Synthesis and Characterization of Nano CuO-NiO Mixed Oxides

A series of single and mixed oxides of nano CuO-NiO were prepared by thermal decomposition of the NiC2O4.2H2O and/or CuC2O4 precursors which obtained via the reaction between alcoholic metal nitrates and oxalic acid solutions. Thermal behavior of the single and mixed salts has been studied using thermal analysis (TG and DTA). Thermal products were characterized using Fouier transform infrared (FTIR), X-ray diffraction (XRD), Electron microscopy (SEM&TEM), Nitrogen adsorption at -196C and surface excess oxygen measurements. The results revealed that, pure NiC2O4.2H2O and CuC2O4 decomposed at 355 and 325C, respectively. Also, it was found that the addition of NiO to CuO brought a significant increase in the degree of crystallinity of the mixed oxides which is accompanying with an increase in their particle size and a decrease in the surface area. Moreover, addition of NiO to CuO, also reduces the amount of surface excess oxygen of CuO due to a synergistic effect.


Introduction
A great deal of fundamental researches has been done on mixed catalysts in the field of heterogeneous catalysis [1][2][3]. These mixed systems may be more active than their separate components [4,5]. The so-called mixed catalysts are often produced by thermal decomposition of the mixed components of their metal salts [6]. It is well known that, nanostructured materials have been extensively explored for the fundamental scientific and technological interests in accessing new classes of functional materials with unprecedented properties and applications [7][8][9]. Thus a reduction in particle size to nanometer scale results in various interesting properties compared with their bulk properties. Therefore, many methods have been studied to synthesize nanostructured copper oxide [10][11][12][13][14][15][16]. CuO is an important oxide in many chemical processes such as degradation of nitrous oxides, selective catalytic reduction of nitric oxide with ammonia and oxidation of CO, hydrocarbons and fine chemicals [17][18][19]. Up to date, many studies have been reported about the preparation of CuO nanomaterials such as nanorods [20], nanowires [21], Shuttke-like structure [22], nanoribbones and nanoparticles [11,16,[23][24][25][26].
It is well known that, the precipitation of oxalates is often used for the elaboration of precursors of high purity and high quality. Whereas preparation of nano crystalline CuO-NiO via their oxalate route, to our knowledge, is not reported. So, The present investigation reports the results of an extensive studies on synthesis, structural and surface characterization of nano-crystalline Cu-Ni mixed oxide systems by a variety of techniques including thermal analysis (TG & DTA), Fourier transform infrared (FTIR), x-ray diffraction (XRD), nitrogen adsorption measurements at -196 o C, surface excess oxygen and electron microscopy (SEM & TEM), Synthesis and Characterization of Nano CuO-NiO Mixed Oxides method as described previously [35,43] with some modifications. In a typical process, analytical grade of copper and nickel nitrates (0.2M) and 1.0M oxalic acid were separately dissolved in absolute ethanol at 50 o C. In the course of magnetically stirred of each metal nitrate solution, the same concentration of warm oxalic acid was added as precipitator dropwise with constant stirring in one direction. Kind of light blue sediments were formed, these precipitates were filtered and washed with ethanol and finally washed with small amount of acetone. The washed precipitates were dried in air overnight at room temperature to get the precursors.
CuO-NiO mixed oxide systems were prepared by co-precipitation method. The calculated amounts of copper and nickel nitrates were dissolved in ethanol at 50 o C, after stirring the mixtures of metal nitrate solutions; ethanolic oxalic acid solution was added dropwise with constant stirring as described above. The precipitates are then filtered off, washed with ethanol and then with acetone and dried in air overnight at room temperature to obtain the precursors. Pure CuO, Pure NiO and CuO-NiO mixed systems with different ratios of NiO (1-50 wt. %) were obtained by annealing these precursors at 400 and 600 o C in muffle furnace under air atmosphere for 3 h.

Thermal Analysis
Thermogravimetery (TG) and Differential thermal analysis (DTA) were carried out using a computerized Shimadzu Thermal analyzer (TGA 60 Japan) apparatus. The samples were studied under air atmosphere with flowing rate of 30 mL min -1 . Constant weights of samples (̴ 5.5 mg) were used in order to avoid the effect of sample weight on peak shape and temperature. α-Al 2 O 3 was used as a reference material in this technique. The heating rate was fixed at 10 o C min -1 .

Fourier transform infrared (FT-IR)
FT-IR spectra of the original and the calcined samples at 400 and 600 o C for 3h were recorded in the 4000-400 cm -1 region with a Nicolet (Madison, WI, USA) 710 FT-IR equipped with data station. Dried samples of about 20 mg each were mixed with 100 mg of dry spectral grade KBr and pressed into disk under hydraulic pressure.

X-ray diffraction (XRD)
An x-ray investigation of the individual and mixed solids precalcined at 400 and 600 o C in air for 3 h was conducted using a Philips diffractometer (Model PW 2103, λ=1.5418Ao, 35 KV and 20 mA) with a source of CuKα1 radiation (Ni Filtered). An on-line data acquisition and handling system facilitated automatic JCPDS library search and match for phase identification purpose.
Crystallite size can be estimated from the full width at half maximum (FWHM) of the X-ray diffraction liner. The broadening of the FWHM is inversely proportional to the average crystallite size (D) as predicted by the well-known Scherer's formula [44] the grain size D is calculated from the following relation: Where β is the observed angular width at half maximum intensity of the peak and calculated from: where β s is the measured line width at half maximum and β o is the instrumental broadening [45]. β o = 0.16 with the apparatus used. K is dimension less number, which is equal to 0.89, λ is the X-ray wavelength (1.548 Å for Cu Kα1) and θ is the diffraction angle.

Nitrogen adsorption
Nitrogen adsorption-desorption isotherms were measured at -196 o C by Quantachrom Instrument Corporation, USA (Model Nova 3200). Test samples were thoroughly out gassed for 2h at 250 o C to a residual pressure of 10 -4 torr and the weight of the out gassed sample was that used in calculations. The porosity of the catalysts was determined from the desorption curve using Nova enhanced data reduction software (Version 2.13). Highly pure nitrogen was used as adsorbent.

Surface excess oxygen measurement
The surface excess oxygen was measured by the method described in reference [46]. This method consists in reducing the surface excess oxygen on the oxide catalyst surface with hydrazine and measuring the amount of nitrogen gas evolved with the aid of gasometric technique which early described by Deren et al. [47].

Electron microscopy (SEM&TEM)
The size and morphology of the investigated solids were characterized by Scanning electron microscope (SEM) and Transmittance electron microscop (TEM) JEOL Model JSM-5400 LV (Jeol, Tokyo, Japan). For SEM imaging, the samples were prepared by the gold sputtering technique [69]. Micrographs were obtained in a secondary electron imaging mode using a potential difference of 25KV. Representative sections from the surface were recorded and developed on monochrome paper. For TEM imaging, the sample powder dispersed in ethanol using ultrasonic radiation for 20 min and a drop of that suspension was place onto the carbon-coated grids. The degree of magnification of TEM images was the same for all the different investigated solids.

Thermal Analysis of Pure CuC 2 O 4 , NiC 2 O 4 .2H 2 O and Their Mixtures
TG and DTA curves of pure copper oxalate, pure nickel oxalate and their mixtures with different ratios (5-50 wt. %

19
NiO) are shown in Fig. (1). TG curve of pure copper oxalate, Fig. (1a), shows one decomposition step in the range of 270-350 o C with a weight loss of 47.8%. This weight loss is attributed to the decomposition of copper oxalate to copper oxide (CuO) which agrees, approximately, with the theoretical value (47.52%). DTA curve shows a sharp exothermic peak maximized at 325 o C which is again corresponding to the decomposition of copper oxalate to CuO. No other peaks were observed either in TG or DTA on increasing the heating temperature up to 1000 o C which confirm that the produced CuO is thermally stable and does not undergo any phase-phase transitions. These results are in agreement with the results obtained by many authors [16,25,26,46]. From the above results, copper oxalate can decompose to copper oxide according to the following equation [25]: TG and DTA curves of pure nickel oxalate dihydrate NiC 2 O 4 .2H 2 O are shown in Fig. (1g). TG curve exhibits two steps. The first step is in the range of 180-250 o C with a weight loss of ̴ 19.8 % and is accompanied with an endothermic peak on the DTA curve centered at 233 o C. Such process was attributed to the removal of the two crystalline water molecules to give the anhydrous oxalate form. The weight loss ( ῀ 19.8 %) in this step is very closely to the theoretical value (19.7%). The second step in the range of 307-370 o C exhibits a 39.7 % weight loss and is accompanied with a sharp exothermic peak maximized at 354 o C. This stage may correspond to the decomposition of anhydrous NiC 2 O 4 to NiO [50,51]. In addition the weight loss (39.7 %) is in good agreement with the theoretical value (39.9 %) From the above results, nickel oxalate can thermally decompose to NiO via the following equations [50].
TG and DTA curves of CuC 2 O 4 mixed with NiC 2 O 4 .2H 2 O are shown in Fig.1(b-f). The TG and DTA curves show that the thermal decomposition of these samples to give the corresponding oxides proceeds via two stages. The first one observed in the range of 170-250 o C with a weight loss of 3.79, 8.9, 10.4, 12.1 and 17. 2 % for the samples 5, 10, 20, 30 and 50 wt. % NiO, respectively. The samples are also exhibit endothermic peaks appear on the DTA curves minimized at (168,176,185,193 and 200 o C) for NiO loadings of (5, 10, 20, 30 and 50 wt. %) respectively. These peaks attributed to the removal of crystalline water molecules from the samples which originally come from nickel oxalate. The elimination of the crystalline water molecules become more easily as the enhancement observed on the DTA curves as illustrated in the Fig. (1). The second thermal stage occurs in the range of 220-370 o C with weight loss of (49.1, 45.2, 41.3, 39.8 and 37.66 %) is accompanied with exothermic peaks maximized at (320, 317 and 322 o C) for the samples containing 5, 10 and 20 wt. % NiO, respectively. It is observed that, on increasing the ratio of nickel oxalate above (20 wt. % NiO), the exothermic peak is splitted into two peaks, the first peak maximized at (317 and 315 o C) and the second peak maximized at (328 and 332 o C). These two peaks are attributed to the simultaneous decomposition of copper and nickel oxalates to the corresponding oxides.

Fourier Transform Infrared (FT-IR)
Fourier transform infrared (FTIR) analysis of the original and the calcined products of pure copper oxalate, pure nickel oxalate and copper-nickel oxalate mixtures was carried out.  [35,52].   [57][58][59]. The broadness of the absorption band indicates that NiO powder is nanocrystalline [51,60]. The other bands present in the spectra which assigned at the same wave number are explained in the CuO spectra.
The spectra of CuO mixed with NiO and calcined at 400 and 600 o C indicate that no bands are assigned corresponding to the formation of new spinel. The observed broad band assigned at 440-530 cm -1 of the mixed oxides may be attributed to the M-O vibration. In addition, this broadness is a proof of the existence of nano-particles exists in the mixed system [60]. The broad band assigned at 3200-3500 cm -1 in all samples may be attributed to vibration of physically adsorbed water. This band is completely different from coordinated water in compounds [51]. The other band which appeared at 1635 cm -1 is due to the bending vibration of H 2 O. This means that the plain band at 3200-3500 cm -1 and the sharp band at 1635-1640 cm -1 assigned to the stretching and bending vibration of OH group of H 2 O [61].

X-Ray Diffraction (XRD)
The   The diffraction patterns for the copper-nickel oxalate mixtures calcined at 400,500,600 and 700 o C were matched with that identified in the JCPDS cards. The results show that at lower NiO mixing ratio (up to 10 wt. % NiO) the predominant diffraction lines are corresponding to CuO.
Above 10 wt. % NiO, the lines corresponding to NiO start to appear and their intensities increase with increasing the percentage content of NiO up to 50 %. Examination of these patterns illustrate that no new peaks corresponding to the solid-solid interactions were observed between the thermally produced NiO and CuO. These results are in good agreement with those obtained from thermal analysis DTA Fig. (1), which shows that, no peaks observed due to the solid-solid interaction between CuO and NiO. The crystallite size of crystalline phases present in the different investigated samples was calculated from the line broadening of the main diffraction lines of these phases using Scherrer equation [44].
The  It is important to mention here that, the effective increase in crystallite sizes of both NiO and CuO could be attributed to conversion of some NiO to Ni 2 O 3 and also some CuO to Cu 2 O phases via a synergistic mechanism [37,[62][63][64] between the nickel and copper oxides as follows: 2CuO

Nitrogen Adsorption
The adsorption-desorption isotherms of nitrogen gas at liquid nitrogen temperature (-196 o C) were measured for pure CuO, NiO and their mixtures calcined at 400 and 600 o C.
All isotherms (not given) of pure and mixed calcined samples are of type II of Brunauer's classification [65]. In addition, the adsorbents under investigation are characterized by being irreversible and exhibit hysteresis loop close in the low relative pressure range. The hysteresis loop belongs to type H4 with little of type E of de Bore classification [66]. The specific surface area S BET was obtained by applying the BET equation in its normal range of applicability (p/p o =0.05 -0.35) with cross-sectional area of N 2 = 16.2 Å 2 . Another series of specific surface area S t was calculated from the V a-t plots. These plots were constructed using standard t-curves depending on the magnitudes of C-constant in the BET-equation (second column of Tables  (2&3)). The computed S BET values of pure and mixed oxide samples calcined at 400 and 600 o C are cited in Table (2,3). Examination of Tables (2&3), the following conclusions can be drawn: i.
At all calcination temperatures, pure CuO possesses the highest value of S BET compared to pure NiO. The S BET value of pure CuO due to mixing with NiO was found to suffer a progressive decrease to an extent that proportional to the amount of NiO added up to 50 wt.. The decrease was, however, more pronounced with the low ratios of NiO (1-10 wt. %). The S BET value decreased from 25.5 to 13.9 m 2 g -1 on doping of CuO with 5 wt. % NiO for the solids calcined at 400 o C. This reflects that about 45.5 % reduction in the S BET value of pure CuO occurred. Further increase in NiO ratio up to 10 wt. % causes a reduction in the S BET value of pure CuO from 25.5 to 7.8 m 2 g -1 which is equal to 69.4 %. Above these ratios, the S BET values are steadily decreased with increasing the amount of NiO added up to 50 %. This decrease in the S BET value might be discussed in terms of (i) the lower value of S BET of NiO added which consequently reduces the surface area of the CuO, (ii) the continuous decrease in the total pore volume with the increase in the NiO content and (iii) blocking of some pores of the host oxide by NiO additions. This blocking may be attributed to the cationic substitution of CuO by NiO. Furthermore, the decrease in S BET due to NiO additions can be discussed in terms of the particle size.
In fact, X-ray analysis of the samples calcined at different temperatures indicated a significant increase in the particle size of the solids by mixing with NiO. This increase in the particle size of the calcined samples should be accompanied by a significant decrease in the surface areas of the mixed solids. ii.
Raising the calcination temperatures from 400 to 600 o C, resulted in a significant and contineous decrease in the S BET values of pure and mixed oxides. This behavior may be attributed to the high temperature sintering process. iii.
The values of S BET and S t are close to each other which justify the correct choice of t-curve for analysis and indicate the absence of ultramicropores.

Determination of Surface Excess Oxygen
The results of experiments carried out for determining the amount of surface excess oxygen for pure CuO, pure NiO and CuO-NiO mixed oxide systems calcined at 400 and 600 o C are cited in Table (4). Raising the calcination temperatures from 500 to 700 o C, resulted in a significant decrease in the amount of surface excess oxygen for the pure CuO and mixed oxide systems. However, it is well known that, CuO is a nonstoichiometric and p-type semiconductor. According to Jeong and Choi [67], CuO was confirmed to be a metal deficient p-type semiconductor with copper vacancies at low temperatures. On the other hand, adsorption of oxygen and change in electrical conductivity of NiO [68,[69][70][71] have shown that, at room temperature, chemisorption of oxygen occurs in the form of negatively charged species. As regards with the nature of negatively charged adsorbed species, the formation of O -2 , O -, O 2ions has been postulated [68]. It is worth mentioning that, the formation of charged chemisorbed oxygen species is accompanied by the promotion of an appropriate number of cations in the oxide crystal lattice to a higher oxidation state. In the case of NiO, this will be the oxidation of Ni 2+ to Ni 3+ [68], which from the viewpoint of the semiconducting properties of the oxides is equivalent to the formation of positive holes [72]. Therefore, the oxidation power of nonstoichiometric NiO can be plausibly ascribed to the creation of surface Ni 3+ . The formation of chemisorbed Ospecies on NiO has been reported [68] to take place at 150-250 o C, at 250 o C chemisorbed oxygen species was found to predominant as O 2-.
For CuO-NiO mixed oxide systems, a synergism [73,[62][63][64] between the two oxides will be occurred. As a result of the syngestic effect,the decrease in the amount of excess oxygen may be attributed to the synergistic effect. The above equations (4&5) revealed that the addition of NiO to CuO is accompanied with consumption of oxygen released from CuO. Moreover, the observed decrease in the specific surface area on mixing CuO with NiO may correspond to such decrease in the surface excess oxygen. The fact that the changes in the values of surface excess oxygen ran almost parallel to the values of specific surface areas of the various adsorbents. This parallelism might suggest the idea that oxygen is uptaken on top of surface layers of the investigated adsorbents [37,73].

Electron Microscopy (SEM&TEM)
The surface morphological study of the pure CuO, pure NiO and CuO mixed with 30 wt.% of NiO catalysts nano-particles calcined at 400 and 600 o C was carried out using SEM and TEM and the images are shown in Figs. (8&9). Fig. (8) shows the SEM images of the solidss under investigation. From the SEM analysis, one can conclude the formation of aggregates of semi-spherical structure with almost uniform distribution. Inspection of these images revealed that: i.
At all calcination temperatures, Pure CuO catalyst has small aggregates size compared to that of pure NiO and 30 wt. % NiO-CuO solids. ii.
The aggregates size increases with increasing the addition of NiO. iii.
The aggregates size increases on increasing the calcination temperature from 400 to 600 o C, this agglomeration resulted in an increase of the crystallite size of the particles. The transmittance electron microscopic (TEM) photographs of the solids have been given in Fig. (9). The size of nano-particles obtained from TEM images show sizes of 12-20 nm for the pure CuO and NiO catalysts and 25-40 nm for the mixed oxides (30 wt. % NiO) calcined at 400 and 600 o C, respectively. These results are in a close agreement with the results that obtained from the XRD diffraction pattern see Table (1). The main conclusions derived from the results obtained can be summarized as follows: 1. Pure and mixed solids of nano CuO and NiO have been synthesized via oxalic acid rout.

CuO, NiO and their mixtures are thermally stable on
No solid-solid interaction was detected between the NiO and CuO oxides. 3. The addition of NiO to CuO resulted in increasing the particle size and decreasing the surface area and surface excess oxygen over the investigated solids.