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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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The Journal of Physical Chemistry

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

53


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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

59

recombination of an electron and hole that are produced by photo-activation in the solid.6 Among

60

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.

73

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

76



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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

80

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

86

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

92

coupled with an oxidizing agent (H2O2) to be much more efficient for desulfurization process.24

93

The eventual sulfur content in the diesel fuel was acquired via the solvent extraction technique.

94

2. Experimental

95

3. 2.1. Feedstock

96

A straight run diesel fuel fraction was conducted from Suez oil processing company (SOCo),

97

Suez-Egypt. The physicochemical characteristics of the feedstock are listed in Table 1.

98 99


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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

106

2.2.1 Materials

107

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

112

chemical precipitation25 and (b) The auto-ignition methods.26

113

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.

115

An aqueous solution of NaOH (0.5N) was afterward added drop-wise to the cadmium solution

116

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

119

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

123

o

124

(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)

129

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

132

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

151



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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.

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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.


275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295

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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



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 16 of 39

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


Page 17 of 39

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


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.

371 372 373 17

ACS Paragon Plus Environment


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Page 18 of 39

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


Page 19 of 39

8 7

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


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



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.%

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 20 of 39

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


Page 21 of 39

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

21 18

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


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



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 22 of 39

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


Page 23 of 39

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


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



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. %

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 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


Page 25 of 39

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.%

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


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.



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


Page 27 of 39

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


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



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 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. %.


Photochemical desulfurization

Page 29 of 39

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


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



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

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 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


Page 31 of 39

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

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


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



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 32 of 39

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


Page 33 of 39

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


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



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

Figure 12 a. FTIR spectrum for Diesel Fuel (Feed Stock).

Page 34 of 39

681 682 683

684 Figure 12 b. FTIR spectrum for diesel fuel after Catalytic photo-chemical reaction.


685 686

Page 35 of 39

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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



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 36 of 39

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.


745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789


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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


Page 39 of 39

827 828 829

TABLE OF CONTENT (TOC)

830 831 832 100 90 80

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


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)


833 834

Photoassisted Desulfurization Induced by Visible ... - ACS Publications

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