Ce nanoparticles supported on reduced graphene oxide for the reduction of 4-nitrophenol and the oxidation of olefins: Experimental and theoretical study

Journal Pre-proof Efficient Cd/Ce nanoparticles supported on reduced graphene oxide for the reduction of 4-nitrophenol and the oxidation of olefins: experimental and theoretical study Maryam Lashanizadegan, Maryam Anafcheh, Hoda Mirzazadeh, Parisa Gholipoor

PII:

S0025-5408(19)31534-X

DOI:

https://doi.org/10.1016/j.materresbull.2020.110773

Reference:

MRB 110773

To appear in:

Materials Research Bulletin

Received Date:

19 June 2019

Revised Date:

3 January 2020

Accepted Date:

6 January 2020

Please cite this article as: Lashanizadegan M, Anafcheh M, Mirzazadeh H, Gholipoor P, Efficient Cd/Ce nanoparticles supported on reduced graphene oxide for the reduction of 4-nitrophenol and the oxidation of olefins: experimental and theoretical study, Materials Research Bulletin (2020), doi: https://doi.org/10.1016/j.materresbull.2020.110773

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Efficient Cd/Ce nanoparticles supported on reduced graphene oxide for the reduction of 4-nitrophenol and the oxidation of olefins: experimental and theoretical study

Maryam Lashanizadegan*, Maryam Anafcheh, Hoda Mirzazadeh and Parisa Gholipoor Department of Chemistry, Faculty of Physics & Chemistry, Alzahra University, P. O. Box 1993893973, Tehran, Iran

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Graphical abstract

Highlights

 Cd/Ce/RGO was synthesized through simple reaction

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 Application of nano-catalysts in the reduction of 4-NP in one minute and oxidation of the styrene were investigated

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 Oxidation reaction of styrene with <99% conversion, and<99% selectivity to benzaldehyde in 1h with green solvent obtained

 Based on the calculated binding energies it is indicated that the product can leave the active sites of graphene oxide surfaces easily and the next 4-NP can be adsorbed successfully

Abstract Reduced graphene oxide-supported cadmium and cerium nanoparticles (Cd/Ce/RGO) were prepared by a simple and economical method, with ethanol as an ecofriendly solvent. Cd/Ce/RGO was employed as catalysts in two different reactions: the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) and

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the oxidation of olefins. The results of the reduction reaction showed that 98% of 4-NP was decreased in the presence of Cd/Ce/RGO (Cd to Ce 3:1 (w/w)) as the catalyst and NaBH4 as the reducing agent with a

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constant rate of 4.9/min and a reaction time of 1 min. Moreover, the catalytic activity of Cd/Ce/RGO in the oxidation reaction of styrene revealed that >99% selectivity towards the benzaldehyde product and

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a conversion of >99% were achieved. The DFT results indicated that the product can leave the active sites of graphene oxide surfaces easily and that the next 4-NP can be adsorbed successfully.

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Key words: Cd/Ce nanoparticles, Reduced graphene oxide, Reduction, 4-nitrophenol, Oxidation, Theory

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1 Introduction

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Running title: An efficient nanocatalyst for reduction and oxidation reaction

An increasingly encountered problem in the production of nitroaromatic compounds in the dye, pigment, pharmaceutical and polymer industries is that the intermediates and side products that are produced are highly toxic and mutagenic carcinogens[1]. Therefore, the reduction of nitroaromatic compounds to amine compounds is industrially important[2]. For more than 100 years, NaBH4 has been

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used as a moderate reducing reagent for this reduction process. However, it is obvious that the use of NaBH4 is insufficient for the reduction of the nitro group alone[3]. Moreover, a previous investigation also revealed that graphene oxide has no catalytic impact on this reaction[4]. Some research has been carried out to reduce the nitro groups of compounds by using a combination of NaBH4 and catalysts[5– 7]. On the other hand, it has been realized that nanoparticles with ultrafine size, such as bimetallic nanoparticles, can increase the surface area and the number of edges and corners of atoms, which greatly improves their catalytic properties[8–13]. Many catalysts that have been used suffer disadvantages such as high catalytic loading, inability to recycle, and high cost[14]. Recently, catalysts

such as nickel, cobalt, cadmium, cerium and copper comprising nanoparticles with NaBH4 are under development and have been widely used[2,15–19]. Additionally, many works have proposed the adsorption of 4-NP on catalysts, but few of them reported the precise model[20–24]. In addition to this reduction reaction, the catalytic oxidation of olefins has gained much attention in recent years[25–28]. According to the literature, styrene oxide and benzaldehyde are the major products of the styrene oxidation process. It is important to synthesize a catalyst that has selectivity towards one product of greater than 99% and a conversion of the substrate of more than 99%.

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Transition metal nanoparticles such as cerium and cadmium exhibit several properties, such as catalytic activity, fluorescence quenching, electrochemical processes, high surface area and oxygen transfer

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ability, that are important for use in biosensors[29–32]. The issues that affect the application of nanoparticles in terms of catalytic activity are aggregation and agglomeration[33].

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It is known that graphene oxide possesses a larger theoretical specific surface area than those of carbon nanotubes and active carbon, and it also exhibits a higher use of surface area because both sides are

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available. Moreover, it can prevent nanoparticles from coalescing. Furthermore, graphene oxide can cause the formation of highly dispersed, high-density and small nanoparticles[34]. Hence, we used it as

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an efficient support for nanoparticles.

In this study, we report the reduction of 4-nitrophenol and the oxidation of olefins with Cd/Ce/RGO as the catalyst. The Cd/Ce/RGO catalyst has high efficiency in both reactions. Various parameters that

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affect these two kinds of reactions are discussed in detail. Moreover, a theoretical study for 4-NP reduction was carried out. It is known that NaBH4 cannot reduce 4-NP alone. A previous investigation also revealed that graphene oxide has no catalytic effects on this reaction[24,27]. On the other hand, it has been recognized that nanoparticles (NPs) with ultrafine size, such as bimetallic NPs, can increase the surface area and the number of edges and corner atoms, which greatly improves their catalytic properties[8,28-30]. Although many works have proposed the adsorption of 4-NP on catalysts, few of

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them suggested a precise model[9,23,31-36]. By utilizing these experimental and theoretical works and to provide a better understanding of the underlying fundamentals describing the microcosmic mechanism of the reaction, we simulated a series of graphene oxide surfaces. These surfaces were supported by bimetallic Cd/Ce nanoparticles with different oxygenated moieties to investigate their influences on the combined interactions with 4-NP. 2 Experimental Sections

2.1 Reagents and Instruments All materials were commercial reagent grade. The surface morphology of the as-synthesized particles was studied by field-emission scanning electron microscopy (FE-SEM, Zeiss, Germany). EDX was also performed using FE-SEM. Fourier transform infrared (FT-IR) spectra of different samples were recorded by using a Bruker Tensor 37 DTGS. UV-Vis spectra were recorded on a Lambda 35 spectrophotometer. Information about the phase and crystallinity was obtained with a Jeol JDX-8030 Xray diffractometer with Cu Kα radiation (λ= 1.540Å) over Bragg angles ranging from 10˚ to 80˚. The

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amount of Cd/Ce in the catalyst was determined using an inductively coupled plasma optical emission spectroscopy (ICP-OES) instrument (Varian, Vista-Pro, Australia). Ultrasonic dispersion was performed

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on a Bandelin electronic, Type: DT 255H (160 W-640 W). GC and GC-MS experiments were carried out using an Agilent Technologies 6890 Series GC System with an HP-5 phenylmethylsiloxane capillary column and an Agilent 5973 Network mass selective detector with an HP-5MS 6989 Network GC system,

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respectively.

2.2 Synthesis of graphene oxide

2.3 Synthesis of Cd/Ce/RGO

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Graphene oxide was prepared according to the procedures described in the literature [35].

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A known amount of graphene oxide (0.05 g) was added to 200 mL of ethanol and 55 mL of doubledistilled water, and then cadmium acetate and cerium nitrate with different mass ratios were added to the mixture according to the amounts listed in Table 1. The obtained solution was vigorously stirred for 2 h. Then, the solution was dispersed in an ultrasonic bath (200 W). In another container, a solution of hydrazine hydrate (0.45 g in 100 mL ethanol) was prepared and then added to the graphene oxide

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solution and stirred vigorously for 1 h. A few drops of hydrogen peroxide were added to remove further hydrazine hydrate from the solution. Finally, the obtained suspension was separated by centrifugation, and the resulting solid was washed with ethanol. 2.4 Application of Cd/Ce/RGO in the reduction of 4-NP to 4-AP 2 mL of 0.1 M NaBH4 solution (freshly prepared), and 0.06 g of catalyst were mixed and poured into 100 mL of deionized water. Then, 2 mL of 4-NP solution (5 mM) was added to the mixture at room temperature (~25 °C). During the reaction process, 1 mL of the reaction mixture was taken as a sample from the reaction system at certain time intervals (0, 15, 30, 45 and 60 min) after the beginning of the

reaction. The samples were quantitatively analyzed by recording the UV-Vis spectra of the solution to determine the concentration of 4-NP from the absorption peak at 400 nm. Table 1 Different mass ratios of cadmium acetate and cerium nitrate for the synthesis of Cd/Ce/RGO a: cadmium acetate(g)

Cd/Ce/RGO

b: cerium nitrate (g )

Cd:Ce (w/w)

a: 0.025

(1:1)

b: 0.025 (1:3)

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a: 0.013 b: 0.038

(3:1)

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a: 0.038

(1:0)

b: 0.050

(0:1)

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a: 0.050

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b: 0.013

2.5 Application of Cd/Ce/RGO in the oxidation of olefins

A mixture of 0.1 g of catalyst, 5 mL of ethanol, 5 mL of hydrogen peroxide and 5 mmol of styrene was

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prepared and added to a round-bottom flask equipped with a condenser. The mixture was heated to 78 °C ± 0.5 °C under vigorous magnetic stirring and maintained at this temperature for 6 h to accomplish the reaction. Samples were taken at specified time intervals (1, 2, 4 and 6 h) and then quantitatively

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analyzed using GC and GC-MS.

3. Results and discussion

3.1 Morphology and structure

The FT-IR spectrum of graphene oxide is shown in Fig. 1a. An intense and broad peak at 3400 cm-1

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resulted from the stretching vibration of OH. The peaks at 1720, 1635, 1170, and 1060 cm

-1

are

attributed to the stretching vibrations C = O, C = C, C-O-C and C-O, respectively. In Fig. 1b (FT-IR spectrum of Cd/Ce/RGO), the peaks at 1170 and 1720 cm-1 decreased, and the absorption peak at 1060 cm-1 disappeared. The peak at 1635 cm-1 remained unchanged. The loss of C = O, C-O-C and C-O groups in Fig. 1b is due to the presence of cerium and cadmium metals on reduced graphene oxide plates [36].

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Fig. 1. FT-IR spectra of a) GO and b) Cd/Ce/RGO

Fig. 2 shows the XRD pattern of the as-prepared nanocatalyst. The peaks at 2θ =20.75°, 24.89°, 28.72°

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and 33.48° are attributed to cerium (JCPDS No 222-9285). Additionally, the peaks at 2θ =12°, 24.91°, 28.72°, 34.82° and 42.50° are attributed to cadmium (JCPDS No 222-3798). The structure of Cd/Ce/RGO

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is monoclinic.

Fig. 2. XRD pattern of Cd/Ce/RGO Surface characterization of Cd/Ce/RGO was performed using field emission scanning electron microscopy (FESEM). Fig. 3 exhibits FESEM images of the as-prepared nanocatalyst. These results show Cd/Ce nanoparticles distributed on the surface of reduced graphene oxide. Energy dispersive spectrometry (EDS) was performed in conjunction with FESEM (Fig. 4). The results show the presence of carbon along with oxygen, cadmium and cerium. Additionally, EDS indicates the absence of any elemental impurities. The Cd/Ce content of Cd/Ce/RGO was determined using ICP-OES.

The results shown in Table 2 indicate that the loading ratios of Cd and Ce in Cd/Ce/RGO for different weight ratios are in good agreement with the initial loading amounts. Table 2 ICP-OES result of Cd/Ce/RGO ICP–OES Cd%

ICP-OES Ce%

1.56 2.79 3.85

1.39 9.91 1.29

Cd/Ce/RGO Ratio of loading 1.1 0.28 2.9

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Fig. 3. FESEM images of Cd/Ce/RGO (a, b)

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Cd/Ce/RGO (Cd:Ce) (w/w) ratio (1:1) 1 (1:3) 0.33 (3:1) 3

Fig. 4. EDS spectra of Cd/Ce/RGO 3.2 Application of Cd/Ce/RGO as a catalyst in 4-NP reduction Fig. 5 shows the UV-Vis spectrum of 4-NP, in the presence of Cd/Ce/RGO and NaBH4 as the catalyst and the reducing agent, respectively, in the range of 250-500 nm. The maximum absorption peak of 4-NP is

at 400 nm. 4-AP has a strong absorption peak at 300 nm. During the reaction, due to the conversion of 4-NP to 4-AP, the peak at 400 nm decreased, and a new peak emerged at 300 nm. It should be noted that through conversion, the concentration of 4-NP in the solution was decreased, while the 4-AP concentration was increased. The results also indicate that 98% of 4-NP was reduced by 1 min after the start of the reaction. 3.3 The proposed mechanism for 4-NP reduction The reduction of 4-NP by NaBH4 is thermodynamically favorable (the standard peak potential difference is greater than zero, ΔE0=E04-NP - E0

H3BO3/BH4-=

- 0.76- (-1.33)=0.67 V[37]. However, in the absence of a

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catalyst, this reaction was kinetically limited.

In the reaction system, the π-π interaction between 4-NP and reduced graphene oxide plates leads to an

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increase in the absorption capacity of graphene. NaBH4 reacts with water at ambient temperature, producing H2 and NaBO2. In the presence of cerium and cadmium nanoparticles, the H-H bond in the H2

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molecule is broken, and each hydrogen atom bonds to the surface of the nanoparticles. Hydrogen with a negative charge in the metal structure can easily attack nitrogen with a positive charge. As a result, the

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nitro group is reduced to a nitroso group, followed by the reduction of the two hydrogen atoms, leading to the production of aminophenol (Fig. 6). It should be mentioned that during 4-NP reduction, three factors are important: 1) the synergistic effects between the bimetallic (Cd, Ce), 2) the specific porous

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structure, which can provide more active sites, and 3) the presence of graphene oxide, which can cause electrostatic attraction by the electron transformation between graphene and the anchored

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catalyst[38–41].

Fig. 5. UV-Vis spectrum of 4-NP in the presence of NaBH4 and Cd/Ce/RGO

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Fig. 6. The proposed mechanism for the reduction of 4-NP

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3.4 Effect of different amounts of as-prepared nanocatalyst

Three different amounts of catalyst, 0.03, 0.06 and 0.12 g, were selected and added to three separate

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reaction mixtures, and the samples were taken 60, 2 and 1 min after the beginning of the reaction for the determination of their reduction percentages. The reduction percentage of 4-NP can be determined from the plots of the absorbance vs. wavelength of the samples, as shown in Fig. 7. The reduction

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percentages are 78%, 91% and 98% for 0.03 g, 0.06 g and 0.12 g of catalyst, respectively. It is apparent

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that the optimum amount of nanocatalyst is 0.12 g (reduction percentage: 98% in 1 min).

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Fig. 7. The absorbance vs. wavelength plots for obtaining the optimum amount of nanocatalyst. (a) 0.06 g, (b) 0.03 g and (c) 0.12 g of catalyst. Reaction conditions: NaBH4: 2 mL (0.1 M), 4-NP: 2 mL (0.5 mM)

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3.5 Effect of different amounts of NaBH4

To study the effect of NaBH4 on 4-NP reduction, three different amounts of NaBH4 were selected (1, 2 and 4 mL). Fig. 8 shows the absorbance plot. The reduction percentage of 4-NP can be obtained according to Fig. 8 and the following equation:

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Reduction percentage = A0-A/A01.00

(1)

where A and A0 are the absorbance of 4-NP (at peak of 400 nm) at time t and t=0, respectively. The reduction percentages were 98% for 2 mL of NaBH4 in 1 min, 66% for 1 mL of NaBH4 in 15 min and 14% for 4 mL of NaBH4 in 60 min. It should be noted that the duration time started at the beginning of the reaction. In regard to the percentage reduction and the shorter duration, 2 mL of NaBH4 with 98% reduction of 4-NP in 1 min was the optimum value.

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Fig. 8. The absorbance vs. wavelength plots in the presence of different amounts of NaBH 4: (a) 2 mL, (b) 1 mL and (c) 4 mL. Reaction conditions: 4-NP: 2 mL (0.5 mM), nanocatalyst: 0.12 g

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3.6 The effect of various catalyst ratios

The effect of various catalyst ratios was studied using Cd/Ce/RGO with different mass ratios of cadmium and cerium (w/w): Cd:Ce (1:1), Cd:Ce (1:3), Cd:Ce (3:1), Cd:Ce (1:0) and Cd:Ce (0:1). The Cd/Ce/RGO catalyst with a Cd:Ce ratio of 1:1 reduced 4-NP by 97% within 1 min after the start of the reaction. The Cd/Ce/RGO catalyst with a Cd:Ce ratio of 3:1 achieved a 98% reduction in 4-NP in 1 min after the start of

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the reaction. Moreover, for the Cd/Ce/RGO catalysts with Cd:Ce ratios of 1:3, 0:1 and 1:0, the reduction percentages were 54%, 60% and 19% after 60 min of the beginning of the reaction, respectively (Fig. S1 shows UV-Vis spectra of different mass ratios of catalysts (see supplementary information)). The results indicate that the Cd/Ce/RGO catalyst with a Cd:Ce ratio of 3:1 had the highest reduction percentage of 4-NP (98%) in the shortest time (1 min). The kinetics of 4-NP reduction over the Cd/Ce/RGO catalyst was determined according to the pseudofirst-order reaction model. By plotting -Ln (Ct/C0) against time (t), a linear correlation was achieved. This

demonstrates that the catalytic activity of Cd/Ce/RGO for the reduction of 4-NP follows pseudo-firstorder kinetics (Fig. S2). The results show that the high rate constant of the catalyst is 4.9 min-1 for a Cd:Ce ratio of 3:1, which shows a good linear correlation (R2 > 0.99). Therefore, the optimum conditions for the reduction of 4-NP were found as follows: 0.12 g of Cd/Ce/RGO, Cd:Ce of 3:1, 2 mL of NaBH4 (0.1 M) and a reaction time of 1 min. The catalytic activity of Cd/Ce/RGO was compared with those reported in the literature for the reduction of 4-NP and is presented in Table 3. As seen in the table, the reaction time and rate constant

Nature of the catalyst

y

Reaction

Reduction of 4-NP Initial

Reactio

rate k

(%)

concentration

n

of 4-NP (mM)

(min)

0.168

2

[42]

(min-1) Co-N-carbon

0.65

>99

composite

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1

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Entr

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Table 3 Comparison of the current work with other studies for 4-NP reduction

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in this study are better than those previously reported.

Ref

time

Pd/TiO2

0.7

>99

0.2

6.8

[43]

3

CuNi

0.79

>99

0.25

5

[44]

4

AuPt@Au NCs/RGO

0.52

>99

0.7

6

[45]

5

Cd/Ce/RGO

4.9

>99

0.5

1

current study

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3.7 Density functional theory study

To understand the underlying mechanism clearly, we simulated a series of graphene oxide surfacesupported bimetallic Cd/Ce nanoparticles with different oxygenated moieties to distinguish their

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influence on the combined interactions with 4-NP. We considered the graphene model that contains 42 carbons, 16 hydrogen atoms and pristine graphene (A), as depicted in Fig. 9. The graphene model is taken from Sidik’s structure, which has been well used in previous simulations[46,47].

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Fig. 9. The optimized geometries of pristine graphene (A), epoxy-attached graphene (B), carboxylattached graphene (C), hydroxyl-attached graphene (D) and alkoxy-attached graphene (E) at the B3LYP/6-311++G(d, p) level of theory.

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Graphene oxide (GO) sheets are emerging materials with abundant oxygenated groups on their surfaces [21]. Their basal plane is covalently surrounded by epoxy and hydroxyl groups, while the edges are

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decorated with carboxyl groups. Even after reduction to achieve graphene, these functional groups are inevitable and cannot be removed completely. In an ideal model, epoxy oxygen will combine with two adjacent carbon atoms, while a hydroxyl group is connected to a single carbon atom of the graphene

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sheet. Their different binding configurations could have different effects on the original electronic structure of graphene, thus affecting the adsorption performance and catalytic activities. Regarding these points, we considered a series of graphene surfaces with different oxygenated moieties, epoxyattached graphene, carboxyl-attached graphene, hydroxyl-attached graphene and alkoxy-attached graphene surfaces (B, C, D, and E in Fig. 9, respectively) to distinguish their influences on the combined

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interactions with bimetallic Cd/Ce nanoparticles. Geometry optimization of pristine graphene and graphene oxide sheets, as well as different GO-supported bimetallic Cd/Ce nanoparticles, was performed using Becke's hybrid three-parameter exchange functional and the nonlocal correlation functional of Lee, Yang, and Parr (B3LYP) [48]. The standard 6-311++G(d, p) basis set was used because it is affordable and accurate enough for geometry optimization of such molecules[49,50]. The optimized geometry calculated for pristine graphene was found to be in excellent agreement with those of previous studies [51]. Relativistic small-core ECPs with the corresponding valence basis sets were employed on the metals: SDD[52–55], i.e., the Stuttgart-Dresden ECP. The optimized geometries of the

GO-supported bimetallic Cd/Ce nanoparticles are shown in Fig. 10. The adsorption energies (AEs) of Cd/Ce/GO were determined using the following relation and are presented in Fig. 10: AE = ECd/Ce/GO -EGO + ECd/Ce

(2)

where GO represents graphene oxide and ECd/Ce is the ground state energy of the isolated Cd/Ce metal alloy cluster. The adsorption energy is negative if the reaction is exothermic. Based on the results obtained for pristine, epoxy-attached, and carboxyl-attached graphene, bimetallic Cd/Ce clusters are far from the basal surfaces with a large separation distance at each interface. For

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these three models, all the obtained adsorption energies were close to 0.0, with separation distances above 3 Å at the interface, demonstrating that bimetallic Cd/Ce clusters are difficult to adsorb on the

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surface for these three cases. On the other hand, for graphene with hydroxyl and alkoxy groups attached, Cd/Ce nanoparticles will be tightly adsorbed on the planar surfaces, showing strong adsorption energies of approximately -12.54 and -13.18 kcal/mol, respectively. As shown in Fig. 10, in

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the most stable models of the graphene oxide surface-supported bimetallic nanoparticles, the Cd/Ce clusters have a configuration slanted to the graphene plane, with Cd and Ce atoms sitting at the center

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of a hexagon. The average Cd–Ce, Cd–C and Ce-C bond lengths were found to be 2.481, 2.242 and 2.382 Å for GO with hydroxyl groups attached and 2.478, 2.184 and 2.330 Å for GO with alkoxy groups

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attached, respectively, which are in agreement with previous reports[50,52,56]. Notably, Cd has a much stronger interaction with graphene than does Ce. Charge transfer is very important for understanding the interaction between bimetallic Cd/Ce nanoparticles and graphene oxide. The charges on atoms in

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the most stable models were derived from NBO analysis, which is an intuitive scheme of dividing molecules into atoms purely based on electronic charge density in real space[57]. NBO charge analysis shows that Cd loses approximately +0.204e or +0.212e and is therefore positively charged, while the natural charge on Ce is determined to be -0.093e or -0.098e. Since the adsorption process is essential and is taken as the initial step in 4-NP reduction, we considered

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bare graphene and graphene-supported Cd/Ce bimetallic nanoparticles, both with attached hydroxyl and alkoxy groups (bare GO and Cd/Ce/GO), to distinguish their influence on the combined interactions with 4-NP. The B3LYP/SDD-optimized structures of 4-NP adsorbed on the most stable configurations of Cd/Ce/GO and pristine GO are shown in Fig. 11. As expected, the C–C bond lengths and the C–C–C bond angles of bare graphene were slightly changed. The bond distances between 4-NP and bare graphene oxide with hydroxyl and alkoxy groups were calculated to be 3.013 and 2.994 Å, respectively, and their corresponding adsorption energies were determined to be -0.44 and -0.58 kcal/mol, respectively.

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Fig. 10. The optimized geometries of the GO-supported bimetallic Cd/Ce nanoparticles at the B3LYP/DSS level of theory.

This indicates that the bare graphene oxide displays very weak sensitivity to the 4-NP molecule and that the adsorption abilities of the 4-NP on the graphene oxide surface will be improved by bimetallic Cd/Ce nanoparticles. Interestingly, for graphene oxide with hydroxyl and alkoxy groups, 4-NP will combine with the Cd atom of bimetallic Cd/Ce clusters, showing strong adsorption by approximately -32.74 and -35.54 kcal/mol, respectively. The calculated results reveal a short separation distance of 2.077 and 2.072 Å

between the Cd atom and 4-NP. For further investigation, adsorption energies were also calculated for the considered models. As expected, their adsorption energies are weaker than those for the adsorption of the reactant 4-NP, and the obtained values were -27.18 and -25.49 kcal/mol on the graphene oxides with hydroxyl and alkoxy groups, respectively. These results demonstrate that the product can leave the

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active sites easily and that the next 4-NP can be adsorbed successfully.

Fig. 11. The B3LYP/SDD-optimized structures of 4-NP adsorbed on the most stable configurations of Cd/Ce/GO and pristine GO.

3.8 Application of Cd/Ce/RGO as a catalyst in the oxidation of olefins Cd/Ce/RGO was applied as a catalyst for the oxidation of olefins in the presence of oxidants such as tertbutyl hydroperoxide (TBHP) and H2O2. In the first step, studies on styrene as an appropriate alkene were performed, and the effects of different factors on conversion and selectivity were investigated to

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other substrates, such as α-methyl styrene and 1-ocetene, was investigated.

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determine the optimal reaction conditions. Then, under optimal conditions, the catalytic activity on

3.9 Effect of reaction time

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To determine the effect of time on the conversion and selectivity, styrene oxidation was studied by the as-prepared catalyst in the presence of hydrogen peroxide at 0.5, 1, 2, 4 and 6 h. The results of the

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effect of time are given in Table S1 and Fig. S3 (see supplementary information). Studies show that at 1 h, the conversion and selectivity are >99%, and with increasing time, the

3.10 Effect of reaction solvent

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selectivity decreases; hence, a time of 1 h was chosen as the optimal time.

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To investigate the effect of the solvent on the styrene oxidation reaction with the Cd/Ce/RGO catalyst, acetonitrile and ethanol solvents were used. The reaction was carried out for an optimal time of 1 h. The results are presented in Table S2. The results show that ethanol with conversion >99% and selectivity >99% for benzaldehyde was chosen as the optimum solvent for the reaction.

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3.11 Effect of different amounts of catalysts The effect of different amounts of catalysts on the styrene oxidation reaction was investigated, and the results are shown in Table S3. According to the results, 0.1 g of catalyst with >99% conversion to benzaldehyde was selected as the optimum amount of catalyst. 3.12 Oxidation of other substrates To investigate the oxidation reaction of other substrates in the presence of a catalyst, substrates such as α-methyl styrene were used, and the results are presented in Table S4. Based on the results, styrene

with >99% conversion to benzaldehyde was introduced as the best substrate for the oxidation of olefins. 3.13 Catalyst recyclability For the reuse test, the catalyst was washed with ethanol. Then, a mixture of catalyst (0.1 g), styrene (5 mmol), hydrogen peroxide (5 mL) and ethanol (5 mL) was refluxed for 1 h. From the reaction mixture, 1 mL was withdrawn, and the product was analyzed by GC. The results showed that the conversion was >99% for 4 consecutive cycles. However, as depicted in Fig. 12, the selectivity decreased from >99% to

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79% after 4 uses. This indicates that the catalyst has good stability.

Fig. 12. The catalytic performance of Cd/Ce/RGO for 4 successive cycles

ethanol (5 mL)

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Reaction Conditions: 0.12 g of Cd/Ce/RGO, Cd:Ce of 3:1, styrene (0.57 mL), hydrogen peroxide (5 mL),

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The catalytic activity of Cd/Ce/RGO was compared with those of other catalysts for styrene oxidation. The results are presented in Table 4. This shows that Cd/Ce/RGO is a good catalyst with good selectivity

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and conversion in a short time.

Table 4 Comparison of the results of this study with those of other studies for olefin oxidation Entry

1

Catalyst

[ZnPc(OC10H14N)4]-

Solvent

DMF

Conversion Selectivity

Reaction

(%)

(%)

time )h)

84.13

88.69

6

Oxidant Ref

TBHP

[58]

ATP 2

MnFeSi

free

3

Cerium-doped cobalt 1,4 dioxane

16.4

45.3

24

H2O2

[59]

32.9

59.6

9

H2O2

[60]

>99

>99

1

H2O2

This

ferrite 4

Cd/Ce/RGO

Ethanol(green solvent)

study

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For olefin oxidation and the effects of different factors on the reaction, it was concluded that, with 0.12 g of Cd/Ce/RGO catalyst with a Cd:Ce ratio of 3:1, 5 mL of ethanol, 5 mL of hydrogen peroxide, 0.57 mL

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of styrene and a reaction time of 1 h, the highest conversion (>99%) and the highest selectivity (>99%) to benzaldehyde were achieved.

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Conclusion

In this study, Cd/Ce/RGO was synthesized through a simple hydrothermal technique and characterized

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by FTIR, EDX, FESEM and XRD. Then, the application of nanocatalysts to the reduction of 4-NP and the oxidation of alkenes was investigated. The results showed that the Cd/Ce/RGO nanocatalyst with a

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Cd:Ce ratio of 3:1 had excellent performance in the 4-NP reduction reaction (98% reduction at 1 min) and the oxidation reaction of styrene, with >99% conversion and >99% selectivity to benzaldehyde in 1 h in a green solvent. Moreover, a theoretical study of the reduction reaction was carried out. Based on

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the calculated binding energies, the product can leave the active sites of graphene oxide surfaces easily, and the next 4-NP can be adsorbed successfully. Our results demonstrated that graphene oxide surfacesupported bimetallic Cd/Ce nanoparticles may act as a new catalyst to be used in the reduction of 4nitrophenol into 4-aminophenol as a model reaction by NaBH4. In addition, Cd/Ce/RGO shows reusability for practical applications.

Jo

conflicts of interest

There is no conflicts of interest in this research.

Acknowledgments

Support of this investigation by Alzahra University is gratefully acknowledged.

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