via the adsorption on
469
the surface of the used photocatalyst or a subsequent extraction step.
470 471
The ratio of H2O2/ feed Hydrogen peroxide with the strength of 48% was then utilized to find out about the effect of
472
different H2O2-to-feed stock ratios ranging from 0.5-3 (V/V) on the efficiency of the
473
photocatalytic-oxidative desulfurization process (Figure 8b). The increase of the hydrogen
474
23 ACS Paragon Plus Environment
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peroxide content in the reaction media had, in turn, elevated the number of the generated free
475
radical (•OH) by which the secondary reaction, oxidation of sulfur, is undertaken.
476
26 25
S removal, Wt. %
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Page 24 of 39
24 23 22 21 20 19 18 0.5
1 2 3 Hydrogen peroxide ratio to Feed (v/v)
The proper H2O2 concentration (1:1) has been found to enhance the desulfurization rate of diesel
477 478 479 480
due to the most efficient generation of hydroxyl radicals (•OH) and the inhibition of the
481
recombination of electron/hole (e-/h+) pairs. It is clear that increasing the ratio of hydrogen
482
peroxide to the feed leads to decrease the degree of sulfur compounds removal. This decrease is
483
may be, when excessive H2O2 is present, H2O2 scavenges reactive oxidative species like •OH
484
radicals and inhibits the subsequent desulfurization reactions.22
485
Figure 8b. Effect of H2O2 to feed ratio on the sulfur removal.
486
The ratio of Acetic acid/ H2O2 Although the oxidative-photocatalytic process had shown reasonable desulfurization level,
487
further treatment was required to reduce the sulfur content of the final diesel fuel. Certain
488
concentrations of the acetic acid were added to the reaction media to improve the quality of the
489
oxidation step and then the desulfurization rate as shown in (Figure 8c). In practical, ratios of
490
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0.5:1 and 1:1 (acetic acid: H2O2) were used at this stage while the oxidizing agent H2O2 to feed
491
ratio was kept at 1:1; the optimum from the prior step.
492
40
36
S removal, wt.%
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The Journal of Physical Chemistry
32
28
24
y = -0.7871x2 + 8.0529x + 17.358 R² = 0.9979
20
(1:0)
(1:0.25)
(1:0.5)
(1:0.75)
(1:1)
Ratio of H2O2 / Acetic acid (v/v)
493
The addition of the acetic acid had significantly influenced the sulfur removal% of the diesel
494 495 496
fuel. Specifically, approximately 38 wt. % in the sulfur compounds in the acquired diesel fuel
497
was noticed as using 1:1 of acetic to H2O2, compared to the peroxide alone. This can be
498
explicitly attributed to the other radicals as well as the extra oxidizing species that could be
499
provided due the presence of acetic acid, as assisting oxidant.
500
3.7 Solvent extraction
501
At the end of the photo-catalytic step, a maximum desulfurization of about 18.65 wt. % was
502
attained in the produced diesel fuel via using 7g CdO/1L diesel fuel for 7h under LHL. A further
503
extraction process via the utilization of acetonitrile (solvent) at 50 oC was then carried out to
504
improve the level of sulfur content in the resulting product. Solvent extraction was tested at
505
different solvent-to-feed (S/F) ratios ranging from (1:1) to (5:1) Figure 9.
506
Figure 8c. Effect of the mixed oxidizing agents on the sulfur removal.
25 ACS Paragon Plus Environment
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68 65.67
66.19
64 61.66
S removal, wt.%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 26 of 39
60 56 54.46 52 48 44
y = -1.5907x2 + 15.023x + 30.904 R² = 0.9999
44.4
40
(1:1)
(2:1)
(3:1)
(4:1)
(5:1)
Solvent to Feed ratio (S/F)
507
Figure 9. Effect of solvent-to-feed ratio on the sulfur removal.
508 509
It shows that the percentage of sulfur removal was increased by increasing the (S/F) ratio, as
510
expected. Also, ratios (4:1) & (5:1) are nearly the same so that the ratio (4:1) is preferable
511
economically. Solvent extraction process also applied to the optimum condition of catalytic
512
photochemical desulfurization via using 7g CdO/1L diesel fuel for 7h under LHL in the presence
513
of the acetic acid with H2O2 as oxidizing agents (1:1) by volume. A maximum removal of
514
97.62% was achieved at S/F ratio of 4:1. This is highly likely due the conversion of the sulfur
515
compounds into oxidized forms during the catalytic photochemical step rather than of being in
516
their original structure. Consequently, the removal of such oxidized compounds would be much
517
easier by using the acetonitrile. The attained high sulfur removal can reflect the success of the
518
designated process during this research study.
519
3.7.1 Cross-Current extraction
520
After the determination of the optimum solvent-to-feed ratio (4/1), the extraction process was
521
then repeated at same proportion while in a multi-stage (cross-current) process. Notably,
522
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The Journal of Physical Chemistry
consecutive four stages extraction was carried out for the diesel fuel which was obtained from
523
the catalytic photochemical reaction. In the four stages cross-current process, a total solvent to
524
feed ratio of 4/1 was used which divided into four equal portions. The sulfur contents of the
525
produced diesel fuels (via direct extraction of the feedstock by acetonitrile or a subsequent
526
extraction after the photochemical stage) are listed in Table 3.
527
Table 3.Effect of solvent extraction on the desulfurization of diesel fuel before and after oxidation using acetonitrile at S/F ratio 4/1
Characterization
Diesel fuel
Oxidized feed
obtained by
(obtained from the catalytic photochemical
solvent
process)
extraction Single stage
Cross current extraction
extraction
(four stages)
83.11%
82.08%
79.42%
1.4755
1.4573
1.4567
Density, 20 C,gm/cm
0.8455
0.8214
0.817
Sulfur content, ppm
7253.6
270.9
45
S Removal, Wt.%.
36.93%
97.64%
99.61%
Aromatic content
18.87%
0.71%
0.13%
Aniline point, C
80.8
91
91.4
Diesel index
62.67
78.66
80.76
alone for raw feedstock Yield, Wt. %. o
Refractive index,20 C o
3
o
528 529 530
531 In general, the percentages of sulfur removal via the subsequent extraction step to the catalytic
532
photochemical process had heavily scored, in comparison to the direct extraction of the diesel
533
fuel feedstock. In particular, the sulfur content in the produced diesel fuel was reduced by nearly
534
three times of magnitude in the combined catalytic-extraction than that of the extraction alone.
535
Which, can reflect the importance of imposing the catalytic photochemical process in the
536
treatment of diesel fuel, before the solvent extraction stage. On the other hand, the extraction via
537
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Page 28 of 39
the cross-current technique had provided further sulfur removal than the single stage extraction.
538
Specifically, the sulfur content was reduced to one-sixth (45ppm) by the cross-current since it
539
was 270 ppm in the single stage extraction. This decreasing of the sulfur content can be referred
540
to the longer interaction time between the solvent and sulfur compounds in the case of cross-
541
current extraction. Moreover, the cross-current technique could provide better selectivity,
542
compared to the single stage extraction, owing the introduction of the solvent as portions (S/F
543
ratio of 1:1) at each step rather than that the ratio of 4:1 at once.
544
3.8 Process economization
545
After the investigation of the desulfurization process was fully accomplished and the optimum
546
conditions were determined, these conditions were used to implement the desulfurization process
547
while under the effect of sunlight. This step was meant to reduce the operational costs via
548
replacing the LHL by the solar energy; hence a less consumption of energy during the photo-
549
based stage. The minimum intensity of the solar energy at the time of performing the experiments is
550
1321 W/m2 as reported.31 The rates of the desulfurization of the acquired diesel fuel as well as their
551
sulfur contents are exhibited in Table 4*.
552
Table 4*. Rates of diesel fuels desulfurization under the effect of solar energy with a subsequent solvent
553
extraction.
554
Catalytic
Diesel
Photo-catalytic
Photo-chemical
fuel
desulfurization
desulfurization
100
82.82
82.95
80.56
Refractive index, 20 oC
1.4866
1.4607
1.4613
1.4572
Density, 20 oC, gm/cm3
0.8575
0.8271
0.8281
0.8207
Sulfur content, ppm
11500
1582.4
1790.1
217.1
Characterization
Yield, Wt. %.
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S Removal, Wt.%. Aniline point, oC Diesel index
zero
86.24
84.43
98.11
74
89
88.8
91.2
54.53
74.95
74.42
79.09
(* the processes were carried between 11 am and 2 pm in June 2015)
555
The obtained diesel fuel by the catalytic photochemical followed by an extraction step has shown
556
a sulfur content of 217 ppm while it was 270 ppm in the case of using the LHL, under the same
557
conditions. Therefore, the desulfurization process can be better attained by the effect of solar
558
energy since a diesel fuel with lower sulfur content was obtained. This choice has been made
559
taking into account the economic perspectives regarding the less energy consumption.
560
3.8.1. Regeneration of the used solvent
561
This step aimed to reduce the operational costs of solvent extraction process via atmospheric
562
distillation of the used solvent (acetonitrile CH3CN). The realization of using the distilled solvent
563
is checked through measuring the refractive index. The utilization of the recovered solvent
564
showed high efficiency in the extraction of sulfur compounds for several times.
565
3.9 Spent catalyst
566
3.9.1 Characterization
567
The spent catalyst which was collected after the completion of the catalytic photochemical
568
desulfurization at the optimum conditions was forwarded for analysis tools to identify its
569
structural and morphological properties. The TEM images, EDX spectrum and the XRD pattern
570
of the spent catalyst are exhibited in Figure 10a.
571 572 573 574 575 576 577 578 579
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The TEM images had shown a rational change in the morphology of the collected catalyst after
580 581 582 583 584 585 586 587 588 589 590 591 592 593 594
the desulfurization process. Ubiquitous non-uniform agglomerated molecules were detected in
595
the whole structure of the spent catalyst. These molecules are the adsorbed sulfur and
596
hydrocarbon compounds onto the CdO particles. The presence of such compounds has been
597
confirmed by the elemental analysis, EDX. The spectrum has exhibited sharp peak of carbon
598
which corresponds to 75 wt. % of the total sample. Indicative peaks for S, Cd, and O, had also
599
been noticed in the EDX. In line with, the XRD pattern had displayed the CdO characteristic
600
peaks which can reflect the conservation of the original catalyst structure after the execution of
601
the process. Noise peaks at 2theta between 8 and 30 have also been detected in the XRD
602
spectrum. These peaks are referring to the existence of the organic hydrocarbon within the
603
catalyst structure.
604
3.9.2 Catalyst sustainability and reusability
605
The as-collected spent catalyst (which had been discussed in Figure 10a) was then subjected to
606
impose the desulfurization process at the determined optimum conditions for a fresh sample of
607
the diesel fuel feedstock. The collected spent catalyst after this run was then used once again for
608
the desulfurization of fresh feedstock. This sequence has been repeated for five times of
609
processing. The catalyst had shown a steady state in the rate of desulfurization over its reusing
610
100 80
CPS
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Page 30 of 39
60 40 20 0 30
40
50 2 Theta (degree)
60
70
Figure 10a. The structural and morphological characteristics of the spent catalyst
30 ACS Paragon Plus Environment
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for these several times. Although the catalyst had some adsorbed sulfur compound onto its
611
structure which could reduce its photo activity, it had exhibited a steady rate of sulfur removal.
612
This, can be attributed to the compromise of the reduced activity by the strong interaction
613
between the adsorbed sulfur within the catalyst structure and that existing sulfur in the feedstock.
614 615
Catalyst Regeneration The spent catalyst after the reusability of five times was forwarded to the recovery stage. The
616
spent catalyst was washed via using benzene followed by ethanol and distilled water. The
617
washed catalyst was left to dry and then transferred to the XRD analysis to confirm its structural
618
formula. The XRD pattern (Figure 10b) of the washed catalyst has been identical to that of the
619
freshly prepared catalyst. This, can plainly reflect the success of regenerating the catalyst in a
620
unpretentious and non-costly way. The similar peaks in Figures 2 & 10b can confirm the ability
621
of the catalyst to maintain its original structure after executing the desulfurization process
622
followed by the regeneration step. The effortless recovery of the catalyst would provide extra
623
economic benefits to the current process.
624 625
120
626
100 80
CPS
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627
60
628
40 20
629
0 30
40
50 2 Theta (degree)
60
70
Figure 10b. XRD spectrum of the regenerated catalyst
630 631
3.10 photo-catalytic oxidation desulfurization mechanism
632
The prospective mechanism of the catalytic photochemical desulfurization of diesel fuel is presented
633
through the following equations as well as Figure 11.
634
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During the photocatalytic oxidative desulfurization process involving the CdO catalyst, there
635
exist both photochemical and photocatalytic oxidation reactions. Under linear halogen lamp
636
irradiation, CdO catalyst can absorb photons with definite wavelength to energize the electrons,
637
which in an excited state then move from the valence band to the conduction band, leaving
638
charged holes in the valence band (Figure 11) which acts as oxidizing agent (Eq.2), so light-
639
generated electrons and holes (h+):
640 CdO (h+) +CdO (e-)
CdO + hƲ
(2)
641 642 643 644 645 646 647 648 649 650 651 652 653 654
Figure 11. Main concept of the photocatalytic reaction
These holes and electrons can easily recombine in the bulk or on the surface of the
655
semiconductor during the migration process, leading to a marked decrease in the photocatalytic
656
reaction rate. The as prepared CdO in this paper by precipitation method possess a significant
657
photo-luminance properties in other words the rate of combination of holes and electron is small
658
which lead to decrease the combination process so the holes can migrate to the surface directly to
659
take part in the oxidation reactions of the sulfur compounds (Eq. 3).
660
CdO (h+) + Sulfur present in diesel fuel
CdO + Oxidation products of Diesel
(3)
661
The addition of hydrogen peroxide in this process has two competing effects: (1) increasing the
662
concentration of hydroxyl radicals in the reaction media, and (2) decreasing the average light
663
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intensity within the solution due to the absorption of visible light by H2O2. Enhancing the
664
desulfurization rate of diesel due to the most efficient generation of hydroxyl radicals (•OH) and
665
the inhibition of the recombination of electron/hole (e-/h+) pairs (Eq.4). So the combination of
666
photochemical oxidation and photocatalytic oxidation leads to enhance the desulfurization
667
reaction efficiently.32
668 2 •OH
H2O2 +hν CdO (e-) + H2O2
669
2 •OH
2 •OH + Sulfur present in diesel fuel
oxidation products of diesel
670 (4)
671
Figure 12a shows the IR spectrum of the diesel fuel feed stock, Figure 12b diesel fuel after the
672
catalytic photochemical reaction and Figure 12c diesel fuel after extraction with acetonitrile.
673
There was an absorption peak at 2924 cm-1, which was attributable to the stretching vibration of
674
(—CH Aromatic) bond. The peak at 2857 cm-1 was due to the stretching vibration of (—CH
675
Aliphatic) bond. There was also an absorption peak at 1620-1630 cm-1 that was due to (C=C Ar.)
676
bond, which Suggested that there conjugated double bond as an aromatic ring. The wavenumber
677
at value ~ 1460 cm-1 CH3 bending (Asym.) appears near the CH2 while CH3 bending (Sym.) ~
678
1375 cm-1.33 The wavenumber at value ~ 1295 cm-1 is corresponding to S=O (Asym.) while S=O
679
(Sym.) ~ 1147 cm-1 which indicated the existence of sulfone group.34
680
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Figure 12 a. FTIR spectrum for Diesel Fuel (Feed Stock).
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681 682 683
684 Figure 12 b. FTIR spectrum for diesel fuel after Catalytic photo-chemical reaction.
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685 686
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687 688 Figure 12 c. FTIR spectrum for diesel fuel after Extraction.
689
Conclusion
690
Two cadmium oxide structures were prepared in the current study via two different techniques;
691
chemical precipitation and auto-ignition. The structural and morphological characteristics of both
692
catalysts were studied through various analysis tools. Both catalysts had shown high crystal linty
693
and micro-porous structures. A little bit different morphology was also detected in both catalysts
694
owing to the different methods of preparation. The UV-visible DRS results showed that the
695
absorption wavelength range of the CdO prepared by precipitation method was extended towards
696
the visible-light region (λ > 400 nm) with band gap energy 2.1 eV. Photoluminescence (PL)
697
spectra, proved that CdO prepared by precipitation method was more active than CdO prepared
698
by auto-ignition method. This indicated a minimum recombination rate. As a result, they possess
699
the highest photocatalytic activity for the desulfurization process due to the effective separation
700
of excited electron/holes. The CdO prepared by precipitation method was then transferred to the
701
catalytic photochemical desulfurization where different parameters were studied and the
702
optimum conditions were determined. The development of catalytic photochemical
703
desulfurization process using CdO nanoparticles was reported at the end of this work. A diesel
704
fuel with a sulfur content of 45 ppm was obtained after the removal of 99.6 wt % of the sulfur
705
compounds from the original feedstock (11500 ppm). Finally, this study reports the development
706
of catalytic photochemical desulfurization process using CdO nanoparticles. The results of this
707
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study are promising in terms of obtaining an increased S removal efficiency compared to only
708
photochemical or photocatalytic process. Thus, the process developed here has potential of being
709
more efficient and economical in removing sulfur from diesel feedstock.
710
Supporting Information The supporting information includes:
711 712 713
Thermal Stabilities of the both Metal Hydroxides and Photo-Catalysts CdO NPs.
714
Tables representing the Physico-Chemical Characteristics of the Obtained Diesel Fuel during this
715
Study as Present from figure (7a to 9) at the Different Parameters:
716
•
Effect of the Light Source on the Sulfur Removal %.
717
•
Effect of CdO-to-Feed Dosage on the Sulfur Removal%.
718
•
Effect of the Operational Time on Sulfur Removal%.
719
•
Effect of Strength of H2O2 on Sulfur Removal % at H2O2 /Feed (1/1).
720
•
Effect of H2O2 to Feed Ratio on the Sulfur Removal%.
721
•
Effect of Acetic acid to H2O2 to Feed Ratio on the Sulfur Removal%.
722
•
Effect of Solvent-to-Feed Ratio on the Sulfur Removal%.
723 724 725
4. References
726 1. Moradi, S.; Vossoughi, M.; Feilizadeh, M.; Zakeri, S. M. E.; Mohammadi, M. M.; Rashtchian, D.; Booshehri, A. Y. Photocatalytic degradation of dibenzothiophene using La/PEG-modified TiO2 under visible light irradiation. Res. Chem. Intermed., 2015, 41, 4151-4167. 2. Song, C.; Ma, X. New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. Appl. Catal., B, 2003, 41, 207-238. 3. Al Zubaidy, I. A.; Tarsh, F. B.; Darwish, N. N.; Majeed, B.; Sharafi, A.; Chacra, L. A. Adsorption process of sulfur removal from diesel oil using sorbent materials. J. Clean Energy Technol., 2013, 1, 66-68. 4. Robertson, J.; Bandosz, T. J. Photooxidation of dibenzothiophene on TiO2/hectorite thin films layered catalyst. J. Colloid Interface Sci., 2006, 299, 125-135. 5. Vargas, R.; Nunez, O. Photocatalytic degradation of oil industry hydrocarbons models at laboratory and at pilot-plant scale. Sol. Energy, 2010, 84, 345-351. 6. Salehi, B.; Mehrabian, S.; Ahmadi, M. Investigation of antibacterial effect of cadmium oxide nanoparticles on staphylococcus aureus bacteria. J. Nanobiotechnol., 2014, 12, 1-8. 7. Su, L.; Grote, N.; Schmitt, F. Diffused planar InP bipolar transistor with a cadmium oxide film emitter. Electron. Lett., 1984, 20, 716 -717. 8. Benko, F.; Koffyberg, F. Quantum efficiency and optical transitions of CdO photoanodes. Solid State Commun., 1986, 57, 901-903. 36 ACS Paragon Plus Environment
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9. Chang, J.; Mane, R .S.; Ham, D.; Lee, W.; Cho, B. W.; Lee, J. K.; Han, S. H. Electrochemical capacitive properties of cadmium oxide films. Electrochim. Acta, 2007, 53, 695-699. 10. Tripathi, R.; Dutta, A.; Das, S.; Kumar, A.; Sinha, T. Dielectric relaxation of CdO nanoparticles. Appl. Nanosci., 2016, 6, 175-181. 11. El Naggar, A. M.; Nassar, I. M.; Gobara, H. M. Enhanced hydrogen production from water via a photo-catalyzed reaction using chalcogenide d-element nanoparticles induced by UV light. Nanoscale, 2013, 5, 9994-9999. 12. Faizullah, A.; Khan, M.; Rahman, M. M. Pyrolized growth of (Al, N) dual doped CdO thin films and study of structural. Surface morphology and opto-electrical properties. Int. J. Mater. Sci. Appl., 2013, 2, 124-127. 13. Nezamzadeh-Ejhieh, A.; Banan, Z. A comparison between the efficiency of CdS nanoparticles/zeolite A and CdO/zeolite A as catalysts in photodecolorization of crystal violet. Desalination, 2011, 279, 146-151. 14. Karimi Andeani, J.; Mohsenzadeh, S .Phytosynthesis of cadmium oxide nanoparticles from achillea wilhelmsii flowers. J. Chem., 2012, 2013. 15. Xiaochun, W.; Rongyao, W.; Bingsuo, Z.; Li, W.; Shaomei, L.; Jiren, X.; Wei, H. Optical properties of nanometer-sized CdO organosol. J. Mater. Res., 1998, 13, 604-609. 16. Han, S.; Feng, X.; Lu, Z.; Johnson, D.; Wood, R. Transparent-cathode for top-emission organic light-emitting diodes. Appl. Phys. Lett., 2003, 82, 2715-2717. 17. Aldwayyan, A.; Al-Jekhedab, F.; Al-Noaimi, M.; Hammouti, B.; Hadda, T.; Suleiman, M.; Warad, I. Synthesis and characterization of CdO nanoparticles starting from organometalic dmphen-CdI2 complex. Int. J. Electrochem. Sci., 2013, 8, 10506-10514. 18. Rajamathi, M.; Seshadri, R. Oxide and chalcogenide nanoparticles from hydrothermal/solvothermal reactions. Curr. Opin. Solid State Mater. Sci., 2002, 6, 337-345. 19. Li, L.; Zhang, J.; Shen, C.; Wang, Y.; Luo, G. Oxidative desulfurization of model fuels with pure nano-TiO2 as catalyst directly without UV irradiation. Fuel 2016, 167, 9-16. 20. Fraile, J. M.; Gil, C.; Mayoral, J. A.; Muel, B.; Roldán, L.; Vispe, E.; Calderón, S.; Puente, F. Heterogeneous titanium catalysts for oxidation of dibenzothiophene in hydrocarbon solutions with hydrogen peroxide: on the road to oxidative desulfurization. Appl. Catal., B, 2016, 180, 680-686. 21. Tao, H.; Nakazato, T.; Sato, S. Energy-efficient ultra-deep desulfurization of kerosene based on selective photo-oxidation and adsorption. Fuel, 2009, 88, 1961-1969. 22. Trongkaew, P.; Utistham, T.; Reubroycharoen, P.; Hinchiranan, N. Photocatalytic desulfurization of waste tire pyrolysis oil. Energies, 2011, 4, 1880-1896. 23. Ristic, M.; Popovic, S.; Music, S. Formation and properties of Cd(OH)2 and CdO particles. Mater. Lett., 2004, 58, 2494-2499. 24. Moradi, S.; Vossoughi, M.; Feilizadeh, M.; Zakeri, S. M. E.; Mohammadi, M. M.; Rashtchian, D.; Booshehri, A. Y. Photocatalytic degradation of dibenzothiophene using La/PEGmodified TiO2 under visible light irradiation. Res. Chem. Intermed., 2014, 1-17. 25. El Sayed, A.; El‐Sayed, S.; Morsi, W.; Mahrous, S.; Hassen, A. Synthesis, characterization, optical, and dielectric properties of polyvinyl chloride/cadmium oxide nanocomposite films. Polym. Compos., 2014, 35, 1842-1851. 26. Hankare, P.; Sanadi, K.; Pandav, R.; Patil, N.; Garadkar, K.; Mulla, I. Structural, electrical and magnetic properties of cadmium substituted copper ferrite by sol–gel method. J. Alloys Compd., 2012, 540, 290-296.
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27. Kalpanadevi, K.; Sinduja, C.; Manimekalai, R. Characterisation of zinc oxide and cadmium oxide nanostructures obtained from the low temperature thermal decomposition of inorganic precursors. ISRN Inorg. Chem., 2013, 2013. 28. Sahoo, S.; Mohapatra, M.; Pandey, B.; Verma, H.; Das, R.; Anand, S. Preparation and characterization of α-Fe2O3–CeO2 composite. Mater. Charact., 2009, 60, 425-431. 29. Gulino, A.; Compagnini, G.; Scalisi, A. A. Large third-order nonlinear optical properties of cadmium oxide thin films. Chem. Mater., 2003, 15, 3332-3336. 30. Subramanyam, T.; Rao, G. M.; Uthanna, S. Process parameter dependent property studies on CdO films prepared by DC reactive magnetron sputtering. Mater. Chem. Phys., 2001, 69, 133142. 31. El Naggar, A. M.; Gobara, H. M.; Nassar, I. M. Novel nano-structured for the improvement of photo-catalyzed hydrogen production via water splitting with in-situ nano-carbon formation. Renewable Sustainable Energy Rev., 2015, 41, 1205-1216. 32. Li, S. W.; Li, Y. Y.; Yang, F.; Liu, Z.; Gao, R. M.; Zhao, J. S. Photocatalytic oxidation desulfurization of model diesel over phthalocyanine/La0.8Ce0.2NiO3. J. Colloid Interface Sci., 2015, 460, 8-17. 33. Colthup, N. Introduction to infrared and raman spectroscopy. Elsevier: 2012. 34. Nisar, A.; Lu, Y.; Zhuang, J.; Wang, X. Polyoxometalate nanocone nanoreactors: magnetic manipulation and enhanced catalytic performance. Angew. Chem., 2011, 123, 3245-3250.
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790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826
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827 828 829
TABLE OF CONTENT (TOC)
830 831 832 100 90 80
S removal, wt.%.
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70 60 50 40 30 Solvent extraction of feed stock
20 Solvent extraction after photocatalytic desulfurization Solvent extraction after photoassisted desulfurization
10 0 (1:1)
(2:1)
(3:1)
(4:1)
(5:1)
Solvent to Feed ratio (S/F)
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833 834