Synthesis and catalytic properties of nickel (II) - copper (II) ferrite

This work studies the process of formation of the spinel structure of nickel (II) -copper (II) ferrite. A possible mechanism for the formation of single-phase spinel samples is considered. It consists of the stage of formation of chelate complexes of nickel (II), copper (II), iron (III) cations with citric acid and their subsequent thermal decomposition. The materials obtained are studied by X-ray phase analysis and the BET technique. The catalytic activity of the synthesized ferrite Cu0.5Ni0.5Fe2O4 in the process of oxidative destruction of methyl orange in the presence of hydrogen peroxide is established. It is shown that the process is significantly accelerated with increasing temperature. The activation energy of the reaction was computed. It was found that the activation energy decreases in the presence of a catalyst. The results can be used to obtain materials suitable for industrial wastewater treatment using organic dyes in production cycles.


Introduction
The modern technologies for improving water quality and water purification from organic and inorganic pollutants require the development of new methods for obtaining materials with desired properties. The ferrites of transition elements with a spinel structure can be used for these purposes. Such materials possess polyfunctional properties [1], so the technological features of obtaining ferrites with a spinel structure should be studied. In particular, it is known that ferrites exhibit the properties of magnetic [2,3], dielectric [4] materials.
Nickel and copper ferrites can be used to obtain active catalysts [5,8], including the synthesis of CO from CO2 [8], electrocatalysts for splitting water to produce hydrogen [8], supercapacitor electrodes [9][10][11][12], sensors [13][14][15]; they possess bactericidal properties [1]. The synthesis of such materials is discussed in a vast number of publications, which indicates the unflagging scientific interest. In recent years, special attention of scientists was given to obtaining fine-crystalline materials with a spinel structure. For synthesis, several technological methods are used, such as precipitation and calcination [16,17], hydrothermal synthesis [18][19][20], synthesis during the decomposition of an organic precursor [4,6,21]. Nanosized materials exhibit increased activity in catalytic and adsorption processes requiring structurally sensitive properties.
This work aims to study the processes of structure formation of copper (II) -nickel (II) ferrite with the composition Cu0.5Ni0.5Fe2O4 and its catalytic properties in the process of oxidative destruction of an organic dye.
The phase compositions of the materials were studied using the ARL X'TRA X-ray diffractometer. Monochromatic Cu-Kα radiation was used. The experiments included point scanning (step 0.01°, accumulation time at point 2 s) in the range of 2θ values from 15° to 70°. The qualitative phase composition was determined using PDF-2 [22] in the Crystallographica software package.
The surface area was determined using the ChemiSorb 2750 V. Isotherms of physical nitrogen adsorption were obtained at 77 K. Before measurements, the samples were degassed.
The catalytic activity of the synthesized materials was studied according to the method described in detail in [21]. For this, a model solution of methyl orange (λmax = 465 nm) with a concentration of 40 mg/L was used. 10 ml of the initial organic dye solution was placed in a reaction vessel, 0.0010 -0.0012 g of catalyst, and 10 ml of hydrogen peroxide solution with a concentration of 3 wt% were added. The dye concentration in the solution was determined by the photocolorimetric method using the KFK-2-UHL 4.2 device at certain time intervals. The experiment was carried out at room temperature and with heating.
The degree of catalytic destruction (P) of methyl orange was calculated by the formula (1) : where C0 is the initial solution concentration, mg/L; C is the registered solution concentration, mg/L. The reaction activation energy (Ea) was calculated using the formula (2): where R=8.314 J/(mol·K) is the universal gas constant ; T1 and T2 are the reaction temperatures, K ; 1 and 2 are the reaction durations, min.

Results and Discussion
In a typical procedure, solutions of nickel (II), copper (II), and iron (III) nitrates were used with a concentration of 1 mol/L. For the synthesis, 25 ml of solutions of nickel (II) nitrate and copper (II) nitrate, 50 ml of iron (III) nitrate were taken. Then, aqueous solutions of ammonia NH3·H2O with a concentration of 25 wt% (15 ml) and citric acid C6H8O7 with a concentration of 6.25 mol/L (25 ml) were added successively with vigorous stirring. The citric acid solution was freshly prepared.
The salt solutions were measured with a graduated cylinder and placed in a stainless steel reaction vessel. Next, an aqueous solution of ammonia was added with vigorous stirring, and a colored flocculent precipitate was observed. The citric acid solution was added to the resulting suspension continuing with stirring. A transparent colored solution was obtained, which was heated until the liquid was completely evaporated. Under these conditions a viscous gel-like substance was formed, and it gradually turned into a solid porous material. Upon further heating, an intense decomposition process took place, accompanied by the release of gaseous substances and the glow of the reaction system. Heating was continued until the organic component was completely decomposed.

Fig. 1. Schematic representation of the complexes.
Upon subsequent heating, the complexes decompose with the formation of a dispersed spinel powder Cu0.5Ni0.5Fe2O4. Figure 2 shows the X-ray diffraction pattern of the synthesized material. Lines belonging to the spinel phase are indexed. The data obtained show that the sample is a cubic spinel based on nickel (II) ferrite (PDF Number 000-54-0964).
The samples were black porous materials. The surface area measured by BET method was 8.7 m 2 /g.
The time dependence of the amount of methyl orange that underwent catalytic destruction at a temperature of 60 °C is shown in Figure 3. The rate of destruction of organic matter was determined at different temperatures. The data on the reaction conditions are shown in Table 1. The catalytic effect of the addition of the synthesized material is manifested in a significant decrease of the reaction activation energy.

Conclusion
The process of formation of the spinel structure of nickel (II) -copper (II) ferrite has been studied. A possible mechanism for the formation of single-phase spinel samples was considered. It consists of the stage of formation of chelate complexes of nickel (II), copper (II), iron (III) cations with citric acid and their subsequent thermal decomposition. The catalytic activity of the synthesized ferrite Cu0.5Ni0.5Fe2O4 in the process of oxidative destruction of methyl orange in the presence of hydrogen peroxide was established. It was shown that the process is significantly accelerated with increasing temperature. The reaction activation energy was computed. It was shown that the activation energy decreases in the presence of a catalyst. The results can be used to obtain materials suitable for industrial wastewater treatment using organic dyes in production cycles.