ZnO nanomaterials

Materials Chemistry and Physics xxx (2016) 1e11

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Effect of preparation method, loading of Co3O4 and calcination temperature on the physicochemical and catalytic properties of Co3O4/ZnO nanomaterials Sahar A. El-Molla, Laila I. Ali, Hala R. Mahmoud*, Marwa M. Ibrahim, Mona A. Naghmash Department of Chemistry, Faculty of Education, Ain Shams University, Roxy, 11757, Cairo, Egypt

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


ZnO catalysts prepared by thermal decomposition, solegel and precipitation methods.
Co3O4/ZnO catalysts prepared by impregnation method using different zinc precursors.
The SBET of Co3O4/ZnO catalysts were much higher than those of the pure oxides.
The decomposition of H2O2 of Co3O4/ ZnO catalysts is higher than of the pure oxides.
The addition of Co3O4 to ZnO catalyst modified the morphology.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 June 2016 Received in revised form 3 September 2016 Accepted 1 October 2016 Available online xxx

ZnO nanomaterials (ZnOT, ZnOS and ZnOP) were successfully synthesized by thermal decomposition, sol egel auto combustion and precipitation methods, respectively. Co3O4/ZnO catalysts were synthesized by impregnation method, nominated as CoZnOT, CoZnOS and CoZnOP, respectively. The as-prepared catalysts were characterized by XRD, N2 adsorptionedesorption isotherms, HR-TEM and FT-IR techniques. The SBET of CoZnO nanomaterials were significantly higher than those of the pure oxides. Further, the Co3O4/ZnO sample in which ZnO synthesized via solegel combustion using citric acid (CoZnOSCI) showed the largest surface area while CoZnOP sample showed the smallest value. The HR-TEM images showed that addition of Co3O4 to ZnO catalyst and the calcination temperature affected the morphology. The catalytic decomposition of H2O2 of the Co3O4/ZnO catalysts is higher than that of the pure oxides. The most catalytically active sample is Co3O4/ZnO in which ZnO synthesized via thermal decomposition of zinc carbonate (CoZnOTC) while CoZnOSCI catalyst being the less active one. © 2016 Elsevier B.V. All rights reserved.

Keywords: Nanostructures Heat treatment Precipitation Sol-gel growth Surface properties

1. Introduction

* Corresponding author. E-mail addresses: [email protected], [email protected] (H.R. Mahmoud).

Hydrogen peroxide (H2O2) is one of the most versatile and environmentally desirable chemical available today. Its oxidation capability enables hydrogen peroxide to be employed as a reactant in chemical synthesis and as a bleaching agent for paper and textile industries, as well as in the treatment of pollutants [1]. In an

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oxidation process, it provides a clean rout without producing any harmful or environmentally unsafe products. It readily decomposes into water and O2. However, self-decomposition process of hydrogen peroxide is usually too slow to be beneficial. The decomposition of hydrogen peroxide in the presence of homo- and heterogeneous catalysts have been widely investigated [2]. Metal oxides of Ag, Cu, Fe, Mn, Ni, Cr and Zr or their supported form on silica, alumina and zeolites, have been the subject of many investigations for decomposition of aqueous hydrogen peroxide [3e6]. Nanostructured transition metal oxides (MOs), a particular class of nanomaterials, are the indisputable prerequisite for the development of various novel functional and smart materials [7]. These nanostructures exhibit significantly novel and distinct chemical, physical, and biological properties, and functionality due to their nanoscale size, have elicited much interest [8]. Among these materials, zinc oxide (ZnO) has received a considerable attention as a cost effective alternative to other oxides. Some interesting properties of ZnO are its non-hazardous nature and thermal stability. In addition to this ZnO has proved to be vital in the fields of catalysts, sensors, antibacterial and magnetism [9e13]. As well as, cobalt oxide (Co3O4) has attracted great interest owing to its potential application in biosensor [14] heterogeneous catalysis [15,16], electrocatalysis [17] and lithium-ion batteries [18], soot oxidation [19] and supercapacitor [20]. The properties of host material ZnO can be controlled or modify by introducing transition metal ions into their lattice [21]. There are two important techniques to control and modify the properties of host material such as size of nanoparticles, and surface area. A first technique is doping and another technique is a synthesis method of nanomaterial. Doping is an effective method to enhance and control the structural, optical, electrical and magnetic properties of host material [22]. Synthesis method also plays an important role in the preparation of nanoparticles [23]. The control of chemical composition, morphology, and particle size is very important to obtain suitable metal-oxide supported ZnO powders for their desired applications. Various methods used to prepare nano-sized ZnO such as auto combustion method [24], ball milling method [25], co-precipitation method [26], hydrothermal process [27], spray pyrolysis [28]. In this paper, we aimed to investigate the influence of preparation methods of ZnO support, loading of Co3O4 on ZnO nanomaterials and calcination temperature toward catalytic decomposition of hydrogen peroxide at 20e40  C. Furthermore, the as-prepared nanomaterials were characterized by X-ray diffraction (XRD), N2 adsorptionedesorption isotherms, high-resolution transmission electron microscope (HR-TEM) and Fourier transform infrared (FT-IR) spectroscopy. 2. Experimental 2.1. Materials All reagents were of analytical grade and they were purchased and used without further purification: zinc nitrate (Loba, 96% purity), zinc carbonate (BDH, 99.9% purity), zinc acetate (Loba, 97% purity), cobalt nitrate (Loba, 96% purity), citric acid (Adwic, 99.5% purity), oxalic acid (Adwic, 99.5% purity), urea (Adwic, 99% purity) and ammonium hydroxide (28 vol%) (Alpha, 99.9% purity).

2.2.1. Thermal decomposition method (T) In a typical synthesis, known amounts of zinc carbonate and zinc acetate were taken as separate samples in a mortar and ground well then calcined at 500  C in air for 3 h. The samples were denoted as ZnOTC and ZnOTA, in which (T ¼ refers to thermal decomposition) and (C and A refers to zinc carbonate and zinc acetate, respectively). 2.2.2. Sol-gel combustion method (S) A solegel auto combustion method using three different fuels; citric acid, oxalic acid and urea were used to synthesize ZnO nanomaterials. In order to describe simply, the final products were denoted as ZnOxy, in which (x ¼ S refers to sol-gel combustion method) and (y ¼ CI, O or U refers to citric acid, oxalic acid and urea fuel); denoted as ZnOSCI, ZnOSO, and ZnOSU samples, respectively. In brief, certain amounts of zinc nitrate and citric acid were dissolved in 100 ml distilled water to form homogeneous solution with a ZnO/fuel molar ratio of 1:1. The solution adjusted using ammonia 28 vol% to reach a pH value of 7 then the solutions refluxed at 80  C for 4 h. Then the solution evaporated at 80  C until the gel was formed. The gel was dried in an electric oven at 80  C for 20 h to give an almost dry powder which was then calcined in muffle furnace at 500  C for 3 h. The produced ZnOSO, and ZnOSU samples were prepared by applying similar conditions using oxalic and urea fuels, respectively. 2.2.3. Precipitation method (P) In this method, 14.857 g of zinc nitrate was dissolved in 100 ml of distilled water while constantly stirring at a temperature of 50  C and then 28% NH4OH was added into this solution until pH reach 8. The obtained precipitate was washed several times using distilled water. The resulting solution was filtered and dried at 80  C. The collected powder then calcined at 500  C for 3 h. This sample was denoted as ZnOP. 2.3. Co3O4/ZnO catalyst preparation An impregnation method using different prepared Zn-samples previously from different precursors were used to synthesize 0.07Co3O4/ZnO catalysts. Zinc precursors were: carbonate, acetate, citrate, oxalate, urea-complex and hydroxide. In a typical route, a given mass of zinc precursor were treated with an aqueous solution containing a fixed amount of cobalt nitrate dissolved in the least amount of distilled water sufficient to make a paste. The resulting paste was dried at 100  C and then calcined at 500  C for 3 h. In order to describe simply, the final products were denoted as 0.07CoZnOTC, 0.07CoZnOTA, 0.07CoZnOSCI, 0.07CoZnOSO, 0.07CoZnOSU, 0.07CoZnOP, respectively. Six samples of Co3O4/ZnO catalysts were prepared by impregnation using known mass of zinc carbonate as precursor with solutions containing different amounts of cobalt nitrate dissolved in the least amount of distilled water. The obtained pastes were dried at 100  C and then calcined at 500  C for 3 h. The nominal compositions of the prepared samples were 0.008, 0.01, 0.03, 0.07, 0.08 and 0.1Co3O4/ZnO. The sample 0.07Co3O4/ZnO was calcined also at different temperatures 450, 500, 550, 600, 700 and 800  C. The samples were denoted as xCoZnOTC, in which (x ¼ refers to the concentration of Co3O4) and (T ¼ refers to thermal decomposition and C ¼ zinc carbonate precursor).

2.2. ZnO support preparation

2.4. Catalyst characterization

ZnO nanomaterials were prepared by three different methods namely: thermal decomposition (T), sol-gel combustion (S) and precipitation (P) methods.

X-ray diffraction (XRD) patterns were obtained using a Philips X-ray diffractometer (Germany) using CuKa1 irradiation (l ¼ 0.15404 nm) at a scan rate of 2 in 2q/min, were used to

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determine the identity of any phase present and their crystallite size. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. Nitrogen sorption experiments were performed at 77 K with a Quantachrome NOVA 3200 automated gassorption apparatus model 10 (USA). The samples were pretreated at 200  C for 2 h under vacuum. The surface areas were calculated using the BrunauereEmmetteTeller (BET) method. The pore volume, Vp was evaluated by converting the volume of nitrogen adsorbed at P/Po of about 0.99 to the volume of liquid nitrogen per gram of the material. The pore size distribution was calculated from desorption branch of the isotherm by the Barrett, Joyner and Halenda (BJH) method. The morphology of the catalysts was observed by high-resolution transmission electron microscope (HR-TEM) microanalysis system (JEM-2100CX (JEOL)). Fourier transform infrared spectroscopy (FT-IR) was recorded on Nicolit IR 6700 spectrometer (USA) using KBr pellets. The catalytic decomposition of H2O2 in presence of the prepared catalysts was determined. 100 mg of a given catalyst sample was taken and 0.5 ml of H2O2 of known concentration diluted to 20 ml with distilled water was used at reaction temperature 20e40  C. The reaction kinetics was monitored by measuring the volume of O2 liberated at different time intervals until no further oxygen was liberated. 3. Results and discussion 3.1. X-ray diffraction analysis The effect of different preparation methods of ZnO nanomaterials namely thermal decomposition, solegel combustion and precipitation on the phase structure of the synthesized catalysts was studied using X-ray diffraction analysis. Fig. 1A shows the XRD of ZnOTC, ZnOTA, ZnOSCI, ZnOSO, ZnOSU and ZnOP samples calcined at 500  C. The peaks at 31.6 , 34.4 , 36.0 , 47.6 , 56.5 , 62.8 and 67.9 correspond to 100, 002, 101, 102, 110, 103 and 112 reflection planes of hexagonal wurtzite structure of ZnO (JCPDS 36-1451) [29], respectively. All prepared samples have the same ZnO peaks with different degree of crystallinity depending on preparation method [30e33]. The result revealed that the ZnOTC catalyst synthesized by thermal decomposition of zinc carbonate shows broader peaks and weaker intensities. This broadening indicates that the size of sample prepared by this method was the smallest compared to the two other methods. The main crystallite size of the prepared samples was calculated from the (101) reflection using Scherrer equation [34]. The structural information and the crystallite size were listed in Table 1. Hence, the XRD results prove that there were some differences between the crystallinity and crystallite sizes of ZnO synthesized by three different methods. In other words, the crystallinity and crystallite size of ZnO samples synthesized via thermal decomposition of zinc carbonate was the smallest and solegel combustion method gave the largest values. The lattice parameters for hexagonal wurtzite structured ZnO ‘a’ and ‘c’ were calculated from the following equations [35]. a ¼ l /√3 sin q

(1)

c ¼ l / sin q

(2)

Where ‘a’ and ‘c’ are the lattice constants for the plane (100) and (002), respectively, and l is the wave length of the incident X-ray (l ¼ 0.15404 nm). The lattice constants (a ¼ b ¼ 0.3249 nm and c ¼ 0.5207 nm) [36]. The variation of lattice parameters ‘a’ and ‘c’ with preparation method were mentioned in Table 1. Interestingly, the lattice parameters ‘a’ and ‘c’ for ZnOSCI catalyst were larger than the obtained values for other samples.

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Fig. 1B shows the XRD of 0.07CoZnOTC, 0.07CoZnOTA, 0.07CoZnOSCI, 0.07CoZnOSO, 0.07CoZnOSU and 0.07CoZnOP samples calcined at 500  C. The peaks at 31.0 , 59.1 and 65.2 correspond to 220, 511 and 440 reflection planes of cubic Co3O4 (JCPDS43-1003) and/or ZnCo2O4 phases (JCPDS 23-1390) in addition to the peaks of ZnO as major phase [36e39]. The peaks of both Co3O4 and ZnCo2O4 were not distinguishable by XRD [29]. The result revealed that the 0.07CoZnOSCI, 0.07CoZnOSO and 0.07CoZnOSU samples synthesized by solegel method shows slightly broader peaks and weaker intensities. This broadening indicates that the size of samples prepared by solegel method was smaller compared to the two other methods. The sequence of crystallite size of ZnO phase for the asprepared catalysts using different preparation methods decreased as follows: 0.07CoZnOSCI (26.5 nm) < 0.07CoZnOSU (35.7 nm) < 0.07CoZnOSO (40.8 nm) < 0.07CoZnOTC (42.6 nm) < 0.07CoZnOTA (49.3 nm) < 0.07CoZnOP (53.9 nm). Furthermore, the crystallite size of Co3O4 and/or ZnCo2O4 phases for all synthesized catalysts obeys nearly the same sequence of the ZnO phase as listed in Table 1. It is clearly observed that the crystallite size of 0.07CoZnO samples synthesized via solegel combustion was the smallest and precipitation method gave the largest sizes. The lattice parameters for 0.07Co3O4/ZnO catalysts prepared by different methods calcined at 500  C were calculated as shown in Table 1. From Table 1 it is found that the lattice parameters for all catalysts relative to that of pure ZnO were decreased except the sample synthesized via solegel combustion method using citric acid (0.07CoZnOSCI) increased. It had been reported that solution combustion synthesis is an effective technique for the preparation of nanoscale material. The sol gel auto combustion method is unique to obtain high porosity and high surface area to volume ratio fine particles. The method is best on the principle that once the reaction is started with low temperature an exothermic reaction occurs that becomes self-sustaining for a certain time interval [40]. So, the increase in lattice parameters of 0.07CoZnOSCI sample can be attributed to the dissolution of Co2þ into the ZnO lattice due to high porosity of ZnOSCI. The expected reaction between Co3O4 and ZnO is the formation of a ZnO.Co3O4 solid solution. This can be explained due to little difference between the ionic radii of Co2þ (0.058 nm) and Zn2þ (0.060 nm) [41]. The XRD of 0.008CoZnOTC, 0.07CoZnOTC and 0.1CoZnOTC catalysts calcined at 500  C were presented in Fig. 1C. It was clearly noticed that the XRD of these catalysts demonstrated peaks due to hexagonal ZnO phase with a moderate degree of ordering [37]. At lower Co3O4 loading, i.e. 0.008CoZnOTC sample, there are no diffraction peaks due to Co3O4 and/or ZnCo2O4 phases. The absence of Co3O4 peaks proves its existence in a highly dispersed state on ZnO surface beside its small amount to be detected by X-ray diffractometer [42,43]. However, with increase of Co3O4 loading, i.e 0.07CoZnOTC and 0.1CoZnOTC, new characteristic peaks were visible of the crystalline Co3O4 and/or ZnCo2O4 phases [38,39]. Moreover, increasing Co3O4 content from 0.008 to 0.07 mol resulted in decreased the crystallinity of ZnO phase due to the presence of scattered Co ions which might have created thermodynamical barriers in-turn slowed down the growth process [44,45]. But further increase in Co3O4 content from 0.07 to 0.1 mol lead to increase in the crystallinity whereas substituted Co2þ ions in tetrahedral coordination of Zn2þ ions in ZnO lattice might have resulted in increase in crystallinity and crystallite size [45]. In other words, the crystallinity of 0.008CoZnOTC sample was the largest value. Increasing of Co3O4 content, increases the crystallite size of ZnO and Co3O4 phases as shown in Table 1. The lattice parameters for 0.008CoZnOTC, 0.07CoZnOTC and 0.1CoZnOTC catalysts calcined at 500  C were calculated as shown in Table 1. Interestingly, the lattice parameters were decreased with

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Fig. 1. A.XRD pattern of ZnO samples calcined at 500  C prepared with different methods. B. XRD pattern of 0.07CoZnO samples calcined at 500  C prepared with different methods. C. XRD pattern of 0.008CoZnOTC, 0.07CoZnOTC and 0.1CoZnOTC catalysts calcined at 500  C. D. XRD pattern of 0.07CoZnOTC calcined at (a) 450  C, (b) 500  C and (c) 800  C.

Table 1 Intensity counts of the main diffraction lines of XRD, crystallite sizes and lattice parameters for pure ZnO and Co3O4/ZnO nanomaterials synthesized using various methods at different calcination temperatures. Catalyst

ZnOSCI ZnOSO ZnOSU ZnOP ZnOTA ZnOTC 0.07CoZnOSCI 0.07CoZnOSO 0.07CoZnOSU 0.07CoZnOP 0.07CoZnOTA 0.008CoZnOTC 0.07 CoZnOTC 0.1 CoZnOTC 0.07 CoZnOTC 0.07 CoZnOTC a b

Calcination temperature ( C)

500 500 500 500 500 500 500 500 500 500 500 500 500 500 450 800

Intensity count (a.u.)

Crystallite size (nm)

Lattice parameter (nm)

ZnO

Co3O4 and/or ZnCo2O4

ZnO

Co3O4 and/or ZnCo2O4

ZnOa

ZnOb

106.0 127.0 99.1 97.5 99.9 50.6 26.0 28.4 24.9 48.4 35.4 107 54.8 56.7 54.7 88.0

e e e e e e 5.04 5.13 4.38 4.55 3.80 e 4.95 5.84 4.30 7.56

39.2 62.0 94.0 50.4 44.3 23.2 26.5 40.8 35.7 53.9 49.3 36.1 42.6 45.7 44.1 69.1

e e e e e e 14.7 27.2 20.5 24.1 12.7 e 16.6 30.9 20.9 60.1

0.3255 0.3253 0.3253 0.3253 0.3252 0.3252 0.3260 0.3255 0.3250 0.3254 0.3248 0.3255 0.3250 0.3250 0.3257 0.3259

0.5218 0.5211 0.5211 0.5213 0.5210 0.5214 0.5238 0.5219 0.5211 0.5213 0.5195 0.5218 0.5205 0.5205 0.5215 0.5215

The standard a value of ZnO is 0.3249 nm. The standard a value of ZnO is 0.5207 nm.

increase Co3O4 loading. It may be due to the substitution of the Co2þ in place of Zn2þ should in fact lead to decrease in the lattice parameters due to smaller ionic radius of divalent Co2þ in tetrahedral coordination [40]. Fig. 1D shows the XRD of 0.07CoZnOTC sample after calcination

at 450  C, 500  C and 800  C for 3 h. All the diffraction peaks in all samples could be indexed to ZnO, Co3O4 and/or ZnCo2O4 phases. It is obvious from Fig. 1D and Table 1 that on increasing the calcination temperature from 450  C to 500  C, the crystallinity and crystallite sizes of all phases did not change. However, on increasing

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the temperature from 500  C to 800  C, both the crystallinity and crystallite sizes of all phases increased which could be explained in the light of the grain growth mechanism or sintering processes [46].

3.2. BET surface area, pore size and pore volume

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nitrogen adsorption and desorption isotherms. Fig. 2A shows the N2 adsorption-desorption isotherms of 0.07CoZnOTC, 0.07CoZnOTA, 0.07CoZnOSCI, 0.07CoZnOSO, 0.07CoZnOSU and 0.07CoZnOP nanomaterials calcined at 500  C. The corresponding pore size distribution was measured using BJH method as shown in Fig. 2B. Furthermore, the BET surface area, pore diameter and total pore volume were summarized in Table 2. It can be seen in Fig. 2A that, all isotherms are of Type IV with an H3-type hysteresis loop, which

The BET analyses and surface porosity were investigated using

Fig. 2. A.Nitrogen adsorptionedesorption isotherms of 0.07CoZnO nanomaterials synthesized with. different methods and calcined at 500  C. B. Pore size distribution of 0.07CoZnO nanomaterials synthesized with different methods and. calcined at 500  C.

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Table 2 Surface characteristics for pure Co3O4 and Co3O4/ZnO nanomaterials synthesized using various methods at different calcination temperatures. Catalyst

Calcination temperature ( C)

SBET (m2/g)

VP (cm3/g)a

Average pore diameter (nm)b

Co3O4 0.07CoZnOSCI 0.07CoZnOSO 0.07CoZnOSU 0.07CoZnOP 0.07CoZnOTA 0.008CoZnOTC 0.07 CoZnOTC 0.1 CoZnOTC 0.07 CoZnOTC 0.07 CoZnOTC

500 500 500 500 500 500 500 500 500 450 800

9.5 22.5 16.7 14.1 14.0 14.1 34.3 20.5 15.0 19.1 10.8

0.027 0.049 0.043 0.037 0.035 0.031 0.065 0.040 0.042 0.041 0.020

11.2 8.6 10.3 10.4 9.9 8.9 7.6 7.9 8.4 8.5 7.4

a b

Pore volume. Average pore diameter obtained from BJH method.

indicates that the materials are mesoporous according to the IUPAC classification [47]. Moreover, the H3-type hysteresis is generally observed for aggregates of plate-like particles giving rise to slitshaped pores [48]. As can be seen in Table 2, the order of surface areas of the nanomaterials synthesized with different preparation methods was as follows: 0.07CoZnOSCI (22.5 m2/g) > 0.07CoZnOTC (20.5 m2/ g) > 0.07CoZnOSO (16.7 m2/g) > 0.07CoZnOSU (14.1 m2/ g) z 0.07CoZnOTA (14.1 m2/g) z 0.07CoZnOP (14.0 m2/g). It could be concluded from above results that the 0.07CoZnOSCI catalyst synthesized via solegel combustion using citric acid as fuel had the largest surface area because it possessed the smallest crystallite size [49,50]. Nevertheless, the 0.07CoZnOP and 0.07CoZnOTA catalysts had smaller value of the SBET due to they had larger crystallite size [51]. Further, the pore volume (VP) of the mesoporous catalyst synthesized using solegel combustion using citric acid as fuel was higher than those prepared by other methods. The corresponding pore size distribution of the 0.07CoZnO nanomaterials synthesized by different methods (Fig. 2B) showed monomodal distributions in the low mesoporous range with average pore diameters around 10 nm. The results of the textural characterization were in agreement with the XRD results discussed earlier. Fig. 3A represents the N2 adsorption-desorption isotherms of Co3O4, 0.008CoZnOTC, 0.07CoZnOTC and 0.1CoZnOTC catalysts calcined at 500  C. It was clearly noticed that all isotherms are of Type IV with an H3-type hysteresis loop, which indicates that the materials are mesoporous in nature [47]. The BET surface area, pore volume and average pore diameter for Co3O4 sample were found to be about 9.5 m2/g, 0.027 cm3/g and 11.2 nm, respectively as shown in Table 2. It was reported that the surface area of ZnO catalyst which prepared by thermal decomposition method was about 8.2 m2/g [52]. As can be observed from Table 2, the specific surface areas of the Co3O4/ZnO nanomaterials were significantly higher than those of the pure oxides. The highest SBET and pore volume values were observed for the catalyst with lower Co3O4 loading, 0.008CoZnOTC. However, the catalyst with higher Co3O4 loading, 0.1CoZnOTC, showed lower SBET value. In other words, increasing the Co3O4 loading led to decrease in the surface area of samples as a result of increasing the crystallite size. Furthermore, the SBET value of the mesoporous 0.1CoZnOTC catalyst was higher than that of Co3O4 and ZnO catalysts which have the smallest value of the SBET. Thus, the addition of Co3O4 to ZnO resulted in an enhancement of its specific surface area. Fig. 3B shows the corresponding pore size distributions of the same samples with monomodal distributions in the low mesoporous range with average pore diameters around 11 nm. The results of the textural characterization were in agreement with the XRD results discussed earlier. The significantly high

specific surface area and the unique pore characteristics of Co3O4/ ZnO nanomaterials compared with the corresponding pure oxides are expected to enhance their catalytic potential. Fig. 4A shows the N2 adsorption-desorption isotherms of the 0.07CoZnOTC catalyst after calcination at 450  C, 500  C and 800  C for 3 h. It also noted that all isotherms are of Type IV with an H3type hysteresis loop. Table 2 showed a limited increase in surface area as a result of increasing the calcination temperature from 450  C to 500  C due to a very small decrease in the crystallite sizes. Furthermore, on increasing the temperature from 500  C to 800  C, a sharp decrease was observed in the surface area value. This can be due to collapse of the pore structure and/or the particle adhesion process (grain growth) [53]. Fig. 4B shows the corresponding pore size distributions of the same samples with monomodal distributions in the low mesoporous range with average pore diameters around 9 nm. 3.3. Morphology study Fig. 5a-c depicts the HR-TEM images of ZnOTC calcined at 500  C and 0.07CoZnOTC samples preheated at 500  C and 800  C, respectively. The aggregates of uniform spherical nanoparticles with average diameter 30.9 nm were observed in ZnOTC nanomaterial. Moreover, on inspection of Fig. 5(b and c), it can be seen that the particles have cube like and irregular shapes with an average diameter of 19.4 and 56.3 nm, respectively. Obviously loading of Co3O4 on ZnO support reduced the particle size. Moreover, the agglomerates of ZnO were denser, which suggested that pure ZnO found it easier to agglomerate than the Co3O4/ZnO nanomaterials. It can be suggested that the aforementioned thermodynamical barrier induced by the Co3O4 causes a slowdown of the nanocrystals' growth. As a consequence, smaller particles are obtained [44]. 3.4. FT-IR analysis The FT-IR spectra of the 0.008CoZnOTC, 0.07CoZnOTC, 0.1CoZnOTC, 0.07CoZnOSCI and 0.07CoZnOP samples calcined at 500  C were showed in Fig. 6. The broad absorption peak at 3600e3200 cm1can be attributed to the hydroxyl groups of hydrated oxide surface and the adsorbed water. All spectra exhibited in the (yOH) region one band with a maximum around 3446 cm1, which characterizes the stretching vibrations of surface hydroxyl groups [54]. The bands observed at 1640e1630 cm1can be assigned to the molecular water bending mode [55]. The bands around 1100 cm1can be attributed to M-OH stretching vibrations [56]. The bands at 1460e1400 cml corresponded to the M-O-M deformation vibrations [57]. The bands at 571 and 663 cm1

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Fig. 3. A. Nitrogen adsorptionedesorption isotherms on pure Co3O4, 0.008CoZnOTC, 0.07CoZnOTC and 0.1CoZnOTC nanomaterials calcined at 500  C. B. Pore size distribution of Co3O4, 0.008CoZnOTC, 0.07CoZnOTC and 0.1CoZnOTC. nanomaterials calcined at 500  C.

corresponds to stretching vibrations of the CoeO bonds [58] while the band at 430 cm1 is correlated to ZneO [59].

3.5. Catalytic decomposition of H2O2 3.5.1. Effect of preparation methods The catalytic decomposition of H2O2 was studied at 20e40  C

over 0.07CoZnOTC, 0.07CoZnOTA, 0.07CoZnOSCI, 0.07CoZnOSO, 0.07CoZnOSU and 0.07CoZnOP samples calcined at 500  C as shown in Fig. 7A and Table 3. This figure shows the first-order plots, its slopes allowed a ready determination of the reaction rate constant (k) at 30  C. Moreover, the catalytic activity of the synthesized catalysts expressed as reaction rate constant k (min1) followed this order: 0.07CoZnOTC > 0.07CoZnOSU > 0.07CoZnOTA>

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Fig. 6. FT-IR spectra of 0.07CoZnO nanomaterials prepared with different methods and CoZnOTC nanomaterials prepared with different Co3O4 loading calcined at 500  C.

Fig. 4. A.Nitrogen adsorptionedesorption isotherms of 0.07CoZnOTC nanomaterials calcined at (a) 450  C, (b) 500  C and (c) 800  C. B. Pore size distribution of 0.07CoZnOTC nanomaterials calcined at (a) 450  C, (b) 500  C and (c) 800  C.

different oxidation states [57]. As we all known that the cobalt ions implying in Co3O4 exist in the mixed valence states of þ2 and þ 3. As a result of high-valent Co ion, oxygen vacancies are created which accumulates a large number of adsorption oxygen on the surface that is the active centre for the oxidation [60]. So, the catalytic activity for 0.07CoZnOTC catalyst is the highest while the 0.07CoZnOSCI is the smallest one. It could be attributed to an effective increase in the concentration of trivalent cobalt ions with increasing of active sites of H2O2 decomposition (Co3þ-Co2þ and Co3þ-Zn2þ ion pairs) [61]. Additionally, the crystallinity and crystallite size of ZnO samples synthesized via thermal decomposition of zinc carbonate was the smallest and solegel combustion method gave the largest values (c.f. Table 1). In other words, ZnO exhibits no catalytic activity in the H2O2

Fig. 5. HR-TEM images of (a) ZnO (b) 0.07CoZnOTC calcined at 500  C and (c) 0.07CoZnOTC. calcined at 800  C.

0.07CoZnOP > 0.07CoZnOSO > 0.07CoZnOSCI. In other words, the 0.07CoZnOTC nanomaterial being the most active while the 0.07CoZnOSCI being the less active one. The catalytic activity of all catalysts remarkably increased as reaction temperature increased from 20 to 40  C. It was reported that the catalytic decomposition of H2O2 is not much sensitive to the catalyst surface area enhancement while it is sensitive to presence transition metal ions with

decomposition reaction. Co3O4 is more active and selective. The combination of both oxides exhibits a catalytic activity that depends on the specific preparation methods and treatment temperature [41]. When Co3O4 nanoparticles disperse on ZnO support, the Co3O4/ZnO systems were catalytically active [41]. Interestingly, the thermal decomposition method using zinc carbonate leaves the Co3O4 active sites dispersed at the ZnO surface which were

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Table 3 Effect of preparation method, Co3O4 loading and calcination temperatures on the values of k (min1)*103 for H2O2 decomposition at 20e40  C. Catalyst

0.07CoZnOSCI 0.07CoZnOSO 0.07CoZnOSU 0.07CoZnOP 0.07 CoZnOTA 0.07 CoZnOTC Co3O4 0.008CoZnOTC 0.01CoZnOTC 0.03CoZnOTC 0.08CoZnOTC 0.1CoZnOTC 0.07 CoZnOTC 0.07 CoZnOTC 0.07 CoZnOTC 0.07 CoZnOTC 0.07 CoZnOTC

Calcination temperature (  C)

500 500 500 500 500 500 500 500 500 500 500 500 450 550 600 700 800

k (min1)*103 20  C

30  C

40  C

4.24 4.80 13.55 8.56 8.56 29.11 3.23 5.35 14.59 20.66 25.33 22.08 10.31 28.94 26.82 15.63 13.99

16.71 21.21 64.70 19.67 25.29 185.42 35.59 68.59 136.81 143.73 173.06 126.19 141.93 171.49 168.77 113.01 84.83

16.71 21.21 64.70 19.67 25.29 185.42 35.59 68.59 136.81 143.73 173.06 126.19 141.93 171.49 168.77 113.01 84.83

synthesized via solegel combustion method using citric acid (0.07CoZnOSCI) catalyst being the less active one, this behavior can be attributed to the dissolution of Co2þ into the ZnO lattice due to high porosity of ZnOSCI as confirmed by the lattice parameters measurements (Table 1). The expected reaction between Co3O4 and ZnO is the formation of a ZnO.Co3O4 solid solution [41]. Thus, the number of free active sites Co3O4 was decreased at the ZnOSCI surface in the system. 3.5.2. Effect of Co3O4 loading Fig. 7B shows the first order plots of H2O2 decomposition at 30  C using CoZnOTC catalysts at different Co3O4 loading calcinated at 500  C. Fig. 7B shows that the ZnO catalyst support exhibits no catalytic activity in the H2O2 decomposition reaction. The catalytic activity of the CoZnOTC catalyst is higher than that of the pure Co3O4 catalyst. A further increase in the cobalt oxide loading from 0.008 to 0.07 mol leads to an increase in the catalytic activity but above 0.07 mol Co3O4 was followed by a decrease in the H2O2 decomposition. The catalytic activity of all catalysts remarkably increased as reaction temperature rose from 20 to 40  C as seen in Table 3. The high activity of 0.07CoZnOTC catalyst can be explained based on its physicochemical properties such as reducing the crystallite size and particle size. As well as, the presence of welldispersed nanosize Co3O4 catalytically active sites at the interface of the system might be responsible for the high activity of the catalyst [41]. The least active catalyst is the 0.008CoZnOTC, which had the largest crystallinity value.

Fig. 7. A. First-order plots of H2O2 decomposition conducted at 30  C over 0.07CoZnO catalysts. calcined at 500  C prepared with different methods. B. First-order plots of H2O2 decomposition at 30  C over pure oxides and CoZnOTC with. different Co3O4 loading calcined at 500  C. C. First-order plots of H2O2 decomposition at 30  C over 0.07 CoZnOTC catalysts at different calcination temperatures.

confirmed by the lattice parameters measurements, thus the 0.07CoZnOTC catalyst being the most active. Whereas the sample

3.5.3. Effect of calcination temperature Fig. 7C shows the first order plots of H2O2 decomposition at 30  C using 0.07CoZnOTC catalysts calcinated at different temperatures from 450 to 800  C. The catalytic activity of 0.07CoZnOTC increases with raising the calcination temperature from 450 to 550  C. Furthermore, the catalytic activity was accompanied by a progressive decrease on increasing the calcination temperature from 500 to 800  C. Thus, the decrease in catalytic activity of the Co3O4/ZnO system after calcination, can be explained as follows: ZnO may interact with Co3O4 and form inactive species such as ZnCo2O4 spinel, decreasing the number of free active sites Co3O4 in the system. The zincecobalt oxide spinel has very little activity, because of the restriction of the active species of cobalt in the tetrahedral sites [41]. It can be noted from the results of XRD and TEM that the crystallinity and crystallite sizes of all phases

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