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Kinetic Analysis of the Reduction of 4-Nitrophenol Catalyzed by CeO2 Nanorods-Supported CuNi Nanoparticles Mona Kohantorabi, and Mohammad Reza Gholami Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04208 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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Industrial & Engineering Chemistry Research

Kinetic Analysis of the Reduction of 4-Nitrophenol Catalyzed by CeO2 Nanorods-Supported CuNi Nanoparticles

Mona Kohantorabi Department of Chemistry, Sharif University of Technology, Tehran, 11365-11155, Iran

Mohammad Reza Gholami* Department of Chemistry, Sharif University of Technology, Tehran, 11365-11155, Iran

(*Corresponding author e-mail: [email protected] )

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Abstract CuxNi100-x (x = 0, 20, 40, 60, 80, and 100) nanoparticles were uniformly grown on surface of CeO2 by liquid impregnation method. The as-prepared nanocomposite abbreviated CuxNi100-xCeO2 was characterized by various techniques including, X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), Brunauert-Emmett-Teller (BET) surface area analyzer, and transmission electron microscopy (TEM). The catalytic activity of CuxNi100-x-CeO2 nanocomposites was investigated in 4-Nitrophenol (4-NP) reduction reaction. Among the synthesized nanocomposites, Cu60Ni40CeO2 exhibited the best catalytic activity (rate constant as 0.1654 s-1) with high recyclability for consecutive 5 runs. The mechanism of the reduction was studied and the adsorption equilibrium constant of 4-NP ( ) and borohydride ( ) calculated by using the LongmuirHinshelwood model. The energy of activation ( ), and thermodynamic parameters such as activation enthalpy (∆ ), entropy (∆ ), and Gibbs free energy (∆ ) have also been determined. Keywords: CuNi nanoparticles, Catalytic reduction, Langmuir-Hinshelwood equation, 4Nitrophenol.


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1. Introduction Metal-organic frameworks (MOFs) are a category of crystalline networks, that created by metal ions or cluster and organic linkers with unique features such as huge surface area, high porosity, and chemical tenability.1-6 These materials have tremendous potential for many practical application including sensing, drug delivery, selective gas storage, biomedical field, and heterogeneous catalysis.7-10 Due to these unparalleled properties, the researcher could be explore novel composites with advanced virtues, by combining MOF with other functional materials such as, metal nanoparticles (NPs), ceramics, polymers, and biomolecules.11-12 MOFs are ideal templates for synthesizing nanomaterial, because presence of open channels and tunable cavities in these compounds would permit to stabilize small metal nanoparticles and increase their catalytic performance.13-15 Due to the specific electronic structure and variable valence states of transition metals, they are widely employed for various kind of catalysis reactions.16-17 The widespread use of NPs as catalyst have major barriers, such as, tendency to aggregate, low recyclability, and recovering difficulty. One of the ideal method to overcome these barriers, is to embed NPs onto the certain support.18-20 MOFs are very attractive as a support for control the growth of metal NPs and increase their catalytic activity in heterogeneous catalysis.21,22 The oxidation of hydrocarbons and alcohols and hydrogenation of alkenes, alkynes, ketones, and aromatics have been catalyzed using MOFs (MIL-100 (Fe), HKUST-1, Cu-MOF, ZIF-8, and MIL-101) supported metal NPs (Pd, Au, and Pt).23-27 As kind of toxic organic compounds, 4nitrophenol (4-NP) is one of the numerous pollutants in wastewaters, while 4-aminophenol (4AP) is an important initial substance in the manufacture of pharmaceuticals.28-31 Because the 4NP is highly stable in natural environment, conversion of 4-NP to 4-AP in the presence of sodium borohydride is generally inefficient at room temperature and needs catalyst. Therefore it is important to extend low cost and green catalyst for reduction of 4-NP.32-34 Bimetallic NPs have 3 ACS Paragon Plus Environment


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potential application in photonics, sensing, catalysis and etc. Due to the strong synergistic effects between two metals, bimetallic alloys NPs have higher catalytic efficiency.35, 36 In recent years, some bimetallic nanoparticles such as PtNi, CuNi, AuAg, CoNi, AuPt. PtPd, AuPd, AgNi, CuAg and etc. have been successfully prepared for the catalytic reduction.37-46 Many supporting materials including graphene38,40, carbon nanotubes (CNTs)47, activated carbon48, and porous materials such as silica49,50, and MOF18 have developed to stabilize noble NPs. Cerium oxide (CeO2) have attracted intense attention, due to its applications in fuel cells, luminescent materials, water treatment, and catalyst.51 The electrochemical and catalytic activity of cerium oxide can be improved by loading noble metal NPs onto the CeO2. There have been a few researches on the MOF and nanostructures derived MOF supported bimetallic NPs. Herein, the CeO2 nanorods were obtained by calcination of Ce-BTC MOF.52, 53 Then CuNi nanoparticles, which have applied in catalyst as a class of transition metal, were synthesized on CeO2 nanorods. The catalytic activity and reusability of CuxNi100-x-CeO2 nanocomposites were studied in reduction of 4-NP. Additionally, the mechanism and kinetic of this reaction were evaluated using Longmuir-Hinshelwood isotherm. 2. Experimental Section 2.1. Materials and instruments Nickel chloride hexahydrate (NiCl2.6H2O), copper chloride (CuCl2), 4-nitrophenol, and sodium borohydride (NaBH4) were purchased from Merck. Cerium nitrate hexahydrate (Ce (NO3)3.6H2O), and benzene-1,3,5-tricarboxylic acid (1,3,5-H3BTC) were obtained from Aldrich. X-ray diffraction (XRD) patterns of products were recorded on a PW3050/60 X' Pert PRO diffractometer equipped with a Cu Kα radiation source (λ = 1.54060 A°). The morphologies of 4 ACS Paragon Plus Environment

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the samples were characterized by the field emission scanning electron microscopy (FE-SEM, Mira3 Tascan), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM, Philips CM30). BET surface area and pore size measurements were investigated by the nitrogen adsorption/desorption isotherm at 77 K on a BELSORB-mini II. A Malvern zetasizer Nano ZS instrument was applied for measuring the surface charge of CeO2 nanorods. The UV-vis Cintra 40 was applied to record the absorbance of reaction solution. 2.2. Preparation of Ce (1,3,5-BTC) (H2O)6 MOF Ce (1,3,5-BTC) (H2O)6 MOF was synthesized by following the reported method.52 In a typical synthesis, 5 mmol of Ce (NO3)3.6H2O (2.17 g) was dissolved in water and then added in to a water-ethanol solution (volume ratio of two solvent 1:1) of 1,3,5-H3BTC (5 mmol, 1.05 g) under stirring. This solution in closed glass container was placed in oil bath at 60 °C for 1 h. The white products were gathered by centrifugation and washed with water and ethanol. Finally, the asobtained MOF was dried at 60 °C for 12 h. 2.3. Synthesis of CeO2 nanorods The Ce (1,3,5-BTC) (H2O)6 MOF was placed in quartz tube, and heated under air to 650 °C at a rate of 2 °C min-1 and maintained at this temperature for 3h. Finally, the yellow CeO2 powder was obtained after cooling to ambient temperature. 2.4. Preparation of CuxNi100-x nanoparticles on CeO2 nanorods Scheme 1A illustrates the preparation mechanism of the CuxNi100-x-CeO2 catalyst by simple liquid impregnation method. In typical procedure, CeO2 (50 mg) was dispersed in 25 ml deionized water for 15 min. Afterward, 0.085 mmol of NiCl2.6 H2O and 0.118 mmol of CuCl2



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solution were added to above mixture. Metal cations were loaded onto the surface of nanostructure with continuous stirring of the solution at 25 °C for 24 h. After this time, the solution was stirred at 0 °C in ice-bath, afterward the mixture was reduced by sodium borohydrid (NaBH4, 0.08 M, 3 ml) solution. The resulting product (Cu60Ni40-CeO2) was washed several times with water by centrifugation, and then was dried at 50 °C. A similar method was used for the preparation of Cu-CeO2, Cu80Ni20-CeO2, Cu40Ni60-CeO2, Cu20Ni80-CeO2, and Ni-CeO2 nanocomposites by modifying the molar ratio of Cu-Ni precursors. For comparison, bare Cu60Ni40 nanoparticles were synthesized in the absence of CeO2 nanorods by applying the analogous procedure. 2.5. Catalytic reduction of 4-Nitrophenol (4-NP) For investigating the performances catalytic activity of the as-prepared catalysts, the reduction of 4-nitrophenol (4-NP) was monitored from 220 to 550 nm using UV-vis Cintra 40 spectrophotometer at a constant temperature of 25 °C. In a typically procedure, 2 ml of 4nitrophenol solution (0.1 mM) and 500 µl of NaBH4 (0.05 M) were appended in quartz cuvette and put in spectrophotometer to record the adsorption maximum of 4-nitrophenolate ions at 400 nm. Afterward, 1 mg of catalyst quickly was transferred into the cuvette and absorbance evolution was recorded after different intervals time, until the absorbance became constant. The stability and reusability of the Cu60Ni40-CeO2 nanocomposite were explored in the reduction of 4-NP with NaBH4. The separated catalyst was reused for five runs in catalytic cycle. 3. Result and discussion 3.1. Characterization of the catalyst


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The phase purity and crystallinity of Ce-BTC MOF, CeO2, bare CuNi nanoparticles, and CuxNi100-x -CeO2 were investigated by X-ray diffraction spectrometer (Figure 1). The diffraction peaks at 28.3, 32.9, 47.3, 56.1, 58.5, 68.9, 76.4, and 78.8 degrees in the pattern of the CeO2 nanorods (Figure 1 B) corresponding to the (111), (200), (220), (311), (222), (400), (331), and (420) plane, respectively (JCPDS no. 01-089-8436). These peaks were observed clearly in all samples which indicated the cubic phase of CeO2 (space group Fm3m, a=b=c= 5.4112 A°) is preserved in all synthesized nanocomposites (Figure 1, C-E). The peak located at 43.19° for CuCeO2 correspond to the (111) plane of the cubic structure for the Cu nanoparticles (JCPDS no. 01-070-3038). The diffraction peak of Ni-CeO2 nanocomposite at 44.02° corresponding to the (111) plane of Ni nanoparticles. The peak at 2Ө = 44° in the pattern of the Cu60Ni40-CeO2 nanocomposite (Figure 1 E) can be attributed to the (111) plane of cubic nanostructure of CuNi nanoparticles (JCPDS no. 98-008-7506), that confirmed the formation of CuNi NPs. The diffraction pattern of the bare CuNi NPs (Figure 1 F) exhibits three characteristic peaks centered at 2Ө = 43.46°, 50.62°, and 74.41° relating to the (111), (002), and (022) plane of cubic structure (JCPDS no. 98-008-7506). The results demonstrated that the CuxNi100-x-CeO2 nanocomposites were successfully synthesized. The average crystalline size (d) for all samples were also .

measured by Scherrer's equation = Ө, in which λ is the X-ray wavelength, β is the full width at half-maximum of the peak, and Ө is the position of the plane peak. The mean crystallite size of Cu, Ni, and CuNi NPs in as-prepared nanocomposites was determined to be 11.1 nm, 12.2 nm, and 10.9 nm, respectively. SEM was applied to study the morphology of the synthesized samples. Figure 2 (A, B) demonstrate the FE-SEM images of Ce-BTC nanorods, with the average dimensions 0.5-1.5 µm in length and 50-150 nm in width. It can be distinguished from the FE-SEM images of CeO2 and CuNi-CeO2 (Figure 2, C-E) that the 7 ACS Paragon Plus Environment


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morphology of the Ce-BTC duplicated, with the dimension of 0.5-1 µm in length and 50-100 nm in width. The CeO2 nanorods have structures similar to Ce-BTC with small different in size length. The EDX spectrum was used to analyze the element profiles of the as-prepared Cu60Ni40CeO2 nanocomposite. The elements including cerium, oxygen, nickel, and cupper were observed in the Cu60Ni40-CeO2 nanocomposite (Figure 2 F). The zeta potential value of CeO2 nanorods was measured to be -16.9 mV when dispersed in DI water (Figure S1, in the Supporting Information). The negative surface charge of CeO2 indicated that the metal cations (Cu2+, and Ni2+) were attached onto surface of nanorods and then reduced by NaBH4. The catalytic performance of nanoparticles is depended to their surface structure. The TEM images of Cu60Ni40-CeO2 (Figure 3 A, and B) indicated that, the nanocomposite was prepared with a nanorod shape. It must be noted that the CuNi nanoparticles (with average size 10 nm) are located on the surface of CeO2 nanorods. The pore size distribution and the surface area of synthesized catalyst were investigated using the Barett-Joyner-Halenda (BJH) theory and Brunauer-Emmett-Teller (BET) method, respectively. N2 adsorption-desorption isotherm of Cu60Ni40-CeO2 was shown in Figure 4. The specific surface area and a total pore volume of nanocomposite were 32.328 m2 g-1 and 0.1268 cm3 g-1, respectively. 3.2. Catalytic properties of CuxNi100-x-CeO2 nanocomposites The catalytic performance of the CuxNi100-x-CeO2 nanocomposites was explored in 4-NP reduction reaction. This reduction process was investigated by evaluating the UV-vis absorption spectra of the reaction solution (Figure S2). A maximum absorption of 4-NP ions appears at 400 nm when NaBH4 is added into the 4-NP solution. After the addition of catalyst, this peak decreased whilst the absorption peak of 4-AP at 300 nm increased, during the reaction. Among of as-prepared nanocomposites, Cu60Ni40-CeO2 showed the best catalytic activity and the 8 ACS Paragon Plus Environment

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reduction reaction completed within 25 sec (Figure 3, C), whereas Ni-CeO2 exhibited the lowest activity. All the CuNi bimetallic decorated on CeO2, demonstrated the best catalytic activity than Ni-CeO2, which confirmed the synergistic effect of two metals for 4-NP reduction reaction. The plots of ln (Ct/C0) (Ct and C0 are 4-NP concentration at time t and 0, respectively) against reaction time (t) which shown in Figure 5 A, with the good linear correlation (R2 ˃ 0.98), confirmed the pseudo-first-order kinetics. The rate constants were calculated from the slopes of ln (Ct/C0)-t, which are reported in Table 1. The kapp values exhibited that the catalytic activity of the catalysts decreases in the following order: Cu60Ni40-CeO2 ˃ Cu-CeO2 ˃ Cu80Ni20-CeO2 ˃ Cu40Ni60-CeO2 ˃ Cu20Ni80-CeO2 ˃ Ni-CeO2. In comparison with bare Cu60Ni40 nanoparticles (kapp = 0.553 × 10-2 s-1, Figure S3) the catalytic activity of Cu60Ni40-CeO2 nanocomposite (kapp = 16.547 × 10-2 s-1) is 30 times higher than bare Cu60Ni40 NPs. The obtained results of rate constants over Cu60Ni40-CeO2 and other catalysts for this reaction were compared and presented in Table 2. The results showed that, the as-prepared catalyst in this study might be one of the best catalyst for 4-NP reduction. The kinetic of the 4-NP reduction reaction was investigated under different conditions. In one way, the concentration of Cu60Ni40-CeO2 was changed from 1 mg to 4 mg at constant concentration of 4-NP [0.1 mM] and NaBH4 [0.05 M] at room temperature. Under these conditions, the reaction rate constant linearly increased with amount of catalyst (Figure 5 B). This behavior is obvious, because by increasing of catalyst more active sites are available for this process. For NaBH4, with addition of concentration from 0.03-0.04 M the rate constant increased, however no change in rate constant was observed when the concentration change from 0.05 to 0.08 M (Figure 5 C). The influence of 4-NP concentration (0.06-0.18 mM) on the reduction of 4-NP is provided in Figure 5 D. At lower concentration of 4-NP the rate constant (kapp) increased with increasing of 4-NP concentration (0.02-0.1 mM), however, in



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concentration higher than 0.1 mM, the rate constant decreased. The nonlinear relation of kapp with NaBH4 concentration and decreasing of rate constant by increasing of 4-NP concentration indicated that, there must be a competition between both reactant in surface of Cu60Ni40-CeO2 for 4-NP reduction. The catalytic efficiency was compared by applying two parameters including turn over number (TON), and turn over frequency (TOF). In heterogeneous catalytic reaction, the number of substrate molecules that can be converted to products by 1g of catalyst, determined using TON. The turn over frequency (TOF) can be obtained by using TON/Time.54 The TOF values of the 4-NP reduction by Cu60Ni40-CeO2 nanocomposite were calculated in different temperatures, as given in Table 3. For this catalyst, the value of TON was estimated to be 2.8676 × 1020 molecules g-1 min-1 at room temperature, which emphasized that 2.8676 × 1020 molecules of 4-NP converted to 4-AP by 1 mg of Cu60Ni40-CeO2 nanocomposite in one minute. The reusability of catalyst is an important subject to determine the worthiness of a catalyst in practical applications. For this purpose, after the each cycle the nanocomposite was collected and washed with water by centrifugation, dried, and used in the new cycle by adding 4-NP and NaBH4 solutions. The ln (Ct/C0) against reaction time (t) was illustrated in Figure 6 A, for the sequential five cycles. The obtained results for these cycles emphasized the high activity of Cu60Ni40-CeO2 nanocomposite, due to slight change in kapp (Figure 6 B). 3.3. Calculated thermodynamic parameters The thermodynamic parameters for the 4-NP reduction reaction were investigated by performing the reaction under the same condition in the temperature range of 5-45 °C. The rate constants (kapp) for the 4-NP reduction were estimated and listed in Table 3. The thermodynamic parameters, including activation of enthalpy (∆ ) and entropy (∆), were computed using the Arrhenius and Eyring equations (eqs. 1, and 2),55, 56 10 ACS Paragon Plus Environment

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=

− + % "#

(1)

∆ ∆ 1 ( ) = ln( ) + − ( ) # ℎ " " #

(2)

in which, T is absolute temperature, R is the gas constant, kB, and h are the Boltzmann and Planck constants, respectively, and Ea is activation energy. The Gibbs energy was obtained by applying eq.3. (3)

∆ = ∆ − #∆

Activation energy is an important parameter in chemical reactions, which can illustrate the temperature dependency of rate constants in catalytic processes. The activation energy of 4-NP reduction catalyzed by Cu60Ni40-CeO2 was calculated to be 39.695 kJ mol-1 from the plot of ln (k) versus 1/T (Figure 7 A). This value is between 8.368-41.84 kJ mol-1 for the surface catalyzed reactions.57, 58 Therefore the obtained value indicated that the reduction reaction occurs on the surface of nanocomposite. From the graph of ln (k/T) versus 1/T (Figure 7 B) the enthalpy and entropy values of 4-NP reduction were estimated to be 37.219 kJ mol-1, and -135.519 J mol-1 K-1, respectively. The Gibbs energy was increased from 77.624 kJ.mol-1 at 298.15 K to 80.334 kJ mol-1 at 318.15 K, which suggests this reaction requires energy. 3.4. Mechanism of 4-NP reduction reaction by Cu60Ni40-CeO2 Borohydrides have a very good potential for the hydrogen generation in many application, therefore the reaction of borohydrides with bimetallic surfaces has become the significant topic research in recent years.59,60 Previous mechanistic investigation demonstrated that metal nanoparticles can be changed by NaBH4 and acts as electron donation in reactions.61 Since the reduction of 4-NP occurs on the surface of the catalyst, Longmuir-Hinshelwood model was 11 ACS Paragon Plus Environment


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applied for explain the mechanism reation.62, 63 A catalysis mechanism by CuxNi100-x-CeO2 was illustrated in Scheme 1 B. The obtained results of variation in kinetics parameters with an alternation in concentration of the catalyst, indicated that the sites available on the surface of Cu60Ni40-CeO2 are effective on the kinetic mechanism. Increasing active sites allows more of 4NP molecules adsorbed on the surface of catalyst. This reaction occurs in two steps: at first step, the borohydride ions react with the surface of the metallic nanoparticles (S) and would give electron to catalyst (eq. 4). Afterward, hydrogen is liberated due to reduction of water to H2. Finally, active hydrogen species are formed on the surface of CuNi NPs (eq. 5). -./

(4)

. , + 42222225 12222223 , 0 -./

8/

(5)

, . + 6 7 422225 ,76 . + .

Where , and are adsorption equilibrium constant of , , and rate of hydrolysis of , , respectively. In second step, 4-NP molecules are adsorbed on the surface of catalyst. Both of these reversible adsorption (, , and 4-NP) can be investigated by the LangmuirHinshelwood model.64 Then, 4-nitrophenolate ions are reduced by the active hydrogen species (AHS) via the formation of 4-hydroxylaminophenol as a stable intermediate, and finally converted into 4-aminophenol (eq. 7). Removal of 4-AP molecules from the surface of catalyst, provide free nanoparticles surface for the catalytic cycle (eq. 8). - 4 − :; + 422222225 122222223 4 − :; - 0 + 6 (% ) 422225 4 − %; > + 6 7


(7)

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4 − %; . 42225 4 − %; +

(8)

The adsorption and desorption of borohydride and 4-NP onto the surface of catalyst are fast, therefore they do not have any effect on the kinetic equation. In the rate determining step, 4-NP molecules are converted to 4-AP by active hydrogen species (eq. 7). The reduction of 4-NP is related to the surface of catalyst coating by , and 4-NP as showing in equation 9: −A = . . C . C B

(9)

in which, k is surface rate constant, is surface area of nanoparticles that was estimated by using the particle size and the volume of reaction solution.65, 66 C and C are surface coverage values of , and 4-NP, respectively, that can be determined from Langmuir isotherm (eq.10).

CD =

(D . AD )EF EI 1 + ∑ HJK(H . AH )

(10)

Where, n is the Langmuir-Freundlich exponent and CD , Ki, and Ci are the surface coverage, adsorption equilibrium constant, and concentration of compound i, respectively. According to this isotherm, the surface coverage of , and 4-NP can be evaluated by the following equations (eqs.11, 12):

C

( . A )L = 1 + ( . A )E + ( . A )L

(11)

( . A )E = 1 + ( . A )E + ( . A )L

(12)

C

Hence, the rate of 4-NP reduction reaction follows as:



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−A = B

MM . A

. . ( . A )E . ( . A )L = (1 + ( . A )E + ( . A )L )6

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(13)

Therefore, the reaction rate (kapp) can be obtained as equation 14.

MM

. . E . A EK . ( . A )L = (1 + ( . A )E + ( . A )L )6

(14)

The surface rate constant (k) and adsorption equilibrium constants calculated by fitting the series of data to Langmuir-Hinshelwood equation (eq. 13) which was shown in Figure 8 (A-H). The values of surface rate constant (k), adsorption constants of 4-NP ( ), and , ( ) were calculated using equation 14. The value of rate constant (k) by Cu60Ni40-CeO2 nanocomposite, which is related to the rate determining step, was higher than the values obtained by other catalysts, however, lower than Pd nanoparticles immobilized on spherical polyelectrolyte brushes (Table 4). The rate of 4-NP reduction was strongly affected by adsorption constant of 4-NP ( ). The value of was 17 times higher than the for 4-NP reduction catalyzed by Cu60Ni40-CeO2. Also, the Freundlich exponent's values m, and n refer to the heterogeneity of the sorbent, were estimated 0.93±0.03 and 0.47±0.02 respectively. 4. Conclusion In summary, CuxNi100-x-CeO2 (x = 0, 20, 40, 60, 80, and 100) nanocomposites were successfully synthesized by a simple method. The as-prepared CeO2-suported CuNi nanoparticles showed a synergetic catalytic effect toward the reduction of 4-nitrophenol. This catalyst displayed the ultrafast response for this reaction with remarkable catalytic activity and stability. The Cu60Ni40CeO2 nanocomposite demonstrated the best catalytic activity in this reaction, which was completed within 25 s. The catalytic reduction with sodium borohydride and its dependence of 14 ACS Paragon Plus Environment

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temperature were described by the Langmuir-Hinshelwood model. The kinetics data was fitted to this equation and the adsorption constants of , ( ), 4-nitrophenol (K4-NP), and Freundlich exponents (m, and n) were obtained. Furthermore, the thermodynamics parameters, including ∆G, ∆S, ∆H, and Ea were successfully calculated from the Eyring as well as Arrhenius equations.



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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel: +98 216 616 5314. Fax: +98 216 600 5718. † Supporting Information: Zeta potential distribution of CeO2 nanorods (Figure S1); UV-vis absorption spectra of the reduction of 4-NP by CuxNi100-x-CeO2 catalysts: (A) Ni-CeO2; (B) Ni80Cu20-CeO2; (C) Ni60Cu40-CeO2; (D) Ni40Cu60-CeO2; (E) Ni20Cu80-CeO2, and (F) Cu-CeO2 (Figure S2); and UV-vis absorption spectra (insert the plot of ln (Ct/C0) of 4-NP against reaction time) for the 4-NP reduction catalyzed by CuNi NPs (Figure S3).


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Table 1. Catalytic rate constants for reduction of 4-nitrophenol with NaBH4 in the presence of CuxNi100-x-CeO2 nanocomposites and the correlation coefficient for the ln (Ct/C0)-t plots.

Sample

Ni-CeO2

kapp × 10-2 (s -1)

0.718

R2

0.99257

Cu20Ni80-CeO2

Cu40Ni60-CeO2

1.107

2.995

0.99466

0.99646

Cu60Ni40-CeO2

Cu80Ni20-CeO2

Cu-CeO2

16.547

4.762

6.170

1

0.9848

0.99386


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Table 2. Comparison of the activity of various catalysts for catalyzing the reduction of 4-NP.

Catalysts

kapp (s-1) × 10-3

References

Cu60Ni40-CeO2

165.47

This work

Graphene/CuNi

136.85

[38]

AuAg-G

8.9

[39]

RGO-Ni25Co75

1.55

[40]

AuPt @BGNs/Fe3O4

165.16

[41]

CeO2-PtAu

108.7

[42]

Fe3O4@C-Pt-Pd

20.2

[43]

SBA-Au0.25Pd0.75

12.21

[44]

Ag50Ni50/RGO

48.40

[45]

Cu/Ag NPs

3.95

[46]



1 2 3 Table 3. Thermodynamics parameters of 4-NP catalytic reduction at different temperatures. 4 5 6 7 Catalyst T kapp TOF × 1020 A×106

∆ ∆ -1 -1 -1 8 (°C) (s ) (Molecule g min ) ( O PQ K ) ( O PQ K ) (O PQ K K ) 9 0.0469 1.6058 5 10 15 0.0929 2.0658 11 12 37.219 -135.519 Cu60Ni40-CeO2 25 0.1654 2.8676 39.695 1.408 13 14 0.2611 6.0220 35 15 45 0.4128 14.4533 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 26 60 ACS Paragon Plus Environment

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∆ ( O PQ K )

74.913 76.268 77.624 78.979 80.334

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Table 4. Langmuir-Hinshelwood parameters and comparison with literature for catalytic reduction of 4-NP. (L mol-1) 11.6±4.1

K4-NP (L mol-1) 195.5±18

References

0.93±0.03

k × 10-5 (mol m-2 s-1) 2.86±0.4

0.37±0.11

0.85±0.08

1.8±0.39

3.6±1.1

90±6.0

[64]

G5-OH(Ru80)

0.56±0.11

0.57±0.15

2.4±0.64

5.4±1.1

87±4.0

[64]

G6-OH(Ru160)

0.54±0.03

0.62±0.05

1.8±0.23

1.1±0.1

94±4.0

[64]

G4-NH2(Pd13)

0.62±0.1

1.0±0.1

0.18±0.02

185±7.0

2101±98

[67]

SPB-Pd

0.6±0.1

1.0±0.1

5.5±0.5

48±5.0

2300±400

[68]

Catalysts

n

m

Cu60Ni40-CeO2

0.47±0.02

G4-OH(Ru40)


This work


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Figure Caption Scheme 1. (A) Mechanism for the preparation of CuxNi100-x-CeO2 nanocomposite and its application to the reduction of 4-nitrophenol (4-NP), and (B) the proposed mechanism for reduction of 4-NP by NaBH4 over the CuxNi100-x-CeO2 nanocomposites. Figure 1. X-ray diffraction patterns of Ce-BTC MOF (A), CeO2 nanorods (B), Cu-CeO2 (C), NiCeO2 (D), Cu60Ni40-CeO2 (E), and CuNi NPs (F). Figure 2. FE-SEM image of Ce-BTC MOF (A, B), CeO2 nanorods (C, D), and Cu60Ni40-CeO2 (E), and EDX spectrum of Cu60Ni40-CeO2 nanocomposite (F). Figure 3. TEM images of Cu60Ni40-CeO2 nanocomposite (A, B), and UV-vis absorption spectra of the catalytic reduction of 4-NP by NaBH4 in the presence of Cu60Ni40-CeO2 nanocomposite (C). Figure 4. Nitrogen adsorption/desorption isotherms of Cu60Ni40-CeO2 nanocomposite at 77 K. Figure 5. (A) Plots of ln (Ct/C0) of 4-NP versus reaction time for the CeO2 nanorods, and CuxNi100-x-CeO2 nanocomposites, (B) Plot between rate constant (kapp) versus amount of Cu60Ni40-CeO2 catalyst at 25 °C , [4-NP] = 0.1 mM, and [NaBH4] = 0.05 M, (C) and (D) the effect of NaBH4 and 4-NP concentration on rate constant at 25 °C, in the presence of Cu60Ni40CeO2 as catalyst, respectively. Figure 6. (A) Plots of ln (Ct/C0) of 4-NP against reaction time for successive 5 cycle reactions employing Cu60Ni40-CeO2 as catalyst. (B) Values of kapp for each cycle with Cu60Ni40-CeO2 as catalyst. Figure 7. (A) Plots of ln (k), and (B) ln (k/T) versus 1/T, for the 4-NP reduction by NaBH4 in the presence of Cu60Ni40-CeO2 nanocomposite.


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Figure 8. (A-H) Plots of Ct/C0 and ln (Ct/C0) of 4-NP versus reaction time in different concentration of 4-NP and NaBH4 according to Langmuir-Hinshelwood mechanism for Cu60Ni40-CeO2 nanocomposite (Surface area 0.2556 m2 L-1) at 298 K.



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Scheme 1.


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Figure 1.



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Figure 2.


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Figure 3.



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 4.


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Figure 5.



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 6.


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Figure 7.



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Figure 8.


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Kinetic Analysis of the Reduction of 4-Nitrophenol ... - ACS Publications

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