Journal of Molecular Liquids

Ionic liquid green synthesis of CeO

₂

nanorods and nano-cubes:

Investigation of the shape dependent on catalytic performance

Mosaed Alhumaimess

^a,

⁎ , Obaid Aldosari

^b,c

, Hamed Alshammari

^d

, Mahmoud M. Kamel

^a

, Mohamed A. Betiha

^e

, Hassan M.A. Hassan

^a,f

aDepartment of Chemistry, College of Science, Jouf University, PO Box 2014, Sakaka, Saudi Arabia

bDepartment of Chemistry, College of Science and Human Studies at Hautat Sudair, Majmaah University, Majmaah, Saudi Arabia

cDepartment of Chemistry, College of Science and Humanities, Prince Sattam bin Abdulaziz University, Alkharj, Saudi Arabia

dDepartment of Chemistry, Faculty of Science, Ha'il University, PO Box 2440, Ha'il 81451, Saudi Arabia

eEgyptian Petroleum Research Institute, Cairo 11727, Nasr City, Egypt

fDepartment of Chemistry, Faculty of Science, Suez University, Suez, Egypt

a b s t r a c t a r t i c l e i n f o

Article history:

Received 5 December 2018

Received in revised form 4 January 2019 Accepted 4 February 2019

Available online 5 February 2019

A new facile and scalable approach for utilizing basic ionic liquid, 1-butyl-3-methylimidazolium hydroxide ([BMIM]⁺OH⁻) for the fabrication of different shapes of ceria nanostructures was adopted. The features of the fabricated ceria and their corresponding gold nanocatalysts were characterized by employing ICP, HRTEM, XRD, XPS, BET and UV–vis spectroscopy. Catalytic performance of CeO2and its dependence on shape was studied in the oxidation of CO and olefins epoxidation. The ratio between the ionic liquid and cerium precursor is one of the valuable factors used to monitor the growth of the particles. The catalytic performance of ceria was found to be dependent on the morphology of the catalysts. The catalytic performance of CeO2in the form of nanorod- shapes is better than that of nanocubes and bulk. The deposition of gold nanoparticles on different shaped CeO2much enhanced their catalytic performance. This enhancement in catalytic performance was, however, more significant in the case of rod-shaped ceria.

© 2019 Elsevier B.V. All rights reserved.

Keywords:

CO oxidation Epoxidation Ionic liquids

CeO2nano-cubes CeO2nano-rods Au/CeO2

1. Introduction

The fabrication of metal oxide nanoparticles with definite sizes and shapes can be tailored through various fabrication conditions. It is very interesting that the shapes and crystal-planes of materials can greatly affect their activity due to different surface energies, along with the ordering of atoms. Nanoparticles exhibit high surface energy, along with a high surface-to-bulk atom ratio, which controls their characteristics [1–5]. A high surface area is valuable in (heterogeneous) catalysis. Therefore, small nanoparticles need to be stabilized; otherwise they will join to thermodynamically favor aggregated particles. More re- cently, ionic liquids (ILs) have been developed in material fabrication owing to the fabrication in ILs usually resulting in materials that are unable to synthesize utilizing other conventional methods [6]. In ionic liquids (ILs) as a novel liquid medium, metal oxide nanoparticles can be fabricated in absence of any additional stabilizers. Principally, ILs may be regarded as a“nano-synthetic template”that stabilizes nano- particles owing to their ionic features, outstanding polarity, and high dielectric constant without the necessity for further capping agents

[7]. However, most of the research has focused on the fabrication of nanoparticles in ILs regardless of their shapes. Thus, it is of great concern that the pliability of ionic liquids should be explored for the fabrication of nanoparticles of different shapes and sizes. Currently, a number of fabrication approaches, including templet agent, gas phase methods, electrochemical deposition, thermal decomposition, chemical vapor deposition, hydrothermal/solvothermal methods and microwave irra- diation treatments, have progressed toward the design and fabrication of nanocatalysts with definite shapes and sizes [8–25]. Among these methods, the microwave irradiation approach is considered to be one of the most promising green and economical approaches compared to conventional methods, as it involves a considerable lowering in the number of reagents and processing steps required for the fabrication of nanoparticles, although it requires heating the reaction mixture uni- formly and rapidly [26]. Microwave irradiation path furnishes facile and prompt courses to the fabrication of nanomaterials without the require- ment high pressure or temperature. Moreover, Energy is transferred to the reactant molecules rapidly than they are able to dangle, forming high immediate temperatures which may boost quality of the fabricated materials. By selection metal salts that exhibit a high microwave absorption as compared to the solvent, very high potential reaction temperatures can be obtained. This permits the quick decomposition

⁎ Corresponding author.

E-mail address:mosaed@ju.edu.sa(M. Alhumaimess).

https://doi.org/10.1016/j.molliq.2019.02.014 0167-7322/© 2019 Elsevier B.V. All rights reserved.

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Journal of Molecular Liquids

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of the starting materials thus forming supersaturated solutions produc- ing the required nanomaterials [27,28].

Ceria with afluorite type structure and oxygen vacancy properties [29,30] have been widely adopted as a ubiquitous constituent in cata- lytic systems in metal oxide fuel cells (SOFCs), water-gas shift reaction, photocatalysis, reforming processes, water splitting, oxygen storage, and the oxidation of volatile organic constituents [31,32]. Most of the applications related to ceria particles depend on not defined ceria morphology, however, various recent investigations have manifested that the redox characteristics of the definite shape and size had an outstanding influence on the catalytic performance [33–35]. Ceria nanoparticles exhibit various defective sites as oxygen vacancies and plenty surface-atoms as compared to the bulk ceria [36,37]. Recent re- search into nanometric CeO2systems has concentrated on the growth of powerful fabrication methods related to size and shape monitored nanoparticles, and the examination of their size and shape-dependent characteristics. Size-controlled CeO2nanoparticles have been synthe- sized by adopting different wet chemical methods as modified precipi- tation, alcohothermal, microemulsion, and sonochemical approaches, and their size has been examined in relation to the UV absorption profile with a view to illustrate the confinement effects in nanometric CeO2

[38]. Li et al. [39] fabricated CeO2in the form of nanorods by using a hydrothermal approach that revealed an enhancement in CO oxidation activity. Chen et al. [40] demonstrated the dependence of the reaction route on size and they exploited the catalytic performance. CeO2in the form of nanowires was prepared using a solution route in the pres- ence of sodium bis(2-ethylhexyl) sulfosuccinate as a template, and the results reveal that Raman spectra depends on the size of ceria. Recent investigations of CeO₂particles have focused on the synthesis of CeO₂ particles with controlled morphologies and emphasize their catalytic performance to set up morphology–activity correlation. However, investigations related to the fabrication of CeO₂nanoparticles with different morphology in the presence of ionic liquids are scarce. Basic

ionic liquids not only exhibit the advantages of classical ILs, but also have the advantage of inorganic bases, such as being stable in aqueous media and air; being facile to recycle and reusable; noncorrosive; non- volatile, and able to be unsettled with various organic solvents. Thus, it is of great interest to explore the applicability of ILs to fabricate and promote a more efficient synthetic strategy of nanostructure materials.

El-Shall et al., prepared Pd and Au nanoparticle deposited on some metal oxides, including ceria nanoparticles, where the results proved that gold deposited on the ceria nanoparticle has the highest activity to- ward CO-oxidation [41–43]. Herein, wefirstly investigated one-step facile microwave-assisted approach for the fabrication of CeO2nanopar- ticles of different shapes using basic ionic liquid. The catalytic activity of the fabricated nanostructure materials was examined in CO oxidation and alkene epoxidation.

2. Experimental section

2.1. Fabrication methods

2.1.1. Fabrication of nanorods and nanocubes CeO2

CeO2 in the form of nanocubes were prepared using a facile microwave-assisted approach. Firstly, the ionic liquid 1-butyl-3- methylimidazolium hydroxide ([BMIM]⁺OH⁻), was evacuated at 80 °C for 12 h. The dried IL was kept in a desiccator to eschew moisture uptake from the atmosphere. A proper amount of [BMIM]⁺OH^– (0.314 g) was sonicated at 40 °C for 30 min in distilled water (20 ml).

Then, Ce(NO3)36H2O (1.303 g) as the cerium precursor were added to the ionic liquid dispersion with continuous sonicating at 40 °C for a further 30 min to form a homogeneous mixture with a pH value amounting to 10. Finally, the clear solution obtained was then trans- ferred to a Teflon reactor placed into a microwave oven. The reaction mixture was then heated at 80 °C for 30 min. The nanorods ceria were collected from ionic liquid dispersion by centrifugation for 20 min at

Scheme 1.Schematic representation the shape-selective fabrication of CeO2nanostructures.

4000 rpm, washed thoroughly with ethanol, and dried at 100 °C over- night (Scheme 1). CeO2nanocubes were fabricated using the same rec- ipe under the conditions of [BMIM]⁺OH^–(0.156 g) and Ce(NO₃)₃6H₂O (1.303 g) at 120 °C for 45 min.

2.1.2. Fabrication of bulk ceria

PEG17000 (1 g) was dissolved in 50 ml of solution containing 1 mmol of cerium (III) nitrate hexahydrate. The solution was sonicated for 30 min at room temperature. Then, NaOH (2 M) was added dropwise with vigorous stirring till pH 10. The mixture was then transferred to a Teflon reactor placed into a microwave oven. The suspension was then heated at 100 °C for 1 h, followed by separation, washing, and drying at 120 °C for 12 h.

2.1.3. Fabrication of Au/CeO2

Gold nanoparticles were deposited onto the surface of the CeO2

nanostructure using the deposition-precipitation (DP) method. Typi- cally, a suitable amount of HAuCl4in HCl was added to the suspension of CeO2in distilled water. The suspension was then sonicated for 5 min. The pH of the suspension was adjusted to 9 using NaOH while performing the stirring. The resulting mixture was then subject to mi- crowave irradiation for 2 min operated in 30-s cycles (on for 10 s, off for 20 s). The catalyst was thenfiltered, washed with distilled water, and dried at 120 °C for 12 h.

2.2. Characterization

The crystalline structure of the prepared samples was obtained using X-ray diffraction (X'Pert Philips Materials Research Diffractometer) with Cu–Kαradiation, and data were recorded at conditions of 45 kV and 40 mA. X-ray photoelectron spectroscopy analysis (XPS) was con- ducted on Thermo Scientific K-ALPHA (USA). The data was recorded for Ce 3d and Au 4f electronic level at 200 W and 12 kV with energy

passed of 50 eV and hγof 1486.6 eV. The Au percent doped CeO2was determined using inductively coupled plasma (ICP). The measurement was achieved by using a Varian (Vista-MPX) Charge Coupled Device (CCD) synchronous ICP-OES instrument. The values of the BET parame- ters (surfaces–area, pore–volume and pore size distribution) of the fabricated nanostructure materials were determined by N₂ adsorption-desorption isotherms at−195 °C, using the quantachrome sorptiometry instrument. High-resolution transmission electron microscopy (HR-TEM) images were collected from a JEOL model 2100 instrument operating at 200 kV.

2.3. Catalytic performance 2.3.1. Carbon monoxide oxidation

The catalytic oxidation of CO was tested by inserting a 50 mg catalyst in a quartz tube reactor. The reactor was placed inside a thermolyne 2100 programmable tube furnace. The catalyst temperature was measured using a thermocouple adjusted near the catalyst [26,44].

The gas mixture composed of CO (4%) and O2(20%) in helium. The reactant gases were adjusted toflow over the catalyst at aflow rate of 50 cm³min⁻¹monitored by an MKSflow meter with a space velocity amount of 15,000 ml h⁻¹ g⁻¹. The conversion of CO to CO2 was measured by using an infrared gas-analyzer (ACS, Automated Customs System Inc). The catalyst was activated at 110 °C in He gas to get rid of the moisture and impurities.

2.3.2. Epoxidation of olefins

Typically, a mixture of alkenes (2 ml), H2O₂(30%, 2 ml), 50 mg catalyst and sodium bicarbonate (0.168 g, 2 mmol) was placed in a Pyrex tube followed by heating it at 80 °C with stirring. The reaction mixture was then collected and analyzed via GC (Agilent Technologies, model6890N, capillary column HP-5, 30 m, containing 5% methyl +95%

phenyl siloxane) after 18 h.

Fig. 1.(A) XRD patterns (B) normalize UV–vis spectra (C) HRTEM for CeO2catalyst in the form of bulk, nanocubes and nanorods.

3. Results and discussion

3.1. Characteristics of CeO₂nanocubes, nanorods, and bulk

The crystalline structure of the fabricated CeO2nanocubes, nanorods and bulk was determined by XRD, as depicted inFig. 1A. Apparently, the obtained powder diffractograms of CeO2nanocubes and nanorods displayed characteristic diffraction peaks corresponding to a cubicfluo- rite structure similar to that presented for the bulk CeO2phase. The diffractograms obtained are nearly comparable to those of cubic CeO2

(JCPDS No. 34-03941, space groupFm-3m) and no diffraction peaks belonging to other foreign elements were detected, indicating that the fabricated materials were pure and successfully prepared. Conse- quently, the three strongest diffraction peaks indexed at 3.067, 1.890, 1.616 Å of ceria, corresponding to the crystal plane (111), (220) and (311), respectively. The broadening of the diffraction peaks for CeO₂ nanocubes and nanorods, clearly confirms their nanocrystalline nature, while the sharper diffraction peaks of CeO2cubes indicates their larger sizes as compared with the former nanocube and nanorod samples.

Moreover, the unit cell parameter aovalues obtained for the fabricated ceria in the form of nanocubes and nanorods are smaller than those measured for bulk CeO2(Table 1). The decrease in the value of unit cell parameters could be attributed to utilizing [BMIM]⁺OH⁻, ionic liquid which may cap the tiny nanoparticles and diminish the surface relaxation of the nanoparticles produced, resulting in the reduction of the lattice parameter.

The normalized UV–vis absorption spectra of ceria nanocubes, nano- rods and bulk, are shown inFig. 1B. The spectra obviously exhibited a

clear absorption band at UV region which may be owing to the charge transfer transition from O 2p to Ce 4f bonds, which surpass the well-known f to f spin-orbit splitting of the Ce 4f state [45]. Moreover, the absorption spectra of the fabricated samples displayed only a single surface Plasmon absorption band at 320 and 309 nm for the ceria in the form of nanorods and nanocubes, respectively. These bands may assist the fabrication of two different CeO2nanostructures. Additionally, no clear absorption bands for bulk CeO2was observed.

To give more prudence into the different CeO2nanostructures, HRTEM measurements were also carried out. Fig. 1C depicts the HRTEM images of the ceria in the form of nanocubes and nanorods, respectively. A large quantity of ceria could be observed in the form of rod structures with widths amounting to 10–15 nm and lengths up to 50 nm. The nanocubes were mostly hexagonal with definite corners and edges. This led to the conclusion that the absorption forces of [BMIM]⁺and the surface of CeO₂nanoparticles decreased. Integrating the growth pathway with that of the obtained results, the transforma- tion processes are emphasized inScheme 1. Two main reactions take place through the microwave irradiation: (i) thefirst step involve the conversion of Ce³⁺ions steaming from the cerium precursor into Ce⁴

+which is main constituents of CeO2(ii) the second step encompass the development of nanorods structure. When the rate of thefirst step is greater than that of the second one the major process involve the for- mation of nanorods ceria followed by deposit of remain Ce⁴⁺on its sur- face. When the rate of the growth is greater than that of thefirst step the Ce³⁺ions prefer to be on the end of nanorods resulted in the formation of CeO2filaments. At higher microwave temperature thesefilaments fractures into nanoparticles which then converted into nanocubes. It is

Fig. 2.(A) Nitrogen adsorption-desorption isotherms (B) pore size distribution for CeO2catalyst in the form of bulk, nanocubes and nanorods.

Table 1

ICP analysis for bulk Au nanoparticles, unit cell parameter (a/nm) and texture characteristics data for CeO2in the form of nanorods, nanocubes and bulk.

Catalysts Calculated Au (wt%) Actual Au (wt%)^a aob/(nm) N2-adsorption isotherm

BET^c(m²g⁻¹) Dp,ads^d/nm Vpe

/(cm³g⁻¹)

CeO2-rods – – 0.5367 37 70 0.0886

CeO2-cubes – – 0.5390 20 135 0.0548

CeO2-bulk – – 0.5411 6 558 0.0294

5%Au/CeO2-nanorods 5 4.86 – – – –

5%Au/CeO2-nanocubes 5 4.67 – – – –

5%Au/CeO2-bulk 5 4.33 – – – –

aObtained by ICP.

b Unit cell parameter.

c Surface area.

d Mean pore diameter(BJH).

eTotal pore volume.

worth noting that the growth attitude constantly assessment the ultimate crystal morphology which is extremely affected by its growth conditions. The spherical particles exhibited two feature facets, namely (111) and (001). The proportion between the developments of these two facets monitored the ultimate morphology of the particles. These facets also displayed distinct surface free energies (γ111bγ100bγ110) [46,47]. These variations in surface energy values were accountable for the various combination energies and reactivity of the crystal faces.

Since (001) is more reactive than (111), it tends to adsorb the ionic liquids over the catalyst surface greater than (111) facets. Ionic liquid acts as a capping agent and inhibited further growth of the (001) facet.

This led to the increasing the growth rate of the [111] face, which led to the fabrication of rod shapes [44]. Therefore, the microwave irradia- tion approach in the existence of basic ionic liquid is promising for the fabrication of large scale CeO2in the form of nanorods and nanocubes.

Obviously, the morphology of the nanostructures ceria could be

monitored by the microwave temperature along with the ILs/Ce³⁺

ratio in the beginning solution.

The BET surface area, pore size and pore volume of CeO₂nanostruc- tures are summarized inTable 1.Fig. 2A shows the N2adsorption– desorption isotherms of the fabricated samples. As can be seen from Fig. 2A, CeO2in the form of nanorods and nanocubes belong to type II isotherms according to IUPAC classification, with hysteresis loop where the desorption required definitively higher energy than adsorp- tion [48,49]. The area of the hysteresis loop of the CeO2nanorods was bigger than those of bulk and nanocubes ceria. The BET specific surface area of CeO2nanorods was much bigger than that of ceria nanocubes.

The pore size distribution curves based on BJH model of the different ceria nanostructures are shown inFig. 2B. The pore size distribution curves of CeO2in the form of nanorods and nanocubes were centered at about 70 and 135 Å respectively, while for bulk CeO2the inflection became smaller owing to the reduction of the pore volume. Accordingly,

Fig. 4.(A) Ce 3d XPS spectrum of Au/CeO2catalyst in the form of nanorods (B) Ce 3d XPS spectrum of Au/CeO2in the form of nanocubes (C) Au 4f XPS spectrum of Au/CeO2in the form of nanorods.

Fig. 3.(A) XRD patterns (B) Normalize UV–vis spectra (C) HRTEM for Au/CeO2catalyst in the form of bulk, nanocubes and nanorods.

the higher surface area of CeO2nanorods compared to those of ceria in the form of nanocubes and bulk can be understood from the pore size distribution and pore volume.

3.2. Characteristics of Au/CeO2nanocubes, nanorods, and bulk

To investigate the support interaction between the metallic nano- particles and CeO2with different morphologies, 5% Au/CeO2was syn- thesized. The bulk gold concentration was estimated by inductive coupled plasma. The results reveal that the Au attained concentration was very close to the gold species introduced over the ceria, confirming the performance of the DP approach for loading gold nanoparticles on the surface of the fabricated ceria (Table 1).Fig. 3A displays the X-ray diffraction (XRD) patterns of 5%Au/CeO₂in the form of nanocubes, nanorods and bulk. Apparently, the ceria nanostructures maintained their original crystal structure after Au deposition. The intensity of the diffraction peak became slightly broader and weaker. Furthermore, loading of CeO2with 5 wt% Au led to good dispersion of Au nanoparti- cles on CeO2surfaces, and only small peaks with very weak intensities could be seen (Fig. 3A). An examination of the diffraction peaks of the gold nanoparticles (JCPDS Card No. 04–0784) in the XRD patterns con- firmed that the gold exists as metallic particles. The absorption spectra of Au/CeO2nanostructures are depicted inFig. 3B. It worth noting that for the Au supported samples, the two Plasmon absorption bands shifted to lower wavelengths at 316 and 297 nm for the ceria in the form of nanorods and nanocubes, respectively. Additionally, the absorp- tion spectra of the fabricated gold samples depicted only a single surface Plasmon absorption band at ~530 and 523 nm for the ceria in the form of nanorods and nanocubes due to the formation of metallic gold in the form of spheres (inset inFig. 3B). The morphology and texture of the fabricated Au/CeO2in the form of nanocubes and nanorods was exam- ined using HRTEM, as depicted inFig. 3C. Obviously, the ceria had their original shapes after gold deposition with no evidence of a change in the shape of the ceria nanostructure.

The catalytic performance was mainly significantly influenced by the surface chemical state owing to most of the reactions being carried out

over the surface of the catalyst. Consequently, the surface chemical state of ceria in the form of nanorods and nanocubes was conducted by XPS spectroscopy, as displayed inFig. 4, and the corresponding XPS data as atomic percentage from integrated peak areas are presented in Table 2. For all the fabricated materials, photoelectron emission bands were examined over Ce 3d and Au 4f binding energy (BE/eV) values only. Consequently, the Ce 3d and Au 4f spectra (Fig. 4) were deconvoluted using CasaXPS version 2.3.1.3 software in order to resolve the overlapping constituents. The Ce 3d spectra (Fig. 4A and B)were deconvoluted into six well-determined peaks, which can be categorized into two groups of spin-orbital. Thefirst peaks located in the range of 899–920 eV were assigned to Ce 3d3/2,while the second peaks located in the range of 880–898 were assigned to Ce 3d_5/2. The BE at 899.7 eV was assigned to Ce³⁺ions, while the other peaks were ascribed to Ce⁴⁺

ions, which confirms the coexistence of Ce³⁺and Ce⁴⁺over Au/CeO2

nanostructure catalysts simultaneously. Additionally, the concentration of Ce³⁺could be determined via the integrated peak area relative to the total Ce, which is summarized inTable 2. Interestingly, Au/CeO2in the form of nanorods showed a higher content of Ce³⁺ than that of nanocubes, which could be ascribed to the formation of oxygen vacancy.

Moreover, the XPS spectrum (Fig. 4C)confirms the presence of Au over CeO2in the form of nanorods. The binding energy ranged from 80 to 95 eV, where double-peak occurred at 83.4 and 87.6 eV, ascribed to Au 4f7/2and Au 4f5/2, respectively, confirming the presence of the Au⁰state as a dominant gold species in the ceria nanorods.

3.3. Catalytic performance 3.3.1. CO oxidation

Generally, the investigations into heterogeneous catalysis comprised nanostructure materials in the form of spheres or nanostructures with indefinite morphology. Recent investigations tested the shape depen- dent on the catalytic performance. The effect of nanoparticle morphol- ogy on the catalytic performance is mainly based on the correlation between the active centers, which results in strong surface reactivity [50–52]. This could be related to the nanocrystals with different morphology having different facets, along with different ordering of atoms at distinct corners, edges, and defect sites [50]. Consequently, nanocatalysts such as Au with definite sizes and shapes show different activities when loaded on metal oxides with different shapes. The ceria catalysts in the form of nanorods and nanocubes were tested for the oxidation of CO. The light-off curves for CO oxidation are displayed inFig. 5. Obviously, the CeO2in the form of nanorods showed greater activity than nanocubes, and nanocubes for the catalytic oxidation of

Fig. 5.Comparison of the CO oxidation over (A) different nanoshaped CeO2nanorods and nanocubes (B) 2% Au/CeO2nanorods, nanocubes and bulk (C) 5% Au/CeO2nanorods, nanocubes and bulk.

Table 2

The surface atomic percentage and atomic ratio obtained by XPS for Au/CeO2in the form of nanorods and nanocubes.

Catalysts Ce³⁺% Ce⁴⁺% Ce³⁺/ Ce³⁺+ Ce⁴⁺

5%Au/CeO2-nanorods 3.32 20.64 13.8

5%Au/CeO2-nancubes 2.31 22.31 9.4

CO in oxygen, as determined by the 50% conversions at temperatures of 359 °C and 408 °C, respectively(Fig. 5A). In addition, the XRD results in- dicate that both CeO2in the form of nanorods and nanocubes have the same cubicfluorite structure. Consequently, the different catalytic performance of the fabricated materials may have manifested from their shapes. The CO oxidation over CeO2catalyst is robustly influenced by the crystal plane owing to the different potency of oxygen formation vacancies which play an important role in oxidation, as presented by Zhou et al. [38]. It seems justifiable to suggest that Au/CeO2in the form of nanorods show a higher content of Ce³⁺as confirmed from the XPS results, compared to that of nanocubes, which could be attrib- uted to the formation of oxygen vacancies. Moreover, the good catalytic performance of CeO2nanorods could be attributed to the large surface area exposing more active sites for CO oxidation. In this manner, there is no doubt that the activity of the catalysts is robustly based on the materials' shapes.

The correlation between catalytic performance and the shape de- pendent on the Au-CeO2catalyst was investigated by CO oxidation.

The CO conversions over 5% Au catalyst supported on CeO2nanostruc- tures are shown inFig. 5B. Obviously, the light off CO oxidation reaction profile allowed the catalytic performance of the three catalysts to be ranked as: Au/CeO2-nanorodsNCeO2-nano cubesNCeO2-bulk. The CO conversion over the 5% Au/CeO₂nanorods was close to 50% at 9 °C, whereas the nanocubes and bulk samples showed 50% at 68 and 138

°C respectively. Since the deposition precipitation conditions (e.g. pH value and microwave time) were kept the same for these gold–ceria catalysts, the above variations in catalytic activity must have stemmed from the shape of the support itself.

3.3.2. Catalytic epoxidation of olefins

The fabrication of novel, extremely powerful heterogeneous catalysts is an energetic and remarkable domain infine chemicals manufacturing. A considerable number display themselves in terms of the significant growth in the fabrication of catalysts, which permit a rich assortment of surface functionalities and characteristics, along with monitoring of the physical characteristics, such as shape, porosity and high surface area. The epoxidation of alkenes, as displayed in Scheme 2(cyclohexene, n-hexene and styrene), in the presence of Au/CeO2in the form of nanorods, nanocubes and bulk using 30%

H2O2–0.2 M NaHCO3as oxidant at 80 °C, produced the epoxy alkane as the dominant product. A control experiment was carried out to obtain additional information related to the epoxidation mechanism.

No obvious conversion of the epoxidation reaction in the absence of a catalyst was observed. This behavior confirms the absence of active sites. The influence of utilizing of Au/CeO2in the form of nanorods, nanocubes and bulk on the catalytic epoxidation of olefin was investigated under the same reaction conditions (2 ml olefin, (2 ml) 30% H2O2, 50 mg catalyst and 2 mmol sodium bicarbonate for 18 h at 80 °C). The reaction conditions adopted in this work were obtained from our previous work [53]. The results of epoxidation conversion, epoxyalkane selectivity percentage and turnover number (TON) are presented inTable 3. It is worth noting that Au/CeO₂in the form of nanorods manifested the highest epoxidation of olefins (cyclohexene, n-hexene and styrene) among all the fabricated gold-ceria catalysts under identical reaction conditions, which can be attributed to (i) the high surface area of Au/CeO2 in the form of nanorods that can feed more available active gold nanoparticles sites for the oxidation of the olefins, resulting in superior conversions; (ii) the different morphologies and crystal plane giving rise to creating oxygen vacancies, which play an important role in oxidation reactions. The results also appear clearly that the catalytic epoxidation conversion ranking as cyclohexeneNstyreneNn-hexene. The olefins containing internal double bonds exhibited higher catalytic epoxidation conversion than those containing terminal ones. This could be attributed to the highest electronic density of the olefin containing terminal double bonds. Fur- thermore, in case of styrene, the steric effects associated with electron density diminished the chance of electrophilic addition, resulting in the epoxidation [53].

The FTIR of the epoxide compounds has been examined. The FTIR spectra of the olefin compounds and the formed epoxy materials by the catalytic reaction are shown in(Fig. S1, Supplementary data).

Obviously, the epoxy spectrum of styrene were characterized by the disappearance of the peak corresponding to the olefin group at 1665 cm⁻¹ (corresponding to aliphatic vinyl group, \\CH_CH2

[54,55] with the appearance of peaks assigned for epoxide-function group at 840 cm⁻¹and 915 cm⁻¹due to the formation of\\C\\O\\C\\

Table 3

Epoxidation of olefin by Au/CeO2in the form of nanorods, nanocubes and bulk utilizing 30% H2O2–0.2 M NaHCO3at 80 °C.

Catalyst Substrate

Cyclohexene^a n-Hexene^a Styrene^a

H2O2-0.2 M NaHCO3 1.6 0.5 1.1^b

Au/CeO2-nanorods 99 (98.5) 340

79 (83) 80

91 (94) 120 Au/CeO2-nanocubes 87 (77)

220

71 (66) 112

81 (78) 144

Au/CeO2-bulk 83 (90)

33

66 (80) 23

76 (88) 19 TON (mmol of epoxide/mmol of catalyst) bold.

aSelectivity in parentheses.

Epoxyalkane Olefin

Au/CeO

₂

/30% H

₂

O

₂

– 0.2M NaHCO

₃

80

^o

C/18h

Scheme 2.Epoxidation of olefin by Au/CeO2catalyst in the form of nanorods, nanocubes and bulk utilizing 30% H2O2–0.2 M NaHCO3at 80 °C for 18 h.

Run 1 Run 2 Run 3 Run 4 Run 5 Run 6 0

20 40 60 80 100

)%(noisrevnoc

Number of cycles

Fig. 6.Recycling ability of Au/CeO2catalyst in the form of nanorods in cyclohexene epoxidation.

bonds [56,57] (Fig. S1a, Supplementary data). Moreover, the peaks at 2950–2820 cm⁻¹(\\CH, stretching) becoming more visible due to sp³ bond formation. The appearance of the peaks at 838 cm⁻¹ and 955 cm⁻¹for CC-6, and the peaks at 842 cm⁻¹and 922 cm⁻¹, confirming the formation of epoxide group (Fig. S1b and c, Supplementary data).

A distinct feature of heterogeneous catalysis is the facile separation from the reaction medium and reusing it for the next reaction until the catalyst is completely disrupted. The recyclability of the catalyst, Au/CeO2in the form of nanorods toward the epoxidation of cyclohexene was investigated at 80 °C using 50 mg of catalyst dose. After each epoxidation run, the catalyst wasfiltrated from the reaction mixture by centrifugation at 3000 rpm, thoroughly washed with ethanol, dried, and reused for the subsequent reaction run under similar condi- tions. The results reveal that the catalyst displayed excellent stability with inconsiderable activity loss compared to thefirst run (Fig. 6). The tenuous reduction in the catalytic performance was probably due to the tiny catalyst loss during the separation process in a consecutive run.

4. Conclusion

Efficient ceria as a green nanocatalyst in the form of nanorods and nanocubes and their corresponding gold catalysts were successfully fabricated by adopting a facile and scalable approach, assisted by microwave irradiation in the presence of economically and ecofriendly basic ionic liquid. This green catalyst was recycled without losing its activity. By adopting this approach, the next generation of catalysts can be designed and developed to discover a novel, exciting chance for heterogeneous catalysis.

Acknowledgment

The authors thank Jouf University for thefinancial support (Project Number: 39/609).

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.

org/10.1016/j.molliq.2019.02.014.

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